Hexakis(dimethylamino)cyclohexaborane, a Boron(I) Compound without Electron Deficiency

Abstract

only evidence for a species of composition BS2 has so far come from mass spectrometry and thermal analysi~[~.'~.During our studies on binary and ternary boron sulf i d e~~' .~' we have now obtained a new boron disulfide of molecular formula B8Sib by two different routes. Rapid heating of a B2S,/S mixture (molar ratio 1 : 1.5) to ca. 300 "C at ca. bar in a sealed quartz glass which has been graphitized or protected by a boron nitride coating, in a twozone furnace with a sharp temperature profile (300/1OO "C) leads to deposition of a crystalline product within a narrow region in the colder zone of the reaction tube. The colorless needles are up to 2 mm long, extremely sensitive to hydrolysis, and decompose above 115 " C in a sealed vessel under normal pressure (Nz).The compound [elemental analysis B : S = 1 : 2.01(2)] forms monoclinic crystals (space-group P2,/c), a = 12.158(3), V = 1040.4 A3, pLalL=1.913, pexp=1.90(2) g cm-'. As shown by the complete X-ray structure analysis (1349 diffractometer data, R = 5.5%), solid boron disulfide is made up of discrete exactly planar BsSi6 molecules whose porphine type skeleton contains four 1,2,4,3,5-trithiadiborolane rings linked by sulfur bridges. Figure 1 shows the molecular structure with distances and angles averaged over chemically equivalent bonding parameters. Within the limits of experimental accuracy the molecule has Dlh symmetry (exact symmetry in the crystal is C,). The average B-S bond length (1.811 A)b=4.089 (1), ~=21.961(4) A, @= 107.65(3)", Z = 16 BS2, Fig. I. BXSlh molecule with bond lengths [A] and bond angles (u=0.003 A and 0.1 ', respectively).corresponds approximately to that in B& (1.808 il") and once again, in agreement with the planarity of the molecule, is indicative of strong ( p -p )~ interactions. The significant difference between the two types of B-S bond lengths in the five-membered rings (Fig. 1) provides the first confirmation of the differing T bond orders estimated by CNDO calculations on trithiadiborolane~l~~. The similarity of the structural characteristics to those of dimethyl-and dichloro-1,2,4,3,5trithiadiborolane in the gas phase161 demonstrates the remarkable, substituent-independent constancy of the bonding in the stable five-membered ring.A second synthetic route leading to BsSlb was found in the thioIysis of the halogenated trithiadiborolanes described by Schmidt and SiebertI7]. Thus reaction of 3,5-dibromo-1,2,4,3,5-trithiadiborolane with trithiocarbonic acid as an HZS generatorl'l in dilute CS, solution according to affords the porphine-like B8Si6 in ca. 6% yield along with polymeric (B2S& chains made up of S-linked B2S3 five-membered rings['l. Cyclization can be promoted in the reaction mixture by addition of d X or d9-transition metal compounds; the geometry of the BXSi6 molecule permits formation of interesting transition metal complexes having square-planar coordination of the tetradentate ligand (transannular S . . . S distance in B,S,,,: 4.667 A).In addition to other fragmentation products, the mass spectrum shows B2S; as f...

Formation and Reactivity of Electron‐Precise B−B Single and Multiple Bonds

“…[13] This method was later significantly improved upon by Schlesinger and co-workers using aglow discharge between mercury electrodes,which enabled the synthesis of B 2 Cl 4 on am ulti-gram scale. [17,18] The p-donor stabilization afforded by dialkylamino substituents enabled the synthesis of the first bench-stable diborane(4), B 2 (NMe 2 ) 4 ,t hrough reductive coupling of (Me 2 N) 2 BCl. [15] Above 0 8 8C, B 2 Cl 4 was found to decompose to B 4 Cl 4, which, rather than ac lassical cyclotetraborane structure,a dopts am ore stable non-classical tetrahedrane structure in the solid state, [16] though 11 BNMR data and calculations suggest the existence of an electronprecise cyclo-tetrachlorotetraborane isomer in solution.…”

“…[15] Above 0 8 8C, B 2 Cl 4 was found to decompose to B 4 Cl 4, which, rather than ac lassical cyclotetraborane structure,a dopts am ore stable non-classical tetrahedrane structure in the solid state, [16] though 11 BNMR data and calculations suggest the existence of an electronprecise cyclo-tetrachlorotetraborane isomer in solution. [17,19] In contrast, kinetically stabilized B 4 tBu 4 , obtained by reduction of BtBuF 2 or B 2 tBu 2 Cl 2 ,p resents an electron-deficient tetrahedrane structure [20] and can be further reduced to the radical anion [B 4 tBu 4 ]C À ,t he EPR spectrum of which suggests ac lassical butterfly structure. [4] Inspired by this methodology,N çth and coworkers succeeded in isolating several higher electron-precise oligoboranes of the formula [B n (NR 2 ) n+2 ]t hrough reductive coupling of various (dialkylamino)haloboranes,c ulminating in the synthesis of hexakis(dimethylamino)cyclohexaborane in 1980.…”

“…[17,19] In contrast, kinetically stabilized B 4 tBu 4 , obtained by reduction of BtBuF 2 or B 2 tBu 2 Cl 2 ,p resents an electron-deficient tetrahedrane structure [20] and can be further reduced to the radical anion [B 4 tBu 4 ]C À ,t he EPR spectrum of which suggests ac lassical butterfly structure. Thel ongest classical oligoboranes isolated to date,l inear B 6 (NMe 2 ) 8 [19] and cyclic B 6 (NR 2 ) 6 (R = Me,E t), [17,23] are only formed as negligible by-products of otherwise unselective reductive coupling reactions.W hile the exact mechanism of these reactions is still unclear, the generation of highly reactive boryl radical and/or borylene intermediates tends to lead to unwanted side-reactions,such as radical hydrogen abstraction from solvent or borylene insertion into CÀHa nd CÀC bonds, [24] as observed, for example,byP ower and co-workers in their attempt to synthesize doubly bonded diborenes stabilized by bulky terphenyl groups (Scheme 2). [22] Despite numerous attempts,n or ational method to predict or control the degree of boron catenation, or the selectivity for classical over non-classical polyboranes,h as been found.…”

Synthese und Reaktivität von Verbindungen mit elektronenpräzisen B‐B‐Einfach‐ und B‐B‐Mehrfachbindungen

“…Die π‐Donorstabilisierung durch Dialkylaminsubstituenten ermöglichte die Synthese des ersten luftstabilen Diborans(4), B 2 (NMe 2 ) 4 , durch die reduktive Kupplung von (Me 2 N) 2 BCl . Durch diese Vorgehensweise inspiriert, schafften es Nöth und Mitarbeiter, eine Reihe längerer, elektronenpräziser Oligoborane der Formel [B n (NR 2 ) n +2 ] durch die reduktive Kupplung von verschiedenen (Dialkylamino)borhalogeniden zu erzeugen, was 1980 in der Synthese von Hexakis(dimethylamino)cyclohexaboran gipfelte . Das sterisch stabilisierte B 4 t Bu 4 , das durch die Reduktion von B t BuF 2 oder B 2 t Bu 2 Cl 2 gewonnen wird, weist jedoch eine verbrückte Tetraederstruktur auf und kann weiter zum Radikalanion [B 4 t Bu 4 ] .− reduziert werden, dessen ESR‐Spektrum eine klassische Schmetterlingstruktur vermuten lässt…”

“…[15] Über 0 8 8Cz erfällt B 2 Cl 4 zu B 4 Cl 4 ,d as im Festkçrper statt einer klassischen cyclo-Tetraboranstruktur eine stabilere,n ichtklassische Te traederstruktur annimmt, [16] wenn auch 11 B-NMR-Daten und theoretische Rechnungen auf die Existenz eines elektronenpräzisen cyclo-Te trachlortetraboran-Isomers in Lçsung hinweisen. [17,18] Die p-Donorstabilisierung durch Dialkylaminsubstituenten ermçglichte die Synthese des ersten luftstabilen Diborans(4), B 2 (NMe 2 ) 4 ,d urch die reduktive Kupplung von (Me 2 N) 2 BCl. [4] Durch diese Vorgehensweise inspiriert, schafften es Nçth und Mitarbeiter,e ine Reihe längerer, elektronenpräziser Oligoborane der Formel [B n (NR 2 ) n+2 ] durch die reduktive Kupplung von verschiedenen (Dialkylamino)borhalogeniden zu erzeugen, was 1980 in der Synthese von Hexakis(dimethylamino)cyclohexaboran gipfelte.…”