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