Spectroelectrochemical studies and molecular orbital calculations on mononuclear complexes [Mo(Tp(Me,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-dimet hylpyrazolyl)borate] with substituted pyridine derivatives (py-R) affords t he 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; 1 2, py-R = isoquinoline]. TWO Of the complexes, 5 and 8, have been crystallo graphically characterised. These neutral, 17-electrlon complexes [formally Mo(I)] were examined by UV/VIS/NIR spectroelectrochemistry. All of them und ergo a chemically reversible one-electron reduction to the 18-electron mono anion at potentials which are sensitive to the nature of the pyridyl substi tuent 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 h ave described before, the reduced forms of the complexes show intense, low- energy transitions ascribable to MLCT transition from the electron-rich met al 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 t hose complexes in which the pyridyl ligand has a strongly electron-withdraw ing substituent at the C-3 position of the pyridyl ligand develop two new M LCT 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). The se spectroscopic results have been rationalised by ZINDO calculations which were used both to calculate the frontier molecular orbitals of the complex es, and to calculate the electronic spectra of the reduced complexes. The s trong near-IR transitions in the reduced complexes with 3-substituted pyrid ines, especially [5](-) and [7](-), are of particular significance for the development of electrochromic dyes for use in electro-optic switching in th is region of the spectrum.