Synthesis of Silyl Aluminum Reagents: Relevance Toward Atomic Layer Deposition of Metal Silicides and the Serendipitous Synthesis of a Novel Al-Hydride Cluster.

The novel mono-silyl [(R3Si)AlX2]2, di-silyl [(R3Si)2AlX]2, tri-silyl (R3Si)3Al·Et2O, and -ate-complex [(R3Si)4Al]-·Li+(Et2O)2 have been synthesized by reaction of AlX3 (X = Cl, Br) with silyl lithium reagents (tBuMe2SiLi, Et3SiLi) in Et2O. Treatment of these compounds with Me3N yields the corresponding amine-coordinated silyl aluminum complexes (R3Si)AlX2·NMe3, (R3Si)2AlX·NMe3, and (R3Si)3Al·NMe3. An intramolecular amine-coordinated mono-silyl aluminum complex Me2N(CH2)3(tBuMe2Si)2SiAlCl2 was prepared by the reaction of Me2N(CH2)3(tBuMe2Si)2SiLi with AlCl3 in Et2O. In addition, reaction of [(tBuMe2Si)2AlBr]2 with LiAlH4 yields the novel aluminum hydride cluster [(tBuMe2Si)2Al(µ-H)AlH3]6 which upon addition of TMEDA yields the ion pair [((tBuMe2Si)2AlH)2(µ-H)]-[AlH2(TMEDA)2]+. The amine-coordinated di- and tri-silyl aluminum complexes possess higher thermal stability than the analogous etherate complexes and are reasonably volatile (100-140 °C, 0.2 Torr). The materials presented herein were analyzed via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) to assess their viability as potential ALD precursors.

Synthesis, structural characterization and thermal properties of copper and silver silyl complexes.

The synthesis of copper and silver silyl complexes containing either N-heterocyclic carbenes or nitrogen donors is described. Alterations made to both the neutral donor ligands as well as the silyl group provided access to a number of different compounds. Many of the complexes synthesized were studied in the solid state and the effect of the donor ligand on the final structure of the complexes was examined. The thermal properties of the complexes were explored using thermogravimetric analysis, differential scanning calorimetry and sublimations. Some of the complexes synthesized were demonstrated to be promising volatile metal precursors.

Kinetic and thermodynamic analysis of processes relevant to initiation of olefin metathesis by ruthenium phosphonium alkylidene catalysts.

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

Generation and spectroscopic characterization of ruthenacyclobutane and ruthenium olefin carbene intermediates relevant to ring closing metathesis catalysis.

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2=CHPR3]+[A]- (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.

Mechanistic studies on 14-electron ruthenacyclobutanes: degenerate exchange with free ethylene.

The phosphonium alkylidene [(NHC)Cl2Ru=CH(PCy3)]+[B(C6F5)4]-, 1, (NHC = N-heterocyclic carbene, Cy = cyclohexyl, C6H11) reacts with 2.2 equiv of ethylene at -50 degrees C to form the 14-electron ruthenacyclobutane (NHC)Cl2Ru(CH2CH2CH2), 2. NMR spectroscopic data indicates that 2 has a C2v symmetric structure with a flat, kite shaped ruthenacyclobutane ring with significant Calpha-Cbeta agostic interactions with the Ru center. Intramolecular exchange of Calpha and Cbeta is fast (14(2) s-1 at 223 K) as measured by EXSY spectroscopy. Intermolecular exchange of Calpha and Cbeta with the methylene groups of free ethylene is much slower and first order in both [Ru] and [H2C=CH2] (4.8(3) x 10-4 M-1 s-1). Activation parameters for this process are DeltaH++ = 13.2(5) kcal mol-1 and DeltaS++ = -15(2) cal mol-1 K-1, also consistent with a rate limiting associative substitution as the key step in this exchange process. On the basis of this observation, mechanisms for the intermolecular exchange process are proposed and the implications for the mechanism of the propagation steps in catalytic olefin metathesis as mediated by Grubbs catalysts are discussed.

Direct observation of a 14-electron ruthenacyclobutane relevant to olefin metathesis.

The 14-electron ruthenium phosphonium alkylidene complex [(IH2Mes)Cl2Ru=CH(PCy3)][B(C6F5)4], 1b, a highly active olefin metathesis catalyst, reacts with stoichiometric quantities of ethylene at -50 degrees C in CD2Cl2 to generate the ruthenacyclobutane complex [(IH2Mes)Cl2RuCH2CH2CH2], 2, and [CH2=CH(PCy3)][B(C6F5)4] in quantitative yield by NMR spectroscopy. 1H and 13C NMR spectroscopies on 2 and 2-13C3 are consistent with a symmetrical C2v structure, providing the first experimental information concerning this crucial intermediate in ruthenium-mediated olefin metathesis. At -50 degrees C, exchange with free ethylene takes place on the chemical time scale. Complex 2 decomposes in solution upon warming to room temperature, generating propene and unknown ruthenium product(s).