Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2.

A novel ß-diketiminate stabilized gallium hydride, (Dipp L)Ga(Ad)H (where (Dipp L)={HC(MeCDippN)2 }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ1 -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C6 F5 )3 , (Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO2 . Under stoichiometric conditions, the addition of one equivalent of B(C6 F5 )3 to (Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(Dipp L)Ga(Ad)][HB(C6 F5 )3 ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO2 to the formaldehyde oxidation level at 60 °C in the presence of Et3 SiH (yielding H2 C(OSiEt3 )2 ). When catalysis is undertaken in the presence of excess B(C6 F5 )3 , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H2 C(OSiEt3 )2 , CH4 and O(SiEt3 )2 . While this system represents proof-of-concept in CO2 hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (Dipp L)Ga(Ad){OC(H)OB(C6 F5 )3 }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(Dipp L)Ga(Ad)]+ cation (AN=69.8).

An N-Heterocyclic Boryloxy Ligand Isoelectronic with N-Heterocyclic Imines: Access to an Acyclic Dioxysilylene and its Heavier Congeners.

Introduced here is a new type of strongly donating N-heterocyclic boryloxy (NHBO) ligand, [(HCDippN)2 BO]- (Dipp=2,6-diisopropylphenyl), which is isoelectronic with the well-known N-heterocyclic iminato (NHI) donor class. This 1,3,2-diazaborole functionalized oxy ligand has been used to stabilize the first acyclic two-coordinate dioxysilylene and its Ge, Sn, and Pb congeners, thereby presenting the first complete series of heavier group 14 dioxycarbene analogues. All four compounds have been characterized by X-ray crystallography and density-functional theory, enabling analysis of periodic trends: the potential for the [(HCDippN)2 BO]- ligand to subtly vary its electronic-donor capabilities is revealed by snapshots showing the gradual evolution of arene π coordination on going from Si to Pb.

Successive Protonation of an N-Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion.

Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a "stable ion" environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N-heterocyclic imine (NHI) derived carbonyl compound (IDippN)2 CO, 2, to the monocation [(IDippN)(IDippNH) CO]+ , [3]+ , and the doubly protonated dication [(IDippNH)2 CO]2+ , [4]2+ . [3]+ represents a rare example of an N-protonated carbonyl cation and [4]2+ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X-ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI-stabilized cation [IDippNCO]+ , [5]+ . Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]+ unit (isoelectronic to CCCO) can be described as an acylium cation "masked" as a cumulene.

A ß-Diketiminate-Stabilized Sila-Acyl Chloride: Systematic Access to Base-Stabilized Silicon Analogues of Classical Carbonyl Compounds.

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel ß-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the SiIV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding SiII systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry.