Synthesis and characterization of rhodium(0) and rhodium(-I) complexes stabilized by 1,1'-bis(diphenylphosphino)ferrocene (dppf). Crystal structure of Rh(dppf)2 and [Na(THF)5][Rh(dppf)2].THF

The rhodium(I) complex stabilised by the organometallic ligand 1,1'-bis(diphenylphosphino)ferrocene (dppf), [Rh(dppf)(2)](+)(1) can be electrochemically reduced to the Rh(0) and Rh(-1) species, [Rh(dppf)(2)] (2) and [Rh(dppf)(2)](-) (3), respectively, in two reversible single-electron processes. These low-valent complexes have been obtained by the chemical reduction of 1 with sodium naphtalenide in tetrahydrofuran solution and their crystal and molecular structures determined by single-crystal X-ray analyses. The anionic complex [Rh(dppf)(2)](-) crystallises as sodium-solvated salt, [Na(THF)(5)][Rh(dppf)(2)]. THF (3) in the monoclinic system. The coordination geometry around the metal in the d(10) configuration is a slightly distorted tetrahedral in which the bite angles of the chelated diphosphines is 103 degrees (average) and the Rh-P bond distances is in the range of 2.230(5)-2.251(5) Angstrom. The sodium cation is surrounded by five molecules of THF in a slightly distorted trigonal-bipyramidal environment. Complex 2 is conveniently prepared by reacting equimolar amount of I and 3 in THF and crystallises in the triclinic system. The dihedral angle defined by the planes containing the atoms P(1)-Rh-P(2) and P(3)-Rh-P(4) is 75.2 degrees, indicating a geometry of the rhodium in the d(9) configuration intermediately between the highly distorted square-planar (dihedral angle 49.7 degrees), found earlier in the cationic d(8) species 1, and in the anionic d(10) complex (dihedral angle 95.0 degrees). Reduction of the metal centre causes a decrease in the Rh-P bond length of 0.034 Angstrom (average) from Rh(I) to Rh(0) and 0.071 Angstrom from Rh(0) to Rh(-1), respectively, with a concomitant increase of the bite angle of the diphosphine which ranges from 93.4 degrees (average) in [Rh(dppf)(2)](+) to 102.6 degrees in [Rh(dppf)(2)](-). The EPR spectroscopic properties of 2 and the analogue iridium derivative, [Ir(dppf)2], have been examined at 20 K in 2-methyltetrahydrofuran matrix. A linear dependence between the hyperfine constants and the M-P (M = Rh, Ir) bond distances has been evidenced. This fact suggests the possibility to use the phosphorus coupling as an indication of the metal-phosphorus bond length in similar complexes.