Organocalcium compounds with catalytic activity for the ring-opening polymerization of lactones
The equimolar reaction of calcium bis[bis(trimethylsilyl)-amide] with 2,2,6,6-tetramethylheptane-3,5-dione (Htmhd) in THF yields liquid mononuclear [Ca(tmhd){N-(SiMe3)(2)}(THF)(3)] (1). A similar reaction in toluene with a stoichiometric ratio of 2:3 gives the dinuclear complex [{(THF)Ca}(2)(tmhd)(2)(mu-tmhd){mu-N(SiMe3)(2)}] (2). The calcium atoms of these complexes are in a distorted octahedral environment. In 2 the complex consists of two octahedra connected by a common face of one nitrogen base and two oxygen atoms; the bridging Ca-N bond lengths are extremely large. The metalation of the tetradentate Jacobsen reagent with calcium bis[bis(trimethylsilyl)amide] in 1,2-dimethoxyethane (DME) gives the corresponding calcium complex 3, nearly quantitatively, as its DME adduct. The calcium atom is in an unusual trigonal prismatic coordination sphere. The metathesis reaction of 3 with tin(II) chloride yields the corresponding yellow tin(II) complex with the metal atom in a distorted square-pyramidal environment. Complexes 2 and 3 show catalytic reactivity in the ring-opening polymerization of cyclic esters such as lactones and lactides.