Armando J. L. Pombeiro

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 synthesis of adducts of 2-aminopyridine or methanol in metal chelates of a N, N, N-tridentate Schiff base ligand. X-ray crystal structures of the Ni(II) and Zn(II) derivatives

Abstract The electrochemical syntheses of series of novel adducts of 2-aminopyridine and methanol in Co(II), Ni(II), Cu(II) or Zn(II) chelates of the N,N,N-tridentate basic form of the Schiff base 2-N-tosylamino(2′-tosylaminobenzylidene)aniline (H2L), [M(L)L′] (M=Co, Ni, Zn, L′=2-aminopyridine; M=Cu, L′=CH3OH), were performed by using the corresponding metal as a sacrificial anode. The compounds were characterized by elemental analysis, IR spectroscopy, FAB mass spectrometry, 1H NMR and magnetic measurements. The crystal structures of the Zn and Ni derivatives as well as of the Schiff base have been determined by X-ray diffraction. In the 2-aminopyridine complexes this ligand is bound to the metal through the endocyclic nitrogen whereas the aminogroup is involved in intramolecular NH⋯O hydrogen bonds with one of the tosyl SO2 groups.

Redox potential, ligand and structural effects in rhodium(I) complexes

The electrochemical behaviour of the set of tetracoordinate rhodium(I) complexes [Rh(O∩O)(CO)L] [O∩O=MeC(O)CHC(O)Me (acac), L=CO (1), P(NC4H4)3 (2), PPh(NC4H4)2 (3), PPh2(NC4H4) (4), PPh3 (5), PCy3 (6), P(OPh)3 (7) or PPh2(C6H4OMe-4) (8); O∩O=PhC(O)CHC(O)Me (bac), L=CO (9) or PPh3 (10); O∩O=PhC(O)CHC(O)CF3(bta), L=CO (11) or PPh3 (12)] and of the pentacoordinate [RhH(CO)L3] [L=P(NC4H4)3 (13), PPh3 (14), P(OPh)3 (15) or P(OC6H4Me-4)3 (16)] and [RhHL4] [L=PPh3 (17) or P(OC6H4Me-3)3 (18)] was studied by cyclic voltammetry and controlled potential electrolysis, in aprotic medium, at a Pt electrode. They present a single-electron oxidation wave (I) (irreversible or quasi-reversible) that can be followed, at a higher potential, by a second and irreversible one (II). The values of first oxidation potential for the tetracoordinate complexes fit the additive Levers electrochemical parameterisation, and the ligand electrochemical Lever EL and Pickett PL parameters were estimated for the N-pyrrolyl phosphines PPhn(NC4H4)3−n (n=0, 1 or 2) and for the organophosphines PCy3 and PPh2(C6H4OMe-4), the former behaving as weaker net electron donors (the electron donor ability decreases with the increase of the number of N-pyrrolyl groups) than the latter phosphines. The pentacoordinate hydride complexes 13–18 fit a distinct relationship which enabled the estimate of the EL ligand parameter for the phosphites P(OC6H4Me-3)3 and P(OC6H4Me-4)3. Electrochemical metal site parameters were obtained for the square planar and the pentacoordinate Rh(I)/Rh(II) couples and, for the former, the redox potential is shown to present a much higher sensitivity to a change of a ligand than the octahedral redox couples investigated so far. Linear relationships were also observed between the oxidation potential and the PL ligand parameter (for the series [Rh(acac)(CO)L]) or the infrared ν(CO) frequency, and a generalisation of the former type of correlation is proposed for series of square-planar 16-electron complexes [M′SL] with a common 14-electron T-shaped binding metal centre {M′S}. Oxidation of 5 by Ag[PF6] leads to the dimerisation of the derived Rh(II) species.

2-Amino-2-oxazoline and trialkylisourea Pt(II) complexes derived from organocyanamides

In the di(organocyanamide) complexes trans-[PtCl2(NCNR2)2] (R = Me 1a, Et 1b), prepared by reaction of PtCl2 with the appropriate NCNR2, the cyanamide ligands, activated by coordination, undergo an unprecedented nucleophilic addition, at room temperature, of an haloalcohol (ClCH2CH2OH/LiBun) or of an alcohol (MeOH) to give the corresponding 2-amino-2-oxazoline cis-[PtCl2{NC(OCH2CH2)NR2}2] (R = Me 2a, Et 2b) or trialkylisourea trans-[PtCl2{NHC(OMe)NR2}2] (R = Me 3a, Et 3b) complexes. The X-ray crystal structure of 2b is also reported, indicating a π-bond delocalization along the NCN group of the aminooxazoline ligands.

Cyanide as a versatile Lewis base ligand at a dinitrogen-binding iron(II) centre: mono- and heteronuclear adducts

Abstract The ligated cyanide in trans-[FeH(CN)(dppe)2] is activated towards aroylation, alkylation and addition of various transition metal Lewis acids, in particular [WCl4(PPh3)2], [ReOCl3(PPh3)2], [PdCl2(PPh3)2] and [PtCl(Ph)(PPh3)2], to give, in the former case, mononuclear isocyanide complexes and, in the latter case, heteronuclear adducts with bridging cyanide. Their syntheses and spectroscopic characterisation are presented, as well as results of the X-ray diffraction analyses of a trinuclear complex with the {FeCNWNCFe} unit and of the parent cyano trans-[FeH(CN)(dppe)2] complex. The cyanide bridge allows the electronic communication between the metal centres as indicated by a cyclic voltammetric study.

The First Observation and Structural Characterization of (Formamide)platinum(IV) Complexes

The reaction of trans-[PtCl4(EtCN)2] and two equivalents of the formamides RR′NCHO (R/R′ = Me/Me, Me/H, tBu/H) in CH2Cl2 led to the formation of the compounds cis-[PtCl4{(O=)CHNRR′}2], two of which (R/R′ = Me/H, tBu/H) were characterized by X-ray single-crystal diffractometry, as well as by elemental analysis, FAB+-MS, and IR, 1H, 13C and 195Pt NMR spectroscopy (including NOE, HMQC and INEPT experiments). The X-ray structures and IR spectroscopic data indicate the coordination of the formamides through the O atom and significant contribution of the bipolar structure −O−C(H)=+NHR in the resonance hybrid of the ligands. The complexes cis-[PtCl4{(O=)CHNRR′}2] represent the first example of (formamide)PtIV species and also a rare case of platinum(IV) complexes with neutral O-donor ligands.

Reactions and electrochemical behaviour of dithiocarbene complexes of platinum(II)

The dithiocarbene complex trans[(PPh3)2PtH{C(SMe)2}][BF4] (1) was obtained from the reaction of trans-[(PPh3)2PtH(CF3)] with CH3SH in the presence of ethereal HBF4. The reactions of 1 with different nucleophiles, such as Cl−, H− and CN− ions, P-donor ligands such as Ph2PCH2CH2PPh2, PPh3, and PMe3, have been investigated and these mostly lead to the decomposition of the dithiocarbene ligand with formation of different sulfured products. From the reaction of 1 with Ph2PCH2CH2PPh2 the complex [Pt{C(H)(SMe)2}(PPh3)(Ph2PCH2CH2PPh2)][BF4] (2), containing an ins dithiocarbene ligand, was isolated in low yield. The electrochemical behaviour of trans-[PtH{C(SMe)2}(PPh3)2][BF4] and the related complexes trans-[(PPh3)2PtH{C(SPh)2}][BF4], , trans-[(PPh3)2PtH{CS(CH2)nS}][BF4] (n=2 or 3) and cis-[(PPh3)2Pt{C(H)S(CH2)2S}][BF4] (with an ‘inserted dithiocarbene’ ligand) was also investigated in 0.2 M [NBu4][BF4]/NCMe by cyclic voltammetry and controlled potential electrolysis, and shown to involve cathodic processes (at potentials dependent on the dithio ligand) with partial reduction of the S-containing ligands to hydrocarbons (ethylene or propylene, although detected in low yields), liberation of PPh3 and dealkylation of the electrolyte, by the reduced metal centre, to give tributylamine, as indicated by GC-MS. The ‘inserted dithiocarbene’ complex also presents an anodic process which involves proton liberation. Evolution of any sulfur-containing product has not been detected electrochemically or by GC-MS. These behaviours are compared with those exhibited by related dioxy-, diamino- or aminooxy-carbene complexes.

Activation of cyanamide by a molybdenum(0) diphosphinic centre. Formation of cyanoimide and its reactivity with electrophiles

Cyanamide NC–NH2, on reaction with trans-[Mo(N2)2(dppe)2] (dppe = Ph2PCH2CH2PPh2) in thf, undergoes reductive dehydrogenation to cyanoimide NCN2− with formation of trans-[Mo(NCN)2(dppe)2] 1 which is susceptible to protonation, alkylation, acylation, aroylation and silylation, on treatment with electrophiles (E+) such as HBF4, [Et3O][BF4], RC(O)Cl (R = Et, Ph) or Me3SiI, affording the corresponding addition products trans-[Mo(NCN)(NCNH)(dppe)2][BF4] 2, trans-[Mo(NCN)(NCNH2)(dppe)2][BF4]23, trans-[Mo(NCN)(NCNEt)(dppe)2][BF4] 4, trans-[Mo(NCN)(NCNHEt)(dppe)2][BF4]27, trans-[Mo(NCN){NCNC(O)R}(dppe)2]Cl (R = Et 5a, Ph 5b) and trans-[Mo(NCN)(NCNSiMe3)(dppe)2]I 6. The electrophilic addition to the exo-N atom is confirmed by the X-ray crystal structures of 2 and 4 which also show a linear multiple-bond coordination of the cyanoimide and the derived NCNE (E = H or Et) ligands, the latter behaving as weaker π-electron donors than the former.