DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1998, Pages 511–512 511 Dioxygen activation by a novel manganese(II) thiolate complex with hydrotris(3,5-diisopropylpyrazol-1-yl)borate ligand Hidehito Komatsuzaki,a,b Yuichi Nagasu,a Kantaro Suzuki,a Takao Shibasaki,a Minoru Satoh,a Fujio Ebina,a Shiro Hikichi,*,b Munetaka Akita b and Yoshihiko Moro-oka *,b a Department of Chemistry and Material Engineering, Ibaraki National College of Technology, 866 Nakane, Hitachinaka 312, Japan b Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Reaction of a MnII thiolate complex bearing hydrotris(3,5- diisopropylpyrazol-1-yl)borate with O2 resulted in O]O bond activation to give a dinuclear MnIII bis(m-oxo) complex and a ligand-oxygenated dinuclear MnIII m-oxo complex, or the dinuclear MnIII,IV m-acetato-bis(m-oxo) complex in the presence of a MnII acetate complex. Dioxygen activation on transition-metal ions is one of the attractive topics from the standpoints of bioinorganic and synthetic chemistry.Manganese–oxygen (O2 2, O2 22, O22, etc.) species are suggested to take part in the physiological dioxygen metabolism and catalytic oxidation of organic compounds.1 By using the hindered tris(pyrazolyl)borate ligand, hydrotris(3,5- diisopropylpyrazol-1-yl)borate (L), we have investigated the chemistry of Mn complexes with dioxygen and its derivatives, for example, synthesis and characterization of the mononuclear MnIII peroxo complex,2 aliphatic C]H bond oxygenation in the dimanganese complex with O2,3 and superoxide anion dismutation by the MnII–carboxylate complexes.4 It is notable that the co-ordinatively unsaturated carboxylate complex, MnIIL(O2CPh),4,5 cannot activate O2, although the FeII derivative shows reversible O2 binding ability to give the corresponding dinuclear FeIII–m-peroxo complex.6 In order to realize O2 activation on a MnL complex, we adopted a thiolate ligand, which is known to be a highly electrondonating soft base compared to such ligands as carboxylate, so as to increase the electron density at metal centers.In this communication, we report the dioxygen activation by a coordinatively unsaturated MnIIL–thiolate complex, and the intermediacy of a Mn–O2 adduct has been confirmed by a trapping experiment. Synthesis of the thiolate complex and its oxygenation reactions are summarized in Scheme 1. The MnII thiolate complex MnIIL(SC6H4NO2-p) 1 † was prepared by reaction of a dinuclear MnII bis(m-hydroxo) complex, LMn(m-OH)2MnL 2,7 with pnitrobenzenethiol under Ar.Formulation of complex 1 is based † Spectroscopic data for complex 1 (Found: C, 59.13; H, 7.40; N, 14.45. Calc. for C33H50BMnN7O2: C, 58.75; H, 7.47; N, 14.53%). IR (KBr pellet, n& /cm21): 2550m (BH), 1586, 1571s (PhC]] C and NO2). Field desorption MS: m/z 675 (M1). The two co-ordinating MeCN molecules are dissociated from the metal center in a non-co-ordinating solvent such as toluene or CH2Cl2, evidenced by the reversible color change from yellow (in toluene) to reddish orange (in MeCN).UV/VIS data: [toluene solution, 23 8C, nm (e/M21 cm21)] 322 (9860); [MeCN solution, 23 8C, nm (e/M21 cm21)] 318 (7580), 487 (9740). In the present study, oxygenation reactions were carried out in toluene to avoid the co-ordination of solvent. The monomeric structure of 1 has been confirmed by X-ray crystallography.Single crystals suitable for analysis have been obtained from MeCN solution. The MnII center is co-ordinated by an N5S donor set including two MeCN molecules. Crystal data for MnL(SC6H4NO2)(MeCN)2?3.5MeCN: C44H68BMnN12.5O2S, M = 901.9, monoclinic, space group C2/c (no. 15), a = 42.99(6), b = 12.475(4), c = 19.686(6) Å, b = 94.85(6)8, U = 10 519(5) Å3, Z = 8, T = 260 8C, Dc = 1.14 g cm21, m(Mo-Ka) = 3.36 cm21, R (R9) = 10.01 (10.98)% for 3728 reflections with 484 parameters.CCDC reference number 186/859. on its IR spectrum, with sharp absorptions around 1590–1570 cm21 arising from the p-nitrophenyl group, and its field desorption MS spectrum [m/z = 675 (M1)]. The Mn center of 1 is assumed to have a co-ordinatively unsaturated geometry as found in the analogous PhO- and RS-LFeII complexes.8 As expected, the thiolate complex 1 readily reacted with dioxygen in a manner similar to the dinuclear MnII bis- (m-hydroxo) complex 2.3 When a toluene solution of 1 was stirred under O2 atmosphere for 1 d, the solution changed from yellow to dark brown.From this dark brown solution, three products were isolated: the dinuclear MnIII bis(m-oxo) complex, LMn(m-O)2MnL 3,7 the ligand-oxygenated dinuclear MnIII complex 4,3 and the corresponding disulfide (O2NC6H4S]SC6- H4NO2).‡ The thiolate complex 1 was not hydrolyzed by treatment with an excess amount of H2O [equation (1)]. We can conclude that the present oxidation reactions proceed via degradation of Mn–O2 species which are formed by reaction of O2 and 1 (not 2) as will be discussed below.LMnII SR + H2O 2 + RSH (1) 1 Scheme 1 H O LMnII MnIIL OH 2 2xRSH 2xH2O LMnII SR 1 O LMnIII MnIIIL O O LMnII O 5 NO2 R = O2 N N N N N B N H L = O LMnIII MnIVL O O O 6 3 + + RS SR N N O N N O O H H N N N N 4 + RS SR B N B N MnIII N MnIII N ‡ The disulfide product was obtained almost quantitatively. The yield (based on complex 1) was determined by GC analysis.Yield of RS]SR in the reaction of 1 with O2 in the absence of 5 88.2%, in the presence of 5 87.4%.512 J. Chem. Soc., Dalton Trans., 1998, Pages 511–512 Although no Mn–O2 species was detected, its participation was supported by the following trapping experiment. Reaction of 1 with dioxygen in the presence of a MnII acetate complex, MnL(OAc) 5,§ resulted in the predominant formation of the MnIII,IV m-acetato-bis(m-oxo) complex, LMn(m-OAc)(m-O)2- MnL 6 ¶ (59% isolated yield based on 1),9 and the disulfide.‡ It is worth noting that the acetate complex 5 is sluggish toward oxidation under similar reaction conditions.When a toluene solution of 5 was stirred under O2, the solution turned from pale yellow to pale brown, but the reaction was very slow (over a week), and neither the Mn–O2 adducts nor the MnIII,IV complex 6 were detected. In addition, reactions of the bis(m-oxo) complex 3 and the acetate complex 5 or aqueous NaOAc or acetic acid under O2 did not yield 6 [equation (2)].Therefore, it is concluded that the dinuclear MnIII,IV complex 6 is formed via a trapping process of the Mn–O2 adduct by 5. Plausible mechanisms for the present O2 activation reactions are summarized in Scheme 2. Reaction of complex 1 with O2 may form a MnIII–superoxo complex 7, which further reacts with another molecule of the MnII complex 1 or 5 to give the corresponding dinuclear MnIII m-peroxo intermediate 8. Metal– superoxo species are known to work as nucleophiles, therefore, the nucleophilic attack of anionic 7 at the positive MnII center of 5 is more favorable than that of 1 and therefore the trapping experiment is successful.Subsequent homolysis of the O]O and Mn]S bonds || results in the formation of 3, 4 and/or 6.10 During the formation of the m-acetato-bis(m-oxo) complex 6, the acetate ligand in 5 bridges the two metal centers (so- Scheme 2 1 O2 – LMnIII SR 7 LMnII X 1: X = SR 5: X = OAc SR LMnIII O O MnIIIL X 8 X = SR X = OAc RS SR RS SR 3 + 4 6 O2 3 + HOAc 5 or or NaOAc(aq) 6 (2) O2 § The acetate complex 5 was obtained by treating Mn(OAc)2?4H2O with KL.Spectroscopic data for 5 (Found: C, 59.84; H, 8.65; N, 14.61. Calc. for C29H49- BMnN6O2: C, 60.11; H, 8.52; N, 14.50%). IR (KBr pellet, n& /cm21): 2545m (BH), 1561s [CO2(asym)]. Field desorption MS: m/z 579 (M1). The acetate ligand is assumed to bind to the MnII center in a bidentate fashion on the basis of the similarity of the n[CO2(asym)] of the benzoate analogue MnL(O2CPh) (1568 cm21), which has a five-co-ordinated distorted trigonal bipyramid MnII center with the bidentate carboxylate ligand established by crystallography (see refs. 4 and 5). The n[CO2(asym)] of 5 is indistinguishable from other peaks arising from the MnL moiety, whereas the unidentate acetatozinc complex with the same ligand gives n[CO2(asym)] and n[CO2(asym)] at 1601 and 1331 cm21, respectively. ¶ The dinuclear MnIII,IV m-acetato-bis(m-oxo) complex 6 was identified by comparison with the data (EPR, field desorption MS, IR and X-ray crystallography) of an authentic sample (see ref. 9). || The O]O bond homolysis of a dinuclear m-peroxo core [Mn1(m-O2 22)Mn1] gives the corresponding two-electron oxidized bis(m-oxo) core [M(n11)1(m-O22)2M(n11)1] and metal]sulfur bond homolysis of a Mn1(SR) core yields a one-electron reduced metal [M(n21)1] center. called ‘carboxylate shift’) as observed in the formation of the dinuclear FeIII m-peroxo complex containing L.11 It is known that reduction of dioxygen to superoxide in a one-electron transfer step has a more negative electrochemical potential than that of the two-electron reduction (O2 to O2 22).12 The O2 activation ability of the co-ordinatively unsaturated thiolate complex 1 may arise from the high electron density at the MnII center as we anticipated.Thiolate complexes with redoxactive metal ions are known to cause homolytic metal–sulfur bond cleavage to give the corresponding disulfides and reduced metal ions, in fact, the thiolate ligand of 1 works as a good leaving group as well as a reductant toward the Mn center.In the case of our previous O2 activation studies by the hydroxo complex 2,3 the dinuclear structure constructed by two fiveco- ordinated MnII centers is advantageous for the two-electron reduction of O2 giving the m-peroxo intermediates, and the hydroxide ligands are proposed to be eliminated as H2O during further O]O bond activation.3 Therefore, it is concluded that a requisite of the O2-activating MnII complex is the presence of co-ordinatively unsaturated metal centers with O2 reducing potential, and good leaving ligands to induce further O]O bond activation.In conclusion, O2 activation has been achieved by a MnII– thiolate complex and the resulting superoxo intermediate reacts with an acetate complex to give a m-peroxo intermediate 8, which is converted into the m-acetato-bis(m-oxo) complex 6 after O]O and Mn]S bond rupture.Further investigations including detection of the O2 adducts and oxidation reactions of external substrates will be performed. Acknowledgements We are grateful to the Ministry of Education, Science, Sports and Culture of the Japanese government for financial support of the research (Grant-in-Aid for Specially Promoted Scientific Research: No. 08102006). References 1 Manganese Redox Enzymes, ed.V. L. Pecoraro, VCH, New York, 1992; K. Wieghardt, Angew. Chem., Int. Ed. Engl., 1989, 28, 1153; V. L. Pecoraro, M. J. Baldwin and A. Gelasco, Chem. Rev., 1994, 94, 807; T. Mukaiyama and T. Yamada, Bull. Chem. Soc. Jpn., 1995, 68, 17. 2 N. Kitajima, H. Komatsuzaki, S. Hikichi, M. Osawa and Y. Morooka, J. Am. Chem. Soc., 1994, 116, 11 596. 3 N. Kitajima, M. Osawa, M. Tanaka and Y. Moro-oka, J. Am. Chem. Soc., 1991, 113, 8952. 4 N. Kitajima, M. Osawa, N. Tamura, Y. Moro-oka, T. Hirano, M. Hirobe and T. Nagano, Inorg. Chem., 1993, 32, 1879. 5 M. Osawa, Y. Moro-oka and N. Kitajima, Yuki Gosei Kagaku Kyokaishi, 1993, 51, 921. 6 N. Kitajima, H. Fukui, Y. Moro-oka, Y. Mizutani and T. Kitagawa, J. Am. Chem. Soc., 1990, 112, 6402; N. Kitajima, N. Tamura, H. Amagai, H. Fukui, Y. Moro-oka, Y. Mizutani, T. Kitagawa, R. Mathur, K. Heerwegh, C. A. Reed, C. R. Randall, L. Que, jun. and K. Tatsumi, J. Am. Chem. Soc., 1994, 116, 9071. 7 N. Kitajima, U. P. Singh, H. Amagai, M. Osawa and Y. Moro-oka, J. Am. Chem. Soc., 1991, 113, 7757. 8 M. Ito, H. Amagai, H. Fukui, N. Kitajima and Y. Moro-oka, Bull. Chem. Soc. Jpn., 1996, 69, 1937. 9 M. Osawa, K. Fujisawa, N. Kitajima and Y. Moro-oka, Chem. Lett., 1997, 919. 10 Oxygen–oxygen bond activation via a dinuclear MnIII–m-peroxo intermediate has been reported recently. Z. Shirin, V. G. Young, jun. and A. S. Borovik, Chem. Commun., 1997, 1967. 11 K. Kim and S. J. Lippard, J. Am. Chem. Soc., 1996, 118, 4914. 12 D. T. Sawyer, in Oxygen Complexes and Oxygen Activation by Transition Metals, eds. A. E. Martell and D. T. Sawyer, Plenum, New York, 1988, p. 131. Received 14th November 1997; Communication 7/08210J