Reversible Switching of the Coordination Modes of a Pyridine-Functionalized Quinonoid Zwitterion; Its Di- and Tetranuclear Palladium Complexes

The
coordination chemistry of a new functional quinonoid zwitterion (E)-3-oxo-4-((2-(pyridin-2-yl)­ethyl)­amino)-6-((2-(pyridin-2-yl)­ethyl)­iminio)­cyclohexa-1,4-dienolate
(2, H2L), in which a CH2CH2 spacer connects the N substituents of the quinonoid core
with a pyridine group, was explored in Pd­(II) chemistry. Different
coordination modes have been observed, depending on the experimental
conditions and the reagents. The reaction of H2L with [Pd­(μ-Cl)­(dmba)]2 (dmba = o-C6H4CH2NMe2-C,N) afforded
the dinuclear complex [{PdCl­(dmba)}2(H2L)] (3) in which H2L acts as a NPy,NPy bidentate ligand. Deprotonation of this complex with NaH
resulted in the formation of the dinuclear complex [{Pd­(dmba)}2(μ-L)] (4) in which a shift of the Pd­(II)
centers from the NPy sites to the N,O donor sites of the
zwitterion core has occurred, resulting in a N2O2 tetradentate behavior of ligand L. Reaction of 4 with
HCl regenerates 3 quantitatively. Chloride abstraction
from 3 with AgOTf (OTf = trifluoromethanesulfonate) resulted
in loss of one of the two dmba ligands and formation of an unusual
tetranuclear Pd­(II) complex, [{Pd­(dmba)}­(μ-L)­Pd]2(OTf)2 (5), in which two dinuclear entities
have dimerized, one pyridine donor group from each dimer forming a
bridge with the other dinuclear entity. This results in a N2, O2, NPy, NPy hexadentate behavior
for the ligand L. Complexes 3 and 4 constitute
an unprecedented reversible, switchable system where deprotonation
or protonation promotes the reversible migration of the [Pd­(dmba)]+ moieties, from the NPy sites in 3, to the N,O donor sites of the quinonoid core in 4,
respectively. This switch modifies the extent of π-delocalization
involving the potentially antiaromatic quinonoid moiety and is accompanied
by a significant color change, from red in 3 to green
in 4. The presence of uncoordinated pyridine donor groups
in 4 allowed the use of this complex for the preparation
of the neutral tetranuclear complex [{Pd­(dmba)}2(μ-L)­{PdCl­(dmba)}2] (6) in which 4 acts as a NPy,NPy-bidentate metalloligand toward two PdCl­(dmba)
moieties. Halide abstraction from 6 afforded the monocationic,
tetranuclear complex [{Pd­(dmba)}2(μ-L)­{Pd­(dmba)}2(μ-Cl)]­PF6 (7) in which the
two Pd­(dmba) moieties are connected by ligand L and a bridging chloride.
By Cl/PF6 anion metathesis, it was possible to switch quantitatively
from complex 6 to 7 and vice versa. All new compounds were unambiguously characterized by IR, NMR,
and mass spectroscopy. Single-crystal X-ray diffraction is also available
for molecules 2–5 and 7.