Mechanism of formation of [(PMe3)3Rh(-C≡C-R)2(H)] via C-H oxidative addition: Isomerization, alkyne exchange, and hydride replacement

Article abstract of DOI:10.1021/om010685r

The mechanism of formation of mer,trans-[(PMe₃)₃Rh(-C=C-R)₂H] from [(PMe₃)₄Rh(Me)] and terminal alkyne has been studied. The initial step of the reaction is the elimination of methane and the formation of the trigonal bipyramidal complex [(PMe₃)₄Rh(-C=C-R)], a reaction that is complete in time of mixing at -78 °C. This intermediate undergoes an oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe₃)₃Rh(-C= C-R)₂H] as the kinetic product. This fac isomer is not stable above -20 °C and isomerizes to the thermodynamic product mer,trans-[(PMe₃)₃Rh(-C=C-R)₂H], fac-[(PMe₃)₃Rh(-C=C-R)₂H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic barrier than the isomerization to mer,trarns-[(PMe₃)₃Rh(-C=C-R)₂H]. Density functional theory studies on all stages of the formation of mer,trans-[(PMe₃)₃Rh(-C=C-R)₂H] have been carried out and give ground state energies in line with those experimentally observed. Once formed, mer,trans-[(PMe₃)₃Rh(-C=C-R)₂H] is configurationally stable and not prone to scrambling, although it will react with chloroform, whereupon the hydride is replaced by chloride. The initial product of this reaction is mer,trans-[(PMe₃)₃Rh(-C=C-R)₂Cl], and this compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe₃)₃-Rh(-C=C-R)₂Cl] isomerizes slowly to mer,cis-[(PMe₃)₃Rh(-C=C-R)₂Cl].

Full text of DOI:10.1021/om010685r

Organometallics 2002, 21, 429-437
429
Mech a n ism of F or m a tion of [(P Me3)3Rh (-CtC-R)2(H)]
via C-H Oxid a tive Ad d ition : Isom er iza tion , Alk yn e
Exch a n ge, a n d Hyd r id e Rep la cem en t
J onathan P. Rourke,*,†,‡ Graham Stringer,§ Pauline Chow,§ Robert J . Deeth,†
Dmitri S. Yufit,| J udith A. K. Howard,| and Todd B. Marder*,§,|,
Department of Chemistry, University of Warwick, Coventry, U.K. CV4 7AL, Department of
Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Department of
Chemistry, University of Durham, South Road, Durham, U.K. DH1 3LE
Received J uly 31, 2001
The mechanism of formation of mer,trans-[(PMe3)3Rh(-CtC-R)2H] from [(PMe3)4Rh(Me)]
and terminal alkyne has been studied. The initial step of the reaction is the elimination of
methane and the formation of the trigonal bipyramidal complex [(PMe3)4Rh(-CtC-R)], a
reaction that is complete in time of mixing at -78 °C. This intermediate undergoes an
oxidative addition reaction with a second equivalent of alkyne to give fac-[(PMe3)3Rh(-Ct
C-R)2H] as the kinetic product. This fac isomer is not stable above -20 °C and isomerizes
to the thermodynamic product mer,trans-[(PMe3)3Rh(-CtC-R)2H]. fac-[(PMe3)3Rh(-CtC-
R)2H] will exchange alkynyl groups with free alkyne, a reaction that has a lower energetic
barrier than the isomerization to mer,trans-[(PMe3)3Rh(-CtC-R)2H]. Density functional
theory studies on all stages of the formation of mer,trans-[(PMe3)3Rh(-CtC-R)2H] have
been carried out and give ground state energies in line with those experimentally observed.
Once formed, mer,trans-[(PMe3)3Rh(-CtC-R)2H] is configurationally stable and not prone
to scrambling, although it will react with chloroform, whereupon the hydride is replaced by
chloride. The initial product of this reaction is mer,trans-[(PMe3)3Rh(-CtC-R)2Cl], and this
compound has been studied by single-crystal X-ray diffraction. In solution mer,trans-[(PMe3)3-
Rh(-CtC-R)2Cl] isomerizes slowly to mer,cis-[(PMe3)3Rh(-CtC-R)2Cl].
In tr od u ction
dendrimers,16,17 molecular scaffolds,18 luminescent
materials,19-22 and as materials for nonlinear optics
(NLO).23-28 Some systems have received more attention
than others, and in particular, those containing
platinum or palladium,11,12,20,21,27,29-33 rhodium,13,34-36
The chemistry of transition metal alkynyls has been
the subject of much interest in recent years.1,2 These
compounds have been investigated for a number of
technological applications, in particular as molecular
wires,3-6 polymeric systems,7,8 catalysts for polymeri-
zation,9,10 liquid crystals,11-14 organometallic catananes,15
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† University of Warwick.
‡ E-mail: j.rourke@warwick.ac.uk.
§ University of Waterloo.
| University of Durham.
E-mail: todd.marder@durham.ac.uk.
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10.1021/om010685r CCC: $22.00 © 2002 American Chemical Society
Publication on Web 12/14/2001

430 Organometallics, Vol. 21, No. 2, 2002
Rourke et al.
ruthenium,37-41rhenium,6,19,42,43 and osmium44 have
received the bulk of the attention.
Sch em e 1
Whereas we were previously able to prepare the
unsymmetrically substituted donor-acceptor trans-
[(PR3)2Pt(-CtC-D)(-CtC-A)] complexes necessary
for useful second-order NLO effects27,28 (others have
recently investigated the mechanism of alkynyl ex-
change in these systems45), we were unsuccessful in
synthesizing related unsymmetrical octahedral Rh(III)
bis(alkynyls) in the absence of their symmetrical coun-
terparts.46 We thus decided to investigate the mecha-
nistic details of the reaction pathway in order to
determine whether it would be possible to make the
desired unsymmetrical rhodium compounds cleanly. We
present the results of these investigations here.
Resu lts a n d Discu ssion
The system we elected to study, mer,trans-[(PMe3)3-
Rh(-CtC-R)2H], is ideally set up for liquid crystalline
behavior13 and one-dimensional polymer formation,7,8,47,48
and if two different alkynyls could be introduced, it
would be appropriate for second-order NLO studies. Our
previously reported route for the synthesis of these
compounds simply involves the addition, at room tem-
perature, of 2 equiv of a terminal alkyne to the rhodium
precursor [(PMe3)4RhMe] (Scheme 1) and results in
essentially quantitative yields.49 The reaction can also
be carried out sequentially, as the trigonal bipyramidal
[(PMe3)4Rh(-CtC-R)] intermediate can be isolated.50
However, when two different alkynes were added se-
quentially, a statistical mixture of bis(alkynyl) com-
plexes resulted.46
To elucidate the reaction pathway, we cooled a solu-
tion of [(PMe3)4RhMe] to -78 °C and added 1 equiv of
a terminal alkyne. With both ethynyltrimethylsilane
(trimethylsilylacetylene, TMSA) and p-methoxyphenyl-
ethyne, an instantaneous reaction occurred, liberating
methane (confirmed by GC-MS) and forming [(PMe3)4-
Rh(-CtC-R)]. While [(PMe3)4Rh(-CtC-R)] com-
pounds are fluxional on the NMR time scale at temper-
atures above -70 °C, they can be unambiguously
assigned a trigonal bipyramidal structure with the
alkynyl ligand in the axial position on the basis of low-
temperature NMR spectroscopy, where two signals are
observed in the 31P spectrum with relative intensities
3:1 and appropriate Rh-P and P-P coupling constants.
Related species have been isolated in previous work, and
their solid state structures confirmed by single-crystal
X-ray diffraction.47,50
(30) Khan, M. S.; Kakkar, A. K.; Long, N. J .; Lewis, J .; Raithby, P.;
Nguyen, P.; Marder, T. B.; Wittmann, F.; Friend, R. H. J . Mater. Chem.
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627, 693.
(35) Stang, P. J .; Tykwinski, R. J . Am. Chem. Soc. 1992, 114, 4411.
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tallics 2001, 20, 2425.
When a second equivalent of the same alkyne was
added at low temperature (-40 °C), a new species was
observed in the 1H and 31P NMR spectra. The new
species had two distinct phosphine ligand environments,
in a 2:1 ratio, and contained a hydride. Thus for R )
-C6H4-4-OMe the hydride signal at -9.42 ppm is split
into a doublet with a large (196 Hz) coupling and then
further split into two apparent quartets with a smaller
(18 Hz) coupling. Assigning the large coupling to a trans
phosphorus and the smaller couplings to two cis phos-
(39) Bruce, M. I.; Hall, B. C.; Kelly, B. D.; Low, P. J .; Skelton, B.
W.; White, A. H. J . Chem. Soc., Dalton Trans. 1999, 3719.
(40) Bruce, M. I.; Low, P. J .; Costaus, K.; Halet, J .-F.; Best, S. P.;
Heath, G. A. J . Am. Chem. Soc. 2000, 122, 1949.
(41) Sun, Y.; Taylor, N. J .; Carty, A. J . Organometallics 1992, 11,
4293.
1
phoruses and the rhodium (2J H-P ) J H-Rh) allows us
to assign the geometry of this new species as fac-
[(PMe3)3Rh(-CtC-R)2H], as we know on the basis of
31P chemical shifts that it is not the previously fully
characterized mer,trans-[(PMe3)3Rh(-CtC-R)2H], for
which the hydride resonates at -9.17 ppm. When the
temperature of the reaction was maintained below -20
°C, this fac species was the only one that was observed
to form, albeit slowly (i.e., after one week at -35 °C,
less than 25% of the starting [(PMe3)4Rh(-CtC-R)]
was converted to this fac compound). When the tem-
perature of the reaction mixture was increased to ca. 0
°C, significant quantities of mer,trans-[(PMe3)3Rh(-Ct
C-R)2H] were also observed, and eventually this be-
came the only species present. This indicates that the
fac isomer is the kinetic product of the reaction, with
the mer,trans isomer being the thermodynamic product
(42) Meyer, W. E.; Amoroso, A. J .; Horn, C. R.; J aeger, M.; Gladysz,
J . A. Organometallics 2001, 20, 1115.
(43) Bartik, T.; Weng, W.; Ramsden, J . A.; Szafert, S.; Fallon, S. B.;
Arif, A. M.; Gladysz, J . A. J . Am. Chem. Soc. 1998, 120, 11071.
(44) Younus, M.; Long, N. J .; Raithby, P. R.; Lewis, J .; Page, N. A.;
White, A. J . P.; Williams, D. J .; Colbert, M. C. B.; Hodge, A. J .; Khan,
M. S.; Parker, D. G. J . Organomet. Chem. 1999, 578, 198.
(45) Osakada, K.; Hamada, M.; Yamamoto, T. Organometallics 2000,
19, 458.
(46) Fyfe, H. B.; Mlekuz, M.; Stringer, G.; Taylor, N. J .; Marder, T.
B. In Inorganic and Organometallic Polymers with Special Properties;
Laine, R. M., Ed.; NATO ASI Series E; 1992; Vol. 206, p 331.
(47) Fyfe, H. B.; Mlekuz, M.; Zargarian, D.; Taylor, N. J .; Marder,
T. B. J . Chem. Soc., Chem. Commun. 1991, 188.
(48) Davies, S. J .; J ohnson, B. F.; Khan, M. S.; Lewis, J . J . Chem.
Soc., Chem. Commun. 1991, 187.
(49) Chow, P.; Zargarian, D.; Taylor, N. J .; Marder, T. B. J . Chem.
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Soc., Chem. Commun. 1989, 540.

Mechanism of Formation of [(PMe3)3Rh(-CtC-R)2(H)]
Organometallics, Vol. 21, No. 2, 2002 431
Sch em e 2
Sch em e 3
rates of scrambling between the two samples was
apparent, and after 15 days ca. 10% of the rhodium
species contain the TMS group. Interestingly, the major
new species contained two TMS groups, not one (Scheme
3). When 3 equiv of PEt3 was added at room tempera-
ture to a solution of mer,trans-[(PMe3)3Rh(-CtC-R)2H]
(R ) p-methoxyphenyl), scrambling of the phosphines
also occurred. The rate of phosphine scrambling was
comparable with that of acetylene scrambling, with little
evidence of scrambling after a week and noticeable
quantities after 15 days. While the species were not
isolated, it is obvious from the NMR spectra that PEt3
incorporates into both phosphine sites, with multiple
incorporation also being observed.
Equimolar quantities of the two symmetric mer,trans-
[(PMe3)3Rh(-CtC-R)2H] (R ) trimethylsilyl and R )
p-methoxyphenyl) compounds were mixed in toluene. At
room temperature, no exchange was observed, even
after one week. However, on heating the solution to 100
°C for 7 h, complete scrambling was observed, with the
mixed alkynyl complex accounting for roughly 50% of
the sample.
(Scheme 2). When the second equivalent of alkyne was
added at room temperature, all of the [(PMe3)4Rh(-Ct
C-R)] was converted smoothly to mer,trans-[(PMe3)3-
Rh(-CtC-R)2H] with no other compounds visible in
the 1H and 31P NMR spectra. That the fac isomer should
be unstable with respect to the mer,trans is not surpris-
ing: simple arguments based on steric congestion
clearly indicate a preference for the mer,trans isomer.
DFT modeling studies (vide infra) allow us to quantify
the relative energies of the isomers: the mer,trans
isomer of [(PMe3)3Rh(-CtC-Me)2H] is some 21 kJ mol-1
more stable than the fac, which is in turn 9 kJ mol-1
more stable than the only other possibility, the mer,cis
isomer.
When a second equivalent of a different alkyne was
added to [(PMe3)4Rh(-CtC-R)] at room temperature,
all three possible mer,trans-hydrido-bis(alkynyl) com-
pounds were formed in statistical proportions. In an-
other experiment, 2 equiv of p-methoxyphenylethyne
was added to [(PMe3)4RhMe] at -78 °C and the sample
was allowed to warm to -35 °C, whereupon only
[(PMe3)4Rh(-CtC-C6H4-OMe)] and fac-[(PMe3)3Rh-
(-CtC-C6H4-OMe)2H] were observed. When 16 equiv
of TMSA was added to this mixture a new set of signals
corresponding to the mixed fac compound was observed.
On cooling to -78 °C to remove the appearance of
fluxionality, signals corresponding to both [(PMe3)4Rh-
(-CtC-R)] (R ) TMS, R ) p-methoxyphenyl) com-
pounds were observed. When this sample was allowed
to warm to room temperature and react completely, the
rhodium-containing species were dominated by mer,-
trans-[(PMe3)3Rh(-CtC-TMS)2H], which accounted for
some 80% of the sample. Thus, most of the -CtC-
C6H4-OMe groups that had been bound to rhodium had
been replaced by -CtC-TMS moieties.
A preference for the nature of the alkynyl group has
also been observed. For example, when a mixture of 2
equiv of each of p-methoxyphenylethyne and p-cyano-
phenylethyne was added to [(PMe3)4RhMe] at room
temperature, a statistical mixture did not result: more
than 90% of the product was mer,trans-[(PMe3)3Rh-
(-CtC-C6H4-CN)2H].
Mech a n istic Im p lica tion s. Considering the fact
that the hydride ligand in the mer,trans-[(PMe3)3Rh-
(-CtC-R)(-CtC-R′)H] lies mutually cis to both alky-
nyl ligands, a simple mechanism that would explain
alkynyl scrambling would be one in which reductive
elimination of either H-CtCR or H-CtCR′ was facile.
This would provide both alkynes as well as both Rh(I)
alkynyls species in solution. However, it is apparent
from the experiments performed on isolated pure samples
that mer,trans-[(PMe3)3Rh(-CtC-R)2H] complexes are
not very labile at room temperature and that any
scrambling of alkynyl ligands that takes place during
the attempted synthesis of the mixed compounds must
involve intermediates rather than the final product.
Conversely, it is also apparent that fac-[(PMe3)3Rh-
(-CtC-R)2H] complexes are labile, even at -35 °C.
Before we address this issue however, we first con-
sider the formation of [(PMe3)4Rh(-CtC-R)] and its
observed lability (alkyne exchange observed even at -35
°C). The reaction of [(PMe3)4RhMe] with alkynes is
remarkably fast: complete in time of mixing at -78 °C.
To investigate further the phenomenon of alkyne
exchange, a number of additional experiments were
carried out. Thus, when 1 equiv of TMSA was added at
room temperature to a solution of mer,trans-[(PMe3)3-
Rh(-CtC-R)2H] (R ) p-methoxyphenyl), both in the
presence of 1 equiv of added PMe3 and with no added
phosphine present, no scrambling of the alkynyls in the
rhodium species was visible in the NMR spectra until
at least a week later. No significant difference in the

432 Organometallics, Vol. 21, No. 2, 2002
Rourke et al.
Sch em e 4
Sch em e 6
Sch em e 7
Sch em e 5
elimination of methane is a factor. Both of the proposed
mechanisms outlined in Schemes 4 and 5 allow for
alkyne scrambling.
That a compound such as [(PMe3)4Rh(-CtC-R)] can
dissociate a phosphine ligand and π-coordinate another
alkyne is not in doubt. Thus, when 1 equiv of 4-CF3-
C6H4-CtC-4-C6H4-CF3 was added to [(PMe3)4Rh-
(-CtC-R)] at room temperature, a new set of reso-
nances appeared in both the 19F and 31P NMR consistent
with the formation of [(PMe3)3Rh(-CtC-TMS)(η2-
CF3-C6H4-CtC-C6H4-CF3)], though this new species
represented less than 10% of the sample. Repeatedly
removing the solvent in vacuo and redissolving the
sample (so as to remove the volatile PMe3 byproduct)
led to the isolation of pure [(PMe3)3Rh(-CtC-TMS)-
(η2-CF3-C6H4-CtC-C6H4-CF3)] (Scheme 7). While we
have been unable to obtain single crystals of this
particular complex suitable for X-ray diffraction, 1H, 19F,
and 31P NMR, IR, and elemental analysis data support
our proposed formulation. The IR stretching frequency
of 1750 cm-1 in addition to that of 2009 cm-1 indicates
an η2-bound alkyne as well as the σ-bonded alkynyl
ligand. We51 and others52 have synthesized related
species with η2-bound alkenes. In these examples, the
π-bound CdC moiety lies in the equatorial plane of the
distorted, but formally trigonal bipyramidal, structure,
as would be expected for a π-acceptor bound to a d8-
ML4 center.53 The 19F NMR evidence suggests that in
the present case both the σ-bound alkynyl group and
Given the fluxionality of the five-coordinate, 18-electron
[(PMe3)4RhMe] on the NMR time scale at temperatures
down to -60 °C and the fact that phosphine scrambling
is known to take place via both intramolecular (pseudo-
rotation) and dissociative mechanisms, the possibility
of π-coordination of an alkyne followed by an oxidative
addition and then a reductive elimination of methane
arises as a reaction route (Scheme 4). Alternatively a
concerted σ-bond metathesis reaction such as that
depicted in Scheme 5 would lead directly to [(PMe3)4-
Rh(-CtC-R)] and methane without phosphine dis-
sociation. The dependence of the rate of this process on
added phosphine has not been examined. A related
process involving formal protonation of the electron-rich
[(PMe3)4RhMe], either at Rh or directly at the Rh-C
bond by the relatively acidic sp tC-H moiety, reductive
elimination of methane, and collapse of the ion pair,
Scheme 6, cannot be ruled out. Once formed, [(PMe3)4-
Rh(-CtC-R)] is observed to exchange alkynyl groups
with free alkyne, though this exchange is not nearly as
fast as the replacement of Me, where, presumably, the
(51) Chow, P.; Taylor, N. J .; Marder, T. B. Unpublished results.
(52) Fine, S. M.; Andersen, R. A.; Muetterties, E. L. Unpublished
results.
(53) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365.

Mechanism of Formation of [(PMe3)3Rh(-CtC-R)2(H)]
Sch em e 8
Organometallics, Vol. 21, No. 2, 2002 433
Ta ble 1. Com p a r ison betw een Obser ved a n d
Ca lcu la ted Bon d Dista n ces (Å) a n d An gles (d eg) in
th e Str u ctu r e of
the η2-bound alkyne are equatorial, as resonances for
two different CF3 groups were observed in a 1:1 ratio.
This arrangement is the same as that found for [(PMe3)3-
Rh(-C6H4-4-CH3)(η2-CF2dCF2)] by Andersen et al.52
A compound such as [(PMe3)3Rh(-CtC-TMS)(η2-H-
CtC-TMS)] can clearly be considered a viable inter-
mediate in the synthesis of our rhodium(III) species, as
it is one oxidative addition step away from it. The
geometry of this intermediate becomes very important;
thus, the fac isomer must arise from the intermediate
with the η2-alkyne in an equatorial and the alkynyl in
an axial site. The three possible geometries of the
intermediate and their resultant products, together with
their ground state energies (R ) Me), as calculated by
DFT, are illustrated in Scheme 8. The relative energies
of the points in Scheme 8 are based on solvated species,
using parameters for THF (the solvent used experimen-
tally). A preliminary comparison of the DFT calculated
structure of the mer,trans-Rh(III) hydride and the
experimentally determined geometry was favorable
(Table 1): the calculated bond lengths are slightly
longer than the observed ones, except for the Rh-H
contact, which is apparently too short by 0.3 Å; however,
this is more likely a reflection of a large experimental
uncertainty in the Rh-H distance. Overall, the chosen
combination of basis set and functional appears ad-
equate, and we are confident that the calculated ground
state energies are reliable.
m er ,tr a n s-[(P Me3)3Rh (-CtC-R)2(H)]
bond length
obs
calc
bond angle
obs
calc
Rh-P(1)
Rh-P(2)
Rh-P(3)
Rh-C(1)
Rh-C(9)
Rh-H(1)
C(1)-C(2)
2.297(1) 2.35 P(1)-Rh-P(2)
2.298(1) 2.34 P(1)-Rh-P(3)
2.348(1) 2.43 P(1)-Rh-C(1)
2.019(4) 2.06 P(1)-Rh-C(9)
2.031(4) 2.06 P(2)-Rh-P(3)
161.43(3) 156.4
98.26(4) 105.0
85.8(1)
92.0(1)
90.1
90.4
99.61(4) 98.5
1.90
1.60 P(2)-Rh-C(1)
88.3(1)
92.7(1)
93.0(1)
87.9(1)
86.5
93.2
93.5
86.0
1.204(6) 1.23 P(2)-Rh-C(9)
C(9)-C(10) 1.213(6) 1.23 P(3)-Rh-C(1)
P(3)-Rh-C(9)
C(1)-Rh-C(9) 178.6(2) 179.4
Rh-C(1)-C(2) 173.9(2) 178.1
Rh-C(9)-C(10) 178.4(2) 177.5
Observed data refer to R ) Ph,49 calculated data refer to R )
Me.
F igu r e 1. Model of the intermediate that would lead to
the mer,trans compound. The methyl groups on the phos-
phines and all hydrogens are omitted for clarity.
The DFT calculations show that the ground state
energies of the potential Rh(I) intermediates leading to
the fac and the mer,trans complexes are the same,
within experimental error, with the energy of the other
intermediate being some 7 kJ mol-1 higher. The geom-
etries that we arrive at for the intermediates are all
distorted away from a pure trigonal bipyramidal struc-
ture and could even be considered as being closer to a
distorted square-based pyramid. Figure 1 shows a
perspective view of the calculated structure of the
intermediate leading to the mer,trans product. The rate
of interconversion of such five-coordinate species is
known to be rapid,54 and isomerization could easily take

434 Organometallics, Vol. 21, No. 2, 2002
Rourke et al.
Sch em e 9
place at the temperature of our reaction, potentially
leading to any of the three possible Rh(III) species.
The calculated energy for mer,cis-[(PMe3)3Rh(-CtC-
R)2H] is higher than that of the lowest intermediates,
suggesting that such a product would be thermodynami-
cally unstable with respect to the intermediates, in
keeping with the absence of any experimental evidence
for its formation. In contrast, the calculated energy for
the fac and mer,trans isomers are, respectively, some 5
and 26 kJ mol-1 lower in energy than the intermediates.
That we observed fac-[(PMe3)3Rh(-CtC-R)2H] as the
kinetic product of the reaction implies that there must
exist a lower energy transition state leading to fac than
there is leading to mer,trans.
Isomerization of the fac isomer to the mer,trans could
conceivably take place via a direct rotation of 120° of a
trigonal (PMe3)2(-CtC-R) face of the octahedron or by
C-H reductive elimination back to the Rh(I) π-complex
followed by axial to equatorial isomerization of the
alkynyl ligand and oxidative addition of C-H. This
second scenario seems more likely, given the known
high barriers to rotation at 18-electron octahedral
compounds and the relatively low temperature (-20 °C)
at which our isomerization occurs. A reductive elimina-
tion of the fac isomer back to the alkyne/alkynyl
intermediate also provides a potential route for alkyne
scrambling, as the intermediate would be in equilibrium
with [(PMe3)4Rh(-CtC-R)]. The relatively slow rate
of scrambling exhibited by the mer,trans in comparison
with the fac isomer could be ascribed simply to its
greater thermodynamic stability with respect to the
alkyne/alkynyl intermediate, but even more important
is the high-energy transition state which must connect
the mer,trans Rh(III) and the (η2-alkyne)Rh(I) species.
While we have no definitive evidence that the mer,-
trans (or, indeed, the fac) isomer does undergo a
reductive elimination back to the alkyne/alkynyl inter-
mediate, we do have some evidence that lends support
to this suggestion. When a free alkyne was added to a
sample of pure mer,trans-[(PMe3)3Rh(-CtC-R)2H] con-
taining a different alkynyl, very slow exchange was
observed, with the major product having had both
alkynyls exchanged (Scheme 3). This suggests that once
the mer,trans product is in a position to exchange one
alkyne, it can then exchange the second one more easily,
and given the relative concentrations of the free alkynes,
this leads to double exchange in the early stages of the
reaction (before equilibrium concentrations are achieved).
The fact that phosphine exchange takes place at a rate
at least qualitatively similar to that of alkynyl exchange,
yet added phosphine does not inhibit alkynyl exchange,
suggests that phosphine dissociation from mer,trans-
[(PMe3)3Rh(-CtC-R)2H] is not a key step in the
alkynyl exchange process. The data, when taken to-
gether, suggest that alkynyl scrambling in the mer,trans
complexes takes place through the [(PMe3)3Rh(-CtC-
R)] species formed by C-H reductive elimination. This
16-electron Rh(I) complex can undergo associative phos-
phine exchange as well as associative reaction with
terminal alkyne.
in which order. Thus, one report noted the precipitation
of a compound with formulation [(PMe3)3Co(-CtC-
C6H5)2H], but was unable to determine whether the
compound was the fac or the mer,trans isomer (though
they were able, on the basis of 31P coupling to the
hydride resonance in the 1H NMR, to eliminate the
possibility of mer,cis).55 A later paper was able to
confirm the structure of this compound as mer,trans-
[(PMe3)3Co(-CtC-C6H5)2H].56 This paper also reported
some mechanistic investigations into the formation of
the mer,trans compound, with an observed ν(CtC)
absorption at 1717 cm-1 in the IR suggesting that
phenylethyne π-coordinates to the metal in an initial
step. However, the suggestion that there were two
subsequent intermediates (mer,cis and then fac) in the
synthesis of the mer,trans isomer of [(PMe3)3Co(-Ct
C-C6H5)2H] was based solely on the observation of IR
bands in the solid state, evidence that cannot be
considered definitive. It is possible that the mer,cis
isomer may be accessible energetically for cobalt, but
our experimental and theoretical results indicate that
it is very high in energy for rhodium and is thus not
observed.
F or m a tion of Ch lor id es. One further reaction of the
mer,trans-[(PMe3)3Rh(-CtC-R)2H] species was ob-
served: dissolution in chloroform exchanged H for Cl
cleanly forming mer,trans-[(PMe3)3Rh(-CtC-R)2Cl],
with complete conversion in 16 h at room temperature
(Scheme 9). The chloride-containing product has been
fully characterized: NMR, IR, and elemental analysis
are consistent with this formulation, which has been
confirmed by single-crystal X-ray diffraction. When the
reaction was followed by 1H NMR in deuterated chloro-
form it was clear that 1 equiv of CHDCl2 was formed
as a byproduct. Such a reaction has many precedents.57
Once formed, mer,trans-[(PMe3)3Rh(-CtC-R)2Cl]
isomerizes slowly to mer,cis-[(PMe3)3Rh(-CtC-R)2Cl],
with complete isomerization taking about 5 weeks at
room temperature (Scheme 9). While a suitable single
crystal for X-ray analysis of the mer,cis isomer has not
yet been obtained, the NMR spectra are definitive, in
that the absence of a relatively large 1J (RhIII-P)
Coba lt An a logu es. Earlier reports on the cobalt
analogues of our rhodium species have also attempted
to deal with the question of which isomers formed and
(55) Habadie, N.; Dartiguenave, M.; Dartiguenave, Y.; Britten, J .
F.; Beauchamp, A. L. Organometallics 1989, 8, 2564.
(56) Klein, H.-F.; Beck, H.; Hammerschmitt, B.; Koch, U.; Koppert,
D.; Cordier, G.; Paulus, H. Z. Naturforsch. 1991, 46b, 147.
(57) Green, M. L. H.; J ones, D. J . Adv. Inorg. Radiochem. 1965, 7,
115.
(54) Shaply, J . R.; Osborn, J . A. Acc. Chem. Res. 1973, 6, 305.

Mechanism of Formation of [(PMe3)3Rh(-CtC-R)2(H)]
Organometallics, Vol. 21, No. 2, 2002 435
F igu r e 3. Crystal structure of mer,trans-[(PMe3)3Rh(-Ct
C-C6H4-4-OMe)2Cl] showing the molecules in infinite
ladder-like chains along the 101 direction.
with angles about C(2), C(1), Rh(1), C(10), and C(11)
being 175.03(15)°, 178.40(13)°, 178.63(6)°, 177.58(12)°,
and 176.41(15)°, respectively.
F igu r e 2. Molecular structure of mer,trans-[(PMe3)3Rh-
(-CtC-C6H4-4-OMe)2Cl] with ellipsoids shown at 50%
probability.
The planes of the two phenyl rings are rotated with
respect to each other by 19.3°. In crystals of mer,trans-
[(PMe3)3Rh(-CtC-R)2Cl], the terminal MeO-C6H4-
groups of adjacent molecules (related by the inversion
centers) are overlapping, with mean interplanar spac-
ings of 3.370 and 3.400 Å between the planes of parallel
Ph-rings. These interactions combine molecules in
infinite ladder-like chains along the 101 direction
(Figure 3).
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) in th e Cr ysta l Str u ctu r e of
m er ,tr a n s-[(P Me3)3Rh (-CtC-C6H4-4-OMe)2(Cl)]
Rh(1)-C(1)
Rh(1)-C(10)
Rh(1)-Cl(1)
C(1)-C(2)
2.035(1)
2.037(1)
2.4493(4)
1.214(2)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-P(3)
C(10)-C(11)
2.3475(4)
2.3436(4)
2.2506(4)
1.207(2)
C(1)-Rh(1)-C(10) 178.63(6)
C(1)-Rh(1)-Cl(1)
C(1)-Rh(1)-P(2)
C(10)-Rh(1)-Cl(1)
C(10)-Rh(1)-P(2)
P(1)-Rh(1)-P(2)
95.17(4)
83.69(4)
86.05(4)
C(1)-Rh(1)-P(1)
C(1)-Rh(1)-P(3)
C(10)-Rh(1)-P(1)
C(10)-Rh(1)-P(3)
P(1)-Rh(1)-P(3)
Cl(1)-Rh(1)-P(1)
Cl(1)-Rh(1)-P(3)
C(1)-C(2)-C(3)
84.32(4)
96.63(4)
95.19(4)
82.16(4)
97.01(4)
Con clu sion s
164.429(14)
94.425(14)
86.457(14)
178.40(13)
96.790(13) P(2)-Rh(1)-P(3)
84.772(13) Cl(1)-Rh(1)-P(2)
168.200(14) Rh(1)-C(1)-C(2)
We have clearly shown that it is not possible to
prepare cleanly the desired unsymmetric mer,trans
hydrido-bis(alkynyl)rhodium compounds in the absence
of their symmetric counterparts via the direct C-H
activation approach. Likewise, the synthesis of well-
defined alternating or block polymers employing differ-
ent dialkynyl linkers would be unfeasible via this route.
While the mer,trans-[(PMe3)3Rh(-CtC-R)2H] com-
plexes, once formed, are configurationally stable and not
prone to scrambling at ambient temperature, they are
synthesized via a route that involves nonconfiguration-
ally stable species which are prone to scrambling. In
addition, the barrier to scrambling of the intermediates
is lower than the activation energy required to make
the final compounds. The facile and high-yield synthesis
of two isomers of [(PMe3)3Rh(-CtC-R)2Cl] may allow
the preparation of novel tris(alkynyl) complexes via the
use of M′-CtC-R′ reagents, and such studies are in
progress.
175.03(15)
Rh(1)-C(10)-C(11) 177.58(12)
C(6)-O(1)-C(9) 116.47(12)
C(10)-C(11)-C(12) 176.41(15)
C(15)-O(2)-C(18) 117.64(11)
coupling constant in the 31P NMR spectrum (cf. 118 Hz
in the mer,trans isomer) allows us to rule out any
structure with a P trans to a Cl, thus leaving the mer,-
cis isomer as the only possibility. IR data confirms the
presence of two different CtC stretching modes, con-
sistent with a cis disposition of the two alkynyls, and
elemental analysis confirms the overall composition.
Crystals of mer,trans-[(PMe3)3Rh(-CtC-R)2Cl] ob-
tained by slow evaporation of CDCl3 confirm the octa-
hedral geometry and the mer,trans-arrangement of the
ligands (Figure 2) suggested by the 31P NMR data. Thus,
the hydride ligand in the precursor compound is re-
placed by chloride with retention of the stereochemistry
about the Rh center. The molecular structure is similar
to that of mer,trans-[(PMe3)3Rh(-CtC-Ph)2H] reported
previously,50 and the symmetry about the Rh center is
very close to C2v. The two Rh-C distances (Table 2) of
2.035(1) and 2.037(1) Å are identical within experimen-
tal error and are similar to those of 2.019(4) and 2.031-
(4) Å in the hydride precursor. The Rh(1)-P(1) and
Rh(2)-P(2) distances of 2.3475(4) and 2.3436(4) Å for
the mutually trans PMe3 ligands are slightly longer
than the related distances of 2.297(1) and 2.298(1) Å in
the hydride analogue, perhaps because the Cl ligand
occupies more space than an H ligand, whereas the
distance for the Rh-P bond trans to the chloride (Rh-
(1)-P(3)) of 2.2506(4) Å is much shorter than that trans
to the hydride (2.348(1) Å) in the hydride analogue,
consistent with the relative trans influence of Cl vs H.
The angles about Rh are close to those expected for
octahedral coordination, with the largest deviation being
the P(1)-Rh(1)-P(2) angle of 164.429(14)° for the
mutually trans PMe3 ligands. This is a result of P(1)
and P(2) being displaced slightly toward C(1), whereas
the unique P(3) leans slightly toward C(2), relieving
steric interactions between pairs of mutually cis PMe3
groups. The alkynyl groups form a nearly linear chain
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All reagents were used as sup-
plied, unless noted otherwise; solvents were distilled under
nitrogen from appropriate drying agents. NMR spectra were
obtained on either Bruker AC 300, Varian Mercury 200, or
Mercury 300 spectrometers and are referenced to external
TMS (1H), CFCl3 (19F), and 85% H3PO4 (31P). IR spectra were
recorded on solid samples on a Perkin-Elmer Paragon 1600
FTIR spectrophotometer. Elemental analyses were performed
by MWH Laboratories, Warwick Analytical Services, and in
the Department of Chemistry, University of Durham. Reac-
tions were carried out, and NMR samples were prepared in
Innovative Technology Inc. gloveboxes under an atmosphere
of dry nitrogen. The [(PMe3)4RhMe] was prepared via a
modification of a literature route58 (MeLi was used instead of
MeMgI).
Syn t h esis of m er ,tr a n s-[(P Me3)3R h (-CtC-C6H 4-4-
OMe)2H]. A solution of p-methoxyphenylethyne (12.5 mg,
0.095 mmol) in THF (5 cm3) was added to a solution of [(PMe3)4-
RhMe] (20 mg, 0.047 mmol) in THF (3 cm3), and the mixture
was stirred for 16 h under a N2 atmosphere. The solvent was
removed in vacuo and the product crystallized from THF/
hexane (yield: 24 mg, 86%). NMR (C6D6): 31P{1H} (80.96
MHz), δ -5.92 (2P, dd, 1J Rh-P ) 93 Hz, 2J P-P ) 27 Hz, P trans

436 Organometallics, Vol. 21, No. 2, 2002
Rourke et al.
Ta ble 3. Cr ysta l Da ta , Da ta Collection , a n d
Refin em en t for
m er ,tr a n s-[(P Me3)3Rh (-CtC-C6H4-4-OMe)2(Cl)]
1
2
to P), -23.43 (1P, dt, J Rh-P ) 76 Hz, J P-P ) 27 Hz, P trans
to H); 1H (200.1 MHz), δ 7.42 (4H, (AB)′, aromatic), 6.76 (4H,
(AB)′, aromatic), 3.27 (6H, s, OMe), 1.42 (18H, vt, J ) 4 Hz,
PMe3 trans to PMe3), 1.20 (9H, d, J ) 8 Hz, PMe3 trans to H),
emp formula
fw
C27H41ClO2P3Rh
628.87
1
2
2
-9.17 (1H, dq, J Rh-H ) J Pcis-H ) 17 Hz, J Ptrans-H ) 195 Hz,
Rh-H). Anal. Found (C27H42O2P3Rh requires): C 54.43 (54.55),
H 7.23 (7.12). IR (solid state): ν(CtC) ) 2088 cm-1, ν(Rh-H)
temp
120.0(2) K
0.71073
monoclinic
P21/c
18.5752(6)
11.7546(4)
13.8703(5)
95.71(1)
λ(Mo KR), Å
cryst syst
space group
a, Å
b, Å
c, Å
) 1943 cm-1
.
Syn th esis of m er ,tr a n s-[(P Me3)3Rh (-CtC-SiMe3)2H].
Ethynyltrimethylsilane (9 mg, 0.09 mmol) was added to a
solution of [(PMe3)4RhMe] (20 mg, 0.047 mmol) in THF (8 cm3),
and the mixture was stirred for 16 h under a N2 atmosphere.
The solvent was removed in vacuo and the product crystallized
from THF/hexane (yield: 24 mg, 95%). NMR (C6D6): 31P{1H}
â, deg
V, Å3
3013.5(2)
Z
D
4
calc, g cm-3
1.386
0.836
1304
1
2
(80.96 MHz), δ -7.75 (2 P, dd, J Rh-P ) 94 Hz, J P-P ) 27 Hz,
µ, mm-1
1
2
P trans to P), -25.37 (1 P, dt, J Rh-P ) 77 Hz, J P-P ) 27 Hz,
P trans to H); 1H (200.1 MHz), δ 1.40 (18 H, vt, J ) 4 Hz,
PMe3 trans to PMe3), 1.18 (9 H, d, J ) 8 Hz, PMe3 trans to H),
F(000)
cryst size, mm
2θ range, deg
index ranges
0.72 × 0.44 × 0.34
2.20-60.72
-24 e h e 25, -16 e k e 15,
-19 e l e 19
34 012
8280 [R(int) ) 0.0484]
471
0.7642 and 0.5843
0.0236
0.0588
1
0.31 (18 H, s, TMS), -9.29 (1H, dq, J Rh-H ) 2J Pcis-H ) 17 Hz,
2J Ptrans-H ) 192 Hz, Rh-H). Anal. Found (C19H46Si2P3Rh
requires): C 43.14 (43.34), H 8.88 (8.81). IR (solid state): ν-
no. of data collected
no. of unique data
no. of params refined
max and min transmn
R1a [I g 2σ(I)]
(CtC) ) 2022 cm-1, ν(Rh-H) ) 1944 cm-1
.
Syn t h esis of m er ,tr a n s-[(P Me3)3R h (-CtC-C6H 4-4-
CN)2H]. A solution of p-cyanophenylethyne (12 mg, 0.095
mmol) in THF (5 cm3) was added to a solution of [(PMe3)4-
RhMe] (20 mg, 0.047 mmol) in THF (3 cm3), and the mixture
was stirred for 16 h under a N2 atmosphere. The solvent was
removed in vacuo and the product recrystallized from THF/
hexane (yield: 23 mg, 82%). NMR (C6D6): 31P{1H} (80.96
MHz), δ -6.69 (2 P, dd, 1J Rh-P ) 92 Hz, 2J P-P ) 26 Hz, P trans
wR2b (all data)
GoFc on F2
1.088
largest diff peak and
0.695 and -0.458
hole, e Å-3
R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
.
a
2
c GOF ) {∑[w(Fo - Fc2)2]/(n - p)}1/2, where n is the number of
1
2
to P), -24.27 (1 P, dt, J Rh-P ) 76 Hz, J P-P ) 26 Hz, P trans
to H); 1H (200.1 MHz), δ 7.06 (8H, s, aromatic), 1.29 (18 H, vt,
J ) 3 Hz, PMe3 trans to PMe3), 1.04 (9 H, d, J ) 7 Hz, PMe3
trans to H), -9.24 (1 H, dq, 1J Rh-H ) 2J Pcis-H ) 16 Hz, 2J Ptrans-H
) 192 Hz, Rh-H). Anal. Found (C27H36N2P3Rh requires): C
55.19 (55.49), H 6.19 (6.21), N 4.81 (4.79). IR (solid state): ν-
(CtN) ) 2219 cm-1, ν(CtC) ) 2080 cm-1, ν(Rh-H) ) 1944
reflections and p is the number of refined parameters.
1
2
) 30 Hz, P trans to P), -17.74 (1 P, dt, J Rh-P ) 83 Hz, J P-P
) 30 Hz, P trans to CtC); 1H (200.1 MHz), δ 7.28 (2 H, (AB)′),
7.12 (2 H, (AB)′), 6.73 (4 H, (AB)′), 3.77 (6 H, s, OMe), 1.72 (18
H, vt, J ) 4 Hz, PMe3 trans to PMe3), 1.54 (9 H, d, J ) 9 Hz,
PMe3 trans to CtC). Anal. Found (C27H41O2P3ClRh requires):
C 51.47 (51.57), H 6.37 (6.57). IR (solid state): ν(CtC) ) 2120,
cm-1
.
2114 cm-1
.
Syn t h esis of m er ,tr a n s-[(P Me3)3R h (-CtC-C6H 4-4-
OMe)2Cl]. mer,trans-[(PMe3)3Rh(-CtC-C6H4-4-OMe)2H] (20
mg, 0.034 mmol) was dissolved in CHCl3 and left to stand for
16 h. The solvent was removed in vacuo, and the product was
recrystallized from THF (yield: 19 mg, 92%). NMR (CDCl3):
Sp ectr oscop ic Da ta for [(P Me3)4Rh (-CtC-C6H4-4-
OMe)]. Compound only observed in solution, not isolated.
NMR ([2H8]THF): 31P{1H} (80.96 MHz, 180 K), δ 0.55 (1 P,
1
2
dq, J Rh-P ) 107 Hz, J P-P ) 44 Hz, axial P trans to CtC),
1
2
31P{1H} (80.96 MHz), δ 11.18 (1 P, dt, J Rh-P ) 118 Hz, J P-P
1
2
-24.94 (3 P, dd, J Rh-P ) 144 Hz, J P-P ) 44 Hz, equatorial
P); 1H (200.1 MHz, 280 K), δ 7.5 (2 H, (AB)′, aromatic), 6.9 (2
H, (AB)′, aromatic), 3.9 (3 H, s, OMe), 1.2-1.8 (36 H, br m,
PMe3).
1
2
) 30 Hz, P trans to Cl), -6.74 (2 P, dd, J Rh-P ) 85 Hz, J P-P
1
) 30 Hz, P trans to P); H (200.1 MHz), δ 7.18 (4 H, (AB)′),
6.73 (4 H, (AB)′), 3.77 (6 H, s, OMe), 1.77 (18 H, vt, J ) 4 Hz,
PMe3 trans to PMe3), 1.72 (9 H, d, J ) 9 Hz, PMe3 trans to
Cl). Anal. Found (C27H41O2P3ClRh requires): C 51.70 (51.57),
H 6.32 (6.57), Cl 5.61 (5.57). IR (solid state): ν(CtC) ) 2108
Sp ect r oscop ic Da t a for [(P Me3)4R h (-CtC-SiMe3)].
Compound only observed in solution, not isolated. NMR ([2H8]-
THF): 31P{1H} (80.96 MHz, 180 K), δ -0.60 (1 P, dq, 1J Rh-P
)
cm-1
.
2
107 Hz, J P-P ) 45 Hz, axial P trans to CtC), -25.00 (3 P,
1
2
X-r a y Str u ctu r a l An a lysis. A fawn crystal (0.72 × 0.44 ×
0.34 mm3), obtained by slow evaporation of a CDCl3 solution,
was used for the X-ray structure determination. Data were
collected at 120.0(2) K on a Bruker SMART CCD 1K diffrac-
tometer equipped with an Oxford Cryostream low-temperature
attachment using graphite-monochromated Mo KR radiation.
Crystal data, data collection, and refinement parameters are
given in Table 3. The structure was solved by direct methods
and refined by full-matrix least squares against F2. All non-
hydrogen atoms were refined anisotropically; H atoms were
located on the difference Fourier maps and refined isotropi-
cally. Final wR2(F2) ) 0.0588 for all data (471 refined
parameters) and R1 ) 0.0236 for 7768 reflections with I g 2σ-
(I).
dd, J Rh-P ) 145 Hz, J P-P ) 45 Hz, equatorial P).
Sp ectr oscop ic Da ta for fa c-[(P Me3)3Rh (-CtC-C6H4-
4-OMe)2H]. Compound only observed in solution, not isolated.
NMR ([2H8]THF): 31P{1H} (80.96 MHz, 180 K), δ -13.25 (2 P,
dd, 1J Rh-P ) 86 Hz, 2J P-P ) 24 Hz, P trans to CtC), -26.97 (1
1
2
1
P, dt, J Rh-P ) 71 Hz, J P-P ) 24 Hz, P trans to H); H (200.1
MHz, 242 K), δ 7.0 (4 H, (AB)′, aromatic), 7.4 (4 H, (AB)′,
aromatic), 3.7 (6 H, s, OMe), 0.8-1.4 (27 H, br m, PMe3), -9.42
1
2
2
(1 H, dq, J Rh-H ) J Pcis-H ) 18 Hz, J Ptrans-H ) 196 Hz, Rh-
H).
Sp ect r oscop ic Da t a for fa c-[(P Me3)3R h (-CtC-Si-
Me3)2H]. Compound only observed in solution, not isolated.
NMR ([2H8]THF): 31P{1H} (80.96 MHz, 242 K), δ -15.33 (2P,
dd, 1J Rh-P ) 85 Hz, 2J P-P ) 25 Hz, P trans to CC), -29.01 (1P,
1
2
Syn th esis of mer ,cis-[(P Me3)3Rh (-CtC-C6H4-4-OMe)2-
Cl]. mer,trans-[(PMe3)3Rh(-CtC-C6H4-4-OMe)2Cl (15 mg,
0.024 mmol) was dissolved in CHCl3 and left to stand for 30
days. The solvent was removed in vacuo, and the product was
recrystallized from THF (yield: 12 mg, 92%). NMR (CDCl3):
dt, J Rh-P ) 72 Hz, J P-P ) 25 Hz, P trans to H); 1H (200.1
MHz, 242 K), δ 0.8-1.4 (27 H, br m, PMe3), 0.27 (9 H, s TMS),
1
2
-9.30 (1 H, dq, J Rh-H ) 2J Pcis-H ) 18 Hz, J Ptrans-H ) 192 Hz,
Rh-H).
Syn th esis of [(P Me3)3Rh (-CtC-TMS)(η2-4-CF 3-C6H4-
CtC-C6H4-4-CF 3)]. Ethynyltrimethylsilane (5 mg, 0.047
1
2
31P{1H} (80.96 MHz), δ -8.30 (2 P, dd, J Rh-P ) 85 Hz, J P-P

Mechanism of Formation of [(PMe3)3Rh(-CtC-R)2(H)]
Organometallics, Vol. 21, No. 2, 2002 437
mmol) was added to a solution of [(PMe3)4RhMe] (20 mg, 0.047
mmol) in THF (5 cm3), and the mixture was stirred for 10 min
under a N2 atmosphere. Bis(4-trifluoromethylphenyl)ethyne
(14.9 mg, 0.047 mmol) was added, and the mixture was stirred
for 5 min. The solvent was removed in vacuo, and the mixture
redissolved in THF and then stirred for 5 min. This step was
repeated four further times (to remove the PMe3) before
collecting the product as a dark orange powder (yield: 29 mg,
92%). NMR (C6D6): 31P{1H} (80.96 MHz), δ -1.29 (2P, dd,
were treated by the frozen core approximation.67 The total
molecular electron density was fitted in each SCF cycle by
auxiliary s, p, d, f, and g STO functions. Coordinates of the
optimized structures are available as Supporting Information.
The later version of ADF (1999.02)68 contains an option for
computing solvation energies using the COSMO formulation.
Unfortunately, bugs in the COSMO routines precluded the use
of ADF1999. Instead, we used the polarized continuum model
implemented in J aguar 3.5.69 The BP functional and lacvp*
basis sets were used at the geometry previously computed
using ADF. The parameters for THF were as follows: density
) 0.8892 kg L-1, molecular weight ) 72.12 g mol-1, and a
dielectric constant of 7.58, which give a calculated probe radius
of 2.524 Å. Default SCF and geometry optimization conver-
gence criteria were used together with a fine grid for the DFT
SCF part of the calculation.
2
1
1J Rh-P ) 95 Hz, J P-P ) 33 Hz), -23.7 (1P, dt, J Rh-P ) 115
2
Hz, J P-P ) 33 Hz); 19F{1H} (188.1 MHz), δ -62.30 (3F, s),
1
-62.37 (3F, s); H (200.1 MHz), δ 8.06 (2H, (AB)′), 7.46 (2H,
(AB)′), 7.43 (4H, s), 1.17 (9H, d, J ) 7 Hz, equatorial PMe3),
0.94 (18H, vt, J ) 3 Hz, axial PMe3), 0.43 (9H, s, TMS). Anal.
Found (C30H44P3F6RhSi requires): C 48.84 (48.52), H 6.11
(5.97). IR (solid state): ν(CtC) ) 2009 cm-1, ν(η2-CtC) ) 1750
cm-1
.
Th eor etica l Stu d ies. DFT geometry optimizations were
based on the numerical Amsterdam Density Functional (ADF)
program system version 2.3.0.59,60 The basis sets comprised set
IV for Rh and set III for all other atoms. Basis set IV is an
uncontracted triple-ú Slater-type-orbital (STO) expansion plus
an additional 5p orbital, while basis set III is a double-ú STO
plus polarization expansion. All ADF calculations were based
on the uniform electron gas local density approximation61
(LDA) and analytical energy gradients.62 The LDA correlation
energy was computed according to Vosko, Wilk, and Nusair’s63
parametrization of electron gas data. Gradient-corrected (GC)
functionals for the LDA exchange and correlation terms
employed the formulations of Becke64 and Perdew,65,66 respec-
tively (BP). The BP functional was used for both geometry
optimization and binding energy calculations with default SCF
and geometry optimization convergence criteria. The lower core
shells on the atoms (1s on C, 1s to 2p on P, 1s to 3d on Rh)
Ack n ow led gm en t. We thank EPSRC (UK), NSERC
(Canada), and the Universities of Warwick and Durham
for financial support, and the Royal Society (London)
and NSERC for a travel grant to J .P.R. under the
Bilateral Exchange scheme. J .A.K.H. thanks EPSRC for
a Senior Research Fellowship. We also thank Professor
R. A. Andersen for a preprint of ref 52, Dr. R. L. Thomas
for samples of p-R-C6H4-CtC-H (R ) MeO, CN) and
bis(4-trifluoromethylphenyl)ethyne, and J ohnson Mat-
they for a generous loan of rhodium chloride.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
crystallographic data, atomic coordinates, interatomic dis-
tances and angles, anisotropic thermal parameters, and
hydrogen atom coordinates for mer,trans-[(PMe3)3Rh(-CtC-
C6H4-4-OMe)2Cl] (cif format) and coordinates of the DFT
optimized structures of the alkyne/alkynyl intermediates and
the Rh(III) products in Scheme 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
(58) Thorn, D. L. Organometallics 1985, 4, 192.
(59) ADF 2.3.0; Theoretical Chemistry, Free University of Amster-
dam, 1997.
(60) te Velde, G.; Baerends, E. J . J . Comput. Phys. 1992, 99, 84.
(61) Slater, J . C. Adv. Quantum Chem. 1972, 6, 1.
(62) Versluis, L.; Ziegler, T. J . Chem. Phys. 1988, 88, 322.
(63) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J . Phys. 1980, 58, 1200.
(64) Becke, A. J . Chem. Phys. 1986, 84, 4524.
OM010685R
(67) Baerends, E. J .; Ellis, D. E.; Ros, P. Theor. Chim. Acta 1972,
27, 339.
(68) ADF 1999.02; Theoretical Chemistry, Free University of Am-
sterdam, 2000.
(65) Perdew, J . P. Phys. Rev. 1986, B33, 8822.
(66) Perdew, J . P. Phys. Rev. 1987, B34, 7406.
(69) J aguar 3.5; Schrodinger Inc.: Portland, OR, 1998.