Copper(II) chelates derived from an N,N,O-tridentate 2-pyridinecarboxaldehyde-N4-phenylsemicarbazone: Synthesis, spectral aspects, crystal structure, FMO and NBO analysis

Seven novel Cu(II) complexes [Cu(HPySc)Cl2] (1), [Cu2(PySc)2(NO3)2]·H2O (2), [Cu(HPySc)(SO4)]·½H2O (3), [Cu2(PySc)2(OAc)2]·CH3OH (4), [Cu2(PySc)2Br2]·H2O (5), [Cu(PySc)(NCS)]·2H2O (6) and [Cu2(PySc)2(N3)2] (7) with 2-pyridinecarboxaldehyde-N4-phenylsemicarbazone (HPySc) have been prepared and characterized using different analytical and spectroscopic techniques. The complex 6 is expected to be square planar while the other complexes are expected to be square pyramidal arrangement, among these, the geometry of complex 4 has been conformed by single crystal XRD study that it is a dimer and adopts distorted square pyramidal geometry around copper(II) centre. All the complexes are found to be paramagnetic and non-conductive in nature. The semicarbazone ligand is coordinated in neutral form in two of the complexes and in ionic form in others. DFT/B3LYP/6-311g**/LANL2DZ computations of the ligand HpySc (PyCHNNCONHPh) and seven Cu(II) complexes were performed to analyze the FMOs and important electron delocalizations that existed in each compound. Molecular stability and bond strengths have been investigated by applying natural bond orbital (NBO) analysis. All the complexes are more stable than the semicarbazone and the complex 2 is the most electronically stable (-5.160 eV) among the complexes. Furthermore, the semicarbazone is the hardest and complex 5 is the softest than all complexes due to the η values. The calculated energy gap between HOMO and LUMO energies show the variations of nucleophilic and electrophilic reactivity regions in the semicarbazone and in complexes 1-7.