ABSTRACT
The heterotrinuclear complexes, [(CpTiCl(2)Cp-PPh(2))(2)Cu](+) (2) (as PF(6)(-) salt) and [CpTiCl(2)Cp-PPh(2)](2)CuCl (3), containing two electron-poor Ti(IV) fragments and one electron-rich Cu(I) center, and a tetrametallic species, [(CpTiCl(2)Cp-PPh(2))Cu(mu-Cl)](2) (4), were synthesized and characterized. The trinuclear nature of 2 and 3 was demonstrated by X-ray crystallography for which the three metallic centers are held together by two CpPPh(2) ligands. Weak Cl...Cu interactions are noted in 3, whereas two stronger Cl-Cu bridges are depicted in the cationic complex 2. The tetranuclear complex 4 contains two Cu atoms bridged together by two chloride ligands and connected to one Ti atom by one CpPPh(2) ligand and one mu-Cl ligand. Despite the short distances in the Ti-Cl-Cu bridges, notably in 2 and 4, the NMR spectra reveals evidence of fluxion in solution attributed to the lability of the mu-Cl ligands. Electrochemical experiments performed on 2 (rotating disk electrode and cyclic voltammograms) demonstrated the presences of two successive 1-electron reductions generating a first structurally unsymmetrical paramagnetic species [(CpTiCl(2)Cp-PPh(2))(2)Cu](0) (2'), confidently characterized by electron paramagnetic resonance (EPR) as a Ti(III)-Cu(I)-Ti(IV) system, and a second one, which is tentatively assigned to a symmetric neutral complex formulated as [CpTiClCp-PPh(2)](2)CuCl (5) with an EPR signature indistinguishable from that of 2' (i.e., Ti(III)-Cu(I)-Ti(III) system with a similar electronic environment). Density functional theory (DFT) computations examining the nature of the frontier orbitals and the geometry confirmed the presence and lack of symmetry in 2 and 2', respectively. The detection of chemical/electrochemical mechanisms in the electrochemical studies provides a clear explanation for the stepwise reduction behavior in these systems through chloride ligand transfer from Ti to Cu prior to the first reduction step. This transfer process is fully reversible upon re-oxidation. The electrochemical properties of complexes 3 and 4 are also reported.
ABSTRACT
After noting that the presence of dihydrogen, generated in situ from the partial hydrolysis of a silane with residual water, significantly enhances the rate of the rhodium-catalysed hydrosilylation of acetophenone, we developed a high speed hydrosilylation reaction under dihydrogen pressure.
ABSTRACT
Catalytic allyltitanation reactions were accomplished from dienes and aldehydes with only 5% of titanocene complexes at the expense of employing stoichiometric amounts of PMHS.
ABSTRACT
The reaction of Cp'(2)TaH(3) (Cp' = C(5)H(4)C(CH(3))(3)) with [MPPh(3)][PF(6)] yields the bimetallic complexes [Cp'(2)TaH(3)MPPh(3)][PF(6)] (M = Au (1),M = Cu (2)). The detailed NMR study of 1 and 2 showed the structure with two bridging hydride ligands. The mutual orientation of the Cp' rings has been determined by NOE experiments. The variable-temperature NMR data showed an intramolecular exchange between two outer hydride ligands. The exchange is faster in 1 (DeltaG()(210 K) = 9.3 kcal/mol) than in 2 (DeltaH() = 8.6 +/- 0.2 kcal/mol, DeltaS() = - 5.0 +/- 0.4 eu, DeltaG() (210 K) = 9.6 +/- 0.2 kcal/mol). In contrast, complex 2 undergoes the faster intermolecular PPh(3)/PPh(3) exchange. It has been demonstrated that T(1min), T(1), T(1sel) and T(1bis) measurements are a powerful instrument for quantitative localization of the hydride ligands in solutions of bimetallic complexes. The determined hydride-hydride and metal-hydride distances reproduce well the structural tendencies in the related niobium trihydride [{Nb(C(5)H(3)RR')(2)H(3)}(2)Au][PF(6)] (R = R' = Si(CH(3))(3)) established by the X-ray method. Tantalum-gold complex 1 showed weak exchange couplings.
ABSTRACT
Reaction of [PPh(2)M(CO)(5)]Li salts (M = Cr or W) toward tungstenocene dichloride occurs via a cyclopentadienyl ring substitution and yields the corresponding binuclear compounds (eta(5)-C(5)H(5))[eta(5)-C(5)H(4)PPh(2)M(CO)(5)]W(H)Cl, 2. They react with LiAlH(4) to give the corresponding dihydride complexes (eta(5)-C(5)H(5))[eta(5)-C(5)H(4)PPh(2)M(CO)(5)]WH(2), 3. These species have been proven to be photosensitive leading to the cyclic heterobimetallic (eta(5)-C(5)H(5))[eta(5)-C(5)H(4)PPh(2)M(CO)(4)]W(&mgr;-H)H compounds, 4; analytical data and spectroscopic measurements on complexes 4 indicate that a hydride group functions as a bridging ligand. Crystals of 4a (M = Cr) were obtained as red needles, grown from toluene solution. An isotropic refinement of only 1243 data (F > 5sigma(F)) from a low resolution data set (3707 data, d(min) = 0.9 Å) indicated significant systematic error. Thus it was possible only to ascertain that the connectivity of the non-hydrogen atoms is not inconsistent with the model proposed from solution NMR and that the Cr.W separation of 3.30 Å precludes a direct Cr-W bond. 4a crystallizes in space group Pbca(No. 61), with a = 19.693(8) Å, b = 20.34(1) Å, c = 11.695(5) Å, V = 46823 Å(3), and Z = 8. Further information on this preliminary structure determination is provided in the Supporting Information. These reactions have been investigated with stereochemical factors in mind using the ring substituted tungtenocene complex (eta(5)-C(5)H(4)Me)(2)WCl(2); the 1-3 regioselectivity of the ring disubstitution reaction is proposed on the basis of (1)H NMR experiments. The temperature dependent relaxation time measured between 295 and 213 K by the inversion recovery method makes it possible to determine a proton-proton distance between the two H ligands of 2.0 Å in 4'a.
