Metal β-diketiminates revisited: ansa-CH2-bridged bis(β-diketiminate)s of lithium and aluminium having diverse structures

The metal β-diketiminato ligand-to-metal binding modes are briefly reviewed, with reference particularly to our previous work on metal complexes using the ligands [{N(R1)C(R2)}2CH] (R1 = SiMe3 = R and R2 = Ph; or R1 = C6H3Pri2-2,6 and R2 = Me). The syntheses of the β-diketimines H[{N(R)C(Ar)}2CH] 1 (Ar = Ph) and 2 (Ar = C6H4Me-4) and the ansa-CH2-bridged bis(β-diketimine)s 3 (Ar = Ph) and 4 (Ar = C6H4Me-4) are reported. Thus, from the appropriate compound Li[{N(R)C(Ar)}2CH] and H2O, (CH2Br)2 or CH2Br2 the product was 2, 3 or 4. Compound 1 was prepared from K[{N(R)C(Ph)}2CH] and (CH2Br)2. Each of 3 or 4 with LiBun surprisingly yielded the bicyclic dilithium compound 5 (Ar = Ph) or 6 (Ar = C6H4Me-4) in which each of the β-diketiminato fragments is an N,N′-bridge between the two lithium atoms and the CH2 moiety joins the two ligands through their central carbon atoms. However, 4 with AlMe3 yielded the expected ansa-CH2-bridged-bis[(β-diketiminato)(dimethyl)alane] 7, which was also obtained from 6 and Al(Cl)Me2. X-ray structures of the known compounds 2 and 3, and of 5, 6 and 7 are presented; the 1H NMR spectra of 6 in toluene-d8 show that there is restricted rotation about the NC–C6H4Me-4 bond.