Trapping of hemiquinone radicals at Mo and P sites by phosphide-bridged dimolybdenum species: Chemistry of complexes [Mo2(η5- C5H5)2(OC6H4OH)(μ- PR2)(CO)4]

Article abstract of DOI:10.1021/ic061344e

The phosphide-bridged dimolybdenum complexes (H-DBU)[Mo₂Cp ₂(μ-PR₂)(CO)₄] (R= Cy, Ph; DBU = 1,8-diazabicyclo[5.4.0.]undec-7-ene) react with p-benzoquinone to give the hemiquinone complexes [Mo₂Cp

Full text of DOI:10.1021/ic061344e

Inorg. Chem. 2006, 45, 9593−9606
Trapping of Hemiquinone Radicals at Mo and P Sites by
Phosphide-Bridged Dimolybdenum Species: Chemistry of Complexes
[Mo2(
[Mo2(
η
η
5-C5H5)2(OC6H4OH)(
µ
-PR2)(CO)4] and
-
5-C5H5)2{µ-PR(OC6H4OH)
}
(CO)4] (R
)
Cy, Ph)
Celedonio M. Alvarez, M. Angeles Alvarez, Mar´ıa Alonso, M. Esther Garc´ıa, M. Teresa Rueda, and
Miguel A. Ruiz*
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo,
E-33071 OViedo, Spain
Patrick Herson
Laboratoire de Chimie Inorganique et Materiaux Moleculaires. UniVersite´ Pierre et Marie Curie,
75252 Paris, Cedex 05, France
Received July 19, 2006
The phosphide-bridged dimolybdenum complexes (H-DBU)[Mo2Cp2(
diazabicyclo[5.4.0.]undec-7-ene) react with p-benzoquinone to give the hemiquinone complexes [Mo2Cp2(OC6H4-
OH)( -PR2)(CO)4]. The latter experience facile homolytic cleavage of the corresponding Mo O bonds and react
readily at room temperature with HSPh or S2Ph2 to give the thiolate complexes [Mo2Cp2( -PCy2)( -SPh)(CO)4] or
[Mo2Cp2( -PR2)( -SPh)(CO)2]. In contrast, PRH-bridged substrates experience overall insertion of quinone into the
H bond to give the anionic compounds (H-DBU)[Mo2Cp2{µ-PR(OC6H4OH) (CO)4], which upon acidification yield
the corresponding neutral hydrides. The cyclohexyl anion experiences rapid nucleophilic displacement of the
hemiquinone group by different anions ER- (ER
OH, OMe, OC4H5, OPh, SPh) to give novel anionic compounds
(H-DBU)[Mo2Cp2{µ-PCy(ER) (CO)4], which upon acidification yield the corresponding neutral hydrides. The structure
µ-PR2)(CO)4] (R) Cy, Ph; DBU ) 1,8-
µ
−
µ
µ
µ
µ
P−
}
)
}
of four of these hydride complexes [PPh(OC6H4OH), PCy(OH), PCy(OMe), and PCy(OPh) bridges] was determined
by X-ray diffraction methods and confirmed the presence of cis and trans isomers in several of these complexes.
In addition, it was found that the hydroxyphosphide anion [Mo2Cp2{µ-PCy(OH)
unprecedented tautomeric equilibrium with its hydride-oxophosphinidene isomer [Mo2Cp2(
}
(CO)4]- displays in solution an
-H){µ-PCy(O)
(CO)4]-.
µ
}
Introduction
of partially hydrogenated aromatic or heteroaromatic com-
pounds.4 The dual role of quinones as redox relays and proton
shuttles has been the subject of numerous electrochemical
studies.5 In solution, quinones are reduced in two successive
one-electron steps to form the radical anion (‚Q-) and
Quinones are an important class of ubiquitous compounds
that have widespread relevance in biology and chemistry.1
Their fundamental role in the biochemistry of living cells is
well established and evolves from their ability to play a
pivotal role in electron- and proton-transfer reactions.2 They
serve as hormones and pigments and are used as pharma-
ceuticals, such as antibiotics and anticancer drugs.3 Quinones
are also suitable dehydrogenation agents for the aromatization
(2) (a) Inaba, K.; Takahashi, Y.-H.; Ito, K.; Hayashi, S. Proc. Natl. Acad.
Sci. U.S.A. 2006, 103, 287. (b) Sazanov, L. A.; Hinchliffe, P. Science
2006, 311, 1430. (c) Larsen, P. L.; Clarke, C. F. Science 2002, 295,
120. (d) Do, T. Q.; Hsu, A. Y.; Jonassen, T.; Lee, P. T.; Clarke, C. F.
J. Biol. Chem. 2001, 276, 18161. (e) Ferreira, K. N.; Iverson, T. M.;
Maghlaoui, K.; Barber, J.; Iwata, S. Science 2004, 303, 1831. (f)
Scewczuk, L. M.; Penning, T. M. J. Nat. Prod. 2004, 67, 1777. (g)
Dowd, P.; Zheng, Z. B. Proc. Natl. Acad. Sci. U.S.A. 1995, 93, 8171.
(3) (a) Vella, F.; Ferry, G.; Delagrange, P.; Boutin, J. A. Biochem.
Pharmacol. 2005, 71, 1. (b) Asche, C. Mini-ReV. Med. Chem. 2005,
5, 449, and references therein.
* To whom correspondence should be addressed. E-mail: mara@uniovi.es.
(1) (a) Voet, D.; Voet, J. G. Biochemistry, 2nd ed.; Wiley: New York,
1995. (b) The Chemistry of the Quinonoid Compounds; Patai, S.,
Rappoport, Z., Eds.; Wiley, New York, 1988; Vol. 2.
10.1021/ic061344e CCC: $33.50
Published on Web 10/17/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 23, 2006 9593

Alvarez et al.
hydroquinone dianion (Q2-). The anions thus formed couple
easily with one or two protons to yield the radical hemiquino-
ne (‚QH) and the hydroquinone molecule (QH2), respectively.
These species can act as ligands by binding to metal centers
either through their oxygen atoms (σ-coordination) or through
their hydrocarbon rings (π-coordination). Both types of
binding can be combined to achieve extended metal-
organometallic coordination networks with potential applica-
tions as functional materials.6 Recently, quinones have been
found to experience interesting reactions, some of them in
the vicinity of metal centers, such as insertion on S-S, P-C,
or P-H bonds. Thus, Adams et al. have recently reported a
heterodinuclear disulfide complex which reacts with p-
benzoquinone to give a p-benzoquinonedithiolate ligand after
S-S and C-H bond cleavages,7 while Bertrand et al. have
reported the insertion of an o-quinone into the P-C bond of
an (amino)(phosphino)carbene.8 Finally, Malisch et al. have
reported the first example of hydrophosphination of p-
benzoquinone, this occurring at a mononuclear iron phos-
phine complex.9
paramagnetic binuclear species of Mo, W, or Fe.11 We then
considered the use of p-benzoquinone as oxidant on some
of the above dinuclear substrates to explore the possibility
of incorporating the hemiquinone radical to these binuclear
substrates. In this paper, we report the preparation and
reactions of the salts [H-DBU][Mo2Cp2(µ-PR2)(CO)4] and
[H-DBU][Mo2Cp2(µ-PHR)(CO)4] with p-benzoquinone (R)
Cy, Ph). The PPh2-bridged anion has been recently reported
to be electrochemically oxidized at quite low potentials to
give first the 33-electron radical [Mo2Cp2(µ-PPh2)(CO)4] and
then the unstable unsaturated cation [Mo2Cp2(µ-PPh2)-
(CO)4]+, a process that can be reproduced using the cation
[FeCp2]+.11a As it will be shown, the oxidation reactions now
being reported result in either O-coordination of the
hemiquinone radical to the metal center or, if a P-H bond
is present in the phosphide bridge, its incorporation at the
phosphorus site. In both cases, the Mo- or P-bound hemiquino-
ne group is easily replaced by other groups such as hydroxo,
alkoxo, or thiolate groups, which adds synthetic value to the
newly formed species.
Recently we have found that the use of p-benzoquinone
as oxidant toward the [H-DBU]+ salt of the anionic complex
Results and Discussion
t
[MoCp(CO)2{P(O)R*}]- (R* ) 2,4,6-C6H2 Bu3, Cp ) η5-
Formation of Starting Anions and their Reactions with
p-Benzoquinone. Hydride complexes of the type [Mo2Cp2-
(µ-H)(µ-PRR′)(CO)4] are easily deprotonated by a variety
of basic or reducing reagents to give the corresponding anions
[Mo2Cp2(µ-PRR′)(CO)4]-.11a,12,13 We have shown that a
strong base such as DBU (DBU ) 1,8-diazabicyclo[5.4.0.]-
undec-7-ene) acts as an efficient deprotonating reagent for
the hydride complex [Mo2Cp2(µ-H)(µ-PPh2)(CO)4] (1b)
while at the same time providing a relatively large cation,
(H-DBU)+, which facilitates the manipulation of the resulting
salt (H-DBU)[Mo2Cp2(µ-PPh2)(CO)4] (3b).11a Analogously,
the hydride compounds [Mo2Cp2(µ-H)(µ-PCy2)(CO)4] (1a)14
and [Mo2Cp2(µ-H)(µ-PHR)(CO)4] [R ) Cy (2a),14 R ) Ph
(2b)15] react with DBU rapidly in THF to give the corre-
sponding salts (H-DBU)[Mo2Cp2(µ-PCy2)(CO)4] (3a) and
(H-DBU)[Mo2Cp2(µ-PHR)(CO)4] [R ) Cy (4a), R ) Ph
(4b)] in quantitative yield (Scheme 1), as shown by IR and
31P NMR spectroscopy. These reactions are faster for PHR-
bridged complexes, compared to the PR2-bridged ones (see
Experimental Section), which seems to point to some steric
C5H5, DBU ) 1,8-diazabicyclo[5.4.0]undec-7-ene) results
in hemiquinone incorporation at the P site of the oxophos-
phinidene ligand to give, after spontaneous H+ transfer from
the (H-DBU)+ counterion, the neutral compound [MoCp-
{κ2-OP(OC6H4OH)R*}(CO)2], which is the first complex
reported to have a P,O-bound phosphonite ligand.10 This is
in contrast with the result of the reaction with an innocent
oxidant such as [FeCp2]+, which results in simple electron
transfer and coupling of the neutral radicals thus generated.10
We have shown previously that simple removal of electrons
with [FeCp2]+ from suitable dinuclear complexes stabilized
by diphosphine or phosphide bridging ligands can be an
excellent synthetic route for highly reactive diamagnetic or
(4) (a) Becker, H.-D.; Turner, A. B. In The Chemistry of Quinonoid
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988;
Vol. 2; Chapter 23, pp 1351-1384. (b) Fu, P. P.; Harvey, R. G. Chem.
ReV. 1978, 78, 317. (c) Ru¨chardt, C.; Gerst, M.; Ebenhoch, J. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1406.
(5) (a) Chamber, J. Q. In The Chemistry of Quinonoid Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1988; Vol. 2; Chapter 12,
pp 719-757. (b) Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997,
119, 6384. (c) Costentin, C.; Robert, M.; Save´ant, J.-M. J. Am. Chem.
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(6) (a) Pierpont, C. G.; Langi, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Pierpont, C. G. Coord. Chem. ReV. 2001, 216, 99. (c) Oh, M.;
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Frake, P. C.; Carpenter, G. B.; Sweigart, D. A. Dalton. Trans. 2006,
2385. (e) Son, S. U.; Reingold, J. A.; Carpenter, G. B.; Sweigart, D.
A. Chem. Commun. 2006, 708. (f) Son, S. U.; Kim, S. B.; Reingold,
J. A.; Carpenter, G. B.; Sweigart, D. A. J. Am. Chem. Soc. 2005, 127,
12238. (g) Moussa, J.; Guyard-Duhayon, C.; Herson, P.; Amouri, H.;
Rager, M. N.; Jutand, A. Organometallics 2004, 23, 6231. (h)
Masaoka, S.; Akiyama, G.; Horike, S.; Kitagawa, S.; Ida, T., Endo,
K. J. Am. Chem. Soc. 2003, 125, 1152.
(11) (a) Alvarez, C. M.; Garc´ıa, M. E.; Rueda, M. T.; Ruiz, M. A.; Sa´ez,
D.; Connelly, N. G. Organometallics 2005, 24, 650. (b) Alvarez, M.
A.; Anaya, Y.; Garc´ıa, M. E.; Ruiz, M. A. Organometallics 2004, 23,
3950. (c) Garc´ıa, M. E.; Riera, V.; Rueda, M. T.; Ruiz, M. A.;
Lanfranchi, M.; Tiripicchio, A. J. Am. Chem. Soc. 1999, 121, 4060.
(d) Alvarez, M. A.; Garc´ıa, G.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.;
Lanfranchi, M.; Tiripicchio, A. Organometallics 1999, 18, 4509. (e)
Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.;
Jeannin, Y. Angew. Chem., Int. Ed. Engl. 1993, 32, 1156.
(12) (a) Vogel, U.; Scheer, M. Z. Anorg. Allg. Chem. 2003, 629, 1491. (b)
Hartung, H.; Walther, B.; Baumeister, U.; Bo¨ttcher, H. C.; Krug, A.;
Rosche, F.; Jones, P. G. Polyhedron 1992, 11, 1563. (c) Petersen, J.
L.; Stewart, R. P., Jr. Inorg. Chem. 1980, 19, 186.
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(8) Merceron-Saffon, N.; Baceiredo, A.; Gornitzka, H.; Bertrand, G.
Science 2003, 301, 1223.
(9) Malisch, W.; Klu¨pfel, B.; Schumacher, D.; Nieger, M. J. Organomet.
Chem. 2002, 661, 95.
(10) Alonso, M.; Alvarez, M. A.; Garc´ıa, M. E.; Ruiz, M. A.; Hamidov,
H.; Jeffery, J. C. J. Am. Chem. Soc. 2005, 127, 15012.
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Organometallics 2002, 21, 5515.
(15) (a) Henrick, K.; McPartlin, M.; Horton, A. D.; Mays, M. J. J. Chem.
Soc., Dalton. Trans. 1988, 1083. (b) Woodward, S.; Curtis, M. D. J.
Organomet. Chem. 1992, 439, 319.
9594 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Scheme 1
Chart 1
insertion process. Instead, they react with p-benzoquinone
to give hydroquinone and biphosphines.16
Even when [NH4]PF6 is added to freshly prepared solutions
of complexes 6, some decomposition of these anions could
not be avoided, which explains the appearance in the
corresponding reaction mixtures of small and variable
amounts of other products such as the hydroxyphosphide
complexes [Mo2Cp2(µ-H){µ-PR(OH)}(CO)4] (8a and 8b)
and the tetranuclear product [{Mo2Cp2(CO)4(µ-H)}2(µ-Cy-
POPCy)] (9) (Chart 1). These minor species could be isolated
as pure materials after chromatography of the reaction
mixture on alumina. The formation of compounds 8 is the
result of the hydrolysis of the anions 6. Indeed, a separate
experiment showed that the addition of water to a basic
solution of 6a followed by acidification with [NH4]PF6 led
to the hydride 8a almost quantitatively; presumably, the
phenyl compound 8b is formed in a similar way. However,
the origin of compound 9 is not so clear. Although this
species might be derived either from 8a by dehydration or
directly from 6a by reaction with oxygen, we have found
no experimental conditions to prepare this compound in a
selective way.
Reaction Pathways. The reaction of p-benzoquinone with
the anionic complexes 3 and 4 are very fast even at 243 K,
and no intermediate species could be detected. However,
since the electrochemical oxidation of 3b occurs at potentials
as low as -0.53 V (PPN+ salt, dichloromethane solution),11a
it is likely that the reactions of compounds 3 and 4 with
p-benzoquinone (Q in Scheme 2) are initiated by an electron-
transfer step to yield the corresponding radicals [Mo2Cp2-
(µ-PRR′)(CO)4] (A) and the hemiquinonate anion (‚Q-)
because the redox potential for the Q/‚Q- couple is above 0
V (although this is significantly dependent on solvent and
pH).5a,b The hemiquinonate anion then would be protonated
by the (H-DBU)+ cation despite unfavorable thermodynamics
(the aqueous pKa values for (H-DBU)+ and for ‚QH are ∼12
and 4, respectively)17,5a to give the hemiquinone radical (‚
QH, Scheme 2). Of course this is only possible if further
reactions (with equilibrium constants above ∼108) take place.
On the basis of the products formed, we conclude that the
evolution of the dimolybdenum radicals thus generated would
be critically dependent on the presence of a P-H bond in
the phosphide ligand. In the absence of such a bond (PR2
bridges), the dimolybdenum radicals would just couple to
hindrance in the approach to the dimetal center of a relatively
bulky base such as DBU. The structures of these anionic
complexes are strongly related to those of their neutral
derivatives and will be discussed later on.
Although the (H-DBU)+ salts 3a, 3b, 4a, and 4b are easily
generated in tetrahydrofuran solution, they are still quite air-
sensitive, and no attempts were made to isolate these
materials. Instead, their reactivity could be easily investigated
using these solutions prepared in situ. Indeed, reactions with
p-benzoquinone proceed rapidly at 243 K, the result being
strongly dependent on the presence of P-H bonds in the
phosphide ligand bridging the dimetal center (Scheme 1).
Thus, the PR2-bridged anions 3a and 3b give the neutral
alkoxo complexes [Mo2Cp2(OC6H4OH)(µ-PR2)(CO)4] [R )
Cy (5a), R ) Ph (5b)], having a p-hydroxyphenoxo (hyd-
roquinone) ligand terminally bound to a Mo atom. In contrast,
the PHR-bridged anions 4a and 4b give the alkoxyphosphide
anionic complexes (H-DBU)[Mo2Cp2{µ-PR(OC6H4OH)}-
(CO)4] [R ) Cy (6a), R ) Ph (6b)]. The neutral complexes
5a and 5b are rather unstable, and only 5b could be isolated
as a reasonably pure solid. Interestingly, they decompose
upon chromatography on alumina even at 253 K to give the
corresponding radicals [Mo2Cp2(µ-PR2)(CO)4] (previously
characterized by us when R ) Ph),11a which is suggestive
of the occurrence of an unexpected homolytic cleavage of
the Mo-O bond, a matter to be discussed later.
For the anionic compounds, 6a and 6b are quite reactive
species, and no attempts to isolate them were made. Instead,
these complexes were converted in situ into the correspond-
ing hydride derivatives [Mo2Cp2(µ-H){µ-PR(OC6H4OH)}-
(CO)4] (7a and 7b) through reaction with [NH4]PF6. Com-
pounds 6 and 7 are formally derived from the insertion of
p-benzoquinone into the P-H bond of a phosphide ligand.
This is a very unusual reaction, and it has been observed
previously only in a secondary phosphine ligand coordinated
to an iron center,9 as mentioned above. Moreover, it should
be noted that free primary phosphines do not experience this
(16) Pudovik, A. N.; Romanov, G. V.; Pozhidaev, V. M. IzV. Akad. Nauk
SSSR, Ser. Khim. 1978, 1660.
(17) Hermecz, I. AdV. Heterocycl. Chem. 1988, 42, 83.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9595

Alvarez et al.
Table 1. IR and 31P{1H} NMR Data for New Non-Hydride
Compounds
Scheme 2. Reaction Pathways Proposed for the Reactions of
p-Benzoquinone (Q) with the Anionic Complexes 3 and 4 (QH )
hemiquinone, Mo ) MoCp(CO)2)
ν(CO)a
(cm-1
compound
)
δ(P)b
210.0
[Mo2Cp2(OC6H4OH)(µ-PCy2)(CO)4] (5a) 1974 (vs),
1912 (vs),
1852 (m)
[Mo2Cp2(OC6H4OH)(µ-PPh2)(CO)4] (5b) 1981 (vs),
182.5 (br)c,d
1922 (vs),
1862 (m)
[Mo2Cp2(µ-PCy2)(µ-SPh)(CO)4] (10)
[Mo2Cp2(µ-PCy2)(µ-SPh)(CO)2] (11a)
[Mo2Cp2(µ-PPh2)(µ-SPh)(CO)2] (11b)
1933 (s),
1854 (m)
1882 (w, sh), 127
1844 (vs)
-71.8
1889 (w, sh), 107.3
1852 (vs)e
a Recorded in tetrahydrofuran solution, unless otherwise stated. b Re-
corded in CD2Cl2 solution at 290 K and 121.50 MHz unless otherwise stated;
δ relative to external 85% aqueous H3PO4. c Recorded in tetrahydrofuran
solution with external D2O. d When recorded at 213 K and 162.0 MHz in
CD2Cl2 solution, two resonances are present at 185.1 (isomer A) and 184.9
ppm (isomer B). e Recorded in dichloromethane solution.
{µ-(R2N)P(CO)P(NR2)}(CO)6] which, upon CO extrusion are
thought to generate phosphorus-centered phosphinidene
diradicals [Fe2{µ-P(NR2)}2(CO)6], which are able to react
with a great variety of single H-X and multiple C-X bonds
(X) O, N, etc.).20 Given the reversible character of the
elementary steps proposed, the formation of the alkoxyphos-
phide compounds 7a and 7b instead of the corresponding
isomeric alkoxocomplexes [Mo2Cp2(OC6H4OH)(µ-PHR)-
(CO)4] (not detected) might have essentially a thermody-
namic origin, since the generation of the latter requires the
formation of a relatively weak Mo-O bond (soft/hard
combination) compared to the strong P-O bond involved
in the formation of hydrides 7a and 7b. Obviously this
difference in not compensated by the distinct strengths of
the respective Mo-H and P-H bonds.
Structure of the Hemiquinone Complexes 5. As stated
above, compounds 5 are rather unstable species, and their
structural characterization has been made only on the basis
of spectroscopic data in solution. Their IR spectra (Table 1)
exhibit C-O stretching bands very similar (both in frequency
and intensity) to those of the chloro-complex [Mo2ClCp2-
(µ-PHPh)(CO)4],18 and therefore a similar structure is as-
sumed for these alkoxocomplexes, with the hemiquinone
ligand positioned cis with respect to the phosphide ligand,
which in turn acts as the unique bridge between the
inequivalent MoCp(CO)2 moieties. In addition, compound
5b was found to exhibit dynamic behavior in solution (no
detailed spectroscopic data were collected for the less-stable
dicyclohexyl compound 5a). This is readily apparent because
the 1H NMR spectrum at room temperature exhibits just one
broad cyclopentadienyl resonance at 5.1 ppm, inconsistent
with the proposed structure. The analysis of the low-
temperature NMR data (Table 1 and Experimental Section)
reveals the presence of two isomers, A and B, interconverting
the hemiquinone radical via the metal and oxygen atoms to
give the hemiquinone complexes 5a and 5b. This is not
surprising in view of the known chemistry and structural data
of [Mo2Cp2(µ-PPh2)(CO)4], which are characteristic of a
metal-centered radical.11c,a Interestingly, this coupling process
seems to be reversible, since the attempted chromatography
of compounds 5 on alumina columns gives the corresponding
dimolybdenum radicals, as stated above. This in turn will
be of help in understanding the dynamic behavior of these
compounds in solution (see below).
When the starting substrate has a PHR bridge, we propose
that the phosphide-bridged radical A would be in equilibrium
with the corresponding hydride-phosphinidene tautomer B,
this following from the oxidative addition of the P-H bond
to the unsaturated metal center. As a result, the unpaired
electron would be likely located at the bridging phosphorus
atom, thus explaining the formation of alkoxyphosphide
derivatives 7a and 7b by radical P-O coupling at this site.
Finally, since DBU is present in the medium, deprotonation
would then occur to give the anions 6a and 6b, as shown by
independent experiments. Although there is no strict prece-
dent in the literature for the proposed phosphide/hydrido-
phosphinidene equilibrium at a paramagnetic substrate, we
recall that a related P-H bond cleavage has been recently
proposed by us to occur at the unsaturated dimolybdenum
cations [Mo2Cp2(µ-PHR)(CO)4]+ (here involving trigonal
phosphinidene groups).18 Interestingly, the reverse reaction
(P-H reductive elimination) seems to be thermodynamically
favored at anionic derivatives containing bent phosphinidene
and hydride bridges, as observed for the cluster [Os3(µ-H)-
(µ2-PPh)(CO)10]-19a,b and proposed for the dimanganese
anion [Mn2(µ-H)(µ-PCy)(CO)8]-.19c On the other hand, the
formation of a bent phosphinidene group bearing an unpaired
electron is itself almost unprecedented. We note, however,
the extensive studies performed on the diiron complexes [Fe2-
(19) (a) Colbran, S. B.; Johnson, B. F. G.; Lewis, J.; Sorrell, R. M. J.
Organomet. Chem. 1985, 296, C1. (b) Deeming, A. J.; Doherty, S.;
Day, M. W.; Hardcastle, K. I.; Minassian, H. J. Chem. Soc., Dalton.
Trans. 1991, 1273. (c) Haupt, H.-J.; Schwefer M.; Egold, H.; Flo¨rke,
U. Inorg. Chem. 1995, 34, 5461.
(18) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa-Vivo, D.; Garc´ıa, M. E.; Ruiz,
M. A.; Sa´ez, D.; Falvello, L. R.; Soler, T.; Herson, P. Dalton. Trans.
2004, 4168.
(20) King, R. B. J. Organomet. Chem. 1998, 557, 29.
9596 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Chart 2
Scheme 3. Dynamic Process Proposed for Hemiquinone Complex 5b
in Solution
rapidly on the NMR time scale (Chart 2). Separate 31P, H,
and 13C NMR resonances could be observed at 243 K, (A/B
ratio ) 5 at this temperature), which revealed that both
isomers display inequivalent cyclopentadienyl resonances,
thus suggesting the terminal coordination of the hemiquinone
ligand in both cases. In agreement with this, the 31P chemical
shifts of ∼180 ppm are the values expected for dicyclo-
hexylphosphide ligands bridging single Mo-Mo bonds.14,21
The 13C NMR spectrum of the major isomer A is fully
consistent with the crystal structure of the model complex
[Mo2ClCp2(µ-PHPh)(CO)4], because it displays four distinct
carbonyl resonances in the terminal region at 226.7 (s, br),
241.6 (s), 241.7 (d, JCP ) 20 Hz), and 248.8 (d, JCP ) 17
Hz) ppm, their multiplicity being consistent respectively with
the presence of two ligands cis and two other trans with
respect to the phosphorus atom.18 From the available data
for the minor isomer, we conclude that its structure would
be quite similar to that of the major species. In fact, we
propose that isomer B has the same structure as A, but with
the hemiquinone ligand and the Cp group of the adjacent
metal center placed on the same side of the Mo2P plane
(Chart 2). The presence of two asymmetric isomers, however,
does not explain the appearance of just a single cyclopen-
tadienyl resonance at room temperature for compound 5b.
Although we have not studied this process in detail, we
propose that a homolytic cleavage of the P-O bond (actually,
the reverse of the formation process) can take place in
solution to a small extent (Scheme 3). This would generate
small amounts of hydroquinone (‚QH) and the tetracarbonyl
radical [Mo2Cp2(µ-PPh2)(CO)4], shown previously to display
a cisoid arrangement of its MoCp(CO)2 metal fragments.11a
Recombination of hemiquinone with that binuclear radical
could happen at either of the molybdenum atoms, thus
justifying the averaging of the inequivalent Cp resonances.
In addition, recombination of hemiquinone could also take
place on either side of the Mo2P plane, thus justifying the
interconversion between isomers A and B (the major isomer
A is proposed to be the one involving minimum repulsions
between hemiquinone and Cp groups, Chart 2). Although
the above hypothesis is consistent with the chemical behavior
of compounds 5 (decomposition on alumina columns and
reactions with HSPh or S2Ph2), other possibilities such as
intramolecular rearrangements cannot be excluded at this
time.
1
Scheme 4
Reaction of the Hemiquinone Complexes 5 with HSPh.
Compounds 5a and 5b react with an excess of HSPh at room
temperature to give the red tetracarbonyl complex [Mo2Cp2-
(µ-PCy2)(µ-SPh)(CO)4] (10) and the green dicarbonyl com-
plex [Mo2Cp2(µ-PPh2)(µ-SPh)(CO)2] (11b), respectively
(Scheme 4). The formation of these products requires the
elimination of hydroquinone (QH2) and, in the case of the
diphenylphosphide compound, the spontaneous loss of two
CO molecules. Decarbonylation also occurs for the dicyclo-
hexylphosphide compound 10 upon prolonged storage at
room temperature under nitrogen or rapidly in toluene
solution at 75 °C to cleanly give the analogous dicarbonyl
[Mo2Cp2(µ-PCy2)(µ-SPh)(CO)2] (11a). From the distinct
conditions required to form dicarbonyls 11a and 11b, we
conclude that electron-releasing substituents on phosphorus,
rather than bulky substituents at that position, favor in this
case the decarbonylation of the corresponding tetracarbonyls,
in agreement with previous findings by Mays et al. on mixed
phosphide or phosphide-arsenide dimolybdenum com-
plexes.13 On the other hand, we note that 11b can be also
prepared in good yield by reacting 5b with diphenyldisulfide
(see Experimental Section); this presumably requires the
(21) Carty, A. J.; MacLaughlin. S. A.; Nucciarone, D. In Phosphorus 31
NMR Spectrscopy in Stereochemical Analysis; Verkade, J. G., Quin,
L. D., Eds.; VCH: New York, 1987; Chapter 16.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9597

Alvarez et al.
Scheme 5
Chart 3
repulsions, we propose for 10 the cis-exo geometry (Chart
3). Incidentally, this is the type of geometry found for the
mentioned dithiolate complex of tungsten23 and the related
dimolybdenum complexes [Mo2Cp*2(µ-ER)2(CO)4] [ER )
SMe, SPh, SeMe, SePh].24 Pyramidal inversion at the sulfur
atom,25 however, has been found to occur rapidly on related
thiolate complexes;26 therefore the possibility of fast inter-
conversion between cis-exo and cis-endo isomers cannot
be completely ruled out.
formation of hydroquinone and quinone. Finally, we recall
that compound 11b was first prepared by us from the reaction
of the 33-electron radical [Mo2Cp2(µ-PPh2)(CO)4] with
HSPh.11c All the above data clearly points to a radical
mechanism for the reactions of these hemiquinone complexes
(Scheme 5), which then seem to behave as effective sources
of the corresponding dimolybdenum radicals. As proposed
above to account for the dynamic behavior of compounds 5
in solution, it is thought that these hemiquinone complexes
experience an homolytic cleavage of the Mo-O bond in
solution, to a small extent, to give the corresponding radicals
[Mo2Cp2(µ-PR2)(CO)4] and hemiquinone (‚QH). The dimo-
lybdenum radicals are themselves reactive toward HSPh and
S2Ph2, as shown by independent experiments on the diphe-
nylphosphide radical mentioned above,11c while hemiquinone
is likely to be converted into hydroquinone by reaction with
the sulfur reagent. The initial metallic product would be in
any case a tetracarbonyl complex that, only in the case of
the diphenylphosphide bridge, experiences fast decarbonyl-
ation to give the 32-electron derivative 11b.
Dicarbonyl complexes 11a and 11b are isoelectronic to
the dithiolate compounds [Mo2Cp2(µ-SR)2(CO)2], which can
display cis or trans geometries,27 and to the bis(phosphide)
complexes [M2Cp2(µ-PR2)(µ-PR′2)(CO)2] (M) Mo, W; R,
R′) Ph, Cy, Et, etc),14,29 for which analogous isomerism is
possible. The spectroscopic data for compounds 11 (Table
1 and Experimental Section) are in fact quite similar to those
of the bis(phosphide) trans-dicarbonyl complexes mentioned
above,14 characterized by a substantially shielded 31P NMR
resonance and C-O stretching bands of weak and strong
intensities (in order of decreasing frequency), as expected
for transoid M2(CO)2 oscillators.30 Thus, a similar trans
geometry is assumed for compounds 11 (Chart 3). In
addition, the presence of the pyramidal phenylthiolate bridge
now makes the carbonyl, cyclopentadienyl, and Cy (11a) or
Ph (11b) groups inequivalent, as observed in the correspond-
1
ing 13C and H NMR spectra (see Experimental Section).
Structure of Thiolate Derivatives. Compound 10 displays
a quite shielded 31P resonance (-71.8 ppm) which suggests
the absence of any metal-metal bond in the molecule.21
According to the EAN rule, this is achieved if the thiolate
ligand adopts a bridging position and then contributes with
three electrons to the dimetal center. The latter can be
accomplished in four different ways depending on the relative
arrangement of the MoCp(CO)2 moieties (cis or trans) and
the position of the phenyl substituent on sulfur (endo or exo)
with respect to the puckered SMo2P ring. The IR spectrum
of 10 in tetrahydrofuran solution displays two C-O stretch-
ing bands at 1933 (s) and 1854 (m) cm-1, these being similar
to those exhibited by related isoelectronic tetracarbonyls such
as [W2Cp2(µ-SPh)2(CO)4],22 cis-[W2Cp2(µ-SCHMe2)2CO)4],23
or trans-[Mo2Cp2(µ-PHPh){µ-P(OEt)2}(CO)4].13 This sug-
gests that the C-O stretching bands are little sensitive to
the relative arrangement of the metal fragments in these
molecules lacking a metal-metal bond. However, by con-
Functionalization at Phosphorus by Hemiquinone Dis-
placement in Compound 6a. The anionic (p-hydroxyphen-
oxy)phosphide complexes 6a and 6b are quite unstable and
reactive species. Despite this, we have been able to show
that the cyclohexyl complex 6a can be an useful synthetic
intermediate since it reacts with a variety of HER molecules
(E ) O, R ) H, Me, Ph, C4H5; E ) S, R ) Ph) in the
presence of DBU to give with good yields the corresponding
anions [Mo2Cp2{µ-PCy(ER)}(CO)4]- [ER ) OH (12, two
tautomers), OMe (13), OC4H5 (14), OPh (15), SPh (16)],
which follow from nucleophilic substitution of QH- by ER-
(24) Schollhammer, P.; Pe´tillon, F. Y.; Pichon, R.; Poder-Guillou, S.;
Talarmin, J. Organometallics 1995, 14, 2277 and references therein.
(25) Abel, E.; Bhargava, S. K.; Orrell, K. G. Prog. Inorg. Chem. 1984,
32, 1.
(26) Courtot-Coupez, J.; Gue´guen, M.; Guerchais, J. E.; Pe´tillon, F. Y.;
Talarmin, J. Mercier, R. J. Organomet. Chem. 1986, 312, 81.
(27) Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J.; Muir, K. W. Coord.
Chem. ReV. 1998, 178-180, 203.
(28) (a) Abasq, M.-L.; Huger, D. L.; Pe´tillon, F. Y.; Pichon, R.; Pickett,
C. J.; Talarmin, J. J. Chem. Soc., Dalton Trans. 1997, 2279. (b)
Benson, I. B.; Killops, S. D.; Knox, S. A. R.; Welch, A. J. J. Chem.
Soc., Chem. Commun. 1980, 1137.
1
sidering that the Cp ligands in 10 give rise to single H or
13C resonances, we must assume a cisoid arrangement of both
metal fragments. Finally, on the basis of minimum steric
(29) Adatia, A.; McPartlin, M.; Mays, M. J.; Morris, M. J.; Raithby, P. R.
J. Chem. Soc., Dalton Trans. 1989, 1555.
(30) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London,
1975.
(22) Weinmam, D. J.; Abrahamson, H. B. Inorg. Chem. 1987, 26, 2133.
(23) Shaver, A.; Lum, B. S.; Bird, P.; Livingstone, E.; Schweitzer, M. Inorg.
Chem. 1990, 29, 1832.
9598 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Scheme 6
Figure 1. ORTEP diagram (30% probability) of compound 7b with H
atoms (except the hydride ligand) omitted for clarity.
anions at the phosphorus site (Scheme 6). As expected, the
latter complexes are readily protonated by [NH4]PF6 to give
the corresponding hydrides [Mo2Cp2(µ-H){µ-PCy(ER)}-
(CO)4] [ER) OH (8a), OMe (17), OC4H5 (18), OPh (19),
SPh (20)], which can be thus isolated in high yield and fully
characterized.
The lability of the phosphorus-hemiquinone bond in
compound 6a is quite unexpected in view of the great
strength of P-O bonds. Even more surprising is the
formation of the thiolatephosphide complex 16, since P-S
bonds are considerably weaker than P-O bonds (201 vs 407
kJ).31 Unfortunately, we have found in the literature no
reactivity data to be used for comparative purposes. A
phosphorus-hemiquinone bond is also present in the iron
cations [FeCpL2{PRH(OC6H4OH)]+, (L2) (CO)2 or diphos-
phine),9 but no reactivity appears to have been reported for
these complexes. Further studies must be carried out, in any
case, to establish the synthetic use of this unexpected lability
of the hemiquinone-phosphorus bond at metal-bound phos-
phide ligands.
Figure 2. ORTEP diagram (25% probability) of compound 8a and the
tetrahydrofuran molecule H-bonded to it with H atoms (except the hydride
and hydroxyl ones) omitted for clarity.
Solid State Structure of Hydride Complexes [Mo2Cp2-
(µ-H){(µ-PR(ER′)}(CO)4]. Upon crystallization in different
solvent mixtures, crystals of 7b, 8a‚OC4H8, 17, and 19
suitable for X-ray diffraction were formed. The molecular
structures are shown in Figures 1-4. All of them exhibit
MoCp(CO)2 fragments bridged by phosphide and hydride
ligands, and are placed in a transoid arrangement with respect
to the average Mo2(µ-H)(µ-P) plane, except compound 17,
which exhibits two almost eclipsed MoCp(CO)2 moieties (cis
geometry). Out of the two possible cis isomers, the crystals
of 17 correspond to the one with the bulkier group (Cy) close
to the Cp ligands. While trans isomers are most commonly
found for phosphide-hydride complexes of type [M2Cp2(µ-
H)(µ-PRR′)(CO)4], the precedents for related cis complexes
are restricted to [Mo2{µ-(η5-C5H4)2SiMe2}(µ-H)(µ-PMe2)-
(CO)4],32 where the cis geometry is forced by the presence
of the linked cyclopentadienyl ligands, and the mesitylphos-
phide complexes [Mo2Cp2(µ-H)(µ-PXMes)(CO)4], (X ) H,
F) recently reported by us.18 However, related cis/trans
Figure 3. ORTEP diagram (30% probability) of compound 17 with H
atoms (except the hydride ligand) omitted for clarity.
isomerism has been previously found and discussed in detail
for the thiolate-bridged complexes [Mo2Cp2(µ-H)(µ-SR)-
t
(CO)4] (R ) Me, Bu, Ph).33
The relevant bond lengths and angles in compounds 7b-
19 are very similar to each other (Table 2), and they also
comparable to those measured previously in related phos-
phide-hydride complexes,18 thus deserving no special com-
ment. The P-O lengths in the alkoxy- or hydroxyphosphide
ligands have values consistent with single bonds, but they
(31) Holleman-Wiberg’s Inorganic Chemistry; Wiberg, N., Holleman, A.,
Wiberg, E., Eds.; Academic Press: London, 2001; p 131.
(32) Heck, J. J. Organomet. Chem. 1986, 311, C5.
(33) Muir, K. W.; Girdwood, S. E.; Pe´tillon, F. Y.; Pichon, R.; Poder-
Guillou, S.; Schollhammer, P.; Talarmin, J. J. Organomet. Chem. 1995,
486, 183.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9599

Alvarez et al.
hemiquinone and the oxygen atom of a carbonyl ligand in
an adjacent molecule.
Solution Structure of Hydride Complexes [Mo2Cp2(µ-
H){(µ-PR(ER′)}(CO)4]. Spectroscopic data in solution for
the title compounds (Table 3 and Experimental Section) are
consistent with the solid-state structures discussed above and
are also comparable to the data measured for related dialkyl-
or diarylphosphide dimolybdenum complexes, which we
have discussed in detail previously,14,18 thereby needing no
detailed discussion. The phenylphosphide compounds 7b and
8b exist in solution exclusively as trans isomers and exhibit
a fluxional process rendering equivalent cyclopentadienyl and
pairs of CO ligands, as found previously for other trans-
[Mo2Cp2(µ-H)(µ-PRR′)(CO)4] complexes.14,15,18 The cyclo-
hexyl products, however, display more complex IR spectra,
which is suggestive of the presence of both cis and trans
isomers in solution (Chart 4). From the previous IR data on
cis and trans isomers in complexes [Mo2Cp2(µ-H)(µ-PRR′)-
(CO)4]18 and [Mo2Cp2(µ-H)(µ-SR)(CO)4],33 we can safely
conclude that compounds 17 and 18 (OMe and OC4H5
substituents on phosphorus) have the highest relative amount
of the cis isomer (presumably, the same as that found in the
crystals of 17), characterized by two strong IR bands at
frequencies ∼15-30 cm-1 above the strong bands of the
trans isomers (Table 3). These isomers interconvert rapidly
on the NMR time scale, so only averaged NMR parameters
are obtained at room temperature. The products with phenoxy
or thiophenoxy substituents on phosphorus, however, have
a smaller proportion of the cis isomer. Indeed, the low-
temperature NMR spectra of compounds 19 and 20 exhibited
separate resonances for both isomers and yielded a trans/cis
ratio of ∼8 in both cases, with no significant dependence
on temperature. Finally, the weak additional bands present
in the IR spectra of the products with hemiquinone (7a),
hydroxyl (8a), and oxo (9) substituents on phosphorus,
suggest the presence of only tiny amounts of the correspond-
ing cis isomers in solution. In fact, the low-temperature NMR
spectra of the hemiquinone compound 7a failed to reveal
separate resonances for any minor isomer. Moreover, we note
that our structural proposal for the side product 9 is based
Figure 4. ORTEP diagram (30% probability) of compound 19 with H
atoms (except the hydride ligand) omitted for clarity.
Table 2. Selected Bond Lengths and Angles for Compounds 7b, 8a,
17, and 19
parametera
trans-7b
trans-8a
trans-19
cis-17
Mo-Mo′
Mo-H
Mo-P
3.2683(5)
1.95(4)
2.395(1)
1.930(6)
1.926(6)
1.69(4)
2.395(1)
1.951(5)
1.957(5)
1.661(3)
1.811(4)
3.2681(6)
1.86(4)
2.405(1)
1.955(4)
1.946(4)
1.87(4)
2.420(1)
1.981(4)
1.937(4)
1.631(2)
1.841(3)
3.2556(4)
1.85(3)
2.411(1)
1.933(2)
1.957(2)
1.84(3)
2.404(1)
1.961(2)
1.949(2)
1.661(1)
1.852(2)
3.2757(6)
1.83(5)
2.415(1)
1.966(4)
1.952(4)
1.82(5)
2.401(1)
1.963(5)
1.937(5)
1.640(3)
1.847(3)
Mo-C1
Mo-C2
Mo′-H
Mo′-P
Mo′-C3
Mo′-C4
P-O
P-C5
Mo-P-Mo′
C1-Mo-C2
P-Mo-C1
P-Mo-C2
C3-Mo′-C4
P-Mo′-C3
P-Mo′-C4
C5-P-O
86.05(3)
77.3(3)
113.1(2)
78.5(2)
79.9(2)
113.2(2)
81.7(2)
85.28(3)
77.5(2)
112.0(1)
78.4(1)
77.6(2)
122.1(1)
81.5(1)
96.5(1)
85.10(2)
78.7(1)
102.2(1)
76.1(1)
77.3(1)
109.6(1)
78.9(1)
85.73(3)
79.6(2)
108.1(1)
77.2(1)
79.3(2)
112.6(1)
80.7(1)
96.3(2)
100.8(2)
101.0(1)
a Bond lengths and angles in angstroms or degrees, respectively,
according to the labeling shown in the figure below.
1
on the strong similitude of its IR, H NMR, and 13C NMR
spectra with those of the hydroxyphosphide complex 8a,
added to the absence of any OH proton resonance or O-H
IR stretch.
are slightly longer (by ∼0.02 Å) for those molecules having
aromatic rings on oxygen (7b and 19). This might be related
to the experimental lability exhibited by the corresponding
bond in the case of the hemiquinone compound 6a.
The 31P chemical shifts of the hydride compounds 7-19
are all above 300 ppm, as expected from the presence of an
electron-withdrawing substituent (OR, OH) on the phosphide
ligand. Although there is a limited number of phosphide
complexes combining alkyl (or aryl) and alkoxy substituents
on phosphorus, we note that similar shifts have been reported
for related ligands, such as those present in the mononuclear
Some of these complexes displayed H-bonding in the
crystal. In the case of the hydroxyphosphide complex 8a,
the crystal contains a tetrahydrofuran molecule H-bonded
to the hydroxyl group (Figure 2). The corresponding inter-
nuclear separations [P-O-H‚‚‚O ) 1.903 Å, P-O‚‚‚O )
2.730 Å] are indicative of the presence of a hydrogen bond
of medium strength.34,35 The structure of hydroquinone
compound 7b features also some intermolecular hydrogen
bonding, now of low strength (O‚‚‚O) 3.03 Å), resulting in
the formation of a 1-D supramolecular organometallic
network running along the c axis. Here, the relevant
interaction is established between the hydroxyl group of the
t
complex [MoCp{P(OMe)Mes*}(CO)2] (Mes*) 2,4,6-C6H2 -
(34) (a) Dekock, R. L.; Gray, H. B. Chemical Structure and Bonding;
Benjamin/Cummings: Menlo Park, CA, 1980; Chapter 7. (b) Green-
wood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-
Heinemann: Oxford, U.K., 1997; Chapter 3.
(35) (a) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; Oxford
University Press: Oxford, U.K., 1999; Chapter 3. (b) Steiner, T.
Angew. Chem., Int. Ed. 2002, 41, 48.
9600 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Table 3. Selected IR and NMR Data for New Hydride [Mo2Cp2(µ-H)(µ-PRR′)(CO)4] Compounds
R
R′
compd
ν(CO)a (cm-1
)
δ(P)b (ppm)
δ(µ-H)b (ppm) [JPH
]
Cy
Ph
Cy
Ph
Cy
Cy
Cy
Cy
OC6H4OH
OC6H4OH
OH
OH
O
OMe
OC4H5
OC6H5
7a
7b
8a
8b
9
17
18
19
1973 (vw), 1959 (vw), 1936 (vs), 1896(w, sh), 1872 (s).
1978 (w,sh), 1943 (vs), 1880 (s).
356.3
319.1
-11.89 [39]
-11.37 [40]
-10.83 [40]
-10.80 [40]d
-11.12 [38]
-11.09 [41]f
-11.22 [41]
-10.90 [42] (cis)
1963 (vw), 1951 (vw), 1930 (vs), 1884 (w, sh), 1867 (s).
337.6
1938 (vs), 1875(s).c
301.8d
1962 (vw), 1947 (vw), 1926 (vs), 1862(s).
1968 (vs),e 1933 (vs),1889 (s),e 1871 (s).
1968 (s),e 1934 (vs), 1889 (s),e 1871 (s)
1974 (vw), 1959 (vw), 1937 (vs), 1897 (w, sh), 1874 (s)
259
358.6f
352.0
358.3 (trans)
351.1 (cis)g
232.9 (trans)
217.7 (cis)h
-11.99 [38] (trans)g
-11.77 [38] (cis)
Cy
SPh
20
1968 (m), 1939 (vs), 1874 (s).
-12.48 [35] (trans)h
a Recorded in tetrahydrofuran solution, unless otherwise stated. b Recorded in CD2Cl2 solutions at 290 K and 300.13 (1H) and 121.50 MHz (31P) unless
otherwise stated; δ in parts per million relative to internal TMS (1H) or external 85% aqueous H3PO4; J in hertz. c In CH2Cl2 solution. d In CDCl3 solution.
e Bands assigned to the cis isomer. f Recorded at 200.1 (1H) or 81.04 MHz (31P). g Recorded at 233 K and 400.13 (1H) or 162.0 MHz (31P); when recorded
at 290 K, only a broad resonance at 353.4 ppm (31P) and at -11.82 ppm (d, JPH) 39) (1H) is observed. h Recorded at 243 K and 400.13 (1H) or 162.0 MHz
(31P).
Table 4. IR and 31P NMR Data for New Anionic Compounds (H-DBU)[Mo2Cp2(µ-PRR′)(CO)4]
R
R′
compd
ν(CO)a (cm-1
)
δ(P)b (ppm) [JPH
]
Cy
Cy
Ph
Cy
Ph
Cy
Cy
H
H
OC6H4OH
OC6H4OH
OH
3a
4a
4b
6a
6b
12d
1869 (w), 1840 (vs), 1777 (s), 1756 (m, sh)
1878 (m), 1836 (vs), 1791 (s), 1768 (m)
1882 (m), 1841 (vs), 1797 (s), 1774 (m)
1881 (w), 1850 (vs), 1792 (s), 1770 (m, sh)
1888 (m), 1850 (vs), 1805 (s), 1785 (m)
1903 (s), 1881 (w), 1836 (vs), 1786 (m), 1765 (w).
248.3
180.6 [303]c
143.0 [327]c
384.5
336.0
353.8 (12A)
314.8 [38] (12B)
376.8
Cy
Cy
Cy
Cy
OMe
OC4H5
OPh
13
14
15
16
1880 (m), 1848 (vs), 1839 (s), 1789 (s), 1770 (m sh)
1880 (w), 1848 (s), 1838 (vs), 1790 (s), 1768 (m, sh)
1884 (d), 1852 (vs), 1794 (s)
373.1
382.5e
SPh
1881 (m), 1846(vs), 1797(m), 1776(w).
257.3
a Recorded in tetrahydrofuran solution, unless otherwise stated. b Recorded in tetrahydrofuran solution with external D2O at 290 K and 121.50 MHz,
unless otherwise stated; δ relative to external 85% aqueous H3PO4. c Recorded in CD2Cl2 solution at 290 K and 81.04 MHz. d This compound appears in
solution as a mixture of phosphide (tautomer 12A) and hydride-oxophosphinidene isomers (tautomer 12B, see text), in similar amounts. e Recorded at 81.04
MHz.
Chart 4
hydrides (mostly exhibiting trans geometries). Indeed, we
note that retention of transoid geometries has been crystal-
lographically verified in the pairs [Mo2Cp2(µ-H)(µ-PR2)(CO)4]/
[Mo2Cp2(µ-PR2)(CO)4]-, (R ) Ph,12b Me12c). However, since
cis and trans isomers in hydride complexes 17-20 are
interconverting rapidly in solution (as previously found for
the isomers in the mentioned complexes [Mo2Cp2(µ-H)(µ-
Bu3, δP ) 350.5)36 or in the cluster anion [FeCo2(CO)9{µ-
PPh(OMe)}]- (δP ) 320.0).37
PXMes)(CO)4]),18 it is clear that the structures of the anions
and those of the corresponding hydrides may differ and
cannot be deduced from each other. An inspection of the IR
spectra of the new anionic complexes described in this work
(Table 4) reveals that most of them exhibit the same pattern,
with four bands at ∼1880 (m to w), 1840 (vs), 1790 (s) and
1770 (m) cm-1. These spectra are thus similar to those of
the diphenylphosphide complexes (H-DBU)[M2Cp2(µ-H)(µ-
PPh2)(CO)4] (M ) Mo, W)11a and to that reported for the
salt (PPN)[Mo2Cp2(µ-H)(µ-PPh2)(CO)4], which displays a
transoid geometry in the solid state.12b Therefore, we deduce
that anions having PCy2 (3a), PCyH (4a), PPhH (4b), PCy-
(OC6H4OH) (6a), PPh(OC6H4OH) (6b), PCy(OPh) (15), and
PCy(SPh) (16) bridges exist in solution essentially as trans
isomers. We note that these are the anions whose hydride
derivatives exhibit trans isomers as unique or very major
species in solution. In contrast, anions with PCy(OMe) (13)
and PCy(OC4H5) (15) bridges (hydride derivatives exhibiting
the highest proportion of cis isomers) display different IR
spectra, characterized by the presence of an additional strong
band at ∼10 cm-1 below the usual strongest band (the
relative intensities of the bands are also somewhat different).
As stated above, cis/trans isomerism in phosphide-hydride
complexes related to compounds 7-20 has been only
observed previously for the mesityl complexes [Mo2Cp2(µ-
H)(µ-PXMes)(CO)4], (X ) H, F).18 By considering the
observed cis/trans ratios, we find the following order:
PCyOMe (∼1) > PMesH (0.5) > PCyOPh ) PCySPh
(∼0.15) > PCy(OC6H4OH) ) PCyOH (∼0). Clearly, there
is not a single electronic or steric parameter governing the
relative amounts of the cis and trans isomers in these
substrates. It rather seems that the cis isomers in these
dimolybdenum complexes are favored by the combination
of a relatively bulky group (Cy, Mes) with a small electron-
withdrawing substituent (H, OMe).
Structure of Anionic Complexes (H-DBU)[Mo2Cp2(µ-
PRX)(CO)4], (X ) R, H, ER). It is generally assumed that
the structures of anionic complexes of type [M2Cp2(µ-PRR′)-
(CO)4]- are strongly related to those of the corresponding
(36) Alonso, M.; Alvarez, M. A.; Garc´ıa, M. E.; Ruiz, M. A.; Hamidov,
H.; Jeffery, J. C. J. Am. Chem. Soc. 2004, 126, 13610.
(37) Don, M.-J.; Richmond, M. G. Inorg. Chem. 1991, 30, 1703.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9601

Alvarez et al.
followed by proton capture from the (H-DBU)+ cation to
give hemiquinone and the corresponding 33-electron radicals
[Mo2Cp2(µ-PRR′)(CO)4]. The latter evolve differently de-
pending on the nature of the phosphorus ligand. Thus, PR2-
bridged radicals just couple to hemiquinone at the metal site
to give the hemiquinone complexes [Mo2Cp2(OC6H4OH)-
(µ-PR2)(CO)4], which exhibit facile homolytic cleavage of
the corresponding Mo-O bonds; then they behave as a
source of their paramagnetic precursors. In contrast, PHR-
bridged substrates experience overall insertion of quinone
into the P-H bond through a reaction sequence thought to
involve an oxidative addition of the P-H bond to the metal
center at the intermediate radicals [Mo2Cp2(µ-PRH)(CO)4],
followed by coupling, at the phosphorus site, of hemiquinone
and the resulting phosphinidene intermediate. The anions (H-
DBU)[Mo2Cp2{µ-PR(OC6H4OH)}(CO)4] thus formed also
exhibit facile release of the hemiquinone group, now via the
heterolytic cleavage of the P-O bond by reaction with ER-
anions (E ) O, S; R ) H, alkyl, aryl), to give derivatives
bridged by novel PR(ER) phosphide ligands.
Scheme 7. Tautomeric Equilibrium Proposed for Compound 12 in
Solution (Mo ) MoCp(CO)2)
We interpret this as an indication of the presence of
substantial amounts of the corresponding cis isomers in the
solutions of these anions. In summary, the isomerism in
anions 3-16 (except the hydroxyphosphide complex 12)
seems to follow the same general trends found for the
corresponding hydride derivatives but with the trans isomers
being more prevalent in the anionic substrates.
Tautomerism in the Hydroxyphosphide Complex 12.
The anionic complex 12 exhibits unique structural features
in solution. Its IR spectrum is completely different from the
other anions since it displays, in addition to the usual C-O
stretching bands, a strong band at quite high frequency (1903
cm-1), while the relative intensities of the rest of bands are
also somewhat modified. This is due to the presence in
solution of a second isomer, which we propose to be the
corresponding hydride-oxophosphinidene tautomer [Mo2Cp2-
(µ-H)(µ-PCyO)(CO)4]- (12B), resulting after proton migra-
tion from the oxygen atom up to the dimetal center (Scheme
7). This expectedly implies a shift of the electronic charge
from the metal to the oxygen atom bound to phosphorus
(PCyO- resonant form), thus explaining the increase of the
C-O stretching frequencies. In agreement with this proposal,
the 31P{1H} NMR spectrum of 12 displays two resonances
of similar intensities. One of them appears at a chemical shift
(354 ppm) comparable to alkoxyphosphide complexes 13 and
14 and is therefore assigned to the hydroxyphosphide
tautomer 12A. The other resonance is significantly lower in
Experimental Section
General Comments. All manipulations and reactions were
carried out using standard Schlenk techniques under an atmosphere
of dry oxygen-free nitrogen (99.995%). Solvents were purified
according to standard literature procedures39 and distilled under
nitrogen prior to use. Petroleum ether refers to that fraction distilling
in the range of 60-65 °C. Compounds [Mo2Cp2(µ-H)(µ-PCy2)-
(CO)4] (1a),14 [Mo2Cp2(µ-H)(µ-PHCy)(CO)4] (2a),14 [Mo2Cp2(µ-
H)(µ-PPh2)(CO)4] (1b),15a [Mo2Cp2(µ-H)(µ-PHPh)(CO)4] (2b),15a
and solutions of [H-DBU][Mo2Cp2(µ-PPh2)(CO)4] (3b),11a were
prepared according to literature procedures. Other reagents were
obtained from the usual commercial suppliers and used without
further purification. Filtrations were carried out using a cannula
or, more generally, through diatomaceous earth, and aluminum
oxide (alumina) for column chromatography was deactivated by
the appropriate addition of water to the commercial material
(Aldrich, neutral, activity I). Low-temperature chromatographic
separations were carried out using jacketed columns refrigerated
by a closed 2-propanol circuit kept at the desired temperature with
a cryostat. NMR spectra were recorded at 300.13 (1H), 121.50 (31P-
{1H}), and 75.47 MHz (13C{1H}) on CD2Cl2 solutions at room
temperature unless otherwise stated. Chemical shifts (δ) are given
in parts per million, relative to internal TMS (1H, 13C) or external
85% H3PO4 aqueous solution (31P), with positive values for
frequencies higher than that of the reference. Coupling constants
(J) are given in hertz.
1
frequency (315 ppm) and splits into a doublet upon H
coupling, with the P-H coupling of 38 Hz being charac-
teristic of the hydride-phosphide couplings in our substrates
(Table 3). Therefore, this resonance is assigned to the
hydride-oxophosphinidene tautomer 12B. It should be noted
that oxophosphinidene complexes are extremely rare species
and their chemistry is virtually unexplored.10,35,38 The anionic
mononuclear complex (H-DBU)[MoCp{P(O)Mes*}(CO)2]
has been shown previously to react with hard electrophiles
such as (Me3O)BF4 at the oxygen site of the oxophosphin-
idene ligand. Compound 12 behaves similarly and reacts
rapidly with the same reagent to give the methoxyphosphide-
hydride complex 17 as unique product (see Experimental
Section). Anion 12, therefore, may have a potential as
synthetic intermediate in the same line as the hemiquinone
compound 7a, a matter to be explored in the future.
Preparation of Solutions of [H-DBU][Mo2Cp2(µ-PCy2)(CO)4]
(3a). Neat DBU (22 µL, 0.147 mmol) was added to a tetrahydro-
furan solution (15 mL) of compound 1a (0.060 g, 0.095 mmol) at
0 °C, and the mixture was stirred at that temperature for 30 min to
give an orange solution of compound 3a which was ready for further
use.
Preparation of Solutions of [H-DBU][Mo2Cp2(µ-PHCy)(CO)4]
(4a). Neat DBU (82 µL, 0.548 mmol) was added to a tetrahydro-
furan solution (15 mL) of compound 1a (0.200 g, 0.363 mmol) at
0 °C, and the mixture was stirred at that temperature for 5 min to
give a cherry red solution of compound 4a which was ready for
further use.
Concluding Remarks
The phosphide-bridged dimolybdenum complexes (H-
DBU)[Mo2Cp2(µ-H)(µ-PRR′)(CO)4] (R) Cy, Ph; R′) R, H)
react readily with p-benzoquinone via electron-transfer
(38) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa, M. E.; Gonza´lez, R.; Ruiz,
(39) Armarego, W. L. F.; Chai, C. Purification of Laboratory Chemicals,
M. A.; Hamidov, H.; Jeffery, J. C. Organometallics 2005, 24, 5503.
5th ed.; Butterworth-Heinemann: Oxford, U.K., 2003.
9602 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Preparation of Solutions of [H-DBU][Mo2Cp2(µ-PHPh)(CO)4]
(4b). Neat DBU (82 µL, 0.548 mmol) was added to a tetrahydro-
furan solution (15 mL) of compound 1a (0.200 g, 0.368 mmol) at
0 °C, and the mixture was stirred at that temperature for 5 min to
give a violet solution of compound 4b which was ready for further
use.
Preparation of Solutions of [Mo2Cp2(OC6H4OH)(µ-PCy2)-
(CO)4] (5a). Solid p-benzoquinone (0.011 g, 0.102 mmol) was
added to a tetrahydrofuran solution of compound 3a (∼0.095
mmol), prepared as described above, and the mixture was stirred
for 10 min to give a brown solution of compound 5a which was
ready for further use. All attempts to isolate this complex as a pure
solid material resulted in its progressive decomposition.
alumina column (activity IV) at 253 K. Elution with dichlo-
romethane/petroleum ether (1:4) gave a yellow fraction yielding,
after removal of solvents under vacuum, compound 9 as a yellow
solid (0.020 g, 10%). Elution with tetrahydrofuran/toluene (1:12)
gave an orange fraction yielding analogously compound 7a as an
orange powder (0.143 g, 60%). Finally, elution with tetrahydrofuran/
toluene (1:6) gave a yellow fraction yielding compound 8a as a
yellow powder (0.51 g, 25%, see later). Data for 7a. Anal. Calcd
for C26H27Mo2O6P: C, 47.43; H, 4.13. Found: C, 47.22; H, 4.30.
1H NMR: δ 6.68 (s, 4H, C6H4), 5.12 (s, br, 10H, Cp), 5.47 (s, 1H,
OH), 2.67-1.27 (m, 11H, Cy), -11.89 (d, JHP ) 39, 1H, µ-H].
13C{1H} NMR (100.63 MHz, 248 K): δ 243.8 (d, JCP ) 24, CO),
240.8 (d, JCP ) 21, CO), 232.9 (s, CO), 231.7 (s, CO), 151.8 (s,
br, C-OH), 147.9 (s, C-OP), 121.4 [s, C2,3(C6H4)], 115.8 [s, br,
C3,2(C6H4)], 89.7, 89.3 (2 × s, 2 × Cp), 50.8 [d, JCP ) 21, C1-
Preparation of [Mo2Cp2(OC6H4OH)(µ-PPh2)(CO)4] (5b). Neat
DBU (6 µL, 0.040 mmol) was added to a tetrahydrofuran solution
(6 mL) of compound 1b (0.020 g, 0.032 mmol) and p-benzoquinone
(0.007 g, 0.065 mmol) at 233 K, and the mixture was stirred at
that temperature for 10 min to give a red-brown solution. The
solvent was then removed under vacuum, the residue washed with
petroleum ether (3 mL) and extracted with tetrahydrofuran (5 mL),
and the extract was filtered using a cannula. The addition of cold
petroleum ether (5 mL) to the filtrate and the removal of solvents
under vacuum gave compound 5b as a brown air-sensitive powder
(Cy)], 30.1 [s, C2,6(Cy)], 28.0 [s, C6,2(Cy)], 26.5-25.8 [m, C3,5
-
(Cy) and C4(Cy)]. Data for 9. Anal. Calcd for C40H44Mo4O9P2:
1
C, 43.11; H, 3.98. Found: C, 43.32; H, 4.10. H NMR (200.13
MHz): δ 5.18 (s, 10H, Cp), 1.90-1.26 (m, 11H, Cy), -11.12 (d,
JHP ) 38, 1H, µ-H). 13C{1H} NMR: δ 243.1 (s, br, 2CO), 237.0
(s, br, 2CO], 90.7 (s, Cp), 31.1 [d, JCP ) 4, C2(Cy)], 27.3 [d, JCP
) 12, C3(Cy)], 26.4 [d, JCP ) 1, C4(Cy)]. The resonance for the
C1(Cy) nucleus was hidden under that of the solvent, and it could
be located at 57.7 ppm (d, JCP ) 24) in toluene-d8 solution.
Preparation of [Mo2Cp2(µ-H){µ-PPh(OC6H4OH)}(CO)4] (7b).
The procedure is similar to that described for 7a, but it uses a
tetrahydrofuran solution of 4b (∼0.368 mmol), 0.040 g of p-
benzoquinone (0.370 mmol), and 0.119 g of [NH4]PF6 (0.730
mmol). Elution with tetrahydrofuran/toluene (1:9) gave a orange
fraction which yielded compound 7b as an orange powder (0.149
g, 62%). Elution with pure tetrahydrofuran gave a yellow fraction
yielding compound 8b as a yellow powder (0.031 g, 15%). The
crystals of 7b used in the X-ray study were grown by slow diffusion
of a layer of petroleum ether into a concentrated toluene solution
of the complex at 253 K. Data for 7b. Anal. Calcd for C26H21-
Mo2O6P: C, 47.87; H, 3.24. Found: C, 47.56; H, 3.15. 1H NMR:
δ 7.39-7.28 (m, 5H, Ph), 6.85, 6.70 (AB syst, JHH ) 9, 4H, C6H4),
4.94 (s, 10H, Cp), 4.78 (s, 1H, OH), -11.37 (d, JHP ) 40, 1H,
µ-H]. 13C{1H} NMR: δ 241.6 (s, br, 2CO), 234.3 (s, br, 2CO),
151.8 (s, C-OH), 151.1 [d, JCP ) 12, C-OP), 149.0 [d, JCP ) 31,
C1(Ph)], 128.9 [d, JCP ) 12, C2,3(Ph)], 128.6 [s, C4(Ph)], 128.0 [d,
1
(0.021 g, 90%). H NMR (293 K, averaged spectrum): δ 7.80-
7.20 (m, 10H, Ph), 6.49 (d, JHH ) 8, 2H, C6H4), 6.00 (d, JHH ) 8,
2H, C6H4), 5.1 (br, 10 H, Cp). 1H NMR (400.13 MHz, 243 K): δ
7.90-7.10 (m, 10H, Ph), 6.48 (d, JHH ) 8, 2H, C6H4, isomers A
and B), 5.97 (d, JHH ) 8, 2H, C6H4, isomers A and B), 5.32, 4.88
(2 × s, Cp, isomer A), 5.31, 4.86 (2 × s, Cp, isomer B). Ratio A/B
) 5. 13C{1H} NMR (100.63 MHz, 213 K, isomer A): δ 248.8 (d,
JCP ) 17, CO), 241.7 (d, JCP ) 20, CO), 241.6 (s, CO), 226.7 (s,
br, CO), 159.3 (s, C-OMo), 149.6 (s, C-OH), 142.7 [d, JCP
)
41, C1(Ph)], 139.0 [d, JCP ) 31, C1(Ph)], 136.5-126.5 (m, Ph),
118.4 [s, br, C2,3(C6H4)], 116.4 [s, C3,2(C6H4)], 95.0, 92.4 (2 × s,
2 × Cp).
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PCy(OC6-
H4OH)}(CO)4] (6a). Although impure solutions of 6a can be
prepared directly from 4a and p-benzoquinone (see preparation of
7a), pure solutions of this anionic complex have to be made from
hydride 7a. Neat DBU (28 µL, 0.187 mmol) was added to a
tetrahydrofuran solution (5 mL) of compound 7a (0.060 g, 0.091
mmol) at 0 °C, and the mixture was stirred at that temperature for
5 min to give a red solution of compound 6a which was ready for
further use.
JCP ) 12, C3,2(Ph)], 120.7 [d, JCP ) 7, C2,3(C6H4)], 115.8 [s, C3,2
-
(C6H4)], 91.4 (s, Cp). Data for 8b. Anal. Calcd for C20H17-
Mo2O5P: C, 42.88; H, 3.06. Found: C, 42.72; H, 3.15. 1H NMR:
δ 7.44-7.31 (m, 5H, Ph), 5.13 (s, br, 10H, Cp), 4.48 (s, 1H, OH),
-10.80 (d, JHP ) 40, 1H, µ-H).
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PPh(OC6-
H4OH)}(CO)4] (6b). Although impure solutions of 6b can be
prepared directly from 4b and p-benzoquinone (see preparation of
7b), pure solutions of this anionic complex have to be made from
hydride 7b. Neat DBU (15 µL, 0.100 mmol) was added to a
tetrahydrofuran solution (5 mL) of compound 7b (0.043 g, 0.065
mmol) at 0 °C, and the mixture was stirred at that temperature for
5 min to give a violet solution of compound 6b which was ready
for further use.
Preparation of [Mo2Cp2(µ-H){µ-PCy(OC6H4OH)}(CO)4] (7a).
A tetrahydrofuran solution (1 mL) of p-benzoquinone (0.055 g,
0.509 mmol) was added to a tetrahydrofuran solution of compound4a
(ca. 0.363 mmol), prepared as described above and cooled at 243
K, and the mixture was stirred at that temperature for 10 min to
give a red solution of compound 6a. Solid [NH4]PF6 (0.115 g, 0.706
mmol) was then added, and the mixture was stirred for 15 min and
allowed to reach room temperature to give an orange solution. The
solvent was then removed under vacuum, the residue was extracted
with dichloromethane and the extract chromatographed on an
Preparation of [Mo2Cp2(µ-PCy2)(µ-SPh)(CO)4] (10). Neat
HSPh (30 µL, 0.285 mmol) was added to a tetrahydrofuran solution
(5 mL) of compound 5a (∼0.095 mmol) prepared as described
above, and the mixture was stirred at room temperature for 4 h.
The solvent was then removed under vacuum; the residue was
extracted with toluene/petroleum ether (1:1, 15 mL), and the extracts
were filtered. Removal of the solvents from the filtrate gave a crude
product which was recrystallized from toluene/petroleum ether to
give compound 10 as red microcrystals (0.035 g, 50%). Anal. Calcd
for C32H37Mo2O4PS: C, 51.90; H, 5.04. Found: C, 51.82; H, 5.13.
1H NMR (200.13 MHz): δ 7.44-6.90 (m, 5H, Ph), 5.08 (s, 10H,
Cp), 2.48-1.30 (m, 22H, Cy). 13C{1H} NMR (50.33 MHz, C6D6):
δ 253.5 [d, JCP ) 20, CO], 252.8 (s, CO), 149.6 [s, C1(Ph)], 95.0
(s, Cp), 50.3 [d, JCP ) 20, C1(Cy)], 43.6 [d, JCP ) 17, C1(Cy)],
34.7-26.6 (m, Cy).
Preparation of [Mo2Cp2(µ-PCy2)(µ-SPh)(CO)2] (11a). Com-
pound 10a, prepared as a crude solid as described above (∼0.095
Inorganic Chemistry, Vol. 45, No. 23, 2006 9603

Alvarez et al.
mmol), was dissolved in toluene (10 mL) and heated at 75 °C for
4 h to give a green solution which was filtered. Removal of the
solvent from the filtrate under vacuum gave a residue which was
recrystallized by slow diffusion of a layer of petroleum ether into
a dichloromethane solution of the product to yield complex 10 as
dark-green microcrystals (0.058 g, 89%). Anal. Calcd for C31H39-
Cl2Mo2O2PS (10‚CH2Cl2): C, 48.39; H, 5.11. Found: C, 48.80;
H, 5.36. 1H NMR (200.13 MHz): δ 7.26-7.13 (m, 5H, Ph), 5.45,
5.29 (2 × s, 2 × 5H, Cp), 2.26-1.28 (m, 22H, Cy). 13C{1H}
NMR: δ 246.4 [d, JCP ) 13, CO], 241.7 [d, JCP )14, CO], 148.3
[s, C1(Ph)], 130.7, 127.5 [2 × s, C2,3(Ph)], 125.4 [s, C4(Ph)], 89.4,
89.2 (2 × s, Cp), 49.8 [d, JCP ) 21, C1(Cy)], 42.6 [d, JCP ) 18,
C1(Cy)], 34.3, 33.5, 32.9, 32.0 [4 × s, C2,6(Cy)], 28.0-27.7 [m,
C3,5(Cy)], 26.1, 25.9 [2 × s, C4(Cy)].
A tetrahydrofuran solution containing ∼0.106 mmol of 6a, prepared
as described above, was then added to this solid, and the mixture
was stirred for 10 min to give a red solution of 13 which was ready
for further use.
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PCy(OC4H5)}-
(CO)4] (14). Neat 3-butyn-1-ol (14 µL 0.18 mmol) was added to a
tetrahydrofuran solution containing ∼0.09 mmol of 6a, prepared
using an excess of DBU as described above, and the mixture was
stirred for 10 min to give a red solution of 14 which was ready for
further use.
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PCy(OPh)}-
(CO)4] (15). The procedure is similar to that described for 14, but
with phenol (0.012 g, 0.128 mmol) instead, to give an orange
solution of 15.
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PCy(SPh)}-
(CO)4] (16). The procedure is similar to that described for 14, but
with thiophenol (20 µL, 0.188 mmol) instead, to give an orange
solution of 16.
Preparation of [Mo2Cp2(µ-PPh2)(µ-SPh)(CO)2] (11b). Neat
HSPh (6 µL, 0.057 mmol, method A) or solid S2Ph2 (0.014 g, 0.063
mmol, method B) was added to a tetrahydrofuran solution (5 mL)
of compound 5b (∼0.032 mmol) prepared in situ as described
above, and the mixture was stirred at room temperature for 4 h to
give a green solution. The solvent was then removed under vacuum;
the residue was extracted with dichloromethane, and the extracts
were filtered through a silica gel column (230-400 mesh, 2 × 20
cm). Removal of the solvent from the filtrate gave compound 11b
as a green powder (method A 0.015 g, 70%; method B 0.012 g,
56%). Anal. Calcd for C30H25Mo2O2PS: C, 53.58; H, 3.75.
Found: C, 53.45; H, 3.67. 1H NMR (200.13 MHz): δ 7.90-7.00
(m, 15H, Cp), 5.52 (s, 10H, Cp). 13C{1H} NMR (50.33 MHz): δ
242.9 [d, JCP ) 14, CO], 239.0 [d, JCP ) 14, CO], 148.3 [s, C1-
(SPh)], 145.5 [d, JCP ) 41, C1(Ph)], 145.0 [d, JCP ) 39, C1(Ph)],
135.5-126.0 (m, Ph), 90.9, 90.7 (2 × s, Cp).
Preparation of [Mo2Cp2(µ-H){µ-PCy(OMe)}(CO)4] (17).
Method A. The solvent was removed from a tetrahydrofuran
solution containing ∼0.071 mmol of salt 12, prepared as described
above, and the residue was dissolved in dichloromethane (5 mL).
Solid [Me3O]BF4 (0.012 g, 0.081 mmol) was then added, and the
mixture was stirred for 1 min to give an orange solution. Method
B. Solid [NH4]PF6 (0.050 g, 0.306 mmol) was added to a
tetrahydrofuran solution containing ∼0.106 mmol of salt 13,
prepared as described above, and the mixture was stirred for 10
min to give an orange solution. In both cases the complex was
purified by removal of the solvent and chromatography of the
residue on an alumina column (2 × 15 cm, activity IV) at 243 K.
A yellow fraction was collected in both cases using dichlo-
romethane/petroleum ether (1:3) as eluant to give, after removal
of solvents, compound 17 as an orange powder (method A 0.036
g, 88%; method B 0.042 g, 68%). The crystals used in the X-ray
study were grown by slow diffusion of a layer of petroleum ether
into a concentrated dichloromethane solution of the complex at 253
K. Anal. Calcd for C21H25Mo2O5P: C, 43.47; H, 4.32. Found: C,
43.22; H, 4.32. IR (Nujol mull, cis-17): ν(CO) 1966 (s), 1921 (s),
Preparation of solutions of [H-DBU][Mo2Cp2{µ-PCy(OH)}-
(CO)4] (12). A tetrahydrofuran solution (5 mL) containing ∼0.061
mmol of the salt 6a and excess DBU was prepared in situ as
described above. Water (4 µL, 0.22 mmol) was then added, and
the mixture was stirred for 5 min to give a red solution shown (by
NMR) to contain an equilibrium mixture of the anionic tautomers
[Mo2Cp2{µ-PCy(OH)}(CO)4]- (12A) and [Mo2Cp2(µ-H){µ-PCy-
(O)}(CO)4]- (12B) in similar amounts.
Preparation of [Mo2Cp2(µ-H){µ-PCy(OH)}(CO)4] (8a). Solid
[NH4]PF6 (0.020 g, 0.122 mmol) was added to a tetrahydrofuran
solution (5 mL) containing ∼0.061 mmol of compound 12, prepared
as described above, and the mixture was stirred for 10 min to give
a yellow solution. The solvent was removed, and the residue was
chromatographed on an alumina column (activity IV) at 253 K.
Elution with tetrahydrofuran/toluene (1:9) gave a yellow fraction
yielding, after removal of solvents, compound 8a as a yellow
powder (0.034 g, 88%). The crystals used in the X-ray study were
grown by slow diffusion of a layer of petroleum ether into a
concentrated tetrahydrofuran solution of the complex at 253 K.
Anal. Calcd for C24H31Mo2O6P (8a‚OC4H8): C, 53.58; H, 3.75.
1
1882 (vs), 1859 (s) cm-1. H NMR (200 MHz): δ 5.23 (s, 10H,
Cp), 2.77 (d, JHP ) 14, 3H, OMe) 2.25-1.31 (m, 11H, Cy), -11.09
(d, JHP ) 41, 1H, µ-H).
Preparation of [Mo2Cp2(µ-H){µ-PCy(OC4H5)}(CO)4] (18).
The preparation and purification of this complex was identical to
that described for compound 17 (method B) but using a solution
containing ∼0.090 mmol of salt 14 instead. This gave compound
18 as an orange solid (0.046 g, 82%). Anal. Calcd for C24H27-
Mo2O5P: C, 46.62; H, 4.40. Found: C, 46.22; H, 4.32. 1H NMR:
δ 5.25 (s, 10H, Cp), 2.92 (q, JHP ) JHH ) 7, 1H, CH2), 2.43 (dt,
JHH ) 7, 3, 1H, CH2), 2.21-1.30 (m, 11H, Cy), 1.98 (t, JHH ) 3,
1H, CH), -11.22 (d, JHP ) 41, 1H, µ-H).
Preparation of [Mo2Cp2(µ-H){µ-PCy(OPh)}(CO)4] (19). The
preparation and purification of this complex was identical to that
described for compound 17 (method B) but with a solution
containing ∼0.090 mmol of salt 15 instead. This gave compound
19 as an orange solid (0.036 g, 61%). The crystals used in the X-ray
study were grown by slow diffusion of a layer of petroleum ether
into a concentrated dichloromethane solution of the complex at 253
K. Anal. Calcd for C26H27Mo2O5P: C, 48.62; H, 4.24. Found: C,
48.72; H, 4.27. 1H NMR: δ 7.26-6.87 (m, 5H, OPh), 5.14 (s, br,
10H, Cp), 2.78-0.86 (m, 11H, Cy), -11.82 (d, JHP ) 39, 1H, µ-H).
1H NMR (400.13 MHz, 233 K): isomer trans-19 δ 7.30 (m, 2H,
OPh), 7.08 (m, 1H, OPh), 6.95 (m, 2H, OPh), 5.37, 4.92 (2 x s, 2
× 5 H, Cp), 3.05-0.69 (m, 11H, Cy), -11.99 (d, JHP ) 38, 1H,
Found: C, 53.45; H, 3.67. IR (Nujol mull): ν(OH) 3270 (br) cm-1
.
1H NMR: δ 5.20 (s, 10H, Cp), 1.91-1.38 (m, 11H, Cy), -10.83
(d, JHP ) 40, 1H, µ-H). 13C{1H} NMR (50.33 MHz): δ 242.0 (sa,
CO), 232.7 (sa, CO), 90.6 (s, Cp), 30.6 [s, C2(Cy)], 27.1 [d, JCP
)
12, C3(Cy)], 25.4 [s, C4(Cy)]. The resonance for the C1(Cy) nucleus
was hidden under that of the solvent, and it could be located at
54.4 ppm (d, JCP ) 24) in CDCl3 solution.
Preparation of Solutions of [H-DBU][Mo2Cp2{µ-PCy(OMe)}-
(CO)4] (13). BuLi (190 µL of a 1.6 M solution in hexanes, 0.304
mmol) was added to a tetrahydrofuran solution (2 mL) of methanol
(10 mL, 0.247 mmol) at 0 °C; the mixture was stirred for 1 min,
and the solvent was then removed under vacuum to give a white
residue of LiMeO which was washed with petroleum ether (5 mL).
9604 Inorganic Chemistry, Vol. 45, No. 23, 2006

Trapping of Hemiquinone Radicals
Table 5. Crystal Data for New Compounds
7b
8a‚OC4H8
19
17
mol formula
mol wt
C26H21Mo2O6P
652.30
C24H31Mo2O6P
638.34
C26H27Mo2 O5P
642.33
C21H25Mo2O5P
580.26
cryst syst
space group
radiation (λ, Å)
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
monoclinic
P21/n
triclinic
P1h
0.71073
triclinic
P1h
0.71073
0.71069
30.009(7)
8.680(9)
19.796(5)
90
99.65(2)
90
5083(5)
8
0.71073
15.252(3)
8.8714(15)
18.616(3)
90
90.517(3)
90
2518.8(8)
4
9.0903(13)
9.9522(15)
15.312(2)
77.434(2)
74.559(2)
80.305(2)
1294.3(3)
2
8.1443(12)
9.2872(14)
15.368(2)
85.330(2)
76.400(2)
82.699(2)
1119.0(3)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
calcd density (g cm-3
)
1.71
1.089
293
1.683
1.096
200(2)
1.648
1.064
292(2)
1.722
1.221
292(2)
abs coeff (mm-1
temp (K)
)
θ range (deg)
index ranges (h, k, l)
reflns collected
1-28
0, 39; 0, 11; -26, 25
6664
1.72-26.44
-19, 19; 0, 11; 0, 23
21 567
1.40-26.40
-10, 11; -11, 12; 0, 19
14 771
1.37-26.42
-9, 10; -11, 11; 0, 19
10 779
independent reflns
reflns with I >2σ(I)
6139
5503
3765
R1 ) 0.035
wR2 ) 0.0757b,c
1.088
5276
4682
R1 ) 0.0191
wR2 ) 0.0511b,d
1.112
4532
3454
R1 ) 0.0295
wR2 ) 0.063b,e
1.060
3845 [I >3σ(I)]
R1 ) 0.0363
wR2 ) 0.0439a
1.093
R (data with I >2σ(I))
GOF
restraints/params
0/317
0.85, -0.49
0/298
0.567, -0.456
0/311
0.316, -0.485
0/266
0.526, -0.456
∆F(max,min) (e Å-3
)
a w ) w′[1 -((||Fo| - |Fc||)/6σ(Fo))2]2 with w′)1/∑rArTr(X) with 3 coefficients 0.884, 0.367, and 0.507 for a Chebyshev Series, for which X is Fc/
Fc(max). b w-1 ) σ2(Fo ) + (aP)2 + (bP), where P ) [max(Fo , 0) + 2Fc ]/3. c a) 0.0345, b) 0.9836. d a ) 0.0249, b ) 0.3897. e a ) 0.0238, b ) 1.1099.
2
2
2
µ-H); isomer cis-19 δ 5.24 (s, 10 H, Cp), -10.90 (d, JHP ) 42,
1H, µ-H); other resonances obscured by those of the major isomer.
trans/cis ratio ) 8.
X-ray Structure Determination of Compounds 8a, 17, and
19. Data were collected at the University of Santiago de Compostela
(Spain) on a Smart CCD-1000 Bruker diffractometer using graphite-
monochromated Mo KR radiation. Cell dimensions and orientation
matrixes were initially determined from least-squares refinements
on reflections measured in 3 sets of 30 exposures collected in 3
different ω regions and eventually refined against all reflections.
The software SMART43 was used for collecting frames of data,
indexing reflections, and determining lattice parameters. The
collected frames were then processed for integration by the software
SAINT,43 and a multiscan absorption correction was applied with
SADABS.44 Using the program suite WinGX,45 we solved the
structures by Patterson interpretation and phase expansion, and they
were refined with full-matrix least squares on F2 with SHELXL97.46
All non-hydrogen atoms were refined anisotropically. All hydrogen
atoms were found in the Fourier maps but were modeled on
idealized positions except for the hydride atoms in all three
compounds and the hydroxyl atom in 8a, which were refined; all
hydrogen atoms were given an overall isotropic parameter. In the
case of 8a, there is a tetrahydrofuran molecule H-bonded to the
hydroxyl group but disordered over two positions (occupancies 0.60
and 0.40) sharing the same oxygen position. The corresponding
carbon atoms were refined isotropically as the temperature factors
were persistently nonpositive definites. For compound 17, the
hydrogens on the methoxy carbon atom were also found to be
Preparation of [Mo2Cp2(µ-H){µ-PCy(SPh)}(CO)4] (20). The
preparation and purification of this complex was identical to that
described for compound 17 (method B) but with a solution
containing ∼0.090 mmol of salt 16 instead. This gave compound
20 as an orange solid (0.043 g, 72%). Anal. Calcd for C26H27Mo2O4-
PS: C, 47.29; H, 4.31. Found: C, 47.43; H, 4.13. 1H NMR (400.13
MHz, 243 K): isomer trans-20 δ 7.63-7.24 (m, 5H, Ph), 5.35,
5.14 (2 × s, 2 × 5 H, Cp), 2.99-0.27 (m, 11H, Cy), -12.48 (d,
JHP ) 35, 1H, µ-H); isomer cis-20 δ 5.22 (s, 10 H, Cp), -11.77
(d, JHP ) 38, 1H, µ-H). The Ph and Cy resonances for the minor
isomer could not been identified because they were hidden under
those of the major isomer. trans/cis ratio ) 8.
X-ray Structure Determination of Compound 7b. A suitable
crystal of 7b was stuck on a glass fiber and mounted on an Enraf-
Nonius MACH-3 automatic diffractometer. Accurate cell dimen-
sions and orientation matrix were obtained by least-squares
refinements of 25 accurately centered reflections. No significant
variations were observed in the intensities of two checked reflections
during data collection. The data were corrected for Lorentz and
polarization effects. Computations were performed by using the
PC version of CRYSTALS.40 Scattering factors and corrections for
anomalous absorption were taken from ref 41. The structure was
solved by direct methods (SHELXS),42 completed by Fourier
techniques and refined by full-matrix least-squares. An empirical
(41) Cromer, D. T. International Tables for X-ray Crystallography; Kynoch
Press: Birmingham, U.K., 1974; Vol. IV.
(42) Sheldrick, G. M. SHELXS 86, Program for Crystal Structures Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(43) SMART & SAINT Software Reference Manuals, version 5.051
(Windows NT version); Bruker Analytical X-ray Instruments: Madi-
son, WI, 1998.
(44) Sheldrick, G. M. SADABS, Program for Empirical Absorption
Correction; University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(45) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(46) Sheldrick, G. M. SHELXL97: Program for the Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
absorption correction based on ψ-scan curve was applied (Tmin
)
0.95, Tmax ) 1). All non-hydrogen atoms were anisotropically
refined. The hydrogen atoms were introduced in calculated positions
and were allocated an overall isotropic thermal parameter.
(40) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
Crystals, Issue 10; Chemical Crystallography Laboratory, University
of Oxford: Oxford, U.K., 1996.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9605

Alvarez et al.
disordered over two positions (occupancies 0.5) related by rotation
around the C-O bond.
Supporting Information Available: Crystallographic data for
the structural analysis of compounds 7b, 8a, 17, and 19 in the CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank the MEC of Spain for a
PhD grant (to M. A.) and the MCYT for financial support
(BQU2003-05471).
IC061344E
9606 Inorganic Chemistry, Vol. 45, No. 23, 2006