Pyridinium-derived N-heterocyclic carbene complexes of platinum: Synthesis, structure and ligand substitution kinetics

Article abstract of DOI:10.1021/ja040075t

A series of [(R-iso-BIPY)Pt(CH₃)L]⁺X^- complexes [R-iso-BIPY = N-(2-pyridyl)-R-pyridine-2-ylidene; (R = 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L = SMe₂, b; dimethyl sulfoxide (DMSO), c; carbon monoxide (CO), d; X = OTf^- = trifluoromethanesulfonate and/or [BPh₄]^-] were synthesized by cyclometalation of the [R-iso-BIPY-H]⁺[OTF]-salts 1a-4a ([R-iso-BIPY-H]⁺ = N-(2-pyridyl)-R-pyridinium) with dimethylplatinum-μ-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c-4c as well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH₃)(DMSO)]⁺ (5c) and (C ₁₁H₈N)Pt(CH₃)(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic carbene complexes display localized C-C and C-N bonds within the pyridinium ligand that are indicative of carbene π-acidity. The significantly shortened platinum-carbon distance, for "parent" complex 1b, together with NMR parameters and the ν(CO) values for carbonyl cations 1d-4d support a degree of Pt-C10 multiple bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene ligands. Exceptionally large second-order rate constants (k₂ = 6.5 ± 0.4 M^-1·s^-1 (3C) to 2300 ± 500 M ^-1·^s-1 (1c)) were measured at 25 °C using ¹H NMR magnetization transfer kinetics and variable temperature line shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series of structurally similar cationic bis(nitrogen)-donor complexes [(N-N)Pt(CH₃)(DMSO)]⁺ reported earlier, and a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated 2-phenylpyridine, (C ₁₁H₈N)Pt(CH₃)(DMSO) (k₂ = 0.21 ± 0.02 M^-1·s^-1 (6C)). The differences in rate constant are discussed in terms of ground state versus transition state energies. Comparison of the platinum-sulfur distances with second order rate constants suggests that differences in the transition-state energy are largely responsible for the range of rate constants measured. The π-accepting ability and trans-influence of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution rate.

Full text of DOI:10.1021/ja040075t

Published on Web 06/12/2004
Pyridinium-Derived N-Heterocyclic Carbene Complexes of
Platinum: Synthesis, Structure and Ligand Substitution
Kinetics
Jonathan S. Owen, Jay A. Labinger,* and John E. Bercaw*
Contribution from the Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received March 5, 2004; E-mail: bercaw@caltech.edu
Abstract: A series of [(R-iso-BIPY)Pt(CH3)L ]+X- complexes [R-iso-BIPY ) N-(2-pyridyl)-R-pyridine-2-
ylidene; (R ) 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L ) SMe2, b; dimethyl sulfoxide
(DMSO), c; carbon monoxide (CO), d; X ) OTf- ) trifluoromethanesulfonate and/or [BPh4]-] were
synthesized by cyclometalation of the [R-iso-BIPY-H]+[OTF]- salts 1a-4a ([R-iso-BIPY-H]+ ) N-(2-pyridyl)-
R-pyridinium) with dimethylplatinum-µ-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c-4c as
well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH3)(DMSO)]+
(5c) and (C11H8N)Pt(CH3)(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic
carbene complexes display localized C-C and C-N bonds within the pyridinium ligand that are indicative
of carbene π-acidity. The significantly shortened platinum-carbon distance, for “parent” complex 1b, together
with NMR parameters and the ν(CO) values for carbonyl cations 1d-4d support a degree of Pt-C10 multiple
bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined
to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene
ligands. Exceptionally large second-order rate constants (k2 ) 6.5 ( 0.4 M-1‚s-1 (3c) to 2300 ( 500 M-1‚s-1
(1c)) were measured at 25 °C using 1H NMR magnetization transfer kinetics and variable temperature line
shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series
of structurally similar cationic bis(nitrogen)-donor complexes [(N-N)Pt(CH3)(DMSO)]+ reported earlier, and
a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated
2-phenylpyridine, (C11H8N)Pt(CH3)(DMSO) (k2 ) 0.21 ( 0.02 M-1‚s-1 (6c)). The differences in rate constant
are discussed in terms of ground state versus transition state energies. Comparison of the platinum-
sulfur distances with second order rate constants suggests that differences in the transition-state energy
are largely responsible for the range of rate constants measured. The π-accepting ability and trans-influence
of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution
rate.
Introduction
derived from deprotonation of pyridinium rather than imidazo-
lium cations would be stabilized by only one mesomeric nitrogen
and thus should be even better σ-donors and π-acceptors.6 These
differences should magnify their trans-influence and trans-effect,
making metal complexes based on this class of N-heterocyclic
carbene ligand more substitutionally labile.
The development of imidazolium-derived N-heterocyclic
carbene ligands has had a major impact on catalysis,1 including
more active ruthenium olefin metathesis catalysts2 and improved
palladium and nickel catalysts for aryl halide cross-couplings.1,3
Enhanced activity for catalysts bearing N-heterocyclic carbene
ligands may be attributable to the strong σ-donating4 and weak
π-accepting properties of imidazole-2-ylidenes.5 Carbene ligands
(1) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 2002, 41, 1290-1309.
(2) Bielawski C. W.; Grubbs, R. H. Angew. Chem. Int. Ed. 2000, 39, 2903-
2906. (b) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956. (c) Chaterjee, A. K.; Morgan, J. P.; Scholl, M.; Grubbs, R.
H. J. Am. Chem. Soc. 2000, 122, 3783-3784. (d) Sanford, M. S.; Ulman,
M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 749-750. (e) Sanford,
M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc. 2001, 123, 6543-
6554.
In recent model studies of the “Shilov cycle”, the rate of
benzene C-H bond activation by [(N-N)M(CH3)L]+ ((N-N)
) R-diimine; M ) Pt, Pd; L ) H2O, 2,2,2-trifluoroethanol )
(3) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. Engl. 2002, 41, 4176-
4211. (b).
(4) Kocher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532 261.
(5) Green, J. C.; Scurr, R. G.; Arnold, P. L.; Cloke, F. G. N. J. Chem. Soc,
Chem. Commun. 1997, 1963. (b) Boehme, C.; Frenking, G. Organometallics
1998, 17, 5801-5809.
(6) Gleiter, R.; Hoffmann, R. J. Am. Chem. Soc. 1968, 90, 5457-5460.
9
10.1021/ja040075t CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 8247-8255
8247

A R T I C L E S
Owen et al.
Scheme 1
Scheme 2
TFE) complexes in TFE solution was found to be strongly
dependent on the position of the pre-equilibrium substitution
of TFE for water.7 Regardless of whether the rate-determining
step is associative benzene displacement of TFE or oxidative
addition of a benzene C-H bond, more electron-rich R-diimine
ligands were found to decrease the thermodynamic preference
for water binding and increase the overall rate of C-H
activation. On the basis of these findings, we reasoned that
electron-rich ligands capable of favoring the binding of TFE
vs water and facilitating ligand substitution should enhance the
propensity for a platinum(II) or palladium(II) complex to activate
C-H bonds. We envisioned that the enhanced σ-donor and
π-acceptor capabilities of pyridinium-derived N-heterocyclic
carbenes might be ideal for facilitating substitution of a trans
ligand.
cyclometalation.9 Pyridinum salts 1a-4a (Scheme 1) were
prepared by heating the appropriately substituted pyridine with
2-pyridyl triflate at 150 °C under argon. These salts react with
[(µ-SMe2)Pt(CH3)2]2 in acetone or dichloromethane solution at
room temperature, with liberation of one equivalent of methane,
to give ∼50-90% isolated yields of the cyclometalated products
1b-4b. Pyridinium salt 1a reacts most rapidly giving an
immediate color change and vigorous evolution of methane gas
upon mixing. The dimethyl sulfide complexes 1b-4b are useful
synthons, providing access to the corresponding DMSO (1c,
2c, 4c) and carbonyl (1d-4d) complexes (Schemes 1 and 2).10
The triflate salts were crystallized directly or were converted
to tetraphenylborate salts (the bromide complex in the case of
3e) by precipitation from methanol to generate more crystalline
complexes suitable for X-ray structural analysis. All complexes
can be handled and stored in air as solids at room temperature;
however, solutions oxidize slowly in air.
We describe herein the synthesis of a series of square planar
platinum(II) complexes having a new class of chelating pyridine/
N-heterocyclic carbene donors, N-(2-pyridyl)-R-pyridine-2-
ylidene (R ) H, 4-tert-butyl, 4-dimethylamino, 5-dimethylami-
no).8 (Because these ligands are isomers of 2,2′-bipyridyl, we
abbreviate them as R-iso-BIPY.) To establish the σ- and
π-bonding properties of this new ligand type, we determined
the X-ray crystal structures for [(R-iso-BIPY)Pt(CH3)(DMS)]+
and [(R-iso-BIPY)Pt(CH3)(DMSO)]+ cations (DMS ) dimethyl
sulfide; DMSO ) dimethyl sulfoxide), examined their NMR
parameters and the carbonyl stretching frequencies for the
corresponding methyl-carbonyl cations, and undertook studies
of the kinetics of DMSO substitution trans to the carbene donor.
Results
To compare the carbene complexes with closely related ones,
[(bipy)Pt(CH3)(DMSO)]+[BPh4]- (5c) and the cyclometalated
2-phenylpyridine adduct, (C11H8N)Pt(CH3)(DMSO) (6c), were
synthesized. Protonolysis of (bipy)Pt(CH3)2 in the presence of
DMSO and precipitation from methanol using NaBPh4 provided
5c. Complex 6c was prepared analogously to 1c-4c, using
2-phenylpyridine as the ligand precursor. The unstable dimethyl
sulfide product was immediately trapped with DMSO and
Synthesis and Isolation of [(R-iso-BIPY)Pt(CH3)L]+ Salts
and Related Complexes. To avoid the need for preparation of
a free, likely very reactive carbene, we designed pyridinium
salts that would provide the desired carbene complexes upon
(7) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124,
1378-1399. (b) Ackerman, L. J.; Sadighi, J. P.; Kurtz, D. M.; Labinger,
J. A.; Bercaw, J. E. Organometallics 2003, 22, 3884-3890.
(8) A few examples of monodentate pyridin-2-ylidene complexes of platinum
and palladium have been prepared by protonation or methylation of the
nitrogen of 2-pyridyl complexes. (a) Crociani, B.; Di Bianca, F.; Giovenco,
A.; Scrivanti, A. J. Organomet. Chem. 1983, 251, 393-411. (b) Crociani,
B.; Di Bianca, F.; Giovenco, A.; Scrivanti, A. J. Organomet. Chem. 1984,
269, 295-304. (c) There is also a report of 4-pyridyl complexes of rhodium
and iridium that could be alkylated: Fanizzi, F. P.; Sunley, G. J.; Wheeler,
J. A.; Adams, H.; Bailey, N. A.; Maitlis, P. M. Organometallics 1990, 9,
131.
(9) This strategy has been documented before: (a) McGuinnes, D. S.; Cavell,
K. J.; Yates, B. F. Chem, Commun. 2001, 355. (b) Herrmann, W. A.;
Ko¨cher, C.; Gooâen, L. J.; Artus, G. R. J. Chem. Eur. J. 1996, 2, 162. (c)
Herrmann, W. A.; Kohl, F. J.; Scwarz, J. Synthetic Methods of Organo-
metallic and Inorganic Chemistry, Vol. 9, Stuttgart; New York: Georg
Thieme Verlag: New York: Thieme Medical Publishers: 2000, p 84.
(10) Isolation of the dimethyl sulfide complex 3b proved troublesome, so this
complex was made in situ as a precursor to 3d.
9
8248 J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004

N-Heterocyclic Carbene Complexes of Platinum
A R T I C L E S
Table 2. Selected Ring Bond Lengths [Å] for Complexes 1b, 3c,
4c, and 6c
N(2)−C(10) C(10)−C(9) C(9)−C(8) C(8)−C(7) C(7)−C(6) C(6)−N(2) Me2N−C
Ar
1b 1.389(3) 1.410(4) 1.356(4) 1.393(4) 1.359(4) 1.369(2)
3c 1.395(3) 1.376(3) 1.408(3) 1.423(3) 1.347(3) 1.370(3) 1.351(3)
4c 1.359(8) 1.405(9) 1.356(9) 1.394(9) 1.400(9) 1.355(8) 1.359(8)
6c 1.407(5)a 1.390(5) 1.386(5) 1.395(5) 1.376(5) 1.397(5)b
a C(11)-C(10). b C(6)-C(11).
Figure 1.
Table 1. Selected Pt-X Bond Lengths [Å] for Complexes 1b, 2c,
3c, 4c, 5c, and 6c
Pt−S(1)
Pt−C(13)
Pt−N(1)
Pt−C(10)
Figure 2.
1b
2c
3c
4c
5c
6c
2.3447(8)
2.2758(19)
2.2693(5)
2.2841(17)
2.1961(11)
2.2759(9)
2.037(3)
2.077(7)
2.046(2)
2.050(6)
2.053(4)
2.047(3)
2.092(2)
2.101(6)
2.100(2)
2.095(5)
2.130(3)
2.114(3)
1.959(3)
1.996(7)
2.011(2)
2.004(6)
2.066(3)a
2.011(3)
Bonding of the sulfur and carbene donors with platinum is
of particular interest. The Pt-S distances for complexes 2c-
4c and 6c are indistinguishable (within 3σ (σAVG) 0.0012Å)),
whereas in complex 5c it is shorter by 0.08 Å. The Pt-C10
distances fall in the order 1b < 2c < 4c < 3c ) 6c < 5c (5c,
the bipy adduct, has no C10; the differences for 2c and 4c are
comparable to the uncertainties). The Pt-C10 bond length of
1.959(3) Å for 1b is significantly shorter than that of the typical
sp2-carbon-platinum single bond (2.01(1) Å),11 whereas the
values for 3c and 6c (2.011(2) and 2.011(3) Å, respectively)
are closer. (It should be noted however that the ligand trans to
C10 is DMS for 1b but DMSO for 3c and 6c.)
a Pt-N(2).
isolated. Attempts to prepare the platinum-carbonyl complex
(C11H8N)Pt(CH3)(CO) unfortunately proved unsuccessful.
The quality of the X-ray structure determinations for 1b, 3c,
and 6c also allows discrimination of C-C and C-N bond
lengths within the ligand. Ring C-C and C-N distances for
complexes 3c, 4c, 6c, and 1b are given in Table 2. Figure 2
shows the bond length patterns in 1b, 3c and 6c.
X-ray Crystal Structures, NMR, and Infrared Spectral
Parameters. X-ray quality crystals of 1b and 2c-6c were grown
by diffusion of petroleum ether or diethyl ether into their acetone
or dichloromethane solutions. Their structures are shown in
Figure 1. In all cases, the platinum methyl group is located trans
to the pyridine nitrogen, and the dimethyl sulfide or dimethyl
sulfoxide ligands trans to the carbene or phenyl donor. Table 1
gives ligand to platinum bond lengths for all complexes.
(11) Several Pt-C (sp2) bond lengths trans to dimethyl sulfide or DMSO donors
have been reported, with an average of 2.019 Å and maximum uncertainty
of ( 0.007 Å. a) 2.036(7) and 2.010(7): Alibrandi, G.; Bruno, G.; Lanza,
S.; Minniti, D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1987, 26, 185-190.
(b) 2.018(5): Zucca, A.; Doppiu, A.; Cinellu, M. A.; Stoccoro, S.;
Minghetti, G.; Manassero, M. Organometallics 2002, 21, 783-785. (c)
2.011(5) and 2.022(6): Unpublished results for (7,8-Benzoquinoline)Pt-
(CH3)(SMe2), CCDC# 170367.
9
J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004 8249

A R T I C L E S
Owen et al.
Table 3. Selected Spectroscopic, Crystallographic, and Kinetics
Parameters
Table 5. Second Order Rate Constants for Complexes 3c, 4c, 5,
and 6 from Magnetization Transfer Kinetics in C2D2Cl4
298 b
c
1J195Pt−13C
[Hz]
r(Pt−C)
[Å]
ν(CO)
k2298
[Pt]a
[DMSO]a
kobs
k2298 c
[k2298
]
ave
-1 a
δ [ppm]
[cm
]
[M-1s-1 b
]
3c
4c
0.0240
0.0570
0.0760
0.0760
0.0387
0.0388
0.0389
0.0530
0.1610
0.1365
0.2795
0.1550
0.0775
0.3114
0.34
1.00
0.94
1.87
4.35
2.16
9.54
6.42
6.21
6.89
6.69
28.1
27.9
30.6
1c
2c
3c
4c
6c
174.0
172.2
164.3
160.9
152.1
nd
nd
2099.4
2097.9
2088.9
2090.2
nd
382(50)
175(25)
6.51(36)
28.8(1.5)
0.21(2)
1231.2
1203.8
1323.8
1070.9
1.996(7)
2.011(2)
2.004(6)
2.011(3)
6.51 ( 0.36
28.8 ( 1.5
0.16 ( 0.01
5cd
6c
a For [OTf-] salts of carbonyl adducts, 1d-4d, in acetone-d6. b In
C2D2Cl4.
0.0414
0.0413
0.0309
1.240
2.063
3.094
0.236
0.419
0.692
0.190
0.203
0.224
Table 4. Second Order Rate Constants and Activation
Parameters for Complexes 1c and 2c from Variable Temperature
Line Shape Simulation
0.21 ( 0.02
a M. b s-1 c M-1 s-1 d In acetone-d6. See ref 14a.
. .
c
298 d
qg
qh
[Pt]b
[DMSO]b T range
kobs
k2298 e
k298 f ∆H
∆S
i
DMSO at room temperature. Simulation of their temperature-
dependent spectra provided first-order rate constants (kobs) and
activation parameters. Plots of kobs versus [DMSO] indicate an
associative substitution mechanism with a first-order DMSO
dependence15 and a contribution from a DMSO-independent
substitution pathway (ki). Complexes 3c, 4c, and 6c undergo
slower associative DMSO exchange and were studied using
magnetization transfer techniques at 298 K.16 At the concentra-
tions studied, the substitution rates display a linear dependence
on DMSO concentration with no evidence for a DMSO-
independent substitution path. The second-order rate constant
for the parent pyridine-2-ylidene complex 1c is 4 orders of
magnitude greater than that measured for its structural isomer,
bipy complex 5c, and 3 orders of magnitude greater than its
neutral 2-phenyl pyridine analogue 6c.
1ca 0.0308 0.0308 255-313 109
0.0416 0.0416 245-312 130
0.0525 0.0525 254-303 159
1ci 0.0197 0.0197 314-347 20.5
0.0504 0.0504 304-338 36.3
11.3 -11.3
11.2 -11.3
10.8 -12.3
10.7 -16.7
10.2 -17.1
11.2 -13.6
11.8 -14.6
10.3 -18.2
10.4 -17.3
8.3 10.3 -17.4
2300 ( 500 37
0.0738 0.0738 304-336 40.8 382 ( 50 14
2ci 0.0527 0.0264 314-348
8.4j
0.0527 0.0527 314-348 17.3
0.0527 0.0791 304-348 22.5
0.0527 0.1054 304-348 26.6 175 ( 25
a In acetone-d6. b M. c K. d s-1, calculated. e M-1 s-1 slope from plot of
obs vs [DMSO]. f s-1, intercept ([DMSO] ) 0) from plot of kobs vs [DMSO].
k
g kcal/mol. h e. u. i In C2D2Cl4. j excluded from determination of k2.
Downfield 13C NMR chemical shifts are characteristic features
of carbene donors. Lappert and co-workers reported a range of
δ ) 175-218 ppm for a series of imidazolium-derived
platinum-carbene complexes.12 In complexes 2c-4c and 6c,
the signal for the carbon bound directly to platinum can be
unequivocally identified by the satellites due to the large 195Pt-
13C coupling constants. 13C{1H} NMR spectra of complexes
Discussion
Previous theoretical studies of complexes bearing imidazo-
lium-derived N-heterocyclic carbenes suggest that they are
primarily strong carbon σ-donor ligands.5 Still unresolved,
however, is whether π-bonding becomes important in the
transition states and intermediates of catalytic reaction cycles,
and has any consequence for catalytic activity. The major
resonance contributors describing the interactions of a transition
metal with N-heterocyclic carbenes derived from imidazolium
(A) and pyridinium (B) cations are shown in Scheme 3. Analogy
to the imidazolium-derived carbenes suggests that the σ-only
form B1 would be the most important contributor for pyri-
dinium-derived carbenes, but as discussed below, our structural
data and fast associative substitution kinetics suggest a signifi-
cant role for π-acidity as well.
2c-4c have peaks in the range of δ ) 160-175 ppm (1JPt-C
≈
1200-1300 Hz), whereas 6c shows a chemical shift of δ )
152 ppm with a significantly smaller 1JPt-C of 1071 Hz.13 13C-
{1H} NMR data for complexes 2c-4c and 6c are included in
Table 3, along with relevant crystallographic, infrared and
kinetics data (see below).
At room temperature, complexes 1c-4c show 1H NMR
spectra consistent with their solid-state structures. The JPt-H
2
satellites observed for the methyl signals (∼81 Hz) are consistent
with methyl trans to the pyridine nitrogen (as observed in the
1
spectrum of 5c). Notably, the 300 MHz H NMR spectrum of
Structural and Spectroscopic Evidence. Bond length pat-
terns in the X-ray structures of 3c and 1b demonstrate the
importance of π-acidity of the carbene ligand. Resonance
structures for complex 3c (Scheme 4) reflect competition
between the N pπ electrons of the dimethylamino substituent,
the pyridinium N pπ electrons, and the dπ electrons of the
platinum for the empty carbon pπ orbital. The X-ray structure
of 3c agrees best with the resonance depiction 3c′, showing a
platinum-carbon single bond with appropriate short/long
alternations for double and single C-C bonds within the ring.
The Pt-C10 distance (2.011(2) Å) indicates a relatively minor
3c shows inequivalent [N(CH3)2] resonances at room temper-
ature, that coalesce at 70 °C.14 In contrast complex 4c shows
one sharp [N(CH3)2] resonance at room temperature.
Ligand Substitution Kinetics. To investigate the trans-
labilizing ability of these pyridine-2-ylidene ligands, DMSO
exchange rates for complexes 1c-4c and 6c were measured in
C2D2Cl4 and acetone-d6 solution, using magnetization transfer
and variable temperature line shape simulation. Kinetic data are
shown in Tables 4 and 5. Complexes 1c and 2c substitute most
1
quickly, giving broad H NMR spectra in the presence of free
(12) Cardin, D. J.; Cetinkay, B.; Lappert, M. F.; Randall, E. W.; Rosenberg, E.
J. Chem. Soc., Dalton Trans. 1973, 19, 1982-1973.
(13) The lower solubility of 1c prevented the determination of its 1JPt-C
coupling constant.
(15) k2 was determined from the slope of the kobs versus [DMSO].
(16) Complex 6c has a substitution rate nearly too slow to study by magnetization
transfer at room temperature. Concentrated samples with several equivalents
of DMSO were used to achieve reliably measurable rates of magnetization
transfer.
(14) The ∆Gq at 70 °C was estimated at 16.8 kcal/mol, and was found to be
invariant to added DMSO.
9
8250 J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004

N-Heterocyclic Carbene Complexes of Platinum
A R T I C L E S
Table 6. Crystala and Refinement Data for Complexes 1b, 2c-4c, 5, 6
1b [BPh4]-
2c [OTf]-
3c [BPh4]-
empirical formula
[C13H17N2SPt]+
[B(C6H5)4]- ‚ (C3H6O)
805.72
11.0611(6)
13.0966(7)
14.2868(7)
114.0150(10)
91.9270(10)
111.0610(10)
1724.46(16)
2
[C17H22N2OSPt]+
[SO3CF3]- ‚ (C6H6)
685.64
21.8153(11)
10.4419(5)
[C15H22BN3OSPt]+
[B(C6H5)4]-
806.72
11.0499(4)
11.3835(4)
14.9527(6)
67.7790(10)
74.5460(10)
88.1200(10)
1673.33(11)
2
formula weight
a, Å
b, Å
c, Å
R, deg
b, deg
22.0778(11)
γ, deg
volume, Å3
Z
5029.2(4)
8
crystal system
space group
d
triclinic
P-1
1.552
orthorhombic
Pbcn
1.811
triclinic
P-1
1.601
calc, g/cm3
θ range, deg
1.60 to 28.30
4.163
1.84 to 28.42
4.291
1.53 to 28.35
4.291
µ, mm-1
abs. correction
GOF
R1,b wR2c [I>2σ(I)]
none
1.215
0.0244, 0.0509
none
1.86
0.0496, 0.0806
none
1.365
0.0203, 0.0470
4c [BPh4]-
5c [BPh4]-
6c
empirical formula
[C15H22N3OSPt]+
[B(C6H5)4]- ‚ (C3H6O)
835.76
11.1872(6)
15.3712(8)
[C13H17N2OPtS]+
[B(C6H5)4]- ‚ (C3H6O)
792.69
23.0688(8)
13.1532(5)
C14H17NOSPt
formula weight
a, Å
b, Å
442.44
10.3677(9)
14.9165(13)
17.3745(15)
c, Å
20.8806(10)
23.9873(9)
R, deg
â, deg
90.4650(10)
115.5560(10)
γ, deg
Volume, Å3
Z
3590.5(3)
4
6566.3(4)
8
2687.0(4)
8
crystal system
space group
dcalc, g/cm3
θ range, deg
µ, mm-1
abs. correction
GOF
monoclinic
P21
1.546
1.65 to 28.39
4.004
none
monoclinic
P21/c
1.604
1.71 to 28.60
4.373
face-indexed Gaussian
1.29
orthorhombic
Pbca
2.187
2.34 to 28.49
10.586
face-indexed Gaussian
1.272
1.074
0.0343, 0.0601
R1,b wR2c [I>2σ(I)]
0.0359, 0.0538
0.0235, 0.0364
2
2
2
a 98(2) K. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]1/2
.
Scheme 3
and ligand C-C and C-N distances (Figure 2) consistent with
significant contributions for both resonance structures B1 and
B2 (Scheme 3). Both structures differ from that of metalated
2-phenylpyridine analogue 6c, which shows delocalized bonding
in the phenyl ring (Table 2, Figure 2), and a platinum-carbon
distance (2.011(3) Å) not significantly different from that for
3c (2.011(2) Å). The Pt-C10 distance of 1.959(3) Å for the
“parent” example, 1b, is significantly shorter than that expected
for a single bond. We conclude, therefore, that the X-ray
structural data support a degree of Pt-C10 π-bonding as
depicted in resonance structure B2 (Scheme 3).
Scheme 4
contribution from 3c′′′. In contrast, the absence of a dimethyl-
amino substituent in 1b leaves the empty carbene orbital to be
stabilized only by interaction with the filled platinum dπ orbital
and the pyridinium N pπ electrons. The former interaction
results in the shortest Pt-C10 bond in the series (1.959(3) Å)
The NMR and infrared parameters for these complexes also
suggest some modest Pt-C10 π-bonding. Although the 13C
NMR chemical shifts for the Pt-bonded carbon for 1c-4c (δ
9
J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004 8251

A R T I C L E S
Owen et al.
160.9-174.0) are significantly downfield of that for the
metalated 2-phenylpyridine complex 6c (δ 152.1), they are not
so far downfield as those for the imidazolium-derived carbenes
(δ ) 175-218) or alkylidenes (δ g 200).17 Moreover, the 1JPt-C
values for 2c-4c are larger (1203.8-1323.8 Hz) than that for
6c (1070.9 Hz), suggesting a higher Pt-C10 bond order for
the (2-pyridyl)-R-pyridine-2-ylidene complexes. The carbonyl
stretching frequencies for 1d-4d lie in the order of increasing
ν(CO): 3c (2088.9 cm-1) < 4c (2090.2 cm-1) < 2c (2097.9
cm-1) < 1c (2099.4 cm-1), approximately the order expected,
as the “parent” ligand of 1c should be most π acidic; 2c and 4c
less so, because of the electron-releasing substituents; and 3c
least, because of the 4-amino nitrogen pπ donation (form 3c′,
Scheme 4).
An additional manifestation of the importance of resonance
structure 3c′ (Scheme 4) is the measurable barrier to rotation
about the [(CH3)2N]-CAr bond in 3c (∆Gq343K ) 16.8 kcal/
mol). The much smaller barrier for 4c is expected, because the
analogous double bonded resonance structure (4c′) is disfavored
by charge separation; the higher CO stretching frequency for
4d vs. 3d is also consistent with this argument.
Figure 3.
diagram of the frontier orbitals of the square planar ground state
and trigonal bipyramidal transition state on an associative
substitution pathway is shown in Figure 3. In this representation,
π-acceptors lower the transition state energy, while strong
σ-donors raise it.
Relative ligand trans-influence can be measured by examining
the bond lengths of the ligand trans to the N-heterocyclic carbene
ligand.21 Thus, any differences in the platinum-sulfur bond
lengths among the series 2c-6c should allow an assessment of
relative ground-state energies on the associative substitution
reaction profile. X-ray crystal structures of the DMSO cations
2c-4c, 6c show very similar platinum-sulfur distances (Table
1), whose average of 2.274 Å is significantly longer (by about
0.08 Å) than the Pt-S distance (2.1961(11) Å) for the bipy
complex 5c. This indicates, as expected, a substantially greater
trans-influence for ligands having a Pt-C bond (2c-4c, 6c) as
compared with a Pt-N bond. The similarity between the
platinum-sulfur distances in 2c-4c and 6c is important and
suggests that formally neutral N-heterocyclic carbene and
anionic sp2-hybridized phenyl carbon donors have a similar
trans-influence.22 This property is maintained despite the lower
basicity of a carbene carbon, and is likely the result of the
π-acceptor character and greater bond order documented above.
However, this is not reflected in the substitution rates: that for
6c is 1-3 orders of magnitude lower than those for the carbene
complexes.23
Structural and Kinetics Evidence. Studies of degenerate
DMSO exchange reveal effects of ligand bonding properties
both on the ground state (trans-influence) and transition state
(trans-effect). Romeo and co-workers have extensively studied
associative, degenerate DMSO exchange at cationic alkyl
platinum centers.18 For [(N-N)Pt(CH3)(DMSO)]+ complexes
(N-N ) a bis(nitrogen donor)) in acetone-d6 solution second-
order rate constants range from [(9.49 ( 0.05) × 10-6 M-1s-1
]
]
for (N-N) ) tetramethylethylenediamine to [0.920 M-1 s-1
for (N-N) ) 1,4-dicyclohexyl-1,4-diazabutadiene.18a,19 Romeo’s
findings can serve as a basis for comparison, in attempting to
quantify the extent to which these new pyridinium-derived
N-heterocyclic carbene ligands accelerate ligand substitution.
Langford and Gray20 and others21 have suggested that strong
σ-bonding between a ligand and the platinum 6s and 5p orbitals
limits the availability of those metal-based orbitals for interacting
with the trans-ligand, thus weakening its bonding interaction, a
phenomenon termed “trans-influence.” Experimentally, stronger
σ-donation results in greater trans-influence, with carbon and
hydride donors being among the strongest.21
We conclude, therefore, that the large acceleration of ligand
substitution for 1c-4c compared to 6c, is primarily due to
stabilization of the transition state energies. This result can be
rationalized using the orbital diagram shown above in Figure 3
where both π-acceptors and weaker σ-donors stabilize the
transition structure. The trend in the associative DMSO substitu-
tion rates (1c > 2c > 4c > 3c) parallels the relative ground
state π-acidities for the N-heterocyclic carbene ligands.
The distinctive σ-donating/π-accepting characteristics of
carbene-donor ligands are undoubtedly quite relevant to their
increasing role in homogeneous catalysis. Structural, spectro-
scopic and kinetics evidence cited here indicates that it is an
augmentation of π-accepting capability that distinguishes the
pyridine-2-ylidene donors from their formally anionic phenyl
analogues, and make them particularly well suited for accelerat-
ing associative ligand substitution. We are currently working
on exploiting this property for enhancing C-H activation and
related chemistry.
π-Bonding, on the other hand, generally exerts its greatest
influence in the transition state structure, with ligands of greater
π-acid character accelerating associative ligand substitution.21
The importance of this π-bonding to the relative energies of
similar trigonal bipyramidal structures can be understood by
examining the equatorial orbital interactions. A qualitative
(17) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons: 2001, pp 265, 298.
(18) Romeo, R.; Scolaro, L. M.; Nastasi, N.; Arena, G. Inorg. Chem. 1996, 35,
5087-5096. (b) Romeo, R.; Nastasi, N.; Scolaro, L. M.; Plutino, M. R.;
Albinati, A.; Macchioni, A. Inorg. Chem. 1998, 37, 5460-5466. (c) Romeo,
R.; Fenech, L.; Scolaro, L. M.; Albinati, A.; Macchioni, A.; Zuccaccia, C.
Inorg. Chem. 2001, 40, 3293. (d) Romeo R.; Fenech L.; Carnabuci, S.;
Plutino, M. R.; Romeo, A. Inorg. Chem. 2002, 41, 2839-2847.
(19) [(2,9-Dimethylphenanthroline)platinum(methyl)(DMSO)]+ had a substitu-
tion rate much faster than the all other complexes in Romeo’s study, but
it was determined to substitute by a different mechanism.
(20) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W. A.
Benjamin, Inc.: New York, 1966.
(21) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry, Prentice Hall,
Upper Saddle River, New Jersey, 1997. (b) Basolo, F.; Pearson, R. G. Prog.
Inorg. Chem. 1962, 4, 381. (c) Wendt, O. F. Platinum(II) and Palladium-
(II) Complexes with Group 14 and 15 Donor Ligands, Ph.D. Thesis, 1997,
Lund University, Lund, Sweden. (d) Crabtree, R. H. The Organometallic
Chemistry of the Transition Metals; John Wiley & Sons: New York, 2001.
(22) Crociani et. al. inferred from ν(Pt-Cl) stretching frequencies that the trans-
influence of 2-pyridyl is less than its ylidene analogue. ref 8b.
(23) Romeo18a has observed that stronger σ-donation need not result in a greater
trans-effect. In the context of the bonding model described in Figure 3,
this would depend on the relative importance of σ-antibonding interactions
in the ground and transition states for substitution.
9
8252 J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004

N-Heterocyclic Carbene Complexes of Platinum
A R T I C L E S
Experimental Section
General Methods. All air and/or moisture sensitive compounds were
manipulated using standard Schlenk techniques or in a glovebox under
a nitrogen atmosphere, as described previously.24 4-tert-butylpyridine,
4-(dimethylamino)pyridine, and pyridine were purchased from Aldrich,
distilled from CaH2 and stored under argon. 2-phenylpyridine, 2,2′-
bipyridyl, and sodium tetraphenylborate were purchased from Aldrich
and used as received. 3-(dimethylamino)pyridine,25 2-pyridyl triflate,26
and [(µ-SMe2)Pt(CH3)2]227 were prepared as described previously. C2D2-
Cl4 and acetone-d6 were purchased from Cambridge Isotope Labora-
tories, distilled from CaH2 and CaSO4, respectively, and stored under
argon. Dimethyl sulfoxide used in kinetic experiments was passed
through an activated alumina column and stored under argon. NMR
spectra were recorded on a Varian UNITYINOVA 500 (499.853 MHz
Figure 4. Magnetization transfer data for (4c) at 298 K in CD2Cl4. (O)
Bound DMSO. (]) Free DMSO. (-) The best fit line from a two-site
1
for H) spectrometer.
exchange model. kobs ) 2.16 s-1; k2 ) 27.9 M-1s-1
.
Magnetization Transfer Experiments and Line Shape Analysis
Experiments. Both types of experiments were carried out in J. Young
NMR tubes under 1 atm of argon. Solutions were prepared in 1.0 mL
volumetric flasks. Reaction temperatures were determined by measuring
the peak separation of an ethylene glycol or methanol standard before
and after the experiment at each temperature.
Magnetization transfer experiments were conducted at 25 °C by
inversion of the signal for free DMSO using a selective DANTE
inversion sequence.28 Relaxation times (T1) for the resonances of interest
were measured before the magnetization transfer experiment using the
inversion recovery method. Signals for free and bound DMSO were
integrated at delay times τ and fit to a two-site exchange model using
the program CIFIT. (Figure 4). Pseudo-first-order rate constants
obtained from these fits were plotted versus DMSO concentration. All
plots showed a linear dependence on DMSO concentration and a zero
intercept within experimental error (Figure 5).
Variable temperature line shape data for complexes 1c and 2c were
recorded in 10-degree increments from the static spectrum (-40 °C)
until the boiling point of the solvent or temperature limitation of the
NMR probe (<100 °C). In all cases, a coalescence point was reached.
Line broadening of the platinum bound DMSO was used to calculate
first-order rate constants in the slow exchange region according to eq
1.29 The temperature dependence of the 195Pt satellites of the platinum-
Figure 5. kobs versus [DMSO] for (4c) at 298 K in C2D2Cl4. k2 ) 27.9
M-1 s-1
.
1/2
kobst ) 1/τt ) π(wt1/2 - wo
)
(1)
bound DMSO signal introduces error in the measured line widths at
half-height (wt1/2). Spectra with significant contributions to the width
at half-height from the platinum satellites were discarded in the Eyring
analysis. Eyring plots of these pseudo-first-order rate constants (Figure
6) provided a method of determining pseudo-first-order rate constants
at 298 K (kobs298). These rate constants were plotted versus DMSO to
determine rate constants for the DMSO dependent (k2) and independent
(ki) substitution paths.
Pyridinium Salts. All pyridinium triflate salts were prepared by a
procedure analogous to the one detailed for 2a below, with exceptions
noted.
N-(2-Pyridyl)-4-tert-butylpyridinium Triflate. (2a) 2-Pyridyl tri-
flate (7.33 g, 32.3 mmol) and 4-tert-butylpyridine (8.73 g, 64.5 mmol)
were combined under argon in a Teflon-stoppered Schlenk tube
Figure 6. Eyring plot for (1c) in acetone-d6 from 255 to 313 K. The value
for k2 was calculated from the linear best-fit at 298 K. (O) kobsT, (0) Data
points excluded due error from 195Pt satellites.
equipped with a stir bar. The solution was degassed, sealed under
vacuum, and heated to 140 °C. After stirring overnight (∼12 h), the
colorless solution turned black and upon cooling partially solidified.
Dissolving the mixture in CH2Cl2 and addition of diethyl ether
precipitated a dark purple solid that was isolated on a glass frit. Several
recrystallizations from hot ethyl acetate provided 6.88 g (59%) of 2a
(24) Burger, B. J.; Bercaw, J. E. New DeVelopments in the Synthesis, Manipula-
tion, and Characterization of Organometallic Compounds; Wayda, A.,
Darensbourg, M. Y., Eds.; American Chemical Society: Washington, DC,
1987; Vol. 357.
1
as purple crystals. H NMR (300 MHz, CDCl3): δ ) 1.47 (s, 9H,
3
3
3
C(CH3)3), 7.64 (dd, 1H, J ) 7.7 Hz, J ) 4.9 Hz), 8.15 (ddd, 1H, J
(25) Giam, C. S.; Hauck, Albert E. Organic Preparations and Procedures
International; 1977, 9(1), 9-11.
3
4
3
) 8.1 Hz, J ) 8.1 Hz, J ) 1.6 Hz), 8.24 (d, 2H, J ) 7.1 Hz), 8.20
(26) Keumi, Takashi; Yoshimura, Kiichiro; Shimada, Masakazu; Kitajima,
Hidehiko. Bull. Chem. Soc. Jpn. 1988, 61(2), 455-9.
(d, 1H, 3J ) 8.2 Hz), 8.65 (dd, 1H, 3J ) 2.8 Hz, 4J ) 1.6 Hz), 9.58 (d,
2H, J ) 7.1 Hz); 13C{1H} (75 MHz, CDCl3): δ ) 24.3, 32.4, 122.1,
3
(27) Hill, G.S.; Irwin, M.J.; Levy, C.J.; Rendina, L.M.; Puddephatt, R.J. Inorg.
Synth. 1998, 32, 149-153.
131.5, 132.7, 148.78, 148.79, 148.9, 158.2, 185.8; Anal. Calcd for
C15H17F3N2O3S: C, 49.72; H, 4.73; N, 7.73. Found: C, 49.65; H, 4.66;
N, 7.66.
(28) Morris, G. A.; Freeman, R. J. Magn. Reson. 1978, 29, 433-462.
(29) Gu¨nther, H. NMR Spectroscopy, Second Edition; John Wiley & Sons
Limited: West Sussex, England, 1995; pp. 335-346.
9
J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004 8253

A R T I C L E S
Owen et al.
N-(2-Pyridyl)-pyridinium Triflate (1a). 2-Pyridyl triflate (7.33 g,
32.3 mmol) and pyridine (10.21 g, 129.1 mmol) were stirred for 18 h
at 140 °C. The crude material was recrystallized from CHCl3 and diethyl
ether. 1.942 g (20%) of 1a as pink plates were obtained. 1H NMR (300
MHz, CD2Cl2): δ ) 7.71 Hz (dd, 1H, 3J ) 7.7 Hz, 3J ) 4.9 Hz), 8.23
(ddd, 1H, 3J ) 8.2 Hz, 3J ) 8.2 Hz, 4J ) 1.6 Hz), 8.35 (t, 2H, 3J ) 7.7
Hz), 8.51 (d, 1H, 3J ) 8.2 Hz), 8.68-8.75 (m, 2H), 9.74 (d, 2H, 3J )
6.0 Hz); 13C{1H} (75 MHz, CD2Cl2): δ ) 118.0, 127.7, 129.2, 141.8,
142.5 (br, 2C), 148.4, 150.3; Anal. Calcd for C11H9F3N2O3S: C, 43.14;
H, 2.96; N, 9.15. Found: C, 43.35; H, 2.99; N, 9.02.
N-(2-Pyridyl)-4-(dimethylamino)-pyridinium Triflate (3a). 2-Py-
ridyl triflate (2.932 g, 12.9 mmol) and 4-(dimethylamino)pyridine (3.154
g, 25.8 mmol) were stirred 5 h at 150 °C. The crude material was
recrystallized from ethanol and diethyl ether giving 3a as a high yield
of pink plates. 1H NMR (300 MHz, CDCl3): δ ) 3.36, 8.23 (ddd, 1H,
dinium triflate (2a) (1.203 g, 3.32 mmol) were allowed to react to yield
1
1.904 g (94%) of 2b as a yellow powder. H NMR (300 MHz, CD2-
Cl2) δ ) 1.22 (s, 3H, 2JPt-H ) 81.3 Hz Pt-CH3), 1.44 (s, 9H), 2.52
3
(br s, 6H, JPt-H ) ∼32 Hz, S(CH3)2), 7.75-7.81 (m, 2H), 8.28 (d,
4
3
3
3
1H, J ) 2.2 Hz, JPt-H ) 54.9 Hz), 8.46 (ddd, 1H, J ) 7.1 Hz, J
) 7.1 Hz, 4J ) 1.7 Hz), 8.63 (d, 1H, 3J ) 8.2 Hz), 9.09 (dd, 1H, 3J )
3
3
3
5.5 Hz, J ) 1.1 Hz, JPt-H ) 7.1 Hz), 9.43 (d, 1H, J ) 7.7 Hz);
13C{1H} NMR (125 MHz, CD2Cl2): δ ) -9.31 (1JPt-C ) 791.8 Hz,
Pt-CH3), 20.77, 30.14, 37.08, 116.2, 120.5, 127.6, 129.9 (JPt-C )
120.8 Hz), 138.7 (JPt-C ) 39.5 Hz), 142.8, 146.7, 156.3 (JPt-C )
49.6 Hz), 169.0 (JPt-C ) 49.6 Hz), 179.2 (1JPt-C ) 1321.6 Hz,
Pt-Ccar); Anal. Calcd for C18H25F3N2O3PtS2: C, 33.28; H, 3.88; N,
4.31. Found: C, 34.03; H, 4.08; N, 4.16.
[(5-(Dimethylamino)-iso-BIPY)Pt(CH3)(DMS)]+ [OTF]- (4b). (µ-
SMe2)Pt(CH3)2]2 (0.758 g, 1.318 mmol) and N-(2-pyridyl)-3-(dimethyl-
amino)-pyridinium triflate (4a) (0.921 g 2.64 mmol) were allowed to
react to yield 1.176 g (72%) of 4b as a yellow powder. 1H NMR (300
3
4
3
3J ) 8.2 Hz, J ) 8.2 Hz, J ) 1.6 Hz), 7.14 (d, 2H, J ) 8.2), 7.45
3
3
4
3
(ddd, 1H, J ) 7.1 Hz, J ) 4.9 Hz, J ) 1.1 Hz), 7.92 (d, 1H, J )
3
3
4
2
8.2 Hz), 8.02 (ddd, 1H, J ) 6.9 Hz, J ) 6.9 Hz, J ) 1.9 Hz), 8.53
MHz, CD2Cl2) δ ) 1.15 (s, 3H, JPt-H ) 80.85 Hz, Pt-CH3), 2.51
(d, 2H, J ) 4.8 Hz), 8.91 (d, 2H, J ) 7.7 Hz); 13C{1H} (125 MHz,
CDCl3): δ ) 40.7, 108.4, 114.6, 124.7, 138.3, 140.6, 149.2, 150.5,
157.3; Anal. Calcd for C13H14F3N3O3S: C, 44.70; H, 4.04; N, 12.03.
Found: C, 44.63; H, 4.01; N, 11.97.
3
3
3
(s, 6H, JPt-H ) 31.5 Hz, S(CH3)2), 3.14 (s, 6H, NMe2), 7.53 (dd,
1H, 3J ) 9.3 Hz, 4J ) 2.8 Hz, JPt-H ) 6.0 Hz), 7.76 (ddd, 1H, 3J )
3
4
3
5.5 Hz, J ) 5.5 Hz, J ) 1.1 Hz), 8.06 (d, 1H, J ) 9.3 Hz, JPt-H
3
) 23.6 Hz), 8.40 (d, 1H, J ) 2.75 Hz, JPt-H ) 4.2 Hz), 8.51 (ddd,
3
3
4
3
1H, J ) 7.7 Hz, J ) 7.7 Hz, J ) 1.6 Hz), 8.67 (d, 1H J ) 8.24
N-(2-Pyridyl)-5-(dimethylamino)-pyridinium Triflate (4a). 2-Py-
ridyl triflate (4.279, 18.8 mmol) and 3-(dimethylamino)pyridine (2.303
g, 18.8 mmol) were heated to 140 °C overnight. Addition of diethyl
ether to a CH2Cl2 solution of the crude product caused a black oil to
separate. Decanting the solvent and removing the volatiles in vacuo
provided a dark brown solid that was recrystallized from CH2Cl2 and
diethyl ether to give 3.444 g (52%) of 4a as very dark yellow crystals.
1H NMR (300 MHz, CDCl3): δ ) 3.23 (s, 6H, N(CH3)2), 7.64 (dd,
Hz), 9.07 (dd, 1H J ) 6.04 Hz, J ) 1.1 Hz); 13C{1H} NMR (125
MHz, CD2Cl2): δ ) -10.31 (1JPt-C ) 789.2 Hz, Pt-CH3), 20.77,
40.5, 116.8 (JPt-C ) 13.6 Hz), 120.3 (JPt-C ) 40.1 Hz), 127.7,
128.8 (JPt-C ) 117.5 Hz), 132.5 (JPt-C ) 117.5 Hz), 142.4, 145.5,
146.2, 156.7 (JPt-C ) 51.2 Hz), 176.1 (1JPt-C ) 1323.3 Hz, Pt-
3
4
Ccar); Anal. Calcd for C16H22F3N3O3PtS2: C,30.97; H, 3.57; N, 6.77.
Found: C, 30.92; H, 3.54; N, 6.58.
3
3
3
3
1H, J ) 7.7 Hz, J ) 4.7 Hz), 7.79 (dd, 1H, J ) 9.1 Hz, J ) 3.0
Substitution of Dimethyl Sulfide by Dimethyl Sulfoxide. The
dimethyl sulfide complex is mixed with sufficient DMSO (∼20 eq) to
fully dissolve the sample, and is left stirring under vacuum for 1 h.
The residue is then dissolved in a small amount of dichloromethane
under argon, and the product precipitated with diethyl ether, filtered
and dried under vacuum.
Hz), 7.95 (dd, 1H, 3J ) 8.8 Hz, 3J ) 6.0 Hz,), 8.17 (ddd, 1H, 3J ) 6.3
3
4
3
Hz, J ) 6.3 Hz, J ) 1.7 Hz), 8.31 (d, 1H, J ) 8.2 Hz), 8.57 (m,
3
4
3
1H), 8.63 (dd, 1H, J ) 2.8 Hz, J ) 1.7 Hz), 8.69 (d, 1H, J ) 6.0
Hz); 13C{1H} (75 MHz, CDCl3): δ ) 40.5, 100.1, 118.3, 123.9, 127.0,
127.5, 128.1, 128.4, 141.5, 148.7, 149.5; Anal. Calcd for
C13H14F3N3O3S: C, 44.70; H, 4.04; N, 12.03. Found: C, 44.57; H,
4.02; N, 11.93.
[(iso-BIPY)Pt(CH3)(DMSO)]+ [OTf]- (1c). The reaction was
carried out according to the above procedure. [(iso-BIPY)Pt(CH3)-
(DMS)]+[OTf]- (1b) (0.257 g, 0.445 mmol), DMSO (0.63 mL, 8.9
Cylclometalation of the Pyridinum Salts 1a-4a. All of the carbene
complexes were prepared and purified according to the procedure
described for 1b below. Complex 3b was only prepared in situ and
was immediately converted to 3d, and isolated as a carbonyl cation.
[(iso-BIPY)Pt(CH3)(DMS)]+ [OTF]- (1b). [(µ-SMe2)Pt(CH3)2]2
(0.984 g, 1.71 mmol) was added, in one portion, to a solution of N-(2-
pyridyl)-pyridinium triflate (1a) (1.049 g 3.43 mmol) in 50 mL of
methylene chloride under argon purge. Evolution of methane was
immediately observed upon mixing, and the solution quickly became
a yellow orange color. After stirring for 1 h a small amount of diethyl
ether was slowly added to precipitate a dark solid. After stirring for 30
min the solution was cannula-filtered to remove the dark solids and
the filtrate concentrated. Addition of diethyl ether precipitated a mustard
colored powder which was isolated on a glass frit and dried under
1
mmol, 20 eq); yield 93%. H NMR (300 MHz, CDCl3) δ ) 0.89 (s,
3H, 2JPt-H ) 80.2 Hz, Pt-CH3), 3.34 (br s, 6H, 3JPt-H ) 21.1 Hz,
S(CH3)2), 7.74 (at, 1H, 3J ) 6.2 Hz), 7.98 (at, 1H, 3J ) 6.2 Hz), 8.10
3
3
(at, 1H, J ) 7.51 Hz), 8.25 (d, 1H, J ) 8.0 Hz, JPt-H ) 23.9 Hz),
8.48 (at, 1H, 3J ) 7.73 Hz), 9.01 (d, 1H, 3J ) 8.7 Hz), 9.85 (d, 1H 3J
) 5.55 Hz), 10.03 (d, 1H 3J ) 5.55 Hz); 13C{1H} (125 MHz, acetone-
d6): δ ) -11.20 (1JPt-C ) 757.8 Hz, Pt-CH3), -43.15 (Pt-OS-
(CH3)2), 116.1, 123.3, 127.6, 133.4, 140.2, 143.0, 144.1, 150.8, 156.4,
174.0; Anal. Calcd for C14H17F3N2O4PtS2: C,28.33; H, 2.89; N, 4.72.
Found: C, 28.07; H, 2.81; N, 4.58.
[(4-tert-Butyl-iso-BIPY)Pt(CH3)(DMSO)]+ [OTf]- (2c). The reac-
tion was carried out according to a procedure similar to the above.
[(tert-butyl-iso-BIPY)Pt(CH3)(DMS)]+[OTf]- (2b) (0.122 g, 0.199
mmol), DMSO (0.30 mL, 3.98 mmol, 20 eq); yield 107 mg (86%). 1H
1
vacuum. Yield 1.887 g (95%). H NMR (500 MHz, acetone-d6) δ )
1.17 (s, 3H, 2JPt-H ) 82.04 Hz, Pt-CH3), 2.60 (br s, 6H, 3JPt-H )
2
NMR (300 MHz, CD2Cl2) δ ) 0.90 (s, 3H, JPt-H ) 80.4 Hz, Pt-
3
3
4
CH3), 1.44 (s, 3H, tert-Bu), 3.30 (s, 6H, 3JPt-H ) 21.2 Hz, OS(CH3)2),
∼28 Hz S(CH3)2), 7.88 (ddd, 1H, J ) 6.9 Hz, J ) 6.9 Hz, J ) 1.4
3
3
4
3
3
4
Hz), 8.03 (ddd, 1H, J ) 5.5 Hz, J ) 5.5 Hz, J ) 0.9 Hz), 8.22-
8.42 (m, 2H), 8.63 (ddd, 1H, 3J ) 7.3 Hz, 3J ) 7.3 Hz, 4J ) 1.4 Hz),
7.74 (at, 1H, J ) 6.8 Hz), 7.87 (dd, 1H, J ) 7.1 Hz, J ) 2.2 Hz),
8.25 (d, 1H, 3J ) 2.3 Hz, JPt-H ) 26.1 Hz), 8.43 (ddd, 1H, 3J ) 7.7
Hz, 3J ) 7.7 Hz, 2J ) 1.8 Hz), 8.68 (d, 1H, 3J ) 8.6 Hz), 9.87 (d, 1H,
3
3
8.7 (d, 1H, J ) 8.4 Hz), 9.22 (br d, 1H J ) 5.06 Hz), 9.6 (br d, 1H
3J ) 6.14 Hz); 13C{1H} (125 MHz, acetone-d6): δ ) -9.40 (1JPt-C
) 792.2, Pt-CH3), 20.39, 116.8, 122.5, 128.8, 133.9 (JPt-C ) 113.8
Hz), 140.5 (JPt-C ) 34.7 Hz), 143.2, 144.42 (JPt-C ) 53.7 Hz),
147.8, 157.0, 176.1 (1JPt-C ) 1317.1 Hz, Pt-Ccar); Anal. Calcd for
C14H17F3N2O3PtS2: C,29.12; H, 2.97; N, 4.85. Found: C, 29.05; H,
2.88; N, 4.77.
3J ) 7.0 Hz), 9.88 (dd, 1H, J ) 5.8 Hz, J ) 1.4 Hz); 13C{1H} (125
MHz, CD2Cl2): δ ) -10.2 (1JPt-C ) 757.4 Hz, Pt-CH3), 30.2, 37.2,
44.3 (-43.15, 1JPt-C ) 44.5 Hz, (Pt-OS(CH3)2), 116.1, 121.5 (1JF-C
) 321.9 Hz, F3CSO3-) 121.9, 127.4, 130.1 (JPt-C ) 112.4 Hz), 139.2
(JPt-C ) 35.4 Hz), 143.3, 151.2, 156.1 (JPt-C ) 47.7 Hz), 169.6-
(JPt-C ) 47.7 Hz), 172.2 (1JPt-C ) 1231.2 Hz, Pt-C); Anal. Calcd
for C18H25F3N2O4PtS2: C, 33.28; H, 3.88; N, 4.31. Found: C, 34.03;
H, 4.08; N, 4.16.
3
4
[(4-tert-Butyl-iso-BIPY)Pt(CH3)(DMS)]+ [OTF]- (2b). [(µ-SMe2)-
Pt(CH3)2]2 (0.954 g, 1.66 mmol) and N-(2-pyridyl)-4-tert-butyl-pyri-
9
8254 J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004

N-Heterocyclic Carbene Complexes of Platinum
A R T I C L E S
[(5-(Dimethylamino)-iso-BIPY)Pt(CH3)(DMSO)]+ [OTF]- (4c).
mixed with 250 mL of methylene chloride and left stirring overnight.
In the morning the bright yellow solution had turned nearly colorless,
and a white precipitate had formed. The mixture was filtered through
Celite, and the volume concentrated to a total of 10 mL. Diethyl ether
was slowly added to give a white precipitate. The suspension was placed
in a -20°C freezer, and the white solids were isolated on a glass frit,
washed with diethyl ether, and dried under high vacuum to give 0.365
The reaction was carried out according to the above procedure to yield
1
1.176 g (72%) of 4c as an orange powder. H NMR (300 MHz, CD2-
Cl2) δ ) 1.15 (s, 3H, 2JPt-H ) 81.0 Hz, Pt-CH3), 2.51 (s, 6H, 3JPt-H
3
) 31.7 Hz, OS(CH3)2), 3.14 (s, 6H, N(CH3)2), 7.53 (dd, 1H, J ) 9.3
Hz, 4J ) 2.8 Hz, JPt-H ) 6.4 Hz), 7.76 (ddd, 1H, 3J ) 5.5 Hz, 3J )
4
3
5.5 Hz, J ) 1.1 Hz), 8.06 (d, 1H, J ) 9.3 Hz, JPt-H ) 24.0 Hz),
3
3
1
8.40 (d, 1H, J ) 2.8 Hz, JPt-H ) 4.3 Hz), 8.51 (ddd, 1H, J ) 7.7
g (92%) of 3c as an off-white powder. H NMR (300 MHz, CD2Cl2)
Hz, 3J ) 7.7 Hz, 4J ) 1.6 Hz), 8.67 (d, 1H 3J ) 8.7 Hz), 9.07 (dd, 1H
2
δ ) 0.70 (s, 3H, JPt-H ) 80.79 Hz Pt-CH3), 3.09 (s, 3H, NCH3),
3J ) 5.5 Hz, J ) 1.1 Hz, JPt-H ) 7.3 Hz); 13C{1H} NMR (125
4
2
3.21 (s, 3H, NCH3), 3.21 (s, 6H, JPt-H ) 20.20 Hz Pt-S(CH3)2),
MHz, CD2Cl2): δ ) -10.31 (1JPt-C ) 789.2, Pt-CH3), 20.77, 40.5,
116.8 (JPt-C ) 13.9 Hz), 120.3 (JPt-C ) 39.8 Hz), 127.7 (JPt-C
) 9.0 Hz), 128.8 (JPt-C ) 117.5 Hz), 132.8 (JPt-C ) 117.5 Hz),
142.7, 145.9, 146.6, 157.0 (JPt-C ) 51.2 Hz), 160.94 (1JPt-C )
1323.3 Hz, Pt-Ccar); Anal. Calcd for C16H22F3N3O4PtS2: C, 30.19; H,
3.48; N, 6.60. Found: C, 30.35; H, 3.47; N, 6.62.
3
3
6.18 (dd, 1H, J ) 8.16 Hz, J ) 3.22 Hz), 6.83 (m, 5H), 6.94-7.04
3
(m, 9H), 7.33-7.47 (m, 10H), 7.90 (m, 1H), 9.61 (dd, 1H J ) 5.72
Hz, J ) 1.29 Hz, JPt-H ) 6.15 Hz); 13C{1H} (125 MHz, CD2Cl2):
δ ) -10.40 (1JPt-C ) 761.3 Hz, Pt-CH3), 105.8, 110.4 (JPt-C )
118.79 Hz), 113.1, 123.7, 136.7, 141.7, 148.9, 153.8 (JPt-C ) 60.5
Hz), 155.1, 163.6 (1JPt-C ) 1202.89 Hz, Pt-Ccar). Anal. Calcd for
C39H42BN3OPtS: C, 58.06; H, 5.25; N, 5.21. Found: C, 57.79; H, 5.40;
N, 5.11.
4
3
Substitution of Tetraphenylborate for Triflate. The triflate salts
were dissolved in a small amount of methanol, and a concentrated
solution of sodium tetraphenylborate (1 eq) was added dropwise,
immediately precipitating the product. Cooling to -40 °C in the freezer
and isolation on a glass frit gave >90% yield of the tetraphenylborate
salts. A representative preparation is shown below.
Synthesis of Carbonyl Cations 1d-4d. The dimethyl sulfide
complexes 1b-4b were dissolved in acetone (1,2,4) or prepared in
situ (3) and degassed. Carbon monoxide (1 atm) was added and the
solutions allowed to stir for 1 h. The carbonyl cations were precipitated
by addition of diethyl ether and isolated on a glass frit. IR spectra were
acquired in acetone. Anal. 2d Calcd for C17H19F3N2O4PtS: C, 34.06;
H, 3.19; N, 4.67. Found: C, 33.73; H, 3.12; N, 4.40. 4d Calcd for
C15H16F3N3O4PtS: C, 30.72; H, 2.75; N, 7.17. Found: C, 31.02; H,
2.73; N, 6.90.
[(5-(Dimethylamino)-iso-BIPY)Pt(CH3)(DMSO)]+ [BPh4]- (4c).
A few drops of DMSO were added to complex (4b) (0.400 g, 0.644
mmol) to dissolve it. After stirring the dark liquid under vacuum for
30 min. methanol (15 mL) was added. A solution of sodium tetraphen-
ylborate (0.221 g, 0.644 mmol) in methanol (20, ml) was added
dropwise with stirring to the orange solution. An orange precipitate
formed upon addition. The mixture was filtered and the orange powder
washed with diethyl ether and dried under vacuum. 0.467 g (yield )
90%). 1H NMR (500 MHz, CD2Cl2) δ ) 0.878 (s, 3H, 2JPt-H ) 80.32
(C11H8N)Pt(CH3)(DMSO) (6c). 2-Phenylpyridine (481 µL, 3.37
mmol) was added dropwise via syringe to a solution of [(µ-SMe2)Pt-
(CH3)2]2 (0.967 g, 1.68 mmol) in 50 mL of acetone at room temperature
in air. The solution quickly became yellow, and small bubbles formed.
After stirring for 1.5 h DMSO (477 µL, 6.73 mmol) was added, and
the solution stirred for another 30 min. The yellow solution was
concentrated, and pentane was added, precipitating the bright yellow
product. The suspension was filtered and dried under vacuum giving
3
Hz, Pt-CH3), 3.03 (s, 6H, NMe2), 3.26 (s, 6H, JPt-H ) 20.9 Hz,
3
3
S(CH3)2), 6.86 (at, 4H, J ) 7.33 Hz), 7.02 (at, 8H, J ) 7.33 Hz),
7.35 (bs, 8H), 7.45 (d, 1H, 3J ) 8.64 Hz), 7.49 (dd, 1H, 3J ) 9.25 Hz,
4J ) 2.75 Hz), 7.63 (at, 1H, 3J ) 6.2 Hz), 7.73 (d, 1H, 3J ) 2.57 Hz),
7.99 (at, 1H, 3J ) 8.44 Hz), 8.03 (d, 1H, 3J ) 9.18 Hz), 9.90 (dd, 1H
1
0.864 g (58%) of 6c as a bright yellow solid. H NMR (300 MHz,
4
3J ) 5.60 Hz, J ) 1.65 Hz); 13C{1H} NMR (125 MHz, CD2Cl2): δ
2
acetone-d6) δ ) 0.66 (s, 3H, JPt-H ) 83.52 Hz, Pt-CH3), 3.21 (s,
) -11.12 (1JPt-C ) 788.8 Hz, Pt-CH3), 39.96, 43.92, 114.6, 118.76
(JPt-C ) 40.92 Hz), 122.0, 125.9, 127.2, 128.1 (JPt-C ) 51.1 Hz),
133.3 (JPt-C ) 110.0 Hz), 136.1, 142.5, 146.0, 146.2, 151.0, 156.0,
159.0, 164.2 (1JB-C ) 49.2 Hz); Anal. Calcd for C39H42BN3OPtS:
C, 58.06; H, 5.25; N, 5.21. Found: C, 57.79; H, 5.40; N, 5.11.
(4-(Dimethylamino)-iso-BIPY)Pt(CH3)Br (3e). [(µ-SMe2)Pt(CH3)2]2
(0.6920 g, 1.20 mmol) was dissolved in 225 mL of methylene chloride
to which was added N-(2-pyridyl)-4-(dimethylamino)-pyridinium bro-
mide (0.6749 g, 2.41 mmol). Upon stirring the solution became yellow,
and methane evolved. The reagents were left stirring for 8 h, and a
crystalline yellow powder slowly precipitated from the solution. The
volume was concentrated to 5 mL, and the solids isolated on a glass
frit to give 0.660 g (56%) of 3e as a bright yellow powder. Further
concentration of the filtrate provided additional material. 1H NMR (300
MHz, CD3NO2) δ ) 0.91 (s, 3H, 3JPt-H ) 83.79 Hz, Pt-CH3), 3.16
(s, 6H, N(CH3)2), 6.72 (dd, 1H, 3J ) 8.19 Hz, 3J ) 3.11 Hz), 7.04 (d,
1H, 3JPt-H ) 70.18 Hz 3J ) 3.27 Hz), 7.39 (m, 1H), 7.67 (d, 1H, 3J
) 8.96 Hz), 8.12 (m, 1H), 8.33 (d, 1H, 3J ) 8.04 Hz, 3JPt-H ) 6.42
2
3
3
6H, JPt-H ) 17.6 Hz, S(CH3)), 7.11 (dt, 1H, J ) 7.69 Hz, J )
3
3
1.65 Hz), 7.21 (dt, 1H, J ) 7.74 Hz, J ) 1.65 Hz), 7.31-7.36 (m,
1H), 7.50 (d, 1H, J ) 1.1 Hz,2JPt-H ) 28.6 Hz), 7.34-7.79 (m,
3
1H), 8.03 (dd, 2H 3J ) 3.85 Hz, 4J ) 1.10 Hz) 9.80 (d, 1H 3J ) 5.60
Hz,2JPt-H ) 7.1 Hz); 13C{1H} (125 MHz, CD2Cl2): δ ) -12.4
(1JPt-C ) 776.1, Pt-CH3), 43.3 (JPt-C ) 34.6), 118.4, 122.5, 123.3
(JPt-C ) 39.3), 124.8, 129.0 (JPt-C ) 66.7 Hz), 132.5 (JPt-C )
88.9 Hz), 137.9, 146.6, 150.3, 150.4 (JPt-C ) 1062.8 Hz, Pt-Cph),
164.0 (1JPt-C ) 74.9 Hz). Anal. Calcd for C14H17NOPtS: C, 38.01;
H, 3.87; N, 3.17. Found: C, 38.12; H, 3.84; N, 3.15.
Acknowledgment. The authors would like to thank Lawrence
M. Henling and Michael W. Day for assistance with X-ray
crystallography and Ola F. Wendt for useful discussions. We
gratefully acknowledge support from the MC2 program in
collaboration with bp.
3
4
3
Hz), 9.61 (dd, 1H J ) 5.51 Hz, J ) 1.60 Hz, JPt-H ) 6.63 Hz);
Anal. Calcd for C13H16BrN3Pt: C, 31.91; H, 3.30; N, 8.59. Found: C,
38.43; H, 4.49; N, 10.11.
Supporting Information Available: Erying plots and kobs vs
[DMSO] plots for 1c-4c and 6 as well as relevant crystal-
lographic data and pictures of structures for 1b, 2c-4c, 5, and
6. This material is available free of charge via the Internet at
http://pubs.acs.org.
[(4-(Dimethylamino)-iso-BIPY)Pt(CH3)(DMSO)]+ [BPh4]- (3c).
N-(2-pyridyl)-4-(dimethylamino)-pyridine-2-ylidene platinum methyl
bromide (3e) (0.2400 g, 0.491 mmol), sodium tetraphenylborate (0.1679
g, 0.491 mmol) and dimethyl sulfoxide (105 µL, 1.484 mmol) were
JA040075T
9
J. AM. CHEM. SOC. VOL. 126, NO. 26, 2004 8255