Cyclam Ring Research Articles

Copper(II) Complexes of Phenolate‐Bridged Dimeric Cyclam Ligands: Synthesis, Coordination and Electrochemical Study

AbstractBis(macrocyclic) complexes are investigated as artificial analogues of natural dinuclear metalloenzymes. Two ligands containing two cyclam rings connected through a p‐cresol or 4‐nitrophenol spacer were synthesized and their complexing properties were investigated. CuII complexes were prepared and their properties were studied by a combination of spectral, diffraction and electrochemical methods. The phenolate group is not or only weakly coordinated to the central CuII ion and the two macrocycles behave mostly as two independent complexing units. All the determined solid‐state structures showed cyclam adopting trans‐III geometry, however, solution study indicated the presence of more isomers in the solution whose abundance changes with pH. Electrochemical reduction of the complexes is irreversible and results in the reductive decomplexation to amalgamated Cu metal. The amalgamated metal is re‐oxidized to CuII during the anodic scan of CV and the metal ion is re‐coordinated. The ligand containing the nitro group shows an irreversible four‐electron reduction of the nitro group. The ligands and the complexes undergo also a slow degradation when incubated in the aqueous solutions for a prolonged time. The degradation relies on the “retro‐Mannich” cleavage of the methylene bridge connecting the cyclam ring with the phenol moiety.

Synthesis, Characterization, and Reactivity Studies of New Cyclam-Based Y(III) Complexes.

[(Bn2Cyclam)Y(N(SiMe3)2)] was prepared by reaction of H2Bn2Cyclam with Y[N(SiMe3)2]3. The protonation of the macrocycle ligand in [(Bn2Cyclam)Y(N(SiMe3)2)] is observed upon reaction with [HNMe3][BPh4] leading to the formation of [(HBn2Cyclam)Y(N(SiMe3)2)][BPh4]. DFT analysis of [(Bn2Cyclam)Y(N(SiMe3)2)] showed that the HOMO is located on the anionic nitrogen atoms of the cyclam ring indicating that protonation follows orbital control. Addition of H2Bn2Cyclam and H2(3,5-tBu2Bn)2Cyclam to a 1:3 mixture of YCl3 and LiCH2SiMe3 in THF resulted in the formation of [((C6H4CH2)BnCyclam)Y(THF)(µ-Cl)Li(THF)2] and [Y{(η3-3,5-tBu2Bn)2Cyclam}Li(THF)], respectively. The reaction of H23,5-tBu2Bn2Cyclam with Y(CH2SiMe3)3(THF)2 was studied and monitored by a temperature variation NMR experiment revealing the formation of [(3,5-tBu2Bn2Cyclam)Y(CH2SiMe3)]. Preliminary catalytic assays have shown that [Y{(η3-3,5-tBu2Bn)2Cyclam}Li(THF)] is a very efficient catalyst for the intramolecular hydroamination of 2,2-diphenyl-pent-4-enylamine.

Synthesis and structural analysis of two cyclam derivatives

Two macrocycle derivatives (1 and 2) from the modification of 1,4,8,11-tetraazacyclotetradecane (cyclam) have been synthesized and structurally characterized. The synthesis was straightforward and led to the production of the final products at high yields and purity. Central rings of both compounds at solid state display a chair conformation with the substituents dangling around the cyclam ring. The binding cavity radii from the crystal structures are 1.349 Å in the unsubstituted precursor cyclam, 1.926 Å in 1, and 2.140 Å in 2. A computational study provided similar bond matrices but slightly larger cavity sizes, likely as a result of the spacious environment and elasticity of the molecules at gas phase. This research provided an intensive look into the modification of polyamine, which further implicates how the molecule could be manipulated to adjust its properties and functions.

Clam-shaped cyclam-functionalized porphyrin for electrochemical reduction of carbon dioxide

A clam-shaped molecule comprising a Zn(II)-porphyrin and a Zn(II)-cyclam is synthesized and characterized. Its electrochemical behavior and catalytic activity for homogeneous electrochemical reduction of carbon dioxide (CO[Formula: see text] are investigated by cyclic voltammetry and compared with those of Zn(II)-meso-tetraphenylporphyrin and Zn(II)-cyclam. Under N2-saturated conditions, cyclic voltammetry of the featured complex has characteristics of its two constituents, but under CO2-saturated conditions, the target compound exhibits significant current enhancement. Iterative reduction under electrochemical conditions indicated the target compound has improved stability relative to Zn(II)-cyclam. Controlled potential electrolysis demonstrates that, without addition of water, methane (CH[Formula: see text] is the only detectable product with 1% Faradaic efficiency (FE). The formation of CH4 is not observed under the catalysis of the Zn(II)-porphyrin benchmark compound, indicating that the CO2-capturing function of the Zn(II)-cyclam unit contributes to the catalysis. Upon addition of 3% v/v water, the electrochemical reduction of CO2 in the presence of the target compound gives carbon monoxide (CO) with 28% FE. Dominance of CO formation under these conditions suggests enhancement of proton-coupled reduction. Integrated action of these Zn(II)-porphyrin and Zn(II)-cyclam units offers a notable example of a molecular catalytic system where the cyclam ring captures and brings CO2 into the proximity of the porphyrin catalysis center.

New Cyclams and Their Copper(II) and Iron(III) Complexes: Synthesis and Potential Application as Anticancer Agents.

New cyclam derivatives (HOCH2 CH2 CH2 )2 (PhCH2 )2 Cyclam and (HOCH2 CH2 CH2 )2 ( PhCH2 )2 Cyclam, as well as their CuII and FeIII complexes, were synthesized and characterized and their stability in cellular media was assessed. The cytotoxic effect of all compounds was examined on human cervical cancer (HeLa) cells, revealing strong anticancer activity. After 24 h, only complexes with the (HOCH2 CH2 CH2 )2 ( PhCH2 )2 Cyclam ligand are cytotoxic, whereas after incubation for 72 h all compounds show significant antiproliferative effects. Notably, compounds containing PhCH2 pendant arms on the cyclam ring revealed the most activity, with cytotoxicity values up to 12 times higher than those of cisplatin. All metal complexes seem to induce cell death through the formation of reactive oxygen species.

New mixed-ligand Ni(II) and Zn(II) macrocyclic complexes with bridged (endo,endo)-bicyclo [2.2.1]hept-5-ene-2,3-dicarboxylate: Synthesis, characterization, antimicrobial and cytotoxic activity

New carboxylate complexes of the tetraazamacrocyclic ligand N,N',N'',N'''-tetrakis(2-pyridilmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc) with Ni(II) and Zn(II) as central ions were prepared. In mixed-ligand complexes (endo,endo)-bicyclo?2.2.1?hept-5-ene-2,3-dicarboxylate dianion (C9H8O4 2-) is also coordinated to metal ions. The complexes were characterized by elemental analysis (C, H, N), FTIR and UV?Vis spectroscopy, molar conductivity determination and magnetic susceptibility measurement at room temperature. The analytical data of the complexes show the formation of binuclear [Ni2(C9H8O4)tpmc](ClO4)2?4H2O and tetranuclear [Zn4(C9H8O4)(tpmc)2](ClO4)6? ?CH3CN?KClO4?4H2O complexes. In tetranuclear Zn(II) complex bicyclic dicarboxylate ligand is most likely to be bridge coordinated, and in binuclear Ni(II) complex it is coordinated in a combined bridged manner with chelate rings formation. In both complexes macrocyclic ligand was exo coordinated, out of cyclam ring and adopts a boat conformation. The Zn(II) complex is one of the rare tetranuclear Zn(II)-tpmc complexes with carboxylate ion bridging two Zn2tpmc units. The complexes were tested for antibacterial activity against Gram-positive bacteria Staphylococcus aureus (ATCC 25923) and Bacillus subtilis (ATCC 6633), Gram-negative bacterium Escherichia coli (ATCC 25922), and yeast Candida albicans (ATCC 10231), and were screened for antiproliferative activity against human cervix adenocarcinoma (HeLa) and human myelogenous leukemia (K562) cell lines.

New macrocyclic Cu(II) complex with bridge terephthalate: Synthesis, spectral properties, in vitro cytotoxic and antimicrobial activity. Comparison with related complexes

New cationic mixed-ligand tetranuclear complex [Cu4(tpht) (tpmc)2](ClO4)6·5H2O (tpmc = N,N′,N″,N‴-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane, H2tpht = terephthalic acid (benzene-1,4-dicarboxylic acid)) was prepared and characterized by elemental analysis (C, H, N, Cu), UV/Vis, IR, EPR (X-band) spectra, conductivity, cyclic voltammetry and magnetic measurements (SQUID). Five coordinated geometry for new Cu(II) complex is proposed. The central metal ion is coordinated with two pyridyl and two cyclam nitrogens and bridged with N(CH2)3N fragments of the cyclam ring and OCO− of the terephthalate anion. Copper (II) ions are coordinated out of cyclam rings and one OCO− group from tpht bridges two Cu(II) inside tpmc unit and two Cu2tpmc units are bridged by dicarboxylate tpht dianion engaging all four oxygen atoms. Weak ferromagnetic interaction between copper (II) ions in new tetranuclear complex is observed, Weiss constant θ = 0.21 K and C = 0.398 cm3Kmol−1.The new complex, related Cu(II) complexes with bridged benzene carboxylate, [Cu2(C6H5COO)tpmc](ClO4)3·CH3OH, [Cu2(Hpht)tpmc](ClO4)3·3H2O (H2pht = phthalic acid), [Cu4(ipht) (tpmc)2](ClO4)6·NaClO4·2CH3CN (H2ipht = isophthalic acid), solvents, starting salts and ligands were in vitro evaluated for antiproliferative and antibacterial activity. Antiproliferative evaluation was carried out using tumour cell lines: HeLa (human adenocarcinoma) and K562 (chronic myelogenous leukaemia). Among all tested compounds, the ipht complex revealed the highest antiproliferative activity. According to antibacterial screening results, most active compound was the benzoate complex against Micrococcus lysodeikticus.

Cooperative Metal-Ligand Hydroamination Catalysis Supported by C-H Activation in Cyclam Zr(IV) Complexes.

Complexes of the type (R2Cyclam)ZrCl2 (where R = CH2═C(H)CH2 (All), CH2═C(Me)CH2 (MeAll), and PhCH2 (Bn)) react with suitable Grignard reagents to produce the corresponding alkyl derivatives (R2Cyclam)ZrR'2 (R' = Me, CH2Ph). Thermally induced double metalation of the pending arms of the cyclam ligand led to the formation of complexes ((CH═C(H)CH2)2Cyclam)Zr, 14, ((CH═C(Me)CH2)2Cyclam)Zr, 15, or ((C6H4CH2)2Cyclam)Zr, 16. These reactions proceed through C(sp2)-H bond activation and R'H elimination and convert the original dianionic tetracoordinated cyclam-based ligands in tetraanionic hexacoordinated ligands that establish two new Zr-C bonds. The cleavage of the Zr-C bonds may be readily achieved by treatment of the bis( ortho-metalated) species 16 with protic substrates ( tert-butanol, phenol, thiophenol, aniline, benzophenone hydrazone, pyrazole, and N, N'-diphenylhydrazine), to give rise to (Bn2Cyclam)ZrX2 complexes (X = OtBu, OPh, SPh, NHPh, NHNCPh2, C3H3N2, N, N'-PhNNPh). In catalytic conditions, complexes (All2Cyclam)Zr(NMe2)2, 14, 15, or 16 convert 2,2-diphenyl-pent-4-enylamine to 2-methyl-4,4-diphenylpyrrolidine with 100% selectivity and conversion values varying between 61 and 88% in 4.5 h, at 115 °C. Complexes 14 and 15, which display metalated allyl and methallyl pending groups on the cyclam ring, are the most active species (1.7 < TOF < 2.0 h-1). The mechanism of this reaction was studied by density functional theory that revealed two competitive paths, one proceeding through an imido intermediate and another that occurs via an amido ligand. Both cases represent cooperative mechanisms with active participation of the cyclam, as proton exchange between the coordinated substrate and the ligand side arm with reversible C-H activation is a crucial feature of the mechanism.

The transformation from 2°-amine to 3°-amine of cyclam ring alters the fragmentation patterns of 1-tosylcytosine-cyclam conjugates.

The novel N-1-sulfonylcytosine-cyclam conjugates 1 and 2 conjugates are ionized by electrospray ionization mass spectrometry (ESI MS) in positive and negative modes (ES+ and ES- ) as singly protonated/deprotonated species or as singly or doubly charged metal complexes. Their structure and fragmentation behavior is examined by collision induced experiments. It was observed that the structure of the conjugate dictated the mode of the ionization: 1 was analyzed in ES- mode while 2 in positive mode. Complexation with metal ions did not have the influence on the ionization mode. Zn2+ and Cu2+ complexes with ligand 1 followed the similar fragmentation pattern in negative ionization mode. The transformation from 2°-amine in 1 to 3°-amine of cyclam ring in 2 leads to the different fragmentation patterns due to the modification of the protonation priority which changed the fragmentation channels within the conjugate itself. Cu2+ ions formed complexes practically immediately, and the priority had the cyclam portion of the ligand 2. The structure of the formed Zn2+ complexes with ligand 2 depended on the number of 3° amines within the cyclam portion of the conjugate and the ratio of the metal:ligand used. The cleavage of the cyclam ring of metal complexes is driven by the formation of the fragment that suited the coordinating demand of the metal ions and the collision energy applied. Finally, it was shown that the structure of the cyclam conjugate dictates the fragmentation reactions and not the metal ions.

Synthesis, characterization, and in vitro antiproliferative and antibacterial studies of tetraazamacrocyclic complexes of Co(II) and Cu(II) with pyromellitic acid

AbstrcatNew cationic tetranuclear Co(II) and neutral binuclear Cu(II) complexes with tpmc (N,N′,N″,N″′-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and bridging pyromellitate ligand pma (tetraanion 1,2,4,5-benzenetetracarboxylic acid) were isolated. The composition of the compounds is proposed based on elemental analyses (C, H, N, M=Cu, Co), molar conductivity determination, UV-Vis, FTIR, EPR, LC-MS and reflectance spectroscopy, magnetic measurements, cyclic voltammetry, as well as TG and DTA. It is proposed that in [Co4(pma)(tpmc)2](ClO4)4·6H2O (1), cobalt(II) is six-coordinate out of cyclam rings and one OCO− from pma participates in coordination with one Co(II). In the case of [Cu2(pma)tpmc]∙8H2O (2), one OCO− from pma bridges two Cu(II). The cytotoxic activity of 1 and 2 was tested against tumor cell lines human cervix adenocarcinoma (HeLa), estrogen-receptor-positive human breast cancer (MCF-7), human myelogenous leukemia (K562), and the human Caucasian Burkitt’s lymphoma (Ramos). The IC50 values for 1 and 2 were within the range 44.66 ± 2.39 to 152.40 ± 2.28 μM, and from 140.88 ± 3.51 to 192.05 ± 2.09 μM, respectively. Both 1 and 2 were tested for antimicrobial activity. We determined that minimal inhibitory concentration for 1 against Staphylococcus aureus, Bacillus subtilis, and Klebsiella pneumoniae was 25 mM. Complex 2 did not express activity against tested microbial strains.

A New Class of Metal-Cyclam-Based Zirconium Metal-Organic Frameworks for CO2 Adsorption and Chemical Fixation.

Metal-organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr6 clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO2 with epoxides, including sterically hindered epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance.

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn2+, Fe2+, and Cu2+ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu2+ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of μ=3.10μB for the Fe2+ complex suggesting that it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu2+ complex, while the μ=5.52μB value for the Mn2+ complex suggests that it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting that catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe2+ complex as the oxidation catalysts most worthy of continued development.

Theoretical investigation of polymer chain stability in the metal coordinated azorubine and cyclam complex

Theoretical investigations have been performed for unit systems with Ni(II) and Zn(II) coordination between azorubine and 1,4,8,11-tetraazacyclotetradecane (cyclam) complexes using the conventional DFT and the DFT-based tight binding (DFTB) methods. Two different geometries (short and long) and spin states (singlet and triplet) of the model system built by two mesylate groups and the cyclam ring together with Ni(II) and Zn(II) ions were energetically characterized. For the Ni(II) coordination complex the triplet geometry is preferred, but one could not exclude also the presence of the singlet spin configuration due to the huge energy barrier defined by the intersystem crossing. The intersystem crossing geometry of the singlet–triplet transition was studied in details and the corresponding spin–orbit couplings were discussed. For the Zn(II) coordination complex only the singlet state was found. Polymer chain build up from four unit systems presents irregular forms with strong coordination bonds between units.

Synthesis, structural studies, and oxidation catalysis of the late-first-row-transition-metal complexes of a 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam.

The first 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam ligand has been synthesized and successfully complexed to Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) cations. X-ray crystal structures were obtained for all six complexes and demonstrate pentadentate binding of the ligand with the requisite cis-V configuration of the cross-bridged cyclam ring in all cases, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of the complexes were evaluated using solid-state magnetic moment determination and acetonitrile solution electronic spectroscopy, which both agree with the crystal structure determination of high-spin divalent metal complexes in all cases. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all but the Ni(2+) complex, suggesting that catalytic reactivity involving electron-transfer processes is possible for complexes of this ligand. Kinetic studies of the dissociation of the ligand from the copper(II) complex under strongly acidic conditions and elevated temperatures revealed that the pyridine pendant arm actually destabilizes the complex compared to the parent cross-bridged cyclam complex. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant for the most biologically relevant Mn(2+), Fe(2+), and Cu(2+) complexes identified the Mn(2+) complex as a potential mild oxidation catalyst worthy of continued development.

Crystal structures of two cross-bridged chromium(III) tetra-aza-macrocycles.

The crystal structure of di-chlorido-(4,10-dimethyl-1,4,7,10-tetra-aza-bicyclo-[5.5.2]tetra-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry (space group P21/n) at 150 K. The structure of the related di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C14H30N4)]PF6, (II), also displays monoclinic symmetry (space group P21/c) at 150 K. In each case, the Cr(III) ion is hexa-coordinate with two cis chloride ions and two non-adjacent N atoms bound cis equatorially and the other two non-adjacent N atoms bound trans axially in a cis-V conformation of the macrocycle. The extent of the distortion from the preferred octa-hedral coordination geometry of the Cr(III) ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octa-hedral geometry.

Preparation, characterisation and study of in vitro biologically active azamacrocyclic Cu(II) dicarboxylate complexes

New cationic Cu(II) complexes with N, N′, N″, N″′-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc) and aliphatic dicarboxylic acids: pentanedioic (glutaric acid=glutH2), hexanedioic acid (adipic acid=adipH2) and decanedioic acid (sebacic acid=sebH2) of general formula [Cu4(L)(tpmc)2](ClO4)6·xH2O, L=glut, x=2; L=adip, x=7; L=seb, x=6 were isolated. Their composition and charges are proposed based on elemental analyses and molar conductivity measurements. By the comparison of their UV–Vis, reflectance, FTIR and EPR spectral data, CV and SQUID magnetic measurements, with those for the complex with butanedioic acid (succinic acid=succH2) of known molecular structure and analysis of LC/MS spectra, geometry with two [Cu2tpmc]4+ units bridged by dicarboxylate dianion engaging all oxygens is proposed. Within units, Cu(II) ions are also bridged with N portion of cyclam ring. All four complexes were screened to in vitro antimicrobial and cytotoxic activity along with free primary and secondary ligands, Cu(II) salt and solvent controls. Detected antibacterial and cytotoxic activity for the complexes was enhanced in most cases than the corresponding controls.

Binuclear biologically active Co(II) complexes with octazamacrocycle and aliphatic dicarboxylates

Four new cationic Co(II) complexes with N,N′,N′′,N′′′-tetrakis (2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tpmc) and dianion of one the aliphatic dicarboxylic acids: butanedioic acid (succinic) acid=succH2, pentanedioic (glutaric) acid=gluH2, hexanedioic acid (adipic) acid=adipH2 or decanedioic acid (sebacic) acid=sebH2 of general formula [Co2(L)(tpmc)](ClO4)2⋅xY, L2−=succ, x=1, Y=H2O; L=glu, x=1, Y=H2O; L=adip, x=1.5, Y=H2O; L=seb, x=1, Y=CH3CN were isolated. The composition and charge are proposed based on elemental analyses (C, H, N) and electrical conductivity measurements. UV–Vis and FTIR spectral data and magnetic moments were in accordance with high-spin Co(II) state. It is proposed that in all complexes Co(II) is hexa-coordinated out of cyclam ring and that both carboxylic groups from dicarboxylate bridge participate in coordination. Oxygens from one group are most likely bonded to the same Co(II) ion thus forming a four-membered ring. The in vitro antibacterial/antiproliferative activities of the complexes were in some cases enhanced compared with the simple Co(II) salt and free ligands, tested as controls.

Magnesium Cyclam complexes with a Mg2(Bn2Cyclam) core: Structural characterization of Mg2(Bn2Cyclam)I2 and Mg2(Bn2Cyclam){μ-H}{μ-F-C6F4}2BC6F5

This work describes the crystal structures of two Mg dimers supported by dianionic trans-disubstituted Bn2Cyclam ligands (1,8-dibenzyl-1,4,8,11-tetraazacyclotetradecane). Mg2(Bn2Cyclam)I2 (1) crystallizes in the monoclinic space group P21/c with half-molecule in the asymmetric unit. The two metal centers and the two Namido donors of the cyclam ring define a four-member Mg2N2 metallacycle. The metals’ coordination spheres are completed by one amine of the macrocycle and one iodide defining a trigonal pyramidal coordination geometry. Mg2(Bn2Cyclam){μ-H}{μ-F-C6F4}2BC6F5 (2) crystallizes in the monoclinic space group P21/c with half-molecule in the asymmetric unit. Compound 2 is a rare example of a zwitterionic species of the type [Mg2(Bn2Cyclam)]2+ with two [HB(C6F5)3]− anions coordinating in a k3-H,F,F tridentate fashion. This compound also displays bridging Namido ligands that bind the two Mg atoms in a Mg2N2 arrangement analogous to 1, promoting a capped square pyramidal coordination geometry. At a supramolecular level C–H⋯I and C–H⋯F hydrogen bonds as well as CH/π and F⋯F interactions are responsible for the crystal packing.

Homogeneous CO2 Reduction by Ni(cyclam) at a Glassy Carbon Electrode

The homogeneous CO(2) reduction activity of several nickel cyclam complexes was examined by cyclic voltammetry and controlled potential electrolysis. CO production with high efficiency from unsubstituted Ni(cyclam) was verified, while the activity was found to be attenuated with methyl substitution of the amines on the cyclam ring. Reactivity with CO(2) was also probed using density functional theory (DFT) calculations. The relative CO(2) binding energies to the Ni(I) state obtained from DFT were found to match well with the experimental results and shed light on the possible importance of the isomeric form of Ni(cyclam) in determining the catalytic activity.

Zinc(ii) complexes of constrained antiviral macrocycles

The configurations of metallocyclams are of interest in relation to protein recognition and anti-HIV activity. We have synthesised four novel zinc(II) complexes with hexyl-Me(2)-cyclam (HMC; 3,14-dimethyl-2,6,13,17-tetraazatricyclo(16.4.0.0(7,12))docosane), 1, and naphthyl-hexyl-Me(2)-cyclam (NHMC; 2,13-bis(1-naphthylmethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo(16.4.0.0(7,12))docosane), 2, as ligands. X-ray crystallographic data for Zn(II)-HMC diacetate, 3 show that zinc is six-coordinate in a distorted octahedral environment bound to four equatorial N atoms from the macrocycle and two axial acetato O atoms. The 14-membered metallo-macrocycle adopts a trans-III (RRSS) configuration with two six-membered rings in chair forms and two five-membered rings in gauche forms. In the chlorido Zn(II)-HMC complex 5, zinc appears to be 5-coordinate with square-pyramidal geometry. Interestingly, the chlorido Zn(II)-NHMC complex 6 crystallised in a trans-I configuration containing 4-coordinate tetrahedral zinc bound to three cyclam ring N atoms, a possible model for intermediates formed during the uptake and release of metals by cyclams. The ligand 1 and the zinc complex 3 were active towards viral strains HIV-1 (III(B)) (IC(50) values of 10.51 ± 0.23 and 3.50 ± 0.33 μM, respectively), and HIV-2 (ROD) (IC(50) values of 133.78 ± 14.10 and >110.67 μM, respectively). 2D [(1)H, (13)C] and [(1)H, (15)N] NMR spectroscopic studies suggested that the types of configurational isomers present in solution depend on the axial ligand.