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Synthesis, characterization and properties of iron(II) complexes with a series of tripodal ligands based on the parent ligand tris(2-pyridylmethyl)amine
Inorganica Chimica Acta 374 (2011) 540–545
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Synthesis, characterization and properties of iron(II) complexes with a series of tripodal ligands based on the parent ligand tris(2-pyridylmethyl)amine Sandra Kisslinger a, Harald Kelm b, Alexander Beitat a, Christian Würtele a, Hans-Jörg Krüger b,⇑, Siegfried Schindler a,⇑ a b
Institut für Anorganische und Analytische Chemie, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 58, 35392 Gießen, Germany Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger Straße, Gebäude 54, 67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Available online 10 March 2011 Dedicated to Professor Dr. Wolfgang Kaim on the occasion of his 60th birthday. Keywords: Iron complexes Tripodal ligands Spin-crossover Crystal structures Mößbauer spectroscopy
a b s t r a c t Iron(II) complexes of the type [Fe(L)(NCS)2] with the tripodal ligand apme (apme = N1-(2-aminoethyl)N1-(2-pyridyl-methyl)-1,2-ethanediamine) as well as with its derivatives were prepared and structurally characterized. The bond distances thus obtained showed that all complexes investigated were high-spin at the respective temperature. Furthermore [Fe(Me4apme)(NCS)2] was analyzed using Mößbauer spectroscopy that showed that this complex remains in its high-spin state over the entire temperature range. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In the last 20 years spin-crossover (SCO) properties have been studied extensively because of their possible future applications in molecular electronics [1–4]. Especially iron(II) complexes have been used in these studies. It is well known that low-spin (HS, 5 T2)–high-spin (LS, 1A1) transitions in iron(II) complexes can be induced by changes in pressure or temperature and by light irradiation [4–23]. Observations that the spin state is quite sensitive to small changes of the ligand field caused intensive research focusing on modifications of the ligands surrounding the metal ion. Iron(II) complexes of the tripodal ligand tris(2-pyridylmethyl)amine (tpa, Scheme 1; alternatively abbreviated as tmpa in the literature) – containing additional co-ligands such as thiocyanate – proved to be quite interesting and useful in these studies [10]. However, only most recently the crystal structure of [Fe(tpa)(NCS)2] has been reported [23,24]. Here it could be demonstrated furthermore, that solvent effects are quite important in regard to the SCO behavior of the complex. Increasing the chelate ring size of one of the chelate rings from 5 to 6 leads to the ligand N2,N2-bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine (pmea, Scheme 1). [Fe(pmea)(NCS)2] also showed SCO behavior (SCO behavior has not been observed in the original ⇑ Corresponding authors. E-mail address: [email protected] (S. Schindler). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.02.062
work on the pmea system) that has been studied previously by some of us [25,26]. Furthermore, Matouzenko et al. replaced one aromatic donor group in tpa by an aliphatic amine arm to obtain (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp, Scheme 1; alternatively abbreviated as DPEA in the literature) [13]. [Fe(unspenp)(NCS)2] exhibited a gradual SCO between 130 and 150 K. Furthermore, this group used (3-aminopropyl)bis(2-pyridylmethyl)amine (DPPA) as a ligand [14]. Compared with uns-penp/ DPEA the chelate ring size of the aliphatic amine arm in the according iron(II) complex was increased from 5 to 6. The resulting complex [Fe(DPPA)(NCS)2] crystallized in three polymorphs, where two of them exhibited SCO (a gradual SCO at about 150 K for one of the polymorphs and a very abrupt SCO at 114 K for the other one). Following up on these results we decided to investigate whether replacing one further pyridyl group by an aliphatic arm in the ligand could induce SCO properties in this type of iron(II) complexes. In principle we did not expect an improvement of the SCO behavior this way because it is well known that the ligand field strength is weakened when a pyridyl arm is replaced by an aliphatic amine arm. However, since it is well known that other effects like crystal packing, solvent molecules in crystals, counter ions can counter balance ligand field changes induced by the donor functions, it seemed to us that it would be worthwhile to actually examine these complexes. Thus we prepared and investigated iron(II) complexes with the ligand apme and its derivatives (Scheme 1).
S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
N
N N
N
N N
N
N
tpa
pmea
NR2 N
N N
N
NR2
N NR2
uns-penp (DPEA) R= H
apme
R=H
Scheme 1. Abbreviations used for tripodal ligands: tris(2-pyridylmethyl)amine (tpa), N2,N2-bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine (pmea), (2-aminoethyl)bis(2-pyridyl-methyl)amine (uns-penp, R = H), N1-(2-aminoethyl)-N1-(2-pyridyl-methyl)-1,2-ethanediamine (apme, R = H) as well as bis[2dimethylamino)ethyl]-(2-pyridylmethyl)amine (Me4apme, R = CH3).
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Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2. Complex 1 crystallizes in a Pna21 orthorhombic space group. The iron(II) cation in 1 is coordinated by six nitrogen donor atoms. Two of them belong to the two thiocyanate ligands and four of them to the tripodal ligand. The iron–nitrogen bond adjoining to the thiocyanate anions are shorter than those adjoining to the pyridin rings and the aliphatic amino groups (see Table 2). This may be due to electrostatic attraction in the case of the thiocyanate anion as well as due to different hybridizations and p-acceptor abilities of the pyridine and the aliphatic amine groups. The coordination sphere of the iron center is best described as a distorted octahedron, because the N–Fe–N angles between the cis and trans nitrogen atoms deviate strongly from the values of 90° and 180° expected for an ideal octahedral environment (see Table 3). N– Fe–N bond angles of 1 are between 75.73° and 114.66° for cis nitrogen donors, whereas bond angles of trans nitrogen donors are between 154.39° and 170.00°. Further three five-membered chelate rings are formed in 1, as shown in Fig. 1. A facial coordination is observed (the amine nitrogen atom together with the two imine nitrogen atoms being considered one donor set) in contrast to 3 and 4 described below. No thiocyanate group is coordinated trans to the aromatic donor arm (Table 4).
2. Results and discussion 2.2. [Fe(apme)(NCS)2] (2) Complexes [Fe(L)(NCS)2] with L = tripodal ligand such as tpa or pmea can be prepared by reacting the respective ligand with the precursor complex [Fe(py)4(NCS)2] in methanol [25]. Yellow to orange powders were obtained by this method. Crystals for X-ray analysis were formed by slow diffusion of diethyl ether into a solution of these complexes in methanol. In contrast, so far attempts to obtain a crystalline iron apme complex this way were not successful. Therefore, the reaction was performed in acetone. 2.1. [Fe(imine2apme)(NCS)2] (1) In contrast to our previous investigations on copper complexes with apme [27], here an imine formation with the solvent acetone occurred instead and N1-(propane-2-ylidene)-N2-(2-(propane-2ylideneamino)ethyl)-N2-((pyridin-2-yl)methyl)ethane-1,2-diamine (imine2apme) coordinated to iron(II) was obtained. This complex, [Fe(imine2apme)(NCS)2] (1), could be isolated and was structurally characterized. The molecular structure of 1 is presented in Fig. 1.
Due to the fact that it was not yet possible to obtain crystalline [Fe(apme)(NCS)2] (2), acetonitrile was used as alternative solvent for recrystallization. Here, a few crystals suitable for crystal structure analysis were obtained. Crystallographic characterization of those crystals examined showed that the target complex 2 has been formed. The molecular structure of 2 is presented in Fig. 2. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2. As already described for 1, complex 2 is coordinated by six nitrogen donor atoms, two of them belonging to the two thiocyanate ligands and four of them to the tripodal ligand. The iron– nitrogen bonds adjoining to the thiocyanate anions are shorter than those adjoining to the pyridines and the aliphatic amino groups. N–Fe–N angles between cis and trans nitrogen atoms deviate strongly from the values of 90° and 180°, thus complex 2 adopts a distorted octahedral geometry too. Three five membered chelate rings are formed in 2, as shown in Fig. 2. Nitrogen-iron– nitrogen bond angles of 1 are between 84.87° and 106.92° for cis-standing nitrogen donors, whereas bond angles of trans-standing nitrogen donors are between 153.85° and 171.19°. The order of bond lengths is 2.080 Å/2.162 Å for NCS-ligands, followed by the nearly equally long bonds to the aromatic (2.205 Å) and aliphatic (2.205/2.213/2.233 Å) nitrogen donor atoms. As observed for 1, only the fac isomer of 2 had crystallized (the amine nitrogen atom together with the two imine nitrogen atoms being considered one donor set). Analysis of the bulk compound unfortunately revealed that the sample is impure. One of the impurities was identified as the starting material, [Fe(py)4(NCS)2], by crystallographic characterization. Since the isolated material is impure no Mößbauer spectroscopy measurements were performed with this compound. Using other solvents such as dichloromethane did not improve the purity of the sample and here we only were able to recover crystalline starting material, [Fe(py)4(NCS)2]. 2.3. [Fe(HMe4apme)(NCS)3] (3)
Fig. 1. Molecular structure of [Fe(imine2apme)(NCS)2] (1).
To avoid the problem of forming imine functions in acetone, the tetra methylated derivative of apme, Me4apme (Scheme 1, R = Me), was used as a ligand. However, as described previously for related
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S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
Table 1 Selected crystallographic data of [Fe(imine2apme)(NCS)2] (1), [Fe(apme)(NCS)2] (2), [Fe(HMe4apme)(NCS)3] (3) and [Fe(Me4apme)(NCS)2] (4). Compound
1
2
3
4
Empiric formula Molecular weight Temperature (K) Crystal size (mm) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z qcalcd. (mg m 3) l (mm 1) F(0 0 0) Scan range h (°) Index ranges
C18H26FeN6S2 446.44 150(2) 0.48 0.24 0.22 orthorhombic Pna21 15.1739(2) 8.9232(1) 32.3826(6) 90 90 90 4384.60(11) 8 1.353 7.410 1872.25 5.14–62.57 11 6 h 6 17 10 6 k 6 10 37 6 l 6 31 16260 5618 0.0350 5618/1/495 0.898 R1 = 0.0313 wR2 = 0.0496 R1 = 0.0252 wR2 = 0.0487 0.170 and 0.216
C12H18FeN6S2 366.29 150(2) 0.39 0.37 0.20 monoclinic P21/c 7.9242(3) 13.2741(4) 15.4981(5) 90 92.338(3) 90 1628.84(9) 4 1.494 9.840 760 4.39–62.65 96h67 15 6 k 6 12 17 6 l 6 17 5520 2583 0.0476 2583/4/202 0.996 R1 = 0.0612 wR2 = 0.1358 R1 = 0.0528 wR2 = 0.1312 0.838 and 0.995
C18H31FeN7OS3 513.53 193(2) 0.16 0.08 0.08 monoclinic P21/c 10.4612(10) 16.9319(17) 14.2204(16) 90 100.284(12) 90 2478.4(4) 4 1.376 0.885 1080 2.54–28.08 13 6 h 6 13 22 6 k 6 22 18 6 l 6 18 21813 5620 0.0900 5620/0/285 0.922 R1 = 0.1134 wR2 = 0.1381 R1 = 0.0529 wR2 = 0.1133 0.643 and 0.766
C16H26FeN6S2 422.40 193(2) 0.52 0.48 0.24 monoclinic P21/n 9.769(2) 15.934(3) 13.057(3) 90 93.23(3) 90 2029.3(7) 4 1.383 0.960 888 2.45–28.15 21 6 h 6 20 22 6 k 6 22 17 6 l 6 17 18253 4870 0.0632 4870/0/230 0.905 R1 = 0.0546 wR2 = 0.0841 R1 = 0.0340 wR2 = 0.0788 0.358 and 0.457
Reflections collected Unique reflections Rint Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices (all data) R indices [I > 2r(I)] Largest difference in peak and hole (e Å
3
)
Table 2 Selected bond lengths [Å] and angles [°] for compounds 1–4. 1 Fe(1)–N(6) Fe(1)–N(5) Fe(1)–N(4) Fe(1)–N(2) Fe(1)–N(1) Fe(1)–N(3) N(6)–Fe(1)–N(5) N(6)–Fe(1)–N(4) N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(2) N(5)–Fe(1)–N(2) N(4)–Fe(1)–N(2) 3 Fe(1)–N(6) Fe(1)–N(7) Fe(1)–N(5) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(1) N(2)–C(6) N(3)–C(8) N(4)–C(13) N(6)–Fe(1)–N(7) N(6)–Fe(1)–N(5) N(7)–Fe(1)–N(5)
2 2.069(3) 2.142(3) 2.221(2) 2.236(3) 2.242(2) 2.252(3) 95.48(11) 114.66(11) 86.14(10) 90.76(11) 88.42(10) 154.39(9)
N(6)–Fe(1)–N(1) N(5)–Fe(1)–N(1) N(4)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(6)–Fe(1)–N(3) N(5)–Fe(1)–N(3) N(4)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(1)–Fe(1)–N(3)
164.11(10) 92.52(10) 79.52(9) 75.73(9) 94.19(11) 170.00(11) 92.16(9) 88.90(9) 77.49(10)
2.091(4) 2.108(3) 2.162(3) 2.231(3) 2.274(3) 2.297(3) 1.339(5) 1.472(5) 1.495(5) 97.16(14) 88.25(14) 101.30(13)
N(6)–Fe(1)–N(2) N(7)–Fe(1)–N(2) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(3) N(7)–Fe(1)–N(3) N(5)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(6)–Fe(1)–N(1) N(7)–Fe(1)–N(1) N(5)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(3)–Fe(1)–N(1)
167.58(13) 93.81(13) 83.88(13) 92.33(13) 87.78(12) 170.76(12) 93.92(12) 95.95(13) 162.78(13) 90.20(12) 74.54(11) 80.57(11)
copper complexes with uns-penp, the amine arm of the ligand was protonated in methanol and therefore did not coordinate to the metal ion [28]. Thus the corresponding iron complex with the mono protonated ligand and an additional thiocyanate anion as co-ligand, [Fe(HMe4apme)(NCS)3] (3), was obtained instead. The molecular structure of 3 is presented in Fig. 3. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2.
Fe(1)–N(5) Fe(1)–N(6) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(4) Fe(1)–N(1) N(5)–Fe(1)–N(6) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(2) N(5)–Fe(1)–N(3) N(6)–Fe(1)–N(3) N(2)–Fe(1)–N(3) 4 Fe(1)–N(5) Fe(1)–N(6) Fe(1)–N(1) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(4) N(5)–Fe(1)–N(6) N(5)–Fe(1)–N(1) N(6)–Fe(1)–N(1) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(2)
2.080(3) 2.162(4) 2.205(3) 2.205(4) 2.213(3) 2.233(3) 93.67(14) 106.92(13) 84.87(13) 91.67(13) 171.19(13) 86.88(12)
N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(4) N(2)–Fe(1)–N(4) N(3)–Fe(1)–N(4) N(5)–Fe(1)–N(1) N(6)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(3)–Fe(1)–N(1) N(4)–Fe(1)–N(1)
98.71(13) 88.50(14) 153.85(13) 97.62(14) 170.19(13) 95.90(13) 76.15(11) 79.11(12) 79.43(12)
2.041(2) 2.1072(19) 2.2133(16) 2.2156(18) 2.3001(18) 2.3499(18) 95.27(8) 166.86(7) 97.23(7) 90.68(8) 173.70(7)
N(1)–Fe(1)–N(2) N(5)–Fe(1)–N(3) N(6)–Fe(1)–N(3) N(1)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(4) N(1)–Fe(1)–N(4) N(2)–Fe(1)–N(4)
76.70(7) 104.08(9) 88.24(7) 80.39(6) 92.36(6) 97.32(9) 89.22(7) 78.85(6) 87.94(6)
The refinement furthermore shows that the complex molecule contains a methanol solvent molecule in the independent unit of the elementary cell which displayed hydrogen bonding to the protonated amine group. The NH and the OH hydrogen atoms were located and isotropically refined. All other hydrogen atoms were positioned geometrically and all non-hydrogen atoms were refined anisotropically.
S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
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Table 3 Range of N–Fe–N angles between cis and trans nitrogen atoms. Complex
Range of N–Fe–N angles between cis nitrogen atoms
Range of N–Fe–N angles between trans nitrogen atoms
[Fe(imine2apme)(NCS)2] [Fe(apme)(NCS)2] [Fe(HMe4apme)(NCS)3] [Fe(Me4apme)(NCS)2]
75.73(9)–114.66(11) 84.87(13)–106.92(13) 74.54(11)–101.30(13) 76.70(7)–104.08(9)
154.39(9)–170.00(11) 153.85(13)–171.19(13) 162.78(13)–170.76(12) 166.86(7)–173.70(7)
Table 4 Fe–C–N angles thiocyanate trans to aliphatic amino and pyridine group. Complex
Fe–C–N angle (thiocyanate trans to aliphatic amino group)
Fe–C–N angle (thiocyanate trans to pyridine group)
[Fe(imine2apme)(NCS)2] [Fe(apme)(NCS)2] [Fe(HMe4apme)(NCS)3] [Fe(Me4apme)(NCS)2]
164.11(10); 170.00(11) 170.19(13); 171.19(13) 162.78(13); 170.76(12) 166.86(7)
none none 167.58(13) 173.70(7)
Fig. 3. Molecular structure of [Fe(HMe4apme)(NCS)3] (3).
The N–Fe–N bond angles in 4 are between 76.70 and 104.08° for cis nitrogen donor atoms, whereas the bond angles of trans-standing nitrogen donor atoms are between 166.86° and 173.70°. As observed for the complexes described above, steric strain impedes the formation of angles of 90° and 180°, respectively, of a perfect octahedron. In contrast to 2, only the meridional isomer of 4 was crystallized. Most likely the steric strain of the methylene groups impedes crystallization of the fac isomer here. Only for the complexes 3 and 4 we found thiocyanate groups in trans position to the coordinated pyridine ring, as shown in Table 4.
2.5. Mößbauer spectroscopic characterization of [Fe(Me4apme)(NCS)2] (4) The crystal structure investigation of all these complexes at the respective temperatures unambiguously showed that the observed iron-nitrogen bond lengths are only consistent with the presence of an iron(II) high-spin state. We only could obtain complex 4 in analytically pure bulk amounts. Therefore, the spin state of complex 4 was investigated in more detail by 57Fe Mößbauer spectroscopy Fig. 2. Molecular structure of [Fe(apme)(NCS)2] (2).
The N–Fe–N bond angles of 3 are between 74.54° and 101.30° for cis nitrogen donor atoms, whereas the bond angles of trans nitrogen donor atoms are between 162.78° and 170.76°. The coordination sphere of the iron center again is best described as a distorted octahedron. Complex 3 forms two five-membered chelate rings. 2.4. [Fe(Me4apme)(NCS)2] (4) The protonation of the ligand Me4apme described above was a consequence of using methanol as a solvent. Therefore, a different solvent was needed for the synthesis of the complex [Fe(Me4apme)(NCS)2]. In contrast to the ligand apme, acetone could now be used because imine formation is not possible with Me4apme. Thus it was possible to obtain [Fe(Me4apme)(NCS)2] (4) in analytically pure form and the molecular structure of this complex is presented in Fig. 4. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2.
Fig. 4. Molecular structure of [Fe(Me4apme)(NCS)2] (4).
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4. Experimental 4.1. Materials and methods Reagents and solvents used were of commercially available reagent grade quality. The complex [Fe(py)4(NCS)2] was synthesized and characterized according to literature methods [35]. Preparation and handling of air-sensitive compounds was carried out in a glove box filled with argon (Braun, Garching, Germany; water and dioxygen less than 1 ppm) or using common Schlenk techniques. Commercially available anhydrous solvents were used that were distilled furthermore under inert conditions. Elemental analyses have been performed at the Institute for Organic Chemistry, University of Gießen. Fig. 5. Mößbauer spectra of [Fe(Me4apme)(NCS)2] (4) at 40 K and 298 K (the isomer shift in this figure is given relative to the radiation source).
4.2. Ligand synthesis The ligands were prepared and characterized according to literature methods [27,36].
at different temperatures. Fig. 5 shows the Mößbauer spectra at 40 K and 298 K. The spectrum obtained at room temperature reveals a doublet with an isomer shift dIS of 1.01 mm s 1 relative to an a-iron foil at room temperature and a quadrupole splitting DEQ of 1.65 mm s 1. These values agree with those obtained for other high-spin iron(II) complexes and support the earlier assignment of the spin state of the iron(II) ion in complex 4 based on the Fe–N bond lengths determined by X-ray structure analysis. Lowering the measurement temperature to 40 K resulted only in a moderate increase of the isomer shift dIS and the quadrupole splitting DEQ to 1.12 mm s 1 and 2.23 mm s 1, respectively. This increase of the values upon lowering the temperatures follows the usually observed temperature dependency of Mößbauer parameters [4]. There are no indications for the presence of a low-spin component at 40 K and therefore for an occurrence of a spin crossover transition. Thus, the iron ion in complex 4 remains in its high-spin state over the entire temperature range.
2.6. IR-spectroscopy Infrared data of the coordinated thiocyanate anions can provide important information on the spin state of the iron(II) ion and this implication in SCO behavior has been discussed in detail in the past [29–34]. IR data of the coordinated thiocyante ligands ions in [Fe (Me4apme)(NCS)2] (4) at room temperature confirm the high spin state of the iron ion.
3. Summary During our efforts to investigate if the replacement of two pyridyl donor arms with aliphatic amine groups in the tripodal ligand tpa would still afford SCO complexes we synthesized and structurally characterized [Fe(imine2apme)(NCS)2], [Fe(apme)(NCS)2], [Fe(HMe4apme)(NCS)3] and [Fe(Me4apme)(NCS)2]. Coordination environment of the iron(II) ion in all these complexes is best described as a distorted octahedron. Bond lengths from the molecular structures indicate that in all complexes the iron(II) ion is in a high spin state. Due to the impurity of the bulk material of [Fe(apme)(NCS)2] it was not possible to investigate this complex in more detail. However, Mößbauer spectroscopic investigations of [Fe (Me4apme)(NCS)2], where Me4apme is the tetra methylated derivative of apme, showed that variation of the tripodal ligand system from tpa to apme does not induce SCO properties at all in complexes with these types of ligands.
4.3. Procedures for the synthesis of iron(II) complexes with the ligands apme, HMe4apme, imine2apme, Me4apme The complexes were prepared according to a similar procedure [13]. 4.3.1. [Fe(imine2apme)(NCS)2] (1) A suspension of [Fe(py)4(NCS)2] (244 mg; 0.4 mmol) in acetone (5 mL) was added dropwise to a solution of the ligand apme (77 mg; 0.4 mmol) in acetone (5 mL) and stirred for 30 min. No precipitate was formed immediately, so the solution was added to 50 mL diethyl ether and yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. A yellow-green powder was obtained, containing 1/3 acetone per molecule according to the elemental analysis. Yield: 82 mg (46%). Anal. Calc. for FeC18.9H27,8N6O0.3S2: C, 48.94; H, 6.04; N; 18.12. Found: C, 49.42; H, 6.15; N; 18.73%. IR (KBr; cm 1): 3440.6, 2940.5, 2919.7, 2864.3, 2071.4 (NCS), 2061.9 (NCS), 1661.5, 1663.5, 1601.3. Yellow crystals suitable for X-ray characterization were obtained by slow diffusion of ether in a solution of acetone. 4.3.2. [Fe(apme)(NCS)2] (2) A suspension of [Fe(py)4(NCS)2] (251 mg; 0.5 mmol) in CH3CN (5 mL) was added dropwise to a solution of the ligand apme (97 mg; 0.5 mmol) in CH3CN (5 mL) and stirred for 15 min. No precipitate was formed immediately from the green solution, so the solution was added to 50 mL diethyl ether and an impure yellow and green powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. Crystals formed from re-crystallization from acetonitrile. 4.3.3. [Fe(HMe4apme)(NCS)3] (3) A solution of [Fe(py)4(NCS)2] (244 mg; 0.4 mmol) in CH3OH (5 mL) was added dropwise to a solution of the ligand Me4apme (100 mg; 0.4 mmol) in CH3OH (5 mL) and stirred for 30 min. After a few minutes yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. Crystals were obtained after re-crystallization from acetone. 4.3.4. [Fe(Me4apme)(NCS)2] (4) A suspension of [Fe(py)4(NCS)2] (251 mg; 0.5 mmol) in acetone (5 mL) was added dropwise to a solution of the ligand Me4apme (125 mg; 0.5 mmol) in acetone (5 mL) and stirred for 30 min. No precipitate from the orange solution was formed immediately, so
S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
the solution was added to 30 mL diethyl ether and yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. A yellow powder was obtained. Yield: 124 mg (59%). Anal. Calc. for FeC16H26N6S2: C, 45.90; H, 6.20; N; 19.90. Found: C, 45.97; H, 6.39; N; 19.96%. IR (KBr; cm 1): 3447.2, 2959.4, 2859.1, 2841.9, 2077.9 (NCS), 2055.2 (NCS), 1603.3. Yellow crystals suitable for X-ray characterization were obtained by slow diffusion of ether in a solution of acetone. 4.4. X-ray The X-ray crystallographic data of compounds 1–2 were collected on a Gemini S-Ultra single crystal CCD diffractometer from Oxford Diffraction equipped with a CryojetHT-temperature system. An Enhance Ultra Cu–X-ray source (k = 1.54184 Å) was used. The crystallographic data of complexes 3–4 were collected on a STOE IPDS-diffractometer equipped with a low temperature system (Karlsruher Glastechnisches Werk). Mo Ka radiation (k = 0.71069 Å) and a graphite monochromator was used. Table 1 summarizes the crystal parameters as well as some details of the data collections and the structure refinements of all crystals. Semiempirical absorption corrections from equivalents (Multiscan) were carried out with the data of 1 and 2 using the program SCALE 3 ABSPACK from the CRYSALISPRO program suite [37]; no absorption corrections were applied to the data sets of 3 and 4. The structures were solved by direct methods in SHELXS97 and refined by using full-matrix least squares in SHELXL97 [38,39]. All non-hydrogen atoms were refined anisotropically. The NH and OH hydrogen atoms in 2 and 3 were found by Fourier difference maps and refined isotropically. The positions of all other hydrogen atoms were calculated using a riding model with isotropic thermal parameters. The structure analyses show two independent complex molecules in the asymmetric unit of the elementary cell in 1 and a methanol solvent molecule in addition to the complex molecule in 3, respectively. 4.5. Mößbauer Mößbauer spectra were recorded using a conventional spectrometer of the Fa. Wissel GmbH in the constant acceleration mode. The temperature can be maintained between 6 K and 400 K by a closed-cycle cryostat unit of Advanced Research Systems Inc. The sample holder is mounted on the tip of the second stage heat station of the expander unit DE204SF inside a radiation shield and a vacuum shroud. The expander unit is decoupled from the vibrations of the compressor ARS-4HW by a DMX20-41 interface. The temperature is controlled by a Lakeshore 331S unit. The windows of the vaccuum shroud are made of mylar foils. The spectra were analyzed by least-square fits using a Lorentzian line shape with the program WinNormus-for-Igor Version 2.0. The isomer shifts are given relative to an a-iron at room temperature.
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Appendix A. Supplementary material CCDC 813778, 813779, 813780 and 813781 contain the supplementary crystallographic data for complexes 1, 2, 3 and 4, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. References [1] O. Kahn, J.P. Launay, Chemtronics, 3 (1988) 140. [2] J.-F. Létard, P. Guionneau, L. Gaux-Capes, Top. Curr. Chem. 235 (2004) 221. [3] L. Salmon, B. Donnadieu, A. Bousseksou, J.-P. Tuchagues, Acad. Sci. Ser. IIc (1999) 305. [4] P. Gütlich, E. Bill, A.X. Trautwein, Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Application, Springer Verlag, Berlin, 2010. [5] P.J. von Koningsbruggen, Y. Maeda, H. Oshio, Top. Curr. Chem. 233 (2004) 259. [6] A.A. Yousif, H. Winkler, H. Toftlund, A.X. Trautwein, R.H. Herber, J. Phys. Condens. Matter 1 (1989) 7103. [7] P. Gütlich, H.A. Goodwin, Top. Curr. Chem. 233 (2004) 1. [8] P. Gütlich, A. Hauser, H. Spiering, Angew. Chem., Int. Ed. 33 (1994) 2024. [9] H. Toftlund, J.J. McGarvey, Top. Curr. Chem. 233 (2004) 151. [10] H. Paulsen, H. Grünstreudel, W. Meyer-Klaucke, M. Gerdan, H.F. Grünsteudel, A.I. Chaumakov, R. Rüffer, H. Winkler, H. Toftlund, A.X. Trautwein, Eur. Phys. J. B 23 (2001) 463. [11] H. Toftlund, Coord. Chem. Rev. 94 (1989) 67. [12] F. Renz, H. Oshio, V. Ksenofontov, M. Waldeck, H. Spiering, P. Gütlich, Angew. Chem., Int. Ed. 39 (2000) 3699. [13] G.S. Matouzenko, A. Bousseksou, S. Lecoq, P.J. von Koningsbruggen, M. Perrin, O. Kahn, A. Collet, Inorg. Chem. 36 (1997) 2975. [14] G.S. Matouzenko, A. Bousseksou, S. Lecoq, P.J. von Koningsbruggen, M. Perrin, O. Kahn, A. Collet, Inorg. Chem. 36 (1997) 5869. [15] T. Buchen, H. Toftlund, P. Gütlich, Chem. Eur. J. 2 (1996) 1129. [16] Z. Yu, Y.F. Hsia, X.Z. You, H. Spering, P. Gütlich, J. Mater. Sci. 32 (1997) 6579. [17] J. Bernarding, G. Buntkowski, S. Macholl, S. Hartwig, M. Burghoff, L. Trams, J. Am. Chem. Soc. 128 (2006) 714. [18] H.A. Goodwin, Top. Curr. Chem. 233 (2004) 59. [19] K.S. Murray, C.J. Kepert, Top. Curr. Chem. 233 (2004) 195. [20] O. Sato, J. Tao, Y.-Z. Zhang, Angew. Chem., Int. Ed. 46 (2007) 2152. [21] A. Hauser, Adv. Polym. Sci. 233 (2004) 49. [22] B. Li, R.-J. Wei, J. Tao, R.-B. Huang, L.-S. Zheng, Inorg. Chem. 49 (2010) 745. [23] B. Li, R.-J. Wei, J. Tao, R.-B. Huang, L.-S. Zheng, Z. Zheng, J. Am. Chem. Soc. 132 (2010) 1558. [24] R.-J. Wei, B. Li, J. Tao, R.-B. Huang, L.-S. Zheng, Z. Zheng, Inorg. Chem. (2010), doi:10.1021/ic102231j. [25] G. Brehm, M. Reiher, B.L. Guennic, M. Leibold, S. Schindler, F.W. Heinemann, S. Schneider, J. Raman Spectrosc. 37 (2006) 108. [26] F. Højland, H. Toftlund, S. Yde-Andersen, Acta Chem. Scand. A 37 (1983) 251. [27] D. Utz, S. Kisslinger, F. Hampel, S. Schindler, J. Inorg. Biochem. 102 (2008) 1236. [28] J. Mandel, B. Douglas, Inorg. Chim. Acta 155 (1989) 55. [29] W.A. Baker, G.J. Long, Chem. Commun. 15 (1965) 368. [30] E. König, K. Majeda, Spectrochim. Acta A 23A (1967) 45. [31] E. König, Coord. Chem. Rev. 3 (1968) 471. [32] R.H. Herber, L.M. Casson, Inorg. Chem. 25 (1986) 847. [33] R.H. Herber, Inorg. Chem. 26 (1987) 173. [34] P. Gütlich, in: G.J. Long (Ed.), Mössbauer Spectroscopy Applied to Inorganic Chemistry, vol. 1, Plenum Press, New York, 1984. [35] N.E. Ericson, N. Sutin, Inorg. Chem. 5 (1966) 1834. [36] G.J.P. Britovsek, J. England, A.J.P. White, Inorg. Chem. 44 (2005) 8125. [37] CRYSALISPRO, 2.31, Bruker Analytical Instruments, 1999. [38] G.M. Sheldrick, Crystal Structure Solution, University of Göttingen, Germany, 1990. [39] G.M. Sheldrick, Crystal Structure Refinement, University of Göttingen, Germany, 1997.
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and properties of iron(II) complexes with a series of tripodal ligands based on the parent ligand tris(2-pyridylmethyl)amine Sandra Kisslinger a, Harald Kelm b, Alexander Beitat a, Christian Würtele a, Hans-Jörg Krüger b,⇑, Siegfried Schindler a,⇑ a b
Institut für Anorganische und Analytische Chemie, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 58, 35392 Gießen, Germany Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger Straße, Gebäude 54, 67663 Kaiserslautern, Germany
a r t i c l e
i n f o
Article history: Available online 10 March 2011 Dedicated to Professor Dr. Wolfgang Kaim on the occasion of his 60th birthday. Keywords: Iron complexes Tripodal ligands Spin-crossover Crystal structures Mößbauer spectroscopy
a b s t r a c t Iron(II) complexes of the type [Fe(L)(NCS)2] with the tripodal ligand apme (apme = N1-(2-aminoethyl)N1-(2-pyridyl-methyl)-1,2-ethanediamine) as well as with its derivatives were prepared and structurally characterized. The bond distances thus obtained showed that all complexes investigated were high-spin at the respective temperature. Furthermore [Fe(Me4apme)(NCS)2] was analyzed using Mößbauer spectroscopy that showed that this complex remains in its high-spin state over the entire temperature range. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In the last 20 years spin-crossover (SCO) properties have been studied extensively because of their possible future applications in molecular electronics [1–4]. Especially iron(II) complexes have been used in these studies. It is well known that low-spin (HS, 5 T2)–high-spin (LS, 1A1) transitions in iron(II) complexes can be induced by changes in pressure or temperature and by light irradiation [4–23]. Observations that the spin state is quite sensitive to small changes of the ligand field caused intensive research focusing on modifications of the ligands surrounding the metal ion. Iron(II) complexes of the tripodal ligand tris(2-pyridylmethyl)amine (tpa, Scheme 1; alternatively abbreviated as tmpa in the literature) – containing additional co-ligands such as thiocyanate – proved to be quite interesting and useful in these studies [10]. However, only most recently the crystal structure of [Fe(tpa)(NCS)2] has been reported [23,24]. Here it could be demonstrated furthermore, that solvent effects are quite important in regard to the SCO behavior of the complex. Increasing the chelate ring size of one of the chelate rings from 5 to 6 leads to the ligand N2,N2-bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine (pmea, Scheme 1). [Fe(pmea)(NCS)2] also showed SCO behavior (SCO behavior has not been observed in the original ⇑ Corresponding authors. E-mail address: [email protected] (S. Schindler). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.02.062
work on the pmea system) that has been studied previously by some of us [25,26]. Furthermore, Matouzenko et al. replaced one aromatic donor group in tpa by an aliphatic amine arm to obtain (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp, Scheme 1; alternatively abbreviated as DPEA in the literature) [13]. [Fe(unspenp)(NCS)2] exhibited a gradual SCO between 130 and 150 K. Furthermore, this group used (3-aminopropyl)bis(2-pyridylmethyl)amine (DPPA) as a ligand [14]. Compared with uns-penp/ DPEA the chelate ring size of the aliphatic amine arm in the according iron(II) complex was increased from 5 to 6. The resulting complex [Fe(DPPA)(NCS)2] crystallized in three polymorphs, where two of them exhibited SCO (a gradual SCO at about 150 K for one of the polymorphs and a very abrupt SCO at 114 K for the other one). Following up on these results we decided to investigate whether replacing one further pyridyl group by an aliphatic arm in the ligand could induce SCO properties in this type of iron(II) complexes. In principle we did not expect an improvement of the SCO behavior this way because it is well known that the ligand field strength is weakened when a pyridyl arm is replaced by an aliphatic amine arm. However, since it is well known that other effects like crystal packing, solvent molecules in crystals, counter ions can counter balance ligand field changes induced by the donor functions, it seemed to us that it would be worthwhile to actually examine these complexes. Thus we prepared and investigated iron(II) complexes with the ligand apme and its derivatives (Scheme 1).
S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
N
N N
N
N N
N
N
tpa
pmea
NR2 N
N N
N
NR2
N NR2
uns-penp (DPEA) R= H
apme
R=H
Scheme 1. Abbreviations used for tripodal ligands: tris(2-pyridylmethyl)amine (tpa), N2,N2-bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine (pmea), (2-aminoethyl)bis(2-pyridyl-methyl)amine (uns-penp, R = H), N1-(2-aminoethyl)-N1-(2-pyridyl-methyl)-1,2-ethanediamine (apme, R = H) as well as bis[2dimethylamino)ethyl]-(2-pyridylmethyl)amine (Me4apme, R = CH3).
541
Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2. Complex 1 crystallizes in a Pna21 orthorhombic space group. The iron(II) cation in 1 is coordinated by six nitrogen donor atoms. Two of them belong to the two thiocyanate ligands and four of them to the tripodal ligand. The iron–nitrogen bond adjoining to the thiocyanate anions are shorter than those adjoining to the pyridin rings and the aliphatic amino groups (see Table 2). This may be due to electrostatic attraction in the case of the thiocyanate anion as well as due to different hybridizations and p-acceptor abilities of the pyridine and the aliphatic amine groups. The coordination sphere of the iron center is best described as a distorted octahedron, because the N–Fe–N angles between the cis and trans nitrogen atoms deviate strongly from the values of 90° and 180° expected for an ideal octahedral environment (see Table 3). N– Fe–N bond angles of 1 are between 75.73° and 114.66° for cis nitrogen donors, whereas bond angles of trans nitrogen donors are between 154.39° and 170.00°. Further three five-membered chelate rings are formed in 1, as shown in Fig. 1. A facial coordination is observed (the amine nitrogen atom together with the two imine nitrogen atoms being considered one donor set) in contrast to 3 and 4 described below. No thiocyanate group is coordinated trans to the aromatic donor arm (Table 4).
2. Results and discussion 2.2. [Fe(apme)(NCS)2] (2) Complexes [Fe(L)(NCS)2] with L = tripodal ligand such as tpa or pmea can be prepared by reacting the respective ligand with the precursor complex [Fe(py)4(NCS)2] in methanol [25]. Yellow to orange powders were obtained by this method. Crystals for X-ray analysis were formed by slow diffusion of diethyl ether into a solution of these complexes in methanol. In contrast, so far attempts to obtain a crystalline iron apme complex this way were not successful. Therefore, the reaction was performed in acetone. 2.1. [Fe(imine2apme)(NCS)2] (1) In contrast to our previous investigations on copper complexes with apme [27], here an imine formation with the solvent acetone occurred instead and N1-(propane-2-ylidene)-N2-(2-(propane-2ylideneamino)ethyl)-N2-((pyridin-2-yl)methyl)ethane-1,2-diamine (imine2apme) coordinated to iron(II) was obtained. This complex, [Fe(imine2apme)(NCS)2] (1), could be isolated and was structurally characterized. The molecular structure of 1 is presented in Fig. 1.
Due to the fact that it was not yet possible to obtain crystalline [Fe(apme)(NCS)2] (2), acetonitrile was used as alternative solvent for recrystallization. Here, a few crystals suitable for crystal structure analysis were obtained. Crystallographic characterization of those crystals examined showed that the target complex 2 has been formed. The molecular structure of 2 is presented in Fig. 2. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2. As already described for 1, complex 2 is coordinated by six nitrogen donor atoms, two of them belonging to the two thiocyanate ligands and four of them to the tripodal ligand. The iron– nitrogen bonds adjoining to the thiocyanate anions are shorter than those adjoining to the pyridines and the aliphatic amino groups. N–Fe–N angles between cis and trans nitrogen atoms deviate strongly from the values of 90° and 180°, thus complex 2 adopts a distorted octahedral geometry too. Three five membered chelate rings are formed in 2, as shown in Fig. 2. Nitrogen-iron– nitrogen bond angles of 1 are between 84.87° and 106.92° for cis-standing nitrogen donors, whereas bond angles of trans-standing nitrogen donors are between 153.85° and 171.19°. The order of bond lengths is 2.080 Å/2.162 Å for NCS-ligands, followed by the nearly equally long bonds to the aromatic (2.205 Å) and aliphatic (2.205/2.213/2.233 Å) nitrogen donor atoms. As observed for 1, only the fac isomer of 2 had crystallized (the amine nitrogen atom together with the two imine nitrogen atoms being considered one donor set). Analysis of the bulk compound unfortunately revealed that the sample is impure. One of the impurities was identified as the starting material, [Fe(py)4(NCS)2], by crystallographic characterization. Since the isolated material is impure no Mößbauer spectroscopy measurements were performed with this compound. Using other solvents such as dichloromethane did not improve the purity of the sample and here we only were able to recover crystalline starting material, [Fe(py)4(NCS)2]. 2.3. [Fe(HMe4apme)(NCS)3] (3)
Fig. 1. Molecular structure of [Fe(imine2apme)(NCS)2] (1).
To avoid the problem of forming imine functions in acetone, the tetra methylated derivative of apme, Me4apme (Scheme 1, R = Me), was used as a ligand. However, as described previously for related
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S. Kisslinger et al. / Inorganica Chimica Acta 374 (2011) 540–545
Table 1 Selected crystallographic data of [Fe(imine2apme)(NCS)2] (1), [Fe(apme)(NCS)2] (2), [Fe(HMe4apme)(NCS)3] (3) and [Fe(Me4apme)(NCS)2] (4). Compound
1
2
3
4
Empiric formula Molecular weight Temperature (K) Crystal size (mm) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z qcalcd. (mg m 3) l (mm 1) F(0 0 0) Scan range h (°) Index ranges
C18H26FeN6S2 446.44 150(2) 0.48 0.24 0.22 orthorhombic Pna21 15.1739(2) 8.9232(1) 32.3826(6) 90 90 90 4384.60(11) 8 1.353 7.410 1872.25 5.14–62.57 11 6 h 6 17 10 6 k 6 10 37 6 l 6 31 16260 5618 0.0350 5618/1/495 0.898 R1 = 0.0313 wR2 = 0.0496 R1 = 0.0252 wR2 = 0.0487 0.170 and 0.216
C12H18FeN6S2 366.29 150(2) 0.39 0.37 0.20 monoclinic P21/c 7.9242(3) 13.2741(4) 15.4981(5) 90 92.338(3) 90 1628.84(9) 4 1.494 9.840 760 4.39–62.65 96h67 15 6 k 6 12 17 6 l 6 17 5520 2583 0.0476 2583/4/202 0.996 R1 = 0.0612 wR2 = 0.1358 R1 = 0.0528 wR2 = 0.1312 0.838 and 0.995
C18H31FeN7OS3 513.53 193(2) 0.16 0.08 0.08 monoclinic P21/c 10.4612(10) 16.9319(17) 14.2204(16) 90 100.284(12) 90 2478.4(4) 4 1.376 0.885 1080 2.54–28.08 13 6 h 6 13 22 6 k 6 22 18 6 l 6 18 21813 5620 0.0900 5620/0/285 0.922 R1 = 0.1134 wR2 = 0.1381 R1 = 0.0529 wR2 = 0.1133 0.643 and 0.766
C16H26FeN6S2 422.40 193(2) 0.52 0.48 0.24 monoclinic P21/n 9.769(2) 15.934(3) 13.057(3) 90 93.23(3) 90 2029.3(7) 4 1.383 0.960 888 2.45–28.15 21 6 h 6 20 22 6 k 6 22 17 6 l 6 17 18253 4870 0.0632 4870/0/230 0.905 R1 = 0.0546 wR2 = 0.0841 R1 = 0.0340 wR2 = 0.0788 0.358 and 0.457
Reflections collected Unique reflections Rint Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices (all data) R indices [I > 2r(I)] Largest difference in peak and hole (e Å
3
)
Table 2 Selected bond lengths [Å] and angles [°] for compounds 1–4. 1 Fe(1)–N(6) Fe(1)–N(5) Fe(1)–N(4) Fe(1)–N(2) Fe(1)–N(1) Fe(1)–N(3) N(6)–Fe(1)–N(5) N(6)–Fe(1)–N(4) N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(2) N(5)–Fe(1)–N(2) N(4)–Fe(1)–N(2) 3 Fe(1)–N(6) Fe(1)–N(7) Fe(1)–N(5) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(1) N(2)–C(6) N(3)–C(8) N(4)–C(13) N(6)–Fe(1)–N(7) N(6)–Fe(1)–N(5) N(7)–Fe(1)–N(5)
2 2.069(3) 2.142(3) 2.221(2) 2.236(3) 2.242(2) 2.252(3) 95.48(11) 114.66(11) 86.14(10) 90.76(11) 88.42(10) 154.39(9)
N(6)–Fe(1)–N(1) N(5)–Fe(1)–N(1) N(4)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(6)–Fe(1)–N(3) N(5)–Fe(1)–N(3) N(4)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(1)–Fe(1)–N(3)
164.11(10) 92.52(10) 79.52(9) 75.73(9) 94.19(11) 170.00(11) 92.16(9) 88.90(9) 77.49(10)
2.091(4) 2.108(3) 2.162(3) 2.231(3) 2.274(3) 2.297(3) 1.339(5) 1.472(5) 1.495(5) 97.16(14) 88.25(14) 101.30(13)
N(6)–Fe(1)–N(2) N(7)–Fe(1)–N(2) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(3) N(7)–Fe(1)–N(3) N(5)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(6)–Fe(1)–N(1) N(7)–Fe(1)–N(1) N(5)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(3)–Fe(1)–N(1)
167.58(13) 93.81(13) 83.88(13) 92.33(13) 87.78(12) 170.76(12) 93.92(12) 95.95(13) 162.78(13) 90.20(12) 74.54(11) 80.57(11)
copper complexes with uns-penp, the amine arm of the ligand was protonated in methanol and therefore did not coordinate to the metal ion [28]. Thus the corresponding iron complex with the mono protonated ligand and an additional thiocyanate anion as co-ligand, [Fe(HMe4apme)(NCS)3] (3), was obtained instead. The molecular structure of 3 is presented in Fig. 3. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2.
Fe(1)–N(5) Fe(1)–N(6) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(4) Fe(1)–N(1) N(5)–Fe(1)–N(6) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(2) N(5)–Fe(1)–N(3) N(6)–Fe(1)–N(3) N(2)–Fe(1)–N(3) 4 Fe(1)–N(5) Fe(1)–N(6) Fe(1)–N(1) Fe(1)–N(2) Fe(1)–N(3) Fe(1)–N(4) N(5)–Fe(1)–N(6) N(5)–Fe(1)–N(1) N(6)–Fe(1)–N(1) N(5)–Fe(1)–N(2) N(6)–Fe(1)–N(2)
2.080(3) 2.162(4) 2.205(3) 2.205(4) 2.213(3) 2.233(3) 93.67(14) 106.92(13) 84.87(13) 91.67(13) 171.19(13) 86.88(12)
N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(4) N(2)–Fe(1)–N(4) N(3)–Fe(1)–N(4) N(5)–Fe(1)–N(1) N(6)–Fe(1)–N(1) N(2)–Fe(1)–N(1) N(3)–Fe(1)–N(1) N(4)–Fe(1)–N(1)
98.71(13) 88.50(14) 153.85(13) 97.62(14) 170.19(13) 95.90(13) 76.15(11) 79.11(12) 79.43(12)
2.041(2) 2.1072(19) 2.2133(16) 2.2156(18) 2.3001(18) 2.3499(18) 95.27(8) 166.86(7) 97.23(7) 90.68(8) 173.70(7)
N(1)–Fe(1)–N(2) N(5)–Fe(1)–N(3) N(6)–Fe(1)–N(3) N(1)–Fe(1)–N(3) N(2)–Fe(1)–N(3) N(5)–Fe(1)–N(4) N(6)–Fe(1)–N(4) N(1)–Fe(1)–N(4) N(2)–Fe(1)–N(4)
76.70(7) 104.08(9) 88.24(7) 80.39(6) 92.36(6) 97.32(9) 89.22(7) 78.85(6) 87.94(6)
The refinement furthermore shows that the complex molecule contains a methanol solvent molecule in the independent unit of the elementary cell which displayed hydrogen bonding to the protonated amine group. The NH and the OH hydrogen atoms were located and isotropically refined. All other hydrogen atoms were positioned geometrically and all non-hydrogen atoms were refined anisotropically.
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Table 3 Range of N–Fe–N angles between cis and trans nitrogen atoms. Complex
Range of N–Fe–N angles between cis nitrogen atoms
Range of N–Fe–N angles between trans nitrogen atoms
[Fe(imine2apme)(NCS)2] [Fe(apme)(NCS)2] [Fe(HMe4apme)(NCS)3] [Fe(Me4apme)(NCS)2]
75.73(9)–114.66(11) 84.87(13)–106.92(13) 74.54(11)–101.30(13) 76.70(7)–104.08(9)
154.39(9)–170.00(11) 153.85(13)–171.19(13) 162.78(13)–170.76(12) 166.86(7)–173.70(7)
Table 4 Fe–C–N angles thiocyanate trans to aliphatic amino and pyridine group. Complex
Fe–C–N angle (thiocyanate trans to aliphatic amino group)
Fe–C–N angle (thiocyanate trans to pyridine group)
[Fe(imine2apme)(NCS)2] [Fe(apme)(NCS)2] [Fe(HMe4apme)(NCS)3] [Fe(Me4apme)(NCS)2]
164.11(10); 170.00(11) 170.19(13); 171.19(13) 162.78(13); 170.76(12) 166.86(7)
none none 167.58(13) 173.70(7)
Fig. 3. Molecular structure of [Fe(HMe4apme)(NCS)3] (3).
The N–Fe–N bond angles in 4 are between 76.70 and 104.08° for cis nitrogen donor atoms, whereas the bond angles of trans-standing nitrogen donor atoms are between 166.86° and 173.70°. As observed for the complexes described above, steric strain impedes the formation of angles of 90° and 180°, respectively, of a perfect octahedron. In contrast to 2, only the meridional isomer of 4 was crystallized. Most likely the steric strain of the methylene groups impedes crystallization of the fac isomer here. Only for the complexes 3 and 4 we found thiocyanate groups in trans position to the coordinated pyridine ring, as shown in Table 4.
2.5. Mößbauer spectroscopic characterization of [Fe(Me4apme)(NCS)2] (4) The crystal structure investigation of all these complexes at the respective temperatures unambiguously showed that the observed iron-nitrogen bond lengths are only consistent with the presence of an iron(II) high-spin state. We only could obtain complex 4 in analytically pure bulk amounts. Therefore, the spin state of complex 4 was investigated in more detail by 57Fe Mößbauer spectroscopy Fig. 2. Molecular structure of [Fe(apme)(NCS)2] (2).
The N–Fe–N bond angles of 3 are between 74.54° and 101.30° for cis nitrogen donor atoms, whereas the bond angles of trans nitrogen donor atoms are between 162.78° and 170.76°. The coordination sphere of the iron center again is best described as a distorted octahedron. Complex 3 forms two five-membered chelate rings. 2.4. [Fe(Me4apme)(NCS)2] (4) The protonation of the ligand Me4apme described above was a consequence of using methanol as a solvent. Therefore, a different solvent was needed for the synthesis of the complex [Fe(Me4apme)(NCS)2]. In contrast to the ligand apme, acetone could now be used because imine formation is not possible with Me4apme. Thus it was possible to obtain [Fe(Me4apme)(NCS)2] (4) in analytically pure form and the molecular structure of this complex is presented in Fig. 4. Crystallographic data and selected bond lengths and angles are given in Tables 1 and 2.
Fig. 4. Molecular structure of [Fe(Me4apme)(NCS)2] (4).
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4. Experimental 4.1. Materials and methods Reagents and solvents used were of commercially available reagent grade quality. The complex [Fe(py)4(NCS)2] was synthesized and characterized according to literature methods [35]. Preparation and handling of air-sensitive compounds was carried out in a glove box filled with argon (Braun, Garching, Germany; water and dioxygen less than 1 ppm) or using common Schlenk techniques. Commercially available anhydrous solvents were used that were distilled furthermore under inert conditions. Elemental analyses have been performed at the Institute for Organic Chemistry, University of Gießen. Fig. 5. Mößbauer spectra of [Fe(Me4apme)(NCS)2] (4) at 40 K and 298 K (the isomer shift in this figure is given relative to the radiation source).
4.2. Ligand synthesis The ligands were prepared and characterized according to literature methods [27,36].
at different temperatures. Fig. 5 shows the Mößbauer spectra at 40 K and 298 K. The spectrum obtained at room temperature reveals a doublet with an isomer shift dIS of 1.01 mm s 1 relative to an a-iron foil at room temperature and a quadrupole splitting DEQ of 1.65 mm s 1. These values agree with those obtained for other high-spin iron(II) complexes and support the earlier assignment of the spin state of the iron(II) ion in complex 4 based on the Fe–N bond lengths determined by X-ray structure analysis. Lowering the measurement temperature to 40 K resulted only in a moderate increase of the isomer shift dIS and the quadrupole splitting DEQ to 1.12 mm s 1 and 2.23 mm s 1, respectively. This increase of the values upon lowering the temperatures follows the usually observed temperature dependency of Mößbauer parameters [4]. There are no indications for the presence of a low-spin component at 40 K and therefore for an occurrence of a spin crossover transition. Thus, the iron ion in complex 4 remains in its high-spin state over the entire temperature range.
2.6. IR-spectroscopy Infrared data of the coordinated thiocyanate anions can provide important information on the spin state of the iron(II) ion and this implication in SCO behavior has been discussed in detail in the past [29–34]. IR data of the coordinated thiocyante ligands ions in [Fe (Me4apme)(NCS)2] (4) at room temperature confirm the high spin state of the iron ion.
3. Summary During our efforts to investigate if the replacement of two pyridyl donor arms with aliphatic amine groups in the tripodal ligand tpa would still afford SCO complexes we synthesized and structurally characterized [Fe(imine2apme)(NCS)2], [Fe(apme)(NCS)2], [Fe(HMe4apme)(NCS)3] and [Fe(Me4apme)(NCS)2]. Coordination environment of the iron(II) ion in all these complexes is best described as a distorted octahedron. Bond lengths from the molecular structures indicate that in all complexes the iron(II) ion is in a high spin state. Due to the impurity of the bulk material of [Fe(apme)(NCS)2] it was not possible to investigate this complex in more detail. However, Mößbauer spectroscopic investigations of [Fe (Me4apme)(NCS)2], where Me4apme is the tetra methylated derivative of apme, showed that variation of the tripodal ligand system from tpa to apme does not induce SCO properties at all in complexes with these types of ligands.
4.3. Procedures for the synthesis of iron(II) complexes with the ligands apme, HMe4apme, imine2apme, Me4apme The complexes were prepared according to a similar procedure [13]. 4.3.1. [Fe(imine2apme)(NCS)2] (1) A suspension of [Fe(py)4(NCS)2] (244 mg; 0.4 mmol) in acetone (5 mL) was added dropwise to a solution of the ligand apme (77 mg; 0.4 mmol) in acetone (5 mL) and stirred for 30 min. No precipitate was formed immediately, so the solution was added to 50 mL diethyl ether and yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. A yellow-green powder was obtained, containing 1/3 acetone per molecule according to the elemental analysis. Yield: 82 mg (46%). Anal. Calc. for FeC18.9H27,8N6O0.3S2: C, 48.94; H, 6.04; N; 18.12. Found: C, 49.42; H, 6.15; N; 18.73%. IR (KBr; cm 1): 3440.6, 2940.5, 2919.7, 2864.3, 2071.4 (NCS), 2061.9 (NCS), 1661.5, 1663.5, 1601.3. Yellow crystals suitable for X-ray characterization were obtained by slow diffusion of ether in a solution of acetone. 4.3.2. [Fe(apme)(NCS)2] (2) A suspension of [Fe(py)4(NCS)2] (251 mg; 0.5 mmol) in CH3CN (5 mL) was added dropwise to a solution of the ligand apme (97 mg; 0.5 mmol) in CH3CN (5 mL) and stirred for 15 min. No precipitate was formed immediately from the green solution, so the solution was added to 50 mL diethyl ether and an impure yellow and green powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. Crystals formed from re-crystallization from acetonitrile. 4.3.3. [Fe(HMe4apme)(NCS)3] (3) A solution of [Fe(py)4(NCS)2] (244 mg; 0.4 mmol) in CH3OH (5 mL) was added dropwise to a solution of the ligand Me4apme (100 mg; 0.4 mmol) in CH3OH (5 mL) and stirred for 30 min. After a few minutes yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. Crystals were obtained after re-crystallization from acetone. 4.3.4. [Fe(Me4apme)(NCS)2] (4) A suspension of [Fe(py)4(NCS)2] (251 mg; 0.5 mmol) in acetone (5 mL) was added dropwise to a solution of the ligand Me4apme (125 mg; 0.5 mmol) in acetone (5 mL) and stirred for 30 min. No precipitate from the orange solution was formed immediately, so
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the solution was added to 30 mL diethyl ether and yellow powder was obtained. The precipitate was filtered, washed with ether and dried under vacuum. A yellow powder was obtained. Yield: 124 mg (59%). Anal. Calc. for FeC16H26N6S2: C, 45.90; H, 6.20; N; 19.90. Found: C, 45.97; H, 6.39; N; 19.96%. IR (KBr; cm 1): 3447.2, 2959.4, 2859.1, 2841.9, 2077.9 (NCS), 2055.2 (NCS), 1603.3. Yellow crystals suitable for X-ray characterization were obtained by slow diffusion of ether in a solution of acetone. 4.4. X-ray The X-ray crystallographic data of compounds 1–2 were collected on a Gemini S-Ultra single crystal CCD diffractometer from Oxford Diffraction equipped with a CryojetHT-temperature system. An Enhance Ultra Cu–X-ray source (k = 1.54184 Å) was used. The crystallographic data of complexes 3–4 were collected on a STOE IPDS-diffractometer equipped with a low temperature system (Karlsruher Glastechnisches Werk). Mo Ka radiation (k = 0.71069 Å) and a graphite monochromator was used. Table 1 summarizes the crystal parameters as well as some details of the data collections and the structure refinements of all crystals. Semiempirical absorption corrections from equivalents (Multiscan) were carried out with the data of 1 and 2 using the program SCALE 3 ABSPACK from the CRYSALISPRO program suite [37]; no absorption corrections were applied to the data sets of 3 and 4. The structures were solved by direct methods in SHELXS97 and refined by using full-matrix least squares in SHELXL97 [38,39]. All non-hydrogen atoms were refined anisotropically. The NH and OH hydrogen atoms in 2 and 3 were found by Fourier difference maps and refined isotropically. The positions of all other hydrogen atoms were calculated using a riding model with isotropic thermal parameters. The structure analyses show two independent complex molecules in the asymmetric unit of the elementary cell in 1 and a methanol solvent molecule in addition to the complex molecule in 3, respectively. 4.5. Mößbauer Mößbauer spectra were recorded using a conventional spectrometer of the Fa. Wissel GmbH in the constant acceleration mode. The temperature can be maintained between 6 K and 400 K by a closed-cycle cryostat unit of Advanced Research Systems Inc. The sample holder is mounted on the tip of the second stage heat station of the expander unit DE204SF inside a radiation shield and a vacuum shroud. The expander unit is decoupled from the vibrations of the compressor ARS-4HW by a DMX20-41 interface. The temperature is controlled by a Lakeshore 331S unit. The windows of the vaccuum shroud are made of mylar foils. The spectra were analyzed by least-square fits using a Lorentzian line shape with the program WinNormus-for-Igor Version 2.0. The isomer shifts are given relative to an a-iron at room temperature.
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