Dehydrogenation of formic acid by Ir-bisMETAMORPhos complexes: Experimental and computational insight into the role of a cooperative ligand

Article abstract of DOI:10.1039/c4sc02555e

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands (La-Lc) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir^I(L^H) species (1a), which convert via formal oxidative addition of the ligand to Ir^III(L) monohydride complexes 2a-c. The rate for this step strongly depends on the ligand employed. Ir^III complexes 2a-c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h^-1, respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by ¹H and ³¹P NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogen-bonding.

Full text of DOI:10.1039/c4sc02555e

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Journal Name
RSCPublishing
ARTICLE
Dehydrogenation of Formic Acid by Ir-bisMETA-
MORPhos Complexes: Experimental and Com-
putational Insight in the Role of a Cooperative Ligand
Cite this: DOI: 10.1039/x0xx00000x
Sander Oldenhof,a Martin Lutz,b Bas de Bruin,a Jarl Ivar van der Vlugt,a Joost N.
H. Reek*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
The synthesis and tautomeric nature of three Xantheneꢀbased bisMETAMORPhos ligands (Laꢀ
Lc) is reported. Coordination of these bis(sulfonamideꢀphosphinamides) to Ir(acac)(cod)
www.rsc.org/
initially led to the formation of IrI( H) species (1a), which convert via formal oxidative
L
addition of the ligand to IrIII(
L) monohydride complexes 2aꢀc. The rates for this step strongly
depend on the ligand employed. IrIII complexes 2aꢀc were applied in the baseꢀfree
dehydrogenation of formic acid, reaching TOFs of 3090, 877 and 1791 hꢀ1, respectively. The
1
dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by H
and 31P NMR spectroscopy and DFT calculations. Besides functioning as an internal base,
bisMETAMORPhos also assists in the preꢀassembly of formic acid within the Irꢀcoordination
sphere and aids in stabilizing the rate determining transition state through hydrogenꢀbonding
.
Introduction
Enzyme active sites are a major source of inspiration for
chemists in the field of synthetic chemistry and homogeneous
catalysis, because of their high activities and selectivities
achieved in chemical transformations and the conceptual
strategies employed by these systems.1 For instance, weak but
highly directional hydrogen bonding stands out as key element
used by enzymes to selectively preꢀassemble and preꢀactivate
substrates and stabilize transition states. Inspired by this,
functional ligands decorated with Hꢀbond donor and/or
acceptor groups have been used to construct supramolecular
ligand structures, enabling modular ligand families2 to be
utilized in transition metal catalysis and also for substrate preꢀ
organization and preꢀactivation via specific ligandꢀsubstrate
interactions.3
Fig 1. General composition of METAMORPhos ligands with
their two tautomeric forms [N
equilibrium.
H H
ꢀP (PIII) and N=P (PV)] in
Formate dehydrogenase metalloenzymes have successfully
been employed for the reduction of CO2 and oxidation of
formate. Although there is no complete consensus concerning
the mechanism of either the reduction or the oxidation,
hydrogenꢀbonding and protonꢀshuttling are suggested to play a
crucial role in the way these enzymes operate.5 We recently
described our initial results with an Irꢀcatalyst bearing a
bisMETAMORPhos ligand in the baseꢀfree catalytic
dehydrogenation of formic acid. A reaction which has attracted
much recent interest in the context of hydrogen storage/release
systems.4f Typically, formic acid (HCOOH) dehydrogenation
catalysts require the addition of subꢀstoichiometric amounts of
base (e.g. 5:2 ratio of HCOOH:NEt3). to facilitate substrate
activation. However, this significantly reduces the overall
In our quest for novel systems able to undergo hydrogenꢀ
bonding interactions to steer reactivity we have developed
sulfonamidophosphine (METAMORPhos) ligands. These
ligands are based on
a PNSO2 scaffold that display
N
H
ꢀP / N=P tautomerism (see Figure 1). They have been
H
employed for monoꢀ and bimetallic Rhꢀcatalyzed
hydrogenation, Ruꢀbased heterolytic H2 cleavage and [2+2+2]
cycloaddition reactions.4
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hydrogen weight percentage of the reaction mixture.6 One
promising strategy to circumvent the use of exogenous base is
to employ catalyst systems bearing cooperative ligands to
access metalꢀligand bifunctional pathways for substrate
activation.7 Hydrogenꢀbonding interactions between ligand and
substrate as (additional) tool to enhance reactivity have
previously been proposed both in the hydrogenation of CO2 and
in the aluminium catalysed dehydrogenation of HCOOH.8
These interactions were also suggested to play a role in the
formation of halfꢀsandwich ruthenium and rhodium hydrides
from formic acid.9 Several studies on the mechanism of
HCOOH dehydrogenation have compared conventional βꢀ
hydride elimination with direct hydrideꢀtransfer (Figure 2A and
B).10 Given the potential Hꢀbonding abilities of
METAMORPhos ligands and the protic nature of formic acid,
we wondered if biomimetic nonꢀcovalent interactions between
the ligand backbone and formic acid could play a role in this
catalytic system. Herein we show that the protonꢀresponsive
ligand not only acts as internal base (Figure 2C), but that its
hydrogenꢀbonding abilities steer substrate preꢀassembly and
stabilize catalytic transition states.
Scheme 1. The synthesis of bisMETAMORPhos ligands La
ꢀLc
and their three tautomeric forms (PIIIꢀPIII, PIIIꢀPV and PVꢀPV).
All three ligands Laꢀ
Lc display PIII/PV tautomerism but in
different ratios for the three possible forms. For ligand La only
the PIIIꢀPIII and PIIIꢀPV tautomers were observed in CD2Cl2 in a
ratio of 1 : 0.4, as determined by 31P NMR spectroscopy. For
both ligands Lb and Lc all three possible tautomers were
observed, with ratios (PIIIꢀPIII: PIIIꢀPV: PVꢀPV) of 1 : 1.8 : 0.2
and 1 : 1.9 : 0.1 for Lb and Lc, respectively. These data show
that the overall PIII/PV ratio is not only determined by the
acidity of the NꢀH bond and basicity of the phosphorus atom
but is also greatly influenced by steric bulk of the substituent on
the sulfon group. The PV tautomer provides stability in these
ligands towards oxidation and hydrolysis, even to the extent
that LaꢀLc can be conveniently purified by column
chromatography. Upon coordination to a metal the tautomeric
behaviour of these ligands is lost.
The addition of Laꢀ
Lc to IrI(acac)(cod) generated complexes
Fig 2. Different proposed mechanisms for the dehydrogenation
of formic acid, BL: bifunctional ligand.
[IrI(LH)] 1aꢀc via a single protonꢀtransfer from the ligand to
acetylacetonate and displacement of cyclooctadiene. These
species show highly fluxional behaviour between the
protonated and deprotonated ligand arms, irrespective of the
specific ligand substitution pattern, which therefore leads to
We will present the synthesis of three bisMETAMORPhos
ligands as well as their coordination to iridium, compare the
activity of these systems in HCOOH dehydrogenation and
explain the dual role of the ligand framework during catalysis
by means of both experimental and computational results.
1
symmetric H and 31P NMR spectra. Formal oxidative addition
of the remaining ligand –NH group in 1a
ꢀ1c generates the
corresponding IrIII(H)( ) complexes 2aꢀc (Scheme 2).‡
L
Results and discussion
Synthesis of ligands and complexes
METAMORPhos ligands are prepared from
a
simple
condensation reaction between a sulfonamide of choice and a
chlorophosphine. They show high stability to both oxidation (at
phosphorus) and hydrolysis (of the PꢀN bond). We attribute this
stability to their prototropic character, resulting in an
equilibrium between the PIII and PV oxidation states that is
influenced by R1 and R2.4e In order to gain insight into the
effect of ligand modification on the coordination and degree of
tuning in HCOOH dehydrogenation catalysis, we prepared
bisMETAMORPhos ligands LaꢀLc via a threeꢀstep synthetic
protocol (Scheme 1) that results in selective formation of only
the respective mesoꢀisomers.‡
Scheme 2. Synthesis of complex 1aꢀc and 2aꢀc from IrI(acac)(cod)
and Laꢀc.
The rates for this overall protonꢀtransfer step vary significantly
for ligands 1aꢀc. IrIII complex 2a (with La) was obtained
quantitatively after 30 hours at room temperature, but no
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formation of complex 2b was observed under the same 3090 (2a), 877 (2b) and 1791hꢀ1
(
2c) reveal a correlation with
conditions. This species could only be obtained after heating the electronic nature of the sulfonamide organic sideꢀgroup, i.e.
the mixture for 40 hours at 70 °C. In contrast, the conversion of high activity is obtained with an electronꢀdonating group in
1c into 2c was significantly faster than the conversion of 1a paraꢀposition (2a
into 2a, and full conversion was observed after already 16
hours at room temperature. We previously reported the
molecular structure of 2a to be dimeric in the solid state (2a2),
with the ‘vacant’ axial site coordinated by an oxygen from the
sulfonamide of a second equivalent of 2a (Figure 3a).4f The
molecular structure determined for the more bulky analogue 2c
was found to be a slightly distorted octahedral mononuclear IrIII
hydride complex, with axial coordination of a water molecule
trans to the hydride to complete the octahedral coordination
environment of IrIII (see Figure 3b). The steric hindrance of the
isopropyl groups in complex 2c seems to effectively prevent the
formation of dinuclear complexes as was also found by
modelling studies. This finding also supports the previously
proposed mononuclear configuration in solution, based on
diffusion NMR data.4f The NꢀS bond lengths of 1.554(2) Å are
in agreement with deprotonated sulfonamide fragments.[4f] The
, nꢀbutyl), whereas an electronꢀwithdrawing
Fig 4. Dehydrogenation curves of 2aꢀc (TOFs are determined
between 4ꢀ30% conversion); total measured volume (mL) consist of
both H2 and CO2.
IrꢀOwater bond length was found to be 2.2427(18) Å, which is in
accordance with transꢀIrIII(H)(OH2) complexes described in
literature.11
group in paraꢀposition (2b, CF3) results in much lower activity
(see computational section for plausible explanation). Also with
sterically encumbered complex 2c a lower activity was obtained
than with 2a. Variable temperature (VT) NMR spectroscopy
was performed to obtain mechanistic insight and detect relevant
intermediates.† The addition of an equimolar amount of
HCOOH to complex 2a at 223 K led to a significant downfield
shift (1.7 ppm) of one of the phosphorus signals (see Figure 5,
left). This might indicate (partial) protonation of one of the
ligand arms. Upon increasing the temperature to 298 K the
original 31P NMR spectrum for species 2a is instantaneously
restored with no observation of any intermediates.δ Addition of
one equivalent of HCOOH to 2a results in an upfield shift at
223 K of the formate proton of 0.13 ppm (Figure 5 right)
compared to free HCOOH, which is an indication of HCOOH
coordination to the axial vacant site (Figure 5 top). Increasing
the temperature leads to decreasing HCOOH signals together
with the formation of H2.
Fig 3. (a) Schematic representation and molecular structure of
2a4f (b) Schematic representation and molecular structure of
2c 14
. Solvent molecules and Hꢀatoms omitted for clarity, except
for hydride and hydrogens of the aqua ligand. Color labels:
purple: iridium, orange: phosphorus, blue: nitrogen, yellow:
sulfur, red: oxygen. Selected bond lengths (Å) and angles (°)
for 2c: N1ꢀS1 1.554(2), N2ꢀS2 1.554(2), S1ꢀO2 1.4974(18), S2ꢀO4
1.5093(18), S1ꢀO3 1.4541(18), S2ꢀO5 1.439(2), Ir1ꢀP1 2.2311(6),
Ir1ꢀP2 2.2315(6), Ir1ꢀO2 2.1639(18), Ir1ꢀO4 2.1752(17), Ir1ꢀO6
(H2O) 2.2427(18), P1ꢀIr1ꢀP2 104.54(2).
Dehydrogenation studies with complexes 2aꢀc
The catalytic dehydrogenation of HCOOH was investigated
with complexes 2aꢀc.§ Catalysis was performed in toluene at 85
°C in the absence of base to generate dehydrogenation curves
that are shown in Figure 4.‡ The turnover frequencies (TOFs) of
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this structure the formate CꢀH bond is preꢀorganized for βꢀ
hydride elimination via high barrier transition state 3II (∆G‡ =
32.2 kcal molꢀ1).¶ This produces cisꢀdihydride structure 3III
,
which is an overall exergonic process (ꢀ5.7 kcal molꢀ1).
Structure 3III can release H2 via protonation of an iridium
hydride by the ligand (NꢀH or OꢀH) or reductive elimination of
the two hydride units. Protonation of the hydride by the NꢀH
moiety of the ligand in transition state 3IV has a rather high
barrier (red profile, 21.4 kcal molꢀ1). No transition state was
found for protonation of the hydride via an OꢀH group.
Transition state 3IV is potentially stabilized by axial
coordination of the ligand via the sulfon group, which is in
proximity to the iridium (2.42 Å). This stabilization cannot
occur upon protonation via the OꢀH moiety, which potentially
explains why no transition state could be found. The reductive
elimination of H2 forms IrI structure
1 (similar to the initially
formed complexes 1aꢀc that lead to the formation of 2aꢀc) via
transition state 3IV’, which is 4.6 kcal molꢀ1 lower in energy
(blue energy profile, 11.1 kcal molꢀ1) than 3IV. After release of
H2 the formed structure
starting structure . This is in agreement with experimental
1
is 6.4 kcal molꢀ1 higher in energy than
2
observations, as iridium(I) complexes 1aꢀc eventually
transform to the thermodynamically more stable iridium(III)ꢀ
hydride complexes 2aꢀc. The above described pathway to cis
ꢀ
dihydride species 3III via transition state 3II (Figure 6, red
line) seems unlikely to occur, as the activation barrier obtained
for CꢀH cleavage is rather high (32.2 kcal molꢀ1). The βꢀhydride
elimination towards the axial position was found to be
energetically more favorable (Figure 6, black energy profile).
Deprotonation of HCOOH by the ligand and formation of κ1ꢀ
formate structure 4I is endergonic by 4.1 kcal molꢀ1 The
.
protonated ligand showed a NꢀH···O hydrogen bonding
interaction between the ligand and the coordinated formate. βꢀ
Hydride elimination from 4I via transition state 4II to yield
4III has an activation barrier of 26.3 kcal molꢀ1. The trans
ꢀ
Fig 5. Proposed coordination of HCOOH to complexes
2 to
dihydride structure 4III is slightly endergonic by 0.1 kcal molꢀ1.
Protonation of the IrꢀH bond by a ligand NꢀH group to release
H2 from 4III via transition state 4IV has a high barrier of 24.0
kcal molꢀ1. This is most likely related to the significant
structural reorganization needed to bring the NꢀH proton in
proximity to the hydride. Under catalytic conditions an excess
of HCOOH is present, so we decided to investigate whether an
additional equivalent of HCOOH could mediate the protonꢀ
transfer from the ligand to the IrꢀH. Indeed, a transition state
was obtained wherein protonation of the IrꢀH with HCOOH
occurred simultaneously with reprotonation of HCOOH by the
ligand NꢀH (Figure 6, green profile). This transition state
form 2aꢀHCOOH adduct (top). VT 31P NMR spectra of 2a with
1
one equivalent of HCOOH (left). VT H NMR spectra of 2a
with one equivalent of HCOOH; arrows indicate HCOOꢀproton
and H2 formed (right).
Computational investigation into βꢀhydride elimination
The CꢀH bond cleavage in the dehydrogenation of HCOOH,
typically the rate determining step in the catalytic cycle, can
either occur via βꢀhydride elimination or via (ligandꢀassisted)
direct hydrideꢀtransfer of the formate hydrogen (HCOOH) atom
(see Figure 2A, B and C). Both possible mechanisms were
computationally investigated using DFT in order to shed light
(
4IV’) turned out to be 13.7 kcal molꢀ1 lower in energy (10.3
on
the
potential
role
of
the
protonꢀresponsive
kcal molꢀ1) compared to transition state 4IV. Similar secondꢀ
sphere interactions were previously proposed in the heterolysis
of H2 assisted by exogenous water.12 In our case, HCOOH
provides a perfect geometrical fit between IrꢀH and the NꢀH
moiety of the ligand for secondꢀsphere assisted proton transfer
to occur.
bisMETAMORPhos ligand. The obtained energy profiles for βꢀ
hydride elimination toward an equatorial and an axial
coordination site of the catalyst are shown in Figure 6 (only the
relevant part of the computed structures is shown).ψ
The first step in the energy profile towards equatorial βꢀhydride
elimination (red profile) is complete protonꢀtransfer of HCOOH
to the cooperative ligand, resulting in the formation of κ1ꢀ
formate complex 3I, which is endergonic by 11.3 kcal molꢀ1. In
4 | J. Name., 2012, 00, 1-3
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H
H
O
O
O
O
O
O
H
Ph
N
Ph
N
Ph
N
Ph
H
P
P
P
P
Ir
N
Ir
O
R
O
R
R
R
O
O
S
S
O
O
S
S
H
H
O
O
5I -3.1 kcal mol-1
6I 3.8 kcal mol-1
R
O
R
O
S
S
O
O
O
O
N
N
Ph
H
H
Ph
N
Ph
Ph
O
P
P
P
P
N
S
Ir
Ir
O
R
R
O
O
O
O
S
H
H
H
O
O
H
7I 11.8 kcal mol-1
8I 13.8 kcal mol-1
Fig 7
. DFT optimized geometries of formic acid adducts 5Iꢀ8I
(BP86, def2ꢀTZVP, ꢁGᴼ298K in kcal molꢀ1), R = phenyl.
Fig 6. Potential energy diagram (DFT, BP86, def2ꢀTZVP) for
The dehydrogenation of HCOOH via a direct hydrideꢀtransfer
pathway was first investigated using the axial HCOOH adducts
5I and 6I. Starting from the energetically most stable structure
5I (see black energy profile in Figure 8), an endergonic
rearrangement to 5II was found (+11.8 kcal molꢀ1), which
orients the formate hydrogen in a favorable position for direct
hydrideꢀtransfer to the metal. In transition state 5III (∆G‡ =
the dehydrogenation of formic acid via βꢀhydride elimination at
the equatorial (red) and axial (black) position ꢁGᴼ
molꢀ1.
is in kcal
298K
Computational investigation into direct hydrideꢀtransfer
An alternative mechanism for the dehydrogenation of formic
acid could involve a direct hydrideꢀtransfer of the formate
hydrogen (HCOOH) to the Irꢀcenter, subsequent to, or in
concert with proton transfer of the acidic HCOOH proton to the
ligand scaffold. In this mechanism a single metal coordination
site is sufficient for effective turnover. To test the validity of
26.8 kcal molꢀ1 and ∆G‡ = 29.9 kcal molꢀ1 with respect to 5I
HCOOH is fully deprotonated by the ligand. This enables facile
expulsion of CO2 leading to 6ꢀmembered
)
a
Ir···H···C···O···H···O transition state. After release of CO2,
dihydride complex 5IV is formed in an overall endergonic
process (+16.9 kcal molꢀ1), whereafter protonation of the
iridiumꢀhydride via transition state 5V (+17.4 kcal molꢀ1)
releases H2. The HCOOH dehydrogenation pathway was also
investigated starting from structure 6I (red profile in Figure 8).
The rearrangement from 6I to 6II (similar to the rotation of 5I
to 5II) is endergonic by 12.8 kcal molꢀ1. However, transition
state 6III was found to be significantly lower in energy (∆G‡ =
20.2 kcal molꢀ1) compared to 5III (∆G‡ = 26.8 kcal molꢀ1),
leading to formation of structure 4III via an unusual
8ꢀmembered Ir···H···C···O···H···N···S···O transition state,
such a pathway, we computed different HCOOHꢀ
2 adducts.
Adducts with either axial of equatorial coordination were all
found to exhibit stabilizing hydrogenꢀbonding interactions with
either the nitrogen atom or the coordinated oxygen atom in the
ligand scaffold, resulting in structures 5Iꢀ8I (see Figure 7).
Axial coordination of HCOOH (5I; interaction with O) is the
only structure found to be exergonic by 3.07 kcal molꢀ1,
compared to the free complex plus HCOOH. Formation of
structure 6I, with an NꢀH interaction, is endergonic by 3.82
kcal molꢀ1. Equatorial coordination of HCOOH, leading to
structure 7I or 8I, is associated with a significant energyꢀ
penalty of 11.8 (7I) or 13.8 (8I) kcal molꢀ1 compared to
formation of the axial adducts.
see Figure 6. Structures 6IꢀIII are all stabilized by hydrogen
bonding interactions between HCOOH/CO2 and the ligand preꢀ
assembling HCOOH and stabilizing the transition state. The
same role of the ligand was found in the energy profile starting
from structure 5I. The IrꢀH and NꢀH bonds in transition state
6III are elongated compared to those found in 4III (IrꢀH: 1.76
Å vs. 1.67 Å; NꢀH: 1.04 Å vs. 1.02 Å), which indicates a late
transition state (Figure 9). A similar interaction was proposed
by Hazari et al. for CO2 insertion of an IrꢀH stabilized by
hydrogen bonding between a ligandꢀbased NꢀH group and
CO2.13 Calculations performed on structures lacking these Hꢀ
bonding interactions (by pointing the NꢀH fragment outwards)
yielded unstable structures and no transition states could be
found. After CO2 release from 6III, structure 4III is formed
that releases H2 with assistance of another molecule of HCOOH
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potentially be explained by the decreased basicity of the
nitrogen atom in the ligand. The electronic effect was also
investigated theoretically by comparing
substituents. Indeed, a higher activation barrier was found for
ꢀCF3 (24 kcal molꢀ1) compared to ꢀCH3 (22.5 kcal molꢀ1), see
pꢀCF3 with pꢀCH3
p
p
ESI for energy profiles. We propose the following overall dualꢀ
role for the bisMETAMORPhos ligand in the mechanism of
formic acid dehydrogenation (Scheme 3). Species
HCOOH at the vacant axial site, aided by hydrogenꢀbonding
with the coordinated sulfonꢀoxygen atom to give structure 5I
2 coordinates
.
Reorientation of the formate group gives a preꢀactivated
HCOOH unit that has hydrogenꢀbonding with the deprotonated
nitrogen of the ligand arm (6II). Release of CO2 is achieved via
transition state 6III (rate determining step) wherein the ligand
deprotonates HCOOH and stabilizes the direct hydride transfer
by a hydrogen bonding interaction. This gives transꢀdihydride
4III that releases H2 aided by an additional equivalent of
Fig 8. Potential energy diagram (DFT, BP86, def2ꢀTZVP) for
dehydrogenation of formic acid by 5I (black profile) and 6I (red HCOOH, which protonates the IrꢀH and in turn is reprotonated
profile); ꢁGᴼ298K is in kcal molꢀ1.
by the ligand, thereby regenerating starting complex
2. The
reversibility of this catalytic system is currently under
investigation.
(structure 4IV’), regenerating the catalyst as described above
(Figure 6, green profile). Direct hydrideꢀtransfer was also
investigated starting from structures 7I and 8I, but these energy
profiles were found to be significantly higher than for 5I and
6I ‡
.
O
O
1.53
O
2.14
H
H
Ph
N
1.04
Ph
1.76
(1.67)
P
P
(1.02)
N
Ir
O
R
R
O
O
S
S
H
O
Fig 9. Transition state 6III with stabilizing hydrogenꢀbond
between ligand and CO2. Relevant computed bond lengths (in
Å) are compared to values found computationally for structure
4III (in parentheses).
Scheme 3. Schematic proposed catalytic cycle of the ligand
These calculations suggest that HCOOH dehydrogenation using
bisMETAMORPhosꢀderived complexes 2aꢀc follows an outerꢀ
sphere direct hydrideꢀtransfer mechanism at the axial vacant
site of these monohydride species. Starting from formic acid
adduct 5I, the energetically most favored pathway requires
rearrangement of 5I to 6I. This is likely a facile process due to
the high protonꢀmobility within the ligand scaffold (from NꢀH
to OꢀH), as was also inferred from the rapid protonꢀshuttling in
complexes 1aꢀc. This results in an overall activation barrier of
23.3 kcal molꢀ1, taking 5I as the resting state. This is in
agreement with the variable temperature 1H NMR observations,
as the change in chemical shift upon addition of HCOOH to 2a
indicates the formation of a 2aꢀHCOOH adduct akin to
structure 5I. Protonation of the ligand takes place in transition
state structure 6III. The hydride ligand already present in
assisted dehydrogenation of HCOOH by bisMETAMORPhos
complexes 2aꢀc
.
Conclusions
We report several bisMETAMORPhos ligands (LaꢀLc) based
on the xanthene backbone bearing two sulfonamidephosphine
units. The prototropic nature of these PN(H)SO2R fragments
results in different ratios for the PIII and PV tautomers,
depending on the electronic and steric nature of the
sulfonamide substituents. Formation of IrI complexes 1aꢀc was
achieved by coordination of LaꢀLc to Ir(acac)(cod). Subsequent
formal oxidative addition of the remaining NꢀH group resulted
in the formation of the corresponding IrIIIꢀmonohydride
complexes IrIII(H)(
L) (2aꢀc). These three complexes are active
catalysts for the baseꢀfree dehydrogenation of formic acid, with
TOFs of 3090, 877 and 1791 hꢀ1 for 2aꢀc, respectively. These
data reflect the influence of subtle electronic and steric changes
in the ligand architecture. The role of the ligand during
complexes 2aꢀc, trans to the axial coordination site of HCOOH,
acts purely as a spectator ligand. The lower activity of complex
2c, bearing electronꢀwithdrawing CF3 groups, can therefore
6 | J. Name., 2012, 00, 1-3
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Chemical Science
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ARTICLE
DOI: 10.1039/C4SC02555E
1
catalysis was investigated by variable temperature H and 31P Therefore, for reactions in solution, the Gibbs free energies for all steps
NMR spectroscopic measurements and DFT calculations. involving a change in the number of species should be corrected. Several
Variable temperature NMR data point to the formation of a 2a
ꢀ
methods have been proposed for corrections of gas phase to solution
HCOOH adduct. DFT calculations indicate that the hydrogenꢀ phase data. The minimal correction term is a correction for the condensed
bonding abilities of the ligand play an important role in the phase (CP) reference volume (1 L mol−1) compared to the gas phase (GP)
mechanism, resulting in an uncommon direct hydrideꢀtransfer reference volume (24.5 L mol−1). This leads to an entropy correction term
mechanism instead of the more commonly proposed βꢀhydride (SCP = SGP + Rln{1/24.5} for all species, lowering the relative free
elimination for HCOOH dehydrogenation. This was also found energies (298 K) of all associative steps with 2.5 kcal mol−1.15 According
to be the rateꢀdetermining step (23.3 kcal molꢀ1), which to some authors, this correction term is too small, and larger correction
confirms experimental observations using DCOOH and terms up to 6.0 kcal mol−1 have been suggested.16 Which correction term
HCOOD.4f A similar mechanism has been proposed for Alꢀ and is best remains a bit debatable.
Feꢀbased systems reported by Berben and Milstein, ψ All calculations were performed with R = phenyl on the sulfon group
8c
respectively.6c,
It was found that the combined hydrogenꢀ for computational simplicity using the diastereomeric form that was
bonding
and
protonꢀresponsive
properties
of
the utilized in VT NMR studies and obtained as crystal structure.
bisMETAMORPhos ligands are essential for the reactivity of Electronic Supplementary Information (ESI) available: [details of any
complexes 2aꢀc. These interactions facilitate the preꢀassembly supplementary information available should be included here]. See
of the HCOOH substrate and the stabilization of catalytically DOI: 10.1039/b000000x/
relevant intermediates and transition states, allowing an
otherwise inaccessible reaction pathway. We thus show that a
single coordination site is effective for the dehydrogenation
reaction to occur when using a functional ligand scaffold. The
protonꢀresponsive and hydrogenꢀbonding features of ligands
1
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(
LaꢀLc) are currently being explored for other reactions. The
mechanistic findings presented herein potentially play a role
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We kindly acknowledge NWO for financial support, Frédéric
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§ Dehydrogenation studies were performed with the diastereomeric
mixtures of complexes 2aꢀc (upon anionic coordination of the sulfon
group these become chiral and give rise to diastereomeric mixtures of the
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¶ Note that these are gasꢀphase calculations for which the translational
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