Crystal structure of Zn 2 [(1,1′-(hexane-1,6-diyl)bis(3-(pyridin-3-yl)urea))·(H 2 O) 2 ·(DMF) 2 ·(SO 4 ) 2 ], C 24 H 50 N 8 O 18 S 2 Zn 2