Mixed Alkyl Hydrido Complexes of Zinc: Synthesis,
Structure, and Reactivity
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Abstract
The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane
(Me<sub>3</sub>TACD, 1,4,7-Me<sub>3</sub>[12]aneN<sub>4</sub>) reacted
with 1 equiv of ZnEt<sub>2</sub> under ethane elimination to give
the mononuclear ethyl complex [(Me<sub>3</sub>TACD)ZnEt] (<b>1</b>). Upon treatment of (Me<sub>3</sub>TACD)H with 2 equiv of ZnEt<sub>2</sub>, the dinuclear complex [(Me<sub>3</sub>TACD)(ZnEt)(ZnEt<sub>2</sub>)] (<b>2</b>) was formed, which was converted with an
additional 1 equiv of (Me<sub>3</sub>TACD)H to <b>1</b>. Reaction
of <b>1</b> with PhSiH<sub>3</sub> led to the formation of a
tetranuclear ethyl hydrido complex [{(Me<sub>3</sub>TACD)ZnEt}<sub>2</sub>(ZnEtH)<sub>2</sub>] (<b>3</b>). Single-crystal X-ray
diffraction study revealed <b>3</b> to be a centrosymmetric
dimer featuring two [(Me<sub>3</sub>TACD)ZnEt] units coordinated to
a [Zn(μ-H)<sub>2</sub>Zn] core via amido nitrogen atoms of the
Me<sub>3</sub>TACD ligands. Substitution of the two [(Me<sub>3</sub>TACD)ZnEt] units in <b>3</b> by N-heterocyclic carbene IMes
[1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] gave [(IMes)ZnEtH]<sub>2</sub> (<b>4b</b>). The mixed alkyl hydrido complexes [(IMes)ZnRH]<sub>2</sub> (R = Me, <b>4a</b>; Et, <b>4b</b>) were alternatively
synthesized in quantitative yield by reacting [(IMes)ZnR<sub>2</sub>] (R = Me, Et) with [(IMes)ZnH<sub>2</sub>]<sub>2</sub> in 2:1 ratio.
Methyl complex <b>4a</b> reacted with CO<sub>2</sub> (<i>p</i>(CO<sub>2</sub>) = 0.5 bar) under facile insertion of CO<sub>2</sub> into Zn–H bonds to give dinuclear formate complex
[(IMes)ZnMe(O<sub>2</sub>CH)]<sub>2</sub> (<b>5a</b>). Treatment
of <b>4b</b> with CO<sub>2</sub> (<i>p</i>(CO<sub>2</sub>) = 0.5 bar) afforded a mixture of di- and trinuclear formate
complexes [(IMes)ZnEt(O<sub>2</sub>CH)]<sub>2</sub> (<b>5b</b>) and [(IMes)<sub>2</sub>Zn<sub>3</sub>Et<sub>3</sub>(O<sub>2</sub>CH)<sub>3</sub>] (<b>6</b>) under elimination of one IMes as
CO<sub>2</sub> adduct <b>IMes</b>·<b>CO</b><sub><b>2</b></sub>
