High yield synthesis and reactivity of a phosphinidene bridged dimolybdenum complex

Article abstract of DOI:10.1021/ja021072o

Protonation of [Mo₂Cp₂(μ-H)(μ-PHR*)(CO)₄] (Cp = η⁵-C₅H₅, R* = 2,4,6-C₆H₂^tBu₃) with HBF₄·OEt₂ gives the hydridophosphinidene complex [Mo₂Cp₂(μ-H)(μ-PR*)(CO)₄]BF₄, which is easily deprotonated with H₂O to give the known phosphinidene complex [Mo₂Cp₂(μ-PR*)(CO)₄] in 95% yield. Reaction of the latter with I₂ gives the unsaturated phosphinidene complex [Mo₂Cp₂I₂(μ-PR*)(CO)₂], which exhibits an intermetallic distance of 2.960(2) A. Irradiation of solutions of [Mo₂Cp₂(μ-PR*)(CO)₄] with UV light gives a mixture of the triply bonded [Mo₂Cp₂(μ-PR*)(μ-CO)₂] and the hydridophosphido derivative [Mo₂Cp₂(μ-H){μ-P(CH₂CMe₂)C₆H₂^tBu₂}(CO)₄] as major species. The latter complex results from an intramolecular C-H bond cleavage from a ^tBu group and has been characterized by spectroscopy and an X-ray study. Irradiation in the presence of HC_≡C(p-tol) results in the insertion of the alkyne into the Mo-P bond to give [Mo₂Cp₂{μ-η¹:η²,κ-C(p-tol)CHPR*}(CO)₄] structurally characterized through an X-ray study. Copyright

Full text of DOI:10.1021/ja021072o

Published on Web 11/08/2002
High Yield Synthesis and Reactivity of a Phosphinidene Bridged
Dimolybdenum Complex
M. Esther Garc´ıa,† V´ıctor Riera,† Miguel A. Ruiz,*,† David Sa´ez,† Jacqueline Vaissermann,‡ and
John C. Jeffery§
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, 33071 OViedo, Spain,
Laboratoire de Chimie Inorganique et Mate´riaux Mole´culaires, UniVersite´ P. Et M. Curie, 75252 Paris, France,
and Inorganic and Materials Chemistry Section, UniVersity of Bristol, Bristol BS8 1TS, U.K.
Received August 12, 2002
Chart 1
The chemistry of complexes having phosphinidene (PR) ligands
is an active field within the current organometallic research.1 The
phosphinidene group is a very versatile ligand and can display
several coordination modes (some shown in Chart 1).
In the terminal mode (A or B), the metal-phosphorus bond has
a considerable multiple character, and this confers a high reactivity
to the corresponding complexes. This is especially true for the bent
t
Scheme 1. (R* ) 2,4,6-C6H2 Bu3)
complexes A, which are comparable to carbene complexes and can
be also classified as either electrophilic (with good π acceptor
ligands around M) or nucleophilic (strong σ donors around M),
this being supported by theoretical calculations.2 These complexes
have been shown to be very useful intermediates for making new
P-E (E ) C, O, N, S, etc.) or P-M bonds and are hence of great
interest also in the field of organophosphorus or cluster chemistry.
By contrast, M-P bonds in the µ3 or µ4 bridging modes are
essentially single, and little reactivity is expected in this case.
Indeed, phosphinidene groups behave usually as good supporting
ligands in clusters, although some reactions involving the M-P
bond can occur.3,4
The situation in the trigonal bridging mode C can be considered
intermediate, as the M-P bond has still a considerable multiple
character and should be reactive. However, while some chemistry
of the open complexes (type C1) has been developed,5 the chemical
behavior of closed phosphinidene complexes (metal-metal bonded,
type C2) remains largely unknown. In fact, the only reactions we
can quote on these complexes are those carried out on the transient
species [Fe2(µ-PNiPr2)2(CO)6]6 or [Fe2(µ-PtBu)(CO)6],7 which can
be generated in situ from suitable precursors. By contrast, no
reactivity studies have been reported on the few stable compounds
of type C2 previously known. This could be due in part to the lack
of good synthetic routes to these compounds. In this paper, we
[PHR*] and [Mo2Cp2(CO)4], followed by protonation of the
resulting green phosphido anion with 85% H3PO4.9 Protonation of
1 with HBF4‚OEt2 occurs instantaneously to give a dark green
intermediate which, treated with water, yields the phosphinidene
complex 2 in 95% yield. The above intermediate can be selectively
and reversibly formed from 2 and HBF4‚OEt2 and has been
identified as the hydrido-phosphinidene complex [Mo2Cp2(µ-H)-
(µ-PR*)(CO)4]BF4 (3).10 The Mo-P in 2 is stable toward I2
addition, which occurs at the dimetal center instead, giving [Mo2-
Cp2I2(µ-PR*)(CO)2] (4).11 In the crystal (Figure 1),12 compound 4
displays a structure closely related to that of 2.8 The Mo-P lengths
are also short (2.294(2) Å) and therefore indicative of multiple bond
character. However, the intermetallic length in 4 (2.960(2) Å) is
much shorter than that in 2 (3.220(3) Å), in agreement with the
double Mo-Mo bond resulting from the application of the EAN
formalism to 4. To our knowledge, compound 4 represents the first
report a new high yield synthesis for Cowley’s complex [Mo2Cp2-
t
(µ-PR*)(CO)4] (2) (Cp ) η5-C5H5; R* ) 2,4,6-C6H2 Bu3).8 This
has allowed us to start a wide study on its reactivity. Our preliminary
results, here presented, indicate that the phosphinidene ligand in 2
behaves in part as a robust supporting group, preserving the
nuclearity of the dimetallic unit through a number of reactions such
as protonation, halogenation, or decarbonylation. However, the
Mo-P bond becomes chemically active under UV conditions, and
intramolecular C-H oxidative addition or intermolecular insertion
of alkynes is induced under these conditions (Scheme 1).
The hydrido-phosphido precursor [Mo2Cp2(µ-H)(µ-PHR*)(CO)4]
(1) can be conveniently prepared (93%) by reaction of [Li(THF)3]-
* To whom correspondence should be addressed. E-mail: mara@
sauron.quimica.uniovi.es.
† Universidad de Oviedo.
‡ Universite´ P. Et M. Curie.
§ University of Bristol.
9
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10.1021/ja021072o CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
potential of compound 2 under photochemical conditions can be
anticipated to be high.
In summary, we have shown that the phosphinidene ligand in
compound 2 is robust enough so as to preserve the dimetal center
from disruption in a number of reactions. It can span not only single,
but also double or triple metal-metal bonds. In contrast, the
phosphinidene bridge becomes quite reactive under UV excitation,
and a rich chemistry centered at the Mo-P bond can be thus
anticipated under the right experimental conditions. This might
follow patterns distinct from those well established for mononuclear
bent phosphinidene complexes.
Acknowledgment. The authors thank the MCYT of Spain for
a grant (to D.S.) and financial support.
Figure 1. Molecular structure of compounds 4 (left) and 5 (right, hydride
ligand not located).
Supporting Information Available: Experimental procedures and
microanalytical data for new complexes (PDF), and crystallographic
data for compounds 4, 5, and 7 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(1) Recent reviews, see: (a) Mathey, F.; Tran Huy, N. H.; Marinetti, A. HelV.
Chim. Acta 2001, 84, 2938. (b) Schrock, R. R. Acc. Chem. Res. 1997, 30,
9. (c) Cowley, A. H. Acc. Chem. Res. 1997, 30, 445.
(2) (a) Ehlers, A. W.; Baerends, E. J.; Lammertsma, K. J. Am. Chem. Soc.
2002, 124, 2831. (b) Ehlers, A. W.; Lammertsma, K.; Baerends, E. J.
Organometallics 1998, 17, 2738. (c) Frison, G.; Mathey, F.; Sevin, A. J.
Organomet. Chem. 1998, 570, 225.
(3) Huttner, G.; Knoll, K. Angew. Chem., Int. Ed. Engl. 1987, 26, 743.
(4) 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.
(5) (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.
Figure 2. Molecular structure of compound 7.
example of a complex exhibiting a phosphinidene ligand bridging
a formally double metal-metal bond.
The stability of the Mo-P bonds in 2 is high, as further illustrated
by the absence of changes even in refluxing xylenes. However,
the situation changes dramatically under UV irradiation, which leads
to a mixture of the hydridophosphido [Mo2Cp2(µ-H){µ-P(CH2-
(6) King, R. B. J. Organomet. Chem. 1998, 557, 29.
(7) Lang, H.; Zsolnai, L.; Huttner, G. Chem. Ber. 1985, 118, 4426.
(8) Complex 2 was first prepared in 17% yield from PR*Cl2 and K[MoCp-
(CO)3], see: Arif, A. M.; Cowley, A. H.; Norman, N. C.; Orpen, A. G.;
Pakulski, M. Organometallics 1988, 7, 309.
t
CMe2)C6H2 Bu2}(CO)4] (5) (major), phosphinidene [Mo2Cp2-
(9) Selected spectroscopic data for 1: ν(CO) (CH2Cl2) 1965 (w, sh), 1940
(µ-PR*)(µ-CO)2] (6), and a third product yet unidentified (δP )
509.7 ppm). Spectroscopic13 and X-ray data for 5 (Figure 1)14 have
allowed us to identify this product as a result of the oxidative
addition of a C-H(tBu) bond to a formally double PdMo bond in
2. This is in contrast with some related C-H cleavages induced
by bent phosphinidenes, which invariably lead to the oxidative
addition at the P atom (then forming a secondary phosphine
ligand).1a,15,16 This could be an indication that closed phosphinidene
bridges might display reactivity patterns different from those of
bent PR complexes.
Complex 6 can be selectively formed upon photolysis of THF
solutions of 2 and a small amount of MeCN. This compound bears
just two carbonyls and a PR* bridge17 and would thus represent
the first example of a complex having a phosphinidene ligand
bridging a formally triple metal-metal bond. A rich chemistry can
be anticipated from the concurrence of both M-P and M-M
multiple bonds within the same molecule, and studies in that
direction are being currently carried out in our laboratory.
The formation of 5 suggests the presence of a reactive, but not
dissociative, photoexcited state for 2. Indeed, photolysis of 2 in
the presence of HCtC(p-tol) gives also a tetracarbonylic species,
this being the metallaphosphaallyl complex [Mo2Cp2{µ-η1: η2,κ-
C(p-tol)CHPR*}(CO)4] (7).18 The structure of 7 (Figure 2)19 reveals
that an alkyne insertion into a Mo-P has occurred, but the piramidal
environment around the P atom is unexpected. The proposal of a
32 e- bent structure for the nondissociative photoexcited state of 2
(type D in Chart 1) is then quite attractive. In any case, the synthetic
(vs), 1865 (s) cm-1. 1H NMR (400.13 MHz, CD2Cl2, 243 K): δ 7.94 (d,
JHP ) 355, 1H, HP), 5.52, 5.19 (2 × s, 2 × 5H, Cp), -12.99 (d, JHP
)
36.5, 1H, µ-H). 31P{1H} NMR (161.99 MHz, CD2Cl2, 243 K): δ 80.5 (s,
µ-PHR*).
(10) Selected spectroscopic data for 3: ν(CO) (CH2Cl2) 2025 (w), 1997 (vs),
1966 (s), 1952 (s, sh) cm-1. 1H NMR (200.13 MHz, CD2Cl2): δ 5.76 (s,
10H, Cp), -9.15 (d, JHP ) 51, 1H, µ-H). 31P{1H} NMR (81.08 MHz,
CD2Cl2): δ 724.9 (s, µ-PR*).
(11) Selected spectroscopic data for 4: ν(CO) (CH2Cl2) 1942 (s, sh), 1925
1
(vs) cm-1. H NMR (200.13 MHz, CD2Cl2): δ 5.24 (d, JHP ) 0.5, 10H,
Cp). 31P{1H} NMR (81.07 MHz, CD2Cl2): δ 596.4 (s, µ-PR*).
(12) X-ray data for 4: black crystals, monoclinic (C2/c), a ) 14.673(5) Å, b
) 14.438(6) Å, c ) 16.653(16) Å, â ) 113.13(5)°, V ) 3244(4) Å3, T )
295 K, Z ) 4, R ) 0.042, GOF ) 1.13.
(13) Selected spectroscopic data for 5: ν(CO) (CD2Cl2) 1959 (w, sh), 1936
(vs), 1862 (s) cm-1
Cp), 3.07 (t, JHP ) JHH ) 14, 1H, CH2), 1.51 (s, 6H, CMe2), 1.40, 1.18
(2 × s, 2 × 9H, tBu), 1.36 (m, 1H, CH2), -12.55 (d, JHP ) 37, 1H, µ-H).
31P{1H} NMR (121.51 MHz): δ 166.8 (s, µ-P).
.
1H NMR (CD2Cl2): δ 5.49, 5.33 (2 × s, 2 × 5H,
(14) X-ray data for 5: red crystals, monoclinic (P21/c), a ) 13.630(2) Å, b )
13.612(3) Å, c ) 16.664(6) Å, â ) 98.05(2)°, V ) 3061(1) Å3, T ) 295
K, Z ) 4, R ) 0.0599, GOF ) 0.99.
(15) Cowley, A. H.; Barron, A. R. Acc. Chem. Res. 1988, 21, 81.
(16) Hey-Hawkins, E.; Kurz, S. J. Organomet. Chem. 1994, 479, 125.
(17) Selected spectroscopic data for 6: ν(CO) (CH2Cl2) 1741 (m), 1709 (s)
cm-1 31P{1H} NMR (121.57 MHz, CD2Cl2): δ 532.1 (s, µ-PR*). 13C-
.
{1H} NMR (100.63 MHz, CD2Cl2, 223 K): δ 296.0 (d, JCP ) 14, µ-CO),
98.0 (s, Cp).
(18) Selected spectroscopic data for 7: ν(CO) (CH2Cl2) 1959 (vs), 1915 (s),
1948 (m), 1806 (w) cm-1 1H NMR (300.13 MHz, CD2Cl2): δ 5.80 (s,
.
1H, CH), 4.77, 4.39 (2 × s, 2 × 5H, Cp). 31P{1H} NMR (121.49 MHz,
CD2Cl2): δ 18.1 (s, κ-P). 13C{1H} NMR (75.48 MHz, CD2Cl2): δ 179.1
(d, JCP ) 39, µ-C), 114.4 (d, JCP ) 64, HCP).
(19) X-ray data for 7. 1/2 C6H12: green crystals, monoclinic (P21/c), a )
10.1070(12) Å, b ) 24.320(3) Å, c ) 16.839(2) Å, â ) 98.224(17)°, V
) 4096.3(8) Å3, T ) 173(2) K, Z ) 4, R ) 0.0368, GOF ) 0.955.
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