Synthesis and X-ray crystal structure of novel cobalt(II) complexes having oligopyridine ligands

Abstract

The quinquepyridine analogue L1, diquinolylterpyridine analogue L3 and a new oligopyridine, quinolylquaterpyridine L2, have been prepared by an iridium-catalyzed synthetic method. The quinquepyridine L1 has five pyridine rings which are connected to each other at the 2- and 6-positions of each pyridine ring. One of the two terminal pyridine rings of L1 was replaced by a quinoline ring to give L2, while both terminal pyridine rings were replaced by two quinoline rings to give L3. All of these oligopyridines produce the corresponding new cobalt(II) complexes. The reaction of cobalt(II) chloride with L1 gave the dinuclear cobalt(II) complex [CoClL1]2(CoCl4) (4a), having an interesting double-stranded helical structure, differing from the double helical dinuclear cobalt complexes previously reported by other groups. The structure of 4a was determined by single-crystal X-ray crystallography. In 4a, the two cobalt centers are equivalent, while the other dinuclear cobalt complexes had non-equivalent cobalt centers. Each cobalt center in 4a displays a distorted octahedral coordination environment, and L1 coordinates to each cobalt with “terpyridyl” and “bipyridyl” fragments. The reaction of L2 with cobalt(II) nitrate gave the dinuclear cobalt(II) complex [Co2L2(NO3)4] (5). Complex 5 has one ligand molecule, while the complex 4a has two ligand molecules. In 5, the terpyridyl fragment of L2 is coordinated to one cobalt center and the quinolylpyridyl fragment is coordinated to the other cobalt center. The two cobalt centers are bridged by one oxygen atom of a nitrate group. The reaction of L3 with cobalt(II) nitrate gave a similar dinuclear cobalt complex, [Co2L3(NO3)4] (6). In 6, the quinolylbipyridyl and quinolylpyridyl fragments each coordinate to a different cobalt center. The bridging nitrate group in 6 is coordinated to the two cobalt centers through two oxygen atoms.