Post on 12-Aug-2020
MScChemistry
ScienceforEnergy&Sustainability
MasterThesis
HeterobinuclearOrganometallicComplexes
AStudyoftheStructure,ElectronicBehaviourandReactivityofNewLow-Valent
Compounds
AnneSofieAbels
10000732
August2018
48ECTS
November2017–August2018
Supervisor/Examiner: Examiner: MScPascalJurt Prof.dr.HansjörgGrützmacher
Prof.dr.BasdeBruin
Dr.ChrisSlootweg
ETHZürich
DepartmentofChemistryandAppliedBiosciences
LaboratoryofInorganicChemistry
TheGrützmacherGroup
1
Acknowledgements
Firstofall,IwouldliketothankProf.Dr.HansjörgGrützmacherfortheopportunitytocarry
out my master thesis in his group. Second, I thank Prof. Dr. Bas de Bruin and Dr. Chris
Slootweg for being my second examiners. Then, a massive thanks goes to my daily
supervisorPascalforallhishelpinthelab,endlessexplanations,encouragingenthusiasmfor
chemistry, enjoyable times in and outside of the lab and of course for showing me the
beauty Switzerlandhas to offer. I am very grateful for the skills and knowledge youhave
taughtmethepastninemonths,whichultimatelyresulted inasuccessfulproject. Iwould
like to thankMatt andGrégoire for their helpwith theNMR experiments and Fabian for
helpingmewiththeXRDmeasurements.DongjunandJanusUrbanus,thankyouforthenice
atmosphereinthelabandLucjethanksforbeingmyNederlandserotsindebranding.Then,
IofcoursewouldliketothanktherestofallthemembersoftheGrützmachergroupforthe
greattimeIhadwiththem,youdefinitelymademystayinZürichunforgettable.Lastbutnot
leastIwouldliketothankmyfamilyandfriendsfortheirloveandsupport,andespeciallymy
parents,whoarealwaysthereformeandgavemetheopportunitytohavethisexperience
abroad.
2
Index
1.INTRODUCTION 4
2.RESULTSANDDISCUSSION 10
2.1SynthesisandCharacterisationoftheTropLigand:PhC≡CTropPPh2 10
2.2SynthesisandCharacterisationoftheRhodiumPhC≡CTropPPh2Dimer 12
2.3HeterobimetallicRh(I)Pt(II)Complexes 152.3.1SynthesisandXRDStudiesofHeterobimetallicRh(I)Pt(II)Complexes 152.3.2NMRStudies 252.3.4ThermogravimetricAnalysis 302.3.3PlatinumInfluence 30
2.4ReactivityofHeterobimetallicRh(I)Pt(II)Complexes 31
2.5HeterobimetallicComplexes–OtherGroup9Metals(Iridium&Cobalt) 37
3.CONCLUSION&OUTLOOK 39
4.EXPERIMENTALSECTION 41
5.APPENDIX 49
ListofAbbreviations 49
6.REFERENCES 50
3
Abstract
Six new heterobimetallic Rh(I)Pt(II) complexes supported by a novel type of chelating
tropPPh2 ligand were synthesised, characterised (figure 1) and tested in nitrous oxide
activation. Heterobinuclear complex 13 proved to be the most efficient homogeneous
catalystforthehydrogenationofthegreenhousegasN2Ouptodate(TON=430),producing
benignwaterandnitrogenundermildreactionconditions.Ligandalterationattherhodium
centre led to fundamental insight into the nature of the bimetallic core: NMR and XRD
studies revealed that cationic compounds contained a shorter metal-metal bond and
stronger 103Rh-195Pt coupling in NMR, while neutral complexes demonstrated a longer
intermetallicdistanceandweaker 1JRhPtcoupling.Furthermore,acorrelationbetween195Pt
satellites in 31P{1H} NMR and the intermetallic bond length was found. In these
heterobimetallic systems, the platinum centre appeared to serve as a supporting ligand,
donatingelectrondensitytotherhodium,whichissuggestedtobeessentialintherhodium
platinumbond.
Figure 1.Novel type of chelating tropPPh2 ligand that enables the coordination of rhodium(I) and
platinum(II). Ligand variation of the rhodium centre gave fundamental insight in the intermetallic
bond.
PPh2
Ph Ph
PPh2
Rh
Ph PhPt
L
+ -X10: L = Cl- 11: L = ACN, X = PF612: L = ACN, X = BArF
13: L = OTf-14: L = µ-Cl, X = BArF
15: L = µ-ClTlCl, X = BArF
8
4
1.Introduction
Nature is a great source of inspiration for the rational design of new catalysts. Since the
presenceofmultimetalliccentresinbiomoleculesarerecognisedtobeofvitalimportancein
enzymatic processes, bimetallic complexes are considered highly interesting for the
activationofdifferentinorganicandorganicsubstrates.[1]Aprominentexampleisagroupof
metalloenzymes that are involved in hydrogen reduction/water oxidation. These so-called
hydrogenasescanbecategoriseddependingontheirmetalactivesitesinmonometallic[Fe],
homobimetallic [FeFe] and heterobimetallic [NiFe] hydrogenases (figure 2). These
metalloenzymes,foundinvariousmicroorganisms,areresponsibleforprovidingenergyand
balancing the redox potential of a cell. In particular, the homobimetallic and
heterobimetallichydrogenasesaremoreactiveintheoxidationofhydrogenandgeneration
ofdihydrogen.[2]
Figure 2. Active site of a) monometallic [Fe] hydrogenase model b) homobimetallic [FeFe]
hydrogenaseandc)heterobimetallic[NiFe]hydrogenase.
Also, in synthetic chemistry there are well-known bimetallic catalysts that show the
importance of catalytic systems containingmetal-metal bonds. The study of intermetallic
bonds has been of great interest for many researchers over half a century,[3] with the
descriptionof the firstmultiplemetal-metalbondsbyProf.F.AlbertCotton in1964.[4]His
work contributed a great deal to the understanding of intermetallic interactions.[5,6]
Although numerous binuclear compounds are described in literature, the catalytic
application of these complexes is not well documented.[7–14] A renowned example of a
bimetalliccatalystistherhodium(II)acetatedimerdiscoveredinthe70’sthatisutilisedasa
carbeneandnitrenetransfercatalystandissuperiortomononuclearanaloguesintermsof
functional group tolerance and catalyst loading (figure 3).[3] Lately, a renewed interest in
Ni Fe
SSCysCys
X
CNCOCN
S
S
Cys
Cys
X: O2-, OH-, OH2, SO, in the reduced form X: H-)
Fe
S
S
N N
COOC Fe Fe
SS
CO
COCN
SCys
OCNC
4Fe-4SNH
a) b) c)
5
bimetallicsystemsresultedintheuseofmetal-metalbondedcompoundsinvariouscatalytic
applications[15]suchashydrogenationreactions[16],smallmoleculeactivation[17,18],coupling
reactions[19]anddiazo-freecyclopropanation.[20]
Figure 3. Schematic representation of rhodium(II) acetate dimer. This dimer is known for carbene
and nitrene transfer reaction and is thought to be one of themost potent catalysts containing a
metal-metalbond.
Thesynthesisandapplicationofhomobimetalliccomplexesaremoreestablishedthan the
preparation of heterobinuclear species, as these compounds, where two different metal
centres are combined within the molecule, are often more challenging to prepare.[21]
Nevertheless the study to heterobimetallic catalysts remains interesting, since the
combinationof twodifferentmetalcentreshasprovento lead tounprecedentedcatalytic
transformations and activities.[8] Most of these heteronuclear transitionmetal complexes
incorporate an early and a late transition metal, bridged by a ligand framework. The
intermetallic bond in these heterobimetallic complexes can be described as a dative
interactionbetweenahard,Lewisacidicearlytransitionmetalormaingroupelementanda
softLewisbasiclatetransitionmetal.[22]ThegroupofLurecentlydescribedaseriesofearly
transition metal-cobalt heterobimetallic complexes capable of nitrogen activation at the
cobaltcentre.Theearly-latemetalsetissupportedbyaheptadentateligandwithchelating
phosphorusandnitrogendonoratoms(figure4).[23]
RhRh
O
O
O
O
OO
OO
6
Figure 4. Structureofearly-lateheterobimetallic complexwhereaheptadentate ligand,containing
phosphorusandnitrogendonoratoms,enforcesthedinuclearM-M’axis.
The intermetallic interaction and the catalytic application of heterobinuclear complexes
bearing two late transitionmetals remain ratherunderexplored.Still, this fieldhasbeena
subjectofsignificantresearchinterest.Forexample,thegroupofChensynthesiseslate-late
heterobimetallic Pd/M (M: Cu, Ag, Au) and Pd(II)/Zn(II) model systems to gain detailed
mechanistic informationon transmetalation reactions inSonogashiraandNegishi coupling
reactions, respectively.[24,25] Furthermore, Nocera and co-workers described a
heterobimetallic complex with a Rh(II)Pt(I) core which is readily oxidised to a Pt(III)Rh(II)
core. Irradiation of the oxidised product promotes the photoelimination of halogen and
regenerates the reducedRh(II)Pt(I) core.Halogen oxidation and halogen photoelimination
chemistry isconsideredpromising for theproductionofdihydrogen fromHXsubstrates to
store solar energy.[26] Additionally, Mankad et al. recently studied a heterobimetallic
ruthenium-silvercompoundforhydrogenactivationandthecatalyticsemi-hydrogenationof
alkynes(figure5).[27]
Figure 5. A ruthenium-silver heterobimetallic complex for the catalytic semi-hydrogenation of
alkynes.
NN
M
P CoPiPr2
NN
PiPr2iPr2
M = V, Cr
N
N
RRH
R
HR
H2 (1atm)Ag-Ru cat. (20%)
xylenes,150 ºC, 24h
N
NAg Ru
COCO
7
Duringthiscatalysisthemetal-metalbondiscleaved,leadingtotwomonometallicspecies.
Toclosethecatalyticcyclethesilverrutheniumbondmustberestored,whichlikelycaused
the high catalyst loading (20%). However, bimetallic cooperation led to the observed cis
selectivity, which only was observed in very few monometallic systems before.[28,29] This
phenomenon of fragmentation during catalysis is common in bimetallic catalysis, since
metal-metalbondsareoftenmorelabilethanorganicbonds.[30]Liganddesigncanbeatool
toavoidbreakingtheintermetallicbondbycreatingasystemthatallowsthecoordinationof
two different metal centres in close proximity, assisting a metal-metal interaction that
remainsintactthroughoutacompletecatalyticcycle.
Inthiscurrentstudy,anewtypeofligandwasdesignedwhichenablesthecoordinationof
two different late transition metals. The newly designed chelating ligand features a trop
(trop = 5-dibenzosuberene) unit, which contains a seven-membered ring, providing the
preparation of robust complexes. A chelating concavely shaped tropPPh2 ligand can be
prepared binding a σ-donor diphenylphosphine to the trop unit, which itself bears a
sterically encumbered double bond that functions as a π-accepting coordination site.[31]
PreviousworkoftheGrützmachergroupshowedthatthisligandallowsstrongcoordination
todifferentlatetransitionmetals,suchasrhodium,palladiumandiridium,andisespecially
valuable for stabilizing low-valent oxidation states.[32–35] Further studies on this tropPPh2
ligandresultedinthedevelopmentofaligand,TMSC≡CTropPPh2,thatisabletocoordinateto
two electron rich group 9 and 10 d8-metals, while leaving open coordination sites at the
metalcentres.Whereasthefirstmetalisboundtothetropmoietyviainteractionwiththe
phosphineandalkene,thesecondmetaliscoordinatedtothealkynemoieties(figure6).[36]
Figure 6. Previous work of the Grützmacher group, where the specially designed TMSC≡CTropPPh2
ligandallowsthecoordinationoftwodifferentlatetransitionmetalsinachelatingfashion.
PPh2
TMS TMS
PPh2
M1Ln
TMS TMSM2Ln
n+Ph2P
TMSTMS
=
M1, M2: Late Transition Metals (group 9 & 10)Ln: Me, Cl, ACN, OTf
8
Inthesynthesisofthisnoveltypeofchelatingligandthealkynegroupswereintroducedbya
cross-coupling Sonogashira reaction. This product required purification by column
chromatography, which involved the use of large amounts of solvents and could not be
scaledupsuccessfully.The last step in this synthesis, the substitutionofa chlorideby the
diphenylphosphine,wasrathertedious,sincedeprotectionofthetrimethylsilyl(TMS)group
canoccurbystronglybasicorproticreactantsorsolvents.Fromligand1asyntheticrouteto
heterobimetallicRh(I)Pt(II)complexeswasdeveloped(figure7).[36]Thesynthesisof2posed
problems, as the isolation and purification required a fine tuned, non-trivial reaction
protocolandpurificationstep.
Figure 7. Previous work of the Grützmacher group: synthesis of heterobimetallic Rh(I)Pt(II)
complexes3and4basedontheTMSC≡CTropPPh2ligand1.
Heterogeneous rhodium-platinum carbonyl catalysts have proven to be active in
hydrogenation reactions of aromatic compounds and CO. In toluene hydrogenation to
methylcyclohexane the presence of platinum appeared to improve the activity of the
catalyst.Howeverbothcatalystsdecomposedafterafewruns.[37]ThehydrogenationofCO
yieldedethyleneglycolandmethanolthatsuggestedthatmixedmetalclusters,whichcould
notbedetermined,wereinvolvedinthecatalyticreactions.[38]
PPh2
TMS TMS
PPh2
Rh
TMSPh2P
Rh
TMSTMS
ClCl
TMS
PPh2
Rh
TMS TMSPt
ClPPh2
Rh
TMS TMSPt
N
+ -PF6
[Rh(C2H4)2Cl]2
DCM
TlPF6
DCM/ACN
[Me2Pt(SMe2)]2, DCM
1 2
4 3
9
The goal of this study was to synthesise a similar chelating trop ligand, allowing the
coordinationoftwodifferentlatetransitionmetals,whileavoidingthepreviouslydescribed
problems.Furthermore, thepreparationofdifferentheterobimetalliccomplexes,basedon
thenewligand,werestudied,usingd8metalsrhodium(I)attheM1positionandplatinum(II)
as M2 (figure 6). Additionally, the coordination of other group 9 metals, iridium(I) and
cobalt(I),tothechelatingtropligandwasinvestigated.Also,theinfluenceofvariousligands
coordinated to theM1metal centre on themetal-metal bondwas explored. Studying the
structureandelectronicpropertiesofthesedifferentheterobinuclearcomplexescouldgive
fundamentalinsightinthisunderdevelopedlatemetal-metalbond.Finally,thereactivityof
this classof compounds towards theactivationof thegreenhousegasnitrousoxide (N2O)
withhydrogenwasstudied.
10
2.ResultsandDiscussion
2.1SynthesisandCharacterisationoftheTropLigand:PhC≡CTropPPh2
Based upon the synthesis of ligand 1 that was described in unpublished results of the
Grützmachergroup,anexperimentalprocedureforthepreparationofanewchelatingtrop
ligand 8 was developed (figure 8).[36] In this new ligand, the TMS groups on the alkyne
moiety were replaced by phenyl groups. First, a Sonogashira coupling was performed to
introducephenylacetyleneinthetropmoietyandbynucleophilicsubstitutionusingthionyl
chloridetheOHgroupisreplacedbyachloride,formingcompound7.Forthelaststepofthe
ligand synthesis, the substitution of the chloride by the phosphine, a different procedure
was applied and optimised to obtain the final product. Diphenylphospine was added to
compound 7 and the reactionmixturewas stirred at 50°Cwhile forming a precipitate of
chlorodiphenylphosphonium salt.UponadditionofNa2CO3 as a base,HClwas eliminated,
leadingtoproduct8witha63%yield.Thisstepdeviates fromthe ligandsynthesisof the
TMS analogue, where n-butyllitihium was used as reagent to deprotonate
diphenylphosphine. The in situ formed lithium diphenylphosphine then replaced the
chloride.However,deprotonationoftheTMSgroupscouldoccurduringthisstep.Theyield
of the synthesis of the PhC≡CTropPPh2 ligand 8was comparable to the TMS analogue and
upscaleofthereactionwaspossible.Therefore,thepreparationofthePhC≡CTropPPh2ligand
wasshowntobemorereliableandscalablecomparedtotheTMSC≡CTropPPh2ligand.Infigure
9 thecrystal structureof thechelating ligand8 isdepicted.This liganddesignenables the
anchoring of twometals using a set of different chelating ligands: phosphine, alkene and
alkynes.
11
Figure8.Three-stepsynthesisof ligand8, includingaSonogashiracouplingto introducethealkyne
moieties in themolecule,yielding6.This is followedbysubstitutionof thealcoholby thechloride
andthefinalproduct8isobtainedbythereactionof7withdiphenylphosphine.
Figure 9.Molecular structureof8.Hydrogenatomsand solventmolecules areomitted for clarity.
Selectedbonddistances(Å):C1-P1:1.890(3),C2=C3:1.373(5),C4≡C5:1.207(5),C6≡C7:1.201(5).
Ph2P
PhPh
HO
PhPh
Br Br
HO
Cl
PhPh
=
PPh2
Ph Ph
PhCCH
5 % Pd0, 5% CuI
Tol/Et3N
HPPh2Na2CO3 (aq)
Tol/n-Hex50ºC, 3 days
SOCl2, pyridineDCM
Yield: 63%
5 6
78
C2
C3P1
C1
C4
C5
C6
C7
12
2.2SynthesisandCharacterisationoftheRhodiumPhC≡CTropPPh2Dimer
Complex 9 was synthesised by reacting ligand 8 with [Rh(C2H4)2Cl]2 in benzene at room
temperature(figure10).Clean9precipitatedfromthereactionmixtureasaredpowderin
good yield. The synthesis of 9 is considerably more convenient compared to theTMSC≡CTropPPh2 trop dimer,where isolation and purification of the product is tedious and
non-scalable.Singlecrystalsofcomplex9wereobtainedbyslowdiffusionofn-hexaneinto
solution of 9 in dichloromethane and analysed by X-ray diffraction. The structure of this
complexisshowninfigure11.Backdonationfromthemetalintotheπ*orbitaloftheolefins
leads to weakening and elongation of the double bond (1.470(3) Å in 9, compared to
1.373(5)Åinfreeligand8).
Figure10.Synthesisofcomplex9fromligand8with[Rh(C2H4)2Cl]2.
Figure11.Molecularstructureof9.Hydrogenatomsandsolventmoleculesareomittedforclarity.
Selectedbonddistances[Å]andangles[°],centroidofC2=C3isindicatedasct1andct1’iscentroid
ofC2’=C3’:Rh1/1’-ct1/1’:2.009(2),Rh1/1’-P1/1’:2.169(6),C2/2’=C3/3’:1.470(3),Rh1-Cl1:2.375(6),
Rh1-Cl1’:2.507(6),ct1-Rh1-P1:92.013(6),ct1-Rh1-Cl1’:100.008(6)Cl1’-Rh1-Cl1:80.76(2)Cl1-Rh1-P1:
87.94(2).
8
[Rh(C2H4)2Cl]2
BenzeneRT, 1 hr
PPh2
Rh
PhPh2P
Rh
PhPh
ClCl
Ph 9
PPh2
Ph Ph
Yield: 79%
Cl1
Rh1
P1
C2C3
C1
Cl1’
Rh1’
P1’
C2’C3’
13
In 1H NMR and 31P NMR spectra of complex 9 broad signals were observed at room
temperature. Line broadening in NMR can be caused by several factors, such as fast
relaxationofthemolecule,thepresenceofaparamagneticspeciesandchemicalexchange.
Cooling and heating experiments of the NMR sample indicated that there is indeed a
dynamicprocess,whichcauses the linebroadening in theNMRspectra.When there isan
equilibriumbetweentwospeciescoolingthesamplecanslowdowntheexchangeandatthe
low-temperaturelimitastaticspectrumcanbeobserved.Heatingthesampleincreasesthe
rateofexchange,whichatthehigh-temperaturelimitleadstothefullyaveragedspectrum.
Betweenthesetwoextremesbroadsignalsareobserved[39].Variabletemperature1HNMR
and 31P NMR spectra from 248 K to 378 K in 1,1,2,2-tetrachloroethane-d2were recorded
(figure12).1HNMRiszoomedinonthesignalofthebenzylicproton.
Figure12revealedthatcompound9waspresentastwospeciesinequilibriuminsolution,
withdifferentRh-Pcouplingsof260Hzand194Hz.NMRstudiesinacoordinatingsolvent,
ACN-d3, resulted in cleavage of the dimer and coordination of the acetonitrile to the
rhodium. Sharp signals were recorded, indicating that there was no chemical exchange.
Basedontheseresultsitisproposedthatthereisanequilibriumbetweenthecis-andtrans-
isomer(figure13).
14
Figure 12. a) Variable temperature 1H NMR and b) 31P NMR of 9 in 1,1,2,2-tetrachloroethane-d2
reveals an equilibrium in chlorinated solvents. Shimming was not carried out before every
experiment,which resulted in broadeningof the signals in someof the experiments. The 1HNMR
spectraarezoomedinonsignalofthebenzylicproton.
4.54.64.74.84.95.0f1 (ppm)
378 K
368 K
358 K
348 K
338 K
328 K
318 K
308 K
298 K
288 K
278 K
268 K
258 K
248 K
a)
102104106108110112114116f1 (ppm)
378 K
368 K
358 K
348 K
338 K
328 K
318 K
308 K
298 K
288 K
278 K
268 K
258 K
248 K
b)
15
Figure13.Equilibriumbetweentrans-isomer9andcis-isomer9ainchlorinatedsolventsledtobroad
signalsinNMRatroomtemperature.
2.3HeterobimetallicRh(I)Pt(II)Complexes
Complex 9 was used as building block for the synthesis of the heterobimetallic species
studied in this thesis. Here we describe the preparation and analysis of these dinuclear
Rh(I)Pt(II)compoundswillbedescribed.
2.3.1SynthesisandXRDStudiesofHeterobimetallicRh(I)Pt(II)Complexes
Thesecondmetal,platinum(II),wasintroducedbyreactingcomplex9with[Me2Pt(μ-SMe2)]2
in dichloromethane to produce complex10. In a one-pot synthesis, complexes11 and12
werepreparedfrom9byabstractingthechloridewithTlPF6orNaBArF,respectively(figure
14).Thissaltmetathesisreactionwascarriedoutinthepresenceofacetonitrile,leadingto
thecoordinationofacetonitriletotherhodiumcentre,withnoncoordinatinganionsPF6and
BArFascounterions.
PPh2
Rh
PhPh2P
Rh
PhPh
ClCl
Ph 9 9a
PPh2
Rh
Ph
Ph2P
Rh
PhPh
ClCl
Ph
16
Figure 14. Synthesis of PhC≡CTropPPh2 heterobimetallic Rh(I)Pt(II) complexes 10, 11 and 12 fromPhC≡CTropPPh2dimer9and[Me2Pt(SMe)]2.
Figure 15. a) Molecular structure of 10. b) Molecular structure of 11 and 12. Hydrogen atoms,
counterionsandsolventmoleculesareomittedforclarity.
PPh2
Rh
Ph PhPt
N
+ -PF6
TlPF6, DCM/ACN NaBArF, DCM/ACN
PPh2
Rh
Ph PhPt
N
+ -BArF
[Me2Pt(SMe2)]2, DCM
PPh2
Rh
Ph PhPt
Cl
PPh2
Rh
PhPh2P
Rh
PhPh
ClCl
Ph
Yield: 58%Yield: 63%
Yield: 85%
9
10
11 12
Rh1 N1
P1
Pt1C7
C8
b)
Rh1
Pt1
P1
C7
C8
Cl1
a)
C1
C2C3
C4
C5
C6
C1
C2
C3
C4
C5
C6
C9
C9
17
Table1.Selectedbonddistances[Å]andangles[°]for10,11and12,wherethecentroidofC2=C3is
indicatedasct1,ct2forC4≡C5andct3forC6≡C7.
Compound 10 11 12
Rh1-ct1 1.953(3) 1.979(3) 1.978(5)
Rh1-P1 2.1850(8) 2.2055(9) 2.1904(12)
C2=C3 1.471(4) 1.467(5) 1.454(6)
Rh1-Cl1/N1 2.3294(7) 2.059(3) 2.02(3)
Rh1-Pt1 2.7657(2) 2.6784(3) 2.6979(4)
C4≡C5 1.227(4) 1.217(5) 1.226(7)
C6≡C7 1.228(4) 1.220(5) 1.225(7)
Pt1-ct2 2.233(3) 2.257(3) 2.272(5)
Pt1-ct3 2.215(3) 2.266(3) 2.247(6)
P1-Rh1-ct1 93.167(8) 93.303(9) 93.524(13)
ct1-Rh1-Pt1 75.701(8) 77.730(10) 77.853(12)
Pt1-Rh1-Cl1/N1 96.34(2) 91.59(8) 92.4(2)
Cl1/N1-Rh1-P1 94.86(3) 97.39(9) 96.1(7)
ct2-Pt1-C8 92.556(12) 94.381(14) 91.708(2)
ct3-Pt1-C9 94.427(11) 93.314(14) 94.128(2)
ct2-Pt1-ct3 93.889(10) 94.747(12) 91.740(18)
C8-Pt1-C9 86.87(13) 87.04(15) 85.9(3)
Rh1-Pt1-ct2 79.832(10) 80.254(9) 79.746(12)
Rh1-Pt1-ct3 78.966(10) 79.815(9) 76.975(12)
Rh1-Pt1-C8 99.80(9) 99.77(11) 101.06(17)
Rh1-Pt1-C9 97.14(9) 98.41(11) 99.69(17)
Thegeometryof10-12canbedescribedasasquareplanargeometryaroundtherhodium
centre, while the geometry around the platinummetal centre can be defined as square
pyramidal,where the rhodium centre occupies the axial position of the square pyramidal
structure(figure15andtable1).Thesignificantlyelongatedolefinbondfor10(1.471(4)Å),
11 (1.467(5)Å)and12 (1.454(6)Å) indicatesstrongbackdonationfromthemetalintothe
anti-bondingorbitalsπ*of theC=Cdoublebond.TheRh-Ptbond is considerably longer in
complex 10 (2.7657(2) Å) than in complexes 11 (2.6784(3) Å) and 12 (2.6979(4) Å).
18
Furthermore,theRh-Cl(2.3294(7)Å)bondissubstantiallylongerthantheRh-N(2.059(3)Å
and 2.02(3) Å) bond. Although the differences in the olefin bond length are rather small
betweencomplexes10-12,itisobservedthatthestrongerπ-donorchlorideresultsinmore
backdonationtoC=Colefintropanti-bondingorbitals,whichindicatesamoreelectronrich
rhodium centre. This interaction can be described as a “push-pull” interaction,where the
olefinacceptselectrondensityintotheπ*anti-orbitaltoreducetheelectrondensityonthe
metal(“pull”).Thisreducesthedestabilisationgeneratedbytheπdonationofthechloride
tothemetal(“push”).Thispush-pullmechanismisknownforsquareplanard8complexes.[40]
Since compounds11 and12 are cationic,while10 is neutral, it is expected that all bond
lengths are shortened in the cationic complexes. This is because the positive charge will
decreaseall orbital energies resulting inbetteroverlapbetween the ligandand themetal
orbitals. Remarkably, this is not observed for the Rh-ct1, whichmay be explained by the
push-pull effect of the chloride compared to the acetonitrile that is solely a σ-donor.
However, the influenceof thepositive charge is strongly reflected in theRh-Pt bond that
indeedbecomesmuchshorterincations11and12comparedtoneutral10.
Furthermore,NMRandcrystallographydataoftheacetonitrileboundcomplexes11and12
wereverysimilartooneanother,whichindicatesthatthecounteriondoesnotinfluencethe
electronicbehaviourandgeometricalfeaturesofthecomplexes.However,BArFconsiderably
increasesthesolubilityofthecomplexinnon-coordinatingsolventscomparedtocompound
11. Furthermore, the use of toxic thallium(I) is avoided in the synthesis of complex 12.
Thereforethesynthesisanduseofcomplex12ismorepreferredcomparedto11.
Consequently, silver triflate was used to abstract the chloride from 10 to bind a weakly
coordinatinganionictriflate,totherhodiumcentre,leadingtocomplex13 (figure16).XRD
studiesrevealedthatthegeometryofcomplex13canbedescribedassquareplanararound
therhodiumcentreandsquarepyramidalaroundtheplatinum(figure17andtable2).The
C=Colefintropbondiselongatedinasimilarfashionasinchloridecomplex10duetoπback
donation (1.473(7)Å in13, 1.471(4)Å in10).Also, themeasuredmetal-metalbondof13
(2.7427(4)Å)israthercomparabletotheRh-Ptbondof10(2.7657(2)Å).Thisdemonstrates
the similar electronic properties for both neutral complexes. Moreover, the metal-metal
bondlengthof13islongerthantheacetonitrilecoordinatedcomplexes11(2.6784(3)Å)and
19
12 (2.6979(4) Å), indicating as well that a neutral complex results in an elongated Rh-Pt
bond.
Figure16.Synthesisofcomplex13from10withAgOTf.
Figure17.Molecularstructureof13.Hydrogenatomsandsolventmoleculesareomittedforclarity.
PPh2
Rh
Ph PhPt
Cl AgOTf
THF
PPh2
Rh
Ph PhPt
OTf
Yield: 69%10 13
P1
Rh1
Pt1
C2
C3
C1
C4
C5
C6
C7
O1
20
Table2.Selectedbonddistances[Å]andbondangles[°]of10and13,wherethecentroidofC2=C3is
indicatedasct1,ct2forC4≡C5andct3forC6≡C7.
Compound 10 13
Rh1-ct1 1.953(3) 1.954(5)
Rh1-P1 2.1850(8) 2.1964(14)
C2=C3 1.471(4) 1.473(7)
Rh1-Cl1/O1 2.3294(7) 2.087(4)
Rh1-Pt1 2.7657(2) 2.7427(4)
C4≡C5 1.227(4) 1.231(7)
C6≡C7 1.228(4) 1.217(8)
Pt1-ct2 2.233(3) 2.235(5)
Pt1-ct3 2.215(3) 2.266(5)
P1-Rh1-ct1 93.167(8) 93.133(14)
ct1-Rh1-Pt1 75.701(8) 76.964(14)
Pt1-Rh1-Cl1/O1 96.34(2) 89.87(10)
Cl1/N1-Rh1-P1 94.86(3) 100.04(11)
ct2-Pt1-C8 92.556(12) 94.085(2)
ct3-Pt1-C9 94.427(11) 91.781(2)
ct2-Pt1-ct3 93.889(10) 91.040(18)
C8-Pt1-C9 86.87(13) 85.1(2)
Rh1-Pt-ct2 79.832(10) 77.785(13)
Rh1-Pt1-ct3 78.966(10) 78.050(14)
Rh1-Pt1-C8 99.80(9) 100.50(16)
Rh1-Pt1-C9 97.14(9) 100.16(15)
Then, the preparation of a T-shaped rhodium complex, with a non-coordinating anion,
leadingtoopencoordinationsitecistotheM-M’bondwasattempted(figure18).SuchaT-
shaped bimetallic complexes is expected to show enhanced reactivity due to the readily
availableopencoordinationsite,asisdescribedformonometallicRh(I)analogues.[41]
21
Figure18.AttemptedpreparationofT-shapedrhodiumcomplexwithanopencoordinationsitecisto
theRh-Ptaxis.NaBArFandTlBArFwereusedaschloridescavengers.
Thereactionof10withNaBArFintoluenedidnotresultinaT-shapedrhodium,butonlyone
chloridecouldbeabstractedanddimerisationofthecomplexwasobserved,leadingto14.
When using TlBArF, another chloride scavenger, to remove the chloride bridged chloride-
thallium-chloride 15 complex was formed (figure 19). This indicated that the chloride is
stronglycoordinatedtotherhodiumcentre.Singlecrystalsof14and15wereanalysedby
XRD (figure 20). These dimeric structures can be as well described as square planar d8
aroundrhodiumandsquarepyramidalfortheplatinumcentre(table3).
Figure19.Synthesisofdimericcomplexes14and15from10,usingdifferentBArFsaltsinattemptto
abstract thechloride from the rhodiumcentre. For15 noyield is reported, since the reactionwas
onlyperformedattestreactionscale.
PPh2
Rh
Ph PhPt
Cl
10
Na/Tl BArF
Toluene
PPh2
Rh
Ph PhPt
NaBArF, Toluene TlBArF, Benzene
PPh2
Rh
Ph PhPt
Cl
PPh2
Rh
Ph PhPt
P
Rh
PhPhPt
Cl
+ -BArFPh2 PPh2
Rh
Ph PhPt
P
Rh
PhPhPt
+ -BArFPh2
Cl ClTl
10
14 15Yield: 38%
22
Figure20.a)Molecularstructureof14.b)Molecularstructureof15.Hydrogenatoms,counterions
andsolventmoleculesareomittedforclarity.
P1
Rh1
Pt1C9
C8
C5
C4
C2C3
C6
C7
C1
Cl1
Pt2
Rh2
P2
a)
Rh1
Pt1
P1
C3
C4
C1
Cl1
Tl1Cl2
Rh2
Pt2
P2
b)
23
Table3.Selectedbonddistances[Å]andbondangles[°]forterminalchloride10,14and15,where
thecentroidofC2=C3isindicatedasct1,ct2forC4≡C5andct3forC6≡C7.
Compound 10 14 15
Rh1-C2=C3 1.953(3) 1.958(5) 1.964(4)
Rh1-P1 2.1850(8) 2.178(13) 2.183(11)
C2=C3 1.471(4) 1.456(7) 1.460(4)
Rh1-Cl1 2.3294(7) 2.403(11) 2.340(10)
Rh1-Pt1 2.7657(2) 2.788(4) 2.750(3)
C4-C5 1.227(4) 1.233(8) 1.227(6)
C6-C7 1.228(4) 1.212(8) 1.225(6)
Pt1-C4-C5 2.233(3) 2.217(6) 2.241(4)
Pt1-C6-C7 2.215(3) 2.256(6) 2.245(4)
P1-Rh1-ct1 93.167(8) 92.933(14) 94.079(11)
ct1-Rh1-Pt1 75.701(8) 76.049(13) 76.140(11)
Pt1-Rh1-Cl1 96.34(2) 97.60(3) 97.60(3)
Cl1-Rh1-P1 94.86(3) 92.85(4) 92.15(4)
ct2-Pt1-C8 92.556(12) 92.847(2) 93.439(19)
ct3-Pt1-C9 94.427(11) 92.816(2) 93.459(18)
ct2-Pt1-ct3 93.889(10) 91.042(18) 92.685(15)
C8-Pt1-C9 86.87(13) 85.5(2) 85.8(2)
Rh1-Pt-C4C5 79.832(10) 76.798(13) 79.407(11)
Rh1-Pt1-C6C7 78.966(10) 79.123(14) 79.066(11)
Rh1-Pt1-C8 99.80(9) 97.816(13) 98.605(14)
Rh1-Pt1-C9 97.14(9) 104.653(16) 99.161(13)
Comparison of bridged compound 15 and terminal chloride complex 10 revealed that 15
matched the trend in the electronic behaviour established previously, inwhich a cationic
compound led to a shorter intermetallic bond. However, 15 can be considered an
intermediate case, since the cationic charge is divided over two bridged molecules.
Therefore,themetal-metalbond isonlyslightlyshorter in15 (2.750(3)Å)comparedto10
(2.7657(2)Å).Sincethesamechlorideorbital isresponsibleforπdonationtotherhodium
centreandσdonationtothethalliumcentrein15,thereislesselectrondensitypresenton
24
therhodiumcentre,resultinginlessbackdonationandashorterolefinbond(1.460(4)Åin
15 compared to 1.471(4) in10). In14 theπ donationof the chloride is dividedonto two
metalcentres,whichwasdemonstratedinacomparableolefinbondlengthas15(1.456(7)
Å).However, theRh-Ptbondwas considerably longer in cationic complex14 (2.788(4)Å),
whichdidnotfittheelectronictrend.Therefore,thiselongatedintermetallicinteractionwas
explainedasastericeffectuptoformationofthedimer.
A survey of the CCDC crystallographic database 1 concerning all reported Rh-Pt bond
distances (214 rhodium-platinumbonds) reveals that themeasuredRh-Pt bond lengths in
this study (2.698 Å – 2.788 Å) are in range with described data in literature. Figure 22a
displays an example of a similar rhodium-platinum heterobimetallic compound with an
intermetallicbonddistanceof2.750(1)Å.[42]Thisreportedvalueiscomparablewiththedata
foundinthisstudy.AnothersimilarheterobimetallicRh-Ptcomplexreportedalongermetal-
metal bond distance (2.949(2) Å) that is explained due to the bridging hydride (figure
22b).[43]
Figure21.PlotofthesurveyresultoftheCCDCcrystallographydatabaseregardingallreportedRh-Pt
bond lengths in Å (X-axis). The Y-axis represents the number of structures for the given bond
distances.TherangeofmeasuredRh-Ptbonddistancesinthisstudyisindicatedinred.
1CSDConquest2018version5.39,accessed14August2018.
25
Figure22.a)HeterobimetalliccomplexwithaRh-Ptbonddistanceof2.750(1)Åb)heterobimetallic
compoundwithaRh-Ptbondlengthof2.949(2)Å.
2.3.2NMRStudies
Figure23.Synthesisedligand8,rhodiumdimer9andheterobimetallicRh(I)Pt(II)complexes10-15.
Table4summarisesselected1HNMRdatafortheligand8,rhodiumdimerprecursor9and
theheterobimetalliccomplexes10-15,thataredisplayedinfigure23.Thebenzylicprotonis
a distinctive feature of this trop system.Up to coordination to a rhodium centre the 2JPH
couplinggetsstronger.Additionally,theprotonsofthemethylgroupsontheplatinumare
observedasasingletaround1.29-1.46ppm,showingthecharacteristic195Ptsatellites.NMR
spectraofcompound14and15showedbroadsignals.Sincethesearedimericstructuresas
well, this could indicate a chemical exchange. The samples were cooled down, which
resulted in a static spectra where two species were observed, demonstrating indeed a
dynamic process.Moreover, sharp signals were observed when NMR spectra of 14 were
recorded in ACN-d3, therefore it was assumed that an equilibrium between the cis- and
Pt RhCO
CO
COCp
F
FF
F
F
F
F F
F
F
a)
Pt RhSi
Ph Ph
H
Si
Me3PPMe3
PMe3
PMe3
Ph
Ph
Cl
b)
H
PPh2
Ph Ph
8
PPh2
Rh
PhPh2P
Rh
PhPh
ClCl
Ph9
PPh2
Rh
Ph PhPt
N
+ -PF6 PPh2
Rh
Ph PhPt
N
+ -BArFPPh2
Rh
Ph PhPt
Cl
10 11 12
PPh2
Rh
Ph PhPt
P
Rh
PhPhPt
Cl
+ -BArFPh2
PPh2
Rh
Ph PhPt
P
Rh
PhPhPt
+ -BArFPh2
Cl ClTl
14 15
PPh2
Rh
Ph PhPt
OTf
13
26
trans-isomer, as seen for 9 (vide supra, figure 13), is responsible for the observed broad
signals in NMRin case of 14 and 15. Although the uncoupled signal from the protons of
methyl groups on the platinumwas sharp, broad signals for the 195Pt satellites were still
observed.Thiscanbeexplainedby195Ptrelaxationeffectsonthemethyl1Hlineshapes.[44]
Therefore,accuratedeterminationofthe2JPtHofcompound14and15wasnotpossible.As
the 1H NMR data was not conclusive about the electronic behaviour of these
heterobimetalliccompounds, furtherNMRstudies including31P, 103Rhand195PtNMRwere
performedtogainmoreinsightintheelectronicbehaviourofthisbinuclearsystem.
Table4.Selected1HNMRspectroscopicdataofcompounds8-15indifferentdeuteratedsolvents.All
datawereobtainedat298Kunlessotherwisestated.
Compound Solvent δHbenzylic
(ppm)
2JPH(Hz)/2JRhH(Hz)
δPt(CH3)2
(ppm)
2JPtH(Hz)
8 CDCl3 4.70 5.7 - -
9 ACN-d3 5.10 14.9 - -
10 THF-d8 4.92 14.4 1.46 92.4
11 CD2Cl2 4.98 15.4/2.3 1.35 87.7
12 CD2Cl2 4.94 15.3/2.4 1.35 87.9
13 THF-d8 5.19 14.8/2.8 1.46 90.1
14[a] THF-d8 4.69 14.3/-[b] 1.42 -[b]
15 THF-d8 5.29 14.4/2.4 1.29 -[b][a]Measuredat193K[b]Broadsignal
In 31P{1H}NMRdata a large change in chemical shiftwas observed upon coordination of
rhodium to diphenylphosphine (table 5). The chemical shift range (88-93 ppm) is
characteristic for these rhodium tropPPh2 complexes.[34,45,46] Introduction of the platinum
didonlyleadtominorchangesofthechemicalshift.Since195PtisalsoanNMRactivenuclei
platinumsatelliteswith33.7%intensityofthe31P{1H}signalcouldbedetected.[44]NMRdata
showed that complexes11 and12have theweakestphosphorus-rhodiumcoupling (197.9
Hzforbothcompounds),whiletheJPPtisstronger(537.4Hzand537.9Hz,respectively).For
theothercomplexesappliedthatiftheJPRhcouplingisstronger,theJPPtisweaker.
27
Table5.Selected31P{1H}NMRspectroscopicdataofcompounds8-15.Chemicalshifts(δ)aregivenin
ppmandcouplingsconstants(J)inHz.
Compound Solvent δ(ppm) 1JPRh(Hz) 2JPPt(Hz)
8 CDCl3 -12.95 - -
9 ACN-d3 93.70 186.5 -
10 THF-d8 89.88 217.5 519.9
11 CD2Cl2 91.40 197.9 537.4
12 C6D6 91.26 197.9 537.9
13 THF-d8 91.60 211.8 519.0
14 THF-d8 88.76 215.6 505.7
15 THF-d8 90.12 218.9 513.6
Additionally,otherNMRactivenuclei,195Ptand103Rh,weremeasuredtostudytheelectronic
behaviouroftheheterobimetalliccomplexes(table6and7).Complex15wasnotstudiedby195Pt and 103Rh NMR, since there was no sufficient amount of compound produced to
measure195Ptand103RhNMRspectra.Especially195PtNMRgivesvaluableinformationabout
the Rh(I)-Pt(II) bond, since the coupling between these two nuclei can bemeasured. The195Pt spectrawereacquiredwith 1Hdecoupling, as this leads to less crowded spectra. For
compound10theweakestPt-Rhcouplingwasmeasured(83.4Hz),whereasforcompounds
11 and12 strongermetal-metal couplingsweredetermined (104.1Hz and109.1Hz). The1JRhPtcouplingforcompound14couldnotbedetected,becauseoffastrelaxationofthe195Pt
nuclei,whichleadstolinebroadening.Asdescribedintheliterature,arelevantcontribution
to the relaxation of heavy nuclei, such as 195Pt, is the chemical shift anisotropy (CSA)
mechanism.[44]Since195Pthasalargechemicalshiftrange,theCSAeffectislargeraswell.[47]
Potentially, this iswhy the 1JRhPtfor14 could not be observed. In literature, one reported1JRhPtof24Hzfora[PtRh5(CO)15]-clusterwasfound,whichisconsiderablyweakerthanthe
measured couplings here.[48] However, the cluster is hardly comparable to the bimetallic
complexespresentedhere,sinceoxidationstateandligandsphereareverydifferent.
28
Table6.195PtNMRspectroscopicdataofcomplex10-15,withexceptionof14.Chemicalshifts(δ)are
giveninppmandcouplingsconstants(J)inHz.
Compound Solvent δ(ppm) 1JPtP(Hz) 2JPtRh(Hz)
10 THF-d8 -3274 530.1 83.4
11 CD2Cl2 -3223 546.7 104.1
12 CD2Cl2 -3196 538.0 109.3
13 THF-d8 -3263 519.4 94.5
14 THF-d8 -3321 502.5 -[a]
[a]Broadsignal
Although103Rhhasanaturalabundanceof100%,directmeasuringof103RhNMRisdifficult,
sinceithasalow(negative)gyromagneticratio,31.8timessmallerthan1H.Therefore,103Rh
NMRisoftendetected indirectlybyheteronuclearcorrelation,where1H iscorrelatedwith103Rh,whichenhances the sensitivityof the rhodiumnucleus[49].Here, 103Rhwasdetected
usingheteronuclearmultiple-quantumcoherence(HMQC).Asforthedatafromthe31P{1H}
spectra,aweakerRh-Pcouplingfortheacetonitrileboundcomplexes11and12,compared
tocomplexes10,13and15,whereananionic ligand isboundtotherhodiumcentre,was
observed.
Table7. 103RhNMRspectroscopicdataofcomplex9-15,withexceptionof14.Chemicalshifts (δ)are
giveninppmandcouplingsconstants(J)inHz.
Compound Solvent δ(ppm) 1JRhP(Hz) 2JRhH(Hz)
9 CDCl3 -6926 237.2 13.5
10 THF-d8 -7045 215.2 13.8
11 CD2Cl2 -7207 196.2 15.0
12 CD2Cl2 -7304 200.3 15.2
13 THF-d8 -6737 210.6 14.6
14 THF-d8 -7491 209.1 14.9
In table 8 NMR and XRD data of the heterobimetallic Rh(I)Pt(II) complexes 10-15 were
compared. It was observed that a shorter metal-metal bond length corresponded to a
29
strongerJRhPtcoupling.Forexample,complexes11and12 includedtheshortestRh-Ptbond
length (2.678Åand2.698Å, respectively)and thestrongestRh-Ptcoupling (104.1Hzand
109.3Hz, respectively),whereasa longerM-M’bond lengthof2.766Åandaweaker JRhPt
coupling(83.4Hz)wasmeasuredfor10.Furthermore,the2JPPtmeasuredin31P{1H}NMRwas
related to the Rh-Pt bond lengths, where a shorter metal-metal bond correlated to a
stronger JPPt coupling (graph 1). Thus, information about the intermetallic distance could
alreadyberetrievedfromanaccessible31P{1H}spectrum.
Table8.SelectedXRDandNMRspectroscopicdataofcomplexes10-15.
Compound dRh-Pt(Å) 1JPtRh(Hz) 2JPPt(Hz)
10 2.766 83.4 519.9
11 2.678 104.1 537.4
12 2.698 109.3 537.9
13 2.743 94.5 519.0
14 2.788 - 505.7
15 2.750 - 513.6
Graph1.CorrelationbetweenP-Ptcoupling(2JPPtinHz)andRh-Ptbondlength(dRhPtinÅ).
y=-294x+1330R²=0.89
490
500
510
520
530
540
550
2.65 2.70 2.75 2.80 2.85
2 JPP
t(Hz
)
dRhPt(Å)
30
2.3.4ThermogravimetricAnalysis
Thermogravimetric analysis (TGA) of compound12 was conducted. Themass changewas
measuredoveraperiodoftime,whilechangingthetemperature.Fromtheseexperiments
moreinformationabouttheformationofaT-shapedrhodiumcompoundcouldberetrieved.
At 171°C there was amass loss of 41 g/mol detected, which corresponds to acetonitrile
(figure24).Afterwards,itwastestedtoremovetheacetonitrilethermallybyheatingatthis
temperature.However,thisledtodecompositionofthecompound.
Figure24.TGAofcomplex12.Bluecurve:thermogravimetry(%),greencurve:differentialscanning
calorimetry,DSC,(μV/mg),pinkcurve:quasimultipleiondetection,QMID,(A).At171°Camassloss
wasdetectedthroughadecreaseintheDSC,whichcorrelatedtothelossofacetonitriledetermined
withQMID.
2.3.3PlatinumInfluence
The coordination environment of the platinummetal was altered to study the electronic
effectoftheplatinumcentreontheintermetallicbond.Methylgroupsarestrongσdonors.
Created with NETZSCH Proteus software
-1.5
-1.0
-0.5
0.0
0.5
DSC /(µV/mg)
50
60
70
80
90
100
TG /%
50 100 150 200 250 300 350 400 450Temperature /°C
2
4
6
8
10
12
QMID *10-12 /A
Main 2018-07-30 16:34 User: abelsa
[1]
s:1, m:41 [2]
[1]
↓ exo
31
Thereforedonationof their electrondensity to theplatinumcentre results in an electron
richmetal.Here, themethyl groupson theplatinumwereprotonatedwith a strong acid,
trifluoroacetic acid to remove themethyl groups and investigate the influence of these σ
donors on the platinum centre (figure 25). In 31P{1H}NMR loss of platinumwas observed
uponprotonation,asthePtsatelliteswerelost,andultimatelycompletedecompositionof
the compound was observed. This could indicate that electron density on the platinum,
donated by the σ donor methyl groups, is essential for the rhodium-platinum bond, by
electrondonationfromtheplatinumtotherhodiumandsuggeststhattheplatinumactsas
aligandtotherhodiumcentre.
Figure25.Protonationofthemethylgroupsontheplatinumbytrifluoroaceticacidledtolossofthe
platinummetal.
2.4ReactivityofHeterobimetallicRh(I)Pt(II)Complexes
Nitrousoxide isagreenhousegaswithawarmingpotential that isabout300timesbigger
thanCO2.[50]Anthropogenicemissionsofnitrousoxide,mainlyduetolarge-scaleapplication
offertilizers,arecausinganincreaseofthelevelofN2Ointheatmosphere.[51]AlthoughN2O
is thermodynamically unstable it is kinetically inert,[52] which necessitates the use of a
catalyst to activate themolecule and convert it to harmlessmolecules. The conversionof
N2Ointonitrogenandwatercouldbeconsideredhighlyinterestingforreducingthenitrous
oxidelevelsinatmospherethatareinvolvedinclimatechangeandozonedepletion(figure
26).
PPh2
Rh
Ph PhPt
N
+ -BArF
F3C OH
O
C6D6
PPh2
Rh
Ph PhPt
N
+ -BArF
Me+ CH4
+ CF3COO-
12 12a
32
Figure26.Schematicoverviewofnitrousoxideemissionsintheatmosphere.Naturalemissionsare
responsiblefor64%ofthenitrousoxideemissions.30%isemittedduetodenitrificationofthesoil,
whereas6%isreleasedintheatmosphereduetoindustrialprocesses,suchasthesynthesisofadipic
acid and nitric acid. Nitrous oxide is a greenhouse gas and related to climate change and ozone
depletion.CatalyticconversionofN2Otoharmlessnitrogenandwatermoleculescould reduce the
levelsinatmosphere.–FigurereprintedwithpermissionfromProf.Gianetti,UniversityofArizona.
N2O isknowntobeaunreactiveandpoor ligand for transitionmetals.[53] InnatureN2O is
activated and reduced in a cooperative fashion to nitrogen by the enzyme nitrous oxide
reductaseata tetranuclear copper sitebinding toadjacent copper centres (figure27a).[54]
Also in a heterogeneous catalytic system, Cu-ZSM-5, N2O is activated by metal-metal
cooperationoftwoadjacentcoppersites(figure27b).[55,56]
Figure27.ActivationofN2Obya)tetranuclearcoppersiteinnitrousoxidereductaseandb)adjacent
coppersitesinCu-ZSM-5.
Cu CuCu Cu
SNHis
NHis NHis
NHisNHis
NHis
NHis
ONN
HNHis
HNLys
Cu CuON N
OSiOAl
OAl OSi
a) b)
33
The development of homogeneous catalytic systems for the hydrogenation of N2O is of
interest,sincethiscouldgivemechanisticunderstanding inthis reaction,which isvaluable
forrationalcatalyticdesign.Recently,Milsteinreportedahomogeneouslycatalysedreaction
of N2O with hydrogen by a PNP ruthenium complex (figure 28), using relatively high
pressures of the gasses, 3 bar ofN2O and 4 bar ofH2, to producewater andnitrogen.[57]
Furthermore, a rhodium(I) carbene complex featuring a trop system was described in
previous work of the Grützmacher group to perform dehydrogenation of alcohols with
nitrous oxide as a hydrogen acceptor.[58] To our knowledge, there is no bimetallic system
identifiedforthisreaction.
Figure 28.HydrogenationofnitrousoxidehomogeneouslycatalysedbyaPNPrutheniumcomplex,
generatingwaterandnitrogenunder3barN2Oand4barH2at65°Cfor48hours.ATONof417was
measuredbasedupontheformationofwater.
In this work the activity of the different Rh(I)Pt(II) complexes (10-14) was systematically
tested for this reaction (figure 29). A solution of the catalyst in benzene or THF was
pressurisedwithonebarN2Oandhydrogen.Afterstirringthereactionmixturefor22hours
atroomtemperature,theturnovernumber(TON)wasdeterminedby1HNMRspectrumof
the reaction solution using mesitylene as an internal standard (figure 30). The results of
theseexperimentsare listed in table9.Acontrolexperimentwithout thepresenceof the
catalystwasperformedandshowednoformationofH2O.
THF, 65º, 48 hTON up to 417
N2O + H23 bar 4 bar H2O + N2
RuN
P
PH
H
CO
P: P(iPr)2
34
Figure29.Thereactivitytowardsthehydrogenationofnitrousoxideissystematicallytestedforthe
synthesized heterobimetallic complexes 10-14 in different solvents at room temperature for 22
hours.
Figure30.1HNMRspectrumofreactionsolutionafterthehydrogenationofnitrousoxideby13.The
amountofH2OformedandtheTONcanbecalculated,usingmesityleneasaninternalstandard.
PPh2
Rh
Ph PhPt
L
+ -X
Solvent, RT, 22 h
N2O + H21 bar 1 bar H2O + N2
10: L = Cl- 11: L = ACN, X = PF612: L = ACN, X = BArF
13: L = OTf-14: L = µ-Cl, X = BArF
1.52.02.53.03.5f1 (ppm)
H2O
Mesitylene
THF THF
35
Table9. OverviewoftheTONsbasedontheamountofH2Oformedafterhydrogenationofnitrous
oxidewithcomplexes10-14indifferentsolvents.
Rh(I)Pt(II)catalyst Solvent TONbasedonH2O
10 THF 11
11 THF Noreaction
12 THF 11
12 Benzene 47
13 THF 62
14 Benzene 6
Compound11 was not able to perform the activation of N2Owith H2, since the complex
formedahighly reactivespeciesunderhydrogenatmosphere,which led to intramolecular
follow-up reactions.Ultimately this resulted indecompositionof thecompound.Since the
non-coordinating counterion BArF increased the solubility in non-coordinating solvents
considerably,thecatalysisreactionsusingcomplexes12and14wereperformedinbenzene.
Thisenhancedthereactivityofthecatalyst12andaTONof54wasobtained.Furthermore,
a side reactionwas observed for12 and14 in benzene,whichwas the hydrogenation of
benzene to cyclohexane. A TON of 9 was calculated for 12 and 14 for 14. This reaction
seemed to compete with the hydrogenation of nitrous oxide and is quite remarkable,
considering the fact that very mild reaction conditions were used.[59] As the other
compounds were not soluble in benzene, the reactions were carried out in THF.Weakly
coordinatingtriflatecomplex13wasshowntobesuperiortoallothercomplexes(TON:62)
13. This could indicate that coordination of nitrous oxide to the complex is necessary.
However,furtherstudiesarenecessarytogainmoreinsightintheactivationofN2OandH2
on the complex. 31P{1H}NMRof the reaction solution revealed that the complexwas still
intactaftercatalysis.
Inanattempttooptimisethereactionconditions,thereactiontimewasprolongedwhile13
wasusedasacatalyst(figure31).ThisresultedinaTONof430basedontheformationof
H2O,whichisthebestperforminghomogeneouscatalystfortheactivationofN2OwithH2at
thismomenttoourknowledge,evenunderremarkablymildreactionconditions.
36
Figure31.Homogeneouslycatalysedhydrogenationofnitrousoxidebycomplex13gavethehighest
TONforthisreaction.Thiscomplexcanbeconsideredasthebestperforminghomogeneouscatalyst
forthisreactionatthismoment.
Althoughnoevidenceispresentyet,acatalyticcyclecouldbeenvisioned,asshowninfigure
32. As in other similar studies on nitrous oxide activation,[57,58] hydrogen activation is
proposed to take place first to afford complexB. The bondingmode of hydrogen on the
metalcomplexwasnotdeterminedyet.ThisstepisfollowedbyanoxygentransferfromN2O
togivecomplexC,andthereleaseofwaterandcoordinationoftheligandcouldregenerate
A. However, thorough investigation on the catalytic mechanism is required to present a
proposalofthegenuinereactionsequenceofthehydrogenationofnitrousoxide.
Figure 32. Envisioned catalytic mechanism for the hydrogenation of nitrous oxide by Rh(I)Pt(II)
complexesinvestigatedinthiswork.
PPh2
Rh
Ph Ph
Pt
OTf
THF, RT, 3 daysTON: 430
N2O + H21 bar 1 bar H2O + N2
H2
H2O
N2O
N2
L
L
RhPt=
PPh2Rh
Ph PhPt
LRhPtRhPt
RhPt
L
H2
H2O
B
CA
37
2.5HeterobimetallicComplexes–OtherGroup9Metals(Iridium&Cobalt)
Besides the synthesis and analysis of Rh(I)Pt(II) dinuclear compounds, the preparation of
other group 9 metal bimetallic complexes was studied. First, the synthesis of iridium
complexes using the TMSC≡CTropPPh2 ligand and different iridium dimer precursors were
examined (figure 33). However, the synthesis of these complexeswas not successful and31P{1H}NMRdatashowedmultiplesignals,whichcouldnotbeexplained,butindicatedthat
iridiumcomplexeswerenotformed.
Figure33.Attemptedsynthesisofhomobimetalliciridium(I)TMSC≡CTropPPh2complexes.
Then,thepreparationofcobalt(I)complexeswasstudied,butthehypothesizedcomplexes
werenotobtainedandNMRdatagavenoconclusionabouttheformedspecies(figure34).It
couldbearguedthatformationofbothcomplexeswasnotpossible,sincereplacingthePPh3
groups by the trop ligand is not favourable. The synthesis of homobimetallic iridium
1 eq [Ir(C2H4)2Cl]2
Benzene
Ph2P
TMSTMS
PPh2
Ir
TMS TMSIr
Ph2P
Ir
TMSTMSIrCl
ClCl
Cl
1 eq [Ir(C2H4)2Cl]2 AgOTf
DCM
Ph2P
TMSTMS
O
OS
PPh2
Ir
TMS TMSIr
O
OSOCF3
CF3
O
1 eq [Ir(COE)2Cl]2
Benzene
Ph2P
TMSTMS
PPh2
Ir
TMS TMSIr
Ph2P
Ir
TMSTMSIrCl
ClCl
Cl
38
PhC≡CTropPPh2complexeswasalsoinvestigated(figure35).However,thesecomplexescould
notbesynthesizedsuccessfully.Apossibleexplanationfortheunsuccessfulpreparationof
theiridiumTMSC≡CTropPPh2andPhC≡CTropPPh2complexesisthattheinsertionofiridiuminthis
chelatingligandisstericallyandelectronicallynotfeasible.
Figure34.Attemptedsynthesisofhomobimetalliccobalt(I)TMSC≡CTropPPh2complexes.
Figure35.Attemptedsynthesisofhomobimetalliciridium(I)PhC≡CTropPPh2complexes.
2 eq [Co(PPh3)2Cl]
THF
Ph2P
TMSTMS
PPh2
Co
TMS TMSCo
Ph2P
Co
TMSTMSCoCl
ClCl
Cl
1 eq [Co(PPh3)2Cl]
THF
Ph2P
TMSTMS
PPh2
Co
Ph Ph
Cl
PPh3
1 eq [Ir(C2H4)2Cl]2
CD2Cl2
Ph2P
PhPh
PPh2
Ir
Ph PhIr
Ph2P
Ir
PhPhIrCl
ClCl
Cl
1 eq [Ir(C2H4)2Cl]2 AgOTf
ACN
Ph2P
PhPh
O
OS
PPh2
Ir
Ph PhIr
O
OSOCF3
CF3
O
39
3.Conclusion&Outlook
Inconclusion,sixnewheterobimetallicRh(I)Pt(II)withanoveltypeofchelatingligandwere
synthesisedandcharacterised.Severalofthesecompoundswerefoundtobeactive inthe
homogeneously catalysed hydrogenation of nitrous oxide under mild conditions, yielding
water and nitrogen. The highest TON was achieved using catalyst 13, which is to our
knowledge the best performing catalyst for this reaction at this moment. This reaction
catalysed by a heterobimetallic Rh(I)Pt(II) complex offers a green, efficient and mild
procedurefortheremovalofthegreenhousegasnitrousoxide.
Furthermore,NMRandXRDstudiesrevealedthat longerRh-Ptbondlengthscorresponded
toaweakerRh-PtcouplinginNMR,whileashortermetal-metalbondlengthisrelatedtoa
strongercoupling.Additionally,alongermetal-metalbondwasinagreementwithaweaker2JPPtmeasuredin31P{1H}NMRandviceversa,whichimpliesthatinformationabouttheRh-Pt
bondlengthcouldalreadyberetrievedfromanaccessible31P{1H}spectrum.
In general, a shorter rhodium-platinum bond was observed for the cationic bimetallic
complexescomparedtotheneutralcompounds.Exceptionfromthiseffectwascomplex14,
wherethelongestRh-Ptbondlengthwasmeasured(2.788(4)Å),whichcouldbeexplained
by steric reasons upon formation of the dimer. Moreover, further studies on the metal-
metal bond suggested that an electron rich Pt(II) is essential for the Rh-Pt bond, which
indicatedthattheplatinummostlikelyactasanelectrondonatingsupportingmetalloligand.
Furtherresearch isrequiredtofindoutwhetheraheterobimetallicsystemissuperiortoa
homo(bi)metallicsystem intheactivationofN2OwithH2.Thiscouldthenbetheevidence
that the presence of two different metal centres indeed enhances the catalytic activity.
Moreover,additionalstudiesarenecessarytogainmoreinsightinthecatalyticmechanism
ofthisreaction.StoichiometricreactionsofthecomplexeswithN2O,H2,H2OandN2could
give insight into the activation of the substrates and the steps in the catalytic cycle.
Additionally, the reaction set-up should be improved in order to determine the precise
amount of gasses used and produced. Furthermore, the internal standard should be
40
calibrated. These actions should then lead to amore accurate determination of the TON
basedontheamountofH2O,determinedby1HNMRandN2,determinedbyGC.Besides,a
homogeneitytestisnecessaryinordertoruleoutthepossibilitythatnanoparticlesandnot
theheterobimetalliccomplexcatalysethehydrogenationofnitrousoxide.Scanningelectron
microscope (SEM) images of the reaction mixture after catalysis should give information
aboutanynanoparticleformation.
41
4.ExperimentalSection
General techniques:Allexperimentswereperformedunderan inertatmosphereofargon
using standard Schlenk and glove-box (argon atmosphere) techniques unless otherwise
specified.DCM,THF,toluene,diethylether,n-hexaneandacetonitrilewerepurifiedusingan
InnovativeTechnologiesPureSolvsystem.DeuteratedsolventswerepurchasedfromEuriso-
Top,degassedanddistilledfromtheproperdryingagent,andstoredovermolecularsieves.
Chemicals:BasicchemicalswereorderedatABCR,Acros,Aldrich,Fluka,LancasterorSTREM.
The following organic compounds and metal precursors were prepared by literature
methods:Br2TropOH[60],[Rh(C2H4)2Cl]2[61],[(Me)2Pt(SMe2)]2[62].PhC≡CTropCl7wassynthesised
accordingtoearlierresultsofthehostgroup.
IRspectrawererecordedonaPerkin-Elmer-Spectrum2000FT-IR-Ramanspectrometer.The
absorptionbandsaredescribedasfollows:strong(s),verystrong(vs),middle(m),weak(w),
orbroad(br).
NMRspectrawererecordedonBrukerAvance200,300,400and500MHzspectrometersat
298 K (if not indicated differently). Chemical shifts (d) are reported in ppm. The absolute
valuesofthecouplingconstants(J)aregiveninHertz(Hz).Multiplicitiesareabbreviatedas
singlet(s),doublet(d),triplet(t),quartet(q)andbroad(br).Spectrawerereferencedwith
externalstandards:for1Hand13CNMRwithTMS,for19FNMRwithCFCl3,for31PNMRwith
H3PO4,for103RhNMRwithRh(acac)3andfor195PtNMRwithK2PtCl6.Quaternarycarbonsare
indicated as Cquat, aromatic carbons as CHarand protons as Har. The benzylic protons and
carbon atoms in the central seven-membered ring are indicated as Hbenz and CHbenz,
respectively.
X-raySinglecrystalssuitableforX-raydiffractionwerecoatedwithpolyisobutyleneoilunder
inertatmosphere,transferredtoanylonloopandthentransferredtothegoniometerofa
BrukerX8APEX2orD8-VenturediffractometeroronanOxfordExcaliburequippedwitha
molybdenumX-ray tube (λ=0.71073Å).Preliminarydatawas collected todetermine the
crystalsystem.ThespacegroupwasidentifiedandthedatawereprocessedusingtheBruker
SAINT+ program and corrected for absorption using SADABS. The structures were solved
usingdirectmethods(SHELXS)onOLEX2completedbyFouriersynthesisandrefinedbyfull-
matrixleast-squaresprocedures.
42
10,11-Di-(phenyl)acetylene-5H-dibenzo[a,d]cyclohepten-5-diphosphine:PhC≡CTropPPh2(8)
MF:C43H29P
MW:576.66PhC≡CTropCl(2.84g,6.64mmol)wasdissolvedintoluene(30mL)andn-hexane(15mL)and
diphenylphosine(1.2mL,1.2g,6.64mmol,)wasaddedtotheclearorangebrownsolution.
The reactionmixturewas stirred at 50°C over theweekend,while yellow precipitatewas
formed.Thesolutionmixturewasheated to reflux for30minutesandafter itwascooled
downdegassedNa2CO3(12mL,10%inH2O)wasadded.Thesolutionwasheatedat50°Cfor
3hoursbefore itwasheatedunderrefluxfor30minutes.Aftercoolingdown,thetoluene
phasewas separated from the aqueous phase and this phasewas extractedwith toluene
twice(2x10mL).Thesolventwasevaporatedandthebrown/orangeoilwastreatedwithn-
hexane. The brown/orange powder was stirred vigorously in acetonitrile (24 mL) and a
yellowpowderwasprecipitatedoutofthesolution.Theproductwasfiltrated,washedwith
hexane(12mL)anddriedundervacuum.Thefiltratewasplacedinthefreezerovernightand
after filtrationasecondbatchof theyellowproductwasobtained,whichwasdriedunder
vacuum.Yield:2.4g(63%).Yellowcrystalswereobtainedbyslowdiffusionofn-hexaneina
solutionof8indiethylether.
1HNMR(300MHz,CDCl3):δ(ppm)4.70(d,2JPH=5.7Hz,1H,Hbenz),6.89(d,3JHH=7.4Hz,2H,
Har),7.01-7.15(m,10H,Har),7.22-7.29(m,10H,Har),7.55-7.59(m,4H,Har),7.82(d,3JHH
=7.5Hz,2H,Har).–13CNMR(75.5MHz,CDCl3):55.41(d,1JCP=21.32Hz,1C,CHbenz),90.99
(s,2C,C≡CPh,Cquat),96.73(s,2C,C≡CPh,Cquat),122.73(s,2C,Cquat),125.73(d,4JCP=1.4
Hz,2C,CHar),126.86(d,3JCP=6.5Hz,4C,CHar),127.33(s,4C,CHar),127.43(d,4JCP=7.4Hz,
2C,CHar),128.10(d,3JCP=3.6Hz,2C,CHar),128.15(s,4C,CHar),129.13(d,4JCP=1.7Hz,2C,
CHar),129.21(d,4JCP=6.3Hz,2C,Colefintrop),130.74(s,4C,CHar),132.54(d,2JCP=19.7Hz,4
C,CHar),133.63(d,3JCP=4.4Hz,2C,Cquat),137.05(d,1JCP=19.6Hz,2C,Cquat),149.99(d,2JCP
=8.9Hz,2C,Cquat).–31PNMR(121.5MHz,CDCl3):δ(ppm)-12.95(s,1P,PPh2).
43
[(PhC≡CTropPPh2)2Rh2(μ-Cl)2](9)
MF:C86H58Cl2P2Rh2
MW:1430.04
[Rh2(μ-Cl)2(C2H4)4](344.9mg,0.89mmol)and8(852.4mg,1.48mmol)werecombinedina
flaskandstirredinbenzene(10mL)for3hoursatroomtemperature.Thesolutionmixture
was placed in a fridge overnight. The orange product was filtrated and washed with n-
hexane(5mL)anddriedinvacuo.Thefiltratewaslayeredwithn-hexaneandasecondbatch
of the product was obtained after recrystallization. Yield: 1000.6 mg (79%). Red crystals
wereattainedbyslowdiffusionofn-hexaneinasolutionof9indichloromethane.
1HNMR(300MHz,CD3CN):δ(ppm)5.10(dd,2JPH=14.9Hz,2JRhH=1.7Hz,1H,Hbenz),7.03(d,3JHH=7.4Hz,2H,Har),7.23–7.30(m,10H,Har),7.35–7.43(m,10H,Har),7.63–7.71(m,4
H,Har),8.56(d,3JHH=7.5Hz,2H,Har).–13CNMR(75.5MHz,CD3CN):δ(ppm)50.57(d,1JCP=
26.1Hz,1C,CHbenz),54.13(m,2C,Colefintrop),94.52(s,2C,C≡CPh),95.71(s,2C,C≡CPh),
125.35(s,2C,Cquat),128.27(s,2C,CHar),128.65(d,3JCP=10.9Hz,4C,CHar),129.25(s,2C,
CHar),129.78(s,4C,CHar),130.19(s,2C,CHar),130.31(d,4JCP=6.4Hz,4C,CHar),131.10(d,1JCP=52.6Hz,2C,Cquat),131.56(d,3JCP=2.5Hz,2C,CHar),131.82(s,4C,CHar),135.20(d,2JCP=9.7Hz,4C,CHar),136.91(d,3JCP=4.5Hz,2C,Cquat),137.80(dd,2JCP=7.7Hz,2C,Cquat).
–31PNMR(121.5MHz,CD3CN):δ(ppm)93.70(d,1JPRh=186.5Hz,1P,tropP).–103RhNMR
(15.8MHz,CD3Cl3):δ (ppm) -6926(d, 1JRhP=237.2Hz).– IR (ATR):3052(w,νCH),3017(w,
νCH),2992(w,νCH),1479(s),1434(s),1260(m),1097(m),1024(m),791(m).
[(PhC≡CTropPPh2)Rh(Cl)Pt(CH3)2](10)
MF:C45H35ClPPtRh
MW:940.17
9 (500.0 mg, 0.35 mmol) and [(Me)2Pt(SMe2)]2 (241.1 mg, 0.42 mmol) were stirred in
dichloromethane(20mL)for3hours.Afterremovalofthesolventtheproductwaswashed
withn-hexaneanddriedinvacuo.Yield:558.7mg(85%).Redcrystalsweregrownbyslow
diffusionofn-hexaneinasolutionof10indichloromethane.
1HNMR(300MHz,CD2Cl2):δ(ppm)1.46(s,2JPtH=92.4Hz,6H,Pt(CH3)2),4.92(d,2JPH=14.4
Hz,1H,Hbenz),7.04(d,3JHH=7.1Hz,2H,Har),7.19–7.42(m,10H,Har),7.45–7.53(m,6H,
44
Har),7.70–7.76(m,4H,Har),7.90–7.93(m,4H,Har),7.13(d,3JHH=7.5Hz,2H,Har).–13C
NMR (75.5 MHz, THF-d8): δ (ppm) 12.08 (s, 2 C, Pt(CH3)2), 40.13 (d, 1JCRh = 2.2 Hz, 1 C,
Colefintrop),40.31(d,1JCRh=2.1Hz,1C,Colefintrop),49.52(d,1JCP=27.2Hz,1C,CHbenz),91.41(s,
2C,C≡CPh),122.89(s,2C,Cquat),126.73(s,2C,C≡CPh),126.96(s,2C,CHar),127.15(d,3JCP
=10.4Hz,4C,CHar),129.25(s,2C,CHar),128.26(s,4C,CHar),128.70(s,2C,CHar),129.11(d,
4JCP=6.5Hz,2C,CHar),129.79(d,4JCP=2.4Hz,4C,CHar),130.53(d,1JCP=46.8Hz,2C,Cquat),
131.56(d,3JCP=2.5Hz,2C,CHar),131.52(s,4C,CHar),133.58(d,2JCP=10.2Hz,4C,CHar),
136.32(d,3JCP=7.2Hz,2C,Cquat),135.39(m,2C,Cquat).–31PNMR(121.5MHz,THF-d8):δ
(ppm)89.88(d,1JPRh=217.5Hz,2JPPt=519.9Hz,1P,tropP).–103RhNMR(15.8MHz,THF-d8):
δ(ppm)-7045(d,1JPRh=215.2Hz).–195PtNMR(64.4MHz,THF-d8):δ(ppm)-3274(dd,2JPtP=
530.1Hz,1JPtRh=83.4Hz)
[(PhC≡CTropPPh2)Rh(CH3CN)Pt(CH3)2]PF6(11):
MF:C47H38F6NP2PtRh
MW:1090.73
9(100mg,0.07mmol)and[(Me)2Pt(SMe2)]2(40.2mg,0.07mmol)werecombinedinaflask
andflushedwithdichloromethane(5mL).Thereactionmixturewasstirredforonehourat
roomtemperature,beforeasolutionofTlPF6(48.9mg,0.14mmol)inacetonitrile(5mL)was
added. After one hour the mixture was filtrated over Celite and washed with
dichloromethane(5mL).Thesolventwasevaporatedandtheredproductwaswashedwith
n-hexane (10mL)anddried invacuo.Yield:96.6mg (63%).Redcrystalswereobtainedby
slowdiffusionofn-hexaneinasolutionof11indichloromethane.
1H NMR (300MHz, CD2Cl2): δ (ppm) 1.35 (s, 2JPtH = 92.4 Hz, 6 H, Pt(CH3)2), 2.14 (s, 3 H,
CH3CN),4.98(dd,2JPH=15.4Hz,2JRhH=2.3Hz,1H,Hbenz),7.13(d,3JHH=7.2Hz,2H,Har),7.29
–7.40(m,10H,Har),7.46–7.62(m,10H,Har),7.89-7.93(m,4H,Har),8.14(d,3JHH=7.3Hz,
2H,Har).–13CNMR(75.5MHz,CD2Cl2):δ(ppm)3.40(s,1C,CH3CN),10.09(s,2C,Pt(CH3)2),
50.43 (d, 1JCP= 27.2Hz, 1 C, CHbenz), 71.79 (d, 1JRhH = 4.0Hz, 2 C, Colefintrop), 89.56 (s, 2 C,
C≡CPh), 95.77 (s, 2 C, C≡CPh), 121.84 (s, 2 C, Cquat), 128.38 (s, 2 C, CHar), 128.88 (s, 1 C,
CH3CN),129.06(d,1JCP=47.1Hz,2C,Cquat),129.37(d,3JCP=10.5Hz,4C,CHar),129.54(s,4
C,CHar),129.91(s,2C,CHar),130.15(d,4JCP=6.7Hz,2C,CHar),131.13(s,4C,CHar),132.34
(d,3JCP=2.6Hz,2C,CHar),132.82(s,4C,CHar),132.97(d,3JCP=7.2Hz,2C,Cquat),133.35(d,
45
2JCP=10.2Hz,4C,CHar),135.70(m,2C,Cquat).–31PNMR(121.5MHz,CD2Cl2):δ(ppm)91.40
(d,1JPRh=197.9Hz,2JPPt=537.4Hz,1P,tropP),-144.45(hept,1JFP=708.5Hz,1P,PF6).–19F
NMR(188.3MHz,CD2Cl2):δ(ppm)-72.92(d,1JFP=710Hz,1P,PF6).–103RhNMR(15.8MHz,
CD2Cl2):δ(ppm)-7207(d,1JPRh=196.2Hz).–195PtNMR(64.4MHz,CD2Cl2):δ(ppm)-3223
(dd,2JPtP=546.7Hz,1JPtRh=104.1Hz).–IR(ATR):3052(w,νCH),2959(m,νCH),2900(w,νCH),
1480(s),1434(s),1260(m),1095(m),1024(m),791(m).
[(PhC≡CTropPPh2)Rh(CH3CN)Pt(CH3)2]BArF(12)
MF:C79H50BF24NPPtRh
MW:1808.98
9 (100.0mg, 0.07mmol) and [(Me)2Pt(SMe2)]2 (36.2mg, 0.06mmol)were combined in a
flask and flushedwith dichloromethane (5mL). The reactionmixturewas stirred for one
houratroomtemperature,beforeasolutionofNaBArF(123.9mg,0.14mmol)inacetonitrile
(5mL)was added. After one hour themixturewas filtrated over Celite andwashedwith
dichloromethane(5mL).Thesolventwasevaporatedandtheredproductwaswashedwith
n-hexane(10mL)anddriedinvacuo.Yield:145.9mg(58%).Redcrystalswereobtainedby
slowdiffusionofn-hexaneinasolutionof12intoluene.
1HNMR(300MHz,CD2Cl2):δ(ppm)1.35(s,2JPtH=87.9Hz,6H,Pt(CH3)2),2.12(s,3H,CH3CN),
4.94(dd,2JPH=15.3Hz,2JRhH=2.4Hz,1H,Hbenz),7.10(d,3JHH=7.3Hz,2H,Har),7.29–7.42
(m,10H,Har),7.42–7.52(m,6H,Har),7.56(brs,4H,BArF),7.58–7.67(m,4H,Har),7.72
(brs,8H,BArF),7.88–7.95(m,4H,Har),8.15(d,3JHH=7.6Hz,2H,Har).–13CNMR(75.5
MHz,CD2Cl2):δ (ppm)3.29 (s,1C,CH3CN),9.97 (s,2C,Pt(CH3)2),45.63 (d,2C,Colefintrop),
50.80(d,1JCP=27.3Hz,1C,CHbenz),95.93(s,2C,C≡CPh),117.86(s,BArF),121.71(s,2C,
Cquat),123.17(s,2C,C≡CPh),126.78(s,1C,CH3CN),128.38(s,2C,CHar),128.66(q,BArF),
128.70(d,1JCP=39.7Hz,2C,Cquat),129.34(d,3JCP=10.6Hz,4C,CHar),129.55(s,4C,CHar),
129.99(s,2C,CHar),130.25(d,4JCP=6.7Hz,2C,CHar),130.89(q,BArF),131.22(s,4C,CHar),
132.35(d,2C,CHar),132.81(s,4C,CHar),132.94(d,3JCP=8.6Hz,2C,Cquat),133.29(d,2JCP=
10.5Hz,4C,CHar),135.18(s,BArF),135.50(d,2C,Cquat),162.13(q,1JBC=49.7Hz,BArF).–31P
NMR(121.5MHz,C6D6):δ(ppm)91.26(d,1JPRH=197.9Hz,2JPPt=537.9.1Hz,1P,tropP).–19FNMR(188.3MHz,C6D6):δ(ppm)-62.83(s,24F,BArF).–11BNMR(C6D6):δ(ppm)-6.60(s,
46
1B,BArF).–103RhNMR(15.8MHz,CD2Cl2):δ(ppm)-7304(d,1JPRh=200.3Hz).–195PtNMR
(64.4MHz,CD2Cl2):δ(ppm)-3196(dd,2JPtP=538.0Hz,1JPtRh=109.3Hz).
[(PhC≡CTropPPh2)Rh(OTf)Pt(CH3)2(13)
MF:C47H39F3O3PPtRhS
MW:1069.84
10(50.0mg,0.053mmol)wasstirredwithAgOTf(13.7mg,0.053mmol)inTHF(3mL)for3
hours.Theprecipitateofsilver(I)chloridewasremovedbyfiltrationoverCelite.Thefiltrate
solution was layered with n-hexane and after recrystallization orange crystals were
obtained.Thesolutionwasremovedandtheproductwaswashedwithn-hexaneanddried
invacuo.Yield:37.0mg(69%).
1HNMR(300MHz,THF-d8):δ(ppm)1.47(d,2JPtH=90.1Hz,6H,Pt(CH3)2),5.14(d,2JPH=14.3
Hz,1H,Hbenz),7.12(d,3JHH=7.6Hz,2H,Har),7.19–7.39(m,10H,Har),7.50–7.57(m,6H,
Har),7.69–7.73(m,4H,Har),7.96–8.02(m,4H,Har),8.22(d,3JHH=7.8Hz,2H,Har).-13C
NMR (75.5 MHz, THF-d8): δ (ppm) 10.11 (s, 2 C, Pt(CH3)2), 45.27 (d, 1JCRh = 2.1 Hz, 2 C,
Colefintrop),50.41(d,1JCP=28.9Hz,1C,CHbenz),91.95(s,2C,C≡CPh),94.66(s,2C,C≡CPh),
123.71(s,2C,Cquat),128.48(s,2C,CHar),128.67(s,2C,CHar),128.95(d,3JCP=10.4Hz,4C,
CHar),128.99(d,4JCP=3.0Hz,4C,CHar),129.26(s,2C,CHar),129.83(d,1JCP=46.6Hz,2C,
Cquat),129.97(s,4C,CHar),130.83(d,4JCP=6.4Hz,2C,CHar),130.92(s,2C,CHar),131.96(d,3JCP=2.7Hz,2C,CHar),133.31(s,4C,CHar),134.95(d,2JCP=10.1Hz,4C,CHar),137.19(d,JPH
=2.8Hz,2C,Cquat).–31PNMR(121.5MHz,THF-d8):δ(ppm)91.60(d,1JPRH=211.8Hz,2JPPt=
519.0Hz,1P, tropP).– 19FNMR(188.3MHz,THF-d8):δ (ppm)-78.4 (s,OTf).– 103RhNMR
(15.8MHz,THF-d8):δ(ppm)-6737(d,1JPRh=210.6Hz).–195PtNMR(64.4MHz,THF-d8):δ
(ppm)-3263(dd,2JPtP=519.4Hz,1JPtRh=94.5Hz).
[(PhC≡CTropPPh2)2Rh2Pt2((CH3)2)2(μ-Cl)]BArF(14)
MF:C122H82BClF24P2Pt2Rh2
MW:2708.09
7(100.0mg,0.11mmol)wasstirredwithNaBArF(47.1mg,0.053mmol)inTHF(5mL).After
1 hour, the reactionmixturewas filtrated over Celite. The solvent was removed and the
47
product was washed with n-hexane and dried under vacuum. Yield: 79.9 mg (38%). Red
crystalsweregrownbyslowdiffusionofn-hexaneinasolutionof9intoluene.
1HNMR(300MHz,THF-d8):δ(ppm)1.47(s,6H,Pt(CH3)2),5.28(d,2JPH=14.6Hz,1H,Hbenz),
7.12–7.38(m,12H,Har),7.48–7.53(m,6H,Har),7.57(brs,4H,BArF),7.79(brs,8H,BArF),
7.80–7.85(m,4H,Har),7.92–8.00(m,4H,Har),8.16(d,3JHH=7.8Hz,2H,Har).–13CNMR
(75.5MHz,THF-d8):δ(ppm)10.12(s,2C,Pt(CH3)2),48.09(d,2C,Colefintrop),49.48(d,1JCP=
27.7Hz,1C,CHbenz),91.54(s,2C,C≡CPh),92.49(s,2C,C≡CPh),117.20(s,BArF),121.29(s,
2C,Cquat),125.62(s,2C,CHar),127.50(d,1JCP=38.7Hz,2C,Cquat),127.85(d,3JCP=10.9Hz,4
C,CHar),129.55(s,4C,CHar),128.77(s,4C,CHar),128.92(q,BArF),129.17(q,BArF),129.48
(s,4C,CHar),129.56(d,4JCP=6.8Hz,2C,CHar),130.60(d,2C,CHar),132.02(s,4C,CHar),
133.79 (d, 2JCP = 10.0 Hz, 4 C, CHar), 132.24 (d, 3JCP = 4.6 Hz, 2 C, Cquat), 134.62 (s, BArF),
135.58(d, 3JCP=2.7Hz,2C,Cquat),161.84(q, 1JBC=49.6Hz,BArF).–31P-NMR(121.5MHz,
THF-d8): δ (ppm) 88.76 (d, 1JPRh= 215.6Hz, 2JPPt= 505.7Hz, 1 P, tropP). – 19F-NMR (188.3
MHz,THF-d8):δ(ppm)-62.87(s,24F,BArF).–11BNMR(THF-d8):δ(ppm)-6.46(s,1B,BArF).
–103RhNMR(15.8MHz,THF-d8):δ(ppm)-7491(d,1JPRh=209.1Hz).–195PtNMR(64.4MHz,
THF-d8):δ(ppm)-3321(br).
[(PhC≡CTropPPh2)2Rh2Pt2((CH3)2)2(μ-ClTlCl)]BArF(15)
MF:C122H82BCl2F24P2Pt2Rh2Tl
MW:2947.94
7 (8.8 mg, 0.0094 mmol) and TlBArF (10.3 mg, 0.0096 mmol) were combined in 1 mL
benzeneandstirredfor1hour.ThereactionmixturewasfilteredoverCeliteandthefiltrate
waslayeredwithn-hexane.Slowdiffusionresultedincrystallisationoftheorangeproduct.
1HNMR(300MHz,THF-d8):δ(ppm)1.50(s,6H,Pt(CH3)2),5.28(d,2JPH=14.6Hz,1H,Hbenz),
7.17(d,3JHH=6.3Hz,2H,Har),7.19–7.41(m,10H,Har),7.48–7.51(m,6H,Har),7.57(brs,
4H,BArF),7.78(brs,8H,BArF),7.83–7.91(m,4H,Har),7.95(m,4H,Har),8.14(d,3JHH=7.8
Hz,2H,Har).–31PNMR(121.5MHz,THF-d8):δ(ppm)90.12(d,1JPRh=218.9Hz,2JPPt=513.6
Hz,1P, tropP). 13C, 195Ptand 103RhNMRwerenotmeasured, since therewasnotenough
compoundtomeasureadecentspectrum.
48
CatalytichydrogenationofN2O
InascrewcapSchlenckwereaddedcompoundX,mesityleneandTHF/benzene(2mL)inthe
quantities described in table x. After degassing the solution by three freeze pump thaw
cyclestheflaskwaschargedwith1barN2O.Thereactionsolutionwasfrozenand1barofH2
wasadded.Afterstirringfor22hoursatroomtemperaturethesolutionwasdegassedand
the amount H2O and TON were determined by 1H NMR of the reaction solution using
mesityleneasinternalstandard(table10).
Table10.Quantitiesofcomplex10-14andmesityleneusedinthecatalytichydrogenationofnitrous
oxide in different solvents. The amount of H2O and TON were determined using mesitylene as
internalstandard.
CompoundX Quantitycompound
(mg,μmol)
Quantitymesitylene
(mg,μmol)
QuantityH2O
(μmol)
TONbased
onH2O
10inTHF 7.5mg,8.0μmol 8.4mg,69.9μmol 85.2μmol 11
11inTHF - - - Noreaction
12inTHF 11.4mg,6.3μmol 7.8mg,64.9μmol 65.2μmol 11
12inBenzene 10.5mg,5.8μmol 8.5mg,70.7μmol 269.2μmol 47
13inTHF 8.1mg,7.6μmol 10.6mg,88.2μmol 464.2μmol 62
14inBenzene 8.7mg,3.2μmol 9.6mg,79.9μmol 21.0μmol 6
13inTHF,3
days
5.2mg,4.9μmol 10.0mg,83.2μmol 2080.0μmol 430
49
5.Appendix
ListofAbbreviations
ACN Acetonitrile
BArF Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
Cu-ZSM-5 CopperZeoliteSoconyMobil–5
DCM Dichloromethane
DSC DifferentialScanningCalorimetry
HMQC Heteronuclearmultiple-quantumcoherence
Me Methyl
n-Hex n-Hexane
NMR Nuclearmagneticresonance
OTf Triflate
Ph Phenyl
QMID Quasimultipleiondetection
RT Roomtemperature
SEM Scanningelectronmicroscope
TGA Thermogravimetricanalysis
THF Tetrahydrofuran
TMS Trimethylsilyl
Tol Toluene
TON Turnovernumber
Trop 5-dibenzosuberene
XRD X-raydiffraction
50
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