Quintuple Bond Research Articles

Cr-Cr Quintuple Bonds: Ligand Topology and Interplay Between Metal-Metal and Metal-Ligand Bonding.

Chromium-chromium quintuple bonds seem to be approaching the lower limit for their bond distances, and this computational density functional theory study tries to explore the geometrical and electronic factors that determine that distance and to find ways to fine-tune it via the ligand choice. While for monodentate ligands the Cr-Cr distance is predicted to shorten as the Cr-Cr-L bond angle increases, with bridging bidentate ligands the trend is the opposite, since those ligands with a larger number of spacers between the donor atoms favor larger bond angles and longer bond distances. Compared to Cr-Cr quadruple bonds, the quintuple bonding in Cr2L2 compounds (with L a bridging bidentate N-donor ligand) involves a sophisticated mechanism that comprises a positive pyramidality effect for the σ and one π bond, but a negative effect for one of the δ bonds. Moreover, the shorter Cr-Cr distances produce a mismatch of the bridging ligand lone pairs and the metal acceptor orbitals, which results in a negative correlation of the Cr-Cr and Cr-N bond distances in both experimental and calculated structures.

Configuring Bonds between First-Row Transition Metals

Alfred Werner, who pioneered the field of coordination chemistry, envisioned coordination complexes as a single, transition metal atom at the epicenter of a vast ligand space. The idea that the locus of a coordination complex could be shared by multiple metals held together with covalent bonds would eventually lead to the discovery of the quadruple and quintuple bond, which have no analogues outside of the transition metal block. Metal-metal bonding can be classified into homometallic and heterometallic groups. Although the former is dominant, the latter is arguably more intriguing because of the inherently larger chemical space in which metal-metal bonding can be explored. In 2013, Lu and Thomas independently reported the isolation of heterometallic multiple bonds with exclusively first-row transition metals. Structural and theoretical data supported triply bonded Fe-Cr and Fe-V cores. This Account describes our continued efforts to configure bonds between first-row transition metals from titanium to copper. Double-decker ligands, or binucleating platforms that brace two transition metals in proximity, have enabled the modular synthesis of diverse metal-metal complexes. The resulting complexes are also ideal for investigating the effects of an "ancillary" metal on the properties and reactivities of an "active" metal center. A total of 38 bimetallic complexes have been compiled comprising 18 unique metal-metal pairings. Twenty-one of these bimetallics are strictly isostructural, allowing for a systematic comparison of metal-metal bonding. The nature of the chemical bond between first-row metals is remarkably variable and depends on two primary factors: the total d-electron count, and the metals' relative d-orbital energies. Showcasing the range of covalent bonding are a quintuply bonded (d-d)(10) Mn-Cr heterobimetallic and the singly bonded late-late pairings, e.g., Fe-Co, which adopt unusually high spin states. A long-term goal is to rationally tailor the properties and reactivities of the bimetallic complexes. In some cases, synergistic redox and magnetic properties were found that are different from the expected sum of the individual metals. Intermetal charge transfer was shown in a Co-M series, for M = Mn to Cu, where the transition energy decreases as M is varied across the first-row period. The potential of using metal-metal complexes for multielectron reduction of small-molecules is addressed by N2 binding studies and a mechanistic study of a dicobalt catalyst in reductive silylation of N2 to N(SiMe3)3. Finally, metal-ion exchange reactions with metal-metal complexes can be selective under appropriate reaction conditions, providing an alternative synthetic route to metal-metal species.

Pushing the Limits of Delta Bonding in Metal-Chromium Complexes with Redox Changes and Metal Swapping.

Into the metalloligand Cr[N(o-(NCH2P((i)Pr)2)C6H4)3] (1, CrL) was inserted a second chromium atom to generate the dichromium complex Cr2L (2), which is a homobimetallic analogue of the known MCrL complexes, where M is manganese (3) or iron (4). The cationic and anionic counterparts, [MCrL](+) and [MCrL](-), respectively, were targeted, and each MCr pair was isolated in at least one other redox state. The solid-state structures of the [MCrL](+,0,-) redox members are essentially the same, with ultrashort metal-metal bonds between 1.96 and 1.74 Å. The formal shortness ratios (r) of these interactions are between 0.84 and 0.74 and are interpreted as triple to quintuple metal-metal bonds with the aid of theory. The trio of (d-d)(10) species [Cr2L](-) (2(red)), MnCrL (3), and [FeCrL](+) (4(ox)) are S = 0 diamagnets. On the basis of M-Cr bond distances and theoretical calculations, the strength of the metal-metal bond across the (d-d)(10) series increases in the order Fe < Mn < Cr. The methylene protons in the ligand are shifted downfield in the (1)H NMR spectra, and the diamagnetic anisotropy of the metal-metal bond was calculated as -3500 × 10(-36), -3900 × 10(-36), and -5800 × 10(-36) m(3) molecule(-1) for 2(red), 3, and 4(ox) respectively. The magnitude of diamagnetic anisotropy is, thus, affected more by bond polarity than by bond order. A comparative vis-NIR study of quintuply bonded 2(red) and 3 revealed a large red shift in the δ(4) → δ(3)δ* transition energy upon swapping from the (Cr2)(2+) to the (MnCr)(3+) core. Complex 2(red) was further investigated by resonance Raman spectroscopy, and a band at 434 cm(-1) was assigned as the Cr-Cr bond vibration. Finally, 4(ox) exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe(I).

The Mo-Mo Quintuple Bond as a Ligand to Stabilize Transition-Metal Complexes.

Herein, we report the employment of the Mo-Mo quintuple bonded amidinate complex to stabilize Group 10 metal fragments {(Et3P)2M} (M=Pd, Pt) and give rise to the isolation of the unprecedented δ complexes. X-ray analysis unambiguously revealed short contacts between Pd or Pt and two Mo atoms and a slight elongation of the Mo-Mo quintuple bond in these two compounds. Computational studies show donation of the Mo-Mo quintuple-bond δ electrons to an empty σ orbital on Pd or Pt, and back-donation from a filled Pd or Pt dπ orbital into the Mo-Mo δ* level (LUMO), consistent with the Dewar-Chatt-Duncanson model.

The MoMo Quintuple Bond as a Ligand to Stabilize Transition‐Metal Complexes

AbstractHerein, we report the employment of the MoMo quintuple bonded amidinate complex to stabilize Group 10 metal fragments {(Et3P)2M} (M=Pd, Pt) and give rise to the isolation of the unprecedented δ complexes. X‐ray analysis unambiguously revealed short contacts between Pd or Pt and two Mo atoms and a slight elongation of the MoMo quintuple bond in these two compounds. Computational studies show donation of the MoMo quintuple‐bond δ electrons to an empty σ orbital on Pd or Pt, and back‐donation from a filled Pd or Pt dπ orbital into the MoMo δ* level (LUMO), consistent with the Dewar–Chatt–Duncanson model.

Binuclear cyclopentadienylosmium hydride chemistry: A stable quadruply bridged structure

The pentamethylcyclopentadienylosmium (Cp*Os) system is of interest in forming a stable binuclear hydride Cp*2Os2(μ-H)4 with four bridging hydrogen atoms as well as a stable mononuclear hydride Cp*OsH5. Density functional theory (DFT) studies on the corresponding unsubstituted systems Cp2Os2Hn (n=8, 6, 4, 2, 0) and CpOsHn (n=5, 3, 1) are reported. The quadruply bridged structure Cp2Os2(μ-H)4 is shown to be the lowest energy Cp2Os2H4 structure. For the other Cp2Os2Hn systems (n=8, 6, 2) the lowest energy structures contain doubly bridged Cp2Os2(μ-H)2 units with decreasing OsOs distances as the numbers of hydride ligands is decreased. A related doubly bridged Cp2Os2(μ-H)2H2 structure is predicted to lie only 2.5kcal/mol above the quadruply bridged Cp2Os2(μ-H)4 isomer. The hydrogen-richer structures Cp2Os2Hn (n=8 and 6) are predicted to lose H2 easily to give Cp2Os2H4 in reactions within ∼8kcal/mol of being thermoneutral. However, the H2 dissociation energy of Cp2Os2H4 to give Cp2Os2H2+H2 is relatively high at ∼42kcal/mol consistent with its stability. Bridging cyclopentadienyl rings are found in low-energy Cp2Os2H2 and Cp2Os2 structures. For Cp2Os2, two structures, each with a bridging μ-Cp ring, are shown to lie more than 14kcal/mol in energy below the singlet and triplet unbridged isomers, which were previously shown by frontier molecular orbital analyses to have formal OsOs quintuple bonds.

Low-Valent Aminopyridinato Chromium Methyl Complexes via Reductive Alkylation and via Oxidative Addition of Iodomethane by a Cr–Cr Quintuple Bond

The reaction of 1 equiv of the deprotonated (sterically demanding) aminopyridine Ap*-H (Ap*-H = (2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)pyridin-2-yl]amine) with [CrCl3(thf)3] led selectively to the monomeric complex [Ap*CrCl2(thf)2]. Methylation of [Ap*CrCl2(thf)2] by methyllithium gave rise to a mixture of a dichromium methylidene and a dichromium methyl complex. Alkylation of [Ap′CrCl2(thf)2] (Ap′-H = (2,6-diisopropylphenyl)-[6-(2,6-dimethylphenyl)pyridin-2-yl]amine) afforded selectively the dimethyl complex [Ap′2CrCr(CH3)2]. In addition to reductive alkylation, oxidative addition can be used to synthesize chromium methyl complexes. Selective oxidative addition was observed when CH3I was reacted with the quintuply bonded CrI dimer [Ap′CrCrAp′]. Here, the Grignard-like Cr2 compound [Ap′2CrCr(μ-I)(μ-CH3)] was formed. DFT calculations were performed to investigate the electronic structure of the organometallic complexes.

The δ Bond and Trigonal Paddlewheels Before the Dawn of the Quintuple Bond

Recognition five decades ago of the existence of a quadruple bond in the (Science 1964, 145, 1305) brought the δ bond into the toolbox of inorganic chemists. This discovery was followed by an explosive growth of the field of metal–metal bonding that led to significant advances in basic chemical knowledge which have transcended the curiosity stage. A few examples are provided here to illustrate how the study of species with a δ bond has contributed to improved understanding of electronic communication, the observation of steps that led to formation of chemical bonds as well as the syntheses of the most easily ionized, isolable chemically stable molecules having closed-shells. Many of these quadruple-bonded molecules have the well-known paddlewheel structure with four bridging ligands spanning two metal units. However, there are also a few dimetal species with only three bridging ligands. An account is given of some aspects of the early work on these trigonal paddlewheels with metal–metal bonds which are now an important motif in some quintuple-bonded species that have two δ bonds.

ChemInform Abstract: The Lengths of the Metal‐to‐Metal Quintuple Bonds and Reactivity Thereof

AbstractReview: 87 refs.

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.

CO2 and SO2 activation by a Cr-Cr quintuple bond.

A quintuply bonded dichromium complex stabilized by aminopyridinato ligands activates CO2 and SO2 by reducing the (formal) bond order of the metal-metal bonds. Oxygen abstraction is observed during CO2 activation. SO2 activation proceeds via formation of a unique dithionite complex/coordination. Furthermore, N2O activation was investigated and the formation of a tetrameric Cr-oxo complex was observed.

ChemInform Abstract: Discovering Complexes Containing a Metal—Metal Quintuple Bond: From Theory to Practice

AbstractReview: 102 refs.

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

Although the field of metal-to-metal multiple bonding has been considered as mature, it was recently reinvigorated by the discovery of quintuple bonded dinuclear complexes. Initiated by theoretical studies, quintuple bonding was promoted by the recognition of the first isolable quintuple bonded chromium phenyl dimer Ar'CrCrAr' (Ar' = 2,6-(2,6-(i)Pr2C6H3)2C6H3) by Power and co-workers in 2005. Soon afterwards, many dinuclear group VI metal-to-metal quintuple bonded complexes stabilized by sterically hindered N-based bidentate ligands were subsequently characterized. All these remarkable compounds feature two or three bridging ligands, so each metal is two to three coordinate with respect to the ligands. Of particular interest is that they all have extremely short metal-metal bond lengths. These low-coordinate homo-univalent quintuple bonded dinuclear species not only display remarkable bonding schemes between two metal atoms, but also show interesting reaction chemistry with unsaturated hydrocarbons and small inorganic molecules. Herein, we review recent advances in quintuple bond chemistry including the synthesis, characterization and reactivity of quintuple bonded complexes.

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.

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

The Mo-Mo quintuple bond was recently applied to catalytic synthesis of benzene from alkynes, which is the first example of the catalytic reaction of the metal-metal multiple bond. This new reaction was studied using DFT and CASSCF/CASPT2 methods. The entire catalytic cycle consists of four steps: [2 + 2], [4 + 2], and [6 + 2] cycloadditions, and reductive elimination of benzene. The symmetry-forbidden [2 + 2] cycloaddition and asymmetric [2 + 2] cycloaddition are two possible pathways for the reaction between an alkyne and the Mo-Mo quintuple bond. Though the barrier of the former pathway is moderate because of the presence of the multi-reference character of the Mo-Mo quintuple bond, the asymmetric pathway is much more favorable because of its symmetry-allowed feature. The C-C bond formation in the next [4 + 2] cycloaddition occurs through charge transfer (CT) from the π orbital of the incoming alkyne to the π* orbital of another alkyne coordinating with the Mo center to afford a novel dimolybdenacyclic species 3. In 3, the δ(d(xz)) and δ(d(xz))* orbitals of the Mo-Mo moiety and four π orbitals of the [C4H4] moiety construct the π and π* orbitals in the six-membered ring. The next [6 + 2] cycloaddition between 3 and one more alkyne affords an eight-membered ring compound 4 which has a Mo-Mo quadruple bond. This is the rate-determining step of the entire catalytic cycle, the ΔG(0‡) value of which is 22.4 kcal mol(-1). The subsequent reductive elimination of benzene easily occurs to yield a μ2-η(2):η(2)-benzene dinuclear Mo complex with a Mo-Mo quintuple bond. On the other hand, further [8 + 2] cycloaddition between 4 and one more alkyne is much more unfavorable than the reductive elimination of benzene. The similar [4 + 2] process between alkyne and a Cr-Cr quadruple bond is calculated to be difficult, which is consistent with the experimental result that only the Mo-Mo quintuple bond was successfully applied to this reaction. It is likely that the crowded coordination environment and the much more stable π(d(yz)) orbital in the Cr-Cr quadruple bond are responsible for the difficulty in the reaction.

Charge Density Analysis and Bond Characterization of 3d‐Transition Metal Complexes

AbstractChemical bond, which makes molecules from independent atoms, is a very important concept in understanding the molecular behavior. Bond characterization is therefore becoming ever needed for predicting the physical and chemical properties of the molecule and the material. Combined experimental and molecular orbital calculation studies have been used for such purpose in terms of deformation density distribution, natural bond orbital analysis and the topological analysis on the total electron density. These analyses provide not only the distribution of density accumulation and density depletion, but also the information such as bond order, bond type etc. Atom domain in molecule or compound can be defined uniquely. Therefore the combined quantum mechanical calculation and high resolution X‐ray crystallography studies will be extremely useful for characterizing the chemical bond precisely. Bonding in 3d‐transition metal complexes is a major topic in coordination chemistry. The complexity of the 3d orbital splittings subjected in the ligand field is well known and this makes the bonding characterization more interesting and challenging. Charge density analysis on a series of Cr compounds, which contain a Cr‐L multiple bond, will be presented. Detail characterization of Cr‐nitrido, ‐oxo, ‐imido, ‐carbyne and ‐carbene bonds will be described. Metal squarates of late 3d transition elements have been investigated systematically. The valence shell charge concentration (VSCC) around metal ion will be demonstrated by the Laplacian topology, which provides the physical basis for the Lewis structure and valence shell electron pair repulsion (VSEPR). The metal‐metal bond is extremely important for metal clusters; the metal‐metal quintuple bond is rare and a detail analysis will be described. Electronic configurations of the 3d metal ion play important role in coordination chemistry; taking the advantage of the x‐ray absorption spectroscopy available in synchrotron radiation facility, the exact electronic configuration can be monitored. The great recent improvements on both experiment and theory have made the charge density analysis more accessible to even more complicated system, for example, the excited state or the meta‐stable states.

Does the metal–metal sextuple bond exist in the bimetallic sandwich compounds Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2?†

Although evanescent at best, the bare dimers of the elements Cr, Mo, and W have been identified as possible candidates for the sextuple metal–metal bond. The corresponding dibenzene sandwich compounds Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, satisfy the ‘18-Electron Rule’, and might achieve high-order metal–metal bonds and longer lifetimes at the same time. Twenty-two different DFT methods have been used to evaluate this possibility. Based on the present Wiberg bond index and molecular orbital analyses, however, only quadruple metal–metal bonds are predicted for the electronic ground states of Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, rather than the sextuple or even quintuple bonds, for both singlet and triplet electronic states. It is possible to force the hypothesised D 6h sextuple bond electron configuration, but the resulting energy is 39 kcal/mol above the D 2h quadruple bond structure for Cr2(C6H6)2. However, the constrained sextuple bond structure for Mo2(C6H6)2 lies 19 kcal/mol above the MoMo singlet. For W2(C6H6)2 the sextuple bond structure is predicted to lie only 3 kcal/mol above the WW structure. Thus the answer to the question raised in our title is ‘almost’ for the tungsten–sextuple bond.

Systematic Variation of Metal–Metal Bond Order in Metal–Chromium Complexes

In the field of metal-metal bonding, the occurrence of stable, multiple bonds between different transition metals is uncommon, and is largely unknown for different first-row metals. Adding to a recently reported iron-chromium complex, three additional M-Cr complexes have been isolated, where the iron site is systematically replaced with other first-row transition metals (Mn, Co, or Ni), while the chromium site is kept invariant. These complexes have been characterized by X-ray crystallography. The Mn-Cr complex has an ultrashort metal-metal bond distance of 1.82 Å, which is consistent with a quintuple bond. The M-Cr bond distances increases across the period from M = Mn to M = Ni, as the formal bond order decreases from 5 to 1. Theoretical calculations reveal that the M-Cr bonds become increasingly polarized across the period. We propose that these trends arise from increasing differences in the energies and/or contraction of the metals' d-orbitals (M vs Cr). The cyclic voltammograms of these heterobimetallic complexes show multiple one-electron transfer processes, from two to four redox events depending on the M-Cr pair.

Cover Picture: The Ligand-Based Quintuple Bond-Shortening Concept and Some of Its Limitations (Chem. Eur. J. 30/2013)

Quintuple bonds between chromium atoms are the first choice for looking at how short a metal–metal bond can be. In their Full Paper on page 9825 ff., L. Gagliardi, R. Kempe et al. describe a rational concept of shortening quintuple bonds based on ligand design. A Cr–Cr distance of 1.7056(12) Å is the value finally reached. Most importantly, the potential limitations of the shortening concept and the formation of other coordination isomers or over-reduction are described.

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.