Aqueous coordination chemistry of H2: Why is coordinated H 2 inert to substitution by water in trans-Ru(P2) 2(H2)H+-type Complexes (P2 )= a Chelating Phosphine)?

Article abstract of DOI:10.1021/ic801884x

The reactivity of a series of trans-Ru(P₂)₂Cl ₂ complexes with H₂ was explored. The complexes reacted with H2 via a stepwise H₂ addition/heterolysis pathway to form the trans-[Ru(P₂)_2<

Full text of DOI:10.1021/ic801884x

Inorg. Chem. 2009, 48, 2976-2984
Aqueous Coordination Chemistry of H2: Why is Coordinated H2 Inert to
Substitution by Water in trans-Ru(P2)2(H2)H+-type Complexes (P2 ) a
Chelating Phosphine)?
Nathaniel K. Szymczak, Dale A. Braden, Justin L. Crossland, Yevgeniya Turov, Lev N. Zakharov,
and David R. Tyler*
Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403
Received October 3, 2008
The reactivity of a series of trans-Ru(P2)2Cl2 complexes with H2 was explored. The complexes reacted with H2 via
a stepwise H2 addition/heterolysis pathway to form the trans-[Ru(P2)2(H2)H]+ dihydrogen complexes. Some of the
resulting η2-H2 complexes were surprisingly inert to substitution by water, even at concentrations as high as 55 M;
however, the identity of the bidentate phosphine ligand greatly influenced the lability of the coordinated η2-H2
ligand. With less electron-donating phosphine ligands, the H2 ligand was susceptible to substitution by H2O, whereas
with more electron-rich phosphine ligands, the H2 ligand was inert to substitution by water. Density functional
theory (DFT) calculations of the ligand substitution reactions showed that the Ru-H2 and Ru-H2O complexes are
very close in energy, and therefore slight changes in the donor properties of the bidentate phosphine ligand can
inhibit or promote the substitution of H2O for H2.
Introduction
for the H-cluster (the most widely modeled site), this involves
a binuclear iron core featuring three CO ligands, two CN-1
ligands, and a thiolate bridging from the Fe4S4 cluster.7,8
Another noteworthy class of molecules that replicate the
proposed aza-dithiolate linkage of the H-cluster are described
by Dubois and co-workers.9 Nickel phosphine complexes
that incorporate pendant amine bases in their periphery were
shown to act as functional hydrogenase mimics, and such
reactivity was proposed to result from the delivery of
exogenous protons through the proton relay system.
From a mechanistic standpoint, the intermediates involved
in conversion of H+ to H2 are unusual. Although several
pathways have been proposed, a common feature is the
presence of a coordinated hydride and a coordinated H2
ligand at biological conditions, namely in water (Figure 1).5
An unusual feature of the mechanism is the competitive
ability of water, H2, and hydrides to coordinate to an iron
center.10 This unique ability of hydrogenase enzymes and
related biomimics was recently highlighted for a heterobi-
The coordination chemistry of dihydrogen is gaining
increased awareness as a result of the recent movement
toward a hydrogen economy.1 To sustain such an economy,
a thorough understanding of how dihydrogen can be pro-
duced and how it interacts with its surroundings on a
molecular level is needed. A particularly attractive route for
H2 generation is inspired by biology. The hydrogenase
enzymes mediate the conversion of protons and electrons to
H2 with impressive efficiency.2 One approach that has been
explored extensively to exploit this function is the construc-
tion of synthetic models of the active site of Fe-Fe and
Fe-Ni hydrogenase enzymes with the goal of synthesizing
a viable catalyst for H2 production.3-6 Such models replicate
key structural features found in the enzyme active site, and
* To whom correspondence should be addressed. E-mail: dtyler@
uoregon.edu.
(1) Kubas, G. J. Chem. ReV. 2007, 107, 4152–4205.
(2) Frey, M. ChemBioChem 2002, 3, 153–160.
(3) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem. ReV. 2000,
206-207, 533–561.
(4) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 3683–3688.
(5) Georgakaki, I. P.; Darensbourg, M. Y. Compr. Coord. Chem. II 2004,
8, 549–568.
(7) Chen, Z.; Lemon, B. J.; Huang, S.; Swartz, D. J.; Peters, J. W.; Bagley,
K. A. Biochemistry 2002, 41, 2036–2043.
(8) Nicolet, Y.; de Lacey, A. L.; Vernede, X.; Fernandez, V. M.;
Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596–1601.
(6) Linck, R. C.; Rauchfuss, T. B. In Bioorganometallics: Biomolecules,
Labeling, Medicine; Jaouen, G., Ed.; Wiley-VCH Verlag GmbH &
Co.: Weinheim, 2006; pp 403-435.
(9) Wilson, A. D.; Shoemaker, R. K.; Miedaner, A.; Muckerman, J. T.;
DuBois, D. L.; DuBois., M. R. Proc. Natl. Acad. Sci. U.S.A. 2007,
104, 6951–6956.
2976 Inorganic Chemistry, Vol. 48, No. 7, 2009
10.1021/ic801884x CCC: $40.75  2009 American Chemical Society
Published on Web 03/02/2009

Aqueous Coordination Chemistry of H2
have since extended the scope of aqueous coordination
chemistry of dihydrogen with the goal of understanding the
key factors that allow a coordinated H2 ligand to be inert to
substitution in 55 M water. Herein, we report the results of
our study focused on the examination of the reactivity of a
variety of ruthenium bidentate phosphine complexes with
H2 in the presence of excess water.
Figure 1. Relevant H2O/H2 interconversion scheme with two likely
intermediates in the hydrogenase-mediated reduction of protons.
metallic Ni-Ru system, where a Ni(µ-H)Ru complex was
formed under ambient conditions by allowing the parent aqua
complex to react with dihydrogen in water.11 Given the
importance of the H2 conversion pathway, it is worthwhile
to understand some of the underlying reactivity trends
associated with this rather elusive class of complexes, namely
water-soluble, stable M-(H2) and M-H complexes.
While the competition between water and H2 in transition-
metal complexes is not completely unprecedented, most
studies thus far have been reported in non-aqueous solvents
(with water as an immiscible phase, thus lowering the
effective concentration), or under high H2 pressures. For
example, Kubas and co-workers showed that at low con-
centrations of water (in hexanes), H2 was not displaced by
water on a tungsten carbonyl fragment. However, the
presence of excess water [in tetrahydrofuran (THF) solution]
resulted in substitution of the H2 ligand.12 In a related study
that highlights similar H2/H2O binding enthalpies in non-
aqueous solutions on RuTp(PPh3)2+ complexes, Lau, Jia, and
co-workers found that in dichloromethane solutions water
could be displaced by H2 at elevated pressures (20 atm).13
Without the excess hydrogen pressure in acidic solution, the
H2 ligand was displaced by H2O. However, when an excess
of water was introduced in dichloromethane solution at a
lower pressure of H2 (10 atm), the substitution reaction was
suppressed and instead, deprotonation of coordinated H2 by
H2O resulted.
Results
Synthesis of the trans-Ru(P2)2Cl2 Complexes. The syntheses
of complexes Ia-f proceeded smoothly with [Ru(COD)Cl2]n
and the appropriate bidentate phosphine in refluxing ethanol
solution using the route previously reported for complex Ic
(eq 2).
Complexes Id, Ie, and If were previously synthesized by
a slightly different route, and the molecules prepared by the
route in eq 2 were spectroscopically identical to the previ-
ously reported spectra.19-21 Solutions containing I exhibit
a singlet in the 31P{1H} NMR spectrum, confirming a trans
geometry (Table 1). Slow cooling of concentrated solutions
of Ia and Ib resulted in the precipitation of yellow crystals
suitable for X-ray crystallographic analysis (Table 2). The
structures (Figure 2 and Supporting Information, Figure S1)
show an octahedral coordination environment around ruthe-
nium with trans chloride ligands. In the case of Ib, hydrogen
bonds (O-H · · · Cl) between two hydroxyl groups and the
coordinated chloride are present, as shown by the close
Cl · · · O contact (3.03 Å).
We recently reported the generation of several water-
soluble η2-H2 hydride complexes of iron and ruthenium (eq
1).14-16 These complexes were formed in water following
heterolytic cleavage of an initially coordinated H2 ligand
under moderate conditions.
(12) Kubas, G. J.; Burns, C. J.; Khalsa, G. R. K.; Van Der Sluys, L. S.;
Kiss, G.; Hoff, C. D. Organometallics 1992, 11, 3390–3404.
(13) Chan, W.; Lau, C.; Chen, Y.; Fang, Y.; Ng, S.; Jia, G. Organometallics
1997, 16, 34–44.
(14) Szymczak, N. K.; Zakharov, L. N.; Tyler, D. R. J. Am. Chem. Soc.
2006, 128, 15830–15835.
(15) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. J. Am. Chem. Soc.
2005, 127, 10184–10185.
(16) Gilbertson, J. D.; Szymczak, N. K.; Tyler, D. R. Inorg. Chem. 2004,
43, 3341–3343.
(17) Li, Z. W.; Taube, H. J. Am. Chem. Soc. 1991, 113, 8946–8947.
(18) Aebischer, N.; Frey, U.; Merbach, A. E. Chem. Commun. 1998, 2303–
2304.
Several features of this system were unexpected, including
the surprising inertness of the coordinated H2 ligand to
substitution by water. Previously, coordination complexes
containing an η2-H2 were exceedingly rare in aqueous
solution,17,18 particularly without a high pressure of H2. We
(10) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chim. Acta
1999, 294, 240–254.
(11) Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.; Menon, S. C.;
Harada, R.; Fukuzumi, S.; Higuchi, Y.; Ohhara, T.; Tamada, T.;
Kuroki, R. Science 2007, 316, 585–587.
(19) Buys, I. E.; Field, L. D.; George, A. V.; Hambley, T. W.; Purches,
G. R. Aust. J. Chem. 1995, 48, 27–34.
(20) Chang, C.; Ting, P.; Lin, Y.; Lee, G.; Wang, Y. J. Organomet. Chem.
1998, 553, 417–425.
(21) Clark, S. F.; Petersen, J. D. Inorg. Chem. 1983, 22, 620–623.
Inorganic Chemistry, Vol. 48, No. 7, 2009 2977

Szymczak et al.
Table 1. Summary of 31P NMR Data for trans-RuCl2(P2)2 Complexesa
Table 2. Crystal Data and Structure Refinement for
trans-Ru(DHMPE)2Cl2 (Ib)
complex
31P{1H}
empirical formula
formula weight
temperature
wavelength
crystal system
space group
C12H36Cl2O9P4Ru
620.26
153(2) K
0.71073 Å
monoclinic
C2/c
Ru(DHPrPE)2Cl2 (Ia)b
Ru(DHMPE)2Cl2 (Ib)b
Ru(DMeOPrPE)2Cl2 (Ic)14
Ru(DEPE)2Cl2 (Id)19
Ru(DPPE)2Cl2 (Ie)20
Ru(DMPE)2Cl2 (If)21
41.2
59.8
44.4
48.0
45.8
37.4
unit cell dimensions
a ) 10.8376(15) Å
b ) 24.915(4) Å
c ) 9.5442(13) Å
R ) 90°
a DHPrPE ) 1,2-bis(dihydroxypropylphosphino)ethane, DHMPE) 1,2-
bis(dihydroxymethylphosphino)ethane, DMeOPrPE) 1,2-bis(dimethox-
ypropylphosphino)ethane, DEPE) 1,2-bis(diethylphosphino)ethane, DPPE)
1,2-bis(diphenylphosphino)ethane, DMPE) 1,2-bis(dimethylphosphino)ethane.
b This work.
ꢀ ) 118.651(2)°
γ ) 90°
volume
2261.6(5) Å3
Z, Z′
4, 0.5
density (calculated)
absorption coefficient
F(000)
1.822 Mg/m3
1.254 mm-1
1272
crystal size
theta range for data collection
index ranges
0.09 × 0.06 × 0.03 mm3
1.63 to 28.26°
-14 e h e 12, -33 e k e30,
-12 e l e 12
reflections collected
9139
independent reflections
completeness to theta ) 28.26°
absorption correction
max. and min. transmission
refinement method
2662 [R(int) ) 0.0389]
94.9%
semiempirical from equivalents
1.000 and 0.554
full-matrix least-squares on F2
2662/0/129
data/restraints/parameters
goodness-of-fit on F2
1.094
final R indices [I > 2sigma(I)]
R indices (all data)
largest diff. peak and hole
R1 ) 0.0497, wR2 ) 0.1251
R1 ) 0.0662, wR2 ) 0.1376
2.188 and -0.822 e Å-3
a coordinated η2-H2 ligand in the short distance regime (0.83
Å-0.86 Å), assuming fast rotation.22 H/D exchange with
methanol-d4 occurred for the η2-H2 ligand over several hours
to form the HD isotopologue. From the isotopologue, the
1JHD was determined and an H-H distance was calculated
using Morris’,23 Heinekey’s24 and Gusev’s25 relationships.
Figure 2. ORTEP representation of trans-Ru(DHMPE)2Cl2 (Ib). Ellipsoids
are shown at 50% probability. Hydrogen atoms and solvent water molecules
are omitted for clarity.
Synthesis of trans-[Ru(P2)2H(H2)]+ Complexes. Solu-
tions of I are stable in the absence of oxygen, even at elevated
temperatures, and ligand substitution reactions did not occur
in organic solvents by simply heating I in the presence of
an incoming ligand. Rather, addition of a strong chloride
abstraction reagent is required to produce an open coordina-
tion site for binding the H2 ligand. Complexes IIa-f were
prepared in toluene or THF with ∼350 psig H2 at 85 °C
using TlPF6 as a chloride abstractor and Proton Sponge as a
proton sequestering reagent (eq 3).
1
According to the JHD values, the H-H bond distances for
all the η2-H2 complexes are estimated between 0.87 Å-0.91
Å, values well within the short regime of dihydrogen
complexes.
Mechanism for the Formation of trans-[Ru(P2)2H-
(η2-H2)]+. Following the addition of H2 to I, there are two
possible pathways for the formation of II (Scheme 1). In
the next step, the H2 can either be heterolytically cleaved
by reaction with a base (top path, Scheme 1) or a second
H2-substitution may occur to give a bis-η2-H2 species (bottom
path, Scheme 1). Note that bis-η2-H2 complexes are uncom-
mon and typically are stabilized by second- and third-row
transition metals such as ruthenium or osmium.28,29
(22) Morris, R. H. Coord. Chem. ReV. 2008, 252, 2381–2394.
(23) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.;
Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc.
1996, 118, 5396–5407.
(24) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396–4399.
(25) Gusev, D. G. J. Am. Chem. Soc. 2004, 126, 14249–14257.
(26) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991,
113, 4876–4887.
(27) Field, L. D.; Hambley, T. W.; Yau, B. C. K. Inorg. Chem. 1994, 33,
2009–2017.
Solutions containing the light-yellow complexes, IIa-f,
gave rise to a single resonance in the 31P{1H} NMR spectrum
1
and displayed H resonances for the η2-H2 and hydride
ligands as a broad singlet and quintet, respectively (Table
3). The 1H T1(min) values of the H2 resonance were
experimentally determined and are also listed in Table 3.
All of the determined 1H T1(min) values were consistent with
(28) Smith, K.-T.; Tilset, M.; Kuhlman, R.; Caulton, K. G. J. Am. Chem.
Soc. 1995, 117, 9473–9480.
(29) Sabo-Etienne, S.; Chaudret, B. Coord. Chem. ReV. 1998, 178-180,
381–407.
2978 Inorganic Chemistry, Vol. 48, No. 7, 2009

Aqueous Coordination Chemistry of H2
Table 3. NMR Data for trans-[Ru(P2)2H(η2-H2)]+ Complexesa
complex
1H NMR
31P{1H}
JH-D, Hz
T1(min), msb
solvent
[Ru(DHPrPE)2H(H2)]+ (IIa)c
[Ru(DHMPE)2H(H2)]+ (IIb)c
[Ru(DMeOPrPE)2H(H2)]+ (IIc)c
[Ru(DEPE)2H(H2)]+ (IId)26
[Ru(DPPE)2H(H2)]+ (IIe)26
[Ru(DMPE)2H(H2)]+ (IIf)27
-6.5 (s, br), -11.3 (q, JP-H ) 20.1 Hz)
65.9
61.3
63.4
60.8
68.6
40.3
32
33
32
32.0
32.9
32.2
20.6 (500 MHz)
16.5 (500 MHz)
21.1 (500 MHz)
16 (400 MHz)
20 (400 MHz)
16.1 (400 MHz)
THF-d8
THF-d8
THF-d8
acetone-d6
acetone-d6
ethanol-d6
2
2
-6.9 (s, br), -13.0 (q, JP-H ) 19.7 Hz)
2
-6.6 (s, br), -11.4 (q, JP-H ) 20.1 Hz)
2
-6.4 (s, br), -11.3 (q, JP-H ) 19.3 Hz)
2
-4.6 (s, br), -10.0 (q, JP-H ) 18 Hz)
2
-7 (s, br), -13 (q, JP-H ) 20.8 Hz)
a DHPrPE ) 1,2-bis(dihydroxypropylphosphino)ethane, DHMPE) 1,2-bis(dihydroxymethylphosphino)ethane, DMeOPrPE) 1,2-bis(dimethoxypropy-
lphosphino)ethane, DEPE) 1,2-bis(diethylphosphino)ethane, DPPE) 1,2-bis(diphenylphosphino)ethane, DMPE) 1,2-bis(dimethylphosphino)ethane.
b Spectrometer frequency (in MHz) listed in parentheses. c This work.
a
species as trans-[Ru(DMeOPrPE)2(H2)Cl]+ (Figure 4). The
Scheme 1. Possible Pathways for the Heterolysis of H2
H-H bond distance was characterized by the T1(min) value
of 35.1 ms (500 mHz), consistent with an H-H distance of
0.94 Å (assuming fast rotation) or 1.18 Å (assuming slow
rotation).31,32 Furthermore, the 1JHD of 25.3 Hz is consistent
with a distance of 1.00 Å based on Morris’ correlation,23
1.01 Å for Heinekey’s correlation,24 and 1.03 Å using
Gusev’s correlation.25 Such an increase in the H-H bond
distance (compared to the trans-[Ru(DMeOPrPE)2(H2)H]+
species) is consistent with the ability of a chloride ligand to
act as a π-base and enhance electron donation to the σ*-
a
orbitals of the coordinated H2, compared to a hydride ligand
that can offer no such interaction. Addition of H2 to a Et2O/
THF solution of trans-[Ru(DMeOPrPE)2(H2)Cl]+, even in
the presence of TlPF6, did not lead to any reaction over
a two week period, thus ruling out the bottom pathway in
Scheme 1. Also when excess triflic acid was added to trans-
[Ru(DMeOPrPE)2(H2)H]+ no reaction was observed, again
ruling out the bottom pathway.
In summary, the formation of trans-[Ru(DMeOPrPE)2-
(H2)H]+ from trans-[Ru(DMeOPrPE)2HCl] demonstrates the
viability of the top pathway in Scheme 1. Furthermore, the
lack of reactivity of trans-[Ru(DMeOPrPE)2(H2)Cl]+ toward
H2, even in the presence of a halide abstractor, strongly
suggests that a bis-η2-H2 intermediate is unlikely under
the reaction conditions used to generate the trans-[Ru-
(DMeOPrPE)2(H2)H]+ complex.
Inertness to Substitution by Water. The dihydrogen
complex IIc can be generated in non-polar organic solvents,
as well as in water buffered at pH 7.14 This intriguing
observation led us to consider the reason why water did not
displace an otherwise easily substitutable H2 ligand. Ac-
cordingly, we sought to evaluate the generality of this
phenomenon and to examine whether most complexes of the
type trans-Ru(P2)2(H2)H+ are unreactive to water substitution.
To our knowledge, there are only a few examples of H2
complexes that are inert to water substitution in neat water,17
and in the case of the [Ru(H2O)5(H2)]2+ complex, stability
is only maintained when the solution is kept under a high
pressure of H2.18,33
In this scheme, B represents a generic base.
To investigate the mechanism of the formation of II, two
key intermediates were synthesized by alternative routes
using the DMeOPrPE ligand as the representative model for
all the bidentate phosphines. One potential key intermediate,
trans-[Ru(DMeOPrPE)2HCl], was prepared from [Ru(COD)-
Cl2]n, lithium hydroxide, and DMeOPrPE in refluxing
methanol (eq 4).
The complex exhibits a single resonance in the 31P{1H}
spectrum at δ 59.1, suggestive of 4 equivalent phosphorus
atoms, while the hydride region of the H NMR spectrum
1
2
gives rise to a quintet at δ -22.1 with a JHP of 20 Hz,
consistent with cis P-H coupling (Figure 3).30 When
solutions of trans-[Ru(DMeOPrPE)2HCl] are treated at room
temperature with 25 psig H2, trans-[Ru(DMeOPrPE)2(H2)H]+
forms within 5 min, as indicated by the appearance of a
singlet at δ 63.4 in the 31P{1H} spectrum and the appearance
1
of new resonances in the H NMR spectrum for the trans-
[Ru(DMeOPrPE)2(H2)H]+ complex at -6.6 (s, br), -11.4
2
(q, JP-H ) 20.1 Hz) (Table 3).
To test the viability of the bottom pathway in Scheme 1,
the trans-[Ru(DMeOPrPE)2(H2)Cl]+ was prepared and re-
acted with H2. Specifically, when solutions of trans-
[Ru(DMeOPrPE)2HCl] are treated with triflic acid
(CF3SO3H), the singlet at δ 59.1 disappears and is replaced
with another singlet at δ 49.9. Likewise, the quintet at δ
-22.1 in the hydride region is replaced with a broad singlet
(or alternatively, when CD3OD is used as the solvent, a 1:1:1
triplet of triplets, representing the HD isotopologue) at δ
-14.3. These data are consistent with assignment of the new
Substitution Patterns of H2. The unusual stability of
complex IIc in water prompted an investigation to probe
whether IIc is substitutionally inert in the presence of other
(31) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126–
4133.
(32) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173–4184.
(33) Grundler, P. V.; Yazyev, O. V.; Aebischer, N.; Helm, L.; Laurenczy,
G.; Merbach, A. E. Inorg. Chim. Acta 2006, 359, 1795–1806.
(30) See references in Table 3.
Inorganic Chemistry, Vol. 48, No. 7, 2009 2979

Szymczak et al.
1
Figure 3. 31P{1H} NMR spectrum of (a) trans-[Ru(DMeOPrPE)2HCl and (b) the hydride region in the H NMR spectrum (b).
1
Figure 4. 31P{1H} NMR spectrum of (a) trans-[Ru(DMeOPrPE)2(H2)Cl]+ and (b) the hydride region in the H NMR spectrum of the HD isotopologue,
trans-[Ru(DMeOPrPE)2(HD)Cl]+. The minor peak at δ 44.4 in spectrum (a) is unreacted trans-Ru(DMeOPrPE)2Cl2.
neutral ligands. It was thought that an examination of the
ligand substitution patterns would lead to a better under-
standing of the reasons governing the substitutional inertness
of the H2 ligand in water. To probe the lability of the
coordinated H2 in the presence of a strong field ligand, the
carbonyl complex trans-[Ru(DMeOPrPE)2H(CO)]+, was
prepared by reacting trans-[Ru(DMeOPrPE)2(H2)H]+ with
CO. As reported previously, the reactions went to completion
within minutes.14
While the reaction with CO confirms that the coordinated
H2 ligand can be easily substituted by a small, strong-field
ligand, it was of interest to examine the effect that entering
ligand size might have on the substitution of H2 because of
the steric constraints imposed by the chelating phosphine
ligand. Restated, if the H2 ligand is substitutionally inert in
the presence of large entering ligands, this may be rational-
ized by the large methoxypropyl sidechains inhibiting
substitution by physically protecting the coordinated H2 from
the entering ligand. Unexpectedly, it was found that substitu-
tion of the H2 ligand was facile even with bulky ligands such
as tert-butylnitrile and norbornadiene (see Supporting In-
formation). The conclusion drawn from these substitution
results is that the stability of the coordinated H2 ligand cannot
be understood solely from a steric standpoint, that is, the
methoxypropyl arms of the DMeOPrPE ligand do not form
a stabilizing pocket around the H2.
Because the donor/acceptor properties of nitriles and
olefins differ vastly from the purely σ-donating H2O ligand,
a substitution reaction with another σ-donating ligand, NH3,
was carried out. Substitution of H2 by NH3 in trans-
[Ru(DMeOPrPE)2(H2)H]+ (IIc) occurred within 10 min, as
noted by the appearance of a new singlet at δ 58.6 (s) in the
31P{1H} NMR spectrum as well as a broad singlet at δ 0.18
(corresponding to the coordinated NH3) and quintet at δ
-20.1 (corresponding to the trans hydride) in the 1H NMR
spectrum,34 concomitant with the disappearance of the start-
ing material resonances. This result shows that the η2-H2
ligand can be easily substituted by a purely σ-donating ligand.
Thus, the stability of the H2 ligand toward substitution by
water is not solely a consequence of the difference in donor/
acceptor abilities. In summary of this section, the substitution
reactions of coordinated H2 in IIc were found to be facile
for ligands of many sizes and donor abilities with the
exception of H2O.
Effect of Phosphine Donicity on H2 Substitution.
Because the dπ-σ* backbonding interaction is a crucial
component that determines the strength of the M-(H2)
(34) This NMR data is consistent with the DMPE analogue for which the
crystal structure is known.35
(35) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.; Bergman, R. G. J. Am.
Chem. Soc. 2002, 124, 4722–4737.
2980 Inorganic Chemistry, Vol. 48, No. 7, 2009

Aqueous Coordination Chemistry of H2
Table 4. Infrared Carbonyl Bands (cm-1) for cis-P2Mo(CO)4
Complexes
complexa
IR (CO) bands
solvent
(DHPrPE)Mo(CO)4
(DEPE)Mo(CO)4
(DMeOPrPE)Mo(CO)4
(DMPE)Mo(CO)4
(DHMPE)Mo(CO)4
(DPPE)Mo(CO)4
2012, 1913, 1896
2012, 1909, 1891, 1873
2013, 1913, 1896
2014, 1927, 1898, 1886
2018, 1926, 1901
2020, 1915, 1892
EtOH
DCE
EtOH
EtOH
EtOH
EtOH
38
bond,36,37 it was hypothesized that the coordinated H2 ligand
is stabilized by relatively electron-rich bidentate phosphine
ligands. To probe this electronic effect, complexes bearing
the same skeletal scaffold (trans-[Ru(P2)2(H2)H]+) with a
variety of bidentate phosphine ligands were compared.
Phosphines of varying donicities were selected. Initially, only
water-soluble phosphines were selected because it was
presumed they were required for the resulting H2 complex
to be soluble in water to discern the aqueous reactivity.
However, the cationic nature of the trans-Ru(P2)2(H2)H+
complexes led to increased solubility in polar solvents, and
complexes formed with non-water-solubilizing phosphines
such as DMPE and DEPE were also found to have an
appreciable solubility in water. To compare the relative
electron-donating ability of the various phosphines used in
the study, the highest energy ν(Ct O) band was compared
in a series of substituted cis-Mo(CO)4P2 complexes (Table
4). The data are consistent with the well-known trend that
the donor strength of the phosphine ligand is governed by
the appending substituents.
Figure 5. ORTEP representation of trans-[Ru(DPPE)2(H2O)H]+. Ellipsoids
are shown at 50% probability. Only the hydrogen atom bonded to the Ru
-
atom is shown for clarity. The PF6 counterion is also omitted for clarity.
Table 5. Crystal Structure and Refinement Data for
trans-[Ru(DPPE)2(H2O)H]+
empirical formula
formula weight
temperature
C56 H51 F6 O2 P5 Ru
1125.89
173(2) K
wavelength
0.71073 Å
crystal system
space group
orthorhombic
Pnma
unit cell dimensions
a ) 16.560(4) Å
b ) 19.896(5) Å
c ) 15.819(3) Å
R ) 90°
Solutions containing trans-Ru(P2)2(H2)H+ (IIa-f) were
treated with an excess of water (>100 equiv) to determine if
the coordinated H2 ligand would be substituted by water.
Surprisingly, complexes IIa, IIc, IId, and IIf were unreactive
to substitution by water (eq 5), even after a week at elevated
temperatures (75 °C).
ꢀ ) 90°
γ ) 90°
volume
5212(2) Å3
Z, Z′
4, 0.5
density (calculated)
absorption coefficient
F(000)
1.435 Mg/m3
0.517 mm-1
2304
crystal size
theta range for data collection
index ranges
0.17 × 0.07 × 0.04 mm3
1.64 to 25.00°
-19 e h e 19, -17 e k e 23,
-18 e l e16
reflections collected
26986
independent reflections
completeness to theta ) 28.26°
Absorption correction
max. and min. transmission
refinement method
4734 [R(int) ) 0.0783]
100.0%
semiempirical from equivalents
0.9796 and 0.9172
full-matrix least-squares on F2
4734/6/301
In contrast, complexes bearing DHMPE or DPPE ligands
(IIb, IIe) were reactive toward substitution by water (eq 6).
This was confirmed by the appearance of a new singlet in
the 31P NMR spectrum at δ 58.9 and δ 64.9 for IIb and IIe,
respectively, as well as a crystal structure in the case of trans-
[Ru(DPPE)2(H2O)H]+ (Figure 5, Table 5).
data/restraints/parameters
goodness-of-fit on F2
1.063
final R indices [I > 2sigma(I)]
R indices (all data)
largest diff. peak and hole
R1 ) 0.0660, wR2 ) 0.1594
R1 ) 0.0908, wR2 ) 0.1724
1.086 and -1.107 e Å-3
phosphines (in IIa, IIc, IId, IIf) confer a more inert
coordinated H2 ligand because increased π-backbonding will
occur to the H2 ligand, which increases the Ru-H2 bond
energy. Conversely, when electron-withdrawing groups were
appended to the phosphine backbone (IIb, IIe), the coordi-
nated H2 ligand was labilized with respect to substitution
bywater.Theaboveresultssuggestthatintrans-Ru(P2)2(H2)H+-
type complexes, the energetics of H2 coordination and H2O
coordination are similar and slight alterations in the electron-
ics at the metal center can shift the favored complex. This
thermodynamic explanation is consistent with the report by
Kubas and co-workers who showed that the enthalpy of H2
The substitution results can be explained by considering
the inductive effects of the substituents appended to the
phosphino ethane scaffold. The more electron donating
(36) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure,
Theory and ReactiVity; Kluwer Academic: New York, 2001.
Inorganic Chemistry, Vol. 48, No. 7, 2009 2981

Szymczak et al.
a
Table 6. Reaction Energies for Substitution of H2
oped for organic molecules, and the application of those
models to charged organometallic complexes is questionable.
The calculated Ru-L bond free energies in Table 7 show
that the Ru-H2bond is very close in energy to the Ru-H2O
bond for IIb and IIf, which explains why the reaction
energies are essentially thermoneutral. The Ru-NH3 bond
in IIf, however, is calculated to be almost 3 kcal/mol stronger
than the Ru-H2 bond, and thus substitution of H2 by NH3
is predicted to be favorable. In summary, these calculations
further support the fact that the Ru-H2 and Ru-H2O
complexes are energetically very similar, and alteration of
the bidentate phosphine ligand can favor one complex or
the other.
reaction
reaction energy reaction free energiesb
[Ru(DMPE)2H(H2)]+ + H2O
[Ru(DHMPE)2H(H2)]+ + H2O
[Ru(DMPE)2H(H2)]+ + NH3
[Ru(DMPE)2H(H2)]+ + CO
0.46
-0.13
-4.63
-22.97
0.82
-0.11
-2.81
-20.27
a Metal-ligand bond energies in kcal/mol. The reaction energies in the
first column are simply the difference in SCF energies, without zero-point
corrections or thermochemical corrections. b Including zero-point energy
and thermochemical corrections at 373 K.
Table 7. Ru-L Bond Energiesa
bond
bond energy bond free energiesb
Ru-(H2) in [Ru(DHMPE)2H(H2)]+
Ru-(H2O) in[Ru(DHMPE)2H(H2O)]+
Ru-(H2) in [Ru(DMPE)2H(H2)]+
Ru-(H2O) in [Ru(DMPE)2H(H2O)]+
Ru-(NH3) in [Ru(DMPE)2H(NH3)]+
Ru-(CO) in [Ru(DMPE)2H(CO)]+
15.37
15.50
14.91
14.45
19.54
37.88
2.04
2.14
1.57
0.75
4.38
21.84
Conclusions
a Metal-ligand bond energies in kcal/mol. The bond energies in the first
column are simply the difference in SCF energies, without zero-point
corrections or thermochemical corrections. b Including zero-point energy
and thermochemical corrections at 373 K.
This study elucidated several key points to be considered
when studying aqueous dihydrogen coordination chem-
istry. It was found that the reaction of trans-[Ru-
(DMeOPrPE)2Cl2] with H2 occurred by a stepwise H2
addition/heterolysis pathway to yield a dihydrogen com-
plex, trans-[Ru(DMeOPrPE)2(H2)H]+. The resulting η2-
H2 complex was surprisingly inert to substitution by water,
even at concentrations as high as 55 M. The identity of
the appended groups on the phosphine ligands influenced
the lability of the coordinated η2-H2 ligand. With less
donating phosphine ligands, the H2 ligand was susceptible
to substitution by H2O, whereas with more electron-rich
phosphine ligands, the H2 ligand was inert to substitution
by water. These results were substantiated by comple-
mentary DFT studies that showed the strengths of the
Ru-H2 and Ru-H2O bonds are quite similar in the
Ru(P2)2H+ fragment. The competitive binding ability of
H2 and H2O for a group 8 transition metal has potential
application in a number of areas.39 Notably, a key step in
the hydrogenase-mediated reduction of protons involves
H2 substitution for H2O on an iron center.
binding was similar to H2O in a tungsten carbonyl fragment,
although in their case, the addition of excess water resulted
in substitution of the H2 ligand.12
An alternative explanation, based on kinetics, comes from
transition-state theory. If a reacting M-L bond is strength-
ened, a higher activation barrier generally results, regardless
of the energy of the products. Accordingly, a more highly
donating phosphine ligand such as DEPE and DMeOPrPE
will contribute to a higher activation barrier for ligand
substitution. This may explain the lack of H2O substitution
in the case of the more electron-donating phosphine ligands.
Energetics of H2 Coordination. To further examine the
energetics of H2 and H2O coordination and the strength of
the Ru-H2 interaction, density functional theory (DFT)
calculations were carried out. The two ruthenium phosphine
complexes [Ru(DHMPE)2H(H2)]+ (IIb) and [Ru(DMPE)2-
H(H2)]+ (IIf) were chosen for the DFT studies because,
experimentally, they have different reactivity toward sub-
stitution by water. The results of the calculations are shown
in Tables 6 and 7. The reaction free energies, which include
zero-point energy corrections and thermochemical corrections
at the experimental temperature of 373 K, are at least
qualitatively consistent with the experimental observations.
Thus, for IIf, substitution of H2 by H2O was calculated to
be slightly thermodynamically unfavorable, while substitution
by NH3 or CO was calculated to be thermodynamically
favorable. For the IIb complex, substitution of H2 by H2O
was calculated to be very slightly favorable. We do not
expect the computed results to be quantitatively accurate
because we have omitted solvation effects. Including solva-
tion effects in this case is problematic for two reasons. First,
most solvation models are based on a dielectric continuum,
and these models have trouble modeling highly structured
solvents like water unless they are heavily parametrized.
Second, parametrized solvation models are generally devel-
Experimental Section
Materials and Reagents. Unless otherwise noted, all manipula-
tions were carried out in either a Vacuum Atmospheres Co.
glovebox (argon filled) or on a Schlenk line under argon or
hydrogen using standard Schlenk techniques. The thallium hexaflu-
orophosphate (Caution! thallium compounds are toxic), Proton
Sponge, triflic acid, and lithium hydroxide were obtained from
commercial vendors and used as received. 1,2-Bis(dihydroxy-
propylphosphino)ethane,40 1,2-bis(dihydroxymethylphosphino)-
42
ethane,41 trans-RuCl2(DMeOPrPE)2 (Ic),14 and [RuCl2(COD)]n
were prepared by published literature procedures. Reagent grade
solvents were dried according to published procedures and deoxy-
genated with either an argon purge or three freeze-pump thaw cycles
prior to use. Water was purified to a resistivity of 17-18 MΩ cm
(39) Szymczak, N. K.; Tyler, D. R. Coord. Chem. ReV. 2008, 252, 212–
230.
(40) Baxley, G. T.; Weakley, T. J. R.; Miller, W. K.; Lyon, D. K.; Tyler,
D. R. J. Mol. Catal., A: Chem. 1997, 116, 191–198.
(41) Nieckarz, G. F.; Weakley, T. J. R.; Miller, W. K.; Miller, B. E.; Lyon,
D. K.; Tyler, D. R. Inorg. Chem. 1996, 35, 1721–1724.
(42) Albers, M. O.; Ashworth, T. V.; Oosthuizen, H. E.; Singleton, E. Inorg.
Synth. 1989, 26, 68–77.
(37) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. ReV. 2000,
100, 601–636.
(38) Chatt, J.; Watson, H. R. J. Chem. Soc. 1961, 4980–4988.
2982 Inorganic Chemistry, Vol. 48, No. 7, 2009

Aqueous Coordination Chemistry of H2
with a Barnstead Ultrapure system and was deoxygenated with an
argon purge before use.
the 6-311G** basis set.48,49 Symmetry was not used for any of the
calculations on the complexes.
Instrumentation and Procedures. 31P{1H} and 1H NMR spectra
were recorded on a Varian Unity/Inova 500 spectrometer at an
operating frequency of 500.62 (1H) and 202.45 (31P) MHz. The 1H
chemical shifts were referenced to an internal TMS standard and
31P chemical shifts were referenced to an external standard of 1%
Synthesis of trans-Ru(DHPrPE)2Cl2 (Ia). To a flask containing
RuCl2COD (0.1871 g, 0.6679 mmol) was added a solution of
DHPrPE (0.4412 g, 1.345 mmol) in ethanol (20 mL). After heating
to reflux for 6 h, the solution was cloudy yellow. Upon cooling, a
solid precipitate formed that was filtered and washed with hexanes
(3 × 10 mL). The filtrate solvent was removed in vacuo, and the
resulting solid was washed with hexanes (3 × 10 mL). A yellow
solid was obtained. Yield: 0.33 g (59%). Crystals were obtained
by cooling a saturated methanol solution in the freezer for 4 days.
31P{1H} NMR (CD3OD): δ 41.2 (s). 1H NMR (CD3OD): δ 4.9 (s,
8H, -OH), 3.57 (t, 16H, CH2OH), 2.17 (m, 16H, -CH2-), 1.93 (m,
8H, PCH2CH2P), 1.85 (m, 16H, PCH2-). 13C{1H} NMR (CD3OD):
δ 64.1 (-CH2OH), 28.9 (-CH2-), 22.9 (PCH2CH2P), 22.1 (PCH2-).
Anal. Calcd for C28H64Cl2O8P4Ru: C, 40.78; H, 7.82; P, 15.02.
Found: C, 41.19; H, 7.61; P, 15.05. Id, Ie, and If were synthesized
by a similar procedure with the appropriate phosphine.
Synthesis of trans-Ru(DHMPE)2Cl2 (Ib). To a flask containing
RuCl2COD (0.4617 g, 1.649 mmol) was added a solution of
DHMPE (0.7062 g, 3.298 mmol) in ethanol (40 mL). After heating
to reflux for 16 h, the solution was yellow with a brown/black solid
lining the flask walls. The solution was filtered, the solvent was
removed in vacuo, and the resulting solid was washed with
petroleum ether (3 × 10 mL), followed by diethyl ether (3 × 10
mL), and tetrahydrofuran (3 × 10 mL). A yellow solid was
obtained. Yield: 0.554 g (56%). Crystals were grown by cooling a
saturated ethanol solution. 31P{1H} NMR (THF-d8): δ 59.8 (s). 1H
NMR (THF-d8): δ 4.4 (m, 8H, CH2OH), 2.2 (m, 4H, -CH2CH2-),
1.9 (br s, 4H, -OH). 13C{1H} NMR (THF-d8): δ 58.58 (CH2OH),
18.88 (-CH2CH2-). Anal. Calcd for C12H32Cl2O8P4Ru: C, 24.01; H,
5.37; P, 20.64. Found: C, 24.11; H, 5.34; P, 20.69.
1
H3PO4 in D2O. Note that the H NMR data for the methyl and
methylene regions in complexes containing the DMeOPrPE ligand
were generally broad and uninformative and therefore are not
reported in the synthetic descriptions below. For spectra acquired
at set temperatures, a 10 min temperature equilibration period was
used. T1 values were determined by plotting the intensity as a
function of delay time and fitting the resultant curve to a 3-parameter
single-exponential function. Unweighted Fourier transforms of each
FID were phased carefully and subjected to baseline correction.
When required, the samples were sealed under argon in 7 mm tubes
fitted with Teflon valves. Mass spectra were obtained using an
Agilent 1100 LC/MS Mass Spectrometer. The samples were
dissolved in THF and introduced into the ionization head (ESI)
using the infusion method. Elemental analyses were performed by
Robertson Microlit Laboratories.
X-ray Crystallography. X-ray diffraction intensities were
collected on a Bruker SMART APEX CCD diffractometer at T )
153(2) K with Mo KR radiation (λ ) 0.71073 Å). The crystal-
lographic data and summary of the data collection and structure
refinement are given in Tables 2 and 5. Absorption correction was
applied by SADABS.43 The structures were solved using direct
methods and completed by subsequent difference Fourier syntheses
and refined by full matrix least-squares procedures on reflection
intensities (F2). All non-hydrogen atoms were refined with aniso-
tropic displacement coefficients. Two -CH2-CH2- groups in trans-
RuCl2(DHMPE)2 (Ia), and one Ph-ring and PF6 anion in trans-
[Ru(DPPE)2(H2O)H]+ are disordered over two positions in a 1:1
ratio. Only one position of the disordered atoms in these structures
is drawn (Figures 2 and 5). The position of the H atom coordinated
to the Ru atom in trans-[Ru(DPPE)2(H2O)H]+ was found on the
F-map and refined. Other H atoms were placed in calculated
positions and were refined in a riding group model. The X-ray
diffraction study of trans-RuCl2(DHPrPE)2 (Ia) showed that, in the
crystal structure, there are two positions of the molecule, and as a
result all atoms in the molecule (except the Ru atom) are disordered
over two positions (Supporting Information, Figure S1). Our
attempts to find a solution for this disorder and to further refine
the structure failed. All software and sources scattering factors are
contained in the SHELXTL (5.10) program package (G. Sheldrick,
Bruker XRD, Madison, WI).
General synthesis of trans-[Ru(P2)2(H2)H]PF6 (II-PF6). To a stain-
less-steel pressure vessel containing trans-Ru(DMeOPrPE)2Cl2
(0.2843 g, 0.3037 mmol), TlPF6 (0.2120 g, 0.6074 mmol), and
Proton Sponge (0.0650 g, 0.3073 mmol) was added toluene (10
mL) and charged with H2 (350 psig) and heated to 85 °C. After
16 h, the solution was cooled to room temperature and filtered
through Celite in an argon-filled glovebox to yield a pale yellow
solution. Following H/D exchange with CD3OD, the HD isotopo-
logue was visualized as a 1:1:1 triplet. Synthetic procedures that
employed different phosphines were identical except that in the
case of hydroxylated phosphines (DHMPE, DHPrPE), THF was
used as the reaction solvent. These complexes were not isolated.
Synthesis of trans-Ru(DMeOPrPE)2HCl. To a flask containing
RuCl2COD (0.0721 g, 0.2567 mmol) and LiOH (0.013 g, 0.5417
mmol) was added a solution of DMeOPrPE (0.220 g, 0.5729 mmol)
in methanol (20 mL). The solution was brought to reflux and within
30 min the cloudy brown solution turned transparent brown-red.
Heating continued for 12 h, after which time solvent was removed
in vacuo, then washed with boiling hexanes (3 × 10 mL), yielding
a brown oil. Attempts to obtain a solid from the brown oil were
unsuccessful. A 31P{1H} spectrum of the reaction mixture showed
one major resonance at δ 59.1 (s). The 1H NMR spectrum (CD3OD)
of the high-field region showed one resonance at δ -22.1 (quint.,
2JHP ) 21 Hz). ESI+: m/z calcd for Ru(DMeOPrPE)2H(Cl), 902.36.
Found: [M - H]+, 901.4.
Computational Methods. All calculations were performed using
Jaguar 6.5.44 Geometry optimizations and the calculation of zero-
point energies were carried out using the popular hybrid density
functional known as B3LYP.45,46 Thermochemical corrections to
the energy were calculated at 373 K because that was the
temperature used in the experimental studies of the substitution
reactions. The basis set was LACV3P**. For the ruthenium atom,
LACV3P** employs the quasi-relativistic Hay-Wadt pseudopo-
tential47 for the core electrons and the associated valence basis set
contracted to triple- form. For non-metals, LACV3P** employs
Generation of trans-[Ru(DMeOPrPE)2(H2)Cl]+. To an NMR
tube containing a solution of trans-[Ru(DMeOPrPE)2HCl] (0.031
g, 0.034 mmol) in CD3OD was added triflic acid (3 µL, 0.034 mol).
(43) Sheldrick, G. M. SADABS (2.01), Bruker/Siemens Area Detector
Absorption Correction Program; Bruker AXS: Madison, WI.
(44) Jaguar, v6.5; Schrodinger, LLC: New York, 2005.
(45) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623–11627.
(46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(47) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310.
(48) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, A. J. J. Chem. Phys.
1980, 72, 650–654.
(49) McLean, A. D.; Chandler, S. G. J. Chem. Phys. 1980, 72, 5639–5648.
Inorganic Chemistry, Vol. 48, No. 7, 2009 2983

Szymczak et al.
1
1
31P{1H} NMR: δ 49.9 (s). H NMR: δ -14.3 (1:1:1 tt; JHD
)
of bond lengths and angles of Ib and trans-[Ru(DPPE)2(H2O)H]+,
ball and stick representation of Ia, experimental procedures for the
preparation of Mo(P2)(CO)4 complexes, experimental details
for [Ru(DMeOPrPE)2(tBuCN)H]+, [Ru(DMeOPrPE)2(C7H8)H]+,
[Ru(DMeOPrPE)2(NH3)H]+, [Ru(DPPE)2(H2O)H]+, and [Ru-
(DHMPE)2(H2O)H]+. This material is available free of charge via
the Internet at http://pubs.acs.org.
25.3 Hz). T1(min) ) 35.05 ms (500 MHz). ESI+: m/z calcd for
[Ru(DMeOPrPE)2(H2)Cl]+, 903.37. Found: [M - H2]+, 901.4.
Acknowledgment. We thank the NSF for funding (NSF-
CHE-0452004 and an NSF IGERT fellowship to N.K.S).
Supporting Information Available: 31P NMR spectra of I and
II, the crystallographic information including the.cif files and tables
IC801884X
2984 Inorganic Chemistry, Vol. 48, No. 7, 2009