Gregory H. Robinson

A Stable Silicon(0) Compound with a Si=Si Double Bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si–Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 ± 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si–Si(Cl):L and L:Si=Si:L.

Carbene-stabilized diphosphorus.

The potassium graphite reduction of L:PCl3 leads to the formation of carbene-stabilized diphosphorus molecules, L:P-P:L, 1 (L: = :C{N(2,6-Pri2C6H3)CH}2) and 2 (L: = :C{N(2,4,6-Me3C6H2)CH}2), respectively. The nature of the bonding in 1 and 2 was delineated by DFT computations.

Planar, twisted, and trans-bent: conformational flexibility of neutral diborenes.

The potassium graphite reduction of R‘BBr3 (R‘ = :C{N(2,4,6-Me3C6H2)CH}2) in Et2O led to the isolation of 3 (R‘(H)BB(H)R‘) and 4 (R‘(H)2B−B(H)2R‘), with BB double and B−B single bonds, respectively. These compounds were characterized by single-crystal X-ray diffraction, 1H and 11B NMR, and elemental analyses. Neutral diborene 3 exhibits polymorphic planar (3a), twisted (3b), and trans-bent (3c) geometries in the solid state.

N-Heterocyclic Carbene—Main-Group Chemistry: A Rapidly Evolving Field

This Award Article targets the evolving, yet surprisingly fruitful, chemistry of N-heterocyclic carbenes with low-oxidation-state main-group elements. Specifically, the chemistry of carbene-stabilized diatomic allotropes, diborenes, gallium octahedra, beryllium borohydride, and a host of related compounds will be presented. Providing a valuable historical perspective, the foundational work concerning the organometallic chemistry of gallium with sterically demanding m-terphenyl ligands from this laboratory will also be discussed.

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)].

A neutral Ga(6) octahedron: synthesis, structure, and aromaticity.

Potassium graphite reduction of L:Ga(Mes)Cl(2) [L: = :C{(i-Pr)NC(Me)}(2), Mes = 2,4,6-Me(3)C(6)H(2)] (1) in hexane yields the organogallium dimer L:(Mes)(Cl)Ga-Ga(Cl)(Mes):L (2), while potassium reduction of 1 in toluene affords the neutral aromatic Ga(6) octahedron L:Ga[Ga(4)Mes(4)]Ga:L (3).

Stabilization of elusive silicon oxides.

Molecular SiO2 and other simple silicon oxides have remained elusive despite the indispensable use of silicon dioxide materials in advanced electronic devices. Owing to the great reactivity of silicon-oxygen double bonds, as well as the low oxidation state of silicon atoms, the chemistry of simple silicon oxides is essentially unknown. We now report that the soluble disilicon compound, L:Si=Si:L (where L: = :C{N(2,6-(i)Pr2C6H3)CH}2), can be directly oxidized by N2O and O2 to give the carbene-stabilized Si2O3 and Si2O4 moieties, respectively. The nature of the silicon oxide units in these compounds is probed by spectroscopic methods, complementary computations and single-crystal X-ray diffraction.

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,

Reaction of trimethylaluminum with a macrocyclic tetradentate tertiary amine. Synthesis and molecular structure of [Al(CH3)3]4[N-tetramethylcyclam]

Abstract Reaction of the macrocyclic tetradentate tertiary amine N -tetramethylcyclam with trimethylaluminum produced the crystalline product [Al(CH 3 ) 3 ] 4 [ N -tetramethylcyclam]. The compound crystallizes in the orthorhombic space group Pbca with unit cell parameters a 13.928(5), b 18.522(6), c 1.4538(6) A, and D calc 0.96 g cm −3 for Z = 4. Least-squares refinement based on 1432 observed reflections led to a final R factor of 0.034, R w = 0.037. The molecule resides on a crystallographic center of symmetry. The four nitrogen atoms are coplanar. The macrocyclic ligand is greatly distorted as the four Al(CH 3 ) 3 units have essentially turned it “inside-out” by forcing the nitrogen atoms from the interior cavity to the macrocyclic perimeter. The independent A1-N distances of 2.093)(3) and 2.102(3) A are among the longest reported.