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Reaction of the pro-ligand α,α,α',α'-tetra(3,5-di-tert-butyl-2-hydroxyphenyl-p-)xylene-para-tetraphenol (p-L(1)H4) with two equivalents of [NbCl5] in refluxing toluene afforded, after work-up, the complex {[NbCl3(NCMe)]2(μ-p-L(1))}·6MeCN (1·6MeCN). When the reaction was conducted in the presence of excess ethanol, the orange complex {[NbCl2(OEt)(NCMe)]2(μ-p-L(1))}·3½MeCN·0.614toluene (2·3½MeCN·0.614toluene) was formed. A similar reaction using [TaCl5] afforded the yellow complex {[TaCl2(OEt)(NCMe)]2(μ-p-L(1))}·5MeCN (3·5MeCN). In the case of the meta pro-ligand, namely α,α,α',α'tetra(3,5-di-tert-butyl-2-hydroxyphenyl-m-)xylene-meta-tetraphenol (m-L(2)H4) only the use of [Nb(O)Cl3(NCMe)2] led to the isolation of crystalline material, namely the orange bis-chelate complex {[Nb(NCMe)Cl(m-L(2)H2)2]}·3½MeCN (4·3½MeCN) or {[Nb(NCMe)Cl(m-L(2)H2)2]}·5MeCN (4·5MeCN). The molecular structures of 1-4 and the tetraphenols L(1)H4 and m-L(2)H4·2MeCN have been determined. Complexes 1-4 have been screened as pre-catalysts for the ring opening polymerization of ε-caprolactone, both with and without benzyl alcohol or solvent present, and at various temperatures; conversion rates were mostly excellent (>96%) with good control either at >100 °C over 20 h (in toluene) or 1 h (neat).

The reaction of the dialkylzinc reagents with acids 2,2-Ph2C(X)(CO2H), where X = NH2, OH, i.e. 2,2′-diphenylglycine (dpgH) or benzilic acid (benzH2), in toluene at reflux afforded the tetra-nuclear ring complexes [RZn(dpg)]4, where R = Me, Et, 2-CF3C6H4, and 2,4,6-F3C6H2

Reaction of R 1 R 2 CHN=CH(3,5-tBu 2 C 6 H 2 -OH-2) (R 1 = R 2 = Me L 1 H; R 1 = Me, R 2 = Ph L 2 H; R 1 = R 2 = Ph L 3 H) with slightly greater than one equivalent of R 3 3 Al (R 3 = Me, Et) afforded the complexes [(L 1-3 )AlR 3 2 ] (L 1 , R 3 = Me 1, R 3 = Et 2; L 2 , R 3 = Me 3, R 3 = Et 4; L 3 R 3 = Me 5, R 3 = Et 6); complex 1 has been previously reported. Use of the N,O-ligand derived from 2,2′-diphenylglycine afforded either 5 or the byproduct [Ph 2 NCH 2 (3,5-tBu 2 C 6 H 2 -O-2)AlMe 2 ] (7). The known Schiff base complex [2-Ph 2 PC 6 H 4 CH 2 (3,5-tBu 2 C 6 H 2 -O-2)AlMe 2 ] (8) and the product of the reaction of 2-diphenylphosphinoaniline 1-NH 2 ,2-PPh 2 C 6 H 4 with Me 3 Al, namely {Ph 2 PC 6 H 4 N[(Me 2 Al) 2 μ-Me]-(μ-Me 2 Al)} (9), were also isolated. For structural and catalytic comparisons, complexes resulting from the interaction of Me 3 Al with diphenylamine (or benzhydrylamine), namely {Ph 2 N[(Me 2 Al) 2 μ-Me]} (10) and [Ph 2 CHNH(μ-Me 2 Al)] 2 ·MeCN (11), were prepared. The molecular structures of the Schiff proligands derived from Ph 2 CHNH 2 and 2,2′-Ph 2 C(CO 2 H)(NH 2 ), together with those of complexes 5, 7 and 9-11·MeCN were determined; 5 contains a chelating imino/phenoxide ligand, whereas 7 contains the imino function outside of the metallocyclic ring. Complex 9 contains three nitrogen-bound Al centres, two of which are linked by a methyl bridge, whilst the [a] Scheme 1. Compounds (proligands) employed herein. Scheme 2. Complexes studied herein.1952 tBu 2 C 6 H 2 -O-2)AlR 3 2 ] (R 1 = R 2 = R 3 = Me 1; R 1 = R 2 = R 3 = Et 2; R 1 = R 3 = Me, R 2 = Ph 3; R 1 = Me, R 2 = Ph, R 3 = Et 4; R 1 = R 2 = Ph, R 3 = Me 5; R 1 = R 2 = Ph, R 3 = Et 6) and compare their behaviour against organoaluminium complexes derived from the amine component only (i.e., minus the phenoxy-containing 3,5-di-tert-butylsalicyl motif, see Schemes 1 and 2). We note that Nomura and colleagues have previously investigated the effect of the imino substituent on the ROP of ε-caprolactone (CL) and observed greatly enhanced activity for aryl substituents (C 6 F 5 , 2,6-iPr 2 C 6 H 3 ) versus aliphatic substituents (adamantyl, tert-butyl). [7] Herein, we initially targeted diphenylglycine (dpg) and derivatives thereof given the tendency of related motifs to form highly crystalline products. [9] However, the loss of CO 2 during the formation L 3 H(dpg), see discussion below, led us to explore the family of proligands with both aliphatic and aromatic substituents bound to the N-bound CH group.