Biomimetic Modeling of Copper Complexes: A Study of Enantioselective Catalytic Oxidation on D-(+)-Catechin and L-( − )-Epicatechin with Copper Complexes

The biomimetic catalytic oxidations of the dinuclear and trinuclear copper(II) complexes versus two catechols, namely, D-(+)-catechin and L-( − )-epicatechin to give the corresponding quinones are reported. The unstable quinones were trapped by the nucleophilic reagent, 3-methyl-2-benzothiazolinone hydrazone (MBTH), and have been calculated the molar absorptivities of the different quinones. The catalytic efficiency is moderate, as inferred by kinetic constants, but the complexes exhibit significant enantio-differentiating ability towards the catechols, albeit for the dinuclear complexes, this enantio-differentiating ability is lower. In all cases, the preferred enantiomeric substrate is D-(+)-catechin to respect the other catechol, because of the spatial disposition of this substrate

Copper(II) compounds with NNO tridentate Schiff base ligands: effect of subtle variations in ligands on complex formation, structures and magnetic properties

The formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1 = (E)-2-((3-aminoethylimino)methyl)phenolate or L2 = (E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monocondensation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) and pyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear derivatives, whose nature depends on the length of the diamine used and hence on the size of the corresponding metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) in compound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decomposition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres by substitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c), with exclusion of pyridine, or the trinuclear [Cu3(L2)3(l3-OH)](ClO4)2 (2d), where pyrazine acts as base generating OH, and it does not appear in the product. Reaction of 1a with pyrazine yields only the trinuclear [Cu3(L1)3(l3-OH)](ClO4)2 (1d). Also with 2,20-bipyridine (2,20-bpy) dinuclear complexes are not formed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear compounds [Cu(L1)(2,20-bpy)](ClO4) (1e) and [Cu(L2)(2,20-bpy)](ClO4) (2e). The crystal structures of compounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities vM(T) of 1a and 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling (J = +0.96(3) cm1) through the short dimeric CuO contacts is detected for 2a (tn, six-membered ring). The X-band EPR spectrum of 2a in a frozen methanol solution at 70 K shows the hyperfine coupling of mononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines

Synthesis, crystal structures and magnetic properties of dinuclear copper(II) compounds with NNO tridentate Schiff base ligands and bridging aliphatic diamine and aromatic diimine linkers

The synthesis and the characterization of new dinuclear copper(II) compounds of general formula [(La–d)2Cu2(m-N–N)](ClO4)2 (1–6) with either neutral aliphatic diamine (N–N = piperazine, pip) or aromatic diimine (N–N = 4,4¢-bipyridine, 4,4¢-bipy) linker are reported. The copper ligands L- (La- = (E)-2-((2-aminoethylimino)methyl)phenolate, Lb- = (E)-2-((2-aminopropylimino)methyl)-phenolate, Lc- = (E)-2-((2-aminoethylimino)methyl)4-nitrophenolate, Ld- = (E)-2-((2-aminoethylimino)methyl)4-methoxyphenolate) are NNO tridentate Schiff bases derived from the monocondensation of a substituted salicylaldehyde 5-G-salH (G = NO2, H, OMe) with ethylenediamine, en, or 1,3-propylenediamine, tn. The crystal structures of compounds [(La)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (1·2MeOH), [(Lb)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (2·2MeOH), [(Ld)2Cu2(m-4,4¢-bipy)](ClO4)2 (4), [(La)2Cu2(m-pip)](ClO4)2 (5) and [(Lb)2Cu2(m-pip)](ClO4)2 (6) have been determined, revealing the preferred (e-e)-chair conformation of the bridging piperazine in compounds 5 and 6. The presence of hydrogen-bond-mediated intermolecular interactions, that involve the methanol molecules, yields dimers of dinuclear units for 1·2MeOH, and infinite zig-zag chains for 2·2MeOH. The temperature dependences of the magnetic susceptibilities cM(T) for all compounds were measured, indicating the presence of antiferromagnetic Cu–Cu exchange. For the compounds 2–4 with 4,4¢-bipy, the coupling constants J are around -1 cm-1, while in compound 1 no interaction could be detected. The compounds 5 and 6 with piperazine display higher Cu–Cu magnetic interactions through the s-bonding backbone of the bridging molecule, with J around -8 cm-1, and the coupling is favoured by the (e-e)-chair conformation of the diamine ring. The non-aromatic, but shorter, linker piperazine gives rise to stronger Cu–Cu antiferromagnetic couplings than the aromatic, but longer, 4,4¢-bipyridine. In the latter case, the rotation along the C–C bond between the two pyridyl rings and the consequent non co-planarity of the two copper coordination planes play an important role in determining the magnetic communication. EPR studies reveal that the dinuclear species are not stable in solution, yielding the solvated [(L)Cu(MeOH)]+ and the mononuclear [(L)Cu(N–N)]+ species; it appears that the limited solubility of the dinuclear compounds is responsible for their isolation in the solid state

A rapid freeze-quench setup for multi-frequency EPR spectroscopy of enzymatic reactions

Electron paramagnetic resonance (EPR) spectroscopy in combination with the rapid freeze-quench (RFQ) technique is a well-established method to trap and characterize intermediates in chemical or enzymatic reactions at the millisecond or even shorter time scales. The method is particularly powerful for mechanistic studies of enzymatic reactions when combined with high-frequency EPR (ν≥90 GHz), which permits the identification of substrate or protein radical intermediates by their electronic g values. In this work, we describe a new custom-designed micro-mix rapid freeze-quench apparatus, for which reagent volumes for biological samples as small as 20 μL are required. The apparatus was implemented with homemade sample collectors appropriate for 9, 34, and 94 GHz EPR capillaries (4, 2, and 0.87 mm outer diameter, respectively) and the performance was evaluated. We demonstrate the application potential of the RFQ apparatus by following the enzymatic reaction of PpoA, a fungal dioxygenase producing hydro(pero)xylated fatty acids. The larger spectral resolution at 94 GHz allows the discernment of structural changes in the EPR spectra, which are not detectable in the same samples at the standard 9 GHz frequency

Copper 1D coordination polymers and dimers: Role of the carboxylate and the ammonium cation, crystal structures and magnetic studies

The 1D coordination polymers [Cu2(syn,syn-g1:g1:l-RCOO)4(anti,anti-g1:g1:l-RCOO)(Et3NH)]n, R =Me (1) and H (2), and the dimer [Cu2(syn,syn-g1:g1:l-PhCOO)4(anti-g1-PhCOO)2(Et3NH)2] (3) are formed by reaction of copper carboxylates with the corresponding carboxylic acid and NEt3. Moreover, the compound [Cu2(syn,syn-g1:g1:l-MeCOO)4(anti-g1-MeCOO)2(Et2NH2)2] (4) can be obtained with the use of NHEt2. The crystal structures of 1CH3CN and 2 comprise linear anionic 1D chains of paddle-wheel copper carboxylates [Cu2(RCOO)4], bridged together by anti,anti-g1:g1:l acetato or formato ions. The negative charges are balanced by hydrogen-bonded triethylammonium countercations, and in compound 1 acetonitrile molecules are hosted in the voids between the chains. In compounds 3 and 4, two anti-g1-benzoato or -acetato anions occupy the apical positions of the paddle-wheel dimers [Cu2(RCOO)4], with Et3NH+ or Et2NH2+ cations balancing the negative charges, without the formation of coordination polymer chains. In compound 4, the dimeric units are held together in a 1D network by hydrogen bonds between the apical carboxylates and the diethylammonium cations. In order to evaluate the effect of the simultaneous presence of syn,syn- and anti,anti-g1:g1:l acetato ions on the Cu–Cu magnetic communication, the magnetic susceptibility of 1 has been measured. Data were fitted to a model based on an alternating chain of copper dimers based on the Heisenberg–Zeeman Hamiltonian (2J Sˆ 1Sˆ 2 convention). The results show the presence of a strong antiferromagnetic coupling within the dimer 2J1 = (310.8 ± 0.1) cm1 and a weak antiferromagnetic coupling between dimers 2J2 = (25 ± 1) cm1. The latter coupling is significant since its introduction in the fit model leads to a well-behaved error surface and to a 35% decrease of the sum of squared residuals. The electron paramagnetic resonance (EPR) spectra of powder sample of 1 have been recorded between 5 and 295 K, and they are typical of the triplet state of an antiferromagnetic copper dimer (zero field splitting |D| = 0.34 cm1)

Unraveling Unidirectional Threading of α‑Cyclodextrin in a [2]Rotaxane through Spin Labeling Approach

We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]­rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu­(I) catalyzed azide–alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed <i>J</i>-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on–off by changing the pH of the solution in the presence of a competing macrocyclic host

Manganese(III)-mediated cyclodimerization of a hydrazinyl derivative generating an unprecedented 1,2,3,5,6-substituted leuco-verdazyl ring

The one-pot reaction of Mn(OAc)3 with two equivalents of 6-(pyridine-2-ylhydrazonomethyl)phenol (Hphp) in methanol leads to a unique leuco-verdazyl-type ring. The remarkable cyclodimerization reaction is proposed to involve a number of one-electron oxidative steps mediated by Mn3+ ions, as well as a very uncommon 1,2-radical aryl migration and a nucleophilic attack of the solvent, i.e. methanol.<br/