Zirconium and hafnium polyhedral oligosilsesquioxane complexes-green homogeneous catalysts in the formation of bio-derived ethers: Via a MPV/etherification reaction cascade

Article abstract of DOI:10.1039/d0cy01864c

The polyhedral oligosilsesquioxane complexes, {[(isobutyl)7Si7O12]ZrOPri·(HOPri)}2 (I), {[(cyclohexyl)7Si7O12]ZrOPri·(HOPri)}2 (II), {[(isobutyl)7Si7O12]HfOPri·(HOPri)}2 (III) and {[(cyclohexyl)7Si7O12]HfOPri·(HOPri)}2 (IV), were synthesized in good yields from the reactions of M(OPri)4 (M = Zr, Hf) with R-POSS(OH)3 (R = isobutyl, cyclohexyl), resp. I-IV were characterized by 1H, 13C and 29Si NMR spectroscopy and their dimeric solid-state structures were confirmed by X-ray analysis. I-IV catalyze the reductive etherification of 2-hydroxy- and 4-hydroxy and 2-methoxy and 4-methoxybenzaldehyde and vanillin to their respective isopropyl ethers in isopropanol as a "green"solvent and reagent. I-IV are durable and robust homogeneous catalysts operating at temperatures of 100-160 °C for days without significant loss of catalytic activity. Likewise, I-IV selectively catalyze the conversion of 5-hydroxymethylfurfural (HMF) into 2,5-bis(isopropoxymethyl)furane (BPMF), a potentially high-performance fuel additive. Similar results were achieved by using a combination of M(OPri)4 and ligand R-POSS(OH)3 as a catalyst system demonstrating the potential of this "in situ"approach for applications in biomass transformations. A tentative reaction mechanism for the reductive etherification of aldehydes catalysed by I-IV is proposed. This journal is

Full text of DOI:10.1039/d0cy01864c

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Zirconium and hafnium polyhedral
oligosilsesquioxane complexes – green
homogeneous catalysts in the formation of bio-
derived ethers via a MPV/etherification reaction
cascade†
Cite this: DOI: 10.1039/d0cy01864c
*
Shipra Garg,
Daniel K. Unruh
and Clemens Krempner
The polyhedral oligosilsesquioxane complexes, {[(isobutyl)7Si7O12]ZrOPri·(HOPri)}2 (I), {[(cyclohexyl)7Si7O12
]
ZrOPri·(HOPri)}2 (II), {[(isobutyl)7Si7O12]HfOPri·(HOPri)}2 (III) and {[(cyclohexyl)7Si7O12]HfOPri·(HOPri)}2 (IV),
were synthesized in good yields from the reactions of M(OPri)4 (M = Zr, Hf) with R-POSS(OH)3 (R =
isobutyl, cyclohexyl), resp. I–IV were characterized by 1H, 13C and 29Si NMR spectroscopy and their dimeric
solid-state structures were confirmed by X-ray analysis. I–IV catalyze the reductive etherification of
2-hydroxy- and 4-hydroxy and 2-methoxy and 4-methoxybenzaldehyde and vanillin to their respective
isopropyl ethers in isopropanol as a “green” solvent and reagent. I–IV are durable and robust homogeneous
catalysts operating at temperatures of 100–160 °C for days without significant loss of catalytic activity.
Likewise, I–IV selectively catalyze the conversion of 5-hydroxymethylfurfural (HMF) into
2,5-bis(isopropoxymethyl)furane (BPMF), a potentially high-performance fuel additive. Similar results were
achieved by using a combination of M(OPri)4 and ligand R-POSS(OH)3 as a catalyst system demonstrating
the potential of this “in situ” approach for applications in biomass transformations. A tentative reaction
mechanism for the reductive etherification of aldehydes catalysed by I–IV is proposed.
Received 22nd September 2020,
Accepted 23rd October 2020
DOI: 10.1039/d0cy01864c
rsc.li/catalysis
secondary alcohol to the carbonyl substrate with high
selectivity, following an outer-sphere mechanism,5 while the
Introduction
The chemical conversion of cellulosic biomass into liquid
fuel, is a subject of current interest from both the industrial
as well as the academic perspective.1 Particularly, alkyl ethers
have become intensively investigated in the last few years
owing to their potential application as high-performance fuel
additives.2 One of the most promising synthetic strategies
for alkyl ethers involves the reductive etherification of
biomass-derived aldehydes via the Meerwein–Ponndorf–
etherification proceeds with elimination of water and can be
catalysed by both, Brønsted or Lewis acid catalysts. Indeed,
some heterogeneous tin, zirconium and hafnium zeolite and
oxide based catalysts have been reported to convert
5-hydroxymethylfurfural (HMF),
a platform chemical for
biofuels,
biochemical
and
biopolymers,6
into
2,5-bis(isopropoxy-methyl)furan (BPMF)7 via reductive
etherification. In contrast, homogeneous catalysts capable of
Verley
(MPV)
reaction
followed
by
acid-catalysed
etherification (Scheme 1).3
This process is operationally simple and environmentally
friendly as it utilizes “green” isopropanol as an inexpensive,
safe and non-toxic solvent and hydrogen transfer reagent in
combination with cheap and abundant acid catalysts. The
MPV reduction,4 the first step in the cascade, proceeds via
Lewis acid-catalysed hydrogen transfer usually from
a
Department of Chemistry and Biochemistry, Texas Tech University, Memorial Dr. &
Boston, Lubbock, TX, 79409, USA. E-mail: clemens.krempner@ttu.edu;
Fax: +806 742 1289; Tel: +806 834 3507
† Electronic supplementary information (ESI) available. CCDC 2009174–2009177.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0cy01864c
Scheme
etherification cascade.
1
Ether formation from aldehydes via
a
reductive
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
converting HMF into BPMF with high selectivity and yields
have not yet been reported.8
Early transition metal “polyhedral” oligosilsesquioxane
(POSS) complexes9 of a tripodal coordination geometry are a
largely overlooked class of robust and durable metal
complexes that have shown some potential as homogeneous
catalysts in alkene epoxidations10 and polymerizations.11 The
electron-withdrawing properties of the POSS ligand combined
with its steric profile give rise to well-defined metal
complexes that possess relatively high Lewis acidity while
maintaining good chemical resistance. Herein, we will
introduce zirconium and hafnium POSS complexes as a new
class of highly efficient and remarkably thermally-stable
homogeneous catalysts for selected biomass transformations
as demonstrated for the reductive etherification of HMF to
biofuel additive, BPMF.
Results and discussion
The synthesis of the targeted zirconium and hafnium POSS
complexes I–IV is illustrated in Scheme 2. Reaction of the
commercially available trisilanols R-POSS(OH)3 (R = isobutyl
or cyclohexyl) with Zr(OPri)4 and Hf(OPri)4, respectively, gave,
after removal of solvent and precipitation or recrystallization
from isopropanol (IPA), compounds I–IV in 58–66% yield. I–
IV are thermally stable crystalline materials, which show
Fig.
1 Solid-state structures of I (top) and III (bottom); isobutyl
substituents at the silicon atoms are omitted for clarity (blue = silicon,
red = oxygen).
types of metal–oxygen bonds that result from this structural
arrangement: bridging M–O bonds [M1–O3, 2.14–2.18 Å],
M–O bonds of the coordinated isopropanol [M1–O7, 2.26–
2.30 Å], and M–O bonds from the supporting POSS ligand
[M1–O1/O5/O9, 1.97–2.02 Å]. As expected, the M1–O7
distances are significantly longer than the bridging M1–O3
and POSS related M–O distances, suggesting a relatively
weak isopropanol to metal coordination.
With the POSS complexes I–IV in hand, we investigated
their catalytic performance in the reductive etherification of
various benzaldehydes along with their precursors Zr(OPri)4
and Hf(OPri)4. Initial screening experiments at 25–70 °C did
not show any substrate conversion. Therefore, the
experiments were carried out in closed reaction vessels at
temperatures ranging from 100 °C to 150 °C with a catalyst
loading of 1 mol%. Isopropanol (IPA) served as solvent and
hydrogen transfer reagent and was used as received (“wet”).
good
solubility
in
toluene,
hexanes,
benzene,
dichloromethane, diethyl ether and THF but are sparingly
soluble in alcohols and DMSO. In addition to be
characterized by 1H, 13C and 29Si NMR spectroscopy and
combustion analysis, the solid-state structures of I–IV were
determined by X-ray analysis. The results are shown for I and
III (Fig. 1); selected bond lengths and angles for I–IV are
listed in Table 1.
In the solid state, all compounds are isostructural dimers
with distorted octahedral coordination environments for
zirconium and hafnium, respectively. The coordination
sphere of each metal centre is completed by coordinated
isopropanol, similar to what is seen for other titanium and
zirconium analogues.12 The dimers are held together via
bridging isopropoxide groups and hydrogen bond
interaction between the OH group of the coordinated
isopropanol and an oxygen from one of the metal binding
siloxy groups [O1⋯O7H, 2.99 to 3.12 Å]. There are three
1
Conversions and product yields were determined by H NMR
Table 1 Selected bond lengths [Å] and angles [°] of I–IV
I (M = Zr)
II (M = Zr)
III (M = Hf)
IV (M = Hf)
M1–O1
M1–O3
M1–O5
M1–O7
2.015(3)
2.177(3)
1.982(3)
2.304(3)
1.966(3)
164.8(1)
1.611(3)
1.997(1)
2.156(1)
1.988(1)
2.303(2)
1.987(1)
170.4(1)
1.612(2)
1.993(5)
2.136(5)
1.967(5)
2.257(5)
1.973(5)
167.9(2)
1.606(5)
1.995(2)
2.146(2)
1.984(2)
2.274(2)
1.982(2)
170.3(1)
1.612(2)
M1–O9
O1–M1–O7
Si–O(M1)a
a
Scheme 2 Synthesis of I–IV.
Average values.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Fig.
4
Reductive etherification of 2-methoxybenzaldehyde: (left
graph) yields of alcohol 4 (purple bars) at various times; (right graph)
yields of alcohol 4 (purple bar) and ether 5 (green bar) after 48 hours.
Fig.
2 MPV reduction of benzaldehyde to benzyl alcohol (1):
conversions (grey bars) and yields of alcohol 1 (purple bars).
spectroscopy using CDCl3 as
a
solvent and 1,3,5-
isopropyl ether (3). While Hf(OPri)4 and Zr(OPri)4
quantitatively converted 4-methoxybenzaldehyde into alcohol
2 only, the hafnium complexes III and IV selectively produced
ether 3 in yields of 90% and 93%, respectively.
trimethylbenzene as internal standard. The catalytic
experiments were triplicated and conversions and product
yields were given as average values.
The moisture sensitive precursors Zr(OPri)4 and Hf(OPri)4
were found to be surprisingly active in selectively reducing
benzaldehyde to benzyl alcohol (1) in “wet” isopropanol (IPA)
with yields of 57% and 80%, respectively, after 24 hours at
100 °C (Fig. 2). The employment of the new POSS complexes
I–IV reproducibly allowed for higher yields with catalysts I
and III each producing 91% of 1. At 150 °C, no differences in
the performance were noted between I–IV, Zr(OPri)4 and
Hf(OPri)4; all quantitatively reduced benzaldehyde to 1 (Table
S1†), but did not form benzyl isopropyl ether.
Next, the reduction of the more electron-rich 4-methoxy-
benzaldehyde was investigated (Fig. 3). Similar to what was
seen in the previous case, all zirconium and hafnium based
catalysts reduced 4-methoxybenzaldehyde to 4-methoxybenzyl
alcohol (2) in good to excellent yields after 24 hours at 100
°C. Hf(OPri)4 and I showed the best catalyst performances
generating 94% of alcohol 2, respectively.
2-Methoxybenzaldehyde was converted much faster and
with better selectivity into its respective alcohol at 100 °C than
4-methoxybenzaldehyde regardless of the catalyst used (Fig. 4
and Tables S3 and S4†). In fact, isobutyl substituted POSS
complex I, Zr(OPri)4 and Hf(OPri)4 quantitatively generated
2-methoxy-benzyl alcohol (4) in only 3 hours at 100 °C. The
cyclohexyl substituted complexes II and IV were slightly less
active presumably due to their poor solubility in IPA at 100 °C.
However, complex III proved to be the most active and selective
catalyst. III not only quantitatively produced alcohol 4 in 1
hour, it also converted alcohol 4 into ether 5 upon increasing
the temperature to 150 °C. In fact, III generated, after 24 hours,
57% of 5 along with 39% of alcohol 4. After 48 hours, 82% of 5
and 16% of 4 were formed (Fig. 4).
Given the structural similarities of I–IV (vide supra), it
remains unclear why only III generates significant amounts
of ether 5. It is known that chelating substrates often cause
metal-catalyzed reactions to be sluggish because of the
inherent strength of the resulting product–metal bonds,
Notably, increasing the temperatures from 100 to 150 °C
not only improved conversions but also changed the
selectivity markedly in favour of the targeted 4-methoxybenzyl
Fig. 3 Reductive etherification of 4-methoxybenzaldehyde: yields of
alcohol 2 at 100 °C (light purple bars); yields of alcohol 2 (dark purple
bars) and ether 3 (green bars) at 150 °C.
Fig.
5
Reductive
etherification
of
4-hydroxybenzaldehyde.
Conversions (grey bars): yields of alcohol 6 (purple bars); yields of
ether 7 (green bars).
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
typically the limiting factor in product liberation from the
metal catalyst. Therefore, even the subtle changes of the
steric profile (isobutyl versus cyclohexyl substituents) and the
identity of the metal (zirconium or hafnium) may account for
the differences in product selectivity.
Encouraged by the ability to selectively catalyse the
formation of ethers, the more challenging substrates
2-hydroxy- and 4-hydroxybenzaldehyde were investigated
(Fig. 5 and 6). To our surprise, complexes I–IV smoothly
catalysed the formation of the corresponding isopropyl ethers
7 and 9, respectively, which were obtained in good to
excellent yields after 24 hours. 2-Hydroxybenzaldehyde was
Fig. 7 Reductive etherification of vanillin: conversions (grey bars) and
yields of ether 10 (green bars).
found to convert into
9 faster and at slightly lower
temperatures (120 °C) than 4-hydroxybenzaldehyde, which
required 125 °C to be converted into ether 7 in acceptable
yields. Notably, also the precursors Zr(OPri)4 and Hf(OPri)4
catalysed the formation of ethers 7 and 9, respectively, from
their respective aldehydes with similar yields but somewhat
lower selectivity.
to their metal chelating properties, resulting in deactivation
of the catalyst.
In comparing the results described above, we noticed that
Zr(OPri)4 and Hf(OPri)4 were fairly active in catalysing the
formation of hydroxy ethers 7, 9 and 10, but did not catalyse
the formation of methoxy ethers 3 and 5. Moreover, 2- and
4-hydroxybenzaldehyde were converted faster and with better
selectivity to the corresponding ethers than 2- and
4-methoxybenzaldehyde regardless of the catalyst used. At
first glance, this appeared to be counter intuitive as
according to the Hammett sigma constants, OH groups are
somewhat stronger electron donors than methoxy groups.
This results in lower carbonyl activities for 2- and
4-hydroxybenzaldehyde, and consequently would lead to
lower rates of reduction.
Having had success in selectively producing hydroxy
ethers 7 and 9, vanillin (4-hydroxy-3-methoxybenzaldehyde),
one of the most widely used aroma chemicals and fragrances
that can be produced from biomass-derived lignin, was
examined (Fig. 7). Astonishingly, all zirconium and hafnium
catalysts converted vanillin into isopropyl ether 10 with high
selectivity and in yields ranging from 68–91%, even though
higher temperatures (150 °C) were required relative to the
reductive etherification of 2- and 4-hydroxybenzaldehyde. Of
all the catalysts used, IV proved to be the most active system
as 91% of ether 10 were formed after 24 hours at 150 °C. The
catalyst performance of IV is similar to that of Rode's dual
heterogeneous catalyst, which is composed of ZrO(OH)2 and
Zr-montmorillonite, and gave 10 in 80% yield after 8 hours at
100 °C.7c
To gain more insights, the kinetic profiles of the reductive
etherification for all the five benzaldehydes were obtained at
100 °C with 1 mol% of III as the catalyst; the results are
shown in Fig. 8 and 9. Consistent with our expectation based
Unfortunately, efforts to reduce the phenolic aldehydes
3,4-dihydroxybenzaldehyde
and
4-hydroxy-3,5-
methoxybenzaldehyde (syringe aldehyde) failed. After 2 days
at 150 °C, no conversion was noted. We attribute the inability
of these multi-functionalized aldehydes to undergo reduction
Fig.
8 Kinetic profiles of the reductive etherification of
Fig.
6
Reductive
etherification
of
4-hydroxybenzaldehyde.
benzaldehyde, 4-MeO-benzaldehyde, 4-HO-benzaldehyde and 2-HO-
benzaldehyde. Reaction conditions: 100 °C, cat. III (1 mol%), 25 eq.
isopropanol (IPA).
Conversions (grey bars): yields of alcohol 8 (purple bars); yields of
ether 9 (green bars).
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
counterparts. One of the key factors that enables rapid and
selective ether formation seems to be the Lewis acidity of the
catalyst system. This is supported by the observation that I–
IV catalyse the etherification of methoxy alcohols 2 and 4 but
not Zr(OPri)4 and Hf(OPri)4, i.e. I–IV are stronger Lewis acids
than their precursors because of the stronger electron-
withdrawing properties of the POSS ligand versus an aliphatic
alkoxide ligand.13
Fig.
9 Kinetic profile of the reductive etherification of 2-MeO-
By contrast, the precursors Zr(OPri)4 and Hf(OPri)4 not
only catalyse the reductive etherification of 2- and
4-hydroxybenzaldehyde and vanillin (Fig. 5–7), their activity
is surprisingly similar to those of I–IV. In addition, the
kinetic results for catalyst III (Fig. 8 and 9) further confirm
that 2- and 4-hydroxybenzaldehyde were converted much
faster into their respective ethers than 2- and
4-methoxybenzaldehyde. It seems that these drastic
differences in rates are not accounted for by the slightly
better ability of the 2- and 4-hydroxy groups to stabilize the
carbo-cationic intermediate than the 2- and 4-methoxy
groups (Scheme 3). Therefore, we propose an alternative
mechanism that involves the formation of ortho- or
para-quinone methide intermediates,14 which are formed via
proton migration of the phenolic proton followed by
cleavage of the benzylic C–O bond (Scheme 4). These
reactive Michael acceptors, which have been generated from
various organic precursors and used extensively in synthetic
organic chemistry,15 can be described as benzylic cations
that are strongly stabilized by the resonance electron-
donating 4-O− or 2-O− substituents.16 As a result, they are
more stable than the cationic methoxybenzyl intermediates
involved in the formation of methoxy ethers 3 and 5,
nonetheless, can readily be attacked by the external
nucleophile isopropanol (present in excess) to form the
corresponding hydroxy ethers 7, 9 and 10, respectively.
We next investigated the reduction of HMF, an important
renewable feedstock for various bio-based organic
compounds such as 2,5-bis(hydroxymethyl)furan (BHMF)
(11), furan-dicarboxylic acid, levulinic acid, ethyl levulinate
and derivatives thereof. Employing Sn-Beta as the catalyst,
Vlachos and co-worker reported the formation of BPMF (12)
from HMF with 87% selectivity and up to 80% overall yield at
180 °C.7b Rode et al. disclosed the conversion of HMF into
benzaldehyde as a function of temperature. Reaction conditions: 100
°C (0–39 hours), 120 °C (39–45 hours), 150 °C (45–99 hours); cat. III (1
mol%), 25 eq. isopropanol (IPA).
on the Hammett constants, the rate of conversion was found
to be in the order C6H5CHO (82%; 16 hours) > 4-MeO–C6H4-
CHO (60%; 16 hours) > 4-HO–C6H4CHO (43%; 16 hours).
The catalytic behaviour of III in the reductive
etherification of 2-methoxybenzaldehyde (Fig. 9) warrants an
additional comment. After having catalysed the quantitative
formation of alcohol 4 in less than an hour, and being
further heated at 100 °C for 39 hours and at 120 °C for
additional 6 hours, III still preserves its catalytic activity. In
fact, upon subsequently increasing the temperature to 150
°C, alcohol 4 slowly converts into ether 5, demonstrating the
catalysts' long-term durability and robustness at high
temperatures over extensive periods of time and in the
presence of water.
We
noticed
that
benzaldehyde
and
2-methoxybenzaldehyde exclusively converted into their
respective alcohols, and only 4-methoxybenzaldehyde
produced small quantities of ether after 30 hours at 100 °C.
Therefore, it is reasonable to assume that the slow step of
the reductive etherification for these substrates is the metal-
catalysed alcohol etherification. Ether formation most likely
proceeds via a metal stabilized intermediate of carbo-cationic
character generated from proton migration of one of the
coordinating IPA molecules (Scheme 3). Additional
stabilization arises from the electron-donating 4-MeO group,
enabling an intramolecular nucleophilic attack of
isopropoxide to form the ether bond.
The active involvement of carbo-cationic intermediates is
supported by the observation that at 150 °C the methoxy
alcohols 2 and 4 convert to the respective ethers but not
benzyl alcohol 1, i.e. the cationic benzyl intermediate is
markedly less stable than the 2- and 4-methoxy substituted
Scheme 4 Proposed alternative formation of hydroxy ether 7 via a
Scheme
cationic intermediate.
3
Proposed ether formation via
a
metal bound carbo-
metal stabilized para-quinone methide intermediate (p-QM
para-quinone methide).
=
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
albeit the latter in fairly low yields (25–30%). Increasing the
reaction time proved to be counter-productive as the yields
of 12 further decreased in favour of humin polymers, a
common problem in this chemistry.17 To suppress the
formation of humin polymers, a series of experiments was
undertaken, where the HMF concentration was gradually
decreased by increasing the amounts of isopropanol, from
25 up to 200 eq. using III as the catalyst at 150 °C. The
most relevant results are summarized in Fig. 11 (see also
Fig. S17–S21†) and revealed the optimum conditions for this
process to be ca. 9–10 hours and 100 eq. of isopropanol.
Gratifyingly, after 9 hours, both HMF and undesired BHMF
(11) were fully consumed, and the reaction mixture only
contained ca. 60% BPMF (12) and polymer, which greatly
facilitated the purification process. Again, increasing the
reaction time led to a slight decrease in the yields of 12
due to polymer formation.
Fig. 10 Reductive etherification of 2-hydroxymethylfurfural (HMF).
Conversions (grey bars) and product yields (purple bars = 11; green
bars = 12) after 4 hours.
BPMF (12) in isopropanol at 150 °C with claimed selectivity
of up to 95% using the dual catalysts, ZrO(OH)2 and Zr-
montmorillonite.7c However, in neither cases have isolated
yields been reported.
To demonstrate the practicality of our homogeneous
catalyst approach, scale-up syntheses of ethers 3, 5, 7, 9, 10
and 12 (1 g substrate) were carried out employing the best
performing catalysts, resp. (Scheme 5). All products except 7
could easily be purified by vacuum distillation with isolated
yields ranging from to 31–94% (ref. 18) without the need of
tedious purification by column chromatography.
As the reductive etherification of HMF is a complex
chemical transformation that involves the formation of
humin
polymers17
and
various
other
synthetic
intermediates,7c we were pleased to see that the simple
precursors Zr(OPri)4 and Hf(OPri)4 selectively generated
BHMF (11) in good yields of 58% and 59%, resp., in only 4
hours at 150 °C (Fig. 10).
In contrast, catalysts I–IV quantitatively converted HMF
into primarily humin polymers and the desired BPMF (12),
The efficacy and robustness of catalyst III in the
conversion of HMF to BPMF (12), prompted us to test the
catalytic activity of Hf(OPri)4 (1 mol%) in the presence of
ligand Bui-POSS(OH)3 (Scheme 6). Gratifyingly, after ca. 10
hours at 150 °C (100 eq. IPA) and subsequent purification by
vacuum distillation, BPMF could be isolated in 69% yield,
similar to what was observed with isolated III as the catalyst.
Since Hf(OPri)4 itself does not produce BPMF (vide supra),
catalyst III appears to have formed during the course of the
reaction. This “in situ” catalyst approach could also be
applied to the catalytic formation of ethers 3, 5 and 10 with
comparable isolated yields but somewhat longer reaction
times (see ESI†).
To study the catalyst structure in the solution, diffusion
experiments for hafnium complex III using DOSY-NMR
spectroscopy were performed in various solvents at room
temperature. The results for THF-D8 and C6D6 suggest III to
be dimeric in solution, which is consistent with its solid-state
structure. In the more polar solvents CD2Cl2 and acetone-D6,
Fig. 11 Reductive etherification of HMF as a function of isopropanol
(IPA) eq.; cond.: 9 hours, 150 °C, cat. III (1 mol%); top: kinetic profile
for 100 eq. of IPA; bottom: conversions (grey bars) and yields of BPMF
12 (green bars) for various eq. of IPA.
Scheme 5 Gram scale synthesis of 3, 5, 7, 9, 10 and 12.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Conclusions
With the goal of developing durable homogeneous catalysts
for selective biomass transformations, we have synthesized
tripodal zirconium and hafnium isopropoxides I–IV. In these
“corner capped” complexes, zirconium and hafnium are
Scheme 6 Synthesis of 10 and 12 employing 1 mol% Hf(OPri)4 and 1
mol% Bui-POSS(OH)3.
3−
rigidly incorporated into the electron-withdrawing POSS(O)3
ligand framework, which provides complexes I–IV with
sufficient Lewis acidity, high kinetic stability and thermal
robustness. In fact, I–IV are efficient homogeneous catalysts
operating at low loadings and high temperatures in the
reductive etherification of electron-rich aromatic aldehydes to
produce various isopropyl ethers. Zr(OPri)4 and Hf(OPri)4 are
active catalysts themselves, showing excellent selectivity
toward the formation of alcohols 2, 4 and 11 but are inactive
regarding ether formation. These selectivity differences can
be interpreted in terms of their different Lewis acidities. In
contrast, I–IV and Zr(OPri)4/Hf(OPri)4 generated the hydroxyl
ethers 7, 9 and 10 with similar activities and selectivities,
which we attribute to the involvement of para- and ortho-
quinone methide intermediates. Most importantly, III proved
to be the first homogeneous catalyst capable of selectively
converting HMF into bio-fuel additive BPMF (12), the latter
could be isolated in good yields and purities upon
distillation. That similar catalytic performances can be
achieved without the need of synthesizing the POSS catalyst,
as demonstrated for the selective conversion of HMF into
BPMF, highlights the potential of our homogeneous
approach for applications in biomass and related
transformations.
Scheme
etherification of aldehydes.
7 Proposed reaction mechanism of the reductive
however, III was found to be monomeric (for more details see
ESI†).
Conflicts of interest
Based on these observations and those discussed above, we
propose a tentative mechanism, where III exists in isopropanol
as a hexa-coordinate monomeric complex of type A (Scheme 7).
For the MPV reduction to occur, complex A, which bears two
coordinated IPA molecules, needs to be in rapid equilibrium
There are no conflicts to declare.
Acknowledgements
with its substrate–metal complex
B to allow for an
This research was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
Catalysis Science Program, under Award DE-SC0019094. We
are grateful to Anthony Cozzolino for calculating the van der
Waals volume of complex III and Shiva Moaven for assisting
in the DOSY-NMR experiments.
intramolecular transfer of hydride from the isopropoxide to the
substrate. Complex C thus generated then converts via
replacement of the coordinated acetone product with excess
IPA into species D. To enable the second step of the overall
process, the etherification, an intramolecular proton transfer
in D from the coordinated IPA to the benzyloxide, needs to
occur. Complex
E thus generated then undergoes an
Notes and references
intramolecular nucleophilic substitution to furnish the ether
bound metal hydroxide F. Liberation of the coordinated ether
product from F via replacement with excess IPA produces metal
hydroxide G. Subsequent intramolecular proton transfer from
the coordinated IPA to the hydroxide generates complex H. The
latter finally converts to catalyst A upon replacement of the
coordinated water molecule by excess IPA. Note that in the case
of 2- and 4-hydroxybenzaldehyde and vanillin as substrates,
proton migration in species D will occur involving the phenolic
proton. This is followed by C–O bond cleavage and formation
of ortho- or para-quinone methide intermediates (see also
Scheme 4).
1 (a) M. J. Climent, A. Corma and S. Iborra, Green Chem.,
2014, 16, 516–547; (b) J. C. Serrano-Ruiz and J. A. Dumesic,
Energy Environ. Sci., 2011, 4, 83–99; (c) D. M. Alonso, J. Q.
Bond and J. A. Dumesic, Green Chem., 2010, 12, 1493–1513.
2 (a) A. Liu, B. Liu, Y. Wang, R. Ren and Z. Zhang, Fuel,
2014, 117, 68–73; (b) M. Balakrishnan, E. R. Sacia and A. T.
Bell, Green Chem., 2012, 14, 1626–1634; (c) M. Musolino,
M. J. Gines-Molina, R. Moreno-Tost and F. Arico, ACS
Sustainable Chem. Eng., 2019, 7, 10221–10226.
3 (a) A. Corma and M. Renz, Angew. Chem., Int. Ed., 2007, 46,
298–300; (b) A. M. Rasero-Almansa, M. Iglesias and F.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Sanchez, RSC Adv., 2016, 6, 106790–106797; (c) G. Li, L. Gao,
Z. Sheng, Y. Zhan, C. Zhang, J. Ju, Y. Zhang and Y. Tang,
Catal. Sci. Technol., 2019, 9, 4055–4065.
4 (a) C. Battilocchio, J. M. Hawkins and S. V. Ley, Org. Lett.,
2013, 15, 2278–2281; (b) B. McNerney, B. Whittlesey, D. B.
Catalysis Science & Technology
H. C. L. Abbenhuis, Angew. Chem., Int. Ed., 2002, 41,
637–639; (g) L. Zhang, H. C. L. Abbenhuis, G. Gerritsen,
N. N. Bhriain, P. C. M. Magusin, B. Mezari, W. Han, R. A.
van Santen, Q. Yang and C. Li, Chem. – Eur. J., 2007, 13,
1210–1221; (h) P. Guillo, M. I. Lipschutz, M. E. Fasulo and
T. D. Tilley, ACS Catal., 2017, 7, 2303–2312.
Cordes and C. Krempner, Chem.
– Eur. J., 2014, 20,
14959–14964; (c) X. Wang, J. Hao, L. Deng, H. Zhao, Q. Liu,
N. Li, R. He, K. Zhi and H. Zhou, RSC Adv., 2020, 10,
6944–6952; (d) A. Corma, M. E. Domine, L. Nemeth and S.
Valencia, J. Am. Chem. Soc., 2002, 124, 3194–3195; (e) K.
Flack, K. Kitagawa, P. Pollet, C. A. Eckert, K. Richman, J.
Stringer, W. Dubay and C. L. Liotta, Org. Process Res. Dev.,
2012, 16, 1301–1306.
11 (a) F. J. Feher, J. F. Walzer and R. L. Blanski, J. Am. Chem.
Soc., 1991, 113, 3618–3619; (b) F. J. Feher and R. L. Blanski,
J. Am. Chem. Soc., 1992, 114, 5886–5887; (c) R. Duchateau,
H. C. L. Abbenhuis, R. A. van Santen, A. Meetsma, S. K.-H.
Thiele and M. F. H. van Tol, Organometallics, 1998, 17,
5663–5673.
12 (a) O. Viotti, A. Fischer, G. A. Seisenbaeva and V. G. Kessler,
Inorg. Chem. Commun., 2010, 13, 774–777; (b) O. Viotti, G. A.
Seisenbaeva and V. G. Kessler, Inorg. Chem., 2009, 48,
9063–9065.
13 (a) F. J. Feher and T. A. Budzichowski, J. Organomet. Chem.,
1989, 379, 33–40; (b) R. West and R. H. Baney, J. Am. Chem.
Soc., 1959, 81, 6145–6148; (c) R. West, R. H. Baney and D. L.
Powell, J. Am. Chem. Soc., 1960, 82, 6269–6272; (d) P. T.
Wolczanski, Polyhedron, 1995, 14, 3335–3362; (e) C.
Krempner, Eur. J. Inorg. Chem., 2011, 1689–1698.
14 (a) Quinone Methides, ed. S. E. Rokita, John Wiley and Sons,
2009; (b) E. Dorrestijn, M. Kranenburg, M. V. Ciriano and P.
Mulder, J. Org. Chem., 1999, 64, 3012–3018; (c) S. J.
Gharpure, A. M. Sathiyanarayanan and P. Jonnalagadda,
Tetrahedron Lett., 2008, 49, 2974–2978; (d) M. M. Toteva, M.
Moran, T. L. Amyes and J. P. Richard, J. Am. Chem. Soc.,
2003, 125, 8814–8819.
15 (a) E. E. Weinert, R. Dondi, S. Colloredo-Melz, K. N.
Frankenfield, C. H. Mitchell, M. Freccero and S. E. Rokita,
J. Am. Chem. Soc., 2006, 128, 11940–11947; (b) R. W. Van de
Water and T. R. R. Pettus, Tetrahedron, 2002, 58, 5367–5405;
(c) W.-J. Bai, J. G. David, Z.-G. Feng, M. G. Weaver, K.-L. Wu
and R. R. Pettus, Acc. Chem. Res., 2014, 47, 3655–3664.
16 M. M. Toteva and J. P. Richard, Adv. Phys. Org. Chem.,
2011, 45, 39–91.
17 G. Tsilomelekis, M. J. Orella, Z. Lin, Z. Cheng, W. Zheng, V.
Nikolakis and D. G. Vlachos, Green Chem., 2016, 18,
1983–1993.
18 The low yield of ether 9 after distillation is the result of a
thermal fragmentation to the ortho-quinone methide
followed by polymerization to undefined products. See also:
(a) S. B. Cavith, H. R. Sarrafizadeh and P. D. Gardner, J. Org.
Chem., 1962, 27, 1211–1216.
5 (a) R. Cohen, C. R. Graves, S. T. Nguyen, J. M. L. Martin and
M. A. Ratner, J. Am. Chem. Soc., 2004, 126, 14796–14803; (b)
O. Eisenstein and R. H. Crabtree, New J. Chem., 2013, 37,
21–27; (c) R. S. Assary, L. A. Curtiss and J. A. Dumesic, ACS
Catal., 2013, 3, 2694–2704.
6 (a) R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B.
Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev.,
2013, 113, 1499–1597; (b) S. Chen, R. Wojcieszak, F.
Dumeignil, E. Marceau and S. Royer, Chem. Rev., 2018, 118,
11023–11117.
7 (a) J. Luo, J. Yu, R. J. Gorte, E. Mahmoud, D. G. Vlachos and
M. A. Smith, Catal. Sci. Technol., 2014, 4, 3074–3081; (b) J.
Jae, E. Mahmoud, R. F. Lobo and D. G. Vlachos,
ChemCatChem, 2014, 6, 508–513; (c) S. Shinde and C. Rode,
ChemSusChem, 2017, 10, 4090–4101.
8 H. Nguyen, N. Xiao, S. Daniels, N. Marcella, J. Timoshenko,
A. Frenkel and D. G. Vlachos, ACS Catal., 2017, 7,
7363–7370.
9 (a) F. J. Feher and T. A. Budzichowski, Polyhedron, 1995, 14,
3239–3253; (b) D. B. Cordes, P. D. Lickiss and F. Rataboul,
Chem. Rev., 2010, 110, 2081–2173; (c) C. Krempner, Eur. J.
Inorg. Chem., 2011, 2011, 1689–1698.
10 (a) H. C. L. Abbenhuis, S. Krijnen and R. A. van Santen,
Chem. Commun., 1997, 331–332; (b) T. Maschmeyer, M. C.
Klunduk, C. M. Martin, D. S. Shephard, J. M. Thomas and
B. F. G. Johnson, Chem. Commun., 1997, 1847–1848; (c) M.
Crocker, R. H. M. Herold and A. G. Orpen, Chem. Commun.,
1997, 2411–2412; (d) F. Carniato, C. Bisio, E. Boccaleri, M.
Guidotti, E. Gavrilova and L. Marchese, Chem. – Eur. J.,
2008, 14, 8098–8101; (e) E. H. Aish, M. Crocker and F. T.
Ladipo, J. Catal., 2010, 273, 66–72; ( f ) M. D. Skowronska-
Ptasinska, M. L.-W. Vorstenbosch, R. A. van Santen and
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020