Imino Pyridine Ligands Research Articles

Crystal structure of {N-[(6-bromo-pyridin-2-yl)(phen-yl)methyl-idene]-2,6-di-methyl-aniline-κ2N,N'}di-chlorido-zinc di-chloro-methane hemisolvate.

The solvated title compound, [ZnCl2(C20H17BrN2)]·0.5CH2Cl2, comprises a bidentate imino-pyridine ligand and two Cl atoms bound to a zinc2+ cation in a distorted tetra-hedral arrangement. The chelate bond lengths are consistent with localized C=N double bonds and a C-C single bond, as expected for an unreduced ligand bound to a closed-shell transition metal cation. Apart from weak nonclassical C-H⋯Cl hydrogen bonds between the complex mol-ecules and the disordered solvent mol-ecules (occupancy = 0.5), no further significant inter-molecular inter-actions are observed.

Heterometallic Cu(I)/Ru(II) halide and pseudohalide hybrid complexes composed of coordination and organometallic sites: Synthesis, spectroscopic characterization, electrochemical and luminescence properties

A new series of heterometallic halide/pseudohalide hybrid complexes of the type [Cu(L1)NC5H4–CC–C6H3–1,3-{CC-trans-RuCl(dppe)2}2X] (1a–d) and [Cu(L2)NC5H4–CC–C6H3–1,3-{CC-trans-RuCl(dppe)2}2X] (2a–d) have been prepared by the reaction of [NC5H4–CC–C6H3–1,3-{CC-trans-RuCl(dppe)2}2] (1) with copper(I) salts in presence of L1 and L2 (where L1=4-methoxy-N-(2-pyridylmethylene) phenylamine and L2=4-nitro-N-(2-pyridylmethylene)phenylamine; X=Cl, I, N3, and NCS) in CH2Cl2/MeOH mixture. Thermal stability of 1a–d and 2a–d has been investigated by using thermogravimetric analysis (TGA). The electrochemical behaviour of 1a–d and 2a–d reveals that all complexes display a quasireversible redox behaviour corresponding to Cu(I)/Cu(II) and Ru(II)/Ru(III) couples. The room temperature luminescence is observed for all complexes in dichloromethane solution indicating that the emission wavelength is finely tuned by the variation of the substituent group on iminopyridine ligand and quenching ability of halides and pseudohalides in the complexes. Further, the excited state of 1a–d and 2a–d are stabilized and red shifted in their emission wavelength with increasing polarity of the solvents.

Rhenium(I) Triscarbonyl Complexes with Redox-Active Amino- and Iminopyridine Ligands: Metal–Ligand Cooperation as Trigger for the Reversible Binding of CO2 via a Dearmomatization/Rearomatization Reaction Sequence

We describe rhenium(I) triscarbonyl compounds (3 and 4) decorated with simple-to-use 2-iminomethyl-pyridine (1, impy) and 2-aminomethyl-pyridine (2, ampy) ligands, respectively, which can serve as cooperative ligand scaffolds enabling CO2 binding via a formal [1,3] addition under Re–O and C–C bond formation. fac-[Re(impy)(CO)3Br] (3) is readily prepared by stirring (1-(pyridin-2-yl)-N-(p-tolyl)methanimine (impy, 1) and [Re(CO)5Br] in refluxing THF. Alternatively, complex 3 can be readily obtained when a mixture of [Re(CO)5Br], p-toluidine, and picolinaldehyde is refluxed in ethanol. Complex 3 is reduced with excess potassium metal in THF (two-electron reduction) to give the anionic amido complex K[Re(amidopy*)(CO)3] (5b, the asterisk indicates the dearomatized ligand). Analysis of the 1H and 13C{1H} NMR spectra of 5b suggest the dearomatization of the pyridine unit. Complex 5b is highly reactive and gives rise to the facile [1,3] addition of CO2. The addition of the CO2 and thus the formation of K[Re(amidopy-COO)(CO)3] (6) is characterized by the concomitant formation of a Re–O and a C–C bond. The addition is triggered by the rearomatization of the pyridine unit in 6. Remarkably, isotopic labeling experiments involving 13CO2 suggest a reversible binding of CO2 to the complex. The related amine complex fac-[Re(ampy)(CO)3Br] (4) is similarly prepared by stirring (4-methyl-N-(pyridin-2-ylmethyl)aniline (2) and [Re(CO)5Br] in THF at 60 °C. Upon addition of excess base (LiHMDS), complex 3 is readily deprotonated twice to give likewise the anionic amido complex Li[Re(amidopy*)(CO)3] (5a). The latter reaction gives rise to a facile access to the reactive dearomatized anionic fragment [Re(amidopy*)(CO)3]− with no need for the application of strong reducing agents. The ion pair M+/[Re(amidopy*)(CO)3]− is highly reactive and combines MLC (metal–ligand cooperation) via a dearomatization/rearomatization scheme and bifunctional reactivity enabled by the nucleophilic nature of the Re complex and the Lewis acidic counter alkali cation.

Optoelectronic properties of four azobenzene-based iminopyridine ligands for photovoltaic application

Because of organic π-conjugated materials’ optoelectronic properties and potential applications in a wide range of electronic and optoelectronic devices, such as organic solar cells, these materials, including both polymers and oligomers, have been widely studied in recent years. This work reposts a theoretical study using the DFT method on four azobenzene-based iminopyridines. The theoretical ground-state geometry, electronic structure and optoelectronic parameters (highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, open-circuit voltage (Voc) and oscillator strengths (O.S)) of the studied molecules were obtained using the density functional theory (DFT) and time-dependent (TDDFT) approaches. The effects of the structure length and substituents on the geometric and optoelectronic properties of these materials are discussed to investigate the relationship between the molecular structure and the optoelectronic properties. The results of this study are consistent with the experimental ones and suggest that these materials as good candidates for use in photovoltaic devices.

Divergent reactivity of a new dinuclear xanthene-bridged bis(iminopyridine) di-nickel complex with alkynes.

The reaction of a dinucleating bis(iminopyridine) ligand L bearing a xanthene linker (L = N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni2(COD)2(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene) leads to the formation of a new dinuclear complex Ni2(L)(DPA). Ni2(L)(DPA) can also be obtained in a one-pot reaction involving Ni(COD)2, DPA and L. The X-ray structure of Ni2(L)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features NiI centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni2(L)(DPA) with one equivalent of ethyl propiolate (HCCCO2Et) forms the Ni2(L)(HCCCO2Et) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerised products by 1H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni2(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5 : 1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerisation products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.

Reductive Cyclization of 1,6- and 1,7-Enynes Catalyzed by Iron Complexes

Iron-catalyzed reductive cyclization of 1,6- and 1,7-enynes was demonstrated by using an oxazoline iminopyridine ligand. Alcohol, ketone, ester, ether, halide, amine, amide, imine, nitrile, silyl, and alkyne groups are tolerated under the mild reaction conditions. A speculative mechanism is proposed on the basis of deuteration studies. A primary enantioselective transformation was also conducted.

Synthesis and (spectro)electrochemical investigations of coordinatively-saturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates

Complexes with the CpRu(PPh3) fragment bound by iminopyridine ligands functionalised by a Hantzsch dihydropyridine donor of hydride ion or by a Hantzsch pyridinium acceptor of hydride ion have been prepared, and their redox chemistry studied by cyclic voltammetry and EPR and UV–Vis spectroelectrochemical investigations. These Ru(II) complexes have a coordinatively saturated, electronically precise (18-electron) ruthenium(II) centre with a non-labile ligand donor set, which suppresses complicating metal-centred reactivity and, thereby, allows the baseline physicochemical properties of the Hantzsch dihydropyridine/pyridinium-functionalised ligands to be investigated. In Ru(II) complexes, the iminopyridine chelate is linked to the Hantzsch pyridine groups by either an ortho-phenyl bridge (electronically delocalized) or by a meta-phenyl bridge (electronically isolated), which leads to notable differences in spectroscopic properties, even for ruthenium centre, and differences in redox reactions. Of note, the primary electrochemical reduction of the Ru(II) complexes with a Hantzsch pyridinium substituent is centred on this group, but did not afford the corresponding Ru(II) complexes with a 1,4-dihydropyridine substituent. Rather it was found that the reduction products were identical to the 1:1 hydroxide adducts formed upon addition of hydroxide ion to the starting Hantzsch pyridinium-substituted Ru(II) complexes. Based on these results and comparisons with data from the literature, the reduction products and hydroxide adducts are tentatively assigned as the corresponding hydroxy-dihydropyridine substituted Ru(II) complexes (during reduction, hydroxide ion was likely formed from the residual water present in the acetonitrile solvent). Implications for the electrochemical cycling of transition metal catalysts with Hantzsch pyridinium/dihydropyridine functional substituents are considered.

Synthesis, characterization and antimicrobial properties of lipophilic palladium complexes bearing iminopyridine ligands

The reaction of [PdCl2(η2-coe)]2 (coe = cis-cyclooctene) with nine lipophilic iminopyridine ligands (NN′, 1a–i) afforded the corresponding metal complexes cis-[PdCl2(NN′)] (2a–i) with concomitant loss of the labile coe ligands. All iminopyridine complexes were characterized by physicochemical and spectroscopic methods. The molecular structures of two complexes, 2a and 2b, were determined by single crystal X-ray diffraction studies. All complexes were tested for their potential antifungal and antibacterial activities.

Synthesis, characterization and photoreactivity of rhenium and molybdenum carbonyl complexes with iminopyridine ligands

In this report a series of novel rhenium tricarbonyl and molybdenum tetracarbonyl complexes containing pyridine-based 1,2-diimine ligands with low-lying π-acceptor orbitals is presented. All compounds display well-resolved chromophoric metal-to-ligand charge transfer (MLCT) bands in the visible spectral region. The rhenium(I) complexes (pyca-2,6-iPr2C6H3)Re(CO)3Cl (1), (pyca-2,6-Me2C6H3)Re(CO)3Cl (2), as well as the molybdenum(0) complexes (pyca-2,6-iPr2C6H3)Mo(CO)4 (3), (pyca-2,6-Me2C6H3)Mo(CO)4 (4) and (pyca-2,4,6-Me3C6H2)Mo(CO)4 (5) derived from pyridine-2-carboxaldehyde (pyca) are characterized by various spectroscopic methods. Solid state structures of the complexes 1–5 determined by X-ray crystallography are described. While the rhenium complexes were found to be weakly luminescent both at 77 K and at 298 K as solid materials or in solution, no luminescence could be detected at 298 K for the molybdenum carbonyl derivatives. Spectral changes indicating ligand substitution processes observed upon steady-state irradiation of the compounds under full Xe-lamp light exposure and upon selective long-wavelength excitation in the MLCT-band region are also reported and briefly discussed.

Crystal structure of di-μ-chlorido-bis(chlorido-{N (1)-phenyl-N (4)-[(pyridin-2-yl-κN)methyl-idene]benzene-1,4-di-amine-κN (4)}mercury(II)).

The whole mol­ecule of the title complex, [Hg2Cl4(C18H15N3)2], is generated by inversion symmetry. It was synthesized from the pyridine-derived Schiff base N-phenyl-N′-[(pyridin-2-yl)methyl­idene]benzene-1,4-di­amine (PPMBD). The five-coordinated Hg2+ ions have a distorted square-pyramidal environment defined by two N atoms, viz. the imine and the other pyridyl [Hg—N = 2.467 (6) and 2.310 (6) Å, respectively] belonging to the bidentate imino­pyridine ligand, and three Cl atoms [Hg—Cl = 2.407 (2), 2.447 (2) and 3.031 (2) Å]. The longest Hg—Cl bond is bridging about the inversion centre. In the ligand, the central ring and pyridine ring are oriented at a dihedral angle of 8.1 (4)°, while the planes of the pyridine ring and the terminal phenyl ring are oriented at a dihedral angle of 53.8 (4)°. In the crystal, mol­ecules are linked by N—H⋯Cl and C—H⋯Cl hydrogen bonds, forming sheets parallel to (001).

Azo-based iminopyridine ligands: synthesis, optical properties, theoretical calculations and complexation studies

We describe herein the full characterization of azobenzene based iminopyridine ligands (L1–L4) synthesized by a condensation reaction between N,N-Dimethyl-4,4′-azodianiline or 4-(4-nitrophenylazo)aniline and 2-pyridinecarboxaldehyde or 2,6-pyridinedicarboxaldehyde. The UV–visible absorption bands of these ligands were fully assigned using DFT and TD-DFT computations. The complexation of ligand L1 and ligand L2 with AgNO3 afforded two neutral silver metal complexes formulated as [AgL1NO3] and [AgL2NO3], respectively. The crystal structure analysis of the two complexes indicate the presence of a Y-shaped tricoordinated silver (I) ion for [AgL1NO3] and a tetracoordinated silver(I) ion displaying rather rare and distorted square planar geometry for [AgL2NO3]. In solid state, for both complexes, a 3D supramolecular architecture is generated via hydrogen bonds of type C–HċO and C–Hċπ. The UV–visible spectrophotometric titration studies of ligands L1–L4, by increasing amount of AgNO3 or of ZnCl2 indicates the possibility of forming transition metal complexes with these ligands.

1,4-Functionalization of 1,3-Dienes With Low-Valent Iron Catalysts

Iron-catalyzed or -mediated transformations of organic substrates have been important throughout the development of organic chemistry due to iron's abundance, low cost, and favorable toxicity profile. Highly reduced iron species, although difficult to isolate and characterize, have proven valuable as catalysts for a variety of C-C and C-heteroatom bond forming processes, as well as cyclization and cycloisomerization reactions. We have developed iminopyridine-ligated low-valent iron catalysts that facilitate selective 1,4-hydrovinylation, hydoboration, hydrosilylation, and polymerization of 1,3-dienes. The catalysts are generated in situ from iron(II) precursors in the presence of activated magnesium metal or trialkylaluminum reductants. The 1,4-addition processes provide access to valuable products such as 1,4-dienes, allylboronic esters, allylsilanes, and highly regioregular polyisoprene. In these transformations, addition is stereoselective, providing (E)-alkene isomers selectively, and (1,2)-addition products are generally not observed. Moreover, modification of steric bulk on the iminopyridine ligand can be used to change selectivity for (1,4)- versus (4,1)-addition to dienes with nonsymmetric substitution. Access to low-valent iron precursor complexes is limited, and we have developed a diaryliron(II) precursor that undergoes smooth reductive elimination in the presence of iminopyridine ligands to provide easy access to low-valent iron catalysts without the use of heterogeneous reductants, which complicate the isolation and study of low-valent iron complexes. We obtained crystal structures of our iron(II) catalyst precursor and an iminopyridine-ligated reduced iron species generated from it. Spectroscopic analysis suggests that although this species is formally iron(0), the redox-active iminopyridine ligands accept electron density from the metal and the complex is more properly formulated as iron(II) coordinated by two radical-anion ligands. We believe that a closely-related set of reaction manifolds is responsible for the 1,4-functionalization reactivity displayed by the iron(iminopyridine) complexes (see text). Kinetics experiments and deuterium-labeling studies provide evidence for the proposed catalytic cycle. The geometry of the double bond remaining after 1,4-addition is set by the requirement that the diene bind to the iron center in an s-cis geometry, and the regioselectivity of addition can be rationalized by the location of steric bulk on the iminopyridine ligand. The transformations presented in this Account utilize iron catalysts to provide access to valuable diene 1,4-addition products such as 1,4-dienes, allylboronate esters, and allylsilanes, as well as highly regioregular polyisoprene. The development of a stable diaryliron(II) precatalyst, structural characterization of an iminopyridine-ligated iron(0) complex, and mechanistic insights into the selective nature of this transformation provide a window into the reactivity profile of low-valent iron.

Synthesis, characterization, and mechanism studies of bis(imino)pyridine ligands and their Cr(III) compounds

Two new bis(imino)pyridine ligands L1, L2 (L1 = 2,6-bis[1-(4-bromophenylimino)]pyridine, L2 = 2,6-bis[1-(4-methylphenylimino)]pyridine) have been prepared. Their compounds [CrCl3L1] (I) and [Cr(Cl)(μ-Cl)(L2′)]2 (II) (L2′ = 6-bis(6-methylquinoline)pyridine) for olefin oligomerization were obtained by hydrothermal methods, during which L2′ was appeared from L2, and characterized by elemental analysis, single-crystal X-ray analysis (CIF files CCDC nos. 1025448 (I),1025447 (II)), IR spectra and melting point test. Compound I is a mononuclear compound, while compound II, bridged with the chlorine atoms, is a binuclear compound. The transformation mechanism between L2 and L2′ is also studied, and the possible reason is due to the presence of the electron-donating groups as substituents on L2. The subsequent studies on that will be carried on.

ChemInform Abstract: Cu‐Iminopyridine Complexes as Catalysts for Carbene and Nitrene Transfer Reactions.

Copper complexes with chiral iminopyridine ligands were screened for their catalytic efficiency in carbene (cyclopropanation) and nitrene transfer reactions (aziridination, CH amidation). Both pre-formed and in situ formed complexes were considered. The results highlighted the poor catalytic efficiency of these complexes in cyclopropanation reactions employing methyl phenyldiazoacetate as the carbene source, whereas better results were obtained in nitrene transfer reactions, particularly in the amidation of CH bonds, albeit the enantioselectivity of the reactions was negligible in nearly all cases. Finally, copper complexes were also found to promote an interesting oxidative functionalization of alkynes with PhI(OAc)2 at room temperature. Copyright © 2014 John Wiley & Sons, Ltd.

Electro- and photocatalytic hydrogen evolution by a cobalt complex based on a tripodal iminopyridine ligand

A cobalt complex based on the tripodal iminopyridine type ligand (2E)-2-N-(pyridin-2-ylmethylidene)-1-N,1-N-bis({2-[(E)-(pyridin-2-ylmethylidene)amino]phenyl})benzene-1,2-diamine (L1), namely, [CoL1](BF4)2 (1a), was synthesized and characterized by FT-IR, UV–Vis spectroscopy, magnetic moment and elemental analysis. X-ray structural study of 1a shows that the complex is coordinatively saturated and contains the uncoordinated bridgehead nitrogen atom. Electrochemical studies reveal that the complex 1a has an electrocatalytic proton reduction activity in acetonitrile, when using acetic acid as a proton source with icat/ip∼3.3. Therefore we estimate that the complex 1a could give the highest TOF of 2.3s−1 for the reduction of proton to hydrogen. Furthermore, under visible-light irradiation, an optimized H2 evolution system containing 1a (0.1mM), [Ir(ppy)2(bpy)](PF6) (0.1mM), and 10vol% TEA in CH3CN–H2O (1/1, v/v) mixed solvents at pH 10 gives a highest turnover number of 34. For comparison, an electron-withdrawing ester group (CO2CH2CH3) is introduced into the pyridine moiety of the ligand as in L2, however, which leads to its corresponding cobalt complex 2a less active than the complex 1a for electro- and photocatalytic H2 evolution. Additionally, the control experiment establishes that the isostructural zinc complex [ZnL1](BF4)2 (1b) is ineffective for H2 generation.

ChemInform Abstract: New Bis(imino)pyridine Complexes of Iron(II) and Iron(III), and Their Catalytic Activity in the Mukaiyama Aldol Reaction.

New iron(II) and iron(III) complexes bearing bis(imino)pyridine ligands were synthesized and successfully applied to the Mukaiyama aldol reaction. The two complexes [FeCl2 L] (L = bis(imino)pyridine ligand, 55% isolated yield) and [LFe(μCl)3FeCl3] (76%) were obtained employing FeCl2 and FeCl3 iron sources, respectively, and characterized by elemental analyses, mass spectrometry, IR spectroscopy and, one example, by X-ray diffraction. The two new iron complexes were subsequently employed as catalysts in the Mukaiyama aldol reaction after abstraction of two chlorides by AgSbF6 to obtain the aldol products in 43% to virtually quantitative yield (CH2Cl2 solvent, room temperature, 3.5 to 16 h reaction time). The impact of the oxidation state of the iron center on the reaction rate and the diastereomeric ratios of the products was investigated.

Cu–iminopyridine complexes as catalysts for carbene and nitrene transfer reactions

Copper complexes with chiral iminopyridine ligands were screened for their catalytic efficiency in carbene (cyclopropanation) and nitrene transfer reactions (aziridination, CH amidation). Both pre‐formed and in situ formed complexes were considered. The results highlighted the poor catalytic efficiency of these complexes in cyclopropanation reactions employing methyl phenyldiazoacetate as the carbene source, whereas better results were obtained in nitrene transfer reactions, particularly in the amidation of CH bonds, albeit the enantioselectivity of the reactions was negligible in nearly all cases. Finally, copper complexes were also found to promote an interesting oxidative functionalization of alkynes with PhI(OAc)2 at room temperature. Copyright © 2014 John Wiley & Sons, Ltd.

Zinc Induced a Dramatic Enhancement of the Nonlinear Optical Properties of an Azo-Based Iminopyridine Ligand

An azobenzene based iminopyridine ligand 1 has been synthesized by a condensation reaction between N,N-dimethyl-4,4′-azodianiline and 2-formylpyridine. The complexation of the ligand with ZnCl2 afforded a tetrahedral neutral zinc metal complex (2) formulated as [(Azo-Imino-Py)ZnCl2]. The crystal structure analysis of the complex indicates the formation of a tetracoordinated zinc(II) ion with the expected tetrahedral geometry. In solid state, a 2D supramolecular architecture reinforced by π–π stacking and hydrogen bonding is observed. The UV–visible spectrophotometric titration studies of the ligand, by increasing amount of ZnCl2, confirm the existence of single complex species in solution. The nonlinear optical (NLO) investigations by Z-scan technique of both ligand 1 and complex 2 indicate a dramatic enhancement of the NLO absorption of the ligand upon complexation with ZnCl2. This enhancement is likely to be due to a large dipole moment found for the complex 2 as calculated by DFT/B3LYP methodology.

Synthesis and application of new iminopyridine ligands in the enantioselective palladium-catalyzed allylic alkylation

A variety of iminopyridines were obtained by condensation of chiral amines with pyridine-2-carboxaldehyde and quinoline-8-carbaldehyde, or of aminoalkylpyridine derivatives with chiral ketones. These ligands were assessed in the enantioselective palladium catalyzed allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate affording the product dimethyl 1,3-diphenylprop-2-enylmalonate in good yields and moderate enantioselectivities (up to 62% ee). Catalytic activity and enantioselectivity were found to be highly dependent upon the steric properties of the ligands. The best enantioselectivity (62% ee) was obtained by an iminopyridine based on a camphane skeleton.

Recyclable chemosensor for oxalate based on bimetallic complexes of a dinucleating bis(iminopyridine) ligand

Herein we describe bimetallic di-nickel and di-copper complexes [Ni2(L)Br4] (1) and [Cu2(L)Br4(NCMe)2] (2) (L = (1E,1'E)-N,N'-(1,4-phenylenebis(methylene))bis(1-(6-(2,4,6-triisopropylphenyl)pyridin-2-yl)methanimine)) that bind oxalate intramolecularly to form [Ni2(L)Br2(C2O4)(NCMe)] (3) and [Cu2(L)Br2(C2O4)] (4). For the di-nickel complex 1, oxalate incorporation is accompanied by a significant colour change, from red-pink (1) to deep green (3). Mass spectrometric experiments demonstrate that the compound 1 is selective for oxalate versus related mono- and di-carboxylates tested. Oxalate can be released by the addition of slight excess of calcium bromide that forms insoluble calcium oxalate and restores the original Ni2(L)Br4 species. The product of the oxalate release was crystallized as [Ni2(L)Br4]·CaBr2(THF)4 species.