Aminoboratabenzenes. An Evaluation of the Exocyclic B−N Interaction

Abstract

The reaction of 1-chloro-1-boracyclohexa-2,5-diene with diisopropylamine or N-benzyl-N-methylamine followed by reaction with LDA in THF affords lithium N,N-diisopropyl-1-aminoboratabenzene (9b) or lithium N-benzyl-N-methyl-1-aminoboratabenzene (9c), respectively. The reaction of 9b with FeCl2 affords bis(N,N-diisopropyl-1-aminoboratabenzene)iron (18b), while the reaction of 9b or 9c with Mn(CO)3(CH3CN)3PF6 affords tricarbonyl[N,N-diisopropyl-1-aminoboratabenzene)manganese (19b) or tricarbonyl-1-(N-benzyl-N-methylamino) boratabenzene manganese (19c), respectively. Photochemical displacement of CO by PMe3 from 19b affords dicarbonyl(trimethylphosphine) (N-benzyl-N-methyl-1-aminoboratabenzene)manganese (20b). The rates of bond rotation about the B−N bonds of 9c, 18b, 19b,c, and 20b have been determined using variable-temperature NMR spectroscopy. The rates depend markedly on the electron-withdrawing power of the coordinating metal group. The boron atom is weakly coordinated to manganese (B−Mn = 2.485(3) Å) and strongly bound to nitrogen (B−N = 1.417 (3) Å).

A Highly Effective Ruthenium System for the Catalyzed Dehydrogenative Cyclization of Amine–Boranes to Cyclic Boranes under Mild Conditions

“…Ac omparative 15 NNMR study of complex 23 with diamine 24,amine-boranes 21 and 25,and complex 26,which possesses ac arbon chain of similarl ength attached to the nitrogen (the Supporting Information, Figures S23-S29), enabled the unambiguous characterization of the pendant NHMe moiety at d = À358 ppm (Table 3), hence validating the structure of 23 in solution.…”

“…The same feature was observedf or the boron-bonded hydrides. Consequently,t he magnetic nonequivalence observed at room temperature for the four hydrides in these complexes is more probably due to as low rotationa bout the BÀNb ond on the NMR timescale, [15] than the result of an intramolecular interaction as depicted in Figures S3, S7, and S12) obtained by 1 H{ 31 P: 11 B} NMR line shape simulations from the two sets of hydride signals over as ignificant range of temperature, allowed us to determine as et of thermodynamic parameters (DG°c oal: , DH°,a nd DS°)a nd to estimatet he activation energy (E a ), respectively,for each complex ( Table 2). This unprecedented study on the BÀNr otational barriers in bis-s-aminoborane ruthenium complexes is discussed further below.…”

“…The same feature was observed for the boron‐bonded hydrides. Consequently, the magnetic nonequivalence observed at room temperature for the four hydrides in these complexes is more probably due to a slow rotation about the BN bond on the NMR timescale,15 than the result of an intramolecular interaction as depicted in Figure 2. Experimentally, the Eyring and Arrhenius plots of the rate constants (the Supporting Information, Figures S3, S7, and S12) obtained by 1 H{ 31 P: 11 B} NMR line shape simulations from the two sets of hydride signals over a significant range of temperature, allowed us to determine a set of thermodynamic parameters (Δ${G{{{\ne}\hfill \atop {\rm coal}.\hfill}}}$ , Δ H ≠ , and Δ S ≠ ) and to estimate the activation energy ( E a ), respectively, for each complex (Table 2).…”

A Binuclear 1,1′‐Bis(boratabenzene) Complex: Unprecedented Intramolecular Metal–Metal Communication through a B−B Bond

“…[2] As seen in Figure 1t his interaction is typically conceived as either a) a borabenzene system, coordinating aLewis base in an effort to stabilize the vacant porbital on boron, [3] or b) ab oratabenzene system, in which atraditional covalent bond between the boron and the exocyclic substituent exists,r esulting in af ormal negative charge for the aromatic system. [4] While borabenzene species are typically thought to feature full 6 pelectron delocalization over all five carbons and the boron atom of the heteroatomic ring, the anionically charged boratabenzene systems feature delocalization primarily across the five carbon atoms of the ring and only partially interact with the boron substituent. Accordingly,b oratabenzenes typically feature B-bound substituents which tend to reduce the electron deficiencyoft he boron atom.…”

Ein zweikerniger 1,1′‐Bis(boratabenzol)‐Komplex: beispiellose intramolekulare Metall‐Metall‐Kommunikation durch eine B‐B‐Bindung

“…[2] Wiei nA bbildung 1g ezeigt, kann diese Wechselwirkung aufgefasst werden als a) ein Borabenzol-System, bei dem eine Lewis-Base das freie p-Orbital am Boratom koordinativ absättigt, [3] sowie als b) ein Boratabenzol-System, bei dem eine kovalente Bindung zwischen dem Boratom und dem exocyclischen Substituenten besteht, woraus eine negative Ladung für das aromatische System resultiert. [4] Während Borabenzolen eine 6 p-Elektronen-Delokalisationü ber den gesamten Ring zugeschrieben wird, erfolgt die Delokalisierung bei den anionischen Boratabenzolen hauptsächlich über die fünf Kohlenstoffatome und nur teilweise über das Boratom. Dementsprechend tragen die Boratome in Boratabenzolen typischerweise elektronendonierende Substituenten, um das Elektronendefizit am Boratom zu kompensieren.…”