Chemical oxidation of binuclear rhodium(I) complexes with silver salts. Synthesis, x-ray crystal structure, and electrochemical properties of the rhodium [Rh24+] mixed-ligand complex Rh2(Form)2(O2CCF3)2(H2O)2.0.5C6H6 (Form = N,N'-di-p-tolylformamidinate anion)

The facile chemical oxidation of the binuclear rhodium(I) formamidinate complex [Rh(C8H12)(Form)]2 (Form = [p-CH3C6H4NC(H)NC6H 4CH3-p]-) with AgO2CCF3 (mole ratio 1:4) yields the mixed-ligand Rh2 4+ complex Rh2(Form)2-(O2CCF3) 2(H2O)2. The complex reacts with Lewis bases such as pyridine, dimethyl sulfoxide, piperidine, and 4-methylimidazole to produce stable 1:2 axial adducts. The crystal structure has been determined by X-ray diffraction analysis. The green crystals are triclinic, space group P1, with a = 10.467 (1) Å, b = 13.151 (1) Å, c = 15.470 (2) Å, α = 70.68 (2)°, β = 88.89 (4)°, γ = 79.16 (3)°, V = 1971.6 Å3, Z = 2, Dcalcd = 1.609 g cm-3, R = 0.032, and Rw = 0.034. The molecule consists of two formamidinate and two trifluoroacetate groups symmetrically disposed about the Rh-Rh unit in the conventional fashion. Two water molecules are axially bonded at 2.315 (3) Å from the rhodium atoms. The striking features of the structure are the lengthening of the Rh-Rh (2.425 (1) Å) and Rh-Oeq (2.082 (3) Å) bond distances with respect to the analogous Rh2(O2CCF3)4(H2O) 2·2DTBN and the cisoid arrangement of the formamidinate groups. The Rh-Rh-Oax angles deviate significantly from linearity (167.9 (1) and 169.3 (1)°) as a consequence of steric interactions between the tolyl fragments and the water molecules that participate in hydrogen-bonding interactions throughout the crystal lattice. The electrochemistry in different nonaqueous solvents points out the ability of the title complex to undergo two subsequent one-electron reversible or quasi-reversible anodic processes, attributable to the RhIIRhII/RhIIRhIII and RhIIRhIII/RhIIIRhIII charge transfers, respectively. As expected, these anodic processes are easier than in the case of the corresponding tetracarboxylato species. Of the two electrogenerable anodic products, only the mixed-valence RhIIRhIII derivative is fully stable, and its stability has been quantitatively evaluated and discussed also with respect to that of the other dirhodium complexes.