Synthesis, vibrational study, crystal structure and density functional calculations of [Ni(dien)2]2+ complexes. Configurational and conformational study of [M(dien)2]n+ complexes
Introduction
Recently the stereochemistry of species with various coordination number and geometries has received increasing attention because of their implication in model compounds for bioinorganic systems, catalysts for enantioselective reactions, the building blocks for supramolecular assemblies and coordination units.
Linear tridentate triaza ligands have attracted interest because of their coordinating ability. According to Zelewsky [1], the diethylenetriamine molecule (dien) can be classified as a N-donor, polycentric, chelate and chiral ligand. Fig. 1 displays the three possible edge-configurations for octahedral [M(dien)2]n+ complexes [2]: configuration 1 (u-fac) is chiral (C2), configuration 2 (s-fac) is achiral (Ci) and configuration 3 (mer) for dien ligand is chiral due to the non-planarity of the central N atom, leaving a C2-symmetry chiral configuration.
Bis(dien) complexes of chromium(III) [3], cobalt(III) [4], rhodium(III) [5], iridium(III) [5], nickel(II) [6], copper(II) [7] and zinc(II) [8] are known. Cobalt and chromium complexes have been useful in various studies concerned with the isomer separations, structures and relative stabilities, optical activity, electrochemistry and isomerisation [9]. Thermally induced solid-state geometrical isomerism in [Ni(dien)2]2+ complexes has been studied [10].
In this contribution, we have focused on the configurational and conformational analyses of [M(dien)2]n+ complexes. For this purpose, the model fragment shown in Fig. 2 was searched in the CSD [11] and a statistical survey of its geometrical features was performed. These results are compared with crystallographic data and density functional calculations.
Section snippets
Materials
Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, Fluka), potassium cyanide (KCN, Merck), sodium hydroxide (NaOH, Fluka), chloranilic acid (C6H2Cl2O4, Aldrich), and diethylenetriamine (C4H13N3, Merck) were used as purchased without further purification. Microanalysis were performed on a Leco CHNS-932 analyser.
Physical measurements
Infrared spectra were obtained on a Nicolet Magna FTIR 550 spectrometer as KBr pellets in the spectral region 4000–400 cm−1. Raman spectra were measured on a Nicolet 950 FT Raman spectrometer
Synthesis and vibrational study
The formation of compound 2 using KCN as a starting material is probably a consequence of the high stability of the [Ni(CN)4]2− complex (log β4=31.3), which leads to the initial formation of the tetracyanonickelate(II) anion when the cyanide salt is added.
The vibrational spectroscopies are useful tools to distinguish between the mer and fac configurations of dien ligands in these metal complexes [25].
These differences can be seen in two regions:
- 1.
Region: 1500–1400 cm−1, CH2-deformations (δ−CH2)
Supplementary material
Further details of the crystal structure investigations may be obtained from the Cambridge Crystallographic Data Centre on quoting the deposition numbers CCDC 104029 (1) and CCDC 104030 (2).
Acknowledgements
This work was supported by the Gobierno Vasco (Grant PI-96/69) and the Universidad del Paı́s Vasco (Grant UPV 169.310-EA 162/98). P.V. acknowledges financial support from MEC (Grant AP95 30650377).
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