MONOMETALLIC AND BIMETALLIC BIPYRIDYL POLYENE COMPLEXES CONTAINING 17-ELECTRON MOLYBDENUM MONONITROSYL CENTERS - ELECTROCHEMICAL, SPECTROSCOPIC, AND MAGNETIC STUDIES

The monometallic bipyridyl complexes [MoTp(NO)X(L-L)] [Tp* = HB(3,5-M e(2)C(3)HN(2))(3); L-L = 3,3'-bipyridine, 4,4'-bipyridine, 1,2-bis(4-p yridyl)ethane and X = Cl; L-L = 4,4'-NC5H4(CH=CH)(n)H4C5N, n = 0, 1, 2 , 3, 4 (all trans) and X = Cl, I] and their bimetallic counterparts [{ (NO)MoTpX}(2)(L-L)] have been synthesized, together with [MoTp*(NO)Cl {4-NC5H4CH=CHC(Me)=CHCH=}](2). The single crystal X-ray structure of [ {MoTp-(No)Cl}(2){4,4'-NC5H4(CH=CH)(4)H4C5N}] confirms that the polyen e chain is in the all-trans configuration and exhibits normal bond len gth alternation with [r(c-c)-r(c=c)] = 0.092 Angstrom. The Mo-Mo dista nce is 20.764(3) Angstrom, and the Mo-N(pyridyl) bond distance of 2.19 6(6) Angstrom indicates a much lower degree of Mo-(N)ligand pi bonding than that found in related amide complexes where Mo-N(amide) distance s are typically some 0.3 Angstrom shorter. Cyclic voltammograms of the monometallic complexes contain a single one-electron reduction proces s, 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 o f 36 mV in the bipyridyl dimethyldecapentene derivative to 765 mV in t he 4,4'-bipyridyl derivative. The isovalent bimetallic complexes exhib it EPR signals (300 K) which show hyperfine coupling to two equivalent metal centers with J much greater than A and are consistent with the unpaired electrons occupying orbitals which are predominantly metal in character. Magnetic measurements indicate the presence of an antiferr omagnetic interaction in [{MoTp(NO)Cl}(2){4,4'-NC5H4-(CH=CH)(4)H4C5N} ] with a coupling -2J estimated to be ca. 80 cm(-1) over a distance of ca. 2 nm.