Hyperfine Interact (2010) 198:229–241 DOI 10.1007/s10751-010-0179-2
Mössbauer spectra of two extended series of basic iron(III) carboxylates [Fe3 O(O2 CR)6 (H2 O)6 ]A (A− = ClO4− , NO3− ) Anastasia N. Georgopoulou · Yiannis Sanakis · Vassilis Psycharis · Catherine P. Raptopoulou · Athanassios K. Boudalis
Published online: 27 July 2010 © Springer Science+Business Media B.V. 2010
Abstract Two series of basic iron(III) carboxylates [Fe3 O(O2 CR)6 (H2 O)3 ]A were prepared, with R = CCl3 , CHBr2 , CH2 F, CH2 Cl, C(OH)Ph2 , H, Ph, (CH2 )3 Cl, Me, CHMe2 , Et, CMe3 . For the former series (1–12) A− = ClO4− and for the latter (13– 24) A− = NO3− . Complexes with R = CF3 were inaccessible for either counteranion, with all synthetic attempts leading to the butterfly complex [Fe4 O2 (O2 CCF3 )8 (H2 O)6 ]. Crystal structures of 1, 2, 5, 7, 9–12, 14, 16, 21 and 23 revealed the close structural similarity of the complexes. Mössbauer studies revealed very similar isomer shifts for all complexes in the region of 0.51–0.54 mm s−1 , and variable quadrupole splittings, ranging from 0.36 to 0.76 mm s−1 . Mössbauer studies of the complexes were carried out in frozen MeCN solutions in order to assess their stability in solution. All complexes proved to be stable in MeCN solutions, except complex 13 (R = CCl3 , A− = NO3− ), which dissociated to a butterfly-type complex. Keywords Cluster compounds · Iron · Mössbauer spectroscopy · Basic iron(III) carboxylates
1 Introduction Basic metal(II/III) carboxylates are complexes of the type [M3 O(O2 CR)6 (L)6 ]0/+ (Fig. 1), where L is a terminal monodentate neutral ligand and unequivocally constitute one of the best studied families of complexes [1]. Their easy and reproducible preparation and their thermodynamic stability, both in the solid state and in solution, render them ideal candidates for use as rigid building-blocks for supramolecular structures [2], and also for the study of various physicochemical phenomena, like magnetic exchange [3], redox reactions [4] and valence trapping/detrapping in mixed
A. N. Georgopoulou · Y. Sanakis · V. Psycharis · C. P. Raptopoulou · A. K. Boudalis (B) Institute of Materials Science, NCSR “Demokritos”, 153 10 Aghia Paraskevi, Attikis, Greece e-mail:
[email protected] URL: http://www.ims.demokritos.gr/people/tbou/index.html
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Fig. 1 Partially labeled POV-Ray plot of the cation of a basic iron(III) carboxylate
Table 1 Formulas of the complexes presented in this work (without solvent molecules)
Complex
Formula
1
[Fe3 O(O2 CCCl3 )6 (H2 O)3 ](ClO4 )
2
[Fe3 O(O2 CCHBr2 )6 (H2 O)3 ](ClO4 )
3
[Fe3 O(O2 CH2 F)6 (H2 O)3 ](ClO4 )
4
[Fe3 O(O2 CCH2 Cl)6 (H2 O)3 ](ClO4 )
5
[Fe3 O(O2 CC(OH)Ph2 )6 (H2 O)3 ](ClO4 )
6
[Fe3 O(O2 CH)6 (H2 O)3 ](ClO4 )
7
[Fe3 O(O2 CPh)6 (H2 O)3 ](ClO4 )
8
[Fe3 O(O2 C(CH2 )3 Cl)6 (H2 O)3 ](ClO4 )
9
[Fe3 O(O2 CMe)6 (H2 O)3 ](ClO4 )
10
[Fe3 O(O2 CCHMe2 )6 (H2 O)3 ](ClO4 )
11
[Fe3 O(O2 CEt)6 (H2 O)3 ](ClO4 )
12
[Fe3 O(O2 CCMe3 )6 (H2 O)3 ](ClO4 )
13
[Fe3 O(O2 CCCl3 )6 (H2 O)3 ](NO3 )
14
[Fe3 O(O2 CCHBr2 )6 (H2 O)3 ](NO3 )
15
[Fe3 O(O2 CH2 F)6 (H2 O)3 ](NO3 )
16
[Fe3 O(O2 CCH2 Cl)6 (H2 O)3 ](NO3 )
17
[Fe3 O(O2 CC(OH)Ph2 )6 (H2 O)3 ](NO3 )
18
[Fe3 O(O2 CH)6 (H2 O)3 ](NO3 )
19
[Fe3 O(O2 CPh)6 (H2 O)3 ](NO3 )
20
[Fe3 O(O2 C(CH2 )3 Cl)6 (H2 O)3 ](NO3 )
21
[Fe3 O(O2 CMe)6 (H2 O)3 ](NO3 )
22
[Fe3 O(O2 CCHMe2 )6 (H2 O)3 ](NO3 )
23
[Fe3 O(O2 CEt)6 (H2 O)3 ](NO3 )
24
[Fe3 O(O2 CCMe3 )6 (H2 O)3 ](NO3 )
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valence complexes [5]. Recently they have also been used as solid-state catalysts for carbon nanotubes production [6]. Iron carboxylate complexes are frequently encountered as the active sites of important enzymes such as hemerythrin, ribonucleotide reductase and methane monoxygenase and have been found to be dinuclear [7, 8]. However, more recent results have also implied the relevance of basic iron(III) carboxylates in the area of biology. It has been found that crystals of bacterial Ribonucleotide Reductase R2 protein, form basic iron(III) carboxylate clusters on the protein’s surface when subjected to iron soaking [9]. It is possible that this finding illustrates the first steps of biomineralization leading to the formation of ferritin, the iron-storage protein of many organisms in the microbe, plant and animal kingdoms. Despite the importance of this family of complexes, a systematic study of their Mössbauer properties is lacking. Although basic iron carboxylates have been prepared with a variety of carboxylate ligands (R), counteranions (A− ) and terminal ligands (L), these synthetic parameters vary independently, with no systematic study of each of them. Here, we describe our Mössbauer studies on two series of basic iron(III) carboxylate complexes [Fe3 O(O2 CR)6 (H2 O)3 ]A comprising twelve members each (Table 1). In this study L is always H2 O, whereas A− is either ClO4− or NO3− , thus making R the only variable synthetic parameter. For better identification of the complexes, single-crystal structural determinations were carried out on any complex that could be obtained in a crystalline form. In this work we attempt to provide an extended and systematic body of experimental data, useful for eventual theoretical studies that will aim at the better understanding of factors influencing Mössbauer spectra.
2 Experimental 2.1 Materials All reagents and solvents were of analytical grade and used as received, except for not commercially available sodium carboxylates. These were prepared from the 1:1 reaction of the respective carboxylic acid with NaHCO3 in H2 O and evaporation to dryness. All syntheses were carried out under aerobic conditions. Warning Although no such tendency was observed during the current work, perchlorate salts are potentially explosive and should be handled with caution and in small quantities. 2.2 Syntheses of [Fe3 O(O2 CR)6 (H2 O)3 ](ClO4 ) (L = H2 O, A− = ClO4− ) Solid NaO2 CR (8.00 mmol) [R = Cl3 C (1.483 g) (1), R = CHBr2 (1.918 g) (2), R = CH2 F (0.801 g) (3), R = CH2 Cl (0.932 g) (4), R = C(OH)Ph2 (2.002 g) (5), R = H (0.544 g) (6), R = Ph (1.153 g) (7), R = Cl(CH2 )3 (1.156 g) (8), R = Me (0.656 g) (9), R = CHMe2 (0.880 g) (10), R = Et (0.768 g) (11), R = Me3 C (0.993 g) (12)] was added to a solution of Fe(ClO4 )3 · 9H2 O (2.065 g, 4 mmol) in MeCN (30 ml). The solution changed from light orange to dark brown-red and was left for slow evaporation. Brown-red crystals of 1·3MeCN·H2 O, 2·MeCN·H2 O,
/
/
/
/
/
Formula Fw T(K) Wavelength (Å) (Radiation) Crystal system Space group a (Å) b (Å) c (Å) α (◦ ) β (◦ ) γ (◦ ) V (Å3 ) Z ρc (g cm−3 ) μ (mm−1 ) 2max (◦ ) Reflections collected/ unique Reflections used/ parameters Reflections with I > 2σ (I) R1, wR2a (all) R1, wR2a (obs.) 6748 0.0404, 0.1008 0.0396, 0.1001
6839 0.0726, 0.1667 0.0625, 0.1579
11.947(4) 13.462(4) 18.254(6) 72.73(1) 81.22(1) 67.15(1) 2581.1(1) 2 1.869 1.884 48.36 8525/8077 [Rint = 0.0105] 8077/609
C14 H17 O21 NBr12 ClFe3 1697.21 180 1.54178 (Cu Kα) Monoclinic P21 /n 10.0971(2) 22.6603(4) 17.9265(3) 90 90.846(1) 90 4101.2(1) 4 2.749 23.375 130 46362/6925 [Rint = 0.0815] 6925/523
2·MeCN·H2 O
C18 H17 Cl19 Fe3 N3 O21 1452.45 298 0.71073 (Mo Kα) Triclinic P1¯
1·3MeCN·H2 O
10997 0.0649, 0.1514 0.0491, 0.1382
14.152(6) 14.198(6) 25.03(2) 89.89(2) 73.72(2) 69.05(2) 4481.29(4) 2 1.392 0.591 48.62 14377/13736 [Rint = 0.0161] 13736/1440
C90 H87 O29 N3 ClFe3 1877.63 298 0.71073 (Mo Kα) Triclinic P1¯
5·3MeCN·3H2 O
7896 0.0539, 0.1293 0.0489, 0.1250
10.8724(2) 13.6848(2) 19.3139(3) 108.582(1) 90.559(1) 90.561(1) 2723.46(8) 2 1.447 7.431 130 36993/8884 [Rint = 0.0709] 8884/808
C48 H45 O20 N3 ClFe3 1186.87 180 1.54178 (Cu Kα) Triclinic P1¯
7·3MeCN
Table 2 Crystallographic data for complexes 1·3MeCN·H2 O, 2·MeCN·H2 O, 5·3MeCN·3H2 O, 7·3MeCN and 9·MeCN 9·MeCN
4434 0.0396, 0.0908 0.0335, 0.0870
C14 H27 O20 NClFe3 732.37 298 0.71073(Mo Kα) Monoclinic P21 /n 13.366(4) 16.116(5) 14.767(5) 90 115.75(1) 90 2865.0(2) 4 1.698 1.675 50.00 5234/5028 [Rint = 0.0170] 5028/377
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Table 3 Crystallographic data for complexes 10·MeCN·H2 O, 11·0.25H2 O and 12·2MeCN·H2 O Formula Fw T(K) Wavelength (Å) (Radiation) Crystal system Space group a (Å) b (Å) c (Å) α (◦ ) β (◦ ) γ (◦ ) V (Å3 ) Z ρc (g cm−3 ) μ (mm−1 ) 2max (◦ ) Reflections collected/unique Reflections used/ parameters Reflections with I >2σ (I) R1, wR2a (all) R1, wR2a (obs.) /
/
/
/
/
10·MeCN·H2 O
11·0.25H2 O
12·2MeCN·H2 O
C26 H53 O21 NClFe3 918.69 180 1.54178 (Cu Kα) Triclinic P1¯
C18 H36.5 ClFe3 O20.25 779.97 298 0.71073(Mo Kα) Tetragonal I 4¯
12.1361(2) 13.9125(2) 14.6817(2) 116.254(1) 96.010(1) 102.955(1) 2106.85(5) 2 1.448 9.414 127.98 25061/6607 [Rint = 0.0812] 6607/531
22.572(7) 22.572(7) 13.230(4) 90 90 90 6741(4) 8 1.537 1.428 50.02 5988/5682 [Rint = 0.0179] 5682/397
C34 H68 ClFe3 N2 O21 1043.90 298 0.71073(Mo Kα) Monoclinic C2/c 19.208(9) 14.476(8) 39.39(2) 90 94.10(2) 90 10925(10) 8 1.269 0.901 48.78 8825/8520 [Rint = 0.0188] 8520/628
5446 0.0566, 0.1361 0.0467, 0.1246
5209 0.0536, 0.1259 0.0473, 0.1203
6533 0.0798, 0.1702 0.0558, 0.1475
3, 4, 5·3MeCN·3H2 O, 6, 7·3MeCN, 8, 9·MeCN, 10·MeCN·H2 O, 11·0.25H2 O and 12·2MeCN·H2 O formed after few days, which were filtered off and dried in vacuo. The yields were 1: 0.69 g, (∼41%), 2: 1.01 g, (∼47%), 3: 0.36 g, (∼35%), 4: 0.41 g, (∼35%), 5: 0.26 g, (∼12%), 6: 0.24 g, (∼31%), 7: 0.44 g, (∼32%), 8: 0.23 g, (∼17%), 9: 0.30 g, (∼34%), 10: 0.93 g, (∼83%), 11: 0.14 g, (∼13%), 12: 0.19 g, (∼15%). Elemental analyses of vacuum-dried solids confirmed the formulas of the fully desolvated complexes (see Supporting Information). 2.3 Syntheses of [Fe3 O(O2 CR)6 (H2 O)3 ](NO3 ) (L = H2 O, A− = NO3− ) Method A Solid NaO2 CR (8.00 mmol) [R = CHBr2 (1.918 g) (14), R = CH2 F (0.801 g) (15), R = CH2 Cl (0.932 g) (16), R = Ph (1.153 g) (19), R = Me (0.656 g) (21), R = CHMe2 (0.880 g) (22), R = Et (0.768 g) (23), R = CMe3 (0.993 g) (24)] was added to a slurry solution of Fe(NO3 )3 ·9H2 O (1.616 g, 4.00 mmol) in MeCN (20 ml). The solution changed from light orange to dark orange and stirred under reflux overnight. After cooling, the solution was filtered off and left for slow evaporation. Brown-red crystals of 14·MeCN·H2 O, 16·3.5H2 O, 19, 21·CH3 COOH and 23·HNO3 and microcrystalline solid of 15, 22 and 24 were formed after few days. Those were collected by decantation of the mother liquor and dried in vacuo. The yields were 14: 0.38 g, (∼18%), 15: 0.17 g, (∼17%), 16: 0.18 g, (∼16%), 19: 0.56 g, (∼42%), 21: 0.57 g, (∼61%), 22: 0.16 g, (∼15%), 23: 0.16 g, (∼14%), 24: 0.18 g, (∼15%). Elemental analyses of vacuum-dried solids confirmed the formulas
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Table 4 Crystallographic data for complexes 14·MeCN·H2 O, 16·3.5H2 O, 21·CH3 COOH and 23·HNO3 Formula Fw T(K) Wavelength (Å) (Radiation) Crystal system Space group a (Å) b (Å) c (Å) α (◦ ) β (◦ ) γ (◦ ) V (Å3 ) Z ρc (g cm−3 ) μ (mm−1 ) 2max (◦ ) Reflections collected/ unique Reflections used/ parameters Reflections with I >2σ (I) R1, wR2a (all) R1, wR2a (obs.) /
/
/
/
/
14·MeCN·H2 O
16·3.5H2 O
21·CH3 COOH
23·HNO3
C14 H17 O20 N2 Br12 Fe3 1659.77 180 1.54178 (Cu Kα)
C12 H25 O22.5 NCl6 Fe3 923.58 180 1.54178 (Cu Kα)
C14 H28 O21 NFe3 713.92 298 0.71073(Mo Kα)
C18 H37 O22 N2 Fe3 801.05 180 1.54178 (Cu Kα)
Monoclinic P21 /n 10.0998(1) 22.5635(4) 17.6117(3) 90 90.001(1) 90 4013.5(1) 4 2.747 23.251 130 26649/6720 [Rint = 0.0998]
Monoclinic P21 /c 12.3958(2) 14.6693(2) 17.7284(3) 90 96.294(1) 90 3204.26(9) 4 1.915 16.172 129.96 20945/5325 [Rint = 0.0796]
Monoclinic P21 /c 11.815(4) 14.729(5) 15.210(5) 90 90.88(1) 90 2646.6(2) 4 1.792 1.716 50.02 4888/4649 [Rint = 0.0101]
Monoclinic I2/m 14.8744(2) 13.2370(2) 18.2556(3) 90 106.971(1) 90 3437.86(9) 4 1.548 10.783 130 11512/2997 [Rint = 0.0548]
6720/498
5325/496
4649/427
2997/264
6316
4646
3943
2491
0.0558, 0.1362 0.0532, 0.1341
0.0591, 0.1414 0.0526, 0.1358
0.0396, 0.0812 0.0306, 0.0761
0.0566, 0.1504 0.0483, 0.1382
of the fully desolvated complexes, except for the acetate complex which analyzed as 21•MeCO2 H (see Supporting Information). Method B Solid Fe(NO3 )3 ·9H2 O (1.616 g, 4.00 mmol) was added to a colorless solution of RCOOH (8.00 mmol) [R = CCl3 (1.307 g) (13), R = C(OH)Ph2 (0.228 g) (17), R = H (0.368 g) (18), R = Cl(CH2 )3 (0.980 g) (20)] in MeCN (20 ml). The solution changed to dark brown-red and was left for slow evaporation. After few days, the brown-red microcrystalline solids of 13, 17, 18 and 20 which formed, were filtered off and dried in vacuo. The yields were 13: 0.50 g, (∼30%), 17: 0.34 g, (∼16%), 18: 0.10 g, (∼13%), 20: 0.78 g, (∼58%). Elemental analyses of vacuum-dried solids confirmed the formulas of the fully desolvated complexes (see Supporting Information). 2.4 X-ray crystallography Diffraction measurements for 1·3MeCN·H2 O, 5·3MeCN·3H2 O, 9·MeCN, 11·0.25 H2 O, 12·2MeCN·H2 O and 21·CH3 COOH were made on a Crystal Logic Dual Goniometer diffractometer using graphite monochromated Mo Kα radiation. Unit cell dimensions were determined and refined by using the angular settings of
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25 automatically centered reflections in the range 11 < 2θ < 23◦ . Intensity data were recorded using a θ–2θ scan. Three standard reflections monitored every 97 reflections showed less than 3% variation and no decay. Lorentz, polarization and psi-scan absorption corrections (for 1·3MeCN·H2 O, 9·MeCN, 12·2MeCN·H2 O and 21·CH3 COOH) were applied using Crystal Logic software. Diffraction measurements for 2·MeCN·H2 O, 7·3MeCN, 10·MeCN·H2 O, 14·MeCN·H2 O, 16·3.5·H2 O, 21·MeCO2 H and 23·HNO3 were made on a Rigaku R-AXIS SPIDER Image Plate diffractometer using graphite monochromated Cu Kα radiation. Data collection (ω-scans) and processing (cell refinement, data reduction and Empirical absorption correction) were performed using the CrystalClear program package [10]. The structure was solved by direct methods using SHELXS-97 [11] and refined by full-matrix least-squares techniques on F2 with SHELXL-97 [12]. Important crystallographic data are listed in Tables 2, 3 and 4. POV-Ray plots of the molecules were drawn using the Diamond 3 program package [13]. 2.5 Physical measurements Elemental analysis for carbon, hydrogen, and nitrogen was performed on a PerkinElmer 2400/II automatic analyzer. Mössbauer spectra were collected with a constant acceleration spectrometer using a 57 Co (Rh) source at RT and a variabletemperature Oxford cryostat. Solution spectra were collected from frozen MeCN solutions of approximate volume of 1 mL and concentrations of 0.04–0.1 M. The solutions of the complexes were flash-frozen in liquid N2 . Spectra were fitted to Lorentzian lines assuming a single asymmetric quadrupole-split doublet.
3 Results and discussion 3.1 Syntheses For the preparation of basic iron(III) carboxylates we employed MeCN as the solvent for several reasons: (1) It gave perchlorate salts of excellent crystallinity. The crystallinity was not as good in the case of the nitrate salts, nevertheless we also obtained several crystalline nitrate salts. (2) Preparation in a weakly coordinating solvent provided complexes with H2 O as the sole terminal ligand in all coordination sites. (3) The complexes were highly soluble and stable in MeCN, which gave us the possibility to prepare concentrated and stable solutions of the complexes for Mössbauer studies in solution. For the perchlorate salts, simple reactions on a 1:2 ratio were carried out between iron(III) perchlorate and the respective sodium carboxylate, by adding the solid carboxylate salt into the iron(III) perchlorate solution. Depending on the solubility of the salts, in several cases evaporation should proceed very near to dryness before solids precipitated. The extremely high solubility of some products was the reason for reduced yields in an otherwise quantitative reaction. However, this drawback was more than compensated by the previously mentioned advantages of MeCN. It was found that this synthetic approach was not possible for the nitrate salts, since iron(III) nitrate is virtually insoluble in MeCN. For this reason, a suspension of iron(III) nitrate and the sodium carboxylate was refluxed, and the filtrate was
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Table 5 Mössbauer parameters of perchlorate salts of various basic iron(III) carboxylates in the solid state Complex (R)
T (K)
δ (mm s−1 )a
EQ (mm s−1 )
+ (mm s−1 )b
− / +
1 (CCl3 ) 2 (CHBr2 ) 3 (CH2 F) 4 (CH2 Cl) 5 [CPh2 (OH)] 6 (H) 7 (Ph) 8 [(CH2 )3 Cl] 9 (Me) 10 (CHMe2 ) 11 (Et) 12 (CMe3 )
78 78 78 78 78 78 78 78 78 78 78 78
0.53(1) 0.54(1) 0.53(1) 0.52(1) 0.53(1) 0.53(1) 0.53(1) 0.53(1) 0.53(1) 0.54(1) 0.53(1) 0.53(1)
0.69(1) 0.59(1) 0.52(1) 0.49(1) 0.63(1) 0.48(1) 0.40(1) 0.55(1) 0.53(1) 0.51(1) 0.60(1) 0.36(1)
0.18(1) 0.17(1) 0.18(1) 0.18(1) 0.21(1) 0.19(1) 0.25(1) 0.19(1) 0.20(1) 0.20(1) 0.16(1) 0.19(1)
0.98(1) 1.09(1) 0.98(1) 0.95(1) 1.01(1) 0.98(1) 0.87(1) 1.08(3) 1.04(1) 1.04(3) 1.02(1) 0.95(1)
a Referenced b Half-width
to metallic iron foil at 293 K at half-maximum
allowed to slowly evaporate (Method A). It is crucial to point out that if the iron(III) nitrate was added first, a small amount of an insoluble ferric product formed. This was avoided by adding the carboxylate salt in the solvent first. Although this approach was adequate for most complexes in the series, for certain carboxylate salts [R = CCl3 (13), C(OH)Ph2 (17), H (18) and (CH2 )3 Cl (20)] it led to intractable oily products after complete evaporation. In these cases it was proven that use of the respective carboxylic acid yielded the required complex (Method B). Attempts to prepare the respective trifluoroacetato complexes were unsuccessful, with the reaction system leading to [Fe4 O2 (O2 CCF3 )8 (H2 O)6 ] irrespective of whether perchlorates or nitrates were used as counteranions. The product was characterized by X-ray crystallography and Mössbauer spectroscopy (Fig. S1). Additional attempts to collect spectra of basic iron(III) trifluoroacetate were made by preparing 1:2 FeIII /− O2 CCF3 solutions in MeCN. However, the formation of the tetranuclear complex persisted even in solution, as indicated by comparing the Mössbauer spectra. 3.2 Mössbauer spectroscopy 3.2.1 Solid state studies The 78 K Mössbauer spectra of the studied complexes exhibit quadrupole-split doublets of varying symmetries. Fits of the data were carried out assuming a single doublet, allowing for a variable − / + ratio to account for the asymmetries of the doublets. Key Mössbauer parameters are given in Tables 5 and 6 and indicative spectra are shown in Fig. 2. Isomer shifts were found to vary very little around 0.51– 0.54 mm s−1 (78 K). Variable-temperature studies of selected complexes showed a decrease of the isomer shift upon heating, attributed to second-order Doppler effects [14]. Interestingly however, quadrupole splittings exhibit large deviations between different complexes, depending on the carboxylate used. Duncan et al. [15] and Long et al. [16] had carried out Mössbauer studies on two similar series of complexes, observing similar variations in EQ values. The recorded linewidths are consistent with the ones observed for this type of clusters [16, 17].
Mössbauer spectra of extended series of iron(III) carboxylates
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Table 6 Mössbauer parameters of nitrate salts of various basic iron(III) carboxylates in the solid state Complex (R)
T (K)
δ (mm s−1 )a
EQ (mm s−1 )
+ (mm s−1 )b
− / +
13 (CCl3 ) 14 (CHBr2 ) 15 (CH2 F) 16 (CH2 Cl) 17 [CPh2 (OH)] 18 (H) 19 (Ph) 20 [(CH2 )3 Cl] 21 (Me) 22 (CHMe2 ) 23 (Et) 24 (CMe3 )
78 78 78 78 78 78 78 90 78 78 78 78
0.53(1) 0.53(1) 0.52(1) 0.53(1) 0.51(1) 0.52(1) 0.53(1) 0.51(1) 0.53(1) 0.52(1) 0.53(1) 0.53(1)
0.76(1) 0.71(1) 0.70(1) 0.67(1) 0.65(1) 0.53(1) 0.52(1) 0.58(1) 0.44(1) 0.64(1) 0.66(1) 0.53(1)
0.19(1) 0.21(1) 0.23(1) 0.20(1) 0.20(1) 0.21(1) 0.24(1) 0.21(1) 0.26(1) 0.19(1) 0.17(1) 0.18(1)
1.00(1) 1.06(7) 0.99(1) 0.87(1) 0.99(3) 0.93(1) 0.88(1) 0.98(3) 0.98(2) 0.99(1) 1.07(1) 1.01(1)
a Referenced b Half-width
to metallic iron foil at 293 K at half-maximum
Fig. 2 Indicative spectra of complexes 12 and 13 at 78 K
3.2.2 Solution studies In order to assess the stability of the complexes in solution, we studied their Mössbauer spectra in flash-frozen MeCN solutions (Tables S1 and S2). To assess the
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Fig. 3 78 K Mössbauer spectra of dibromoacetate (R = CHBr2 ) complexes 2 (perchlorate, top) and 14 (nitrate, bottom) in the solid state and in frozen MeCN solutions. The vertical lines indicate the positions of the singlet peaks of the smallest EQ doublet (solid perchlorate). The arrows indicate the reversibility of the dissolution-drying process in the identity of the products
reversibility of the dissolution process, we allowed the MeCN solutions to evaporate and remeasured the resulting solids for selected complexes (Figs. 3 and 4). The spectra of these solids were identical to those of the original solids, confirming that the basic iron(III) carboxylates in question are stable in MeCN solutions. The only exception to this rule was complex 13 (R = CCl3 , A− = NO3− ). Whereas dissolution of complex 1 (R = CCl3 , A− = ClO4− ) yielded a stable solution of the triferric complex, dissolution of 13 did not. Instead, the solution spectrum of 13 (shown in Fig. S2) appeared very similar to that of [Fe4 O2 (O2 CCF3 )8 (H2 O)6 ], consisting of two nested quadrupole-split doublets of approximately equal intensities, which we attribute to the formation of a butterfly complex containing the {Fe4 O2 }8+ core (see Fig. S1). It is noteworthy that a similar behavior was only observed in the case of the trifluoroacetate complexes.
4 Conclusions During this work we prepared and studied the Mössbauer properties of two extended series of basic iron(III) carboxylates. In each series we varied only one parameter, i.e. the carboxylate ligand, while the counteranion (ClO4− or NO3− ) and terminal ligand (H2 O) were “fixed” parameters. From our synthetic endeavours we concluded that trifluoroacetates exhibit a distinctly different behavior from the other carboxylates
Mössbauer spectra of extended series of iron(III) carboxylates
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Fig. 4 78 K Mössbauer spectra of benzoate (R = Ph) complexes 7 (perchlorate, top) and 19 (nitrate, bottom) in the solid state and in frozen MeCN solutions. The vertical lines indicate the positions of the singlet peaks of the smallest EQ doublet (solid perchlorate). The arrows indicate the reversibility of the dissolution-drying process in the identity of the products
we tested, not leading to the respective cationic basic iron(III) carboxylate, but to a neutral butterfly complex, irrespective of the counteranion A− . The isomer shift is a direct measure of the electron density at the iron nucleus which depends on the bonding properties of the cluster. The average isomer shift at 78 K is 0.53 mm s−1 and this value is typical for FeIII (S = 5/2) in an octahedral environment comprising O/N ligands. The same average value is found in the series of complexes studied by Duncan et al. [15] and Long et al. [16], in smaller series of complexes. The narrow distribution (±0.01 mm s−1 ) around 0.53 mm s−1 suggests that the bonding properties of basic carboxylates do not depend on the nature of the carboxylate ligand but exhibit a relatively well “conserved” trend. On the contrary, quadrupole splittings show a strong dependence on the carboxylate ligand ranging from 0.36 to 0.76 mm s−1 . A similar trend had been observed in the series of complexes studied by Duncan et al. [15] and Long et al. [16], although in much smaller series of complexes. The main contribution in the quadrupole splitting in FeIII (S = 5/2) complexes arises from the asymmetry of the lattice around the ferric ion. We have considered several geometrical elements that could account for this variation, i.e. the Fe–Ooxo , Fe–OH2O and Fe–Ocarboxylate , distances. No systematic and statistically reliable correlation with the quadrupole splitting along the series is found. Apparently, other electronic properties intrinsically related
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to each carboxylate ligand are responsible for this variation and detailed theoretical calculations are required to elucidate the relevant mechanism. Regarding the solution (MeCN) behaviour of the complexes, it was found that, except for complex 13 (R = CCl3 , A− = NO3− ), which we assume dissociates to a butterfly-type structure, all complexes are stable upon dissolution in MeCN. Although their isomer shifts remain practically unchanged, their quadrupole splittings exhibit variations in the frozen solution. This is probably due to solid-state effects (e.g. packing) whose influence diminishes upon dissolution. A final comment should be made on the reversibility of this process; drying of the solutions led to solids with identical parameters to those of the initially studied complexes. This attests to both the stability of the basic iron(III) carboxylates in solution and to the reversibility of the process. Supporting Information Crystallographic data for 1, 2, 5, 7, 9–12, 14, 16, 21 and 23 in CIF format have been deposited at the Cambridge Crystallographic Data Centre as CCDC 754790–754801. Elemental analyses for all complexes; Fig. S1 showing the 78 K Mössbauer spectrum of complex [Fe4 O2 (O2 CCF3 )8 (H2 O)6 ]; Fig. S2 showing the 78 K Mössbauer spectrum of a frozen MeCN solution of complex 13; Tables S1 and S2 showing the Mössbauer parameters of the complexes (1–12 and 13–24, respectively) in frozen MeCN solutions.
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