A series of dinucleating bis(β-diketiminate) ligands with rigid bridges has been prepared. In all cases the β-diketiminate unit is 2,6-iPr2C6H3NC(Me)C(H)C(Me)N-(bridge), and the bridges are either para-phenylene, meta-phenylene or 2, 6-pyridylene (the dinucleating ligands are abbreviated as PARA-H2, META-H2 and PYR-H2, respectively). These ligands have been converted to heteroleptic bimetallic calcium and zinc complexes. For calcium, only the PARA-phenylene bridged ligand led to a heteroleptic bimetallic calcium amide complex PARA-[CaN(SiMe3)2·THF]2. For the other ligands, homoleptic complexes have been isolated: META-Ca and PYR-Ca. For zinc, the whole range of heteroleptic amides could be isolated: PARA-[ZnN(SiMe3)2]2, META-[ZnN(SiMe3)2]2, and PYR-[ZnN(SiMe3)2]2. Analogue ethylzinc complexes have been prepared in quantitative yields: PARA-(ZnEt)2, META-(ZnEt)2, and PYR-(ZnEt)2. Reactions of the ethylzinc complexes with SO2 gave access to the ethylsulfinate complexes META-(ZnO2SEt)2 and PYR-(ZnO2SEt)2; for the para-phenylene bridged ligand no products could be isolated. Crystal structures of the following complexes are presented: (META-Ca)2, PARA-[CaN(SiMe3)2·THF]2, PARA-[ZnN(SiMe3)2]2, META-[ZnN(SiMe3)2]2, PYR-(ZnEt)2, and PYR-(ZnO2SEt)2. All heteroleptic complexes have been tested for activity in the copolymerization of cyclohexene oxide (CHO) and CO2. The bimetallic complex PARA-[CaN(SiMe3)2·THF]2 is not active. For zinc, the PARA and META complexes were found to be active under highly concentrated conditions (Zn/CHO ratio of 1/1000; no solvent), but no significant polymer yields could be achieved with PYR complexes. This is either due to conformational changes of the complex or to coordination of the pyridylene N atom to the catalytic centers. The order of activity found in the bimetallic zinc complexes is META > PARA. This is likely due to a more advantageous Zn···Zn distance in the meta-phenylene bridged bimetallic catalysts. There are several indications for bimetallic action. First of all, the ethylzinc systems PARA-(ZnEt)2 and META-(ZnEt)2 initiate CHO/CO2 copolymerization, whereas monometallic ethylzinc catalysts are not reactive. Second, the bimetallic zinc catalysts are also highly active under diluted conditions: a low metal/CHO ratio of 1/3000 gave high-MW polymers (M n > 100.000, PDI = 1.33) with essentially only carbonate linkages. In contrast, monometallic systems show a drastic loss of activity upon dilution. Finally, the bimetallic catalyst META-[ZnN(SiMe3)2]2 shows a significantly higher activity than a comparable monometallic model system.

The diprotic bis(salicylaldimine) ligand N,N′‐bis(3,5‐di‐tBu‐salicylidene)diimine, (1‐NN)H2, was smoothly deprotonated by reaction with two equivalents of a metal amide M[N(SiMe3)2]2 (M = Mg, Ca or Zn) to form a bimetallic metal amide complex (1‐NN)[MN(SiMe3)2]2. This heteroleptic complex was only stable in the case of Mg and (1‐NN)[MgN(SiMe3)2·thf]2 was structurally characterized by X‐ray diffraction. For Ca and Zn homoleptic products could be obtained: (1‐NN)Ca·(thf)2 and (1‐NN)Zn. The latter crystallized as a trimer with approximate C3‐symmetry. The diprotic N–N coupled bis(β‐diketimine) ligand [RHN–C(Me)=C–C(Me)=N–N=C(Me)–C=C(Me)–NHR, R = 2,6‐di‐iPr‐C6H3], which is abbreviated as (2‐NN)H2, can be deprotonated only under much harsher reaction conditions. Attempted synthesis of (2‐NN)[Mg(nBu)]2 gave after ligand exchange the homoleptic species (2‐NN)Mg and Mg(nBu)2. For Ca, the complexes [(2‐NN)Ca]2 and (2‐NN)[CaN(SiMe3)2·thf]2 both have been prepared and structurally characterized by X‐ray diffraction. The latter heteroleptic complex is in benzene solution in equilibrium with [(2‐NN)Ca]2 and Ca[N(SiMe3)2]2·(thf)2. Reaction of (2‐NN)H2 with excess of Et2Zn gave the heteroleptic complex (2‐NN)[ZnEt]2 and is stable towards ligand exchange reactions. The complex is not active in CO2/cyclohexene oxide copolymerization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Keywords: Alkaline earth metals / Magnesium / Calcium / Zinc / Polylactide Four dinucleating ligands based on sterically protected salicylaldimine units bridged by a variety of rigid groups have been prepared: 1,4-bis [(3,5-

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Silver-doped calcium phosphate ceramics were prepared in discontinuous and continuous processes with different amounts of incorporated silver (up to 1.8 wt% silver).