Copper(II)-assisted hydrolysis of 2,4,6-tris(2-pyridyl)-1,3,5-triazine. Part 3. Crystal structures of diaqua[bis(2-pyridylcarbonyl)amido]copper(II) nitrate dihydrate and aquabis(pyridine-2-carboxamide)copper(II) nitrate monohydrate

Abstract

Copper(II) has been found to promote the hydrolysis of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) to yield [bis(2-pyridylcarbonyl)amido]copper(II) and free pyridine-2-carboxamide, NC₅H₄CONH₂. The compounds [Cu{(NC₅H₄CO)₂N}(H₂O)₂]NO₃·2H₂O (1) and [Cu(NC₅H₄CONH₂)₂(H₂O)][NO₃]₂·H₂O (2), where (NC₅H₄CO)₂N^– is the bis(2-pyridylcarbonyl)amide anion (L), were grown from aqueous solutions of tptz and copper(II) nitrate. Crystals of (1) and (2) are triclinic, space group P and Z= 2, with a= 6.811(1), b= 10.211(1), c= 12.039(1)Å, α= 88.65(1), β= 82.32(1), and γ= 89.79(1)° for (1) and a= 11.622(3), b= 11.280(3), c= 8.007(3)Å, α= 90.33(2), β= 101.85(4), and γ= 115.28(3)° for (2). The copper(II) ion is five-co-ordinate in both structures and its co-ordination geometry is near square pyramidal. The three nitrogen atoms of the (NC₅H₄CO)₂N^– ligand and the oxygen atom of a water molecule form a distorted square plane around the copper ion in complex (1), whereas such a basal plane is built by the pyridine nitrogen and oxygen atoms of two carboxamides in (2). The axial position is filled by a water molecule in both cases. The proton association and acid dissociation constants of the complex [CuL(H₂O)]⁺[equations (i) and (ii)] have been determined by potentiometry in aqueous solution: log K₁=[CuL(H₂O)]⁺+ H⁺ [graphic omitted] [Cu(HL)(H₂O)]²⁺(i), [CuL(H₂O)]⁺ [graphic omitted] [CuL(OH)]+ H⁺(ii) 1.83(1) and log K₂=–8.80(1)(25 °C, 0.1 mol dm^–3 NaNO₃). The co-ordination of L to copper(II) decreases the basicity of the imide nitrogen atom as shown by the low value of log K₁. The value of the hydroxo stability constant falls within the range established for hydroxocopper(II) complexes with N-donor tridentate ligands. A discussion of the factors which determine the nature of the isolated products of this copper(II)-promoted hydrolysis is made in the light of the available structures.