Dominique Lucas

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 Cue5fbCue5fbO, to give the niobium(V) complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Br(R 1 R 2 Cue5fbCue5fbOue5f8 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 Cue5fbO 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 HCCue5fbO)] + ( 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 Cue5fb Cue5fbOue5f8 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 Cue5fbCue5fbO)] − 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 (Cue5f8O) 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 Cue5f8O ketene bond are considered, the Nb atom displays a distorted tetrahedral coordination.

Synthesis and electrochemistry of niobium complexes incorporating asymmetrically substituted ansa-ligands

Abstract The following asymmetric ansa -niobocene(IV) dichloride complexes have been prepared: [Nb{Me 2 Si(η 5 -C 5 Me 4 )(η 5 -C 5 H 3 R)}Cl 2 ] (R=H ( 1a ), Me ( 2a ), SiMe 3 ( 3a ), Pr i ( 4a ), PPh 2 ( 5a )). The ESR spectra of 1a – 5a have been recorded. Oxidation and reduction potentials have also been measured and compared with non ansa -niobocene complexes. The new niobocene complexes [Nb(η 5 -C 5 H 4 R) 2 }Cl 2 ] (R=Pr i ( 6a ), PPh 2 ( 7a )) have been also synthesized. In addition, reduction of 1a in the presence of ligand (RCue606CR) (R=Me, Ph) to give [Nb{Me 2 Si(η 5 -C 5 Me 4 )(η 5 -C 5 H 4 )}Cl(RCue606CR)] (R=Me ( 8a ), Ph ( 9a )) is described.

Electrochemical studies on organometallic compounds: XXXIX. Electrochemical bahavior of bis-silylated cyclopentadienyl niobium trihydride. ESR spectroscopic characterization of the dimeric hydride niobium(IV) complex [Nb(η5-Me3SiC5H4)2H]2(μ-H)2

Abstract The one-electron oxidation of NbCp2′H3, 1 (Cp′ = Me3SiC5H4), has been found to give the dimeric niobium(IV) complex (NbCp2′H2)2, 3, which has been characterized by ESR spectroscopy. The cationic niobium(V) species [NbCp2′H2(THF)]+, 4, has been prepared through a two-electron oxidation of 1. 3 can also be obtained by one-electron reduction of 4. The same behaviour has been observed for Nb(η5-tBuC5H4)2H3.

Electrochemical studies on organometallic compounds: XLI. Electrogeneration and ESR characterization of [Nb(η5C5H4SiMe3)2Cl{P(OMe)3}]+☆

In tetrahydrofuran with 0.2 M NaBPh4 as supporting electrolyte, the one-electron oxidation at −0.4 V relative to an aqueous saturated calomel electrode (SCE) of the electrogenerated species [Nb(η5-C5H4SiMe3)2{ClP(OMe)3}] (3) yields the cationic niobium(IV) complex [Nb(η5-C5H4SiMe3)2-Cl{P(OMe)3}]+ (4), which has been characterized by ESR spectroscopy. 4 can also be obtained by chemical oxidation of 3.

Electrochemical studies on organometallic compounds XXXVII. Unusual paramagnetic bis(silylated cyclopentadienyl)niobium(IV) cationic complexes

Abstract Electrochemical oxidation of Cp′ 2 NbHL (Cp′ = C 5 H 4 SiMe 3 , L = P(OMe) 3 ) yields Cp′ 2 NbHL + and Cp′ 2 NbL 2 2+ , which have been characterized by ESR spectroscopy. These species represent the first niobium(IV) monocationic hydride and dicationic niobiocene complexes.

Electrochemical and spectroscopic studies on dicarboxylato niobocene complexes

Abstract Reaction of sodium oxalate or malonate onto the niobocene dichloride Nb(η5-C5H4SiMe3)Cl2 (1) yields the dicarboxylato complexes Nb(η5-C5H4SiMe3)2(O,O′-OCOOCO) (4a) and Cp′2Nb(O,O′-OCOCH2OCO) (5a) where the dicarboxylato ligand is proved to be chelating. An alternative way to yield the dicarboxylato niobium (IV) complexes consists in the electrochemical reduction of 1 in the presence of the corresponding carboxylic acid followed by the oxidation of the electrogenerated complex. In addition to 4a and 5a the latter method has been shown to yield the succinato and orthophtalato analogs Nb(η5-C5H4SiMe3)2(O,O′-OCOCH2CH2OCO) (6a) and Nb(η5-C5H4SiMe3)2(O,O′-OCOC6H4OCO) (7a). The structure of the complexes as well as the mechanism of their formation is discussed above electrochemical and spectroscopic data. A special emphasis is made on the ESR data which strongly depends on the length of the dicarboxylato chain.

Electrochemical studies on organometallic compounds: XLII. Electrogeneration and spectroscopic characterization of [Nb(η-C5H4SiMe3)2(PhC CPhC,C)]

Abstract The two-electron reduction of [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 ] in the presence of PhCue5fcCPh yields the first acetylene niobium(IV) complex, [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (PhCue5fcCPhue5f8 C , C )].

Synthesis, electrosynthesis and structural studies of bis(silylcyclopentadienyl) niobium complexes with acetylene ligands

Abstract The reduction of [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Cl 2 ] ( 1 ) with one equivalent of sodium amalgam (10%) in the presence of acetylenes yields [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Cl(η 2 -RC 1 ue5fcCR 2 )], (R 1 = H, R 2 = COOMe, 3c ; R 1 = Me, R 2 = COOMe, 3d ; R 1 = R 2 COOMe, 3e ) The structural characterization of these compounds has been carried out by NMR spectroscopy. The reduction of the (acetylene)niobium(V) complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Cl(η 2 -R 1 Cue5fcCR 2 )] with one equivalent of sodium amalgam (10%) gives the paramagnetic niobium(IV) complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 -R 1 Cue5fcCR 2 )] (R 1 = R 2 = Ph, 4a ; R 1 = H, R 2 = Ph, 4b ; R 1 = H, R 2 = COOMe, 4c ; R 1 = Me, R 2 = COOMe, 4d ; R 1 = R 2 = COOMe, 4e ), which have been characterized by ESR spectroscopy. However, electroreduction of 1 in the presence of the acetylenes gives the complexes 3 or 4 depending both on the nature of the acetylene and on the experimental conditions. In the electrolysis of 1 in the presence of acetylenes c or d a slow production of 4c and 4d , respectively, occurs to yield the alkenyl complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 Cl(η 1 -R 1 C = CHR 2 )], 4c and 4d . Finally, the addition of HCl to solutions of 4a gives a mixture of 1 and cis -stilbene.

Reactivity of niobocene dihalogenides toward nitroso derivatives. EPR and IR characterization of the first niobium(IV) complexes containing an ArNO-N,O ligand

Abstract The reaction of [Nb(η 5 -C 5 H 4 R) 2 X 2 ] [ 1: R = SiMe 3 , X = Cl ; 2: R = SiMe 3 , X = Br ; 3: R = H , X = Cl ; 4: R = t , X = Cl ] with nitroso derivatives ArNO [ a : Ar = Ph; b : Ar = o -CH 3 -C 3 H 4 ; c : Ar = p -(CH 3 ) 2 NC 6 H 4 ] yields paramagnetic complexes formulated as [Nb(η 5 -C 5 H 4 R)(η 3 -C 5 H 4 R)X 2 (ArNO- N , O ) 1a, 1b, 1c, 2a, 3a, 4a and 4c , which have been characterized by ESR and IR spectroscopy.

Electrocatalytic process in the reduction of Nb[η5-C5H4(SiMe3)]2(Cl)(NHPh)BF4

Abstract Protonation of the biscyclopentadienyl imido niobium complex ( η 5 -C 5 H 4 SiMe 3 ) 2 Nb(Cl)(=NPh) affords the corresponding cationic amido complex [( η 5 -C 5 H 4 SiMe 3 ) 2 Nb(Cl)(NHPh)] + , 1 . Starting from 1 , an interesting amido group elimination reaction has been found to take place, reaction which must be electrochemically induced.