Inorg. Chem. 1980, 19, 1400-1402
1400
Contribution from the Department of Chemistry, University of Nevada, Reno, Nevada 89557, and the Institut National de Recherche Chimique AppliquOe, 91710 Vert- 1e-Petit, France Isomerization Energetics for Palladium(I1) Complexes of 1-Substituted 3,4-Dimethylphospholes J. Jeffrey MacDougall,la Francois Mathey,lb and John H. Nelson*la
Received July 17, 1979
There has been a great deal of interest in square-planar bis(phosphine)(transition metal) complexes, especially since many of these complexes homogeneously catalyze a wide variety of organic reactions. The bis(phosphine) complexes of palladium(I1) and platinum(I1) have been widely used as models for studying this catalytic behavior and are themselves excellent catalysts.2 Although bis(phosphine)platinum(II) complexes are rather robust, palladium(I1) bis(phosphine) complexes are so labile that they often exist as mixtures of cis and trans isomers in ~ o l u t i o n ~and - ~ are believed to isomerize spontaneously at ambient temperatures without catalysk6 An investigation into the cis-trans isomerization thermodynamics and mechanism is important because in several proposed mechanisms for the catalytic behavior of complexes of these metals, cis-trans isomerism is p o ~ t u l a t e d . ~ ~ ~ Recent investigation^^^"^ of the geometrical isomerization of complexes of the type L2PdX2 have indicated that the isomerization process is associative, proceeding through pentacoordinate transition states as illustrated in eq 1. The 11
I L, L’ M X I X
various cmditions under which each pathway occurs have been discussed previou~ly.’~At the moment there is not much information on the solution behavior of pentacoordinate palladium(I1) species. In addition, there have been few pentacoordinate complexes of palladium(I1) with monodentate ligands and all but involve sterically undemanding phosphole ligands; however, each of these complexes is extensively dissociated in solution. There is evidence26 to suggest that the relative stabilities of ML2X2and ML3X2 species depend upon a subtle interplay of steric and electronic effects, although there are no clear reasons why this may be so. We have recently shown27that the sterically undemanding 1-substituted 3,4-dimethylphospholes (I) form stable L2MX2
I
R I
phosphole structure and ring numbering scheme complexes with palladium(I1). Spectroscopic and X-ray data27 indicate that the Pd-P bonds in these complexes are as strong as, if not stronger than, Pd-P bonds in similar phosphine complexes. With this in mind we have investigated, via variable-temperature 31P(1H)N M R , the solution behavior of L2PdX2complexes of l-R-3,4-dimethylphospholes(R = CH3, n-C4H9, t-C4Hg,C6H5,and CH2C6H5;X = C1-, Br-, N3-) in three .solvents in order to better understand the interplay of ligand size and electronic character and their effects on the geometrical isomerization process for palladium(I1). Experimental Section Proton-decoupled phosphorus-31 NMR spectra were recorded at 40.26 MHz on a JEOL FX-100 NMR spectrometer in Fourier
transform mode, equipped with a variable-temperature probe. The preparation of the c~mplexes,~’ as well as the preparation of the saturated solutions for NMR,5 has been previously described. Conductivity studies were performed at 25 f 0.1 OC as previously described.’6J8 Conductance ranges for electrolytes were taken from published values.28 ~
(1) (a) University of Nevada, Reno. (b) Institut National de Recherche
Chimique Appliquk. (2) H. Bruner and J. C. Bailar, Inorg. Chem., 12, 1465 (1973). (3) R. L. Keiter, Ph.D. Thesis, University of Maryland, College Park, Md.,
1967. (4) S. 0. Grim and R. L. Keiter, Inorg. Chim. Acta, 4, 56 (1970). (5) D. A. Redfield and J. H. Nelson, Inorg. Chem., 12, 15 (1973); A. W. Verstuyft, J. H. Nelson, and L. W. Cary, ibid., 15, 732 (1976); A. W. Verstuyft, L. W. Cary and J. H. Nelson, ibid., 14, 1495 (1975). (6) F. Basolo and R. G.Pearson, “Mechanisms of Inorganic Reactions”, 2nd ed., Wiley, New York, 1971, pp 423-7. (7) J. Kwaitek in “Transition Metals in Homogeneous Catalysis”, G.N. Schrauzer, Ed., Marcel Dekker, New York, 1971, p 13. (8) P. M. Maitlis, “The Organic Chemistry of Palladium”, Vols. I and 11, Academic Press, New York, 197 1. (9) P. Haake and R. M. Pfeiffer, Chem. Commun., 1330 (1969). (10) P. Haake and R. M. Pfeiffer, J . Am. Chem. SOC.,92, 4996 (1970). (1 1) P. Haake and R. M. Pfeiffer, J . Am. Chem. Soc., 92, 5243 (1970). (12) D. G. Cooper and J. Powell, J. Am. Chem. Soc., 95, 1102 (1973). (13) D. G. Cooper and J. Powell, Can. J . Chem., 51, 1634 (1973). (14) J. Powell and D. G. Cooper, J . Chem. Soc., Chem. Commun., 749 (1974). (15) D. A. Redfield and J. H. Nelson, J. Am. Chem. SOC.,96,6219 (1974). (16) D. A. Redfield, J. H. Nelson, R. A. Henry, D. W. Moore, and H. B. Jonassen, J . Am. Chem. Soc., 96, 6298 (1974). (17) W. J. Louw, J . Chem. SOC.,Chem. Commun., 353 (1974). (18) D. A. Redfield, L. W. Cary, and J. H. Nelson, Inorg. Chem., 14, 50 (1975). (19) A. W. Verstuyft and J. H. Nelson, Inorg. Chem., 14, 1501 (1975).
0020-1669/80/1319-1400$01.00/0
Results The 31P{1H)N M R spectra are straightforward in that only one singlet is observed for each isomer present in solution. When both cis and trans isomers are present in solution, the cis 31P(1H)resonance is broader than the trans resonance and appears downfield of the trans resonance in all cases. The ratios of isomers in solution were determined primarily by cutting and weighing, with integration by the FX-100 computer employed as an accuracy check. Each temperature (20) D. W. Allen, I. T. Millar, and F. G. Mann, J . Chem. SOC.A, 1101 (1969). (21) D. W. Allen, F. G. Mann, and I. T. Millar, Chem. Znd. (London),2096 (1966). (22) . . J. W. Collier, F. G.Mann, D. G.Watson, and H. R. Watson, J. Chem. Soc., 1803 (1964). (23) J. W. Collier and F. G.Mann, J . Chem. Soc., 1815 (1964). (24) . . D. W. Allen. F. G. Mann, and I. T. Millar, J . Chem. SOC.C,3937 (1971). (25) . . R. L. Bennet. M. I. Bruce, and F. G.A. Stone, J . Or.?anornel. Chem., 38, 325 (1972). (26) B. B. Chastain, E. A. Rick, R. L. Pruett, and H. B. Gray, J. Am. Chem. Soc., 90, 3994 (1968); M. Bressan and P. Rigo, Inorg. Chem., 14, 38
,(.,
