Allyl and Pentadienyl Carbanion Complexes of Alkali Metals: Metal- and Functionality-directed Structure and Bonding

Abstract

Five ansa-tris(allyl) complexes [(PhSi{C3H3(SiMe3)}3)(Li•tmeda)3] (2.1), [(MeSi{C3H3(SiMe3)}3)(Li•pmdeta)3] (2.2), [(MeSi{C3H3(SiMe3)}3)-(Na.tmeda)3] (2.3), [(PhSi{C3H3(SiMe3)}3)(Na•tmeda)2Na]2 [2.4]2 and [(MeSi-{C3H3(SiMe3)}3)(K•OEt2)2(KLi{OtBu})]2 [2.5]2 have been synthesised, and studied by X-ray crystallography and NMR spectroscopy. A collaboration was undertaken to study some of the complexes by DFT. Crystallographic studies have shown that the overall structure of the complex is dependent on a combination of several factors: the metal cation; the substituent on the central silicon atom for the ansa-tris(allyl) ligands; and the co-ligand, tmeda or pmdeta. (tmeda = N,N,N',N'¬-tetramethylethylenediamine and pmdeta = N,N,N',N',N''-pentamethyldiethylene-triamine). Solution studies of the ansa-tris(allyl) complexes showed that the solid-state structures are maintained in solution. The first examples of donor-functionalised allyl pro-ligands have been synthesised and coordinated to a variety of s-block metals; [Li{(SiMe3)2C3H2(1-CH2C4H7O)}]2 [4.1]2, [Li{(SiMe3)2C3H2(1-CH2CH2OCH3)}]2 [4.2]2, [(thf)K{(SiMe3)2C3H2(1-CH2C4H7O)}]2 [4.5]infinity and [Mg{(SiMe3)2C3H2(1-CH2C4H7O)}2] (4.6). As with the ansa-tris(allyl) complexes, both X-ray crystallographic and NMR spectroscopy studies have been undertaken, and the structures of the donor-functionalised allyl complexes were found to be dependent on the metal cation, with each cation coordinated in a different manner by the allyl ligand. For the potassium allyl complex 4.5 there is complete delocalisation of the allyl negative charge, and it is η3-coordinated in a polymeric structure. However for lithium complexes, [4.1]2 and [4.2]2, the donor-functionalised allyl ligand is η2-coordinated, and the negative charge is only partially delocalised. The magnesium complex 4.6 has the allyl ligand coordinated via a sigma-bond to the metal and the allyl has localised single and double bonds.Finally, the synthesis of the first two donor-functionalised pentadienyl ligands and their lithium complexes are reported. Complexes [(tmeda)Li{1,5-(SiMe3)2C5H4(CH2C4H7O)}] (6.1) and [(tmeda)Li{1,5-(SiMe3)2C5H4(CH2CH2OCH3)}] (6.2) are the first structurally characterised lithium pentadienyl complexes, and are the first donor-functionalised pentadienyl complex of any metal. As well as structural characterisation, complexes 6.1 and 6.2 have been investigated by NMR spectroscopy and collaborative DFT studies. X-ray crystallography revealed that both complexes have the W-conformation of the pentadienyl ligand η2-coordinated to the lithium cation, as well as the ether oxygen atom and the tmeda nitrogen atoms. DFT studies showed that the most stable gas-phase structure of the 1,5-bis(trimethylsilyl)-pentadienyl anion is the W-conformation, but its lithium complex is most stable in the U-conformation. The [Li{1,5-(SiMe3)2C5H4(CH2CH2OCH3)}]- anion has the W-conformation and the U-conformation is isoenergetic, but the addition of tmeda gives the W-conformation as the most stable in both the gas-phase and in toluene. Finally NMR spectroscopy studies showed that in solution complexes 6.1 and 6.2 are either in the symmetrical U-conformation or in fluxional process with a very low activation energy.

Date of Award	1 Aug 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Richard Layfield (Supervisor)