Hydrogen-Bonding and Polar Group Effects on Redox Potentials in Mo[HB(Me₂pz)₃](NO)(SR)₂

A series of Mo thiolate complexes with the formula Mo[HB(Me₂pz)₃](NO)(SR)2, R = Et (1), Buⁿ (2), CH₂CONHCH₃ (3), CH₂CON(CH₃)₂ (4), C₂H₄CONHCH₃ (5), and C₂H₄CON(CH₃)₂ (6), have been studied using the methods of cyclic voltammetry, IR and resonance Raman spectroscopy, and for 3 and 5, X-ray crystallography. The polar groups of the thiolate ligands exert an influence on the redox potentials reflected in the E_1/2 series for the Mo²⁺/Mo³⁺ redox couple recorded in CH₃CN: 2, -0.960; 1, -0.940; 5, -0.820; 6, -0.750; 4, -0.740; 3, -0.643 V (relative to SCE). The corresponding frequencies for the v(NO) band increase in the same order: 2, 1658; 1, 1661; 5, 1663; 6,1664; 4,1669; 3,1671 cm^-1, showing a correlation between the redox potential and the nitrosyl frequency. Complex 3 belongs to space group PI with a = 10.564(4) Å, b = 12.160(5) Å, c = 12.478(6) Å, α = 110.27(3)°, β = 92.64(4)°, γ = 105.96(3)°, V = 1427.8(10) ų, and Z = 2. Complex 5 belongs to space group PI with a = 9.565(8) Å, b = 11.480(9) Å,c= 18.510(20) Å, α = 73.35(8)°, β = 77.38(8)°, γ = 88.17(7)°, v = 1899.2(31) ų, and Z = 2. Complex 3 exhibits a single intraligand N-H-S hydrogen bond in the solid state, with even more extensive N-H-S bonding evident in solution. By contrast 5 forms an interligand N-H-O hydrogen bond which precludes the formation of N-H…S hydrogen bonds. Within the series of thiolate complexes examined, simple charge-dipole interactions appear to induce redox potential shifts of several hundred millivolts. The largest shift is observed in 3, which forms intraligand hydrogen bonds. These results underscore the likely importance of charge-dipole interactions involving ligated thiolate sulfur in iron-sulfur redox proteins. From this perspective, hydrogen bonding and its effects on redox potential should be viewed in terms of electrostatic influence exerted on the electronic structure of the redox center.