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steric influence on metal–ligand binding
Inorganica Chimica Acta 357 (2004) 202–206 www.elsevier.com/locate/ica
Tuning redox and spin state properties of Fe(II) N-heterocyclic complexes via electronic/steric influence on metal–ligand binding Tim Ayers a, Rebekah Turk a, Chris Lane a, James Goins a, Donald Jameson b, Spencer J. Slattery a,* a
Department of Chemistry, State University of West Georgia, 1601 Maple Street, Carrollton, GA 30118, USA b Department of Chemistry, Gettysburg College, Gettysburg, PA 17325-1486, USA Received 4 June 2003; accepted 7 June 2003
Abstract A series of complexes with the general formula [Fe(L)2 ]2þ , where L represents the tridentating 6-(N-3,5-dimethylpyrazolyl)2,20 bipyridine (L4 ); 6-(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine (L5 ); and 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine (L6 ), were prepared and characterized. The room temperature solution magnetic susceptibility and redox properties of these compounds were investigated as a function of stepwise variation in the ligand structure. The Fe(III/II) couple was characterized by way of cyclic voltammetry using aprotic solvent conditions (acetonitrile) where each complex was observed to have reversible behavior. NMR methodology was used for measuring the magnetic susceptibilities where both [Fe(L4 )2 ]2þ and Fe(L5 )2 ]2þ exhibited diamagnetic low spin behavior; however, [Fe(L6 )2 ]2þ measured a leff of 4.1 Bohr-magnetons indicating spin equilibrium predominantly in the high spin state. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Electrochemistry; Iron complexes; N-heterocyclic ligand complexes; Magnetic susceptibility; Steric hindrance
1. Introduction Numerous studies have shown that the electronic behavior, such as redox and spin state properties, on transition metal complexes can be significantly influenced by small changes in the metal–ligand interaction [1–6]. Studies of this nature are of interest due to its use in molecular electronics, information recording, and catalysis [7–9]. Recently we reported the redox and spin state properties for two series of complexes with the general formulas [Co(L)2 ]2þ and [Fe(L)2 ]2þ , where L represents the ligands shown in Fig. 1. In that study we were attempting to quantify the ability to regulate electronic behavior by progressively altering the number of coordinated pyrazoles relative to pyridines in a synthetically convenient manner. An overall decrease in the *
Corresponding author. Tel.: +1-7708366550; fax: +1-7708364656. E-mail address: [email protected] (S.J. Slattery).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0020-1693(03)00378-5
separation between the d-orbital energy levels was observed when coordinated pyridines are replaced with pyrazoles resulting in an increasing stability of the high spin configuration relative to the low spin state. The change in the electronic properties was rationalized on the basis of pyrazole being a weaker p-acceptor and rdonor than pyridine. Of particular interest was the room temperature spin state behavior of Fe(II) which was low spin when coordinated to L1 or L2 but switched to predominantly high spin behavior with the L3 ligand. From the results of that study the [Fe(L2 )2 ]2þ complex was presumed to have a relatively low lying high spin excited state; hence, we decided to explore small changes in the L2 structure (see Fig. 2) that would incorporate both inductive and steric factors when coordinated onto the metal center in order to approach the spin crossover. The preparation and characterization of the Fe(II) bis-L4 through L6 compounds are presented, and the
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Fig. 1. A series of tridentating ligands is illustrated where the pyridyl/pyrazole ratio is varied for redox and spin state regulation.
Fig. 2. A family of tridentating ligands is shown where the relative number of pyridine/pyrazole ratio remains the same, but the overall structure is varied via pyrazole substituent and/or methylene (–CH2 –) spacer.
variation in the Fe(II) spin state and Fe(III/II) redox behavior as a function of ligand structure are discussed.
2. Experimental 2.1. Materials and techniques Reagent grade chemicals and solvents were used in the synthesis of the iron and cobalt complexes. Tetra-nbutyl ammonium hexafluorophosphate (TBAH) was synthesized from a literature procedure and recrystallized twice from ethanol [10]. Acetonitrile used in electrochemical measurements was distilled from P2 O5 and molecular sieves. stored over 4 A Electrochemical measurements of the iron and cobalt complexes were obtained using a Princeton Applied Research model 250-Versastat/Potentiostat. The working electrode was a platinum disk electrode (Bioanalytical Systems) for electrochemistry carried out in organic solvents. The working electrode was polished routinely with 0.5 lm alumina polish. A platinum electrode served as the auxiliary electrode. The potentials were measured using a saturated sodium calomel electrode (SSCE). The reference electrode was periodically checked with the Fcþ /Fc couple [11]. The half wave potential (E1=2 ) was calculated using the formula E1=2 ¼ ðEp;a þ Ep;c Þ=2, where Ep;a and Ep;c are the peak anodic and peak cathodic potentials, respectively. Magnetic susceptibility measurements in solution were made by standard NMR methodology on a 200 MHz Varian Gemini Spectrometer using d6 -acetone solutions with 1% v/v tetramethylsilane (TMS) [12,13]. The equation for determining the mass susceptibility (xg in cm3 /g) using modern NMR instrumentation which has the external magnetic field coaxial with the sample tube, is given below (Eq. (1)):
xg ¼ ½ð3Df Þ=4pFm þ x0 þ ½x0 ðd0 ds Þ=m;
ð1Þ
where Df is the observed frequency shift of the reference resonance (e.g., TMS signal), F is the fixed probe frequency in Hz of the NMR spectrophotometer, x0 is the mass susceptibility of the solvent, d0 and ds are the, respective, densities (g/cm3 ) for the solvent and solution, and m is the mass in grams of the complex per cm3 of solution. The last two terms in the equation correct for the solvents diamagnetic contribution by taking into account the change in its density after adding the solute. In all cases within this study the complex concentration was less than 9 mM which allows for the solution density (ds ) to be approximated by d0 þ m since the amount of solvent displaced by the added solute is very small; hence, the mass susceptibility equation can be simplified to the following relationship (Eq. (2)) [14]: xg ¼ ½ð3Df Þ=4pFm:
ð2Þ 3
The molar susceptibility (cm /mol) for each complex was calculated by simply multiplying xg by the molar mass (g/mol). This was followed by taking into account diamagnetic corrections using tabulated values and PascalÕs constants [15,16] resulting with the molar susceptibility of the complex due to paramagnetic contributions (para xm ). From this the effective magnetic moment (leff ) in Bohr-magnetons was calculated from the relationship below (Eq. (3)), where T is the temperature in Kelvin. leff ¼ 2:828ðpara xm T Þ1=2 :
ð3Þ
2.2. Syntheses 2.2.1. Ligand synthesis from literature procedures The syntheses of 6-(N-3,5-dimethylpyrazolyl)2,20 -bipyridine (L4 ), 6-(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine
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(L5 ), and 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 bipyridine (L6 ) ligands were carried out from reported procedures [17]. 2.2.2. Synthesis of the complex [Fe(L4 )2 ](PF6 )2 Fe(NH4 )2 (SO4 )2 6H2 O (87.0 mg; 0.222 mmol) was dissolved in H2 0 (15 ml). In a separate flask, the 6-(N3,5-dimethylpyrazolyl)2,20 -bipyridine ligand (101 mg; 0.401 mmol) was added to ethanol (15 ml). The solutions were combined and stirred at room temperature while gently heating for 75 min. During this time the reaction color changed from colorless to intense purple. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The maroon colored solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 120 mg (0.142 mmol; yield 70.2%). Anal. Calc. for [Fe(L4 )2 ](PF6 )2 : C, 42.57; H, 3.33. Found: C, 42.73; H, 3.33%. 2.2.3. Synthesis of the complex [Fe(L5 )2 ](PF6 )2 Fe(NH4 )2 (SO4 )2 6H2 O (89.2 mg; 0.228 mmol) was dissolved into 20 ml of H2 O. In a separate flask, the 6(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (87.5 mg; 0.465 mmol) was added to 20 ml of ethanol. The two solutions were combined and stirred at room temperature for approximately 1 h. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 mL of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 88.9 mg (0.109 mmol; yield 47.8%). Anal. Calc. for [Fe(L5 )2 ](PF6 )2 : C, 41.10; H, 2.96. Found: C, 40.73; H, 2.97%. 2.2.4. Synthesis of the complex [Fe(L6 )2 ](PF6 )2 Excess Fe(NH4 )2 (SO4 )2 6H2 O (177 mg; 0.451 mmol) was dissolved into 20 ml of H2 O. In a separate flask, the 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (237 mg; 0.897 mmol) was added to 20 ml of ethanol. The solutions were combined and stirred at room temperature for 75 min. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 196 mg (0.224 mmol; yield 49.9%). Anal. Calc. for [Fe(L6 )2 ](PF6 )2 : C, 43.95; H, 3.69. Found: C, 43.73; H, 3.72%.
solutions were combined and stirred at room temperature for approximately 90 min. Next, NH4 PF6 (400 mg: 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration and washed with water and washed/dried with diethylether yielding 141 mg (0.158 mmol; 45.0%). Anal. Calc. for [Co(L6 )2 ](PF6 )2 H2 O: C, 43.80; H, 3.68. Found: C, 43.52; H, 3.65%.
3. Results and discussion 3.1. Synthesis All of the ligands (L4 , L5 , and L6 ) were prepared from literature procedures with no modifications. All of the iron complexes were conveniently prepared by reacting the appropriate metal salt with the desired ligand in approximate 1:2 molar ratio, respectively. Each of the metal compounds were isolated as PF6 salts under aqueous conditions and found to have good purity where no further purification was necessary. 3.2. Magnetic susceptibility We previously reported that the [Fe(L2 )2 ]2þ complex exhibited no paramagnetic behavior that correlates with low spin, diamagnetic behavior (S ¼ 0) where the energy separation between the low and high spin states are greater than room temperature kT [1]. However, we introduced minor variations to the 6-(N-pyrazolyl)2,20 bipyridine ligand (L2 ) to explore the possibility of decreasing the energy separation between the high and low spin states in order to drive it near or past the spin crossover point. For each compound, solution magnetic susceptibility measurements were carried out in d6 acetone/1% TMS using standard NMR methodology at room temperature. From the data shown in Table 1, no room temperature paramagnetic contributions from Table 1 Fe(III/II) redox potentials (E1=2 ) and Fe(II) effective magnetic moments (leff ) Complex 2
[Fe(L )2 ] [Fe(L4 )2 ]2þ [Fe(L5 )2 ]2þ [Fe(L6 )2 ]2þ [Co(L6 )2 ]2þ a
6
2.2.5. Synthesis of the complex [Co(L )2 ](PF6 )2 H2 O CoCl2 6H2 O (92.1 mg; 0.387 mmol) was dissolved in H2 O (20 mL). In another flask the 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (0.186 mg; 0.702 mmol) was added to 20 ml of ethanol. The
leff (Bohr-magnetons)a
2þ
4.1 4.6
E1=2 (V)b 1.14c 1.06 0.94 0.93
The magnetic moments (leff ) of the Fe(II) and Co(II) complexes were carried out under solution conditions in d6 -acetone/1% TMS at 25 °C. b The voltammograms were measured in 0.1 M TBAH/acetonitrile using a platinum working electrode, a SSCE reference electrode, and a scan rate of 100 mV/s. c Ref. [1].
T. Ayers et al. / Inorganica Chimica Acta 357 (2004) 202–206
possible intermediate or high spin configurations were observed except with the [Fe(L6 )2 ]2þ complex, where the effective magnetic moment (leff ) was measured at 4.1 Bohr-magnetons. Orbital contributions from the ground state is expected to be quenched for the [Fe(L6 )2 ]2þ due to its low symmetry (C2 ) that fully removes d-orbital degeneracy; however, we have observed values for leff that are greater than the calculated spin only value for a high spin cobalt system, [Co(L2 )2 ]2þ , with similar C2 symmetry [1]. This is presumed to be from the mixing of higher energy states that have orbital contributions [16]. If orbital contributions from the [Fe(L6 )2 ]2þ complex is neglected, the leff is calculated to be 4.5 Bohr-magnetons for the S ¼ 2 high spin state. This is greater than the experimentally observed 4.1 Bohr-magnetons which signifies contributions from the low (S ¼ 0) and/or intermediate (S ¼ 1) spin states. Furthermore, we investigated the behavior of the analogous d7 cobalt complex, [Co(L6 )2 ]2þ , which measured a leff of 4.6 Bohr-magnetons. This is considerably greater than the expected spin only contribution that is calculated to be 3.5 Bohr-magnetons for the S ¼ 3=2 high spin state; hence, orbital contributions are presumed to be involved. Assuming that both the iron and cobalt bis-L6 complexes have similar structures, the higher leff exhibited by Co(II) further supports spin equilibrium conditions with the [Fe(L6 )2 ]2þ system where the high spin arrangement is the ground state. 3.3. Electrochemistry Cyclic voltammetry was used to characterize the Fe(III/II) redox potentials where the peak to peak separation (DEp ) was between 80 and 100 mV which can be interpreted as reversible to borderline between reversible and quasi-reversible behavior. In addition, each complex exhibited an equivalent amount of current passed for the oxidation and reduction processes. The voltammograms of the iron complexes coordinated to the ligands L2 , L4 , L5 , and L6 are shown in Fig. 3. The highest Fe(III/II) redox potential is exhibited by the [Fe(L2 )2 ]3þ=2þ couple followed by [Fe(L4 )2 ]3þ=2þ , where both the L2 and L4 ligands are identical in structure except the L4 molecule consists of two methyl substituents on the pyrazolyl group (see Figs. 1 and 2). The methyls should have a relatively weak electron donating influence consistent with the 80 mV cathodic shift observed in the [Fe(L4 )2 ]3þ=2þ redox potential. A similar comparison can be made between the [Fe(L5 )2 ]3þ=2þ and [Fe(L6 )2 ]3þ=2þ couples where the L5 and L6 ligands are identical in structure except the latter ligand has two methyls on the pyrazolyl group. Hence, a lower potential of approximately 80 mV is expected for the L6 complex due to the additional methyl substituents; however, a shift of only 10 mV was measured.
205
Fig. 3. Cyclic voltammograms of the Fe(III/II) couple are shown where L2 represents [Fe(L2 )2 ]3þ=2þ , L4 represents [Fe(L4 )2 ]3þ=2þ , L5 represents [Fe(L5 )2 ]3þ=2þ , and L6 represents [Fe(L6 )2 ]3þ=2þ . The electrochemistry was measured in 0.1 M TBAH/acetonitrile using a SSCE reference electrode and scan rate of 100 mV/s.
In this latter comparison the –CH2 – spacer extends the pyrazolyl group from the 2,20 -bipyridine in such a manner that the added methyl substituents may be causing steric crowding between the ligands. This would hinder the ligandÕs ability to approach the metal center resulting in a decreased metal–ligand bonding interaction which will diminish the separations in the d-orbital energy levels, and shift the Fe(III/II) couple towards higher potentials. In addition there is the possibility of having a tetrahedral arrangement where the metal center is coordinated only to the 2,20 -bipyridyl portion; however, this is unlikely since: (1) the metal localized redox behavior is well behaved (reversible), (2) the E1=2 potential (volts) is in the proximity expected for the pseudo-octahedral coordination composed of four pyridines and two dimethyl-pyrazoles, and (3) the tetrahedral coordination will have considerable steric hindrance where the coordinated 2,20 -dipyridyls will have the – CH2 -N-3,5-dimethylpyrazolyl group hanging off the 60 position. Interestingly, the L2 and L5 ligands have no methyl substituents on the pyrazolyl group but they differ only in the –H2 – spacer on the L5 ligand where a considerable drop in the redox potential (200 mV) is observed indicating no significant steric crowding. The cathodic shift is presumed to be due to the electron donating affects of the –CH2 – spacer between the pyrazole and pyridine, as well as the change in bite angle. Once the methyl substituents are added to the pyrazolyl with the –CH2 – spacer (L6 ), steric factors are large enough to noticeably counter the inductive influence of the methyl substituents. A reported example of this type of steric phenomena was reported with a series of Fe(II) complexes composed of the ligands shown in Fig. 4. This family of complexes exhibited an unexpected increase in potential (anodic shift) with the Fe(III/II)
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Grant #9452084 from the National Science FoundationÕs Instrumentation and Laboratory Improvement Program. We also wish to thank the State University of West Georgia for their financial support with Faculty Research Grants.
Fig. 4. General structure of the 2,6-bis(pyrazol-1-ylmethyl)pyridine ligand.
couple as methyl substituents were added [18]. This behavior was rationalized on the basis that adding methyl groups enhanced the steric crowding between the ligands to an extent that over-compensates the electron donating influence typically observed for methyl substituents. However, when a larger metal cation was used such as Ru(II), the steric crowding between the ligands was reduced so that the Ru(III/II) redox couple was observed to undergo the expected cathodic shift to lower potentials as the methyl groups were added [19]. We are currently making additional modifications to the L2 ligands pyrazole functionality which offers a broad range of pendent groups with convenient synthetic methods. A single substituent can be prepared in either the carbon four or five position, or two substituents in the carbon three and five positions such as the location of the methyls on the L4 ligand. Variation in the substituents inductive and/or steric properties are being investigated to further tune spin state properties and oxidation potentials of Fe(II).
Acknowledgements Acknowledgment is made to the donors of The Petroleum Research Fund for support of this research (ACS-PRF# 34835-B3). Purchase of the Gemini-2000 variable temperature NMR spectrometer at the State University of West Georgia was partially funded by
References [1] T. Ayers, S. Scott, J. Goins, N. Caylor, D. Hathcock, S.J. Slattery, D. Jameson, Inorg. Chim. Acta 307 (2000) 7. [2] D.J. Hathcock, K. Stone, J. Madden, S.J. Slattery, Inorg. Chim. Acta 282 (1998) 131, and references therein. [3] S.J. Slattery, N. Gokaldas, T. Mick, K.A. Goldsby, Inorg. Chem. 33 (1994) 3621. [4] S.J. Slattery, W. Bare, K.A. Goldsby, J. Chem. Soc., Dalton Trans. 8 (1999) 1347. [5] P. Gutlich, A. Hauser, H. Hartmut, Angew. Chem. Int. Ed. Engl. 33 (1994) 2024, and references therein. [6] J.K. McCusker, A.L. Rheingold, D.N. Hendrickson, Inorg. Chem. 35 (1996) 2100. [7] J.S. Miller, A.J. Epstein, Chem. Eng. News (Oct. 2) (1995) 30. [8] H.C. Hurrell, H.D. Abruna, Inorg. Chem. 29 (1990) 736. [9] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewsky, Coord. Chem. Rev. 84 (1988) 85. [10] T. Sawyer, Experimental Electrochemistry for Chemists, Wiley, New York, 1976. [11] R.R. Gagne, C.A. Koval, G.C. Lisenski, Inorg. Chem. 19 (1980) 2854. [12] E.M. Schubert, J. Chem. Ed. 69 (1992) 62, and references therein. [13] J. Woolcock, A. Zafar, J. Chem. Ed. 69 (1992) A176, and references therein. [14] M.L. Naklicki, C.A. White, L.L. Plante, C.E. Evans, R.J. Crutchley, Inorg. Chem. 37 (1998) 1880. [15] R.C. Weast, M.J. Astle (Eds.), CRC Handbook of Chemistry and Physics, 63rd ed., CRC, Boca Raton, FL, 1982. [16] B.N. Figgis, Ligand field theory, in: G. Wilkinson (Ed.), Comprehensive Coordination Chemistry, vol. 1, Pergamon, Oxford, 1987, Chapter 6. [17] A.J. Downard, G.E. Honey, P.J. Steel, Inorg. Chem. 30 (1991) 3733. [18] S. Mahapatra, N. Gupta, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1991) 2911. [19] S. Mahapatra, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1992) 2337.
Tuning redox and spin state properties of Fe(II) N-heterocyclic complexes via electronic/steric influence on metal–ligand binding Tim Ayers a, Rebekah Turk a, Chris Lane a, James Goins a, Donald Jameson b, Spencer J. Slattery a,* a
Department of Chemistry, State University of West Georgia, 1601 Maple Street, Carrollton, GA 30118, USA b Department of Chemistry, Gettysburg College, Gettysburg, PA 17325-1486, USA Received 4 June 2003; accepted 7 June 2003
Abstract A series of complexes with the general formula [Fe(L)2 ]2þ , where L represents the tridentating 6-(N-3,5-dimethylpyrazolyl)2,20 bipyridine (L4 ); 6-(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine (L5 ); and 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine (L6 ), were prepared and characterized. The room temperature solution magnetic susceptibility and redox properties of these compounds were investigated as a function of stepwise variation in the ligand structure. The Fe(III/II) couple was characterized by way of cyclic voltammetry using aprotic solvent conditions (acetonitrile) where each complex was observed to have reversible behavior. NMR methodology was used for measuring the magnetic susceptibilities where both [Fe(L4 )2 ]2þ and Fe(L5 )2 ]2þ exhibited diamagnetic low spin behavior; however, [Fe(L6 )2 ]2þ measured a leff of 4.1 Bohr-magnetons indicating spin equilibrium predominantly in the high spin state. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Electrochemistry; Iron complexes; N-heterocyclic ligand complexes; Magnetic susceptibility; Steric hindrance
1. Introduction Numerous studies have shown that the electronic behavior, such as redox and spin state properties, on transition metal complexes can be significantly influenced by small changes in the metal–ligand interaction [1–6]. Studies of this nature are of interest due to its use in molecular electronics, information recording, and catalysis [7–9]. Recently we reported the redox and spin state properties for two series of complexes with the general formulas [Co(L)2 ]2þ and [Fe(L)2 ]2þ , where L represents the ligands shown in Fig. 1. In that study we were attempting to quantify the ability to regulate electronic behavior by progressively altering the number of coordinated pyrazoles relative to pyridines in a synthetically convenient manner. An overall decrease in the *
Corresponding author. Tel.: +1-7708366550; fax: +1-7708364656. E-mail address: [email protected] (S.J. Slattery).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0020-1693(03)00378-5
separation between the d-orbital energy levels was observed when coordinated pyridines are replaced with pyrazoles resulting in an increasing stability of the high spin configuration relative to the low spin state. The change in the electronic properties was rationalized on the basis of pyrazole being a weaker p-acceptor and rdonor than pyridine. Of particular interest was the room temperature spin state behavior of Fe(II) which was low spin when coordinated to L1 or L2 but switched to predominantly high spin behavior with the L3 ligand. From the results of that study the [Fe(L2 )2 ]2þ complex was presumed to have a relatively low lying high spin excited state; hence, we decided to explore small changes in the L2 structure (see Fig. 2) that would incorporate both inductive and steric factors when coordinated onto the metal center in order to approach the spin crossover. The preparation and characterization of the Fe(II) bis-L4 through L6 compounds are presented, and the
T. Ayers et al. / Inorganica Chimica Acta 357 (2004) 202–206
203
Fig. 1. A series of tridentating ligands is illustrated where the pyridyl/pyrazole ratio is varied for redox and spin state regulation.
Fig. 2. A family of tridentating ligands is shown where the relative number of pyridine/pyrazole ratio remains the same, but the overall structure is varied via pyrazole substituent and/or methylene (–CH2 –) spacer.
variation in the Fe(II) spin state and Fe(III/II) redox behavior as a function of ligand structure are discussed.
2. Experimental 2.1. Materials and techniques Reagent grade chemicals and solvents were used in the synthesis of the iron and cobalt complexes. Tetra-nbutyl ammonium hexafluorophosphate (TBAH) was synthesized from a literature procedure and recrystallized twice from ethanol [10]. Acetonitrile used in electrochemical measurements was distilled from P2 O5 and molecular sieves. stored over 4 A Electrochemical measurements of the iron and cobalt complexes were obtained using a Princeton Applied Research model 250-Versastat/Potentiostat. The working electrode was a platinum disk electrode (Bioanalytical Systems) for electrochemistry carried out in organic solvents. The working electrode was polished routinely with 0.5 lm alumina polish. A platinum electrode served as the auxiliary electrode. The potentials were measured using a saturated sodium calomel electrode (SSCE). The reference electrode was periodically checked with the Fcþ /Fc couple [11]. The half wave potential (E1=2 ) was calculated using the formula E1=2 ¼ ðEp;a þ Ep;c Þ=2, where Ep;a and Ep;c are the peak anodic and peak cathodic potentials, respectively. Magnetic susceptibility measurements in solution were made by standard NMR methodology on a 200 MHz Varian Gemini Spectrometer using d6 -acetone solutions with 1% v/v tetramethylsilane (TMS) [12,13]. The equation for determining the mass susceptibility (xg in cm3 /g) using modern NMR instrumentation which has the external magnetic field coaxial with the sample tube, is given below (Eq. (1)):
xg ¼ ½ð3Df Þ=4pFm þ x0 þ ½x0 ðd0 ds Þ=m;
ð1Þ
where Df is the observed frequency shift of the reference resonance (e.g., TMS signal), F is the fixed probe frequency in Hz of the NMR spectrophotometer, x0 is the mass susceptibility of the solvent, d0 and ds are the, respective, densities (g/cm3 ) for the solvent and solution, and m is the mass in grams of the complex per cm3 of solution. The last two terms in the equation correct for the solvents diamagnetic contribution by taking into account the change in its density after adding the solute. In all cases within this study the complex concentration was less than 9 mM which allows for the solution density (ds ) to be approximated by d0 þ m since the amount of solvent displaced by the added solute is very small; hence, the mass susceptibility equation can be simplified to the following relationship (Eq. (2)) [14]: xg ¼ ½ð3Df Þ=4pFm:
ð2Þ 3
The molar susceptibility (cm /mol) for each complex was calculated by simply multiplying xg by the molar mass (g/mol). This was followed by taking into account diamagnetic corrections using tabulated values and PascalÕs constants [15,16] resulting with the molar susceptibility of the complex due to paramagnetic contributions (para xm ). From this the effective magnetic moment (leff ) in Bohr-magnetons was calculated from the relationship below (Eq. (3)), where T is the temperature in Kelvin. leff ¼ 2:828ðpara xm T Þ1=2 :
ð3Þ
2.2. Syntheses 2.2.1. Ligand synthesis from literature procedures The syntheses of 6-(N-3,5-dimethylpyrazolyl)2,20 -bipyridine (L4 ), 6-(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine
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(L5 ), and 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 bipyridine (L6 ) ligands were carried out from reported procedures [17]. 2.2.2. Synthesis of the complex [Fe(L4 )2 ](PF6 )2 Fe(NH4 )2 (SO4 )2 6H2 O (87.0 mg; 0.222 mmol) was dissolved in H2 0 (15 ml). In a separate flask, the 6-(N3,5-dimethylpyrazolyl)2,20 -bipyridine ligand (101 mg; 0.401 mmol) was added to ethanol (15 ml). The solutions were combined and stirred at room temperature while gently heating for 75 min. During this time the reaction color changed from colorless to intense purple. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The maroon colored solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 120 mg (0.142 mmol; yield 70.2%). Anal. Calc. for [Fe(L4 )2 ](PF6 )2 : C, 42.57; H, 3.33. Found: C, 42.73; H, 3.33%. 2.2.3. Synthesis of the complex [Fe(L5 )2 ](PF6 )2 Fe(NH4 )2 (SO4 )2 6H2 O (89.2 mg; 0.228 mmol) was dissolved into 20 ml of H2 O. In a separate flask, the 6(N-pyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (87.5 mg; 0.465 mmol) was added to 20 ml of ethanol. The two solutions were combined and stirred at room temperature for approximately 1 h. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 mL of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 88.9 mg (0.109 mmol; yield 47.8%). Anal. Calc. for [Fe(L5 )2 ](PF6 )2 : C, 41.10; H, 2.96. Found: C, 40.73; H, 2.97%. 2.2.4. Synthesis of the complex [Fe(L6 )2 ](PF6 )2 Excess Fe(NH4 )2 (SO4 )2 6H2 O (177 mg; 0.451 mmol) was dissolved into 20 ml of H2 O. In a separate flask, the 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (237 mg; 0.897 mmol) was added to 20 ml of ethanol. The solutions were combined and stirred at room temperature for 75 min. Next, NH4 PF6 (400 mg; 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration, washed with water, and washed/dried with diethyl-ether yielding 196 mg (0.224 mmol; yield 49.9%). Anal. Calc. for [Fe(L6 )2 ](PF6 )2 : C, 43.95; H, 3.69. Found: C, 43.73; H, 3.72%.
solutions were combined and stirred at room temperature for approximately 90 min. Next, NH4 PF6 (400 mg: 2.45 mmol) was dissolved in 100 ml of water and added to the reaction mixture resulting in the precipitation of the product. The solid was isolated by suction filtration and washed with water and washed/dried with diethylether yielding 141 mg (0.158 mmol; 45.0%). Anal. Calc. for [Co(L6 )2 ](PF6 )2 H2 O: C, 43.80; H, 3.68. Found: C, 43.52; H, 3.65%.
3. Results and discussion 3.1. Synthesis All of the ligands (L4 , L5 , and L6 ) were prepared from literature procedures with no modifications. All of the iron complexes were conveniently prepared by reacting the appropriate metal salt with the desired ligand in approximate 1:2 molar ratio, respectively. Each of the metal compounds were isolated as PF6 salts under aqueous conditions and found to have good purity where no further purification was necessary. 3.2. Magnetic susceptibility We previously reported that the [Fe(L2 )2 ]2þ complex exhibited no paramagnetic behavior that correlates with low spin, diamagnetic behavior (S ¼ 0) where the energy separation between the low and high spin states are greater than room temperature kT [1]. However, we introduced minor variations to the 6-(N-pyrazolyl)2,20 bipyridine ligand (L2 ) to explore the possibility of decreasing the energy separation between the high and low spin states in order to drive it near or past the spin crossover point. For each compound, solution magnetic susceptibility measurements were carried out in d6 acetone/1% TMS using standard NMR methodology at room temperature. From the data shown in Table 1, no room temperature paramagnetic contributions from Table 1 Fe(III/II) redox potentials (E1=2 ) and Fe(II) effective magnetic moments (leff ) Complex 2
[Fe(L )2 ] [Fe(L4 )2 ]2þ [Fe(L5 )2 ]2þ [Fe(L6 )2 ]2þ [Co(L6 )2 ]2þ a
6
2.2.5. Synthesis of the complex [Co(L )2 ](PF6 )2 H2 O CoCl2 6H2 O (92.1 mg; 0.387 mmol) was dissolved in H2 O (20 mL). In another flask the 6-(N-3,5-dimethylpyrazolyl-1-ylmethyl)-2,20 -bipyridine ligand (0.186 mg; 0.702 mmol) was added to 20 ml of ethanol. The
leff (Bohr-magnetons)a
2þ
4.1 4.6
E1=2 (V)b 1.14c 1.06 0.94 0.93
The magnetic moments (leff ) of the Fe(II) and Co(II) complexes were carried out under solution conditions in d6 -acetone/1% TMS at 25 °C. b The voltammograms were measured in 0.1 M TBAH/acetonitrile using a platinum working electrode, a SSCE reference electrode, and a scan rate of 100 mV/s. c Ref. [1].
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possible intermediate or high spin configurations were observed except with the [Fe(L6 )2 ]2þ complex, where the effective magnetic moment (leff ) was measured at 4.1 Bohr-magnetons. Orbital contributions from the ground state is expected to be quenched for the [Fe(L6 )2 ]2þ due to its low symmetry (C2 ) that fully removes d-orbital degeneracy; however, we have observed values for leff that are greater than the calculated spin only value for a high spin cobalt system, [Co(L2 )2 ]2þ , with similar C2 symmetry [1]. This is presumed to be from the mixing of higher energy states that have orbital contributions [16]. If orbital contributions from the [Fe(L6 )2 ]2þ complex is neglected, the leff is calculated to be 4.5 Bohr-magnetons for the S ¼ 2 high spin state. This is greater than the experimentally observed 4.1 Bohr-magnetons which signifies contributions from the low (S ¼ 0) and/or intermediate (S ¼ 1) spin states. Furthermore, we investigated the behavior of the analogous d7 cobalt complex, [Co(L6 )2 ]2þ , which measured a leff of 4.6 Bohr-magnetons. This is considerably greater than the expected spin only contribution that is calculated to be 3.5 Bohr-magnetons for the S ¼ 3=2 high spin state; hence, orbital contributions are presumed to be involved. Assuming that both the iron and cobalt bis-L6 complexes have similar structures, the higher leff exhibited by Co(II) further supports spin equilibrium conditions with the [Fe(L6 )2 ]2þ system where the high spin arrangement is the ground state. 3.3. Electrochemistry Cyclic voltammetry was used to characterize the Fe(III/II) redox potentials where the peak to peak separation (DEp ) was between 80 and 100 mV which can be interpreted as reversible to borderline between reversible and quasi-reversible behavior. In addition, each complex exhibited an equivalent amount of current passed for the oxidation and reduction processes. The voltammograms of the iron complexes coordinated to the ligands L2 , L4 , L5 , and L6 are shown in Fig. 3. The highest Fe(III/II) redox potential is exhibited by the [Fe(L2 )2 ]3þ=2þ couple followed by [Fe(L4 )2 ]3þ=2þ , where both the L2 and L4 ligands are identical in structure except the L4 molecule consists of two methyl substituents on the pyrazolyl group (see Figs. 1 and 2). The methyls should have a relatively weak electron donating influence consistent with the 80 mV cathodic shift observed in the [Fe(L4 )2 ]3þ=2þ redox potential. A similar comparison can be made between the [Fe(L5 )2 ]3þ=2þ and [Fe(L6 )2 ]3þ=2þ couples where the L5 and L6 ligands are identical in structure except the latter ligand has two methyls on the pyrazolyl group. Hence, a lower potential of approximately 80 mV is expected for the L6 complex due to the additional methyl substituents; however, a shift of only 10 mV was measured.
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Fig. 3. Cyclic voltammograms of the Fe(III/II) couple are shown where L2 represents [Fe(L2 )2 ]3þ=2þ , L4 represents [Fe(L4 )2 ]3þ=2þ , L5 represents [Fe(L5 )2 ]3þ=2þ , and L6 represents [Fe(L6 )2 ]3þ=2þ . The electrochemistry was measured in 0.1 M TBAH/acetonitrile using a SSCE reference electrode and scan rate of 100 mV/s.
In this latter comparison the –CH2 – spacer extends the pyrazolyl group from the 2,20 -bipyridine in such a manner that the added methyl substituents may be causing steric crowding between the ligands. This would hinder the ligandÕs ability to approach the metal center resulting in a decreased metal–ligand bonding interaction which will diminish the separations in the d-orbital energy levels, and shift the Fe(III/II) couple towards higher potentials. In addition there is the possibility of having a tetrahedral arrangement where the metal center is coordinated only to the 2,20 -bipyridyl portion; however, this is unlikely since: (1) the metal localized redox behavior is well behaved (reversible), (2) the E1=2 potential (volts) is in the proximity expected for the pseudo-octahedral coordination composed of four pyridines and two dimethyl-pyrazoles, and (3) the tetrahedral coordination will have considerable steric hindrance where the coordinated 2,20 -dipyridyls will have the – CH2 -N-3,5-dimethylpyrazolyl group hanging off the 60 position. Interestingly, the L2 and L5 ligands have no methyl substituents on the pyrazolyl group but they differ only in the –H2 – spacer on the L5 ligand where a considerable drop in the redox potential (200 mV) is observed indicating no significant steric crowding. The cathodic shift is presumed to be due to the electron donating affects of the –CH2 – spacer between the pyrazole and pyridine, as well as the change in bite angle. Once the methyl substituents are added to the pyrazolyl with the –CH2 – spacer (L6 ), steric factors are large enough to noticeably counter the inductive influence of the methyl substituents. A reported example of this type of steric phenomena was reported with a series of Fe(II) complexes composed of the ligands shown in Fig. 4. This family of complexes exhibited an unexpected increase in potential (anodic shift) with the Fe(III/II)
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Grant #9452084 from the National Science FoundationÕs Instrumentation and Laboratory Improvement Program. We also wish to thank the State University of West Georgia for their financial support with Faculty Research Grants.
Fig. 4. General structure of the 2,6-bis(pyrazol-1-ylmethyl)pyridine ligand.
couple as methyl substituents were added [18]. This behavior was rationalized on the basis that adding methyl groups enhanced the steric crowding between the ligands to an extent that over-compensates the electron donating influence typically observed for methyl substituents. However, when a larger metal cation was used such as Ru(II), the steric crowding between the ligands was reduced so that the Ru(III/II) redox couple was observed to undergo the expected cathodic shift to lower potentials as the methyl groups were added [19]. We are currently making additional modifications to the L2 ligands pyrazole functionality which offers a broad range of pendent groups with convenient synthetic methods. A single substituent can be prepared in either the carbon four or five position, or two substituents in the carbon three and five positions such as the location of the methyls on the L4 ligand. Variation in the substituents inductive and/or steric properties are being investigated to further tune spin state properties and oxidation potentials of Fe(II).
Acknowledgements Acknowledgment is made to the donors of The Petroleum Research Fund for support of this research (ACS-PRF# 34835-B3). Purchase of the Gemini-2000 variable temperature NMR spectrometer at the State University of West Georgia was partially funded by
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