Guanidinate Complexes of Heavier Alkaline‐Earth Metals (Ca, Sr): Syntheses, Structures, Styrene Polymerization and Unexpected Reaction Behaviour

The guanidinate complex [Ca{(Cy)N–C{N(SiMe 3 ) 2 }–N(Cy)} 2 ] can be prepared in good yields and in crystalline purity by two different routes: 1) addition of [Ca{N(SiMe 3 ) 2 } 2 ] to (Cy)N=C=N(Cy) or 2) addition of KN(SiMe 3 ) 2 to (Cy)N=C=N(Cy) followed by a metathesis reaction of the obtained potassium guanidinate complex with CaI 2 . Crystallization from Et 2 O yielded [Ca{(Cy)N–C[N(SiMe 3 ) 2 ]–N(Cy)} 2 · (Et 2 O)] ( 1 ), which shows a C 2 ‐symmetric structure in the crystal. A Sr analogue could be prepared likewise and also crystallizes as the mono‐etherate [Sr{(Cy)N–C{N(SiMe 3 ) 2 }–N(Cy)} 2 · (Et 2 O)] ( 2 ) with a very similar crystal structure. The Schlenk equilibrium between ( 1 ) and [Ca{α‐(Me 3 Si)‐ o ‐(Me 2 N)‐benzyl} 2 · (THF) 2 ] ( 3 ) in benzene is not completely on the heteroleptic side; however, an excess of ( 1 ) yields predominantly the heteroleptic benzylcalcium complex [Ca{(Cy)N–C{N(SiMe 3 ) 2 }–N(Cy)}{α‐(Me 3 Si)‐ o ‐(Me 2 N)‐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 1 and 3 gives the crystalline decomposition product [Ca{(Cy)N–C{N(SiMe 3 ) 2 }–N(Cy)}{(Cy)(TMS)N}] ( 5 ), which crystallizes as a dimer with bridging amide ligands. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)