An S = 1/2 Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe-H Parameters

Article abstract of DOI:10.1021/jacs.8b02603

Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N₂ binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated Fe^III(H)(N₂) complex, which releases H₂ upon warming to yield an Fe^II-N₂-Fe^II complex. Bimolecular reductive elimination of H₂ from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe-H bond with a computationally determined BDFE of 55.6 kcal/mol.

Full text of DOI:10.1021/jacs.8b02603

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An S = ½ iron complex featuring N2, thiolate, and hydride ligands:
Reductive elimination of H2 and relevant thermochemical Fe-H parameters.
Nina X. Gu, Paul H. Oyala, and Jonas C. Peters
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02603 • Publication Date (Web): 23 Apr 2018
Downloaded from http://pubs.acs.org on April 23, 2018
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An S = ½ iron complex featuring N2, thiolate, and hydride ligands:
Reductive elimination of H2 and relevant thermochemical Fe-H pa-
rameters.
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Nina X. Gu, Paul H. Oyala, and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, United States
ABSTRACT: Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strong-
ly donating hydrides have been proposed to facilitate N2 binding to Fe and may also participate in the hydrogen evolution process
concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated FeIII(H)(N2) complex,
which releases H2 upon warming to yield an FeII-N2-FeII complex. Bimolecular reductive elimination of H2 from metal hydrides is
pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and
usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride
species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe-H bond with a computa-
tionally-determined BDFE of 55.6 kcal/mol.
INTRODUCTION
nitrogenase state, where the hydride ligands have been as-
signed as iron-ligated, has been proposed to undergo H2 elimi-
nation concurrent with nitrogen uptake and subsequent reduc-
tion.9a,17
Despite a wealth of recent progress towards functional
models of biological N2-to-NH3 conversion,1,2 many questions
remain as to how the active site cofactors of nitrogenase en-
zymes, as in the iron-molybdenum cofactor (FeMoco) of Mo-
Fe-nitrogenase,3 manage N2 binding and the subsequent bond-
breaking and making steps en route to NH3 formation. While
the community has primarily focused on iron as the most like-
ly site/s of N2 binding in recent years,4 a particular conundrum
concerns the presence of relatively weak-field sulfide (S2-)
ligands around the candidate iron sites,5 as yet unknown lig-
ands in Fe-N2 model chemistry, and the related requirement
that N2 binding must occur at a biologically accessible redox
potential.6
H2 elimination steps from metal-bound hydride states are al-
so presumed to be relevant to iron-rich hydrogenase en-
zymes,18,19 and often in synthetic molecular catalysts for the
hydrogen evolution reaction (HER),19,20 including examples
featuring Fe, Co, and Ni.20bc,21,22 When considering the key H-
H bond formation step from metal hydride complexes, there
are a number of pathways one might consider, including bond
formation via direct protonation of the hydride ligand,23 lig-
and-facilitated protonation of metal hydrides,20b,24 and reduc-
tive elimination from a metal polyhydride species.25 In particu-
lar, although bimolecular reductive elimination of H2 from
metal hydride species has been demonstrated to play a role in
electrocatalytic systems for proton reduction,22 examples of
such reactivity from well-defined terminal metal hydride com-
plexes are limited.26 There is thus substantial motivation to
map the reactivity patterns and fundamental thermochemical
parameters of Fe-H (and other M-H) species in the presence of
thiolate and N2 ligands, particularly in systems where H2 evo-
lution is viable.
One plausible scenario to help account for N2 binding at
iron at the FeMoco is that redox leveling of successive elec-
tron transfer steps may be achieved by concomitant proton
transfers7 to generate, for example, protonated sulfides (e.g.,
SH–)8 and/or iron hydrides (e.g. Fe-H, Fe--H-Fe).9 Such a
scenario would not only alleviate local charge build-up, there-
by buffering redox potential, but would also install compara-
tively strong-field hydrides that are compatible with, and
might even facilitate, Fe-N2 bound states.4,10 Hence, there is
substantial motivation to prepare synthetic model complexes
featuring Fe-N2 with a combination of thioether11,12/thiolate13,14
and hydride ligands within the immediate iron coordination
sphere.15
In this study, we report the synthesis and characterization of
structurally unusual Fe(H)(N2)(thiolate) complexes in two
redox states,
S S = ½
= 0 FeII(H)(N2)(thiolate) and
FeIII(H)(N2)(thiolate), which have each been characterized by
numerous techniques, including by XRD analysis and pulse
electron-nuclear double resonance (ENDOR) spectroscopy for
the S = ½ state. EPR data for terminally bound and open-shell
Fe-H species, regardless of the other ligands in the coordina-
tion sphere, is highly limited.2,27,28 Furthermore, prior to this
report, Fe-N2 complexes have, to our knowledge, only been
characterized in oxidation states of 2+ and lower;29 our finding
Relatedly, spin-active model complexes of these types, S =
½ systems especially, may provide needed spectroscopic pa-
rameters to help guide reliable assignments of intermediate
states within the biological systems. Accordingly, various EPR
techniques have proven effective for observing S = ½ interme-
diate states during nitrogenase turnover (e.g., hydride- and
NxHy-bound states),4,9,16 and an EPR-active, hydride-bound
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that Fe(III) binds a weakly activated N2 ligand in the presence
of thiolate and hydride ligands is hence noteworthy.
Synthesis and characterization of FeII(H)(N2)(thiolate), 7-H
and FeIII(H)(N2)(thiolate), 8-H.
1
The FeIII(H)(N2)(thiolate) species persists at low tempera-
tures in solution but undergoes bimolecular conversion to a
diiron FeII-N2-FeII product upon warming, along with associat-
ed loss of H2. Such reactivity has strong precedence for sec-
ond- and third-row transition metal hydrides,26 but is still unu-
sual among first-row metal hydrides,30,31,32,33,34 and among
paramagnetic hydrides in general.26ab,27 Thus, the present
FeII/FeIII hydride system offers an opportunity to map kinetic
parameters for H2 evolution and fundamental Fe-H thermo-
chemical (H+, H•, H−) parameters of broad current interest in
the context of HER and small molecule reduction catalysis.35
To template a thiolate-supported iron(N2) complex, we en-
visioned the incorporation of a thiolate group within a poly-
phosphino silyl framework, a motif our lab has previously
utilized to gain entry into Fe(N2) chemistry (Scheme 1).36 Al-
kylation of 2-phenylbenzenethiol (1)37 followed by directed
ortho lithiation provides the aryl lithium salt (3) as a TMEDA
adduct (88%). Treatment of bis(o-diisopropylphosphino-
phenyl)-chlorosilane (4)12 with 3 affords the ligand HSiP2S (5)
(Scheme 1a; 66%). A singlet corresponding to the two phos-
phines is observed by 31P NMR spectroscopy at 1.10 ppm, and
IR spectroscopy reveals an Si-H stretch at 2228 cm-1. Com-
plexation of 5 using FeCl2 and subsequent addition of excess
MeMgCl promotes cleavage of the Si-H and S-iPr bonds to
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RESULTS AND DISCUSSION
afford
the
thiolate-bound,
yellow-brown
complex
SH
SiPr
[(SiP2S)Fe]2(-N2) (6) in moderate yield (Scheme 1b). The
two previous reports of thiolate-coordinated Fe(N2) species
exhibit terminally bound N2 ligands.13,14
iPrBr
nBuLi
Ph
Ph
K2CO3
TMEDA
Compound 6 exhibits a bridging N2 ligand coordinated end-
on to each iron center. This is confirmed by X-ray crystallog-
raphy, which elucidates two similar but crystallographically
distinct iron centers in the solid state with a bridging N≡N
bond length of 1.138(2) Å (Fig. 1a, Table 1). A weak N2
stretch at 1888 cm-1 is observed in the solid-state by IR spec-
troscopy, consistent with the absence of a rigorous inversion
center. The 80 K 57Fe Mössbauer spectrum of 6 can be fit to
one unique iron center (δ = 0.447 mm/s, ΔEQ = 1.776 mm/s)
with an isomer shift similar to other five-coordinate, S = 1 iron
species ligated by related polyphosphine ligands.38 The room
temperature solution-state magnetic moment of compound 6
(eff = 4.8B) suggests an overall S = 2 species, and magnetic
susceptibility data collected between 25 ˚C and -75 ˚C obey
1
2
iPr2P
iPr2P
H
Si
Cl
iPr2P
iPr2P
SiPr
SiPr
H
Ph
Li
4
Si
Ph
· 0.5 TMEDA
(a)
5
3
Ph
iPr2
P
S
PiPr2
MeMgCl
5
0.5
+
Si Fe
S
N
N
Fe Si
FeCl2
P
iPr2
iPr2P
6
Ph
- 0.5 H2
- 0.5 N2
LiEt3BH, N2
THF
N
N
N
[Cp2Co][PF6]
or
[Cp*2Fe][PF6]
N
Li(THF)2
Ph
iPr2P
iPr2P
H
Fe
Si
H
Fe
Si
S
S
Ph
iPr2P
iPr2P
12-crown-4
8-H
7-H
Li(12-c-4)(THF)
N
N
iPr2P
H
Fe
Si
S
Ph
iPr2P
Figure 1. X-ray structures of (a) 6 (b) 7-H(crown) (c) 8-H. C-H
hydrogen atoms, solvent molecules, and the counterion of 7-
H(crown) are omitted for clarity. Ellipsoids are depicted at 50%
occupancy.
(b)
7-H(crown)
Scheme 1. (a) Synthesis of ligand precursor 5. (b) Metallation
procedure of 5 to yield diiron 6, and the synthesis of species 7-H,
7-H(crown) and 8-H.
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Table 1. Select bond metrics and angles of complexes 6, 7-H(crown) and 8a
1
2
d(Fe-N)
d(N-N)
d(Fe-P)
d(Fe-S)
d(Fe-H)
∠(P-Fe-P)
∠(P-Fe-S)
3
4
5
6
6
1.871(1)
1.138(2)
2.3059(6)
2.3132(7)
2.2314(5)
-
130.72(2)
110.53(2)
112.59(2)
1.865(1)
-
2.3444(6)
2.3189(5)
2.2323(6)
-
131.34(2)
110.75(2)
111.98(2)
7
8
9
7-H(crown)
8-H
1.810(4)
1.882(3)
1.117(6)
1.077(4)
2.16(1)
2.339(2)
-
146.5(3)
104.21(6)
106.3(3)
2.180(2)
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2.2541(8)
2.2422(8)
2.2185(7)
1.54(4)
138.70(3)
106.98(3)
111.19(3)
a Bond distances in listed in Å and bond angles listed in degre
the Curie-Weiss law. These data are consistent with a descrip-
tion for 6 featuring two S = 1 centers with strong ferromagnet-
ic exchange. Strong ferromagnetic coupling has been observed
in an S = 3/2, dinitrogen-bridged Fe(I)/Fe(II) species,12 and
weak ferromagnetic exchange in an S = 3 Fe(I)/Fe(I) species,39
both previously described by our lab. Alternatively, Fe-NN-
Fe species are known to exhibit antiferromagnetic coupling
between iron centers,2 or bear iron centers antiferromagneti-
um triethylborodeuteride. IR spectroscopy of compound 7-H
reveals three N2 stretches (2020, 1976, 1935 cm-1) that pre-
sumably arise from distinct coordination modes of the lithium
cation,41 and an Fe-H stretch at 1864 cm-1 that is absent in the
IR spectrum of 7-D. Treatment of 7-H with 12-crown-4 af-
fords
[(SiP2S)FeII(H)(N2)][Li(12-crown-4)(THF)]
(7-
H(crown)), which is isolated as a red solid. IR spectroscopy of
7-H(crown) shows a single N2 stretch at 1971 cm-1 ((Fe-H)
at 1886 cm-1), and XRD analysis of 7-H(crown) confirms its
relation to 7-H (Fig. 1b, Table 1).
2-
cally coupled to an S = 1 dinitrogen (N2 ) unit.40
To install the desired hydride ligand on iron, we found that
treatment of diiron 6 with lithium triethylborohydride affords
red, diamagnetic [(SiP2S)FeII(H)(N2)]Li(THF)2 (7-H). X-ray
diffraction data confirms the assignment of 7-H and reveals a
coordinated dinitrogen with an N-N bond length of 1.128(9)
Å. Although the hydride ligand could not be resolved from the
XRD analysis of 7-H, a wide P-Fe-P angle of 145.24(7)° is
consistent with the presence of a hydride located between the
two phosphine groups. Furthermore, a hydridic resonance
coupled to the two 31P nuclei is observed by 1H NMR spectros-
copy at -19.32 ppm (t, 3JH, P = 71.3 Hz), absent in the 1H NMR
spectrum of [(SiP2S)FeII(D)(N2)]Li(THF)2 (7-D); the latter
species is obtained in an analogous fashion to 7-H with lithi-
Room temperature cyclic voltammetry measurements of 7-
H in THF reveal a reversible oxidation event at -1.63 V vs.
Fc/Fc+, corresponding to the FeIII/II couple (Fig. 2a). Chemical
oxidation of the Fe(II) precursor 7-H with [Cp2Co][PF6] af-
fords a dark blue solution that persists for several hours at
room temperature, enabling the characterization of the title
complex (SiP2S)FeIII(H)(N2) (8-H). Neutral 8-H co-
crystallizes with crystals of the cobaltocene byproduct; despite
repeated attempts, 8-H and Cp2Co could not be separated,
owing to very similar solubility. Nevertheless, high quality X-
ray data confirms the solid-state structure of 8-H (Fig. 1, Table
1).
Figure 2. (a) Cyclic voltammetry of 7-H at various scan rates depicting the FeIII/II couple at -1.63 V vs. Fc/Fc+ (0.4 M [NBu4][PF6] in
THF). (b) Thin-film IR spectra of isotopologues 8-H and 8-D. (c) 57Fe Mꢀssbauer spectrum of 8-H collected at 80 K with a 50 mT applied
parallel field. (d) 77 K CW X-band EPR spectra of isotopologues 8-H and 8-D in 2-MeTHF.
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the observed correlation between spin state and Fe-P bond
length noted on related polyphophino Fe complexes. The
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hydride ligand can be located in the difference map of 8-H and
freely refined to an Fe-H bond length of 1.54(4) Å, compara-
ble to the crystallographically determined Fe-H bond lengths
of other paramagnetic, terminal iron hydride species.28a,b The
isotopologue (SiP2S)FeIII(D)(N2) (8-D) is prepared in an anal-
ogous fashion to 8-H via oxidation of 7-D. Spectroscopic
characterization of charge-neutral 8-H and 8-D is carried out
on samples generated in situ at -78 °C. Furthermore, manipu-
lations with 8-H and 8-D are carried out at low temperature
due to their thermal instability (vide infra).
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The IR spectrum of 8-H reveals N2 and Fe-H stretches at
2123 cm-1 and 1852 cm-1, respectively (Fig. 2b). Compared to
7-H(crown), there is a 152 cm-1 shift in the N2 stretching fre-
quency upon oxidation, consistent with a metal-centered oxi-
dation. The observed Fe-D IR stretch of 8-D is in good agree-
ment with the values predicted by the simple harmonic oscilla-
tor model (calculated: 1321 cm-1; observed: 1333 cm-1). Addi-
tionally, the 57Fe Mössbauer spectrum of 8-H is consistent
with a single iron-containing species (δ = 0.227 mm/s, ΔEQ =
1.734 mm/s); these parameters correspond to an approximate
fit due to asymmetric line broadening of the spectrum (Fig.
2c).44 Broad, paramagnetically shifted peaks are observed by
1H NMR spectroscopy of 8-H at -78 °C, and the S = ½ spin
state is corroborated by continuous-wave (CW) EPR spectros-
copy (Fig. 2d, vide infra).45
Figure 3. Spin density map of gas-phase optimized structure of
compound 8-H (Isovalue: 0.005 e-/Å3; M06-L functional:
def2tzvp (Fe), def2svp (all other atoms)). Atom colors: Fe (pur-
ple), S (yellow), H (white), P (orange), C (gray), N (blue), Si
(light blue).
Compared to the crystallographic data for 7-H(crown), the
structure of 8-H reveals elongation of the Fe-N≡N bond (from
1.810(4) Å to 1.882(3) Å) and contraction of the N≡N bond
(1.117(6) Å to 1.077(4) Å) upon oxidation, consistent with
poorer π-donicity from the more oxidized iron center. The
crystallographically determined N≡N bond length of 8-H of
1.077(4) Å is close to the bond length of free N2 (1.0975).42
Furthermore, the Fe-P bond lengths elongate upon conversion
of diamagnetic 7-H(crown) to doublet 8-H, consistent with
Figure 4. Field-dependent Q-band Davies ENDOR of (a) 8-H and (b) 8-D in 2-MeTHF with simulations. Simulation parameters: g = [2.07,
2.0475, 2.02]; A(31Pα) = ±[31, 36, 27] MHz; A(31Pβ) = ±[28, 25, 23] MHz; A(1H’) = ±[6.8, 10, 6.8] MHz. Fig. 4a was simulated with addi-
tional coupling to Fe-H (A(1H) = ±[15, 56, 58] MHz), whereas Fig. 4b was simulated with additional coupling to Fe-D (A(2H) = ±[2.3, 8.6,
8.9] MHz). Summation of individual component ENDOR simulations is displayed in red. Experimental conditions: microwave frequency
= 33.674 GHz; MW π pulse length = 40 ns; interpulse delay τ = 300 ns; 휋푅퐹 pulse length = 15 µs; TRF delay = 1 µs; shot repetition time
(srt) = 5 ms; temperature = 18.5 K; RF frequency randomly sampled. 77 K X-band CW EPR spectra of (c) 8-H and (d) 8-D in 2-MeTHF
with simulations. The CW EPR spectra were simulated with the same parameters as the corresponding ENDOR spectra.
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Figure 5. (a) 1H NMR spectra of 8-H in THF-d8 collected at -78 °C (top) and after stirring at 25 °C overnight (middle), with 1H NMR spec-
trum of 6 in THF-d8 (bottom). (b) UV-vis spectra showing the decay of 8-H and the growth of 6 at 50 °C. Spectra were collected every 30
min. (c) Plot of [8-H]-1 (M-1) versus time, using the UV-vis data shown in Fig. 5b. (d) Eyring plot of the conversion of 8-H to 6. (T in K, k
in M-1·min-1).
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In addition to the coordination of a strong field hydride lig-
and, the ferric Fe-N2 species 8-H is additionally stabilized by
partial spin delocalization onto the aryl thiolate ligand;46 this
attenuates the perturbation to Fe-N2 and Fe-H bonding upon
oxidation. There is significant contraction in the Fe-S bond of
the more oxidized 8-H (2.2185(7) Å) compared to 7-
H(crown) (2.339(2) Å), which contrasts with the observed
ligand; this coupling is also present in the spectra of 8-D.
ENDOR data on 8-D are additionally simulated (Fig. 4b) with
coupling to two 31P nuclei with identical parameters as that of
2
8-H and a H nucleus; almost complete disappearance of the
1H hydride signal is also evident. The 2H signal can be suitably
simulated by scaling A(1H) by the ratio of the 2H/1H gyromag-
netic ratios, (A(2H) = ±[2.3, 8.6, 8.9] MHz), and the X-band
CW EPR spectra of both 8-H and 8-D are simulated well by
using the hyperfine coupling constants obtained via ENDOR
spectroscopy (Fig. 4c,d).
elongation of the Fe-P bonds in the ferric analogue (Fe-Pavg
:
2.17 Å in 7-H(crown), 2.25 Å in 8-H).
The spin density map of the gas-phase optimized 8-H struc-
ture (M06-L: def2tzvp (Fe), def2svp (all other atoms))47 in-
deed reveals partial spin leakage onto the sulfur atom, with
lesser delocalization onto the other coordinated fragments
(Fig. 3). However, the calculated distribution of unpaired spin
suggests the oxidation is predominantly metal based, which is
consistent with the significant shift in N2 stretching frequency
observed between 7-H(crown) and 8-H. There is a calculated
spin density of 0.92 e- (58%) localized at Fe and 0.18 e- (11%)
at sulfur. Additionally, the calculated -0.029 e- (2%) localized
on the hydride fragment is consistent with the experimentally
observed hyperfine coupling obtained via EPR and ENDOR
spectroscopies (vide infra).
Pulse EPR data on an S = ½ freeze-trapped state of MoFe
nitrogenase, which has been observed during FeMoco-
catalyzed proton reduction9 and nitrogen fixation,48 are con-
sistent with the accumulation of hydride intermediates at the
1
cofactor under turnover conditions. Two H nuclei (H1: aiso
=
24.3 MHz, T = [-13.3, 0.7, 12.7] MHz; H2: aiso = 22.3 MHz, T
= [10.7, -12.3, 1.7] MHz) are assigned as Fe-(μ-H)-Fe moie-
1
ties.9a The H signals are believed to arise from bridging hy-
drides, as opposed to terminal, due to the approximate rhom-
bic symmetry of the dipolar tensor; a point-dipole model pre-
dicts a dipolar tensor of approximate axial symmetry for a
terminally bound hydride.49
EPR and ENDOR characterization of 8-H.
Decomposition of the hydridic 1H coupling constants of 8-H
to the isotropic and dipolar contributions yields an isotropic
value of aiso = ± 43 MHz and an approximately axial dipolar
tensor of T = ± [-28, 13, 15] MHz, in good agreement with the
predicted tensor symmetry for a terminal hydride. Notably,
this experimental aiso value indicates that ± 0.030 e- is local-
ized at the hydride ligand,50 which is consistent with the DFT-
calculated value of -0.029 e-. The greater aiso value observed
for 8-H compared to that of the hydride-bound form of FeMo-
co correlates with greater spin density localized at the hydro-
gen atom of 8-H, presumably due to a greater degree of spin
delocalization within the cofactor.
The 77 K X-band CW EPR spectrum of 8-H (Fig. 2d)
shows a rhombic signal, which is simulated with slight g ani-
sotropy (g = [2.07, 2.0475, 2.02]). Q-band Davies ENDOR
spectra collected at 18.5 K on 8-H across the EPR envelope
1
are simulated well (Fig. 4a) with coupling to the hydridic H
nucleus (A(1H) = ±[15, 56, 58] MHz) with a small Euler rota-
1
tion β = 25° of the H A tensor relative to the g-tensor, and
two similar but inequivalent 31P nuclei (A(31Pα) = ±[31, 36,
27] MHz; A(31Pβ) = ±[28, 25, 23] MHz). Additional coupling
1
to a H nucleus is also observable (A(1H’) = ±[6.8, 10, 6.8]
MHz), likely arising from coupling to hydrogen atom(s) of the
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N
N
1
2
3
4
5
6
7
8
Bimolecular H2 elimination from 8-H.
- H2 , N2
THF
Although FeIII(H)(N2)(thiolate) 8-H could be spectroscopi-
cally characterized, it is thermally unstable in solution and
liberates H2. Monitoring a THF-d8 solution of 8-H at room
temperature by 1H NMR spectroscopy reveals the near quanti-
tative conversion of dark blue 8-H to yellow-brown 6 over-
night, which corresponds to formal loss of an H• and half an
N2 molecule per Fe (Fig. 5a). H2 liberation was confirmed by
GC analysis of the headspace.
[Fe]
N
N
[Fe]
[Fe]
H
8-H
6
- H2
- N2
+ N2
9
H
[Fe]
N2
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Monitoring the decay of 8-H at 50 °C by UV-vis spectros-
copy shows that 8-H, with an absorption at 607 nm, decays in
concert with the growth of 6 (Fig. 5b). The decay of 8-H fol-
lows second-order kinetics (Fig. 5c), consistent with a bimo-
lecular H2 reductive elimination pathway to all-ferrous diiron
6. While we think such a pathway is most plausible, other
scenarios, such as a fast monomer-dimer pre-equilibrium with
release of H2 from a dimeric species, could also be consistent
with the observed rate dependence on [8-H]. The UV-vis
timecourse data display isosbestic behavior, and the absence
of observable intermediates indicates that transformations
prior to the rate determining step are both endergonic and re-
versible. At 25 °C, the conversion of 8-H to 6 proceeds with a
second-order rate constant of k = 0.068 M-1·min-1. Monitoring
the decay of 8-D reveals a kinetic isotope effect of 1.7 at 25
°C, suggesting a role for the hydride ligand in the transition
state of the rate determining step. An early transition state
featuring substantial Fe-H character in a bimolecular reductive
elimination step is consistent with these data.51
[Fe]
H
[Fe]
Figure 6. Proposed mechanism for the conversion of 8-H to 6,
where [Fe] = (SiP2S)Fe.
Thermochemical parameters for the Fe-H subunit in 7-
H(crown) and 8-H.
The bimolecular elimination of H2 from 8-H is indicative of
an Fe-H bond with a relatively low homolytic bond strength,
consistent with spin density localized on the hydride ligand as
1
evidenced by the large coupling constants to the hydridic H
nucleus.27,28 Hence, our use of the term “hydride” masks radi-
cal H• character present in the Fe-H subunit. We therefore
sought to explore this idea in greater detail.
The immediate product of H• loss from 8-H is the iron(II)
species (SiP2S)Fe(N2), but this species is not experimentally
observed, presumably because the formation of diiron 6 is too
facile. However, we have been able to generate and spectro-
scopically characterize the one- and two-electron reduced rela-
tives of this species, (SiP2S)Fe(N2)n- (n = 1, 2, Scheme 2a).
Under an N2 atmosphere, reduction of 6 with excess Na(Hg) in
THF followed by addition of 12-crown-4 affords the S = ½
iron(I) species [(SiP2S)Fe(N2)][Na(12-crown-4)2] (9) as a dark
red solid ((N2) = 1963 cm-1). The gas-phase optimized struc-
ture of 9 (M06-L: def2tzvp (Fe), def2svp (all other atoms))
indicates less unpaired spin density localized at the sulfur at-
om (0.02 e-, 1% overall spin density) than for 8-H (0.18 e-,
11% overall spin density). Alternatively, treatment of 6 with
excess potassium metal in THF under N2 provides the dianion-
Eyring plot analysis of the conversion of 8-H to diiron 6
provides the following activation parameters: ∆H‡ = 31(4)
kcal/mol; ∆S‡ = 39(13) cal/(mol·K); ∆G‡ = 19 kcal/mol (25
°C) (Fig. 5d). The large and positive ∆S‡ value is surprising as
the transition state of two iron centers interacting in an ordered
manner is likely to incur high entropic cost. For comparison,
Wayland’s classic study of Rh(II)-porphyrin metalloradical M-
H/M-R species shows that bimolecular release of R-H pro-
vides negative entropies of activation correlated with highly
ordered, tertiary transition states (e.g., {[Rh]-HR-[Rh]}‡).52
For the present iron system, we suggest that the large and
positive ∆S‡ value may be rationalized if a requisite N2-
dissociation step precedes the rate determining step via pre-
equilibration of an N2-bound and an N2-dissociated state. A
plausible mechanism consistent with this scenario and the data
in hand is depicted in Fig. 6. Accordingly, an N2 ligand of 8-H
first dissociates, giving rise to free N2 and an unobserved, re-
active hydride intermediate, (SiP2S)FeIIIH. This Fe-H interme-
diate is intercepted by 8-H, present at much higher concentra-
tion, via a transition state containing one N2 ligand and two
Fe-H subunits, then decaying to H2 and the final product,
diiron 6. While in principle it may also be possible for two
molecules of (SiP2S)FeH to react directly to form H2 without a
coordinated N2 ligand in the transition state, we favor the sce-
nario shown in Fig. 6.
ic
and
diamagnetic
iron(0)
complex
[(SiP2S)Fe(N2)][K(THF)x]2 (10) as a dark brown species
((N2) = 1805 cm-1). The availability of 9 and 10 provides
access to additional data needed to assess the thermochemical
properties of the hydride ligand in 8-H.
Complex 9 shows a reversible FeII/I redox couple in THF at
-1.75 V vs. Fc/Fc+, but data collection in MeCN reveals in-
stead an irreversible oxidation event at -1.71 V vs. Fc/Fc+,
presumably due to rapid solvent substitution for the N2 ligand
upon oxidation to produce (SiP2S)Fe(MeCN), 11 (see below).
Additionally, a reversible reductive couple (FeI/0) is observed
at -2.94 V vs. Fc/Fc+ in MeCN, corresponding to the conver-
sion of [(SiP2S)Fe(N2)]- to [(SiP2S)Fe(N2)]2-.
Cyclic voltammograms of the FeII(H)(N2)(thiolate) 7-
H(crown) derivative in MeCN shows an irreversible oxidative
feature at -0.58 V vs. Fc/Fc+. This feature is significantly
shifted from the reversible oxidative FeIII/II couple that is rec-
orded in THF (-1.63 V vs. Fc/Fc+). The irreversibility and
extreme solvent dependence of the oxidation potential of 7-
H(crown) indicates a process that is more complex than outer-
sphere electron transfer may be occurring in MeCN. Thus, for
the thermochemical calculations presented below, we have
utilized -1.63 V vs. Fc/Fc+ as the value for the 7-H(crown)/8-
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Journal of the American Chemical Society
H couple, because it is a well-defined, reversible feature as
obtained in THF.
Utilizing available thermodynamic values that relate H+, H•
and H– in MeCN (eq. 1, 2),35b the upper bounds for the free
energies of H+/H•/H– transfer from 7-H(crown) and H+/H•
transfer from 8-H can be related (Fig. 7, eq. 3-6). The approx-
imated upper bounds for homolytic and heterolytic values of
Fe-H bond cleavage from 7-H(crown) and 8-H are shown in
Table 2. Although absolute free energy values for the H+/H•/
H– transfers (∆G) are not established, the difference between
any two such free energies (∆∆G) can be determined from eq.
1-6. The Fe-H bond of 7-H(crown) is estimated to have a
bond dissociation free energy (BDFE) of < 57 kcal/mol and a
pKa of < 53. Compared to the Fe-H bond of 7-H(crown), the
Fe-H bond of 8-H is considerably more acidic, by more than
22 pKa units, with an estimated upper bound of pKa < 30.
1
2
3
4
5
6
7
8
Na(12-c-4)2
N
N
Na(Hg), N2
iPr2P
iPr2P
Fe
Si
S
Ph
12-crown-4
9
9
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
K2(THF)x
N
N
K (s)
N2
iPr2P
iPr2P
Fe
Si
S
Ph
(a)
10
CD3
C
N
1 atm CO2
MeCN-d3
- [HCO2][Li(12-c-4)(THF)]
iPr2P
iPr2P
Fe
Si
7-H(crown)
S
Ph
Table 2. Oxidation potentials and thermochemical parameters
pertaining to Fe-H bond cleavage of 7-H(crown) and 8-H
(b)
11-d3
Scheme 2. (a) Synthesis of 9 and 10 via reduction of 6. (b) Hy-
dride transfer upon treatment of 7-H(crown) with CO2.
Exposing a degassed MeCN-d3 solution of 7-H(crown) to
an atmosphere of CO2 results in complete hydride transfer to
yield the formate salt [Li(12-crown-4)][HCO2] and the solvent
complex (SiP2S)Fe(CD3CN) (11-d3, Scheme 2b). Defined as
the heterolytic dissociation energy of M-H to M+ and H– (∆GH-
),35b the hydricity of 7-H(crown) in MeCN must therefore be
close to, or less than, the hydricity of formate (∆GH- = 44
kcal/mol in MeCN).35b Loss of H– from 7-H(crown) should
generate (SiP2S)Fe(N2), which is not observed due to facile
ligand substitution by MeCN to produce the MeCN adduct 11-
d3 instead. Thus, the hydricity of 7-H(crown) can be estimated
to have an upper bound of ~44 kcal/mol in MeCN.53 There are
two previous reports of the hydricity of terminally-bound Fe-H
a values in kcal/mol b potentials reported in V vs. Fc/Fc+
The estimated upper bound for the BDFE of the Fe-H bond
in FeIII(H)(N2)(thiolate) 8-H of 56 kcal/mol is in close agree-
ment with the DFT-predicted value of 55.6 kcal/mol (M06-L;
def2tzvp (Fe), def2svp (all other atoms) in MeCN solvation).
It has been previously suggested that M-H bonds with bond
dissociation enthalpies of < 56 kcal/mol56 are competent to
release H2 at room temperature (BDFE(H2) in MeCN = 102.3
kcal/mol).57 Interestingly, eq. 6 indicates that the Fe-H BDFE
of Fe(II) 7-H(crown) is only 2.8 kcal/mol greater than that of
Fe(III) 8-H in THF, but 7-H(crown) is observed to be stable
in solution at room temperature, whereas 8-H undergoes clean
conversion to 6 with release of H2.
,
species.54 55 A study by our lab of a related five-coordinate
iron(II) hydride complex, (SiP3)Fe(H)(H2), estimated a hy-
dricity of 54.3 ± 0.9 kcal/mol in MeCN,54 significantly less
hydridic than 7-H(crown), likely in part reflective of the dif-
ferent charges.
Although there is only a small difference in BDFE between
the ferrous and ferric hydrides, the Keq for homolytic Fe-H
bond cleavage is thus about 100 times greater at room temper-
ature for 8-H than 7-H(crown); this is further biased by the
thermodynamic stability afforded by the formation of 6 rather
than (SiP2S)FeN2, the direct product of Fe-H bond homolysis
from 8-H. Additionally, electrostatic penalties associated with
a biomolecular reaction between two anionic species may be a
kinetic impediment to H2 release. Finally, assuming N2 disso-
ciation would need to precede an H2 evolution in 7-H(crown),
as we have suggested is likely for 8-H (Fig. 6), the stability of
7-H(crown) may be correlated to a greater kinetic barrier to
N2 dissociation from 7-H(crown), which features a more
strongly coordinated N2 ligand.
+ e-
+ e-
-
2-
(SiP2S)FeN2
(SiP2S)FeN2
9
(SiP2S)FeN2
10
- e-
- e-
-
+
H
+
H -
H
-
+
- H• + H•
- H• + H•
+
+
+
H
H -
H
-
+
+ e-
- e-
-
(SiP2S)Fe(H)N2
8-H
(SiP2S)Fe(H)N2
7-H(crown)
Figure 7. Thermochemical scheme relating H+, H• and H– trans-
fers from 7-H(crown) and 8-H.
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CONCLUSION
1
The synthesis of a trivalent, thiolate-FeIII(H)(N2) complex
has been accomplished and serves as an unusual Fe-N2 species
from the perspective of both oxidation state and the presence
of both hydride and thiolate ligands. Its spin is primarily iron-
centered but some leakage onto the thiolate ligand likely con-
tribute to its modest stability. This is the first example of a
ferric dinitrogen complex, and it is sufficiently persistent at
low temperature to be amenable to characterization by X-ray
crystallography and various spectroscopic techniques, includ-
ing CW EPR and ENDOR spectroscopies. Additionally, we
have demonstrated via kinetic measurements that this ferric
Fe-H complex undergoes bimolecular reductive elimination to
liberate H2 with a primary kinetic deuterium isotope effect. To
the best of our knowledge, this is the first example of a well-
defined, bimolecular H2 elimination process from a terminal
Fe-H species. Surprisingly, the entropy of activation for H2
elimination is positive (∆S‡ = 39(13) cal/(mol·K)), whereas a
large and negative value would be anticipated based solely on
the prediction of a highly ordered transition state ([Fe-HH-
Fe]‡). Pre-dissociation of N2 from thiolate-FeIII(H)(N2) to af-
ford thiolate-FeIII(H), which is then captured by thiolate-
FeIII(H)(N2) to proceed to an H-H bond-forming transition
state, is invoked to accommodate the collective data. Finally,
synthetic access to thiolate-FeII(H)(N2) and thiolate-
FeIII(H)(N2) species, in addition to the reduced derivatives
thiolate-FeI(N2)- and thiolate-Fe0(N2)2-, has provided access to
the physical detail needed to estimate important thermochemi-
cal H+, H•, and H– parameters of broad current interest, and
within an S = ½ Fe system that evolves H2.
2
3
4
5
6
7
8
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ASSOCIATED CONTENT
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Supporting Information
Experimental procedures and compound characterization data.
The Supporting Information is available free of charge on the
ACS Publications website at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
jpeters@caltech.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
The authors are grateful to the NIH for support via grant GM-
O70757 and to the National Science Foundation for support of the
Caltech EPR Facility via grant NSF-153194. N.X.G. acknowledg-
es an NSF Graduate Research Fellowship. Dr. Gaël Ung provided
technical assistance towards the synthesis of 2 and 3. Dr. Michael
Takase and Larry Henling are thanked for providing crystallo-
graphic assistance. We thank Dr. Jay Winkler for helpful discus-
sions.
24 Bullock, R. M.; Appel, A. M.; Helm, M. L. Chem. Commun. 2014,
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44 An asymmetric 57Fe Mössbauer spectrum has been similarly ob-
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See ref. 28c; (ii) polyphosphine-bound iron complexes studied by
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10
11
12
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15
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33
34
35
36
37
38
39
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no, T. J.; Mankad, N. P. J. Am. Chem. Soc. 2013, 135, 17258-17261.
(b) Parmelee, S. R.; Mazzacano, T. J.; Zhu, Y.; Mankad, N. P.; Keith,
J. A. ACS Catal. 2015, 5, 3689-3699.
45 Unless otherwise noted, [Cp*2Fe][PF6] was utilized as the oxidant
to prepare EPR and ENDOR samples of 8-H and 8-D. Decamethylfer-
rocenium was favored over cobaltocenium because the resultant
Cp*2Fe byproduct is diamagnetic. However, the 77 K X-band EPR
spectrum obtained via oxidation with [Cp2Co][PF6] was indiscernible
from that obtained utilizing [Cp*2Fe][PF6] as the oxidant (see SI for
complete details).
31 Koelle, U.; Ohst, S. Inorg. Chem. 1986, 25, 2689-2694.
32 For related examples of unimolecular reductive elimination from
multimetallic, first-row transition metal hydride complexes, see: (a)
Vollhardt, K. P. C.; Cammack, J. K.; Matzger, A. J.; Bauer, A.;
Capps, K. B.; Hoff, C. D. Inorg. Chem. 1999, 38, 2624-2631. (b) Lee,
Y.; Anderton, K. J.; Sloane, F. T.; Ermert, D. M.; Abboud, K. A.;
García-Serres, R.; Murray, L. J. J. Am. Chem. Soc. 2015, 137, 10610-
10617. (c) Manz, D.-H.; Duan, P.-C.; Dechert, S.; Demeshko, S.;
Oswald, R.; John, M.; Mata, R. A.; Meyer, F. J. Am. Chem. Soc.
2017, 139, 16720-16731. (d) Bellows, S. M.; Arnet, N. A.;
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R.; Holland, P. L. J. Am. Chem. Soc. 2016, 138, 12112-12123. (e) Yu,
Y.; Sadique, A. R.; Smith, J. R.; Dugan, T. R.; Cowley, R. E.; Bren-
nessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland,
P. L. J. Am. Chem. Soc. 2008, 130, 6624-6638. (f) Ding, K.; Brennes-
sel, W. W.; Holland, P. L. J. Am. Chem. Soc. 2009, 131, 10804-
10805.
46 (a) Chang, Y.-H.; Su, C.-L.; Wu, R.-R.; Liao, J.-H.; Liu, Y.-H.;
Hsu, H.-F. J. Am. Chem. Soc. 2011, 133, 5708-5711. (b) Broering, E.
P.; Dillon, S.; Gale, E. M.; Steiner, R. A.; Telser, J.; Brunold, T. C.;
Harrop, T. C. Inorg. Chem. 2015, 54, 3815-3828.
47 The gas-phase DFT-optimized structure of 8-H is calculated to
have an N2 stretch of 2208 cm-1 and an Fe-H stretch of 1907 cm-1.
48 Lukoyanov, D.; Khadka, N.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L.
C.; Hoffman, B. M. J. Am. Chem. Soc. 2016, 138, 10674-10683.
49 Kinney, R. A.; Saouma, C. T.; Peters, J. C.; Hoffman, B. M. J. Am.
Chem. Soc. 2012, 134, 12637-12647.
50 The spin density at the hydride ligand is calculated using the aiso
value for a free hydrogen atom of 1420 MHz, see: Wittke, J. P.;
Dicke, R. H. Dicke Phys. Rev. 1956, 103, 620-631.
33 The elimination of H2 from two M-H species (2 M-H → H2 + M2)
may also occur through a M-M bonded species, whereby the interme-
diate prior to reductive elimination is a dinuclear M2H2 species, see:
(a) Halpern, J.; Pribaníc, M.; Inorg. Chem. 1970, 9, 2616-2618. (b)
Ungváry, F.; Markó, L. J. Organomet. Chem. 1969, 20, 205-209. (c)
Trinquier, G.; Hoffmann, R. Organometallics 1984, 3, 370-380.
34 It has been reported that CpFe(CO)2H (FpH) undergoes reductive
elimination of H2 to yield {CpFe(CO)2}2. See: Green, M. L. H.;
Street, C. N.; Wilkinson, G. Z. Naturforschg. 1959, 14, 738. However,
subsequent studies provided evidence to suggest that the H2 evolution
was catalyzed by the presence of a trace oxidant. See: Shackleton, T.
A.; Mackie, S. C.; Fergusson, S. B.; Johnston, L. J.; Baird, M. C.
Organometallics 1990, 9, 2248-2253. The reverse bimolecular oxida-
tive addition of H2 (Fp2 + H2 → 2 FpH) is known to occur at high
pressures of H2 (Fp2 + H2 → 2 FpH). See: Chang, B.-H.; Coil, P. C.;
Brown, M. J.; Barnett, K. W. J. Organomet. Chem. 1984, 270, C23-
C25.
51 Gómez-Gallego, M.; Sierra, M. A. Chem. Rev. 2011, 111, 4857-
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52 Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113,
5305-5311.
53 Following the reaction of 7-H(crown) with CO2 by 1H NMR re-
veals direct conversion of 7-H(crown) to 11; a N2-bound intermediate
is not observed in the reaction mixture. It may be possible that the
thermodynamic stability afforded by MeCN coordination drives the
hydride transfer reaction forward, even if the hydricity of 7-H(crown)
is slightly greater than 44 kcal/mol. Thus, 44 kcal/mol is determined
to be an approximate upper bound for the hydricity of 7-H(crown).
54 Fong, H.; Peters, J. C. Inorg. Chem. 2015, 54, 5124-5135.
55 Estes, D. P.; Vannucci, A. K.; Hall, A. R.; Lichtenberger, D. L.;
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56 Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D. J. Am. Chem. Soc.
1990, 112, 5657-5658.
57 Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110,
6961-7001.
35 (a) Waldie, K. M.; Ostericher, A. L.; Reineke, M. H.; Sasayama, A.
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36 (a) Whited, M. T.; Mankad, N. P.; Lee, Y.; Oblad, P. F.; Peters, J.
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37 McGinley, P. L.; Koh, J. T. J. Am. Chem. Soc. 2007, 129, 3822-
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38 Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem.
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39 Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.;
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40 Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland, P. L.;
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10192.
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