Conformational stabilities, PEP, EPR and NQR studies of the diaqua tris(ethylenediamine) copper

Five-membered ethylenediamine chelate rings have two possible conformations known as δ and λ. When three such chelates form a tris complex, the metal center is chiral and will have two possible enantiomeric configurations known as Δ and Λ. Combining all the structural possibilities there results, even for the simple, symmetrical chelate ethylene-diamine(en), a total of eight isomers [1] : Λ(δδδ), Λ(δδλ), Λ(δλλ), Λ(λλλ) and Δ(δδδ) ,Δ(δδλ) ,Δ(δλλ) ,Δ(λλλ). In a nonchiral medium the discussion can be limited to the first four of these isomers with the realization that all arguments apply equally well to their appropriate mirror images. In agreement with theoretical energy calculation [2] the Λ(δδδ) isomer has been found to be the most abundant. More recently, however, it has been argued that hydrogen bonding [3] or crystal packing forces [4] may cause any of the four isomers to be the most stable.It has also been argued that entropy effects would cause the Λ(δδλ) isomer to have the lowest free energy [5] In order to test this latter contention it would be necessary to determine the structure of a [M(en)3]m+ complex ion in the presence of a large, nonhydrogen bonding counterion.Conformational stabilities of the transition metal complex of the Cu(en)3H2O is studied using Density Functional Theory (DFT). The deformational potential energy profiles (PEPS), pathways between the different isomeric conformational energies were calculated using DFT/B3LYP/6-311++G**. Relative conformational energies of the Δ(λλλ), Δ(λλδ), Δ(λδδ) and Δ(δδδ) are 2.3,1.5,0.79,0.0 kcal/mol respectively. Which are small compared to the barrier heights for reversible phase transitions 58.2,58.6,59.4 kcal/mol respectively. EPR g-tensor of the Cu(en)32+ with two unpaired electron is calculated for electronic structure determination of Cu. Hydrogen bonds (HB) properties were studied in Cu(en)3 2H2O by DFT calculations of solid-state NQR parameters including quadrupole coupling constants at oxygen, nitrogen, andhydrogen nuclei. The ring geometry observed in complex is correlated with such parameters as intermolecular hydrogen bonding. It is suggested that the Λ(δδλ) configuration is the lowest energy isomer for these complex cations .