“…It is worth to mention that the nitrogen atom is not protonated and takes part in coordination. This is different from those in the reported compounds [40][41][42][43][44][45][46][47]. In addition, there are four hydrogen bondings in this structure: (1) the protonated 1,4-butanediamine provides three hydrogen atoms to form hydrogen bondings with adjacent phosphonate groups (N2 Á Á Á O5, 2.816(6)Å; N3 Á Á Á O3, 2.816(6)Å; N3 Á Á Á O8d, 2.892(7)Å); (2) the protonated phosphonate oxygen atom (O7) interacts with the neighboring phosphonate oxygen atom (O2e) through a strong hydrogen bonding (O7 Á Á Á O2e, 2.460(6)Å).…”