Syntheses, structures, and reductive elimination studies of six-membered diaryl platinacycle complexes

Article abstract of DOI:10.1021/om901033z

The six-membered platinacycles PtL₂(C₆H ₄XC₆H₄) (X = CH₂, O, NMe; L = PEt₃, L₂ = 1,3bis(diphenylphosphino)propane (dppp), 4,4'-bis-rerr-butyl-2,2'-bipyridine (^tBu ₂bpy)), have been prepared from. Cw-PtL₂Cl₂ and the appropriate dilithio reagents. Reductive elimination studies on the platinacycles with L = PEt₃ show that the bridging group (X) dramatically influences the reductive elimination rate. Thermodynamic activation parameters were determined for the platinacycles and showed a ΔH* trend X = NMe O < CH₂ with an essentially zero value for ΔS*. Rate constants at 95 °C show over a million-fold increase on going from X = CH₂ to X = NMe. DFT calculations support direct elimination without phosphine ligand loss and indicate a progressively earlier transition state in the series X = CH₂,O, NMe. The earlier transition state and the accelerated rate are associated with the beginning of aromatization in the eliminating organic unit. Computed thermodynamic activation parameters are in good agreement with the experimental results.

Full text of DOI:10.1021/om901033z

1388 Organometallics 2010, 29, 1388–1395
DOI: 10.1021/om901033z
Syntheses, Structures, and Reductive Elimination Studies of
Six-Membered Diaryl Platinacycle Complexes
Robert Robinson Jr. and Paul R. Sharp*
125 Chemistry, University of Missouri;Columbia, Columbia, Missouri 65211
Received November 30, 2009
The six-membered platinacycles PtL2(C6H4XC6H4) (X = CH2, O, NMe; L = PEt3, L2 = 1,3-
bis(diphenylphosphino)propane (dppp), 4,40-bis-tert-butyl-2,20-bipyridine (tBu2bpy)), have been
prepared from cis-PtL2Cl2 and the appropriate dilithio reagents. Reductive elimination studies on
the platinacycles with L = PEt3 show that the bridging group (X) dramatically influences the
reductive elimination rate. Thermodynamic activation parameters were determined for the
platinacycles and showed a ΔHq trend X = NMe , O < CH2 with an essentially zero value for
ΔSq. Rate constants at 95 °C show over a million-fold increase on going from X = CH2 to X =
NMe. DFT calculations support direct elimination without phosphine ligand loss and indicate a
progressively earlier transition state in the series X = CH2, O, NMe. The earlier transition state and
the accelerated rate are associated with the beginning of aromatization in the eliminating organic
unit. Computed thermodynamic activation parameters are in good agreement with the experi-
mental results.
Introduction
electron donating and even higher rates when one aryl
group has an electron-withdrawing substituent and the
other an electron-donating substituent.5-7
Reductive elimination is an important and vital reaction
in chemistry and is often the crucial step for product
formation in metal-catalyzed organic synthesis reactions.1
Both palladium(0) and nickel(0) complexes are commonly
used as catalysts because of their high activity.2,3 High
activity implies short-lived intermediates, making isolation
and study of intermediates very difficult. Analogous
platinum(0) complexes are usually more stable and amen-
able to study. Diaryl Pt complexes of the formula cis-
PtL2(C6H4R)(C6H4R0) (L2 = phosphines) can be isolated
and undergo reductive elimination to give biaryls,
RC6H4-C6H4R0 (eq 1), whereas the analogous Pd com-
plexes cannot be isolated.4 Reductive elimination studies
of these Pt complexes have revealed interesting aryl sub-
stituent features. Increased reductive elimination rates are
observed in symmetric complexes when the R groups are
These studies are informative for simple diaryl complexes
and the formation of biaryls. However, the formation of ring
systems incorporating biaryls requires reductive elimination
from metallacycle complexes.8-12 Linking the aryl groups
together in a metallacycle complex introduces additional
factors in the reductive elimination process. Our group has
shown that for Pt and Pd four- and five-membered metalla-
cycles where the aryl groups have been linked into a polycyclic
framework simple reductive elimination is shut down.13,14
Instead, apparent bimolecular reductive elimination occurs
catalytically on colloidal metal surfaces after transfer of the
metallacycle ring from the complex to the metal colliod sur-
face. In the presence of alkynes cycloaddition reactions also
occur on the colloid surface. In this paper we report our work
on reductive eliminations from six-membered platinacycles
*Corresponding author. E-mail: SharpP@missouri.edu.
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4 ed.; Wiley: New York, 2005.
(2) Brandsma, L.; Vasilevsky, S. F.; Verkruijsse, H. D. Application of
Transition Metal Catalysts in Organic Synthesis; Springer-Verlag:
New York, 1999.
(3) Tsuji, J. Palladium Reagents and Catalysts: New Perspectives for
the 21st Century, 2nd ed.; J. Wiley: Chichester, 2004.
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Trans. 1976, 1306–1310.
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1845–56.
(10) Campeau, L. C.; Parisien, M.; Jean, A.; Fagnou, K. J. Am.
Chem. Soc. 2006, 128, 581–590.
(11) Campo, M. A.; Huang, Q.; Yao, T.; Tian, Q.; Larock, R. C.
J. Am. Chem. Soc. 2003, 125, 11506–11507.
(7) Shekhar, S.; Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 13016–
13027.
(8) Ma, S.; Gu, Z. Angew. Chem., Int. Ed. 2005, 44, 7512–7517.
(9) Garcia-Cuadrado, D.; Braga, A. A. C.; Maseras, F.; Echavarren,
A. M. J. Am. Chem. Soc. 2006, 128, 1066–1067.
(12) Catellani, M.; Motti, E.; Faccini, F.; Ferraccioli, R. Pure Appl.
Chem. 2005, 77, 1243–1248.
(13) Begum, R. A.; Chanda, N.; Ramakrishna, T. V. V.; Sharp, P. R.
J. Am. Chem. Soc. 2005, 127, 13494–13495.
(14) Chanda, N.; Sharp, P. R. Organometallics 2007, 26, 1635–1642.
r
pubs.acs.org/Organometallics
Published on Web 02/23/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 6, 2010 1389
Table 1. Selected 195Pt, 31P, and 13C NMR Data for 1-5a
δ 195Pt
δ 31P (JPt-P
)
δ C1d (1JC-Pt, 2JC-Ptrans
)
1b,c
2b
-4574
-3112
-4700
-3079
-3646
4.8 (1833)
5.0 (2014)
7.3 (1979)
3.6 (1902)
161.5 (806.5, 116.7)
145.3 (769.7, 105.7)
not resolved
not resolved
not resolved
3e
4b
5b
a Chemical shifts (δ) in ppm and coupling constants (J) in Hz. b In
15
CDCl3. c Data from ref
.
d C1 = Pt-bonded carbon atom. e In THF
at -40 °C.
where the aryl groups have been linked together by a bridging
group in the ortho positions. We find that simple reductive
elimination is turned back on and that, similar to the ring
substituents in the diaryl complexes (eq 1), the bridging group
influences the reductive elimination rate, but the effect is many
orders of magnitude larger.
Figure 1. Molecular structure of Pt(PEt3)2(C6H4OC6H4) (2)
(50% probability ellipsoids, hydrogen atoms omitted for clarity).
Results
Syntheses and NMR Spectra. Platinacycles 1-5 were
synthesized by the reaction of cis-PtL2Cl2 (L = PEt3, L2 =
t
dppp or Bu2bpy) with the appropriate dilithio reagents
(eq 2). The synthesis of 1 by this method was previously
reported.15 With the exception of 3, which could not be
isolated due to its instability above -30 °C, the platinacycles
were isolated in >80% yield. Complex 3 was characterized
by NMR spectroscopy and by its decomposition products
(see below). All other new platinacycles were fully character-
ized by NMR spectroscopy, X-ray crystallography, and
elemental analysis. For comparison purposes, previously
reported data for 1 are included below.
Figure 2. Molecular structure of Pt(dppp)(C6H4NMeC6H4) (4)
(50% probability ellipsoids, hydrogen atoms omitted for clarity).
all show triplets from 31P coupling with no discernible trend in
the shifts.
All complexes exhibit the expected four aromatic proton
and six aromatic carbon NMR signals for the two equivalent
aryl rings. 13C NMR data for the Pt-bonded carbon atoms
(C1) of 1 and 2 are included in Table 1. Coupling constants are
consistent with the trans influence trend in the 31P-195Pt
coupling observed in the 195Pt and 31P NMR spectra. Both
the 13C-195Pt and the 13C-31P coupling decrease from 1 to 2,
indicating a weaker donor on going from X = CH2 to X = O.
X-ray Structures. Thesolid-statestructures of2, 4, and5 are
depicted in Figures 1-3, with selected metrical parameters
listed in Table 2. These three complexes and 115 differ only in
the bridging group X and/or the ancillary ligand, and the
overall structures are very similar. All of the complexes have
slightly distorted square-planar environments surrounding
the Pt center. In 2, the L-Pt-L angle (P1-Pt1-P2) is
98.43(3)o, nearly identical to the previously reported value
for 1 of 98.8(1)o. Smaller L-Pt-L angles are observed in 4
(90.58(3)o) and 5 (76.56(10)o). The N-Pt-N angle in 5 is
31P and 195Pt NMR spectroscopic data for the platinacycles
are shown in Table 1. 31P NMR spectra of 1-4 show singlets
with 195Pt satellites. Chemical shifts for the PEt3 derivatives
1-3 lie in a narrow region (greatest difference (3.2 ppm),
consistent with nearly equivalent chemical environments for
the 31P nuclei. 31P-195Pt coupling constants are typical for a
phosphine ligand trans to an aryl group in Pt(II) complexes.16
For 1-3 a trend is detected where the coupling constant
increases in the order X = CH2 < NMe < O, indicative of a
decrease in the trans influence of the aryl group with increasing
electronegativity of the bridging group.17,18 195Pt NMR spectra
(15) Alcock, N. W.; Bryars, K. H.; Pringle, P. G. J. Chem. Soc.,
Dalton Trans. 1990, 1433–1439.
(16) Dietrich, G.; Vimal, K. J.; Axel, K.; Thilo, S.; Stanislav, Z. L.
Eur. J. Inorg. Chem. 2005, 2005, 4056–4063.
(17) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335–422.
t
controlled by the geometry of the bidentate Bu2bpy ligand.
The smaller P-Pt-P angle in 4 is probably not enforced by
the dppp ligand but rather may result from steric interac-
tions between the PPh2 groups and the platinacycle aryl rings.
The C-Pt-C angle in 2 (80.31(11)o) is slightly smaller than
(18) Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738–47.

1390 Organometallics, Vol. 29, No. 6, 2010
Robinson and Sharp
Table 3. Experimental (exptl)a and Computational (DFT)b
Thermodynamic Data
ΔHq (kcal/mol)
ΔSq (cal/K mol)
3
exptl
DFT
exptl
DFT
1 or 10 (X = CH2)
2 or 20 (X = O)
3 or 30 (X = NMe)
29.0(9)
25.9(6)
21(2)
27.4
23.9c
18.4
-4.2(3)
-3.1(2)
6(2)
3.8
0.8c
4.3
a In toluene. b T = 298 K, gas phase, with PMe3 in place of PEt3.
c Optimization not converged (see text).
Scheme 1
Figure 3. Molecular structure of Pt(tBu2bpy)(C6H4NMeC6H4)
(5) (50% probability ellipsoids, hydrogen atoms omitted for
clarity).
“stern”. The fold angle, as defined by the acute angle between
the two planes (Pt1, C1, C2, X and Pt1, C7, C8, X), decreases
in the order 1 > 2 . 4 > 5. The two platinacycles 4 and 5
allow us to examine structural features associated with the
NMe group that are not possible with unstable 3. Both
complexes show a substantially smaller angle than the
X = O and CH2 derivatives, 1 and 2, indicating a greater
tendency toward planarity for the X = NMe complexes.
This is attributed to nitrogen lone pair donation into the
aromatic rings of the platinacyclic system and is consistent
with the near planarity of the N centers (sum of angles
˚
Table 2. Selected Metrical Parameters (distances in A and angles
in deg) for Platinacycles 1, 3, 4, and 5a
1 (X = CH2)
2 (X = O) 4 (X = NMe) 5 (X = NMe)
Pt-C
2.062(11)
2.038(11)
2.062(3)
2.059(3)
2.046(3)
2.058(3)
1.993(3)
1.992(3)
av = 2.050(17) av = 2.060(2) av = 2.052(8) av = 1.992(1)
Pt-L
2.303(3)
2.323(3)
2.321(1)
2.330(1)
2.3072(7)
2.3144(7)
av = 2.313(14) av = 2.326(6) av = 2.311(5) av = 2.120(0)
2.120(3)
2.120(3)
C2-X
C1---C7
1.531(17)
1.512(14)
av = 1.522(13) av = 1.410(1) av = 1.419(1) av = 1.414(8)
1.410(3)
1.411(4)
1.418(4)
1.419(3)
1.408(5)
1.420(4)
˚
∼350°), the short average C-N distance (1.414 A), and the
small angle between the aryl ring planes (48.6°).
2.67(2)
2.658(4)
80.32(11)
98.43(3)
109.7(2)
66.6
2.697(3)
82.17(10)
90.58(3)
116.6(2)
58.0
2.688(6)
84.82(13)
76.56(10)
116.8(3)
51.4
C1-Pt-C7 81.3(4)
L-Pt-L 98.8(1)
C2-X-C8 107.5(9)
fold angleb 69.5
ring anglec 74.7
Reductive Elimination Studies. All three platinacycles 1-3
thermally decompose by reductive elimination with C-C
bond formation (eq 3). This process occurs at comparable
rates at -30 °C for 3, 70 °C for 2, and 110 °C for 1. The
products are fluorene (X = CH2), dibenzofuran (X = O),
and N-methylcarbazole (X = NMe) and the fragment Pt-
(PEt3)2. For subsequent kinetic studies PhCCPh was added
to trap the Pt(PEt3)2 fragment, giving the stable and readily
detected adduct Pt(PEt3)2(η2-PhCCPh) (6). Control experi-
ments indicated that PhCCPh, PEt3, water, and O2 additions
to the reactions do not affect the reductive elimination rates.
Reactions were followed by 31P NMR spectroscopy over
three half-lives, and plots of the concentration against time
revealed first-order kinetics for all three platinacycles.
69.4
53.8
48.6
a Numbers in parentheses indicate the estimated error in the last
significant digit or for the averages (av) the standard deviation. b Acute
angle between planes Pt1, C1, C2, X and Pt1, C7, C8, X. c Acute angle
between C6 ring planes C1-C6 and C7-C12.
that in 1 (81.3(4)o) but increases in 4 (82.17(10)o) and 5
(84.82(13)o). With both 2 and 4 containing phosphine ligands
˚
the average Pt-C (2.060(2) and 2.052(8) A) and Pt-P
(2.326(6) and 2.311(5) A) bond distances are very similar
˚
˚
and comparable to those in 1 (2.050(17) and 2.313(14) A).
Similarly, the average Pt-N and Pt-C bond lengths of 2.12
19-24
˚
and 1.99 A in 5 are typical of bpy phenyl complexes.
As previously observed for 1,15 the Pt-containing six-
membered ring of 2, 4, and 5 adopts a boat conformation
with the Pt atom and the bridging group X at the “bow” and
(19) De Crisci, A. G.; Lough, A. J.; Multani, K.; Fekl, U. Organo-
metallics 2008, 27, 1765.
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R.-Y.; Macartney, D.; Wang, S. J. Am. Chem. Soc. 2007, 129, 7510.
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Rate constants for the reductive elimination reactions
were determined at three different temperatures (Supporting
Information). Extracted thermodynamic parameters are
listed in Table 3. As expected from the reductive elimination

Article
Organometallics, Vol. 29, No. 6, 2010 1391
a
˚
Table 4. Selected Metrical Parameters (distances in A and angles in deg) for DFT Optimized Structures
10
20
30
10-TS
20-TSb
30-TS
(X = CH2)
(X = O)
(X = NMe)
(X = CH2)
(X = O)
(X = NMe)
Pt-Cb
2.068
2.420
1.521
2.717
82.1
96.4
108.1
72.9
2.059
2.420
1.394
2.699
81.9
95.5
113.1
61.2
2.057
2.419
1.427
2.682
81.4
95.6
114.0
62.2
2.153
2.372
1.519
1.844
50.7
111.1
103.4
60.2
2.124
2.381
1.378
1.870
52.2
108.8
108.8
52.5
2.113
2.382
1.399
1.916
53.9
107.8
111.1
53.8
Pt-Pb
X-Cb
C1–C7
C1-Pt-C7
P-Pt-P
C2-X-C8
ring anglec
a Optimization not converged (see text). b Average distance. c Acute angle between C6 ring planes C1-C6 and C7-C12.
temperatures, the ΔHq values follow the trend 1 > 2 . 3,
where the 2 and 3 gap is nearly twice that of 1 and 2. The
ΔSq values are small, as expected for a unimolecular reaction.
Overall, these activation parameters are quite comparable to
those for biaryl elimination from Pt((C6F5)2PCH2CH2P-
(C6F5)2)(Ph)2 in benzene (ΔHq = 27.6 ( 1 kcal/mol, ΔSq =
-1.5 ( 1 cal/K mol)25 and fall into the range reported for
3
elimination from a number of complexes of formula cis-
Pt(P(C6H4X)3)2(C6H4Y)(C6H4Z) (ΔHq = 23.4 ( 0.4 to
34 ( 2 kcal/mol, ΔSq = -5.9 ( 4.6 to 21 ( 7 cal/K mol).26
3
Computational Studies. The platinacycle reductive elimi-
nation process was evaluated computationally in the gas
phase using DFT. Model structures (10, 20, and 30), where the
PEt3 ligand of 1, 2, and 3 was replaced with PMe3, were
optimized. A comparison of the calculated structures with
the crystallographic structures showed reasonable agree-
Figure 4. GaussView34 drawing of 30-TS.
smoothly for 10-TS and 30-TS but failed to converge for 20-TS
due to low-energy ((0.06 kcal) PMe3 group oscillations. Opti-
mization with force constant calculation at each step did not
correct the problem. Therefore, a minimum energy structure
was selected as representative of 20-TS. A frequency calculation
for this structure and for 10-TS and 30-TS showed a single
imaginary frequency with the expected vibrational mode. Com-
puted activation parameters are listed in Table 3, and although
ΔHq values are lower by a few kilocalories, the parameters
match the experimental results very well including the larger gap
in energy between 2 and 3 than between 1 and 2. Selected
metrical parameters for the optimized structures are given in
Table 4, and a drawing of 30-TS is provided in Figure 4.
As would be expected, the transition-state structures all
show a decrease in the C1-Pt-C7 angle, a lengthening of the
Pt-C distance, and a reduction in the C1–C7 distance.
Simultaneously, there is a reduction in the Pt-P distance
and a widening of the P-Pt-P angle consistent with the
incipient formation of the linear Pt(PMe3)2 fragment. These
features are very similar to those of transition-state struc-
tures calculated for biphenyl reductive elimination from
diaryl Pt and Pd complexes27-29 and account for the much
greater stability of 4 and 5 over 3. The chelating ligands of 4
and 5 prevent widening of the L-Pt-L angle, thereby
destabilizing the reductive elimination transition state.
The boat configuration with the Pt atom at the “bow” and
X atthe “stern”is still observed, but the Ptatom has moved up
and back toward the “stern”. This has moved the Pt atom into
position for bonding with the arylπ-systems and begins to free
the C sp2 orbital for formation of the incipient C-C bond. In
the eliminating organic component there is an increase in
planarity, as shown by the decrease in the acute angle between
the aryl rings and a decrease in the C2-X-C8 angle consis-
tent with the beginning formation of the five-membered ring.
There is also a decrease in the X-C2, C8 distance though it is
ment. A notable difference is the trend in fold angles:
66.17° for 10, 61.01° for 20, and 62.21° for 30, as compared
to 69.5°, 66.6°, and 58° from the corresponding X-ray
structures. However, the last X-ray value is based on the
dppp structure of 4 as a surrogate for that of 3.
Two mechanistic pathways were considered for the reductive
elimination process: (1) phosphine ligand loss followed by
reductive elimination from a three-coordinate intermediate
and (2) reductive elimination from the four-coordinate com-
plexes. The first process has a much greater total activation
energy than the second and is inconsistent with the absence of a
free phosphine influence on the reductive elimination rates. The
second process is also consistent with the majority of studies on
related complexes.4,26-33 Direct reductive elimination from the
four-coordinate complexes is therefore considered to be opera-
tive (Scheme 1).
Transition states (10-TS, 20-TS, and 30-TS) were readily
located using potential energy scans over the Pt-bonded carbon
atom distance (C1--C7). Subsequent optimization proceeded
(25) Merwin, R. K.; Schnabel, R. C.; Koola, J. D.; Roddick, D. M.
Organometallics 1992, 11, 2972–2978.
(26) Brune, H. A.; Falck, M.; Hemmer, R.; Alt, H. G. Chem. Ber.
1984, 117, 2803–14.
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(27) Perez-Rodrıguez, M.; Braga, A. A. C.; Garcia-Melchor, M.;
Perez-Temprano, M. H.; Casares, J. A.; Ujaque, G.; de Lera, A. R.;
ꢀ
ꢀ
Alvarez, R.; Maseras, F.; Espinet, P. J. Am. Chem. Soc. 2009, 131, 3650–
3657.
(28) Yi-Luen, H.; Chia-Ming, W.; Fung, E. H. Chem.-Eur. J. 2008,
14, 4426–4434.
(29) Ananikov, V. P.; Musaev, D. G.; Morokuma, K. Organometallics
2005, 24, 715–723.
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1392 Organometallics, Vol. 29, No. 6, 2010
Robinson and Sharp
very small in 10-TS (X = CH2). For 20-TS (X = O) and 30-TS
(X= NMe) the greater decrease is attributed toincreased lone
pair donation into the aromatic rings.
primarily with the larger ring size. We know from oxidative
addition studies of biphenylene to Pt(0) complexes that
the energies for biphenylene reductive elimination from
five-membered biphenyl platinacycle are unfavorable.31,38
Direct elimination from 8 would require the formation of a
strained four-membered biphenylene, even more strained
than parent biphenylene. Direct elimination from 7 would
form a three-membered ring that would be extremely un-
favorable.
In comparing the three transition states there are trends in
the metrical parameters indicating that the transition state
occurs earlier in the reductive elimination process in the order
30-TS earlier than 20-TS earlier than 10-TS. This is seen in the
order of the Pt-C1, C7 distances, 30-TS < 20-TS < 10-TS, the
C1–C7 distances, 30-TS > 20-TS > 10-TS, and the P-Pt-P
angles, 30-TS < 20-TS < 10-TS. Earlier stabilization is likely
due to lone pair donation into the π-system with incipient
aromatization on C1-C7 bond formation. Donation from the
NMe group is stronger and the final product, N-methylcarba-
zole, is more aromatic than dibenzofuran, the final product
with X = O. With X = CH2, there is no possibility of
aromaticity in the ring system. These arguments are consistent
with the Hammond postulate, where greater stabilization of
the product will shift the transition state toward the reactant.35
Reductive elimination from 1-3 should share features
with reductive elimination from the diaryl complexes
cis-PtL2(C6H4R)2. While the best comparison would be
with 2-substituted cis-PtL2(C6H4-2-R)2, the complexes
that have been most studied are 3- and 4-substituted cis-
PtL2(C6H4-4-R)2. Strong similarities should still be ob-
served and the behavior of cis-PtL2(C6H4-4-R)2 with R =
CH3, OMe, and NMe2 would be expected to show parallels
to 1-3, where X = CH2, O, NMe. A reductive elimina-
tion kinetic study of this series with L2 = dppf (1,10-bis
(diphenylphosphino)ferrocene) was recently reported.7
Activation parameters were not determined, but first-order
rate constants at 95 °C are 15.5 ꢀ 10-5 s-1 (R = CH3),
5.35 ꢀ 10-5 s-1 (R = OMe), and 32.3 ꢀ 10-5 s-1 (R =
NMe2). As with 1-3, the fastest rate is observed for the
nitrogen derivative, but the rates for the oxygen and carbon
derivatives are inverted from that for 1-3. For compar-
ison, calculated rate constants for 1-3 at 95 °C are 0.466 ꢀ
10-5 s-1 (X = CH2), 71.0 ꢀ 10-5 s-1 (X = O), and
2 800 000 ꢀ 10-5 s-1 (X = NMe). Similar substituent
effects are observed, but the changes are vastly greater
with 1-3. The rates for 1 and the diaryl complexes are
similar, but the rate for 3 is millions of times greater than
for the R = NMe2 diaryl complex.
A larger substituent effect with a trend matching that of
1-3 is observed for the asymmetric diaryl complexes cis-Pt-
(dppf)(C6H4-4-R)(C6H4-4-CF3), where the rate constants
at 95 °C are 17.6 ꢀ 10-5 s-1 (R = CH3), 23.3 ꢀ 10-5 s-1
(R = OMe), and 159 ꢀ 10-5 s-1 (R = NMe2).7 The effect is
greater than with the symmetric diaryl complexes but is still
orders of magnitude less than for 1-3. We believe these
larger effects are due to lone pair electron delocalization in
the transition states. The very large effect observed in 3 and
to a lesser extent in 2 is due to the beginning of aromatiza-
tion of the ring system with the formation of the C-C bond.
The N lone pair begins to delocalize over the entire ring
system, strongly stabilizing the transition state. A similar,
though less dramatic stabilization of the transition state
occurs for the asymmetric diaryl cis-Pt(dppf)(C6H4-4-R)
(C6H4-4-CF3) when R = NMe2, where the formation of the
C-C bond allows electron density from the electron-rich
C6H4-4-NMe2 ring to delocalize into the relatively electron-
poor C6H4-4-CF3 ring. Alternatively, this can be viewed in
terms of the Hammond postulate, where greater stabiliza-
tion of the product by electron delocalization shifts the
transition state toward the reactant.35
Discussion
Our initial interest in the six-membered platinacycles 1-3
was in the possibility of colloidal Pt catalysis of their reductive
elimination reactions. We have shown previously that alkyne
complex 6 in the presence of even traces of O2 suffers oxidative
deligation at 120 °C to produce colloidal Pt and that the
colloidal Pt catalyzes reductive eliminations and alkyne cy-
cloadditions of four- and five-membered platinacycles (eqs 4
and 5) and palladacycles.13,14 The lack of any change in the
reductive elimination kinetics of eq 3 for 1 in the presence of O2
indicates that if colloidal Pt can participate in reductive elim-
ination from 1, the process is energetically too high to compete
with direct reductive elimination. For the planar four- and five-
membered metallacycles 7 and 8 the rate-limiting step in the
colloidal Pt-catalyzed process appears to be transmetalation of
the metallacycle to the colloid surface. Given the nonplanarity
of 1, the barrier to transmetalation may be higher than for the
planar metallacycles. Unfortunately, little is known about
factors associated with transmetalation to metal surfaces, a
process that is also significant in OMCVD36,37 and of potential
application in the manipulation of carbon nanotubes.
(34) GaussView 3.0; Gaussian, Inc., 2003.
(35) Hammond, G. S. J. Am. Chem. Soc. 1955, 77, 334–338.
(36) Zinn, A.; Niemer, B.; Kaesz, H. D. Adv. Mater. 1992, 4, 375–378.
(37) Mark, J.; Hampden-Smith, T. T. K. Chem. Vap. Disposition
1995, 1, 8–23.
(38) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Simhai, N.;
Iverson, C. N.; Muller, C.; Satoh, T.; Jones, W. D. J. Mol. Catal. A:
Chem. 2002, 189, 157–168. Simhai, N.; Iverson, C. N.; Edelbach, B. L.;
Jones, W. D. Organometallics 2001, 20, 2759–2766.
The relative ease of direct reductive elimination from
platinacycles 1-3 as compared to 7 and 8 is associated

Article
Organometallics, Vol. 29, No. 6, 2010 1393
Conclusion
the stirred colorless solution over a 10 min period. The resulting
dark orange solution was stirred another 10 min and then
replaced in the refrigerator for ca. 30 min. This orange solution
was slowly added dropwise into a stirred suspension of cis-Pt-
(PEt3)2Cl2 (46.4 mg, 0.0924 mmol) in 2 mL of THF, previously
cooled to -30 °C in the refrigerator. The resulting bright orange
mixture was stirred for 1 h and allowed to warm to room
temperature. This solution was filtered through diatomaceous
earth and the solvent removed under reduced pressure to give an
orange oil. This oily product was washed twice with Et2O and
dried under reduced pressure to give 1 as a white solid. Yield:
16 mg (30.7%). 31P NMR (CDCl3, 101 MHz): 4.8 (s with
satellites, JPt-P = 1833 Hz). 195Pt NMR (CDCl3, 64 MHz):
-4574 (s with satellites, JP-Pt = 1886 Hz).
Six-membered diaryl platinacycle complexes Pt(C6H4X-
C6H4)L2 (X = CH2, O, or NMe and L = PEt3, L2 = 1,3-bis
(phenylphosphino)propane (dppp) or 4,40-bis-tert-butyl-
2,20-bipyridine (tBu2bpy)) are readily synthesized in >80%
yield. Similar to related diaryl complexes PtL2(C6H4-R)2,
direct reductive elimination from the L = PEt3 derivatives
1-3 is observed with rates that depend on the ring substi-
tuent. However, the rates for the platinacycles 1-3 are
dramatically more dependent on the bridging group X.
The more than million-fold increase in the rate from 1
(X = CH2) to 3 (X = NMe) is attributed to aromatic
stabilization of the transition state for 3 by nitrogen lone pair
donation. A much weaker effect is observed for 2 (X = O).
Preparation of 2,20-Dilithiodiphenyl Ether. This procedure is
based on a reported in situ preparation.42 A colorless solution of
diphenyl ether (1073 mg, 6.304 mmol) in 5 mL of THF was
cooled to -30 °C in a refrigerator. The solution was removed
from the refrigerator, and while being stirred, nBuLi (5.30 mL,
13.3 mmol, 2.5 M in hexanes) was added dropwise into the
stirred Ph2O solution over a 10 min period. The resulting bright
orange solution was stirred and allowed to warm to room
temperature overnight. A white solid formed within the bright
orange solution. The solution was decanted and the white solid
was washed twice with cold hexane. The white solid product was
dried in vacuo. Yield: 411.9 mg (35.8%). A few milligrams were
treated with D2O, and the mixture was extracted with CDCl3.
The 1H NMR indicated the formation of 2,20-dideuterodiphenyl
Experimental Section
All procedures were performed under a dinitrogen atmo-
sphere in a Vacuum Atmospheres Corporation drybox unless
otherwise stated. All solvents used were dried by standard
techniques and degassed, then stored under dinitrogen over
˚
4 A molecular sieves or sodium metal. N-Methyldiphenylamine,
2,20-dibromobenzophenone, n-butyllithium, diphenylacetylene,
triethylphosphine (10 wt % in hexane), and diphenyl ether were
obtained from commercial sources (Aldrich, Acros, or Strem)
and used as received. cis-PtCl2(PEt3)2,39 Pt(dppp)Cl2,40 Pt(4,40-
bis-tert-butyl-2,20-bipyridine)Cl2,41 2,20-dilithiodiphenyl-N-
methylamine,42 and 2,20-dibromodiphenylmethane43,44 were
prepared by literature methods.
1
ether: H NMR (CDCl3, 250 MHz): 7.31 (m, 4H, m-H), 7.08
(td, JH-H = 7.4 and 1.2 Hz, 2H, o-H), 7.01 (dd, JH-H = 8.4 and
1.2 Hz, 2H, p-H). The bright orange solution remaining after
separation of the product was dried under reduced pressure, and
the resulting orange residue was also treated with D2O. The
NMR spectra were recorded on Bruker AMX-250, -300, or
-500 spectrometers at ambient probe temperature unless other-
wise stated. Chemical shifts (δ) are given in ppm and coupling
constants (J) in Hz. The 1H shifts are relative to an internal TMS
(0 ppm) standard (referenced to protio solvent signals), and the
1
mixture was extracted with CDCl3 for H NMR, which indi-
cated that the remaining material was mostly the product.
Synthesis of Pt(C6H4OC6H4)(PEt3)2 (2). A colorless solution
of 2,20-dilithiodiphenyl ether (152 mg, 0.835 mmol) in 2 mL of
THF was cooled to -30 °C in a refrigerator. The cold solution
was slowly added dropwise to a stirred suspension of Pt(PEt3)2-
Cl2 (425 mg, 0.846 mmol) in 2 mL of THF, previously cooled to
-30 °C. The resulting light yellow solution was stirred for 2 h
and allowed to warm to room temperature. The yellow solution
was filtered through diatomaceous earth and the solvent
removed under reduced pressure to give a yellow oil. This was
washed twice with MeOH and dried under reduced pressure to
give 2 as a white solid. Yield: 425 mg (85.0%). Colorless single
crystals for X-ray analysis were grown by slow vapor diffusion
of MeOH into a C6H6 solution at room temperature. Anal.
Calcd (%) for C24H38OP2Pt: C, 48.08; H, 6.39. Found: C, 47.82;
H, 6.06.
13C shifts are relative to internal solvent signal standard. The 31
P
shifts are relative to an external 85% H3PO4 (0 ppm) standard,
and the 195Pt shifts are relative to an external K2PtCl4/D2O
(-1624 ppm) standard. Columbia Analytical Services, Inc.
preformed the microanalyses.
1H NMR (C6D6, 300 MHz): 7.69 (td with satellites, JH-H
0
7.2 and 1.8 Hz, JPt-H = 60.5 Hz, 2H, H2,2 ), 7.53 (dd, JH-H
=
=
Preparation of Pt(C6H4CH2C6H4)(PEt3)2 (1). This is a mod-
ification of the reported procedure.15 A colorless solution of
2,20-dibromodiphenylmethane (28.4 mg, 0.0871 mmol) in 2 mL
of THF was cooled to -30 °C in a refrigerator. The color-
less solution was removed from the refrigerator, and n-BuLi
(0.070 mL, 0.18 mmol, 2.5 M in hexanes) was added dropwise to
0
7.8 and 1.5 Hz, 2H, H5,5 ), 7.06 (td, JH-H = 7.2 and 1.5 Hz, 2H,
0
0
H
4,4 ), 6.99 (t, JH-H = 6.9 Hz, 2H, H3,3 ) 1.37 (m, 6H, CH2), 0.76
(m, 9H, CH3). 13C NMR (C6D6, 125 MHz): 160.9, 138.9, 128.4,
124.2, 122.4, 117.8, 17.1-16.6 (m), 8.2 (s with satellites, JPt-C =
19.1 Hz). 13C NMR (CDCl3, 75 MHz): 159.8 (s with satellites,
0
Pt-C = 26.4 Hz, C2,2 ), 145.3 (dd with satellites, JP-C = 105.7
J
and 10.6 Hz, JPt-C = 769.7 Hz, C1,1 ), 138.4 (t, JP-C = 7.5 Hz,
0
(39) Parshall, G. W. Inorg. Synth. 1970, 12, 26–33.
(40) Appleton, T. G.; Bennett, M. A.; Tomkins, I. B. J. Chem. Soc.,
Dalton Trans. 1976, 439–46.
(41) Rendina, L. M.; Vittal, J. J.; Puddephatt, R. J. Organometallics
1995, 14, 1030–1038.
(42) Braddock-Wilking, J.; Corey, J. Y.; French, L. M.; Choi, E.;
Speedie, V. J.; Rutherford, M. F.; Yao, S.; Xu, H.; Rath, N. P.
Organometallics 2006, 25, 3974–3988.
(43) Bickelhaupt, F.; Jongsma, C.; De Koe, P.; Lourens, R.; Mast,
N. R.; Van Mourik, G. L.; Vermeer, H.; Weustink, R. J. M. Tetrahedron
1976, 32, 1921–1930.
0
0
5,5 ), 123.5 (s, C4, 4 ), 122.2 (t with satellites, JPt-C = 58.1 Hz,
C
JP-C = 3.0 Hz, C6,6 ), 116.8 (s with satellites, JPt-C = 18.8,
C
0
0
3,3 ), 17.0-16.3 (m, PEt3-CH2), 8.2 (s with satellites, JPt-C
=
18.8 Hz, PEt3-CH3). 31P NMR (CDCl3, 101 MHz): 5.0 (s with
satellites, JPt-P = 2014 Hz). 31P NMR (C6D6, 101 MHz): 5.3 (s
with satellites, JPt-P = 2002 Hz). 195Pt NMR (CDCl3, 64 MHz):
-3112 (s with satellites, JP-Pt = 2015 Hz).
Synthesis of Pt(C6H4NMeC6H4)(PEt3)2 (3). A suspension of
Pt(PEt3)2Cl2 (45.2 mg, 0.0900 mmol) in 2 mL of THF was cooled
to -78 °C in an acetone/dry ice bath. Then, a light yellow solution
of 2,20-dilithiodiphenyl-N-methylamine (35.8 mg, 0.0837 mmol)
(44) Lee, W. Y.; Park, C. H.; Kim, Y. D. J. Org. Chem. 1992, 57,
4074–4079.

1394 Organometallics, Vol. 29, No. 6, 2010
Robinson and Sharp
in 2 mL of THF, previously cooled to -78 °C, was quickly added
into the stirred suspension. The resulting yellow mixture was
slowly warmed to -40 °C. Due to the instability of 3 above
-30 °C, a sample could not be isolated and characterization is
limited to in situ NMR spectroscopy and reactivity.
125 MHz): 162.0, 156.1, 152.2, 150.0, 137.5, 128.3, 123.3,
123.0, 120.1, 118.4, 111.5, 36.6, 35.6, 30.3. 195Pt NMR (CDCl3,
64 MHz): -3646.
Thermolysis of Pt(C6H4CH2C6H4)(PEt3)2 (1) and Pt(C6H4-
OC6H4)(PEt3)2 (2). Solutions for kinetic studies of both 1 and 2
were prepared similarly by dissolving 5.0 ( 0.3 mg of the platina-
cycle (0.0084 mmol of 1 or 0.0083 mmol for 2) and 14.0 ( 0.5 mg of
PhCCPh (0.079 mmol) into 0.300 ( 0.001 mL of toluene. The
colorless solutions containing 0.028 M platinacycle were placed
into 5 mm NMR tubes, topped with a plastic cap, and further
sealed with wax film to prevent exposure to air during the experi-
ment. A 31P NMR spectrum was taken at room temperature to
validate purity and the relative concentrations of the sample. The
sample was then inserted into the preheated probe of the Bruker
AMX-250 spectrometer at the appropriate temperature, and 31P
NMR spectra were recorded every 15 min until at least 90% of the
platinacycle had decomposed. The final solutions were homoge-
neous and yellow. Experiments without PhCCPh addition showed
that the added PhCCPh had no effect on the reaction rates.
Decomposition of Pt(C6H4NMeC6H4)(PEt3)2 (3). cis-Pt-
(PEt3)2Cl2 (11.0 ( 0.3 mg, 0.022 mmol) suspended in 0.30 (
0.001 mL of toluene was placed into a 5 mm screw-capped NMR
tube. The NMR tube was capped with a septum cap and further
sealed with wax film. A light yellow solution of 2,20-dilithiodiphe-
nyl-N-methylamine (10.0 ( 0.3 mg, 0.023 mmol) dissolved in
0.20 ( 0.001 mL of toluene was loaded into a 1.0 mL syringe. The
NMR tube was cooled to -78 °C in an acetone/dry ice bath, and
thenthe Lisolutionwasslowlyadded, beingcarefulnottoincrease
the resulting solution’s temperature. (Assumedconcentrationof3:
0.044 M at 100% yield.) The NMR tube was again sealed with wax
film to prevent exposure to air during the experiment. The sample
was then inserted into the precooled probe of a Bruker AMX-250
spectrometer at the appropriate temperature, and 31P NMR
spectra were recorded every 15 min until at least 90% of the
platinacycle had decomposed, resulting in a yellow solution.
Thermolysis of 1 with Added O2 and Air. (a) O2: A 5 mm screw-
capped NMR tube was charged with 5.0 ( 0.3 mg of 1 (0.0084
mmol) and 14.0 ( 0.5 mg of PhCCPh (0.079 mmol) dissolved into
0.3 (0.01 mL of toluene. The NMR tube was capped with a septum
cap and sealed with wax film. Molecular oxygen (5.0 ( 0.1 mL, 0.22
mmol) was injected into the NMR tube using a syringe. The NMR
tube was again sealed with more wax film. (b) Air: A solution of
5.0 ( 0.3 mg of 1 (0.0084 mmol) dissolved into 0.3 ( 0.01 mL of
toluene was exposed to air for several minutes. The solution was
then placed into a 5 mm NMR tube and capped. The samples were
inserted into the preheated probe of the Bruker AMX-250 spectro-
meter at 120 or 130 °C, and 31P NMR spectra were recorded every
15 min until at least 90% of the platinacycle had decomposed.
Thermolysis of 2 with Added H2O and PEt3. (a) H2O: A 5 mm
NMR tube was charged with 5.0 ( 0.3 mg of 2 (0.0083 mmol)
dissolved in 0.300 ( 0.01 mL of toluene. One drop of DI H2O
was added. (b) PEt3: A 5 mm NMR tube was charged with 5.0 (
0.3 mg of 2 (0.0083 mmol) dissolved in 0.22 ( 0.01 mL of
toluene. PEt3 (10 wt % in hexane, 0.080 ( 0.001 mL, 0.045
mmol) was added. The samples containing 0.028 M 2 were
capped and sealed with wax film. The samples were then inserted
into the preheated probe of the Bruker AMX-250 spectrometer
at 90 °C, and 31P NMR spectra were taken every 15 min until at
least 90% of the platinacycle had decomposed.
13C NMR (THF, 75 MHz, -40 °C): 151.7, 144.2, 137.3, 121.7,
118.7, 111.6, 35.3, 16.0-15.6 (m), 7.6. 31P NMR (THF,
121 MHz, -30 °C): 7.3 (s with satellites, JPt-P = 1979 Hz).
31P NMR (toluene, 101 MHz, -20 °C): 5.7 (s with satellites,
J
Pt-P = 1998 Hz). 195Pt NMR (THF, 64 MHz, -40 °C): -4700
(s with satellites, JP-Pt = 2007 Hz).
Synthesis of Pt(C6H4NMeC6H4)(1,3-bis(diphenylphosphino)
propane) (4). A light yellow solution of 2,20-dilithiodiphenyl-
N-methylamine (21.1 mg, 0.0494 mmol) in 2 mL of THF was
cooled to -30 °C in a refrigerator. The solution was slowly added
dropwise into a stirred suspension of Pt(dppp)Cl2 (38.8 mg,
0.0572 mmol) in 2 mL of THF, previously cooled to -30 °C in
a refrigerator. The resulting light yellow solution was stirred for
30 min and allowed to warm to room temperature. The yellow
solution was filtered through diatomaceous earth and the solvent
removed under reduced pressure to give a bright yellow precipi-
tate. The precipitate was washed twice with ether and dried under
reduced pressure to give 4 as a bright yellow solid. Yield: 32.7 mg
(84.1%). Bright yellow single crystals for X-ray analysis were
grown by slow vapor diffusion of Et2O into a C6H6 solution at
room temperature. Anal. Calcd (%) for C40H37NP2Pt: C, 60.91;
H, 4.73; N, 1.78. Found: C, 60.50; H, 4.46; N, 2.17.
1H NMR (CDCl3, 300 MHz): 7.96 (b, 4H, p-Ph), 7.40 (b, 8H,
0
m-Ph), 7.09 (b, 8H, o-Ph), 7.01 (d, JH-H = 7.2 Hz, 2H, H5,5 ),
6.77 (d with satellites, JH-H = 8.1 Hz, JPt-H = 74.1 Hz, 2H,
0 0
2,2 ), 6.55 (t, JH-H = 7.8 Hz, 2H, H3,3 ), 5.89 (t, JH-H = 7.2 Hz,
H
0
2H, H4,4 ), 3.37 (s, 3H, NMe), 2.46 (b, 2H, propyl), 2.19 (b, 4H,
propyl). 13C NMR (C6D6, 125 MHz): 149.7, 141.9, 135.3, 132.6,
130.2, 127.3, 123.3, 120.6, 112.8, 67.8, 45.9, 37.3, 25.8, 20.1.
31P NMR (CDCl3, 101 MHz): 3.6 (s with satellites, JPt-P
=
1902 Hz). 31P NMR (C6D6, 101 MHz): 4.1 (s with satellites,
J
Pt-P = 1884 Hz). 195Pt NMR (CDCl3, 64 MHz): -3079 (s with
satellites, JP-Pt = 1905 Hz).
Synthesis of Pt(C6H4NMeC6H4)(4,40-bis-tert-butyl-2,20-bi-
pyridine) (5). A light yellow solution of 2,20-dilithiodiphenyl-
N-methylamine (20.6 mg, 0.0482 mmol) in 2 mL of THF was
cooled to -30 °C in a refrigerator. The solution was slowly
added dropwise into a stirred suspension of Pt(tBu2bpy)Cl2
(32.0 mg, 0.0599 mmol) in 2 mL of THF, previously cooled to
-30 °C in the refrigerator. The resulting dark purple solution
was stirred for 30 min and allowed to warm to room tempera-
ture. The solution was filtered through diatomaceous earth and
the solvent removed under reduced pressure. The dark residue
was washed twice with MeOH and dried under reduced pressure
to give 5 as a dark purple solid. Yield: 26.4 mg (84.9%). Bright
red crystals for the X-ray analysis were grown by slow vapor
diffusion of EtOH into a C6H6 solution at room temperature.
Anal. Calcd (%) for C31H35N3Pt: C, 57.75; H, 5.47; N, 6.52.
Found: C, 57.86; H, 5.71; N, 6.05.
Computational Details. Gaussian 0345 with the B3LYP46
functional was used for all calculations (gas phase). The
LANL2DZ47-50 basis set was employed for Pt and P with added
(45) Gaussian 03, Revision B.04; Gaussian, Inc., 2003 (see SI for full
reference ).
(46) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(47) Dunning, T. H.; Hay, J. P. Modern Theoretical Chemistry;
Plenum: New York, 1976; Vol. 3.
(48) Hay, P. T.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(49) Wadt, W. R.; Hay, P. T. J. Chem. Phys. 1985, 82, 284.
(50) Hay, P. T.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
1H NMR (CDCl3, 500 MHz): 9.07 (d, JH-H = 6.0 Hz, 2H,
0
bpy-H6,6 ), 7.98 (d, JH-H = 1.5 Hz, 2H, bpy-H3,3 ), 7.48 (d with
0
0
satellites, JH-H = 7.0 Hz, JPt-H ≈ 61.3 Hz, 2H, H2,2 ), 7.47 (dd,
0
JH-H = 6.0 and 2.0 Hz, 2H, bpy-H5,5 ), 7.00 (t, J = 8.0 Hz, 2H,
0
0
H
H
3,3 ), 6.85 (d, J = 8.0 Hz, 2H, H5,5 ), 6.79 (t, J = 7.0 Hz, 2H,
0
4,4 ), 3.31 (s, 3H, NMe), 1.42 (s, 18H, Bu). 13C NMR (CDCl3,
t

Article
Organometallics, Vol. 29, No. 6, 2010 1395
d-diffuse functions for Pt (R = 0.05) and d-diffuse (R = 0.364)
and p-polarization (R = 0.0298) functions for P.51 The 6-31G(d,
p) basis set was used for all other atoms. All geometries were
optimized (no symmetry constraints) and energies zero-point-
corrected. Free energies, enthalpies, and entropies were calcu-
lated at 298.15 K and 1 atm. Analytical frequency calculation
gave no imaginary frequencies for the reactants and one ima-
ginary frequency for the transition states. Examination of the
corresponding vibrational modes for the imaginary frequencies
with GaussView34 showed these to be consistent with the
products and reactants. In cases where the nature of the vibra-
tional mode was unclear an IRC calculation or geometry
optimization of the structure after slight displacement along
the vibrational mode direction demonstrated connection with
the reactant and product.
Acknowledgment. We thank NSF for early, initial,
partial support of this work (CHE-0406353) and the
University of Missouri for a Gus T. Ridgel Fellowship
(R.R.). A grant from the National Science Foundation
(9221835) provided a portion of the funds for the pur-
chase of the NMR equipment. We also thank Dr. C.
Barnes for X-ray support, Dr. W. Wei for NMR support,
and Dr. C. Deakyne for DFT support.
Supporting Information Available: CIF files for the crystal
structures, kinetic data, and DFT results. This material is
available free of charge via the Internet at http://pubs.acs.org.
(51) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert,
T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111.