Synthesis and characterization of a series of structurally and electronically diverse Fe(II) complexes featuring a family of triphenylamido-amine ligands

Article abstract of DOI:10.1021/ic9015838

A family of triphenylamido-amine ligands of the general stoichiometry L^xH₃=[R-NH-(2-C₆H₄)]₃N (R=4-t-BuPh (L¹H₃), 3,5-t-Bu₂Ph (L ²H₃), 3,5-(CF₃)₂Ph (L ₃H₃), CO-Z-Bu (L⁴H₃), 3,5-Cl ₂Ph (L⁵H₃), COPh (L⁶H₃), CO-i-Pr (L⁷H₃), COCF₃ (L⁸H ₃), has been synthesized and characterized, featuring a rigid triphenylamido-amine scaffold and an array of stereoelectronically diverse aryl, acyl, and alkyl substituents (R). These ligands are deprotonated by potassium hydride in THF or DMA and reacted with anhydrous FeCI2 to afford a series of ferrous complexes, exhibiting stoichiometric variation and structural complexity. The prevalent [(L^x)Fe(II)-SoIv]^- structures (L^x=L¹, L², L³, L⁵, Solv=THF; L^x=L⁸, solv=DMA; L^x=L⁶, L⁸, Solv=MeCN) reveal a distorted trigonal bipyramidal geometry, featuring ligand-derived [N₃,_amidoN_amine] coordination and solvent attachment trans to the N^amine atom. Specifically for [(L⁸)Fe(ll)-DMA]^-, a N_amido residue is coordinated as the corresponding N_imino moiety (Fe-N(Ar)=C(CF3)-O^-). In contrast, compounds [(L⁴)Fe(ll)] [(L⁶)₂Fe(ll)₂]^2-, [K(L ⁷)2Fe(ll)2]22-, and [K(L⁹)Fe]₂ are all solvent-free in their coordination sphere and exhibit four-coordinate geometries of significant diversity. In particular, [(L⁴)Fe(ll)]^- demonstrates coordination of one amidato residue via the O-atom end (Fe-O-C(t-Bu)=N(Ar)). Furthermore, [(L⁶)₂Fe(ll) ₂]^2- and [K(L⁷)₂Fe(ll) ₂]₂^2- are similar structures exhibiting bridging amidato residues (Fe-N(Ar)C(R)=O-Fe) in dimeric structural units. Finally, the structure of [K(L⁹)Fe]₂ is the only example featuring a minimal [N₃,_amidodoN_amine] coordination sphere around each Fe(II) site. All compounds have been characterized by a variety of physicochemical techniques, including Moessbauer spectroscopy and electrochemistry, to reveal electronic attributes that are responsible for a range of Fe(ll)/Fe(lll) redox potentials exceeding 1.0 V.

Full text of DOI:10.1021/ic9015838

108 Inorg. Chem. 2010, 49, 108–122
DOI: 10.1021/ic9015838
Synthesis and Characterization of a Series of Structurally and Electronically
Diverse Fe(II) Complexes Featuring a Family of Triphenylamido-Amine Ligands
Patrina Paraskevopoulou,†,‡ Lin Ai,†,§ Qiuwen Wang,† Devender Pinnapareddy,† Rama Acharyya,†
Rupam Dinda,† Purak Das,† Remle C-elenligil-C-etin, Georgios Floros,‡ Yiannis Sanakis,^ Amitava Choudhury,†
Nigam P. Rath,# and Pericles Stavropoulos*,†
†Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409,
‡Department of Inorganic Chemistry, Faculty of Chemistry, University of Athens, Panepistimioupoli
Zographou 15771, Athens, Greece, §College of Chemistry, Beijing Normal University, Beijing 100875, China,
Department of Chemistry, Boston University, Boston, Massachusetts 02215, ^Institute of Materials Science,
NCSR “Demokritos”, Ag. Paraskevi 15310, Greece, and #Department of Chemistry and Biochemistry,
University of Missouri—St. Louis, St. Louis, Missouri 63121
Received August 7, 2009
A family of triphenylamido-amine ligands of the general stoichiometry LxH3 =[R-NH-(2-C6H4)]3N (R=4-t-BuPh
(L1H3), 3,5-t-Bu2Ph (L2H3), 3,5-(CF3)2Ph (L3H3), CO-t-Bu (L4H3), 3,5-Cl2Ph (L5H3), COPh (L6H3), CO-i-Pr (L7H3),
COCF3 (L8H3), and i-Pr (L9H3)) has been synthesized and characterized, featuring a rigid triphenylamido-amine
scaffold and an array of stereoelectronically diverse aryl, acyl, and alkyl substituents (R). These ligands are
deprotonated by potassium hydride in THF or DMA and reacted with anhydrous FeCl2 to afford a series of ferrous
complexes, exhibiting stoichiometric variation and structural complexity. The prevalent [(Lx)Fe(II)-solv]- structures
(Lx=L1, L2, L3, L5, solv=THF; Lx=L8, solv=DMA; Lx=L6, L8, solv=MeCN) reveal a distorted trigonal bipyramidal geometry,
featuring ligand-derived [N3,amidoNamine] coordination and solvent attachment trans to the Namine atom. Specifically for
[(L8)Fe(II)-DMA]-, a Namido residue is coordinated as the corresponding Nimino moiety (Fe-N(Ar)dC(CF3)-O-).
In contrast, compounds [(L4)Fe(II)]-, [(L6)2Fe(II)2]2-, [K(L7)2Fe(II)2]22-, and [K(L9)Fe]2 are all solvent-free in their
coordination sphere and exhibit four-coordinate geometries of significant diversity. In particular, [(L4)Fe(II)]-
demonstrates coordination of one amidato residue via the O-atom end (Fe-O-C(t-Bu)dN(Ar)). Furthermore,
[(L6)2Fe(II)2]2- and [K(L7)2Fe(II)2]2 are similar structures exhibiting bridging amidato residues (Fe-N(Ar)-
2-
C(R)dO-Fe) in dimeric structural units. Finally, the structure of [K(L9)Fe]2 is the only example featuring a minimal
[N3,amidoNamine] coordination sphere around each Fe(II) site. All compounds have been characterized by a variety of
€
physicochemical techniques, including Mossbauer spectroscopy and electrochemistry, to reveal electronic attributes
that are responsible for a range of Fe(II)/Fe(III) redox potentials exceeding 1.0 V.
Introduction
storing and transferring redox equivalents. Typical examples
of this trend include ligand scaffolds with nitrogen-rich re-
sidues (Chart 1) such as diiminates,2 bis(imino)pyridines,3
PNP pincer ancillaries,4 and even N-heterocyclic carbenes.5
Although the aforementioned ligands, and those described
in the present work, are not biomimetic in nature, it is worth
Recent advances in metal-mediated catalysis and enzymol-
ogy1 have provided a plethora of examples in which ligand
residues contribute as redox-active partners during turnover.
Ligand frameworks, which were in the past considered to
solely convey stereoelectronic attributes to metal sites, are now
scrutinized with greater frequency as potential participants in
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*To whom correspondence should be addressed. Tel.: (þ1) 573-341-7220.
Fax: (þ1) 573-341-6033. E-mail: pericles@mst.edu.
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pubs.acs.org/IC
Published on Web 12/01/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 109
Chart 1
Chart 2
Chart 3
noting that redox-active elements are also ubiquitous
in enzymological functions6 (ribonucleotide reductase,7
cytochrome c peroxidase,8 galactose oxidase,9 prostaglandin
H synthase,10 lipoyl synthase,11 biotin synthase,12 diol dehy-
drase,13 pyruvate-formate lyase,14 DNA photolyase,15 and
lysine-2,3-aminomutase),16 combining metal sites, in a mani-
fold of oxidation states, and radicalresidues (Tyr•, Cys•, Gly•,
Trp•, and TrpH•þ) toward executing fundamental catalytic
steps (hydrogen-atom abstraction, proton transfer, and
single-electron transfer).1,6 While electron-rich, radical-stabi-
lizing oxygen and sulfur moieties have traditionally prevailed
metal compounds with N/O/S-containing polydentate
ligands has led to the storage of oxidation equivalents onto
the ligand rather than the metal and has drawn attention to
the care needed in correctly evaluating the role of noninno-
cent ligands in redox processes of a stoichiometric or catalytic
nature.24 However, metalloradicals involving N-donor
moieties,25 especially those that can be generated by redox
processes to afford incipient N-centered radicals, remain
scarce and require further exploration in order to understand
their reactivity in the context of a rapidly growing field of
nitrogen-rich ligands.
as research targets,17,18 nitrogen-centered radicals19,20 (NR2
•
and NR3•þ) are currently emerging as worthy competitors.
Protonation or metalation (especially with hard metal
centers) of these nitrogen-centered radicals can give rise to
potent electrophilic species involved, for instance, in typical
olefin addition and H-atom abstraction reactions.21 When
the metal site is redox-active, challenges arise in establishing
the correct electronic state of the M-NR2 fragment, either as
a bona fide amido unit (Mnþ-NR2-) or as an aminyl radical-
In previous work,26 we have shown that tripodal reagents
(M=Fe(II), Mn(II), Cr(III)), featuring the rigid triphenyla-
mido-amine core and electron-rich aryl arms (Chart 2, R=4-
t-BuPh-), can be oxidized by dioxygen and other oxidants to
generate a transient aminyl radical at an equatorial Namido
atom. The metal-bound aminyl radical causes rearrangement
of the ligand, followed by storage of the oxidizing equivalent
in a newly formed ortho-diiminosemiquinonato residue
(o-disq-1, Chart 3), which can be further oxidized or reduced
by one electron. In the present work, we set the stage for
further exploration of the redox chemistry of ferrous tripodal
compounds, by providing a whole family of reagents bearing
the triphenylamido-amine core and a variety of pendant arms
(R).27 The ligands examined in this work are cousins of the
•
bearing moiety (M(n-1)þ-NR2 ).22 Wieghardt et al.’s work23
has provided numerous instances in which the oxidation of
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110 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
TREN family28-30 but, owing to the triphenylamido-amine
core, are significantly more rigid and electronically variable.
Depending on the arm substituents, the reagents can be
infinitely tuned to display a continuum ranging from non-
innocent to redox-robust ligand behavior and, thus, permit a
systematic analysis of the metal/ligand synergism in support-
ing atom- and group-transfer chemistry. Elements of struc-
ture, characterization, and redox behavior are explored in
this work, whereas aspects of reductive and oxidative reac-
tivity will be communicated separately in due course. Parts of
the present work have been previously documented in two
Ph.D. theses.27
general, settings for the ion optics were determined automati-
cally during the regular tuning and calibration of the instru-
ment. For high-resolution data, the Orbitrap analyzer is set to a
resolution of 100 000. Microanalyses were done by Galbraith
Laboratories, Knoxville, Tennessee; Quantitative Technologies
Inc., Whitehouse, New Jersey; and on an in-house Perkin-Elmer
2400 CHN analyzer.
Ligand Synthesis. [(3,5-di-tert-Butyl-C6H3)-NH-(2-C6H4)]3N
(L2H3, 2a). A 250-mL Schlenk flask was charged with
(2-NH2-C6H4)3N (1.59 g, 5.46 mmol), 3,5-di-tert-butylbromo-
benzene (4.28 g, 16.00 mmol), and tert-Bu-ONa (1.78 g, 18.50
mmol). The slurry was stirred in anhydrous toluene. In another
Schlenk flask, [Pd2(dba)3] (tris(dibenzylideneacetone)dipalladium,
0.075 g, 0.08 mmol) and BINAP (2,20-bis(diphenylphosphino)-
1,10-binaphthyl, 0.150 g, 0.24 mmol) were stirred in anhydrous
toluene and heated until BINAP dissolved. The resulting cherry-
red solution was transferred into the former flask via a cannula.
The slurry was stirred at 100 ꢀC for 36 h to give a brown solution,
with precipitation of a solid. The solution was allowed to cool to
room temperature followed by the addition of diethylether. The
brown solution was filtered, and the filtrate was evaporated under
a vacuum to give a brown oily residue. The residue was dissolved
in 10.0 mL of hexane to give a brown solution, which upon
standing at room temperature gave a tan-colored solid. The solid
was filtered under a vacuum and washed with a minimum amount
of cold hexane. The filtrates were treated in a similar manner to
obtain the product in a combined yield of 58% (2.63 g). 1H NMR
(CDCl3, 7.27 ppm): δ 7.22 (d, 3H, J=7.6 Hz, aryl), 7.06 (m, 6H,
aryl), 6.95 (t, 3H, J=1.6 Hz, aryl), 6.78 (t, 3H, J=7.2 Hz, aryl),
6.63 (d, 6H, J=1.6 Hz, aryl), 5.89 (s, 3H, NH), 1.16 (s, 54H, tert-
butyl). 13C NMR (CDCl3, 77 ppm): δ 151.81, 141.89, 140.53,
134.33, 126.08, 126.38, 126.06, 116.88, 116.06, 115.85, 34.89,
31.56. IR (KBr, cm-1): 3402, 3367, 3063, 2962, 2867, 1588,
1527, 1481, 1455, 1362, 1330, 1272, 1205, 1134, 1043, 977, 754,
711, 629. MS-EI (m/z) calcd: 855.31. Found: 852.4 ([M] - 3H).
Elem anal. calcd for C60H78N4: C, 84.26; H, 9.19; N, 6.55. Found:
C, 84.14; H, 9.28; N, 6.47.
Experimental Section
General Considerations. All operations were performed under
anaerobic conditions under a pure dinitrogen or argon atmo-
sphere using Schlenk techniques on an inert gas/vacuum mani-
fold or in a drybox (O2, H2O < 1 ppm). Anhydrous diethyl
ether, methylene chloride, acetonitrile, tetrahydrofuran, hex-
ane, pentane, benzene, toluene, dimethylformamide, dimethy-
lacetamide, and dimethylsulfoxide were purchased from
Sigma-Aldrich. Ethanol and methanol were distilled over the
corresponding magnesium alkoxide, and acetone was distilled over
drierite. Solvents were degassed by three freeze-pump-thaw
cycles. Unless otherwise noted, all other reagents were pur-
chased at the highest purity available. Potassium hydride was
provided as a dispersion in mineral oil and was thoroughly
washed prior to use with copious amounts of tetrahydrofuran
followed by hexane. Compounds L1H3 (1),26a L7H3 (3c),31 and
[K(THF)3][(L1)Fe(II)-THF] 0.5THF (5)26b have been pre-
3
pared according to literature methods. 1H NMR and 13C
NMR spectra were recorded on Varian XL-400, Varian IN-
OVA/UNITY 400 MHz, and Varian 300 Unity Plus NMR
spectrometers. 19F NMR spectra were recorded on a Varian
Mercury spectrometer (at 188 MHz), and chemical shifts are
referenced to trifluoroacetic acid (TFA, external standard). IR
spectra were obtained on a Perkin-Elmer 883 IR spectrometer
and FT-IR spectra on Nicolet Nexus 470 and 670 and Magna
750 FT-IR ESP spectrometers. UV-vis spectra were obtained
on Hewlett-Packard 8452A diode array, Varian Cary 50, and
Varian Cary 300 spectrophotometers. Electron ionization (EI)
and fast-atom bombardment (FAB) mass spectra were obtained
on a Finnigan MAT-90 mass spectrometer. Electrospray ioni-
zation mass spectrometry (ESI) and atmospheric pressure che-
mical ionization mass spectra were obtained on a Thermo-
Finnigan TSQ7000 triple-quadrupole mass spectrometer,
equipped with the API2 source and Performance Pack
(ThermoFinnigan, San Jose, CA). HRMS data were collected
on a Thermo Fisher Scientific LTQ-Orbitrap XL hybrid mass
spectrometer, using the Orbitrap analyzer for acquisition of
high-resolution accurate mass data. Samples were infused using
the integrated syringe pump at 3 μL/min, and ionization was via
the electrospray source with source settings at their defaults. In
[(3,5-Bis-trifluoromethyl-C6H3)-NH-(2-C6H4)]3N (L3H3,
2b). A 250-mL Schlenk flask was charged with (2-NH2-
C6H4)3N (1.59 g, 5.46 mmol), 3,5-bis-trifluoromethyl-bromo-
benzene (4.79 g, 16.38 mmol), and Cs2CO3 (8.15 g, 25.0 mmol).
The slurry was stirred in anhydrous toluene. In another Schlenk
flask, [Pd2(dba)3] (0.125 g, 0.137 mmol) and BINAP (0.255 g,
0.41 mmol) were stirred in anhydrous toluene and heated until
BINAP dissolved. The resulting cherry-red solution was trans-
ferred into the former flask via a cannula. The slurry was stirred
at 100 ꢀC for 36 h to give a brown solution. At the end of the
reaction, the solution was allowed to cool to room temperature,
followed by the addition of diethylether. The brown solution
was filtered, and the filtrate was evaporated to give a brown-
black oily residue. The addition of hexane precipitated a white
solid, which was filtered and recrystallized from hot hexane to
afford the product as a pure crystalline solid (2.62 g, 52%). 1H
NMR (CDCl3, 7.27 ppm): δ 7.27 (s, 6H, aryl), 7.19 (m, 3H, aryl),
7.08 (d, 6H, J=4.0 Hz, aryl), 6.84 (s, 3H, aryl), 5.46 (s, 3H, NH).
13C NMR (CDCl3, 77 ppm): δ 144.29, 137.98, 135.03, 132.73 (q,
2JCF=33.4 Hz, aryl), 126.47, 125.83, 125.10, 122.90 (q, 1JCF
=
(28) (a) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9–16. (b) Verkade, J. G.
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272.6 Hz, CF3), 121.83, 115.54. 19F NMR (CDCl3, TFA, -77
ppm): δ -63.03. IR (KBr, cm-1): 3398, 3359, 3300, 3278, 3062,
1624, 1591, 1531, 1489, 1471, 1421, 1385, 1282, 1188, 1130, 999,
964, 881, 758, 729, 702, 683, 625. MS-FAB (m/z) calcd: 926.65.
Found: 926.17. HRMS (ESI), m/z, [M þ H]þ calcd. for
C42H25N4F18: 927.1786. Found: 927.1788. Elem anal. calcd
for C42H24N4F18: C, 54.44; H, 2.61; N, 6.05. Found: C, 54.24;
H, 2.58; N, 6.17.
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[(tert-Butyl-CO)-NH-(2-C6H4)]3N (L4H3, 3a). To a stirred
solution of (2-NH2-C6H4)3N (2.90 g, 10.0 mmol) and triethy-
lamine (5.10 mL, 36.6 mmol) in THF (40 mL) was slowly added
trimethylacetyl chloride (3.62 g, 30.0 mmol) at 0 ꢀC by means of
(31) Jones, M. B.; MacBeth, C. E. Inorg. Chem. 2007, 46, 8117–8119.

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 111
a syringe, and the mixture was allowed to warm up to room
temperature and then stirred overnight. The solvent was re-
moved under reduced pressure, and the crude residue was
purified by column chromatography with petroleum ether/ethyl
acetate (3:1) to give the product as a white solid (4.34 g, 80%).33
1H NMR (CDCl3, 7.27 ppm): δ 8.08 (s, 3H, NH), 7.73 (d, 3H, J=
7.6 Hz, aryl), 7.06 (t, 3H, J=7.6 Hz, aryl), 6.97 (t, 3H, J=7.6,
[(CF3CO)-NH-(2-C6H4)]3N (L8H3, 3d). To a stirred solu-
tion of (2-NH2-C6H4)3N (2.90 g, 10.0 mmol) and triethylamine
(5.10 mL, 36.6 mmol) in THF (40 mL) was slowly added
trifluoroacetic anhydride (4.17 mL, 30.0 mmol) at 0 ꢀC, and
the mixture was allowed to warm up to room temperature
and then stirred overnight. The solvent was removed under
reduced pressure, and the crude residue was purified by column
chromatography with petroleum ether/ethyl acetate (3:1) to give
the product as a white solid (5.09 g, 88%). Colorless crystals
suitable for X-ray structural analysis were grown from aceto-
nitrile at room temperature. 1H NMR (CDCl3, 7.27 ppm): δ 8.61
(s, 3H, NH), 7.65 (d, 3H, J=6.8 Hz, aryl), 7.21 (m, 6H, aryl),
6.86 (d, 3H, J=15.6 Hz, aryl). 13C NMR (CDCl3, 77 ppm): δ
aryl), 6.79 (d, 3H, J=8.0 Hz, aryl), 0.93 (s, 27H, tert-butyl). 13
C
NMR (CDCl3, 77 ppm): δ 176.42, 137.71, 131.31, 125.77,
125.46, 124.52, 124.17, 39.19, 26.98. IR (KBr, cm-1): 3389,
3308, 3282, 3259, 3062, 3034, 2974, 2960, 2909, 2869, 1680,
1653, 1591, 1516, 1492, 1480, 1439, 1396, 1365, 1302, 1264, 1228,
1161, 1115, 1052, 1028, 930, 771, 757, 733, 624, 490, 478. MS-
FAB (m/z) calcd: 542.33. Found: 542.32. Elem anal. calcd for
C33H42N4O3: C, 73.03; H, 7.80; N, 10.32. Found: C, 72.94; H,
7.68; N, 10.54.
2
154.93 (q, JCF =37.9 Hz, C(O)CF3), 138.05, 128.51, 127.75,
1
126.28, 125.97, 124.31, 115.37 (q, JCF =288.5 Hz, CF3). 19F
NMR (CDCl3, TFA, -77 ppm): δ -75.13. IR (KBr, cm-1):
3266, 3136, 3070, 1727, 1703, 1540, 1287, 909, 759. HRMS
(ESI), m/z, [M þ H]þ calcd. for C24H16N4F9O3: 579.1073.
Found: 579.1071. Calcd for C24H15N4F9O3: C, 49.84; H, 2.61;
N, 9.69. Found: C, 49.88; H, 2.57; N, 9.44.
[(3,5-Dichloro-C6H3)-NH-(2-C6H4)]3N (L5H3, 2c). A mixture
of BINAP (0.089 g, 0.15 mmol), [Pd2(dba)3] (0.046 g, 0.05
mmol), and Cs2CO3 (3.26 g, 10 mmol) was stirred in 10.0 mL
of anhydrous toluene at ambient temperature for 30 min in
a 50.0 mL Schlenk flask in the drybox. To this mixture were
added 1-bromo-3,5-dichloro-benzene (1.36 g, 6.0 mmol) and
(2-NH2-C6H4)3N (0.58 g, 2.0 mmol). The flask was attached to
a Schlenk line, and the mixture was heated under reflux under a
nitrogen atmosphere for 42 h. The mixture was then allowed to
cool down to room temperature and filtered under a vacuum.
The solution was concentrated under a vacuum, and the residue
was purified by column chromatography on silica gel (ethyl
acetate/petroleum ether=1:20) to afford the product as a pale
yellow solid. The solid was recrystallized from chloroform/
hexane to afford pure product as a white solid (0.834 g, 1.15
mmol) in 57.5% isolated yield. 1H NMR (CDCl3, 7.24 ppm): δ
7.30 (d, 6H, J=7.6 Hz, aryl), 7.13-7.18 (m, 3H, aryl), 7.00-7.02
(m, 6H, aryl), 6.78 (t, 3H, J=1.6 Hz, aryl), 6.38 (d, 6H, J=1.6
Hz, aryl), 5.25 (s, 3H, NH). 13C NMR (CDCl3, 77 ppm): δ
144.88, 137.78, 135.66, 126.26, 125.87, 124.25, 121.79, 120.70,
114.56. IR (KBr, cm-1): 3395, 1578, 1505, 1468, 1442, 1306,
1253, 1109, 987, 954, 829, 759, 756, 657, 563. MS-FAB (m/z)
calcd: 722.01. Found: 722.01. HRMS (ESI), m/z [M]þ calcd. for
C36H24N4Cl6: 722.0127. Found: 722.0130. Elem anal. calcd for
C36H24N4Cl6: C, 59.61; H, 3.34; N, 7.72. Found: C, 59.44; H,
3.38; N, 7.61.
[iso-Propyl-NH-(2-C6H4)]3N (L9H3, 4). In an ACE pressure
tube under an inert atmosphere, Shvo’s catalyst (0.16 g, 0.15
mmol) and (2-NH2-C6H4)3N (2.90 g, 10.0 mmol) were dis-
solved in toluene (7.25 mL) and isopropylamine (5.11 mL, 60.0
mmol). The pressure tube was then heated at 150 ꢀC for 24 h in
an oil bath. The solvent was removed in vacuo, and the crude
product was purified by column chromatography with petro-
leum ether/ethyl acetate (100:1) as an eluent to afford the
product as a white solid (3.75 g, 90%). Colorless crystals suitable
for X-ray structural analysis were grown from acetonitrile at
room temperature. 1H NMR (CDCl3, 7.24 ppm): δ 6.99 (td, J=
7.8 Hz, J=1.6 Hz, 3H, aryl), 6.87 (dd, J=7.8 Hz, J=1.6 Hz, 3H,
aryl), 6.61 (dd, J=7.8 Hz, J=1.2 Hz, 2H, aryl), 6.53 (td, J=7.8
Hz, J=1.2 Hz, 4H, aryl), 3.78 (d, J=8.0 Hz, 3H, NH), 3.52 (m,
3H, CHMe2), 0.95 (br, 18H, CHMe2). 13C NMR (CD3Cl, 77
ppm): δ 142.37, 132.31, 125.66, 125.53, 116.19, 111.33, 43.61,
22.81. IR (KBr, cm-1): 3266, 3136, 3070, 1727, 1703, 1541, 1287,
1154, 909, 758. HRMS (ESI), m/z, [M þ H]þ calcd. for
C27H37N4: 417.3013. Found: 417.3012. Calcd. for C27H36N4:
C, 77.84; H, 8.71; N, 13.45. Found: C, 77.54; H, 8.61; N, 13.63.
Synthesis of Ferrous Complexes. [K(THF)3][(L2)Fe(II)-
THF] (6). The ligand L2H3 (0.285 g, 0.33 mmol) was dissolved
in degassed THF (15 mL), and KH (0.040 g, 1.0 mmol) was
added to this solution. The mixture was allowed to stir over-
night, followed by the addition of anhydrous FeCl2 (0.042 g,
0.33 mmol). This mixture was stirred for an additional 12 h to
afford a dark yellow-brown solution. The solution was filtered,
and the filtrate was reduced under a vacuum to 3.0 mL. Pentane
(20 mL) was carefully layered over the THF, and the system was
allowed to slowly mix at -30 ꢀC to afford yellow-brown crystal-
line material of the title compound (0.235 g, 58%). Crystals
suitable for X-ray analysis can be obtained as long yellow-
brown needles from concentrated THF solutions. 1H NMR
(CD3CN, 1.95 ppm): δ 47.28, 36.23, 31.93, 18.67, 13.60, 8.23,
7.6-6.46, 6.05, 3.66, 2.12, 1.82, 1.22, 1.17, 0.22, -1.82, -2.98,
-8.56, -23.43, -26.76, -32.20. IR (KBr, cm-1): 3385, 3043,
2954, 2895, 2859, 1579, 1474, 1442, 1427, 1388, 1358, 1326, 1307,
1275, 1244, 1200, 1153, 1123, 1109, 1039, 995, 972, 902, 861, 849,
744, 707, 620. UV-vis (THF): λmax (ε (M-1 cm-1)) 368 (23 000).
Elem anal. calcd for C76H107N4FeKO4: C, 73.87; H, 8.73; N,
4.53. Found: C, 74.11; H, 8.68; N, 4.59.
[(Phenyl-CO)-NH-(2-C6H4)]3N (L6H3, 3b). To a stirred
solution of (2-NH2-C6H4)3N (2.90 g, 10.0 mmol) and triethy-
lamine (5.10 mL, 36.6 mmol) in THF (35 mL) was slowly added
benzoyl chloride (4.22 g, 30.0 mmol) at 0 ꢀC by means of a
syringe, and the mixture was allowed to warm up to room
temperature and then stirred overnight. The solvent was re-
moved under reduced pressure, and the crude residue was
purified by recrystallization from hexane/ethyl acetate (10:1)
1
to give the product as a white solid (5.37 g, 89%). H NMR
(CDCl3, 7.24 ppm): δ 8.82 (s, 3H, NH), 7.57 (m, 3H, aryl), 7.42
(m, 9H, aryl), 7.24 (t, 6H, J=7.6 Hz, aryl), 6.97 (m, 9H, aryl). 13
C
NMR (CDCl3, 77 ppm): δ 165.54, 137.96, 133.77, 131.50,
131.10, 127.97, 127.13, 126.27, 125.73, 124.79, 124.29. IR
(KBr, cm-1): 3350, 3277, 3058, 3025, 1674, 1660, 1638, 1597,
1578, 1525, 1490, 1477, 1453, 1438, 1311, 1263, 1160, 1121, 1073,
1047, 1025, 946, 926, 902, 828, 797, 756, 715, 700, 688, 624, 592,
496, 477. MS-FAB (m/z) calcd: 602.23. Found: 603.23 ([M þ
H]). HRMS (ESI), m/z [M þ H]þ calcd. for C39H31N4O3:
603.2391. Found: 603.2393. Elem anal. calcd for C39H30N4O3:
C, 77.72; H, 5.02; N, 9.30. Found: C, 77.84; H, 5.08; N, 9.44.
[K(THF)2][(L3)Fe(II)-THF] (7). The ligand L3H3 (0.400 g,
0.43 mmol) was dissolved in degassed THF (30 mL), and KH
(0.052 g, 1.3 mmol) was added to this solution. The mixture was
allowed to stir overnight, followed by the addition of anhydrous
FeCl2 (0.055 g, 0.43 mmol). This mixture was stirred for an
additional 12 h to afford a dark yellow-brown solution. The
solvent was reduced under a vacuum to 10.0 mL, and the
solution was refrigerated (-30 ꢀC) overnight. The insoluble
(32) (a) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575–5580. (b) Wolfe, J. P.;
Buchwald, S. L. J. Org. Chem. 2000, 65, 1144–1157.
(33) A new synthesis of ligand L4H3 appeared in the literature while this
manuscript was in an advanced stage of preparation. A previous synthesis
was communicated in ref 27b: Jones, M. B.; Hardcastle, K. I.; MacBeth, C. E.
Polyhedron 2009, DOI: 10.1016/j.poly.2009.06.013.

112 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
salts were filtered off on an anaerobic frit in the drybox, and the
filtrate was further reduced to 5.0 mL. Pentane (20 mL) is care-
fully layered over the THF, and the system is allowed to slowly
mix at -30 ꢀC to afford yellow-brown crystalline material of the
title compound (0.330 g, 62%). Crystals suitable for X-ray
diffraction are obtained by the slow diffusion of pentane into
concentrated THF solutions of the bulk material at -30 ꢀC. 1H
NMR (CD3CN, 1.95 ppm): δ 32.82, 15.66, 7.20, 7.05, 6.83, 3.65,
1.80, -1.82, -5.27, -29.41, -35.00. IR (KBr, cm-1): 3385,
3049, 2966, 2872, 1618, 1587, 1518, 1477, 1412, 1379, 1274, 1172,
1128, 1042, 995, 959, 869, 858, 754, 727, 701, 682, 665, 623.
UV-vis (THF): λmax (ε (M-1 cm-1)): 339 (38 000). Elem anal.
calcd for C54H45N4F18FeKO3: C, 52.52; H, 3.67; N, 4.54.
Found: C, 52.28; H, 3.56; N, 4.34.
6H, J=8.3 Hz, aryl), 7.35 (t, 3H, J=6.8 Hz, aryl), 7.20 (t, 6H, J=
7.1 Hz, aryl), 7.04 (t, 6H, J=3.6 Hz, aryl), 6.92 (m, 3H, aryl). IR
(KBr, cm-1): 3049, 3016, 2970, 2865, 1523, 1477, 1445, 1379,
1309, 1257, 1130, 1090, 928, 899, 793, 752, 703, 621, 585, 416.
UV-vis (THF): λmax (ε (M-1 cm-1)) 290 (sh). Elem anal. calcd
for C106H110N8Fe2K2O13: C, 67.22; H, 5.85; N, 5.92. Found: C,
67.47; H, 5.91; N, 5.84.
[K(NCMe)][(L6)Fe(II)-NCMe] 2MeCN (11). The ligand
3
L6H3 (0.242 g, 0.40 mmol) was dissolved in degassed THF
(30 mL), and KH (0.048 g, 1.20 mmol) was added to this
solution. The mixture was allowed to stir overnight, followed
by the addition of anhydrous FeCl2 (0.051 g, 0.40 mmol). This
mixture was stirred for an additional 12 h to afford a yellow-
orange solution. The solvent was reduced under a vacuum to
10.0 mL, and the solution was refrigerated (-30 ꢀC) overnight.
The insoluble salts were filtered off on an anaerobic frit in the
drybox, and the filtrate was evaporated under a vacuum to
dryness. The residue was dissolved in MeCN (30 mL); the yellow
solution was filtered and reduced to 10.0 mL. Yellow-orange
crystals (0.222 g, 65%), suitable for X-ray analysis, were depos-
[K(DMA)2][(L4)Fe(II)] (8). The ligand L4H3 (0.398 g, 0.73
mmol) was dissolved in degassed DMA (10 mL), and KH (0.088
g, 2.19 mmol) was added to this solution. The mixture was
allowed to stir overnight, followed by the addition of anhydrous
FeCl2 (0.093 g, 0.73 mmol). This mixture was stirred for an
additional 12 h to afford a light green-brown solution. The
solution was refrigerated (-30 ꢀC) overnight, and the insoluble
salts were filtered off on an anaerobic frit in the drybox. Diethyl
ether (20 mL) was carefully layered over the DMA solution, and
the system is allowed to slowly mix at -30 ꢀC to afford light
brown crystalline material of the title compound as sticky
crystalline material suitable for X-ray diffraction (0.290 g,
1
ited from the MeCN solution overnight. H NMR (CD3CN,
1.95 ppm): δ 28.28, 13.93, 12.66, 7.59, 7.60-7.42, 7.38, 7.36,
7.33, 7.23, 7.21, 7.18, 6.93, 6.92, 6.81, 6.79, 6.78, 6.66, 5.71, 4.93,
3.04, 2.10, 1.98, 1.29. IR (KBr, cm-1): 3047, 3014, 2915, 2847,
1593, 1579, 1554, 1473, 1442, 1371, 1241, 1157, 1131, 1093, 1069,
1038, 1022, 929, 796, 755, 722, 699, 621, 593. UV-vis (MeCN):
λmax (ε (M-1 cm-1)) 271 (49500). Elem anal. calcd for
C47H39N8FeKO3: C, 65.73; H, 4.58; N, 13.05. Found: C,
65.49; H, 4.66; N, 13.33.
1
48.9%). H NMR (CD3CN, 1.95 ppm): δ 56.77, 47.85, 34.92,
20.95, 19.70, 13.12, 12.49, 11.97, 11.69, 7.99, 7.65, 7.53, 6.92,
6.59, 2.97, 2.84, 1.98, 0.91, -3.00, -10.82, -14.80. IR (KBr,
cm-1): 3266, 3048, 2943, 2916, 2859, 1634, 1591, 1557, 1534,
1473, 1442, 1389, 1338, 1311, 1265, 1240, 1212, 1169, 1106, 1037,
1014, 958, 931, 914, 752, 626, 589. UV-vis (THF): λmax (ε (M-1
cm-1)) 294 (14 000). Elem anal. calcd for C41H57N6FeKO5: C,
60.88; H, 7.10; N, 10.39. Found: C, 60.48; H, 7.21; N, 10.52.
[K(L5)Fe(II)-THF] (9). The ligand L5H3 (0.354 g, 0.49 mmol)
was dissolved in degassed THF (30 mL), and KH (0.059 g, 1.47
mmol) was added to this solution. The mixture was allowed to
stir overnight, followed by the addition of anhydrous FeCl2
(0.062 g, 0.49 mmol). This mixture was stirred for an additional
12 h to afford a light green-brown solution. The solvent was
reduced under a vacuum to 10.0 mL, and the solution was
refrigerated (-30 ꢀC) overnight. The insoluble salts were filtered
off on an anaerobic frit in the drybox, and the filtrate was further
reduced to 5.0 mL. Pentane (20 mL) is carefully layered over the
THF, and the system is allowed to slowly mix at -30 ꢀC to
afford off-white crystalline material of the title compound
(0.380 g, 87%). Essentially colorless crystals suitable for X-ray
diffraction are obtained by the slow diffusion of pentane into
concentrated THF solutions of the bulk material at -30 ꢀC. 1H
NMR (CD3CN, 1.95 ppm): δ 15.60, 7.25, 6.88, 6.61, 3.65, 1.81,
-1.91, -5.10. IR (KBr, cm-1): 3621, 3386, 3051, 2968, 2868,
1578, 1504, 1473, 1440, 1309, 1275, 1255, 1156, 1109, 1064, 1042,
987, 978, 954, 831, 794, 756, 76, 705, 665, 657, 622, 469, 430.
UV-vis (THF): λmax (ε (M-1 cm-1)) 374 (25 000). Elem anal.
calcd for C40H29N4Cl6FeKO: C, 54.02; H, 3.29; N, 6.30. Found:
C, 54.18; H, 3.26; N, 6.48.
[K2(DMA)4][K(L7)2Fe(II)2]2 2DMA (12). The ligand L7H3
3
(0.274 g, 0.55 mmol) was dissolved in degassed DMA (10 mL),
and KH (0.066 g, 1.65 mmol) was added to this solution. The
mixture was allowed to stir overnight, followed by addition of
anhydrous FeCl2 (0.070 g, 0.55 mmol). This mixture was stirred
for an additional twelve hours to afford a yellow-brown solu-
tion. The solution was refrigerated (-30 ꢀC) overnight, and the
insoluble salts were filtered off on an anaerobic frit in the
Drybox. Diethyl ether (20 mL) was carefully layered over the
DMA solution, followed by 10 mL of pentane, and the system
was allowed to slowly mix at -30 ꢀC to afford light yellow
crystalline material of the title compound, which is suitable for
X-ray diffraction (0.265 g, 67%). 1H NMR (CD3CN, 1.95 ppm):
δ 7.72 (d, 12H, J=7.5 Hz, aryl), 7.12 (td, 12H, J=1.5 Hz, J=7.3
Hz, aryl), 7.05 (td, 12H, J=1.5 Hz, J=7.3 Hz, aryl), 6.72 (d,
12H, J=7.6 Hz, aryl), 2.94 (s, 18H, DMA), 2.81 (s, 18H, DMA),
2.21 (sept, 12H, J=6.7 Hz, CH), 1.97 (s, DMA), 0.93 (s, 36H,
CH3), 1.51 (s, 36H, CH3). IR (KBr, cm-1): 3615, 3225, 3174,
3105, 3048, 3017, 2958, 2923, 2862, 1636, 1588, 1541, 1515, 1477,
1443, 1410, 1393, 1304, 1258, 1216, 1196, 1159, 1089, 1034, 1014,
963, 944, 914, 887, 745, 621, 590, 548, 486, 476. UV-vis
(MeCN): λmax (ε (M-1 cm-1)) 293 (53 000). Elem anal. calcd
for C144H186N22Fe4K4O18: C, 59.78; H, 6.48; N, 10.65; Fe 7.72.
Found: C, 59.42; H, 6.46; N, 10.53, Fe 7.78.
[K(DMA)][(L8)Fe(II)-DMA] (13) and [K(NCMe)][(L8)Fe(II)-
NCMe] (14). The ligand L8H3 (0.250 g, 0.43 mmol) was dis-
solved in degassed DMA (10 mL), and KH (0.052 g, 1.30 mmol)
was added to this solution. The mixture was allowed to stir
overnight, followed by the addition of anhydrous FeCl2 (0.055
g, 0.43 mmol). This mixture was stirred for an additional 12 h to
afford a light yellow-brown solution. The solution was refriger-
ated (-30 ꢀC) overnight, and the insoluble salts were filtered off
on an anaerobic frit in the drybox. Diethyl ether (20 mL) was
carefully layered over the DMA solution, and the system was
allowed to slowly mix at -30 ꢀC to afford off-white crystalline
material of the title compound (0.212 g, 58%). Crystals suitable
for X-ray diffraction analysis were obtained from slow diffusion
of diethyl ether into concentrated DMA solutions of the com-
[K2(THF)5][(L6)2Fe(II)2] 2THF (10). The ligand L6H3 (0.323
3
g, 0.54 mmol) was dissolved in degassed THF (30 mL), and KH
(0.065 g, 1.62 mmol) was added to this solution. The mixture was
allowed to stir overnight, followed by the addition of anhydrous
FeCl2 (0.068 g, 0.54 mmol). This mixture was stirred for an
additional 12 h to afford a yellow-orange solution. The solvent
was reduced under a vacuum to 10.0 mL, and the solution was
refrigerated (-30 ꢀC) overnight. The insoluble salts were filtered
off on an anaerobic frit in the drybox, and the filtrate was further
reduced to 5.0 mL and refrigerated at -30 ꢀC. Large yellow-
orange crystals (0.264 g, 52%), suitable for X-ray analysis, were
deposited from the THF solution after several days. 1H NMR
(d8-THF, 1.73, 3.58 ppm): δ 7.80 (t, 3H, J=4.7 Hz, aryl), 7.49 (d,
1
pound. H NMR (CD3C(O)N(CD3)2, 2.10 ppm): δ 7.66, 7.31,
6.98, 3.14, 2.97, 2.10. IR (KBr, cm-1): 3487, 3061, 3019, 2928,

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 113
2869, 1726, 1627, 1483, 1453, 1402, 1258, 1163, 1019, 966, 756,
726, 594. UV-vis (DMA): λmax 286 (sh). Elem anal. calcd for
C32H30N6F9FeKO5: C, 45.51; H, 3.58; N, 9.95. Found: C, 45.23;
H, 3.47; N, 9.77.
and University of Missouri-St. Louis by using a Bruker
SMART CCD (charge-coupled device)-based diffractometer
equipped with an Oxford Cryostream low-temperature appara-
tus operating at variable low temperatures. A suitable crystal
was chosen and mounted on a glass fiber using grease. Data were
measured using ω scans of 0.3ꢀ per frame for 30 s, such that a
hemisphere was collected. A total of 1271 frames were collected
with a maximum resolution of 0.75 A. The first 50 frames were
recollected at the end of data collection to monitor for decay.
Cell parameters were retrieved using SMART software and
refined using SAINT on all observed reflections. Data reduction
was performed using the SAINT software, which corrects for Lp
and decay. The structures are solved by the direct method using
the SHELXS-97 program and refined by the least-squares
method on F2, SHELXL-97, incorporated in SHELXTL-PC
V 5.10. All non-hydrogen atoms were refined anisotropically.
Hydrogens were calculated by geometrical methods and refined
as a riding model. The crystals used for diffraction studies
showed no decomposition during data collection. All drawings
are done at 30% ellipsoids unless otherwise stated. Pertinent
crystallographic data are collected in Tables 1 (compounds
6-15) and S1 (compounds 2a, 3a-d, 4; Supporting Informa-
tion). Selected bond distances and angles for compounds 6-15
are given in Tables 2 and 3, respectively.
The acetonitrile adduct 14 was synthesized by dissolving 13 in
MeCN, filtering, and concentrating the solution to saturation.
Colorless crystals of the compound were obtained overnight at
1
-30 ꢀC (85%). H NMR (CD3CN, 1.95 ppm): δ 20.64, 11.19,
2.74. IR (KBr, cm-1): 3431, 3057, 3021, 2994, 2929, 2579, 2307,
2281, 2255, 1630, 1587, 1478, 1451, 1416, 1368, 1303, 1253, 1225,
1146, 1104, 1035, 949, 938, 870, 865, 824, 773, 762, 746, 727, 697,
665, 626, 611, 560, 496, 464, 428, 418. UV-vis (MeCN): λmax
(ε (M-1 cm-1)) 250 (35 700). Elem anal. calcd For C28H18N6-
F9FeKO3: C, 44.70; H, 2.41; N, 11.17. Found: C, 44.52; H, 2.44;
N, 11.32.
[K(L9)Fe(II)]2 (15). The ligand L9H3 (0.223 g, 0.54 mmol) was
dissolved in degassed THF (30 mL), and KH (0.065 g, 1.62
mmol) was added to this solution. The mixture was allowed to
stir overnight, followed by the addition of anhydrous FeCl2
(0.068 g, 0.54 mmol). This mixture was stirred for an additional
12 h to afford a green-brown solution. The solvent was reduced
under a vacuum to 10.0 mL, and the solution was refrigerated
(-30 ꢀC) overnight. The insoluble salts and a green-brown oily
residue were filtered off on an anaerobic frit in the drybox, and
the orange filtrate was further reduced to 5.0 mL. Pentane
(20 mL) is carefully layered over the THF, and the system is
allowed to slowly mix at -30 ꢀC to afford orange crystalline
material of the title compound (0.140 g, 51%). Crystals suitable
for X-ray diffraction are obtained by the slow diffusion of
pentane into concentrated THF solutions of the bulk material
at -30 ꢀC. 1H NMR (CD3CN, 1.95 ppm): δ 68.00, 50.22, 46.12,
41.68, 35.32, 6.90, 6.84, 3.65, 2.96, 2.84, 2.22, 1.81, -7.22, -8.15,
-14.62, -26.22. IR (KBr, cm-1): 3490, 3386, 3054, 3026, 2955,
2919, 2860, 1595, 1580, 1510, 1461, 1433, 1380, 1361, 1316, 1260,
1228, 1173, 1153, 1120, 1042, 740, 626, 576, 417. UV-vis (THF):
Results and Discussion
Ligand Synthesis. Scheme 1 summarizes the ligands
employed in the present study. Their synthesis (Scheme 2)
makes use of a common precursor, 2,20,200-triamino-
triphenylamine (LH3), previously reported by us,26a and
by MacBeth and Jones31 in a more concise preparation.
Aryl, acyl, and alkyl arms are then appended to the ortho-
NH2 moieties via a variety of C-N coupling methodo-
logies.
Aryl arms can be readily installed according to the
general Hartwig-Buchwald, Pd-mediated, coupling meth-
odology for the arylation of amines.32 Substituents at aryl
positions 4- (1) and 3,5- (2) can be easily accommodated
to afford a series of electronically and sterically diverse
variants (Scheme 2). The electron-rich ligand 1 has been
previously reported26 and shown to engage in oxidative
ligand rearrangement in several Fe(II), Mn(II), and
Cr(III) complexes.
Acyl arms can be attached via straightforward con-
densations of the aniline -NH2 moieties with acyl chlor-
ides or anhydrides in the presence of a base (3, Scheme 2).
Better yields are obtained by using Et3N in organic
solvents rather than NaOH in aqueous media. The former
method was employed by MacBeth and Jones in the
synthesis of the previously reported ligand L7H3.31 Alkyl
arms proved to be the most difficult to append but are
amenable to installation via condensation of the aniline
-NH2 units with alkylamines, mediated by Shvo’s cata-
lyst34 (4, Scheme 2). The procedure, which leads to
ammonia formation, was pioneered by Beller et al.35
and is applicable only to alkylamines possessing R-H
atoms, by means of the “borrowing hydrogen” methodo-
logy.36 A single example with iso-propyl substituents is
λ
max (ε (M-1 cm-1)) 313 (23 000). Elem anal. calcd for C54H66-
N8Fe2K2: C, 63.77; H, 6.54; N, 11.02. Found: C, 63.53; H, 6.47;
N, 10.94.
Other Physical Measurements. Cyclic voltammetry was car-
ried out with an Eco Chemie Autolab PGSTAT100 electroche-
mical workstation fitted in a drybox and controlled with the
General Purpose Electrochemical Software or with a Bipoten-
tiostat AFCBP1 from Pine Instrument Company fitted in a
drybox and controlled with the PineChem 2.7.9 software.
Experiments were performed using a gold disk working elec-
trode (2 mm diameter) and a Ag/Agþ (0.01 M AgNO3 and 0.1 or
0.5 M [nBu4N]PF6 in acetonitrile or DMSO) nonaqueous re-
ference electrode (Bioanalytical Systems, Inc.) with a prolonged
bridge (0.1 or 0.5 M [nBu4N]PF6 in acetonitrile or DMSO). A
thin Pt foil or gauge (8 cm2, Sigma-Aldrich) was employed as
counterelectrode. The working electrode was polished using
successively 6, 3, and 1 μm diamond paste on a DP-Nap
polishing cloth (Struers, Westlake, OH), washed with water
and acetone, and air-dried. The Pt foil and gauge electrodes were
cleaned in a H2O2/H2SO4(conc) solution (1:4 v/v) and oven-
dried. The concentrations of the samples were between 1 and
3 mM, and that of [nBu4N]PF6 (supporting electrolyte)
was 0.1 or 0.5 M. The potential sweep rate varied between
10-1000 mV/s. All potentials arereported versus the ferrocenium/
ferrocene (Fcþ/Fc) couple.
€
Mossbauer measurements were recorded on a constant accel-
eration conventional spectrometer with a source of 57Co (Rh
matrix) located at the Institute of Materials Science, NCSR
“Demokritos”, Greece. Variable-temperature spectra were ob-
tained by using an Oxford cryostat. Isomer shift values are
quoted relative to iron foil at 293 K.
(34) (a) Karvembu, R.; Prabhakaran, R.; Natarajan, N. Coord. Chem.
Rev. 2005, 249, 911–918. (b) Prabhakaran, R. Synlett 2004, 2048–2049.
€
(35) Hollmann, D.; Bahn, S.; Tillack, A.; Beller, M. Angew. Chem., Int.
Ed. 2007, 46, 8291–8294.
Crystallographic data were collected at the Departments of
Chemistry at Harvard University, Missouri University of
Science and Technology, University of Missouri-Columbia,
(36) Hamid, M. H. S. A.; Allen, C. L.; Lamb, G. W.; Maxwell, A. C.;
Maytum, H. C.; Watson, A. J. A.; Williams, J. M. J. J. Am. Chem. Soc. 2009,
131, 1766–1774.

114 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
Table 1. Summary of Crystallographic Data for Fe(II) Compounds 6-15
6
7
8
9
10
formula
Mr
cryst syst
space group
a (A)
C
76H107FeKN4O4
C54H45F18FeKN4O3
1234.89
triclinic
P1
12.732(4)
14.506(4)
16.574(5)
111.680(6)
92.961(6)
98.996(5)
2789.7(14)
2
1.470
223(2)
0.71073
0.451
0.1170
C41H57FeKN6O5
808.88
triclinic
P1
11.054(4)
11.624(4)
17.167(6)
101.488(6)
91.197(6)
95.524(6)
2149.77
2
1.250
223(2)
0.71073
0.496
0.0791
C40H29Cl6FeKN4O
889.32
monoclinic
C2/c
27.8793(15)
13.0277(7)
21.2649(11)
90
102.3540(10)
90
C212H220Fe4K4N16O26
3787.84
triclinic
P1
13.4826(12)
14.9649(15)
23.685(2)
80.439(6)
85.325(6)
79.324(6)
4624.6(7)
1
1.360
100(2)
0.71073
0.473
0.1385
1235.61
monoclinic
P2(1)/n
13.683(2)
35.079(5)
15.003(2)
90
100.479(2)
90
7081.1(18)
4
1.159
130(2)
0.71073
0.321
0.0870
0.1819
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Z
7544.6(7)
8
1.566
Dcalcd (g cm-3
T (K)
)
130(2)
λ (A)
0.71073
0.975
0.0389
μ (mm-1
)
R1a (I > 2σ(I))
wR2b (I > 2σ(I))
0.2760
0.2142
0.1048
0.3259
11
12
13
14
15
formula
Mr
cryst syst
space group
a (A)
C47H39FeKN8O3
858.81
C144H186Fe4K4N22O18
2892.95
triclinic
P1
20.365(3)
20.450(3)
21.442(3)
64.440(2)
81.256(2)
67.663(2)
7450.8(19)
2
1.289
130(2)
0.71073
0.562
0.0786
C32H30F9FeKN6O5
844.57
monoclinic
Cc
18.2046(14)
10.7576(8)
19.0952(15)
90
94.4560(10)
90
C28H18F9FeKN6O3
C54H66Fe2K2N8
1017.05
triclinic
P1
9.520(3)
13.226(4)
20.248(6)
89.225(5)
80.427(5)
86.355(5)
2508.9(12)
2
1.346
130(2)
0.71073
0.789
0.0754
752.43
monoclinic
P2(1)/c
10.6910(18)
18.772(4)
15.681(3)
90
101.933(5)
90
3079.0(10)
4
1.623
130(2)
0.71073
0.721
0.0707
0.1425
monoclinic
P2(1)/n
10.208(3)
23.532(7)
18.691(5)
90
103.422(5)
90
4367(2)
4
1.306
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Z
3728.3(5)
4
1.505
Dcalcd (g cm-3
T (K)
)
223(2)
130(2)
λ (A)
0.71073
0.491
0.0672
0.71073
0.608
0.0715
μ (mm-1
)
R1a (I > 2σ(I))
wR2b (I > 2σ(I))
0.1497
0.1917
0.1887
0.1520
P
P
P
P
a R1 =
Fo| - |Fc
/
|Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
Table 2. Selected Interatomic Distances (A) for Compounds 6-15
6
7
8
9
10
11
12
13
14
15
Fe(1)-N(1)
2.244(3) 2.301(6) 2.203(3)
2.063(3) 2.050(7) 3.350(nb) 2.0808(18) 2.083(10)
2.2246(17) 2.459(n.b.) 2.280(3) 2.506(n.b.) 2.437(5) 2.378(4) 2.132(6)
Fe(1)-N(2)
2.096(3) 2.085(5)
2.097(3) 2.099(4)
2.079(3) 2.103(4)
1.984(3)
2.149(8) 2.109(4) 1.968(6)
2.082(5) 2.125(4) 2.047(6)
2.100(5) 2.101(4) 1.990(7)
Fe(1)-N(3)
Fe(1)-N(4)
2.029(3) 2.038(7) 2.054(3)
2.016(3) 2.059(7) 2.047(3)
2.157(2) 2.167(6) 1.923(3)
2.0910(18) 2.081(11)
2.0218(18) 2.092(10)
2.1435(15) 1.967(11)
Fe(1)-O(1)/O(6)
Fe(1)-O(4)/N(5)
Fe(2)-N(5)
2.106(3)
1.994(5) 2.102(5)
2.140(7)
1.976(6)
2.464(n.b.)
2.075(12)
2.083(11)
2.089(12)
1.981(10)
2.625(n.b.)
Fe(2)-N(6)
2.124(4)
2.079(5)
2.116(4)
1.999(4)
0.6743
Fe(2)-N(7)
1.984(7)
2.012(7)
Fe(2)-N(8)
Fe(2)-O(2)/O(3)
Fe(1)-[N(2), N(3), N(4)]a 0.3794
Fe(2)-[N(6), N(7), N(8)]a
0.4795
0.0783b
0.4120
0.6565
0.6517
0.5251
0.6573
0.6223
0.2764
0.2422
0.7613
a Distance of Fe from mean plane. b Fe(1)-[O(1), N(3), N(4)]
presented here. The structures of the precursor amine
LH3 and ligand L1H3 (1) have been previously reporte-
d.26a The structures of ligands L2H3 (2a), L4H3 (3a), L6H3
(3b, cocrystallized with PPh4Cl), L7H3 (3c), L8H3 (3d),
and L9H3 (4) are shown in Figures S1-S6 (Supporting
Information). Crystallographic data for these ligands are
collected in Table S1 (Supporting Information). With the
exception of L6H3, which features a propeller-type struc-
ture that minimizes repulsion among the electron-rich
aryl groups, all other ligand structures exhibit a preorga-
nized C3-symmetric cavity (albeit distorted), which is
predisposed to capture metal ions via a minimal trigonal
pyramidal geometry after deprotonation of the ligand.
Synthesis of Fe(II) Complexes. The general methodol-
ogy used for the preparation of the Fe(II) compounds
calls for deprotonation of the tripodal ligands with 3
equiv of KH, followed by the addition of anhydrous
FeCl2 under an inert atmosphere. The reaction is usually

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 115
Table 3. Selected Angles (deg) for Compounds 6-15
6
7
8
9
10
11
12
13
14
15
N(1)-Fe(1)-N(2)
77.23(11)
80.00(11)
80.69(12)
77.8(2)
75.3(2)
76.8(2)
76.91(7)
78.16(7)
80.39(7)
74.76(10)
75.89(10)
75.66(10)
72.4(3)
71.88(18)
71.44(17)
73.26(13)
72.90(13)
72.37(13)
83.1(3)
80.3(2)
82.6(3)
N(1)-Fe(1)-N(3)
79.77(11)
80.63(12)
N(1)-Fe(1)-N(4)
N(1)-Fe(1)-O(1)
168.44(10) 160.4(2) 102.19(11) 172.45(6)
N(1)-Fe(1)-O(4)/N(5)
N(2)-Fe(1)-N(3)
177.89(10)
174.4(2)
178.05(15)
110.74(12) 106.6(3)
127.70(12) 105.0(3)
112.83(7) 104.0(4) 113.22(10) 112.06(17) 114.8(3)
128.91(7) 125.0(4) 109.97(11) 108.01(17) 112.0(3)
115.93(14) 112.9(3)
110.70(14) 123.2(3)
107.38(14) 118.2(3)
N(2)-Fe(1)-N(4)
N(3)-Fe(1)-N(4)
111.17(13) 131.6(3) 117.90(12) 106.24(7) 101.6(4) 118.49(11) 110.40(17) 105.5(2)
96.65(11) 121.8(2) 97.98(7)
96.5(3) 107.88(12) 109.12(6)
96.9(2) 133.75(12) 98.99(7)
N(2)-Fe(1)-O(1)
N(3)-Fe(1)-O(1)
111.47(11)
95.84(11)
N(4)-Fe(1)-O(1)
N(2)-Fe(1)-O(1)/O(6)
N(3)-Fe(1)-O(1)/O(6)
N(4)-Fe(1)-O(1)/O(6)
N(2)-Fe(1)-O(4)/N(5)
N(3)-Fe(1)-O(4)/N(5)
N(4)-Fe(1)-O(4)/N(5)
N(5)-Fe(2)-N(6)
112.6(4)
111.3(4)
101.8(4)
111.05(16)
104.43(16)
110.91(16)
103.92(11)
106.19(11)
103.42(11)
105.1(3)
113.7(2)
105.5(2)
104.86(15)
107.74(16)
109.01(15)
83.3(3)
N(5)-Fe(2)-N(7)
83.4(3)
82.3(3)
122.5(3)
115.1(3)
117.9(3)
N(5)-Fe(2)-N(8)
N(6)-Fe(2)-N(7)
105.8(5)
105.0(5)
120.8(4)
110.9(4)
111.6(4)
102.5(5)
109.08(18)
111.18(17)
102.75(18)
107.89(17)
107.02(17)
118.48(16)
N(6)-Fe(2)-N(8)
N(7)-Fe(2)-N(8)
N(6)-Fe(2)-O(2)/O(3)
N(7)-Fe(2)-O(2)/O(3)
N(8)-Fe(2)-O(2)/O(3)
general stoichiometry [(Lx)Fe(II)-NCMe]- (Lx = L6
(11), L8 (14)) has always been obtained. Finally, the
extreme case of [K(L9)Fe(II)]2 (15) is noted, in which
the strong equatorial ligand field imposed by the arylalk-
ylamido residues weakens the axial ligation and leads to a
compound devoid of any solvent, coordinated or sol-
vated. Also noteworthy is the contribution of the potas-
sium counterion, which via association with electron-rich
ligand moieties leads frequently to the generation of
polymeric units in the solid state. Further details of the
solid-state structures are provided below.
Scheme 1
Solid-State Structures. [(Lx)Fe(II)-solv]- (Lx = L2
(6), L3 (7), L5 (9), solv = THF; Lx = L8 (13), solv =
DMA; Lx =L6 (11), L8 (14), solv = MeCN). Structures
that feature a solvent molecule coordinated to the Fe(II)
site exhibit a trigonal bipyramidal (TBP) geometry, which
is distorted to various degrees. [(L3)Fe(II)-THF]- (7)
proved to possess a structure (Figure 1) most deviating
conducted in tetrahydrofuran (THF), but for ligands with
more limited solubility, such as L4H3, L7H3, and L8H3,
dimethylacetamide (DMA) can be used instead. After the
removal of insoluble KCl, isolation of the Fe(II) products
is achieved largely by layering pentane over concentrated
THF, or diethyl ether over DMA, which provides good
yields of bulk material of high-purity for further explora-
tion. Crystalline materials suitable for X-ray diffraction
analysis usually require the slow diffusion of pentane into
THF, or diethyl ether into DMA, at low temperatures.
The coordination sphere of the ferrous site in the solid
state can vary significantly (Scheme 3). On a number of
occasions, the simple stoichiometry [(Lx)Fe(II)-THF]-
(Lx=L1 (5), L2 (6), L3 (7), L5 (9)) or [(Lx)Fe(II)-DMA]-
(Lx=L8 (13)) is obtained. However, when acyl arms are
involved, the carbonyl oxygen of the ligand, either of the
same ferrous site or an adjacent one, can effectively
compete for coordination at the expense of THF or
DMA. This situation is encountered with ligands Lx =
L4 (8), L6 (10), and L7 (12), with the additional caveat that
L4 is O-atom-coordinated to Fe(II) in 8 via a N(Ar)dC-
(R)-O- residue rather than the more typical N-atom
coordinated -N(Ar)-C(R)dO isomer. On the other
hand, when acetonitrile is used for recrystallization, the
from TBP geometry, as exemplified by the N(1)amine
-
Fe(1)-O(1)THF angle along the axial dimension, and a
combination of two acute and one wide angle in the
equatorial coordination plane. Moreover, the Fe(II)
atom lies significantly off the equatorial plane of the three
amido nitrogens, due to the structural requirements of
the five-membered metalated rings (average Namine
-
Fe(1)-Namido=76.6(2)ꢀ). The structures of [(L2)Fe(II)-
THF]- (6), [(L5)Fe(II)-THF]- (9) (Figure 1), and
[(L1)Fe(II)-THF]- (5)26b are by comparison less dis-
torted, especially along the axial coordination, while the
angle distortion in the equatorial plane and the deviation
of the Fe(II) atom from the mean plane of the three
Namido atoms remain pronounced. The role of Kþ as a
structural element will be detailed in a subsequent section.
The structure of [(L8)Fe(II)-DMA]- (13) (Figure 2) is
more distorted from the C3 symmetry, whereas the corre-
sponding acetonitrile adducts [(L8)Fe(II)-NCMe]- (14)
and [(L6)Fe(II)-NCMe]- (11) (Figure 2) are the least

116 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
Scheme 2
perturbed C3-symmetric structures noted in this study.
Nevertheless, the significantly longer Fe(1)-N(2) bond in
[(L8)Fe(II)-DMA]- (Figure 2) is accompanied by a short
(14), and [K(DMA)][(L8)Fe(II)-DMA] (13) demonstrate
the same potassium-induced, three-dimensional architec-
ture, featuring K(solv)þ ions that are coordinated by all
three amidato carbonyl moieties (O(1), O(2), and O(3)),
each belonging to a different molecule (best shown for
[K(NCMe)][(L8)Fe(II)-NCMe] (14) in Figure 2).
N(2)-C(7) (1.168(12) A) and
a long C(7)-O(1)
(1.336(12) A) bond, suggesting -O-C(CF3)d(Ar)N-Fe
coordination. The metrical parameters associated with
the other two Namido residues, as well as those observed
for all three Namido units of [(L8)Fe(II)-NCMe]-, are
consistent with OdC(CF3)-(Ar)N-Fe coordination.
The deviation of the Fe(II) atom from the mean plane
defined by the three Namido atoms is very significant for all
three compounds.
Structures of [(L4)Fe(II)][K(DMA)2] (8), [K2(THF)5]-
[(L6)2Fe(II)2] 2THF (10), and [K2(DMA)4][K(L7)2Fe(II)2]2
3
3
2DMA (12). The common characteristic of these struc-
tures is the absence of a metal-coordinating solvent
molecule, and its replacement by an amidato residue37
through the carbonyl oxygen atom. The structure of
[K(DMA)2][(L4)Fe(II)] (8) (Figure 3) is the simplest of
all three, but also unique, inasmuch as one of the equa-
torial Namido atoms remains outside the coordination
The average Fe-Namido bond distances (2.030 (L1),
2.036 (L2), 2.049 (L3), 2.065 (L5), 2.091 (L6), and 2.111 A
(L8)) reflect the strength of the equatorial ligand field in
accordance with the electron-donating capabilities of the
ligands noted above. However, deviations from the aver-
age Fe-Namido bond distances are significant and can be
attributed to the presence of Kþ ions that make close
contacts with aromatic and other electron-rich residues
associated with specific Namido atoms, leading to elonga-
tion of the corresponding Fe-Namido bond distance. For
instance, the presence of a Kþ-(η6-arene) interaction
involving the phenylene group between atoms N(4) and
N(1) in [(L3)Fe(II)-THF]- (7, Figure 1) is responsible for
the longer Fe(1)-N(4) bond in this compound.
sphere (Fe(1) N(2)=3.350 A), while the corresponding
3 3 3
amidato oxygen atom is preferentially bonded to the four-
coordinate metal site via the (Ar)NdC(t-Bu)-O- coor-
dination mode. The Fe(II) site deviates only slightly from
the [O(1), N(3), N(4)] mean plane. The structure is in
reality polymeric, because each DMA-solvated Kþ ion is
also associated with carbonyl atoms O(2) and O(3) be-
longing to adjacent molecules.
The structure of [K2(THF)5][(L6)2Fe(II)2] 2THF (10)
3
(Figure 4) features a slightly asymmetric dimer with two
ferrous sites differing in their metrical parameters by a
small margin (average Fe(1)-Namido = 2.085 A; Fe-
(2)-Namido=2.082 A). The coordination sphere of each
Fe(II) is essentially similar to that of the trigonal bipyr-
The presence of Kþ ions is also responsible for tertiary
structures observed in the packing of the unit cell.
Whereas [K(THF)3][(L1)Fe(II)-THF] 0.5THF (5)26b
3
and [K(THF)3][(L2)Fe(II)-THF] (6) are strictly mono-
meric, [K(THF)2][(L3)Fe(II)-THF] adopts a centro-
symmetric dimeric structure by virtue of mutual close
contacts between the Kþ ion of each molecule and a
fluorine atom of the partner molecule (K-FCF2Ar =
2.735 A, Figure 1). In contrast, [K(L5)Fe(II)-THF] (9)
exhibits an array of Kþ ions connecting ligand residues of
adjacent molecules, giving rise to a one-dimensional
polymeric structure. Furthermore, [K(NCMe][(L6)Fe(II)-
amidal [K(NCMe)][(L6)Fe(II)-NCMe] 2MeCN (11)
3
(Figure 2), save for two major modifications: (a) the axial
Namine atoms make very weak contacts, if at all, with the
metal sites, and (b) the axial coordination sites trans to
those Namine atoms are occupied by carbonyl moieties
provided by an amidato residue of the partner monomer.
The metrical parameters associated with the amidato
(37) Leitch, D. C.; Beard, J. D.; Thomson, R. K.; Wright, V. A.; Patrick,
B. O.; Schafer, L. L. Eur. J. Inorg. Chem. 2009, 2691–2701.
NCMe] 2MeCN (11), [K(NCMe)][(L8)Fe(II)-NCMe]
3

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 117
Scheme 3
units are consistent with an Fe(1,2)-N(Ar)-C(Ph)d
O-Fe(2,1) coordination mode. The mutual amidato
contacts are responsible for holding the dimer together,
further assisted by both Kþ counterions, each coordi-
nated, at least, by an amidato carbonyl moiety from each
ligand, and also by one (K(1)) and four (K(2)) THF
molecules, respectively (Figure 5). These solvent mole-
cules are responsible for terminating the dimeric mole-
cular entity and not allowing any dimer-dimer inter-
actions via Kþ bridging.
Namido =2.096 A; Fe(2)-Namido =2.106 A), the Namine
atoms are essentially noncoordinated, and the axial co-
ordination is provided by the amidato carbonyl molecule
of the partner ligand.
The Fe(1)/Fe(2) and Fe(3)/Fe(4) dimers of the repeat-
ing unit ([Fe(1)/Fe(2) K(1)-O(10) Fe(3)/Fe(4)
3 3 3 3 3 3
3 3 3
K(2)-O(1)]) differ slightly in their metrical parameters
(average Fe(3)-Namido=2.097 A; Fe(4)-Namido=2.099
A) and are interconnected via one of their respective Kþ
ions (K(1) or K(2); Figure 5). Each of these Kþ ions is
attached to carbonyl residues and other electron-rich
entities of one dimer, and to a unique amidato carbonyl
residue of the second dimer (K(1)-O(10) and K(2)-O-
(1)). The N-atom ends of these unique amidato residues,
N(14) and N(2), are connected to atoms Fe(1) and Fe(4),
respectively. These Kþ ions are also very similar in
The structure of [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA
3
(12) (Figure 4) is the most complicated of all, featuring a
dimer of dimers within an overall polymeric construct.
The individual dimer is not unlike the one discussed
with regard to the structure of [K2(THF)5][(L6)2Fe(II)2]
2THF (10); it is similarly asymmetric (average Fe(1)-
3

118 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
Figure 1. Solid-state structures of [K(THF)3][(L2)Fe(II)-THF] (6,
top), [K(THF)2][(L3)Fe(II)-THF] (7, middle), and [K(L5)Fe(II)-THF]
(9, bottom), showing 30% probability ellipsoids and the atom labeling
Figure 2. Solid-state structures of [K(DMA)][(L8)Fe(II)-DMA] (13,
top), [K(NCMe)][(L8)Fe(II)-NCMe] (14, middle), and [K(NCMe)][(L6)-
Fe(II)-NCMe] 2MeCN (11, bottom), showing 30% probability ellip-
3
scheme. K F interaction between adjacent molecules is also shown
3 3 3
soids and the atom labeling scheme.
for 7.
coordination to that of the K(1) ion of [K2(THF)5][(L6)2-
Fe(II)2] 2THF (10) that carries the single THF molecule,
with the exception that the THF molecule is here replaced
by the unique amidato moiety. However, the present
Kþ-induced, three-dimensional structure is more compli-
cated, because the remaining two Kþ ions (K(3) and K(4))
form an interesting [K2(DMA)4] dimer, composed of
two bridging DMA molecules and two terminal DMA
3

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 119
molecules attached to K(4). The coordination is com-
pleted by a bridging atom (O(7)) of an amidato carbonyl
moiety belonging to the Fe(3)/Fe(4) dimer, and a terminal
atom (O(4)) of an amidato carbonyl residue belonging to
the Fe(1)/Fe(2) dimer. The N-atom ends (N(10) and N(6))
of these two amidato residues are attached to atoms Fe(3)
and Fe(2), respectively. The overall architecture is thus
one in which the pseudo 1-D polymer with the repeat-
ing [Fe(1)/Fe(2) K(1) Fe(3)/Fe(4) K(2)] motif is
3 3 3
3 3 3
3 3 3
linked to identical 1-D arrays via lateral connections
provided by the K(3)/K(4) dimers.
Structure of [K(L9)Fe(II)]2 (15). The tripodal arylalk-
ylamido-amine ligand L9 affords a ferrous compound,
whose structure is unique. Although crystallized from
THF/pentane, the compound is devoid of any solvent
residue in the unit cell. Two ferrous sites are observed
as being interconnected by means of Kþ contacts to
electron-rich moieties of adjacent [(L9)Fe(II)] units to
generate an -[Fe(1)-K(1)-Fe(2)-K(2)]n- sequence
(Figure 6). More specifically, K(1) is shown to be involved
in Kþ-(η6-arene) interactions between adjacent pheny-
lene moieties, as well as in long-range contacts with N(4)
and C(20) (K(1)-N(4)=3.127(7), K(1)-C(20)=3.535(8)
A). Similarly, K(2) is engaged in Kþ-(η6-arene) contacts
(phenylene rings between N(5)/N(8) and N(1)/N(2)) and
in long-range interactions with atoms N(3), N(7), and
C(37) (K(2)-N(3)=3.089(7), K(2)-N(7)=3.067(7), and
Figure 3. Solid-state structure of [K(DMA)2][(L4)Fe(II)] (8), showing
30% probability ellipsoids and the atom labeling scheme.
Figure 4. Solid-state structures of the anions of [K2(THF)5][(L6)2Fe(II)2] 2THF (10, left) and [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA (12, right; only one
dimer shown), showing 30% probability ellipsoids and the atom labeling scheme.
3
3
Figure 5. Solid-state structures of [K2(THF)5][(L6)2Fe(II)2] 2THF (10, left) and [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA (12, right), showing the respective
3
3
positions of the Kþ ions with regard to the diferrous units.

120 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
Figure 6. Solid-state structure of [K(L9)Fe(II)]2 (15), showing 30%
probability ellipsoids and the atom labeling scheme.
K(2)-C(37)=3.425(7) A). However, these contacts are
arbitrarily allowed by means of the rather generous upper
limits set by the sum of Bondi’s intermolecular van der
Waals radii (K, 2.75; C, 1.70; and K, 1.55 A),38 and not all
of them can be considered as genuine interactions. In
particular, the long-distance Kþ contacts to C(20) and
C(37) are doubtful. The two ferrous sites differ slightly in
their metrical parameters and feature a tight four-coor-
dinate ligand field generated by the electron-rich arylalk-
ylamido residues (average Fe(1)-Namido = 2.002 A;
Fe(2)-Namido = 1.991 A) and the apical Namine atom.
As expected, the strong trigonal-pyramidal ligand field
weakens considerably the axial coordination trans to the
Namine atom, which remains free of any solvent. In
acetonitrile solutions, compound 15 readily releases the
ligand L9H3 via an unidentified pathway.
€
Figure 7. The 78 K Mossbauer spectra from powdered samples of
compounds [K(THF)2][(L3)Fe(II)-THF] (7), [K2(THF)5][(L6)2Fe(II)2]
3
2THF (10), [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA (12), and [K(L9)-
Fe(II)]2 (15), as indicated. Solid lines are theoretical simulations as
described in the text.
3
It has not been possible to ascertain whether the
structures of compounds 6-15 are retained in solution.
Efforts to obtain ESI-MS data have been hampered by
the sensitivity of these compounds to oxidative events.
NMR data provided in the Experimental Section suggest
that the low symmetry observed in the solid state (absence
of a strict C3 axis in all structures) is reflected in fluid
matrices, but the situation is further complicated due to
the uncertainty regarding the coordination of the potas-
sium ions in different solvents. Therefore, the NMR data
reported should be treated as markers for identification
purposes and not necessarily as true representatives of the
solid structures.
indicative of high-spin Fe(II) (S=2) species39 in a N/O
ligand environment.40 For all ferrous complexes that
feature a solvent molecule in the coordination sphere
(5-7, 9, 13, and 14), the value for the isomer shift41 is
consistent with that reported for other five-coordinate
ferrous sites42 and is narrowly distributed around 1.0 mm/s.
Compound 13 has the highest isomer shift value in this
series, since, by comparison to 14, it possesses a weaker
ligand field, owing to the imino N-atom residue and the
coordinated DMA.
The variation of the quadrupole splitting for the fer-
rous sites is noteworthy, but with the exception of the
ΔEQ values for 15 (see below), the range is much nar-
rower, in agreement with the values reported for the vast
majority of high-spin Fe(II) (S = 2) complexes (ΔEQ =
2.5-3.5 mm/s).43 The variation of the quadrupole split-
ting for the solvated ferrous sites reflects small differences
in the coordination environment, especially local stereo-
chemical twists that induce asymmetry, which cannot be
fully accounted for at the present stage of the investiga-
tion.
€
€
Mossbauer Spectroscopy. Zero-field Mossbauer spec-
tra from powdered samples were recorded at liquid nitro-
gen temperatures. Parameters are shown in Table 4.
Representative spectra are shown in Figure 7. The
remaining spectra are shown in Figures S7-S12 (Sup-
porting Information). For mononuclear complexes
[(Lx)Fe(II)-solv]- (Lx = L1 (5), L2 (6), L3 (7), L5 (9),
solv=THF; L8, solv=DMA (13), MeCN (14)) as well as
[K(DMA)2][(L4)Fe(II)] (8), the spectra consist of a unique
quadrupole doublet. For complexes [K2(THF)5][(L6)2-
Fe(II)2] 2THF (10), [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA
For dimeric compound 10, the existence of two differ-
ent doublets is in agreement with the slightly different
geometric characteristics of the two ferrous sites. For 12,
which is structurally a dimer of dimers, only two doublets
3
3
(12), and [K(L9)Fe(II)]2 (15), the spectra are more compli-
cated, in agreement with the structural data.
€
In all cases, the Mossbauer parameters, and especially
the isomer shifts (ranging from 0.75 to 1.14 mm/s), are
ꢀ
(40) (a) Weber, B.; Kaps, E. S.; Desplanches, C.; Letard, J.-F.; Achter-
hold, K.; Parak, F. G. Eur. J. Inorg. Chem. 2008, 4891–4898. (b) Weber, B.;
ꢀ
Carbonera, C.; Desplances, C.; Letard, J.-F. Eur. J. Inorg. Chem. 2008, 1589–
1598.
(38) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441–451. (b) Rowland, R. S.;
Taylor, R. J. Phys. Chem. 1996, 100, 7384–7391. (c) Mantina, M.; Chamberlin,
A. C.; Valero, R.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. A 2009, 113,
5806–5812.
(41) Andres, H.; Bominaar, E. L.; Smith, J. M.; Eckert, N. A.; Holland, P.
€
L.; Munck, E. J. Am. Chem. Soc. 2002, 124, 3012–3025.
(42) (a) Hu, C.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem.
2008, 47, 8884–8895. (b) Hachem, I.; Belkhiria, M. S.; Giorgi, M.; Schulz, C. E.;
Nasri, H. Polyhedron 2009, 28, 954–958.
€
(39) Munck, E. Physical Methods in Inorganic and Bioinorganic Chem-
istry; University Science Books: Sausalito, CA, 2000; Chapter 6, pp 287-319.
(43) Kurtz, D. M., Jr. Chem. Rev. 1990, 90, 585–606.

Article
Inorganic Chemistry, Vol. 49, No. 1, 2010 121
€
Table 4. Mossbauer Parameters at 78 K and Electrochemical Data at Room Temperature for Compounds 5-15
δ (mm/s) ΔEQ (mm/s) Area (%)
E1/2 (or Ep,a) (V vs Fcþ/Fc)
ΔE (mV) ip,a/ip,c
[K(THF)3][(L1)Fe(II)-THF] 0.5THF (5)26a
0.96
0.96
2.60
3.16
100
100
-1.302 (DMSO)26a
-1.298 (DMF)26a
-1.326 (DMSO)
-1.348 (DMF)
-0.791 (DMSO)
-0.640 (DMF)
81
71
88
71
92
1.23
1.16
1.05
1.09
1.14
3
[K(THF)3][(L2)Fe(II)-THF] (6)
[K(THF)2][(L3)Fe(II)-THF] (7)
[K(DMA)2][(L4)Fe(II)] (8)
[K(L5)Fe(II)-THF] (9)
0.95
0.96
0.96
2.57
2.86
2.60
100
100
100
-0.841 (DMSO)
-0.859 (DMF)
72
70
69
1.15
1.53
1.38
[K2(THF)5][(L6)Fe(II)]2 2THF (10)
site 1 1.00
site 2 1.14
2.83
2.28
50
50
-0.567 (DMSO)
-0.93, -0.76, -0.534 (THF)
-0.572 (DMSO)
-0.904, -0.720 (DMA)
-0.705 (DMSO)
-1.19, -0.92, -0.684 (DMF)
0.013 (DMA)
0.140 (MeCN)
0.017 (DMF)
-1.185, -0.913 (DMF)
3
[K(NCMe)][(L6)Fe(II)-NCMe] 2MeCN (11)
56
73, 72
1.27
3
[K2(DMA)4][K(L7)2Fe(II)2]2 2DMA (12)
site 1 1.04
site 2 1.04
site 3 1.01
1.12
3.34
2.97
1.69
3.34
2.98
37
49
12
100
100
3
[K(DMA)][(L8)Fe(II)-DMA] (13)
[K(NCMe)][(L8)Fe(II)-NCMe] (14)
1.04
165
2.54
[K(L9)Fe(II)]2 (15)
site 1 0.75
site 2 0.76
0.91
1.43
47
53
could be safely discerned and simulated, without taking
the risk of overparametrization. Apparently, the two
€
dimers in 12 are indistinguishable by Mossbauer spec-
troscopy. It should also be noted that an unidentified
ferrous site, which accounts for approximately 12% of the
signal area and exhibits a significantly different ΔEQ, has
been encountered in all analytically pure samples of 12
tested. It is currently unknown if this minority site is due
to a different phase that is not represented by the structure
of 12. The four Fe(II) (S = 2) sites are magnetically
isolated at room temperature, as judged by a χMT value
of 11.2 emu K mol-1, measured on an Evans magnetic
balance.
For complex 15, the two ferrous sites exhibit the
smallest isomer shift and quadrupole splitting values in
the series. The isomer shift values are in agreement with
the very strong ligand field in 15 and within the range
expected for four-coordinate Fe(II) species,44 as well as
comparable with the isomer shift of [FeS4]6- moieties
encountered in proteins such as reduced Rubredoxin45
and synthetic analogs.46 On the other hand, the ferrous
sites in 15 exhibit substantially smaller ΔEQ values, out-
side the typical range for ferrous sites. Nevertheless, a
small but significant number of ferrous compounds with
similarly low ΔEQ values (and analogous δ values) have
Figure 8. Cyclic voltammograms (Fe(II)/Fe(III)) of compounds
[K(THF)3][(L1)Fe(II)-THF] 0.5THF (5) and [K(THF)3][(L2)Fe(II)-
3
THF] (6) in DMF/[(n-Bu)4N]PF6, [K(THF)2][(L3)Fe(II)-THF] (7),
[K(L5)Fe(II)-THF] (9) and [K(NCMe)][(L6)Fe(II)-NCMe] 2MeCN
3
(11) in DMSO/[(n-Bu)4N]PF6, [K2(DMA)4][K(L7)2Fe(II)2]2 2DMA
3
(12) in DMA/[(n-Bu)4N]PF6, and [K(NCMe)][(L8)Fe(II)-NCMe] (14)
in MeCN/[(n-Bu)4N]PF6, as indicated, with a Au disk electrode (1.6 mm
in diameter); scan rate, 0.1 V/s. Differences in current are due, in part, to
differences in concentration (1.0-3.0 mM).
apparently both positive and negative, to the electric field
gradient.
been discussed by Munck and co-workers41 in association
€
with planar three-coordinate [(β-diketiminate)Fe(II)(X)]
Electrochemistry. Electrochemical data are collected in
Table 4. Cyclic voltammetry experiments on [(L1)Fe(II)-
solv]- (solv=THF (5),26b DMF,26a and DMSO26a) under
anaerobic conditions have previously provided a semire-
versible Fe(II)/Fe(III) couple at very negative potentials
(Figure 8 and Figure S13, Supporting Information).
The structurally and electronically related [(L2)Fe(II)-
THF]- (6) also furnishes a similar semireversible wave
(Figure 8 and Figure S14, Supporting Information) at
even more negative potentials, as expected due to the
strong electron-donating character of the ligand. In
DMSO, the wave has been obtained cathodically, after
stoichiometric oxidation of Fe(II) to Fe(III), due to the
extreme sensitivity of 6 to oxidation in DMSO. As
expected, the related [(L3)Fe(II)-THF]- (7) and [(L5)-
Fe(II)-THF]- (9) in DMSO and DMF, which bear the
electron-withdrawing aryl substituents 3,5-(CF3)2 and
€
species. The reader is referred to Munck et al.’s
paper for further discussion of the ligand contributions,
(44) (a) Cowley, R. E.; Elhaık, J.; Eckert, N. A.; Brennessel, W. W.; Bill,
´
E.; Holland, P. L. J. Am. Chem. Soc. 2008, 130, 6074–6075. (b) Cowley, R. E.;
Bill, E.; Neese, F.; Brennessel, W. W.; Holland, P. L. Inorg. Chem. 2009, 48,
4828–4836. (c) Stoian, S. A.; Smith, J. M.; Holland, P. L.; M€unck, E.; Bominaar,
E. L. Inorg. Chem. 2008, 47, 8687–8695. (d) Khusniyarov, M. M.; Weyherm€uller,
T.; Bill, E.; Wieghardt, K. J. Am. Chem. Soc. 2009, 131, 1208–1221. (e) Lu,
C. C.; Bill, E.; Weyherm€uller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc.
2008, 130, 3181–3197. (f) Muresan, N.; Lu, C. C.; Ghosh, M.; Peters, J. C.; Abe,
M.; Henling, L. M.; Weyherm€uller, T.; Bill, E.; Wieghardt, K. Inorg. Chem. 2008,
47, 4579–4590.
€
(45) Debrunner, P. G.; Munck, E.; Que, L.; Schultz, C. E. Iron Sulfur
Proteins; Lovenberg, W., Ed; Academic Press: New York, 1977; Vol. III, Chapter
10 and refs therein.
(46) (a) Lane, R. W.; Ibers, J. A.; Frankel, R. B.; Papaefthymiou, G. C.;
Holm, R. H. J. Am. Chem. Soc. 1977, 99, 84–98. (b) Coucouvanis, D.;
Swenson, D.; Baenziger, N. C.; Murphy, C.; Holah, D. G.; Sfarnas, N.;
Simopoulos, A.; Kostikas, A. J. Am. Chem. Soc. 1981, 103, 3350–3362.

122 Inorganic Chemistry, Vol. 49, No. 1, 2010
Paraskevopoulou et al.
3,5-Cl2, respectively, exhibit a substantial anodic shift of
their Fe(II)/Fe(III) redox couple (Figure 8 and Figures
S15 and S17, Supporting Information) and are in agree-
ment with the expected electronic character of these
substituents.
arms has been synthesized and utilized in the
synthesis of the corresponding ferrous complexes.
(b) In spite of the unique ligand framework, a multitude
of structures are obtained for the synthesized Fe(II)
complexes. Although the general stoichiometry
[LFe(II)-solv]- (solv=THF, DMA,and MeCN)
is encountered in many instances, signific-
ant distortions of the coordination sphere
are apparent, with one extreme case indicating
imino (Fe-N(Ar)dC(R)-O-) rather than amido
(Fe-N(Ar)-C(R)dO) coordination.
(c) Other significant structural variants are devoid of
any coordinated solvents and include O-atom
rather than N-atom donor contributions via
Fe-O-C(R)dN(Ar) coordination modes, as
well as bridging amidato elements by means of
Fe-N(Ar)-C(R)dO-Fe units. The latter give
rise to dimeric structures in the absence of
strongly nucleophilic solvents. The simpler
[LFe(II)]- case is also observed in one instance,
supported by a strong N-donor ligand field.
The acyl-substituted ligands further accentuate the
anodic shift of the Fe(II)/Fe(III) potential, as best illu-
strated by [(L6)Fe(II)-NCMe]- (11), which demon-
strates an almost reversible Fe(II)/Fe(III) wave in
DMSO (Figure 8 and Figure S18, Supporting Infor-
mation). The dimeric version of this compound,
[(L6)2Fe(II)2]2- (10), exhibits an almost identical cyclic
voltammogram, suggesting that both compounds are
monomeric in DMSO. The dimer 10 is crystallized in
THF, and the cyclic voltammogram in this solvent only
shows broad irreversible features (Figure S19, Support-
ing Information). In contrast, the structurally related
[K(L7)2Fe(II)2]22- (12) exhibits two closely spaced waves
with narrow ΔE values in DMA (Figure 8 and Figure S20,
Supporting Information), presumably due to the presence
of the dimeric iron unit, whereas only irreversible features
are observed in both DMSO and DMF (Figures S21 and
S22, Supporting Information). Similarly, the structurally
unique [K(DMA)2][(L4)Fe(II)] (8) affords only a broad
irreversible feature (Figure S16, Supporting Informa-
tion). The strongly electron-withdrawing COCF3 substi-
tuent of [K(DMA)][(L8)Fe(II)-DMA] (13) and [K(NC-
Me)][(L8)Fe(II)-NCMe] (14) furnishes an irreversible
anodic wave for 13 in DMA (Figure S23, Supporting
(d) While mononuclear and dinuclear molecular entities
of the type [K(L)Fe(solv)x]n (n=1, 2) have been
observed, Kþ ions are also involved in intricate
intermolecular links, resulting in 1-D, 2-D, and
3-D polymers. The usual Kþ contacts involve
electron-rich aromatic entities and O/N-atom
donor residues.
€
(e) Mossbauer and electrochemical data provide
Information; coupled to three cathodic waves at Ep,c
=
reference values for typical high-spin Fe(II) sites
and reveal a great variety of residue-derived
electronic contributions to the metal center that
effect a range of Fe(II)/Fe(III) redox potentials
on the order of 1.3 V.
-0.116, -0.625, and -1.083 V) and an anodic/cathodic
wave for 14 in MeCN (Figure 8 and Figure S24, Support-
ing Information) or DMF (Figure S25, Supporting In-
formation; coupled to two cathodic waves at Ep,c
=
-0.114, -0.614 V) shifted to much more positive poten-
tial values. Accordingly, compound 14 proved to be the
most difficult to oxidize. Finally, the unique, alkyl-sub-
stituted [K(L9)Fe(II)]2 (15) shows two broad irreversible
anodic waves (Figure S26, Supporting Information),
which denote substantial stabilization of the ferric state
as expected due to the electron-rich nature of the arylalk-
ylamido-amine ligand, and may also reflect the presence
of two slightly distinct iron sites. The irreversible char-
acter of the waves noted in this section makes the assign-
ment of the Fe(II)/Fe(III) potential tentative (albeit
reasonable with respect to the electronic character of each
ligand) and cannot presently exclude contributions from
ligand-centered events.
Acknowledgment. We thank Drs. Ekk Sinn, Richard
Staples, and Charles Barnes for assistance with X-ray
diffraction analysis; Dr. Nathan Leigh for obtaining and
interpreting mass spectra; Dr. Victoria Magrioti for
obtaining the 19F NMR spectra; and Dr. Spyros Koinis
for helpful discussions on the spectroscopic data. We
gratefully acknowledge generous financial support (to
P.S.) by the NSF (CHE-0412959) and by the NIH/
NIEHS (5 P42 ES007381). We also acknowledge NSF
funding (CHE-0420497) for the purchase of the diffract-
ometer at the University of Missouri-St. Louis.
Supporting Information Available: Solid-state structures of
compounds 2a, 3a-d, and 4 (Figures S1-S6). Summary of
X-ray crystallographic data for the same compounds (Table S1).
X-ray crystallographic data for all structurally characterized
Conclusions
The following are the salient observations and findings of
this work:
€
compounds in CIF format. Mossbauer spectra of compounds 5,
(a) A family of C3-symmetric triphenylamido-amine
ligands, possessing a rigid tris(2-NH-phenyl)-
amine core and a selection of aryl, acyl, and alkyl
6, 8, 9, 13, and 14 (Figures S7-S12). Cyclic voltammograms of
compounds 5-15 (Figures S13-S26). The material is available
free of charge via the Internet at http://pubs.acs.org.