Reactivity of Organozinc Complexes

Reactivity of organozinc complexes

Synthesis of N,N'-chelating complexes

Organometallic complexes of s-, p-, d-and f-block metals with anionic N, N'-chelate ligands, such as b-diketiminate, guanidinate- and amidinate ligands have been studied for a long time.[1-4]. Their steric and electronic properties can be tuned by variation of the substitution pattern. They show a remarkable bond variability and can act as bidentate (η2) and bridging monodentate (μ-η11) four-electron donors. b-diketiminate zinc complexes LZnR (L = β-diketiminate) are intensely studied due to their promising potential as highly active catalysts in the ring-opening polymerization (ROP) of cyclic esters,[5] particularly of lactides,[6] as well as in the copolymerization of CO2 with epoxides.[7]

Synthesis and structure of amidinate- and β-diketiminate zinc complexes

Hetero- and homoleptic amidinate zinc complexes of the general type LZnX and L2Zn (L = amidinate, ß-diketiminate; X = alkyl, aryl, halide, amide) are synthesized by salt elimination reactions.[8,9]

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Scheme 1. Synthesis of mono- und bisamidinate zinc complexes.

Monoamidinate zinc complexes typically form centrosymmetric dimers with bridging nitrogen atoms in which the amidinate ligand coordinates as a chelating (η2) or bridging (μ-η11) unit. [t-BuC(NDipp)2Zn(μ-I)]2, containing the sterically demanding Dipp substituent (Dipp = 2,6-i-Pr2C6H3), forms a iodine-bridged dimeric complex.[8,9,10]

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Figure 1: Structures of halogenated zinc amidinate complexes.

We established a straightforward pathway for the synthesis of heteroleptic amidinate and b-diketiminate zinc complexes based on the reaction of ZnMe2 and LH.[10,11]

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Scheme 2: Synthesis of amidinate und ß-diketiminate zinc complexes.

[t-BuC(NDipp)2]ZnMe coordinates in a rather unexpected η1-N,η3-arene fashion. While this coordination mode has been previously observed for large and, according to the HSAB concept, rather soft metals such as K, In and Tl,[12,13] it is rather unexpected for Zn(II) centers. Addition of the Lewis base t-butyl pyridine leads to a structural rearrangement to the common N,N'-chelating binding mode.[10]

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Figure 2: Structures of methylated and base stabililated amidinate zinc complexes.

The heteroleptic complexes LZnX are suitable for substitution reactions. MesnacnacZnMe (Mesnacnac = [HC{C(Me)N(2,4,6-Me3C6H2)}2]) reacts with alcohols ROH (R = Me, Et, i-Pr) with elimination of methane and subsequent formation of the corresponding zinc alkoxides, which typically adopt dimeric structures with bridging OR-groups in the solid state.[14,15]

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Scheme 3: Synthesis of β-diketiminate zinc alkoxides.

MesnacnacZnCl reacts with NaH or KN(i-Pr)HBH3 with formation of the hydride complex MesnacnacZnH. Insertion reactions of heterocumulenes such as CO2, C(Ni-Pr)2 and t-BuNCO into the Zn–H bond yields the corresponding formato, formamido and formamidinato complexes.[16-18] The formato complex forms an unsymmetrically bridged dimer exhibiting a syn-syn-(μ-η2) and a bridging (μ-η1) HCO2 unit, whereas MesnacnacZnCH(i-PrN)2 with the sterically demanding i-Pr groups is a monomer with a chelating η2-(Ni-Pr)2CH. Comparable insertion reactions of heterocumulenes into Zn-carbon bonds were observed.[19-21]

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Figure 3: Structures of insertion products.


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