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

Abstract

Four desoxo Mo(IV) bis(dithiolene) complexes, [Et₄N][Mo^IV(PPh₃)(SC₆H₄-p-Me)(mnt)₂] (1) (mnt = maleonitrile dithiolate), [PNP][Mo^IV(PPh₃)(SC₆H₄-o-COOH)(mnt)₂]·CH₃CN·ⁱPrOH (2) (PNP is [Ph₃PNPPh₃]⁺), [Et₄N][Mo^IV(PPh₃)(NCS)(mnt)₂] (3) and [Et₄N]₂[Mo^IV(NCS)₂(mnt)₂] (4) are synthesized and characterized structurally. Complexes 1 and 2 are found to release the pentacoordinated species, [Mo^IV(SR)(mnt)₂]¹⁻ (R = –C₆H₄-o-COOH, –C₆H₄-p-Me) due to the dissociation of coordinated PPh₃ in solution and this pentacoordinated species can then reduce nitrate to nitrite. But unlike the model complex of nitrate reductase, [Et₄N][Mo^IV(PPh₃)(SPh)(mnt)₂] (5) (A. Majumdar, K. Pal and S. Sarkar, J. Am. Chem. Soc. 2006, 128, 4196), these reactions remain incomplete. Such inefficiency of complexes 1 and 2 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 3 does not release a pentacoordinated species in solution and hence is unable to reduce nitrate. Although 4 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 1, 2 and 5 can be converted to 3 or 4 depending on the amount of thiocyanate employed. Complexes 3 and 4 undergo a facile interconversion among themselves. Substitution of essential thiolate coordination rather than replacing the dissociable PPh₃ in 1 and 2 by thiocyanate resulted in inactivation (formation of 3) 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.