A monomeric manganese(II) catecholato complex: Synthesis, crystal structure, and reactivity toward molecular oxygen

Accepted Manuscript A Monomeric Manganese(II) Catecholato Complex: Synthesis, Crystal Structure, and Reactivity toward Molecular Oxygen Shin-ichiro Agake, Hidehito Komatsuzaki, Minoru Satoh, Tomohiro Agou, Yuya Tanaka, Munetaka Akita, Jun Nakazawa, Shiro Hikichi PII: DOI: Reference:

S0020-1693(18)30920-4 https://doi.org/10.1016/j.ica.2018.09.013 ICA 18471

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

15 June 2018 1 September 2018 5 September 2018

Please cite this article as: S-i. Agake, H. Komatsuzaki, M. Satoh, T. Agou, Y. Tanaka, M. Akita, J. Nakazawa, S. Hikichi, A Monomeric Manganese(II) Catecholato Complex: Synthesis, Crystal Structure, and Reactivity toward Molecular Oxygen, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.09.013

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A Monomeric Manganese(II) Catecholato Complex: Synthesis, Crystal Structure, and Reactivity toward Molecular Oxygen

Shin-ichiro Agake a, Hidehito Komatsuzaki a,*, Minoru Satoh a, Tomohiro Agou b, Yuya Tanaka c, Munetaka Akita c, Jun Nakazawa d, and Shiro Hikichi d,*

a

National Institute of Technology, Ibaraki Collage, Hitachinaka 312-8508, Japan

b

Ibaraki University, Hitachi 316-8511, Japan

c

Tokyo Institute of Technology, Yokohama 226-8503, Japan

d

Kanagawa University, Yokohama 221-8686, Japan

*Corresponding author Tel.: +81-29-271-2984, fax: +81-29-271-2992. Email address: [email protected] (H. Komatsuzaki)

1

[Abstract] A

monomeric

manganese(II)

catecholato

hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate

(TptBu,iPr)

complex was

synthesized

with and

characterized. X-ray analysis of the compound shows a tetrahedral manganese coordinated to one oxygen atom of the catecholato ligand and three nitrogen atoms of the TptBu,iPr ligand. The Mn-O distance (1.952(5)Å) is longer than the corresponding Fe-O distance (1.874(7)Å) in the analogous iron(II) TptBu,iPr complex due to the weaker Lewis acidity of manganese(II). Upon reacting of the manganese(II) complex with molecular oxygen, a manganese(II) semiquinonato complex was obtained, thus serving as an important model for catechol dioxygenase. This differs from the reaction of the iron(II) analog, which generates an iron(III) catecholato complex.

[Keywords] Manganese complex; Hydrotris(pyrazolyl)borate; Catecholato ligand; Dioxygen activation; Catechol dioxygenase

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[Introduction] Catechol dioxygenase (CDO) is an enzyme which activates molecular oxygen to cleave the aromatic C-C bond of catechols [1-5]. There are two types of CDO, classified based on the position of the catechol bond cleaved: the intradiol type that cleaves the C1-C2 position and the extradiol type that cleaves C2-C3 position. The former utilizes an iron(III) cofactor in the active center, whereas the latter utilizes either iron(II) or manganese(II) cofactors. For both types, it has been proposed that formation of a catechol-bound metal complex (metal-substrate complex) and activation of molecular oxygen by the metal-substrate complex are key steps in the CDO enzymatic process [1-13]. To date, only a limited number of manganese-catecholato (and one-electron oxidized semiquinonato) complexes have been reported as models of the manganese-substrate complex for the manganese-dependent catechol dioxygenase enzyme (Mn-CDO) [13-20]. However, information for Mn-CDO is scarce compared to that for iron(II) or (III)-dependent catechol dioxygenases. We previously reported the synthesis, structural characterization, and reactivity toward molecular oxygen of a monomeric manganese(II) semiquinonato complex with hydrotris(3,5-di-isopropyl-1-pyrazolyl)borate (TpiPr2; Figure 1), [TpiPr2MnIIDTBSQ] (1SQ; where DTBSQ denotes 3,5-di-tert-butylsemiquiononate) [14].

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Figure 1 Hydrotris(3,5-dialkyl-1-pyrazolyl)borate ligand

The semiquinonato ligand of complex 1SQ coordinates to the manganese(II) center in a bidentate fashion, and 1SQ reacts with molecular oxygen to give the C-C bond cleavage products of the catechol moiety and 3,5-di-tert-butyl-1,2-benzoquinone. We believe that a manganese(II) catecholato complex 1CatH is formed prior to generation of 1SQ. Therefore, we tried to synthesize the catecholato complex 1CatH by reaction of a bis(-hydroxo) manganese(II, II) complex and 3,5-di-tert-butylcatechol (3,5-DTBCH2), but not isolated because of the reactivity of 1CatH toward O2, and subsequent rapid conversion into 1SQ. In addition, we have synthesized monomeric iron(III) and iron(II) catecholato model compounds with TpiPr2 and the bulkier TptBu,iPr ligand (TptBu,iPr = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate) as shown in Figure 1 [21]. When using the TptBu,iPr ligand instead of the TpiPr2 ligand, a monomeric manganese(II) catecholato complex, [TptBu,iPrMnIIDTBCH] (2CatH; where DTBCH denotes 3,5-di-tert-butylcatecholato), resulted in which the catecholato is bound to the manganese(II)

4

center via unidentate coordination. Here, we report the synthesis, molecular structure, and reactivity toward molecular oxygen of the compound 2CatH.

[Experimental] Materials and methods All solvents used in these experiments were purified by the methods as reported in literature [22]. Toluene, CH2CH2 and MeCN were treated with appropriate drying agents, distilled, and stored under nitrogen. The highest grade of reagents commercially available was used without further purification process. Hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate (TptBu,iPr) and [TptBu,iPrMnIICl] (3) were prepared using the methodology reported so far [23, 24]. All manipulations were performed under nitrogen using standard Schlenk techniques. IR measurements were carried out using a JASCO FT/IR-6100 spectrometer. UV-Vis spectra were recorded on a JASCO V-650. EPR spectra were recorded on a JEOL JES-X310 spectrometer as a frozen toluene solution at low temperature in quartz tube. Field desorption mass spectrometry (FD-MS) was performed on a JEOL JMS-700 mass spectrometer.

Crystal and data collections Single crystals of 2CatH were obtained by recrystallization from the colorless CH2Cl2 solution. In

5

case of a semiquinonato complex, [TptBu,iPrMnIIDTBSQ] (5), single crystals were obtained from dark green MeCN/CH2Cl2 solution. The intensity data was collected on a Rigaku XtaLabMini diffractometer or on a Rigaku Saturn 724+ CCD diffractometer with a Rigaku VariMax Mo Optic System using Mo K radiation ( = 0.71073 Å). The reflection data were processed by using the CrysAlisPro program (version 1.171.38.43) [25]. The structure was solved by a direct method (SHELXT-2014/5) and refined by full-matrix least square method on F2 for all reflections (SHELXTL-2014/7) [26]. All hydrogen atoms were placed using AFIX instructions, while all the other atoms were refined anisotropically. CCDC 1827461 and 1865255 contain the supplementary crystallographic data for 2CatH and 5, which can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data_request/cif. A summary of the cell parameters, data collection conditions and refinement results is provided in Table 1. Crystal

data

for

0.210×0.150×0.100 mm3,

2CatH:

C45H75N6O2BCl2Mn, ,

FW

868.76,

T

173(2)

K,

, a = 16.6194(5), b = 10.5088(3), c =

30.0042(6) Å,  =  =  = 90°, V = 5240.2(2) Å3, Z = 4, Dcalcd = 1.101 F(000) = 1868, 2.665°≤≤25.493°, Reflection collected

,  = 0.392 mm-1,

, Independent reflections 5037

(Rint = 0.0359), Completeness to max 99.5%, Data/restraints/parameters

, GOF on

F2 1.090, R1 [I>2(I)] 0.0677, wR2 (all data) 0.1939, Largest diffraction peak and hole 0.700 and –0.732 e Å-3. Crystal data for 5: C48H78N8O2BMn, FW 864.93, T 123(2) K,

6

0.080×0.070×0.050 mm3,

, P2 2 2 , a = 15.4530(10), b = 17.1281(13), c =

19.1395(14) Å,  =  =  = 90°, V = 5060.0(6) Å3, Z = 4, Dcalcd = 1.135

,  = 0.304 mm-1,

F(000) = 1872, 1.595°≤≤25.500°, Reflection collected 17923, Independent reflections 9397 (Rint = 0.0800), Completeness to max 99.9%, Data/restraints/parameters 9397

, GOF on

F2 0.996, R1 [I>2(I)] 0.0705, wR2 (all data) 0.1416, Largest diffraction peak and hole 0.626 and –0.296 e Å-3. We encountered severe problems in the X-ray crystallographic analysis of complex 2CatH. After a number of attempts, we could obtain only twinned crystals of this complex, which lost their crystallinity soon after removal from the mother liquor even when mounted in pre-cooled mineral oil, owing to the loss of the solvent molecule (CH 2Cl2). These problems significantly complicated the data collection and the structural refinement. In particular, several iPr and tBu groups in complex 2CatH were highly disordered. We could not account for all of the disordered substituents, resulting in the unusually large anisotropic displacement parameters for some iPr and tBu groups. These problems might not affect the structural parameters of the central part of complex 2CatH.

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Table 1 Crystal data and data collection details of [TptBu,iPrMnIIDTBCH] (2CatH･CH2Cl2) and [TptBu,iPrMnIIDTBSQ] (5･2MeCN)

[TptBu,iPrMnIIDTBCH] (2CatH･CH2Cl2)

[TptBu,iPrMnIIDTBSQ] (5･2MeCN)

Formula

C45H75N6O2BCl2Mn

C48H78N8O2BMn

Formula weight

868.76

864.93

Temperature

173(2) K

123(2) K

Wavelength

0.71073 Å

0.71073 Å

Crystal system

Orthorhombic

Orthorhombic

Space group

Cmc21

P212121

Unit cell dimensions

a = 16.6194(5) Å = 90°.

a = 15.4350(10) Å = 90°

b = 10.5088(3) Å = 90°.

b = 17.1281(13) Å = 90°.

c = 30.0042(6) Å  = 90°.

c = 19.1395(14) Å  = 90°.

5240.2(2) Å3

5060.0(6) Å3

Z

4

4

Density (calculated)

1.101 Mg/m3

1.135 Mg/m3

Absorption coefficient

0.392 mm-1

0.304 mm-1

F(000)

1868

1872

Crystal size

0.210 x 0.150 x 0.100 mm3

0.080 x 0.070 x 0.050 mm3

Theta range for data collection

2.665 to 25.493°.

1.595 to 25.500°.

Index ranges

-20<=h<=20, -12<=k<=12, -36<=l<=36

-18<=h<=15, -20<=k<=16, -15<=l<=23

Reflections collected

45861

17923

Independent reflections

5037 [R(int) = 0.0359]

9397 [R(int) = 0.0800]

Completeness to theta = 25.242°

99.50%

99.90%

Refinement method

Full-matrix least-squares on F2

Full-matrix least-squares on F2

Data / restraints / parameters

5037 / 24 / 359

9397 / 18 / 564

Goodness-of-fit on F2

1.09

0.996

Final R indices [I>2sigma(I)]

R1 = 0.0677, wR2 = 0.1917

R1 = 0.0705, wR2 = 0.1200

R indices (all data)

R1 = 0.0695, wR2 = 0.1939

R1 = 0.1171, wR2 = 0.1416

Absolute structure parameter

0.009(6)

-0.04(3)

Extinction coefficient

n/a

n/a

Largest diff. peak and hole

0.700 and -0.732 e.Å-3

0.626 and -0.296 e.Å-3

  Volume

8

Synthesis of [TptBu,iPrMnIIDTBCH] (2CatH) Complex 2CatH was obtained when a toluene solution of a mixture of 3,5-di-tert-butylcatechol (39.1mg, 0.1759mmol) and sodium hydride (60% purity, 7.7mg, 0.1917mmol) was added slowly to 10mL of CH2Cl2 solution of a manganese(II) chloride complex (0.1053g, 0.1761mmol), [TptBu,iPrMnIICl] (3) [24] (Scheme 1). After filtration and following recrystallization from MeCN/CH2Cl2, colorless single crystals of 2CatH were obtained (60.6mg, –1

0.0773mmol, 43.9% yield). Data for 2CatH: IR (KBr pellet,  / cm ): 3441 (w, OH), 2966 (vs, CH), 2908 (m, CH), 2870 (m, CH), 2571 (w, BH), 1588 (m, C=CPh), 1536 (m), 1465 (s), 1422 (m), 1364 (m), 1302 (m), 1260 (m), 1240 (m), 1175 (s, C-O), 1127 (w), 1100 (w), 1063 (m), 1049 (m), 1026 (m), 985 (m), 798 (m), 752 (m), 644 (m). FD-MS: calcd. for C44H73N6O2BMn (M+) 784 m/z, found (M+) 784 m/z (Figure S1 in the Supporting Information). Elemental analysis: calcd (%) for C45H75N6O2BCl2Mn (2CatH ·CH2Cl2): C, 62.21; H, 8.70; N, 9.67; found: C, 62.64; H, 9.02; N, 10.34.

Scheme 1 Synthesis of catecholato complex 2CatH

9

Reaction of 2CatH and O2 Complex 2CatH was dissolved in 5 mL of toluene (0.357 mmol/L) and this solution was stirred at ambient temperature under O2 (1 atom).

At the end of the reaction period after 24h, the

reaction mixture was degassed under vacuum. Recrystallization from MeCN/CH2Cl2, green –1

single crystals of 5 were obtained (42.8mg, 0.0543mmol, 70.7% yield). IR (ATR,  / cm ): 2960 (s, CH), 2908 (m, CH), 2868 (m, CH), 2560 (w, BH), 1584 (m, C=C Ph), 1533 (m), 1465 (s), 1442 (s), 1359 (s), 1296 (m), 1240 (m), 1173 (s), 1127 (w), 1096 (w), 1057 (m), 1044 (s), 1025 –1

–1

(m), 984 (m), 794 (vs), 751 (s), 641 (s). UV-vis (toluene, 292K, nm, (/ M cm )): 298 nm (sh,  = 8825), 316 (sh, 11211), 326 (12129), 380 (7720), 406 (sh, 626), 426 (sh, 647), 466 (1345) and 762 nm (462 M-1cm-1). FD-MS: calcd. for C44H72N6O2BMn (M+) 783 m/z, found (M+) 783 m/z (Figure S2 in the Supporting Information). Elemental analysis: calcd (%) for C45H74N6O2BCl2Mn (5·CH2Cl2 which was purified by only CH2Cl2.): C, 62.28, H, 8.60; N, 9.68; found: C, 62.07; H, 8.42; N, 10.15.

Changes with time for the reaction of 2CatH and O2 A toluene solution of 2CatH (0.357 mmol/L) was prepared under an N2 atmosphere and transferred to a quartz cell filled with N2 gas. After removal of the N2 gas in vacuo, O2 gas at 1 atom was purged into the quartz cell. The reaction of 2CatH with O2 at 295K was monitored via

10

UV-Vis, with a spectrum recorded every minute.

[Results and discussion] Spectroscopic analyses An IR spectrum of 2CatH showed a strong absorption at 3441cm-1. This is similar to the ν(OH) peak (3460cm-1) of an analogous iron(II) monomeric catecholato complex with the TptBu,iPr ligand, [TptBu,iPrFeIIDTBCH] (4CatH), in which the catechol binds to the iron(II) in unidentate mode [21]. In addition, absorptions at 2571cm-1 based on the ν(BH) and at 1588cm-1 based on the ν(PhC=C) are similar to that of complex 4CatH (ν(BH) = 2565cm-1 and ν(C=CPh) = 1589cm-1 ). The measurement result of field desorption mass spectrometry (FD-MS) of 2CatH showed a signal at m/z = 784 (M+ for C44H73N6O2BMn, calcd. 784 m/z), indicating a monomeric structure similar to the previously-reported manganese(II) 1SQ [14] and iron(II) 4CatH complexes [21].

Crystal structure description The structural characterization of complex 2CatH was carried out via X-ray single crystal analysis, and the molecular structure is shown in Figure 2. The bond distances and angles are summarized in Table 2.

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Figure 2 Crystal structure of 2CatH･CH2Cl2 drawn at the 50% probability level. All hydrogen atoms except for BH and OH, and CH2Cl2 are omitted for clarity.

Table 2 Selected bond distances (Å) and angles (°) for the complex 2CatH･CH2Cl2

Complex 2CatH has monomeric structure with an N3O1 ligand donor set, in which three nitrogen atoms are of the TptBu,iPr ligand and one oxygen atom is of the unidentate catecholato ligand. Another oxygen atom of the catecholato ligand is located far from the

12

manganese ion, and there is a non-bonding OH group in the catecholato ligand of 2CatH as identified in the IR spectrum. There is a mirror surface on the manganese ion and the N2 of the TptBu,iPr ligand due to the unidentate coordination of the catecholato ligand. The average Mn-N bond length in 2CatH is 2.146 Å, similar to that in the manganese(II) complex 3 (av. 2.138 Å) [24] and those of the manganese(II) complexes with TpiPr2 [14, 27, 28] or TptBu,iPr ligands [23]. Two C-O bond lengths (1.359(12) Å and 1.361(10) Å) are longer than those in the semiquinonato complex 1SQ (1.283(3) Å and 1.284(3) Å) [14]. The aromatic C-C bond lengths are characteristic of the catecholato ligand, and differ from those of the semiquinonato ligand of 1SQ. In addition, the fact that there is no counter ion based on the X-ray analysis, and that complex 2CatH is colorless and has no absorption band in UV-Vis spectrum, support the hypothesis that the charge of the central manganese ion is +2. Therefore, complex 2CatH is a manganese(II) complex coordinated by a monoanionic catecholato ligand and not a manganese(III) complex coordinated to a dianionic catecholato ligand. Furthermore, the Mn-O bond distance (1.952(5) Å) is longer than the Fe-O bond (1.874(7) Å) in complex 4CatH due to the weaker Lewis acidity of manganese(II) compared to that of iron(II) [21]. Complex 2CatH has a monoanionic catecholato ligand, as in the proposed manganese-catecholato complex for the Mn-CDO enzyme [13], although the catecholato ligand cannot coordinate in a bidentate mode due to steric hindrance of the bulky alkyl substituents of the Tp tBu,iPr and catecholato ligands.

13

The EPR spectral pattern of 2CatH in toluene solution at 77K (Figure S3 in the Supporting Information) is similar to that of the manganese(II) thiolate complex with TpiPr2 ligand, [TpiPr2MnIISC5F5], which is a high-spin complex (s = 5/2) with distorted tetrahedral structure [29]. In addition, this spectral pattern of 2CatH seems to be similar to that of the Mn-CDO enzyme [11]. To our knowledge, 2CatH is the first report of a unique manganese(II) catecholato complex with unidentate catecholato coordination.

Reactivity of the complex 2CatH toward O2 When complex 2CatH in toluene was exposed to molecular oxygen at room temperature, the solution color changed instantly to green (Scheme 2). Since the new solution color was sustained for 24h under O2, we assume that this product is stable at least 24h.

Scheme 2 Reaction of complex 2CatH with O2

14

After removing the solvent and following recrystallization from MeCN/CH2Cl2, a green compound 5 was obtained. In the IR spectrum of 5, the O-H peak at 3441cm-1 of complex 2CatH disappeared, suggesting that the catecholato ligand may be oxidized to the semiquinonate upon reaction with O2. Moreover, compound 5 in toluene solution at 292K, UV-Vis absorption bands (298 nm (sh,  = 8825), 316 (sh, 11211), 326 (12129), 380 (7720), 406 (sh, 626), 426 (sh, 647), 466 (1345) and 762 nm (462 M-1cm-1)) were observed as shown in Figure 3. The bands are characteristic of metal-semiquinonato complexes, and the similar bands were observed in manganese(II) semiquinonato complex 1SQ (toluene solution, 293K, 334 nm (= 6227), 384 (3945), 444 (sh, 887), 486 (1178), 750 nm (420 M-1cm-1)) [14] as well as in other semiquinonato complexes [15, 18, 19, 30, 31] as summarized in Table S1 in the Supporting Information. Therefore, we speculated that the manganese(II) semiquinonato complex 5 is produced as the main product of 2CatH and O2.

15

Figure 3 UV-Vis spectrum of semiquinonato complex 5 (toluene, 292K).

In addition, after 2CatH reacted with O2 in toluene solution (resulting in the semiquinonato compound 5), the EPR spectral pattern of 2CatH at 77K changed quickly and the peak intensity decreased as shown in Figure S4 in the Supporting Information. It might be also suggested that either the formation of monomeric manganese(III) complex with catecholate dianion or the occurrence of the antiferromagnetic interaction between manganese(II) (s = 5/2) and semiquinone radical (s = 1/2). As results of UV-Vis spectrum and the structural characterization by X-ray analysis (described below) of the semiquinonato complex 5, we speculate that the antiferromagnetic interaction occurs under this condition, as reported in metal semiquinonato complexes [30]. There was no change in the signal even after at least several

16

hours, which is consistent with the results of the time-dependent change of the UV-Vis spectra (described later). Finally, the molecular structure of the semiquinonato complex 5 was determined by X-ray crystallographic analysis. The molecular structure of complex 5 is shown in Figure 4 and the selected bond distances and angles are summarized in Table 3. As result, the central manganese ion of 5 has a distorted trigonal bipyramidal geometry ( = 0.72) constructed by N3O2 ligand donor set in which three nitrogen atoms are from TptBu,iPr ligand and two oxygen atoms are from semiquinonate moiety coordinated by an asymmetric bidentate mode (Figure 4). Two Mn-O bond distances (2.145(4) Å and 2.091(4) Å) and two C–O bond distances (1.278(6) Å and 1.288(6) Å) of 5 are similar to those of the reported manganese(II)-semiquinonato complexes [14, 18, 19]. In addition, unequal C-C bond distances of the six-membered ring moiety (1.456(8), 1.414(8), 1.365(8), 1.435(8), 1.367(8) and 1.455(8) Å) are characteristic of the semiquinonato ligand [14, 18, 19, 30]. There is no counter ion in the crystal structure of 5, therefore, we concluded that 5 is manganese(II)-semiquinonato complex.

17

Figure 4 Crystal structure of 5･2MeCN drawn at the 50% probability level. All hydrogen atoms except for BH and two MeCN molecules are omitted for clarity.

Table 3 Selected bond distances (Å) and angles (°) for the complex 5･2MeCN

18

Up to now, synthesis and structural analysis of manganese(II) catecholato complex is few compared with that of manganese(II) semiquinonato complex because the catecholate is easily oxidized to the corresponding semiquinonate rather than oxidation of manganese(II) [14, 16, 18, 19]. When using a dimeric manganese(III, III) bis--oxo complex in order to prepare a manganese(III) catecholato complex with TpiPr2 ligand, the manganese(II) semiquinonato complex 1SQ is obtained but not the manganese(III) catecholato complex due to occurrence of one-electron transfer between manganese(III) and the catecholato ligand [14]. Similarly, in case of the cobalt(II) and zinc(II) complexes having TpR ligands, the catecholato ligand is one-electron oxidized to obtain the semiquinonato complexes, respectively [30, 31]. In contrast, in iron(II) complexes with TptBu,iPr or TpiPr2 ligand, oxidation of metal ions is more likely to proceed than oxidation of the catecholato ligand [21]. By using the TpiPr2 ligand, we were unable to characterize and isolate the manganese(II) catecholato complex 1CatH which is in prior to generation of 1SQ. In this research, complex 2CatH possessing unique unidentate coordination is obtained by using bulkier TptBu,iPr ligand. It would be thought that the unidentate coordination of catecholato ligand of 2CatH is an important point to synthesize and to structurally characterize the 2CatH. If non-bonding OH group of 2CatH is located near the manganese(II), the OH group would be bound to the manganese(II) to be weak O-H bond and the catecholato ligand would be oxidized to the semiquinonato state as case of complex 1SQ.

19

Changes with time for the reaction of 2CatH and O2 As shown in Figure 5, by the monitoring changes in the UV-Vis spectrum, it is apparent that the reaction of complex 2CatH and O2 toward the semiquinonato complex 5 is complete within 10 min, and the spectral change of the produced green species did not occur for at least several hours. In the reaction of the iron(II) catecholato complex 4CatH with O2, the monoanionic catecholato ligand was oxidized to the corresponding dianionic catecholato ligand, resulted in iron(III) complex [TptBu,iPrFeIIIDTBC] (4Cat) [21]. In contrast, in the reaction of complex 2CatH with O2, one electron oxidation is carried out in the catecholato moiety. Preliminary kinetics experiments were attempted, monitoring changes in the UV-Vis spectrum without stirring because of the lability of complex 2CatH toward molecular oxygen. The characteristic absorption bands of the semiquinonato complex 5 increased over time, and the reaction terminated under 10 min. However, a good linear fitting was not obtained neither first-order nor second-order. Therefore, overall mechanistic details concerning the generation of 5 remain unclear at present. We have been focusing our efforts on trying to elucidate the mechanism that molecular oxygen is activated on the manganese(II) center generating a manganese-dioxygen species and that H2O2 is produced from the reaction [31-33].

20

Figure 5 UV-Vis spectra of reaction of 2CatH with O2 (1atm) in toluene solution recorded at 295K at 1-minute intervals. The absorbance intensity increases at ~380nm and ~762nm terminated within 10 min.

[Conclusions] In summary, the manganese(II) monoanionic catecholato complex 2CatH was synthesized and was characterized by FT-IR, FD-MS, UV-vis, EPR and X-ray analysis. We found that complex 2CatH has a tetrahedral structure similar to iron(II) model complex 4CatH. In an excellent model of the CDO enzymatic process, complex 2CatH reacts with oxygen to produce the manganese(II) semiquinonato complex 5, despite the fact that the analogous iron complex 4CatH is converted to iron(III) catecholato complex 4Cat after dioxygen activation. Studies concerning generation of 21

hydrogen peroxide after dioxygen activation and oxidation of the catecholato ligand are ongoing researches in our laboratory.

[Acknowledgement] JSPS KAKENHI (Grant Nos. JP 17K05822

Dyah Sulistyanintyas of National Institute of Technology, Ibaraki Collage.

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A Manganese (II)

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[Highlights]  A

monomeric

manganese(II)

catecholato

complex

with

hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate (TptBu,iPr), 2CatH, was synthesized and characterized.  Upon reacting of the manganese(II) complex 2CatH with molecular oxygen, a manganese(II) semiquinonato complex was 5 obtained, thus serving as an important model for catechol dioxygenase.  The manganese(II) semiquinonato complex was also structurally characterized.

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