Organoiridium pyridonates and their role in the dehydrogenation of alcohols

Article abstract of DOI:10.1021/om100901b

New derivatives of 2-hydroxypyridine (2-hpH) and Cp*Ir(III) are described. Under conditions for catalytic dehydrogenation of 1-phenylethanol catalyzed by Cp*IrCl(Κ²-2-hp) (1), the main species observed are [Cp*₂Ir₂H₂(2-hp)]Cl ([2]Cl) and Cp*IrHCl(Κ¹-2-hpH) (3). Crystallographic analysis confirms that the cation in [2]PF₆ consists of a Cp* ₂Ir₂(μ-H)_x²⁺ core complemented by a pyridonate ligand that bridges via O and N centers. Although [2]Cl is catalytically highly active, the related salt [2]PF₆ is not. Addition of chloride sources reactivates [2]PF₆. Collectively, our experiments indicate that [2]Cl is a resting state that reverts to a more active species, which we propose is 1 itself. In situ NMR observations and PPh ₃ trapping experiments show that under catalytically relevant conditions 1 rapidly converts to 3, which can be observed spectroscopically. Compound 3 was independently generated by transfer hydrogenation of 1. In other experiments, 1 was found to ring-open upon treatment with PPh₃ to give Cp*IrCl(Κ¹-2-hp)(PPh₃), which in turn was found to react with AgPF₆ to give [Cp*Ir(Κ²- 2-hp)(PPh₃)]PF₆. Both PPh₃ derivatives proved catalytically inactive for dehydrogenation. Cp*IrCl(Κ²-2- hp-6-Me) was also prepared but could not be converted to Κ¹-2- hpH-6-Me derivatives. The complex Cp*IrCl(C₅H₃O ₂NH), nominally derived from the conjugate base of 2,6-dihydroxypyridine, features the novel ligand η³-C ₃H₃(CO)₂NH.

Full text of DOI:10.1021/om100901b

Organometallics 2010, 29, 6763–6768 6763
DOI: 10.1021/om100901b
Organoiridium Pyridonates and Their Role in the Dehydrogenation
of Alcohols
Aaron M. Royer, Thomas B. Rauchfuss,* and Danielle L. Gray
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801,
United States
Received September 17, 2010
New derivatives of 2-hydroxypyridine (2-hpH) and Cp*Ir(III) are described. Under conditions for
catalytic dehydrogenation of 1-phenylethanol catalyzed by Cp*IrCl(κ2-2-hp) (1), the main species
observed are [Cp*2Ir2H2(2-hp)]Cl ([2]Cl) and Cp*IrHCl(κ1-2-hpH) (3). Crystallographic analysis
confirms that the cation in [2]PF6 consists of a Cp*2Ir2(μ-H)x2þ core complemented by a pyridonate
ligand that bridges via O and N centers. Although [2]Cl is catalytically highly active, the related salt
[2]PF6 is not. Addition of chloride sources reactivates [2]PF6. Collectively, our experiments indicate
that [2]Cl is a resting state that reverts to a more active species, which we propose is 1 itself. In situ
NMR observations and PPh3 trapping experiments show that under catalytically relevant conditions
1 rapidly converts to 3, which can be observed spectroscopically. Compound 3 was independently
generated by transfer hydrogenation of 1. In other experiments, 1 was found to ring-open upon
treatment with PPh3 to give Cp*IrCl(κ1-2-hp)(PPh3), which in turn was found to react with AgPF6 to
give [Cp*Ir(κ2-2-hp)(PPh3)]PF6. Both PPh3 derivatives proved catalytically inactive for dehydro-
genation. Cp*IrCl(κ2-2-hp-6-Me) was also prepared but could not be converted to κ1-2-hpH-6-Me
derivatives. The complex Cp*IrCl(C5H3O2NH), nominally derived from the conjugate base of 2,6-
dihydroxypyridine, features the novel ligand η3-C3H3(CO)2NH.
Introduction
Both the 2-hydroxypyridine and the tautomeric 2-pyridone
are known to serve as unidentate ligands, binding metals
through N and O, respectively.2 Complexes with O-bonded
pyridones are common with hard metals, for example, [Fe-
(O-2-C5H4NH)6]2þ,8 whereas softer metals prefer N-bonded
2-hydroxypyridine (2-hpH).9 The ligand properties of the
pyridonate anion are usefully referenced to the behavior of
carboxylates. With a pKa of 17.0 (DMSO),10 2-hydroxypyr-
idine is much less acidic than acetic acid, with a pKa of 12.3,
DMSO.11 Pyridonate can serve as either an O- or an N-bonded
ligand,12 the latter being more common for soft metals, for
example, Ru(NC5H4O)2(terpy)(H2O).13 Most commonly,
pyridonates serve as N,O-bridging ligands in polymetallic
compounds.1,14 Monometallic complexes of bidentate pyr-
idonates (κ2-pyridonates) are rarer.15,16
The coordination chemistry of the tautomeric pair
2-hydroxypyridine/2-pyridone is well established1,2 and is rich
with opportunities for catalytic reactions that would benefit
from the these bifunctional ligands in proton-transfer reac-
tions. Ligand-facilitated proton transfer is pervasive in bio-
inorganic and industrial catalysis, being recently highlighted
through studies of the [FeFe]-hydrogenases3 and alkyne
hydration,4 respectively. A 2-hydroxypyridine-derived cofac-
tor has been discovered at the active site of the hydrogenase
enzyme Hmd.5 Furthermore, the oxygen center of the pyr-
idone/hydroxypyridine projects toward the proposed cata-
lytic coordination site (Figure 1).6 It is likely that the 2-oxo
functionality on the pyridyl ring participates in the hydrogen-
transfer reactions catalyzed by this enzyme.7
The hydroxyl substituent of 2-pyridinols is known to inter-
act with anionic ligands via hydrogen bonding. For example,
hydrogen bonding between 2-pyridinol and hydride ligands
*To whom correspondence should be addressed. E-mail: rauchfuz@
uiuc.edu.
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r
2010 American Chemical Society
Published on Web 11/19/2010
pubs.acs.org/Organometallics

6764 Organometallics, Vol. 29, No. 24, 2010
Royer et al.
Results
Hydrides Derived from Cp*Ir(K2-2-pyridonate)Cl. The rel-
evant catalysts or precatalysts are Cp*Ir(κ1-2-hydroxypyr-
idine)Cl2 and Cp*Ir(κ2-2-pyridonate)Cl (1). In the presence
of the base (K2CO3), the dichloride was found to give the
same results as 1, which was the starting point for our studies.
The related pyridonate complex (cymene)Ru(κ2-2-pyrido-
nate)Cl16 was also tested but found inactive. Treatment of
solutions of 1 with secondary alcohols was found to give
hydrides. For example, a CD2Cl2 solution of 1 with 5ꢀ excess
PhCH(OH)Me at 25 °C initially gave a hydride, previously
unobserved, which converted cleanly to a more stable hydride
over the course of several hours. ESI-MS analysis of reaction
mixtures indicated that the stable hydride has the formula
[Cp*2Ir2( μ-H)2( μ-2-hp)]þ ([2]þ).
Figure 1. Structure of the dithiothreitol-modified active site of
Hmd, showing the binding of the pyridone/pyridinol cofactor.
has been observed by Crabtree and Morris.17 2-Hydroxy-
pyridine is also known to form hydrogen bonds to halide and
other ligands (eq 1).18 More complicated ligands that incor-
porate the 2-pyridone functionality also participate in intra-
molecular H-bonding.19
[Cp*2Ir2( μ-H)2( μ-2-hp)]Cl ([2]Cl) was independently gen-
erated by treatment of Cp*2Ir2H2Cl2 with Na2-hp. Its un-
symmetrical structure is indicated by two equally intense 1H
NMR signals assigned to the nonequivalent Cp* ligands.
Anion exchange with aqueous NaPF6 converted the chloride
salt into [2]PF6, which was obtained in analytical purity. Crys-
tallographic analysis confirmed that the cation in [2]PF6 is
unsymmetrical, with the pyridonate ligand bridging the two
Ir centers (Figure 2). The structure is related to the symmetri-
Yamaguchi et al. reported that the complex Cp*Ir(2-hp)Cl
is a highly active catalyst for the acceptorless dehydrogena-
tion of secondary alcohols.20 With 1-phenylethanol in reflux-
ing toluene TONs up to 700 were observed. In terms of activ-
ity for alcohol dehydrogenation, Cp*Ir(κ2-2-pyridone)Cl is
one of the best (see Supporting Information).21 Dehydroge-
nation was proposed to proceed via ring-opening of the Ir(κ2-
2-pyridinoate) center to give a pyridinol-alkoxide complex
that undergoes β-hydride elimination.20 Pursuant to our
interests in transfer hydrogenation,22,23 we sought further
insight into the Cp*Ir-2-pyridonate system with the goal of
identifying intermediates and improving the efficiency of the
catalyst.
24
˚
cal compound [Cp*2Rh2H2( μ-OAc)]PF6 (rRh-Rh=2.60(1) A),
but the dihedral angle (between the Cp* groups) is smaller. In
[Cp*2Ir2H3]NO3 ([4]NO3) the Cp* groups are perpendicular
to the Ir---Ir vector.25
A hydride observed in all reactions of 1 with hydrogen
donors is proposed to be the 2-hydroxypyridine complex
Cp*IrHCl(2-hpH) (3). The compound was not isolated in
1
pure form, but the H NMR data are consistent with the
proposed stoichiometry. Compound 3 was independently
generated by treatment of 1 with the transfer hydrogenation
catalyst Cp*IrH(TsDPENH) (TsDPENH = racemic H2N-
CHPhCHPhNTs-).26 This process occurs rapidly even below
0 °C (eq 3, the enantioselectivity of the intermetallic hydro-
gen transfer was not investigated).
(15) Grotjahn, D. B.; Lo, H. C.; Groy, T. L. Acta Crystallogr., Sect. C
1996, C52, 300–302. Grotjahn, D. B.; Joubran, C. Organometallics 1995,
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2000, 19, 2310–2317.
(16) Lahuerta, P.; Latorre, J.; Sanau, M.; Cotton, F. A.; Schwotzer,
W. Polyhedron 1988, 7, 1311–1316. Morrison, E. C.; Palmer, C. A.; Tocher,
D. A. J. Organomet. Chem. 1988, 349, 405–411.
(17) Peris, E.; Lee, J. C., Jr.; Rambo, J. R.; Eisenstein, O.; Crabtree,
R. H. J. Am. Chem. Soc. 1995, 117, 3485–3491. Park, S. H.; Lough, A. J.;
Yap, G. P. A.; Morris, R. H. J. Organomet. Chem. 2000, 609, 110–122.
(18) Ragaini, F.; Gallo, E.; Cenini, S. J. Organomet. Chem. 2000,
593-594, 109–118.
(19) Miyabayashi, T.; Hara, T.; Yamagata, T.; Mashima, K. Acta
Crystallogr., Sect. E 2007, E63, m576–m578. Tomon, T.; Koizumi, T.-a.;
Tanaka, K. Eur. J. Inorg. Chem. 2005, 285–293. Royer, A. M.; Rauchfuss,
T. B.; Wilson, S. R. Inorg. Chem. 2008, 47, 395–397.
(20) Fujita, K.; Tanino, N.; Yamaguchi, R. Org. Lett. 2007, 9, 109–111.
(21) Zhao, J.; Hartwig, J. F. Organometallics 2005, 24, 2441–2446.
Adair, G. R. A.; Williams, J. M. J. Tetrahedron Lett. 2005, 46, 8233–8235.
van Buijtenen, J.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof, L. A.;
Kooijman, H.; Spek, A. L. Organometallics 2006, 25, 873–881. Lin, Y.; Ma,
D.; Lu, X. Tetrahedron Lett. 1987, 28, 3115–3118. Junge, H.; Loges, B.;
Beller, M. Chem. Commun. 2007, 522–524. Johnson, T. C.; J., M. D.; Wills,
M. Chem. Soc. Rev. 2010, 39, 81–86. Friedrich, A.; Schneider, S. Chem-
CatChem 2009, 1, 72–73.
(22) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2006, 128,
13048–13049.
(23) Heiden, Z. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 2009, 131,
3593–3600.
In the presence of excess MeOH, Cp*IrH(TsDPENH) (as
well as (cymene)RuH(TsDPENH)) slowly catalyzes this same
(24) Nutton, A.; Bailey, P. M.; Maitlis, P. M. J. Organomet. Chem.
1981, 213, 313–332.
(25) Ogo, S.; Nakai, H.; Watanabe, Y. J. Am. Chem. Soc. 2002, 124,
597–601.
(26) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300–1308.

Article
Organometallics, Vol. 29, No. 24, 2010 6765
Table 1. Turnover Numbers (TON) for Various Catalysts for
Conversion of PhCH(OH)Me into PhC(O)Mea
catalyst
TON 5 h
TON 21 h
Cp*IrCl(2-hp) (1)
Cp*IrCl(2-hp) (1) þ
237
253
571
640
5 equiv 2-hpH
[Cp*2Ir2(μ-H)2(2-hp)]Cl ([2]Cl)
[Cp*2Ir2(μ-H)2(2-hp)]PF6 ([2]PF6)
[Cp*2Ir2(μ-H)2(OAc)]PF6
[Cp*2Ir2(μ-H)3]PF6 ([4]PF6)
Cp*2Ir2H2Cl2
167
7
38
57
44
23
408
107
83
130
114
167
Cp*Ir(2-hp)2
a Conditions: 6 mL of refluxing toluene, 2.4 mL of 1-phenylethanol,
0.1 mol % Ir complex.
Figure 2. Structure of the cation in [2]PF6. Selected bond
˚
lengths (A) and angles (deg): Ir(1)-N(1), 2.095(9); Ir(1)-N(1B),
2.101(9); Ir(1)-H(1A), 1.59(6); Ir(1)-H(1B), 1.73(7); Ir(2)-O-
(1), 2.033(6); Ir(2)-H(1A), 1.74(7); Ir(2)-H(1B), 1.59(6); Ir-
(1)-Ir(2), 2.675(3); Cpcentroid-Ir(1), 1.836; Cpcentroid(B)-Ir(1),
1.814; Cpcentroid-Ir(2), 1.807; Cpcentroid(B)-Ir(1), 1.792; Ir(1)-
Ir(2)-H(1A), 35(2); Ir(1)-Ir(2)-H(1B), 38(2).
conversion, the slow step being the regeneration of Cp*IrH-
(TsDPENH). Addition of Et3N to a solution of 3 resulted in
the complete and immediate formation of [2]þ. Solutions of 3
were also found to react with PPh3 to give Cp*IrClH(PPh3)
(eq 4).
Figure 3. TON (cumulative) vs time for the dehydrogenation of
PhCH(OH)Me (0.1 mol % Ir, refluxing toluene) by [2]Cl and
[2]PF6. The squares mark the results for the PF6- salt before and
after addition of 2 equiv of PPNCl (at 21 h).
very poor catalyst, even though [2]þ is the dominant species
in solutions of active catalysts (see above). 19F NMR analysis
of the mixture after 21 h verified that the majority (∼95%) of
the PF6- remained intact; thus the inactivity of [2]PF6 is not
attributable to degradation of the counterion. Addition of
PPNCl (PPNþ = N(PPh3)2þ) to this solution gave activity
comparable to that of [2]Cl (Figure 3).
Otherwise, further equivalents of PPNCl, which has good
solubility under catalytic conditions, had only a modest effect
on catalysis by 1. The addition of a 5 equiv excess of 2-hpH
to 1 improves the stability of the catalyst system. Greater ex-
cesses of 2-hpH were found to suppress activity, consistent
with our finding that the bis(pyridonate) Cp*Ir(κ2-2-hp)-
(κ1-2-hp) is a modest catalyst.
We also showed that [2]Cl converted to 1 in the presence of
2-hpH in refluxing xylene. Thus, treatment of Cp*2Ir2H2Cl2
with 2-hpH at room temperature initially gave [2]þ, while
heating such solutions gave the monomer 1.
PPh3 Derivatives. Phosphine adducts of 1 were examined
in order to expand the range of these unusual catalysts and
prevent dimerization to species such as [2]þ. We found that
Cp*IrCl2(2-hpH) reacts with PPh3 to give Cp*IrCl2(PPh3),
consistent with the high lability of the κ1-hpH ligand. Treat-
ment of 1 with PPh3 cleanly gave Cp*IrCl(κ1-2-hp)(PPh3)
(5). The 1H NMR signals at δ 5.60, 5.87, 6.78, and 8.12 are
diagnostic for a N-bonded pyridonate.28 Treatment of 5 with
AgPF6 gave a new single product, [6]PF6, which we propose
ꢁ
ꢁ
Cp IrClHð2-hpHÞ þ PPh3 f Cp IrClHðPPh3Þ þ 2-hpH
3
ð4Þ
The fate of 1 was determined under conditions approx-
imating those used in catalysis,20 that is, 100 °C with 5 equiv
of PhCH(OH)Me vs 110 °C (refluxing toluene) with 1000
equiv of PhCH(OH)Me. We observed the initial formation
of a small quantity of 3 followed within minutes by the
appearance of [2]þ as the predominant species, which per-
sisted for hours at 100 °C. In contrast to its rapid reaction
with PhCH(OH)Me, it is interesting neither phenol nor
2,4-dinitrophenol reacts with 1; thus proton-induced ring-
opening is not facile. Similarly, 1 is unreactive toward
methanol (25 °C, 24 h).
Solutions of 1 in CD2Cl2 were also found to react readily
with H2. Again, 3 was the first detectable hydride, being
observed after a few seconds at -30 °C. Conversion of 1 was
complete within minutes at room temperature, the main prod-
uct being [2]þ together with a trace of Cp*2Ir2H2Cl2, as
observed by Yamaguchi under similar conditions.27 Upon
prolonged exposure of these solutions to H2, only [2]þ and
[Cp*2Ir2H3]þ ([4]þ) (δ -15.5) were observed.
Catalytic Role of 2þ. We assessed the relative activity of
several Cp*Ir-hp complexes for the catalytic dehydrogena-
tion of PhCH(OH)Me (Table 1). Compound 1 was found to
be superior to [2]Cl, but both were far more active than
related compounds. Striking was the finding that [2]PF6 is a
(28) Grotjahn, D. B.; Lo, H. C. Organometallics 1995, 14, 5463–5465.
Chuchuryukin, A. V.; Chase, P. A.; Mills, A. M.; Lutz, M.; Spek, A. L.;
van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2006, 45, 2045–2054.
(27) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K.-i. J. Am.
Chem. Soc. 2009, 131, 8410–8412.

6766 Organometallics, Vol. 29, No. 24, 2010
Royer et al.
is [Cp*Ir(κ2-2-hp)(PPh3)]PF6 (eq 5). Neither 5 nor [6]PF6
showed any reactivity toward H2 or toward PhCH(OH)Me.
The above experiments showed that PPh3 efficiently (i)
replaces 2-hydroxypyridine in 3 and (ii) opens the Ir(κ2-2-hp)
chelate ring to produce stable adducts. In view of these
results, we repeated the experiments involving hydrogena-
tion of 1 (-25 °C, 1 atm H2) followed by quenching the mix-
tures with PPh3. 31P NMR analysis of these PPh3-treated
mixtures revealed the presence of significant amounts of
Cp*IrHCl(PPh3). This hydrido chloride is proposed to be
derived from displacement of κ1-hydroxypyridine in the
transiently formed 3; under these conditions, Cp*2Ir2H2Cl2
and PPh3 react relatively slowly.29 We also observed Cp*Ir-
Cl2(PPh3) as well as Cp*IrCl(2-hp)(PPh3) (5), the latter
arising from 1. These same species were generated by the ad-
dition of PPh3 to a cooled catalytic reaction mixture. In this
case, a large amount of unreacted PPh3 was detected, reflect-
ing the low reactivity of [2]þ toward PPh3.
Complexes of Other Pyridones. Given the high catalytic
activities seen for 1, we examined related complexes using
6-methyl-2-hydroxypyridine (6-Me-2-hpH).Thenewcomplexes
were prepared by combining Cp*2Ir2Cl4 with the sodium
salts of these pyridonates. Spectroscopically, the complexes 1
and Cp*Ir(κ2-6-Me-2-hp)Cl (7) are similar. We were unable,
however, to prepare Cp*Ir(κ1-6-Me-2-hpH)Cl2, the analogue of
Cp*Ir(κ1-2-hpH)Cl2, via cleavage of Cp*2Ir2Cl4 by 6-MehpH.
We propose that the increased steric bulk of this ligand inhibits
κ1-coordination. Furthermore, 7proved to be a poor catalyst for
dehydrogenation of PhCH(OH)Me.
Figure 4. Crystallographically determined structure of (C5Me4-
3
˚
Et)IrCl(η -2,6-pyridionate). Selected bond distances (A): Ir1-
Cl1, 2.3919(12); Ir1-C12, 2.081(42); Ir1-C13, 2.219(4); Ir1-C16,
2.227(4); C12-C13, 1.436(5); C12-C16 1.417(5); C13-C14
1.464(5); N1-C14 1.387(5); N1-C15 1.391(4); C15-C16
1.463(5); C14-O2, 1.228 (4); C15-O1, 1.229 (4); Cp centroid
- Ir1, 1.833.
being Ir(η3-C3H5)I[N(SiMe2CH2PPh2)2] with a C2-Ir of
2.113 A.
30
˚
With a TON of 230 (standard conditions, 21 h), complex 8
is an effective catalyst (or precatalyst) for the dehydrogena-
tion of PhCH(OH)Me, but it is inferior to 1.
Conclusions
Our results confirm that Cp*IrCl(κ2-2-hp) is highly reac-
tive toward hydrogen donors. Under catalytic conditions,
the dominant species in solution is the μ-pyridonato salt
[Cp*2Ir2H2(2-hp)]Cl. Our results indicate that this diiridium
dihydride arises from the hydroxypyridine hydride complex
(eq 6).
1
ꢁ
ꢁ
2Cp IrHClðK -2-hpHÞ f ½Cp 2Ir2H2ð2-hpÞꢂCl þ ½2-hpH2ꢂCl
3
½2ꢂCl
ð6Þ
Required for this conversion is the high substitutional labi-
lity of the 2-hpH ligand in Cp*IrHCl(κ1-2-hpH), which was
confirmed by a variety of experiments. Although [Cp*2Ir2-
H2(2-hp)]þ is the dominant complex in solution during
catalysis, it is catalytically inactive. Instead, in the presence
of chloride, this diiridium cation converts to a highly active
monometallic catalyst system that consists of Cp*IrHCl(κ1-
2-hpH) and its dehydro precuror Cp*IrCl(κ2-2-hp).
From the reaction of Cp*2Ir2Cl4 with the monodeproto-
nated derivative of 2,6-dihydroxypyridine, we prepared a
species with the nominal formula Cp*Ir(C5H4NO2)Cl (8). 1H
NMR measurements, even at -60 °C, indicated that 8 adopts
a symmetrical structure. The low-field signal was found to
rapidly exchange with D2O. Crystallographic analysis of the
(C5Me4Et)Ir analogue of 8 established the presence of an
allyl-like ligand, that is, Cp*Ir(η3-2,6-pyridionate)Cl (Figure 4).
Experimental Section
General Considerations. Unless otherwise indicated, reactions
were conducted using standard Schlenk techniques (N2) at room
temperature with stirring. Solvents were dried and degassed
prior to use. The following were prepared according to literature
methods: Cp*Ir(κ2-2-hp)Cl,20 [Cp*2Ir2H3]PF6,31 Cp*2Ir2H2Cl2,29
[Cp*2Ir2H2( μ-OAc)]PF6,29 Cp*IrCl2(PPh3),32 Cp*2Ir2Cl4,33 and
Cp*IrH(TsDPENH).23 The reaction of Cp*2Ir2H2Cl2 with
˚
The Ir1-C13 and Ir1-C16 distances of 2.219 and 2.227 A
are within the normal distances for an Ir(III)-allyl com-
˚
plex. The Ir1-C12 distance of 2.081 A is the shortest
Ir(III)-C bond among these structures, the closest example
(29) Gill, D. S.; Maitlis, P. M. J. Organomet. Chem. 1975, 87, 359–364.
(30) Fryzuk, M. D.; Huang, L.; McManus, N. T.; Paglia, P.; Rettig,
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Article
Organometallics, Vol. 29, No. 24, 2010 6767
PPh3 to produce Cp*IrClH(PPh3) is much slower than origi-
nally described,29 requiring about 24 h for completion. 2-Hydro-
xypyridine, 6-methyl-2-hydroxypyridine, 2,6-dihydroxypyridine
hydrochloride, NaOMe, Et3N, and AgPF6 were purchased from
Aldrich. Racemic 1-phenylethanol was obtained from Alfa-Aesar.
Electrospray ionization-mass spectra (ESI-MS) were acquired
using a Micromass Quattro QHQ quadrupole-hexapole-quadru-
pole instrument. 1H, 19F, and 31P NMR spectra were acquired on
Varian UNITY INOVA TM 500NB and UNITY 500 NB in-
struments. Elemental analyses were performed by the School of
Chemical Sciences Microanalysis Laboratory utilizing a model
CE 440 CHN analyzer.
and κ1-2-hp groups appear as an average in the spectrum, as seen
previously for a related complex.17 1H NMR (500 MHz, CDCl3
25 °C): δ 1.68 (s, 15H, Cp*), 5.97 (d, 2H, 8.7 Hz, aryl-CH), 6.15
(d of d of d, 2H, 3.1 Hz, 10.4 Hz, 12.7 Hz aryl-CH), 7.13 (d of d of
d, 2H, 2.0 Hz, 6.8 Hz, 8.7 Hz, aryl-CH), 7.75 (d of d, 2H, 1.7 Hz,
5.9 Hz, aryl-CH).
Cp*Ir(2-hp)Cl(PPh3) (5). A solution of 329 mg (1.26 mmol) of
PPh3 in 5 mL of CH2Cl2 was transferred to a orange solution of
574 mg (1.26 mmol) of 1. After stirring for 10 min, the reaction
solution was concentrated under vacuum and diluted with hex-
anes to precipitate yellow crystals, which were dried under vacuum.
Yield: 721 mg (1.01 mmol, 81%). Alternatively, the product could
be obtained by reaction of Cp*IrCl2(PPh3) with Na2-hp in sim-
ilar yield. 1H NMR (500 MHz, CD2Cl2): δ 1.35 (d, 15H, 2.1 Hz,
Cp*), 5.60 (d of d, 1H, 1.5 Hz, 8.7 Hz, pyr-aryl-CH), 5.87 (t of d,
1H, 1.5 Hz, 6.5 Hz, pyr-aryl-CH), 6.78 (t of d, 1H, 2.2 Hz,
7.5 Hz, pyr-aryl-CH), 7.00 (m, 2H, Ph-CH), 7.14 (m, 3H, Ph-
CH), 7.33 (m, 4H, Ph-CH), 7.45 (m, 4H, Ph-CH), 7.97 (m, 2H,
Ph-CH), 8.12 (d of d, 1H, 2.2 Hz, 5.5 Hz, pyr-aryl-CH). 31P
NMR (202 MHz, CD2Cl2): δ 6.53 (s, Ir-PPh3). Anal. Calcd for
C33H34ClIrNOP (found): C, 55.10 (54.96); H, 4.76 (4.71); N,
1.95 (2.10).
[Cp*2Ir2H2( μ-2-hp)]X ([2]Cl and [2]PF6). A solution of 400 mg
(7.4 mmol) of NaOMe in 5 mL of MeOH was transferred to a
solution of 704 mg (7.4 mmol) of 2-hpH in 5 mL of MeOH to
give a colorless homogeneous solution. Solvent was removed
under vacuum at 60 °C overnight to give an air-stable hygro-
scopic white powder, assumed to be Na2-hp, which was stored in
a desiccator. Yield:774 mg (6.6 mmol,89%). 1H NMR(500 MHz,
CD3OD): δ 6.33 (t, 1H, 6.2 Hz, aryl-CH), 6.40 (d, 1H, 8.5 Hz,
aryl-CH), 7.35 (m, 1H, aryl-CH), 7.67 (d, 1H, 5.4 Hz, aryl-CH).
A solution of 24 mg (0.205 mmol) of Na2-hp in 2 mL of MeOH
was transferred to a blue solution of 150 mg (0.205 mmol) of
Cp*2Ir2H2Cl2 in 10 mL of CH2Cl2 to immediately give a red
solution. After stirring 5 min, the solvent was removed under
vacuum. The residue was extracted into 10 mL of CH2Cl2, and
the slurry was cannula-filtered to remove NaCl. The filtrate was
concentrated to ∼3 mL and diluted with hexanes to produce a
[Cp*Ir(K2-2-hp)(PPh3)]PF6 ([6]PF6). A solution of 40 mg
(0.158 mmol) of AgPF6 in 3 mL of CH2Cl2 was transferred to
an orange solution of 114 mg (0.158 mmol) of 5 in 4 mL of
CH2Cl2 to give an immediate colorless precipitate. The solu-
tion mixture was filtered to give a pale orange solution. Addi-
tion of hexanes to the filtrate gave yellow crystals, which were
1
1
brown powder. Yield: 138 mg (0.176 mmol, 86%). H NMR
dried under vacuum. Yield: 103 mg (0.125 mmol, 79%). H
(500 MHz, CD2Cl2): δ -14.10 (s, 2H, Ir-H-Ir) 1.92 (s, 15 H,
Cp*), 1.99 (s, 15 H, Cp*’), 6.57 (d of d of d, 1H, 5.0 Hz, 8.2 Hz,
13.7 Hz aryl-CH), 7.28 (m, 2H, aryl-CH, aryl-CH0), 8.33 (d, 1H,
6.1 Hz, aryl-CH). ESI-MS: m/z = 750.2 ([Cp*2Ir2H2(2-hp)]þ).
Red crystals of [Cp*2Ir2H2( μ-2-hp)]PF6 precipitated upon the
addition of 0.5 mL of a saturated aqueous solution of NaPF6 to
an acetone solution of [2]Cl. The solid was filtered off, washed
with water, and dried under vacuum overnight. Yield: 350 mg
NMR (500 MHz, CD2Cl2): δ 1.45 (d, 15H, 2.1 Hz, Cp*), 5.46
(d, 1H, 8.7 Hz, pyr-aryl-CH), 6.52 (d of d of d, 1H, 1.0 Hz,
5.8 Hz, 6.9 Hz, pyr-aryl-CH), 7.11 (t of d, 1H, 1.3 Hz, 7.9 Hz,
pyr-aryl-CH), 7.41 (m, 12H, Ph-CH), 7.49 (m, 3H, Ph-CH),
7.77 (d of d, 1H, 0.8 Hz, 5.8 Hz, pyr-aryl-CH). 31P NMR (202
MHz, CD2Cl2): δ 15.72 (s, Ir-PPh3), -145.2 (p, 710 Hz, PF6).
ESI-MS: m/z=684.4 ([Cp*Ir(2-hp)(PPh3)]þ), 625.3 ([Cp*Ir-
Cl(PPh3)]þ), 589.3 ([Cp*Ir(PPh3)]þ).
1
(0.39 mmol, 68%). H NMR (500 MHz, CD2Cl2): δ -14.10
Cp*IrCl(6-Me-2-hp) (7). The salt “Na6-Me-2-hp” was synthe-
sized analogously for Na2-hp. H NMR (500 MHz, CD3OD):
1
(s, 2H, Ir-H-Ir), 1.92 (s, 15 H, Cp*), 1.99 (s, 15 H, Cp*0), 6.57
(d of d, 1H, 5.3 Hz, 8.9 Hz aryl-CH), 7.28 (m, 2H, aryl-CH, aryl-
CH0), 8.33 (d, 1H, 6.3 Hz, aryl-CH). ESI-MS: m/z = 750.2
([Cp*2Ir2H2(2-hp)]þ). Anal. Calcd for C25H36F6Ir2NOP (found):
C, 33.51 (33.37); H, 4.05 (3.97); N, 1.56 (1.63). Crystals suitable
for X-ray diffraction were obtained by layering a solution of
60 mg of [3]PF6 in 2 mL of CH2Cl2 with 30 mL of Et2O. Crystals
grew over the course of 2 h at room temperature.
δ 2.24 (s, 3H, CH3), 6.18 (d, 1H, 6.9 Hz, aryl-CH), 6.24 (d, 1H,
8.7 Hz, aryl-CH), 7.30 (m, 1H, aryl-CH). A colorless solution of
110 mg (0.656 mmol) of Na6-Me-2-hp in 5 mL of MeOH was
transferred to an orange solution of 261 mg (0.328 mmol) of
Cp*2Ir2Cl4 in 10 mL of CH2Cl2. After the solution was stirred
for 1 h, the solvent was removed by vacuum. The product was
extracted into 5 mL of CH2Cl2, and this extract was filtered to
remove NaCl. The filtrate was concentrated to ∼2 mL and then
diluted with 10 mL of hexanes to produce a yellow precipitate.
Yield: 101 mg (0.213 mmol, 65%). 1H NMR (500 MHz,
CD2Cl2): δ 1.72 (s, 15H, Cp*), 2.28 (s, 3H, 6-CH3), 5.87 (d, 1H,
8.5 Hz, aryl-CH), 6.34 (d, 1H, 7.2 Hz, aryl-CH), 7.29 (d of d, 1H,
7.3 Hz, 8.4 Hz, 4-CH). Anal. Calcd for C16H21ClIrNO (found):
C, 40.81 (40.58); H, 4.49 (4.43); N, 2.97 (3.02). A CH2Cl2 solu-
tion of 7 was treated with 1 atm of H2 at room temperature over
24 h; 1H NMR analysis revealed ∼95% unreacted 7 as well as a
small amount of free 6-Me-hpH.
Cp*IrH(Cl)(2-hpH) (3) via Transfer Hydrogenation. A solu-
tion of 7.9 mg (0.017 mmol) of 1 in 0.8 mL of CD2Cl2 was treated
with 12.0 mg (0.017 mmol) of Cp*IrH(TsDPENH) at -27 °C.
After the addition, the mixture was immediately allowed to warm
to room temperature. 1H NMR analysis of the dark red solution
1
verified that the reaction was complete. H NMR (Figure S5)
(500 MHz, CD2Cl2): δ -15.8 (s, 1H, Ir-H), 1.54 (s, 15H, Cp*),
6.29 (t, 1H, 6.3 Hz, aryl-CH), 6.54 (d, 1H, 7.8 Hz, aryl-CH), 7.24
(m, 1H, aryl-CH), 8.03 (d of d, 1H, 1.8 Hz, 6.3 Hz, aryl-CH). For
catalytic hydrogenation see the Supporting Information.
Cp*Ir(2-hp)2. A colorless solution of 88.2 mg (0.753 mmol) of
Na2-hp in 6 mL of MeOH was transferred to an orange solution
of 150 mg (0.188 mmol) of Cp*2Ir2Cl4 in 5 mL of CH2Cl2. After
stirring 1 h the solvent was removed by vacuum. The product
was extracted into 5 mL of CH2Cl2 and filtered to remove NaCl.
The filtrate was diluted with 25 mL of hexanes, and this mixture
was concentrated under vacuum to 10 mL, producing a yellow
powder. The product was collected by filtration and washed
with 5 mL of hexanes. Yield: 142 mg (0.274 mmol, 73%). In the
(C5Me4R)IrCl(η3-C3H3-2,6-(CO)2NH) (R=Me (8) and R=Et).
A solution of 562 mg (10.41 mmol) of NaOMe in 5 mL of
MeOH was transferred to a solution of 768 mg (5.2 mmol) of
2,6-dihydroxypyridine HCl in 5 mL of MeOH to give a homo-
geneous colorless solution. Solvent was removed under vacuum
3
at 60 °C overnight to give an air-sensitve hygroscopic white
powder. Yield of “NaC5H4NO2 NaCl”: 945 mg (4.94 mmol,
3
95%). 1H NMR (500 MHz, CD3OD): δ 5.40 (d, 2H, 8.2 Hz, aryl-
CH), 7.23 (t, 1H, 8.2 Hz, aryl-4-CH). A solution of 151 mg
(0.791 mmol) of NaC5H4NO2 NaCl in 3 mL of MeOH was
1
room-temperature H NMR spectrum, signals for the κ2-2-hp
3
added to a solution of 300 mg (0.377 mmol) of Cp*2Ir2Cl4
in 3 mL of CH2Cl2. After stirring for 1 h, the solution was
concentrated under vacuum. The residue was extracted into
5 mL of CH2Cl2, and this extract was filtered to remove NaCl.
(32) Kang, J. W.; Moseley, K.; Maitlis, P. M. J. Am. Chem. Soc. 1969,
91, 5970–5977.
(33) White, C.;Yates, A.;Maitlis, P. M. Inorg. Synth. 1992, 29, 228–234.

6768 Organometallics, Vol. 29, No. 24, 2010
Royer et al.
The filtrate was concentrated to a small volume and diluted with
hexanes to produce a yellow powder, which was dried under vac-
uum. Yield: 280 mg (0.592 mmol, 79%). H NMR (500 MHz,
Cp*IrCl(2-hp)(PPh3) (δ 6.5, 47%). A signal at δ 16.3 (14%) could
not be assigned.
1
High-Temperature PhCH(OH)Me Reaction. A solution of
53 mg (0.12 mmol) of 1 in 2 mL of toluene in a three-necked
flask fitted with a condenser was heated in a 130 °C oil bath, then
treated with 0.7 mL (5.8 mmol) of PhCH(OH)Me. After 30 min,
the red solution was cooled in an ice bath and treated with
33.5 mg (0.128 mmol) of PPh3. After 20 min stirring, 0.7 mL of
solution was removed and analyzed by 31P NMR spectroscopy.
The following species were observed (% relative amounts based
on 31P NMR analysis): Cp*IrHCl(PPh3) (δ 12.5, 4.2%), Cp*Ir-
Cl2(PPh3) (δ 2.1, 18%), PPh3 (δ -4.8, 70%), Cp*IrCl(2-hp)-
(PPh3) (δ 6.3, 4.9%). A signal at δ 19.0 (2.8%) could not be
assigned.
X-ray Cystallography of Compounds [2]PF6 and (CpMe4Et)-
IrCl(η3-C5H3-2,6-(CO)2NH). The crystallographic analysis of
[2]PF6 and (C5Me4Et)IrCl(η3-C5H3-2,6-(CO)2NH) were con-
ducted in the usual way (Supporting Information), but the
cation 2þ suffered from severe disorder of the ligands and PF6
CD2Cl2): δ 1.84 (s, 15H, Cp*), 4.36 (d of d, 2H, 1.6 Hz 5.4 Hz,
aryl-CH; exchange in D2O), 5.61 (t, 1H, 5.4 Hz, 4-aryl-CH), 6.69
(s, 1H, NH; exchange in D2O). Anal. Calcd for C15H19ClIrNO2
(found): C, 38.09 (38.30); H, 4.05 (4.47); N, 2.96 (2.56). The
(C5Me4Et)Ir derivative was obtained similarly. Yield: 128 mg
(0.263 mmol, 79%). 1H NMR (500 MHz, CDCl3): δ 1.14 (t, 3H,
7.6 Hz, CH2CH3),1.86 (s, 6H, CH3), 1.90 (s, 6H, CH3), 2.17
(q, 2H, 7.6 Hz, CH2CH3), 4.44 (d of d, 2H, 1.6 Hz, 5.4 Hz, aryl-
CH exchanged in D2O), 5.62 (t, 1H, 5.4 Hz, 4-aryl-CH), 6.69
(s, 1H, NH; exchange in the presence of D2O). Single crystals
suitable for X-ray diffraction were obtained by diffusion of Et2O
into a concentrated CH2Cl2 solution of (C5Me4Et)IrCl(C3H3-
2,6-(CO)2NH).
Catalytic Dehydrogenation of 1-Phenylethanol (see Table 1).
In a 100 mL three-necked flask equipped with a nitrogen inlet
and reflux condenser was mixed 6 mL of toluene, 2.4 mL of
1-phenylethanol, 0.5 mL of CH2Ph2, and 0.1 mol % (Ir) catalyst.
The mixture was heated at a vigorous reflux in a 130 °C oil
bath. Samples of 100 μL were withdrawn at hourly intervals
via syringe-septum techniques for the first 5 h, and a final
sample was removed after 21 h. Conversions were analyzed by
1H NMR spectroscopy versus CH2Ph2 as an internal integration
standard.
Conversion of Cp*2Ir2H2Cl2 to 1 in the Presence of 2-hpH.
Low-Temperature Reaction. A solution of 7.5 mg (0.01 mmol) of
Cp*2Ir2H2Cl2 and 2 mg (0.02 mmol) of 2-hpH in 0.75 mL of
CD2Cl2 was prepared in a J. Young tube. After 4 h, the char-
acteristic purple color of the Cp*2Ir2H2Cl2 had faded. 1H NMR
analysis of this solution confirmed that [2]þ was the major
product and remained such for 7 days.
High-Temperature Reaction. A solution of 204 mg (0.28 mmol)
of Cp*2Ir2H2Cl2 and 58.6 mg (0.62 mmol) of 2-hpH in 10 mL of
p-xylenes was heated at reflux for 40 min. An orange precipitate
appeared. After cooling to room temperature, the mixture was fil-
tered to give 112 mg of brownish-yellow 1 containing traces of [2]þ.
Chloride Rescue of Catalysis by [2]PF6. Reaction Conditions. A
mixture of 6 mL of toluene, 2.4 mL of PhCH(OH)Me, 0.1 mol % Ir
catalyst, and 0.5 mL of CH2Ph2 was refluxed in a 130 °C oil bath.
After 21 h, 0.7 mL of the reaction mixture was analyzed by 19F
NMR spectroscopy, which showed minimal, ∼5% decomposition
of the PF6-. Solid PPNCl (11.4 mg, 0.020 mmol) was added to the
mixture. Over the next 4 h, 0.1 mL samples were withdrawn from
the reaction mixture, diluted with CDCl3, and assayed by 1HNMR
spectroscopy in the usual manner.
anion. Our model converged with wR2 = 0.0914 and R1
=
0.0548 for 631 parameters with 1198 restraints against all 6133
data. The main residue had both Cp* ligands positionally dis-
ordered. The Ir-C distances between an individual Ir-Cp*
ligand were constrained to be similar (esd 0.01). The pyridonate
ligand was also positionally disordered. Like C-O and Ir-N,
distances on the pyrdinate ligand were restrained to be similar
(esd 0.01). Rigid-bond restraints (esd 0.01) were imposed on
displacement parameters for all disordered sites, and similar
displacement amplitudes (esd 0.01) were imposed on disordered
sites overlapping by less than the sum of van der Waals radii.
Methyl H-atom positions, R-CH3, were optimized by rotation
about R-C bonds with idealized C-H, R-C, and H
H
3 3 3
distances. Both hydride, H-Ir, atoms were located in the dif-
ference map in asymmetric positions. Full positional refinement
of the hydride atoms was not possible because of the heavy Ir
atoms and the sever disorder in the structure, so restraints had to
be applied. The short Ir1-H1a and Ir2-H1b distances were
restrained to be similar (esd 0.01) as well as the longer Ir1-H1b
and Ir2-H1a distances (esd 0.01). The remaining H atoms were
included as riding idealized contributors. Methyl H atom U’s
were assigned as 1.5 times Ueq of the carrier atom, hydride H
atom U’s were assigned as 1.5 times Ueq of the carrier Ir atom
(the carrier atom being the Ir atom that H shared the shortest
bond distance with), and remaining H atom U’s were assigned as
1.2 times carrier Ueq.
PPh3-Trapping Experiments. Low-Temperature H2 Reaction.
A yellow solution of 81 mg (0.18 mmol) of 1 in 5 mL of CH2Cl2 at
-25 °C was flushed with 1 atm of H2. After 40 min, the solution
becomes a deep red color and was treated with 46 mg (0.18 mmol)
of PPh3. The flask was flushed with N2. After stirring the solution
for 20 min at -25 °C, 0.8 mL of sample was removed and analyzed
by 31P NMR spectroscopy: Cp*IrHCl(PPh3) (δ 12.9, 9.4%),
Cp*IrCl2(PPh3) (δ 2.4, 19%), PPh3 (δ -4.96, 12%), as well as
Acknowledgment. This research was sponsored by the
U.S. Department of Energy.
Supporting Information Available: Experimental details and
spectra. Crystallographic data for [2]PF6 and (C5Me4Et)IrCl-
(η3-2,6-dhpH) are provided. This material is available free of
charge via the Internet at http://pubs.acs.org.