Interconversion between Polymeric and Monomeric Forms of Oxovanadium(IV) Complexes with Tetradentate Schiff Base Ligands Derived from (R,R)-2,4-Pentanediamine

New tetradentate Schiff base−oxovanadium(IV) complexes, [VO{3-Xsal-( R,R )-2,4-ptn}] (H 2 {3-Xsal( R,R )-2,4-ptn}: N,N ‘-di-3-Xsalicylidene-( R,R )-2,4-pentanediamine; X = EtO (ethoxy), MeO (methoxy), and H) were prepared and characterized. Only an orange polymeric form was obtained for the 3-methoxy and nonsubstituted (X = H) complexes. Both orange polymeric ( 1 ) and green monomeric ( 2 ) crystals were obtained for the 3-ethoxy substituted complex. X-ray structure analyses of 1 and 2 were carried out. Crystal data: [VO{3-EtOsal-( R,R )-2,4-ptn}]·H 2 O ( 1 ), VO 6 N 2 C 23 H 30, orthorhombic, P 2 1 2 1 2 1, a = 13.764(4) Å, b = 21.544(4) Å, c = 7.682(5) Å, Z = 4; [VO{3-EtOsal-( R,R )-2,4-ptn}] ( 2 ), VO 5 N 2 C 23 H 28, orthorhombic, P 2 1 2 1 2 1, a = 10.393(2) Å, b = 22.278(2) Å, c = 9.886(2) Å, Z = 4. The orange crystals ( 1 ) have water of crystallization, and the complexes take a polynuclear linear chain structure (···V O···V O···) with a six-membered N−N chelate ring in a chair form. The green crystals ( 2 ) have no water of crystallization, and the complexes take a mononuclear square-pyramidal structure with a six-membered N−N chelate ring in a flattened boat form. Upon heating at 170 °C for 10 min, orange 1 turned to green 2, which reverted to orange 1 by suspension in water−acetonitrile. Mechanism of the interconversion between the polymeric and monomeric forms is discussed on the basis of their crystal structures.