J. Am. Chem. SOC. 1995,117, 3529-3538 3529 EPR and NMR Spectra as Probes of Spin-Density Distribution in Heterocyclic Ligands Coordinated in trun~-[L(Im)(NH3)4Ru'~~] : Implications for Long-Range Electron Transfer. Crystal Structure of truns-[(Im)2(NH3)4R~]C13*H20 K. J. LaChance-Galang? Peter E. Doan,*$* M. J. Clarke,"?? U. Rae,? A. Yamano? and Brian M. Hoffman$ Contribution from the Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 021 67, and Department of Chemistry, Northwestem University, Evanston, Illinois 60208 Received August 2, 1994@ Abstract: Spectroscopic studies of trans-[(L)(Im)(NH3)&un1], where Im = imidazole and L = isonicotinamide (Isn), pyridine (Py), Im, NH3, C1-, and S042-, indicate that n-bonding by the trans ligand significantly affects mixing of the d,-n (imidazole) orbitals. Analysis of the EPR spectra provides a description of the frontier d, orbitals involved in electron transfer and estimates of A and V (the tetragonal and rhombic distortion parameters, respectively), all of which vary with the n-donor abilities of L. As A and V are.of the same magnitude as the the spin-orbit coupling parameter, A, there is extensive spin-orbit mixing of the dxz and dyz and (to a lesser extent) the dq orbitals. Reduction potentials and energies of imidazole - Ru"' charge transfer transitions correlate linearly with the n-donorlacceptor ability of L so that a correlation is also evident between these properties and the ligand field splitting of the tzg manifold, which leads to an unsuspected correlation between the difference between the two largest g values, Ag12, and E". Electronic perturbations appear to be transmitted to C5 on the imidazole ring, which is the site linked to Ru-modified proteins used as probes of long-range electron transfer. This implies that variations of the ligand in the trans position to modify the E" for the Runm couple can also affect the superexchange coupling involved in electron transfer. trans-[(Im)2(NH3)&~~~~]C13-H20 crystallizes in the monoclinic space group, P21h (No. 14), with cell parameters a = 18.111(9) A, b = 7.187(2) A, c = 14.352(7) A, ,8 = 113.26(4)", and Z = 4 and exhibits an eclipsed conformation of the imidazole rings. MM2 and IEHT calculations suggest why the eclipsed conformation is slightly favored over the staggered and that the imidazole rings freely rotate in solution. Introduction An effective method of probing the distance dependence of long-range electron transfer (ET) in proteins has been to attach ammineruthenium centers to histidyl imidazoles at predeter- mined distances from the native redox site. This approach also allows the electron transfer rates to be studied as a function of driving force by varying n-acceptor ligands on the ruthenium.' The electron transfer rate between the two redox centers is expressed by where AGO is the electrochemical driving force, A is the reorganization energy, and Hm is the electronic coupling, whose magnitude varies with the separation and medium intervening between the donor/acceptor pair.2 HAB also depends on the delocalization of the frontier ruthenium d, orbital onto the histidylimidazole, which links the metal to the protein. Since histidylimidazoles are bound to the peptide chain through C5, variations in the coefficient of the donor/acceptor MO at this position must affect long-range electron transfer rates. NMR studies of [L(NH3)5Ru1"] (d5; S = l/2), where L = imidazole, pyridine, purine, and pyrimidine derivatives, indicate that the paramagnetic shifts of ring protons are strongly Boston College. * Northwestem University. @Abstract published in Advance ACS Abstracts, February 15, 1995. (1) Winkler, J. R.; Gray, H. B. Chem. Rev. 1992, 92, 369. (2) Wishart, J. F.; Zhang, X. a,; Isied, S. S.; Potenza, J. A,; Schugar, H. J. Inorg. Chem. 1992, 31, 3179-3181. 0002-7863/95/15 17-3529$09.00/0 dependent on ring substitutions. Consequently, it is reasonable to expect that the ligands used to adjust the reduction potential of the ruthenium center' also transmit n-electronic effects into the imidazole ring that would affect coupling through C5. In this study, the ligand trans to the imidazole in trans+,@@- (NH3)4RuIn] has been systematically varied so as to modulate E". These changes are correlated with parallel changes in the charge transfer transitions, EPR spectra, and ligand field splittings derived from these spectra and 'H NMR spectra. In addition, the unexpected finding of an eclipsed conformation for the n-donor and n-acceptor heterocyclic ligands in trans- [(Isn)(Im)(NH3)&u](CF3C0~)3~ prompted an examination of the arrangement between the two n-donor ligands in trans-[(Im)z- (NH3)4Ru]C13 and a consideration of the relative energies of the eclipsed and staggered conformations. Experimental Section Syntheoes. RuCl3 was purchased from Johnson Matthey. Imidazole (Im), isonicotinamide (Isn), and pyridine (Py) were purchased from Aldrich and used without further purification. The compounds trans- [Cl(SOz)(NH~)4Ru]CI,3 truns-[(so4)(Py)(NH3)4Ru]cl, truns-[(SO4)(Isn)- (NH3)4Ru]Cl, and trans-[(S04)(Im)(NH3)4Ru]Cl were prepared by literature methods? hans-[(Isn)(Im)(NH~)~u]Cl~ was synthesized by dissolving trans- [(S04)(Isn)(NH~)aRu]Cl in a minimum of water and reducing it with zinc amalgam for 20 min under an argon atmosphere. A 2:l molar ratio of imidazole was added to the solution, and reduction was continued for 3 h to give a dark reddish-brown solution. The zinc amalgam and undissolved ligand were filtered off, and a 50/50 mixture (3) Vogt, L. H.; Katz, J. L.; Wiberly, S. E. Inorg. Chem. 1965.4, 1157. (4) Isied, S.S.; Taube, H. Inorg. Chem. 1976, 15, 3070-3075. 0 1995 American Chemical Society