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Po/yhedron Vol. 13, No. H/16, pp. 245S2458, 1994 Copyright 0 1994 Elsevier Science Ltd

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COMMUNICATION

A CASE OF SEVERE STERIC AGGRAVATION: X-RAY STRUCTURAL ANALYSIS OF

cis-Mo(CO)~(PC~,), (Cy = cycle-C&)

MARCUS WATSON and SIMON WOODWARD*

School of Chemistry, University of Hull, Kingston-upon-Hull, HU6 7RX, U.K.

and

GRbNNE CONOLE,* MARGALITH KESSLER and GEORGIA SYKARA

School of Applied Chemistry, University of North London, Holloway Road, London N7 8DB, U.K.

(Received 20 January 1994 ; accepted 29 March 1994)

Abstract-Despite being extremely sterically congested the molecule cis-Mo(C0)4(PCy3)2 is readily synthesized and characterized by X-ray crystallography which reveals that the primary method of relieving the steric strain is by substantial increase of the molybdenum- phosphorus bond distances [MO-P,,. = 2.654(4) A].

There is a popular piece of folklore in inorganic chemistry that placement of two highly bulky phos- phine ligands mutually cis in a metal complex is not viable. Such intuitive steric arguments can be semi- quantified by appealing to Tolman’s cone angle concept’ where the spatial requirements for ligands such as PPr’; (0,,,, = 160”), PCy, (&,,, = 170”) and PBu\ (e,,,, = 182”) are well above the idealized P-M-P angle of 90” in a cis complex. Because of these properties such phosphines, particularly PCy,, have been used to stabilize low coordination number geometries with large P-M-P angles. Some recent examples are : M(PCy3)2 (M = Pd, 158.4” ; M = Pt, 160.5°),2,3 Cr(CO)3(PCy3)2 (1 60.2”),4 and w(CO)3(PCy3)2 (1 60.94”),5 where the numbers in parentheses are the P-M-P angles. In conjunction with another project we recently had occasion to attempt the synthesis of cis-Mo (CO)4(PCy3), from cis-Mo(CO),(CSHlONH), by the method of Darensbourg.6 Surprisingly the com-

* Authors to whom correspondence should be addressed.

pound turns out to be easily isolated and highly crystalline. Apparently no other complex with two cis disposed PCy, ligands is crystallographically characterized. Only a few trigonal NiL(PCy3)2 com- plexes with small P-M-P angles are available: L = 1,2-q2-anthracene (1 18.3°),7 q2-CO2 (122.6”)* and q2-PhCHO (118.9’).’ Similarly, some related structures including: Ni(CO)2(PCy3)2 (122.8”),” Pt(q3-C3HS)(PCy3), (111 .2’),11 Pt(q2-CF3C=CCF3) (PCy3)2 (1 10.23”)12 and Pt(q2-CH&Hz)(PCy& (1 16.33’)13 are known.

RESULTS AND DISCUSSION

The experimental protocol presented here is opti- mized and represents the highest yield of compound that may be isolated, two byproducts are Mo(CO), and Mo(CO),(PCy,). An ORTEP representation of the compound appears in Fig. 1 together with summarized bond length and angle data. In addition a number of crystallographic parameters are defined to allow comparison of the structure with other c~~-Mo(CO)~(PR,)~ complexes. The

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metal adopts a distorted octahedral geometry. To avoid steric clashes the cyclohexyl rings interleave somewhat. However, the cone angle of the phos- phine is not greatly reduced as judged by the C-P-C angles in the coordinated phosphine [99.8(7)-107.1(7)“], compared to that of the free phosphine (C-P-C = 103.1, 103.2, 105.1”),‘4 rather steric relief is attained by dramatic increases in the Mo-P distances [2.649(4), 2.659(4) A]. Con- sistent with these elongated phosphine bonds the complex completely converts to its truns isomer after brief reflux in toluene while at room tem- perature in solution it shows a distinct tendency to dissociate one phosphine. As expected the Ma-CO bond lengths vary according to the a-acidity of the trans ligand. ” Shorter Mo-C(carbony1) bond distances are observed for the two carbonyl func- tions which lie tram to the weaker rc-acid PCy, groups compared to those which lie tram to one another and cis to the phosphines (Fig. 1). All four carbonyls bound to molybdenum are essentially linear with a MO-C-O range of 172(1)-178(l)“.

The complex cis-M0(C0)~(PCy,), is of con- siderable theoretical interest as the family of com- plexes Mo(CO),(PR,), are frequently used to assess

steric factors on phosphine o-donor/rr-acceptor properties.” Some years ago Cotton and Dar- ensbourg obtained the crystal structures of a num- ber of these complexes. These data together with two other related species are summarized in Table 1 together with phosphine cone angle, PK,,*~ and Mo-P bond strength values where known.17 While no relationship between electronic factors (as rep- resented by pK,) and geometrical factors is appar- ent the correlation of the bond distance and angle data to the cone angle is clear. In particular the fit of PR,(&,,) to the average Mo-P distance is essentially a straight line.

EXPERIMENTAL

General

Spectroscopic instrumentation has been described in a previous publication.23 The complex c~~-Mo(CO)&H,,,NH)~ is available uiu literature procedures. Cone angles for the phosphines PPhCl(NHPf) and PPh,(CH,NHTol-4) were cal- culated by the method of Tolman from con- formationally minimized energy structures

Fig. 1. Molecular structure of cis-Mo(CO)~(PC~,), and definition of the parameters in Table 1. The

atom numbering scheme is included but hydrogen atoms are omitted for clarity. Some important bond distances are: Mo-P(1) 2.649(4), Mo-P(2) 2.659(4), Ma-C(l) 1.967(18), Ma-C(2) 1.970(16),

Ma-C(3) 2.007(17), Mo-C(4) 2.022(17) A; bond angles can be found in Table 1.

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generated by the programmes CHARMm and QUANTA developed by Molecular Simulations Inc.

Preparation ofcis-Mo(CO),(PCy&

Solid PCy, (0.75 g, 2.68 mmol) was added to a solution of cis-Mo(C0)4(CSH10NH)2 (0.50 g, 1.32 mmol) in CH$& (40 cm3) and the mixture refluxed (20 min). The brown solution was allowed to cool to room temperature, filtered through Celite and evaporated to a dark oil which was re-dissolved in CH,Cl, (10 cm3). Methanol (25 cm’) was added, the last 5 cm3 slowly, and the solution cooled (- 5°C 2 h). The bright yellow crystals of cis-Mo(CO), (PCy,), were isolated by filtration and dried under a stream of nitrogen, 0.40-0.50 g (42-53%). M.p. 113-114” (dec.); m/z(e.i.) 715 (M+ 1-2CO); v,,/cm-’ (KBr disc) 2941m (CH), 2860m (CH), 2002s (CO), 1898s (CO), 1881s (CO), 1851s (CO) and 1450m ; CT&, (270 MHz, CDCl,) 1.12-2.02 (m, 66 H, Cy) ; & (67.8 MHz, CDC13) 26.5 (CH,), 28.1 (CH,), 29.8 (CH,), 37.3 (CH), 213.6 (apparent t, Jpc = 9 Hz, CO) and 216.4 (apparent t, Jpc = 8 Hz, CO); 6, (109.3 MHz, CDC13) 37.0 (PCy,). The cis-Mo(C0)4(PCy3), prepared by this route gave elemental analyses comparable to literature prep- arations of the tram complex.”

Structure determination and reJinemen?

Data for cis-Mo(CO),(PCy,), were collected on a Philips PW 1100 four circle diffractometer in the B-range 3-21”, with a scan width of 0.80”, using the technique described previously.26 The data were corrected for Lorentz and polarization factors and equivalent reflections were merged to give a total of 1980 unique data with Z/o(Z) > 3.0. Absorption corrections were applied to the data after initial refinement with isotropic thermal parameters for all atoms*’ to give the following data. Cis-Mo (CQ)4(pCy3),, C40H66M~P2Q4, M = 768.9, mono- clinic, space group P2,/n, a = 10.868(2), b = 25.386, c = 14.847(3) A, /3 = 94.82(2)“, U = 4081.74 A3, D, = 1.251 g crx3, Z= 4, F(OO0) = 1640. A bright yellow crystal of size 0.11 x 0.12 x 0.24 mm, ~(Mo-Z&) = 4.1 cm-’ was used in the data collection. The structure was solved by standard Patterson techniques. All atoms except the cyclohexyl hydrogens were detected in sub- sequent difference-Fourier syntheses. The former were geometrically constrained to reside on the rel- evant carbon atoms with fixed thermal parameters of 0.08 A3, but their parameters were not refined.

The molybdenum atom, both phosphorus atoms, and the carbon and oxygen atoms of the four car-

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bony1 ligands were assigned anisotropic thermal 12. parameters in the final cycles of the full-matrix refinement which converged at R = 0.0675 and 13. R, = 0.0650 with weight of w = l/a’ F, assigned to individual reflections.

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15. AcknowledgementsWe thank the School of Chemistry and for generous financial support of this project and to the SERC mass spectroscopy service for FAB mass spectra. 16.

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REFERENCES

C. A, Tolman, Chem. Rev. 1977,77,313. A. Immirzi and A. Musco, J. Chem. Sot., Chem. Commun. 1974,400. A. Immirzi, A. Musco, P. Zambelli and G. Carturan, Znorg. Chim. Acta 1975, 13, L13. K. Zhang, A. A. Gonzalez, S. L. Mukerjee, S.-J. Chou, C. D. Hoff, K. A. Kubat-Martin, D. Barnhart and G. J. Kubas, J. Am. Chem. Sot. 1991,113,9170. H. J. Wasserman, G. J. Kubas and R. R. Ryan, J. Am. Chem. Sot. 1986,108,2294. D. J. Darensbourg and R. L. Kump, Znorg. Chem. 1978,17,2680. D. J. Brauer and C. Kruger, Znorg. Chem. 1977, 16, 884. A. Dohring, P. W. Jolly, C. Kruger and M. J. Romao, Z. Naturforsch. B 198540,484. J. Kaiser, J. Sieler, D. Walther, E. Dinjus and L. Golic, Acta Cryst., Sect. B 1982,38, 1584. A. Del Pra, G. Zanotti, L. Pandolfo and P. Segala, Cryst. Struct. Commun. 1981, 10,7. J. D. Smith and J. D. Oliver, Znorg. Chem. 1978, 17, 2585.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

J. F. Richardson and N. C. Payne, Can. J. Chem. 1977,55,3203. H. C. Clark, G. Ferguson, M. J. Hampden-Smith, B. Kaitner and H. Ruegger, Polyhedron 1988,7, 1349. J. A. Davies, S. Dutremez and A. A. Pinkerton, Znorg. Chem. 1991, 30, 2380. S. Woodward, in Comprehensive Organometallic Chemistry (Edited by E. Abel, F. G. A. Stone and G. Wilkinson), Vol. 5, 2nd edn. Pergamon, Oxford, in press. M. M. Rahman, H.-Y. Liu, K. Eriks, A. Prock and W. P. Giering, Organometallics 1989, 8, 1. S. P. Nolan and C. D. Hoff, J. Organomet. Chem. 1985,290,365. T. G. Hill, R. C. Haltiwanger, T. R. Prout and A. D. Norman, Znorg. Chem. 1989,28,3461. F. A. Cotton, D. J. Darensbourg, S. Klein and B. W. S. Kolthammer, Znorg. Chem. 1982, 21, 2661. F. A. Cotton, D. J. Darensbourg, S. Klein and B. W. S. Kolthammer, Znorg. Chem. 1982, 21,294. F. A. Cotton, D. J. Darensbourg, S. Klein and B. W. S. Kolthammer, Znorg. Chem. 1982, 21, 1651. D. L. Davies, F. I. Healey, J. Howarth, D. R. Russell and L. J. S. Sherry, J. Organomet. Chem. 1989,376, c31. J. Green, E. Sinn, S. Woodward and R. Butcher, Polyhedron 1993, 12, 99 1. F. G. Moers and J. G. A. Reuvers, Reel. Trav. Chim. Pays-Bus 1974,93,246. G. M. Sheldrick, SHELX-76, Program for Crystal Structure Determination, University of Cambridge (1976). M. K. Cooper, P. J. Guemey, H. J. Goodwin and M. McPartlin, J. Chem. Sot., Dalton Trans. 1982, 757. N. Walker and D. Stuart, Acta Cryst., Sect. A 1983, 39, 158.