Carbon monoxide promoted reductive elimination of hydrogen from Tp′ platinum complexes

Article abstract of DOI:10.1021/ja056766m

Acid-assisted reductive elimination of hydrogen from TpPtH₃ and of methane and hydrogen from Tp′PtMeH₂ (Tp′ = hydridotris(3,5-dimethylpyrazolyl)borate) is examined herein. Loss of H ₂ is observed from solutions containing platinum(IV) complexes of the type Tp′Pt(R)(H)₂ (R = Me, H) upon protonation and addition of a ligand such as CO. Results of kinetic studies on reductive elimination of H₂ and formation of [κ²-(HTp′)Pt(R)(L)] [BAr′₄] products from intermediates derived from Tp′Pt(R)(H)₂ precursors are described. Elimination appears to occur from cationic 6-coordinate [κ²-(HTp′)Pt(R)(H) ₂(L)][BAr′₄] species.

Full text of DOI:10.1021/ja056766m

Published on Web 01/24/2006
Carbon Monoxide Promoted Reductive Elimination of
Hydrogen from Tp′ Platinum Complexes
Nathan M. West, Stefan Reinartz, Peter S. White, and Joseph L. Templeton*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received October 3, 2005; E-mail: joetemp@unc.edu
Abstract: Acid-assisted reductive elimination of hydrogen from Tp′PtH3 and of methane and hydrogen
from Tp′PtMeH2 (Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate) is examined herein. Loss of H2 is observed
from solutions containing platinum(IV) complexes of the type Tp′Pt(R)(H)2 (R ) Me, H) upon protonation
and addition of a ligand such as CO. Results of kinetic studies on reductive elimination of H2 and formation
of [κ2-(HTp′)Pt(R)(L)][BAr′4] products from intermediates derived from Tp′Pt(R)(H)2 precursors are described.
Elimination appears to occur from cationic 6-coordinate [κ2-(HTp′)Pt(R)(H)2(L)][BAr′4] species.
Introduction
Activation of H2 by and reductive elimination of H2 from
transition metal complexes are two fundamental reactions in
organometallic chemistry; these are crucial steps in many
catalytic reactions.1-5 Of the two processes, H2 activation has
been more widely investigated than has hydrogen elimination.6-9
Hydrogen production currently involves steam reformation
of hydrocarbons produced from fossil fuels, and consequences
of this are high cost and dependence on a limited resource.
Significant research has focused on the synthesis of catalysts
capable of producing hydrogen from biomass, water, or other
renewable resources.10-14 To move toward a hydrogen economy
an inexpensive route for the production of hydrogen is needed.
One well-known method of either producing or purifying H2
is the water gas shift reaction (WGSR). This reaction is
thermodynamically favorable at standard temperature and pres-
sure, but without a catalyst high temperatures and high pressures
are required for rapid reaction.2,15 Some homogeneous catalysts
accomplish this process under mild conditions, but they have
not been adapted to industrial processes.15-19 Lowering both
temperature and pressure for WGSR conversion of water and
CO to hydrogen and CO2 is highly desirable.
Figure 1. Tp′ ligand (Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate).
Here we report initial studies of CO-assisted, acid-induced
elimination of hydrogen from Tp′Pt di- and trihydride complexes
(Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate) (Figure 1)
which are in turn synthesized by WGS chemistry. The resulting
Pt(II) carbonyl complexes can then be reoxidized to Pt(IV)
polyhydrides thus completing the WGSR in stepwise fashion.
Examination of the individual steps of this reaction may provide
insight for future catalyst development in the growing field of
aqueous organoplatinum chemistry.20
Results and Discussion
Synthesis of Tp′PtH3 (1). In the original synthesis of Tp′PtH3
(1)21 (Scheme 1) heating Tp′PtMe2H (2)22 in HSiEt3 produced
the key intermediate Tp′Pt(SiEt3)(H)2 in ca. 20% yield. Replace-
ment of the triethylsilyl group by a proton was effected by
refluxing in MeOH/CH2Cl2. While this method required only
two steps from reagent 2, the product was obtained in a 13%
overall yield. An alternative synthetic route was therefore sought
in order to make 1 a viable starting material for future reactions.
A sequence of clean reactions converts Tp′PtMe2H (2) into
Tp′PtMeH2 (3),23 (Scheme 2) and we hypothesized that it might
(1) Landis, C. R.; Brauch, T. W. Inorg. Chim. Acta 1998, 270, 285.
(2) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry; Prentice Hall:
Upper Saddle River, NJ, 1997.
(3) Frey, M. ChemBioChem 2002, 3, 153.
(4) Mondal, J. U.; Blake, D. M. Coord. Chem. ReV. 1982, 47, 206.
(5) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd. ed.; Wiley: New
York, 1992.
(6) Deutsch, P. P.; Eisenberg, R. J. Am. Chem. Soc. 1990, 112, 714.
(7) Matsubara, T.; Hirao, K. Organometallics 2002, 21, 4482.
(8) Alikhani, M. E.; Minot, C. J. Phys. Chem. A 2003, 107, 5352.
(9) Koch, A.; Bargon, J. Inorg. Chem. 2001, 40, 533.
(10) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Science 2003, 300, 2075.
(11) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Nature 2002, 418, 964.
(12) Sayama, K.; Nomura, A.; Zou, Z.; Abe, R.; Abe, Y.; Arakawa, H. Chem.
Commun. 2003, 23, 2908.
(13) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004,
303, 993.
(14) Forsberg, C. W. Int. J. Hydrogen Energy 2003, 28, 1073.
(15) Laine, R. M.; Crawford, E. J. J. Mol. Catal. 1988, 44, 357.
(16) Yoshida, T.; Ueda, Y.; Otsuka, S. J. Am. Chem. Soc. 1978, 100, 3941.
(17) Taqui Khan, M. M.; Halligudi, S. B.; Shukla, S. Angew. Chem., Int. Ed.
1988, 27, 1735.
(18) Ziessel, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 844.
(19) Kubiak, C. P.; Eisenberg, R. J. Am. Chem. Soc. 1980, 102, 3637.
(20) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R. J.
Am. Chem. Soc. 2006, 128, 82.
(21) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. Organometallics
2000, 19, 3748.
(22) O’Reilly, S. A.; White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1996,
118, 5684.
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J. AM. CHEM. SOC. 2006, 128, 2059-2066
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A R T I C L E S
West et al.
Scheme 1. Previous Synthesis of Tp′PtH3 (1)
Scheme 2. Synthesis of Tp′PtH3 (1) from Tp′PtMe2H (3)
Figure 2. X-ray structure of Tp′Pt(H)(CO) (5). Ellipsoids are drawn with
50% probability. Selected bond distances in angstroms (Å): Pt1-C2 )
1.827, Pt1-N5 ) 2.139, Pt1-N12 ) 2.489, Pt1-N19 ) 2.088, C2-O3
) 1.138. Selected bond angles in degrees (deg): C2-Pt1-N5 ) 102.0,
C2-Pt1-N12 ) 115.4, N5-Pt1-N12 ) 81.5, C2-Pt1-N19 ) 163.6,
N5-Pt1-N19 ) 85.5, N12-Pt1-N19 ) 79.8.
be possible to repeat this sequence to access the trihydride (1).
This method required the synthesis of Tp′Pt(H)(CO) (5)
followed by oxidation of 5 in H2O/acetone to form the trihydride
1 in a manner similar to that used by Keinan to synthesize
TpPtMeH2 from TpPt(Me)(CO).24
Note that the weakly bound third Tp′ arm is approximately
trans to the vacant site. In other words the weak interaction is
positioned axially relative to a square-planar point of departure
for this d8 complex. The strong trans influence of the CO ligand
produces a Pt-N bond distance of 2.088 Å while the strong
trans influence of the hydride ligand produces a Pt-N bond
distance of 2.139 Å.
Reflux of Tp′Pt(H)(CO) (5) in acetone/water (1:1) under
slightly basic conditions yields Tp′PtH3 (1) quantitatively. This
method is analogous to that used to synthesize Tp′PtMeH2 from
Tp′Pt(Me)(CO).23,24 This reaction likely occurs by a WGS type
mechanism in which water or a hydroxide ion attacks the
carbonyl carbon forming a CO2 linkage so that the complex
can then lose CO2 and form the [Tp′Pt(R)H] anion, such that
addition of a proton yields Tp′Pt(R)H2 (R ) Me, H). Heating
Tp′Pt(H)(13CO) (5*) in acetone-d6/D2O with catalytic NaOH
in an NMR tube yielded Na13CO3H which was converted to
13CO2 which was observed by 13C NMR after addition of one
drop of HBF4‚Et2O.
The new synthesis of 1 (Scheme 2), a simple extension of
the synthesis of the dihydride (3), requires four steps, but the
overall yield from 1 (64%) is significantly improved from the
two step route (13%). Many dihydride complexes have been
reported in the literature; most of them are synthesized from
hydride sources or from activation of H2.6,30-33 The fact that
the hydrides in complexes 3 and 1 come from H2O is significant
because H2 elimination from these complexes would represent
completion of the water gas shift reaction.
Protonation of Tp′PtH3 (1). In the absence of added ligands,
reaction of Tp′PtH3 (1) with [H(OEt2)2][BAr′4] at -78 °C in
CD2Cl2 results in a color change from colorless to light orange,
but 1H NMR analysis shows no signals corresponding either to
dihydrogen or to the hydride associated with the [κ2-(HTp′)Pt-
Tp′Pt(H)(CO) (5) is synthesized by protonation of Tp′PtMeH2
with [H(OEt2)2][BAr′4] [BAr′4 ) tetrakis(3,5-trifluorometh-
ylphenyl)borate] at -78 °C in CH2Cl2 followed by purging the
reaction mixture with CO while warming to room temperature.
Loss of methane and trapping with CO generates [κ2-(HTp′)-
Pt(H)(CO)][BAr′4] (4), which is then deprotonated in situ by
reaction with NEt3 at -78 °C to form Tp′Pt(H)(CO) (5).
Solution phase IR (CH2Cl2) for 5 shows one B-H absorption
at 2526 cm-1 indicating that the Tp′ ligand is bound in κ3
fashion,25 one Pt-H absorption at 2216 cm-1, and one CO
absorption at 2070 cm-1, suggesting the presence of only one
geometric isomer. Most platinum(II) complexes are square
planar and hence 4-coordinate, but the π acidity of the CO ligand
makes the metal center sufficiently electron deficient here that
it adopts a 5-coordinate structure. The closely related complex
Tp′Pt(Me)(CO) was shown to be present in both κ2 and κ3 forms
in solution.26 The analogous TpPt(Me)(CO) complex with no
methyls on the pyrazole rings also is fluxional in solution with
both 4-coordinate and 5-coordinate geometries easily acces-
sible,24,27,28 but has a solid-state structure that is square planar
with the Tp binding in a bidentate fashion.29 X-ray quality
crystals of the neutral Tp′Pt(H)(CO) complex 5 were obtained
by layering a CH2Cl2 solution of 5 with hexanes at 4 °C. The
X-ray crystal structure shows Tp′ to be tridentate, and thus the
complex is 5-coordinate. The three Pt-N bond lengths of 2.088,
2.139, and 2.489 Å (Figure 2) reflect weak binding of the third
Tp′ arm and suggest that the geometry of the complex lies
between square planar and trigonal bipyramidal. The hydride
ligand position was not refined.
(23) Reinartz, S.; Baik, M.-H.; White, P. S.; Brookhart, M.; Templeton, J. L.
Inorg. Chem. 2001, 40, 4726.
(24) Haskel, A.; Keinan, E. Organometallics 1999, 18, 4677.
(25) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka, Y. Organo-
metallics 1997, 16, 4121.
(26) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. Organometallics
2000, 19, 3854.
(27) Clark, H. C.; Manzer, L. E. Inorg. Chem. 1974, 13, 1996.
(28) Clark, H. C.; Manzer, L. E. J. Am. Chem. Soc. 1973, 95, 3812.
(29) Oliver, J. D.; Rush, P. E. J. Organomet. Chem. 1976, 104, 117.
(30) Ingleson, M. J.; Brayshaw, S. K.; Mahon, M. F.; Ruggiero, G. D.; Weller,
A. S. Inorg. Chem. 2005, 44, 3162.
(31) Packett, D. L.; Jensen, C. M.; Cowan, R. L.; Strouse, C. E.; Trogler, W.
C. Inorg. Chem. 1985, 24, 3578.
(32) Vaska, L. Acc. Chem. Res. 1968, 1, 335.
(33) Puddephatt, R. J. Coord. Chem. ReV. 2001, 219-221, 157.
9
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CO Promoted Reductive Elimination of H
A R T I C L E S
1
Figure 3. 500 MHz H NMR spectra (193 K) showing hydride signals for [κ2-(HTp′)Pt(H)3(CO)][BAr′4] (6) (t, -12.33; d, -17.51) and [κ2-(HTp′)Pt(H)-
(CO)][BAr′4] (4) (s, -14.64).
Scheme 3. Products of Protonation of Tp′PtH3 (1) under CO
(H)(solv)][BAr′4] complex which is visible by NMR after loss
of methane from the methyl dihydride (3).23 The only species
visible by 1H NMR at low temperature is the neutral trihydride
reagent. Upon warming to room-temperature some dihydrogen
loss was detected along with formation of Tp′H and platinum
black. The observed H2 must be formed after dissociation of
doublets at δ -17.51 ppm (dd, 2H, 2JC-H ) 6 Hz, 2JH-H ) 4.4
Tp′H from Pt otherwise the Tp′Pt(II) monohydride cation would
Hz, 1JPt-H ) 1108 Hz). The large 2JC-H coupling for the hydride
be observed. NMR suggests that Tp′PtH3 is difficult to protonate
and only a small amount of [HTp′PtH3]+ is ever generated.
Typically, not more than 50% of the starting material is
consumed in this reaction.
at δ -12.33 ppm reinforces the notion that it is located trans to
CO, and the small 13CO coupling to the hydrides at δ -17.51
ppm indicates that they are cis to CO. As the reaction mixture
warms to room temperature, loss of dihydrogen can be detected
Dramatically different results are obtained if protonation of
Tp′PtH3 (1) is conducted under a CO atmosphere. This reaction
ultimately yields [κ2-(HTp′)Pt(H)(CO)][BAr′4] (4), a product
compatible with loss of H2. This result resembles reactivity
observed by Keinan and co-workers in their study of TpPt-
MeH2.34 They reported H2 loss from TpPtMeH2 upon addition
of PMe3 at room temperature, but no loss of H2 without added
phosphine even when heated to 70 °C. They proposed two
possible mechanisms for this process, both involving an initial
dissociation of one Tp arm. In one pathway, subsequent PMe3
coordination forms a 6-coordinate species which then eliminates
H2. The second pathway involves formation of a 4-coordinate
η2-H2 species followed by loss of H2 and coordination of PMe3.
Low-Temperature NMR Studies. Efforts to probe the
mechanism of hydrogen formation in the Tp′PtH3 system were
undertaken. Proton NMR experiments under a CO atmosphere
at 193 K show a single initial product upon protonation of the
trihydride (1): coupled hydride signals appear at δ -12.33 ppm
1
by H NMR spectroscopy as a singlet appears at δ 4.57 ppm,
the chemical shift where H2 appears in CD2Cl2. If the solution
is subjected to a freeze pump thaw cycle, this peak disappears.
Formation of the H2 elimination product [κ2-(HTp′)Pt(H)(CO)]-
[BAr′4] (4) can be detected by the growth of a hydride peak at
-14.64 ppm (1JPt-H ) 1122 Hz); the yield is ca. 90%. At low
temperatures this process is slow, making it possible to monitor
the reaction by NMR.
Trapping Ligands Other Than CO. When similar NMR
experiments are performed with R-olefins (ethylene, propylene,
1-butene, or 1-pentene) added to the solution instead of CO
gas, H2 loss is observed at even lower temperatures. No
6-coordinate olefin bound species, analogous to [κ2-(HTp′)Pt-
(H)3(CO)][BAr′4], is observed by NMR even at 150 K; the
observed products are H2 and [κ2-(HTp′)Pt(H)(L)][BAr′4] (L
) H2CdCH2,23 H2CdCHCH3, H2CdCHCH2CH3, H2CdCHCH2-
CH2CH3). With these olefins no 6-coordinate cationic trihydride
adducts are detected as stable intermediates; likely the olefin
trihydride cations are higher in energy than the CO trihydride
cation and cannot be seen by NMR. This change in reaction
coordinate precludes a kinetic study of the rate of disappearance
of an intermediate. No reaction with internal olefins is observed,
probably because of their increased steric bulk.
2
1
(t, 1H, JH-H ) 4.4 Hz, JPt-H ) 1120 Hz) and δ ) -17.51
2
1
ppm (d, 2H, JH-H ) 4.4 Hz, JPt-H ) 1108 Hz) (Figure 3).
Given the presence of three hydride ligands reflected in the
NMR data, we formulate the product as 6-coordinate Pt(IV)
carbonyl complex [κ2-(HTp′)Pt(H)3(CO)][BAr′4] (6) in which
the unique hydride is trans to CO and the two equivalent
hydrides are cis to CO. Similar types of trapping experiments
using nitriles have been used to access other cationic Pt(IV)
hydride species.35,36 Experiments using 13CO gas yield [κ2-
(HTp′)Pt(H)3(13CO)][BAr′4] (6*). The hydrides of 6* appear
When acetonitrile is used as a trapping ligand after protona-
tion, the 6-coordinate nitrile-bound species, [κ2-(HTp′)Pt(H)3-
(NCMe)][BAr′4] (7), is observed by NMR at low temperature.
1
The cationic acetonitrile adduct has hydride signals in the H
1
2
NMR spectrum at δ -19.94 ppm (s, JPt-H ) 1166 Hz, 2H,
as a doublet of triplets at δ -12.33 ppm (dt, 1H, JC-H ) 90
1
2
1
Pt-H), and at δ -24.08 ppm (s, JPt-H ) 1423 Hz, 1H, Pt-H).
Hz, JH-H ) 4.4 Hz, JPt-H ) 1120 Hz) and a doublet of
Upon warming to room temperature no H2 loss is observed and
the only platinum species visible by NMR is the starting
material, neutral Tp′PtH3 (ca. 50%) with the rest of the material
having undergone decomposition.
(34) Lo, H. C.; Haskel, A.; Kapon, M.; Keinan, E. J. Am. Chem. Soc. 2002,
124, 3226.
(35) Norris, C. M.; Templeton, J. L. Organometallics 2004, 23, 3101.
(36) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124, 12116.
9
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A R T I C L E S
West et al.
Scheme 4. Possible Pathways for H2 Elimination from the Trihydride Carbonyl Cation (6)
Addition of either PMe3 or PMe2Ph as the trapping reagent
results in a small but detectable amount of H2 loss, but
undesirable side reactions of the phosphines with acid precluded
detailed study. Use of 3,5-dimethylphenyl isocyanide or iso-
propyl isocyanide as trapping ligands after protonation results
in observation of a 6-coordinate species. Upon warming only a
small amount of hydrogen loss is observed; the major species
observable at room temperature is starting material. The
isocyanide ligands may be too bulky to coordinate to the metal
strongly, or they may be sufficiently basic to buffer the solution
acidity.
Mechanistic Studies. Although we have shown that CO binds
to the trihydride before H2 elimination to form complex 6, there
are still several pathways by which H2 elimination could occur
from this complex (Scheme 4). Most reductive eliminations from
octahedral Pt(IV) occur via coordinatively unsaturated 5-coor-
dinate intermediates;26,37-44 there are multiple ways in which a
5-coordinate intermediate could be implicated here. Pathway
A is pre-dissociation of CO from 6, elimination of H2, then
recoordination of CO; pathway B is migration of a hydride to
CO, elimination of H2, followed by deinsertion of the CO from
the formyl ligand to yield the Pt(II) product 4. Another
possibility which does not involve direct reductive elimination
from Pt(IV) is pathway C in which one of the hydrides couples
with the proton on nitrogen, H2 is lost, and the Pt(IV) dihydride
is deprotonated by nitrogen yielding complex 4. The simplest
possibility is direct elimination of H2 from the 6-coordinate
complex 6, pathway D. Yet another possibility is dechelation
of a Tp′ arm from 6 resulting in a 5-coordinate intermediate
which could eliminate H2, followed by coordination of the arm.
Experiments were conducted to probe whether CO binding
is reversible. Protonation of 1 under 12CO followed by removal
of gas via three freeze, pump, thaw cycles and refilling with
13CO yields a significant amount of the 6-coordinate trihydride-
13CO complex, indicating facile exchange between bound and
free CO gas in solution. The reversibility of the hydrogen
elimination reaction was also investigated. Protonation of
Tp′PtD3 (1-d3) at low temperature under CO and H2 leads to
elimination of D2 with no incorporation of H into the platinum
product. No HD or Pt-D formation is detected from stirring [κ2-
(HTp′)Pt(H)(CO)][BAr′4] (4) in CD2Cl2 under D2 at room
temperature, indicating that H2 loss is irreversible.
Experiments at various CO pressures and constant temperature
provide further support for facile reversible loss of CO. An
increase in the rate of hydrogen production with increasing CO
pressure is observed. Elimination of hydrogen from the five
coordinate product of CO loss from 6 would yield an inverse
rate relationship with CO pressure. This qualitative rate data,
coupled with the fact that no H2 loss is seen in the absence of
CO, allows us to rule out pathway A as a possibility. Protonation
of Tp′PtH3 (1) with [D(OEt2)2][BAr′4] or protonation of Tp′PtD3
(1-d3) with [H(OEt2)2][BAr′4] under CO resulted in no elimina-
tion of HD, ruling out Pathway C.
Pathway B, the insertion-deinsertion pathway, could explain
why H2 loss is faster using olefins than CO. Insertion of olefins
into metal hydride bonds to form alkyl ligands is typically
facile,45,46 while CO insertion to form stable formyl ligands is
unlikely.47,48 Attempted synthesis of Tp′Pt(H)2(CHO) from
Tp′Pt(H)(CO) by stepwise H-, H+ addition yielded the desired
product along with Tp′PtH3 as determined by 1H NMR
spectroscopy.49 This illustrates that Tp′Pt(H)2(CHO) does not
lose H2 but rather can lose CO to form the neutral trihydride 3.
In the case of ethylene addition, if a 6-coordinate trihydride
olefin cation is formed, then it could undergo insertion leading
to a five coordinate ethyl dihydride cation. This intermediate
would seem more likely to lose ethane than hydrogen, based
on known reactions with [HTp′PtMeH2]+.23 The NMR for
Tp′PtH3 plus acid and ethylene contains a large signal due to
hydrogen, but no signal for ethane is detected, so an ethyl
(37) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E. J. Chem. Soc., Dalton
Trans. 1974, 2457.
(38) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem. Soc. 2000,
(45) Chatt, J.; Coffey, R. S.; Gough, A.; Thompson, D. T. J. Chem. Soc. A
1968, 190.
(46) Brookhart, M.; Hauptman, E.; Lincoln, D. M. J. Am. Chem. Soc. 1992,
114, 10394.
122, 1456.
(39) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. Organometallics 2001, 20,
2669.
(40) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125,
(47) Gladysz, J. A. AdV. Organomet. Chem. 1982, 20, 1.
(48) Reinartz, S.; Brookhart, M.; Templeton, J. L. Organometallics 2002, 21,
247.
9442.
(41) Jensen, M. P.; Wick, D. D.; Reinartz, S.; White, P. S.; Templeton, J. L.;
Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 8614.
(49) Reaction products determined by 1H NMR. The formyl hydride anion
[Tp′Pt(H)(CHO)][K] displays coupled formyl proton and hydride signals
at 13.93 (2JPt-H ) 268 Hz, 3JH-H ) 7 Hz), and -16.09 (2JPt-H ) 1620 Hz,
3JH-H ) 7 Hz) respectively (ppm, THF-d8, 283 K). The neutral formyl
(42) Roy, S.; Puddephatt, R. J.; Scott, J. D. J. Chem. Soc., Dalton Trans. 1989,
2121.
(43) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999, 18,
1408.
dihydride Tp′Pt(H)2(CHO) displays coupled formyl proton and hydride
3
(44) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998,
120, 2843.
signals at 12.85 (2JPt-H ) 182 Hz, JH-H ) 4 Hz), and -17.96 (2JPt-H
)
3
1340 Hz, JH-H ) 4 Hz) respectively (ppm, THF-d8, RT).
9
2062 J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006

CO Promoted Reductive Elimination of H
A R T I C L E S
Figure 4. Eyring plot of H2 loss from 6 under 18.7 psi CO.
dihydride intermediate seems unlikely. If a 3-coordinate plati-
num ethyl species resulted from H2 elimination, then it could
be trapped by excess ethylene, but we see no evidence for a
platinum ethyl ethylene complex. Reaction of [D(OEt2)2][BAr′4]
with Tp′PtD3 (1-d3) under C2H4 ethylene resulted in [Tp′DPt-
(D)(C2H4)][BAr′4] with no incorporation of D into the ethylene
ligand and no formation of a platinum hydride. This evidence
rules out pathway B as a viable mechanism for H2 loss.
Dechelation of a Tp′ arm to form a reactive 5-coordinate
intermediate is kinetically indistinguishable from direct elimina-
tion; however dechelation of Tp′ from Pt(IV) is unprecedented
at the low temperatures used in these reactions. Since the
platinum center is electron deficient (νco ) 2130 cm-1) the Tp′
arms are probably tightly bound. Direct elimination of H2 from
the cationic 6-coordinate intermediate 6, pathway D, remains
the mechanism of choice.
Kinetic Studies. Kinetic experiments performed with 1H
NMR spectroscopy between 225 and 245 K monitored the
disappearance of the hydride signals of the platinum(IV)(CO)
adduct (6). Disappearance of intermediate 6 and formation of
platinum(II) product 4 display first order kinetics over several
half-lives, probably because the concentration of CO in solution
is effectively constant. The rate at 239 K under 18.7 psi CO
was found to be 4 × 10-4 s-1, giving rise to a ∆Gq value of
17.2 (( 0.1) kcal/mol. An Eyring plot (Figure 4) was constructed
using rate constants from this admittedly narrow temperature
range at 18.7 psi CO; ∆Hqand ∆Sq are 23.0 (( 1) kcal/mol and
24.5 (( 4) eu, respectively.
Figure 5. Possible reaction coordinate diagram for elimination of H2 from
6.
The normal isotope effect and positive entropy of transition
state formation suggest that the barrier to reductive coupling of
H2 is lower than the barrier to dissociation of H2 (Figure 5). In
such a case the transition state is becoming less ordered resulting
in positive entropy of activation. It has been shown that
dissociation of H2 can account for a normal isotope effect of
this magnitude (vida supra). A lower barrier for reductive
coupling to form H2 in the coordination sphere than for
dissociation of H2 is consistent with the mechanism proposed
for elimination of CH4 from Tp′Pt systems. The barrier for
reductive coupling of methane was previously found to be lower
than the energy barrier for dissociation of methane.23,26,34,41,61
A reaction profile of this type would indicate that the observed
isotope effect of 2.2 is a combination of a kinetic isotope effect
due to the irreversible dissociation of H2, a secondary kinetic
effect due to the additional M-D bond, and an equilibrium
effect from the reversible reductive coupling.
π-Acid Induced H2 Elimination? A possible explanation
for generation of hydrogen in the presence of CO is that with
an electron-withdrawing CO ligand bound to platinum there may
be too little electron density at the metal to maintain a Pt(IV)
dihydride formulation, and formation of an η2-H2 Pt(II) species
is favored. If the metal is electron rich a dihydride will be
favored, but with decreasing electron density an η2-H2 ligand
will be more likely to form. The protonated CO trihydride
complex [κ2-(HTp′)Pt(H)3(CO)][BAr′4] (6) is highly electron-
deficient as judged by the C-O stretching frequency determined
by in situ FTIR spectroscopy (νCO ) 2130 cm-1).
Measurement of the rate of D2 loss from [κ2-(HTp′)Pt(D)3-
(CO)][BAr′4] (6-d3), formed by protonation of Tp′PtD3 (1-d3),
at 239 K under 18.7 psi CO allows for calculation of an isotope
effect of kH/kD ) 2.2 (( 0.1) by comparison with the rate of
H2 loss from 6 under the same conditions. Isotope effects of
similar magnitude have been reported for H2 elimination from
other systems.6,50,51 It has been demonstrated previously that
oxidative addition of H2 to a metal center results in an inVerse
equilibrium isotope effect,50-56 and this has also been shown
to hold for the coordination of H2 in η2-H2 complexes.57-60
Because of the stronger affinity of the metal to bind D2 than
H2 a normal isotope would be expected for dissociation of D2
relative to H2 as is observed in this case.
The surprising low-temperature hydrogen generation with
ethylene present could be due to the larger size of ethylene
relative to CO, perhaps increasing unfavorable steric interactions
with the Tp′ ligand and increasing the energy of the 6-coordinate
adduct. It could also be that the ethylene π* orbitals have more
effective overlap with a single filled metal dπ orbital, thus
making ethylene a single faced π-acid uniquely suited to increase
the rate of H2 elimination. There is relevant experimental
evidence regarding ethylene as a strong single-faced π-acid. The
complexes Tp′Pt(Me)(CO)26 and Tp′Pt(Ph)(CO)62 both exhibit
(50) Rabinovich, D.; Parkin, G. J. Am. Chem. Soc. 1993, 115, 353.
(51) Hascall, T.; Rabinovich, D.; Murphy, V. J.; Beachy, M. D.; Friesner, R.
A.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 11402.
(52) Janak, K. E.; Parkin, G. Organometallics 2003, 22, 4378.
(53) Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 13219.
(54) Abu-Hasanayn, F.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem.
Soc. 1993, 115, 8019.
(55) Shin, J. H.; Parkin, G. J. Am. Chem. Soc. 2002, 124, 7652.
(56) Yan, S. G.; Brunschwig, B. S.; Creutz, C.; Fujita, E.; Sutin, N. J. Am.
Chem. Soc. 1998, 120, 10553.
(57) Bender, B. R.; Kubas, G. J.; Jones, L. H.; Swanson, B. I.; Eckert, J.; Capps,
K. B.; Hoff, C. D. J. Am. Chem. Soc. 1997, 119, 9179.
(58) Gusev, D. G.; Bakhmutov, V. I.; Grushin, V. V.; Vol’pin, M. E. Inorg.
Chim. Acta 1990, 177, 115.
(59) Hauger, B. E.; Gusev, D.; Caulton, K. G. J. Am. Chem. Soc. 1994, 116,
208.
(60) Bakhmutov, V. I.; Bertran, J.; Esteruelas, M. A.; Lledos, A.; Maseras, F.;
Modrego, J.; Oro, L. A.; Sola, E. Chem. Eur. J. 1996, 2, 815.
9
J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006 2063

A R T I C L E S
West et al.
Scheme 5. Isomerization Following Protonation of Tp′PtMeH2 (3)
under CO
solution IR and NMR data consistent with rapid interconversion
between κ2 and κ3 binding modes of Tp′. However, the
analogous ethylene complexes, Tp′Pt(Me)(H2CdCH2)26 and
Tp′Pt(Ph)(H2CdCH2),62 have spectroscopic data consistent with
only κ3 bound Tp′. These complexes are aptly described as
octahedral Pt(IV) metallocyclopropane complexes,26 and this
is reflected in the crystal structure of Tp′Pt(Ph)(H2CdCH2) that
has Pt-N bond lengths of 2.140, 2.168, and 2.220 Å.62 In
comparison, the bond lengths for Tp′Pt(H)(CO) are 2.088, 2.139,
and 2.489 Å clearly showing that the third pyrazolyl is bound
much more weakly, and thus indicating that this complex is
more similar to square planar d8 Pt(II), than to octahedral Pt-
(IV).
Protonation of Tp′PtMeH2 (3). When reactions analogous
to the protonation of trihydride (1) are performed with the
methyl dihydride (3) under CO, it is possible to observe similar
6-coordinate species, namely [κ2-(HTp′)Pt(Me)(H)2(CO)][BAr′4]
(8) and [κ2-(HTp′)Pt(Me)(H)2(13CO)][BAr′4] (8*). In the absence
of added ligand, protonation of Tp′PtMeH2 induces rapid
methane loss even at 193 K.26 However, by having CO in
solution before acidification it was possible to trap the 5-coor-
dinate intermediate that leads to methane loss. Complex 8 has
hydride signals at δ -12.33 (d, 2JH-H ) 5.5 Hz, 1JPt-H ) 1166
Hz) and δ -17.84 (d, 2JH-H ) 5.5 Hz, 1JPt-H ) 1193 Hz). The
13C labeled carbonyl complex 8* resembles complex 6* in that
solution. Methane loss, but not dihydrogen loss, is observed
from complex 8c. The hydride signal for 8c shows 4 Hz 2JC-H
coupling when 13CO is used so cis CO, hydride ligands are
indicated. An attractive geometry with CO trans to N for 8c is
presented in Scheme 5. There is precedence for a trans dihydride
carbonyl structure being thermodynamically favored over a cis
dihydride carbonyl structure,63 but this formulation requires
coincidental equivalence of the trans hydride NMR signals in
order to fit the limited data available for 8c at this point. Only
methane loss seems feasible from a trans-dihydride isomer in
accord with our observations.
Protonation of Tp′PtMeD2 under CO at 193K results in
immediate isotope scrambling in the 6-coordinate cationic
carbonyl species. Observation of CH3, CH2D, and CHD2
isotopomers of the H2 loss product [κ2-(HTp′)Pt(Me)(CO)]-
[BAr′4] (9) upon warming indicates that this scrambling is a
result of reversible reductive coupling to form methane in the
coordination sphere. The isotope scrambling is complete in the
initial 1H NMR, thus it was not possible to monitor this process
by NMR.
2
the downfield hydride has a 94 Hz JC-H coupling while the
upfield signal has only a 3.2 Hz 13C-H coupling. No 3J coupling
to the CO carbon is detected for the methyl protons. These data
indicate that 8 has one hydride trans to CO, leaving the other
hydride and the methyl ligand cis to CO. This octahedral species,
8, appears to lose either H2 or CH4 to form either 4 or [κ2-
(HTp′)Pt(Me)(CO)][BAr′4] (9).26 This result is surprising be-
cause in previous experiments we observed only loss of CH4
from Tp′PtMeH2 (3) upon protonation and trapping with an
added ligand. However, the solution of protonated 3 in those
experiments was allowed to warm slightly before trapping ligand
was added. It has previously been shown that complex 3 will
easily lose methane from the 5-coordinate species when
protonated in the absence of a trapping ligand.23 When CO is
present, the protonated species is trapped. Either CH4 or H2 can
be eliminated from the resulting cationic 6-coordinate species,
with a ratio of CH4 to H2 loss about ∼5:1. Complex 8 has a
carbonyl trans to hydride configuration that is similar to the
(dppe)Ir(Et)(H)2(CO) complex reported by Eisenberg and co-
workers, from which C-H and H-H coupling and elimination
are competitive,6 as in this case.
If the methyl dihydride is protonated at 193K and CH3CN is
quickly added, then it is again possible to trap the 5-coordinate
methyl dihydride cation and form the 6-coordinate cation [κ2-
(HTp′)Pt(Me)(H)2(NCMe)][BAr′4] (10) which has inequivalent
hydride signals at -20.67 (s, 1JPt-H ) 1265 Hz, 2H, Pt-H) and
1
-24.08 (s, JPt-H ) 1496 Hz, 1H, Pt-H). Warming of this
There is a small amount of another isomer of 8 present (8b)
complex results in elimination of methane only and formation
of [κ2-(HTp′)Pt(H)(NCMe)][BAr′4],23 once again showing the
necessity of a π-acid for the elimination of H2.
1
(Scheme 5) which has a hydride signal at δ -16.72 (s, JPt-H
) 1201 Hz) and a methyl signal at δ 0.53 (s, 2JPt-H ) 55 Hz).
Presumably, this is the isomer with CO trans to methyl. This
isomer formulation is further confirmed when 13CO is used:
the 2JC-H coupling from the 13CO to the two equivalent hydrides
is only 4 Hz, indicating that they are both located cis to CO.
Complexes 8 and 8b slowly convert to a third species which
has a single hydride signal at δ -17.05 (s, 1JPt-H ) 1212 Hz).
This third species (8c), the most thermodynamically stable of
the three isomers, eventually becomes the major species in
Conclusions
Low-temperature NMR spectra reported here show that after
protonation opens a coordination site, carbon monoxide adds
to Pt, and reductive elimination of H2 occurs from the resulting
6-coordinate complex. The isotope effect (2.2 ( 0.1) and
reaction entropy (24.5 ( 4 eu) are consistent with a mechanism
involving reversible reductive coupling to form coordinated
dihydrogen followed by rate-limiting irreversible dissociation
(61) Lo, H. C.; Haskel, A.; Kapon, M.; Keinan, E. J. Am. Chem. Soc. 2002,
124, 7041.
(62) MacDonald, M. G.; Kostelansky, C. N.; White, P. S.; Templeton, J. L.,
manuscript in preparation.
(63) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16,
3786.
9
2064 J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006

CO Promoted Reductive Elimination of H
A R T I C L E S
using D2O/acetone-d6. 1H NMR (CD2Cl2, 298 K, δ): 5.85 (s, 3H,
Tp′CH), 2.38, 2.12 (s, 9H, Tp′CH3), -20.03 (s, 1JPt-H ) 1168 Hz, 1H,
Pt-H).
of dihydrogen. Significant hydrogen elimination occurs when
either CO or ethylene is used as a trapping ligand. We postulate
that the π-acid character of these ligands withdraws sufficient
electron density from the metal so that it is reluctant to support
the platinum(IV) dihydride structure. Surprisingly, elimination
of H2 occurs at lower temperatures with ethylene than with CO
even though CO would typically be considered the stronger
π-acid. However, examination of the structures of Tp′Pt(R)CO
(R ) Me, H) complexes compared to the structures of Tp′Pt-
(R)(H2CdCH2) (R ) Me, Ph) provides relevant data. The Pt-
(II) carbonyl complexes have only weak coordination of the
third Tp′ arm and are thus more similar to Pt(II) square planar
complexes. The Tp′Pt(II) ethylene complexes have strong
coordination of all three Tp′ arms and resemble octahedral Pt-
(IV) complexes. For these Tp′Pt complexes the single faced
π-acid ethylene ligand seems to be a uniquely effective π-acid.
This feature may explain why it facilitates H2 loss at such low
temperatures compared to the cylindrically symmetrical CO
π-acid. Also, it is important to note that the neutral hydride
complexes were generated by WGS chemistry and thus elimina-
tion of H2 from Tp′PtH3 represents net production of H2 from
H2O and CO, a stoichiometric water gas shift reaction.
[κ2-(HTp′)Pt(H)(CO)][BAr′4] (4). In the drybox, Tp′PtMeH2 (3)
(100 mg, 0.196 mmol) and [H(OEt2)2][BAr′4] (223 mg, 0.213 mmol)
were added to a 100 mL Schlenk flask. The flask was capped with a
septum, removed from the box, and cooled to -78 °C. CH2Cl2 (15
mL) was syringed into the flask and was allowed to stir for 5 min. The
cold bath was removed and CO was bubbled into solution for 30 min.
The solvent was removed in vacuo. The product was recrystallized from
CH2Cl2/pentane at -30 °C, yielding colorless crystals. Yield: 217 mg
(80%). [κ2-(HTp′)Pt(H)(13CO)][BAr′4] (4*) was synthesized in an
analogous fashion using 13CO gas. 1H NMR (CD2Cl2, 298 K, δ): 9.59
(s, 1H, pz′NH), 6.18 6.14, 6.14 (s, 1H each, Tp′CH), 2.42, 2.42, 2.34,
1
2.32, 2.32, 1.77 (s, 3H each, Tp′CH3), -14.69 (s, JPt-H ) 1120 Hz,
1H, Pt-H). 13C NMR (CD2Cl2, 298 K, δ): 163.0 (1JPt-C ) 1790 Hz,
Pt-CO); 155.6, 154.0, 150.8, 150.8, 149.8, 145.3 (HTp′CCH3); 110.6,
108.9, 108.8 (HTp′CH); 16.7, 15.2, 13.3, 13.0, 11.5, 11.3 (HTp′CH3).
IR (CH2Cl2): νBH ) 2524 cm-1, νPtH ) 2225 cm-1, νCO ) 2107 cm-1
.
Anal. Calcd for C48H36N6F24B2OPt: C, 41.61; H, 2.62; N, 6.07.
Found: C, 41.62; H, 2.66; N, 6.18.
Tp′Pt(H)(CO) (5). [κ2-(HTp′)Pt(H)(CO)][BAr′4] (4) (217 mg, 0.157
mmol) was dissolved in CH2Cl2 mixture and run down an alumina
column. The solvent was removed and the resulting white powder was
crystallized in 10:1 hexanes/CH2Cl2 giving colorless X-ray quality
Experimental Section
1
crystals. Yield: 78 mg (95%). H NMR (CD2Cl2, 298 K, δ): 5.85 (s,
3H, Tp′CH), 2.33, 2.26 (s, 9H, Tp′CH3), -15.95 (s, 1JPt-H ) 1057 Hz,
Materials and Methods. All reactions were performed under an
atmosphere of dry nitrogen or argon using standard Schlenk and drybox
techniques. Argon and nitrogen were purified by passage through
columns of BASF R3-11 catalyst and 4 Å molecular sieves. All
glassware was flame dried under vacuum and cooled under N2 before
use. Diethyl ether, methylene chloride, acetonitrile, toluene, and pentane
were purified under an argon atmosphere and passed through a column
packed with activated alumina.64 Tetrahydrofuran was distilled from
sodium/benzophenone ketyl. Methylene chloride-d2 was vacuum trans-
ferred from calcium hydride and degassed by several freeze-pump-
thaw cycles.
Tp′PtMe2H (1),22 Tp′PtMeH2 (2),23 Tp′PtMeD2 (2-d2),23 Tp′PtPhH2,65
[HTp′Pt(SiEt3)H2][BAr′4],66 and [H(OEt2)2][BAr′4]67 were synthesized
using published procedures. Carbon monoxide was obtained from
Matheson Gas Products, Inc. and 13C labeled carbon monoxide from
Cambridge Isotope Laboratories, Inc. All other reagents were used as
received.
1H, Pt-H). IR (CH2Cl2): νBH ) 2526 cm-1, νPt-H ) 2216 cm-1, νCO
)
2070 cm-1 13C NMR (CD2Cl2, 193 K, δ): 165.1 (Pt-CO), 149.4 (JPt-C
.
) 38 Hz, Tp′CCH3), 144.9 (Tp′CCH3), 105.9 (3JPt-C ) 14 Hz, Tp′CH),
15.3 (JPt-C ) 28 Hz, Tp′CCH3), 12.8 (Tp′CCH3). Anal. Calcd for
C16H23N6BOPt: C, 36.86; H, 4.45; N, 16.12. Found: C, 36.74; H, 4.46;
N, 15.88.
In Situ Generation of [κ2-(HTp′)Pt(H)3(CO)][BAr′4] (6). Tp′PtH3
(1) (10 mg, 0.02 mmol) and [H(OEt2)2][BAr′4] (23 mg, 0.02 mmol)
were added to an NMR tube in the drybox. The tube was capped with
a septum and removed from the box. The tube was purged and filled
with the desired pressure of CO 3 times. It was cooled to -78 °C and
CD2Cl2 (0.7 mL) was added. It was then placed in the NMR with the
probe cooled to 193 K. [κ2-(HTp′)Pt(D)3(CO)][BAr′4] (6-d3) was
synthesized in an analogous fashion using Tp′PtD3 (1-d3). [κ2-(HTp′)-
Pt(H)3(13CO)][BAr′4] (6*) was synthesized in an analogous fashion
1
using 13CO gas. H NMR (CD2Cl2, 193 K, δ): 9.76 (s, 1H, pz′NH),
1H NMR and 13C NMR spectra were recorded on Bruker AMX 300
MHz, Bruker Avance 400 MHz, or Bruker DRX 500 MHz spectrom-
eters. 1H NMR and 13C NMR chemical shifts were referenced to residual
1H and 13C signals of the deuterated solvents. Infrared spectra were
recorded on an ASI ReactIR 1000.
6.15, 6.15, 6.07 (s, 1H, Tp′CH), 2.40, 2.40, 2.24, 2.20, 2.20, 1.46 (s,
2
1
3H, Tp′CH3), -12.33 (t, JH-H ) 5 Hz, JPt-H ) 1126 Hz, 1H, Pt-H),
-17.51 (d, 2JH-H ) 5 Hz, 1JPt-H ) 1114 Hz, 1H, Pt-H). IR (CH2Cl2):
νBH ) 2522 cm-1, νCO ) 2130 cm-1. Spectral data for [κ2-(HTp′)Pt-
1
(D)3(CO)][BAr′4] (6-d3). H NMR (CD2Cl2, 193 K, δ): 9.76 (s, 1H,
1
Representative [BAr′4]- NMR Data. H and 13C NMR data for
pz′NH), 6.15, 6.15, 6.07 (s, 1H, Tp′CH), 2.40, 2.40, 2.24, 2.20, 2.20,
1.46 (s, 3H, Tp′CH3), -12.33 (t, 2JH-H ) 5 Hz, 1JPt-H ) 1126 Hz, 1H,
Pt-H), -17.51 (d, 2JH-H ) 5 Hz, 1JPt-H ) 1114 Hz, 1H, Pt-H). Spectral
the [BAr′4]- counterion are reported separately for simplicity. 1H NMR
(CD2Cl2, 193 K, δ): 7.77 (br, 8H, o-Ar′), 7.60 (br, 4H, p-Ar′). 13C
NMR (CD2Cl2, 193 K, δ): 162.2 (1:1:1:1 pattern, 1JB-C ) 50 Hz, Cipso),
1
data for [κ2-(HTp′)Pt(H)3(13CO)][BAr′4] (6*). H NMR (CD2Cl2, 193
2
4
135.3 (Cortho), 129.4 (qq, JC-F ) 30 Hz, JC-F ) 5 Hz, Cmeta), 125.1
K, δ): 9.76 (s, 1H, pz′NH), 6.15, 6.15, 6.07 (s, 1H, Tp′CH), 2.40,
1
2
(q, JC-F ) 270 Hz, CF3), 117.9 (Cpara).
2.40, 2.24, 2.20, 2.20, 1.46 (s, 3H, Tp′CH3), -12.33 (dt, JC-H ) 90
Hz, 2JH-H ) 5 Hz, 1JPt-H ) 1126 Hz, 1H, Pt-H), -17.51 (dd, 2JC-H
)
Tp′PtH3 (1). Tp′Pt(H)(CO) (5) (78 mg, 0.149 mmol) was placed in
a Schlenk flask and purged with N2. A 1:1 mixture of acetone/H2O
(50 mL) was then added along with NaOH (5 drops). The solution
was refluxed for 2 h and then cooled to 0 °C. A white powder of
Tp′PtH3 was filtered from the mixture and dried under vacuum. Yield:
73 mg (98%). Tp′PtD3 (1-d3) was synthesized in an analogous fashion
4 Hz, 2JH-H ) 5 Hz, 1JPt-H ) 1114 Hz, 1H, Pt-H). 13C NMR (CD2Cl2,
193 K, δ): 171.0 (dt, 1JPt-C ) 570 Hz, 2JC-H trans ) 90 Hz, 2JC-H cis
4 Hz, CO).
)
In Situ Generation of [κ2-(HTp′)Pt(H)3(NCCH3)][BAr′4] (7).
Tp′PtH3 (1) (10 mg, 0.02 mmol) and [H(OEt2)2][BAr′4] (23 mg, 0.02
mmol) were added to an NMR tube in the drybox. The tube was capped
with a septum and removed from the box. It was then cooled to -78
°C and CD2Cl2 (0.7 mL) was added. Acetonitrile (6.5 µL, 0.2 mmol)
was syringed in and the tube was shaken, it was then placed in the
(64) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518.
(65) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. Organometallics
2001, 20, 1709.
(66) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem.
Soc. 2001, 123, 6425.
1
NMR at 193 K. H NMR (CD2Cl2, 193 K, δ): 9.77 (s, 1H, pz′NH),
(67) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920.
6.10, 6.10, 6.04 (s, 1H, Tp′CH), 2.39, 2.39, 2.26, 2.26, 2.22, 2.17, 1.48
9
J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006 2065

A R T I C L E S
West et al.
1
(s, 3H, Tp′CH3, NCCH3), -19.94 (s, JPt-H ) 1166 Hz, 2H, Pt-H),
the NMR at 193 K. 1H NMR (CD2Cl2, 193 K, δ): 10.56 (s, 1H, pz′NH),
6.04, 6.03, 6.03 (s, 1H, Tp′CH), 2.34, 2.34, 2.30, 2.30, 2.22, 2.20, 1.45
1
-24.08 (s, JPt-H ) 1423 Hz, 1H, Pt-H).
In Situ Generation of [κ2-(HTp′)Pt(Me)(H)2(CO)][BAr′4] (8).
Tp′PtMeH2 (3) (10 mg, 0.02 mmol) and [H(OEt2)2][BAr′4] (23 mg,
0.02 mmol) were added to an NMR tube in the drybox. The tube was
capped with a septum and removed from the box. The tube was purged
and filled with the desired pressure of CO 3 times. It was cooled to
-78 °C and CD2Cl2 (0.7 mL) was added. It was then placed in the
NMR with the probe cooled to 193 K. [κ2-(HTp′)Pt(Me)(H)2(13CO)]-
[BAr′4] (8*) was synthesized in an analogous fashion using 13CO gas.
1H NMR (CD2Cl2, 193 K, δ): 10.01 (s, 1H, pz′NH), 6.10, 6.09, 6.09
(s, 1H, Tp′CH), 2.37, 2.36, 2.27, 2.26, 2.15, 1.49 (s, 3H, Tp′CH3),
(s, 3H, Tp′CH3, NCCH3), -20.67 (s, JPt-H ) 1265 Hz, 1H, Pt-H),
1
1
-24.08 (s, JPt-H ) 1496 Hz, 1H, Pt-H).
Structural Data for 5. Crystals from CH2Cl2/hexanes at 0 °C;
C16H23B1N6OPt, M ) 521.30; monoclinic, space group P21/n; Z ) 4;
a ) 13.5767(5), b ) 7.8292(3), c ) 18.9222(6) Å; R ) 90, â )
110.744(2), γ ) 90°; U ) 1880.94(12) Å3; Dc ) 1.84 Mg m-3; T )
-100 °C; max 2θ: 56°; Mo KR radiation (λ ) 0.710 73 Å); 4538 unique
reflections were obtained and 4525 of these with I > 2.5 σ(I) were
used in the refinement; data were collected on a Bruker SMART
diffractometer, using the ω scan method. For significant reflections
merging R-value: 0.0006; Residuals: RF: 0.065; RW: 0.170 (significant
reflections); GoF: 1.092. The hydride could not be located in the
Fourier difference map and was placed in a calculated position. Further
structural data is presented in the Supporting Information.
Kinetic Studies. Complex 6 was prepared in situ as described above,
then the NMR spectrometer was adjusted to the desired temperature
and 1H NMR spectra were taken after specific time intervals. The
hydride peaks for 6 were integrated relative to the solvent peak. The
data were plotted as a function of ln(integration)/(initial integration)
vs time and resulted in a straight line from which the pseudo first-
order rate constant was determined. The kinetic isotope effect was
determined by comparison of the rate of decay of 6 and 6-d3 at 239 K
and 18.7 psi CO. The rate of decay of 6-d3 was monitored by integration
of one of the Tp′ methyl peaks relative to the solvent peak.
2
1
-12.33 (d, JH-H ) 5.5 Hz, JPt-H ) 1166 Hz, 1H, Pt-H), -17.84 (d,
2JH-H ) 5.5 Hz, 1JPt-H ) 1193 Hz, 1H, Pt-H). IR (CH2Cl2): νBH
)
2522 cm-1, νCO ) 2124 cm-1. Spectral data for [κ2-(HTp′)Pt(Me)(H)2-
(13CO)][BAr′4] (8*). 1H NMR (CD2Cl2, 193 K, δ): 10.01 (s, 1H,
pz′NH), 6.10, 6.09, 6.09 (s, 1H, Tp′CH), 2.37, 2.36, 2.27, 2.26, 2.15,
2
2
1.49 (s, 3H, Tp′CH3), -12.33 (dd, JC-H ) 94 Hz, JH-H ) 5.5 Hz,
1JPt-H ) 1166 Hz, 1H, Pt-H), -17.84 (dd, JC-H ) 3.2 Hz, JH-H
)
2
2
5.5 Hz, JPt-H ) 1193 Hz, 1H, Pt-H). Spectral data for [κ2-(HTp′)Pt-
1
1
(Me)(H)2(CO)][BAr′4] (8b). H NMR (CD2Cl2, 193 K, δ): 0.53 (s,
2JPt-H ) 55 Hz, 3H, Pt-Me), -16.72 (s, 1JPt-H ) 1201 Hz, 2H, Pt-H).
Spectral data for [κ2-(HTp′)Pt(Me)(H)2(13CO)][BAr′4] (8b*). 1H NMR
2
(CD2Cl2, 193 K, δ): 0.53 (s, JPt-H ) 55 Hz, 3H, Pt-Me), -16.72 (d,
2JC-H ) 4 Hz, JPt-H ) 1201 Hz, 2H, Pt-H). Spectral data for [κ2-
1
(HTp′)Pt(Me)(H)2(CO)][BAr′4] (8c). 1H NMR (CD2Cl2, 193 K, δ):
-17.05 (s, JPt-H ) 1212 Hz, Pt-H). Spectral data for [κ2-(HTp′)Pt-
1
Acknowledgment. This research was supported by funding
from the National Science Foundation (CHE-0414726).
(Me)(H)2(13CO)][BAr′4] (8c*). H NMR (CD2Cl2, 193 K, δ): -17.05
1
2
1
(d, JC-H ) 3.7 Hz, JPt-H ) 1212 Hz, Pt-H).
In Situ Generation of [κ2-(HTp′)Pt(Me)(H)2(NCCH3)][BAr′4] (10).
Tp′PtMeH2 (3) (10 mg, 0.02 mmol) and [H(OEt2)2][BAr′4] (23 mg,
0.02 mmol) were added to an NMR tube in the drybox. The tube was
capped with a septum and removed from the box. It was then cooled
to -78 °C and CD2Cl2 (0.7 mL) was added. Acetonitrile (6.5 µL, 0.2
mmol) was syringed in and the tube was shaken, it was then placed in
Supporting Information Available: Structure properties for
complex 5 are listed in the Supporting Information along with
a cif file. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA056766M
9
2066 J. AM. CHEM. SOC. VOL. 128, NO. 6, 2006