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Synthesis and Crystal Structures of <i>P</i>-Iron-Substituted Phosphinoborane Monomers
A family of <i>P</i>-iron-substituted phosphinoboranes,
CpÂ(CO)<sub>2</sub>FeÂ{PÂ(Ar)ÂBMes<sub>2</sub>} (Ar = Ph, Mes, Tipp, Mes*),
have been prepared from the reaction of CpÂ(CO)<sub>2</sub>FeCl and
(Li)Â(Ar)ÂPBMes<sub>2</sub>. All the complexes have been characterized
successfully by <sup>1</sup>H, <sup>11</sup>B, and <sup>31</sup>P
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 <sup>31</sup>PÂ{<sup>1</sup>H} 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 <sup>1</sup>H 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 Pî—»B double-bond
character is balanced by steric and electronic effects of the substituents
around the phosphorus