A method of selectively extracting anions from an aqueous solution using an anion encapsulating aggregate formed upon the addition of a solvent, a copper contributor, a hydroxide contributor, an optional counterion contributor, and at least one encapsulating anion to an aqueous solution containing the anions, or upon the addition of a solvent, a polymeric chain of [CuII(μ-OH)(μ-ea)]∞ and optionally, a counterion contributor to the aqueous solution. The aggregates include compounds of the formula cis-CuIIx(OR1)y(R2ea)z, where R1 is H or an alkyl group, R2 is H, or an alkyl group or a charged group, ea is an encapsulating anion, and each of x, y, and z is equal to an integer between about 1 and 40, inclusive. In addition, anion encapsulating aggregates can be formed by synthesis with alternate encapsulating anions by substituting alternate encapsulating anions for the encapsulating anion incorporated in the aggregate.