Structural preferences and isomerism in nickel(II) and copper(II) complexes with 3-hydroxypicolinic acid

Abstract

Recrystallization of [Ni(3-OHpic)2(H2O)2] (1) from pyridine (py) affords trans-[Ni(3-OHpic)2(py)2] (2), while cis-[Ni(3-OHpic)2(4-pic)2] (3) is obtained from 4-picoline (4-pic). The nickel(II) ion is octahedrally coordinated by two N,O-bidentate 3-hydroxypicolinate ligands in both 2 and 3, and by two pyridine molecules in trans positions (2) or two 4-picoline molecules in cis positions (3). [Cu(3-OHpic)2] reacted with 4-picoline to yield the pentacoordinated species [Cu(3-OHpic)2(4-pic)] (4), but in the presence of 3-picoline (3-pic) the octahedral trans-[Cu(3-OHpic)2(3-pic)2] (5), similar to 2, was isolated. The crystal structures of 2–5 exhibit strong intramolecular O–H···O hydrogen bonds, but the intermolecular interactions are weak (C–H···O hydrogen bonds, π–π and C–H···π interactions). DFT calculations showed a preference for octahedral coordination for both the nickel(II) and copper(II) complexes, and a very small preference for a cis arrangement of the monodentate ligands, as observed in 2 and 5. The cooperative effect of weak intermolecular interactions should be responsible for reversing the preferences, but no major driving force could be identified. Calculated (DFT) and experimental IR spectra for 2–5 were compared and characteristic bands assigned. Thermogravimetric studies showed the initial loss of two coordinated pyridine, 4-picoline or 3-picoline molecules in 2, 3 and 5, respectively.