Crystal structure of cyclo[diaqua-bis(μ2-3′,5-dicarboxy-[1,1′-biphenyl]-3,4′-dicarboxylato-κ4O,O′:O′′,O′′′)-bis(4,4′-bis(pyrid-4-yl)biphenyl-K1N)dicadmium(II)], C76H52Cd2N4O18