Dioxomolybdenum(VI) Complexes of Tripodal Nitrogen-Donor Ligands: Syntheses and Spectroscopic, Structural, and Electrochemical Studies, Including the Generation of EPR-Active Molybdenum(V) Species in Solution

Abstract

The dioxo-Mo(VI) complexes LM0O2X [L = hydrotris(3,5-dimethylpyrazol-1 -yl)borate (La), hydrotris(3isopropylpyrazol-1 -yl)borate (Lb), hydrotris(3,5-dimethyl-1,2,4-triazol-1 -yl)borate (Lc); X = Cl, Br, NCS, OMe, OEt, OPh, SPr', SPh, SCH2Ph] have been synthesized and characterized by spectroscopic and structural methods.The infrared spectra of the complexes exhibit ( 2) bands at 940-920 and 910-890 cm-1, and the NMR spectra are indicative of molecular C, symmetry in solution. The X-ray crystal structures of three complexes are reported. LaMo02(SPh): monoclinic space group P2\lc, a = 18.265( 6) Á, b = 8.110(3) Á, c = 18.299(3) Á, ß = 117.06(2)°, V = 2414(1) Á* 123 with Z = 4. LbMo02(OMe): monoclinic space group Cllc, a = 30.365(4) Á, b = 8.373(1) Á, c = 19.646( 2) Á, ß = 113.28(1)°, V = 4588(1) Á3 5with Z = 8. LcMo02(SPh): orthorhombic space group P2,2,2,, a = 7.9302(13) k,b = 16.627(2) k,c= 17.543(2) k,V= 2313.1(9) Á3 with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043,0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries. The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMovC>2X]-. The requirements for reversible, one-electron electrochemical reduction of dioxo-Mo(VI) complexes appear to be (i) minimal conformational change, restricting substitution trans to the oxo groups, upon reduction and (ii) a steric or electrostatic barrier to the close approach and dinucleation of the reduced species. A number of oxo-hydroxo-Mo(V) complexes of the type LMovO-(OH)X were generated by protonation of the anions [LMov02X]~. Chemical reduction by Bun4NSH results in the sequential generation of [LMov02X]-and [LMovOSX]-anions (except for X = OPh, SPh, and SPr1, when only [LMov02X]is formed). The Mo(V) complexes have been characterized by EPR spectroscopy.

Oxidomolybdenum(IV), ‐(V), ‐(VI) Complexes with Relevance to Molybdenum Enzymes: Oxygen Atom Transfer, Redox Chemistry and EPR Spectroscopy

“…1 H, 13 C and 15 N NMR spectra clearly reveal two sets of chelate ligand atoms due to the reduction of symmetry from C 2 (1) to C 1 (5a-5c). Exemplarily, characteristic 1 H and 15 N resonances of 5a-5c are considered: the resonance of the imine protons H 7 (δ = 7.93 ppm, 1) splits into two resonances (H 7 , H 7Ј ) at δ = 7.98, 7.50 ppm (5a), [16] [9] Coordination of the phosphane ligand to the molybdenum atom in 5a-5c is directly seen by the characteristic 31 P NMR chemical shifts at δ = 2.4 ppm (5a), [16] 13.7 ppm (5b) and 29.9 ppm (5c), respectively, and the cross peaks to the protons H 7 , H 7Ј and H…”

Heptacoordinated Molybdenum(VI) Complexes of Phenylenediamine Bis(phenolate): A Stable Molybdenum Amidophenoxide Radical

“…The MOS calculated for the experimental structure 1 is 1.55 (14). Corresponding values for the DFT optimized structures 1'-S and 1'-DR are 1.58(17) and 1.47(15), respectively.…”

Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

“…Complexes 1, 2, 5, and 6 exhibit two successive cath-A C H T U N G T R E N N U N G odA C H T U N G T R E N N U N G ic waves as well as their corresponding anodic waves (Table 4 and Supporting Information, Figure S1), which are assigned to the Mo VI !Mo V and Mo V !Mo IV redox processes, respectively. [29,31,[47][48][49] However, complexes 3 and 4 show only one cathodic and one anodic wave (Table 4) ) plot into the Randles-Sevcik equation [50] are of the same order as those observed for metal complexes undergoing two consecutive one-electron reduction processes. [31,37,39,41] The E 1/2 values of the Mo VI /Mo V redox potentials of the complexes from CV vary in the order 6 > 5 > 1 > 2 > 3 > 4 (Table 4), suggesting a decrease in Lewis acidity (ten- (Figure 2).…”