A planar carboxylate-rich tetrairon(II) complex and its conversion to linear triiron(II) and paddlewheel diiron(II) complexes

Inorg. Chem. 2007, 46, 10754−10770

A Planar Carboxylate-Rich Tetrairon(II) Complex and Its Conversion to

Linear Triiron(II) and Paddlewheel Diiron(II) Complexes

Erwin Reisner,^†Joshua Telser,^‡and Stephen J. Lippard*^,†

Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts

02139, and Department of Biological, Chemical, and Physical Sciences, RooseVelt UniVersity, 430

S. Michigan AVenue, Chicago, Illinois 60605

Received August 23, 2007

We report a series of oligonuclear carboxylate-rich high-spin iron(II) complexes with three different [Fe^II

O₂Cbiph)_2n(L)_m] (n 4; m

2 or 4) structural motifs, where ^-O₂Cbiph is 2-biphenylcarboxylate and L is an

exogenous ligand bound to terminal iron atoms. Solid compounds were isolated and their structural, spectroscopic,

and magnetic properties thoroughly investigated. The discrete tetranuclear complexes [Fe₄( -O₂Cbiph)₈(L)₂] crystallize

in a planar tetrairon(II) motif in which two diiron paddlewheel units are linked in an unprecedented manner involving

₃-1,1,3-bridging mode. X-ray crystallography reveals average Fe O_antibond lengths of 2.081[2] Å at the dimer

_n(µ-

)

2−

)

µ

a

µ

−

dimer interface. Terminal axial positions are capped by ligands L, where L is tetrahydrofuran (THF) (1), indazole

(2), pyrazole (3), 3,5-dimethylpyrazole (4), or acetamide (5). Reaction of 1 with an excess of acetonitrile affords the

linear compound [Fe₃(µ-O₂Cbiph)₆(MeCN)₄] (6). The acetonitrile ligands in 6 can be replaced by THF or

dimethoxyethane at elevated temperatures with retention of the structure to afford 7 and 8, respectively. Reaction

of 1 or 6 with pyridine or 1-methylimidazole results in the isolation of paddlewheel dimers 9 and 10, respectively,

with [Fe₂(µ-O₂Cbiph)₄(L)₂] composition. Mo¨ssbauer spectroscopy confirms the presence of high-spin ferrous ions

and indicates that the two iron sites of the dimer are geometrically indistinguishable. For the tri- and tetrairon

compounds, two quadrupole doublets are observed, suggesting that the iron centers do not have identical geometries.

Plots of magnetic susceptibility versus temperature reveal intramolecular antiferromagnetic exchange coupling for

all complexes under study. The magnetic data were fit to a theoretical model incorporating exchange coupling,

single-ion zero-field splitting, and g-tensor anisotropy. The resulting magnetic parameters reveal in most cases

weak antiferromagnetic exchange coupling (J typically <3 cm^-¹) and dominant zero-field-splitting parameters.

Introduction

site of the hydroxylase component of this enzyme (MMOH)

features a non-heme diiron center coordinated by four

glutamate and two histidine residues. The reduced di-

iron(II) form (MMOH_red; Chart 1) reacts directly with

dioxygen, activating it for methane hydroxylation.³

Metalloproteins with redox-active diiron cores coordinated

to bridging carboxylate ligands are relatively common in

biological systems. These diiron proteins perform multifac-

eted chemically and biologically intriguing functions includ-

ing hydrocarbon oxidation and desaturation in bacterial

multicomponent monooxygenases and stearoyl-acyl carrier

protein ∆⁹desaturase, respectively, radical generation in a

class I ribonucleotide reductase R2 subunit, dioxygen

transport in hemerythrin, and iron storage in ferritin.¹There

is much current interest in C-H bond activation,²stimulated

in part by detailed investigations of methane hydroxylation

by soluble methane monooxygenase (sMMO). The active

Recently, a variety of carboxylate-rich diiron(II) complexes

have been prepared to mimic the active sites of non-heme

(1) (a) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;

University Science Books: Mill Valley, CA, 1994. (b) Bertini, I.; Gray,

H. B.; Stiefel, E. I.; Valentine, J. S. Biological Inorganic Chemistry:

Structure and ReactiVity; University Science Books: Sausalito, CA,

2007.

(2) (a) Bergman, R. G. Nature 2007, 446, 391-393. (b) Mas-Balleste´,

R.; Que, L., Jr. Science 2006, 312, 1885-1886.

(3) (a) Merkx, M.; Kopp, D. A.; Sazinsky, M. H.; Blazyk, J. L.; Mu¨ller,

J.; Lippard, S. J. Angew. Chem., Int. Ed. 2001, 40, 2782-2807. (b)

Sazinsky, M. H.; Lippard, S. J. Acc. Chem. Res. 2006, 39, 558-566.

(c) Murray, L. J.; Lippard, S. J. Acc. Chem. Res. 2007, 40, 466-474.

(d) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625-2657.

* To whom correspondence should be addressed. E-mail: lippard@

mit.edu. Fax: 617-258-8150.

^†Massachusetts Institute of Technology.

^‡Roosevelt University.

10754 Inorganic Chemistry, Vol. 46, No. 25, 2007

10.1021/ic701663j CCC: $37.00

Published on Web 11/13/2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

Figure 1. Oligonuclear structural motifs in carboxylate-rich ferrous complexes containing a {Fe_n(O₂CR)_2n} core: (a) diiron paddlewheel; (b) windmill;

(c) linear triiron; (d) the unprecedented planar tetranuclear ferrous dimer of dimers motif; (e) the yet unreported corresponding polymeric assembly.

Chart 1

Chart 2

ligand such as pyridine, which facilitates the positioning of

a functional group proximal to the diiron center. Sulfoxida-

tion occurs in this manner and is controlled by the Fe‚‚‚S

distance; phosphine oxidation takes place only when the

substrate is positioned in the ortho position of the pyridine

ring.⁶

diiron centers like that in MMOH. The introduction of

sterically hindered m-terphenyl-based carboxylates facilitates

the assembly of discrete dinuclear cores with stoichiometries

identical to that at the active site of the enzyme. These

complexes emulate the hydrophobic protein active site cavity,

and the bulky carboxylates prevent bimolecular decomposi-

tion of the assembled diiron(II) complexes. Diferrous com-

plexes containing different m-terphenyl carboxylates and

N-donor ligands display a variety of structural motifs; doubly

bridged (windmill), triply bridged, and quadruply bridged

(paddlewheel) diiron(II) complexes are commonly encoun-

tered (Figure 1a,b).⁴

Paddlewheel compounds such as [Fe₂(µ-O₂CAr^Tol)₄L₂] (L

) 4-cyano- or 4-acetylpyridine; ^-O₂CAr^Tol) 2,6-di-p-

tolylbenzoate; Chart 2) can convert to aquated windmill

forms upon the addition of water to generate [Fe₂(µ-

O₂CAr^Tol)₂(O₂CAr^Tol)₂L₂(H₂O)₂] complexes. The oxygenation

rates of the aquated windmill complexes are 10-20 times

higher than those of their nonaquated paddlewheel ana-

logues.⁵Carboxylate-rich diiron(II) complexes have been

successfully utilized in substrate oxidation by dioxygen,

especially when the substrates are tethered to an N-donor

Because of the structural diversity of iron complexes with

m-terphenylcarboxylate ligands, we were interested in in-

vestigating whether the sterically less demanding, asymmetric

2-biphenylcarboxylate ^-O₂Cbiph (Chart 2) would provide

sufficient steric bulk to avoid polymerization while maintain-

ing the ability to facilitate the assembly of discrete oligo-

nuclear iron(II) complexes with novel structural features and

motifs. We speculated that the diminished steric crowding

and possible rotation around the ^-O₂C-C_Rbond axis of the

asymmetric carboxylate would enhance structural flexibility

and possibly modulate the properties of the resulting

constructs. Here we describe the use of ^-O₂Cbiph to prepare

a planar tetranuclear [Fe₄(µ-O₂Cbiph)₈(THF)₂] (THF )

tetrahydrofuran) complex with a dimer of dimers structure,

the geometry of which is unprecedented in iron chemistry.

The tetranuclear construct can be converted into linear tri-

iron(II) and paddlewheel diiron(II) complexes having [Fe_n(µ-

O₂Cbiph)_2n(L)_m] (n ) 2-4; m ) 2 or 4) stoichiometries. The

syntheses, structures, spectroscopic and magnetic properties,

and interconversion chemistry of these compounds are

discussed in detail.

(4) (a) Tshuva, E. Y.; Lippard, S. J. Chem. ReV. 2004, 104, 987-1012.

(b) Tolman, W. B.; Que, L., Jr. J. Chem. Soc., Dalton Trans. 2002,

653-660. (c) Lee, D.; Lippard, S. J. In ComprehensiVe Coordination

Chemistry II; McCleverty, J., Meyer, T. J., Eds.; Elsevier: Oxford,

U.K., 2003; Vol. 8, pp 309-342.

(5) (a) Yoon, S.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 8386-

8397. (b) Zhao, M.; Song, D.; Lippard, S. J. Inorg. Chem. 2006, 45,

6323-6330.

(6) (a) Carson, E. C.; Lippard, S. J. J. Am. Chem. Soc. 2004, 126, 3412-

3413. (b) Carson, E. C.; Lippard, S. J. Inorg. Chem. 2006, 45, 828-

836. (c) Carson, E. C.; Lippard, S. J. Inorg. Chem. 2006, 45, 837-

848. (d) Reisner, E.; Abikoff, T. C.; Lippard, S. J. Inorg. Chem. 2007,

in press. (e) Yoon, S.; Lippard, S. J. Inorg. Chem. 2006, 45, 5438-

5446.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10755

Reisner et al.

C

₁₁₂H₈₈O₁₈Fe₄(M_r) 1945.31 g mol^-1): C, 69.15; H, 4.56.

Experimental Section

Found: C, 68.93; H, 4.42. Mp: 131-134 °C (dec). IR (KBr): 3054

(m), 3022 (m), 2976 (w), 1945 (w), 1617 (s), 1479 (m), 1449 (m),

1438 (m), 1416 (s), 1264 (w), 1264 (m), 1103 (w), 1075 (m), 1051

(w), 1032 (m), 1018 (m), 1008 (m), 918 (m), 859 (s), 844 (m), 776

(m), 747 (s), 700 (s), 665 (s), 523 (s), 477 (m), 453 (m).

Method 2. Powdered yellow crystals of [Fe₃(O₂Cbiph)₆(MeCN)₄]

(6; 100 mg, 65 µmol) were dissolved in THF (5 mL), and the

resulting green solution was stirred for 30 min. The solution was

concentrated to dryness and the crude product recrystallized from

CH₂Cl₂by pentane vapor diffusion. The colorless-green dichroic

blocks were washed with pentane and dried under high vacuum.

Yield: 80 mg (80%).

General Considerations. All reagents were obtained from

commercial suppliers and used as received unless otherwise noted.

Dichloromethane, tetrahydrofuran (THF), and pentane used for the

preparation of the iron(II) complexes were saturated with dinitrogen

and purified by passage through activated Al₂O₃columns under

argon.⁷Anhydrous dimethoxyethane was purchased from Aldrich

and deoxygenated by bubbling argon through the solution prior use.

Chlorobenzene (distilled over P₄O₁₀) and acetonitrile (predried over

molecular sieves and distilled over CaH₂) were freshly dried prior

to use. Sodium hydroxide pellets were ground and dried at 40 °C

under high vacuum overnight. Fe(OTf)₂‚2MeCN was prepared

according to a literature procedure.⁸All ferrous complexes were

prepared under dinitrogen in an Mbraun drybox.

Physical Measurements. ¹H and ¹³C NMR spectra were

measured with a Varian 300 or 500 MHz spectrometer at the MIT

Department of Chemistry Instrumentation Facility. Chemical shifts

were referenced to residual solvent peaks.⁹Electrospray ionization

mass spectrometry (ESI-MS) spectra were obtained in methanol

solutions with an Agilent 1100 series LC/MSD mass spectrometer.

Fourier transform IR spectra were recorded on a Thermo Nicolet

Avatar 360 spectrometer with the OMNIC software. UV-vis spectra

were recorded on a Hewlett-Packard 8453 diode array spectropho-

tometer under anaerobic conditions. Melting points were measured

on an electrothermal Mel-Temp melting point apparatus. When

compounds were prepared by two different methods, the composi-

tion of the material synthesized by Method 2 was confirmed by IR

spectroscopy and by a unit cell determination of the crystalline

material.

[Fe₄(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₆(indazole)₂] (2). Indazole (11.8

mg, 100 µmol) dissolved in CH₂Cl₂(2 mL) was added to a stirred

yellow solution of 6 (100 mg, 65 µmol) in CH₂Cl₂(6 mL). The

resulting pale-green solution was stirred for 30 min, and the product

was crystallized by pentane vapor diffusion to give colorless-green

dichroic blocks within several days. The crystals were washed with

pentane and dried under high vacuum. Yield: 72 mg (69%). X-ray

diffraction quality single crystals were selected directly from the

reaction vessel, yielding 2‚2CH₂Cl₂. Anal. Calcd for 2‚CH₂Cl₂,

C

₁₁₉H₈₆Cl₂Fe₄N₄O₁₆(M_r) 2122.26 g mol^-1): C, 67.35; H, 4.08;

N, 2.64. Found: C, 67.37; H, 4.21; N, 2.82. Mp: 92-94 °C (dec).

IR (KBr): 3401 (m), 3054 (m), 3021 (w), 2922 (s), 2851 (w), 1608

(s), 1590 (s), 1573 (m), 1557 (m), 1478 (m), 1450 (w), 1437 (m),

1401 (s), 1267 (w), 1154 (w), 1100 (w), 1074 (w), 1051 (w), 1008

(m), 955 (m), 859 (m), 844 (m), 777 (m), 745 (s), 699 (s), 664 (s),

521 (m), 469 (m).

[Fe₄(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₆(pyrazole)₂] (3). 6 (100 mg,

65 µmol) was dissolved in CH₂Cl₂(4 mL), and pyrazole (6.7 mg,

99 µmol) dissolved in CH₂Cl₂(4 mL) was added to the stirred

yellow solution. After 30 min, the pale-green solution was set up

for crystallization by pentane vapor diffusion to give green-

colorless dichroic crystals within several days, which were washed

with pentane and dried under high vacuum. Yield: 78 mg (80%).

X-ray diffraction quality single crystals were selected directly from

the reaction vessel. Anal. Calcd for 3‚¹/₂CH₂Cl₂, C_110.5H₈₁-

ClFe₄N₄O₁₆(M_r) 1979.68 g mol^-1): C, 67.04; H, 4.12; N, 2.83.

Found: C, 67.10; H, 3.93; N, 2.82. Mp: 114-117 °C (dec). IR

(KBr): 3367 (m), 3054 (m), 3021 (w), 1607 (s), 1591 (m), 1571

(w), 1479 (m), 1449 (m), 1440 (m), 1395 (s), 1349 (w), 1155 (m),

1057 (m), 1043 (m), 1008 (m), 859 (m), 845 (m), 776 (m), 745

(s), 700 (s), 666 (s), 523 (m), 453 (m).

[Fe₄(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₆(3,5-dimethylpyrazole)₂] (4).

6 (100 mg, 65 µmol) was dissolved in CH₂Cl₂(5 mL), and 3,5-

dimethylpyrazole (9.5 mg, 99 µmol) dissolved in CH₂Cl₂(3 mL)

was added dropwise to the stirred yellow solution. After 30 min,

the pale-green solution was set aside for crystallization by pentane

vapor diffusion to give large green blocks within several days. The

crystals were washed with pentane and dried under high vacuum.

Yield: 68 mg (66%). X-ray diffraction quality single crystals were

selected directly from the reaction vessel, yielding 4a‚4CH₂Cl₂and

4b‚2CH₂Cl₂. Anal. Calcd for 4‚CH₂Cl₂, C₁₁₅H₉₀N₄Cl₂Fe₄O₁₆(M_r

) 2078.25 g mol^-1): C, 66.46; H, 4.36; N, 2.70. Found: C, 66.48;

H, 4.08; N, 2.92. Mp: 88-90 °C (dec). IR (KBr): 3342 (m), 3057

(m), 3020 (m), 2920 (w), 1611 (s), 1590 (m), 1570 (m), 1475 (m),

1450 (w), 1438 (w), 1403 (s), 1279 (w), 1264 (w), 1036 (w), 1008

(w), 777 (m), 746 (s), 699 (s), 664 (s), 518 (m), 464 (m).

[Fe₄(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₆(acetamide)₂] (5). 6 (100 mg,

65 µmol) was dissolved in acetonitrile (5 mL), and acetamide (6.9

mg, 99 µmol) was added to the stirred yellow solution. After 2 h,

the product was stripped to dryness and the solid was recrystallized

Synthesis. Sodium 2-Biphenylcarboxylate (NaO₂biph). 2-Bi-

phenylcarboxylic acid (1.50 g, 7.57 mmol) and sodium hydroxide

(0.31 g, 7.57 mmol) in MeOH (40 mL) were heated at 50 °C for

3 h. The solution was cooled to room temperature, concentrated to

dryness, and kept at 60 °C overnight under high vacuum. Yield:

1

1.67 g, quant. H NMR (300 MHz, CD₃OD) δ: 7.54 (dt, J ) 6.9

and 1.8 Hz, 2H); 7.48 (dt, J ) 6.3 and 1.5 Hz, 1H); 7.38-7.23 (m,

6H). ¹³C NMR (300 MHz, CD₃OD) δ: 179.2, 143.4, 142.7, 140.0,

130.9, 129.8, 129.2, 128.9, 128.2, 127.9, 127.9. Mp: >300 °C. ESI-

MS (m/z, MeOH). Calcd for [M - Na]^-197.1, [2M - Na]^-417.1,

[3M - Na]^-637.2, [4M - Na]^-857.2. Found: 196.9 (4%), 417.0

(23%), 637.1 (80%), 857.1 (100%). IR (KBr): 3067 (m), 3022 (w),

2963 (w), 1601 (m), 1584 (s), 1572 (s), 1560 (s), 1475 (w), 1448

(w), 1436 (w), 1408 (s), 1391 (m), 1262 (m), 1101 (w), 1074 (w),

1009 (w), 848 (w), 812 (w), 778 (w), 746 (s), 702 (m), 664 (m),

615 (w), 518 (w), 407 (w).

[Fe₄(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₆(THF)₂] (1). Method 1. A

portion of Fe(OTf)₂‚2MeCN (500 mg, 1.15 mmol) and NaO₂Cbiph

(505 mg, 2.29 mmol) were suspended in THF (15 mL) and stirred

at room temperature. After 15 min, an olive-green solution was

formed, which was concentrated to dryness after 2 h under reduced

pressure at room temperature. The product was then extracted into

CH₂Cl₂(10 mL). A white solid (NaOTf) was filtered off, and the

product was recrystallized from CH₂Cl₂by pentane vapor diffusion.

After 4 days, the green crystals that formed were washed with

pentane and dried under high vacuum at room temperature. Yield:

522 mg (47%). X-ray diffraction quality single crystals were

selected directly from the reaction vessel. Anal. Calcd for 1,

(7) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;

Timmers, F. J. Organometallics 1996, 15, 1518-1520.

(8) Hagen, K. S. Inorg. Chem. 2000, 39, 5867-5869.

(9) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62,

7512-7515.

10756 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

from chlorobenzene by pentane vapor diffusion. After several days,

the resulting yellow-green dichroic blocks were washed with

pentane and dried under high vacuum. Yield: 48 mg (51%). X-ray

diffraction quality single crystals were selected directly from the

reaction vessel, yielding 5‚2C₆H₅Cl. Anal. Calcd for 5, C₁₀₈H₈₂N₂-

Fe₄O₁₈(M_r) 1919.19 g mol^-1): C, 67.59; H, 4.31; N, 1.46.

Found: C, 67.95; H, 4.32; N, 1.56. Mp: 108-113 °C (dec). IR

(KBr): 3478 (m), 3056 (m), 3021 (w), 2929 (w), 1606 (s), 1588

(s), 1575 (m), 1478 (m), 1450 (m), 1439 (m), 1408 (s), 1266 (w),

1153 (w), 1078 (w), 1020 (w), 1008 (w), 775 (w), 746 (s), 700 (s),

664 (m), 601 (w), 515 (m), 469 (m).

[Fe₃(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₄(MeCN)₄] (6). Acetonitrile (8

mL) was added to a stirred green suspension of 1 (200 mg, 103

µmol) in CH₂Cl₂(4 mL). The reaction mixture was heated at 40

°C for ca. 5 min until an intense yellow solution was formed, which

was allowed to stand at room temperature. Yellow blocks of the

product were isolated after 4 days, and the crystals were washed

with pentane. Yield: 174 mg (83%). X-ray diffraction quality single

crystals were selected directly from the reaction vessel, yielding

vapor diffusion. Several days later, the yellow crystals that formed

were washed with pentane and dried at room temperature under

high vacuum. Yield: 49 mg (86%). X-ray diffraction quality single

crystals were selected directly from the reaction vessel, yielding

9‚CH₂Cl₂. Anal. Calcd for 9‚CH₂Cl₂, C₆₃H₄₈N₂Cl₂O₈Fe₂(M_r)

1143.66 g mol^-1): C, 66.16; H, 4.23; N, 2.45. Found: C, 66.32;

H, 4.03; N, 2.44. ¹H NMR (500 MHz, CD₂Cl₂) δ: 93.1, 29.6, 14.2,

13.0, 10.0, 7.8, 7.4, 6.2, 4.9, 4.6. Mp: 133-136 °C (dec). IR

(KBr): 3053 (m), 3014 (m), 1622 (s), 1601 (s), 1566 (m), 1477

(m), 1445 (s), 1397 (s), 1268 (w), 1211 (w), 1150 (w), 1068 (m),

1042 (m), 1002 (m), 841 (m), 781 (m), 763 (s), 748 (s), 715 (m),

695 (s), 664 (s), 549 (m). UV-vis [CH₂Cl₂, λ_max, nm (ꢀ, M^-1

cm^-1)]: 374 (1900).

Method 2. Pyridine (12 mg, 152 µmol) in CH₂Cl₂(1 mL) was

added dropwise to a stirred suspension of 6 (50 mg, 33 µmol) in

CH₂Cl₂(3 mL) at room temperature. After 30 min, the yellow

solution was exposed to pentane vapor diffusion. Yellow crystals

formed after 2 days and were washed with pentane and dried at

room temperature under high vacuum. Yield: 50 mg (88%).

[Fe₂(µ₂-O₂Cbiph)₄(1-MeIm)₂] (10). 1-Methylimidazole (12 mg,

146 µmol) in CH₂Cl₂(2 mL) was added to a stirred suspension of

1 (49 mg, 25 µmol) in CH₂Cl₂(5 mL). After 30 min, the resulting

yellow solution was carefully layered with pentane (3 mL) and

allowed to stand for crystallization. Yellow crystals were harvested

after 3 days, washed with pentane, and dried at room temperature.

Yield: 48 mg (88%). X-ray diffraction quality single crystals were

selected directly from the reaction vessel, yielding 10‚CH₂Cl₂. Anal.

Calcd for 10‚0.25CH₂Cl₂, C_60.25H_48.5Cl_0.5N₄Fe₂O₈(M_r) 1085.97

g mol^-1): C, 66.64; H, 4.50; N, 5.16. Found: C, 66.37; H, 4.31;

6a and 6b‚CH₂Cl₂. Anal. Calcd for 6‚0.25CH₂Cl₂, C_86.25H_66.5Cl_0.5

-

Fe₃N₄O₁₂(M_r) 1536.23 g mol^-1): C, 67.43; H, 4.36; N, 3.65.

Found: C, 67.49; H, 4.27; N, 3.45. Mp: 89-92 °C (dec). IR

(KBr): 3053 (m), 3019 (w), 2988 (w), 2928 (w), 2304 (m), 2276

(m), 1606 (s), 1592 (s), 1576 (w), 1530 (m), 1479 (w), 1450 (w),

1437 (w), 1400 (s), 1155 (w), 1076 (w), 1008 (w), 864 (w), 807

(w), 778 (m), 745 (s), 728 (m), 702 (s), 662 (m), 516 (m).

[Fe₃(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₄(THF)₄] (7). Powdered yellow

crystals of 6 (50 mg, 33 µmol) were dissolved in THF (4 mL), and

the solution was gently heated to boiling for 2 min. The resulting

yellow solution was layered with pentane. After several days, yellow

blocks were isolated and washed with pentane. X-ray diffraction

quality single crystals were selected directly from the reaction

vessel. Yield: 38 mg (69%). Anal. Calcd for 7‚0.25CH₂Cl₂,

1

N, 5.09. H NMR (500 MHz, CD₂Cl₂) δ: 29.8, 23.6, 20.2, 13.9,

13.6, 10.6, 9.6, 7.4, 5.9, 5.0, 1.3, 0.9. Mp: 110-112 °C (dec). IR

(KBr): 3128 (m), 3044 (m), 3014 (m), 1629 (s), 1594 (m), 1533

(m), 1477 (m), 1436 (m), 1394 (s), 1281 (w), 1233 (w), 1111 (s),

1091 (m), 1008 (m), 947 (m), 843 (s), 748 (s), 699 (s), 664 (s),

617 (m).

C

_94.25H_86.5Cl_0.5Fe₃O₁₆(M_r) 1660.45 g mol^-1): C, 68.18; H, 5.25.

Found: C, 67.94; H, 5.04. Mp: 93-97 °C (dec). IR (KBr): 3056

(m), 3022 (w), 2975 (m), 2874 (m), 1604 (s), 1590 (s), 1576 (w),

1478 (m), 1449 (m), 1437 (m), 1402 (s), 1262 (w), 1155 (w), 1036

(m), 1008 (w), 876 (w), 778 (w), 745 (s), 700 (s), 663 (m), 519

(w), 443 (w).

X-ray Crystallographic Studies. Intensity data were recorded

on a Bruker APEX CCD diffractometer with graphite-monochro-

mated Mo KR radiation (λ ) 0.710 73 Å), controlled by a Pentium-

based PC running the SMART software package.¹⁰Single crystals

were mounted in Paratone N oil on the tips of glass fibers or loops

and cooled under a stream of dinitrogen maintained by a KRYO-

FLEX low-temperature apparatus. A total of 2800 frames were

measured for each compound. The structures were solved by direct

methods for 1-5 and 9-11 and by the Patterson method for 6-8

and were refined on F ²by using the SHELXTL software package.¹¹

Empirical absorption corrections were applied with SADABS,¹²and

the structures were validated using the PLATON software.¹³All

non-hydrogen atoms were located, and their positions were refined

with anisotropic thermal parameters. Hydrogen atoms were placed

at calculated positions, except the two amide protons in 5, which

were located in the difference Fourier map. Hydrogen atoms were

assigned thermal parameters equal to either 1.5 (methyl hydrogen

atoms) or 1.2 (non-methyl hydrogen atoms) times the thermal

parameters of the atom to which they were attached.

[Fe₃(µ₃-O₂Cbiph)₂(µ₂-O₂Cbiph)₄(DME)₂] (8). Method 1. 6 (100

mg, 65 µmol) was dissolved in MeCN (8 mL), dimethoxyethane

(8 mL), and CH₂Cl₂(4 mL) under gentle heating. The resulting

yellow solution was allowed to cool slowly to room temperature,

and the yellow diamond-shaped blocks that formed within several

days were washed with pentane and dried under high vacuum.

Yield: 68 mg (67%). X-ray diffraction quality single crystals were

selected directly from the reaction vessel. Anal. Calcd for 8, C₈₆H₇₄-

Fe₃O₁₆(M_r) 1531.03 g mol^-1): C, 67.47; H, 4.87. Found: C,

67.04; H, 4.82. Mp: 111-113 °C (dec). IR (KBr): 3060 (w), 3019

(w), 2941 (w), 1590 (s), 1577 (m), 1479 (m), 1451 (m), 1401 (s),

1101 (m), 1065 (m), 1009 (m), 862 (m), 779 (w), 742 (s), 698 (m),

662 (m), 513 (w).

Method 2. A solution of 1 (50 mg, 26 µmol) was stirred for 10

min in MeCN (4 mL), after which dimethoxyethane (4 mL) was

added to the reaction mixture. After 5 min, the yellow suspension

was heated at 50 °C and the resulting yellow solution was allowed

to cool slowly to room temperature. The yellow blocks formed were

washed with pentane and dried under high vacuum. Yield: 42 mg

(80%).

(10) SMART, Software for the CCD Detector System, version 5.6; Bruker

AXS: Madison, WI, 2000.

(11) (a) Sheldrick, G. M. SHELXTL00: Program for Refinement of Crystal

Structures; University of Go¨ttingen: Go¨ttingen, Germany, 2000. (b)

SHELXTL, Program Library for Structure Solution and Molecular

Graphics, version 6.10; Bruker AXS: Madison, WI, 2001.

(12) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;

University of Go¨ttingen: Go¨ttingen, Germany, 2001.

[Fe₂(µ₂-O₂Cbiph)₄(py)₂] (9). Method 1. Pyridine (12 mg, 152

µmol) in CH₂Cl₂(2 mL) was added dropwise to a stirred suspension

of 1 (49 mg, 25 µmol) in CH₂Cl₂(7 mL) at room temperature.

After 1 h, the resulting yellow solution was exposed to pentane

(13) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht

University: Utrecht, The Netherlands, 2000.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10757

Reisner et al.

Table 1. Crystal Data and Details of Data Collection for Compounds 1-5

1

2‚2CH₂Cl₂

3

4a‚4CH₂Cl₂

5‚2C₆H₅Cl

chemical formula

C

₁₁₂H₈₈Fe₄O₁₈

C

₁₂₀H₈₈Cl₄Fe₄N₄O₁₆

C

₁₁₀H₈₀Fe₄N₄O₁₆

C

₁₁₈H₉₆Cl₈Fe₄N₄O₁₆

C

₁₂₀H₉₂Cl₂Fe₄N₂O₁₈

fw (g mol^-1

space group

a (Å)

)

1945.27

P2₁/n

13.1530(19)

25.159(4)

14.097(2)

2207.14

P1h

1937.18

P2₁/n

13.559(6)

24.633(11)

13.817(6)

2332.99

P1h

2144.26

P1h

14.0208(15)

14.2725(15)

15.7331(16)

109.966(2)

96.596(2)

115.812(2)

2531.4(5)

1

13.2698(7)

13.7009(7)

29.8852(16)

93.2150(10)

94.7490(10)

91.0210(10)

5404.8(5)

2

13.955(3)

14.181(3)

15.806(3)

109.421(3)

95.956(3)

116.237(3)

2527.1(9)

1

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

Z

93.854(3)

96.444(9)

4654.3(12)

2

1.388

4586(4)

2

1.403

F

_calc(g cm^-3

cryst size (mm)

)

1.448

1.434

1.409

0.25 × 0.25 × 0.20

0.30 × 0.25 × 0.20

0.16 × 0.14 × 0.12

0.40 × 0.40 × 0.40

0.18 × 0.12 × 0.12

s frame^-1

10

5

10

T (K)

110

0.683

2.17-27.88

79 075

11 104

604

110

0.739

2.08-25.68

37 310

9579

667

99.6

0.0434

110

0.692

2.16-25.02

63 384

8106

604

99.9

0.0463

0.0995

1.026

110

0.792

2.12-27.10

87 883

23 695

1380

99.4

0.0345

110

0.688

2.08-26.37

39 327

10 295

665

λ(Mo KR) (mm^-1

Θ range (deg)

total no. of data

)

no. of unique data

no. of param

completeness to θ (%)

R1^a

100

99.6

0.0414

0.0944

1.131

0.0495

0.1015

1.044

wR2^b

0.1075

1.038

0.0817

1.029

GOF^c

2

^aR1 ) ∑||F_o| - |F_c||/∑|F_o|. ^bwR2 ) {∑[w(F_o- F_c)²]/∑[w(F_o)²]}^1/2

.

^cGOF ) {∑[w(F_o- F_c)²]/(n - p)}^1/2, where n is the number of reflections

and p is the total number of parameters refined.

Table 2. Crystal Data and Details of Data Collection for Compounds 6-10

6a

7

8

9‚CH₂Cl₂

10‚CH₂Cl₂

chemical formula

C₈₆H₆₆Fe₃N₄O₁₂

1514.98

P1h

C₉₄H₈₆Fe₃O₁₆

1639.18

P1h

C₈₆H₇₄Fe₃O₁₆

1530.98

P1h

C₆₃H₄₈Cl₂Fe₂N₂O₈

1143.63

C2/c

24.960(7)

10.012(3)

21.217(6)

C₆₁H₅₀Cl₂Fe₂N₄O₈

1149.6

C2/c

24.769(10)

10.586(6)

20.607(10)

fw (g mol^-1

space group

a (Å)

)

12.598(2)

12.617(2)

13.368(2)

71.827(3)

83.311(3)

64.997(3)

1829.3(5)

1

13.776(2)

14.849(2)

22.949(5)

94.328(4)

105.652(3)

117.224(2)

3909.6(11)

2

12.222(13)

12.345(9)

13.650(11)

104.55(2)

115.45(2)

90.96(3)

1782(3)

1

b (Å)

c (Å)

R (deg)

â (deg)

γ (deg)

V (Å³)

Z

92.282(5)

91.00(4)

5298(3)

4

1.434

5402(4)

4

1.414

F

_calc(g cm^-3

cryst size (mm)

)

1.375

1.392

1.427

0.40 × 0.20 × 0.10

0.32 × 0.24 × 0.10

0.30 × 0.10 × 0.05

0.35 × 0.25 × 0.25

0.40 × 0.30 × 0.04

s frame^-1

10

7

10

7

10

T (K)

110

0.654

2.07-25.68

26 938

6924

477

99.6

0.0381

110

0.620

2.10-26.37

60 798

15 918

1036

99.5

0.0417

110

0.675

2.35-27.91

30 602

8407

477

98.8

0.0415

110

0.697

2.09-27.02

42 158

5907

350

99.8

0.0386

0.0916

1.120

λ(Mo KR) (mm^-1

Θ range (deg)

total no. of data

)

0.709

2.19-27.88

44 243

6321

348

100

0.0328

0.0804

1.081

no. of unique data

no. of param

completeness to θ (%)

R1^a

wR2^b

0.0988

1.046

0.0954

1.022

0.0916

1.065

GOF^c

^aR1 ) ∑||F_o| - |F_c||/∑|F_o|. ^bwR2 ) {∑[w(F_o- F_c)²]/∑[w(F_o)²]}^1/2

.

^cGOF ) {∑[w(F_o- F_c)²]/(n - p)}^1/2, where n is the number of reflections

2

and p is the total number of parameters refined.

X-ray diffraction quality single crystals were selected directly

from the reaction vessels. Complex 2 contains two CH₂Cl₂solvent

molecules per tetrairon unit. Polymorphism was encountered when

mounting several crystals of 4 and 6. Complex 4a crystallizes with

four CH₂Cl₂molecules per tetranuclear unit, of which one is

disordered over two positions, and 4b contains one disordered

2-biphenylcarboxylate ligand with one CH₂Cl₂solvent molecule

per asymmetric unit. Compound 6a contains half a molecule of

the complex in the asymmetric unit, and no solvent molecules could

be located. Two half-molecules were identified in the asymmetric

unit of 6b, which contains one CH₂Cl₂molecule per triiron unit.

One fully occupied chlorobenzene solvent molecule per asymmetric

unit was encountered in 5. One phenyl ring in 9 is involved in 1:1

positional disorder. In 9 and 10, one dichloromethane half-molecule

lies on a center of symmetry, resulting in one solvent molecule per

complex in the crystal lattice of these compounds. No solvent

molecules were located in the crystal lattices of 1, 3, 7, 8, and 11.

Crystal data, data collection parameters, and structure refinement

details for 1-10 are given in Tables 1 and 2 and those for 4b, 6b

and 11 in the Supporting Information. ORTEP diagrams, atom-

labeling schemes, CIF files, and tables containing interatomic

distances and angles for complexes 1-11 can be found in the

Supporting Information.

⁵⁷Fe Mo1ssbauer Spectroscopy. Zero-field Mo¨ssbauer spectra

were obtained on an MS1 spectrometer (WEB Research Co.)

with a ⁵⁷Co source in a rhodium matrix maintained at room

10758 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

Scheme 1

temperature housed in the MIT Department of Chemistry. Solid

samples were prepared by suspending powdered samples in Apiezon

grease and packing the mixture into a nylon sample holder. All

samples were prepared and loaded in an Mbraun glovebox and

transported to the spectrometer in liquid nitrogen. All data were

collected at 4.2 K, and the isomer shift (δ) values are reported with

respect to natural iron foil that was used for velocity calibration at

room temperature. Spectra were fit by using the WMOSS program.¹⁴

Magnetic Susceptibility Studies. Magnetic susceptibility data

for polycrystalline powders of the complexes were measured

between 2 and 300 K at the MIT Center for Materials Science and

Engineering on a Quantum Design MPMS-5S SQUID suscepto-

meter in direct current mode. In a typical experiment, magnetic

fields of 0.1, 10, and 50 kOe were applied and 82 data points were

collected per temperature sweep. No temperature-dependent hys-

teresis was observed for any of the compounds when collecting

data during the warm-up or cool-down periods. Samples were

loaded into capsules in a glovebox and mounted in a plastic straw

under a dinitrogen atmosphere. The samples were transported to

the susceptometer in liquid nitrogen, and care was taken to minimize

exposure to air. On several occasions, samples were examined for

color changes in a glovebox and analyzed by ⁵⁷Fe Mo¨ssbauer

spectroscopy following magnetic measurements. In no case could

significant oxidation impurities be detected. Data were corrected

for the magnetism of the sample holder, which was independently

determined at the same temperature range and fields. The underlying

diamagnetism of the samples was subtracted from the experimental

susceptibilities and estimated by using Pascal’s constants.¹⁵The

values were calculated to be -1.12 × 10^-3(2), -1.05 × 10^-3(3),

-1.10 × 10^-3(4), -8.08 × 10^-4(6), -8.55 × 10^-4(8), -5.87 ×

variable-temperature susceptibility data collected at several magnetic

fields. A standard spin Hamiltonian for coupled spin systems was

employed, which includes, in order of increasing interaction energy,

anisotropic electronic Zeeman coupling to each Fe^IIspin, zero-field

splitting (zfs) for each Fe^II, and anisotropic exchange coupling

between the two to four electronic spins.^18,19Further information

on the fitting procedure is given in the Supporting Information.

4

H )

Bg_iS_i+

S_iDS_i+

S_iJ_ijS_j

∑

i)1

i)1,j)2,i*j

Electrochemistry. Cyclic voltammograms (CVs) were measured

by using a 0.2-mm-diameter glassy-carbon or platinum-disk working

electrode, a platinum auxiliary electrode, and a silver-wire pseu-

doreference or a Ag/AgNO₃(0.10 M in MeCN) reference electrode.

Measurements were performed at room temperature using a PAR

263 potentiostat. All electrochemical measurements were performed

in CH₂Cl₂or MeCN in an Mbraun glovebox under a dinitrogen

atmosphere. The redox potentials of 2 mM concentrations of the

samples were measured in the presence of 0.50 M ⁿBu₄N(PF₆), using

[Fe(η⁵-C₅H₅)₂]^+/0(Cp₂Fe⁺/Cp₂Fe) as internal standard (∆E_p) 80

mV at 100 mV s^-1). All redox potentials are quoted relative to

Cp₂Fe⁺/Cp₂Fe.

Results and Discussion

Synthesis and Optical Spectroscopic Characteriza-

tion: Conversion between Tetra-, Tri-, Di-, and Mono-

iron(II) Complexes. The synthetic routes for the preparation

of complexes 1-12 and their interconversion are depicted

in Scheme 1. Complex 1 was prepared via self-assembly by

reacting Fe(OTf)₂‚2MeCN with NaO₂Cbiph in a 1:2 ratio in

THF. Warming the powdered tetranuclear complex 1 in a

mixture of MeCN/CH₂Cl₂allowed the linear trinuclear

iron(II) complex 6 to be obtained in 83% yield. The two

medium-intensity absorptions at 2304 and 2276 cm^-1in the

IR spectrum of 6 are assigned as the symmetric and asym-

metric combination CtN stretching bands for the two

10^-4(9), and -5.91 × 10^-4(10) cm³mol^-1

.

Magnetic Susceptibility Fitting Methodology. Magnetic sus-

ceptibility data were fit by using locally written programs that

involved solving the spin Hamiltonian matrix for Fe^II(n ) 1-4)

n

systems.^16,17The procedure involved simultaneous best fits to

(14) Kent, T. A. WMOSS: Mo¨ssbauer Spectral Analysis Software, version

2.5; WEB Research Co.: Minneapolis, MN, 1998.

(15) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders:

Orlando, FL, 1992.

(16) Krzystek, J.; Park, J.-H.; Meisel, M. W.; Hitchman, M. A.; Stratemeier,

H.; Brunel, L.-C.; Telser, J. Inorg. Chem. 2002, 41, 4478-4487.

(17) Goldberg, D. P.; Telser, J.; Bastos, C. M.; Lippard, S. J. Inorg. Chem.

1995, 34, 3011-3024.

(18) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of

Transition Ions; Dover Publications, Inc.: New York, 1986.

(19) Bocˇa, R. Coord. Chem. ReV. 2004, 248, 757-815.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10759

Reisner et al.

even when reacting 4-6 equiv (with respect to the products)

of the donors with 6 in CH₂Cl₂. The N-H stretching

frequencies at 3401, 3367, 3342, and 3478 cm^-1for 2-5,

respectively, confirm the presence of the coordinated and

protonated heterocycles or acetamide ligand in the com-

plexes. Strong ν_asymcarboxylate absorption bands from the

^-O₂Cbiph ligand dominate the IR spectrum of 5, obscuring

the carbonyl stretch of acetamide.

The paddlewheel complexes [Fe₂(O₂Cbiph)₄(L)₂] (9 and

10) were prepared by reacting 2-3 equiv (with respect to

the products) of the strong N-donors (N_strong) pyridine (py)

or 1-methylimidazole (1-MeIm) with 1 or 6 in CH₂Cl₂.

Yellow blocks were isolated in 86 and 88% yields, respec-

tively. Following the reaction by UV-vis spectroscopy

indicated the conversion to be complete within 2 min. A low-

energy band at 843 (ꢀ ∼ 60 M^-1cm^-1) or 912 nm (ꢀ ∼ 70

M^-1cm^-1) for 9 and 10, respectively, appeared during the

reaction. Red-shifted bands that move to lower energy with

increasing donor strengths of the axial ligands were previ-

ously reported for paddlewheel dirhodium(II) acetate com-

plexes.²⁰Although reactions of 1 or 6 in neat py under similar

experimental conditions gave compound 9 in high yield

(84%), reacting 1, 6, or 10 with excess or neat 1-MeIm

resulted in only some crystals of the mononuclear species

[FeCl₂(1-MeIm)₄] (11). In this complex, the X-ray structure

of which is given in the Supporting Information, the chloro

ligands most likely originate from chlorine abstraction from

the solvent CH₂Cl₂. A homoleptic octahedral 1-methylimi-

dazolemonoiron complex (12) was also identified by X-ray

crystallography, but the structure was not good enough for

us to assign the counterions in the lattice. These results

indicate that a large excess of 1-MeIm should be avoided in

the synthesis of 10. The reaction of 2.2 equiv of 1-MeIm

with [Fe₂(µ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(THF)₂] resulted in the

formation of [Fe₂(µ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(1-MeIm)₂],²¹and

the addition of 1-MeIm to [(Mes₂ArCO₂)Li(Et₂O)]₂and FeCl₂

or Fe(OTf)₂in CH₂Cl₂gave the mononuclear complex

[(Mes₂ArCO₂)₂Fe(1-MeIm)₂] as colorless crystals, irrespec-

tive of the amount of 1-MeIm used in the reaction (1 equiv

to a large excess).²²

Figure 2. Electronic absorption spectra of 1-4 measured 2 min after the

addition of the azole ligands (2-4) or THF (1) to a 6 mM solution of 6 in

CH₂Cl₂. The UV-vis spectrum of 6 is also shown.

acetonitrile ligands per terminal iron atom, as observed in

the X-ray crystal structure of 6 (see below).

Reactions of 6 for several minutes in THF or in a mixture

of dimethoxyethane/acetonitrile/dichloromethane with heat-

ing resulted in ligand displacement to afford 7 or 8,

respectively, with preservation of the overall triiron(II)

composition. Yellow blocks were isolated upon crystalliza-

tion of the trinuclear product in 69 and 67% yield for 7 and

8, respectively. Stirring complex 6 in THF at room temper-

ature gave a pale-green solution, indicative of the formation

of a tetranuclear species. Upon concentration to dryness,

complex 1 crystallized from a green CH₂Cl₂solution with

pentane vapor diffusion as pale-green blocks in 80% yield,

demonstrating the reversible interconversion of 1 and 6.

However, the initial crystallization of 1 produced a yellow

supernatant, which upon further recrystallization from the

same solvent mixture gave yellow crystals of 7 in small yield

(∼10%). This result indicates that the trinuclear species forms

as a minor byproduct that is more soluble than the tetra-

nuclear species 1.

The tetrairon(II) complexes [Fe₄(O₂Cbiph)₈(L)₂] (2-5)

were isolated by reacting 6 with 2 equiv (with respect to the

products) of the weak azole ligands (N_weak) indazole (Hind),

pyrazole (Hpz), and 3,5-dimethylpyrazole (Hdimepz) and

acetamide. In these reactions, the color of the reaction

mixture changed from yellow to pale green. The formation

of complexes 1-4 was followed by electronic spectroscopy;

we monitored the appearance of broad absorption bands at

824 (50), 836 (70), 847 (65), and 862 nm (ꢀ ∼ 70 M^-1cm^-1),

respectively (Figure 2). The significantly smaller intensity

for the absorption in 1 might be explained by the formation

of 7 as a minor byproduct; 7 has no absorption band in the

low-energy region of the UV-vis spectrum. For all com-

plexes, no further spectral changes were observed after 2

min. Compounds 1-4 also display high-intensity absorption

bands in the UV region (λ < 350 nm) originating from the

2-biphenylcarboxylate ligands. Pale-green crystals were

isolated in all cases from CH₂Cl₂and pentane vapor diffusion

in 69, 80, 66, and 51% yield for 2-5, respectively. For 2-5,

no complexes with different nuclearity could be isolated,

The pK_avalues for protonation at the metal-binding sites

of the heterocycles (azole/azolium couple) are 1.25, 2.48,

4.06, 5.30, and 7.12 for Hind, Hpz, Hdimepz, py, and

1-MeIm, respectively (Table 3).²³The pK_avalue is a measure

of the σ-donor strength for a given ligand and correlates with

the net electron-donor properties, expressed by the ligand

electrochemical parameter E_L, for a series of azole ligands.²⁴

E_Lparameters were also reported to correlate with Hammett

substituent parameters (σ).²⁵These parameters allow the three

(20) Johnson, S. A.; Hunt, H. R.; Neumann, H. M. Inorg. Chem. 1963, 2,

960-962.

(21) Lee, D.; Lippard, S. J. J. Am. Chem. Soc. 1998, 120, 12153-12154.

(22) Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1998,

120, 13531-13532.

(23) Catalan, J.; Abboud, J. L. M.; Elguero, J. AdV. Heterocycl. Chem.

1987, 41, 187-274.

(24) Reisner, E.; Arion, V. B.; Eichinger, A.; Kandler, N.; Giester, G.;

Pombeiro, A. J. L.; Keppler, B. K. Inorg. Chem. 2005, 44, 6704-

6716.

(25) Masui, H.; Lever, A. B. P. Inorg. Chem. 1993, 32, 2199-2201.

10760 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

Table 3. Selected Bond Distances (Å) and Angles (deg) for Complexes 1-10

1

2‚2CH₂Cl₂

3

4a‚4CH₂Cl₂5‚2C₆H₅Cl

Atom1-Atom2 (Å)

6a

7

8

9‚CH₂Cl₂10‚CH₂Cl₂

Fe(1)-Fe(2)

2.8360(5) 2.8239(5) 2.8506(12) 2.878[3]

3.4658(6) 3.4266(7) 3.4579(15) 3.402[13]

2.0694(12) 2.061(2)

2.8656(7) 3.5790(6) 3.565 [22] 3.606(3)

3.4569(9)

2.8912(7) 2.9348(13)

2.1181(12) 2.0947(18)

Fe(2)-Fe(3)

Fe(1)-L(1)

2.051(3)

2.068[6]

2.040(2)

2.1388(18) 2.110[15] 2.264(2)

2.246(2) 2.205[19] 2.131(2)

Fe(1)-L(2)

Fe(2)-O(1,syn)

Fe(2)-O(1A,anti)

Fe-O_min

2.1997(12) 2.2131(16) 2.196(2)

2.0841(12) 2.0845(16) 2.082(2)

1.9973(13) 2.0012(16) 1.993(2)

2.1997(12) 2.2131(16) 2.196(2)

2.071[27] 2.066[17] 2.082[21]

2.097 2.101 2.095

2.070[36] 2.076[37] 2.071[35] 2.080[28]

2.073 2.073 2.073 2.073

2.229[1]

2.075[1]

1.999(1)

2.229(1)

2.2783(19)

2.0775(18)

1.9964(18) 1.9856(14) 2.0114(14) 1.989(2)

2.2783(19) 2.2915(14) 2.2847(14) 2.2637(18) 2.0938(11) 2.1609(15)

2.080[5]

2.073

2.082[51] 2.114[35] 2.119[28] 2.129[12]

2.073 2.140 2.140 2.140

2.0467(11) 2.0381(16)

Fe-O_max

Fe(1)-O_meanobs^a

Fe(1)-O_meancalcd^b

Fe(2)-O_meanobs^a

Fe(2)-O_meancalcd^b

2.081[8]^e

2.073

2.121[63] 2.126[34] 2.115[56] 2.062[11] 2.076[29]

2.163 2.140 2.140 2.080 2.087

Atom1-Atom2-Atom3 (deg)

Fe(1)-Fe(2)-Fe(3) 112.45(1) 112.09(2) 113.46(3) 111.9(4)

Fe(2)-O(1)-Fe(3/1) 107.97(5) 105.71(7) 107.84(9) 104.4(6)

108.95(2) 180

104.96(7) 106.99(6) 105.9[22] 110.93(8)

73.62(4) 37.76(4) 37.7[11] 35.17(5)

148.16(5) 142.24(4) 142.5[14] 144.83(5)

158.67(6) 158.26(5) 161.81[6] 155.53(4) 164.29(3) 161.05(4)

111.97(5) 102.5[18] 130.47(4)

175.82(1) 180

Fe(1)-Fe(2)-O(1)

77.64(3)

76.76(4)

78.57(5)

76.0(7)

Fe(1)-Fe(2)-O(1A) 149.42(4) 149.48(5) 150.49(6) 150.6(1)

Fe(2)-Fe(1)-L(1)

Fe(2)-Fe(1)-L(2)

τ₁,^cFe(1)

166.35(4) 166.24(6) 166.90(7) 162.4(7)

0.16

0.15

0.09

0.07

1.25

0.32

0.27

2.48

0.26

0.23

4.06

0.02

0.01

0.02

τ₂,^cFe(2)

pK_a(N-heterocycle)^d

5.30

7.12

^aThe observed unweighted mean value and its standard deviation for Fe-O(carboxylate) are given.^{26 b}Calculated Fe-O distances based on bond valence

sum analysis (see the Supporting Information). ^cτ ) (â - R)/60, where â and R are the largest and second largest bond angles, respectively. For a perfectly

square-pyramidal geometry, τ is zero, and it becomes unity for a perfectly trigonal-bipyramidal geometry. Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn,

J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356. ^dTaken from ref 23. ^eSee ref 26.

sets of N donors used in this study to be distinguished. The

weak N donors (Hind, Hpz, and Hdimepz) are not capable

of breaking the doubly single-atom bridged {Fe₂O₂} center,

and the tetranuclear paddlewheel association therefore re-

mains intact in complexes 2-4. Strong N donors (py and

1-MeIm) break the dimer of dimers structure within several

minutes, affording the yellow paddlewheel dimers 9 and 10.

When a large excess of the very strong N-donor 1-MeIm is

used, all 2-biphenylcarboxylate ligands are replaced and only

mononuclear species 11 and 12 are isolated.

days, as indicated by a color change to brown. Elemental

analyses after drying a sample of 6 under high vacuum

indicate a loss of all acetonitrile ligands (% N ≈ 0).

Structural Characterization. The dimer of dimers struc-

tures of 1 and 4 are depicted in Figures 3 and 4, and Table

3 lists selected bond lengths and angles for 1-5. All

molecules except 4b have a crystallographically required

center of symmetry, and the four metal atoms lie in one

geometric plane with a dihedral angle of Θ_{Fe(1)-Fe(2)-Fe(2A)-Fe(1A)}

) 180° as required by symmetry. The {Fe₂(O₂Cbiph)₄}₂core

is held together by two µ₂-1,1 bridges from the syn,syn,anti-

µ₃-3,1,1-biphenylcarboxylate ligands. As expected, the 2.851-

[10]²⁶Å Fe(1)-Fe(2) (quadruply bridged) distances are

considerably shorter than the 3.442[12] Å Fe(2)-Fe(2A)

(doubly bridged) distances, and the mean Fe^II-O(carboxy-

late) bond lengths fall in the expected range between

1.993(2) and 2.2783(19) Å in 1-5. The axial pockets

generated by four bridging 2-biphenylcarboxylates circum-

scribe the apical positions of the exogenous ligands in 1-5.

The Fe-O bond distance to the terminally bound THF

molecule in 1 is slightly shorter than the Fe-O(THF) bond

Complexes 1-10 are very sensitive to air, and both

dichloromethane solutions and the crystalline material turn

brown within seconds or minutes when exposed to the

atmosphere. Complex 9 shows an absorption band at

382 nm (ꢀ_max) 2100 M^-1cm^-1) in CH₂Cl₂, and the

corresponding spectral changes were followed by UV-vis

spectroscopy after bubbling dry dioxygen through a solution

of the compound at -78 °C. This oxygenation produces a

metastable derivative of 9, as indicated by the appearance

of a new band in the visible spectrum, at 477 nm (ꢀ_max

)

1200 M^-1cm^-1), which starts to decay after several hours

(see the Supporting Information). Although crystalline blocks

of 6 are stable under a dinitrogen atmosphere for several

weeks at room temperature, pulverizing the sample or drying

it under high vacuum results in decomposition within several

(26) Parentheses refer to unique values and square brackets to average

values calculated by the following equations. Considering a sample

of n observations x_i, the unweighted mean value (x_u) with its standard

deviation (σ) was calculated using the following equations: x_u) ∑_ix_i/n

and σ ) {∑_i(x_i- x_u)²/[n(n - 1)]}^1/2

.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10761

Reisner et al.

The X-ray crystal structure of 5 reveals infinite

chains of molecules held together by an intermolec-

ular acetamide hydrogen-bonding network along the

a

axis having N(1)-O(9B) ) 2.9773(34) Å and

N(1)-O(4B) ) 2.8951(32) Å (Figure S6 in the Sup-

porting Information). The hydrogen bonding between

two amide ligands consists of an eight-membered

N(1)-H(2N)‚‚‚O(9B)-C(1B)-N(1B)-H(2NB)‚‚‚O(9)-C(1)

ring. The respective O(9)dC(1) and C(1)-N(1) bond lengths

of 1.248(3) and 1.312(4) Å are consistent with reported

distances for the acetamide tautomer found in an allenedi-

carboxylic acid/acetamide complex, where CdO and N-C

are 1.251(2) and 1.316(3) Å, respectively.²⁸The coordination

via O(9) instead of the acetamide nitrogen atom or its imine

tautomer can be rationalized as a consequence of the

stabilizing influence of the strong hydrogen-bonding net-

work involving the amide ligand, which would be com-

promised by N donation to iron. The Fe(1)-O(amide) bond

distance of 2.040(2) Å is significantly longer than that in a

dithiocarbamate complex [Fe(OC₄H₈dtc)₂(DMF)], where

Fe^II-O(DMF) is 1.983(7) Å.²⁹The N-H stretching fre-

quency at 3478 cm^-1confirms the presence of the amide.

To the best of our knowledge,³⁰5 is the first structurally

characterized iron(II) complex containing a coordinated

acetamide.

Figure 3. ORTEP diagram of 1 (top) and 4a (bottom) showing 50%

probability thermal ellipsoids for all non-hydrogen atoms. The solvent

molecules of 4a and the aromatic rings of the ^-O₂Cbiph ligands (except

the R-carbon atom) for both compounds were omitted for clarity.

A closer examination of compounds 1-5 reveals that the

asymmetric 2-phenylbenozate ligand facilitates a tetranuclear

arrangement. In these compounds, the phenyl rings in ortho

positions of the benzoate groups are oriented outward, away

from the linked paddlewheel subunits, preventing further self-

association through the anti lone pairs of the bridging

carboxylate oxygen atoms and allowing for the isolation of

discrete tetranuclear complexes. None of the 2-biphenylcar-

boxylate ligands in complexes 1-5 points toward the center

of the molecule (Figure 4 and the Supporting Information).

More sterically hindered symmetric terphenyl-based car-

boxylates shield both sides of the diiron core, preventing

association of the dinuclear subunits into tetranuclear species.

A large number of publications describe tetranuclear iron-

sulfur³¹and organometallic clusters, but only a relatively

small number of compounds have been reported that contain

carboxylate-bridged tetranuclear iron clusters, despite the

importance of iron-oxo clusters in bioinorganic chemistry.

The reported {Fe₄} cores can be roughly categorized into

four structural types: adamantane {Fe₄O₆} cores,³²face-to-

face {Fe₄O₆} cores,³³cubane {Fe₄O₄} cores,³⁴and butterfly

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Figure 4. Space-filling view of 1 along the b axis (top) and orientated

parallel to the ac plane showing the disposed Fe^IIcenter (bottom).

lengths of 2.0941(19) Å in [Fe₂(µ-O₂CAr^Tol)₂(O₂CAr^Tol)₂-

(THF)₂]‚2CH₂Cl₂.²¹The Fe^II-N bond distances in 2-4 are

comparable with values previously reported for carboxylate-

rich iron compounds.^21,22,27

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10762 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

Chart 3

discrete dimer of dimers structures has been reported for

rhodium(II),⁴⁰copper(II),⁴¹and chromium(II)⁴²complexes.

In all previously studied compounds, the association via the

monoatom-bridging oxygen is usually weak, with M-O_anti

distances between 2.22 and 2.61 Å.^41,43This result is in

agreement with the generally accepted higher basicity of

syn rather than anti lone pairs in carboxylate ligands.⁴⁴

Examples with M-O_synlonger than the M-O_antidistances

are not known for homometallic carboxylate-bridged

dimer of dimers constructs. Complexes 1-5 feature a strong

self-association, with coordinative covalent bonds display-

ing mean Fe(2)-O(1A) bond distances (anti coordination)

of 2.081[2] Å, compared to the considerably longer

Fe(2)-O(1) bond lengths of 2.223[15] Å (Chart 3) for the

syn binding mode. The strong interaction between the two

{Fe₂(O₂CR)₄L} dimers explains the unchanged tetranuclear

arrangement in complexes 2-5 even when azole (N_weak) and

acetamide are used as the exogenous ligands. The donor

properties of azole and acetamide are weaker than the strong

dimeric interaction, and therefore they serve only as exo-

genous terminal ligands in the tetranuclear compounds,

replacing the weaker THF ligands in 1. All dimer of dimers

structures reported so far in the literature have only either

very weak (e.g., THF) or no exogenous ligands. The dimer-

dimer interactions in these molecules are weak, as indicated

by long Fe-O_antibond lengths (see above).⁴⁵Use of a

stronger exogenous ligand would probably break these weak

dimer-dimer interactions, preventing their isolation as

tetranuclear species. Only dimer of dimers complexes with

strong interdimer interactions, as found in 2-4, can accom-

modate relatively strong exogenous ligands such as azoles,

preventing them from disrupting the association of paddle-

wheels.

Figure 5. Capped-sticks core representations of tetranuclear ferrous

complexes containing carboxylate bridging ligands with three distinct

geometries: (a) [Fe₄^II(bpg)₃(O₂CPh)₃](ClO₄)₂with an iron atom sitting on

t

a C₃-symmetry axis; (b) cubane core of [Fe^II₄(µ-OH)₄(µ-O₂CAr^{4- BuPh})₂(µ-

OTf)₂(4-^tBupy)₄]; (c) planar carboxylate-bridged tetrairon(II) cluster 1. Color

code: Fe, dark red; O, red; C, gray; N, blue; S, yellow.

{Fe₄O₂} cores.³⁵Although most of these complexes contain

mainly ferric ions, some tetranuclear ferrous complexes with

bridging carboxylate ligands have also been described. A

II

tetrairon(II) aggregate [Fe₄(bpg)₃(O₂CPh)₃](ClO₄)₂with

three identical {Fe(bpg)(O₂CPh)} units coordinated to a

central Fe^IIion displaying threefold symmetry and an

Fe-Fe(center) distance of 3.393(7) Å has been synthesized

(Figure 5a).³⁶Cubane-type Fe^II₄clusters have been isolated

and crystallographically characterized.³⁷[Fe₄(µ-OH)₄(µ-

t

O₂CAr^{4- BuPh})₂(µ-OTf)₂(4-^tBupy)₄] (^-O₂CAr^{4- BuPh}) 2,6-bis-

(4-tert-butylphenyl)benzoate) with a {Fe₄(µ-OH)₄}⁴⁺core

is depicted in Figure 5b. The Fe^II-Fe^IIdistances in this

tetrairon cube vary between 3.0605(8) and 3.2508(8) Å.³⁸

Iron paddlewheel associations with a {Fe₄(µ-O₂CR)₈} core

(1-5; Figures 1, 3, and 5c) have not been previously

reported.³⁰

Examples of iron carboxylate polymers include [Fe^II(µ-

CF₃CO₂)₂(CF₃CO₂H)₂]_n, which has bridging trifluoroacetate

The association of paddlewheel dimers into polymers is

known,³⁹and the isolation and structural characterization of

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2004, 43, 3219-3224. (d) Alarco´n, C. J.; Lahuerta, P.; Peris, E.;

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Inorganic Chemistry, Vol. 46, No. 25, 2007 10763

Reisner et al.

ligands arranged in a windmill configuration,⁴⁶and a mixture

of µ₃-syn,syn,anti- and µ₂-syn,syn-bridging carboxylates in

[Fe^II(O₂CH)₂‚¹/₃HCO₂H]_n,⁴⁷[Fe^II(µ-O₂CMe)₆(κ-O-DMSO-

3

d₆)₂]_n,⁴⁸and [Fe^II(µ-O₂CMe)₄(py)₃]_n.⁴⁸In addition, oxalate

2

ligands form iron(II) polymers with µ₂-anti,anti-⁴⁹and µ₃-

syn,anti,anti-binding^49,50modes. Tetrairon complexes often

result from the formal dimerization of two diiron cores, as

we previously observed in the oxo-bridged tetrairon(III)

complexes (Et₄N)[Fe^III₄O₂(O₂CR)₇(H₂B(pz)₂)₂],³⁵where R )

-

Me or Ph and H₂B(pz)₂) dihydrobis(1-pyrazolyl)borate,

and in [Fe^III₄O₂(BICOH)₂(BICO)₂(O₂CPh)₄]Cl₂, where

BICOH ) bis(N-methylimidazol-2-yl)carbinol.⁵¹The ferric

complexes were formulated as the 2 + 2 condensation

products of two {Fe^III₂O}⁴⁺cores, resulting in {Fe^III₄O₂}⁸⁺

associations (eq 1). The related dimerization of two

{Fe(O₂CR)}⁺cores in 1-5 is shown in eq 2.

Figure 6. ORTEP diagrams of 6a (top) and 8 (bottom) showing 50%

probability thermal ellipsoids for all non-hydrogen atoms. All atoms of the

^-O₂Cbiph ligands, except for the carboxylate group and the R-carbon atom,

were omitted for clarity.

The carboxylate ion has one delocalized negative charge,

and each sp²oxygen atom has two lone pairs disposed at an

angle of 120° with respect to each other. The syn-coordina-

tion mode is more frequently observed⁵²and has a higher

Lewis basicity than binding through the anti lone pair.⁵³

Although a preference for syn coordination of metal ions to

carboxylate ligands has been noted in a survey of the

Cambridge Structural Database, a syn preference for the

carboxylate ligand is not demonstrated by all metal ions.⁴⁴

For all metal complexes evaluated, the distribution is 63%

syn, 23% anti, and 14% direct binding (i.e., bidentate

coordination to a single metal center). From a large number

of transition metals, only Fe^II, Mn^III, Co^III, and Pt^IIpreferably

adopt the anti-coordination mode.⁴⁴This empirical survey

explains the unprecedented shorter M-O_antithan M-O_syn

bond distances and the strong dimer interaction in the

paddlewheel associations within 1-5. In addition, it shows

the importance of steric shielding around the carboxylate

center whether in model compounds or in a protein environ-

ment for diiron(II) systems. Both iron(III), and presumably

iron(IV), have a large preference for syn coordination,⁴⁴and

the enforced syn-coordination mode of the carboxylate

ligands to the Fe^IIions in MMOH_redmight help to store

energy to reach the active diiron(IV) intermediate Q in the

catalytic cycle of MMOH, which is ultimately responsible

for the conversion of methane to methanol. Syn coordination

might act as an energy reservoir to destabilize the low-valent

intermediate. The compression of the Fe^II₂core, possibly by

the positioning of the γ subunit in MMOH_red(Fe-Fe ∼ 3.3

Å),⁵⁴was recently revealed by computational studies. This

work demonstrated how the protein matrix can affect the

active site geometry, destabilizing intermediates that lie closer

in energy to reduced forms of the enzyme and facilitating

the formation of the methane-oxidizing diiron(IV) species

Q.⁵⁵

The solid-state structures of 6 and 8 are depicted in Figure

6. Selected bond lengths and angles are given in Table 3.

The central atom in 6 and 8 is located on a crystallographic

inversion center, resulting in a symmetry-required linear

disposition of iron atoms. The corresponding angle in 7 is

175.82(1)°, giving a slightly bent array of iron atoms. The

compositions of 6-8 involve three Fe^IIions; six bidentate

carboxylate ligands bridge two iron atoms flanking the central

atom. Four biphenylcarboxylate ligands bridge in the com-

mon µ₂-syn,syn mode and two link the three iron atoms in

a syn,syn,anti-µ₃-3,1,1 configuration, completing the octa-

hedral coordination sphere at Fe(2). The termini of the

{Fe^II(µ-O₂Cbiph)₃Fe^II(µ-O₂Cbiph)₃Fe^II} units are capped

either by four acetonitrile or THF ligands in 6 and 7,

respectively, or by two dimethoxyethane (8) molecules that

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10764 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

Table 4. Pertinent Structural Parameters for 1-8

MMOH^b

[Fe(O₂CH)₂‚ protomer protomer

a

1

2

3

4a

5

6a

7

8

¹/₃HO₂CH]_n

A

B

Fe_non-Fe¹

A (Å)

B (Å)

C (Å)

D (Å)

3.4658(6) 3.4266(7) 3.4579(15) 3.402[13] 3.4569(9) 3.5790(6) 3.565 [22] 3.606(3)

3.1973(10)

2.145(4)

3.31

2.44

2.45

3.27

2.31

2.26

int

2.0841(12) 2.0845(16) 2.082(2)

2.1997(12) 2.2131(16) 2.196(2)

2.0821(13) 2.1193(16) 2.075(2)

2.075[1]

2.229[1]

2.059[1]

2.0775(18) 2.160(1)

2.2783(19) 2.2915(14) 2.265[20] 2.2238(19) 2.134(4)

2.0772(18)

2.202[18] 2.153(2)

2.072(4)

3.506

3.3688(13) 3.4508(16) 3.3413(22) 3.440[22] 3.5266(19) 2.1155(15) 2.124[19] 2.192(2)

2.44

2.37

R (deg)

â (deg)

γ (deg)

Θ (deg)

τ ) R - â

(deg)

124.98(11) 125.87(14) 125.99(19) 127.44[30] 124.57(17) 130.01(12) 152.0[38] 131.92(13) 127.91

126.86(11) 128.30(14) 126.16(18) 128.01[81] 129.72(16) 85.82(11) 86.41[90] 89.00(13) 133.21

130.81

91.23

91.82

85.26

39.58

144.94

92.75

88.05

91.10

52.19

68.88(9)

66.92(12) 69.26(16) 67.60[48] 67.40(15) 94.03(12) 93.28[88] 90.94(12) 63.08

107.97(5) 105.71(7) 107.84(9) 104.39[62] 104.96(7) 106.99(6) 105.9[22] 110.93(8) 96.71(16)

-1.88

-2.43

-0.17

-0.57

-5.15

44.19

65.59

42.92

-5.30

category^c

class 1

class 3

class 1

^aReference 47; the values for the {Fe₂(µ-1,1-O₂CH)₂(µ-1,3-O₂CH)} unit are listed. ^bReference 54 from PDB code 1FYZ on MMOH_redat 2.15 Å resolution;

measurements taken in Coot modeling software. ^cClassification according to ref 59.

Chart 4

render the terminal iron atoms hexacoordinate. The Fe-N

bond distances and C-N triple bond lengths of 1.137(3) and

1.116(3) Å in 6a are similar to those previously reported

for iron(II) acetonitrile complexes.^21,22The Fe-NtC

angles of 173.85(18) and 159.1(2)° are slightly bent, and

the NtC-C angles are essentially linear at 179.5(3) and

177.3(3)°. The remaining bond distances for the carboxylate

ligands to the iron centers in 6-8 lie between 1.977(3) and

2.2847(14) Å (Table 3). The monoatomic-bridged carboxy-

late motif has been previously observed in diferrous com-

pounds. An example is [Fe₂(µ-L)(µ-O₂CR)(O₂CR)(N)₂],

where L is a dinucleating bis(carboxylate) ligand based

on m-xylylenediamine bis(Kemp’s triacid imide) and N is a

py- or imidazole-derived ligand.⁵⁶Others include those in

polymeric complexes, such as {[Fe(µ-O₂CH)₂]‚¹/₃HO₂CH}_n,⁴⁷

and the MMOH active site.⁵⁴In addition, similar linear triiron

by a strong interaction between the dangling oxygen atom

(O_din Table 4; O(2) in the X-ray nomenclature in Figures 3

and 6) and the interacting iron center (Fe¹_int), resulting in

relatively large Fe-Fe distances and a tilt value τ around

50° (for the definition of the tilt angle τ, see Table 4).

Conversely, class 1 compounds have essentially no or very

weak interactions between O_dand Fe¹_int, resulting in shorter

Fe-Fe distances and R ∼ â. Class 2 compounds are grouped

in between and are characterized by a weak Fe¹_int‚‚‚O_d

interaction. Compounds 6-8 with mean Fe-Fe and Fe-O_d

distances of 3.383[12] and 2.144[24] Å, respectively, fall

into class 3. Complexes 1-5 belong to class 1 because they

have significantly shorter Fe-Fe distances, 3.442[12] Å,

and longer Fe¹_int-O_dcontacts, 3.426[33] Å. The class 1

configuration of 1-5 allows atom O_dto interact with another

metal atom, Fe₂^int, and ultimately results in the observed

dimer of dimers structures. The interconversion of purely

syn,anti-µ₂-1,1- (class 1), syn,syn,anti-µ₂-3,1,1- (class 3), and

syn,syn-µ₂-1,3-bridging modes is shown in Chart 4.

arrays occur in acetate-bridged complexes, e.g., [Fe^II(µ-

3

O₂CMe)₆(BIPhMe)₂] [BIPhMe ) 2,2′-bis(1-methylimidazol-

yl)(phenyl)methoxymethane]⁵⁷and (Et₄N)₂[Fe^II(µ-OAc)₈],⁵⁸

3

the structures of which have been thoroughly described.⁵⁷

The monoatomic bridging motif of carboxylate ligands has

been studied in detail, leading to the definition of the

“carboxylate shift”.⁵⁹Compounds with a monoatomic car-

boxylate bridge can be grouped into three categories depend-

ing on several parameters. Class 3 compounds are defined

The structures of compounds 9 and 10 are shown in Figure

7, and selected bond lengths and angles are provided in Table

3. There are two crystallographically equivalent square-

pyramidal iron atoms and four bridging carboxylate ligands.

The average Fe-Fe and Fe-N distances of 2.913[22] and

2.106[12] Å, respectively, compare well with those in

previously reported diiron(II) paddlewheel compounds. The

Fe-Fe and Fe-N distances of [Fe₂(µ-O₂CAr^Tol)₄(4-^tBupy)₂]²⁷

(56) LeCloux, D. D.; Barrios, A. M.; Mizoguchi, T. J.; Lippard, S. J. J.

Am. Chem. Soc. 1998, 120, 9001-9014.

(57) (a) Rardin, R. L.; Bino, A.; Poganiuch, P.; Tolman, W. B.; Liu, S.;

Lippard, S. J. Angew. Chem., Int. Ed. 1990, 29, 812-814. (b) Rardin,

R. L.; Poganiuch, P.; Bino, A.; Goldberg, D. P.; Tolman, W. B.; Liu,

S.; Lippard, S. J. J. Am. Chem. Soc. 1992, 114, 5240-5249.

(58) Reynolds, R. A., III; Dunham, W. R.; Coucouvanis, D. Inorg. Chem.

1998, 37, 1232-1241.

(59) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,

417-430.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10765

Reisner et al.

Table 5. Zero-Field Mo¨ssbauer Parameters at 4.2 K for

Carboxylate-Rich Iron(II) Complexes 2-4, 6, and 8-10 and Related

Enzyme Active Sites

compd

δ (mm s^-1

)

∆E_Q(mm s^-1

)

Γ (mm s^-1

)

site^a

Fe_oSp, NO₄

Fe_iSp, O₅

Fe_oSp, NO₄

Fe_iSp, O₅

Fe_oSp, NO₄

Fe_iSp, O₅

Fe_oOh, N₂O₄

Fe_iOh, O₆

Fe_oOh, O₆

CE^b

2

3

4

6

8

9

1.16(2)

1.14(2)

1.17(2)

1.18(2)

1.27(2)

1.36(2)

1.30(2)

1.40(2)

1.15(2)

1.13(2)

1.30

2.98(2)

1.95(2)

3.64(2)

2.48(2)

3.14(2)

2.29(2)

2.96(2)

2.59(2)

3.13(2)

2.67(2)

2.88(2)

2.53(2)

2.87

0.25(2)

0.26(2)

0.23(2)

0.29(2)

0.26(2)

0.30(2)

0.32(2)

0.28(2)

0.33(2)

0.30(2)

0.32(2)

0.24(2)

Fe_i

Oh, O₆

Sp, NO₄

10

MMOH^c

MMOH^d

RNR-R2^e

∆9D^f

1.3

1.26

1.30

2.4-3.1

3.13

3.04-3.36

^aFe_o) terminal (outer) iron; Fe_i) central (inner) iron. ^bCE )

coordination environment (geometry and donor set); Sp ) square pyramidal;

Oh ) octahedral. ^cMethylococcus capsulatus (Bath). Liu, K. E.; Valentine,

A. M.; Wang, D.; Huynh, B. H.; Edmondson, D. E.; Salifoglou, A.; Lippard,

S. J. J. Am. Chem. Soc. 1995, 117, 10174-10185. ^dMethylosinus tricho-

sporium OB3b. Pulver, S.; Froland, W. A.; Fox, B. G.; Lipscomb, J. D.;

Solomon, E. I. J. Am. Chem. Soc. 1993, 115, 12409-12422. ^eCoates Pulver,

S.; Tong, W. H.; Bollinger, J. M.; Stubbe, J.; Solomon, E. I. J. Am. Chem.

Soc. 1995, 117, 12664-12678. ^fFox, B. G.; Shanklin, J.; Somerville, C.;

Mu¨nck, E. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 2486-2490.

Figure 7. ORTEP diagrams of 9 (top) and 10 (bottom) showing 50%

probability thermal ellipsoids for all non-hydrogen atoms. All atoms of the

^-O₂Cbiph ligand, except for the carboxylate group and the R-carbon atom,

were omitted for clarity.

and [Fe₂(µ-O₂CPh)₄(py)₂]‚MeCN⁶⁰are 2.8229(9) and

2.851(2) Å and 2.105(4) and 2.092(8) Å, respectively.

The corresponding terphenylcarboxylate compounds

[Fe₂(µ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(L)₂] (L ) py or 1-MeIm)

crystallize with windmill structures.²⁷Although a large

number of paddlewheel compounds have been reported,

diiron paddlewheel complexes are relatively scarce. Addi-

tional structural analyses of 1-10 including (i) a prediction

of the Fe-O bond lengths based on bond valence sum

analyses and (ii) the distribution of biphenylcarboxylate

torsion angles, φ, with different structural composition, can

be found in the Supporting Information.

Mo1ssbauer Spectroscopy. The zero-field Mo¨ssbauer

spectra of 2-4, 6, and 8-10 were recorded at 4.2 K in the

absence of an external field, and the resulting parameters

are listed in Table 5. The spectra of the high-spin diiron(II)

complexes 9 and 10 display one sharp and symmetric

quadrupole doublet, as is expected for two indistinguishable

iron sites related by a center of symmetry (Figure 8).

Complexes 6 and 8 contain two chemically distinguishable

iron atoms, and quadrupole doublets in a 2:1 ratio are

observed in the Mo¨ssbauer spectra for these triiron(II)

compounds (Figure 8). This ratio allows for peak assign-

ments, with the more intense, outer quadrupole doublet

corresponding to the outer [terminal, Fe(1), and Fe(3)] iron

atoms (Fe_o) and the inner doublet to the inner [center and

Figure 8. Zero-field Mo¨ssbauer spectra at 4.2 K [experimental data (|)

and calculated fits (gray solid line)] for solid samples of 10 (top), 6 (middle),

and 3 (bottom).

Fe(2)] iron atoms (Fe_i). The observed isomer shifts (δ) of

1.14[1] mm s^-1for the pentacoordinate complexes 9 and 10

and 1.33[3] mm s^-1for the hexacoordinate complexes 6 and

8 agree well with the trend expected for high-spin iron(II)

complexes.^5a

(60) Randall, C. R.; Shu, L.; Chiou, Y.-M.; Hagen, K. S.; Ito, M.; Kitajima,

N.; Lachicotte, R. J.; Zang, Y.; Que, L., Jr. Inorg. Chem. 1995, 34,

1036-1039.

The tetrairon(II) complexes 2-4 display two quadrupole

doublets in a 1:1 ratio. Their assignments in Table 5 can be

10766 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

rationalized by considering the isomer shift and quadrupole

splitting (∆E_Q) parameters. Both the degenerate two inner

(Fe_i) and outer (Fe_o) absorption peaks produce a quadrupole

doublet with an isomer shift in the expected range for

pentacoordinate species, ca. 1.16 mm s^-1for the three

complexes (Figure 8). If one inner peak and one outer peak

formed a single quadrupole doublet, then δ values as low as

0.88 mm s^-1and as high as 1.46 mm s^-1would be obtained

for 3. Values lower than 1.0 and higher than 1.3 are very

unlikely for pentacoordinate high-spin iron(II) species, and

this assignment can therefore be excluded. Less obvious is

the identification of the quadrupole doublets belonging to

the terminal [Fe(1) and Fe(4)] and central [Fe(2) and Fe(3)]

iron atoms in complexes 2-4. However, the electric field

gradient is expected to be higher for a more asymmetric NO₄

donor set than for an O₅donor set when the iron atoms have

comparable coordination environments. Indeed, both the

terminal and central iron atoms have similar square-

pyramidal coordination spheres with similar τ values (Table

3), justifying the assignment of the outer quadrupole doublet

to the outer (terminal) iron atoms and the inner doublet to

the inner (center) iron ions.

Magnetic Susceptibility Studies. Plots of the magnetic

susceptibility (ø_M) and effective moment (µ_eff) versus tem-

perature for 2-4, 6, and 8-10 are depicted in Figure 9 and

in the Supporting Information; the µ_effvalues are summarized

in Table 6. The theoretical effective moments are 9.80 µ_B

for 2-4 with four independent S ) 2 ions, 8.49 µ_Bfor 6

and 8 with three independent S ) 2 ions, and 6.93 µ_Bfor 9

and 10 with two independent S ) 2 ions and g ) 2.0. The

experimental effective moments at 300 K are consistent with

these calculated values (Table 6). At low temperature, all

compounds show a significant decrease in µ_effrelative to their

values at 300 K. For complexes containing an even number

of Fe^IIions (2-4, 9, and 10), fully antiparallel spin alignment

would yield an effective moment of 0 µ_B. For the complexes

with three Fe^IIions (6 and 8), the alignment would yield a

spin-only value of 4.90 µ_B(assuming for g ) 2.0). All of

these complexes approach their corresponding theoretical

value, as shown in Table 6.

The magnetic properties of paddlewheel diiron and linear

triiron complexes have been studied previously.²⁷The two

ferrous ions in the windmill complexes [Fe₂(µ-O₂CAr^Tol)₂-

(O₂CAr^Tol)₂(py)₂] and [Fe₂(µ-O₂CAr^Tol)₂(O₂CAr^Tol)₂(1-MeIm)₂]

and in the paddlewheel complex [Fe₂(µ-O₂CAr^Tol)₄(4-

^tBupy)₂] are antiferromagnetically coupled. The exchange

coupling constants (J; the J formalism is used here, H )

JS_iS_j, rather than -2J or -J) for the windmill complexes

were determined to be ∼2 cm^-1for the py and 1-MeIm

species. However, zfs was not accounted for when fitting

these data.²⁷Intramolecular ferro- and antiferromagnetic

interactions occur in linear triiron(II) complexes depending

on the interaction of the dangling oxygen of the second

oxygen from the monobridging carboxylate.¹⁷The magnetic

properties correlate with structural parameters. The com-

pounds [Fe₃(OAc)₆(BIPhMe)₂], [Fe₃(OAc)₆(BIPhOH)₂]

Figure 9. Plot of experimental data (b) and theoretical fits (solid line) of

molar susceptibility, ø_M, and effective moment, µ_eff, for 10 (top), 6 (middle),

and 2 (bottom): J ) 0.15 cm^-1, D ) -9 cm^-1, g ) 1.7, and g_|) 2.7 for

10; J₁₂) J₂₃) 1.8 cm^-1, D₁) D₃) -1.0 cm^-1, D₂) -17 cm^-1, g_1,iso

) g_3,iso) 2.13, and g_2,iso) 1.95 for 6; J₁₂) J₃₄) 2.26 cm^-1, J₂₃

-50 cm^-1, D ) 0, and g_iso) 2.32 for 2.

)

1,1-bis(1-methyl-2-imidazolyl)-1-(3,5-di-tert-butyl-4-hydroxy-

phenyl)ethane], grouped into class 1 (see above), show a

parallel spin alignment (J ) -2 to -5 cm^-1). Complexes

[Fe₃(O₂CPh)₆(ⁱPrOx)₂] [ⁱPrOx ) bis[2-((4S)-(1-methylethyl)-

1,3-oxazolinyl)]methane] and [Fe₃(O₂CPh)₆(PheMeEda)₂]

[PheMeEda ) N,N,N′-trimethyl-N′-[4,4-dimethyl-4-(3,5-di-

tert-butyl-4-hydroxyphenyl)butyl]ethylenediamine], both of

which are in class 3, display antiferromagnetic coupling

(J ∼ 1 cm^-1).¹⁷The antiferromagnetic behavior of class 3

compounds 6-8 (Tables 4 and 6) is in agreement with these

results.

Quantitative Analysis of the Magnetic Data. (a) Di-

nuclear Complexes 9 and 10. In principle, these complexes

should exhibit magnetic behavior that is easiest to interpret

[BIPhOH

) bis(1-methyl-2-imidazolyl)phenylhydroxy-

methane], and [Fe₃(OAc)₆(BIDPhEH)₂] [BIDPhEH )

Inorganic Chemistry, Vol. 46, No. 25, 2007 10767

Reisner et al.

Table 6. Summary of Susceptibility Data for Complexes 2-4, 6, and

8-10

negligible effect on the fit quality. The only moderately

successful fits, as defined by both matching the data and

yielding believable single-ion parameters, were those in

which either the exchange coupling was allowed to be

anisotropic (uniaxial) or the zfs axes were noncollinear. The

latter cases typically yielded J ≈ 22 cm^-1, D_i≈ -8 cm^-1,

g_iso≈ 2.3, and with D₂rotated with respect to D₁by Euler

angles R and â ≈ 45°, which yielded a very rhombic zfs.

Regardless of the exact magnitude of the magnetic param-

eters in the two complexes, there was a dramatic difference

between the two. There is strong antiferromagnetic coupling

in 9, which may be very anisotropic, whereas in 10, the two

Fe^IIions are almost noninteracting. That such a difference

results solely from the difference in axial ligation, with

ligands differing relatively little as donors (py and 1-MeIm),

is truly remarkable, and at this point, we have no explanation

for the observation.

complex

µ

_effat T_min^a/µB

µ

_effat T_max^b/µ_B

Planar Tetrairon(II) Complexes

calcd^cfor AF coupling

2

3

4

0^d

9.80^e

10.6

4.2

2.0

1.5

9.89

Linear Triiron(II) Complexes

calcd^cfor AF coupling

6

8

4.90^f

4.99

5.09

8.49^g

8.81

9.26

Paddlewheel Diiron(II) Complexes

calcd^cfor AF coupling

9

10

0^d

6.93^h

6.99

7.27

0.4

3.1

^aThe minimum temperature is 2 K. ^bThe maximum temperature is 300

K. ^cCalculated with µ_eff) [g²S(S + 1)]^1/2with g ) 2.0. ^dTotal spin S )

0. ^eFour independent S ) 2 ferrous ions. ^fTotal spin S ) 2. ^gThree

independent S ) 2 ferrous ions. ^hTwo independent S ) 2 ferrous ions.

(b) Trinuclear Complexes 6 and 8. In these complexes,

the physical model becomes far more complicated because

there are potentially three exchange coupling constants (J₁₂,

J₁₃, and J₂₃) that could be anisotropic and there are similarly

three zfs systems that could have any orientation with respect

to each other. To make the problem at all tractable, we

imposed the symmetry constraint that the single-ion param-

eters for the two terminal Fe^IIions be identical, which is

supported by the structural and Mo¨ssbauer data given above.

We also disregarded any noncollinearity of the zfs coordi-

nates for these ions. Last, we considered only exchange

coupling between adjacent ions and related these by sym-

metry as well so that J₁₂≡ J₂₃and J₁₃≡ 0.

because they are the simplest spin systems among those

described herein, with only one exchange coupling parameter

and two ions with presumed symmetry equivalence. The

temperature-dependent susceptibility behavior for 10 can be

described solely by a model including single-ion zfs without

exchange coupling, yielding D_i≈ -4.5 cm^-1(a negative

value is more successful than positive D_i) with g_iso≈ 2.1,

values consistent with single Fe^IIions in these environ-

ments.¹⁹This model accounts for the high-temperature

plateau of µ_effand for its sharp drop at low temperature (T

< 20 K; Figure 9). However, the model does not explain

the slight “bump” in µ_effseen near 30 K. Inclusion of

exchange coupling improves the fit, although this improve-

ment may simply reflect the increase in the parameters.

Taken at face value, such fits yield J ≈ 0.1-0.2 cm^-1, with

zfs again reasonable for the single ions, D_i≈ -9 cm^-1.

Inclusion of exchange coupling does not reproduce the

“bump” near 30 K. Inclusion of g-value anisotropy repro-

duces this feature to a slight extent, resulting in g ≈ 1.75-

(5) and g_|≈ 2.65(2). This spread is much larger than

expected, and the low value for g may not be realistic,

although the average (g_avg) is 2.05. Inclusion of temperature

independent paramagnetism (TIP) and/or a small amount of

Fe^IIIimpurity has little effect on the overall fit quality.

The situation with complex 9 is dramatically different.

Qualitatively, the decrease in µ_effversus T (Supporting

Information) is indicative of antiferromagnetic coupling

between the two Fe^IIions. However, the shape of the curves

is unlike that seen for typical paramagnets (vide supra). The

magnetic data for this complex proved surprisingly difficult

to analyze quantitatively. As might be expected, it was

impossible to fit the data with only exchange coupling

between the two Fe^IIions, whereas the use of only zfs (i.e.,

no exchange coupling) was also unsuccessful, in contrast to

10. The combination of these effects did not improve matters

much. In many fit attempts with the use of an isotropic g

value, the fit maximum was generally reached for this

parameter. The use of axial g values led to their divergence,

resulting in both unrealistically large and small values,

generally the allowed fit extrema. Correction factors such

as TIP and inclusion of an Fe^IIIimpurity generally had a

Complexes 6 and 8 exhibit qualitatively the same magnetic

susceptibility behavior (Figure 9 and the Supporting Infor-

mation), namely, a decrease in µ_eff(or øT) as T decreases. A

reasonable fit is obtained for 6 with the use of J₁₂) J₂₃

≈

1.8(2) cm^-1, g_iso(Fe_1,3(o)) ≈ 2.13(3), g_iso(Fe_2(i)) ≈ 1.95(5),

D(Fe_1,3(o)) ≈ -1.0(5), and -1 > D(Fe_2(i)) > -18 cm^-1. The

fit value for D(Fe₂), in contrast to the other parameters, is

highly variable; it is very sensitive to initial parameters and

to the inclusion of the additional terms, TIP and/or the

amount of Fe^IIIimpurity, despite the fact that neither is large

[e.g., TIP ≈ (1-4) × 10^-3; mole fraction of Fe^III≈ 0.07, a

value that can hardly be considered as equivalent to an

analytical impurity but merely a fit result]. Complex 8 was

analyzed first with the use of a model in which all three Fe^II

ions were held equivalent. In this model, the exchange

coupling (J₁₂) J₂₃≈ 1.5 cm^-1) is quite small and the zfs of

each single ion is negligible (D_i< 0.01 cm^-1). Indeed, a fit

with only exchange coupling is reasonably successful,

yielding J₁₂) J₂₃≈ 1.7 cm^-1and g_iso≈ 2.3. The more

physically meaningful model, where the outer Fe^IIions are

held the same but the inner Fe^IIion is allowed to differ, may

suffer from too broad a parameter space but did give results

for 8 resembling those for 6, at least in terms of exchange

coupling, although with much lower precision: J₁₂) J₂₃

≈

1.2(2) cm^-1, g_iso(Fe_1,3(o)) ≈ 1.9(1), g_iso(Fe_2(i)) ≈ 2.4(2),

D(Fe_1,3(o)) ≈ -10(5), and 1 < |D(Fe_2(i))| < 20 cm^-1(a

negative value for D(Fe_2(i)) yields slightly better fits, but a

positive value cannot be ruled out).

10768 Inorganic Chemistry, Vol. 46, No. 25, 2007

Carboxylate-Rich Oligonuclear Iron(II) Complexes

One must be circumspect about the quantitative aspects

of these fits, yet it is clear that, in these trinuclear complexes,

there is very little exchange coupling. It also appears that

the zfs of the outer Fe ions is relatively small, whereas that

of the central ion may be larger. The Fe^IIions in 8 have an

O₆donor set, and in 6, it is O₄N₂but the N donor is the

weak ligand MeCN. Such a symmetrical environment might

lead to small zfs, although this may be a coincidence. The

complex [Fe(Im)₆](NO₃)₂, with an N₆donor set, has very

large zfs (D ≈ 20 cm^-1), which is poorly described by an S

) 2 spin Hamiltonian. A full ligand-field treatment is

needed.⁶¹

romagnetic coupling between the terminal (outer) and internal

(inner) ions but strong ferromagnetic coupling between the

internal ions: J₁₂) J₃₄≈ 2.3 cm^-1, J₂₃≈ -50 cm^-1, and

g_iso≈ 2.3. For complex 3, all of the coupling was antifer-

romagnetic: J₁₂) J₃₄≈ 3.8 cm^-1, J₂₃≈ 14 cm^-1, and g_iso

≈ 2.3. Relatively small values for exchange coupling

between inner and outer Fe ions are consistent with the

results for the dinuclear complex 10, which represent the

“dimer”, whereas 2 and 3 are the dimer of dimers.

Complex 4 presented the least successful situation, in that

in all cases the fits required unrealistically large g values (g

> 3.0). This situation was obtained even with inclusion of

zfs, for which the fit yielded negligibly small values (D_i<

0.05 cm^-1). When the g values were disregarded, the fits

invariably yielded large magnitude antiferromagnetic cou-

pling of internal to terminal ions [J₁₂) J₃₄≈ 60(20) cm^-1]

but smaller magnitude ferromagnetic coupling between the

internal ions [J₂₃≈ 5(3) cm^-1]. Complex 4 would thus seem

to resemble the diiron complex 9, in terms of strong exchange

coupling within the “dimer”.

The use of single-ion parameters as described above for

the terminal Fe^IIions was moderately successful in fitting

data for 2 and 3, but this model was totally unsuccessful for

4, providing additional evidence as to the impenetrable nature

of its magnetic behavior. Qualitatively, it is possible that there

is a combination of ferro- and antiferromagnetic exchange

interactions in these tetranuclear high-spin Fe^IIclusters

because this situation occurs in iron-sulfur proteins.⁶²A key

difference between the systems studied here versus proteins

and model compounds containing Fe₄S₄clusters is the

symmetrical linkage (via bridging sulfides) of all Fe ions

with each other and their roughly equivalent coordination

spheres.

(c) Tetranuclear Complexes 2-4. These complexes

present a very difficult case for the quantitative analysis of

magnetic susceptibility behavior, and we are not aware of

others who have attempted to extract a full set of spin

Hamiltonian parameters. The outer Fe ions of the tetranuclear

complexes (2-4) are structurally very similar to those in

the diiron complexes 9 and 10. This result suggests that the

single-ion parameters of these outer Fe ions would resemble

those of the dinuclear Fe ions, which should facilitate fitting

of the magnetic data of the more complex tetrairon systems.

Unfortunately, the fit results for 9 and 10 are quite different,

which suggests that the nature of the terminal N-donor ligand

is crucial in determining both the single-ion properties and

the exchange coupling. Complexes 2-4 also differ in their

axial ligands so that generalization of single-ion parameters

among these complexes may not be justified. Consistent with

an axial ligand effect, 3 and 4 exhibit significantly different

magnetic behavior (see the Supporting Information) despite

their axial ligands differing by only methyl groups. As

described above, there are six exchange coupling constants;

however, these will be simplified by the assumption of no

coupling between nonadjacent ions and symmetrical coupling

between terminal and internal ions, so that J₁₂≡ J₃₄* J₂₃

* 0 and J₁₃) J₁₄) J₃₄) 0. In principle, the single-ion

parameters for the terminal ions, Fe_1,4, should be equivalent,

as should those for the internal ions, Fe_2,3, but the internal

and terminal ions need not be equivalent (see the Mo¨ssbauer

data above). In practice, however, in order to restrict the

parameter space, we employed two approaches: in one case,

we kept all of the parameters for the single ions equivalent;

in the other case, we kept the parameters for the terminal

Fe^IIions the same as those in 10: -8 > D > -10 cm^-1and

2.0 < g_iso< 2.1, which are representative for Fe^II. We did

not consider anisotropy or noncollinearity among these Fe^II

ions. The results described here are not definitive but at least

shed some light on the magnetic interactions within these

complicated multinuclear systems. The magnetic susceptibil-

ity data for these complexes are shown in Figure 9 and in

the Supporting Information. All of them exhibit qualitatively

the same behavior, with a gradual decrease in µ_eff(or øT) as

T decreases. Complexes 2 and 3, however, display an

inflection at ∼10 K that is absent in the data for 4.

Electrochemistry. The CVs of the diiron(II) complexes

n

9 and 10 in 0.50 M Bu₄N(PF₆)/CH₂Cl₂solutions collected

at a scan rate of 0.20 V s^-1display one quasireversible

oxidation wave (Figure 10), I^ox, which is assigned⁶³to an

Fe^IIFe^IIf Fe^IIFe^IIIoxidation. The redox potentials [E_1/2

)

(E_p,a+ E_p,c)/2] of 0.02 and -0.32 V for 9 and 10,

respectively, are comparable with that of [Fe₂(µ-O₂CAr^Tol)₄-

(^tBupy)₂], -0.22 V vs Cp₂Fe⁺/Cp₂Fe, and agree well with

the expected⁶⁴net electron-donor character of the exogenous

N-donor ligands: 1-MeIm > tert-butylpyridine > py. In

contrast to [Fe₂(µ-O₂CAr^Tol)₄(^tBupy)₂]⁺, the mixed-valent

oxidation products of 9 and 10 have an ^Ii_p^red/^Ii_p^oxratio for I^ox

of 0.3 and 0.4 at 0.20 V s^-1, respectively, indicating

incomplete chemical reversibility. At lower scan rates, the

ox

^Ii_p^red/^Ii_pratio decreases and approaches zero for 9 at scan

rates lower than 50 mV s^-1. Upon potential scan reversal

following the formation of wave I^ox, one quasireversible

reduction wave, II^red, at -0.55 (9) and -0.73 V (10) vs

Cp₂Fe⁺/Cp₂Fe was detected, which we attribute to the

reduction of products formed in the anodic process at I^ox

(62) Noodleman, L.; Peng, C. Y.; Case, D. A.; Mouesca, J.-M. Coord.

Chem. ReV. 1995, 144, 199-244.

For 2, it was possible to fit the data moderately well with

a parameter set excluding zfs. The results indicate antifer-

(63) Lee, D.; Krebs, C.; Huynh, B. H.; Hendrich, M. P.; Lippard, S. J. J.

Am. Chem. Soc. 2000, 122, 5000-5001.

(61) Carver, G.; Tregenna-Piggott, P. L. W.; Barra, A.-L.; Neels, A.; Stride,

(64) (a) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271-1285. (b) Pombeiro,

A. J. L. Eur. J. Inorg. Chem. 2007, 1473-1482.

J. A. Inorg. Chem. 2003, 42, 5771-5777.

Inorganic Chemistry, Vol. 46, No. 25, 2007 10769

Reisner et al.

ligands, which precludes the isolation of a large variety of

structural motifs. Furthermore, the commonly favored syn-

coordination mode normally prevents strong dimer of dimers

interactions for non-ferrous paddlewheel associations. The

iron carboxylate complexes described in this study have

avoided this situation, and their structural diversity can be

attributed to the flexible biphenylcarboxylate ligand and the

easily accessible syn- and anti-binding modes of the car-

boxylates at the ferrous centers. Flexibility of carboxylate

ligands in a biological context is thought to be an important

factor for the proper function of non-heme diiron enzymes.

The complexes were investigated by variable-temperature

magnetic susceptibility studies, which showed a degree of

intramolecular antiferromagnetic exchange coupling at low

temperatures. Quantitative analysis of this phenomenon was

very difficult because of the complex electronic structures

of the constituent high-spin Fe^IIions. However, it appears

that, in most cases, there is relatively weak antiferromagnetic

exchange coupling between adjacent ions, 0 < J e 5 cm^-1,

despite the extensive carboxylate bridging between them.

Exceptions are the diiron complex 9, which exhibits relatively

strong antiferromagnetic coupling (J > 20 cm^-1) yet is

structurally almost the same as complex 10, and the tetrairon

complex 4, in which there is likely also strong coupling

between dimer units. The magnetic properties depend on

effects that we cannot identify, and similarity in the

coordination environment is not a sufficient determinant of

these properties. Possibly, differences in the internal packing

of the ligands, which give rise to disparate Fe-O bond

lengths for the bridging carboxylates, are an important factor.

A full understanding of the relationship between the structure

and magnetism of oligomeric high-spin Fe^IIions remains to

be determined.

n

Figure 10. CV of 2 mM solutions in CH₂Cl₂with 0.5 M Bu₄N(PF₆) of

9 at a platinum working electrode at a scan rate of 0.20 V s^-1. The Fe^IIFe^II

f Fe^IIFe^IIIredox process is indicated by I^ox, and upon scan reversal after

the oxidation process, the appearance of a new partially reversible species,

II^red, is observed.

(Figure 10). It is assumed that the less sterically hindered

biphenylcarboxylate ligand allows for a structural rearrange-

ment upon oxidation that results in only partial reversibility

of wave I, which is not seen in the CV of the more sterically

shielded [Fe₂(µ-O₂CAr^Tol)₄(^tBupy)₂]⁺. Only negligible geo-

metric changes were observed for [Fe^II(µ-O₂CAr^Tol)₄(^t-

2

Bupy)₂] upon one-electron oxidation to the corresponding

Fe^IIFe^IIIspecies, as revealed by X-ray crystallography.⁶³The

triiron(II) complex 6 under the same experimental conditions

in MeCN displayed one quasireversible (E_1/2) -0.36 V)

oxidation wave (see the Supporting Information), which

became irreversible in CH₂Cl₂, indicating oxidative decom-

position due to loss of MeCN ligands and complex rear-

rangement. In addition, one irreversible (E_p) -0.10 V vs

Cp₂Fe⁺/Cp₂Fe) oxidation wave was observed. Scan reversal

upon the first oxidation wave showed the appearance of one

irreversible reduction wave at -0.85 V vs Cp₂Fe⁺/Cp₂Fe.

Acknowledgment. This work was supported by Grant

GM032134 from the National Institute of General Medical

Sciences. E.R. is the recipient of a Schro¨dinger fellowship

(Grant J2485-B10) from the Austrian Science Foundation.

We thank Ms. Simone Friedle for assistance in acquiring

the Mo¨ssbauer spectra and Dr. Sebastian Stoian for helpful

discussions.

Conclusion

A series of oligonuclear iron(II) complexes containing

flexible and asymmetric 2-phenylbenzoate ligands with three

different structural motifs were prepared and thoroughly

characterized. Included are (i) the novel planar tetrairon(II)

dimer of dimers complexes 1-5 with an unprecedented

strong axial interaction between the two dimer subunits, (ii)

the linear triiron(II) complexes 6-8, and (iii) the paddlewheel

diiron(II) complexes 9 and 10. The steric pocket of the

biphCO₂^-ligand allows for the isolation of discrete oligomers

by preventing polymerization. Transition-metal complexes

usually have a preferred coordination mode for carboxylate

Supporting Information Available: Structural details, crystal-

lographic tables, ORTEP diagrams with atom-numbering schemes,

and X-ray crystallographic files in CIF format for complexes 1-11,

IR spectra for 1-10, Mo¨ssbauer spectra of 2, 4, 8, and 9, magnetic

plots of 3, 4, 8, and 9, CV of 6 in MeCN, and time-resolved UV-

vis spectra of the oxygenation of 9. This material is available free

of charge via the Internet at http://pubs.acs.org.

IC701663J

10770 Inorganic Chemistry, Vol. 46, No. 25, 2007

Article Doi

A planar carboxylate-rich tetrairon(II) complex and its conversion to linear triiron(II) and paddlewheel diiron(II) complexes

DOI: 10.1021/ic701663j

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Article abstract of DOI:10.1021/ic701663j

Full text of DOI:10.1021/ic701663j

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