N-Heterocyclic Carbene–Carbodiimide (“NHC–CDI”) Adduct or Zwitterionic-Type Neutral Amidinate-Supported Magnesium(II) and Zinc(II) Complexes

A series of structurally characterized magnesium and zinc complexes of the form L^4-tBuPh-M{N(SiMe₃)₂}₂ [M = Mg (1) and Zn (2); L^4-tBuPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-tert-butylphenyl)amidinate}], L^4-iPrPh-M{N(SiMe₃)₂}₂ [M = Mg (3) and Zn (4); L^4-iPrPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-isopropylphenyl)amidinate}], and L^4-iPrPh-ZnEt₂ (5) bearing a zwitterionic-type neutral amidinate or N-heterocyclic carbene-carbodiimide ("NHC-CDI") adduct and monoanionic amido or alkyl ligands have been reported. The synthesis of compounds 1-5 was achieved by the direct addition of a "NHC-CDI" adduct to a corresponding metal bis(amide) or dialkyl reagent. All compounds 1-5 exist as monomers in the solid state. In all cases, the metal (magnesium or zinc) centers adopt a distorted four-coordinate tetrahedral geometry bonded to one N,N'-chelated neutral zwitterionic ligand and two monoanionic amido or alkyl moieties. In contrast, sterically bulky zwitterionic amidinate 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(2,6-diisopropylphenyl)amidinate} (L^Dipp) upon treatment with lithium bis[(trimethylsilyl)amide], Li{N(SiMe₃)₂}, affords the NHC-lithium complex ^MeIEt-[Li{N(SiMe₃)₂}]₂ (6), in which one molecule of NHC (^MeIEt = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) coordinates to one of the two lithium centers. In a similar way, the reaction between L^Dipp and Mg{N(SiMe₃)₂}₂ allowed the formation of a NHC adduct of metal bis(amide), ^MeIEt-Mg{N(SiMe₃)₂}₂ (7), instead of a zwitterionic adduct of metal bis(amide). Alternatively, the synthesis of both compounds 6 and 7 was achieved by the direct addition of 1 equiv of NHC, i.e., ^MeIEt to Li{N(SiMe₃)₂} (2.0 equiv) and Mg{N(SiMe₃)₂}₂ (1.0 equiv) in benzene-d₆, respectively. All compounds (1-7) were characterized by multinuclear {¹H, ¹³C, and ²⁹Si (for 1-4, 6, and 7) and ⁷Li (for compound 6)} magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structural analysis. In addition, preliminary reactivity studies of zwitterion-supported metal complexes have been investigated. Furthermore, density functional theory calculations have been carried out to obtain the energetics of zwitterion-supported lithium and magnesium complexes.