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(μ-dppm)}2+ (Pt2dppm; HL: 2-phenyl-6-(1H-pyrazol-3-yl)-pyridine) demonstrates interesting reversible “pivot-hinge”-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+ (Pt2dchpm) 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 Pt2dppm. In the case of Pt2dchpm, 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 Pt2dppm 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 Pt2dchpm was 6 kcal mol−1 less stable than its alternative closed configuration.