Synthesis and reactivity of transition metal complexes supported by oxazolinyl borato ligands and the functionalization of silica surface

Abstract

<p>Oxidative addition of a nonpolar Si–H bond to the rhodium dicarbonyl {PhB(OxMe2)2ImMes}Rh(CO)2 is an unusual example of this kind, as it is accompanied by thermal dissociation of CO. The reaction was found to be reversible, and kinetic measurements model the approach to equilibrium. The oxidative addition is first-order in both {PhB(OxMe2)2ImMes}Rh(CO)2 and primary silane, with rate constants for oxidative addition of PhSiH3 and PhSiD3 revealing kH/kD ~ 1. The reaction is significantly inhibited in the presence of externally added CO. The reverse reaction, reductive elimination of Si–H from {PhB(OxMe2)2ImMes}RhH(SiH2R)CO, also follows a second-order rate law (first-order in both the reactants). Equilibrium constants are measured from equilibrium concentrations of all the species in the reaction mixture over a 30° C temperature range, provide ΔH° = –5.5 ± 0.2 kcal/mol and ΔS° = –16 ± 1 cal·mol–1K–1. Activation entropy for both forward and reverse reaction are negative (ΔS‡ = –26 ± 3 cal·mol–1·K–1 and ΔS‡ = –10 ± 3 cal·mol–1·K–1 respectively), suggests a highly associated transition state containing {PhB(OxMe2)2ImMes}Rh(CO)2 and RSiH3.</p> <p>RuHCl(PPh3)3 catalyzes cross-dehydrogenative coupling between various silane-alcohol and silane-amine pairs in homogeneous reaction conditions under ambient temperature and low catalyst loading (1 mol%). Catalysis was further extended to functionalize silica material. Surface silanol (SiOH) successfully reacts with hydrosilanes (primary, secondary and tertiary) in presence of a catalytic amount of RuHCl(PPh3)3 under ambient reaction conditions to produce SiOSiR3. Infra-red spectroscopy measurement confirmed the disappearance of isolated SiOH (3747 cm-1) and the appearance of new Si–H stretching in the case of primary and secondary hydrosilanes. The amount of silane grafted was measured via elemental analysis, 29Si DPMAS study and titration of the remaining SiOH with Mg(CH2Ph)2(O2C4H8)2. Newly created Si–H bonds further successfully reacted with N–H of amines in the presence of RuHCl(PPh3)3 to produce unique Si¬–O–Si–N– bonded functionalized materials. Notably, the synthesis of surface silazanes from silica material was successfully achieved in a one-pot technique.</p> <p>A new ruthenium tris(oxazolinyl)borato complex is synthesized by stirring TlToM and Ru(NCPh)4Cl2 at 80° C. Resulting complex shows a Cs symmetric geometry around the ruthenium center. Further, the ruthenium complex efficiently catalyzes cycloisomerization of diethyl diallylmalonate to produce exo-methylenecyclopentane isomer selectively. A significant solvent effect was observed as the only cycloisomerization occurred in protic solvent (iPrOH) and a side product resulting from olefin isomerization was found in CH2Cl2. In addition, a range of different 1,6-heptadienes with different functional groups was tested to successfully produce cycloisomerization products in high yield.</p>