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Research Interests

Main-group and transition metal organometallic chemistry: Challenging conventional perspective on bonding, structure, stability and reactivity

My research group investigates the chemistry of low-valent p- and d-block complexes with emphasis on their unprecedented electronic structures and reactivities, which were thought to be inexistent and unstable at room temperature, leading to possible application in catalysis, materials chemistry, hydrogen storage, etc. Our key achievement is highlighted in the following:

Carbon Dioxide Fixation

Efforts towards using more environmentally benign and abundant elements to replace precious and toxic transition metals in catalysis are of paramount importance. Considerable progress has been made with the design of sustainable main-group element compounds, which are capable of mimicking the behavior of precious transition metals. Catalytic cycles for transition metals are typically based upon redox processes, owing to their flexible oxidation states. For p-block elements such as silicon, aluminum and boron, a range of oxidation states are available, however, the ability to stabilize low oxidation states requires kinetic stabilization from sterically hindered supporting ligands. Upon oxidative addition, these reactive low-valent species tend to form very stable products rendering the reductive elimination step highly challenging, thus, inhibiting catalytic turnover. To prove that p-block main-group elements can replace precious transition metals in industrial catalysis, several key challenges are under investigation: (1) synthesis of stable low oxidation state main group complexes with an open coordination site; (2) activation of common small molecules such as CO2, H2; (3) elimination of a functionalized product; and (4) regeneration of the active catalyst.

We reported the first use of a divalent silicon complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]I (1, IMe = :C{N(Me)C(Me)}2) as a versatile catalyst in organic synthesis (J. Am. Chem. Soc. 2019, 141, 17629). Complex 1 was as an efficient catalyst for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC(=O)H]. Its catalytic activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form their respective borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions. This is an significant milestone in main-group chemistry as the first example to experimentally show that the divalent silicon centre in complex 1 is able to exhibit  transition metal-like properties in substrate activation for catalytic hydroboration reactions.

Scheme 1.jpg

Scheme 1. NHC-parent silyliumylidene cation-catalyzed hydroboration of unsaturated compounds.

In addition, we reported the first use of a multiply bonded boron complex, N-phosphinoamidinato diborene 2 as a versatile catalyst in organic synthesis (J. Am. Chem. Soc. 2020, in preparation). Complex 2 was an efficient catalyst to conduct hydroboration of CO2 (Conversion: >99%) with HBpin in C6D6 to afford methoxyborane [pinBOMe] and diborate ether [(pinB)2O]. This is another significant step forward in main-group chemistry as it demonstrates that multiply bonded main-group compounds are capable of behaving like transition metal catalyst to activate CO2. Complex 2 was found to be recoverable after catalysis.

Scheme%202_edited.jpg

Scheme 2. Diborene-catalyzed hydroboration of unsaturated compounds

Subnanometre Main-group Element Clusters

Subnanometre main-group element clusters are structures with typically 2 to <15 metal atoms. They preserve molecular-like electronic properties, in contrast to metal nanoparticles (NPs). They have well-defined HOMO and LUMO, with an energy and shape that easily varies with atomicity, topology and electronic environment of subnanometre main-group element clusters. In addition, subnanometre main-group element clusters possess a variety of distinct characteristics including very high ratio of surface-to-bulk atoms, electronic shell closings, geometric shell closings, superatomic character in which electrons are shared among atoms differently from bulk materials, and quantum confinement. As a result, subnanometre main-group element clusters exhibit a range of fascinating reactive, optical, electronic and magnetic properties, which are not observed in the corresponding bulk sample and NPs. To prove the concept, several key challenges are being investigated: (1) synthesis of subnanometre main-group element clusters; (2) functionalization of subnanometre main-group element clusters and (3) the fabrication of nanoparticles through bottom-up approach.

We synthesized stable singlet diatomic germanium {:Ge=Ge:} in zero oxidation state, which is stabilized by the N-heterocyclic silylene ligands through a weak synergistic donor-acceptor interaction.  This is a significant piece of work as it demonstrates novel chemistry of the N-heterocyclic silylene ligands.  In addition, the ability to synthesize stable diatomic semiconducting material (such as germanium) in zero oxidation state is fundamental to ‘bottom-up’ molecular assembly of electronic devices, which potentially can lead to smaller electronic devices than the current ‘top-down’ electronics manufacturing processes based on surface engineering of bulk materials.  (Angew. Chem. Int. Ed. 2014, 53, 13155).

Scheme 3.JPG

Scheme 3. NHSi-stabilized digermanium(0) compound

Since silicon-based compounds are predominant in semiconducting industry, silicon monotelluride is envisioned to be a new generation material for electronic applications. However, silicon monotelluride (SiTe) cannot exist in nature and its synthesis is a formidable challenge, being supported by theoretical studies.

We developed a strategy to synthesize silicon monotelluride from a NHC-disilicon(0) complex 4 (Scheme 3, Angew. Chem. Int. Ed. 2017, 56, 11565). The disilicon(0) cluster 4 was reacted with two equivalents of tellurium to form the first di(silicon telluride) cluster 5 under ambient conditions. It can further be functionalized with elemental sulphur to form the disilicon sulphide ditelluride cluster 6, which is the first ternary silicon heterodichalcogenide comprising two different chalcogens in the form of an adduct with NHC. To the best of our knowledge, bulk ternary silicon heterodichalcogenides are still unknown. This research shows that novel electronic materials (SiTe, Si2STe2) obtained through the bottom-up approach lead to material compositions and structures, which are impossible to synthesize through the top-down approach from cylindrical single-crystal-silicon ingots.

Scheme 4.JPG

Scheme 4. Synthesis of silicon chalcogenides that cannot be obtained through the top-down approach.

Main-group Element-Supported Transition Metal Complexes

Catalysis is a key technology in industry and offers the potential to increase the material output of chemical synthesis without an unreasonable increase in the energy necessary for the production of new materials. Therefore, fundamental research on improving the performances of existing industrial catalyses is essential, which would result in a further reduction of global energy demands and production costs in industrial processes. Currently, homogeneous catalysis, in which the catalyst and substrates are in the same phase, can produce sophisticated structurally diverse or isomerically pure products in high yields and turnover rates. In order to design an effective metal-catalyzed process, the choice of an ancillary ligand is crucial; in fact, it can be as critical as the choice of the metal itself. This is due to the extraordinary control that ligands exert over the reactivity of the resulting catalysts and, not less important, over the product selectivity of the catalyzed processes. We are interested in developing novel functionalized ligands based on heavier group 14 elements, which exhibit unique electronic and structural effects. These properties enhance the activity of transitional metal, where catalysis is able to perform at milder conditions, along with increasing efficiency.

 

We successfully prepared a base-stabilized environmentally friendly cobalt-silicon catalyst (Scheme 5), which is capable of regio- and stereoselectively promoting the addition of the ortho-C-H bond in arylpyridines with the C-C triple bonds in alkynes to form (Z)-isomers (E/Z >> 1/99) (Chem. - Eur. J. 2018, 24, 14329-14334: Selected as Hot Paper and Invited for the Cover of the journal). In contrast, when CoBr2 was used, a mixture of isomers was isolated. This shows novel functionalized ligands based on heavier group 14 elements should be able to provide new (regio- and stereoselective) reaction pathways and smaller activation barriers for organic transformation, which enhance catalytic activity.

Scheme 5.JPG

Scheme 5. A cobalt/silicon cluster-catalysed regio- and stereoselective C-C bond formation

Main-group Element Radical Complexes

The synthesis of stable low-coordinate main-group element and transition metal complexes has attracted our attention due to their unique structural and electronic properties. The metal centre in these complexes has a very high degree of coordinative unsaturation with several open or singly occupied valence orbitals, which facilitates a rich coordination chemistry spanning a wide variety of substitution, addition, or oxidation reactions. This enables possible application in small molecules activation and catalytic transformation.

 

In the past three years, we have successfully synthesized novel low-valent silicon complexes such as the cAAC-stabilized silicon(I) cation (shown below), which possesses multi-functionality: a lone pair of electrons, a radical and vacant orbital on the silicon centre. Such electronic properties resemble transition metals, which enable possible application in small molecules activation and catalyses (see above)

Scheme 6.JPG

Scheme 6. Base-stabilized silicon(I) radical

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