Synthesis, characterization and magnetic properties of dinuclear complexes of manganese(III) and manganese(II) with oxo and benzoate derivatives as bridging ligands

PolyhedronVol. 15, No. 1, pp. 91 96, 1996

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Pergamon 0277-5387 (95)00193-X

SYNTHESIS,

CHARACTERIZATION

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277-5387/96 $9.50+0.00

AND MAGNETIC

PROPERTIES OF DINUCLEAR COMPLEXES OF MANGANESE(III) AND MANGANESE(II) WITH OXO AND B E N Z O A T E D E R I V A T I V E S AS B R I D G I N G L I G A N D S

BELEN ALBELA, MONTSTERRAT CORBELLA and JOAN RIBAS* Departament de Quimica Inorg~mica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain

(Received 20 February 1995 ; accepted 19 April 1995) Abstract--Four new dinuclear Mn m complexes were synthesized: [{Mn(bipy)(H20)}2(#O)(#-2-CIC6H4COO)2](NO3)2 (1), [{Mn(bipy)(H20)}2(#-O)(#-3-CIC6H4COO)2](NO3)2 (2), [{Mn(bipy)(H20)}2(/~-O)(/~-4-C1C6H4COO)2](NO3)2 (3) and [{Mn(bipy)(H20)}2(~O) (/.t-2-CIC6H4COO)2](CIO4)2 (4), where bipy is 2,2'-bipyridyl and 2-, 3- and 4-C1C6H4COO are 2-, 3- and 4- chlorobenzoate. Magnetic susceptibility measurements were carried out on this series of complexes and the data were interpreted using the Heisenberg Hamiltonian -J12SIS2 and led to small ferromagnetic coupling for 1 and 2 and small antiferromagnetic coupling for 3 and 4, respectively. By reaction of a solution of these complexes with H202 in the presence of C10 4- anion, three new dinuclear Mn n complexes, [{Mn(bipy)2}2 (#-2-CLC6H4COO)2](C104)2 (5), [{Mn(bipy)2}2(/t-3-ClC6H4COO)2](C104)2 (6) and [{Mn(bipy)2}2(#-4-C1C6H4COO)2](CIO4)2 (7), were obtained. Magnetic susceptibility data were interpreted with the same Hamiltonian, but for Mn u ions instead of Mn u~. The experimental data indicate small antiferromagnetic coupling in all three complexes. EPR spectra in powder samples and in frozen solutions agree with the formulae proposed.

The non-heine catalases containing manganese catalyse the disproportionation of H202 into H20 and 02. The catalases isolated from Lactobacillus plantarum, Thermoleophilum album and Thermus thermophilus contain a dinuclear manganese cluster on the active site. The Thermus thermophilus enzyme has been crystallized and a low resolution structure places the manganese atoms within 3.6 & of each other.I The Thermus thermophilus and Lactobaeillus pkmtarum enzymes exhibit similar optical absorption spectra to that seen for Mn In model complexes. 2 Based on these data, an [Mn2(p-O)(#RCOO)2] structure has been postulated for the active site. An EXAFS study3 of the reduced Mn n Mn" and the superoxidized MnmMn TM derivatives of the Lactobacillus plantarum enzyme reports more information about the active centre. (a) The

Mn" Mn" catalase shows longer Mn--N,O distances on reduction, and the bridging structure probably has another #-OOCR or a/a-OH, but not the #-oxo bridge. EPR spectroscopy shows that the manganese ions are weakly coupled. (b) The superoxidized catalase probably contains an [Mn2(~-O)2(/aRCOO)] core and it is unable to oxidize H202. The addition of another/~-oxo bridge stabilizes the MnmMn TM with respect to reduction. Thermus thermophilus in the reduced form Mn"Mn n shows an EPR signal with 11 resolved hyperfine lines attributed to the coupling with the N-imidazole ligand, for the excited state S = 1.4 The aim of this study was to obtain new dinuclear Mn l" complexes with the [Mn2(/~-O)(/2-RCOO)2] core to study the reaction with H202. We used a bidentate amine (2,2'-bipyridyl, bipy) as terminal ligand and to complete the manganese coordination there is one water molecule on each manganese. H20 can act as a labile ligand and favours the interaction with H202. We synthesized the new

*Author to whom correspondenceshould be addressed. 91

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92

[{Mn(bipy)(H20)}2(#-O)(#-RCOO)2]X2 with R = 2-, 3-, 4-chlorobenzoate and X = NO3- and CIO4-. By reaction of these compounds with H202, we obtained three new complexes formulated as [{Mn(bipy)2} 2(#-RCOO)2] (CIO4)2.

EXPERIMENTAL

Synthesis of the manganese(III) complexes [{M n(b i p y)(H20)}2(#-O)(#-2-C 1C6H4C O 0)2 ] (NO3)2 (1), [{Mn(bipy)(n20)}2(#-O)(#-3-flfrH4 COO)2](NO3)z (2) and [{Mn(bipy)(H20)}2(#-O) (#--4-C1C6H4COO)2](NO3)2(3) were synthesized as follows: 0.40 g (2.5 mmol) of the corresponding chlorobenzoic acid (Aldrich, recrystallized in hot water), dissolved in 25 cm 3 of MeCN, were added with stirring to a solution containing 0.36 g (2 mmol) of Mn(NO3)2 (aq) in 20 cm 3 of MeCN. An MeCN solution of 0.18 g (0.50 mmol) of (NBu4)MnO45 was added to these colourless solutions with constant stirring. The solutions became dark brown, yielding a small quantity of precipitate, which redissolved on addition of a MeCN solution (20 cm 3) of 0.39 g (2.5 mmol) of 2,2'-bipyridyl. Very dark solutions were obtained, which were filtered to eliminate any insoluble impurity. The solutions were left undisturbed for several hours, giving small brown crystals which were filtered off, washed with ether and dried in air. All attempts to recrystallize these three new complexes to obtain suitable single X-ray crystals were unsuccessful. In contrast, and especially with the 2-chloro derivative, the recrystallization gave rise to the formation of polymeric species. Compound 1. Yield : 60%. Found : C, 45.2 ; H, 3.0; N, 9.1 ; C1, 8.0. Compound 2. Yield: 65%. Found: C, 44,7; H, 3.2; N, 9.1; C1, 8.0%. Compound 3. Yield : 60%. Found : C, 44.8 ; H, 3.1 ; N, 9.4 ; C1, 7.8. Calc. for C34H28C12Mn2N6013 : C : 44.9 ; H, 3.1; N, 9.2;C1, 7.8%. [{M n(b i p y)(H20)}2(#-O)(#-2-C 1C6H4C O 0)2] (CIO4)z (4). Compound 1 (0.45 g, 0.5 mmol) was dissolved in 25 cm 3 of MeCN, giving a dark suspension. NaCIO4" H20 (0.28 g, 2 mmol), dissolved in 20 cm 3of MeCN, was added with constant stirring. A dark solution was formed immediately and after several days a red microcrystalline powder was formed, which was filtered and discarded: elementary analysis indicated it was a mixture of several forms, predominantly a tetranuclear species which could not be obtained with sufficient purity. This red species was filtered, and the clear solution gave rise, after several days, to dark microcrystals whose analysis correspond to the formula proposed. Compound 4. Yield: 30%. Found: C,

40.8; H, 2.8; N, 5.5; C1, 14.7. Calc. for C34H28CI4Mn2N4015: C, 41.5; H, 2.9; N, 5.7; C1, 14.4%.

Synthesis of the manganese(II) complexes [{Mn(bipy)2} 2(#-2-C1C6H4COO)2] (C104)2 (5), [{Mn(bipy)2}2(#-3-C1C6H4COO)2](C104)2 (6) and [{Mn(bipy)2} 2(#-4-C1C6H4COO)2] (CIO4)2 (7). H202 (40 cm 3, 30%) was added dropwise to a suspension of 1 g (1.1 mmol) of 1, 2 and 3 in 50 cm 3 of MeCN. The reaction was highly exothermic, and the temperature of the reaction was kept below 30°C. The solution quickly became pale yellow, with evolution of O2. When the reaction finished, the solution was kept at room temperature for 30 min. Afterwards, 0.31 g (2.2 mmol) of NaC104" H20 dissolved in 20 cm 3 of MeCN was added. The solution was stirred until there was no further evolution of gas and filtered to eliminate any visible insoluble impurity. Then it was left undisturbed for 24 h, after which a microcrystalline yellow product appeared. Compound 5. Yield: 35%. Found: C, 51.7; H, 3.2; N, 9.0; C1, 10.9; Mn, 8.8. Compound 6. Yield: 30%. Found: C, 51.7; H, 3.2; N, 9.2; CI, 11.3; Mn, 8.8. Compound 7. Yield: 35%. Found: C, 51,7; H, 3.2; N, 9.1 ; C1, 11.4; Mn, 8.9. Calc. for C54H40C14Mn2NsOI2: C, 52.1 ; H, 3.2; N, 9.0; C1, 11.4; Mn, 8.8%.

Physical measurements IR spectra (4000-200 cm -~) were measured on a Nicolet 520 FT-IR spectrometer using KBr pellets. Magnetic susceptibility of powdered samples was measured in a 15 kG field on a MANICS-DSM8 susceptometer equipped with an Oxford Instruments liquid helium cryostat, working down to 4.2 K. The susceptometer was calibrated with a standard ferrite. Diamagnetic corrections were calculated from Pascal Tables. The data were reproducible on samples taken from different preparations. EPR measurements were carried out with a Bruker ESP 300E, working in X-band from room temperature to 4 K.

RESULTS AND DISCUSSION

Man#anese(III) complexes The reaction of Mn(NO3)z with NBu4MnO4 in the presence of 2,2'-bipyridyl and 2-, 3- and 4-chlorobenzoic acids gave rise to the synthesis of new dinuclear species [{Mn(bipy) (H20) }2(#-O) (#CIC6H4COO)2] (NO3)2. Similar syntheses have been

Dinuclear complexes of Mn m and Mn n with bridging ligands reported in the literature for other benzoate analogues. 6 In the absence of structural data, analysis, IR and magnetic properties allow us to assume a dinuclear structure with one p-oxo ligand, two pchlorobenzoate ligands and one 2,2'-bipyridyl as terminal ligand for each Mn In centre. To complete the hexacoordination of each metallic centre, we have to assume one water molecule, as is found in other similar carboxylato derivatives. 2b In the IR spectra of all these new complexes, a broad, intense band centred between 3400 and 3300 cm -~, characteristic of water, is clearly shown. The typical band for free NO3 ion is also shown: an intense sharp band at 1385 c m - l . 7 For the perchlorate derivative of 2-chlorobenzoate, the two typical bands for free C104- are shown : 1130 cm ~(partially split, broad) and another, less intense, but sharp at 625 cm-l.7 The presence of 2,2'-bipyridyl ligand is shown by four sharp intense bands in the 1610-1435 cm -~ region. The C1C6H4COO- anion acting as bridging ligand is shown by two intense bands, corresponding to the symmetric and asymmetric stretching of C O O - . 7"8 vs(COO) appears between 1410 and 1380 c m - ~and Vasbetween 1620 and 1550 c m - ~. In the nitrate complexes, the vs(COO) band is difficult to detect owing to the overlap with the NO3 -3 bands, which appear sharp and intense at 1385 cm -~. In the C104- complex, the vs(COO) band is clearly shown, sharp and intense. The A(v) value, defined as Vas(COO)- vs(COO), allows us to try to distinguish between the carboxylate ligand acting as a bridge between two manganese ions or acting as a chelating bidentate ligand for only one manganese. Deacon and Phillips s have examined this A(v) for acetato and trifluoroacetato complexes: (a) separations substantially greater than ionic are indicative of unidentate carboxylate coordination; (b) separations significantly less than ionic values are indicative of chelating and/or bringing carboxylate groups; (c) very low separations generally indicate chelation. According to the same authors, there appear to be no substantial reasons why these conclusions should not also apply to coordination of other simple carboxylate ions, at least in general terms. In all our cases, this difference is between 160 and 180 cm -~, very similar to that observed for the free carboxylate group, 7"8 indicating the possible bridging character of the chlorobenzoate ligand. Without the X-ray crystal structure it is not possible to rule out any other coordination, but according to the literature data, s all these dinuclear Mn "1 complexes present the carboxylato as bridging ligands. F'inally, all complexes show a group of less intense bands in the 670-640 cm-l region, which correspond to va~ ( M n - - O - - M n ) . 9

93

Magnetic properties The susceptibility measurements were carried out in the 300-4.2 K temperature range. The ;~MT plots vs T for 2 and 4 are shown in Fig. 1. Complex 1 behaves like complex 2 and complex 3 like complex 4. For 1 and ] xMT increases slightly when the temperature decreases, from a value close to 6 c m 3 mol-~ K (characteristic of two independent d 4 ions) to 7.2 cm 3 mol -~ K at 25 K, indicating low ferromagnetic coupling. From 25 to 4 K, there is a clear decrease in ZMT owing to intermolecular interactions and single ion zero field splitting, important in a d 4 ion. In contrast, complexes 3 and 4 have different behaviour : ; ~ M T decreases when the temperature decreases, indicating antiferromagnetic coupling. The spin Hamiltonian H --- - J ~ 2S1S 2 was used to fit the magnetic results, assuming that the two Mn m ions have the same g value. From the Van-Vleck formula, the sus-

o ¢o

E 6-

x 5"

4'

3 0

i 1~

t 2~

3~

6-

5"

o~

E

4"

E o 3'

x 2-

1-

0 TIK

Fig. 1. Plot of the temperature dependence of the product )~MTfor compounds 2 (top) and 4 (bottom). The solid lines represent a least-squares fit of the data. For 1 and 3, the plots are, respectively, very similar.

94

B. ALBELA et al.

ceptibility expression for two Mn "j (d 4) ions is given as:"'

2Ng2f12

kT

similar for the other two dinuclear complexes. In all cases, the curves ZM VS T show a maximum

30 exp (10x) + 14 exp (6x) + 5 exp (3x) + exp (x) 9exp(lOx)+7exp(6x)+5exp(3x)+3exp(x)+l"

For the two ferromagnetic complexes, taking into account the decrease in ZMT at low temperature, a new parameter J ' was added (in which the two phenomena, intermolecular interactions and zero field spliting, are taken into account). This was introduced as a phenomenological Weiss parameter : 2Ng2 flZ F (J, = kT-J'F(J,

X=

T) T)'

x = J/kT.

very near to 4 K ; the ZM T VS T curves start at approximately 8 c m 3 mol -l K and decrease continuously to 0: both characteristics indicate small antiferromagnetic coupling. The spin Hamiltonian H = -J~2S~$2 was used to fit the magnetic results assuming that the two Mn n ions have the same g value. F r o m the VanVleck formula, the susceptibility expression for two Mn" (d 5) ions is given as :lO

2Ng2 fl 2 55 exp (15x) + 30 exp (10x) + 14 exp (6x) + 5 exp (3x) + exp (x) kT llexp(15x)+9exp(lOx)+7exp(6x)+5exp(3x)+3exp(x)+l'

where F ( J , T) is the Van-Vleck expression for an Mn H~ dinuclear complex. II The magnetic parameters are listed in Table 1. In the absence of crystal structure determination, we cannot correlate these magnetic values with structural data. However, all Mn"~Mn "l complexes with /~-oxo and two /~-carboxylato ligands are weakly ferro or antiferromagnetically coupled. ~2A small modification in angles and distances in/~-oxo and/or/~-carboxylato bridging ligands modifies the J values, even changing the sign. Redox reactivity oJmanganese(III) dinuclear complexes

All the dinuclear Mn "~ complexes reported above reacted easily with H202, with evolution of 02. The dark solution in M e C N became pale yellow and, according to the counteranion present in the solution, new Mn" complexes may be obtained. With NO3- the new complexes were not pure enough to be characterized. However, by adding NaCIO4, new dinuclear M n " M n " compounds were prepared and characterized by analysis (see Experimental), IR and magnetic measurements (susceptibility and ESR). IR spectra show the characteristic bands due to the bipy ligand and C 1 0 4 ion. The bands due to the carboxylate ions are shown in the same region as for the Mn "l analogues, indicating that they are acting as bidentate ligands. From magnetic data we can also deduce that we are dealing with dinuclear complexes: magnetic susceptibility measurements for complex 5 are given in Fig. 2 and the curves are

x = J/kT.

The J values are listed in Table 1. All values are small, as is assumed taking into account the nature of the two carboxylato bridges. To improve the fitting, it is convenient to introduce a J ' parameter in the Van Vleck formula (see above for Mn m complexes), which considers the intermolecular interactions that may be of the same magnitude as the intramolecular ones.

E S R spectra

The three new M n H dinuclear complexes show a similar pattern, either in the solid state or in frozen solution (Figs 3 and 4). At room temperature in the solid state they present a broad band close to # = 2.00. When the temperature is lowered, this

Table 1. Least-squares values for the magnetic parameter fitting of the magnetic susceptibility data of compounds 1 ~ (Jand J ' in cm -~) Complex

J

1

3.7

2 3 4 5 6 7

3.5 -5.0 --2.8 - 1.5 - 1.6 -- 1.5

J'

g

-0.3

1.95 1.96 1.95 1.95 1.99 2.00 1.99

--0.3 ----0.2 --0.1 --0.1

Dinuclear complexes of Mn ~" and Mn u with bridging ligands

95

10

T

Y

E E o

f

x 4-

20

i 100

! 200

:300

T/K

Fig. 2. Plot of the temperature dependence of the product gMT for compound 5. The solid line represents a leastsquares fit of the data. For 6 and 7, the plots are very similar.

A

J

l

i

f

i

1

2

3

4

5

kG

Fig. 4. ESR spectra for complex 5 for frozen solution (in M e C N - D M F at 4 K, v = 9.43 GHz, P w = 2.0 mV, RG = 1600). For 6 and 7, the spectra are very similar.

solid state became only one when working in frozen solution. Taking into a c c o u n t the weak antiferromagnetic coupling in these complexes, at high temperature the signal centred a t g = 2.00 is characteristic for an M n n complex, which is split in frozen solution, due to hyperfine coupling (IMn = 5/2). The splitting o f these six bands m a y be due to the coupling with the N(bipy) (IN = 1). This spectrum is similar to the S_, state in the O E C o f PSII. 13 The bands appearing at low field m a y be explained by the population o f states S = 1, S = 2 . . . . . even at low temperature.

Catalytic activity B

As indicated above, the new M n ' " M n IH comp o u n d s react rapidly with H202 with 02 evolution, but the reduced f o r m MnI~Mn n is not able to reduce the H202 to H20 to close the catalytic cycle. The reaction M n m M n m + H202

o15

l:s

2:s

i.s

4:s

s:s

~:s kG

Fig. 3. ESR spectra for complex 5 in powdered samples (A, room temperature, v = 9.78 GHz, Pw = 63.2 mV and RG = 200; B, 4 K, v = 9.44 GHz, Pw = 63.2 mV and RG = 800). For 6 and 7, the spectra are very similar.

signal is split. O n the other hand, in the zone between 1000 and 2000 Gauss, two new b r o a d signals appear when the temperature is lowered. In frozen solution, the signal centred at g = 2.00 appears split into six components, each split into two small bands. The two signals at low field in the

) M n " M n " + 02

involves, in our case, a structural change in the dinuclear c o m p o u n d . The M n H M n " complexes have two bidentate bipy ligands for each m a n g a nese; therefore, the M n " is m o r e stable and more difficult to oxidize. This result suggests that using less stabilizing n o n - a r o m a t i c ligands, the dinuclear M n " complexes would be easier to oxidize than the analogous c o m p o u n d s with the aromatic bipy.

Acknowledgements--We are grateful to DGICYT (grant PB90/0029) for financial support and to "Serveis Cientifico-T+cnics de la U.B." for manganese analysis. B. A. also appreciates a fellowship from the Spanish Ministerio de Educaci6n y Ciencia.

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