1) Transition and rare earth metallaboranes and metallaheteroboranes:

Our research group is best known for the chemistry of polyhedral boranes, metallaboranes, and their metal-based derivatives containing 1-4/ or more metals and 1-12/ or more boron atoms that include the reaction of monocyclopentadienyl metal halides or hydrides with monoborane reagents (BH₃·THF, LiBH₄·THF or BHCl₂·SMe₂). Metallaboranes that incorporate early transition metals or rare earth are sparsely explored area of cluster chemistry and one of the key challenges is to find suitable synthetic routes. The characterisation typically involves the use of multinuclear NMR spectroscopy such as ¹H, ¹³C, and ¹¹B NMR and X-ray diffraction study. For bonding and electronic structure determination, we collaborate with Prof. E. D. Jemmis, Indian Institute of Science, Prof. Jean-Francois Halet, UniVersite´ de Rennes 1, Bruce King, Department of Chemistry, University of Georgia.

(i) Higher vertices metallaborane clusters

While large metal clusters have been known with the order of 100 atoms for decades, the search for boron clusters with more than 12 atoms met with little success. Even through computational studies suggest stability for clusters with more than 16 vertices, all attempts till date to prepare these clusters have failed to date. Thus, the development of new synthetic routes for the generation of higher vertex polyhedral borane/ metallaborane clusters of larger size and unique structures is of great interest. In 2013, we isolated, for the first time, higher nuclearity (15 and 16 vertices) metallaborane clusters. Although Lipscomb predicted in 1977 that supra-icosahedral boron clusters would be viable, their synthesis has been impeded by the unavailability of appropriate synthetic methodologies. Herein, we reported the first examples of the open 16-vertex oblato–hypho-titanaborane clusters [(Cp^*Ti)₂[B₁₄H₁₇R] (R = H; R = Me) having a non-Wadean 19-skeletal-electron-pair count in 2023. Interestingly, these clusters show a six-membered [Ti₂B₄] open face, which could lead to close-19-vertex clusters. Along with that, Isolation of planar [B₆H₆] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B₆H₆] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple-decker sandwich complex of titanium [(Cp*Ti)₂(μ-η⁶:η⁶-[B₆H₆])(μ-H)₆], which features the first-ever experimentally achieved nearly planar six-membered [B₆H₆] ring, albeit within a [B₆H₁₂] borate.

(ii) Metal-rich metallaboranes containing borylene and boride ligands

Metal-rich metallaborane clusters containing borylene and boride ligands are of significant interest due to their unique structures, diverse reactivity, and applications. We have synthesized a series of such type of homo- and heterometallic clusters (see picture) and our quest for the isolation of them in high yield is in progress.

 

(iii) Metallaheteroboranes

An area of continuing importance in polyhedral metallaborane chemistry is the development of new efficient methods in which atom-insertion reactions lead to formation of expanded-cage clusters. Although the structural variety of polyhedral metallaborane chemistry is in theory also accessible to any combinations of main-group elements that have the same numbers of valence electrons, this chemistry is dominated typically by carboranes and metallacarboranes. Therefore, development of new metallahetero-borane clusters is of significant interest and practically all actual and projected applications of metallaborane/ or metallaheteroboranes require substituted derivatives in which organic or inorganic functional groups are introduced to the metal, boron or heteroatom of the core. This, in turn, makes it essential to develop practical methods for derivatization using different synthetic approaches (see picture).

 

2) Transition metal diborane complexes:

The chemistry of diboranes has experienced a renaissance in recent times, particularly the development of B₂H₆, B₂H₄ and B₂H₂ species in the coordination sphere of early and late transition metals. The diboration reaction stands out as “the most-used organometallic reaction in organic synthesis” of diborane(4) as it offers a valued synthetic route for the introduction of boryl group across the unsaturated organic species. Although the synthesis of transition metal diborane complexes is challenging, they have been utilized as relevant starting materials to generate electron precise metal–boron bonds. Recently, we have synthesized a dimolybdenum–diborane(4) species, which is a true mimic of alkyne complexes of molybdenum reported by Cotton and co-workers in 1978. Further, for the first time, we have isolated a classical [B₂H₅]^– species at the coordination sphere of transition-metal template.

 

3) Homo and Heterobimetallic complexes for Hydrogen Evolution Reactions (HER) :

Dinuclear complexes with early to late transition metals (TM) are crucial for their unique structures, bonding, reactivities, and catalytic activities. Chalcogenate-bridged complexes have a distinctive reactivity and structural resemblance to metalloenzymes like nitrogenases or hydrogenases. The active site of chalcogenate-bridged heterodinuclear [NiFe] hydrogenase plays a vital role in biological systems, converting protons to H₂ through a bridging hydride moiety. Whilst numerous thiolate-bridged homo/heterobimetallic complexes have been employed in catalytic H₂ generation, there aren’t many heavier chalcogenate parallels. From this perspective, we have synthesized several homo- and heterobimetallic complexes bridged by light as well as heavier chalcogenates. On top of that, we have also synthesized hydride-bridged dichalcogenate heterobimetallic complexes. As hydride-bridged species such as Ni-R and Ni-C had been identified in the [NiFe]H₂ases, these heterodinuclear hydride-bridged complexes can be a boon to get deep insight into the detailed mechanism of HER shown by these hydrogenase enzymes. Our current area of interest is the investigation of these chalcogenate bridging homo- and heterobimetallic complexes in hydrogen evolution processes as well as their mechanistic pathway.

 

4) Tri/tetra-coordinated boron complexes for small molecule activation :

The transition metal (TM) σ complexes of hydroboranes and hydroborates, for example, H-BR₂, H-BR₃, (H-)₂BR₂, are somewhat less well studied. However, they have relevance in catalytic borylation reactions which are of significant interest in organic synthesis. We and other groups have put continuous effort to explore the stynthesis and reacitivity of different types of TM-borane/borate complexes. One of the key challenges for this study is the design of robust hetercyclic borate ligands. The most possible outcome of these reactions are different types of σ-borane/borate, bis(σ)-borane/borate, agostic borate, boratrane species stabilised in the coordination sphere of TM. We have made several metal precursors and borate ligands that led the formation of various tri- and tetrametallic σ-borane/borate species (see picture).

The reactivity of these TM-borane/borate complexes are very interesting as they are potential to react with various small molecules.
• Current research interest is mostly focused on the reactivity of these TM-borane/borate complexes with unsaturated hydrocarbons that typically led to the formation of borataallyl, alkene-borane and vinyl borane species.

• Our research interest is also focused on the reactivity of these TM-borane/borate complexes with various boranes, silanes, phosphines, CO, isocyanide and dichalcogenides ligands which is very fascinating and longstanding interest.

• Boron is well known to play a role in CO₂ activation either as a reductant in the form of hydroborane or hydroborate or as Lewis acid activator as an active part of non-metallic catalyst in the form of borane. Reduction of CO₂ using hydroborates (HBR₃) are well known from 1950s. Recently, in 2010, it was reported that hydroboranes (HBR₂) add to CO₂ in presence of various catalysts. Our research interest mainly lies on the utilization of both the Lewis acid and hydride donor ability of boron-based reagents to activate and transform CO₂. Presence of a large library of hydroborate complexes, we now aim at designing and synthesizing boron-based compounds with pendant Lewis base moiety.