Synthesis and Structural Characterization of Phosphine–Borane-Stabilized Dicarbanions with either Rigid or Flexible Spacers

Abstract

The reaction between 1,2-C6H4(CH2Cl)2 and 2 equiv of in situ generated [R2P(BH3)]Li in THF gives the corresponding o-phenylene-bridged bis(phosphine–boranes) 1,2-C6H4{CH2P(BH3)R2}2 (R = iPr (1a), Ph (2a), Cy (3a)). Treatment of 1a–3a with 2 equiv of nBuLi and 2 equiv of tmeda yields the corresponding phosphine–borane-stabilized carbanion (PBC) complexes [1,2-C6H4{CHP(BH3)R2}2][Li(tmeda)]2·nL (R = iPr, n = 0 (1b); R = Ph, nL = THF (2b); R = Cy, nL = 2PhCH3 (3b)). In contrast, treatment of 1a with 2 equiv of MeK, followed by 2 equiv of pmdeta, yields the monodeprotonation product [1,2-C6H4{CHP(BH3)iPr2}{CH2P(BH3)iPr2}][K(pmdeta)] (1c), due to a competing side reaction with the solvent. Treatment of 1a and 3a with the less aggressive metalating agent PhCH2K gives the corresponding dipotassium salts, the latter of which was isolated as the adduct [1,2-C6H4{CHP(BH3)Cy2}2][K(pmdeta)]2 (3c). X-ray crystallography reveals that 1b–3b adopt similar structures in which the lithium ions are coordinated by the carbanion centers and the borane hydrogen atoms of the phosphine–borane-stabilized carbanions. The potassium ion in 1c is coordinated by the carbanion center and by B–H···K contacts with both borane groups, whereas the two potassium ions in 3c exhibit multihapto interactions with the aromatic ring of the PBC ligand, along with B–H···K contacts. The reaction between ClSiMe2CH2CH2SiMe2Cl and 2 equiv of in situ generated [R2P(BH3)CH2]Li gives the bis(phosphine–boranes) [CH2SiMe2CH2P(BH3)R2]2 (R = Me (4a), Ph (5a)). Treatment of 4a or 5a with 2 equiv of nBuLi in THF readily yields the 1,6-dicarbanion complexes [CH2SiMe2CHP(BH3)R2]2[Li(THF)2]2 (R = Me (4b), Ph (5b)). A similar reaction of 5a, 2 equiv of PhCH2K, and 2 equiv of pmdeta in THF gives the potassium complex [CH2SiMe2CHP(BH3)Ph2]2[K(pmdeta)]2 (5c). Complex 5b adopts a linear structure in the solid state, while 5c adopts an unusual polycyclic structure by virtue of bridging K···H–B–H···K contacts.

Synthesis and structure of an acyclic dialkylstannylene

“…The lithium ion is bound by the carbanion centre and one of the hydrogen atoms of the BH 3 group to give a pseudo-four-membered chelate ring, and the coordination of Li is completed by two nitrogen atoms from a molecule of TMEDA, affording a pseudo-tetrahedral geometry about the lithium ion. The Li-C(1) distance [2.213(3) Å] is similar to the corresponding distances in previously reported lithium complexes of phosphine-borane-stabilised carbanions (Izod et al, 2004(Izod et al, , 2006c(Izod et al, , 2007(Izod et al, , 2010(Izod et al, , 2014, while the Li…H(1B) distance [2.02(2) Å] lies in the normal range for η 1 -BH n …Li contacts (Kimblin et al, 2000;Franz et al, 2011), although we note that such contacts are rare in comparison with η 2 -and η 3 -BH n …Li interactions. The carbanion centre adopts a pyramidal geometry (sum of angles in the CHSiP framework = 350.7°).…”

“…The 1 H, 13 C{ 1 H}, 11 B{ 1 H}, 31 P{ 1 H}, and 7 Li{ 1 H} NMR spectra of 6a are typical of such species: the 31 P{ 1 H} NMR spectrum consists of a broad quartet at 33.4 ppm, while the 11 B{ 1 H} NMR spectrum consists of a broad doublet at -39.6 ppm. We have previously noted that α-metalation of phosphine-borane adducts results in a significant increase in the 11 B-31 P coupling constant; consistent with this, the 11 B-31 P coupling constants of 5 and 6a are 59 and 90 Hz, respectively (Izod et al, 2004(Izod et al, , 2006b(Izod et al, ,c, 2007(Izod et al, , 2010(Izod et al, , 2014.…”

Boron and other Triel Lewis Acid Centers: From Hypovalency to Hypervalency

“…[16] Thus, the sp 2 trivalent electron-deficient B atom that does not obey the octet rule changes its electron structure and for the product this rule is obeyed. [18] Frustrated Lewis pairs, which are inter-and intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C 6 F 5 ) 2 moieties, were analyzed, and the possibility of tetravalency of the B centers was considered. Interaction of the ZF 3 trigonal structure with a single HCN or N 2 ligand may lead to a tetrahedral structure with the tetravalent triel atom, whereas interaction of ZF 3 with two ligands results in a trigonal bipyramidal structure with a pentacoordinate Z atom.…”

“…[13] The hydridic character of the hydrogen atoms of the ÀBH 3 group attached to the Lewis base centers, that is, at which the boron is tetravalent, was analyzed recently in numerous structures. [18] Frustrated Lewis pairs, which are inter-and intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C 6 F 5 ) 2 moieties, were analyzed, and the possibility of tetravalency of the B centers was considered. [19] The combinations of the boron moieties in frustrated Lewis pairs are often the subject of investigations, and tetravalent boron is often the Lewis acid center in such systems.…”

Investigation of interactions in Lewis pairs between phosphines and boranes by analyzing crystal structures from the Cambridge Structural Database

“…The phosphine-borane pairs, especially frustrated ones, have found great use in different domains: from synthesis and catalysis to hydrogen storage and materials (Imamoto, 1993;Imamoto et al, 1992;Mimeau et al, 2004;Denis et al, 2003;Heiden et al, 2011;Izod et al, 2014;Ö zgö n, Chen et al, 2016;Lim et al, 2013;Thomas & Peters, 2004;Welch et al, 2007;Agou et al, 2006;Lough & Stephan, 2009). Many reports deal with: calculations on phosphine-borane systems, particularly their interaction energies and related distances between acid and base centres; the influences of the substituents on the boron and phosphorus atoms on their reactivity; the catalytic reaction mechanisms, as well as the importance of non-covalent interactions (Ö zgö n, Ye et al, 2016;Skancke & Skancke, 1996;Bannwarth et al, 2015;Horvá th et al, 2003;Rokob et al, 2009;Wang et al, 2010;Gao et al, 2011;Wu et al, 2012).…”