Synthesis, structure, and magnetic properties of tetranuclear cubane-like and chain-like iron(II) complexes based on the N4O pentadentate dinucleating ligand 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol

The tetranuclear complexes [Fe-4(pypentO)(pym)(3)(Oac)(NCS)(3)].1.5EtOH (1), [Fe-4(pypentO)(pym)(Oac)(2)(NCS)(2)(MeO)(2)-(H2O)].H2O (2), [Fe-2(pypentO)(NCO)(3)](2) (3), and [Fe-2(pypentO)(N-3)(3)](2) (4) have been prepared, and their structure and magnetic properties have been studied (pypentOH = 1,5-bis[(2-pyridylmethyl)amino]pentan-3-ol, pymH = 2-pyridylmethanol). The X-ray diffraction analysis of 1 (C43H53N10O7.5S3F4 monoclinic, P2(1)/n, a = 11.6153(17) Angstrom, b = 34.391(17) Angstrom, c = 14.2150(18) Angstrom, beta = 110.88(5)degrees, V = 5305(3) Angstrom(3), Z = 4) and 2 (C31H45N7O10S2Fe4, monoclinic, C2/c, a = 19.9165(17) Angstrom, b = 21.1001(12) Angstrom, c = 21.2617(19) Angstrom, beta = 104.441 (10)degrees, V = 8652.7(12) Angstrom(3), Z = 8) showed a Fe4O4 cubane-like arrangement of four iron(II) atoms, four mu(3)-O bridging ligands, one (1) or two (2) syn-syn bridging acetates. The X-ray diffraction analysis of 3 (C40H46N14O8Fe4, monoclinic, P2(1)/c, a = 11.7633(18) Angstrom, b = 18.234(3) Angstrom, c = 10.4792(16) Angstrom, beta = 99.359(18)degrees, V = 2217.7(6) Angstrom(3), Z = 2) and 4 (C34H46N26O2Fe4, monoclinic, P2(1)/c, V = 4412.4(10) Angstrom(3), a = 23.534(3) Angstrom, b = 18.046(2) Angstrom, c = 10.4865(16) Angstrom, beta = 97.80(2)degrees, Z= 4) showed a zigzag bis-dinuclear arrangement of four iron(II) cations, two mu(2)-0 bridging pypentO ligands, four mu(2)-N-cyanato bridging ligands (3) or four end-on azido bridging ligands (4): they are the first examples of cyanato and azido bridged discrete polynuclear ferrous compounds, respectively. The Mossbauer spectra of 1 are consistent with four different high-spin iron(II) sites in the Fe4O4 cubane-type structure. The Mossbauer spectra of 3 are consistent with two high-spin iron(II) sites (N5O and N4O). Below 190 K, the Mossbauer spectra of 4 are consistent with one N5O and two N4O high-spin iron(II) sites. The temperature dependence of the magnetic susceptibility was fitted with J(1) similar to 0 cm(-1), J(2) = -1.3 cm(-1), J(3) = 4.6 cm(-1), D = 6.4 cm(-1), and g = 2.21 for 1; J(1) = 2.6 cm(-1), J(2) = 2.5 cm(-1), J(3) = - 5.6 cm(-1), D = 4.5 cm(-1), and g = 2.09 for 2; J(1) = 1.5 cm(-1), J(2) = 0.2 cm(-1), D = - 5.6 cm(-1), D' = 4.5 cm(-1), and g = 2.14 for 3; and J(1) = -2.6 cm(-1), J(2) = 0.8 cm(-1), D = 6.3 cm(-1), D' = 1.6 cm(-1), and g = 2.18 for 4. The differences in sign among the J(1), J(2), and J(3) super-exchange interactions indicate that the faces including only mu(3)-OR bridges exhibit ferromagnetic interactions. The nature of the ground state in 1-3 is confirmed by simulation of the magnetization curves at 2 and 5 K. In the bis-dinuclear iron(II) compounds 3 and 4, the J(2) interaction resulting from the bridging of two Fe-2(pypentO)X-3 units through two pseudo-halide anions is ferromagnetic in 3 (X = mu(2)-N-cyanato) and may be either ferro- or antiferromagnetic in 4 (X = end-on azido). The J(1) interaction through the central O-alkoxo and pseudo-halide bridges inside the dinuclear units is ferromagnetic in 3 (X = mu(2)-N-cyanato) and antiferromagnetic in 4 (X = end-on azido). In agreement with the symmetry of the two Fe-II sites in complexes 3 and 4, D' (pentacoordinated sites) is larger than 9 (octahedral sites).