Electrocatalytic Reduction of CO2 and H2O with Zn(II) Complexes Through Metal-Ligand Cooperation.

Air and water-stable zinc (II) complexes of neutral pincer bis(diphenylphosphino)-2,6-di(amino)pyridine ("PN3P") ligands are reported. These compounds, [Zn(κ2-2,6-{Ph2PNR}2(NC5H3))Br2] (R=Me, 1; R=H, 2), were shown to be capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to selectively yield CO in mixed water/acetonitrile solutions. These complexes also electrocatalytically generate H2 from water in acetonitrile solutions, at the same potential, with Faradaic efficiencies of up to 90 %. DFT computations support a proposed mechanism involving the first reduction of 1 or 2 occurring at the PN3P ligand. Furthermore, computational analysis suggested a mechanism involving metal-ligand cooperation of a Lewis acidic Zn(II) and a basic ligand.

Nickel-Catalyzed Homologation of Vinylidene Difluoride (CH2═CF2): Selective ß-F vs ß-H Elimination.

Hydrofluoroolefins (HFOs) constitute the newest generation of fluorocarbon refrigerants and foam-blowing agents due to their reduced global warming potential vs their saturated analogues. To identify new synthetic routes to HFOs, we show that reactions of bulky Ni(0) phosphine and -NHC complexes with vinylidene difluoride (VF2) afford µ-fluoro-1,1,3-trifluorobut-3-enyl Ni complexes. Moreover, addition of triisopropylsilane allows for reductive elimination of the reduced product─2,4,4-trifluoro-1-butene─demonstrating the Ni-catalyzed hydrodefluorodimerization of VF2. Accompanying DFT calculations identify the T-shaped nickelacyclopentane intermediate that spontaneously undergoes selective intramolecular ß-F (vs ß-H) elimination.

Electrocatalytic reduction of CO2 to CO and HCO2- with Zn(II) complexes displaying cooperative ligand reduction.

Air-stable zinc(II) pyridyl phosphine complexes, [Zn(κ2-2,6-{Ph2PNMe}2(NC5H3))Br2] (1) and [Zn(κ2-2-{Ph2PNMe}(NC5H3))Br2] (2) are reported and 1 was capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to yield CO/HCO2H in mixed water/acetonitrile solutions. DFT computations support a proposed mechanism involving electron transfer reactions from a species with the anionic PN3P ligand ("L-/Zn(II)").

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in "PN3 P" Pincer-Supported NiII Complexes.

Water is the most sustainable source for H2 production, and the efficient electrocatalytic production of H2 from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ3 -2,6-{Ph2 PNR}2 (NC5 H3 )Br2 ] (R=H I, Me II) is reported. Hydrogen generation from H2 O/CH3 CN solutions is initiated at -2 V against Fc+/0 , and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s-1 . A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H2 formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni0I species bonded to an anionic PN3 P ligand ("L- /Ni0I ") and a NiI -hydride ("Ni-H"). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pKa .

Asymmetric Ring Opening in a Tetrazine-Based Ligand Affords a Tetranuclear Opto-Magnetic Ytterbium Complex.

We report the formation of a tetranuclear lanthanide cluster, [Yb4 (bpzch)2 (fod)10 ] (1), which occurs from a serendipitous ring opening of the functionalised tetrazine bridging ligand, bpztz (3,6-dipyrazin-2-yl-1,2,4,5-tetrazine) upon reacting with Yb(fod)3 (fod- =6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octandionate). Compound 1 was structurally elucidated via single-crystal X-ray crystallography and subsequently magnetically and spectroscopically characterised to analyse its magnetisation dynamics and its luminescence behaviour. Computational studies validate the observed MJ energy levels attained by spectroscopy and provides a clearer picture of the slow relaxation of the magnetisation dynamics and relaxation pathways. These studies demonstrate that 1 acts as a single-molecule magnet (SMM) under an applied magnetic field in which the relaxation occurs via a combination of Raman, direct, and quantum tunnelling processes, a behaviour further rationalised analysing the luminescent properties. This marks the first lanthanide-containing molecule that forms by means of an asymmetric tetrazine decomposition.

Shedding Light on the Diverse Reactivity of NacNacAl with N-Heterocycles.

The aluminum(I) compound NacNacAl (NacNac=[ArNC(Me)CHC(Me)NAr]- , Ar=2,6-iPr2 C6 H3 , 1) shows diverse and substrate-controlled reactivity in reactions with N-heterocycles. 4-Dimethylaminopyridine (DMAP), a basic substrate in which the 4-position is blocked, induces rearrangement of NacNacAl by shifting a hydrogen atom from the methyl group of the NacNac backbone to the aluminum center. In contrast, C-H activation of the methyl group of 4-picoline takes place to produce a species with a reactive terminal methylene. Reaction of 1 with 3,5-lutidine results in the first example of an uncatalyzed, room-temperature cleavage of an sp2 C-H bond (in the 4-position) by an AlI species. Another reactivity mode was observed for quinoline, which undergoes 2,2'-coupling. Finally, the reaction of 1 with phthalazine produces the product of N-N bond cleavage.

Two heads are better than one: improving magnetic relaxation in the dysprosium metallocene upon dimerization by use of an exceptionally weakly-coordinating anion.

Partial metathesis between two weakly-coordinating anions in the archetypical dysprosium metallocene results in the first example of [BPh4]- as a bridging ligand in 4f metals, with a unique η2,η2:η2,η2-bridge. Magnetic susceptibility and relaxation dynamics studies along with ab initio calculations reveal improved slow relaxation of the magnetization in over its mononuclear congener, resulting in an energy barrier of 490 K/340 cm-1 and waist-restricted hysteresis up to 6.5 K.

Ge(0) Compound Stabilized by a Diimino-Carbene Ligand: Synthesis and Ambiphilic Reactivity.

The germylone dimNHCGe (5, dimNHC = diimino N-heterocyclic carbene) was successfully prepared via the reduction of the germanium cation [dimNHCGeCl]+ with KC8. The molecular structure of 5 was unambiguously established by both NMR spectroscopy and single-crystal X-ray diffraction. The reactivity of 5 was investigated, revealing that it undergoes oxidative addition of HCl, CH3I, and PhI, accompanied by an unusual migration of the H, Me, and Ph groups from germanium to the carbene ligand. Related chemistry was also observed with C5F5N, which results in the migration of the fluorinated pyridine moiety to the carbene ligand. Compound 5 also undergoes cycloaddition with tetrachloro-o-benzoquinone to afford a Ge(IV) adduct.

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3-Iodoethynylpyridine and 3-Iodoethynylbenzoic Acid.

The halogen bond has previously been explored as a versatile tool in crystal engineering and anion coordination chemistry, with mechanochemical synthetic techniques having been shown to provide convenient routes towards cocrystals. In an effort to expand our knowledge on the role of halogen bonding in anion coordination, here we explore a series of cocrystals formed between 3-iodoethynylpyridine and 3-iodoethynylbenzoic acid with halide salts. In total, we report the single-crystal X-ray structures of six new cocrystals prepared by mechanochemical ball milling, with all structures exhibiting C≡C-I⋅⋅⋅X- (X=Cl, Br) halogen bonds. Whereas cocrystals featuring a pyridine group favoured the formation of discrete entities, cocrystals featuring a benzoic acid group yielded an alternation of halogen and hydrogen bonds. The compounds studied herein were further characterized by 13C and 31P solid-state nuclear magnetic resonance, with the chemical shifts offering a clear and convenient method of identifying the occurrence of halogen bonding, using the crude product obtained directly from the mechanochemical ball milling. Whereas the 31P chemical shifts were quickly able to identify the occurrence of cocrystallization, 13C solid-state NMR was diagnostic of both the occurrence of halogen bonding and of hydrogen bonding.

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3-Iodoethynylpyridine and 3-Iodoethynylbenzoic Acid.

Invited for this month's cover picture is the group of Professor David Bryce at the University of Ottawa. The cover picture shows a stylized depiction of the mechanochemical halogen-bond induced cocrystallization of 3-iodoethynylpyridine with tetraphenylphosphonium bromide. Read the full text of their Full Paper at 10.1002/open.201900194.

Sequential Oxidation and C-H Bond Activation at a Gallium(I) Center.

In situ oxidation of the GaI compound NacNacGa by either N2 O or pyridine oxide results in the generation of a labile monomeric oxide, NacNacGa(O), which can easily cleave the C-H bonds of aliphatic and aromatic substrates featuring good donor sites. The products of this reaction are gallium organyl hydroxides. DFT calculations show that these reactions start with the formation of NacNac-Ga(O)(L) adducts, the oxo ligand of which can easily abstract protons from nearby C-H bonds, even for sp2 -hybridized carbon centers. Aliphatic amines do not enter this reaction for kinetic reasons, presumably because of the unfavorable sterics.

An integrated Re(i) photocatalyst/sensitizer that activates the formation of formic acid from reduction of CO2.

The complex cis-[Re(bpy)2(CO)2]+OTf- (1+OTf-) is an integrated photosensitizer/catalyst for the selective visible light promoted photocatalytic reduction of CO2. The formation of formic acid is unique among this class of Re catalysts, which yield CO as the selective product. A supplemental photosensitizer, Ru(bpy)32+, considerably enhanced the performance of this catalyst.

A Luminescent Thermometer Exhibiting Slow Relaxation of the Magnetization: Toward Self-Monitored Building Blocks for Next-Generation Optomagnetic Devices.

The development and integration of Single-Molecule Magnets (SMMs) into molecular electronic devices continue to be an exciting challenge. In such potential devices, heat generation due to the electric current is a critical issue that has to be considered upon device fabrication. To read out accurately the temperature at the submicrometer spatial range, new multifunctional SMMs need to be developed. Herein, we present the first self-calibrated molecular thermometer with SMM properties, which provides an elegant avenue to address these issues. The employment of 2,2'-bipyrimidine and 1,1,1-trifluoroacetylacetonate ligands results in a dinuclear compound, [Dy2(bpm)(tfaa)6], which exhibits slow relaxation of the magnetization along with remarkable photoluminescent properties. This combination allows the gaining of fundamental insight in the electronic properties of the compound and investigation of optomagnetic cross-effects (Zeeman effect). Importantly, spectral variations stemming from two distinct thermal-dependent mechanisms taking place at the molecular level are used to perform luminescence thermometry over the 5-398 K temperature range. Overall, these properties make the proposed system a unique molecular luminescent thermometer bearing SMM properties, which preserves its temperature self-monitoring capability even under applied magnetic fields.

Linear End-On Coordination Modes of CO2.

The second case of linear end-on and evidence for an unprecedented bridging end-on coordination mode of CO2 have been discovered for vanadium aryloxide complexes of the tetradentate ligand system (ONNO)2- (ONNO=2,4-Me2 -2-(OH) C6 H2 CH2 ]2 N(CH2 )2 NMe2 ). The reaction of divalent (ONNO)VII (TMEDA) with CO2 and under the appropriate reaction conditions affords the trivalent (ONNO)VIII (OH)(η1 -CO2 ) resulting from an intermediate CO2 deoxygenation pathway followed by H-atom abstraction from the aromatic solvent, and CO2 fixation. In contrast, the reduction of trivalent (ONNO)VIII Cl(THF) with K, followed by exposure to CO2 in ethereal solvent, afforded the dinuclear [(ONNO)VII ]2 (µ, η1 -CO2 ).

Single-Crystal NMR Characterization of Halogen Bonds.

Oxygen-17-enriched triphenylphosphine oxide and three of its halogen-bonded cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene as halogen bond donors have been characterized by 31P and 17O single-crystal NMR spectroscopy. Single-crystal NMR allows for the measurement of not only the magnitudes of various NMR interaction tensors, but also their orientations relative to the crystal lattice and therefore relative to the halogen bonds themselves. 31P chemical shift tensors, 17O chemical shift tensors, 17O quadrupolar coupling tensors, and 31P-17O indirect nuclear spin-spin (J) coupling tensors are reported here for P═O···I halogen bonds. The angular deviations in the directions of the pseudo-unique components of the 31P chemical shift tensors, the 17O chemical shift tensors, and the 17O quadrupolar coupling tensors from the direction of the oxygen-iodine halogen bond correlate with the deviations in linearity of the P═O···I halogen bond. There is also a clear decrease in anisotropy and an increase in asymmetry of the J(31P,17O) coupling tensors attributable to the formation of iodine-oxygen halogen bonds. The small but quantifiable changes in the tensors are consistent with the weak nature of these halogen bonds relative to the P═O motif. Overall, this work establishes single-crystal NMR as a novel probe of halogen bonds in solids. Analysis of the results has provided insights into the correlations between the magnitude and orientation of various NMR interaction tensors and the local geometry of the halogen bond. Gauge-including projector-augmented wave computations corroborate the experimental findings.

Selective Copper Complex-Catalyzed Hydrodefluorination of Fluoroalkenes and Allyl Fluorides: A Tale of Two Mechanisms.

The transition to more economically friendly small-chain fluorinated groups is leading to a resurgence in the synthesis and reactivity of fluoroalkenes. One versatile method to obtain a variety of commercially relevant hydrofluoroalkenes involves the catalytic hydrodefluorination (HDF) of fluoroalkenes using silanes. In this work it is shown that copper hydride complexes of tertiary phosphorus ligands (L) can be tuned to achieve selective multiple HDF of fluoroalkenes. In one example, HDF of the hexafluoropropene dimer affords a single isomer of heptafluoro-2-methylpentene in which five fluorines have been selectively replaced with hydrogens. DFT computational studies suggest a distinct HDF mechanisms for L2CuH (bidentate or bulky monodentate phosphines) and L3CuH (small cone angle monodentate phosphines) catalysts, allowing for stereocontrol of the HDF of trifluoroethylene.

Interaction of Multiple Bonds with NacNacGa: Oxidative Cleavage vs Coupling and Cyclization.

The reactivity of the gallium(I) compound NacNacGa (4) to a variety of unsaturated compounds has been studied. Whereas 4 proved surprisingly unreactive toward olefins, ketones, guanidines, and thioureas, it reacts with isocyanates and carbodiimides to furnish the products of coupling of two heterocumulenes. With isothiocyanate, the C═S cleavage occurs, leading to the dimer (NacNacGa)2(µ-S)(µ-CNPh) and the cyclization product NacNacGa(S2C═NPh). These compounds were characterized by multinuclear NMR spectroscopy and X-ray crystal structure analyses. Oxidative addition of S═PPh3 occurs at elevated temperatures and results in the known dimer [NacNacGa(µ-S)]2.

Visible-Light Photocatalytic Reduction of CO2 to Formic Acid with a Ru Catalyst Supported by N,N'-Bis(diphenylphosphino)-2,6-diaminopyridine Ligands.

Visible-light photocatalytic CO2 reduction is carried out by using a RuII complex supported by N,N'-bis(diphenylphosphino)-2,6-diaminopyridine ("PNP") ligands, an unprecedented molecular architecture for this reaction that breaks the longstanding domination of α-diimine ligands. These competent catalysts transform CO2 into formic acid with high selectivity and turnover number. A proposed mechanism, with combined electron transfer and catalytic cycles, models the experimental rate of formic acid production.

7-Metalla-1,4-diphosphanorbornadienes: cycloaddition of monovalent group 13 NacNac complexes to a stable 1,4-diphosphinine.

This work presents the first example of a dienophilic property of the monovalent group-13 NacNac complexes, which are well-known for their carbene-like reactivity, e.g., addition to multiple bonds, insertion into single bonds, and oxidative cleavage of some multiple bonds. In reactions with imidazole-2-thione based tricyclic 1,4-diphospinine 4, the monovalent compounds NacNacM (M = Al,Ga) showed dienophilic behaviour and produced the corresponding 7-metalla-1,4-diphosphanorbornadienes (5-6), leaving the C[double bond, length as m-dash]S functionality of the imidazole-2-thione intact, while the NacNacIn complex did not show any reactivity because of unfavourable thermodynamics. DFT calculations revealed that for NacNacAl, the cycloaddition was kinetically more favoured (due to a small reaction barrier) than the oxidative cleavage of the C[double bond, length as m-dash]S functionality, despite the fact that the product of the C[double bond, length as m-dash]S bond addition has a somewhat higher stability. For NacNacGa, the cycloaddition reaction was both kinetically and thermodynamically favourable than the cleavage of C[double bond, length as m-dash]S. Furthermore, these reactions were found to be reversible in nature and diphosphinine 4 showed a clear preference towards Al than towards Ga which reflects the better inclination of Ga to stay monovalent than be in the trivalent state.

Imino-stabilised phosphinidene (or azaphosphole?) and some of its derivatives.

Diiminophenyl (dimph) proved to be an excellent ligand platform to stabilise a low-valent phosphorus centre. The resultant compound dimphP (3), which can be rationalized as an imino-stabilised phosphinidene or benzoazaphosphole, shows remarkable chemical stability: it withstands treatment with water and oxidizers (O2 and pyridine oxide) and only reacts with excess strong acid (e.g. HCl) to generate the P(iii) chloride (dimHph)PCl (7), where dimHph stands for a diiminophenyl group hydrogenated at one imine position. Surprisingly, substitution of the chloride under some nucleophilic (KOBut) and electrophilic conditions (Me3SiOTf) regenerates the parent compound 3, by proton removal from the weakly acidic CH2N position. A related species (dimH2ph)P (14), where dimH2ph stands for diiminophenyl hydrogenated to imino(aminomethyl)phenyl, is produced upon thermal rearrangement of the hydride (dimHph)PH (13). X-ray structures of compounds 3, 14, and (dimHph)PO2CH (17) are also discussed.