Metal-metal and carbon-carbon bonds as potential components of molecular batteries

The reductive coupling of [M(salophen)] derivatives, where M is an early transition metal and salophen is N,N'-o-phenylenebis(salicylideneaminato) dianion, led to the formation of dimers linked through C-C and M-M bonds. Both of these bonds can potentially function as electron reservoirs: each bond can be used as a reversible source of a pair of electrons under the condition that it is not chemically transformed by the incoming substrate which functions as an electron acceptor. To explore this potential function as well as the competition in the redox processes between C-C and M-M bonds within the same molecular framework, we investigated the reduction of [(tBu4-salophen)NbCl3] (1) and [(tBu4-salophen)MoCl2] (7) as model compounds. In the former case, the reduction led to [(Nb-Nb)(tBu4-*salophen2*)] (2) which contains both a Nb-Nb bond (2.6528(7) A) and two C-C bonds across two imino groups of the ligand. Complex 2 can be reduced further to a transient compound 5 that contains an Nb=Nb bond. In the second case, the reduction of 7 by two electrons led to [(Mo[triplebond]Mo)(tBu4-salophen)2] (8), which does not contain any C-C linkages between the two salophen units. Complexes 2 and 5 are able to transfer one pair and two pairs of electrons, respectively, to give compounds 3, 4, and 6, with the consequent cleavage of the Nb-Nb and Nb=Nb bonds. In the present case, it is surprising that the C-C bonds do not participate in the reduction of the substrates. A careful theoretical treatment anticipates, both in the case of 1 and 7, the preferential formation of metal-metal bonds upon reduction. This is indeed the case for 7, but not for 1, where the formation of C-C bonds competes with that of M-M bonds, the latter being the first ones, however, to be involved in electron-transfer reactions. The theoretical approach allowed us to investigate the possibility of intramolecular electron transfer from C-C bonds to M-M bonds and vice versa.     

Mechanism of carbon-halogen bond reductive cleavage in activated alkyl halide initiators relevant to living radical polymerization: theoretical and experimental study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(?-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(?-), which is shown here to be nonexistent.     

Carbon-carbon bond formation from azaallyl and imine couplings about metal-metal bonds

Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation.     

Topographical analyses of homonuclear multiple bonds between main group elements

Recent experiments have resulted in the completion of the series of Group 14 and Group 15 element double-bond systems, R(n)E=ER(n) (E = C - Pb, n = 2; E = N - Bi, n = 1). Furthermore, new families of multiple-bonded species have been discovered, such as the radical anion [RSnSnR](-) , the close ion pairs [RE(mu2Na)2ER] (E = Ge,Sn), and a digallyne [RGa(mu2Na)2GaR] for which a Ga=Ga triple bond was formulated. Some of these compounds show classical multiple bond features (i.e. the dipnictogens RE=ER, E=N-Bi) in the sense that planar structures with short E-E distances are observed. However, many (i.e. R2E=ER2, E = Si - Pb) do not behave as expected for compounds with multiple bonds. They have trans bent structures, show enormous variation in their E-E distances, and some dissociate easily under E-E bond cleavage in solution. These properties raised doubts as to whether these compounds can be formulated as multiple-bonded systems. Using the electron localization function (ELF) it is possible to clearly show the topographical similarities between classical and nonclassical multiple bonds; ELF divides these systems into unslipped (classical) and slipped (nonclassical) systems. ELF can also be employed to confirm the nonexistence of multiple bonds. Therefore, topographical analyses using ELF are useful to categorize a bonding system. In particular, the bonds in the heavier Group 14 double systems and the Ga-Ga bond in digallyne are clearly shown by this method as slipped double and triple bonds, respectively.     

Stereoelectronic effects and general trends in hyperconjugative acceptor ability of sigma bonds

A systematic study of general trends in sigma acceptor properties of C-X bonds where X is a main group element from groups IVa-IIa is presented. The acceptor ability of the C-X sigma bonds in monosubstituted ethanes increases when going to the end of a period and down a group. Enhancement of acceptor ability of C-X sigma bonds as one moves from left to right in periods parallels the increase in electronegativity of X, whereas augmentation of acceptor ability in groups is opposite to the changes in electronegativity of X and in the C-X bond polarization, following instead the decrease in the energy of sigma(C)(-)(X) orbitals when one moves from the top to the bottom within a group. This simple picture of acceptor ability of sigma bonds being controlled by electronegativity in periods and by sigma orbital energy in groups is changed in monosubstituted ethenes where the role of electronegativity of the substituent X becomes more important due to increased overlap between sigma orbitals. The combination of several effects of similar magnitude influences acceptor ability of sigma bonds in monosubstituted ethenes in a complex way. As a result, the acceptor ability of sigma bonds can be significantly modified by substitution and is conformer dependent. Stereoelectronic effects displayed by C-X bonds with X from second and third periods are highly anisotropic. For example, C-chalcogen bonds are excellent sigma acceptors at the carbon end but poor sigma acceptors at the chalcogen end. This effect can be relied upon in the design of molecular diodes with sigma bridges with unidirectional electron conductivity. While the general trends revealed in this work should be useful for the qualitative understanding of stereoelectronic effects, one should bear in mind that the magnitude of hyperconjugative effects is extremely sensitive to small variations in structure and in substitution. This advocates for the increased role of theoretical methods in analysis of stereoelectronic effects.     

Topological insights into the nature of the halogen-carbon bonds in dimethylhalonium ylides and their cations

In this study the nature of the bonding in a series of dimethylhalonium ylides (fluoronium, chloronium, bromonium and iodonium) was analyzed by means of topological methodologies (AIM and ELF analysis), to document the changes in the nature of the C-X bonds (X = F, Cl, Br, I) upon the series. For the sake of comparison the same study was performed on the corresponding dimethylhalonium cations (XC 2H 6 (+)) and the XCH 3 series. The wave functions used for the topological analysis were obtained at B3LYP level using extended triple-zeta basis sets. The formation of the cationic XC 2H 6 (+) structures can be interpreted to arise from the interaction between the XCH 3 and CH 3 (+) moieties. The resultant structures can be explained in terms of the superposition of two electrostatically interacting and two dative mesomeric structures. The halogen-carbon bonds have all the characteristics of the charge-shift (CS) bonds. The analysis of the C-X bond in the XC 2H 5 series shows a progressive reinforcing of the CH 3X-CH 2 bond, from FC 2H 5 that can be considered as formed from two fragments, FCH 3 and CH 2, to IC 2H 5, in which the CH 3I-CH 2 bond has all the features of a multiple bond involving atoms bearing lone pairs. Particularly interesting is BrC 2H 5, in which a special type of bond (hybrid covalent-dative double bond) has been characterized. The energetic stability of the XC 2H 5 structures with respect to the dissociation into the XCH 2 + CH 3 and XCH 3 + CH 2 ground-state fragments was studied in detail.     

Electron deficient carbon-titanium triple bonds: formation of triplet XC/TiX3 methylidyne complexes

Laser-ablated titanium atoms react with CX4 (X = F and Cl) to produce triplet state XC/TiX3 complexes trapped in an argon matrix. Products are identified by their infrared spectra and comparison to theoretically predicted vibrations. Density functional theory calculations converge to C(3v) symmetry structures for these lowest-energy products. The two unpaired electrons in the carbon 2p orbitals are shared with empty titanium d orbitals leading to degenerate singly occupied pi molecular orbitals and an electron-deficient triple bond between the carbon and titanium centers, on the basis of DFT bonding analysis and spin density calculations. The carbon-titanium distances are near typical C=Ti double bond lengths, and the C-X bonds in the resulting products are shorter than in the CX4 precursors. It appears that X lone-pair conjugation contributes to the C-X bond strength and somewhat to the C-Ti bond, and Cl does better in this regard than F.     

Photodecomposition profiles of beta-bond cleavage of phenylphenacyl derivatives in the higher triplet excited states during stepwise two-color two-laser flash photolysis

Photochemical properties of p-phenylphenacyl derivatives (PP-X) having C-halide, C-S, and C-O bonds in the lowest (T 1) and higher (T n ) triplet excited states were investigated in solution by using single-color and stepwise two-color two-laser flash photolysis techniques. PP-Xs (X = Br, SH, and SPh) undergo beta-bond dissociation in the lowest singlet excited states (S 1) while the C-X bonds of other PP-Xs are stable upon 266-nm laser photolysis. The T 1(pi,pi*) states of PP-X were efficiently produced during 355-nm laser photolysis of benzophenone as a triplet sensitizer. Triplet PP-Xs deactivate to the ground state without photochemical reactions. Upon 430-nm laser photolysis of the T 1 states of PP-X (X = Br, Cl, SH, SPh, OH, OMe, and OPh), decomposition of PP-X in the T n states was found. On the basis of the changes in the transient absorption, quantum yields (Phi dec) of the decomposition of PP-X in the T n states were determined, while bond dissociation energies (BDE) of the C-X bonds were calculated by computations. According to the relationship between the Phi dec and BDE values, it was shown that the decomposition of PP-X in the T n state is due to beta-cleavage of the corresponding C-X bond, and that the state energy of the reactive T n for the C-O bond cleavage differs from that for the C-halide and C-S bond cleavage. The reaction profiles of the C-X bond cleavage of PP-X in the T n states were discussed.     

Bonding analysis of metal-metal multiple bonds in R3M-M'R3 (M, M' = Cr, Mo, W; R = Cl, NMe2)

The bonding situation of homonuclear and heteronuclear metal-metal multiple bonds in R(3)M-M'R(3) (M, M' = Cr, Mo, W; R = Cl, NMe(2)) is investigated by density functional theory (DFT) calculations, with the help of energy decomposition analysis (EDA). The M-M' bond strength increases as M and M' become heavier. The strongest bond is predicted for the 5d-5d tungsten complexes (NMe(2))(3)W-W(NMe(2))(3) (D(e) = 103.6 kcal/mol) and Cl(3)W-WCl(3) (D(e) = 99.8 kcal/mol). Although the heteronuclear molecules with polar M-M' bonds are not known experimentally, the predicted bond dissociation energies of up to 94.1 kcal/mol for (NMe(2))(3)Mo-W(NMe(2))(3) indicate that they are stable enough to be isolated in the condensed phase. The results of the EDA show that the stronger R(3)M-M'R(3) bonds for heavier metal atoms can be ascribed to the larger electrostatic interaction caused by effective attraction between the expanding valence orbitals in one metal atom and the more positively charged nucleus in the other metal atom. The orbital interaction reveal that the covalency of the homonuclear and heteronuclear R(3)M-M'R(3) bonds is due to genuine triple bonds with one σ- and one degenerate π-symmetric component. The metal-metal bonds may be classified as triple bonds where π-bonding is much stronger than σ-bonding; however, the largest attraction comes from the quasiclassical contribution to the metal-metal bonding. The heterodimetallic species show only moderate polarity and their properties and stabilities are intermediate between the corresponding homodimetallic species, a fact which should allow for the experimental isolation of heterodinuclear species. CASPT2 calculations of Cl(3)M-MCl(3) (M = Cr, Mo, W) support the assignment of the molecules as triply bonded systems.     

Metal-thiolate bonds in bioinorganic chemistry

Metal-thiolate active sites play major roles in bioinorganic chemistry. The M--S(thiolate) bonds can be very covalent, and involve different orbital interactions. Spectroscopic features of these active sites (intense, low-energy charge transfer transitions) reflect the high covalency of the M--S(thiolate) bonds. The energy of the metal-thiolate bond is fairly insensitive to its ionic/covalent and pi/sigma nature as increasing M--S covalency reduces the charge distribution, hence the ionic term, and these contributions can compensate. Thus, trends observed in stability constants (i.e., the Irving-Williams series) mostly reflect the dominantly ionic contribution to bonding of the innocent ligand being replaced by the thiolate. Due to high effective nuclear charges of the Cu(II) and Fe(III) ions, the cupric- and ferric-thiolate bonds are very covalent, with the former having strong pi and the latter having more sigma character. For the blue copper site, the high pi covalency couples the metal ion into the protein for rapid directional long range electron transfer. For rubredoxins, because the redox active molecular orbital is pi in nature, electron transfer tends to be more localized in the vicinity of the active site. Although the energy of hydrogen bonding of the protein environment to the thiolate ligands tends to be fairly small, H-bonding can significantly affect the covalency of the metal-thiolate bond and contribute to redox tuning by the protein environment.     

Ligand-free Ni-catalyzed reductive cleavage of inert carbon-sulfur bonds

A catalytic reductive cleavage of C(sp(2))- and C(sp(3))-SMe bonds under ligandless conditions is presented. The method is characterized by its wide scope and high chemoselectivity profile including challenging substrate combinations, allowing the design of orthogonal and site-selectivity approaches.     

含杂三元环化合物键强变化规律

用键强参数f~i, ~A~-B, f~A~-~B可定量地表达含杂三元环化合物CH2-CH2-X(X=Be, BH, CH2, NH, O及S)中成键情况及键强变化规律: 1. 无论在三元环中, 还是在C-X-C中, X=Be→O时, f~c~-~x随X中重原子的核电荷数Z递增而增大。其原因: X中重原子实对于价电子的有效核电荷数随其核电荷数Z的增大而增大。C-X的"键电荷"也随X的Z值递增。2. 当X=Be→O, 在形成CH2-CH2-X时, 较强键被削弱, 较弱键则增强, 致环内各-键键强和键长都有平均化的趋势。其原因为: 三元环分子中形成σ共轭的三中心键, C-X与C-C键共享"键电荷"。这种"σ-共轭效应"与π-共轭效应有相似处。3.由于硫的价电子云平均半径较大, 可向c-C提供更多共享电荷, 故在含S三元环化合物CH2-CH2-S中C-S键受到更大程度的削弱, C-C则更被增强。     

Metal-catalyzed nucleophilic carbon-heteroatom (C-X) bond formation: the role of M-X intermediates

Many important reactions that lead to carbon-heteroatom bond formation involve attack of anionic heteroatom nucleophiles, such as hydroxides, alkoxides, amides, thiolates and phosphides, at carbon. Related catalytic transformations are mediated by late transition metal complexes of these groups, which remain nucleophilic on metal coordination as a result of repulsive filled-filled interactions between the heteroatom lone pairs and metal d-orbitals and/or of polarization of the bonds Mdelta+-Xdelta-. This Perspective presents examples of catalytic nucleophilic C-X bond formation in both biological and synthetic systems and describes how changes in the metal, ancillary ligands and X groups may be used to tune nucleophilic reactivity.     

Density functional analysis of group 10 organometallic diphosphinito complexes for catalytic formation of C‐P bonds

New group 10 metalloorganic complexes are proposed as the basis of new catalysts for the formation of carbon‐phosphorous bonds. Density functional theory (DFT) is applied, using multiple DFT functionals, to model molecular geometry as well as electron density distribution in the highest occupied molecular orbitals (HOMOs) expected to carry out a reductive catalytic cycle. DFT/M06 analysis predicts a robust planar geometry, regardless of alteration of major components. Precursors for rapid catalyst generation should begin with an electron‐withdrawing monodentate ligand. Palladium and platinum catalysts have lower chemical hardness, but the electron distribution in the HOMO of the nickel‐based catalyst is preferred for reductive catalytic mechanisms. Both electron density and chemical hardness, however, are affected by the choice of metal ion and the composition of the monodentate ligand bound to it. Group 10 metalloorganic complexes are modeled as precursors for generating new catalysts for a minimally wasteful method of forming bonds commonly found in biochemically active compounds. Suitable precursors have an accessible metal center, as well as significant the HOMO/LUMO involvement at the metal center. All complexes studied offer similar geometries, but precursor transformation into catalyst depends on the electron‐withdrawing ligand being exchanged. Catalyst turn over number is predicted to depend primarily on the central metal. © 2016 Wiley Periodicals, Inc.     

Binuclear manganese and rhenium carbonyls M2(CO)n (n = 10, 9, 8, 7): comparison of first row and third row transition metal carbonyl structures

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic binuclear rhenium carbonyls Re2(CO)n (n = 10, 9, 8, 7) were determined using the MPW1PW91 and BP86 methods from density functional theory (DFT) with the effective core potential basis sets LANL2DZ and SDD. In all cases triplet structures for Re2(CO)n were found to be unfavorable energetically relative to singlet structures, in contrast to corresponding Mn2(CO)n derivatives, apparently owing to the larger ligand field splitting of rhenium. For M2(CO)10 (M = Mn, Re) the unbridged structures (OC)5M-M(CO)5 are preferred energetically over structures with bridging CO groups. For M2(CO)9 (M = Mn, Re) the two low energy structures are (OC)4M(micro-CO)M(CO)4 with an M-M single bond and a four-electron donor bridging CO group and (OC)4M[double bond, length as m-dash]M(CO)5 with no bridging CO groups and an M[double bond, length as m-dash]M distance suggesting a double bond. The lowest energy structures for Re2(CO)8 have Re[triple bond, length as m-dash]Re distances in the range 2.6-2.7 A suggesting the triple bonds required to give the Re atoms the favored 18-electron configuration. Low energy structures for Re2(CO)7 are either of the type (OC)(4)M[triple bond, length as m-dash]M(CO)3 with short metal-metal distances suggesting triple bonds or have a single four-electron donor bridging CO group and longer M-M distances consistent with single or double bonds. The 18-electron rule thus appears to be violated in these highly unsaturated Re2(CO)7 structures.     

Characteristics of multiple N,O bonds

The electron distribution for N,O bonds in a wide range of molecules is analyzed using Bader's atoms in molecules theory and measures of bond order. It is shown that electron density derived parameters correlate very well with bond length and bond orders. An unexpectedly large number of bond orders are near to or greater than 2, indicating that nitrogen is effectively pentavalent in a many simple molecules. This conclusion is substantiated by examination of the Laplacian of the electron density. Such hypervalence indicates that nitrogen does not always obey the classical octet rule.     

Roussin红盐和Roussin黑盐的电子结构

Roussin黑盐簇阴离子及其"元件化合物"Roussin红盐簇阴离子,是固氮酶活性中心福州模型I(网兜状原子簇模型)的模型物.本文用闭壳层CNDO/2(S,D方案)法计算了它们的电子结构.根据计算所得的Mulliken重叠集居,电荷密度,分子轨道能量和轨道特征等数据,对成键性质进行了分析,得出如下主要结论:两种簇阴离子骨架电子的非定域性都比较强,桥硫原子Sb在由红盐形成黑盐的电子转移过程中起施主作用,两种簇阴离子中都存在M-M键,强度与M-Sb键相近,其主要贡献都来源于金属的s,pz,dz2轨道与硫原子的s,pz轨道之间的σ作用,金属d轨道的π作用对整个骨架的成键贡献很小.     

Ab-inito Calculations in Reductive Bond-breaking Reaction of C-X Bond in CH3X and CH2X2 with X = F and CI

MP4/6-31+G* level calculations are performed to study the reductive bond-breaking reaction of the C-X bond in halomethanes, CH₃X and CH₂X₂ where X is a fluorine atom or chlorine atom. This type of reaction involves a radical anion, after attaching an extra electron to the halomethane molecule, in which a C-X bond-breaking takes place. Products are a radical and a halogen anion. The equilibrium geometry and bond dissociation energy of the C-X bond thus found are in good agreement with previous theoretical and experimental results. The anomeric effect, electrostatic effect, and radical re-stabilization effect, are investigated to find their influences on bond length and bond dissociation energy in CH₃X and CH₂X₂. Potential energy curves are calculated for the reductive bond-cleavage process, and trends in activation energy for various cases are discussed.     

N−N Bond Forming Reductive Elimination via a Mixed‐Valent Nickel(II)–Nickel(III) Intermediate

Natural products containing N–N bonds exhibit important biological activity. Current methods for constructing N?N bonds have limited scope. An advanced understanding of the fundamental N?N bond formation/cleavage processes occurring at the transition‐metal center would facilitate the development of catalytic reactions. Herein we present an N?N bond‐forming reductive elimination, which proceeds via a mixed‐valent Ni^II–Ni^III intermediate with a Ni–Ni bond order of zero. The discrete Ni^II–Ni^III oxidation states contrast with the cationic dimeric Ni analogue, in which both Ni centers are equivalent with an oxidation state of 2.5. The electronic structures of these mixed‐valent complexes have implications for the fundamental understanding of metal–metal bonding interactions.     

Intermolecular amidation of unactivated sp2 and sp2 C-H bonds via palladium-catalyzed cascade C-H activation/nitrene insertion

This communication describes the Pd(OAc)2-catalyzed intermolecular amidation reactions of unactivated sp2 and sp3 C-H bonds using primary amides and potassium persulfate. The substrates containing a pendent oxime or pyridine group were amidated with excellent chemo- and regioselectivities. It is noteworthy that reactive C-X bonds were well-tolerated and a variety of primary amides can be effective nucleophiles for the Pd-catalyzed C-H amidation reactions. For the reaction of unactivated sp3 C-H bonds, beta-amidation of 1 degrees sp3 C-H bonds versus 2 degrees C-H bonds is preferred. The catalytic reaction is initiated by chelation-assisted cyclopalladation involving C-H bond activation. Preliminary mechanistic study suggested that the persulfate oxidation of primary amides should generate reactive nitrene species, which then reacted with the cyclopalladated complex.