Necessity of fine tuning in Mo(iv) bis(dithiolene) complexes to warrant nitrate reduction

Four desoxo Mo(iv) bis(dithiolene) complexes, [Et(4)N][Mo(IV)(PPh(3))(SC(6)H(4)-p-Me)(mnt)(2)] () (mnt = maleonitrile dithiolate), [PNP][Mo(IV)(PPh(3))(SC(6)H(4)-o-COOH)(mnt)(2)].CH(3)CN.(i)PrOH () (PNP is [Ph(3)PNPPh(3)](+)), [Et(4)N][Mo(IV)(PPh(3))(NCS)(mnt)(2)] () and [Et(4)N](2)[Mo(IV)(NCS)(2)(mnt)(2)] () are synthesized and characterized structurally. Complexes and are found to release the pentacoordinated species, [Mo(IV)(SR)(mnt)(2)](1-) (R = -C(6)H(4)-o-COOH, -C(6)H(4)-p-Me) due to the dissociation of coordinated PPh(3) in solution and this pentacoordinated species can then reduce nitrate to nitrite. But unlike the model complex of nitrate reductase, [Et(4)N][Mo(IV)(PPh(3))(SPh)(mnt)(2)] () (A. Majumdar, K. Pal and S. Sarkar, J. Am. Chem. Soc. 2006, 128, 4196), these reactions remain incomplete. Such inefficiency of complexes and to mediate a complete reduction of nitrate is attributed to the steric bulk exerted by the para-methyl and ortho-carboxylic acid functionalities thereby retarding the formation of the nitrate bound Mo(iv) complex necessary for nitrate reduction. Complex does not release a pentacoordinated species in solution and hence is unable to reduce nitrate. Although dissociates one coordinated NCS ligand in solution to release a penta-coordinated species {[Mo(IV)(NCS)(mnt)(2)]}(1-), which remains inactive towards nitrate reduction indicating the necessity of thiolate coordination and hence the necessity of fine electronic tuning to effect nitrate reduction. Complexes , and can be converted to or depending on the amount of thiocyanate employed. Complexes and undergo a facile interconversion among themselves. Substitution of essential thiolate coordination rather than replacing the dissociable PPh(3) in and by thiocyanate resulted in inactivation (formation of ) towards nitrate reduction which is somewhat similar to the dead-end type inhibition often encountered in native systems. These results followed by theoretical calculations at DFT level establish the necessity of fine stereoelectronic tuning at the axial position of desosxo molybdenum bis(dithiolene) complexes to warrant nitrate reduction.