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.

Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

Probing the Boundaries between Lewis-Basic and Redox Behavior of a Parent Borylene

The parent borylene (CAAC)(Me3 P)BH, 1 (CAAC=cyclic alkyl(amino)carbene), acts both as a Lewis base and one-electron reducing agent towards group 13 trichlorides (ECl3 , E=B, Al, Ga, In), yielding the adducts 1-ECl3 and increasing proportions of the radical cation [1]•+ for the heavier group 13 analogues. With boron trihalides (BX3 , X=F, Cl, Br, I) 1 undergoes sequential adduct formation and halide abstraction reactions to yield borylboronium cations and shows an increasing tendency towards redox processes for the heavier halides. Calculations confirm that 1 acts as a strong Lewis base towards EX3 and show a marked increase in the B-E bond dissociation energies down both group 13 and the halide group.

Bonding in M(NHBMe)2 and M[Mn(CO)5]2 complexes (M=Zn, Cd, Hg; NHBMe=(HCNMe)2B): divalent group 12 metals with zero oxidation state

Quantum chemical studies using density functional theory were carried out on M(NHBMe)2 and M[Mn(CO)5]2 (M=Zn, Cd, Hg) complexes. The calculations suggest that M(NHBMe)2 and M[Mn(CO)5]2 have D2d and D4d symmetry, respectively, with a 1A1 electronic ground state. The bond dissociation energies of the ligands have the order of Zn > Cd > Hg. A thorough bonding analysis using charge and energy decomposition methods suggests that the title complexes are best represented as NHBMe⇆M0⇄NHBMe and Mn(CO)5⇆M0⇄Mn(CO)5 where the metal atom M in the electronic ground state with an ns2 electron configuration is bonded to the (NHBMe)2 and [Mn(CO)5]2 ligands through donor–acceptor interaction. These experimentally known complexes are the first examples of mononuclear complexes with divalent group 12 metals with zero oxidation state that are stable at ambient condition. These complexes represent the rare situation where the ligands act as a strong acceptor and the metal center acts as strong donor. The relativistic effect of Hg leads to a weaker electron donating strength of the 6s orbital, which explains the trend of the bond dissociation energy.

Filling the void: controlled donor-acceptor interaction facilitates the formation of an M-M single bond in the zero oxidation state of M (M = Zn, Cd, Hg)

The intriguing question of whether it is possible to form a genuine M0-M0 single bond for the M2 species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively MI-MI. Herein, we present viable M2(NHBMe)2 (M = Zn, Cd, Hg; NHBMe = (HCNMe)2B) complexes in which the controlled donor-acceptor interaction leads to an M0-M0 single bond. In these complexes, M2 in the 1∑g ground state with the (nσg+)2(nσu+)2 (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor-acceptor bonding with singlet 2NHBMe ligands where a combined effect of dominant (+,-) σ-backdonation from the antibonding (nσu+)2 orbital of M2 to the 2NHBMe ligands and a somewhat weaker (+,+) σ-donation from the 2NHBMe ligands to the bonding (n + 1)σg+ orbital leads to the unorthodox bonding situation of forming an M-M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHBMe ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M2(PhDipp)2 complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M2(NHBMe)2 complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M2(NHBMe)2 → M + M(NHBMe)2 and M2(NHBMe)2 → M2 + (NHBMe)2. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5-39.5 kcal mol-1. The experimental characterization of M2(PhDipp)2 despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M2(NHBMe)2 complexes. Furthermore, the ligand exchange reaction considering M2(PhMe)2 as the starting material also turned out to be feasible. Therefore, the M2(NHBMe)2 complexes are the first cases that feature a neutral M2 moiety with a single M0-M0 covalent bond, where M is a Group 12 metal.

Single and Double Hydroboration of B-B Triple Bonds and Convergent Routes to a Cationic Tetraborane

A compound with a boron-boron triple bond is shown to undergo stepwise hydroboration reactions with catecholborane to yield an unsymmetrical hydro(boryl)diborene and a 2,3-dihydrotetraborane. Abstraction of H^- from the latter compound produces an unusual cationic, planar tetraborane with a hydrogen atom bridging the central B₂ moiety. Spectroscopic and crystallographic data and DFT calculations support a "protonated diborene" structure for this compound, which can also be accessed via direct protonation of the corresponding diborene.

Diborane heterolysis: breaking and making B-B bonds at magnesium

Reaction of the dimeric β-diketiminato magnesium hydride [(BDI)MgH]2 (BDI = HC{(Me)CN-2,6-i-Pr2C6H3}2) with bis-pinacolatodiborane (B2pin2) resulted in B-O bond activation and formation of a magnesium complex of an unusual borylborohydride anion. In contrast, similar treatment of the mononuclear organomagnesium [{BDI}Mg(n-Bu)] with 4,4,4',4',6,6'-hexamethyl-2,2'-bi(1,3,2-dioxaborinane) (B2hex2) provided a B(sp2)-B(sp3) diborane anion, [(hex)BB(n-Bu)(hex)]-, with a constitution which is analogous to that formed in the previously reported reaction with bis(pinacolato)diboron (B2pin2). Subsequent addition of 4-dimethylaminopyridine to a solution of this compound induced alkylborane displacement and provided a magnesium boryl derivative containing a terminal Mg-B(hex) interaction (Mg-B 2.319(3) Å), a result which reinforces the generality of this approach for the synthesis of boryl anions by B-B bond heterolysis. Further studies of the reactivity of the initially formed B(sp2)-B(sp3) anions with diborane small molecules also resulted in alkylborane displacement and the production of triboron anions, which are propagated by contiguous and electron precise (2c-2e) B-B-B interactions.

Magnesium Boryl Reactivity with 9-BBN and Ph3 B: Rational B-B' Bond Formation and Diborane Isomerization

Reactions of a magnesium‐based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9‐BBN and Ph₃B, provide facile B−B′ single bond formation. Although the Ph₃B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9‐BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn‐diboration of the resultant alkenyl carbocycle.

Borylation of fluorinated arenes using the boron-centred nucleophile B(CN)32- - a unique entry to aryltricyanoborates

The potassium salt of the boron-centred nucleophile B(CN)32- (1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K+ salts of the respective borate anions with one {B(CN)3} unit bonded to the (hetero)arene. An excess of K21 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C-F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C6F6-n H n (n = 1-5), generally react with K21 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R-C6F5 (R = -CN, -OMe, -Me, or -CF3) react via C-F/C-B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an SNAr mechanism. The reaction of 1,4-F2C6H4 with K21 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.

Facile Access to Unprecedented Electron-Precise Monohydrodiboranes(4), cis-1,2-Dihydrodiboranes(4), and a 1,1-Dihydrodiborane(5)

2,3-Bis(dimethylamino)-substituted B₂ N₂ C₂ heterocycles underwent selective dimethylamino/hydride exchange with either one or two equivalents of BH₃ ⋅SMe₂ to give the corresponding cyclic monohydrido- or (cis)1,2-dihydridodiboranes(4), respectively. Upon either heating or irradiation in solution, the latter underwent ring contraction to the corresponding five-membered BN₂ C₂ heterocycles, whereas irradiation of the 1,2-dimethylaminoethene-supported 1,2-dihydridodiborane(4) in the presence of PEt₃ gave an unprecedented unsymmetrical 1,1-dihydrodiborane(5) phosphine adduct.

Isolation and Characterization of Radical Anions Derived from a Boryl-Substituted Diphosphene

Radical anions of a diphosphene with two boryl substituents were isolated and characterized by single-crystal X-ray diffraction, electron spin resonance (ESR), and UV/Vis absorption spectroscopy as well as DFT calculations. Structural analysis of the radical anions revealed an elongation of the P=P bond and a contraction of the B-P bonds relative to the neutral diphosphene. The UV/Vis spectra of these radical anions showed a strong absorption in the visible region, which was assigned to SOMO-related transitions on the basis of DFT calculations. The ESR spectra revealed that the hyperfine coupling constant with the phosphorus nuclei is the smallest that has been reported thus far. The results of the DFT calculations furthermore suggest that this should be attributed to a soaking of electron spin to the vacant p orbitals of the boryl substituents.

A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction

A boryl-substituted diphosphene was synthesized through the nucleophilic borylation of PCl3 with a borylzinc reagent, followed by a reduction with Mg. A combined analysis of the resulting diboryldiphosphene by single-crystal X-ray diffraction, DFT calculations, and UV/Vis spectroscopy revealed a σ-electron-donating effect for the boryl substituent that was slightly weaker than that of the 2,4,6-tri-tert-butylphenyl (Mes*) ligand. The reaction of this diboryldiphosphene with (n) BuLi afforded a boryl-substituted phosphinophosphide that was, in comparison with the thermally unstable Mes*-substituted diaryldiphosphene, stabilized by a π-electron-accepting effect of the boryl substituent.

sp(2)-sp(3) diboranes: astounding structural variability and mild sources of nucleophilic boron for organic synthesis

Despite the widespread use of organoborane reagents in organic synthesis and catalysis, a major challenge still remains: very few boron-centered nucleophiles exist for the direct construction of B-C bonds. Perhaps the most promising emerging solution to this problem is the use of sp(2)-sp(3) diboranes, in which one boron atom of a conventional diborane(4) is quaternised by either a neutral or anionic nucleophile. These compounds, either isolated or generated in situ, serve as relatively mild and convenient sources of the boryl anion [BR2](-) for use in organic synthesis and have already proven their efficacy in metal-free as well as metal-catalysed borylation reactions. This Feature article documents the history of sp(2)-sp(3) diborane synthesis, their properties and surprising structural variability, and their burgeoning utility in organic synthesis.

Beryllium bis(diazaborolyl): old neighbors finally shake hands

The synthesis of a linear beryllium bis(diazaborolyl) compound featuring the first non-cluster bond between boron and beryllium has been achieved through the reaction of Yamashita's lithium diazaborolide and BeCl2. In accord with the established chemistry of beryllium, the bonding is polar covalent in character, as determined by structural and spectroscopic analysis, as well as reactivity studies.

Lowering the reduction potential of a boron compound by means of the substituent effect of the boryl group: one-electron reduction of an unsymmetrical diborane(4)

We have clarified and observed the high electron affinity of pinB-BMes2 (1; Mes = mesityl, pin = pinacolato). By using electrochemistry, it was shown that 1 has a higher electron affinity than those of B2pin2 and Mes3B. One-electron reduction of 1 gave the corresponding radical anion. The ESR spectroscopy and DFT calculation revealed the unsymmetrical distribution of electron density over the B-B bond. UV/Vis spectroscopy showed that the SOMO-related absorption supports the deep purple color of the radical anion. DFT studies on the torsion angle dependency of the LUMO levels and relative energies revealed the reason why 1 has high electron affinity as a result of the substituent effect of the Bpin group.