First Example of Equatorial−Equatorial Disposition of End-to-End Thiocyanate Bridges in a Polynuclear Copper(II) Complex and Its Relation to the Very Efficient Transmission of the Magnetic Interaction

The preparation, X-ray structure, and spectroscopic and magnetic
properties of
[Cu(NCS)(μ-NCS)(Hmtpo)(H2O)]2
(where Hmtpo=
4H,7H-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine)
are described. This compound crystallizes
in triclinic space group P1̄, with unit cell dimensions
a = 8.408(2) Å, b = 10.581(3) Å,
c = 7.927(2) Å, α =
89.45(2)°, β = 77.44(2)°, γ = 69.75(2)°, and
Z = 1 (binuclear unit). The structure was refined to
final R =
0.033 for 2143 data with I ≥ 3.0σ(I).
The structure is comprised of polymeric 1-D copper(II)
complexes supported
by two different kinds of thiocyanate bridges. The coordination
geometry around each of the metal centers is a
distorted octahedron (N3S + O + S*). The
equatorial positions are occupied by a monodentate Hmtpo
ligand,
coordinated via N3, two bridging thiocyanate groups coordinated in a
“symmetric” end-to-end fashion, and the
N-donor atom of an asymmetric end-to-end thiocyanate bridge.
Finally, a water molecule and a weakly bonded
S-donor atom from the asymmetric end-to-end thiocyanate bridge, with a
Cu···S separation of 2.959(1) Å, occupy
the apical sites. The presence of two “symmetric” thiocyanate
bridges with short Cu−SCN and Cu−NCS distances
of 2.402(1) and 1.938(3) Å, respectively, results in a
Cu−Cu separation of 5.499(1) Å. The magnetic
susceptibility
data measured in the 11−290 K temperature range shows a strong
antiferromagnetic coupling between the copper
ions through the “symmetric” thiocyanate bridges. Fitting the
data to the Bleaney−Bowers equation gives the
exchange parameter 2J = −148.2
cm-1, which is many times higher than those
previously found in other copper(II) thiocyanate bridged complexes, and g = 2.20. The
unprecedented equatorial−equatorial disposition of the
thiocyanate bridging entities appears to be responsible for the
magnetic behavior of this compound.