Mono- and di-nuclear nickel(II) complexes with Schiff bases derived from 4-substituted 2,6-diformylphenol and 7-amino-4-methyl-5-azahept-3-en-2-one; crystal and molecular structure of [4-chloro-2,6-bis(4-methyl-2-oxo-5,8-diazanona-3,8-dienyl)phenolato(3–)]dinickel(II) bromide hemihydrate