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Counter cation-controlled air oxidation of manganese derivatives of tetrachlorocatechol
www.elsevier.nl/locate/inoche Inorganic Chemistry Communications 3 (2000) 76–79
Counter cation-controlled air oxidation of manganese derivatives of tetrachlorocatechol Rafael Ruiz a, Andrea Caneschi a, Dante Gatteschi a,*, Claudio Sangregorio a, Lorenzo Sorace a, b ´ Miguel Vazquez b
a Dipartimento di Chimica, Universita` di Firenze, 50144 Florence, Italy ´ ´ Departamento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain
Received 13 December 1999
Abstract The potassium salt of the triphenylphosphine oxide adduct of bis(tetrachlorocatecholate)manganese(III) complex reacts with O2 in the presence of Kq-sequestering agents at room temperature in MeCN resulting in the formation of the corresponding MnIV-tris(catecholate) species. q2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Catecholato complexes; Hypervalent compounds; Manganese; O–O activation; Oxygenation; Potassium
Manganese catecholate chemistry is of great current interest in diverse areas ranging from redox chemistry and electron transfer to catechol oxidation and dioxygen reactivity [1,2]. Catechol acting as a bidentate ligand through the strong electron donating deprotonated oxygen atoms is able to stabilize tetravalent manganese [3–9]. Further, being a redox-active non-innocent ligand, it can also give rise to several possible redox isomers for which the formal oxidation states of the metal center are different (Scheme 1). In certain cases, intramolecular electron transfer between the metal center and the catechol ligand occurs giving rise to an equilibrium between metal catecholate and semiquinone forms (resonance forms I and II, respectively) [6–10]. This new phenomenon, known as valence tautomerism, has certainly further stimulated the recent interest in this field [11]. Likewise, manganese–catechol complexes can interact with dioxygen either in a reversible manner forming Mn–O2 adducts [12,13], whose identity is still a matter of controversy [14,15], or through a wide variety of nonreversible reaction pathways affording manganese-o-semiquinonecomplexes [6,10], free benzoquinone [16,17], or even oxidative carbon–carbon bond cleavage products of catechol [10,18]. These studies have acquired an additional biological relevance since the discovery of manganese dioxygenase * Corresponding author. Tel.: q39 055 354 841; fax: q39 055 354 845; e-mail: [email protected]
Scheme 1.
enzymes, a new class of extradiol dioxygenases with Mn(II) at the active site [19–22]. Much of the investigation in this field involved the oxygenation of Mn derivatives of 3,5-ditert-butylcatechol (dtbcatH2), but little work has been devoted to the dioxygen activation by manganese complexes of other catechols. We describe herein the formation of a new high-valent manganese(IV) complex of tetrachlorocatechol (Cl4catH2) from the reaction of lower-valent manganese precursors with dioxygen.
Reaction of a manganese(II) salt with tetrachlorocatechol, prepared in situ from tetrachloro-1,2-benzoquinone and triphenylphosphine, in basic tetra-n-butylammonium hydroxide acetone/water solution under aerobic conditions yields
1387-7003/00/$ - see front matter q2000 Published by Elsevier Science S.A. All rights reserved. PII S 1 3 8 7 - 7 0 0 3 ( 0 0 ) 0 0 0 1 0 - 1
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the manganese(IV) catecholate complex (Bu4N)2[Mn(Cl4cat)3]PMe2CO (1) 1. The electronic spectrum of the dark blue complex 1 in acetonitrile (Fig. 1) exhibits an intense visible absorption band at 615 nm (´f7600 My1 cmy1), with two distinct shoulders at 685 and 425 nm, which is typical of high-valent manganese(IV) catecholate species [3–9]. In fact, this band can be assigned to ligand-to-metal charge-transfer (LMCT) transitions of type catecholato ™MnIV on the basis of comparision to previously reported tris(catecholate)manganese(IV) complexes with cat and dtbcat ligands [3–5]. Interestingly, under similar reaction conditions but with KOH in the place of Bu4NOH, Pierpont and co-workers obtained the manganese(III) catecholate complex K[Mn(Cl4cat)2(OPPh3)]PH2OPMe2CO (2) [23]. The structure of 2 consists of two bis(tetrachlorocatecholate)(triphenylphosphine oxide)manganese(III) complex anions, [Mn(Cl4cat)2(OPPh3)]y, which are linked together by relatively weak out-of-plane manganese–catecholate oxygen bonds. In addition, potassium cations are tightly bound to catecholate oxygen atoms from adjacent molecules (see Scheme 2), in a similar manner to that found in the manganese(IV)–tris(catecholate) complex K2[Mn(dtbcat)3]P6MeCN [5]. The electronic spectrum of the light green complex 2 in CH3CN (Fig. 1) shows only a weak visible band at 635 nm (´s150 My1 cmy1) on the lowenergy tail of an UV absorption band, which corresponds to a ligand field (d–d) transition of MnIII (d4, high spin). In an attempt to understand this remarkable cation selectivity for the attainment of a particular oxidation state of the manganese ion in the corresponding Mn–catecholate complex, we have studied the oxygenation of 2 with potassium complexing agents. Thus, the reaction of complex 2 with O2 1
Synthesis and selected spectral data for 1. A deaerated acetone solution (25 cm3) of Cl4catH2, prepared in situ by stoichiometric mixing of tetrachloro-1,2-benzoquinone (0.50 g, 2.0 mmol) and triphenylphosphine (0.52 g, 2.0 mmol), previously neutralized with a methanolic solution of Bu4NOH 1.0 M (4.5 cm3, 4.5 mmol) was added dropwise under vigorous stirring in an inert atmosphere to a solution of MnCl2P4H2O (0.20 g, 1.0 mmol) in deaerated water (5 cm3). A light pink precipitate was formed initially (presumably the neutral (tetrachlorocatecholate)manganese(II) complex), that redissolved to give a light brown solution which was extremely air sensitive (it slowly became greenish-brown even when stirred under argon atmosphere). The reaction mixture was further stirred under gentle reflux for 15 min in air. The resulting dark green solution was filtered under vacuum to eliminate a red–brown insoluble solid (fundamentally MnO2), washed copiously with acetone, and reduced in volume on a rotatory evaporator to yield a dark green crystalline solid and a dark brown mother liquor. The dark brown solution was separated from the dark green solid, which was then dissolved in acetone. The initial dark green acetone solution soon turned greenish-blue and became deep blue overnight, while a brown powder appeared in the bottom of the flask. Small dark blue crystals of complex 1 not suitable for X-ray analysis were obtained from the filtered deep blue solution after several days of slow evaporation in air at room temperature. They were filtered on paper and further recrystallized from acetone (30%). Satisfactory chemical analysis was obtained for C50H72Cl12MnN2O6PC3H6O (1335.6): calc.: C, 47.66; H, 5.89; N, 2.10; found: C, 47.70; H, 6.03; N, 2.07%. nmax/cmy1 (KBr): 1716 (CO) from Me2CO, 1423 (CO) from Cl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685 (sh) (6550) (MeCN).
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Fig. 1. Electronic absorption spectra of complexes 1 and 2 in acetonitrile. Inset: time course of the absorbance variation at 615 nm of CH3CN solutions of 2 (0.5 mM) with or without 18-crown-6 (curves (a) and (b), respectively) in air at room temperature.
in the presence of the Kq-specific chelator 18-crown-6 has been followed by UV–Vis spectroscopy at room temperature in CH3CN (inset Fig. 1, curve (a)). The appearance of the intense absorption band in the visible region (lmax 615 nm), associated with the blue color developed by the initial pale green solution with time, is indicative of the formation of a high-valent manganese(IV) catecholate species. Indeed, the electronic spectrum of the final deep blue solution is identical to that of complex 1. In the absence of a potassium-chelator the oxidation of 2 proceeds to some extent, but with a dramatically lower reaction rate (inset Fig. 1, curve (b)). In fact, complex 2 is rather stable in acetonitrile solution at room temperature, but the addition of excess tetraphenylphosphonium chloride (a general alkali metal chelator) produces an immediate color change from pale green to greenish-blue, to ultimately yield a deep blue solution after a period of several hours in air. From the final deep blue solution, we have isolated the alkali metal free manganese(IV) catecholate complex (Ph4P)2[Mn(Cl4cat)3] (3) 2, and its structure has been determined through single-crystal X-ray analysis 3. 2 Synthesis and selected spectral data for 3. To a solution of complex 2 (0.94 g, 1.0 mmol) in MeCN (25 cm3) was added solid PPh4Cl (0.56 g, 1.5 mmol) under stirring at room temperature. The initial pale green solution soon turned greenish-blue and became deep blue after several hours in air. Dark blue crystals of complex 3, contaminated with a small amount of redbrick powder, were deposited from the final deep blue solution overnight. Recrystallization from MeCN yields small dark blue hexagonal prisms of 3 suitable for X-ray analysis after a few days of slow evaporation at room temperature. They were filtered on paper and air-dried (60%). Satisfactory chemical analysis was obtained for C66H40Cl12MnP2O6 (1471.3): calc.: C, 53.88; H, 2.74; found: C, 53.69; H, 2.73%. nmax/cmy1 (KBr): 1423vs (CO) from Cl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685 (sh) (6550) (MeCN). 3 Crystal data for 3: C66H40Cl12MnP2O6, Ms1471.26, monoclinic, space ˚ bs group C2/c, as23.273(5), bs14.152(5), cs20.138(5) A, ˚ 3, Ts293 K, Ds1.481 g cmy3, Zs4, 95.790(5)8, Us6598.8(32) A m(Mo-Ka)s0.785 mmy1, 4037 unique reflections, 2040 assumed as
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Scheme 2.
The structure of 3 consists of discrete mononuclear propeller-like tris(tetrachlorocatecholate)manganese(IV)complex anions, [Mn(Cl4cat)3]2y, with a two-fold symmetry (Fig. 2), and tetraphenylphosphonium cations. The manganese atom is coordinated to six oxygen atoms from the bidentate catecholate ligands in a trigonally distorted octahedral geometry. The Mn–O bond lengths are nearly identical (aver˚ and they are comparable to age bond distance of 1.88 A), those found for the other two structurally characterized manganese(IV)–tris(3,5-di-tert-butylcatecholate) complexes [4,5]. The O–Mn–O bond angles deviate from an ideal octahedron, owing to the constraints imposed by the fivemembered chelate rings (average bite angle of 85.68), resulting in the observed trigonal distortion of the manganese coordination sphere (average twist angle of 53.98). All structural features are consistent with a manganese(IV)– catecholate formulation for 3 (i.e., the absence of Jahn–Teller distortion typical of MnIII for an alternative manganese(III)– semiquinone charge distribution). As a matter of fact, the two independent catecholate moieties have almost equivalent C–O and C–C bond lengths within the 3s criterion (average ˚ respectively), which are bond distances of 1.32 and 1.39 A, in the range observed for other manganese catecholate complexes [4–10,23]. In summary, we have found a novel alkali metal chelating agent-promoted aerial oxidation of a manganese(III) dimer of tetrachlorocatechol to a high-valent manganese(IV) catecholate monomer (Scheme 2). As far as we are aware, there is only one precedent in the literature of this kind of reaction by O’Halloran and co-workers who reported the air oxidation of a Mn(III)–bis(amidoalkoxo) dimer to a Mn(V)–oxo complex [24]. It appears that loss of Kq bridges observed with I)2s(I) which were used in all calculations. The structure was solved using direct methods with subsequent full-matrix least-squares method refinement on F2. The hydrogen atoms were calculated at fixed distances and refined with an overall isotropic thermal parameter. Refinement of 393 variables with anisotropic thermal parameters for all nonhydrogen atoms gave Rs0.063 and Rws0.150 with Ss1.123.
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Fig. 2. Perspective view of the anionic mononuclear unit of 3 with the atomnumbering scheme (thermal ellipsoids are drawn at the 30% probability ˚ and angles (8) with standard devialevel). Selected bond distances (A) tions in parentheses: Mn(1)–O(11) 1.892(6), Mn(1)–O(12) 1.885(5), Mn(1)–O(21) 1.877(6); O(11)–Mn(1)–O(12) 85.6(2), O(11)– Mn(1)–O(21) 90.8(2), O(11)–Mn(1)–O(11I) 92.9(4), O(11)–Mn(1)– O(12I) 90.8(2), O(11)–Mn(1)–O(21I) 175.1(3), O(12)–Mn(1)–O(21) 92.7(2), O(12)–Mn(1)–O(12I) 174.8(3), O(12)–Mn(1)–O(21I) 91.2(2), O(21)–Mn(1)–O(21I) 85.7(3) (symmetry code: Isyx, y, yzq1/2).
in 2 leads to formation of a O2-reactive Mn species. Although the exact nature of this intermediate species is unknown, a monomeric five-coordinate MnIII–bis(catecholate) triphenylphosphine oxide adduct or a dissociation product of 2 such as MnIII–tris(catecholate) seem to us the more probable candidates to interact with O2. An investigation of the mechanistic details of the air oxidation reaction is planned in order to provide further insight into the process of dioxygen activation by this new family of manganese tetrachlorocatecholate complexes.
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Supplementary material Tables of atomic coordinates, bond lengths and angles, and thermal parameters are available from the authors on request.
Acknowledgements This work was supported by the 3MD-EU Network (contract no. ERB 4061, PL 97-0197). We would like to express our gratitude to Prof. A. Dei for fruitful discussions and continuous interest in this work.
References C.G. Pierpont, R.M. Buchanan, Coord. Chem. Rev. 38 (1981) 45. C.G. Pierpont, C.W. Lange, Prog. Coord. Chem. 41 (1993) 331. S.E. Jones, D.H. Chin, D.T. Sawyer, Inorg. Chem. 20 (1981) 4257. D.H. Chin, D.T. Sawyer, W.P. Schaefer, C.J. Simmons, Inorg. Chem. 22 (1983) 752. [5] J.R. Hartman, B.M. Foxman, S.R. Cooper, Inorg. Chem. 23 (1984) 1381. [6] M.W. Lynch, D.N. Hendrickson, B.J. Fitzgerald, C.G. Pierpont, J. Am. Chem. Soc. 106 (1984) 2041.
[1] [2] [3] [4]
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[7] A.S. Attia, O.S. Jung, C.G. Pierpont, Inorg. Chim. Acta 226 (1994) 91. [8] A.S. Attia, C.G. Pierpont, Inorg. Chem. 34 (1995) 1172. [9] A.S. Attia, C.G. Pierpont, Inorg. Chem. 36 (1997) 6184. [10] A. Caneschi, A. Dei, Angew. Chem., Int. Ed. Engl. 37 (1998) 3005. ¨ A. Dei, Angew. Chem., Int. Ed. Engl. 36 (1997) 2734 and [11] P. Gutlich, Refs. therein. [12] K.D. Magers, C.G. Smith, D.T. Sawyer, J. Am. Chem. Soc. 100 (1978) 989. [13] K.D. Magers, C.G. Smith, D.T. Sawyer, Inorg. Chem. 19 (1980) 492. [14] S.R. Cooper, J.R. Hartman, Inorg. Chem. 21 (1982) 4315. [15] D.H. Chin, D.T. Sawyer, Inorg. Chem. 21 (1982) 4317. [16] C.A. Tyson, A.E. Martell, J. Am. Chem. Soc. 94 (1972) 939. [17] U. Russo, M. Vidali, B. Zarli, R. Purrello, G. Maccarrone, Inorg. Chim. Acta 120 (1986) L11. [18] R. Ruiz, A. Caneschi, D. Gatteschi, A.B. Gaspar, J.A. Real, I. ˜ Fernandez, M.C Munoz, Inorg. Chem. Commun. 2 (1999) 521. [19] L. Que Jr., J. Widom, R.L. Crawford, J. Biol. Chem. 256 (1981) 10941. [20] Y.R. Boldt, M.J. Sadowsky, L.B.M. Ellis, L. Que Jr., L.P. Wackett, J. Bacteriol. 177 (1995) 1225. [21] A.K. Whiting, Y.R. Boldt, M.P. Hendrich, L.P. Wackett, L. Que Jr., Biochemistry 35 (1996) 160. [22] N.A. Law, M.T. Caudle, V.L. Pecoraro, Adv. Inorg. Chem. 46 (1998) 305 and Refs. therein. [23] S.K. Larsen, C.G. Pierpont, G. De Munno, G. Dolcetti, Inorg. Chem. 25 (1986) 4828. [24] F.M. MacDonnell, N.L.P. Fackler, C. Stern, T.V. O’Halloran, J. Am. Chem. Soc. 116 (1994) 7431.
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Counter cation-controlled air oxidation of manganese derivatives of tetrachlorocatechol Rafael Ruiz a, Andrea Caneschi a, Dante Gatteschi a,*, Claudio Sangregorio a, Lorenzo Sorace a, b ´ Miguel Vazquez b
a Dipartimento di Chimica, Universita` di Firenze, 50144 Florence, Italy ´ ´ Departamento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain
Received 13 December 1999
Abstract The potassium salt of the triphenylphosphine oxide adduct of bis(tetrachlorocatecholate)manganese(III) complex reacts with O2 in the presence of Kq-sequestering agents at room temperature in MeCN resulting in the formation of the corresponding MnIV-tris(catecholate) species. q2000 Published by Elsevier Science S.A. All rights reserved. Keywords: Catecholato complexes; Hypervalent compounds; Manganese; O–O activation; Oxygenation; Potassium
Manganese catecholate chemistry is of great current interest in diverse areas ranging from redox chemistry and electron transfer to catechol oxidation and dioxygen reactivity [1,2]. Catechol acting as a bidentate ligand through the strong electron donating deprotonated oxygen atoms is able to stabilize tetravalent manganese [3–9]. Further, being a redox-active non-innocent ligand, it can also give rise to several possible redox isomers for which the formal oxidation states of the metal center are different (Scheme 1). In certain cases, intramolecular electron transfer between the metal center and the catechol ligand occurs giving rise to an equilibrium between metal catecholate and semiquinone forms (resonance forms I and II, respectively) [6–10]. This new phenomenon, known as valence tautomerism, has certainly further stimulated the recent interest in this field [11]. Likewise, manganese–catechol complexes can interact with dioxygen either in a reversible manner forming Mn–O2 adducts [12,13], whose identity is still a matter of controversy [14,15], or through a wide variety of nonreversible reaction pathways affording manganese-o-semiquinonecomplexes [6,10], free benzoquinone [16,17], or even oxidative carbon–carbon bond cleavage products of catechol [10,18]. These studies have acquired an additional biological relevance since the discovery of manganese dioxygenase * Corresponding author. Tel.: q39 055 354 841; fax: q39 055 354 845; e-mail: [email protected]
Scheme 1.
enzymes, a new class of extradiol dioxygenases with Mn(II) at the active site [19–22]. Much of the investigation in this field involved the oxygenation of Mn derivatives of 3,5-ditert-butylcatechol (dtbcatH2), but little work has been devoted to the dioxygen activation by manganese complexes of other catechols. We describe herein the formation of a new high-valent manganese(IV) complex of tetrachlorocatechol (Cl4catH2) from the reaction of lower-valent manganese precursors with dioxygen.
Reaction of a manganese(II) salt with tetrachlorocatechol, prepared in situ from tetrachloro-1,2-benzoquinone and triphenylphosphine, in basic tetra-n-butylammonium hydroxide acetone/water solution under aerobic conditions yields
1387-7003/00/$ - see front matter q2000 Published by Elsevier Science S.A. All rights reserved. PII S 1 3 8 7 - 7 0 0 3 ( 0 0 ) 0 0 0 1 0 - 1
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the manganese(IV) catecholate complex (Bu4N)2[Mn(Cl4cat)3]PMe2CO (1) 1. The electronic spectrum of the dark blue complex 1 in acetonitrile (Fig. 1) exhibits an intense visible absorption band at 615 nm (´f7600 My1 cmy1), with two distinct shoulders at 685 and 425 nm, which is typical of high-valent manganese(IV) catecholate species [3–9]. In fact, this band can be assigned to ligand-to-metal charge-transfer (LMCT) transitions of type catecholato ™MnIV on the basis of comparision to previously reported tris(catecholate)manganese(IV) complexes with cat and dtbcat ligands [3–5]. Interestingly, under similar reaction conditions but with KOH in the place of Bu4NOH, Pierpont and co-workers obtained the manganese(III) catecholate complex K[Mn(Cl4cat)2(OPPh3)]PH2OPMe2CO (2) [23]. The structure of 2 consists of two bis(tetrachlorocatecholate)(triphenylphosphine oxide)manganese(III) complex anions, [Mn(Cl4cat)2(OPPh3)]y, which are linked together by relatively weak out-of-plane manganese–catecholate oxygen bonds. In addition, potassium cations are tightly bound to catecholate oxygen atoms from adjacent molecules (see Scheme 2), in a similar manner to that found in the manganese(IV)–tris(catecholate) complex K2[Mn(dtbcat)3]P6MeCN [5]. The electronic spectrum of the light green complex 2 in CH3CN (Fig. 1) shows only a weak visible band at 635 nm (´s150 My1 cmy1) on the lowenergy tail of an UV absorption band, which corresponds to a ligand field (d–d) transition of MnIII (d4, high spin). In an attempt to understand this remarkable cation selectivity for the attainment of a particular oxidation state of the manganese ion in the corresponding Mn–catecholate complex, we have studied the oxygenation of 2 with potassium complexing agents. Thus, the reaction of complex 2 with O2 1
Synthesis and selected spectral data for 1. A deaerated acetone solution (25 cm3) of Cl4catH2, prepared in situ by stoichiometric mixing of tetrachloro-1,2-benzoquinone (0.50 g, 2.0 mmol) and triphenylphosphine (0.52 g, 2.0 mmol), previously neutralized with a methanolic solution of Bu4NOH 1.0 M (4.5 cm3, 4.5 mmol) was added dropwise under vigorous stirring in an inert atmosphere to a solution of MnCl2P4H2O (0.20 g, 1.0 mmol) in deaerated water (5 cm3). A light pink precipitate was formed initially (presumably the neutral (tetrachlorocatecholate)manganese(II) complex), that redissolved to give a light brown solution which was extremely air sensitive (it slowly became greenish-brown even when stirred under argon atmosphere). The reaction mixture was further stirred under gentle reflux for 15 min in air. The resulting dark green solution was filtered under vacuum to eliminate a red–brown insoluble solid (fundamentally MnO2), washed copiously with acetone, and reduced in volume on a rotatory evaporator to yield a dark green crystalline solid and a dark brown mother liquor. The dark brown solution was separated from the dark green solid, which was then dissolved in acetone. The initial dark green acetone solution soon turned greenish-blue and became deep blue overnight, while a brown powder appeared in the bottom of the flask. Small dark blue crystals of complex 1 not suitable for X-ray analysis were obtained from the filtered deep blue solution after several days of slow evaporation in air at room temperature. They were filtered on paper and further recrystallized from acetone (30%). Satisfactory chemical analysis was obtained for C50H72Cl12MnN2O6PC3H6O (1335.6): calc.: C, 47.66; H, 5.89; N, 2.10; found: C, 47.70; H, 6.03; N, 2.07%. nmax/cmy1 (KBr): 1716 (CO) from Me2CO, 1423 (CO) from Cl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685 (sh) (6550) (MeCN).
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Fig. 1. Electronic absorption spectra of complexes 1 and 2 in acetonitrile. Inset: time course of the absorbance variation at 615 nm of CH3CN solutions of 2 (0.5 mM) with or without 18-crown-6 (curves (a) and (b), respectively) in air at room temperature.
in the presence of the Kq-specific chelator 18-crown-6 has been followed by UV–Vis spectroscopy at room temperature in CH3CN (inset Fig. 1, curve (a)). The appearance of the intense absorption band in the visible region (lmax 615 nm), associated with the blue color developed by the initial pale green solution with time, is indicative of the formation of a high-valent manganese(IV) catecholate species. Indeed, the electronic spectrum of the final deep blue solution is identical to that of complex 1. In the absence of a potassium-chelator the oxidation of 2 proceeds to some extent, but with a dramatically lower reaction rate (inset Fig. 1, curve (b)). In fact, complex 2 is rather stable in acetonitrile solution at room temperature, but the addition of excess tetraphenylphosphonium chloride (a general alkali metal chelator) produces an immediate color change from pale green to greenish-blue, to ultimately yield a deep blue solution after a period of several hours in air. From the final deep blue solution, we have isolated the alkali metal free manganese(IV) catecholate complex (Ph4P)2[Mn(Cl4cat)3] (3) 2, and its structure has been determined through single-crystal X-ray analysis 3. 2 Synthesis and selected spectral data for 3. To a solution of complex 2 (0.94 g, 1.0 mmol) in MeCN (25 cm3) was added solid PPh4Cl (0.56 g, 1.5 mmol) under stirring at room temperature. The initial pale green solution soon turned greenish-blue and became deep blue after several hours in air. Dark blue crystals of complex 3, contaminated with a small amount of redbrick powder, were deposited from the final deep blue solution overnight. Recrystallization from MeCN yields small dark blue hexagonal prisms of 3 suitable for X-ray analysis after a few days of slow evaporation at room temperature. They were filtered on paper and air-dried (60%). Satisfactory chemical analysis was obtained for C66H40Cl12MnP2O6 (1471.3): calc.: C, 53.88; H, 2.74; found: C, 53.69; H, 2.73%. nmax/cmy1 (KBr): 1423vs (CO) from Cl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685 (sh) (6550) (MeCN). 3 Crystal data for 3: C66H40Cl12MnP2O6, Ms1471.26, monoclinic, space ˚ bs group C2/c, as23.273(5), bs14.152(5), cs20.138(5) A, ˚ 3, Ts293 K, Ds1.481 g cmy3, Zs4, 95.790(5)8, Us6598.8(32) A m(Mo-Ka)s0.785 mmy1, 4037 unique reflections, 2040 assumed as
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Scheme 2.
The structure of 3 consists of discrete mononuclear propeller-like tris(tetrachlorocatecholate)manganese(IV)complex anions, [Mn(Cl4cat)3]2y, with a two-fold symmetry (Fig. 2), and tetraphenylphosphonium cations. The manganese atom is coordinated to six oxygen atoms from the bidentate catecholate ligands in a trigonally distorted octahedral geometry. The Mn–O bond lengths are nearly identical (aver˚ and they are comparable to age bond distance of 1.88 A), those found for the other two structurally characterized manganese(IV)–tris(3,5-di-tert-butylcatecholate) complexes [4,5]. The O–Mn–O bond angles deviate from an ideal octahedron, owing to the constraints imposed by the fivemembered chelate rings (average bite angle of 85.68), resulting in the observed trigonal distortion of the manganese coordination sphere (average twist angle of 53.98). All structural features are consistent with a manganese(IV)– catecholate formulation for 3 (i.e., the absence of Jahn–Teller distortion typical of MnIII for an alternative manganese(III)– semiquinone charge distribution). As a matter of fact, the two independent catecholate moieties have almost equivalent C–O and C–C bond lengths within the 3s criterion (average ˚ respectively), which are bond distances of 1.32 and 1.39 A, in the range observed for other manganese catecholate complexes [4–10,23]. In summary, we have found a novel alkali metal chelating agent-promoted aerial oxidation of a manganese(III) dimer of tetrachlorocatechol to a high-valent manganese(IV) catecholate monomer (Scheme 2). As far as we are aware, there is only one precedent in the literature of this kind of reaction by O’Halloran and co-workers who reported the air oxidation of a Mn(III)–bis(amidoalkoxo) dimer to a Mn(V)–oxo complex [24]. It appears that loss of Kq bridges observed with I)2s(I) which were used in all calculations. The structure was solved using direct methods with subsequent full-matrix least-squares method refinement on F2. The hydrogen atoms were calculated at fixed distances and refined with an overall isotropic thermal parameter. Refinement of 393 variables with anisotropic thermal parameters for all nonhydrogen atoms gave Rs0.063 and Rws0.150 with Ss1.123.
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Fig. 2. Perspective view of the anionic mononuclear unit of 3 with the atomnumbering scheme (thermal ellipsoids are drawn at the 30% probability ˚ and angles (8) with standard devialevel). Selected bond distances (A) tions in parentheses: Mn(1)–O(11) 1.892(6), Mn(1)–O(12) 1.885(5), Mn(1)–O(21) 1.877(6); O(11)–Mn(1)–O(12) 85.6(2), O(11)– Mn(1)–O(21) 90.8(2), O(11)–Mn(1)–O(11I) 92.9(4), O(11)–Mn(1)– O(12I) 90.8(2), O(11)–Mn(1)–O(21I) 175.1(3), O(12)–Mn(1)–O(21) 92.7(2), O(12)–Mn(1)–O(12I) 174.8(3), O(12)–Mn(1)–O(21I) 91.2(2), O(21)–Mn(1)–O(21I) 85.7(3) (symmetry code: Isyx, y, yzq1/2).
in 2 leads to formation of a O2-reactive Mn species. Although the exact nature of this intermediate species is unknown, a monomeric five-coordinate MnIII–bis(catecholate) triphenylphosphine oxide adduct or a dissociation product of 2 such as MnIII–tris(catecholate) seem to us the more probable candidates to interact with O2. An investigation of the mechanistic details of the air oxidation reaction is planned in order to provide further insight into the process of dioxygen activation by this new family of manganese tetrachlorocatecholate complexes.
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Article: 328
R. Ruiz et al. / Inorganic Chemistry Communications 3 (2000) 76–79
Supplementary material Tables of atomic coordinates, bond lengths and angles, and thermal parameters are available from the authors on request.
Acknowledgements This work was supported by the 3MD-EU Network (contract no. ERB 4061, PL 97-0197). We would like to express our gratitude to Prof. A. Dei for fruitful discussions and continuous interest in this work.
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StyleTag -- Journal: INOCHE (Inorganic Chemistry Communications)
Article: 328