Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters

Article abstract of DOI:10.1021/acs.inorgchem.8b02684

Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm^R)₃ that can bind [Fe₄S₄]²⁺ or [Fe₆S₆]³⁺ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm^R)₃ ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm^Me)₃ (tris(1,3,5-(3-(N,N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] and 3 equiv of NaBPh₄ furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] provides (L(NIm^Me)₃)Fe₆S₆Cl₆ in high yield. Formation of an [Fe₆S₆]³⁺ cluster using L(NIm^Tol)₃ (tris(1,3,5-(3-(N,N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe₄S₄]²⁺ cluster [(L(NIm^Tol)₃)(Fe₄S₄Cl)][BPh₄] is cleanly generated when 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe₆S₆]³⁺ clusters but not [Fe₄S₄]²⁺ clusters. Thus, the structure and nuclearity of L(NIm^R)₃-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.

Full text of DOI:10.1021/acs.inorgchem.8b02684

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters
Alex McSkimming and Daniel L. M. Suess*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: Obtaining rational control over the structure
and nuclearity of metalloclusters is an ongoing challenge in
synthetic Fe−S cluster chemistry. We report a new family of
tridentate imidazolin-2-imine ligands L(NImR)3 that can bind
[Fe4S4]2+ or [Fe6S6]3+ clusters, depending on the steric profile
of the ligand and the reaction stoichiometry. A high-yielding
synthetic route to L(NImR)3 ligands (where R is the
imidazolyl N substituents) from trianiline and 2-chloroimida-
zolium precursors is described. For L(NImMe)3 (tris(1,3,5-(3-
(N,N-dimethyl-4,5-diphenylimidazolin-2-imino)-
phenylmethyl))benzene), metalation with 1 equiv of [Ph4P]2[Fe4S4Cl4] and 3 equiv of NaBPh4 furnishes a mixture of products,
but adjusting the stoichiometry to 1.5 equiv of [Ph4P]2[Fe4S4Cl4] provides (L(NImMe)3)Fe6S6Cl6 in high yield. Formation of an
[Fe6S6]3+ cluster using L(NImTol)3 (tris(1,3,5-(3-(N,N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))-
benzene) is not observed; instead, the [Fe4S4]2+ cluster [(L(NImTol)3)(Fe4S4Cl)][BPh4] is cleanly generated when 1 equiv of
[Ph4P]2[Fe4S4Cl4] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings
whereby long N-imidazolyl substituents preclude formation of [Fe6S6]3+ clusters but not [Fe4S4]2+ clusters. Thus, the structure
and nuclearity of L(NImR)3-bound Fe−S clusters may be selectively controlled through rational modification the ligand’s
substituents.
form.30 Examples of rationally controlling the structure and
INTRODUCTION
■
nuclearity of Fe−S clusters are limited. Removal of the unique
Synthetic metal−chalcogenide clusters have been actively
studied for decades owing to their applications in a number of
Fe from a site-differentiated Fe4S4 cluster bound to a chelating
trithiolate ligand31,32 allowed for the preparation of the first
diverse settings. For example, they serve as models for the active
synthetic open-cuboidal Fe3S4 cluster;33 much like a protein, this
sites of Fe−S proteins,1−3 as building blocks in nanomateri-
rigid scaffold prevents conversion to linear Fe3S4 clusters, which
als,4−7 and as catalysts in their own right.8−17 Their broad utility
are favored in the absence of such a scaffold.34,35
in both biological and abiological contexts derives in part from
the diversity of their geometric and electronic structures.
Whereas nature has evolved complex biosynthetic pathways
Examples of rational synthetic cluster design are more
abundant outside of Fe−S cluster chemistry. Betley et al.
showed that a hexaamido ligand platform could support 3- or 6-
for controlling the structure and nuclearity of biological Fe−S
clusters,18−20 synthetic Fe−S clusters (as well as other metal−
Fe clusters depending on the metalation conditions (i.e., the
identity of the Fe precursor and the presence or absence of
chalcogenide clusters) are often prepared through multi-
additional phosphine ligands)36,37 and that 8-Fe clusters could
component self-assembly reactions that can be difficult to
rationally control.1−3,21,22 For example, the cuboidal cluster
be accommodated by expanding the size of the ligand cavity and
introducing an amine donor into the ligand framework.38 In
[R4E]2[Fe4S4Cl4] ([R4E]+ = [Bu4N]+ or [Ph4P]+) may be
prepared from simple precursors (Scheme 1a).23−25 Even a
addition, reactive trimetallic clusters have been prepared using a
rigid tris(diketiminate) scaffold developed by Murray et al.39−42
seemingly minor perturbation such as substitution of [Bu4N]+ or
[Ph4P]+ by [Et4N]+ results in the generation of a cluster of
and Agapie et al. have employed a ligand framework with three
different nuclearity, [Et4N]3[Fe6S6Cl6].26,27 Moreover, the
products of such self-assembly reactions can be subject to facile
dipyridyl alkoxide groups anchored to a central phenyl group to
selectively construct, for example, Mn3O4M (M = Ca2+, Ln3+,
etc.) cubanes as models for the oxygen-evolving complex in
redistribution reactions. For example, [Et4N]3[Fe6S6Cl6] is itself
photosystem II.43−45
thermally unstable and converts to [Et4N]2[Fe4S4Cl4] upon
mild heating,28 which, upon oxidation, undergoes a redistrib-
ution reaction to afford [Et4N]2[Fe6S6Cl6].29 In addition, all-
ferrous, phosphine-ligated Fe−S clusters readily rearrange to
higher-nuclearity clusters: Fe4S4(PR3)4 clusters convert to
Fe8S8(PR3)6 or Fe16S16(PR3)8 clusters depending on the
phosphine substituents (Scheme 1, b), and as such the parent
Fe4S4(PR3)4 clusters have not been isolated in analytically pure
We encountered challenges associated with controlling Fe−S
cluster nuclearity in our efforts to generate site-differentiated
Fe−S clusters using new donor sets. In particular, we targeted
Fe−S clusters bound to tridentate, guanidine-like imidazolin-2-
Received: September 20, 2018
© XXXX American Chemical Society
A
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Article
were refined anisotropically and hydrogen atoms were refined using a
riding model.
Scheme 1. Preparation of Fe−S Clusters of Variable
a
Nuclearity
Materials. THF was distilled from purple Na/benzophenone and
distilled prior to use. Other solvents were sparged with argon and dried
by passage through 2× columns packed with activated alumina. All
solvents were stored over activated 3 Å molecular sieves for at least 24 h
prior to use. All reagents were purchased from commercial suppliers
and used without further purification unless otherwise noted.
[Ph4P]2[Fe4S4Cl4],25 N,N-dimethyl-4,5-diphenylimidazol-2-one,59
and N,N-di(p-tolyl)-4,5-diphenylimidazol-2-one60 were prepared
according to literature procedures.
Synthesis of 1. 3-Nitroacetophenone (20.3 g, 0.123 mol) and
dimethylformamide dimethyl acetal (29.3 g, 0.246 mol) were heated
with stirring at 120 °C for 1 h. Excess solvent was removed under a
stream of N2 to afford a dark crystalline residue. Recrystallization from
boiling methanol gave 1 as dark-orange crystals. Yield: 19.1 g (71%).
Spectral data were consistent with that reported.61
Synthesis of 2. 1 (17.0 g, 0.0772 mol) was heated with stirring at
vigorous reflux in acetic acid (80 mL) for 16 h. The reaction mixture
was cooled to room temperature and ethanol (100 mL) was slowly
added with rapid stirring to precipitate 2 as a yellow-brown solid. The
product was collected on a frit, washed with ethanol until the washings
were colorless, and dried under reduced pressure. Yield: 8.80 g, 65%.
Spectral data were consistent with that reported.62
Synthesis of 3. 2 (5.00 g, 9.52 mmol) was suspended in methanol
(50 mL) and the solution heated to boiling. NaBH4 was added in ∼200
mg portions until the solution became homogeneous (∼1 g (26 mmol)
required for complete conversion). Caution: This reaction is exothermic
with vigorous evolution of H2. The reaction mixture was cooled to room
temperature, and water (500 mL) was added causing a brown oil to
separate. After standing for 1 h the solvent was decanted, and the
residue was taken up in diethyl ether (100 mL). The diethyl ether
solution was washed with HCl (aqueous, 10%), washed with water,
dried over sodium sulfate, and filtered through cotton wool. Removal of
the solvent under reduced pressure gave a brown oil, which was
extracted with several ∼20 mL portions of diethyl ether. These were
quickly filtered through a plug of cotton wool leaving a small amount of
tarry brown residue. The solvent was removed thoroughly under
reduced pressure to give an off white powder. Trituration with diethyl
ether afforded 3 as a cream solid. Yield: 3.55 g, 71%. 1H NMR
(CD3CN): δ 8.14 (t, J = 2.0 Hz, 3H, 3 × ArH), 8.05 (dd, J = 8.0, 2.0 Hz,
3H, 3 × ArH), 7.70 (d, J = 8.0 Hz, 3H, 3 × ArH), 7.51 (t, J = 8.0 Hz, 3H,
3 × ArH), 7.35 (s, 3H, 3 × ArH), 5.86 (d, J = 3.4 Hz, 3H, CHOH), 4.17
(d, J = 3.4 Hz, 3H, OH). 13C{1H} NMR (CD3CN): 149.23, 147.94,
145.71, 133.55, 130.51, 124.68, 122.97, 121.71 (8 × ArC), 74.75
(COH). DART-MS (+): m/z 514.1235; calcd for [M − OH]+: m/z
514.1256.
Synthesis of 4. 3 (3.00 g, 5.69 mmol) was suspended in
dichloromethane (50 mL) and triethylsilane (3.6 mL, 22.5 mmol),
then BF3·Et2O (2.9 mL, 23.1 mmol) added with stirring. Stirring was
continued at room temperature for 24 h at which point the reaction
mixture was nearly homogeneous. The reaction mixture was filtered
through a pad of Celite, transferred to a separatory funnel and washed
with water (50 mL). The organic layer was dried over sodium sulfate
and passed through a short (∼5 cm) pad of silica. The silica was washed
with dichloromethane until the product had fully eluted (as determined
by TLC; CH2Cl2, Rf ∼0.6). Solvent was removed under reduced
pressure and the residue recrystallized from dichloromethane-ethanol
to afford 4 as pale yellow crystals. Although minor impurities were
visible by NMR spectroscopy, this material was sufficiently pure for the
next step. Yield: 2.45 g (89%). 1H NMR (CDCl3): δ 8.05 (d, J = 7.6 Hz,
3H, 3 × ArH), 7.99 (s, 3H, 3 × ArH), 7.48 (d, J = 7.6 Hz, 3H, 3 × ArH),
7.45 (t, J = 7.6 Hz, 3H, 3 × ArH), 6.89 (s, 3H, 3 × ArH), 4.03 (s, 6H,
CH2). 13C{1H} NMR (CDCl3): δ 148.53, 142.84, 140.74, 135.14,
129.60, 128.16, 123.72, 121.60 (8 × ArC), 41.33 (CH2). DART-MS
(+): m/z 484.1502; calcd for [M + H]+: m/z 484.1509.
a
Examples of (a and b) Fe−S core rearrangement reactions of
synthetic clusters28,30 and (c) rational control over cluster nuclearity
presented in this work.
imines ligands L(NImR)3 (Scheme 1c),46−52 which we expected
to be strongly binding, weak-field ligands that would mimic the
donor properties of cysteine thiolates in proteins. We herein
describe the synthesis of site-differentiated Fe−S clusters ligated
by imidazolin-2-imines, demonstrate that the selectivity for
cluster size may be rationally controlled by modifying the steric
properties of the L(NImR)3 ligands, and show that these ligands
impart substantial steric protection to site-differentiated Fe4S4
clusters.
EXPERIMENTAL SECTION
■
General Methods. All manipulations involving metal complexes
were carried out in an N2 atmosphere glovebox. Glassware was oven-
dried for at least several hours at 160 °C prior to use.
Spectroscopy and Spectrometry. NMR spectra were recorded
on a Bruker 400 MHz spectrometer. H and 13C chemical shifts are
1
reported in ppm relative to tetramethylsilane using residual solvent as
an internal standard. Solution-phase effective magnetic moments were
determined by the method described by Evans53 and are corrected for
diamagnetic contributions.54 Zero-field Fe Mossbauer spectra were
57
̈
measured with a constant acceleration spectrometer at 90 K. Isomer
shifts are quoted relative to Fe foil at room temperature (RT). Data was
analyzed and simulated with WMOSS v. 4.55 Mass spectrometry data
were collected on a JEOL AccuTOF spectrometer with a DART source
or a Waters Q-TOF spectrometer with an ESI source. FTIR spectra
were recorded on solid samples using a Bruker Alpha II FTIR
Spectrometer operating at 2 cm−1 resolution. Elemental analysis were
performed by Midwest Microlab.
X-ray Crystallography. X-ray intensity data were collected on a
Bruker APEX CCD detector employing graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å) at 100(1) K. Absorption and other
corrections were applied using SADABS.56 The structures were solved
by direct methods using SHELXT57 and refined against F2 on all data by
full-matrix least-squares with SHELXL-2015.58 Non-hydrogen atoms
Synthesis of 5. 4 (2.00 g, 4.17 mmol) was suspended in THF (30
mL) and heated to 70 °C with stirring until the mixture became
homogeneous. Solid ammonium formate (∼6 g, ∼100 mmol) was
added followed by absolute ethanol (30 mL). The mixture was heated
B
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to 80 °C and Pd/C (5%, ∼100 mg) added cautiously (vigorous gas
evolution). The reaction mixture was maintained at 80 °C until the
reaction was complete (as determined by TLC; EtOAc-Et2O (1:3), Rf
∼0.4), cooled to room temperature, and filtered through a pad of Celite.
The solution was diluted with water (200 mL) and diethyl ether (50
mL). The organic layer was separated and washed with 3 × 50 mL
portions of water. The organic layer was dried over sodium sulfate and
passed through a ∼5 cm pad of silica, which was washed with diethyl
ether until all of the product had eluted (as determined by TLC). The
solvent was removed under reduced pressure and the residue
recrystallized from dichloromethane-hexanes to afford 5 as a white
crystalline solid. Yield: 1.41 g (86%). 1H NMR (CD3CN): δ 6.97 (t, J =
7.6 Hz, 3H, 3 × ArH), 6.90 (s, 3H, 3 × ArH), 6.45 (m, 9H, 9 × ArH),
4.04 (s, br, 6H, NH2), 3.73 (s, 6H, CH2). 13C{1H} NMR (CD3CN): δ
148.95, 143.41, 142.80, 130.03, 127.85, 118.60, 115.65, 113.10 (8 ×
ArC), 42.34 (CH2). DART-MS (+): m/z 394.2288; calcd for [M + H]+:
m/z 394.2283.
Synthesis of 6. 1,2-Dimethyl-4,5-diphenylimidazol-2-one (15.1 g,
0.0571 mol) was heated in POCl3 (30 mL) at 110 °C for 16 h. Excess
POCl3 was distilled from the reaction mixture to yield a pale blue oil
which was dissolved in methanol (∼50 mL). To this solution was slowly
added a saturated aqueous solution of KPF6 (∼100 mL), which caused
a white solid to precipitate. Additional water (∼300 mL) was added to
the mixture, and the solid was collected by filtration, washed with
copious water, and dried under reduced pressure. Recrystallization from
acetone−isopropanol afforded 6 as colorless crystals. Yield: 14.8 g, 61%.
1H NMR (CD3CN): δ 7.44−7.53 (m, 6H, 6 × ArH), 7.35−7.37 (m,
4H, 4 × ArH), 3.64 (s, 6H, 2 × N−CH3). 13C{1H} NMR (CD3CN): δ
133.42 (C−Cl), 132.97, 131.72, 131.52, 130.01, 125.90 (5 × ArC),
34.68 (N−CH3). 31P NMR (CD3CN): δ −144.6 (sept, JPF = 712 Hz,
[PF6]−). DART-MS (+): m/z 283.0997; calcd for [M]+: m/z 283.1002.
Synthesis of 7. 1,2-Di(p-tolyl)-4,5-diphenylimidazol-2-one (4.00 g,
9.60 mmol) was heated in POCl3 (8 mL) at 110 °C for 16 h. Excess
POCl3 was distilled from the reaction mixture to yield a pale yellow oil,
which was dissolved in methanol (∼20 mL). To this solution was slowly
added a saturated aqueous solution of KPF6 (∼20 mL), which caused a
white solid to precipitate. Additional water (∼200 mL) was added to
the mixture and the solid collected by filtration, washed with copious
water, and dried under reduced pressure. Recrystallization from
dichloromethane−isopropanol afforded 7 as colorless crystals. Yield:
4.95 g, 89%. 1H NMR (CD3CN): δ 7.45 (d, J = 8.2 Hz, 4H, 4 × ArH),
7.36 (d, J = 8.2 Hz, 4H, 4 × ArH), 7.24−6.34 (m, 10H, 10 × ArH), 2.38
(s, 6H, 2 × Ar−CH3). 13C{1H} NMR (CD3CN): δ 143.07 (C−Cl),
134.42, 134.31, 132.00, 131.48, 131.22, 130.97, 129.57, 128.31, 125.98
(9 × ArC), 21.24 (Ar−CH3). 31P NMR (CD3CN): δ −144.6 (sept, JPF
= 712 Hz, [PF6]−). DART-MS (+): m/z 435.1632; calcd for [M]+: m/z
435.1625.
864w, 841w, 790w, 763w, 743w, 696s, 651w, 599w, 505w, 449w. ESI-
MS (+): m/z 1132.5736; calcd for [M + H]+: m/z 1132.5754.
Synthesis of L(NImTol)3 (9). In a glovebox, a solution of LiHMDS
(584 mg, 3.49 mmol) in THF was cooled to −35 °C and added
dropwise with stirring to a cold (−35 °C) suspension of 5 (208 mg,
0.529 mmol) and 7 (968 mg, 1.67 mmol) in THF (10 mL). The
mixture was allowed to come to RT and stirred for a further 1 h to give a
yellow solution. The following operations were performed in air. The
solvent was removed under reduced pressure to give a yellow solid,
which was partitioned between water (∼2 mL) and benzene (5 mL).
The organic layer was separated, dried over sodium sulfate, and filtered,
and the solvent removed under reduced pressure. The oily residue was
dissolved in dichloromethane (∼1 mL) and diluted with acetonitrile
(20 mL), and the solution was concentrated to ∼10 mL, which caused
the product to separate as a flocculent, pale-yellow solid. This solid was
collected by filtration, washed with acetonitrile (3 × 5 mL), and dried
under reduced pressure. Yield: 756 mg, 90%. 1H NMR (C6D6): δ 7.85
(d, J = 8.2 Hz, 12H, 12 × ArH), 7.07 (d, J = 8.2 Hz, 12H, 12 × ArH),
6.98 (s, 3H, 3 × ArH), 6.78−6.87 (m, 21H, 21 × ArH), 6.64−6.66 (m,
18H, 18 × ArH), 6.49 (d, J = 7.4 Hz, 3H, 3 × ArH), 3.61 (s, 6H, 3 ×
CH2), 1.91 (s, 18H, 6 × Ar−CH3). 13C{1H} NMR (C6D6): δ 150.17
(CN), 144.85, 141.85, 140.88, 136.27, 135.17, 130.89, 130.17,
129.08, 128.97, 127.54, 124.38, 123.54, 120.61, 119.83 (15 × ArC; 2 ×
concealed by C6D6 resonances), 42.40 (CH2), 20.98 (Ar−CH3). FTIR:
cm−1 3032w, 2920w, 1651s, 1586s, 1575s, 1513s, 1502m, 1482w,
1445m, 1375s, 1313w, 1279w, 1239m, 1211w, 1177w, 1160w, 1106m,
1073w, 1033w, 1021w, 982w, 960w, 937w, 916w, 889w, 809m, 794m,
760m, 735w, 722w, 695s, 638w, 615w, 587w, 536m, 523m, 506w,
491w, 476w, 456w, 444w. ESI-MS (+): m/z 1588.7614; calcd for [M +
H]+: m/z 1588.7632.
Synthesis of (L(NImMe)3)Fe6S6Cl3 (11). To a stirred suspension of
NaBPh4 (385 mg, 1.13 mmol) in dichloromethane (3 mL) was rapidly
added a solution of L(NImMe
) (411 mg, 0.363 mmol) and
3
[Ph4P]2[Fe4S4Cl4] (639 mg, 0.545 mmol) in dichloromethane (6
mL). Stirring was continued for 2 h. The dark-crimson reaction mixture
was then filtered through a pad of Celite. Solvent was removed under
reduced pressure and the crude solid was washed with acetonitrile (5 ×
2 mL) to afford the complex as black-red microcrystals (545 mg, 85%).
Crystals suitable for XRD studies were grown by layering a THF
solution with hexanes. Elemental analysis was conducted on a sample
recrystallized from toluene−acetonitrile. Evans method (CDCl3): 3.6
μB. UV−vis (THF): λmax (nm) εmax (M−1 cm−1) 348 (sh, 2.3 × 104),
496 (1.7 × 104). 1H NMR (CDCl3): δ 10.32 (t (partially resolved), J = 7
Hz, 3H, 3 × 5-bridgePhH), 7.46 (12H, 12 × ArH), 7.28 (6H, 6 × ArH),
7.07 (12H, 12 × ArH), 7.02 (s, 3H, 3 × basalPhH), 5.46 (s, 18H, 3 × N−
CH3), 4.55 (3H, 3 × 2-bridgePhH), 2.38 (d (partially resolved), J = 7 Hz,
3H, 3 × 4/6-bridgePhH), 2.12 (s, 6H, 3 × CH2), 2.00 (d, J = 7.3 Hz, 3H, 3
× 4/6-bridgePhH). 13C{1H} NMR (CDCl3) δ 213.93, 208.69, 191.65,
162.28, 154.10, 138.47, 136.45, 131.64, 130.55, 130.44, 128.62, 125.00,
121.93, 118.87 (14 × ArC), 46.42 (CH2), 29.43 (N−CH3). FTIR: cm−1
3052w, 2948w, 2899w, 1601m, 1573m, 1517s, 1482s, 1441s, 1418m,
1359w, 1290m, 1228m, 1156m, 1090w, 1076w, 1053w, 1043w, 1023w,
997w, 971w, 920w, 874m, 854m, 809w, 764s, 733w, 721w, 699s, 616w,
597w, 549w, 509w. Elemental analysis for C78H69Cl3Fe6N9S6·1.5C7H8·
CH3CN: calcd C, 55.87; H, 4.35; N, 7.30. Found C, 56.24; H, 4.32; N,
7.23.
Synthesis of L(NImMe)3 (8). In a glovebox, a solution of LiHMDS
(603 mg, 3.60 mmol) in THF (2 mL) was cooled to −35 °C and added
dropwise with stirring to a cold (−35 °C) suspension of 5 (215 mg,
0.546 mmol) and 6 (738 mg, 1.72 mmol) in THF (6 mL). The mixture
was allowed to come to RT and stirred for a further 1 h to give an orange
solution. The following operations were performed in air. The solvent
was removed under reduced pressure to give an orange oil, which was
partitioned between water (∼2 mL) and benzene (5 mL). The organic
layer was separated, dried over sodium sulfate, and filtered, and the
solvent was removed under reduced pressure. The oily residue was
dissolved in boiling acetonitrile (5 mL), then cooled to −10 °C to
induce separation of a yellow oil. The orange supernatant was decanted
and the residue was dried thoroughly under reduced pressure.
Trituration with hexanes gave the product as a pale yellow powder.
Yield: 571 mg, 92%. 1H NMR (C6D6): δ 7.08−7.15 (m, 9H, 9 × ArH),
6.75−7.02 (m, 33H, 33 × ArH), 6.75 (d, J = 7.2 Hz, 3H, 3 × ArH), 3.89
(s, 6H, 3 × CH2), 2.85 (s, 18H, 3 × N−CH3). 13C{1H} NMR (C6D6) δ
151.93 (CN), 149.25, 142.59, 142.01, 130.05, 129.84, 129.14,
128.80, 124.32, 121.97, 120.15, 119.59 (11 × ArC; 2 × concealed by
C6D6 resonances), 42.78 (CH2), 32.53 (N−CH3). FTIR: cm−1 3051w,
2919w, 1615m, 1564s, 1503w, 1481w, 1452m, 1420m, 1377m, 1318w,
1230w, 1178w, 1155w, 1072w, 1040w, 1020m, 994w, 917w, 885w,
Synthesis of [(L(NImTol)3)(Fe4S4Cl)][BPh4] (12). To a stirred
suspension of L(NImTol
)
(414 mg, 0.261 mmol) and
3
[Ph4P]2[Fe4S4Cl4] (306 mg, 0.261 mmol) in THF (5 mL) was added
a solution of NaBPh4 (277 mg, 0.809 mmol) in THF (3 mL) dropwise.
Stirring was continued for 2 h. The orange-brown reaction mixture was
filtered through a pad of Celite. Solvent was removed under reduced
pressure and the crude solid washed with MeCN (3 × 2 mL) then
diethyl ether (3 × 5 mL) to afford the complex as dark-brown
microcrystals (478 mg, 80%). Crystals suitable for XRD studies were
grown by layering a chloroform solution with hexanes. Elemental
analysis was conducted on a sample recrystallized from from CH2Cl2−
hexanes. Evans method (CDCl3): 2.7 μB. UV−vis (THF): λmax (nm)
1
εmax (M−1 cm−1) 468 (1.1 × 104), 736 (sh, 2.0 × 103). H NMR
(CDCl3): δ 8.15 (3H, 3 × 5-bridgePhH), 7.48 (m, 8H, 8 × o-PhH4B),
C
DOI: 10.1021/acs.inorgchem.8b02684
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Article
Scheme 2. Synthesis of Ligands L(NImMe)3 (8) and L(NImTol)3 (9)
Scheme 3. Metalation Reactions of L(NImR)3 Presented in
This Work
7.16 (20H, 20 × ArH), 7.08 (t, J = 7.1 Hz, 8H, 8 × m-PhH4B), 6.96
(17H, 17 × ArH), 6.90 (t, J = 7.1 Hz, 4H, 4 × p-PhH4B), 6.75 (12H, 12
× ArH), 5.98 (3H, 3 × basalPhH), 5.60 (3H, 3 × 4/6-bridgePhH), 5.38
(3H, 3 × 4/6-bridgePhH), 3.55 (3H, 3 × 2-bridgePhH), 3.01 (s, 6H, 3 ×
CH2), 2.28 (s, 18H, 3 × Ar−CH3). 13C NMR (126 MHz, CDCl3) δ
169.86 (br, ArC), 166.85 (br, ArC), 164.98 (br, ArC), 164.53 (q, JBC
40 Hz, ipsoC−BPh4), 150.59 (ArC), 140.67 (ArC), 139.58 (ArC), 136.51
metaC−BPh4), 135.19 (ArC), 134.67 (ArC), 132.48 (br, ArC), 132.21
=
(
(ArC), 130.29 (ArC), 129.52 (ArC), 128.61 (ArC), 128.56 (ArC),
128.45 (br, ArC), 125.65 (ArC), 125.57 (q, JBC = 21 Hz, orthoC−BPh4),
125.52 (ArC), 121.63 (paraC−BPh4), 43.41 (CH2), 21.69 (Ar−CH3); 1
× ArC not observed. FTIR: cm−1 3053w, 3031w, 2981w, 2917w,
1600w, 1575w, 1515m, 1479s, 1447m, 1422s, 1377w, 1309w, 1264w,
1227w, 1179w, 1158w, 1114w, 1088w, 1072w, 1031w, 1019w, 999w,
958w, 916w, 868w, 817m, 791w, 782w, 731m, 696s, 678s, 639w, 611m,
574w, 534m, 523m, 494w, 475w, 475w. Elemental analysis for
C138H113BClFe4N9S4·2CH2Cl2: calcd C, 68.21; H, 4.78; N, 5.11.
Found C, 68.24; H, 4.99; N, 4.78.
the reaction stoichiometry (i.e., using 1.5 equiv of
[Ph4P]2[Fe4S4Cl4]). Unlike [Et4N]3[Fe6S6Cl6], complex 11 is
thermally stable (as a CDCl3 solution at 60 °C over 24 h.).
The 1H NMR spectrum of 11(Figure 1) shows paramagneti-
cally shifted resonances, the most dramatically affected being
those for the 1H nuclei on the phenylene group directly bound to
the N donor (δ 10.32, 4.55, 2.38, and 2.00), which were
identified by a 1H−1H COSY experiment (Figure S17). At RT,
11 displays C3v symmetry on the NMR time scale (400 MHz).
The ground spin state is S = 1/2 as shown by its EPR spectrum
(Figure S24), which resembles that of other known [Fe6S6]3+
RESULTS
■
The tridentate L(NImR)3 ligands employed in this work adopt
the general geometry imposed by Holm’s seminal, site-
differentiating trithiolate ligand,31,32 analogs of which have
been fruitfully employed by other research groups.63−65 The
new ligands presented in this work were prepared as shown in
Scheme 2. First, condensation of 3-nitroacetophenone with
N,N-dimethylformamide dimethyl acetal followed by thermal
cyclization of resulting enone 1 provided trinitrotrione 2.
Stepwise reduction of the nitro and ketone groups furnishes
trianiline 5,which can be readily prepared on a multigram scale.
Trianiline 5 may be reacted with a variety of 2-chloroimidazo-
liums in the presence of strong base to furnish ligands including
L(NImMe)3 (8) and L(NImTol)3 (9) in >90% yield; 5 is therefore
a useful precursor for introducing different imidazolyl groups in
the final synthetic stages.
clusters.28 The zero-field 57Fe Mossbauer spectrum of solid 11
̈
(Figure 3a) shows two quadrupole doublets in a 1:1 ratio with
isomer shifts of 0.46 and 0.52 mm s−1 (ΔEQ = 0.89 and 1.25 mm
s−1, respectively), consistent with two chemically distinct sets of
three Fe centers (one set Cl ligated, the other NIm ligated). The
̈
Mossbauer parameters for 11 are similar to those reported for
[Fe6S6X6]3− clusters (X = halide, RS−, RO−; e.g., X = Cl, δ = 0.50
mm s−1, ΔEQ = 1.08 mm s−1), suggesting a similar electronic
structure in which charge is delocalized over the six Fe sites to
give an average Fe oxidation state of +2.5.28
Attempts to prepare an L(NImMe)3-ligated [Fe4S4]2+ cluster
by metalation of L(NImMe)3 with 1 equiv of [Ph4P]2[Fe4S4Cl4]
and 3 equiv of NaBPh4 resulted in a mixture of species including
[(L(NImMe)3)Fe4S4Cl][BPh4] (10) and the higher-nuclearity
cluster (L(NImMe)3)Fe6S6Cl3 (11) in a roughly 4:1 ratio
(Scheme 3 and Figure S26). Cationic cluster 10 could not be
The molecular structure of 11 determined by X-ray
crystallography (Figure 3, a) shows a distorted hexagonal
prismane geometry for the Fe−S cluster akin to that of other
reported [Fe6S6]3+ complexes.26−28,66 The L(NImMe)3 ligand
binds three Fe centers on one face of the prismane with an
average Fe−N distance of 1.98 Å. This is somewhat shorter than
those reported for NIm-ligated, tetrahedral high-spin Fe2+
complexes (2.05 Å avg),67 likely a result of the effective Fe
oxidation state of +2.5 (vide supra), which results in a slight
1
isolated but was identified by comparison with the H NMR
spectrum of [(L(NImTol)3)Fe4S4Cl][BPh4] (vide infra). Com-
plex 11 could be cleanly generated by appropriately adjusting
D
DOI: 10.1021/acs.inorgchem.8b02684
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Figure 1. 1H NMR spectra of (a) (L(NImMe)3)Fe6S6Cl3 and (b) [(L(NImTol)3)Fe4S4Cl][BPh4] recorded in CDCl3 at 400 MHz. Peaks marked with
circles were assigned to the bridge-phenylene protons. Asterisks mark the peak due to CDCl3.
contraction of the Fe−N bonds. The average interdonor N−N
distance is 6.86 Å, indicative of splaying of the ligand arms away
from the basal phenyl ring compared with [(L(NImTol)3)-
Fe4S4Cl][BPh4] (vide infra). The imidazolyl rings are positioned
nearly upright with respect to the molecular 3-fold rotational axis
(∠(Saxial−Fe−N−Cimid) ≈ 9° (avg) where the Saxial−Fe bond is
parallel to the molecular pseudo-C3 axis; see Figures 3a and 4a).
On the basis of the structure of 11 and some computational
modeling, we made several predictions relevant to achieving
clean synthetic access to Fe4S4 clusters bound to L(NImR)3
ligands: First, that L(NImR)3 ligands based on this tri-
(arylmethyl)aryl framework should also accommodate Fe4S4
clusters; second, and most importantly, that unlike for the Fe6S6
cluster 11, the imidazolyl rings in L(NImR)3-bound Fe4S4
clusters would be substantially tilted with respect to the
molecule’s pseudo-3-fold rotational axis. Thus, we predicted
that the substituents on the imidazolyl N atoms would
determine the ability of L(NImR)3 ligands to accommodate an
Fe6S6 cluster: Although shorter substituents (e.g., methyl in
L(NImMe)3) clearly allow for Fe6S6 cluster binding, longer
substituents (e.g., p-tolyl in L(NImTol)3) would clash with
neighboring substituents and disfavor Fe6S6 cluster formation
(Figure 4a). However, longer substituents should be tolerated in
L(NImTol)3-bound Fe4S4 clusters owing to the tilt of the
imidazolyl rings, which allows for the donor arms to pack more
tightly around the Fe4S4 cluster (Figure 4b).
resonances attributed to the donor-N aryl rings (δ 8.15, 6.60,
5.38 and 3.55) are less dramatically shifted compared with those
of 11 owing to the diamagnetic ground state of 12.1,68,68
However, they are somewhat shifted owing to the population of
paramagnetic excited states; the RT solution magnetic moment
determined by the Evans method (2.7 μB) is consistent with this
proposal and with those of other [Fe4S4]2+ clusters.1,68,69 The
57
̈
zero-field Fe Mossbauer spectrum of solid 12 was best
simulated as two quadrupole doublets in a 3:1 ratio of roughly
the same isomer shift (0.48 mm s−1), reflecting both (i) the
distinct coordination environment of the apical Fe center70,71
and (ii) charge delocalization over the four Fe sites resulting in
effective Fe oxidations state of +2.5.1,72−75
The structure of 12 shows the anticipated ligand binding
mode with three Fe atoms bound by L(NImTol)3 and the unique
Fe atom bound by Cl (Figure 3b). As anticipated, the imidazolyl
rings are significantly tilted (∠(Sbasal−Fe−NCimid) ≈ 50°
(avg) where Sbasal is located on the pseudo-C3 axis; see Figures
3b and 4b), thereby allowing the sterically demanding p-tolyl
groups to fit around the cluster core. The dramatic descent in
symmetry from solution (C3v) to the solid state (C3) indicates
that in solution the imidazolyl rings are rapidly flipping about the
pseudo σv planes. The Fe−N distances in 12 are similar to those
for 11 (1.98 Å, avg). The average interdonor N−N distance in
12 (5.93 Å) is ∼15% shorter than that in 11 as accommodation
of the smaller cluster core requires less splaying of the ligand
arms.
To test these hypotheses, we metalated L(NImTol)3 with 1
equiv of [Ph4P]2[Fe4S4Cl4] and 3 equiv of NaBPh4, which
resulted in clean formation of the desired complex [(L-
(NImTol)3)Fe4S4Cl][BPh4] (12)(Scheme 3). Under no con-
ditions have we been able to generate an Fe6S6 cluster of
L(NImTol)3 (for example, by adjusting the stoichiometry to use
1.5 equiv of [Ph4P]2[Fe4S4Cl4] (Figure S26) or heating 12 in the
presence of 0.5 equiv of [Ph4P]2[Fe4S4Cl4]). Thus, through
simple and rational modification of the steric profile of the
L(NImR)3 ligand, site-differentiated [Fe4S4]2+ or [Fe6S6Cl3]3+
clusters may be generated with high selectivity.
DISCUSSION
■
The encapsulation of clusters of specific nuclearity in chelating
ligands presents additional challenges compared with binding
chelating ligands to mononuclear metal sites. Clusters require
larger ligands that typically have more degrees of freedom. As
such, multiple small twists in the dihedral angles of the ligand
backbone can cumulatively lead to a broad distribution of
possible interdonor distances and orientations, which can lead to
a binding pocket that accommodates a number of cluster
structures. This challenge is illustrated by the ability of the
L(NImMe)3 ligand to bind both [Fe4S4]2+ and [Fe6S6]3+ clusters
Like complex 11, complex 12 displays C3v symmetry on the
NMR time scale (400 MHz) at RT (Figure 1). The 1H
E
DOI: 10.1021/acs.inorgchem.8b02684
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Figure 3. Thermal-ellipsoid (50%) plots of the solid-state structures of
(a) (L(NImMe)3)Fe6S6Cl3 and (b) [(L(NImTol)3)Fe4S4Cl][BPh4].
Orange, yellow, green, blue, and gray ellipsoids represent Fe, S, Cl,
N, and C, respectively. H atoms, counterions, 4,5-phenyl substituents,
and cocrystallized solvent molecules are omitted for clarity.
Figure 2. Zero-field 57Fe Mossbauer spectra of solid (a) (L(NImMe)3)-
Fe6S6Cl3 and (b) [(L(NImTol)3)Fe4S4Cl][BPh4] at 90 K. Black circles
represent experimental data, solid lines are simulations. Simulation
parameters: (a) Doublet 1: δ = 0.52 mm s−1, ΔEQ = 1.25 mm s−1, Γ =
0.29 mm s−1, rel. area = 0.5; Doublet 2: δ = 0.46 mm s−1, ΔEQ = 0.89
mm s−1, Γ = 0.34 mm s−1, relative area = 0.5. (b) Doublet 1: δ = 0.48
mm s−1, ΔEQ = 1.12 mm s−1, Γ = 0.35 mm s−1, rel. area = 0.75; Doublet
2: δ = 0.48 mm s−1, ΔEQ = 0.70 mm s−1, Γ = 0.25 mm s−1, relative area =
0.25. An alternate simulation for panel a is shown in Figure S25.
̈
by modestly expanding or contracting its binding pocket. Our
work demonstrates that this inherent challenge can be overcome
by rationally modulating the sterics of the ligand, in this case
using the steric bulk of the L(NImTol)3 ligand to preclude
formation of an [Fe6S6]3+ cluster.
Another challenge in site-differentiated Fe−S cluster
chemistry is modulating the steric environment of the unique
Fe center(s). In 3:1 site-differentiated Fe4S4 clusters, the
environment of the unique Fe center can only be controlled
remotely by tuning the ligands to the other three Fe sites. In 12,
the orientation of the imidazolyl groups positions one of the N-
tolyl groups on each donor arm in a manner so as to confer
substantial steric protection to the unique Fe site (Figure 5).
Moreover, the structure of 12 suggests further ways to tune the
steric environment of the unique Fe site through simple
modification of the imidazolyl ring substituents.
Figure 4. Simplified diagrams demonstrating (a) the upright positions
of the imidazolyl substutents for (L(NImMe)3)Fe6S6Cl3 and the
resulting steric clash for the hypothetical molecule (L(NImTol)3)-
Fe6S6Cl3 and (b) tilt of the imidazolyl substituents for [(L(NImTol)3)-
Fe4S4Cl]+. Ph groups are omitted for clarity.
for σ bonding with Fe. The imidazolyl rings are oriented nearly
perpendicularly to the remaining N lone pair, thereby
minimizing delocalization into the imidazolyl π system and
making the lone pair available for Fe−N π bonding.47−49 As
such, the NIm donors are best represented by their zwitterionic
forms (Figure 6b).79 We anticipate that the strong σ and π donor
properties of NIm ligands will find further utility in Fe−S cluster
chemistry.
NIm donors have not been previously utilized in Fe−S cluster
chemistry and are emerging as a useful ligand class in transition
metal50−52,76,77 and main group78 chemistry. In complexes 11
and 12, one lone pair of electrons on the donor N atom is used
F
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AUTHOR INFORMATION
Corresponding Author
*E-mail: suess@mit.edu.
■
ORCID
Daniel L. M. Suess: 0000-0002-0916-1973
Funding
This work was supported in part by the MIT Skoltech Program.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Prof. Theodore Betley (Harvard University) for use of
■
̈
his Mossbauer spectrometer, Kevin Anderton (Harvard
̈
University) for assistance in acquiring the Mossbauer spectro-
scopic data, and Drs. Peter Muller and Charlene Tsay for
̈
assistance with X-ray crystallographic experiments.
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■
Figure 5. Space-filling model of the cationic portion of [(L(NImTol)3)-
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CONCLUSIONS
■
In conclusion, tridentate ligands featuring imidazolin-2-imine
donors with different steric properties may be prepared in
synthetically useful quantities from a trianiline precursor.
Metalation with the Fe−S cluster precursor [Ph4P]2[Fe4S4Cl4]
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02684.
■
S
NMR spectra of new compounds, 1H−1H COSY spectra,
UV−vis and EPR data for metal clusters, additional
̈
Mossbauer data, NMR spectra of reaction mixtures
(PDF)
Accession Codes
CCDC 1868895−1868896 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
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