Alkaline Earth Metal–Carbene Complexes with the Versatile Tridentate 2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine Ligand

Diffusion
of 2,6-bis­(3-mesitylimidazol-2-ylidene)­pyridine (CarMesPyCarMes, 2) into a solution of
CaI2 in THF leads to microcrystalline [(CarMesPyCarMes)­(thf)­CaI2] (3), in one
case containing a few single crystals of the unique separated ion
pair [(CarMesPyCarMes)2(thf)­Ca]­I2 (4) with four Ca–C bonds. However, isolation
of single-crystalline [(CarMesPyCarMes)­(thf)­CaI2] (3) succeeds via the addition of 2 to a solution of [(thf)5CaI]+[BPh4]− due to a subsequent dismutation. Two modifications
with the shapes of needles and cubes crystallize simultaneously. In
contrast to this finding, the reaction of [(thf)5CaI]+[AlPh4]− with 2 yields
solvent separated [(CarMesPyCarMes)­(thf)2CaI]+[AlPh4]− (5). The Ca–CNHC bond lengths lie in a typical
range of Ca–CAryl σ bonds and represent the
shortest Ca–CNHC bonds known to date. Soluble [(CarMesPyCarMes)­(thf)­Ca­(NPh2)2] (6) can be prepared via a metathetical approach from 4 and KNPh2 as well as via the addition of 2 to Ca­(NPh2)2 in tetrahydrofuran. The
bulkier amido ligands lead to elongated bonds between the calcium
center and the ligand 2. Furthermore, the reactions of
MI2 (M = Sr, Ba) with 2 yield [(CarMesPyCarMes)­(thf)2MI2] (M = Sr (7), Ba (8)).