Alessandro Bismuto

The reaction of 1,3-diisopropylimidazolin-2-ylidene (iPr₂Im) with diphenyldichlorosilane (Ph₂SiCl₂) leads to the adduct (iPr₂Im)SiCl₂Ph₂ 1. Prolonged heating of isolated 1 at 66 °C in THF affords the backbone-tethered bis(imidazolium) salt [(^aHiPr₂Im)₂SiPh₂]²⁺ 2 Cl⁻ 2 (“^a” denotes “abnormal” coordination of the NHC), which can be synthesized in high yields in one step starting from two equivalents of iPr₂Im and Ph₂SiCl₂. Imidazolium salt 2 can be deprotonated in THF at room temperature using sodium hydride as a base and catalytic amounts of sodium tert-butoxide to give the stable N-heterocyclic dicarbene (^aiPr₂Im)₂SiPh₂ 3, in which two NHCs are backbone-tethered with a SiPh₂ group. This easy-to-synthesize dicarbene 3 can be used as a novel ligand type in transition metal chemistry for the preparation of dinuclear NHC complexes, as exemplified by the synthesis of the homodinuclear copper(I) complex [{^a(ClCuiPr₂Im)}₂SiPh₂] 4.

Show some backbone: NHC to aNHC rearrangement, instead of NHC ring expansion, as was observed for Ph₂SiH₂, is the predominant pathway for the reaction of the N-heterocyclic carbene 1,3-diisopropylimidazolin-2-ylidene with Ph₂SiCl₂ (see scheme; aNHC denotes “abnormal” coordination of the NHC). This rearrangement can be utilized for novel, easy-to-synthesize, backbone-tethered dicarbene ligands.

Enantiopure silanediols derived from BINOL are an innovative family of stereoselective hydrogen-bond donor (HBD) catalysts. Silanediols incorporated into a BINOL framework are attractive catalysts, as they are readily accessible and highly customizable. Structural modifications of the BINOL backbone affect the reactivity and selectivity of the silanediol catalysts in the additions of silyl ketene acetals to N-acyl isoquinolinium ions. The best results were obtained when the silanediol scaffold was substituted at the 4,4′- and 6,6′-positions. This report includes details regarding the properties of selected BINOL-based silanediol catalysts, including their acidities, binding constants, and X-ray crystal structures.

A family of BINOL-derived enantiopure silanediols has been prepared and studied. The effect of the substitution pattern of the BINOL backbone on catalyst performance is detailed. The acidities and chloride binding constants are also reported for this new and exciting class of hydrogen-bond donor (HBD) catalysts.

Non-Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural and electronic characterization. The main focus concerns the ability of the ligands in these complexes to mediate proton transfer reactions. As examples, aluminum–ligand cooperation in the activation of polar bonds is described, as is the importance of hydrogen bonding to stabilization of a transition state for β-hydride abstraction. Taken together these reactions enable catalytic processes such as the dehydrogenation of formic acid.

Al in tandem with cooperative ligands: Aluminum complexes of non-innocent ligands participate in various reactions where metal–ligand cooperation promotes proton transfer to and from substrates. The properties of these complexes are outlined, followed by a discussion of OH and NH bond activation and the catalysis of dehydrogenation reactions that can result from the bond activation step.

A new class of exceptionally stable asymmetric N-heterocyclic germylenes, stannylenes, and plumbylenes has been successfully isolated and characterized by single-crystal X-ray diffraction analysis and multinuclear NMR spectroscopy. Their stability results from tetrameric supramolecular aggregation through strong intermolecular N_pyE^II (E=Ge, Sn, Pb) interactions involving the nitrogen atom of a neighboring pyridine moiety. The electronic structures and stabilities of the prepared divalent derivatives of Ge, Sn, and Pb in monomeric and aggregated forms are discussed based on theoretical investigations.

Remarkably stable N-heterocyclic derivatives of divalent germanium, tin, and lead have been successfully isolated. Structural evaluations revealed the tetrameric supramolecular aggregation of these molecules through strong intermolecular NE^II interactions. Theoretical studies demonstrate significant energy gain upon the formation of tetramers compared to monomeric or dimeric units (see figure).

Carbon dioxide is an abundant and renewable C₁ source. However, mild transformations with carbon dioxide at atmospheric pressure are difficult to accomplish. Silanediols have been discovered to operate as effective hydrogen-bond donor organocatalysts for the atom-efficient conversion of epoxides to cyclic carbonates under environmentally friendly conditions. The reaction system is tolerant of a variety of epoxides and the desired cyclic carbonates are isolated in excellent yields.

Life in the silane: Silanediols operate in conjunction with iodide to catalyze the incorporation of carbon dioxide into epoxides. The reaction system benefits from mild reaction conditions and the ability to prepare a wide array of cyclic carbonates in excellent yields. The excellent hydrogen-bonding abilities of silanediols render them useful metal-free catalysts in reactions of carbon dioxide and epoxides.