3,​5-​Pyridinedicarboxylic acid, 1,​4-​dihydro-​2,​6-​dimethyl-​4-​(3-​nitrophenyl)​-​, 3-​[2-​[4-​(diphenylmethyl)​-​1-​piperazinyl]​ethyl] 5-​methyl ester, hydrochloride (1:2)