Kenneth G. Caulton

Characterization of aluminium isopropoxide and aluminosiloxanes

Abstract Aluminium isopropoxide, in the form of a crystalline solid of melting point 127°C, has been shown by X-ray diffraction to be a tetrameric molecular species of formula Al[(μ-OiPr)2Al(OiPr)2]3, consistent with earlier proposals and spectroscopic data. A central aluminium achieves coordination number six via two bridging alkoxide groups from each of three Al(OiPr)4− groups. The resulting planar Al(μ2-O)2Al rings have widened (∼ 132°) angles at oxygen to increase the non-bonded AlAl distance. While the terminal OiPr groups show the shortest (1.70 A) AlO distances in the molecule, they are nevertheless strongly bent (∼ 140°) at oxygen. The 1H NMR spectrum of Al4(OiPr)12 is analysed based on the structure reported here. Characterization of aluminosiloxane derivatives Al(OiPr)3-x(OSiMe3)x (x = 1−3) by 1H and 27Al NMR, mass spectrometry and IR spectroscopy is also reported. For x = 1 and 2, these show co-existence of several oligomers. The effect of incorporation of OSiMe3 groups is to favour smaller oligomers.

Mechanism of Heterolysis of H2 by an Unsaturated d8 Nickel Center: via Tetravalent Nickel?

Collision of H(2) with the unusual nickel complex, (PNP)Ni(+), where PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N, forms a rare dihydrogen complex of the d(8) configuration which then rearranges to heterolytically cleave the H-H bond. Experimental studies support a short H/H distance in the coordinated diatomic, and DFT calculations show that the transition state for heterolysis, in spite of the fact that this involves an amide nitrogen located trans to the H(2), has the H/H bond fully split, and has all the geometric features of Ni(IV), but this is a local maximum, not a minimum.

Factors governing the equilibrium between metalalkyl, alkylidene and alkylidyne: MCX2R, XMCXR and X2MCR

Abstract The reactions of vinyl ethers, H2CCH(OR), with RuHClL2 (L=PiPr3) furnish the carbene complexes RuHCl[C(CH3)(OR)]L2 by H migration. Os(H)3ClL2 serves as a surrogate for unknown OsHClL2, to give the analogous carbene, but this transforms further for RPh to give the carbyne OsHCl(OPh)(CCH3)L2. DFT calculations furnish insight into the relative thermodynamic stability of the various isomeric species, and are consistent with the major influence of π-donation by OR, as well as the preference of Os (versus Ru) for saturation and higher oxidation state. Comparison of the reactivity of H2CCHD0 (D0=π-donor) towards MHClL2 versus Cp2ZrHCl shows the dominant influence of metal π-donor power. Ruthenium and osmium complexes containing an MCF3 subunit show remarkably facile isomerization to FMCF2 carbenes.

Coordinated carbenes from electron-rich olefins on RuHCl(PPr3i)2

Dehydrohalogenation of RuH2Cl2L2 (L=PPr3i) gives (RuHClL2)2, shown to be a halide-bridged dimer by X-ray crystallography; the fluoride analog is also a dimer. (RuHClL2)2 reacts with N2, pyridine and C2H4 (L′) to give RuHClL′L2, but with vinyl ether and vinyl amides, H2CCH(E) [E=OR, NRC(O)R′] such olefin binding is followed by isomerization to the heteroatom-substituted carbene complex L2HClRuC(CH3)(E). The reaction mechanism for such rearrangement is established by DFT(B3PW91) computations, for C2H4 as olefin (where it is found to be endothermic), and the structures of intermediates are calculated for H2CC(H)(OCH3) and for cyclic and acyclic amide-substituted olefins. It is found, both experimentally and computationally, that the amide oxygen is bonded to Ru, with a calculated bond energy of approximately 9 kcal mol−1 for an acyclic model. Less electron-rich vinyl amides or amines form η2-olefin complexes, but do not isomerize to carbene complexes. Calculated ΔE values for selected ‘‘ competition’’ reactions reveal that donation by both Ru and the heteroatom-substituted X are necessary to make the carbene complex L2HClRuC(X)(CH3) more stable than the olefin complex L2HClRu(η2-H2CCHX). This originates in part from a diminished endothermicity of the olefin→carbene transformation when the sp2 carbon bears a π-donor substituent. The importance of a hydride on Ru in furnishing a mechanism for this isomerization is discussed. The compositional characteristics of Schrock and Fischer carbenes are detailed, it is suggested that reactivity will not be uniquely determined by these characteristics, and these new carbenes RuHCl[C(X)CH3]L2 are contrasted to Schrock and Fischer carbenes.

Preparation, crystal and molecular structure of a hydrocarbon soluble, volatile oxo-alkoxide of barium: H4Ba6(µ6-O)(OCH2CH2OCH3)14

Barium granules suspended in toluene react rapidly and exothermically with 2-methoxyethanol to yield a hydrocarbon-soluble, volatile (160 °C, 10–1 Torr; 1 Torr = 133.322 Pa), crystalline oxo-alkoxide H4Ba6(µ6-O)(OCH2CH2OCH3)14 which has been characterized by a single-crystal X-ray study; a central octahedral Ba6(µ6-O) unit is supported by eight µ3-η2-OCH2CH2OCH3, four η2-OCH2CH2OMe and two η1-OCH2CH2OMe groups and intermolecular hydrogen bonds.

Preparation and Characterization of Mono-cyclopentadienylvanadium Dihalide Bis-phosphine Complexes; Crystal Structure of (η5-C5H5)VCl2(PMe3)2

Abstract Mono-cyclopentadienyl complexes CpVX 2 (PR 3 ) 2 and Cp′VX 2 (PR 3 ) 2 (Cp = η 5 - C 5 H 5 ; Cp′ = η 5 -C 5 H 4 Me; R = Me, Et; X = Cl, Br) have been prepared by reaction of VX 3 (PR 3 ) 2 with CpM (M = Na, T1, SnBu n 3 , 1/2 Mg) or Cp′Na. Attempts to prepare analogous complexes with other phosphine ligands, PPh 3 , PPh 2 Me, PPhMe 2 , Pcy 3 , DMPE and DPPE failed. Reduction of CpVCl 2 (PEt 3 ) 2 with zinc or aluminium under CO (1 bar) offers a simple method for the preparation of CpV(CO) 3 (PEt 3 ). The crystal structure of the trimethylphosphine complex CpVCl 2 (PMe 3 ) 2 is reported.

Spin State, Structure, and Reactivity of Terminal Oxo and Dioxygen Complexes of the (PNP)Rh Moiety

[Rh(III)H{(tBu(2)PCH(2)SiMe(2)NSiMe(2)CH(2)PtBu{CMe(2)CH(2)})}], ([RhH(PNP*)]), reacts with O(2) in the time taken to mix the reagents to form a 1:1 eta(2)-O(2) adduct, for which O--O bond length is discussed with reference to the reducing power of [RhH(PNP*)]; DFT calculations faithfully replicate the observed O-O distance, and are used to understand the oxidation state of this coordinated O(2). The reactivity of [Rh(O(2))(PNP)] towards H(2), CO, N(2), and O(2) is tested and compared to the associated DFT reaction energies. Three different reagents effect single oxygen atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at and above -60 degrees C and by DFT calculations, is a ground-state triplet, is nonplanar, and reacts, above about +15 degrees C, with its own tBu C--H bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe(2)CH(2)PtBu(2))(SiMe(2)CH(2)PtBu{CMe(2)CH(2)})}].

Ligand redistribution and carbonyl insertion: The effect of competitive π-donation

Abstract Electron deficient CpTiCl 3 reacts with the bis-alkoxide CpCl TiOCMe 2 CMe 2 O to produce the diolato dimer (CpTiCl 2 )OCMe 2 CMe 2 O(TiCl 2 Cp). CpTiCl 3 reacts with Cp 4 Ti 4 Cl 4 (μ 2 -O) 4 , which also has two oxo ligands on each titanium atom, to produce [CpTiCl 2 ] 2 O. These and other redistribution reactions indicate that destabilization results from internal competition for metal d -orbitals by strong π-donor (e.g. alkoxide) ligands. Equilibrium constants for the formation of the η 2 -acetyl complexes Cp 2 Zr[C(O)Me]X by Co insertion into Cp 2 ZrMeX decrease in the order X = Me>Cl>OEt. This reflects internal competition of π-donor orbitals on X with the oxygen donor orbital in the η 2 -acetyl functionally. The significance of this effect for Fischer-Tropsch syntheses in both homogeneouus and heterogeneous media is discussed.

Potassium triphenylsiloxide -ATE compounds of TIN(II) : molecular and separated ion forms and variable potassium coordination numbers

Abstract Neutral bis-silyloxidetin(II) dimers [Sn(OSiPh3)(μ-OSiPh3)]2 (1) and [Sn{OSi(tBu)Me2}{μ-OSi(tBu)Me2}] 2 (2) were synthesized in one step from [Sm{N(SiMe3)2}2] and 2 equiv. of silanol. Complexes 1 and 2 were characterized by IR, NMR (1H, 13C and 119Sn) and combustion analysis. The bis-triphenylsilyloxidetin(II) dimer [Sn(OSiPh3 (μ-OSiPh3)]2 (1) reacted with 2 equiv. of [K(THF)n(OSiPh3)], followed by crystallization from dimethoxyethane (DME) to afford [K(DME)2Sn(μ-OSiPh3)3] (3). 1H NMR studies on [K(DME)2Sn(μ-OSiPh3)3] demonstrated that this complex was kinetically labile on the NMR time-scale with respect to exchange with free, added DME. Single crystal X-ray diffraction studies revealed that [K(DME)2Sn(μ-OSiPh3)3] exists in two crystalline states which formed under apparently identical conditions. Structure determinations showed Sn(μ-OSi Ph3)3K units with K+ further coordinated by two bidentate DME ligands (monoclinic form) or by one bidentate and one monodentate DME (triclinic form). Crystal data : Monoclinic (at −135°C): a = 13.904(3), b = 22.431(5), c = 19.200(4) A, β = 102.23(1)°, with Z = 4 in space group P21/n. Triclinic (−154°C): a = 21.250(4), b = 22.258(5), c = 13.271(3) A, α = 93.26 (1)°, β = 104.44(1)°, γ = 75.79(1)° with Z = 4 in space group P 1 . Reaction of [Sn(OSiPh3)(μ-OSiPh3)]2 with 2 equiv. of [K(18-crown-6)(OSiPh3)] in toluene gave [K(18-crown-6)(η2-toluene)2][K(18-crown-6){Sn(OSiPh3)3}2] (4) in high yields. An X-ray diffraction study revealed a salt-like formulation [(η2-toluene)2K+ (18-crown-6)][{(Ph3SiO)3Sn} 2K(18-crown-6)−]. Crystal data (at −155°C): a = 13.850(4), b = 18.510(5), c = 27.475(7) A, β = 102.47(1)° with Z = 2 (asymmetric units) in space group P21/c. Similarly, [Sn{OSi(tBu)Me2}{μ-OSi(tBu)Me2}]2 (2) reacted with 2 equiv. of [K(OSiMe2tBu)]n, in toluene to give [KSn(μ-OSiMe2tBu)3] (5) as a crystalline solid in good yields. The electronic structure, geometry and nucleophilicity of the model anion, Sn(OH)3−, have been probed using Fenske-Hall methods.

R-Group reversal of isomer stability for RuH(X)L2(CCHR) s. Ru(X)L2(CCH2R): access to four-coordinate ruthenium carbenes and carbynes

NaOPh converts equimolar RuHClL2(CCHR) (L = PPr3i and PCy3) first to RuH(OPh)L2(CCHR), but then, only for R = H, these isomerize to the more stable carbynes Ru(OPh)L2(C–CH3); the rate of isomerization is slowed considerably by THF. RuH(OPh)L2(CCHR) can also be synthesized by reaction of RuCl2L2[CH(CH2R)] with 2 NaOPh; again, only when R = H does the hydrido vinylidene isomerize to the carbyne. While phenoxide converts RuCl2L2(CHPh) to Ru(OPh)L2(CPh), ia the observable intermediates RuCl2−n(OPh)nL2(CHPh), alkoxides OBut and OAdamantyl cause phosphine displacement to give the four-coordinate carbenes Ru(OR)2L(CHPh). DFT (B3PW91) calculations show these d6 species have a traditional cis-divacant octahedral structure with trans OR groups.