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A Ni(ii) complex with tridentate phenoxy–amidate–aNHC donors was synthesized, which was efficient in Suzuki–Miyaura coupling with additional IMes·HCl.

Co(dia)1.5(Hsip)(H2O)·H2O (1) and Zn2(μ-OH)(dia)2(sip)·2H2O (2) were prepared from the same set of ligand precursors. They exhibited bnn and dia topologies, respectively. Factors that contributed to the higher proton conductivity of 1 were presented.

Ligand precursors for amido/amidate-functionalized triazole-based MIC ligands were synthesized. An initial theoretical calculation confirmed that triazole-based MIC ligands were promising ligands in terms of their σ-donating and π-acidic properties. Based on these ligand precursors, three different types of palladium complexes were successfully obtained, namely (1) a PEPPSI-type MIC complex, (2) a complex containing both a bidentate ligand of a MIC and an amidate donor and a mondentate NHC derived from nitron, and (3) a complex featuring a tridentate ligand of a MIC, an amidate, and a phenoxy donor. The structures of all these complexes were established by single-crystal X-ray diffraction studies. Imidazole derivatives are important heterocycles with enormous medicinal value. The catalytic activities of these new palladium complexes in the green direct C-H arylation of imidazoles with aryl halides were investigated and compared to those delivered from palladium complexes with IMes and triazole-based normal NHC ligands. Among the new complexes, the PEPPSI-type palladium complex with the monodentate triazole-based MIC ligand was found to be a very promising precatalyst which was capable of utilizing electron-deficient aryl chlorides as coupling partners in the reaction.

Mononuclear and tetranuclear palladium complexes with functionalized “abnormal” N-heterocyclic carbene (aNHC) ligands based on 3-phenylimidazo[1,5-a]pyridine were synthesized. All of the new comple...

C20H17N4I, monoclinic, P21/n (no. 14), a = 7.304(3) Å, b = 14.830(7) Å, c = 16.482(7) Å, β = 98.202(12)°, V = 1767.2(14) Å3, Z = 4, R gt (F) = 0.0568, wR ref (F 2) = 0.1359, T = 150 K.

A series of “abnormal” NHC ligand precursors based on imidazopyridines and their corresponding NHC-palladium diiodide complexes were prepared. Our study included the first report on a palladium complex with an abnormal NHC ligand derived from imidazo[1,5-a]pyridine. DFT calculation on the free ligands confirmed the higher electron-donating properties of the abnormal NHC ligands than their normal NHC isomers. The structural properties of all the complexes were studied in detail by single-crystal X-ray diffraction studies, revealing that, in general, Pd–carbene bond distances in palladium complexes with abnormal NHCs were longer than those in their isomers with normal NHCs. A significant trans influence is observed in the palladium complexes with abnormal NHC ligands. But in the complexes with normal NHC ligands, both Pd–carbene and Pd–N bond distances are short. Catalytic studies revealed that the palladium complexes were very efficient in catalyzing direct C–H arylation between 1,2-dimethylimidazole and a...

Two dinuclear oxidovanadium(V) complexes [LiVVO(µ2-O)VVO(Li)] (i = 1, H2L1, complex 1 and i = 2 for H2L2, complex 2) of two ONNO donor amine-bis(phenolate) ligands have been synthesized and characterized by X-ray diffraction studies which exhibited distorted octahedral geometry around each V center. In MeCN the complexes exist as dimers as indicated by HRMS studies, however, in the presence of 2 or more equivalents of H+ the dimers turned into monomers, ([LiVV = O]+ which exists in equilibrium with ([LiVV = OH]2+ and evidenced from the shift in λmax from 685 nm to 765 nm for complex 1 and 600 to 765 nm for complex 2. The complexes 1 and 2 efficiently catalyze the oxidative bromination of salicylaldehyde in the presence of H2O2 to produce 5-bromo-salicylaldehyde as the major product with TONs 405 and 450, respectively in the mixed solvent system (H2O:MeOH:THF = 4:3:2, v/v). The kinetic analysis of the bromide ion oxidation reaction indicates a mechanism which is first order in peroxidovanadium complex and bromide ion and limiting first-order on [H+]. The evaluated kBr and kH values are (8.82 ± 0.35) and (65.0 ± 2.23) M−1 s−1 for complex 1 and (6.74 ± 0.19) and (61.87 ± 2.27) M−1 s−1 for complex 2, respectively. The Ka of protonated species ([LiVV = OH]2+ are: Ka = (4.3 ± 0.40) × 10−3 (pKa = 2.37) and (4.7 ± 0.50) × 10−3 (pKa = 2.33) for complex 1 and 2 respectively. On the basis of the chemistry displayed by these model compounds, a mechanism of bromide oxidation and a tentative catalytic cycle have been framed which might be relevant to vanadium haloperoxidase enzymes and supported by DFT calculations.

Two series of ligand precursors, based on imidazo[1,2-a]pyridine and C2-phenyl substituted imidazole moieties, were developed and synthesized in high yields, featuring an N-CH2(C═O)Ar substituent on the imidazole ring. Upon reacting with palladium acetate, both series of ligands underwent double C–H bond activations at the methylene and o-aryl carbon sites on the N-CH2(C═O)Ar substituent, yielding C,C-type palladacycles bearing five-membered chelate rings. A dimeric palladium complex with bridging bromides was obtained from the ligand precursor with the bromide anion, whereas an ionic palladium complex with two “throw away” pyridine ligands was formed with the precursor of the tetrafluoroborate anion. All complexes are air-stable and were characterized by 1H and 13C NMR spectroscopy and elemental analysis. The structures of three of the new complexes were further established by single-crystal X-ray diffraction studies. These complexes have been screened for catalyzing Mizoroki–Heck coupling reaction using...

Four series of new push–push–pull chromophores based on two types of heterocyclic cores, namely imidazo[5,1,2-cd]indolizines and naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines, and two types of linear π spacers, namely diphenylacetylene and biphenyl, were prepared. The two isomeric heterocyclic compounds were prepared by palladium-catalyzed double C–H functionalization reactions. Further palladium-catalyzed C–C coupling reactions allow the construction of diphenylacetylene and biphenyl spacers on the chromophores. Different electron-donors including NMe2, NPh2, and OMe and CN acceptors were incorporated into the chromophores. The corresponding donor- and acceptor-free compounds were also prepared for comparative studies. In the case of chromophores with strong NMe2 or NPh2 donors, a strong intramolecular charge-transfer (ICT) process occurs leading to fluorescence emission quenching in polar solvents. The use of a slightly weaker OMe donor together with the naphtho[1′,2′:4,5]imidazo[1,2-a]pyridine core allows to obtain an intense blue fluorescence with high quantum yield of up to 0.825. DFT and TD-DFT calculations offer satisfactory explanations to the observed photophysical data.

An efficient recyclable catalyst system for Suzuki-Miyaura coupling reactions in water was developed based on a zwitterionic palladium complex bearing an imidazolium-based carbon donor and tricyclohexylphopshine. The complex served as an effective metal precursor for the formation of catalytically active palladium nanoparticles. The addition of excess n-tetrabutylammonium bromide (TBAB) was essential for stabilizing the nanoparticles in water. After several catalytic runs, a phase separation was observed and TBAB became an ionic liquid phase, supporting the palladium nanoparticles. The nanoparticles was recyclable, which allowed up to nine consecutive catalytic runs to be completed without significant loss of activity. The recyclable catalyst system allowed the use of unreactive aryl chlorides as substrates, changes of substrates between catalytic runs, manipulation of the reactions in air, and upscaling of the catalytic reaction.

An effective palladium-catalyzed direct arylation of 2-alkyl-imidazo[1,2-a]pyridine with alkynes towards imidazo[5,1,2-cd]indolizines through double C–H functionalization was developed. The catalyst system is an ionic palladium(II) complex bearing amido-functionalized N-heterocyclic carbene and triphenylphosphine ligands. A mere 2.5 mol-% loading of palladium was sufficient for catalyzing the reactions. Copper acetate and n-tetrabutylammonium bromide were employed as oxidant and phase-transfer catalyst, respectively. Beside imidazo[1,2-a]pyridine, desirably, less electron-rich derivatives could be employed as coupling partners. Also a wide range of diarylalkynes was used. The structure of one 2-alkyl-imidazo[5,1,2-cd]indolizine was established by single-crystal X-ray diffraction analysis. UV and PL spectra of the compounds confirmed that some are highly fluorescent. The method provides an easy synthetic route for the preparation of these new fluorescent materials.

The mono- and dinuclear oxidovanadium(V) complexes [VVO(L1)(Cl)] (1) and [L1VVO(μ2-O)VO(L1)] (2) of ONNO donor amine-bis(phenolate) ligand (H2L1) were readily synthesized by the reaction between H2L1 and VCl3.(THF)3 or VO(acac)2 in MeOH or MeCN, respectively, and then characterized through mass spectroscopy, 1H-NMR and FTIR techniques. Both the complexes possess distorted octahedral geometry around each V centre. Upon the addition of 1 equivalent or more acid to a MeCN solution of complex 1, it immediately turned into the protonated form, which might be in equilibrium as: [L1ClVVOH]+ ↔ [L1ClVV–OH]+ (in the case of [L1ClVVOH]+ oxo-O is just protonated, whereas in [L1ClVV–OH]+ it is a hydroxo species), with the shift in λmax from 610 nm to 765 nm. Similar was the case for complex 2. The complexes 1 and 2 could efficiently catalyze the oxidative bromination of salicylaldehyde in the presence of H2O2 to produce 5-bromo salicylaldehyde as the major product with TONs of 405 and 450, respectively, in the mixed solvent system (H2O : MeOH : THF = 4 : 3 : 2, v/v). The kinetic analysis of the bromide oxidation reaction indicated a first-order mechanism in the protonated peroxidovanadium complex and a bromide ion and limiting first-order mechanism on [H+]. The evaluated kBr and kH values were 5.78 ± 0.20 and 11.01 ± 0.50 M−1 s−1 for complex 1 and 6.21 ± 0.13 and 20.14 ± 0.72 M−1 s−1 for complex 2, respectively. The kinetic and thermodynamic acidities of the protonated oxido species of complexes 1 and 2 were pKa = 2.55 (2.35) and 2.16 (2.19), respectively, which were far more acidic than those reported by Pecoraro et al. for peroxido-protonation instead of oxido protonation. On the basis of the chemistry observed for these model compounds, a mechanism of halide oxidation and a detailed catalytic cycle are proposed for the vanadium haloperoxidase enzyme and these were substantiated by detailed DFT calculations.

A series of new Pd(0) complexes of N -heterocyclic carbene (NHC) ligands with dangling NMe(C O) functionalities (L Me ) is reported, with yields in the range 56–73%. The reactivity of one of the Pd(0) complexes, bearing bulky N -mesityl groups, towards the oxidative addition of pentafluoroiodobenzene was studied, revealing the formation of decafluorobiphenyl, a diiodo Pd(II) NHC complex and an intriguing Pd(II) complex with four carbon-donor ligands coordinated to the metal center. The latter complex contains two new NHC ligands derived from L Me . One of the NHC ligands is a bidentate ligand, resulting from an addition reaction between L Me and the coordinated maleic anhydride. The other NHC ligand is derived from the N -protonation of L Me . A pentafluorophenyl ligand completes the fourth coordination site. A mechanism for the formation of this intriguing Pd(II) complex is proposed. The structures of these two new Pd(II) complexes were established by single-crystal X-ray diffraction studies. The catalytic activities of the new Pd(0) complex with L Me in the Mizoroki–Heck coupling reaction between 4-chloroacetophenone and styrene were compared and they contrast with those reported for the Pd(0) NHC complex with dangling NH(C O) groups (L H ).

A series of new imidazolium salts was synthesized by simple quaternization reactions of 1-benzyl-1H-imidazole or 1-benzyl-2-methyl-1H-imidazole and their derivatives with 2-chloro-N-(pyridin-2-ylmethyl)acetamide. These resulting imidazolium salts were successfully employed as ligand precursors for the syntheses of novel palladium(II) complexes bearing tridentate ligands of N-heterocyclic carbene, amidate, and pyridine donor moieties. Simple tuning of the reaction conditions allowed selective coordination of the N-heterocyclic carbene moiety in either “normal” or “abnormal” binding modes. An isomeric pair of palladium complexes with “normal” and “abnormal” N-heterocyclic carbenes was successfully characterized by single-crystal X-ray diffraction studies. Theoretical calculations indicated that the palladium complex with “normal” NHC coordination was more thermally stable than its isomeric complex with “abnormal” NHC binding. The potential of these two sets of palladium complexes as anticancer drugs was eva...

New ligand precursors with different pendent alky chain length were obtained in a single step by reacting appropriate 1-alkyl-2-methyl-1H-imidazoles with chloroacetone in THF at 80 °C overnight. Zwitterionic palladium complexes were prepared by reacting PdCl2 with these new imidazole derivatives in DMF with the presence of pyridine and Cs2CO3 as base at room temperature for 5 h. These new compounds were characterized by 1H and 13C{1H} NMR spectroscopy, elemental analysis, and electrospray ionization mass spectrometry. They were effectively applied as recyclable catalysts for mediating Mizoroki–Heck reactions in ionic liquids. Transmission emission microscopy revealed that palladium nanoparticles were formed during the catalytic process.

A new ionic salt, cis-[Pd(L)(PPh3)(H2O)2](SO3CF3)2 (A), in which L is an amido-functionalized N-heterocyclic carbene (NHC) ligand, has been prepared by the salt metathesis reaction between [Pd(L)(PPh3)Cl2] and AgOTf in wet acetonitrile. It was successfully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction analysis. Complex A was found to be effective in catalyzing the double C–H functionalization of various nitrogen- and sulfur-containing heteroarenes to form fused polycyclic heteroaromatic compounds with alkynes in the presence of Cu(OAc)2 and tetra-n-butylammonium bromide. Indoles, thiophenes, imidazo[1,2-a]pyridine, and imidazoles were used as substrates. In particular, intriguing imidazo[5,1,2-cd]indolizine compounds could be obtained under the conditions employed.

Three new two-dimensional coordination polymers were constructed from 2,6-bis(imidazole-1-yl)pyridine (2.6-bip), terephthalic acid (1,4-bdcH 2 ), and M(NO 3 )∙nH 2 O (M = Co, Cd). [M(2,6-bip)(1,4-bdc)] ( 1 : M = Co; 2 : M = Cd) were prepared under hydrothermal condition, whereas [Cd(2,6-bip)(1,4-bdc)∙ x DMF] ( 3 ) was obtained under solvothermal condition in DMF solvent. Single-crystal X-ray analyses revealed the first example of a 2D 4-connected net of 6 6 topology in 1 and 2 and a rare 2D 4,5-connected net of 4,5 L20 topology in 3 . All these complexes were thermally robust and emissive in solid-states. This work confirmed the versatility of using hingelike 2,6-bip as a linker and the importance of solvent effect in the process of framework formation.

A series of new imidazolium chlorides were straightforwardly prepared from the reactions between chloroacetone and imidazole derivatives. Deprotonation of the methylene proton next to the ketone group in these salts by pyridine led to the formation of a monodentate ligand that coordinated to palladium, readily forming zwitterionic anionic palladium pyridine complexes bearing a formal positive charge on the ligand ancillary. The pyridine ligand in the zwitterionic complexes can be facilely replaced by phosphine ligands. Seven of these new complexes were successfully characterized by X-ray crystallography. The zwitterionic phosphine complexes were highly efficient in catalyzing room-temperature Suzuki–Miyaura reactions between sterically hindered aryl chlorides and arylboronic acids in an aqueous medium.

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The preparation of a series of ligand precursors that feature imidazo[1,2-a]pyridine and amido functional groups is reported. The reactions between the new ligand precursors, pyridine, and palladium acetate in DMF afforded monodentate PdII abnormal (mesoionic) carbene complexes. An isomeric PdII normal carbene complex with a formally identical steric environment to that of the abnormal carbene counterpart was also prepared. The isomeric pairs were characterized by X-ray structural studies. Under different complexation conditions, in which PdCl2 was employed as metal precursor, a zwitterionic CC-type palladacycle formed by the C–H activation at the ortho-N-phenyl and methylene carbon atoms was obtained. The use of a similar ligand precursor with an NH instead of an NMe group produced a zwitterionic palladalactam complex. In contrast to the previous work in the literature, owing to the rigidity of the fused heterocyclic ring, the palladalactam complex did not undergo intramolecular proton transfer through the coordinated N atom. Overall, we demonstrated that proper modification of the structures of ligand precursors and complexation conditions allowed us to obtain a full range of intriguing PdII complexes including normal and abnormal carbene complexes, and zwitterionic palladalactam and CC-type palladacycles by means of A–H bond (A = N, C) activations.

Solvothermal syntheses involving the hybrid ligands {3-pyridin-3-ylimidazo[1,2-a]pyridine (L1) and 3-pyridin-4-ylimidazo[1,2-a]pyridine (L2)}, aromatic polycarboxylates [1,4-benzenedicarboxylate (1,4-bdc), 1,3-benzenedicarboxylate (1,3-bdc), and 1,3,5-benzenetricarboxylate (1,3,5-btc)], and transition metal ions [Ni(II) and Zn(II) ions] afforded five new coordination polymers, namely, [Ni(1,4-bdc)(L1)(H2O)2]n (1), [Ni2(1,3-bdc)2(L1)2(H2O)]n (2), [Zn2(1,4-bdc)1.5(L1)(OH)]n (3), and [Zn2(1,3,5-btc)(L1)O0.5(H2O)0.5·0.5H2O]n (4), and [Zn2(1,3,5-btc)(L2)(OCOH)(H2O)]n (5). A diversity of net topologies was observed including a rare 2D 4-connected net with the point symbol of (44·63·8) in 1, a dia net in 2, a twofold interpenetrating pcu net in 3, a rare 3D (3,8)-connected binodal 3,8T35 net in 4, and sra and rtl nets in 5. Four different secondary building units were involved in the construction of the 3D nets. The three Zn(II) complexes exhibit photoluminescence.

Two oxidovanadium(V) complexes, [VVO(L1)(OMe)] (1) and [VVO(L2)(OMe)] (2), with ONNO donor ethylenediamine-bis(phenolate) ligands (H2L1 and H2L2) have been readily synthesized by the reaction between the ligand precursors and VOSO4·5H2O in MeOH, and characterized by physico-chemical techniques and single crystal X-ray diffraction studies. Both complexes 1 and 2 are hexa-coordinated with a pseudo-octahedral geometry in an N2O4 coordination environment and are found to catalyze the oxidation of toluene to benzoic acid and isomers of xylene to the corresponding hydroxy acids, with turnover numbers (TON) over 200, except for o-xylene. On changing the ligand fragments and coordination geometry around the metal centre there is an improvement in the catalytic efficiency, selectivity and also TON of the reaction as compared with the previously reported systems. The reactions were monitored using 51V NMR spectroscopy. Based on mass spectra analysis and 51V NMR studies, the mechanism of the catalytic process has been proposed, employing the formation of oxido-hydroperoxido/hydroxido-peroxido [V(ONNO)(O)(OOH)] ⇔ [V(OH)(ONNO)(O2)] intermediates.