The allenylidene complexes trans- [FeBr(dCdCdCRR′) (depe) 2 ][Y] (R ) Me, R′ ) Ph, 1; R ) R′ ) Ph, 2; R ) R′ ) Et, 3; depe ) Et 2 PCH 2 CH 2 PEt 2 ; Y ) BF 4 , BPh 4 ) were obtained by reaction of trans-[FeBr 2 (depe) 2 ] with the appropriate alkynol HCtCCRR′(OH), in MeOH and in the presence of Na[BF 4 ] or Na[BPh 4 ]. Deprotonation of 3 or nucleophilic γ-addition to 2 led to the neutral enynyl and alkynyl complexes trans- [FeBr{-CtCC(dCHMe)Et}-, in acetonitrile solution, upon reaction with NHMe 2 , NH 2 Me, and PMe 3 , respectively. The complexes have been characterized by multinuclear NMR and IR spectroscopy, FAB-MS, and elemental analysis and, in the cases of 5a and 6a, also by X-ray diffraction analysis. Controlled-potential electrolysis of 2 yields the alkynyl species trans-[FeBr{-CtCCPh 2 (H)}(depe) 2 ] (8) via a 2e -/H + process, and the oxidation potential of the complexes, measured by cyclic voltammetry, has allowed us to estimate the electrochemical Pickett (P L ) and Lever (E L ) ligand parameters for the cumulenic ligands. These are then ordered (together with related ligands) according to their net π-electron acceptor minus σ-donor ability as follows: carbynes > aminocarbyne > CO > vinylidenes > aryl allenylidene > alkyl allenylidene > NCR . phosphonium alkynyl > cyanoalkynyl, Br -, NCO -> alkynyl, enynyl, aminoalkynyl.
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 Tl[BF(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.
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