还原条件下非对称钳形PNN钴配合物的反应性研究

Reactivities of Nonsymmetric PNN Pincer Cobalt Complex Under Reductive Conditions

本工作报道了还原条件下,非对称钳形钴配合物PNNCoCl(1)(PNN:[6-(^(t)Bu_(2)PN)C_(5)H_(3)N-2-(3-Mes)C_(3)H_(2)N_(2)])和有机分子的反应性.利用PNNCoCl、还原剂与异腈或一氧化碳反应,实现了异腈和一氧化碳配位一价钴配合物2和3的合成.通过更换还原剂,实现了偶氮苯C—H、N=N键的选择性断裂反应.当使用石墨钾作为还原剂时,PNNCoCl(1)与偶氮苯在四氢呋喃中反应,得到偶氮苯C—H键断裂产物5;而当使用KHBEt_(3)作为还原剂时,PNNCoCl与偶氮苯反应,则能得到偶氮苯N=N键断裂产物PNNCoNHPh(6).此外,PNNCoCl(1)还可以与卡宾或邻苯醌反应,得到了相应配合物4和7.这些配合物均通过了X射线衍射、元素分析、红外光谱和溶液相磁化率测试等表征.

Nonsymmetric pincer complexes have received much attention due to their hemilability and more easily modu-lated spatial and electronic effects.However,their synthesis and reactivity studies remain considerably limited in comparison to symmetric pincer complexes.Common strategies to enhance the reactivity of complexes,such as reducing coordination sites or lowering metal center valences,invariably complicate synthesis.In this work,employing the strategy of introducing reducing agents,the PNNCoCl(1)(PNN:[6-(^(t)Bu_(2)PN)C_(5)H_(3)N-2-(3-Mes)C_(3)H_(2)N_(2)])reacts with various small molecules to yield a variety of nonsymmetric pincer cobalt complexes,which were characterized by single-crystal X-ray diffractometry analysis and spectroscopy.The reaction of PNNCoCl,potassium graphite(or KHBEt3)with 1 equiv.of CNXyl(2,6-dimethylphenyl isocyanide)or carbon monoxide successfully yielded the monovalent cobalt complexes 2(PNNCoCNXyl)and 3(PNNCo-CO)in 64%and 41%yields,respectively.Moreover,the reaction of complex 1 with N-heterocyclic carbene(1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene),involving C—H bond cleavage and Co—C bond formation,afforded the corresponding divalent cobalt complex 4.Furthermore,by using different reducing agents,we achieved the modulation of the selective reaction of azobenzene C—H or N=N bonds.When KC8(potassium graphite)was used as the reducing agent,the reaction of PNNCoCl and azobenzene in tetrahydrofuran afforded the azobenzene C—H bond-breaking product 5 in 53%yield.The cobalt atom is pentacoordinate in a tetragonal pyramid geometry.The N—N lengths of 0.12835(17)nm in complex 5 are in the range of normal N=N double bonds(0.122~0.130 nm).In contrast,the reaction of 1 with azobenzene in the presence of KHBEt3 gave the cobalt complex PNNCoNHPh(6),as confirmed by X-ray diffraction analysis.The structure suggests N=N double bond cleavage of azobenzene in the formation of the aniline complex.The N—H stretching bands of 6 were recorded at 3131 cm^(-1),and the solution magnetic moment wasµeff=2.2µB.In addition,the treatment of complex 1 with 3,5-di-tert-butyl-o-benzoquinone in the presence of 1.2 equiv.of KC8 gave rise to the cobalt complex 7.X-ray diffrac-tion analysis of 7 revealed that the PNN ligand and o-benzoquinone parts are linked by forming a P—O single bond.The o-benzoquinone was reduced to the catechol ion.Complex 7 exhibits a high-spin electronic structure(S=3/2),as evidenced by the solution magnetic moment of 4.0µB.