Formation and structure of Rh(0) complexes of phosphinine-containing macrocycles: EPR and DFT investigations

Electrochemical and chemical reductions of Rh(I) complexes of LP4 (a macrocycle containing four phosphinine rings) and of LP2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with 31P nuclei. An additional coupling (27 MHz) with 103Rh is detected, in the liquid state, for the spectrum obtained with [LP2S2Rh(0)]; moreover, resolved 31P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh(I) complexes of model macrocycles L‘P4 and L‘P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for [L‘P2S2Rh(0)], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (ρRh =0.35 for [L‘P2S2Rh(0)]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle