Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand.

Cu systems feature prominently in aerobic oxidative catalysis in both biology and synthetic chemistry. Metal ligand cooperativity is a common theme in both areas as exemplified by galactose oxidase and by aminoxyl radicals in alcohol oxidations. This has motivated investigations into the aerobic chemistry of Cu and specifically the isolation and study of Cu-superoxo species that are invoked as key catalytic intermediates. While several examples of complexes that model biologically relevant Cu(II) superoxo intermediates have been reported, they are not typically competent aerobic catalysts. Here, we report a new Cu complex of the redox-active ligand tBu,TolDHP (2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) that activates O2 to generate a catalytically active Cu(II)-superoxo complex via ligand-based electron transfer. Characterization using ultraviolet (UV)-visible spectroscopy, Raman isotope labeling studies, and Cu extended X-ray absorption fine structure (EXAFS) analysis confirms the assignment of an end-on κ1 superoxo complex. This Cu-O2 complex engages in a range of aerobic catalytic oxidations with substrates including alcohols and aldehydes. These results demonstrate that bioinspired Cu systems can not only model important bioinorganic intermediates but can also mediate and provide mechanistic insight into aerobic oxidative transformations.

Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence.

Metal ligand cooperativity is a powerful strategy in transition metal chemistry. This type of mechanism for the activation of O2 is best exemplified by heme centers in biological systems. While aerobic oxidations with Fe and Cu are well precedented, Ni-based oxidations are frequently less common due to less-accessible metal-based redox couples. Some Ni enzymes utilize special ligand environments for tuning the Ni(II)/(III) redox couple such as strongly donating thiolates in Ni superoxide dismutase. A recently characterized example of a Ni-containing protein, however, suggests an alternative strategy for mediating redox chemistry with Ni by utilizing ligand-based reducing equivalents to enable oxygen binding. While this mechanism has little synthetic precedent, we show here that Ni complexes of the redox-active ligand tBu,TolDHP (tBu,TolDHP = 2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) activate O2 to generate a Ni(II) superoxo complex via ligand-based electron transfer. This superoxo complex is competent for stoichiometric oxidation chemistry with alcohols and hydrocarbons. This work demonstrates that coupling ligand-based redox chemistry with functionally redox-inactive Ni centers enables oxidative transformations more commonly mediated by metals such as Fe and Cu.

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover.

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well-characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt2-) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt2-. UV-vis-near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt2- π-manifold from the Fe(II)-centered spin-crossover. The TTFtt2--centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.

Ligand-Based Storage of Protons and Electrons in Dihydrazonopyrrole Complexes of Nickel.

A newly developed dihydrazonopyrrole ligand and corresponding Ni complexes have been synthesized and thoroughly characterized. Electrochemical studies and chemical reactivity tests show that these complexes can reversibly store both electrons and protons, or equivalently H-atoms, via ligand-based events. The stored H-atom equivalent can be transferred to small molecules such as acetonitrile or oxygen. Furthermore, this series of complexes can adopt a variety of different coordination modes. In addition to one e- reactivity, the two e- electrophilic oxidation of phosphines is also demonstrated. Taken together, these results show that dihydrazonopyrrole complexes represent a geometrically and electronically flexible scaffold for controlling the flow of both electrons and protons.

Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium.

Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis( N-cyclohexyl-1-( p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,CyDIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)-Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligand-based radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.

Direct observation of ß-chloride elimination from an isolable ß-chloroalkyl complex of square-planar nickel.

Reported here are the isolation, structural characterization, and decomposition kinetics of the four-coordinate pentachloroethyl nickel complex, NiCl(CCl2CCl3)(CNAr(Mes2))2 (Ar(Mes2) = 2,6-(2,4,6-Me3C6H2)2C6H3). This complex is a unique example of a kinetically persistent ß-chloroalkyl in a system relevant to coordination-insertion polymerization of polar olefins. Kinetic analysis of NiCl(CCl2CCl3)(CNAr(Mes2))2 decomposition indicates that ß-chloride (ß-Cl) elimination proceeds by a unimolecular mechanism that does not require initial dissociation of a CNAr(Mes2) ligand. The results suggest that a direct ß-Cl elimination pathway is available to four-coordinate, Group 10 metal vinyl chloride polymerization systems.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity.

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J P-Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh2CH2BF3K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh4][SePPh2CH2BF3] and [TEA][SePPh2CH2BF3], and the Rh carbonyl complex [PPh4][Rh(acac)(CO)(PPh2(CH2BF3))]. Solvent-dependent changes in J P-Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh4][SePPh2CH2BF3] relative to SePPh2Et, while controls with [TEA][SePPh2CH2BF3] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J P-Se method was extended to [PPh4][SePPh2(2-BF3Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh3. The use of PPh2CH2BF3K also accelerates C-F oxidative addition reactivity with Ni(COD)2 by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt3 and PCy3. This enhanced reactivity prompted the investigation of catalytic fluoroarene C-F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

Hydrazine Energy Storage: Displacing N2 H4 from the Metal Coordination Sphere.

Hydrogen carriers, such as hydrazine (N2 H4 ), may facilitate long duration energy storage, a vital component for resilient grids by enabling more renewable energy generation. Lanthanide coordination chemistry with N2 H4 as well as efforts to displace N2 H4 from the metal coordination sphere to develop an efficient catalytic production cycle were detailed. Modeling the equilibrium of different ligand coordination, it was predicted that strong sigma donor molecules would be required to displace N2 H4 . Monitoring competition experiments with nuclear magnetic resonance confirmed that trimethyl phosphine oxide, dimethylformamide, and dimethyl sulfoxide displaced N2 H4 in large or small lanthanide complexes.

Catalytic hydrogenation enabled by ligand-based storage of hydrogen.

Biology employs exquisite control over proton, electron, H-atom, or H2 transfer. Similar control in synthetic systems has the potential to facilitate efficient and selective catalysis. Here we report a dihydrazonopyrrole Ni complex where an H2 equivalent can be stored on the ligand periphery without metal-based redox changes and can be leveraged for catalytic hydrogenations. Kinetic and computational analysis suggests ligand hydrogenation proceeds by H2 association followed by H-H scission. This complex is an unusual example where a synthetic system can mimic biology's ability to mediate H2 transfer via secondary coordination sphere-based processes.

Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds.

Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH4, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt4-. Capping of TTFtt4- with Bu2Sn2+ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1'-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical-radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.