A hydrothermal reaction of a mixture of iron oxalate, arsenic pentoxide, hydrofluoric acid and triethylenetetramine (TETA) at 150 degrees C for 48 h gives rise to anew iron arsenate [C6N(4)H(21)][Fe-3(HAsO4)(6)], 1. The structure consists of a network of FeO(6)andAsO(4) building units connected through their vertices giving rise to a new secondary building unit, SBU-5. The SBU-5 units are through their corners forming a three-dimensional structure possessing one-dimensional channels bound by 8-T atoms (T = Fe, As). The formation of SBU-5 units is noteworthy. Variable temperature magnetic studies indicate antiferromagnetic interactions between the Fe centers with T-N of 21.9 K. Crystal data: M = 1156.36, monoclinic, space group = C2/c (no. 15), a = 18.422(3) angstrom, b = 8.8527(13) angstrom, c = 16.169(2) angstrom, beta = 111.592(2)degrees, V = 2451.9(6) angstrom(3), Z = 8, rho(calc) = 3.037 g cm(-3), mu(Mo K alpha) = 9.903 mm(-1), = 0.0358, wR(2) = 0.0763, S = 1.140 for 234 parameters. (C) 2006 Elsevier Ltd. All rights reserved.