Mariham Y. Abraham

A Viable Anionic N-Heterocyclic Dicarbene

The first anionic N-heterocyclic dicarbene, polymeric [:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHCLi(THF)}](n) 1, containing both normal (C2) and abnormal carbene (C4) centers in the same five-membered imidazole ring (III), has been prepared by lithiation of the imidazole monocarbene, :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2). The dicarbene nature of 1 was unambiguously demonstrated by the formation of the group 13 Lewis acid adducts (THF)(2)Li:C{[N(2,6-Pr(i)(2)C(6)H(3))](2)CHC(LA)}, where LA = AlMe(3) [2·(THF)(2)] and BEt(3) [3·(THF)(2)].

Oxidation of carbene-stabilized diarsenic: diarsene dications and diarsenic radical cations.

Oxidation of carbene-stabilized diarsenic, L:As-As:L [L: = :C{N(2,6-(i)Pr(2)C(6)H(3))CH}(2)] (1), with gallium chloride in a 1:4 ratio in toluene affords the dicationic diarsene complex [L:As═As:L](2+)([GaCl(4)](-))(2) (2(2+)[GaCl(4)](2)), while oxidation of 1 with GaCl(3) in a 1:2 ratio in Et(2)O yields the monocationic diarsenic radical complex [L:AsAs:L](•+)[GaCl(4)](-) (2(•+)[GaCl(4)]). Strikingly, complex 2(•+) is the first arsenic radical to be structurally characterized in the solid state. The nature of the bonding in these complexes was probed computationally and spectroscopically.

NHC‐Stabilized Triorganozincates: Syntheses, Structures, and Transformation to Abnormal Carbene–Zinc Complexes

The chemistry of organozinc compounds has been inextricably entwined with the development of organic synthetic methods since Frankland s seminal discovery of zinc alkyls in 1849. Compared to diorganozinc reagents, the corresponding anionic zincate derivatives are considerably better nucleophiles. Consequently, triorganozincates [R3Zn]M (M = alkali metals) and tetraorganozincates [R4Zn]Mn (M = alkali metals, n = 2; M = alkali earth metals, n = 1) have been extensively utilized in organic transformations such as halogen–metal exchanges, nucleophilic additions, deprotonative metalations, and epoxide ring-openings. Recently, Mulvey et al. employed TMP-zincates (TMP = 2,2,6,6-tetramethylpiperidide) in developing a zincation– anion trapping strategy and in realizing the unusual transformation of a diamine into an unsaturated diazaethene through a lithium/zinc bimetallic system. The advantages of organozincates may be attributed to their intrinsic “bimetallic” character. Indeed, due to their synergic reactivity, bimetallic systems often outperform their monometallic components and have shown greater potential in organic transformations. Lithium zincate investigations reveal that not only the Li/Zn ratio 4, 9] but also the substituents may have remarkable effects on their reactivity. Thus, the TMP ligand has contributed to the renaissance of metallation chemistry involving organozincate compounds. We show here that extensions of the ligand systems can result in organozincates with novel reactivity. While N-heterocyclic carbenes (NHCs) have become ubiquitous ligands for organic and transition-metal catalysis, they are also capable of stabilizing highly reactive main group molecules. NHC-zinc complexes may act as polymerization catalysts. However, all the reported NHC-based zinc compounds are neutral and C2-bound. Considering the unique application of C4-bound NHC [i.e., abnormal carbene (aNHC)] complexes in catalysis, synthesis of NHC-based anionic zincates, especially those involving C4 carbene center, is intriguing. A common synthetic route to zincates involves the reaction of organolithium reagents with either diorganozinc or zinc halides. We recently synthesized the first anionic N-heterocyclic dicarbene [NHDC; 1, R = 2,6-iPr2C6H3; Scheme 1] through C4lithiation of the NHC ligand. Significantly, ligand 1 provides a convenient platform to access the unexplored NHCbased zincate chemistry. Herein, we report the syntheses,