Syntheses, Structures, and Reactivity of Barium Carbene Complexes with Chelating Bis-iminophosphorano Arms

Reaction of (Me3SiNPPh2)2CH2 with Ba[N(SiMe3)2]2 in 1:1 ratio gave only monodeprotonation and the homoleptic product [(Me3SiNPPh2)2CH]2Ba (3-Ba) was formed. Reaction with the strong base dibenzylbarium gave fast and clean 2-fold deprotonation and the dimeric complex [(Me3SiNPPh2)2CBa·THF]2 (1-Ba) was isolated. Also the bulkier ligand (2,6-iPr2C6H3−NPPh2)2CH2 was doubly deprotonated by this reagent, which gave clean formation of the monomeric complex (2,6-iPr2C6H3−NPPh2)2CBa·(THF)3 (2-Ba). Although organobarium compounds generally show high reactivity, the monomeric barium carbene complex 2-Ba reacted sluggishly with benzophenone or adamantyl cyanide and reaction products could not be identified. Reaction of 2-Ba with cyclohexyl isocyanate, however, cleanly gave the [2+2]-cycloaddition product, which crystallized as a dimer (4). Crystal structures of monomers 2-Ba and 3-Ba and dimers 1-Ba and 4 show in all cases tridentate (C, N, N‘)-coordination of the bis-iminophosphorano ligand. In some cases the large coordination sphere of Ba2+ was saturated by agostic iPr···Ba2+ and π-Ph···Ba contacts.将双(三甲基硅基膦亚胺基)甲烷[(Me3SiN=PPh2)2CH2]与双(三甲基硅基)氨基钡(Ba[N(SiMe3)2]2)以1:1摩尔比反应，仅发生单去质子化过程，生成均配型配合物[(Me3SiN=PPh2)2CH]2Ba（3-Ba）。该配体与强碱二苄基钡反应时，可快速且完全地发生双去质子化，分离得到二聚体配合物[(Me3SiN=PPh2)2CBa·THF]2（1-Ba）。大位阻配体双[(2,6-二异丙基苯基)二苯基膦亚胺基]甲烷[(2,6-iPr2C6H3−N=PPh2)2CH2]可被该二苄基钡试剂双去质子化，顺利生成单体配合物(2,6-iPr2C6H3−N=PPh2)2CBa·(THF)3（2-Ba）。尽管有机钡化合物通常具有较高反应活性，但单体钡卡宾配合物2-Ba与二苯甲酮或金刚烷基氰的反应较为迟缓，无法鉴定其反应产物。然而，2-Ba与环己基异氰酸酯反应时，可干净地得到[2+2]环加成产物，该产物以二聚体形式结晶析出（4）。单体2-Ba、3-Ba与二聚体1-Ba、4的晶体结构均表明，双膦亚胺配体以(C,N,N')三齿配位模式与Ba²+结合；部分情况下，Ba²+的大配位球可通过异丙基的agostic相互作用与苯基π体系的钡配位接触实现配位饱和。