Boryl/Borane Interconversion and Diversity of Binding Modes of Oxygenous Ligands in PBP Pincer Complexes of Rhodium

A series of Rh complexes
derived from a PBP-type pincer ligand
have been synthesized and characterized. It was previously reported
that reaction of [(COD)­RhCl]2 with 2,2′-bis­(diisopropylphino)­triphenylborane
(1) resulted in a mixture of complexes containing a Z-type borane interaction (2-Cl), a boryl pincer
(3a-Cl), and a η2 binding of the B–Ph
bond to Rh (4-Cl). In this work, we demonstrate that
analogous complexes are accessible by replacement of chloride with
potentially bidentate acetylacetonate, carboxylate, and trifluoromethanesulfonate
ligands. In addition, a new type of isomer was observed in complexes
with acetate and pivalate, where the carboxylate bridges between Rh
and B (3b-OAc, 3b-OPiv). All of these types
of complexes are isomeric, and the preference for particular isomers
for different anionic ligands varies. These isomers differ and are
related by a change in the coordination mode of the oxygenous ligands
and the migration of the Ph group between B and Rh.