Electropositive metal N-heterocyclic carbene complexes

Abstract

The first chapter is an introduction to the f-elements, with a focus on the synthesis and chemistry of tetravalent cerium complexes. The synthesis, characterisation and reactivity of carbenes, particularly N-heterocyclic carbenes (NHCs), and anionic-functionalised NHC ligands is discussed. The synthesis and reactivity of s-block, Group three and fblock NHC complexes is reviewed. The synthesis of the alcohol-functionalised unsaturated imidazolium proligand, [H2L]I [H2L = HOCMe2CH2(1-CH{NCHCHNiPr})], is extended to saturated imidazolinium analogues, [H2LR]X, [HOCMe2CH2(1-CH{NCH2CH2NR})]X (R = iPr, abbreviated to P; R = Mes, abbreviated to M; R = Dipp, abbreviated to D, X = Cl, I). Mono-deprotonation results in the isolation of bicyclic imidazolidines HLR, which can be ring-opened and silylated by treatment with Me3SiI, to afford [HLR, OSiMe3]I, R = iPr and Mes. Further deprotonation of HLR with MN"2 (M = Mg, Zn; N" = N(SiMe3)2) affords LRMN" (M = Mg and Zn) and ZnLR 2. These complexes proved active for the polymerisation of raclactide at room temperature without the need for an initiator. The third chapter focuses on cerium chemistry. Repetition of the literature synthesis of CeIV tert-butoxide compounds affords the unsolvated Ce(OtBu)4 and also the cluster Ce3(OtBu)11. Treatment of Ce(OtBu)4 with HL did not yield a CeIV-NHC complex, but one bearing a tethered imidazolium group, (OtBu)3Ce( -OtBu)2( -HL)Ce(OtBu)3. The synthesis of a CeIII-NHC complex, CeL3, and the oxidation of this forms an unprecedented CeIV-NHC complex, CeL4, via ligand redistribution; the complex contains two bidentate ligands and two alkoxide-tethered free NHC groups. Functionalisation of the free NHCs with boranes affords the adducts Ce(L)2(L-B)2, where B = BH3 or 9-BBN (9-Borabicyclo[3.3.1]nonane). A number of cerium complexes of the saturated-backbone NHC, LR, LRCeN"2 and LR 2CeN", have been synthesised and their oxidation chemistry and reactivity investigated. The final chapter contains an NMR study of the synthesis of a series of yttrium LR adducts, LP xYN"(3-x) (x = 1, 2 or 3), and of the synthesis of uranium complexes LRUN"2, R = Mes or Dipp, including a range of small molecule reaction chemistry. The uranyl NHC complexes, UO2LR 2, R = Mes or Dipp, have also been synthesised and characterised; these possess very high frequency carbene carbon chemical shifts.