Synthesis of asymmetric dirhodium(4+) complexes. X-ray crystal structure of Rh2(form)3(NO3)(PPh3).0.5CH2Cl2 and Rh2(form)3(NO3)(C5H5N) (form = N,N'-di-p-tolylformamidinate)

The reaction of the paramagnetic Rh2 5+ complex Rh2(form)3(NO3)2 (form = N,N′-di-p-tolylformamidinate) with an excess of the neutral ligands PPh3, pyridine, and dimethylamine leads, via reductive elimination of one nitrate group, to the formation of the asymmetric Rh2 4+ complexes Rh2(form)3(NO3)L (L = PPh3 (1), pyridine (2), N(CH3)2H (3)). The two rhodium atoms in complexes 1-3 display different coordinations and reduced average formal oxidation states. The complexes have been characterized by IR, 31P NMR, and 103Rh NMR spectroscopic methods, which point out the nonequivalence of the two rhodium atoms. Complexes 1 and 2 have been also characterized by single-crystal X-ray analysis. Complex 1 belongs to the monoclinic space group system P21/n, with a = 21.529 (3) Å, b = 23.990 (3) Å, c = 11.944 (1) Å, β = 102.13 (4)°, Z = 4, and R = 0.057 for 2046 reflections with I > 3σ(I). The crystal structure reveals that one of the three formamidinate groups spanning the dimetal center (Rh-Rh = 2.498 (2) Å) is bonded in an unusual way, namely, σ,σ-N,N′ with a localized double bond. Complex 2 forms monoclinic crystals in space group P21/c with a = 10.238 (1) Å, b = 16.567 (2) Å, c = 28.356 (4) Å, β = 95.56 (3)°, Z = 4, and R = 0.041 for 3392 reflections with I > 3σ(I). In complexes 2 and 3 all of the bridging ligands are bonded in the usual fashion, namely σ,σ-N,N′ with delocalized double bonds.