Marie-Hélène Desbois

Coupling electrocatalysis (electron transfer chain) with organometallic catalysis: polymerisation of terminal alkynes catalysed by [W(CO)3(NCMe)3] and [Fe(cp)2]+PF6–(cp =η5-cyclopentadienyl)

Terminal alkynes (RC2H; R = Ph, Bun) are instantaneously polymerized at 20 °C by 1% of W(CO)3(NCMe)3(1) and 0.01% of [Fe(cp)2]+PF6–(2) in a system which couples electrocatalysis (electron transfer chain) with organometallic catalysis (cp =η5-cyclopentadienyl).

Design of bifunctional organometallic ‘electron reservoirs’: 35- to 38-electron sandwiches including the first stable, localized mixed valence complexes containing FeI

The new bimetallic complex (cpFe)(cp–cp)(FeC6Me6)+PF6–, (cp = C5H5)(2), synthesized from biferrocene and C6Me6, shows four oxidation states (FeIIIFeII)2+, (FeIIFeII)+, FeIIFeI, and (FeIIFe0)– reversibly connected by cyclic voltammetry; the first three are stable and can be isolated and Mossbauer spectroscopy was used to investigate the electronic structure of the localized mixed valence FeIIFeI.

Electron-transfer chain catalysed chelation of [Fe(η5-C5Me5)(CO)2{σ-SC(S)NMe2}]: a reaction which can be initiated either by oxidation or by reduction

The electron-transfer chain catalysed chelation of the dithiocarbamate ligand in Fp*(σ-dtc)(1)[Fp*= Fe(η5-C5Me5)(CO)2; dtc = SC(S)NMe2] can be initiated by cathodic reduction or by naphthyl-sodium (as well as by ferricinium salts) to give [Fe(C5Me5)(CO)(n;2-dtc)](2) and CO; the choice of the reducing agent is crucial because of counter-ion effects in the propagation cycle, and the oxidative and reductive modes of initiation are compared.

How to Design Fast Two-Electron Transfer Reagents: 37-Electron Mixed Valence FeIFeII-Bisandwiches as Key Intermediates

The 38-electron complexes [(C5R5Fe)2(μ2,η12-biphenyl)] ++(PF6 -)2 (R = H, la; CH3, lb) are reduced by 2 electrons to the neutral bicyclohexadienylidene complexes [(C5R5Fe)2(η10-biphenyl) (R = H, 2a; CH3, 2b), structurally defined for 2b (double bond between the two phenyl rings: 1.37 A folding angle: 25°). The cyclic voltammetry shows a fast two-electron cathodic wave for la (Eo = - 1.12 V vs SCE, DMF, Pt) and two close one-electron waves for lb (EO: - 1.3 and ” 1.43 V vs SCE, DMF, Pt). The thermodynamically stable average valence 37-electron complex [(C5Me5-FeII)2(diphenyl)]+PF[6 - 3b can be isolated and its X-ray crystal structure shows that the diphenyl llgand is almost flat (folding angle: 5°) and coordinated in an hexahapto fashion to each ironas in the precursors 1. By comparison, the isomeric series [Fe2(fulvalene) (C6R6)2]++ (R = H, 4a; CH3, 4b) show fours well separated one-electron waves without structural rearrangement after at least two one-electron transfers separated by 0.5 V. Thus the gain of energy provided by the structural rearrangement FeIFeIdiphenyl → FeIIFeII bicyclohexadienylidene compensates the electrostatic factor responsible for the Eo value (0.5 V in the isomeric series 4) which brings the two-one electron waves into a single two-electron wave for 1a. The knowledge of the CV’s and of the exact structures of the three components 1, 2 and 3 together with the slight difference between the Cp and C5Me5 series allows for the first time to demonstrate how and why a fast-two electron transfer proceeds and thus how to design it rationally in any area of molecular chemistry.

Structural Consequences of Electron-Transfer in Dinuclear Iron Polyaromatic Complexes

Binuclear complexes of polyaromatics have been synthesized in order to examine their electron transfer (ET) chemistry and the stereoelectronic consequences of ET. The electrochemistry of known complexes bearing two (FeIICp)+ units (Cp = C5H5) shows a single 2-e− wave for diphenyl in DMF on Hg cathode at - 30°C, two close one-electron waves for dihydrophenanthrene and four one-electron waves with pyrene, triphenylene and phenanthrene. Since reduced states are not stable, FeIICp* analogues were made (Cp* = C5Me5). The diphenyl complex now has two one-e− reductions and the X-ray crystal structures of both the mono- reduced (average valence on the Mossbauer time scale) and of the bi-reduced complexes show that chemical coupling intervenes in the course of the 2nd ET to give. the new bi-cyclohexadienylidene ligand. Delocalized mixed valence FeI FeII complexes are obtained for all the polyaromatics under study.

Chelation of iron(II) dithiocarbamates: an electrocatalytic process with an endergonic cross electron-transfer propagation step

The electrocatalysed chelation of the dimethyldithiocarbamate ligand (dtc) in [Fe(η5-C5R5)(CO)2(σ-dtc)](R = H or Me) is shown to involve a dual role of the oxidant in an efficient process involving an endergonic (ΔG° 12 kcal mol–1) cross electron-transfer propagation step; the preparative yields are subject to a ‘specific anion effect’ which controls the importance of the deactivation of the overall chain conversion.

Single electron reduction of tetracyanoquinodimethane, TCNQ, and phenazine and two electron reduction of TCNQ by organo-iron electron reservoir complexes

cp(C6Me6)FeI, (1), (cp = C5H5) reacts with one equivalent of phenazine and tetracyanoquinodimethane, TCNQ, to give the single electron transfer salts cp(C6Me6)Fe+, phenazine–, (2), and cp(C6Me6)Fe+, TCNQ–, (3), whereas addition of TCNQ to 2 equivalents of (1) or cp(PriPh)FeI gives the crystalline salts {cp(arene)Fe+}2 TCNQ2–, (4) and (5); (C6Me6)2Fe0, (7), also reacts with one equivalent of these acceptors to give (C6Me6)2Fe+, phenazine–, (8) and (C6Me6)2Fe+ TCNQ–, (9).