P[triple bond, length as m-dash]B and As[triple bond, length as m-dash]B triple bonds in the linear PB2O- and AsB2O- species

Due to its electron deficiency, boron triple bonds are relatively scarce. We use high-resolution photoelectron imaging to investigate the structures and bonding of the EB2O- (E = P, As) type of clusters, which are found to have [E[triple bond, length as m-dash]B-B[triple bond, length as m-dash]O]- closed-shell linear structures with E[triple bond, length as m-dash]B triple bonds. The B atoms in the linear EB2O- species undergo sp hybridization, while the E atoms also undergo sp hybridization to form a σ bond with the sp orbital of B along with two π bonds formed by the p x and p y orbitals. The high-resolution photoelectron imaging data reveal detachment transitions from the EB2O- (1Σ+) anions to the EB2O (2Π) neutrals. The electron affinities of PB2O and AsB2O are measured to be 3.592(1) eV and 3.432(1) eV, respectively; the vibrational frequencies for the E-B, B-B, and B-O stretching modes are measured for both species. The spin-orbit splitting of the 2Π state to 2Π3/2 and 2Π1/2 is measured to be 153 cm-1 and 758 cm-1 for PB2O and AsB2O, respectively.

Metalized Borylene: A New Inorganic Cousin of Borylenes

Borylenes have become a heated topic of intense interest due to their fascinating transition metal-mimicking reactivity and their role as key intermediates in synthesizing complex boron-containing molecules. However, the practical utilization of borylenes has been limited by their instability and complicated methodologies. Very recently, metalized borylenes based on boron-metal covalent interactions and stabilization strategies involving boron-metal multicenter bonding have been proposed. Spectroscopic and theoretical evidence has substantiated that metal-stabilized metalized borylenes possess molecular activation abilities comparable to or even superior to those of classical organoborylenes; however, these species are yet to be isolated or realized experimentally. This brief concept presents an overview of the definition, structural and bonding characteristics, stabilization strategies, amphoteric reactivity, experimental cases, and future perspectives of metal-stabilized metalized borylene compounds. As a new member of the borylene family, metal-stabilized metalized borylenes unlock new access to hyperactive amphoteric borylene, paving the way for synthesizing novel chemicals and developing new catalytic reactions.

Harnessing transient CAAC-stabilized mesitylborylenes for chalcogen activation

Newly synthesized adducts of CAAC-bound mesitylborylene with carbon monoxide (CO) and trimethylphosphine (PMe3) are established as efficient precursors for the in situ generation of the dicoordinate borylene [(CAAC)BMes] (CAAC = cyclic(alkyl)(amino)carbene), as demonstrated by their ability to activate elemental chalcogens. Upon thermal or photolytic activation, these precursors readily react with sulfur and selenium, yielding boron chalcogenides characterized by terminal boron-chalcogen double bonds. In contrast, the reaction with tellurium leads to the formation of an unusual diradical ditelluride species with a Te-Te bond. Quantum chemical calculations of its electronic structure indicate an open-shell singlet ground state characterized by significant diradical character. Further investigations into the redox behavior of these boron chalcogenides reveal intriguing transformations, including the redox-induced formation and cleavage of E-E bonds.

Heterocycles in reactions with boradigermaallyl

Reactions of boradigermaallyl, which can also be regarded as a chloroborylene stabilzed by two germylene donors, with thiophene, furan, pyridazine and 2,2'-bipyridine are presented. Insertion of the boron atom into the heterocycles is observed and the resulting heterocycles continue to react with the bis(germylene) molecule.

Monodentate σ-Accepting Boron-Based Ligands Bearing Square-Planar Ni(0) Centers

Transition metals are known to work as electron donors toward electron-accepting heavier-group-13 elements (Al, Ga, and In), called Z-type ligands. However, complexes with boron-based Z-type ligands are stable only in the presence of additional coordination units (the so-called "supported-ligand" strategy). Here, we report the synthesis and characterization of square-planar Ni(0) complexes that bear tris(perfluoroaryl)boranes as monodentate Z-type ligands, even though such coordination geometry has been traditionally associated with Ni(II) species based on the well-established ligand-field theory. A combined theoretical and experimental approach revealed a mixed covalent/dative character for the Ni-B bonds. This strategy uses frustrated L/Z-ligand pairs that combine sterically encumbered electron-donating (L-type) and electron-accepting ligands to form noncovalent interactions over L-M-Z units to achieve unprecedented low-valent transition metal species with monodentate Z-type ligands.

Reactivity of a Stable and Highly Electron-Rich (η6-Diborabenzene)nickel(0) Synthon

The reaction of the diborabenzene (DBB) nickel(0) pogo-stick complex [(η6-DBB)Ni(CO)] (II) with a large excess of [Ni(CO)4] yields the dark green, unstable dinickel(0) complex [(η6-DBB)Ni(μ-CO)Ni(CO)3] (1), which loses one CO ligand to yield the purple, bimetallic Ni02 half-sandwich complex [(η6-DBB)Ni2(μ-CO)(η1-CO)2] (2). The addition of the chromium aminoborylene complex [(OC)5Cr{BN(TMS)2}] (TMS = trimethylsilyl) to II does not result in the expected borylene transfer but in the formation of the black Ni0-Cr0 complex [(η6-DBB)Ni(μ-CO)Cr(CO)5] (3), alongside the dimeric iminoborane [(TMS)BN(TMS)]2 (4), which results from the rearrangement of the released BN(TMS)2 aminoborylene moiety. Furthermore, the oxidative addition of methyl triflate (MeOTf) to II leaves the (η6-DBB)Ni moiety intact and provides the ionic NiII half-sandwich complex [(η6-DBB)NiMe(CO)]OTf (5), while reaction with pentaphenylborole (PPB) yields the unique, dark-blue, unsymmetrical sandwich complex [(η6-DBB)Ni2(μ-CO)2(η5-PPB)] (6). DFT calculations point toward 6 being a dinickel(0) complex with a neutral aromatic DBB and a neutral antiaromatic PPB ligand.

Ambient Temperature Isolation of a Monatomic Boron(0) Complex

The first bottleable example of a neutral Group 13 atom bound only by neutral donor ligands (L) has been fully characterized by spectroscopic methods and its structure determined by a single-crystal X-ray diffraction study. A two-coordinate paramagnetic L2B0 complex can readily be accessed through a facile reduction reaction and is stabilized by π-accepting cyclic (alkyl)(amino)carbene (CAAC) ligands. Further reduction of (CAAC)2B leads to the isolation of a stable diamagnetic boride anion. In turn, oxidation leads to the putative formation of a transient two-coordinate cationic borylene, which has been trapped to form a stable boron(I) complex. Density functional theory calculations support the formulation of (CAAC)2B as a boron(0) complex stabilized by strong multiple bonding.

Metalized Borylene in Boron-Gold Carbonyl Complexes: Infrared Spectra and Theoretical Calculations

Borylenes (:B-R) that are built on a single B-R bond between boron and another nonmetallic atom or group are a heated subject of special interest due to their intriguing transition-metal-mimicking reactivity, but the relative lack of understanding for the electronic structure and chemical bonding of transition metal borides leads to lingering neglect of metalized borylenes (:B-M) based on covalent B-M bonding. Here we use infrared photodissociation spectroscopy in combination with density functional calculations to study the geometric structure and chemical bonding of boron-gold carbonyl complex cations. The structure and bonding analyses demonstrated that the BAu(CO)3 + and BAu2(CO)4 + complexes can be described as bis-carbonyl-trapped borylene adducts. While the metal-rich BAu3(CO)4 + complex represents an unusual multicenter-bond-stabilized borylene cation with excellent σ-acidity and π-backbonding capability for CO activation, featuring Cs symmetry with a quasi-T-shaped BAu3 + core. It is manifested that BAu3 + presents greater amphoteric reactivity and improved stability compared to BAu1,2 + due to the presence of the three-center-two-electron Au-B-Au bond. This study discloses a conceptually new platform for accessing reactive metalized borylenes by exploiting the boron-mediated multicenter-bond stabilization strategy and using more bench-stable and ubiquitous metal carbonyl fragments as starting materials, thus providing a broader opportunity for the design of novel chemical structures and catalytic reactions.

Evidence of boride-borylene ligand-tautomerism leading to a remote C-C-bond and concomitant boryl ligand formation

The formation of a rhodium pincer-type complex with a boron-based donor ligand and its reactivity are reported. The starting complex contains a formal borylene moiety, stabilised by two pyridine substituents. Quantum chemical investigations indicate the possibility of deprotonation of the central donor group of the type py2BH in this complex. Efforts to isolate the resulting formal boride species, however, led to a boryl complex with concomitant formation of a new C-C-bond, accompanied by a loss of aromaticity. Mechanistic investigations indicate the presence of tautomerism between two deprotonated species, giving rise to a ligand-stabilised boride and a ligand-stabilised borylene motif.

Isolable Si=B Analogue of a Vinyl Halide: A Building Block for Facile Access toward Silicon-Boron Multiple Bonded Species

Compared to the outstanding development in the synthesis of Si-B single bonded species, borylsilanes and their application to organic synthesis, the chemistry of Si=B double bonded species, borasilenes and boratasilenes have only made little progress, first of all, due to the difficulties in accessing such double bonds. Herein we report the synthesis of the first Si=B analogue of a vinyl halide, a bromoboratasilene, via formal borylene insertion to the coordination sphere of a monoatomic Si(0) complex, using a dihaloborane as the borylene source. The treatment of bromoboratasilene toward neutral or anionic Lewis bases gives access to new boratasilenes, all of which were proved to possess significant Si=B double bond character by XRD analysis and DFT calculations. These results demonstrate exciting strategies to synthesize new types of Si=B double bonded species which should further progress the chemistry of boron, silicon-containing molecules.

Reactivity of Diruthenium Bisborylene Complexes: Formation of B-C and B-H Bonds via Borylene Ligand Coupling

Thermal or photoinduced isomerization of diruthenium bridging bisborylene complexes [{Cp*(H)2Ru}2(μ-BAr)2] (1a, Ar = Ph; 1b, Ar = 3,4,5-F3C6H2) led to nido-ruthenacarboranes 2a and 2b with newly formed B-C and B-H bonds. The reaction mechanism was analyzed by deuterium-labeling experiments and density functional theory calculations. Additionally, 2-fold B-H coupling between borylene and two hydrido ligands of 1a can be achieved, assisted by Lewis base IPr2Me2 to generate a dinuclear bridging borylene complex [(Cp*Ru)2(μ-H)2(μ-BPh)] (3). Our results provide new reactivity patterns for borylene-based functionalizations.

Arene extrusion as an approach to reductive elimination at boron: implication of carbene-ligated haloborylene as a transient reactive intermediate

Herein, we report boron-centered arene extrusion reactions to afford putative cyclic(alkyl)(amino) carbene (CAAC)-ligated chloroborylene and bromoborylene intermediates. The borylene precursors, chloro-boranorbornadiene (ClB(C6Me6), 2Cl) and bromo-boranorbornadiene (BrB(C6Me6), 2Br) were synthesized through the reaction of the corresponding 1-halo-2,3,4,5-tetramethylborole dimer (XBC4Me4)2 (X = Cl, 1Cl; X = Br, 1Br) with 2-butyne. Treatment of 2Cl with CAACs resulted in the release of di-coordinate chloro-borylene (CAAC)BCl from hexamethylbenzene (C6Me6) at room temperature. In contrast, the reaction of 2Br with CAAC led to the formation of a boronium species [(CAAC)BC6Me6]+Br- (7) at room temperature. Heating 7 in toluene promoted the release of di-coordinate bromo-borylene (CAAC)BBr as a transient species. Surprisingly, heating 7 in dichloromethane resulted in the C-H activation of hexamethylbenzene. The conversion of a CAAC-stabilized bromo-borepin to a borylene, a boron-centered retro Büchner reaction, was also investigated.

Construction of a Boron Chain on a Single Metal by Dehydrocoupling of Borane Ligands

Borane coordination, B-H borane bond activation, and borane catenation via metal-mediated dehydrocoupling to form electron-precise B-B bonds are reported. The reaction of trans-[M(IMes)2Cl4] (M = W, Mo) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with borates Li[BH3R] (R = Mes, Dur; Mes = 2,4,6-Me3C6H2 and Dur = 2,3,5,6-Me4C6H) afforded the complexes [M(IMes)(η2-H2BR)2(η1-H2BR)] (M = W: R = Mes 1, R = Dur 3; M = Mo: R = Mes 2, R = Dur 4). Three borane ligands are coordinated in 1-4 to the group 6 metal atom via five (σ-B-H) bonds. Reaction of 1 with the phosphines PMe3 and PEt3, respectively, led to the elimination of one of the borane ligands and afforded the hydrido (σ-B-H)-boryl bis(σ-B-H)-borane complexes trans-[W(IMes)(PR3)(η1-HBMes)(η2-H2BMes)(H)] (R = Me 5, R = Et 6), in which the metal atom inserted into one of the remaining σ-B-H bonds of the borane ligands. Reaction of 1 with an additional equivalent borane BH2Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η4-BH2Mes-BMes-BMes-BH2Mes)] 7, featuring a unique B4 chain as a ligand. Reaction of trans-[W(IMes)2Cl4] with NaBH4 also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH4)}2(B5H9)] 9 was formed, in which two tungsten atoms bridge a B5 chain.

A Coppoborylene Stabilized by Multicenter Covalent Bonding and Its Amphoteric Reactivity to CO

A cationic copper-stabilized coppoborylene was prepared and structurally characterized via infrared photodissociation spectroscopy and density functional theory calculations. This structure exemplifies a new class of borylenes stabilized by three-center-two-electron metal-boron-metal covalent bonding interaction, displaying exceptional σ-acidity and unparalleled π-donor capability for CO activation that outperforms all of the known transition metal cations and is comparable or even superior to the documented base-trapped borylenes. Its neutral form represents a monovalent boron compound with a strongly reactive amphoteric boron center built on transition-metal-boron bonds, which inspires the design and synthesis of new members of the borylene family.

Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM2(CO)8+ (M = Co, Rh, Ir) Complexes

Investigations of transition-metal boride clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The geometric structures and bonding features of heteronuclear boron-containing transition metal carbonyl cluster cations BM(CO)6+ and BM2(CO)8+ (M = Co, Rh, and Ir) are studied by a combination of the infrared photodissociation spectroscopy and density functional calculations at B3LYP/def2-TZVP level. The completely coordinated BM2(CO)8+ complexes are characterized as a sandwich structure composed of two staggered M(CO)4 fragments and a boron cation, featuring a D3d symmetry and 1Eg electronic ground state as well as metal-anchored carbonyls in an end-on manner. In conjunction with theoretical calculations, multifold metal-boron-metal bonding interactions in BM2(CO)8+ complexes involving the filled d orbitals of the metals and the empty p orbitals of the boron cation were unveiled, namely, one σ-type M-B-M bond and two π-type M-B-M bonds. Accordingly, the BM2(CO)8+ complexes can be described as a linear conjugated (OC)4M═B═M(CO)4 skeleton with a formal B-M bond index of 1.5. The three delocalized d-p-d covalent bonds render compensation for the electron deficiency of the cationic boron center and endow both metal centers with the favorable 18-electron structure, thus contributing much to the overall structural stability of the BM2(CO)8+ cations. As a comparison, the saturated BRh(CO)6+ and BIr(CO)6+ complexes are determined to be a doublet Cs-symmetry structure with an unbridged (OC)2B-M(CO)4 pattern, involving a two-center σ-type (OC)2B → M(CO)4+ dative single bond along with a weak covalent B-M half bond. This work offers important insight into the structure and bonding of late transition metal boride carbonyl cluster cations.

Infrared Photodissociation Spectroscopy of Mass-Selected Dinuclear Transition Metal Boride Carbonyl Cluster Cations

The transition-metal-boron bonding interactions and geometric structures of heterodinuclear transition metal carbonyl cluster cations BM(CO)n+ (M = Co, Ni, and Cu) are studied by a combination of the infrared photodissociation spectroscopy and density functional theory calculations at the B3LYP/def2-TZVP level. The BCu(CO)5+ and BCo(CO)6+ cations are characterized as an (CO)2B-M(CO)3/4+ structure involving an σ-type (OC)2B → M(CO)3,4+ dative bonding with end-on carbonyls, while for BNi(CO)5,6+ complexes with a bridged carbonyl, a 3c-2e bond involving the 5σ electrons of the bridged carbonyl and an electron-sharing bond between the B(CO)2 fragment and the Ni(CO)2,3+ subunits were revealed. Moreover, the fundamental driving force of the exclusive existence of a bridged carbonyl group in the boron-nickel complexes has been demonstrated to stem from the desire of the B and Ni centers for the favorable 8- and 18-electron structures.

Hetero-trimetallic complexes comprising bridging boryl and borylene ligands: an experimental and theoretical study

In an effort to explore the coordination chemistry of the coordinative sulfur centers in arachno-ruthenaborane [(Cp*Ru)2(B3H8)(CS2H)] (arachno-1), we have thermolyzed arachno-1 with group-6 metal carbonyls [M(CO)5·THF] (M = Cr, Mo and W). The reaction of arachno-1 with [Cr(CO)5·THF] resulted in the formation of hetero-trimetallic triply bridging borylene [(Cp*Ru)2(μ-CO)(μ3-CH2S2-κ2S':κ2S''){Cr(CO)3}(μ3-BH)] (2), bridging boryl-borylene [(Cp*Ru)2(μ-CO){(μ3-BH(CH2S2)-κ2B:κ2S':κ1S'')}{Cr(CO)3}(μ3-BH)] (3), and sulfido bridged hetero-trimetallic complex [(Cp*Ru)2(μ-CO)3{Cr(CO)3}(μ3-S)] (4). In 2, one side of Ru2Cr-triangle features a μ3-BH ligand while the other side is quadruply bridged by a methanedithiolato ligand in an unsymmetrical fashion. Unlike 2, in complex 3, one side of the Ru2Cr-triangle has a μ3-BH ligand while the opposite side is bridged by a boryl ligand BH(CH2S2) in an unsymmetrical way (μ3-κ2:κ2:κ1) to the metal centers. Interestingly, when the similar reactions of arachno-1 were performed with heavier group-6 metal carbonyls [M(CO)5·THF] (M = Mo and W), it led to the formation of methanedithiolato bridged hetero-trimetallic chain complexes, [{Cp*Ru(CO)}2(μ-CO)2(μ3-CH2S2-κ2S':κ2S''){M(CO)2}] (5, M = Mo; 6, M = W) and sulfido-bridged hetero-trimetallic complexes [(Cp*Ru)2(μ-CO)3{M(CO)3}(μ3-S)] (7, M = Mo; 8, M = W), analogous to 4. In complexes 5 and 6, a Ru2M-chain is symmetrically bridged by a methanedithiolato ligand. On the other hand, in complexes 4, 7, and 8, a sulfido ligand coordinates to two ruthenium and one group-6 metal atoms in μ3-fashion. All the complexes have been characterized by 1H NMR, 13C NMR, UV-vis, IR spectroscopy, and mass spectrometry and their structural architectures have been unambiguously established by single crystal X-ray diffraction studies. In addition, theoretical investigations provided valuable insights into their electronic structures and bonding properties.

Mono(Lewis Base)-Stabilized Gallium Iodide: An Unexplored Class of Promising Ligands

Quantum-chemical (DFT) calculations on hitherto unknown base(carbene)-stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p-orbital - both centered at the central gallium atom - and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor-acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non-innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.

Observation of an electron-precise metal boryne complex: [BiBH]. Chem Commun (Camb) 2023;59:12431-12434. [PMID: 37768059 DOI: 10.1039/d3cc04235a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Metal-boron triple bonds are rare due to the electron deficiency of boron. This study uncovers a simple electron-precise metal boryne complex, [BiBH]-, which is produced within an ion trap through chemical reactions of the open-shell BiB- anion with H2. Photoelectron imaging is used to investigate the electronic structure and chemical bonding of the BiBH- complex. The B atom in the linear closed-shell BiBH- is found to undergo sp hybridization, forming a B-H single bond and a BiB triple bond. Photoelectron imaging reveals three detachment transitions from the BiBH- (1Σ+) anion to the neutral BiBH, including the ground state (2Π3/2) and two excited states (2Σ+ and 2Π1/2). Strong vibronic coupling is observed between the 2Π3/2 and 2Σ+ states, evidenced by the appearance of bending vibrations and their unique photoelectron angular distributions. The BiBH- complex not only stands as the simplest metal boryne complex, but also serves as an ideal molecular system to investigate both spin-orbit and vibronic couplings.

Bonding and stability of elusive silaboryne (SiB) and germaboryne (GeB) with donor base ligands

Stabilizing the exotic chemical species possessing multiple bonds is often extremely challenging due to insufficient orbital overlap, especially involving one heavier element. Bulky aryl groups and/or carbene as ligand have previously stabilized the SiSi, GeGe, and BB triple bonds. Herein, theoretical calculations have been carried out to shed light on the stability and bonding of elusive silaboryne/germaboryne (Si/GeB triple bond) stabilized by donor base ligands ((cAAC)BE(Me)(L); E = Si, L = cAAC^Me , NHC^Me , PMe₃ ; E = Ge, L = cAAC^Me ). The heavier analogues (Sn, Pb) have been further studied for comparison. Additionally, the effects of bulky substituents at the Si and N atoms on the structural parameters and stability of those species have been investigated. Energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV; for Si) showed that cAAC/NHC ligands could stabilize the exotic BSi-Me species more efficiently than PMe₃ ligands. The BSi partial triple bond of the corresponding species possesses a mixture of one covalent electron sharing BSi σ-bond and two dative π-bonds (B ← Si, B → Si).

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt₂)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr₃(SMe₂)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC₈) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C₆F₅)₃), forming a frustrated radical pair [(SiNSi)B]^•+[B(C₆F₅)₃]^•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C₆F₅)₃] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.

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.

Stabilisation and reactivity studies of donor-base ligand-supported gallium-phosphides with stronger binding energy: a theoretical approach

Gallium phosphide is a three-dimensional polymeric material of the hetero-diatomic GaP unit, which has a wurtzite type structure, and captivating application as a light emitting diode (LED). As a result, there is a constant search for suitable precursors to synthesise GaP-based materials. However, the corresponding monomeric species is exotic in nature due to the expected Ga[triple bond, length as m-dash]P multiple bond. Herein, we report on the theoretical studies of stability, chemical bonding, and reactivity of the monomeric gallium phosphides with two donor base ligands having tuneable binding energies. We have performed detailed investigations using density functional theory at three different levels (BP86/def2-TZVPP, B3LYP/def2-TZVPP, M06-2X/def2-TZVPP), QTAIM and EDA-NOCV (BP86-D3(BJ)/TZ2P, M06-2X/TZ2P) to analyse various ligand-stabilised GaP monomers, which revealed the synthetic viability of such species in the presence of stable singlet carbenes, e.g., cAAC, and NHC as ligands [cAAC = cyclic alkyl(amino) carbene, NHC = N-heterocyclic carbene] due to the larger bond dissociation energy compared to a phosphine ligand (PMe₃). The calculated bond dissociation energies between a pair of ligands and the monomeric GaP unit are found to be in the range of 87 to 137 kcal mol^-1, predicting their possible syntheses in the laboratory. Further, the reactivity of such species with metal carbonyls [Fe(CO)₄, and Ni(CO)₃] have been theoretically investigated.

Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl4] (Cp* = ɳ5-C5Me5) with four equivalents of Li[BH2E3] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)3(μ3-Se)3(BH)(μ-Se)3] (1) and the homocubane-like cluster [(NbCp*)3(μ3-Se)3(μ-Se)3(BH)(μ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se3BH}- ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH2S3] afforded the trimetallic clusters [(NbCp*)3(μ-S)4{μ-S2(BH)}] (3) and [(NbCp*)3(μ-S)4{μ-S2(S)}] (4). Both clusters 3 and 4 have an Nb3S6 core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by 11B{1H}, 1H, and 13C{1H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb3 framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.

On Neutral Unsaturated Ouroboric Borylenes

The search is on for stable isolated borylenes. Potential roles in modern synthetic chemistry for boron analogues of carbenes continue to motivate interest in locating them. Using density functional and ab initio methods, we posit and examine the thermochemistry, and chemical bonding, including aromaticity, of several classes of 5- and 6-membered borylenic rings. In these systems, cyclization relies on dative bonding (ouroboric coordination) and π donation to a monovalent boron center from an adjacent O center. Certain neutral five-membered rings (heterocyclic cyclopentadienyl analogues) in particular are found to exhibit exceptionally strong preferences for the singlet multiplicity, each with singlet-triplet (S-T) gaps in excess of 40 kcal·mol^-1. The singlet five-membered rings with the largest S-T gaps and some of the six-membered rings show evidence of weak aromaticity. Relationships of the form N = A·r^-b, in line with Gordy's and other functions linking bond order, N, and covalent bond length, are identified for dative B←O contacts, r, reinforced in rings by π-delocalization.

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.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

EDA-NOCV analysis of carbene-borylene bonded dinitrogen complexes for deeper bonding insight: A fair comparison with a metal-dinitrogen system

Binding of dinitrogen (N₂ ) to a transition metal center (M) and followed by its activation under milder conditions is no longer impossible; rather, it is routinely studied in laboratories by transition metal complexes. In contrast, binding of N₂ by main group elements has been a challenge for decades, until very recently, an exotic cAAC-borylene (cAAC = cyclic alkyl(amino) carbene) species showed similar binding affinity to kinetically inert and non-polar dinitrogen (N₂ ) gas under ambient conditions. Since then, N₂ binding by short lived borylene species has made a captivating news in different journals for its unusual features and future prospects. Herein, we carried out different types of DFT calculations, including EDA-NOCV analysis of the relevant cAAC-boron-dinitrogen complexes and their precursors, to shed light on the deeper insight of the bonding secret (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence). The hidden bonding aspects have been uncovered and are presented in details. Additionally, similar calculations have been carried out in comparison with a selected stable dinitrogen bridged-diiron(I) complex. Singlet cAAC ligand is known to be an exotic stable species which, combined with the BAr group, produces an intermediate singlet electron-deficient (cAAC)(BAr) species possessing a high lying HOMO suitable for overlapping with the high lying π*-orbital of N₂ via effective π-backdonation. The BN₂ interaction energy has been compared with that of the FeN₂ bond. Our thorough bonding analysis might answer the unasked questions of experimental chemists about how boron compounds could mimic the transition metal of dinitrogen binding and activation, uncovering hidden bonding aspects. Importantly, Pauling repulsion energy also plays a crucial role and decides the binding efficiency in terms of intrinsic interaction energy between the boron-center and the N₂ ligand.

The unique sandwich K6Be2B6H6 cluster with a real borozene B6H6 core

Theoretical evidence is reported for a boron-based K₆Be₂B₆H₆ sandwich cluster, showing a perfectly D _6h B₆H₆ ring, being capped by two tetrahedral K₃Be ligands. Due to the comfortable charge transfer, the sandwich is viable in [K₃Be]³⁺[B₆H₆]^6-[BeK₃]³⁺ ionic complex in nature. The [B₆H₆]^6- core with 6π aromaticity vividly imitates the benzene (C₆H₆), occurring as a real borozene. In contrast, the tetrahedral [K₃Be]³⁺ ligand is 2σ three-dimensional aromatic, acting as the simple superatom. Thus, this complex possesses a collectively three-fold 2σ/6π/2σ aromaticity. The interlaminar interaction is governed by the robust electrostatic attraction. The unique chemical bonding gives rise to interesting dynamic fluxionality.

EDA-NOCV Analysis of Donor-Base-Stabilized Elusive Monomeric Aluminum Phosphides [(L)P-Al(L'); L, L' = cAACMe, NHCMe, PMe3]

Herein, we report on the stability and bonding analysis of donor-base-stabilized monomeric AlP species (1-6) of the general formula (L)P-Al(L'); [L = cAACMe, L' = cAACMe, NHCMe, PMe3, (N i Pr2)2 (1-4); L = L' = NHCMe, PMe3 (5 and 6); cAAC = cyclic alkyl(amino) carbene; NHC = N-heterocyclic carbene]. Energy decomposition analysis coupled with natural orbitals for chemical valence (EDA-NOCV) analysis indicates the synthetic viability of this class of species, stabilized in their singlet ground state, in the laboratory. The CL-P bond is found to be a partial double bond (WBI ∼ 1.45), while the CL/PL-Al bond is a single bond (WBI ∼ 0.42-0.69). These bonds are mostly covalent or dative σ/π bonds depending upon the ligands attached. The central P-Al bond is an electron-sharing covalent polar single bond (WBI ∼ 0.80; P-Al) for 1-4 and a dative σ bond for 5 and 6 (WBI ∼ 0.89-0.93; P-Al). The calculated intrinsic interaction energies of the central P-Al bonds are found to be in the range from -116 to -216 kcal/mol (1-3 and 5 and 6). This value is the highest for compound 3, possibly due to the push and pull effects from the ligands PMe3 and cAAC, respectively.

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.

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.

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.

Reactivity of cyano- and isothiocyanatoborylenes: metal coordination, one-electron oxidation and boron-centred Brønsted basicity

Doubly base-stabilised cyano- and isothiocyanatoborylenes of the form LL'BY (L = CAAC = cyclic alkyl(amino)carbene; L' = NHC = N-heterocyclic carbene; Y = CN, NCS) coordinate to group 6 carbonyl complexes via the terminal donor of the pseudohalide substituent and undergo facile and fully reversible one-electron oxidation to the corresponding boryl radical cations [LL'BY]˙+. Furthermore, calculations show that the borylenes have very similar proton affinities, both to each other and to NHC superbases. However, while the protonation of LL'B(CN) with PhSH yielding [LL'BH(CN)+][PhS-] is fully reversible, that of LL'B(NCS) is rendered irreversible by a subsequent B-to-CCAAC hydrogen shift and nucleophilic attack of PhS- at boron.

Transition-metal-like bonding behaviors of a boron atom in a boron-cluster boronyl complex [(η7-B7)-B-BO]. Chem Sci 2021;12:8157-8164. [PMID: 34194706 PMCID: PMC8208299 DOI: 10.1039/d1sc00534k]

Boron displays many unusual structural and bonding properties due to its electron deficiency. Here we show that a boron atom in a boron monoxide cluster (B₉O⁻) exhibits transition-metal-like properties. Temperature-dependent photoelectron spectroscopy provided evidence of the existence of two isomers for B₉O⁻: the main isomer has an adiabatic detachment energy (ADE) of 4.19 eV and a higher energy isomer with an ADE of 3.59 eV. The global minimum of B₉O⁻ is found surprisingly to be an umbrella-like structure (C_6v, ¹A₁) and its simulated spectrum agrees well with that of the main isomer observed. A low-lying isomer (C_s, ¹A′) consisting of a BO unit bonded to a disk-like B₈ cluster agrees well with the 3.59 eV ADE species. The unexpected umbrella-like global minimum of B₉O⁻ can be viewed as a central boron atom coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other side, [(η⁷-B₇)-B-BO]⁻. The central B atom is found to share its valence electrons with the B₇ unit to fulfill double aromaticity, similar to that in half-sandwich [(η⁷-B₇)-Zn-CO]⁻ or [(η⁷-B₇)-Fe(CO)₃]⁻ transition-metal complexes. The ability of boron to form a half-sandwich complex with an aromatic ligand, a prototypical property of transition metals, brings out new metallomimetic properties of boron.

The global minimum of the B₉O⁻ cluster is found to have an umbrella-like structure, where the central B atom exhibits transition-metal-like bonding properties, coordinated by a η⁷-B₇ ligand on one side and a BO ligand on the other.

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].

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

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.

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).

F2BMF (M = V, Nb, and Ta) and FBMF2 (M = Nb and Ta): A Combined Matrix Isolation Infrared Spectroscopic and Quantum Chemical Investigation

Through matrix isolation infrared spectrometry and quantum chemical calculations, the reactions of laser ablated V, Nb, and Ta with boron trifluoride were investigated in excess solid neon at 4 K. The possible reaction products FBMF₂, F₂BMF, and BMF₃ (M = V, Nb, and Ta) were calculated at the B3LYP, BPW91, and CCSD(T) levels of theory. The B-M bond strength in FBMF₂ molecules is confirmed by energy decomposition analysis-natural orbitals for chemical valence calculations, CASSCF calculation, and natural bond orbital analysis, which favors one σ bond and two half π bonds.

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.