Copper(II) Carboxylate
Dimers Prepared from Ligands
Designed to Form a Robust π···π Stacking
Synthon: Supramolecular Structures and Molecular Properties
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Abstract
The reactions of bifunctional carboxylate ligands (1,8-naphthalimido)propanoate,
(<b>L</b><sub><b>C2</b></sub><sup><b>–</b></sup>), (1,8-naphthalimido)ethanoate, (<b>L</b><sub><b>C1</b></sub><sup><b>–</b></sup>), and (1,8-naphthalimido)benzoate,
(<b>L<sub>C4</sub><sup>–</sup>)</b> with Cu<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub> in methanol or ethanol at room temperature lead to the formation
of novel dimeric [Cu<sub>2</sub>(<b>L</b><sub><b>C2</b></sub>)<sub>4</sub>(MeOH)<sub>2</sub>] (<b>1</b>), [Cu<sub>2</sub>(<b>L</b><sub><b>C1</b></sub>)<sub>4</sub>(MeOH)<sub>2</sub>]·2(CH<sub>2</sub>Cl<sub>2</sub>) (<b>2</b>), [Cu<sub>2</sub>(<b>L</b><sub><b>C4</b></sub>)<sub>4</sub>(EtOH)<sub>2</sub>]·2(CH<sub>2</sub>Cl<sub>2</sub>) (<b>3</b>) complexes.
When the reaction of <b>L</b><sub><b>C1</b></sub><sup><b>–</b></sup> with Cu<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub> was carried out
at −20 °C in the presence of pyridine, [Cu<sub>2</sub>(<b>L</b><sub><b>C1</b></sub>)<sub>4</sub>(py)<sub>4</sub>]·2(CH<sub>2</sub>Cl<sub>2</sub>) (<b>4</b>) was produced.
At the core of complexes <b>1</b>–<b>3</b> lies
the square Cu<sub>2</sub>(O<sub>2</sub>CR)<sub>4</sub> “paddlewheel”
secondary building unit, where the two copper centers have a nearly
square pyramidal geometry with methanol or ethanol occupying the axial
coordination sites. Complex <b>4</b> contains a different type
of dimeric core generated by two κ<sup>1</sup>-bridging carboxylate
ligands. Additionally, two terminal carboxylates and four trans situated
pyridine molecules complete the coordination environment of the five-coordinate
copper(II) centers. In all four compounds, robust π···π stacking interactions of the naphthalimide rings organize
the dimeric units into two-dimensional sheets. These two-dimensional
networks are organized into a three-dimensional architecture by two
different noncovalent interactions: strong π···π
stacking of the naphthalimide rings (also the pyridine rings for <b>4</b>) in <b>1</b>, <b>3</b>, and <b>4</b>,
and intermolecular hydrogen bonding of the coordinated methanol or
ethanol molecules in <b>1</b>–<b>3</b>. Magnetic
measurements show that the copper ions in the paddlewheel complexes <b>1</b>–<b>3</b> are strongly antiferromagnetically
coupled with –<i>J</i> values ranging from 255 to
325 cm<sup>–1</sup>, whereas the copper ions in <b>4</b> are only weakly antiferromagnetically coupled. Typical values of
the zero-field splitting parameter <i>D</i> were found from
EPR studies of <b>1</b>–<b>3</b> and the related
known complexes [Cu<sub>2</sub>(<b>L</b><sub><b>C2</b></sub>)<sub>4</sub>(py)<sub>2</sub>]<b>·</b>2(CH<sub>2</sub>Cl<sub>2</sub>)<b>·</b>(CH<sub>3</sub>OH), [Cu<sub>2</sub>(<b>L</b><sub><b>C3</b></sub>)<sub>4</sub>(py)<sub>2</sub>]<b>·</b>2(CH<sub>2</sub>Cl<sub>2</sub>) and [Cu<sub>2</sub>(<b>L</b><sub><b>C3</b></sub>)<sub>4</sub>(bipy)]<b>·</b>(CH<sub>3</sub>OH)<sub>2</sub><b>·</b>(CH<sub>2</sub>Cl<sub>2</sub>)<sub>3.37</sub> (<b>L</b><sub><b>C3</b></sub><sup><b>–</b></sup> = (1,8-naphthalimido)butanoate)),
while its abnormal magnitude in [Cu<sub>2</sub>(<b>L</b><sub><b>C2</b></sub>)<sub>4</sub>(bipy)] was qualitatively rationalized
by structural analysis and DFT calculations
