C-X bond formation and cleavage in the reactions of the ditungsten hydride complex [W2(η5-C5H5) 2(H)(μ-PCy2)(CO)2] with small molecules having multiple C-X bonds (X = C, N, O)

Article abstract of DOI:10.1039/c1dt10871a

Two molecules of C₂(CO₂Me)₂ or isocyanides could be added to the title hydride complex under mild conditions to give dienyl-[W₂Cp₂{μ-η¹,κ: η²-C(CO₂Me)C(CO₂Me)C(CO₂Me) CH(CO₂Me)}(μ-PCy₂)(CO)₂] (Cp = η⁵-C₅H₅), diazadienyl-[W₂Cp ₂{μ-κ,η:κ,η-C{CHN(4-MeO-C₆H ₄)}N(4-MeO-C₆H₄)}(μ-PCy₂)(CO) ₂] or aminocarbyne-bridged derivatives [W₂Cp ₂{μ-CNH(2,6-Me₂C₆H₃)}(μ- PCy₂){CN(2,6-Me₂C₆H₃)}(CO)]. In contrast, its reaction with excess (4-Me-C₆H₄)C(O)H gave the C-O bond cleavage products [W₂Cp₂{CH ₂(4-Me-C₆H₄)}(O)(μ-PCy₂)(CO) ₂] and [W₂Cp₂{μ-η:η,κ-C(O) CH₂(4-Me-C₆H₄)}(O)(μ-PCy₂)(CO)]. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10871a

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COMMUNICATION
C–X bond formation and cleavage in the reactions of the ditungsten hydride
complex [W2(g5-C5H5)2(H)(l-PCy2)(CO)2] with small molecules having
multiple C–X bonds (X = C, N, O)†
M. Angeles Alvarez, M. Esther Garc´ıa, Miguel A. Ruiz* and M. Fernanda Vega
Received 9th May 2011, Accepted 29th June 2011
DOI: 10.1039/c1dt10871a
Two molecules of C2(CO2Me)2 or isocyanides could
be added to the title hydride complex under mild
conditions to give dienyl-[W2Cp2{l-g1,j:g2-C(CO2Me)
C(CO2Me)C(CO2Me) CH(CO2Me)}(l-PCy2)(CO)2] (Cp =
hydride and a semi-bridging carbonyl (T, minor isomer).1 It was
thus thought that, in addition to the differences originated from the
change in the metal (W instead of Mo), the presence of the isomer
T in the solutions of 1 might imply a significant modification in the
reactivity of this ditungsten complex, compared to the behaviour
observed for [Mo2Cp2(m-H)(m-PCy2)(CO)2]. We thus initiated a
systematic study on the reactivity of 1 aimed at examining these
differences. In this paper we report our preliminary studies on
the reactions of 1 with different unsaturated organic molecules
such as alkynes, isocyanides and aldehydes (Scheme 1). From this
we conclude that the reactivity of the ditungsten complex indeed
exhibits features unparalleled by its dimolybdenum analogue,
notably its ability to incorporate two, rather than one, molecules
of reagent, then promoting C–C coupling processes at room
g5-C5H5),
diazadienyl-[W2Cp2{l-j,g:j,g-C{CHN(4-MeO–
C6H4)}N(4-MeO–C6H4)}(l-PCy2)(CO)2] or aminocarbyne-
bridged derivatives
[W2Cp2{l-CNH(2,6-Me2C6H3)}(l-
PCy2){CN(2,6-Me2C6H3)}(CO)]. In contrast, its reaction
with excess (4-Me–C6H4)C(O)H gave the C–O bond cleavage
products [W2Cp2{CH2(4-Me–C6H4)}(O)(l-PCy2)(CO)2] and
[W2Cp2{l-g:g,j-C(O)CH2(4-Me–C6H4)}(O)(l-PCy2)(CO)].
Recently we reported the preparation of the unsaturated ditung-
sten hydride [W2Cp2(H)(m-PCy2)(CO)2] (1),1 a new member of the
scarce family of 30-electron binuclear complexes having bridging
hydride ligands, a circumstance providing these electron-deficient
molecules with a high reactivity towards many different reagents
under mild conditions, and hence with a substantial synthetic
potential, as illustrated by the wide chemistry developed around
6
the polyhydrides [M2Cp*2(m-H)4] (M = Fe, Ru),2 and [M2(h -
C6Me6)2(m-H)3]+ (M = Ru, Os),3 and the molybdenum analogue of
1, the monohydride [Mo2Cp2(m-H)(m-PCy2)(CO)2] (2).4 Unlike the
latter species, which displays solely an H-bridged structure,4a,f the
ditungsten complex 1 exists in solution as a mixture of two isomers
in equilibrium (Chart 1): the expected one with a bridging hydride
ligand (B, major isomer) and a second one having a terminal
Chart 1
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, Universidad
de Oviedo, E-33071, Oviedo, Spain. E-mail: mara@uniovi.es
† Electronic supplementary information (ESI) available: Experimental
procedures and spectroscopic and microanalytical data for new com-
pounds (PDF), and crystallographic data in CIF format. CCDC reference
numbers 825086–825089 For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10871a
Scheme 1 Reactions of complex 1 with small unsaturated molecules (R1 =
CO2Me: R2 = 2,6-Me2C6H3; R3 = 4-MeO–C6H4; R4 = 4-Me–C6H4).
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temperature, and its unusual ability to cleave the C–O bond of
an aldehyde under moderate thermal activation.
Compound 1 is able to add two molecules of alkyne under
the right conditions, a circumstance not met by its molybdenum
analogue 2.4e For instance, the reaction with excess dimethyl
acetylenedicarboxylate at 273 K gives a mixture of the alkenyl
alkenyl products,2,4e,5,6 apparently none of these previous reactions
involved an alkenyl–alkyne coupling as observed in the formation
of 4, and further studies on the reactivity of 1 towards different
alkynes are now under way. We note, however, that the dihydride
[Os3(m-H)2(CO)10] has been shown to induce alkyne–alkyne cou-
pling of C2Ph2 and C2H2,5,7 and that a comparable C–C coupling
has been previously observed in the reactions of anionic diiron
1
2
complex
cis-[W2Cp2{m-h :h -C(CO2Me) CH(CO2Me)}(m-
1
2
PCy2)(CO)2] (3), and the 1,3-dienyl derivative [W2Cp2{m-
alkenyl complexes of the type [Fe2(m-h :h -CRCHR¢)(CO)6]- with
1
2
h ,k:h - C(CO2Me) C(CO2Me)C(CO2Me) CH(CO2Me)}(m -
PCy2)(CO)2] (4) (Scheme 1). Spectroscopic data for 3 (see the
ESI†) indicate a cisoid arrangement of the carbonyl ligands
alkynes.8
The reactivity of the hydride 1 towards isocyanides CNR was
found to be strongly dependent both on the nature of R and
on the experimental conditions, and it again involved products
which cannot be afforded using the molybdenum hydride 2. For
instance, the reaction of 1 with excess CN(4-MeO–C6H4) at room
temperature gives, with good yield, the C–C coupled diazadienyl
complex [W2Cp2{m-k,h:k,h-C{CHN(4-MeO–C6H4)}N(4-MeO–
C6H4)}(m-PCy2)(CO)2] (5), whereas the analogous reaction
with CN(2,6-Me2C6H3) gives the aminocarbyne–isocyanide
1
2
and are not very different from those of cis-[Mo2Cp2{m-h ,k:h -
C(CO2Me) CH(CO2Me)}(m-PCy2)(CO)2], minor isomer
a
formed in the reactions of the dimolybdenum hydride 2 with
C2(CO2Me)2 and exhibiting coordination of the O atom of
a carboxylate group to one of the Mo atoms, as determined
crystallographically.4e However, the relatively high value of
the 31P–183W couplings in 3 (av. 303 Hz) are characteristic of
coordinatively unsaturated complexes, so we trust that the alkenyl
ligand in this complex is exclusively bound to the ditungsten
centre via the C atoms.
derivative
[W2Cp2{m-CNH(2,6-Me2C6H3)}(m-PCy2){CN(2,6-
Me2C6H3)}(CO)] (6). Spectroscopic and microanalytical data for
5 (see the ESI†) confirmed the incorporation of two isocyanide
molecules into the complex and the migration of the former
hydride ligand to a carbon atom, but otherwise are comparable
to those of the formimidoyl complexes [Mo2Cp2(m-k,h:k,h-
CHNR)(m-PCy2)(CO)2] formed in the reactions of 2 with CNR
The spectroscopic data for 4 clearly establish the presence of
two C2(CO2Me)2 units in this complex, and the migration of the
former hydride ligand to one of the C atoms of the added alkyne.
An X-ray analysis of this compound confirmed the presence of a
dienyl chain s-bound to one of the W atoms through the alkenyl
t
(R = Bu, 4-Me–C6H4).4a,d Moreover, the low 31P–183W couplings
˚
carbon (W1–C3 = 2.17(1) A) and one of the carboxylate groups
in 5 (av. 193 Hz) are fully consistent with the proposed m-k,h:k,h
coordination of the diazadienyl ligand, this leading to the full
saturation of the dimetal centre.
˚
(W1–O7 = 2.24(1) A), while p-bound to the other W atom through
˚
the second double C C bond (W–C lengths ca. 2.25 A) (Fig. 1).‡
The spectroscopic data for 6 denoted the presence of two
isomers in solution, and were for both clearly indicative of the
incorporation of two molecules of isocyanide, migration of the
former hydride ligand to a nitrogen atom, and formation of an
aminocarbyne ligand (dC ca. 335 ppm, see the ESI†). However,
these data could not define the actual binding of the second CNR
molecule. An X-ray study on this complex‡ (Fig. 2) revealed
that the latter molecule was coordinated in a relatively unusual
terminal-bent mode (C3–N2–C12 139(2)◦), surely due to steric
crowding, thus explaining the absence in solution of C–N stretches
in the usual (linear) terminal region (around 2100 cm-1). The
˚
intermetallic length in 6 is quite short (2.677(2) A), as expected
for carbyne-bridged 32-electron complexes of the type [M2Cp2(m-
CX)(m-PR2)L2], for which a double metal–metal bond should be
formulated.9
It is well established that reactions of complexes having
unsaturated M2(m-H)x centres with isocyanides can lead either
to formimidoyl or to aminocarbyne derivatives, depending on
whether the hydride ligand is transferred to the carbon or to the
nitrogen atoms of the incoming ligand.4d,5,10 The formation of 5
requires, however, the additional insertion of a second isocyanide
molecule into the W–C bond of the formimidoyl intermediate
that would follow from insertion of the first isocyanide molecule
into the W–H bond of the hydride 1, with concomitant C–C
coupling. This coupling process is rather unusual. Actually, the
first example of such a process involving a binuclear hydride
complex has just been described while our work was in progress,11a
and the related reductive coupling of two isocyanide ligands at a
dimetal centre is also a rare event.11b Interestingly, the monitoring
by 31P NMR spectroscopy of the reactions leading to compounds
Fig. 1 ORTEP drawing (30% probability) of compound 4, with H atoms
(except H12) and cyclohexyl rings (except the C1 atoms) omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): W1–W2 = 3.0273(6), W1–P1 =
2.433(3), W1–C1 = 1.99(1), W1–C3 = 2.17(1), W1–O7 = 2.24(1), W2–P1 =
2.475(3), W2–C2 = 1.94(1), W2–C9 = 2.24(1), W2–C12 = 2.26(1), C3–C6 =
1.35(1), C6–C9 = 1.50(1), C9–C12 = 1.44(1), C9–C10 = 1.46(1), C10–O7 =
1.24(1); W2–W1–C1 = 73.2(3), W1–W2–C2 = 118.4(3).
The formation of 4 can be viewed as resulting from the
insertion of a C2(CO2Me)2 molecule into the W–C s-bond in
an alkenyl intermediate following from the insertion of the first
C2(CO2Me)2 molecule into the W–H bond of the hydride 1. A
separate experiment, however, revealed that the alkenyl complex 3
is not such an intermediate. Although the reactions of complexes
having unsaturated M2(m-H)x centres with alkynes typically give
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Fig. 4 ORTEP drawing (30% probability) of compound 8, with H atoms
(except the methylenic ones), CH3 group and cyclohexyl rings (except
1
˚
the C atoms) omitted for clarity. Selected bond lengths (A) and angles
(deg): W1–W2 = 2.8510(8), W1–P1 = 2.364(3), W1–C1 = 1.96(1), W1–C2 =
2.16(1), W1–O3 = 2.166(7), W2–P1 = 2.422(3), W2–O2 = 1.729(7),
W2–C2 = 2.102(9); W2–W1–C1 = 93.0(3), O2–W2–W1 = 110.6(2).
Fig. 2 ORTEP drawing (30% probability) of compound 6, with H atoms
(except H1) and cyclohexyl rings (except the C1 atoms) omitted for clarity.
˚
Selected bond lengths (A) and angles (deg): W1–W2 = 2.677(2), W1–P1 =
2.403(6), W1–C1 = 1.93(3), W1–C2 = 1.97(3), W2–P1 = 2.356(7), W2–C2 =
2.06(2), W2–C3 = 1.94(2), C2–N1 = 1.31(3), N1–C4 = 1.46(3), C3–N2 =
1.23(3), N2–C12 = 1.42(3), W2–W1–C1 = 79.6(8), W1–W2–C3 = 95.9(7).
C2–N1–C4 = 127(2), C3–N2–C12 = 139(2).
and 4). The structures of these compounds are comparable to
those recently determined by us for the isomeric pair of dinitrosyl
complexes [Mo2Cp2(m-PCy2)R(CO)(NO)2] (R = Me, CH2Ph) and
[Mo2Cp2{m-h:h,k-C(O)Me}(m-PCy2)(NO)2],12 although there are
differences in the corresponding metric parameters, and in the fact
that the ditungsten isomers do not interconvert in refluxing toluene
solutions, as shown by independent experiments. The formation
of compounds 7 and 8 requires a C–O bond cleavage in the
hypothetical alkoxide complex that would follow from the initial
insertion of the aldehyde into the W–H bond of 1. The latter
reaction is a process reported previously for some unsaturated
hydride-bridged complexes.13 Indeed a 31P NMR monitoring of the
above reaction allowed the detection of an unstable intermediate
identified by a resonance at 58.8 ppm (JPW 300 Hz) that could
well correspond to a complex of formula [W2Cp2{m-OCH2(4-
Me–C6H4)}(m-PCy2)(CO)2] (cf. dP 89.3 ppm, with JPW = 292
Hz for [W2Cp2(m-I)(m-PCy2)(CO)2]).1 The subsequent oxidative
addition of the C–O bond of the alkoxide group to yield oxo and
alkyl ligands is a rather unusual event,14 although we can quote
at least one example presumably involving a bridging alkoxide
ligand.15
5 and 6 revealed the presence of at least an unstable intermediate
characterized in both cases by a similar resonance at ca. 30 ppm,
and further work is now in progress to fully elucidate the complex
pathways under operation in these reactions.
Compound 1 did not react with the aldehyde (4-Me–
C5H4)C(O)H at room temperature even using a large excess of
reagent, but does react slowly above ca. 303 K. This reaction can
be more conveniently completed (ca. 1 h) in refluxing toluene
solutions and by using ca. 3 equiv of reagent or above, to give
a mixture containing as major products the oxo–alkyl com-
plex [W2Cp2{CH2(4-Me–C6H4)}(O)(m-PCy2)(CO)2] (7) and its
oxo–acyl isomer [W2Cp2{m-h:h,k-C(O)CH2(4-Me–C6H4)}(O)(m-
PCy2)(CO)] (8), along with small amounts of other products as
yet uncharacterized. Fortunately, the isomers 7 and 8 could be
separated chromatographically and fully characterized (Fig. 3
In summary, we have shown that the replacement of molyb-
denum with tungsten in the 30-electron hydrides [M2Cp2(H)(m-
PCy2)(CO)2] has significant effects not only on the structure but
also on the reactivity and nature of the corresponding derivatives.
The ditungsten hydride 1 exhibits enhanced reactivity towards
small unsaturated organic molecules such as alkynes, isocyanides
and aldehydes under mild conditions. Up to two molecules of the
organic molecule can be easily incorporated into the unsaturated
ditungsten centre, therefore enabling different C–C coupling
processes to take place under mild conditions. At the same time,
the unsaturated nature of the dimetal centre seems also to be
critical to induce a rare C–O bond cleavage process in the reaction
with (4-Me–C6H4)C(O)H. Further studies are now in progress to
fully explore the synthetic potential of this 30-electron ditungsten
hydride.
Fig. 3 ORTEP drawing (30% probability) of compound 7, with H
atoms (except the methylenic ones), CH3 group and cyclohexyl rings
(except the C1 atoms) omitted for clarity. Selected bond lengths (A)
˚
and angles (deg): W1–W2 = 2.9532(6), W1–P1 = 2.385(3), W1–C1 =
1.97(1), W1–C2 = 1.94(1), W2–P1 = 2.469(3), W2–O3 = 1.723(7), W2–C3 =
2.23(1); W2–W1–C2 = 90.5(3), C1–W1–P1 = 77.8(3), C3–W2–P1 = 80.0(3),
O3–W2–W1 = 94.9(2).
We thank the DGI of Spain (Projects CTQ2006-01207,
CTQ2009-09444, and a grant to M. F. V.) for supporting this
work.
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Notes and references
‡ Crystal data for 4: dark green crystals, C37H47Cl2O10PW2 (4·CH2Cl2),
¯
M = 1121.32, triclinic (P1), a = 9.8445(4), b = ◦9.9405(5), c = 20.4221(9)
3
˚
˚
A, a = 76.143(3), b = 84.799(3), g = 76.694(3) , V = 1886.9(2) A , T =
100 K, Z = 2, 60960 reflections measured, 8399 unique (Rint = 0.1597).
Final R = 0.0585 (observed data with I > 2s(I)), GOF = 1.000. Crystal
data for 6: dark green crystals, C41H51N2OPW2, M = 986.51, triclinic
¯
˚
(P1), a = 10.806(3), b = 11.238(3), c = 16.928(6) A, a = 99.745(18), b =
◦
3
˚
105.113(18), g = 105.295(13) , V = 1850.8(10) A , T = 100 K, Z = 2,
21272 reflections measured, 4486 unique (Rint = 0.1572). Final R = 0.0674
(observed data with I > 2s(I)), GOF = 1.022. Crystal data for 7: orange
crystals, C32H41O3PW2, M = 872.32, monoclinic (P21/c), a = 13.9983(6),
◦
3
˚
˚
b = 12.3286(6), c = 17.0691(9) A, b = 92.294(2) , V = 2943.4(2) A , T = 100
K, Z = 4, 43580 reflections measured, 5784 unique (Rint = 0.1242). Final
R = 0.0513 (observed data with I > 2s(I)), GOF = 1.040. Crystal data for
8: orange crystals, C33H43Cl2O3PW2 (8·CH2Cl2), M = 957.22, monoclinic
◦
˚
(P21/c), a = 11.693(4), b = 12.758(5), c = 22.391(8) A, b = 102.867(5) , V =
3
˚
3256(2) A , T = 100 K, Z = 4, 36132 reflections measured, 6420 unique
(Rint = 0.0680). Final R = 0.0483 (observed data with I > 2s(I)), GOF =
1.105.
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This journal is
The Royal Society of Chemistry 2011
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©