Controlling the ligating behaviour of biologically important p-hydroxybenzoate towards copper(II) by the use of nitrogen bases: synthesis, characterization and single crystal X-ray structure determination of [trans-Cu(en)2(H20)2](L1)2.2H20 and [cis-Cu(L1)2(L2)2] where en=ethylenediamine, L1=p-hydroxybenzoate, L2= 3-picoline