Hepta-Coordinated Alkaline-Earth Complexes Featuring E^N^N^N^E (E = BH₃, S, Se) Ligands as Efficient Catalysts for...

Herein, we synthesize and characterize three diprotic hybrid E^N^N^N^E (E = BH₃, S, Se) amidophosphineborane and chalcogenide multidentate ligands [{Ph₂P(E)NHCH₂}₂C₅H₃N] (E = BH₃(1-H2); S (2-H2); Se (3-H2)) and probed their reactions with alkaline earth (Ae) metal amides [Ae{N(SiMe₃)₂}₂(THF)₂] [Ae = Mg, Ca, Sr] in THF. The Ae metal silylamides smoothly trigger the deprotonation of 1-H2, 2-H2 and 3-H2, which produce complexes represented by generic notation [κ⁵-({Ph₂P(E)NCH₂}₂C₅H₃N)-Ae(THF)₂]; [E = BH₃; Ae = Mg (1a), Ca (1b), E = S; Ae = Mg (2a), Ca (2b), Sr (2c), and E = Se; Ae = Mg (3a), Ca (3b), Sr (3c)] in 90-98% yield with the elimination of volatile [NH(SiMe₃)₂]. Additionally, the barium analogue, 2d, was obtained in good yield by a salt metathesis reaction of the diprotic ligand 2-H2, potassium benzyl (KCH₂Ph), and barium iodide (BaI₂). Solid state structures of ligands (2-H2, and 3-H2) and metal complexes (1a, 1b, 2b, 2c, 3a, and 3b) reveal that non-planar coordinating sites of ligands (N, P, E) enforce a co-planar penta-coordination to alkaline earth metals (Mg, Ca, Sr). Moreover, the Ae metal complexes exhibited multitasking catalytic activity for the hydroboration of heterocumulenes (e.g., isocyanates and carbodiimides) and the ROP polymerization of cyclic esters (rac-lactide, ε-caprolactone).