Phosphine-borane-stabilised carbanion complexes of main group elements

Abstract

An array of phosphorus-stabilised-carbanion complexes of group 1, 2 and 14 elements and their behaviour in the solid state is reviewed, along with a brief overview of the chemistry of phosphine-boranes, stannyl-stannylenes, heteroleptic stannylenes and oxidative addition. The synthesis and characterisation of a range of novel phosphine-borane precursors is described: [Me2P(BH3)CH2SiMe2CH2]2 (1), [Ph2P(BH3)CH2Si(Me2)CH2]2 (2), 1,2- C6H4{CH2PiPr2(BH3)}2 (3), 1,2-C6H4{CH2PPh2(BH3)}2 (4), 1,2-C6H4{CH2PCy2(BH3)}2 (5), C6H5CH2PiPr2(BH3) (6), C6H5CH2PPh2(BH3) (7), C6H5CH2PCy2(BH3) (8), C6H5{CH2SiMe3PCy2(BH3) (9), {CH2CH2PCy2(BH3)}2 (10). Treatment of 1 and 2 with two equivalents of nBuLi and tetramethylethylenediamine (tmeda) in THF yields the corresponding complexes [Li(tmeda)]2[Me2P(BH3)CHSi(Me2)CH2]2 (1a) and [Li(THF)2]2[Ph2P(BH3)CHSi(Me2)CH2]2 (2a), respectively, as monomeric units where the metal centre is bound to the carbanion along with agostic-type B-H···Li interactions. Similarly, the reaction between 2 and two equivalents of BnK (PhCH2K) in the presence of the pentamethyldiethylenetriamine (pmdeta), followed by recrystallisation from hot toluene yields the potassium complex [K(pmdeta)]2[CH2SiMe2CHP(BH3)Ph2]2 (2b), which has an unusual cyclic structure. The o-phenylene-bridged bis(phosphine-boranes) 3-5 undergo α-metalation with two equivalents of nBuLi and tmeda to yield the corresponding phosphine-borane-stabilised carbanions [1,2-C6H4{CHP(BH3)R2}2][Li(tmeda)]2·nL (R = iPr, n = 0 (3a); R = Ph, nL = THF (4a); R = Cy, nL = 2PhCH3 (5a)) after recrystallisation. Compounds 3a-5a adopt similar structures where each Li is positioned on the opposite sides of an essentially planar C6H4(CP)2 fragment and has interactions with the carbanion and BH3 unit. Treatment of 3-5 with two equivalents of PhCH2K and pmdeta yields the corresponding phosphine-borane-stabilised carbanions [K(pmdeta)]2[1,2-C6H4{CHPR2(BH3)}2] (R = iPr (3b), Ph (4b) and (5b)). Compound 5b crystallises as a discrete monomer, in which each potassium is bound to the carbanion, the pmdeta co-ligand, an η1 contact to the BH3 unit and has additional short contacts with an ipso carbon atom in the o-phenylene linker. The benzyl-substituted phosphine-boranes 6, 7 and 8 undergo α-metalation with one equivalent of nBuLi and tmeda to yield the corresponding phosphine-borane-stabilised carbanions [C6H5CHP(BH3)R2][Li(tmeda)] (R = iPr (6a), Ph (7a) and Cy (8a)). Compound 8a ii crystallises as a discrete molecular species with coordination between the Li and the carbanion centre, BH3 hydrogens in an η2-fashion, the hydrogen atom associated with the carbanion centre and with the coordination sphere completed by two contacts to the two nitrogen atoms in one molecule of tmeda. Treatment of compound 8 with an excess of BnK in THF results in the phosphine-borane-stabilised carbanion complex K[C6H5CHPCy2(BH3)] (8b); which crystallises as a sheet polymer, The synthesis of group 2 complexes of 6 and 8 was investigated using three different methods in which included the use of nBuLi followed by the group 2 halide, BnK followed by the group 2 halide and the use of previously prepared Bn2M (M = Ca, Sr, Ba). Treatment of in situ-generated 5a with one equivalent of Cp2Sn in toluene yields pale yellow crystals of the unusual agostically-stabilised stannyl-stannylene [[1,2- C6H4{CHP(BH3)Cy2}2]Sn]2·1½PhMe (11). Compound 11 crystallises as a discrete molecular species containing two different Sn centres with formal oxidation states of I and III, joined by a formal Sn-Sn σ-bond. Treatment of one equivalent of SnCl2 with two equivalents of in situ-generated [Li(tmeda)][C6H5CHPR2(BH3)] (R = iPr (6a) and Cy (8a)) in diethyl ether yields Sn[C6H5CHPR2(BH3)]2 (R = iPr (rac-6c) and Cy (rac-8c)). Both rac-6c and rac-8c have interactions between the Sn centre and one H atom from each of two separate BH3 units in an η1-fashion. Treatment of one equivalent of SnCl2 with one equivalent of in situ-generated Li[C6H5CHPR2(BH3)] (R = iPr (6a), Cy (8a)) in diethyl ether yields SnCl[C6H5CHPR2(BH3)] (R = iPr (6d), Cy (8d)). Treatment of a diethyl ether solution of 6c with one equivalent of methyl iodide or benzyl bromide yields the corresponding oxidative addition products [C6H5CHPiPr2(BH3)]2Sn(Me)I (12) and [C6H5CHPiPr2(BH3)]2Sn(Bn)Br (13), respectively. Compound 12 crystallises as a discrete monomer where the tin has contacts with two phosphine-borane fragments via the carbanion centre, a bond to the methyl carbon atom and a contact with an iodide in a distorted tetrahedral geometry; this compound has no agostic-type interactions between the borane hydrogens and the tin. The solution state structure of 12 was determined using multi-element and multi-nuclear NMR spectroscopy.