The Controlled Formation and Cleavage of an Intramolecular d⁸–d⁸ Pt–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-(1H-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-d⁸ Pt-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-metal-to-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 1-pyrazolyl-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^-1 more stable than its alternative "closed" configuration. On the other hand, the open configuration of the deprotonated Pt₂·dchpm was 6 kcal mol^-1 less stable than its alternative closed configuration.