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1.
2.
The ligand 1,3‐bis[3‐oxo‐3‐(2‐hydroxyphenyl)propionyl]benzene (H4L), designed to align transition metals into tetranuclear linear molecules, reacts with MII salts (M=Ni, Co, Cu) to yield complexes with the expected [MM???MM] topology. The novel complexes [Co4L2(py)6] ( 2 ; py=pyridine) and [Na(py)2][Cu4L2(py)4](ClO4) ( 3 ) have been crystallographically characterised. The metal sites in complexes 2 and 3 , together with previously characterised [Ni4L2(py)6] ( 1 ), favour different coordination geometries. These have been exploited for the deliberate synthesis of the heterometallic complex [Cu2Ni2L2(py)6] ( 4 ). Complexes 1 , 2 , 3 and 4 exhibit antiferromagnetic interactions between pairs of metals within each cluster, leading to S=0 spin ground states, except for the latter cluster, which features two quasi‐independent S=1/2 moieties within the molecule. Complex 4 gathers the structural and physical conditions, thus allowing it to be considered as prototype of a two‐qbit quantum gate.  相似文献   

3.
Photoredox catalysts are integral components of artificial photosystems, and have recently emerged as powerful tools for catalysing numerous organic reactions. However, the development of inexpensive and efficient earth-abundant photoredox catalysts remains a challenge. We here present the photochemical and photophysical properties of a Ni–Mabiq catalyst ([NiII(Mabiq)]OTf (1); Mabiq = 2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6)—and of a Zn-containing analogue ([ZnII(Mabiq)OTf] (2))—using steady state and time resolved optical spectroscopy, time-dependent density functional theory (TDDFT) calculations, and reactivity studies. The Ni and Zn complexes exhibit similar absorption spectra, but markedly different photochemical properties. These differences arise because the excited states of 2 are ligand-localized, whereas metal-centered states account for the photoactivity of 1. The distinct properties of the Ni and Zn complexes are manifest in their behavior in the photo-driven aza-Henry reaction and oxidative coupling of methoxybenzylamine.

The development of earth-abundant photoredox catalysts remains a challenge. Studies of Ni- and Zn-Mabiq complexes demonstrate how the coordinating metal ion influences the photochemistry, photodynamics and reactivity of photocatalysts.  相似文献   

4.
The reaction of [Ni(Mes2Im)2] (1) (Mes2Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of 1 and [Ni(iPr2Im)2] (1ipr) (iPr2Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C–F bond of C6F6 is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes2Im)2(F)(ArF)] (ArF = 4-CF3-C6F42, C6F53, 2,3,5,6-C6F4N 4, 2,3,5,6-C6F4H 5, 2,3,5-C6F3H26, 3,5-C6F2H37) in fair to good yields with the exception of the formation of the pentafluorophenyl complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C6F6, in line with a radical pathway for the C–F bond activation step using 1. The difluoride complex trans-[Ni(Mes2Im)2(F)2] (9), the bis(aryl) complex trans-[Ni(Mes2Im)2(C6F5)2] (15), the structurally characterized nickel(i) complex trans-[NiI(Mes2Im)2(C6F5)] (11) and the metal radical trans-[NiI(Mes2Im)2(F)] (12) were identified. Complex 11, and related [NiI(Mes2Im)2(2,3,5,6-C6F4H)] (13) and [NiI(Mes2Im)2(2,3,5-C6F3H2)] (14), were synthesized independently by reaction of trans-[Ni(Mes2Im)2(F)(ArF)] with PhSiH3. Simple electron transfer from 1 to C6F6 was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes2Im)2]+, which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(iPr2Im)2] (1ipr) and [Ni(Mes2Im)2] (1) into the C–F bond of C6F6. For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.

A combined experimental and theoretical study on the mechanism of the C–F bond activation of C6F6 with [Ni(NHC)2] is provided.  相似文献   

5.
Metalation of the polynucleating ligand F,tbsLH6 (1,3,5-C6H9(NC6H3−4-F−2-NSiMe2tBu)3) with two equivalents of Zn(N(SiMe3)2)2 affords the dinuclear product (F,tbsLH2)Zn2 ( 1 ), which can be further deprotonated to yield (F,tbsL)Zn2Li2(OEt2)4 ( 2 ). Transmetalation of 2 with NiCl2(py)2 yields the heterometallic, trinuclear cluster (F,tbsL)Zn2Ni(py) ( 3 ). Reduction of 3 with KC8 affords [KC222][(F,tbsL)Zn2Ni] ( 4 ) which features a monovalent Ni centre. Addition of 1-adamantyl azide to 4 generates the bridging μ3-nitrenoid adduct [K(THF)3][(F,tbsL)Zn2Ni(μ3-NAd)] ( 5 ). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S = ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K2(THF)9][(F,tbsL)Zn2Ni(μ3-NAd)] ( 6 ) was isolated and characterized while the neutral redox isomer was observed to undergo both intra- and intermolecular H-atom abstraction processes. Ni K-edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2Ni] nitrenoid complexes. However, DFT analysis suggests Ni-borne oxidation for 5 .  相似文献   

6.
Four tridentate ONS ligands, namely 2-hydroxyacetophenonethiosemicarbazone (H2L1), the 2-hydroxyacetophenone Schiff base of S-methyldithiocarbazate (H2L2), the 2-hydroxy-5-nitrobenzaldehyde Schiff base of S-methyldithiocarbazate (H2L3), and the 2-hydroxy-5-nitrobenzaldehyde Schiff base of S-benzyldithiocarbazate (H2L4), and their complexes of general formula [Ni(HL1)2], [ML] (M?=?NiII or CuII; L?=?L1, L2, L3 and L4), [Co(HL)(L); L?=?L1, L2, L3 and L4] and [ML(B)] (M?=?NiII or CuII; L?=?L2 and L4; B?=?py, PPh3) have been prepared and characterized by physico-chemical techniques. Spectroscopic evidence indicates that the Schiff bases behave as ONS tridentate chelating agents. X-ray crystallographic structure determination of [NiL2(PPh3)] and [CuL4(py)] indicates that these complexes have an approximately square-planar structure with the Schiff bases acting as dinegatively charged ONS tridentate ligands coordinating via the phenoxide oxygen, azomethine nitrogen and thiolate sulfur atoms. The electrochemical properties of the complexes have been studied by cyclic voltammetry.  相似文献   

7.
A new coordination polymer of the formula [Ni(NIT4py)2(ip)(H2O)]n(NIT4py = 2‐(4′‐pyridinyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide and ip = isophthalate dianion) has been synthesized and characterized by elemental analyses, IR spectrum, and single‐crystal X‐ray diffraction. The coordination about each Ni2+ ion is a distorted octahedra. Each isophthalate dianion binds two Ni2+ ions in monodentate‐bidentate mode, leading to a 1‐D chain. Among the chains, the coordinated water molecules and carboxylato oxygen atoms form hydrogen bonds, generating an infinite 1‐D ladder structure of a double‐chain. The magnetic study shows that the decrease of χMT value in the low temperature for the complex is mainly ascribed to the zero‐field splitting of the distorted octahedral Ni2+ ions.  相似文献   

8.
The characterization of the unstable NiII bis(silylamide) Ni{N(SiMe3)2}2 ( 1 ), its THF complex Ni{N(SiMe3)2}2(THF) ( 2 ), and the stable bis(pyridine) derivative trans‐Ni{N(SiMe3)2}2(py)2 ( 3 ), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric NiI species, [Ni{N(SiMe3)2}]4 ( 4 ), also obtainable from LiN(SiMe3)2 and NiCl2(DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of NiII to NiI and the stabilization of 4 through dispersion forces.  相似文献   

9.
The synthetic investigation of the NiII/M(NO3)3·6H2O/di-2-pyridyl ketone [(py)2CO] tertiary reaction system in EtOH has yielded triangular Ni2M cationic complexes (M = lanthanide, Y). The reaction between Ln(NO3)3·6H2O, Ni(ClO4)2·6H2O, (py)2CO and base (1:3:3:3) in EtOH under gentle heating gave the isostructural complexes [Ni2Ln{(py)2C(OEt)(O)}3{(py)2C(OH)(O)}(NO3)(H2O)](ClO4)2 (Ln = Gd, 2; Ln = Tb, 3) in high yields. The ligands (py)2C(OEt)(O) and (py)2C(OH)(O) are the monoanions of the hemiketal and gem-diol derivatives of (py)2CO, respectively, formed in situ in the presence of the metal ions. The cations of 2 and 3 consist of one 8-coordinate LnIII and two distorted octahedral NiII atoms in an essentially isosceles, triangular arrangement capped by a central μ3 atom of the unique 3.3011 (Harris notation) (py)2C(OH)(O) ligand. Each metal-metal edge is bridged by the deprotonated O atom of one 2.2011 (py)2C(OEt)(O) ligand. The isostructural complexes [Ni2M{(py)2C(OEt)(O)}4(NO3)(H2O)]2[M(NO3)5](ClO4)2 (M = Y, 4 ; M = Tb, 5 ; M = Dy, 6) were prepared by the 1:1 reaction of the mononuclear “metalloligand” [Ni(O2CMe){(py)2CO}{(py)2C(OH)2}](ClO4) (1) and M(NO3)3·6H2O in EtOH under mild heating in moderate to good yields. The structures of the dications of 4-6 are similar to those in 2 and 3, the only difference being the replacement of the unique 3.3011 (py)2C(OH)(O) ligand of the latter by one 3.3011 (py)2C(OEt)(O) group in the former. The YIII, TbIII and DyIII atoms in [M(NO3)5]2− are coordinated by five bidentate chelating nitrato groups. Characteristic IR bands of the complexes are discussed in terms of the known structures and the coordination modes of the ligands. Variable temperature, solid-state direct current magnetic susceptibility and magnetization studies were carried out on dried samples of 2-4. The data indicate ferromagnetic Ni?Ni and Ni?Gd exchange interactions, and an ST = 11/2 ground state for 2. Complex 3 is characterized by a high-spin ground state while the ferromagnetic Ni?Ni interaction for 2 is independently supported by the study of 4. No out-of-phase, alternating current susceptibility signals have been detected for 3 that would be indicative of SMM behavior.  相似文献   

10.
Reaction of Ni(OTf)2 with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [NiII4L6]8+. By selectively exchanging the bound triflate from [OTf⊂NiII4L6](OTf)7 (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [MIIX4⊂NiII4L6](OTf)6, where MIIX42− = MnCl42− (2), CoCl42− (5), CoBr42− (6), NiCl42− (7), and CuBr42− (8) or [MIIIX4⊂NiII4L6](OTf)7, where MIIIX4 = FeCl4 (3) and FeBr4 (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN6} vertexes within a single Ni4L6 unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [NiII4] complex, and between the host and the MX4n guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX4n guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [NiII4L6]8+ cage can reversibly bind paramagnetic MX41/2− guests, inducing magnetic exchange interactions between host and guest.  相似文献   

11.
A new tetradentate N2O2-type Schiff base, bis(2-hydroxypropiophenone)-1,2-propanediimine (L), was synthesized by the reaction of 1,2-propanediamine with 2-hydroxypropiophenone in EtOH. The Schiff base is able to extract CoII, NiII, CuII and ZnII ions in aqueous NaNO3 media into a CH2Cl2 organic phase via a cation exchange mechanism. The observed extraction order was as follows: CuII > NiII > CoII > ZnII. Reaction of nickel acetate with the Schiff base in EtOH afforded the neutral complex Ni · L. Single crystals of this complex were obtained from mixed CHCl3-EtOH (3:1) solvent and its structure was determined by X-ray diffraction. Crystal data for Ni · L · CHCl3: triclinic, space group Pī, with a = 9.005(2) Å, b = 9.625(2) Å, c = 14.212(4) Å, V = 1136.8(5) Å3, α = 106.06(2)°, β = 106.06(2), γ = 105.10(2)°, and Z = 2. A near square planar structure is observed for the studied complex.  相似文献   

12.
The synthesis, crystal structure and electrochemical properties of a Ni(II) Schiff base complex, [Ni(L)]PF6 (where L is 2,4,9,11,11-pentamethyl-2,3,4 triaza-1-one-4-amine) are reported herein. The complex has been characterized by its electrochemical behavior, X-ray crystallographic structural analysis, physio-chemical methods and spectroscopic techniques. Electrospray mass spectroscopic analysis gives a dominant ion peak with m/z = 296 which corresponds to the {[Ni(L)]PF6–HPF6}+ fragment. Cyclic voltammograms for [Ni(L)]PF6, obtained in DMF (0.1 M Bu4NPF6) at a glassy carbon electrode with a scan rate of 100 mV s?1, exhibit reversible ([NiII(L)]+/[NiI(L)]) reduction and chemically irreversible ([NiII(L)]+/[NiIII(L)]2+→ electroactive product) oxidation processes at ?2.05 and 0.62 V, respectively. The diffusion coefficient, calculated using the Randles–Sevcik relationship, is 9.7 × 10?6 cms?1. Electrochemical studies reveal that the NiI reduced form of the complex is capable of catalyzing CO2 reduction at a potential that is thermodynamically more favorable than for the reduced [Ni(N,N′-ethylenebis(acetylacetoneiminato)]complex. Spectroelectrochemical analyses following bulk electrolysis of [Ni(L)]PF6 under CO2 revealed the formation of oxalate and bicarbonate.  相似文献   

13.
Hydrogen production technology by water splitting has been heralded as an effective means to alleviate the envisioned energy crisis. However, the overall efficiency of water splitting is limited by the effectiveness of the anodic oxygen evolution reaction (OER) due to the high energy barrier of the 4e process. The key to addressing this challenge is the development of high-performing catalysts. Transition-metal hydroxides with high intrinsic activity and stability have been widely studied for this purpose. Herein, we report a gelatin-induced structure-directing strategy for the preparation of a butterfly-like FeNi/Ni heterostructure (FeNi/Ni HS) with excellent catalytic performance. The electronic interactions between Ni2+ and Fe3+ are evident both in the mixed-metal “torso” region and at the “torso/wing” interface with increasing Ni3+ as a result of electron transfer from Ni2+ to Fe3+ mediated by the oxo bridge. The amount of Ni3+ also increases in the “wings”, which is believed to be a consequence of charge balancing between Ni and O ions due to the presence of Ni vacancies upon formation of the heterostructure. The high-valence Ni3+ with enhanced Lewis acidity helps strengthen the binding with OH to afford oxygen-containing intermediates, thus accelerating the OER process. Direct evidence of FeNi/Ni HS facilitating the formation of the Ni–OOH intermediate was provided by in situ Raman studies; the intermediate was produced at lower oxidation potentials than when Ni2(CO3)(OH)2 was used as the reference. The Co congener (FeCo/Co HS), prepared in a similar fashion, also showed excellent catalytic performance.

A butterfly-like FeNi/Ni HS featuring a “torso” of Ni-doped FeOOH and two “wings” of Ni2(CO3)(OH)2 showed excellent activity in electrocatalytic oxygen evolution reaction attributable to the increase of higher-valance Ni3+ in the heterostructure.  相似文献   

14.
The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me6tren)2MII2(C6H4O22−)]2+ (Me6tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me6tren)2MII2(C6H4O2˙)]3+. Single-crystal X-ray diffraction shows that the Me6tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone–transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < −600 cm−1, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal–semiquinone exchange constants of J = −144(1) and −252(2) cm−1, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of Ueff = 22 and 46 cm−1, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear NiII complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.

A semiquinone radical bridging two trigonal bipyramidal metal centers facilitates strong magnetic exchange and single-molecule magnet behavior.  相似文献   

15.
Abstract

The novel high spin Ni2+ complexes of the topologically constrained tetraazamacrocycles (1–4) [4,11-dimethyl-1,4,8,11 - tetraazabicyclo[6.6.2]hexadecane (1); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3); racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4)] show striking properties. Potentiometric titrations of the ligands 2 and 4 revealed them to be proton sponges, as reported earlier for 1 [1]. Ligand 3 is less basic, losing its last proton with a pK = 11.3(2). Despite high proton affinities, complexation reactions in the absence of protons successfully yielded Ni2+ complexes in all cases. The X-ray crystal structures of Ni(1)(acac)+, Ni(3)(acac)+ and Ni(1)(OH2)2 2+ demonstrate that the ligands enforce a distorted octahedral geometry on Ni2+ with two cis sites occupied by other ligands. Magnetic measurements and electronic spectroscopy on the corresponding Ni(L)Cl2 (L = 1–3) complexes reveal that all are high spin and six-coordinate with typical magnetic moments. In contrast, [Ni(4)Cl+] is five-coordinate with a slightly higher magnetic moment and its own characteristic electronic spectrum. The extra methyl groups on ligand 4 define a shallow cavity, sterically allowing only one chloride ligand to bind to the nickel(II) ion.  相似文献   

16.
The new complexes mer-Cr(py)3(N3)3, NaCr(py)4(N3)4, KCr(py)4(N3)4, and RbCr(py)3(N3)4 (py = pyridine) have been prepared. Infrared (4000-50 cm−1) and diffuse reflectance spectra (region 300-77 K) of powdered samples have been measured and discussed on bases of the known structures of these complexes. Single crystal absorption spectra for the mer-complex were obtained in the temperature range from 300 to 10 K revealing extensive vibronic structure associated with the 2Eg(Oh) and 2T1g(Oh) electronic origins. Crystal field calculations were used to assign the bands in the vibronic region and to obtain estimates for the crystal field and Racah parameters for this class of substances. The parameters found for the mer-complex at 10 K are 10 Dq = 17906 cm−1, B = 387 cm−1 and C = 3381 cm−1.  相似文献   

17.
Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorption via a metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H2 and liquid-product H2O2 under visible light, with evolution rates reaching 26.6 and 24.0 μmol g−1 h−1, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splitting via a two-electron reaction pathway.

Single-atom Ni terminating agent is introduced to coordinate with sp2 or sp3 N atoms in the heptazine units of PCN, realizing visible-light photocatalytic overall water splitting to H2O2 and H2 without additional cocatalyst.  相似文献   

18.
Convergent paired electrosynthesis is an energy-efficient approach in organic synthesis; however, it is limited by the difficulty to match the innate redox properties of reaction partners. Here we use nickel catalysis to cross-couple the two intermediates generated at the two opposite electrodes of an electrochemical cell, achieving direct arylation of benzylic C–H bonds. This method yields a diverse set of diarylmethanes, which are important structural motifs in medicinal and materials chemistry. Preliminary mechanistic study suggests oxidation of a benzylic C–H bond, Ni-catalyzed C–C coupling, and reduction of a Ni intermediate as key elements of the catalytic cycle.

A direct arylation of benzylic C–H bonds is achieved by integrating Ni-catalyzed benzyl–aryl coupling into convergent paired electrolysis.

Electrochemical organic synthesis has drawn much attention in recent years.1 Compared to processes using stoichiometric redox agents, electrosynthesis can potentially be more selective and safe, generate less waste, and operate under milder conditions.1b In the majority of examples, the reaction of interest occurs at one electrode (anode for oxidation or cathode for reduction), while a sacrificial reaction occurs at the counter electrode to fulfil electron neutrality.1a,2 Paired electrolysis uses both anodic and cathodic reactions for the target synthesis, thereby maximizing energy efficiency.1a,3 However, there are comparatively few examples of paired electrolysis for organic synthesis.1a,3,4Paired electrolysis might be classified into three types: parallel, sequential, and convergent (Fig. 1).1a,3a In parallel paired electrolysis (Fig. 1a), the two half reactions are simultaneous but non-interfering. In sequential paired electrolysis (Fig. 1b), a substrate is oxidized and reduced (or vice versa) sequentially. In convergent paired electrolysis (Fig. 1c), intermediates generated by the anodic and cathodic processes react with one another to yield the product.1a,3a,4b,c,5 The activation mode of all three types of paired electrolysis is based on the innate redox reactivity of substrates. As a result, the types of reactions that could be conducted by paired electrolysis remain limited. We proposed a catalytic version of convergent paired electrolysis, where a catalyst is used to cross-couple the two intermediates generated at the two separated electrodes (Fig. 1d). Although mediators have been used in paired electrosynthesis,3a,4c,6 catalytic coupling of anodic and cathodic intermediates remains largely undeveloped. This mode of action will leverage the power of cross-coupling to electrosynthesis, opening up a wide substrate and product space. Here we report the development of such a process, where cooperative nickel catalysis and paired electrolysis enable direct arylation of benzylic C–H bonds (Fig. 1e).Open in a separate windowFig. 1Different types of paired electrolysis: (a) parallel paired electrolysis, (b) sequential paired electrolysis, (c) convergent paired electrolysis, (d) catalytic convergent paired electrolysis and (e) this work.Our method can be used to synthesize diarylmethanes, which are important structural motifs in bioactive compounds,7 natural products8 and materials.9 Direct arylation of benzylic C–H bonds has been recognized as an efficient strategy to synthesize diarylmethanes, and methods using metal catalysis10 and in particular combined photoredox and transition-metal catalysis have been reported.11 Electrosynthesis provides a complementary approach to these methods, with the potential advantages outlined above. The groups of Yoshida12 and Waldvogel13 previously developed synthesis of diarylmethanes via a Friedel–Crafts-type reaction of a benzylic cation and a nucleophile. The benzylic cations were generated by anodic oxidation of benzylic C–H bonds.14 To avoid the overoxidation of products and to stabilize the very reactive benzylic cations, the reactions had to be conducted in two steps, where the benzylic cations generated in the anodic oxidation step had to be trapped by a reagent. We thought a Ni catalyst could be used to trap the benzyl radical to form an organonickel intermediate, which is then prone to a Ni-catalyzed C–C cross-coupling reaction. Although such a coupling scheme was unprecedented, Ni-catalyzed electrochemical reductive coupling of aryl halides was well established.15 We were also encouraged by a few recent reports of combined Ni catalysis and electrosynthesis for C–N,16 C–S,17 and C–P18 coupling reactions.We started our investigations using the reaction between 4-methylanisole 1a and 4-bromoacetophenone 2a as a test reaction (Table 1). Direct arylation of benzylic C–H bonds was challenging and was typically conducted using toluene derivatives in large excess, e.g., as a solvent.11a,b,11df To improve the reaction efficiency, we decided to use only 3 equivalents of 4-methylanisole 1a relative to 2a. After some initial trials, we decided to conduct the reaction in an undivided cell using a constant current of 3 mA. These conditions are straightforward from a practical point of view. After screening various reaction parameters, we found that a combination of 4,4′-dimethoxy-2-2′-bipyridine (L1) and (DME)NiBr2 as a catalyst, THF/CH3CN (4 : 1) as a solvent, fluorine-doped tin oxide (FTO) coated glass as an anode and carbon fibre as a cathode gave a 50% GC yield of 1-(4-(4-methoxybenzyl)phenyl)ethanone 3a after 18 h (Table 1, entry 1). Extending the reaction time to 36 h improved the yield to 76% (isolated yield) (entry 2). The target products were formed in a diminished yield with other bipyridine type ligands (entries 3–5). Solvents commonly used in Ni-catalyzed cross-coupling reactions, such as DMA and DMF, were less effective (entries 7–8). Replacing carbon fibre by nickel foam or platinum foil as the cathode was detrimental to the coupling, but substantial yields were still obtained (entries 9–10). On the other hand, FTO could not be replaced as the anode. Using carbon fibre as the anode shut down the reaction (entry 11). Likewise, using Pt foil as the anode gave only a 7% GC yield (entry 12). The sensitivity of the reaction outcomes to the electrodes originates from the electrode-dependent redox properties of reaction components (see below). Additional data showing the influence of other reaction parameters such as nickel sources, current, concentration, and electrolytes are provided in the ESI (Table S1, ESI).Summary of the influence of key reaction parametersa
EntryLigandAnodeCathodeSolventYield (%)
1 L1 FTOCarbon fibreTHF/CH3CN = 4 : 156
2 L1 FTOCarbon fibreTHF/CH3CN = 4 : 176b
3 L2 FTOCarbon fibreTHF/CH3CN = 4 : 143
4 L3 FTOCarbon fibreTHF/CH3CN = 4 : 146
5 L4 FTOCarbon fibreTHF/CH3CN = 4 : 121
6 L1 FTOCarbon fibreCH3CN4
7 L1 FTOCarbon fibreDMA15
8 L1 FTOCarbon fibreDMF6
9 L1 FTONi foamTHF/CH3CN = 4 : 145
10 L1 FTOPt foilTHF/CH3CN = 4 : 128
11 L1 Carbon fibre (1 cm2)Carbon fibreTHF/CH3CN = 4 : 10
12 L1 Pt foil (cm2)Carbon fibreTHF/CH3CN = 4 : 17
Open in a separate windowaReaction conditions: 1a (0.6 mmol), 2a (0.2 mmol), (DME)NiBr2 (6 mol%), ligand (7.2 mol mol%), LutHClO4 (0.1 M), and lutidine (0.8 mmol) in solvent (2 mL) at 40 °C. GC yield.bReaction time: 36 h. Isolated yield.With the optimized reaction conditions in hand, we explored the substrate scope (Table 2). A large number of aryl and heteroaryl bromides could be coupled (3a–3x). These substrates may contain electron-rich, neutral, or poor groups. For aryl bromide bearing electron-donating groups, replacing (DME)NiBr2 by Ni(acac)2 gave higher yields (3k–3o). The method tolerates numerous functional groups in the (hetero)aryl bromides, including for example ketone (3a), nitrile (3b, 3u, and 3v), ester (3c, 3m, 3n, and 3s), amide (3d), aryl-Cl (3q), CF3(3i, 3t, and 3w), OCF3(3e), aryl-F(3x), pyridine (3w and 3x), and arylboronic ester (3g). We then probed the scope of benzylic substrates using 4-bromoacetophenone 2a as the coupling partner (3aa–3ai). Toluene and electron-rich toluene derivatives were readily arylated (3aa–3ac). Toluene derivatives containing an electron-withdrawing group such as fluoride (3ad) and chloride (3ae) could also be arylated, although a higher excess of them (10 equiv.) was necessary. More elaborated toluene derivatives containing an additional ester (3af, 3ai) or ether (3ag, 3ah, and 3ai) were also viable.Substrate scopea
Open in a separate windowaReaction conditions: 1 (0.6 mmol), 2 (0.2 mmol), (DME)NiBr2 (6 mol%), L1 (7.2 mol mol%), LutHClO4 (0.1 M), and lutidine (0.8 mmol) in THF/CH3CN (4 : 1, 2 mL) at 40 °C. Isolated yield.b(DME)NiBr2 (5 mol%) and L1 (6 mol%) were used as the catalysts.cNi(acac)2 was used instead of (DME)NiBr2.dSolvent: THF/CH3CN (3 : 1, 2 mL).e2 mmol toluene or its derivative was used as the substrate.fReaction time: 60 h.Linear sweep voltammetry (LSV) was applied to probe the possible processes at both the anode and cathode. The measurements were made in THF/CN3CN (4 : 1, 2 mL) using [LutH]ClO4 (0.1 M) as the electrolyte and lutidine (0.4 M) as an additional base to mimic the coupling conditions. The LSV curves of individual reaction components indicate that only 4-methylanisole 1a and the Ni catalyst may be oxidized at the anode (Fig. 2a). The current at 3 mA appears to be the sum of the oxidation currents of 1a and the Ni catalyst. Meanwhile, LSV curves indicate that only the Ni catalyst might be reduced at the cathode (Fig. 2b).Open in a separate windowFig. 2The LSV curves of different reaction components at the anode or cathode. The components were dissolved in THF/CN3CN (4 : 1, 2 mL); the solution also contained [LutH]ClO4 (0.1 M) and lutidine (0.4 M). Scan rate: 50 mV s−1. (a) The LSV curves of different reaction components at the FTO anode; (b) the LSV curves of different reaction components at the carbon fibre cathode; (c) the LSV curves of different reaction components at the carbon fibre anode.It was observed that FTO was an essential anode for the reactions. If FTO was replaced by a carbon fibre anode, no coupling product was obtained. LSV was performed to probe the oxidation of 1a and the Ni catalyst on a carbon fibre anode (Fig. 2c). The oxidation of the Ni catalyst was much easier on carbon fibre than on FTO. At 3 mA, the oxidation is exclusively due to the Ni catalyst. This result suggests that the absence of coupling on the carbon fibre anode is due to no oxidation of 1a. The different redox properties of 1a and the Ni catalyst observed on different electrodes might be attributed to the different nature of surface species which influence the electron transfer. Although FTO is rarely used in electrosynthesis, it is widely used in electrocatalysis and photoelectrocatalysis for energy conversion.19 FTO is stable, commercially available and inexpensive. In our reactions, the FTO anode could be reused at least three times.The LSV curves in Fig. 2 revealed the issue of “short-circuit” of catalyzed/mediated paired electrolysis in an undivided cell, as the catalyst or mediator can be reduced and oxidized at both the cathode and anode. When carbon fibre or graphite was used as the anode, the short-circuit problem was very severe so that nearly no current was used for electrosynthesis. However, by using an appropriate anode such as FTO, the short-circuit problem was alleviated and around half of the current was used to oxidize the substrate (1a) while the other half was used to oxidize the nickel complex. The remaining short-circuit is one of the reasons why the current efficiencies of the reactions are low (<10%). Another factor contributing to the low current efficiency is the instability of the benzyl radical, which can abstract hydrogen from the solvent to regenerate the substrate. Nevertheless, useful products could be obtained in synthetically useful yields under conditions advantageous to previous methods.For the test reaction (Table 1), a small amount of homo-coupling product bis(4-methoxyphenyl)methane (<2%) was detected by GC-MS under the optimized conditions. In the absence of ligand L1, the yield of the homo-coupling products increased (∼8%). In the presence of a radical acceptor, the electron-withdrawing alkene vinyl benzoate, the product originating from the addition of a benzyl radical to the olefin was obtained in about 12% GC yield (ESI, Scheme S1). These data support the formation of a benzyl radical intermediate. As bromide existed in our reaction system, it is possible to be oxidized to form a bromine radical. Previous studies showed that a bromine radical can react with a toluene derivative to give a benzyl radical.11b,g,20 To probe the involvement of the Br radical, we conducted a coupling of 4-methylanisole 1a with 4′-Iodoacetophenone, using Ni(acac)2 instead of (DME)NiBr2 as the Ni source. We obtained a GC yield of 24% for the coupling after 18 h (Scheme S2). This result suggests that a Br-free path exists for the coupling, although a non-decisive involvement of Br/Br˙ cannot be ruled out.Based on the data described above, we propose a mechanism for the coupling (Scheme 1). The oxidation of a toluene derivative at the anode gives a benzyl radical. This radical is trapped by a LNi(ii)(Ar)(Br) species (B) in the solution to give a LNi(iii)(Ar)(benzyl)(Br) intermediate (C). The latter undergoes reductive elimination to give a diarylmethane and a LNi(i)(Br) species (A). There are at least two ways A can be convert to B to complete the catalytic cycle: either by oxidative addition of ArBr followed by a 1-e reduction at the cathode or by first 1-e reduction to form a Ni(0) species followed by oxidation addition of ArBr. In addition to a toluene derivative, a Ni species is oxidized at the anode. We propose that this oxidation is an off cycle event, which reduces the faradaic and catalytic efficiency but does not shut down the productive coupling.Open in a separate windowScheme 1Proposed mechanism of the direct arylation of benzylic C–H bonds.  相似文献   

19.
Single-crystal optical absorption spectra of NiO, NiTiO3, NiWO4, NiV2O6, NiNb2O6, Ni2SiO4, Ni3V2O8, LiNiPO4, Li2Ni2Mo3O12, SrNiTeO6, LiScSiO4:Ni, MgSiO3:Ni, and (Mg,Ni)2SiO4 are presented for the purpose of comparing the spectra of yellow and green Ni2+ compounds. Powder spectra of NiTiO3, NiWO4, NiV2O6, NiNb2O6, and Ni3V2O8 in the ultraviolet region help elucidate the more intense charge transfer bands. Bright yellow color results when Ni2+ is in a six-coordinated site significantly distorted from octahedral symmetry. Increased absorption intensity occurs when the metal ion d-d bands are in proximity to an ultraviolet charge transfer band.  相似文献   

20.
A terminal FeIIIOH complex, [FeIII(L)(OH)]2− (1), has been synthesized and structurally characterized (H4L = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene). The oxidation reaction of 1 with one equiv. of tris(4-bromophenyl)ammoniumyl hexachloroantimonate (TBAH) or ceric ammonium nitrate (CAN) in acetonitrile at −45 °C results in the formation of a FeIIIOH ligand radical complex, [FeIII(L˙)(OH)] (2), which is hereby characterized by UV-visible, 1H nuclear magnetic resonance, electron paramagnetic resonance, and X-ray absorption spectroscopy techniques. The reaction of 2 with a triphenylcarbon radical further gives triphenylmethanol and mimics the so-called oxygen rebound step of Cpd II of cytochrome P450. Furthermore, the reaction of 2 was explored with different 4-substituted-2,6-di-tert-butylphenols. Based on kinetic analysis, a hydrogen atom transfer (HAT) mechanism has been established. A pKa value of 19.3 and a BDFE value of 78.2 kcal/mol have been estimated for complex 2.

One-electron oxidation of an FeIII–OH complex (1) results in the formation of a FeIII–OH ligand radical complex (2). Its reaction with (C6H5)3C˙ results in the formation of (C6H5)3COH, which is a functional mimic of compound II of cytochrome P450.  相似文献   

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