Macrocycle spectroscopic properties

For the macrotricyclic complexes (573)-(576), which retain the same 14-membered macrocyclic framework, the size of the subring moieties slightly affects the spectroscopic properties of the complexes.1465,1467 There is a qualitative relationship between Ni—N bond distances and Amax values. As the Ni—N bond distance increases, the Amax value increases.91... 

Many new Ni(II) complexes of aza-type macrocycles have been synthesized, and their redox chemistries have been explored. In particular, complexes of macropolycyclic ligands and bismacrocyclic ligands have been prepared. Complexes with uncommon oxidation states of nickel (Nim and Ni1 complexes) have also been synthesized by employing a specially designed macrocycle, and their characteristic spectroscopic properties and X-ray structures reported. These nickel(II) complexes... 

The chemical and spectroscopic properties of the cofactor F-430 have been reviewed [98,99], The structure of the macrocycle (Figure 4) was elucidated by x-ray crystallography and NMR spectroscopy [100], The free cofactor, which is present in substantial amounts in the cells, has an absorption maximum at 430 nm, hence its name. In the enzyme, the absorption maximum is blue shifted to 420 nm. The pentamethyl ester of F-430 is soluble in organic solvents and can be reduced to the Ni(I) state under aprotic conditions, resulting in an absorption peak shift to 382 nm [101], or can be oxidized to the Ni(III) state, giving an absorption peak at 368 nm [102],... 

Porphyrin (1), the quintessential pyrrolic macrocycle, is one of the favorite structural motifs of organic chemistry [1], Its biological relevance, combined with a range of useful properties such as the rich electronic absorption spectra and the ability to coordinate metal ions, makes 1 a versatile building block for the synthetic chemist, as well as an important subject for physical investigations. Among the most conspicuous features of the porphyrin macrocycle is its aromatic character, which has a strong influence on the spectroscopic properties and chemical reactivity of 1 and its derivatives. 

Disregarding this aspect, and since cyclam is an interesting core for constructing den-drimers because it can be easily functionalized and because despite its absence of spectroscopic properties, it can interact in such a way with dendrons as to modify their photophysical properties, the interaction of lanthanide ions with cy clam-based dendrimers has been investigated. The dendrimers are constmcted from the cyclam core fitted with four dimethoxyben-zene and eight naphthyl units (generation 1, fig. 81) second generation introduces a total of 12 dimethoxybenzene and 16 naphthyl moieties. Coordination to Lnm ions occurs in acetoni-trile/methylene chloride (Ln = Nd, Eu, Gd, Tb, and Dy), but no sensitized Ln-luminescence was observed (Saudan et al., 2004). Another example of a macrocycle-based dendrimer is discussed below in section 3.3.2. 

One of the variables in the structures of the porphyrins present in heme proteins is the presence or absence of vinyl substituents on the periphery of the macrocycle. For example, b hemes have vinyl substituents whereas c hemes do not. Because of the sensitivity of such vinyl substituents during synthetic transformations, it has often been desirable to use octa-alkyl porphyrins in model studies of the spectroscopic properties of heme systems. The development of improved methods for the preparation of octa-alkyl porphyrins has likewise increased the availability of such porphyrins for model studies (20, 21). To assess the effect that replacement of the two vinyl substituents in protoporphyrin IX with alkyl (ethyl) groups has on the MCD properties of the heme system, an extensive and systematic study of the MCD properties of mesoheme IX-reconstituted myoglobin and horseradish peroxidase in comparison with the spectra of the native protoheme-bound proteins has been carried out (22). The structures of these two porphyrins are shown in Figure 3. 

With the exception of the Mn(IV) adamantane, all the macrocyclic complexes discussed above are at oxidation levels much below those likely to be present in functional OEC states. For this reason they are unlikely to mimic the spectroscopic properties of the active states unless they can be oxidized (an unfavorable process for seven-coordinate Mn(II) 248)). However, the variable and sometimes irregular geometries observed make these complexes valuable as calibrants for X-ray absorption studies 233, 239). The low symmetry observed in some of the macrocyclic tetramanganese structures and predicted for the OEC highlights that the differences among the four structural classes illustrated in Fig. 50 are not as great as it first appears and interconversions may be possible without major disruption of the coordination environment. The planar dimer-of-dimers structure, twisted or folded slightly, becomes a butterfly structure and if this is folded sufficiently a cubane results. The disposition of the Mn atoms in the cubane is identical to that in the adamantane structure. 

Bernhardt, Comba, and Hambley have applied this technique to predict the spectroscopic properties of Co(III), Cr(III), and Ni(II) complexes of ds-6,13-dimethyl-1,4,8,1 l-tetraazacyclotetradecane-6,13-diamine (cfs-diammac) macrocycle.In the same study, molecular mechanics calculations were used to predict the relative concentrations of conformers in solution. The cis-(ob)2 conformer of [Co(ds-diammac)] was found to be the lowest energy conformer, which was 6.5 kj/mol more stable than the cis-lel,ob conformer. The calculated ratio of cis- ob)2 conformer to the cis-lel,ob conformer is —9 1, which is in agreement with the H NMR spectrum. The lowest energy trans-Kb conformer is —2.6 kJ/mol more stable than the trans-hh conformer. The calculated ratio of trans-kh conformer to trans-hh conformer is —3 2. The H NMR data gave an average of these two conformers. Of course, the paramagnetic Co(III) and Ni(II) complexes could not be analyzed by NMR techniques, but molecular mechanics predicts that the cis- ob)2 and trans-hh conformers would be —6 kJ/mol more stable than other conformers. This series of papers contains several interesting and creative uses of transition metal MM. 

Calixarenes, in which only some of the hydroxy functions are O-alkylated or O-acylated, are important derivatives by themselves and interesting intermediates for the construction of more sophisticated compounds. Selected H NMR spectroscopic properties of partial ethers of 2a have been summarized in Table 4. The distinction of regioiso-mers and conformational isomers becomes more and more complicated for the higher macrocycles. 

Two mononuclear Wemer-type complexes based on Pd(III) have been characterized by X-ray crystallography. The X-ray crystal structure of mononuclear Pd (III) complex 1, in which the Pd(III) center is supported by two 1,4,7-trithiacyclo-nonane ligands, was reported in 1987 (Fig. 2) [37-39]. An analogous complex (2), supported by 1,4,7-triazacyclonOTiane ligands, was reported a year later [40]. Both complexes contain distorted octahedral Pd centers, as expected for low-spin (f Pd (III) [6]. Detailed examinatiOTi of the electrochemical and spectroscopic properties of Pd(III) complexes supported by macrocyclic polydentate ligands has indicated that the unpaired electron in 1 and 2 resides predominantly in the orbital [41 6]. 

The complementary segment, bis(3-dimethylaminoenone) 14 was also prepared from 9 via dibenzylidene derivative 13. Macrocyclization was effected by treating a 1 1 mixture of 12 and 14 with trifluoromethane-sulfonic acid in hot acetic acid followed by ammonium acetate. Neutralization of the reaction mixture with lithium hydroxide and chromatography of the chloroform extract gave torand 15 as the calcium triflate complex in 12 yield. Calcium is introduced as a 0.3% impurity in the triflic acid used in this experiment. The spectroscopic properties of 15 are similar to those reported for the unsubstituted parent system. 

The synthesis and characterization of new crown ethers and crown ether complexes is by now a relatively mature field which clearly has benefitted from application of solid state NMR techniques. One difference between solid-state and solution NMR is the fact that at room temperature in solution many crown molecules are conformationally mobile, whereas in the solid the conformations are frozen in ( However, below see the effect of the " merry-go-round " motion on the NMR spectroscopic properties of simple crown molecules and complexes). The main use has been to deduce the size of the asymmetric unit from the splitting pattern of the resonances, and hence, some information on the macrocycle symmetry, and also to use chemical shift information to derive information... 

Aspicilin (37), the first macrocyclic lactone isolated from a lichen 189), would appear to be biogenetically related to the hydroxyacids (36). Huneck and ScHREiBER 189) isolated aspicilin from Aspicilia gibbosa and established the structure (37) by analysis of the spectroscopic properties, chemical interconversions and properties of the derivatives so obtained (Scheme 6). Hence oxidation of aspicilin (37) gave the diacid (38) while catalytic reduction gave dihydroaspicilin (39). Subsequent saponification of (39) gave 4,5,6,17-tetrahydroxystearic acid (40). 

Chen, R., Nossarev, G.G., and Hogen-Esch, T.E. (2004) Synthesis and thermal and spectroscopic properties of macrocyclic vinyl aromatic polymers. Journal of Polymer Science Part A-Polymer Chemistry, 42,5488 503. 

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