FULL PAPER DOI: 10.1002/ejic.201200183 β-Diketiminato–Alkaline Earth Cationic Complexes: Synthesis, Structures, Lactide Polymerization and Unusual Oxidative Reactivity of the Ancillary Ligand Bo Liu, [a] Vincent Dorcet, [a] Laurent Maron, [b] Jean-François Carpentier,* [a] and Yann Sarazin* [a] Keywords: Calcium / Strontium / Alkaline earth metals / Ring-opening polymerization / Oxidation Well-defined (β-diketiminate)calcium and -strontium cat- ionic complexes paired with weakly coordinating anions have been prepared in high yields by straightforward proto- cols. These highly electrophilic species are efficient catalysts Introduction Interest in the organometallic and coordination chemis- try of large alkaline earth elements (Ae = Ca, Sr and Ba) has experienced a remarkable resurgence in recent years, not least because the well-defined heteroleptic complexes of these elements have displayed remarkable activity as cata- lysts in a variety of organic transformations. [1] Thus, several examples of kinetically stable neutral Ae complexes sup- ported by nitrogen-containing [2] or phenolate [3] ligands have now been reported. The β-diketiminato complex {BDI iPr }CaN(SiMe 3 ) 2 (THF) [{BDI iPr }H = H 2 C{C(Me)N- 2,6-(iPr) 2 C 6 H 3 } 2 ], which was originally developed by Chish- olm for the ring-opening polymerization (ROP) of cyclic esters, [4] occupies a prominent place in this range of hetero- leptic Ae complexes, notably owing to the pioneering work of Hill and co-workers who have employed it very success- fully to catalyze hydroamination [2d,2f,2g,5] and hydrophos- phination [6] reactions. [7] Moreover, the {BDI iPr } – frame- work has allowed for the preparation of several other stable heteroleptic Ae-based complexes that display unusual reac- tivity, as illustrated by Harder, [8] H. Roesky [9] and Hill. [10] Heteroleptic complexes of large Ae metals (ionic radii for six-coordinate centers: Ca 2+ , 1.00 Å; Sr 2+ , 1.18 Å; Ba 2+ , 1.35 Å) [11] are typically plagued by deleterious Schlenk-type equilibria in solution. In this respect, the ability of the {BDI iPr } – ligand to prevent decomposition of Ca-based [a] Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS – Université de Rennes 1, Campus de Beaulieu, 263 avenue du Général Leclerc, 35042 Rennes Cedex, France E-mail: jean-francois.carpentier@univ-rennes1.fr yann.sarazin@univ-rennes1.fr [b] LPCNO, UMR 5215 CNRS – Université de Toulouse, 135 avenue de Rangueil, 31077 Toulouse, France Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201200183. Eur. J. Inorg. Chem. 2012, 3023–3031 © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3023 for the immortal ring-opening polymerization of lactide and display unusual reactivity by providing a unique example of oxidation of a {BDI iPr } core to give a diiminoalkoxide. complexes is remarkable; however, this property could not be extended to protect complexes of the large Sr and Ba ions. In fact, whereas {BDI iPr }CaN(SiMe 3 ) 2 (THF) is stable in solution even under relatively harsh conditions, its Sr and Ba congeners are readily prone to ligand scrambling. [10b] It was only very recently that our group has described the syntheses of {BDI iPr }AeN(SiMe 2 H) 2 (THF) n (Ae = Ca, n = 1; Sr, n = 2; Ba, n = 2) compounds, which are a family of kinetically stable complexes that feature internal β-agostic Ae···H–Si interactions. [12] Well-defined cationic complexes of group 3 and 4 metals are well known, they represent potential intermediates or models of active species in, for instance, the polymerization of α-olefins. [13] By contrast, occurrences of discrete Zn, Mg and Ae cationic complexes remain rather scarce. Recent achievements [14] with discrete cations include the cyclohy- droamination of aminoalkenes catalyzed by (aminotrop- oniminato)Zn complexes, [15] the synthesis of allylcalcium cations, [16] and the ROP of cyclic esters promoted by Zn– Mg [17] and Ca–Ba [18] cationic complexes bearing neutral (bis)phosphinimine [17a–17e] or monoanionic amino ether phenolate ancillary ligands. [17f,18] Somewhat surprisingly, well-defined Ae cations supported by the ubiquitous {BDI iPr } – β-diketiminate have not been reported to date. We describe here the syntheses and characterization of [{BDI iPr }Ae(Pyr) 3 ] + cations (Ae = Ca, Sr; Pyr = pyridine) paired with weakly coordinating anions, [X] – , and show that upon association with excess alcohol as a transfer agent, they make valuable catalytic systems for the controlled ROP of lactide. The reactivity of these cations with low coordina- tion numbers was high, as illustrated by the formation of [(Ca{OCH(C(Me)=NC 6 H 3 iPr 2 ) 2 }) 2 ] 2+ ·2[H 2 N{B(C 6 F 5 ) 3 } 2 ] – , a unique dinuclear complex formed by rare oxidation of the {BDI iPr } – ligand, the structure of which has been elucidate by XRD analysis.