Curtis macrocycle

In early studies, reaction of the Ni(n) complexes (59) and (60) of the trans and cis (diimine) isomers of the Curtis macrocycle with nitric acid yielded the tetraimine species (294) and (295), respectively. There is strong evidence that these reactions proceed via Ni(m) intermediates... 

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). 

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

Macrocyclic coordination compounds formed bv condensation of metal amine complexes with aliphatic carbonyl compounds. N. F. Curtis, Coord. Chem. Rev., 1968, 3, 3-47 (78). 

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... 

Reactions of selected metal complexes of multidentate amines with formaldehyde and a range of carbon acids (such as nitroethane) have led to ring-closure reactions to yield a series of three-dimensional cage molecules (see Chapter 3). Condensations of this type may also be used to produce two-dimensional macrocycles (Comba et al., 1986) - see [2.20], In such cases, it appears that imine intermediates are initially produced by condensation of the amines with formaldehyde as in the Curtis reaction. This is followed by attack of the conjugate base of the carbon acid on an imine carbon. The resulting bound (new) carbon acid then reacts with a second imine in a cis site to yield chelate ring formation. 

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. 

The location of the induced unsaturation in the macrocyclic system is metal-ion dependent. This is illustrated by the examples given in Figure 8.2. In the Fe(n) complex, the imine functions form as conjugated pairs (Dabrowiak, Lovecchio, Goedken Busch, 1972 Goedken Busch, 1972) - such a-diimine species have long been known to have a special affinity for Fe(ii). In contrast, Ni(n) promotes formation of a product in which the respective imine functions are in electronically isolated positions (Curtis, 1968 1974). 

One of the most spectacular and useful template reactions is the Curtis reaction , in which a new chelate ring is formed as the result of an aldol condensation between a methylene ketone or inline and an imine salt. The initial example of this reaction was the formation of a macrocyclic nickel(II) complex from tris(l,2-diaminoethane)nickel(II) perchlorate and acetone (equation 53).182 The reaction has been developed by Curtis and numerous other workers and has been reviewed.183 In mechanistic terms there is some circumstantial evidence to suggest that the nucleophile is an uncoordinated aoetonyl carbanion which adds to a coordinated imine to yield a coordinated amino ketone (equation 54). If such a mechanism operates then the template effect is largely, if not wholly, thermodynamic in nature, as described for imine formation. Such a view is supported by the fact that the free macrocycle salts can be produced by acid catalysis alone. However, this fact does not... 

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... 

Metal template syntheses of complexes incorporating the p-amino imine fragment have been introduced by Curtis as a result of his discovery that tris(l,2-diaminoethane)nickel(II) perchlorate reacted slowly with acetone to yield the macrocyclic complexes (40) and (41) (equation 8).81-83 In this macrocyclic structure the bridging group is diacetone amine imine, arising from the aldol condensation of two acetone molecules. This reaction is widely general, in the same way that the aldol reaction is, and can be applied to many types of amine complexes. The subject has been reviewed in detail with respect to macrocyclic complexes by Curtis.84... 

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... 

The reaction of the nickel(II) perchlorate complex (89) with acetone and related ketones yields simple hydrazone complexes rather than macrocyclic complexes resulting from a Curtis-type reaction. Acetylacetone also fails to bridge the bis(hydrazine) and instead yields a pyrazole derivative (Scheme 37).197 198 In contrast to this result, 1,4-dihydrazinophthalazine undergoes a template reaction with 2,2-dimethoxypropane, a source of acetone, in the presence of nickel(II) fluoroborate and a trace of fluoroboric acid (Scheme 38).199... 

Donald Cram attempted synthesis of cyclophane charge transfer complexes with (NC)2C=C(CN)2 1961 - N.F. Curtis first Schiff s base macrocycle from acetone and ethylene diamine 1964 - Busch and Jager Schiff s base macrocycles... 

Most of the template syntheses of nonbenzenoid macrocycles originated with Curtis (39) and involve the condensation of metal-amine complexes with aliphatic carbonyl compounds, e.g., the reaction of acetone with tris(diaminoethane)nickel(II) perchlorate at ambient temperature leads to the isolation of three products, two of which may be represented as cts-XLIX and trans-L and the other is formed by a further interconversion of complex L in solution (39,143). With Cu(II) diaminoperchlorates, a mixture of cis and trans complexes analogous to XLIX and L is formed, but with Co(II) only the trans analog of L has been isolated. When ketones containing bulky groups are used, the reaction is much slower, e.g., there is only a small yield of LI from... 

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... 

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." ... 

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 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). 

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... 

Perhaps the most clearly defined example involves the macrocyclic complex of Curtis, Ni(CT)X2. The solid, violet, paramagnetic, anhydrous chloride and bromide must be carefully protected from moisture for they readily revert to yellow, diamagnetic dihydrates. The fact that two molecules of water are taken up to produce a diamagnetic nickel(II) ion is especially interesting for displacement of the halide, and coordination by the water molecules would surely produce a violet, paramagnetic species... 

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... 

Curtis has discussed the preparation of complexes of this general class in a comprehensive review. Condensation of several nickel and copper diamine complexes with a range of aliphatic aldehydes and ketones has led to the synthesis of a wide variety of macrocycles containing varying substituents and/or varying macrocyclic ring sizes. " The mechanism of the general reaction is not understood. 

Curtis, N. F. Structural Aspects. In Coordination Chemistry of Macrocyclic Compounds Melson, G. A., Ed. Plenum New York, 1979 pp 219-344. 

Curtis and coworkers first reported the amine-imine linkage formation of the type shown in Figure 1 by condensation of two molecules of acetone with ethylenediamines in the presence of nickel(II) or copper(II) ions ( 1, 2 3). It was also shown that this type of reaction generally occurs with several carbonyl compounds (4, 5). Later, the mechanism of the linkage formation and the stereoisomerism and some reactions of the metal complexes with macrocycles were investigated in detail (5, 6, 7 ). 

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