Synthesis, reactivity, and theoretical studies of the η2(4e)-bonded phosphaalkyne complex [CpMo{P(OMe)3}2{η2(4e)-P≡CBut }][B(C6F5)4] and the molybdenum-mediated cycloc

Article abstract of DOI:10.1021/om020169g

Synthesis of complexes containing [CpMo{P(OMe)₃}₂{η²(4e)-P≡CBu^t }][B(C₆F₅)₄] was discussed. Single crystal X-ray crystallography was used. It was found that the cations do not re

Full text of DOI:10.1021/om020169g

3076
Organometallics 2002, 21, 3076-3078
Syn th esis, Rea ctivity, a n d Th eor etica l Stu d ies of th e
η2(4e)-Bon d ed P h osp h a a lk yn e Com p lex
[Cp Mo{P (OMe)3}2{η2(4e)-P tCBu t}][B(C6F 5)4] a n d th e
Molybd en u m -Med ia ted Cyclocotr im er iza tion of Alk yn e
a n d P h osp h a a lk yn e Liga n d s
Andrew D. Burrows,† Nicholas Carr,† Michael Green,*,†,‡ J ason M. Lynam,†,‡
Mary F. Mahon,† Martin Murray,‡ Boggavarapu Kiran,§ Minh T. Nguyen,§,| and
Cameron J ones
Department of Chemistry, University of Bath, Claverton Down, Bath, U.K. BA2 7AY,
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, U.K. BS8 1TS,
Department of Chemistry, University of Leuven, Celestijueleua 200F, B-3001 Leuven, Belgium,
and Department of Chemistry, University of Wales, Cardiff, P.O. Box 912,
Park Place, Cardiff, U.K. CF1 3TB
Received February 26, 2002
Summary: Whereas the cations [CpMo{P(OMe)3}2{η2(4e)-
alkyne}]+ do not react with alkynes or PtCBut, the
newly synthesized isostructural phosphaalkyne complex
[CpMo{P(OMe)3}2{η2(4e)-PtCBut}][B(C6F5)4], which is
unreactive towards PhC2Ph, readily reacts via an as-
sociative stepwise process with PtCBut to give [CpMo-
the neutral complex7 CpMoCl(CO)3 react (room temper-
ature and 40 °C, respectively) with PtCBut to give only
η4-1,3-diphosphacyclobutadiene complexes, we reasoned
that in order to avoid the coupling of two coordinated
phosphaalkyne ligands it was important to devise a
product-forming step which occurred at room temper-
ature or below. Such a synthetic pathway was suggested
by our recent observation8 that the η2(3e)-vinyl complex
{P(OMe)3}2{η4-1,3-P2C2But }][B(C6F5)4]. A further in-
2
teresting difference in alkyne and phosphaalkyne chemis-
try was observed when it was found that CpMoCl(CO)-
{η2(4e)-PhC2Ph} reacts with TlPF6 and PtCBut to give
the unusual 16-electron cyclocotrimerization product
CpMo{dC(Ph)CHPh}{P(OMe)3}2 (2) (Scheme 1) reacts
(CH2Cl2, -50 °C f +25 °C) with [PhNHMe2][X] (X )
B(C6F5)4-, BF4-) to give the labile cationic complexes
[CpMo{P(OMe)3}2{η2(2e)-trans-stilbene}][X], which in
turn react (-78 °C f +25 °C) with PhCtCPh to afford
[CpMo{P(OMe)3}2{η2(4e)-PhC2Ph}][X] (3). Thus, we re-
acted (CH2Cl2, -78 °C) [PhNHMe2][X] with 2 and
warmed the reaction mixture to room temperature
before recooling to -78 °C and adding 1 mol equiv of
PtCBut. On subsequent warming of the resulting
reaction mixture to room temperature, the color changed
(2 h) from red to green and, on addition of hexane, the
sought-for cationic complexes [CpMo{P(OMe)3}2{η2(4e)-
PtCBut}][X] (1a , X- ) B(C6F5)4-; 1b, X ) BF4-; 86%
yield) precipitated as green microcrystalline powders.
The 13C{1H} and 31P{1H} NMR spectra (CD2Cl2) of
1a showed resonances consistent4 with the presence of
an η2(4e)-bonded PtCBut ligand. The contact carbon
appeared as a doublet (1J PC ) 122.7 Hz) at δ 334.8 and
the contact phosphorus as a triplet (2J PP ) 33.4 Hz) at
very low field, δ 491.6. Interestingly the appearance of
the contact phosphaalkyne 31P signal as a triplet and
the presence of only one P(OMe)3 resonance at δ 174.1
suggests the occurrence at room temperature of a
“windscreen wiper” motion by the phosphaalkyne, analo-
gous to that observed5a with alkyne complexes such as
[CpMo{dC(But)PC(But)dPC(Ph)dC(Ph)}(CO)][PF6], iden-
tified by single-crystal X-ray crystallography.
While the isolobal (P¶CR) and diagonal (P/C)
relationships have been used1 to relate the structural
chemistries of alkyne and phosphaalkyne transition-
metal complexes, these simple ideas are of less value
in predicting and rationalizing relative reactivity pat-
terns. During investigations2-4 to develop the chemistry
of η2(4e)-bonded phosphaalkyne complexes, we have
extended our studies to molybdenum complexes and, on
comparing these compounds with the corresponding
alkyne derivatives, we have observed interesting dif-
ferences in reaction chemistry.
Our initial objective was to synthesize complexes
containing the cation [CpMo{P(OMe)3}2{η2(4e)-PtC-
But}]+ (1), to compare the chemistry of these species
with that of the alkyne-substituted5 cations [CpMo-
{P(OMe)3}2{η2(4e)-alkyne}]+. Since it has been shown
that both the cation6 [(η5-C9H7)Mo(NCMe)2(CO)2]+ and
* To whom correspondence should be addressed at the University
of Bristol. E-mail: michael.green@bris.ac.uk.
† University of Bath.
‡ University of Bristol.
§ University of Leuven.
| E-mail: minh.nguyen@chem.kuleuven.ac.be.
University of Wales.
(5) (a) Allen, S. R.; Beevor, R. G.; Green, M.; Norman, N. C.; Orpen,
A. G.; Williams, I. D. J . Chem. Soc., Dalton Trans. 1985, 435. (b) Allen,
S. R.; Beevor, R. G.; Green, M.; Orpen, A. G.; Paddick, K. E.; Williams,
I. D. J . Chem. Soc., Dalton Trans. 1987, 591.
(1) Dillon, K. B.; Mathey, F.; Nixon, J . F. Phosphorus: The Carbon
Copy; Wiley: New York, 1998.
(2) Brauers, G.; Green, M.; J ones, C.; Nixon, J . F. J . Chem. Soc.,
Chem. Commun. 1995, 1125.
(3) Carr, N.; Green, M.; Mahon, M. F.; J ones, C.; Nixon, J . F. J .
Chem. Soc., Chem. Commun. 1995, 2191.
(4) Burrows, A. D.; Dransfeld, A.; Green, M.; J effrey, J . C.; J ones,
C.; Lynam, J . M.; Nguyen, M. T. Angew. Chem., Int. Ed. 2001, 40, 3221.
(6) Hitchcock, P. B.; Maah, M. J .; Nixon, J . F.; Green, M. J .
Organomet. Chem. 1994, 466, 153.
(7) Weller, A. S.; Andrews, C. D.; Burrows, A. D.; Green, M.; Lynam,
J . M.; Mahon, M. F.; J ones, C. Chem. Commun. 1999, 2147.
(8) Beddows, C. J .; Burrows, A. D.; Connelly, N. G.; Green, M.;
Lynam, J . M.; Paget, T. J . Organometallics 2001, 20, 231.
10.1021/om020169g CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/26/2002

Communications
Organometallics, Vol. 21, No. 15, 2002 3077
Sch em e 1a
1,3-P2C2But }][B(C6F5)4] (4). The room-temperature 31P-
2
{1H} NMR (CH2Cl2) spectrum of 4 showed a triplet
2
resonance at δ 162.2 (P(OMe)3, J PP ) 2.8 Hz) and a
broad signal at δ 58.5 due to the η4-1,3-P2C2But2 ligand.
When the temperature is lowered to -60 °C, this broad
signal7 collapsed to give two sharp resonances at δ 59.7
and 49.2, implying that the η4-1,3-P2C2But ligand is
2
undergoing restricted rotation on the NMR timescale
and allowing us to measure, for the first time, the
barrier to rotation (Tc ) 273 K, ∆ν ) 1691 Hz, ∆Gq )
48.0 kJ mol-1).12
a
L ) P(OMe)3. Legend: (i) +[PhNHMe2][X], +PtCBut,
-trans-stilbene, -PhNMe2, CH2Cl2.
In a previous study2 we had obtained evidence for the
stepwise formation of the η4-1,3-P2C2But ligand; how-
2
3. This latter NMR spectrum was essentially invariant
on cooling to -78 °C, indicating that the fluxional
process was still rapid at this temperature.
ever, our observation of the reaction 1a f 4 provided
an opportunity to examine more closely this type of
reaction as a prelude to a more detailed study of the
mechanism using DFT techniques. This was important,
because in our earlier theoretical study (DFT) of the
In the absence of an X-ray structure the optimum
geometry of [CpMo{P(OH)3}2{η2(4e)-PtCMe}]+ (A) was
calculated at the B3LYP/TZVP level using the Turbo-
mole program,9 and for purposes of comparison the
geometry of the related cation [CpMo{P(OH)3}2{η2(4e)-
HCtCMe}]+ (B) was also calculated. The computed
topologies of A and B are essentially identical with that
observed5b for the solid-state structure of the cation
[CpMo{P(OMe)3}2{η2(4e)-HC2But}]+, where the alkyne
is orientated parallel to one of the Mo-P vectors. The
calculated Mo-C(Me) distance in the phosphaalkyne
complex A is short, and since this structural feature is
also found10 within a range of transition-metal η2(4e)-
bonded alkynes, it is reasonable to infer that 1a and
1b can be depicted as η2(4e)-PtCBut complexes. More-
over, this thesis is consistent with our recent4 findings
that a short tantalum to carbon contact distance, Ta-
CBut (2.079(9) Å), occurs in the complex Cp*TaCl2{η2-
(4e)-PtCBut}.
EHMO calculations11 based on the DFT-calculated
bond parameters for A and B of the π|, π , π|*, and π *
orbital occupancies gave further structural insight. The
phosphaalkyne complex A has the orbital occupancies
π| (1.866), π (1.898), π|* (1.064), and π * (0.312) and
the alkyne complex B occupancies π| (1.724), π (1.796),
π|* (0.894), and π * (0.081). This indicates that the
phosphaalkyne ligand in A is a poorer donor and better
acceptor than the alkyne ligand in B. Of particular
interest is the relative occupancy of the π * orbital in
each complex. The alkyne ligands in the cations [CpMo-
{P(OMe)3}2{η2(4e)-alkyne}]+ align themselves parallel
to one of the Mo-P vectors to maximize π-back-bonding
from a metal d orbital into the π * orbital; clearly in
the case of the A this back-bonding is more efficient than
in B, possibly due to 3p-4d and to 2p-4d overlap,
respectively.
formation of CpCo(η4-1,3-P2C2But ) it was concluded
2
that cobaltadiphosphacyclopentadienes were unlikely to
be intermediates in these reactions, although there was
an interesting possibility that a tilted 1,3-diphosphabi-
cyclo[1.1.0]butanediyl ligand was involved.13
When the reaction of 1a with PtCBut in CD2Cl2 was
studied by NMR, no evidence for the formation on the
NMR time scale of any intermediates was obtained. In
addition, the reaction was observed to follow second-
order kinetics (k ) (6.79 ( 0.05) × 10-3 dm3 mol-1 s-1).
Further mechanistic insight was gained from a series
of 31P magnetization transfer experiments. When a
solution (CD2Cl2) containing approximately equimolar
amounts of 1a and P(OMe)3 was prepared and a 31P-
{1H} spectrum of the solution acquired with preirra-
diation (2 s) of the resonance due to uncomplexed
P(OMe)3, the spectrum did not exhibit the characteristic
doublet resonance for 1a at δ 174.1. Similarly, preirra-
diation of this latter resonance resulted in the bleaching
of the signal for the uncomplexed P(OMe)3. These
analyses imply that exchange of free and coordinated
P(OMe)3 was occurring, and since the signals for the
free and complexed P(OMe)3 did not show any signifi-
cant broadening, the rate of exchange (kex) must be less
than 9.0 × 10-3 dm3 mol-1 s-1. The same result was
obtained with the alkyne complex [CpMo{P(OMe)3}2-
{η2(4e)-PhC2Ph}][BF4]5a (5) and P(OMe)3; however, in
this case the signals for both free and coordinated
P(OMe)3 showed significant broadening (ω1/2(P{OMe}3)
) 30.4 Hz). Addition of a further 5 equiv of P(OMe)3
caused increased broadening (ω1/2(P{OMe}3) ) 77.7 Hz),
implying that exchange between free and coordinated
P(OMe)3 is more rapid in the case of 5 and the reaction
occurs via an a ssocia tive mechanism. These results
suggest that in the exchange of phosphite ligands within
the coordination sphere of molybdenum the phos-
phaalkyne or alkyne switches its bonding mode from
η2(4e) to η2(2e) to allow coordination of a third phosphite.
Overall, our findings suggest that both in terms of
bonding and structure there is a close relationship
between the phosphaalkyne-substituted cation 1 and the
alkyne-substituted cations [CpMo{P(OMe)3}2{η2(4e)-
alkyne}]+. However, whereas the alkyne cations are
unreactive toward excess alkyne, 1a was found to react
(1d) at room temperature with PtCBut in CH2Cl2 to
give a high yield of the complex [CpMo{P(OMe)3}2{η4-
(12) The low-temperature (-50 °C) 31P NMR spectrum (CH2Cl2) did
not show any evidence for an interaction between the η4-1,3-P2C2But
2
ligand and B(C6F5)4-; this makes an interesting comparison with the
findings6 that, in the solid-state structure of the complex [(η5-C9H7)-
Mo(CO)2{η4-1,3-P2C2But2}][BF4], the BF4- anion is interacting with one
of the phosphorus centers, this species undergoing reversible dissocia-
tion in solution.
(9) Ahlrichs, R.; Bar, M.; Huser, M.; Horn, H.; Kolmel, C. Chem.
Phys. Lett. 1989, 162, 165.
(10) Templeton, J . L. Adv. Organomet. Chem. 1989, 29, 1.
(11) EHMO calculations were performed with the CACAO 98
program: Mealli, C.; Prosierpio, D. M. J . Chem. Educ. 1990, 67, 399.
(13) Creve, S.; Nguyen, M. T.; Vanquickenborne, L. G. Eur. J . Inorg.
Chem. 1999, 1281.

3078 Organometallics, Vol. 21, No. 15, 2002
Communications
Intramolecular exchange of the phosphite environments
then occurs, followed by dissociation of phosphite.
It is, therefore, reasonable to postulate that in the
formation of 4 by reaction of 1a with PtCBut, the
coordinated phosphaalkyne already present in 1a
switches its bonding mode (η2(4e) f η2(2e)) so as to
facilitate coordination of a second PtCBut. Significantly,
when 31P magnetization transfer experiments with a
solution of 1a and PtCBut in CH2Cl2 were attempted,
no exchange was observed, which suggests that, once
formed, the presumed intermediate [CpMo{P(OMe)3}2-
{η2(2e)-PtCBut}2][B(C6F5)4] rapidly reacts further to
give 4 by a process which remains to be elucidated.
When we found that PtCBut did not react with 5 and
that 1a did not react with PhC2Ph, we speculated that
it might be possible to synthesize interesting mixed
alkyne/phosphaalkyne complexes such as the cation
[CpMo(CO)(η2-PhC2Ph)(η2-PtCBut)]+. However, the
subtle nature of this chemistry was revealed when we
examined the reaction of PtCBut with CpMoCl{η2(4e)-
PhC2Ph}(CO) (6) in CH2Cl2 solution in the presence of
TlPF6.
F igu r e 1. Molecular structure of the cation of complex 7,
with hydrogen atoms removed for clarity.
ered and C(6) exhibiting a deviation of 0.384 Å from the
plane of the atoms P(1), P(2), C(11), and C(12). Interest-
ingly, the carbon-carbon double bond present in the 1,3-
diphosphacyclopentadiene ring is not coordinated to the
molybdenum, presumably due to the geometric con-
straints imposed on the ring by the carbene unit;
therefore, the molybdenum is a 16-electron center.
This remarkable reaction is without precedent and
serves to emphasize not only the versatility of the Pt
CBut ligand but also how its reaction chemistry differs
from that of alkynes. The only previous examples of
cyclocotrimerization reactions at a metal center between
an alkyne and a phosphaalkyne are the formation of
Addition at room temperature of PtCBut (2 molar
equiv) to a stirred suspension of TlPF6 and 6 in CH2Cl2
led, within 1 h, to the formation of TlCl and a change
in color of the reaction mixture. Filtration (Celite) gave
a red solution which on addition of Et2O and cooling
gave thermolabile, air- and moisture-sensitive red crys-
tals of 7 (85% yield). The IR (CH2Cl2) spectrum of 7
showed one terminal carbonyl band at 2053 cm-1, and
examination of the 1H and 31P{1H} NMR spectra
(CDCl3) revealed resonances corresponding to the link-
ing of two phosphaalkynes and one PhC2Ph ligand to
form a P2C4But Ph2 fragment coordinated onto a [Mo-
2
[Fe{η4(4e)-1,3-P2C2But }{η6-2,4-di-tert-butyl-1,3-diphos-
2
(CO)(η5-C5H5)]+ center. Although the presence of two
doublet resonances in the 31P{1H} NMR spectrum at δ
47.7 (J PP ) 20 Hz) and δ -86.8 (J PP ) 20 Hz) implied
that a novel coupling reaction had occurred, attempts
to obtain further structural insight by recording a 13C-
{1H} NMR spectrum were frustrated by the low solubil-
ity of the cation in inert solvents and by its thermal
sensitivity in solution. However, the structural identity
phabenzene)] on reaction of [Fe(η2-C2H4)2(η6-toluene)]
with PtCBut and HCtCH and the stepwise rhodium-
mediated formation of (η5-C9H7)Rh(η4-2,4-di-tert-butyl-
5,6-diphenyl-1,3-diphosphabenzene) from RhCl(η2-PhC2-
Ph)(PCy3)2 and PtCBut.15,16
Ack n ow led gm en t. We thank the EPSRC (N.C.,
J .M.L.), the Ramsay Memorial Fellowship Trust and the
University of Bristol (for a fellowship to J .M.L.), and
the Leverhulme Trust for an Emeritus Professorship
(M.G.); the Leuven group is indebted to the KULeuven
Research Council (GOA program) for financial support.
We also thank Dr. C. J . Beddows for preliminary
experiments.
of 7 as [CpMod{C(But)PC(But)dPC(Ph)dC(Ph)}(CO)]-
[PF6] was established by a single-crystal X-ray diffrac-
tion study.14 This revealed (see Figure 1) that a [Mo-
(CO)(η5-C5H5)]+ fragment is doubly bonded to a ButC
group (Mo-C(26) ) 1.881(8) Å), which is linked by a
single carbon-phosphorus bond (P(1)-C(26) ) 1.920-
(7) Å) to the phosphorus center P(1) of a 1,3-diphos-
phacyclopentadiene ring system. This ring system is η2-
bonded via the ring phosphaalkene to the molybdenum
center (Mo-P(2) ) 2.587(2) Å, Mo-C(6) ) 2.151(7) Å,
P(2)-C(6) ) 1.780(7) Å), the ring being slightly puck-
Su p p or tin g In for m a tion Ava ila ble: Experimental and
selected characterization details for all of the complexes
(including crystallographic data for complex 7), as well as the
coordinates for the optimized geometries of A-C. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM020169G
(14) Crystal data for 7: empirical formula C36.7H33F6MoOP3, T )
293(2) K, λ ) 0.710 70 Å, space group P21/c, a ) 13.526(3) Å, b )
11.326(3) Å, c ) 23.276(5) Å, R ) 90°, â ) 104.41(2)°, γ ) 90°, V )
3453.6(14) Å3, Z ) 4, R indices (I > 2σ(I)) R1 ) 0.0511 and wR2 )
0.1251.
(15) Bo¨hm, D.; Knoch, S.; Kummer, U.; Schmidt, U.; Zenneck, U.
Angew. Chem., Int. Ed. Engl. 1995, 34, 198.
(16) Binger, P.; Haas, J .; Betz, P.; Kru¨ger, C. Chem. Ber. 1995, 128,
737.