Spectroelectrochemical studies and molecular orbital calculations on mononuclear complexes [Mo(TpMe,Me)(NO)Cl(py)] (where py is a substituted pyridine derivative):: electrochromism in the near-infrared region of the electronic spectrum

Reaction of [Mo(Tp(Me,Me))(NO)Cl-2] [where Tp(Me,Me) is hydrotris(3,5-dimethylpyrazolyl)borate] with substituted pyridine derivatives (py-R) affords the complexes [Mo(Tp(Me,Me))(NO)Cl(py-R)] [1, R = 4-(1-butylpentyl); 2, R = 4-Ph; 3, R = 3-Ph; 4, R = 4-benzoyl; 5, R = 3-benzoyl; 6, R - 4-acetyl; 7, R = 3-acetyl; 8, R = 4-cyano; 9, R = 3-cyano; 10, R = 4-Cl; 11, R = 3-Cl; 12, py-R = isoquinoline]. TWO Of the complexes, 5 and 8, have been crystallographically characterised. These neutral, 17-electrlon complexes [formally Mo(I)] were examined by UV/VIS/NIR spectroelectrochemistry. All of them undergo a chemically reversible one-electron reduction to the 18-electron monoanion at potentials which are sensitive to the nature of the pyridyl substituent R. Whereas the neutral (17-electron) forms of the complexes only show transitions in the UV and visible regions of the spectrum of the type we have described before, the reduced forms of the complexes show intense, low-energy transitions ascribable to MLCT transition from the electron-rich metal centre [formally Mo(0)] to the pi* orbitals of the pyridyl ligands. The 4-substituted pyridines show only one such low-energy transition (e.g. for [1](-), lambda(max) = 830 nm; epsilon = 12 000 dm(3) mol(-1) cm(-1)), but those complexes in which the pyridyl ligand has a strongly electron-withdrawing substituent at the C-3 position of the pyridyl ligand develop two new MLCT transitions on reduction of which one is well into the near-IR region: e.g. for complex [5](-), lambda(max) = 717 nm; epsilon = 5300 dm(3) mol(-1) cm(-1) and lambda(max) = 1514 nm; epsilon = 2500 dm(3) mol(-1) cm(-1). These spectroscopic results have been rationalised by ZINDO calculations which were used both to calculate the frontier molecular orbitals of the complexes, and to calculate the electronic spectra of the reduced complexes. The strong near-IR transitions in the reduced complexes with 3-substituted pyridines, especially [5](-) and [7](-), are of particular significance for the development of electrochromic dyes for use in electro-optic switching in this region of the spectrum.