Diborabutatrien: ein elektronenarmes Cumulen

Abstract

Die Komplexierung von zwei ¾quivalenten eines cyclischen (Alkyl)(amino)carbens (CAAC) an Tetrabromdiboran führte unter nachfolgender Reduktion mit vier ¾qui-valenten Natriumnaphthalid zur Bildung des CAAC-stabilisierten Diboracumulens (CAAC) 2 B 2 . Gegenüber dem bereits bekannten Komplex, der eine von cyclischen Di(amino)carbenen stabilisierte B B-Bindung aufweist, führt das Vermçgen des CAAC-Liganden, p-Elektronendichte aufzunehmen (B 2 ! CAAC), zu signifikanten Unterschieden, wie etwa einer län-geren BÀB-und kürzeren BÀC-Bindungen. Die Analyse der Grenzorbitale lässt auf eine Verteilung der Valenzelektronen entlang der gesamten linearen C-B-B-C Einheit in (CAAC) 2 B 2 schließen, was durch natürliche Populationsanalyse und Cyclovoltammetrie weiter bestätigt wird.

Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

“…A comparably large shift difference of 20–24 ppm is observed for B 2 I 4 (SMe 2 ) 2 and B 2 Br 4 (SMe 2 ) 2 ( δ =−20.0 and −0.3 ppm, respectively), as well as B 2 I 4 (PCy 3 ) 2 and B 2 Br 4 (PCy 3 ) 2 ( δ =−28.0 ppm and −4.3 ppm, respectively) . A smaller highfield shift of 5–8 ppm is observed between pairs of B 2 Cl 4 L 2 and B 2 Br 4 L 2 analogues, for example, for B 2 Cl 4 (SMe 2 ) 2 and B 2 Br 4 (SMe 2 ) 2 ( δ =7.3 and −0.3 ppm, respectively), B 2 Cl 4 (PMe 3 ) 2 and B 2 Br 4 (PMe 3 ) 2 ( δ =0.3 and −7.3 ppm, respectively) as well as B 2 Cl 4 (CAAC Me ) 2 and B 2 Br 4 (CAAC Me ) 2 ( δ =2.3 and −2.3 ppm, respectively) . This is in line with the greater increase in relative electronegativity from I to Br (ca.…”

“…It is the first example of such a structural motif for Lewis base‐stabilized tetrahalodiboranes. Given that the addition of CAAC Me to the lighter tetrahalodiboranes(4) results in the exclusive formation of the neutral bis‐adducts (Scheme ) we propose that the formation of [B 2 I 3 (CAAC Me ) 2 ]I is driven by a release of steric strain because the sterically demanding CAAC Me ligand displaces one of the large iodide ligands.…”

“…Interestingly, the outcome of the ligand‐exchange reaction of B 2 Br 4 (SMe 2 ) 2 with CAAC Me exclusively yielded bromide‐bridged [B 2 Br 3 (CAAC Me ) 2 ]Br ( δ =7.8 ppm, Scheme c), whereas the direct addition of CAAC Me to B 2 Br 4 affords the classical diborane(4) bis‐adduct only ( δ =−2.3 ppm, Scheme b) . Isolated samples of [B 2 Br 3 (CAAC Me ) 2 ]Br and B 2 Br 4 (SMe 2 ) 2 are thermally stable in toluene solutions up to 110 °C, thus precluding interconversion of the two species.…”

“…Following a computational study by Frenking and Jones on the suitability of B 2 X 4 (IMe) 2 adducts (X=H, Cl; IMe=1,3‐dimethylimidazol‐2‐ylidene) as precursors for B 2 X 2 (IMe) 2 diborenes and the linear B 2 (IMe) 2 diboryne, our group showed that B 2 Br 4 (IDip) 2 could indeed be selectively reduced first to the corresponding diborene, B 2 Br 2 (IDip) 2 and ultimately to the first isolable diboryne, B 2 (IDip) 2 (Scheme a) . Since then a handful of symmetrical low‐valent diboron compounds have been synthesized by reduction of B 2 Br 4 L 2 precursors (L=carbene), including a diboracumulene stabilized by strongly π‐accepting cyclic (alkyl)(amino)carbene (CAAC) ligands (Scheme b) …”

Titanium Imido Complexes by Displacement of –SiMe3 and C–H Bond Activation in a TiIII Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

“…Inspired by recent advances in the utilization of strong σ-donating and π-accepting cyclic (alkyl)(amino) carbenes (cAACs) 11 as ligands to stabilize highly reactive main group species, such as phosphorous and boron-based radicals, borylene (:BH), and the B≡B fragment, 12 and unusually low-coordinate complexes M(cAAC) 2 (M = Au, Cu, Co, Fe, Ni, Mn, Zn), 13 attempts were made to synthesize low-valent titanium imido complexes supported by cAAC ligands.…”