Synthesis and Crystal Structures of <i>P</i>-Iron-Substituted Phosphinoborane Monomers

Abstract

A family of <i>P</i>-iron-substituted phosphinoboranes, Cp­(CO)₂Fe­{P­(Ar)­BMes₂} (Ar = Ph, Mes, Tipp, Mes*), have been prepared from the reaction of Cp­(CO)₂FeCl and (Li)­(Ar)­PBMes₂. All the complexes have been characterized successfully by ¹H, ¹¹B, and ³¹P NMR; IR spectroscopy; and X-ray crystallography. In the IR spectra, all the complexes display similar carbonyl stretching frequencies that are remarkably higher than those of closely related phosphide complexes. These observations indicate that a repulsive interaction between the filled d orbital on the iron and the lone pair on the phosphorus is less severe in the studied iron-phosphinoboranes, which is most likely because of the P→B π interaction that occurs in them. The ³¹P­{¹H} NMR chemical shifts of the phosphinoborane phosphorus move upfield with the increasing steric bulk of the Ar groups in the order Ph (−51.4 ppm) < Mes (−68.8 ppm) < Tipp (−84.9 ppm). However, the phosphorus bearing the most sterically demanding Mes* group appears at an unexpectedly downfield value of −44.9 ppm, which is probably reflective of its structural peculiarities. The ¹H NMR spectrum of each complex displays two sets of signals, assignable to inequivalent Mes groups on the boron atom, as a consequence of a hindered rotation around the P–B bond. This high rotational barrier most likely results from the significant double-bond character in the P–B bond. The X-ray diffraction studies have confirmed the iron-phosphinoboranes considered herein to be monomeric species. Each molecule consists of a nearly planar phosphinoborane fragment with a short P–B bond. The Fe–P bond is notably elongated as the Ar group becomes larger, demonstrating its somewhat vulnerable nature with respect to steric congestion. In contrast, the variation in the P–B bond distance is relatively small throughout the series of iron-phosphinoboranes, suggesting that the PB double-bond character is balanced by steric and electronic effects of the substituents around the phosphorus