Recent advances in boron-centered ligands and their transition metal complexes

Borataalkenes, boraalkenes, and the η2-B,C coordination mode in coordination chemistry and catalysis

Borataalkenes and boraalkenes are the boron-containing isoelectronic analogues of alkenes and vinyl cations respectively. Compared with alkenes, the borataalkene and boraalkene ligand motifs in transition metal coordination chemistry are relatively underexplored. In this review, the synthesis of borataalkene and boraalkene complexes and other transition metal complexes featuring the η2-B,C coordination mode is described. The diversity of coordination modes and geometry in these complexes, and the spectroscopic and structural evidence supporting their assignments is disclosed as well as computational analysis of bonding. The applications of the borataalkene ligand motif in synthetic organic homogeneous catalysis, especially those involving geminal bis(pinacolatoboronates) and 1,4-azaborines, are discussed.

Synthesis, reduction and C-H activation chemistry of azaborinines with redox-active organoboryl substituents

The 2,3,4,5,6-pentaphenyl-1,2-azaborinin-1-yl (PPAB) potassium complex 1 undergoes facile salt metathesis with 9,10-dibromo-9,10-dihydroboraanthracene (DBABr2), 5-bromodibenzo[b,d]borole (DBBBr), 1-chlorotetraphenylborole (TPBCl) and dibromo(phenyl)borane (BBr2Ph) to yield the corresponding N-borylated azaborinines N-DBABr-PPAB (2, which hydrolyses and dimerises to the oxo-bridged N,N'-O(DBA)2-(PPAB)2, 3), N,N'-DBA-(PPAB)2 (4), N-DBB-PPAB (5), N-PPB-PPAB (7) and N-BBrPh-PPBA (9). Stepwise reduction of 4 yields the corresponding stable radical anion 4˙- and dianion 42-. One-electron reduction of 5 with KC8 yields the purple radical anion 5˙-, which forms a highly insoluble coordination polymer. 5˙- undergoes very slow radical intramolecular ortho-C-H activation at the C4-phenyl substituent of the PPAB moiety, yielding a BN-analogue of the 5,5'-spiro-bi[dibenzoborole] anion, [6]K. Compound 7 cannot be isolated and undergoes spontaneous and diastereoselective 2,5-anti-addition of the ortho-C-H bond of the PPAB C4-phenyl substituent to yield a novel BNB-analogue of the triply fused dihydrocyclopenta[l]phenanthrene cation, compound 8. Finally the one-electron reduction of 9 results in the ortho-C-H activation of the PPAB C4-phenyl substituent at an in situ-generated dicoordinate boryl anion (10), resulting in the formation of a BNB-analogue of 9H-fluorene, the borate 11-. DFT calculations provide a rationale for the diverse C-H activations observed in these reactions.

Isolation and Reactivity of Homoleptic Diphosphene Lead Complexes

Due to their limited capacity for π-backdonation, isolation of π-complexes of main-group elements remains a great challenge. We report herein the synthesis of a homoleptic diphosphene lead complex (2) from the degradation of P4 with a bis(germylene)-stabilized Pb(0) complex. Structural and computational studies showed that 2 possesses significant π bonding interactions between Pb atom and diphosphene ligands, which is reminiscent of transition-metal diphosphene complexes. Consistent with its unique electronic structure, complex 2 can deliver Pb(0) atoms to perform redox reaction with an iminoquinone to produce a cyclic plumbylene (4) and perform 2,5-dimethyl-3,4-dimethylimidazol-1-ylidene (IMe2 Me2 ) induced phosphorus cation abstraction to give an anionic PbP3 complex (6).

Diboriranide σ-Complexes of d- and p-Block Metals

Diboriranides are the smallest conceivable monoanionic aromatic cycles, yet only limited examples have been reported and their reactivity and complexation behavior remain completely unexplored. We report a straightforward synthesis of the first peraryl diboriranide c-(DurB)2 CPh- as its lithium salt in three steps via the corresponding non-classical diborirane from a readily available 1,2-dichlorodiborane(4) (Dur=2,3,5,6-tetramethylphenyl). With the preparation and complete characterization of representative complexes with tin, copper, gold and zinc, we demonstrate the strong preference of the diboriranide for σ-type coordination modes towards main group and transition metal centers under unperturbed retention of the three-membered B2 C-ring's 2e- π-system.

C-C and C-N Bond Activation, Lewis-Base Coordination and One- and Two-Electron Oxidation at a Linear Aminoborylene

A cyclic alkyl(amino)carbene (CAAC)-stabilized dicoordinate aminoborylene is synthesized by the twofold reduction of a [(CAAC)BCl₂ (TMP)] (TMP=2,6-tetramethylpiperidyl) precursor. NMR-spectroscopic, X-ray crystallographic and computational analyses confirm the cumulenic nature of the central C=B=N moiety. Irradiation of [(CAAC)B(TMP)] (2) resulted in an intramolecular C-C bond activation, leading to a doubly-fused C₁₀ BN heterocycle, while the reaction with acetonitrile resulted in an aryl migration from the CAAC to the acetonitrile nitrogen atom, concomitant with tautomerization of the latter to a boron-bound allylamino ligand. One-electron oxidation of 2 with CuX (X=Cl, Br) afforded the corresponding amino(halo)boryl radicals, which were characterized by EPR spectroscopy and DFT calculations. Placing 2 under an atmosphere of CO afforded the tricoordinate (CAAC,CO)-stabilized aminoborylene. Finally, the twofold oxidation of 2 with chalcogens led, in the case of N₂ O and sulfur, to the splitting of the B-C_CAAC bond and formation of the 2,4-diamino-1,3,2,4-dichalcogenadiboretanes and CAAC-chalcogen adducts, whereas with selenium a monomeric boraselenone was isolated, which showed some degree of B-Se multiple bonding.

Leveraging Metal and Ligand Reactive Sites for One Pot Reactions: Ligand-Centered Borenium Ions for Tandem Catalysis with Palladium

Tandem catalysts that perform two different organic transformations in a single pot are highly desirable because they enable rapid and efficient assembly of simple organic building blocks into more complex molecules. Many examples of tandem catalysis rely on metal-catalyzed reactions involving one or more metal complexes. Remarkably, despite surging interest in the development of chemically reactive (i. e., non-innocent) ligands, there are few examples of metal complexes that leverage ligand-centered reactivity to perform catalytic reactions in tandem with separate catalytic reactions at the metal. Here we report how multifunctional Pd complexes with triaminoborane-derived diphosphorus ligands, called TBDPhos, appear to facilitate borenium-catalyzed cycloaddition reactions at the ligand, and Pd-catalyzed Stille and Suzuki cross-coupling reactions at the metal. Both transformations can be accessed in one pot to afford rare examples of tandem catalysis using separate metal and ligand catalysis sites in a single complex.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C₆F₅, C₂SiMe₃), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of k_r = 1.3-14 × 10⁷ s^-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S₁ state to be observed, while the spin-orbit coupling in the T₁ state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Carbene-Anchored Boryl- and Stibanyl-Phosphaalkenes as Precursors for Bis-Phosphaalkenyl Dichlorogermane and Mixed-Valence AgI/AgII Phosphinidenide

Cyclic alkyl(amino) carbene (cAAC)-anchored boryl- and stibanyl-phosphaalkenes with general formula cAAC = P-ER₂ [E = B, R = (NⁱPr₂)₂ (3a-c); E = Sb, R = 2,4,6-triisopropylphenyl (5a-b)] have been synthesized and utilized as precursors for the bis-phosphaalkenyl dichlorogermane [(cAAC = P)₂GeCl₂] (6) and the first molecular example of a neutral polymeric mixed-valence Ag^I/Ag^II phosphinidenide complex [(cAACP)₂Ag₄^IAg^IICl₄]_n (7). All compounds have been characterized by single-crystal X-ray diffraction and further investigated by nuclear magnetic resonance (NMR), mass spectrometric analysis, and UV-vis/fluorescence measurements. The paramagnetic complex 7 has been characterized by ESR spectroscopy. Cyclic voltammetry studies of compounds 3/5 have suggested possible one-electron quasi-reversible reductions, indicating their redox noninnocent behavior in solution. Quantum chemical studies revealed the electron-sharing nature of the P-B and P-Sb σ bonds in compounds 3 and 5, and the polar C_cAAC = P bonds in compounds 3, 5, and 6 prevailing their phosphaalkene structures over phosphinidenes.

From trihydroborates to bisborylenes: a route to dinuclear bisborylene complexes

A new route for the synthesis of dinuclear bisborylene complexes was described. A series of novel diruthenium bisborylenes were prepared through unprecedented triple B-H oxidative addition of trihydroborates with concomitant hydrogen liberation. Conversion of trihydroborates to bisborylenes involved the formation of tris(σ-B-H) borate as the crucial intermediate stage.

A neutral, acyclic, borataalkene-like ligand for group 11 metals: L- and Z-type ligands side by side

The overall neutral α-borylated phosphorus ylide Ph₃PC(Me)BEt₂ behaves like a polar borataalkene and can act as acyclic, ambiphilic π-type ligand with L- and Z-type functionalities side by side. In the complexes [MX{η²-Ph₃PC(Me)BEt₂}] (M = Cu, (Ag), Au; X = Cl, NTf₂), the bonding is dominated by the highly nucleophilic ylidic carbon atom (L-type ligand). The Lewis acidic boron atom furnishes nonetheless a small but significant bonding contribution (Z-type ligand).

Trinuclear anionic gold clusters bridged by dialkylsilylene and dialkylgermylene

While the chemistry of gold clusters has been developed rapidly, the knowledge of trinuclear gold clusters with low-valent gold remains scarce. Herein, we report the synthesis, structural characterization, and density...

Assessing the donor ability of boratabenzene and 9-borataphenanthrene anions through metal complexes with carbonyl ligands

A series of anionic group 6 tricarbonyl and neutral rhodium dicarbonyl complexes featuring substituted boratabenzene and borataphenanthrene ligands are prepared and the donor strengths of the boracyclic ligands are assessed.

Bringing out the potential of organoboron compounds by designing the chemical bonds and spaces around boron

Since the structures, reactivity and properties of organoboron compounds stem from the electron deficiency and low electronegativity of boron, the design of the chemical bonds attached to boron as well...

From a Möbius-aromatic interlocked Mn2B10H10 wheel to the metal-doped boranaphthalenes M2@B10H8 and M2B5 2D-sheets (M = Mn and Fe): a molecules to materials continuum using DFT studies

The design of (1) Möbius aromatic interlocked boron wheel Mn2B10H10, (2) Hückel aromatic boron analogs of naphthalene (M2@B10H8; M = Mn and Fe), and (3) metal boride monolayers (FeB5 and Fe2B5), creating a molecules to materials continuum.

Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)–carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)–metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM–E and TM–H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid–hydrogen and metalloid–metalloid bonds to unsaturated compounds; activation and functionalization of C–H bonds and C–X bonds with hydrometalloids and bismetalloids; activation and functionalization of C–H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

A Free Aluminylene with Diverse σ-Donating and Doubly σ/π-Accepting Ligand Features for Transition Metals*

We report herein the synthesis, characterization, and coordination chemistry of a free N-aluminylene, namely a carbazolylaluminylene 2 b. This species is prepared via a reduction reaction of the corresponding carbazolyl aluminium diiodide. The coordination behavior of 2 b towards transition metal centers (W, Cr) is shown to afford a series of novel aluminylene complexes 3-6 with diverse coordination modes. We demonstrate that the tri-active ambiphilic Al center in 2 b can behave as: 1. a σ-donating and doubly π-accepting ligand; 2. a σ-donating, σ-accepting and π-accepting ligand; and 3. a σ-donating and doubly σ-accepting ligand. Additionally, we show ligand exchange at the aluminylene center providing access to the modulation of electronic properties of transition metals without changing the coordinated atoms. Investigations of 2 b with IDippCuCl (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) show an unprecedented aluminylene-alumanyl transformation leading to a rare terminal Cu-alumanyl complex 8. The electronic structures of such complexes and the mechanism of the aluminylene-alumanyl transformation are investigated through density functional theory (DFT) calculations.

A Ni0 σ-Borane Complex Bearing a Rigid Bidentate Borane/Phosphine Ligand: Boryl Complex Formation by Oxidative Dehydrochloroborylation and Catalytic Activity for Ethylene Polymerization

While of interest, synthetically feasible access to boryl ligands and complexes remains limited, meaning such complexes remain underexploited in catalysis. For bidentate boryl ligands, oxidative addition of boranes to low-valent Ir^I or Pt⁰ are the only examples yet reported. As part of our interest in developing improved group 10 ethylene polymerization catalysts, we present here an optimized synthesis of a novel, rigid borane/phosphine ligand and its Ni⁰ σ-borane complex. From the latter, an unprecedented oxidative dehydrochloroborylation, to give a Ni^II boryl complex, was achieved. Furthermore, this new B/P ligand allowed the nickel-catalyzed polymerization of ethylene, which suggests that Ni⁰ σ-hydroborane complexes act as masked Ni^II boryl hydride reagents.

Bimetallic Ru-Pd and Trimetallic Ru-Pd-Cu Assemblies on the Carborane Cluster Surface

The synthesis of well-defined heterometallic complexes remains a frontier challenge in inorganic chemistry. We report an approach that relies on the sequential insertion of electrophilic metal fragments into electron-rich Ru-B bonds of the η²-BB-carboryne complex (POBBOP)Ru(CO)₂ [POBBOP = 1,7-OP(ⁱPr)₂-m-2,6-dehydrocarborane]. Utilizing this synthetic strategy, bimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)] and trimetallic (POBBOP)(Ru)(CO)₂[Pd(P^tBu₃)](CuBr) complexes were selectively prepared. Structural and theoretical analysis of the features of chemical bonding within Ru-B-B-Cu and Ru-B-B-Pd fragments is presented.

Access to cationic polyhedral carboranes via dynamic cage surgery with N-heterocyclic carbenes

Polyhedral boranes and heteroboranes appear almost exclusively as neutral or anionic species, while the cationic ones are protonated at exoskeletal heteroatoms or they are instable. Here we report the reactivity of 10-vertex closo-dicarbadecaboranes with one or two equivalents of N-heterocyclic carbene to 10-vertex nido mono- and/or bis-carbene adducts, respectively. These complexes easily undergo a reaction with HCl to give cages of stable and water soluble 10-vertex nido-type cations with protonation in the form of a BHB bridge or 10-vertex closo-type cations containing one carbene ligand when originating from closo-1,10-dicarbadecaborane. The reaction of a 10-vertex nido mono-carbene adduct with phosphorus trichloride gives nido-11-vertex 2-phospha-7,8-dicarbaundecaborane, which undergoes an oxidation of the phosphorus atom to P = O, while the product of a bis-carbene adduct reaction is best described as a distorted C₂B₆H₈ fragment bridged by the (BH)₂PCl₂⁺ moiety.

In comparison to their neutral or anionic counterparts, examples of cationic boron clusters remain scarce. Here, the authors prepare a variety of cationic polyhedral boranes by reacting closo-10-vertex carboranes with N-heterocyclic carbenes; the resulting open-cage cationic nido- arachno- or closo- derivatives are water soluble, which may enable unprecedented applications.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Synthesis and characterisation of boranediyl- and diboranediyl-bridged diplatinum A-frame complexes

A series of boranediyl-bridged diplatinum A-frame complexes, [Pt2X2(μ-BY)(μ-dmpm)2] (X = Cl, Br, I; Y = aryl, alkyl, amino, halo; dmpm = bis(dimethylphosphino)methane), were synthesised by the twofold oxidative addition of BX2Y to [Pt2(nbe)2(μ-dmpm)2] (nbe = norbornene) or to the paddlewheel complex [Pt2(μ-dmpm)3]. Similarly, the addition of B2X2(NMe2)2 (X = Cl, Br) to [Pt2(nbe)2(μ-dmpm)2] provided access to the diborane-1,2-diyl-bridged A-frame complexes [Pt2X2(μ-1,2-B2(NMe2)2)(μ-dmpm)2]. X-ray crystallographic studies of these (BY)n-bridged complexes show structural trends depending on the steric demands of Y and the nature of X. Analysis of higher-order 31P NMR satellites provided information on JP-Pt and JPt-Pt coupling constants, the latter correlating with the PtPt distance. All (di)boranediyl complexes also proved unstable towards (successive) loss of the bridging "BY" unit(s), resulting in the formation of [Pt2X2(μ-dmpm)2].

Highly Colored Boron-Doped Thiazolothiazoles from the Reductive Dimerization of Boron Isothiocyanates

Reduction of (CAAC)BBr2 (NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.

Synthesis, characterization, and biological applications of pyrazole moiety bearing osmium(IV) complexes

Osmium (IV) complexes with pyrazole nucleus containing ligands were synthesized. Os(IV) compounds were characterized using ESI-MS, ICP-OES, IR spectroscopy, electronic spectroscopy, conductance, and magnetic measurements. Whereas, ligands were characterized by heteronuclear spectroscopy, (1H and 13C), IR spectroscopy, and elemental analysis. All the compounds were tested for their potential to interact with HS-DNA by absorption titration, fluorescence spectroscopy, viscosity measurement, and docking study. The quenching constant and Stern Volmer constant values were calculated using fluorescence study. The synthesized compounds were studied for in-vitro bacteriostatic and cytotoxic activities. The cancer cell line studies of all the synthesized complexes were carried out on human lung cancer cells (A549).Supplemental data for this article is available online at https://doi.org/10.1080/15257770.2021.1921795 .

Stepwise B–H bond activation of a meta-carborane

Stepwise multiple B–H bond activation is a major challenge in synthetic chemistry.

Theoretical investigation of the nature of π(BB)⋯M interactions in coinage metal π-diborene complexes

π(BB)⋯M interactions are strong covalent bonds and their strengths increase in the sequence of M = Ag, Cu, and Au, for the same number of MCl. Theoretical investigation on the nature of π(BB)⋯M interactions in coinage metal π-diborene complexes is outlined here.

Intermetallic transfer of unsymmetrical borylene fragments: isolation of the second early-transition-metal terminal borylene complex and other rare species

Transition metal borylene complexes of the type [(OC)5M[double bond, length as m-dash]BN(SiMe3)(tBu)] (M = Cr, Mo, W) have been synthesised by salt elimination of the corresponding dibromoborane and the dianionic metallates Na2[M(CO)5]. The borylene complexes have been characterised by multinuclear solution-state NMR spectroscopy and solid-state molecular structure determination. The group 6 borylene complexes can be used to effectively transfer the borylene ligand to other transition metal complexes by replacing one or two carbonyl ligands upon irradiation of the reaction mixture with UV light. This borylene transfer reaction led to the formation of new terminal and bridging borylene complexes which cannot be formed by the corresponding salt elimination reactions, including a rare example of a bis(terminal borylene) complex and only the second reported terminal borylene complex of an early transition metal (vanadium).

Functionalization of N2 via Formal 1,3-Haloboration of a Tungsten(0) σ-Dinitrogen Complex

Boron tribromide and aryldihaloboranes were found to undergo 1,3-haloboration across one W-N≡N moiety of a group 6 end-on dinitrogen complex (i.e. trans-[W(N₂ )₂ (dppe)₂ ]). The N-borylated products consist of a reduced diazenido unit sandwiched between a W^II center and a trivalent boron substituent (W-N=N-BXAr), and have all been fully characterized by NMR and IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Both the terminal N atom and boron center in the W-N=N-BXAr unit can be further derivatized using electrophiles and nucleophiles/Lewis bases, respectively. This mild reduction and functionalization of a weakly activated N₂ ligand with boron halides is unprecedented, and hints at the possibility of generating value-added nitrogen compounds directly from molecular dinitrogen.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Phosphinoborylenes as stable sources of fleeting borylenes

Base-stabilised borylenes that mimic the ability of transition metals to bind and activate inert substrates have attracted significant attention in recent years. However, such species are typically highly reactive and fleeting, and often cannot be isolated at ambient temperature. Herein, we describe a readily accessible trimethylphosphine-stabilised borylborylene which was found to possess a labile P-B bond that reversibly cleaves upon gentle heating. Exchange of the labile phosphine with other nucleophiles (CO, isocyanide, 4-dimethylaminopyridine) was investigated, and the binding strength of a range of potential borylene "ligands" has been evaluated computationally. The room-temperature-stable PMe3-bound borylenes were subsequently applied to novel bond activations including [2 + 2] cycloaddition with carbodiimides and the reduction of dichalcogenides, revealing that PMe3-stabilised borylenes can effectively behave as stable sources of the analogous fleeting dicoordinate species under mild conditions.

Boranediyl- and Diborane(4)-1,2-diyl-Bridged Platinum A-Frame Complexes

Diplatinum A-frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)-bridged Pt0 2 complex. While structurally analogous to well-known μ-borylene complexes, in which delocalized dative three-center-two-electron M-B-M bonding prevails, theoretical investigations into the nature of Pt-B bonding in these A-frame complexes show them to be rare dimetalla(di)boranes displaying two electron-sharing Pt-B σ-bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.

Transition-metal-carbene-like intermolecular insertion of a borylene into C-H bonds

We demonstrate that, in analogy to transition-metal carbene chemistry, [(OC)5Mo[double bond, length as m-dash]BN(SiMe3)2] facilitates intermolecular transfer of the borylene [:BN(SiMe3)2], which ultimately undergoes insertion into C-H bonds under very mild conditions. The one-pot multiple functionalization of the cyclopentadienyl rings of tungstenocene dihydride is demonstrated using this approach.

Crystalline Boragermenes

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene-PMe₃ adduct 1 with Cl₂ BN(SiMe₃ )₂ followed by reductive dehalogenation with KC₈ . Addition of a Lewis base (^Me NHC) to 3 leads to the formation of the corresponding adduct 4, which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H₂ concomitant with a complete scission of the Ge=B bond.

Linking Low-Coordinate Ge(II) Centers via Bridging Anionic N-Heterocyclic Olefin Ligands

We introduce a large-scale synthesis of a sterically encumbered N-heterocyclic olefin (NHO) and illustrate the ability of its deprotonated form to act as an anionic four-electron bridging ligand. The resulting multicenter donating ability has been used to link two low oxidation state Ge(II) centers in close proximity, leading to bridging Ge-Cl-Ge and Ge-H-Ge bonding environments supported by Ge₂C₂ heterocyclic manifolds. Reduction of a dimeric [RGeCl]₂ species (R = anionic NHO, [(MeCNDipp)₂C═CH]^-; Dipp = 2,6-ⁱPr₂C₆H₃) did not give the expected acyclic RGeGeR analogue of an alkyne, but rather ligand migration/disproportionation transpired to yield the known diorganogermylene R₂Ge and Ge metal. This process was examined computationally, and the ability of the reported anionic NHO to undergo atom migration chemistry contrasts with what is typically found with bulky monoanionic ligands (such as terphenyl ligands).