Synthesis of Bis(phosphino)silyl Pincer-Supported Iron Hydrides for the Catalytic Hydrogenation of Alkenes

Article abstract of DOI:10.1021/acs.organomet.8b00807

The synthesis and characterization of Fe pincer complexes supported by a bis(phosphino)silyl (PSiP) ligand are described. While four-coordinate species of the type (PSiP)FeX (X = halide) proved challenging to access, examples of five-coordinate (PSiP)Fe(II) and (PSiP)Fe(I) species were prepared and crystallographically characterized. In studying the reactivity of such (PSiP)Fe precursors, a variety of iron hydride species were observed and characterized, and interconversion among such complexes facilitated by the coordination of N₂ was noted. The structures and spectroscopic features of several such diamagnetic Fe(II) hydrides were elucidated, including that of a unique and highly stable η²-(Si-H)Fe(II) dihydride complex. A surrogate for a low coordinate (PSiP)FeH species in the form of its bis(dinitrogen) adduct was found to be an effective precatalyst for the direct hydrogenation of alkenes, including various mono- and disubstituted aliphatic alkenes, as well as a trisubstituted example. Esters and ethers were found to be well-tolerated by the catalyst, and alkyne hydrogenation was also demonstrated.

Full text of DOI:10.1021/acs.organomet.8b00807

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Bis(phosphino)silyl Pincer-Supported Iron Hydrides for
the Catalytic Hydrogenation of Alkenes
Luke J. Murphy,† Michael J. Ferguson,‡ Robert McDonald,‡ Michael D. Lumsden,†
and Laura Turculet*,†
†Department of Chemistry, Dalhousie University, 6274 Coburg Road, P.O. Box 15000, Halifax, Nova Scotia, Canada B3H 4R2
‡X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: The synthesis and characterization of Fe pincer
complexes supported by a bis(phosphino)silyl (PSiP) ligand are
described. While four-coordinate species of the type (PSiP)FeX (X =
halide) proved challenging to access, examples of five-coordinate
(PSiP)Fe(II) and (PSiP)Fe(I) species were prepared and crystallo-
graphically characterized. In studying the reactivity of such (PSiP)Fe
precursors, a variety of iron hydride species were observed and
characterized, and interconversion among such complexes facilitated by
the coordination of N2 was noted. The structures and spectroscopic
features of several such diamagnetic Fe(II) hydrides were elucidated,
including that of a unique and highly stable η2-(Si−H)Fe(II) dihydride
complex. A surrogate for a low coordinate (PSiP)FeH species in the
form of its bis(dinitrogen) adduct was found to be an effective
precatalyst for the direct hydrogenation of alkenes, including various
mono- and disubstituted aliphatic alkenes, as well as a trisubstituted example. Esters and ethers were found to be well-tolerated
by the catalyst, and alkyne hydrogenation was also demonstrated.
Chart 1. Fe Catalysts for the Hydrogenation of Alkenes
1. INTRODUCTION
The hydrogenation of alkenes is one of the most important and
widely applied chemical transformations known, with
applications ranging from the synthesis of fine chemicals to
the processing of petrochemicals, and it is routinely employed
on large scales.1 Traditionally, alkene hydrogenation processes
have been dominated by the use of transition metal catalysts,
most often of the platinum group metals. However, there has
been recent success in the development of systems utilizing
more abundant and less costly first-row transition metals,2
particularly cobalt,3 though well-defined examples of readily
accessible and easily handled homogeneous catalysts with high
activity and functional group tolerance still remain relatively
scarce, especially in the case of iron. Yet iron is in many ways
an ideal metal to employ in catalysis,4 due primarily to its low
cost, comparatively low toxicity, and high abundance.
In 2004, Chirik and co-workers5 developed an Fe-based
alkene hydrogenation catalyst, (iPrPDI)Fe(N2)2, supported by
a bis(imino)pyridinyl scaffold (Chart 1). The discovery of this
catalyst marked an important step forward in the field of Fe-
catalyzed olefin hydrogenation, as prior to this the only
examples of such a transformation involved the use of
Fe(CO)5 as a catalyst at high temperature and pressure or
under photocatalytic conditions.3f,6 Around the same time
Peters and co-workers7 reported alkene hydrogenation
catalyzed by a tris(phosphino)borato-supported Fe complex
under mild conditions (1−4 atm H2, 23 °C), albeit at higher
(10 mol %) catalyst loadings.
Received: November 3, 2018
© XXXX American Chemical Society
A
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In Chirik’s initial Fe system,5 (iPrPDI)Fe(N2)2 was found to
hydrogenate a variety of terminal, internal, and gem-
disubstituted alkenes under very mild conditions and at low
catalyst loadings. However, trisubstituted alkenes could not be
hydrogenated under these conditions. In a subsequent report8
functional group tolerance in the catalytic system was
investigated, with examples of amino, ether, ketone, and ester
functionalized alkenes undergoing chemoselective alkene
hydrogenation under varying conditions (some requiring
elevated temperatures or a higher catalyst loading). However,
the authors also identified C−O bond cleavage of ethers and
esters by the catalyst as a problematic side-reaction resulting in
catalyst deactivation if substrate and catalyst were not kept
frozen at liquid N2 temperature until H2 addition.9 More
recently, investigations into ligand modifications of the original
ligand structure10 ultimately led to a bis(arylimidazol-2-
ylidene)pyridinyl scaffold (Chart 1).11 Such (RCNC)Fe(N2)2
(R = Me, Mes) complexes were found to be active for the
hydrogenation of a handful of trisubstituted alkenes and even a
tetrasubstituted alkene (2,3-dimethyl-1H-indene, 60−68%
conversion over 48 h).11b
In other recent contributions, monoanionic PNP pincer
ligation has been explored in the context of Fe-catalyzed alkene
hydrogenation. Jones and co-workers12 reported the hydro-
genation of styrene derivatives using a (PNP)Fe(H)(CO)
precatalyst (PNPN(CH2CH2PiPr2)2, Chart 1). While
various styrenes, including substrates featuring ester and nitrile
substitution on the arene ring, were reduced with high
conversion and selectivity, nonpolar aliphatic alkenes such as
1-hexene were not hydrogenated, even at increased temper-
ature and pressure. DFT calculations support a bifunctional
mechanism involving stepwise transfer of Fe-hydride from
(PNHP)Fe(H)2(CO) to the CC bond, followed by proton
transfer from the N−H in the pincer backbone. This
mechanism is in agreement with the observed rate dependence
of the reaction on the polarity of the CC bond.
Subsequently, Gade and co-workers13 reported high-spin,
bis(phosphino)pyrrole (PNP)Fe(alkyl) complexes that func-
tioned as precatalysts for alkene hydrogenation (Chart 1).
Secondary aliphatic alkenes, such as 2-pentene and 2-hexene,
were hydrogenated effectively, while trans-stilbene only
reached 34% conversion after 72 h, and the tetrasubstituted
alkene 2,3-dimethylbut-2-ene proved unreactive.
The use of a bis(anthracene) metalate of iron, as well as
other related arene metalate iron complexes, for catalytic
alkene hydrogenation was also recently explored.14 However,
outside of high conversions for a select number of terminal
alkenes (styrene derivatives, 1-octene, and 1-dodecene)
catalytic performance was quite poor for these systems, while
related cobalt complexes were found to be more active. The
hydrogenation of α,β-unsaturated carbonyl compounds by an
Fe(II)/EDTA catalyst has also been reported.15 Outside the
realm of well-defined homogeneous catalysts, recent examples
of effective Fe-hydrogention catalysts include MOF-supported
species,16 as well as systems involving in situ generated Fe
hydride clusters and Fe nanoparticles. In one such report from
Jacobi von Wangelin and co-workers,17 a combination of 5 mol
% FeCl3 with 5−10 mol % LiAlH4 was employed to generate
the catalyst species. The substrate scope featured predom-
inately terminal or disubstituted alkenes, with a trisubstituted
alkene (1-phenylcyclohexene) giving <20% conversion. The
authors proposed a homogeneous catalytic cycle at the onset of
catalysis, giving way to insoluble Fe(0) species that exhibit
lower catalytic activity. A second report from this group18
invokes generation of Fe hydride nanoclusters upon reaction of
Fe[N(SiMe3)2]2 (5 mol %) with DIBAL-H (10 mol %), which
enables a broad variety of alkene hydrogenations, with
terminal, disubstituted, trisubstituted, and even several
tetrasubstituted alkenes hydrogenated in moderate to excellent
yields.
While the above examples illustrate significant advances in
the field of Fe-catalyzed alkene hydrogenation, the identi-
fication of new ligands that are capable of supporting effective
Fe catalysts for alkene hydrogenation remains an important
goal. In particular, further effort is needed to develop catalysts
that not only exhibit functional group tolerance but are also
easily handled and not susceptible to decomposition in the
presence of substrate. Investigation into the stoichiometric
chemistry of ligand-supported Fe hydride complexes is key to
this effort, as such Fe hydride complexes are expected to be
vital intermediates in a catalytic cycle where alkene
coordination and subsequent migratory insertion likely
represent fundamental mechanistic steps. In this regard,
fundamental studies examining the role of ancillary ligation
in supporting low-coordinate Fe hydride complexes are also
important, as the steric requirements for the coordination of
weakly donating or sterically congested alkenes must be
suitably balanced with the electronic requirements for the
activation and eventual reduction of an unsaturated substrate
at the Fe center.
Our group has investigated the utility of tridentate
bis(phosphino)silyl (PSiP) ligation for group 8, 9, and 10
transition metals, with the aim of uncovering new routes to
challenging bond activation processes as well as catalytic
applications.19,20 In this regard we have identified routes to the
C−H activation of benzene,21 the N−H activation of
ammonia,22 in addition to catalytic reduction of CO2 to
methane.23 Most recently we have become interested in
exploring the behavior of PSiP ligand scaffolds in conjunction
with the more abundant, and consequently less costly, first-row
transition metals, and we have demonstrated a highly selective
reduction of CO2 to the formaldehyde level catalyzed by a
(PSiP)Ni complex.24 We have also pursued the synthesis and
reactivity of (PSiP)Co species, including their application in
alkene hydrogenation catalysis.25 Comparatively, (PSiP)Fe
chemistry is remarkably underexplored, with three reports
appearing in the literature to date, in which only saturated 18-
electron Fe(II) complexes for use outside of alkene hydro-
genation are described.26 Inspired by the progress made to
date regarding homogeneous Fe-catalyzed alkene hydro-
genation (vide supra), we sought to prepare an active Fe
catalyst based on the PSiP ligand scaffold, in anticipation that
the strong trans-influence of the anionic silyl donor would
promote generation of a reactive, low-coordinate Fe species
poised to undergo the necessary steps to facilitate such a
transformation.
Herein we report significant progress in the field of (PSiP)Fe
chemistry, including the application of (PSiP)Fe(II)-hydride
species in alkene hydrogenation catalysis. In the course of these
studies, (PSiP)Fe-hydride chemistry has been explored in
detail, including the synthesis of a unique η2-silane complex of
an Fe(II) dihydride, as well as a bis(dinitrogen) Fe(II) hydride
(inset, Chart 1), which was found to be an isolable and easily
handled precatalyst for the hydrogenation of a variety of
alkenes.
B
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2. RESULTS AND DISCUSSION
Scheme 1. Synthetic Routes for the Preparation of 2·PMe3
and 2·(CO)2
Synthesis of (PSiP)Fe-Halide Complexes. Previous
efforts to access (PSiP)Fe chemistry, as reported by the Sun,
Iwasawa, and Nishibayashi groups, have focused on the
synthesis of saturated, 18-electron derivatives of the form (R-
PSiPR′)FeH(PMe3)(L) (R-PSiPR′ = κ3-(2-R2PC6H4)2SiR′; R =
Cy, Ph; R′ = Me, Ph; L = PMe3, N2).26a,c In efforts to uncover
new and relevant reactivity toward substrate molecules
featuring E-H bonds (e.g., H2, silanes, boranes), we first
sought to generate four-coordinate Fe(II) complexes of the
form (Cy-PSiPMe)FeX (X = halide). While examples of Co
analogues of this type have been successfully isolated and
characterized,20i,25,26c to date no evidence for the correspond-
ing Fe complexes has been observed.
In an analogous fashion to the preparation of (R-PSiPMe)-
CoX,20i,25,26c compounds of the form (Cy-PSiPMe)FeX were
targeted by treatment of the tertiary silane (Cy-PSiPMe)H with
an FeX2 precursor and base. While many attempts were made
to induce Si−H metalation by varying the base (a broad range
of bases were studied in this regard, including amines) and
FeX2 source, these attempts were largely unsuccessful, as the
resulting paramagnetic products proved difficult to isolate in
pure form. In some cases, samples of crystalline material
obtained from such reactions revealed the formation of Fe(I)
and Fe(III) species, implicating undesired one-electron redox
processes in the observed reactivity. In one case, a minute
amount of X-ray quality crystals of monomeric, four-coordinate
(Cy-PSiPMe)FeBr (1-Br, Figure S1) were obtained from
treatment of (Cy-PSiPMe)H with FeBr2 and one equiv of
MeMgBr at low temperature. While the obtained crystals and
corresponding X-ray data likely represent only a small portion
of the resultant product mixture, the observations nonetheless
support the feasibility of complexes of the desired formulation.
Details of these attempts, including X-ray crystal structures
obtained in the course of our investigations, are summarized in
the Supporting Information.
Figure 1. Crystallographically determined structures of 2·PMe3 (left)
and 2·(CO)2 (right) with thermal ellipsoids shown at the 50%
probability level (in each case, one of two crystallographically
independent molecules is shown); hydrogen atoms have been omitted
for clarity. Selected interatomic distances (Å) and angles (deg): for 2·
PMe3 Fe−Cl 2.2731(6), Fe−P1 2.23224(6), Fe−P2 2.3346(6), Fe−
P3 2.3113(6), Fe−Si 2.3436(6), Cl−Fe−Si 173.20(2), P1−Fe−P2
122.82(2), P1−Fe−P3 121.93(2), P2−Fe−P3 110.38(2); for 2·
(CO)2 Fe−Cl 2.3435(4), Fe−P1 2.2786(4), Fe−P2 2.2761(4), Fe−Si
2.3284(5), Fe−C1 1.7228(17), Fe−C2 1.8203(16), O1−C1
1.159(2), O2−C2 1.145(2), Cl−Fe−C1 167.71(5), P1−Fe−P2
162.500(17), Si−Fe−C2 173.93(5).
As complexes of the type (Cy-PSiPMe)FeX proved
challenging to isolate, stabilization of these reactive com-
pounds with an additional ligand was pursued. In previous
reports by Sun,26a as well as Nihibayashi and Iwasawa,26c
(PSiP)M (M = Fe, Co, Ni) complexes were accessed through
the use of the zerovalent M(PMe3)4 starting materials, and
thus the resulting saturated complexes all feature one or two
equiv PMe3 bound to the metal center. With this in mind, we
treated a mixture of (Cy-PSiPMe)H, FeCl2(THF)1.5 and one
equiv PMe3 with BnMgCl at −35 °C (Scheme 1). The five-
coordinate Fe(II) complex (Cy-PSiPMe)Fe(PMe3)Cl (2·
PMe3) was obtained as an analytically pure orange solid in
94% yield by this route. Use of CO (1 atm) in place of PMe3
generated the dicarbonyl complex 2·(CO)2.
Solution magnetic moment measurements for 2·PMe3
(Evans method, benzene-d6, 300 K) resulted in a calculated
μeff value of 3.0 μB (S = 1 ground state). The solid state
structure of 2·PMe3 (Figure 1) confirms the formulation of
this complex as a 16-electron PMe3 adduct that features
distorted trigonal bipyramidal coordination geometry at iron
with κ3-coordination of the PSiP ligand. The pincer ligand
phosphino donors and the PMe3 ligand are bound in the
equatorial plane, while Si and Cl take up the axial positions.
The related diamagnetic five-coordinate Ru complex (Cy-
PSiPMe)Ru(PMe3)Cl adopts a slightly different geometry in
the solid state, with one pincer ligand phosphino donor and
the PMe3 ligand bound in the axial positions of a distorted
trigonal bipyramidal structure.27 Complex 2·PMe3 is also
closely related in structure to the tris(phosphino)silyl iron
chloride complexes (SiRP3)FeCl (SiRP3 = κ4-(2-R2PC6H4)3Si;
R = Ph or iPr) prepared by Peters and co-workers,28 which also
feature slightly distorted trigonal bipyramidal geometry in the
solid state and exhibit similar μeff values (μeff = 2.9 μB and 3.3
i
μB for R = Ph and Pr, respectively).
Complex 2·PMe3 is the first example of an isolable,
electronically unsaturated (PSiP)Fe complex. While similarities
exist between 2·PMe3 and related iron complexes prepared by
Peters and co-workers that feature tetradentate tris-
(phosphino)silyl ligation, the use of a tridentate ligand herein
allows for potential dissociation or displacement of PMe3 from
the metal center, which may provide access to reactive low-
coordinate iron species more readily. For example, exposure of
a THF solution of 2·PMe3 to an atmosphere of CO led to
displacement of the PMe3 ligand and clean conversion to the
dicarbonyl complex 2·(CO)2 (Scheme 1). Complex 2·(CO)2 is
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diamagnetic and forms as a single Cs-symmetric isomer in
solution, as indicated by the presence of a single 31P{1H} NMR
resonance at 86.2 ppm. The solid state structure of 2·(CO)2
(Figure 1) and the presence of two νCO IR stretches (1969,
1909 cm−1), are consistent with the proposed formulation for
this complex. The 6-coordinate complex features octahedral
coordination geometry, with mer-κ3-coordination of the PSiP
ligand.
Reduction of 2·PMe3. Having previously observed
evidence for redox processes involving putative (Cy-PSiPMe)-
FeX species, we considered that 2·PMe3 might undergo a more
controlled single-electron reduction to a corresponding Fe(I)
complex. Although the +1 oxidation state is not common for
iron, there are several well-characterized examples of Fe(I)
complexes that have been previously reported, including
related tris(phosphino)silyl species prepared by Peters and
co-workers.28 While Co(I) complexes of the form (R-
PSiPMe)Co(PMe3)(N2) have been synthesized and applied
toward N2 reduction26c and alkene hydrogenation catalysis,25
thus far there have been no well-defined examples of analogous
five-coordinate PSiP-supported Fe(I) complexes. Such (PSiP)-
Fe(I) complexes could be susceptible to oxidative addition
reactions of E-H bonds, leading to Fe(III) species of the type
(PSiP)Fe(PMe3)(E)(H). Toward this end, a solution of 2·
PMe3 in THF was stirred with excess magnesium overnight
under a N2 atmosphere, leading to the formation of the desired
Figure 2. Crystallographically determined structure of 3 with thermal
ellipsoids shown at the 50% probability level; hydrogen atoms have
been omitted for clarity. Selected interatomic distances (Å) and
angles (deg) for 3 (one of two crystallographically independent
molecules): Fe−P1 2.2845(4), Fe−P2 2.2878(5), Fe−P3 2.2253(5),
Fe−Si 2.3145(5), Fe−N1 1.8191(15), N1−N2 1.119(2), Si−Fe−N1
177.17(5), P1−Fe−P2 118.122(18), P1−Fe−P3 135.000(19), P2−
Fe−P3 103.155(18).
paramagnetic product mixtures from which no pure material
could be isolated. In select cases, the formation of a variety of
distinct iron hydride species, including (Cy-PSiPMe)Fe(PMe3)-
(N2)H (4·N2), which has been previously reported by
Nishibayashi, Iwasawa, and co-workers,26c was observed. A
description of these observations is included in the Supporting
Information. As these hydride species were typically formed as
mixtures by what often appeared to be complex mechanisms,
the preparation of iron hydride species by more direct routes
was pursued.
paramagnetic complex (Cy-PSiPMe)Fe(PMe3)(N2) (3; μeff
=
1.9 μB, S = 1/2), which was isolated as a yellow solid in 61%
yield (eq 1).
Toward this end, 2·PMe3 was treated with one equiv
NaEt3BH under an N2 atmosphere. Monitoring of this reaction
1
mixture by H and 31P{1H} NMR spectroscopy revealed the
formation of a hydride complex formulated as (Cy-PSiPMe)-
Fe(PMe3)H (4, Scheme 2). Complex 4 gives rise to a Fe-H 1H
As in the case of 2·PMe3, and the Co(I) analogue (Cy-
PSiPMe)Co(PMe3)(N2), the solid state structure of 3 (Figure
2) features trigonal bipyramidal coordination geometry at the
iron center, with κ3-coordination of the PSiP ligand. The
pincer ligand phosphino donors and the PMe3 ligand are
bound in the equatorial plane, while Si and N2 take up the axial
positions. The structure of 3 is also similar to that of
(SiiPrP3)Fe(N2) reported by Peters and co-workers.28 The N−
N bond distance of 1.119(2) Å in 3 is elongated relative to that
of 1.065(5) Å observed for (SiiPrP3)Fe(N2), which suggests
increased electron backdonation from the iron center to the N2
ligand (cf. N−N bond length of 1.0975 Å for free N2).
However, the νNN stretching frequency of 2011 cm−1
observed for 3 is nearly identical to the value of 2008 cm−1
obtained for (SiiPrP3)Fe(N2), indicating minimal electronic
differences between the two complexes (cf. νNN = 2331 cm−1
for free N2). As such, the N2 ligand in 3 is best described as
weakly activated.29
Scheme 2. Synthesis of PMe3 Adducts of Fe(II)−Hydride
Complexes
PMe3 Adducts of Fe(II)-Hydride Complexes. Having
successfully reduced 2·PMe3 to an Fe(I) species, the reactivity
of 3 was first probed with respect to oxidative addition of E−H
bonds. While in most cases (e.g. H2, PhSiH3, H3N·BH3, HC
CPh) evidence of a reaction was observed (based on color
change) over the course of ca. 18 h at room temperature, most
of these reactions led to the formation of complicated
2
2
NMR resonance at −18.53 ppm (td, JPH = 64.2 Hz, JPH2
=
24.0 Hz), as well as 31P{1H} NMR resonances at 90.0 (d, JPP
= 55 Hz) and 3.6 (t, 2JPP = 55 Hz) ppm, assigned to the pincer
phosphino donors and PMe3, respectively. The IR spectrum of
4 does not feature a band corresponding to the νNN stretch of
coordinated N2. These spectroscopic signatures are distinct
D
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from those reported for 4·N2 (1H NMR: − 14.84 ppm Fe-H;
31P{1H} NMR: 84.8 PSiP, 11.5 ppm PMe3; νNN = 2041
cm−1).26c In their report on the prepartion of 4·N2 (accessed
directly from reaction of (Cy-PSiPMe)H with Fe(PMe3)4)
Nishibayashi and Iwasawa do not report N2 loss from 4·N2,
which suggests that the latter complex corresponds to a
thermodynamic product from which N2 loss is not facile. We
observed that while complex 4 was formed as the major
product from the reaction of 2·PMe3 with NaEt3BH, 4
coordinates N2 over time to form trace 4·N2 (ca. 30%
conversion to 4·N2 after 3 days in solution under an N2
atmosphere). Removal of the N2 atmosphere by subjecting a
solution of 4 and 4·N2 to sequential freeze−pump−thaw cycles
did not lead to regeneration of 4.
features broad 1H NMR resonances at −9.4, − 9.7, and −14.6
ppm (1:1:1 ratio). While full conversion to this product was
not observed directly from 4 or 4·N2 (ca. 10% conversion to 5
after 3 days), treatment of a mixture of 4 and trace 4·N2 with
H2 (1 atm) led to quantitative (by 31P NMR) formation of 5
after heating at 65 °C for 18 h. X-ray crystallographic analysis
of 5 revealed an apparent η2-(Si−H) complex of an Fe(II)
dihydride (Figure 3).
Complex 5 adopts distorted octahedral geometry in the solid
state, featuring mer-κ3-coordination of the PSiP ligand. The
Fe−Si distance of 2.2806(8) Å is well within the range of
2.2472(4) − 2.2844(4) Å observed for all (Cy-PSiPMe)Fe-
hydride complexes reported in this work. The acute Si−Fe−
H1 angle of 50.7(14)° is consistent with a likely interaction
between Si and H1. The measured Si···H1 distance of ca. 1.73
Å is much less than the sum of the van der Waals radii of the
two atoms (3.4 Å), strongly supporting an Si−H interaction in
the solid state. The Si···H2 distance of 2.34 Å, while also less
than the sum of the van der Waals radii of Si and H, is
significantly longer than the Si···H1 distance. Based on these
metrical parameters 5 can be formulated as an η2-(Si−H)
complex (through Si−H1−Fe) of an Fe dihydride, with a
potential secondary interaction between Si and H2 based on
their proximity.31
Complex 4 features distorted square pyramidal coordination
geometry in the solid state, on the basis of X-ray diffraction
analysis, such that the silyl donor occupies the axial position
(Figure 3). The Si−Fe−H1 angle of 75(2)° is acute, and the
In solution, each of H1, H2, and H3 (as labeled in Figure 3)
exist in unique environments, as evidenced by three unique
1
Fe−H H NMR resonances (a complex multiplet centered at
−9.7 ppm and two broad multiplet resonances centered at
−9.4 and −14.6 ppm, respectively; a detailed discussion of
assignments for these resonances and additional NMR data for
5 can be found in the Supporting Information). At room
temperature, selective 1D NOESY experiments (0.5 s mixing
time, Figure S6C) indicate exchange between all three hydride
environments. The resonance at −9.7 ppm (1JSiH = 65 Hz) was
assigned as H1 on the basis of a H−29Si HMQC experiment
1
(29Si chemical shift: 32.7 ppm). No correlation between Si and
Figure 3. Crystallographically determined structures of 4 (left) and 5
(right) with thermal ellipsoids shown at the 50% probability level;
hydrogen atoms not coordinated to Fe have been omitted for clarity.
Selected interatomic distances (Å) and angles (deg): for 4 Fe−P1
2.2081(10), Fe−P2 2.2044(18), Fe−Si 2.2727(17), Fe−H1 1.43(6),
P1−Fe−P1′ 146.98(7), P2−Fe−Si 128.08(8), P2−Fe−H1 157(2),
Si−Fe−H1 75(2); for 5 (one of two crystallographically independent
molecules) Fe−P1 2.2034(8), Fe−P2 2.1984(8), Fe−P3 2.2053(8),
Fe−Si 2.2806(8), Fe−H1 1.49(4), Fe−H2 1.57(3), Fe−H3 1.54(4),
P1−Fe−P2 147.56(3), P3−Fe−Si 129.01(3), Si−Fe−H1 50.7(14),
Si−Fe−H2 69.6(13), H2−Fe−H3 83.9(19).
1
either H2 or H3 was observed using H−29Si HMQC or
1H−29Si HMBC NMR techniques. While there can be
considerable ambiguity in the assignment of Si−H-M non-
classical interactions,30c the value of 65 Hz for JSiH is consistent
with a possible η2-(Si−H) interaction in 5 that persists in
solution.
Due to the relatively broad nature of the Fe−H resonances
1
in 5 at room temperature, variable temperature H NMR
experiments were carried out. Upon cooling (Figure S3), the
signals in the hydride region of the 1H NMR spectrum
gradually sharpened, with the best resolved coupling observed
at 253 K. Cooling below this temperature did not lead to
further resolution and line shape patterns generally remained
consistent between 193 and 273 K. At higher temperatures
(Figure S4), the broadened resonances begin to coalesce, with
the signals corresponding to H2 and H3 coalescing at
approximately 343 K. The signal for H1 also broadens
significantly with increasing temperature, but no coalescence
was observed up to 353 K.
Si···H1 distance of ca. 2.48 Å is less than the sum of the van
der Waals radii for these atoms (3.4 Å). However, the JSiH
value of 19 Hz obtained for 4 falls within a range where it is
not possible to make a definitive assignment of η2-(Si−H)
coordination (while accepted values of JSiH for η2-silane
complexes range from ca. 40−70 Hz, lower values are possible
in some circumstances).30 The solid state structure of the
related complex (Cy-PSiPMe)Ru(PMe3)H has previously been
reported27 and features similar coordination geometry.
Interestingly, single crystal X-ray analysis of material obtained
from treatment of 3 with HBPin revealed cocrystallized 4
(87.5%) and 4·N2 (12.5%), further substantiating the viability
of both iron hydride species (Figure S2 in the Supporting
Information).
Overall, the solution state structure of 5 appears to be
generally consistent with the solid state structure determined
31
́
through X-ray crystallographic analysis. Sabo-Etienne has
previously described the importance of secondary interactions
in silane complexes of ruthenium and has noted that Si−H
distances between 1.9−2.4 Å may indicate the presence of a
SISHA (secondary interaction between a silicon and a
hydrogen atom). Thus, as the Si−H2 distance of 2.27 Å falls
Surprisingly, mixtures of 4 and 4·N2 were found to react
further upon standing in benzene solution (Scheme 2), to
generate a third diamagnetic iron hydride species 5, which
E
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within this range, the presence of such an interaction cannot be
discounted, though no through-bond coupling could be
measured between Si and H2. Measurement of νFeH and
νSiH values by infrared spectroscopy has been cited as a useful
tool for the determination of the presence of a nonclassical
interaction, and for 5 νMH values of 1925 cm−1, 1871 cm−1,
and 1846 cm−1 were obtained. Unfortunately as the hydride
positions cannot be selectively deuterated (rapid exchange
intermediate that features a classical silyl group and a
dihydrogen ligand can be invoked for hydride exchange in 5.
Fe(II)-Hydride Complexes Lacking Exogenous Phos-
phine Coordination. In an effort to access increasingly
reactive iron hydride species, we targeted hydride complexes
that did not feature PMe3 stabilization. Toward this end,
treatment of (Cy-PSiPMe)H with py4FeCl2 and one equiv of
BnMgCl was found to generate (Cy-PSiPMe)Fe(py)Cl (2·py).
Treatment of 2·py with one equiv of NaEt3BH (Scheme 3) led
occurs, vide infra), values of isotopically shifted IR stretches are
31
́
of limited value. Sabo-Etienne cites the presence of a broad
and intense IR band in the range 1650−1800 cm−1 as a good
indicator of the presence of a σ-(Si−H)-type interaction;
however, none of the values for 5 lie within this range. While
the IR band at 1846 cm−1 is indeed intense and quite broad, it
cannot be definitively assigned as corresponding to a σ-(Si−
H).
Scheme 3. Synthesis of Fe(II)-Hydride Complexes Lacking
Exogenous Phosphine Coordination
A related Fe-based comparison to 5 is perhaps that of
[(PhBPiPr3)Fe(H)(η3-H2SiRR′)] (R = Me; R′ = Ph or Mes;
PhBPiPr3 = PhB(CH2PiPr2)3), which as the formula implies has
been assigned as an η3-silane Fe(II) adduct by Peters and co-
workers (binding of the silane through two H atoms, and Si, to
the Fe center).32 The bonding scenario for 5 differs
significantly, however, in that the Si−H−Fe interaction in 5
involves a tertiary silane and the metal coordination sphere
does not feature a similar phosphino-borate component. The
Si−H distances in Peters’ complexes (1.464(1) and 1.552(2)
Å) are both very reasonable for a significant Si−H interaction,
while in 5 the Si−H2 distance of ca. 2.27 Å is considerably
elongated by comparison and more in line with a SISHA.
Furthermore, the 29Si chemical shift of the η3-H2SiRR′−Fe
complex is 162 ppm (for R′ = Ph), which is dramatically
downfield relative to the 32.7 ppm value measured for 5, a
feature cited by Peters as reflective of the η3 nature of the
silane ligand. As such, complex 5 is likely best described as an
η2-silane adduct of an Fe(II) dihydride. While an Fe(IV) silyl
trihydride formulation may also be invoked for 5, this high
oxidation state would be unusual for a 3d metal. The closely
related complex Fe(H)2(H2)(PEtPh2)3 has been unambigu-
ously characterized as an Fe(II)-dihydride dihydrogen adduct
that features a similar cis-dihydride mer-tris(phosphine)
coordination geometry to 533 and is likely the most closely
matched comparison to the present study.
Complex 5 is a remarkably stable compound, showing little
to no evidence of thermal degradation upon extended periods
of heating (days, 80 °C) and also appears to be at least
moderately air and moisture stable. No reaction of 5 was
observed upon addition of CO2, and no alkene isomerization
was observed upon addition of 1-octene. Addition of BPh3 to 5
did not lead to any apparent generation of Ph3B·PMe3,
suggesting that PMe3 does not dissociate readily from the Fe
center. However, all three hydride positions in 5 undergo
exchange with D2 (1 atm) to generate 5-d3. This exchange is
relatively slow at room temperature (no 2H incorporation after
2 h at room temperature, by 1H NMR) but can be accelerated
1
to the clean (by 31P and H NMR) formation of a single
diamagnetic product (6·N2), featuring a 31P{1H} NMR
resonance at 87.3 ppm, as well as an Fe-H 1H NMR resonance
2
at −16.31 ppm (t, 1 H, JPH = 58.5 Hz). Unexpectedly, upon
attempted isolation of this hydride species, partial conversion
to a new diamagnetic iron hydride (6·py) was observed. This
newly formed hydride complex gives rise to a 31P{1H} NMR
1
resonance at 80.2 ppm and an Fe-H H NMR resonance
centered at −15.35 ppm (t, 1 H, JPH = 60.3 Hz). We suspect
2
that the reversible formation of a pyridine-triethylborane
adduct is implicated in this unusual interconversion. Upon
attempted workup of the initially generated hydride complex 6·
N2, the py·BEt3 byproduct partially dissociates, allowing
pyridine to recoordinate to the Fe center. Exposure of the
reaction mixture to vacuum leads to removal of free BEt3 and
drives the conversion of 6·N2 to the pyridine adduct 6·py. To
circumvent this process, we considered that a more Lewis
acidic borane might better sequester pyridine and facilitate the
isolation of 6·N2. Indeed, addition of one equiv of BPh3 to the
reaction mixture after the addition of NaEt3BH (Scheme 3)
was sufficient to trap pyridine as a py·BPh3 adduct, allowing for
the isolation of 6·N2. X-ray crystallographic analysis of both
iron hydrides revealed six-coordinate complexes in both cases,
where 6·N2 features two mutally cis-N2 ligands, while 6·py
features one N2 ligand and one pyridine coordinated to (Cy-
PSiPMe)FeH.
Both 6·py and 6·N2 feature a distorted octahedral geometry
in the solid state with κ3-coordination of the PSiP ligand in a
mer-configuration (Figure 4). In each complex the hydride
ligand occupies a coordination site cis to Si, with acute Si−Fe−
H(1) angles of 75.8(7)° and 78.8(7)° for 6·py and 6·N2,
respectively. The Si···H(1) distances of ca. 2.35 and 2.44 Å (for
6·py and 6·N2, respectively) are both less than the sum of the
van der Waals radii for the two atoms, and the measured values
of JSiH for the two complexes (67 and 70 Hz, respectively) are
with heating to 65 °C. From a mechanistic perspective, Sabo-
34
́
Etienne and co-workers have shown that in the case of
Ru(H)2[(η2-H-SiMe2)CH2CH2(η2-H-SiMe2)](PCy3)2, which
features two classical dihydride ligands and two η2-(Si−H)
interactions, facile exchange of all M−H occurs via a process
involving equilibria between the parent structure and isomers
that feature, in turn, one dihydrogen ligand, one η2-(Si−H)
ligand, and a classical silyl group, or two dihydrogen ligands
and two classical silyl groups. A similar process involving an
2
somewhat high for typical values of JSiH for classical cis-
F
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Alkene Hydrogenation Catalyzed by 6·N2. As described
in the Introduction, relatively few examples of homogeneous
Fe-catalyzed alkene hydrogenation have been described in the
literature, and with two potentially labile N2 ligands, similar to
Chirik’s catalysts,3f 6·N2 is anticipated to readily coordinate
incoming substrate molecules. To survey the effectiveness of 6·
N2 as an alkene hydrogenation precatalyst, 1-octene was first
investigated as a substrate. Using 6·N2 at a 5 mol % loading, 1-
octene was hydrogenated to n-octane in 98% conversion over 4
h (10 atm H2, room temperature; Table 1, Entry 1). Under 1
Table 1. Hydrogenation of Alkenes Catalyzed by 6·N2
Figure 4. Crystallographically determined structures of 6·N2 (left)
and 6·py (right) with thermal ellipsoids shown at the 50% probability
level; hydrogen atoms not coordinated to Fe have been omitted for
clarity. Selected interatomic distances (Å) and angles (deg): for 6·N2
Fe−H1 1.418(19), Fe−P1 2.22268(3), Fe−P2 2.2147(3), Fe−Si
2.2844(4), Fe−N1 1.8368(12), Fe−N3 1.8654(12), N1−N2
1.1111(18), N3−N4 1.1065(18), P1−Fe−P2 149.831(13), Si−Fe−
N3 174.21(4), N1−Fe−H1 171.4(7), Si−Fe−H1 78.8(7); for 6·py
Fe−H1 1.441(18), Fe−P1 2.2238(3), Fe−P2 2.2196(3), Fe−Si
2.2472(4), Fe−N1 1.7949(11), Fe−N3 2.0664(11), N1−N2
1.1228(16), P1−Fe−P2 156.068(14), Si−Fe−N3 171.52(3), N1−
Fe−H1 168.6(7), Si−Fe−H1 75.8(7).
disposed silyl hydrides, indicating the potential for nonclassical
interactions between H and Si in these complexes. Conversely,
the 29Si chemical shifts are shifted downfield, to 72.3 and 69.0
ppm, respectively, which are more characteristic of a metal silyl
complex rather than, for example, an η2-silane. Further support
for the assignment of these complexes as silyl hydrides is
provided by infrared spectroscopy, where values of 1949 and
2012 cm−1 for νFeH were measured for 6·py and 6·N2,
respectively, neither of which are typical of a nonclassical Si−H
interaction.31
Both complexes feature an N2 ligand coordinated trans to
the Fe−H. The N2 ligand in 6·py features a N−N bond length
of 1.1228(16) Å, and a value of 2012 cm−1 was measured for
the N−N stretching frequency by IR spectroscopy, indicating
moderate activation. Comparatively in 6·N2, the N2 ligand
bound trans to Si features a N−N bond distance of 1.1065(18)
Å, while the N2 trans to the Fe-H features a comparable bond
distance of 1.1111(18) Å. By use of IR spectroscopy, N−N
stretching frequencies of 2123 and 2063 cm−1 were measured
for 6·N2. The propensity of 6·N2 to recoordinate pyridine, as
observed during its preparation, consistently leads to only one
isomer of 6·py with displacement of the N2 ligand trans to Si.
Complex 6·N2 can also be accessed from 2·PMe3 through an
analogous route; however, the concomitant formation of small
amounts of 4, 4·N2, and 5 was typically observed, among other
byproducts.
Complex 6·N2 is particularly intriguing, as it can be thought
of as a surrogate for the low-coordinate Fe hydride, (Cy-
PSiPMe)FeH, which does not appear to be isolable under a N2
atmosphere. The N2 ligands (particularly the N2 bound trans
to Si) are anticipated to be displaced readily by a variety of
incoming donors. We considered that 6·N2 appeared poised
for coordination and subsequent insertion of an unsaturated
substrate, and thus we turned our attention toward catalytic
alkene hydrogenation using 6·N2 as a precatalyst.
a
Conversion on the basis of 1H NMR integration using 1,3,5-
trimethoxybenzene as an internal standard (average of two runs).
b
c
Reaction performed at room temperature. Isolated yield denoted in
parentheses.
atm H2, 5 mol % 6·N2 was sufficient to afford n-octane in 99%
conversion over 18 h at room temperature (Table S1, Entry 1),
a time that could be decreased to 4 h by heating at 65 °C
(Table S1, Entry 2). Lastly, the catalyst loading could be
reduced to 0.5 mol %, affording 97% conversion to n-octane
after heating for 4 h 65 °C (10 atm H2; Table S1, Entry 3).
Encouraged by these initial results, the substrate scope was
expanded to include a variety of substituted alkenes. While
moderately elevated temperature (65 °C) and pressure (10
atm H2) were required to afford high conversion for such
substrates, a wide variety of alkenes were found to be active
toward hydrogenation. Both cis- and trans-4-octene were
hydrogenated quantitatively (Table 1, Entries 2 and 3), as was
cis-stilbene (Table 1, Entry 6). For 4-vinylcyclohexene, at 65
°C, both the internal double bond and the vinyl group were
reduced, yielding ethylcyclohexane with 94% conversion
(Table 1, Entry 5). When this substrate was hydrogenated at
G
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room temperature, the internal double bond was left intact,
and 4-ethylcyclohexene could be obtained selectively in 93%
conversion (Table 1, Entry 4). A 1,1-disubstituted alkene, α-
methylstyrene, was fully converted to cumene (Table 1, Entry
7), and even a trisubstituted alkene, ethylidenecyclohexane,
was hydrogenated to ethylcyclohexane in 97% conversion
(Table 1, Entry 8).
substrates, no special precautions were required to prevent
undesired C−O bond cleavage chemistry involving the
substrate;5,9 all substrates were combined with the precatalyst
at room temperature prior to introduction of H2. Ketones
appear to not be tolerated by this system, with 11% conversion
obtained in the hydrogenation of 5-hexen-2-one (Table 2,
Entry 1); no evidence for reduction of the carbonyl was
observed. Hydrogenation of a terminal alkyne, phenyl-
acetylene, was also attempted; however, a complex mixture
of products was obtained (by 1H NMR), and while no
unreacted phenylacetylene was observed, the only products
that could be identified were ethylbenzene (ca. 19%) and 1,4-
diphenylbutane (ca. 14%). The generation of 1,4-diphenylbu-
tane implicates a homocoupling process, followed by hydro-
genation. Such a process could yield a variety of hydrogenated
or partially hydrogenated derivatives and may in part explain
the large product distribution obtained. A 1,2-disubstituted
alkyne, diphenylacetylene, was better tolerated by the catalyst,
and under standard conditions 36% conversion to bibenzyl was
observed, with 42% conversion to trans-stilbene (Table 2,
Entry 5).35 However, by increasing the temperature to 90 °C
and the reaction time to 6 h, selective formation of bibenzyl in
92% conversion was obtained (Table 2, Entry 6).
While (PDI)Fe(N2)2 and (CNC)Fe(N2)2 alkene hydro-
genation catalysts of the type developed by Chirik and co-
workers5,11b are apparently more active than 6·N2, the latter
PSiP complex is notable in that it represents a rare example of
an Fe catalyst that is capable of effectively hydrogenating a
comparable range of alkene substrates, including aliphatic
alkenes and a trisubstituted example, under relatively mild
conditions. Moreover, the reactivity of 6·N2 is complementary,
in that side reactions involving C−O bond cleavage have not
been observed, allowing for facile reduction of alkenes that
feature ether and ester functionality. Further efforts to fine-
tune the PSiP ligand scaffold may lead to increased activity and
selectivity, and such strategies are currently being investigated
by our group, as are the mechanistic details of these
transformations.
Although the reaction of (Cy-PSiPMe)Fe(PMe3)(N2) (3)
with H2 afforded an apparent mixture of paramagnetic species
(vide supra), the reactivity of 3 was tested under alkene
hydrogenation conditions. Using the same conditions
employed with 6·N2 (5 mol % 3, 10 atm H2, 65 °C), only
35% conversion of 1-octene and 17% conversion of trans-4-
octene to n-octane were observed, respectively, after 4 h. No
isomerization of 1-octene was observed under these con-
ditions. By comparison, the Co analogue (Cy-PSiPMe)Co-
(PMe3)(N2) achieved 95% conversion of 1-octene to n-octane
under comparable reaction conditions.25 As (Cy-PSiPMe)FeH-
(PMe3)(N2)n (4, n = 0; 4·N2, n = 1) converts to 5 with the
addition of hydrogen (vide supra), hydrogenation of trans-4-
octene using 5 was also attempted for direct comparison with
6·N2. Under the typical hydrogenation conditions employed (5
mol % 5, 10 atm H2, 65 °C) <5% conversion to n-octane was
observed after 4 h. Collectively, these observations establish 6·
N2 as being superior among the bis(phosphino)silyl Fe
precatalysts examined herein.
Notably, hydrogenation catalyzed by 6·N2 was found to
proceed with functional group tolerance toward ester and ether
functionalities. Ethyl crotonate was hydrogenated to ethyl
butanoate in 80% conversion under the standard conditions
(Table 2, Entry 2), while allyl butyl ether and allyl phenyl ether
were hydrogenated with 96 and 98% conversions, respectively
(Table 2, Entries 3 and 4). Notably for these C−O-containing
Table 2. Hydrogenation of Functionalized Alkenes and
Alkynes Catalyzed by 6·N2
3. CONCLUSION
Thus far, literature examples of iron complexes supported by
phenylene-based PSiP pincer ligation have been limited to only
two examples of coordinatively saturated 18-electron com-
plexes of Fe(II).26a,c We have expanded the Fe chemistry of
this versatile ligand class, providing examples of coordinatively
unsaturated Fe(II) and Fe(I) complexes supported by Cy-
PSiPMe. While isolable four-coordinate Fe(II) complexes
remain elusive, possibly due to their inherent instability
toward one-electron redox processes, we have demonstrated
that five-coordinate analogues are readily prepared and
isolated.
A variety of diamagnetic iron hydride species have been
observed and characterized herein, and interconversion among
such complexes facilitated by the coordination of N2 has been
documented. We have also observed evidence for η2-silane
coordination in such iron hydride species involving the PSiP
ligand, and such interactions may play a role in the subsequent
reactivity of these complexes. A unique example of an η2-(Si−
H) complex of an Fe dihydride (5) was investigated
thoroughly by use of a suite of 1D- and 2D-NMR techniques
over a range of temperatures to elucidate the probable bonding
situation in this complex. We also successfully prepared a PSiP-
supported iron hydride complex stabilized only by N2
a
Conversion on the basis of 1H NMR integration using 1,3,5-
trimethoxybenzene as an internal standard (average of two runs).
b
Conversion to trans-stilbene; 36% conversion to bibenzyl also
c
observed; no cis-stilbene observed. Reaction performed in toluene-d8,
for 6 h, at 90 °C.
H
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benzene and filtered through Celite. The filtrate solution was
collected, and benzene was removed in vacuo. The remaining residue
was triturated with 3 × 2 mL pentane and washed with 3 × 1 mL cold
(−35 °C) Et2O to afford 2·(CO)2 as a tan solid (0.18 g, 58% yield).
Method 2: A 250 mL resealable Schlenk flask equipped with a Teflon
stopcock was charged with a solution of 2·PMe3 (0.10 g, 0.13 mmol)
in ca. 7 mL of THF. The solution was degassed via three freeze−
pump−thaw cycles and then placed under an atmosphere of CO. A
color change from red-orange to light orange was observed in under 1
min. After stirring for a further 10 min, the volatile components of the
reaction mixture were removed in vacuo. The remaining residue was
triturated with ca. 2 mL of pentane and subsequently washed with 3 ×
2 mL of pentane to afford 2·(CO)2 (0.081 g, 84% yield) as a tan solid.
X-ray quality crystals of 2·(CO)2 were obtained from a concentrated
Et2O solution −35 °C. 1H NMR (300 MHz, benzene-d6): δ 8.13 (d, J
= 7 Hz, 2 H, Harom), 7.47 (m, 2 H, Harom), 7.27−7.15 (4 H, Harom),
2.61−2.55 (6 H, PCy), 2.20−1.95 (8 H, PCy), 1.80−1.05 (30 H,
PCy), 0.95 (s, 3 H, SiMe). 13C NMR (75.5 MHz, benzene-d6): δ
218.1 (Fe-CO), 212.2 (Fe-CO), 156.6 (apparent t, J = 21 Hz, Carom),
142.5 (apparent t, J = 24 Hz, Carom), 133.7 (apparent t, J = 9 Hz,
CHarom), 131.0 (CHarom), 129.4 (CHarom), 127.6 (CHarom), 42.8
(apparent t, J = 9 Hz, CHCy), 41.4 (apparent t, J = 10 Hz, CHCy), 33.6
(CH2Cy). 29.9 (CH2Cy), 29.0−28.8 (overlapping resonances, CH2Cy),
28.2 (CH2Cy), 27.7 (CH2Cy), 27.3 (CH2Cy), 26.6 (CH2Cy), 26.3
(CH2Cy), 9.2 (SiMe). 31P{1H} NMR (121.5 MHz, benzene-d6): δ
86.2. 29Si NMR (59.6 MHz, benzene-d6): δ 64.3. IR (thin film, cm−1):
1969 (s, νCO), 1909 (s, νCO). Anal. Calcd for C39H55O2P2SiClFe: C,
63.54; H, 7.52. Found: C, 63.29; H, 7.87.
coordination (6·N2), which can be thought of as a surrogate
for the unobserved four-coordinate species (Cy-PSiPMe)FeH.
This complex was found to be an effective, easily manipulated
precatalyst for the hydrogenation of a range of terminal and
multiply substituted alkenes. Esters and ethers were found to
be well-tolerated by the catalyst, and alkyne hydrogenation was
also demonstrated. This represents a relatively rare example of
Fe-catalyzed alkene direct hydrogenation, a reaction of
fundamental importance in organometallic catalysis.
4. EXPERIMENTAL SECTION
General Considerations. All experiments were conducted under
nitrogen in a glovebox or using standard Schlenk techniques.
Tetrahydrofuran and diethyl ether were distilled from Na/
benzophenone ketyl. Benzene, toluene, and pentane were first sparged
with nitrogen and subsequently dried by passage through a double-
column (one activated alumina column and one column packed with
activated Q-5) solvent purification system. All purified solvents were
stored over 4 Å molecular sieves. Benzene-d6 and cyclohexane-d12
were degassed via three freeze−pump−thaw cycles and stored over 4
Å molecular sieves. The ligand precursor (Cy-PSiPMe)H21 and
36
FeCl2(THF)1.5 were prepared by previously reported methods. All
other reagents were purchased from commercial suppliers and used
without further purification. Unless otherwise stated, 1H, 13C, 11B, 31P,
and 29Si characterization data were collected at 300 K, with chemical
1
shifts reported in parts per million downfield of SiMe4 (for H, 13C,
1
and 29Si), BF3·OEt2 (for 11B), or 85% H3PO4 in D2O (for 31P). H
(Cy-PSiPMe)Fe(PMe3)N2 (3). Mg powder (0.027 g, 1.1 mmol) was
added to a solution of 2·PMe3 (0.085 g, 0.11 mmol) in ca. 5 mL THF.
The resulting mixture was stirred for 18 h at room temperature and
was subsequently filtered through Celite. The filtrate solution was
evaporated to dryness in vacuo, and the remaining residue was
extracted with pentane (5 mL) and filtered through Celite. The filtrate
was collected, and the volatile components were removed in vacuo.
The resulting red solid was washed with Et2O (5 × 0.25 mL) and
dried under vacuum to afford paramagnetic 3 (0.052 g, 61% yield) as
a bright yellow solid. Crystals of 3 suitable for X-ray diffraction
analysis were obtained from a concentrated Et2O solution at −35 °C.
and 13C NMR chemical shift assignments are based on data obtained
from 13C{1H}, 13C-DEPTQ, 1H−1H COSY, 1H−13C HSQC, and
1H−13C HMBC NMR experiments. 29Si NMR assignments are based
on 1H−29Si HMBC and 1H−29Si HMQC experiments. Solution
magnetic moment measurements were determined by use of the
Evans method.37 Infrared spectra were recorded as thin films between
NaCl plates at a resolution of 4 cm−1.
Cy-PSiPMe)Fe(PMe3)Cl (2·PMe3). A solution of (Cy-PSiPMe)H (0.25
g, 0.42 mmol) in ca. 3 mL of THF was added to a suspension of
FeCl2(THF)1.5 (0.099 g, 0.42 mmol) in ca. 3 mL of THF. Neat PMe3
(0.064 g, 0.85 mmol) was added to the resulting suspension. The
reaction mixture was stirred at room temperature until all solids were
dissolved to give a green solution (ca. 2−3 min). The reaction mixture
was subsequently cooled to −35 °C, and BnMgCl (0.30 mL, 1.4 M in
THF, 0.42 mmol) was added dropwise. The resulting red solution was
stirred for 2 h at room temperature, at which point the volatile
components were removed under vacuum. The resulting residue was
extracted with ca. 5 mL benzene and filtered through Celite. The
filtrate solution was evaporated to dryness, and the remaining residue
was triturated with 3 × 5 mL of pentane to afford paramagnetic 2·
PMe3 (0.30 g, 94% yield) as a red-orange solid. Crystals of 2·PMe3
suitable for X-ray diffraction analysis were obtained from a
concentrated Et2O solution at −35 °C. μeff (benzene-d6): 3.04 μB
1
μeff (benzene-d6): 1.88 μB (S = 1/2). H NMR (300 MHz, benzene-
d6): δ 9.00, 8.20, 8.07, 7.66, 7.32, 7.22, 6.81, 5.68, 5.09, 4.41, 3.85,
3.27, 0.79−2.64 (overlapping resonances). IR (thin film, cm−1): 2011
(s, νN2). Anal. Calcd for C40H64N2P3SiFe: C, 64.07, H, 8.60, N: 3.74.
Found: C, 63.75; H, 8.83; N; 3.38.
(Cy-PSiPMe)Fe(PMe3)H (4). A solution of 2·PMe3 (0.030 g, 0.040
mmol) in ca. 3 mL benzene was treated with NaEt3BH (0.040 mL, 1.0
M in toluene, 0.040 mmol) added via microsyringe. A color change
from red to dark brown was observed, and the resulting mixture was
allowed to stir for 18 h at room temperature. The volatile components
of the reaction mixture were removed in vacuo, and the remaining
residue was triturated with 3 × 1 mL pentane and extracted with 5 mL
of a ca. 1:1 pentane:benzene mixture. The extracts were combined
and filtered through a glass microfiber filter. The filtrate solution was
evaporated to dryness in vacuo and triturated with 3 × 1 mL pentane
1
(S = 1). H NMR (benzene-d6, 300 MHz) δ: 73.65, 62.73, 22.18,
6.56, 6.24, 5.29, 4.67, 3.90, −0.40−2.10 (overlapping resonances),
−1.43, −2.32, −3.39, −6.76, −51.01. Anal. Calcd for C40H64P3SiClFe:
C, 63.44; H, 8.52. Found: C, 63.08; H, 8.74.
1
to afford 4 as an off-white solid (0.021 g, 74% yield). H NMR (500
MHz, benzene-d6): δ 8.36 (d, J = 7 Hz, 2 H, Harom), 7.01 (d, J = 8 Hz,
2 H, Harom), 7.33 (apparent t, J = 7 Hz, 2 H, Harom), 7.23 (apparent t, J
= 7 Hz, 2 H, Harom), 2.38 (m, 2 H, PCy), 2.16 (m, 4 H, PCy), 2.04 (m,
2 H, PCy), 1.05−1.90 (overlapping resonances, 45 H, PCy + PMe3;
the PMe3 resonance was identified as a doublet (2JPH = 6 Hz) at 1.49
(Cy-PSiPMe)Fe(CO)2Cl (2·(CO)2). Method 1. A 250 mL resealable
Schlenk flask equipped with a Teflon stopcock and containing a
magnetic stirbar was charged with a FeCl2(THF)1.5 (0.099 g, 0.42
mmol). A solution of (Cy-PSiPMe)H (0.25 g, 0.42 mmol) in ca. 10 mL
THF was added. The reaction mixture was degassed through three
sequential freeze−pump−thaw cycles and then placed under an
atmosphere of CO. The mixture was then cooled to 0 °C, and a
solution of BnMgCl (0.30 mL, 1.4 M in THF, 0.42 mmol) in ca. 2 mL
of THF was added dropwise via syringe under a flow of CO, resulting
in a gradual color change to green-yellow. The reaction mixture was
allowed to warm to room temperature and stir for 2 days under a
static CO atmosphere. The volatile components of the reaction
mixture were subsequently removed in vacuo, and the resulting
residue was triturated with 3 × 2 mL pentane and extracted with 5 mL
2
2
ppm), 0.96 (s, 3 H, SiMe), −18.53 (td, JPH = 64 Hz, JPH= 24 Hz, 1
H, FeH). 13C NMR (125.8 MHz, benzene-d6): δ 160.5 (apparent t, J
= 21 Hz, Carom), 149.6 (m, Carom), 132.0 (apparent t, J = 8 Hz,
CHarom), 128.5 (CHarom), 128.4 (CHarom), 127.0 (CHarom), 42.5
(CHCy), 32.0 (CHCy), 29.9 (CH2Cy), 29.6 (CH2Cy), 29.1 (CH2Cy),
28.8 (CH2Cy), 28.6−26.6 (overlapping resonances, CH2Cy), 24.3 (d,
1JCP = 16 Hz, PMe3), 6.2 (SiMe). 31P{1H} NMR (202.5 MHz,
benzene-d6): δ 90.0 (d, 2JPP = 55 Hz, 2 P, PSiP), 3.6 (t, 2JPP = 55 Hz, 1
P, PMe3). 29Si NMR (99.4 MHz, benzene-d6): δ 77.4 (d, JSiH = 19.5
Hz). IR (thin film, cm−1): 1815 (m, νFeH). This complex was
I
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observed to convert to both (Cy-PSiPMe)FeH(PMe3)(N2) (4·N2)26c
and 5 over time (as described fully in the text; Scheme 2), both in the
solid state and in solution. As such, satisfactory combustion analysis
could not be obtained.
and the collected extracts were filtered through Celite, combined with
ca. 5 equiv pyridine (20 μL), and allowed to stir briefly (5 min) before
all volatile components were removed in vacuo. The resulting solid
was triturated with 3 × 2 mL pentane and washed with 3 × 0.5 mL
cold (−35 °C) pentane to afford a mixture of 6·py and 6·N2 (0.024 g;
ca. 1:1 ratio of 6·py to 6·N2 typically obtained). A slight excess of
pyridine was thus added to the mixture to minimize the amount of 6·
N2 for solution spectroscopic analysis. In one instance, a minute
quantity of X-ray quality crystals of 6·py was obtained from a
concentrated Et2O solution of a mixture of 6·py and 6·N2, as prepared
above, at room temperature. 1H NMR (300 MHz, benzene-d6): δ 9.35
(broad, 1 H, Hpy), 8.31 (d, J = 7 Hz, 2 H, Harom), 8.14 (broad, 1 H,
Hpy), 7.75 (d, J = 7 Hz, 2 H, Harom), 7.35 (apparent t, J = 7 Hz, 2 H,
[Cy-PSi(μ-H)PMe]FeH2(PMe3) (5). A solution of 2·PMe3 (0.067 g,
0.089 mmol) in ca. 5 mL of benzene was treated with NaEt3BH
(0.089 mL, 1.0 M in toluene, 0.089 mmol) added via microsyringe.
The resulting mixture was stirred for 2 h at room temperature and
then evaporated to dryness in vacuo. The remaining residue was
triturated with 3 × 2 mL pentane and was subsequently redissolved in
ca. 3 mL benzene and filtered through Celite. The filtrate solution was
transferred to a 50 mL thick-walled reaction vessel equipped with a
Teflon stopcock and was degassed through three sequential freeze−
pump−thaw cycles and placed under an atmosphere of H2. The
reaction mixture was allowed to stir for 18 h at 65 °C. The volatile
components of the reaction mixture were then removed in vacuo. The
remaining residue was triturated with 3 × 2 mL pentane and then
washed with 3 × 1 mL pentane to yield 5 as a beige-yellow solid
(0.052 g, 81% yield). X-ray quality crystals were grown from a
H
arom), 7.26 (apparent t, J = 7 Hz, 2 H, Harom), 6.81 (broad, 1 H, Hpy),
6.55−6.30 (overlapping resonances, 2 H, Hpy), 2.62 (m, 2 H, PCy),
2.10−0.90 (overlapping resonances, 42 H, PCy), 0.75 (s, 3 H, SiMe),
−15.34 (t, JPH = 61 Hz, 1 H, FeH). 13C{1H} NMR (125.8 MHz,
2
benzene-d6): δ 162.2 (apparent t, J = 23.9 Hz, Carom), 159.8 (CHpy),
153.7 (CHpy), 147.1 (apparent t, J = 25 Hz, Carom), 133.4 (CHpy),
132.6 (t, J = 8.8 Hz, CHarom), 129.0 (CHarom), 128.9 (CHpy), 128.8
(CHarom), 127.2 (CHarom), 123.3 (CHpy), 40.4 (CHCy), 32.3 (CHCy),
29.9 (CH2Cy), 29.4 (CH2Cy), 29.2 (CH2Cy), 28.9 (CH2Cy), 28.4−28.1
(overlapping resonances, CH2Cy), 28.0 (CH2Cy), 27.7 (CH2Cy), 27.5
(CH2Cy), 27.3 (CH2Cy), 6.2 (SiMe). 31P{1H} NMR (121.5 MHz,
benzene-d6): δ 80.2. 29Si NMR (99.4 MHz, benzene-d6): δ 72.3 (2JSiH
= 67 Hz). IR (thin film, cm−1): 2012 (νN2), 1949 (νFeH).
1
concentrated pentane/THF solution of 5 at room temperature. H
NMR (300 MHz, benzene-d6; ABXY2Z spin system, where Y2Z =
(RPCy2)2(PMe3)): δ 8.22 (d, J = 6.9 Hz, 2 H, Harom), 7.42 (d, J = 7
Hz, 2 H, Harom), 7.26 (apparent t, J = 7.2 Hz, 2 H, Harom), 7.12
(apparent t, J = 7 Hz, 2 H, Harom), 2.53 (m, 2 H, PCy), 2.30−1.05
(overlapping resonances, 50 H, PCy + PMe3 + SiMe; the PMe3
resonance was identified as a doublet at 1.36 ppm (2JPH = 7 Hz); the
SiMe resonance was identified as a singlet at 1.13 ppm, 0.79 (m, 2 H,
(Cy-PSiPMe)FeH(N2)2 (6·N2). A solution of 2·py (0.11 g, 0.14 mmol)
in ca. 7 mL of benzene was treated with NaEt3BH (0.14 mL, 1.0 M in
toluene, 0.14 mmol) added via microsyringe. A gradual color change
from dark red-orange to yellow-orange was observed, and the reaction
mixture was allowed to stir for 18 h at room temperature. A solution
of BPh3 (0.034 g, 0.14 mmol) in ca. 1 mL benzene was then added,
and the reaction mixture was allowed to stir for a further 2 h. The
volatile components of the reaction mixture were removed in vacuo,
and the resulting residue was triturated with 3 × 3 mL pentane and
then extracted with ca. 15 mL pentane, and the combined extracts
were filtered through Celite. The filtrate solution was evaporated to
dryness, and the remaining residue was subsequently washed with 3 ×
1 mL pentane to yield 6·N2 as a tan solid (0.051 g, 61% yield). X-ray
quality crystals were obtained from a concentrated solution of 6·N2 in
1:1 pentane: Et2O at room temperature. 1H NMR (500 MHz,
benzene-d6): δ 8.14 (d, J = 7 Hz, 2 H, Harom), 7.58 (d, J = 7 Hz, 2 H,
2
PCy), 0.42 (m, 2 H, PCy), A = −9.5 ppm (1 H, FeH, JHH = −9.18
Hz, 2JHH = −11.63 Hz, 2JH‑PCy2 = 51.07 Hz, 2JH‑PMe3 = 70.40 Hz), B =
−9.8 ppm (1 H, SiHFe, 2JHH = −9.18 Hz, 2JH‑PCy2 = 16.42 Hz, 2JH‑PMe3
= 41.07 Hz), X = −14.5 ppm (1 H, FeH, 2JHH = −11.63 Hz, 2JH‑PCy2
=
59.36 Hz, 2JH‑PMe3 = 15.31 Hz). 13C{1H} NMR (75.5 MHz, benzene-
d6): δ 160.1 (Carom), 148.1 (Carom), 131.8 (CHarom), 127.7 (CHarom),
127.3 (CHarom), 125.8 (CHarom), 39.0 (CHCy), 34.5 (apparent t, J =
13.6 Hz, CHCy), 29.6 (CH2Cy), 29.1 (CH2Cy), 28.4−27.3 (overlapping
resonances, CH2Cy), 26.7 (CH2Cy), 9.13 (SiMe). 31P{1H} NMR
2
(121.5 MHz, benzene-d6): δ 100.8 (d, JPP = 29 Hz, 2 P, PSiP), 18.7
2
(t, JPP = 29 Hz, 1 P, PMe3). 29Si NMR (59.6 MHz, benzene-d6): δ
32.7. IR (KBr pellet, cm−1): 1925 (νFeH), 1871 (νFeH), 1846
(νFeH). Anal. Calcd For C40H67P3SiFe: C, 66.28; H, 9.32. Found: C,
65.94; H, 9.21.
Generation of (Cy-PSiPMe)FeCl(py) (2·py). To a suspension of
(pyridine)4FeCl2 (0.15 g, 0.34 mmol) in ca. 15 mL THF was added
(Cy-PSiP)H (0.20 g, 0.34 mmol). The yellow suspension was stirred
for 1 h at room temperature. BnMgCl (0.24 mL, 0.34 mmol) as a 1.4
M solution in THF was diluted to approximately 2 mL in THF and
added dropwise to the stirring suspension at room temperature. A
color change from bright yellow to dark red was observed, and the
resulting mixture was allowed to stir for 18 h at room temperature.
The volatile components of the reaction mixture were removed in
vacuo and the resulting residue was triturated with 3 × 3 mL pentane
and extracted with ca. 10 mL benzene and filtered through Celite. The
filtrate solution was collected and solvent was removed in vacuo. The
residue was triturated with 3 × 3 mL pentane and washed with 3 × 2
mL pentane to afford 2·py as a red solid (0.21 g, 80% crude yield).
We have not been able to obtain satisfactory elemental analysis for
this compound. Nonetheless, crude 2·py, prepared and isolated as
described above, was used directly and with success for subsequent
H
arom), 7.30 (apparent t, J = 7 Hz, 2 H, Harom), 7.19 (apparent t, J = 7
Hz, 2 H, Harom), 2.49 (m, 2 H, PCy), 2.39 (m, 2 H, PCy), 2.23 (m, 2
H, PCy), 2.02 (m, 2 H, PCy), 1.91 (m, 2 H, PCy), 1.86−1.75
(overlapping resonances, 4 H, PCy), 1.70 (m, 2 H, PCy), 1.65−1.46
(overlapping resonances, 12 H, PCy), 1.44−1.04 (overlapping
2
resonances, 16 H, PCy), 0.71 (s, 3 H, SiMe), −16.31 (t, JPH = 59
Hz, 1 H, FeH). 13C NMR (125.8 MHz, benzene-d6): δ 159.9
(apparent t, J = 24 Hz, Carom), 145.1 (apparent t, J = 26 Hz, Carom),
132.5 (apparent t, J = 9 Hz, CHarom), 129.2 (CHarom), 128.7 (CHarom),
127.4 (CHarom), 39.6 (apparent t, J = 5 Hz, CHCy), 38.6 (apparent t, J
= 10 Hz, CHCy), 29.6 (CH2Cy), 29.5 (CH2Cy), 28.9 (CH2Cy), 28.2−
28.0 (overlapping resonances, CH2Cy), 27.6 (CH2Cy), 27.3 (apparent
t, J = 6 Hz, CH2Cy), 27.1 (CH2Cy), 26.7 (CH2Cy), 5.9 (SiMe). 31P
NMR (202.5 MHz, benzene-d6): δ 87.3. 29Si NMR (99.4 MHz,
benzene-d6): δ 69.0 (d, JSiH = 70 Hz). IR (thin film, cm−1): 2123
(νN2), 2063 (νN2), 2012 (νFeH). Anal. Calcd for C37H56N4P2SiFe:
C, 63.24; H, 8.03; N, 7.97. Found: C, 62.91; H, 7.88; N, 7.80.
General Procedure for Catalytic Hydrogenation Experi-
ments. All hydrogenations were performed on a 0.057 mmol
substrate scale using a 250 μL total reaction volume. All substrates
and catalysts were delivered via microsyringe as stock solutions in the
indicated solvent (typically benzene-d6). A 1 dram vial containing
preweighed internal standard (1,3,5-trimethoxybenzene) was charged
with 0.057 mmol substrate (as a stock solution in benzene-d6) and 6·
N2 (as a stock solution in benzene-d6). Additional benzene-d6 was
then added to bring the total reaction volume to 250 μL. The vial was
then equipped with a stirbar and closed with a PTFE-sealed cap, with
a needle inserted through the seal to allow for introduction of H2 gas.
1
syntheses. μeff (benzene-d6): 2.93 μB (S = 1). H NMR (benzene-d6,
300 MHz): δ 61.15, 42.00, 16.00, 15.97, 10.76, 8.75, 5.66, 5.15, 3.41,
2.50−0.40 (overlapping resonances), −0.19, −0.56, −1.04, −2.96,
−3.29, −4.84, −9.68, −61.0.
Generation of (Cy-PSiPMe)FeH(py)(N2) (6·py). A solution of 2·py
(0.037 g, 0.049 mmol) in ca. 3 mL of benzene was treated with
NaEt3BH (0.049 mL, 1.0 M in toluene, 0.049 mmol) added via
microsyringe. A gradual color change from dark red-orange to yellow-
orange was observed, and the mixture was stirred for 18 h at room
temperature. The volatile components of the reaction mixture were
removed in vacuo, and the remaining residue was triturated with 3 × 2
mL pentane. The residue was then extracted with ca. 3 mL benzene,
J
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The vial was then transferred to a Parr reactor which was sealed and
purged with H2 and subsequently pressurized to 10 atm. The Parr
reactor was heated to 65 °C in an oil bath for the duration of the
reaction time. Afterward, the Parr reactor was removed from heat,
allowed to cool to room temperature, and depressurized of H2. In the
glovebox, the sample was transferred to an NMR tube for data
acquisition. For calculation of NMR conversions, a chosen diagnostic
product signal was integrated relative to that of the internal standard.
An excessively long relaxation delay (60 s) was used to ensure
accurate integration. All reactions were performed in duplicate, and
reported conversions are the average of both runs. The same
procedure was followed for all hydrogenations at 10 atm, with
variations in solvent, catalyst, or temperature, as indicated. For
hydrogenations at 1 atm (at room temperature or 65 °C), reactions
were performed in a 50 mL thick-walled reaction vessel equipped with
a Teflon stopcock, from which headspace N2 was removed via one
freeze−pump−thaw cycle.
attached atom. Additional crystallographic detail is available in the
deposited CIFs (CCDC 1870820−1870830).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00807.
■
S
Experimental details for synthesis of all complexes and
hydrogenation procedures, crystallographic refinement
details, supplementary hydrogenation data, detailed
discussion and supplementary spectroscopic data for
complex 5, and selected NMR spectra for reported
complexes and hydrogenation experiments (PDF)
Accession Codes
Procedure for Catalytic Hydrogenation of α-Methylstyrene.
A 1 dram vial containing 1,3,5-trimethoxybenzene (9.7 mg) was
charged with 0.057 mmol α-methylstyrene (82.7 μL of a 0.69 M stock
solution in benzene-d6) and 6·N2 (as 100 μL of a 0.028 M stock
solution in benzene-d6). Benzene-d6 (67.3 μL) was then added to
bring the total reaction volume to 250 μL. The vial was then equipped
with a stirbar and closed with a PTFE-sealed cap, with a needle
inserted through the seal to allow for the introduction of H2 gas. The
vial was then transferred to a Parr reactor which was sealed and
purged with H2 and subsequently pressurized to 10 atm. The Parr
reactor was heated at 65 °C in an oil bath for 4 h. Afterward, the Parr
reactor was removed from heat, allowed to cool to room temperature,
and depressurized of H2. In the glovebox, the sample was transferred
to an NMR tube for data acquisition. 98.6% conversion to cumene
CCDC 1870820−1870830 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.
AUTHOR INFORMATION
Corresponding Author
*E-mail for L.T.: laura.turculet@dal.ca.
■
ORCID
Michael J. Ferguson: 0000-0002-5221-4401
Robert McDonald: 0000-0002-4065-6181
Laura Turculet: 0000-0002-2499-5343
1
(0.055 mmol) was calculated relative to the internal standard by H
NMR spectroscopy. This value represents one of two runs averaged to
give the value reported in Table 1.
Notes
Procedure for Catalytic Hydrogenation of cis-Stilbene and
Isolation of Bibenzyl. A 1 dram vial was charged with 0.020 g of 6·
N2 (0.028 mmol) and 2.4 mL of benzene. cis-Stilbene (102 μL, 0.57
mmol) was added via microsyringe. The vial was then equipped with a
stirbar and closed with a PTFE-sealed cap with a needle inserted
through the seal to allow for the introduction of H2 gas. The vial was
then transferred to a Parr reactor which was sealed and purged with
H2 and subsequently pressurized to 10 atm. The Parr reactor was
heated at 65 °C in an oil bath for 4 h. Afterward, the Parr reactor was
removed from heat, allowed to cool, and depressurized of H2. In the
glovebox, the sample was evaporated to dryness in vacuo and the
residue was extracted with 3 mL of pentane and filtered through a
plug of silica gel. The filtrate solution was concentrated in vacuo and
cooled to −35 °C. Bibenzyl was isolated as a white crystalline solid
(0.095 g, 2 crops, 0.52 mmol by this method.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Killam Trusts, and
Dalhousie University for their support of this work.
REFERENCES
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parameters based on the isotropic displacement parameter of the
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