Bis{μ-2-[(4,6-bis{(2-hydroxy-5-methylphenyl)[(pyridin-2-yl)methyl]amino}-1,3,5-triazin-2-yl)[(pyridin-2-yl-κN)methyl]amino-κN]-4-methylphenolato-1:2κ2 O:O}bis[(nitrato-κ2 O,O′)zinc]–acetonitrile–water (2/4/1)