Hetero-bimetallic alkali titanosilicates [MOTi{OSi(OtBu)3}3]2 (M = Li-Cs) with terminal Ti-O- groups.

The molecular titanosilicate [(tBuO3)3SiO]3TiNEt2 (1) was obtained from the reaction between silanol (tBuO3)3SiOH and titanium amide Ti(NEt2)4. The reaction of 1 with alkali metal hydroxides MOH (M = Li, Na, K, Rb, Cs) offers a straightforward route to the alkaline salts of titanosilicates [MOTi{OSi(OtBu)3}3]2 with a terminal Ti-O- moiety. All compounds were characterised by single-crystal X-ray diffraction studies. Hirshfeld atom refinement and QTAIM analysis of the electron density in 1 and in the Rb salt 5 revealed the D-A nature of the Ti-O and Ti-N bonds and the presence of agostic C-H⋯Rb interactions.

Reactivity patterns for the activation of CO2 and CS2 with alumoxane and aluminum hydrides.

Carbon dioxide is readily fixed when reacting with either alumoxane dihydride [{MeLAl(H)}2(µ-O)] (1) or aluminum dihydride [MeLAlH2] (2) (MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2-) to produce bimetallic aluminum formates [(MeLAl)2(µ-OCHO)2(µ-O)] (3) and [(MeLAl)2(µ-OCHO)2(µ-H)2] (5), respectively. Furthermore, [(MeLAl)2(µ-OCHO)2(µ-OH)2] (4) is easily obtained upon the reaction of 3 or 5 with H2O. The stability of the unusual dialuminum diformate dihydride core observed in 5 stems from the proximity of the Al centers allowing the formation of two Al-HAl bridges and precluding further hydride transfer to the HCO2 moieties. Contrary to this behavior, 1 and 2 react with CS2 giving cyclic alumoxane and aluminum sulfides [(MeLAl)2(µ-S)(µ-O)] (6) and [{MeLAl(µ-S)}2] (7), respectively. The molecular structures of 3-7 were characterized by IR, Raman, solution or solid-state (MAS) NMR spectroscopy and mass spectrometry and for 4-7 were characterized by X-ray diffraction studies. NMR kinetic studies and DFT calculations suggest that the mechanisms for the formation of 6 and 7 involve the transfer of a hydride group forming transient aluminum thioformate intermediates which proceed to form Al-S-Al moieties through the cleavage of C-S bonds and insertion of a sulfur atom, followed by the elimination of thioformaldehyde.

Synthesis of Cyclic and Cage Borosilicates Based on Boronic Acids and Acetoxysilylalkoxides. Experimental and Computational Studies of the Stability Difference of Six- and Eight-Membered Rings.

A series of borosilicates was synthesized, where the structure of the borosilicate core was easily modulated using two strategies: blocking of condensation sites and controlling the stoichiometry of the reaction. Thus, on the one hand, the condensation of phenylboronic or 3-hydroxyphenylboronic acid with diacetoxysilylalkoxide [(tBuO)(Ph3CO)Si(OAc)2] led to the formation of borosilicates (tBuO)(Ph3CO)Si{(µ-O)BPh}2(µ-O) (1), [{(tBuO)(Ph3CO)Si(µ-O)BPh(µ-O)}2] (2), and [{(tBuO)(Ph3CO)Si(µ-O)B(3-HOPh)(µ-O)}2] (3) with a cyclic inorganic B2SiO3 or B2Si2O4 core, respectively. On the other hand, the reaction of phenylboronic acid with triacetoxysilylalkoxide (Ph3CO)Si(OAc)3 in 3:2 ratio resulted in the formation of a cagelike structure [{(Ph3CO)Si(µ-O)2BPh(µ-O)}2] (4) with B4Si4O10 core, while the reaction of the boronic acid with silicon tetraacetate generated an unusual 1,3-bis(acetate)-1,3-diphenyldiboraxane PhB(µ-O)(µ-O,O'-OAc)2BPh (5). Additionally, compound 1 was used to evaluate the possibility to form N→B donor-acceptor bond between the boron atom in the borosilicates and a nitrogen donor. Thus, coordination of 1 with piperazine yielded a tricyclic [{(tBuO)(Ph3CO)Si(OBPh)2(µ-O)}2·C4H10N2] compound 6 with two borosilicate rings bridged by a piperazine molecule. Finally, the processes involved in the formation of the six- and eight-membered rings (B2SiO3 and B2Si2O4) in compounds 1 and 2 were explored using solution 1H NMR studies and density functional theory calculations. These molecules represent to the best of our knowledge first examples of cyclic molecular borosilicates containing SiO4 units.

Diastereoselective addition of diazomethane to zaluzanin A.

The diastereoselectivity of diazomethane addition to the conjugated double bond of alpha,beta-unsaturated sesquiterpene lactones was explored using zaluzanin A (1) as a model. Thus, the absolute configuration of 1 was assured by X-ray diffraction analysis including evaluation of Flack and Hooft parameters, and by vibrational circular dichroism spectroscopy of its diacetyl derivative 2, while the absolute configuration of the diazomethane addition product, zaluzanin A pyrazoline (3), was determined by evaluation of the 1H NMR chemical shift changes with respect to 1, and confirmed by X-ray diffraction analysis, again including evaluation of Flack and Hooft parameters.