The first acceptor-free heavier germanium analogue of an acylium ion, [RGe(O)(NHC)2]X (R = MesTer = 2,6-(2,4,6-Me3C6H2)2C6H3; NHC = IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = (Cl or BArF = {(3,5-(CF3)2C6H5)4B}), was isolated by reacting [RGe(NHC)2]X with N2O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh3)] and corresponding heavier analogues ([RGe(S)(NHC)2]X and [RGe(Se)(NHC)2]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO2 and silane, with the former found to be an example of reversible activation of CO2, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO2 hydrosilylation and reductive N-functionalization of amines using CO2 as the C1 source. Mechanistic studies were undertaken both experimentally and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh2)].

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N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations.

The one-electron reduction of (CAACMe)BCl3 (CAACMe = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields the dichloroboryl radical [(CAACMe)BCl2]˙. Furthermore, the twofold reduction of (CAACMe)BCl3 in the presence of a range of Lewis bases (L = CAACMe, N-heterocyclic carbene, phosphine) yields a series of doubly base-supported (CAACMe)LBCl chloroborylenes, all of which were structurally characterised. NMR and UV-vis spectroscopic and electrochemical data for (CAACMe)LBCl show that the boron centre becomes more electron-rich and the HOMO–LUMO gap widens as L becomes less π-accepting. A [(CAACMe)BCl2]− boryl anion coordination polymer was isolated as a potential intermediate in these reductions. In most cases the reduction of the chloroborylenes resulted in the formation of the corresponding hydroborylenes or derivatives thereof, as well as ligand C–H activation products.

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Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes.

The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR3) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR3). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR2, H-BR3, (H-)2BR2) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes.

The room-temperature reaction of [Cp*TaCl4 ] with LiBH4 ⋅THF followed by addition of S2 CPPh3 results in pentahydridodiborate species [(Cp*Ta)2 (μ,η2 :η2 -B2 H5 )(μ-H)(κ2 ,μ-S2 CH2 )2 ] (1), a classical [B2 H5 ]- ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B2 H5 ]- in 1 is mainly due to the stabilization of sp2 -B center by electron donation from tantalum. Reactions to replace the hydrogens attached to the diborane moiety in 1 with a 2 e {M(CO)4 } fragment (M=Mo or W) resulted in simple adducts, [{(Cp*Ta)(CH2 S2 )}2 (B2 H5 )(H){M(CO)3 }] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.

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Stabilization of Classical [B 2 H 5 ] − : Structure and Bonding of [(Cp*Ta) 2 (B 2 H 5 )(μ‐H)L 2 ] (Cp*=η 5 ‐C 5 Me 5 ; L=SCH 2 S)

Contemporary developments in transition metal boryl complexes: An overview

Trinuclear complexes of group 6, 8, and 9 transition metals with a (μ3 -BH) ligand [(μ3 -BH)(Cp*Rh)2 (μ-CO)M'(CO)5 ], 3 and 4 (3: M'=Mo; 4: M'=W) and 5-8, [(Cp*Ru)3 (μ3 -CO)2 (μ3 -BH)(μ3 -E)(μ-H){M'(CO)3 }] (5: M'=Cr, E=CO; 6: M'=Mo, E=CO; 7: M'=Mo, E=BH; 8: M'=W, E=CO), have been synthesized from the reaction between nido-[(Cp*M)2 B3 H7 ] (nido-1: M=Rh; nido-2: M=RuH, Cp*=η(5) -C5 Me5 ) and [M'(CO)5 ⋅thf] (M'=Mo and W). Compounds 3 and 4 are isoelectronic and isostructural with [(μ3 -BH)(Cp*Co)2 (μ-CO)M'(CO)5 ], (M'=Cr, Mo and W) and [(μ3 -BH)(Cp*Co)2 (μ-CO)(μ-H)2 M''H(CO)3 ], (M''=Mn and Re). All compounds are composed of a bridging borylene ligand (B-H) that is effectively stabilized by a trinuclear framework. In contrast, the reaction of nido-1 with [Cr(CO)5 ⋅thf] gave [(Cp*Rh)2 Cr(CO)3 (μ-CO)(μ3 -BH)(B2 H4 )] (9). The geometry of 9 can be viewed as a condensed polyhedron composed of [Rh2 Cr(μ3 -BH)] and [Rh2 CrB2 ], a tetrahedral and a square pyramidal geometry, respectively. The bonding of 9 can be considered by using the polyhedral fusion formalism of Mingos. All compounds have been characterized by using different spectroscopic studies and the molecular structures were determined by using single-crystal X-ray diffraction analysis.

New Trinuclear Complexes of Group 6, 8, and 9 Metals with a Triply Bridging Borylene Ligand.

A series of novel isocloso-diiridaboranes [(Cp*Ir)2B6H6], 1, 2; [1,7-(Cp*Ir)2B8H8], 4; [1,4-(Cp*Ir)2B8H8], 5; [(Cp*Ir)2B9H9], 8; isonido-[(Cp*Ir)2B7H7], 3; and 10-vertex cluster [5,7-(Cp*Ir)2B8H12], 6 (Cp* = η5-C5Me5) have been isolated and structurally characterized from the pyrolysis of [Cp*IrCl2]2 and BH3·thf. On the other hand, the corresponding rhodium system afforded 10- and 11-vertices clusters [5-(Cp*Rh)B9H13)], 7, and [(Cp*Rh)2B9H9], 9, respectively. Clusters 1 and 2 are topological isomers. The geometry of 1 is dodecahedral, similar to that of its parent borane [B8H8]2–, in which two of the [BH] vertices are replaced by two [Cp*Ir] fragments. The geometry of 2 can be derived from a nine-vertex tricapped trigonal prism by removing one of the capped vertices. Compounds 4 and 5 are 10-vertex isocloso clusters based on a 10-vertex bicapped square antiprism structure. The only difference between them is the presence of a metal–metal bond in 5. The solid-state structures of 8 and 9 attain an 11-vertex...

Hypoelectronic 8–11-Vertex Irida- and Rhodaboranes

Irradiation of [Re2(CO)10] in the presence of BH3·thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)4Re}{Re(CO)3}2(μ3-η2:η2:η2-B2H6)(μ-H)], 1; [{(OC)4Re}2{Re(...

Rini Prakash

Papers

New Trinuclear Complexes of Group 6, 8, and 9 Metals with a Triply Bridging Borylene Ligand.

Hypoelectronic 8–11-Vertex Irida- and Rhodaboranes

Diborane(6) and Its Analogues Stabilized by Mono-, Bi-, and Trinuclear Group 7 Templates: Combined Experimental and Theoretical Studies.

Chlorination of the terminal hydrogen atoms in the hydrogen-rich group 5 dimetallaboranes (Cp*M) 2 (B 2 H 6 ) 2 (M = Nb, Ta)

Metal rich metallaboranes of group 9 transition metals