New cationic iridium(III) complexes of diiodobenzene as electrophilic catalysts: Using chelation and lability in concert [4]

Full text of DOI:10.1021/ja016127l

J. Am. Chem. Soc. 2001, 123, 12091-12092
Scheme 1
12091
New Cationic Iridium(III) Complexes of
Diiodobenzene as Electrophilic Catalysts:
Using Chelation and Lability in Concert
Paul J. Albietz, Jr., Brian P. Cleary, Witold Paw, and
Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester
Rochester, New York 14627
ReceiVed May 2, 2001
Cationic complexes of the platinum group elements that are
soluble in less polar, aprotic media represent fertile targets for
the synthesis and development of electrophilically driven systems
for catalysis and bond activation. Notable achievements in this
regard include PdMe(sol)(L-L)+, where L-L ) diimine or di-
(phosphine) and sol ) coordinated solvent for CO + C2H4
copolymerization,1,2 related PdMe(sol)(Ar2DAB)+ systems, where
Ar2DAB ) diaryldiazabutadiene for ethylene and R-olefin
polymerization,3-5 and PtMe(sol)(NN)+ complexes, where NN
) Me4en or Ar2DAB for C-H bond activation.6-9 All of these
complexes have weakly bound solvent molecules as ligands and
carried out with 3 equiv of NaBARF in CH2Cl2 in the presence
of excess DIB, formation of a single Ir species13 is observed. After
filtration of the NaOTf precipitate and subsequent solvent
evaporation, thermally stable white solids are obtained from the
reactions. Complexes 3a and 3b demonstrate clean spectroscopic
features in their 31P, 19F, and 1H NMR spectra.14 Complete
metathesis of the OTf- ions from 3 is evidenced by the absence
of triflate resonances in the 19F NMR spectrum and by the
presence of that for the CF3 groups of the BARF anion at 0.24
ppm. Likewise, 1:1 resonances in the 31P NMR spectra of 3a and
3b indicate that the complexes each have two different 31P
environments resulting from different ligands trans to the dppe
phosphine donors.15 Although the coordinated DIB resonances
are partially masked by the dppe phenyl groups, dissolution of
the complexes in CD3CN results in rapid conversion to the
corresponding bis acetonitrile complexes16 4a and 4b with
concomitant liberation of 1 equiv of DIB.
A closer investigation of the MeCN exchange reaction at low
temperatures indicates that DIB displacement occurs in two steps
for both of the complexes. For 3a, the initial exchange begins at
-40 °C, approximately 15 °C lower than that for complex 3b.
Additionally, reactions with 13CO as the incoming ligand verify
that the first exchange occurs trans to the alkyl group followed
by equatorial exchange resulting in formation of the tricarbonyl
complex IrR(CO)3(dppe)2+ and free DIB.17 For both 3a and 3b,
the reactions with CO and MeCN appear to proceed via the same
pathway as evidenced by similar chemical shifts in the 31P NMR
spectra of the intermediate species. Facile substitution of DIB by
simple carboxylate esters further supports the notion of the lability
of 3. Two equivalents of methyl acetate displace DIB from 3a
irreversibly to generate IrCH3(CO)(dppe)(CH3COOCH3)22+ in situ,
whereas methyl acrylate yields an equilibrium mixture of the ester
disubstitution product and the starting complex. There is no
evidence, however, of interaction between 3a and the olefinic
bond of methyl acrylate. Complex 3a does not show evidence of
-
essentially noncoordinating anions such as B(3,5-(CF3)2C6H3)4
or BARF-. As a d6 metal ion, Ir(III) is thought to possess the
electrophilic character of Pd2+ but its configuration in a hexaco-
ordinate environment confers inertness on its cationic complexes.
The ability to overcome this inertness has been demonstrated by
(Cp*)IrMe(PMe3)(CH2Cl2)+, which in the presence of 13CH4 gives
evidence of facile loss of weakly bound CH2Cl2, C-H bond
activation, and methane exchange.10 In studies designed to
generate Ir(III) complexes having adjacent labile sites, we have
previously reported11 the synthesis of the bis(triflate) complex
IrMe(CO)(dppe)(OTf)2 (dppe ) 1,2-bis(diphenylphosphino)-
ethane, OTf ) triflate), which is a weak electrolyte in dichlo-
romethane, and the dicationic acetyl complex Ir(C(O)Me)(dppe)-
2+
-
(MeCN)3 as its PF6 salt. In their reaction chemistry, both of
these complexes show evidence of facile ligand dissociation that
leads to electrophilic behavior. In this report, we describe two
new Ir(III) cations that exhibit greatly enhanced electrophilic
reactivity, including the initiation of a variety of cationic
polymerizations. The complexes are rendered stable enough for
isolation and characterization by use of the labile chelating ligand
1,2-diiodobenzene (DIB), which was first observed to function
in this way by Crabtree in 1982.12
Synthesis of the DIB adducts 3a and 3b (Scheme 1) can be
accomplished in two steps starting from the Ir(III) diiodide
complexes IrR(CO)(dppe)I2, where R ) CH3 (1a) and CF3 (1b).
As found for 1a,11 halide abstraction from lb with 2 equiv of
AgOTf in CH2Cl2 generates Ir(CF3)(CO)(dppe)(OTf)2 (2b). When
metathesis of the coordinated OTf ligands of either 2a or 2b is
(13) For complex 3a, longer reaction times result in increased formation
of a symmetrical product arising from isomerization of 3a, 31P{1H} NMR
(CD2Cl2): δ 35.20 (s).
(14) Complex 3a: 1H NMR (CD2Cl2) δ 7.02-7.83 (overlapping, 48H,
phenyl, DIB, BARF), 3.60 (overlapping m, 2H, PCH2CH2P), 2.97 (overlapping
m, 2H, PCH2CH2P), 1.05 (dd, JP-H ) 5.3 and 3.1 Hz, 3H, Ir-CH3); 31P{1H}
NMR (CD2Cl2) δ 27.28 (d, JP-P ) 3.6 Hz, 1P, cis to CO), 3.95 (d, JP-P ) 3.6
Hz, 1P, trans to CO); 19F NMR (CD, Cl2) δ 0.24 (s, BARF). Complex 3b:
1H NMR (CD2Cl2) δ 6.86-7.93 (overlapping, 48H, phenyl, DIB, BARF),
3.75 (overlapping m, 2H, PCH2CH2P), 3.38 (m, 1H, PCH2CH2P), 3.13 (m,
(1) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663-681.
(2) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118,
4746-4764.
(3) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995,
117, 6414-6415.
(4) Tempel, D. J.; Johnson, L. K.; Huff, R. L.; White, P. S.; Brookhart, M.
J. Am. Chem. Soc. 2000, 122, 6686-6700.
1H, PCH2CH2P); 31P{1H} NMR (CD2Cl2) δ 25.74 (qd, JP-P ) 1.5 Hz, JF-P
)
(5) Ittel, D. F.; Johnson, L. K. Chem. ReV. 2000, 100, 1169-1203.
(6) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1997, 119, 848-849.
4.8 Hz, 1P, cis to CO), -1.20 (qd, JP-P ) 1.5 Hz, JF-P ) 4.1 Hz, 1P, trans
to CO); 19F NMR (CD2Cl2) δ 70.45 (dd, JP-F ) 4.8 and 4.1 Hz, 3F, Ir-CF3),
0.24 (s, 48F, BARF).
(7) Johansson, L.; Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1999, 121,
1974-1975.
(15) An unsymmetrical orientation of CO relative to dppe is also supported
by the JP-C cis (ca. 6 Hz) and trans (ca. 120 Hz) magnitudes. See: Deutsch,
P. P.; Eisenberg, R. J. Am. Chem. Soc. 1990, 112, 714-721.
(16) Generation of 4a,b may also be accomplished by reacting 2a,b which
thus NaBARF in the presence of MeCN and supports the assignments.
(17) Conversion of methyl complex 3a to the corresponding tricarbonyl is
complete. However, the CF3 analogue 3b yields only ca. 10% (by NMR) of
the tricarbonyl species with the remainder decomposing over time into
numerous unidentified species.
(8) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 2000, 122, 10846-10855.
(9) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739-740.
(10) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970-1973.
(11) Cleary, B. P.; Eisenberg, R. Inorg. Chim. Acta 1995, 240, 135-143.
(12) Crabtree, R. H.; Faller, J. W.; Mellea, M. F.; Quirk, J. M. Organo-
metallics 1982, 1, 1361-1366.
10.1021/ja016127l CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/07/2001

12092 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Communications to the Editor
Scheme 2
the solutions demonstrates conversion of the monomers to
polymerized/oligomerized olefin in all cases.21 Polystyrene and
poly-â-pinene products are isolated as solids, whereas polyisobu-
tylene polymerizations yield viscous oils. For norbornene as the
olefin, polymerization occurs more slowly and gives an oily
product that is soluble in low-polarity solvents, such as CH2Cl2
or CHCl3. The extremely weak olefinic resonances in the 1H NMR
spectrum indicate that ring-opening processes22,23 are not occur-
ring. Furthermore, the complexity of a H,H-COSY of the isolated
oligomer suggests that both 2,3- (Scheme 2) and 2,7-additions
are happening due to norbornyl cation formation (vide infra).
The fact that 3a does not actively polymerize ethylene whereas
both 3a and 3b promote polymerization of isobutylene, nor-
bornene, styrene, and â-pinene suggests that polymerization
proceeds via a cationic mechanism with 3 serving as the initiator
rather than by a coordination/insertion mechanism. The wide range
of observed polydispersities supports this notion. Furthermore,
quenching of styrene polymerization with tert-butyl alcohol 5 s
after monomer addition to a CH2Cl2 solution of 3a results in
incorporation of a tert-butyl ether end group into the polymer
chain, as seen by a large singlet at δ 1.26 in the 1H NMR
spectrum. Since initiation by the metal complex can occur by
either direct reaction with the olefin or indirect reaction with trace
water to generate H+, definitive evidence supporting either
pathway under normal reaction conditions cannot be offered.
However, metal complex end groups24 in chromatographed poly-
â-pinene and polystyrene have been detected by 31P NMR
spectroscopy for reactions conducted in the presence of the
hindered base 2,6-di-tert-butylpyridine (>4 equiv xs). Although
monomer consumption is expectedly slower when base is present,
consumption of the metal complex is observed indicating that
direct initiation is occurring. While cationic promotion and
polymerization by metal complexes has been seen previously for
a few late metal complexes,25 the reactivity reported here for DIB
complexes 3a and 3b is unprecedented for Ir(III) systems. Further
work examining the reaction chemistry of these complexes is in
progress.
reaction with stoichiometric amounts of ethylene, but does exhibit
reaction when the latter is present in large excess, leading to the
formation of a fluxional complex having equivalent phosphines
and ethylene ligands at 298 K. However, no evidence of
polymerization or oligomerization is seen. With l-pentene as the
olefin, double bond isomerization occurs over the period of several
days at room temperature to yield cis- and trans-2-pentene, as
determined spectroscopically and by GC analysis. The reaction
leads to a product distribution of 2% 1-pentene, 14% cis-2-
pentene, and 84% trans-2-pentene that is slightly enriched in the
trans isomer relative to the thermodynamic ratio under these
conditions.18 During the isomerization, complex 3a is observed
as the only organometallic species in solution. When the same
reaction is repeated in the presence of ethylene (olefin ratio ≈
1:1), retardation of isomerization is seen indicating that ethylene
and 1-pentene coordinations are competitive. The mechanism most
likely proceeds via olefin binding to Ir(III) followed by allylic
proton dissociation and subsequent reprotonation to give the
isomerized olefin. The fact that oligomerized olefin19 is also
observed in trace amounts from the reaction suggests that 3a has
the ability to generate a secondary carbocation with 1-pentene, a
notion supported by the polymerization reactions discussed below.
The most notable evidence of electrophilic reactivity of 3 is
given by polymerizations the complexes are found to promote
(Scheme 2). For isobutylene, â-pinene, and styrene,20 addition
of the respective olefin to dichloromethane solutions of the DIB
complexes results in exothermic reactions where the monomers
are rapidly consumed and the solvent begins to boil. Analysis of
Acknowledgment. We thank the National Science Foundation (Grant
CHE-0092446) for support of this work. We also wish to thank Professors
Frank Feher and Robert G. Bergman for invaluable discussions regarding
this chemistry in addition to Eastman Kodak for conducting the polymer
analyses.
Supporting Information Available: Experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA016127L
(21) Polystyrene: [3a, Mw ) 22 200, PDI ) 5.9] and [3b, Mw ) 23 800,
PDI ) 4.9]. Poly-â-pinene: [3a, Mw ) 21 900, PDI ) 2.5] and [3b, Mw
)
12 900, PDI ) 2.91]. Polyisobutylene: [3a, Mw ) 1940, PDI ) 2.6] and [3b,
Mw ) 3430, PDI ) 3.8]. Polynorbornylene: [3a, Mw ) 234, PDI ) 1.2] and
[3b, Mw ) 262, PDI ) 1.2].
(22) Jeremic, D.; Wang, Q.; Quyoum, R.; Baird, M. C. J. Organomet. Chem.
1995, 497, 143-147.
(23) Gilliom, L. R. J. Am. Chem. Soc. 1986, 108, 733-742.
(24) 31P NMR spectra for poly-â-pinene (3 h) and for polystyrene (>12 h)
revealed resonances of phosphine donors; for PS, two broad resonances around
11 ppm were seen, one of which had coupling to 13C using carbonyl-labeled
3a.
(18) Bond, G. C.; Hellier, M. J. Catal. 1965, 4, 1-5.
(19) Peng, Y. X.; Cun, L. F.; Dai, H. S.; Liu, J. L. J. Polym. Sci.: A:
Polym. Chem. 1996, 34, 903-906.
(20) Similar reactions conducted with R-methylstyrene result in indan
formation supporting cationic initiation by 3. See ref 25 for more details.
(25) Baird, M. C. Chem. ReV. 2000, 100, 1471-1478.