Synthesis, spectroscopic and electrochemical properties of some heteroleptic tris-chelates of ruthenium (II) involving 2,2′-bipyridine (bpy) and N-(aryl) pyridine-2-aldimine(L): X-ray crystal structures of [Ru(bpy)(L2)2](ClO4)2 · H2O and 3-N(4-tolyl) imidazo [1,5a] pyridinium perchlorate