Enhancement of London Dispersion in Frustrated Lewis Pairs: Towards a Crystalline Encounter Complex

The encounter complex, i.e., the pre-organized assembly consisting of a Lewis acid and a Lewis base, is a fundamental concept in frustrated Lewis pair (FLP) chemistry. However, this donor acceptor complex is challenging to study due to its transient nature. Here, we present a combined theoretical and experimental investigation on the potential isolation of an encounter complex enabled by enhancement of London dispersion forces between a sterically encumbered Lewis acid and base pair. Guided by computational analyses, the FLP originating from the bulky triarylamine N(3,5-tBu2C6H3)3 and the novel triarylborane B(3,5-tBu2C6H3)3 was investigated, leading to the isolation of at 1:1 co-crystal of both FLP components

Low-Valence Anionic α-Diimine Iron Complexes: Synthesis, Characterization, and Catalytic Hydroboration Studies

The synthesis of rare anionic heteroleptic and homoleptic alpha-diimine iron complexes is described. Heteroleptic BIAN (bis(aryl)iminoacenaphthene) complexes 1-[K([18]c-6)-(thf)(0.5)] and 2-[K([18]c-6)(thf)(2)] were synthesized by reduction of the [(BIAN)FeBr2] precursor complex using stoichiometric amounts of potassium graphite in the presence of the corresponding olefin. The electronic structure of these paramagnetic species was investigated by numerous spectroscopic analyses (NMR, EPR, Fe-57 Mossbauer, UV-vis), magnetic measurements (Evans NMR method, SQUID), and theoretical techniques (DFT, CASSCF). Whereas anion 1 is a low-spin complex, anion 2 consists of an intermediate-spin Fe(III) center. Both complexes are efficient precatalysts for the hydroboration of carbonyl compounds under mild reaction conditions. The reaction of bis(anthracene) ferrate(1-) gave the homoleptic BIAN complex 3-[K([18]c-6)(thf)], which is less catalytically active. The electronic structure was elucidated with the same techniques as described for complexes 1-[K([18]c-6)(thf)(0.5)] and 2-[K([18]c-6)(thf)(2)] and revealed an Fe(II) species in a quartet ground state

Revisiting the Electronic Structure of Cobalt-Porphyrin Nitrene and Carbene Radicals with NEVPT2-CASSCF Calculations: Doublet versus Quartet Ground States

Cobalt-porphyrin complexes are established catalysts for carbene and nitrene radical group transfer reactions. The key carbene, mono- and bis-nitrene radical complexes coordinated to [Co(TPP)] (TPP = tetraphenylporphyrin) have previously been investigat-ed with a variety of experimental techniques and supporting (single-reference) DFT calculations that indicated doublet (S = ½) ground states for all three species. In this contribution we revisit their electronic structures with multireference NEVPT2-CASSCF calculations to investigate possible multireference contributions to the ground state wavefunctions. The carbene ([CoIII(TPP)(•CHCO2Et)]) and mono-nitrene ([CoIII(TPP)(•NNs)]) radical complexes were confirmed to have uncomplicated doublet ground states, although a higher carbene or nitrene radical character and a lower Co‒C/N bond order was found in the NEVPT2-CASSCF calculations. Supported by EPR analysis and spin counting, paramagnetic molar susceptibility determination and NEVPT2-CASSCF calculations, we report that the cobalt-porphyrin bis-nitrene complex ([CoIII(TPP•)(•NNs)2]) has a quartet (S = 3/2) spin ground state, with a thermally assessable multireference & multideterminant ‘broken-symmetry’ doublet spin excited state. A spin flip on the porphyrin-centered unpaired electron allows for interconversion between the quartet and broken-symmetry doublet spin states, with an approximate 10- and 200-fold higher Boltzmann population of the quartet at room tempera-ture or 10 K, respectively.<br /

Electronically Asynchronous Transition States for C-N Bond Formation by Electrophilic [Coᴵᴵᴵ(TAML)]-Nitrene Radical Complexes Involving Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered Spin Shuttle

The oxidation state of the redox non-innocent TAML (Tetra-Amido Macrocyclic Ligand) scaffold was recently shown to affect the formation of nitrene radical species on cobalt(III) upon reaction with PhI=NNs [J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b11715]. For the neutral [CoIII(TAMLsq)] complex this leads to the doublet (S = ½) mono-nitrene radical species [CoIII(TAMLq)(N•Ns)], while a triplet (S = 1) bis-nitrene radical species [CoIII(TAMLq)(N•Ns)2]‒ is generated from the anionic [CoIII(TAMLred)]‒ complex. The one-electron reduced Fischer-type nitrene radicals (N•Ns‒) are formed through single (mono-nitrene) or double (bis-nitrene) ligand-to-substrate single-electron transfer (SET). In this work we describe the reactivity and mechanisms of these nitrene radical complexes in catalytic aziridination. We report that [CoIII(TAMLsq)] and [CoIII(TAMLred)]‒ are both effective catalysts for chemoselective (C=C versus C‒H bonds) and diastereoselective aziridination of styrene derivatives, cyclohexene and 1-hexene under mild and even aerobic (for [CoIII(TAMLred)]‒) conditions. Experimental (Hammett plots, radical inhibition, catalyst decomposition tests) and computational (DFT, CASSCF) studies reveal that [CoIII(TAMLq)(N•Ns)], [CoIII(TAMLq)(N•Ns)2]‒ and [CoIII(TAMLsq)(N•Ns)]– are key electrophilic intermediates in the aziridination reactions. Surprisingly, the electrophilic one-electron reduced Fischer-type nitrene radicals do not react as would be expected for nitrene radicals (i.e. via radical addition and radical rebound). Instead, nitrene transfer proceeds through unusual electronically asynchronous transition states, in which (partial) styrene substrate to TAML ligand (single) electron transfer precedes C-N coupling. The actual C-N bond formation processes are best described as involving a nucleophilic attack of the nitrene (radical) lone pair at the thus (partially) formed styrene radical cation. These processes are coupled to TAML-to-cobalt and cobalt-to-nitrene single-electron transfer, effectively leading to formation of an amido-[gamma]-benzyl radical (Ns–N–CH2–•CH–Ph) bound to an intermediate spin (S = 1) cobalt(III) center. Hence, the TAML moiety can be regarded to act as a transient electron acceptor, the cobalt center behaves as a spin shuttle and the nitrene radical acts as a nucleophile. Such a mechanism for (cobalt catalyzed) nitrene transfer was hitherto unknown and complements the known concerted and stepwise mechanisms for N-group transfer

Direct synthesis of an anionic 13-vertex closo-cobaltacarborane cluster

Reaction of 1,2-bis(diphenylphosphino)-ortho-carborane (L) with [K(thf){((Mes)BIAN)Co(eta(4)-cod)}] (1, (Mes)BIAN = bis(mesityliminoace-naphthene)diimine, cod = 1,5-cyclooctadiene) affords an anionic 13-vertex closo-cobaltacarborane cluster (2) in one step. The mechanism of this transformation has been studied by experimental and quantum chemical techniques, which suggest that a series of outer-sphere electron transfer and isomerisation processes occurs. This work shows that low-valent metalate anions are promising reagents for the synthesis of anionic metallacarborane clusters

Ligand-Mediated Spin State Changes in a Cobalt-Dipyrrin-Bisphenol Complex

ABSTRACT: The influence of a redox-active ligand on spin changing events induced by coordination of exogenous donors is investigated within the cobalt complex [CoII(DPP•2‒)], bearing a redox-active DPP2‒ ligand (DPP = dipyrrin-bis-(o,p-di-tert-butylphenolato) with a pentafluorophenyl moiety on the meso-position. This square planar complex was subjected to coordination of THF, pyridine, tBuNH2 and AdNH2 (Ad = 1‑adamantyl), and the resulting complexes were analyzed with a variety of experimental (XRD, NMR, UV-Vis, HRMS, SQUID, Evans’ method) and computational (DFT, NEVPT2-CASSCF) techniques to elucidate the respective structures, spin states and orbital compositions of the corresponding octahedral bis-donor adducts, relative to [CoII(DPP•2‒)]. This starting species is best described as an open-shell singlet complex containing a DPP•2‒ ligand radical that is antiferromagnetically coupled to a low-spin (S = ½) cobalt(II) center. The redox-active DPPn‒ ligand plays a crucial role in stabilizing this complex, and in its facile conversion to the triplet THF-adduct [CoII(DPP•2‒)(THF)2] and closed-shell singlet pyridine and amine adducts [CoIII(DPP3‒)(L)2] (L = py, tBuNH2 or AdNH2). Coordination of the weak donor THF to [CoII(DPP•2-)] changes the orbital overlap between the DPP•2‒ ligand radical π-orbitals and the cobalt(II) metalloradical d-orbitals, which results in a spin-flip to the triplet ground state without changing the oxidation states of the metal or DPP•2‒ ligand. In contrast, coordination of the stronger donors pyridine, tBuNH2 or AdNH2 induces metal-to-ligand single-electron transfer, resulting in formation of low-spin (S = 0) cobalt(III)-complexes [CoIII(DPP3‒)(L)2] containing a fully reduced DPP3‒ ligand, thus explaining their closed-shell singlet electronic ground states

A low-valent dinuclear ruthenium diazadiene complex catalyzes the oxidation of dihydrogen and reversible hydrogenation of quinones

The dinuclear ruthenium complex [Ru2H(mu-H)(Me(2)dad)(dbcot)(2)] contains a 1,4-dimethyl-diazabuta-1,3-diene (Me(2)dad) as a non-innocent bridging ligand between the metal centers to give a [Ru-2(Me(2)dad)] core. In addition, each ruthenium is bound to one dibenzo[a,e]cyclooctatetraene (dbcot) ligand. This Ru dimer converts H-2 to protons and electrons. It also catalyzes reversibly under mild conditions the selective hydrogenation of vitamins K-2 and K-3 to their corresponding hydroquinone equivalents without affecting the C=C double bonds. Mechanistic studies suggest that the [Ru-2(Me(2)dad)] moiety, like hydrogenases, reacts with H-2 and releases electrons and protons stepwise.Swiss National Science Foundation; ETH Zurich; China Scholarship Council; ETH Zurich Postdoctoral Fellowship Program - ETH Zurich-Marie Curie action for people [FEL-14 15-1]Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Combining Metal-Metal Cooperativity, Metal-Ligand Cooperativity and Chemical Non-Innocence in Diiron Carbonyl Complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand’s naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)–H bonds by ~25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = 1/2) complex containing a mixed-valent iron(0)-iron(I) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity

Convenient Transfer Semihydrogenation Methodology for Alkynes Using a Pd^II-NHC Precatalyst

A convenient and easy-to-use protocol for the <i>Z</i>-selective transfer semihydrogenation of alkynes was developed, using ammonium formate as the hydrogen source and the easily prepared and commercially available, highly stable complex PdCl­(η³-C₃H₅)­(IMes) (<b>1</b>) as the (pre)­catalyst. Combined with triphenyl posphine as an additional ligand, this system provides a robust catalytic synthetic method that shows little to no over-reduction or isomerization after full substrate conversion. The system allows the direct use of solvents and reagents, as received from the supplier without drying or purification, thus providing a practical method for semihydrogenation of a broad range of alkynes. The mechanism behind these high and enhanced selectivities was determined through a set of kinetic experiments

Styrene Aziridination with [Co^III(TAML^red)]– in Water; Understanding and Preventing Epoxidation via Nitrene Hydrolysis

Enabling (radical-type) nitrene transfer reactions in water can open up a wide range of (novel) applications, such as the in vivo synthesis of medicines. However, these reactions typically suffer from oxygen-containing side-product formation, of which the origin is not fully understood. Therefore, we investigated aqueous styrene aziridination using a water-soluble [CoIII(TAMLred)]– catalyst known to be active in radical-type nitrene transfer in organic solvents. The cobalt-catalyzed aziridination of styrene in water (pH = 7) yielded styrene oxide as the major product, next to minor amounts of aziridine product. Based on 18O-labeling studies, catalysis and mass spectrometry experiments, we demonstrated that styrene oxide formation proceeds via hydrolysis of the formed nitrene radical complexes. Computational studies support that this process is facile and yields oxyl radical complexes active in oxygen atom transfer to styrene. Based on these mechanistic insights, the pH was adjusted to afford selective aziridination in water