Cyclam Cycle

Cyclam, i.e., 1,4,8,11-tetraazacyclotetradecane, linked to PTh chain [139], has been used to complex Ni(II), obtaining effective redox matching (see Fig. 3.32). The 

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Before a 1/1 /70 FDA ban (rescission proposed in early 1990), cyclamate noncaloric sweeteners were the major derivatives driving cycloliexylamine production. The cyclohexylsulfamic acid sodium salt (39) [139-05-9J and mote thermally stable calcium cyclohexylsulfamic acid (40) [139-06-1] salts were prepared from high purity cyclohexylamine by, among other routes, a reaction cycle with sulfamic acid. 

The 14-membered macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam or [14]aneN4), unlike cyclen, is capable of encircling most transition metal ions and in the case of Co111 the trans configuration is much preferred by comparison with the folded cis isomer. Electrochemical reduction of A,v-[Co(cyclam)(OI I)2]+ in 3M NaOH leads to rapid isomerization to the trans form, and the relative stabilities of the trans and cis isomers of the di- and trivalent complexes were determined from a thermodynamic cycle.702 This preference for trans orientation of the non-macrocyclic donors has enabled the isolation and investigation of many Co complexes without the complications of isomerization. Some novel examples include /r[Pg.61]

As a catalyst Nin(cyclam)Br2 is employed which in a proposed cycle is reduced to Ni (cyclam)+ coordinated to C02. In the following, the catalyst undergoes an oxidative addition with the bromoaryl compound to form a Nini species which in a radical like reaction inserts into the double bond. It follows another le" reduction to give a Ni11 species which undergoes C02 uptake to form the nickel carboxyl-ate 171 as the final product. 

The spin state of the six-coordinate complexes [Fe(cyclam)X2]+ [cyclam = 1,4,8,11-tetraaza-cyclotetradecane (105)] is governed by the geometrical configuration of the 14-membered macro-cycle, being high-spin for the cis complexes and low-spin for the trans complexes. However, frans-[Fe(cyclam)Br2][Cl04] with Arr— 3.90 BM at 295 K. may exist in a high-spinlow-spin equilibrium.392... 

Since publication of CHEC-II(1996), the field covered by this chapter has expanded enormously. The development is driven mainly by an extensive use of the cycles in metal and/or anion complexation, in modeling of enzyme reactions, in catalysis, and, mostly, in medicine as contrast agents (CAs) for magnetic resonance imaging (MRI), radiopharmaceuticals, or drugs. The chemistry is mainly focused on derivatives of two of the most important macrorings 1,4,7,10-tetraazacyclododecane (cyclen) 1 and 1,4,8,11-tetraazacyclotetradecane (cyclam) 2. From related compounds, tetrakis(acetic acid) derivatives DOTA 3 (DOTA= 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and TETA 4 (TETA = 1,4,8,1 l-tetraazacyclotetradecane-l,4,8,ll-tetraacetic acid) and their analogs/derivatives are of the main interest. 

This chapter does not cover cyclic amides and peptides, since their number would enormously expand this text. They are reviewed only if they serve as reaction intermediates during synthesis of cyclic amines. In addition, metal ions complexation will be presented in required minimum, for example, if it serves for template formation during ring synthesis or as a main topic in some application. In this chapter, most of the sections deal with the literature data for all cycle types, except Section 14.11.6, which focuses mainly on chemistry of cyclen and cyclam and their analogs and derivatives. In Section 14.11.8, we give only a brief overview of the utilizations and provide a reader with reviews where more detailed information may be found. 

If nonprotected cycles are used in the reaction, a number of substituents, and a place of reaction, are often controlled by conditions (mainly by stoichiometry and solvent used). Direct reaction with amines is simple to run but may lead to inseparable reaction mixture. Selective protection adds several extra synthetic steps. Overall yields of both ways can be, therefore, comparable as well as time consumption. Isomers can be prepared with two or more substituents, (depending on the symmetry of the macrocycle). For cyclen and cyclam, they are commonly called trans ... 

However, little is known as to why selective generation of CO takes place in the electrochemical reduction by these Ni macrocycle species adsorbed on the surface of an Hg electrode in H2O. Sakaki et al. carried out SCF ah initio calculations for [NiF(NH3)4] as the model of [Ni(cyclam)] adsorbed on Hg (Fig. 5), and a NiX7ji-C02 adduct with an OCO angle of 135.3° is suggested as the active species during CO2 reduction (66, 67). The increase in electron density of an 0-atom of the terminal CO2 from -0.33e (free CO2) to -0.58e upon coordination to Ni supports the proposed mechanism for the catalytic cycle involving an hydroxycarbonyl intermediate. 

The mixed system of 16, [Ni(cyclam)] (23, cyclam = 1,4,8,11-tetraazacyclo-tetradecane), and ascorbic acid (H2A) was irradiated tmder a CO2 atmosphere using 340-600-nm light. This mostly evolved H2 along with a small amount of CO. The TN and quantum yield for CO production were TNco < 1 and co = 0.0006 [39, 40]. Control experiments carried out in the dark or without 16, 23, H2A or CO2 produce no CO [39]. A labeling experiment using " C02 revealed that the CO is produced from CO2 [40]. This photocatalytic reaction cycle is also initiated by reductive quenching of the MLCT excited state of 16 by HA. ... 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

Yebra et al. [83] used a continuous-flow procedure for the indirect determination of sodium cyclamate by flame atomic absorption spectrometry (FAAS). This method is based on oxidation of the sulfamic group derived from cyclamate to sulfate in acidic conditions and in the presence of sodium nitrite. The procedure is adapted to a flow system with precipitate dissolution (Figure 24.11), where sulfate formed is continuously precipitated with lead ion. The lead sulfate formed is retained on a filter, washed with diluted ethanol, and dissolved in ammonium acetate (because of the formation of soluble lead acetate) for online FAAS determination of lead, the amount of which in the precipitate is proportional to that of cyclamate in the sample. In this work a home-made filtration device was used made of a Teflon tubing packed with a cotton pulp and the ends of the filter column were plugged with filter paper (chamber inner volume 141 J,L). This precipitate collector was effective in retaining the precipitate and did not produce excessive back-pressure if the precipitate was dissolved following each precipitation cycle. 

Fig. 17.9. Postulated mechanistic cycle for the electrocatalytic reduction of CO2 into CO by Ni cyclam in water (BelQ etaL, J. Chem. Soc. 108,7461-7467). Reproduced with permission.

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