Different Coordination Modes of the Ph<sub>2</sub>PC<sub>sp<sup>3</sup></sub>PPh<sub>2</sub> Pincer Ligand in Rhodium
Complexes as a Consequence of C<sub>sp<sup>3</sup></sub>–H
Metal Interaction
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Abstract
Starting
from commercially available 4,4′-di-<i>tert</i>-butyldiphenylmethane
the pincer ligand bis(4-<i>tert</i>-butyl-2-(diphenylphosphino)phenyl)methane
(<b>PCP</b>) was
prepared in two steps in moderate yield. Treatment of a solution of
RhCl<sub>3</sub>·3H<sub>2</sub>O in a mixture of isopropyl alcohol
and toluene with equimolar amounts of <b>PCP</b> gave the dimeric
rhodium complex <b>1</b>. In an electrophilic metalation a facially
coordinated pincer complex is formed. When <b>PCP</b> is treated
with [CODRhCl]<sub>2</sub> in a solution of pyridine, the square-pyramidal
complex <b>2</b> is generated where the bis-phosphine <b>PCP</b> acts as bidentate ligand that coordinates in a cis fashion.
SnCl<sub>2</sub> inserts into the Rh–Cl bond of <b>2</b>, which results in an oxidative addition of one of the methylene
C–H bonds to form the Rh(III) complex <b>3</b>, where
the <b>PCP</b> ligand coordinates in a meridional way. A 2 equiv
portion of <b>PCP</b> reacts with 1 equiv of [CODRhCl]<sub>2</sub> in the presence of the electron-donating ligands HPhPC<sub>6</sub>H<sub>4</sub>NMe<sub>2</sub>, PPh<sub>2</sub>Py, and PPh<sub>3</sub>, respectively, as well as with stanna- and germa-<i>closo</i>-dodecaborate to give the octahedral Rh(III) complexes <b>4</b>–<b>8</b>. Attempts to remove the HCl with KO<sup>t</sup>Bu from complexes <b>4</b>–<b>6</b> produces the
planar Rh(I) compounds <b>9</b> and <b>10</b>. No carbene
formation has been observed
