Yves Mugnier

Electrochemical studies of organometallic compounds : I. On the reversibility of the first reduction stage of titanocene dichloride

The 1e reduction of titanocene dichloride (η5-Cp2TiCl2) in various solvents has been examined in detail by polarography, voltammetry on a disk electrode, and linear potential sweep voltammetry. In all cases, the electron uptake is followed by a fast loss of Cl−, which is replaced by a neutral ligand (which can be a solvent molecule). In tetrahydrofuran, whose binding ability is weak, a pseudo reversible behaviour is observed due to a return of Cl− to the molecule. In dimethylformamide, which is strongly bonded, the reaction is irreversible. In pyridine, it can be reversible or irreversible according to the conditions.

Exploring the redox reactivity of magnesium porphine. Insight into the origins of electropolymerisation

Magnesium(II) porphine, MgP (1), was synthesised according to the Lindsey procedure allowing to isolate and crystallise 1-formyldipyrromethane (2) as a synthetic intermediate. Unprecedented X-ray diffraction studies revealed multiple intermolecular associations in the crystal between neighbouring units of 2, namely hydrogen bonds and CH...pi. The electrochemical behaviour of 1 was examined by means of cyclic voltammetry. In oxidation, two well-defined and distinct steps are assigned to macrocycle concerned electron transfers yielding initially the pi-cation radical and pi-dication, respectively. The highly reactive dication condenses neutral magnesium porphine to form a diprotonated di-isoporphine species, which is assumed to be a key intermediate at the origin of the electropolymerisation. Electrolyses were performed at the potential of the pi-cation radical generation. Investigation of the electrolysed solution by UV-vis spectroscopy and MALDI-TOF spectrometry revealed the presence of several oligoporphyrins, with diporphine as an important product. In all the oligomers series, extensive demetallation is evidenced, due to the increasing acidity of the medium as oligomerisation progresses. This demetallation could be prevented by addition of 2,6-lutidine as a base prior to electrolysis. In the oligomer series, the porphine units seem to be connected through meso-meso bonds, an argument supported by certain features of the UV-vis spectrum. Finally, the mechanism of oxidative oligomerisation is discussed, from the point of view of the initial steps of the electropolymerisation.

Early-transition-metal ketene complexes: Synthesis, reactivity and structure of ketene complexes of bis(trimethylsilyl)niobocene, X-ray structure of [Nb(η5-C5H4SiMe3)2Br(Ph2CCOC,O)

Abstract The “carbenoid-like” complex [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Br] 1a reacts with 1 equivalent of several ketenes, R 1 R 2 CCO, to give the niobium(V) complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Br(R 1 R 2 CCO C,O )] ( 2a , R 1 = R 2 = Ph; 3a , R 1 = R 2 = Me; 4a , R 1 = Ph, R 2 = Me; 5a , R 1 = Ph, R 2 = Et) with the expected CO bonding mode found in several early-transition-metal moieties. The protonation of these complexes with 1 equivalent of H + (an ethereal solution of HBF 4 ) affords the acyl cationic niobocene [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Br(R 1 R 2 HCCO)] + ( 6a , R 1 = R 2 = Ph; 7a , R 1 = R 2 = Me; 8a , R 1 = Ph, R 2 = Me; 9a , R 1 = Ph, R 2 = Et). The ketene complexes 2a and Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Cl(R 1 R 2 C CO C,O )] 2b (hereafter b refers to the chloro-complexes) undergo a two-electron reduction without transformation of the ketene moiety to give the same anionic niobium(III) species [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 -(C,O)R 1 R 2 CCO)] − 10 by an ECE process. The structure of 2a was determined by X-ray diffraction methods. The crystals are triclinic, space group P 1 with Z = 4 in a unit cell of dimensions a = 16.063(7), b = 19.108(8), c = 10.696(6) A, α = 99.89(2), β = 94.64(2), γ = 111.95(2)°. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by blocked full-matrix least-squares on the basis of 9706 observed reflections, to R and R w values of 0.0391 and 0.0567 respectively. The diphenylketene and the two Cp′ rings (Cp′ = C 5 H 4 SiMe 3 ) are η 2 (CO) and η 5 respectively. The niobium atom is also bonded to a Br atom. If the centroids of the Cp′ rings and the midpoint of the CO ketene bond are considered, the Nb atom displays a distorted tetrahedral coordination.

Electrochemical studies on organometallic compounds

Abstract The electrochemical oxidation of titanocene monochloride, Cp 2 TiClL (L=tetrahydrofuran or dimethylphenylphosphine), has been studied by voltammetry on disc electrode, by linear potential sweep voltammetry and by controlled potential electrolysis. A first one electron step yields Cp 2 TiClL + , which then reacts with Cp 2 TiClL to give Cp 2 TiCl 2 and Cp 2 TiL 2 + . The latter is oxidized to Cp 2 TiL 2 2+ .

Metal telluride clusters composed of niobocene carbonyl, telluride, and cobalt carbonyl units: syntheses, structures, and reactivity

Abstract: The reaction of [Cp#2NbTe2H] (1#; Cp# = Cp* (C5Me5) or Cp(x) (C5Me4Et)) with two equivalents of [Co2(CO)8] gives a series of cobalt carbonyl telluride clusters that contain different types of niobocene carbonyl fragments. At 0 degrees C, [Cp#2NbTe2CO3(CO)7] (2#) and [Co4Te2(CO)10] (3) are formed which disappear at higher temperatures: in boiling toluene a mixture of [cat2][Co9Te6(CO)8] (5#) (cat= [Cp#2Nb(CO)2]+) and [cat2][Co11Te7(CO)10] (6#) is formed along with [cat][Co(CO)4] (4#). Complexes 6# transform into [cat][Co11Te7(CO)10] (7#) upon interaction with HPF6 or wet SiO2. The molecular structures of 2(Cp(x)), 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) have been determined by X-ray crystallography. The structure of the neutral 2(Cp(x)) consists of a [Co3(CO)6Te2] bipyramid which is connected to a [(C5Me4Et)2Nb(CO)] fragment through a mu4-Te bridge. The ionic structures of 4(Cp(x)), 5(Cp*), 6(Cp*) and 7(Cp*) each contain one (4, 7) or two (5, 6) [Cp#2Nb(CO)2]+ cations. Apart from 4, the anionic counterparts each contain an interstitial Co atom and are hexacapped cubic cluster anions [Co9Te6(CO)8]2- (5) or heptacapped pentagonal prismatic cluster anions [Co11Te7(CO)10]n- (n=2: [6]2- , n=1: [7]-), respectively. Electrochemical studies established a reversible electron transfer between the anionic clusters [Co11,Te7(CO)10]- and [Co11Te7(CO)10]2in 6# and 7# and provided evidence for the existence of species containing [Co11Te7(CO),0] and [Co11Te7(CO)0]3-. The electronic structures of the new clusters and their relative stabilities are examined by means of DFT calculations.

Electrochemical studies on organometallic compounds: IX. Electrogeneration of an unusual specias of niobium(III)

Abstract The one-electron reduction in THF/(Bu 4 N + PF 6 − ) of the complex Cp 2 NbCl 2 (Cp = η 5 -C 5 H 5 ) at a low temperature (−30°C) yields the anionic dimer (Cp 2 NbClCl 2 NbCp 2 ) − which remains stable for several hours.

Reduction electrochimique du ferrocene

Abstract A well characterized 1 e − reversible polarographic wave is observed at low temperature (⋍ −30°C) for ferrocene and methylferrocene in dimethylformamide. At higher temperatures, a 2 e reductive cleavage of the molecule occurs.

Investigation into the reactivity of oxoniobocene complexes [Cp*2Nb(O)R] (Cp*=η5-C5Me5; R=H, OH, OMe) towards heterocumulenes: formation of carbamato and thiocarbamato complexes and catalytic cyclization of PhNCO

Abstract The reaction of [Cp*2NbCl2] (Cp*=η5-C5Me5) with KOH or Ba(OH)2·8H2O in THF was investigated under slightly modified conditions. In addition to the known complex [Cp*2Nb(O)H] (1), the new compound [Cp*2Nb(O)OH] (2) was formed. The reaction of 2 with PhNCS gave yellow [Cp*2Nb(O){SC(O)NHPh}] (3), while PhNCO formed yellow [Cp*2Nb(O){OC(O)NHPh}] (4). Complexes 3 and 4 were analytically and spectroscopically characterized. X-ray diffraction analyses of both compounds show that they contain either η1-S-thiocarbamato (3) or η1-O-carbamato ligand (4) along with a terminal NbO group. The reaction of 1 with one equivalent of PhNCO mainly gave orange [Cp*2NbH{OC(O)NPh}] (5) along with some 4. The molecular structure of 5 contains a niobocene unit comprising η2-N,O-carbamato chelate, which formally is a [2+2] cycloaddition product of the NbO group and the heterocumulene. A hydride ligand completes the coordination sphere around Nb. Reaction of 1 with excess PhNCO gave a mixture of heterocycles (PhNCO)2 (6) and (PhNCO)3 (7) in the approximate ratio 3:2. By contrast, the reaction of [Cp*2Nb(O)OMe] with PhNCO in molar ratios from 1:3 to 1:100 gave nearly pure triphenylisocyanurate 7.

Synthesis, electrochemistry and reactivity of formato– and acetato–niobocene complexes

The complex [Nb(η5-C5H4SiMe3)2H3]1 reacted with CO2 to give the formato complex [Nb(η5-C5H4SiMe3)2{OC(O)H-O,O′}]2, which can alternatively be prepared from a two-electron reduction of [Nb(η5-C5H4SiMe3)2Cl2]3 in the presence of formic acid. The reaction of 2 with different π-acids or heterocumulene molecules resulted in opening of the bidentate formate ligand giving rise to the monodentate formato-containing complexes [Nb(η5-C5H4SiMe3)2{OC(O)H-O}L], L = CS24, CO 5 or 2,6-Me2C6H3NC 6. On the other hand, [Nb(η5-C5H4SiMe3)2{OC(O)Me-O,O′}]8 was prepared from either the reaction of [Nb(η5-C5H4SiMe3)2Cl]7 with 1 equivalent of Tl(O2CMe) or from a two-electron reduction of 3 in the presence of acetic acid. The complex [Nb(η5-C5H4SiMe3)2(MeCOCHCOMe-O,O′)]9 can also be isolated from the reaction of 7 with 1 equivalent of Tl(acac)(acac = acetylacetonate). Complex 8 reacted with oxygen to give [Nb(η5-C5H4SiMe3)2O{OC(O)Me-O}]10, and with π-acids or heterocumulenes to give products with a monodentate OC(O)Me ligand, namely [Nb(η5-C5H4SiMe3)2{OC(O)Me-O}L], L = CS211, SCNPh 12, PhNCCPhEt 13, MeO2CCCCO2Me 14, ButNC 15 or CO 16. The structures of all complexes have been established by spectroscopic methods.

Synthese et etude electrochimique de complexes dithiolenes du dicyclopentadienyl niobium

Abstract Dithiolene complexes (Cp2NbvS2C2R2)+ are synthesized and characterized by spectroscopic data. They show two reversible monoelectronic reduction steps. A controlled potential reduction on the plateau of the first wave leads to stable niobium(IV) complexes Cp2NbS2C2R2.