996 Inorg. Chem. 1986, 25, 996-999 Contribution from the Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405 Use of Tetra-n-butylammonium Permanganate for Inorganic Syntheses in Nonaqueous Solvents. Preparation and Structure of a Manganese(II1) Dimer Containing Bridging Phenoxo Oxygen Atoms John B. Vincent, Kirsten Folting, John C. Huffman, and George Christou* Received August 21, 1985 The use of N-n-Bu4Mn04for the synthesis of inorganic complexes in nonaqueous solvents is explored. A reaction system leading to the preparation of known Mn(pi~)~.H,0(1) (picH = picolinic acid) is described. 1 can be synthesized in high yields (40-70%) by using EtOH, MeCN, DMF, and Me2C0 as solvents. With salicylic acid (salH,) in pyridine (pyr) the product, after re- crystallization from EtOH/petroleum spirits, is the new compound [Mn(EtOH),] [Mn,(sal),(pyr),] (2). Complex 2 crystallizes in the monoclinic space group P2,/n with a = 13.142 (3) A, b = 16.742 (5) A, c = 10.842 (2) A, p = 97.15 (1)O, and Z = 2 at T = -160 OC. The anion consists of two Mn(II1) atoms bridged by two p-phenoxo oxygen atoms from salicylate groups whose carboxylates are also bound to the metal. Each Mn(II1) additionally contains another salicylate and a pyridine; the dimer has Cisymmetry with each metal octahedrally coordinated. The cation is a Mn(I1) atom possessing four EtOH and two trans oxygen ligands, the latter belonging to salicylate carboxyl groups in the anion. The Mn(I1) is thus octahedrally coordinated and serves to link the anionic dimers to yield an infinite network in the crystal. 2 is the first example of a structurally characterized Mn(II1) dimer containing phenoxide bridges. Magnetic moment measurements in Me2S0 yield values of 3.14 pB/Mn(III), indicating that the dimer remains intact in solution and that the two d4 Mn(II1) atoms are antiferromagnetically coupled. Introduction the results of one approach, namely the use of the permanganate Biological water oxidation during plant photosynthesis repre- ion, Mn04-, as a reagent for inorganic synthesis in nonaqueous sents a source of electrons for the light-driven reactions of pho- solvents. We also describe the structure of the first new complex tosystem I1 (PS 11) of the photosynthetic apparatus and leads also obtained from this route, containing the manganese(II1) dimer to the evolution of molecular dioxygen (eq l).',, The requirement unit [Mn2(sal)4(pyr)2]2- (salH, = salicylic acid; pyr = pyridine). Experimental Section 2H20- 0, + 4H+ + 4e- (1) Picolinic acid (picH), salicyclic acid (salH,), Mn(OAc),.4H20, pyr- for manganese (Mn) atoms in this water oxidation reaction seems idine, absolute ethanol, and petroleum spirits were used as received. well established, and it is thought that they may represent the N-n-Bu,MnO, was prepared from KMnO, and NBu,Br as described9 site of binding and oxidation of the water molecules. Because and used without recrystallization. All operations were carried out under of this, much effort has been concentrated on elucidating the aerobic conditions at ambient temperature. identity of the manganese site, and significant progress has been Warning! There have been reports in the literature of the detonation made. In particular, the manganese atoms seem to be located of quaternary ammonium permanganates during drying at elevated tem- perature~.'~'~A systematic studyI3 has shown that salts containing tetranuclear sites with distances approxi- in di- or Mn-Mn of unsaturated groups in the cation (Et3NCH2Ph+, Me3NCH2Ph+, mately 2.7 and a coordination shell comprised of 0 and/or N A Ph3NMe+,Ph3PCH2CH2PPh32+, Ph,P+) will explode at 80 OCor above. atoms (possibly including bridging oxygen atoms between the N-n-Bu,' and NEt4+salts will also decompose at these temperatures, but In the known absence of porphyrin rings, the oc- not explosively. We recommend appropriate care be taken in the use of currence of amino acid side-group ligation is suggested. The organic permanganates. Further, the NEt4+ or N-n-Bu,+ salts should be manganese complex is capable of cycling between five distinct used where possible and dried in vacuo at room temperature, as we oxidation levels, labelled So through S4 in the pioneering work routinely do. We have also found that N-n-Bu,MnO, is pure enough for of Kok,* and it is widely accepted that metal oxidation states of use without recrystallization and that storage in a refrigerator increases 11,111, and/or IV are involved, although the precise nature of the its stability to a slow decomposition over several weeks at room tem- perature to yield a brown, sticky solid. redox reactions during the water oxidation cycle are by no means established with certainty. Syntheses Our objective in these laboratories is to synthesize an inorganic Mn(pic),.H,O (1). A solution of Mn(OAc),.4H20 (1.47 g, 6.0 mmol) model of the native manganese complex to assist in elucidating and picolinic acid (3.0 g, 24.4 mmol) in EtOH (30 mL) was rapidly the nature and mode of action of this unit during water oxidation. stirred while solid N-n-Bu,MnO, (0.72 g, 2.0 mmol) was added in small A necessary preliminary in this approach, however, is a better portions over about 10 min. The resulting bright red solution soon understanding of the chemistry of higher oxidation state man- deposited red crystals, which were collected by filtration, washed thor- ganese with the type of ligands likely to be binding to the metal oughly with EtOH, and dried in vacuo. Yield: 1.88g (54%). Anal. within the natural system. This area of manganese chemistry is Calcd for C,8H,,N30,Mn: C, 49.22; H, 3.21; N, 9.57. Found: C, 49.27; H, 3.54; N, 9.14. currently poorly investigated. We are seeking to develop this area [Mn(EtOH),][Mn2(sal)4(pyr)2](2). Salicylic acid (1 SO g, 10.86 and have been much concerned to date with exploring synthetic mmol) and Mn(OAc),.4H20 (1.00 g, 4.08 mmol) were dissolved in procedures to the desired type of compound. We herein report pyridine (20 mL) to give a clear yellow solution. This was stirred while solid (N-n-Bu,)Mn04 (0.57 g, 1.58 mmol) was added in small portions over approximately 20 min; the solution turned a deep brown-green. Renger, G.; Govindjee. Photosynth. Res. 1985, 6, 33. Overnight storage gave a brown-green precipitate, which was collected Sauer, K. Acc. Chem. Res. 1980, 13, 249. by filtration and redissolved in absolute EtOH (30 mL). Layering of an Kirby, J. A.; Robertson, A. S.; Smith, J. P.; Thompson, A. C.; Cooper, equal volume of petroleum spirits gave, after several days, dark green S. R.; Klein, M. P.J. Am. Chem. SOC.1981, 103, 5529. Dismukes, G. C.; Siderer, Y.Proc. Natl. Acad. Sci. U.S.A.1981, 78, 274. Dismukes, G. C.; Ferris, K.; Watnick, P. Photobiochem. Photobiophys. (9) Sala, T.; Sargent, M. V. J. Chem. SOC.,Chem. Commun. 1978, 253. 1982.1 3.1~ 243.~ (10) Morris, J. A,; Mills, D. C. Chem. Br. 1978, 14, 326. Abimowicz, D. A.; Dismukes, G. C. Biochim. Biophys. Acta 1984, (1 1) Jager, M.; Lutolf, J.; Meyer, M. W. Angew. Chem., Int. Ed. Engl. 1979, 765, 318. 18, 786. Goodin, D. B.; Yachandra, V. K.; Britt, R. D.; Sauer, K.; Klein, M. P. (12) Schmidt, H.-J.; Schafer, H. J. Angew. Chem., Znt. Ed. Engl. 1979, 18, Biochim. Biophys. Acta 1984, 767, 209. 781. Forbush, B.; Kok, B.; McGloin, N. P. Photochem. Photobiol. 1971, 14, (13) Leddy, B. P.; McKervey, M. A,; McSweeney, P. Tetrahedron Lett. 307. 1980, 21, 2261. 0020- 1669/86/ 1325-0996$01.50/0 0 1986 American Chemical Society Inorganic Syntheses with N-n-Bu,MnO, Inorganic Chemistry, Vol. 25, No. 7, 1986 997 Table I. Summary of Crystal Data, Intensity Collection, and weaker field ligands. Presumably this is due to extensive ligand Structure Refinement for 2 protonation at the low pHs hampering reaction or instability of parameter value generated products to the acidic aqueous medium causing hy- drolysis. It thus seemed desirable to avoid acidic aqueous solutions formula C46H50N2016Mn3 entirely and perform the reactions in organic solvents. The M1 1051.74 solubilization using crown ethers of KMn04for organic oxidations cryst syst monoclinic in benzene is documented,” but more convenient seemed the report space group P2,ln temp, OC -160 of tetraalkylammonium permanganates and their use for organic a, A 13.142 (3)“ oxidations in pyridine, acetic acid, and dichloromethane solu- b, A 16.742 (5) tion~.~,~~The potential for inorganic synthesis using these ma- c, A 10.842 (2) terials in such solvents seemed attractive to us. We were further 0,deg 97.15 (1) encouraged by the report’’ that in DMF the first reduction po- v,A3 2366.89 tential of Mn0,- is -0.73 V vs. SCE, a shift of approximately l Z 2 V compared to the value in aqueous solution (+0.32 V)! This cryst dimens, mm 0.32 X 0.20 X 0.20 suggested that the weaker oxidizing capability in organic solvents radiation (Mo Kcu), A 0.710 69b abs coeff, cm-l 10.708 might be manifested as cleaner, less vigorous, more controlled reactions, possibly to yield our desired products. scan speed, deg min-l 4(8120) scan width, deg 1.8 + dispersion Having thus decided to investigate the potential of N-n- data collcd‘ 60 5 2e 5 450 Bu,Mn04 for inorganic synthesis, our first priority was to de- total no. of data 4090 termine whether known, stable compounds available by other no.