The forgotten nitroaromatic phosphines as weakly donating P-ligands: An N-aryl-benzimidazolyl series in RhCl(CO) complexes
Résumé
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 RhI complexes. From a set of seven such phosphines L=Lxyz(′) (x, y, z=0 or 1=number of NO2 substituents at the 5, 6 and N‐Ph para positions, respectively), including the non‐nitrated parent L000 and its dicationic N‐methyl counterpart L000′, three LRhCl(COD) and seven L2RhCl(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 νCO varies in the expected sense, from 1967±1 cm−1 for Lxy0 to 1978±1 cm−1 for Lxy1, and 2005 cm−1 for L000′. The 103Rh NMR chemical shift δRh varies from −288 ppm for L000 to −316±1 ppm for L10z or L01z, and −436 ppm for L000′. The νCO and δRh 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 L000′, a tight electrostatic sandwiching of the Rh‐Cl bond by the benzimidazole moities is evidenced by X‐ray crystallography (RhClδ−⋅⋅⋅CN2+ ≈3.01 Å). Along with the LRhCl(CO) complexes, dinuclear side‐products (μ‐CO)(RhClL)2 were also obtained in low spectroscopic yield: for the dinitro ligand L=L011, a unique 1:6.7 clathrate structure, with dichloromethane as solvate, is also revealed by X‐ray crystallography.