B. Chiari et al., FERROMAGNETIC MN(II)...CU(II) EXCHANGE IN THE NEW BIMETALLIC QUASI-2-D COMPOUND CU(OP)(2)MNCL4 (OP=1,4-DIAZACYCLOHEPTANE), Inorganic chemistry, 35(25), 1996, pp. 7413-7418
The synthesis, crystal structure, and magnetic properties are reported
for the new bimetallic compound Cu(op)(2)MnCl4, where op HN(CH2)(5)NH
. The compound, C10H24N4Cl4CuMn, crystallizes in the monoclinic space
group P2(1)/n. Cell dimensions are as follows: a = 15.316(3) Angstrom,
b = 16.608(3) Angstrom, c = 7.141(2) Angstrom, beta = 100.01(5)degree
s, Z = 4. The structure consists of well-separated and magnetically eq
uivalent layers which are composed of chloride-bridged Cu(op)(2)MnCl4
binuclear units connected by rather loose Cu-N-H...Cl-Mn contacts. The
MnCl4 fragment approximates tetrahedral symmetry. The Cu(II) geometry
is (4 + 1) square-pyramidal with the apical position occupied by a br
idging chloride ligand and the basal ones by the nitrogen atoms from t
he organic ligands. The shortest interlayer M...M separations, similar
to 7 Angstrom, are of the Mn Cu type. Magnetic susceptibility and sin
gle-crystal EPR measurements for the compound have been carried out ov
er the range 4-300 K. At room temperature the T-chi product (per MnCu
unit) has a value of 4.84 emu . mol(-1). K, close to that expected for
uncoupled S = 5/2 and S = 1/2 spins. When the temperature is lowered,
T-chi remains almost constant until 80-90 K, slightly increases to re
ach a maximum at similar to 13 K (5.21 emu . mol(-1). K), and then rap
idly decreases. Comparison between theory and experiment, made with us
e of both a mean field corrected dimer model and an approximate 2-D mo
del, indicates that Mn(II)...Cu(II) exchange is ferromagnetic within t
he dimers (J(1) similar to 2.6 cm(-1)) and antiferromagnetic among dim
ers, with J values between -0.07 and -0.03 cm(-1) (the interaction Ham
iltonian is of the form H = -2JS(A) . S-B). Single-crystal EPR spectra
recorded along the a, b, and c axes show a large temperature depende
nce of the g factors: at 4.2 K, g(a) = 2.10, g(b) = 1.96, and g(c) =
2.01. This pattern substantiates the presence of a 2-D magnetic struct
ure with ferromagnetic intradimer exchange and interdimer antiferromag
netic exchange of weaker magnitude. The opposite signs of the interact
ions are ascribed to the local symmetries of the Cu(II) and Mn(II) ion
s.