Metal-hydride bond activation and metal-metal interaction in dinuclear iron complexes with linking dinitriles: a synthetic,electrochemical, and theoretical study

The dinuclear iron(II)-hydride complexes FeH(dppe)(2)](2)(mu-LL)]BF(4)](2) (LL = NCCH=CHCN (1a), NCC(6)H(4)CN (1b), NCCH(2)CH(2)CN (1c); dppe = Ph(2)PCH(2)CH(2)PPh(2)) and the corresponding mononuclear ones, trans-FeH(LL)(dppe)(2)]BF(4)] (2a-c) were prepared by treatment of trans-FeHCl(dppe)(2)], in tetrahydrofuran (thf) and in the presence of TlBF(4)], with the appropriate dinitrile (in molar deficiency or excess, respectively). Metal-metal interaction was detected by cyclic voltammetry for 1a, which, upon single-electron reversible oxidation, forms the mixed valent Fe(II)/Fe(III) 1a(+) complex. The latter either undergoes heterolytic Fe-H bond cleavage (loss of H(+)) or further oxidation, at a higher potential, also followed by hydride-proton evolution, according to ECECE or EECECEC mechanistic processes, respectively, which were established by digital simulation. Anodically induced Fe-H bond rupture was also observed for the other complexes and the detailed electrochemical behavior, as well as the metal-metal interaction (for 1a), were rationalized by ab initio calculations for model compounds and oxidized derivatives. These calculations were used to generate the structural parameters (full geometry optimization), the most stable isomeric forms, the ionization potentials, the effective atomic charges, and the molecular orbital diagrams, as well as to predict the nature of the other electron-transfer induced chemical steps, i.e. geometric isomerization and nucleophilic addition, by BF(4)(-), to the unsaturated iron center resulting from hydride-proton loss. From the values of the oxidation potential of the complexes, the electrochemical P(L) and E(L) ligand parameters were also estimated for the dinitrile ligands (LL) and for their mononuclear complexes 2 considered as ligands toward a second binding metal center.