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1:2 Cocrystals of piperazine (PPN) with o- and p-nitrophenol (oNPH and pNPH) were obtained from aqueous solution. The co-crystal structure of PPN, 2pNPH, and 2H(2)O is triclinic space group P (1) over bar: a = 6.401(1) Angstrom, b = 6.7515(1) Angstrom, c = 11.219(1) Angstrom, alpha = 100.37(1)degrees, beta = 97.10(1)degrees, gamma = 99.99(1)degrees,V= 465.5(1) Angstrom(3), Z = 2. Refinement led to a final conventional R value of 0.0365 for 2081 reflections. PPN, 2oNPH, and 2H(2)O cocrystallize in the monoclinic space group P2(1) : a = 7.753(1) Angstrom, b = 10.888(2) Angstrom, c = 11.378(2) Angstrom, beta = 92.89(1)degrees, V = 953.1(3) Angstrom(3), Z = 2. Refinement led to a final conventional R value of 0.0347 for 1978 reflections. It was found in both cocrystals that the hydroxyl H-atom of pNPH and oNPH was transferred to a N-atom of PPN, forming new ionic complexes PPNH22+.2(oNP(-)) and PPNH22+.2(pNP(-)), respectively. (Less)