Synthesis, Structure, and Solution Behavior of a Phosphine−Borane-Stabilized 1,3-Dicarbanion

The reaction between the phosphine−borane {(Me3Si)CH2}2P(BH3)Ph (2) and 2 equiv of n-BuLi in THF yields the 1,3-dicarbanion complex (THF)2Li{(Me3SiCH)2P(BH3)Ph}Li(THF)3 (3), which crystallizes as an unusual contact ion pair. In solution 3 is subject to dynamic exchange which interconverts the two lithium environments

Acyclic Dialkylstannylene and -Plumbylene Compounds That Are Monomeric in the Solid State

The new phosphine-borane adduct (Me2PhSi)CH2P(BH3)Me2 is prepared by the reaction between Me2PhSiCl and in situ-generated Me2P(BH3)CH2Li; the adduct undergoes clean deprotonation on treatment with n-BuLi to give the phosphine-borane-stabilized carbanion complex [(Me2PhSi){Me2P(BH3)}CH]Li. The reaction between 2 equiv of [(Me2PhSi){Me2P(BH3)}CH]Li and either Cp2Sn or Cp2Pb gives the acyclic dialkylstannylene and -plumbylene compounds rac-[(Me2PhSi){Me2P(BH3)}CH]2E [E = Sn (13), Pb (14)]. Similarly, the reaction between 2 equiv of [(Me3Si){Me2P(BH3)}CH]Li and either Cp2Sn or Cp2Pb yields rac-[(Me3Si){Me2P(BH3)}CH]2E [E = Sn (15), Pb (16)]. X-ray crystallography reveals that compounds 13−16 crystallize as discrete monomers that are stabilized by two agostic-type B−H···E contacts in each case; multielement NMR spectroscopy and UV/visible spectroscopy indicate that these agostic-type contacts are preserved in solution. DFT calculations reveal that these B−H···E contacts stabilize compounds 13−16 by between 38.0 and 43.7 kcal mol−1. Calculations suggest that the dimerization of 15, which is isoelectronic with the archetypal dialkylstannylene {(Me3Si)2CH}2Sn, to the corresponding distannene [(Me3Si){Me2P(BH3)}CH]2SnSn[CH{P(BH3)Me2}(SiMe3)]2 (152) is disfavored by some 30.5 kcal mol−1

Alkaline Earth Metal Complexes of a Phosphine-Borane-Stabilized Carbanion: Synthesis, Structures, and Stabilities

The reaction between either MgI2 or CaI2 and 2 equiv of [(Me3Si)2{Me2(H3B)P}C]K (2) in toluene gives the corresponding organo-alkaline earth metal compounds [(Me3Si)2{Me2(H3B)P}C]2M in moderate to good yields [M = Mg (3), Ca (4)]. Compound 3 crystallizes solvent-free, whereas X-ray quality crystals of 4 could not be obtained in the absence of coordinating solvents; crystallization of 4 from cold methylcyclohexane/THF gives the solvate [(Me3Si)2{Me2(H3B)P}C]2Ca(THF)4 (4a). The corresponding heavier alkaline earth metal complexes [(Me3Si)2{Me2(H3B)P}C]2M(THF)5 [M = Sr (7), Ba (8)] are obtained from the reaction between MI2 and 2 equiv of 2 in THF, followed by recrystallization from cold methylcyclohexane/THF. Compound 3 degrades over a period of several weeks at room-temperature both in the solid state and in toluene solution to give the free phosphine-borane (Me3Si)2{Me2(H3B)P}CH (5) as the sole phosphorus-containing product. In addition, compounds 3, 4, and 4a react rapidly with THF in toluene solution, yielding 5 as the sole phosphorus-containing product; in contrast, compounds 7 and 8 are stable toward this solvent

Heavier Alkali Metal Complexes of a Silicon- and Phosphine-Borane-Stabilized Carbanion

Treatment of the phosphine-borane adduct (Me3Si)2{Me2P(BH3)}CH (5) with either MeNa or MeK in diethyl ether yields the corresponding alkali metal salts [[(Me3Si)2{Me2P(BH3)}C]ML]∝ [ML = Na(THF)2·1/2PhMe (6a); K (7)] after recrystallization. Metathesis reactions between in situ prepared [(Me3Si)2{Me2P(BH3)}C]Li and either rubidium or cesium 2-ethylhexoxide give the compounds [[(Me3Si)2{Me2P(BH3)}C]ML]n [ML = Rb, n = ∞ (10); Cs(pmdeta), n = 2 (11)] after recrystallization. Compounds 6a, 7, 10, and 11 are the first heavier alkali metal complexes of a phosphine-borane-stabilized carbanion to be isolated in the solid state. Compounds 5, 6a, 7, 10, and 11 have been characterized by elemental analyses, multielement (1H, 13C{1H}, 11B{1H}, and 31P{1H}) NMR spectroscopy, and X-ray crystallography. In the solid state compounds 6a, 7, 10, and 11 adopt dimeric or polymeric structures in which the alkali metal cations are associated with both the carbanion centers and the borane hydrogen atoms

Metalation of Trimethylphosphine−Borane. Unexpected Reactivity of a Simple Phosphine−Borane-Stabilized Carbanion toward Siloxanes

The reaction between trimethylphosphine−borane (3) and 1 equiv of MeNa in diethyl ether gives the polymeric complex [{Me2P(BH3)CH2}Na(THF)]∞ (5a) in good yield after recrystallization from THF. In contrast, the reaction between 3 and 1 equiv of n-BuLi in diethyl ether gives the unusual complex {Me2P(BH3)CHSiMe2OLi}4Li4(Et2O)2.75(THF)1.25 (6), which crystallizes as a cluster containing [Me2P(BH3)CHSiMe2O]2- siloxy-functionalized carbanions formed via the reaction between the α-lithiated intermediate {Me2P(BH3)CH2}Li (4) and silicone grease. Although the intermediate 4 could not be isolated, reactions between in situ-generated 4 and sources of Me2SiO (Dow Corning silicone grease, silicone oil, hexamethylcyclotrisiloxane (8)) proceed cleanly to give good yields of 6; no reaction was observed between 4 and hexamethyldisiloxane. Reactions between 4 and 8 generate half an equivalent of 3 per equivalent of 4 consumed, and the cluster 6 reacts with excess siloxane to give 3 and unidentified products that contain silicon, but not phosphorus; thus, the optimum ratio of 4 to 8 in these reactions is 6:1. Variable-temperature 1H, 7Li, 11B{1H}, and 31P{1H} NMR spectroscopy suggests that 6 is subject to dynamic equilibria in solution that involve deaggregation of the cluster

Seven-Membered Cyclic Dialkylstannylene and -Plumbylene Compounds Stabilized by Agostic-type B−H···E Interactions [E = Sn, Pb]

The reaction between [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 and either Cp2Sn or Cp2Pb in toluene cleanly gives the compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E in moderate yield [E = Sn (10), Pb (11)]. NMR spectra of crude samples indicate that 10 is predominantly formed as the rac isomer; for 11 there is no evidence for the formation of the meso diastereomer at all. Crystallization from diethyl ether yields the solvates 10·Et2O and 11·Et2O; X-ray crystallography reveals that these compounds crystallize as discrete rac-dialkylstannylene and -plumbylene species in which there are two short agostic-type B−H···E contacts. A DFT study suggests that these agostic-type interactions stabilize 10 and 11 by 47.7 and 42.7 kcal mol−1, respectively. Calculations on the corresponding meso diastereomers of 10 and 11 suggest that the tin and lead centers in these compounds have close contacts to just one hydrogen atom of a BH3 group, although a second, weaker B−H···E contact is observed in each case. These contacts afford an overall stabilization of 41.3 and 32.6 kcal mol−1, respectively, for the meso-stannylene and -plumbylene

Seven-Membered Cyclic Dialkylstannylene and -Plumbylene Compounds Stabilized by Agostic-type B−H···E Interactions [E = Sn, Pb]

The reaction between [[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]Li(THF)3]2 and either Cp2Sn or Cp2Pb in toluene cleanly gives the compounds rac-[{Me2P(BH3)}(Me3Si)C{(SiMe2)(CH2)}]2E in moderate yield [E = Sn (10), Pb (11)]. NMR spectra of crude samples indicate that 10 is predominantly formed as the rac isomer; for 11 there is no evidence for the formation of the meso diastereomer at all. Crystallization from diethyl ether yields the solvates 10·Et2O and 11·Et2O; X-ray crystallography reveals that these compounds crystallize as discrete rac-dialkylstannylene and -plumbylene species in which there are two short agostic-type B−H···E contacts. A DFT study suggests that these agostic-type interactions stabilize 10 and 11 by 47.7 and 42.7 kcal mol−1, respectively. Calculations on the corresponding meso diastereomers of 10 and 11 suggest that the tin and lead centers in these compounds have close contacts to just one hydrogen atom of a BH3 group, although a second, weaker B−H···E contact is observed in each case. These contacts afford an overall stabilization of 41.3 and 32.6 kcal mol−1, respectively, for the meso-stannylene and -plumbylene

Acyclic Dialkylstannylene and -Plumbylene Compounds That Are Monomeric in the Solid State

The new phosphine-borane adduct (Me2PhSi)CH2P(BH3)Me2 is prepared by the reaction between Me2PhSiCl and in situ-generated Me2P(BH3)CH2Li; the adduct undergoes clean deprotonation on treatment with n-BuLi to give the phosphine-borane-stabilized carbanion complex [(Me2PhSi){Me2P(BH3)}CH]Li. The reaction between 2 equiv of [(Me2PhSi){Me2P(BH3)}CH]Li and either Cp2Sn or Cp2Pb gives the acyclic dialkylstannylene and -plumbylene compounds rac-[(Me2PhSi){Me2P(BH3)}CH]2E [E = Sn (13), Pb (14)]. Similarly, the reaction between 2 equiv of [(Me3Si){Me2P(BH3)}CH]Li and either Cp2Sn or Cp2Pb yields rac-[(Me3Si){Me2P(BH3)}CH]2E [E = Sn (15), Pb (16)]. X-ray crystallography reveals that compounds 13−16 crystallize as discrete monomers that are stabilized by two agostic-type B−H···E contacts in each case; multielement NMR spectroscopy and UV/visible spectroscopy indicate that these agostic-type contacts are preserved in solution. DFT calculations reveal that these B−H···E contacts stabilize compounds 13−16 by between 38.0 and 43.7 kcal mol−1. Calculations suggest that the dimerization of 15, which is isoelectronic with the archetypal dialkylstannylene {(Me3Si)2CH}2Sn, to the corresponding distannene [(Me3Si){Me2P(BH3)}CH]2SnSn[CH{P(BH3)Me2}(SiMe3)]2 (152) is disfavored by some 30.5 kcal mol−1

Synthesis, Crystal Structure, and Solution Behavior of a Sterically Hindered α-Metalated Phosphine-Borane

Treatment of the phosphine-borane adduct (Me3Si)2CHPMe2(BH3) (4) with n-BuLi in THF, followed by recrystallization from diethyl ether/methylcyclohexane, gives (THF)3Li{(Me3Si)2CPMe2(BH3)}2Li (5), which crystallizes as a contact ion multiple ate complex. In solution 5 is subject to a dynamic equilibrium between the ate complex and a second, possibly monomeric species

Alkaline Earth Metal Complexes of a Phosphine-Borane-Stabilized Carbanion: Synthesis, Structures, and Stabilities

The reaction between either MgI2 or CaI2 and 2 equiv of [(Me3Si)2{Me2(H3B)P}C]K (2) in toluene gives the corresponding organo-alkaline earth metal compounds [(Me3Si)2{Me2(H3B)P}C]2M in moderate to good yields [M = Mg (3), Ca (4)]. Compound 3 crystallizes solvent-free, whereas X-ray quality crystals of 4 could not be obtained in the absence of coordinating solvents; crystallization of 4 from cold methylcyclohexane/THF gives the solvate [(Me3Si)2{Me2(H3B)P}C]2Ca(THF)4 (4a). The corresponding heavier alkaline earth metal complexes [(Me3Si)2{Me2(H3B)P}C]2M(THF)5 [M = Sr (7), Ba (8)] are obtained from the reaction between MI2 and 2 equiv of 2 in THF, followed by recrystallization from cold methylcyclohexane/THF. Compound 3 degrades over a period of several weeks at room-temperature both in the solid state and in toluene solution to give the free phosphine-borane (Me3Si)2{Me2(H3B)P}CH (5) as the sole phosphorus-containing product. In addition, compounds 3, 4, and 4a react rapidly with THF in toluene solution, yielding 5 as the sole phosphorus-containing product; in contrast, compounds 7 and 8 are stable toward this solvent