Tripodal P3XFe-N2Complexes (X = B, Al, Ga): Effect of the Apical Atom on Bonding, Electronic Structure, and Catalytic N2-to-NH3Conversion

Article abstract of DOI:10.1021/acs.inorgchem.0c03354

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P3X ligands (X = B, C, Si) have previously been shown to mediate catalytic N2-to-NH3 conversion (N2RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P3B) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and N2RR activity, Fe-N2 complexes supported by analogous group 13 tris(phosphino)alane (P3Al) and tris(phosphino)gallane (P3Ga) ligands are synthesized. The series of P3XFe-N2[0/1-] compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N2 activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)2][P3XFe-N2] (X = Al, Ga) with excess acid/reductant in the form of HBArF4/KC8 generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH3 per Fe, respectively. Similarly, the use of [H2NPh2][OTf]/Cp*2Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH3. Preliminary reactivity studies confirming P3XFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N2RR may be responsible for the attenuated yields of NH3 observed for P3AlFe and P3GaFe relative to P3BFe.

Full text of DOI:10.1021/acs.inorgchem.0c03354

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X
Tripodal P3 Fe−N2 Complexes (X = B, Al, Ga): Effect of the Apical
Atom on Bonding, Electronic Structure, and Catalytic N2‑to-NH3
Conversion
Javier Fajardo, Jr. and Jonas C. Peters*
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ABSTRACT: Terminal dinitrogen complexes of iron ligated by tripodal,
X
tetradentate P3 ligands (X = B, C, Si) have previously been shown to
mediate catalytic N2-to-NH3 conversion (N2RR) with external proton and
electron sources. From this set of compounds, the tris(phosphino)borane
B
(P3 ) system is most active under all conditions canvassed thus far. To
further probe the effects of the apical Lewis acidic atom on structure,
bonding, and N2RR activity, Fe−N2 complexes supported by analogous
Ga
group 13 tris(phosphino)alane (P3Al) and tris(phosphino)gallane (P3
)
ligands are synthesized. The series of P3 Fe−N2[0/1−] compounds (X = B,
Al, Ga) possess similar electronic structures, degrees of N2 activation, and
geometric flexibility as determined from spectroscopic, structural,
electrochemical, and computational (DFT) studies. However, treatment
X
X
of [Na(12-crown-4)2][P3 Fe−N2] (X = Al, Ga) with excess acid/
reductant in the form of HBArF /KC8 generates only 2.5 0.1 and 2.7
4
0.2 equiv of NH3 per Fe, respectively. Similarly, the use of [H2NPh2][OTf]/Cp*2Co leads to the production of 4.1 0.9 (X = Al)
X
and 3.6 0.3 (X = Ga) equiv of NH3. Preliminary reactivity studies confirming P3 Fe framework stability under pseudocatalytic
conditions suggest that a greater selectivity for hydrogen evolution versus N2RR may be responsible for the attenuated yields of NH3
B
observed for P3AlFe and P3GaFe relative to P3 Fe.
component to gaining a better understanding of key N2RR
catalyst design principles.
In this context, extensive investigations carried out by our
INTRODUCTION
■
While it was not long ago that well-defined molecular N2-to-
NH3 conversion (N2RR) catalysts were scarce and limited to
Fe- and Mo-based transition metal complexes,1−3 significant
progress in the development of N2RR catalysts in recent years
has resulted in their increase in number, diversity, and
efficiency.4 These advancements have largely been made
possible by a combination of systematic reactivity studies, in-
depth mechanistic investigations, and structure−activity
surveys. Notably, the latter have been instrumental in
improving the performance of Fe and Mo N2RR catalysts, as
well as in the discovery of new catalytic systems based on other
metals. For instance, building upon their first report on
(PNP)Mo-mediated N2RR,2 Nishibayashi and co-workers have
performed systematic pincer ligand variations to develop a
number of related (PXP)Mo complexes (X = N, P, C) capable
of catalyzing N2RR with more mild proton and electron
sources, higher turnover numbers, and/or greater selectivities.5
Similarly, metal center substitution of known Fe N2-to-NH3
conversion catalysts by V6 or the nonbiologically relevant
metals Co,7 Ru,8 and Os8 has led to new catalytic systems
supported by the same ligand platforms and featuring
comparable N2RR efficiencies. These systematic comparative
studies (see Chart 1 for group 8 systems) are a critical
group on the dinitrogen coordination chemistry and N2RR
Chart 1. Group 8 Metallotrane Systems for Exploring
Factors That Impact Catalytic N2RR Efficacy
Received: November 12, 2020
Published: January 7, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03354
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 1220−1227
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Inorganic Chemistry
catalysis by tripodal P3 -ligated Fe complexes (P3
pubs.acs.org/IC
Article
X
B
X
[0/1−]
=
=
Scheme 1. Synthesis of P3 Fe−N2
Complexes
(o-iPr2PC6H4)3B; P3 = [(o-iPr2PC6H4)3C]−; P3
C
Si
[(o-iPr2PC6H4)3Si]−) have established that the identity of the
apical atom (X) influences the observed N2RR versus HER
(hydrogen evolution reaction) selectivity.3,9−13 Notably, from
B
this series, the P3 Fe system is the most efficient N2RR catalyst
under all conditions canvassed thus far, highlighting the
privileged role of the group 13 Lewis acidic borane. This
enhanced efficacy is in part attributed to the electron-accepting
nature and geometric flexibility of the Fe → B interaction,
which helps stabilize both low-valent and Fe−N multiply
B
bonded species proposed to be intermediates of a P3 Fe-
mediated N2 fixation cycle.14−20
Given the importance of the boron center, a common
question that has surfaced concerns the effect of X(III) apical
X
atom substitution in the P3 Fe framework, with respect to
bonding, electronic structure, and N2RR activity (Chart 1). To
address this issue, herein we report the synthesis and
X
X
[0/1−]
characterization of a series of P3 Fe−Br and P3 Fe−N2
compounds supported by analogous group 13 tris(phosphino)-
21
alane (P3 = (o-iPr2PC6H4)3Al) and tris(phosphino)gallane
(P3Ga = (o-iPr2PC6H4)3Ga)22 ligands. Structural, spectroscopic,
electrochemical, and computational studies reveal that all three
Al
X
P3 Fe systems (X = B, Al, Ga) possess similar electronic
structures, degrees of N2 activation, and geometric flexibility.
However, P3AlFe and P3GaFe display significantly lower N2RR
B
efficiencies than P3 Fe. Preliminary reactivity studies show that
X
all P3 Fe frameworks are robust under the catalytic conditions
employed, suggesting that a greater selectivity for competing
HER may be responsible for the attenuated NH3 yields
observed for the former.
RESULTS AND DISCUSSION
Synthesis and Characterization of P3 Fe−N2
Complexes. As an entry point into the desired P3 Fe
■
X
[0/1−]
X
dinitrogen chemistry, we sought to prepare an Fe(I) halide
B
precursor similar to our approach in the related P3 Fe system.
B
Whereas P3 Fe−Br (2a) can readily be prepared by heating a
14
mixture of P3GBa(1a), Fe(0), and FeBr2 to 90 °C in THF, P3
Al
X
(1b) and P3 (1c) proved thermally unstable under these
conditions. Instead, stirring 1b and 1c with FeBr2 in benzene,
followed by one-electron reduction with sodium amalgam
dinitrogen complexes of the form P3 Fe−N2 (X = Al (3b), X =
Ga (3c)) (Scheme 1). 3b and 3c display intense, diagnostic
infrared (IR) absorption bands at ν(NN) = 2003 and 1997
cm−1, respectively, where the end-on N2 ligand is only slightly
X
(Na(Hg)), afforded the corresponding P3 Fe−Br complexes
B
2b and 2c as bright green solids in moderate isolated yield
more activated than in the related P3 Fe−N2 (3a, 2011
(Scheme 1). Like P3 Fe−Br, both P3 Fe−Br derivatives feature
paramagnetically shifted 1H NMR resonances, solution
magnetic moments of 4.3 μB (2b) and 4.2 μB (2c), and low-
temperature rhombic EPR spectra (toluene glass, 10 K),
consistent with an S = 3/2 ground state.
cm−1)14 and iron alumatrane (AltraPhos)Fe−N2 (6, 2010
cm−1, AltraPhos = Al[N(o-C6H4NCH2PiPr2)3])24 complexes
and is moderately labile under vacuum.
B
X
The terminal nature of the N2 moiety was further
corroborated by X-ray diffraction (XRD) analysis of single
crystals of 3b and 3c (Figure 1). The dinitrogen ligand in 3b
and 3c exhibits a d(N−N) of 1.0616(17) and 1.118(6) Å,
respectively, consistent with minimal N2 activation. In the solid
state, 3b and 3c feature two short (3b: 2.3781(4), 2.3843(4)
Å; 3c: 2.3898(5), 2.3969(3) Å) and one comparable (3b:
2.4220(4) Å; 3c: 2.4342(5) Å) Fe−P contact compared to 2b
and 2c. This contraction in the Fe−P bonds is in line with
increased π-backdonation from the Fe center to the
phosphines upon reduction, as well as the S = 1 ground
state adopted by 3b (3.1 μB) and 3c (2.8 μB) in room
temperature C6D6 solutions. XRD characterization of 3b and
3c allows for a structural comparison across the series of
compounds 2−4 (vide infra) that span the formal oxidation
The long Fe−P bond lengths in the solid-state structures of
2b (2.4603(3) Å) and 2c (2.4690(3) Å) are also consistent
with this high-spin configuration assignment (Figure 1). It is
noteworthy that the Fe−Br bond in 2b and 2c remains intact,
Al
Ga
as previous examples of P3 and P3 coordination to CuCl
and AuCl(SMe2) have instead yielded the zwitterionic
complexes P3Al−ClCu and P3X−ClAu (X = Al, Ga) resulting
from halide abstraction by the apical X atom.21,22 For
B
comparison, reaction of P3 with these metal precursors
instead yields P3 Cu−Cl and P3 Au−Cl.23 These differences in
B
B
reactivity suggest that Al(III) and Ga(III) are stronger Lewis
acids than B(III) within the P3 framework.
X
Reduction of 2b and 2c with Na(Hg) in benzene under a
nitrogen atmosphere results in the formation of mononuclear
B
states Fe(I) to Fe(−I). This contrasts with the P3 Fe system,
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Figure 1. Solid-state structures of 2b, 3b, and 4b with thermal ellipsoids set at 50% probability. Hydrogen atoms and cocrystallized solvents are
omitted for clarity. The XRD structures of 2c, 3c, and 4c are analogous and are provided in the Supporting Information. See Table 1 for select
structural data.
where the XRD structure of 3a could not be authenticated due
Fe−N multiply bonded species that may be traversed along an
Fe-mediated nitrogen fixation cycle. Storage of additional
electron density in the Fe−B manifold stabilizes the low-valent
states, and shuttling of the iron center between trigonal
bipyramidal and pseudotetrahedral geometries affords access to
intermediates with different electronic structures.14−20 Accord-
to severe twinning.14
X
X
Treatment of P3 Fe−Br or P3 Fe−N2 with excess Na(Hg)
in the more polar solvent THF leads exclusively to the anionic
X
S = 1/2 complex [Na(THF)3][P3 Fe−N2] (X = Al (4b), X =
B
Ga (4c)) (Scheme 1). Like [Na(THF)3][P3 Fe−N2] (4a), 4b
14,18
[0/1−/2−]
and 4c feature a phase-dependent Lewis acid−base interaction
between Nβ and the Na+ counterion that can be monitored by
IR spectroscopy.
ingly, the catalytically relevant species Fe−N2
,
FeNNH2
,
17−19 and FeN[1+]18 have proven accessible
[1+/0]
B
on the P3 scaffold. Given the significant role of the Fe → B
interaction in this respect, we sought to determine whether the
Fe→Al and Fe→Ga bonds would behave similarly.
Whereas in the solid state 4b and 4c exhibit a single ν(NN)
at 1883 and 1879 cm−1, in THF two bands (4b: 1879, 1922
cm−1; 4c: 1878, 1920 cm−1) are observed as a result of
solvation of the Na+ ion to yield some population of the free
B
[0/1−]
The bonding and electronic structures of P3 Fe−N2
[0/1−]
and (AltraPhos)Fe−N2
have previously been elucidated
anion P3 Fe−N2 . Encapsulation of Na+ by treatment of 4b
and 4c with two equivalents of 12-crown-4 (12-c-4) yields
[Na(12-c-4)2][P3AlFe−N2] (5b) and [Na(12-c-4)2][P3GaFe−
N2] (5c), respectively, which give rise to only the higher
energy vibration. Compound 4a (1877, 1918 cm−1),14
compound 5a (1918 cm−1),14 and the reduced iron
alumatrane, [K(18-crown-6)][(AltraPhos)Fe−N2] (7, 1925
cm−1),25 display similar degrees of N2 activation to 5b and 5c.
The intimate ion pair in salts 4b and 4c was also
authenticated by their corresponding solid-state structure,
where the capping Na+ is coordinated by three additional THF
molecules (Figure 1). As was the case in going from the
formally Fe(I) to Fe(0) complexes, the Fe−P bond lengths in
4b (2.2751(6) Å) and 4c (2.2824(5) Å) are significantly
contracted from those of 3b and 3c, respectively. Increased π-
backbonding into the N2 unit also leads to shorter Fe−N (4b:
1.769(3) Å; 4c: 1.759(5) Å) and longer N−N (4b: 1.134(4)
Å; 4c: 1.141(4) Å) distances in 4b and 4c. Complexes 4a
(d(Fe−N) = 1.775 Å; d(N−N) = 1.149 Å) and 7 (d(Fe−N) =
1.783 Å; d(N−N) = 1.135 Å) possess comparable Fe−N2
bond metrics, in line with their similar IR profiles.
through spectroscopic and computational methods.14,24−26 In
short, the bonding in these complexes is best described as
involving an Fe → X(III) dative interaction where the apical
Lewis acidic atom serves to lower the energy of the σ(Fe−X)
X
−
2
2
− y2
orbital of Fe dz and X pz parentage below that of the dxy, x
set in a typical trigonal bipyramidal orbital scheme. Therefore,
reduction of 3 to 4a/5a and 6 to 7 is primarily Fe centered,
giving rise to formally Fe(−I) species with electronic
3
configurations (dxz, yz)4(σ(Fe−X))2(dxy, x − y ) and a substan-
2
2
tial Fe-to-X backbond.27
A consequence of having three electrons in a set of
2
− y2
degenerate orbitals of dxy and dx
parentage is a Jahn−
Teller active state. Distortion from C3 symmetry is clearly
reflected by the asymmetry of the P−Fe−P angles in the solid-
state structures of 4a (two independent molecules present in
asymmetric unit; 107.3°, 110.3°, 134.6° and 113.4°, 114.1°,
124.7°), 5a (105.4°, 112.3°, 135.0°), and 7 (107.1°, 111.9°,
131.1°). In contrast, 4b and 4c crystallize in the trigonal space
group R3, being 3-fold symmetric about the apical axis defined
by the X−Fe−N2 unit. While this may suggest that 4b and 4c
possess an electronic structure that differs from that of 4a/5a,
density functional theory (DFT) geometry optimization and
single-point energy calculations on the anions P3AlFe−N2− and
X
Electronic Structure and Bonding in P3 Fe Systems.
While isostructural alkyl (X = C),9,10 silyl (X = Si),10 and
borane (X = B)3,10,11,13 P3 Fe−N2 complexes have all
P3GaFe−N2 reveal a relaxation to a Jahn−Teller distorted
X
−
−
demonstrated the capacity to facilitate catalytic N2RR, the
P3 Fe system affords the highest efficiencies under all
configuration with computed P−Fe−P angles of 111°, 112°,
125° and 111°, 112°, 126°, respectively. Corresponding spin
density plots show that this distortion also arises from
localization of the unpaired electron in an Fe-based d-orbital
of xy symmetry (Figure 2; Mulliken spin density on Fe: X = B
B
conditions canvassed. This can be attributed, in part, to the
greater electronic and geometric flexibility of the dative Fe →
B interaction, which allows access to both reduced Fe−N2 and
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X
[0/1−]
Figure 3. Cyclic voltammograms depicting the P3 Fe−N2
redox
couple of 3a (red), 3b (blue), and 3c (green) at a scan rate of 40 mV/
s in THF with 0.1 M [Bun N][PF6] supporting electrolyte.
4
[0/1−]
The ca. 200 mV anodic shift in the Fe−N2
reduction
potential for 3b and 3c relative to 3a suggests that exchange of
the apical Lewis acidic element in the P3 scaffold results in
X
only modest electronic perturbations at the iron center. This is
X
[0/1−]
in line with the ν(NN) vibrational data for P3 Fe−N2
complexes, which reveal similar degrees of N2 activation, as
well as with electronic structure calculations, which show that
the Fe → X interaction directly lowers the energy of the Fe dz
2
orbital but only indirectly influences the energy of the singly
occupied molecular orbital (SOMO) through σ-inductive
withdrawal of electron density from the Fe center. Interest-
[0/1−]
ingly, the reversible (AltraPhos)Fe−N2
reduction wave
occurs at −2.08 V vs Fc[1+/0]
,
24 suggestive of a weaker Fe → Al
interaction than that in 3b. We attribute this difference to
competition between both Fe and Napical for donation into the
empty Al pz orbital in the former.
Figure 2. DFT-computed molecular orbital diagram (α-manifold,
−
isovalue = 0.06 au) for P3AlFe−N2 (top) and spin density plots
To determine whether geometric flexibility is also conserved
in the P3AlFe and P3GaFe platforms, we turned our attention to
the XRD structures of 2−4. With the exception of the Fe → X
interaction, all isostructural complexes exhibit comparable
bond distances and angles, irrespective of the identity of the
apical Lewis acidic atom (Table S3). However, inspection of
the Fe−X distances reveals that all three systems possess
similar degrees of flexibility (Table 1). The overall changes in
(isovalue = 0.003 au) for P3AlFe−N2− (bottom left) and P3GaFe−N2
−
(bottom right). Hydrogen atoms are omitted for clarity. The
calculated frontier molecular orbitals for P3GaFe−N2 are analogous.
−
= 1.08; X = Al = 1.09; X = Ga = 1.10). X-band EPR spectra of
4b and 4c collected in 2-methyltetrahydrofuran glass (77 K)
display axial signals with no discernible hyperfine coupling to
27Al or 69/71Ga nuclei. Negligible spin leakage onto the B, Al,
and Ga atoms is detected computationally. In conjunction with
the measured solution magnetic susceptibilities and computed
frontier molecular orbital schemes (Figure 2), these data
Table 1. Select Structural Data for Complexes 2−4, 6, and 7
a
b
b
c
Fe−X
∑C−X−C
∑P−Fe−P
r
d
e
h
h
h
X
[0/1−]
2a
3a
4a
2.458
2.417
2.309
2.662
2.539
2.485
2.666
2.544
2.489
2.809
2.574
341.8
332.8
330.1
340.8
336.2
334.8
341.9
337.8
335.8
351.6
344.4
342.7
345.8
352.3
344.8
347.8
349.6
345.8
348.7
350.4
335.0
350.1
1.14
1.12
1.07
1.05
1.00
0.98
1.05
1.00
0.98
1.11
1.02
indicate that P3 Fe−N2
(X = Al, Ga) possess electronic
structures similar to those of P3 Fe−N2[0/1−] and (AltraPhos)-
B
14,24−26
d
[0/1−]
Fe−N2
.
The perfect trigonal symmetry exhibited
2b
3b
4b
2c
3c
4c
by 4b and 4c in the solid state is likely the result of crystal
packing effects.28
Having concluded that significant Fe → X σ-backdonation is
operative in P3AlFe−N2
and P3GaFe−N2[0/1−], we next
[0/1−]
sought to determine the strength of this interaction relative to
27
[0/1−]
B
that in P3 Fe−N2
.
To do so, we explored the redox
behavior of 3b and 3c by cyclic voltammetry. 3b and 3c display
f
6
g
[0/1−]
7
reversible Fe−N2
redox couples centered at −1.97 V and
−1.99 V vs Fc[1+/0] (Fc = ferrocene), respectively (Figure 3).
Under identical conditions, the analogous reduction event for
3a occurs at the slightly more negative potential of −2.19 V.14
The scan rate dependence of all three couples is consistent
with a diffusion-controlled process. For comparison, the
a
b
c
Units of angstroms (Å). Units of degrees (°). Ratio of the Fe−X
bond length to the sum of the covalent radii (Fe: 1.32 Å; B: 0.84 Å;
Al: 1.21 Å; Ga: 1.22 Å; from ref 32). From ref 14. For 4a, values are
d
an average of two independent molecules in the asymmetric unit.
e
f
Values obtained from a DFT geometry optimized structure. From
X
[0/1−]
observed trend in the P3 Fe−N2
redox potentials is also
ref 24. In solution, the terminal dinitrogen species (AltraPhos)Fe−N2
is believed to be in equilibrium with its dinitrogen-bridged analogue,
[(AltraPhos)Fe]2(μ-N2). The structural data presented here is for the
in accordance with the theoretically29,30 and experimentally31
determined acceptor ability of XPh3 according to the order X =
Al > Ga > B.
g
h
latter complex. From ref 25. See footnote in ref 33.
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X
X
these axial bond lengths upon reduction of P3 Fe−Br to
Table 2. N2RR Mediated by [Na(12-c-4)2][P3 Fe−N2]
X
a
[Na(THF)3][P3 Fe−N2] are 0.149 Å (X = B), 0.177 Å (X =
Complexes
Al), and 0.177 Å (X = Ga). Moreover, the value calculated for
the ratio (r) of the Fe−X bond length to the sum of the
respective covalent radii,32,33 which accounts for the differing
sizes of B, Al, and Ga, bares an overall net difference of Δr =
0.07 in each case, suggesting that the Fe−X bond is equally
flexible in all three systems.
acid
reductant
(equiv)
NH3/Fe
(equiv)
Yield NH3/H+
(%)
cat
(equiv)
b
d
e
1
2
3
4
5
6
5a
5b
5c
-
5b
5c
46
50
7.0 1.0
2.5 0.1
2.7 0.2
12.8 0.5
4.1 0.9
3.6 0.3
46
17
17
72
27
24
7
1
1
3
6
2
d
e
46
50
d
e
46
50
c
f
g
54
108
50
50
The decreasing values of r in the order 2 > 3 > 4 are also
indicative of a stronger Fe−X bond upon reduction.34 As
expected, such an increase in the dative interaction is
accompanied by a greater pyramidalization at the X(III) apical
atom and more trigonal planar geometry about Fe (Table 1).
Interestingly, while the r values of P3AlFe and P3GaFe are
f
g
46
f
g
46
a
Catalyst, acid, reductant, and Et2O sealed in a Schlenk tube at −196
°C under an N2 atmosphere, warmed to −78 °C, and stirred. For runs
utilizing HBArF , reactions were stirred at −78 °C for 1 h, followed by
4
stirring at room temperature for 45 min. For other runs, reactions
were allowed to stir and warm to room temperature overnight. Yields
B
consistently lower than those of P3 Fe, alluding to a stronger
b
c
Fe → Al and Fe → Ga interaction relative to Fe → B, the
slightly more pyramidalized geometry about boron across the
series would seem to imply it forms the strongest Fe−X bond.
This discrepancy likely stems from the difference in atomic
sizes between B, Al, and Ga, as well as steric constraints
imposed by the cage structure. This has also been observed in
are reported as an average of three runs. From ref 3. From ref 11.
d
e
f
B
P3 Fe+ was used as the precatalyst. HBArF . KC8. [H2NPh2][OTf].
4
g
Cp*2Co.
complexes to 10 equiv of acid and 12 equiv of reductant was
examined. P3 Fe is known to be rather robust under both sets
of catalytic conditions, with substantial active (pre)catalyst
remaining at the conclusion of reactions as evidenced by
B
X
related P3 Pd compounds (X = B, In), where the larger size of
In prevents pyramidalization that accurately reflects the
strength of the donor−acceptor interaction. While computa-
̈
Mossbauer spectroscopic studies and substrate reloading
In
experiments.10,11 Indeed, NMR and IR analysis of the
tions and the smaller r = 0.93 value for P3 Pd indicate it has a
B
postreaction of 5a with HBArF /KC8 reveals [M(solv)x]-
stronger Pd → X interaction than P3 Pd (r = 1.01), the indium
4
B
B
center resides in a less pyramidalized environment than boron
(∑C−In−C = 354.9°; ∑C−B−C = 341.8°).23,35 Therefore,
in accord with the electrochemical data, we favor a bonding
scheme where the Fe → X donor−acceptor interaction is
stronger for X = Al, Ga than for X = B. This assignment is also
[P3 Fe−N2] (solv = solvent) and (P3 )(μ-H)Fe(L)(H) (L =
B
H2, N2) as the major Fe-containing products. Similarly, P3 Fe−
B
N2 and (P3 )(μ-H)Fe(N2)(H) are obtained when [H2NPh2]-
[OTf]/Cp*2Co are employed (see Supporting Information for
full procedures). Free or decomposed P3 is not observed in
either instance. It is worth noting that while the hydride-
borohydride complex (P3 )(μ-H)Fe(L)(H) is an off-path
B
̈
validated computationally, where normalized Lowdin bond
B
orders (with respect to the corresponding Fe−B bond) of 1.08,
−
1.05, 1.05, and 1.02 are calculated for 3b, 3c, P3AlFe−N2 , and
B
species of P3 Fe-mediated N2RR catalysis, in the presence of
P3GaFe−N2 , respectively.
−
B
−
protons and electrons it can revert back to P3 Fe−N2 and
X
serve as a competent precatalyst.10
N2RR Activity. Having concluded that the P3 Fe platforms
(X = B, Al, Ga) exhibit similar electronic structures, flexibility,
and degrees of N2 activation, we reasoned that 5b and 5c
might function as competent catalysts for N2RR. Thus, we
subjected these compounds to the catalytic conditions that
Analogous experiments utilizing 5b and 5c yield slightly
different results depending on the conditions. In the case of
reaction with 10 equiv of HBArF and 12 equiv of KC8,
4
X
[M(solv)x][P3 Fe−N2] (X = Al, Ga) is the major terminal Fe-
B
have proved most fruitful for the P3 Fe system, namely,
containing product, with very little ligand decomposition
observed by 31P{1H} NMR spectroscopy. On the other hand,
use of [H2NPh2][OTf] and Cp*2Co yields a mixture of
treatment with excess [H(OEt2)2][BArF ] (HBArF , BArF −
=
or
4
4
4
3,10
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)/KC8
[H2NPh2][OTf]/Cp*2Co11 as the acid/reductant source at
−78 °C in diethyl ether (Et2O). Under the former conditions,
5b and 5c generate 2.5 0.1 (17 1% selectivity based on
H+) and 2.7 0.2 (17 1% efficiency) equiv of NH3 per Fe,
respectively (Table 2, entries 2 and 3). Moving to the
[H2NPh2][OTf]/Cp*2Co combination, which is in the activity
regime with the highest N2-to-NH3 selectivity observed for
X
compounds that includes P3 Fe−N2 (X = Al, Ga) and
X
products of P3 Fe decomposition. The latter is evidenced by
the presence of an intense 31P{1H} NMR signal consistent
with uncoordinated phosphine (see Supporting Information).
While the observation of substantially more decomposition
when 5b and 5c are reacted with [H2NPh2][OTf]/Cp*2Co
appears to be at odds with the milder nature of these reagents,
P3 Fe,13 results in only modest improvements, with 5b and 5c
B
and the higher NH3 yields obtained relative to HBArF /KC8 in
4
producing 4.1 0.9 (27 6% based on acid) and 3.6 0.3
(24 2% efficiency) equiv of NH3 per Fe (Table 2, entries 5
and 6). Hydrazine is not detected under either set of
conditions. These efficiencies are appreciably lower than
catalytic reactions, this can be rationalized by the differing
X
strength of the reductants and reactivity of P3 Fe−N2 species
with H2. Because of the highly reducing nature of KC8 (E ≤ −
3.0 V vs Fc[1+/0]), and its slight excess in the reactions, after
complete acid consumption, any residual KC8 is anticipated to
B
those of P3 Fe under similar conditions (Table 2, entries 1
and 4),3,11 and suggest that, in addition to the extent of N2
X
X
−
reduce P3 Fe−N2 to P3 Fe−N2 , which is unreactive toward
B
activation and flexibility of the Fe−X linkage, other factors
H2. Additionally, conversion of (P3 )(μ-H)Fe(L)(H) to
10
P3 Fe−N2 is viable under the HBArF /KC8 conditions. In
X
B
−
associated with the identity of the apical atom in the P3
4
scaffold play an influential role in the nitrogen fixation process.
In order to discern whether the divergent N2RR catalytic
profiles of 5a−c are a consequence of catalyst deactivation/
decomposition, the Fe speciation present after exposure of the
contrast, the reduction potential of Cp*2Co (E = −1.96 V vs
Fc[1+/0]) is very close to that of the P3 Fe−N2[0/1−] couples (X
X
B
= B, Al, Ga). While Cp*2Co is capable of reducing P3 Fe−N2
11
B
−
B
to P3 Fe−N2 at −78 °C, reaction of P3 Fe−N2 with excess
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Article
B
−
Accession Codes
Cp*2Co at room temperature does not produce P3 Fe−N2
(as judged by IR spectroscopy). Such observations are
consistent with a temperature-dependent redox equilibrium
CCDC 2043743−2043748 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Si
[0/1−]
analogous to that reported for P3 Os−N2
(E = −1.97 V
vs Fc[1+/0]).8 As a consequence, upon allowing the catalyst
speciation reactions to warm and stir at room temperature,
X
X
P3 Fe−N2 (rather than [M(solv)x][P3 Fe−N2]) is expected to
be the dominant species in solution when Cp*2Co is employed
X
AUTHOR INFORMATION
Corresponding Author
as the reductant. Reaction of P3 Fe−N2 with H2 formed from
■
background and/or catalyzed HER may therefore account for
the remaining terminal products observed in the reaction of
Jonas C. Peters − Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States; orcid.org/0000-0002-
6610-4414; Email: jpeters@caltech.edu
B
5a−c with [H2NPh2][OTf]/Cp*2Co. P3 Fe−N2 is known to
B
react with H2 to cleanly generate (P3 )(μ-H)Fe(L)(H) (L =
H2, N2).36 In turn, treatment of P3AlFe−N2 and P3GaFe−N2
with H2 gives rise to product profiles that match those
observed at the end of catalysis model reactions (see
Supporting Information).
Author
Javier Fajardo, Jr. − Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States
The above results suggest that P3AlFe and P3GaFe platforms
are relatively robust under both sets of N2RR catalytic
conditions explored. Although minor catalyst decomposition/
deactivation does occur (and is anticipated to be slower at −78
°C), this does not correlate well with the dramatically lower
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03354
N2RR efficiencies of P3AlFe and P3GaFe versus P3 Fe, or with
B
Notes
the higher NH3 yields obtained with the [H2NPh2][OTf]/
The authors declare no competing financial interest.
Cp*2Co conditions. Instead, our findings are suggestive that
P3AlFe and P3GaFe exhibit a greater selectivity for HER versus
ACKNOWLEDGMENTS
This work was supported by the NIH (GM070757). We thank
Dr. Michael K. Takase, Lawrence M. Henling, Prof. Gael Ung,
[1+/0]
■
N2RR. Bimolecular coupling of FeNNH2
N2RR
intermediates featuring weak N−H bonds is predicted to be
B
̈
an operative unproductive HER pathway for the P3 Fe
system.12 In this respect, it is worth noting that Fe-imido
(FeNR; R = adamantyl,17 p-methoxyphenyl14) and Fe-
disilylhydrazido (FeNNSi2)15,25 complexes that are acces-
and Prof. Jonathan Rittle for crystallographic assistance. We
also thank Dr. Matthew J. Chalkley and Dr. Nina X. Gu for
feedback in the preparation of this manuscript. J.F.J. acknowl-
edges the support of the NSF for a Graduate Fellowship
(GRFP).
B
sible and stable on the P3 Fe and (AltraPhos)Fe platforms
could not be isolated on the P3AlFe and P3GaFe scaffolds
despite our attempts to do so.
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■
In summary, the series of P3AlFe and P3GaFe complexes 2−4
spanning the formal oxidation states Fe(I) to Fe(−I) have
been synthesized, isolated, and thoroughly characterized.
These complexes expand upon and complement known
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03354.
Experimental details, synthetic procedures, and com-
pound characterization data, including NMR spectra,
EPR spectra, IR spectra, cyclic voltammograms, XRD
structures, XRD tables, and DFT optimized coordinates
(PDF)
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