Phenylamino derivatives of tris(2-pyridylmethyl)amine: hydrogen-bonded peroxodicopper complexes

Iron-Catalyzed C-H Oxygenation Using Perchlorate Enabled by Secondary Sphere Hydrogen Bonds

Perchlorate (ClO4-) is a groundwater pollutant that is challenging to remediate. We report a strategy to use Fe(II) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended aniline hydrogen bonds (H-bonds) to promote ClO4- reduction. These complexes facilitate oxygen atom transfer from ClO4- to PPh3 and C-H oxygenation reactions of organic substrates. Catalytic reactions using 15 mol % afforded excellent yields for oxygenation of anthracene and cyclic alkyl aromatics, and this methodology tolerates aryl halides as well as heterocycles containing either O, S, or N.

Bioinspired complexes confined in well-defined capsules: getting closer to metalloenzyme functionalities

Reproducing the key features offered by metalloprotein binding cavities is an attractive approach to overcome the main bottlenecks of current open artificial models (in terms of stability, efficiency and selectivity). In this context, this featured article brings together selected examples of recent developments in the field of confined bioinspired complexes with an emphasis on the emerging hemicryptophane caged ligands. In particular, we focused on (1) the strategies allowing the insulation and protection of complexes sharing similarities with metalloprotein active sites, (2) the confinement-induced improvement of catalytic efficiencies and selectivities and (3) very recent efforts that have been made toward the development of bioinspired complexes equipped with weakly binding artificial cavities.

What Is the Right Level of Activation of a High-Spin {FeNO}7 Complex to Enable Direct N-N Coupling? Mechanistic Insight into Flavodiiron NO Reductases

Flavodiiron nitric oxide reductases (FNORs), found in pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N₂O) to detoxify NO released by the human immune system. Previously, we reported the first FNOR model system that mediates direct NO reduction (Dong, H. T.; J. Am. Chem. Soc. 2018, 140, 13429-13440), but no intermediate of the reaction could be characterized. Here, we present a new set of model complexes that, depending on the ligand substitution, can either mediate direct NO reduction or stabilize a highly activated high-spin (hs) {FeNO}⁷ complex, the first intermediate of the reaction. The precursors, [{Fe^II(MPA-(RPhO)₂)}₂] (1, R = H and 2, R = ^tBu, Me), were prepared first and fully characterized. Complex 1 (without steric protection) directly reduces NO to N₂O almost quantitatively, which constitutes only the second example of this reaction in model systems. Contrarily, the reaction of sterically protected 2 with NO forms the stable mononitrosyl complex 3, which shows one of the lowest N-O stretching frequencies (1689 cm^-1) observed so far for a mononuclear hs-{FeNO}⁷ complex. This study confirms that an N-O stretch ≤1700 cm^-1 represents the appropriate level of activation of the FeNO unit to enable direct NO reduction. The higher activation level of these hs-{FeNO}⁷ complexes required for NO reduction compared to those formed in FNORs emphasizes the importance of hydrogen bonding residues in the active sites of FNORs to activate the bound NO ligands for direct N-N coupling and N₂O formation. The implications of these results for FNORs are further discussed.

A guide to secondary coordination sphere editing

This tutorial review showcases recent (2015-2021) work describing ligand construction as it relates to the design of secondary coordination spheres (SCSs). Metalloenzymes, for example, utilize SCSs to stabilize reactive substrates, shuttle small molecules, and alter redox properties, promoting functional activity. In the realm of biomimetic chemistry, specific incorporation of SCS residues (e.g., Brønsted or Lewis acid/bases, crown ethers, redox groups etc.) has been shown to be equally critical to function. This contribution illustrates how fundamental advances in organic and inorganic chemistry have been used for the construction of such SCSs. These imaginative contributions have driven exciting findings in many transformations relevant to clean fuel generation, including small molecule (e.g., H⁺, N₂, CO₂, NO_x, O₂) reduction. In most cases, these reactions occur cooperatively, where both metal and ligand are requisite for substrate activation.

A caged tris(2-pyridylmethyl)amine ligand equipped with a Ctriazole-H hydrogen bonding cavity

A capped bioinspired ligand built from a tris(2-pyridyl-methyl)amine (TPA) unit and surmounted by a triazole-based intramolecular H-bonding secondary sphere, was prepared. The resulting cage structure describes a well-defined cavity combining...

Hydrogen bonds and dispersion forces serving as molecular locks for tailored Group 11 bis(amidine) complexes

A flexible polydentate bis(amidine) ligand LH2 operates as a molecular lock for various coinage metal fragments and forms the dinuclear complexes [LH2(MCl)2], M = Cu (1), Au (2), the coordination...

Effects of Protonation State on Electrocatalytic CO2 Reduction by a Cobalt Aminopyridine Macrocyclic Complex

A critical component in the reduction of CO₂ to CO and H₂O is the delivery of 2 equiv of protons and electrons to the CO₂ molecule. The timing and sequencing of these proton and electron transfer steps are essential factors in directing the activity and selectivity for catalytic CO₂ reduction. In previous studies, we have reported a series of macrocyclic aminopyridine cobalt complexes capable of reducing CO₂ to CO with high faradaic efficiencies. Kinetic investigations reveal a relationship between the observed rate constant (k_obs) and the number of pendant amine hydrogen bond donors minus one, suggesting the presence of a deprotonated active catalytic state. Herein, we investigate the feasibility of these proposed deprotonated complexes toward CO₂ reduction. Two deprotonated derivatives, Co(L^4-) and Co(L^2-), of the tetraamino macrocycle Co(L) were independently synthesized and structurally characterized revealing extensive delocalization of the negative charge upon deprotonation. ¹H nuclear magnetic resonance spectroscopy and ultraviolet-visible titration studies confirm that under catalytic conditions, the active form of the catalyst gradually becomes deprotonated, supporting thus the n_donor - 1 relationship with k_obs. Electrochemical studies of Co(L^4-) reveal that this deprotonated analogue is competent for electrocatalysis upon addition of an exogenous weak acid source, such as 2,2,2-trifluoroethanol, resulting in faradaic efficiencies for CO₂-to-CO conversion identical to those observed with the fully protonated derivative (>98%).

Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O-H, N-H, and C-H Substrates

Instability of end-on superoxocopper(II) complexes, with respect to conversion to peroxo-bridged dicopper(II) complexes, has largely constrained their study to very low temperatures. This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of bulky ligands, Ar₃-TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O₂ to yield [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ species, which are stable against dimerization at all temperatures. Binding of O₂ saturates at subambient temperatures and can be reversed by warming. The onset of oxygenation for the Ar = tpb and dpb systems is observed at 25 °C, and all three [Cu^II(η¹-O₂^•-)(Ar₃-TMPA)]⁺ complexes are stable against self-decay at temperatures of ≤-20 °C. This provides a wide temperature window for study of these complexes, which was exploited by performing extensive reaction kinetics measurements for [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ using a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants (k₂) versus oxidation potentials (E_ox) for a range of phenols, construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicate that reaction with all substrates proceeds via H atom transfer (HAT), reaction with the phenols proceeds with significant charge transfer, and full tunneling of both H and D atoms occurs in the case of 1,2-diphenylhydrazine and 4-methoxy-2,6-di-tert-butylphenol. Oxidation of C-H bonds proved to be kinetically challenging, and whereas [Cu^II(η¹-O₂^•-)(tpb₃-TMPA)]⁺ can oxidize moderately strong O-H and N-H bonds, it is only able to oxidize very weak C-H bonds.

Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump-probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [(^R3P₃tren)Cu^IIN₃]BAr₄^F (1^PR3, ^R3P₃tren = tris[2-(phosphiniminato)ethyl]amine, BAr₄^F = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR₃ ligand are systematically varied across the series (PR₃ = PMe₃, PMe₂Ph, PMePh₂). The asymmetric stretching mode of azide in the 1^PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump-probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ₂). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure-function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

Controlling the Reactivity of a Metal-Hydroxo Adduct with a Hydrogen Bond

The enzymes manganese lipoxygenase (MnLOX) and manganese superoxide dismutase (MnSOD) utilize mononuclear Mn centers to effect their catalytic reactions. In the oxidized Mn^III state, the active site of each enzyme contains a hydroxo ligand, and X-ray crystal structures imply a hydrogen bond between this hydroxo ligand and a cis carboxylate ligand. While hydrogen bonding is a common feature of enzyme active sites, the importance of this particular hydroxo-carboxylate interaction is relatively unexplored. In this present study, we examined a pair of Mn^III-hydroxo complexes that differ by a single functional group. One of these complexes, [Mn^III(OH)(PaPy₂N)]⁺, contains a naphthyridinyl moiety capable of forming an intramolecular hydrogen bond with the hydroxo ligand. The second complex, [Mn^III(OH)(PaPy₂Q)]⁺, contains a quinolinyl moiety that does not permit any intramolecular hydrogen bonding. Spectroscopic characterization of these complexes supports a common structure, but with perturbations to [Mn^III(OH)(PaPy₂N)]⁺, consistent with a hydrogen bond. Kinetic studies using a variety of substrates with activated O-H bonds, revealed that [Mn^III(OH)(PaPy₂N)]⁺ is far more reactive than [Mn^III(OH)(PaPy₂Q)]⁺, with rate enhancements of 15-100-fold. A detailed analysis of the thermodynamic contributions to these reactions using DFT computations reveals that the former complex is significantly more basic. This increased basicity counteracts the more negative reduction potential of this complex, leading to a stronger O-H BDFE in the [Mn^II(OH₂)(PaPy₂N)]⁺ product. Thus, the differences in reactivity between [Mn^III(OH)(PaPy₂Q)]⁺ and [Mn^III(OH)(PaPy₂N)]⁺ can be understood on the basis of thermodynamic considerations, which are strongly influenced by the ability of the latter complex to form an intramolecular hydrogen bond.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

Coupled dinuclear copper oxygen cores (Cu₂ O₂ ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O₂ transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (^C P) have been proposed as key structures in the early stages of O₂ binding in these proteins; their reversible isomerization to other Cu₂ O₂ cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic ^C P cores in biological and synthetic systems, the properties and reactivity of ^C P Cu₂ O₂ species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (^T P) and ^C P dicopper cores. Ca^II mediates this process by reversible binding at the Cu₂ O₂ core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O₂ activation in biomimetic systems.

Late-stage ligand functionalization via the Staudinger reaction using phosphine-appended 2,2'-bipyridine

The ability of a phosphine-appended-2,2'-bipyridine ligand ((Ph2P)2bpy) to serve as a platform for late-stage ligand modifications was evaluated using tetrahedral (Ph2P)2bpyFeCl2. We employed a post-metalation Staudinger reaction to install a series of functionalized arenes, including those containing Brønsted and Lewis acidic groups. This reaction sequence represents a versatile strategy to both tune the ligand donor properties as well as directly incorporate appended functionality.

Hydrogen-bonded nickel(I) complexes

A series of nickel(ii) tris(2-pyridylmethyl)amine (TPA) complexes featuring appended hydrogen bonds (H-bonds) to halides (F, Cl, Br) was synthesized and charcterized. Reduction to the nickel(i) state provided access to an unusual nickel(i) fluoride complex stabilized by H-bonds, enabling structural and spectroscopic characterization.

Generation and Reactivity of a NiIII2(μ-1,2-peroxo) Complex

High-valent transition metal-oxo, -peroxo, and -superoxo complexes are crucial intermediates in both biological and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the isolation of such an unusual high-valent species in a thermally unstable Ni^III₂(μ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. Reactivity studies show that this complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization

We outline the generality and requirements for cooperative N₂H₄ capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH₂ stabilization. While the Lewis acids are required to bind N₂H₄, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

Theory Demonstrated a "Coupled" Mechanism for O2 Activation and Substrate Hydroxylation by Binuclear Copper Monooxygenases

Multiscale simulations have been performed to address the longstanding issue of "dioxygen activation" by the binuclear copper monooxygenases (PHM and DβM), which have been traditionally classified as "noncoupled" binuclear copper enzymes. Our QM/MM calculations rule out that Cu_M(II)-O₂^• is an active species for H-abstraction from the substrate. In contrast, Cu_M(II)-O₂^• would abstract an H atom from the cosubstrate ascorbate to form a Cu_M(II)-OOH intermediate in PHM and DβM. Consistent with the recently reported structural features of DβM, the umbrella sampling shows that the "open" conformation of the Cu_M(II)-OOH intermediate could readily transform into the "closed" conformation in PHM, in which we located a mixed-valent μ-hydroperoxodicopper(I,II) intermediate, (μ-OOH)Cu(I)Cu(II). The subsequent O-O cleavage and OH moiety migration to Cu_H generate the unexpected species (μ-O^•)(μ-OH)Cu(II)Cu(II), which is revealed to be the reactive intermediate responsible for substrate hydroxylation. We also demonstrate that the flexible Met ligand is favorable for O-O cleavage reactions, while the replacement of Met with the strongly bound His ligand would inhibit the O-O cleavage reactivity. As such, the study not only demonstrates a "coupled" mechanism for O₂ activation by binuclear copper monooxygenases but also deciphers the full catalytic cycle of PHM and DβM in accord with the available experimental data. These findings of O₂ activation and substrate hydroxylation by binuclear copper monooxygenases could expand our understanding of the reactivities of the synthetic monocopper complexes.

Impact of Intramolecular Hydrogen Bonding on the Reactivity of Cupric Superoxide Complexes with O-H and C-H Substrates

The dioxygen reactivity of a series of TMPA-based copper(I) complexes (TMPA=tris(2-pyridylmethyl)amine), with and without secondary-coordination-sphere hydrogen-bonding moieties, was studied at -135 °C in 2-methyltetrahydrofuran (MeTHF). Kinetic stabilization of the H-bonded [( ( X 1 ) ( X 2 ) TMPA)Cu^II (O₂ ^.- )]⁺ cupric superoxide species was achieved, and they were characterized by resonance Raman (rR) spectroscopy. The structures and physical properties of [( ( X 1 ) ( X 2 ) TMPA)Cu^II (N₃ ^- )]⁺ azido analogues were compared, and the O₂ ^.- reactivity of ligand-Cu^I complexes when an H-bonding moiety is replaced by a methyl group was contrasted. A drastic enhancement in the reactivity of the cupric superoxide towards phenolic substrates as well as oxidation of substrates possessing moderate C-H bond-dissociation energies is observed, correlating with the number and strength of the H-bonding groups.

Early Metal Di(pyridyl) Pyrrolide Complexes with Second Coordination Sphere Arene-π Interactions: Ligand Binding and Ethylene Polymerization

Early metal complexes supported by hemilabile, monoanionic di(pyridyl) pyrrolide ligands substituted with mesityl and anthracenyl groups were synthesized to probe the possibility of second coordination sphere arene-π interactions with ligands with potential for allosteric control in coordination chemistry, substrate activation, and olefin polymerization. Yttrium alkyl, indolide, and amide complexes were prepared and structurally characterized; close contacts between the anthracenyl substituents and Y-bound ligands are observed in the solid state. Titanium, zirconium, and hafnium tris(dimethylamido) complexes were synthesized, and their ethylene polymerization activity was tested. In the solid state structure of one of the Ti tris(dimethylamido) complexes, coordination of Ti to only one of the pyridine donors is observed pointing to the hemilabile character of the di(pyridyl) pyrrolide ligands.

Dioxygen-Derived Nonheme Mononuclear FeIII(OH) Complex and Its Reactivity with Carbon Radicals

A new tetradentate, monoanionic, mixed N/O donor ligand (BNPA^Ph2O^-) with second coordination sphere H-bonding groups has been synthesized for stabilization of a terminal Fe^III(OH) complex. The complex Fe^II(BNPA^Ph2O)(OTf) (1) reacts with O₂ to give a mononuclear terminal Fe^III(OH) complex, Fe^III(OH)(BNPA^Ph2O)(OTf) (2), both of which were characterized by X-ray diffraction, electrospray ionization mass spectrometry, UV-vis, ¹H and ¹⁹F nuclear magnetic resonance, ⁵⁷Fe Mössbauer, and electron paramagnetic resonance spectroscopies. Treatment of 2 with carbon radicals (Ar₃C·) gives Ar₃COH and the Fe^II complex 1, in direct analogy with the elusive radical "rebound" process proposed for nonheme iron enzymes.

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe(ii)-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable S = 3/2 reduction product. The borane-capped Fe(ii)-dihydride: (1) rapidly deprotonates E-H (E = N, O, P, S) bonds to afford borane-stabilized Fe adducts and (2) releases H₂ upon exposure to π-acids. The Lewis acids provide an avenue for redox-leveling in analogy to the near constant operating potential for N₂ reduction in nitrogenase.

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins

Interconversion between dioxygen (O₂) and hydrogen peroxide (H₂O₂) has attracted much interest because of the growing importance of H₂O₂ as an energy source. There are many reports on O₂ conversions to H₂O₂; however, no example has been reported on O₂/H₂O₂ interconversion. Herein, we describe successful achievement of a reversible O₂/H₂O₂ conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O₂ to afford the corresponding porphyrin and H₂O₂; conversely, the porphyrin also reacted with excess H₂O₂ to reproduce the corresponding isophlorin and O₂. The isophlorin-O₂/porphyrin-H₂O₂ interconversion was repeatedly proceeded by alternate bubbling of Ar or O₂, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O₂ as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H₂O₂. The intriguing isophlorin-O₂/porphyrin-H₂O₂ interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O₂/porphyrin-H₂O₂ states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O₂/porphyrin-H₂O₂ interconversion using ¹H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H₂O₂ as an intermediate.

Stoichiometric and electrocatalytic production of hydrogen peroxide driven by a water-soluble copper(ii) complex

Herein, a water-soluble molecular copper complex was investigated as a catalyst for O₂ reduction in both water and an organic solvent.

Chirality transfer in a cage controls the clockwise/anticlockwise propeller arrangement of the tris(2-pyridylmethyl)amine ligand

A predictable control of the propeller arrangement of the tris(2-pyridylmethyl)amine (TPA) ligand was achieved in the smallest hemicryptophane 1. Coordination of Cu(i) result in a rare T-shaped complex with controlled helicity of the TPA-Cu core.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Manganese-Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Hydrogen bonds (H-bonds) within the secondary coordination sphere are often invoked as essential noncovalent interactions that lead to productive chemistry in metalloproteins. Incorporating these types of effects within synthetic systems has proven a challenge in molecular design that often requires the use of rigid organic scaffolds to support H-bond donors or acceptors. We describe the preparation and characterization of a new hybrid tripodal ligand ([H₂pout]^3-) that contains two monodeprotonated urea groups and one phosphinic amide. The urea groups serve as H-bond donors, while the phosphinic amide group serves as a single H-bond acceptor. The [H₂pout]^3- ligand was utilized to stabilize a series of Mn-hydroxido complexes in which the oxidation state of the metal center ranges from 2+ to 4+. The molecular structure of the Mn^III-OH complex demonstrates that three intramolecular H-bonds involving the hydroxido ligand are formed. Additional evidence for the formation of intramolecular H-bonds was provided by vibrational spectroscopy in which the energy of the O-H vibration supports its assignment as an H-bond donor. The stepwise oxidation of [Mn^IIH₂pout(OH)]^2- to its higher oxidized analogs was further substantiated by electrochemical measurements and results from electronic absorbance and electron paramagnetic resonance spectroscopies. Our findings illustrate the utility of controlling both the primary and secondary coordination spheres to achieve structurally similar Mn-OH complexes with varying oxidation states.

Hydrogen Bonds Dictate O2 Capture and Release within a Zinc Tripod

Six directed hydrogen bonding (H-bonding) interactions allow for the reversible capture and reduction of dioxygen to a trans-1,2-peroxo within a tripodal zinc(II) framework. Spectroscopic studies of the dizinc peroxides, as well as on model zinc diazides, suggest H-bonding contributions serve a dominant role for the binding/activation of these small molecules.

Large-bite diboranes for the μ(1,2) complexation of hydrazine and cyanide

As part of our interest in the chemistry of polydentate Lewis acids as hosts for diatomic molecules, we have investigated the synthesis and coordination chemistry of bidentate boranes that feature a large boron-boron separation. In this paper, we describe the synthesis of a new example of such a diborane, namely 1,8-bis(dimesitylboryl)triptycene (2) and compare its properties to those of the recently reported 1,8-bis(dimesitylboryl)biphenylene (1). These comparative studies reveal that these two diboranes feature some important differences. As indicated by cyclic voltammetry, 1 is more electron deficient than 2; it also adopts a more compact and rigid structure with a boron-boron separation (4.566(5) Å) shorter by ∼1 Å than that in 2 (5.559(4) Å). These differences appear to dictate the coordination behaviour of these two compounds. While 2 remains inert toward hydrazine, we observed that 1 forms a very stable μ(1,2) hydrazine complex which can also be obtained by phase transfer upon layering a solution of 1 with a dilute aqueous hydrazine solution. The stability of this complex is further reflected by its lack of reaction with benzaldehyde at room temperature. We have also investigated the behaviour of 1 and 2 toward anions. In MeOH/CHCl3 (1/1 vol) both compounds selectively bind cyanide to form the corresponding μ(1,2) chelate complexes with a B-C[triple bond, length as m-dash]N-B bridge at their cores. Competition experiments in protic media show that the anionic cyanide complex formed by 1 is the most stable, with no evidence of decomplexation even in the presence of (C6F5)3B.

Intramolecular Hydrogen Bonding Enhances Stability and Reactivity of Mononuclear Cupric Superoxide Complexes

[(L)CuII(O2•-)]+ (i.e., cupric-superoxo) complexes, as the first and/or key reactive intermediates in (bio)chemical Cu-oxidative processes, including in the monooxygenases PHM and DβM, have been systematically stabilized by intramolecular hydrogen bonding within a TMPA ligand-based framework. Also, gradual strengthening of ligand-derived H-bonding dramatically enhances the [(L)CuII(O2•-)]+ reactivity toward hydrogen-atom abstraction (HAA) of phenolic O-H bonds. Spectroscopic properties of the superoxo complexes and their azido analogues, [(L)CuII(N3-)]+, also systematically change as a function of ligand H-bonding capability.