Characterization, nucleolytic, cytotoxic and antibacterial property of tetrakis(μ-4-chloro-2-nitrobenzoato-κ2O,O′)bis[aquacopper(II)], tetrakis(μ-2-chloro-6-fluorobenzoato-κ2O,O′)bis[aquacopper(II)] and tetrakis(μ-2-chlorobenzoato-κ2O,O′)bis[aquacopper(II)]