Flexible coordination of diphosphine ligands leading to cis and trans Pd(0), Pd(II), and Rh(I) complexes.

A series of diphosphine ligands (i)Pr2P-C6H4-X-C6H4-P(i)Pr2 (for ligand L(1), X = CH2; for ligand L(2), X = CH2CH2) was investigated to determine the preference for cis/trans coordination to palladium(0), palladium(II), and rhodium(I). Increasing the length of the bridging alkyl backbone from one to two carbons changes the geometry of the resulting palladium(II) complexes, with L(1) coordinating preferentially cis, while L(2) coordinates in a trans fashion. Coordination to Pd(0) leads to L(1)Pd(dba) and L(2)Pd(dba), in which both ligands accommodate a P-M-P angle close to 120°. L(2) was found to coordinate cis in a rhodium(I) complex ([L(2)Rh(nbd)][BF4], where nbd = norbornadiene).