SYNTHESIS AND CHARACTERIZATION OF A SERIES OF NOVEL RHODIUM AND IRIDIUM COMPLEXES CONTAINING POLYPYRIDYL BRIDGING LIGANDS - POTENTIAL USES IN THE DEVELOPMENT OF MULTIMETAL CATALYSTS FOR CARBON-DIOXIDE REDUCTION

A series of bis chelate rhodium(III) and iridium(III) complexes containing polypyridyl bridging ligands have been prepared. Complexes of the type [Rh(L)2Br2]+and [Ir(L)2CI2]+(where L = 2, 2′-bipyrimidine (bpm), 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3-bis(2-pyridyl)benzoquinoxaline (dpb)) have been prepared, and their syntheses and characterization are reported herein. The complex [Rh(dpq)2Br2](PF6)-CH3CN and the ligand 2,3-bis(2-pyridyl)quinoxaline have been crystallized and the crystal structures determined. [Rh(dpq)2Br2](PF6)·CH3CN crystallizes in the triclinic Pī space group with a = 12.3039 (32) Å,b = 12.8995 (28) Å, c = 13.6792 (34) Å, α = 98.856 (19)°, β = 105.344 (19)°, γ = 107.937 (18)°, Z = 2, R = 0.0773, and Rw= 0.0569. The ligand 2,3-bis(2-pyridyl)quinoxaline crystallizes in the P21/cspace group with a = 6.2166 (11) Å, b = 15.0398 (30) Å, c = 15.4147 (28) Å, β = 98.911 (15)°, Z = 2, R = 0.0518, and Rw= 0.0436. The electrochemical and spectroscopic properties of these novel bridging ligand complexes differ considerably from those of the previously prepared (2,2′-bipyridine)rhodium and iridium complexes. All of the bridging ligands used are easier to reduce than bipyridine. Thus, the complexes reported here are easier to reduce and exhibit electronic transitions at lower energy. A number of these new polypyridyl bridging ligand rhodium and iridium complexes catalyze the reduction of carbon dioxide to formate. Remote nitrogens on the polypyridyl bridging ligands make these complexes ideal building blocks for the development of multimetal systems. © 1990, American Chemical Society. All rights reserved.