Synthesis and Crystal Structures of P-Iron-Substituted Phosphinoborane Monomers

A family of P-iron-substituted phosphinoboranes, Cp(CO)(2)Fe{P(Ar)BMes(2)} (Ar = Ph, Mes, Tipp, Mes*), have been prepared from the reaction of Cp(CO)(2)FeCl and (Li)(Ar)PBMes(2). All the complexes have been characterized successfully by H-1, B-11, and P-31 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 pi interaction that occurs in them. The P-31{H-1} 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 H-1 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 anearly 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 are the phosphorus.