Synthesis and characterisation of rhodium(0) and rhodium(−1) complexes stabilised by 1,1′-bis(diphenylphosphino)ferrocene (dppf). Crystal structures 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 d10 configuration is a slightly distorted tetrahedral in which the bite angles of the chelated diphosphines is 103° (average) and the RhP bond distances is in the range of 2.230(5)–2.251(5) Å. 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 1 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°, indicating a geometry of the rhodium in the d9 configuration intermediately between the highly distorted square-planar (dihedral angle 49.7°), found earlier in the cationic d8 species 1, and in the anionic d10 complex (dihedral angle 95.0°). Reduction of the metal centre causes a decrease in the RhP bond length of 0.034 Å (average) from Rh(I) to Rh(0) and 0.071 Å from Rh(0) to Rh(−1), respectively, with a concomitant increase of the bite angle of the diphosphine which ranges from 93.4° (average) in [Rh(dppf)2]+ to 102.6° 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.