Syntheses, structures and redox properties of tetrakis-(μ-benzamidato)dirhodium(II) complexes

Abstract

The reaction of Rh₂(O₂CCh₃)₄ with an excess of molten PhCONH₂ gave Rh₂(PhCONH)₄(PhCONH₂)₂ (1) which contains a dirhodium unit having four bridging benzamidato anions and two axial, neutral benzamide ligands. The compound has been characterized by elemental analysis and infrared spectroscopic data. Reactions of (1) with SbPh₃ and pyridine in CH₂Cl₂ (2) and Rh₂(PhCONH)₄(py)₂ (3), respectively, The structures (2) and (3) were determined from single-crystal X-ray diffraction data. Compound (2) crystallizes in the triclinic space group P1 with half of the dinner and half of the CH₂Cl₂ molecule constituting the asymmetric unit. The cell dimensions for (2) are: a = 12.927(7) Å, b = 13.661(5) Å, c = 10.242(6) Å α = 103.45(4)°, β = 99363(4)°, γ = 67.44(4)°, V = 1622(1) Å3, and Z = 1. The molecule, which has a crystallographic center of inversion, consists of a dirhodium unit bridged by four benzamidato ligands. Each rhodium has one axial ligand, SbPh₃. The Rh---Rh distance is 2.463(1) Å and the Rh---Sb bond length is 2.681(1) Å. The mean Rh---O and Rh---N distances are 2.029[4] Å and 2.056[3] Å, respectively. The Rh---Rh---Sb angle is essentially linear having a value of 176.40(2)°. The compound (3) crystallizes in the orthorhombic space group Pbca with the following unit cell dimensions: a = 16.395(2) À, b = 18.250(2) À, c = 11.754(1) Å, α = β = γ = 90.0°, V = 3517(1) ų, and Z = 4. Half of the molecule has two pyridine ligands in the axial positions. The equatorial positions are occupied by four anionic benzamidato ligands as in (2). The Rh---Rh distance is 2.437(1) Å. The distance between the rhodium and the axial pyridine nitrogen atoms is 2.227(7) Å and the angle Rh---Rh---N(3) is 175.5(2)°. The averages of the Rh---O and Rh---N bond lengths are 2.059[6] Å and 2.050[7] Å, respectively. Electron transfer behavior of (1) in different solvents and in the presence of strong neutral donor ligands has been investigated. The compound (1) in THF, DMF and CH₃CN exhibits a prominent oxidation wave at E_½ = 0.42 δ (δE_p = 100 m V), = 0.23 V (58 mV), and +0.36 V (65 mV), respectively. The pyridine, dithiane, PPh3 adducts of (1) in CH₂Cl₂ show the first oxidation at +0.39 V (60 m V), +0.48 V (90 mV), +0.46 V (65 mV) and +0.47 V (200 mV), respectively. The PPh₃ adduct displays another quasireversible oxidation at +1.1 V (90 mV). In the THF, the pyridine and PPh₃ adducts undergo oxidation at +0.35 V and +0.60 V, respectively. In DMF, the PPh3 adduct show oxidation at +0.50 V. In most of the cases, a second, less prominent oxidation wave was observed at slightly higher potential than the prominent oxidation peak. The electronic spectra of the pyridine, PPh3 and SbPh3 adducts show low energy absorption at 460 nm (shoulder), 445 nm (shoulder), and 508 nm (ε = 1520 M^-1 cm^-1, respectively. The electronic spectra of (1) studied in several polar solvents display bands in the range 440-563 nm.