Chemical and electrochemical oxidation of diphenylphosphide-bridged hydrides [M2(n5-C5H5) 2(μ-H)(μ-PPh2)(CO)4] and Anions [M 2(n5-C5H5)2(μ-PPh 2)(CO)4]-(M = Mo, W)

Article abstract of DOI:10.1021/om0494824

Reaction of the hydride complexes [M₂Cp₂(μ-H) (μ-PPh₂)(CO)₄] (M = Mo, W; Cp = n⁵-C ₅H₅) with [FeCp₂]BF₄ proceeds in a 1:2 ratio to give the unsaturated tetracarbonyl salts [M₂Cp ₂-(μ-PPh₂)(CC₄]BF₄, which spontaneously (M = Mo) or in the presence of CO (M = W) transform into the corresponding electron-precise pentacarbonyls [M₂Cp ₂(μ-PPh₂](BAe′)(Ar′ = ₄]BF ₄. In contrast, reaction of the above hydride complexes with [FeCp₂]BAr₄) (Ar′ = 3,5-C₅H ₃(CF₃)₂) proceeds in a 1:1 ratio to give the paramagnetic salts [M₂Cp₂(μ-H(μ-PPh ₂)(CO)₄(BAr′₄). No further reaction takes place with [FeCp₂](BAr′₄) unless a proton acceptor such as water or tetrahydrofuran is present, in which case the corresponding salts [M₂Up₂-(μ-PPh₂CO)₄] (BAr′₄ or [M₂Cp₂(μ-PPh ₂)(μ-CO)(CO)₄] KBAr′) are rapidly formed. Reactions of the anions [M₂Cp₂(μ-PPh ₂)(CO)₄]- (as (H-DBU)⁺ salts, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) with [FeCp₂]BF₄ proceed stepwise to give first the 33-electron radicals [M₂Cp ₂(μ-PPh₂)(CO)₄], which can be isolated as very air-sensitive green solids, and then the unsaturated salts [M ₂Cp₂(μ-PPh₂)(CO)₄]BF ₄, which are identical to those formed from the neutral hydrides. The structure and dynamic behavior of the new compounds are analyzed on the basis of solution IR, ¹H, P, and ¹³C NMR or ESR spectroscopic data. In addition, the redox and other chemical transformations connecting all the new compounds are discussed on the basis of the synthetic and structural data, as well as cyclic voltammetry measurements carried out in dichloromethane on the neutral hydrides [M₂Cp₂)(μ-H)(μ-PPh ₂)(CO)₄] and the anions [M₂Cp ₂(μ-PPh₂)(CO)₄] (as [N(PPh ₂]salts).

Full text of DOI:10.1021/om0494824

650
Organometallics 2005, 24, 650-658
Chemical and Electrochemical Oxidation of
Diphenylphosphide-Bridged Hydrides
[M2(η5-C5H5)2(µ-H)(µ-PPh2)(CO)4] and Anions
[M2(η5-C5H5)2(µ-PPh2)(CO)4]- (M ) Mo, W)
Celedonio M. Alvarez, M. Esther Garc´ıa, M. Teresa Rueda, Miguel A. Ruiz,* and
David Sa´ez
Departamento de Quı´mica Orga´nica e Inorga´nica/IUQOEM, Universidad de Oviedo,
E-33071 Oviedo, Spain
Neil G. Connelly
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
Received July 10, 2004
Reaction of the hydride complexes [M2Cp2(µ-H)(µ-PPh2)(CO)4] (M ) Mo, W; Cp ) η5-C5H5)
with [FeCp2]BF4 proceeds in a 1:2 ratio to give the unsaturated tetracarbonyl salts [M2Cp2-
(µ-PPh2)(CO)4]BF4, which spontaneously (M ) Mo) or in the presence of CO (M ) W)
transform into the corresponding electron-precise pentacarbonyls [M2Cp2(µ-PPh2)(µ-CO)-
(CO)4]BF4. In contrast, reaction of the above hydride complexes with [FeCp2](BAr′4) (Ar′ )
3,5-C6H3(CF3)2) proceeds in a 1:1 ratio to give the paramagnetic salts [M2Cp2(µ-H)(µ-PPh2)-
(CO)4](BAr′4). No further reaction takes place with [FeCp2](BAr′4) unless a proton acceptor
such as water or tetrahydrofuran is present, in which case the corresponding salts [M2Cp2-
(µ-PPh2)(CO)4](BAr′4) or [M2Cp2(µ-PPh2)(µ-CO)(CO)4](BAr′4) are rapidly formed. Reactions
of the anions [M2Cp2(µ-PPh2)(CO)4]- (as (H-DBU)+ salts, DBU ) 1,8-diazabicyclo[5.4.0]undec-
7-ene) with [FeCp2]BF4 proceed stepwise to give first the 33-electron radicals [M2Cp2(µ-
PPh2)(CO)4], which can be isolated as very air-sensitive green solids, and then the
unsaturated salts [M2Cp2(µ-PPh2)(CO)4]BF4, which are identical to those formed from the
neutral hydrides. The structure and dynamic behavior of the new compounds are analyzed
1
on the basis of solution IR, H, 31P, and 13C NMR or ESR spectroscopic data. In addition,
the redox and other chemical transformations connecting all the new compounds are
discussed on the basis of the synthetic and structural data, as well as cyclic voltammetry
measurements carried out in dichloromethane on the neutral hydrides [M2Cp2(µ-H)(µ-PPh2)-
(CO)4] and the anions [M2Cp2(µ-PPh2)(CO)4]- (as [N(PPh3)2]+ salts).
Introduction
chemistry.2,4 The scarce data available, however, sug-
gest that these paramagnetic molecules react with
different molecules much more rapidly than their
related diamagnetic substrates do and also that they
are able to experience transformations not observed for
diamagnetic substrates. For example, the 33-electron
compounds [Fe2(µ-PPh2)(CO)7] and [CoTa(µ-CH2)2Cp3]
react very rapidly under mild conditions with phos-
phines5 or organic disulfides,6 respectively, while
[Mo2Cp2(µ-PPh2)(CO)4] can induce an unusual C-H(Cp)
oxidative addition when reacting with [Re2(CO)10],1 and
the 31-electron cation [W2Cp2(µ-CO)(CO)2(µ-Ph2PCH2-
PPh2)]+ experiences an unprecedented H atom abstrac-
tion at the oxygen atom of the carbonyl bridge to yield
the hydroxycarbyne cation [W2Cp2(µ-COH)(CO)2-
(µ-Ph2PCH2PPh2)]+.7 To explore this promising chem-
Recently we reported that oxidation of the dimolyb-
denum anion [Mo2Cp2(µ-PPh2)(CO)4]- with [FeCp2]BF4
proceeded stepwise, thus allowing the synthesis and
isolation of the 33-electron radical [Mo2Cp2(µ-PPh2)-
(CO)4] and then the unsaturated diamagnetic cation
[Mo2Cp2(µ-PPh2)(CO)4]+.1 There are several points of
interest concerning these reactive products. As for the
paramagnetic [Mo2Cp2(µ-PPh2)(CO)4], we note that isol-
able binuclear radicals having metal-metal bonds of
order higher than 1 are still relatively rare in the field
of organometallic radicals.2,3 As a result, little is known
about their chemical behavior apart from their electro-
* Correspondence author. E-mail: mara@fq.uniovi.es.
(1) Garc´ıa, M. E.; Riera V.; Rueda, M. T.; Ruiz, M. A.; Lanfranchi,
M.; Tiripicchio, A. J. Am. Chem. Soc. 1999, 121, 4060.
(2) (a) Paramagnetic Organometallic Species in Activation/Selectiv-
ity, Catalysis; Chanon, M., Julliard, M., Poite, J. C., Eds.; Kluwer
Academic Publishers: Dordrecht, 1989. (b) Organometallic Radical
Processes; Trogler, W. C., Ed.; Elsevier: Amsterdam, 1990. (c) Astruc,
D. Electron Transfer and Radical Processes in Transition-Metal
Chemistry; VCH: New York, 1995.
(4) (a) Connelly, N. G. Chem. Soc. Rev. 1989, 18, 153. (b) Geiger,
W. E.; Connelly, N. G. Adv. Organomet. Chem. 1985, 24, 87.
(5) Baker, R. T.; Calabrese, J. C.; Krusic, P.J.; Therien, M. J.;
Trogler, W. C. J. Am. Chem. Soc. 1988, 110, 8392.
(6) (a) Aubart, M. A., Bergman, R. G. J. Am. Chem. Soc. 1996, 118,
1793. (b) Aubart, M. A., Bergman, R. G. J. Am. Chem. Soc. 1998, 120,
8755.
(3) (a) Sun, S.; Sweigart, D. A. Adv. Organomet. Chem. 1996, 40,
171. (b) Hoff, C. D. Coord. Chem. Rev. 2000, 206, 451.
10.1021/om0494824 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/14/2005

Diphenylphosphide-Bridged Hydrides
Organometallics, Vol. 24, No. 4, 2005 651
Chart 1
istry, however, it is necessary to be able to prepare
suitable paramagnetic complexes stable enough to allow
further reactivity studies.
On the other hand, our interest in the 32-electron
diamagnetic cation [Mo2Cp2(µ-PPh2)(CO)4]+ stems from
the fact that this species combines the presence of a
multiple metal-metal bond with a positive charge, a
circumstance that should lead to enhanced electrophilic
behavior and potential Lewis acid catalysis.8 For ex-
ample, the unsaturated ditungsten cation [W2Cp2-
(µ-CO)(CO)2(µ-Ph2PCH2PPh2)]2+ induces the cleavage of
P-H bonds of HPRR′ phosphines at low temperatures
and acts as a catalyst for the polymerization of olefins,9
while the diruthenium cation [Ru2(CO)4(µ-L4)]2+ (L4 )
tetradentate P-donor ligand) has been found to be a good
precatalyst for the hydroformylation of 1-alkenes.10
Taking into account the above considerations, we
decided to study in more detail the redox reactions
leading to the mentioned unsaturated species [Mo2Cp2-
(µ-PPh2)(CO)4]n+ (n ) 0, 1) and to extend these studies
to the corresponding ditungsten substrates. A prelimi-
nary experiment showed that related unsaturated cat-
ions could be obtained through oxidation of the neutral
hydrides [M2Cp2(µ-H)(µ-PPh2)(CO)4] (1a,b) [M ) Mo (a),
W (b)]; these were therefore included as substrates to
be studied. These studies have been carried out using
[FeCp2]BF4 as oxidizing agent,11 complemented with
cyclic voltammetry measurements on all the starting
substrates. Moreover, to analyze the expected influence
of the counteranion on the behavior of the unsaturated
cations to be generated,9 we have used also the ferro-
cenium salt [FeCp2](BAr′4) as oxidizing agent (Ar′ ) 3,5-
C6H3(CF3)2).12 This tetraarylborate anion13 is a robust
and extremely weak coordinating ligand,14 and the salts
of this anion with reactive cations have been found by
us and others to be more stable than the corresponding
salts having more conventional weakly coordinating
Chart 2
and [FeCp2]BF4 in dichloromethane solution proceed in
a 1:2 molar ratio at either room temperature or -30 °C,
even when using less than 2 equiv of oxidant. The initial
products are, in all cases, the corresponding tetracar-
bonyl cations [M2Cp2(µ-PPh2)(CO)4]BF4 (2a,b) (Chart 1).
The dimolybdenum complex 2a is unstable and decom-
poses rapidly in solution at room temperature to give
the pentacarbonyl complex [Mo2Cp2(µ-PPh2)(µ-CO)(CO)4]-
BF4 (3a) (Chart 1). This decomposition reaction is fast
enough so as to preclude the recording of a satisfactory
IR spectrum of 2a, a circumstance that led us to assign
erroneously a CO-bridged structure to this unsaturated
cation in our preliminary report.1 Of course, 2a reacts
rapidly with CO even at low temperature to give the
pentacarbonyl complex 3a quantitatively.
The ditungsten complex 2b is more stable than 2a,
and complete spectroscopic analysis can be carried out
at room temperature. In fact, although 2b is rapidly
carbonylated to give the corresponding pentacarbonyl
compound [W2Cp2(µ-PPh2)(µ-CO)(CO)4]BF4 (3b), partial
decarbonylation of the latter occurs in solution simply
by removing the CO atmosphere.
-
- 9,15
anions such as BF4 or PF6
.
As will be shown, the
external anion plays an unanticipated but critical role
in the oxidation reactions studied, depending on its
proton-acceptor ability.
Reactions of the Hydride Complexes 1 with
[FeCp2](BAr′4). The formation of complexes 2 from 1
and 2 mol of [FeCp2]BF4 implies that spontaneous
deprotonation occurs after oxidation. Under the experi-
mental conditions used (dichloromethane solution) the
proton is presumably transferred as solvated HBF4.
Thus, using a different salt of the oxidizing reagent
might change not only the stability of the unsaturated
cations 2 but also the extent of the proton-transfer
reactions.
In fact, the hydride complexes 1a,b experience only
one-electron oxidation even in the presence of excess
[FeCp2](BAr′4), to give the paramagnetic cations
[M2Cp2(µ-H)(µ-PPh2)(CO)4](BAr′4) (4a,b) (Chart 2). Ac-
tually, an equilibrium between the cation and the neu-
tral precursor is attained for the molybdenum substrate,
Results and Discussion
Reactions of the Hydride Complexes 1 with
[FeCp2]BF4. Reactions between the dimolybdenum or
ditungsten hydrides [M2Cp2(µ-H)(µ-PPh2)(CO)4] (1a,b)
(7) Alvarez, M. A.; Anaya, Y.; Garc´ıa, M. E.; Ruiz, M. A. Organo-
metallics 2004, 23, 3950
(8) (a) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.;
Jeannin, Y. Angew. Chem., Int. Ed. Engl. 1993, 32, 1156. (b) Alvarez,
M. A.; Garc´ıa, G.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Lanfranchi,
M.; Tiripicchio, A. Organometallics 1999, 18, 4509.
(9) (a) Alvarez, M. A.; Anaya, Y.; Garc´ıa, M. E.; Riera, V.; Ruiz, M.
A.; Vaissermann, J. Organometallics 2003, 22, 456. (b) Alvarez, M.
A.; Anaya, Y.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A. Organometallics
2004, 23, 433.
(10) (a) Mattews, R. C.; Howell, D. K.; Peng, W.-J.; Train, S. G.;
Treleaven, W. D.; Stanley, G. G. Angew. Chem., Int. Ed. Engl. 1996,
35, 2253. (b) Aubry, D. A.; Bridges, N. N.; Ezell, K.; Stanley, G. G. J.
Am. Chem. Soc. 2003, 125, 11180.
(11) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(12) Cha´vez, I.; Alvarez-Carena, A.; Molins, E.; Roig, A.; Manink-
iewicz, W.; Arancibia, A.; Arancibia, V.; Brand, H.; Manr´ıquez, J. M.
J. Organomet. Chem. 2000, 601, 126.
(15) For some recent work, see for example: (a) Jime´nez-Tenorio,
M.; Puerta, M. C.; Salcedo, I.; Valerga, P.; Costa, S. I.; Gomes, P. T.;
Mereiter, K. Chem. Commun. 2003, 1168. (b) Leatherman, M. D.;
Svejda, S. A.; Johnson, L. K.; Brookhart, M. J. Am. Chem. Soc. 2003,
125, 3068. (c) Norris, C. M.; Reinartz, S.; White, P. S.; Templeton, J.
L. Organometallics 2002, 21, 5649. (d) Taw, F. L.; Mellows, H.; White,
P. S.; Hollander, F. J.; Bergman, R. G.; Brookhart, M.; Heinekey, D.
M. J. Am. Chem. Soc. 2002, 124, 5100. (e) Voges, M. H.; Bullock, R.
M. J. Chem. Soc., Dalton Trans. 2002, 759.
(13) Nishida, H.; Takada, N.; Yoshimura, M.; Sonoda, T.; Kobayashi,
H. Bull. Chem. Soc. Jpn. 1984, 57, 2600.
(14) (a) Strauss, S. H. Chem. Rev. 1993, 93, 927. (b) Seppelt, K.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1025. (c) Krossing, I.; Raabe,
I. Angew. Chem., Int. Ed. 2004, 43, 2066.

652 Organometallics, Vol. 24, No. 4, 2005
Table 1. Selected IR and 31P{1H} NMR Spectroscopic Data for New Compounds
Alvarez et al.
c
compounda
ν(CO)b
δP (JPW)
[Mo2Cp2(µ-PPh2)(CO)4]BF4
[Mo2Cp2(µ-PPh2)(CO)4](BAr′4)
[W2Cp2(µ-PPh2)(CO)4]BF4
2a
158.9d
2a′
2b
2b′
3a
2011 (m), 1975 (vs), 1954 (s)
159.7e
2011 (m), 1972 (vs), 1945 (s)
112.8 (243)
111.9 (245)
119.9e
[W2Cp2(µ-PPh2)(CO)4](BAr′4)
[Mo2Cp2(µ-PPh2)(µ-CO)(CO)4]BF4
[Mo2Cp2(µ-PPh2)(µ-CO)(CO)4](BAr′4)
[W2Cp2(µ-PPh2)(µ-CO)(CO)4]BF4
[W2Cp2(µ-PPh2)(µ-CO)(CO)4](BAr′4)
[Mo2Cp2(µ-H)(µ-PPh2)(CO)4](BAr′4)
[W2Cp2(µ-H)(µ-PPh2)(CO)4](BAr′4)
(H-DBU)[Mo2Cp2(µ-PPh2)(CO)4]
(H-DBU)[W2Cp2(µ-PPh2)(CO)4]
(PPN)[W2Cp2(µ-PPh2)(CO)4]
[Mo2Cp2(µ-PPh2)(CO)4]
2008 (m), 1971 (vs), 1947 (vs)
2057 (vs), 2009 (s), 1763 (m, br)
3a′
3b
3b′
4a′
4b′
5a
2058 (vs), 2009 (s), 1770 (m, br)
121.7
2053 (vs), 1998 (s), 1718 (m, br)
31.1 (168)f
28.7 (162)
2054 (vs), 1999 (s), 1720 (m, br)
2051 (vs), 2033 (w), 1998 (vs), 1965 (s,br)
2043 (s), 2023 (w), 1989 (vs), 1969 (m, sh), 1947 (m, br)
1878 (m), 1829 (vs), 1774 (s), 1755 (m)
1868 (m), 1822 (vs), 1774 (s), 1745 (m)g
1874 (m), 1833 (vs), 1785 (s), 1762 (m)h
1951 (m), 1905 (vs), 1879 (s)
5b
5b′
6a
[W2Cp2(µ-PPh2)(CO)4]
6b
1943 (m), 1895 (vs), 1865 (s)
a Ar′ )3,5-C6H3(CF3)2; DBU ) 1,8-diazabicyclo[5.4.0]undec-7-ene; PPN ) N(PPh3)2. b Recorded in dichloromethane solution, unless
otherwise stated; ν in cm-1 c Recorded at 121.50 MHz and 291 K in CD2Cl2 solutions unless otherwise stated; δ in ppm relative to external
.
85% aqueous H3PO4; J in hertz. d Recorded at 243 K. e Recorded at 203 K. f Recorded under a CO atmosphere. g 1873 (m), 1831 (vs), 1785
(s), and 1760 (m) when recorded in tetrahydrofuran solutions. h In tetrahydrofuran solution.
a fact that can be justified by considering the redox
potentials of these compounds, as will be discussed
later on.
All four pentacarbonyl complexes 3 display similar IR
spectra in the C-O stretching region, with one medium
band in the bridging region and two strong bands in
the region for terminal carbonyls, with the relative
intensities expected for CO oscillators defining CMC
angles below 90°.17 This pattern is identical to those
found for the above-mentioned complexes [Mo2Cp2-
(µ-PHR)(µ-CO)(CO)4]BF4. An X-ray study on the latter
(R ) Cy) revealed that the cation displays two almost
eclipsed MoCp(CO)2 moieties bridged by the cyclohex-
ylphosphide and carbonyl ligands.16 This is therefore
also the case for compounds 3, which might be described
as cis isomers (Cp ligands placed on the same side of
the M2(µ-P) plane). This observation is relevant since
compounds 1, and almost all known hydrides of the
type [M2Cp2(µ-H)(µ-PRR′)(CO)4] (M ) Mo, W), display
MCp(CO)2 moieties in a relative trans arrangement with
respect to the Mo2(µ-P) plane, as confirmed crystallo-
graphically on PMe2, PtBu2, and PEtPh derivatives.18
Thus, we conclude that trans to cis isomerization takes
place at some step on the way from the hydrides 1 to
the pentacarbonyls 3.
The radicals 4 are not deprotonated by weak bases
such as water or tetrahydrofuran and do not react with
[FeCp2](BAr′4) themselves. However, an immediate
reaction of 4 occurs with the latter oxidant in the
presence of water or tetrahydrofuran, to give the tetra-
carbonyl salts [M2Cp2(µ-PPh2)(CO)4](BAr′4) (2a′,b′)
cleanly. This result shows that the second oxidation of
the hydrides 1 using the ferrocenium ion is only possible
if coupled to a proton-transfer step, in agreement with
electrochemical data to be discussed later on. The
tetraarylborate ion is expected to have a proton affinity
- 13,14c
lower than BF4
,
and thus an external proton
acceptor is required in that case. On the other hand,
the stabilizing effect of the BAr′4- anion on the unsatur-
ated cations 2 is modest, as the dimolybdenum com-
pound 2a′ still decomposes in solution at room temper-
ature to give the pentacarbonyl 3a′. However, a
reasonably clean IR spectrum (Table 1) can now be
recorded for this complex. Of course, both 2a′ and 2b′
are rapidly carbonylated by CO to yield the correspond-
ing pentacarbonyls 3a′ and 3b′. As deduced from the
The 13C{1H} NMR spectrum of 3a recorded at 233 K
is in agreement with the proposed structure. As ex-
pected, three different carbonyl resonances are observed,
with the most deshielded (δ ) 275.2, JCP ) 25) corre-
sponding to the bridging carbonyl. The resonances
appearing in the terminal region are safely assigned on
the basis of their P-C couplings, by recalling that 2JPC
couplings in complexes of the type [MCpX(CO)2(PR3)]
-
corresponding spectroscopic data, these BAr′4 salts
have the same structure as their BF4- counterparts, as
discussed next.
Structural Characterization of Compounds 2
and 3. The IR and 31P NMR data for all the tetracar-
bonyls 2 and pentacarbonyls 3 are summarized in Table
1, while 1H and 13C NMR data are collected in the
Experimental Section. The proposed structures (Chart
1) are based on these data and a comparison with those
from the related PHR-bridged cations [Mo2Cp2(µ-PHR)-
(CO)n]+ (n ) 4, 5), recently prepared through protona-
tion reactions of the phosphide-hydride precursors
[Mo2Cp2(µ-H)(µ-PHR)(CO)4] (R ) Ph, Cy, Mes).16
In the first place, we note that IR and NMR spectro-
(M ) Mo, W; X ) halogen, alkyl, hydride, etc.) usually
19,20
follow the order Jcis > Jtrans
.
Thus, the doublet
resonance at 226.9 ppm (JPC ) 15) is assigned to the
carbonyl ligands cis to the phosphide bridge, whereas
the singlet resonance at 221.1 ppm then corresponds to
those CO groups trans to that bridge. According to the
proposed structure of the cations 3, the phenyl groups
(17) Braterman, P. S. Metal Carbonyl Spectra; Academic Press:
London, UK, 1975.
-
-
scopic data for the BF4 and BAr′4 salts are similar;
we thus conclude that there are no significant anion-
cation interactions in the dichloromethane solutions of
compounds 2 and 3.
(18) (a) Petersen, J. L.; Dahl, L. F.; Williams, J. M. J. Am. Chem.
Soc. 1974, 96, 6610. (b) Jones, R. A.; Schwab, S. T.; Stuart, A. L.;
Whittlesey, B. R.; Wright, T. C. Polyhedron 1985, 4, 1689. (c) Bridge-
man, A. J.; Mays, M. J.; Woods, A. D. Organometallics 2001, 20, 2076.
(19) Todd, L. J.; Wilkinson, J. R.; Hickley, J. P.; Beach, D. L.;
Barnett, K. W. J. Organomet. Chem. 1978, 154, 151.
(20) Wrackmeyer, B.; Alt, H. G.; Maisel, H. E. J. Organomet. Chem.
1990, 399, 125.
(16) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa-Vivo´, D.; Garc´ıa, M. E.;
Falvello, L. R.; Sa´ez D.; Soler, T.; Herson, P. Dalton Trans. 2004,
4168.

Diphenylphosphide-Bridged Hydrides
Organometallics, Vol. 24, No. 4, 2005 653
Scheme 1. Fluxional Process Proposed for the
Cation in the Solutions of Compounds 2a
fluxional behavior of hydride compounds of type
[M2Cp2(µ-H)(µ-PRR′)(CO)4],21-24 involving a trigonal bi-
pyramidal intermediate. As a result, all four carbonyls
of the cations 2 become equivalent in the fast exchange
limit, as well as both phenyl rings on phosphorus, in
agreement with the room-temperature spectra of 2a and
2b.
Structural Characterization of the Paramag-
netic Cations 4. Compounds 4 display C-O stretching
bands at frequencies ca. 90 cm-1 higher than their
neutral precursors, as expected due to the presence of
a positive charge on the molecule (average ν(CO) values
in dichloromethane solution are 1995 (4a′) and 1987
cm-1 (4b′), cf. 1910 (1a) and 1898 cm-1(1b)). However,
the pattern of the spectra of compounds 4 is very
different from that of the parent hydrides 1. Instead of
one weak and two strong C-O stretching bands (i.e.,
1959 (w), 1926 (vs), and 1853 (s) cm-1 for 1b), the IR
spectrum of 4b′ exhibits four strong to medium bands
at 2043 (s), 1989 (vs), 1969 (m, sh), and 1947 (m, br).
The same is observed for 4a′, with the exception that
the two bands at lower frequency are degenerate (1965
cm-1). This pattern is very similar to that found for the
compound [Mo2{µ-(η5-C5H4)2SiMe2}(µ-H)(µ-PMe2)(CO)4]
(1959 (vs), 1925 (s), 1874 (vs), 1860 (sh) cm-1),25 which
has cis Mo(CO)2 moieties due to the presence of the
linked cyclopentadienyl ligands. Therefore, we conclude
that cations 4 exhibit a cis geometry in dichloromethane
solution (Chart 2). However, the IR spectra of com-
pounds 4 always exhibit an additional weak band at
2033 (4a′) or 2023 (4b′) cm-1 with constant relative
intensity. The position of this band would be ap-
proximately that one expected for a trans isomer (Chart
2). Thus, it is not unlikely that the dominant cis isomers
of 4 might be in equilibrium with a small amount of
the trans isomers in dichloromethane solution. Indeed,
we have recently shown that cis and trans isomers
coexist in solution for the mesitylphosphide complexes
[Mo2Cp2(µ-H)(µ-PMesX)(CO)4] (X ) H, F)16 and that
trans to cis isomerization occurs after one-electron
oxidation in the related diiron hydride trans-[Fe2Cp2-
(µ-H)(µ-PPh2)(CO)2].26
a An asterisk labels one in each pair of groups related by
the process.
of the diphenylphosphide bridge are not equivalent. In
agreement with this, the 13C NMR spectrum of 3a at
233 K exhibits eight phenyl resonances and the 1H NMR
spectrum also displays two multiplets corresponding to
the ortho-protons of inequivalent phenyl rings.
The unsaturated tetracarbonyls 2 exhibit 31P NMR
resonances considerably more deshielded than the cor-
responding pentacarbonyls (by ca. 40 ppm (Mo) or 80
ppm (W)) and display three well-separated IR bands in
the stretching region of the terminal carbonyls. These
spectroscopic features are very similar to those exhibited
by the PHR-bridged tetracarbonyl cations [Mo2Cp2-
(µ-PHR)(CO)4]+ (R ) Cy, Mes), which were shown by
1H and 13C NMR spectroscopy to display a cis arrange-
ment of their MoCp(CO)2 moieties with respect to the
Mo2P plane and be rigid on the NMR time scale.16
A
similar structure is therefore proposed for compounds
2 (Chart 1). However, the 13C NMR spectra of com-
pounds 2 clearly reveal the presence of dynamic effects
in solution. Thus, the 13C{1H} NMR spectrum of the
molybdenum complex 2a at 243 K exhibits only one
broad carbonyl resonance and one set of phenyl reso-
nances, while the 1H NMR spectrum displays broad
phenyl resonances. A similar situation is found for the
tungsten compound 2b; at room temperature the 13C-
{1H} NMR spectrum displays only one carbonyl reso-
nance at 214.9 ppm (JCP ) 8) and also one set of phenyl
resonances. At 233 K a single (but now broad) carbonyl
resonance was still observed. At 203 K two broad
resonances at 216.2 and 211.2 ppm were finally ob-
served, presumably corresponding to the carbonyls
placed either cis or trans with respect to the phosphide
bridge, respectively. At this temperature, however, a
single set of phenyl resonances was still present in the
13C or 1H NMR spectra. These spectra became severely
broadened and uninformative at lower temperatures, so
that the limiting spectra, displaying the expected two
sets of resonances corresponding to the inequivalent
phenyl rings, could not be observed. This is not totally
unexpected, since the kinetic barrier for the dynamic
process must be quite low; temperatures close to 203 K
were needed to distinguish considerably separated (by
ca. 500 Hz) carbonyl resonances.
ESR spectra were recorded for the paramagnetic
molybdenum cation (generated in situ at low tempera-
ture from 1a and [FeCp2]PF6) at different temperatures.
Best resolution was reached at ca. 180 K, when the
spectrum exhibits a well-resolved triplet resonance at
g ) 2.029 due to the hyperfine isotropic coupling to the
31P and 1H nuclei (AH ) AP ) 11.7 G) (Figure 1). A
weaker and broader triplet resonance at g ) 2.006 (AH
) AP ) 13.5 G) is tentatively assigned to the minor trans
isomer suspected to be present in solution. The very
weak and broad features to low field of the main
resonance are satellites due to hyperfine coupling to the
95Mo and 97Mo (I ) 5/2) nuclei.27 Although there are not
(21) Casey, C. P.; Bullock, R. M. Organometallics 1984, 3, 1100.
(22) Henrick, K.; McPartlin, M.; Norton, A. D.; Mays, M. J. J. Chem.
Soc., Dalton Trans. 1988, 1083.
(23) Curtis, M. D.; Woodward, S. J. Organomet. Chem. 1992, 439,
319.
(24) Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sa´ez, D.
Organometallics 2002, 21, 5515.
(25) Heck, J. J. Organomet. Chem. 1986, 311, C5.
(26) Alvarez, C. M.; Garc´ıa, M. E.; Ruiz, M. A.; Connelly, N. G.
Organometallics 2004, 23, 4750.
To account for the above observations for compounds
2 in solution, we propose the fluxional process depicted
in Scheme 1, based on that used to account for the
(27) Connelly, N. G.; Metz, B.; Orpen, A. G.; Rieger, P. H. Organo-
metallics 1996, 15, 729.

654 Organometallics, Vol. 24, No. 4, 2005
Alvarez et al.
state. For CV measurements (discussed below) the
(PPN)+ salts were more easy to handle. The (PPN)+ salt
having the same anion as 5a has been described
previously.29 We have prepared the ditungsten complex
by a similar route, involving first the reaction of 1b with
K[HBsBu3] to give the potassium salt K[W2Cp2(µ-PPh2)-
(CO)4] and then metathesis with (PPN)Cl. This gives a
good yield of the salt (PPN)[W2Cp2(µ-PPh2)(CO)4] (5b′).
The IR spectrum of 5b′ exhibits C-O stretching bands
with a pattern and frequencies identical to those of the
(H-DBU)+ salt 5b, indicating that the structure of the
anion is not modified significantly by the cation.
Reactions of the Anions 5 with [FeCp2]BF4. When
stoichiometric amounts of [FeCp2]BF4 and compounds
5a,b are mixed in dichloromethane at -30 °C, an
immediate reaction occurs to give deep-green solutions
of the neutral 33-electron radicals [M2Cp2(µ-PPh2)(CO)4]
(6a,b), which can be isolated in good yields as very air-
sensitive powders after chromatography. As expected,
compounds 6 are easily oxidized, reacting with a second
equivalent of [FeCp2]BF4 to give the corresponding
cationic tetracarbonyls 2a,b, identical to the products
obtained by direct oxidation of the hydrides 1.
Figure 1. ESR spectrum of compound 4a. Recorded at 180
K in CH2Cl2/THF (2:1).
Chart 3
The IR spectra of compounds 6 display three C-O
stretching bands with a pattern very similar to that of
cations 2 but with all the bands shifted by ca. 60-80
cm-1 to lower frequencies, as expected due to the
increased electron density when going from a cation to
a neutral molecule (Table 1). In fact, the average ν(CO)
for the radicals 6 is quite similar to that of the
corresponding diamagnetic hydrides 1, as expected. This
also indicates that the electron density in compounds 6
is evenly distributed between both metal fragments, in
agreement with electrochemical data to be discussed
later on. As a result, we propose that the 33-electron
radicals 6 have a cisoid structure very similar to that
for the 32-electron cations 2. From this, we conclude
that a trans to cis isomerization occurs upon one-
electron oxidation of the anions 5, thus paralleling the
result of the one-electron oxidation of hydrides 1.
ESR spectra were recorded for both 6a and 6b
(generated in situ at low temperature from the [PPN]+
salts and [FeCp2]PF6) at different temperatures. At 300
K the dimolybdenum radical gave an isotropic broad
doublet resonance at g ) 2.048 (AP ) 12.0 G). Unfor-
tunately, the spectra recorded on frozen solutions were
either too broad or too complex to be analyzed. The
ditungsten radical at 300 K gave a broad singlet
resonance at g ) 2.114. However, hyperfine couplings
to 31P were partially resolved in the anisotropic spectra
recorded on frozen solutions. For example, at 100 K
three well-separated resonances were observed at g
values of 2.270, 2.123, and 1.959, with the last split by
13.4 G due to hyperfine coupling to 31P (Figure 2). From
the magnitude of the observed 31P couplings, which are
similar to that for 4a and the mentioned diiron radicals
[Fe2(µ-PPh2)(CO)6L],5,28 we conclude that the unpaired
electron density in radicals 6 is also largely localized at
the metal atoms and carbonyl ligands.
many data available for comparison, we note that the
observed hyperfine couplings to P in 4a are similar to
those measured for the diphenylphosphide-bridged radi-
cals [Fe2(µ-PPh2)(CO)6L] (AP ) 20-24 G, L ) CO, PR3,
P(OR)3 ligand).5,28 For these iron complexes, the un-
paired electron density was found by EHMO calcula-
tions to be largely localized at the metal and carbonyl
ligands, with little participation of the phosphorus
orbitals.28 This might also be the case for cations 4. As
1
for the H hyperfine coupling, we have found no data
on related complexes, but the value of ca. 12-14 G found
for 4a seems to indicate little unpaired electron density
on the hydride ligand.
Preparation of Anions [M2Cp2(µ-PPh2)(CO)4]-.
The hydride complexes [M2Cp2(µ-H)(µ-PR2)(CO)4] are
easily deprotonated by a variety of reducing or basic
reagents to give the corresponding anions [M2Cp2(µ-
PR2)(CO)4]-. We have found that a strong base such as
DBU (DBU ) 1,8-diazabicyclo[5.4.0.]undec-7-ene) acts
as an efficient deprotonating reagent while at the same
time providing a relatively large cation, (H-DBU)+,
which facilitates manipulation of the resulting salt.
Thus, compounds 1a,b react in a few minutes with DBU
to give the corresponding carbonylates (H-DBU)[M2Cp2-
(µ-PPh2)(CO)4] (5a,b) in quantitative yield. The IR
spectrum of 5a is similar to that reported for the (PPN)+
salt of the same anion,29 an X-ray study on which
showed a gauche arrangement of the MoCp(CO)2 moi-
eties, so that the Cp ligands lie on opposite sides of the
Mo2P plane. The anions in compounds 5a,b are there-
fore thought to display similar transoid geometries
(Chart 3). We note that retention of transoid geometries
has been crystallographically verified in the pairs
[Mo2Cp2(µ-H)(µ-PR2)(CO)4]/[Mo2Cp2(µ-PR2)(CO)4]- (R )
Ph,29 Me30).
Although the (H-DBU)+ salts 5a,b are easily gener-
ated in situ, they are still quite air-sensitive in the solid
Electrochemical Studies. To gain further insight
into the chemical transformations following the oxida-
tion of hydrides 1 and anions 5 discussed above, we
carried out cyclic voltammetry measurements in dichlo-
(28) Krusic, P. J.; Baker, R. T.; Calabrese, J. C.; Morton, J. R.;
Preston, K. F.; Le Page, Y. J. Am. Chem. Soc. 1989, 11, 1262.
(29) Hartung, H.; Walther, B.; Baumeister, U.; Bo¨ttcher, H. C.; Krug,
A.; Rosche, F.; Jones, P. G. Polyhedron 1992, 11, 1563.
(30) Petersen, J. L.; Stewart, R. P., Jr. Inorg. Chem. 1980, 19, 186.

Diphenylphosphide-Bridged Hydrides
Organometallics, Vol. 24, No. 4, 2005 655
Figure 4. CV of compound 1a in CH2Cl2. Two cycles were
Figure 2. ESR spectrum of compound 6b. Recorded at 100
K in CH2Cl2/THF (2:1).
recorded from - 0.9 to 0.9 V at a scan rate of 200 mV s-1
.
Table 2. Electrochemical Data for Compounds 1
and 5 in Dichloromethane Solutionsa
change in the potentials, so a reversible wave is
observed. There are precedents of this sort of behavior
in diphenylphosphide-bridged complexes. For example,
a fully reversible oxidation wave was observed for the
complexes [M2(µ-PPh2)2(CO)6(PPh2H)2] (M ) Mo, W),
despite the reorganization of the carbonyl ligands (from
mer to fac) occurring after one-electron oxidation.31
Moreover, the great separation between the successive
oxidations of compounds 5 (∆E ) 0.61 V (Mo), 0.47 V
(W)) is indicative of great electronic delocalization
between metal centers at the intermediate radicals
6,2c,32 in agreement with the IR data discussed above.
Although we are not aware of related electrochemical
studies of the oxidation of phosphide-bridged dimolyb-
denum or ditungsten complexes related to compounds
5, we note that similar CVs have been reported for the
isoelectronic thiolate anions [Mo2Cp2(µ-SR)(CO)4]- (R )
Me, Ph, tBu), although the nature of the oxidized species
was not investigated at the time.33
compound
(E°′)1
(E°′)2
(Ep)2
1a
+0.45
+0.35
-0.53
-0.60
+0.76
+0.55
1b
5a′
5b′
+0.08
-0.13
a From cyclic voltammetry at room temperature; measurements
were routinely carried out at v ) 0.05, 0.1, and 0.2 V s-1 using a
saturated calomel electrode (SCE) as reference and FeCp2 as
internal standard. E°′ are the formal reduction potentials corre-
sponding to the reversible processes, while Ep are the peak
potentials of the irreversible processes. All values are given in volts
vs SCE. The subindexes 1 and 2 refer respectively to the first and
second oxidation process in each case.
The CVs of the hydride complexes 1a and 1b when
scanned from -0.9 V to positive potentials were similar
to each other (Figure 4), showing a reversible wave at
ca. 0.4 V and then an irreversible oxidation at 0.76 V
(Mo) or 0.55 V (W). In the return sweep, to negative
potentials, new waves also appear, which, by cycling the
CV between -0.9 and 0.9 V, are identified as the two
waves observed in the CVs of the corresponding anions
5a,b (Figure 3).
The CVs are therefore consistent with the observed
chemical transformations. The first wave corresponds
to the oxidation of the hydride 1 to the paramagnetic
cation 4. We have shown through IR spectroscopy that
this is accompanied by a trans to cis isomerization, and
we assume again that this does not imply a significant
change in the potentials, so that a reversible wave is
observed. The second oxidation wave is irreversible,
consistent with a proton loss from the dication, giving
the tetracarbonyl cation 2 (i.e., 4+ f 2 + H+), which
gives rise to the two new waves in the return sweep,
i.e., to the transformations 2 f 6 f 5. Incidentally, we
note that the successive oxidation waves of the neutral
hydrides 1 are less separated from each other than those
Figure 3. CV of compound 5a in CH2Cl2, from - 0.9 to
0.8 V at a scan rate of 200 mV s-1 (oxidation waves of
[FeCp2], added as an internal calibrant, marked *).
romethane on both 1a and 1b as well as on the (PPN)+
salts 5a′ and 5b′. In each case, the behavior of the
dimolybdenum or ditungsten analogues was similar,
except for the easier oxidation of the latter (by 0.05 to
0.20 V, Table 2), as expected on the basis of the higher
electron density of tungsten complexes (also reflected
in lower C-O stretching frequencies). The CVs of
compounds 5 display two well separated reversible
waves at ca -0.5 and 0.0 V (Figure 3), each correspond-
ing to the removal of one electron; the two waves
correspond to the transformations 5 f 6 f 2. As
discussed above, compounds 5 display transoid struc-
tures, whereas radicals 6 are proposed to display cisoid
structures (Chart 3). Therefore, we assume that the
trans to cis isomerization occurring after removal of one
electron from anions 5 does not imply a significant
(31) Keiter, R. L.; Keiter, E. A.; Rust, M. S.; Miller, D. R.; Sherman,
E. O.; Cooper, D. E. Organometallics 1992, 11, 487.
(32) (a) Crutchley, R. J. Adv. Inorg. Chem. 1994, 41, 273. (b) Creutz,
C. Progr. Inorg. Chem. 1983, 30, 1.
(33) Schollhammer, P.; Pe`tillon, F. Y.; Poder-Guillon, S.; Talarmin,
J.; Muir, K. W.; Yufit, D. S. J. Organomet. Chem. 1996, 513, 181.

656 Organometallics, Vol. 24, No. 4, 2005
Alvarez et al.
in a range of hydrocarbon-bridged complexes.4a More-
over, an increase in acidity of about 20 pKa units on one-
electron oxidation has been calculated for the mononu-
clear hydrides [MCp(CO)3H] (M ) Cr, Mo, W).34
Scheme 2. Reaction Pathways in the Oxidation of
Compounds 1 and 5a
Deprotonation from dications 4+ lead to the tetracar-
bonyl cations 2, which retain the cis geometry dominant
in radicals 4. The cations 2 are now connected to the
corresponding anions through two independent and
reversible one-electron reduction steps. In the first step,
the isolable radicals 6 are formed, with retention of the
cisoid structure, as discussed above. However, by con-
sidering that the anions 5 display transoid structures,
it is reasonable to suppose that the radicals 6 might be
in equilibrium with very small amounts of the corre-
sponding trans isomers (as proposed for the paramag-
netic hydrides 4), the reduction of which would give
reversibly the pertinent carbonylates 5. There are,
however, other possible explanations for the observed
reversibility of the transformation 6/5. In any case, our
data suggest that while trans geometries are favored
for the saturated 34-electron substrates, cis geometries
are preferred for the 33-electron radicals 4 and 6, as
well as the unsaturated 32-electron cations 2.
Experimental Section
a Species shown in parentheses have not been detected.
General Comments. All reactions were carried out
under an atmosphere of nitrogen. Solvents were purified
according to standard procedures35 and distilled under
nitrogen prior to use. Petroleum ether refers to that
fraction distilling in the range 65-70 °C. Reagents
[FeCp2]BF4 and [FeCp2](BAr′4) were prepared according
to the literature procedure.11,12 DBU (1,8-diazabicy-
clo[5.4.0.]undec-7-ene) was purchased from Aldrich
and used as received. Compounds [Mo2(µ-H)(µ-PPh2)-
Cp2(CO)4] (1a),22 [W2(µ-H)(µ-PPh2)Cp2(CO)4] (1b),24 and
(PPN)[Mo2(µ-PPh2)Cp2(CO)4] (5a′)29 were prepared as
reported before. Low-temperature reactions were per-
formed using jacketed Schlenk tubes, refrigerated by a
closed propan-2-ol circuit kept at the desired tempera-
ture with a cryostat. Filtrations were carried out using
dry diatomaceous earth under nitrogen. Aluminum
oxide (activity I, 150 mesh) was purchased from Aldrich
and degassed under vacuum prior to use. It was mixed
afterward under nitrogen with the appropriate amount
of water to reach the activity desired.
involving the neutral radicals 6, the ∆E values being
ca. 0.35 V (Mo) and 0.25 V (W).
Reaction Pathways in the Oxidation Reactions
of Hydrides 1 and Anions 5. By combining informa-
tion from chemical and electrochemical studies just
discussed, we can define the transformations induced
by oxidation of the diphenylphosphide-bridged com-
pounds 1 and 5 (Scheme 2).
The ferrocenium ion (E°′ ) +0.47 V in dichlo-
romethane solutions)11 is a strong enough oxidant to
remove one electron from the ditungsten hydride 1b (E°′
) 0.35 V), but its E°′ is only marginally more positive
than that for the oxidation of 1a (E°′ ) 0.45 V). This
explains why [FeCp2](BAr′4) (E°′ ) 0.46 V in CH2Cl2
solution)12 gives 4b from 1b but only an equilibrium
mixture of 4a and 1a for the dimolybdenum complex
(Keq is calculated from the Nernst equation to be ca. 2).
The first oxidation of compounds 1 should initially give
the trans isomer of radicals 4 (trans-4), but must rapidly
transform into their cis isomers 4, which are dominant
in solution. The spectroscopic data suggest that the
trans/cis equilibrium for radicals 4 is strongly shifted
toward the cis isomers.
The measured potentials also explain why [FeCp2]-
(BAr′4) cannot remove an electron from radicals 4. To
overcome the unfavorable thermodynamics (the second
oxidation occurs at 0.1 to 0.25 V more positive than the
E°′ value of the ferrocenium ion), proton transfer must
be coupled to oxidation. This is facilitated by the
presence of a weak base such as water or tetrahydro-
furan or by the external anion BF4-, if using [FeCp2]-
BF4 as oxidizing agent. All these species act as proton
acceptors to the hydride dication derived from the
second oxidation (4+), which displays a greatly enhanced
acidity due to its high positive charge. This is not
unusual. For example, a similar double-oxidation depro-
tonation sequence results in the cleavage of C-H bonds
NMR spectra were recorded at 300.13 (1H), 121.50
(31P{1H}), or 75.47 MHz (13C{1H}) at room temperature,
unless otherwise indicated. Chemical shifts (δ) are given
in ppm, 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. 13C{1H} NMR
spectra were routinely recorded on solutions containing
a small amount of tris(acetylacetonato)chromium(III) as
a relaxation reagent.
X-band ESR spectra were recorded on a Bruker
ESP300 spectrometer equipped with a Bruker variable-
temperature accessory and a Hewlett-Packard 5350B
microwave frequency counter. The field calibration was
(34) (a) Ryan, O. B.; Tilset, M.; Parker, V. D. J. Am. Chem. Soc.
1990, 112, 2618. (b) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res.
1993, 26, 287.
(35) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, U.K., 1988.

Diphenylphosphide-Bridged Hydrides
Organometallics, Vol. 24, No. 4, 2005 657
checked by measuring the resonance of the diphenylpi-
crylhydrazyl (dpph) radical before each series of spectra.
Electrochemical studies were carried out using an
EG&G model 273A potentiostat linked to a computer
using EG&G Model 270 Research Electrochemistry
software in conjunction with a three-electrode cell. The
auxiliary electrode was a platinum wire, and the work-
ing electrode a platinum disk. The reference was an
aqueous saturated calomel electrode separated from the
test solution by a fine-porosity frit and an agar bridge
saturated with KCl. Solutions were 5.0 × 10-4 or 1.0 ×
10-3 mol dm-3 in the test compound and 0.1 mol dm-3
in [NBu4][PF6] as the supporting electrolyte in CH2Cl2.
Under the conditions used, E°′ for the one-electron
oxidation of [Fe(η5-C5H5)2] added to the test solutions
as an internal calibrant is 0.47 V.
Preparation of [Mo2Cp2(µ-PPh2)(CO)4]BF4 (2a).
Compound 1a (0.025 g, 0.04 mmol) was added slowly
to a CH2Cl2 solution (1 mL) containing [FeCp2]BF4
(0.023 g, 0.08 mmol) at -30 °C, and the mixture was
stirred for 10 min at that temperature. Petroleum ether
(6 mL) was then added at -30 °C, which caused the
precipitation of compound 2a as a brown solid, which
was dried under vacuum. The complex is thermally
unstable at room temperature; therefore, satisfactory
IR spectra and microanalytical data could not be
obtained. 1H NMR (400.14 MHz, CD2Cl2, 243 K): δ
7.80-6.70 (m, br, 10H, Ph), 5.80 (s, 10 H, Cp). 13C{1H}
NMR (100.63 MHz, CD2Cl2, 238 K): δ 226.1 (br, CO),
139.3 [d, JCP ) 49, C1(Ph)], 132.1 [d, JCP ) 10, C2(Ph)],
131.6 [s, C4(Ph)], 129.6 [d, JCP ) 10, C3(Ph)], 96.4 (s,
Cp).13C{1H} NMR (100.63 MHz, CD2Cl2, 290 K): δ 226.8
(d, JCP ) 12, CO), 140.0 [d, JCP ) 48, C1(Ph)], 132.3 [d,
JCP ) 11, C2(Ph)], 131.5 [s, C4(Ph)], 129.7 [d, JCP ) 12,
C3(Ph)], 96.3 (s, Cp).
[d, JCP ) 58, C1(Ph)], 133.2 [d, JCP ) 12, C2(Ph)], 131.6
[s, C4(Ph)], 129.6 [d, JCP ) 12, C3(Ph)], 94.3 (s, Cp).
13C{1H} NMR (100.63 MHz, CD2Cl2, 203 K): δ 216.2
(br, CO), 211.2 (br, CO), 140.2 [d, JCP ) 59, C1(Ph)],
132.7 [d, JCP ) 12, C2(Ph)], 131.6 [s, C4(Ph)], 129.5 [d,
JCP ) 12, C3(Ph)], 94.7 (s, Cp).
Preparation of [W2Cp2(µ-PPh2)(CO)4](BAr′4) (2b′).
Compound 1b (0.040 g, 0.05 mmol) was added slowly
to a CH2Cl2 solution (6 mL) of [FeCp2](BAr′4) (0.105 g,
0.1 mmol) at -30 °C, and the mixture was stirred for
15 min. Tetrahydrofuran (0.1 mL) was then added to
the dark green resulting solution, and the mixture was
stirred at room temperature for 15 min to give a brown
solution, which was filtered. Removal of solvent under
vacuum from the filtrate gave a dark oily solid, which
was washed with a mixture of diethyl ether and
petroleum ether (1:2, 3 × 5 mL) and dried under vacuum
to give compound 2b′ (0.075 g, 90%) as a brown solid.
Anal. Calcd for C58H32BF24O4PW2: C, 41.98; H, 1.93.
Found: C, 41.52; H, 1.97.
Preparation of [Mo2Cp2(µ-PPh2)(µ-CO)(CO)4]BF4
(3a). A CH2Cl2 solution (6 mL) of compound 2a (ca.
0.040 mmol) was prepared in situ at -30 °C as described
above. The nitrogen atmosphere was then replaced by
CO, and stirring was continued for 10 min to give a red
solution, which was filtered. Removal of solvent under
vacuum from the filtrate gave a red solid, which was
washed with toluene (2 × 5 mL) to remove ferrocene.
Recrystallization of this product from CH2Cl2/petroleum
ether at -20 °C gave compound 3a (0.026 g, 90%) as
dark red crystals. Anal. Calcd for C27H20BF4Mo2O5P: C,
44.14; H, 2.72. Found: C, 44.60; H, 2.78. 1H NMR
(400.14 MHz, CD2Cl2, 233 K): δ 7.72 (m, 1H, Ph), 7.67
(m, 2H, Ph), 7.46-7.40 (m, 5H, Ph), 6.71(m, 2H, Ph),
5.37 (s, 10 H, Cp). 13C{1H} NMR (100.63 MHz, CD2Cl2,
233 K): δ 275.2 (d, JCP ) 25, µ-CO), 226.9 (d, JCP ) 15,
CO), 221.1(s, CO), 144.3 [d, JCP ) 33, C1(Ph)], 132.6 [d,
JCP ) 52, C1(Ph)], 135.6 [d, JCP ) 9, C2(Ph)], 132.4, 130.8
(2 × s, 2 × C4(Ph)], 129.8 [d, JCP ) 11, 2 × C3(Ph)], 129.7
[d, JCP ) 8, C2(Ph)], 94.7 (s, Cp).
Preparation of [Mo2Cp2(µ-PPh2)(µ-CO)(CO)4]-
(BAr′4) (3a′). A CH2Cl2 solution (6 mL) of compound
2a′ (ca. 0.04 mmol) was prepared in situ as described
above and then stirred under a CO atmosphere for 10
min at room temperature to give a red solution, which
was filtered. Removal of solvent from the filtrate under
vacuum gave a red solid, which was washed with
petroleum ether (2 × 5 mL) and recrystallized from
CH2Cl2/petroleum ether at -20 °C to give compound 3a′
(0.050 g, 83%) as dark red crystals. Anal. Calcd for
C59H32BF24Mo2O5P: C, 46.89; H, 2.12. Found: C, 47.40;
H, 2.08.
Preparation of [Mo2Cp2(µ-PPh2)(CO)4](BAr′4)
(2a′). Compound 1a (0.030 g, 0.05 mmol) was added
slowly to a CH2Cl2 solution (10 mL) containing [FeCp2]-
(BAr′4) (105 mg, 0.10 mmol) at -30 °C, and the mixture
was stirred for 10 min at that temperature and then
10 min more at room temperature after adding 0.1 mL
of tetrahydrofuran. Solvents were then removed under
vacuum, and the resulting oily solid was washed with
petroleum ether (3 × 5 mL) to give a dark brown solid.
This crude product was shown (by 31P{1H} NMR) to be
a 10:1 mixture of compounds 2a′ and 3a′. The complex
2a′ is thermally unstable and slowly decomposes at
room temperature to give 3a′. Therefore, satisfactory
microanalytical data could not be obtained.
Preparation of [W2Cp2(µ-PPh2)(CO)4]BF4 (2b).
Compound 1b (0.048 g, 0.06 mmol) was added slowly
to a CH2Cl2 solution (6 mL) of [FeCp2]BF4 (0.032 g,
0.12 mmol), and the mixture was stirred for 15 min
and then filtered. Removal of solvent under vacuum
from the filtrate gave a dark oily solid, which was
washed with toluene (3 × 5 mL) and petroleum ether
(2 × 5 mL) and dried under vacuum to yield compound
2b (0.048 g, 90%) as a brown solid. Anal. Calcd for
C26H20BF4O4PW2: C, 35.37; H, 2.27. Found: C, 35.06;
Preparation of [W2Cp2(µ-PPh2)(µ-CO)(CO)4]BF4
(3b). A CH2Cl2 solution (1 mL) of compound 2b (ca.
0.05 mmol) was prepared in situ as described above
and then stirred under a CO atmosphere for 10 min at
room temperature to give a red solution. A layer of
petroleum ether (3 mL) was then added over the
CH2Cl2 solution at -20 °C under CO. After complete
diffusion at this temperature, red crystals of compound
3b were separated (0.027 g, 60%). Anal. Calcd for
C27H20BF4O5PW2: C, 35.60; H, 2.20. Found: C, 35.35;
1
H, 2.37. H NMR (200.13 MHz, CD2Cl2): δ 7.52 (s, br,
6H, Ph), 7.20 (m, br, 4H, Ph), 5.68 (s, 10H, Cp). 1H NMR
(400.13 MHz, CD2Cl2, 213 K): δ 7.61 (s, br, 6H, Ph),
7.20 (m, br, 4H, Ph), 5.82 (s, 10H, Cp). 13C{1H} NMR
(100.63 MHz, CD2Cl2): δ 214.9 (d, JCP ) 8, CO), 141.5
H, 2.05. 1H NMR (acetone-d6, CO atmosphere):
7.60-7.30 (m, br, 10H, Ph), 5.85 (s, 10 H, Cp). 31P{1H}
δ

658 Organometallics, Vol. 24, No. 4, 2005
Alvarez et al.
(acetone-d6, CO atmosphere): δ 33.8 [s, JPW ) 176,
µ-PPh2]. 13C{1H} NMR (acetone-d6, CO atmosphere): δ
135.0-129.0 (m, br, Ph), 93.9 (s, Cp).
0.14 mmol) was added to a tetrahydrofuran solution (25
mL) of compound 1b (0.103 g, 0.13 mmol), and the
mixture was stirred for 1 h to give a dark purple
solution. Solid (PPN)Cl (0.086 g, 0.15 mmol) was then
added, and the mixture was stirred overnight and
filtered. The solvent was then removed from the filtrate
under vacuum to give a purple residue, which was
washed with methanol (3 × 5 mL) and dried under
vacuum to give 5b′ as a dark purple solid. Recrystalli-
zation from CH2Cl2/petroleum ether (1:3) gave com-
pound 5b′ as a microcrystalline solid (0.156 g, 90%).
Anal. Calcd for C62H50NO4P3W2: C, 55.80; H, 3.75.
Found: C, 55.60; H, 3.70.
Preparation of [Mo2Cp2(µ-PPh2)(CO)4] (6a). Solid
[FeCp2]BF4 (0.026 g, 0.1 mmol) was added to a solution
of compound 5a (ca. 0.1 mmol, prepared as described
above) in dichloromethane (15 mL) at -30 °C, and the
mixture was stirred and allowed to reach room temper-
ature for 15 min to give a green solution. Solvent was
then removed under vacuum to give a green residue,
which was dissolved in 10 mL of petroleum ether/
CH2Cl2 (4:1) and put on the top of an alumina column
(activity IV, 20 × 2 cm) cooled at -20 °C. Elution with
petroleum ether gave a yellow band of ferrocene. Elution
with petroleum ether/ CH2Cl2 (3:1) gave a green frac-
tion. Removal of solvents from the latter under vacuum
yielded compound 6a as a dark green, very air-sensitive
powder (0.050 g, 83%). Satisfactory microanalytical data
could not be obtained for this complex. ESR (toluene,
300K): g ) 2.048 (d, AP ) 12.0 G). ESR (toluene, 190
K): g1 ) 2.087, g2 ) 2.045, g3 ) 2.006.
Preparation of [W2Cp2(µ-PPh2)(µ-CO)(CO)4]-
(BAr′4) (3b′). A CH2Cl2 solution (2 mL) of compound
2b′ (0.05 mmol) was prepared in situ as described
above and then stirred under a CO atmosphere for 10
min at room temperature to give a red solution. Petro-
leum ether (5 mL) was then added, which caused the
precipitation of compound 3b′ as a red crystalline
solid, which was separated from the solution and
dried under vacuum (0.080 g, 95%). Anal. Calcd for
C59H32BF24O5PW2: C, 41.99; H, 1.90. Found: C, 42.10;
H, 1.95.
Preparation of Solutions of [Mo2Cp2(µ-H)(µ-
PPh2)(CO)4](BAr′4) (4a′). Compound 1a (0.030 g, 0.05
mmol) and [FeCp2](BAr′4) (0.053 g, 0.05 mmol) were
stirred in dichloromethane (10 mL) at room temperature
for 3 min to give a dark green solution containing an
equilibrium mixture of compounds 1a and 4a′. Almost
complete disappearance of the starting material 1a was
achieved by using a 3-fold excess of oxidizing agent. ESR
-
(PF6 salt, CH2Cl2/THF (2:1), 180 K): g ) 2.029 (t, AP
) AH ) 11.7 G, cis isomer), g ) 2.006 (t, AP ) AH
13.5 G, trans isomer).
)
Preparation of Solutions of [W2Cp2(µ-H)(µ-PPh2)-
(CO)4](BAr′4) (4b′). Compound 1b (0.040 g, 0.05 mmol)
and [FeCp2](BAr′4) (0.053 g, 0.05 mmol) were stirred in
dichloromethane (10 mL) at room temperature for 3 min
to give a dark green solution shown (by IR) to contain
essentially pure compound 4b′.
Preparation of Solutions of (H-DBU)[Mo2Cp2(µ-
PPh2)(CO)4] (5a). Neat DBU (16 µL, 0.11 mmol) was
added to a CH2Cl2 solution (15 mL) of compound 1a
(0.060 g, 0.10 mmol), and the mixture was stirred for
15 min to give a dark purple solution of compound 5a,
which was ready for further use.
Preparation of [W2Cp2(µ-PPh2)(CO)4] (6b). Solid
[FeCp2]BF4 (0.020 g, 0.08 mmol) was added to a solution
of compound 5b (ca. 0.08 mmol) in dichloromethane (15
mL) at -30 °C, and the stirred mixture was allowed to
reach room temperature for 15 min to give a green
solution. Workup as described for 6a (elution with
petroleum ether/CH2Cl2 (4:1)) gave compound 6b as a
dark green, very air-sensitive powder (0.044 g, 74%).
Satisfactory microanalytical data could not be obtained
for this complex. ESR (CH2Cl2/tetrahydrofuran, (2:1),
300 K): g ) 2.114. ESR (CH2Cl2/tetrahydrofuran, (2:
1), 100 K): g1 ) 2.270, g2 ) 2.123, g3 ) 1.959 (d, AP )
13.4 G).
Preparation of (H-DBU)[W2Cp2(µ-PPh2)(CO)4]
(5b). Neat DBU (13 µL, 0.087 mmol) was added to a
CH2Cl2 solution (15 mL) of compound 1b (0.064 g, 0.08
mmol) and the mixture stirred for 15 min to yield a dark
purple solution. Solvent was then removed under vacuum
to give a purple residue, which was washed with
petroleum ether (3 × 5 mL) and dried under vacuum to
give essentially pure compound 5b as a dark purple
solid (0.068 g, 90%). Satisfactory microanalytical data
could not be obtained for this air-sensitive complex. IR
(THF): ν(CO) 1873 (m), 1831 (vs), 1785 (s), 1760 (m)
Acknowledgment. We thank the Ministerio de
Ciencia y Tecnolog´ıa of Spain for financial support
(Projects BQU2000-0944 and BQU2003-05471) and for
a grant to D.S. We also thank the FICYT of Asturias
for a grant to M.T.R. We finally thank Luc´ıa Ferna´ndez
for her collaboration in studies of the oxidation of
compound 1b by [FeCp2](BAr′4).
1
cm-1. H NMR (CD2Cl2): δ 9.96 (s, br, 1H, NH), 7.80-
6.90 (m, 10H, Ph), 4.85 (s, br, 10H, Cp), 3.40-1.60 (4 ×
m, 16H, CH2).
Preparation of (PPN)[W2Cp2(µ-PPh2)(CO)4] (5b′).
K[HBsBu3] (140 µL of a 1 M solution in tetrahydrofuran,
OM0494824