Some new dipyridyl and diphenol bridging ligands containing oligothienyl spacers, and their dinuclear molybdenum complexes: electrochemical, spectroscopic and luminescence properties

Abstract

A series of new ligands has been prepared in which pyridyl (L¹–L⁵) or phenol (H₂L⁷) end-groups are linked by oligothienyl chains: 2,5-bis(4-pyridyl)thiophene (L¹); 5,5′-bis(4-pyridyl)-2,2′-bithiophene (L²); 5,5″-bis(4-pyridyl)-2,2′:5′,2″-terthiophene (L³); 5,5‴-bis(4-pyridyl)-2,2′:5′,2″:5″,2‴-quaterthiophene (L⁴); 5,5′-bis[2-(4-pyridyl)ethenyl]-2,2′-bithiophene (L⁵) and 5,5′-bis(4-hydroxyphenyl)-2,2′-bithiophene (H₂L⁷). The compounds L¹–L⁵ undergo two one-electron reductions at potentials which converge as the compounds lengthen; H₂L⁷ shows an irreversible oxidation ascribed to the formation of a quinone. They all have very strong π–π* transitions in their electronic spectra and are all strongly luminescent. Attachment of {Mo[HB(dmpz)₃](NO)Cl} termini (dmpz = 3,5-dimethylpyrazolyl) to these compounds gives paramagnetic (for L¹–L⁵) or diamagnetic (for L⁷) dinuclear complexes which show strong electrochemical interactions between the termini: the (formally) 17-electron/18-electron metal-based couples in the complex of L¹, for example, are separated by 450 mV, which may be ascribed to a substantial degree of delocalisation of the added electrons onto the bridging ligands. The electrochemical interactions across the ligands are significantly stronger than across those of comparable length with polyene bridges, which confirms the more effective ability of the oligothienyl groups to transmit electronic interactions. The EPR spectra of the paramagnetic dinuclear complexes of L¹–L⁵ reveal the presence of magnetic exchange interactions between the unpaired spins. The complexes are all weakly luminescent, due to residual ligand-based luminescence which is not entirely quenched by the metal centres.