A DFT Study on the Stabilization of the B≡B Triple Bond in a Metallaborocycle: Contrasting Electronic Structures of Boron and Carbon Analogues

Abstract

The electronic structure of (η -Cp) Zr(NH -BB-NH ) (3 b) suggests that it could be a candidate for having a boron-boron triple bond in the cyclic system; however, computational studies shows that 3 b is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η -Cp) Zr[B(PH ) -BB-B(PH ) ] (3 c) reduces the relative energy dramatically. The B≡B triple bond arises through the donation of two electrons from the metal fragment, ZrCp , to the in-plane π-bonding orbital of the central B-B unit. Strong σ-donating and chelating bis-phosphine ligands (Me P(CH ) PMe ), which stabilize donor-acceptor bonding interaction in gem-diborene L B-BBr (10), would be a good choice along the synthetic path towards 3 d, (η -Cp) Zr[B (Me P(CH ) PMe ) ]. A comparison of the energetics of the reaction leading to a cyclic boryne system (3 d), with the linear boryne isomer [(B NHC ) ] shows that the angle strain from cyclization is compensated by stabilization from the metal.

Unsymmetrische, cyclische Diborene und thermische Umlagerung zu einem Borylborylen

“…Close inspection of the calculated partial charges (qB1 = -0.243, qB2 = -0.038) indicates a highly chargeseparated state in 8, in which the polarity is inversed with respect to 6 and the negative charge localized on B1. This analysis favors the borylborylene configuration proposed by Jemmis, [18] where B1 carries a lone pair. Furthermore, the frontier molecular orbitals shown in Figure 3 indicate a marked difference in the HOMO compared to the trans-diborenes.…”

“…[17] A recent computational study by Ghorai and Jemmis predicted that a simplified diborene, B2Br2(PH3)2, would be more stable as its gem isomer than as its cis and trans isomers (derivatives of both of which have been realized experimentally), while its NHC derivative favors the trans conformation. [18] The authors proposed a borylborylene type structure, involving a large donoracceptor contribution to the boron-boron bond, from the P2B unit to the BBr2 boron atom. Having isolated such a compound, we again turned to DFT for more insight into the formation and structure of 8.…”

“…Die Reaktion von 3a mit kleinen Lewis-Basen (L) führte zu deren Koordination an das verbleibende sp 2 -Boratom unter Bildung von sp 3 -sp 3 [18] In einer kürzlich verçffentlichten, theoretischen Studie zeigten Ghorai und Jemmis,dass das gem-Isomer eines vereinfachten Diborens,B 2 Br 2 (PH 3 ) 2 ,s tabiler sein sollte als die cis-u nd trans-Isomere (von denen Derivate bereits experimentell realisiert wurden), während das entsprechende NHC-Derivat die trans-Konformation bevorzugt. [19] Die Autoren schlugen eine Borylborylen-Struktur vor,i nd er die Bor-Bor-Bindung einen hohen Anteil von einer Donor-Akzeptor-Wechselwirkung zwischen der P 2 B-Einheit und dem BBr 2 -Boratom aufweist. Mit dem Vorliegen einer solchen Verbindung wandten wir uns wieder DFT-Rechnungen zu, um einen Einblick in die Bildung und Struktur von 8 zu bekommen.…”

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu)

“…Very recently, we have developed two neutral asymmetric diborene derivatives 1 a and 1 b (Figure B), which potentially possess boryl borylene‐like natures ( 1 a′ , 1 b′ ) . The groups of Braunschweig and Jemmis propose that gem ‐diborenes would be regarded as boryl borylenes …”

“…[17] The groups of Braunschweig and Jemmis propose that gem-diborenes would be regardeda sb oryl borylenes. [18,19] In this article, we present the synthesis andX -ray diffraction analysiso faseries of asymmetricd iborene metal (Au, Ag, and Cu) complexes. The coordination properties of asymmetric diborenes to these metals are systematically discussed on the basis of theoretical calculations.…”