Reactions of a coordinatively unsaturated rhodium hydride complex with isocyanides. Preparation and dynamic behavior of (μ-H)2(RNC)Rh2[P(O-i-Pr)3]4 and (μ-RNC)2Rh2[P(O-i-Pr)3]4

Article abstract of DOI:10.1021/ic00255a021

The reaction of the coordinatively unsaturated binuclear complex (μ-H)₂Rh₂[P(O-i-Pr)₃]₄ with 1 equiv of an isocyanide RNC yields the monoadduct (μ-H)₂(RNC)Rh₂[P(O-i-Pr)₃]₄. These complexes adopt an unsymmetrical structure with one trigonalbipyramidal and one square-pyramidal rhodium center. In the case of the relatively compact isocyanides with R = 4-ClC₆H₄, PhCH₂, and n-Bu, the isocyanide symmetrically bridges the two rhodium centers, while for larger isocyanides such as t-BuNC, steric interactions force the isocyanide into a position in which it is strongly associated with the trigonal-bipyramidal Rh and only weakly bonded to the other Rh. For all the monoadducts, a rapid fluxional process exchanges the Rh and P environments and generates a symmetrical time-averaged structure at room temperature. Variable-temperature ³¹P{¹H} NMR spectroscopy was used to obtain the activation parameters for this process; the value of ΔH? ranges from 9.7 to less than 7.7 kcal/mol, while ΔS? is nearly zero. Treatment of (μ-H)₂(RNC)Rh₂[P(O-i-Pr)₃]₄ with additional RNC results in a complex fragmentation reaction, with the product distribution dependent upon the nature of the isocyanide. In addition to mononuclear fragmentation products, the zerovalent binuclear compound (μ-RNC)₂Rh₂[P(O-i-Pr)₃]₄ is formed, particularly in the case of R = 4-ClC₆H₄. The compound (μ-4-ClC₆H₄NC)₂Rh₂[P(O-i-Pr) ₃]₄ crystallizes in the monoclinic space group C2/c with a = 15.526 (2) A, b = 20.351 (4) A, c = 20.975 (3) A, β = 94.34 (1)°, and Z = 4. Its X-ray crystal structure (R = 0.030, R_w = 0.038 for 3045 data with I > 3σ) shows a nonplanar geometry at each Rh atom with a bent pair of bridges (dihedral angle of 135.8° between the Rh₂C planes) and a Rh-Rh separation of 2.640 (1) A. The chemistry of the isocyanide compounds is compared and contrasted with that of their CO analogue.