Recent advances in transition metal diborane(6), diborane(4) and diborene(2) chemistry

Lewis Acid Decorated Hexacyanodiborane(6) Dianion

First examples of diborane(6) dianions decorated with weakly coordination B(C6F5)3 (BCF) groups and SiEt3 + moieties have been synthesized demonstrating the synthetic potential of the [B2(CN)6]2- dianion. [B2{CNB(C6F5)3}6]2- (1) was isolated as potassium and tetrabutylammonium salt. 1 is a rare example for a weakly coordinating dianion and it was used for the stabilization of the carbocation [Ph3C]+ and the oxonium acid [H(OEt2)2]+. Reaction of [Ph3C]21 with HSiEt3 resulted in the silylated neutral diborane(6) [B2{CNB(C6F5)3}4(CNSiEt3)2] (2) in which two BCF groups have been selectively replaced by SiEt3 + substituents, underscoring the stability and chemical versatility of the [B2(CN)6]2- dianion. The chemical properties and physicochemical data of 1 and 2 provide insight into electronic, coordinating, and steric properties of theses novel diborane(6) compounds.

Doubly Base-Stabilized Diborane(4) and Borato-Boronium Species and Their Chemistry with Chalcogens

In an effort to isolate diborane(4) derivatives, we have developed an efficient and uncatalyzed approach using [BH3·THF] and the mercaptopyridine ligand. Thermolysis of 2-mercaptopyridine, in the presence of [BH3·THF], afforded a doubly base-stabilized diborane(4) species 1, [HB(μ-C5H4NS)]2, along with the formation of its isomeric species 2, [HB(μ-C5H4NS)]2, albeit in less yield. Based on the coordination of the boron with the mercaptopyridine ligand in 2 and its spectroscopic data, compound 2 has been designated as a borato-boronium species, in which the anionic borate and cationic boronium units are covalently bonded to each other. Furthermore, we have demonstrated the oxidative insertion of chalcogen atoms (S and Se) through the B-B bond of the base-stabilized diborane(4), 1, that yielded chalcogenido-diboron species, 3(S) and 4(Se).

Stabilization of Diborynes versus Destabilization of Diborenes by Coordination of Lewis Bases: Unravelling the Dichotomy

We have quantum chemically investigated the boron-boron bonds in B2 , diborynes B2 L2 , and diborenes B2 H2 L2 (L=none, OH2 , NH3 ) using dispersion-corrected relativistic density functional theory at ZORA-BLYP-D3(BJ)/TZ2P. B2 has effectively a single B-B bond provided by two half π bonds, whereas B2 H2 has effectively a double B=B bond provided by two half π bonds and one σ 2p-2p bond. This different electronic structure causes B2 and B2 H2 to react differently to the addition of ligands. Thus, in B2 L2 , electron-donating ligands shorten and strengthen the boron-boron bond whereas, in B2 H2 L2 , they lengthen and weaken the boron-boron bond. The aforementioned variations in boron-boron bond length and strength become more pronounced as the Lewis basicity of the ligands L increases.

Transition Metal Triple-decker Sandwich Complexes Containing Group 13 Elements

Transition metal triple-decker complexes are an interesting class of sandwich complexes that engrossed great attention due to their structures and properties. Over the decades, synthesis of triple-decker complexes featuring homocyclic, heterocyclic or π-conjugated rings as middle decks have been abundantly reported. In this regard, the chemistry of such complexes bearing boron in the middle deck are well explored due to the ability of boron-containing cycles to readily coordinate bifacially with metal atoms thereby forming triple-decker complexes. On the other hand, electron counting rules and theoretical calculations have strengthened our knowledge of the structure and bonding in these complexes. Further, these complexes can be used as synthons to generate organometallic polymers having interesting electronic, optical and magnetic properties that can be appropriately tuned to cater to a wide range of applications. In our quest for novel metallaboranes and metallaheteroboranes, we have been successful in isolating various triple-decker complexes that feature boron in the middle deck. This review explained elaborately the synthesis, structures, and bonding in such complexes reported by us and others.

Diborane and Triborane Species in the Coordination Sphere of Group-8 Transition Metals

The synthesis and structural elucidation of a series of ruthenium diborane and triborane compounds are described. Treatment of [Cp*Ru(PPh3)2Cl] (1) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; PPh3 = triphenylphosphine) with [BH3·THF] (THF = tetrahydrofuran) at 60 °C led to the formation of the hydrogen-rich ruthena-octahydrotetraborane arachno-[2-{Cp*Ru(PPh3)B3H8}] (2). The chemistry of 2 is explored with [Fe2(CO)9] at room temperature, which resulted in the formation of a metal-stabilized triborane species, [{Cp*Ru(PPh3)}(μ3-η1:η2:η2-B3H6){Fe2(CO)7}] (3). Compound 3 can be considered as a triborane analogue [B3H6]3- that stabilizes in the coordination sphere of two iron and one ruthenium atoms. Further, the photolysis of nido-[1,2-(Cp*Ru)2(μ-H)2(B3H7)] (4) with [M(CO)5·THF] (M = Mo and W) afforded an arachno-[1,2-(Cp*Ru)(Cp*RuCO)(μ-H)(B3H8)] (5), in which the [M(CO)5·THF] acted as a CO source. In an attempt to convert arachno-5 into a closo or nido species, we have pyrolyzed arachno-5 in toluene at 90 °C for 20 h that afforded nido-[2,3-(Cp*Ru)2(μ-CO)(μ3-H)(B3H6)] (6) having two ruthenium atoms at the basal position. Irradiation of arachno-5 with an intermediate generated from CS2 and [LiBH4·THF] in THF afforded the diborane(5) species [(Cp*RuCO)(Cp*Ru)(μ-H)(μ-η1:η2-B2H4)(CS2H)] (7) in which a dithioformato ligand (SHC═S) is attached to one Ru-B bond. Compound 7 can be considered as a diborane(5) species, which is stabilized by a dithioformato ligand. All the synthesized compounds have been characterized by employing electrospray ionization-mass spectrometry, multinuclear NMR, and IR spectroscopy techniques. The single-crystal X-ray diffraction studies of compounds 2, 3, 6, and 7 helped to establish the structural integrity of these compounds. Further, density functional theory studies were performed to provide insight into the bonding of these metal-stabilized diborane and triborane species.

Stereoselective formation of boron-stereogenic organoboron derivatives

Four-coordinate organoboron derivatives present interesting chemical, physical, biological, electronical, and optical properties. Given the increasing demand for the synthesis of smart functional materials based on chiral organoboron compounds, the exploration of stereoselective synthesis of boron-stereogenic organo-derivatives is highly desirable. However, the stereoselective construction of organoboron compounds stereogenic at boron has been far less studied than other elements of the main group due to configurational stability concerns. Nowadays, these species are no longer elusive and configurationally stable compounds have been highlighted. The idea is to show the potential of the stereoselective building of the four-coordinate boron centre and encourage future endeavors and developments in the field.

Chemistry of CS2 and CS3 Bridged Decaborane Analogues: Regular Coordination Versus Cluster Expansion

In an effort to synthesize metallaheteroborane clusters of higher nuclearity, the reactivity of metallaheteroboranes, nido-[(Cp*M)₂B₆S₂H₄(CS₃)] (Cp* = C₅Me₅) (1: M = Co; 2: M = Rh) with various metal carbonyls have been investigated. Photolysis of nido-1 and nido-2 with group 6 metal carbonyls, M'(CO)₅.THF (M' = Mo or W) were performed that led to the formation of a series of adducts [(Cp*M)₂B₆S₂H₄(CS₃){M'(CO)₅}] (3: M = Co, M' = Mo; 4: M = Co, M' = W; 5: M = Rh, M' = Mo; 6: M = Rh, M' = W) instead of cluster expansion reactions. In these adducts, the S atom of C=S group of di(thioboralane)thione {B₂CS₃} moiety is coordinated to M'(CO)₅ (M = Mo or W) in η¹-fashion. On the other hand, thermolysis of nido-1 with Ru₃(CO)₁₂ yielded one fused metallaheteroborane cluster [{Ru(CO)₃}₃S{Ru(CO)}{Ru(CO)₂}Co₂B₆SH₄(CH₂S₂){Ru(CO)₃}₂S], 7. This 20-vertex-fused cluster is composed of two tetrahedral {Ru₃S} and {Ru₂B₂}, a flat butterfly {Ru₃S} and one octadecahedron {Co₂RuB₇S} core with one missing vertex, coordinated to {Ru₂SCH₂S₂} through two boron and one ruthenium atom. On the other hand, the room temperature reaction of nido-2 with Co₂(CO)₈ produced one 19-vertex fused metallaheteroborane cluster [(Cp*Rh)₂B₆H₄S₄{Co(CO)}₂{Co(CO)₂}₂(μ-CO)S{Co(CO)₃}₂], 8. Cluster 8 contains one nido-decaborane {Rh₂B₆S₂}, one butterfly {Co₂S₂} and one bicapped square pyramidal {Co₆S} unit that exhibits an intercluster fusion with two sulfur atoms in common. Clusters 3-6 have been characterized by multinuclear NMR and IR spectroscopy, mass spectrometry and structurally determined by XRD analyses. Furthermore, the DFT calculations have been carried out to gain insight into electronic, structural and bonding patterns of the synthesized clusters.

F2BMF (M = B and Al) Molecules: A Matrix Infrared Spectra and Theoretical Calculations Investigation

Reactions of laser-ablated B and Al atoms with BF₃ have been explored in the 4 K excess neon through the matrix isolation infrared spectrum, isotopic substitutions and quantum chemical calculations. The inserted complexes F₂BMF (M = B, Al) were identified by anti-symmetric and symmetric stretching modes of F-B-F, and the F-¹¹B-F stretch modes are at 1336.9 and 1202.4 cm^-1 for F₂¹¹B¹¹BF and at 1281.5 and 1180.8 cm^-1 for F₂¹¹BAlF. The CASSCF analysis, EDA-NOCV calculation and the theory of atoms-in-molecules (AIM) are applied to investigate the bonding characters of F₂BBF and F₂BAlF molecules. The bonding difference between boron and aluminum complexes reveals interesting chemistries, and the FB species stabilization by a main group atom was first observed in this article.

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.

Cluster Growth Reactions: Structures and Bonding of Metal-Rich Metallaheteroboranes Containing Heavier Chalcogen Elements

In an effort to synthesize cobalt-rich metallaheteroboranes from decaborane(14) analogues, we have studied the reaction of 10-vertex nido-[(Cp*Co)₂B₆H₆E₂] (Cp* = η⁵-C₅Me₅, 1: E = Se and 2: E = Te) with [Co₂(CO)₈] under thermolytic conditions. All of these reactions yielded face-fused clusters, [(Cp*Co)₂B₆H₆E₂{Co(CO)}(μ-CO){Co₃(CO)₆}] (3: E = Se and 4: E = Te). Further, when clusters 3 and 4 were treated with [Co₂(CO)₈], they underwent further cluster buildup reactions leading to the formation of 16-vertex doubly face-fused clusters [(Cp*Co)₂B₆H₆E₂{Co₂(CO)₂}(μ-CO)₂{Co₄(CO)₈}] (5: E = Se and 6: E = Te). Cobaltaheteroboranes 3 and 4 comprise one icosahedron {Co₄B₆E₂} and one square pyramidal {Co₃B₂} moiety, whereas 5 and 6 are made with one icosahedron {Co₄B₆E₂} and two square pyramidal {Co₃B₂} cores. In an attempt to generate heterometallic metal-rich clusters, we have explored the reactivity of decaborane(14) analogue nido-[(Cp*Co)₂B₇TeH₉] (7) with [Ru₃(CO)₁₂] at 80 °C, which afforded face-fused 13-vertex cluster [(Cp*Co)₂B₇H₇Te{Ru₃(CO)₈}] (8). Cluster 8 is a rare example of a metal-rich metallaheteroborane in which one icosahedron {Co₂Ru₂B₇Te} and a tetrahedron {Ru₂B₂} units are fused through a common {RuB₂} triangular face. Further, the treatment of nido-[(Cp*Co)₂B₆S₂H₄(CH₂S₂)] (9) with [Fe₂(CO)₉] afforded 11-vertex nido-[(Cp*Co)₂B₆S₂H₄(CH₂S₂){Fe(CO)₃}] (10). The core structure of 10 is similar to that of [C₂B₉H₁₁]^2- with a five-membered pentahapto coordinating face. All of the synthesized metal-rich metallaheteroboranes have been characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy, ESI-MS, and structurally solved by single-crystal X-ray diffraction analysis. Furthermore, theoretical investigations gave insight into the bonding of such higher-nuclearity clusters containing heavier chalcogen atoms.

A (μ-hydrido)diborane(4) anion and its coordination chemistry with coinage metals

A tetra(o-tolyl) (μ-hydrido)diborane(4) anion 1, an analogue of [B₂H₅]^- species, was facilely prepared through the reaction of tetra(o-tolyl)diborane(4) with sodium hydride. Unlike common sp²-sp³ diborane species, 1 exhibited a σ-B-B bond nucleophilicity towards NHC-coordinated transition-metal (Cu, Ag, and Au) halides, resulting in the formation of η²-B-B bonded complexes 2 as confirmed by single-crystal X-ray analyses. Compared with 1, the structural data of 2 imply significant elongations of B-B bonds, following the order Au > Cu > Ag. DFT studies show that the diboron ligand interacts with the coinage metal through a three-center-two-electron B-M-B bonding mode. The fact that the B-B bond of the gold complex is much prolonged than the related Cu and Ag compounds might be ascribed to the superior electrophilicity of the gold atom.

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.

Derivatization of an especially electron-rich diborane

Starting with electron-rich ditriflato-diborane B₂(hpp)₂(OTf)₂ (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate), novel symmetric and unsymmetric diboranes B₂(hpp)₂X₂ and B₂(hpp)₂XY with X,Y = Br, NCS, N₃ or OTf are synthesized by substitution reactions with S_N1 mechanisms. The stability of the unsymmetric diboranes with respect to dismutation equilibria is evaluated.

Low-Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments

Low-valent titanium species were prepared by reaction of [TiCp*X₃ ] (Cp*=η⁵ -C₅ Me₅ ; X=Cl, Br, Me) with LiEH₄ (E=Al, B) or BH₃ (thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X₃ ] with LiAlH₄ in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(μ-H)}₂ {(μ-H)₂ AlX(L)}₂ ] (L=thf, X=Cl, Br; L=OEt₂ , X=Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH₂ XL fragments, which behave as σ-accepting (Z-type) ligands. Analogous reactions of [TiCp*X₃ ] with LiBH₄ (2 and 3 equiv.) in tetrahydrofuran at room temperature and at 85 °C lead to the titanium(III) compounds [{TiCp*(BH₄ )(μ-X)}₂ ] (X=Cl, Br) and [{TiCp*(BH₄ )(μ-BH₄ )}₂ ], respectively. The treatment of [TiCp*Me₃ ] with 4 and 5 equiv. of BH₃ (thf) produces the diamagnetic [{TiCp*(BH₃ Me)}₂ (μ-B₂ H₆ )] and paramagnetic [{TiCp*(μ-B₂ H₆ )}₂ ] complexes, respectively.

Directed Syntheses of CS2- and CS3-Bridged Decaborane-14 Analogues

To establish a procedure for single-cage cluster expansion of open cage dimetallaoctaboranes(12), we have investigated the chemistry of nido-[(Cp*M)₂B₆H₁₀] (η⁵-C₅Me₅ = Cp*, 1: M = Co; 2: M = Rh), with diverse chalcogen-based borate ligands. As a result, treatment of nido-1 and nido-2 with Li[BH₂E₃] (E = S, Se, or Te) yielded 10-vertex nido-[(Cp*Co)₂B₇EH₉] (3: E = S; 4: E = Se; 5: E = Te) along with known 10-vertex nido-[(Cp*M)₂B₆H₆E₂] (6: E = S, M = Co; 7: E = Se, M = Co; 8: E = Te, M = Co; 9: E = Se, M = Rh). The geometries of dimetallachalcogenaboranes, 3-9, are isostructural with decaborane(14). Thermolysis of nido-1 and nido-2 with an intermediate, generated from CS₂ and [LiBH₄]·THF reaction in THF, produced nido-[(Cp*M)₂B₆S₂H₄(CH₂S₂)] (10: M = Co; 11: M = Rh) and nido-[(Cp*M)₂B₆S₂H₄(CS₃)] (12: M = Co; 13: M= Rh). Clusters 10-13 are rare species in which one of the B-B bonds is coordinated with a {CS₂}^2- or {CS₃}^2- ligand, generating di(thioborolane) {B₂S₂CH₂} or di(thioboralane)-thione {B₂CS₃} moieties. To examine further the coordination chemistry of CS₂-bridged decaborane(14) analogue nido-10, photolysis was carried out with {M(CO)₅·THF} (M = Mo or W) that led to the isolation of [(Cp*Co)₂B₆S₂H₄(CH₂S₂){M(CO)₅}] (14: M = Mo; 15: M = W), where the {CH₂S₂} moiety is coordinated with one {M(CO)₅} moiety in η¹-fashion. All the synthesized clusters have been characterized by ESI-mass, multinuclear NMR spectroscopy, and IR spectroscopy and structurally solved by single-crystal XRD. Furthermore, DFT calculations probe the bonding of these CS₂- and CS₃-bridged decaborane analogues.

Construction of σ-Aromatic AlB2 Ring via Borane Coupling with a Dicoordinate Cyclic (Alkyl)(Amino)Aluminyl Anion

Since the groundbreaking discovery in 2018 that the synthesis of a bottleable nucleophilic aluminyl anion is feasible, a handful of derivatives have been developed to date, which are, however, limited to diamino- and dialkyl-substituted species. Herein, we report the synthesis of a cyclic (alkyl)(amino)aluminyl anion based on a five-membered framework. The dicoordinate aluminum center features both a lone pair of electrons and an unoccupied 3p orbital, thus genuinely making it isoelectronic with carbenes. We show the bond formation and bond activation at the Al sphere: thus, not only does it undergo electron redistribution with borane to furnish a heteroatomic group 13 ring exhibiting a σ-aromatic nature concomitant with a three-center two-electron AlB₂ bond but also the ambiphilic nature allows for oxidative addition of Si-H, N-H, and even C-C bonds at the aluminum center.

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

Irradiation of [Re₂(CO)₁₀] in the presence of BH₃·thf resulted in the formation of several rhenium diborane(6) species, for example, [{(OC)₄Re}{Re(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1; [{(OC)₄Re}₂{Re(CO)₃}(μ₃-η²:η²:η¹-B₂H₆)(μ-H)], 2; and [{(OC)₄Re}₂(μ-η²:η²-B₂H₆)], 3, comprising diverse coordination modes of the [B₂H₆]^2- ligand. Compound 1 contains a tris(bidentate) [B₂H₆]^2- unit, whereas 2 consists of an unsymmetrically bound [μ₃-η²:η²:η¹-B₂H₆]^2- ligand. In contrast, the irradiation of [Mn₂(CO)₁₀] with BH₃·thf yielded only the Mn analogue of 1, [{(OC)₄Mn}{Mn(CO)₃}₂(μ₃-η²:η²:η²-B₂H₆)(μ-H)], 1'. In an attempt to generate the bimetallic Mn-diborane(6), we have carried out the reaction of 1' with PCy₃ under photolytic conditions. The reaction led to the formation of two single base stabilized unsymmetrical diborane(5) species, [{Mn(CO)₃}{Mn(CO)₂PCy₃}(μ-η²:η²-B₂H₅·PCy₃)(μ-H)], 4, and [{Mn(CO)₂PCy₃}(η³-B₂H₅·PCy₃)], 5. As [B₂H₆]^2- and [B₂H₅·PCy₃]^- are isoelectronic, the bondings in 4 and 5 are analogous to that of diborane(6) species 1-3. All the new species have been characterized spectroscopically, and their structures were further confirmed by single-crystal X-ray diffraction studies. DFT-type quantum chemical calculations were carried out that provided insight into the bonding interaction of [B₂H₆]^2- and [B₂H₅·PCy₃]^- with the M(CO)_n fragments.