Coordination Of Lewis Acid Research Articles

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 BCy 4 )2 ] (P2 BCy 4 =1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [NiII (P2 BCy 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 BCy 4 )2 ], a "boron-trapped" 16-electron κ1 -diphosphine Ni(0) complex. Moreover, formation of [NiII (P2 BCy 4 )(Ph)(I)] is inherent to the P2 BCy 4 secondary coordination sphere: treatment of the Lewis adduct, [Ni0 (P2 BCy 4 )2 (DMAP)8 ] with PhI provides [NiII (P2 BCy 4 )2 (DMAP)8 (I)]I via iodine-atom abstraction and not a [NiII (Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni0 (P2 BCy 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.

Meso-Oxoisocorroles: Tunable Antiaromaticity by Metalation and Coordination of Lewis Acids as Well as Aromaticity Reversal in the Triplet Excited State.

The corrole derivative meso-oxoisocorrole has been theoretically predicted to be antiaromatic, despite its formally cross conjugated electronic system. In this study, this prediction has been experimentally proven by the facile preparation of meso-oxoisocorrole via the oxidation of a meso free corrole with MnO2 and its comprehensive characterization using NMR, UV/vis absorption, FT-IR, and transient-absorption spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Furthermore, the free base meso-oxoisocorrole was metalated by treatment with Ni(acac)2, PdCl2(PhCN)2, and Zn(OAc)2 to give the corresponding metal complexes. These complexes are more strongly antiaromatic, and their degree of paratropicity depends on their planarity. Thus, fine tuning of their antiaromaticity was achieved with concomitant modulation of their HOMO-LUMO gaps. In the presence of tris(pentafluorophenyl)borane, their antiaromaticity is significantly enhanced due to the elongation of the C═O bond, which promotes the polarized C+-O- resonance state. Furthermore, a distinct frequency shift of the C═O vibrational mode in the triplet state was observed in the time-resolved IR spectra in accordance with the Baird rule, which indicates aromaticity reversal in the excited state.

Unveiling KuQuinone Redox Species: An Electrochemical and Computational Cross Study.

The study of the electrochemical properties of variegated quinones is a fascinating topic in chemistry. In fact, redox reactions occurring with quinoid scaffolds are essential for most of their applications in biological systems, in photoelectrochemical devices, and in many other fields. In this paper, a detailed investigation of KuQuinones’ redox behavior is presented. The distinctiveness of such molecules is the presence in the structure of two condensed naphthoquinone units, which implies the possibility to undergo multiple one-electron reduction processes. Solvent, supporting electrolyte, and hydrogen bond donor species effects have been elucidated. Changing the experimental parameters provoked significant shift of the redox potential for each reduction process. In particular, additions of 2,2,2-trifluoroethanol as a hydrogen bond donor in solution as well as Lewis acid coordination were crucial to obtain important shifts of the redox potentials toward more favorable values. UV–vis–NIR spectroelectrochemical experiments and DFT calculations are also presented to clarify the nature of the reduced species in solution.

Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids.

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙-), and doubly reduced benzoquinone ([Q]2-) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol-1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid-Q/[Q]˙-/[Q]2- adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol-1, and the capacity to transfer a hydride by about 30 kcal mol-1.

Perturbation of 1JC,F Coupling in Carbon-Fluorine Bonds on Coordination to Lewis Acids: A Structural, Spectroscopic, and Computational Study.

A lithiated m-terphenyl ligand bearing fluorine atoms at the ortho positions of the flanking aryl rings was synthesized and characterized using single crystal X-ray diffraction, variable-temperature multinuclear NMR spectroscopy, and computational methods. Changes in 1JC,F on coordination to lithium as a spectroscopic observable parametrizing the strength of the C-F···Li interaction are described, and a general, qualitative relationship between C-F bond lengths, Δ1JC,F values, and the extent of C-F bond activation as a result of Lewis acid coordination is proposed.

Single-Electron Transfer in Frustrated Lewis Pair Chemistry.

Frustrated Lewis pairs (FLPs) are well known for their ability to activate small molecules. Recent reports of radical formation within such systems indicate single‐electron transfer (SET) could play an important role in their chemistry. Herein, we investigate radical formation upon reacting FLP systems with dihydrogen, triphenyltin hydride, or tetrachloro‐1,4‐benzoquinone (TCQ) both experimentally and computationally to determine the nature of the single‐electron transfer (SET) events; that is, being direct SET to B(C6F5)3 or not. The reactions of H2 and Ph3SnH with archetypal P/B FLP systems do not proceed via a radical mechanism. In contrast, reaction with TCQ proceeds via SET, which is only feasible by Lewis acid coordination to the substrate. Furthermore, SET from the Lewis base to the Lewis acid–substrate adduct may be prevalent in other reported examples of radical FLP chemistry, which provides important design principles for radical main‐group chemistry.

Single‐Electron Transfer in Frustrated Lewis Pair Chemistry

AbstractFrustrated Lewis pairs (FLPs) are well known for their ability to activate small molecules. Recent reports of radical formation within such systems indicate single‐electron transfer (SET) could play an important role in their chemistry. Herein, we investigate radical formation upon reacting FLP systems with dihydrogen, triphenyltin hydride, or tetrachloro‐1,4‐benzoquinone (TCQ) both experimentally and computationally to determine the nature of the single‐electron transfer (SET) events; that is, being direct SET to B(C6F5)3 or not. The reactions of H2 and Ph3SnH with archetypal P/B FLP systems do not proceed via a radical mechanism. In contrast, reaction with TCQ proceeds via SET, which is only feasible by Lewis acid coordination to the substrate. Furthermore, SET from the Lewis base to the Lewis acid–substrate adduct may be prevalent in other reported examples of radical FLP chemistry, which provides important design principles for radical main‐group chemistry.

Lewis Base/Acid Coordination Stabilizes Planar 6π Aromatic Si6/P6

We have developed a strategy for designing planar all-Si and all-P 6π aromatics based on experimentally available building blocks. A Si6 hexagon coordinated by three phosphines at the 1,3,5-positions, Si6(PH3)3, which has three lone-pair-carrying Si atoms is nearly planar and aromatic. Replacing PH3 by NH3 induces perfect planarity. Two-electron reduction of Si6(PH3)3 results in doubly aromatic Si62– with the dissociation of Si–P bonds, instead of a nonaromatic system such as the benzene dianion. Similarly to base-coordinated Si6, Lewis acid coordination to P6 can give planar 6π aromatics. P6(BH3)3 is a nearly planar minimum. Replacing BH3 with a stronger Lewis acid, B(CN)3, gives a perfectly planar structure. A PH3-coordinated all-Si diene resembling butadiene is predicted. A concerted 4 + 2 cycloaddition between this all-Si diene and ethylene is proposed here and has a computed barrier of only 22 kcal/mol.

Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs.

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO2) is of interest to the chemical industry, as many synthetic materials can be derived from CO2. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO2 to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO2 into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO2 insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO2 adduct of tBu3P and B(C6F5)3 resulted in converting the sequestered CO2 molecule to CO. Reduction of the CO2 adduct of tBu3P and B(C6F5)3 with both electrons and protons resulted in only proton reduction.

Remote coordination approach for electronic tuning of a rhodium(I)-N-heterocyclic carbene (NHC)-complex

A simple Lewis acid coordination at the remote binding site installed within a modular N-heterocyclic carbene (NHC) ligated rhodium(I) complex could induce desired electronic influence and thereby enhance the activity of the complex in a model styrene hydroboration reaction.

Reversal of reaction type selectivity by Lewis acid coordination: the ortho photocycloaddition of 1- and 2-naphthaldehyde.

The value of a specific substrate class for synthetic applications is greatly enhanced if different types of reactions can be performed selectively upon a judicious choice of reaction conditions. In the present study it was shown that the typical photochemical behaviour of naphthaldehydes is completely altered in the presence of Lewis acids. Without Lewis acids, reactions at the carbonyl group are exclusively observed while Lewis acids facilitate a visible light-mediated cycloaddition at the arene core providing access to products of aromatic C-H functionalization via cyclobutane intermediates.

Stable Sn/Pb-Based Perovskite Solar Cells with a Coherent 2D/3D Interface

SummaryLow-band-gap metal halide perovskite semiconductor based on mixed Sn/Pb is a key component to realize high-efficiency tandem perovskite solar cells. However, the mixed perovskites are unstable in air due to the oxidation of Sn2+. To overcome the stability problem, we introduced N-(3-aminopropyl)-2-pyrrolidinone into the CH3NH3Sn0.5Pb0.5IxCl3-x thin film. The carbonyl group on the molecule interacts with Sn2+/Pb2+ by Lewis acid coordination, forming vertically oriented 2D layered perovskite. The 2D phase is seamlessly connected to the bulk perovskite crystal, with a lattice coherently extending across the two phases. Based on this 2D/3D hybrid structure, we assembled low-band-gap Sn-based perovskite solar cells with power conversion efficiency greater than 12%. The best device was among the most stable Sn-based organic-inorganic hybrid perovskite solar cells to date, keeping 90% of its initial performance at ambient condition without encapsulation, and more than 70% under continuous illumination in an N2-filled glovebox for over 1 month.

Enantioselective Lewis Acid Catalyzed ortho Photocycloaddition of Olefins to Phenanthrene-9-carboxaldehydes.

Visible‐light irradiation (λ=457 nm) enabled the enantioselective ortho photocycloaddition of olefins to phenanthrene‐9‐carboxaldehydes (15 examples, 46–93 % yield, 82–98 % ee). A chiral oxazaborolidine Lewis acid (20 mol %) was employed as the catalyst. It operates by coordination to the aldehyde inducing a bathochromic absorption shift beyond the nπ* absorption of the uncomplexed aldehyde. At long wavelengths the Lewis acid complex is exclusively excited; within the complex, one enantiotopic face of the aromatic aldehyde is efficiently shielded. Lewis acid coordination also alters the type selectivity and the simple diastereoselectivity of the photocycloaddition.

Playing with Pearson's concept: orthogonally functionalized 1,4-diaza-1,3-butadienes leading to heterobinuclear complexes.

By reacting 1,2-diketones and ortho- diphenylphosphinoyl aniline in the presence of zinc(ii) as a templating agent, cationic zinc(ii) complexes of novel phosphine oxide functionalized 1,4-diaza-1,3-butadiene ligands are acessible. Herein the zinc(ii) site is bound to all four donor atoms of the ligand. Depending on the flexibility of the 1,4-diaza-1,3-butadiene backbone, the bonds to zinc(ii) from the 1,4-diaza-1,3-butadiene donors can be broken. Reaction with oxalate cleaves the zinc(ii) coordination completely and makes accessible the free ligands possessing orthogonal (N,N: soft; O,O: hard) sets of donor sites. This allows for the specific coordination of soft and hard Lewis acids and thus for the generation of heterobimetallic complexes, here exemplarily shown for the combination of palladium(ii) (soft) and zinc(ii) (hard) centres.

Cascade covalent and coordination bond formation for Ti-based cage assembly: catalysis and coordination bifunctionality of TiCl4.

The metallosupramolecular Ti-based M4L4 cage of [Ti4O2L4Cl8] (L = 4-acetamidobenzoic acid) is synthesized via the one-pot solvothermal reaction of 4-aminobenzoic acid, acetic acid and TiCl4. The Ti(iv) ion functions as a Lewis acid catalyst for acylation condensation, and then coordinates with the resultant 4-acetamidobenzoic acid to afford Ti-based cage formation in a cascade fashion.

Dramatic Increase in the Rate of Olefin Insertion by Coordination of Lewis Acids to the Oxo Ligand in Oxorhenium(V) Hydrides

In this work we show that classic coordination of the oxo group in an oxorhenium hydride complex to M(C6F5)3 (M = Al, B) leads to dramatic increases in the rate of migratory olefin insertion. Combined experimental and computational studies have been utilized to understand the reasons for the rate enhancement upon coordination of the oxo group to the Lewis acid. The mechanism for migratory insertion involves coordination of the olefin to rhenium in the equatorial plane. This induces mixing of the rhenium–hydride σ bond with a rhenium–oxygen π* orbital. This results in an accumulation of electron density on the oxo ligand. The Lewis acid lowers the barrier for migratory insertion by diminishing the electron density on the oxo ligand in the transition state.

Enantioselective photochemistry through Lewis acid-catalyzed triplet energy transfer.

Relatively few catalytic systems are able to control the stereochemistry of electronically excited organic intermediates. Here we report the discovery that a chiral Lewis acid complex can catalyze triplet energy transfer from an electronically excited photosensitizer. We applied this strategy to asymmetric [2 + 2] photocycloadditions of 2'-hydroxychalcones, using tris(bipyridyl) ruthenium(II) as a sensitizer. A variety of electrochemical, computational, and spectroscopic data rule out substrate activation by means of photoinduced electron transfer and instead support a mechanism in which Lewis acid coordination dramatically lowers the triplet energy of the chalcone substrate. We expect that this approach will enable chemists to more broadly apply their detailed understanding of chiral Lewis acid catalysis to stereocontrol in reactions involving electronically excited states.

Effect of Lewis acids on the stereoregularity of N,N-dimethyl acrylamide: A computational approach

In this study, the effect of Lewis acid coordination (ScCl3) in controlling the stereoregularity during the free radical polymerization of N,N-dimethyl acrylamide (DMAM) has been investigated by Density Functional Theory (DFT). Experimentally, ScCl3, Sc(OTf)3 and Yb(OTf)3 have been used to increase the isotactic percentage in the polymerization of another acrylamide derivative, N-isopropyl acrylamide (NIPAM) (Habaue et al., 2002). The relative orientation of the terminal and penultimate side chains is expected to determine the stereoregularity in free radical polymerization reactions (Noble et al., 2014). We have analyzed the mechanistic details of the propagation reaction by considering all coordination types of the Lewis acid to the propagating species. Calculations have shown the bridging of the Lewis acid between the terminal side chain and the monomer to be the most probable pathway, which is in favor of the pro-meso propagation during the free radical polymerization of DMAM. In this case, it is the bridging capacity of the catalyst along the less crowded direction that dictates the preference for isotacticity. Overall, the strategy suggested in this study can be easily used by experimentalists in their endeavour of choosing the catalysts in order to end-up with the desired stereoregulation of the polymer chain.

EPR Study of Substrate Binding to Mn(II) in Hydroxynitrile Lyase from Granulicella tundricola

GtHNL from Granulicella tundricola is a Mn(II) containing hydroxynitrile lyase with a cupin fold. The quasi-octahedral manganese is pentacoordinated by the enzyme. It catalyzes the enantioselective addition of HCN to aldehydes, yielding R-cyanohydrins. On the Lewis acidic vacant coordination site the Mn binds either substrate or the product, leading to a hexacoordinated 17 electron complex. EPR spectra of the active enzyme are unusually wide with a zero-field splitting approximately equal to the X-band microwave energy. A spectral change is induced by incubation with either one of the substrates/products HCN, benzaldehyde, and/or mandelonitrile. This points toward Mn(II) catalyzed cyanohydrin synthesis.