The low J values observed for the two compounds are in
agreement with previously discussed inability of the present
bridging ligands to support sizeable magnetic exchange of
either sign.13
Acknowledgements
Financial support from the Ministero per l’Università e la
Ricerca Scientifica e Tecnologica (MURST) is gratefully
acknowledged.
As to the D parameters, the seeming inconsistency between
the values obtained for the exchange coupled Co() dimers 1
and 6 (2–3 cmϪ1) and those (∼11 cmϪ1) observed for the
uncoupled Co() ions in 8 and 11, is not too surprising. Indeed,
the problem of relating zero-field splittings to single ion (D0)
and anisotropic exchange contributions in exchange coupled
systems (DCoCo), is a difficult one, both from an experimental
and a theoretical point of view. At present, reliable information
on this matter is available24 only for systems in which the
Ϫ2J(S1ؒS2) magnetic exchange term dominates any other term
in the spin Hamiltonian. If so states with different values of the
total spin quantum number, S are so well separated in energy
that, in principle, the zero field spitting of each S multiplet, Ds,
can be measured using EPR techniques (but not magnetic sus-
ceptibility measurements) and, under fortunate circumstances,
can be quantitatively related to anisotropic contributions.24,25
Clearly, relationships between Ds and DCoCo and Dco parameters
lose any significance when, as in the case of our compounds, the
J and zfs terms in the spin Hamiltonian are of comparable
magnitude. In such a situation, states with different S can mix
since S is no longer a good quantum number.
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Conclusions
The binuclear nature of the fourteen new Co complexes,
together with the previously obtained results for the related
Cu() compounds,13 provide convincing evidence that “short”
salen homologues strongly favour the formation of bridging
structures. The apparent reluctance of these ligands to form
mononuclear complexes, similar to those usually observed with
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structures involve several interesting features. First the rather
unusual coordination geometry of Co(): tetrahedral with a
relevant distortion arising from the obliged bite angle of
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point is the different behaviour towards oxidation of the salmen
and salben derivatives. Such difference could arise from a dif-
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Interestingly, the isolation of the mixed valence type A com-
pounds offers the opportunity of comparing magnetic data for
magnetically dilute and exchange coupled Co() ions in similar
local environments. Such an analysis, in particular, illustrates
the difficulty in determining reliable zero field splittings in
weakly exchange coupled systems.
Finally, as stated above, the finding that different oxidation
products can be isolated using low I2/Co ratios, opens the
possibility of obtaining a number of other compounds, includ-
ing polynuclear derivatives.20 This novel and rich chemistry is
currently under thorough investigation.
25 O. Kahn, in Molecular Magnetism, VCH, New York, 1993, appendix 6.
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3616
J. Chem. Soc., Dalton Trans., 2001, 3611–3616