Metal−Metal Bonding in Mixed Valence Ni25+ Complexes and Spectroscopic Evidence for a Ni26+ Species

Polarized metal-metal multiple bonding and reactivity of phosphinoamide-bridged heterobimetallic group IV/cobalt compounds

Heterobimetallic complexes are studied for their ability to mimic biological systems as well as active sites in heterogeneous catalysts. While specific interest in early/late heterobimetallic systems has fluctuated, they serve as important models to fundamentally understand metal-metal bonding. Specifically, the polarized metal-metal multiple bonds formed in highly reduced early/late heterobimetallic complexes exemplify how each metal modulates the electronic environment and reactivity of the complex as a whole. In this Perspective, we chronicle the development of phosphinoamide-supported group IV/cobalt heterobimetallic complexes. This combination of metals allows access to a low valent Co-I center, which performs a rich variety of bond activation reactions when coupled with the pendent Lewis acidic metal center. Conversely, the low valent late transition metal is also observed to act as an electron reservoir, allowing for redox processes to occur at the d0 group IV metal site. Most of the bond activation reactions carried out by phosphinoamide-bridged M/Co-I (M = Ti, Zr, Hf) complexes are facilitated by cleavage of metal-metal multiple bonds, which serve as readily accessible electron reservoirs. Comparative studies in which both the number of buttressing ligands as well as the identity of the early metal were varied to give a library of heterobimetallic complexes are summarized, providing a thorough understanding of the reactivity of M/Co-I heterobimetallic systems.

Torsion Effects Beyond the δ Bond and the Role of π Metal-Ligand Interactions

Previous studies on bimetallic paddlewheel compounds have established a direct correlation between metal-metal distance and ligand torsion angles, leading to the rule that higher torsion results in longer metal-metal bond distances. Here, the new discovery based on diarylformamidinate Ru₂⁵⁺ paddlewheel compounds [Ru2Cl(DArF)4] that show an opposite behavior is reported: higher torsions lead to shorter metal-metal distances. This discovery challenges the assumption that internal rotation solely impacts the δ bond. By combining experimental and theoretical techniques, it is demostrated that this trend is associated with previously overlooked π metal-ligand interactions. These π metal-ligand interactions are a direct consequence of the paddlewheel structure and the conjugated nature of the bidentate ligands. This findings offer far-reaching insights into the influence of equatorial ligands and their π-conjugation characteristics on the electronic properties of paddlewheel complexes. That this effect is not exclusive of diruthenium compounds but also occurs in other bimetallic cores such as ditungsten or dirhodium is demonstrated, and with other ligands showing allyl type conjugation. These results provide a novel approach for fine-tuning the properties of these compounds with significant implications for materials design.

An Isolable Radical Anion Featuring a 2-Center-3-Electron π-Bond without a Clearly Defined σ-Bond

Featuring an extra electron in the π* antibonding orbital, species with a 2-center-3-electron (2c3e) π bond without an underlying σ bond are scarcely known. Herein, we report the synthesis, isolation and characterization of a radical anion salt [K(18-C-6)]+ {[(HCNDipp)2 Si]2 P2 }⋅- (i.e. [K(18-C-6)]+ 3⋅- ) (18-C-6=18-crown-6, Dipp=2,6-diisopropylphenyl), in which 3⋅- features a perfectly planar Si2 P2 four-membered ring. This species represents the first example of a Si- and P-containing analog of a bicyclo[1.1.0]butane radical anion. The unusual bonding motif of 3⋅- was thoroughly investigated via X-ray diffraction crystallography, electron paramagnetic resonance spectroscopy (EPR), and calculations by density functional theory (DFT), which collectively unveiled the existence of a 2c3e π bond between the bridgehead P atoms and no clearly defined supporting P-P σ bond.

Lantern-type dinickel complexes: An exploration of possibilities for nickel-nickel bonding with bridging bidentate ligands

Many binuclear nickel complexes have NiNi distances suggesting NiNi covalent bonds, including lantern-type complexes with bridging bidentate ligands. This DFT study treats tetragonal, trigonal, and digonal lantern-type complexes with the formamidinate, guanidinate, and formate ligands, besides some others. Formal bond orders (ranging from zero to two) are assigned to all the NiNi bonds on the basis of MO occupancy considerations. A VB-based electron counting approach assigns plausible resonance structures to the dinickel cores. Model tetragonal complexes with the dimethylformamidinate and the dithioformate ligands have singlet ground states whose non-covalently bonded NiNi distances are close to those in their experimentally known counterparts. Trigonal dinickel complexes are unknown, but are predicted to have quartet ground states with NiNi bonds of order 0.5. The model digonal complexes are predicted to have triplet ground states, but the predicted NiNi bond lengths are longer than those found in their experimentally known counterparts. This could owe to inadequate treatment of electron correlation by DFT in these short NiNi bonds with their multiconfigurational character. All the NiNi bond distances here are categorized into ranges according to the NiNi bond orders of 0, 0.5, 1, 1.5, and 2, no NiNi bonds of order higher than two being identified. The NiNi bonds of given order in these lantern-type complexes are consistently shorter than the corresponding NiNi bonds in dinickel complexes having carbonyl ligands, attributable to the metalmetal bond lengthening effect of CO ligands.

Three-Electron Nickel(I)/Nickel(0) Half-Bond

An (N-heterocyclic carbene)nickel(I) cation precursor reacts with the corresponding nickel(0) complex to form a dinickel(I,0) monocation. The Ni···Ni distance in this cation is 0.93 Å shorter than in the analogous dinickel(0) complex. Although the solid-state structure shows equivalent Ni centers, density functional theory calculations indicate significant electronic localization. Reactions with CO and NO form mononuclear carbonyl and nitrosyl complexes. Oxidative addition of an aryl bromide results in C-arylation of the carbene ligands.

Electronic Structure of Ru26+ Complexes with Electron-Rich Anilinopyridinate Ligands

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru₂(Xap)₄Cl]⁺ species in the Ru₂⁶⁺ oxidation state. Despite paramagnetism of the compounds, ¹H NMR spectroscopy proved highly informative for determining the isomerism of the Ru₂⁵⁺ and Ru₂⁶⁺ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru₂(ap)₄Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru₂⁶⁺ oxidation state. The resulting Ru₂⁶⁺ monocations were found to have the expected (π*)² ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru₂⁵⁺ and Ru₂⁶⁺ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru₂⁶⁺ will be ligand-centered rather than metal-centered.

A tetranuclear nickel cluster isolated in multiple high-valent states

We report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(ii) ([Ni₄]) neutral cluster. Electrochemical analysis of [Ni₄] reveals three reversible sequential oxidations at 0.248 V (1e^-), 0.678 V (1e^-), and 0.991 V (2e^-) vs. Fc/Fc⁺ corresponding to mono-, di-, and tetra-oxidized species, [Ni₄]⁺, [Ni₄]²⁺, [Ni₄]⁴⁺, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(iii) cluster, [Ni₄]⁴⁺.

Mechanisms of Nickel-Catalyzed Coupling Reactions and Applications in Alkene Functionalization

Nickel complexes exhibit distinct properties from other group 10 metals, including a small nuclear radius, high paring energy, low electronegativity, and low redox potentials. These properties enable Ni catalysts to accommodate and stabilize paramagnetic intermediates, access radical pathways, and undergo slow β-H elimination. Our research program investigates how each of these fundamental attributes impact the catalytic properties of Ni, in particular in the context of alkene functionalization.Alkenes are versatile functional groups, but stereoselective carbofunctionalization reactions of alkenes have been underdeveloped. This challenge may derive from the difficulty of controlling selectivity via traditional two-electron migratory insertion pathways. Ni catalysts could lead to different stereodetermining steps via radical mechanisms, allowing access to molecular scaffolds that are otherwise difficult to prepare. For example, an asymmetric alkene diarylation reaction developed by our group relies upon the radical properties of Ni(III) intermediates to control the enantioselectivity and give access to a library of chiral α,α,β-triarylethane molecules with biological activity.Mechanistic studies on a two-component reductive 1,2-difunctionalization reaction have shed light on the origin of the cross-electrophile selectivity, as C sp² and C sp³ electrophiles are independently activated at Ni(I) via two-electron and radical pathways, respectively. Catalyst reduction has been identified to be the turnover-limiting step in this system. A closer investigation of the radical formation step using a (Xantphos)Ni(I)Ar model complex reveals that Ni(I) initiates radical formation via a concerted halogen-abstraction pathway.The low redox potentials of Ni have allowed us to develop a reductive, trans-selective diene cyclization, wherein a classic two-electron mechanism operates on a Ni(I)/Ni(III) platform, accounting for the chemo- and stereoselectivity. This reaction has found applications in the efficient synthesis of pharmaceutically relevant molecules, such as 3,4-dimethylgababutin.The tendency of Ni to undergo one-electron redox processes prompted us to explore dinuclear Ni-mediated bond formations. These studies provide insight into Ni-Ni bonding and how two metal centers react cooperatively to promote C-C, C-X, and N-N bond forming reductive elimination.Finally, isolation of β-agostic Ni and Pd complexes has allowed for X-ray and neutron diffraction characterization of these highly reactive molecules. The bonding parameters serve as unambiguous evidence for β-agostic interactions and help rationalize the slower β-H elimination at Ni relative to Pd. Overall, our research has elucidated the fundamental properties of Ni complexes in several contexts. Greater mechanistic understanding facilitates catalyst design and helps rationalize the reactivity and selectivity in Ni-catalyzed alkene functionalization reactions.

Non-classical covalent bonds

The possibility of multiple bond formation between Periodic Table Group 13 – 15 elements is considered. The ways of triple bond formation between these elements are discussed; particular attention is paid to the B≡B triple bonds. New non-linear compounds with triple bonds and their molecular structures are considered. The causes are given for the formation of compounds with unusually short distances between chemically non-bonded atoms. The grounds of the theory of two-centre three-electron bonds are presented and conditions of existence of isolated square planar carbon clusters are analyzed.

The bibliography includes 181 references.

Metal-Metal (MM) Bond Distances and Bond Orders in Binuclear Metal Complexes of the First Row Transition Metals Titanium Through Zinc

This survey of metal-metal (MM) bond distances in binuclear complexes of the first row 3d-block elements reviews experimental and computational research on a wide range of such systems. The metals surveyed are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc, representing the only comprehensive presentation of such results to date. Factors impacting MM bond lengths that are discussed here include (a) the formal MM bond order, (b) size of the metal ion present in the bimetallic core (M₂) ⁿ⁺, (c) the metal oxidation state, (d) effects of ligand basicity, coordination mode and number, and (e) steric effects of bulky ligands. Correlations between experimental and computational findings are examined wherever possible, often yielding good agreement for MM bond lengths. The formal bond order provides a key basis for assessing experimental and computationally derived MM bond lengths. The effects of change in the metal upon MM bond length ranges in binuclear complexes suggest trends for single, double, triple, and quadruple MM bonds which are related to the available information on metal atomic radii. It emerges that while specific factors for a limited range of complexes are found to have their expected impact in many cases, the assessment of the net effect of these factors is challenging. The combination of experimental and computational results leads us to propose for the first time the ranges and "best" estimates for MM bond distances of all types (Ti-Ti through Zn-Zn, single through quintuple).

Contrasting the group 6 metal-metal bonding in sodium dichromate(ii) and sodium dimolybdate(ii) polymethyl complexes: synthetic, X-ray crystallographic and theoretical studies

Extending the class of group 6 metal-metal bonded methylate compounds supported by alkali metal counter-ions, the first sodium octamethylmolybdate(ii) complex [(TMEDA)Na]₄Mo₂Me₈ and heptamethylchromate(ii) relations [(donor)Na]₃Cr₂Me₇ (donor is TMEDA or TMCDA) are reported. The former was made by treating [(Et₂O)Li]₄Mo₂Me₈ with four equivalents of NaOtBu/TMEDA in ether; whereas the latter resulted from introducing TMEDA or TMCDA to ether solutions of octamethyldichromate [(Et₂O)Na]₄Cr₂Me₈. X-ray crystallography revealed [(TMEDA)Na]₄Mo₂Me₈ is dimeric with square pyramidal Mo centres [including a short Mo-Mo interaction of 2.1403(3) Å] each with four methyl groups in a mutually eclipsed conformation. In dinuclear [(TMCDA)Na]₃Cr₂Me₇ trigonal bi-pyramidal Cr centres each bond to three terminal methyl groups and one common Me bridge, that produces a strikingly short Cr-Cr contact of 1.9136(4) Å. Broken symmetry density functional theoretical calculations expose the multiconfigurational metal-metal bonding in these compounds with a Mo-Mo bond order of 3 computed for octamethylmolybdate(ii). This is contrasted by the single Cr-Cr bond in heptamethylchromate(ii) where the singlet ground state is derived by strong antiferromagnetic coupling between adjacent metal ions.

Redox-Initiated Reactivity of Dinuclear β-Diketiminatoniobium Imido Complexes

High-valent dichloride and dimethylniobium complexes 1 and 2 bearing tert-butylimido and N,N'-bis(2,4,6-trimethylphenyl)-β-diketiminate (BDI^Ar) ligands were prepared. The dimethyl complex reacted with dihydrogen to release methane and generate the hydride-bridged diniobium(IV) complex 3 in high yield. One-electron oxidation of 3 with silver salts resulted in the release of dihydrogen and conversion to a mixed-valent Nb^III-Nb^IV complex, 4, that displayed a frozen-solution X-band electron paramagnetic resonance signal consistent with a slight dissymmetry between the two Nb centers. Spectroscopic and computational analysis supported the presence of Nb-Nb σ-bonding interactions in both 3 and 4. Finally, one-electron reduction of 4 resulted in conversion to the highly dissymmetric Nb^V-Nb^V dimer 5 that formed from the reductive C-N bond cleavage of one of the BDI^Ar supporting ligands.

Two-Center/Three-Electron Sigma Half-Bonds in Main Group and Transition Metal Chemistry

First proposed in a classic Linus Pauling paper, the two-center/three-electron (2c/3e) σ half-bond challenges the extremes of what may or may not be considered a chemical bond. Two electrons occupying a σ bonding orbital and one electron occupying the antibonding σ* orbital results in bond orders of ∼0.5 that are characteristic of metastable and exotic species, epitomized in the fleetingly stable He2(+) ion. In this Account, I describe the use of coordination chemistry to stabilize such fugacious three-electron bonded species at disparate ends of the periodic table. A recent emphasis in the chemistry of metal-metal bonds has been to prepare compounds with extremely short metal-metal distances and high metal-metal bond orders. But similar chemistry can be used to explore metal-metal bond orders less than one, including 2c/3e half-bonds. Bimetallic compounds in the Ni2(II,III) and Pd2(II,III) oxidation states were originally examined in the 1980s, but the evidence collected at that time suggested that they did not contain 2c/3e σ bonds. Both classes of compounds have been re-examined using EPR spectroscopy and modern computational methods that show the unpaired electron of each compound to occupy a M-M σ* orbital, consistent with 2c/3e Ni-Ni and Pd-Pd σ half-bonds. Elsewhere on the periodic table, a seemingly unrelated compound containing a trigonal bipyramidal Cu3S2 core caused a stir, leaving prominent theorists at odds with one another as to whether the compound contains a S-S bond. Due to my previous experience with 2c/3e metal-metal bonds, I suggested that the Cu3S2 compound could contain a 2c/3e S-S σ half-bond in the previously unknown oxidation state of S2(3-). By use of the Cambridge Database, a number of other known compounds were identified as potentially containing S2(3-) ligands, including a noteworthy set of cyclopentadienyl-supported compounds possessing diamond-shaped Ni2E2 units with E = S, Se, and Te. These compounds were subjected to extensive studies using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, density functional theory, and wave function-based computational methods, as well as chemical oxidation and reduction. The compounds contain E-E 2c/3e σ half-bonds and unprecedented E2(3-) "subchalcogenide" ligands, ushering in a new oxidation state paradigm for transition metal-chalcogen chemistry.

Copper(I) Complexes of Zwitterionic Imidazolium-2-Amidinates, a Promising Class of Electroneutral, Amidinate-Type Ligands

The first complexes containing imidazolium-2-amidinates as ligands (betaine-type adducts of imidazolium-based carbenes and carbodiimides, NHC-CDI) are reported. Interaction of the sterically hindered betaines ICyCDI(DiPP) and IMeCDI(DiPP) [both bearing 2,6-diisopropylphenyl (DiPP) substituents on the terminal N atoms] with Cu(I) acetate affords mononuclear, electroneutral complexes 1a and 1b, which contain NHC-CDI and acetate ligands terminally bound to linear Cu(I) centers. In contrast, the less encumbered ligand ICyCDI(p-Tol), with p-tolyl substituents on the nitrogen donor atoms, affords a dicationic trigonal paddlewheel complex, [Cu2(μ-ICyCDI(p-Tol))3](2+)[OAc(-)]2 (2-OAc). The nuclear magnetic resonance (NMR) resonances of this compound are broad and indicate that in solution the acetate anion and the betaine ligands compete for binding the Cu atom. Replacing the external acetate with the less coordinating tetraphenylborate anion provides the corresponding derivative 2-BPh4 that, in contrast with 2-OAc, gives rise to sharp and well-defined NMR spectra. The short Cu-Cu distance in the binuclear dication [Cu2(μ-ICyCDI(p-Tol))3](2+) observed in the X-ray structures of 2-BPh4 and 2-OAc, ca. 2.42 Å, points to a relatively strong "cuprophilic" interaction. Attempts to force the bridging coordination mode of IMeCDI(DiPP) displacing the acetate anion with BPh4(-) led to the isolation of the cationic mononuclear derivative [Cu(IMeCDI(DiPP))2](+)[BPh4](-) (3b) that contains two terminally bound betaine ligands. Compound 3b readily decomposes upon being heated, cleanly affording the bis-carbene complex [Cu(IMe)2](+)[BPh4(-)] (4) and releasing the corresponding carbodiimide (C(═N-DiPP)2).

The usefulness of EPR spectroscopy in the study of compounds with metal–metal multiple bonds

A discussion of how EPR spectroscopy has contributed to the understanding of the electronic structure of paddlewheel compounds with multiple bonds between metal atoms is presented while commemorating the 50th anniversary of the paper describing the quadruple bond and the identification of the delta bond in the Re₂Cl₈²⁻ anion.

Chloro and azido diruthenium complexes bearing electron-rich N,N',N''-triphenylguanidinate ligands

The reaction of Ru(2)(OAc)(4)Cl with N,N',N''-triphenylguanidine (HTPG) produces one of two different compounds depending on the reaction conditions. In acetone in the presence of triethyl amine, the reaction produces tri-substituted Ru(2)(TPG)(3)(OAc)Cl, and in refluxing xylene, the tetra-substituted Ru(2)(TPG)(4)Cl is produced. Both of these new complexes can be cleanly converted into their corresponding azido analogues by reaction with sodium azide in methanol. The X-ray crystal structures of Ru(2)(TPG)(3)(OAc)Cl, Ru(2)(TPG)(3)(OAc)N(3), and Ru(2)(TPG)(4)Cl are presented, along with magnetic, electrochemical, and spectral measurements for each compound. Studies in solution show that, in contrast to Ru(2)(TPG)(3)(OAc)Cl, Ru(2)(TPG)(4)Cl is sterically hindered at the axial positions, and readily dissociates a chloride ion at high ionic strength. Equilibrium constants for chloride association and dissociation have been estimated. Mass spectrometric data suggest that the two azido complexes are precursors to new diruthenium nitrido species.

Remarkable regioselectivity in the preparation of the first heterotrimetallic Mo[quadruple bond]W...Cr chain

Addition of CrCl(2) to the dinuclear synthon MoW(dpa)(4) yields a regioselectively formed heterotrimetallic Mo[quadruple bond]W...Cr chain; computational studies suggest that the polarization of the Mo[quadruple bond]W quadruple bond partially accounts for this unexpected selectivity.

A Fractional Bond Order of 1/2 in Pd25+−Formamidinate Species; The Value of Very High-Field EPR Spectra

Reaction of Pd(2)(DAniF)(4), 1, (DAniF = di-p-anisylformamidinate) with 1 equiv of AgPF(6) in CH(2)Cl(2) at or below -10 degrees C produces the paramagnetic species [Pd(2)(DAniF)4]PF(6), 1-PF(6), that has been studied by X-ray crystallography, UV-vis spectroscopy, electrochemistry, and multifrequency (9.5, 34.5, 110, and 220 GHz) EPR spectroscopy. Upon oxidation of the precursor, the Pd-Pd distance decreases by 0.052 Angstrom from 2.6486(8) to 2.597(1) Angstrom. The EPR spectra show broad signals with line widths of about 1000 G. The spectra collected at high field show a large spread of g tensor components ( approximately 0.03), but these are masked at lower frequencies (9.5 and 34.5 GHz). A reinvestigation using high-field EPR of the p-tolyl analogue, which is the only other structurally characterized Pd(2)(5+) species (Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988, 110, 1144), shows that this species, which had been reported to give an isotropic 9.5 GHz EPR spectrum, also gives anisotropic 110 and 220 GHz EPR spectra with a similarly large spread of g tensor components consistent with the unpaired electron residing in a metal-based MO. The results of these studies and calculations using density functional theory are consistent with the oxidation being metal-based, resulting in an uncommon Pd(2)(5+) species with a Pd-Pd bond order of 1/2.

High Yield Syntheses of Stable, Singly Bonded Pd26+ Compounds

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.