Synthesis and structure of CpMo(CO)(dppe)H and its oxidation by Ph 3C+

Article abstract of DOI:10.1021/ic060111k

The reaction of CpMo(CO)(dppe)Cl (dppe = Ph₂PCH ₂CH₂PPh₂) with Na⁺[AlH ₂(OCH₂CH₂OCH₃)₂] ^- gives the molybdenum hydride complex CpMo(CO

Full text of DOI:10.1021/ic060111k

Inorg. Chem. 2006, 45, 4712−4720
Synthesis and Structure of CpMo(CO)(dppe)H and
+
Its Oxidation by Ph3C
Tan-Yun Cheng, David J. Szalda,† Jie Zhang, and R. Morris Bullock*
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000
Received January 19, 2006
The reaction of CpMo(CO)(dppe)Cl (dppe
molybdenum hydride complex CpMo(CO)(dppe)H, the structure of which was determined by X-ray crystallography.
)
Ph2PCH2CH2PPh2) with Na+[AlH2(OCH2CH2OCH3)2]- gives the
Electrochemical oxidation of CpMo(CO)(dppe)H in CH3CN is quasi-reversible, with the peak potential at
−0.15 V
(vs Fc/Fc+). The reaction of CpMo(CO)(dppe)H with 1 equiv of Ph3C+BF4 in CD3CN gives [CpMo(CO)(dppe)-
(NCCD3)]+ as the organometallic product, along with dihydrogen and Gomberg’s dimer (which is formed by dimerization
of Ph3C•). The proposed mechanism involves one-electron oxidation of CpMo(CO)(dppe)H by Ph3C+ to give the
radical-cation complex [CpMo(CO)(dppe)H]•+. Proton transfer from [CpMo(CO)(dppe)H]•+ to CpMo(CO)(dppe)H,
loss of dihydrogen from [CpMo(CO)(dppe)(H)2]+, and oxidation of Cp(CO)(dppe)Mo• by Ph3C+ lead to the observed
products. In the presence of an amine base, the stoichiometry changes, with 2 equiv of Ph3C+ being required for
each 1 equiv of CpMo(CO)(dppe)H because of deprotonation of [CpMo(CO)(dppe)H]•+ by the amine. Protonation
of CpMo(CO)(dppe)H by HOTf provides the dihydride complex [CpMo(CO)(dppe)(H)2]+OTf-, which loses dihydrogen
to generate CpMo(CO)(dppe)(OTf).
-
Introduction
ionic hydrogenation of ketones.11 Knowledge of the factors
that influence the hydride transfer ability of metal hydrides
is useful in the design of such reactions in stoichiometric or
catalytic cycles. DuBois and co-workers have reported
extensive studies on the thermodynamics of hydride transfer
from a series of metal hydrides.12 We studied the kinetics
of hydride transfer for a series of metal carbonyl hydrides
Transition-metal hydrides are involved in numerous ho-
mogeneously catalyzed reactions, performing the critical role
of delivering hydrogen to organic substrates from a metal
complex. Metal hydrides exhibit versatile reactivity patterns,
undergoing cleavage of the M-H bond in the form of a
proton (H+),1 a hydrogen atom (H•),2,3 or a hydride (H-).4
Hydride transfers from metal hydrides to cationic organic
compounds constitute a key step in stoichiometric ionic
hydrogenations5 of alkenes,6 alkynes,7 ketones,8 acetals,9 and
acyl chlorides,10 as well as the analogous steps in the catalytic
-
in their reaction with Ph3C+BF4 (eq 1).13,14
kH
-
M-H + Ph3C+BF4
-8 M-F-BF3 + Ph3C-H (1)
The rate constants for kinetic hydricity in CH2Cl2 at 25 °C
* To whom correspondence should be addressed. E-mail: bullock@bnl.gov.
† Research Collaborator at Brookhaven National Laboratory. Permanent
address: Department of Natural Sciences, Baruch College, New York, NY
10010.
(6) (a) Bullock, R. M.; Rappoli, B. J. J. Chem. Soc., Chem. Commun.
1989, 1447-1448. (b) Bullock, R. M.; Song, J.-S. J. Am. Chem. Soc.
1994, 116, 8602-8612.
(7) Luan, L.; Song, J.-S.; Bullock, R. M. J. Org. Chem. 1995, 60, 7170-
(1) For a review of proton-transfer reactions of metal hydrides, see:
Kristja´nsdo´ttir, S. S.; Norton, J. R. In Transition Metal Hydrides;
Dedieu, A., Ed.; VCH: New York, 1991; Chapter 9, pp 309-359.
(2) For a review of hydrogen-atom transfer reactions of metal hydrides,
see: Eisenberg, D. C.; Norton, J. R. Isr. J. Chem. 1991, 31, 55-66.
(3) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J.
Am. Chem. Soc. 1991, 113, 4888-4895.
(4) For a review of nucleophilic reactions of metal hydrides, see: Labinger,
J. A. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH: New York,
1991; Chapter 10, pp 361-379.
7176.
(8) (a) Gaus, P. L.; Kao, S. C.; Youngdahl, K.; Darensbourg, M. Y. J.
Am. Chem. Soc. 1985, 107, 2428-2434. (b) Song, J.-S.; Szalda, D.
J.; Bullock, R. M.; Lawrie, C. J. C.; Rodkin, M. A.; Norton, J. R.
Angew. Chem., Int. Ed. Engl. 1992, 31, 1233-1235. (c) Song, J.-S.;
Szalda, D. J.; Bullock, R. M. Organometallics 2001, 20, 3337-3346.
(9) Song, J.-S.; Szalda, D. J.; Bullock, R. M. J. Am. Chem. Soc. 1996,
118, 11134-11141.
(10) Song, J.-S.; Szalda, D. J.; Bullock, R. M. Inorg. Chim. Acta 1997,
259, 161-172.
(5) For reviews of ionic hydrogenations using metal hydrides, see: (a)
Bullock, R. M. Chem.sEur. J. 2004, 10, 2366-2374. (b) Bullock, R.
M. In The Handbook of Homogeneous Hydrogenations: de Vries, J.
G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2006; in
press.
(11) (a) Tooley, P. A.; Ovalles, C.; Kao, S. C.; Darensbourg, D. J.;
Darensbourg, M. Y. J. Am. Chem. Soc. 1986, 108, 5465-5470. (b)
Bullock, R. M.; Voges, M. H. J. Am. Chem. Soc. 2000, 122, 12594-
12595. (c) Voges, M. H.; Bullock, R. M. J. Chem. Soc., Dalton Trans.
2002, 759-770.
4712 Inorganic Chemistry, Vol. 45, No. 12, 2006
10.1021/ic060111k CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/16/2006

Synthesis and Structure of CpMo(CO)(dppe)H
spanned a range of over 106. Replacement of one CO with
a phosphine results in a substantially increased kinetic
hydricity because of the strong electronic effect of the
-
phosphine. The rate constant kH for hydride transfer from
2
Cp(CO)3MoH was kH ) 3.8 × 10 M-1 s-1, while the rate
-
constant for hydride transfer from trans-Cp(CO)2(PPh3)MoH
was about 1000 times higher, kH ) 5.7 × 10 M-1 s-1. These
phosphine-substituted hydrides Cp(CO)2(PR3)MoH exist as
interconverting mixtures of cis/trans isomers, and the trans
isomers were much more reactive than the cis isomers, which
have the hydride cis to the phosphine. The rate constant for
hydride transfer from trans-Cp(CO)2(PCy3)MoH is about 3
orders of magnitude higher than that of cis-Cp(CO)2(PCy3)-
MoH. This indicates that a phosphine cis to a hydride exerts
a substantial steric effect that decreases the rate of hydride
transfer, compared to the electronic effect of phosphines,
which greatly enhances the rate of hydride transfer. Evalu-
ation of our data suggests13 that hydride transfer from metal
5
-
tried proved less satisfactory. Substitution of one CO of
CpMo(CO)3H by PR3 has long been known to provide a good
synthetic route to Cp(CO)2(PR3)MoH for PPh3,17 PMe3,18 and
PCy3.13 These ligand substitution reactions were shown by
Brown and co-workers to proceed by radical chain mecha-
nisms.19 Our attempts to use this type of reaction to
synthesize CpMo(CO)(dppe)H were unsuccessful. The pre-
dominant product appears to be CpMo(CO)2(η1-dppe)H in
which only one “arm” of the diphosphine is coordinated,
but other products were also observed, and it became clear
that this route was not attractive for the synthesis of CpMo-
(CO)(dppe)H. CpMo(CO)(dppe)Cl did not appear to react
at all with LiBH4 (60 °C, 1 h). The stronger hydride donor
LiBHEt3 did convert CpMo(CO)(dppe)Cl to CpMo(CO)-
(dppe)H, but other products were formed, including free
dppe. Reduction of CpMo(CO)(dppe)Cl with NaK in THF
gave evidence for the molybdenum anion K+[CpMo-
(CO)(dppe)]- [ν(CO) 1793 (s) cm-1] and protonation with
EtOH generated CpMo(CO)(dppe)H, but neither the yield
nor the purity was as high as that obtained using Na+[AlH2-
(OCH2CH2OCH3)2]-.
Although there is evidence for protonation of the anion
[CpMo(CO)(dppe)]- to give the metal hydride, we have not
attempted to determine the pKa of CpMo(CO)(dppe)H. This
hydride would likely have a very low acidity, considering
the presence of a diphosphine ligand as an electron donor
that would lower the stability of the metal anion. A rough
estimate of the acidity of CpMo(CO)(dppe)H is pKa > 25
in CH3CN, based on the pKa of CpMo(CO)3H (13.9 in CH3-
CN)1,20 and the substantially lower acidity of Cp(CO)2-
(PMe3)WH (pKa ) 26.6 in CH3CN) compared to that of
CpW(CO)3H (pKa ) 16.1 in CH3CN).20
-
hydrides to Ph3C+BF4 proceeds by a single-step hydride
transfer rather than an initial electron transfer followed by
hydrogen-atom transfer. Smith et al. also found single-step
hydride transfer from Cp(CO)2(PPh3)MoH to protonated
acetone.15
These studies have shown that a phosphine can have a
dramatic effect on the reactivity of metal hydrides and that
both steric and electronic factors influence the reactivity. In
this paper, we report the synthesis and structure of CpMo-
(CO)(dppe)H (dppe ) Ph2PCH2CH2PPh2), its electrochemi-
cal oxidation, its reaction with Ph3C+, and its protonation
by acid to give a dihydride complex. The chelating diphos-
phine in this complex places the hydride cis to one phosphine
and trans to the other. We report evidence that the reaction
of this metal hydride complex with Ph3C+BF4- proceeds by
an initial electron-transfer reaction rather than single-step
hydride transfer.
Results and Discussion
Synthesis and Characterization of CpMo(CO)(dppe)H.
The reaction of CpMo(CO)(dppe)Cl16 with Na+[AlH2(OCH2-
CH2OCH3)2]- led to replacement of the chloride with hy-
dride (eq 2), and CpMo(CO)(dppe)H was isolated in 64%
yield. Other potential routes to CpMo(CO)(dppe)H that were
The 31P{1H} NMR spectrum of CpMo(CO)(dppe)H at -87
°C appears as a quartet due to the two phosphorus atoms
being nonequivalent. This pattern, which was verified by
computer simulation, indicates that the two phosphorus atoms
of the diphosphine are at slightly different chemical shifts
(δ 95.0 and 95.6) with 2JPP ) 38 Hz. The hydride resonance
(12) (a) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc.
1999, 121, 11432-11447. (b) Price, A. J.; Ciancanelli, R.; Noll, B.
C.; Curtis, C. J.; DuBois, D. L.; DuBois, M. R. Organometallics 2002,
21, 4833-4839. (c) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois,
D. L. J. Am. Chem. Soc. 2002, 124, 1918-1925. (d) Ciancanelli, R.;
Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc. 2002,
2984-2992. (e) Ellis, W. W.; Ciancanelli, R.; Miller, S. M.; Raebiger,
J. W.; DuBois, M. R.; DuBois, D. L. J. Am. Chem. Soc. 2003, 125,
12230-12236. (f) Raebiger, J. W.; Miedaner, A.; Curtis, C. J.; Miller,
S. M.; Anderson, O. P.; DuBois, D. L. J. Am. Chem. Soc. 2004, 126,
5502-5514. (g) Ellis, W. W.; Raebiger, J. W.; Curtis, C. J.; Bruno,
J. W.; DuBois, D. L. J. Am. Chem. Soc. 2004, 126, 2738-2743.
(13) Cheng, T.-Y.; Brunschwig, B. S.; Bullock, R. M. J. Am. Chem. Soc.
1998, 120, 13121-13137.
(17) Bainbridge, A.; Craig, P. J.; Green, M. J. Chem. Soc. A 1968, 2715-
2718.
(18) Kalck, P.; Pince, R.; Poilblanc, R.; Roussel, J. J. Organomet. Chem.
1970, 24, 445-452.
(19) (a) Byers, B. H.; Brown, T. L. J. Am. Chem. Soc. 1975, 97, 947-
948. Hoffman, N. W.; Brown, T. L. Inorg. Chem. 1978, 17, 613-
617. (b) Brown, T. L. In Organometallic Radical Processes; Trogler,
W. C., Ed.; Elsevier: Lucerne, 1990; Chapter 3, pp 67-107.
(20) Faller, J. W.; Anderson, A. S. J. Am. Chem. Soc. 1970, 92, 5852-
5860.
(14) (a) Cheng, T.-Y.; Bullock, R. M. J. Am. Chem. Soc. 1999, 121, 3150-
3155. (b) Cheng, T.-Y.; Bullock, R. M. Organometallics 2002, 21,
2325-2331.
(15) Smith, K.-T.; Norton, J. R.; Tilset, M. Organometallics 1996, 15,
4515-4520.
(16) Sta¨rker, K.; Curtis, M. D. Inorg. Chem. 1985, 24, 3006-3010.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4713

Cheng et al.
Table 1. Crystallographic Data and Refinement Information
formula
fw
temp (K)
cryst syst
space group
a (Å)
C39H38MoOP2
680.57
293(2)
triclinic
P1h
10.830(3)
11.307(3)
14.309(4)
96.65(2)
105.50(2)
96.43(2)
1658.3(8)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
m (mm-1
λ (Å)
)
0.522
0.710 73
1.363
F
calc (g cm-3
)
total no. of reflns
7590
no. of param
397
Figure 1. 1H NMR spectrum of the Mo-H resonance of CpMo(CO)-
(dppe)H at -87 °C. The upper spectrum is the experimental spectrum, and
the lower trace is the simulated spectrum.
final R indices [I > 2σ(I)]a
R indices (all data)a
R1 ) 0.0579, wR2 ) 0.1196
R1 ) 0.2393, wR2 ) 0.1578
2
2 2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|; wR2 ) {Σ[w(|Fo | - |Fc2|)2]/Σ[w|Fo | ]}1/2
.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
CpMo(CO)(dppe)H
Mo-C(1)
Mo-P(1)
Mo-P(2)
1.916(8)
2.3923(18)
2.4161(17)
P(1)-Mo-P(2)
P(2)-Mo-C(1)
P(1)-Mo-C(1)
78.4(1)
81.3(2)
100.23(19)
and refinement. The crystal has one molecule of toluene (not
shown in Figure 2). Selected bond distances and angles are
provided in Table 2.
The P(1)-Mo-P(2) angle of 78.4(1)° in CpMo(CO)-
(dppe)H is slightly larger than the P-Mo-P angle of 75.3-
(1)° found in the chloride complex CpMo(CO)(dppe)Cl.22
Poli and co-workers found23 a larger P-Mo-P angle of
80.60(3)° in Cp*Mo(dppe)Cl2, a 17-electron Mo complex
that has an unusual trans geometry of the two phosphine
ligands. The molybdenum trihydride complex Cp*Mo(dppe)-
H3 was found24 to have a P-Mo-P angle of 81.21(4)°.
Electrochemical Oxidation of CpMo(CO)(dppe)H. Elec-
trochemical oxidation of CpMo(CO)(dppe)H was studied by
Figure 2. Molecular structure of CpMo(CO)(dppe)H showing 30%
probability ellipsoids.
1
in the H NMR spectrum at -87 °C exhibits the pattern
shown in the upper part of Figure 1. This spectrum was also
simulated (lower part of Figure 1), providing values of 2JPH
) 16 Hz for the coupling of the hydride to the trans
phosphine and 2JPH ) 69 Hz for the coupling of the hydride
to the cis phosphine. The magnitude of these coupling
constants is comparable to those directly observed in
interconverting cis/trans isomer mixtures of related phosphine
hydride complexes CpMo(CO)2(PPh3)H (2JPH ) 21 Hz for
trans; 2JPH ) 64 Hz for cis)21 and CpMo(CO)2(PCy3)H (2JPH
-
cyclic voltammetry in CH3CN with a 0.1 M Bu4N+PF6
electrolyte. The peak potential for the oxidation wave was
found at -0.15 V (vs the Fc/Fc+ couple); the cyclic
voltammetry trace is shown in the Supporting Information.
The oxidation is only quasi-reversible and does not provide
a reliable value for E1/2. For comparison, Tilset and co-
workers reported that the peak potential for oxidation of
CpMo(CO)3H was found at Eox ) +0.800 V (vs Fc/Fc+).25
As expected, substitution of one CO by a phosphine makes
the metal hydride much more readily oxidized; electrochemi-
cal oxidation of CpMo(CO)2(PPh3)H was reported at +0.26
V (vs Fc/Fc+) by Smith and Tilset.26 Poli and co-workers
reported27 +0.23 V (vs Fc/Fc+) for the anodic peak potential
of CpMo(CO)2(PPh3)H, in good agreement with Tilset’s
2
) 20 Hz for trans; JPH ) 60 Hz for cis).13
The IR spectrum of CpMo(CO)(dppe)H exhibits one ν-
(CO) band. The location of ν(CO) often varies somewhat
with different solvents, but we found an unusually large
solvent dependence on the energy of the ν(CO) band of
CpMo(CO)(dppe)H, ranging from 1859 cm-1 in hexane, to
1840 cm-1 in toluene, to 1821 cm-1 in CH2Cl2. We were
unable to find a band that could be confidently assigned to
the ν(M-H) stretching vibration, even in concentrated
solutions.
Structural Determination of CpMo(CO)(dppe)H. The
structure of CpMo(CO)(dppe)H was determined by single-
crystal X-ray diffraction. An ORTEP diagram is shown in
Figure 2, and Table 1 gives details of the data collection
(22) Bush, M. A.; Hardy, A. D. U.; Manojlovic-Muir, L.; Sim, G. A. J.
Chem. Soc. A 1971, 1003-1009.
(23) Fettinger, J. C.; Keogh, D. W.; Pleune, B.; Poli, R. Inorg. Chim. Acta
1997, 261, 1-5.
(24) Pleune, B.; Poli, R.; Fettinger, J. C. Organometallics 1997, 16, 1581-
1594.
(21) Moore, E. J.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1986,
108, 2257-2263.
(25) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112,
2618-2626.
4714 Inorganic Chemistry, Vol. 45, No. 12, 2006

Synthesis and Structure of CpMo(CO)(dppe)H
Scheme 1
value. Substitution of another CO by a phosphine also has a
large effect: our value for the oxidation potential of CpMo-
(CO)(dppe)H shows that it is easier to oxidize (by roughly
0.4 V) than CpMo(CO)2(PPh3)H. As expected, substitution
of the last CO for a phosphine produces a metal hydride
that is very readily oxidized: Poli and co-workers found that
electrochemical oxidation of CpMo(PMe3)3H was reversible,
with E1/2 ) -1.46 V (vs Fc/Fc+).28
Reaction of CpMo(CO)(dppe)H with Ph3C+. We previ-
ously found that a variety of metal carbonyl hydrides cleanly
-
We propose the mechanism shown in Scheme 1, which
begins with oxidation of CpMo(CO)(dppe)H by Ph3C+BF4 .
transfer hydride to Ph3C+BF4 in CH2Cl2, producing Ph3-
-
CH and metal complexes with weakly bound FBF3- ligands
-
Although Cp(CO)2(PPh3)MoH and other metal hydrides have
been shown13 to transfer hydride to Ph3C+ or to protonated
acetone15 in a single step, the second phosphine ligand in
dppe makes CpMo(CO)(dppe)H much easier to oxidize, so
one-electron oxidation of the metal hydride by Ph3C+ occurs
rather than direct hydride transfer. The electrochemical
(eq 1).13,14 Reaction of CpMo(CO)(dppe)H with Ph3C+BF4
in CD2Cl2 gave several organometallic products, as indicated
1
by H and 31P NMR. The organic products of this reaction
were Ph3CH (49%, based on NMR integration versus an
internal standard) and Gomberg’s dimer (0.20 equiv; 40%
yield of Ph3C•). The formation of Ph3CH indicates that
hydride is transferred from the metal to Ph3C+, and this
product was expected because many metal hydrides react
cleanly according to eq 1. Gomberg’s dimer,29 shown in eq
3, is formed through dimerization of two Ph3C• radicals.
Formation of Gomberg’s dimer implies that Ph3C• was
formed by one-electron oxidation of CpMo(CO)(dppe)H by
Ph3C+.
-
reduction of Ph3C+PF6 by cyclic voltammetry in CH3CN
studied was reported to be partially reversible with E1/2
≈
+0.26 V (vs SCE),30 in agreement with an earlier report of
E1/2 ) +0.27 V (vs SCE) determined by polarography.31
Conversion to a Fc/Fc+ reference (by subtraction of 380
mV)32 gives an estimate of E1/2 ≈ -0.12 V (vs Fc/Fc+) for
the reduction potential of Ph3C+. A comparison of this value
to the electrochemical oxidation potentials for molybdenum
hydrides discussed above reveals that while oxidation of Cp-
(CO)2(PPh3)MoH by Ph3C+ is thermodynamically unfavor-
able by about 0.37 V, one-electron transfer to Ph3C+ from
CpMo(CO)(dppe)H is roughly thermoneutral.
Because a mixture of products were formed from reactions
-
of CpMo(CO)(dppe)H with Ph3C+BF4 in CD2Cl2, experi-
Oxidation of metal hydrides is well-known to produce
transient radical cations (MH•+),25,26,33-37 which are generally
very reactive. Studies by Tilset and co-workers have shown
that MH•+ complexes are about 19-22 pKa units more acidic
than the corresponding MH.25,34,35 We suggest that the second
step of the mechanism (Scheme 1) is proton transfer from
ments were conducted in CD3CN because in that case the
solvent could bind to unsaturated molybdenum intermediates.
-
The reaction of CpMo(CO)(dppe)H with Ph3C+BF4 in
CD3CN (eq 4) led to the formation of [CpMo(CO)(dppe)-
(NCCD3)]+[BF4]- (88%) along with dihydrogen. Gomberg’s
dimer was formed in 86% yield [calculated with the
stoichiometry shown in eq 4; i.e., 0.86 equiv of “Ph3C•”
produced for each 1 equiv of CpMo(CO)(dppe)H], with a
smaller amount (13%) of Ph3CH being detected.
(27) Quadrelli, E. A.; Kraatz, H.-B.; Poli, R. Inorg. Chem. 1996, 35, 5154-
5162.
(28) Fettinger, J. C.; Kraatz, H.-B.; Poli, R.; Quadrelli, E. A.; Torralba, R.
C. Organometallics 1998, 17, 5767-5775.
(29) (a) Gomberg, M. J. Am. Chem. Soc. 1900, 22, 757-771. (b) Lankamp,
H.; Nauta, W. T.; MacLean, C. Tetrahedron Lett. 1968, 2, 249-254.
(c) McBride, J. M. Tetrahedron 1974, 30, 2009-2022.
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N. J. Organometallics 1985, 4, 1020-1024.
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1993, 32, 3816-3821.
The reaction was also carried out in CD2Cl2 solvent with
∼6 equiv of CH3CN present. Using those conditions,
[CpMo(CO)(dppe)(NCCH3)]+[BF4]- (79%) was formed,
along with Gomberg’s dimer (66%), Ph3CH (34%), and
dihydrogen.
(34) Ryan, O. B.; Tilset, M.; Parker, V. D. Organometallics 1991, 10, 298-
304.
(35) (a) Tilset, M. J. Am. Chem. Soc. 1992, 114, 2740-2741. (b) Skagestad,
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8681-8689.
(26) Smith, K.-T.; Tilset, M. J. Organomet. Chem. 1992, 431, 55-64.
(37) Pedersen, A.; Tilset, M. Organometallics 1994, 13, 4887-4894.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4715

Cheng et al.
the radical cation [CpMo(CO)(dppe)H]•+ to the neutral
hydride CpMo(CO)(dppe)H, producing the 17-electron mo-
lybdenum radical Cp(CO)(dppe)Mo• and the cationic dihy-
dride [CpMo(CO)(dppe)(H)2]+ (vide infra). Dihydrogen is
observed as a product of decomposition of the dihydride
complex. Oxidation of molybdenum radical Cp(CO)(dppe)-
Mo• by Ph3C+ produces the metal cation, which is trapped
by MeCN to give the observed product, [CpMo(CO)(dppe)-
(NCCH3)]+. Proton transfer from [CpMo(CO)(dppe)H]•+ to
CpMo(CO)(dppe)H may occur by direct metal-to-metal
proton transfer, but it is possible that this reaction could occur
by a more complicated pathway. Tilset and co-workers
carried out detailed electrochemical studies of Cp(PPh3)2-
RuH36 and found that its radical cation [MH]•+ dispropor-
tionates to give the dicationic hydride [MH]2+ and the neutral
hydride MH, with subsequent proton transfer from the highly
acidic [MH]2+ to MH leading to [M]+ and the dihydride
[MH2]+. Further experiments would be required to determine
whether a complex mechanism of this type could be operative
in our Mo example studied here.
In contrast to the high reactivity of radical cations formed
through one-electron oxidation of metal carbonyl hydrides,
Poli and co-workers have found that some 17-electron
molybdenum(III) complexes of metal halides are sufficiently
stable for isolation and complete characterization.38 For
example, CpMo(CO)(dppe)Cl and Cp*Mo(CO)(dppe)Cl are
reversibly oxidized in electrochemical experiments, and a
crystal structure was reported for [Cp*Mo(CO)(dppe)Cl]•+.
Reaction of CpMo(CO)(dppe)H with Ph3C+ with a
Base Present. When the oxidation of CpMo(CO)(dppe)H
by Ph3C+ is carried out in the presence of an amine base, 2
equiv of Ph3C+ are consumed for each 1 equiv of metal
hydride (eq 5). The proposed mechanism is shown in Scheme
Scheme 2
Tilset and co-workers reported several examples in which
oxidation of metal hydrides in the presence of an amine base
required 2 equiv of the oxidizing agent.25,26,34,37 One complex
exhibiting this behavior is the molybdenum complex Cp-
(CO)2(PPh3)MoH, which was shown26 to react with (p-
MeOC6H4)Ph2C+ in CH3CN to give [trans-Cp(CO)2(PPh3)-
Mo(NCCH3)]+ by a single-step hydride transfer. In contrast,
oxidation of Cp(CO)2(PPh3)MoH by Cp2Fe+ in CH3CN in
the presence of an amine base required 2 equiv of Cp2Fe+
and produced [cis-Cp(CO)2(PPh3)Mo(NCCH3)]+. Poli and
co-workers reported similar observations when Cp(CO)2-
(PPh3)MoH was oxidized in the presence of water as a base.27
Reaction of CpMo(CO)(dppe)H with (p-MeOC6H4)3C+.
Substitution of one or more Ph groups on Ph3C+ with an
electron-donating MeO group provides substituted trityl
cations that are far less reactive.39 The rate constant for
hydride transfer (eq 1) from Cp*Mo(CO)3H to Ph3C+ is k
) 6.5 × 103 M-1 s-1 (CH2Cl2 solvent at 25 °C), while the
hydride transfer of the same hydride to (p-MeOC6H4)3C+ is
3 orders of magnitude lower (k ) 1.4 M-1 s-1).13 The
reaction of CpMo(CO)(dppe)H with Ph3C+BF4- in CH3CN
discussed above is complete within minutes, but the reaction
of (p-MeOC6H4)3C+ with CpMo(CO)(dppe)H (eq 6) occurs
much more slowly. The reaction was about 74% complete
after 4 h and over 90% complete after 24 h. In contrast to
the reaction with Ph3C+, which gives Gomberg’s dimer, the
reaction with (p-MeOC6H4)3C+ leads to C-H bond forma-
tion, giving (p-MeOC6H4)3CH. This reaction proceeds by
hydride transfer, as found for other hydrides (eq 1).
Electrochemical studies31,40 show that Ph3C+ is easier to
reduce than (p-MeOC6H4)3C+, by about 0.5 V. We interpret
these results to indicate that the increased barrier for one-
2. In contrast to Scheme 1, where intermolecular proton
transfer from the radical cation [CpMo(CO)(dppe)H]•+ occurs
to the metal hydride as a base, the proton transfer in Scheme
2 occurs to the nitrogen of the amine base, producing a metal
radical Cp(CO)(dppe)Mo• that is subsequently oxidized by
Ph3C+ to give the metal cation.
(38) Fettinger, J. C.; Keogh, D. W.; Poli, R. J. Am. Chem. Soc. 1996, 118,
3617-3625.
(39) (a) Deno, N. C.; Jaruzelski, J. J.; Schriesheim, A. J. Am. Chem. Soc.
1955, 77, 3044-3051. (b) Deno, N. C.; Schriesheim, A. J. Am. Chem.
Soc. 1955, 77, 3051-3054.
(40) Feldman, M.; Flythe, W. C. J. Am. Chem. Soc. 1969, 91, 4577-4578.
4716 Inorganic Chemistry, Vol. 45, No. 12, 2006

Synthesis and Structure of CpMo(CO)(dppe)H
electron oxidation of CpMo(CO)(dppe)H by (p-MeOC6H4)3C+
(compared to oxidation by Ph3C+) changes the mechanism
to hydride transfer. This hydride transfer could occur in a
single step or by oxidation of the metal hydride to give
[CpMo(CO)(dppe)H]•+, which then transfers a hydrogen
atom to (p-MeOC6H4)3C•. Because the para position of (p-
MeOC6H4)3C+ is substituted by a MeO group, it would not
be possible to form an analogue of Gomberg’s dimer in this
reaction.
Attempted Hydrogen-Atom Abstraction from CpMo-
(CO)(dppe)H. Hydrogen-atom transfers from metal hydrides
to organic substrates are involved in certain types of
hydrogenations,41 hydroformylations,42 and polymerizations.43
The kinetics of hydrogen-atom transfer from a series of metal
hydrides to a substituted trityl radical were reported by
Norton and co-workers.3 A metal-metal-bonded complex
forms through dimerization of the metal radical that results
from hydrogen-atom transfer from the metal hydride. Reac-
tion of the molybdenum hydride Cp(CO)2(PPh3)MoH with
Gomberg’s dimer was reported to be complete within 5 min,
providing the Mo-Mo-bonded dimer [Cp(CO)2(PPh3)Mo]2.44
We repeated this reaction and found it to proceed as reported.
In contrast, attempted reaction of Gomberg’s dimer with
CpMo(CO)(dppe)H in CD2Cl2 for 7 days at room temper-
ature gave no evidence for hydrogen-atom transfer; a small
amount of free dppe was observed as a decomposition
product. Similar observations were made when a solution
of CpMo(CO)(dppe)H and Gomberg’s dimer were heated
in C6D6 for 7 days at 80 °C. Thus, we find no evidence for
hydrogen-atom transfer from CpMo(CO)(dppe)H to trityl
radical, even with prolonged heating.
doublets at δ -4.26 (JPH ) 43.0 Hz; JP′H ) 23.0 Hz) is
observed for the dihydride hydrogens at -87 °C. The T1 for
the dihydride resonance at -87 °C was 244 ms, indicating
a dihydride rather than a dihydrogen complex. The dihydride
resonance becomes a triplet (JPH ) 32.7 Hz) at -58 °C. In
the 31P{1H} NMR at -87 °C, two resonances are observed
at δ 69.6 and 68.2 (JPP ) 10.7 Hz) for the inequivalent
phosphines. These resonances broaden with the loss of
observable coupling at -58 °C, and a 31P{1H} NMR
spectrum at -35 °C showed coalescence to a broad singlet.
These data do not definitively indicate the geometry of
[CpMo(CO)(dppe)(H)2]+[OTf]-; the geometry shown in eq
7 is based on the crystal structure of the related dihydride
complex [CpW(CO)2(PMe3)(H)2]+[OTf]-, which has one of
the hydrides trans to the Cp ligand.46 The NMR data indicate
that a fluxional process makes the two phosphorus atoms
equivalent; at all temperatures studied, the two hydrides
remained equivalent.
Protonation of CpMo(CO)(dppe)H with DOTf gives a
doublet of doublets (JPH ) 43.0 Hz; JP′H ) 23.0 Hz) for
[CpMo(CO)(dppe)(H)(D)]+[OTf]- at δ -4.285 at -87 °C,
which differs by -0.030 ppm from the resonance for
[CpMo(CO)(dppe)(H)2]+[OTf]- at δ -4.255. Precedent for
1
an upfield H NMR chemical shift upon partial deuteration
of a dihydride or polyhydride complex comes from several
studies. Crabtree and co-workers found upfield chemical
shifts upon successive replacement of hydrogen by deuterium
in polyhydride complexes.47 For example, 1H NMR spectra
of a series of rhenium heptahydride complexes ReH7L2 (L2
) bidentate phosphine) were compared with those of the
partially labeled complexes ReH7-xDxL2 (x ) 0-6), and they
found that replacing hydrogen by deuterium led to an upfield
shift of -0.002 to -0.009 ppm for each deuterium substi-
tuted. Heinekey and co-workers found larger upfield isotope
effects on chemical shifts (up to -0.075 ppm) when one
hydrogen in the trihydride complex [Cp(PPh3)Ir(H)3]+ was
substituted by deuterium.48 The change in chemical shift upon
deuteration that we observed for [CpMo(CO)(dppe)-
(H)(D)]+[OTf]- is further evidence that the protonated
complex is a dihydride rather than a dihydrogen complex
because M(η2-HD) complexes exhibit a 1:1:1 triplet, which
is often used in the characterization of dihydrogen com-
Protonation of CpMo(CO)(dppe)H. No reaction was
observed by NMR upon the addition of HOAc to CpMo-
(CO)(dppe)H. Addition of the stronger acid HOTf (OTf )
OSO2CF3) to a CD2Cl2 solution of CpMo(CO)(dppe)H at
-87 °C leads to protonation of the metal hydride complex
to give a cationic dihydride complex, [CpMo(CO)(dppe)-
(H)2]+[OTf]-, which is formed in 99% yield (eq 7). It is
(44) Drake, P. R.; Baird, M. C. J. Organomet. Chem. 1989, 363, 131-
136.
(45) (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure,
Theory, and ReactiVity; Kluwer Academic/Plenum Publishers: New
York, 2001. (b) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993,
93, 913-926. (c) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV.
1992, 121, 155-284.
possible that the initial site of protonation is at the metal
hydride bond, initially producing a cationic dihydrogen
complex,45 [CpMo(CO)(dppe)(η2-H2)]+, though no evidence
for a dihydrogen complex was obtained. A doublet of
(46) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996, 15,
2504-2516.
(47) (a) Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 4813-
4821. (b) Hamilton, D. G.; Luo, X.-L.; Crabtree, R. H. Inorg. Chem.
1989, 28, 3198-3203. (c) Luo, X.-L.; Schulte, G. K.; Demou, P.;
Crabtree, R. H. Inorg. Chem. 1990, 29, 4268-4273. (d) Luo, X.-L.;
Baudry, D.; Boydell, P.; Charpin, P.; Nierlich, M.; Ephritikhine, M.;
Crabtree, R. H. Inorg. Chem. 1990, 29, 1511-1517. (e) Luo, X.-L.;
Crabtree, R. H. J. Chem. Soc., Dalton Trans. 1991, 587-590. (f) Luo,
X.-L.; Michos, D.; Crabtree, R. H. Organometallics 1992, 11, 237-
241.
(41) (a) Sweany, R. L.; Halpern, J. J. Am. Chem. Soc. 1977, 99, 8335-
8337. (b) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990,
112, 6886-6898.
(42) Nalesnik, T. E.; Orchin, M. J. Organomet. Chem. 1981, 222, C5-
C8. Klingler, R. J.; Rathke, J. W. J. Am. Chem. Soc. 1994, 116, 4772-
4785.
(43) (a) Gridnev, A. A.; Ittel, S. D. Chem. ReV. 2001, 101, 3611-3659.
(b) Tang, L.; Papish, E. T.; Abramo, G. P.; Norton, J. R.; Baik, M.-
H.; Friesner, R. A.; Rappe´, A. J. Am. Chem. Soc. 2003, 125, 10093-
10102.
(48) (a) Heinekey, D. M.; Millar, J. M.; Koetzle, T. F.; Payne, N. G.; Zilm,
K. W. J. Am. Chem. Soc. 1990, 112, 909-919. (b) Heinekey, D. M.;
Hinkle, A. S.; Close, J. D. J. Am. Chem. Soc. 1996, 118, 5353-5361.
Inorganic Chemistry, Vol. 45, No. 12, 2006 4717

Cheng et al.
plexes.45 No JHD was detected in our experiments on
[CpMo(CO)(dppe)(H)(D)]+[OTf]-. In cases where they can
be measured,49 JHH values are small, usually 5 Hz or less,
so the corresponding JHD might not be easily observed.
While the dihydride can be observed at low temperatures
by NMR, at higher temperatures it decomposes, releasing
dihydrogen and forming the metal triflate complex CpMo-
(CO)(dppe)(OTf) (eq 8). This triflate complex was isolated
as a red solid in 80% yield.
sodium benzophenone, and CH2Cl2 was distilled from P2O5. CpMo-
(CO)(dppe)Cl16 was synthesized by a previously reported route.
HOTf was purified by distillation. Other reagents were obtained
from Aldrich and used without further purification.
Preparation of CpMo(CO)(dppe)H. A 50-mL Schlenk flask
was charged with CpMo(CO)(dppe)Cl (500 mg, 0.803 mmol), and
THF (20 mL) was added. To this red solution was added
Na[AlH2(OCH2CH2OCH3)2] (“Red-Al” obtained from Aldrich; 1.00
mmol, 0.3 mL, of a 65% solution in toluene, diluted to ∼8 mL
with toluene). The reaction mixture was stirred at ambient tem-
perature and gradually turned dark red. An IR spectrum showed
that the reaction was complete after 2.5 h. Ethanol (0.5 mL) was
added with vigorous stirring, and dihydrogen evolution was
observed. The solvent was removed under vacuum from the
yellowish-brown solution, and the residue was extracted with
benzene (15 mL) and filtered through an alumina column (2.5 × 4
cm). The yellow filtrate was concentrated to 5 mL, and hexane
(15 mL) was added. The solution mixture was concentrated and
triturated to give a yellow solid. The product was isolated by
filtration and washed with hexane (300 mg, 64% yield). The product
1
can be recrystallized by slow evaporation from toluene. H NMR
(CD2Cl2, 25 °C): δ 7.75-7.22 (br m, 20 H, Ph), 4.45 (s, 5 H, Cp),
2.74-1.81 (br m, 4 H, CH2), -6.44 (1 H, MoH; see text and Figure
1 for more details; simulations were computed using gNMR). 31P-
{1H} NMR (CD2Cl2, -87 °C): δ 95.3 (apparent quartet, analyzed
and simulated with nonequivalent chemical shifts of δ 95.0 and
Conclusions
-
Oxidation of CpMo(CO)(dppe)H by 1 equiv of Ph3C+BF4
2
95.6; JPP ) 38 Hz). 13C{1H} NMR (CD2Cl2, -35 °C): δ 243.6
in CD3CN gives [CpMo(CO)(dppe)(NCCD3)]+, along with
dihydrogen and Gomberg’s dimer. The proposed mechanism
involves an initial one-electron oxidation of CpMo(CO)-
(dppe)H by Ph3C+. This initial step of electron transfer from
CpMo(CO)(dppe)H contrasts with the reactivity of related
complexes such as Cp(CO)2(PPh3)MoH, which were previ-
ously shown to react with Ph3C+ by single-step hydride
transfer, forming Ph3CH. When the oxidation of CpMo(CO)-
(dppe)H by Ph3C+ is carried out in the presence of an amine
base, 2 equiv of Ph3C+ are consumed for each metal hydride.
The proposed mechanism for this reaction involves proton
transfer from the initially generated radical cation [CpMo-
(CO)(dppe)H]•+ to the amine base. Protonation of CpMo-
(CO)(dppe)H by HOTf produces the dihydride complex
[CpMo(CO)(dppe)(H)2]+OTf-, which is characterized in
solution but decomposes to release dihydrogen and form the
triflate complex CpMo(CO)(dppe)(OTf).
(dd, CO, JCP ) 18.1 Hz, JCP′ ) 4.8 Hz), 132.6 (dd, J ) 10.2 and
9.6 Hz, Ph), 130.8 (d, J ) 9.7 Hz, Ph), 130.2 (d, J ) 9.1 Hz, Ph),
129.4-127.5 (multiple resonances, Ph), 86.1 (s, C5H5), 30.5 (dd,
CH2, JCP ) 28.1 Hz, JCP′ ) 24.6 Hz), 28.7 (t, CH2, JCP ) JCP′
)
19.0 Hz). IR (hexane): ν(CO) 1859 (s) cm-1. IR (toluene): ν(CO)
1841 (s) cm-1. IR (THF): ν(CO) 1840 (s) cm-1. IR (CH2Cl2): ν-
(CO) 1821 (s) cm-1. Anal. Calcd for C32H30OP2Mo: C, 65.31; H,
5.14. Found: C, 65.34; H, 5.49.
Electrochemical Experiments. Cyclic voltammetry was carried
out using a BAS 100B electrochemical analyzer using glassy carbon
as the working electrode, Pt wire as the counter electrode, and Ag/
Ag+ (0.1 M AgNO3) as the reference electrode in a CH3CN solution
containing 0.1 M Bu4N+PF6-. The concentration of the Mo complex
was 0.01 M CpMo(CO)(dppe)H, and sweep rates up to 2 V s-1
were used. The peak potential for the quasi-reversible oxidation
was found at -0.15 V (vs the Fc/Fc+ couple). The potential was
calibrated using the Cp2Fe/Cp2Fe+ (Fc/Fc+) couple. A trace from
the cyclic voltammetry experiment is shown in the Supporting
Information.
Reactions of CpMo(CO)(dppe)H with Ph3C+BF4-. (a) In
CD2Cl2. A 5-mm NMR tube was loaded with CpMo(CO)(dppe)H
(7.4 mg, 0.013 mmol, 1 equiv), CD2Cl2 (∼0.4 mL), and 1,2-
dichloroethane (1.5 µL, internal integration standard). After the
initial spectrum was recorded, this solution was added to a solution
Experimental Section
General Procedures. All manipulations were carried out under
an atmosphere of argon using Schlenk or vacuum-line techniques
or in a Vacuum Atmospheres drybox. 1H NMR chemical shifts were
referenced to the residual proton peak of CD2Cl2 at δ 5.32.
Elemental analyses were carried out by Schwarzkopf Microana-
lytical Laboratory (Woodside, NY). NMR spectra were recorded
on a Bruker AM-300 spectrometer (300 MHz for 1H NMR).
of Ph3C+BF4 (4.2 mg, 0.013 mmol, 1 equiv) in CD2Cl2 (∼0.1
-
mL), and the reaction mixture turned from yellow to orange. The
1H NMR spectrum of the resulting solution showed the formation
of Gomberg’s dimer (40% yield; i.e., 0.40 equiv of “Ph3C•” based
on HMo), Ph3CH (49%), and several organometallic products
(including ∼8% [CpMo(CO)(dppe)(H)2]+[BF4]-).
Ph3C+BF4 was purchased from Aldrich and purified by recrys-
-
tallization from CH2Cl2/Et2O. (p-MeOC6H4)3C+BF4- was prepared
as previously reported.50 Et2O and hexane were distilled from
(b) In CD3CN. The analogous procedure described above was
1
used except CD3CN was the solvent. The H NMR spectrum of
(49) (a) Johnson, C. E.; Fisher, B. J.; Eisenberg, R. J. Am. Chem. Soc.
1983, 105, 7772-7774. (b) Fryzuk, M. D.; MacNeil, P. A. Organo-
metallics 1983, 2, 682-684. (c) Lee, J. C., Jr.; Peris, E.; Rheingold,
A. L.; Crabtree, R. H. J. Am. Chem. Soc. 1994, 116, 11014-11019.
(d) Peris, E.; Lee, J. C., Jr.; Rambo, J. R.; Eisenstein, O.; Crabtree, R.
H. J. Am. Chem. Soc. 1995, 117, 3485-3491.
the resulting solution showed the formation of [CpMo(CO)(dppe)-
(NCCD3)]+[BF4]- (88%), Gomberg’s dimer (0.86 equiv of “Ph3C•”
(50) Olah, G. A.; Svoboda, J. J.; Olah, J. A. Synthesis 1972, 544.
4718 Inorganic Chemistry, Vol. 45, No. 12, 2006

Synthesis and Structure of CpMo(CO)(dppe)H
formed per HMo), and Ph3CH (δ 5.59 for CH resonance, 13%). A
resonance for dihydrogen at δ 4.56 was also observed. IR (CD3-
CN) for [CpMo(CO)(dppe)(CD3CN)]+[BF4]-: ν(CO) 1871 (s)
[OTf]-: δ 70.0 (s). An NMR spectrum recorded after the tube had
been kept at room temperature for 6 h showed that the dihydride
complex, [CpMo(CO)(dppe)(H)2]+[OTf]-, had completely con-
verted to CpMo(CO)(dppe)(OTf).
cm-1
.
(c) In CD2Cl2 with CH3CN Added. CpMo(CO)(dppe)H (6.1
mg, 0.010 mmol) was dissolved in CD2Cl2 (∼0.4 mL) containing
1,2-dichloroethane (0.5 µL, internal integration standard) and CH3-
CN (3 µL, 0.0574 mmol). After measurement of the initial spectrum,
this solution was added to a solution of Ph3C+BF4- (3.5 mg, 0.010
mmol) in CD2Cl2 (∼0.2 mL). The 1H NMR spectrum of the
resulting solution showed the formation of Gomberg’s dimer (0.66
equiv of “Ph3C•” formed per HMo), Ph3CH (34%), and [CpMo(CO)-
(dppe)(NCCH3)]+[BF4]- (86%); a signal for dihydrogen was
observed at δ 4.62. 1H NMR (CD2Cl2, 25 °C) of [CpMo(CO)(dppe)-
(NCCH3)]+[BF4]-: δ 4.98 (d, 5 H, Cp, JPH ) 2.0 Hz), 3.01-2.18
(br m, 4 H, CH2), 1.79 (d, 3 H, CH3, JPH ) 2.4 Hz). 31P{1H} NMR
(CD2Cl2, 25 °C) of [CpMo(CO)(dppe)(NCCH3)]+[BF4]-: δ 85.8
(d, JPP ) 37.7 Hz), 74.9 (d, JPP ) 37.7 Hz).
Preparation of CpMo(CO)(dppe)OTf. A two-neck, 25-mL
round-bottomed flask attached to a frit assembly was charged with
CpMo(CO)(dppe)H (100 mg, 0.17 mmol), and CH2Cl2 (10 mL)
was vacuum transferred into this flask. HOTf (14 µL, 0.16 mmol)
was added at room temperature through a syringe (this reaction is
carried out with a slight molar deficiency of HOTf because excess
HOTf causes a further reaction that is being investigated). The
reaction mixture gradually turned from yellow to red. After 30 min,
the solvent was reduced to 2 mL and Et2O (∼3 mL) was added,
followed by hexane (10 mL), to give a red precipitate. The product
was collected by filtration and dried under vacuum (yield 93 mg,
1
0.13 mmol, 80% based on HOTf). H NMR (CD2Cl2, 25 °C): δ
7.69-7.26 (m, 20 H, Ph), 4.66 (d, 5 H, Cp, JPH ) 2.5 Hz), 3.20-
2.78 (m, 2 H, CH2), 2.41-2.20 (m, 1 H, CH2), 1.80-1.58 (m, 1 H,
CH2). 31P{1H} NMR (CD2Cl2, 25 °C): δ 90.7 (d, JPP ) 37.0 Hz),
68.3 (d, JPP ) 37.0 Hz). IR (CH2Cl2): ν(CO) 1858 cm-1. Anal.
Calcd for C33H29F3O4P2SMo: C, 53.81; H, 3.97. Found: C, 53.83;
H, 4.20.
-
Reaction of CpMo(CO)(dppe)H with 2 equiv of Ph3C+BF4
in the Presence of 2,6-Di-tert-butyl-4-methylpyridine. CpMo-
(CO)(dppe)H (5.0 mg, 0.0085 mmol, 1 equiv) and 2,6-di-tert-butyl-
4-methylpyridine (2.0 mg, 0.010 mmol, 1.2 equiv) were placed in
an NMR tube, along with CD3CN (0.4 mL) and 1,2-dichloroethane
(1 µL, internal integration standard). The initial 1H NMR spectrum
Protonation of CpMo(CO)(dppe)H To Give [CpMo(CO)-
(dppe)(H)2]+[OTf-] at Low Temperature. HMo(CO)(dppe)Cp
(16.0 mg, 0.027 mmol, 1 equiv) was weighed in a screw-capped
NMR tube along with CD2Cl2 (∼0.6 mL) and 1,2-dichloroethane
(3 µL, internal integration standard). The initial spectrum was
recorded at -87 °C. The solution was cooled to -78 °C, and HOTf
(4 µL, 0.045 mmol, 1.7 equiv) was added. The tube was shaken
and then quickly inserted into a precooled NMR probe (-87 °C).
The NMR spectrum of the resulting solution showed the formation
1
was recorded. H NMR (CD3CN, 25 °C) of 2,6-di-tert-butyl-4-
methylpyridine: δ 7.01 (s, 2 H, CH), 2.28 (s, 3 H, CH3), 1.30 (s,
18 H, CCH3). This solution was mixed with a solution of
Ph3C+BF4- (5.6 mg, 0.017 mmol, 2 equiv) in CD3CN (∼0.3 mL).
1
The reaction was complete in less than 10 min. The H NMR
spectrum showed the formation of [CpMo(CO)(dppe)-
(NCCD3)]+[BF4]- (89%), Gomberg’s dimer (1.82 equiv of “Ph3C•”
formed per HMo), Ph3CH (10%), and protonated 2,6-di-tert-butyl-
1
of [CpMo(CO)(dppe)(H)2]+[OTf]- (99% yield). H NMR (CD2-
1
4-methylpyridine (89%). H NMR (CD3CN, 25 °C) of protonated
Cl2, -87 °C): δ 8.0-7.4 (m, 20 H, Ph), 4.51 (s, 5 H, Cp), 3.0-
2,6-di-tert-butyl-4-methylpyridine: 2.59 (s, 3 H, CH3), 1.49 (s, 18
1.9 (m, 4H, CH2), -4.26 (dd, 2 H, MoH, JPH ) 43.0 Hz, JP′H )
H, CCH3), signals for CH obscured by the phenyl region.
Reaction of CpMo(CO)(dppe)H with (p-MeOC6H4)3C+BF4
23.0 Hz). 31P{1H} NMR (CD2Cl2, 186 K): δ 69.6 (d, JPP ) 10.7
Hz), 68.2 (d, JPP ) 10.7 Hz). The temperature was increased to
-58 °C, and the two hydride resonances became a triplet. 1H NMR
(CD2Cl2, -58 °C): δ 4.55 (s, 5 H, Cp), -4.19 (t, 2 H, MoH, JPH
) 32.7 Hz). 31P{1H} NMR (CD2Cl2, -58 °C): δ 69.9 (br s), 68.3
(br s). When the temperature was increased to -35 °C, the two
phosphorus resonances of dppe coalesced to one broad singlet at δ
69.7 in the 31P{1H} spectrum. This dihydride complex was stable
at -12 °C for about 30 min without noticeable decomposition.
Attempted Reaction of CpMo(CO)(dppe)H with Gomberg’s
Dimer. CpMo(CO)(dppe)H (5.9 mg, 0.010 mmol) was weighed in
an NMR tube along with CD2Cl2 (∼0.4 mL) and 1,2-dichloroethane
(0.5 µL, internal integration standard). After the initial spectrum
was taken, this NMR solution was added to a solution of Gomberg’s
dimer (2.4 mg, 0.010 mmol calculated as Ph3C•) in CD2Cl2 (∼0.2
mL); the color of the mixture remained yellow. After 10 min, the
NMR spectrum showed no reaction. After 7 days, only a small
amount of decomposition (free dppe) was observed in the NMR
spectrum. A similar experiment in C6D6 was heated for 4 days at
80 °C and also showed a small amount of decomposition, giving
free dppe.
-
.
CpMo(CO)(dppe)H (5.9 mg, 0.010 mmol, 1 equiv) and hexameth-
ylbenzene (0.5 mg, internal integration standard) were weighed in
an NMR tube along with CD3CN (∼0.4 mL). After the initial
spectrum was taken, this solution was added to an orange-red
-
solution of (p-MeOC6H4)3C+BF4 (4.2 mg, 0.010 mmol, 1 equiv)
in CD3CN (∼0.3 mL); the solution turned lighter orange. The
reaction was monitored by NMR spectroscopy and showed [CpMo-
(CO)(dppe)(CD3CN)]+ (23% after 15 min; 74% after 4 h). After
24 h, a 95% yield of [CpMo(CO)(dppe)(CD3CN)]+ was observed,
with 5% CpMo(CO)(dppe)H remaining. The yield of (p-MeOC6H4)3-
CH after 24 h was 85%.
NMR Tube Reaction of CpMo(CO)(dppe)H with HOTf.
CpMo(CO)(dppe)H (9.5 mg, 0.016 mmol, 1 equiv) was weighed
in an NMR tube along with CD2Cl2 (∼0.4 mL) and 1,2-dichloro-
ethane (1 µL, internal integration standard). After the initial
spectrum was recorded, HOTf (1.5 µL, 0.017 mmol, 1.1 equiv)
was added, and the tube was shaken. The color of the solution
turned from yellow to orange, and gas (dihydrogen) bubbling was
observed. The NMR spectrum of the resulting solution showed
CpMo(CO)(dppe)(OTf) (73%) and [CpMo(CO)(dppe)(H)2]+[OTf]-
Collection and Reduction of X-ray Data. Crystals of CpMo-
(CO)(dppe)H were yellow prisms. A crystal was mounted on a glass
fiber and used for data collection. Some crystals appeared to be
twinned, and others seemed to quickly lose solvent molecules and
turn white and powdery, while others, like the crystal used for data
collection, retained their yellow color. Diffraction data obtained
using an Enraf Nonius CAD-4 diffractometer with molybdenum
radiation and a graphite monochromator indicated triclinic symmetry
1
(27%) as the products. H NMR (CD2Cl2, 25 °C) of CpMo(CO)-
1
(dppe)(OTf): δ 4.65 (d, 5 H, Cp, JPH ) 2.5 Hz). H NMR (CD2-
Cl2, 25 °C) of [CpMo(CO)(dppe)(H)2]+[OTf]-: δ 4.64 (s, 5 H,
Cp), -4.07 (t, 2 H, MoH, JPH ) 32.8 Hz) (signal for dihydrogen
at δ 4.60 was observed). 31P{1H} NMR (CD2Cl2, 25 °C) of CpMo-
(CO)(dppe)(OTf): δ 90.7 (d, JPP ) 37.1 Hz), 68.3 (d, JPP ) 37.1
Hz). 31P{1H} NMR (CD2Cl2, 25 °C) of [CpMo(CO)(dppe)(H)2]+-
Inorganic Chemistry, Vol. 45, No. 12, 2006 4719

Cheng et al.
consistent with the space groups P1 and P1h. Crystal data and
information about the data collection and refinement are provided
in Table 1.
the other hydrogen atoms. A Gaussian absorption correction was
applied.
Acknowledgment. Research at Brookhaven National
Laboratory (BNL) was carried out under Contract DE-AC02-
98CH10886 with the U.S. Department of Energy and was
supported by its Division of Chemical Sciences, Office of
Basic Energy Sciences, and by BNL LDRD funding. We
thank Dr. Etsuko Fujita and Dr. Carol Creutz for helpful
discussions.
Determination and Refinement of the Structure. The structure
was solved by standard heavy-atom Patterson methods. Space group
P1h was used for the solution and refinement of the structure. In
the least-squares refinement, anisotropic temperature parameters
were used for all of the non-hydrogen atoms. The hydrogen atoms
were placed at calculated positions and allowed to “ride” on the
atom to which they were attached, except for the hydrogen atoms
on the cylcopentadiene ligand and the toluene of solvation. Both
of these were disordered so the hydrogen atoms on these groups
were not included in the refinement. The hydride atom was located
on a difference Fourier map and included in the final cycles of
refinement and its coordinates and isotropic thermal parameter were
refined. A common isotropic thermal parameter was refined for
Supporting Information Available: Plot of the cyclic volta-
mmetry experiment, crystal data and structural refinement, and other
X-ray crystallographic data (CIF format). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC060111K
4720 Inorganic Chemistry, Vol. 45, No. 12, 2006