New Ligand Systems Incorporating Two and Three 4,4‘-Bipyridine Units. Characterization of Bi- and Trimetallic Rhodium and Iridium Complexes

The synthesis of new ligand systems based on the bipyridine unit for bi- and trimetallic complexes, including a rare
example of a chiral bimetallic complex, is presented. Ligands BBPX (bis-bipyridine-xylene, 3) and TBPBX (tris-bipyridine-bis-xylene, 4) were prepared in one step by reacting α,α‘-dibromo-o-xylene (2) with 2 equiv of the
monolithiated derivative of 4,4‘-dimethyl-2,2‘-bipyridine. Dilithium (S)-binaphtholate (5) reacted with 2 equiv of
4-bromomethyl-4‘-methyl-2,2‘-bipyridine (6), affording ligand (S)-BBPBINAP (bis-bipyridine-binaphtholate, 7). These
ligands reacted cleanly with 1, 1.5, and 1 equiv of the rhodium dimer [Rh2Cl2(HD)2] (HD = 1,5-hexadiene), respectively.
Chloride abstraction led to the isolation of the cationic complexes BBPX[Rh(HD)BF4]2 (8), TBPBX[Rh(HD)BF4]3
(10), and (S)-BBPBINAP[Rh(HD)BF4]2 (12). When BBPX (3), TBPBX (4), and (S)-BBPBINAP (7) were added to 2,
3, and 2 equiv of [Rh(NBD)2]BF4 or [Rh(NBD)(CH3CN)2]BF4 (NBD = norbornadiene), respectively, clean formation
of BBPX[Rh(NBD)BF4]2 (9), TBPBX[Rh(NBD)BF4]3 (11), and (S)-BBPBINAP[Rh(NBD)BF4]2 (13) was observed. The
neutral iridium complex (S)-BBPBINAP[IrCl(COD)]2 (14) was obtained by reaction of (S)-BBPBINAP (7) with 1
equiv of [Ir2Cl2(COD)2] (COD = cyclooctadiene). The complexes were fully characterized including X-ray structural
studies of 8, 9, and 13, and preliminary studies on their catalytic activity were performed.