SYNTHESIS AND COMPUTATIONAL STUDIES OF 1-4-η-CYCLOHEXA-1,3-DIENE DERIVATIVES OF IRONTRICARBONYL COMPLEXES

Abstract

Metal complexes have interesting properties and varied applications in light emitting diodes. A number of substituted pyridine metal (Rhenium, Ruthenium and Iridium) complexes have been synthesised and characterised. However, the synthesis and characterisation of substituted pyridine iron complexes such as Tricarbonyl (1-4-η-pyridino-cyclohexa-1,3-diene) iron complexes for its light emitting property remain scanty. Therefore, this research was designed to synthesise, characterise and calculate the electronic properties of Tricarbonyl (1-4-η-5-exo-N-pyridino-2-cyclohexa-1,3-diene) iron complexes. Twelve Tricarbonyl (1-4-η-5-exo-N-pyridino-cyclohexa-1,3-diene) iron complexes were synthesised by the addition of pyridines to the dienyl ring of Tricarbonyl-1-5- η-cyclohexadienyl iron tetrafluoroborate at room temperature, in solutions of acetonitrile. They were characterised using elemental analysis, infra-red (IR) and proton-nuclear magnetic resonance (1H NMR) spectroscopy. Their theoretical studies were carried out using Molecular Mechanics Force Field (MMFF), semi-empirical Parameterization Method three (PM3), Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TDDFT) with Becke three, Lee, Yang and Parr at 6-31G* level. The MMFF was used to obtain the stable conformer before subjecting each complex to geometry calculations. The geometrical and thermodynamic properties were obtained from PM3 while the electronic energy [Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO)], chemical potential, chemical hardness and electrophilicity index were obtained from DFT. The calculation of electronic-excited state was carried out using TDDFT. The calculated elemental compositions of the synthesised complexes were in agreement with the observed values. All the complexes had very sharp IR peaks at 1980 and 2055 cm-1 corresponding to the stretching vibrations of carbonyl bond attached to the metal. The 1H NMR spectra displayed well separated overlapping multiplets which are characteristic of the outer diene protons H1 and H4 at 3.2 ppm while the inner protons H2 and H3 appeared at 4.7 and 5.3 ppm respectively. The geometrical parameters ranged from 1.374 to 1.795 Å (bond lengths), 105.1 to 128.8 o (bond angles) and -8.9 to 128.5 o (dihedral angles). These properties were functions of the substituents on the rings. The thermodynamic parameters were: free energy change (-412.6 to -188.5 kJmol-1), enthalpy change (-237.3 to +4.7 kJmol-1) and entropy change (+595.2 to +697.3 Jmol-1K-1) indicating that the formation of these complexes was spontaneous. The electronic properties were within the range of -9.8 to -9.1 eV and -6.1 to -4.8 eV (HOMO and LUMO energies) respectively, 3.3 to 4.5 eV (energy band gap), -6.9 to -7.9 eV (chemical potential), 1.6 to 2.2 eV (chemical hardness), 11.4 to 18.1 eV (electrophilicity index). These values were indicative of reactivities of these molecules. The electronic-excited state of methyl and dimethyl substituted complexes were triplets having 3.46, 3.42 and 3.40 eV excitation energies with 0.022, 0.039 and 0.019 oscillator strength. The amino substituted complexes were singlets with 4.34 eV excitation energy and 0.024 oscillator strength indicating their light emitting ability. The electronic-excited state of synthesised methyl, dimethyl, and amino substituted Tricarbonyl (1-4-η- pyridino-cyclohexa-1,3-diene) iron complexes are capable of emitting light via phosphorescence and fluorescence, and may be useful in light emitting diodes.