Tetra cyclam

Strategies for selective partial alkylation of cyclam have been developed and Ni11 complexes of the variously substituted ligands prepared.1489 In a series of tetra-A-alkylated cyclams, Ni11 incorporation was only observed for smaller substituents such as Me, Et, and Pr, while no... 

The zinc complex of the saturated macrocycle (1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19, 20,21,22-docosahydrodibenzo-[B,I][l,4,8,l 1] tetra-azacyclotetradecine), which is a 14[ane]N4 (cyclam) ring incorporating two cyclohexyl rings into the macrocycle, has been prepared and characterized.688 Two isomers of the ligand were separately complexed with zinc and characterized by NMR. 

Ni2+ was very popular in the early days of the investigation of mechanisms of complex formation, since the time-scale for its reactions with simple ligands was so convenient for the then recently developed stopped-flow technique. However, interest has now moved on to other first-row cations, especially to Cu2+. A review of the kinetics and mechanisms of formation of tetraazamacrocyclic complexes concentrates on Ni2+ and Cu2+, and their reactions with cyclam and similar ligands (267). The tetra(4 -sulfonatophenyl)porphyrin complexes of Ni2+ and of Cu2+ react immeasurably slowly with cyanide, but their IV-methyl derivatives do react, albeit extremely slowly. The relevant time scales are hours for removal of Ni2+, months for the removal of Cu2+, by 10-4 M cyanide at pH 7.4 (268). 

The complexes [LCo(p-02)(p-OH)CoL] [L = en, trien, dien, tetra-ethylenepentamine, or tris-(2-aminoethyl)amine] have been studied, and the new complexes [[Co(imidazole)(gly)2 202],4H20 [ Co2(imidazole)2-(gly)402 0H],3H20, and [Co(imidazole)(gly)2(02)H20] have been prepared The spectroscopic properties of various p-peroxo- and p-superoxo-cobalt(iii) complexes have been examined. The singly-bridged p-peroxo-compounds have a strong band at 300 nm, whereas this falls at 350 nm for p-peroxo-p-hydroxo-complexes and two peaks at 480 and 700 nm are observed for p-superoxo-species. The i.r. spectra of p-peroxo-bridged complexes of cobalt(iii)-cyclam have been reported. ... 

Bis-iV-alkylated complexes of Me2-9 and Me2-ll, as well as the tetra-methylated Ni(II)cyclam (NinTMC) derivatives, have been synthesized by the deprotonation of secondary amines followed by alkylation (34, 47,48). When EtI or other alkyl halide with /3-hydrogen was added to the deprotonated Ni(II) complex of cyclam or 11, HX elimination occurred instead of SN2 reaction. Therefore, ethylene gas was produced instead of -ethylated complex formation when EtI was added to the deprotonated complex of cyclam or 11. However, in the case of 8, bis- -ethylated Ni(II) complex was isolated. This may be because HX elimination is slower than SN2 reaction. The - -alkylated Ni(II) complexes of 9 (Me2-9 and Et2 9) and Me2-ll were stable against ligand dissociation in acidic aqueous solutions. The -alkylated complexes were dealkylated when the complexes were heated in aqueous solutions (34, 47). 

Grant et al. studied a similar system using [Ni(cyclam)]2+ as the catalyst (cyclam = 1,4,8,11-tetra-azacyclo tetradecane), [Ru(bpy)3]2+ as the photosensitizer, and ascorbic acid as the sacrificial reductant [34], and observed a pH dependence on CO/H2 ratios, with the best ratio of 0.83 1 at pH 5. When Kimura etal. prepared pyridine derivatives of [Ni(cyclam)]2+ [35], the best complex, in C02-saturated ascorbate buffer at pH 5.1 and [Ru(bpy)3]2+ as the photosensitizer, produced 5.8-fold more CO than [Ni(cyclam)]2+. 

Beley et al. and others studied tetra-azamacrocyclic Ni complexes, similar to those presented in Section 11.2, in aqueous and organic solvents for the mediated reduction of C02 [98-100], In aqueous 0.1 M KN03 solution at a potential of-1.2 V (versus SCE), Ni(II)-cyclam dichloride (cyclam = 1,4,8,11-tetra-azatetradecane) reduced C02 to CO with 96% faradaic efficiency at Hg electrodes. The mechanism involved a first electron reduction of the species which coordinated C02, followed by C02 protonation, and a second electron transfer to yield CO and OH (as dis-... 

Metal ion Cyclam Tetra-N-methyl cyclam Ionic radius (A)... 

Scheme 5.2 Synthesis of tetra (hydroxy propyl) cyclam (5.21) via Michael addition followed by reduction, and X-ray crystal structure of its 1 1 copper(II) acetate complex.19...

As an example, Fig. 4 illustrates the Ni11 complexation equilibria in water involving 2.3.2-tet (1), champion of non-cyclic tetra-amines, and cyclam (2),... 

Fig. 4 Thermodynamic macrocyclic effect complexation equilibria involving the Nin aqua.ion and the champions of non-cyclic tetra-amines 1 (2.3.2-tet) and of the tetra-amine macrocycles 2 (cyclam)...

Sequestering in a cyclic environment imparts to the metal novel properties and favours redox activity. For instance, [Nin(cyclam)]2+ in 1 M HC1 is oxidised to the indefinitely stable [Nira(cyclam)]3+ complex, at a moderately positive potential (0.72 V vs. NHE) [9]. The acidic medium is required to prevent intramolecular electron transfer processes, leading to decomposition [10]. Moreover, [Nin(cyclam)]2+ can be electrochemically reduced to NiVcyclam) + at a mercury electrode, where it catalyses the reduction of CO2 to CO and HCOO (in an aqueous solution buffered to pH 5) [11]. This is nothing especially new encircling by tetra-aza macrocycles (e.g. porphyrins) is a trick known to Nature for billions of years to favour and control the redox activity of metal ions. 

The IR and Raman spectra of Cr(pic)3, where Hpic = 2-picolinic acid, include vCrN at 305 cm-1 and vCrO at 379/364 cm-1 (IR), 357 cm-1 (Raman).159 The complex Cr(N)(quin)2, where quin = 8-hydroxo-quinolinate, gives an IR band at 1015 cm-1, as expected for vCr=N in a five-coordinate complex.160 Skeletal mode assignments were proposed, from the IR and Raman spectra of [Cr(ox)(cyclam)]+, where ox = oxalate, cyclam = 1,4,8, 11-tetra-azacyclotetradecane. These were consistent with the presence of bidentate oxalate, and a cis-V geometry for the cyclam ligand.161... 

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