Running head: New Mn(III) complexes
Synthesis and characterisation of new manganese(III) complexes with the Schiff base N,N’-(1,2-phenylene)-bis(3-hydroxysalicylidenimine)
M. J. RODRÍGUEZ-DOUTÓN†, M. I. FERNÁNDEZ∗†, A. M. GONZÁLEZ-NOYA†, M. MANEIRO†, R. PEDRIDO‡ and M. J. ROMERO‡ † Dpto. de Química Inorgánica, Facultade de Ciencias, Universidade de Santiago de Compostela, 27002 Lugo, Spain ‡ Dpto. de Química Inorgánica, Facultade de Química, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
∗
M. Isabel Fernández Departamento de Química Inorgánica Facultade de Ciencias Universidade de Santiago de Compostela 27002 Lugo, Spain Telephone number: + 34 982883996 Fax number: + 34 982285872 E-mail address:
[email protected]
1
Four novel manganese(III) complexes have been prepared using the tetradentate ONNO symmetrical Schiff base N,N’-(1,2-phenylene)-bis(3-hydroxysalicylidenimine) and employing different manganese(II) and manganese(III) starting salts (chloride, acetate, acetylacetonate and perchlorate). These complexes were thoroughly characterised by elemental analysis, ES mass spectrometry, infrared and paramagnetic 1
H NMR spectroscopies, magnetic susceptibility measurements and molar
conductivities. Electrochemical studies by cyclic and normal pulse voltammetry were also performed. All complexes obtained are manganese(III) ones, in which the anions belonging to the starting salts have diverse coordinating tendencies; this fact seems to determine the nuclearity of the final complexes, with empirical formulae [Mn(H2L)(H2O)2]Cl(H2O)3 1, [Mn(H2L)(OAc)](H2O) 2, [Mn(H2L)(acac)](H2O)3 3, and [Mn(H2L)(H2O)(CH3OH)](ClO4) 4.
Keywords: Acetate; Acetylacetonate; Chloride; Manganese; Perchlorate; Schiff bases
1. Introduction The role played by manganese in a number of biological systems has driven the research efforts of many scientists from a variety of fields [1-3]. Inorganic chemists have been contributed to the understanding of the structural and mechanistic key-motifs of these processes in different ways, but one of the most common approaches has been the modelling of biological systems with artificial inorganic complexes [4-6]. This made possible much of the current knowledge about the manganese enzymes, but besides it led to the development of a number of catalysts of interest for industrial oxidation processes [7-12].
2
Over the last decade we have focused part of our research program on the manganesesalen derivatives [13-19], which constitute one of the more versatile systems in order to mimic biological and industrial catalyses. The activity of these models strongly depends on the nature of the salen derivative, in the sense that minor changes in the design of the Schiff base may have an important incidence on that, but also the use of different ancillary ligands or counterions determines the structure and, in the end, the activity, of the final complexes. Herein we report the manganese complexes obtained with the Schiff base N,N’-(1,2phenylene)-bis(3-hydroxysalicylidenimine), H4L [20], which incorporates a bulkier and very rigid phenyl ring between the imine nitrogen atoms, instead of the distinctive ethylene chain of the original salen ligand. H4L also contains six potential donor atoms, two imine nitrogen and four phenolic oxygen atoms, being a possible two compartimental ligand (see scheme 1). Moreover we employ different starting manganese salts, in order to try out the influence of the diverse nature of the anions: from poor coordinating as perchlorates, to great tendency to bind the metal ion as some carboxylates.
[Insert scheme 1 about here]
2. Results and discussion
2.1. Synthesis and characterisation of the ligand The symmetrical H4L ligand has been prepared by condensation of 2,3-dihydroxybenzaldehyde
and
1,2-phenylenediamine,
3
as
detailed
in
the
Experimental Part. The obtained Schiff base has been satisfactorily characterised by elemental analysis, mass spectrometry, IR, and 1H and 13C NMR spectroscopies.
2.2. Synthesis and characterisation of complexes All complexes have been prepared as detailed in Experimental Part and they are brown powders with low solubility in most common organic solvents, but are quite soluble in polar coordinating solvents such as DMF and DMSO. They appear to be air-stable in the solid state and in solution. Complexes 1-3 melt over 300 °C. Elemental analyses were satisfactory (see table 1) and show that the different manganese salts react with this Schiff base to afford solvated complexes of the following stoichiometry: [Mn(H2L)(H2O)2]Cl(H2O)3 for 1; [Mn(H2L)(OAc)](H2O) for 2; [Mn(H2L)(acac)](H2O)3 for 3; and [Mn(H2L)(H2O)(CH3OH)](ClO4) for 4. These formulations are in agreement with molar conductivity measurements in 10−3 M DMF solutions (see table 2), which are in the range 70-75 Ω−1 cm2 mol−1 for 1 and 4, indicating a typical behaviour of 1:1 electrolytes [21], whereas the scarcely conductivity (3-4 Ω−1 cm2 mol−1) exhibited by 2 and 3 is consistent with their nonelectrolyte nature. Furthermore, other spectroscopic techniques support such formulae and give insights on both solid and solution structure of the complexes.
[Insert table 1 about here]
2.2.1. IR spectroscopy and mass spectrometry studies The most significant IR bands for the complexes are recorded in table 1. They verify the coordination of H4L to the manganese ion, since in all cases we observe a negative shift (8-14 cm−1) of the ν(CNimine) band and also a positive shift (10-34 cm−1)
4
of the ν(COphenol) mode with respect to the free ligand. These data suggest the coordination of the Schiff base in its dianionic form through the inner phenol oxygen and the imine nitrogen atoms [17, 18, 22]. The sharp band attributed to the phenol OHinner groups in the ligand, disappears in the complexes spectra, and a very broad band at ca. 3400 cm−1, which is associated with a combination of the ν(OH) modes of coordinated and lattice water/methanol, is now present in the complexes. A sharp band corresponding to the remaining outer hydroxyl groups would be also expected, but it is obscured by the presence of water or methanol. Some other IR spectroscopical features allow understanding how the former manganese salt anions are bound to the central ion or even indicating their coordination behaviour: -the absence of ν(Mn−Cl) bands in the far IR spectrum of 1 indicates that the chloride is not coordinated to the metal centre [19]; -the appearance of two new bands in 2 at 1551 and 1380 cm−1 corresponds to the asymmetrical and symmetrical CO2− vibrational modes, respectively, and it is in accordance with the acetate ion coordinated to the manganese ion; besides, the difference between these two bands (∆ν = 171 cm−1) indicates the acetate is acting as a bridge between neighbouring molecules [8, 23]; -two new bands at 1574 and 1551 cm−1 in 3 are assigned to the ν(C=C) and ν(C=O) acetylacetonate bidentate modes [15] suggesting a possible behaviour as bridge between neighbouring molecules; -the broad unsplit band at 1120cm−1 in 4 is indicative of the presence of the uncoordinated perchlorate anion, its characteristic ν4 stretching mode at 637 cm−1 can be also easily identified [16].
5
These data suggest that the Schiff base is coordinated in its dianionic form to the metal, through both inner phenol oxygen and imine nitrogen atoms. The anions of the starting manganese salts may occupy the axial positions in function of their ability to bind to the metal ion. Thus, carboxylates, which have great tendency to coordinate manganese, should be bound to the metal ions in the case of 2 and 3, according to their conductivity measurements. It is likely that bidentate acetate and acetylacetonate groups act as bridges between two manganese ions [14, 15, 23], increasing the nuclearity of the complexes. Nevertheless the poor coordinating chloride and perchlorate ions originate 1:1 electrolytes for 1 and 4, respectively, remaining out of the metal first coordination sphere. ES mass spectra (see table 1) further corroborates coordination of the ligand to the manganese ion in the complexes, as they show, in all cases, a peak due to the fragment [Mn(H2L)]+. Furthermore, carboxylate complexes 2 and 3 also present other minor peaks which could be attributed to dimeric [Mn2(H2L)2]+ (and even trimeric [Mn3(H2L)3]+ species for the case of 2), which may suggest a dimeric or even polymeric nature for these compounds [14, 15].
2.2.2. Magnetic studies Room-temperature magnetic moments were corrected for diamagnetism of the sample using Pascal constants. Values obtained are shown in table 1. In all cases the results are close to 4.9 B. M., typical for octahedral, high spin manganese(III) complexes, indicating little or no antiferromagnetic interaction [16].
6
2.2.3. 1H NMR studies Paramagnetic 1H NMR allows us to carry out a complementary solution study for 1-4, and serves to substantiate the formation of the manganese(III) complexes. The spectra were undertaken using DMSO-d6 as solvent, and 1H NMR shifts are listed in table 2. All complexes show at least two proton upfield resonances that lie outside the diamagnetic region (ca. 0-10 ppm) at ca. −16 and −28 ppm, which must arise from the H5 and H4 protons of the aromatic rings (see scheme 1), respectively. These assignments are in accordance with previous findings for manganese(III) complexes with related Schiff base ligands [19, 24, 25]. Moreover, we have prepared the manganese(III) complex with N,N’-bis(4-hydroxysalicylidene)ethane-1,2-diamine*, in order to corroborate the assignment proposed. This complex only shows the H5 signals at –19.95 ppm because hydroxy groups occupy the H4 positions (see figure 1). The protons ortho to the donor groups of the Schiff base ligand are not observed (noted as H6 and H2’ in scheme 1), being an usual behaviour previously reported [8, 26]. But the protons in para positions (H3’) to the imine nitrogens of the phenyl spacer of the Schiff base may be observed. We only see the signals from these protons for the case of 2, and their appearance very close to the H5 protons values (–13.08 and –14.67 ppm) prevents to distinguish which signal exactly corresponds to each one.
*
This complex was prepared by reaction of MnCl2.4H2O with N,N’-bis(4hydroxysalicylidene)ethane-1,2-diamine, following the same procedure as described for 1. C16H16ClMnN2O5 (406.5). Anal. Calcd. (%):C, 47.2; H, 3.9; N, 6.9. Found: C, 47.5; H, 3.8; N, 7.1; µ = 4.8 B. M.; ΛM = 60 Ω-1 cm2 mol-1. The 1H NMR was recorded in DMSO-d6.
7
[Insert table 2 about here] [Insert figure 1 about here]
2.2.4. Electrochemical studies The electrochemical behaviour of these complexes in DMF was studied by cyclic and normal pulse voltammetry. Cyclic voltammograms of these complexes exhibit a quasi-reversible one-electron reduction-oxidation wave (see figure 2a, table 2). The oxidation state +III for the manganese ions in DMF solutions was confirmed by NPV. When the initial potentials were more negative than the wave voltage range, anodic and cathodic currents were observed (see figure 2b). However, when the initial potentials were more positive than the wave range only cathodic currents were observed (see figure 2c). This behaviour suggests that the oxidation state of manganese in solution is +III, being the process responsible for the redox wave MnIII→MnII, as we have previously found for related systems [19].
[Insert figure 2 about here]
In conclusion, the overall study of the MnIII-Schiff base complexes seems to indicate that H4L is behaviouring as a tetracoordinated ligand in all present complexes, using the inner N2O2 donor set. This behaviour would be influenced by the well-known stability of the manganese ion inside of the salen derivative cage [27], and very probably also by the rigidity and short length of the phenyl spacer [28]. Such spacer in H4L does not favour the accommodation of two metal ions in the inner and outer
8
cages of the same ligand unit or a suitable conformation of two units of the latter to bind the formers in the O2N + O2N compartments. We have recently reported a similar coordination for the ligand N,N'-bis(3-hydroxysalicylidene)ethane-1,2-diamine [19]. Therefore, the outer phenoxy groups would be limited to intermolecular hydrogen bonds interactions. In this sense, the presence of additional groups with suitable lone pairs (for example, solvent molecules such as water or methanol, or also chloride and perchlorate anions) usually promotes supramolecular assemblies in solid state via hydrogen bonding [16, 19, 28, 29].
3. Experimental
3.1. Materials All
the
solvents,
phenylenediamine
(Aldrich),
2,3-dihydroxybenzaldehyde manganese(II)
chloride
(Sigma-Aldrich), tetrahydrate
1,2-
(Probus),
manganese(II) perchlorate hexahydrate (Fluka), manganese(III) acetate dihydrate (Fluka) and manganese(III) acetylacetonate (Aldrich) were used as received without further purification.
3.2. Physical measurements Elemental analyses were performed on a Carlo Erba EA 1108 analyser. The IR spectra were recorded as KBr discs on a Bio-Rad FTS 135 spectrophotometer in the range 4000-200 cm-1. 1H NMR spectra were recorded on a Bruker WH 300 MHz spectrometer using CHCl3-d as solvent for the ligand and DMSO−d6 for the complexes. The electro-spray mass spectra of the complexes were measured on a Hewlett-Packard model LC-MSD 1100 instrument (positive ion mode, 98:2 MeOH-
9
HCOOH as mobile phase, 30 to 100 V). Electrochemical measurements were performed using an EG&G PAR model 273 potentiostat, controlled by EG&G PAR model 270 software. A Metrohm model 6.1204.000 graphite disc coupled to a Metrohm model 628-10 rotating electrolyte device was used as a working electrode. A saturated calomel electrode was used as a reference and a platinum wire as an auxiliary electrode. Measurements were made with ca. 10−3 M solutions of complexes in dimethylformamide using 0.1 M tetraethylammonium perchlorate as a supporting electrolyte. Cyclic voltammetry measurements were performed with a static graphite electrode, whilst direct-current and pulse voltammograms were recorded with the graphite disc rotating at 2000 revolutions per minute. ΛM was measured by using a WTW type LF conductivemeter. Room-temperature magnetic susceptibilities were measured using Digital Measurement system MSB-MKI, calibrated using tetrakis(isothiocyanato)cobaltate(II).
3.3. Preparation and characterisation
3.3.1. Synthesis of the ligand H4L 2.0 g (14.48 mmol) of 2,3-dihydroxybenzaldehyde was dissolved in 100 mL of chloroform, and 0.78 g (7.24 mmol) of 1,2-phenylenediamine was added. This mixture was refluxed in a round bottom flask fitted with a Dean-Stark trap to remove the water produced during the reaction. After refluxing for 3 h, the solution was concentrated to yield a red solid. The product was collected by filtration, washed with diethyl ether and dried in air. Yield was almost quantitative. M. p. 186 ºC. Anal. Cacld. for C20H16N2O4(%): C, 68.9; H, 4.6; N, 8.0. Found: C, 68.7; H, 4.6; N, 8.0. ES MS (m/z): 349. IR (KBr, cm−1): ν(O-H) 3422 (m), ν (C=N) 1615 (vs), ν (C−O) 1238
10
(s). 1H NMR (DMSO−d6, ppm): δ 6.79 (t, 2H, Hd), 6.95 (d, 2H, Hc), 7.12 (d, 2H, He), 7.42 (m, 4H, Hg), 8.88 (s, 2H, Hf), 9.33 (s, 2H, Hb), 12.93 (s, 2H, Ha).
13
C NMR
(DMSO−d6, ppm): δ 119.5-127.7 (Car), 142.1 (C−OHinner), 145.6 (C−OHouter), 164.7 (C=N).
3.3.2. [Mn(H2L)(H2O)2]Cl(H2O)3, 1 H4L (0.86 mmol, 0.30 g) was dissolved in 40 mL of methanol and MnCl2·4H2O (0.86 mmol, 0.17 g) dissolved in 20 ml of methanol was added to the initial yellow solution, which changed to green. After stirring for 10 min, 4M NaOH aqueous solution (1.72 mmol, 0.43 mL) was added and the mixture turned dark. The reaction mixture was refluxed for 2.5 hours, and then concentrated in vacuo until half volume. The complex was obtained as a brown powder after slow concentration of the solution. The brownish solid was isolated by filtration, washed with diethyl ether and dried in air, yield: 0.38 g, 85 %.
3.3.3. [Mn(H2L)(OAc)](H2O), 2 To a solution of H4L (0.86 mmol, 0.30 g) in 40 mL of methanol, Mn(OAc)3.2H2O dissolved in 20 mL of methanol was added (0.86 mmol, 0.23 g). The resultant brown solution was stirred at room temperature for 12 h, and then concentrated in vacuo until half volume. The brown complex was isolated after filtration, washed with diethyl ether and dried in air, yield: 0.33 g, 80 %.
3.3.4. [Mn(H2L)(acac)](H2O)3, 3 H4L (0.86 mmol, 0.30 g) was dissolved in 40 mL of methanol and Mn(acac)3 (0.86 mmol, 0.30 g) dissolved in 20 mL of methanol was added and the initial yellow
11
mixture turned brown. The reaction mixture was refluxed for 45 min, and a precipitate was formed. The brown-reddish complex was isolated by filtration, washed with diethyl ether and dried in air, yield: 0.31 g, 65 %.
3.3.5. [Mn(H2L)(H2O)(CH3OH)](ClO4), 4 To a solution of H4L (0.86 mmol, 0.30 g) in a 1:1 methanol:ethanol mixture (100 mL), Mn(ClO4)2.6H2O was added (0.86 mmol, 0.31 g) [CAUTION: Although no problems have been encountered in this work, perchlorates are potentially explosive and should be handled in only small quantities and with care]. After ten minutes, NaOH 4M (1.72 mmol, 0.43 mL) was poured into the solution. The progress of the reaction was followed by TLC, and after five days of stirring, all the Schiff base has reacted. The resultant solution was left to evaporate in air, and the microcrystalline brown solid produced was isolated by filtration, washed with diethyl ether and dried in air. Yield: 0.38 g, 80 %.
Acknowledgements We thank Ministerio de Ciencia y Tecnología of Spain (BQU2003-00167) for financial support.
References [1] J.D. Crowley, D.A. Traynor, D.C. Watherburn. Met. Ions Biol. Syst., 37, 209 (2000). [2] S. Iwata, J. Barber. Curr. Opin. Struc. Biol., 14, 447 (2004). [3] K.A. Campbell, E. Yikilmaz, C.V. Grant, W. Gregor, A.F. Miller, R.D. Britt. J. Am. Chem. Soc., 121, 4714 (1999).
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[4] M. Maneiro, W.F. Ruettinger, E. Bourles, G.L. McLendon, G.C. Dismukes. Proc. Nat. Acad. Sci. USA, 100, 3707 (2003). [5] D. Pursche, M.U. Triller, C. Slinn, N. Reddig, A. Rompel, B. Krebs. Inorg. Chim. Acta, 357, 1695 (2004). [6] I. Batinic-Haberle, I. Spasojevic, R.D. Stevens, P. Hambright, P. Neta, A. OkadaMatsumoto, I. Fridovich. Dalton Trans., 1696 (2004). [7] L. Canali, D.C. Sherrington. Chem. Soc. Rev., 28, 85 (1999). [8] C. Palopoli, M. Gonzalez-Sierra, G. Robles, F. Dahan, J.P. Tuchagues, S. Signorella. Dalton Trans., 3813 (2002). [9] M. Hoogenraad, K. Ramkisoensing, S. Gorter, W.L. Driessen, E. Bouwman, J.G. Haasnoot, J. Reedijk, T. Mahabiersing, F. Hartl. Eur. J. Inorg. Chem., 377 (2002). [10] T.K. Paine, T. Weyhermuller, E. Bothe, K. Wieghardt, P. Chaudhuri. Dalton Trans., 3136 (2003). [11] H. Nishikori, C. Ohta, T. Katsuki. Synlett, 1557 (2000). [12] I. Kuzniarska-Biernacka, A.R. Silva, R. Ferreira, A.P. Carvalho, J. Pires, M.B. de Carvalho, C. Freire, B. de Castro. New. J. Chem., 28, 853 (2004). [13] N. Aurangzeb, C.E. Hulme, C.A McAuliffe, R.G. Pritchard, M. Watkinson, M.R. Bermejo, A. Garcia-Deibe, M. Rey, J. Sanmartin, A. Sousa. J. Chem. Soc., Chem. Comm., 1153 (1994). [14] C.E. Hulme, M. Watkinson, M. Haynes, R.G. Pritchard, C.A. McAuliffe, N. Jaiboon, B. Beagley, A. Sousa, M.R. Bermejo, M. Fondo. J. Chem. Soc., Dalton Trans., 1805 (1997). [15] M. Watkinson, M. Fondo, M.R. Bermejo, A. Sousa, C.A. McAuliffe, R.G. Pritchard, N. Jaiboon, N. Aurangzeb, M. Naeem. J. Chem. Soc., Dalton Trans., 31 (1999).
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[16] M. Maneiro, M.R. Bermejo, A. Sousa, M. Fondo, A.M. Gonzalez, A. SousaPedrares, C.A. McAuliffe. Polyhedron, 19, 47 (2000). [17] M. Maneiro, M.R. Bermejo, M.I. Fernandez, A.M. Gonzalez-Noya, A.M. Tyryshkin, R.G. Pritchard. Z. Anorg. Allg. Chem., 629, 285 (2003). [18] M. Maneiro, M.R. Bermejo, M.I. Fernandez, E. Gomez-Forneas, A.M. GonzalezNoya, A.M. Tyryshkin. New J. Chem., 27, 727 (2003). [19] M.R. Bermejo, M.I. Fernandez, A.M. Gonzalez-Noya, M. Maneiro, R. Pedrido, M.J. Rodriguez, M. Vazquez. Eur. J. Inorg. Chem., 2769 (2004). [20] P. Cassoux, A. Gleizes. Inorg. Chem., 19, 665 (1980). [21] W.J. Geary. Coord. Chem. Rev., 7, 81 (1971). [22] R. Karmakar, C.R. Choudhury, G. Bravic, J.P. Sutter, S. Mitra. Polyhedron, 23, 949 (2004). [23] N. Aurangzeb, C.E. Hulme, C.A. McAuliffe, R.G. Pritchard, M. Watkinson, A. Garcia-Deibe, M.R. Bermejo, A. Sousa. J. Chem. Soc., Chem. Comm., 1524 (1992). [24] J. Bonadies, M. Maroney, V.L. Pecoraro. Inorg. Chem., 28, 2044 (1989). [25] M.R. Bermejo, A.M. González-Noya, V. Abad, M.I. Fernández, M. Maneiro, R. Pedrido, M. Vazquez. Eur. J. Inorg. Chem., 3696 (2004). [26] M.R. Bermejo, A.M. Gonzalez, M. Fondo, A. Garcia-Deibe, M. Maneiro, J. Sanmartin, O.L. Hoyos, M. Watkinson. New J. Chem., 24, 235 (2000). [27] S.W. Lee, S. Chang, D. Kossakovski, H. Cox, J.L. Beauchamp. J. Am. Chem. Soc., 121, 10152 (1999). [28] J. Sanmartin, M.R. Bermejo, A. Garcia-Deibe, O.R. Nascimento, A.J. CostaFilho. Inorg. Chim. Acta, 318, 135 (2001). [29] M.J. Hannon, L.J. Childs. Supramol. Chem., 16, 7 (2004).
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Table 1. Elemental analysis, mass spectrometry (ES), IR spectroscopy and magnetic moments at room temperature data for the complexes.
Complex
ES a) (m/z)
Found (Calcd.) (%)
µ (BM)
ν(C=N)
ν(C−O)
401
1607 vs
1252 s
5.2
4.1 (4.0)
401 (801, 1201)
1601 vs
1272 s
5.0
53.9 (54.2)
4.6 (4.9)
401 (801)
1606 s
1264 s
5.2
46.0 (45.8)
3.6 (3.7)
401 (801)
1607 vs
1248 s
5.3
%N
%C
%H
1
5.2 (5.3)
45.4 (45.6)
4.3 (4.6)
2
5.9 (5.9)
55.3 (55.2)
3
5.2 (5.1)
4
5.2 (5.1)
a)
IR spectrab) (cm−1)
fragment [MnL]+, in parentheses [Mn2L2]+ and, in the case of 2, also [Mn3L3]+; b) vs = very strong;
s = strong.
15
Table 2. Conductivity measurements, 1H NMR and cyclic voltammetry data for the complexes.
Complex
ΛMa)
1
Redox potentialsb)
H NMR
H4
H5
Eox(V)
Ered(V)
1
70
−27.80
−18.90
−0.154
−0.237
2
3
−28.78
−13.08, −14.67
0.044
−0.038
3
4
−29.43
−12.27
0.036
−0.052
4
75
−29.40
−18.05
−0.065
−0.149
a)
in Ω−1 cm2 mol−1; b) potentials vs. SCE
16
OH (b)
OH OH (a)
(c)
H4
HO
(d) (e)
(f)
N
H5
N H6 H2'
g (Harom) H3'
Scheme 1
17
Figure 1
18
Figure 2 19
Captions
Figure 1. 1H NMR spectra in DMSO−d6 at room temperature for: (a) complex described in footnote *; (b) complex 1.
Figure 2. Cyclic voltammogram for 1 at a scan rate of 0.02 V s−1 (a); normal pulse voltammogram (NPV) with anodic scan (b); and NPV with cathodic scan (c).
20
