Cycloaddition Reactions of a Chromium-Chromium Quintuple Bond

Quadruple C-H Bond Activations of Methane by Dinuclear Rhodium Carbide Cation [Rh2C3]. JACS AU 2021;1:1631-1638. [PMID: 34723266 PMCID: PMC8549038 DOI: 10.1021/jacsau.1c00265]

The structure of the [Rh₂C₃]⁺ ion and its reaction with CH₄ in the gas phase have been studied by infrared photodissociation spectroscopy and mass spectrometry in conjunction with quantum chemical calculations. The [Rh₂C₃]⁺ ion is characterized to have an unsymmetrical linear [Rh-C-C-C-Rh]⁺ structure existing in two nearly isoenergetic spin states. The [Rh₂C₃]⁺ ion reacts with CH₄ at room temperature to form [Rh₂C]⁺ + C₃H₄ and [Rh₂C₂H₂]⁺ + C₂H₂ as the major products. In addition to the [Rh₂C]⁺ ion, the [Rh₂ ¹³C]⁺ ion is formed at about one-half of the [Rh₂C]⁺ intensity when the isotopic-labeled ¹³CH₄ sample is used. The production of [Rh₂ ¹³C]⁺ indicates that the linear C₃ moiety of [Rh₂C₃]⁺ can be replaced by the bare carbon atom of methane with all four C-H bonds being activated. The calculations suggest that the overall reactions are thermodynamically exothermic, and that the two Rh centers are the reactive sites for C-H bond activation and hydrogen atom transfer reactions.

Coordination Chemistry of Bulky Aminopryridinates with Main Group and Transition Metals

The coordination chemistry of bidentate aminopyridinato ligands (ApH), in particular 2-aminopyridines, is a highly popular area of research. Due to easy accessibility and versatility, 2-aminopyridines have played a prominent role as alternatives to cyclopentadienyl ligands in coordination chemistry. Easily modifiable steric bulks and the ability for fine-tuning of electronic effects have allowed researchers to control not only the metal-to-ligand stoichiometry but also the properties of their metal complexes. Previously, ligand redistribution was frequently observed for ligands of small steric demands. Bulky aminopyridinato ligands refer to ligands that incorporate alkyl-substituted phenyl groups at the amine/amido nitrogen and at the sixth position of the pyridine ring. The steric crowding allowed the stabilization of transition metals in unusually low oxidation conditions. One of the remarkable developments, for example, is the stabilization of metal-metal quintuple bonds by these ligands, thus providing a diamagnetic platform to study such systems chemically. Application of metal aminopyridinates in homogeneous catalysis has also broadened considerably in recent years. This review provides a comprehensive account of advances made with such ligands since their development for main group and transition elements.

Formation of a dimeric tungsten(i) complex via C-H activation

An aminopyridinato ligand stabilized and coordinatively unsaturated dimeric tungsten(0) complex having an electronic structure with six metal centred HOMOs (σ²,π²,π²,δ²,δ², and δ*²) has been isolated and structurally characterized. Its reaction with a cryptand leads to a C-H bond activation of one of the isopropyl groups of the N-ligand to form a dimeric tungsten(i) complex.

Neutral nano-polygons with ultrashort Be–Be distances

DFT calculations revealed that neutral polygons (E-Be₂H₃)_n are the viable targets for realizing ultrashort metal–metal distances between main group metals.

A coordination strategy to realize a sextuply-bonded complex

The synthesis of higher-order multiple bonds is a great challenge in chemistry. However, no stable compound with a sextuple bond has been reported, except for Mo₂ in an inert matrix at low temperatures. Herein, we propose a strategy to construct a sextuple bond in a dinuclear transition metal complex based on complete active space second-order perturbation theory (CASPT2) and density functional theory calculations. When the dinuclear core M₂ (M = W, Mo, and Re⁺) is capped by two neutral electron-donating ligands at both M-M ends, a sextuple bond can be realized. The proposed ligands stabilize the M₂ core by the coordination, conserve the six bonding orbitals in the occupied space, and suppress the weight of the δ-δ* excited electronic configuration. Calculated large formation energies of these complexes indicate the large possibility of the synthesis. Electronic structures and sextuply bonding interactions were analyzed in detail.

Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η⁶ :η⁶ -1,3,5-(Me₃ Si)₃ C₆ H₃ )[Cr{κ² -HC(N-2,6-ⁱ Pr₂ C₆ H₃ )(N-2,6-R₂ C₆ H₃ )}]₂ (R=ⁱ Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me₃ C₆ H₂ CN, the bridging arene 1,3,5-(Me₃ Si)₃ C₆ H₃ in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.

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 (CT_Mo→XH) from the Mo-Mo quintuple bond to the X-H (X = H, C, or O) bond and that (CT_XH→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 CT_Mo→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.

Methane Activation by Tantalum Carbide Cluster Anions Ta2C4. J Phys Chem Lett 2017;8:605-610. [PMID: 28088857 DOI: 10.1021/acs.jpclett.6b02568]

Methane activation by transition metals is of fundamental interest and practical importance, as this process is extensively involved in the natural gas conversion to fuels and value-added chemicals. While single-metal centers have been well recognized as active sites for methane activation, the active center composed of two or more metal atoms is rarely addressed and the detailed reaction mechanism remains unclear. Here, by using state-of-the-art time-of-flight mass spectrometry, cryogenic anion photoelectron imaging spectroscopy, and quantum-chemical calculations, the cooperation of the two Ta atoms in a dinuclear carbide cluster Ta₂C₄^- for methane activation has been identified. The C-H bond activation takes place predominantly around one Ta atom in the initial stage of the reaction and the second Ta atom accepts the delivered H atom from the C-H bond cleavage. The well-resolved vibrational spectra of the cryogenically cooled anions agree well with theoretical simulations, allowing the clear characterization of the structure of Ta₂C₄^- cluster. The reactivity comparison between Ta₂C₄^- cluster and the carbon-less analogues (Ta₂C₃^- and Ta₂C₂^-) demonstrated that the cooperative effect of the two metal atoms can be well tuned by the carbon ligands in terms of methane activation and transformation.

From chromium-chromium quintuple bonds to molecular squares and porous coordination polymers

Reaction of the quintuply bonded chromium(I) dimer [ApCrCrAp] (Ap = sterically demanding 2-aminopyridinate) with pyrazine yields a chromium(II) complex with a η(4):η(4) face-on coordinated pyrazine dianion. Reaction with 4,4'-bipyridine, on the other hand, completely cleaves the metal-metal bond, leading to a chromium(II)-based molecular square. XRD and magnetic measurements show ligand radical anions and a ferrimagnetic alignment of alternating metal and ligand magnetic moments. Controlled polymerization of the molecular square with pyrazine yields a porous coordination polymer featuring both reduced and nonreduced linkers.

Binding and activation of small molecules by a quintuply bonded chromium dimer

The quintuply bonded [(H)L(iPr)Cr]2 reacts with various small molecules, revealing a pattern of two kinds of transformations. Unsaturated molecules that are neither polar nor oxidizing form binuclear [2+n] cycloaddition products retaining Cr-Cr quadruple bonds. In contrast, polar or oxidizing molecules effect the complete cleavage of the Cr-Cr bond.

Theory, synthesis and reactivity of quintuple bonded complexes

Recent achievements in the area of metal–metal quintuple bonding are highlighted, including synthesis of quintuple bonded complexes, metal-to-metal bonding schemes, and their reactivity.

The important role of the Mo–Mo quintuple bond in catalytic synthesis of benzene from alkynes. A theoretical study

The reaction mechanism of catalytic synthesis of benzene from alkynes by the Mo–Mo quintuple bond and the electronic structure and bonding nature of dimetallacyclobutadiene and dimetallabenzyne were studied theoretically.

Discovering complexes containing a metal–metal quintuple bond: from theory to practice

Recent advances in quintuple bonding are highlighted based on theoretical predictions and experimental achievements.

The ligand-based quintuple bond-shortening concept and some of its limitations

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.

Stepwise construction of the Cr-Cr quintuple bond and its destruction upon axial coordination

Give me five! Terdentate 2,6-diamidopyridyl ligands were used to stabilize the Cr-Cr quintuple bond and have made it possible to isolate and characterize not only the Cr-Cr quintuple-bonded complex, but also the mixed-valent intermediates (Cr(I) and Cr(II)), which are important species in the formation of type I quintuple-bonded complexes.

The Quest for Metal-Metal Quadruple and Quintuple Bonds in Metal Carbonyl Derivatives: Nb2(CO)9 and Nb2(CO)8

The synthesis by Power and co-workers of the first metal-metal quintuple bond (Science2005, 310, 844) is a landmark in inorganic chemistry. The 18-electron rule suggests that Nb2(CO)9 and Nb2(CO)8 are candidates for binary metal carbonyls containing metal-metal quadruple and quintuple bonds, respectively. Density functional theory (MPW1PW91 and BP86) indeed predicts structures having very short Nb-Nb distances of ∼2.5 Å for Nb2(CO)9 and ∼2.4 Å for Nb2(CO)8 as well as relatively large Nb-Nb Wiberg bond indices supporting these high formal Nb-Nb bond orders. However, analysis of the frontier molecular orbitals of these unbridged structures suggests formal Nb≡Nb triple bonds and 16-electron metal configurations. This contrasts with an analysis of the frontier orbitals in a model chromium(I) alkyl linear CH3CrCrCH3, which confirms the generally accepted presence of chromium-chromium quintuple bonds in such molecules. The presence of Nb≡Nb triple bonds rather than quadruple or quintuple bonds in the Nb2(CO)n (n = 9, 8) structures frees up d(xy) and d(x(2)-y(2)) orbitals for dπ→pπ* back-bonding to the carbonyl groups. The lowest energy Nb2(CO)n structures (n = 9, 8) are not these unbridged structures but structures having bridging carbonyl groups of various types and formal Nb-Nb orders no higher than three. Thus, the two lowest energy Nb2(CO)9 structures have Nb≡Nb triple bond distances of ∼2.8 Å and three semibridging carbonyl groups, leading to a 16-electron configuration rather than an 18-electron configuration for one of the niobium atoms. The lowest energy structure of the highly unsaturated Nb2(CO)8 is unusual since it has a formal single Nb-Nb bond of length ∼3.1 Å and two four-electron donor η(2)-μ-CO groups, thereby giving each niobium atom only a 16-electron configuration.