Optical Absorption and Computational Studies of [Ni]-Bacteriochlorophyll-a. New Insight into Charge Distribution between Metal and Ligands
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- 作者：Dror Noy ; Roie Yerushalmi ; Vlad Brumfeld ; Idan Ashur ; Hugo Scheer ; Kim K. Baldridge ; and Avigdor Scherz
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：April 26, 2000
- 年：2000
- 卷：122
- 期：16
- 页码：3937 - 3944
- 全文大小：93K
- 年卷期：v.122,no.16(April 26, 2000)
- ISSN：1520-5126
文摘
The relation between electronegativity and the electronic chemical potential provides new avenuesfor investigating chemical entities and their dynamics. One particular application concerns the tuning of biologicalredox centers consisting of metals and different ligands, where the effective charge at the metal center and theassociation and dissociation of the ligands play a key role. To quantify these factors we have recently synthesizeda set of metal-substituted bacteriochlorophylls ([M]-BChls), whereby the caged metal can bind various axialligands of biological significance and the BChl 
-system is used as a "molecular potentiometer" to estimatethe metal's effective charge. Here, we have concentrated on modifying this charge by axial ligation. Wespecifically 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 equalizationand 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 fromimidazole and pyridine, respectively, into the [Ni]-BChl central core upon monoligation. An additional, similaramount was transferred with the second ligation. High-level hybrid density functional theory (HDFT) calculationsperformed for [Ni]-BChl and [Ni]-BChl·Im in the gas phase were in very good agreement with the empiricalresults, suggesting that the [Ni]-BChl central core is enriched by 0.21 electron charge units upon ligation toa single Im molecule. Moreover, the Ni(II) covalent radius expanded by 0.07 and 0.09 Å upon monoligationand by 0.13 and 0.18 Å upon biligation with pyridine and imidazole, respectively. These results are in goodagreement 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 (0.085Å expansion upon Im monoligation). Line shape analyses of the Qy bands indicated that the initial excited-state 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 causeddissociation of the ligands (in ~100 ps), which recovered after a much longer time.
-system is used as a "molecular potentiometer" to estimatethe metal's effective charge. Here, we have concentrated on modifying this charge by axial ligation. Wespecifically 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 equalizationand 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 fromimidazole and pyridine, respectively, into the [Ni]-BChl central core upon monoligation. An additional, similaramount was transferred with the second ligation. High-level hybrid density functional theory (HDFT) calculationsperformed for [Ni]-BChl and [Ni]-BChl·Im in the gas phase were in very good agreement with the empiricalresults, suggesting that the [Ni]-BChl central core is enriched by 0.21 electron charge units upon ligation toa single Im molecule. Moreover, the Ni(II) covalent radius expanded by 0.07 and 0.09 Å upon monoligationand by 0.13 and 0.18 Å upon biligation with pyridine and imidazole, respectively. These results are in goodagreement 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 (0.085Å expansion upon Im monoligation). Line shape analyses of the Qy bands indicated that the initial excited-state 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 causeddissociation of the ligands (in ~100 ps), which recovered after a much longer time.