Synthesis and structural characterisation of heavy alkaline earth N,N'-bis(aryl)formamidinate complexes

Treatment of calcium or strontium with 2.0 equivalents of N,N-bis(o-methylphenyl)formamidine (o-TolFormH), N,N-bis(2,6-dimethylphenyl)formamidine (XylFormH) or N,N-bis(o-phenylphenyl)formamidine (o-PhPhFormH) in the presence of 1.0 equivalent of Hg(C6F5)2 in tetrahydrofuran (thf) affords the bis(formamidinate) complexes Ca(o-TolForm)2(thf)2] (1), Ca(XylForm)2(thf)2] (2), Ca(o-PhPhForm)2(thf)2].thf (3), Sr(o-TolForm)2(thf)3] (4), Sr(XylForm)2(thf)3].3thf (5) and Sr(o-PhPhForm)2(thf)3].2thf (6). Analogous reactions with barium were generally unsatisfactory but Ba(o-PhPhForm)2(thf)3].2thf (7) was successfully prepared. Compounds 1-7 have been characterised by various spectroscopic methods (1H, 13C{1H} NMR and IR), elemental analyses and, for 1, 2 and 4-6, X-ray crystallography. The calcium complexes are monomeric and six-coordinate with either transoid octahedral or trigonal prismatic geometry, whilst the larger radius of strontium accommodates an additional thf solvent donor to give seven-coordinate structures with two types of coordination polyhedra.