Mono-alkylation of cyanoimide at a molybdenum(IV) diphosphinic center by alkyl halides: synthesis, cathodically induced isomerization and theoretical studies

Treatment of trans-[Mo(NCN)2(dppe)2] with alkyl halides (RX) affords the alkylated cyanoimido-complexes trans-[Mo(NCN)(NCNR)(dppe)2]X [R = Me, X = I (1); R = Et, X = I (2); R = Pr, X = I (3); R = iPr, X = I (4); R = CH2Ph, X = Br (5); R = CH2C6H4NO2-4, X = Br (6)], while its reaction with the trimethyloxonium salt [Me3O][BF4] affords trans-[Mo(NCN)(NCNMe)(dppe)2][BF4] (7). The reactions are accelerated by microwave irradiation. Complexes 1-7 were fully characterized by elemental analyses, IR and NMR spectroscopies, FAB-MS spectrometry, cyclic voltammetry and controlled potential electrolysis. The electrophilic addition to the exo-N atom of one of the cyanoimide ligands was confirmed by single crystal X-ray crystal analysis of 1. In aprotic medium and at a Pt electrode, compounds 1-7 undergo, apart from two consecutive single-electron reversible oxidations, also two successive single-electron reductions at different potentials, involving a cathodically induced trans-to-cis isomerization, following a double square ECEC-type mechanism which was studied in detail by digital simulation of the cyclic voltammograms. Quantum-chemical calculations indicate that the oxidations and reductions are mainly metal centered (although the latter with some involvement of the cyanoimide moieties), and that the reduction leads to a decrease of the relative stability of the trans isomer vs. the cis one.