Synthesis and Characterization of a Terminal Borylene (Boranediyl) Complex

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.

Molecular pyramids - from tetrahedranes to [6]pyramidanes

The study of 3D architectures at a molecular scale has fascinated chemists for generations. This includes molecular pyramids with all-carbon frameworks, such as trigonal, tetragonal and pentagonal pyramidal geometries. A small number of substituted tetrahedranes and all-carbon [4]-[5]pyramidanes have been experimentally generated and studied. Although the hypothetical unsubstituted parent [3]-[6]pyramidanes have only been explored computationally, the formal replacement of carbon vertices with isolobal main group element fragments has provided a number of examples of stable hetero[m]pyramidanes, which have been isolated and amply characterized. In this Review, we highlight the synthesis and chemical properties of [3]-[6]pyramidanes and summarize the progress in the development of chemistry of pyramid-shaped molecules.

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.

C-PCM study on the electronic and optical properties of Fe(CO)4B12N12 complexes

We explored solvent effect on the stability, dipole moment, polarizability and first hyperpolarizability of Fe(CO)4B12N12 complexes at MPW1PW91/6-311G(d,p) level of theory. These complexes were considered in the low spin states. The self-consistent reaction field theory (SCRF) based on conductor-like polarizable continuum model (PCM) was employed to illustration of the solvent influences. The relations between the parameters with solvent polarity functions (McRae and Suppan functions) were given. Also, relations of the wavenumbers values of the stretching of carbonyl ligands with the Kirkwood–Bauer–Magat equation (KBM) were provided.

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.

Boron-lead multiple bonds in the PbB2O- and PbB3O2- clusters

Despite its electron deficiency, boron can form multiple bonds with a variety of elements. However, multiple bonds between boron and main-group metal elements are relatively rare. Here we report the observation of boron-lead multiple bonds in PbB₂O^- and PbB₃O₂^-, which are produced and characterized in a cluster beam. PbB₂O^- is found to have an open-shell linear structure, in which the bond order of B☱Pb is 2.5, while the closed-shell [Pb≡B-B≡O]^2- contains a B≡Pb triple bond. PbB₃O₂^- is shown to have a Y-shaped structure with a terminal B = Pb double bond coordinated by two boronyl ligands. Comparison between [Pb≡B-B≡O]^2-/[Pb=B(B≡O)₂]^- and the isoelectronic [Pb≡B-C≡O]^-/[Pb=B(C≡O)₂]⁺ carbonyl counterparts further reveals transition-metal-like behaviors for the central B atoms. Additional theoretical studies show that Ge and Sn can form similar boron species as Pb, suggesting the possibilities to synthesize new compounds containing multiple boron bonds with heavy group-14 elements.

Reduction and Rearrangement of a Boron(I) Carbonyl Complex

The one-electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)-stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].- radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate.

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).

Quadruple bonding of bare group-13 atoms in transition metal complexes

Density functional theory calculations at the M06-D3/def2-TZVPPD level of the group-13 anion complexes EFe(CO)3- (E = B-Tl) and the isoelectronic neutral and charged boron adducts BTM(CO)3q (TMq = Fe-, Ru-, Os-, Co, Rh, Ir, Ni+, Pd+, Pt+) give tetrahedral (C3v) geometries in the 1A1 electronic ground state as equilibrium structures. The analysis of the bonding situation with the energy decomposition analysis in combination with natural orbital for chemical valence method suggests that the E-TM(CO)3q bonds possess four bonding components: (a) one strong electron-sharing σ bond E-TM(CO)3q; (b) two π backdonations E⇇TM(CO)3q and (c) one weak σ donation E→TM(CO)3q. The relative strength of the four bonding components depends on the charge of the system, the transition metal TM and the group-13 atom E. The σ donation E→TM(CO)3q is in all systems rather weak while the associated charge migration is not negligible. A similar situation of the bonding of terminal group-13 atoms Ga and In is found in Ga-TM(GaCp)4+ and E-Pt(PMe3)3+ (TM = Ni, Pd, Pt; E = Ga, In), which are model compounds for the stable complexes GaTM(GaCp*)4+ (TM = Ni, Pt) and InPt(PPh3)3+. The quadruple bonds E→TML2 are hybrids of electron-sharing and dative bonds.

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.

Observation of Transition-Metal-Boron Triple Bonds in IrB2 O- and ReB2 O. Angew Chem Int Ed Engl 2020;59:15260-15265. [PMID: 32424965 DOI: 10.1002/anie.202006652]

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π →BR back-donation, despite the electron deficiency of boron. An electron-precise metal-boron triple bond was first observed in BiB₂ O^- [Bi≡B-B≡O]^- in which both boron atoms can be viewed as sp-hybridized and the [B-BO]^- fragment is isoelectronic to a carbyne (CR). To search for the first electron-precise transition-metal-boron triple-bond species, we have produced IrB₂ O^- and ReB₂ O^- and investigated them by photoelectron spectroscopy and quantum-chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB₂ O^- has a closed-shell bent structure (C_s , ¹ A') with BO^- coordinated to an Ir≡B unit, (^- OB)Ir≡B, whereas ReB₂ O^- is linear (C_∞v , ³ Σ^- ) with an electron-precise Re≡B triple bond, [Re≡B-B≡O]^- . The results suggest the intriguing possibility of synthesizing compounds with electron-precise M≡B triple bonds analogous to classical carbyne systems.

Heterometallic Triply-Bridging Bis-Borylene Complexes

Triply-bridging bis-{hydrido(borylene)} and bis-borylene species of groups 6, 8 and 9 transition metals are reported. Mild thermolysis of [Fe₂ (CO)₉ ] with an in situ produced intermediate, generated from the low-temperature reaction of [Cp*WCl₄ ] (Cp*=η⁵ -C₅ Me₅ ) and [LiBH₄ ⋅THF] afforded triply-bridging bis-{hydrido(borylene)}, [(μ₃ -BH)₂ H₂ {Cp*W(CO)₂ }₂ {Fe(CO)₂ }] (1) and bis-borylene, [(μ₃ -BH)₂ {Cp*W(CO)₂ }₂ {Fe(CO)₃ }] (2). The chemical bonding analyses of 1 show that the B-H interactions in bis-{hydrido (borylene)} species is stronger as compared to the M-H ones. Frontier molecular orbital analysis shows a significantly larger energy gap between the HOMO-LUMO for 2 as compared to 1. In an attempt to synthesize the ruthenium analogue of 1, a similar reaction has been performed with [Ru₃ (CO)₁₂ ]. Although we failed to get the bis-{hydrido(borylene)} species, the reaction afforded triply-bridging bis-borylene species [(μ₃ -BH)₂ {WCp*(CO)₂ }₂ {Ru(CO)₃ }] (2'), an analogue of 2. In search for the isolation of bridging bis-borylene species of Rh, we have treated [Co₂ (CO)₈ ] with nido-[(RhCp*)₂ (B₃ H₇ )], which afforded triply-bridging bis-borylene species [(μ₃ -BH)₂ (RhCp*)₂ Co₂ (CO)₄ (μ-CO)] (3). All the compounds have been characterized by means of single-crystal X-ray diffraction study; ¹ H, ¹¹ B, ¹³ C NMR spectroscopy; IR spectroscopy and mass spectrometry.

Boron-Transition-Metal Triple-Bond FB≡MF2 Complexes

The boron-transition-metal triple-bond complexes FB≡MF₂ (M= Ir, Os, Re, W, Ta) were trapped in excess solid neon and argon through metal atom reactions with boron trifluoride and identified by matrix isolation infrared spectroscopy and quantum chemical calculations. The FB≡MF₂ molecule features very high ¹¹B-F stretching frequencies at 1586.6 cm^-1 (Ir), 1526.6 cm^-1 (Os), 1505.5 cm^-1 (Re), and 1453.2 cm^-1 (W), respectively. The very high strength of B≡M bonds with triple-bonding character is confirmed by EDA-NOCV calculations and the active molecular orbital and NBO analysis. The experimental observation of FB stabilization by heavy transition-metal atoms with triple bonds opens the door to design new boron-transition-metal complexes.

Coupling of fluoroborylene ligands in manganese carbonyl chemistry to give a difluorodiborene ligand

A bridging difluorodiborene μ-B₂F₂ ligand has been observed in two of the three lowest energy Mn₂(BF)₂(CO)₇ structures as well as in the lowest energy Mn₂(BF)₂(CO)₆ structure.

Coordination and Insertion: Competitive Channels for Borylene Reactions

Monovalent boron, free borylene species of the form B-R are notoriously unstable. Consequently, there are substantial gaps in the literature concerning the potential utility of those species in organic and inorganic synthesis either as ligands or as critical intermediates in reactions. We show that the relative stability of borylene complexes varies widely, depending on the electron donating ability of the R group. We find that borylenes can form, in the gas phase, weak sigma hole type interactions to saturated carbon centers and stronger dative bonds to tetravalent silicon and germanium. An insertion reaction of the form FH₃M + BR → FH₂MBHR competes against dative bonding, however, and the reaction is barrierless in several cases when M = Si and in a few cases when M = Ge. For M = C, the barriers are high enough to stabilize monovalent boron complexes. In each case, the barrier heights to M-H bond activation and BR insertion are very sensitive to the nucleophilicity of BR. We confirm, at the MP2(full) and CCSD(T) levels, a substantial preference in borylenes for the singlet over the triplet state. An account is provided at the B3LYP-D3 and MP2(full) levels for the facile insertion reaction on the singlet surface when M = Si and for the stability of FH₃M·BR type complexes and the higher barriers to insertion when M = C and Ge.

Boryl substituted group 13 metallylenes: complexes with an iron carbonyl fragment

The first examples of boryl substituted aluminylene and gallylene complexes, [(DAB)B(THF)Al{Fe(CO)₃(μ-CO)}]₂ and [(DAB)BGa{μ-Fe(CO)₄}]₂ (DAB = {(C₆H₃Prⁱ₂-2,6)NCH}₂) have been prepared by reduction of MX₂(THF){B(DAB)} (M = Al or Ga, X = Cl or Br) with K₂[Fe(CO)₄]. Spectroscopic and crystallographic analyses of the compounds show them to be structurally distinct dimers, the latter of which possesses a close GaGa separation that computational analyses reveal has negligible bonding character.

Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds

MP2/aug'-cc-pVTZ calculations were performed to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)₂ (HB):ClX and (N₂ )₂ (HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH₃ , and H. Equilibrium halogen-bonded complexes with boron as the electron-pair donor are found on all of the potential surfaces, except for (CO)₂ (HB):ClCH₃ and (N₂ )₂ (HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)₂ (HB):ClF, (CO)₂ (HB):ClCl, (N₂ )₂ (HB):ClCl, and (N₂ )₂ (HB):ClOH, which are stabilized by chlorine-shared halogen bonds. These complexes have increased binding energies and shorter B-Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl-A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge-transfer interaction occurs in (CO)₂ (HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants, ^1x J(B-Cl), across the halogen bonds are also indicative of the changing nature of this bond. ^1x J(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)₂ (HB)-Cl]⁺ and [(N₂ )₂ (HB)-Cl]⁺ . Changes in ¹¹ B chemical shieldings upon complexation correlate with changes in the charges on B.

Donor–acceptor bonding in novel low-coordinated compounds of boron and group-14 atoms C–Sn

Donor–acceptor complexes of one, two or three atoms E = B, Si–Sn which are stabilized by σ-donor ligands L are discussed.

Reductive borylene-CO coupling with a bulky arylborylene complex

Partial metal-boron bond cleavage and coupling of a borylene with two CO ligands was observed upon reduction of a new bulky arylborylene complex. Both the borylene precursor and dianionic product were structurally and spectroscopically characterized. In contrast, reduction of an aminoborylene complex led to complete loss of the borylene ligand and classical Hieber reduction. A rationale for these differences based on DFT methods is presented.

Syntheses and characterization of new vinyl-borylene complexes by the hydroboration of alkynes with [(μ3-BH)(Cp*RuCO)2(μ-CO)Fe(CO)3]

Room temperature photolysis of a triply-bridged borylene complex, [(μ(3)-BH)(Cp*RuCO)(2)(μ-CO)Fe(CO)(3)] (1 a; Cp* = C(5)Me(5)), in the presence of a series of alkynes, 1,2-diphenylethyne, 1-phenyl-1-propyne, and 2-butyne led to the isolation of unprecedented vinyl-borylene complexes (Z)-[(Cp*RuCO)(2)(μ-CO)B(CR)(CHR')] (2: R, R' = Ph; 3: R = Me, R' = Ph; 4: R, R' = Me). This reaction permits a hydroboration of alkyne through an anti-Markovnikov addition. In stark contrast, in the presence of phenylacetylene, a metallacarborane, closo-[1,2-(Cp*Ru)(2)(μ-CO)(2){Fe(2)(CO)(5)}-4-Ph-4,5-C(2)BH(2)] (5 a), is formed. A plausible mechanism has been proposed for the formation of vinyl-borylene complexes, which is supported by density functional theory (DFT) methods. Furthermore, the calculated (11)B NMR chemical shifts accurately reflect the experimentally measured shifts. All the new compounds have been characterized in solution by mass spectrometry and IR, (1)H, (11)B, and (13)C NMR spectroscopies and the structural types were unequivocally established by crystallographic analysis of 2, 5 a, and 5 b.