"Broken-hearted" carbon bowl via electron shuttle reaction: energetics and electron coupling.

Unprecedented one-step C[double bond, length as m-dash]C bond cleavage leading to opening of the buckybowl (π-bowl), that could provide access to carbon-rich structures with previously inaccessible topologies, is reported; highlighting the possibility to implement drastically different synthetic routes to π-bowls in contrast to conventional ones applied for polycyclic aromatic hydrocarbons. Through theoretical modeling, we evaluated the mechanistic pathways feasible for π-bowl planarization and factors that could affect such a transformation including strain and released energies. Through employment of Marcus theory, optical spectroscopy, and crystallographic analysis, we estimated the possibility of charge transfer and electron coupling between "open" corannulene and a strong electron acceptor such as 7,7,8,8-tetracyanoquinodimethane. Alternative to a one-pot solid-state corannulene "unzipping" route, we reported a nine-step solution-based approach for preparation of novel planar "open" corannulene-based derivatives in which electronic structures and photophysical profiles were estimated through the energies and isosurfaces of the frontier natural transition orbitals.

Multiple activations of CH bonds in arenes and heteroarenes.

The activation of CH bonds in arenes and heteroarenes has attracted considerable attention in recent years. Examples of the activation of two or more CH bonds in arenes or heteroarenes are rare. In recent studies it has been found that certain polynuclear metal carbonyl complexes, such as Re2(CO)8(µ-C6H5)(µ-H), 1 and Os3(CO)10(NCMe)2, can react two or more times with selected arenes and heteroarenes through a series of CH activations to yield interesting new multiply-CH activated arenes and heteroarenes and can lead to the opening of ring systems in the case of heteroarenes. A summary of these novel reactions and new products is presented in this Frontier article.

Activation of Heteroaromatic C-H Bonds in Furan and 2,5-Dimethylfuran.

The reaction of Re2(CO)8(µ-C6H5)(µ-H) (1) with furan in CH2Cl2 at 40 °C yielded two new isomeric dirhenium compounds, Re2(CO)8(µ-η2-2,3-C4H3O)(µ-H) (2) and Re2(CO)8(µ-η2-3,2-C4H3O)(µ-H) (3), which contain a bridging (σ + π)-coordinated furyl ligand formed by activation of the C-H bond at the 2 and 3 positions of furan, respectively. Compound 3 is the first example of a compound formed by C-H bond activation at the 3 position of a furan ring. Compound 3 was isomerized to 2 by heating to 80 °C for 7 days. The reaction of compound 2 with a second 1 equiv of 1 in CH2Cl2 at 40 °C yielded the doubly metalated furan-bis[Re2(CO)8(µ-H)] product [Re2(CO)8(µ-H)]2(µ-η2-2,3-µ-η2-5,4-C4H2O) (4) containing a 2,5-furdiyl ligand that is (σ + π)-coordinated to two Re2(CO)8(µ-H) groups by activation of the C-H bond at the 5 position of the furyl ligand in 2. Compound 2 can also be formed as the major isomer, together with a small quantity of 4 by heating furan with Re2(CO)8(µ-H)[µ-η2-C(H)═C(H)Bu n] in toluene to reflux. An isomer of 4, [Re2(CO)8(µ-H)]2(µ-η2-2,3-µ-η2-4,5-C4H2O) (5), was obtained from the reaction of 3 with 1 through C-H activation at the 4 position of the furyl ring in 3. Compound 4 was also obtained from 5 by heating to 110 °C for 24 h. The reaction of 1 with 2,5-dimethylfuran (DMFUR) in CH2Cl2 at 40 °C yielded the new compound [Re2(CO)8(µ-η2-3,2-(CH3)2C4H2O)](µ-H) (6), which contains a bridging (σ + π)-coordinated 2,5-dimethylfuryl ligand formed by activation of the C-H bond at the 3 position of DMFUR. All of the new compounds were characterized structurally by single-crystal X-ray diffraction analysis.

Multiple C-H Bond Activations in Corannulene by a Dirhenium Complex.

The reaction of Re2 (CO)8 (µ-C6 H5 )(µ-H), 1 with corannulene (C20 H10 ) yielded the product Re2 (CO)8 (µ-H)(µ-η2 -1,2-C20 H9 ), 2 (65 % yield) containing a Re2 metalated corannulene ligand formed by loss of benzene from 1 and the activation of one of the CH bonds of the nonplanar corannulene molecule by an oxidative-addition to 1. The corannulenyl ligand has adopted a bridging η2 -σ+π coordination to the Re2 (CO)8 grouping. Compound 2 reacts with a second equivalent of 1 to yield three isomeric doubly metalated corannulene products: Re2 (CO)8 (µ-H)(µ-η2 -1,2-µ-η2 -10,11-C20 H8 )Re2 (CO)8 (µ-H), 3 (35 % yield), Re2 (CO)8 (µ-H)(µ-η2 -2,1-µ-η2 -10,11-C20 H8 )Re2 (CO)8 (µ-H), 4 (12 % yield), and Re2 (CO)8 (µ-H)(µ-η2 -1,2-µ-η2 -11,10-C20 H8 )Re2 (CO)8 (µ-H), 5 (12 % yield), by a second CH activation on a second rim double bond on the corannulene molecule. The isomers differ by the relative orientations of the coordinated Re2 (CO)8 (µ-H) groupings. All new products were characterized structurally by single crystal X-ray diffraction analysis.

Multiple Aromatic C-H Bond Activations by an Unsaturated Dirhenium Carbonyl Complex.

Reactions of Re2(CO)8(µ-C6H5)(µ-H), 1, with naphthalene and anthracene have yielded the first multiply-CH activated arene products through the reductive elimination of benzene from 1 and multiple oxidative-additions of the dirhenium octacarbonyl grouping to these polycyclic aromatic compounds under very mild conditions. In addition, compound 1 was found to react with itself to yield the bis-Re2-metalated C6H4 bridged compound Re2(CO)8(µ-H)(µ-1,µ-3-C6H4)Re2(CO)8(µ-H), 3. Reaction of 1 with naphthalene yielded two doubly CH activated isomers, Re2(CO)8(µ-H)(µ-η2-1,2-µ-η2-3,4-C10H6)Re2(CO)8(µ-H), 4, 41% yield, and Re2(CO)8(µ-H)(µ-η2-1,2-µ-η2-5,6-C10H6)Re2(CO)8(µ-H), 5, via the mono CH activated intermediate Re2(CO)8(µ-η2-C10H7)(µ-H), 2. Compound 4 contains two Re2(CO)8(µ-H) groups on one C6 ring formed by CH activations at the 2- and 4-positions. Compound 5 contains two Re2(CO)8(µ-H) groups; one formed by CH activation at the 2-position on one C6 ring and the other formed by CH activation at the 6-position (or centrosymmetrically related 2'-position) on the second C6 ring. The Re2(CO)8(µ-H) groups are coordinated to the C6 rings by binuclear σ + π coordination to two adjacent carbon atoms in the rings. Compound 1 reacts with anthracene to yield the mono-CH activated compound Re2(CO)8(µ-η2-1,2-C14H9)(µ-H), 6, and two doubly CH activated compounds, Re2(CO)8(µ-H)(µ-η2-1,2-µ-η2-3,4-C14H8)Re2(CO)8(µ-H), 7, and Re2(CO)8(µ-H)(µ-η2-1,2-µ-η2-5,6-C14H8)Re2(CO)8(µ-H), 8. Compounds 7 and 8 are isomers that are structurally similar to 4 and 5. Compounds 7 and 8 can also be obtained in good yields from the reaction of 6 with 1. In the presence of a 5/1 ratio of 1/anthracene, a small amount (5% yield) of the tetra-substituted anthracene product [Re2(CO)8(µ-H)]4(µ-η2-1,2-µ-η2-3,4-µ-η2-5,6-µ-η2-7,8-C14H6), 9, was formed. Compound 9 contains four σ + π coordinated Re2(CO)8(µ-H) groups formed by oxidative additions of the CH bonds of anthracene to the Re2(CO)8 groups at the 2, 4, 6, and 8 positions of the three ring system. Molecular orbital calculations have been performed for all new compounds in order to develop an understanding of the bonding of the ring systems to the Re2(CO)8(µ-H) groups. All new compounds were characterized by single-crystal X-ray diffraction analyses.

Multiple C-H Bond Activations and Ring-Opening C-S Bond Cleavage of Thiophene by Dirhenium Carbonyl Complexes.

The reaction of Re2(CO)8(µ-C6H5)(µ-H) (1) with thiophene in CH2Cl2 at 40 °C yielded the new compound Re2(CO)8(µ-η2-SC4H3)(µ-H) (2), which contains a bridging σ-π-coordinated thienyl ligand formed by the activation of the C-H bond at the 2 position of the thiophene. Compound 2 exhibits dynamical activity on the NMR time scale involving rearrangements of the bridging thienyl ligand. The reaction of compound 2 with a second 1 equiv of 1 at 45 °C yielded the doubly metalated product [Re2(CO)8(µ-H)]2(µ-η2-2,3-µ-η2-4,5-C4H2S) (3), formed by the activation of the C-H bond at the 5 position of the thienyl ligand in 2. Heating 3 in a hexane solvent to reflux transformed it into the ring-opened compound Re(CO)4[µ-η5-η2-SCC(H)C(H)C(H)][Re(CO)3][Re2(CO)8(µ-H)] (4) by the loss of one CO ligand. Compound 4 contains a doubly metalated 1-thiapentadienyl ligand formed by the cleavage of one of the C-S bonds. When heated to reflux (125 °C) in an octane solvent in the presence of H2O, the new compound Re(CO)4[η5-µ-η2-SC(H)C(H)C(H)C(H)]Re(CO)3 (5) was obtained by cleavage of the Re2(CO)8(µ-H) group from 4 with formation of the known coproduct [Re(CO)3(µ3-OH)]4. All new products were characterized by single-crystal X-ray diffraction analyses.

Biosorption of Heavy Metals from Aqueous Solution by Bacteria Isolated from Contaminated Soil.

This study was carried out to analyze the heavy metals biosorption potential of bacteria isolated from soil contaminated with electroplating industrial effluents. Bacterial isolates were screened for their multi-metal biosorption potential against copper, nickel, lead, and chromium. Bacterial isolate CU4A showed the maximum uptake of copper, nickel, lead, and chromium in aqueous solution, with a biosorption efficiency of 87.16 %, 79.62%, 84.92%, and 68.12%, respectively. The bacterial strain CU4A was identified as Bacillus cereus, following 16S rRNA gene sequence analysis. The surface chemical functional groups of bacterial biomass were identified by Fourier transform infrared (FTIR) spectroscopy as hydroxyl, carboxyl, amine, and halide, which may be involved in the biosorption of heavy metals. Analysis with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) confirmed the adsorption of metals on the bacterial cell mass. The results of this study are significant and could be further investigated for the removal of heavy metals from contaminated environments.

Multiple aromatic C-H bond activations by a dirhenium carbonyl complex.

Multiple aromatic CH activations by a dirhenium complex have provided meta-related dimetallated arene rings in the complexes Re2(CO)8(µ-H)(µ-η2-1,2-µ-η2-3,4-C14H8)Re2(CO)8(µ-H), 3 and Re2(CO)8(µ-H)(µ-η1-1,µ-η1-3-C6H4)Re2(CO)8(µ-H), 4. Both products were characterized structurally by single crystal X-ray diffraction and DFT molecular orbital analyses.

Synthesis and Reactivity of Electronically Unsaturated Dirhenium Carbonyl Compounds Containing Bridging Gold-Carbene Groups.

The electronically unsaturated compounds Re2(CO)8[µ-Au(NHC)](µ-Ph), 1, and Re2(CO)8[µ-Au(NHC)]2, 2, were obtained from the reaction of Re2(CO)8[µ-η2-C(H)═C(H)Bun](µ-H) with MeAu(NHC), NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. Compound 1 was converted to the new compound Re2(CO)8[µ-Au(NHC)](µ-H), 3, by reaction with H2. Addition of CO to 3 yielded the new compound Re2(CO)9[Au(NHC)](µ-H), 4, which contains a terminally coordinated Au(NHC) group on one of the rhenium atoms, and the hydrido ligand was shifted to bridge the Re-Au bond. The mechanism of the formation of 4 was established by DFT computational analyses. Compound 3 also reacted with C2H2 by an addition with insertion into the Re-H bonds to yield the compound Re2(CO)8[µ-Au(NHC)](µ-C2H3), 5, which contains a σ-π coordinated, bridging C2H3 ligand. The stereochemistry of the insertion was found to proceed preferentially with a cis- (syn-) stereochemistry. Compound 1 reacted with HCl to yield Re2(CO)8[µ-Ph](µ-H), 6, and ClAu(NHC) by selective removal of the bridging Au(NHC) group. All new compounds were characterized by single-crystal X-ray diffraction analyses.