Formation and cleavage of P-C, Mo-C, and C-H bonds involving arylphosphinidene and cyclopentadienyl ligands at dimolybdenum centers

Article abstract of DOI:10.1021/om0605229

The phosphinidene-bridged complex [Mo₂Cp₂(μ -PR*)(CO)₄] (R = 2,4,6- CoH₂^t Bu³) experiences an intramolecular C-H bond cleavage from a ^tBu group to give the phosphide-hydride derivative [Mo₂-Cp₂(μ-H) {μ-P(CH₂CMe₂)C₆H₂ ^tBu₂}(CO)₄] in refluxing diglyme (ca. 438 K) or under exposure to near-UV-visible light. In contrast, its exposure to UV light yields two different dicarbonyl derivatives depending on the reaction conditions, either the triply bonded [Mo₂Cp₂(μ- PR*)(μ-CO)₂] (Mo-Mo = 2.5322(3) A) or its isomer [Mo₂Cp₂(μ-κ¹:κ¹, η⁶-PR*)(CO)₂], in which the phosphinidene ligand bridges asymmetrically the metal centers while binding its aryl group to one of the molybdenum atoms in a η⁶-fashion. The latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene-phosphinidene derivative [Mo₂Cp(μ- κ¹:κ¹, η⁵-PC₅H ₄)(η⁶-R*H)(CO)₂]. Carbonylation of the η⁶-phosphinidene complex proceeds stepwise through the η⁴-tricarbonyl complex [Mo₂Cp₂(μ- κ¹:κ¹,η⁴-PR*)(CO) ₃] and then to the starting tetracarbonyl compound, whereas its reaction with CN^tBu yields only the η⁴-complex [Mo₂Cp₂(μ-κ¹:κ¹, η⁴-PR*)(CN^tBu)(CO)₂], which was characterized through an X-ray study. The η⁴-tricarbonyl species reacts with CN^tBu in tetrahydrofuran to give the metal-metal bonded derivative [Mo₂Cp₂(μ-PR*)(CN^tBu)(CO) ₃]. In petroleum ether, however, this reaction yields the bis(isocyanide) derivative [Mo₂Cp₂(μ-PR*)(CN ^tBu)₂(CO)₃], which has an asymmetric trigonal phosphinidene bridge and no metal-metal bond. All the above results can be explained by assuming the operation of two primary processes in the photolysis of [Mo₂Cp₂(μ-PR*)(CO)₄], one of them involving a valence tautomerization of the phosphinidene ligand, from the trigonal (four-electron donor) to the pyramidal (two-electron donor) coordination mode. The carbonylation reaction of the η⁶-complex is accelerated by the presence of CuCl, due to the formation of the trimetal species [CuMo₂(Cl)Cp₂(μ-κ¹: κ¹:κ¹, η⁶-PR*)(CO) ₂] and [CuMo₂(Cl)-Cp₂(μ-κ¹: κ¹:κ¹,η⁴-PR*)(CO) ₃]. The latter complexes were also characterized by single-crystal X-ray studies.

Full text of DOI:10.1021/om0605229

Organometallics 2006, 25, 4857-4869
4857
Formation and Cleavage of P-C, Mo-C, and C-H Bonds
Involving Arylphosphinidene and Cyclopentadienyl Ligands at
Dimolybdenum Centers
Inmaculada Amor,† M. Esther Garc´ıa,† Miguel A. Ruiz,*,† David Sa´ez,†
Hayrullo Hamidov,‡ and John C. Jeffery‡
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, 33071 OViedo,
Spain, and School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K.
ReceiVed June 14, 2006
t
The phosphinidene-bridged complex [Mo2Cp2(µ-PR*)(CO)4] (R ) 2,4,6- C6H2 Bu3) experiences an
t
intramolecular C-H bond cleavage from a Bu group to give the phosphide-hydride derivative [Mo2-
t
Cp2(µ-H){µ-P(CH2CMe2)C6H2 Bu2}(CO)4] in refluxing diglyme (ca. 438 K) or under exposure to near-
UV-visible light. In contrast, its exposure to UV light yields two different dicarbonyl derivatives depending
on the reaction conditions, either the triply bonded [Mo2Cp2(µ-PR*)(µ-CO)2] (Mo-Mo ) 2.5322(3) Å)
or its isomer [Mo2Cp2(µ-κ1:κ1,η6-PR*)(CO)2], in which the phosphinidene ligand bridges asymmetrically
the metal centers while binding its aryl group to one of the molybdenum atoms in a η6-fashion. The
latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene-
phosphinidene derivative [Mo2Cp(µ-κ1:κ1,η5-PC5H4)(η6-R*H)(CO)2]. Carbonylation of the η6-phosphin-
idene complex proceeds stepwise through the η4-tricarbonyl complex [Mo2Cp2(µ-κ1:κ1,η4-PR*)(CO)3]
and then to the starting tetracarbonyl compound, whereas its reaction with CNtBu yields only the η4-
complex [Mo2Cp2(µ-κ1:κ1,η4-PR*)(CNtBu)(CO)2], which was characterized through an X-ray study. The
η4-tricarbonyl species reacts with CNtBu in tetrahydrofuran to give the metal-metal bonded derivative
[Mo2Cp2(µ-PR*)(CNtBu)(CO)3]. In petroleum ether, however, this reaction yields the bis(isocyanide)
derivative [Mo2Cp2(µ-PR*)(CNtBu)2(CO)3], which has an asymmetric trigonal phosphinidene bridge and
no metal-metal bond. All the above results can be explained by assuming the operation of two primary
processes in the photolysis of [Mo2Cp2(µ-PR*)(CO)4], one of them involving a valence tautomerization
of the phosphinidene ligand, from the trigonal (four-electron donor) to the pyramidal (two-electron donor)
coordination mode. The carbonylation reaction of the η6-complex is accelerated by the presence of CuCl,
due to the formation of the trimetal species [CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η6-PR*)(CO)2] and [CuMo2(Cl)-
Cp2(µ-κ1:κ1:κ1,η4-PR*)(CO)3]. The latter complexes were also characterized by single-crystal X-ray studies.
Chart 1
Introduction
The phosphinidene ligand (PR, with R ) alkyl, aryl, etc.) is
an extremely versatile four-electron donor that can bind ef-
ficiently up to four metal atoms in many different ways (Chart
1). The nature of the M-P bond changes dramatically among
these coordination modes, and this gives rise in turn to quite
different chemical behaviors. For example, the M-P bonds in
the µ3 or µ4 bridging modes (F, G) are essentially single bonds,
and little reactivity is expected for them. Indeed, phosphinidene
groups behave usually as good supporting ligands in clusters,
although some reactions involving their M-P bonds can
occur.1,2 As opposed to this, the M-P bonds in the terminal
complexes have a considerable multiple character and should
be reactive centers, especially in the case of bent complexes
(A) where a lone pair at the phosphorus atom is also present.
In fact, the reactivity of mononuclear phosphinidene complexes
is the subject of intense current research.3,4 The carbene-like
* To whom correspondence should be addressed. E-mail: mara@
uniovi.es.
† Universidad de Oviedo.
‡ University of Bristol.
(3) Reviews: (a) Lammertsma, K. Top. Curr. Chem. 2003, 229, 95. (b)
Lammerstma, K.; Vlaar, M. J. M. Eur. J. Org. Chem. 2002, 1127. (c)
Mathey, F.; Tran Huy, N. H.; Marinetti, A. HelV. Chim. Acta 2001, 84,
2938. (d) Stephan, D. W. Angew. Chem., Int. Ed. 2000, 39, 314. (e) Schrock,
R. R. Acc. Chem. Res. 1997, 30, 9. (f) Cowley, A. H. Angew. Chem., Int.
Ed. Engl. 1997, 30, 445.
(1) Huttner, G.; Knoll, K. Angew. Chem., Int. Ed. Engl. 1987, 26, 743.
(2) For some recent reactions, see: (a) Wang, W.; Corrigan, J. F.; Enright,
G. D.; Taylor, N. J.; Carty, A. J. Organometallics 1998, 17, 427. (b) Scoles,
L.; Yamamoto, J. H.; Brissieux, L.; Sterenberg, B. T.; Udachin, K. A.; Carty,
A. J. Inorg. Chem. 2001, 40, 6731.
10.1021/om0605229 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/24/2006

4858 Organometallics, Vol. 25, No. 20, 2006
Amor et al.
Scheme 1. Photochemical Derivatives of Compound 1
behavior of the bent phosphinidene complexes, a type of
molecules that can also be classified as either electrophilic or
nucleophilic ones, has turned these compounds into useful
intermediates in the synthesis of new organophosphorus mol-
ecules.3,5
t
(R* ) 2,4,6-C6H2 Bu3)
In contrast, the chemistry of phosphinidene-bridged binuclear
complexes has been comparatively little explored (especially
those having M-M bonds) even when the presence of multiple
M-P bonding (types C and D) or a lone pair at phosphorus
(type E) should make these molecules quite reactive. The
chemical behavior of the pyramidal-phosphinidene-bridged
complexes (E) is likely to be dominated by the high nucleo-
philicity at the phosphorus site,6 while the behavior of the
trigonal phosphinidene bridges is less straightforward, especially
in the presence of M-M bonds. Actually little research in this
last area has been carried out after the pioneering work by
Huttner.7,8 Following our high-yield synthesis of Cowley’s
complex [Mo2Cp2(µ-PR*)(CO)4] (1) (Cp ) η5-C5H5; R* )
t
2,4,6-C6H2 Bu3), we initiated a systematic study on the reactivity
of this trigonal phosphinidene-bridged species.9 The photo-
chemical decarbonylation reactions of complex 1 were particu-
larly puzzling and interesting since they gave mixtures of
products arising from several unusual processes such as C-H
bond activation and η-coordination of the aryl group.9,10 In this
paper we report our full studies on the decarbonylation reactions
of compound 1 and the structural characterization of the products
derived thereof. This has enabled us to find selective preparative
routes for each of the possible products and to disclose a
completely unprecedented isomerization involving the cleavage
and formation of several P-C, Mo-C, and C-H bonds in these
phosphinidene-bridged species.
hours without noticeable change. Upon refluxing in diglyme
solution (ca. 438 K), a slow isomerization takes place to give
the phosphide-hydride derivative [Mo2Cp2(µ-H){µ-P(CH2CMe2)-
t
C6H2 Bu2}(CO)4] (2) in medium yield. The formation of 2
requires formally the addition of a C-H bond from a tBu group
across the multiple M-P bond in the phosphinidene bridge, and
it can be accomplished more efficiently using photochemical
methods.
The photolysis of compound 1, however, turned out to be
far more complex than suspected, and it can yield one or several
of the following products: the above tetracarbonyl hydride 2,
the triply bonded dicarbonyl [Mo2Cp2(µ-PR*)(µ-CO)2] (3), its
tautomer [Mo2Cp2(µ-κ1:κ1,η6-PR*)(CO)2] (4) with no metal-
metal bond, and the tricarbonyl complex [Mo2Cp2(µ-κ1:κ1,η4-
PR*)(CO)3] (6a). The relative amounts of the above products
were strongly dependent on the experimental conditions such
as wavelength of light, solvent, temperature, and reaction time,
and almost selective conditions could be found to prepare each
of them (Scheme 1). The tricarbonyl compound 6a was always
obtained in low yield, this decreasing with reaction time due to
its facile conversion to the corresponding dicarbonyl 4 under
photolytic conditions, as shown by independent experiments.
Compound 2 could be selectively formed (87% after purifica-
tion) through visible-UV irradiation in toluene at 288 K using
ordinary Pyrex glassware (this filters off most of the high-energy
UV radiation). In contrast, removal of CO, as required to form
compound 3 or 4, was accomplished in THF or THF/MeCN
solutions using quartz glassware. In addition, the formation of
3 was favored over that of 4 (or 6) at low temperatures, whereas
the presence of MeCN had dramatic effects: (a) it suppressed
the formation of 2 at all concentrations, (b) it favored all
decarbonylation processes in general, and (c) at high concentra-
tions it favored the formation of 4 (or 6) over that of 3. Thus,
compound 3 could be formed almost selectively (84% after
purification) by carrying out the photolytic experiment at 263
K in a THF/MeCN (50:1) mixture. In contrast, the photolysis
of 1 at 318 K in THF/MeCN (1:2) gave mainly a mixture of
the aryl-bonded complexes 4 and 6 (3:1), from which compound
4 could be obtained in good overall yield (68%).
Results and Discussion.
Decarbonylation Reactions of Compound 1. Loss of carbon
monoxide from the title compound cannot be induced thermally.
Actually, this complex can be refluxed in toluene for several
(4) For recent papers of the groups working in the field, see: (a) Wolf,
R.; Hey-Hawkins, E. Eur. J. Inorg. Chem. 2006, 1348. (b) Compain, C.;
Donnadieu, B.; Mathey, F. Organometallics 2006, 25, 540. (c) Graham, T.
W.; Udachin, K. A.; Carty, A. J. Chem. Commun. 2005, 5890. (d) Borst,
M. L. G.; Bulo, R. E.; Gibney, D. J.; Alem, Y.; de Kanter, F. J. J.; Ehlers,
A. W.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammerstma, K. J. Am. Chem.
Soc. 2005, 127, 16985. (e) Ionescu, E.; von Frantzius, G.; Jones, P. G.;
Streubel, R. Organometallics 2005, 24, 2237. (f) Bailey, B. C.; Huffman,
J. C.; Mindiola, D. J.; Weng, W.; Ozerov, O. V. Organometallics 2005,
24, 1390. (g) Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003,
125, 13350.
(5) (a) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon
Copy; Wiley: New York, 1998. (b) Shah, S.; Protasiewicz, J. D. Coord.
Chem. ReV. 2000, 210, 181.
(6) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa, M. E.; Gonza´lez, R.; Ruiz,
M. A.; Hamidov, H.; Jeffery, J. C. Organometallics 2005, 24, 5503, and
references therein.
(7) (a) Huttner, G.; Evertz, K. Acc. Chem. Res. 1986, 19, 406. (b) Huttner,
G.; Lang, H. In Multiple Bonds and Low Coordination in Phosphorus
Chemistry; Regitz, M., Scherer, O. J., Eds.; Georg Thieme Verlag: Sttugart,
1990; p 48.
(8) For recent work, see: (a) Bai, G.; Pingrong, W.; Das, A. K.; Stephan,
D. W. Dalton Trans. 2006, 1141. (b) Graham, T. W.; Udachin, K. A.; Carty,
A. J. Chem. Commun. 2005, 4441. (c) Shaver, M. P.; Fryzuk, M. D.
Organometallics 2005, 24, 1419. (d) Termaten, A. T.; Nijbacker, T.; Ehlers,
A. W.; Schakel, M.; Lutz, M.; Spek, A. L.; Mckee, M. L.; Lammertsma,
K. Chem. Eur. J. 2004, 10, 4063. (e) Sanchez-Nieves, J.; Sterenberg, B.
T.; Udachin, K. A.; Carty, A. J. Inorg. Chim. Acta 2003, 350, 486. (f)
Blaurock, S.; Hey-Hawkins, E. Eur. J. Inorg. Chem. 2002, 2975.
(9) (a) Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Sa´ez, D.; Vaissermann, J.;
Jeffery, J. C. J. Am. Chem. Soc. 2002, 124, 14304. (b) 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.
(10) Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Sa´ez, D.; Hamidov, H.;
Jeffery, J. C.; Riis-Johannessen, T. J. Am. Chem. Soc. 2003, 125, 13044.
The decarbonylation reactions leading to dicarbonyls 3 and
4 can be reversed under a CO atmosphere to yield the starting

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4859
Table 1. IR and 31P{1H} NMR Data for New Compounds
compound
[Mo2Cp2(µ-PR*)(CO)4] (1)c
νst(CO)a/cm-1
δP/ppmb
1958(w), 1921(vs), 1880 (s), 1856 (s)
1958(w, sh), 1936(vs), 1862(s)
1741(m), 1709(vs)
685.6
166.8
532.1
509.7
519.0
476.1
524.5
439.1
424.4e
675.3
445.8g
t
[Mo2Cp2(µ-H){µ-P(CH2CMe2)C6H2 Bu2}(CO)4] (2)
[Mo2Cp2(µ-PR*)(µ-CO)2] (3)
[Mo2Cp2(µ-κ1:κ1,η6-PR*)(CO)2] (4)
1890(vs), 1815(s)
1908(vs), 1827(s)
[Mo2Cp(µ-κ1:κ1,η5-PC5H4)(η6-R*H)(CO)2] (5)
[Mo2Cp2(µ-κ1: κ1,η4-PR*)(CO)3] (6a)
[Mo2Cp2(µ-κ1:κ1,η4-PR*)(CNtBu)(CO)2] (6b)
[CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η6-PR*)(CO)2] (7)
[CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η4-PR*)(CO)3] (8)
[Mo2Cp2(µ-PR*)(CNtBu)(CO)3] (9)
1961(s), 1899(vs), 1823(s)
2129(m),d 1881(s), 1804(s)
1918(s), 1844(s)
1980(m), 1926(vs), 1858(s)
2109(m),d 2068(w),d 1913(vs), 1859(vs), 1828(vs)
2100 (w),d 2068(w),d1931(s), 1873(s), 1852(vs)f
[Mo2Cp2(µ-PR*)(CNtBu)2(CO)3] (10)
a Recorded in dichloromethane solution, unless otherwise stated. b Recorded in CD2Cl2 solution at 290 K and 121.50 MHz unless otherwise stated; δ
d
relative to external 85% aqueous H3PO4. c Data taken from ref 9b. νst(C-N). e CDCl3 solution. f Recorded in petroleum ether solution. g Toluene-d8 solution.
material 1. The reaction of the triply bonded 3 with CO occurs
in a few minutes at 263 K, thus resembling the behavior of the
diphosphine-bridged, triply bonded complexes [M2Cp2(CO)2(µ-
R2PCH2PR2)] (R ) Ph, Me; M ) Mo, W).11 In contrast, the
carbonylation of 4 was slower and more complex and will be
discussed later on.
Structural Characterization of Compounds 2-4. The
structure of compound 2 was confirmed through a single-crystal
X-ray diffraction study.9a The molecule displays two MoCp-
(CO)2 moieties placed in a transoid relative arrangement and
bridged by hydride and phosphide ligands, a common geometry
for compounds of the type [Mo2Cp2(µ-H)(µ-PRR′)(CO)4].9b,12
The intermetallic separation in 2, 3.250(1) Å, falls in the usual
range for these compounds. However, the cyclic phosphide
t
ligand P(CH2CMe2)C6H2 Bu2 displays Mo-P lengths of 2.475-
(2) Å, ca. 0.05 Å longer than the values usually found for
comparable noncyclic PRR′ bridges, this being perhaps indica-
Figure 1. ORTEP diagram of the molecular structure of compound
1, with hydrogen atoms and one of the Bu groups omitted for
clarity. Selected bond lengths (Å): Mo(1)-Mo(2) ) 2.5322(3),
Mo(1)-P(3) ) 2.2970(6), Mo(2)-P(3) ) 2.3028(5), Mo(1)-C(31)
) 2.099(2), Mo(2)-C(31) ) 2.141(2), Mo(1)-C(32) ) 2.128(2),
Mo(2)-C(32) ) 2.123(2), P(3)-C(1) ) 1.831(2). Bond angles
(deg): Mo(1)-P(3)-Mo(2) ) 66.8(1), Mo(1)-P(3)-C(1) ) 138.8-
(1), Mo(2)-P(3)-C(1) ) 154.2(1).
t
tive of the presence of some strain in the PC4 ring. A similar
effect has been previously recognized in the dirhodium complex
t
i
[Rh2(µ-H){µ-P(CH2CMe2)C6H2 Bu2}(dippe)2] (dippe ) Pr2-
PCH2CH2PiPr2).13 Spectroscopic data in solution for 2 (Table
1 and Experimental Section) are consistent with its solid-state
structure and are similar to those for related [Mo2Cp2(µ-H)(µ-
PRR′)(CO)4] species,9b,12 thus deserving no further comments.
(µ-PCy2)(µ-CO)] (2.478(1) Å),15a [Mo2Cp2(µ-PPh2)2(µ-CO)]
(2.515(2) Å),15b and [Mo2Cp2(µ-PhPC6H4PPh)2(µ-CO)] (2.532-
(1) Å).15c The Mo-P lengths in 3 (ca. 2.30 Å) are essentially
identical to those measured in 1 and ca. 0.1 Å shorter than single
Mo-P bond lengths in the above phosphide complexes. This
is indicative of substantial multiple-bonding character in the
phosphinidene binding to the dimolybdenum center, not affected
by the formal bond order of the intermetallic interaction. We
should note that compound 3 is the first example of a complex
having a phosphinidene ligand bridging a triple metal-metal
bond. The simultaneous presence in the same molecule of both
M-P and M-M multiple bonding could lead to enhanced
reactivity, and studies in that direction are now in progress in
our laboratory.
Spectroscopic data in solution for 3 are fully consistent with
its solid-state structure (see Table 1 and Experimental Section).
The presence of two equivalent bridging carbonyls is denoted
by the appearance of a highly deshielded doublet resonance (δC
) 294.0 ppm, JCP ) 13 Hz) in the 13C{1H} NMR spectrum of
3 and two low-frequency C-O stretching bands in its IR
The structure of 3 has now been determined through a single-
crystal X-ray diffraction study and is shown in Figure 1, along
with some relevant structural data. The molecule displays two
cyclopentadienyl molybdenum moieties symmetrically bridged
by two carbonyls and a trigonal phosphinidene ligand (the sum
of Mo-P-Mo and Mo-P-C angles being 359.8°). According
to the EAN formalism, a triple Mo-Mo bond must be
formulated for this 30-electron complex, which is in agreement
with the very short intermetallic length, 2.5322(3) Å, ca. 0.7 Å
shorter than the corresponding distance in the electron-precise
complex 1 (3.220(3) Å)14 and similar to the values found for
related triply bonded cyclopentadienyl complexes bridged by
dialkyl- or diarylphosphide ligands such as [Mo2Cp2(µ-COEt)-
(11) (a) Alvarez, C.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A. Organome-
tallics 1997, 16, 1378. (b) Garc´ıa, G.; Garc´ıa, M. E.; Melo´n, S.; Riera, V.;
Ruiz, M. A.; Villafan˜e, F. Organometallics 1997, 16, 624. (c) Alvarez, M.
A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Falvello, L. R.; Bois, C.
Organometallics 1997, 16, 354.
(12) Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sa´ez. D.
Organometallics 2002, 21, 5515, and references therein.
(13) Stradiotto, M.; Fujdala, K. L.; Tilley, T. D. HelV. Chim. Acta 2001,
84, 2958.
(14) Arif, A. M.; Cowley, A. H.; Norman, N. C.; Orpen, A. G.; Pakulski,
M. Organometallics 1988, 7, 309.
(15) (a) Garc´ıa, M. E.; Melon, S.; Ramos, A.; Riera, V.; Ruiz, M. A.;
Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983.
(b) Adatia, T.; McPartlin, M.; Mays, M. J.; Morris, M. J.; Raithby, P. R. J.
Chem. Soc., Dalton Trans. 1989, 1555. (c) Kyba, E. P.; Mather, J. D.; Hasset,
K. L.; McKennis, J. S.; Davis, R. E. J. Am. Chem. Soc. 1984, 106, 5371.

4860 Organometallics, Vol. 25, No. 20, 2006
Amor et al.
shielding of the carbon nuclei measured in the 13C NMR
spectrum, this being characteristic of arene complexes.18 Thus,
the ortho, meta, or para resonances of the aryl ring appear ca.
40-50 ppm shielded with respect to those of the nonbonded
ring in 3. This effect is even stronger for the ipso-carbon atom
(ca. 65 ppm shielded), which can be attributed to the fact that
this atom is placed ca. 0.1 Å closer to molybdenum than are
the other ring carbon atoms. This nucleus is also strongly
coupled to phosphorus (JCP ) 74 Hz, to be compared with a
value of 30 Hz in 3), and this seems to be related to the
significant pyramidalization imposed on carbon by the simul-
taneous coordination of the phosphorus atom and the aryl ring
to the same metal center. As it will be seen below, this
spectroscopic feature is found in all other complexes incorporat-
ing an aryl-bonded phosphinidene ligand described in this work.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 4, 6b, 7, and 8
parametera
4b
6bc
7
8
Mo-P
Mo′-P
1-P
1-Mo
2-Mo
3-Mo
4-Mo
5-Mo
6-Mo
2.3630(6)
2.2480(6)
1.833(2)
2.202(2)
2.314(2)
2.307(2)
2.328(2)
2.301(2)
2.317(2)
1.459(2)
1.417(2)
1.420(2)
1.419(2)
1.409(2)
1.459(2)
141.5(1)
61.8(1)
2.355(2)
2.256(1)
1.783(5)
2.261(5)
2.233(5)
2.273(5)
2.446(5)
3.194(5)
3.188(5)
1.478(7)
1.434(7)
1.430(7)
1.490(7)
1.337(7)
1.510(7)
143.1(2)
64.5(2)
2.361(1)
2.294(1)
1.839(3)
2.190(3)
2.302(3)
2.300(3)
2.335(3)
2.321(3)
2.350(3)
1.453(4)
1.404(2)
1.411(4)
1.421(4)
1.410(4)
1.458(4)
141.3(1)
61.4(1)
2.370(1)
2.303(1)
1.789(4)
2.273(3)
2.244(2)
2.301(4)
2.517(4)
3.227(4)
3.179(4)
1.487(5)
1.429(4)
1.418(5)
1.487(5)
1.335(5)
1.487(5)
141.6(1)
64.5(1)
1-2
2-3
3-4
4-5
5-6
6-1
The Unexpected Rearrangement of Compound 4: From
a 10- to a Nine-Electron-Donor Cyclopentadienylidene-
Phosphinidene Ligand. While optimizing the synthesis of
complex 4 through photolysis of 1 in warm solutions, we
detected the formation of small amounts of a new compound
characterized by a 31P NMR resonance at ca. 519.0 ppm. Further
experiments allowed us to prove that this new species was in
fact an air-sensitive isomer of 4, the cyclopentadienylidene-
phosphinidene complex [Mo2Cp(µ-κ1:κ1,η5-PC5H4)(η6-R*H)-
(CO)2] (5). Actually the transformation 4/5 does not require
participation of the UV-visible light, but takes place on a
preparative scale by just stirring solutions of 4 at 313 K (CH2-
Cl2 solution) or 353 K (THF solutions), provided that trace
amounts of water are available. Independent experiments proved
that no isomerization of 4 occurs under rigorously anhydrous
conditions even at 353 K. However, addition of a small amount
of a weak acid such as benzoic acid to a dichloromethane
solution of 4 caused its steady transformation into 5 even at
room temperature. We should note that no intermediate species
could be detected in any of the above transformations.
1-P-Mo′
1-P-Mo
Mo-P-Mo′
7-Mo
Cu-P
Cu-Mo′
Cu-Cl
156.54(2)
152.3(1)
2.076(6)
157.04(4)
152.9(1)
2.003(4)
2.259(1)
2.636(1)
2.135(1)
2.240(1)
2.682(1)
2.162(1)
a Bond lengths and angles in Å and deg, respectively, according to the
labeling shown in the figure.
b Data taken from ref 10. c Data corresponding to one of the two independent
molecules in the unit cell; those for the second molecule are similar.
spectrum, the latter with relative intensities (medium and strong,
in order of decreasing frequency) consistent with the angle
defined by the C-O oscillators (ca. 122° in the crystal).16 The
phosphinidene 31P nucleus in compound 3 is quite deshielded
(δP ) 532.1 ppm), as usually found for trigonal phosphinidene
bridges,3,17 but not as much as in the starting compound 1. The
relative shielding in 3 (ca. 150 ppm with respect to 1) might be
related to the presence of the triple Mo-Mo bond or the acute
Mo-P-Mo angle derived from it. Interestingly, the doubly
bonded [Mo2Cp2I2(µ-PR*)(CO)2] (Mo-Mo ) 2.960(2) Å)
exhibits a 31P shielding (δP ) 596.4 ppm)9a intermediate between
those of 1 and 3.
The structure of 4, the first complex displaying a 10-electron
donor arylphosphinidene ligand, was determined through a
single-crystal X-ray diffraction study.9b The molecule displays
a strongly asymmetric phosphinidene bridge, tightly bound to
a MoCp(CO)2 moiety (Mo-P ) 2.2480(6) Å) and bound to a
MoCp moiety both through its P atom (Mo-P ) 2.3630(6) Å)
and the aryl ring, in a η6-fashion (Table 2). The phosphorus,
ipso-carbon of the aryl ring, and molybdenum atoms are placed
in the same plane, which is in turn a symmetry plane of the
molecule. The spectroscopic data in solution for 4 are consistent
with this structure (Table 1 and Experimental Section): for
example, the carbonyl ligands give rise to a single 13C NMR
doublet resonance in the terminal region (δC ) 242.2 ppm, JCP
) 7 Hz) and to a pair of C-O stretching bands of similar
intensity, as expected for CO ligands defining an angle close
to 90° (87.2(1) Å in the crystal).16 Evidence for the coordination
of the aryl ring to the molybdenum atom comes from the strong
Our proposal for the above acid-catalyzed rearrangement is
depicted in Scheme 2 and is further supported by additional
experiments. Protonation of compound 4 with water or other
weak acid (HX in Scheme 2) would generate a small amount
of the phosphido-bridged salt [Mo2Cp(η6-R*H)(µ-P)Mo-
(CO)2Cp]X (B). We have shown previously that this unusual
reaction can be driven to full term by using strong acids such
as HBF4‚OEt2 or HBAr′4‚2OEt2 [Ar′ ) 3,5-C6H3(CF3)2] and
that the proton first attacks the MoCp(CO)2 moiety (A in
Scheme 2), then migrates to the carbon atom of the aryl group,
thus completing the P-C cleavage in the phosphinidene ligand.10
In a second stage, the cation B would be deprotonated by its
own counterion (OH- in the case of the water-catalyzed process)
specifically at the cyclopentadienyl ring. This generates a
carbanionic cyclopentadienylidene group that would attack the
electrophilic phosphorus atom, to finally give compound 5.
Indeed, we have found in a separate experiment that addition
of a drop of an aqueous KOH solution to a stirred THF solution
of the stable salt [Mo2Cp(η6-R*H)(µ-P)Mo(CO)2Cp](BAr′4)
leads slowly to compound 5 as the only organometallic product.
The implied acidity of the cyclopentadienyl protons at the
cationic complex B is quite remarkable since deprotonation of
metal-bonded Cp ligands, a reaction useful to synthesize
substituted cyclopentadienyl complexes (C5H4R),19 usually
requires the use of very strong bases such as BuLi.
(16) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London,
U.K., 1975.
(17) Carty, A. J.; MacLaughlin, S. A.; Nucciarone, D. In Phosphorus-
31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J. G., Quin,
L. D., Eds.; VCH: New York, 1987; Chapter 16.
(18) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: London, U.K., 1981.
(19) Coville, N. J.; du Plooy, K. E.; Pickl, W. Coord. Chem. ReV. 1992,
116, 3.

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4861
Scheme 2. Reaction Mechanism for the Proton-Catalyzed
Isomerization of Compound 4
Scheme 3. Ring-Slippage Reactions of Compound 4
presence in the molecule of a symmetry plane bisecting this
group and containing the Mo and P atoms. Unfortunately, the
resonance for the carbon atom directly bonded to phosphorus
could not be located in the spectrum. The binding of the C5H4
moiety to the P atom, however, has been crystallographically
confirmed on several derivatives of 5.20 Finally, a standard
NOESY experiment carried out on a C6D6 solution of 5 at 298
K revealed the presence of some positive nuclear Overhausser
t
enhancement (NOE) from the Bu proton resonances to those
from the C5H4 and C5H5 groups, but not between the latter ones.
This is consistent with a conformation derived from that of 4,
that is, with the cyclopentadienyl rings placed away from each
other.
To our knowledge, compound 5 is the first complex reported
to have a cyclopentadienylidene-phosphinidene ligand. More-
over, this molecule seems to be quite reactive, and further
experiments are now in progress to examine in detail its
chemical behavior.20 Of interest to the present discussion is the
observation that compound 5 reacts instantaneously with
HBAr′4‚2OEt2 in dichloromethane solution at room temperature
to give cleanly the stable phosphido complex [Mo2Cp(η6-R*H)-
(µ-P)Mo(CO)2Cp](BAr′4), thus proving the reversible character
of the deprotonation step proposed to explain the formation of
5. Reactions of phosphinidene ligands with coordinated cyclo-
pentadienyl rings are rare processes that have been previously
reported to occur only on some transient mononuclear com-
plexes such us [M(PR)(CO)5] (M ) Cr, Mo, W),21 [W(C5H4-
Me)2(PR*)],22 or [ZrCp2(PR)].23 The formation of compound 5
then represents the first example of cyclopentadienyl C-H
cleavage involving a bridging phosphinidene ligand.
Ring Slippage of the η6-Bound Arylphosphinidene: η4-
Derivatives. To better understand the elemental steps leading
from 1 to the unique coordination mode of the arylphosphinidene
ligand present in complex 4, we analyzed the reverse reaction,
that is, the carbonylation reactions of 4. We also studied the
t
reactions of 4 with BuNC, the latter being a ligand of
characteristics similar to those of carbon monoxide.
Complex 4 reacts quite slowly with CO (ca. 4 atm) at room
temperature in dichloromethane solution. After 3 days, the
reaction mixture contains trace amounts of the starting complex
4 and only small amounts of the product of full carbonylation,
the tetracarbonyl compound 1. The major species in the solution
at this stage, however, is the tricarbonyl complex [Mo2Cp2(µ-
κ1: κ1,η4-PR*)(CO)3] (6a) (Scheme 3).
The incorporation of a CO molecule to the molybdenum atom
bonded by the aryl ring in 4 causes a change in the hapticity of
the latter, from η6- to η4-coordination, this causing the phos-
phinidene ligand to become an eight-electron donor in 6a, as
supported by spectroscopic and crystallographic data discussed
later on. Full conversion of tricarbonyl 6a into the tetracarbonyl
1 requires ca. 3 more days under the same conditions (pCO ) 4
atm, 293 K). No other intermediate species could be detected
in these carbonylation reactions by IR or 31P NMR monitoring
of the corresponding reaction mixtures. We recall here that
complex 6a can be detected as a minor product in the
photochemical reactions converting complex 1 into the dicar-
bonyl species 4. Thus, we conclude that the tricarbonyl 6a is a
genuine intermediate connecting the phosphinidene complexes
1 and 4.
The structure of 5 in solution is related to that of 4 after
formally exchanging P-C and C-H bonds between the aryl
and cyclopentadienyl groups. The chemical environments around
the phosphorus atom and the MoCp(CO)2 moiety are thus
similar in 4 and 5, in agreement with their similar 31P and
13C(CO) NMR shifts or C-O stretching bands (Table 1 and
t
Experimental Section). The presence of the η6-bonded C6H3 Bu3
1
ring is clearly denoted by the appearance of just one H and
two 13C NMR aryl resonances, all of them strongly shielded.
The cyclopentadienylidene group gives rise to pairs of 13C and
1H NMR resonances for the CH units, thus revealing the
(20) Alvarez, M. A.; Amor, I.; Garc´ıa, M. E.; Ruiz, M. A. Unpublished
results.
(21) Mathey, F.; Svara, J. Organometallics 1986, 5, 1159.
(22) Arif, A. M.; Cowley, A. H.; Nunn, C. M.; Pakulski, M. J. Chem.
Soc., Chem. Commun. 1987, 994.
(23) (a) Ho, J.; Hou, Z.; Drake, R. J.; Stephan, D. W. Organometallics
1993, 12, 3145 (b) Breen, T. L.; Stephan, D. W. Organometallics 1996,
15, 4509.
t
Complex 4 reacts analogously with BuNC. The reaction is
now faster and can be completed in ca. 5 h at 333 K using
THF as solvent. However, only one molecule of the isocyanide
ligand is incorporated to the dimetal substrate even after
prolonged reaction times, to yield the dicarbonyl isocyanide

4862 Organometallics, Vol. 25, No. 20, 2006
Amor et al.
derivative [Mo2Cp2(µ-κ1:κ1,η4-PR*)(CNtBu)(CO)2] (6b), struc-
turally related to the tricarbonyl 6a.
significant multiple character in this bond, a question to be
addressed later on. In addition, the molybdenum, phosphorus,
and ipso-carbon atoms remain almost in the same plane, as they
are in 4. Although there are no structural studies in the literature
concerning related PMCu frameworks, the internuclear distances
around Cu suggest a moderately strong binding of the CuCl
molecule. Thus the Cu-P length of 2.240(1) Å is close to the
usual values found for Cu(I) phosphine complexes,25 while the
Mo-Cu length of 2.682(1) Å is similar or shorter than the
corresponding lengths in the tungsten carbyne-copper clusters
[{WCp(µ-CR)(CO)2}2Cu2(µ-Cl)2] (R ) NMeEt, W-Cu )
2.610(1) Å)26 and (NEt4)[W2Cu(µ-CCtCtBu)2(CO)4(η5-C2B9H9-
Me2)2] (W-Cu ) 2.764(1), 2.602(1) Å).27 The Cu-Cl distance,
2.162(1) Å, is quite short when compared to normal single-
bond lengths, for example 2.322(2) Å in [Cu2(µ-Cl)2(PCy3)2]28
or 2.307(4) and 2.324(4) Å in the above ditungsten-copper
compound. This suggests the presence of significant pπ-dπ
bonding from Cl to Cu to mitigate the unsaturation around the
trigonal copper atom. There is also a weak semibridging
interaction from a carbonyl ligand to copper (Mo-C) 1.958(4)
Å, Cu‚‚‚C) 2.352(3) Å, Mo-C-O ) 170.5(3)°) that can be
understood along the same lines.
Compound 6b results from the sideways addition of CNtBu
to the molybdenum atom bearing the η6-bonded aryl ligand in
4. This causes almost no effect upon the Mo-P bonds, but
forces the detachment from the metal of the C5 and C6 carbon
atoms, which are now better described as joined by an
uncoordinated, normal double bond (C5-C6 ) 1.337(7) Å). As
a result, there is a strong deviation from planarity in the aryl
ring, the folding angle being ca. 139°. The C1 to C4 carbon
atoms remain almost in the same plane and are even closer
(except for the C4 atom) to the molybdenum atom than they
are in 4. This justifies the description of the aryl ring coordina-
tion as of the η4-type, as it would have been anticipated on the
basis of the usual electron-counting schemes.
The structure of compound 8 combines the geometrical effects
derived from the presence of CuCl bonded to the Mo-P multiple
bond (as in 7) and from the incorporation of a new ligand (here
CO) to the arene-bonded molybdenum atom (as in 6b). As a
result, the aryl ring displays an η4-coordination mode to
accommodate the new CO ligand (C5 and C6 aryl atoms
uncoordinated) and the CuCl molecule adds to the multiple
Mo-P bond sideways (the angle between Mo2P and MoPCu
planes is ca. 110°). The coordination environment around the
copper atom is essentially trigonal with a short Cu-Cl bond,
as observed in 7, and there is also a weak semibridging
interaction from one of the carbonyls to copper [Cu‚‚‚C )
2.481(4) Å]. We note finally that compound 8 displays both
the CuCl group and the CO ligand placed at the same side of
the Mo2P plane. This is possibly the less crowded side, because
of the detachment of the C6 atom and its associated tBu group.
The other possible isomer, with the CuCl group placed on the
other side of the Mo2P plane, seems to be never formed even
in solution.
The carbonylation reaction of 4 cannot be speeded up
efficiently by carrying out the experiments at higher tempera-
tures, since this leads to substantial decomposition. However,
we found that the addition of CuCl to dichloromethane solutions
of 4 greatly increased the rate of carbonylation, an effect that
is due to the formation of well-defined and isolable CuCl adducts
(Scheme 3). In fact, CuCl reacts rapidly with 4 to give
quantitatively the trinuclear complex [CuMo2(Cl)Cp2(µ-κ1:κ1:
κ1,η6-PR*)(CO)2] (7), which incorporates a CuCl moiety bridg-
ing the multiple Mo-P bond present in 4, as shown by an X-ray
diffraction study (see later). Compound 7 is quite robust
thermally (it can be heated in refluxing toluene without
significant decomposition), but it is much more reactive toward
CO than 4. Carbonylation of 7 can now be completed in just 3
h at 323 K (toluene, pCO ) 4 atm) to give the tricarbonyl
complex [CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η4-PR*)(CO)3] (8) in high
yield. The incorporation of the CO molecule to 7 causes the
same effect observed in the formation of compounds 6, that is,
a shift from η6 to η4 in the coordination of the aryl ring, also
substantiated crystallographically. Finally, CuCl can be detached
from 8 in a number of ways to yield almost quantitatively the
tricarbonyl 6a, for example by just passing dichloromethane
solutions of 8 through an alumina column. This process is
reversible, and compound 8 can be quantitatively formed by
reacting CuCl with 6a in dichloromethane solutions.
Apart from its preparative utility, the formation of compound
7 has some points of interest. In the first place, since the CuCl
molecule can be considered as a soft Lewis acid, then its binding
to the Mo-P multiple bond in 4 thus reveals that this is a
nucleophilic site of the molecule under conditions of orbital
control. Second, this complex could serve as a model for some
of the intermediate species involved in the proton-induced P-C
cleavage of complex 4 (A to B in Scheme 2). Finally, compound
7 might be related to intermediate species involved in the CuCl-
catalyzed decomposition of 7-phosphanorbornadiene M(CO)5
complexes (M ) Cr, Mo, W). Indeed, the latter is a well-
established route for the “in situ” generation or transient, very
reactive electrophilic phosphinidene complexes of type [M(PR)-
(CO)5].3c The role of CuCl in those reactions has been addressed
recently through DFT calculations on the reaction profile, and
it seems to involve the binding of CuCl to the M-P bond at
some transient stages.24
Structural Characterization of Compounds 6-8. The
structures of complexes 6b, 7, and 8 have been determined
through single-crystal X-ray studies and are shown in Figures
2 to 4. The most significant features in these structures concern
the coordination of the phosphorus atom and the aryl ring of
the phosphinidene ligand, and the relevant structural data are
assembled in Table 2 for comparative purposes.
Compound 7 results from CuCl addition to the multiple
Mo-P bond in 4, sideways with respect to the Mo2P plane (the
angle between the Mo2P and MoCuP planes is ca. 112°). This
causes almost no change in the η6-coordination of the aryl ring,
which remains almost perfectly flat with Mo-C lengths very
similar to those of 4, ca. 2.31 Å for all carbons except the
pyramidalyzed ipso-carbon, strongly bound to molybdenum at
2.190(3) Å. No change occurs in the long Mo-P bond, now
2.361(1) Å, but the short Mo-P bond (P-Mo′ in Table 2) is
elongated by ca. 0.05 Å as a result of CuCl coordination, as
expected. Yet, the value of 2.294(1) Å denotes the retention of
A Comment on the Mo-P Bonding in Complexes 4 to 8.
Throughout this paper, the asymmetric coordination of the
phosphinidene ligand in compounds 4 to 8 is depicted as
(25) Hathaway, B. J. In ComprehensiVe Coordination Chemistry; Wilkin-
son, G., Gillard, R. D., McCleverly, J. A., Eds.; Pergamon Press: Oxford,
UK, 1987; Vol. 5, Chapter 53.
(26) Albano, V. G.; Busetto, L.; Cassani, M. C.; Sabatino, P.; Schmitz,
A.; Zanotti, V. J. Chem. Soc., Dalton Trans. 1995, 2087.
(27) Cabioch, J. L.; Dosset, S. J.; Hart, I. J.; Pilotti, M. U.; Stone, F. G.
A. J. Chem. Soc., Dalton Trans. 1991, 519.
(24) Lammertsma, K.; Ehlers, A. W.; McKee, M. L. J. Am. Chem. Soc.
2003, 125, 14750.
(28) Churchill, M. R.; Rotella, F. I. Inorg. Chem. 1979, 18, 166.

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4863
Figure 5. Canonical forms describing the metal-phosphinidene
bonding in binuclear compounds 4-8 (D1 to D4) and in nucleo-
philic (A1) and electrophilic (A2) terminal complexes.
Figure 2. ORTEP diagram of the molecular structure of compound
6b, with hydrogen atoms omitted for clarity.
sented by A1 (double M-P bond), whereas those species with
good π-acceptor ligands around M (electrophilic complexes)
are best represented by A2, then displaying essentially single
M-P bonds (Figure 5).3a,c
Unfortunately, the number of structurally characterized
complexes having asymmetric PR bridges is extremely limited.
Actually there is only one case involving group 6 metals, this
being the ditungsten compound [W2Cp2(µ-PMes)(CO)4(PH2-
Mes)] (Mes ) 2,4,6-C6H2Me3), which displays phosphinidene
W-P lengths of 2.555(3) and 2.281(7) Å.29 The first value can
be taken as a reference figure for a single M-P bond (M )
Mo, W). In contrast, the phosphine W-P length in the above
compound is 2.440(3) Å, a figure that can be considered as
normal for a single dative PfM bond. In fact, the corresponding
lengths in the electrophilic bent-phosphinidene complexes
[MCp*(CO)3(PNiPR2)][AlCl4] are similar, 2.4506(4) and
2.4503(6) Å for the Mo and W derivates, respectively.30 Clearly,
the long Mo-P bond in compounds 4 to 8 is at least 0.1 Å
shorter than the reference figures for single Mo-P bonds, and
hence some double-bond character must be attributed to that
bond (represented by the canonical from D). This is in agreement
with our preliminary DFT calculation on 4, revealing the
presence of a π-bonding interaction delocalized along the Mo-
P-Mo chain.10
Figure 3. ORTEP diagram of the molecular structure of compound
7, with hydrogen atoms omitted for clarity.
The analysis of the short Mo-P′ interaction in compounds
4 to 8 is not so straightforward. Triple Mo-P bonds are expected
to be just below 2.20 Å in length, as exemplified by the linear
phosphinidene complex [WCl2(PR*)(CO)(PMePh2)2] (2.169(1)
Å)31 or the phosphido-bridged compounds [L3WtPfW(CO)5]
(L3 ) tridentate N- or O-donor ligand; W-P ) 2.13-2.20 Å)32
and [Mo2Cp2(µ-P)(CO)2(η6-PR*)](BAr′4), (2.1685(9) Å).10 Double
Mo-P bonds, however, seem to span a much wider range of
lengths. For example, the nucleophilic phosphinidene complexes
[MCp2(PR*)] display relatively long M-P separations of
2.370(2) and 2.349(5) Å for the Mo and W complexes,
respectively.33 In contrast, three-electron donor dialkylphosphide
or related ligands display substantially shorter M-P lengths,
e.g., 2.284(4) Å for [WCp(PtBu2)(CO)2]34 or even 2.204(1) Å
Figure 4. ORTEP diagram of the molecular structure of compound
8, with hydrogen atoms omitted for clarity.
implying Mo-P bonds of orders one and three. It should be
kept in mind that this is just one of the canonical forms that
can be written to describe the Mo-P bonds in these complexes
(D1 in Figure 5). Other forms are D2 and D3, implying bond
orders of one and two, and D4, this implying bond orders of
two and two. It is not possible to deduce the relative contribu-
tions of all these forms from structural data since Mo-P lengths
span quite wide ranges for a given formal bond order. In
addition, a “normal” bond (M-P) implies somewhat longer
internuclear separations than a dative bond (MfP). Finally, we
note that the differences in the pairs D1/D2 and D4/D3 are
similar to those distinguishing the extreme types of bent-
phosphinidene complexes. For the latter, it has been found that
those compounds having strong σ-donors around the metal
(nucleophilic complexes) have an M-P interaction best repre-
(29) Malisch, W.; Hirth, U. A.; Bright, T. A.; Ka¨b, H.; Ertel, T. J.;
Hu¨ckmann, S.; Bertagnolli, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1525.
(30) Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organometallics
2001, 20, 2657.
(31) Cowley, A. H.; Pellerin, B.; Atwood, J. L.; Bott, S. G. J. Am. Chem.
Soc. 1990, 112, 6734.
(32) Johnson, B. P.; Balazs, G.; Scheer, M. Top. Curr. Chem. 2004, 232,
1.
(33) (a) Hitchcock, P. B.; Lappert, M. F.; Leung, W. P. J. Chem. Soc.,
Chem. Commun. 1987, 1282. (b) Bohra, R.; Hitchcock, P. B.; Lappert, M.
F.; Leung, W. P. Polyhedron 1989, 8, 1884.
(34) Jo¨rg, K.; Malisch, W.; Reich, W.; Meyer, A.; Schubert, U. Angew.
Chem., Int. Ed. Engl. 1986, 25, 92.

4864 Organometallics, Vol. 25, No. 20, 2006
Amor et al.
for the fluorophosphide complex [MoCp(PFR*)(CO)2].35 There-
fore, the value of ca. 2.25 Å for the short Mo-P′ interaction in
compounds 4 and 6b is probably consistent with Mo-P bond
orders between 2 and 3, this being reduced by their coordination
to the Lewis acid CuCl (compounds 7 and 8) as expected. In
summary, the Mo-P bonding in compounds 4, 5, and 6 cannot
be described satisfactorily using just one of the canonical forms
D1 to D4, and further theoretical calculations will have to be
carried out in order to gain deeper insight into the bonding within
these asymmetric phosphinidene complexes. While forms D2
to D4 might relate the implied multiple M-P bonding to those
present in mononuclear dialkyl-phosphide complexes or sym-
metrically bridged phosphinidene species, the form D1 estab-
lishes a relationship with carbyne complexes. It is interesting
to note here that the triple MtC bonds of transition metal
carbyne compounds react with CuCl much in the same way as
the multiple Mo-P bond in compounds 4 and 6b.26,27,36 It
remains to be seen whether this phosphinidene/carbyne analogy
can become a useful tool when rationalizing the chemical
behavior of asymmetric phosphinidene complexes such as the
dicarbonyls 4 and 5 or related species.
Scheme 4. Aryl-Detachment Reactions of Compound 6a
Solution Structure of Compounds 6-8. The spectroscopic
data available for compounds 6-8 (Table 1 and Experimental
Section) are fully consistent with the solid-state structures
determined for 6b and the copper derivatives 7 and 8. The
presence of the added ligand terminally bound in compounds 6
is clearly denoted by the appearance of extra C-O and C-N
stretching bands or 13C NMR resonances in the terminal region
of the corresponding spectra, while the CO bands or 13C and
1H NMR resonances corresponding to the MoCp(CO)2 frag-
ments are similar to those of 4, except that these carbonyl ligands
are now inequivalent. This is also the case of the 31P NMR
resonance of compounds 6, appearing at chemical shifts (ca.
500 ppm) similar to that for 4. The η4-coordination of the aryl
ring of the phosphinidene ligand is denoted by the appearance
of six different aryl resonances in the corresponding 13C NMR
spectra, four of them strongly shielded (ca. 40-50 ppm for C2,
C3, and C4; ca. 70 ppm for C1) and two of them with chemical
shifts (ca. 128 and 157 ppm) similar to those for substituted
cyclohexenes, which are thus assigned to the two nonbonded
inequivalent due to the presence of the CuCl moiety positioned
sideways with respect to the Mo2P plane.
Finally, the spectroscopic data for 8 show trends similar to
those just discussed for compounds 6 (effects of η4-coordination
of the aryl ring) and 7 (effects of CuCl binding to the Mo-P
multiple bond) and need then no further comments. We only
note that these data establish the presence of a single isomer in
solution.
Ring Slippage of the η4-Complex 6a: Recovering the
Metal-Metal Bond. As stated above, the tricarbonyl compound
6a reacts slowly with CO to yield the starting compound 1. No
intermediates could be detected in this necessarily complex
transformation, which requires complete detachment of the aryl
ring and regeneration of the Mo-Mo bond. To obtain further
information concerning these last steps, we examined (as a
model for carbonylation) the reactions of 6a with CNtBu. The
latter proceeded smoothly at room temperature, but, to our
surprise, turned out to be strongly dependent on the solvent used
(Scheme 4). In tetrahydrofuran, just one isocyanide molecule
is incorporated to the dimetal substrate, to give the metal-metal
bonded tricarbonyl [Mo2Cp2(µ-PR*)(CO)3(CNtBu)] (9) as the
unique product. In petroleum ether, however, two isocyanide
molecules are incorporated, thus giving the tricarbonyl bis-
(isocyanide) derivative [Mo2Cp2(µ-PR*)(CO)3(CNtBu)2] (10),
which has no metal-metal bond. Separate experiments showed
that compound 9 did not react with further CNtBu to give 10.
1
aryl carbon atoms. Full assignment of the 13C and H NMR
resonances was made on the basis of standard direct and long-
range 1H-13C correlations. This proved that the uncoordinated
carbon atoms were those in positions 5 and 6, as found in the
crystal for 6b.
As discussed above, the structure of 7 was found to be similar
to that of 4 except for the presence of a CuCl fragment bound
to the Mo-P multiple bond and causing a lengthening of the
latter. In agreement with this, the phosphinidene P nucleus
becomes considerably more shielded (by ca. 100 ppm), as
expected for µ3-phosphinidene ligands,17 and the C-O stretching
bands in the IR spectrum increase by ca. 30 cm-1, thus
indicating a considerable flux of electron density from the
P-Mo bond to the copper atom. This might be a major cause
for the higher reactivity of 7 toward CO (when compared to 4),
since the arene-bonded molybdenum atom in 7 becomes
doubtlessly less electron-rich. The η6-coordination of the aryl
ring of the phosphinidene ligand in 7 is denoted by the strong
shielding of all six carbon atoms (ca. 35-50 ppm for the
C2-C6 atoms and 76.8 ppm for the C1 atom), which are now
The structure of compound 9 is derived from that of 1 by
just replacing a CO ligand by a CNtBu group (this substitution,
however, cannot be accomplished directly). Thus, it is not
surprising that their 31P chemical shifts are so similar (Table
1). The presence of just a single isocyanide ligand in 9 is
1
deduced from the analysis of the pertinent H and 13C NMR
resonances. Besides, we note that only one of the carbonyl 13
C
NMR resonances exhibits coupling to phosphorus. By recalling
2
that the absolute values of JPC couplings in complexes of the
type [MCpX(CO)2(PR3)] (M ) Mo, W; X ) halogen, alkyl,
hydride, etc.)37 and [M2Cp2(µ-H)(µ-PR2)(CO)4]9b,12 usually
follow the order Jcis > Jtrans, we can deduce that two carbonyls
(35) Alonso, M.; Garc´ıa, M. E.; Ruiz, M. A.; Hamidov, H.; Jeffery, J.
C. J. Am. Chem. Soc. 2004, 126, 13610.
(36) Clark, G. R.; Cochrane, C. M.; Marsden, K.; Roper, W. R.; Wright,
L. J. J. Organomet. Chem. 1986, 315, 211.
(37) (a) Todd, L. J.; Wilkinson, J. R.; Hickley, J. P.; Beach, B. L.; Barnett,
K. W. J. Organomet. Chem. 1978, 154, 151. (b) Wrackmeyer, H. G.; Alt,
H. G.; Maisel, H. E. J. Organomet. Chem. 1990, 399, 155.

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4865
Scheme 5. Proposed Pathways for the Decarbonylation
are placed trans to the PR* bridge and one is placed cis to it.
As a result, we conclude that compound 9 is formed specifically
as the isomer having the CNtBu ligand placed in a cis-position
with respect to the phosphinidene ligand. We finally note that
the IR spectrum of 9 shows, in addition to the main C-N
Reactions of Compound 1 [M ) Mo(η5-C5H5)]
stretching band at 2109 cm-1, a weak C-N band at 2068 cm-1
.
This is not unusual for terminal isocyanide ligands, which can
adopt slightly bent conformations in solution.38
The structure of compound 10 is formally derived from the
addition of a second CNtBu molecule to complex 9 (actually, a
process not observed). The molecule then displays no metal-
metal bond, but an asymmetric trigonal phosphinidene bridge
related to those present in compounds 4 to 6 or in the ditungsten
compounds [W2L2(µ-PMes)(CO)4(PH2Mes)] (L ) η5-Cp or
C5Me5).29 In fact, the 31P chemical shift of 10 (445.8 ppm) is
intermediate between the values exhibited by our dimolybdenum
complexes (520-475 ppm) and the above ditungsten compounds
(313.9 and 397.4 ppm). It seems thus that asymmetric trigonal
phosphinidene bridges (D in Chart 1) would be characterized
by 31P chemical shifts substantially lower than those of
symmetrical trigonal phosphinidene bridges (C in Chart 1).
Among the latter, moreover, there seems to be a shielding effect
upon increasing the metal to metal bond order (BO), as it can
be appreciated in the series [W2(µ-PR*)(CO)10] (δP ) 961 ppm;
BO ) 0)39 > 1 (δP ) 685.6 ppm; BO ) 1)9b > [Mo2Cp2I2(µ-
PR*)(CO)2] (δP ) 596.4 ppm; BO ) 2)9a > 4 (δP ) 532.1
ppm; BO ) 3).
The IR spectrum of 10 exhibits three strong C-O stretching
bands and two weak and well-separated C-N stretches of
similar intensity, the latter suggesting the presence of two
terminal isocyanide ligands arranged in a cis-relative position
on the same metal atom. This yields an asymmetric molecule,
in agreement with the inequivalence of all tBu groups that can
be deduced from the 1H NMR spectrum of this complex.
Overall, the transformations 4 f 6a f 10 imply the complete
replacement of the η6-bonded aryl ring of the phosphinidene
ligand in 4 by one carbonyl and two isocyanide ligands.
C-H (tBu) bond on the phosphorus atom, to give the unsaturated
phosphine intermediate C. This sort of cleavage of the tBu group
in the PR* ligand to yield free or bonded phosphine PH(CH2-
Reaction Pathways. The structures and synthetic relation-
ships connecting the complexes 2 to 10 just discussed allow us
to grasp the main reaction pathways governing the photolytic
reactions of compound 1 and the unusual rearrangements
involved (Scheme 5).
t
CMe2)C6H4 Bu2 has been previously reported to occur on
transient mononuclear phosphinidene complexes of Fe41 and
Zr,42 or even on the transient free ligand.43 More recently,
Lammerstma et al. have reported a similar C-H cleavage
t
leading to the bridging (µ-P(CH2CMe2)C6H4 Bu2) ligand, pre-
In the first place, there seems to be two main primary
processes. The first one is dissociative in nature and would be
favored by using UV light and low temperatures. This process
would lead to an unstable tricarbonyl intermediate A and then
finally to the triply bonded dicarbonyl 3 much in the same way
as proposed to occur in the photolysis of the dimers [M2Cp2-
(CO)4L2] (M ) Mo, W; L2 ) (CO)2 or µ-R2PCH2PR2).11,40 The
second primary process would be nondissociative in nature, and
it would be favored at higher temperatures (even by just heating)
and by the absorption of near-visible rather than UV light,
according to our experimental findings already discussed. For
this process we propose a valence tautomerization of the
phosphinidene bridge, from trigonal to the pyramidal (two-
electron donor) coordination mode. This would yield an
unsaturated intermediate B (not detected) that would be reactive
enough so as to induce an irreversible oxidative addition of a
sumably induced by transient Rh2 or Ir2 dimers having trigonal
PR* bridges.8d,44 In our case, the phosphine ligand present in
the unsaturated intermediate C would experience rapidly a new
oxidative addition, now of its P-H bond on the dimetal center,
to yield finally the electron-precise hydride complex 2. We note
that this last step is similar to those invoked to explain the
formation of [M2Cp2(µ-H)(µ-PR2)(CO)4] complexes using pri-
mary and secondary phosphines.12 The proposed rearrangement
1/B is also consistent with the results of the reaction of 1 with
HCtC(p-tol) under photolytic conditions, which leads to the
metallaphosphaallyl complex [Mo2Cp2{µ-η1:η2,κ-C(p-tol)-
CHPR*}(CO)4], with its phosphorus atom bearing a nonbonded
electron pair.7a We note finally that related valence tautomer-
ization processes have been occasionally invoked to explain the
(41) Arif, A. M.; Cowley, A. H.; Pakulski, M.; Pearsall, M. A.; Clegg,
W.; Norman, N. C.; Orpen, A. G. J. Chem. Soc., Dalton Trans. 1988, 2713.
(42) Hey-Hawkins, E.; Kurz, S. J. Organomet. Chem. 1994, 479, 125.
(43) (a) Shah, S.; Simpson, M. C.; Smith, R. C.; Protasiewick, J. D. J.
Am. Chem. Soc. 2001, 123, 6925. (b) Smith, R. C.; Shah, S.; Protasiewick,
J. D. J. Organomet. Chem. 2002, 646, 255.
(38) Singleton, E.; Oosthuizen, H. E. AdV. Organomet. Chem. 1983, 22,
209.
(39) Lang, H.; Orama, O.; Huttner, G. J. Organomet. Chem. 1985, 291,
293.
(40) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.;
Jeannin, Y. J. Am. Chem. Soc. 1995, 117, 1324, and references therein.
(44) Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.; Spek, A.
L.; Lammertsma, K. Chem. Eur. J. 2003, 9, 2200.

4866 Organometallics, Vol. 25, No. 20, 2006
Amor et al.
reactivity of several binuclear complexes having “E-inidene”
bridges (E ) P, As, Sb).7
expected η4-complex, which was never found as a local
minimum.47 Our results, then, provide some experimental
evidence for the classical η6f η4f η2 ring-slippage process
concerning the mentioned arene substitution reactions.
The presence of MeCN in the reaction medium during the
photolysis of 1 has the dramatic effect of suppressing the C-H
cleavage step leading to 2, thus enabling the selective formation
of 3. This might be due to the formation of unstable phosphin-
idene-MeCN adducts, in a similar way to those formed by
mononuclear bent-phosphinidene complexes.3,45 At high MeCN
concentrations and warm temperatures, however, a new pathway
appears that leads finally to the dicarbonyl 4 instead of its isomer
3 (separate experiments showed that 3 and 4 do not interconvert
in refluxing toluene or under the photolytic conditions used).
For this last path we propose that a molecule of MeCN actually
coordinates to the unsaturated intermediate B to give the
asymmetrically bridged intermediate D (S ) MeCN), of which
compound 10 is a model. This implies a new rearrangement of
the phosphinidene bridge, now from the pyramidal to the
asymmetric trigonal mode. Under UV irradiation, D would in
turn dissociate the MeCN and then a CO ligand, this being
compensated by the progressive coordination of the aryl ring
of the phosphinidene group, adopting the η2- (intermediate E,
not detected) and η4-modes (complex 6a), respectively. Further
decarbonylation then leads from 6a to 4, as shown by
independent experiments, this being accompanied by a final
change in the coordination of the aryl ring, from η4- to the η6-
mode, so as to keep the electronic balance of the molecule
constant.
Conclusions
Visible-UV irradiation of compound 1 induces two primary
processes. The first process is decarbonylation and leads to the
triply bonded [Mo2Cp2(µ-PR*)(µ-CO)2] in a reversible way. The
second process is proposed to be a valence tautomerization to
yield an unstable intermediate having a pyramidal phosphinidene
bridge, which in turn evolves in two different ways: either
t
through an irreversible C-H bond cleavage of a Bu group, to
t
give the hydride [Mo2Cp2(µ-H){µ-P(CH2CMe2)C6H2 Bu2}-
(CO)4], or through a reversible rearrangement involving a new
valence tautomerization of the phosphinidene ligand, to adopt
an asymmetric trigonal coordination mode while its aryl group
gets progressively coordinated in the η2-, η4-, and η6-modes as
the decarbonylation proceeds, to give finally the dicarbonyl
[Mo2Cp2(µ-κ1:κ1,η6-PR*)(CO)2]. The latter experiences an
unprecedented tautomerization, giving the arene complex [Mo2-
Cp(µ-κ1:κ1,η5-PC5H4)(CO)2(η6-R*H)], which contains the new
nine-electron-donor cyclopentadienylidene-phosphinidene ligand.
This reaction is catalyzed by weak acids and is proposed to
involve the phosphide-bridged cation [Mo2Cp2(µ-P)(CO)2(η6-
R*H]+ as intermediate species. The reactions of the η6-
complexes with CO and CNtBu prove the reversibility of the
above aryl ring slippages and the facile valence tautomerization
between symmetric and asymmetric trigonal phosphinidene
bridges, which also involve the formation or cleavage of metal-
metal bonds. The new asymmetric trigonal phosphinidene
complexes display short Mo-P bonds, which can be described
as having a character intermediate between pure double and
triple bonds. As a result, they might exhibit reactivity in some
ways related to that of transition metal carbyne complexes.
The elemental steps proposed to explain the formation of the
η6-complex 4 from the tetracarbonyl 1 are also of help to
understand not only the reverse reaction, i.e., the carbonylation
of 4 to regenerate 1, but also the reactions of the tricarbonyl 6a
with CNtBu (Scheme 4). Thus, while the addition of two CO
molecules to 4 would lead stepwise to 6a and then to the η2-
t
bonded E, addition of BuNC on 6a would give first a similar
η2-bonded complex [Mo2Cp2(µ-κ1:κ1,η2-PR*)(CO)3(NCtBu)]
(E′). In tetrahydrofuran, a solvent molecule would displace the
η2-bound aryl ring to yield first solvates of type D (S )
tetrahydrofuran), which afterward would rearrange by ejecting
the solvent molecule and regenerating the metal-metal bond,
then forming the symmetrically bridged compound 1 or 9. This
last step has its precedent in the ditungsten complex [W2Cp*2-
(µ-PMes)(CO)4(PH2Mes)], which above 233 K ejects a PH2Mes
molecule and rearranges into the metal-metal bonded derivative
[W2Cp2*(µ-PMes)(CO)4].29 Finally, when the reaction of 6a
with CNtBu is carried out in petroleum ether, intermediate E′
cannot incorporate a solvent molecule; then it reacts with a
second CNtBu ligand to give compound 10. It is possible that
a similar pentacarbonyl complex [Mo2Cp2(µ-PR*)(CO)5] might
be formed in the carbonylation reactions of 4 or 6a, but in that
case it would rapidly dissociate CO to give 1.
Experimental Section
General Procedures and Starting Materials. All manipulations
and reactions were carried out under a nitrogen (99.995%)
atmosphere using standard Schlenk techniques. Solvents were
purified according to literature procedures48 and distilled prior to
use. Compound 1 was prepared as described previously.9b All other
reagents were obtained from the usual commercial suppliers and
used as received. Photochemical experiments were performed using
jacketed quartz or Pyrex Schlenk tubes, cooled by tap water (ca.
288 K) or by a closed 2-propanol circuit, kept at the desired
temperature with a cryostat; for temperatures above 288 K, a closed
water circuit was used instead. A 400 W mercury lamp (Applied
Photophysics) placed ca. 1 cm away from the Schlenk tube was
used for all these experiments. Chromatographic separations were
carried out using jacketed columns cooled as described above.
Commercial aluminum oxide (activity I, 150 mesh) was degassed
under vacuum prior to use. The latter was mixed under nitrogen
with the appropriate amount of water to reach the activity desired.
Carbonylation experiments were carried out using Schlenk tubes
equipped with Young’s valves. Filtrations were performed using
diatomaceous earth. Nuclear magnetic resonance (NMR) spectra
were routinely recorded at 300.13 (1H), 121.50 (31P{1H}), or 75.47
MHz (13C{1H}) in CD2Cl2 solutions at 290 K unless otherwise
stated. Chemical shifts (δ) are given in ppm, relative to internal
Although the stepwise de-coordination of the arene ring in 4
to give 1 as the incorporation of CO molecules proceeds can
be viewed as a logic or expected process, the detection and
stability of the η4-intermediate is remarkable. Indeed, similar
steps have been long proposed for the arene exchange reactions
in [Cr(η6-arene)(CO)2L] complexes, a much studied process
because of its synthetic applications, but the η4- or η2-
intermediates have been never detected.46 Actually, a recent DFT
study on the carbonylation products of [Cr(η6-C6H6)(CO)3]
predicted the formation of [(η1-C6H6)Cr(CO)4] rather than the
(45) Streubel, R. Coord. Chem. ReV. 2002, 227, 175.
(47) Cohen, R.; Weitz, E.; Martin, J. M. L.; Ratner, M. A. Organo-
metallics 2004, 23, 2315.
(48) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, U.K., 1988.
(46) See, for example: (a) Semmelhack, M. F.; Chlenov, A.; Ho, D. M.
J. Am. Chem. Soc. 2005, 127, 7759, and references therein. (b) Traylor, T.
G.; Stewart, K. J.; Goldberg, M. J. J. Am. Chem. Soc. 1984, 106, 4445.

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4867
tetramethylsilane (TMS) or external 85% aqueous H3PO4 solutions
(31P). Coupling constants (J) are given in hertz. Assignments of
the 13C NMR resonances for the aryl groups follow the labeling
shown in the figure of Table 2 and are reported as Cn(C6H2) (n )
1 to 6); the resonances for the tBu groups are analogously reported
as Cm(tBu) (m ) 1, 2).
(0.015 g, 3%). The compound 6a obtained above could be almost
quantitatively converted into 4 by visible-UV irradiation in
tetrahydrofuran (15 mL) at 288 K for 30 min (quartz Schlenk tube).
Filtering of the resulting solution and removal of solvent from the
filtrate gave additional compound 4 (0.075 g, 96% from 6a). The
overall yield of 4 was thus increased up to 68% (0.315 g). Anal.
Calcd for C30H39Mo2O2P: C, 55.05; H, 6.01. Found: C, 54.85; H,
t
Preparation of [Mo2Cp2(µ-H){µ-P(CH2CMe2)C6H2 Bu2}-
1
6.10. H NMR: δ 5.59 (s, 2H, C6H2), 5.37 (s, 5H, Cp), 5.29 (d,
(CO)4] (2). A toluene solution (25 mL) of compound 1 (0.100 g,
0.141 mmol) was irradiated with visible-UV light in a Pyrex-
jacketed Schlenk tube refrigerated by tap water for 1 h 30 min,
while keeping a gentle N2 purge. Solvent was removed under
vacuum from the deep orange resulting solution, and the residue
was dissolved in a minimum of toluene and chromatographed on
alumina (activity IV, 30 × 2.5 cm) at 288 K. Elution with
dichloromethane/petroleum ether (1:5) gave an orange fraction,
which yielded, after removal of solvents, compound 5 as an orange
microcrystalline solid (0.087 g, 87%). Anal. Calcd for C32H39-
Mo2O4P: C, 54.09; H, 5.53. Found: C, 54.19; H, 5.62. 1H NMR:
δ 7.41 (d, JHP ) 3, 1H, C6H2), 7.28 (d, JHP ) 2, 1H, C6H2), 5.49,
5.33 (2 × s, 2 × 5H, Cp), 3.07 (t, JHH ) JHP ) 13, 1H, CH2), 1.50
t
JHP ) 2, 5H, Cp), 1.31 (s, 18H, 2 × Bu), 1.18 (s, 9H, tBu). 13C{1H}
NMR: δ 242.2 (d, JCP ) 7, CO), 112.1 [s, C2(C6H2)], 99.3 [s,
C4(C6H2)], 92.5, 88.0 (2 × s, 2 × Cp), 84.2 [d, JCP ) 74, C1(C6H2)],
80.2 [s, C3(C6H2)], 35.7 [s, 2 × C1(tBu)], 35.1 [s, 2 × C2(tBu)],
34.5 [s, C1(tBu)], 31.3 [s, C2(tBu)].
Preparation of [Mo2Cp(µ-κ1:κ1,η5-PC5H4)(η6-R*H)(CO)2] (5).
Wet dichloromethane (0.5 mL of a solution prepared by mixing
10 µL of H2O with 30 mL of dry dichloromethane) was added to
a dichloromethane solution (2 mL) of compound 4 (0.020 g, 0.031
mmol), and the mixture was heated at 313 K for 2 h to give a deep
red solution shown (by IR and NMR) to contain essentially pure
compound 5. The solvent was then removed under vacuum to give
compound 5 as a red, very air-sensitive microcrystalline solid, which
is assumed to be formed almost quantitatively, and it is thus ready
for further use. No further purification or elemental analysis was
t
(s, 6H, CMe2), 1.40 (s, 9H, Bu), 1.36 (m, 1H, CH2), 1.18 [s, 9H,
tBu], -12.54 (d, JHP ) 37, 1H, µ-H). 13C{1H} NMR: δ 244.1 (d,
JCP ) 23, CO), 243.3 (d, JCP ) 30, CO), 237.1, 234.6 (2 × s, 2 ×
CO), 158.5, 153.2 [2 × d, JCP ) 15, C2,6(C6H2)], 153.1 [s,
C4(C6H2)], 137.0 [d, JCP ) 14, C1(C6H2)], 122.6 [d, JCP ) 6,
C3,5(C6H2)], 118.8 [d, JCP ) 8, C5,3(C6H2)], 92.5, 91.0 (2 × s, 2 ×
Cp), 56.1 (d, JCP ) 26, CH2), 43.9 (s, CMe2), 38.1, 35.2 [2 × s,
C1(tBu)], 32.8, 31.2 [2 × s, C2(tBu)], 32.5, 30.2 [2 × s, 2 × Me].
1
attempted for this sensitive product. H NMR (C6D6): δ 5.35 (s,
5H, Cp), 5.03 (d, JHP ) 4, 3H, C6H3), 4.76 (false t, JHH + JHH′
)
4, 2H, C5H4), 4.47 (m, JHH + JHH′ ) 4, JHP ) 3, 2H, C5H4) 1.15
t
(s, 27 H, Bu). 13C{1H} NMR: δ 241.6 (d, JCP ) 13, CO), 111.1
[s, C2,4,6(C6H3)], 91.2 (s, Cp), 89.2 [d, JCP ) 3, C2(C5H4)], 78.9 [d,
JCP ) 3, C3(C5H4)], 78.8 [s, C1,3,5(C6H3)], 34.6 [s, C1(tBu)], 31.9
[s, C2(tBu)]. The resonance for the C1(C5H4) carbon atom could
not be located in the spectrum.
Preparation of [Mo2Cp2(µ-PR*)(µ-CO)2] (3). A tetrahydro-
furan/acetonitrile solution (51 mL of a 50:1 mixture) of compound
1 (0.400 g, 0.563 mmol) was irradiated with visible-UV light for
2 h in a quartz-jacketed Schlenk tube at 263 K, while bubbling N2
through the solution gently. Solvents were then removed from the
brown-orange solution, and the residue was dissolved in dichloro-
methane/petroleum ether (4 mL of a 1:1 mixture) and chromato-
graphed on alumina (activity IV, 20 × 2 cm) at 253 K. Elution
with dichloromethane/petroleum ether (1:2) gave trace amounts of
compound 4. Elution with a 2:1 solvent mixture gave a brown
fraction, which yielded, after removal of solvents, compound 3 as
a brown microcrystalline solid (0.310 g, 84%). Anal. Calcd for
Preparation of [Mo2Cp2(µ-κ1:κ1,η4-PR*)(CO)3] (6a). Method
A: A toluene solution (5 mL) of compound 4 (0.075 g, 0.115 mmol)
was placed in a bulb equipped with Young’s valve. The bulb was
cooled at 77 K, evacuated, and then refilled with CO. The valve
was then closed, and the solution was allowed to reach room
temperature and further stirred for 72 h. Solvent was then removed
under vacuum, and the residue was dissolved in a minimum of
dichloromethane and chromatographed on alumina (activity IV, 30
× 3 cm) at 288 K. Elution with dichloromethane/petroleum ether
(1:6) gave a black band containing a small amount of compound 1
(0.012 g, 15%). Elution with a 1:3 solvent mixture gave a yellow
fraction, which yielded, after removal of solvents, compound 6a
as a microcrystalline orange solid (0.035 g, 45%). Method B:
Compound 4 (0.050 g, 0.076 mmol) and CuCl (0.020 g, 0.202
mmol) were stirred in dichloromethane (5 mL) for 1 min, and the
solution was then filtered. Solvent was removed under vacuum from
the filtrate, the residue was dissolved in toluene (5 mL), and this
solution was transferred into a bulb equipped with a Young’s valve
and reacted for 3 h with CO as described in method A, but at 323
K. The solution was concentrated under vacuum and chromato-
graphed on alumina (activity IV, 2 × 15 cm) at 288 K. Elution
with dichloromethane/petroleum ether (1:9) gave a fraction contain-
ing trace amounts of [Mo2Cp2(CO)6]. Elution with a 1:3 solvent
mixture yielded complex 6a as a microcrystalline orange solid
(0.043 g, 83%). Anal. Calcd for C31H39Mo2O3P: C, 54.56; H, 5.76.
1
C30H39Mo2O2P: C, 55.05; H, 6.01. Found: C, 54.69; H, 5.76. H
NMR: δ 7.42 (d, JHP ) 4, 2H, C6H2), 5.79 (s, 10H, Cp), 1.38 (d,
JHP ) 1, 18H, tBu), 1.35 (s, 9H, tBu). 13C{1H} NMR: δ 294.0 (d,
JCP ) 13, µ-CO), 152.6 [d, JCP ) 3, C2(C6H2)], 152.0 [d, JCP ) 3,
C4(C6H2)], 149.3 [d, JCP ) 30, C1(C6H2)], 122.9 [d, JCP ) 9,
C3(C6H2)], 98.8 (s, Cp), 38.4 [s, 2 × C1(tBu)], 35.6 [s, C1(tBu)],
33.1 [s, 2 × C2(tBu)], 31.1 [s, C2(tBu)]. The crystals used in the
X-ray study were grown by slow diffusion of a layer of petroleum
ether into a concentrated solution of the complex in diethyl ether
at 253 K.
Preparation of [Mo2Cp2(µ-κ1:κ1,η6-PR*)(CO)2] (4). A tetrahy-
drofuran/acetonitrile solution (24 mL of a 1:2 mixture) of compound
1 (0.500 g, 0.704 mmol) was irradiated with visible-UV light for
1 h 30 min in a quartz-jacketed Schlenk tube at 318 K, while
bubbling N2 through the solution gently. Solvents were then
removed from the deep orange solution, and the residue was washed
with petroleum ether (4 × 20 mL). The residue was then dissolved
in dichloromethane (20 mL) and filtered. Removal of solvent from
the filtrate gave compound 4 as an orange microcrystalline solid
(0.240 g, 52%). The petroleum ether washings were collected and
the solvent was removed under vacuum. This new residue was then
chromatographed on alumina (activity IV, 30 × 2.5 cm) at 253 K.
Elution with dichloromethane/petroleum ether gave trace amounts
of [Mo2Cp2(CO)6]. Elution with a 1:2 solvent mixture gave a yellow
fraction yielding, after removal of solvents, compound 6a as an
orange solid (0.081 g, 17%). Elution with a 2:1 solvent mixture
gave a brown fraction containing a small amount of compound 3
1
Found: C, 54.48; H, 5.91. H NMR: 6.19, 5.74 (2 × s, 2 × 1H,
C6H2), 5.51, 5.45 (2 × s, 2 × 5H, Cp), 1.43, 1.13, 1.03 (3 × s, 3
× 9H, tBu). 13C{1H} NMR: δ 243.9 (d, JCP ) 16, CO), 238.9 (d,
JCP ) 17, CO), 234.5 (d, JCP ) 13, CO), 158.5 [s, C6(C6H2)], 126.3
[s, C5(C6H2)], 109.8 [s, C2(C6H2)], 108.9 [s, C4(C6H2)], 91.7, 91.4
(2 × s, 2 × Cp), 87.2 [s, C3(C6H2)], 78.8 [d, JCP ) 65, C1(C6H2)],
39.9, 35.3, 35.2 [3 × s, C1(tBu)], 33.7, 31.7, 30.3 [3 × s, C2(tBu)].
Preparation of [Mo2Cp2(µ-κ1:κ1,η4-PR*)(CNtBu)(CO)2] (6b).
A tetrahydrofuran solution (5 mL) of compound 4 (0.075 g, 0.115
mmol) was treated with CNtBu (5 mL of a 0.05 M solution in
petroleum ether, 0.250 mmol), and the mixture was heated at 333

4868 Organometallics, Vol. 25, No. 20, 2006
Table 3. Crystal Data for Compounds 3, 6b, 7, and 8
Amor et al.
3
6b
7‚1/2C7H8
8
mol formula
mol wt
C30H39Mo2O2P
654.46
C35H48Mo2N O2P
737.59
C33.5H43ClCuMo2O2P
799.02
C31H39ClCuMo2O3P
781.46
cryst syst
space group
radiation (λ), Å
a, Å
b, Å
c, Å
triclinic
P1h
0.71073
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/n
0.71073
31.069(6)
11.668(2)
18.673(4)
90
92.56(3)
90
6763(2)
8
0.71073
10.559(2)
22.180(4
14.337(3)
90
96.19(3)
90
3338.1(12)
4
1.590
1.527
100
1.70 to 27.48
-13,13; -24,28;
-18, 14
23 745
0.71073
13.958(3)
15.885(3)
14.011(3)
90
90.10(3)
90
3106.6(11)
4
8.0711(9)
10.0012(11)
19.182(2)
86.711(2)
85.086(2)
68.912(2)
1438.8(3)
2
1.511
0.951
173
2.13 to 27.49
-10,10; -12,12;
-24, 24
15 396
R, deg
â, deg
γ, deg
V, Å3
Z
calcd density, g cm-1
absorp coeff, mm-1
temperature, K
θ range, deg
index ranges (h, k, l)
1.449
0.819
100
1.671
1.641
100
1.31 to 27.49
-40,0; -15,5;
-23, 4
1.94 to 27.50
-17,13; -14,20;
-4, 18
no. of reflns collected
45 271
7560
no. of indep reflns (Rint
)
6548 (0.0241)
5422
R1 ) 0.0230
45271
38015
R1 ) 0.0888
7648 (0.0478)
5548
R1 ) 0.0361
5721 (0.0235)
4770
R1 ) 0.0358
no. of reflns with I > 2σ(I)
R indexesa [data with
I > 2σ(I)]
wR2 ) 0.0520b
R1 ) 0.0330,
wR2 ) 0.0551b
0.996
wR2 ) 0.2277c
R1 ) 0.0967
wR2 ) 0.2326c
1.043
wR2 ) 0.0612d
R1 ) 0.0558
wR2 ) 0.0653d
0.857
wR2 ) 0.0841e
R1 ) 0.0464
wR2 ) 0.0896e
1.020
R indexesa (all data)
GOF
no. of restraints/params
0/472
0.419, -0.374
777/733
2.771, -2.342
0/380
0.787, -0.679
0/361
0.835, -0.804
∆F(max,min), e Å-3
2
2
2
a w-1 ) σ2(Fo ) + (aP)2 + (bP), where P ) [max(Fo , 0) + 2Fc ]/3. b a ) 0.0305, b ) 0.000. c a ) 0.1540, b ) 0.000. d a ) 0.0199, b ) 0.000. e a )
0.0429, b ) 3.4885.
K for 5 h to give an orange solution. Solvent was then removed
under vacuum, and the residue was dissolved in a minimum of
toluene (ca. 1.5 mL) and chromatographed on alumina (activity
IV, 30 × 2.5 cm) at 288 K. Elution with dichloromethane/petroleum
ether (1:2) gave an orange fraction, which yielded, after removal
of solvents, compound 6b as an orange microcrystalline solid (0.068
g, 80%). The crystals used in the X-ray study were grown by slow
diffusion of a petroleum ether solution of CNtBu (0.05 M) into a
concentrated dichloromethane solution of the complex at 253 K.
Anal. Calcd for C35H48Mo2NO2P: C, 56.99; H, 6.56; N, 1.90.
Found: C, 56.78; H, 6.41; N, 1.78. 1H NMR: δ 6.06, 5.65 (2 × s,
2 × 1H, C6H2), 5.39 (s, 5H, Cp), 5.22 (d, JHP ) 1, 5H, Cp), 1.46,
Preparation of [CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η4-PR*)(CO)3] (8).
Solid CuCl (0.020 g, 0.202 mmol) was added to a solution of
compound 6a (0.040 g, 0.059 mmol) in dichloromethane (4 mL),
and the mixture was stirred for 1 min to give a deep orange solution,
which was filtered. Removal of solvent from the filtrate under
vacuum gave compound 8 as a red microcrystalline solid (0.042 g,
92%). The crystals used in the X-ray study were grown by slow
diffusion of a layer of petroleum ether into a concentrated solution
of the complex in toluene. Anal. Calcd for C31H39ClCuMo2O3P:
1
C, 47.64; H, 5.03. Found: C, 47.98; H, 5.12. H NMR (200.13
MHz, CDCl3): δ 6.06, 5.85 (2 × s, 2 × 1H, C6H2), 5.28, 5.22 (2
× s, 2 × 5H, Cp), 1.44, 1.14, 1.04 (3 × s, 3 × 9H, tBu). 13C{1H}
NMR (100.63 MHz, CDCl3, 233 K): δ 239.3 (d, JCP ) 16, CO),
237.2, 236.3 (2 × s, 2 × CO), 157.4 [s, C6(C6H2)], 127.4 [s,
C5(C6H2)], 111.1 [s, C2(C6H2)], 107.4 [s, C4(C6H2)], 92.8, 91.8 (2
× s, 2 × Cp), 85.9 [s, C3(C6H2)], 40.4, 35.5, 35.3 [3 × s, 3 ×
C1(tBu)], 33.7, 30.9, 30.4 [3 × s, 3 x C2(tBu)]. The resonance for
the C1(C6H2) carbon atom could not be located in the spectrum.
t
1.32, 1.13 (3 × s, 3 × 9H, Bu), 1.03 (s, 9H, CNtBu). 13C{1H}
NMR: δ 245.3, 240.6 (2 × s, 2 × CO), 165.1 (s, CN), 154.7 [s,
C6(C6H2)], 127.7 [s, C5(C6H2)], 102.7 [s, C2(C6H2)], 98.2 [s,
C4(C6H2)], 91.2, 90.2 (2 × s, 2 × Cp), 86.8 [s, C3(C6H2)], 78.3 [d,
JCP ) 61, C1(C6H2)], 57.6 [s, NC1(tBu)], 39.9, 35.0, 34.5 [3 × s, 3
× C1(tBu)], 33.5, 31.3, 30.3, 30.2 [4 × s, 4 × C2(tBu)].
Preparation of [CuMo2(Cl)Cp2(µ-κ1:κ1:κ1,η6-PR*)(CO)2] (7).
Solid CuCl (0.020 g, 0.202 mmol) was added to a solution of
compound 4 (0.050 g, 0.076 mmol) in dichloromethane (5 mL),
and the mixture was stirred for 1 min to give a deep red solution.
Petroleum ether (5 mL) was then added to that solution, and the
mixture was filtered. Removal of solvents from the filtrate under
vacuum gave compound 7 as a purple microcrystalline solid (0.047
g, 82%). The crystals 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. Anal. Calcd for C33.5H43-
ClCuMo2O2P (7‚1/2C7H8): C, 50.32; H 5.42. Found: C, 49.98; H
Preparation of [Mo2Cp2(µ-PR*)(CNtBu)(CO)3] (9). A 0.05 M
solution of CNtBu in petroleum ether (2.5 mL, 0.125 mmol) was
added to a solution of compound 6a (0.025 g, 0.036 mmol) in
tetrahydrofuran (5 mL), and the mixture was stirred at room
temperature for 8 h. The solvent was then removed under vacuum,
and the residue was dissolved in 0.5 mL of dichloromethane/
petroleum ether (1:1) and chromatographed on alumina (activity
IV, 20 × 2.5 cm) at 258 K. Elution with a 1:4 solvent mixture
gave a dark green fraction. Removal of solvent from the latter
yielded complex 9 as a dark green solid (0.014 g, 51%). Anal. Calcd
for C36H48Mo2NO3P: C, 56.48; H, 6.32; N, 1.83. Found: C, 56.72;
H, 6.43; N, 1.64. 1H NMR (200.13 MHz): δ 7.51, 7.46 (2 × m, 2
× 1H, C6H2), 5.40, 5.28 (2 × s, 2 × 5H, Cp), 1.49, 1.46, 1.33 (3
1
5.30. H NMR: δ 5.74 (s, 2H, C6H2), 5.45 (s, 5H, Cp), 5.33 (d,
t
t
JHP ) 2, 5H, Cp), 1.39 (s, 18H, Bu), 1.23 (s, 9H, Bu). 13C{1H}
NMR: δ 238.6 (s, CO), 237.6 (d, JCP ) 14, CO), 118.1, 113.2 [2
× s, C2,6(C6H2)], 103.1 [s, C4(C6H2)], 93.0, 90.4 (2 × s, 2 × Cp),
82.1 [d, JCP ) 14, 2 × C3,5(C6H2)], 72.5 [d, JCP ) 70, C1(C6H2)],
38.1 [s, 2 × C1(tBu)], 34.8 [s, C1(tBu)], 35.5, 34.2, 31.0 [3 × s, 3
× C2(tBu)].
t
× s, 3 × 9H, Bu), 1.14 (s, 9H, CNtBu).13C{1H} NMR: δ 237.9,
237.6 (2 × s, 2 × CO), 237.4 (d, JCP ) 7, CO), 151.0 [d, JCP
)
35, C1(C6H2)], 150.4, 149.8, 149.1 [3 × s, C2,4,6(C6H2)], 122.5, 121.9
[2 × d, JCP ) 13, C3,5(C6H2)], 89.1 (s, 2 × Cp), 57.2 [s, NC1(tBu)],
38.2, 38.0, 34.2 [3 × s, 3 × C1(tBu)], 32.5, 32.2, 30.1, 29.8 [4 ×

Dimolybdenum Centers
Organometallics, Vol. 25, No. 20, 2006 4869
s, 4 × C2(tBu)]. The resonance for the CNtBu carbon atom could
methods and refined by least squares on weighed F2 values for all
reflections.51 All non-hydrogen atoms were assigned anisotropic
displacement parameters and refined without positional constraints
except for a number of carbon atoms in 6b, which were refined
isotropically as they acquired nonpositive definites after refinement
cycles. For compound 3, all hydrogen atoms were located in the
electron density difference map, assigned isotropic displacement
parameters, and refined without positional constraints. For com-
pounds 6b, 7, and 8 all hydrogen atoms were constrained to ideal
geometries and refined with fixed isotropic displacement parameters.
Refinement proceeded in all cases smoothly to give the residuals
shown in Table 3. Complex neutral-atom scattering factors were
used.52 For compound 6b there are two independent molecules in
the unit cell, both displaying similar structural parameters; one of
the Cp rings is disordered over two positions with occupancy factors
of 0.55 and 0.45. For compound 7, there is a toluene molecule
located at an inversion center in the unit cell (thus giving an overall
composition 7‚1/2C7H8), so that the methyl group is disordered in
two positions with half-occupancy.
not be located in the spectrum.
Preparation of [Mo2Cp2(µ-PR*)(CNtBu)2(CO)3] (10). A 0.05
M solution of CNtBu in petroleum ether (2.5 mL, 0.125 mmol)
was added to a solution of compound 6a (0.020 g, 0.029 mmol) in
petroleum ether (3 mL), and the mixture was stirred at room
temperature for 4 h. After workup similar to that described for 9,
a pale green fraction was eluted using a 1:8 solvent mixture.
Removal of solvents from the latter under vacuum gave compound
10 as a pale green microcrystalline solid. Anal. Calcd for C41H57-
Mo2N2O3P: C, 58.02; H, 6.77; N, 3.30. Found: C, 57.79; H, 6.60;
1
N, 3.19. H NMR (200.13 MHz, toluene-d8): δ 7.52 [d, JHP ) 7,
2H, C6H2], 5.28, 5.22 (2 × s, 2 × 5H, Cp), 1.58, 1.55 (2 × s, 2 ×
t
t
9H, Bu), 1.28 (s, 18H, Bu and CNtBu), 0.81 (s, 9H, CNtBu).
X-ray Structure Determination for Compounds 3, 6b, 7, and
8. Full crystallographic data for the title compounds are given in
CIF format, while Table 3 collects the most relevant crystal data
for these structural studies. In each case a single crystal was coated
in paraffin oil and mounted on a glass fiber. X-ray measurements
were made using a Bruker SMART CCD area-detector diffracto-
meter with Mo KR radiation (λ ) 0.71073 Å).49 Intensities were
integrated (SAINT)49 from several series of exposures, each
exposure covering 0.3° in ω, and the total data being a sphere (3)
or a semisphere (6b, 7, 8). Absorption corrections were applied,
based on multiple and symmetry equivalent measurements,50 except
for 6b, for which no corrections were applied, as the crystal was
non-merohedrally twinned; in that case GEMINI49 was used to
prepare data in HKLF 5 format. The structures were solved by direct
Acknowledgment. This work is dedicated to Prof. F. G. A.
Stone in recognition of his outstanding contributions to modern
inorganic chemistry. We thank the MEC of Spain for a grant
(to D.S.) and financial support (Project BQU2003-05478), and
the University of Oviedo for a grant to I.A.
Supporting Information Available: Crystallographic data for
the structural analysis of compounds 3, 6b, 7, and 8 in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0605229
(49) SMART & SAINT Software Reference Manuals, Version 5.051
(Windows NT Version); Bruker Analytical X-ray Instruments: Madison WI,
1998.
(51) SHELXTL program system Version 5.1; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(50) Sheldrick, G. M. SADABS: A program for absorption correction
with the Siemens SMART system; University of Gottingen: Germany, 1996.
(52) International Tables for Crystallography; Kluwer: Dordrecht, 1992;
Vol. C.