Stabilization of Double Sandwich Structure of Mercury(II) Porphyrins: Hg•••Hg•••Hg Interactions and Structure-Function Correlation

Abstract

A series of stable trinuclear "double sandwich" complexes of mercury(II) porphyrins having linear Hg3 core have been stabilized successfully utilizing both flexible and rigid porphyrin dimer frameworks. The gross structural patterns are indeed similar: two terminal Hg(II) centers are above and below the porphyrin rings, whereas the middle Hg(II) center is sandwiched between the two rings. The mercury-nitrogen distances are largely diversified in the complexes also. Mercurophilic interactions play crucial role in stabilizing this unique structure with linear Hg•••Hg•••Hg unit overcoming the inherent instability arises out of two coplanar aromatic (porphyrin) rings placed on the exact top of each other with eclipsed conformations, a hallmark of these double sandwich complexes reported here. Interestingly, strongest mercurophilic interactions (with Hg•••Hg distances of 3.1251(11)Å and 3.1333(16)Å) are observed with highly flexible ethane-bridged porphyrin dimer. Extensive DFT calculations demonstrates that the mercurophilic interaction is evident when relativistic and dispersion effects are included and the distances are also in excellent match with the X-ray structure of the complexes. NBO and QTAIM analyses revealed distinct bond paths and bond critical points (BCPs), that are commonly recognized as key indicators of mercurophilic interactions. The absorption (with MMLCT band ~350 nm) and photoluminescent properties of the complexes display direct correlation with the strength of the Hg•••Hg interactions. Fluorescence decays at the blue end (related to the mercurophilic interactions) of the emission spectra are faster than those at the red end (associated with ligand emission) for all the complexes at both 298 K and 77 K.