Metal atom lability in polynuclear complexes

Article abstract of DOI:10.1021/ic302694y

The asymmetric oxidation product [(^PhL)Fe₃(μ-Cl)] ₂ [^PhLH₆ = MeC(CH₂NHPh-o-NHPh) ₃], where each trinuclear core is comprised of an oxidized diiron unit [Fe₂]⁵⁺ and an isolated trigonal pyramidal ferrous site, reacts with MCl₂ salts to afford heptanuclear bridged structures of the type (^PhL)₂Fe₆M(μ-Cl) ₄(thf)₂, where M = Fe or Co. Zero-field, ⁵⁷Fe Mo?ssbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl₄(thf)₂ position indicating Co migration into the trinuclear subunits has occurred. Reaction of [(^PhL)Fe₃(μ-Cl)]₂ with CoCl ₂ (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by ¹H NMR, ⁵⁷Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M₂]⁵⁺), while the M²⁺ site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo]⁵⁺) or S = 1/2 ([Co₂]⁵⁺) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [( ^PhL)Fe₃(μ-Cl)]₂ with isotopically enriched ⁵⁷FeCl₂(thf)₂, the results of which suggest rapid equilibration of ⁵⁷Fe into both the M²⁺ site and oxidized diiron site, achieving a 1:1 mixture.

Full text of DOI:10.1021/ic302694y

Article
pubs.acs.org/IC
Metal Atom Lability in Polynuclear Complexes
Emily V. Eames, Raul Hernandez Sanchez, and Theodore A. Betley*
́
́
́
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge Massachusetts 02138, United
States
S
* Supporting Information
ABSTRACT: The asymmetric oxidation product [(PhL)-
Fe3(μ-Cl)]2 [PhLH6 = MeC(CH2NHPh-o-NHPh)3], where
each trinuclear core is comprised of an oxidized diiron unit
[Fe2]5+ and an isolated trigonal pyramidal ferrous site, reacts
with MCl2 salts to afford heptanuclear bridged structures of the
type (PhL)2Fe6M(μ-Cl)4(thf)2, where M = Fe or Co. Zero-
field, 57Fe Mossbauer analysis revealed the Co resides within
̈
the trinuclear core subunits, not at the octahedral, halide-
bridged MCl4(thf)2 position indicating Co migration into the
trinuclear subunits has occurred. Reaction of [(PhL)Fe3(μ-
Cl)]2 with CoCl2 (2 or 5 equivalents) followed by
precipitation via addition of acetonitrile afforded trinuclear
products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified
1
by H NMR, 57Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic
analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2]5+), while the M2+ site remains iron-
substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a
strongly coupled S = 1 unit ([FeCo]5+) or S = 1/2 ([Co2]5+) that is weakly antiferromagnetically coupled to the high spin (S = 2)
ferrous site. The kinetic pathway for metal substitution was probed via reaction of [(PhL)Fe3(μ-Cl)]2 with isotopically enriched
57FeCl2(thf)2, the results of which suggest rapid equilibration of 57Fe into both the M2+ site and oxidized diiron site, achieving a
1:1 mixture.
metal or alkaline ions,10 or using polynucleating ligands that
I. INTRODUCTION
have different elemental binding affinities.11 The selective
Polynuclear, heterometallic clusters are employed in nature to
exchange of transition metals into a polynuclear cluster could
facilitate small molecule activation.1−3 Two prototypical
be a potential method for the preparation of polymetallic
enzymes that feature heteropolynuclear metallocofactors are
clusters in a systematic fashion, allowing for rigorous study of
the water oxidizing center in photosystem II2 and the nitrogen
the electronic property perturbations of the newly formed
fixing cofactors of nitrogenase.1 In water oxidation, the
polymetallic species. In this contribution, we present the metal
presence of Ca within the Mn4O4Cl cofactor is critical for
atom metathesis of cobalt for iron within a preformed, all-iron
oxygen evolution, though the mechanistic role Ca plays is still a
topic of debate.4 The composition of the nitrogen-reducing
trinuclear species to afford isostructural bimetallic cluster
cofactor has been observed to contain Fe alone5 in a fused
analogues.
We have observed that low-spin and high-spin iron-based
clusters mediate redox processes in two distinct pathways. In
cubane structure with an iron−sulfur Fe8S9 cluster or
heterometallic clusters where a single Fe is replaced with
Mo1 or V6 in the FeMo and FeV cofactors, respectively. The
the low-spin regime, core-delocalized redox behavior was
observed where intracore bonding is enhanced.12 In contrast,
cofactor composition has been reported to have significant
ramifications on the enzymatic chemoselectivity and catalytic
13
the chemical oxidation of high-spin (PhL)Fe3(thf)3 [PhLH6 =
efficacy.7 The desirable properties afforded by polymetallic
MeC(CH2NHPh-o-NHPh)3 binds as a hexadentate, hexa-
anionic ligand when fully deprotonated] leads to cluster
assemblies extend beyond enzymatic function, as mixed-metal
oxidation where the trinuclear core distorts featuring a diiron
assemblies have impacted applications in metallurgy, electro-
site as the locus of oxidation [Fe2]5+, separate from the site of
halide capture at a formally +2 iron site (Scheme 1).14
Oxidation is accompanied by a significant ligand rearrangement
to accommodate the distorted core. Typically, such ligand
reorganization presents a large energy barrier; thus we
chemistry, and heterogeneous catalysis.8
Research into the synthesis of abiological, polymetallic
clusters is aimed at understanding how metal atom substitution
alters the cluster electronic properties and reactivity profiles of
the resulting clusters. Synthetic effort targeting polymetallic
clusters falls into three general categories: formation using
nonselective, self-assembly processes,9 reaction of incomplete
clusters (e.g., partially formed cubanes) with binary transition
Received: December 7, 2012
Published: April 23, 2013
© 2013 American Chemical Society
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1.8−10 K under fields of 1, 2, 3, 4, 5, 6, and 7 T. Susceptibility data
were corrected for the diamagnetic contribution of a blank sample
consisting of the wax, capsule, and straw at the correct field and
temperature. The magnetic susceptibilities were adjusted for
diamagnetic contributions using the constitutive corrections from
Pascal’s constants. The molar magnetic susceptibility (χm) was
calculated by converting the magnetization (M) obtained from the
magnetometer to a molar susceptibility using the multiplication factor
[molecular weight (MW)]/[sample weight (m) × field strength (H)].
All samples were checked for ferromagnetic impurities by collecting a
field dependence curve at 100 K, and samples were rejected if any
deviation from linearity was observed.
Scheme 1
proposed that the maximally high-spin formulation of the all-
ferrous precursor creates an inherently labile system. In this
contribution, we present data to suggest the notion that lability
extends beyond ligand reorganization to include the facile
exchange of metal atoms out of the polynuclear core.
Zero-field, 57Fe Mossbauer spectra were measured on a constant
̈
acceleration spectrometer (SEE Co, Minneapolis, MN) with a Janis
SVT-100 cryostat. Isomer shifts are quoted relative to α-Fe foil (<25
μm thick) at room temperature. Samples were prepared using
approximately 30 mg of sample suspended in paratone-N oil.
Temperatures were controlled using a LakeShore 321 autotuning
temperature controller. Temperature variations were no greater than
10 K, and were generally within 2 K. Data were analyzed using an
in-house package written by E. R. King and modified by E. V. Eames in
Igor Pro (Wavemetrics).
II. EXPERIMENTAL SECTION
Materials and Methods. All manipulations involving metal
complexes were carried out using standard Schlenk line or glovebox
techniques under a dinitrogen atmosphere. All glassware was oven-
dried for a minimum of 4 h and cooled in an evacuated antechamber
prior to use in the drybox. Benzene, diethyl ether, acetonitrile
(NCCH3), and tetrahydrofuran (THF) were dried and deoxygenated
on a Glass Contour System (SG Water USA, Nashua, NH) and stored
over 4 Å molecular sieves (Strem) prior to use. Benzene-d6, NCCH3-
d3, and THF-d8 were purchased from Cambridge Isotope Laboratories
and were degassed and stored over 4 Å molecular sieves prior to use.
Solvents were typically tested with a standard purple solution of
sodium benzophenone ketyl in THF in order to confirm effective
oxygen and moisture removal. MeC(CH2NHPh-o-NHPh)3 (PhL),13
(PhL)Fe3(thf)3, and [(PhL)Fe3(μ-Cl)]2 (1) were prepared following
published methods.13,14 57Fe powder was purchased from Cambridge
Isotope Laboratories and converted to 57Fe-enriched FeCl2 by
conproportionation with FeCl3 following the procedure outlined by
Wilkinson.15 All other reagents were purchased from commercial
vendors and used without further purification unless explicitly stated.
Physical Measurements. All of the measurements for the metal
complexes except XRF analysis were made under anaerobic conditions.
Elemental analyses were performed by Complete Analysis Labo-
ratories, Inc., Parsippany, New Jersey. 1H NMR spectra were recorded
on a Varian Unity/Inova 500B NMR spectrometer with chemical shifts
(δ ppm) referenced to residual NMR solvent. UV/visible spectra were
recorded on a Varian Cary 50 UV/Visible spectrometer using quartz
cuvettes. NIR spectra were recorded on a Perkin Elmer Lambda 750
high-performance UV−vis spectrometer. X-ray fluorescence analyses
were recorded on a Bruker Tracer III-SD XRF analyzer with the yellow
filter (composition 12 mil Al and 1 mil Ti, passes 12−40 keV), and
data were collected on each sample for at least 2 min. Samples for the
calibration curve were prepared by grinding together cobalt(II)-
chloride hexahydrate and ferrous chloride tetrahydrate. The spectra of
pure Fe and Co were each fit to two Voigt lineshapes. The calibration
samples were then fit to four Voigt lineshapes, with the areas of each
peak varying freely but all other parameters held to the reference
values. Fits are substantially poorer if the Kα:Kβ peak area ratios are
fixed, though the overall results are unchanged. Quantification was
performed using only the Kα peak areas, but there is no difference in
result if the Kα and Kβ areas are summed. Fe/Co ratios in samples
were determined by fitting using the calibration data and calculating
the Fe/Co ratio from the peak area ratios using the linear fit to the
calibration curve.
Preparation of (PhL)2Fe7(μ-Cl)4(thf)2 (2). A chilled (−35 °C)
solution of [(PhL)Fe3(μ-Cl)]2 (1; 100 mg, 61.3 μmol, 1 equiv) in
tetrahydrofuran (15 mL) was added to solid FeCl2 (8 mg, 63.5 μmol,
1.04 equiv) and stirred for 3 h, allowing the reaction to warm to room
temperature. The volatiles were removed under vacuum conditions.
Benzene (5 mL) was added with three drops of tetrahydrofuran to the
solids, and the resulting solution was filtered through a pipet fitted
with filter paper and stored at room temperature. The resulting
precipitate was washed with diethyl ether and dried under vacuum
conditions. Yield: 60 mg (52%). X-ray quality crystals were grown
from a concentrated solution in benzene at room temperature. Anal.
Calcd for C90H88Fe7N12Cl4O2: C, 56.82; H, 4.66; N, 8.83. Found: C,
56.57; H, 4.61; N, 8.71.
Preparation of (PhL)2Fe6Co(μ-Cl)4(thf)2 (3). A chilled (−35 °C)
solution of 1 (60 mg, 37 μmol, 1 equiv) in tetrahydrofuran (15 mL)
was added to solid CoCl2 (4.8 mg, 37 μmol, 1 equiv) and stirred for 3
h, allowing the reaction to warm to room temperature. The volatiles
were removed under vacuum conditions. Benzene (5 mL) was added
with three drops of tetrahydrofuran to the solids, and the resulting
solution was filtered through a pipet fitted with filter paper and stored
at room temperature. The resulting precipitate was washed with
benzene and dried under vacuum conditions. Yield: 35 mg (51%). X-
ray quality crystals were grown from a concentrated solution in
benzene at room temperature. Anal. Calcd for C90H88Fe6CoN12Cl4O2:
C, 56.73; H, 4.65; N, 8.82. Found: C, 56.69; H, 4.58; N, 8.75.
Preparation of (PhL)Fe2CoCl(NCCH3) (4). A solution of 1 (130
mg, 80 μmol, 1 equiv) in thawing tetrahydrofuran (20 mL) was added
to solid CoCl2 (21.2 mg, 164 μmol, 2.06 equiv) and stirred for 3 h,
allowing the reaction to warm to room temperature. The volatiles were
removed under vacuum conditions. Acetonitrile (2 mL) was added,
briefly dissolving the solid and rapidly precipitating a crystalline
product. The crystals were collected on a fritted glass funnel and
washed with acetonitrile (3 mL), then dissolved in benzene and
filtered to remove any insoluble material. The volatiles were removed
under vacuum conditions, and four drops of tetrahydrofuran were
added, followed by acetonitrile (2 mL). The product, which
precipitated from acetonitrile, was collected on a fritted glass funnel
and washed with more acetonitrile, then dried under vacuum
conditions. Recrystallized yield: 58 mg (42%). X-ray quality crystals
were grown from a concentrated solution in acetonitrile at room
1
temperature. H NMR (C6D6 with 3 drops NCCH3-d3, 500 MHz, δ,
Magnetic measurements were recorded using a Quantum Design
MPMS-XL magnetometer. Powdered samples were placed in Lilly #4
gel capsules and thoroughly saturated with melted eicosane wax to
prevent particle torquing in the magnetic field. Samples were
suspended in the magnetometer using plastic straws. Samples were
prepared under a dinitrogen atmosphere in a glovebox. DC magnetic
susceptibility data were collected in the temperature range 5−300 K
under fields of 0.1, 0.5, 1, and 2 T. Magnetization data were acquired at
ppm): 54, 48, 46, 45, 41, 23, 17, 16, 11, 5, 2, −2, −5, −14, −29, −39,
−72, −77, −90. Anal. Calcd for C43H39Fe2CoN7Cl: C, 60.06; H, 4.57;
N, 11.40. Found: C, 59.96; H, 4.59; N, 11.42.
Preparation of (PhL)FeCo2Cl(NCCH3) (5). A solution of 1 (140
mg, 86 μmol, 1 equiv) in thawing tetrahydrofuran (20 mL) was added
to solid CoCl2 (57.1 mg, 443 μmol, 5.1 equiv) and stirred for 3 h,
allowing the reaction to warm to room temperature. The volatiles were
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removed under vacuum conditions. Acetonitrile (2 mL) was added,
briefly dissolving the solid and rapidly precipitating a crystalline
product. The crystals were collected on a fritted glass funnel and
washed with acetonitrile (3 mL), then dissolved in benzene and
filtered to remove any insoluble material. The volatiles were removed
under vacuum conditions, and four drops of tetrahydrofuran were
added, followed by acetonitrile (2 mL). The product, which
precipitated from acetonitrile, was collected on a fritted glass funnel
and washed with more acetonitrile, then dried under vacuum
conditions. Recrystallized yield: 60.3 mg (41%). X-ray quality crystals
were grown from a concentrated filtered solution in acetonitrile at
Scheme 2
1
room temperature. H NMR (C6D6 with three drops NCCH3-d3, 500
MHz, δ, ppm): 48, 46, 30, 28, 26, 24, 17, 15, 11, 10, 5.5, −2, −4, −8,
−16, −19, −29, −39, −42, −56, −72. Anal. Calcd for
C43H39FeCo2N7Cl: C, 59.85; H, 4.55; N, 11.36. Found: C, 59.75;
H, 4.66; N, 11.28.
Preparation of [(PhL)FeCo2(μ-Cl)]2 (6). A solution of 1 (140 mg,
86 μmol, 1 equiv) in thawing tetrahydrofuran (20 mL) was added to
solid CoCl2 (57.1 mg, 443 μmol, 5.1 equiv) and stirred for 5 h,
allowing the reaction to warm to room temperature. The volatiles were
removed under vacuum conditions. Acetonitrile (2 mL) was added,
briefly dissolving the solid, and rapidly precipitated a crystalline
product. The crystals were collected on a fritted glass funnel and
washed with acetonitrile (3 mL), then dissolved in benzene and
filtered to remove any insoluble material. The volatiles were removed
under vacuum conditions, followed by dissolution in benzene (5 mL),
and the solution was stored overnight. The crystalline material was
collected on a fritted glass funnel and washed with benzene, then dried
under vacuum conditions. Recrystallized yield: 52 mg (37%). X-ray
quality crystals were grown from a concentrated solution in benzene at
room temperature. Anal. Calcd for C41H36FeCo2N6Cl: C, 59.91; H,
4.41; N, 10.22. Found: C, 59.94; H, 4.47; N, 10.08.
nearly the expected ratio of 4:2:1, and the metrical parameters
for the trinuclear cores match well with those reported for 1.14
For complex 3, three quadrupole doublets are present with the
following metrical parameters (δ, |ΔEQ| (mm/s)): 0.21, 2.69
(38.9%); 0.73, 1.39 (38.9%); and 1.16, 2.35 (22%), which
coincide with spectral features for the [Fe2]5+, Fe2+, and
FeCl4(thf)2 sites, respectively, in 2 (see Figure 1b). If the Co
atom occupied the tetrachloro-bridge position MCl4(thf)2, we
would anticipate the spectral features for the trinuclear subunits
to be unchanged and no spectral features for the MCl4(thf)2
site. However, the data are consistent with two iron atoms in
the trinuclear cores and iron, not cobalt, residing in the
MCl4(thf)2 site, suggesting Co atom migration into the
trinuclear subunits has occurred. This metal atom substitution
is striking in that a minimum of four Fe−N bonds must be
broken concomitant with the formation of four Co−N bonds.
To explore this substitution reaction further, we investigated
the reactivity of complex 1 with two or five equivalents of
CoCl2 (giving Fe/Co ratios of 3:1 and 3:2.5, respectively) in
tetrahydrofuran for 3 h at room temperature. Following
removal of the volatiles in vacuo, acetonitrile was added to
the solid. The solids briefly dissolved and rapidly precipitated a
fraction of the material as crystals. The crystallized material was
collected on a fritted glass funnel and washed with acetonitrile.
The collected product was recrystallized from a mixture of
tetrahydrofuran and acetonitrile at room temperature. The
reaction products obtained from these two reactions are
consistent with the following compositions: (PhL)Fe2CoCl-
(NCCH3) (4) from the reaction of 1 with two equivalents of
CoCl2, and (PhL)Co2FeCl(NCCH3) (5) from the reaction of 1
with five equivalents of CoCl2 (see Scheme 2). Excess CoCl2 is
used in the latter case to ensure full substitution during the time
scale of the reaction, while in the former reaction precisely one
equivalent of CoCl2 per equivalent of (PhL)Fe3Cl is used to
prevent oversubstitution to form 5. Without the addition of
extra acetonitrile during crystallization from benzene, the
halide-bridged species was obtained [(PhL)Co2Fe(μ-Cl)]2 (6)
from the reaction with excess CoCl2 (Figure 2c). Although the
Preparation of (PhL)Fe3Cl(NCCH3) (7). To a solution of 1 (50 mg,
31 μmol) in benzene (5 mL) was added 25 drops of acetonitrile. The
solution was filtered and the volatiles removed under vacuum
conditions. No crystals of this material were ever obtained, despite
numerous attempts. Yield: 51 mg (97%). 1H NMR (C6D6 with 3 drops
NCCH3-d3, 500 MHz, δ, ppm): 53, 36, 33, 28, 19, 13.5, 12, 6, −16,
−47, −80, −98. Anal. Calcd for C43H39Fe3N7Cl: C, 60.28; H, 4.59; N,
11.44. Found: C, 60.26; H, 4.54; N, 11.42.
57Fe Exchange Experiments. Degenerative exchange of 57Fe-
enriched FeCl2(thf)2 was observed by zero-field, 57Fe Mossbauer
̈
spectroscopy. Enriched FeCl2(thf)2 (2.5 mg, 0.5 equiv) was dissolved
in tetrahydrofuran (2 mL) with stirring, cooled to −35 °C in the
glovebox freezer, and added to 1 (30 mg, 18.4 μmol) frozen in 5 mL of
tetrahydrofuran. This experiment was performed in triplicate, and the
resulting solutions were filtered through a pipet fitted with filter paper
and evacuated to dryness (with constant stirring) under vacuum
conditions after 0, 2, and 15 h had elapsed. Mossbauer samples were
̈
prepared from the collected solids (∼15 mg).
III. RESULTS AND DISCUSSION
While exploring the incorporation of the oxidized trinuclear
species (PhL)Fe3X unit into larger assemblies, we observed the
propensity for metal atom substitution within the core. For
example, stirring [(PhL)Fe3(μ-Cl)]2 (1) with FeCl2 or CoCl2 in
tetrahydrofuran afforded new complexes that could be
crystallized from benzene yielding (PhL)Fe7(μ-Cl)4(thf)2 (2)
and (PhL)Fe6Co(μ-Cl)4(thf)2 (3), respectively (Scheme 2).
The solid-state molecular structures for 2 and 3 both feature
two trinuclear cores bridged by a single metal atom via four
bridging chloride ligands (see Figure 1c for a representative
molecular structure of 2 and Figures S2 and S3).15 Zero-field,
57Fe Mossbauer analysis of 2 was fit to four quadrupole
̈
doublets (δ, |ΔEQ| (mm/s), Figure 1a): two quadrupole
doublets for [Fe2]5+ (0.28, 2.38; 0.17, 2.67; 54% of the total
Fe), the ferrous site in the trinuclear subunit (0.72, 1.32; Fe2+;
29%), and the bridge position (1.18, 2.37; FeCl4(thf)2; 17%), in
1
complexes 4−6 feature paramagnetically shifted H NMR, the
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Figure 1. Zero-field 57Fe Mossbauer spectrum obtained at 90 K and spectral fits (δ, |ΔE | (mm/s)) for (a) (PhL) Fe (μ-Cl) (thf) (2; (blue, 28.7%)
̈
Q
2
7
4
2
0.28, 2.38; (green, 24.9%) 0.17, 2.67; (gold, 29%) 0.72, 1.32; (pink, 17.4%) 1.18, 2.37); (b) (PhL)2Fe6Co(μ-Cl)4(thf)2 (3; (blue, 38.9%) 0.21, 2.69;
(green, 38.9%) 0.73, 1.39; (gold, 22.3%) 1.16, 2.35). Solid-state structures for (PhL)2Fe7(μ-Cl)4(thf)2 (2) with the thermal ellipsoids set at the 50%
probability level (hydrogen atoms, and solvent molecules omitted for clarity; Fe, orange; C, gray; N, blue; O, red; Cl, green).
Figure 2. Solid-state structures for (a) (PhL)M1M2FeCl(NCCH3) (4), (b) (PhL)FeCo2Cl(NCCH3) (5), and (c) [(PhL)FeCo2(μ-Cl)]2 (6) with the
thermal ellipsoids set at the 50% probability level (hydrogen atoms and solvent molecules omitted for clarity; Fe, orange; Co, aquamarine; C, gray;
N, blue; O, red; Cl, green). Bond lengths (Å) for 4: Fe1−M2, 2.5391(7); Fe1−M3, 2.5493(8); M2−M3, 2.2934(8); Fe1−Cl, 2.3393(9); Fe1−NACN
,
2.134(3). For 5: Fe1−Co2, 2.5253(6); Fe1−Co3, 2.5348(6); Co2−Co3, 2.2971(5); Fe1−Cl, 2.2348(9); Fe1−NACN, 2.129(3). For 6: Fe1A−Co2A,
2.5120(14); Fe1A−Co3A, 2.5319(14); Co2A−Co3A, 2.2860(13); Fe1A−Cl1, 2.349(2); Fe1A−Cl2, 2.441(2); Fe1−Fe1, 3.4474(14); Fe1B−Co2B,
2.5009(14); Fe1B−Co3B, 2.5334(14); Co2B−Co3B, 2.2862(14); Fe1B−Cl1, 2.349(2); Fe1B−Cl2, 2.428(2); Fe1−Fe1, 3.4474(14).
spectra are diagnostic and distinguishable (see Figure S18). The
spectra for the acetonitrile adducts 4 and 5 are distinct from
each other, as well as (PhL)Fe3Cl(NCCH3) (7), prepared by
the addition of acetonitrile to 1, suggesting 4 and 5 are distinct
molecular species with integer Fe/Co ratios, rather than a
mixture of substitution products. The evolution of FeCl2 as an
acetonitrile solvate was observed by analysis of the reaction
in the M2 and M3 positions, whereas the data for 5 were
refined with Co exclusively in the dinuclear [M2]5+ unit. The
four-coordinate sites (M2, M3) are each bound to four ligand
anilide ligands and feature a close M−M contact (M2−M3,
2.2934(8) Å in 4, Co2−Co3, 2.2971(5) Å in 5) in the axial site.
The two remaining M−M contacts between the dinuclear
[M2]5+ unit and the trigonal pyramidal site are shorter
compared to M−M separation observed in (PhL)Fe3Cl(py)
(data compiled in Table 1), although the solvent ligands are
bound in different positions.14 In (PhL)Fe3Cl(py), the pyridine
ligand completes the trigonal plane of Fe1, whereas this
supernatant by 57Fe Mossbauer spectroscopy (δ, |ΔE | (mm/
̈
Q
s): 1.24, 2.16), which matches the metrical parameters for
FeCl2(NCCH3)2 prepared independently (Figure S12).17
The molecular structures of the reaction products 4−6 were
obtained by single crystal X-ray diffraction analysis (see Figure
2). For the acetonitrile-bound, trinuclear complexes 4 and 5,
the molecular structures are analogous to the previously
described oxidized trinuclear complex (PhL)Fe3Cl(py),14
maintaining the overall (PhL) coordination mode. The
molecular structures feature two metal sites in an intermediate
geometry between square planar and tetrahedral (M2 and M3
in Figure 2a) and one metal in a trigonal, monopyramidal
geometry (neglecting M−M interactions) bound by chloride,
acetonitrile, and two anilide ligands (Fe1 in Figure 2a). As Fe
and Co are indistinguishable by X-ray diffraction, the data for
complex 4 was refined with an equal population of Co and Fe
Table 1. Selected Bond Distances
M1
M1−M2
M2
M1−M3
M3
M2−M3
1
6
Fe
2.5889(5)
2.512(1)
2.501(1)
2.7303(8)
2.5391(7)
2.5253(6)
Fe
2.5801(5)
2.532(1)
2.533(1)
2.6534(8)
2.5493(8)
2.5348(6)
Fe
2.3410(5)
2.286(1)
2.286(1)
2.2955(8)
2.2934(8)
2.2971(5)
FeA
FeB
Fe
CoA
CoB
Fe
CoA
CoB
Fe
a
Fe3Cl
4
5
Fe
M1
Co
M2
Co
Fe
a
(PhL)Fe3Cl(py).14
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Figure 3. Zero-field 57Fe Mossbauer spectrum obtained at 90 K and spectral fits (δ, |ΔE | (mm/s)) for (a) (PhL)Fe CoCl(NCCH ) (4; component
̈
Q
2
3
1 (blue): 0.83, 1.41, 56.7%; component 2 (green): −0.01, 2.36, 22.1%; component 3 (magenta): 0.21, 2.92, 21.1%); (b) [(PhL)Co2Fe(μ-Cl)]2 (6;
0.69, 1.38). (c) X-ray fluorescence spectra (data black circles, fits represented as lines): of 1 (red), 4 (green), 6 (blue). (d) Variable-temperature
magnetic susceptibility data for 4 (circles) and 5 (triangles) collected in an applied dc field of 0.1 T. Solid lines represent fits to the data as described
in the text.
position is occupied by Cl in 4 and 5 which may contribute to
some of the trinuclear core distortion observed.
Scheme 3. Frontier Molecular Orbital Interactions of
[MAMB]5+ Unit in 1 ,4, and 5
The chloride-bridged complex 6 maintains the connectivity
present in its all-iron precursor 1, providing the best
comparison to illustrate the intracore distortions resulting
from substitution of iron by cobalt. All three M−M interactions
are contracted for the mixed-metal cluster 6 as compared to the
metrics found in the all-iron complex 1 (see Table 1). As with
the tri-iron congeners, the bond metrics within the (PhL) ligand
o-phenylenediamide units are consistent with the closed-shell
dianion state (see Tables S3−S7), indicating ligand-redox
participation is not involved.18
The zero-field, 57Fe Mossbauer spectra for complexes 4, 6,
̈
and 7 obtained at 90 K are shown in Figures 3a,b and S11,
respectively. Like 1 and the pyridine bound congener
(PhL)Fe3Cl(py), the spectrum of 7 features three distinct
quadrupole doublets (δ, |ΔEQ| (mm/s): 0.71, 1.27, 33.3%; 0.31,
2.48, 33.3%; 0.19, 2.68, 33.3%). The two low-velocity doublets
correspond to the iron sites within the oxidized dinuclear unit
[Fe2]5+, while the high-velocity doublet corresponds to the Fe2+
pyramidal site. The spectrum for 4 (Figure 3a) shows little
perturbation in the metrical parameters (δ, |ΔEQ| (mm/s): 0.83,
1.41, 56.7%; −0.01, 2.36, 22.1%; 0.21, 2.92, 21.1%), except the
ratio of high-velocity to low-velocity doublets has changed from
almost 1:2 in 7 to 1.3:1 in 4, signifying the Co substitution is
occurring within the oxidized dinuclear unit ([M2]5+) rather
than the M2+ site. The spectrum for 6 (Figure 3b) shows
complete substitution of both iron atoms by cobalt within the
oxidized dinuclear subunit [M2]5+, leaving only a single
quadrupole doublet observed with metrical parameters
consistent with the Fe2+ site (δ, |ΔEQ| (mm/s): 0.69, 1.38).
manifold in a quartet configuration prevented population of
2
(M−N)σ* and (M−M)σ *(z
interactions, providing a
based)
u
stabilizing interaction (via M−M bonding) within the oxidized
dinuclear unit. With complexes 3−6, substitution of Co for Fe
within the oxidized dinuclear unit does not structurally perturb
the trinuclear core (vide supra) as to suggest reconfiguration of
these orbital interactions. Thus, for complex 4 which features a
[FeCo]5+ dinuclear unit, we would anticipate a triplet
formulation for [MAMB]5+, as the number of valence electrons
in the orbital manifold has increased to 12. For complexes 5
and 6, we would anticipate a doublet configuration for the 13
electron [Co2]5+ subunit.
To probe the magnetic behavior of complexes 4 and 5
further, variable temperature dc susceptibility data were
collected in the temperature range of 5−300 K (Figure 3d).
In the case of 4, χMT decreases from a value of 5.78 cm3 K
mol−1 at 300 K to a minimum value of 1.55 cm3 K mol−1 at 5 K
(see Figure 3d). Below 50 K, the data undergo a downturn,
likely the result of Zeeman and zero-field splitting. In the case
of 5, χMT decreases from a value of 4.53 cm3 K mol−1 at 300 K
to a minimum value of 2.23 cm3 K mol−1 at 5 K (see Figure
3d).
The composition suggested by the Mossbauer analysis was
̈
corroborated by X-ray fluorescence analysis. The superposition
of Fe and Co Kα and Kβ emission lines of crystalline samples
for complexes 3, 4, and 6 suggest Fe/Co ratios of 6:1, 2:1, and
1:2 (see Figures 3c and S13−S17).
As previously reported, magnetic data acquired for the all-
iron oxidation product 1 revealed that the oxidized diiron unit
[Fe2]5+ is comprised of a strongly coupled S = 3/2 unit that is
weakly ferromagnetically coupled to the high spin (S = 2)
ferrous site, giving an overall S = 7/2 ground state for the
trinuclear unit. The close M−M separation within the oxidized
dimetal unit [M2]5+ suggests significant M−M orbital overlap,
dictated by the orbital overlap permitted for two edge-sharing
square planar ions (Scheme 3).14 For two Fe atoms comprising
the [M2]5+ unit, population of the 11 valence electrons into this
The foregoing Mossbauer analysis suggests iron uniquely
̈
resides in the M2+ site of the trinuclear core following Co
substitution; thus we modeled the magnetic data using the two-
spin Hamiltonian shown below, where S1 = 2 for the ferrous
unit and S2 represents the mixed-valent dinuclear unit spin.
̂
H = −2J(S S ) +
DS2 + gμ S·B
∑
1 2
i i
B
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dx.doi.org/10.1021/ic302694y | Inorg. Chem. 2013, 52, 5006−5012

Inorganic Chemistry
Article
Following the orbital analysis presented above, the spin state of
the oxidized dinuclear [M2]5+ unit S2 was modeled as a triplet
for the [FeCo]5+ unit in 4 and as a doublet for the [Co2]5+ unit
in 5. The corresponding simulation using the program
MAGPACK19 that best reproduces the susceptibility and
reduced magnetization data (see Figures S19−22) affords
parameters of J = −4.25 cm−1, D1 = 5.25 cm−1, D2 = −20 cm−1,
and g = 2.46 for complex 4 and J = −6.25 cm−1, D1 = 2.5 cm−1,
and g = 2.35 for complex 5. Unlike the all-iron oxidized
clusters,14 substitution of Co into the oxidized dinuclear unit is
best modeled as weak antiferromagnetic coupling between the
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data for 2−6; selected
crystallographic data and bond lengths for 2−6; CIF file for 2−
6. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: betley@chemistry.harvard.edu.
Notes
■
ferrous site and the oxidized dinuclear unit unit giving rise to S
The authors declare no competing financial interest.
1
= 1 and S =
/ ground states for complexes 4 and 5,
2
respectively.
ACKNOWLEDGMENTS
The authors thank Harvard University and NIH (GM 098395)
for financial support, Prof. R. H. Holm for the generous use of
■
With the compositions for the metal atom substitution
reactions confirmed by a variety of spectroscopic techniques,
we sought to probe how the metal substitution occurs
kinetically. While a myriad of routes may be possible for the
CoCl2 salt to engage the (PhL)Fe3Cl(thf) core formed in situ,
we propose two such possibilities for consideration: Formation
of an extended core (similar to heptanuclear products 2 and 3),
followed by metathesis of Fe2+ for cobalt. Subsequent
rearrangement of the trinuclear core with respect to the (PhL)
ligand would position the Co within the oxidized dinuclear site.
An alternative pathway could involve CoCl2 association to the
exposed axial face of either of the distorted square planar Fe
sites within the oxidized dinuclear core and direct metal atom
metathesis occurs into that position. To probe these
possibilities we investigated the reaction of complex 1 with
57Fe-labeled 57FeCl2(thf)2 to produce heptanuclear 2 in
tetrahydrofuran at room temperature. The reaction was
his Mossbauer spectrometer, the George W. Merck Fellowhsip
̈
́ ́
(T.A.B.), the CONACYT and Fundacion Mexico en Harvard
A. C. (R.H.S.), and the Harvard University Center for the
Environment for funding (E.V.E.).
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(16) Compound 2: C90H88Fe7N12Cl4O2 1.25(C6H6), Mr = 2171.95,
triclinic, P1, a = 12.761(3), b = 13.232(3), c = 15.826(4) Å, α =
̅
79.669(5)°, β = 74.646(5)°, γ = 79.387(5)°, V = 2508.9(10) Å3, Z =
1, ρcalcd = 1.438 Mg/m3, μ = 1.147 mm−1, R1 = 0.0698, wR2 = 0.1983.
Compound 3: C90H88Fe6CoN12Cl4O2 1.25(C6H6), Mr = 2175.03,
triclinic, P1, a = 12.761(3), b = 13.232(3), c = 15.826(4) Å, α =
̅
79.669(5)°, β = 74.646(5)°, γ = 79.387(5)°, V = 2508.9(10) Å3, Z = 1,
ρcalcd = 1.440 Mg/m3, μ = 1.168 mm−1, R1 = 0.0709, wR2 = 0.1588.
Compound 4: C43H39Fe2CoN7Cl 1.5(C2H3N), Mr = 1842.95, triclinic,
P1, a = 12.754(1), b = 18.156(2), c = 19.233(2) Å, α = 72.238(1)°, β =
̅
86.489(1)°, γ = 76.050(1)°, V = 4115.8(6) Å3, Z = 2, ρcalcd = 1.487
Mg/m3, μ = 1.204 mm−1, R1 = 0.0413, wR2 = 0.1007. Compound 5:
C43H39FeCo2N7Cl 1.5(C2H3N), Mr = 1849.11, triclinic, P1, a =
̅
11.183(1), b = 11.844(1), c = 17.588(2) Å, α = 73.418(1)°, β =
82.232(1)°, γ = 67.098(1)°, V = 2055.8(3) Å3, Z = 1, ρcalcd = 1.494
Mg/m3, μ = 1.256 mm−1, R1 = 0.0411, wR2 = 0.0948. Compound 6:
C82H72Fe2Co4N12Cl2 9.5(C6H6), Mr = 2385.86, triclinic, P1, a =
̅
18.084(1), b = 19.073(1), c = 19.329(1) Å, α = 74.288(1)°, β =
67.149(1)°, γ = 80.502(1)°, V = 5900.6(5) Å3, Z = 2, ρcalcd = 1.343
Mg/m3, μ = 0.891 mm−1, R1 = 0.1010, wR2 = 0.2914. CCDC-894769
(2), 894770 (3), 894771 (4), 894772 (5), 894773 (6) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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dx.doi.org/10.1021/ic302694y | Inorg. Chem. 2013, 52, 5006−5012