Metal-metal Bond Research Articles

Diverse bimetallic mechanisms emerging from transition metal Lewis acid/base pairs: development of co-catalysis with metal carbenes and metal carbonyl anions.

The rational development of catalytic reactions involving cooperative behavior between two catalytic reactive sites represents a frontier area of research from which novel reactivity and selectivity patterns emerge. Within this context, this Feature highlights the development of a cooperative system involving transition metal Lewis acid/base pairs. Bimetallic systems consisting of copper carbene Lewis acids and metal carbonyl anion Lewis bases, (NHC)Cu-[MCO], are easily synthesized from readily available organometallic building blocks (NHC = N-heterocyclic carbene; [MCO]- = metal carbonyl anion, e.g. [FeCp(CO)2]-, [Mn(CO)5]-, etc.). Stoichiometric reactivity studies indicate that the dative Cu←M bonds in these systems are labile towards heterolysis under mild conditions, thus providing in situ access both to polar metal-metal bonds and to "frustrated" transition metal Lewis acid/base pairs as dictated by reaction conditions. Catalytic transformations ranging from C-C and C-B coupling reactions to hydrogenation and other reductions have been developed from both manifolds: bimetallic catalysis involving (a) binuclear intermediates engaging in cooperative bond activation and formation, and (b) orthogonal mononuclear intermediates that operate in either tandem or co-dependent manners. Preliminary indications point to the future emergence of novel reactivity and selectivity patterns as these new motifs undergo continued development, and additionally demonstrate that the relative matching of two reactive sites provides a method for controlling catalytic behavior. Collectively, these results highlight the fundamental importance of exploring unconventional catalytic paradigms.

Regioselective B-H/C-H activation and metal-metal bond formation induced by half-sandwich metals complexes at hydroxy-substituted o-carboranes.

A binuclear iridium complex, (Cp2*Ir2(CH2O)C2B10H8) (6), with a unique metal-metal bond has been synthesized and fully characterized. Importantly, this complex is constructed via selective C-H and B(3)-H bond activation on the carborane precursor. Additionally, when the proligand (2-pyridine)(o-carboranyl)methanol ligand was combined with a half-sandwich iridium complex, selective B(6)-H bond activation or metal-carbon bond formation can be induced by the use of different bases. And the rhodium complex constructed from (2-pyridine)(o-carboranyl)methanol ligand containing a metal-carbon bond has been obtained and fully characterized.

Structural diversity of homobinuclear transition metal complexes of the phenazine ligand: theoretical investigation

DFT/BP86 calculations have been carried out on a series of hypothetical binuclear compounds of general formula (L3M)2(C12N2H8) (M = Sc–Ni, L3 = (CO)3, (PH3)3 and Cp−, and C12N2H8 = phenazine ligand-denoted Phn). The various structures with syn and anti configurations have been investigated, in order to determine the phenazine’s coordination to first-row transition metals of various spin states with syn and anti conformations. The lowest energy structures depend on the nature of the metal, the spin state, and the molecular symmetry. This study has shown that the electronic communication between the metal centers depends on their oxidation state and the attached ligands. The tricarbonyl and the triphosphine ligands gave rise to comparable results in terms of stability order of isomers, metal-metal bond distances, and the coordination modes. Metal-metal multiple bonding has been evidenced for Sc, Ti, and V complexes to compensate the electronic deficiency. The Cr, Mn, Fe, Co, and Ni-rich metals prefer the anti conformation due to the enhancement of the metal valence electron count. The spin density values calculated for the triplet and quintet spin structures point out that the unpaired electrons are localized generally on the metal centers. The Wiberg bond indices are used to evaluate the metal-metal bonding. Furthermore, calculations using the BP86-D functional which take into account the attractive part of the van der Waals type interaction potential between atoms and molecules that are not directly connected to each other gave comparable results to those obtained by BP86 functional in terms of coordination modes, HOMO-LUMO gaps, metal-metal bond orders, and the stability order between isomers, but with slight deviation of M–C, M–N, and M–M bond distances not exceeding 3%.

Electronic structure and bonding of the dinuclear metal M2(CO)10 decacarbonyls: applications of natural orbitals for chemical valence.

The nature of the chemical metal-metal bond in M2(CO)10 (M = Mn, Re, Tc) dinuclear decacarbonyls complexes was investigated for the first time using the natural orbital chemical valence (NOCV) approach combined with the extended transition state (ETS) for energy decomposition analysis (EDA). The optimized geometries carried out at different levels of theory BP86, BLYP, BLYPD and BP86D, showed that the latter method, i.e., BP86D, led to the best agreement with X-ray experimental measurements. The BP86D/TZP results revealed that the computed covalent contribution to the metal-metal bond are 60.5%, 54.1% and 52.0% for Mn-Mn, Re-Re and Tc-Tc, respectively. The computed total interaction energies resulting from attractive terms (ΔE orb and ΔE eles), correspond well to experimental predictions, based on bond lengths and energy interaction analysis for the studied complexes.

Synthesis, characterization and single crystal structures of chiral Schiff base and its tetranuclear palladium complex with Pd[sbnd]O[sbnd]Pd bridging and Pd[sbnd]Pd bonds

A new chiral Schiff base ligand 2-{N-[(2S)-(1-hydroxy-3-phenylpropan-2-yl]ethanimidoyl} phenol ((S)H2L) was obtained by acid catalyzed condensation of (2S)-(−)-2-amino-3-phenyl-1-propanol with 2′-hydroxyacetophenone. The palladium complex was prepared by treating a solution of (S)H2L in acetone with a solution of Na2PdCl4 in water in 1:1 M ratio. The new ligand and its complex were characterized by FT-IR, 1H, 13C{1H} NMR spectroscopy, polarimetry and elemental analysis and their molecular structures were determined by single crystal X-ray diffraction. Both the compounds crystallizes in monoclinic system in the space group P21. There exists an intra [OH⋯N (1.62(5) Å)] and intermolecular [OH⋯O (1.53(5) Å) and CH⋯O (2.59 Å)] hydrogen bonding and secondary interactions in the crystal of (S)H2L. The structure of the palladium complex was found very interesting wherein the ligand coordinated to metal center as tridentate dianionic (O−, N, O−) fashion, (S)-L, resulting in a tetranuclear palladium cluster, [Pd4((S)-L)4]. In these supramolecular structures phenolate oxygen coordinated to Pd(II) ion as PdO terminal bonds [1.934(12) - 1.977(11) Å] and the alkoxide oxygen coordinated as PdOPd bridging bonds [1.993(11) - 2.012(12) Å]. The PdN bond lengths found were in the range of 1.949(13) to 1.919(12) Å. There exists two asymmetric tetranuclear complex molecules in its crystal lattice. There exists very strong metal-metal bond interaction, Pd(2)Pd(3) [3.0410(18) Å] and Pd(6)Pd(7) [3.0517(19) Å] respectively in the two asymmetric units.

Bond-Energy-Driven, Low- or High-Angle-Grain-Boundary-Movement-Mediated Synthesis of Porous Se-Te for Use in Water-Splitting Reactions.

Herein, for the first time, we applied the metal-metal-bond-energy factor to the evolution of a porous Se-Te alloy. The porous Se-Te material has been prepared from the constituents' elemental states, through only a heating-cooling process in silicone oil without the use of any reagent, surfactant, or capping agent. Surprisingly, the reaction occurred at a much lower temperature (240 °C) than the mp (450 °C) of Te0. The reaction's nucleation and growth by means of varied bond energy have been clarified for the first time. A difference in the bond energies of a hetero metal-metal bond (Se-Te) and a homo metal-metal bond (Se-Se) directs nucleation and growth toward the fabrication of a porous structure, even from the constituents' elemental states, in which low-angle-grain-boundary (LAGB) and high-angle-grain-boundary (HAGB) movements play governing roles. Proper band-gap alignment of Se and Te makes the alloy composite applicable to water-splitting reactions under Xe-arc-lamp illumination. PEC efficiency of Se-Te was found to be higher than those reported for Se and other composite materials.

Energy decomposition analysis of single bonds within Kohn–Sham density functional theory

An energy decomposition analysis (EDA) for single chemical bonds is presented within the framework of Kohn-Sham density functional theory based on spin projection equations that are exact within wave function theory. Chemical bond energies can then be understood in terms of stabilization caused by spin-coupling augmented by dispersion, polarization, and charge transfer in competition with destabilizing Pauli repulsions. The EDA reveals distinguishing features of chemical bonds ranging across nonpolar, polar, ionic, and charge-shift bonds. The effect of electron correlation is assessed by comparison with Hartree-Fock results. Substituent effects are illustrated by comparing the C-C bond in ethane against that in bis(diamantane), and dispersion stabilization in the latter is quantified. Finally, three metal-metal bonds in experimentally characterized compounds are examined: a [Formula: see text]-[Formula: see text] dimer, the [Formula: see text]-[Formula: see text] bond in dizincocene, and the Mn-Mn bond in dimanganese decacarbonyl.

Influence of Relativistic Effects on Bonding Modes in M(II) Dinuclear Complexes (M = Au, Ag, and Cu).

The stability and bonding in dinuclear group 11 metal complexes (M = Au, Ag, and Cu) in their +2 oxidation state has been investigated by quantum chemical methods. Two model complexes were selected as representatives of different bonding situations in the dinuclear M(II) complexes, a direct metal-metal bond between two ligand stabilized monomers and ligand-mediated bridged dimer system, making them interesting for a direct comparison and to study the influence of relativistic effects. Relativity substantially stabilizes the direct metal-metal bonded system obtaining the sequence in M-M bond stability Au > Ag > Cu. In the ligand-bridged structure, an asymmetric bonding situation is obtained for gold, resulting in two stronger/covalent and two weaker/ionic bonds per gold atom. Here we observe the opposite trend in stability Cu > Ag > Au. Our analysis nicely corroborates with what is known from experimental observation.

Observation of Vacancies, Faults, and Superstructures in Ln5Mo2O12 (Ln = La, Y, and Lu) Compounds with Direct Mo-Mo Bonding.

Among oxide compounds with direct metal-metal bonding, the Y5Mo2O12 (A5B2O12) structural family of compounds has a particularly intriguing low-dimensional structure due to the presence of bioctahedral B2O10 dimers arranged in one-dimensional edge-sharing chains along the direction of the metal-metal bonds. Furthermore, these compounds can have a local magnetic moment due to the noninteger oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when oxide compounds with direct metal-metal bonding have integer oxidation states resulting from the lifting of orbital degeneracy typically induced by the metal-metal bonding. Although a monoclinic C2/m structure has been previously proposed for Ln5Mo2O12 (Ln = La-Lu and Y) members of this family based on prior single crystal diffraction data, it is found that this structural model misses many important structural features. On the basis of synchrotron powder diffraction data, it is shown that the C2/m monoclinic unit cell represents a superstructure relative to a previously unrecognized orthorhombic Immm subcell and that the superstructure derives from the ordering of interchangeable Mo2O10 and LaO6 building blocks. The superstructure for this reason is typically highly faulted, as evidenced by the increased breadth of superstructure diffraction peaks associated with a coherence length of 1-2 nm in the c* direction. Finally, it is shown that oxygen vacancies can occur when Ln = La, producing an oxygen deficient stoichiometry of La5Mo2O11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, a result confirmed by magnetic susceptibility measurements. This represents the first observation of oxygen vacancies in this family of compounds and provides an important means of continuously tuning the magnetic interactions within the one-dimensional octahedral chains of this system.

Green and facial combustion synthesis of Sr3Al2O6 nanostructures; a potential electrochemical hydrogen storage material

Nanoscale strontium aluminum oxides (Sr3Al2O6) were successfully synthesized via a combustion method in combination with thermal treatment using single fuel. Maltose, glucose and urea were used separately as a fuel in order to obtain pure crystalline phase. The lattice structures of as-prepared nanoparticles were identified from XRD patterns. Scherrer equation was confirmed the nanocrystallite formation of the materials. The narrow distribution of the particles with normal probability of average grain sizes of 26.5 nm (in the range of 20–35 nm) were observed in microscopic analysis. The FTIR results clearly confirmed the formation of metal-oxygen and metal-metal bonds of the product. The band gap energy (Eg) of the sample was calculated about 3.30 eV. The photoluminescence spectrum was gained a broad peak at 360 nm (Eg = 3.49 eV). Physical and structural intrinsic features of strontium aluminates have rendered the nanomaterials might be suitable for energy storage applications. The discharge capacity of the sample was recorded after 15 cycles at about 2500 mA hh/g.

Suppression of magnetism in Ba5AlIr2O11 : Interplay of Hund's coupling, molecular orbitals, and spin-orbit interaction

The electronic and magnetic properties of Ba$_5$AlIr$_2$O$_{11}$ containing Ir-Ir dimers are investigated using the GGA and GGA+SOC calculations. We found that strong suppression of the magnetic moment in this compound recently found in [J. Terzic {\it et al.}, Phys. Rev. B {\bf 91}, 235147 (2015)] is not due to charge-ordering, but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intra-atomic Hund's rule exchange interaction to reduce total magnetic moment of the dimer. We argue that the same mechanism could be relevant for other $4d$ and $5d$ dimerized transition metal compounds.

Single molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene

Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y2@C80 and Dy2@C80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy2@C80(CH2Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy2@C80(CH2Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 μB with a dysprosium-electron exchange constant of 32 cm−1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped.

Suppressing Isomerization of Phosphine-Protected Au9 Cluster by Bond Stiffening Induced by a Single Pd Atom Substitution.

The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au9(PPh3)8]3+ with a crown motif (Au9(C)) and that with a butterfly motif (Au9(B)) induced by association and dissociation with compact counteranions (NO3-, Cl-). However, structural isomerization was suppressed by substitution of the central Au atom of the Au9 core in [Au9(PPh3)8]3+ with a Pd atom: [PdAu8(PPh3)8]2+ with a crown motif (PdAu8(C)) did not isomerize to that with a butterfly motif (PdAu8(B)) upon association with the counteranions. Density functional theory calculation showed that the energy difference between PdAu8(C) and PdAu8(B) is comparable to that between Au9(C) and Au9(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye-Waller factors obtained by X-ray absorption fine-structure analysis revealed that the intracluster bonds of PdAu8(C) were stiffer than the corresponding bonds in Au9(C). Natural bond orbital analysis suggested that the radial Pd-Au and lateral Au-Au bonds in PdAu8(C) are stiffened due to the increase in the ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal-metal bonds by Pd atom doping inhibits the isomerization from PdAu8(C) to PdAu8(B).

Cupriphication of gold to sensitize d10–d10 metal–metal bonds and near-unity phosphorescence quantum yields

Outer-shell s0/p0 orbital mixing with d10 orbitals and symmetry reduction upon cupriphication of cyclic trinuclear trigonal-planar gold(I) complexes are found to sensitize ground-state Cu(I)-Au(I) covalent bonds and near-unity phosphorescence quantum yields. Heterobimetallic Au4Cu2 {[Au4(μ-C2,N3-EtIm)4Cu2(µ-3,5-(CF3)2Pz)2], (4a)}, Au2Cu {[Au2(μ-C2,N3-BzIm)2Cu(µ-3,5-(CF3)2Pz)], (1) and [Au2(μ-C2,N3-MeIm)2Cu(µ-3,5-(CF3)2Pz)], (3a)}, AuCu2 {[Au(μ-C2,N3-MeIm)Cu2(µ-3,5-(CF3)2Pz)2], (3b) and [Au(μ-C2,N3-EtIm)Cu2(µ-3,5-(CF3)2Pz)2], (4b)} and stacked Au3/Cu3 {[Au(μ-C2,N3-BzIm)]3[Cu(µ-3,5-(CF3)2Pz)]3, (2)} form upon reacting Au3 {[Au(μ-C2,N3-(N-R)Im)]3 ((N-R)Im = imidazolate; R = benzyl/methyl/ethyl = BzIm/MeIm/EtIm)} with Cu3 {[Cu(μ-3,5-(CF3)2Pz)]3 (3,5-(CF3)2Pz = 3,5-bis(trifluoromethyl)pyrazolate)}. The crystal structures of 1 and 3a reveal stair-step infinite chains whereby adjacent dimer-of-trimer units are noncovalently packed via two Au(I)⋯Cu(I) metallophilic interactions, whereas 4a exhibits a hexanuclear cluster structure wherein two monomer-of-trimer units are linked by a genuine d10-d10 polar-covalent bond with ligand-unassisted Cu(I)-Au(I) distances of 2.8750(8) Å each-the shortest such an intermolecular distance ever reported between any two d10 centers so as to deem it a "metal-metal bond" vis-à-vis "metallophilic interaction." Density-functional calculations estimate 35-43 kcal/mol binding energy, akin to typical M-M single-bond energies. Congruently, FTIR spectra of 4a show multiple far-IR bands within 65-200 cm-1, assignable to vCu-Au as validated by both the Harvey-Gray method of crystallographic-distance-to-force-constant correlation and dispersive density functional theory computations. Notably, the heterobimetallic complexes herein exhibit photophysical properties that are favorable to those for their homometallic congeners, due to threefold-to-twofold symmetry reduction, resulting in cuprophilic sensitization in extinction coefficient and solid-state photoluminescence quantum yields approaching unity (ΦPL = 0.90-0.97 vs. 0-0.83 for Au3 and Cu3 precursors), which bodes well for potential future utilization in inorganic and/or organic LED applications.

On bonding in bis(dimethylglyoximato)nickel(II)

The Ni(II) complex of dimethylglyoxime (dmgH where H is dissociable oxime proton) occurs as an infinite chain of the discrete molecule Ni(dmgH)2 with a NiNi distance of 3.24Å. Density Functional Theoretical calculations, using LanL2DZ and 6-31+G(2d,p) as two sets of basis functions and M06-2X as the functional, show that the dimeric unit [Ni(dmgH)2]2 has a cohesive energy more than 20kcalmol−1. This energy is almost same in the corresponding hypothetical zinc analogue. Since there cannot be any metal-metal bond in [Zn(dmgH)2]2, possibility of a NiNi bond in [Ni(dmgH)2]2 is precluded. This is supported by qualitative m.o. analyses also. Further the NiNi bond distance is almost same as the sum of the van der Waals radii of the two Ni atoms as designated by Bondi which is 3.26Å. It is found that the binding energy in [Ni(dmgH)2]2 arises from aromatic-aromatic interactions and that in [Zn(dmgH)2]2 due to π-π interactions between the ligand frameworks. An energy decomposition analysis show that electrostatic and dispersion effects govern the cohesive energy in [Ni(dmgH)2]2 and [Zn(dmgH)2]2. Their numerical values are comparable. The orbital interactions in them are relatively less. [Zn(dmgH)2]n is not stable for n>2. There are two types of chelate rings in Ni(dmgH)2 monomer – a five membered one with six π electrons and a six membered one having ten σ electrons. Consequently the smaller chelate ring is π-aromatic and the other one σ-aromatic. Such a conclusion is drawn from considerably negative values of NICS(0) (Nucleus Independent Chemical Shift at the centroid of a ring) for the two chelates. The corresponding chelates in [Zn(dmgH)2]2 are not aromatic.

Copolymerization of Norbornene and Norbornadiene Using a cis-Selective Bimetallic W-Based Catalytic System.

The bimetallic cluster Na[W2(μ-Cl)3Cl4(THF)2]·(THF)3 ({W2}, {W 3 W}6+, a′2e′4), which features a triple metal-metal bond, is a highly efficient room-temperature initiator for ring opening metathesis polymerization (ROMP) of norbornene (NBE) and norbornadiene (NBD), providing high-cis polymers. In this work, {W2} was used for the copolymerization of the aforementioned monomers, yielding statistical poly(norbornene)/poly(norbornadiene) PNBE/PNBD copolymers of high molecular weight and high-cis content. The composition of the polymer chain was estimated by 13C CPMAS NMR data and it was found that the ratio of PNBE/PNBD segments in the polymer chain was relative to the monomer molar ratio in the reaction mixture. The thermal properties of all copolymers were similar, resembled the properties of PNBD homopolymer and indicated a high degree of cross-linking. The morphology of all materials in this study was smooth and non-porous; copolymers with higher PNBE content featured a corrugated morphology. Glass transition temperatures were lower for the copolymers than for the homopolymers, providing a strong indication that those materials featured a branched-shaped structure. This conclusion was further supported by viscosity measurements of copolymers solutions in THF. The molecular structure of those materials can be controlled, potentially leading to well-defined star polymers via the “core-first” synthesis method. Therefore, {W2} is not only a cost-efficient, practical, highly active, and cis-stereoselective ROMP-initiator, but it can also be used for the synthesis of more complex macromolecular structures.

(Invited) Metal-Bonding Electrons inside the Fullerene Cage: Electrochemical, Quantum Chemical and EPR Studies

Electronic properties of endohedral metallofullerenes (EMFs) and especially their behavior in the charge transfer processes have been in the focus since the middle of 1990s, when sufficient amounts of EMFs became available. The Closed carbon shell of EMF represents itself as a suitable environment for stabilization of different exotic metal species, including dimetallic clusters with metal-metal bond. Here we report the electrochemical properties of M2@C82 clustersfullerenes (2 cage isomers of C82: with C3v and Cs symmetry; M=Lu, Er, Sc) . UV-Vis and NIR absorption spectroscopy data and theoretical calculations expose the unique structure of M2@C82 EMFs with two M2+ions inside the cage, connected with metal-metal bonding (despite the fact that in majority of EMFs lanthanides demonstrate oxidation state 3+). Electrochemical experiments along with theoretical calculations have proved that metal-metal bonding orbital in EMFs is always one of the frontier orbitals, and its energy to a great extant determines the redox behavior of the molecule. For M2@C82 family metal-metal bonding orbital presents itself as a HOMO. The cyclic voltammetry has revealed the cathodic shift* of the first oxidation potential for Sc2- and Er2-EMFs in comparison with corresponding isomers of Lu2@C82, caused by the significant difference in position of the HOMO. Monocation of Sc2@C82-C3v, generated by chemical oxidation, has been studied with EPR spectroscopy as well. Due to the spd-character of metal-metal bonding orbital, the spin density at the nuclei is extremely high, that provides the second order effects in line position and variations of the signal line width.

Solvent-Induced Assembly of Sliver Coordination Polymers (CPs) as Cooperative Catalysts for Synthesizing of Cyclopentenone[b]pyrroles Frameworks.

A series of solvent-induced Ag-based coordination polymers (CPs) (1-3), which were synthesized by regulating the size, shape, and polarity of solvent molecules (EtOH, iPrOH, 1,4-dioxane) as structure-directing agents, has been used as heterogeneous catalysts to explore how the metal-metal bonds could serve as cooperative catalysis for tandem acylation-Nazarov cyclization reaction to prepare cyclopentenone[b]pyrroles frameworks. Structural conversions from a three-dimensional (3D) framework of 1 to the structure of 2 with Ag-Ag-Ag bonds and to the two-dimensional (2D) framework of 3 with Ag-Ag bonds, arising from the different sizes, shapes, and polarities of the solvent molecules, were observed. Futhermore, the effective cooperative catalysis for tandem acylation-Nazarov cyclization reaction has been evidenced by the significantly transformed connection modes of Ag-Ag interactions in 1-3. As a result, the unique structural characteristics of 2, especially containing the Ag-Ag-Ag bonds, endow 2 with intact multinuclear reaction pathways and well-defined multinuclear platforms that can execute tandem acylation-Nazarov cyclization reaction through the cooperative effect of Ag-Ag-Ag interactions during the catalytic process.

Mo-Mo Quintuple Bond is Highly Reactive in H-H, C-H, and O-H σ-Bond Cleavages Because of the Polarized Electronic Structure in Transition State.

The recently reported high reactivity of the Mo-Mo quintuple bond of Mo2(N∧N)2 (1) {N∧N = μ-κ2-CH[N(2,6-iPr2C6H3)]2} in the H-H σ-bond cleavage was investigated. DFT calculations disclosed that the H-H σ-bond cleavage by 1 occurs with nearly no barrier to afford the cis-dihydride species followed by cis-trans isomerization to form the trans-dihydride product, which is consistent with the experimental result. The O-H and C-H bond cleavages by 1 were computationally predicted to occur with moderate (ΔG°⧧ = 9.0 kcal/mol) and acceptable activation energies (ΔG°⧧ = 22.5 kcal/mol), respectively, suggesting that the Mo-Mo quintuple bond can be applied to various σ-bond cleavages. In these σ-bond cleavage reactions, the charge-transfer (CTMo→XH) from the Mo-Mo quintuple bond to the X-H (X = H, C, or O) bond and that (CTXH→Mo) from the X-H bond to the Mo-Mo bond play crucial roles. Though the HOMO (dδ-MO) of 1 is at lower energy and the LUMO + 2 (dδ*-MO) of 1 is at higher energy than those of RhCl(PMe3)2 (LUMO and LUMO + 1 of 1 are not frontier MO), the H-H σ-bond cleavage by 1 more easily occurs than that by the Rh complex. Hence, the frontier MO energies are not the reason for the high reactivity of 1. The high reactivity of 1 arises from the polarization of dδ-type MOs of the Mo-Mo quintuple bond in the transition state. Such a polarized electronic structure enhances the bonding overlap between the dδ-MO of the Mo-Mo bond and the σ*-antibonding MO of the X-H bond to facilitate the CTMo→XH and reduce the exchange repulsion between the Mo-Mo bond and the X-H bond. This polarized electronic structure of the transition state is similar to that of a frustrated Lewis pair. The easy polarization of the dδ-type MOs is one of the advantages of the metal-metal multiple bond, because such polarization is impossible in the mononuclear metal complex.

Metal-metal multiple bond in low-valent diuranium porphyrazines and its correlation with metal oxidation state: A relativistic DFT study

To explore the uranium-uranium bonding nature, a ligated diuranium complex that could be experimentally possible would show features with no bridging ligand constraints (i.e. discrete or unsupported ligand), rigid ligand skeleton and suitable U-ligand bond. In this respect, we have designed and examined a series of diuranium diporphyrazines (Um2Pz2, m=II, III and IV) using relativistic density functional theory. Optimizations on all possible electron-spin isomers find that the triplet, quintet and quintet states are energetically lowest for Um2Pz2 (m=II, III and IV), respectively. They possess bond lengths of UU at 2.37, 2.46 and 2.91Å, bond orders of 3.48, 3.33 and 2.11, and stretching vibrational frequencies of 239, 172 and 108cm−1. Associated with the electronic-structure and QTAIM (quantum theory of atoms in molecules) analyses, a weak quadruple bond is suggested for the triplet state of UII2Pz2, and the triple and double bonds for UIII2Pz2, and UIV2Pz2, respectively. It shows that the uranium oxidation state approximately correlates with the number of multiple bonds.