Dihydrogen complexes of electrophilic metal centers: Observation of Cr(CO)5(H2), W(CO)5(H2) and [Re(CO)5(H2)]+

Article abstract of DOI:10.1021/ja0433370

Photolysis of dichloromethane solutions of M(CO)₆ (M = Cr, W) at low temperature in the presence of hydrogen gas affords W(CO)₅(H₂) (1) and Cr(CO)₅(H₂) (2). Complexes 1 and 2 are characterized as dihydrogen complexes based on short T₁ values for the hydride resonances and a large HD coupling of 35.3 Hz (W) and 35.8 Hz (Cr) in the HD derivatives. A cationic analogue, [Re(CO)₅(H₂)]⁺ (3), was prepared by reaction of Re(CO)₅Cl with [Et₃Si][B(C₆F₅)₄] in fluorobenzene under hydrogen. Complex 3-d₁ exhibits J_HD = 33.9 Hz. Complex 3 is strongly acidic, with complete deprotonation by diethyl ether; complexes 1 and 2 are moderately acidic. Deprotonation of 1 is complete in the presence of one equivalent of triethylamine. Copyright

Full text of DOI:10.1021/ja0433370

Published on Web 12/24/2004
Dihydrogen Complexes of Electrophilic Metal Centers: Observation of
Cr(CO)5(H2), W(CO)5(H2) and [Re(CO)5(H2)]+
Steven L. Matthews, Vincent Pons, and D. Michael Heinekey*
Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700
Received November 4, 2004; E-mail: heinekey@chem.washington.edu
The first examples of isolable transition metal dihydrogen
complexes were reported by Kubas and co-workers 20 years ago.1
These complexes are exemplified by tungsten complexes such as
i
W(CO)3(PR3)2(H2) (R ) Pr; Cy) and their Mo analogues, which
bind hydrogen relatively weakly and are prepared by displacement
of an agostic interaction with H2 gas. Subsequently, a Cr analogue
was reported, which binds hydrogen more weakly.2 Relatively weak
binding of H2 in these complexes is signaled by only slight
elongation of the H-H distance (dHH) compared to H2 gas. Values
of dHH ) 0.85(1) Å (Cr)2, 0.87(1) Å (Mo)3, and 0.89(1) Å (W)3 for
these complexes have been determined by solid-state NMR
measurements of dipolar coupling. A closely related class of
dihydrogen complexes consists of molecules that are not isolable
but have been studied in detail under matrix isolation conditions,4
in liquid xenon,5 or by fast spectroscopy methods.6 For example,
the prototypical molecules M(CO)5(H2) (M ) Cr, Mo, and W) have
been observed in liquid Xe and in supercritical Xe, and are
definitively characterized as dihydrogen complexes based on
infrared spectroscopy, including studies of isotopically substituted
Figure 1. Partial (hydride region) 1H NMR spectra (CD2Cl2, 500 MHz,
derivatives. While vibrational data such as the detection in the
240 K ) of 1 (bottom) and 1-d1(top).
infrared spectrum of the H-H stretching mode can confirm the
presence of a dihydrogen ligand, quantitative structural information
is lacking. The key structural parameter is the H-H distance (dHH),
which is best determined by diffraction or NMR methods. Since
-3.88 ppm with JHW ≈ 35 Hz. Measurement of the relaxation time
T1 of this resonance at temperatures between 190 and 220 K gives
values between 25 and 60 ms, respectively (500 MHz). The
the limited stability of these complexes precludes isolation, solution
relaxation time was still decreasing at the lowest temperature
NMR spectroscopy becomes the method of choice. Hydrogen
accessible, and the maximum rate of relaxation could not be
complexes with only CO as co-ligands are of fundamental interest
measured. Although a quantitative analysis of these data is not
in that these very electrophilic metal centers may bind hydrogen
possible, these observations are suggestive of a dihydrogen structure
relatively weakly but provide substantial activation of the H-H
bond toward heterolysis. These simple molecules have also been
the focus of significant computational effort. The best computational
results give dHH of ca. 0.8 Å for M(CO)5(H2), slightly shorter than
that for the computed structures for M(CO)3(PH3)2(H2), which are
models for the isolable phosphine-containing complexes. In this
work, we present investigations of the structure and reactivity of
such complexes in conventional solvents using NMR spectroscopy,
which allows for a direct experimental test of the computed
structures.
for complex 1.7 This was confirmed by carrying out the photolysis
reaction under 15 psi of HD gas, which led to the spectrum shown
in Figure 1. Complex 1 is formulated as a dihydrogen complex
W(CO)5(H2).8 The measured value of JHD is 35.3 Hz, which is
consistent with dHH) 0.85 Å.9 This coupling can be compared to
the value of 34.0 Hz (dHH ) 0.87 Å) reported for W(CO)3(PiPr3)2-
(HD).10 Measurement of dHH by determination of the dipolar
coupling in the bound H2 ligand of W(CO)3(PCy3)2(H2) by solid-
state NMR gives a value of 0.890(6) Å.3 The HD coupling data
for complex 1 indicates that the interaction of H2 with the metal
center results in less activation of H2 than in the original Kubas
complex. Due to the narrower line width exhibited by the hydride
signal in 1-d1, it was also possible to accurately measure JHW
)
38.2 Hz (Figure 1), which is larger than the value of JHW ) 33.6
Hz reported for W(CO)3(PiPr3)2(H2).10
The chromium analogue Cr(CO)5(H2) (2) has been similarly
prepared and exhibits a single hydride resonance at δ ) -7.45
ppm. When the preparative reaction was carried out with HD gas,
complex 2-d1 is obtained, with JHD ) 35.8 Hz, only slightly different
from that observed in 1-d1.This is the largest value for JHD reported
to date, corresponding to dHH ) 0.84 Å, which is the shortest value
of dHH thus far observed for a dihydrogen complex. This observation
Photolysis (Hg lamp, 195 K) of W(CO)6 solutions (CD2Cl2, 5
mM) under 20 psi of hydrogen gas affords a thermally labile
dihydrogen complex (1). The 1H NMR spectrum of 1 in the hydride
region is shown in Figure 1. A single resonance is observed at δ )
9
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J. AM. CHEM. SOC. 2005, 127, 850-851
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C O M M U N I C A T I O N S
not affect dHH. This confirms the crucial role of the trans CO ligand
in controlling the degree of H2 activation, as noted previously by
Kubas.19
The acidity of complex 3 is greater than that of the neutral
tungsten analogue. In contrast to complexes 1 and 2, isotopically
pure 3-d1 could not be cleanly prepared and invariably contained
significant amounts of 3 (Figure 2). We attribute this to the greater
acidity of 3 and surmise that 3 arises from 3-d1 by intermolecular
proton (deuteron) exchange catalyzed by traces of adventitious bases
such as water, as previously demonstrated for cationic dihydrogen
complexes of Re.20 This acidity was confirmed by reaction of 3
with excess Et2O, which led to complete deprotonation to the neutral
Re(CO)5H.
The preparation of these simple dihydrogen complexes demon-
strates that binding of hydrogen to very electrophilic metal centers
leads to dramatic activation of the bound hydrogen toward
heterolysis. This is despite the H-H distance becoming only slightly
elongated in comparison to the distance in free hydrogen gas,
suggesting a relatively weak interaction with the metal center. We
are continuing to investigate the preparation and reactivity of σ-bond
complexes of highly electrophilic metal centers.
1
Figure 2. Partial (hydride region) H NMR spectrum (fluorobenzene-d5,
500 MHz, 260 K ) of 3-d1. The spectrum shown was obtained using a 180-
τ-90 pulse sequence with τ ) 16 ms.
is consistent with a weak interaction of H2 with this highly
electrophilic metal center. Complex 2 had been previously prepared
by photolyis of alkane solutions of Cr(CO)6 under H2 pressure and
found to decay with a half-life of ca. 25 s at ambient temperature.11
The experimentally determined values of dHH for complexes 1
and 2 can be compared to the values reported from computational
studies. A study using DFT methods gave values for dHH of ca.
0.81-0.83 Å for M(CO)3(PH3)2(H2), which are computational
models for the Cr, Mo, and W phosphine complexes of Kubas.12
Using the same methodology, dHH of ca. 0.79-0.80 Å was reported
for M(CO)5(H2), slightly shorter than the values for the phosphine
complexes, as expected for less basic metal centers. The calculations
seem to significantly underestimate dHH in the complexes of Kubas
and also in the new complexes 1 and 2.
The bound H2 ligand in complex 1 is significantly activated with
respect to heterolytic cleavage, with deprotonation to the known13
anion [HW(CO)5]- by one equivalent of Et3N or excess (10 equiv)
of H2O. As expected, complex 1 is significantly more acidic than
the phosphine-containing analogues reported by Kubas and co-
workers.14
Acknowledgment. This research was supported by the National
Science Foundation.
References
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(CO)5Cl with [Et3Si][B(C6F5)4]17 in fluorobenzene solution under
H2 gas. An immediate reaction occurs to give a new species 3 with
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generated by reaction of Re(CO)5H with excess triflic acid. A
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(8) We had previously reported that similar experimental conditions led to a
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107, 7000-7006), but subsequent higher resolution data gives JHD ) 34.0
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,
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are identical. Replacement of a PPh3 ligand with a CO presumably
leads to much reduced electron density at the metal center but does
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