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 potentia l provides new avenues for investigating chemical entities and their dynami cs. One particular application concerns the tuning of biological redox cent ers consisting of metals and different ligands, where the effective charge at the metal center and the association and dissociation of the ligands pla y a key role. To quantify these factors we have recently synthesized a set of metal-substituted bacteriochlorophylls ([M]-BChls), whereby the caged me tal can bind various axial ligands of biological significance and the BChl pi-system is used as a "molecular potentiometer" to estimate the metal's ef fective charge. Here, we have concentrated on modifying this charge by axia l ligation. We specifically selected [Ni]-BChl because (I) it forms three s tates of coordination with nitrogenous ligands, (2) Ni(II) has biological s ignificance. and (3) [Ni]-porphyrins are extensively used for modeling [Fe] -porphyrins. The pure spectrum of each state of coordination and the equili brium constants for monoligation (K-1 = 5.6 +/- 0.2 and 29.6 =/- 1.1 M-1) a nd biligation (K-2 = 35.1 +/- 0.9 and 26.8 +/- 0.9 M-1) of pyridine (Py) an d imidazole (Im), respectively, were determined by factor analysis. Followi ng the principle of electronegativity equalization and the model described in our previous paper (Noy, D.; Fiedor, L.; Hartwich, G.; Scheer, H.; Scher z, A. J. Am. Chan. Sec. 1998, 120, 3684-3693), we estimated that 0.30 and 0 .27 electron charge units migrated from imidazole and pyridine, respectivel y, into the [Ni]-BChl central core upon monoligation. An additional, simila r amount was transferred with the second ligation. High-level hybrid densit y functional theory (HDFT) calculations performed for [Ni]-BChl and [Ni]-BC hl Im in the gas phase were in very good agreement with the empirical resul ts, 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) c ovalent radius expanded by 0.07 and 0.09 Angstrom upon monoligation acid by 0.13 and 0.18 Angstrom upon biligation with pyridine and imidazole, respec tively. 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.; S cheidt, W. R,; Shelnutt, J. A. Inorg. Chem. 1998, 37, 4402-4412) and our HD FT calculations (0.085 Angstrom expansion upon Im monoligation). Line shape analyses of the Q(y) bands indicated that the initial excited-state lifeti mes of [Ni]-BChl were 75; 153, and 184 fs when ligated with zero, one, or t wo molecules of imidazole. The lifetimes for the analogous complexes with p yridine were 50% longer. Excitation of [Ni]-BChl . Py-2 caused dissociation of the ligands (in similar to 100 ps), which recovered after a much longer time.