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

Abstract

The reaction of Rh 2(O 2CCh 3) 4 with an excess of molten PhCONH 2 gave Rh 2(PhCONH) 4(PhCONH 2) 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 SbPh 3 and pyridine in CH 2Cl 2 ( 2) and Rh 2(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 CH 2Cl 2 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) Å 3, 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, SbPh 3. 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) Å, b = 18.250(2) Å, c = 11.754(1) Å, α = β = γ = 90.0°, V = 3517(1) Å 3, 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 CH 3CN exhibits a prominent oxidation wave at E 1/2 = 0.42 Λ (Δ E p = 100 m V), = 0.23 V (58 mV), and +0.36 V (65 mV), respectively. The pyridine, dithiane, PPh3 adducts of ( 1) in CH 2Cl 2 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 PPh 3 adduct displays another quasireversible oxidation at +1.1 V (90 mV). In the THF, the pyridine and PPh 3 adducts undergo oxidation at +0.35 V and +0.60 V, respectively. In DMF, the PPh 3 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, PPh 3 and SbPh 3 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.