Allenylidene Iron(II) Complexes and Their Deprotonation, Nucleophilic Addition Reactions, and Cathodic Protonation toward Alkynyl Derivatives: A Chemical and Electrochemical Study

The allenylidene complexes trans-[FeBr(CCCRR‘)(depe)2][Y] (R = Me, R‘ = Ph, 1; R
= R‘ = Ph, 2; R = R‘ = Et, 3; depe = Et2PCH2CH2PEt2; Y = BF4, BPh4) were obtained by
reaction of trans-[FeBr2(depe)2] with the appropriate alkynol HC⋮CCRR‘(OH), in MeOH
and in the presence of Na[BF4] or Na[BPh4]. Deprotonation of 3 or nucleophilic γ-addition
to 2 led to the neutral enynyl and alkynyl complexes trans-[FeBr{−C⋮CC(CHMe)Et}(depe)2] (4) and trans-[FeBr(−C⋮CCPh2R‘ ‘)(depe)2] (R‘ ‘ = CN (5a), MeO (5b)), respectively.
Complex 2 (Y = BPh4) also leads to the cationic alkynyl compounds trans-[Fe(NCMe){−C⋮CCPh2(X)}(depe)2][BPh4] (X = NMe2 (6a), NHMe (6b)) and trans-[Fe(NCMe){−C⋮CCPh2(PMe3)}(depe)2]Y2 (Y2 = [BPh4]2 (7a), [BPh4]2-xBrx (7b)), in acetonitrile solution, upon reaction
with NHMe2, NH2Me, and PMe3, 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{−C⋮CCPh2(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 (PL) and Lever (EL) 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.