Synthesis, characterization, and electrochemical properties of a series of sterically varied iron(II) alkoxide precursors and their resultant nanoparticles

Article abstract of DOI:10.1021/ic200423m

A new family of iron(II) aryloxide [Fe(OAr)₂(py)_x] precursors was synthesized from the alcoholysis of iron(II) mesityl [Fe(Mes)₂] in pyridine (py) using a series of sterically varied 2-alkyl phenols (alkyl = methyl (H-oMP), isopropyl (H-oPP), tert-butyl (H-oBP)) and 2,6-dialkyl phenols (alkyl = methyl (H-DMP), isopropyl (H-DIP), tert-butyl (H-DBP), phenyl (H-DPhP)). All of the products were found to be mononuclear and structurally characterized as [Fe(OAr)₂(py)_x] (x = 3 OAr = oMP (1), oPP (2), oBP (3), DMP (4), DIP (5); x = 2 OAr = DBP (6), DPhP (7)). The use of tris-tert-butoxysilanol (OSi(OBu^t)₃ = TOBS) led to isolation of [Fe(TOBS)₂(py)₂] (8). The new Fe(OAr)₂(py)_x (1-6) were found, under solvothermal conditions, to produce nanodots identified by PXRD as the γ-maghemite phase. The model precursor 3 and the nanoparticles 6n were evaluated using electrochemical methods. Cyclic voltammetry for 3 revealed multiple irreversible oxidation peaks, which have been tentatively attributed to the loss of alkoxide ligand coupled with the deposition of a solid Fe-containing coating on the electrode. This coating was stable out to the voltage limits for the acetonitrile solvent.

Full text of DOI:10.1021/ic200423m

ARTICLE
pubs.acs.org/IC
Synthesis, Characterization, and Electrochemical Properties of a Series
of Sterically Varied Iron(II) Alkoxide Precursors and Their Resultant
Nanoparticles
Timothy J. Boyle,* Leigh Anna M. Ottley, Christopher A. Apblett, Constantine A. Stewart, Sarah M. Hoppe,
Krista L. Hawthorne, and Mark A. Rodriguez
Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Boulevard, Southeast, Albuquerque, New Mexico 87106,
United States
S Supporting Information
b
ABSTRACT: A new family of iron(II) aryloxide [Fe(OAr)2(py)x]
precursors was synthesized from the alcoholysis of iron(II) mesityl
[Fe(Mes)2] in pyridine (py) using a series of sterically varied 2-alkyl
phenols (alkyl = methyl (H-oMP), isopropyl (H-oPP), tert-butyl (H-
oBP)) and 2,6-dialkyl phenols (alkyl = methyl (H-DMP), isopropyl
(H-DIP), tert-butyl (H-DBP), phenyl (H-DPhP)). All of the products
were found to be mononuclear and structurally characterized as
[Fe(OAr)2(py)x] (x = 3 OAr = oMP (1), oPP (2), oBP (3), DMP
(4), DIP (5); x = 2 OAr = DBP (6), DPhP (7)). The use of tris-tert-
butoxysilanol (OSi(OBut)3
=
TOBS) led to isolation of
[Fe(TOBS)2(py)2] (8). The new Fe(OAr)2(py)x (1ꢀ6) were found,
under solvothermal conditions, to produce nanodots identified by
PXRD as the γ-maghemite phase. The model precursor 3 and the
nanoparticles 6n were evaluated using electrochemical methods. Cyclic voltammetry for 3 revealed multiple irreversible oxidation
peaks, which have been tentatively attributed to the loss of alkoxide ligand coupled with the deposition of a solid Fe-containing
coating on the electrode. This coating was stable out to the voltage limits for the acetonitrile solvent.
’ INTRODUCTION
and low lithium-ion diffusion coefficient. Recent results
indicate that production of nanoscaled [Fe(PO4)]19ꢀ21 and
[LiFe(PO4)]19,20,22ꢀ26 may overcome these disadvantages.
It was reasoned that if the Fe-based phosphates were improved
by moving to the nanoregime, the Fe-based oxides may also
benefit from a similar reduction in size. It has been reported
in several studies where the nanoparticles of FeOx (200ꢀ
250 mAh/g, 425ꢀ500 mWh/g)14,27ꢀ29 and LizFeyOx (140ꢀ
150 mAh/g)30,31 demonstrated high capacities.13,32ꢀ37 There-
fore, we were interested in exploring the electrochemical beha-
vior of FeOx nanoparticles. A wide number of commercially
available and designer precursors have been used to generate a
variety of morphologically varied FeOx-based nanomaterials.
Interestingly, the use of iron alkoxide ([Fe(OR)n], where n = 2
or 3) precursors for production of FeOx nanomaterials has been
surprisingly understudied.38ꢀ48 These compounds were of inter-
est since we previously demonstrated that the structure of the
metal alkoxide [M(OR)x] precursor plays a vital role in controlling
the properties (i.e., morphology, phase, size, etc.) of the final
nanomaterials.49ꢀ51 Therefore, we undertook the synthesis and
characterization of a series of sterically varied [Fe(OR)x]
The literature on structurally characterized, complex iron
alkoxides [Fe(OR)x] is voluminous with over 2000 species re-
ported that contain at least one Fe and two OR groups.1 However,
fromthis largefamilyof compounds, surprisingly few are the simple
[Fe(OR)x] (x = 2 or 3) species.2ꢀ9 These simple [Fe(OR)x]
molecules are important for a diverse number of applications
ranging from biological processes to power storage to magnetic
devices to fundamental research appeal. Our interest in these
compounds lies in the utility of iron-based materials for lithium-
ion batteries due to their low cost, low hazard concerns, ‘green’
nature of the material, and potential for high theoretical energy
density. Of these, bulk iron oxide (FeOx) and lithium iron oxide
(LiFeOx) materials have been previously explored as potential
lithium-ion battery materials but were quickly abandoned due to
their poor performance.10ꢀ14
The olivine structure of lithium iron phosphate [LiFe(PO4)]
along with the delithiated phase of iron phosphate [Fe(PO4)]
have garnered recent interest not only for their ‘green’ properties
but also for their reported high power, high specific capacity, high
natural abundance, high specific energy, long cycle life, stable
structure, low toxicity, low pollution, and low cost.15ꢀ26 As these
materials in the bulk phase were explored some issues arose that
could limit their utility, such as poor electronic conductivity
Received: March 1, 2011
Published: June 03, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
focusing on aryloxide (OAr) ligands. Several synthetic approaches
were explored with the alkylꢀalcohol exchange ultimately becom-
ing the preferred route due to a higher yield and purer crude
products. For this study, iron(II) mesityl ([Fe(Mes)2]; Mes =
C6H2(CH3)3-2,4,6) was reacted with a series of 2-alkyl phenols
(alkyl = methyl (H-oMP), isopropyl (H-oPP), tert-butyl (H-
oBP)) and 2,6-dialkyl phenols (alkyl = methyl (H-DMP), iso-
propyl (H-DIP), tert-butyl (H-DBP), phenyl (H-DPhP)) as
shown in eq 1. The products were crystallographically character-
ized exclusively as monomers: [Fe(OAr)2(py)3], where OAr =
oMP (1), oPP (2), oBP (3), DMP (4), DIP (5), [Fe(OAr)2(py)2],
where OAr = DBP (6), DPhP (7), and [Fe(TOBS)2(py)2] (8),
where TOBS = OSi(OCMe3)3. Once isolated, compounds 1ꢀ6
were used to generate nanoparticles following a solvothermal
(SOLVO) process. The model precursor 3 and the 6n nanopar-
ticles were preliminarily studied using electrochemical methods
to establish their baseline properties for lithium-ion battery
applications
[Fe(oPP)2(py)3] (2). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-oPP
(0.466 g, 3.42 mmol), and ∼5 mL of py. Yield 62.2% (0.600 g). FTIR
(KBr pellet, cmꢀ1): 3059(br, w), 2960(s), 2866(w), 1603(m), 1590(m),
1480(s), 1444(s), 1382(w), 1342(w), 1359(w), 1289(m), 1261(m),
1190(w), 1150(w), 1082(m), 1069(m), 1034(m), 931(w), 895(w),
853(m), 827(w), 803(w), 752(s), 700(s), 619(br, m), 484(br, m). Anal.
Calcd for C33H37FeN3O2 (MW = 563.51): C, 70.33; H, 6.62; N, 7.46.
Found: C, 69.88; H, 6.51; N, 7.54.
[Fe(oBP)2(py)3] (3). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-oBP
(0.514 g, 3.42 mmol), and ∼5 mL of py. Yield 34.7% (0.350 g). FTIR
(KBr pellet, cmꢀ1): 3069(br, w), 2996(s), 2950(m), 2905(w), 2866
(w), 1596(m), 1522(m), 1477(s), 1435(s), 1383(s), 1348(s), 1306(s),
1286(w), 1263(w), 1235(w), 1213(s), 1151(s), 1121(s), 1085(s),
1068(s), 1058(s), 1045(s), 1035(s), 1005(s), 926(m), 871(s), 842(s),
823(s), 756(s), 739(s), 699(s), 652(s), 632(m), 620(s), 599(s), 571(w),
505(s), 444(s), 422(s). Anal. Calcd for C35H41FeN3O2 (MW = 591.56):
C, 71.06; H, 6.99; N, 7.10; for C30H36FeN2O2 (MW = 512.46, equivalent
to ꢀ1 py): C, 70.31; H, 7.08; N, 5.47. Found: C, 69.52; H, 5.54; N, 6.11.
[Fe(DMP)2(py)3] (4). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-DMP
(0.418 g, 3.42 mmol), and ∼5 mL of py. Yield 43.6% (0.40 g). FTIR
(KBr pellet, cmꢀ1): 2944(s), 2924(s), 1589(m), 1467(s), 1445(s),
1422(s), 1292(m), 1263(m), 1236(w), 1213(m), 1087(s), 1070(s),
1040(s), 861(m), 803(m), 759(s), 701(s), 663(w), 540(m), 468(br, m).
Anal. Calcd for C31H33FeN3O2 (MW = 535.45): C, 69.54; H, 6.21; N,
7.85. Found: C, 64.00; H, 6.16; N, 5.11.
[Fe(DIP)2(py)3] (5). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-DIP
(0.611 g, 3.42 mmol), and ∼5 mL of py. Yield 64.0% (0.710 g). FTIR
(KBr pellet, cmꢀ1): 3055(m), 3003(w), 2960(s), 2945(sh), 2866(m),
1690(m), 1599(m), 1431(s), 1380(w), 1358(sh), 1338(m), 1262(s),
1211(m), 1130(w), 1097(m), 1067(m), 1039s), 890(m), 857(m),
803(m), 750(s), 700(s), 469(br, m). Anal. Calcd for C39H49FeN3O2
(MW = 647.66): C, 72.32; H, 7.63; N, 6.49. Found: C, 71.59; H, 7.63;
N, 6.46.
[Fe(DBP)2(py)2] (6). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-DBP
(0.706 g, 3.42 mmol), and ∼5 mL of py. Yield 46.7% (0.510 g). FTIR
(KBr pellet, cmꢀ1): 2956(s), 2916(sh), 2874(sh), 1601(m), 1509(w),
1443(s), 1405(s), 1381(m), 1358(sh, m), 1282(m), 1262(s), 1232(m),
1216(m), 1151(m), 1094(s), 1069(m), 1040(m), 1025(m), 873(m),
797(m), 748(s), 704(s), 661(m), 499(m) 459(br, m). Anal. Calcd for
C38H52FeN2O2 (MW = 624.67): C, 73.06; H, 8.39; N, 4.48. Found: C,
72.93; H, 7.85; N, 5.73.
py
s
f
½FeðMesÞ2ꢁ þ 2HꢀOAr
½FeðOArÞ ðpyÞ ꢁ þ 2HꢀMes
2
x
ð1Þ
n = 3: OAr = oMP (1), oPP (2), oBP (3), DMP (4), DIP (5).
n = 2: OAr = DBP (6), DPhP (7), TOBS (8).
’ EXPERIMENTAL SECTION
All compounds and reactions described were handled with rigorous
exclusion of air and water using standard Schlenk line and argon-filled
glovebox techniques. All solvents were stored under argon and used as
received in (Aldrich) Sure/Seal bottles, including toluene (tol) and
pyridine (py). The following chemicals were used as received (Aldrich
and Alfa Aesar): [BrMg(Mes)] (1.0 M in THF), FeBr2, H-oMP, H-oPP,
H-oBP, H-DMP, H-DIP, H-DBP, H-DPhP, and H-TOBS. [Fe(Mes)2]
was synthesized from the reaction of FeBr2 in THF/2,4-dioxane (5:1)
with 2 equiv of [BrMg(Mes)].52,53 FTIR data were obtained for KBr
pressed pellets using a Bruker Vector 22 Instrument under an atmo-
sphere of flowing nitrogen. Elemental analyses were performed on a
Perkin-Elmer 2400 CHN-S/O Elemental Analyzer. Gas chromotogra-
phy-mass spectrum (GC-MS) data were obtained on a Hewlett-Packard
model 5890/5972A Series with a HP-1 cross-linked methylsilicone gum
column 60 m ꢂ 0.25 mm (0.25 μm film thickness) (HP part no. 19091Z-
436). Typical run conditions were as follows: (i) preheated oven at 50 °C
with a ramp rate of 4.0 °C/min to a final temperature of 200 °C and held
for 2.0 min. The mass spectrometer conditions for evaluation were (i)
solvents from 10 to 450 atomic mass units (amu) to detect for water and
(ii) all other samples from 45 to 450 amu.
General Synthesis. In an argon-filled glovebox, the appropriate
HꢀOAr previously dissolved in pyridine was added to a vial of Fe(Mes)2
in pyridine. After stirring for 12 h, the volume of the reaction mixture was
drastically reduced by vacuum distillation. The resulting mixtures were
set aside with their caps loose until crystals formed. The crystals that
were isolated were used for all subsequent analyses. Yields were not
optimized.
[Fe(oMP)2(py)3] (1). Used Fe(Mes)2 (0.500 g, 1.71 mmol), H-oMP
(0.370 g, 3.42 mmol), and ∼10 mL of py. Yield 75.4% (0.655 g). FTIR
(KBr pellet, cmꢀ1): 3054(m), 3006(m), 2829(m), 1591(s), 1571
(m,sh), 1482(s), 1444(s), 1295(s), 1262(s), 1212(s), 1150(m),
1112(m), 1069(m), 1034(m), 1004(m), 950(w), 918(w), 866(m),
803(w), 753(s), 715(s), 698(s), 663(m), 619(m), 598(m), 459(br,m).
Anal. Calcd for C29H29FeN3O2 (MW = 507.42): C, 68.65; H, 5.76; N,
8.28. Found: C, 68.21; H, 5.88; N, 7.86.
[Fe(DPhP)2(py)2] (7). Used Fe(Mes)2 (1.00 g, 3.40 mmol), H-DPhP
(1.68 g, 6.81 mmol), and ∼10 mL of py. Yield 72.1% (1.73 g). FTIR
(KBr pellet, cmꢀ1): 3519(s), 3103(m), 3058(s), 3036(m), 3022
(m), 2959(m), 2923(s), 2588(s), 1638(s), 1599(s), 1580(m), 1561
(w), 1493(w), 1483(w) 1458(sh), 1454(s), 1445(s), 1412(s),
1364(w), 1328(w), 1311(m), 1293(s), 1272(m), 1255(m), 1223(m),
1214(w), 1172(w), 1157(w), 1103(w), 1084(w), 1069(m), 1038(m),
1028(m), 1010(m), 997(w), 915(w), 862(s), 852(w), 802(m), 756
(s), 749(s), 700(s), 627(m), 608(m), 598(m), 588(m), 515(w),
482(w), 436(w), 425(m). Anal. Calcd. for C46H36FeN2O2 (MW =
704.62): C, 78.41; H, 5.15; N, 7.93. Found: C, 75.93; H, 5.44; N, 2.69.
[Fe(TOBS)2(py)2] (8). Used Fe(Mes)2 (0.250 g, 0.848 mmol),
H-TOBS (0.450 g, 1.70 mmol), and ∼5 mL of py. Yield 64.0% (0.403 g).
FTIR (KBr pellet, cmꢀ1): 2972(s), 2927(m), 2870(w), 1560(w),
1541(w), 1508(w), 1491(w), 1450(m), 1386(m), 1362(s), 1260(m),
1241(m), 1195(s), 1087(sh), 1053(br,s), 1017(br,s), 820(s), 758(m),
703(s), 518(w), 475(w), 433(w). Anal. Calcd for C34H64FeN2O8Si2
(MW = 740.90): C, 55.12; H, 8.71; N, 3.78; for C63H123Fe2N3O16Si4
(MW = 1401.67; equivalent of ꢀ1/2 py): C, 53.94; H, 8.84; N, 3.00.
Found: C, 53.51 H, 8.73; N, 3.96.
General X-ray Crystal Structure Information.31 Crystals were
mounted onto a glass fiber from a pool of Fluorolube and immediately
placed in a cold N2 vapor stream on a Bruker AXS diffractometer
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Inorganic Chemistry
ARTICLE
equipped with a SMART 1000 CCD detector using graphite-mono-
chromatized Mo KR radiation (λ = 0.7107 Å). Lattice determination and
data collection were carried out using SMART Version 5.054 software.
Data reduction was performed using SAINTPLUS Version 6.01 soft-
ware and corrected for absorption using the SADABS program within
the SAINT software package.
ceramic frit (BASi), (ii) a 1 M Bu4NþPF6ꢀ in CH3CN sample blank
solution, (iii) a polished (0.05 μm alumina paste (Beuhler)) 3 mm
platinum working electrode, and (iv) a Pt wire (BASi) counter
electrode. All sample solutions were prepared, and all cyclic voltam-
magrams (CV) were collected under an argon atmosphere. For 3,
initially CV scans were collected on a blank solution for 5 cycles to
obtain a meaningful background.
Structures were solved by direct methods that yielded the heavy
atoms along with a number of lighter atoms or using the PATTERSON
method. Subsequent Fourier syntheses yielded the remaining light-
atom positions. The hydrogen atoms were fixed in positions of ideal
geometry and refined using SHELXS software. The final refinement of
each compound included anisotropic thermal parameters for all non-
hydrogen atoms. It should be noted that crystal structures of M(OR)x
often contain disorder within the atoms of the ligand chain causing higher
than normal final correlations.54ꢀ58 All final CIF files were checked at
http://www.iucr.org/. Additional information concerning data collec-
tion and final structural solutions can be found in the 14 information files
or by accessing CIF files through the Cambridge Crystallographic Data
Base. Data collection parameters for 1ꢀ8 are given in Table 1.
Specific Issues. For compound 1, a disordered py in the lattice was
squeezed using PLATON. Both compounds 5 and 8 displayed disorder
in the pendant carbon chains of the Pri and But moieties, respectively.
This was modeled using standard crystallographic restraints; however;
the final quality of the structure was not significantly improved, and further
efforts were not pursued. Compound 6 had disorder in the coordinated py
ring, and all attempts to resolve the disorder were abandoned since no
improvement in the final structure was found.
Beryllium Dome X-ray Powder Patterns (BeD-XRD). De-
tailed information pertaining to the beryllium dome XRD (BeD-XRD)
analyseshasbeen previously disseminated.59ꢀ61 All sampleswere prepared
in an argon-filled glovebox using a 1 cm quartz disk (zero-background
plate), where the sample was pressed into the specimen cavity, leveled,
sealed, and carefully loaded into a Siemens D500 diffractometer (scan
settings, 40 kV and 30 mA with a 0.04° step size, 1 s count time, range
of 5ꢀ30° 2θ, 1° divergence and receiving slits; goniometer radius =
250 mm). Note: Only trained personnel wearing the appropriate personal
protective equipment should handle the BeD equipment. If the BeD is
accidentally shattered, proper safety cleanup and disposal protocols must
be followed.
Nanoparticle Synthesis. A SOLVO route was used to generate
nanomaterials from dissolution of the desired crystalline material in py
(0.50 g/20 mL) under an atmosphere of argon. This solution was then
sealed in a Teflon-lined Parr acid digestion bomb and then heated to
200 °C for 24 h. After cooling to room temperature each reaction bomb
was opened in air, the reaction mixture transferred to centrifuge tubes
and centrifuged, and the supernatant decanted from the precipitate.
The precipitate was then washed three times with hexanes, and the
resulting powder was allowed to dry at room temperature under ambient
conditions.
For transmission electron microscopy (TEM) analyses, the de-
sired powder was slurried in MeOH and then a small aliquot was
placed directly onto a holey-carbon copper-coated TEM grid (200
mesh from Electron Microscopy Sciences) and allowed to dry in air.
The resultant particles were studied using a Philips CM 30 TEM
operating at 300 kV accelerating voltage. In addition, the powder was
characterized using powder XRD on a PANalytical X’Pert Pro using
Cu KR radiation with a step size of 0.0167°, with a 0.152°/s dwelling
time. Dried crystalline powders were mounted directly onto a zero
background holder.
Electrode preparation for cyclic voltammetry measurements in water
was undertaken using a solution of 5 wt % sulfonated fluorine-sub-
stituted polyethylene or Nafion (Sigma-Aldrich) dissolved in a mixture
of low-weight aliphatic alcohols. To this solvent mixture graphite (Timrex
KS6, Timcal Corp.) was added to generate a solids ratio of 25:75 wt %
polymer:graphite followed by stirring (10 min at room temperature) until
a low-viscosity mixture formed. Approximately 100 μL of this slurry was
subsequently drop cast onto a glassy carbon electrode (Bioanalytical
Systems). To prepare the nanoparticle electrode, a similar preparation
was synthesized and processed as noted above with inclusion of 0.2 g of
6n in the slurry. Electrochemical measurements in water were performed
using a Bioanalytical Systems Epsilon potentiometer at 25 mV/s at room
temperature. A saturated Ag/AgCl electrode was used as a reference, Pt
as the counter electrode, and 1 M H2SO4 in DI water as the electrolyte.
Solutions were measured in both aerated and deaerated (nitrogen purge
for 12 h) conditions.
’ RESULTS AND DISCUSSION
Prior to initiating a study on the electrochemical behavior of
FeOx nanoparticles, the use of [Fe(OR)x] precursors with well-
understood structural properties was desired. This is due to the
fact that the size, shape, and composition of the final nanomater-
ials have been shown to be directly impacted by the structure of
the [M(OR)x] precursor.49ꢀ51 Unfortunately, there is a surpris-
ingly small number of structures reported for this family of
compounds including the following: (i) the oxo derivatives
[Fe9(μ3-O)3(μ-OEt)5(OEt)6(HOEt)]9 (OCH2CH3 (OEt)) and
[Fe5(μ5-O)(μ-OR)8(OR)5] (where OR = OEt,5,6 OCH(CH3)2
(OPri)3); (ii) the alkoxides [Fe(OPri)3]8 and [Fe(μ-OR)(OR)2]2
[where OR = {OC(CH3)2R: R = CH3,4,7 CH2CH3 }, OC(CH3)2-
3
(C6H5),5 OCH(C6H5)248], [Fe(OCPh3)(THF)2],2 and (iii) the
aryloxides [Fe(μ-DBP-But-4)(DBP-But-4)]2,2 [Fe(OC6H3-
[C6H3(CH(CH3)2)-2,6]-2,6],46 [(OR)cFe(μc-OR)]2 (OR =
OC6H4(CH2N(CH3)2)-2; c = chelating] and [(py)2Fe(OC6H3-
c
(CH2N(CH3)2)2 -2,6].47 One reason for this crystallographic
lacuna may be due to the reported insolubility of Fe(II) alkoxides.62
Therefore, it became necessary to synthesize highly soluble [Fe-
(OR)x] nanoparticle precursors which could be crystallographi-
cally characterized. From the wide array of alkoxide ligands
available, based on previous experience, the ortho-substituted
phenols were ideal ligands for this study due to their commercial
availability, variations in steric bulk of the phenol, and known
value in reducing oligomerization.54ꢀ58,63ꢀ67
Several synthetic routes to the [Fe(OAr)x] of interest were
explored. Initially, the aminolysis (eqs 2a and 2b in THF) route
was used for both Fe(II) and Fe(III) metal centers. From eq 2a,
the following crystals were of high enough quality to yield a
structure solution (see Table 1): [Fe(DBP)2(THF)(py)] (1S),
[Fe(DPhP)3] (2S), or [Fe(DPhP)2(THF)2] (3S); figures are
available in the Supporting Information. Unfortunately, these
and other products isolated were (i) contaminated (with alkali
metal and/or halides), attributed to the difficulty in preparing
pure Fe(NR2)n, (ii) variable in final oxidation states, and/or
(iii) insoluble. Due to these inconsistent results, these routes
(eqs 2a and 2b) were abandoned.
Electrochemical Analysis. All cyclic voltammetry (CV) analyses
were performed using a PAR2273 in potentiostatic mode at room
temperature using a sweep rate of 25 mV/s. The sweep ran from the
reference potential following an anodic sweep with the following
setup: (i) Ag/AgNO3 (0.1 M in CH3CN) reference electrode with a
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ARTICLE
Table 1. Data Collection Parameters for 1ꢀ8 and 1Sꢀ3S
compound
1c
2
3
4
chemical formula
fw
C
29H29Fe N3O2
C33H37FeN3O2
563.51
C35H41FeN3O2
591.56
C31H33FeN3O2
535.45
507.40
temp (K)
space group
a (Å)
173(2)
173(2)
173(2)
173(2)
orthorhombic, Fddd
13.493(2)
28.102(5)
29.703(5)
monoclinic, C2/c
34.823(4)
16.292(2)
17.320(2)
110.544(3)
9201(2)
monoclinic, C2/c
37.654(6)
13.255(2)
25.791(4)
128.674(5)
10049(3)
12
monoclinic, C2/c
13.5680(19)
11.6850(16)
19.236(3)
104.329(2)
2954.8(7)
4
b (Å)
c (Å)
β (deg)
V (Å3)
11263(3)
16
Z
12
Dcalcd(Mg/m3)
μ (Mo, KR) (mmꢀ1
R1a (%) (all data)
wR2b (%) (all data)
1.197
1.220
1.173
1.204
)
0.563
0.524
0.483
0.540
5.87(6.90)
12.37(12.90)
5.49(8.44)
12.14(14.83)
3.62(4.93)
8.24(8.92)
3.16(3.67)
7.74(8.05)
compound
5
6
7
8
chemical formula
fw
C
39H49FeN3O2
C38H52FeN2O2
C46H36FeN2O2
C34H64FeN2O8Si2
647.66
624.67
704.62
740.90
temp (K)
space group
a (Å)
173(2)
173(2)
173(2)
173(2)
orthorhombic, Pbca
14.461(3)
18.341(3)
27.896(5)
monoclinic, P2(1)/c
21.884(3)
16.018(2)
20.621(3)
104.327(2)
7003.4(15)
8
monoclinic, P2(1)/n
13.131(2)
15.622(3)
19.182(4)
109.558(3)
3707.9(12)
4
tetragonal, P4(3)2(1)2
10.7648(6)
10.7648(6)
37.937(3)
b (Å)
c (Å)
β (deg)
V (Å3)
7399(2)
8
4396.2(5)
4
Z
Dcalcd(Mg/m3)
1.163
1.185
1.262
1.119
μ,(Mo, KR) (mmꢀ1
R1a (%) (all data)
wR2b (%) (all data)
)
0.443
0.464
0.447
0.440
6.38(13.47)
12.07(14.77)
5.59(6.78)
12.16(12.97)
2.73(2.92)
7.37(7.53)
4.63(5.33)
11.19(11.74)
compound
1S
2S
3S
chemical formula
fw
C37H55FeNO3
617.67
C54 H39FeO3
C44H42FeO4
690.63
791.70
temp (K)
space group
a (Å)
173(2)
173(2)
173(2)
monoclinic, P2(1)/n
9.740(2)
22.662(5)
16.299(3)
101.432(4)
3526.2(13)
4
rhombohedral, R3
10.5954(5)
10.5954(5)
10.5954(5)
97.2140(10)
1158.57(9)
1
monoclinic, C2/c
64.054(13)
9.4340(19)
23.507(5)
100.035(7)
13988(5)
16
b (Å)
c (Å)
β (deg)
V (Å3)
Z
Dcalcd(Mg/m3)
μ,(Mo, KR) (mmꢀ1
R1a (%) (all data)
wR2b (%) (all data)
1.163
1.135
1.312
)
0.461
0.365
0.475
3.71(4.66)
7.29 (7.35)
17.18(18.39)
28.62 (29.15)
9.42(10.03)
2
19.05 (19.14)
a R1 = Σ Fo| ꢀ |Fc /Σ|Fo| ꢂ 100. b wR2 = [Σw(Fo ꢀ Fc2)2/Σ(w|Fo|2)2]1/2 ꢂ 100. c Molecule of solvent was squeezed out.
Since THF and the aminolysis routes proved problematic, we
elected to investigate py as a solvent using the alkyl/alcoholysis
exchange (eq 1). The synthesis and characterization of these
Fe(OAr)2 precursors, the subsequent γ-Fe2O3 nanoparticles,
and the initial electrochemical properties of model precursors
and nanoparticles are presented below.
½FeðNðSiMe3Þ2Þ2ꢁ þ 2HꢀOAr f ½FeðOArÞ2ꢁ þ 2HꢀNðSiMe3Þ2
ð2aÞ
½FeðNMe2Þ3ꢁ þ 3HꢀOAr f ½FeðOArÞ3ꢁ þ 3HꢀNMe2 ð2bÞ
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Synthesis. Using the [Fe(Mes)2] with HOAr in pyridine
proved to be a rapid, convenient, high-yield, and reproducible
route to the desired family of soluble nanoparticle precursors.
For each reaction the dark-red [Fe(Mes)2] powder was dissolved
in pyridine and the appropriate aryl alcohol was added. The
reaction mixtures darkened considerably over time with no pre-
cipitate observed after stirring for 12 h. After drastically reducing
the volume of the reaction, each was set aside and allowed to
slowly evaporate until crystals formed.
Single-Crystal Structures. As crystals of the products from
eq 1 were obtained, single-crystal structures were solved to assist
in determining their identity. Interestingly, for 1ꢀ5, all the OAr
derivatives isolated proved to be monomers with each Fe
adopting a distorted square base pyramidal geometry (τ = 0.21
(1), 0.22 (2), 0.21 (3), 0.21(4), 0.37 (5)).68 Apy solventmolecule
occupies the apical position for each of these compounds with
two trans OAr ligands and two trans py molecules in the basal
positions. Compounds 6ꢀ8 were also found to be monomeric
but adopted a distorted tetrahedral geometry. Figures 1 and 2
show the structure of representative compounds 3 and 6,
respectively. The remaining structural plots are available in the
Supporting Information. The reduced number of bound solvent
molecules for these compounds is presumably due to the
increased steric bulk of the DBP, DPhP, or TOBS ligands. The
metrical data for these [Fe(OAr)2(py)x] are tabulated in Table 2
and were found to be consistent within this family of compounds
as well as literature species.2ꢀ9
Bulk Powder Analyses. Analyses of the bulk powders of 1ꢀ8
were undertaken; however, the paramagnetic nature of the
iron(II) metal center yields extremely broad NMR spectra, which
makes structural interpretation difficult and verification of
the purity tenuous. The majority of the compounds’ elemental
analyses for 1ꢀ8 were outside the range of the expected values
and were not consistent with fully desolvated compounds. This
incongruity has been previously noted for a number of solvated
M(OR)x and is often attributed to preferential loss of solvent
upon analysis (when possible, these are noted in in the
Experimental Section) or complex thermal decomposition of
the precursor.54ꢀ58,63ꢀ67,69,70 Therefore, additional analytical
methods were investigated to assist in characterizing these
compounds.
Figure 1. Structural plot of 3. Heavy-atom thermal ellipsoids drawn at
the 30% level, and carbon atoms drawn as ball and stick for simplicity.
Hydrogen atoms omitted for clarity. Two molecules per unit cell.
For the FTIR spectra of 1ꢀ7, the strong stretches above
1500 cmꢀ1 (tentatively assigned as the FeꢀC stretch)71 were
absent, with stretches and bends associated with the respec-
tive OAr ligands now present. Since the bonding mode around the
Fe metal centers of 1ꢀ7 consists of FeꢀO and FeꢀN bonds only,
the fingerprint region of the FTIR spectra of these compounds is
nearly identical. Two major stretches around 750 and 700 cmꢀ1
and two minor stretches located around 860 and 800 cmꢀ1 were
observed. The remainder of the spectrum consists of OAr and py
stretches/bends. For 8, the spectrum is greatly simplified,
especially in the fingerprint region with only two major stretches
at 803 and 703 cmꢀ1. There is a minor stretch at 758 cmꢀ1, and
the other one may be obscured by the broad peak at 803 cmꢀ1
The changes in bond stretches must be attributed to the
differences of the siloxide ligand in comparison to the aryloxides.
.
Figure 2. Structural plot of 6. Heavy-atom thermal ellipsoids drawn at
the 30% level, and carbon atoms drawn as ball and stick for simplicity.
Two molecules per unit cell.
Table 2. Select Averaged Metrical Data for 1ꢀ8
distance (Å)
angle (deg)
compound
Fe-OAr/OSiR3(av Å)
FeꢀNpy(av Å)
OꢀFeꢀO (av deg)
NꢀFeꢀN (av deg)
OꢀFeꢀN (av deg)
1
2
3
4
5
6
7
8
1.948(2)
1.94
2.19
157.25(15)
160.61(12)
165.68(7)
159.23(11)
150.91(9)
132.47(9)
152.49(5)
134.77(18)
133.53
134.20
132.10
132.36
133.59
86.14
93.29
92.80
92.60
93.45
94.24
107.70
99.12
105.88
2.21
1.94
2.30
1.913(2)
1.93
2.21
2.23
1.89
2.18
1.91
2.15
95.33(5)
89.35(16)
1.865(3)
2.138(3)
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Figure 3. PXRD pattern of the representative spectrum of 8n and library pattern for γ-maghemite (39-1348, γ-Fe2O3).
In general, it can be concluded that the spectra of these
compounds are in agreement with each other, which implies
that the bulk powders are in agreement with the single-crystal
X-ray structures.
Since the precursors are all monomers and form the same
γ-Fe2O3, based on the precursor structure argument, the general
shape of the precursor can be used to generate preferred
morphologies,49ꢀ51 the nanoparticles should all look similar. A
polydispersed set of nanodots is observed for each [Fe(OR)2]
precursor. The largest and most uniform nanodots appear to be
generated for 1n (Figure 4a), 6n (Figure 4f), 7n (Figure 4g), and
8n (Figure 4h), which range from 10 to 60 nm in size with the
majority appearing around 40 nm in size. For 2n (Figure 4b), 3n
(Figure 4c), and 4n (Figure 4d), the particles observed had some
similar sized nanoparticles noted for the previous compounds
but the majority of particles are substantially smaller. This is
especially true for 5n (Figure 4e), where the nanoparticle appears
to be uniform 12 nm sized nanoparticles. In order to determine
whether the size and shape formation of the nanoparticles was a
result of the thermodynamically favored species forming inde-
pendent of the starting material, the Fe(Mes)2 precursor was also
run under similar conditions. The particles are shown in Figure 4i
and appear to be more square-like in shape while maintaining the
γ-Fe2O3 phase; however, repetition of the synthesis of these
nanoparticles led to less regular square-shaped material. There-
fore, the alkoxides can be attributed to favoring the formation of
dots where the size variations may be attributed to slight differences
in processing time and concentrations of the selected precursors.
The need for more complex shaped precursors is necessary to
probe this process further, and work in this area is currently
underway.
While the FTIR data is informative, it is not conclusive in
terms of purity. To address this issue, we opted to focus on
beryllium dome PXRD (termed BeD-XRD),59ꢀ61 which allows
for X-ray analyses of air-sensitive compounds. A comparison
between the experimental data with the theoretical patterns
(derived from the single-crystal structure) can lend some insight
into the purity of the bulk powders. It is of note that the theoretical
patterns are generated based upon an idealized random sample,
whereas crystal orientation for the actual powders will favor
certain d-spacing patterns. Further, to prepare a random sample,
it is often necessary to grind the powder, which can alter the
degree of solvation and affect the final powder pattern observed.
Therefore, dried crystalline material that was not ground was
used for each sample. The experimental patterns of the crystalline
material appear to be in-line with the calculated spectra (see
Supporting Information) for 3, 4, 6, 7, and 8, whereas, the spectra
of compounds 1, 2, and 5 are less definitive due to the texture
issues noted above. Combined, the FTIR and BeD-XRD data
indicate that the bulk materials are in agreement with the single-
crystal structures obtained.
Nanoparticle Synthesis. With this family of well-character-
ized precursors in hand, it was of interest to determine their utility
in producing nanomaterials of FeOx. Using a SOLVO route,
nanoparticles were successfully isolated from each precursor. For
convenience, these are named based on the assigned precursor
number followed by n (i.e., nanoparticles generated from com-
pound 1 are named 1n). Powder X-ray diffraction indicated that
each sample produced the iron deficient γ-maghemite (PDF 39-
136, γ-Fe2O3). Since all the XRD patterns were identical a
representative PXRD pattern (8n) is shown in Figure 3, while the
TEM images of the resultant nanomaterials for each precursor
are shown in Figure 4aꢀi.
Electrochemical Characterization. Little information is
available concerning the electrochemical characterization of
Fe(OAr)2 compounds or the nanoparticles derived from them.
Since the precursors were all monomeric solvated species and the
nanoparticles all adopted the same phase and general mor-
phology, only one sample from each was evaluated. Initially, a
control electrolyte solution was prepared and tested to collect
background voltammagrams, which foundno observable reduction
oxidation (redox) waves. This setup was then used to explore the
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Figure 4. TEM images of (a) 1n, (b) 2n, (c) 3n, (d) 4n, (e) 5n, (f) 6n, (g) 7n, (h) 8n, and (i) nanoparticles generated from Fe(Mes)2 using the SOLVO
preparation route.
electrochemical results for the representative precursor (3) and
nanoparticle (6n).
Compound 3was added to an electrolyte solution of (Bu4N)PF6
in CH3CN and scanned with the resulting cyclic voltammagram
(CV) curves (oxidative to reductive) shown in Figure 5. During
the initial electrochemical evaluation several oxidative peaks were
observed: the final oxidative current was unresolved as a separate
reaction due to the limits of the electrochemical window for
the solvent chosen (acetonitrile, stability limits ∼1 V to ∼ꢀ2 V
vs Ag/Agþ). There were no reduction peaks present. Five cycles
were run, and the intensity of these peaks decreased as the number
of cycles increased. It was reasoned that as the current was applied, a
coating was being deposited on the electrode and subsequent
oxidation cycles yielded additional material deposition, which
ultimately suppressed further oxidation of the reagents. After 5
cycles the test was stopped and the electrode visually inspected.
Figure 5. Cyclic voltammetry of 3 in CH3CN.
The electrode was now a dull, brown color instead of the initial
shiny metal platinum. Unfortunately, the film generated on the
electrode did not yield an interpretable PXRD pattern (see
Supporting Information). This can be attributed to too small of a
sample to allow for meaningful data, the size of the materials
formed is subnanometer in size, which significantly broadens the
signal, and/or the final material being amorphous. Further analyses
of the material using micro-X-ray fluorescence (XRF; see Support-
ing Information) indicated that Fe was present in the sample.
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Inorganic Chemistry
ARTICLE
the nanoparticles (and the higher energy of the surface atoms
residing on the nanoparticle surface).
’ SUMMARY AND CONCLUSION
A novel family of [Fe(OAr)2(py)x] precursors has been
synthesized and isolated from the alcoholysis of [Fe(Mes)2]
in py. On the basis of the degree of solvation, the metal centers
of 1ꢀ5 were found to adopt a distorted square base pyramidal
geometry. As more sterically demanding ligands were intro-
duced, compounds 6ꢀ8 adopted a distorted tetrahedral geo-
metry around the Fe metal center. This new family of com-
pounds was used in a SOLVO approach to generate nanodots
of γ-Fe2O3. Electrochemical investigation of 3 revealed that the
precursor undergoes several oxidation steps that have been
preliminarily assigned to the loss of OR ligand and oxidation of
the Fe2þ metal center. Oxidation and reduction of the nano-
particles (6n) was observed in an aqueous electrolyte bound
into a porous electrode, although potentials higher than the
literature values for standard oxidation potentials were ob-
served. More work needs to be done to see if this is due to
the nano-characteristics of the material. With the baseline
established to the electrochemical behavior of these nanomater-
ials, additional work to probe their capability to intercalate Li ions
for use as lithium-ion battery cathodes is underway.
Figure 6. Porous electrode cyclic voltammetry of blank carbon porous
electrode vs carbon porous electrode with 6n added. Conditions: room
temperature, 25 mV/s sweep rate, 1 M H2SO4 electrolyte.
Understanding the various transformations and assigning the
CV peaksto specific aspects of the precursor were attempted. Initial
investigations focused on determining the structural properties of 3
in CH3CN. Recrystallization of 3 from hot CH3CN yielded an
identical crystal (3a). This indicates the compound is stable in
hot CH3CN and any changes that occur in the electrochemical
study must be attributed to the electrochemical process only and
not the solvent. The electrolyte of the cell after cycling was
evaluated by GC-MS, and the full experimental data set is shown
in the Supporting Information. Initially, a set of standards was
run to verify retention time and the MS spectrum in the presence
of the salt, yielding py at 3.87 min, CH3CN at 5.14 min, and
H-oBP at 25.1 min. The analysis of the resultant elute from the
redox yielded three peaks at 3.87, 5.14, and 25.1 min, which were
consistent with the elution time of the solvents. An initial GC-MS
analysis of the 3 in CH3CN indicates the alcohol was present as
well, which may be due to the thermal break down of 3 upon
injection into the GCMS. On the basis of these data and crystal-
lization of the same structure from CH3CN (vide infra 3a), the first
two peaks have been tentatively assigned to oxidation of the oBP
ligand. This eventually leads to the oxide forming (vide infra
PXRD) and the Fe-containing deposit found on the electrode.
Additional studies are underway to fully characterize the various
redox properties of these interesting precursors and explore how
the ligands would effect these transformations.
As expected, a blank of polymer and graphite showed no
electrochemical behavior in the voltage window explored. With
the addition of 6n, a distinct oxidation/reduction peak pair was
observed (Figure 6). Deaeration of the reaction media did not
significantly change the extent of oxidation and reduction
observed. The midpoint potential between oxidation and reduc-
tion occurs at around 450 mV vs Ag/Agþ, which is higher than
the tabulated standard potential for iron oxidation of þ240 mV
vs Ag/Agþ.72 However, the relatively broad peaks for both
oxidation and reduction peaks coupled with the porous nature of
the electrode makes definitive assessment of the exact chemical
potential difficult. The large separation between oxidation and
reduction peaks is associated with the diffusion within the porous
electrodes used in this study. What is clear from the study is that
the nanoparticle iron is electroactive within the aqueous voltage
window and that clear quasi-reversible oxidation and reduction is
occurring. Additional work is needed to prepare nonporous
electrodes that can be studied without diffusional limitations in
order to better understand the oxidation and reduction of the
nanoparticles themselves. In particular, the relatively large shift in
standard potential observed may be due to some surface effect of
’ ASSOCIATED CONTENT
S
Supporting Information. Additional data including BeD-
b
XRD setup and experimental BeD-XRD patterns for 1ꢀ8, GC
patterns of nanoparticle electrochemical solutions, GIXRD pat-
terns of electrochemically generated materials, and structural plots
of 1ꢀ8 and S1ꢀS3, are available. The crystal structures of 1ꢀ8
and S1ꢀS3 have been deposited at the Cambridge Crystallo-
graphic Data Centre and allocated the deposition numbers CCDC
816497ꢀ816507. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: (505)272-7625. Fax: (505)272-7336.E-mail tjboyle@
Sandia.gov
’ ACKNOWLEDGMENT
The authors thank Dr. S. D. Bunge, M. Trujillo, and H. D.
Pratt III for technical assistance. This work was supported by the
Laboratory Directed Research and Development (LDRD) pro-
gram at Sandia National Laboratories. Sandia is a multiprogram
laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy’s Na-
tional Nuclear Security Administration under Contract DE-
AC04-94AL85000.
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