Thermodynamic hydride donor abilities of [HW(CO)4L]- complexes (L = PPH3, P(OMe)3, CO) and their reactions with [C5Me5Re(PMe3)(NO)(CO)]+

Article abstract of DOI:10.1021/ja036524r

Abstract: The thermodynamic hydride donor abilities of [HW(CO) ₅]^- (40 kcal/mol), [HW(CO)₄P(OMe ₃)]^- (37 kcal/mol), and [HW(CO)₄(PPh ₃)]- (36 kcal/mol) have been measured in acet

Full text of DOI:10.1021/ja036524r

Published on Web 09/13/2003
Thermodynamic Hydride Donor Abilities of [HW(CO)4L]-
Complexes (L ) PPh3, P(OMe)3, CO) and Their Reactions with
[C5Me5Re(PMe3)(NO)(CO)]+
William W. Ellis,† Rebecca Ciancanelli,‡ Susie M. Miller,§ James W. Raebiger,†
M. Rakowski DuBois,*,‡ and Daniel L. DuBois*,†
Contribution from the National Renewable Energy Laboratory, 1617 Cole BouleVard,
Golden, Colorado 80401, and Department of Chemistry and Biochemistry,
UniVersity of Colorado, Boulder, Colorado 80309
Received June 5, 2003; E-mail: ddubois@tcplink.nrel.gov; Mary.Rakowski-dubois@Colorado.edu
Abstract: The thermodynamic hydride donor abilities of [HW(CO)5]- (40 kcal/mol), [HW(CO)4P(OMe3)]-
(37 kcal/mol), and [HW(CO)4(PPh3)]- (36 kcal/mol) have been measured in acetonitrile by either equilibrium
or calorimetric methods. The hydride donor abilities of these complexes are compared with other complexes
for which similar thermodynamic measurements have been made. [HW(CO)5]-, [HW(CO)4P(OMe3)]-, and
[HW(CO)4(PPh3)]- all react rapidly with [Cp*Re(PMe3)(NO)(CO)]+ to form dinuclear intermediates with
bridging formyl ligands. These intermediates slowly form [Cp*Re(PMe3)(NO)(CHO)] and [W(CO)4(L)(CH3-
CN)]. The structure of cis-[HW(CO)4(PPh3)]- has been determined and has the expected octahedral
structure. The hydride ligand bends away from the CO ligand trans to PPh3 and toward PPh3.
Introduction
this goal requires quantitative measurements of this property
for a range of compounds. Previous studies of transition-metal
hydrides have focused mainly on the hydricity of five- and
seven-coordinate complexes.1-8 In this paper, hydride donor
abilities are reported for six-coordinate octahedral complexes,
one of the most common classes of transition-metal hydrides.
Six-coordinate, anionic cis-[HW(CO)4(L)]- complexes react
with a variety of organic electrophiles, including alkyl halides,
acyl chlorides, ketones, aldehydes, and epoxides.11-13 Each of
these reactions involves a formal transfer of the hydride ligand
to the substrate. In competitive studies of reactions involving
hydride transfer from W to alkyl halides and organic carbonyl
compounds, cis-[HW(CO)4P(OMe3)]- was found to react faster
than [HW(CO)5]- in all cases except for reactions with tertiary
The tendency of metal hydride complexes, HMLn, to undergo
heterolytic cleavage of the M-H bond to form a solvated
hydride ion (H-) is an important fundamental property of these
complexes. Indeed, it is for this property that they are named.
The free energy associated with this heterolytic cleavage
-
reaction, ∆G°H , is a measure of the hydricity of a metal hydride,
and only recently have such thermodynamic measurements been
reported.1-8 There are many important reactions in which a
formal hydride transfer reaction occurs. The hydricities of
NADH and NADPH determine the thermodynamic driving
forces of a large number of biological reactions involved in
metabolism. The heterolytic cleavage of hydrogen by hydro-
genases involves a hydride transfer to iron or nickel.9 Stoichio-
metric reductions of a variety of organic compounds also involve
formal hydride transfers. Finally, many catalytic processes
involve heterolytic M+-H- bond cleavage at some point in the
catalytic cycle.10 As a result, understanding those features which
control a compound’s hydricity is of great interest. Achieving
(9) (a) Nicolet, Y.; de Lacey, A. L.; Verne`de, X.; Fernandez, V. M.; Hatchikian,
E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596. (b)
Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998,
282, 1853. (c) Pereira, A. S.; Tavares, P.; Moura, I.; Moura, J. J. G.; Huynh,
B. H. J. Am. Chem. Soc. 2001, 123, 2771. (d) Peters, J. W. Curr. Opin.
Struct. Biol. 1999, 9, 670. (e) Volbeda, A.; Garcin, E.; Piras, C.; de Lacey,
A. L.; Fernandez, V. M.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps,
J. C. J. Am. Chem. Soc. 1996, 118, 12989. (f) Higuchi, Y.; Ogata, H.;
Miki, K.; Yasuoka, N.; Yagi, T. Structure 1999, 7, 549. (g) Garcin, E.;
Vernede, X.; Hatchikian, E. C.; Volbeda, A.; Frey, M.; Fontecilla-Camps,
J. C. Structure 1999, 7, 557.
(10) (a) Magee, M. P.; Norton, J. R. J. Am. Chem. Soc. 2001, 123, 1778. (b)
Song, J.-S.; Szalda, D. J.; Bullock, R. M. Organometallics 2001, 20, 3337.
(c) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana, M.
J. Am. Chem. Soc. 2001, 123, 1090. (d) Yamakawa, M.; Ito, H.; Noyori,
R. J. Am. Chem. Soc. 2000, 122, 1466. (e) Jiang, Y.; Jiang, Q.; Zhang, X.
J. Am. Chem. Soc. 1998, 120, 3817. (f) Lee, D.-H.; Patel, B. P.; Clot, E.;
Eisenstein, O.; Crabtree, H. Chem. Commun. 1999, 297. (g) Abdur-Rashid,
K.; Lough, A. J.; Morris, R. H. Organometallics 2000, 19, 2655.
(11) Kao, S. C.; Spillett, C. T.; Ash, C.; Lusk, R.; Park, Y. K.; Darensbourg,
M. Y. Organometallics 1985, 4, 83.
† National Renewable Energy Laboratory.
‡ University of Colorado.
§ Department of Chemistry, Colorado State University.
(1) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999, 121,
11432.
(2) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem. Soc.
2002, 124, 1918.
(3) Ciancanelli, R. F.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem.
Soc. 2002, 124, 2984.
(4) Price, A. J.; Ciancanelli, R.; Noll, B. C.; Curtis, C. J.; DuBois, D. L.;
DuBois, M. R. Organometallics 2002, 21, 4833.
(5) Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; DuBois, M. R.;
DuBois, D. L. Organometallics 2001, 20, 1832.
(6) Curtis, C. J.; Miedaner, A.; Ciancanelli, R. F.; Ellis, W. W.; Noll, B. C.;
DuBois, M. R.; DuBois, D. L. Inorg. Chem. 2003, 42, 216.
(7) Sarker, N.; Bruno, J. W. J. Am. Chem. Soc. 1999, 121, 2174.
(8) Sarker, N.; Bruno, J. W. Organometallics 2001, 20, 55.
(12) Kao, S. C.; Gaus, P. L.; Youngdahl, K.; Darensbourg, M. Y. Organome-
tallics 1984, 3, 1601.
(13) Gaus, P. L.; Kao, S. C.; Youngdahl, K.; Darensbourg, M. Y. J. Am. Chem.
Soc. 1985, 107, 2428.
9
12230
J. AM. CHEM. SOC. 2003, 125, 12230-12236
10.1021/ja036524r CCC: $25.00 © 2003 American Chemical Society

Hydride Donor Abilities of [HW(CO)4L]- Complexes
A R T I C L E S
(0.00 ppm). All 31P NMR spectra were proton decoupled. Isothermal
titration calorimetry was performed on a Calorimetry Sciences Corpora-
tion ITC 4200 calorimeter. Solvents were reagent grade and were
purchased from Aldrich. Acetonitrile was vacuum transferred from CaH2
and stored in a glovebox. The tungsten hydride complexes, [HW(CO)5]-
(1a), [HW(CO)4P(OMe)3]- (1b), and [HW(CO)4(PPh3)]- (1c), were
prepared by literature methods,19-21 as were [Cp*Re(PMe3)(NO)(CO)]-
[PF6] (2)22 and [CpRe(PMe3)(NO)(CHO)].18 The acetonitrile complexes
[W(CO)5(CH3CN)] (4a), [W(CO)4(CH3CN)P(OMe)3] (4b), and [W(CO)4-
(CH3CN)(PPh3)] (4c) were also prepared using literature methods.23-27
X-ray quality crystals of [HW(CO)4(PPh3)][(PPh3)2N] were grown by
cooling a saturated acetonitrile solution to -40 °C in a glovebox.
Equilibration reactions were performed at room temperature (21 °C).
Synthesis of Cp*Re(PMe3)(NO)(CHO). The following preparation
was performed under an atmosphere of N2 on a Schlenck line.
Tetramethylammonium borohydride (108 mg; 1.21 mmol) in methanol
(6 mL) was added by cannula to a solution of [Cp*Re(PMe3)(NO)-
(CO)]PF6 (420 mg; 0.700 mmol) in methanol (8 mL) in a centrifuge
tube. A white precipitate formed quickly. The mixture was stirred for
10 min and then centrifuged. The yellow supernatant was removed by
cannula. The precipitate was subsequently washed with 5 mL of
methanol. The wash was combined with the supernatant, and the solvent
was removed to form a yellow solid. The solid was extracted with 5
mL of toluene, and the resulting solution was filtered through Celite,
followed by two small washes. Removal of the solvent in vacuo gave
285 mg of a yellow solid (89% yield). Because of the low thermal
stability of this class of compounds, this compound was stored in a
freezer at -40 °C under nitrogen. This compound is pure on the basis
of 1H NMR and 31P NMR spectra. 1H NMR (400 MHz, CD3CN): 16.26
halides, where steric demands favor reactions with less bulky
hydrides.11 It has been suggested that cis-[HW(CO)4P(OMe)3]-
is similar in reactivity to the powerful nucleophile LiBEt3H,
Super Hydride.11 Because these tungsten hydrides are well-
characterized and display a variety of synthetically useful
hydride transfer reactions, it was of interest to determine the
thermodynamic hydride donor ability of these compounds. This
would permit comparison of kinetic and thermodynamic mea-
surements of hydricity for these complexes and a thermody-
namic comparison with other classes of hydride donors.
Recently, three independent methods have been developed
for measuring the thermodynamic hydride donor ability, or
hydricity, of transition-metal hydrides. One method is based on
an equilibrium measurement of the heterolytic cleavage of
hydrogen to form the metal hydride of interest.2-4 A second
method requires a measurement of the pKa of the hydride and
the two-electron oxidation potential of the conjugate base of
the metal hydride.1,5,6 This method is based on an extension of
a thermodynamic cycle used by a number of research groups
to determine homolytic bond dissociation energies.14-16 These
two methods have been shown to agree within experimental
error ((2 kcal/mol),2 and both provide free-energy values that
do not require a reference hydride donor. The third method is
intuitively the simplest. It relies on equilibrium measurements
of hydride transfer reactions between hydride donors, LnMH,
and hydride acceptors, A, as shown in reaction 1. In this case,
-
eq 2 can be used to calculate ∆G°H from the equilibrium
4
ppm (s, 1H, C(O)H), 1.97 ppm (d, JPH ) 0.4 Hz, 15 H, C5(CH3)5),
constant and the hydride donor ability of the reference com-
pound, AH-.
1.49 ppm (d, 3JPH ) 10 Hz, 9 H, P(CH3)3). 31P NMR (161 MHz, CD3-
CN): -24.36 ppm (s). IR (Nujol mull): νNO 1643 cm-1, νCO 1533 cm-1
.
Closely related formyl complexes have been described previously.28-30
Equilibration of [HW(CO)4P(OMe)3]- and [Cp*Re(PMe3)(NO)-
(CO)]+. Forward Reaction. [Cp*Re(PMe3)(NO)(CO)]PF6 (2) (26.8
mg; 0.0446 mmol) and [HW(CO)4P(OMe)3][(PPh3)2N] (1b) (42 mg;
0.044 mmol) were measured into a calibrated NMR tube in a glovebox.
The solids were dissolved in CD3CN (total volume ) 600 µL). The
LnMH + A h LnM+ + AH-
(1)
(2)
∆G°(LnMH) ) ∆G°H-(AH-) - RT ln K1
This approach has been used to determine the relatiVe hydride
donor abilities of NADH model compounds,17 transition-metal
hydrides,1,7,8 and formyl complexes.18 Unlike the first two
methods, it requires that the hydricity of a reference compound
be known to obtain absolute hydricity values.
Of these three methods, the one involving direct hydride
transfer is most suitable for the study of cis-[HW(CO)4(L)]-
complexes because the appropriate tungsten derivatives do not
react with hydrogen, nor do they display reversible electron-
transfer waves in cyclic voltammetric experiments. This paper
describes both equilibrium and calorimetric measurements of
the hydricity of these complexes using hydride transfer reactions.
1
reaction was monitored by H NMR and 31P NMR spectroscopy, and
the Cp* signals of [Cp*Re(PMe3)(NO)(CO)]PF6 and its products were
used as an internal standard to determine concentrations. Formation of
an intermediate [Cp*Re(PMe3)(NO)(µ-CHO)W(CO)4P(OMe)3] (3b)
was observed immediately (∼10 min after mixing). [1H NMR of 3b
(CD3CN): 14.76 ppm (s, 1 H, C(O)H), 3.65 ppm (d, 9 H, P(OCH3)3,
4
3JPH ) 11 Hz), 2.01 ppm (d, 14 H, C5(CH3)5, JPH ) 0.8 Hz), 1.54
ppm (d, 9 H, P(CH3)3, 2JPH ) 10 Hz). 31P NMR of 3b (CD3CN): 152.7
1
ppm (s, P(OCH3)3, JPW ) 382 Hz), -23.6 ppm (s, P(CH3)3).] The
equilibrium constant for this conversion (K ) [3b]/[1b][2]) was
calculated to be 230 M-1 by integration of the 1H NMR and 31P NMR
spectra. This value was constant for the first 3 h of the reaction, after
which the concentrations of 1b and 2 were too low to measure.
Concurrent formation of [W(CO)4(CD3CN)P(OMe)3] (4b) and Cp*Re-
(PMe3)(NO)(CHO) (5) was observed. The equilibrium constant for this
Experimental Section
General Procedures and Materials. NMR spectra were recorded
on a Varian 400-MHz spectrometer. Proton chemical shifts were
recorded relative to residual protons in CD3CN (1.93 ppm). 31P NMR
chemical shifts were reported relative to an external sample of H3PO4
(19) Darensbourg, M. Y.; Slater, S. J. Am. Chem. Soc. 1981, 103, 5914.
(20) Darensbourg, M. Y.; Slater, S. Inorg. Synth. 1983, 22, 181.
(21) Slater, S. G.; Lusk, R.; Schumann, B. F.; Darensbourg, M. Organometallics
1982, 1, 1662.
(22) Meyer, W. E.; Amoroso, A. J.; Jaeger, M.; Le Bras, J.; Wong, W.-T.;
Gladysz, J. A. J. Organomet. Chem. 2000, 616, 44.
(23) Bleijerveld, R. H. T.; Vrieze, K. Inorg. Chim. Acta 1976, 19, 195.
(24) Dickson, C. A.; McFarlane, A. W.; Coville, N. J. Inorg. Chim. Acta 1989,
158, 205.
(25) Strohmeier, W.; Schonauer, G. Chem. Ber. 1961, 94, 1346.
(26) Schenk, W. A.; J. Organomet. Chem. 1979, 179, 253.
(27) Peterson, L. K.; Huang, S. Inorg. Chim. Acta 1993, 203, 87.
(28) Casey, C. P.; Andrews, M. A.; McAlister, D. R.; Rinz, J. E. J. Am. Chem.
Soc. 1980, 102, 1927.
(29) Tam, W.; Lin, G. Y.; Wong, W. K.; Kiel, W. A.; Wong, V. K.; Gladysz,
J. A. J. Am. Chem. Soc. 1982, 104, 141.
(30) Sweet, J. R.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104, 2811.
(14) (a) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711. (b) Tilset,
M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112, 2843. (c) Parker, V. D.;
Handoo, K. L.; Roness, F.; Tilset, M. J. Am. Chem. Soc. 1991, 113, 7493.
(d) Ryan, B. O.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112,
2618.
(15) Wang, D.; Angelici, R. J. J. Am. Chem. Soc. 1996, 118, 935.
(16) Wayner, D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287.
(17) (a) Anne, A.; Moiroux, J. J. Org. Chem. 1990, 55, 4608. (b) Ostovic, D.;
Lee, I. S. H.; Roberts, R. M. G.; Kreevoy, M. M. J. Org. Chem. 1985, 50,
4206.
(18) Ellis, W. W.; Miedaner, A.; Curtis, C. J.; Gibson, D. H.; DuBois, D. L. J.
Am. Chem. Soc. 2002, 124, 1926.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12231

A R T I C L E S
Ellis et al.
Table 1. Crystallographic Data for [HW(CO)4(PPh3)][(PPh3)2Na
conversion (K ) [4b][5]/[3b]) reached a constant value of 16 M over
one week of observation.
empirical formula
formula weight
crystal system
space group
C58H46NO4P3W
1097.72
monoclinic
P2(1)/c
Reverse Reaction. Cp*Re(PMe3)(NO)(CHO) (5) (30 mg; 0.066
mmol) and [W(CO)4(CH3CN)P(OMe)3] (4b) (30 mg; 0.065 mmol) were
added to a calibrated NMR tube in a glovebox. The solids were
dissolved in CD3CN (total volume ) 600 µL). As above, the reaction
unit cell dimensions
a (Å)
b (Å)
11.389(2)
26.182(4)
16.820(3)
90
101.849(12)
90
was monitored by H NMR and 31P NMR spectroscopy, and the Cp*
1
signals of Cp*Re(PMe3)(NO)(CHO) and its products were used as an
internal standard to determine concentrations. After one week of
reaction, the intermediate complex 3b could be observed, and the
equilibrium constant for the reaction (K ) [4b][5]/[3b]) had reached a
constant value of 24 M.
c (Å)
R (deg)
â (deg)
γ (deg)
volume (Å3)
4908.9(14)
4
Z
Equilibration of [HW(CO)5]- and [Cp*Re(PMe3)(NO)(CO)]+.
NMR tube reactions were studied using known amounts of [Cp*Re-
(PMe3)(NO)(CO)]PF6 and [HW(CO)5][(PPh3)2N]. An intermediate,
[Cp*Re(PMe3)(NO)(µ-CHO)W(CO)5] (3a), was observed immediately
(∼10 min after mixing). 1H NMR of 3a (CD3CN): 15.01 ppm (s, 1 H,
C(O)H), 2.00 ppm (d, 15 H, C5(CH3)5, 4JPH ) 0.4 Hz), 1.55 ppm (d, 9
H, P(CH3)3, 2JPH ) 10 Hz). The equilibrium constant for this conversion
(K ) [3a]/[1a][2]) was calculated to be 2.8 M-1 by integration of the
1H NMR spectra. Over a 20-h period, the formation of Cp*Re(PMe3)-
(NO)(CHO) was observed.
density (calcd) (Mg/m3)
crystal size (mm3)
final R indices [I > 2σ(I)]
R indices (all data)
extinction coefficient
largest diff. peak and hole (e‚Å-3
1.485
0.60 × 0.45 × 0.40
R1 ) 0.0340, wR2 ) 0.0799
R1 ) 0.0382, wR2 ) 0.0819
0.0193(4)
)
1.985 and -0.575
a
Definitions of R1 and wR2: R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 )
{∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
2
2
2
monochromator). Selected details related to the crystallographic experi-
ment are listed in Table 1. Unit cell parameters were obtained from a
least-squares fit to the angular coordinates of all reflections, and
intensities were integrated from a series of frames (3.35-23.33° ω
rotation) covering more than a hemisphere of reciprocal space.
Absorption and other corrections were applied by using SADABS.31
The structure was solved by using direct methods and refined (on F2,
using all data) by a full-matrix, weighted least-squares process. All
nonhydrogen atoms were refined by using anisotropic displacement
parameters. Hydrogen atoms were placed in idealized positions and
refined by using a riding model. Standard Bruker Nonius control
(SMART) and integration (SAINT) software was employed, and Bruker
Nonius SHELXTL32 software was used for structure solution, refine-
ment, and graphics.
Reaction of [HW(CO)4(PPh3)]- and [Cp*Re(PMe3)(NO)(CO)]+.
NMR tube reactions were studied using known amounts of [Cp*Re-
(PMe3)(NO)(CO)]PF6 and [HW(CO)4(PPh3)][(PPh3)2N]. An intermedi-
ate, [Cp*Re(PMe3)(NO)(µ-CHO)W(CO)4(PPh3)] (3c), was observed
1
immediately (∼10 min after mixing). H NMR of 3c (CD3CN): 14.7
ppm (s, 1 H, C(O)H), 1.96 ppm (s, 15 H, C5(CH3)5), 1.50 ppm (d, 9 H,
P(CH3)3, 2JPH ) 10 Hz). 31P NMR of 3c (CD3CN): 30.6 ppm (s, PPh3,
1JPW ) 236 Hz), -22.4 ppm (s, P(CH3)3). This reaction goes to
completion, and no equilibrium constant could be determined. Over a
20-h period, the formation of [W(CO)4(CD3CN)(PPh3)] and Cp*Re-
(PMe3)(NO)(CHO) was observed.
Equilibration of [CpRe(PMe3)(NO)(CHO)] and [Cp*Re(PMe3)-
(NO)(CO)]+. [Cp*Re(PMe3)(NO)(CO)](PF6) (9.2 mg; 0.015 mmol) and
[CpRe(PMe3)(NO)(CHO)] (8.0 mg; 0.021 mmol) were measured into
an NMR tube in the glovebox. The solids were dissolved in CD3CN
(600 µL), and the reaction was followed by 1H NMR for 5 h.
Equilibrium was reached after 30 min. The equilibrium constant of
the reaction was determined to be 0.035 by integration of the 1H NMR
and 31P NMR spectra for this reaction. The reverse reaction was studied
in a similar manner. ∆G° for this reaction was determined to be 2.0
Results
Equilibrium Studies. The tungsten hydride complexes,
[HW(CO)5]- (1a), cis-[HW(CO)4P(OMe)3]- (1b), and cis-[HW-
(CO)4(PPh3)]- (1c), react with [Cp*Re(PMe3)(NO)(CO)]+ (2)
(where Cp* ) pentamethylcyclopentadiene), as shown in
reaction 3. In each case, an intermediate formyl-bridged
dinuclear complex, [Cp*Re(PMe3)(NO)(µ-CHO)W(CO)4(L)]
-
kcal/mol. A ∆G°H value of 44.1 ( 1 kcal/mol for CpRe(PMe3)(NO)-
(CHO)18 results in a ∆G°H value of 42.1 ( 1 kcal/mol for Cp*Re-
-
1
(3), is formed. The H NMR spectra of these intermediates
(PMe3)(NO)(CHO).
exhibit a singlet at 14-15 ppm that is characteristic of a formyl
resonance.28-30 These resonances are 1-2 ppm upfield from
the resonance of [Cp*Re(PMe3)(NO)(CHO)]. In addition, new
1H NMR resonances are observed for the trimethylphosphine
and Cp* ligands on rhenium, with corresponding resonances
for the trimethyl phosphite and triphenylphosphine ligands on
tungsten (see Experimental Section). These resonances appear
and disappear in unison, and they have integration ratios
consistent with the formation of mixed bimetallic species. 31P
NMR results are also consistent with the formation of bridged
species (see Experimental Section). Two coordination modes
are reasonable for the bridging formyl ligand of these intermedi-
ates. In the one shown in structure 3, the relatively basic oxygen
atom of the formyl moiety is bound to tungsten. A second
possible mode involves a C-H-W linkage (structure 6). This
is not unrealistic given the propensity of tungsten to form
Calorimetry. In a typical experiment, the reaction cell of the
calorimeter was purged with a stream of N2 for 15 min. Subsequently,
it was loaded by syringe with a 5.4 mM solution of [(p-Me2NC6H4)3C]-
[PF6] in dry acetonitrile (cell volume ) 1.3 mL). The reference cell of
the calorimeter was loaded with neat acetonitrile. Stirring was com-
menced at 300 rpm, and the calorimeter was allowed to equilibrate for
1.5 h. Next, a 250 µL syringe containing a 10.6 mM solution of [HW-
(CO)4P(OMe)3][(PPh3)2N] in acetonitrile was inserted into the cell.
(Both solutions were prepared in the glovebox.) After 20 min of
equilibration at 25.6 °C, 10 injections (5 µL) of the hydride solution
were added into the cell, with an 8-min delay between injections. The
first data point was discarded, and the average enthalpy/mol for the
subsequent nine injections was 34.1 kcal/mol. Next, 18 injections (10
µL) of the hydride solution were added to the cell with the same 8-min
delay. Again, after discarding the first data point, the average enthalply/
mol for the following 17 injections was 34.1 kcal/mol. At least three
independent experiments were performed in this manner for each of
the three hydrides studied.
(31) Sheldrick, G. M. SADABS: A program for area detector absorption
X-ray Diffraction Studies. X-ray diffraction data from a crystal of
[HW(CO)4(PPh3)][(PPh3)2N] were recorded on a Bruker Nonius
SMART CCD diffractometer employing Mo KR radiation (graphite
corrections.
(32) Sheldrick, G, M. SHELXTL, version 6.12; Bruker AXS: Madison, WI,
1999.
9
12232 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Hydride Donor Abilities of [HW(CO)4L]- Complexes
A R T I C L E S
Table 2. Thermodynamic, Kinetic, and Spectroscopic Parameters for cis-[HW(CO)4(L)]- Complexes
b
-1 d
∆H°(4) (kcal/mol)a
∆G°H (kcal/mol)
k × 103 (s-1 M-1 c
3.3
50
)
νCO (cm
)
−
compound
[HW(CO)5]-
-31
-34
-35
40
37
36
2029, 1889, 1858
1986, 1856, 1822
1974, 1851, 1815
[HW(CO)4P(OMe)3]-
[HW(CO)4(PPh3)]-
a
b
Enthalpy associated with reaction 4 at 298.6 K. Free energy (at 294 K) associated with the heterolytic cleavage of the W-H bond to form solvated
H- and cis-[HW(CO)4(L)(CH3CN)]-. Second-order rate constants taken from ref 11 for the reaction of cis-[HW(CO)4(L)]- complexes with n-bromobutane
to form n-butane (299 K in THF). IR data taken from refs 21 and 45.
c
d
bridging hydride complexes21 and the likelihood that a C-H-W
interaction is involved in the initial formation of the C-H bond.
However, the lack of 183W satellites for the formyl resonances
and the relatively small differences in chemical shifts between
the free formyl complex and that of the intermediate argue
against a direct W-H interaction in the observed intermediate.
In addition, in previous hydride transfer reactions of these
complexes with CO2 and aldehydes, products or intermediates
with W-O bonds were observed, e.g., WOC(O)H and
WOCH3.13,21 Efforts to isolate intermediates 3a-c have been
unsuccessful, and no parent ion was observed by electrospray
mass spectroscopy.
one week. At this time the equilibrium constant (K ) [4b][5]/
[3b], where the activity of the solvent, acetonitrile, is taken as
unity) for the conversion of intermediate 3b to products is 24
M. To establish that this reaction is an equilibrium, the reverse
reaction between [Cp*Re(PMe3)(NO)(CHO)] and cis-[W(CO)4P-
(OMe)3(CH3CN)] was studied. After one week, a small quantity
of complex 3b was observed. An equilibrium constant of 16 M
for the second step of reaction 3 was determined from these
data, with an average from the forward and reverse reactions
of 20 M.
The overall equilibrium constant for reaction 3, starting from
cis-[HW(CO)4P(OMe)3]- and [Cp*Re(PMe3)(NO)(CO)]+ to
form [Cp*Re(PMe3)(NO)(CHO)] and cis-[W(CO)4P(OMe)3-
(CH3CN)] is 4600, the product of the equilibrium constants for
steps 1 and 2. This corresponds to a free energy of -5 ( 1
kcal/mol for the hydride transfer from cis-[HW(CO)4P(OMe)3]-
to [Cp*Re(PMe3)(NO)(CO)]+ in acetonitrile. The free energy
of hydride transfer for [Cp*Re(PMe3)(NO)(CHO)] was deter-
mined to be 42 ( 1 kcal/mol (see Experimental Section). An
-
addition of -5 kcal/mol to this value gives a ∆G°H value of
37 ( 2 kcal/mol for cis-[HW(CO)4P(OMe)3]- in acetonitrile.
Calorimetric Studies. It is likely that there is also an
equilibrium in the dissociation of intermediate 3a. However,
After the formation of intermediate 3, slow release of the
formyl complex, [Cp*Re(PMe3)(NO)(CHO)] (5), occurs as
shown in the second step of reaction 3. Formation of the
corresponding cis-[W(CO)4(L)(CH3CN)] complexes, 4, also
occurs in this step. Both of the products were confirmed by
the concentration of the acetonitrile complex, [W(CO)
5(CD3-
CN)], cannot be easily measured by NMR, nor can it be assumed
to be equal to the concentration of formyl product, [Cp*Re-
(PMe3)(NO)(CHO)], because of the formation of bridging
hydride complexes of tungsten.21
To overcome this problem,
1
comparison of the H NMR spectra and 31P NMR spectra of
authentic compounds synthesized by literature methods.23-27 For
the reaction of the triphenylphosphine complex, [HW(CO)4-
(PPh3)]-, complete conversion to the formyl-bridged intermedi-
ate, 3c, occurs in less than 10 min, followed by a slow
dissociation to the ultimate reaction products, cis-[W(CO)4-
(PPh3)(CD3CN)] and [Cp*Re(PMe3)(NO)(CHO)], over a period
of 20 h. However, for [HW(CO)5]- and cis-[HW(CO)4P-
(OMe)3]-, only partial conversion to the corresponding formyl-
bridged intermediates (3a and 3b) occurs initially. These
reactants establish a measurable equilibrium with the intermedi-
ates within 10 min. For the reaction of cis-[HW(CO)4P(OMe)3]-
with [Cp*Re(PMe3)(NO)(CO)]+, the equilibrium constant was
calorimetric studies were undertaken to determine the relative
-
hydride donor abilities of [HW(CO)
5] , cis-[HW(CO)4P-
-
-
(OMe)3] , and cis-[HW(CO)4(PPh3)] . Reactions of these
hydrides with crystal violet, [(p-Me2NC6H4)3C]+ (reaction 4),
were found to be fast and clean by NMR spectroscopy, a
necessary requirement for obtaining useful calorimetric data.
Enthalpy measurements of reaction 4 were made using an
isothermal calorimeter. In a typical experiment, 10-20 aliquots
of a dilute acetonitrile solution of one of the hydrides were
injected into a cell containing a more concentrated solution of
the acceptor at 25.6 °C (see Experimental Section for details),
and the heat produced was measured. The results of these
experiments are shown in the second column of Table 2. The
accuracy of these measurements is estimated to be (2 kcal/
mol.
1
measured to be 230 M-1 at 21 °C in CD3CN by H and 31P
NMR. The same equilibrium constant is obtained over a period
of 3 h, after which the concentrations of cis-[HW(CO)4P-
(OMe)3]- become too small to measure because of the slow
conversion of the intermediate to the final products cis-
[W(CO)4P(OMe)3(CH3CN)] and [Cp*Re(PMe3)(NO)(CHO)].
The reaction of [HW(CO)5]- and [Cp*Re(PMe3)(NO)(CO)]+
behaves in a similar manner. In this case, the equilibrium
cis-[HW(CO)4(L)]- + [(p-Me2NC6H4)3C]+ CH CN8
3
cis-[W(CO)4(L)(CH3CN)] + (p-Me2NC6H4)3CH (4)
constant for the formation of the intermediate is 2.8 M-1
.
The conversion of intermediate 3b to final products [Cp*Re-
(PMe3)(NO)(CHO)] and cis-[W(CO)4P(OMe)3(CH3CN)] is par-
ticularly interesting. Although the reaction proceeds nearly to
completion, intermediate 3b persists in low concentration after
The hydride donor ability of cis-[HW(CO)4P(OMe3)]- was
determined from the equilibrium measurements described above
to be 37 kcal/mol. The observed enthalpy change for reaction
4 for cis-[HW(CO)4P(OMe3)]- is -34 kcal/mol, which indicates
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12233

A R T I C L E S
Ellis et al.
that addition of 71 kcal/mol is required to convert from the
-
enthalpy of reaction 4 to ∆G°H . The free energies shown in
column 3 of Table 2 were obtained using this same correction
for all three complexes and the heats of reaction shown in
column 2. The estimated uncertainties on these values is (2
kcal/mol.
Using the same correction factor to convert from the heat of
-
reaction 4 to ∆G°H , we assume that the entropy contributions
for hydride transfer for the three different hydride complexes
are the same. This assumption seems reasonable. All of the
complexes have the same charge, all of the reactions involve
formation of a tungsten acetonitrile complex, and all three
complexes are of roughly the same size. If acetonitrile binds to
the carbonium ion on the left-hand side of reaction 4, as it does
to tungsten on the right-hand side, then reaction 4 is a simple
interchange of the hydride and acetonitrile ligands, with a
corresponding change in charge on carbon and tungsten. For
these reasons, hydride transfer reactions appear to be very similar
to electron-transfer reactions. Entropy changes associated with
individual redox couples can be calculated for metal complexes
using a semiempirical equation developed by Weaver.33 T∆S°
values for reaction 4 of 2.3, 2.1, and 1.9 kcal/mol were
calculated for [HW(CO)5]-, cis-[HW(CO)4P(OMe3)]-, and cis-
[HW(CO)4(PPh3)]-, respectively, using estimated radii of 6.2,
4.5, 5.2, and 6.5 Å for the radii of (Me2NC6H4)3CH, [HW(CO)5]-,
cis-[HW(CO)4P(OMe3)]-, and cis-[HW(CO)4(PPh3)]-, respec-
tively. The similarity of these calculated values suggests that
assuming the same entropy corrections for all three tungsten
hydrides for reaction 4 is reasonable.
From the hydride donor ability of (p-Me2NC6H4)3CH in
acetonitrile (74.2 kcal/mol)34,35 and cis-[HW(CO)4P(OMe3)]-
(37 kcal/mol), a free energy for reaction 4 of -37 kcal/mol is
expected. The observed enthalpy change of -34 kcal/mol
indicates that T∆S° is approximately 3 kcal/mol. This value
agrees well with the 1.9-2.3 kcal/mol range calculated in the
preceding paragraph. Although this agreement may be fortuitous,
good agreement between calculated and experimentally deter-
mined T∆S° values has also been observed in hydride transfer
reactions of [HNi(diphosphine)2]+ complexes.1
Structure of [HW(CO)4PPh3]-. Crystals of cis-[HW(CO)4-
(PPh3)][(PPh3)2N] were grown from cold acetonitrile solutions.
Crystallographic data for this complex are given in Table 1. A
perspective drawing is shown in Figure 1, and selected bond
angles and distances are shown in Table 3. The structure of the
cis-[HW(CO)4(PPh3)]- anion is octahedral with the hydride and
triphenylphosphine ligands occupying cis positions. The bond
distances to the coordinated ligands are in the normal range for
W(0) complexes.36 The P(1)-W-C(3) bond angle is nearly
linear (177.4°), but the H(1)-W-P(1) angle is significantly less
than 90° (80.6°). These angles indicate a bending of the hydride
ligand away from the carbonyl ligand C(3)-O(3) toward
triphenylphosphine. In addition, the two carbonyl ligands cis
to the hydride ligand, C(1)-O(1) and C(2)-O(2), bend toward
Figure 1. Drawing of the cis-[HW(CO)4(PPh3)]- anion showing the atom
numbering scheme.
Table 3. Selected Bond Distances (Å) and Bond Angles (deg) for
cis-[HW(CO)4(PPh3)]-
bond distances (Å)
bond angles (deg)
H(1)-W(1)-P(1)
W(1)-H(1)
1.682(6)
1.961(5)
1.967(5)
2.003(5)
2.012(6)
2.5014(10)
1.148(6)
1.169(6)
1.170(5)
1.154(6)
80.61(17)
90.07(17)
99.9(2)
89.03(16)
96.4(2)
W(1)-C(4)
W(1)-C(3)
W(1)-C(2)
W(1)-C(1)
W(1)-P(1)
O(1)-C(1)
O(2)-C(2)
O(3)-C(3)
O(4)-C(4)
C(4)-W(1)-C(3)
C(4)-W(1)-C(2)
C(3)-W(1)-C(2)
C(4)-W(1)-C(1)
C(3)-W(1)-C(1)
C(2)-W(1)-C(1)
C(4)-W(1)-P(1)
C(3)-W(1)-P(1)
C(2)-W(1)-P(1)
C(1)-W(1)-P(1)
91.37(16)
163.8(2)
91.48(13)
177.39(12)
88.63(11)
90.55(12)
the hydride, as indicated by a C(1)-W-C(2) bond angle of
163.8°. It has been shown previously that ligands cis to a
π-acceptor ligand and a σ-only ligand, such as the hydride, will
tend to move away from the π-acceptor ligand toward the
hydride.37 This distortion is expected to reduce the hydricity of
the hydride ligand compared to that of an undistorted octahedral
complex because of a redistribution of negative charge from
the hydride ligand to the trans carbonyl ligand.38
Discussion
Comparison of Thermodynamic Hydricity and Reactivity
of cis-[HW(CO)4(L)]- Complexes. Although numerous pKa
values for transition-metal hydrides have been determined,39-44
relatively few thermodynamic measurements of the hydricity
of these complexes have been reported.1-8 In this study, both
(37) (a) DuBois, D. L.; Hoffmann, R. NouV. J. Chim. 1977, 1, 479. (b) Elian,
M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058. (c) Albright, T. A.;
Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J. Am. Chem. Soc. 1979, 101,
3810.
(38) Wander, S. A.; Miedaner, A.; Noll, B. C.; DuBois, D. L. Organometallics
1996, 15, 3360.
(39) (a) Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides; Dedieu,
A., Ed.; VCH: New York, 1991; pp 309-359. (b) Jordan, R. F.; Norton,
J. R. J. Am. Chem. Soc. 1982, 104, 1255. (c) Moore, E. J.; Sullivan, J. M.;
Norton, J. R. J. Am. Chem. Soc. 1986, 108, 2257. (d) Kristja´nsdo´ttir, S. S.;
Moody, A. E.; Weberg, R. T.; Norton, J. R. Organometallics 1988, 7, 1983.
(40) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155. (b)
Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.;
Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000, 122,
9155. (c) Morris, R. H. Inorg. Chem. 1992, 31, 1471.
(33) Hupp, J. T.; Weaver, M. J. Inorg. Chem. 1984, 23, 3639-3644. The
2
equation is ∆S°rc ) 21.9 - 0.58(AN) + 20.7(Zox - Zred2)/r where AN
represents the acceptor number of the solvent (19.3 for acetonitrile), Zox
and Zred represent the charges on the complexes in their oxidized and
reduced forms, and r represents the radius of the molecule in Å.
(34) Arnett, E. M.; Flowers, R. A., II.; Ludwig, R. T.; Meekhof, A. E.; Walek,
S. A. J. Phys. Org. Chem. 1997, 10, 499.
(35) Zhang, X.-M.; Bruno, J. W. J. Org. Chem. 1998, 63, 4671.
(36) Bau, R.; Teller, R. G.; Kirtley, S. W.; Koetzle, T. F. Acc. Chem. Res. 1979,
12, 176.
(41) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996, 15, 2504.
(42) Davies, S. C.; Henderson, R. A.; Hughes, D. L.; Oglieve, K. E. J. Chem.
Soc., Dalton Trans. 1998, 425.
9
12234 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Hydride Donor Abilities of [HW(CO)4L]- Complexes
A R T I C L E S
equilibrium and calorimetry measurements were used to obtain
information on the energetics of hydride transfer for [HW(CO)5]-,
cis-[HW(CO)4P(OMe)3]-, and cis-[HW(CO)4(PPh3)]-. These
data are summarized in columns 2 and 3 of Table 2. Darens-
bourg first prepared and studied the reactivity and relative
reaction rates of these tungsten hydrides.11-13 In competitive
studies of reactions of [HW(CO)5]- and cis-[HW(CO)4P-
(OMe)3]- with n-bromobutane to form n-butane, cis-[HW-
(CO)4P(OMe)3]- reacted approximately 20 times faster than
[HW(CO)5]-.11 Similarly, on the basis of the stretching frequen-
cies of the carbonyl ligands, the expected hydride donor ability
is [HW(CO)5]- < cis-[HW(CO)4P(OMe)3]- e cis-[HW(CO)4-
(PPh3)]-.21,45 These kinetic and spectroscopic data are shown
in columns 4 and 5 in Table 2, and they correlate well with our
thermodynamic measurements of hydricity shown in column
3. The observation that tert-butyl bromide reacts more slowly
with cis-[HW(CO)4P(OMe)3]- than with [HW(CO)5]- has been
attributed to steric interactions,13 and this is supported by our
structural studies of the cis-[HW(CO)4(PPh3)]- anion. Not only
is the hydride cis to the bulky phosphine ligand, there is also a
significant bending of the hydride ligand toward phosphorus
by about 10°. Although this bending may be less for a phosphite
ligand, significant bending is still expected because CO is a
better acceptor than trimethyl phosphite.37 This distortion will
increase the steric screening of the hydride ligand by the cis
phosphine or phosphite ligands.
Comparison of Thermodynamic Hydricity and Reactivity
of cis-[HW(CO)4(L)]- Complexes with Other Transition-
Metal Hydrides. Thermodynamic measurements of hydricity
have now been reported for more than 20 transition-metal
hydride complexes, and a hydride donor scale for these
complexes covers a range of over 50 kcal/mol.1-8 These values
may be correlated to observed reactions of metal hydrides, as
discussed above, and used to predict reactivity.6,18 In general,
the hydride donor abilities of metal hydride complexes are found
to increase as the ligands become more electron donating or
poorer π-acceptors, as the charge on the metal complex
decreases, and as one moves from right to left in the Periodic
Table.48 In Table 2, a lower ∆G°H value indicates a better
-
hydride donor, and cis-[HW(CO)4(PPh3)]- is a better hydride
donor than [HW(CO)5]- by 4 kcal/mol. This difference in
hydride donor ability upon substitution of CO with triphen-
ylphosphine is of similar magnitude to those observed for other
systems. For example, for the rhenium formyl complexes, CpRe-
-
(NO)(CO)CHO (∆G°H ) 55 kcal/mol) and CpRe(NO)(PPh3)-
-
CHO (∆G°H ) 46 kcal/mol), the substitution of CO with
triphenylphosphine increases the hydride donor ability by 9 kcal/
mol.18 Similarly, CpMo(CO)3H (∆G°H ) 89 kcal/mol) and
-
-
CpMo(CO)2(PPh3)H (∆G°H ) 82 kcal/mol) differ by 7 kcal/
mol.7
Comparison of the hydride donor ability of CpW(CO)3H
7
-
-
-
(∆G°H ) 83 kcal/mol) with [HW(CO)5] (∆G°H ) 40 kcal/
mol) suggests that charge and geometry have very large effects
on hydride donor abilities. The only other octahedral metal
hydride for which the hydride donor ability has been determined
The hydricity of these tungsten complexes and their clean
reactions with organic carbonyl complexes12,13 suggested that
they might also react cleanly with metal carbonyl complexes.
As discussed above, [HW(CO)5]-, cis-[HW(CO)4P(OMe)3]-,
and cis-[HW(CO)4(PPh3)]- all react with [Cp*Re(PMe3)(NO)-
(CO)]+ as shown in reaction 3 to first form observable
intermediates assigned structures 3a-c. Qualitatively, the rate
and extent of formation of these intermediates parallels the
hydricity of the reactants, i.e., [HW(CO)5]- < cis-[HW(CO)4P-
(OMe)3]- < cis-[HW(CO)4(PPh3)]-. Intermediates 3a-c ulti-
mately convert into the final products, cis-[W(CO)4(L)(CH3CN)]
and [Cp*Re(PMe3)(NO)(CHO)]. Intermediates 3a-c are similar
to those observed in other laboratories. In Darensbourg’s work,
alkoxide intermediates were formed as the first products
resulting from hydride transfer to aldehydes.13 Reactions of CO2
with these hydrides resulted in the formation of formate
complexes, cis-[HCOOW(CO)4(L)]-.21 Similarly, in the reaction
of the hydride acceptor 1-benzyl-3-carbamoylpyridinium cation
with HRe(bpy)(CO)3, an intermediate is observed after initial
hydride transfer to the pyridinium ring.46 This intermediate
contains a bond between rhenium and the oxygen atom of the
carbamoyl group. Similar intermediates have been proposed in
catalytic reductions of NAD+.47 These reactions all appear to
occur by a common pathway of hydride transfer followed by
rearrangement to form an intermediate with an M-O bond and
then slow solvolysis of the M-O bond.
is [H2Co(dppe)2]+, for which ∆G°H is 65 kcal/mol.3 The
-
octahedral tungsten hydrides in this study are better hydride
donors by nearly 30 kcal/mol. Certainly the fact that they are
negatively charged, whereas [H2Co(dppe)2]+ is positively
charged, is a major contributor to the observed differences. The
most surprising and interesting comparison is that of HRh(dppb)2
-
(∆G°H ) 34 kcal/mol, where dppb is 1,2-bis(diphenylphos-
phino)benzene)4 with cis-[HW(CO)4(PPh3)]- (∆G°H ) 36 kcal/
-
mol). The observation that a neutral rhodium hydride containing
a nonbasic diphosphine ligand is as hydridic as an anionic
tungsten hydride indicates that making judgments about hy-
dricity on the basis of the position of a metal in the Periodic
Table may lead to conclusions that are not even qualitatively
correct.
Comparison of Thermodynamic Hydricity and Reactivity
of cis-[HW(CO)4(L)]- Complexes with Other Hydride Do-
nors. Compared with other transition-metal hydrides, the cis-
[HW(CO)4(L)]- complexes are among the best hydride donors.
From Table 4, it can be seen that these tungsten hydrides are
also more hydridic than the better organic hydride donors.49,50
For example, NADH model compounds are generally considered
to be good hydride donors and the benzyl derivative, 1-benzyl-
1,4-dihydronicotinamide, shown in Table 4, has a hydride donor
ability of 59 kcal/mol in dimethyl sulfoxide.49 Outside of some
very unstable radical intermediates,51 the formyl complex,
[Cp*Re(PMe3)(NO)(CHO)], prepared in this study is the best
(43) (a) Nataro, C.; Thomas, L. M.; Angelici, R. J. Inorg. Chem. 1997, 36, 6000.
(b) Nataro, C.; Angelici, R. J. Inorg. Chem. 1998, 37, 2975. (c) Nataro,
C.; Chen, J.; Angelici, R. J. Inorg. Chem. 1998, 37, 1868. (d) Wang, D.;
Angelici, R. J. Inorg. Chem. 1996, 35, 1321. (e) Siclovan, O. P.; Angelici,
R. J. Inorg. Chem. 1998, 37, 432.
(44) Pearson, R. G.; Kresge, C. T. Inorg. Chem. 1981, 20, 1878.
(45) Darensbourg, M. Y.; Slater, S. J. Am. Chem. Soc. 1981, 103, 5914.
(46) Kobayashi, A.; Takatori, R.; Kikuchi, I.; Konno, H.; Sakamoto, K.; Ishitani,
O. Organometallics 2001, 20, 3361.
(47) Lo, H. C.; Leiva, C.; Buriez, O.; Kerr, H. B.; Olmstead, M. M.; Fish, R.
H. Inorg. Chem. 2001, 40, 6705.
(48) Labinger, J. A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New
York, 1991; pp 361-379.
(49) Cheng, J.-P.; Lu, Y.; Zhu, X.; Mu, L. J. Org. Chem. 1998, 63, 6108.
(50) Cheng, J.-P.; Handoo, K. L.; Parker, V. D. J. Am. Chem. Soc. 1993, 115,
2655.
(51) Handoo, K. L.; Cheng, J.-P.; Parker, V. D. J. Am. Chem. Soc. 1993, 115,
5067.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12235

A R T I C L E S
Ellis et al.
Table 4. Hydride Donor Abilities of Different Classes of
Compounds
rates of reactions of these complexes with organic substrates
except for cases where steric interactions are expected to
contribute. The tungsten complexes are relatively powerful
hydride donors, and a relative ordering of these complexes
compared to other classes of metal hydrides (in kcal/mol) is as
follows: CpMo(CO)2(L)H (79-89) < CpW(CO)2(L)H (81-
83) < H2 (76) < [HNi(diphosphine)2]+ (51-67) < [HPt-
(diphosphine)2]+ (42-55) ≈ CpRe(NO)(L)(CHO) (42-55) ≈
HCo(dppe)2 (49) < cis-[HW(CO)4(L)]- (36-40) < HRh(dppb)2
(34). Although transition-metal hydrides are generally much
better hydride donors than organic, organometallic, and metal
hydroxide compounds, all of these classes of compounds exhibit
large ranges of hydride donor abilities, and large overlaps occur.
Some of the factors that appear to contribute to the wide range
of hydricities for transition-metal hydrides are metal geometry
and coordination number, charge on the complex, electron-
donating ability of the ligands, chelate bite size, periodic row
of the transition metal, and solvation of the product formed upon
hydride transfer. As a result, it should be possible to tune the
hydricities of transition-metal hydrides to meet specific applica-
tions. The cis-[HW(CO)4(L)]- described in this study are some
of the most powerful hydride donors reported to date for
transition-metal hydrides.
∆G°H− (kcal/mol)
ref
Heterolytic Cleavage of C-H Bonds
Ph3CH
96
59
47
42
50
49
49
1-benzyl-1,4-dihydronicotinamide
N-(4-methoxyphenyl)fluoreneamine
Cp*Re(NO)(PMe3)C(O)H
Heterolytic Cleavage of M-H Bonds
CpMo(CO)3H
87
7
6
[HNi(Et2PCH2CH2CH2PEt2)2]+
[HW(CO)4(L)]-
66
36-40
HRh(dppb)2
34
4
Heterolytic Cleavage of O-H Bonds
[(phen)2Mn(µ-O)(µ-OH)Mn(phen)2]3+
[(phen)2Mn(µ-OH)2Mn(phen)2]2+
120
78
52
52
Heterolytic Cleavage of B-H Bonds
∼45
NaBH3(CN)
34
hydride donor known involving cleavage of a C-H bond with
-
a ∆G°H value of 42 kcal/mol. As expected, the cis-[HW(CO)4-
(L)]- complexes are much more hydridic than manganese
complexes with bridging hydoxo ligands,52 but it is interesting
to note that the data in Table 4 suggest that [(phen)2Mn(µ-
OH)2Mn(phen)2]2+ is a better hydride donor than some transi-
tion-metal hydride complexes. Finally, the cis-[HW(CO)4(L)]-
complexes and HRh(dppb)2 are better hydride donors than the
commonly used reducing agent, sodium cyanoborohydride.33 As
the rhodium complex can be generated from hydrogen, this
raises interesting possibilities for using hydrogen under basic
conditions to carry out a variety of difficult reduction processes
in a catalytic fashion.
Acknowledgment. D.L. DuBois, W.W. Ellis, and J. W.
Raebiger would like to acknowledge the support of the U.S.
Department of Energy, Office of Science, Chemical and
Biological Sciences Division under DOE Contract No. DE-
AC36-99GO10337. C. Ciancanelli and M. Rakowski DuBois
would like to acknowledge the support of the National Science
Foundation.
Summary
Supporting Information Available: Tables of crystal data,
data collection parameters, structure solution and refinement,
atomic coordinates and equivalent isotropic displacement pa-
rameters, bond lengths, bond angles, anisotropic thermal
parameters, hydrogen coordinates, and isotropic displacement
parameters for [HW(PPh3)(CO)4](PPN) (CIF). This information
is available free of charge via the Internet at http://pubs.acs.org.
The hydride donor abilities of [HW(CO)5]-, cis-[HW(CO)4P-
(OMe)3]-, and cis-[HW(CO)4(PPh3)]- have been measured in
acetonitrile, and the reactions of these complexes with [Cp*Re-
(PMe3)(NO)(CO)]+ have been studied. The thermodynamic
hydride donor abilities of these tungsten complexes parallel the
(52) Larsen, A. S.; Wang, K.; Lockwood, M. A.; Rice, G. L.; Won, T.-J.; Lovell,
S.; Sad´ılek, M.; Turcek, F.; Mayer, J. M. J. Am. Chem. Soc. 2002, 124,
10112.
JA036524R
9
12236 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003