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Simple Solution-Phase Syntheses of Tetrahalodiboranes(4) and their Labile Dimethylsulfide Adducts
(2017)
Convenient, solution-phase syntheses of tetrahalodiboranes(4) B\(_2\)F\(_4\), B\(_2\)Cl\(_4\) and B\(_2\)I\(_4\) are presented herein from common precursor B\(_2\)Br\(_4\). In addition, the dimethylsulfide adducts B\(_2\)Cl\(_4\)(SMe\(_2\))\(_2\) and B\(_2\)Br\(_4\)(SMe\(_2\))\(_2\) are conveniently prepared in one-step syntheses from the commercially-available starting material B\(_2\)(NMe\(_2\))\(_4\). The results provide simple access to the full range of tetrahalodiboranes(4) for the exploration of their untapped synthetic potential.
Recent years have seen rapid advances in the chemistry of small molecules containing electron-precise boron-boron bonds. This review provides an overview of the latest methods for the controlled synthesis of B–B single and multiple bonds as well as the ever-expanding range of reactivity displayed by the latter.
Despite the prevalence of stable π-complexes of most d\(^{10}\) metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d\(^{10}\) Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ion's limited capacity for π-backdonation. By employing more strongly donating π- ligands, namely neutral diborenes with a high-lying π(B=B) or- bital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π- complexes were synthesized in good yields. Metal–B2 π- interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB2 planes significantly twist out of the MX\(_2\) planes about the M-centroid(B–B) vector, with angles rang- ing from 47.0° to 85.5°, depending on the steric interactions be- tween the diborene ligand and the MX\(_2\) fragment.
The self-stabilizing, tetrameric cyanoborylene [(cAAC)B(CN)]4 (I, cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) and its diborene relative, [(cAAC)(CN)B=B(CN)(cAAC)] (II), both react with disulfides and diselenides to yield the corresponding cAAC-supported cyanoboron bis(chalcogenides). Furthermore, reactions of I or II with elemental sulfur and selenium in various stoichiometries provided access to a variety of cAAC- stabilized cyanoboron-chalcogen heterocycles, including a unique dithiaborirane, a diboraselenirane, 1,3-dichalcogena-2,4-diboretanes, 1,3,4-trichalcogena- 2,5-diborolanes and a rare six-membered 1,2,4,5-tetrathia-3,6-diborinane. Stepwise addition reactions and solution stability studies provided insights into the mechanism of these reactions and the subtle differences in reactivity observed between I and II.
The heteroarene 1,4-bis(CAAC)-1,4-diborabenzene (1; CAAC = cyclic (alkyl)(amino)carbene) reacts with [(MeCN)\(_3\)M(CO)\(_3\)] (M = Cr, Mo, W) to yield half-sandwich complexes of the form [(η\(^6\)-diborabenzene)M(CO)\(_3\)] (M = Cr (2), Mo (3), W (4)). Investigation of the new complexes with a combination of X-ray diffraction, spectroscopic methods and DFT calculations shows that ligand 1 is a remarkably strong electron donor. In particular, [(η\(^6\)-arene)M(CO)\(_3\)] complexes of this ligand display the lowest CO stretching frequencies yet observed for this class of complex. Cyclic voltammetry on complexes 2-4 revealed one reversi- ble oxidation and two reversible reduction events in each case, with no evidence of ring-slippage of the arene to the η\(^4\) binding mode. Treatment of 4 with lithium metal in THF led to identification of the paramagnetic complex [(1)W(CO)\(_3\)]Li·2THF (5). Compound 1 can also be reduced in the absence of a transition metal to its dianion 1\(^{2–}\), which possesses a quinoid-type structure.
CO\(_2\) is found to undergo room-temperature, ambient- pressure reactions with two species containing boron-boron multiple bonds, leading to incorporation of either one or two CO\(_2\) molecules. In one case, a thermally-unstable intermediate was structurally characterized, indicating the operation of an initial 2+2 cycloaddition mechanism in the reaction.
The two-fold reduction of (cAAC)BHX\(_2\) (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene; X = Cl, Br) provides a facile, high-yielding route to the dihydrodiborene (cAAC)\(_2\)B\(_2\)H\(_2\). The (chloro)hydroboryl anion reduction intermediate was successfully isolated using a crown ether. Overreduction of the diborene to its dianion [(cAAC)\(_2\)B\(_2\)H\(_2\)]\(^{2−}\) causes a decrease in the B–B bond order whereas the B–C bond orders increase.
B≡N and B≡B triple bonds induce C-H activation of acetone to yield a (2-propenyloxy)aminoborane and an unsymmetrical 1-(2- propenyloxy)-2-hydrodiborene, respectively. DFT calculations showed that, despite their stark electronic differences, both the B≡N and B≡B triple bonds activate acetone via a similar coordination-deprotonation mechansim. In contrast, the reaction of acetone with a cAAC-supported diboracumulene yielded a unique 1,2,3-oxadiborole, which according to DFT calculations also proceeds via an unsymmetrical diborene, followed by intramolecular hydride migration and a second C-H activation of the enolate ligand.
Reaction of bis-catecholatodiboron-NHC adducts, B\(_2\)Cat\(_2\)(NHC), (NHC = IMe (tetramethylimidazol-2-ylidene), IMes (1,3-dimesitylimidazol-2-ylidene) or IDIPP (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)) with BCl3 results in the replacement of the catecholato group bound to the four coordinate boron with two chlorides to yield diboron(5) Lewis acid-base adducts of formula CatB-BCl\(_2\)(NHC). These compounds are precursors to diboron(5) monocations, accessed by adding AlCl\(_3\) or K[B(C\(_6\)F\(_5\))\(_4\)] as halide abstraction agents in the presence of a Lewis base. The substitution of the chlorides of CatB-BCl\(_2\)(NHC) for hydrides is achieved using Bu\(_3\)SnH and a halide abstracting agent to form 1,1-dihydrodiboron(5) compounds, CatB-BH\(_2\)(NHC). Attempts to generate diboron(4) monocations of formula [CatB-B(Y)(NHC)]\(^+\) (Y = Cl or H) led to the rapid formation of CatBY.
Molecules containing multiple bonds between atoms—most often in the form of olefins—are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.