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Despite the synthetic importance and widespread use of Grignard reagents, and the fact that magnesium alkyls have been known since the middle of the 19th century, 1 the chemistry of well characterized simple mono-alkyl magnesium systems is surprisingly undeveloped. 2 Indeed, it appears that only a handful of halide-free, uncharged mono-nuclear systems are known, these being based on pyrazolylborate, 3 carbazolyl 4 or triazinyl 5 ligands. 6 None of these systems is formally coordinatively unsaturated. Furthermore, alkyl-bridged dimeric systems are virtually unknown for simple mono-alkyl magnesium. 7,8 Our studies have been directed toward the formation of coordinatively unsaturated, and also alkyl bridged or solvent coordinated mono-alkyl magnesium complexes (i.e. systems that have a vacant coordination site or effectively a protected vacant site), these being designed as potential catalytic agents. In such systems a suitable ancilliary ligand could both stabilize the desired molecular arrangement and also influence the activity and selectivity of the reagent. A bulky β-diketiminate ligand 9 has been used to this effect in the present work. Here we report the first examples, for halide-free monoalkyl magnesium systems, of: (i) a coordinatiVely unsaturated mononuclear complex and (ii) an alkyl-bridged dimer, 7 as well as a solvent coordinated derivative.

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The Schiff base ligands [3,5-Bu t 2 -2-(OH)C 6 H 2 CH᎐ ᎐ NL] [L = CH 2 CH 2 NMe 2 (1a), 2-(PhO)C 6 H 4 (1b), 2-CH 2 C 5 H 3 N (1c), 8-C 9 H 6 N (quinoline) (1d) and 2-(PPh 2 )C 6 H 4 (1e)] are accessed in good yields (>85%) via standard imine condensation reactions. Reaction of 1a-e with Me 3 Al at room temperature affords the corresponding complexes [(3,5-Bu t 2 -2-(O)C 6 H 2 CH᎐ ᎐ NL)AlMe 2 ] (2a-e); in the case of L = 8-quinoline, the same reaction conducted in refluxing toluene affords binuclear {[3,5-Bu t 2 -2-(O)C 6 H 2 CHMeN-8-C 9 H 6 N]AlMe} 2 (3) by methyl migration from metal to ligand. Further reaction of the dimethyl compounds with B(C 6 F 5 ) 3 in CD 2 Cl 2 or C 6 D 6 affords the cationic systems [The crystal structures of 2a, 2c, 2e and 3 have been determined. In 2a and 2c the respective ligands bind to the metal centre via all three heteroatoms, the aluminium having a trigonal bipyramidal geometry, whereas in 2e coordination is via nitrogen and oxygen only, and the aluminium is tetrahedral. Complex 3 has a dimeric structure with the ligand adopting both tridentate and binucleating roles; the aluminium centres are trigonal bipyramidal.

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