SOME NEW DIPYRIDYL AND DIPHENOL BRIDGING LIGANDS CONTAINING OLIGOTHIENYL SPACERS, AND THEIR DINUCLEAR MOLYBDENUM COMPLEXES - ELECTROCHEMICAL SPECTROSCOPIC AND LUMINESCENCE PROPERTIES

A series of new ligands has been prepared in which pyridyl (L(1)-L(5)) or phenol (H(2)L(7)) end-groups are linked by oligothienyl chains: 2, 5-bis(4-pyridyl)thiophene (L(1)); 5,5'-bis(4-pyridyl)-2,2'-bithiophene (L(2)); 5,5'-bis(4-pyridyl)-2,2':5',2 ''-terthiophene (L(3)); 5,5'''- bis(4-pyridyl)-2,2':5',2 '':5 '',2'''-quaterthiophene ));5,5'-bis[2-(4 -pyridyl)ethenyl]-2,2'-bithiophene (L(5)) and 5,5'-bis(4-hydroxyphenyl )-2,2'-bithiophene (H(2)L(7)). The compounds L(1)-L(5) undergo two one -electron reductions sit potentials which converge as the compounds le ngthen; H(2)L(7) shows an irreversible oxidation ascribed to the forma tion of a quinone. They all have very strong pi-pi transitions in the ir electronic spectra and are all strongly luminescent, Attachment of {Mo[HB(dmpz)(3)](NO)Cl} termini (dmpz = 3,5-dimethylpyrazolyl) to thes e compounds gives paramagnetic (for L(1)-L(5)) or diamagnetic (for L(7 )) dinuclear complexes which show strong electrochemical interactions between the termini: the (formally) 17-electron/18-electron metal-base d couples in the complex of L(1), for example, are separated by 450 mV , which may be ascribed to a substantial degree of delocalisation of t he added electrons onto the bridging ligands. The electrochemical inte ractions across the ligands are significantly stronger than across tho se of comparable length with polyene bridges, which confirms the more effective ability of the oligothienyl groups to transmit electronic in teractions. The EPR spectra of the paramagnetic dinuclear complexes of L(1)-L(5) reveal the presence of magnetic exchange interactions betwe en the unpaired spins. The complexes are all weakly luminescent, due t o residual ligand-based luminescence which is not entirely quenched by the metal centres.