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J. Am. Chem. Soc. 1997, 119, 6670-6671
Synthesis and Characterization of Mn4O4L6 Complexes with Cubane-like Core Structure: A New Class of Models of the Active Site of the Photosynthetic Water Oxidase
Scheme 1. Proposed Mechanisms for Water Oxidation by the WOC of PSII (ref 2)
Wolfgang F. Ruettinger,1a Charles Campana,1b and G. Charles Dismukes*,1a Chemistry Department, Hoyt Laboratory Princeton UniVersity, Princeton, New Jersey 08544 Siemens Analytical X-ray Systems Incorporated Madison, Wisconsin 53719-1173 ReceiVed NoVember 12, 1996 The oxo-manganese cubane core structure, [Mn4O4]X+, is particularly interesting because it has not been previously isolated as a discrete complex and because it was proposed to be part of the catalytic cycle for dioxygen production by the photosynthetic water oxidizing complex (WOC) in two independent hypotheses, depicted in Scheme 1.2 In one of these proposals, oxide ions from water are incorporated into a butterfly-type core, Mn4O2, resulting in the formation of an “activated” cubane core which releases dioxygen with regeneration of the butterfly core. This proposal was based on the established structure of several butterfly core complexes.3b In the second proposal, oxide ions from water were proposed to add to a cubane core, forming an “activated” Mn4O6-adamantane core which in turn eliminates dioxygen and reverts to the cubane. This proposal was based on the established adamantane structure of the Mn4O6 core.3a Neither of these proposed reactions has been experimentally demonstrated, nor do the adamantane or butterfly-type complexes characterized so far exhibit spectral properties like those of the WOC of PSII.3 Herein we report the synthesis of the first example of the Mn4O4 cubane core. Several tetranuclear Mn complexes with distorted cubane-like cores have been synthesized before, including several Mn4X4 alkoxo complexes in low oxidation states4 or having incomplete core structures, Mn4O3X,5 or being part of a larger Mn cluster.6 Synthesis7 of Mn4O4(O2P(Ph)2)6 1, was done by fusing together two pre-formed Mn2O23+ units. A dimerization ap(1) (a) Princeton University. (b) Siemens. (2) (a) Brudvig, G. W.; Crabtree, C. W. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 4586-4588 . (b) Christou, G.; Vincent, J. B. Inorg. Chim. Acta 1987, 136, L41-43. (3) (a) Wieghardt, K. Angew. Chem. Int. Ed. Engl. 1989, 28, 11531172. (b) Vincent, J. B.; Christou, G. AdV. Inorg. Chem. 1989, 33, 197257 (4) (a) Horn, E.; Snow, M. R.; Zeleny, P. C. Aust. J. Chem. 1980, 33, 1659. (b) McKee, V.; Shepard, W. B. J. Chem. Soc, Chem. Commun. 1985, 158-159. (c) Brooker, S.; McKee, V.; Metcalfe, T. Inorg. Chim. Acta 1996, 246, 171-179. (d) Taft, K. L.; Caneschi, A.; Pence, L. E.; Delfs, C. D.; Papaefthymiou, G. C.; Lippard,S. J. J. Am. Chem. Soc. 1993, 115, 1175311766. (e) Pence, L. E.; Caneschi, A.; Lippard, S. J. Inorg. Chem. 1996, 35, 3069-3072. (5) (a) Gedye, C.; Harding, C.; McKee, V.; Nelson, J.; Patterson, J. J. Chem. Soc., Chem. Commun. 1992, 392-394. (b) Bashkin, S.; Chang, H.R.; Streib, W. E.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc. 1987, 109, 6502-6504. (c) Wang, S.; Folting, K.; Steib, W. E.; Schmitt, E. A.; McCusker, J. K.; Hendrickson, D. N.; Christou, G. Angew. Chem. Int. Ed. Engl. 1991, 30, 305-306. (d) Wang, S.; Tsai, H.L.; Streib, W. E.; Christou, G.; Hendrickson, D. N. J. Chem. Soc., Chem. Commun. 1992, 1427-1429. (e) Wang, S.; Tsai, H.-L.; Hagen, K. S.; Hendrickson, D. N.; Christou, G. J. Am. Chem. Soc. 1994, 116, 83768377. (6) Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804-1816. (7) Diphenylphosphinic acid (Aldrich) (0.8 mmol) suspended in 5 mL of methanol is added under stirring to 0.8 mL of 1 M tetra-N-butylammium hydroxide in methanol (Aldrich). After stripping solvent in vacuum, the residue is dissolved in 10 mL acetonitrile/acetone (2/1 v/v) and added to a solution of 0.2 mmol of [Mn2O2(bpy)4](ClO4)3 in 10 mL of acetonitrile and stirred overnight. The resulting red-brown precipitate is filtered and washed with acetonitrile and with methanol. Yield: 38%. Single crystals were isolated from methylene chloride solvent.
S0002-7863(96)03902-9 CCC: $14.00
proach was previously successful for the synthesis of a Co4O4 cubane complex starting from a complex with a dimeric CoIII2(OH)2 core and using bidentate ligands with shorter bridging atom separation.8 This method was suggested as a possible route to the Mn4O4 cubane core but was not realized until now. 1 forms after addition of diphenylphosphinate salts to a solution of [Mn2O2(bpy)4](ClO4)3 in acetonitrile.9 The IR spectrum of 1 shows mainly vibrations due to the phosphinate group10 and confirms the absence of perchlorate counterions or bpy ligands. MALDI-TOF mass spectral data11 indicate a parent molecular mass of 1583-1584 D, corresponding to the formulation Mn4O4L6 (1586.7 D), which is also supported by elemental analysis.12 Figure 1 shows the structure of 1 determined by X-ray diffraction13 together with selected bond distances and angles. The molecule shown is one of four independent molecules in each unit cell with slightly different bond distances and angles. The molecules differ mainly in the orientation of the phenyl rings, probably due to packing effects. Diphenylphosphinate ligands bridge between Mn pairs across each of the six Mn2O2 faces of the cube. This feature distinguishes 1 from the more compact Co4O4 cubane, which has two µ-bridging acetates on opposite faces,8 and the pseudo-cubanes, Mn4O3X, which have three faces bridged by carboxylates (MnIV-MnIII, 2.8 Å) and three open faces (MnIII-MnIII, 3.2-3.3 Å). Each independent cluster of 1 contains one unique and three symmetry-related Mn ions. No large (Jahn-Teller) distortions are observed in the Mn-O bond lengths, and this absence appears not to be due to a superposition of inequivalent Mn valences. Further support comes from the absence of appreciable differences in the anisotropic displacement factors for different O atoms, unlike the mixed valence complex Mn2O2(phen)43+, where these are 5-8 times larger for the axial metal ligand bonds.14c These features suggest that the valence electrons may be delocalized, yielding a rare example of a class III (delocalized) mixedvalence MnIIIMnIV compound.14a This conclusion is supported (8) Dimitrou, K.; Folting, K.; Streib, W. E.; Christou, G. J. Am. Chem. Soc. 1993, 115, 6432-6433. (9) Synthesized as previously described in: Cooper, S. R.; Dismukes, G. C.; Klein, M. P.; Calvin, M. J. Am. Chem. Soc. 1978, 100, 7248. (10) IR (KBr disk): 1438, 1132, 1109, 1032, 1007, 985, 754, 732, 693, 551, 526 cm-1. (11) Available as Supporting Information. (12) Found (calcd for Mn4O16P6C72H60) (%): C, 53.19 (54.49); H, 3.78 (3.78); N, 0 (
