Curtis ligands

By using combinations of hydrogenation and dehydrogenation reactions it has been possible to obtain nickel derivatives of the Curtis macrocycle containing from zero to four imine groups (Curtis, 1968 1974). Related reactions in the presence of a variety of other central metal ions have been described. The electrochemical oxidation of the Cu(ii) complex of the reduced Curtis ligand proceeds initially via a two-electron step to yield the monoimine complex (296) (Olson Vasilevskis, 1971). 

Interest in complexes of new synthetic macrocyclic ligands (and especially amines) was sparked by the discovery of the so-called Curtis ligands (111) first observed893 as the product from the... 

Carbonyl compounds are commonly used precursors for the polyaza macrocycles, as in the classic synthesis of the Curtis ligand from the condensation of acetone with Ni(en)3 (Scheme 2). 2,6-Diacetylpyridine has provided the... 

The complex [Co(NH3)5(OS02CF3)]2+ undergoes triflate substitution by cyanate and condensation with acetone to form the unusual bis-bidentate imine complex (213)917 in a reaction reminiscent of the classic Curtis condensation of acetone with ethylenediamine. Apart from the novelty of the synthesis, the reported crystal structure was only the second of a CoN6 complex bearing a cyanate ligand. 

Apart from this one-reaction type, the routine use of metal template procedures for obtaining a wide range of macrocyclic systems stems from 1960 when Curtis discovered a template reaction for obtaining an isomeric pair of Ni(n) macrocyclic complexes (Curtis, 1960). Details of this reaction are discussed later in this chapter. The template synthesis of these complexes marked the beginning of renewed interest in macrocyclic ligand chemistry which continues to the present day. 

As mentioned already, Curtis reported the first (Curtis, 1960) of a number of pioneering template reactions for macrocyclic systems which were published in the period 1960 to 1965. In the Curtis synthesis, a yellow crystalline product was observed to result from the reaction of [Ni(l,2-diaminoethane)3]2+ and dry acetone. This product was initially thought to be a bis-ligand complex of the diimine species (58). However, the stability of the product in the presence of boiling acid or alkali was... 

Lawrance Curtis, 1975) in this case the trans-diene ligand was isolated, for example, in greater than 80% yield, as its dihydrobromide. 

M(iii) and Cn(m) complexes. In early classic studies the redox chemistry of tetraaza macrocyclic complexes of Ni(n) and Cu(n) (of the Curtis and reduced Curtis type) was investigated in acetonitrile (Olson Vasilevskis, 1969 1971). These authors were the first to report the electrochemical generation of Ni(m) and Cu(m) complexes of such N4-cyclic ligands. Since this time, a considerable number of related studies, involving both nickel and copper macrocyclic species, have been reported. 

Figure 8.2. The metal-ion dependence of ligand oxidation in complexes of the reduced Curtis macrocycle.

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

Curtis-type macrocyclic complexes are sufficiently stable to undergo a variety of oxidation and reduction processes to yield a range of complexes containing from zero to four imine linkages (Scheme 10).40 66 84 85 96 The oxidative dehydrogenation process is metal-ion dependent, as illustrated by the alternative ligand structure developed from the iron(II) complex (equation 14) 97 9S... 

Thomson CG, Beer MS, Curtis NR, Diggle HJ, Handford E, Kulagowski JJ. Thi-azoles and thiopyridines novel series of high affinity h5HT(7) ligands. Bioorg Med Chem Lett 2004 14 677-680. 

Curtis and Eisenstein355 have made a molecular orbital analysis of the regioselectivity of the addition of nucleophiles to 77-allyl complexes and on the conformation of the 773-allyl ligand in [MoX(CO)2L2(773-allyl)] type complexes. A detailed study of the chirality retention in rearrangements of complexes of the type [MX(CO)2(dppe)(rj3-C3H5)] has been made.356 Studies of the photoelectron spectra,357 electrochemical properties,358 infrared spectroelectrochemistry,359 and fast atom bombardment mass spec-... 

Metal ion template mediation in macrocyclic synthesis has been a part of the field since its inception, its importance having been realized early in the development of this area. Two specific roles for the metal ion in template reactions have been proposed. These are, in turn, kinetic and thermodynamic in origin." In the kinetic template effect, the arrangement of ligands already coordinated to the metal ion provides control in a subsequent condensation during which the macrocycle is formed. The thermodynamic effect serves to promote stabilization of a structure which would not be favored in the absence of a metal ion. Schiff base condensations tend to be dependent on this latter type of template effect. Some of the more routine and general synthetic procedures will be described here. A more in-depth treatment can be found in a review by Curtis, with particular emphasis on general methods as well as modifications of preformed macrocycles." ... 

The report by Curtis of the condensation of [Ni(en)3] + with acetone to give the open-chain condensation product and subsequently the macrocyclic complex (equation 8) was the first example of the synthesis of N-donor macrocycles by a metal template, and predates work on the oxycrown ligands such as 18-crown-6 ([18]ane06). 

Gupta AK, Beny C, Gupta M, Curtis A (2003) Receptor-mediated targeting of magnetic nanoparticles using insulin as a surlace ligand to prevent endocytosis. IEEE Trans Nanobioscience 2 255—261. 

It is instructive to begin with a consideration of systems that are not necessarily poised but may be shifted far toward a single, spin state. The macrocyclic ligand reported by Curtis (5), 2,4,4,9,11,11-hexamethyl-1,5,8,12-tetraazacyclotetradecane (Ni + complex. Structure I, hereafter... 

This ligand belongs to a group of widely studied synthetic macrocycles discovered by Curtis. The first published procedure, involving the reaction of tris-(ethylenediamine)nickel(II) perchlorate with acetone, is relatively arduous because (1) the condensation reaction occurs slowly over several days and (2) the major products " of the reaction are the chemically very stable positional isomers 5,7,7,12,14,14-hexamethyl-l, 4,8,1 l-tetraazacyclotetradeca-4,11-diene (Me(j[14]-4,1 l-dieneN4) and 5,7,7,12,12,14-hexamethyl-l,4,8,l 1-tetraazacyclo-tradeca-4,14-diene (Me6[14]-4,14-dieneN4). These isomers, which are similar in... 

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