One-Dimensional Cobalt Diphosphonates Exhibiting Weak Ferromagnetism and Field-Induced Magnetic Transitions

This paper reports two new cobalt phosphonate compounds [NH3(CH2)nNH3]Co2(hedpH)2·2H2O [n = 4 (1), 5 (2)],
where hedp is (1-hydroxyethylidene)diphosphonate [CH3C(OH)(PO3)2]. Both contain ladderlike chains with the same
composition {Co2(hedpH)2}n
2
n
-, where edge-shared {CoO6} octahedra are each bridged by O−P−O units. The
chains are linked by very strong hydrogen bonds to form a three-dimensional open network with channels in which
the diammonium counterions and lattice water reside. Magnetic studies reveal dominant antiferromagnetic interactions
between the Co centers within the chain in both compounds. The field dependent magnetization confirms the
occurrence of the field-induced magnetic transitions, with the critical fields ca. 25 kOe for 1 and 27.5 kOe for 2,
respectively. In the case of compound 2, weak ferromagnetism is also observed at very low temperatures, possibly
arising from spin canting of the antiferromagnetically coupled Co(II) ions in the chain. Crystal data:  1, monoclinic,
P21/c, a = 5.4868(8), b = 12.9116(18), and c = 15.251(2) Å, β = 98.843(2)°, V = 1067.6(3) Å3, Z = 2; 2,
monoclinic, P21/c, a = 5.4757(7), b = 12.7740(16), and c = 15.794(2) Å, β = 98.797(2)°, V = 1091.7(2) Å3,
Z = 2.