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

Abstract

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.

Copper complexes with new oxaaza-pendant-armed macrocyclic ligands: X-ray crystal structure of a macrocyclic copper(II) complex

“…Interest in pendant-armed macrocycles is growing on account of their unique coordination and structural properties [1], their bioinorganic applications [2], their utility as contrast reagents in MRI [3], their use as tumor-directed radioisotope carriers [4], and their ability to carry out controlled molecular movements and translocations [5]. Furthermore, this type of functionalized amino pendant-armed macrocycle provides opportunities for the synthesis of a wide range of derivatives, by a Schiff-base condensation with a suitable carbonyl compound, leading to precursors for polynuclear complexation.…”

Synthesis and characterization of nickel(II) complexes with three potentially hexadentate Schiff-base ligands and polyamines: X-ray crystal structure determination of one nickel(II) complex

“…The IR spectra of the three polyamine complexes all show similar features. As in the free ligands, there are bands associated with m(NH 2 ) and m(NH) from the primary and secondary amines, which have undergone a shift toward low wavenumber on complexation, suggesting an interaction between the metal ions and the amine nitrogen atoms [15]. All of the complexes exhibit a broad mediumintensity band at 2258-2265 cm -1 assigned to the C-N group.…”

“…The solution was then concentrated in a rotary evaporator to a volume of ca. 15 General synthesis for the polyamine complexes A methanolic solution (50 mL) of Ni(ClO 4 ) 2 Á6H 2 O (0.37 g, 1 mmol) was added to a methanol/acetonitrile (2:1) solution of the appropriate polyamine (1 mmol …”

Synthesis and characterization of three heptaaza manganese(II) macrocyclic Schiff base complexes containing two 2-pyridylmethyl pendant arms

“…Previously, we and others have reported a one-step synthesis of Schiff base macrocyclic complexes bearing a primary amino group in the pendant arm via a template condensation between 2,6-diacetylpyridine and tripodal aliphatic tetraamines [6][7][8][9][10]. In the last decade, we have used 2,6-diacetylpyridine, 2,6-diformylpyridine, and 2,9-dicarboxaldehyde-1,10-phenanthroline as precursors to achieve several kinds of cyclocondensation reactions with a series of multi-amine derivatives, and a number of pendant-armed macrocycles have been prepared in this way [11][12][13].…”