Activation and deactivation of Cp*Ir(TsDPEN) hydrogenation catalysts in water

Article abstract of DOI:10.1002/ejic.200900780

The addition of H₃PO₄ to Cp*Ir(TsDPEN-H), where TsDPEN = H₂NCHPhCHPhNT_s, is a simple method to obtain a water-soluble hydrogenation catalyst capable of reducing aromatic ketones to their corresponding alcohols in aqueous solutions. Key to the reactivity is the low affinity of the coordinatively unsaturated [Cp*Ir(TsDPEN)]⁺ for H₂PO₄. Catalyst degradation proceeds via the protonation of the tosylamido ligand, as was established by the crystallographic characterization of the tosylamine complex [Cp*Ir(NCMe)(HTsDPEN)] ²⁺.

Full text of DOI:10.1002/ejic.200900780

SHORT COMMUNICATION
DOI: 10.1002/ejic.200900780
Activation and Deactivation of Cp*Ir(TsDPEN) Hydrogenation Catalysts in
Water
Christopher S. Letko,[a] Zachariah M. Heiden,[a] and Thomas B. Rauchfuss*[a]
Keywords: Homogeneous catalysis / Aqueous catalysis / Catalyst degradation / Hydrogenation / Hydrides
The addition of H3PO4 to Cp*Ir(TsDPEN-H), where TsDPEN
= H2NCHPhCHPhNTs–, is a simple method to obtain a water-
soluble hydrogenation catalyst capable of reducing aromatic
ketones to their corresponding alcohols in aqueous solutions.
Key to the reactivity is the low affinity of the coordinatively
unsaturated [Cp*Ir(TsDPEN)]+ for H2PO4–. Catalyst degrada-
tion proceeds via the protonation of the tosylamido ligand, as
was established by the crystallographic characterization of
the tosylamine complex [Cp*Ir(NCMe)(HTsDPEN)]2+
.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
active in the presence of H2.[2,4] Recently it was discovered
that these amido complexes can be easily converted into
catalysts for direct hydrogenation.[5–9]
Introduction
Numerous amine-supported hydrogenation catalysts
have been reported in recent years, many being inspired by
Ikariya and Noyori’s pioneering work on transfer hydrogen-
ation (TH) catalysis involving the (cymene)RuH(TsDPEN)
motif, where TsDPEN = H2NCHPhCHPhNTs–.[1,2] These
catalysts efficiently catalyze the transfer hydrogenation of
polar substrates such as ketones from H2-donors such as 2-
propanol and formic acid[3] but are found to be minimally
The key insight is that protonation of 16e diamido com-
plexes generates cationic derivatives that are sufficiently
electrophilic to activate H2 directly.[10,11] Upon exposure to
H2, these cationic complexes convert to amino hydrides that
in turn reduce ketones and imines following the pathway for
traditional TH catalysis (Scheme 1).[7,8,11] The new cationic
catalysts offer significantly broadened flexibility to the hy-
drogenation of polar substrates. In specific examples, triflic
acid (HOTf) converts (cymene)Ru(S,S-TsDPEN-H) and
Cp*Ir(S,S-TsDPEN-H) into [(ring)M(S,S-TsDPEN)]OTf,
which activates H2 but will not abstract H2 from traditional
hydrogen donors.[8,9]
Results and Discussion
The present report is based from the following observa-
tion: addition of ca. 1 equiv. of H3PO4 to a two-phase
CH2Cl2/H2O solution containing Cp*Ir(TsDPEN-H) (1)
causes the metal complex to transfer to the aqueous phase.
The transfer of the iridium complex to the aqueous solution
is indicated by the color change, consistent with the forma-
tion of [Cp*Ir(TsDPEN)]+ ([1H]H2PO4) (Figure 1).[12]
These observations are described by Equation (1).
Cp*Ir(TsDPEN-H) + H3PO4 Ǟ [Cp*Ir(TsDPEN)]H2PO4
(1)
Scheme 1. Proposed mechanism for the catalytic hydrogenation of
ketones by [(cymene)Ru(S,S-TsDPEN)]+.
The 31P NMR spectrum of the D2O solution of [1H]-
H2PO4 indicated that the phosphate anion is non-coordi-
nated, its chemical shift being identical to that seen for
aqueous NaH2PO4 (δ = 0.87 for [1H]H2PO4 vs. δ = 1.07 for
NaH2PO4). The 1H NMR spectrum is consistent with a
five-coordinate complex, as indicated by the CHPhCHPh
signals that are diagnostic of trans-diaxial phenyl substitu-
[a] Department of Chemistry, University of Illinois,
Box 12-6 MC 712, 600 S. Mathews Ave, Urbana, Illinois 61801,
USA
E-mail: rauchfuz@illinois.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900780.
Eur. J. Inorg. Chem. 2009, 4927–4930
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C. S. Letko, Z. M. Heiden, T. B. Rauchfuss
Table 1. Aqueous hydrogenation experiments.[a]
SHORT COMMUNICATION
Catalyst
Time S/C TON[b] TOF Conversion Notes
[h]
[h–1]
[%]
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[1H]H2PO4
[2H]H2PO4
[3H(H2PO4)]
6
6
14
12
6
12
6
18
12
24
250
250
250
1000
250
1000
250
200
1000
200
150
202
225
650
175
0
200
0
590
0
25
34
16
54
29
0
33
0
49
0
60
81
90
65
70
0
80
0
59
0
Figure 1. Solution of 1 in a 1:1 mixture of water and CH2Cl2 before
[c]
(left) and after addition of one drop of H3PO4 (right).
[d,e]
[f]
ents. As we have reported,[10] [Cp*Ir(TsDPEN)]+ tends not
to bind oxyanions. When the reaction in Equation (1) was
conducted in CH2Cl2/Et2O solution, we obtained an orange
precipitate that dissolved in water to give NMR spectra
identical with those seen for the in-situ protonation. Ele-
mental analysis indicates that this solid corresponds to
[Cp*Ir(TsDPEN)]H2PO4·H2O. As evidence for its coordi-
natively unsaturated character, [1H]H2PO4 was found to re-
act in solution with CO and PMe3 to rapidly give the ad-
ducts [1(CO)]+ [νCO (KBr) = 2057 cm–1] and [1(PMe3)]+
[δ (31P, CDCl3) = –28.09].[10,13] Analogous to the prepara-
tion of [1H]H2PO4, we generated [Cp*Ir(MsDPEN)]-
H2PO4, [2H]H2PO4, by the reaction of H3PO4 with
Cp*Ir(MsDPEN-H).[5] This salt is also catalytically active
in water.
[g]
[h]
[i]
[d,e]
[d,e]
[a] Unless otherwise noted, reactions were conducted with [1H]-
H2PO4 developed in situ using 1 and 1.1 equiv. H3PO4, along with
1.5 mmol of 2-hydroxyacetophenone and 5 mL H2O in a glass au-
toclave pressurized with 10 atm H2. [b] Determined by 1H NMR
spectroscopy. [c] Using isolated [1H]H2PO4. [d] Performed in a
stainless steel autoclave. [e] Pressure: 30 atm. [f] Temperature:
40 °C. [g] Employing 200 equiv. of H3PO4. [h] pH = 6, NaH2PO4
buffer solution was employed. [i] DMSO employed as solvent.
Table 2. Effect of water/methanol ratio on hydrogenation of 2-hy-
–
droxyacetophenone by [1H]H2PO4.[a]
The hydrophilic nature of the H2PO4 is apparently criti-
Conversion [%][b]
cal to the water solubility of [1H]H2PO4, which contains an
otherwise hydrophobic organometallic cation. For example,
[Cp*Ir(TsDPEN)]OTf is insoluble in water. Sulfuric acid re-
acts with 1 to produce a bisulfate salt that is poorly water-
soluble, indicating the diminished hydrophilicity of the
HSO4– anion. Nitric acid also does not afford a water-solu-
ble salt. Although (cymene)Ru(TsDPEN-H) reacts with
H3PO4, the resulting product proved insoluble in water. It
MeOH [mL]
H2O [mL]
5
2.5
0
0
2.5
5
35
58
70
[a] Reactions were performed for 6 h at 40 °C under 10 atm of H2
in a glass autoclave. [b] Determined by H NMR spectroscopy.
1
is known, however, that the [(cymene)Ru(TsDPEN)]+ center The degradation product, [Cp*2Ir2(µ-H)3]+,[15] was indepen-
is more Lewis acidic than the related [Cp*Ir(TsDPEN)]+.[12] dently generated by treating an aqueous solution of [1H]-
Thus [(cymene)Ru(TsDPEN)H2PO4], [3H(H2PO4)], is prob- H2PO4 with 10 atm of H2 at 60 °C. Also formed in this
ably non-ionic. Severin and co-workers have previously hydrogenolysis is HTsDPEN+. We found that [Cp*2Ir2(µ-
shown that phosphonates bind to [(arene)Ru(diamine)]+ H)3]H2PO4 hydrogenates ACP-2-OH to the diol, but it is a
centers.[14]
[1H]H2PO4 catalyzes the hydrogenation of 2-hydroxyace-
relatively poor catalyst.
We propose that the degradation of [1H]H2PO4 is initi-
tophenone (ACP-2-OH) in aqueous solutions (Table 1). In ated by intramolecular proton transfer from a dihydrogen
a typical procedure, a suspension of 4 mg of 1 in 1 mL of ligand to the tosylamido nitrogen, which is usually consid-
H2O is treated with 1.1 equiv. of H3PO4 to afford a homo- ered nonbasic (Scheme 2).[16] The hydrogenolytic deactiva-
geneous red-orange solution. The solution is added to a tion of [1H]+ may be generally applicable to amido-based
suspension of 200 mg of ACP-2-OH in 4 mL of water and TH catalysts under acidic conditions.
pressurized with 10 atm of H2. Catalytic hydrogenation of
Protonation of the TsDPEN ligand would weaken the
ACP-2-OH employing S,S-[1H]H2PO4 afforded the (R)-diol Ir-NTs bond. Xiao and co-workers have proposed that the
with an ee of 75%, thereby establishing the integrity of the TsDPEN ligand becomes hemi-labile under acidic condi-
Ir-DPEN ensemble during catalysis.
tions and have isolated [(cymene)Ru(dppe)(κ1-TsDPEN)]2+
,
The pH of a completed hydrogenation reaction was wherein the tosylamine (RNHTs) is pendant by virtue of
found to be 3. When the reaction solution was buffered to the bidentate nature of dppe.[17b] The ability of tosylamines
pH 6 using NaH2PO4/Na2HPO4, the rate was only margin- to coordinate to these soft metal centers remains unproven.
ally affected. Catalysis was also observed in MeOH, but
We found that treatment of the acetonitrile adduct
conversions were higher in pure aqueous solution (Table 2). [1H(MeCN)]+ with HOTf afforded a salt containing the
Catalyst [1H]H2PO4 degrades as evidenced by decreased proposed tosylamino ligand [Equation (2)].[17] The soft
TOF concomitant with the bleaching of the solution color. MeCN ligand enhances the basicity of the dicationic Ir site.
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Activation and Deactivation of Hydrogenation Catalysts
vents, the pKa of the tosylamine proton of [1HH-
(NCMe)]2+ can be estimated to be fairly acidic at 1.77 in
H2O and 2.12 in MeOH, with the pKa being about two pKa
units lower for [1HH]2+ [19]
The lability of the HTsDPEN
.
ligand in the unsaturated [1HH]2+ is attributed to the high
acidity of the dicationic tosylamine complex.
Conclusions
The present report further demonstrates the versatile be-
havior of the soft Lewis acid [1H]+, which is now suitable
for studies in aqueous solution. The work also clarifies the
degradation pathway of this emerging class[20] of cationic
catalysts.
Scheme 2. Proposed catalyst deactivation mechanism.
Experimental Section
[Cp*Ir(TsDPEN)]H2PO4·H2O [1H]H2PO4: Cp*Ir(TsDPEN-H)
(250 mg, 0.36 mmol) was dissolved in a mixture of CH2Cl2 (5 mL)
and Et2O (50 mL). An aqueous solution of H3PO4 (25 µL, 85%)
was added to the red-orange solution to afford a light orange pre-
cipitate. The precipitate was isolated via filtration to afford a light
(2)
1
orange solid (205 mg, 72%). H NMR (500 MHz, D2O): δ = 1.72
(s, 15 H, Cp*), 2.14 (s, 3 H, SO2C6H4-4-CH3), 4.20 (br. s, 1 H,
H2NCHPhCHPhNTs), 4.44 (br. s, 1 H, H2NCHPhCHPhNTs),
6.78–7.31 (14 H, aromatic) ppm. H NMR (500 MHz, CD3OD): δ
Single crystals suitable for X-ray crystallography of
[Cp*Ir(MeCN)(HTsDPEN)]2+, [1HH(NCMe)](OTf)2, were
isolated as the OTf– salt and were analyzed (Figure 2).
1
= 1.90 (s, 15 H, Cp*), 2.27 (s, 3 H, SO2C6H4-4-CH3), 4.09 (d, J
= 3.8 Hz, 1 H, H2NCHPhCHPhNTs), 4.59 (d, J = 3.8 Hz, 1 H,
H2NCHPhCHPhNTs), 6.88–7.24 (14 H, aromatic) ppm. 13C
(125 MHz, CD3OD): δ = 5.41, 16.47, 65.69, 71.85, 85.80, 122.98,
123.48, 123.69, 123.72, 124.12, 124.32, 124.56, 125.17, 134.20,
136.42, 138.60 ppm. 31P NMR (500 MHz, D2O): δ = 1.00 (s) ppm.
31P NMR (500 MHz, CD3OD):
δ
=
2.21 (s) ppm.
IrC31H40N2O7PS·H2O (807.91): calcd. C 46.09, H 4.99, N 3.47;
found C 45.83, H 4.67, N 3.39.
[Cp*Ir(NCMe)(HTsDPEN)](OTf)2 [1HH(NCMe)](OTf)2: A CH2Cl2
solution of (100 mg, 145 µmol) Cp*Ir(TsDPEN-H) was treated
with (40 µL, 450 µmol) of triflic acid resulting in an immediate
color change from reddish-purple to a bright yellow color. The
solution was stirred for 30 min, and the solvent was removed under
reduced pressure. A yellow slightly oily solid was obtained and
washed with n-hexane. No further work up was attempted due to
the extreme instability of the complex, which resulted in
[Cp*Ir(TsDPEN)]OTf upon exposure to Et2O or THF. Exposure
of the solid to air longer than 24 hours resulted in a slight discolor-
Figure 2. Molecular structure showing [Cp*Ir(NCMe)(HTs-
DPEN)](OTf)2. The thermal ellipsoids are drawn at 50% prob-
ability and are omitted on the Cp* and phenyl groups for clarity.
ation to
a reddish-orange color indicative of formation of
The crystallographic analysis shows that all Ir–ligand
bonds in the dication are elongated relative to the conjugate
base [1H(NCMe)]+.[10] The ca. 0.1 Å elongation of the Ir–
NHTs bond is striking. One triflate anion hydrogen-bonds
(rN–O = 2.876 Å) to the NHTs (Figure 1) (see Supporting
Information). In this dicationic MeCN adduct, the phenyl
groups are diequatorial as observed in related pseudo-octa-
hedral complexes.
To determine the relative acidity of the tosylamine, pKa
measurements were undertaken in MeCN. Using urea as a
base (pKa = 7.7 in MeCN),[18] the tosylamine proton was
found to have a pKa of 8.55Ϯ0.32, which is 13 orders of
magnitude more acidic than the NH2 proton of [1H]+.[11]
1
[Cp*Ir(TsDPEN)]OTf. H NMR (500 MHz, CD3CN): δ = 1.60 (s,
15 H, Cp*), 2.28 (s, 3 H, SO2C6H4-4-CH3), 4.11 (t, J = 10 Hz, 1 H,
H2NCHPhCHPhNHTs), 4.52 (t, J = 10 Hz, 1 H, H2NCHPhCH-
PhNHTs), 4.73 (br. t, J = 10 Hz, 1 H, HHNCHPhCHPhNHTs),
5.49 (br. d, J = 10 Hz, 1 H, HHNCHPhCHPhNHTs), 6.73–7.52
(m, 14H and H2NCHPhCHPhNHTs) ppm.
Procedure for Hydrogenation of 2-Hydroxyacetophenone:
Cp*Ir(TsDPEN-H) (4 mg, 5.8 µmol) was suspended in deionized
water (1 mL), followed by the addition of an aqueous solution of
H3PO4 (25 µL, 0.29 ). Vigorous shaking of the suspension af-
forded a red-orange solution, which was transferred into a 80 mL
silated glass autoclave along with 2-hydroxyacetophenone (200 mg,
1.47 mmol), deionized H2O (4 mL), and a magnetic stir bar. The
Using the correlations of pKa’s of pyridines in other sol- autoclave was pressurized with 14 atm H2 and depressurized to
Eur. J. Inorg. Chem. 2009, 4927–4930
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4929

C. S. Letko, Z. M. Heiden, T. B. Rauchfuss
SHORT COMMUNICATION
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Supporting Information (see also the footnote on the first page of
this article): Molecular structure data of [Cp*Ir(NCMe)-
(HTsDPEN)](OTf)2 and [TsDPENH]OTf, synthetic preparations,
HPLC traces and other spectroscopic data.
CCDC-743230 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: August 11, 2009
Published Online: October 14, 2009
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