Guanidinate complexes of heavier alkaline-earth metals (Ca, Sr): Syntheses, structures, styrene polymerization and unexpected reaction behaviour

The guanidinate complex [Cal (Cy)N-C{N(SiMe3)(2))-N(Cy))(2)] can be prepared in good yields and in crystalline purity by two different routes: 1) addition of [Ca(N(SiMe3)(2))(2)] to (CY)N=C=N(Cy) or 2) addition of KN(SiMe3)2 to (Cy)N=C=N(Cy) followed by a metathesis reaction of the obtained potassium guanidinate complex with CaI2- Crystallization from Et2O yielded [Ca((Cy)N-C[N(SiMe3)(2)]-N(Cy)}(2)center dot(Et2O)] (1), which shows a C-2-symmetric structure in the crystal. A Sr analogue could be prepared likewise and also crystallizes as the monoetherate [Sr{(Cy)N-C{N(SiMe3)(2))-N(Cy))(2)center dot(Et2O)] (2) with a very similar crystal structure. The Schlenk equilibrium between (1) and [Ca{alpha-(Me3Si)-o-(Me2N)-benzyl}(2)center dot(THF)(2)] (3) in benzene is not completely on the heteroleptic side; how- ever, an excess of (1) yields predominantly the heteroleptic benzylcalcium complex [Ca{(Cy)N-CIN(SiMe3)(2))-N(Cy))alpha-(Me3Si)-o-(Me2N)-benzyl)] (4). Styrene polymerization with this mixture results largely in atactic polystyrene of which the molecular weight distribution shows a tailing in the lower range. This could be due to decomposition of the guanidinate ligand under the given polymerization conditions. It is shown that heating a mixture of I and 3 gives the crystalline decomposition product [Ca{(Cy)N-C{N(SiMe3)(2))-N(Cy))I(Cy)(TMS)N) (5), which crystallizes as a dimer with bridging amide ligands. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).