Stabilisation of heavier carbene analogues with bulky phosphides and related systems

Abstract

An array of monodentate alkali metal phosphides was prepared, for use as precursors to diphosphatetrylenes. Treatment of (Dipp)2PH (111) with either nBuLi, PhCH2Na or PhCH2K in THF gives [(Dipp)2P]Li(THF)3 (128a), {[(Dipp)2P]Na(THF)2}2 (129a) and [(Dipp)2P]K(THF)4 (130a), respectively; the alternative adduct [(Dipp)2P]Na(PMDETA) (129c) was prepared by the treatment of 129a with PMDETA. Treatment of either (Dipp)(Mes)PH (109) or (Dipp){(Me3Si)2CH}PH (113) with nBuLi gives [(Dipp)(Mes)P]Li(THF)3 (131a) and [(Dipp){(Me3Si)2CH}P]Li(THF)3 (134), respectively. Treatment of (Mes)2PH (108) with nBuLi gives [(Mes)2P]Li(THF)3 (132a) after recrystallization from n-hexane or the alternative solvate [(Mes)2P]2Li2(THF)2(OEt2) (132b) after recrystallization from Et2O. Exposure of compounds 128a, 129a, 130a, 131a, 132a and 132b to vacuum leads to the loss of coordinated solvent, yielding the alternative solvates [(Dipp)2P]Li(THF)2 (128b), [(Dipp)2P]Na(THF)2]1.5 (129b), [(Dipp)2P]K (130b), [(Dipp)(Mes)P]Li(THF)2 (131b) and [(Mes)2P]Li(THF) (132c). Variable-temperature 7Li and 31P{1H} NMR spectroscopy indicates that 128b, 129b, 131b and 134 are subject to a monomer-dimer equilibrium in solution, while 132c is subject to a dynamic equilibrium between a dimer and cyclic trimer in solution. An array of diphosphatetrylenes was prepared by reactions between two equivalents of the corresponding lithium phosphides discussed above and the group 14 dihalide. In the solidstate, compounds {(Dipp)2P}2Sn (104Sn), {(Tripp)2P}2Ge∙(C7H14) (145Ge∙(C7H14)) and [{(Me3Si)2CH}2P]2Sn (148Sn) are stabilised by π interactions from a planar phosphorus centre, while {(Tripp)2P}2Sn (145Sn), {(Dipp)(Mes)P}2Ge∙(C6H14)0.5 (146Ge∙(C6H14)0.5), {(Dipp)(Mes)P}2Sn (146Sn) and [(Dipp){(Me3Si)2CH}P]2Sn (147Sn) adopt the alternative configuration with two pyramidal phosphorus centres, which stabilises the tetrel centre with up to two short Ge/Sn∙∙∙Cipso contacts. Variable-temperature 31P{1H} NMR spectroscopy indicates that 104Sn, 145Ge and 145Sn are subject to a dynamic equilibria in solution, which interconverts between the planar and pyramidal centre and between two possible configurations. The reaction between two equivalents of [(Mes)2P]K and GeCl2(1,4-dioxane) in THF initially gives a blue solution of the putative diphosphagermylene {(Mes)2P}2Ge (160Ge). However, this solution degrades after three days at room temperature to the diphosphine (Dipp)2P-P(Dipp)2 (150) and the tetranuclear Ge(I) cluster [{(Mes)2P}Ge]4 (162). Treatment of {(Mes)2P}2SiHCl (174) with either (Me3Si)2NLi or TMPLi gives a mixture of unidentified products. The reaction between {(Mes)2P}2SiCl2 (175) and lithium or potassium metal gives the corresponding alkali metal phosphide [(Mes)2P]M [M = Li, K] as the major product, although a small number of crystals of the potassium triphosphasilanate [{(Mes)2P}3Si]K(THF)3 (177∙THF) were isolated from the reaction with the potassium phosphide and characterised by X-ray crystallography. The reaction between four equivalents of [(Mes)2P]Li and SiBr4 in Et2O gives the tetraphosphadisilene {(Mes)2P}2Si=Si{P(Mes)2}2 (181). The reaction between two equivalents of [(Dipp)2P]K and either SiHCl3 or SiCl4 gives {(Dipp)2P}2SiHCl (189) or {(Dipp)2P}2SiCl2 (190), respectively, as impure mixtures. Compound 189 proved to be inert towards a range of strong bases, while the reduction of 190 with lithium or potassium metal in THF gives (Dipp)2P-P(Dipp)2 (150). Treatment of the base stabilised dichlorosilylene [{CH2N(Dipp)}2C]SiCl2 (191) with two equivalents of [(Dipp)2P]K gives the phosphasilene {(Dipp)2P}Si(Dipp){P(Dipp)} (192), rather than the corresponding diphosphasilylene, due to a 1,2-aryl migration. The diphosphaarsenium cation [{(Dipp)2P}2As][Al{OC(CF3)3}4] (208) was prepared by the reaction between {(Dipp)2P}2AsCl (205) and Li[Al{OC(CF3)3}4] in fluorobenzene. In the solid-state, compound 208 is stabilised by a π interaction from a planar phosphorus centre, while in solution a dynamic equilibrium is in operations, which interconverts the planar and pyramidal phosphorus centres, similarly to the diphosphagermylenes 104Ge and 145Ge. The reaction between {(Mes)2P}2PCl (213) and Na[B{3,5-(CF3)2C6H3}4] in fluorobenzene gives the cyclic diphosphaphosphonium salt [{μ-(Mes)P}2P(Mes)2][B{3,5- (CF3)2C6H3}4] (216). Compound 216 arises from a 1,2-aryl migration of the putative diphosphaphosphenium cation {(Mes)2P}2P + , followed by a cyclization. The reaction between two equivalents of [(Dipp)2As]Li and GeCl2(1,4-dioxane) or SnCl2 gives {(Dipp)2As}2Ge∙C7H8 (234Ge) and {(Dipp)2As}2Sn∙C7H8 (234Sn), respectively. Compounds 234Ge and 234Sn each adopt a configuration with two pyramidal arsenic centres in the solid state and are stabilised by two short Ge/Sn∙∙∙Cipso contacts. A series of bidentate lithium phosphides were prepared, for use as potential precursors to P-heterocyclic tetrylenes. Treatment of CH2(PHDipp)2 (246) with two equivalents of nBuLi, followed by TMEDA gives [CH2(PLiDipp)2](TMEDA)2 (257). Treatment of {CH2PHDipp)2 (253) with two equivalents of nBuLi gives the solvate iii {CH2(PLiDipp)}2(THF)(OEt2)1.5 (258b), from this material the alternative solvate {CH2(PLiDipp)}2(THF)(OEt2) (258a) was characterised by X-ray crystallography. Treatment of DippPH(CH2)nPHDipp [n = 3 (254), 4 (255), 5 (256)] with two equivalents of nBuLi in THF gives [CH2{CH2(PLiDipp)}2(THF)4 (259), {CH2CH2(PLiDipp)}2(THF)6 (260) and [CH2{CH2CH2P(Dipp)}2]Li2(THF)6(C7H8) (261a). Compound 261a rapidly loses coordinated solvent on exposure to vacuum, yielding the alternative solvate [CH2{CH2CH2(PLiDipp)}2](THF)2.5 (261b). Compounds 258b, 259, 260 and 261b exhibit complex variable-temperature 31P{1H} and 7Li NMR behaviour indicating dynamic equilibria between monomeric and oligomeric species in solution. The reaction between {CH2(PLiDipp)}2 and SnCl2 gives [{CH2(PDipp)}2Sn]2 (cis/trans-265) as a mixture of cis and trans stereoisomers, while the reaction between [CH2{CH2(PLiDipp)}2 and SnCl2 gives the cis stereoisomer of [CH2{CH2(PDipp)}2Sn]2 (cis266c). The solvates [{CH2(PDipp)}2Sn]2.Et2O (trans-265a), [CH2{CH2(PDipp)}2Sn]2.(THF)2 (cis-266b) and [CH2{CH2(PDipp)}2Sn]2.(THF)3 (cis-266c) were characterised by X-ray crystallograph