Accessing Ni(0) to Ni(IV) via nickel-carbon-phosphorus bond reorganization

Two-electron oxidation of a NiIIPh(PCP) pincer complex initiates phosphine ligand insertion, generating an η6-arylphosphonium moiety coordinated to NiII. The reaction is fully reversible under reducing conditions. X-ray crystallography, NMR/EPR spectroscopy, electrochemistry, and DFT calculations support the proposed Ni-C-P bond reorganization mechanisms, which access oxidation states from Ni0 to NiIV.

Microporous Metal-Phosphonates with a Novel Orthogonalized Linker and Complementary Guests: Insights for Trivalent Metal Complexes from Divalent Metal Complexes

The reliable self-assembly of microporous metal-phosphonate materials remains a longstanding challenge. This stems from, generally, more coordination modes for the functional group allowing more dense structures, and stronger bonding driving less crystalline products. Here, a novel orthogonalized aryl-phosphonate linker, 1,3,5-tris(4'-phosphono-2',6'-dimethylphenyl) benzene (H₆ L3) has been used to direct formation of open frameworks. The peripheral aryl rings of H₆ L3 are orthogonalized relative to the central aromatic ring giving a tri-cleft conformation of the linker in which small aromatic molecules can readily associate. When coordinated to magnesium ions, a series of porous crystalline metal-organic, and hydrogen-bonded metal-organic frameworks (MOFs, HMOFs) are formed (CALF-41 (Mg), HCALF-42 (Mg), -43 (Mg)). While most metal-organic frameworks are tailored based on choice of metal and linker, here, the network structures are highly dependent on the inclusion and structure of the guest aromatic compounds. Larger guests, and a higher stoichiometry of metal, result in increased solvation of the metal ion, resulting in networks with connectivities increasingly involving hydrogen-bonds rather than direct phosphonate coordination. Upon thermal activation and aromatic template removal, the materials exhibit surface areas ranging from 400-600 m² /g. Self-assembly in the absence of aromatic guests yields mixtures of phases, frequently co-producing a dense 3-fold interpenetrated structure (1). Interestingly, a series of both more porous (530-900 m² /g), and more robust solids is formed by complexing with trivalent metal ions (Al, Ga, In) with aromatic guest; however, these are only attainable as microcrystalline powders. The polyprotic nature of phosphonate linkers enables structural analogy to the divalent analogues and these are identified as CALF-41 analogues. Finally, insights to the structural transformations during metal ion desolvation in this family are gained by considering a pair of structurally related Co materials, whose hydrogen-bonded (HCALF-44 (Co)) and desolvated (CALF-44 (Co)) coordination bonded networks were fully structurally characterized.

Coordination-Induced Weakening of a C(sp3)-H Bond: Homolytic and Heterolytic Bond Strength of a CH-Ni Agostic Interaction

The scission of a C(sp³)-H bond to form a new metal-alkyl bond is a fundamental step in coordination chemistry and catalysis. However, the extent of C-H bond weakening when this moiety interacts with a transition metal is poorly understood and quantifying this phenomenon could provide insights into designing more efficient C-H functionalization catalysts. We present a nickel complex with a robust adamantyl reporter ligand that enables the measurement of C-H acidity (pK_a) and bond dissociation free energy (BDFE) for a C(sp³)-H agostic interaction, showing a decrease in pK_a by dozens of orders of magnitude and BDFE decrease of about 30 kcal/mol upon coordination. X-ray crystallographic data is provided for all molecules, including a distorted square planar Ni^III metalloradical and "doubly agostic" Ni^II(κ²-CH₂) complex.

A redox-active Mn(0) dicarbene metalloradical

A rare redox-active Mn(0) dicarbene anion with solvent-dependent electrochemical behaviour has been synthesized and thoroughly characterized.

Lewis Acid-Promoted Oxidative Addition at a [Ni0 (diphosphine)2 ] Complex: The Critical Role of a Secondary Coordination Sphere

Oxidative addition represents a critical elementary step in myriad catalytic transformations. Here, the importance of thoughtful ligand design cannot be overstated. In this work, we report the intermolecular activation of iodobenzene (PhI) at a coordinatively saturated 18-electron [Ni⁰ (diphosphine)₂ ] complex bearing a Lewis acidic secondary coordination sphere. Whereas alkyl-substituted diphosphine complexes of Group 10 are known to be unreactive in such reactions, we show that [Ni⁰ (P₂ B^Cy ₄ )₂ ] (P₂ B^Cy ₄ =1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [Ni^II (P₂ B^Cy ₄ )(Ph)(I)]. This difference in oxidative addition reactivity has been scrutinized computationally - an outcome that is borne out in ring-opening to provide the reactive precursor - for [Ni⁰ (P₂ B^Cy ₄ )₂ ], a "boron-trapped" 16-electron κ¹ -diphosphine Ni(0) complex. Moreover, formation of [Ni^II (P₂ B^Cy ₄ )(Ph)(I)] is inherent to the P₂ B^Cy ₄ secondary coordination sphere: treatment of the Lewis adduct, [Ni⁰ (P₂ B^Cy ₄ )₂ (DMAP)₈ ] with PhI provides [Ni^II (P₂ B^Cy ₄ )₂ (DMAP)₈ (I)]I via iodine-atom abstraction and not a [Ni^II (Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni⁰ (P₂ B^Cy ₄ )₂ ] with 4-iodopyridine was surveyed, which resulted in a pyridyl-borane linked oligomer. The implications of these outcomes are discussed in the context of designing strongly donating, and yet labile diphosphine ligands for use in a critical bond activation step relevant to catalysis.

Preparation of a borane-appended Co(III) hydride: evidence for metal-ligand cooperativity in O-H bond activation

Cobalt hydrides are known to mediate a number of important chemical transformations including proton (H+), hydride (H-), and hydrogen-atom (H˙) transfer. Central to the tunability of such frameworks is judicious ligand design, which offers the flexibility to alter fundamental properties relevant to reactivity. Herein, we report the preparation of one such cobalt(iii) hydride: [Cp*CoIII(P2BCy4)(H)]BPh4 (Cp* = C5Me5-, P2BCy4 = 1,2-bis(di(3-dicyclohexylborane)propylphosphino)ethane) that is encircled by a boron-based Lewis-acidic secondary coordination sphere. The structure of this species is supported by synchrotron-radiation crystallography, evidencing a terminal Co(iii) hydride with four sp2-hybridized boranes that invite Lewis base coordination. To this end, electrochemical reactivity studies performed using [Cp*CoIII(P2BCy4)Cl]+ or an "all-akyl" model, [Cp*CoIII(dnppe)Cl]+ (dnppe = 1,2-bis(di-n-propylphosphino)ethane) with benzoic or 4-pyridylbenzoic acid show divergent responses for protonation of electrochemically-generated Co(i) to give a Co(iii) hydride. For [Cp*CoIII(P2BCy4)Cl]+, this process is complex, not only involving protonation, but also engagement of the pendant borane moieties in Lewis acid/base interactions. For protonation by benzoic acid, for example, borane-benzoate contacts are substantiated by variable temperature NMR spectroscopic measurements and theoretical calculations, pointing to a cooperative Co-H/B-O bond forming process. These data are discussed in the context of designing new molecular catalysts for ligand-assisted hydrogen evolution reactivity.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O₂ to H₂O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O₂ to water via Co(III)-O-O-Co(III) peroxo intermediates.

Amphiphile-like self assembly of metal organic polyhedra having both polar and non-polar groups

Mixed linker metal-organic polyhedra (MOPs) with polar and non-polar groups on the same MOP have been synthesized. This yields two types of MOPs, one where the ligands are evenly and symmetrically distributed over each polyhedron, as confirmed crystallographically and the other where respective groups segregate. The segregation is confirmed by the amphiphile-like behavior of the latter MOP in different polarity solvents, as seen through transmission electron microscopy (TEM) even though the anchor points of the functional groups are ∼10 Å apart on the MOP surface.

Facile hydrogen atom transfer to iron(iii) imido radical complexes supported by a dianionic pentadentate ligand

Facile hydrogen atom transfer from toluene.

A dianionic tetrapodal pentadentate diborate ligand is introduced. This ligand forms a high spin neutral iron(ii) complex that reacts with a variety of organoazides to yield transient Fe(iii) imido radicals that are extremely potent hydrogen atom abstractors. The nature of these species is supported by full characterization of the Fe(iii) amido products, kinetic studies, density functional computations and Mössbauer spectroscopy on the –C₆H₄-p-^tBu substituted derivative.

Activation of Si–H bonds across the nickel carbene bond in electron rich nickel PCcarbeneP pincer complexes

Silicon–hydrogen bonds are shown to add to a nickel carbon double bond to yield nickel α-silylalkyl hydrido complexes.

Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PCcarbeneP ligands

The donor properties of five different PC_carbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations.

Mechanistic studies on the addition of hydrogen to iridaepoxide complexes with subsequent elimination of water

Iridium complexes of the PC_sp²P ligand in which the donors are linked by 2,3-benzo[b]thiophene groups engage in the cooperative activation of N₂O and the resulting iridaepoxides can be treated with dihydrogen to effect elimination of water and regeneration of the starting iridium complex.

Iridium complexes of the PC_sp²P ligand in which the donors are linked by 2,3-benzo[b]thiophene groups engage in the cooperative activation of N₂O and the resulting iridaepoxides can be treated with dihydrogen to effect elimination of water and regeneration of the starting iridium complex. The mechanism of the steps in this reaction have been investigated using low temperature NMR investigations that reveal H/D exchange processes that point to a highly reactive kinetic product of hydrogen addition to the iridaepoxide. This intermediate is also involved in the water elimination pathway, and model compounds have been synthesized to provide further evidence for the mechanistic proposals for water elimination. The adaptable donor properties of the PC_sp²P ligand framework, particularly the anchoring carbene donor, plays a significant role in the ability of these compounds to mediate the transformation of N₂O in this way.

Synthesis of unsymmetrical 5,6-POCOP′-type pincer complexes of nickel(ii): impact of nickelacycle size on structures and spectroscopic properties

Different structures and reactivities observed with the unsymmetrical (5,6-POCOP) and symmetrical (5,5-POCOP) pincer complexes of Ni(ii) underline the importance of the metallacycle ring size and its degree of flexibility.

Characterization of divalent and trivalent species generated in the chemical and electrochemical oxidation of a dimeric pincer complex of nickel

The electrolytic and chemical oxidation of the dimeric pincer complex [κ(P),κ(C),κ(N),μ(N)-(2,6-(i-Pr(2)POC(6)H(3)CH(2)NBn)Ni](2) (1; Bn = CH(2)Ph) has been investigated by various analytic techniques. Cyclic voltammetry measurements have shown that 1 undergoes a quasi-reversible, one electron, Ni-based redox process (ΔE(0)(1/2) = -0.07 V vs Cp(2)Fe/[Cp(2)Fe](+)), and spectroelectrochemical measurements conducted on the product of the electrolytic oxidation, [1](+•), have shown multiple low-energy electronic transitions in the range of 10,000-15,000 cm(-1). Computational studies using Density Functional Theory (B3LYP) have corroborated the experimentally obtained structure of 1, provided the electronic structure description, and helped interpret the experimentally obtained absorption spectra for 1 and [1](+·). These calculations indicate that the radical cation [1](+·) is a dimeric, mixed-valent species (class III) wherein most of the spin density is delocalized over the two nickel centers (Ni(+2.5)(2)N(2)), but some spin density is also present over the two nitrogen atoms (Ni(2+)(2)N(2)·). Examination of alternative structures for open shell species generated from 1 has shown that the spin density distribution is highly sensitive toward changes in the ligand environment of the Ni ions. NMR, UV-vis, electron paramagnetic resonance (EPR), and single crystal X-ray diffraction analyses have shown that chemical oxidation of 1 with N-Bromosuccinimide (NBS) follows a complex process that gives multiple products, including the monomeric trivalent species κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr(2) (2). These studies also indicate that oxidation of 1 with 1 equiv of NBS gives an unstable, paramagnetic intermediate that decomposes to a number of divalent species, including succinimide and the monomeric divalent complexes κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr (3) and κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH(2)N(H)Bn)}NiBr(2) (4); a second equivalent of NBS then oxidizes 3 and 4 to 2 and other unidentified products. The divalent complex 3 was synthesized independently and shown to react with NBS or bromine to form its trivalent homologue 2. The new complexes 2 and 3 have been characterized fully.

Monomeric and dimeric nickel complexes derived from a pincer ligand featuring a secondary amine donor moiety

Reaction of NiBr(2)(CH(3)CN)(x) with the unsymmetrical pincer ligand m-(i-Pr(2)PO)(CH(2)NHBn)C(6)H(4) (Bn = CH(2)Ph) gives the complex (R,S)-kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NHBn)-C(6)H(3)}Ni(II)Br, 1, featuring an asymmetric secondary amine donor moiety. Deprotonation of the latter with methyl lithium gave a dark brown compound that could not be characterized directly, but fully characterized derivatives prepared from this compound indicate that it is the LiBr adduct of the 14-electron amido species [kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NBn)-C(6)H(3)}Ni], 2. Thus, 2.LiBr reacts with water to regenerate 1, while reaction with excess benzyl or allyl bromide gave the POCN-type pincer complexes 3 and 4, respectively, featuring tertiary amine donor moieties. On the other hand, heating 2.LiBr at 60 degrees C led to loss of LiBr and dimerization to generate the orange crystalline compound [mu(N);kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NBn)-C(6)H(3)}Ni](2), 5. Solid state structural studies show that 1, 3, and 4 are monomeric, square planar complexes involving one Ni-N interaction, whereas complex 5 is a C(2)-symmetric dimer involving four Ni-N interactions and a Ni(2)N(2) core featuring a short Ni-Ni distance (2.51 A). Preliminary reactivity tests have shown that 5 is stable toward weak nucleophiles such as acetonitrile but reacts with strong nucleophiles such as CO or 2,6-Me(2)(C(6)H(3))NC. Reactions with protic reagents showed that phthalimide appears to break the dimer to generate a monomeric species, whereas alcohols appear to leave the dimer intact, giving rise instead to adducts through N...H...O interactions. These ROH adducts of 5 were found to be active precatalysts for the alchoholysis of acrylonitrile with up to 2000 catalytic turnover numbers.