Amidophosphine complexes of electron-poor metals
Creators: name: Giesbrecht, Garth Ronald
Publisher: name: University of British Columbia
Year: 2009

description: The preparation of new scandium phosphine complexes that contain two hydrocarbyl groups is reported. Reaction of the amidodiphosphine ligand precursor LiN(SiMe2CH2PPri2)2 with ScCl3(THF)3 leads to the formation of ScCl2(THF)[N(SiMe2CH2PPri2)2]. Addition of alkyl lithium reagents RLi (R = Me, Et, and CH2SiMe3) results in bis(hydrocarbyl) derivatives, ScR2[N(SiMe2CH2PPri2)2]. A number of reactions with small molecules such as CO, CO2 and H2 were attempted but generally led to decomposition, or reaction at the scandium-amide bond. A chloride ligand was replaced with a cyclopentadienyl ligand to yield Sc(η5- C5H5)Cl[N(SiMe2CH2PPri2)2]. Metathesis of the chloride ligand with RLi (R = Me, Ph, BH4) yields Sc(η5-C5H5)R[N(SiMe2CH2PPri2)2]. The methyl complex was inert to H2, but found to react with amines to produce amide complexes. The borohydride complex reacts with PMe3 to form a putative scandium-hydride complex. To study the effect the amide linkage has on reactivity, ScCl2[η5-C5H3-1,3-(SiMe2CH2PPri2)2] was synthesized. Substitution of the chlorides with bulky alkyl groups (neopentyl or trimethylsilylmethyl) proceeds; however these dialkyls decompose when exposed to small molecules as a result of α-hydrogen abstraction. The preparation of new four- and five-coordinate aluminum amidodiphosphine complexes is reported. The reaction of LiN(SiMe2CH2PPri2)2 with AICI3 leads to the formation of AlCl2[N(SiMe2CH2PPri2)2]- Addition of the alkyl lithium reagents RLi (R = Me, Et) or dialkyl magnesium reagents R2Mg (R = Me, CH2Ph) yields AlR2[N(SiMe2CH2PPri2)2]. Variable temperature NMR studies are consistent with rapidly fluxional species at ambient temperature. These compounds are inert to transformation. The reaction of the lithium salt LiN(SiMe2CH2PPri2)2 with RAICI2 (R = Me, Et) produces mixtures containing predominantly Al(R)X[N(SiMe2CH2PPri2)2] (R = Me, Et). Treatment of the monoethyl derivative with excess AICI3 in an effort to generate a cationic species instead affords the adduct, AlCl2[N(SiMe2CH2PPri2)2] • AICI3. The preparation of LiCl, LiAlMe4, LiAlEt4, LiBEt4 and NaBEt4 adducts of the lithium salt of the tridentate ligand precursor LiN(SiMe2CH2PPri2)2 is reported. The reaction of HN(SiMe2CH2Cl)2 with LiPPri2 leads to the isolation of {LiN(SiMe2CH2PPri2)2}2LiCl, under certain conditions. The X-ray crystal structure shows it to exist as a 2:1 adduct with pseudo C2 symmetry in which a LiCl molecule is sandwiched between two LiN(SiMe2CH2PPri2)2 monomers. Variable temperature 31P and 7Li NMR spectroscopy indicate that the basic structural features of this compound are maintained in solution. The addition of LiAlMe4 to LiN(SiMe2CH2PPri2)2 results in the formation of {LiN(SiMe2CH2PPri2)2*LiAlMe4}2. The X-ray crystal structure of this product indicates that a 2:2 dimer of C2 symmetry is present. Variable temperature NMR studies are consistent with a highly fluxional molecule under ambient conditions. The addition of NaBEt4 to LiN(SiMe2CH2PPri2)2 affords {LiN(SiMe2CH2PPri2)2*NaBEt4}x. The X-ray crystal structure of this compound shows it to be an infinite one-dimensional polymer. Comparison of the three crystal structures illustrates that even with varying adducts (i.e., LiCl, LiAlMe4, and NaBEt4) the basic geometry of the LiN(SiMe2CH2PPri2)2 unit remains similar. Investigations on the preparation of four- and five-coordinate aluminum, gallium and indium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li2(THF)2[P2N2] ([P2N2] = [PhP(CH2SiMe2NSiMe2CH2)2PPh]) with AICI3 or GaCl3 leads to the formation of the four-coordinate species anti-MCl[P2N2] (M = Al, Ga). The X-ray crystal structure of anti-GaCl[P2N2] shows the [P2N2] ligand bound through a single phosphorus donor, resulting in the retention of the anti-conformation. The addition of AICI3, GaCl3 and InCl3 to syn-Li2(dioxane)[P2N2] yields the five-coordinate complexes syn- MCI[P2N2] (M = Al, Ga, In). The X-ray crystal structure of syn-GaCl[P2N2] reveals the coordination of both phosphorus atoms, with retention of the syn- conformation. Heating the anti- complexes results in the clean conversion to the syn- complexes, with pyramidal inversion observed at phosphorus. Barriers to pyramidal inversion (ΔG‡) were calculated to be 29.1 and 30.1 kcal/mol for aluminum and gallium, respectively; these are ~2 to 3 kcal/mol lower than that determined for the metal-free compounds anti- and syn-H2[P2N2]. The complexes syn- MCl[P2N2] (M = Al, Ga, In) were suitable starting materials for the generation of monomelic aluminum and gallium hydrides syn-MH[P2N2] (M = Al, Ga). Reduced forms of gallium and indium were also accessible; syn-MCl[P2N2] (M = Ga, In) could be reduced with KC8 to yield the dimers {syn-M[P2N2] }2 (M = Ga, In). [Scientific formulae used in this abstract could not be reproduced.]