Syntheses, electrochemistry, and spectroscopy of dirhodium(II) tetra-acetamidate and tetrakis(trifluoroacetamide) complexes with axial Group 15 substituents. The X-ray crystal structures of [Rh2(CH3CONH)4(AsPh3)2] and [Rh2(CH3CONH)4 –n(CH3CO2)n(MPh3)2], M = As or Sb, n≈ 1

Abstract

The complexes [Rh₂(CH₃CONH)₄(MPh₃)₂] and [Rh₂(CF₃CONH)₄(MPh₃)₂], M = P, As, or Sb, have been synthesised, studied by electronic and Raman spectroscopy, and their electrochemical properties investigated. Single-crystal X-ray crystallographic studies of [Rh₂(CH₃CONH)₄(AsPh₃)₂](1) and [Rh₂(CH₃CONH)_{4 –n}(CH₃CO₂)_n(MPh₃)₂], M = As or Sb, n≈ 1 (2) and (3) respectively, have been carried out and, in each case, the structures are found to refine in the space group P with one centrosymmetric molecule per unit cell. For (2) and (3) the acetate group randomly replaces acetamidate to give an effective symmetry of C_i. The dirhodium tetra-bridged cage has the typical, near D_4h, symmetry with axially co-ordinated Group 15 ligands and Rh–Rh bond lengths ranging from 2.471 (2) to 2.461 (2)Å. The Rh–As bond lengths are 2.540(2) and 2.553(4)Å. for (1) and (2), respectively, and the Rh–Sb bond length is 2.699(3)Å for (3). The Rh–N and Rh–O bond lengths are significantly different from one another in (1)[2.014(6)vs. 2.077 (6)Å]; it is inferred from this that, in the tetra-acetamidate, there is no scrambling of the arrangement of dimers within the crystal, and that each dimer has no two oxygen or nitrogen atoms trans to each other. The wavenumber of the rhodium–rhodium stretching mode [ν(RhRh)] has been identified from the resonance Raman spectra and found to be 273.5, 283.5, and 294 cm^–1 for the phosphine, arsine, and stibine tetra-acetamidate complexes, respectively; ν(RhRh) is insensitive to trifluoroacetamidate for acetamidate substitution despite the fact that the oxidation potential differs by ca. 0.5 V for the differently bridged species. As far as the dirhodium fragment is concerned, the order of σ-donor ability of the bridging ligand is acetamide > acetate ≈ trifluoroacetamide, and of π-acceptor ability is acetamide ≈ trifluoroacetamide > acetate. The RhRh stretching frequency is more sensitive to the σ-donor than the π-acceptor properties of the axial ligand but to the π-acceptor rather than the σ-donor properties of the bridging ligand.