mössbauer spectra of two extended series of basic iron(iii ... · hyperfine interact (2010)...

Hyperfine Interact (2010) 198:229–241DOI 10.1007/s10751-010-0179-2

Mössbauer spectra of two extended series of basiciron(III) carboxylates [Fe3O(O2CR)6(H2O)6]A(A− = ClO −

4 , NO −3 )

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 [Fe3O(O2CR)6(H2O)3]A wereprepared, with R = CCl3, CHBr2, CH2F, CH2Cl, C(OH)Ph2, H, Ph, (CH2)3Cl, Me,CHMe2, Et, CMe3. For the former series (1–12) A− = ClO −

4 and for the latter (13–24) A− = NO −

3 . Complexes with R = CF3 were inaccessible for either counteran-ion, with all synthetic attempts leading to the butterfly complex [Fe4O2(O2CCF3)8

(H2O)6]. Crystal structures of 1, 2, 5, 7, 9–12, 14, 16, 21 and 23 revealed theclose structural similarity of the complexes. Mössbauer studies revealed very similarisomer shifts for all complexes in the region of 0.51–0.54 mm s−1, and variablequadrupole splittings, ranging from 0.36 to 0.76 mm s−1. Mössbauer studies of thecomplexes were carried out in frozen MeCN solutions in order to assess their stabilityin solution. All complexes proved to be stable in MeCN solutions, except complex 13(R = CCl3, A− = NO −

3 ), 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 [M3O(O2CR)6(L)6]0/+(Fig. 1), where L is a terminal monodentate neutral ligand and unequivocally consti-tute one of the best studied families of complexes [1]. Their easy and reproduciblepreparation and their thermodynamic stability, both in the solid state and in solution,render them ideal candidates for use as rigid building-blocks for supramolecularstructures [2], and also for the study of various physicochemical phenomena, likemagnetic 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, Greecee-mail: [email protected]: http://www.ims.demokritos.gr/people/tbou/index.html

230 A.N. Georgopoulou et al.

Fig. 1 Partially labeledPOV-Ray plot of the cation ofa basic iron(III) carboxylate

Table 1 Formulas of thecomplexes presented in thiswork (without solventmolecules)

Complex Formula

1 [Fe3O(O2CCCl3)6(H2O)3](ClO4)

2 [Fe3O(O2CCHBr2)6(H2O)3](ClO4)

3 [Fe3O(O2CH2F)6(H2O)3](ClO4)

4 [Fe3O(O2CCH2Cl)6(H2O)3](ClO4)

5 [Fe3O(O2CC(OH)Ph2)6(H2O)3](ClO4)

6 [Fe3O(O2CH)6(H2O)3](ClO4)

7 [Fe3O(O2CPh)6(H2O)3](ClO4)

8 [Fe3O(O2C(CH2)3Cl)6(H2O)3](ClO4)

9 [Fe3O(O2CMe)6(H2O)3](ClO4)

10 [Fe3O(O2CCHMe2)6(H2O)3](ClO4)

11 [Fe3O(O2CEt)6(H2O)3](ClO4)

12 [Fe3O(O2CCMe3)6(H2O)3](ClO4)

13 [Fe3O(O2CCCl3)6(H2O)3](NO3)

14 [Fe3O(O2CCHBr2)6(H2O)3](NO3)

15 [Fe3O(O2CH2F)6(H2O)3](NO3)

16 [Fe3O(O2CCH2Cl)6(H2O)3](NO3)

17 [Fe3O(O2CC(OH)Ph2)6(H2O)3](NO3)

18 [Fe3O(O2CH)6(H2O)3](NO3)

19 [Fe3O(O2CPh)6(H2O)3](NO3)

20 [Fe3O(O2C(CH2)3Cl)6(H2O)3](NO3)

21 [Fe3O(O2CMe)6(H2O)3](NO3)

22 [Fe3O(O2CCHMe2)6(H2O)3](NO3)

23 [Fe3O(O2CEt)6(H2O)3](NO3)

24 [Fe3O(O2CCMe3)6(H2O)3](NO3)

Mössbauer spectra of extended series of iron(III) carboxylates 231

valence complexes [5]. Recently they have also been used as solid-state catalysts forcarbon nanotubes production [6]. Iron carboxylate complexes are frequently encoun-tered as the active sites of important enzymes such as hemerythrin, ribonucleotidereductase 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 bacterialRibonucleotide Reductase R2 protein, form basic iron(III) carboxylate clusters onthe protein’s surface when subjected to iron soaking [9]. It is possible that this findingillustrates the first steps of biomineralization leading to the formation of ferritin, theiron-storage protein of many organisms in the microbe, plant and animal kingdoms.

Despite the importance of this family of complexes, a systematic study of theirMössbauer properties is lacking. Although basic iron carboxylates have been pre-pared with a variety of carboxylate ligands (R), counteranions (A−) and terminalligands (L), these synthetic parameters vary independently, with no systematic studyof each of them.

Here, we describe our Mössbauer studies on two series of basic iron(III)carboxylate complexes [Fe3O(O2CR)6(H2O)3]A comprising twelve members each(Table 1). In this study L is always H2O, whereas A− is either ClO −

4 or NO −3 ,

thus making R the only variable synthetic parameter. For better identification of thecomplexes, single-crystal structural determinations were carried out on any complexthat could be obtained in a crystalline form. In this work we attempt to provide anextended and systematic body of experimental data, useful for eventual theoreticalstudies that will aim at the better understanding of factors influencing Mössbauerspectra.

2 Experimental

2.1 Materials

All reagents and solvents were of analytical grade and used as received, except fornot commercially available sodium carboxylates. These were prepared from the 1:1reaction of the respective carboxylic acid with NaHCO3 in H2O and evaporation todryness. All syntheses were carried out under aerobic conditions.

Warning Although no such tendency was observed during the current work, perchlo-rate salts are potentially explosive and should be handled with caution and in smallquantities.

2.2 Syntheses of [Fe3O(O2CR)6(H2O)3](ClO4) (L = H2O, A− = ClO −4 )

Solid NaO2CR (8.00 mmol) [R = Cl3C (1.483 g) (1), R = CHBr2 (1.918 g) (2),R = CH2F (0.801 g) (3), R = CH2Cl (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 = Me3C(0.993 g) (12)] was added to a solution of Fe(ClO4)3 · 9H2O (2.065 g, 4 mmol) inMeCN (30 ml). The solution changed from light orange to dark brown-red andwas left for slow evaporation. Brown-red crystals of 1·3MeCN·H2O, 2·MeCN·H2O,


Tab

le2

Cry

stal

logr

aphi

cda

tafo

rco

mpl

exes

1·3M

eCN

·H2O

,2·M

eCN

·H2O

,5·3M

eCN

·3H2O

,7·3M

eCN

and

9·MeC

N

1·3M

eCN

·H2O

2·MeC

N·H

2O

5·3M

eCN

·3H2O

7·3M

eCN

9·MeC

N

For

mul

aC

18H

17C

l 19F

e 3N

3O

21C

14H

17O

21N

Br 1

2C

lFe 3

C90

H87

O29

N3C

lFe 3

C48

H45

O20

N3C

lFe 3

C14

H27

O20

NC

lFe 3

Fw

1452

.45

1697

.21

1877

.63

1186

.87

732.

37T

(K)

298

180

298

180

298

Wav

elen

gth

(Å/

)(R

adia

tion)

0.71

073

(Mo

Kα)

1.54

178

(Cu

Kα)

0.71

073

(Mo

Kα)

1.54

178

(Cu

Kα)

0.71

073(

Mo

Kα)

Cry

stal

syst

emT

ricl

inic

Mon

oclin

icT

ricl

inic

Tri

clin

icM

onoc

linic

Spac

egr

oup

P1̄

P2 1

/nP

1̄P

1̄P

2 1/n

a(Å

/

)11

.947

(4)

10.0

971(

2)14

.152

(6)

10.8

724(

2)13

.366

(4)

b(Å

/

)13

.462

(4)

22.6

603(

4)14

.198

(6)

13.6

848(

2)16

.116

(5)

c(Å

/

)18

.254

(6)

17.9

265(

3)25

.03(

2)19

.313

9(3)

14.7

67(5

)α

(◦)

72.7

3(1)

9089

.89(

2)10

8.58

2(1)

90β

(◦)

81.2

2(1)

90.8

46(1

)73

.72(

2)90

.559

(1)

115.

75(1

)γ

(◦)

67.1

5(1)

9069

.05(

2)90

.561

(1)

90V

(Å/

3)

2581

.1(1

)41

01.2

(1)

4481

.29(

4)27

23.4

6(8)

2865

.0(2

)Z

24

22

4ρ

c(g

cm−3

)1.

869

2.74

91.

392

1.44

71.

698

μ(m

m−1

)1.

884

23.3

750.

591

7.43

11.

675

2�m

ax(◦

)48

.36

130

48.6

213

050

.00

Ref

lect

ions

colle

cted

/85

25/8

077

4636

2/69

2514

377/

1373

636

993/

8884

5234

/502

8un

ique

[Rin

t=

0.01

05]

[Rin

t=

0.08

15]

[Rin

t=

0.01

61]

[Rin

t=

0.07

09]

[Rin

t=

0.01

70]

Ref

lect

ions

used

/80

77/6

0969

25/5

2313

736/

1440

8884

/808

5028

/377

para

met

ers

Ref

lect

ions

wit

hI

>2σ

(I)

6839

6748

1099

778

9644

34R

1,w

R2a

(all)

0.07

26,0

.166

70.

0404

,0.1

008

0.06

49,0

.151

40.

0539

,0.1

293

0.03

96,0

.090

8R

1,w

R2a

(obs

.)0.

0625

,0.1

579

0.03

96,0

.100

10.

0491

,0.1

382

0.04

89,0

.125

00.

0335

,0.0

870


Table 3 Crystallographic data for complexes 10·MeCN·H2O, 11·0.25H2O and 12·2MeCN·H2O

10·MeCN·H2O 11·0.25H2O 12·2MeCN·H2O

Formula C26H53O21NClFe3 C18H36.5ClFe3O20.25 C34H68ClFe3N2O21

Fw 918.69 779.97 1043.90T(K) 180 298 298Wavelength (Å

/

) (Radiation) 1.54178 (Cu Kα) 0.71073(Mo Kα) 0.71073(Mo Kα)

Crystal system Triclinic Tetragonal MonoclinicSpace group P1̄ I4̄ C2/ca (Å

/

) 12.1361(2) 22.572(7) 19.208(9)b (Å

/

) 13.9125(2) 22.572(7) 14.476(8)c (Å

/

) 14.6817(2) 13.230(4) 39.39(2)α (◦) 116.254(1) 90 90β (◦) 96.010(1) 90 94.10(2)γ (◦) 102.955(1) 90 90V (Å

/

3) 2106.85(5) 6741(4) 10925(10)Z 2 8 8ρc (g cm−3) 1.448 1.537 1.269μ (mm−1) 9.414 1.428 0.9012�max (◦) 127.98 50.02 48.78Reflections 25061/6607 5988/5682 8825/8520

collected/unique [Rint = 0.0812] [Rint = 0.0179] [Rint = 0.0188]Reflections used/ 6607/531 5682/397 8520/628

parametersReflections with I >2σ (I) 5446 5209 6533R1, wR2a (all) 0.0566, 0.1361 0.0536, 0.1259 0.0798, 0.1702R1, wR2a (obs.) 0.0467, 0.1246 0.0473, 0.1203 0.0558, 0.1475

3, 4, 5·3MeCN·3H2O, 6, 7·3MeCN, 8, 9·MeCN, 10·MeCN·H2O, 11·0.25H2O and12·2MeCN·H2O 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%). Elementalanalyses of vacuum-dried solids confirmed the formulas of the fully desolvatedcomplexes (see Supporting Information).

2.3 Syntheses of [Fe3O(O2CR)6(H2O)3](NO3) (L = H2O, A− = NO −3 )

Method A Solid NaO2CR (8.00 mmol) [R = CHBr2 (1.918 g) (14), R = CH2F(0.801 g) (15), R = CH2Cl (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)3·9H2O (1.616 g, 4.00 mmol) inMeCN (20 ml). The solution changed from light orange to dark orange and stirredunder reflux overnight. After cooling, the solution was filtered off and left for slowevaporation. Brown-red crystals of 14·MeCN·H2O, 16·3.5H2O, 19, 21·CH3COOHand 23·HNO3 and microcrystalline solid of 15, 22 and 24 were formed after fewdays. 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


Table 4 Crystallographic data for complexes 14·MeCN·H2O, 16·3.5H2O, 21·CH3COOH and23·HNO3

14·MeCN·H2O 16·3.5H2O 21·CH3COOH 23·HNO3

Formula C14H17O20N2Br12Fe3 C12H25O22.5NCl6Fe3 C14H28O21NFe3 C18H37O22N2Fe3

Fw 1659.77 923.58 713.92 801.05T(K) 180 180 298 180Wavelength (Å

/

) 1.54178 (Cu Kα) 1.54178 (Cu Kα) 0.71073(Mo Kα) 1.54178 (Cu Kα)

(Radiation)Crystal system Monoclinic Monoclinic Monoclinic MonoclinicSpace group P21/n P21/c P21/c I2/ma (Å

/

) 10.0998(1) 12.3958(2) 11.815(4) 14.8744(2)b (Å

/

) 22.5635(4) 14.6693(2) 14.729(5) 13.2370(2)c (Å

/

) 17.6117(3) 17.7284(3) 15.210(5) 18.2556(3)α (◦) 90 90 90 90β (◦) 90.001(1) 96.294(1) 90.88(1) 106.971(1)γ (◦) 90 90 90 90V (Å

/

3) 4013.5(1) 3204.26(9) 2646.6(2) 3437.86(9)Z 4 4 4 4ρc (g cm−3) 2.747 1.915 1.792 1.548μ (mm−1) 23.251 16.172 1.716 10.7832�max (◦) 130 129.96 50.02 130Reflections 26649/6720 20945/5325 4888/4649 11512/2997

collected/ [Rint = 0.0998] [Rint = 0.0796] [Rint = 0.0101] [Rint = 0.0548]unique

Reflections used/ 6720/498 5325/496 4649/427 2997/264parameters

Reflections with 6316 4646 3943 2491I >2σ (I)

R1, wR2a (all) 0.0558, 0.1362 0.0591, 0.1414 0.0396, 0.0812 0.0566, 0.1504R1, wR2a (obs.) 0.0532, 0.1341 0.0526, 0.1358 0.0306, 0.0761 0.0483, 0.1382

of the fully desolvated complexes, except for the acetate complex which analyzed as21•MeCO2H (see Supporting Information).

Method B Solid Fe(NO3)3·9H2O (1.616 g, 4.00 mmol) was added to a colorlesssolution 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). Thesolution changed to dark brown-red and was left for slow evaporation. After fewdays, the brown-red microcrystalline solids of 13, 17, 18 and 20 which formed, werefiltered 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-driedsolids confirmed the formulas of the fully desolvated complexes (see SupportingInformation).

2.4 X-ray crystallography

Diffraction measurements for 1·3MeCN·H2O, 5·3MeCN·3H2O, 9·MeCN, 11·0.25H2O, 12·2MeCN·H2O and 21·CH3COOH were made on a Crystal Logic DualGoniometer diffractometer using graphite monochromated Mo Kα radiation. Unitcell dimensions were determined and refined by using the angular settings of


25 automatically centered reflections in the range 11 < 2θ < 23◦. Intensity datawere recorded using a θ–2θ scan. Three standard reflections monitored every 97reflections showed less than 3% variation and no decay. Lorentz, polarization andpsi-scan absorption corrections (for 1·3MeCN·H2O, 9·MeCN, 12·2MeCN·H2O and21·CH3COOH) were applied using Crystal Logic software. Diffraction measure-ments for 2·MeCN·H2O, 7·3MeCN, 10·MeCN·H2O, 14·MeCN·H2O, 16·3.5·H2O,21·MeCO2H and 23·HNO3 were made on a Rigaku R-AXIS SPIDER Image Platediffractometer using graphite monochromated Cu Kα radiation. Data collection(ω-scans) and processing (cell refinement, data reduction and Empirical absorptioncorrection) were performed using the CrystalClear program package [10]. The struc-ture was solved by direct methods using SHELXS-97 [11] and refined by full-matrixleast-squares techniques on F2 with SHELXL-97 [12]. Important crystallographicdata are listed in Tables 2, 3 and 4. POV-Ray plots of the molecules were drawnusing the Diamond 3 program package [13].

2.5 Physical measurements

Elemental analysis for carbon, hydrogen, and nitrogen was performed on aPerkinElmer 2400/II automatic analyzer. Mössbauer spectra were collected with aconstant acceleration spectrometer using a 57Co (Rh) source at RT and a variable-temperature Oxford cryostat. Solution spectra were collected from frozen MeCNsolutions of approximate volume of 1 mL and concentrations of 0.04–0.1 M. Thesolutions of the complexes were flash-frozen in liquid N2. Spectra were fitted toLorentzian 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 thesolvent for several reasons: (1) It gave perchlorate salts of excellent crystallinity. Thecrystallinity was not as good in the case of the nitrate salts, nevertheless we alsoobtained several crystalline nitrate salts. (2) Preparation in a weakly coordinatingsolvent provided complexes with H2O as the sole terminal ligand in all coordinationsites. (3) The complexes were highly soluble and stable in MeCN, which gave usthe possibility to prepare concentrated and stable solutions of the complexes forMössbauer studies in solution.

For the perchlorate salts, simple reactions on a 1:2 ratio were carried out betweeniron(III) perchlorate and the respective sodium carboxylate, by adding the solidcarboxylate salt into the iron(III) perchlorate solution. Depending on the solubilityof the salts, in several cases evaporation should proceed very near to dryness beforesolids precipitated. The extremely high solubility of some products was the reasonfor reduced yields in an otherwise quantitative reaction. However, this drawbackwas 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 suspensionof iron(III) nitrate and the sodium carboxylate was refluxed, and the filtrate was


Table 5 Mössbauer parameters of perchlorate salts of various basic iron(III) carboxylates in thesolid state

Complex (R) T (K) δ (mm s−1)a �EQ (mm s−1) �+ (mm s−1)b �−/�+1 (CCl3) 78 0.53(1) 0.69(1) 0.18(1) 0.98(1)2 (CHBr2) 78 0.54(1) 0.59(1) 0.17(1) 1.09(1)3 (CH2F) 78 0.53(1) 0.52(1) 0.18(1) 0.98(1)4 (CH2Cl) 78 0.52(1) 0.49(1) 0.18(1) 0.95(1)5 [CPh2(OH)] 78 0.53(1) 0.63(1) 0.21(1) 1.01(1)6 (H) 78 0.53(1) 0.48(1) 0.19(1) 0.98(1)7 (Ph) 78 0.53(1) 0.40(1) 0.25(1) 0.87(1)8 [(CH2)3Cl] 78 0.53(1) 0.55(1) 0.19(1) 1.08(3)9 (Me) 78 0.53(1) 0.53(1) 0.20(1) 1.04(1)10 (CHMe2) 78 0.54(1) 0.51(1) 0.20(1) 1.04(3)11 (Et) 78 0.53(1) 0.60(1) 0.16(1) 1.02(1)12 (CMe3) 78 0.53(1) 0.36(1) 0.19(1) 0.95(1)aReferenced to metallic iron foil at 293 KbHalf-width 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. Thiswas avoided by adding the carboxylate salt in the solvent first.

Although this approach was adequate for most complexes in the series, for certaincarboxylate salts [R = CCl3 (13), C(OH)Ph2 (17), H (18) and (CH2)3Cl (20)] it ledto intractable oily products after complete evaporation. In these cases it was proventhat use of the respective carboxylic acid yielded the required complex (Method B).

Attempts to prepare the respective trifluoroacetato complexes were unsuccess-ful, with the reaction system leading to [Fe4O2(O2CCF3)8(H2O)6] irrespective ofwhether perchlorates or nitrates were used as counteranions. The product was char-acterized by X-ray crystallography and Mössbauer spectroscopy (Fig. S1). Additionalattempts to collect spectra of basic iron(III) trifluoroacetate were made by preparing1:2 FeIII/−O2CCF3 solutions in MeCN. However, the formation of the tetranuclearcomplex 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-splitdoublets of varying symmetries. Fits of the data were carried out assuming a singledoublet, allowing for a variable �−/�+ ratio to account for the asymmetries of thedoublets. Key Mössbauer parameters are given in Tables 5 and 6 and indicativespectra 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 adecrease of the isomer shift upon heating, attributed to second-order Doppler effects[14]. Interestingly however, quadrupole splittings exhibit large deviations betweendifferent complexes, depending on the carboxylate used. Duncan et al. [15] and Longet al. [16] had carried out Mössbauer studies on two similar series of complexes,observing similar variations in �EQ values. The recorded linewidths are consistentwith the ones observed for this type of clusters [16, 17].


Table 6 Mössbauer parameters of nitrate salts of various basic iron(III) carboxylates in the solidstate

Complex (R) T (K) δ (mm s−1)a �EQ (mm s−1) �+ (mm s−1)b �−/�+13 (CCl3) 78 0.53(1) 0.76(1) 0.19(1) 1.00(1)14 (CHBr2) 78 0.53(1) 0.71(1) 0.21(1) 1.06(7)15 (CH2F) 78 0.52(1) 0.70(1) 0.23(1) 0.99(1)16 (CH2Cl) 78 0.53(1) 0.67(1) 0.20(1) 0.87(1)17 [CPh2(OH)] 78 0.51(1) 0.65(1) 0.20(1) 0.99(3)18 (H) 78 0.52(1) 0.53(1) 0.21(1) 0.93(1)19 (Ph) 78 0.53(1) 0.52(1) 0.24(1) 0.88(1)20 [(CH2)3Cl] 90 0.51(1) 0.58(1) 0.21(1) 0.98(3)21 (Me) 78 0.53(1) 0.44(1) 0.26(1) 0.98(2)22 (CHMe2) 78 0.52(1) 0.64(1) 0.19(1) 0.99(1)23 (Et) 78 0.53(1) 0.66(1) 0.17(1) 1.07(1)24 (CMe3) 78 0.53(1) 0.53(1) 0.18(1) 1.01(1)aReferenced to metallic iron foil at 293 KbHalf-width at half-maximum

Fig. 2 Indicative spectra ofcomplexes 12 and 13 at 78 K

3.2.2 Solution studies

In order to assess the stability of the complexes in solution, we studied theirMössbauer spectra in flash-frozen MeCN solutions (Tables S1 and S2). To assess the


Fig. 3 78 K Mössbauer spectraof dibromoacetate (R =CHBr2) complexes 2(perchlorate, top) and 14(nitrate, bottom) in the solidstate and in frozen MeCNsolutions. The vertical linesindicate the positions of thesinglet peaks of the smallest�EQ doublet (solidperchlorate). The arrowsindicate the reversibility of thedissolution-drying process inthe identity of the products

reversibility of the dissolution process, we allowed the MeCN solutions to evaporateand remeasured the resulting solids for selected complexes (Figs. 3 and 4). Thespectra of these solids were identical to those of the original solids, confirming thatthe basic iron(III) carboxylates in question are stable in MeCN solutions. The onlyexception to this rule was complex 13 (R = CCl3, A− = NO −

3 ). Whereas dissolutionof complex 1 (R = CCl3, A− = ClO −

4 ) yielded a stable solution of the triferriccomplex, dissolution of 13 did not. Instead, the solution spectrum of 13 (shown inFig. S2) appeared very similar to that of [Fe4O2(O2CCF3)8(H2O)6], consisting oftwo nested quadrupole-split doublets of approximately equal intensities, which weattribute to the formation of a butterfly complex containing the {Fe4O2}8+ core (seeFig. S1). It is noteworthy that a similar behavior was only observed in the case of thetrifluoroacetate complexes.

4 Conclusions

During this work we prepared and studied the Mössbauer properties of two extendedseries of basic iron(III) carboxylates. In each series we varied only one parameter, i.e.the carboxylate ligand, while the counteranion (ClO −

4 or NO −3 ) and terminal ligand

(H2O) were “fixed” parameters. From our synthetic endeavours we concluded thattrifluoroacetates exhibit a distinctly different behavior from the other carboxylates


Fig. 4 78 K Mössbauer spectraof benzoate (R = Ph)complexes 7 (perchlorate, top)and 19 (nitrate, bottom) in thesolid state and in frozenMeCN solutions. The verticallines indicate the positions ofthe singlet peaks of thesmallest �EQ doublet (solidperchlorate). The arrowsindicate the reversibility of thedissolution-drying process inthe identity of the products

we tested, not leading to the respective cationic basic iron(III) carboxylate, but to aneutral butterfly complex, irrespective of the counteranion A−.

The isomer shift is a direct measure of the electron density at the iron nucleuswhich depends on the bonding properties of the cluster. The average isomer shiftat 78 K is 0.53 mm s−1 and this value is typical for FeIII (S = 5/2) in an octahedralenvironment comprising O/N ligands. The same average value is found in the seriesof complexes studied by Duncan et al. [15] and Long et al. [16], in smaller series ofcomplexes. The narrow distribution (±0.01 mm s−1) around 0.53 mm s−1 suggeststhat the bonding properties of basic carboxylates do not depend on the nature of thecarboxylate ligand but exhibit a relatively well “conserved” trend.

On the contrary, quadrupole splittings show a strong dependence on the carboxy-late ligand ranging from 0.36 to 0.76 mm s−1. A similar trend had been observed inthe series of complexes studied by Duncan et al. [15] and Long et al. [16], althoughin much smaller series of complexes. The main contribution in the quadrupolesplitting in FeIII (S = 5/2) complexes arises from the asymmetry of the latticearound the ferric ion. We have considered several geometrical elements that couldaccount for this variation, i.e. the Fe–Ooxo, Fe–OH2O and Fe–Ocarboxylate, distances.No systematic and statistically reliable correlation with the quadrupole splittingalong the series is found. Apparently, other electronic properties intrinsically related


to each carboxylate ligand are responsible for this variation and detailed theoreticalcalculations 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− = NO −

3 ), which we assume dissociates toa butterfly-type structure, all complexes are stable upon dissolution in MeCN. Al-though their isomer shifts remain practically unchanged, their quadrupole splittingsexhibit variations in the frozen solution. This is probably due to solid-state effects(e.g. packing) whose influence diminishes upon dissolution. A final comment shouldbe made on the reversibility of this process; drying of the solutions led to solids withidentical parameters to those of the initially studied complexes. This attests to boththe stability of the basic iron(III) carboxylates in solution and to the reversibility ofthe process.

Supporting Information Crystallographic data for 1, 2, 5, 7, 9–12, 14, 16, 21 and 23 in CIF formathave 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[Fe4O2(O2CCF3)8(H2O)6]; Fig. S2 showing the 78 K Mössbauer spectrum of a frozen MeCNsolution of complex 13; Tables S1 and S2 showing the Mössbauer parameters of the complexes (1–12and 13–24, respectively) in frozen MeCN solutions.

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