Metal Complexes with Cis alpha Topology from Stereoselective Quadridentate Ligands with Amine, Pyridine, and Quinoline Donor Groups.
Though the principles governing quadridentate topology and metal stereochemistry have been known for some time, the cis alpha topology has been little exploited in designing catalysts for asymmetric reactions. Investigation of the inorganic chemistry of labile metal cis alpha complexes was undertaken as a prelude to exploring their potential to serve as catalysts for a variety of different reactions. The synthesis of a series of first row transition metal complexes of quadridentate ligands with ethylenediamine (en) and S-propylenediamine (S-pn) backbones that have been alkylated at nitrogen with either pyridine (py) or quinoline (qn) donor groups as well as with noncoordinating benzyl (Bn) or pentafluorobenzyl (F(5)Bn) groups was undertaken. The steric and electronic properties vary throughout the ligand series, en(Bn)py, 1, en(F(5)Bn)py, 2, S-pn(F(5)Bn)py, 3, and S-pn(F(5)Bn)qn, 4. These ligands were reacted with MCl(n)() salts (n = 2, M = Mn, Fe, Co, Ni, Cu, Zn; n = 3, M = Fe) to generate, in most cases, octahedral complexes with the targeted cis alpha topology. UV/vis, NMR, IR, cyclic voltammetry (CV), and conductivity analysis are described for the metal compounds. X-ray structural analysis of [Cu{en(F(5)Bn)py}Cl]Cl reveals a five coordinate square pyramidal geometry. Single or major diastereomers were obtained for all diamagnetic Zn(II) complexes as well as for Co(III) analogues that were prepared by oxidation of Co(II) species using Br(2) as the oxidant. Electronic differences among ligands are reflected in the oxidation potentials of the respective metal complexes as determined by CV, with fluorinated systems showing greater resistance to oxidation, as expected.
Ng
Christina
C
Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22901.
Sabat
Michal
M
Fraser
Cassandra L.
CL