Well‐Defined Cationic Alkyl– and Alkoxide–Aluminum Complexes and Their Reactivity with ε‐Caprolactone and Lactides

We describe the synthesis, structure, and reactivity of low‐coordinate Al–alkyl and –alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6‐(CH 2 NMe 2 )‐2‐CPh 3 ‐4‐Me‐C 6 H 2 O − ( N,O ). These complexes are derived from the ionization of neutral dialkyl Al complexes ( N,O )AlR 2 ( 1 a , R=Me; 1 b , R= i Bu), readily obtained by alkane elimination between AlR 3 and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C 6 F 5 ) 3 and [Ph 3 C][B(C 6 F 5 ) 4 ]. The reactions of 1 a , b with B(C 6 F 5 ) 3 yield complicated mixtures or decomposition products, however the ionization of the Al–diisobutyl derivative 1 b with [Ph 3 C][B(C 6 F 5 ) 4 ] affords a stable four‐coordinate AlPhBr cationic adduct [( N,O )Al( i Bu)(PhBr)] + ( 3 + ), as deduced from elemental analysis data. Complex 3 + readily coordinates Lewis bases such as THF to form the corresponding adduct [( N,O )Al( i Bu)(thf)] + ( 4 + ), and also rapidly chain‐transfers with 1‐hexene to yield the three‐coordinate Al–hexyl cation [( N,O )Alhexyl] + ( 5 + ). Both cations 3 + and 5 + slowly dimerize to form unprecedented organoaluminum dications [( N,O )AlR + ] 2 ( 3′ ++ , R= i Bu; 5′ ++ , R=hexyl) as deduced from X‐ray crystallographic analysis. Cation 3 + reacts quickly with i PrOH to form a stable Lewis acid/base adduct [( N,O )Al( i Bu)(HO i Pr)] + ( 6 + ), which constitutes the first X‐ray characterized adduct between an Al–alkyl complex and a simple ROH. The AlRO H proton in 6 + is readily abstracted by NMe 2 Ph to form the neutral isopropoxide Al complex [( N,O )Al( i Bu)(O i Pr)] ( 7 ). Upon reaction with THF, cation 6 + undergoes an intramolecular proton transfer to yield the ammonium AlTHF complex [(η 1 ‐H N,O )Al( i Bu)(O i Pr)(thf)] ( 8 b + ), in which the aminophenolate is η 1 ‐coordinated to the Al center. Cation 8 b + can then be converted to the desired Al–alkoxide derivative [( N,O )Al(O i Pr)(thf)] + ( 10 + ), by an intramolecular protonolysis reaction, as confirmed by X‐ray crystallography. The synthesized Al–alkyl cations form robust four‐coordinate adducts in the presence of cyclic esters such as ε‐caprolactone and ( d,l )‐lactide, but no insertion chemistry occurs, illustrating the poor ability of the AlR + moiety to ring‐open. In contrast, the Al–alkoxide cation 10 + polymerizes ε‐caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of ( d,l )‐lactide and L ‐lactide. Control experiments with L ‐lactide show that cation 10 + ring‐opens L ‐lactide to yield a robust five‐coordinated Allactate cation [( N,O )Al(η 2 ‐ L ‐lactate‐O i Pr)(thf)] + ( 13 + ), derived from a monoinsertion of L ‐lactide into the AlO i Pr bond of 10 + , that does not further react. Cation 13 + may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX − =bidentate monoanionic ligand and L=labile ligand) metal complex initiators.