Mono- and dinuclear zinc(II) complexes of biological relevance. Crystal structures of [L2Zn](PF6)2, [L'Zn(O2CPh)2(H2O)], [L'2Zn2(.mu.-OH)2](ClO4)2, and [L'2Zn2(.mu.-OH)(.mu.-CH3CO2)2](ClO4).cntdot.H2O (L = 1,4,7-triazacyclononane, L' = 1,4,7-trimethyl-1,4,7-triazacyclononane)