Interaction of metal ions with humic acid-like models. Part 4. Synthesis, spectral properties, and crystal and molecular structure of tetrakis(.mu.-2,6-dimethoxybenzoato)diaquadicopper(II) and of bis(.mu.-2,6-dimethoxybenzoato)bis(.mu.-acetato)diaquadicopper(II), a case of a dimeric copper(II) carboxylate complex with mixed bridges