Phosphanyl-substituted tetrylenes and ditetrelenes

Abstract

A series of aryl-substituted phosphines R2PH [R = Duryl (56), Dep (57), Xylyl (58), tBu-Xylyl (59)] were synthesised by metathesis reactions between the corresponding Grignard reagents RMgBr and PCl3, followed by reaction with LiAlH4 [Duryl = 2,3,5,6-Me4C6H; Dep = 2,6- Et2C6H3; Xylyl = 2,6-Me2C6H3, t-Bu-Xylyl = 2,6-Me2-4-tBuC6H2]. Both (Duryl)2PH (56) and (Dep)2PH (57) were characterised by X-ray crystallography. The lithium phosphides [[(Duryl)2P]Li(THF)]2 (56Li), [(Dep)2P]Li(OEt2)]2 (57Li), [[(Xylyl)2P]Li(OEt2)]2 (58Li), and [[(tBu-Xylyl)2P]Li(OEt2)]2 (59Li) were synthesised by treatment of the corresponding phosphines with n BuLi. While 57Li and 59Li were successfully characterised by X-ray crystallography as ether-solvated dimers, single crystals of satisfactory quality were unavailable for 56Li and 58Li. Variable-temperature (VT) 31P{1 H} and 7 Li NMR spectroscopy revealed that 56Li is subject to dynamic exchange in solution, possibly involving a monomer-dimer equilibrium. Conversely, 57Li and 58Li both appear to persist as dimers across the measured temperature range. The sodium and potassium phosphides [(Duryl)2P]Na(THF)0.5 (56Na) and solvent-free [(Duryl)2P]K (56K) were synthesised by metalating the phosphine 56 with PhCH2Na and PhCH2K, respectively, in THF. While 56Na is subject to dynamic exchange in solution, 56K persists as a monomer. Similarly, solvent-free [(Xylyl)2P]K (58K) was prepared by treatment of 58 with PhCH2K in THF; this compound persists as a monomer in solution, according to VT 31P{1 H} NMR spectroscopy. The digermene (Mes2P)2Ge=Ge(PMes2)2 (62) was synthesised by the reaction of two equivalents of in situ generated [(Mes)2P]Li with GeCl2(dioxane) in diethyl ether at -78°C; crystalline material of 62 was obtained from cold n-pentane. X-ray crystallography reveals an unprecedented trans-bent and highly twisted structure (62A) in the solid state. VT 31P{1 H} NMR spectroscopy of 62 indicates that at temperatures lower than 233 K, the digemene persists as the twisted conformer 62A, displaying a characteristic AA¢BB¢ coupling pattern. At higher temperatures, a conformational equilibrium between twisted 62A and non-twisted conformer 62B was observed. DFT calculations confirmed that the conformational equilibrium between 62A and 62B is the lowest energy process. Digermene 62 undergoes spontaneous decomposition to afford two equivalents of the diphosphine (Mes)2P-P(Mes)2 (60¢) and one equivalent of a unique Ge(I) cluster [{(Mes)2P}Ge]4(CyMe)5 (68) per two equivalents of 62. This decomposition process is solvent dependent and accelerates drastically in polar solvents such as THF. DFT calculations on the decomposition process suggest that decomposition occurs via a low-lying excited triplet state of 62 (62T). However, EPR spectroscopy of digermene 62 did not provide convincing evidence of 62T. UV/VIS spectroscopy was also employed to monitor the decomposition of 62 but no definitive data were collected due to the extremely sensitive nature of 62. The reaction of two equivalents of 57Li with one equivalent of GeCl2(dioxane) in diethyl ether gave digermene {(Dep)2P}2Ge=Ge{P(Dep)2}2 (73) as a deep red oil which could not be crystallised. VT 31P{1 H} NMR spectroscopy revealed that at temperatures lower than 233 K, 73 persists as a twisted conformer 73A displaying an AA¢BB¢ coupling pattern similar to that of 62A. At higher temperatures, 73 dissociates into monomeric germylene {(Dep)2P}2Ge (73D) which is subject to a further equilibrium between a germylene with one planar and one pyramidal phosphorus centre (73Dplan) and one with two pyramidal phosphorus centres (73Dpyr). Due to the monomer-dimer equilibrium, 73 does not undergo spontaneous decomposition. The disilene {(Xylyl)2P}2Si=Si{P(Xylyl)2}2 (76) was synthesised from the reaction between two equivalents of 58Li and one equivalent of SiCl2•DippNHC; single crystals were obtained from cold n-pentane. Similar to the previously reported {(Mes)2P}2Si=Si{P(Mes)2}2 (54), 76 exhibits a highly symmetrical trans-bent structure with aryl p-stacking across the Si=Si bond; however, unlike 54, 76 has good solubility in organic solvents. A heterogeneous reaction between digermene 62 and an excess of S8 in toluene yielded the decasulfide [{(Mes)2P(S)}S]2Ge(µ-S)]2 (91), while attempted coordination of 73 with {cisMo(CO)4(piperidine)2} yielded an unexpected bridged diphosphagermylene {[Dep2PClGe][GePDep2(OSiMe2(n-Bu)]} (92). The reaction between 1,1-diphenylethylene and lithium metal in THF gave the Schlenk dimerisation product [Li(THF)4][(Ph2CCH2CH2CPh2)Li(THF)] (95Li). The solid state structure of 95Li shows a separated ion pair containing an unusual dialkyllithate anion. The sodium analogue [(Ph2CCH2CH2CPh2)Na2(THF)]¥ (95Na) was synthesised by the reaction between 1,1-diphenylethylene and elemental sodium in THF. The solid state structure of 95Na shows a complex 2D polymeric structure. The potassium analogue [(Ph2CCH2CH2CPh2)K(THF)n] (95K) was not prepared using potassium metal but rather a metathesis reaction between 95Li and KOt Bu. Although 95K could not be crystallised, its 12- crown-4-ether coordinated analogue [(Ph2CCH2CH2CPh2)K2(THF)2(12-crown-4)2] (95K¢) was isolated; this compound exhibits no short contacts between the [K(12-crown-4)(THF)]+ cations and the [Ph2CCH2CH2CPh2] carbanion centres, but rather exhibits multihapto interactions between the potassium ions and the phenyl rings of the dicarbanion ligand. The 12-crown-4 coordinated lithium analogue [(Ph2CCH2CH2CPh2)][Li(THF)(12-crown-4)]2 (95Li¢) also shows no Li-C contacts, but crystallises as a separated ion pair.