Base-free Complex Research Articles

Silylene-assisted hydride transfer to CO2 and CS2 at a [P2Si]Ru pincer-type complex.

The synthesis and characterization of base-stabilized and base-free pincer-type bis(phosphine)/silylene [P2Si]Ru complexes are reported. The base-free complex readily reduces CO2 and CS2via silylene-assisted hydride transfer, affording structurally distinct products with silicon-to-ruthenium formate and dithioformate bridges.

Solvent-resistant structures of base-free lithium and potassium allyl complexes, M[(SiMe3)nC3H5-n] (M = Li,n= 3; M = K,n= 2)

The structures of unsolvated {Li[1,1′,3-(SiMe3)3C3H2]}2 ([LiA″]2) and {K[1,3-(SiMe3)2C3H3]}∞ ([KA′]∞) are reported. The base-free complex LiA″ is a dinuclear species in the solid state in which the two Li atoms are bridged by the allyls in a μ2-η1, η2 fashion. The Li[sbnd]C σ-bonded distance is 2.232(7) A; the Li[sbnd]C η2-interaction occurs at distances of 2.230(7) A and 2.241(6) A. Density functional theory calculations on [Li(C3H5)]2 and {Li[1,1′,3-(SiH3)3C3H2]}2 indicate that inclusion of the silyl groups is critical to reproducing the distortions observed in [LiA″]. The structure of the solvated Li(C3H5)(thf)2 complex is calculated to be stable as a dimer, consistent with previous measurements on allyllithium in THF solution. Like related DME and THF solvates, [KA′]∞ is a coordination polymer with potassium ions linked by bridging π-allyl ligands; in [KA′]∞ the polymer takes the form of helical chains with a broad range of K[sbnd]C distances (2.87–3.15 A).

Nonchelated d0 Zirconium−Alkoxide−Alkene Complexes

The reaction of Cp'2Zr(O(t)Bu)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex [Cp'2Zr(O(t)Bu)][B(C6F5)4] (6), which exists as Cp'2Zr(O(t)Bu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'2Zr(O(t)Bu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C(int) and negative charge buildup at C(term). These features arise due to the lack of d-pi* back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K(eq) = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M(-1)) >> ethylene (7.0) approximately alpha-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.

Exploration of the Suitability of Bicyclic Guanidinates as Ligands in Catalytic Chemistry Mediated by Titanium

The synthesis, structure, and reactivity of titanium complexes supported by the bicyclic guanidinate ligand derived from 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) are described. In situ reaction between a THF solution of the lithium salt, (hppLi)n, and titanium chloride starting materials afforded the series of compounds Ti(hpp)nCl4-n(THF)x [n = 1, x = 1 (1); n = 2, x = 0 (3); n = 3, x = 0 (4)]. The dimeric, base-free complex [Ti(hpp)Cl2(-Cl)]2 (2) was synthesized from the reaction of the silylated ligand precursor, hppSiMe3, with TiCl4 in CH2Cl2. Preliminary olefin polymerization studies of 1-3 using MAO as activator indicated low activities. The reaction between 3 and 2 equiv of AlMe3 resulted in formation of a novel trimetallic Ti(III) species, Ti{AlMe2(hpp)2}Cl(-Cl)AlMe3 (5), highlighting the noninnocent behavior of the ligands. The bis(benzyl) complex Ti(hpp)2(CH2Ph)2 (6) was synthesized from the alkane elimination reaction between the neutral ligand precursor, hppH, and Ti(CH2Ph)4. Attempted generation of the corresponding mono-benzyl cation through reaction with the neutral borane B(C6F5)3 again afforded reduced species. The synthesis of titanium tert-butylimido compounds [Ti(-NtBu)(hpp)Cl]2 (7) and [Ti(NtBu)(hpp)(-hpp)]2 (8) from the salt metathesis reaction of 1 and 2 equiv of (hppLi)n, respectively, with Ti(NtBu)Cl2(py)3 is reported. The molecular structures of compounds 2 and 3 and 5-8 are reported, and the distribution of -electron density throughout the guanidinate ligands is discussed in detail.

Synthesis and molecular structures of uranium and thorium diamidoamine complexes

Treatment of UCl 4 or ThCl 4 with Li 2[Me 3SiN{CH 2CH 2NSiMe 3} 2] affords the mono-diamidoamine complexes [M(Me 3SiN{CH 2CH 2NSiMe 3} 2)Cl 2(THF)] (M=U, Th), and also the bis-diamidoamine complexes [M(Me 3SiN{CH 2CH 2NSiMe 3} 2) 2] (M=U, Th). The base-free complex [U(Me 3SiN{CH 2CH 2NSiMe 3} 2)Cl 2] 2 was obtained by treatment of [U(Me 3SiN{CH 2CH 2NSiMe 3} 2)Cl 2(THF)] with hot toluene. The X-ray structures of [Th(Me 3SiN{CH 2CH 2NSiMe 3} 2)Cl 2(THF)] and [U(Me 3SiN{CH 2CH 2NSiMe 3} 2)Cl 2] 2 reveal a distorted octahedral geometry, whereas [M(Me 3SiN{CH 2CH 2NSiMe 3} 2) 2] (M=U, Th) display trigonal bipyramidal geometry at the metal centre and the amino group on one of the diamidoamine ligands is non-bonding.

Reactivity of the Base-Stabilized Bis(silylene)iron Complex (η5-C5H5)Fe(CO)(η2-SiMe2-OtBu-SiMe2): Elevated Temperature Trapping of SiMe2 by R3EH (R = Me3Si, E = Si, Ge) and Elimination of Me2(OtBu)SiSiMe2H by n-Bu3SnH

The base-stabilized silylene complex (η5-C5H5)Fe(CO)(η2-SiMe2-OtBu-SiMe2) is unreactive toward (Me3Si)3EH (E = Si, Ge) under photochemical irradiation or at room temperature. However, at 80 °C it reacts, presumably via the equilibrium concentration of base-free complex (η5-C5H5)Fe(CO)(SiMe2OtBu)(SiMe2), to transfer the silylene group and form (Me3Si)3ESiMe2H. Attempts to transfer the SiMe2 group to tributyltin hydride led to formation of bis(tributylstannyl)iron complexes.

Uranium alkoxide chemistry. 3. tert-Butylisocyanide adducts of tris(2,6-di-tert-butylphenoxy)uranium(III)

The solution reactions of a uranium(III) phenoxide complex, U(O-2,6, t-Bu{sub 2}C{sub 6}H{sub 3}){sub 3}, with tert-butyl isocyanide (t-BuCN), a sterically unencumbered ligand, are described. The tert-butyl isocyanide has been reported to form both 1:1 and 1:2 adducts in solution with the cerium(III) complex, Ce(O-2, 6,-t-Bu{sub 2}C{sub 6}H{sub 3}){sub 3}. Results of {sup 1}H NMR spectral studies indicated that the U complex is capable of binding one or two t-BuCN ligands in solution. At room temperature, the isonitile exchange between the base-free complex and the 1:1 adduct was noted to occur at a rate close to the NMR time scale, and the 1:1 adduct was found to rapidly exchange with the free t-BuCN in solution. 11 refs., 4 figs.