The controlled formation and cleavage of an intramolecular d⁸-d⁸Pt-Pt interaction in a dinuclear cycloplatinated molecular "pivot-hinge"

The bis(diphenylphosphino)methane (dppm)-bridged dinuclear cycloplatinated complex {[Pt(L)]2(μdpprn)}2+(Pt2· dppm; HL: 2-phenyl-6(1 H-pyrazol-3-yl)-pyridine) demonstrates interesting reversible "pivothinge"-like intramolecular motions in response to the protonation/deprotonation of L. In its protonated "closed" configuration, the two platinum(II) centers are held in position by intramolecular d8-d8Pt-Pt interaction. In its deprotonated "open" configuration, such Pt-Pt interaction is cleaved. To further understand the mechanism behind this hingelike motion, an analogous dinuclear cycloplatinated complex, {[Pt(L)]2(μ-dchpm)}2+(Pt2·dchpm) with bis(dicyclohexylphosphino) methane (dchpm) as the bridging ligand, was synthesized. From its protonation/deprotonation responses, it was revealed that aromatic π-π interactions between the phenyl moieties of the μ-dppm and the deprotonated pyrazolyl rings of L was essential to the reversible cleavage of the intramolecular Pt-Pt interaction in Pt2·dppm. In the case of Pt2·dchpm, spectroscopic and spectrofluorometric titrations as well as X-ray crystallography indicated that the distance between the two platinum(II) centers shrank upon deprotonation, thus causing a redshift in its room-temperature triplet metal-metalto-ligand charge-transfer emission from 614 to 625 nm. Ab initio calculations revealed the presence of intramolecular hydrogen bonding between the deprotonated and negatively charged 1pyrazolyl-N moiety and the methylene CH and phenyl C-H of the μ-dppm. The "open" configuration of the deprotonated Pt2·dppm was estimated to be 19 kcal mol-1more stable than its alternative "closed" configuration. On the other hand, the open configuration of the deprotonated Pt2·dchpm was 6 kcal mol-1less stable than its alternative closed configuration. KGaA.