A new open framework iron(III) phosphite with formula (C5H18N3)[Fe-3(HPO3)(6)]center dot 3H(2)O has been prepared by hydrothermal synthesis with N-(2-aminoethyl)-1,3-propanediamine as a templating agent. The crystal structure was solved from single-crystal X-ray diffraction data in the trigonal space group R (3) over cap. The unit cell parameters are a= 8.803(1) angstrom and c= 25.292(2) angstrom with Z= 3. The complex pillared structure can be described as two interpenetrating subnets, one organic, [(C5H18N3)center dot 3H2O](3+), and one inorganic, [Fe-3(HPO3)(6)](3-). In the inorganic subnet, the pillars are formed by FeO6 trimers linked by vertex sharing phosphite groups, while in the cationic subnet the organic molecules act like pillars. With increasing temperature, the flexibility of the structure allows contraction due to dehydration followed by thermal expansion before reaching the thermal stability limit. The Dq and Racah parameters calculated for (C5H18N3)[Fe-3(HPO3)(6)]center dot 3H(2)O are Dq = 965, B = 1080, and C = 2472 cm(-1). Mossbauer spectroscopy confirms the trivalent oxidation state of iron cations and the crystallographic multiplicities of their sites. The ESR spectra show isotropic signals with a g-value of 2.00(1). Specific-heat measurements show a three-dimensional (lambda-type) peak at a critical temperature T-c = 32 K. The value of the entropy at saturation is 46 J/mol K, very near the expected value of 44.7 J/mol K for the iron(III) cations with S = 5/2. Magnetic measurements indicate a three-dimensional antiferromagnetic ordering below 32 K and a reorientation of spins below 15 K with an incomplete cancellation of spins due to triangular interactions inherent to the structure.