A nonclassical dihydrogen adduct of S = 1/2 Fe(I)

Article abstract of DOI:10.1021/ja207003m

We have exploited the capacity of the "(SiP^iPr ₃)Fe(I)" scaffold to accommodate additional axial ligands and characterized the mononuclear S = 1/2 H₂ adduct complex (SiP ^iPr₃)Fe^I(H₂). EPR and ENDOR data, in the context of X-ray structural results, revealed that this complex provides a highly unusual example of an open-shell metal complex that binds dihydrogen as a ligand. The H₂ ligand at 2 K dynamically reorients within the ligand-binding pocket, tunneling among the energy minima created by strong interactions with the three Fe-P bonds.

Full text of DOI:10.1021/ja207003m

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A Nonclassical Dihydrogen Adduct of S = 1/2 Fe(I)
||
Yunho Lee,†,§,|| R. Adam Kinney,‡, Brian M. Hoffman,*,‡ and Jonas C. Peters*,†
†Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
‡Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
S Supporting Information
b
Scheme 1
ABSTRACT: We have exploited the capacity of the
“(SiPiPr3)Fe(I)” scaffold to accommodate additional axial
1
ligands and characterized the mononuclear S = /2 H2
adduct complex (SiPiPr3)FeI(H2). EPR and ENDOR data,
in the context of X-ray structural results, revealed that this
complex provides a highly unusual example of an open-shell
metal complex that binds dihydrogen as a ligand. The H2
ligand at 2 K dynamically reorients within the ligand-binding
pocket, tunneling among the energy minima created by
strong interactions with the three FeꢀP bonds.
ow-valent iron has been proposed to play a prominent role in
the function of hydrogenase and nitrogenase enzymes, a
L
resonance (EPR)/electronꢀnuclear double resonance (ENDOR)
study might prove most effective in characterizing the product
formed by reaction of H2 with the S = 1/2 Fe(I) complex. The
present report provides the results of such a study and introduces
an S = 1/2 FeI(H2) adduct complex. Evidence for its related but
cationic S = 1 FeII(H2)+ analogue is also provided.
hypothesis that has inspired the study of formally low-valent iron
model complexes that afford access to N2 and H2 ligation.1ꢀ3
Whereas model systems have historically focused on Fe(0) and
Fe(II) systems in this context, the coordination chemistry of
Fe(I) has more recently come into focus.4 This is in part due to
its suggested intermediacy in enzymatic hydrogenase activity.
Fe(I) is a key formal oxidation state to consider for N2 and H+
reduction cycles, and understanding interactions between Fe(I)
and N2/H2 is essential.
As previously reported, treatment of the methyl complex
(SiPiPr3)FeꢀCH3 (1) with HCl followed by reduction generates
the S = 1/2 Fe(I)ꢀ N2 adduct (SiPiPr3)Fe(N2) (2).4a,b The N2
ligand is sufficiently labile to be displaced by H2 in benzene to
afford a yellow species formulated as S = 1/2 “(SiPiPr3)Fe(H2)” or
“(SiPiPr3)Fe(H)2” (3) (μeff = 1.9 μB by the Evans method;
Scheme 1) in nearly quantitative yield. The optical spectrum of 3
[see the Supporting Information (SI)] features a low-energy
dꢀd transition at 1350 nm (ε ≈ 100 cmꢀ1 Mꢀ1) similar to that
of 2 (1250 nm, ε ≈ 300 cmꢀ1 Mꢀ1).
In several recent studies using a tripodal tetradenate tris-
(phosphino)silyl ligand XL3 (e.g., SiPiPr = [Si(o-C6H4P-
3
iPr2)3]ꢀ),3b,4 it has been established that five-coordinate iron
in a trigonal-bipyramidal (TBP) geometry (XL3FeꢀL0) can
accommodate a terminally bonded N2 (or CO) ligand in the
axial site across three formal oxidation states (XL3Fe0ꢀN2
,
ꢀ
XL3FeIꢀN2, and XL3FeIIꢀN2 ). It is noteworthy that the spin
states of these TBP systems vary such that an S = 0 state is favored
for Fe(0), an S = 1/2 state for Fe(I), and an S = 1 state for Fe(II).
These findings encouraged us to explore the affinity of dihydro-
gen for the axial site, particularly for the S = 1/2 Fe(I) or S = 1
Fe(II) states. Whereas it is common for H2 to occupy the same
coordination site as N2, thoroughly characterized dihydrogen
adducts of open-shell complexes for transition metals remain
exceptionally rare.5,6 Oxidative addition to form a dihydride and
heterolytic cleavage to form a monohydride are well-established
alternatives. NMR techniques, including measurement of T1
values and JHD coupling constants, have been the methods of
choice for characterizing closed-shell H2 adducts and distinguish-
ing between the dihydrogen/dihydride extrema on the H2
bonding continuum.7 Open-shell H2 adduct complexes are not
amenable to this approach because of the extreme line broadening
of resonances expected for H atoms directly coordinated to a
metal center with spin S > 0. Because the S = 1/2 state is EPR-
active, we reasoned that a combined electron paramagnetic
+
Displacement of the N2 ligand of 2 by H2 can be conveniently
monitored by 1H NMR spectroscopy in C6D6 (Figure 1).
As shown in Figure 1, the H2 adduct 3 is stable to vacuum over
prolonged periods (even at 60 ꢀC over 12 h) but converts back to
2 upon exposure to N2, which would be consistent with an
associative exchange process proceeding through a formally 19-
electron intermediate. Alternatively, and perhaps more likely, is a
process wherein a labile phosphine donor exposes a site for N2
binding via a 15-electron intermediate, followed by H2 loss and
recoordination of the phosphine donor to provide 2. We also
prepared and similarly characterized the D2 derivative of 3 (see
the SI). The 1H NMR spectrum of 3-D2 is analogous to that for 3,
and an 2H NMR spectrum does not reveal any deuterium reso-
nances for 3-D2 (see the SI). This fact is consistent with
1
deuterons that are directly bonded to an S = /2 iron center.
Received: July 26, 2011
Published: September 28, 2011
r
2011 American Chemical Society
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Figure 2. (A) Displacement ellipsoid (50%) representation of 3
obtained from a crystal grown under a H2 atmosphere. H atoms of
SiPiPr3 have been omitted for clarity. (B) Overlay of the closely related
core structures of 2 (blue) and 3 (red).
Figure 1. 1H NMR spectra of (A) (SiPiPr3)Fe(N2) (2), (B) (SiPiPr3)-
Fe(H2) (3), (C) 3 under full vacuum, and (D) a mixture of 2 and 3
measured in C6D6 at RT.
Table 1. Selected Data for Complexes Discussed in the Text
complex
dFeꢀSi (Å)
dFeꢀP (Å) —PFeP (deg)
μeff (μB)a
Incubation of 2 under HD instead of H2 at RT for 15 h caused no
HD/H2/D2 scrambling, suggesting that 3 binds an intact H2
ligand rather than undergoing heterolytic activation (e.g., to
generate a SiꢀH or PꢀH bond and an FeꢀH bond).
Fe(N2) 2
2.2713(6)
2.2657(5)
2.2841(7)
2.3244(6)
2.2442(9)
2.260(1)
111.82(2)
113.59(2)
128.64(2)
113.31(3)
118.07(3)
122.36(4)
113.157(9)
117.63(1)
123.16(1)
1.90
Fe(H2) 3b
Fe(H2) 3c
2.254(1)
1.88
The well-behaved NMR properties of 2 and 3 allow for
the direct measurement of the equilibrium constant for H2/N2
exchange at the S = 1/2 Fe center. Using sealed J-Young vessels
with known N2/H2 gas mixtures and exploiting the well-defined
resonances of the SiPiPr3 ligand at ca. 10 and 8 ppm to integrate
the respective concentrations of 2 and 3, we estimated the
equilibrium constant to be 50 ( 20 in favor of H2 binding in
C6D6 at room temperature.
2.2631(9)
2.2418(3)
2.2577(3)
2.2613(2)
2.2478(3)
a Magnetic moments by the Evans method in C6D6 at 22 ꢀC. b Under
vacuum. c Under H2.
X-ray structures of 3 were obtained from crystals under
vacuum and under an atmosphere of H2. While both structures
are of high quality, neither shows the positions of the H2/
hydride. Nonetheless, it is clear that the two structures are
essentially identical and are likewise very similar to that of the
N2 adduct complex 24b and its CO congener (SiPiPr3)FeI(CO)4c
(Figure 2 and Table 1), except for the absence of an axially bound
diatomic ligand for 3. We were unable to locate a vibration
consistent with either an FeꢀH or an Feꢀ(HꢀH) motif.
Whereas terminal MꢀH vibrations are readily assigned, ν-
(HꢀH) vibrations for H2 adduct complexes are not as reliably
discerned.6a,7 The absence of ν(SiꢀH) or ν(PꢀH) stretches in
the IR spectrum is likewise inconsistent with heterolytic genera-
tion of FeꢀH and SiꢀH/PꢀH bonds but consistent with an
intact H2 ligand.
We examined the EPR spectrum of 3 in both the solid state and as
a frozen glass. Treatment of 2(g= [2.364, 2.036, 2.003]) with H2 gas
leads to complete loss of the starting material and the appearance of
3with an S=1/2 EPR signal (g = [2.275, 2.064, 2.015]), as measured
for frozen solutions (9:1 THF/Me-THF) and pure powders (see
the SI). The unique magnetic direction g1 (g1 = 2.275) is assigned to
the pseudo-C3 symmetry axis of the molecule. These low-tempera-
ture measurements of 3, combined with NMR Evans method
(Table 1) show that the coordination sphere and low-spin d7
character of the Fe(I) center are maintained under all conditions.
The g values of 3 (and 2) can be described in terms of a “pseudo-
JahnꢀTeller” (PJT) effect wherein spinꢀorbit coupling competes
with vibronic coupling to lower the energy of the molecule by a
distortion from a C3-symmetric structure.8 The idealized trigonal
Fe(I) geometry created by the [SiPiPr3]ꢀ ligand leads to a doubly
degenerate 2E ground state in which the (dxy, dx2ꢀy2) orbital doublet
is triply occupied, and therefore, the complex is subject to a JT
distortion. The most obvious distortion in the X-ray structure of 3
(and2) is inthe PꢀFeꢀP angles, with an increase in one angle from
the symmetric value of 120ꢀ and a decrease in the other two (e.g., for
3, P2ꢀFeꢀP3 = 123.2ꢀ and P1ꢀFeꢀP3 ≈ P1ꢀFeꢀP2 ≈ 115.4ꢀ).
Although the hydrogenic ligand of 3 is not visible by X-ray
diffraction (XRD), we directly characterized this ligand by 1,2H
ENDOR spectroscopy. Figure 3 displays a part of the 2D fieldꢀ
frequency 1H ENDOR pattern comprising spectra collected across
the EPR envelope of 3. The spectrum collected at g3 shows a doublet
centered at the 1H Larmor frequency that is split by the hyperfine
2
interaction |A3(1H)| = 37.8 MHz. The equivalent H ENDOR
spectrum from [SiPiPr3]Fe(2H2) (30) shows a corresponding
doublet; its splitting, |A3(2H)| = 5.6 MHz, matches that of the 1H
doublet upon scaling by the respective nuclear g values with a small
isotope-effect correction. Likewise, the 2D fieldꢀfrequency pattern
of the 2H ENDOR spectra corresponds to the 1H pattern of Figure 3
(see the SI). The complete 2D pattern is exceptionally well
simulated by hyperfine coupling to a single type of 1H, with coupling
tensor A = +[2.3, ꢀ40.6, ꢀ37.8] MHz, isotropic coupling aiso
=
ꢀ25.4 MHz, and anisotropic dipolar hyperfine coupling tensor
T = +[27.7, ꢀ15.2, ꢀ12.4] MHz; the tensor orientation relative to g
is given by the rotation angles (α, β, γ) = (0, 6, 0). The absolute sign
of the 1,2H hyperfine coupling was determined experimentally using
the PESTRE technique (see the SI).9
The simplest chemical species compatible with a single type of
1
1
interacting H whose Feꢀ H vector lies close to g1 is a neutral
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Figure 3. Q-band stochastic 2D fieldꢀfrequency continuous-wave
ENDOR pattern for 3 (black) with simulations of the exogenous “H2”
1H ENDOR response (red; see the text for details). The corresponding
2H ENDOR spectrum collected from 3-D2 at g3 = 2.015 (blue) is scaled
by the ratio of the 1H and 2H nuclear g values. ENDOR responses from
Figure 4. Simulations of 1H ENDOR spectra for models of 3, for the
spectrum at g = 2.146. The simulations are based on the hyperfine tensor
determined through a fit to the 2D fieldꢀfrequency plot of Figure 3
(A = [2.3, ꢀ40.6, ꢀ37.8] MHz) but with the dipolar interaction rotated
by the angle β between the H nuclei and the g1 axis, which is taken to lie
along the FeꢀSi bond (labeled as “g1”).
31P of the [SiPiPr ꢀ ligand are indicated by (*).
]
3
complex in which a terminal hydride bound to Fe(I) is generated
by heterolytic cleavage of the H2 molecule with delivery of the
proton to the Si of the SiPiPr3 ligand. However, this model requires
cleavage of the FeꢀSi bond with rearrangement of the geometry at
Si and a substantial increase in dFeꢀSi relative to 2, whereas the
crystal structure of 3 reveals a negligible change in dFeꢀSi relative
to 2 (see above).10 This conclusion was corroborated by EPR
measurements showing that the solution, polycrystalline powder
(see the SI), and single-crystal forms of 3 have the same g tensor11
and thus the same structure. The ENDOR pattern of Figure 3
must therefore arise from a neutral complex that has been
generated by the addition of H2, either in the form of an Fe(I)ꢀH2
complex or via oxidative addition to an Fe(III) dihydride.
The observed g values provide a powerful argument against
the formulation of 3 as an Fe(III) dihydride. The PJT effect
predicts that the g values for an Fe(III) d5 ion must be less than
ge = 2, contrary to observation.12
Fe(I)ꢀH2 center (dHꢀH = 0.85 Å; dFeꢀH = 1.62 Å),14 the H
atoms would exhibit a geometrically determined β value of ∼15ꢀ.
As shown in Figure 4 (also see the SI), the 2D ENDOR pattern of
Figure 3 cannot be described by such a static structure. The
electronꢀnuclear dipolar interaction for H2 must therefore be
modulated by rapid reorientation of H2 about the FeꢀH2 bond axis.
The H2 cannot undergo rotation that is only weakly hindered by
the binding pocket environment because in that case the ground state
would correspond to the free-rotor ground rotational state (m=0).15
The total wave function of a rotating 1H2 must be antisymmetric with
respect to exchange of the two 1H nuclei. For 1H2 bound to the Fe(I)
center of a statically distorted 3, this wave function is the product of
1
the H2 rotational and spin functions. The rotor ground state is
Elimination of the Fe(III) dihydride model for 3 was con-
firmed by analysis of the 1H ENDOR results. The 2D ENDOR
pattern for 3 can be compared with that predicted for this species
on the basis of a consensus geometry of similar complexes (dHꢀH
> 1.6 Å; dFeꢀH = 1.54 Å; Figure 4).13 The critical parameter in the
ENDOR simulations is the value of the angle β between the g1
principal axis and the FeꢀH vectors. In an Fe(III) dihydride
symmetric with respect to exchange of the two H nuclei (the “para”
state) and thus must be associated with the antisymmetric I = 0 total
nuclear spin state, which cannot exhibit a 1H ENDOR signal.16 If 3
instead undergoes a dynamic PJT distortion, the total H2 wave
function would have the vibronic electronꢀnuclear wave function as
an additional factor. For 3 this factor is symmetric with respect to
exchange, so the conclusion remains.
1
complex, β g 30ꢀ. H ENDOR simulations using β ≈ 30ꢀ
Instead, the measurements are probably best understood as a
consequence of H2 tunneling among localized states set up by a
strong sixfold barrier associated with rotation of the “dumbell-
shaped” H2 within the threefold-symmetric molecular potential
(Figure 4B, β = 30ꢀ; also see the SI) showed this to be
incompatible with experiment. Figure 4B illustrates this with
simulations for a spectrum collected at a field where each
component of the doublet observed at g3 (and g2) in Figure 3
is split into an intense peak and a less intense “shoulder”.
The splitting for β ≈ 30ꢀ is far larger than seen experimentally.
Rejection of the assignment of 3 as an Fe(I) monohydride or an
Fe(III) dihydride implies by elimination that 3 is an Fe(I)ꢀH2
adduct. The similarity of the solution NMR and UVꢀvis data for 2
and 3 are also highly consistent with this notion (see above). The
ENDOR simulations further show that even at 2 K, the H2 of
the Fe(I)ꢀH2 adduct undergoes dynamic reorientation within the
ligand-binding pocket of 3. In a consensus geometry for the
1
(Figure 5).5 In this limit, the H ENDOR response is allowed,
and tunneling would average the dipolar interaction, causing
the unique hyperfine axis to lie along the axis of rotation. We
attribute the nonzero β to tensor noncollinearity caused by the
distortion from trigonal symmetry that is introduced by the PJT
effect and observed in the X-ray structure (Table 1).
It is of interest to compare the stable (SiPiPr3)FeI(H2) complex
described here to that assigned as a dihydrogen adduct of Mo(III),
“[HIPTN3N]MoIII(H2)”.6b In a recent report, it was shown that
the latter species undergoes facile heterolytic cleavage of H2,
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suggestions about low-temperature dynamics. Dr. Peter M€uller
provided assistance with XRD analyses. Professor George R.
Rossman provided access to a near-IR spectrometer. Henry Fong
probed the deprotonation of 5.
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Figure 5. Schematic representation of H2 hopping/tunneling on the
potential energy surface for PJT-distorted 3.
delivering Hꢀ to Mo to afford [[HIPTN3N]MoIII(H)]ꢀ and H+,
with the H+ presumably delivered to sacrificial [HIPTN3N]3ꢀ at
reduced temperature.6c In contrast, the Si atom of the (SiPiPr3)Fe
scaffold is insufficiently basic to accept H+ from the coordinated
H2, thus stabilizing the H2 adduct complex 3 against heterolytic
cleavage. The stability of H2 bound to the “(SiPiPr3)Fe” scaffold
also extends to its corresponding cation, “(SiPiPr3)Fe+”. Treating
(4) (a) Lee, Y.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558.
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previously reported S = 1 {(SiPiPr3)Fe(N2)}{BArF } (4) with H2
4
reversibly generates what we have assigned as the S = 1 complex
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{(SiPiPr3)Fe(H2)}{BArF } (5) (Scheme 1). Complex 5 is EPR/
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4
ENDOR-silent because of its integer-spin triplet state, but solution
NMR and UVꢀvis data (see the SI) provide evidence for a species
that is highly similar to the cationic N2 adduct 4. Moreover,
addition of anexogenous base(e.g., NiPr2Et) to5under N2 cleanly
effects heterolytic cleavage, affording the neutral Fe(II) complex
(SiPiPr3)Fe(N2)(H) (6) (Scheme 1).
In summary, the mononuclear S = 1/2 FeI(H2) adduct complex 3
provides a highly unusual example of a well-characterized open-
shell metal complex that binds dihydrogen as a ligand. Combined
XRD, EPR, and ENDOR data are consistent with a PJT-distorted
d7 configuration and a H2 ligand that at 2 K tunnels among the
energetic minima created by the FeꢀP bonds. The S = 1/2 title
complex (SiPiPr3)Fe(H2) can be formally oxidized to its S = 1
cation, {(SiPiPr3)Fe(H2)}+, and the latter species binds H2 as an
intact ligand that is subject to heterolytic cleavage upon addition
of exogenous base.
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(SiPiPr3)Fe(H2) or (SiPiPr3)Fe(H)2 isomer.
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’ ASSOCIATED CONTENT
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Supporting Information. Experimental procedures, char-
b
acterization and crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
bmh@northwestern.edu; jpeters@caltech.edu
Present Addresses
§Department of Chemistry, School of Molecular Science, Korea
Advanced Institute of Science and Technology, Daejon, Republic
of Korea.
Author Contributions
These authors contributed equally.
’ ACKNOWLEDGMENT
We acknowledge the NIH (GM-070757 to J.C.P.; HL 13531
to B.M.H.). We thank Prof. Harden McConnell for insightful
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