Photoactivation of water by Cp'2Mo and photochemical studies of Cp2MoO. Investigation of a proposed water-splitting cycle and preparation of a water-soluble molybdocene dihydride

Article abstract of DOI:10.1016/S0020-1693(99)00541-1

Irradiation (λ > 350 nm) of Cp'₂MoH₂ (Cp'= η⁵-C₅H₄CH₃) dissolved in 3:5 H₂O-CH₃CN (v/v) results in the quantitative formation of Cp'₂MoO and 2 equiv. of H₂. In light of this result, the photochemistry of the Cp'₂MoO and Cp₂MoO complexes was re-examined to determine the feasibility of using these molybdocene complexes as sensitizers in a photochemical water-splitting scheme. The metal-containing products formed by irradiation of Cp₂MoO were [Cp₂MoO₂(MoO₂)]₂, {(η-C₅H₅)(μ-[η¹:η⁵-C₅H₄])Mo}₂ (C₂₀H₁₈Mo₂) and Cp₂MoPPh₃ (in the presence of PPh₃), but gas chromatographic and mass spectroscopic analyses showed that free O₂ was not a product. Variations in the temperature, pH of the solution, and wavelength of the irradiating light did not yield any O₂. Experiments showed that O₂ reacted with Cp₂MoO to form [Cp₂MoO₂(MoO₂)]₂. A sensitive apparatus was therefore built to remove and quantitate any O₂ generated in solution (as little as 0.O₂ μmol of O₂) before it could react with Cp₂MoO, but no O₂ was detected in experiments using this apparatus. It is concluded that O₂ is not produced by irradiation of Cp₂MoO. Electron spin resonance experiments in the presence of α-phenyl-tert-butylnitrone, a radical spin trap, demonstrated that Cp radicals form when Cp₂moO is irradiated, and it is proposed that this photoprocess may be responsible for the observed photochemistry. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(99)00541-1

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Inorganica Chimica Acta 300–302 (2000) 102–112
Photoactivation of water by Cp%Mo and photochemical studies of
2
Cp2MoO. Investigation of a proposed water-splitting cycle and
preparation of a water-soluble molybdocene dihydride
Gregory T. Baxley, Alfred A. Avey, Tim M. Aukett, David R. Tyler *
Department of Chemistry, Uni6ersity of Oregon, Eugene, OR 97403-1253, USA
Received 15 July 1999; accepted 27 October 1999
Abstract
Irradiation (u\350 nm) of Cp%2MoH2 (Cp%=h5-C5H4CH3) dissolved in 3:5 H2O–CH3CN (v/v) results in the quantitative
formation of Cp%2MoO and 2 equiv. of H2. In light of this result, the photochemistry of the Cp%MoO and Cp2MoO complexes was
2
re-examined to determine the feasibility of using these molybdocene complexes as sensitizers in a photochemical water-splitting
scheme. The metal-containing products formed by irradiation of Cp2MoO were [Cp2MoO2(MoO2)]2, {(h-C5H5)(m-[h1:h5-
C5H4])Mo}2 (C20H18Mo2) and Cp2MoPPh3 (in the presence of PPh3), but gas chromatographic and mass spectroscopic analyses
showed that free O2 was not a product. Variations in the temperature, pH of the solution, and wavelength of the irradiating light
did not yield any O2. Experiments showed that O2 reacted with Cp2MoO to form [Cp2MoO2(MoO2)]2. A sensitive apparatus was
therefore built to remove and quantitate any O2 generated in solution (as little as 0.02 mmol of O2) before it could react with
Cp2MoO, but no O2 was detected in experiments using this apparatus. It is concluded that O2 is not produced by irradiation of
Cp2MoO. Electron spin resonance experiments in the presence of a-phenyl-tert-butylnitrone, a radical spin trap, demonstrated
’
that Cp radicals form when Cp2MoO is irradiated, and it is proposed that this photoprocess may be responsible for the observed
photochemistry. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Photochemistry; Molybdocene complexes; Water activation
hw
H O
2
1. Introduction
Cp2*WH2 ꢀꢀꢀꢀꢁ Cp2*W ꢀꢀꢀꢀꢁ Cp2*WH(OH)
−H
2
“Cp2*WO +H2 (1)
A vast majority of the published research working
toward photochemical water-splitting has focused on
systems based on outer-sphere electron transfer [1,2].
An alternative route for the activation of water involves
oxidative addition to an organometallic complex. There
are, however, relatively few examples of this type of
activation, presumably due to the strength of the
HꢀOH bond (:119 kcal mol−1) [3]. In a recent paper,
we reported that irradiation of Cp*2 WH2 (Cp*=h5-
C5(CH3)5) in aqueous media results in the activation of
water and the subsequent formation of H2 via an
oxidative addition route (Eq. (1)) [4]. Two equivalents
of H2 are formed in this reaction because the initial
photoprocess is loss of H2 to form Cp2*W.
What made this finding especially intriguing was that,
in 1988, we reported that the closely-related Cp2MoO
molecule gave some O2 on irradiation (Eq. (2)) [5]. The
Cp2M-based molecules (M=Mo or W) thus appear to
be likely candidates for water-splitting catalysts. We
therefore decided to re-examine the photochemistry of
Cp2MoO and Cp%MoO (Cp%=h5-C5H4CH3) with the
goals of quantifying the amount of O2 produced and of
determining if O2 and H2 can be photochemically gen-
erated from water using the same catalyst. This paper
2
(2)
* Corresponding author. Tel.: +1-541-346 4601; +1-541-346
0487.
E-mail address: dtyler@oregon.uoregon.edu (D.R. Tyler)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00541-1

G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
103
details our reinvestigation of the photochemistry of the
experiments conducted in the dark without Cp2MoO. It
is reasonable to assume that the minute amounts of O2
observed in these experiments were due to air leakage
through the septa or into the syringe needle.
Cp2MoO and Cp%MoO complexes.
2
As detailed in Section 4, mass spectroscopic analysis
of the samples did not detect the presence of O2.
2. Results and discussion
2.1. Acti6ation of water by Cp%MoH2
2
2.3. Irradiation of Cp2MoO in aqueous solution
A reaction analogous to Eq. (1) occurred with
If Cp2MoO were acting as a catalyst for the water-
splitting reaction then the amount of O2 produced in an
aqueous solvent system should be greater than the
amount of O2 produced in a dry organic solvent. To test
this theory, Cp2MoO (0.04 M) in 5:2 CH3CN–H2O3 was
irradiated (u\350 nm) for 26 h. In these experiments,
a brown precipitate of [Cp2MoO2(MoO2)]2 formed and
the solution color changed from green to brown. Again,
however, no O2 above background levels was detected.
The headspace gases were analyzed by GC and the peak
area for O2 was calculated to represent 3 mmol of O2 (a
3% yield), similar to that found (3.8 mmol) in a control
sample that was stored in the dark.
Cp%MoH2 in aqueous solution (Eq. (3)). Thus, irradia-
2
tion of Cp%MoH2 (u\350 nm) in 3:5 H2O–CH3CN (v/v)
2
formed Cp%MoO in quantitative yield (as determined by
2
NMR) and 2 equiv. of H2 (as determined by gas
manometric measurements). The rates of H2 and
Cp%MoO formation were monitored as a function of
2
time, and H2 was produced at twice the rate of Cp%MoO.
2
hw
Cp%MoH2 ꢀꢀꢀꢁ Cp%MoO+2H2
(3)
2
2
H O
2
2.2. Irradiation of Cp2MoO in nonaqueous sol6ents
To determine the feasibility of completing the water-
splitting cycle started in Eq. (3), we reinvestigated the
2.4. The effect of wa6elength
photochemistry of Cp2MoO and Cp%MoO. Unless spe-
2
cifically noted, the results obtained with each complex
were identical, and therefore the abbreviation Cp will be
used throughout to indicate both the Cp and Cp% ligands.
Our initial goal was to quantify the amount of O2
generated in reaction 2 because the original detection of
O2 by mass spectroscopy was only qualitative. For the
quantitative analyses, gas chromatography (GC) was
used to analyze the headspace gases above irradiated
solutions of Cp2MoO. In a typical experiment, a 0.39 M
deoxygenated benzene solution of Cp2MoO was placed
in a septum-sealed 5-ml flask and irradiated (u\350 nm)
for 15 h1. During this time the solution turned from
green to brown, accompanied by the formation of a
reddish–brown precipitate (previously identified as
[Cp2MoO2(MoO2)]25) that was insoluble in common
organic solvents. A gas tight syringe2 was used to
withdraw 250 ml of the headspace gases, which were
immediately analyzed by GC. Peak areas corresponding
to O2 were calculated to represent 0.3 mmol, or a 0.1%
The effect of decreasing wavelength on the photochem-
istry of Cp2MoO was explored by irradiating with light
(u\200 nm). By extending the wavelength to 200 nm,
a higher energy absorption band with umax at 250 nm also
absorbed the radiation. The electronic absorption spec-
trum of Cp2MoO shows bands at approximately 650, 430
(sh) and 250 nm [5]. In this experiment, a 5:2 CH3CN–
H2O solution of Cp2MoO (0.04 M) was irradiated
(u\200 nm) for 18 h, and the headspace gases were
analyzed by GC. The peak area corresponding to O2 was
calculated to be identical to that found in a control
sample.
2.5. Analysis of O2 with the sweeping-gas apparatus
There are few compelling arguments that explain the
discrepancy between our current results and the earlier
experiment [5] in which O2 was apparently formed. One
reason might be that our earlier mass spectrometer had
a lower detection limit than our current instrument.
However, even if this were the case, the O2 would be
forming in such small yields as to make the reaction
useless in a solar conversion process. Alternatively, it is
noteworthy that Cp2MoO is oxygen sensitive, reacting
with O2 to yield [Cp2MoO2(MoO2)]2 as one of several
oxygenated molybdocene products (Eq. (4)) [5,6]4.
yield. Repeated experiments (with both Cp%MoO and
2
Cp2MoO) showed that the apparent yield of O2 (approx-
imately 0.1% with 200 mg Cp2MoO) could not be
distinguished from the amount of O2 found in control
1 The original experiment in which O2 was detected was conducted
by irradiating Cp2MoO at u\350 nm. The photoreaction was at-
tributed to the lowest energy electronic excited state, which has a
broad absorption centered with umax at 650 nm [5].
3 A mixed solvent system was used due to the low solubility (:10
mM) of Cp2MoO in pure H2O.
4 (a) IR absorption bands of [Cp2MoO2(MoO2)]2 are assigned at
911, 881 and 850 cm−1. (b) A crystal structure of the (n-Bu)Cp
derivative of [Cp2MoO2(MoO2)]2 has been determined; this complex
has similar IR bands in the parent complex (see Ref. [6]).
2 A syringe with a small needle dead volume (Alltech catalog
number 010031), designed especially for gas sampling and analysis,
was purged with the appropriate inert gas prior to use in an effort to
reduce atmospheric contamination.

104
G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
Fig. 1. Sweeping gas apparatus used for the removal and determination of O2 from solution. The components are: (a) N2 purification column of
BASF R3-11 oxygen removal catalyst and molecular sieves; (b) gas washing bottle with 10% Cr2+ and Hg/Zn amalgam; (c) benzene bubbler; (d)
reaction vessel; (e) aliquot of 0.09585 M Cr2+; (f) oil bubbler; (g) Cr2+ dispensing buret; (h) storage vessel for 0.09585 M Cr2+ solution.
Thus, O2 might be reacting with either the starting
material or some intermediate before it can escape from
the solution and enter the gas phase.
With this device, the plan was to sweep away molec-
ular oxygen from the reaction solution with the flow of
inert gas before it could react with any species in
solution. In a typical experiment, a 4:1 benzene–
pyridine solution of Cp2MoO (0.24 M) was syringed
under nitrogen into the sparging vessel of the N2 purg-
ing apparatus. The solution was cooled to 10°C in a
water bath to reduce evaporation of the solvent. The
solution was simultaneously purged with N2 and irradi-
ated (u\350 nm) for 4 h. The Cr2+ solution was
treated with deoxygenated Fe3+ solution and then ti-
trated with Ce4+. However, in none of the experiments
was O2 detected above the level of uncertainty in the
concentration of the Cr2+ solution (0.0958590.00002
M)5. This uncertainty corresponds to 1 mmol of O2, or
a 0.2% yield. In summary, these experiments confirm
the GC and MS results: free O2 is not a photoproduct
in the irradiation of Cp2MoO.
(4)
A method for circumventing both the sensitivity and
reactivity problems was developed by combining a sen-
sitive method for the detection of O2 in inert gas
streams with a method for separating O2 from the
starting materials and other products [7]. In this
method (Fig. 1), solutions of Cp2MoO were sparged
with a strong flow of purified N2 during irradiation.
This gas stream was subsequently bubbled through an
aliquot of Cr2+, which is rapidly oxidized by O2 (Eq.
2.6. Effect of temperature increases on the reacti6ity of
Cp2MoO
(5)). Each equivalent of O2 oxidizes 4 equiv. of Cr2+
,
providing for a very sensitive method of detection. Any
remaining Cr2+ is oxidized by the addition of Fe3+
(Eq. (6)), and the resulting Fe2+ is titrated with Ce4+
(Eq. (7)), allowing the amount of Cr2+ oxidized by O2
to be calculated.
Our earlier study [5] reported that Cp2MoPPh3
formed when Cp2MoO was irradiated in the presence of
PPh3 (Eq. (8)), from which it was inferred that Cp2Mo
was an intermediate in the photochemical reaction.
4CrCl2+4HCl+O2“4CrCl3+2H2O
2CrCl2+Fe2(SO4)3+2HCl
(5)
5 Control experiments showed that the system was effective in
trapping 50% of the O2 in 10 ml of air (0.09 mmol O2) injected via
syringe into the sparging vessel over a period of 30 seconds, and 80%
was trapped during a 3 min injection. The photochemical reactions
would likely form O2 at a much slower rate than these air injections,
so the apparatus should be capable of detecting all of the O2
produced in a photoreaction.
“2CrCl3+2FeSO4+H2SO4 (6)
Ce4+ +Fe2+ “Ce3+ +Fe3+
(7)

G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
105
Table 1
Selected spectroscopic data
b
Complex
1H NMR (l) a
31P NMR (l) a
IR (cm−1
)
Reference
[5a]
Cp2MoO
Cp2MoPPh3
5.11 (s)
800 [w(MoꢂO)]
7.86 (m, 6H), 7.0 (m, 9H), 3.88
(m, Cp, 10H, 4.2 Hz)
80.2 (s)
this work, [5a,10]
[Cp2MoO2(MoO2)]2
C20H18Mo2
911, 881, 850
[5a,6]
[5a]
5.40 (m, 2H), 4.84 (m, 2H), 4.54 (s, 10H),
4.05 (m, 2H), 3.02 (m, 2H)
I
4.55 (t, 2H, Cp), 4.25 (t, 2H, Cp),
2.41 (m, 2H), 2.14 (m, 2H), 2.09
(m, 6H), −8.4 (s, MoꢀH)
this work
this work
II c
4.88 (t, Cp), 4.74 (t, Cp), 3.3–3.0 (m, br)
a Benzene-d6.
b Mineral oil mull.
c D2O.
Cp2MoO+PPh3“Cp2MoPPh3
(8)
products showed only resonances for Cp2MoPPh3.
Thus, all of the Cp2MoO had reacted without forming
any detectable O2, from which it is concluded that O2
does not form thermally from Cp2MoO. The fate of the
oxo ligand is discussed in Section 2.7.
These trapping experiments were repeated in order to
quantify the amount of Cp2MoPPh3 formed. H NMR
1
spectra of the soluble products formed during a 12 h
irradiation (u\350 nm) of a benzene-d6 solution of
Cp2MoO (0.02 M) and PPh3 (0.03 M) at 22°C showed
a 5% conversion to Cp2MoPPh3 (l=3.88 ppm, Table
1). A small amount of [Cp2MoO2(MoO2)]2 was also
formed. The reaction to form Cp2MoPPh3 was ther-
mally enhanced, as shown by irradiating (u\350 nm) a
solution of Cp2MoO (0.02 M) and PPh3 (0.03 M) at
40°C. In this case, the yield of Cp2MoPPh3 increased to
33% (10 h). The other major soluble product (21%) of
this reaction was {(h-C5H5)(m-[h1:h5-C5H4])Mo}2
(henceforth abbreviated as C20H18Mo2 in this paper),
which is known to form from the reactive Cp2Mo
species (Eq. (9)) [8]. Control reactions held at 40°C in
2.7. Fate of the oxo ligand
No molecular oxygen was detected in the head space
gas or in the swept-gas under any conditions in any of
the photochemical or thermal reactions of Cp2MoO
described above. Clearly, however, Cp2MoO did react
to form non-oxygenated products (Cp2MoPPh3 and
C20H18Mo2) in its thermal reactions, and the question
was, therefore, where did the oxygen atom go? In all
cases where Cp2MoO reacted to a reasonable extent, a
red–brown precipitate of [Cp2MoO2(MoO2)]2 formed
(Table 1) [6], and it is reasonable to propose that this
compound is the oxo-ligand sink6.
Triphenylphosphine oxide is known to form from
PPh3 and O2 under UV irradiation, and the reaction is
catalyzed in the presence of certain transition metals [9].
An interesting finding is that OꢂPPh3 was not observed
by infrared or NMR spectroscopy in any of the photo-
chemical or thermal experiments. Prolonged heating of
Cp2MoO (0.04 M) in benzene-d6 with 1–3 equiv. of
PPh3 revealed no OꢂPPh3 resonances in the 31P NMR
spectra recorded after 4, 10, 20 and 70 h7. No reactions
1
the dark showed H NMR resonances for Cp2MoPPh3
(35%), with minimal C20H18Mo2 (B5%, 24 h).
(9)
The larger yields of Cp2MoPPh3 at higher tempera-
tures suggested that elevated temperatures might facili-
tate the photochemical reactions of Cp2MoO. To check
this possibility, the photochemical experiments to detect
O2 were all repeated at 50–60°C. The control experi-
ments were likewise repeated at higher temperature.
However, as was the case at room temperature, no O2
was detected by GC, MS, or the sweeping-gas appara-
tus in any of these photochemical experiments. Simi-
larly, no O2 was detected above background levels in
experiments run at 50°C in the dark. In these experi-
ments 1H NMR analysis of the benzene-d6-soluble
6 The loss of Cp in forming the tetrameric oxo species photochem-
ically may be explained by an excited state that has partial Cp-to-
metal charge-transfer character [5]. This is discussed in a later section
of the paper.
7 In these experiments, the free PPh3 (l= −5.6) and Cp2MoPPh3
(l=80.2) resonances were the only two phosphorus-containing spe-
cies present in solution. The chemical shift observed for Cp2MoPPh3
in this study (80.2 ppm) differed significantly from the value reported
in the literature [10]. However, preparation of the complex by the
same route reported in the literature gave an identical spectrum to the
one obtained in the photochemical and thermal experiments reported
herein.

106
G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
Table 2
Assorted ESR data and hyperfine coupling constants
’
R
Source
a(N) (G)
a(H) (G)
g
Ref.
’
’
’
’
’
’
C5H5
C5H5
C5H5
C5H5
C5H5
C6H5
Cp2MoO a
Cp2MoO a
Cp2MoO b
14.67
14.62
14.6
14.68
14.5
3.90
3.90
3.4
3.98
5.7
2.007
2.007
2.006
2.007
2.007
NA
this work
this work
this work
this work
[12]
a
HgCp2
HgCp2
c
a
Sn(C6H5)Cl2
13.81
2.12
[11b]
a In C6H6.
b In THF.
c In toluene.
occurred (as monitored by 1H or 31P NMR) when C6H6
solutions of Cp2MoO or Cp2MoH2 were either heated
or irradiated in the presence of OꢂPPh3, demonstrating
that [Cp2Mo] does not abstract the oxo group from
OꢂPPh3.
irradiation of Cp2MoO with PBN. HgCp2 undergoes
facile photo-cleavage of Cp , and this radical has been
trapped with numerous nitroso- and nitrone-functional-
ized spin traps [11].
’
An additional point to note is the dissociation of two
generally strongly bonded Cp ligands from Cp2MoO to
form [Cp2MoO2(MoO2)]2. No cyclopentadiene was de-
2.8. Interpretation of the photochemical and thermal
reactions
1
tected by H NMR in any of the experiments described
Although free O2 is not a product of Cp2MoO irradi-
ation, it might be argued that, because [Cp2MoO2-
(MoO2)]2 forms by reaction of Cp2MoO with O2 and
because [Cp2MoO2(MoO2)]2 is a product of all the
photoreactions, the O2 photoproduct reacts rapidly
with Cp2MoO before it can be detected. However, two
points suggest this explanation is unlikely. First, it
would be necessary to stipulate that all of the O2 is
reacting with the Cp2MoO before it can be swept out of
the reaction solution. The strong flow of the nitrogen
purge gas and the sensitivity of the detection apparatus
suggest that, were any O2 to form, some of it would be
swept from the solution and detected by the apparatus.
Second, only low yields of products were obtained that
can be construed as forming from the Cp2Mo interme-
diate that would result from MoꢀO bond cleavage. We
recall, only small amounts of Cp2Mo(PPh3) and
C20H18Mo2 formed in the photochemical reactions.
Higher yields of these products were formed in the
thermal reactions of Cp2MoO, which suggests that
MoꢀO bond cleavage is a thermal process.
above. To investigate the mechanism of Cp loss, irradi-
ated solutions of Cp2MoO were monitored by ESR for
the formation of stable free radical species (i.e. Cp ).
’
No radical signals were observed during irradiation
(u\340 nm) of Cp2MoO (0.1 M in C6H6) in the ESR
cavity over a period of 0.5–22 h, but this was not
surprising considering the low quantum yields (0.01–
0.05) [5]. However, in the presence of the spin trap
a-phenyl tert-butyl nitrone (PBN, 0.13M), a solution of
Cp2MoO (0.013 M) in C6H6 that was irradiated (u\
350 nm) directly in the ESR cavity for 55 min gave an
ESR spectrum consistent with that of other CpꢀPBN
adducts (Table 2). Note that PBN is reportedly stable
towards photolysis in C6H6 [12]; this observation was
confirmed during this investigation by irradiating C6H6
solutions of PBN in the ESR cavity. No radicals were
detected over a time period of 5–120 min. Unfortu-
nately, the spin trapping technique with PBN is un-
available for use in the thermolysis studies due to
substantial spontaneous nitroxide radical formation
when solutions of PBN are heated [11]. Note that PBN
showed no signs of reactivity with Cp2MoO in C6H6
after 3 days at 23°C in the dark as monitored by
The results in Section 2.7 suggest that [Cp2-
MoO2(MoO2)]2 is formed in a pathway involving pho-
toinduced MoꢀCp bond cleavage. The photoinduced
1
’
UV–Vis and H NMR.
loss of Cp may be explained by low-energy ligand-to-
The b-1H and [9] N hyperfine splitting constants
(Table 2) for the observed radical species formed during
the irradiation of Cp2MoO are similar to those reported
for the CpꢀPBN nitroxide radical in toluene [12] (the
solvent polarity has a large effect on the hyperfine
splitting) [13]. Spectra obtained during the irradiation
of mercurocene (HgCp2) with PBN in C6H6 had nearly
identical hyperfine splitting to those found during the
metal charge-transfer (LMCT) bands predicted by
molecular orbital calculations (umax=640–665 nm, m=
140–200 M−1 cm−1) [5]. This type of reactivity is not
uncommon; several Group IV metallocenes [14] and
CpReO3 [15] have been shown to undergo photo-
’
lytic cleavage of Cp using the spin-trapping tech-
nique. CpReO3 has also been shown to undergo
’
thermal cleavage to yield Cp [16]. A low energy

G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
107
LMCT band was suggested to be responsible for the
photo-cleavage of Cp from CpReO3, [15,16], which is
and spectroscopic properties similar to phosphines
without the electronegative groups [19].
’
similar to the mechanism proposed here for Cp2MoO.
Subsequent reaction steps that lead to [Cp2MoO-
2(MoO2)]2 remain unknown, but in this system it is
reasonable to propose that this oxidized species is a
thermodynamic sink, and its formation therefore in-
evitable.
The ammonium complex (II) is highly water soluble
but only slightly soluble in CH3OH and CH3CN. Note
that only 0.8 equiv. of either dimethyl sulfate or CH3I
is used for the conversion of I to II. An excess of
dimethyl sulfate or of CH3I gives products that have
additional hydride resonances in the 1H NMR spec-
trum. These products may be methylated metal species,
possibly molybdocene methyl hydrides [20]8. Addition
of CH3I was done dropwise at 0°C, and the precipitate
immediately filtered and washed to avoid the replace-
ment of hydrido ligands by iodide.
2.9. Preparation and photochemistry of
[{(CH3)3NCH2CH2Cp}2MoH2][I]2 (II)
One of the long term goals in studying the Cp2MoO
molecule is to probe the effect that water has as a
solvent on organometallic reactions. While many water-
soluble organometallic complexes have been reported,
the vast majority of these must incorporate hydrophilic
phosphines [17]. This approach is of limited scope in
metallocene chemistry, and it was necessary therefore to
develop routes to provide water-soluble cyclopentadi-
enyl complexes. As an example of a water-soluble
metallocene complex, the derivatized Cp2MoH2 com-
plex with structure II was prepared. The synthetic route
is shown in Eq. (10). This scheme is analogous to that
using unsubstituted Cp ligands, albeit in a lower yield
(18%).
3. Summary
If O2 is produced by irradiation of Cp%MoO, it is
2
either at a level below our detection limits (approxi-
mately 1 mmol of O2) or it is reacting too rapidly with
other species in solution to be observe by these meth-
ods. However, neither of these explanations seems valid
in view of the sensitivity of the O2-detection apparatus
and the nature of the products that form in the pho-
toreaction. ESR spin-trapping experiments showed that
irradiation of Cp2MoO homolytically cleaves the
’
MoꢀCp bonds to produce Cp , and it is suggested that
this process is responsible for the observed
photochemistry.
The water soluble [{CpCH2CH2N(CH3)3}2MoH2][I]2
complex was prepared and its photochemistry is similar
to the parent dihydride. While this particular complex
is difficult to prepare and purify, the dimethyl-
aminoethyl cyclopentadiene ligand is a convenient lig-
and to prepare. It is readily converted to the quaternary
ammonium salt, in which form it renders complexes
water soluble. Use of this derivatized cyclopentadiene
ligand should allow for more extensive development of
water-soluble metallocene-type complexes.
(10)
4. Experimental
1
The H NMR chemical shifts of the Cp ring protons
in I (4.55 and 4.25) and Cp%MoH2 (4.45 and 4.20 ppm)
All manipulations of air-sensitive materials were car-
ried out in an inert (N2 or Ar) atmosphere by using
standard Schlenk or vacuum line techniques or by
handling the materials and solutions in a Vacuum
Atmospheres glove-box.
2
are essentially identical, which implies that the incorpo-
ration of the amine onto the Cp ring has minimal
impact on the electronic properties of complex I com-
pared to Cp%MoH2. This conclusion is consistent with
2
previous studies. For example, the electronic and spec-
troscopic properties of Cp% and fluoroalkyl–Cp com-
plexes are very similar if two or more methylene groups
separate the Cp ring from any electron withdrawing
groups on the fluoroalkyl chain [18]. In addition, we
recently showed that phosphines with at least three
methylene units between the P atom and an electron
withdrawing group (e.g. OH, SO3−) have coordination
4.1. Materials
Cp2MoO [Cp=h5-C5H5] and Cp%MoO [Cp%=h5-
C5H4(CH3)] were prepared and purified according to
2
8 Cp2W(CH3)H and Cp*2W(CH3)H are known complexes.

108
G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
literature methods [21]. Solvents were purified and rig-
orously deoxygenated by standard methods [22].
Molybdenum(V) chloride (Johnson–Matthey Electron-
ics) and other chemicals were used as received.
4.4. Calibration of McLeod gauge and measurements
of hydrogen gas
A linear-scale McLeod compression gauge connected
with a high vacuum manifold and a standard 25-ml
Schlenk tube was calibrated by determining the total
enclosed volume (V1), with deionized water. Prior to
irradiation, the entire system was evacuated (with the
reaction solution in a Schlenk tube frozen in liquid N2)
B1 mm Hg, the system was sealed under static vacuum,
and the initial pressure was measured (P1). After allow-
ing the solution to return to room temperature, it was
irradiated for a specific amount of time, and the solu-
tion was again frozen in N2. The pressure of the system
was then measured (P2), any increase in pressure was
due to evolved headspace gases. The amount of gas
generated (V2) was determined using Boyle’s Law
(P1V1=P2V2).
4.2. Instrumentation
1H NMR spectra were collected either on GE QE-
300 (300.15 MHz) or Varian Unity/Inova 300 (299.95
MHz) spectrometers. 13C (75.43 MHz) and 31P (121.42
MHz) spectra were both collected using the Varian
instrument. Signals were referenced to TMS, DSS, or
1% H3PO4 in D2O. Infrared (IR) spectra were obtained
on a Nicolet Magna IR 550 spectrometer with 0.03-mm
path length sodium chloride sealed cells or with mineral
oil mulls on NaCl plates. Mass spectra were obtained
by electron impact (EI) or fast atom bombardment
(FABS) on a VG-ZAB 2F-HF instrument. Electronic
absorption spectra (UV–Vis) were acquired using a
Perkin–Elmer Lambda 6 spectrophotometer and quartz
cells especially designed for air sensitive work. Potentio-
metric titrations were conducted with Corning 105 pH
meter (in mV mode), with Pt and saturated calomel
reference electrodes, and pH titrations were conducted
with a Corning 220 pH meter.
ESR spectra were obtained on a Bruker ESP 300e
operating at 9.6 GHz with a Hewlett–Packard fre-
quency counter. Typical analyses were conducted with
a 50 G sweep width (center field 3430 G), a modulation
frequency of 50–100 KHz, modulation amplitude 1–5
G, conversion time 20.5 ms, time constant 2.56 ms and
a field resolution of 1024 bits. Solid diphenyl picryl
hydrazine (DPPH) was used as an external standard for
the measurement of g tensors.
4.5. Molecular oxygen detection
For mass spectrometric analyses, solutions of
Cp2MoO or Cp%MoO were degassed by three freeze–
2
pump–thaw cycles on a high vacuum line in the dark
and were left under static vacuum during irradiation.
Mass spectra of head space gases were obtained by first
evacuating a small glass tube with Teflon valves at
either end on a high vacuum line. Head space gases
were then collected in the tube by opening it to the
irradiated sample after freezing the sample at −78°C.
The trapped gases were directly introduced into the
mass spectrometer. Samples for GC analyses were pre-
pared in special round-bottom Schlenk-type flasks un-
der N2 or Ar gas. The flasks had stopcock-sealed side
arms that were double-sealed with two septa.
Headspace gases were sampled by opening the stopcock
and inserting a gas tight syringe through the septa and
the stopcock tap. Quantification of GC peak areas was
based on calibration curves made from the appropriate
gas.
A third method of oxygen detection involved the
continuous purging of Cp2MoO solutions with purified
N2 gas (see Fig. 1). Any generated O2 contained in the
sweeping gas would react with a measured amount of
Cr2+, and a subsequent potentiometric titration deter-
mined the amount of O2 [7]. The N2 stream was
purified by passing it over activated R3-11 (BASF)
Gas chromatographs were obtained on a Shimadzu
GC-9A equipped with a thermal conductivity detector
and a CR-3A integrator. A 12 ft×1/8%% stainless steel
column was packed with 80/100 mesh molecular sieves
,
(5 A) and pretreated at 360°C for 24 h under an Ar
flow to remove residual water [23]. The GC was oper-
ated under the following conditions: Ar or He carrier
gas (flow rate 18 ml min−1), detector temperature
120°C, oven temperature 50°C and detector current 80
mA.
4.3. General irradiation procedure
Irradiations were conducted using an Oriel 200 W
high pressure Hg arc lamp equipped with a Corning
Glass 0-52 UV cutoff filter (u\350 nm). Irradiations in
the UV region (u\200 nm) were carried out in quartz
cells without the filter. Prolonged irradiation of
,
catalyst and 5 A molecular sieves (Fig. 1(a)) and then
sparging it through a 10% Cr2+ solution in 1 M H2SO4
over 20 g of amalgamated zinc (Fig. 1(b), used to
reduce Cr3+). The N2 was then bubbled through 20 ml
of benzene (Fig. 1(c), to prevent evaporation of the
reaction solution). Solutions of Cp2MoO were syringed
under N2 into a 10-ml glass bottle fitted with a fritted-
glass sparging tube (Fig. 1(d)). The bottle was placed in
Cp2MoO or Cp%MoO solutions produced a color
2
change in the solution from green to brown along with
the formation of a red–brown precipitate. The solu-
1
tions were filtered before H NMR analysis.

G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
109
a thermostated oil or H2O bath that was in a position
for irradiation. The purified, benzene-saturated N2 gas
purged solutions of Cp2MoO at rates of either 20 or 50
ml min−1. The N2 stream (containing any generated
O2) was then bubbled through an aliquot of 0.09585 M
CrCl2 (15.00 ml, 1.438 mmol, the Cr2+ solution had
been acidified to pH 1 with HCl), which was dispensed
from a buret (Fig. 1(g)) under a blanket of N2 into a
100-ml pear-shaped flask (Fig. 1(e)). Any O2 passed
were sampled with a gas tight syringe and analyzed by
GC. The peak area corresponding to O2 was calculated
to represent 0.3 mmol of the gas, or a 0.1% yield of O2.
The amount of O2 observed in the irradiated sample
was similar to that found in an identically prepared
sample that was stored in the dark.
4.9. Irradiation of Cp%MoO in aqueous solutions:
2
analysis for O2 by GC
through this solution would oxidize 4 equiv. of Cr2+
.
Unreacted Cr2+ was oxidized by the addition of 0.3 M
Fe2(SO4)3 (5 ml, 1.5 mmol). The air-stable Fe2+ was
then titrated potentiometrically in air with 0.04985 M
(NH4)2Ce(NO3)6. Blank determinations were conducted
in order to both standardize the Cr2+ solutions and to
test for leaks. The solutions of Cr2+ were stored over
several grams of amalgamated zinc to prevent or re-
verse oxidation to Cr3+ (Fig. 1(h)).
2
A 0.04 M solution of Cp%MoO (57 mg, 0.2 mmol) in
5 ml of 5:2 CH3CN–H2O was placed in a septum-
sealed 10-ml round-bottom flask. The solution was
irradiated (u\350nm), and the headspace gases were
sampled and analyzed by GC after 0.5, 6.5 and 26 h of
irradiation. After 26 h, the peak area corresponding to
O2 was calculated to represent 3 mmol of the gas, or a
3% yield of O2. The amount of O2 observed in the
irradiated sample was similar to that found (3.8 mmol)
in an identically prepared sample that was stored in the
dark.
4.6. Irradiation of Cp%MoH2 in H2O/CH3CN
2
A 0.04 M solution of Cp%MoO (90 mg, 0.33 mmol) in
2
A solution of Cp%MoH2 (22 mg, 0.087 mmol) in 4 ml
2
8 ml of 5:2 CH3CN–H2O was placed in a septum-
sealed 12-ml quartz cuvette. The solution was irradiated
(u\200 nm) for 18 h, and 500 ml of the headspace
gases were analyzed by GC. The peak area correspond-
ing to O2 was calculated to represent 6 mmol of the gas,
or a 3.6% yield of O2. The amount of O2 observed in
the irradiated sample was identical to that found (6
mmol) in an identically prepared sample that was stored
in the dark.
of 3–5 water–acetonitrile (v/v) was degassed by three
freeze–pump–thaw cycles and irradiated (u\350 nm).
Electronic absorption spectra were collected and mano-
metric measurements were performed every hour for
the first 6 h of irradiation and at the 24 h mark. After
1
24 h, the H NMR spectrum (in benzene-d6 after re-
moval of the solvent in vacuo) showed only Cp%MoO.
2
Analysis by UV–Vis indicated a 94% yield of Cp%MoO
2
from Cp%MoH2. Manometric measurements showed
2
that 2.2 equiv. of H2 were generated. EI MS of the
headspace gas showed a parent ion for hydrogen [M+]
at m/z 2. No reaction occurred with an identically
prepared sample stored in the dark for 24 h.
4.10. Irradiation of Cp2MoO in aqueous solutions:
analysis of O2 by GC
A 0.10 M solution of Cp2MoO (125 mg, 0.52 mmol)
in 5 ml of 3:2 CH3CN–H2O was placed in a septum-
sealed 10-ml round-bottom flask. The N2 atmosphere in
the cuvette was exchanged with Ar. The solution was
irradiated (u\350 nm) for 14 h, and GC analysis of
the headspace gases showed no O2 present above back-
ground levels (B0.02 mmol).
A 0.16 M solution of Cp2MoO (154 mg, 0.64 mmol)
in 4 ml of 4:1 CH3CN–H2O that had been adjusted to
pH 12 with 6 M NaOH was placed in a septum-sealed
20-ml quartz cuvette. The N2 atmosphere in the cuvette
was exchanged with Ar. The solution was irradiated
(u\200 nm) for 10 h, and analysis of the headspace
gases showed no O2 present above background levels
(B0.02 mmol). A similar experiment adjusted to pH 3
with 1 M HCl also yielded no O2.
4.7. Irradiation of Cp%MoH2 in D2O–CD3CN
2
A solution of Cp%MoH2 (20.0 mg, 0.078 mmol) in 4
2
ml of 3:5 deuterium oxide–acetone-d3 (v/v) was irradi-
ated (u\350 nm) for 24 h. The color of the solution
turned from golden-yellow to brown. Electronic ab-
sorption spectra were collected every hour for the first 6
h of irradiation and then at the 24 h mark. After 24 h,
1
the H NMR spectrum (in benzene-d6 after removal of
the solvent in vacuo) showed mostly the deuterated
starting material Cp%MoD2 and a small amount (B5%)
2
of C22H24Mo2. An identical sample stored in the dark
also showed that the hydrides exchanged in D2O.
4.8. Irradiation of Cp%MoO: analysis for O2 by GC
2
4.11. Irradiation of Cp2MoO at ele6ated temperatures:
analysis of O2 by GC
A 0.39 M solution of Cp%MoO (105 mg, 0.39 mmol)
2
in 1 ml of benzene was placed in a septum-sealed 5-ml
round-bottom flask. The solution was irradiated (u\
350 nm) for 15 h, and 250 ml of the headspace gases
A 0.20 M solution of Cp2MoO (455 mg, 1.9 mmol) in
9.5 ml of 4:1 benzene–pyridine was placed in a septum-

110
G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
sealed 25-ml Schlenk tube. The tube was warmed to
60°C in an oil bath and irradiated (u\350 nm) for 5 h.
Analysis of the headspace gases showed O2 to be
present in 0.1% yield (1.3 mmol), similar to that found
(0.5 mmol) in an identically prepared sample that was
stored in the dark.
were identical to those recorded by the spectrometer
prior to exposure of the headspace gases.
A 0.25 M solution of Cp2MoO (500 mg, 2.4 mmol) in
8 ml of benzene and 1.5 ml of pyridine was placed
along with PPh3 (630 mg, 2.4 mmol) in a 25-ml Schlenk
tube, which was then placed in a 55°C oil bath in the
dark. MS analysis showed no significant O2 production.
The solvent from this reaction was removed in vacuo
and a portion of the brown residue was redissolved in
4.12. Analysis of O2 by GC in non-irradiated solutions
at ele6ated temperatures
1
benzene-d6. H NMR analysis showed resonances due
A 0.29 M solution of Cp2MoO (210 mg, 0.87 mmol)
in 3 ml of 4:1 benzene–pyridine was placed along with
PPh3 (260 mg, 1 mmol) in a septum-sealed 25-ml
Schlenk tube. The N2 atmosphere in the tube was
exchanged with Ar. The tube was warmed to 50°C in
an oil bath in the dark for 12 h. Analysis of the
headspace gases showed no O2 present above back-
ground levels (B0.02 mmol).
to Cp2MoPPh3 (d, l=3.88, 95%) and C20H18Mo2 (5%,
1
Table 1). No Cp2MoO or OꢂPPh3 was observed in H,
31P NMR or IR spectra.
4.16. Analysis of O2 with purging/potentiometric
titration apparatus
A 0.24 M solution of Cp2MoO (290 mg, 1.2 mmol) in
5 ml of 4:1 benzene–pyridine was syringed under nitro-
gen into the sparging vessel in the N2 purging appara-
tus. The sparging vessel was cooled to 10°C in a water
bath. The solution was simultaneously purged with N2
and irradiated (u\350 nm) for 4 h. The Cr2+ solution
was then titrated with Ce4+. O2 was not detected above
the level of uncertainty in the concentration of the
Cr2+ solution (corresponding to 1 mmol of O2, or a
0.2% yield).
4.13. Irradiation of Cp%MoO: analysis of O2 by MS
2
A 0.12 M solution of Cp%MoO (81 mg, 0.3 mmol) in
2
2 ml of benzene and 0.5 ml of pyridine in a 25-ml
Schlenk tube was degassed by three freeze–pump–thaw
cycles. The solution was irradiated (u\350 nm) for 15
h under static vacuum, and the headspace gases were
collected and analyzed by MS as described above.
Intensities of the m/z 28 and 32 peaks were identical to
those recorded by the spectrometer prior to exposure of
the headspace gases. Similarly, a 0.24 M solution of
A 0.24 M solution of Cp2MoO (230 mg, 0.95 mmol),
along with PPh3 (400 mg, 1.5 mmol) in 4 ml of 4:1
benzene–pyridine was syringed under N2 into the
sparging vessel in the N2 purging apparatus. The sparg-
ing vessel was warmed to 50°C in an oil bath while the
solution was simultaneously purged with N2 for 3 h. A
water-cooled cold trap was used to condense solvent
vapors that had evaporated from the heated solution of
Cp2MoO. The Cr2+ O2 trapping solution was then
titrated with Ce4+. O2 was not detected above the level
of uncertainty in the concentration of the Cr2+ solu-
tion (1 mmol of O2, or a 0.2% yield). Analysis of the
Cp%MoO (600 mg, 2.2 mmol in 8 ml benzene and 1 ml
2
pyridine) also resulted in no observable increase in O2
after 65 h.
4.14. Irradiation of Cp2MoO: analysis of O2 by MS
A 0.17 M solution of Cp2MoO (750 mg, 3.1 mmol) in
15 ml of benzene and 3 ml of pyridine in a 25-ml
Schlenk tube was degassed by three freeze–pump–thaw
cycles. The solution was irradiated (u\350 nm) for 4.5
h under static vacuum, and the headspace gases were
collected and analyzed by MS. Intensities of the m/z 28
and 32 peaks were identical to those recorded by the
spectrometer prior to exposure of the headspace gases.
1
benzene-d6-soluble products in the purging vessel by H
NMR showed only Cp2MoPPh3.
4.17. Irradiation of Cp2MoO: formation of
Cp2MoPPh3 and (Cp2Mo)2
4.15. Irradiation of Cp2MoO at ele6ated temperatures:
analysis of O2 by MS
A solution of Cp2MoO (10 mg, 0.041 mmol) and
PPh3 (15 mg, 0.057 mmol) in 2 ml of benzene-d6 was
irradiated (u\350 nm) for 12 h. H NMR analysis of
the filtered solution showed Cp2MoPPh3 (5%, l 3.88),
Cp2MoO (95%), and a minor amount of C10H18Mo2
(B5%). The relative amounts of Cp2MoPPh3 and
Cp2MoO did not change when the amounts of
Cp2MoO and PPh3 were doubled to 50 (0.21 mmol)
and 100 mg (0.38 mmol) in the same volume of solvent.
A 0.25 M solution of Cp2MoO (500 mg, 2.4 mmol) in
8 ml of benzene and 1.5 ml of pyridine was placed
along with PPh3 (200 mg, 0.8 mmol) in a 25-ml Schlenk
tube. The solution was degassed by three freeze–
pump–thaw cycles and then placed in an oil bath held
at 55°C and irradiated (u\350 nm) for 3 h under static
vacuum. The headspace gases were collected and ana-
lyzed by MS. Intensities of the m/z 28 and 32 peaks
1

G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
111
4.18. Reaction of Cp2MoO at ele6ated temperatures
A control reaction with PBN (24 mg, 0.13 mmol) in
1 ml C6H6 was irradiated (u\340 nm) in the ESR
cavity for 60 min. No radical signals were observed
from this solution. After heating the solution for 30
min at 50°C, a complex series of lines were observed in
the ESR spectrum, none of which showed the charac-
teristic triplet of doublets pattern seen when Cp2MoO
was present.
A solution of Cp2MoO (25 mg, 0.10 mmol) and PPh3
(22 mg, 0.10 mmol) in 2 ml of benzene-d6 in a 10-ml
Schlenk tube was placed in a constant-temperature oil
1
bath at 60°C in the dark for 10 h. H NMR analysis of
the filtered solution showed Cp2MoPPh3 (50%),
Cp2MoO (48%) and C10H18Mo2 (2%). The IR spectrum
(mineral oil mull) of the red–brown precipitate formed
in the above reaction consisted of bands at 912 (s), 881
(m), and 842 (w) cm−1, consistent with the formation
of the oxygenated product [Cp2MoO2(MoO2)]2 [6].
4.21. Synthesis of [(CH3)2NCH2CH2Cp]2MoH2 (I)
Dimethylaminoethyl cyclopentadiene was prepared
according to a literature procedure [24], modified by the
use of 1,3-dimethyl-2-imidazolidinone instead of
HMPA. A solution of 9.0 g (0.066 mol) of the above in
150 ml of THF was deprotonated at 0°C with 27 ml of
n-butyl lithium (2.5 M in pentane), added in 9 ml
portions. A mixture of 35 ml of hexanes, 4.5 g MoCl5
and 2.9 g of NaBH4 was cooled to −78°C. The
deprotonated Cp was added to this mixture via an
addition funnel over a period of 1 h. The mixture was
allowed to warm to room temperature and was then
refluxed for 4.5 h. The rest of the synthesis was com-
pleted as reported, except that the last filtration must be
stopped before any oily brown–yellow residue passes
through the frit. After benzene extraction, the product
was concentrated in vacuo to 2.8 g of a yellow brown
oil (46%). The product was purified by loading 0.5 g in
5 ml of benzene on a 1×6%% in long column of specially
treated alumina [5], with the modification of eluting the
column with benzene. The collected fractions were con-
centrated in vacuo to yield 0.2 g of purified material.
Yield: 18%. 1H NMR (C6D6): l 4.55, 4.25 (t, Hꢀ
CpCH2CH2N(CH3)2), 2.41 (m, CpCH2CH2N(CH3)2),
2.14 (m, CpCH2CH2N(CH3)2), 2.09 (m, CpCH2-
CH2N(CH3)2), −8.4 (s, MoꢀH). 13C{H} NMR (C6D6):
l 101.87 (s, CpCH2CH2N(CH3)2), 78.89, 74.01 (s,
HꢀCpCH2CH2N(CH3)2), 62.42 (s, CpCH2CH2N-
(CH3)2), 45.97 (m, CpCH2CH2N(CH3)2), 29.67 (s,
CpCH2CH2N(CH3)2). Mass spectra (EI) analysis shows
the parent ion at 370 m/z.
1
No triphenyl phosphine oxide was detected by H or
31P NMR or by IR. Cp2MoPPh3 has a singlet in the 31
P
NMR spectrum at 80.2 ppm. The identity of this
species was confirmed by preparing Cp2MoPPh3 via the
irradiation (u\350 nm, 10 h) of Cp2MoH2 (15 mg,
0.07 mmol) in the presence of PPh3 (17 mg, 0.07 mmol)
in 1 ml benzene-d6.
A solution of Cp2MoO (25 mg, 0.10 mmol) and PPh3
(22 mg, 0.10 mmol) in 2 ml of benzene-d6 in a 10-ml
Schlenk tube was placed in a 60°C oil bath and irradi-
1
ated (u\350 nm) for 10 h. H NMR analysis of the
filtered solution showed product ratios of Cp2MoPPh3
(33%), Cp2MoO (46%) and C10H18Mo2 (21%).
4.19. Irradiation of Cp2MoPPh3
Cp2MoPPh3 (50 mg, 0.10 mmol) was isolated via
silica gel column chromatography from the above reac-
tion and dissolved in 2 ml of benzene in a 10-ml
Schlenk tube. The solution was irradiated (u\350 nm)
for 16 h in an oil bath at 60°C. 1H NMR analysis of the
solution indicated a 45% conversion of Cp2MoPPh3 to
C10H18Mo2, with 55% of the starting material remain-
ing. An identically prepared sample showed no reaction
when heated to 60°C in the dark.
4.20. Analysis of irradiated solutions of Cp2MoO by
ESR
Cp2MoO (10 mg, 0.041 mmol) in 1 ml C6H6 con-
tained in a quartz ESR tube was irradiated (u\350
nm) in the ESR cavity. ESR spectra (64–128 scans)
obtained at 15, 30 and 60 min, and at 4, 8, and 22 h
showed no radical signals in the 3100–3700 G range
(where carbon centered radicals are typically located).
Cp2MoO (10 mg, 0.041 mmol) and a-phenyl tert-
butyl nitrone (PBN, 73 mg, 0.41 mmol) in 1 ml C6H6
contained in a quartz ESR tube were irradiated (u\
350 nm) in the ESR cavity. ESR spectra (64 scans)
collected after 55 min showed a triplet of doublets,
centered at g=2.007, with a(N)=14.67 G and a(H)=
3.90 G. An identical experiment performed in THF
solution provides a similar signal, with g=2.006,
a(N)=14.6 G and a(H)=3.4 G.
4.22. Synthesis of [{(CH3)3NCH2CH2Cp}2MoH2][I]2
(II)
A total of 180 mg (0.49 mmol) of I was dissolved in
10 ml of THF. Addition of 0.4 mmol CH3I dropwise
via syringe to the above solution produced a yellow
precipitate. The product was immediately filtered
through a sintered glass frit, rinsed with THF and dried
in vacuo to yield 0.15 g of a pale yellow solid. Yield:
58%. 1H NMR (D2O): l 4.88, 4.74 (t, HꢀCpCH2-
CH2N(CH3)3), 3.3–3.0 (m br, CpCH2CH2N(CH3)2).
Mass spectra (FABS) analysis shows the parent ion at
654 m/z.

112
G.T. Baxley et al. / Inorganica Chimica Acta 300–302 (2000) 102–112
Katakis, C. Mitsopoulou, J. Konstantatos, J. Photochem. Pho-
tobiol. A, Chem. 68 (1992) 375.
[8] (a) M. Berry, N.J. Cooper, M.L.H. Green, A.H. Lynch, S.J.
Simpson, J. Chem. Soc., Dalton Trans. (1980) 29. (b) M.
Berry, S.G. Davies, M.L.H. Green, J. Chem. Soc., Chem.
Commun. (1978) 99.
[9] G.L. Geoffroy, D.A. Denton, C.W. Eigenbrot, Inorg. Chem.
15 (1976) 2310.
[10] (a) G.L. Geoffroy, M.G. Bradley, J. Organomet. Chem. 134
(1977) C27. (b) G.L. Geoffroy, M.G. Bradley, Inorg. Chem.
17 (1978) 2410.
4.23. Irradiation of [{(CH3)3NCH2CH2Cp}2MoH2][I]2
(II)
A 3.5 ml CH3OH solution of II (95 mg, 0.15 mmol)
was placed in a quartz cuvette. The yellow solution was
irradiated (u\200 nm) for 48 h, and a 250 ml sample of
the headspace gas was withdrawn with a syringe. Anal-
ysis by GC showed that 0.034 mmol of H2 was pro-
duced (23%).
[11] (a) E.G. Janzen, Acc. Chem. Res.
4 (1971) 31. (b) E.G.
Janzen, B.J. Blackburn, J. Am. Chem. Soc. 91 (1969) 4481.
[12] P.B. Brindley, A.G. Davies, J.A. Hawari, J. Organomet.
Chem. 250 (1983) 247.
Acknowledgements
[13] C. Heller, H.M. McConnell, J. Chem. Phys. 32 (1960) 1535.
[14] (a) A.T. Tsai, C.H. Brubaker, J. Organomet. Chem. 107 (1979)
1990. (b) R.W. Harrigan, G.S. Hammond, H.B. Gray, J.
Organomet. Chem. 81 (1974) 79.
Acknowledgement is made to the DOE for the sup-
port of this work through grant DE-FG06-94ER14406.
[15] W.A. Herrmann, F.E. Kuhn, D.A. Fiedler, M.R. Mattner,
M.R. Geisberger, H. Kunkely, A. Vogler, S. Steenken,
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