N‑Heterocyclic Carbene−Carbodiimide (“NHC−CDI”) Adduct or Zwitterionic-Type Neutral Amidinate-Supported Magnesium(II) and Zinc(II) Complexes Ashim Baishya, Lokesh Kumar, Milan Kr. Barman, Himansu S. Biswal, † and Sharanappa Nembenna* School of Chemical Sciences, National Institute of Science Education and Research (NISER), Homi Bhabha National Institute (HBNI), Bhubaneswar 752 050, India *S Supporting Information ABSTRACT: A series of structurally characterized magnesium and zinc complexes of the form L 4‑tBuPh −M{N(SiMe 3 ) 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(SiMe 3 ) 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 −ZnEt 2 (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(SiMe 3 ) 2 }, affords the NHC−lithium complex Me IEt−[Li{N(SiMe 3 ) 2 }] 2 (6), in which one molecule of NHC ( Me IEt = 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(SiMe 3 ) 2 } 2 allowed the formation of a NHC adduct of metal bis(amide), Me IEt−Mg{N(SiMe 3 ) 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., Me IEt to Li{N(SiMe 3 ) 2 } (2.0 equiv) and Mg{N(SiMe 3 ) 2 } 2 (1.0 equiv) in benzene-d 6 , respectively. All compounds (1−7) were characterized by multinuclear { 1 H, 13 C, and 29 Si (for 1−4, 6, and 7) and 7 Li (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. ■ INTRODUCTION The coordination chemistry of anionic amidinate [RNC(R′)- NR] − ligands has been widely investigated in s-, p-, d-, and f- block metal chemistry. 1 Amidinate anions can bind to the metal center as monodentate, bidentate, and bridging modes. 2 Metal amidinate complexes are important in many areas, including catalysis, material science, and synthesis. 3 In this context, chemists synthesized several magnesium and zinc complexes bearing monoanionic amidinate ligand systems. 4 Generally, these metal complexes are prepared by following any of three methods: (i) metalation of a neutral amidine with MR 2 (R = amide, alkyl, Cp*, etc.), (ii) the addition of MR 2 to carbodiimide, and (iii) a salt metathesis reaction, metalation reaction of an alkali-metal amidinate to MX 2 (X = halide). 3a Furthermore, Mulvey and Robertson et al. reported a heteroleptic amidinate magnesium complex by the reaction of magnesium bis(alkyl) and sodium amide with benzonitrile. 5 In the first method, metalation of a neutral amidine with MR 2 led to the formation of LML or LMR (depending on the stoichiometry used), in which L − acts as a monoanionic ligand. However, a N-heterocyclic carbene−carbodiimide (NHC− CDI) adduct or zwitterionic-type amidinate ligand upon coordination with MR 2 affords L′ → MR 2 complex, in which L′ acts as a neutral ligand (see Scheme 1). This new class of neutral NHC−CDI adducts is different from that of neutral monodentate NHC and bidentate monoanionic amidinate and guanidinate ligands. Compared to monodentate NHC ligands, bidentate NHC−CDI ligands can give greater stabilities in their metal complexes if they bind to the metal center in a N,N′-chelated fashion (vide infra). As far as the stability of the ancillary ligands in their metal complexes are concerned, the chelate effect is highly useful and desirable. 6 Like amidinates, two resonance structures are Received: April 9, 2017 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b00879 Inorg. Chem. XXXX, XXX, XXX−XXX