Definitive evidence for a pairwise addition of hydrogen to a platinum bis(phosphine) complex using parahydrogen-induced polarization

Article abstract of DOI:10.1021/om960295i

Through the use of parahydrogen-induced polarization (PHIP), conclusive evidence has been obtained that H₂ addition to Pt(Ph₂PCH₂CH(Me)OPPh₂) proceeds in a concerted, pairwise manner. PHIP has also been used in conjunction with a 2D HMQC sequence to obtain ³¹P and ¹⁹⁵Pt NMR spectra from submilligram samples in 2-5 min.

Full text of DOI:10.1021/om960295i

Organometallics 1996, 15, 2863-2865
2863
Defin itive Evid en ce for a P a ir w ise Ad d ition of Hyd r ogen
to a P la tin u m Bis(p h osp h in e) Com p lex Usin g
P a r a h yd r ogen -In d u ced P ola r iza tion
Meehae J ang,† Simon B. Duckett,‡ and Richard Eisenberg*,†
Departments of Chemistry, University of Rochester, Rochester, New York 14627, and
University of York, Heslington, York, England YO1 5DD
Received April 18, 1996X
Summary: Through the use of parahydrogen-induced
polarization (PHIP), conclusive evidence has been ob-
tained that H2 addition to Pt(Ph2PCH2CH(Me)OPPh2)
proceeds in a concerted, pairwise manner. PHIP has
also been used in conjunction with a 2D HMQC sequence
to obtain 31P and 195Pt NMR spectra from submilligram
samples in 2-5 min.
(diphos) were employed,13-16 but it was found that many
of the PtH2(diphos) systems reversibly lost hydrogen
with concomitant formation of [PtH(diphos)]2 dimers.
In a recent investigation of these systems, Andersen
observed that, in addition to monomer-dimer equilibria,
mixtures of PtH2(diphos) and PtD2(diphos) led to facile
generation of the d1 isotopomer.17 Far from establishing
the concerted addition of H2 to Pt(0), the studies of PtH2-
(diphos) systems have generated uncertainty about the
presumed mechanism of addition.
In this communication, we present definitive evidence
for the pairwise addition of dihydrogen to a platinum
center using parahydrogen-induced polarization (PHIP)
and show that with the aid of PHIP extremely rapid
detection of 31P and 195Pt NMR resonances can be
achieved from submilligram samples of complex. As
described previously,18-25 PHIP arises when H2 enriched
in the para spin state adds to a metal center in a
manner such that spin correlation is maintained be-
tween the two protons, leading to enhanced absorptions
and emissions in the product dihydride resonances. A
key to observing PHIP is that the transferred protons
become magnetically distinct in the product.
In order to remove the equivalence of the hydride
ligands in PtH2L2 systems, the unsymmetrical bidentate
ligand Ph2PCH2CH(Me)OPPh2((dpp)2mop) has been pre-
pared from propylene oxide, LiPPh2, and PClPh2, and
the corresponding Pt dihydride complex 1 has been
synthesized according to eq 1.26 The identity of 1 is
established unequivocally by 1H, 31P, and 195Pt NMR
spectroscopies.27 When 0.4 mg of 1 is dissolved in
toluene-d8 under ca. 2 atm of para-enriched H2, the
spectrum shown in Figure 1 is obtained within 67 s after
The activation of dihydrogen by metal complexes in
solution is a key step in homogeneous hydrogenation.1-3
For most late metal catalysts, the activation proceeds
by oxidative addition in a concerted manner leading to
a dihydride product of metal oxidation state two greater
than in the starting compound. For d8 complexes such
as IrCl(CO)(PPh3)2 and IrBr(CO)(dppe) (dppe ) bis-
(diphenylphosphino)ethane), oxidative addition of H2
has been studied extensively and the dihydride products
have been shown conclusively to form in a concerted,
pairwise manner with hydride ligands mutually cis.4-9
For d10 complexes of Pt(0), the addition of H2 is
presumed to take place in an analogous manner, but
evidence in this regard is less compelling. While Pt(II)
dihydrides have long been known,10 their formation
often involved reductions using hydride sources, and in
cases where the reaction was done under H2, the
possible addition of hydrogen was not studied mecha-
nistically. However, from Trogler’s study of the cis-
trans iosmerism of PtH2(PMe3)2 and subsequent reduc-
tive elimination of H2,11,12 a pairwise addition mechanism
can be inferred mainly on the basis of a kinetic isotope
effect for the elimination. In other studies designed to
maintain a cis disposition of the hydride ligands in
PtH2L2 complexes, chelating bis(phosphine) ligands
† University of Rochester.
(13) Yoshida, T.; Yamagata, T.; Tulip, T. H.; Ibers, J . A.; Otsuka, S.
J . Am. Chem. Soc. 1978, 100, 2063-2073.
‡ University of York.
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) J ames, B. R. Homogeneous Hydrogenation; J ohn Wiley and
Sons: New York, 1973.
(2) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(3) J ames, B. R. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New York,
1982; Vol. 8, Chapter 51.
(14) Clark, H. C.; Smith, M. J . H. J . Am. Chem. Soc. 1986, 108,
3829-3830.
(15) Carmichael, D.; Hitchcock, P. B.; Nixon, J . F.; Pidcock, A. J .
Chem. Soc., Chem. Commun. 1988, 1554-1556.
(16) Scrivanti, A.; Campostrini, R.; Carturan, G. Inorg. Chim. Acta
1988, 142, 187-189.
(17) Schwartz, D. J .; Andersen, R. A. J . Am. Chem. Soc. 1995, 117,
4014-4025.
(18) Bowers, C. R.; Weitekamp, D. P. Phys. Rev. Lett. 1986, 57,
2645-2648.
(4) Vaska, L.; DiLuzio, J . W. J . Am. Chem. Soc. 1962, 84, 679-680.
(5) Chock, P. B.; Halpern, J . J . Am. Chem. Soc. 1966, 88, 3511-
3514.
(19) Bowers, C. R.; Weitekamp, D. P. J . Am. Chem. Soc. 1987, 109,
5541.
(6) Halpern, J . Acc. Chem. Res. 1970, 3, 386-392.
(7) J ohnson, C. E.; Fisher, B. J .; Eisenberg, R. J . Am. Chem. Soc.
1983, 105, 7772-7774.
(20) Eisenschmid, T. C.; Kirss, R. U.; Deutsch, P. P.; Hommeltoft,
S. I.; Eisenberg, R.; Bargon, J .; Lawler, R. G.; Balch, A. L. J . Am. Chem.
Soc. 1987, 109, 8089-8091.
(8) J ohnson, C. E.; Eisenberg, R. J . Am. Chem. Soc. 1985, 107,
3148-3160.
(21) Kirss, R. U.; Eisenschmid, T. C.; Eisenberg, R. J . Am. Chem.
Soc. 1988, 110, 8564-8566.
(9) Deutsch, P. P.; Eisenberg, R. Chem. Rev. 1988, 88, 1147-1161.
(10) Moulton, C. J .; Shaw, B. L. J . Chem. Soc., Chem. Commun.
1976, 365-366.
(22) Bargon, J .; Kandels, J .; Kating, P.; Thomas, A.; Woelk, K.
Tetrahedron Lett. 1990, 31, 5721-5724.
(23) Bargon, J .; Kandels, J .; Woelk, K. Angew. Chem., Int. Ed. Engl.
1990, 29, 58-59.
(11) Packett, D. L.; Trogler, W. C. J . Am. Chem. Soc. 1986, 108,
5036-5038.
(24) Eisenberg, R. Acc. Chem. Res. 1991, 24, 110-116.
(25) Eisenberg, R.; Eisenschmid, T. C.; Chinn, M. S.; Kirss, R. U.
Adv. Chem. Ser. 1992, No. 230, 47-74.
(12) Packett, D. L.; Trogler, W. C. Inorg. Chem. 1988, 27, 1768-
1775.
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2864 Organometallics, Vol. 15, No. 13, 1996
Communications
F igu r e 1. 1H PHIP NMR spectrum of ((dpp)2mop)PtH2,
1, obtained from the reaction of 0.4 mg of 1 and para-
enriched H2 in toluene-d8 at 298 K, showing antiphase
doublets due to Ha and Hb. Other peaks due to solvent (s)
and dissolved orthohydrogen (*) are labeled.
shaking the sample and introducing it into the spec-
trometer. The signals corresponding to Ha and Hb at δ
-0.282 and -0.014 ppm, respectively, exhibit doublet
of doublets patterns due to 31P coupling, 195Pt satellites,
and enhanced absorption/emission (A/E) polarization
with a separation between absorption and emission
extrema of 2.4 Hz corresponding to J HH. The phase of
the polarization (A/E) indicates that the sign of the J HH
coupling constant is positive.19 Hydride Ha also exhibits
an additional coupling to one of the methylene protons
of the (dpp)2mop ligand.
F igu r e 2. Cross peaks and projections from 1H-X cor-
relation spectra of 1 obtained with para-enriched H2 in
toluene-d8 at 298 K: (a) 1H-31P HMQC spectrum, acquired
transPHa
with τ ) 1/(2J
) and an acquisition time of 2 min; (b)
1H-195Pt correlation spectrum acquired with τ ) 1/(2J PtH
)
and an acquisition time of 5 min, showing cross peakas
connecting 31P-coupled Ha and Hb resonances to 31P-coupled
195Pt resonances.
1
In Figure 1, there are no other resonances observable
of polarization, which is a function of H spin-lattice
1
except for residual H in toluene-d8 and the signal for
relaxation times, the rates of H2 oxidative addition and
reductive elimination, and relaxation from a “higher
order” spin state resulting from parahydrogen addi-
tion,19 follows clean first-order kinetics with a rate
constant of 4.03 × 10-3 s-1. At t ) 2.3 min, the observed
polarization enhancement was greater than 900-fold. By
extrapolation, the magnitude of enhancement is esti-
mated to be greater than 1600-fold, which is close to
the theoretical maximum obtainable under our experi-
mental condition (1750) and is the largest PHIP en-
hancement yet reported. The results establish un-
equivocally a pairwise mechanism for H2 addition to
form 1.
dissolved orthohydrogen. The magnitude of the polar-
ization was estimated by measuring the rate of polar-
ization decay and extrapolating back to t ) 0. The decay
(26) The ligand (dpp)2mop was purified by chromatography over
alumina using benzene (yield, 20%). 31P{1H} NMR (benzene-d6):
δ
+107 (s, Po), -23 (s, Pc). 1H and 1H COSY NMR: δ 7.2-7.6 (m, 24H,
2
2
Ph), 2.68 and 2.24 (dd, J HH ) 4.5 Hz, J HP ) 13.8 Hz, 1H, CH2), 4.22
(qdd, CH), 1.35 (d, 3J HH ) 6.1, CH3). The complex ((dpp)2mop)PtCl2 is
isolated as a white powder from CH2Cl2-cyclohexane (yield, 60%). 31P-
2
1
{1H} NMR (methylene chloride-d2): δ +82.7 (d, J PP ) 17.8 Hz, J PPt
) 3842 Hz, Po), +0.48 (d, J PPt ) 3504 Hz, Pc). 1H NMR: δ 7.2-7.6
1
(m, 24H, Ph), 1.10 (d, 3J HH ) 5.6 Hz, CH3), 2.31 (m, 2H, CH2), 3.71 (m,
1H, CH). 195Pt{1H} NMR (methylene chloride-d2): δ -14.8 (dd).
(27) Complex 1 is isolated as pale yellow powder from THF and
n-hexane under H2 (yield, 70%). Both hydride and phosphorus
resonances are flanked by 195Pt satellites. 1H NMR (methylene
When a solution of 1 is placed under D2, rapid loss of
hydride resonances occurs with initial observation of H2
and a small amount of HD. Upon standing, the relative
amount of HD increases while that of H2 diminishes.
These observations indicate that reductive elimination
of H2 from 1 takes place in a pairwise manner, consis-
tent with the PHIP results described above for the
reverse addition process.
3
chloride-d2): δ 6.7-8.3 (m, 24H, Ph), 1.09 (d, J HH ) 5.5 Hz, CH3),
2.04 (m, 2H, CH2), 3.96 (m, 1H, CH), -0.282 (ddd, 4J HH ) 2.8 Hz, 2J
cisPH
1
) 18.6 Hz, 2J
) 170 Hz, J HPt ) 1204 Hz, 1H, Ha), -0.014 (dd,
transPH
2
2
) 199 Hz, 1J HPt ) 1076 Hz, 1H, Hb). 31P{1H}
PH
2 1
cis
trans
J
) 17.6 Hz, J
PH
NMR (tetrahydrofuran-d8): δ +125.9 (d, J PP ) 22 Hz, J PPt ) 2300
Hz, Po), +14.5 (d, J PPt ) 2005 Hz, Pc). The 1H-coupled 31P NMR
1
spectrum shows two doublets at +125.9 and +14.5 ppm with 2J
transPH
values identical to those obtained from the 1H NMR spectrum. 195Pt-
{1H} NMR (toluene-d8): δ -820.0 (dd).
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Communications
Organometallics, Vol. 15, No. 13, 1996 2865
Previously, parahydrogen-induced polarization has
been used in conjunction with pulse sequences such as
INEPT+ to enhance signals of less sensitive nuclei.28
Recently, it has been shown that indirect detection of
these nuclei can be achieved rapidly through utilization
of a 2D-HMQC sequence via parahydrogen polarization
195Pt spectrum of 1 in which phase cycling removes
signals from products containing other isotopes of
platinum is shown in Figure 2b. In the 195Pt dimension,
doublet of doublets multiplicity due to coupling to Po
and Pc is centered at -820 ppm.
The results we have described show PHP in platinum
hydride complexes for the first time and provide defini-
tive evidence for pairwise H2 addition to form 1. Ad-
ditionally, PHIP has been used in conjunction with 2D
1
transfer.29,30 In Figure 2, we show 2D 1H-31P and H-
195Pt correlation spectra of 1, acquired in 2-5 min using
less than 1 mg of sample. For these spectra, an initial
45° 1H pulse is employed with delays for magnetization
transfer set as in the Figure 2 caption. The 1H-31P
correlation spectrum of 1 is shown in Figure 2a. Four
major cross peaks connecting the phosphorus nuclei Po
(δ ) +125.9) and Pc (δ ) +14.5) to their proton coupling
partners, Ha and Hb, are observed. Additional cross
peaks for the 195Pt satellites are clearly visible in this
spectrum (195Pt, I ) 1/2, 33.8%). The corresponding 1H-
HMQC pulse sequences to give indirect detection of 31
P
and 195Pt in extraordinarily short amounts of spectrom-
eter time.
Ack n ow led gm en t. Financial support of this work
from the National Science Foundation (Grant No. CHE
94-09441) and from NATO is gratefully recognized. In
addition, a generous loan of platinum salts from
the J ohnson-Mathey Aesar/Alfa Co. is thankfully
acknowledged.
(28) Duckett, S. B.; Newell, C. L.; Eisenberg, R. J . Am. Chem. Soc.
1993, 115, 1156-1157.
(29) Duckett, S. B.; Barlow, G. K.; Partridge, M. G.; Messerle, B. A.
J . Chem. Soc., Dalton Trans. 1995, 3427-3429.
(30) Duckett, S. B.; Mawby, R. J .; Partridge, M. G. J . Chem. Soc.,
Chem. Commun. 1996, 383-384.
OM960295I
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