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Solution structure of a seven coordinated manganese(II) complex via electrospray ionization mass spectrometry
Spectrochimica Acta Part A 75 (2010) 1168–1170
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Solution structure of a seven coordinated manganese(II) complex via electrospray ionization mass spectrometry Mohammad Mahdi Najafpour a,b , Davar M. Boghaei a,∗ , Per J.R. Sjöberg c a b c
Department of Chemistry, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran Department of Chemistry, Institute for Advanced Studies in Basic Sciences, 45195-1159, Gava Zang, Zanjan, Iran Analytical Chemistry, Department of Physical and Analytical Chemistry, Uppsala University, Biomedical Centre, Box 599, SE-751 24 Uppsala, Sweden
a r t i c l e
i n f o
Article history: Received 19 November 2008 Received in revised form 25 December 2009 Accepted 31 December 2009 Keywords: Manganese Oxygen evolving complex ESI(+)–MS/MS tptz
a b s t r a c t The mononuclear complex [Mn(tptz)(CH3 COO)(OH2 )2 ]NO3 (1) was investigated by electrospray ionization mass spectrometry in aqueous solution at pH 4.5. Electrospray ionization mass spectrometry shows that mononuclear and dinuclear manganese cationic species are present in solution, probably in equilibrium with neutral 1. An experiment showed that the most important reaction in the presence of oxone (2KHSO5 ·KHSO4 ·K2 SO4 ) is decoordination. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The coordination chemistry of manganese(II) is dominated by the coordination numbers four and six. Five and seven-coordination are less frequently observed [1,2]. There has been considerable interest in the coordination chemistry of manganese complexes because of the significant involvement of manganese in various biological systems. Water oxidation to evolve oxygen is an important and fundamental chemical reaction in photosynthesis catalyzed by a Mn4 Ox Ca complex housed in a special protein environment [3]. Mn4 Ox Ca complex may serve as a model for technical approaches to split water by sunlight, which is a prerequisite for a sustainable hydrogen economy [4]. Numerous complexes of terpyridine and oligopyridines with manganese have been investigated because of their interesting structural, redox, photochemical, catalytic and water oxidation properties [5,6]. Recently, we reported the preparation, characterization and single-crystal X-ray diffraction of a seven coordinated manganese(II) complex with 2,4,6-tris(2-pyridyl)-1,3,5-triazine [7]. The complex acted as an oxygen evolving complex with oxone (2KHSO5 ·KHSO4 ·K2 SO4 ) as primary oxidant in aqueous solution at pH 4.5 [7]. Polar solvents, such as water, able to promote ligand exchange which would drastically change the structure of the complex in solid state, while promoting equilibria with other species stabilized by solvation
∗ Corresponding author. Tel.: +98 21 6616 5306; fax: +98 21 6601 2983. E-mail address: [email protected] (D.M. Boghaei). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.086
[8]. We now report solution structure of the seven coordinated manganese(II) in complex via electrospray ionization–mass spectrometry (ESI–MS). 2. Experimental All reagents and materials used for synthesis were reagent grade and used without further purification. Elemental analyses were performed on a Perkin-Elmer 2400 CHNS/O elemental analyzer. The acetate buffer was prepared by adjusting the pH of a 0.1 M KOAc solution to 4.5 with concentrated acetic acid. The detection of the Mn-complex was made in the positive electrospray mode using a PE-Sciex API III+ triple quadrupole mass spectrometer (Concord, ON, Canada) with an ion spray interface. A syringe pump (Syringe Infusion Pump 22, Harvard Apparatus Inc., Cambridge, MA, USA) operated at a flow rate of 5 L/min was utilized to pump the solution through a fused silica capillary (Polymicro Technologies, Phoenix, AZ, USA) into the ion source of the mass spectrometer. The fused silica spray capillary entering the mass spectrometer was centered in a stainless steel capillary auxiliary assembly delivering 40 psi of dry nitrogen (boil off from liquid nitrogen) for pneumatically assisted ESI–MS. The flow rate of dry nitrogen counter-current curtain gas (heated to 63 ◦ C) was 1.2 L/min over the sampling orifice. The mass spectrometric parameters were as follows: ion spray voltage (ISV) 4500 V, interface plate voltage (IN) 650 V, orifice lens voltage (OR) 50 V, and AC entrance rod (R0) 30 V. Q1 scans between m/z 100 and 1400 (step size 0.1), with a dwell time of 1 ms, were recorded in the multi-channel acquisition (MCA,
M.M. Najafpour et al. / Spectrochimica Acta Part A 75 (2010) 1168–1170
1169
Fig. 1. View of the complex with the atom-numbering scheme.
summation of 10 scans) mode. Furthermore, in MS/MS mode the collision energy was set to 20 eV with a collision gas thickness of 2.00 × 1015 molecules/cm2 . Data were collected on a Macintosh computer by the Tune2.5-FPU software. 2.1. [Mn (tptz)(CH3 COO)(OH2 )2 ]NO3 The complex was prepared by dissolving manganese(II) acetate (1 mmol, 245 mg) and tptz (1 mmol, 312 mg) in water (10 mL); the mixture was stirred for about 2 h at room temperature. Then HNO3 acid (0.2 M, 0.1 mL) was added to this solution. This solution yielded yellow crystals of the complex after 10 d (446 mg, 87% yield). C20 H19.20 MnN7 O7.10 Anal. Calc. for the complex: C, 45.6; H, 3.6; N, 18.6; Found: C, 45.3; H, 3.9; N, 18.4. 3. Results and discussion It is assumed that a structure in the solid state is preserved in solution, which is likely to hold for most solvents with low polarity and limited coordination abilities. But for polar solvents able to promote ligand exchange (water or DMSO), significant structural modifications may occur, which would drastically change the structure of the solvated complex, while promoting equilibria with other species stabilized by solvation. View of the complex with the atom-numbering scheme in solid state is shown in Fig. 1. In the crystal structure, each manganese atom is heptacoordinated, with a coordination polyhedron close to a pentagonal bipyramid. Heptacoordination is achieved by means of three N atoms belonging to the tptz ligand (N1, N4 and N5) and two oxygen atoms from the bidentate acetato ligand (O21 and O22), which determine the equatorial plane and finally the oxygen atoms from water molecules (O1W, O2W) are in a trans disposition (yielding O1W–Mn1–O2W as the trans-axial angle 178.86(4)◦ ). The hydrogen atoms of the coordinated water molecules were located from difference maps and refined with a fixed thermal parameter, while those of the 10% occupancy water molecule were not included in the model. The uncoordinated nitrate anion is disordered and was modelled with 90:10% occupancy of two overlapping positions; the minor component is hydrogen-bonded to the partial occupancy water molecule [7]. At room temperature, 1 is soluble in H2 O. To evaluate whether the solid state structure of 1 would be stable in solution, the ESI(+) mass spectrum (Fig. 2) of 1 in water/acetonitrile (2.5/1) at pH 4.5
was acquired, since ESI–MS has been shown to be an useful tool to characterize coordination compounds in solution [8–16]. Acetonitrile was added to facilitate spraying. Fig. 2 shows proposals for the major detected species. Owing to the gentle ionization process and based on ESI–MS/MS experiments, the assigned ions are unambiguously detected from solution and have been directly transferred (no gas-phase dissociation) to the gas-phase by ESI. The MS data indicates therefore that structure of 1 is not fully preserved in aqueous solution at pH 4.5, hence its possible activities of the complex in water should not be limited to the structure of the solid state. In water/acetonitrile (2.5/1) solution, 1 is in equilibrium with some cationic species, those proposed in Fig. 2. Note that, being a neutral species, 1 is not detected in the ESI(+)–MS/MS. The ESI(+)–MS/MS detection of the ion of m/z 911 reveals the presence, in solution, of a cationic species containing two manganese ions and two ligand molecules. The ESI(+)–MS/MS detection of the ion of m/z 1223 also reveals the presence, in solution, of a cationic species containing two manganese ions and three ligand molecules. Other species containing one manganese ion and two (m/z = 739) or three (m/z = 1050) ligands are also detected. This observation rationalizes also the presence in solution and hence the ESI(+)–MS/MS detection of the ion of m/z 351, 663 and 975 containing one potassium
Fig. 2. ESI (+)–MS for a water/acetonitrile (2.5/1) solution of 1 (L = tptz).
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of NH group is located from Fourier difference maps and refined in isotropic approximation with constrained NH distance. 4. Conclusions In summary, the mononuclear complex [Mn(tptz)(CH3 COO) (OH2 )2 ]NO3 (1) was investigated by ESI(+)–MS/MS in aqueous solution at pH 4.5. The MS data indicated that structure of the complex is not fully preserved in aqueous solution at pH 4.5. In water/acetonitrile solution, the complex is in equilibrium with some cationic species and a dissociative Mn-ligand process occurs in the water/acetonitrile solution. The experiment also showed that the most important reactions in the presence of oxone is decoordination. Acknowledgements
Fig. 3. ESI (+)–MS for reacting of solution of 1 and oxone.
The authors are grateful to the Research Council of Sharif University of Technology and National Elite Foundation for their financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2009.12.086. References
Fig. 4. ORTEP structure of 2 with partial atom labelling at the 50% probability level.
and one, two or three ligand molecules (Fig. 2). These cations indicate therefore that, to some extension, an interesting dissociative Mn-ligand process occurs in the water/acetonitrile solution (Fig. 2). There were reported that reactions between manganese complexes and oxone led to homogeneous catalytic O2 evolution [17,5]. To follow the speciation of Mn during reaction with oxone, we looked at the reaction of the complex and oxone in solution. The addition of oxone to the complex suppresses the other signals quite a lot making it difficult to detect any new signals and a series of peaks differing 152 are seen that corresponds to adducts of KHSO5 (m/z = 191.0, 343.0, 494.7) (Fig. 3). The experiment showed that the most important reaction in the presence of oxone is decoordination (m/z = 313.4). Decoordination was also observed in reacting of oxone and [Mn2 III/IV O2 (bpy)4 ](ClO4 )3 [18] that a heterocyclic Noxide (2) was detected (Fig. 4). In the structure of 2, distances of N1–C1 (1.360 Å), N1–C5 (1.371 Å), N2–C6 (1.353 Å) and N2–C10 (1.349 Å) are consistent with carbon–nitrogen double bonds in the structure of 2; similarly, the bond lengths of N1–O1 (1.328 Å) is also consistent with typical nitrogen–oxygen double bonds. The H atom
[1] A. Majumder, G. Pilet, M. Teresa, G. Rodriguez, S. Mitra, Polyhedron 25 (2006) 2550–2558. [2] A. Abakumov, V.K. Cherkasov, A.I. Poddelsky, M.P. Bubnov, L.G. Abakumova, G.K. Fukin, Dokl. Chem. 399 (2004) 207–210. [3] Y. Junko, J. Kern, K. Sauer, M.J. Latimer, Y. Pushkar, B. Jacek, B. Loll, W. Saenger, J. Messinger, A. Zouni, V.K. Yachandra, Science 314 (2006) 821–825. [4] A.F. Collings, C. Critchley, Artificial Photosynthesis: From Basic Biology to Industrial Application, first ed., Wiley-VCH, 2005. [5] J. Limburg, G.W. Brudvig, R.H. Crabtree, J. Am. Chem. Soc. 119 (1997) 2761–2762. [6] S. Das, C.D. Incarvito, R.H. Crabtree, G.W. Brudvig, Science 312 (2006) 1941–1943. [7] M.M. Najafpour, D.M. Boghaei, V. McKee, Inorg. Chim. Acta, in press. [8] J.A. Lessa, A. Horn Jr., C.B. Pinheiro, L.L. Farah, M.N. Eberlin, M. Benassi, R.R. Catharino, C. Fernandes, Inorg. Chem. Commun. 10 (2007) 863–866. [9] G. Cerchiaro, P.L. Saboya, A.M. da Costa Ferreira, D.M. Tomazela, M.N. Eberlin, Trans. Met. Chem. 29 (2004) 495–504. [10] A. Horn Jr., L. Fim, A.J. Bortoluzzi, B. Szpoganicz, M.de.S. Silva, M.A. Novak, M. Benassi Neto, L.S. Eberlin, R.R. Catharino, M.N. Eberlin, C. Fernandes, J. Mol. Struct. 797 (2006) 154–164. [11] H.E. Toma, A.D.P. Alexiou, A.L.B. Formiga, M. Nakamura, S. Dovidauskas, M.N. Eberlin, D.M. Tomazela, Inorg. Chim. Acta 358 (2005) 2891–2899. [12] B.C. Gilbert, J.R. Smith, A. Payeras, J. Oakes, Org. Biomol. Chem. 2 (2004) 1176–1180. [13] A.L. Rosen, G.M. Hieftje, Spectrochim. Acta Part B 59 (2004) 135–146. [14] B. Zeng, X. Zhu, X. Yu, S. Cai, Z. Chen, Spectrochim. Acta Part A 69 (1) (2008) 117–122. [15] Z. Chen, Y. Wu, F. Huang, D. Gu, F. Gan, Spectrochim. Acta Part A 66 (4) (2007) 1024–1029. [16] X. Yu, S. Cai, Z. Chen, Spectrochim. Acta Part A 60 (1) (2004) 391–396. [17] P. Kurz, G. Berggren, M.F. Anderlund, S. Styring, J. Chem. Soc. Dalton Trans. (2007) 4258–4261. [18] S.R. Cooper, M. Calvin, J. Am. Chem. Soc. 99 (1977) 6623–6630.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Solution structure of a seven coordinated manganese(II) complex via electrospray ionization mass spectrometry Mohammad Mahdi Najafpour a,b , Davar M. Boghaei a,∗ , Per J.R. Sjöberg c a b c
Department of Chemistry, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran Department of Chemistry, Institute for Advanced Studies in Basic Sciences, 45195-1159, Gava Zang, Zanjan, Iran Analytical Chemistry, Department of Physical and Analytical Chemistry, Uppsala University, Biomedical Centre, Box 599, SE-751 24 Uppsala, Sweden
a r t i c l e
i n f o
Article history: Received 19 November 2008 Received in revised form 25 December 2009 Accepted 31 December 2009 Keywords: Manganese Oxygen evolving complex ESI(+)–MS/MS tptz
a b s t r a c t The mononuclear complex [Mn(tptz)(CH3 COO)(OH2 )2 ]NO3 (1) was investigated by electrospray ionization mass spectrometry in aqueous solution at pH 4.5. Electrospray ionization mass spectrometry shows that mononuclear and dinuclear manganese cationic species are present in solution, probably in equilibrium with neutral 1. An experiment showed that the most important reaction in the presence of oxone (2KHSO5 ·KHSO4 ·K2 SO4 ) is decoordination. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The coordination chemistry of manganese(II) is dominated by the coordination numbers four and six. Five and seven-coordination are less frequently observed [1,2]. There has been considerable interest in the coordination chemistry of manganese complexes because of the significant involvement of manganese in various biological systems. Water oxidation to evolve oxygen is an important and fundamental chemical reaction in photosynthesis catalyzed by a Mn4 Ox Ca complex housed in a special protein environment [3]. Mn4 Ox Ca complex may serve as a model for technical approaches to split water by sunlight, which is a prerequisite for a sustainable hydrogen economy [4]. Numerous complexes of terpyridine and oligopyridines with manganese have been investigated because of their interesting structural, redox, photochemical, catalytic and water oxidation properties [5,6]. Recently, we reported the preparation, characterization and single-crystal X-ray diffraction of a seven coordinated manganese(II) complex with 2,4,6-tris(2-pyridyl)-1,3,5-triazine [7]. The complex acted as an oxygen evolving complex with oxone (2KHSO5 ·KHSO4 ·K2 SO4 ) as primary oxidant in aqueous solution at pH 4.5 [7]. Polar solvents, such as water, able to promote ligand exchange which would drastically change the structure of the complex in solid state, while promoting equilibria with other species stabilized by solvation
∗ Corresponding author. Tel.: +98 21 6616 5306; fax: +98 21 6601 2983. E-mail address: [email protected] (D.M. Boghaei). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2009.12.086
[8]. We now report solution structure of the seven coordinated manganese(II) in complex via electrospray ionization–mass spectrometry (ESI–MS). 2. Experimental All reagents and materials used for synthesis were reagent grade and used without further purification. Elemental analyses were performed on a Perkin-Elmer 2400 CHNS/O elemental analyzer. The acetate buffer was prepared by adjusting the pH of a 0.1 M KOAc solution to 4.5 with concentrated acetic acid. The detection of the Mn-complex was made in the positive electrospray mode using a PE-Sciex API III+ triple quadrupole mass spectrometer (Concord, ON, Canada) with an ion spray interface. A syringe pump (Syringe Infusion Pump 22, Harvard Apparatus Inc., Cambridge, MA, USA) operated at a flow rate of 5 L/min was utilized to pump the solution through a fused silica capillary (Polymicro Technologies, Phoenix, AZ, USA) into the ion source of the mass spectrometer. The fused silica spray capillary entering the mass spectrometer was centered in a stainless steel capillary auxiliary assembly delivering 40 psi of dry nitrogen (boil off from liquid nitrogen) for pneumatically assisted ESI–MS. The flow rate of dry nitrogen counter-current curtain gas (heated to 63 ◦ C) was 1.2 L/min over the sampling orifice. The mass spectrometric parameters were as follows: ion spray voltage (ISV) 4500 V, interface plate voltage (IN) 650 V, orifice lens voltage (OR) 50 V, and AC entrance rod (R0) 30 V. Q1 scans between m/z 100 and 1400 (step size 0.1), with a dwell time of 1 ms, were recorded in the multi-channel acquisition (MCA,
M.M. Najafpour et al. / Spectrochimica Acta Part A 75 (2010) 1168–1170
1169
Fig. 1. View of the complex with the atom-numbering scheme.
summation of 10 scans) mode. Furthermore, in MS/MS mode the collision energy was set to 20 eV with a collision gas thickness of 2.00 × 1015 molecules/cm2 . Data were collected on a Macintosh computer by the Tune2.5-FPU software. 2.1. [Mn (tptz)(CH3 COO)(OH2 )2 ]NO3 The complex was prepared by dissolving manganese(II) acetate (1 mmol, 245 mg) and tptz (1 mmol, 312 mg) in water (10 mL); the mixture was stirred for about 2 h at room temperature. Then HNO3 acid (0.2 M, 0.1 mL) was added to this solution. This solution yielded yellow crystals of the complex after 10 d (446 mg, 87% yield). C20 H19.20 MnN7 O7.10 Anal. Calc. for the complex: C, 45.6; H, 3.6; N, 18.6; Found: C, 45.3; H, 3.9; N, 18.4. 3. Results and discussion It is assumed that a structure in the solid state is preserved in solution, which is likely to hold for most solvents with low polarity and limited coordination abilities. But for polar solvents able to promote ligand exchange (water or DMSO), significant structural modifications may occur, which would drastically change the structure of the solvated complex, while promoting equilibria with other species stabilized by solvation. View of the complex with the atom-numbering scheme in solid state is shown in Fig. 1. In the crystal structure, each manganese atom is heptacoordinated, with a coordination polyhedron close to a pentagonal bipyramid. Heptacoordination is achieved by means of three N atoms belonging to the tptz ligand (N1, N4 and N5) and two oxygen atoms from the bidentate acetato ligand (O21 and O22), which determine the equatorial plane and finally the oxygen atoms from water molecules (O1W, O2W) are in a trans disposition (yielding O1W–Mn1–O2W as the trans-axial angle 178.86(4)◦ ). The hydrogen atoms of the coordinated water molecules were located from difference maps and refined with a fixed thermal parameter, while those of the 10% occupancy water molecule were not included in the model. The uncoordinated nitrate anion is disordered and was modelled with 90:10% occupancy of two overlapping positions; the minor component is hydrogen-bonded to the partial occupancy water molecule [7]. At room temperature, 1 is soluble in H2 O. To evaluate whether the solid state structure of 1 would be stable in solution, the ESI(+) mass spectrum (Fig. 2) of 1 in water/acetonitrile (2.5/1) at pH 4.5
was acquired, since ESI–MS has been shown to be an useful tool to characterize coordination compounds in solution [8–16]. Acetonitrile was added to facilitate spraying. Fig. 2 shows proposals for the major detected species. Owing to the gentle ionization process and based on ESI–MS/MS experiments, the assigned ions are unambiguously detected from solution and have been directly transferred (no gas-phase dissociation) to the gas-phase by ESI. The MS data indicates therefore that structure of 1 is not fully preserved in aqueous solution at pH 4.5, hence its possible activities of the complex in water should not be limited to the structure of the solid state. In water/acetonitrile (2.5/1) solution, 1 is in equilibrium with some cationic species, those proposed in Fig. 2. Note that, being a neutral species, 1 is not detected in the ESI(+)–MS/MS. The ESI(+)–MS/MS detection of the ion of m/z 911 reveals the presence, in solution, of a cationic species containing two manganese ions and two ligand molecules. The ESI(+)–MS/MS detection of the ion of m/z 1223 also reveals the presence, in solution, of a cationic species containing two manganese ions and three ligand molecules. Other species containing one manganese ion and two (m/z = 739) or three (m/z = 1050) ligands are also detected. This observation rationalizes also the presence in solution and hence the ESI(+)–MS/MS detection of the ion of m/z 351, 663 and 975 containing one potassium
Fig. 2. ESI (+)–MS for a water/acetonitrile (2.5/1) solution of 1 (L = tptz).
1170
M.M. Najafpour et al. / Spectrochimica Acta Part A 75 (2010) 1168–1170
of NH group is located from Fourier difference maps and refined in isotropic approximation with constrained NH distance. 4. Conclusions In summary, the mononuclear complex [Mn(tptz)(CH3 COO) (OH2 )2 ]NO3 (1) was investigated by ESI(+)–MS/MS in aqueous solution at pH 4.5. The MS data indicated that structure of the complex is not fully preserved in aqueous solution at pH 4.5. In water/acetonitrile solution, the complex is in equilibrium with some cationic species and a dissociative Mn-ligand process occurs in the water/acetonitrile solution. The experiment also showed that the most important reactions in the presence of oxone is decoordination. Acknowledgements
Fig. 3. ESI (+)–MS for reacting of solution of 1 and oxone.
The authors are grateful to the Research Council of Sharif University of Technology and National Elite Foundation for their financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2009.12.086. References
Fig. 4. ORTEP structure of 2 with partial atom labelling at the 50% probability level.
and one, two or three ligand molecules (Fig. 2). These cations indicate therefore that, to some extension, an interesting dissociative Mn-ligand process occurs in the water/acetonitrile solution (Fig. 2). There were reported that reactions between manganese complexes and oxone led to homogeneous catalytic O2 evolution [17,5]. To follow the speciation of Mn during reaction with oxone, we looked at the reaction of the complex and oxone in solution. The addition of oxone to the complex suppresses the other signals quite a lot making it difficult to detect any new signals and a series of peaks differing 152 are seen that corresponds to adducts of KHSO5 (m/z = 191.0, 343.0, 494.7) (Fig. 3). The experiment showed that the most important reaction in the presence of oxone is decoordination (m/z = 313.4). Decoordination was also observed in reacting of oxone and [Mn2 III/IV O2 (bpy)4 ](ClO4 )3 [18] that a heterocyclic Noxide (2) was detected (Fig. 4). In the structure of 2, distances of N1–C1 (1.360 Å), N1–C5 (1.371 Å), N2–C6 (1.353 Å) and N2–C10 (1.349 Å) are consistent with carbon–nitrogen double bonds in the structure of 2; similarly, the bond lengths of N1–O1 (1.328 Å) is also consistent with typical nitrogen–oxygen double bonds. The H atom
[1] A. Majumder, G. Pilet, M. Teresa, G. Rodriguez, S. Mitra, Polyhedron 25 (2006) 2550–2558. [2] A. Abakumov, V.K. Cherkasov, A.I. Poddelsky, M.P. Bubnov, L.G. Abakumova, G.K. Fukin, Dokl. Chem. 399 (2004) 207–210. [3] Y. Junko, J. Kern, K. Sauer, M.J. Latimer, Y. Pushkar, B. Jacek, B. Loll, W. Saenger, J. Messinger, A. Zouni, V.K. Yachandra, Science 314 (2006) 821–825. [4] A.F. Collings, C. Critchley, Artificial Photosynthesis: From Basic Biology to Industrial Application, first ed., Wiley-VCH, 2005. [5] J. Limburg, G.W. Brudvig, R.H. Crabtree, J. Am. Chem. Soc. 119 (1997) 2761–2762. [6] S. Das, C.D. Incarvito, R.H. Crabtree, G.W. Brudvig, Science 312 (2006) 1941–1943. [7] M.M. Najafpour, D.M. Boghaei, V. McKee, Inorg. Chim. Acta, in press. [8] J.A. Lessa, A. Horn Jr., C.B. Pinheiro, L.L. Farah, M.N. Eberlin, M. Benassi, R.R. Catharino, C. Fernandes, Inorg. Chem. Commun. 10 (2007) 863–866. [9] G. Cerchiaro, P.L. Saboya, A.M. da Costa Ferreira, D.M. Tomazela, M.N. Eberlin, Trans. Met. Chem. 29 (2004) 495–504. [10] A. Horn Jr., L. Fim, A.J. Bortoluzzi, B. Szpoganicz, M.de.S. Silva, M.A. Novak, M. Benassi Neto, L.S. Eberlin, R.R. Catharino, M.N. Eberlin, C. Fernandes, J. Mol. Struct. 797 (2006) 154–164. [11] H.E. Toma, A.D.P. Alexiou, A.L.B. Formiga, M. Nakamura, S. Dovidauskas, M.N. Eberlin, D.M. Tomazela, Inorg. Chim. Acta 358 (2005) 2891–2899. [12] B.C. Gilbert, J.R. Smith, A. Payeras, J. Oakes, Org. Biomol. Chem. 2 (2004) 1176–1180. [13] A.L. Rosen, G.M. Hieftje, Spectrochim. Acta Part B 59 (2004) 135–146. [14] B. Zeng, X. Zhu, X. Yu, S. Cai, Z. Chen, Spectrochim. Acta Part A 69 (1) (2008) 117–122. [15] Z. Chen, Y. Wu, F. Huang, D. Gu, F. Gan, Spectrochim. Acta Part A 66 (4) (2007) 1024–1029. [16] X. Yu, S. Cai, Z. Chen, Spectrochim. Acta Part A 60 (1) (2004) 391–396. [17] P. Kurz, G. Berggren, M.F. Anderlund, S. Styring, J. Chem. Soc. Dalton Trans. (2007) 4258–4261. [18] S.R. Cooper, M. Calvin, J. Am. Chem. Soc. 99 (1977) 6623–6630.