A Stable Aluminum Tris(dithiolene) Triradical.

A stable aluminum tris(dithiolene) triradical (3) was experimentally realized through a low-temperature reaction of the sterically demanding lithium dithiolene radical (2) with aluminum iodide. Compound 3 was characterized by single-crystal X-ray diffraction, UV-vis and EPR spectroscopy, SQUID magnetometry, and theoretical computations. The quartet ground state of triradical 3 has been unambiguously confirmed by variable-temperature continuous wave EPR experiments and SQUID magnetometry. Both SQUID magnetometry and broken-symmetry DFT computations reveal a small doublet-quartet energy gap [ΔEDQ = 0.18 kcal mol-1 (SQUID); ΔEDQ = 0.14 kcal mol-1 (DFT)]. The pulsed EPR experiment (electron spin echo envelop modulation) provides further evidence for the interaction of these dithiolene-based radicals with the central aluminum nucleus of 3.

A wealth of structures for the Ge2H2+ radical cation: comparison of theory and experiment.

Five structures of Ge2H2 and Ge2H2+ are investigated in this study. Optimized geometries at the CCSD(T)/cc-pwCVQZ-PP level of theory were obtained. Focal point analyses were performed on these optimized geometries to determine relative energies using the CCSD(T) method with polarized basis sets up to quintuple-zeta. Energy corrections include full T and pertubative (Q) coupled-cluster effects plus anharmonic corrections to the zero-point vibrational energy. Relative ordering in energy from lowest to highest of the five Ge2H2+ structures is butterfly, germylidene, monobridged, trans, then linear. In neutral Ge2H2, the monobridged structure lies lower in energy than the germylidene structure. Fundamental vibrational frequencies and IR intensities were computed for the minima at the CCSD(T)/cc-pwCVTZ-PP level of theory to compare with experimental research. Partial atomic charges and natural bonding orbital analyses indicated that the positive charge of Ge2H2+ is contained in the region of the Ge-Ge bond.

Structures and Energetics of E2H3+ (E = As, Sb, and Bi) Cations.

E2H2 (E = As, Sb, Bi) structures involving multiple bonds have attracted much attention recently. The E2H3+ cations (protonated E2H2) are predicted to be viable with substantial proton affinities (>180 kcal/mol). Herein, the bonding characters and energetics of a number of E2H3+ isomers are explored through CCSD(T) and DFT methods. For the As2H3+ system, the CCSD(T)/cc-pVQZ-PP method predicts that the vinylidene-like structure lies lowest in energy, with the trans and cis isomers higher by 6.7 and 9.3 kcal/mol, respectively. However, for Sb2H3+ and Bi2H3+ systems, the trans isomer is the global minimum, while the energies of the cis and vinylidene-like structures are higher, respectively, by 2.0 and 2.4 kcal/mol for Sb2H3+ and 1.6 and 15.0 kcal/mol for Bi2H3+. Thus, the vinyledene-like structure is the lowest energy for the arsenic system but only a transition state of the bismuth system. With permanent dipole moments, all minima may be observable in microwave experiments. Besides, we have also obtained transition states and planar-cis structures with higher energies. The current results should provide new insights into the various isomers and provide a number of predictions for future experiments.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species.

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

Germanium(II) Dithiolene Complexes.

The 1 : 2 reaction of the imidazole-based dithiolate (2) with GeCl2 • dioxane in THF/TMEDA gives 3, a TMEDA-complexed dithiolene-based germylene. Compound 3 is converted to monothiolate-complexed (5) and N-heterocyclic carbene-complexed (7) germanium(II) dithiolene complexes via Lewis base ligand exchange. A bis-dithiolene-based germylene (8), involving a 3c-4e S-Ge-S bond, has also been synthesized through controlled hydrolysis of 7. The bonding nature of 3, 5, and 8 was investigated by both experimental and theoretical methods.

Activation of Ammonia by a Carbene-Stabilized Dithiolene Zwitterion.

A carbene-stabilized dithiolene zwitterion (3) activates ammonia, affording 4• and 5, through both single-electron transfer (SET) and hydrogen atom transfer (HAT). Reaction products were characterized spectroscopically and by single-crystal X-ray diffraction. The mechanism of the formation of 4• and 5 was probed by experimental and computational methods. This discovery is the first example of metal-free ammonia activation via HAT.

Carbene-Stabilized Dithiolene (L0 ) Zwitterions.

A series of reactions between Lewis bases and an imidazole-based dithione dimer (1) has been investigated. Both cyclic(alkyl)(amino)carbene (CAAC) (2) and N-heterocyclic carbene (NHC) (4), in addition to N-heterocyclic silylene (NHSi) (6), demonstrate the capability to cleave the sulphur-sulphur bonds in 1, giving carbene-stabilized dithiolene (L0 ) zwitterions (3 and 5) and a spirocyclic silicon-dithiolene compound (7), respectively. The bonding nature of 3, 5, and 7 are probed by both experimental and theoretical methods.

Reduction of Dinitrogen via 2,3'-Bipyridine-Mediated Tetraboration.

A new molecular system for nitrogen reduction, involving a 2,3'-bipyridine-anchored, end-on-bridging dinitrogen complex of the Me2B-BMe2 intermediate (4), has been explored by theoretical methods. The 2,3'-bipyridine-mediated cleavage of the Bsp3-Bsp3 bond in 4 may lead to transient electron-rich sp3-hybridized boron species and subsequent activation of the strong N≡N triple bond of the complexed N2. Through a boryl transfer sequence, a catalytic cycle may be achieved for the reductive addition of diboranes to a dinitrogen molecule with an energy span of 23 kcal/mol. In addition, the reaction is exothermic by 80.5 kcal/mol, providing a substantive chemical driving force.

A Stable Naked Dithiolene Radical Anion and Synergic THF Ring-Opening.

Reaction of the lithium dithiolene radical 2• with the imidazolium salt [{(Me)CN(i-Pr)}2CH]+[Cl]- (in a 1:1 molar ratio) gives the first stable naked anionic dithiolene radical 3•, which, when coupled with hexasulfide, [{(Me)CN(i-Pr)}2CH]+2[S6]2- (4), and N-heterocyclic silylene 5, unexpectedly results in synergic THF ring-opening via a radical mechanism.

Unusual Structures of the Parent Molecules Diarsene, Distibene, and Dibismuthene: Toward Their Observation.

There is considerable interest, from both experimental and theoretical perspectives, in molecules incorporating multiple bonds between main group elements. Herein, we not only consider the parent molecules HE=EH (E=As, Sb, Bi), but also a number of their isomers. For each E2 H2 molecule, a number of different structures were optimized with four different DFT methods. Final structures were determined with the coupled cluster method CCSD(T) using large basis sets, namely cc-pVQZ-PP, incorporating relativistic psuedopotentials. All feasible dissociation pathways are examined. For all three E2 H2 molecules the trans isomer lies lowest in energy, with the cis isomer higher by 2.7 (As), 2.1 (Sb), and 1.8 (Bi) kcal mol-1 , respectively. However, both cis and trans structures should be observable, as large barriers (27.7, 20.5, and 17.7 kcal mol-1 ) separate them. For both the cis and trans structures, in the infrared the strong E-H stretching frequencies should also be observable. Only the cis structures have dipole moments (0.62, 0.01, and 0.83 debye, respectively), and their observation by microwave spectroscopy would be stunning. Also considered were the higher energy vinylidene-like, pyramidal, monobridged, and linear structures. We conclude that molecules such as HSb=SbH-Fe(CO)4 , HBi=BiH-Fe(CO)4 , and related systems, should be feasible synthetic targets.

Carbene-Stabilized Disilicon as a Silicon-Transfer Agent: Synthesis of a Dianionic Silicon Tris(dithiolene) Complex.

Reaction of carbene-stabilized disilicon (1) with the lithium-based dithiolene radical (2. ) affords the first dianionic silicon tris(dithiolene) complex (3). Notably, the formation of 3 represents the unprecedented utilization of carbene-stabilized disilicon (1) as a silicon-transfer agent. The nature of 3 was probed by multinuclear NMR spectroscopy, single-crystal X-ray diffraction, and DFT computations.

Stable Boron Dithiolene Radicals.

Whereas low-temperature (-78 °C) reaction of the lithium dithiolene radical 1. with boron bromide gives the dibromoboron dithiolene radical 2. , the parallel reaction of 1. with (C6 H11 )2 BCl (0 °C) affords the dicyclohexylboron dithiolene radical 3. . Radicals 2. and 3. were characterized by single-crystal X-ray diffraction, UV/Vis, and EPR spectroscopy. The nature of these radicals was also probed computationally. Under mild conditions, 3. undergoes unexpected thiourea-mediated B-C bond activation to give zwitterion 4, which may be regarded as an anionic dithiolene-modified carbene complex of the sulfenyl cation RS+ (R=cyclohexyl).

Facile Conversion of Bis-Silylene to Cyclic Silylene Isomers: Unexpected C-N and C-H Bond Cleavage.

Reaction of thiolate 1 with carbene-stabilized diiodo-bis-silylene (2) (in a 2:1 ratio) in THF unexpectedly gives both the first five-membered, sulfur-containing, zwitterionic silylene ring (3) via insertion of the "SiI2" unit of 2 into the olefinic C-H bond of the imidazole ring of 1 and four-membered cyclic silylene (4) via insertion of a silicon(I) atom of 2 into the Cphenyl-N bond of the carbene ligand. The parallel reaction in toluene only gives 3 as the major product. The nature of the bonding in isomeric 3 and 4 was probed by experimental and theoretical methods.

A Stable Anionic Dithiolene Radical.

Sulfurization of anionic N-heterocyclic dicarbene, [:C{[N(2,6-Pri2C6H3)]2CHCLi}]n (2), with elemental sulfur (in a 1:2 ratio) in Et2O at low temperature gives 3 by inserting two sulfur atoms into the Li-C (i.e., C2 and C4) bonds in polymeric 2. Further reaction of 3 with 2 equiv of elemental sulfur in THF affords 4• via unexpected C-H bond activation, which represents the first anionic dithiolene radical to be structurally characterized in the solid state. Alternatively, 4• may also be synthesized directly by reaction of 1 with sulfur (in a 1:4 ratio) in THF. Reaction of 4• with GeCl2·dioxane gives an anionic germanium(IV)-bis(dithiolene) complex (5). The nature of the bonding in 4• and 5 was probed by experimental and theoretical methods.

Push-Pull Stabilization of Parent Monochlorosilylenes.

While reaction of carbene-stabilized disilicon L:Si═Si:L (L: = C{N(2,6-(i)Pr2C6H3)CH}2) (8) with HCl·NC5H5 results in carbene-stabilized Si2Cl2 (2) and substituted 1H-imidazole (9), combination of the corresponding Fe(CO)4-modified disilicon carbene complex L:Si═Si[Fe(CO)4]:L (6) with pyridine hydrochloride gives a species containing two push-pull-stabilized parent monochlorosilylenes that are bridged by an Fe(CO)3 unit (7). The nature of 7 was further elucidated by spectroscopic, crystallographic, and computational methods. Spectroscopic data suggest that 7 exists as two diastereoisomers.

Stabilization of Silicon-Carbon Mixed Oxides.

The first carbene-stabilized silicon-carbon mixed oxide, (SiO2)2CO2 (4), was synthesized by CO2 oxidation of either carbene-stabilized disilicon, L:Si═Si:L (L: = :C{N(2,6-Pr(i)2C6H3)CH}2) (1), or carbene-stabilized Si2O3 (2) (which can be obtained via N2O oxidation of 1). The structure and bonding of 4 was probed by both experimental and computational methods.

Transition-Metal-Mediated Cleavage of a Si=Si Double Bond.

Reaction of carbene-stabilized disilicon (1) with Fe(CO)5 gives the 1:1 adduct L:Si=Si[Fe(CO)4 ]:L (L:=C{N(2,6-Pr(i) 2 C6 H3 )CH}2 ) (2) at room temperature. At raised temperature, however, 2 may react with another equivalent of Fe(CO)5 to give L:Si[µ-Fe2 (CO)6 ](µ-CO)Si:L (3) through insertion of both CO and Fe2 (CO)6 into the Si2 core, which represents the first experimental realization of transition metal-carbonyl-mediated cleavage of a Si=Si double bond. The structures and bonding of both 2 and 3 have been investigated by spectroscopic, crystallographic, and computational methods.

Dynamic complexation of copper(I) chloride by carbene-stabilized disilicon.

Reaction of N-heterocyclic-carbene (NHC)-stabilized disilicon (1) with CuCl gave a carbene-stabilized disilicon-copper(I) chloride complex (2). The nature of the structure and bonding in 2 has been investigated by crystallographic, spectroscopic, and computational methods. The dynamic complexation behavior of 2 was experimentally explored by variable-temperature NMR analysis.

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.

Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane.

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.