The Forgotten Nitroaromatic Phosphines as Weakly Donating P-ligands: An N-Aryl-benzimidazolyl Series in RhCl(CO) Complexes.

The coordination chemistry of a priori weakly σ-donating nitroaromatic phosphines is addressed through a series of nitro-substituted (N-phenyl-benzimidazol-1-yl)diphenylphosphines in Rh complexes. From a set of seven such phosphines L=L (x, y, z=0 or 1=number of NO substituents at the 5, 6 and N-Ph para positions, respectively), including the non-nitrated parent L and its dicationic N-methyl counterpart L ', three LRhCl(COD) and seven L RhCl(CO) complexes have been obtained in 72-95 % yield. Despite of a cis orientation of the L and CO ligands, the C=O IR stretching frequency ν varies in the expected sense, from 1967±1 cm for L to 1978±1 cm for L , and 2005 cm for L '. The Rh NMR chemical shift δ varies from -288 ppm for L to -316±1 ppm for L or L , and -436 ppm for L '. The ν and δ probes thus reveal moderate but systematic variations, and act as "orthogonal" spectroscopic indicators of the presence of nitro groups on the N-Ph group and the benzimidazole core, respectively. For the dicationic ligand L ', a tight electrostatic sandwiching of the Rh-Cl bond by the benzimidazole moities is evidenced by X-ray crystallography (RhCl ⋅⋅⋅CN ≈3.01 Å). Along with the LRhCl(CO) complexes, dinuclear side-products (μ-CO)(RhClL) were also obtained in low spectroscopic yield: for the dinitro ligand L=L , a unique 1:6.7 clathrate structure, with dichloromethane as solvate, is also revealed by X-ray crystallography.