M.Amélia N.D.A. Lemos

Electron-transfer induced isomerizations of coordination compounds

Abstract Geometrical isomerization processes induced electrochemically are reviewed for octahedral-type complexes, in particular with π-electron acceptor ligands, with focus on those whose mechanisms have been established in terms of square ECEC-type schemes and electron-transfer chain catalysis. Rationalizations based on the isomeric redox potential differentiation are also presented.

Electrochemical behaviour of trans-[FeH(CNR)(dppe)2]+. Kinetic parameters determined by digital simulation of cyclic voltammetry

A new digital simulation program, based on the box method, has been developed and applied to the cyclic voltammetric ECECE anodic behaviour of trans-[FeH(CNR)(dppe)2]+ (R  Me, Et, tBu, C6H4Me-4, C6H4NO2-4, or C6H4OMe-4; dppe = Ph2PCH2CH2PPh2), allowing us to estimate relevant kinetic parameters, in particular the homogeneous rate constant for electroinduced proton loss upon metal-hydride bond cleavage. This is shown to correlate with the electron donor/acceptor properties of the trans isocyanide ligand, as expressed by the electrochemical PL ligand parameter, and the Taft σ★ polar constant (for the alkyl isocyanides) or the Hammett σP+ constant (for the aromatic isocyanides).

Mononuclear alkynyl, alkenyl, alkylidyne and alkylidene complexes of molybdenum and tungsten from reactions of 1-alkynes with hydride complexes. Crystal Structure of [WH2(CCCO2Me)2(Ph2PCH2CH2PPh2)2]

Abstract [WH2(CCR)2(dppe)2] (R = Ph or CO2Me; dppe = Ph2PCH2CH2PPh2) react with HBF4 to give [WF(CCH2CO2Me)(dppe)2] and [WF(CHCH2Ph)(dppe)2]BF4; the X-ray structure of [WH2(CCCO2Me)2(dppe)2] is reported. With HBF4, [MoH4(dppe)2] reacts with R′CO2CCH (R′ = Me or Et) t give [ MoH 2 {CHCC(O )OR′}(dppe)2]BF4, but with HBr, [MoBrH2(CHCHR′)(dppe)2] are formed.

Electron-transfer activation of the aminocarbyne and the hydrogen isocyanide complexes trans-[ReCl(CNHn)(Ph2PCH2CH2PPh2)2][BF4]n−1 (n=2 or 1). Interconversion of coordinated CNH2 and CNH

Abstract The aminocarbyne complex trans-[ReCl(CNH2)(dppe)2][BF4] (dppe=Ph2PCH2CH2PPh2) undergoes, by cyclic voltammetry in 0.2 M [NBu4][BF4]/NCMe, at a Pt electrode, a single-electron oxidation involving anodically induced deprotonation to give the hydrogen isocyanide compound trans-[ReCl(CNH)(dppe)2]+ which exhibits, at a higher potential, a similar anodic process. Both trans-[ReCl(CNH2)(dppe)2][BF4] and trans-[ReCl(CNH)(dppe)2] can undergo proton dissociation in solution and, by cathodic reduction, dihydrogen is evolved with conceivable formation of cyanide species. The mechanism of the anodic processes has been investigated by digital simulation of cyclic voltammograms which allowed the rate constants of the deprotonation reactions induced by electron transfer, and the acid dissociation constants for the aminocarbyne species and its oxidized form, to be estimated.

Novel synthesis of a phosphinidene oxide-κP(RPO, R = ButCH2–) complex of rhenium(I) from a phosphaalkyne precursor. Crystal and molecular structure of [ReCl(Ph2PCH2CH2PPh2)2{P(O)CH2But}]

The first example of the transformation of a phosphaalkyne to a phosphinidene oxide-κP ligand is reported.

Syntheses and properties of isocyanide complexes of iron, trans-[FeH(CNR)(Ph2PCH2CH2PPh2)2][A] (A BF4 or PF6)

Abstract The complexes trans -[FeH(CNR)(dppe) 2 ][A] (R  Me, Et, t Bu, C 6 H 4 OMe-4, C 6 H 4 Me-4, C 6 H 5 , or C 6 H 4 NO 2 -4; A  PF 6 or BF 4 ; dppe  Ph 2 PCH 2 CH 2 PPh 2 ) have been prepared either by treatment of a tetrahydrofuran solution of trans -[FeHCl(dppe) 2 ] under argon with the appropriate isocyanide, in the presence of Tl[A], or upon N 2 replacement in trans -[FeH(N 2 )(dppe) 2 ][A] by CNR. The dinitrogen complexes were obtained by reaction of trans -[FeHCl(dppe) 2 ] in THF with Tl[A] under N 2 .

Electrochemical metalhydride bond cleavage at the dinitrogen-binding iron centre {FeH(Ph2PCH2CH2PPh2)2}+, and its electroactivation towards nucleophilic attack

Abstract Complexes trans -[FeHL(dppe) 2 ][BF 4 ] (II: L  CNR; R  Me, Bu t , C 6 H 4 OMe-4 or C 6 H 4 NO 2 -4. III: L  N 2 ; dppe  Ph 2 PCH 2 CH 2 PPh′ 2 ) have been obtained from the reaction of trans -[FeHCl(dppe) 2 ] ( I ) with the appropriate ligand L in the presence of TlBF 4 . In cyclic voltammetry, at a sufficiently low scan rate, compound II (R  Me) undergoes an anodic ECE process, with metalhydride bond rupture (proton loss) and metal fluorination, to give trans -[FeF(CNMe)(dppe) 2 ][BF 4 ], which has been electrosynthesized by CPE.

Synthesis and X-ray crystal structure of trans-[MoF(CCH2 tBu)(Ph2PCH2CH2PPh2)2][BF4], a paramagnetic alkylidynefluorocomplex

Abstract Treatment of the alkynylhydridocomplex [MoH 3 (CC t Bu)(dppe) 2 ](dppePh 2 PCH 2 CH 2 PPh 2 ) with [Et 2 OH][BF 4 ] gives trans -[MoF(CCH 2 t Bu)(dppe) 2 ], the first example of a stable paramagnetic alkylidyne complex, the X-ray structure of which is reported.

Conversion of alk-1-ynes into alkyne, alkynyl, alkylidyne and alkylidene complexes of molybdenum and tungsten

A variety of alkyne, alkynyl, alkylidyne and alkylidene complexes has been obtained from the alkynes RCCH (R = Ph, CO2Me or CO2Et). The reactions of these alkynes with trans-[M(N2)2(dppe)2](M = Mo or W, dppe = Ph2PCH2CH2PPh2) and trans-[Mo(N2)2(depe)2](depe = Et2PCH2CH2PEt2) give the alkyne complex [Mo(HCCPh)2(dppe)2]A, the alkynyl complexes [MH2(CCR)2(L–L)2]B(M = Mo or W; R = Ph, CO2Me or CO2Et; L–L = dppe; M = Mo, R = Ph, L–L = depe) and trans-[M(CCR)2(dppe)2]C(M = Mo or W; R = Ph, CO2Me or CO2Et) depending on the conditions. The redox properties and interconversion of B and C are described and the X-ray crystal structures of B(M = W, R = CO2Me, L–L = dppe) and C(M = Mo, R = Ph) are presented. The complexes [WH2(CCR)2(dppe)2]B(R = CO2Me or Ph) react with HBF4 to give [WF(CCH2CO2Me)(dppe)2]D and [WF(CHCH2Ph)(dppe)2]BF4E respectively. The polymerisation of RCCH (R = H or Ph) at these metal centres is also discussed.

Mechanism of the electrochemical reduction of [Fe(η5-C6H7)(CO)3][PF6] — a theoretical approach to the intermediates

Abstract A mechanism is proposed for the electrochemical reduction of [Fe(η 5 -C 6 H 7 )(CO) 3 ][PF 6 ] based on cyclic voltammetry and simulation techniques. In [NBu 4 ][X]/CH 3 CN (X=BF 4 or ClO 4 ) but not in [NBu 4 ][BF 4 ]/CH 2 Cl 2 , a rapid equilibrium prior to the electron transfer process is identified between [Fe(η 5 -C 6 H 7 )(CO) 3 ][PF 6 ] and a species formulated as [Fe(η 3 -C 6 H 7 )(CO) 3 (NCMe)] + . The formation of the species under equilibrium involves solvent coordination and η 5 to η 3 ring slippage of the cyclohexadienyl ligand as the response of the system to the high electron count. Electrochemical electron transfer to [Fe(η 3 -C 6 H 7 )(CO) 3 (NCMe)] + affords a highly reactive 19-electron intermediate exhibiting chemical reactivity (ECE mechanism) that leads to the formation of dimer-type species. A ‘father–son’ type mechanism is proposed for the formation of the products of the electrochemical reduction of [Fe(η 5 -C 6 H 7 )(CO) 3 ][PF 6 ]. All the species involved in the mechanism were analysed by theoretical means and are proposed on the basis of calculations made with the B3LYP HF/DFT hybrid functional.