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Coordination Chemistry with Quinaldinate and Secondary Amines

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Coordination Chemistry with Quinaldinate and Secondary Amines molecules Article Beyond the Simple Copper(II) Coordination Chemistry with Quinaldinate and Secondary Amines Barbara Modec * , Nina Podjed * and Nina Lah y Department of Chemistry and Chemical Technology, University of Ljubljana, Veˇcnapot 113, 1000 Ljubljana, Slovenia * Correspondence: [email protected] (B.M.); [email protected] (N.P.) Current address: Lek Pharmaceuticals, 1000 Ljubljana, Slovenia; [email protected]. y Academic Editor: Catherine Housecroft Received: 18 March 2020; Accepted: 28 March 2020; Published: 30 March 2020 Abstract: Copper(II) acetate has reacted in methanol with quinaldinic acid (quinoline-2-carboxylic acid) to form [Cu(quin) (CH OH)] CH OH (1) (quin = an anionic form of the acid) with 2 3 · 3 − quinaldinates bound in a bidentate chelating manner. In the air, complex 1 gives off methanol and binds water. The conversion was monitored by IR spectroscopy. The aqua complex has shown a facile substitution chemistry with alicyclic secondary amines, pyrrolidine (pyro), and morpholine (morph). trans-[Cu(quin)2(pyro)2](2) and trans-[Cu(quin)2(morph)2](4) were obtained in good yields. The morpholine system has produced a by-product, trans-[Cu(en)2(H2O)2] (morphCOO)2 (5) (morphCOO− = morphylcarbamate), a result of the copper(II) quinaldinate reaction with ethylenediamine (en), an inherent impurity in morpholine, and the amine reaction with carbon dioxide. (pyroH)[Cu(quin)2Cl] (3) forms on the recrystallization of [Cu(quin)2(pyro)2] from dichloromethane, confirming a reaction between amine and the solvent. Similarly, a homologous amine, piperidine (pipe), and dichloromethane produced (pipeH)[Cu(quin)2Cl] (11). The piperidine system has afforded both mono- and bis-amine complexes, [Cu(quin)2(pipe)] (6) and trans-[Cu(quin)2(pipe)2](7). The latter also exists in solvated forms, [Cu(quin) (pipe) ] CH CN (8) and [Cu(quin) (pipe) ] CH CH CN (9). Interestingly, only the 2 2 · 3 2 2 · 3 2 piperidine system has experienced a reduction of copper(II). The involvement of amine in the reduction was undoubtedly confirmed by identification of a polycyclic piperidine compound 10, 6,13-di(piperidin-1-yl)dodecahydro-2H,6H-7,14-methanodipyrido[1,2-a:10,20-e][1,5]diazocine. Keywords: copper(II) complexes; quinaldinate; secondary amines; pyrrolidine; morpholine; piperidine; reduction; crystal structure 1. Introduction The coordination chemistry of copper is very rich because of its biological roles [1–3] and diverse practical applications, e.g., as catalysts, fungicides, and pesticides [4]. The chemistry of no other transition metal surpasses that of divalent copper with N- and O-donor ligands [4]. The reactivity of copper in its metalloenzymes and proteins rests mostly in its redox-active character, as they are involved in electron transfer, oxygen transport, and oxidation of important substrates such as amines, L-ascorbic acid, galactose, etc. [4,5]. Under oxidizing conditions in the cell, copper exists as Cu2+, whereas reducing conditions favor the Cu+ form. Different behavior of the two states may be traced to differences in their hardness: whereas the Cu2+ ion is classified as a borderline Lewis acid, the reduced counterpart, the Cu+ ion, is an exemplary soft acid [6]. Hard N- and O-donors dominate the coordination chemistry of divalent state, whereas the Cu+ ion favors ligands with soft donor atoms, such as phosphorus, sulfur, or iodine. Because of the spherical symmetric d10 configuration, the Cu+ ion lacks any LFSE and it has, as a consequence, no preference for a specific coordination environment [5]. On the opposite, the Cu2+ Molecules 2020, 25, 1573; doi:10.3390/molecules25071573 www.mdpi.com/journal/molecules Molecules 2020, 25, x FOR PEER REVIEW 2 of 24 2+ environmentMolecules 2020, 25 [5]., 1573 On the opposite, the Cu ion usually displays a distorted octahedral environment2 of 24 with four tightly bound donors in a plane and two occupying more distant sites above and below this plane. In the limit, the elongation of the axial bonds often results in a square-planar geometry [5].ion The usually described displays distortions a distorted of octahedralthe coordination environment polyhedra with are four due tightly to the bound operating donors Jahn in a–Teller plane effect,and two a characteristic occupying more of a metal distant complex sites above with and the belowd9 electron this plane.configuration In the limit, [7]. The the ligands elongation and oftheir the spatialaxial bonds distribution often results can ininduce a square-planar changes in geometry the reduction [5]. The potential described of distortions the metal of ion the and coordination thereby influencepolyhedra its are oxidation due to thestate operating [8]. The Jahn–Tellerresearch on e theffect, Cu a2+ characteristic and Cu+ model of asystems metal complex with a common with the 9 ligandd electron environment configuration can give [7]. information The ligands about and their the spatialmetal’s distribution mode of action can induceat the changesactive sites in thein enzymesreduction [8]. potential of the metal ion and thereby influence its oxidation state [8]. The research on the 2+ + Cu Ourand current Cu model study systems involves with copper(II) a common complexes ligand environment with quinald caninate give and information alicyclic secondary about the aminesmetal’s modeas auxiliary of action ligands. at the activeQuinaldinic sites in acid, enzymes with [ 8a]. rational name quinoline-2-carboxylic acid (shownOur in current Scheme study 1), is involves a biological copper(II) molecule, complexes mostly with known quinaldinate for its role and in alicyclic tryptophan secondary metabolism amines [9].as auxiliaryIts anionic ligands. form, abbreviat Quinaldiniced as acid, quin with–, readily a rational forms name complexes quinoline-2-carboxylic with many transition acid metal (shown ions in andScheme has1, ),therefore is a biological, found molecule, use in their mostly quantitative known for gravimetric its role in tryptophan determination metabolism [10,11]. [ 9 Structurally]. Its anionic characterizedform, abbreviated complexes as quin −with, readily quinaldinate forms complexes reveal a with bidentate many transition chelating metal manner ions through and has, p therefore,yridine nitrogenfound use and in theircarbox quantitativeylate oxygen gravimetric as a prevailing determination coordination [10,11 mode]. Structurally [12–22]. characterizedExamples of a complexes bridging modewith quinaldinatethrough two revealor all athree bidentate donor chelating atoms are manner not very through common pyridine [23–26 nitrogen]. The quinaldinate and carboxylate was introducedoxygen as ainto prevailing our reaction coordination system through mode [ 12the–22 [Cu(quin)]. Examples2(H2O)] of astarting bridging material, mode throughone of the two rare or copper(II)all three donorquinaldinate atoms arecompounds not very known common prior [23 to–26 this]. report The quinaldinate [27]. The choice was of introduced auxiliary ligands, into our pyrrolidine,reaction system morpholine through, the and [Cu(quin) piperidine2(H (2SchemeO)] starting 1), was material, governed one by of thetheir rare ability copper(II) to bind quinaldinate uniformly incompounds a monodenta knownte manner. prior to The this group report shares [27]. Thea highly choice basic of auxiliary character ligands, and complete pyrrolidine, miscibility morpholine, with a largeand piperidine number of (Scheme solvents.1), Besides, was governed their NH by theirmoiety ability makes to bindthem uniformly good hydrogen in a monodentate bond donors. manner. Such anThe ensemble group shares of ligands a highly is of basic interest character also andfrom complete the viewpoint miscibility of crystal with aengi largeneering number and of chemical solvents. recognition.Besides, their NH moiety makes them good hydrogen bond donors. Such an ensemble of ligands is of interestHerein, also fromwe present the viewpoint products of of crystal the [Cu(quin) engineering2(H2 andO)] chemicalreactions recognition.with the selected amines. The compoundsHerein, were we presentcharacterized products by ofX- theray [Cu(quin)structure 2(Hanalysis2O)] reactionson a single with crystal the selected and infrared amines. vibrationalThe compounds spectroscopy. were characterized A series of no byvel X-ray compounds structure may analysis be divided on a singleinto two crystal groups. and The infrared first onevibrational is comprise spectroscopy.d of the desired A series amine of novel complexes, compounds [Cu(quin) may be2(pyro) divided2] (2), into [Cu(quin) two groups.2(morph) The2] first(4), [Cu(quin)one is comprised2(pipe)] of( the6), desired[Cu(quin) amine2(pipe) complexes,2] (7), [Cu(quin) [Cu(quin)2 (pyro)2(pipe)2](2]∙CH), [Cu(quin)3CN 2(8(morph)), and2] (4), [Cu(quin) (pipe)] (6), [Cu(quin) (pipe) ](7), [Cu(quin) (pipe) ] CH CN (8), and [Cu(quin) [Cu(quin)2(pipe)2 2]∙CH3CH2CN (9). The2 second2 group, where2 products2 · 3 of several unexpected2 (pipe) ] CH CH CN (9). The second group, where products of several unexpected reactions were reactions2 · we3re assembled,2 illustrated both the reactivity of copper(II) and, as a consequence, the unpredictabilityassembled, illustrated of the bothreaction the reactivity outcome. of [Cu(quin) copper(II)2Cl] and,–, a as complex a consequence, with chloride, the unpredictability was obtained of as the a pyrrolidiniumreaction outcome. (3) [Cu(quin)or a piperidinium2Cl]−, a complex (11) withsalt, chloride,when the was corresponding obtained as a pyrrolidiniumamine
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