Iron(II) Complexes of a Hemilabile SNS Amido Ligand: Synthesis, Characterization, and Reactivity

Article abstract of DOI:10.1021/acs.inorgchem.7b01802

We report an easily prepared bis(thioether) amine ligand, S^MeN^HS^Me, along with the synthesis, characterization, and reactivity of the paramagnetic iron(II) bis(amido) complex, [Fe(κ³-S^MeNS^Me)₂] (1). Binding of the two different thioethers to Fe generates both five- and six-membered rings with Fe-S bonds in the five-membered rings (av 2.54 ?) being significantly shorter than those in the six-membered rings (av 2.71 ?), suggesting hemilability of the latter thioethers. Consistent with this hypothesis, magnetic circular dichroism (MCD) and computational (TD-DFT) studies indicate that 1 in solution contains a five-coordinate component [Fe(κ³-S^MeNS^Me)(κ²-S^MeNS^Me)] (2). This ligand hemilability was demonstrated further by reactivity studies of 1 with 2,2′-bipyridine, 1,2-bis(dimethylphosphino)ethane, and 2,6-dimethylphenyl isonitrile to afford iron(II) complexes [L₂Fe(κ²-S^MeNS^Me)₂] (3-5). Addition of a Br?nsted acid, HNTf₂, to 1 produces the paramagnetic, iron(II) amine-amido cation, [Fe(κ³-S^MeNS^Me)(κ³-S^MeN^HS^Me)](NTf₂) (6; Tf = SO₂CF₃). Cation 6 readily undergoes amine ligand substitution by triphos, affording the 16e^- complex [Fe(κ²-S^MeNS^Me)(κ³-triphos)](NTf₂) (7; triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; ¹H NMR, M?ssbauer, IR, and UV-vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine-borane dehydrogenation catalysis show complex 7 to be a selective and particularly robust precatalyst.

Full text of DOI:10.1021/acs.inorgchem.7b01802

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Iron(II) Complexes of a Hemilabile SNS Amido Ligand: Synthesis,
Characterization, and Reactivity
Uttam K. Das,† Stephanie L. Daifuku,‡ Theresa E. Iannuzzi,‡ Serge I. Gorelsky,† Ilia Korobkov,†
Bulat Gabidullin,† Michael L. Neidig,*,‡ and R. Tom Baker*,†
†Department of Chemistry and Biomolecular Sciences and Centre for Catalysis Research and Innovation, University of Ottawa,
Ottawa, Ontario K1N 6N5, Canada
‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: We report an easily prepared bis(thioether) amine ligand, SMeNHSMe,
along with the synthesis, characterization, and reactivity of the paramagnetic iron(II)
bis(amido) complex, [Fe(κ3-SMeNSMe)2] (1). Binding of the two different thioethers to
Fe generates both five- and six-membered rings with Fe−S bonds in the five-
membered rings (av 2.54 Å) being significantly shorter than those in the six-
membered rings (av 2.71 Å), suggesting hemilability of the latter thioethers.
Consistent with this hypothesis, magnetic circular dichroism (MCD) and computa-
tional (TD-DFT) studies indicate that 1 in solution contains a five-coordinate
component [Fe(κ3-SMeNSMe)(κ2-SMeNSMe)] (2). This ligand hemilability was
demonstrated further by reactivity studies of 1 with 2,2′-bipyridine, 1,2-bis(dimethylphosphino)ethane, and 2,6-dimethylphenyl
isonitrile to afford iron(II) complexes [L2Fe(κ2-SMeNSMe)2] (3−5). Addition of a Brønsted acid, HNTf2, to 1 produces the
paramagnetic, iron(II) amine−amido cation, [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)](NTf2) (6; Tf = SO2CF3). Cation 6 readily
undergoes amine ligand substitution by triphos, affording the 16e− complex [Fe(κ2-SMeNSMe)(κ3-triphos)](NTf2) (7; triphos =
bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; 1H NMR,
Mossbauer, IR, and UV−vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine−borane
̈
dehydrogenation catalysis show complex 7 to be a selective and particularly robust precatalyst.
In our efforts to develop new bifunctional iron catalysts, we are
investigating sterically svelte tridentate ligands with a mixture of
hard nitrogen and soft sulfur donors capable of stabilizing a
range of metal oxidation states,34−48 and hemilabile arms that
allow for substrate coordination.49 Recently, we reported a
series of mono-, di- and trinuclear iron(II) complexes
containing an easily prepared tridentate thiolate ligand with
thioether and imine donors, [SMeNS−].50 In this work,
reduction of a similar [SMeNSMe] ligand affords an amine
derivative that is used to prepare new iron amido complexes
containing hemilabile thioether donors. Thioether ligands
usually bind weakly to first row transition metals,38,42,48,51−54
and their hemilability has been demonstrated previously.38,54−56
Amido groups typically form strong bonds to metals, may serve
as terminal or bridging ligands, and can thus form mononuclear
or multimetallic compounds. Additionally, late-metal-bound
amido groups have reactive lone-pair electrons available for
bifunctional substrate activation.
INTRODUCTION
■
Iron amido complexes have attracted much interest in recent
years as a result of their utility in many catalytic processes
including asymmetric hydrogenation,1,2 dehydrogenation,3−5
hydroamination,6,7 and cross-coupling8 reactions. For example,
in asymmetric hydrogenation and transfer hydrogenation
catalysis, an iron amido complex is the key intermediate in
some catalytic cycles, reacting with isopropanol (in asymmetric
transfer hydrogenation) or dihydrogen (in asymmetric hydro-
genation) to generate an iron hydride and a secondary amine in
a bifunctional mechanism.2 Iron-mediated chemical trans-
formations such as dinitrogen reduction,9−12 C−H bond
amination,13,14 and olefin hydroamination15,16 also involve
iron amido species. Moreover, iron amido complexes are
fundamentally important due to their diverse structural
features,17−20 unusual reactivity,21,22 and interesting magnetic
properties.23−26 To date, numerous iron amido and imido
complexes have been isolated and structurally characterized
with a wide range of iron oxidation states from + I to
+V.5,17,19,20,24,27−30 In general, sterically bulky amido ligands are
used to stabilize low-coordinate iron amido complexes whereas
multidentate chelating amido ligands are able to stabilize
coordinatively saturated compounds.
A variety of tridentate sulfur-containing amido ligands are
known, including [SRN−SR],40,41,47 [S−N−S−],44 [O−N−SR],48,56
[NRN−S−],57 and [N−SRN−] examples42,58 (see corresponding
secondary amines in Figure 1). It is surprising that nearly all of
these sulfur-based amido ligands have been studied with
The proliferation of bifunctional catalysts has been largely
due to the popularity of pincer ligands that allow for tight
binding and a variety of donors including P, S, N, and C.31−33
Received: July 14, 2017
© XXXX American Chemical Society
A
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Figure 1. Sulfur-containing amines.
7.16; CDCl3, δ 7.26; CD2Cl2, δ 5.32; and for 13C{1H} NMR, C6D6, δ
128.06 ppm). 19F NMR spectra were referenced to internal 1,3-
bis(trifluoromethyl)benzene (BTB) (Aldrich, 99%, deoxygenated by
purging with nitrogen, stored over activated 4 Å molecular sieves), set
to δ −63.5 ppm. 31P NMR data were referenced to external H3PO4
(85% aqueous solution), set to δ 0.0 ppm. UV−vis spectra were
recorded on an Agilent Cary 7000 universal measurement
spectrophotometer, using sealable quartz cuvettes (1.0 cm path
length) and dry CH2Cl2 or THF. IR data were collected on a Thermo
Scientific Nicolet 6700 FT-IR spectrometer. Elemental analyses were
transition metals other than iron: Deng’s group reported a few
high- and low-spin iron(II) complexes employing the bulky
N,N′-dimesityl-2,2′-diamidophenyl sulfide ligand, [N−SRN−],42
and Mascharak and co-workers prepared and characterized an
iron(III) complex bearing the N-2-mercaptophenyl-2′-pyridi-
necarboxamide ligand, [NRN−S−], which served as a structural
model for nitrile hydratases.57 Gusev et al. reported highly
efficient ruthenium catalysts using a pincer-type SNS ligand,
HN(C2H4SEt2), for bifunctional ester hydrogenation.59 Given
the importance of sulfur-based amido ligands and the well-
known hemilability of thioether groups in synthetic and
biological coordination chemistry as well as in bifunctional
catalysis, we describe herein the synthesis and characterization
of a series of iron(II) amido complexes derived from an easily
prepared, unsymmetrical bis(thioether) amine ligand.
performed by Elemental Analysis Service, Universite
́ ́
de Montreal,
Montreal, Quebec, and by CENTC Elemental Analysis Facility,
́
́
University of Rochester, Rochester, NY 14627. For electron impact
(EI), solid samples were prepared by drying products under vacuum,
and a Kratos Concept S1 (Hres 7000−10000) mass spectrometer was
used. [Fe{N(SiMe3)2}2] was prepared by following a previously
reported literature procedure.60 The spin-only magnetic moment in
solution at room temperature was obtained by the Evans method.61
Synthesis of the [SMeNHSMe] Ligand. First Step: Synthesis of 2-
(2-Methylthiobenzylidene)methylthioaniline. 2-(Methylthio)-
benzaldehyde (1.7 mL, 13.14 mmol) and 2-(methylthio)-aniline (1.8
mL, 14.45 mmol, 1.1 equiv) were added to a 100 mL round-bottom
Schlenk flask, followed by addition of 20 mL of dry EtOH. The
resulting brown solution was refluxed for 18 h under dynamic nitrogen
(vented to an oil bubbler) after which no further color change was
observed. The reaction mixture was cooled at −35 °C overnight after
which the product precipitated as a yellow solid. Finally, the yellow
product was filtered, washed with cold EtOH, and dried in vacuo.
Yield: 3.30 g, 92% based on 2-(methylthio)-benzaldehyde. The
product was used directly in the second step without further
purification.
EXPERIMENTAL SECTION
■
General Considerations. Experiments were conducted under
nitrogen, using Schlenk techniques or an MBraun glovebox unless
otherwise stated. All solvents were deoxygenated by purging with
nitrogen. Toluene, hexanes, diethyl ether, and THF were dried on
columns of activated alumina using a J. C. Meyer (formerly Glass
Contour) solvent purification system. [d6]-Benzene (C6D6) was dried
by standing over activated alumina (ca. 10 wt %) overnight, followed
by filtration. Dichloromethane, [d2]-dichloromethane (CD2Cl2),
chloroform, and d-chloroform (CDCl3) were dried by refluxing over
calcium hydride under nitrogen. After distillation, CDCl3 and
dichloromethane were further dried by filtration through activated
alumina (ca. 5−10 wt %). CD2Cl2 was vacuum-transferred before use.
Ethanol (EtOH) was dried by refluxing over Mg/I2 under nitrogen,
followed by distillation. All solvents were stored over activated (heated
at ca. 250 °C for >10 h under vacuum) 4 Å molecular sieves except
ethanol which was stored over activated 3 Å molecular sieves.
Glassware was oven-dried at 150 °C for >2 h. The following chemicals
were obtained commercially, as indicated: 2-(methylthio)-
benzaldehyde (Aldrich, 90%), 2-methylthioaniline (Alfa Aesar, 98%),
ammonia−borane (NH3−BH3, Scitix, 91%), dimethylamine−borane
((CH3)2NH−BH3, Aldrich, 97%), bipyridine (bpy, Aldrich, 98%), 2,6-
dimethylphenyl isonitrile (CNxylyl, Aldrich, 96%), 1,2-bis-
(dimethylphosphino)ethane (dmpe, Strem, 98%), and trimethylphos-
1H NMR (300 MHz, C6D6 at 25 °C) δ 1.88 (s, 3H, SMe), 2.02
(s, 3H, SMe), 6.84−7.01 (m, 7H, ArH), 8.41 (d, 1H, ArH),
9.02 (s, 1H, NCH). 13C NMR (101 MHz, C6D6) δ 14.54 (CH3),
16.71 (CH3), 117.90 (ArC), 125.03 (ArC), 125.38 (ArC),
125.84 (ArC), 126.64 (ArC), 127.97 (ArC), 129.61 (ArC),
131.49 (ArC), 135.05 (ArC), 135.29 (ArC), 141.12 (ArC),
1
150.14 (ArC), 157.62 (NC). Figures S1 and S2 contain the H
and 13C NMR spectra.
Second Step: Synthesis of 2-(2-Methylthiobenzyl)-
methylthioaniline. 2-(2-Methylthiobenzylidene)-methylthioaniline
(1.00 g, 3.66 mmol) was added to a 100 mL ampule charged with a
stir bar. A 20 mL portion of THF was added to form a yellow solution,
followed by 0.45 g (14.64 mmol, 4 equiv) of NH3−BH3. The ampule
was sealed, and the resulting yellow solution was heated to 65 °C for
24 h over which time the color of the reaction mixture turned from
yellow to colorless. THF was removed using vacuum, and the residue
was purified using column chromatography (hexane/ethyl acetate, 4:1)
1
phite [P(OMe)3, Strem, 97%]. H, 19F, and 31P NMR spectra were
recorded on either a 300 MHz Bruker Avance or a 300 MHz Bruker
Avance II instrument at room temperature (21−25 °C). 13C{1H}
NMR spectra were recorded on a 400 MHz Bruker Avance
instrument. NMR spectra were referenced to the residual proton
1
peaks associated with the deuterated solvents (for H NMR, C6D6, δ
B
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to afford a white solid. Yield: 0.950 g, 94% based on 2-(2-
methylthiobenzylidene)-methylthioaniline.
mL) and dried in vacuo to give 4. Yield: 0.229 g, 80% based on [Fe(κ3-
SMeNSMe)2] (1).
1H NMR (300 MHz, C6D6 at 25 °C) δ 1.72−2.43 (m, 24H, SMe
and MeCNxylyl), 4.29 (d, 1H, CH2), 4.55−4.75 (m, 2H, 
CH2), 5.31−5.45 (m, 1H, CH2), 6.23−7.98 (m, 22H, ArH). 13C
NMR (75 MHz, C6D6) δ 14.38 (CH3), 15.34 (CH3), 15.61 (CH3),
18.13 (CH3), 18.60 (CH3), 18.89 (CH3), 46.11 (NC), 65.92 (N
C), 109.82 (ArC), 110.13 (ArC), 110.93 (ArC), 115.33 (Ar
C), 117.66 (ArC), 120.32 (ArC), 123.96 (ArC), 124.13 (Ar
C), 125.29 (ArC), 124.28 (ArC), 125.27 (ArC), 125.94 (Ar
C), 126.04 (ArC), 126.17 (ArC), 127.12 (ArC), 127.17 (Ar
C), 127.35 (ArC), 127.55 (ArC), 127.87 (ArC), 127.92 (Ar
C), 127.94 (ArC), 128.23 (ArC), 130.05 (ArC), 132.41 (Ar
C), 134.89 (ArC), 135.04 (ArC), 135.39 (ArC), 135.49 (Ar
C), 136.42 (ArC), 136.73 (ArC), 137.19 (ArC), 137.49 (Ar
C), 139.55 (ArC), 140.39 (ArC), 148.69 (ArCN), 164.33
(ArCN). UV−vis (THF) λmax/nm (ε/M−1 cm−1): 230 (56 300),
239 (53 600), 249 (49 200), 285 (15 700), 310 (1700), 396 (3400). IR
(ATR, cm−1): 2054 (CN), 2107 (CN). Anal. Calcd for
C48H50FeN4S4: C 66.49, H 5.81, N 6.46. Found: C 65.78, H 5.98,
N 6.50. It is noted that due to high air sensitivity, the elemental
analysis data of 4 are <2% low. Figures S12−S14 contain the 1H NMR,
13C NMR, and IR spectra.
1H NMR (300 MHz, C6D6 at 25 °C) δ 1.96 (s, 3H, S−Me), 2.00 (s,
3H, S−Me), 4.29 (d, 2H, 3J = 5.9 Hz, −CH2), 5.46 (t, 1H, 3J = 5.9 Hz,
N−H), 6.53 (dd, 1H, Ar−H), 6.59 (td, 1H, Ar−H), 6.89 (m, 1H, Ar−
H), 6.99 (m, 3H, Ar−H), 7.23 (dq, 1H, Ar−H), 7.48 (dd, 1H, Ar−H).
13C NMR (101 MHz, C6D6) δ 15.36 (CH3), 18.13 (CH3), 46.12 (N−
C), 110.94 (Ar−C), 117.67 (Ar−C), 120.32 (Ar−C), 125.29 (Ar−C),
125.99 (Ar−C), 127.88 (Ar−C), 127.94 (Ar−C), 130.04 (Ar−C),
134.89 (Ar−C), 137.24 (Ar−C), 137.51 (Ar−C), 148.71 (Ar−C). IR
(ATR, cm−1): 3390 (N−H), 752 (S−Me). UV−vis (THF) λmax/nm
(ε/M−1 cm−1): 291 (4300), 313 (6900). HRMS (EI, 70 eV): calcd for
C15H17NS2 m/z 275.0802 [M+], found 275.0815 [M+]. Anal. Calcd for
C15H17NS2: C 65.41, H 6.22, N 5.09, S 23.28. Found: C 64.89, H 6.46,
1
N 5.04, S 23.13. Figures S3−S7 contain the H, 13C NMR, UV−vis,
EI-MS, and IR spectra.
Synthesis of [Fe(κ3-SMeNSMe)2] (1). A 100 mL round-bottom
Schlenk flask was charged with 2-(2-methylthiobenzyl)-methylthioani-
line, [SMeNHSMe] (1.00 g, 3.63 mmol), and 15 mL of hexane, yielding a
white suspension. A graduated dropping funnel was charged with
[Fe{N(SiMe3)2}2] (0.684 g, 1.815 mmol, 0.5 equiv) and 10 mL of
hexane giving a clear green solution. The dropping funnel was then
connected to the Schlenk flask, and the green solution was added
dropwise to the white suspension. Upon addition of [Fe{N-
(SiMe3)2}2], the color of the reaction mixture instantly turned to
yellow-green. The resulting solution was stirred for 6 h at room
temperature over which period a yellow solid deposited in the bottom
of the flask. After decanting the supernatant yellow-green solution, the
yellow precipitate of the title complex 1 was collected by filtration
using a frit followed by washing with hexane (ca. 3 × 10 mL) and
diethyl ether (3 × 5 mL), and drying in vacuo. Yield: 1.09 g, 92% based
on [Fe{N(SiMe3)2}2]. Crystals of 1 suitable for X-ray crystallography
were obtained from a concentrated toluene solution at −35 °C.
1H NMR (300 MHz, C6D6 at 25 °C) δ −43.45 (br s, Δν1/2 = 117
Hz), −6.35 (br s, Δν1/2 = 251 Hz), −0.80 (br s, Δν1/2 = 89 Hz), 6.53−
6.69 (br m), 17.50 (br s, Δν1/2 = 451 Hz), 33.35 (br s, Δν1/2 = 249
Hz), 43.50 (br s, Δν1/2 = 145 Hz), 66.42 (br s, Δν1/2 = 622 Hz),
118.30 (br s, Δν1/2 = 790 Hz). UV−vis (THF) λmax/nm (ε/M−1
cm−1): 314 (14 100), 365 (2200), 444 (1000). μeff (C6D6) = 4.80 μB.
Anal. Calcd for C30H32FeN2S4: C 59.59, H 5.33, N 4.63, S 21.21.
Found: C 58.48, H 5.38, N 4.53, S 20.43. It should be noted that the
elemental analysis data for 1 are <2% low due to its high air sensitivity.
Synthesis of [Fe(κ3-SMeNSMe)2(dmpe)] (5). A J-young NMR tube
was charged with [Fe(κ3-SMeNSMe)2] (1) (0.020 g, 0.033 mmol) and
0.5 mL of CD2Cl2 affording a yellow solution. A 5.5 μL portion of 1,2-
bis(dimethylphosphino)ethane, dmpe (0.005 g, 0.033 mmol, 1 equiv),
was added to the yellow solution using a glass microsyringe. Upon
addition of dmpe, no color change was observed which was confirmed
1
by both H and 31P{1H} NMR experiments at room temperature.
Upon cooling to −40 °C, the color of the reaction mixture turned to
brown. However, attempts to isolate and characterize the expected
brown product by recrystallization at lower temperatures were
unsuccessful. As a result, we could not obtain its elemental analysis.
However, both 1H and 31P{1H} NMR spectra clearly demonstrate
formation of 5 at low temperature in solution.
1H NMR (300 MHz, CD2Cl2 at −40 °C) δ 1.11 (br t, 12H, −CH3
dmpe), 1.97 (ov s, 3H, S−Me), 1.99 (ov s, 3H, S−Me), 2.10 (s, 3H,
2
S−Me), 2.47 (s, 3H, S−Me), 2.99 (d, 1H, J = 18 Hz, −CH2 dmpe),
3.58 (d, 1H, 2J = 18 Hz, −CH2 dmpe), 3.85 (d, 1H, 2J = 18 Hz, −CH2
dmpe), 4.21 (d, 1H, 2J = 18 Hz, −CH2 dmpe), 5.71 (br s, 1H, −CH2),
6.01 (br m, 3H, −CH2), 6.52 (br s, 1H, Ar−H), 6.69 (br s, 2H, Ar−
H), 6.88 (br d, 4H, Ar−H), 7.02 (br d, 4H, Ar−H), 7.16 (br s, 2H,
Ar−H), 7.34 (br d, 2H, Ar−H). 31P{1H} NMR (121 MHz, CD2Cl2 at
1
Figures S8−S9 contain the H NMR and UV−vis spectra.
Synthesis of [Fe(κ3-SMeNSMe)2(bpy)] (3). A 20 mL scintillation vial
was charged with [Fe(κ3-SMeNSMe)2] (1) (0.200 g, 0.33 mmol), 2,2′-
bipyridine (0.051 g, 0.33 mmol, 1 equiv), and THF (10 mL) affording
instantly a red-brown solution. The solution was then stirred for 6 h at
room temperature over which period no further color change was
observed. The solution was concentrated using vacuum and left to
stand at room temperature overnight to yield dark red-brown crystals
(1st crop) of 3. The red-brown filtrate was further concentrated which
gave a second crop of the crystals. Finally, the dark red crystals were
collected by filtration, washed with cold THF (ca. 3 × 5 mL) and
diethyl ether (ca. 3 × 5 mL), and dried in vacuo. Combined yield was
0.201 g, 80% based on [Fe(κ3-SMeNSMe)2] (1).
2
2
−40 °C) δ 44.36 (d, JPP = 30 Hz, dmpe), 56.07 (d, JPP = 30 Hz,
dmpe). Figures S15−S16 contain the H and 31P{1H} NMR spectra.
1
Synthesis of [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)](NTf2) (6). A 50 mL
round-bottom Schlenk flask was charged with [Fe(κ3-SMeNSMe)2] (1)
(0.300 g, 0.496 mmol) and 10 mL of dichloromethane, yielding a
yellow solution. A graduated dropping funnel (10 mL) was charged
with bis(trifluoromethane)sulfonimide, HNTf2 (0.139 g, 0.496 mmol,
1 equiv), and 5 mL of dichloromethane giving a clear colorless
solution. The dropping funnel was then connected to the Schlenk
flask, and the HNTf2 solution was added dropwise to 1 affording a
brick red solution. The resulting solution was stirred for 2 h at room
temperature, and the solvent was removed under vacuum. The
remaining red solid was then washed with diethyl ether (ca. 5 × 5 mL)
and dried in vacuo. Yield: 0.360 g, 82% based on [Fe{N(SiMe3)2}2].
Crystals of 7 suitable for X-ray crystallography were obtained from a
concentrated dichloromethane solution at −35 °C.
1H NMR (300 MHz, C6D6 at 25 °C) δ −43.20 (br s), −37.20 (br
s), −13.70 (br s), −6.32 (br s), 0.25 (br s), 8.70 (br s), 17.10 (br s),
32.10 (br s), 43.73 (br s), 66.72 (br s), 118.70 (br s). UV−vis (THF)
λmax/nm (ε/M−1 cm−1): 218 (45 200), 247 (39 900), 252 (37 800),
283 (15 900), 313 (8300). μeff (C6D6) = 4.33 μB. Anal. Calcd for
C40H40FeN4S4: C 63.14, H 5.30, N 7.36. Found: C 63.31, H 5.98, N
1H NMR (300 MHz, CD2Cl2 at 25 °C) δ −69.22 (br s), −42.56 (br
s), −12.02 (br s), −11.05 (br s), −7.31 (br s), 0.45 (br s), 2.32 (br s),
4.30 (br s), 7.07 (br s), 8.63 (br s), 10.54 (br s), 13.68−14.58 (br m),
22.08 (br s), 25.55 (br s), 36.58 (br s), 40.96 (br s), 54.19 (br s), 81.70
(br s), 86.40 (br s), 125.70 (br s), 132.10 (br s), 152.70 (br s). 19F
NMR (282 MHz, CDCl3) δ −66.80 (br s, Δν1/2 = 215 Hz, Tf). UV−
vis (CH2Cl2) λmax/nm (ε/M−1 cm−1): 312 (6800), 351 (2000). IR
(ATR, cm−1): 3210 (N−H). μeff (CDCl3) = 5.37 μB Anal. Calcd for
C32H33F6FeN3O4S6: C 43.39, H 3.75, N 4.74. Found: C 42.92, H 3.54,
1
6.60. Figure S11 shows the H NMR spectrum.
Synthesis of [Fe(κ2-SMeNSMe)2(CNxylyl)2] (4). A 20 mL scintillation
vial was charged with [Fe(κ3-SMeNSMe)2] (1) (0.200 g, 0.33 mmol),
2,6-dimethylphenyl isonitrile, CNxylyl (0.087 g, 0.66 mmol, 2 equiv),
and THF (10 mL) yielding instantly a brown solution. The solution
was stirred for 6 h at room temperature over which period no further
color change was observed. THF was removed under vacuum, and the
resulting brown residue was washed with cold diethyl ether (ca. 3 × 5
1
N 4.32. Figures S17−S18 contain the H and 19F NMR spectra.
C
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Synthesis of [Fe(κ3-SMeNSMe)(κ3-triphos)] (7). A 20 mL scintillation
vial was charged with [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)](NTf2) (6)
(0.060 g, 0.068 mmol) and triphos (0.036 g, 0.068 mmol, 1 equiv).
Upon addition of 5 mL of THF, the color of the reaction mixture
instantly turned to magenta. The resulting solution was stirred for 6 h
at room temperature over which period no further color change was
observed. THF was removed under vacuum, and the remaining dark
solid was washed with benzene (ca. 4 × 5 mL) and diethyl ether (3 ×
5 mL) and dried in vacuo. Yield: 0.051 g, 65% based on [Fe(κ3-
SMeNSMe)(κ3-SMeNHSMe)](NTf2). Crystals of 7 suitable for X-ray
crystallography were obtained from a concentrated benzene solution
layered with hexane at room temperature.
Scheme 1. Preparation of [SMeNHSMe] Ligand
1H NMR (300 MHz, THF-d8 at 25 °C) δ 2.20 (s, 3H, S−Me), 2.39
(s, 3H, S−Me), 3.11 (br m, 6H, −CH2 triphos), 4.10 (br s, 2H, −CH2
triphos), 5.89 (br s, 1H, −CH2), 6.15 (d, 1H, −CH2), 6.48−7.25 (m,
26H, Ar−H), 7.38−7.74 (m, 7H, Ar−H). 31P{1H} NMR (121 MHz,
Scheme 2. Synthesis of Bis(amido) Iron(II) Complex
2
THF-d8 at 25 °C) 75.60 (br s, triphos), 83.15 ppm (t, JPP ≈ 22 Hz,
2
triphos); at −40 °C, 81.54 (dd, Jpp = 21 Hz, 28 Hz, triphos), 74.74
(dd, 2Jpp = 21 Hz, 28 Hz, triphos), 74.12 (t, 2Jpp = 21 Hz, triphos). 19
F
NMR (282 MHz, THF-d8) δ −78.50 (br s, Δν1/2 = 140 Hz, Tf). UV−
vis (THF) λmax/nm (ε/M−1 cm−1): 251 (22 000), 311 (6600), 519
(2400). Anal. Calcd for C54H52F6FeN2O4P3S4: C, 54.78; H, 4.43; N,
2.37. Found: C, 54.63; H, 4.42; N, 2.29. Figures S19−S21 contain the
1H, 31P{1H}, and 19F NMR spectra.
Procedure for Amine−Borane Dehydrogenation Catalysis.
In a J. Young NMR tube, 3 mg of ammonia−borane (AB, 32 equiv)
was mixed with 3.5 mg of 7 in THF (0.3 mL). The NMR solution was
then heated at 60 °C at which time the color of the solution changed
from magenta to orange. The progress of the reaction was monitored
by 11B, 11B{1H}, and 31P{1H} NMR spectroscopy. After 23 h, the
resulting solution was filtered, and THF was removed using vacuum.
The resulting solid was further characterized by 1H and 31P{1H} NMR
spectroscopy. For dehydrogenation catalysis using dimethylamine−
borane (DMAB), in a J. Young NMR tube, 7.7 mg of DMAB (20
equiv) was mixed with 7.5 mg of 7 in THF (0.3 mL). The NMR
solution was then heated at 60 °C in which time the color of the NMR
solution changed from magenta to orange. The progress of the
reaction was monitored by 11B, 11B{1H}, and 31P{1H} NMR
spectroscopy. After 23 h, the resulting solution was filtered, and
THF was removed using vacuum. The resulting solid was then
amido ligand resulting from a dynamic exchange process in
solution (vide infra). Room temperature magnetic measure-
ment in solution (Evans’ method)61 gave an effective magnetic
moment, μeff, of 4.80 μB consistent with an S = 2 ground state.
The molecular structure of 1 was determined by single-
crystal X-ray diffraction (Figure 2). Crystals of 1 were grown
from a saturated toluene solution at −35 °C. The structure
contains two [κ3-SMeNSMe] amido ligands meridionally bound
to the iron center. The iron center adopts a distorted
octahedral geometry with trans-N donors. The sum of angles
about N(1) and N(2) are 356.38° and 356.45°, respectively,
consistent with the near planarity of the amido ligands. The
1
characterized by H and 31P{1H} NMR spectroscopy.
RESULTS AND DISCUSSION
■
Synthesis of the [SMeNHSMe] Ligand. We have targeted
ligands that can be synthesized rapidly in one or two steps from
inexpensive materials, in contrast to many of the previously
reported tridentate N,S-donor ligands.40−44,47,57,58 The triden-
tate ligand 2-(2-methylthiobenzyl)-methylthioaniline,
[SMeNHSMe], was prepared in two steps. Condensation of
commercially available 2-(methylthio)-benzaldehyde and 2-
(methylthio)-aniline in ethanol at room temperature afforded
an imine ligand that was subsequently reduced to the amine in
excellent yield using an excess of ammonia−borane (Scheme
1). Formation of the pure ligand was confirmed by H, 13C
1
NMR, UV−vis, and IR spectroscopy; EI-MS; and elemental
analysis.
Synthesis and Characterization of Iron(II) Bis(amido)
Complex, 1. The iron complex [Fe(κ3-SMeNSMe)2] (1) was
prepared by a transamination reaction of the [SMeNHSMe] ligand
with the low-coordinate bis(trimethylsilyl)amido iron complex,
[Fe{N(SiMe3)2}2] (Scheme 2). Slow addition of solid
[Fe{N(SiMe3)2}2] to a hexane suspension containing 2 equiv
of the [SMeNHSMe] ligand at room temperature afforded 1 as a
Figure 2. Molecular structure of [Fe(κ3-SMeNSMe)2] (1) with 40%
thermal ellipsoids. H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)−S(1) 2.5389(7), Fe(1)−S(2)
2.7156(7), Fe(1)−S(3) 2.5431(7), Fe(1)−S(4) 2.7014(8), Fe(1)−
N(1) 2.015(2), Fe(1)−N(2) 2.031(2), S(1)−Fe(1)−S(2) 166.63(2),
S(3)−Fe(1)−S(4) 160.67(3), N(1)−Fe(1)−N(2) 177.60(8), N(1)−
Fe(1)−S(1) 80.72(6), N(1)−Fe(1)−S(2) 86.80(6), N(2)−Fe(1)−
S(3) 78.48(6), N(2)−Fe(1)−S(4) 95.63(6).
1
yellow solid in 92% yield. The H NMR spectrum of 1 shows
broadened and shifted resonances ranging from 118.1 to −43.4
ppm, consistent with a paramagnetic iron center. The spectrum
exhibits one set of resonances for the unsymmetrical [SMeNSMe]
D
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Fe−S distances in 1 (av Fe- S = 2.62 Å) are longer than those
observed previously in high-spin iron(II) complexes with
thioether ligation.42,46,62,63 More importantly, the Fe−S
distances of the six-membered metallacycle rings are signifi-
cantly longer than those of the five-membered rings [e.g.,
2.7156(7) and 2.7014(8) Å for Fe(1)−S(2) and Fe(1)−S(4)
versus 2.5389(7) and 2.5431(7) Å for Fe(1)−S(1) and Fe(1)−
S(3)]. In spite of their trans arrangement, the Fe−Namido bond
lengths (av 2.02 Å) are comparable to those found in related
iron(II) amido complexes.17−19,42 The S(1)−Fe(1)−S(2) and
S(3)−Fe(1)−S(4) angles of 166.63(2)° and 160.67(3)° show
significant deviation from 180° for an ideal octahedral
geometry.
Since the solid-state structural parameters and DFT-obtained
relative energies of five- and six-coordinate structures were
suggestive of possible hemilability of the SNS ligand in 1, near-
infrared magnetic circular studies (NIR MCD) of 1 in frozen
solution were performed. The 5 K, 7 T NIR MCD spectrum of
1 in 1/1 THF/2-Me-THF (Figure 4) contains ligand-field (LF)
transitions at ∼7000 and ∼14 900 cm−1.
In order to further evaluate the electronic structure of
complex 1, Mossbauer spectroscopic and electrochemical
̈
studies were performed. Upon exposure to air, the yellow
color of 1 turns initially to purple and finally to brown. The
electrochemical study of 1 reveals an irreversible oxidation at
0.59 V versus ferrocene in dichloromethane solution (Figure
Figure 4. 5 K, 7 T NIR MCD spectrum of [Fe(κ3-SMeNSMe)2] (1) in
1/1 THF/2-MeTHF.
S10). The 80 K Mossbauer spectrum of solid 1 exhibits a single
̈
quadrupole doublet with observed parameters of δ = 0.87 mm/
s and ΔEQ = 0.95 mm/s, where the isomer shift is consistent
with a high-spin iron(II) species (Figure 3).64−66
The observed transition energies are indicative of the
presence of a distorted square pyramidal five-coordinate (5C)
high-spin iron(II) component being present in solution,67,68
consistent with TD-DFT calculations for 5C which predict
electronic transitions at 8300 and 14200 cm−1. Thus, it is clear
that 1 can exist as square pyramidal complex 2, [Fe(κ3-
SMeNSMe)(κ2-SMeNSMe)], in solution, consistent with hemil-
ability of the SNS ligand (Figure 5). However, it should be
Figure 3. 80 K Mossbauer spectrum of [Fe(κ3-SMeNSMe) ] (1).
̈
2
In order to probe the potential hemilability of the six-
membered ring thioether donors, DFT optimizations of four-,
five-, and six-coordinate structures of 1 were performed at the
PBE/TZVP level of theory with Grimme’s dispersion
corrections (GD3) in the gas phase. The Gibbs free energies
of five- and six-coordinate structures of 1 are very similar, with
the five-coordinate structure being 0.8 kcal mol−1 higher in
energy. The four-coordinate structure was found to be 2.5 kcal
mol−1 higher in energy. The structural parameters for the six-
coordinate structure of 1 obtained from DFT are similar to
those from the X-ray structure [e.g., 2.74 and 2.73 Å for Fe(1)−
S(2) and Fe(1)−S(4), 2.500 and 2.539 Å for Fe(1)−S(1) and
Fe(1)−S(3)]. The calculated Mayer bond orders for Fe−S
bonds are 0.25−0.27 and 0.43−0.48 for the longer and shorter
bonds, respectively. Thus, the covalency of the Fe(1)−S(2) and
Fe(1)−S(4) bonds is very weak. In comparison, the Fe−N
bonds have bond orders of 0.61 and 0.63. As can be expected,
the metal−ligand bond distances in the optimized five-
coordinate structure of 1 show a small contraction relative to
the six-coordinate structure, with Fe−N distances being
reduced from 2.033 and 2.038 Å to 1.98 and 2.00 Å. The
Fe−S distances show smaller contraction to 2.491, 2.514, and
2.74 Å. Overall, the loss of one weak covalent Fe−S bond does
not cause major changes in the lengths of the remaining metal−
ligand bonds. The Mayer valence index for the Fe atom in the
five- and six-coordinate structures remains virtually the same
(5.99 vs 6.01).
Figure 5. Proposed structure of [Fe(κ3-SMeNSMe)(κ2-SMeNSMe)] (2)
present in solution.
noted that since five-coordinate (5C) species generally exhibit
much larger Δε values in MCD than distorted six-coordinate
(6C) species, combined with the potential overlap of 6C LF
transitions in the 14 000−16 000 cm−1 region (from TD-DFT
calculations), the presence of a 6C component as well as the
relative amounts of 5C versus 6C species in solution cannot be
unambiguously determined.
To further address the amount of 5C versus 6C species that
might be present in solution, frozen-solution Mossbauer studies
̈
of 57Fe-enriched 1 in 1/1 THF/2-MeTHF were performed.
While a reasonable fit to a single species with Mossbauer
̈
parameters nearly identical to those of 1 in the solid state was
possible (Figure S28), the increased broadness in solution,
slight doublet asymmetry, and reduced quality fit overall were
consistent with the presence of a second, minor component at
<3% of the total iron. Thus, combined with the MCD studies,
the majority of 1 is 6C in solution though a minor, 5C species
is also present. Even though both the MCD and TD-DFT
calculations are most consistent with the square pyramidal
E
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complex 2, we do not observe this species in the 1H NMR (vide
supra) as exchange is presumably too fast at room temperature
and cooling the solution just gives complex 1.
Reactivity Studies of Iron(II) Bis(amido) Complex, 1.
Reactivity studies of complex 1 with a variety of mono- and
bidentate neutral donor ligands further confirmed the
hemilability of the six-membered ring thioethers. While no
reaction was observed with acetonitrile, addition of 1 equiv of
2,2′-bipyridine (bpy) to 1 in THF gave an instant color change
from yellow to red-brown with formation of mononuclear iron
complex, [Fe(κ2-SMeNSMe)2(bpy)] (3), in 80% yield (Scheme
1
3). The H NMR spectrum of 3 displays broad resonances
Scheme 3. Synthesis of Paramagnetic Iron(II) Bis(SNS)
Complex
Figure 7. Molecular structure of [Fe(κ2-SMeNSMe)2(bpy)] (3) with
40% thermal ellipsoids. H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)−S(1) 2.6146(4), Fe(1)−S(3)
2.6158(5), Fe(1)−N(1) 2.0678(13), Fe(1)−N(2) 2.0597(13),
Fe(1)−N(3) 2.2539(13), Fe(1)−N(4) 2.2030(13), N(1)−Fe(1)−
S(3) 175.95(4), N(2)−Fe(1)−N(3) 156.58(5), N(4)−Fe(1)−S(1)
159.88(4), N(2)−Fe(1)−S(1) 105.08(4), N(2)−Fe(1)−N(1)
104.27(5), S(1)−Fe(1)−S(3) 97.154(14), N(2)−Fe(1)−N(4)
94.95(5), N(3)−Fe(1)−S(1) 88.95(4).
spanning a chemical shift range of 119 to −32 ppm, confirming
a high-spin iron center. The room temperature magnetic
measurement in solution showed an effective magnetic moment
of μeff = 4.33 μB consistent with an S = 2 ground state. The 80
hand, are comparable to those previously reported for other
high-spin iron(II) bipyridine complexes.70−72 The N(2)−
Fe(1)−N(3) and S(1)−Fe(1)−N(4) angles of 156.58(5)°
and 159.88(4)°, respectively, deviate significantly from the ideal
octahedral geometry similar to that observed in 1. The sums of
the angles about N(1) and N(2) of the amido ligands are
359.85° and 353.68°, respectively.
K Mossbauer spectrum of 3 is characterized by a single
̈
quadrupole doublet with observed parameters of δ = 0.96 mm/
s and ΔEQ = 2.81 mm/s, consistent with a high-spin iron(II)
complex (Figure 6).64−66
Next we examined the reactivity of 1 toward CNxylyl (2,6-
dimethylphenyl isonitrile) and a chelating phosphine (i.e., 1,2-
bis(dimethylphosphino)ethane, dmpe). Addition of 2 equiv of
CNxylyl to a THF solution of 1 resulted in an immediate color
change from yellow to brown. Removal of solvent in vacuo
yielded a mononuclear complex, [Fe(κ2-SMeNSMe)2(CNxylyl)2]
1
(4), in 80% yield (Scheme 4). The H NMR spectrum of 4
only has resonances in the diamagnetic region, consistent with
the formation of a low-spin iron(II) complex. Furthermore, the
IR spectrum of 4 in the solid state shows two strong, sharp
signals at 2054 and 2104 cm−1 which are assigned to the C−N
stretching vibrations from the two cis-disposed isonitrile
Figure 6. 80 K Mossbauer spectrum of [Fe(κ2-SMeNSMe) (bpy)] (3).
̈
2
Crystals of 3 suitable for X-ray diffraction were grown from a
saturated THF solution at room temperature. The structure
contains two [κ2-SMeNSMe] amido ligands and a κ2-bipyridine
ligand (Figure 7) in a distorted octahedral geometry about the
iron. The Fe−S distances of the five-membered metalacycle
rings are longer than those observed in 1 [e.g., 2.6146(4) Å for
Fe(1)−S(1) and 2.6158(5) Å for Fe(1)−S(3) in 3 versus
2.5389(7) Å for Fe(1)−S(1) and 2.5431(7) Å for Fe(1)−S(3)
in 1] as well as in previously reported high-spin Fe(II)
complexes with thioether ligation.42,46,63 In contrast to 1, the
amido nitrogens in 3 are in a cis arrangement and the two Fe−
Namido distances (Fe(1)−N(1) = 2.0678(13) and Fe(1)−N(2)
= 2.0597(13) Å) are also slightly longer than those observed in
1 and other related iron(II) amido complexes.17,19,42,69 The
Fe−Nbpy distances (2.2534(1) and 2.203(1) Å), on the other
Scheme 4. Synthesis of Diamagnetic Iron(II) Bis(SNS)
Complexes
F
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ligands. The 80 K Mossbauer spectrum of 4 exhibits a single
̈
doublet with observed parameters of δ = 0.14 mm/s and ΔEQ =
0.67 mm/s consistent with a low-spin, six-coordinate iron(II)
complex (Figure 8).64,73−75
Figure 9. 80 K Mossbauer spectrum of [Fe(κ3-SMeNSMe)(κ3-
̈
SMeNHSMe)](NTf2) (6).
Crystals of 6 suitable for X-ray diffraction were grown from a
saturated dichloromethane solution at −35 °C. The molecular
structure consists of meridional κ3-amido and κ3-amine SNS
ligands in a distorted octahedral geometry about iron (Figure
10). The Fe−S bond lengths of the amine ligand are longer
Figure 8. 80 K Mossbauer spectrum of [Fe(κ2-SMeNSMe) (CNxylyl) ]
(4).
̈
2
2
However, addition of 1 equiv of dmpe to a THF solution of 1
resulted in no color change at room temperature. The 1H NMR
spectrum exhibited paramagnetically shifted resonances of the
starting material and 31P{1H} NMR spectrum showed
resonances of free dmpe. Upon cooling to −40 °C, the color
of the reaction mixture changed from yellow to red-brown. The
1H NMR spectrum of 5 at −40 °C showed resonances in the
diamagnetic region, and the 31P{1H} NMR spectrum displayed
two doublets at 56.1 and 44.4 ppm (2JPP = 30 Hz), which
disappeared on warming the solution to room temperature
accompanied by changing color back to yellow suggesting the
reversible coordination of dmpe ligand to 1.
Formation of the Iron(II) Amine−Amido Cation, 6.
Deprotonated amines are usually strong bases, and therefore,
one expects the basic amido donors of 1 to react with Brønsted
acids. When 1 equiv of bis(trifluoromethane)sulfonimide,
HNTf2, was added to a solution of 1 in dichloromethane, the
color changed instantly from yellow to red, yielding a cationic
iron(II) complex, [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)](NTf2) (6),
1
in 82% yield (Scheme 5). The H NMR spectrum shows the
Figure 10. Molecular structure of [Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)]-
(NTf2) (6) with 40% thermal ellipsoids. H atom in the amine is
shown, and other H atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe(1)−S(1) 2.6047(6), Fe(1)−S(2)
2.6573(5), Fe(1)−S(3) 2.5200(5), Fe(1)−S(4) 2.5387(5), Fe(1)−
N(1) 2.2763(15), Fe(1)−N(2) 1.9974(16), S(3)−Fe(1)−S(4)
163.403(19), N(1)−Fe(1)−S(4) 100.24(4), N(2)−Fe(1)−S(1)
108.18(5), S(3)−Fe(1)−S(1) 103.930(18), N(3)−Fe(1)−S(1)
88.95(4), S(1)−Fe(1)−S(2) 154.924(19), N(1)−Fe(1)−N(2)
173.25(6).
Scheme 5. Synthesis of Cationic Iron(II) Amine-Amido
Complex
than those of the amido ligand [e.g., 2.6047(6) Å for Fe(1)−
S(1) and 2.6573(5) Å for Fe(1)−S(2) versus 2.5200(5) Å for
Fe(1)−S(3) and 2.5387(5) Å for Fe(1)−S(4)]. However, both
Fe−S distances of the amido [SMeNSMe] ligand in 6 are shorter
than those found in complexes 1 and 3 (vide supra). The Fe−
Namine distance of 2.2763(15) Å is expectedly longer than the
Fe−Namido distance, 1.9974(16) Å. The coordination angles of
the amine nitrogen in [SMeNHSMe] are consistent with the
anticipated sp3 hybridization (average of all four angles around
N atom being 111.21°). The S(3)−Fe(1)−S(4) and S(1)−
Fe(1)−S(2) angles of 163.40(19)° and 154.92(19)° deviate
significantly from the ideal octahedral geometry. The elongated
Fe−S and Fe−Namine bonds of the [SMeNHSMe] moiety in 6
paramagnetically shifted resonances of a high-spin Fe(II)
complex. The 19F NMR spectrum consisted of a broad singlet
at −77.25 ppm assigned to the NTf2 anion in the outer
coordination sphere. The IR spectrum of 6 in the solid state
shows a broad signal at 3488 cm−1 which can be attributed to
an N−H stretching vibration supportive of the protonation of
the amide group. Attempts to further protonate 6 were
unsuccessful. The 80 K Mossbauer spectrum is also consistent
̈
with a high-spin iron(II) complex with observed parameters of
δ = 0.96 mm/s and ΔEQ = 1.92 mm/s (Figure 9).64−66
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suggest that this ligand could be easily displaced by stronger
donor ligands (vide infra).
Reactivity Studies of Iron(II) Amine−Amido Complex, 6.
Treatment of cation 6 with 3 equiv of P(OMe)3 or CNxylyl
failed to give the expected diamagnetic iron SNS amido
c o m p l e x . A d d i t i o n o f
1
e q u i v o f b i s ( 2 -
diphenylphosphinoethyl)phenyl phosphine (triphos) to a
THF solution of 6 at room temperature, however, gave an
instant color change from red to magenta. From this reaction, a
16e− diamagnetic iron amido complex, [Fe(κ2-SMeNSMe)(κ3-
triphos)](NTf2) (7), was isolated in 65% yield (Scheme 6).
Scheme 6. Synthesis of Iron(II) Amido SNS Complex
Figure 11. Molecular structure of [Fe(κ2-SMeNSMe)(κ3-triphos)]-
(NTf2) (7) with 35% thermal ellipsoids. H atoms and NTf2 anion
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Fe(1)−S(1) 2.2506(18), Fe(1)−N(1) 1.921(4), Fe(1)−P(1)
2.1665(18), Fe(1)−P(2) 2.2205(18), Fe(1)−P(3) 2.2395(19),
N(1)−Fe(1)−P(2) 169.35(14), S(1)−Fe(1)−P(3) 158.13(7),
S(1)−Fe(1)−P(1) 96.78(7), N(1)−Fe(1)−P(1) 103.76(15), P(3)−
Fe(1)−P(1) 101.48(7), P(2)−Fe(1)−P(1) 84.74(7), S(1)−Fe(1)−
P(2) 87.83(7), S(1)−Fe(1)−N(1) 84.83(14), N(1)−Fe(1)−P(3)
102.22(14), P(2)−Fe(1)−P(3) 82.00(7).
1
The H NMR spectrum shows two singlets at δ 2.21 and 2.39
due to iron-bound and unbound thiomethyl groups of the SNS
amido ligand. The 31P{1H} NMR spectrum shows a broad
singlet and triplet at δ 77.5 and 85.0 (integration ratio: 2:1),
2
respectively, with Jpp ≈ 22 Hz. On cooling to −40 °C, the
spectrum shows three different signals: two doublet of doublets
at 81.5 and 74.7 ppm (2Jpp = 21 Hz, 28 Hz) and a parent triplet
at 74.1 ppm (2Jpp = 21 Hz). It should be noted that the
spectrum also contains resonances due to free triphos (10%).
angles being 119.29°). The S(1)−Fe(1)−P(3) and N(1)−
Fe(1)−P(2) angles of 158.13(7)° and 169.35(14)° deviate
distinctly from the ideal square pyramidal geometry. Among
three Fe−P bond distances, the apical Fe−P bond [Fe(1)−
P(1) 2.1665(18) Å] is shorter than those in the basal plane
[i.e., Fe(1)−P(2) 2.2205(18) and Fe(1)−P(3) 2.2395(19) Å].
Amine−Borane Dehydrogenation Catalysis. In order to
explore the relevance of the hemilability of the [SMeNHSMe]
ligand and potential bifunctional behavior of the iron(II) amido
complexes, complex 7 was assessed as a precatalyst for amine−
borane dehydrogenation catalysis. At 60 °C, gas evolution was
observed when a solution of 7 in THF was mixed with
ammonia−borane (AB; 32 equiv). The reaction mixture
changed color from magenta to orange accompanied by
formation of an off-white precipitate of polyaminoborane.76
After 23 h of heating at 60 °C, the 11B NMR spectrum (Figure
S22) still shows unreacted AB confirming that the dehydrogen-
ation is slow. Product resonances include a doublet at 30.7 ppm
(borazine) and a doublet, triplet, and quartet from −5 to −25
ppm which are assigned to the aminoborane tetramer, B-
(cyclotriborazanyl)amine−borane (BCTB).4 Interestingly, the
31P{1H} NMR spectrum displays only a doublet and a triplet at
The 80 K Mossbauer spectrum of 7 shows a single doublet with
̈
parameters of δ = 0.22 mm/s and ΔEQ = 1.77 mm/s consistent
with a low-spin, five-coordinate iron(II) complex (Table 1 and
Figure S29).64,73−75
Table 1. Experimentally Determined 80 K 57Fe Mossbauer
̈
a
Parameters for Fe(II) Complexes 1−7
ΔEQ
Fe(II) complexes
δ (mm/s)
(mm/s)
γ (mm/s)
[Fe(κ3-SMeNSMe)2] (1)
0.87
0.96
0.14
0.96
0.95
2.81
0.67
1.92
0.27
0.25
0.39
0.26
[Fe(κ2-SMeNSMe)2(bpy)] (3)
[Fe(κ2-SMeNSMe)2(CNxylyl)2] (4)
[Fe(κ3-SMeNSMe)(κ3-SMeNHSMe)]
(NTf2) (6)
[Fe(κ2-SMeNSMe)(κ3-triphos)](NTf2)
(7)
0.22
1.77
0.30
a
The error bars for the fit analyses were δ 0.02 mm/s and ΔEQ
3%.
2
The molecular structure of the cation 7 was determined by
88.7 and 144.6 ppm, respectively, with Jpp ≈ 30 Hz. These
X-ray diffraction (Figure 11). Crystals of 7 were grown from a
saturated benzene solution layered with hexane at room
temperature. The molecular structure contains a κ2-SNS
amido and a κ3-triphos ligand in a distorted square pyramidal
geometry about iron. Both Fe−N [1.921(4) Å] and Fe−S bond
lengths [Fe(1)−S(1) 2.2506(18) Å] to the amido ligand are
significantly shorter than those found in complexes 1, 3, and 6
(vide supra). The Fe−N distance of 1.921(4) Å is also shorter
than those in complexes 1, 3, and 6. The sum of three angles
around N in SNS amido ligand is 357.88° (average of all three
resonances are correlated (gated proton decoupled 31P NMR
spectrum; Figure S23) with a quartet hydride resonance at δ
1
−24.5 (3JHP ≈ 58 Hz) in the H NMR spectrum (Figure S24),
suggestive of bifunctional AB activation. The isolation and
characterization of this iron hydride complex is currently in
progress to check for formation of the N−H bond.
In contrast, complex 7 shows improved catalytic activity with
N,N-dimethylamine−borane (DMAB; 20 equiv in THF at 60
°C) affording predominantly the aminoborane cyclic dimer.
1
While the H NMR spectrum of the resulting orange solution
H
DOI: 10.1021/acs.inorgchem.7b01802
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
again showed the quartet resonance at δ −24.1, the 31P{1H}
NMR spectra showed a number of multiplet resonances,
indicating less stability of the catalyst resting state with the
DMAB substrate (Figure S26). Importantly, no evidence of a
phosphine−borane decomposition byproduct or black solids
was observed.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: neidig@chem.rochester.edu.
*E-mail: rbaker@uottawa.ca.
CONCLUSIONS
■
ORCID
In summary, an unsymmetrical amine, 2-(2-methylthiobenzyl)-
methylthioaniline, [SMeNHSMe], is readily prepared in excellent
yield in two steps from commercially available starting
materials. The derived pseudo-octahedral Fe(II) bis(κ3-
amido) complex, 1, is shown by X-ray diffraction to contain
two meridional tridentate ligands in the solid state with trans-
nitrogens and two long Fe−S distances associated with six-
membered metalacycle rings. Further studies using MCD and
DFT calculations support the presence of a five-coordinate
isomer, 2, in solution containing one κ2-amido ligand due to
cleavage of one of the six-membered rings. Reactivity studies of
1 with a variety of donor ligands such as 2,2′-bipyridine,
CNxylyl, and dmpe produced a series of high- and low-spin
iron(II) bis(κ2-amido) SNS complexes, 3−5, due to cleavage of
both six-membered rings. Interestingly, stable products are only
formed with those ligands that can accept electron density from
the iron center; the dmpe analogue is only stable at low
temperatures.
R. Tom Baker: 0000-0002-1133-7149
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSERC, the Canada Research Chairs Program,
and Los Alamos National Laboratory’s LDRD program for
generous financial support and the University of Ottawa,
Canada Foundation for Innovation, and Ontario Ministry of
Economic Development and Innovation for essential infra-
structure. Dr. Glenn Facey and Dr. Eric Ye are thanked for
assistance with NMR experiments. M.L.N. acknowledges
research support from the National Science Foundation
(CHE-1454370) and from an Alfred P. Sloan Research
Fellowship.
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DOI: 10.1021/acs.inorgchem.7b01802
Inorg. Chem. XXXX, XXX, XXX−XXX