Rhodium phosphine olefin complexes of the weakly coordinating anions [BAr4F]- and [1-closo-CB11H 6Br6]-. Kinetic versus thermodynamic factors in anion coordination and complex reactivity

Article abstract of DOI:10.1021/om060975d

Solution and solid-state structures for the pair of complexes Rh{P(Cyp ₂)(η²-C₅H₇)}{η⁶- (C₆H₃(CF₃)₂)BAr₃ ^F} and Rh{P(Cyp₂)(η²-C₅H ₇)}(I-closo-CB₁₁H₆Br₆), which contain bound weakly coordinating anions, are reported. While thermochemical data show that enthalpically [I-closo-CB₁₁H₆Br ₆]^- binds less strongly with the metal fragment and it is the large entropy loss for the overall process of coordination of the [BAr ₄^F]^- anion that results in the latter anion being thermodynamically more weakly coordinating. Qualitative kinetic data arising from reaction with H₂ indicates that the carborane anion is displaced more readily, attributable to the ability of the carborane to lift a Rh-Br interaction.

Full text of DOI:10.1021/om060975d

Organometallics 2007, 26, 463-465
463
Rhodium Phosphine Olefin Complexes of the Weakly Coordinating
Anions [BArF ]- and [1-closo-CB11H6Br6]-. Kinetic versus
4
Thermodynamic Factors in Anion Coordination and Complex
Reactivity
Thomas M. Douglas, Eduardo Molinos, Simon K. Brayshaw, and Andrew S. Weller*
Department of Chemistry, UniVersity of Bath, Bath BA2 7AY, U.K.
ReceiVed October 23, 2006
Chart 1
Summary: Solution and solid-state structures for the pair of
complexes Rh{P(Cyp2)(η2-C5H7)}{η6-(C6H3(CF3)2)BArF } and
3
Rh{P(Cyp2)(η2-C5H7)}(1-closo-CB11H6Br6), which contain bound
weakly coordinating anions, are reported. While thermochemical
data show that enthalpically [1-closo-CB11H6Br6]- binds less
strongly with the metal fragment and it is the large entropy
loss for the oVerall process of coordination of the [BArF ]-
4
anion that results in the latter anion being thermodynamically
more weakly coordinating. QualitatiVe kinetic data arising from
reaction with H2 indicates that the carborane anion is displaced
more readily, attributable to the ability of the carborane to lift
a Rh-Br interaction.
The [1-closo-CB11H6Br6]-- and [BArF ]--type anions (Chart
4
1; ArF ) C6H3(CF3)2) are commonly referred to as being among
the least nucleophilic and most stable anions known.1 Com-
parisons between these classes of anions have been reported in
terms of the synthesis of free silylium cations2 and protonated
arenes,3 in relative anion basicities,4 and in catalysis when
partnered with cationic transition-metal fragments.5 The solid-
state structures of Ag+ salts of these two anions are also
known.6,7 However, systems where a direct structural and
thermochemical comparison can be made between the coordina-
tion properties of these counterions when they are partnered
with transition-metal fragments, as far as we are aware, have
not been reported. This is because, in order to do this, each
anion must cleanly form a complex in both the solution and
the solid state with the transition-metal fragment, and this is
understandably difficult to achieve, given their weakly coordi-
nating properties. Thermochemical data are well established for
early-transition-metal (metallocene) cations partnered with
Scheme 1
[MeB(ArF )]- anions where complexes between cation and
3
anion can be systematically made.8 We communicate here the
synthesis and solid-state and solution structures of a cationic
rhodium phosphine fragment partnered with either a coordinated
* To whom correspondence should be addressed. E-mail: a.s.weller@
bath.ac.uk. Fax: +44 (0)1225 383394.
[BArF ]- or [1-closo-CB11H6Br6]- anion. This affords, for the
(1) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066-2090.
Reed, C. A. Acc. Chem. Res. 1998, 31, 133-139. Reed, C. A. Chem.
Commun. 2005, 1669-1677.
4
first time we believe, a comparison of simple structural,
thermodynamic, and kinetic factors that influence anion coor-
dination between these two anions.
(2) Kim, K. C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.;
Lin, L. J.; Lambert, J. B. Science 2002, 297, 825-827. Lambert, J. B.;
Zhao, Y. Angew. Chem., Int. Ed. 1997, 36, 400-401.
(3) Reed, C. A.; Kim, K. C.; Stoyanov, E. S.; Stasko, D.; Tham, F. S.;
Mueller, L. J.; Boyd, P. D. W. J. Am. Chem. Soc. 2003, 125, 1796-1804.
Reed, C. A.; Fackler, N. L. P.; Kim, K. C.; Stasko, D.; Evans, D. R.; Boyd,
P. D. W.; Rickard, C. E. F. J. Am. Chem. Soc. 1999, 121, 6314-6315.
(4) Stoyanov, E. S.; Kim, K. C.; Reed, C. A. J. Am. Chem. Soc. 2006,
128, 8500-8508.
We have recently reported that addition of Na[BArF ] to
4
Rh(nbd)(PCyp3)Cl (PCyp3 ) tricyclopentylphosphine) in fluo-
robenzene solvent results in the elimination of NaCl and the
facile dehydrogenation of one of the cyclopentyl groups to afford
[Rh(η6-C6H5F){P(Cyp2)(η2-C5H7)}][BArF ] (A[BArF ]; Chart
4
4
1), which has a chelating phosphine-olefin ligand and coor-
dinated fluorobenzene ligand.9 We reasoned that repeating this
reaction in CH2Cl2, a solvent much less likely to coordinate to
(5) Macchioni, A. Chem. ReV. 2005, 105, 2039-2073.
(6) Powell, J.; Lough, A.; Saeed, T. J. Chem. Soc., Dalton Trans. 1997,
4137-4138.
(7) Xie, Z. W.; Jelinek, T.; Bau, R.; Reed, C. A. J. Am. Chem. Soc.
1994, 116, 1907-1913.
(8) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998,
120, 1772-1784.
(9) Douglas, T. M.; Le Notre, H.; Brayshaw, S. K.; Frost, C. G.; Weller,
A. S. Chem. Commun. 2006, 3408-3410.
10.1021/om060975d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/20/2006

464 Organometallics, Vol. 26, No. 3, 2007
Communications
Scheme 2
of 3) results in the dehydrogenation of one cyclopentyl ring
and formation of Rh{P(Cyp2)(η2-C5H7)}(1-closo-CB11H6Br6) (2)
in 69% isolated yield. By comparison, the analogous complex
using the [1-closo-CB11H6Cl6]- anion could not be prepared
cleanly by this route. The solid-state structure of 2 is shown in
Figure 2. The carborane anion coordinates with the rhodium
fragment through three Rh-Br interactions. One of these is
significantly longer than the other two (2.9514(6) Å versus
2.6504(5), 2.6702(5) Å), and we describe the geometry of 2 as
pseudo trigonal bipyramidal with one weaker equatorial
Rh‚‚‚Br interaction. In solution (CD2Cl2) the anion is fluxional,
as C5V symmetry is observed for the boron cage, in contrast to
the approximate Cs symmetry observed in the solid state. As
with 1, addition of 1 equiv of fluorobenzene to 2 results in an
equilibrium between 2 and A[1-closo-CB11H6Br6], being ob-
served in a 1:2.7 ratio, respectively. Addition of an excess of
fluorobenzene results in the quantitative formation of A[1-closo-
CB11H6Br6], which has been spectroscopically characterized by
independent synthesis (see the Supporting Information).
With complexes 1 and 2 in hand, we have a unique
opportunity to determine the relative strengths of binding of
these two anions with a late-transition-metal fragment. Addition
Figure 1. Solid-state structure of 1. Thermal ellipsoids are shown
at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Rh-C(31) )
2.426(3), Rh-C(32) ) 2.308(3), Rh-P ) 2.2608(8), Rh-C(4) )
2.131(3), Rh-C(3) ) 2.158(3); B-C(31)-C(34) ) 167.3(2).
the metal center, would afford the new complex Rh{P(Cyp2)-
(η2-C5H7)}{η6-(C6H3(CF3)2)BArF } (1), in which the [BArF ]-
3
4
anion is now a ligand coordinated through one of its aryl rings.
This is indeed the case, and complex 1 can be isolated in 54%
yield (Scheme 1). Solutions of 1 slowly decompose to unidenti-
fied products, some of which appear to have undergone B-C
cleavage of the anion.10 The solid-state structure of 1 is shown
in Figure 1 and clearly demonstrates the η6 coordination of
the weakly coordinating [BArF ]- anion through one of its aryl
4
rings (d(Rh-Caryl) ) 2.263(3)-2.426(3) Å), with the longest
Rh-Caryl interaction being not surprisingly with the ipso carbon,
C(31). Solution NMR data demonstrate that the anion also
remains coordinated in CD2Cl2 solution. In particular the
1
of 1 equiv of [NBu4][BArF ] to 2 slowly (hours at 298 K)
coordinated aryl protons are shifted upfield in the H NMR
4
spectrum and the 19F NMR spectrum displays two environments
in the ratio 1:3. As far as we are aware, there are only two
establishes an equilibrium between 1 and 2 (Scheme 3).
Measuring the relative ratios of these two complexes over the
other examples of [BArF ]- coordinated to a metal center, with
4
{Rh(cod)}+ and Ag+ fragments,6 while there are only a handful
of crystallographically characterized examples of the analogous
[B(C6F5)4]- anion coordinated through M‚‚‚F interactions, all
with early transition metals or lanthanides.11 Although it is η6-
coordinated, the [BArF ]- anion in 1 can be displaced by weak
4
ligands. Addition of 1 equiv of fluorobenzene establishes an
equilibrium between 1 and A[BArF ], so that a ratio of 1:1,
4
respectively, is observed at 298 K. Addition of excess fluo-
robenzene affords A[BArF ] quantitatively.
4
In order to compare the two anions, we have developed a
synthesis of the complex directly analogous to 1 with the
[1-closo-CB11H6Br6]- anion (Scheme 2). Reaction of the hy-
drogen acceptor tert-butylethene with Rh(PCyp3)H2(1-closo-
CB11H6Br6) (3; see the Supporting Information for the synthesis
Figure 2. Solid-state structure of 2. Thermal ellipsoids are shown
at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Rh-Br(11)
) 2.6504(5), Rh-Br(12) ) 2.6702(5), Rh-Br(10) ) 2.9514(6),
Rh-P ) 2.197(1), Rh-C(4) ) 2.077(5), Rh-C(3) ) 2.109(5);
P-Rh-centC(3)/C(4) ) 84.0(2), Br(12)-Rh-centC(3)/C(4) )
153.8(2), P-Rh-Br(11) ) 171.32(4).
(10) Konze, W. V.; Scott, B. L.; Kubas, G. J. J. Chem. Soc., Chem.
Commun. 1999, 1807-1808.
(11) Bouwkamp, M. W.; Budzelaar, P. H. M.; Gercama, J.; Del, Hierro,
Morales, I.; de Wolf, J.; Meetsma, A.; Troyanov, S. I.; Teuben, J. H.; Hessen,
B. J. Am. Chem. Soc. 2005, 127, 14310-14319. Yang, X. M.; Stern, C. L.;
Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015-10031. Yang, X. M.;
Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840-842.

Communications
Organometallics, Vol. 26, No. 3, 2007 465
Scheme 3a
a The insert gives a van’t Hoff plot for the equilibrium established between 1 and 2.
temperature range 275-313 K13 affords a van’t Hoff plot from
which ∆H° ) -19.0 ( 0.3 kJ mol-1, ∆S° ) -87.6 ( 0.8 J
K-1 mol-1, and ∆G°(298) ) +7.0 ( 0.5 kJ mol-1 were
determined. Interestingly, this shows that although enthalpically
tive structural, thermodynamic, and reactivity data with the
weakly coordinating carborane anion [1-closo-CB11H6Br6]-
in a transition-metal system. These show that although the
[BArF ]- anion forms a slightly weaker complex than [1-closo-
4
the [BArF ]- anion binds more strongly than the carborane anion
CB11H6Br6]-, kinetically it is substitutionally less labile with
the weak ligand H2 in the weakly coordinating solvent CH2Cl2.
4
(∆H° is negative), it is the large negative entropic contribution
involved in the coordination of the [BArF ]- anion/dissociation
This is a consequence of the η6 binding of the [BArF ]-
4
4
and solvation of the carborane anion that overall results in the
[1-closo-CB11H6Br6]- anion being marginally favored (∆G° is
small but positive). Addition of 1 equiv of [NBu4][1-closo-
CB11H6Cl6], an anion which is ranked as being less basic that
its bromo congener,4 to 2 did not result in an equilibrium being
established to the detection limits of 1H and 31P NMR
spectroscopy after 24 h at room temperature, with 2 remaining
unchanged and no decomposition observed. This demonstrates
that, in this system at least, [1-closo-CB11H6Cl6]- must be more
compared with the carborane anion that can act in a hemilabile
manner. Given that there are a growing number of organic
transformations mediated by cationic late-transition-metal frag-
ments in weakly coordinating solvents such as CH2Cl2 (e.g.
hydrogenation, hydroacylation, cyclizations, and cycloaddi-
tions12) such subtle differences between anions as we disclose
here may prove to be important.
Acknowledgment. We thank the Royal Society and the
EPSRC for support.
weakly coordinating even than [BArF ]-.
4
Reaction of H2 (1 atm, 298 K) with 2 affords 3 in 3 h, whereas
no reaction is observed with 1, even after 12 h. We explain
Supporting Information Available: Text, tables, figures, and
CIF files giving full experimental data for the synthesis of
complexes 1, 2, and 3, details of the anion exchange experiments,
NMR spectra for complex 1, and crystallographic data for com-
plexes 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
this by the [BArF ]- anion being unable to create a vacant site
4
on the 18-electron metal center in 1 required for addition of
H2, whereas the [1-closo-CB11H6Br6]- can presumably lift the
already weak Rh-Br interaction to reveal a reactive 16-electron
rhodium center. With stronger ligands such as THF or MeCN
reaction is effectively instantaneous for both to form the
previously reported metal fragment [Rh{P(Cyp2)(η2-C5H7)}-
(L)2]+ (L ) MeCN, THF).9
OM060975D
(12) Modern Rhodium-Catalysed Organic Reactions; Evans, P. A., Ed.;
Wiley-VCH: Weinheim, Germany, 2005.
(13) The collection of equilibrium data over a wider temperature range
was frustrated by the very slow time taken to reach equilibrium at each
temperature: e.g., 16 h at 275 K.
In conclusion we report a rare example of [BArF ]- coordi-
4
nated to a transition-metal center and present the first compara-