Synthesis and NMR investigation of dirhodium(4+) formamidinate complexes containing 2-(diphenylphosphino)pyridine as a bridging ligand. X-ray crystal structure of the complex Rh2(form)2(μ-PPh2Py)2(O2CCF3)2 (form = N,N'-di-p-tolylformamidinate)

The Rh2 4+ complex Rh2(form)2(O2CCF3) 2(H2O)2 (form = N,N′-di-p-tolylformamidinate) reacts with 2-(diphenylphosphino)pyridine in molar ratios 1:1 and 1:2, giving the complexes Rh2(form)2(μ-O2CCF 3)(μ-PPh2Py)(O2CCF3) (1) and Rh2(form)2(μ-PPh2Py)2(O 2CCF3)2 (2), respectively. The complexes were characterized by IR and 1H and 31P NMR spectroscopy. Both complexes exhibit a "lantern" type structure with the axial sites occupied by one or two monoligated trifluoroacetate groups, respectively. Complex 2 was also characterized by single-crystal X-ray diffraction analysis, crystallizing in the monoclinic space group P21/n, with a = 19.608 (2) Å, b = 19.384 (2) Å, c = 20.576 (3) Å, β = 117.16 (3)°, V = 6958.34 Å3, Z = 4, dc = 1.343 g/cm3. The two phosphorus ligands are observed in a cisoid arrangement, head-to-tail fashion. The Rh-Rh bond distance [2.5406 (6) Å] is appreciably greater than those observed in other formamidinate derivatives as consequence of the P⋯N separation. Conductivity measurements indicate that in acetonitrile solution the monoligated trifluoroacetate groups dissociate, giving rise to the cationic species [Rh2(form)2(μ-O2CCF 3)(μ-PPh2Py)]+ (3) and [Rh2(form)2(μ-PPh2Py)2] 2+ (5) that were isolated by reacting acetonitrile solutions of 1 and 2 with TlPF6. The same experiment carried out in acetone gives the monocationic complex [Rh2(form)2(μ-PPh2Py)2(O 2CCF3)]PF6 (4). The monoligated trifluoroacetate groups of 1 and 2 are displaced by halide ions, giving the complexes Rh2(form)2(μ-O2CCF 3)(μ-PPh2Py)X (X = Cl (6), Br (7)), Rh2(form)2(μ-PPh2Py)2(O 2CCF3)Cl (8), and Rh2(form)2(μ-PPh2Py)2I 2 (9). The reactions between Rh2(form)2(O2CCF3) 2(H2O)2 and PPh2Py, molar ratios 1:1 and 1:2, have been also investigated by 31P NMR spectroscopy. The 31P NMR spectra in CDCl3 at 225 K of the 1:1 mixture reveal the quasiquantitative formation of a monoaxial adduct together with small quantities of bisaxial adduct. On warming of solutions to 310 K, the monoadduct isomerizes to give a mixture of equatorial isomers. The same experiments carried out in acetonitrile lead to the formation of only one isomer. When the experiment is performed with the ratio of 1:2, the 31P NMR spectra, recorded at 225 K, show the formation of a bisadduct (δp = -96.05) that, on warming to 310 K, isomerizes to give an equatorial isomer that exhibits a broad doublet centered at δ 29.01. Both the 1:1 and 1:2 equatorial adducts are stable and do not undergo transformation into the axial isomers upon cooling the samples at 225 K.