Synthesis, x-ray crystal structure, and electrochemical properties of the dirhodium(4+) complex Rh2(form)4 (form = N,N'-di-p-tolylformamidinate anion)

The reaction of the dirhodium(II) mixed-ligand complex Rh2(form)2(O2CCF3) 2(H2O)2 (form = N,N′-di-p-tolylformamidinate anion) with free N,N′-di-p-tolylformamidine (mole ratio 1:4) leads to the formation of the complex Rh2(form)4 (1) in excellent yield. The complex was characterized by a single-crystal X-ray diffraction analysis, crystallizing in the space group Pn3n, with a = 20.990 (2) Å, V = 9248 Å3, Z = 6, R = 0.036, and Rw = 0.0378. The Rh-Rh [2.4336 (4) Å] and the Rh-N [2.050 (5) Å] bond distances are well in the range observed for this class of compounds. The molecule is in a staggered conformation, with N-Rh-Rh-N torsion angles of 16.7 (2)°. Complex 1 reacts with carbon monoxide, giving the monocarbonyl adduct Rh2-(form)4(CO) (2), characterized by elemental analysis and IR, 13C NMR, and electrochemical measurements. Unlike other dirhodium(II) carbonyl derivatives, 2 is remarkably stable. In the 13C NMR spectrum a dramatic upfield shift is seen for the carbonyl resonance (δ 144.06); the IR spectrum shows v(CO) at 2040 cm-1. These data coupled to electrochemical measurements suggest that the π back-bonding is reasonably strong in 2. The electrode behavior of 1 has been investigated in different nonaqueous solvents. Complex 1 undergoes two anodic redox changes, Rh(II)-Rh(II)/Rh(II)-Rh(III) (reversible) and Rh(II)-Rh(III)/ Rh(III)-Rh(III) (irreversible), and one cathodic change, Rh(II)-Rh(II)/Rh(II)-Rh(I) (irreversible), under cyclic voltammetry. The ESR spectrum at 100 K of the species [Rh2(form)4]+ displays g⊥ = 2.062 and g∥ = 1.957. The g∥ is split into a 1:2:1 triplet by two equivalent rhodium nuclei (A∥ = 18.9 × 10-4 cm-1). The generated Rh(II)-Rh(III) species is quite stable, and it belongs to the completely delocalized "class III" (Robin-Day) mixed-valent derivatives.