Cu4CH3C (OH) (PO3) 22 (C4H4N2) (H2O) 4: a novel, three-dimensional copper diphosphonate with metamagnetismElectronic supplementary information (ESI) available: views of structure 1, temperature dependence of ac magnetic susceptibility and field dependence of magnetization of 1. See http: //www. rsc. org/suppdata/cc/b1/b106780j/