The Controlled Formation and Cleavage of an Intramolecular d(8)-d(8) Pt-Pt Interaction in a Dinuclear Cycloplatinated Molecular Pivot-Hinge

The bis(diphenylphosphino)methane (dppm)-bridged dinuclear cycloplatinated complex {[Pt(L)](2)(mu-dppm)}(2+) (Pt-2-dppm; 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 d(8)-d(8) 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)(mu-dchpm)}(2+) (Pt-2 center dot chpm) with bis(dicyclohexylphos-phino)methane (dchpm) as the bridging ligand, was synthesized. From its protonation/deprotonation responses, it was revealed that aromatic pi-pi interactions between the phenyl moieties of the mu-dppm and the deprotonated pyrazolyl rings of L was essential to the reversible cleavage of the intramolecular Pt-Pt interaction in Pt-2 center dot dppm. In the case of Pt-2 center dot dchpm, spectroscopic and spectrofluorometric titrations as well as X-ray crystallography indicated that the distance between the two plati-num(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 mu-dppm. The open configuration of the deprotonated Pt-2 center dot 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 Pt-2 center dot dchpm was 6 kcal mol(-1) less stable than its alternative closed configuration.