Zero valent iron complexes as base partners in frustrated Lewis pair chemistry

Article abstract of DOI:10.1039/d0dt03551c

The prototypical iron(0) complex [Fe(CO)3(PMe3)2] (1) forms a frustrated Lewis pair (FLP) with B(C6F5)3 (BCF). In this FLP, the iron complex acts as the Lewis base partner, and the borane as the Lewis acid partner. This FLP is able to cleave H-H, H-Cl, H-O and H-S bonds in H2, HCl, H2O and HSPh. The FLP 1/BCF is shown to catalyze the hydrogenation of alkenes under mild conditions, where terminal alkenes are preferentially reduced. Mechanistic studies using D2 gas suggest that a branched intermediate in an alkene insertion cycle or an ionic cycle is favored for this catalytic reaction.

Full text of DOI:10.1039/d0dt03551c

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DOI: 10.1039/D0DT03551C
Hendrik Tinnermann, Craig Fraser, Rowan D. Young*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
ABSTRACT: The prototypical iron(0) complex [Fe(CO)3(PMe3)2] (1) forms a frustrated Lewis pair (FLP) with B(C6F5)3 (BCF). In this FLP,
the iron complex acts as the Lewis base partner, and the borane as the Lewis acid partner. This FLP is able to cleave H-H, H-Cl, H-O
and H-S bonds in H2, HCl, H2O and HSPh. The FLP 1/BCF is shown to catalyze the hydrogenation of alkenes under mild conditions,
where terminal alkenes are preferentially reduced. Mechanistic studies using D2 gas suggest that a branched intermediate in an
alkene insertion cycle or an ionic cycle is favored for this catalytic reaction.
Small molecule activation for functionalization purposes
has become highly developed for noble metal catalysts. How-
ever, the relative cost and toxicity of noble metals has driven
chemists to derive new means to activate and functionalize
small molecules.1 This is perhaps most true for the hydrogen
molecule, and its subsequent use in hydrogenation reactions.2
To circumvent the need for noble metals in hydrogenation
reactions, chemists have looked to employ earth-abundant el-
ements in catalysis. Two strategies to accomplish this that have
been employed are (i) to enhance first row transition metal re-
activity through combination with weak field, strong field or
redox active ligands or metal-metal and metal-ligand coopera-
tivity,3 and (ii) to use main-group elements in Frustrated Lewis
Pair (FLP) chemistry.4 While both approaches have obvious
benefits in moving away from noble metal catalysis, neither
has accomplished comparable activities to noble metal cata-
lysts.
Many reported main-group FLP systems are known to
cleave dihydrogen in a facile manner. However, such systems
generally perform poorly as hydrogenation catalysts for
imines, ketones, alkenes and alkynes, requiring high loadings
to achieve complete conversions.5 Furthermore, the reactivity
Figure 1. A Previous transition metals used as Lewis bases in FLP
chemistry. B The well-studied iron(0) complex [Fe(CO)3(PMe3)2] (1)
has been used as a Lewis base in forming iron-Lewis acid adduct, but
of such systems is generally restricted to ionic hydrogenations,
with classical hydride insertion chemistry rare.6
Conversely, many base metal systems are able to exploit
the activation chemistry of these adducts has not been explored, nor
their ability to partake in organometallic type reactivity (e.g.
carbonyl migratory insertion, olefin insertion) but require ele-
vated temperatures and/or high pressures to cleave dihydro-
gen.7
has 1 been used in FLP chemistry. BCF = B(C6F5)3.
is capable of H2 activation in the absence of BCF or gold(I), and
hydrogen activation in these systems may have been due to
separate H2 activation and hydride abstraction steps, although
computational analysis of various reaction pathways does sug-
gest Campos’ Au(I)/Pt(0) system activates H2 in its FLP form.11b
Furthermore, in Ison’s system, the Re metal center isn’t di-
rectly involved in H2 activation, with the Re oxo-ligand acting
as the base partner.
Systems that utilize base metals as Lewis base partners in
Lewis adducts are well known.13 Indeed, Braunschweig and
Bigorgne have utilized the prototypical zero valent iron com-
plex [Fe(CO)3(PMe3)2] (1) to report a number of Fe(0) adducts
with gallium, silver and mercury (Figure 1B).14 Surprisingly,
none of these systems have been exploited for small molecule
activation.
To counter the limitations of base metal and FLP H2 activa-
tion/hydrogenation, efforts have been made to combine tran-
sition metal and FLP reactivity in a synergistic fashion. To date,
most metal based FLP systems have focused on utilizing tran-
sition metals as the Lewis acid partner.8 For example, both
Wass and Erker have reported inter- and intra-molecular zirco-
nium-phosphine FLP systems.9 These systems are capable of
hydrogen activation and reaction with carbon dioxide, ke-
tones, alkenes, alkynes and ethers (inter alia). Generally, such
systems generate very basic hydrides which are prone to hy-
drolysis and the formation of inactive hydroxide or oxide by-
products.
Systems that utilize metals as the Lewis base partner are
restricted to a report by Wass of H2 activation by platinum(0)
and [B(C6F5)3] (BCF),10 a platinum(0)/gold(I) all-metal reversible
FLP system reported by Campos,11 and a rhenium oxide com-
plex reported by Ison that was able to hydrogenate alkenes in
cooperation with BCF (Figure 1A).12 It must be noted that Pt(0)
We reasoned that unique umpolung reactivity might be ob-
served employing base metals as Lewis base partners in FLP
activations. Furthermore, the activity of base metal catalysis

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DOI: 10.1039/D0DT03551C
Figure 2. Activation of H2 with the FLP 1/BCF yields the salt [1-
H][BH(C6F5)3]. The reverse reaction to release hydrogen was not found
to proceed.
Figure 4. Molecular structure of [1-H][BCl(C6F5)3]. Hydrogen atoms (ex-
cept H1) omitted, thermal ellipsoids shown at 50%. H1 was located in
the Fourier Difference map. Selected bond distances (Å) and angles (°):
Fe1-P1, 2.256(1); B1-Cl1, 1.910(2); H1-Cl1 (closest contact), 7.62(3); P1-
Fe1-P2, 166.6(1).
Figure 3. The FLP 1/BCF was found to activate HCl, H2O and HSPh.
may be improved and/or better understood through coopera-
tive use in FLP chemistry. Herein, we report on the activation
of hydrogen and other small molecules utilizing an iron(0) sys-
tem as a Lewis base partner in cooperation with BCF.
To examine the ability of iron metal bases to activate H2 in
combination with main group Lewis acids, we chose to utilize
the prototypical Fe(0) complex, [Fe(CO)3(PMe3)2]15 (1) in our in-
vestigations (Figure 2). Mixing complex 1 with BCF in a 1:1 ratio
resulted in no apparent change in the 31P NMR signals of 1 nor
the 11B NMR signal of BCF. FTIR analysis of the reaction mix-
tures also resulted in no change from pure samples of 1, indi-
cating the pairing of complex 1 with BCF is an FLP.
Figure 5. Molecular structure of [1-H][(μ-OH){B(C6F5)3}2]. Hydrogen at-
oms (except H1 and H2) omitted, thermal ellipsoids shown at 50%. H1
was located in the Fourier Difference map. Selected bond distances (Å)
and angles (°): Fe1-P1, 2.252(1); B1-O1, 1.557(2); H1-O1 (closest con-
tact), 8.06(3); P1-Fe1-P2, 170.8(1).
Addition of H2 at 77 K to a sample of 1 and BCF in chloro-
benzene (PhCl) resulted in an immediate colour change from
light pink to light yellow upon thawing. 31P NMR analysis
showed the formation of a new signal at 16.1 ppm correspond-
ing to the iron hydride complex, [FeH(CO)2(PMe3)2]+ {[1-H]+}
with an NMR yield of 63% after 24 hours at room temperature.
The formation of the iron hydride was confirmed through the
Figure 6. Molecular structure of [1-H][B(SPh)(C6F5)3]. Hydrogen atoms
(except H1) omitted, thermal ellipsoids shown at 50%. H1 was located
in the Fourier Difference map. Selected bond distances (Å) and angles
(°): Fe1-P1, 2.245(1); B1-S1, 1.963(2); H1-S1 (closest contact), 3.31(3);
P1-Fe1-P2, 170.7(1).
1
observation of a high field signal in the H NMR spectrum ap-
pearing as a triplet at δH -10.3 (2JPH = 35.8 Hz). Concurrently, a
1
signal in the 11B NMR spectrum at δB -25.0 (d, JBH = 96.1 Hz)
signified the formation of [BH(C6F5)3]-. The formation of the ion
pair [1-H][BH(C6F5)3] was found to be highly favored, with de-
gassing the solution resulting in little to no regeneration of 1
or BCF. Indeed, independently prepared samples of [1-
H][BArF ] {BArF = B(3,5-(CF3)2-C6H3)4} and [nBu4N][BH(C6F5)3]
species. Secondly, the addition of [Ph3C][BArF ] to 1 and hydro-
4
gen did not result in any observation of [1-H]+ or triphenylme-
thane, ruling out the possibility that 1 activates hydrogen re-
versibly, and that BCF simply perturbs this equilibrium by re-
acting with small concentrations of an iron dihydride species.
Confirmation that heterolytic cleavage of dihydrogen was
the H-atom source for the formation of [1-H][BH(C6F5)3] was
obtained via reaction of 1 and BCF with D2. In a similar reaction
to that described above, addition of D2 to a sample of 1 and
BCF at 77 K in PhCl once again resulted in a new signal at 16.1
4
4
failed to generate any 1 or BCF when mixed, even after freeze-
pump-thaw degassing and heating at 100 °C for 24 hours.
The synergistic role of complex 1 and BCF in the activation
of hydrogen was confirmed from a number of control experi-
ments. Firstly, it was observed that exposing 1 to H2 in the ab-
sence of BCF did not result in [1-H]+ or any other iron hydride

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described above, however, the 11B NMR spectrum of the reac-
tion revealed a signal at δB -6.6 arising from the anion
[BCl(C6F5)3]-. The formation and molecular structure of the ion
pair [1-H][BCl(C6F5)3] were confirmed via single crystal X-ray
diffraction (Figure 4).
The FLP 1/BCF was found to readily activate O–H and S–H
bonds. Stoichiometric quantities of complex 1 with two equiv-
alents of BCF activated water to generate [1-H][(μ-
OH){B(C6F5)3}2] quantitatively. Similarly, equal quantities of 1
and BCF generated [1-H][B(SPh)(C6F5)3] in 62% yield when
mixed with benzenethiol. This could be improved to 91% yield
based on iron when two equivalents of BCF were employed. In
contrast to the molecular structure of [1-H][(μ-OH){B(C6F5)3}2]
(Figure 5), which exhibits two BCF motifs coordinated to the
[OH]- fragment, a molecular structure of the product from the
reaction with PhSH (Figure 6) revealed only a single BCF pre-
sent in the anion {i.e. [PhS–BCF]-}, suggesting the higher con-
version with two equivalents of BCF may arise from perturba-
tion of an equilibrium. Addtionally, the salts [1-H][BCl(C6F5)3]
and [1-H][(μ-OH){B(C6F5)3}2] display large minimum distances
between the iron hydride position and any of the anion B–Cl or
Figure 7. Hydrogenation of terminal alkenes and attempted hydro-
genation of internal alkenes with 1/BCF. Yields determined by 1H
NMR analysis.
ppm in the reaction’s 31P NMR spectrum, with a NMR yield of
65% after 3 hours at 60 °C. The 11B NMR spectrum of the reac-
tion showed the appearance of a broad signal (FWHM = 75 Hz)
at δB -25.0. However, explicit evidence for the formation of the
isotopologues came from the 2H NMR spectrum of the reaction
mixture that showed a sharp triplet signal at δD -10.4 (2JPD = 5.6
Hz) and a broad signal at δD 3.9, assigned to the iron deuteride
and borodeuteride signals respectively.
Activation of other small molecules using 1 and BCF was
also achieved (Figure 3). While, addition of HCl to 1 resulted in
no reaction, in the presence of one equivalent of BCF, the
product [1-H][BCl(C6F5)3] was formed within minutes quantita-
tively. The spectroscopic data for [1-H]+ are identical to those
B–O positions (Figures
4 and 5), while complex [1-
H][B(SPh)(C6F5)3] displays a Fe–H···S–B distance of 3.31(3) Å,
indicating a weak electrostatic interaction between the anion
and iron hydride (Figure 6).
It was found that reaction of [1-H][BH(C6F5)3] with a stoichi-
ometric amount of 1,1-diphenylethylene generated the hydro-
gen transfer product 1,1-diphenylethane in 35% yield after 48
hours at 80 °C with concurrent formation of 1 (as adjudged by
31P NMR analysis). As such, the FLP 1/BCF was utilized in the
catalytic hydrogenation of unactivated alkenes under mild
Figure 8. Proposed hydride insertion and ionic hydrogenation mechanisms leading to observed deuteration ratios in 3a when 2a is reacted with 1/BCF
and D2.

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conditions. In isolation, complex 1 and BCF were found to be
react with [BH(C6F5)3]- to give 3a-1-D, thus also providing a
plausible mechanism for the enrichment of deuterium in the
1-position of product 3a.
catalytically inactive for the hydrogenation of 1,1-diphe-
nylethene (Figure 7). Additionally, the iron hydride [1-H][BArF ]
4
was tested under hydrogenation conditions and found to be
inactive.
The ionic hydrogenation mechanism provides a simple ex-
planation for the deuterium enrichment in the 1-position of 3a,
and it would also not require the dissociation of any ligands to
generate a vacant site for the alkene to coordinate. However,
it is observed that terminal alkenes (2a-d) gave much better
results than internal alkenes (2e-h). This selectivity is normally
observed for transition metal hydride insertion reactions. DFT
and kinetic mechanistic studies to understand this discrepancy
are ongoing.
In summary, we have developed the first FLP system that
employs a base metal as the Lewis base partner. In contrast to
previously reported systems that may activate H2 without di-
rect intervention by then metal center (i.e. [Re=O]/BCF) or in a
non-FLP mechanism (i.e. [Pt] systems), this iron based system
is shown to activate hydrogen and other small molecules in an
FLP manner where iron acts as the Lewis base partner.
We have also shown basic proof-of-principle that the com-
bination of FLP chemistry with base metal chemistry can im-
prove the catalytic efficiency of catalytic hydrogenation. Im-
portnatly, the use of an FLP strategy allows the catalytically in-
active complex 1 to be utilized in olefin hydrogenation. Given
that we have shown activation of other small molecules, the
opportunity exists to explore a number of other catalytic reac-
tions employing this FLP system and small molecules that can
be activated by it.
In contrast, the combination of 1 and BCF at 5 mol % load-
ing under 4 atm of hydrogen was capable of reducing 1,1-di-
phenylethene (2a) to 1,1-diphenylethane (3a). Heating this re-
action for 24 h at 60 °C resulted in a 29% yield of 3a after 24
hours, however, increasing the reaction temperature to 130 °C
resulted in a yield of 76% after 24 hours. Although this result is
not noteworthy when compared to the activity of noble metal
catalysts, it is comparable in yield and conditions to other FLP
catalyzed alkene hydrogenation reactions.16 For example, the
rhenium oxide system reported by Ison operates at 100 °C with
10 mol % loading of BCF to hydrogenate 1,1-diphenylethlene
in 84% yield.12 Further, main group FLP systems capable of
ionic hydrogenation of alkenes using BCF combined with either
electron poor phosphines16a,b or ethers16c operate between 20-
40 mol % catalyst loading, and electron poor alkenes can be
reduced with BCF combined with 1,4-diazabicyclo[2.2.2]oc-
tane (DABCO)16d,e or 2,2,6,6-tetramethylpiperidine (TMP)16f in
5-20 mol % loadings.
The above reaction conditions were applied to a range of
alkenes (Figure 7). It was found that terminal olefins 2a-d pro-
vided average yields of the corresponding reduced products af-
ter 24 hours. However, very little to no product was observed
for internal alkenes 2e-h.
Insight into the mechanism of hydrogenation was sought
using deuterium (D2) in place of hydrogen (Figure 8). A sample
of 2a was loaded with 5 mol % 1/BCF and 1 atm of D2 was in-
troduced at -196 °C. The sample was then thawed and heated
at 60 °C for three days, after which time it was analyzed by 1H,
2H and 31P NMR spectroscopies.
Acknowledgements
We thank the Singapore Ministry of Education (WBS R-143-
000-A05-112) and the Singapore Agency for Science, Technol-
ogy and Research (A*STAR grant No. A1983c0033) for financial
support.
Analysis of the 2H NMR spectrum of the reaction revealed
deuteration of 2a at both the 1- and 2-positions in a ratio of
10:4 respectively representing formation of a range of isotop-
ologues of 3a. Furthermore, deuterium scrambling had re-
sulted in a substantial concentration of the isotopologue 2a-2-
D, where a deuterium had substituted a 2-position hydrogen
atom in alkene 2a.
Conflict of Interests
The authors declare no competing financial interest.
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Table of Contents artwork