Stereodynamics of the ligand-metal bond. Phosphorus-31 DNMR studies of internal rotation about rhodium-phosphorus bonds in trans-[RhX(CO)L2] complexes (X = halogen; L = tertiary phosphine)

Abstract

The "P('H) DNMR spectra of a series of rhodium(1) square-planar (d8) complexes of the type rrans-[RhX(CO)L,]. in which L = (t-C4H9)2PCH3, or (t-C4H9)2P(C6H~) and X = C1, Br, or I show changes consistent with slowing intramolecular conformational exchange. The 31P(lH) NMR spectrum of each complex at slow exchange can be interpreted in terms of different conformers. DNMR line-shape analyses of the exchange-broadened spectra speak for a specific stepwise interconversion among the various conformers rather than a nonselective exchange pathway. Such a stepwise itinerary is consistent with simple internal rotation about the rhodium-phosphorus bonds.Previous research in our laboratory has shed some light on the intramolecular stereodynamics of tertiary phosphines2 and bis(ph~sphines).~ In addition, we have examined the effects of complexation on carbon-phosphorus bond rotation in tertiary phosphine boranes4 and LM(C0)5 complexes (L = tertiary phosphine; M = Cr, Mo, W).s This work led us to examine the dynamic stereochemistry of a series of transbis(phosphine) halocarbonylrhodium( I) systems.Planar four-coordinate Rh(1) complexes play key roles in catalysis of the hydroformylation reaction (e.g., Rh [P-(c6H5)3]2(cO)cl) and the homogeneous hydrogenation of alkenes and acetylenes (e.g., Rh[P(C6H5)3]3C1).6 Solution conformational preferences and conformational exchange rates may be factors in determining the stereospecificity of asymmetric induction by related complexes.' Many studies of rhodium complexes using techniques such as electronic spectroscopy,* 'H and 31P nuclear magnetic resonance spectroscopy? infrared spectroscopy? and X-ray diffractionlo have shed light on the gross structure and the nature of metal-ligand bonding in these systems. There have been reports concerning "dynamic" N M R (DNMR) studies of ligand exchange and restricted internal rotation for complexes of various metals including Rh( I). l , I 2 However, for metal-phosphine complexes, there is generally a dearth of information regarding (a) University of Vermont; Alfred P. Sloan Research Fellow; Camille and Henry Dreyfus Teacher-Scholar. (b) Worcester Polytechnic Institute. (c) E. I. du Pont de Nemours and Co. (d) University of Michigan. Bushweller, C. H.; Brunelle, J. A. J. Am. Chem. SOC. 1973, 95, 5949. Brunelle, J. A,; Bushweller, C. H.; English, A. D. J. Phys. Chem. 1976, 80, 2598. Bushweller, C. H.; Brunelle, J. A. Tetrahedron Lerr. 1974, 893. Bushweller, C. H.; Lourandos, M. Z. Inorg. Chem. 1974, 13, 2514. (a) Livingstone, S. E. Compr. Inorg. Chem. 1973,3, 1237. (b) Osborn, J. A.; Jardine, F. H.; Young, F. H.; Wilkinson, G. J . Chem. SOC. A 1966, 1711. Tolman, C. A. Chem. Reo. 1977, 77, 313. Brady, R.; Flynn, B. R.; Geoffroy, G. L.; Gray, H. B.; Peone, J.; Vaska, L. Inorg. Chem. 1976, 15, 1485. (a) Brown, T. H.; Green, P. J. J . Am. Chem. Soc. 1970, 92,2359. (b) Brown, T. H.; Green, P. J. Ibid. 1%9,91 3378. (c) Mann, B. E.; Shaw, B. L.; Slade, R. M. J. Chem. SOC. A 1971, 2976. (d) Tolman, C. A.; Meakin, P. A.; Lindner, D. L.; Jesson, J. P. Meak...

Special effects of ortho-isopropylphenyl groups. Diastereoisomerism in platinum(ii) and palladium(ii) complexes of helically chiral PAr3 ligands

“…Tri(o-tolyl)phosphine (1a) was purchased from Strem and all other starting materials were purchased from Aldrich. Ligand 1c 2c and complexes 2a and 3a 11 have been previously reported. NMR spectra were measured on a Jeol Eclipse 300, Jeol Eclipse 400 or Jeol GX 400 spectrometer.…”

“…Restricted M-PR 3 rotations are common for bulky phosphines and barriers of up to 58 kJ mol −1 have been previously reported for square planar complexes of the type trans-[RhCl(CO)(PR 3 ) 2 ]. 11 The 31 P NMR spectrum of 2c at room temperature showed two singlets at d 26.4 and 24.7 (intensity ratio 3 : 1) which were assigned to trans-PtP 2 Cl 2 species on the basis of the 1 J(PtP) values of 2555 and 2603 Hz. The 1 H NMR spectrum of 2c was sharp and showed the signals for a 3 : 1 mixture of similar species.…”

Orientation and Restricted Rotation of Lopsided Aromatic Ligands. Octahedral Complexes Derived from cis-RuCl2(Me2SO)4

“…An example of hindered rotation about metal−phosphorus bonds has been reported for a square planar Rh(I) complex with bulky phosphines . Another particularly well-studied example concerns hindered rotation of dialkylamido ligands in some four-coordinate molybdenum dimers where, however, the Mo−NR 2 bond has a partial double-bond character. , …”

Observation of an Unusual Molecular Switching Device. The Position of One 1,2-Dimethylimidazole Switched “On” or “Off” the Rotation of the Other 1,2-Dimethylimidazole in cis,cis,cis-RuIICl2(Me2SO)2(1,2-dimethylimidazole)2

“…The phenomenon of restricted rotation about metal−ligand σ bonds in simple coordination compounds has been studied in detail in a few square-planar complexes − but has seldom been investigated in complexes with 6 or higher coordination number. , Higher-coordinate inorganic stereodynamic systems typically studied involve fluxional isomerism in π-bound ligands or scrambling in carbonyl clusters. , …”