Polyhedron Vol. 12, No. 4, pp. 427-434, 1993 Printed in Great Britain

0277-5387/93 $6.00+ .I0 0 1993 Pergamon Press Ltd

OLIGOPHOSPHINE LIGANDS-XXXVII.” STEREOCHEMICAL FEATURES OF COMPLEXES

CONTAINING THE cis-L,Ru FRAGMENT : MOLECULAR STRUCTURES OF (Me3P)4RuH2 AND {P[(CH2),PMe2],}RuH2

LUTZ DAHLENBURGT and KAY-MICHAEL FROSIN

Institut fiir Anorganische Chemie der Universitat Hamburg, Martin-Luther-King-Platz 6, D-2000 Hamburg 13, Germany

(Received 20 August 1992; accepted 29 September 1992)

Abstract-The title compounds have been studied by single-crystal X-ray diffraction. Both structures can be described as distorted octahedra. P-Ru-P angles between phosphorus atoms at mutually cis positions are considerably larger than 90”, with the sum of the cis angles amounting to 495.6” for (Me,P),RuH, (1) and to 486.2” for (P[(CHJ3PMe&}RuHZ (2). These structural parameters and the observed bending over of the two axial donor sets toward the hydrido ligands, which results in tram P-Ru-P angles of 151.5” for complex 1 and of 153” for structure 2 averaged over four crystallographically independent molecules, suggest a considerably higher degree of steric encumbrance in cis-(Me3P),Ru than in {P[(CHJ3PMe,],}Ru systems. They thus correspond to both the well-known ability of the {P[(CH,)3PMe&}Ru(0) fragment to insert across C-H bonds of non-coordinated hydrocarbons and the propensity of cis-(Me,P),Ru(O) to undergo spontaneous cyclo- metallation. Selected bond lengths of compound 1 are : Ru-P (tram ligand H), 2.306(l) and 2.306(2) A; Ru-P (tram to each other), 2.276(l) and 2.289(2) A. For the averaged structure 2, the mean distances between the central metal and the donor atoms are 2.270(5) A for the bridging phosphorus atom, 2.292(6) 8, for the PMe, group tram to hydrogen, and 2.281(7) and 2.284(10) w for the PMe, substituents situated at mutually truns positions.

Within the scope of our studies with the (ppJRu metal/ligand system (pp3 = P[(CHJ3PMe2],)“” we also came across several complexes derived from the closely related cis-(Me,P),Ru fragment, which have been synthesized in great variety, first by Wil- kinson, 12-’ 7 then by Schmidbaur,‘* Werner” and Green,” and recently by Andersen and Bergman’ 1-27 as well as Osakada and Yamamoto.28 The &uc- turally characterized ruthenium complexes cis- L,Ru(X)(Y), with trimethylphosphine as the common ligand L, include the alkyl L4Ru(H)(Et),17

* For parts XXXIV to XXXVI, see refs l-3. TAuthor to whom all correspondence should be

addressed at his new address at the Institut fiir Anor- ganische Chemie der Friedrich-Alexander-Universitkit Erlangen-Niirnberg, EgerlandstraBe 1, D. 8520 Erlangen, Germany.

the aryl L4Ru(OH)(Ph),23b the arylamide and -oxide L4Ru(H)(NHPh)26 and L4Ru(H)(OC6H4 Me-4), 26*28 the mercury-bridged heterotrimetallic

[LdRu(Me)l *Hg, I6 as well as a series of further derivatives where the cis-L,Ru moiety is part of a three- to five-membered metallaheterocvcle. in- cluding L4mH2),17 L~Ru(C~H$“~<~~

[CH2C(0)CHJ,27b’d L4Ru[eH2C(=CHCMe3)b]27d and several ortho-metallated systems, such as L4kU(o-C6H4C;H2),29 L4ku(o-C6H40cH2),30 L4 ku[o-C6H4C(=CH2)~]27C and L4ku(o-C6H4C(Ph) a).23b

Here we report the results of two X-ray structure determinations performed on the parent dihydride of the above derivatives, ci+(Me3P)4RuH2 (l), and its closely related analogue, (pp3)RuH2 (2). Dihy- dride 1 represents a key-compound in ruthenium trimethylphosphine chemistry. The complex has

427

428 L. DAHLENBURG and K.-M. FROSIN

thus been shown to undergo H/D exchange with C6Ds solvent,24 to assist proton-transfer processes with OH acids26,28 and even with the weaker NH acids,26 presumably on a pathway that involves the reversible formation of a non-classical cation [(Me3P)4RuH3]+ containing both a hydride and a dihydrogen ligand. 26,28 (Me3P)4R~H2 furthermore serves as an efficient catalyst for the homo-dimer- ization of terminal alkynes, IRGCH, to yield 1,4- disubstituted but-1-en-3-yne$, RCH=CHC=CR,3’ as well as for the co-dimerization of 1-alkynes with buta-1,3-diene to afford substituted hex-3- en-5-ynes, CH3CH2CH=CHC=CR.32 Upon photolytic conversion with Me,SiH to (Me,P), RuH3(SiMe3), dihydride I finally mediates the dehydrogenative coupling of trialkylsilanes to generate carbosilanes as a net result of silane Si-H and C-H activation and organosilicon Si-C bond formation. 33

The two structural analyses were performed on the principal expectation that their results would help to improve our understtanding of factors that govern the competition between intro- and inter- molecular C-H oxidative addition reactions occurring on coordinatively unsaturated phosphine complexes of ruthenium : Aicommon problem with reactive poly(trimethylphosphine) systems such as (Me3P),Ru is their propensity for undergoing ligand cyclometallation rather than hydrocarbon C-H bond activation. “Sag Metal/ligand fragments with chelating phosphines, e.g. (Me2PCH2CH2 PMe2)2Ru34’35 or (P[(CH2)3PMe2]3}Ru,3*6,g-11 on the other hand, are in general capable of inter- molecular reactivity. The control of balance be- tween intra- and intermolecular C-H cleavage on transition metal complexes has been attributed to arise from two opposing effects ; (1) confor- mational strain within the ligand sphere, retard- ing the metallation of ligand C-H bonds due to an extra amount of activation energy required to bend the ligand over in the proper orientation ; and (2) steric congestion about the metal centre, which prevents additional molecules from enter- ing into the coordination sphere but may bring co- ordinated ligands a bit closer to the central metal atom and may therefore b+e thought to drive the sterically more encumbered systems toward intra- molecular reactivity. 36 Both 1 and 2 appear to be particularly suited for evaluating the degree of steric encumbrance and/or conformational strain in chelated and non-chelated L4Ru systems, as it can be anticipated that the additional two hydrido ligands required to stabiliae the otherwise highly reactive 16e fragments will: distort the phosphine coordination less than sterically more demanding substituents such as halides:or alkyl groups.

EXPERIMENTAL

(Me3P)4RuH2’5~‘6*‘g*24 (1) was prepared from (Me,P),ku(H)(CH,PMe,) by reaction with meth- anol as described by Werner and Wemer’ga,C and was isolated in 71% yield by repeated high-vacuum sublimation at 5G8O”C. Dihydride 2 was obtained from (pp3)R~Clz and LiAlH4, as reported previously,5 and was also purified by sublimation at ca 70°C in the dynamic vacuum of a mercury diffusion pump. Single crystals of 1 grew when hex- ane solutions were allowed to evaporate slowly at ambient conditions ; crystals of 2 suitable for X-ray analysis were collected from re-sublimed samples of the compound.

X-ray measurements were carried out at 20 &- 2°C on a Syntex P2, diffractometer using MO-K, radi- ation, as described elsewhere. 37 Data were corrected for Lorentz and polarization effects, but not for absorption or extinction.

In each structure the ruthenium, as well as several of the phosphorus atoms, were readily located by comparison of the Patterson map with the results of the TREF direct-methods attempts of SHELXS- 86. 38 Subsequent alternate cycles of difference- Fourier synthesis and least-squares refinement (full- matrix for 1, block-diagonal for 2), employing the SHELX-76 program package3’ with scattering fac- tors from the sources given therein, revealed the positions of the remaining non-hydrogen atoms. The structure models resulting for the two com- plexes were refined with allowance for anisotropic thermal motion of all non-hydrogen atoms. In both structures, many of the hydrogen atoms present were also located experimentally, although most of those belonging to the phosphine methyl and methylene groups did not refine well. Hence, only the ruthenium-bound hydrogens (as far as detec- ted) were freely refined with isotropic thermal parameters, whilst those on the phosphine ligands were included into the molecular models in geo- metrically idealized positions with C-H distances constrained to 0.96 A. No attempts were made to resolve the disorder of the methyl groups attached to the phosphorus atom P(4) in structure 1, which became obvious from high values of the dis- placement factor coefficients characterizing the anisotropic thermal motion of the respective carbon atoms.

A summary of relevant crystallographic infor- mation and results is collected in Table 1. Atomic positional coordinates, thermal parameters, full lists of bond lengths and angles and lists of observed and calculated structure factors have been deposited as supplementary material with the Editor, from whom copies are available upon request.

Oligophosphine ligands-XXXVII

Table 1. Summary of crystal data, intensity collection and structure refinement

1 2

429

Crystal data

Empirical formula Molecular mass Crystal system Space group

a (A) b (A) c (A) v (A’) z

& (g cm- ‘) ~(Mo-Kb) (cm- ‘)

Intensity collection

2@,, ; 3Lax (deg.) Recip. latt. segment

Number of data collected unique used (IF01 > 40)

Structure refinement

Data/parameter ratio R

RW Weighting scheme Largest diff. peak (e A- 3, Largest diff. hole (e A- ‘)

G~H~J’~Ru C,J-IJ’& 407.40 443.43

Orthorhombic

p2,2,21 Pn2,a 9.359(2) 18.150(9) 15.141(4) 30.165(20) 15.260(4) 15.857(8) 2162.3(9) 8682(8) 4 16 1.252 1.357 9.90 9.92

4.5; 65.0 4.5 ; 55.0 h:0+15 h:O+24 k:O-+23 k: O-+40 1: O-24 1: 0+21

4417 10,902 4390 9553 3440 5194

21.10 7.00 0.047 0.064 0.57 0.068 [a~(F,)+0.001(F0)2]-’ 1.09 1.08 1.05 0.84

Atomic coordinates have also been deposited with the Cambridge Crystallographic Data Centre.

RESULTS

Complex 1 crystallized in the space group P2,2 t2 1 (No. 19). Selected bond lengths and angles

are collected in Table 2. A SCHAKAL drawing4’ of the molecule is shown in Fig. 1. Dihydride 2 was found to crystallize in space group Pn2,a (uncon- ventional setting of Pna2,, No. 33) with four crys- tallographically independent molecules that had structures similar to each other. A perspective view of one of these molecules is given in Fig. 2. Table 3

Table 2. Selected bond lengths and angles for dihydride I

Lengths Angles (“)

(A) Ru P(2) R(3) P(4) H(1) H(2)

2.306( 1) P(1) 98.8( 1) 98.4(l) 99.2(l) 175.8(4) 87.3(3) 2.306(2) P(2) 97.4(l) 101.8(l) 80.9(4) 173.9(3) 2.276( 1) P(3) 151.5(l) 77.6(4) 82.1(3) 2.289(2) R(4) 84.9(3) 76.6(3) 1.507(9) H(1) 93.0(5) 1.659( 10) H(2)

430 L. DAHLENBURG and K.-M. FROSIN

Fig. 1. Structure of (Me3P),RuH, (1) with atom labelling (methyl hydrogen atoms omitted).

summarizes relevant interatomic distances and bond angles. For simplicity, only averaged values of the latter, such as also presented in Table 3, will be used throughout further discussion.

DISCUSSION

In both structures, the four phoshorus atoms and the two hydrogens bonded to the ruthenium atom are situated at the coordination sites of distorted octahedra.

For molecule 1 the distances between the central metal and the phosphorus atoms P( 1) and P(2) tram to the hydride ligands are equally long, 2.306(2) A, and are significantly elongated relative to those between the ruthenium centre and the phosphorus atoms P(3) and P(4), situated at mutually tram pos- itions [2.276(l) and 2.289(2) A]. Although this is expected from the usual tram influencing properties of metal-bound hydride and phosphine, respec- tively, all these distances remain remarkably short as compared to the Ru-P bond lengths for PMe,

ligands located tram to Ru-H or tram to a second phosphine in other hydrido(trimethylphosphine)- ruthenium complexes of the composition cis- (Me,P),Ru(H)(X) ; X = Et, ” NHPh,26 OC6H4Me- 4.26,28 In these latter systems, the Ru-P sep- arations for the phospine tram to the hydride sub- stituent range from 2.326(3) A in the ethyl com- pound ” to 2.359(5) 8, (averaged for two inde- pendent molecules) in the anilido complex26 to 2.371(5) A (mean value from two independent structure determinations) in the p-cresolato deriva- tive 26,28 with the metal-phosphorus bond lengths for ‘the phosphines trans to each other varying between 2.313(3) A in (Me,P),Ru(H)(Et)” and 2.329(6) A (average of 10 values) in (Me,P), Ru(H)(NHPh)26 and (Me,P),Ru(H)(OC61-I,Me-

4), 26,28 respectively. The trans P-Ru-P distances measured for 1 are approximately as short as those reported for the Ru-P bonds opposite to the poor trans influence ligand PhNH(-) in the crys- tallographically independent molecules of cis- (Me,P),Ru(H)(NHPh), 2.268(2) and 2.271(2) A,

Oligophosphine ligands-XXXVII

P

431

Fig. 2. Perspective view of one of the four independent molecules of (P[(CH,),PMeJ,}RuHZ (2) defining the atomic numbering scheme (phosphine hydrogen atoms removed for clarity).

respectively.26 The contraction of the metal-phos- phine separations in 1 is attributed to the stereo- chemically undemanding nature of hydride sub- stituents relative io that of other anionic groups linked to ruthenium, since the wealth of structural data available for trimethylphosphine derivatives, cis-(Me,P),Ru(X)(Y), suggest that the separations d(Ru-P) for the mutually trans-arranged phos- phines are particularly sensitive to the steric demand of the two additional ligands X and Y, increasing in the order X, Y = H, H [2.283(7) A] < H, Et [2.313(3) i&II7 < -CH2CHz-- [2.326(3) A]” x H, NHPh [2.329(9) A]‘” z H, O&H.,Me-4 [2.330(3) A12”,‘” < -CH2C(=CHR)O-* [2.343(2)

* Ligand portion of a flat four-membered metallacycle (dihedral angle, 1 .9°).27d

TLigand portion of a tilted four-membered metal- lacycle (dihedral angle, 45.6”).27d

$ Ligand portion of a twisted five-membered metal- lacycle (twist angle, 3.0°).27c

A]27d < -CH,C(O)CH,-_t [2.355(3) A]27d x -o- &,H.&(dH&)-$ [2.357(l) A]:7c as shown by the distances given parenthetically as mean values for the two tram bonds.

For the four independent molecules of structure 2, the average separation between the central atom and the terminal MerP group tram to one of the hydride ligands, 2.292(6) A, is longer than that of 2.270(5) 8, between the metal and the bridging phos- phorus atom of the tripodal-tetradentate ppj system, but is barely elongated when set against the average length of the Ru-P bonds in the tram P-Ru-P moieties of the four molecules, 2.282(9) A. These parameters can be compared directly with those of several aryl(hydrido)ruthenium(II) deriva- tives of the pp3 ligand, as there exist only marginal differences in tram influencing properties between the hydride and aryl groups. For a series of (pp,)Ru(H)(Ar) compounds with Ar = C6H5, C,H,Me-3, C,H,Me,-3,4, C,H,Me,-3,5 and C6H4CFz-3, the length of the bond between the

432 L. DAHLENBURG and K-M. FROSIN

Table 3. Selected bond lengths and angles for dihydride 2

Molecule 1 Lengths Angles (“)

(A) Ru(1) P(12) P(13) P(14) H(I2)

2.275(4) P(l1) 94.3(2) 96.0(2) 90.9(2) 90(l) 2.283(5) P(12) 102.9(2) 102.0(2) 176(l) 2.291(4) P(13) 153.5(2) 76(2) 2.276(4) F(l4) 79(2) H( 11) not localized 1.65(l) H(12)

Molecule 2 Lengths Angles (“)

(A) ku(2) P(22) Pt23) ~(24) H(21) H(22)

2.262(5) P(21) 93.0(2) 96.1(2) 91.2(2) 170(l) 91(l) 2.299(5) P(22) 102.4(2) 102.2(2) 94(2) 171(l) 2.292(4) ~(23) 153.9(2) 76(2) 78(2) 2.291(5) ~(24) 93(2) 77(2) 1.75(4) H(21) 77(2) 1.78(6) H(22)

Molecule 3 Lengths Angles (“)

(A) Ru(3) P(32) P(33) P(34)

2.273(4) P(31) 93.4(2) 2.296(5) P(32) 2.267(5) P(33) 2.272(4) P(34) H(31) not well refined H(32) not localized

96.2(2) 91.7(2) 103.6(2) 103.3(2)

151.4(2)

Molecule 4 Lengths Angles (“)

(A) Ru(4) P(42) P(43) PW)

2.269(4) P(41) 93.2(l) 2.288(5) P(42) 2.284(4) ,P(43) 2.285(5) PW H(41) not well refined H(42) not localized

96.4(2) 90.8(2) 102.4(2) 102.1(2)

154.0(2)

Averaged values : Lengths Angles CY

(A) Ru P(2) P(3) P(4)

2.270(5) P(1) 2.292(6) P(2) 2.284(10) P(3) 2.281(7) P(4)

93.5(5) 96.3(2) 91.2(4) 102.8(5) 102.4(5)

153.2(1.0)

Oligophosphine ligands-XXXVII 433

ruthenium atom and the MezP substituent opposite to Ru-H has * previously been determined as 2.342(5) A, on average. 9-1 ’ Thus, the distance of the Me,P group truns to hydrogen from the central metal is considerably shorter in (pp3)RuH2 than in (pp,)Ru(H)(Ar), and the same applies to the aver- age values obtained for d(Ru-PMe,) in the trans

P-Ru-P portions of the different molecules, 2.282(9) A for 2 but 2.310(6) A for the aryl hy- drides,” ’ as well as to the separations between the ruthenium centre and the bridging phosphorus atom trans to hydride or aryl, 2.270(5) A for dihy- dride 2 vs 2.289(3) 8, for the various aryl hydrido complexes. *’ ’ Ru-PMe2 bonds tram to hydrogen or phosphorus that are significantly longer than in structure 2 have also been observed for (pp,)Ru(H)(Cl), where the mutually trans-coor- dinated Me,P substituents and the MezP group tram to the Ru-H linkage are at distances from the ruthenium atom of 2.310(2) and 2.343(2) & respectively.7 Thus, the two families of complexes cis-(Me,P),Ru(H)(X) and (pp3)Ru(H)(X) (X = al- kyl, aryl, arylamide, aryloxide, halide) are struc- turally similar in that in both series of compounds it is the dihydride that is characterized by the shortest Ru-P distances, as long as bonds opposite to the very weak trans influence ligands containing highly electronegative donor atoms are not considered.

In structures 1 and 2 all P-Ru-P angles between phosphorus atoms at mutually cis pos- itions are considerably larger than 90”, while the axial Ru-P bonds trans to each other are bent towards the hydride ligands with P-Ru-P angles much smaller than 180”. Both deformations result from steric repulsions that are larger between the methyl substituents on the phosphorus atoms cis to each other than those between the metal-bound hydrogens and the cis-located PMe3 ligands and MezP substituents, respectively. In agreement with the stereochemically undemanding nature of the hydride, as compared to other anionic ligands, these distortions are much more pronounced in dihy- drides 1 and 2 than in cis-(Me,P),Ru(H)(X) or (pp3)Ru(H)(X), where X = alkyl, aryl, amide, alkoxide or halide. Thus, the sum of cis angles, Z(PRuP),,, which would be 450” for an ideal octa- hedron, adds up to 495.6” for compound 1 and 486.2” for the averaged structure 2. Values of Z(PRuP),, found for other cis-L,Ru complexes with trimethylphosphine ligands are w 479” for

* The corresponding angle in molecule 3 of structure 2 is smaller, 15 1.4”, presumably due to solid-state effects rather than different steric crowding within the coor- dination spheres of the individual molecules.

LqI&$%$HZ),‘7 482-483” for L,Ru(H)(Et)‘7 and L,Ru(H)(NHPh),26 and cu 484” for L,Ru (H)(OC6H4Me-4).26*28 In the structures of (pp,)R~(H)(cl)~ and (pp,)Ru(H)(Ar)“’ ’ (for aryl ligands Ar, see above), cis P-Ru-P angles, result- ing in approximately constant sums of 47&477”, have been observed in previous work.

The finding that the angle sum Z(PRuP),, obtained for dihydride 1 exceeds that of its chelated analogue 2 by nearly lo”, and the observation that the angle trans P-Ru-P determined as 15 1.5” for cis-(Me,P),RuH, tends to be some two to three degrees smaller than that of 153.5-l 54.0” measured for three of the four independent molecules of struc- ture 2*, are in agreement with the view that there is a higher degree of steric crowding about the central metal in cis-(Me,P)4Ru than in (pp3)Ru systems.6 Hence, a 16e fragment, cis-L,Ru(O), coordinated by the tripodal-tetradentate P[(CH2),PMe2] 3 ligand can indeed be thought of as being more open and more accessible at the central metal than a formally related fragment with four monodentate PMe, ligands bonded to the ruthenium atom. This in turn corresponds to the observed propensity of cis-

(Me,P),Ru(O) to stabilize by metal atom insertion across C-H bonds of its own ligands, ‘y,24 as con- trasted with the ability of (pp,)Ru(O) to form stable products by oxidative addition of solvent C-H bonds_%6,9- 1

In view of the limited accuracy obtained for the positions of the hydrido ligands of both 1 and 2, it is difficult to discuss and compare Ru-H bond lengths and angles.

Acknowledgements--We gratefully acknowledge gen- erous support by the Volkswagen-Stiftung (Hannover) and the Fonds der Chemischen Industrie (Frank- furt/Main). We are furthermore indebted to Degussa AG (Hanau) for generous gifts of previous metal salts.

4.

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