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.