Ferromagnetic Coupling Through the End-to-End Thiocyanate Bridge in Cobalt(II) and Nickel(II) Chains

The preparation, spectroscopic characterization,
and X-ray crystal
structure of two novel one-dimensional compounds of formula [MII(tppz)­(NCS)­(μ-1,3-NCS)]n [tppz = 2,3,5,6-tetrakis­(2-pyridyl)­pyrazine and
M = Co­(1) and Ni (2)] are reported. 1 and 2 are isomorphous compounds, and they crystallize
in the P21/n space group.
Their structures are made up of zigzag chains of cobalt­(II) (1) and nickel­(II) ions (2) bridged by single
end-to-end thiocyanate groups with a tridentate tppz molecule and a terminally bound thiocyanate-N ligand
achieving distorted MN5S octahedral surroundings around
each metal center. The main source of the distortion of the ideal
octahedron is due to the geometrical constraints issued from the occurrence
of two fused five-member chelate rings of the tridentate tppz ligand, the values of the N–M–N bond angles covering
the range 75.58(9)–78.66(9)°. The M–N bond lengths
vary in the range 2.025(3)–2.116­(29 (1) and 2.001(2)–2.079(2)
Å (2), and they are shorter than the M–S
bond distance [2.6395(10) (1) and 2.5225(9) Å (2)]. The values of the intrachain metal–metal separation
are 6.4197(7) (1) and 6.3257(5) Å (2). The magnetic properties of 1 and 2 have
been investigated in the temperature range 1.9–300 K. Both
compounds exhibit intrachain ferromagnetic interactions with values
of the magnetic coupling (J) of +4.60 (1) and +7.82 cm–1 (2) [the spin Hamiltonian
being defined as Ĥ = −JΣi = 1n–1ŜiŜi+1].