La-NMR-spektroskopie an allyllanthan(III)-komplexen

¹³⁹La-NMR chemical shifts were measured for several anionic complexes of formulae Li(C₄H₈O₂)_3/2 [La(ν³-C₃H₅)₄], [Li(C₄H₈O₂)₂][Cp′_nLa(ν³-C₃]H₅)_4−n] (Cp′ = Cp(ν⁵-C₅H₅); n = 1, 2 and Cp′ = Cp * (ν⁵-C₅Me₅); N = 1) and Li[R_nLa(ν³-C₃H₄)₄−n] (R = N(SiMe₃)₂; n = 1, 2 and R = CCsIMe₃; n = 4), as well as for neutral compounds for formulae La(ν³-C₃H₅)₃L_n (L = (C₄H₈O₂)_1.5, (HMPT)₂, TMED), Cp′_nLa(ν³-C₃H₅)_3−n (Cp′= Cp(ν⁵-Cp₅H₅), Cp *(ν⁵-C₅Me₅); n = 1, 2) and La(ν³-C₃H₂)₂X(THF)₂ X = Cl, Br, I). Typical ranges of the ¹³⁹La-NMR chemical shifts were found for the different types of complex independent of number and kind of organyl groups directly bonded to lanthanum.

Zusammenfassung

¹³⁹La-NMR-Spektroskopie wurde an einer Reihe anionischer Allyllanthanat(III)-Komplexe der Zusammensetzung ]- [La)ν³-C₃H₅)₄, [Li(C₄H₈)₂][Cp′_nLa(ν³-C₃H₅)_4−n(Cp′ = Cp(ν⁵-C₅H₅); n = 1, 2 und Cp′ = Cp * (ν⁵-C₅Me₅); N = 1) und Li[R_nLa(ν³-C₃H₅)_4−n (R = B(SiMe₃)₂; n = 1, 2 und R = CCSiMe₃; n = 4 sowie neutraler Allyllanthan(III)-Komplexe der Zusammensetzung La(ν³-C₃H₅)₃L_n (L_n = (C₄H₈O₂)_1.5, (HMPT)₂, TMED), Cp′_n, La(ν³-C₃H₅)_3−n (Cp′ = Cp(ν⁵-C₅H₅), Cp * (ν⁵- Cp₅Me₅); n = 1, 2) und La(ν³-Cp₃H₅)₂X(THF)₂ (X = Cl, Br, I) durchgefürt. In Abhängikeit von der Anzahl und der Art der am Lanthan gebundenen Gruppen wurden für die verschieden Komplextypen charakteristische Resonanzbereiche ermittelt.     

A case study in performance evaluation of Density Functional Tight Binding method in two-layer ONIOM method

A performance evaluation of Density Functional Tight Binding (DFTB) in the two-layer ONIOM method is presented in an effort to estimate DFTB effectiveness as an inexpensive low level quantum mechanical layer. Ground state geometries, geometry error, S-values and energy error for: (H₂O)_x(MeOH)_y, [(η⁵-C₅Me_nH_5−n)₂Ti]₂(μ₂, η²,η²-N₂), n = 4, and complexes of Cu⁺ with tyrosine, were compared to target calculations at B3LYP level of theory for all three of the systems and second order Moller-Plesset (MP2) target level of theory for the first two systems. The calculated root-mean-square errors (RMS) of the ONIOM optimized geometries relative to the target are found to be small. The DFTB level of theory was unable to reproduce the target geometry structure for one of the isomers of tyrosine–Cu⁺ complex, while the ONIOM combinations were able to reproduce all target structures. The absolute value of the geometry error was determined to be smaller then the corresponding energy error except for the (H₂O)_x(MeOH)_y system at the ONIOM(MP2/6-31G(d,p):DFTB) level of theory. The S-values were relatively small and close in value contributing to relatively small energy errors. Both method combinations ONIOM(MP2:DFTB) and ONIOM(DFT:DFTB) show similar performance compared to the corresponding target level of theory. The results also suggest that it is safe to use ONIOM(DFT:DFTB) for investigations of [(η⁵-C₅Me_nH_5−n)₂Ti]₂(μ₂, η²,η²-N₂) complexes.     

Group 3 and 4 metal alkyl and hydrido complexes containing a linked amido-cyclopentadienyl ligand: “constrained geometry” polymerization catalysts for nonpolar and polar monomers

In order to understand the nature of the putative cationic 12-electron species [M(η⁵:η¹-C₅R₄SiMe₂NR′)R″]⁺ of titanium catalysts supported by a linked amido-cyclopentadienyl ligand, several derivatives with different cyclopentadienyl C₅R₄ and amido substituents R′ were studied systematically. The use of tridentate variants (C₅R₄SiMe₂NCH₂CH₂X)²⁻ (C₅R₄=C₅Me₄, C₅H₄, C₅H₃^tBu; X=OMe, SMe, NMe₂) allowed the NMR spectroscopic observation of the titanium benzyl cations [Ti(η⁵:η¹-C₅Me₄SiMe₂NCH₂CH₂X)(CH₂Ph)]⁺. Isoelectronic neutral rare earth metal complexes [Ln(η⁵:η¹-C₅R₄SiMe₂NR′)R″] can be expected to be active for polymerization. To arrive at neutral 12-electron hydride and alkyl species of the rare earth metals, we employed a lanthanide tris(alkyl) complex [Ln(CH₂SiMe₃)₃(THF)₂] (Ln=Y, Lu, Yb, Er, Tb), which allows the facile synthesis of the linked amido-cyclopentadienyl complex [Ln(η⁵:η¹-C₅Me₄SiMe₂NCMe₃)(CH₂SiMe₃)(THF)]. Hydrogenolysis of the linked amido-cyclopentadienyl alkyl complex leads to the dimeric hydrido complex [Ln(η⁵:η¹-C₅Me₄SiMe₂NCMe₃)(THF)(μ-H)]₂. These complexes are single-site, single-component catalysts for the polymerization of ethylene and a variety of polar monomers such as acrylates and acrylonitrile. Nonpolar monomers such as -olefins and styrene, in contrast, give isolable mono-insertion products which allow detailed studies of the initiation process.     

Photochemical reactions of the isostructural 17- and 18-electron complexes (η-C5H5)M(Me)PPh3 (M=Co, Ni)

The photochemical reactions of the title complexes were studied in air-free benzene solution. In both cases photolysis leads to the production of complexes of the formula (η⁵-C₅H₅)M(PPh₃)₂. Both reactions are the result of the initial loss of a methyl radical from the excited state. The primary photoproduct, (η⁵-C₅H₅)MPPh₃ (M=CO, Ni), then scavenges neutral ligands from the solution to yield, in the case of PPh₃, (η⁵-C₅H₅)M(PPh₃)₂. In the absence of uncoordinated ligand in the reaction solution, the cobalt derivative reacts with the starting material to yield (η⁵-C₅H₅)Co(PPh₃)₂, a methyl radical and (η⁵-C₅H₅)Co(solvent)_n.     

Synthesis and thermal properties of mesomorphic 1,1'-bis[ω-(4'-cyano-4-biphenyloxy)alkyl] ferrocenes

A new series of 1,1'-disubstituted ferrocene compounds of the type [(η⁵-C₅H₄(CH₂)_nOC₆H₄C₆H₄CN]₂Fe (3a-d, n = 5, 6, 8, 11) incorporating a variable length alkyloxy cyanobiphenyl unit has been prepared and their mesomorphic properties have been investigated. Compounds 3b, c and d exhibit a thermotropic smectic C phase and 3c also exhibits a monotropic smectic A phase over a fairly wide range near ambient temperature.     

Organometalloidal derivatives of the transition metals : XXX. Mass spectrometry of transition metal substituted disilanes

Mass spectral fragmentation of a series of transition metal substituted disilanes, LMSiMe₂SiMe₂ML, LM = (η⁵-C₅H₅)Fe(CO)₂- (Fp), (η⁵-C₅H₅)Fe(η⁵-C₅H₄)- (Fc), RFe(CO)₂(η⁵-C₅H₄), are reported. They exhibit significant distinctions depending on the nature of LM. Direct cleavage of the Si---Si bond occurs in the order Fc å Fp å RFe(CO)₂(η⁵-C₅H₄) owing to the capacity of the LM fragment to stabilize positive charge. For complexes containing a direct Fe---Si bond, i.e. Fp-SiMe₂SiMe₂ML, disilene complexed ions are observed, and those complexes containing both an Fp group and a (η⁵-C₅H₄-SiMe₂SiMe₂) group exhibit significant formation of (C₅H₄=Si=SiMe₂) complexed ions. Little disproportionation is observed for any of the complexes studied, in contrast to organodisilanes.     

双核茂金属催化剂的合成、表征与应用

使用桥连配体锂盐与MCl₄络合, 合成了4个不同结构的双核茂金属化合物[μ,μ-(CH₂)₃]{[C(H)·(η⁵-C₅H₄)(η⁵-C₁₃H₈)](MCl₂)}₂[M=Zr or Ti](4, 5)和[μ,μ-(CH₂)₃]{[C(H)(η⁵-C₅H₄)(η⁵-C₉H₆)]·(MCl₂)}₂[M=Zr or Ti](6, 7), 配体和化合物都经过核磁氢谱(¹H NMR)、 碳谱(¹³C NMR)、 红外光谱(IR)及元素分析等表征, 确认了化学结构. 以甲基铝氧烷(MAO)为助催化剂, 化合物4~7为催化剂催化丙烯聚合, 考察了聚合温度、 乙烯压力、 铝钛或铝锆比对催化剂活性及聚合物分子量的影响. 结果表明, 多亚甲基桥连双核茂金属是高活性乙烯和丙烯聚合催化剂, 乙烯聚合活性最高达到7.5× 10⁶ g PE/(mol Zr·h)(化合物6), 丙烯聚合活性达 10 × 10⁵ g sPP/(mol Zr·h)(化合物4). 所得间规聚丙烯(sPP)的间规度指数(SI, r) 达到90%. 在同样条件下, 双核化合物的催化活性、 聚合物分子量M_w(> 100000)以及分子量分布(MWD>2.5)均比相应的单核化合物高(M_w<70000, MWD≤2), 表明该体系中存在较强的核效应.     

Mehrfachbindungen zwischen Hauptgruppenelementen und Übergangsmetallen : XLVI. Organoeisen-Komplexe mit Selen-Brücken

The nucleophilicity of the bridging atom of the selenium complex (μ-Se)[(η⁵-C₅H₅)Fe(CO)₂]₂ (1) has been demonstrated by addition of the complex cation [(η⁵-C₅H₅)Fe(CO)₂]⁺: Reaction of 1 with the ionic complex [(η⁵-C₅H₅)Fe(CO)₂-(THF)][BF₄] cleanly yields the ionic trinuclear complex [(μ₃-Se)(η⁵-C₅H₅)-Fe(CO)₂₃][BF₄] (3). This addition reaction converts the bridging selenium atom from a bent FeSeFe structure into a flattened Fe₃Se pyramid (X-ray diffraction studies), without significant changes in the iron-selenium bond lengths (244.9(<1) pm and 242.7(1)/243.3(1)/244.8(1) pm, respectively). These bonds are considered to be single bonds in accord with the EAN rule.     

Formation, solution structure and reactivity of alkylperoxo complexes of titanium

A detailed in situ ¹³C and ¹H NMR spectroscopic characterization of the following families of alkylperoxo complexes of titanium is presented: Ti(η²-OOtBu)_n(OiPr)_4−n, where n = 1–4; binuclear complexes [(iPrO)₃Ti(μ-OiPr)₂Ti(OiPr)₂(η²-OOtBu)] and [(η²-OOtBu)(iPrO)₂Ti(μ-OiPr)₂Ti(OiPr)₂(η²-OOtBu)]; complexes with β-diketonato ligands: Ti(LL)₂(OEt)(η²-OOtBu), Ti(LL)₂(OiPr)(η²-OOtBu), Ti(LL)₂(η²-OOtBu)₂, Ti(LL)₂(OtBu)(η¹-OOtBu), where HLL = acetylacetone, dipivaloylmethane. These alkylperoxo complexes could not be isolated due to their instability and were studied in situ at low temperatures. Whereas the side-on (η²) coordination mode of tert-butylperoxo ligand is generally preferable, the end-on (η¹) coordination caused by spatial hindrance from surrounding bulky ligands is found in two cases. The quantitative data on the reactivity of alkylperoxo complexes found towards sulfides and alkenes were obtained. The system TiO(acac)₂/tBuOOH in C₆H₆ was reinvestigated using ¹³C and ¹H NMR spectroscopy. The structure of the complex Ti(acac)₂{CH₃C(O)(OOtBu)COO} actually formed in this system was elucidated. Four types of titanium(IV) alkylperoxo complexes were detected in the Sharpless–Katsuki catalytic system using ¹³C NMR spectroscopy.     

Linked amido-indenyl complexes of titanium

Titanium complexes Ti(η⁵ : η¹-C₉H₆SiMe₂NCMe₃)X₂(X = Cl, Me, CH₂SiMe₃, CH₂Ph) containing the tert-amino-functionalized indenyl ligand C₉H₆SiMe₃NCMe₃ have been synthesized by the reaction of the dilithium derivative Li₂[C₉H₆SiMe₂NCMe₃ ] with TiCl₃ (THF)₃ followed by oxidation or by the alkylation of the dichloro derivative. Unexpectedly, the reaction of C₉H₆(SiMe₃)(SiMe₂Cl) with TiCl₄ does not give Ti(η⁵-C₉H₆SiMe₂Cl)Cl₃.     

Synthesis and structural characterization of sulfonates, phosphinates and carboxylates of organometallic Group 4 metal fluorides

Reaction of [Cp^*TiF₃] (Cp^* = (ν⁵-C₅Me₅)) with Me₃SiOSO₂- p-C₆H₄CH₃, Me₃SiOPOPh₂ and 1,2-(Me₃SiOCO)₂C₆H₄ yields the dinuclear complexes [{Cp^*TiF(μ-F)(μ-OSO₂-p-C₆H₄CH₄)}₂] (1), [{Cp^*TiF(μ-F)(μ-OPOPh₂)}₂] (2) and [{Cp^*TiF(μ-F)(μ-OCO-o-C₆H₄CO₂SiMe₃)}₂] (3). The molecular structures of 1 and 2 have been determined by single-crystal X-ray analysis. In complexes 1-3, the two titanium atoms are connected by bridging fluorine atoms as well as bridging sulfonate, phosphinate and carboxylate groups respectively. Each titanium atom is also bonded to a terminal fluorine atom. Reaction of [Cp₂^*ZrF₂] with 1,2-(Me₃SiOCO)₂C₆H₄ leads to the mononuclear pentacoordinate 18-electron species [Cp₂^*ZrF(μ-OCO-o-C₆H₄CO₂SiMe₃)] (4) and its structure was determined by X-ray crystallographic methods.     

Synthesis and X-ray crystal structures of [Ph2PMe2][(η-C5H4Bu)2Li] and [(η-C5H4Bu)2Yb(Cl)CH2P(Me)Ph2]

The interaction of [(η⁵-C₅H₄Bu^t)₂YbCl · LiCl] with one equivalent of Li[(CH₂) (CH₂)PPh₂] in tetrahydrofuran gave [Ph₂PMe₂][(η⁵-C₅H₄Bu^t)₂Li] (1) and [(η⁵-C₅H₄Bu^t)₂Yb(Cl)CH₂P(Me)Ph₂] (2) in 10% and 30% yields, respectively. 1 could also be prepared in 70% yield from the reaction of [Ph₂PMe₂][CF₃SO₃] with two equivalents of (C₅H₄Bu^t)Li. Both compounds have been fully characterized by analytical, spectroscopic and X-ray diffraction methods. The solid state structure of 1 reveals a sandwich structure for the [(η⁵-C₅H₄Bu^t)₂Li]⁻ anion.     

New ferrocenyl heterometallic complexes of 2,7-diethynylfluoren-9-one

A new series of rigid-rod alkynylferrocenyl precursors with central fluoren-9-one bridge, 2-bromo-7-(2-ferrocenylethynyl)fluoren-9-one (1b), 2-trimethylsilylethynyl-7-(2-ferrocenylethynyl)fluoren-9-one (2) and 2-ethynyl-7-(2-ferrocenylethynyl)fluoren-9-one (3), have been prepared in moderate to good yields. The ferrocenylacetylene complex 3 can provide a direct access to novel heterometallic complexes, trans-[(η⁵-C₅H₅)Fe(η⁵-C₅H₄)CCRCCPt(PEt₃)₂Ph] (4), trans-[(η⁵-C₅H₅)Fe(η⁵-C₅H₄)CCRCCPt(PBu₃)₂CCRCC(η⁵-C₅H₄)Fe(η⁵-C₅H₅)] (5), [(η⁵-C₅H₅)Fe(η⁵-C₅H₄)CCRCCAu(PPh₃)] (6) and [(η⁵-C₅H₅)Fe(η⁵-C₅H₄)CCRCCHgMe] (7) (R=fluoren-9-one-2,7-diyl), following the CuI-catalyzed dehydrohalogenation reactions with the appropriate metal chloride compounds. All the new complexes have been characterized by FTIR, ¹H-NMR and UV–vis spectroscopies and fast atom bombardment mass spectrometry. The solid state molecular structures of 3, 5, 6 and 7 have been established by X-ray crystallography. The redox chemistry of these mixed-metal species has been investigated by cyclic voltammetry and oxidation of the ferrocenyl moiety is facilitated by the presence of the heavy metal centre and increased conjugation in the chain through the ethynyl and fluorenone linkage units.     

π-Olefin-Iridium-Komplexe : XXI. Synthese und Kristallstrukturen heterobinuclearer Komplexe mit wannenförmiger, synfacial gebundener η: η-Benzolbrücke

CpIr(η⁴-C₆H₆) (2) has been obtained in high yield by a four-step synthesis. Thermal reaction of 2 with CpCO(C₂H₄)₂ and photochemical reaction of 2 with CpRh(C₂H₄)₂ or CpRh(C₂H₄)₂ give the compounds μ-(η³: η³-C₆H₆)CoIrCp₂ (3), μ-(η³: η³-C₆H₆)RhIrCp₂ (4), and μ-(η³: η³-C₆H₆)(RhCp)(IrCp) (5), respectively. The X-ray crystallography data of 3 and 4 reveal a boat-shaped conformation of the synfacially bridging benzene ligand with a rather long Co---Ir bond distance in 3 and a relatively short Rh---Ir bond length in 4 which are caused by almost constant folding angles of the benzene unit. The dynamic behaviour of the benzene bridge was investigated by NMR spectrometry.     

A study of ortho- and para-siloxyanilines for the synthesis of mono-, bi-, and tetra-nuclear early transition metal–imido complexes

The siloxyanilines o-Me₃SiOC₆H₄NH₂ (1) and p-RMe₂SiOC₆H₄NH₂ (R=H (2); R=Me (3)), and their N-silylated derivatives p-Me₃SiOC₆H₄NHSiMe₃ (4) and p-Me₃SiOC₆H₄N(SiMe₃)₂ (5) have been prepared from ortho- or para-aminophenol and used in the synthesis of imido complexes. Thus, binuclear [{Ti(η⁵-C₅H₅)Cl}{μ-NC₆H₄(p-OSiMe₃)}]₂ (6) and mononuclear [TiCl₂{NC₆H₄(p-OSiMe₃)}(py)₃] (7) imido complexes have been obtained from the reaction of 3 and [Ti(η⁵-C₅H₅)Cl₃] or [TiCl₂(N^tBu)(py)₃], respectively. In contrast, the reaction of 1 with TiCl₄ and ^tBupy affords the titanocycle [TiCl₂{OC₆H₄(o-NH)---N,O}(^tBupy)₂] (8). Compound 5 has also been used to prepare the niobium imide complex [NbCl₃{NC₆H₄(p-OSiMe₃)}(MeCN)₂] (9), by its reaction with NbCl₅ in CH₃CN. These findings have been applied to the synthesis of polynuclear systems. Thus, chlorocarbosilane Si[CH₂CH₂CH₂Si(Me)₂Cl]₄ (CS–Cl) has been functionalized with the ortho- and para-aminophenoxy groups to give 10 and 11, respectively. The use of 11 has allowed the formation of the tetranuclear compound 12. Attempts to synthesize terminal imido titanium complexes from 10 and TiCl₄ in the presence of ^tBupy and Et₃N, give complex 8 and carbosilane CS–Cl.     

Group 4 metallacarboranes of constrained geometries derived from B(cage)- and C(cage)-silylamido-substituted carborane ligands: a synthetic and structural investigation

The reactions of RNHSi(Me)₂Cl (1, R=t-Bu; 2, R=2,6-(Me₂CH)₂C₆H₃) with the carborane ligands, nido-1-Na(C₄H₈O)-2,3-(SiMe₃)₂-2,3-C₂B₄H₅ (3) and Li[closo-1-R′-1,2-C₂B₁₀H₁₀] (4), produced two kinds of neutral ligand precursors, nido-5-[Si(Me)₂N(H)R]-2,3-(SiMe₃)₂-2,3-C₂B₄H₅, (5, R=t-Bu) and closo-1-R′-2-[Si(Me)₂N(H)R]-1,2-C₂B₁₀H₁₀ (6, R=t-Bu, R′=Ph; 7, R=2,6-(Me₂CH)₂C₆H₃, R′=H), in 85, 92, and 95% yields, respectively. Treatment of closo-2-[Si(Me)₂NH(2,6-(Me₂CH)₂C₆H₃)]-1,2-C₂B₁₀H₁₁ (7) with three equivalents of freshly cut sodium metal in the presence of naphthalene produced the corresponding cage-opened sodium salt of the “carbons apart” carborane trianion, [nido-3-{Si(Me)₂N(2,6-(Me₂CH)₂C₆H₃)}-1,3-C₂B₁₀H₁₁]³⁻ (8) in almost quantitative yield. The reaction of the trianion, 8, with anhydrous MCl₄ (M=Ti and Zr) in 1:1 molar ratio in dry tetrahydrofuran (THF) at −78 °C, resulted in the formation of the corresponding half-sandwich neutral d⁰-metallacarborane, closo-1-M[(Cl)(THF)_n]-2-[1′-η¹σ-N(2,6-(Me₂CH)₂C₆H₃)(Me)₂Si]-2,4-η⁶-C₂B₁₀H₁₁ (M=Ti (9), n=0; M=Zr (10), n=1) in 47 and 36% yields, respectively. All compounds were characterized by elemental analysis, ¹H-, ¹¹B-, and ¹³C-NMR spectra and IR spectra. The carborane ligand, 7, was also characterized by single crystal X-ray diffraction. Compound 7 crystallizes in the monoclinic space group P2₁/c with a=8.2357(19) Å, b=28.686(7) Å, c=9.921(2) Å; β=93.482(4)°; V=2339.5(9) ų, and Z=4. The final refinements of 7 converged at R=0.0736; wR=0.1494; GOF=1.372 for observed reflections.     

Substituted cyclopentadienyl ligands—10. Intramolecular steric interactions in [(η-C5H3(SiMe3)2)Mo(CO)2(L)I]

Reaction of C₅H₄(SiMe₃)₂ with Mo(CO)₆ yielded [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₃]₂, which on addition of iodine gave [(η⁵-C₅H₃(SiMe₃)₂Mo(CO)₃I]. Carbonyl displacement by a range of ligands: [L = P(OMe)₃, P(OPrⁱ)₃,P(O-o-tol)₃, PMe₃, PMe₂Ph, PMePh₂, PPh₃, P(m-tol)₃] gave the new complexes [(η⁵-C₅H₃(SiMe₃)₂ MO(CO)₂(L)I]. For all the trans isomer was the dominant, if not exclusive, isomer formed in the reaction. An NOE spectral analysis of [(η⁵-C₅H₃(SiMe₃)₂)Mo(CO)₂(L)I] L = PMe₂Ph, P(OMe)₃] revealed that the L group resided on the sterically uncongested side of the cyclopentadienyl ligand and that the ligand did not access the congested side of the molecule. Quantification of this phenomenon [L = P(OMe)₃] was achieved by means of the vertex angle of overlap methodology. This methodology revealed a steric preference with the trans isomer (less congestion of CO than I with an SiMe₃ group) being the more stable isomer for L = P(OMe)₃.     

Mechanism of formation of (η-C5H5)2NbH3 from the reaction of (η-C5H5)2NbCl2 with hydridoaluminate reducing agents

The mechanism of the transformation of (η⁵-C₅H₅)₂NbCl₂ to (η⁵-C₅H₅)₂NbH₃ by hydridoaluminate reducing agents has been investigated. Results suggest disproportionation of a niobium(IV) hydrite, leading to the trihydride product and a niobium(III) hydridoaluminate, (η⁵-C₅H₅)₂NbH₂AlR₂, which in turn is converted to the trihydride on hydrolysis. (η⁵-C₅H₅)₂NbH₂AlH₂ has been isolated; deuterium labelling shows that hydrogens exchange between ring and metal-bridging positions in this molecule.     

Synthesis and crystal structure of the complex [MoW(OEt)(μ-CH2){μ-C(C6H4Me-4)C(Me)O}(η-C7H7)(η-C2B9H10Me)]

The complex [MoW(μ-CC₆H₄Me-4)(CO)₂(η⁷-C₇H₇)(η⁵-C₂B₉H₁₀Me)] reacts with diazomethane in Et₂O containing EtOH to afford the dimetal compound [MoW(OEt)(μ-CH₂){μ-C(C₆H₄Me-4)C(Me)O}(η⁷-C₇H₇)(η⁵-C₂B₉H₁₀Me)]. The structure of this product was established by X-ray diffraction. The Mo---W bond [2.778(4) Å] is bridged by a CH₂ group [μ-C---Mo 2.14(3), μ-C---W 2.02(3) Å] and by a C(C₆H₄Me-4)C(Me)O fragment [Mo---O 2.11(3), W---O 2.18(2), Mo---C(C₆H₄Me-4) 2.41(3), W---C(C₆H₄Me-4) 2.09(3), Mo---C(Me) 2.26(3) Å]. The molybdenum atom is η⁷-coordinated by the C₇H₇ ring and the tungsten atom is η⁵-coordinated by the open pentagonal face of the nido-icosahedral C₂B₉H₁₀Me cage. The tungsten atom also carries a terminally bound OEt group [W---O 1.88(3) Å]. The ¹H and ¹³C-{¹H} NMR data for the dimetal compound are reported and discussed.     

Electrochemical behaviour of some polyoxo clusters

We describe the redox behaviour in non-aqueous solvents of some cyclopentadienyl(oxo)titanium derivatives. The derivative [Ti₄{η⁵-C₅H₄(SiMe₃)}₄(μ-O)₆] shows an electrochemically and chemically reversible le reduction process, followed by a multi-electron, chemically complicated reduction at a fairly cathodic potential. On the basis of the overall electrochemical features and the comparison with the redox behaviour of the quasi-planar compound [[Ti{η⁵-C₅H₄(SiMe₃)}Cl(μ-O)]₄] we propose an EECCEE mechanism for the first derivative, where the second electron-transfer induces a cascade of chemical reactions giving rise to irreversible cluster breakdown. The electrochemically induced fragmentation can be viewed as a retrosynthetic pathway. The heterometallic derivative [{Ti(η⁵-C₅H₄Me)₂(μ₂-MoO₄)₂}₂] shows two consecutive reduction processes; the first is chemically reversible, and the second quasi-reversible. The molybdate bridges apparently increase the stability of the electrogenerated anions. However none of these poly-oxo clusters can be considered as good models of electron ‘sinks’.