Structural Trends in First-Row Transition-Metal Bis(benzimidazole) Complexes † Pramatha Payra, Huichang Zhang, Wai Him Kwok, Maosheng Duan, Judith Gallucci, and Michael K. Chan* Departments of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210 ReceiVed September 8, 1999 Dimethyl-substituted bis(benzimidazole) (Me 2 BBZ) is a novel macrocyclic ligand that possesses an intrinsic nonplanarity. To examine how metal-ion binding affects the magnitude of this nonplanarity, we have determined the structures of a periodic series of Me 2 BBZ complexes bound to Mn(II), Fe(II), Co(II), Ni(II), and Cu(II). These studies demonstrate that the extent of ligand ruffling and metal doming is indeed influenced by the nature of the metal. Concomitant with the periodic decrease of the ionic radii of the encapsulated divalent metal ion, a decrease in the magnitude of both the ligand nonplanarity and the metal out-of-the-plane distance is observed. For the metal out-of-the-plane distance, this correlation persists until the metal finally moves into the mean ligand plane. For the nonplanar distortion, the extent of the nonplanarity decreases to a limiting value that is intrinsic to the Me 2 BBZ ligand due to steric factors. These observations indicate that the relative sizes of the metal ion and the Me 2 BBZ ligand cavity have profound effects on the structural features of the metal-ligand complex. Introduction In metalloporphyrins, the conformation of the porphyrin ligand has been shown to influence their axial bonding affini- ties, 1 redox potentials, 2 electron-transfer rates, 3,4 and photo- physical properties. 5-7 Indeed, nonplanar distortions of heme cofactors in proteins are thought to be important for tuning their functional properties, suggesting that similar features might be used to optimize the properties of synthetic tetrapyrroles for practical applications in photodynamic therapy, 8 biomimetic solar energy conversion, 9 and catalysis. 10 Hence, studies of the general factors that influence the nonplanarity of porphyrins and related ligands are of interest. We recently completed the synthesis of a new class of porphyrin analogue, the dimethyl-substituted bis(benzimidazole) ligand (Me 2 BBZ), and its manganese complex (Mn-Me 2 BBZ). 11 Structures of both the diprotonated Me 2 BBZ ligand, [H 2 (Me 2 - BBZ)](ClO 4 ) 2 (1), and the Mn-Me 2 BBZ complex, [Mn(Me 2 - BBZ)Cl]Cl (2), revealed an inherent ruffling of the ligand resulting from its distinct steric and geometric features. 11 As a consequence of this ruffling, significant metal-doming of the Mn-Me 2 BBZ complex was also observed. Comparison of the diprotonated Me 2 BBZ ligand and Mn- Me 2 BBZ complex revealed differences in the magnitude of their nonplanarities. On the basis of the distances of the ligand atoms from the mean plane of the four-liganding nitrogens (N 4 -plane), the Mn-Me 2 BBZ complex appears less distorted. Each of the out-of-plane distances of the ligand atoms in the Mn-Me 2 BBZ complex is smaller than in the diprotonated Me 2 BBZ ligand. These features are important since they demonstrate that the observed nonplanarity is an inherent feature of the Me 2 BBZ ligand and that the metal ion can modulate the magnitude of this nonplanarity. To evaluate the extent to which metals modulate the ligand ruffling in metal Me 2 BBZ complexes, and to elucidate the mechanism by which this control is achieved, we have synthesized (Scheme 1) and crystallographically characterized four related metal-Me 2 BBZ complexes: [Fe(Me 2 BBZ)Cl]- (BPh 4 ), (Fe-Me 2 BBZ, 3), [Co(Me 2 BBZ)Cl]Cl, (Co-Me 2 BBZ, 4), [[Ni(Me 2 BBZ)(H 2 O)(CH 3 CN)][Ni(Me 2 BBZ) (H 2 O) 2 ](ClO 4 ) 4 , (Ni-Me 2 BBZ, 5), and [Cu(Me 2 BBZ)(ClO 4 ) 2 ], (Cu-Me 2 BBZ, 6). Structural comparison of these complexes along with the Mn- Me 2 BBZ complex yields insight into the structural features that control metal-bis(benzimidazole) geometries. * To whom correspondence should be addressed. Telephone: 614-292- 8375. Fax: 614-292-6773. E-mail: chan@chemistry.ohio-state.edu. † Abbreviations used: Me2BBZ, dimethyl-substituted bis(benzimidazole) ligand; Mn-Me2BBZ, manganese(II) Me2BBZ complex; Fe-Me2BBZ, iron- (II) Me2BBZ complex; Co-Me2BBZ, cobalt(II) Me2BBZ complex; Ni-Me2- BBZ, nickel(II) Me2BBZ complex; Cu-Me2BBZ, copper(II) Me2BBZ complex; TPP, tetraphenylporphyrin; TPpivP, meso-5,10,15,20-tetrakis(o- pivalamidophenyl)porphyrin; OEP, octaethylporphyrin; DMAP, 4-(di- methylamino)pyridine; F 28TPP, 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20- tetrakis(pentafluorophenyl)porphyrin; py, pyridine; TPP(Br)4(CN)4, 2,3,12,13- tetrabromo-7,8,17,18-tetracyano-5,10,15,20-tetraphenylporphyrin; TPP- (CN)4, 7,8,17,18-tetracyano-5,10,15,20-tetraphenylporphyrin; OETPP, 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin. (1) David, S.; James, B. R.; Dolphin, D.; Traylor, T. G.; Lopez, M. A. J. Am. Chem. Soc. 1994, 116,6-14. (2) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am. Chem. 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(10) Collman, J. P.; Zhang, X.; Lee, V. J.; Uffelman, E. S.; Brauman, J. I. Science 1993, 261, 1404-1411. (11) Kwok, W. H.; Zhang, H.; Payra, P.; Duan, M.; Hung, S.-C.; Johnston, D. H.; Gallucci, J.; Skrzypczak-Jankun, E.; Chan, M. K. Submitted for publication. 1076 Inorg. Chem. 2000, 39, 1076-1080 10.1021/ic991075a CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000