Synthesis, crystal structures and reactions of sila- and germa-β-diketiminates

Abstract

Addition of ArCN or PhCN to LiSiR₃(thf)_n (n = 0 or 3) or LiGeR₃(thf)₃ gave the lithium 3-sila- or 3-germa-β-diketiminates [Li{(N(R)C(Ar))₂SiR}D] 1 or [Li{(N(R)C(Ph))₂GeR}]₂2, respectively [Ar = 2,6-Me₂C₆H₃; R = SiMe₃; D = (thf)₂ (1a), NCAr (1b), or (NCAr)₂ (1c)]. Compound 1a with an equivalent portion of the heavier alkali metal tert-butoxide gave the appropriate dinuclear metal complex [M{(N(R)C(Ar))₂SiR}]D]₂ [M = Na, D absent (3), or M = Rb, D = NCAr (5)] or [K{μ-(N(R)C(Ar))₂SiR}]₂4; while 1a with Hg₂Cl₂ gave Hg[SiR{C(Ar)=NR}₂]₂6. Thus, the ligand is (i) N,N′-chelating in the mononuclear lithium complexes 1a–1c, (ii) Si-terminally bonding in the Hg(II) complex 6, (iii) not only N,N′-chelating but also Si- (3, 5) or Ge- (2) bridging in the Na, Rb or Li dinuclear complexes, and (iv) N,N′,Si-bridging in the K complex 4. Treatment of the lithium 3-sila-β-diketiminate 1a with H–C₅H₅, Cl–TiCp₂Cl, Br–CH₂CH₂Br or Br–HgBr, Me₃Si–Cl, or Me₃Sn–Cl yielded the appropriate silane Si(R)X[C(Ar)=NR]₂ (X = H 7, Cl 8, Br 9, SiMe₃10 or SnMe₃11); whereas with two equivalents of methanol the product was Si(R)OMe[C(Ar)=NR][C(H)(Ar)–N(H)R] 12. The reaction between 2 and half an equivalent of NiCl₂ or PdCl₂ afforded the digermane [Ge(R){C(Ph)NR}₂]₂13. The X-ray structures of the crystalline compounds 1b, 5, 6, 8, 9, 11 and 13 are reported; those for 1c, 2, 3 and 4 were described in preliminary publications. For the four latter, as well as 5, it is evident that the alkali metal 3-sila- or 3-germa-β-diketiminates are zwitterionic (unlike the 3-carba-π-delocalised analogues) having significant negative charge localised at the 3-Si or 3-Ge atom and localised CN double and E–N (E = Si or Ge) single bonds. Reaction pathways and solution NMR data are discussed.