氮杂环卡宾在三吡唑基硼酸阴离子为支撑配体的钌中心上的环金属化

Cyclometalation of N-Heterocyclic Carbene on Ruthenium Center Supported by Tris( pyrazolyl) borate Anion

鉴于三吡唑基硼酸阴离子(tris(pyrazolyl)borate anion,Tp)的钌配合物TpRuCl(PPh_3)_2在金属有机合成和催化反应中的多种用途以及氮杂环卡宾(N-heterocyclic carbene,NHC)配体所表现出的优势,试图合成与TpRuCl(PPh_3)_2类似的TpRu(NHC)骨架的配合物.采用TpRuCl(PPh_3)_2与NHC配体1,3-N,N-双(2,4,6-三甲基苯基)-4,5-二氢咪唑-2-卡宾(1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene,H_2IMes)反应,未能得到预期产物TpRuCl(H_2IMes)(PPh3),而是发生了H_2IMes配体N-侧臂上邻位甲基的C—H键活化,生成H_2IMes配体环金属化的产物TpRu(H_2IMesH)(PPh3)(其中H_2IMes-H表示H_2IMes的一个N-侧臂上邻位的甲基脱除了1个质子).分离得到的该化合物在二氯甲烷溶液中加热可解离PPh3配体,促成H_2IMes-H配体环金属化侧臂上进一步发生苄基C—H键的α-H消除,转化为螯合的双卡宾H_2IMes-2H配位的Ru—H中间体物种(其中H_2IMes-2H表示H_2IMes的一个N-侧臂上邻位的甲基脱除了2个质子),再和二氯甲烷溶剂发生H/Cl交换,生成最终产物TpRuCl(H_2IMes-2H).

Inspired by the rich reactivity and catalytic properties of the tris( pyrazolyl) borate anion( Tp) ruthenium complex TpRuCl( PPh_3)_2 and the advantage of N-heterocyclic carbene( NHC) over the traditional phosphine ligand,the reaction of TpRuCl( PPh_3)_2 with the NHC ligand 1,3-bis( 2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene( H_2IMes) was investigated to obtain the TpRu( NHC) complex analogous to the versatile precursor TpRuCl( PPh_3)_2. The reaction proceeded to give the NHC-cyclometalated pro-duct,TpRu( H_2IMes-H)( PPh3)( H_2IMes-H = the deprotonated form of H_2IMes) instead of the expected TpRuCl( H_2IMes)( PPh3),in which the C—H bond activation of the o-methyl on the N-bound wingtip of the H_2IMes ligand occurred. Heating a solution of the isolated compound TpRu( H_2IMes-H)( PPh3) in CH_2Cl_2 led to the dissociation of the PPh3 ligand and further activation of the metalated benzyl C—H on the ruthenium center viaα-H elimination,which was then followed by a hydride/chloride metathesis between the Ru—H intermediate and CH_2Cl_2 solvent to result in the bis-carbene chelated compound TpRuCl( H_2IMes-2H).