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Journal of Organometallic Chemistry 700 (2012) 41e47

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Journal of Organometallic Chemistry

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Reductive NO dimerization to trans-hyponitrite in diruthenium complexes:Intramolecular attack of hyponitrite on a CO ligand

Tobias Mayer, Peter Mayer, Hans-Christian Böttcher*

Department of Chemistry, Ludwig Maximilian University, Butenandtstraße 5�13, 81377 Munich, Germany

a r t i c l e i n f o

Article history:Received 23 September 2011Received in revised form12 November 2011Accepted 15 November 2011

Dedicated to Prof. Max Herberhold on theoccasion of his 75th birthday

Keywords:RutheniumHyponitrito ligandsNOR model complexesCrystal structure

* Corresponding author. Tel.: þ49 89218077422; faE-mail address: hans.boettcher@cup.uni-muenche

0022-328X/$ e see front matter � 2011 Elsevier B.V.doi:10.1016/j.jorganchem.2011.11.012

a b s t r a c t

The reaction of the electronically and coordinatively unsaturated compounds [Ru2(CO)4(m-H)(m-PBut2)(m-

L2)] (1) (Ru]Ru) with nitric oxide afforded four new trans-hyponitrito complexes of the general formula[Ru2(CO)4(m-H)(m-PBut

2)(m-L2)(m-h2-ONNO)] (2) in good yields. Thus the complexes 2aee were obtainedusing the following bridging biphosphane ligands (m-L2): dppm (2a, dppm ¼ Ph2PCH2PPh2), dppen (2b,dppen ¼ Ph2PC(]CH2)PPh2), dpppha (2c, dpppha ¼ Ph2PN(Ph)PPh2), dpppra (2d, dpppra ¼ Ph2PN(Pr

n)PPh2), and dppbza (2e, dppbza ¼ Ph2PN(CH2Ph)PPh2). The molecular structures of the complexes 2aeewere determined by X-ray diffraction. Furthermore the molecular structure of the known compound 2awas reinvestigated in light of a better refinement of the short intramolecular contact between thehyponitrite O atom in close vicinity to the C atom of a neighbouring carbonyl group. This unusual effect incrystals of all compounds 2aee can be interpreted as a nucleophilic intramolecular attack of a hyponitritoligand towards an electrophilic carbon center of a carbonyl ligand.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Some years ago we reported the synthesis and the X-ray crystalstructure analysis of the unusual trans-hyponitrito complex[Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNO)] (2a, dppm ¼Ph2PCH2PPh2) which was formed by a reductive dimerization ofnitric oxide in the coordination sphere of the unsaturatedcompound [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)] (1a) [1]. Solid 2aexhibited a very close intramolecular contact between one C atomof a carbonyl group and the non-coordinating O atom of the trans-hyponitrito ligand. We could show that this contact is broken inreactions with electrophiles such as Hþ (from HBF4), and CH3

þ (from[Me3O][BF4]), affording the new complex salts [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNOX)][BF4] (X ¼ H and X ¼ CH3) [2]. Thecomplex [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNOH)]þ (3)reacted in solution at elevated temperatures under loss of nitrousoxide to the complex [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-OH)]þ (4)[2]. Moreover, we reported that [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNOCH3)]þ can be deprotonated by the base DBU(DBU ¼ 1.8-diazabicyclo[5.4.0]undec-7-ene) at the metalemetalbond affording a neutral complex containing a deprotonated

x: þ49 89218077407.n.de (H.-C. Böttcher).

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monoester of the hyponitrous acid as the ligand, namely[Ru2(CO)4(m-PBut2)(m-dppm)(m-h2-ONNOCH3)] [3]. The compound2a belongs to the still little explored class of complexes containingthe hyponitrito ligand, N2O2

2�, and two reviews on the restrictedfield of complexes containing these ligands were given recently [4].Species bearing hyponitrito ligands got currently more in the focusof bioinorganic investigations since they may serve as possiblemodel compounds of active sites in the processes of enzymaticnitric oxide reductases (NOR) [5]. It is assumed that NOR-typereaction sequences could include the coupling of two NO mole-cules on transition metal centers (e.g. Fe/Fe or Fe/Cu in enzymes)resulting in NeN coupling as the key step. From these systems ina subsequent step N2O is released following the overall reaction:2NO þ 2e� þ 2Hþ / N2O þ H2O. A closely related reactionsequence could be realized starting from our diruthenium complex1a (see below).

A remarkable result in the field of NOR model complexesrepresents the report on the synthesis and X-ray crystal structure ofthe trans-hyponitrito complex [(oep)Fe(m-ONNO)Fe(oep)](oep¼ octaethylporphyrinato) by Richter-Addo and co-workers [6].Very recently the novel unusual yttrium complex [{Y[(Me3Si)2N]2}4(m3-ONNO)2(thf)2] containing trans-hyponitritoligands was described [7]. The latter compound was formedunexpectedly by the reaction of [{Y[(Me3Si)2N]2}2(m-h2-NO)(thf)2]with nitric oxide in toluene. Interestingly, syntheses of transition

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e4742

metal complexes containing trans-N2O22� ligands were reported till

now frequently by pathways using nitric oxide as the sole source ofthe hyponitrite. Exceptions in this light represent e.g. the complex[(oep)Fe(m-ONNO)Fe(oep)] [6] and some compounds containingmain group elements like Ph3E(m-ONNO)EPh3 (E ¼ Sn, Pb) andMe3Si(m-ONNO)SiMe3 respectively [8]. The latter compounds wereprepared by the use of Ag2N2O2 as the origin of the hyponitriteexploiting the method of salt metathesis.

In this paper we describe the synthesis, the spectroscopiccharacterization, and the X-ray crystal structures of new trans-hyponitrito complexes of the general formula [Ru2(CO)4(m-H)(m-PBut2)(m-L2)(m-h2-ONNO)] (2bee, L2 ¼ biphosphanes) and the roleof these species as conceivable model complexes in NOR relatedprocesses are discussed.

2. Experimental

2.1. General considerations

All synthetic operations were performed under a dry argonatmosphere using conventional Schlenk techniques. Solvents weredried over sodium-benzophenone ketyl or molecular sieves andwere distilled under argon prior to use. Compound 1awas preparedas described previously [9]. The ligands dpppha, dpppra, anddppbza were prepared by a modified literature procedure [10]. Thesynthesis of the compounds 1be1e as well as of complexes 4 and 5,respectively, were reported elsewhere [11,2]. Chemicals werepurchased commercially from Aldrich, nitric oxide (purity 2.5) wasobtained from Air liquide. IR spectra were recorded as solid witha JASCO FT/IR-460 plus spectrometer. NMR spectra were obtainedusing Jeol Eclipse 270 and 400 instruments operating at 270 and400 (1H) and at 109 MHz (31P), respectively. Chemical shifts aregiven in ppm from SiMe4 (1H) or 85% H3PO4 (31P{1H}). Microanal-yses (C, H, N) were performed by the Microanalytical Laboratory ofthe Department of Chemistry, LMU Munich, using a Heraeus Ele-mentar Vario EI instrument.

2.2. Synthesis of [Ru2(CO)4(m-H)(m-PBut2)(m-dppen)(m-h

2-ONNO)](2b)

Asolutionof1b (428mg, 0.5mmol) in toluene (25mL)was cooledto�60 �C, and a slow streamof nitric oxidewas bubbled through thesolution for about 15 min. During this time the color of the solutionchanged from deep violet to orange. The mixture was warmed toroom temperature without stirring for 5 h. During this time theproduct precipitated as yelloweorange crystals from the solution.

2b: Yield: 358 mg (71%). Anal. Calcd for C45H49N2O6P3Ru2 (2b $

toluene) (1010.09): C, 53.57; H, 4.90; N, 2.78. Found: C, 53.36; H,4.93; N, 2.74%. IR (solid, cm�1): n(CO): 2036s, 1958vs, 1756s; n(NN):1402w, 1379w; n(NO): 1041s, 985m, 970m. 1H NMR (270 MHz,CD2Cl2): d 7.53e7.00 (m, 20H, PC6H5), 6.45 (d, 1H, 3JPH ¼ 15.3 Hz,PeC]CH2), 6.32 (d, 1H, 3JPH ¼ 14.9 Hz, PeC]CH2), 1.43 (d, 18H,3JPH ¼ 13.4 Hz, PC4H9), �11.81 (m, 1H, m-H). 31P{1H} NMR (109 MHz,CD2Cl2): 232.9 (dd, 2JPP¼178.4Hz, 2JPP¼160.8Hz,m-PBut

2), 49.0 (dd,2JPP ¼ 160.8 Hz, 2JPP ¼ 125.6 Hz, m-dppen), 34.6 (dd, 2JPP ¼ 178.4 Hz,2JPP¼ 125.6 Hz, m-dppen). Suitable single crystals of 2b for the X-raydiffraction study were obtained from the reaction solution.

2.3. Synthesis of [Ru2(CO)4(m-H)(m-PBut2)(m-dpppha)(m-h

2-ONNO)](2c)

A similar procedure (using 0.5 mmol of 1c) as described for thepreparation of 2b afforded yellow crystals of 2c. Yield: 275 mg(56%). Anal. Calcd for C42H44N3O6P3Ru2 (981.89): C, 51.38; H, 4.52;N, 4.28. Found: C, 50.99; H, 4.63; N, 4.32%. IR (solid, cm�1): n(CO):

2025vs, 1965vs, 1735s; n(NN): 1390w, 1367w; n(NO): 1027s, 999sh.1H NMR (270MHz, CD2Cl2): d 7.72e7.05 (m, 20H, PC6H5), 6.88e5.87(m, 5H, NC6H5), 1.43e1.38 (d, 18H, 3JPH ¼ 14.3 Hz, PC4H9), �11.25(ddd, 1H, 2JPH ¼ 24.0, 20.0, 17.3 Hz, m-H). 31P{1H} NMR (109 MHz,CD2Cl2): 237.6 (dd, 2JPP ¼ 192.5 Hz, 2JPP ¼ 173.9 Hz, m-PBut2), 112.1(dd, 2JPP ¼ 173.9 Hz, 2JPP ¼ 121.4 Hz, m-dppha), 92.4 (dd,2JPP ¼ 192.5 Hz, 2JPP ¼ 121.4 Hz, m-dppha). Suitable single crystals of2c for the X-ray diffraction study were grown from CH2Cl2/ethanolat room temperature.

2.4. Synthesis of [Ru2(CO)4(m-H)(m-PBut2)(m-dpppra)(m-h

2-ONNO)](2d)

Ananalogousprocedure (using0.5mmolof1d) asdescribed for thepreparation of 2b afforded yellow crystals of 2d. Yield: 299 mg (63%).Anal. Calcd for C39H46N3O6P3Ru2 (947.87): C, 49.42; H, 4.89; N, 4.43.Found: C, 49.12; H, 4.63; N, 4.40%. IR (solid, cm�1): n(CO): 2026vs,1964vs, 1731s; n(NN): 1390w, 1368w; n(NO): 1032s, 1000sh. 1H NMR(270MHz, CD2Cl2): d 7.68e7.14 (m, 20H, PC6H5), 3.12 (m, 2H, NeCH2),2.72 (m, 2H, CH2eCH3),1.21 (d,18H, 3JPH¼ 14.8Hz, PC4H9), 0.13 (m, 3H,CH2eCH3),�11.42 (m,1H,m-H). 31P{1H}NMR(109MHz,CD2Cl2): 232.8(dd, 2JPP¼ 191.0 Hz, 2JPP¼ 171.8 Hz, m-PBut2),118.1 (dd, 2JPP¼ 171.8 Hz,2JPP ¼ 114.9 Hz, m-dpppr), 84.1 (dd, 2JPP ¼ 191.1 Hz, 2JPP ¼ 114.9 Hz, m-dpppr). Suitable single crystals of 2d for the X-ray diffraction studywere grown from CH2Cl2/ethanol at room temperature.

2.5. Synthesis of [Ru2(CO)4(m-H)(m-PBut2)(m-dppbza)(m-h

2-ONNO)](2e)

A similar procedure (0.5 mmol of 1e) as described for thepreparation of 2b afforded yellow crystals of 2e. Yield: 314 mg(63%). Anal. Calcd for C43H46N3O6P3Ru2 (995.92): C, 51.86; H, 4.66;N, 4.22. Found: C, 52.10; H, 4.60; N, 4.14%. IR (solid, cm�1): n(CO):2024s, 1967vs, 1730s; n(NN): 1398w, 1390w; n(NO): 1039s, 981m,954m. 1H NMR (270 MHz, CD2Cl2): d 7.72e6.97 (m, 20H, PC6H5),7.35e6.60 (m, 5H, CH2eC6H5), 4.76 (dt, 1H, 3JPH ¼ 26.4 Hz,3JPH ¼ 9.9 Hz, CH2eC6H5), 4.08 (dt, 1H, 3JPH ¼ 21.1 Hz, 3JPH ¼ 4.6 Hz,CH2eC6H5), 1.36 (d, 18H, 3JPH ¼ 13.9 Hz, PC4H9),�11.39 (m,1H, m-H).31P{1H} NMR (270 MHz, CD2Cl2): 230.7 (dd, 2JPP ¼ 191.3 Hz,2JPP ¼ 173.7 Hz, m-PBut2), 117.5 (dd, 2JPP ¼ 173.7 Hz, 2JPP ¼ 113.9 Hz,m-dppbza), 90.0 (dd, 2JPP ¼ 191.3 Hz, 2JPP ¼ 113.9 Hz, m-dppbza).Suitable single crystals for the X-ray diffraction study of 2e weregrown from CH2Cl2/ethanol at room temperature.

2.6. X-ray structural determination

Suitable single crystals for X-ray diffraction of the compounds2aee, 4, and 5were obtained as described in the experimental part.Crystals were selected by means of a polarization microscope,mounted on the tip of a glass fiber, and investigated on an OxfordXCalibur and a Bruker Nonius-Kappa CCD diffractometer, respec-tively, using Mo-Ka radiation (l ¼ 0.71073 Å). The structures weresolved by direct methods (SHELXS) [12] and refined by full-matrixleast-squares calculations on F2 (SHELXL-97) [13]. Details of thecrystal data, data collection, structure solution, and refinementparameters of compounds 2aee are summarized in Tables 1 and 2.The data of 4 and 5 are listed in the Table 3.

3. Results and discussion

3.1. Preparation and characterization of new hyponitrito complexes

Some years ago we described the coordinatively unsaturatedcomplex [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)] (Ru]Ru) (1a, dppm ¼Ph2PCH2PPh2) [9]. Compound 1awas obtained in a kind of one-pot

Table 3Crystal data and structure refinement details for compounds [4]BF4 and 5.

Compound [4]BF4 5

Empirical formula C37H42BF4O5P3Ru2 C37H41O5P3Ru2Formula weight 948.59 860.78Temperature (K) 200(0) 200(0)Crystal system Triclinic TriclinicSpace group P1 P1a (Å) 11.8844(4) 12.2378(5)b (Å) 12.4377(4) 13.1234(6)c (Å) 13.4349(5) 13.5432(5)a (�) 94.578(3) 61.582(4)b (�) 96.488(3) 78.897(4)g (�) 105.639(3) 82.443(4)Volume (Å3) 1933.88(11) 1875.15(13)Z 2 2rcalcd (g cm�3) 1.62905(9) 1.52455(11)m/mm�1 0.966 0.973q range for data collection (�) 3.76e26.33 3.76e26.32Reflections measured 14 927 14 754Rint 0.0260 0.0233Observed reflections 5916 5179Reflections, unique 7808 7565Parameters/restraints 477/0 433/2R (Fobs) 0.0305 0.0272Rw(F2) 0.0682 0.0576S 0.933 0.933Max electron density (e Å�3) 1.024 0.802Min electron density (e Å�3) �1.012 �0.424

Table 1Crystal data and structure refinement details for compounds 2aec.

Compound 2a 2b $ toluene 2c $ 0.5 EtOH

Empirical formula C37H41N2O6P3Ru2 C45H49N2O6P3Ru2 C43H50N3O6.5P3Ru2

Formula weight 904.77 1008.91 1004.89Temperature (K) 173(2) 173(2) 173(2)Crystal system Monoclinic Monoclinic MonoclinicSpace group P21/c P21/c C2/ca (Å) 12.5828(2) 10.6856(3) 23.1863(12)b (Å) 12.3114(2) 16.6837(6) 10.3367(2)c (Å) 25.7381(4) 24.6548(13) 36.1329(6)a (�) 90 90 90b (�) 106.450(1) 94.607(3) 93.415(1)g (�) 90 90 90Volume (Å3) 3823.93(11) 4381.1(3) 8644.6(3)Z 4 4 8rcalcd (g cm�3) 1.572 1.530 1.544m/mm�1 0.962 0.848 0.861q range for data

collection (�)3.25e27.52 4.22e26.29 3.16e26.40

Reflections measured 60 208 19 975 28 887Rint 0.0578 0.0235 0.0595Observed reflections 7022 7059 6055Reflections, unique 8764 8865 8692Parameters/restraints 461/1 534/0 535/0R (Fobs) 0.0308 0.0297 0.0422Rw(F2) 0.0694 0.0734 0.1014S 1.026 0.999 1.032Max electron

density (e �3)0.490 0.882 0.871

Min electrondensity (e �3)

�0.668 �0.626 �0.855

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e47 43

synthesisby reactionof [Ru3(CO)12]withanexcessofPBut2H including[Ru2(CO)4(m-H)(m-PBut2)(PBut2H)2] (Ru]Ru) as the intermediate insitu [14]. The latter species reacted with dppm in refluxing tolueneaffording 1a in good yield. After the investigation of the unusualreactivity of 1a towards nitric oxidewhich resulted in the unexpectedtrans-hyponitrito complex [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNO)] (2a) [1],wewantednowto explore the circumstances leadingto the formation of this unprecedented species. First of all we studiedthe influence of the bidentate bridging ligand L2 across the metal-emetal bond on the course of the reaction. Thus we prepared some

Table 2Crystal data and structure refinement details for compounds 2d and 2e.

Compound 2d $ 0.5 EtOH 2e $ CH2Cl2

Empirical formula C40H49N3O6.5P3Ru2 C44H48Cl2N3O6P3Ru2

Formula weight 970.87 1080.80Temperature (K) 173(2) 173(2)Crystal system Monoclinic MonoclinicSpace group C2/c P21/ca (Å) 39.9466(12) 16.8882(7)b (Å) 10.2167(3) 11.2862(4)c (Å) 23.3147(7) 25.0406(3)a (�) 90 90b (�) 115.911(2) 101.668(3)g (�) 90 90Volume (Å3) 8558.7(4) 4674.2(3)Z 8 4rcalcd (g cm�3) 1.507 1.536m/mm�1 0.867 0.912q range for data collection (�) 3.25e23.10 4.24e26.26Reflections measured 22 594 22 056Rint 0.2343 0.0331Observed reflections 3003 6872Reflections, unique 6013 9387Parameters/restraints 502/0 571/4R (Fobs) 0.0604 0.0333Rw(F2) 0.1366 0.0784S 0.909 0.917Max electron density (e Å�3) 0.998 0.761Min electron density (e Å�3) �0.629 �0.456

new complexes [Ru2(CO)4(m-H)(m-PBut2)(m-L2)] (m-L2 ¼ bridgingbiphosphane; type of complex 1) using a similar synthetic pathway asapplied to 1a [11]. During this procedure the bridging biphosphanesdppen, Ph2PC(]CH2)PPh2, dpppha, Ph2PN(Ph)PPh2, dpppra,Ph2PN(Prn)PPh2, and dppbza, Ph2PN(CH2Ph)PPh2, were investigated.These ligands reacted with in situ prepared [Ru2(CO)4(m-H)(m-PBut2)(PBut2H)2] with substitution of both terminal phosphanestowards the corresponding bridging biphosphane affording the newcompounds1beeasviolet crystals ingoodyields [11]. Thecompounds1bee reacted in a similarmanner as used for the synthesis of 2awithnitric oxide in toluene (�60 �C) resulting inaquick colour change fromdeep violet to orange. After workup at room temperature thecomplexes 2bee were obtained as pale yellow solids in good yields(Scheme 1). The new trans-hyponitrito complexeswere characterizedby analytical and spectroscopic methods (see Experimental Section).The 31P{1H} NMR spectra of 2bee are comparable to those of theknown compound 2a. Thus for all complexes a pattern of three signalgroups (dd) was found with the corresponding couplings indicatingthe chemical inequivalency of the three phosphorus nuclei. The 1HNMR spectra of 2bee include a characteristic multiplet resonance in

Scheme 1. Synthesis of new hyponitrito complexes.

Scheme 2. NOR related reaction steps across the diruthenium core.

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e4744

the upfield region attributable to a bridging hydrido ligand. Thereforethemolecular structures of2bee in solutionmeet the same structuralfeatures like the known complex 2a.

The mechanism of the formation of these unusual trans-hypo-nitrito complexes is still unknown. Currentlywe assume that at firstin toluene a dimerization of the NO could be occur [15]. Then ina subsequent step the N2O2 is reduced to N2O2

2� by the ruth-enium(I) species resulting in two ruthenium(II) centers. The twoelectrons for this process could be supported from the RueRudouble bond which is formally expected in the sense of the 18-electron rule for this organometallic complex. Interestingly, asconfirmed by the X-ray crystallographic studies of all fivecompounds, all molecules in 2aee exhibit an unusual close contactbetween the non-coordinated O atom of the trans-hyponitritoligand and the carbon atom of one neighbouring carbonyl group(see Table 4). We would like to interpret this as an intramolecularnucleophilic attack of the terminal hyponitrito O atom towards anelectrophilic carbon center. In a mechanistic point of view, thisobservation is comparable e.g. with the intermolecular attack ofa nitrite ion towards a carbonyl ligand which is coordinated inametal complex [16]. In these cases this attack results usually in theformation of a nitrosyl ligand with a simultaneous loss of carbondioxide. Such a kind of reaction pathway is however not observedin the present case. That is, our hyponitrito complexes do notrelease CO2, neither upon treatment under elevated temperaturesnor under irradiation with UV light in their solutions. Moreoversuch a reaction pathway could proceed with a loss of carbondioxide together with nitrous oxide, and thus no stable complexshould result from the remaining dimetal core.

3.2. Investigations with respect to NOR model compounds

In light of mechanistic aspects relating to NOR modelcompounds, we investigated the transformation of the trans-hyponitrito complexes to the corresponding hydroxido-bridgedspecies with elimination of nitrous oxide. For the known complex2a we confirmed such a pattern of reactivity since the protonationof 2a (with HBF4) resulted in the cationic complex [Ru2(CO)4(m-H)(m-PBut2)(m-dppm)(m-h2-ONNOH)]þ (3), isolated as [3]BF4. Thelatter reacted in refluxing ethanol with a loss of N2O affording thecomplex [Ru2(CO)4(m-H)(m-OH)(m-PBut2)(m-dppm)]þ (4) [2]. Finally,this species can be deprotonated in THF, e.g. by the base DBU,yielding the neutral complex [Ru2(CO)4(m-OH)(m-PBut2)(m-dppm)](5), see Scheme 2. Interestingly, during our deprotonation experi-ments of 4 with DBU we found no indication of the oxido-bridgedspecies [Ru2(CO)4(m-H)(m-O)(m-PBut2)(m-dppm)], i.e., obviouslya deprotonation on the hydroxido ligand of 4 in the first step couldnot be evidenced. On the other hand, the analogous sulfido-bridgedcomplex [Ru2(CO)4(m-H)(m-S)(m-PBut2)(m-dppm)] could be ob-tained by us and its molecular structure confirmed by X-ray crys-tallography [17]. To ensure the findings concerning thecorresponding NOR cycle steps, nowwe confirmed additionally the

Table 4Selected bond lengths (Å) and angles (�) for 2aee and the calculated complex trans-[Ru2(CO)4(m-H)(m-PH2)(m-H2PCH2PH2)(m-h2-ONNO)] (by DFT) [1].

Compound 2a 2b 2c 2d 2e DFT

Ru1eO1 2.125(2) 2.153(2) 2.168(3) 2.157(6) 2.162(2) 2.174Ru2eN1 2.014(2) 2.020(2) 1.993(3) 1.989(8) 2.005(2) 2.062N1eN2 1.265(3) 1.267(3) 1.260(4) 1.268(10) 1.260(3) 1.256N1eO1 1.329(3) 1.330(2) 1.335(4) 1.339(9) 1.346(3) 1.336N2eO2 1.370(3) 1.374(3) 1.373(4) 1.401(9) 1.374(3) 1.351O2eC1 1.581(3) 1.602(3) 1.543(5) 1.527(11) 1.541(4) 1.573C1eO3 1.187(3) 1.172(3) 1.188(5) 1.212(11) 1.198(4)Ru2eC1eO3 142.8(2) 144.1(3) 141.7(4) 140.0(9) 140.6(3) 140.8

crystal and molecular structures of the species 4 and 5, since wedescribed these compounds in our previous report only by spec-troscopic means [2]. Furthermore, in the course of the crystal andmolecular structure determination of the compounds 2bee, wereinvestigated the crystal structure of complex 2a to obtain a betterrefinement of the structural parameters around the hyponitrito Oatom and the neighbouring carbonyl group (see below).

In light of some interesting results with respect to a similarcoupling of two NO molecules in the coordination sphere of a dir-uthenium complex, some findings related to NOR model processeswere reported recently [18]. Thus the complex [(TpRuNO)2(m-Cl)(m-pz)]2þ (Tp ¼ hydrotris(pyrazolyl)borate; pz ¼ pyrazolato) could bereduced by 2 electrons to the species [(TpRu)2(m-Cl)(m-pz){meN(O)eN(O)-k2}]. The latter compound reacted in the presenceof Hþ to the oxido-bridged complex [(TpRu)2(m-Cl)(m-pz)(m-O)]. Thereaction was accompanied by releasing nitrous oxide. On the otherhand, protonation of the oxido-bridged species with HBF4 gave thecorresponding hydroxido-bridged complex which reacted subse-quently with NO (in the presence of Hþ) back to the dicationicdinitrosyl compound. Thus the NO reduction cycle on this dir-utheniummetal core was completed. In contrast to our compoundsreported herein, the coupled N2O2 ligand in [(TpRu)2(m-Cl)(m-pz){meN(O)eN(O)-k2}] can not be described as a hyponitrito ligand,N2O2

2�. Therefore, a reaction starting from our compound [4]BF4with nitric oxide in the presence of acid to remove the hydroxidoligand (as H2O eliminated), should lead back to the hyponitritocomplex 2a only by a simultaneous 2e� reduction. Inspired by thefindings of Arikawa et al. [18], we examined the reaction of thecompound [4]BF4 with NO in CH2Cl2 in the presence of tetra-fluoroboric acid. Indeed we could confirm that without thereduction step no complex 2awas reformed. On the other hand, thereaction of [4]BF4 with H2N2O2, prepared from Ag2N2O2 in anetheric HCl solution as described in a similar manner by Richter-Addo and co-workers [6], reacted also not back to the hyponitritocomplex 2a as indicated by NMR spectroscopy.

3.3. Molecular structures of new trans-hyponitrito complexes

The results of the structural identification of 2bee establishedby X-ray crystallography afforded unambiguously the presence ofhyponitrito ligands across the RueRu bond in trans configuration. Arepresentation of the molecule of 2b is shown in Fig. 1, selected

Fig. 1. Molecular structure of 2b in the crystal. Thermal ellipsoids were drawn at the50% probability level. The solvent molecule and hydrogen atoms (except hydridoligand) are omitted for clarity.

Fig. 2. Molecular structure of 2c in the crystal. Thermal ellipsoids were drawn at the50% probability level. Hydrogen atoms are omitted for clarity (except hydrido ligand).

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e47 45

bond lengths and angles are summarized in the Table 4. Thecompound exhibits a diruthenium tetracarbonyl core bridged bya hydrido ligand, a phosphanido group, the dppen, and the bridgingtrans-hyponitrito ligand. The overall structural features are in goodagreement with the observed bonding characteristics of the knowncompound 2a. The molecules of compounds 2cee exhibit a similardiruthenium core with a closely related arrangement of ligands asfound for 2a and 2b respectively. Therefore only a view of themolecule of 2c is depicted in Fig. 2 representing the similar struc-tural features of the related molecules 2d and 2e respectively.

In the course of the X-ray crystal structure determination ofcompounds 2bee the following aspect raised up: these moleculesexhibit in the crystal a very close contact of the free termination ofthe ONNO group from the oxygen O(2) to the carbon C(1) of thenext-neighbouring carbonyl group in each case. These interactionsare clearly indicated by the very short distances C(1)eO(2) of1.521 Å in the average. This value agrees very well with the calcu-lated one by DFT methods at the B3LYP level for the modelcompound [Ru2(CO)4(m-H)(m-PH2)(m-H2PCH2PH2)(m-h2-ONNO)][1]. However, these results were in disagreement with the corre-sponding interatomic distance found for the compound 2a, namely2.062(3) Å (molecule A) and 2.244(4) Å (molecule B) in our previouscrystal structure report [1]. Unfortunately in the latter case weobtained only crystals exhibiting large channels filled withconsiderable amounts of disordered solvent molecules. Obviously,this resulted in the remarkable difference of the CeO distance ofinterest. Therefore we carried out a new X-ray crystal structurestudy on crystals of 2a. Although suitable crystals were grown fromthe same solvent and under similar conditions as described in ourprevious report [1], now a better refinement of the distance ofinterest was possible. The main problem of our previous crystalgrowing efforts of 2a was that we obtained the compound only asethanol solvate in each case. But now we were able to overcomethis troublesome problem and we crystallized 2a without anysolvent molecules. Thus during the repeated crystal structuredetermination of 2a, a distance C(1)eO(2) of 1.581(3) Å was found.This is in a very good agreement with the other corresponding

values observed for complexes 2bee. As mentioned above, wewould interpret this close intramolecular contact as a nucleophilicattack of the bound hyponitrito ligand to the neighbouring carbonylgroup. The corresponding distances C(1)eO(2) lie close to the rangeof CeO single bonds and one should interpret this as a bondingdistance in the crystal. Thus an unusual coupled cycle of ligands, i.e.carbon monoxide and hyponitrito group, results. A further indica-tion for this unusual carbonyl group in these molecules is given bytheir IR spectra: all complexes 2aee exhibit in the solid in theirspectra a n(CO) band about 1740 cm�1 whereas the other CO ligandsare found in the range of 2035e1964 cm�1 which is indicative ofterminal carbonyl groups (see Experimental Section).

3.4. Molecular structures of complexes 4 and 5

To ensure our findings with respect to the conversion cycle ofNOR model compounds, we carried out the X-ray crystal structuredeterminations on the complexes 4 and 5. Suitable single crystals of4[BF4] were grown from acetone/diethyl ether at room tempera-ture. Pale yellow (almost colorless) crystals of the latter compoundbelonging to the triclinic space group P1 were obtained. Fig. 3shows a selected ORTEP view of the molecule of 4, selected bondlengths and angles are listed in the Table 5. The molecular structureof 4 is closely related to that of the complex [Ru2(CO)4(m-H)(m-Cl)(m-PBut2)(m-dppm)]þ (6) [19]. The molecule 4 consists of a four-fold bridged diruthenium tetracarbonyl core with a bridgingphosphanido, a dppm, a hydrido, and a hydroxido ligand. Incontrast to the cationic complex 6, the molecule 4 exhibits nomirror plane passing through the four bridging ligands, thereforeno CS symmetry is found. The RueRu distance of 2.7355(3) Åcorresponds well to a single bond between the two metal atomswhich is also in accordance with the 18-electron rule by valenceelectron counting. The metal to metal bond separation in 6 was

Fig. 3. Molecular structure of the complex cation 4 in the crystal. Thermal ellipsoidswere drawn at the 50% probability level. Hydrogen atoms are omitted for clarity(except hydrido and hydroxido ligand respectively).

Fig. 4. Molecular structure of 5 in the crystal. Thermal ellipsoids were drawn at the50% probability level. Hydrogen atoms are omitted for clarity (except on hydroxidoligand).

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e4746

found to be 2.7608(5) Å and is therefore longer because of the sizeof the chloride bridge.

A comparison of the bonding parameters between 4 and 6afforded a good agreement of the corresponding bond lengths andangles. Thus the following data of 6 agree well with those of 4 (seeTable 5), for 6: RueH(1), 1.78(2); RueP(1), 2.4055(6); RueP(2),2.4088(7) Å; RueP(2)eRu’, 69.93(2)�. Suitable single crystals ofcompound 5 were grown from acetone/ethanol at room tempera-ture. Yellow crystals of 5 were obtained belonging to the triclinicspace group P1. Fig. 4 shows a selected view of the molecule of 5,selected bond lengths and angles are summarized in Table 5. Themolecular structure of 5 is closely related, and therefore compa-rable, to that of [Ru2(CO)4(m-Cl)(m-PBut2)(m-dppm)] (7) [17]. Themolecule 5 consists of a threefold bridged diruthenium tetra-carbonyl core with a bridging phosphanido, a dppm, anda hydroxido ligand.

In the past we observed frequently a bond shortening effect ofthe metalemetal bond upon protonation by comparison of closelyrelated dinuclear metal cores [19]. In the present case, on goingfrom 5 to 4 (protonation of the MeM bond), we observed theusually expected lengthening effect of a metalemetal bond uponintroducing the hydrido ligand. Thus, the MeM bond in 4 is about0.01 Å longer than in 5 (see Table 5). As found for 4, also the

Table 5Selected bond lengths (Å) and angles (�) for [4]BF4 and 5.

Compound [4]BF4 5

Ru1eRu2 2.7355(3) 2.7259(3)Ru1eO5 2.100(2) 2.1260(18)Ru2eO5 2.108(2) 2.1182(19)Ru1eP3 2.3863(8) 2.3735(9)Ru2eP3 2.3890(8) 2.3608(7)Ru1eP1 2.3908(8) 2.3424(8)Ru2eP2 2.3960(8) 2.3658(8)Ru1eH1 1.71(3) e

Ru2eH1 1.77(3) e

Ru1eO5eRu2 81.10(8) 79.92(6)Ru1eP3eRu2 69.90(3) 70.31(2)

molecule of 5 exhibits no CS symmetry which is for the molecule 7the case [17]. Except of the RueRu distance, which is in 7 elongatedbecause of the size of the chlorido bridge, the following data of 7agree well with those of 5 (see Table 5). For 7 the following bondingparameters were found: RueP(1), 2.3948(2); RueP(2), 2.3845(2);RueP(3), 2.364(2) Å; RueP(2)eRu’, 71.64(5)�. Furthermore,a comparison of some bonding parameters with other dinuclearruthenium complexes containing m-hydroxido bridges should begiven here. Thus for [Ru2(CO)2(PMe3)2(m-H)(m-OH){m-k2eC(O)O}]the following data were reported: Ru(1)eRu(2), 2.788(1); Ru(1)eO(1), 2.089(5); Ru(2)eO(1), 2.089(5) Å [20]. The bonding parame-ters found for 4 and 5, respectively, fall also within the range ofother observed hydroxido bridge bonds in diruthenium complexes.For the closely related molecule [Ru2(CO)2Cl2(m-H)(m-OH)(m-dppm)2] the following datawere reported: Ru(1)eRu(2), 2.8620(7);Ru(1)eO(3), 2.161(4); Ru(2)eO(3), 2.182(5); Ru(1)eH(1), 1.797(10);Ru(2)eH(1), 1.748(10) Å [21]. In the cationic complex [Ru2(m-Cl)2(m-OH)(h6eC6H6)2]þ the distances Ru(1)eO(1), 2.084(3) and Ru(2)eO(1), 2.083(3) Å within the Ru2(m-OH) core were found [22].

4. Conclusions

Herein we described the synthesis and characterization of fournew diruthenium complexes containing unusual trans-hyponitritoligands which were formed in the reaction of coordinativelyunsaturated dimetal complexes with nitric oxide. All thesecomplexes exhibit in the solid state a very close contact betweenthe terminal hyponitrito terminus and a neighbouring carbonylgroup resulting in an unusual cyclic ligand. This structural featureshows a relation to the nucleophilic attack of the nitrite ion towardsa bound carbonyl ligand e a state which is only postulated froma mechanistic point of view and remains speculative in the case ofmetal carbonyl complexes. However, in the case of complexes 2aeesuch an intermediate structural motif is frozen in the solid state andsupports such mechanistic assumptions. Furthermore, for thefuture we want to elucidate which kind of other factors allow alsoa reductive coupling of two NO molecules in the coordination

T. Mayer et al. / Journal of Organometallic Chemistry 700 (2012) 41e47 47

sphere of dimetal complexes. Herein we demonstrated that somevariations in the nature of bridging biphosphane ligands arepossible without significant changes in the course of the titlereaction. In the future we want to examine a further tuning in theequipment of the ligand sphere and, furthermore, we want toinclude the element iron in our studies because the latter is morerelevant for biochemical investigations. For this purpose the elec-tronically and coordinatively unsaturated complex [Fe2(CO)4(m-H)(m-PBut2)(m-dppm)] [23] is available in our hands and wewant toextend our investigations in the field of the analogous diironcomplexes.

Acknowledgments

The authors are grateful to the Department of Chemistry of theLudwig Maximilian University Munich for supporting these inves-tigations. T. M. thanks Prof. P. Klüfers for financial support.Furthermore we thank S. Albrecht for collecting the X-ray crystaldata. The Johnson Matthey plc, Reading, UK, is grateful acknowl-edged for a generous loan of hydrated RuCl3.

Appendix A. Supplementary material

CCDC-823991 (2a), CCDC-823990 (2b), CCDC-823992 (2c),CCDC-823993 (2d), CCDC-823989 (2e), CCDC-824759 ([4]BF4), andCCDC-824758 (5) contain the supplementary crystallographic datafor this paper. Copies of these data can be obtained from TheCambridge Crystallographic Data Centre viadeposit@ccdc.cam.ac.ukor http://www.ccdc.cam.ac.uk/. Supplementary data associatedwith this article can be found, in the online version.

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