Hemilability Research Articles

Palladium(II) Complexes with N-Phosphine Oxide-Substituted Imidazolylidenes (PoxIms): Coordination Chemistry and Catalysis

The coordination chemistry of N-phosphine oxide-substituted imidazolylidenes (PoxIms) as heteroditopic, hemilabile ligands toward palladium(II) has been investigated. Both bis-carbene and mono-carbene complexes can be prepared, but the latter are accessible only in the presence of ancillary ligands that do not easily dissociate from the metal center. The phosphanyl oxide group was found to often coordinate to palladium(II) as well, though this interaction appears to be weak as this group is indeed easily replaced by, e.g., a solvent molecule. The resulting complexes have been tested as catalysts in the low-temperature Suzuki reaction of 4-chloroanisole with phenylboronic acid and they show promise for further optimization studies.

Catalytic Asymmetric C-H Arylation of (η6-Arene)Chromium Complexes: Facile Access to Planar-Chiral Phosphines.

A catalytic asymmetric direct C–H arylation of (η6-arene)chromium complexes to obtain planar-chiral compounds is reported. The use of the hemilabile ligand H8-BINAP(O) is key to providing high enantioselectivity in this transformation. We show that this methodology opens the door to the synthesis of a variety of planar-chiral chromium derivatives which can be easily transformed into planar chiral mono- or diphosphines. Mechanistic studies, including synthesis and characterization of Pd and Ag complexes and their detection in the reaction mixture, suggest a Pd-catalyzed/Ag-promoted catalytic system where Ag carries out the C–H activation step.

Utilization of BozPhos as an Effective Ligand in Enantioselective C-H Functionalization of Cyclopropanes: Synthesis of Dihydroisoquinolones and Dihydroquinolones.

The bisphosphine monoxide ( R, R)-BozPhos enables enantioselective C-H functionalization of cyclopropanes in a palladium-catalyzed cyclization. The synthesis of a broad spectrum of dihydroisoquinolones and dihydroquinolones in good yields and high enantiomeric excess was achieved through the use of this hemilabile ligand. Furthermore, the isolation of an intermediary palladium(II)-BozPhos complex after oxidative addition was successful and a second complex provided further insight into bond length and angles through a crystal structure.

Synthesis and coordination properties of phospholanopyridinium hydrochlorides

Pyridyl containing phospholanes became promising templates for construction of novel hemilabile ligands. Their N–H derivatives, namely phospholanopyridinium hydrochlorides, were firstly synthesized and utilized as P-monodentate ligands towards copper(I) iodide.GRAPHICAL ABSTRACT

Molybdenum(vi) complexes of hemilabile aroylhydrazone ligands as efficient catalysts for greener cyclooctene epoxidation: an experimental and theoretical approach

The focus of this work was on the synthesis, characterization and catalytic activity of Mo(vi) complexes with hemilabile 2-aminobenzhydrazone ligands.

Catalytic Au(i)/Au(iii) arylation with the hemilabile MeDalphos ligand: unusual selectivity for electron-rich iodoarenes and efficient application to indoles.

The ability of the hemilabile (P,N) MeDalphos ligand to trigger oxidative addition of iodoarenes to gold has been thoroughly studied. Competition experiments and Hammett correlations substantiate a clear preference of gold for electron-enriched substrates both in stoichiometric oxidative addition reactions and in catalytic C-C cross-coupling with 1,3,5-trimethoxybenzene. This feature markedly contrasts with the higher reactivity of electron-deprived substrates typically encountered with palladium. Based on DFT calculations and detailed analysis of the key transition states (using NBO, CDA and ETS-NOCV methods in particular), the different behavior of the two metals is proposed to result from inverse electron flow between the substrate and metal. Indeed, oxidative addition of iodobenzene is associated with a charge transfer from the substrate to the metal at the transition state for gold, but opposite for palladium. The higher electrophilicity of the gold center favors electron-rich substrates while important back-donation from palladium favors electron-poor substrates. Facile oxidative addition of iodoarenes combined with the propensity of gold(iii) complexes to readily react with electron-rich (hetero)arenes prompted us to apply the (MeDalphos)AuCl complex in the catalytic arylation of indoles, a challenging but very important transformation. The gold complex proved to be very efficient, general and robust. It displays complete regioselectivity for C3 arylation, it tolerates a variety of functional groups at both the iodoarene and indole partners (NO2, CO2Me, Br, OTf, Bpin, OMe…) and it proceeds under mild conditions (75 °C, 2 h).

Benefit of a hemilabile ligand in deoxygenation of fatty acids to 1-alkenes.

One of the most important tasks for chemistry in our time is to contribute to sustainable chemical production. A green industrial process for linear α-olefins, the arguably most important class of petrochemical intermediates, from renewable resources would be a major contribution to this end. Plant oils are attractive renewable feedstocks for this purpose because their triglycerides can be hydrolyzed to fatty acids that contain valuable long-chain hydrocarbons (C16-C22). These hydrocarbons may, in turn, be converted to α-olefins by the deoxygenation of the fatty acids. For the most selective of these deoxygenation reactions, transition-metal catalyzed decarbonylative dehydration, the density functional theory (DFT) calculations have just started to offer valuable mechanistic insight, and the use of this insight in rational catalyst design has been facilitated by the arrival of the first well-defined precatalyst for this reaction, Pd(cinnamyl)Cl(DPEphos) (1). Here, we present DFT calculations showing how, in 1, the hemilability of DPEphos, a classical P-O-P diphosphine, contributes to a low overall barrier and high α-selectivity. DPEphos facilitates decarbonylation by first switching from bidentate to monodentate binding to create a coordination site for CO. The recoordination of the dangling phosphine displaces the Pd-bound CO, a co-product that must leave the reactor for the reaction to proceed, and the escaping CO is here modelled using a low pressure in the calculation of its thermochemical corrections. Finally, the role of the hemilabile ligand suggests that further improvements in the decarbonylative dehydration of fatty acids to α-olefins might be achieved by exploring new, potentially asymmetric, hemilabile ligands.

Synthesis of palladium tellurolate complexes derived from hemi-labile tellurolate ligands and studies their reactivity as gas sensing materials

The reaction of [PdCl2(dppf)] with the sodium salt of pyridyl/pyrimidyl tellurolate resulted a mononuclear cis configured complex [Pd(TeAr)2(dppf)] (Ar = C5H4N, C4H3N2). These complexes were characterized by NMR (1H, 31P) spectroscopy and elemental analysis. In addition, the thin film of this complex was fabricated on the glass using solution process and it was found to be sensitive towards detection of H2S gas. The chemi-resistive response of the film was ascribed to the fact that an auxiliary tellurolate ligand attached to palladium metal centre substituted reversibly by thiolate (HS−) species of hydrogen sulphide gas.

New arylselanylpyrazole-copper catalysts: Highly efficient catalytic system for C[sbnd]Se and C[sbnd]S coupling reactions

We describe herein the use of arylselanylpyrazole–copper complexes as versatile catalysts for CSe and CS coupling reactions. The performance of these complexes for CSe reactions was investigated in chalcogenoacetylene synthesis. The reactions were carried out under mild and aerobic conditions and afforded selanylalkynes bearing a variety of electron-withdrawing and electron-donating groups. The performance of these catalysts for CS coupling was investigated through the reaction of aryl halides with thiols and products were obtained in moderate to excellent yields. A plausible mechanism for selenoacetylene synthesis is also suggested, and the 77Se NMR results show that these arylselanylpyrazole ligands act as hemilabile ligands. High-resolution mass spectrometry was used to investigate the intermediates and also to corroborate the proposed catalytic cycle.

Hydrogenation of N-benzylideneaniline by palladium (II) catalysts with phosphorus-nitrogen ligands using formic acid as a renewable hydrogen source

This paper reports the hydrogenation of N-benzylideneaniline catalysed by Pd(II) compounds containing heterobidentated phosphorus-nitrogen (PN) ligands. The general formulas of the catalysts are cis-[PdCl2(L1)] and [PdCl(PPh3)(L1)]Cl (where L1= Ph2PCH2py, Ph2PNCH3py, Ph2PNHpy, Ph2PNHpym and Ph2Pqn). The hydrogenation of N-benzylideneaniline was carried out using propan-2-ol or formic acid as the hydrogen source. The catalytic activity was studied under different conditions by varying the base (NaOH and tert-butoxide) and the base/catalyst ratio (B/C = 20/1 and 50/1.Additionally, triethylamine was also used with a base/catalyst ratio of B/C = 500/1. It was found that palladium catalysts containing hemilabile heterobidentated ligands showed high catalytic activity and selectivity for the hydrogenation of N-benzylideneaniline. The catalysts cis-[PdCl2(Ph2PNHPy)] (2) and [PdCl(Ph2Pqn)(PPh3)]Cl (6) maintained the activities when the reactor was recharged with a fresh substrate (1000/1 substrate/catalyst ratio). It was also found that catalytic hydrogenation works well with only formic acid, with its activities ranging close to 100% for the 2 and 6 complexes.

A Redox-Switchable, Allosteric Coordination Complex.

A redox-regulated molecular tweezer complex was synthesized via the weak-link approach. The PtII complex features a redox-switchable hemilabile ligand (RHL) functionalized with a ferrocenyl moiety, whose oxidation state modulates the opening of a specific coordination site. Allosteric regulation by redox agents gives reversible access to two distinct structural states-a fully closed state and a semi-open state-whose interconversion was studied via multinuclear NMR spectroscopy, cyclic voltammetry, and UV-vis-NIR spectroscopy. Two structures in this four-state system were further characterized via SCXRD, while the others were modeled through DFT calculations. This fully reversible, RHL-based system defines an unusual level of electrochemical control over the occupancy of a specific coordination site, thereby providing access to four distinct coordination states within a single system, each defined and differentiated by structure and oxidation state.

Phosphorus Coordination Chemistry in Catalysis: Air Stable P(III)-Dications as Lewis Acid Catalysts for the Allylation of C–F Bonds

Modification of C–F bonds with main-group catalysts has typically employed electron-deficient Lewis superacids in high oxidation states, and the challenges of preparing and handling such species have prevented broader adoption of metal-free reduction protocols. Here, we show that a hemilabile ligand coordinated to an easily accessed P(III) center imparts air stability without sacrificing the ability to activate C–F bonds. Catalytic C–C coupling of benzyl fluorides with allylsilanes was achieved using a P(III) complex under benchtop conditions. This application of coordination chemistry principles to main-group Lewis acids reveals a new strategy for controlling catalysis.

Solvatochromism, thermochromism and density functional theory studies of Copper(II) complexes containing hemilabile tetradentate ligand

Two complexes of [Cu(L)H2O](ClO4)2,1 and [Cu(L)H2O](NO3)2, 2 containing hemilabile tetradentate ligand of 3,3'-(piperazine-1,4-diyl)dipropanamide, L, were synthesized and characterized by elemental analyses, spectroscopic (IR, UV–Vis, X-ray) data, molar conductance measurements and X-ray crystallography. The solvatochromic behavior of complex 1 was examined in the selected solvents (nitromethane, acetonitrile, propionitrile, acetone, tetrahydrofuran, ethanol, methanol, dimethylformamide, dimethylsulfoxide, and hexamethylphosphorictriamide), observing positive solvatochromism. The observed solvatochromism was analyzed by a statistical method. The obtained results confirm that the donor number (DN) parameter has the dominant contribution in the observed solvatochromism. For cationic complex 2 density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations was carried out to explore the mechanism of the solvatochromic behavior of the complex and satisfactory theoretical–experimental agreement was achieved. Additional information about the relative stability of CuN(amide) over CuO(amide) isomers was obtained by DFT analysis. The complexes were also observed to exhibit thermochromism in the solvent of dimethyl sulfoxide.

An Allosterically Regulated, Four-State Macrocycle.

Macrocycles capable of host-guest chemistry are an important class of structures that have attracted considerable attention because of their utility in chemical separations, analyte sensing, signal amplification, and drug delivery. The deliberate design and synthesis of such structures are rate-limiting steps in utilizing them for such applications, and coordination-driven supramolecular chemistry has emerged as a promising tool for rapidly making large classes of such systems with attractive molecular recognition capabilities and, in certain cases, catalytic properties. A particularly promising subset of such systems are stimuli-responsive constructs made from hemilabile ligands via the weak-link approach (WLA) to supramolecular coordination chemistry. Such structures can be reversibly toggled between different shapes, sizes, and charges based upon small-molecule and elemental-anion chemical effectors. In doing so, one can deliberately change their recognition properties and both stoichiometric and catalytic chemistries, thereby providing mimics of allosteric enzymes. The vast majority of structures made to date involve two-state systems, with a select few being able to access three different states. Herein, we describe the synthesis of a new allosterically regulated four-state macrocycle assembled via the WLA. The target structure was made via the stepwise assembly of ditopic bidentate hemilabile N-heterocyclic carbene thioether (NHC,S) and phosphino thioether (P,S) ligands at PtII metal nodes. The relatively simple macrocycle displays complex dynamic behavior when addressed with small-molecule effectors, and structural switching can be achieved with several distinct molecular cues. Importantly, each state was fully characterized by multinuclear NMR spectroscopy and, in some cases, single-crystal X-ray diffraction studies and density functional theory computational models. This new structure opens the door to complex multicue switching reminiscent of multistate chemoswitches that could be important in controlling stoichiometric and catalytic transformations as well as generating molecular logic systems.

Cooperative catalytic methoxycarbonylation of alkenes: uncovering the role of palladium complexes with hemilabile ligands.

Mechanistic studies of the catalyst [Pd2(dba)3/1,1'-bis(tert-butyl(pyridin-2-yl)phosphanyl)ferrocene, L2] for olefin alkoxycarbonylation reactions are described. X-ray crystallography reveals the coordination of the pyridyl nitrogen atom in L2 to the palladium center of the catalytic intermediates. DFT calculations on the elementary steps of the industrially relevant carbonylation of ethylene (the Lucite α-process) indicate that the protonated pyridyl moiety is formed immediately, which facilitates the formation of the active palladium hydride complex. The insertion of ethylene and CO into this intermediate leads to the corresponding palladium acyl species, which is kinetically reversible. Notably, this key species is stabilized by the hemilabile coordination of the pyridyl nitrogen atom in L2. The rate-determining alcoholysis of the acyl palladium complex is substantially facilitated by metal-ligand cooperation. Specifically, the deprotonation of the alcohol by the built-in base of the ligand allows a facile intramolecular nucleophilic attack on the acyl palladium species concertedly. Kinetic measurements support this mechanistic proposal and show that the rate of the carbonylation step is zero-order dependent on ethylene and CO. Comparing CH3OD and CH3OH as nucleophiles suggests the involvement of (de)protonation in the rate-determining step.

Germylene-sulfoxide as a potential hemilabile ligand: application in coordination chemistry.

We describe here the synthesis of heteroleptic organogermylenes containing a sulfoxide donor function for their application in coordination chemistry. While complexation reaction with [W(cod)(CO)4] and [Mo(nbd)(CO)4] afforded bis(germanium)(ii) transition-metal complexes, a bidentate complex coordinated by germanium(ii) and the oxygen atom of the sulfinyl group was obtained from [Ru(PPh3)3Cl2].

Synthesis and characterization of an iron complex bearing a hemilabile NNN-pincer for catalytic hydrosilylation of organic carbonyl compounds.

A low-coordinate iron(ii) complex (CztBu(PztBu)2)Fe[N(SiMe3)2], 1 bearing an NNN-pincer ligand was prepared and fully characterized. Intramolecular C-H activation on the 5-position of a pyrazole at elevated temperatures was observed. Complex 1 was found to be an efficient and chemoselective pre-catalyst for the hydrosilylation of organo carbonyl substrates.

Interplay of hemilability and redox activity in models of hydrogenase active sites

The hydrogen evolution reaction, as catalyzed by two electrocatalysts [M(N2S2)·Fe(NO)2]+, [Fe-Fe]+ (M = Fe(NO)) and [Ni-Fe]+ (M = Ni) was investigated by computational chemistry. As nominal models of hydrogenase active sites, these bimetallics feature two kinds of actor ligands: Hemilabile, MN2S2 ligands and redox-active, nitrosyl ligands, whose interplay guides the H2 production mechanism. The requisite base and metal open site are masked in the resting state but revealed within the catalytic cycle by cleavage of the MS-Fe(NO)2 bond from the hemilabile metallodithiolate ligand. Introducing two electrons and two protons to [Ni-Fe]+ produces H2 from coupling a hydride temporarily stored on Fe(NO)2 (Lewis acid) and a proton accommodated on the exposed sulfur of the MN2S2 thiolate (Lewis base). This Lewis acid-base pair is initiated and preserved by disrupting the dative donation through protonation on the thiolate or reduction on the thiolate-bound metal. Either manipulation modulates the electron density of the pair to prevent it from reestablishing the dative bond. The electron-buffering nitrosyl's role is subtler as a bifunctional electron reservoir. With more nitrosyls as in [Fe-Fe]+, accumulated electronic space in the nitrosyls' π*-orbitals makes reductions easier, but redirects the protonation and reduction to sites that postpone the actuation of the hemilability. Additionally, two electrons donated from two nitrosyl-buffered irons, along with two external electrons, reduce two protons into two hydrides, from which reductive elimination generates H2.

Making C-N and C-P bonds on the quinone derivatives through the assistance of silver-mediated C-H functionalization processes

Several multi-substituted 1,4-benzoquinone (BQ) derivatives, 4, 5, and 6, possessing C-N and C-P bonds were prepared through the assistance of silver-mediated C-H functionalization processes and characterized by spectroscopic methods. The crystal structures of some selected products were determined by X-ray diffraction methods as well. Reaction pathways are proposed to account for the formations of products from 4 to 5 then to 6. Three rather structurally distinctive compounds, 8b, 9, and 10, were obtained individually from the reactions of di-substituted compound 4aa under specified conditions. Crystal structures of these three compounds have shown that the scaffolding of each compound contains multiple fused rings. Attempts to reduce specifically the PO bond of the -P(O)Ph2 on a di-substituted 1,4-naphthoquinone N4aa to a P,N-bidentate hemilabile ligand resulted in an unexpected over-reduced compound 11. As revealed from the crystal structure of it, one of the ketone groups was completely reduced to CH2 and the other one was reduced to C-OH; yet, the targeted PO bond remains intact.

Gold(III) NHC Complexes for Catalyzing Dihydroalkoxylation and Hydroamination Reactions.

A gold(III) complex of an N-heterocyclic carbene based hemilabile ligand with two pendant pyrazole arms (1,3-bis((1H-pyrazol-3-yl)methyl)-2,3-dihydro-1H-imidazole, LH) was synthesized. Complex [LAu(III)Cl3] is an excellent catalyst for promoting dihydroalkoxylation at room temperature, even catalyzing this reaction at 0 °C. [LAu(III)Cl3] is one of the most efficient catalysts reported to date for the spirocyclization of alkynyl diols. Furthermore, [LAu(III)Cl3] catalyzed intra- and intermolecular hydroamination reactions, achieving good to excellent conversions. [LAu(III)Cl3] is a more efficient catalyst than a gold(I) analogue, [LAu(I)Cl]. The dependence of the quantity of weakly coordinating anion [BArF4]- ((3,5-trifluoromethyl)phenyl borate) present on catalysis efficiency was probed for the dihydroalkoxylation reaction. X-ray diffraction analysis of single crystals demonstrated the solid-state structure of gold complexes [LAu(III)Cl3] and [LAu(I)Cl], which displayed the expected square-planar and linear coordination geometries, respectively.