Mixed Alkyl Hydrido Complexes of Zinc: Synthesis, Structure, and Reactivity

The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane (Me(3)TACD, 1,4,7-Me-3[12]aneN(4)) reacted with 1 equiv of ZnEt2 under ethane elimination to give the mononuclear ethyl complex [(Me(3)TACD)ZnEt] (1). Upon treatment of (Me(3)TACD)H with 2 equiv of ZnEt2, the dinuclear complex [(Me(3)TACD)(ZnEt)(ZnEt2)] (2) was formed, which was converted with an additional 1 equiv of (Me(3)TACD)H to 1. Reaction of 1 with PhSiH3 led to the formation of a tetranuclear ethyl hydrido complex [{(Me(3)TACD)ZnEt}(2)(ZnEtH)(2)] (3). Single-crystal X-ray diffraction study revealed 3 to be a centrosymmetric dimer featuring two [(Me(3)TACD)ZnEt] units coordinated to a [Zn(mu-H)(2)Zn] core via amido nitrogen atoms of the Me(3)TACD ligands. Substitution of the two [(Me(3)TACD)ZnEt] units in 3 by N-heterocyclic carbene IMes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] gave [(IMes)ZnEtH](2) (4b). The mixed alkyl hydrido complexes [(IMes)ZnRH](2) (R = Me, 4a; Et, 4b) were alternatively synthesized in quantitative yield by reacting [(IMes)ZnR2] (R = Me, Et) with [(IMes)ZnH2](2) in 2:1 ratio. Methyl complex 4a reacted with CO2 (p(CO2) = 0.5 bar) under facile insertion of CO2 into Zn-H bonds to give dinuclear formate complex [(IMes)ZnMe(O2CH)](2) (5a). Treatment of 4b with CO2 (p(CO2) = 0.5 bar) afforded a mixture of di- and trinuclear formate complexes [(IMes)ZnEt(O2CH)](2) (5b) and [(IMes)(2)Zn3Et3(O2CH)(3)] (6) under elimination of one IMes as CO2 adduct IMes center dot CO2.