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

The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane
(Me3TACD, 1,4,7-Me3[12]­aneN4) reacted
with 1 equiv of ZnEt2 under ethane elimination to give
the mononuclear ethyl complex [(Me3TACD)­ZnEt] (1). Upon treatment of (Me3TACD)H with 2 equiv of ZnEt2, the dinuclear complex [(Me3TACD)­(ZnEt)­(ZnEt2)] (2) was formed, which was converted with an
additional 1 equiv of (Me3TACD)H to 1. Reaction
of 1 with PhSiH3 led to the formation of a
tetranuclear ethyl hydrido complex [{(Me3TACD)­ZnEt}2(ZnEtH)2] (3). Single-crystal X-ray
diffraction study revealed 3 to be a centrosymmetric
dimer featuring two [(Me3TACD)­ZnEt] units coordinated to
a [Zn­(μ-H)2Zn] core via amido nitrogen atoms of the
Me3TACD ligands. Substitution of the two [(Me3TACD)­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)2Zn3Et3(O2CH)3] (6) under elimination of one IMes as
CO2 adduct IMes·CO2.