Mono- and Bimetallic Bipyridyl Polyene Complexes Containing 17-Electron Molybdenum Mononitrosyl Centers: Electrochemical, Spectroscopic, and Magnetic Studies

Abstract

The monometallic bipyridyl complexes [MoTp*(NO)X(L-L)] [Tp* = HB(3,5-Me2C3HN2)3; L-L = 3,3‘-bipyridine, 4,4‘-bipyridine, 1,2-bis(4-pyridyl)ethane and X = Cl; L-L = 4,4‘-NC5H4(CHCH)nH4C5N, n = 0, 1, 2, 3, 4 (all trans) and X = Cl, I] and their bimetallic counterparts [{(NO)MoTp*X}2(L-L)] have been synthesized, together with [MoTp*(NO)Cl{4-NC5H4CHCHC(Me)CHCH}]2. The single crystal X-ray structure of [{MoTp*(NO)Cl}2{4,4‘-NC5H4(CHCH)4H4C5N}] confirms that the polyene chain is in the all-trans configuration and exhibits normal bond length alternation with [rCC−rCC] = 0.092 Å. The Mo−Mo distance is 20.764(3) Å, and the Mo−N(pyridyl) bond distance of 2.196(6) Å indicates a much lower degree of Mo−(N)ligand π bonding than that found in related amide complexes where Mo−N(amide) distances are typically some 0.3 Å shorter. Cyclic voltammograms of the monometallic complexes contain a single one-electron reduction process, whereas those of the bimetallic complexes contain two one-electron processes. The separation between the two reduction potentials varies almost linearly with polyene chain length from the statistical limit of 36 mV in the bipyridyl dimethyldecapentene derivative to 765 mV in the 4,4‘-bipyridyl derivative. The isovalent bimetallic complexes exhibit EPR signals (300 K) which show hyperfine coupling to two equivalent metal centers with J ≫ A and are consistent with the unpaired electrons occupying orbitals which are predominantly metal in character. Magnetic measurements indicate the presence of an antiferromagnetic interaction in [{MoTp*(NO)Cl}2{4,4‘-NC5H4(CHCH)4H4C5N}] with a coupling −2J estimated to be ca. 80 cm-1 over a distance of ca. 2 nm.