Acyclic complexes

In the macrocydic structure the metal binding site within the ligand is more encapsulated and the entropy is decreased upon metal incorporation. As a result the stability of the majority of macrocydic metal chelates is higher than that of acyclic complexes (Table 3) [27,28]. Generally the macrocydic complexes exhibit a higher kinetic stability. 

Compound 1 exhibits significantly different reactivity than the acyclic analogue. The metallacycle is more stable than the acyclic complex and whereas Cp2Ti"Bu2 decomposes via the expected (3-1I elimination pathway to produce butenes and butane, the thermal decomposition products of 1 are ethylene and 1-butene. In addition, the metallacycle is observed to be significantly more reactive towards CO than Cp2Ti"Bu2 Reaction of 1 with carbon monoxide at —55 °C yields the titanium acyl species, based on infrared data, which then rapidly converts to cyclopentanone at 0 °C (Scheme l).13... 

The formation of macrocyclic ligands by template reactions frequently involves the reaction of two difunctionalised precursors, and we have tacitly assumed that they react in a 1 1 stoichiometry to form cyclic products, or other stoichiometries to yield polymeric open-chain products. This is certainly the case in the reactions that we have presented in Figs 6-8, 6-9, 6-10, 6-12 and 6-13. However, it is also possible for the difunctionalised species to react in other stoichiometries to yield discrete cyclic products, and it is not necessary to limit the cyclisation to the formal reaction of just one or two components. This is represented schematically in Fig. 6-19 and we have already observed chemical examples in Figs 6-4, 6-11 and 6-18. We have already noted the condensation of two molecules of 1,2-diaminoethane with four molecules of acetone in the presence of nickel(n) to give a tetraaza-macrocycle. Why does this particular combination of reagents work Again, why are cyclic products obtained in relatively good yield from these multi-component reactions, rather than the (perhaps) expected acyclic complexes We will try to answer these questions shortly. 

An x-ray crystal structure was reported for 250. Selected parameters are recorded in Table VIII. The nickel complex is similar to its platinum counterpart in that it has a nearly planar coordination geometry, and most of the parameters are very similar for the two. The most striking difference is in the M-C and M-P bond distances, differences that are comparable to those found in acyclic complexes.94 In the IR spectrum, the triple bond... 

The macrocyclic Schiff base 5,9,14,18-tetramethyl-l,4,10,13-tetraazacyclo-octadeca-5,8,14,17-tetraene-7,16-dione (I) (H4daen) can serve as a precursor to the formation of either mononuclear macrocyclic complexes or mononuclear acyclic complexes, depending on the reaction conditions. The mononuclear complexes may then be used in the synthesis of homo- or heterobinuclear complexes. The synthetic procedures described have been extended to include Schiff bases derived from a range of symmetrical and nonsymmetrical tri-... 

Gd-based clinical CAs can be divided into two groups according to their structure. The two groups are acyclic complexes and cyclic complexes, the structures of which are shown in Figures 10.1 and 10.2, respectively, and the properties of which are summarized in Table 10.1. 

The 1,2-azoniaboratolidines 26 are internal amine-borane complexes. Analogous acyclic complexes have been studied since the classical work of H. C. Brown in the 1940s <1947JA1332>. Qualitative data suggest that many 1,2-azoniaboratolidines are in equilibrium with their open-chain isomers 130, as illustrated in Equation (5). The position of the equilibrium is a function of the Lewis acidity of the boron atom and steric hindrance about both boron... 

Dimetallacycles are commonly synthesized by substitution of a geminal dihalide (Equation (27)) <86JOM(3l4)C43, 89JOM(359)205>, (Equation (28)) <88JA7868>. The osmium compound can also be prepared by elimination of methane from an acyclic complex (Equation (29)) <82JA7325>. Many dimetallics have been synthesized by additions to metal carbenes <82AOC(20)159>. Transmetallation reactions are also known (Equation (30)) <90ICAll>. 

Fig. 7.12 Comparison of a cyclic and an acyclic complex with a bridging phosphanide ligand...

Lemma 2.2.5.1. The right-F-acyclic complexes in J are the objects of a localizing subcategory ( 1.7). Moreover, the restriction of F to this subcategory transforms quasi-isomorphisms into isomorphisms in other words, if the complex X is both exact and right-F-acyclic, then F X) = 0 see (1.5.1)). 

In particular, the direct sum of two right-F-acyclic complexes is right-F-acyclic, because the direct sum is the summit of a triangle whose base is the zero-map from one to the other, see (1.4.2.1). Also, 0 G J is clearly right-F-acyclic. We see then that the right-F-acyclic complexes are the objects of a A-subcategory of J. 

We leave it to the reader to establish a corresponding statement for left-F-acyclic complexes. 

Proof Clearly, right-F-acyclic complexes are right-(GF)-acyclic. It follows then from (2.2.4) and (2.2.5) that the assertion need only be proved for the restriction of F to the subcategory of right-F-acyclic complexes, in which case it follows fi-om (2.1.3). Q.E.D. 

We leave the corresponding statements for left-F-acyclic complexes and left-derived functors to the reader. 

In other words, if Z is the complex which looks like A B in degrees —1 and 0 and which vanishes elsewhere, then the quasi-isomorphism Z C given by the exact sequence 0—>C —>0 becomes, upon application of , an isomorphism in D( ) and hence, by (2.2.5.2) (dualized), Z is a acyclic complex. 

Reaction with O2 produces OJ. A series of rigidly structured Ni(II) macrocyclic and acyclic complexes has been shown to react rapidly with hydroxyl radicals and aquated electrons.Addition of e generates a Ni(I) transient except when the ligand incorporates a nitro group. Reaction with OH leads to a longer-lived Ni(III) species. OH radicals have also been shown to react with Pu(III) with a specific rate of oxidation of 1.83 0.78 x 10 Ms at pH Comparison has been... 

T (H2O) = 1.54 ms, T (D2O) = 2.61 ms, < ,ot 0.32, < ,ot (H2O) = 0.32. (Eu)36 r (H2O) = 0.62 ms, x (D2O) = 2.25 ms, brightness (relative to (Eu)DTPA-csl24) = 0.57, quantum yield was not substantially increased in this DOTA derivative compared to previously reported acyclic complexes (DTPA-csl24). One important feature of this article is the discussion about the lanthanide quantum yield ( Ln). This parameter has been often poorly determined however, it is a key parameter in LRET experiment since the distance R, inferred from energy transfer experiments via donor luminescence decay or intensity measurements, depends on knowing the donor ( Ln) quantum yield R oc Very often this has been explicitly or implicitly... 

R. Oliver, Fixed-point sets of group actions on finite acyclic complexes. Comment. Math. Helv. 50 (1975), pp. 155 177. 

One route to T -dienyl complexes is by hydride abstraction from readily available -diene complexes using a hydride acceptor, such as the trityl cation (Scheme 10.23). For an additional example involving a cobalt diene complex, see Scheme 11.23. Hydride abstraction is most easily done using cyclic complexes, which have the required Z-geometry. Acyclic complexes can be made in this way, but the Z-geometry is less easily obtained. 

---