Optical Absorption and Computational Studies of [Ni]-Bacteriochlorophyll-a. New Insight into Charge Distribution between Metal and Ligands

The relation between electronegativity and the electronic chemical potential provides new avenues for investigating chemical entities and their dynamics. One particular application concerns the tuning of biological redox centers consisting of metals and different ligands, where the effective charge at the metal center and the association and dissociation of the ligands play a key role. To quantify these factors we have recently synthesized a set of metal-substituted bacteriochlorophylls ([M]-BChls), whereby the caged metal can bind various axial ligands of biological significance and the BChl π-system is used as a “molecular potentiometer” to estimate the metal’s effective charge. Here, we have concentrated on modifying this charge by axial ligation. We specifically selected [Ni]-BChl because (1) it forms three states of coordination with nitrogenous ligands, (2) Ni(II) has biological significance, and (3) [Ni]-porphyrins are extensively used for modeling [Fe]-porphyrins. The pure spectrum of each state of coordination and the equilibrium constants for monoligation (K1 ) 5.6 ( 0.2 and 29.6 ( 1.1 M-1) and biligation (K2 ) 35.1 ( 0.9 and 26.8 ( 0.9 M-1) of pyridine (Py) and imidazole (Im), respectively, were determined by factor analysis. Following the principle of electronegativity equalization and the model described in our previous paper (Noy, D.; Fiedor, L.; Hartwich, G.; Scheer, H.; Scherz, A. J. Am. Chem. Soc. 1998, 120, 3684-3693), we estimated that 0.30 and 0.27 electron charge units migrated from imidazole and pyridine, respectively, into the [Ni]-BChl central core upon monoligation. An additional, similar amount was transferred with the second ligation. High-level hybrid density functional theory (HDFT) calculations performed for [Ni]-BChl and [Ni]-BChl‚Im in the gas phase were in very good agreement with the empirical results, suggesting that the [Ni]-BChl central core is enriched by 0.21 electron charge units upon ligation to a single Im molecule. Moreover, the Ni(II) covalent radius expanded by 0.07 and 0.09 Å upon monoligation and by 0.13 and 0.18 Å upon biligation with pyridine and imidazole, respectively. These results are in good agreement with X-ray data for ligated [Ni]-porphyrins (Jia, S. L.; Jentzen, W.; Shang, M.; Song, X. Z.; Ma, J. G.; Scheidt, W. R.; Shelnutt, J. A. Inorg. Chem. 1998, 37, 4402-4412) and our HDFT calculations (0.085 Å expansion upon Im monoligation). Line shape analyses of the Qy bands indicated that the initial excitedstate lifetimes of [Ni]-BChl were 75, 153, and 184 fs when ligated with zero, one, or two molecules of imidazole. The lifetimes for the analogous complexes with pyridine were 50% longer. Excitation of [Ni]-BChl‚Py2 caused dissociation of the ligands (in ∼100 ps), which recovered after a much longer time.