High-Spin Iron Imido Complexes Competent for C-H Bond Amination

Article abstract of DOI:10.1021/jacs.7b06682

Reduction of previously reported (^ArL)FeCl with potassium graphite furnished a low-spin (S = 1/2) iron complex (^ArL)Fe which features an intramolecular η⁶-arene interaction and can be utilized as an Fe^I synthon (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin). Treatment of (^ArL)Fe with adamantyl azide or mesityl azide led to the formation of the high-spin (S = 5/2), three-coordinate imidos (^ArL)Fe(NAd) and (^ArL)Fe(NMes), respectively, as determined by EPR, zero-field ⁵⁷Fe M?ssbauer, magnetometry, and single crystal X-ray diffraction. The high-spin iron imidos are reactive with a variety of substrates: (^ArL)Fe(NAd) reacts with azide yielding a ferrous tetrazido (^ArL)Fe(κ²-N₄Ad₂), undergoes intermolecular nitrene transfer to phosphine, abstracts H atoms from weak C-H bonds (1,4-cyclohexadiene, 2,4,6-^tBu₃C₆H₂OH) to afford ferrous amido product (^ArL)Fe(NHAd), and can mediate intermolecular C-H amination of toluene [PhCH₃/PhCD₃ k_H/k_D: 15.5(3); PhCH₂D k_H/k_D: 11(1)]. The C-H bond functionalization reactivity is rationalized from a two-step mechanism wherein each step occurs via maximal energy and orbital overlap between the imido fragment and the C-H bond containing substrate.

Full text of DOI:10.1021/jacs.7b06682

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Article
High spin iron imido complexes competent for C-H bond amination
Matthew J.T. Wilding, Diana A. Iovan, and Theodore A. Betley
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06682 • Publication Date (Web): 04 Aug 2017
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High spin iron imido complexes competent for C-H bond ami-
nation
Matthew J. T. Wilding, Diana A. Iovan, and Theodore A. Betley†*
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†Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United
States
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide
significant keywords to aid the reader in literature retrieval.
ABSTRACT: Reduction of previously reported (ArL)FeCl with potassium graphite furnished a lowꢀspin (S = ½) iron complex
(ArL)Fe which features an intramolecular η6ꢀarene interaction and can be utilized as an FeI synthon (ArL = 5ꢀmesitylꢀ1,9ꢀ(2,4,6ꢀ
Ph3C6H2)dipyrrin). Treatment of (ArL)Fe with adamantyl azide or mesityl azide led to the formation of the highꢀspin (S = 5/2), threeꢀ
coordinate imidos (ArL)Fe(NAd) and (ArL)Fe(NMes), respectively, as determined by EPR, zeroꢀfield 57Fe Mössbauer, magnetomeꢀ
try, and single crystal Xꢀray diffraction. The highꢀspin iron imidos are reactive with a variety of substrates: (ArL)Fe(NAd) reacts
with azide yielding a ferrous tetrazido (ArL)Fe(κ2ꢀAdN4Ad); undergoes intermolecular nitrene transfer to phosphine; abstracts Hꢀ
atoms from weak C–H bonds (1,4ꢀcyclohexadiene, 2,4,6ꢀtBu3C6H2OH) to afford ferrous amido product (ArL)Fe(NHAd); and can
mediate intermolecular C–H amination of toluene [PhCH3/PhCD3 kH/kD: 15.5(3); PhCH2D kH/kD: 11(1)]. The C–H bond functionaliꢀ
zation reactivity is rationalized from a twoꢀstep mechanism wherein each step occurs via maximal energy and orbital overlap beꢀ
tween
the
imido
fragment
and
the
C–H
bond
containing
substrate.
literature examples and determine the impact of the electronic
structure for group transfer processes.
1. INTRODUCTION
Coordination complexes featuring metal ligand multiple
bonds (MLMBs) are implicated as intermediates in a variety
of catalytic transformations both biologically1ꢀ2 and abiologiꢀ
cally.3ꢀ5 The pursuit of MLMBs is driven by their capacity to
facilitate atomꢀ and groupꢀtransfer catalysis to unreactive C–H
bond and olefinic functionalities, key to biological metaboꢀ
lism. Factors that govern MLMB reactivity and, conversely,
stability include metal oxidation state, molecular oxidation
level, and electronic structure.1, 5ꢀ20 Efficacy for group transfer
reactivity is often reliant on accessing high molecular oxidaꢀ
tion21 levels to provide the driving force for substrate funcꢀ
tionalization (e.g., cytochrome P450), though the direct impact
of differing electronic structures has not yet been fully exꢀ
plored. We hypothesized that targeting highꢀspin configuraꢀ
tions could provide a similar driving force: electronic populaꢀ
tion of MLMB antibonding orbitals limits the MLMB covaꢀ
lency, enhancing its electrophilicity and reactivity group transꢀ
fer processes.22ꢀ26 We have demonstrated enhanced reactivity
for group transfer from cobalt imido27 and iron iminyl comꢀ
plexes22, 24, 26 capable of activating and functionalizing C–H
bonds, respectively. The dipyrrin’s weak ligand field strength
engenders the highꢀspin electronic structure despite the inherꢀ
ent strong field MLMB component. Ferric imido complexes
have been isolated in S = 1/2 and 3/2 configurations,28ꢀ34 but the
2. EXPERIMENTAL SECTION
General experimental considerations, reaction assays, characꢀ
terization and physical measurements, data and spectra, and Xꢀ
ray diffraction techniques are all provided in the Supplemenꢀ
tary Information.
(ArL)FeCl (1): The preparation of this complex has been modꢀ
ified from the previously reported method22: To a 50 mL presꢀ
sure vessel was added (ArL)Li (0.500 g, 0.57 mmol, 1 equiv.),
FeCl2 (0.078 mg, 0.61 mmol, 1.07 equiv.), and 20 mL toluene.
The vessel was equipped with a magnetic stir bar, capped,
removed from the glovebox, and placed in an oil bath at 150
°C for 14 h. The dark purple mixture was cooled to room temꢀ
perature, returned to the glovebox, and filtered through a
coarse glass frit with Celite to remove lithium chloride and
excess iron. The Celite was washed with benzene until the
filtrate was nearly colorless. The solvent was removed in vacꢀ
uo and the resulting purple material recrystallized overnight at
–40 °C from 18 mL of 2:1 ratio of hexanes to benzene. The
mother liquor was decanted and the crystals washed with 4 mL
cold (–40 °C) hexanes and dried in vacuo to afford (ArL)FeCl
as purple crystals (0.400 g, 73%). Characterization data (Figꢀ
ure S1–S3) for this compound matches that previously reportꢀ
5
putative highꢀspin /2 configuration has thus far eluded isolaꢀ
ed.22 ꢁeff (295 K, SQUID, Figure S3) = 5.3 ꢁB
.
tion. Thus, we sought to prepare and evaluate the reaction
chemistry of a highꢀspin iron(III) imido complex against the
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+
(ArL)Fe (2): In a 20 mL vial, potassium graphite (KC8, 0.055
84.73, H 5.89, N 4.09. HRMS (ESI+) m/z calcd C66H49FeN2
[M+]: 1074.4444, found: 1074.4421.
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g, 0.41 mmol, 1.2 equiv.) was slurried in 1 mL benzene and
frozen. A thawing solution of 1 (0.322 g, 0.33 mmol, 1.0
equiv.) in benzene was pipetted on top of the frozen slurry and
the reaction allowed to stir while warming to room temperaꢀ
ture, during which time the color changed from pink to bright
purple. After 3 hours, the reaction was filtered through Celite
and the filter cake eluted with excess benzene until the filtrate
was nearly colorless. The solvent was removed in vacuo and
the solids were dissolved in benzene and again filtered through
Celite. The solvent was removed in vacuo and the resulting
material dissolved in minimal benzene and transferred to a 20
mL vial. The solution was frozen and the solvent removed via
sublimation in vacuo to afford 2 as a purple powder (0.191 g,
62%). Crystals suitable for Xꢀray diffraction were grown from
(ArL)Fe(NMes) (5): A solution of mesityl azide (15 mg, 0.09
mmol, 1.05 equiv) in 1 mL benzene was added all at once to a
solution of 2 (83 mg, 0.089 mmol, 1 equiv.) in 5 mL benzene,
causing an immediate color change from purple to pink. The
solvent was removed in vacuo and the resulting material reꢀ
crystallized from a concentrated solution of toluene layered
with pentane at –35 °C. The mother liquor was decanted the
resulting crystals washed with pentane to yield 5 as purple
crystals (70 mg, 74 %). Crystals were suitable for Xꢀray difꢀ
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fraction prior to pentane washing. H NMR (400 MHz, C6D6)
δ 60.95, 50.58, 38.18, 32.24, 17.51, 14.66, 12.57, 11.13, 5.61,
ꢀ16.90, ꢀ22.01, ꢀ49.31. Zeroꢀfield 57Fe Mössbauer (200 K) (δ,
|ꢂEQ| (mm/s)): 0.47, 0.00 (γ = 0.82 mm/s) (Figure S10). %CHN
Calc. for C75H60FeN3•C7H8: C 85.55, H 5.95, N 3.65; Found: C
85.46, H 6.28, N 3.73 (one molecule of toluene comes from
crystal unit cell).
(ArL)Fe(HNAd) (6): A solution of 1 (25.8 mg, 26.8 ꢁmol, 1.0
eq) in 5 mL benzene was added to a stirring slurry of lithium
adamantyl amide (4.3 mg, 27.4 ꢁmol, 1.02 eq) in 1 mL benꢀ
zene. After stirring at room temperature for 8 hours, the reacꢀ
tion was filtered through Celite and the filter cake washed with
excess benzene until the eluent was nearly colorless. The solꢀ
vent was frozen and removed in vacuo to yield 6 as a purple
powder (22.2 mg, 77 %). Crystals suitable for Xꢀray diffracꢀ
tion were grown from a concentrated solution of toluene layꢀ
1
a 2:1 nꢀhexane/benzene mixture at −38 °C. H NMR (400
MHz, 295 K, C6D6): no observable peaks. ꢁeff (150 K,
SQUID, Figure S4): 1.7 ꢁB. EPR (toluene, 77 K): geff = 2.24,
2.04, 1.98 (Figure 1b). Zeroꢀfield 57Fe Mössbauer (90 K) (δ,
|ꢂEQ| (mm/s)): 0.64, 0.86 (γ = 0.22 mm/s) (Figure 1a). %CHN
Calc. for C66H49FeN2: C 85.61, H 5.33, N 3.03; Found: C
+
85.38, H 5.13, N 2.89. HRMS (ESI+) m/z calcd C66H49FeN2
[M+]: 925.3240, found: 925.3219.
(ArL)Fe(κ2ꢀN4Ad2) (3): In a 20 mL vial 2 (80 mg, 0.09 mmol,
1 equiv.) was dissolved in 10 mL of benzene. A solution of
adamantyl azide in benzene (3.21 g, 1% w/w, 0.18 mmol, 2
equiv) was added resulting in an immediate color change from
dark purple to dark red. After stirring for 30 min the solvent
was removed in vacuo, and the residue washed with nꢀhexane
to give the tetrazene complex 3 as a dark red powder (98.3 mg,
91%). Crystals suitable for Xꢀray diffraction were grown from
a concentrated toluene solution layered with pentane at −38
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ered with pentane at –40 °C. H NMR (400 MHz, C6D6) δ
71.27, 50.96, 49.63 (br), 47.26, 36.07, 25.61, 25.36, 17.53,
13.63, 11.69, 10.97, 9.96, 7.76, 2.26, ꢀ12.30, ꢀ22.49, ꢀ54.93
(br). Zeroꢀfield 57Fe Mössbauer (200 K) (δ, |ꢂEQ| (mm/s)): 0.61,
0.83 (γ = 0.17 mm/s) (Figure S19). %CHN Calc. for
1
°C. H NMR (400 MHz, C6D6) δ 31.05, 20.84, 15.40, 15.05
C76H65FeN3: C 84.82, H 6.09, N 3.90; Found: C 84.68, H 5.96,
+
N
3.76. HRMS (ESI+) m/z calcd C66H49FeN2
(br s), 14.64, 13.63, 8.76 (br s), 6.73 (d, J = 27.2 Hz), 5.67 (t, J
= 7.0 Hz), 5.40 (t, J = 7.2 Hz), 5.30, 4.79, 3.56 (d, J = 7.3 Hz),
1.76, 1.60, 1.55, 1.32 (q, J = 11.4 Hz), ꢀ3.49, ꢀ5.40, ꢀ5.96 (br
s), ꢀ7.08, ꢀ8.68, ꢀ9.52, ꢀ18.01 (br s), ꢀ27.70, ꢀ30.73. ꢁeff (295 K,
SQUID, Figure S7) 4.1 ꢁB. EPR (toluene, 4 K): geff = 6.25,
2.07. Zeroꢀfield 57Fe Mössbauer (90 K) (δ, |ꢂEQ| (mm/s)): 0.70,
2.74 (γ = 0.27 mm/s) (Figure S6). %CHN Calc. for C86H79FeN6:
C 82.47, H 6.36, N 6.71; Found: C 82.30, H 6.21, N 6.57.
[M+THF+MeOH+H]+: 1180.54435, found: 1180.5554. (Note:
ionization of this molecule was very poor, which contributed
to the 9.3 ppm error.)
(ArL)Fe(HNMes) (7): A solution of (ArL)FeCl (100 mg, 0.089
mmol, 1.0 eq) in 5 mL benzene was added to a stirring slurry
of lithium mesityl amide (14.7 mg, 0.1 mmol, 1.16 eq) in 1
mL benzene. After stirring at room temperature for 8 hours,
the reaction was filtered through Celite and the filter cake
washed with excess benzene until the eluent was nearly colorꢀ
less. The solvent was frozen and removed in vacuo to yield 7
as a purple powder (105 mg, 95 %) 1H NMR (400 MHz,
C6D6) δ 114.03, 93.91, 86.90, 79.20, 49.81, 38.01, 26.62,
14.47, 12.69, 11.62, 10.17, ꢀ18.90, ꢀ20.26, ꢀ34.42. Zeroꢀfield
57Fe Mössbauer (90 K) (δ, |ꢂEQ| (mm/s)): 0.66, 0.46 (Figure
S11). %CHN Anal. Calc. for C75H61FeN3: C 84.97, H 5.80, N
3.96; Found: C 83.78, H 5.93, N 3.96.
HRMS (ESI+) m/z calcd C86H80FeN6 [M+]: 1252.5791 found:
+
1252.5780.
(ArL)Fe(NAd) (4): A solution of 2 (101 mg, 94 ꢁmol, 1
equiv.) in 50 mL benzene was transferred to an ovenꢀdried
100 mL round bottom flask. A solution of adamantyl azide in
benzene (7.90 g, 0.2% w/w, 89 ꢁmol, 0.95 equiv) was added
dropwise over the course of 1.5 hours, during which time the
color changed from purple to pink. The solvent was removed
in vacuo and the resulting solid washed extensively with hexꢀ
anes to remove unreacted azide and (ArL)Fe, yielding 4 as a
pink/purple solid (81 mg, 85 %). To remove trace tetrazido 3,
4 was recrystallized from a concentration toluene solution
layered with pentane at –40 °C to afford purple microcrystals
3. RESULTS AND DISCUSSION
Scheme 1
1
suitable for Xꢀray diffraction analysis. H NMR (500 MHz,
C6D6) δ 136.50 (br s), 56.28 (br s), 51.87, 41.60, 37.00, 32.96,
17.89, 10.60, 9.85, 9.43, 2.12, ꢀ4.19 (br s), ꢀ6.81, ꢀ11.79, ꢀ
26.25, ꢀ82.68 (br s). ꢁeff (295 K,SQUID) 6.20 ꢁB. EPR (toluꢀ
ene, 4 K): geff = 8.62, 5.35, 3.10. Zeroꢀfield 57Fe Mössbauer
(200 K) (δ, |ꢂEQ| (mm/s)): 0.37, 0.00 (γ = 0.50 mm/s). %CHN
Calc. for C76H64FeN3: C 84.90, H 6.00, N 3.91; Found: C
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instead it manifests a gradual increase at ca. 250 K that conꢀ
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2
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tinues to the temperature limit of the magnetometer. Thus, we
assign the ground state of 2 as a lowꢀspin (S = ½) FeI that may
thermally populate higher spin states.
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3.1 Synthesis and Structural Characterization. The synꢀ
thesis of metal imido complexes typically utilize oxidative
group transfer from organic azides or hypervalent iodine reaꢀ
gents to a reactive coordination complex capable of nitrene
capture.5 Thus, we prepared a suitable iron(I) reagent followꢀ
ing chemical reduction of the three coordinate ferrous dipyrrin
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complex
(ArL)FeCl
(1)
(ArL:
5ꢀmesitylꢀ1,9ꢀ(2,4,6ꢀ
Ph3C6H2)dipyrrin).22 Cyclic voltammetry on 1 in dichloroꢀ
methane revealed a reversible FeII/III couple at –460 mV and an
irreversible reduction event at –1.80 V (both versus [Cp2Fe]0/+
,
Figure S1) which was attributed to an ironꢀcentered reduction.
Accordingly, treatment of 1 with a slight excess (1.05 equiv)
of potassium graphite in thawing benzene cleanly afforded the
reduced complex (ArL)Fe (2) (Scheme 1), as evidenced by the
single quadrupole doublet (δ: 0.62 mm/s, |ꢂEQ|: 0.86 mm/s) in the
57Fe Mössbauer spectrum (Figure 1a). Structural elucidation of
2 reveals an η6ꢀinteraction between the iron and an orthoꢀ
phenyl substituent of the dipyrrin 2,4,6ꢀPh3C6H2 aryl flanking
unit (Figure 2a). The arene binding interaction is inert to disꢀ
placement by coordinating ligands (THF, diethyl ether, pyriꢀ
dine, trimethylphosphine, primary amines).
Figure 1. 90 K Zeroꢀfield 57Fe Mössbauer (a) and frozen toluꢀ
ene Xꢀband EPR (b) at 77 K of 2 (red lines provide fits to data
with the parameters indicated; EPR fit represents simulation
with Easyspin35 using labeled geff values and S = ½).
1
Although the H NMR displays no observable resonances
Dropwise addition of a dilute (0.03 % w/w) solution of adꢀ
amantyl azide to (ArL)FeI was sufficient to suppress the forꢀ
mation of the ironꢀtetrazinato complex (ArL)Fe(κ2ꢀN4Ad2) (3),
and provide the targeted ironꢀimido complex (ArL)Fe(NAd) (4)
in good yield (85%). Treatment of 2 with mesitylazide cleanly
affords the arylimido complex (ArL)Fe(NMes) (5, 74%) withꢀ
out the competing tetrazinato formation, presumably due to
the steric protection of the imido aryl ortho substituents. The
molecular structures of 4 and 5 were obtained by single crystal
Xꢀray diffraction studies on single crystals grown from a conꢀ
centrated toluene solution layered with pentane at –35 °C
(Figure 2b and c, respectively).
(295 K), the frozen toluene EPR spectrum of 2 at 77 K conꢀ
sists of a rhombic signal (geff = 2.24, 2.04, 1.98), consistent for
an S = 1/2 spin state (Figure 1b). SQUID magnetometry corꢀ
roborates this assignment with ꢁeff = 1.7 ꢁB at 75 K, complete
overlay of the isofield curves by reduced magnetization, and
good fit of the fieldꢀdependence of lowꢀtemperature magnetiꢀ
1
zation to the S = /2 Brillouin function (Figure S4). A singleꢀ
point DFT calculation16 on a truncated structural model of 2
suggests a high degree of covalency between the iron and η6ꢀ
bound arene (Figure S5), although the predominance of spin
density (75 % based on Mulliken spin population analysis) is
localized on the metal center and no brokenꢀsymmetry soluꢀ
tion was found (e.g., SFe = 1, Sarene = –1/2). The VT magnetic
susceptibility data for 2 does not plateau at higher temperature,
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Figure 2. Solidꢀstate structure of (a) (ArL)Fe (2), (b) (ArL)Fe(NAd) (4) at 100 K with 20% probability ellipsoids and (c)
(ArL)Fe(NMes) (5) and (d) (ArL)Fe(NHAd) (6) at 100 K with 35 % probability ellipsoids. H atoms and solvent molecules have been
omitted for clarity. Colors: Fe, orange; C, gray; H, white; N, blue.
Figure 3. (a) Zeroꢀfield 57Fe Mössbauer of 4 (red) and 5 (blue) (δ, |ꢂEQ|, Γ (mm/s): 4, 0.37, 0.00, 0.50; 5, 0.47, 0.00, 0.82), collected
at 200 K. (b) Xꢀband EPR of 4 (fit parameters: S = 5/2, g = 2.0, |E/D| = 0.145), collected at 4 K. (c) VT dc magnetic susceptibility for
4 collected at 0.5 T over the temperature range 5 – 300 K (blue circles). Inset: VTVH magnetization data collected on increasing
temperature from 1.8 to 10 K at increasing field for 1, 4, and 7 T with the fit parameters as described in the text.
The molecular structures for imidos 4 and 5 feature distortꢀ
ed trigonal planar iron(III) centers supported by the dipyrrin
ligand and single imido moieties. The Fe–N3 distance of
1.674(11) Å in 4 and 1.708(2) Å in 5 are consistent with the
reported range for threeꢀ and fourꢀcoordinate ironꢀalkylimido
species [1.622 – 1.700(5) Å] with established multiple bond
character.22, 28ꢀ31, 33ꢀ34, 36ꢀ44 In addition, the ∠Fe–N3–Cimido angle
of 174.7° in 4 and 172.3(2)° in 5 suggest biaxial π contribuꢀ
tions, supporting the view of 4 and 5 as iron(III) complexes
featuring doublyꢀbonded imidos.
broadening and low signalꢀtoꢀnoise ratio, likely due to incomꢀ
plete resolution of hyperfine splitting at 90 K or more complex
relaxation broadening phenomena45 often observed in highꢀ
spin FeIII compounds.46ꢀ47
In accordance with the observed temperature dependent
phenomena (vide supra), 4 and 5 do not exhibit an EPR signal
above ca. 50 K. The frozen toluene EPR spectrum of 4 (Figure
3b) obtained at 4 K displays a rhombic signal (geff = 8.62, 5.35,
3.10), well reproduced by an S = 5/2 simulation treating each
intraꢀdoublet transition as an effective S = 1/2 system (in acꢀ
cordance with the weakꢀfield limit) and including the effects
of both rhombicity (E/D) and Dꢀstrain (simulation parameters: g
= 2.0, |E/D| = 0.145, linewidth = 15 G). The inclusion of Dꢀ
strain is necessary to account for the randomness in spatial
confirmation of the frozen solution and provide an effective
3.2 Spectroscopic Characterization. The Mössbauer specꢀ
tra of 4 and 5 feature single broad transitions (δ, |ꢂEQ|, Γ
(
mm/s): 4, 0.37, 0.00, 0.50; 5, 0.47, 0.00, 0.82) at 200 K (Figure
3a). The isomer shift values are close to those previously reꢀ
ported for iminyl radicals (ArL)FeIIICl(•NAr) (δ, |ꢂEQ|
E
powder pattern for a given S and /D.48 No satisfactory agreeꢀ
(
mm/s): 0.29, 2.29),22 tBuL)FeIIICl(•NAr) (δ, |ꢂEQ| (mm/s): 0.37,
(
2.17),24 and the bimolecularlyꢀcoupled [(ArL)FeIIICl]2(κ2ꢀ
N(Ph)(C6H5)N) (δ, |ꢂEQ| (mm/s): 0.33, 2.15)22 suggesting a
common iron oxidation state assignment among these moleꢀ
cules and 4, whereas the parameters for 5 are presumably afꢀ
fected by the conjugation to the imido mesityl functionality.
Spectra obtained below ca. 150 K suffer from significant
ment with experiment was found for simulations employing
the S = 3/2 assignment. Accordingly, solidꢀstate magnetometry
of 4 is consistent with a high spin state as evidenced by a ꢁeff
of 5.3 ꢁB (
value of MT over the temperature range surveyed is consistent
with an S = /2 configuration (spinꢀonly value anticipated is
χ
MT = 4.76 cm3 K/mol) at 295 K (Figure 3c). The
χ
5
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4.38 cm3 K/mol). The spin state suggested by the magnetic
susceptibility data is further corroborated by variableꢀ
temperature, variableꢀfield magnetization data collected on
heating from 1.8 to 10 K and at increasing fields of 1–7 T
(Figure 3c inset). Magnetization saturation occurs at 4.86 ꢁB at
1
2
3
4
5
6
7
8
1.8 K and 7 T. The lower than expected saturation (5.92 ꢁB for
5
an ideal S =
/ with g = 2) and the observation of nonꢀ
2
superimposable isofield curves indicates the presence of zeroꢀ
field splitting, which was quantified by fitting the data to the
2
spin Hamiltonian Ĥ = DŜz2 + E(Ŝx2 – Ŝy ) + gisoꢁB S•H. The fit
9
5
parameters considering an S = /2 that best reproduce the data
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are g = 2.02, D = 1.0 cm–1, |E/D| = 0.01 (Figure 3c inset, conꢀ
tinuous black traces).
A singleꢀpoint DFT calculation49 on 4 corroborates the S =
5/2 ground state assignment and the calculated Mössbauer paꢀ
rameters match well with those observed for 4 (calculated δ
(
mm/s): 0.35). The calculated spin density plot (α – β) for 4,
shown in Figure 4a, illustrates the contributions to the total
spin contributed by iron (78%) and the imide nitrogen atom
(16%). The molecular orbital ordering indicated in Figure 4b
is governed by the type of bonding and overlap between the
metal center, the dipyrrin ligand, and the imido fragment, each
singly populated in the highꢀspin configuration: 286α (dxz: σ*
to dipyrrin; π* to imido Npx), 285α (dyz2: π* to imido Npy), 280α
(dz2: σ* to imido Npz), 273α–274α (dx –y2, dxy: nominally nonꢀ
bonding) (Figure S20). The density functional modeling and
magnetic data acquired for imido complexes 4 and, by extenꢀ
Figure 4. The calculated spin density population (α – β) for 4
(S = /2) (a), the qualitative orbital energetic ordering (b), and
the highest singly occupied orbitals for (Feyz – NRpy, 285α) and
(Fexz – NRpx, 286α) (c) as suggested by DFT (B3LYP/6ꢀ
31g(d); Gaussian 09).49
5
5
The reactivity of three coordinate 4 and 5 should be conꢀ
trasted with the quartet imido reported by Holland that does
not undergo intermolecular HAT reactivity except in the presꢀ
ence of a Lewis base (e.g. pyridine, lutidine).34 The authors
sion, 5 are assigned as the first highꢀspin (S = /2) complexes
thus far observed on iron.
3.3 Reactivity Assessment. With the isolated highꢀspin imꢀ
idos 4 and 5 in hand, we were thus interested in assessing
whether the reactivity of these two complexes diverge from
that reported for the lowꢀ and intermediate spin analogues.
Lowꢀspin (S = ½) complexes have been shown to exhibit
nitrene transfer to tertiary phosphines and CO,28 whereas the
intermediate spin imido complexes have displayed this reacꢀ
tivity in addition to facilitating Hꢀatom abstraction from weak
C–H bonds.37, 50ꢀ52 A reaction between 4 and excess PMe2Ph
(100 equivalents) proceeded instantaneously at room temperaꢀ
ture to produce the phosphinimide Me2PhP(NAd) and regenerꢀ
ate 2. Heating a mixture of 4 and 1,4ꢀcyclohexadiene at 45 °C
resulted in slow consumption of 4 and formation of a new
paramagnetic complex as evidenced by 1H NMR (Figure S13),
to afford the amido product of Hꢀatom abstraction
(ArL)Fe(NHAd). This assignment was verified by comparison
5
propose the intermediacy of an S = /2 pyridine adduct that
possesses sufficient potency to engender reactivity towards C–
H bonds. While imide variants that do not necessitate pyridine
3
coordination prior to HAT have since been prepared (S = /2),
their rate of reaction with 1,4ꢀcyclohexadiene, for example, is
between one and four orders of magnitude slower than the
postulated S = 5/2 adduct complex.32
While 4 is stable in toluene solutions below or at room temꢀ
perature, heating toluene solutions of 4 to 80 °C resulted in
amination of the solvent benzylic C–H bond. Analysis of the
reaction mixture by LCꢀMS revealed intermolecular C–H
amination and formation of Nꢀadamantylꢀbenzylamine (14 %),
Nꢀadamantylꢀbenzylimine (3 %), and significant amounts of
1,2ꢀdiphenylethane. Kinetic isotope effects (kH/kD) of 15.5(3)
and 11(1) were determined for intermolecular (C7H8 vs. C7D8)
and intramolecular (d1ꢀtoluene C6H5CH2D activation) competiꢀ
tion experiments, respectively, based on analysis of the resultꢀ
ing insertion products by LCꢀMS (see Supporting Inforꢀ
mation). These data are reminiscent of the intermolecular C–H
amination chemistry with (AdL)FeCl(OEt2) reported previously
(KIE: 12.8(5), presence of 1,2ꢀdiphenylethane at elevated
temperatures as a result of solvent cage breakdown and couꢀ
pling of two benzylic radicals), and are similarly consistent
with a stepwise abstraction/recombination mechanism. Altꢀ
hough 4 does not possess radical density on the imido fragꢀ
ment as proposed for (ArL)FeCl(•NAr),22 C–H functionalizaꢀ
tion chemistry is still possible with 4.
1
of the reaction mixture H NMR spectrum (Figure S13) with
an authentic sample of (ArL)Fe(NHAd) (6) afforded by the
direct metathesis of 1 with LiNHAd in benzene. The amide is
not stable to prolonged heating, preventing the full consumpꢀ
tion of 4 prior to decomposition. Nevertheless, a significant
amount of product can be observed after 7 days incubating at
45 °C. In a similar fashion, 5 partially (ca. 50% completion)
converts to the amide (ArL)Fe(NHMes) following treatment
with 2,4,6ꢀtBu3C6H2OH (BDE O–H: 81 kcal/mol)53 at 60 °C
for 6 h, as shown in the 1H NMR overlay with authentic
(ArL)Fe(NHMes) (7) (Figure S14). While production of the
amide 7 is evident, further reaction between 7 and the aryloxꢀ
ide radical is also apparent (Figure S14).
3.4 Molecular Orbital Considerations. The large KIE obꢀ
served in the reaction of 4 with toluene suggests that the C–H
bond breaking event contributes significantly to the rate limitꢀ
ing step of the reaction sequence illustrated in Figure 5a. A
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consideration of the frontier molecular orbitals (FMOs) that largely unchanged and could proceed by similar mechanisms.
*
1
2
3
4
5
6
7
8
complete this process includes the two iron imido π interacꢀ
tions and the toluene C–H σ bond (see Figure 5). The two N π
The πx/πx , however, do change between the two ancillary
ligands. The stronger σꢀdonating βꢀdiketiminate destabilizes
πx* which remains unpopulated in the ground state. Similarly,
the corresponding bonding πx would be lowered in energy with
respect to 4. Thus the radical recombination step could be
affected by the orbital energy changes and diminish the probaꢀ
bility for the recombination step.
FMOs consist of the nonꢀdegenerate π interactions: πx [σ*
(dxz –
Lσ) ± Npx] which includes a Fe–Ldipyrrin σ* component, and the
orthogonal πy (Fe dyz ± Npy). We propose the πy* FMO dictates
the primary orbital interaction with the incoming toluene C–H
substrate given its steric accessibility and more favorable enꢀ
ergetic overlap. The Hꢀatom abstraction step is highlighted in
Figure 5b, where the homolyzed C–H orbital (yielding PhC•H2
Akin to (ArL)Co(NR) imido complexes which thermally acꢀ
cess open shell configurations permitting C–H bond activation
to occur,27 the highꢀspin FeIII imido complexes reported herein
can facilitate nitrene transfer including C–H bond functionaliꢀ
zation. Furthermore, the highꢀspin imido complexes can be
synthesized utilizing both alkyl and arylꢀimido fragments,
suggesting the sextet configuration is energetically preferred
even when the MLMB unit ligand field strength is increased.
Comparing the results demonstrated herein with the known
reactivity of ferric imido complexes spanning electronic conꢀ
figurations of S = ½ → 5/2, the sextet configuration is the only
electronic structure that permits both Hꢀatom abstraction and
the subsequent recombination reaction to occur. Thus, one
could envision catalytic intraꢀ or intermolecular nitrene delivꢀ
ery reactions to be developed, so long as the highꢀspin elecꢀ
tronic structure is maintained for the MLMB fragment. Given
that both the ferric imido and iminyl radical species have been
shown to be competent for intermolecular nitrene delivery into
C–H bonds, we are now interested in understanding how the
oxidative potency is varied between the species. Research is
currently underway to probe this factor for group transfer caꢀ
talysis.
9
*
and H 1s) combine with πy/πy to yield four orbital combinaꢀ
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0
tions: (σN–H)2(Fe 3dyz)2(PhC•H2)1(σ*
)
N–H
constituting a oneꢀ
,
electron reduction at the iron center to FeII. The Hꢀatom abꢀ
straction step conserves total spinꢀangular momentum as the
resulting radical pair complex remains in a sextet spin configꢀ
uration. The recombination step (Figure 5c) should proceed
combining the benzylic radical with the remaining πx [σ*
(dxz –
± Npx] pair yielding three product orbitals: (σN–C)2(Fe
Lσ)
3dxz)2(σ*N–C)0, requiring a spin inversion to avoid population of
σ*N–C, and thus constituting a net reduction of the iron center to
FeI.
While 4 is operative for C–H bond amination, the (βꢀ
3
diketiminate)FeIII(NAd) (S = /2) congener described by Holꢀ
land was reported to be active for the Hꢀatom abstraction reacꢀ
tion but not for the radical rebound mechanism.32, 34 The prinꢀ
cipal difference between the two nearly isostructural materials
is the spin ground states resulting from the ligand field
strengths of the two ancillary ligand sets. From an orbital perꢀ
spective, the πy components are largely the same between the
βꢀdiketiminate and dipyrrin complexes, thus the HAA step is
Figure 5. Frontier molecular orbital description of FeIII(NAd) reaction profile for Hꢀatom abstraction from PhCH3 and the subseꢀ
quent radical rebound event.
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(27) King, E. K.; Sazama, G.; Betley, T. A. J. Am. Chem. Soc. 2012, 134,
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ASSOCIATED CONTENT
Supporting Information. Crystallographic data, Xꢀray crystal
structure images, magnetic data, Mössbauer, and 1H NMR
spectra. This material is available free of charge via the ACS
Publications website at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
betley@chemistry.harvard.edu
Funding Sources
This work was supported by grants from the NSF (CHEꢀ
0955885) and NIH (GMꢀ115815), and from Harvard Uniꢀ
versity. M.W. would like to thank the NSF for a Predoctoral
Graduate Fellowship.
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