Studies of pendant-arm macrocyclic ligands. Part 5. Synthesis of two pyridine-containing penta-aza macrocycles with single pyrrolidinyl pendant arms and characterisation of their nickel(<scp>II</scp>) and copper(<scp>II</scp>) complexes. Crystal structure of perchlorato{7-[2′-(1″-pyrrolidinyl)ethyl]-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene}nickel(<scp>II</scp>) perchlorate

Alcock, Nathaniel W.; Balakrishnan, K. P.; Moore, Peter; Omar, Hadi A. A.

doi:10.1039/dt9870000545

Cited by 16 publications

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“…In contrast, for I , d (Ni−O21) is short enough (2.66 Å) to be considered a weak coordinative bond, making the overall coordination number 5 + 1. The X-ray structure of I is very similar to that described previously for an analogous nickel(II) complex having a pyrrolidinyl group in the side arm . Although all Ni−N distances are somewhat shorter for I , the Ni−O(ClO 4 ) distance is 0.27 Å longer in I than in Alcock's Schiff base complex .…”

Section: Resultssupporting

confidence: 74%

“…The X-ray structure of I is very similar to that described previously for an analogous nickel(II) complex having a pyrrolidinyl group in the side arm . Although all Ni−N distances are somewhat shorter for I , the Ni−O(ClO 4 ) distance is 0.27 Å longer in I than in Alcock's Schiff base complex . The Cu−N(macrocycle) bonds in II are significantly shorter than the corresponding Ni−N(macrocycle) bonds in I , while the axial Cu−N(pendant arm) bond (2.186 Å) is significantly elongated due to Jahn−Teller distortion and is ca.…”

Section: Resultssupporting

confidence: 68%

“…The pH range for the proton-dependent dissociation of the coordinated aminopropyl pendant arms is very convenient as it allows for switching between two different ligand binding modes under mild conditions. Shorter side arms, which form five-membered chelate rings with the central metal ion, are usually less susceptible to protonation and can only be protonated in acidic media (pH of about 2). , The same qualitative trend in relative stabilities of the coordinated N , N -dimethylaminopropyl and N , N -dimethylaminoethyl groups attached to the pyridine-containing macrocycles has also been reported by Alcock and Moore. ,, …”

Section: Resultssupporting

confidence: 55%

“…These include 3 + 2 condensation leading to cryptand formation (observed, for example, in analogous condensations between 2,6-diformylphenol and trpn) and several kinds of polymerization processes. Extensive investigations of monofunctionalized pyridine-containing complexes by Kaden , and Alcock and Moore − did not reveal these difficulties, because, in their rigorous synthetic approach, the functional group in the pendant arm (such as 2-methylpyridine or N , N -dimethylethylamine) was chosen to be unreactive toward the carbonyl groups of 2,6-diformylpyridine.…”

Section: Resultsmentioning

confidence: 99%

“…(4) It is difficult to satisfy the diverse coordination requirements of different metal ions incorporated into rigid Schiff base pyridine-containing macrocycles with an additional donor atom in the side arm. Analogous reduced ligands are flexible and adopt a variety of conformations, producing both tetragonal-pyramidal and trigonal-bipyramidal metal complexes. − How will the unsaturated ligands accommodate different metal ions, using highly restricted conformational space? In this regard, the structures of Schiff base complexes formed in template condensations are of particular interest.…”

Section: Resultsmentioning

confidence: 99%

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Unusual Ligand Isomerization Dictated by Coordination Requirements of the Metal Ion: A Double-Bond Shift in Cu(II)-Assisted Template Condensation between Diacetylpyridine and a Tripodal Tetramine

1999

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This paper reports a new effect in template-directed Schiff base condensation, a "fine-tuning" of the macrocyclic structure in accordance with the precise coordination requirements of the metal ion. The nature of the metal template determines the outcome of Schiff base condensation between a difunctional carbonyl component (2,6-diacetylpyridine) and a trifunctional amine component (tris(3-aminopropyl)amine, trpn). Both Cu(II) and Ni(II) facilitate [1 + 1] condensation with the formation of macrocyclic products in 50-60% yield, but single-crystal X-ray diffraction reveals that the chemical structure of the major product is metal-ion dependent. In the nickel(II) complex [Ni(L(sym))](2+), the C=N double bonds are conjugated with the pyridine ring, as expected from the structure of the starting diacetylpyridine. In contrast, the copper(II) ion caused isomerization of the macrocyclic ligand, with one of the C=N double bonds migrating into the initially saturated six-membered chelate ring forming [Cu(L(asym))](2+). The second product isolated, in small quantities, from the copper(II)-templated condensation has been characterized as an isomeric symmetric macrocycle [Cu(L(sym))](2+). In the case of the zinc(II) template, no macrocyclic product has been isolated; instead, a zinc(II) complex of the starting tetramine, Zn(trpn)Cl(2).2H(2)O, was recovered and structurally characterized. An unusual double-bond migration in the course of Cu(II)-directed template condensation is governed by the coordination requirements of the central metal ion. The asymmetric ligand isomer is more flexible than its symmetric counterpart and can therefore better accommodate the five-coordinate central metal ion. This effect is more pronounced for the copper(II) complex than for the nickel(II) complex, because of shorter in-plane M-N distances in the former, as follows from the X-ray structure determination. The position of the double bond in the macrocyclic ring influences the spectral properties of the copper(II) complexes and the ionization constants of the amino group in the side arm.

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Section: Resultssupporting

confidence: 74%

Section: Resultssupporting

confidence: 68%