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

Abstract

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^I complexes. From a set of seven such phosphines L=L_xyz^(') (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 L₂ 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_CO varies in the expected sense, from 1967±1 cm^-1 for L_xy0 to 1978±1 cm^-1 for L_xy1 , and 2005 cm^-1 for L₀₀₀ '. The ¹⁰³ Rh NMR chemical shift δ_Rh varies from -288 ppm for L₀₀₀ to -316±1 ppm for L_10z or L_01z , and -436 ppm for L₀₀₀ '. The ν_CO and δ_Rh 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 (RhCl^δ- ⋅⋅⋅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.

Keywords: P ligands; coordination bond; nitroaromatic chemistry; nitroarylphosphines; phosphines; rhodium carbonyl complexes.