Stabilization of Calcium Hydride Complexes by Fine Tuning of Amidinate Ligands

A range
of symmetric amidinate ligands RAmAr (R is backbone
substituent, Ar is N substituent) have been investigated for their
ability to stabilize calcium hydride complexes of the type RAmArCaH. It was found that the precursors of the type RAmArCaN­(SiMe3)2 are only stable toward
ligand exchange for Ar = DIPP (2,6-diisopropylphenyl). The size of
the backbone substituent R determines aggregation and solvation. The
following complexes could be obtained: [RAmDIPPCaN­(SiMe3)2]2 (R = Me, p-Tol),
RAmDIPPCaN­(SiMe3)2·Et2O (R = Np, tBu), AdAmDIPPCaN­(SiMe3)2·THF, and AdAmDIPPCaN­(SiMe3)2. Reaction of these heteroleptic calcium amide
complexes with PhSiH3 gave only for larger backbone substituents
(R = tBu, Ad) access to the dimeric calcium hydride
complexes (RAmArCaH)2. (N,aryl)-coordination
of the amidinate ligand seems crucial for the stability of these complexes,
and the aryl···Ca interaction is found to be strong
(17 kcal/mol). Addition of polar solvents led to a new type of trimeric
calcium hydride complex exemplified by the crystal structures of (tBuAmDIPPCaH)3·2Et2O and (AdAmDIPPCaH)3·2THF. The overall
conclusion of this work is that minor changes in sterics (tBu vs Ad) or coordinated solvent (THF vs Et2O) can have large consequences for product formation and stability.

本研究系统探究了一系列对称型脒基配体(amidinate ligand) RAmAr（其中R为骨架取代基，Ar为氮取代基）稳定RAmArCaH型氢化钙配合物的能力。研究发现，仅当Ar为2,6-二异丙基苯基(DIPP, 2,6-diisopropylphenyl)时，RAmArCaN(SiMe3)2型前驱体才具备抗配体交换的稳定性。骨架取代基R的空间尺寸决定了配合物的聚集状态与溶剂化行为。本研究成功获得如下配合物：[RAmDIPPCaN(SiMe3)2]₂（R=甲基、对甲苯基）、RAmDIPPCaN(SiMe3)2·二乙基醚(Et₂O)（R=新戊基、叔丁基）、AdAmDIPPCaN(SiMe3)2·四氢呋喃(THF)以及AdAmDIPPCaN(SiMe3)2。将上述杂配位型酰胺钙配合物与苯基硅烷(PhSiH₃)进行反应后，仅当骨架取代基位阻较大（R=叔丁基、金刚烷基）时，方可得到二聚型氢化钙配合物(RAmArCaH)2。脒基配体的(N,芳基)双配位模式对这类配合物的稳定性至关重要，且芳基与钙中心的相互作用较强（键合能约为17 kcal/mol）。向体系中加入极性溶剂后，可得到新型三聚型氢化钙配合物，其晶体结构可通过(tBuAmDIPPCaH)3·2Et₂O与(AdAmDIPPCaH)3·2THF得以佐证。本研究的整体结论为：位阻环境的细微调整（如叔丁基与金刚烷基的差异）或配位溶剂的更换（四氢呋喃与二乙基醚的区别），均可对产物的生成与稳定性产生显著影响。