Synthesis and reactivity of dimethyl platinum(IV) hydrides in water

Article abstract of DOI:10.1021/ja046310p

New hydrophilic ligands of the di(2-pyridyl)methanesulfonate family, L = dpms and Me-dpms, enable the synthesis of methyl platinum(IV) hydrides, LPtMe₂H, the study of very fast CH reductive coupling, and reductive elimination of these complexes in water. In dichloromethane solutions, ¹³CH₄ reacts with (Me-dpms)PtMe₂H to produce isotopomeric complexes. Copyright

Full text of DOI:10.1021/ja046310p

Published on Web 08/18/2004
Synthesis and Reactivity of Dimethyl Platinum(IV) Hydrides in Water
Andrei N. Vedernikov,* James C. Fettinger, and Fabian Mohr
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received June 22, 2004; E-mail: avederni@umd.edu
Alkane CH bond oxidative addition to transition-metal complexes
{L′M} (eq 1a) is one of the important reactions utilized in
hydrocarbon functionalization.1
In contrast to the exo-conformer, the endo-conformer is preor-
ganized16 for protonation. The sulfonate group in the latter is
proximal to the metal atom and poised to form a bond with Pt when
Pt changes the formal oxidation state from +2 to +4.
In the case where {L′M} is a platinum(II) precursor, alkyl
hydrido platinum(IV) complexes L′M(Alk)H 2 may emerge as stable
products of reaction 13,4 or as plausible intermediates.5 The stability
and reactivity of these species is primarily a function of their
structure.2,4,6,7 Exposed to a highly reactive solvent such as water,
L′PtIV(Me)H may undergo proton reductive elimination (eq 1b)8,9
or methane reductive elimination (reversal of eq 1a).
Studies of the mechanism of methane activation in aqueous
solutions of platinum(II) salts9 and search for evidence of inter-
mediacy of PtIV(Alk)H in these solutions were the subject of a
number of publications.10 Recent results8 suggest that methane CH
bond cleavage by aqueous platinum(II) compounds involves alkane
oxidative addition to platinum(II) (eq 1a) with consecutive proton
reductive elimination to form a platinum(II) alkyl (eq 1b). Despite
a number of efforts, formation of LPtMe2H in purely aqueous
solutions was never observed directly.11,12
Methanol-d4 and its mixtures with D2O caused fast reVersible
H/D exchange in the Pt-Me fragments of [LPtMe2]- as opposed
to slow H/D exchange accompanied by irreVersible methanolysis
reported earlier for a number of dimethylplatinum(II) complexes.11,17
The obserVed pseudo-first-order rate constants (e.g., 2.10(4) × 10-3
s-1 at 62.3 mol % D2O at 296 K) increased linearly with a molar
concentration of water in solution leading to a second-order rate
constant k ) 8.5(3) × 10-5 M-1 s-1 and effective Gibbs activation
q
energy of the H/D exchange ∆Gex ≈ 22.8 kcal/mol. These results
imply both facile protonation of [LPtMe2]- by D2O to produce
two isomeric LPtMe2D (see below) and Very fast D-C reductiVe
coupling to form an intermediate methane complex without
q
elimination of methane (eq 2; ∆Grc < ∆Greq, Figure 1) in contrast
to earlier observations.11,18
We report here the synthesis and the reactivity of a series of
new methyl hydrido platinum(IV) complexes LPtMe2H featuring
the ability to exchange their methyl groups with 13CH4 and
persistence in water, which is sufficient for observation of their
reactivity and even for their isolation from aqueous solutions of
corresponding anionic platinum(II) precursors. This wet hydrido
platinum(IV) chemistry became possible by using the new hydro-
philic anionic fac-chelating ligand L, di(2-pyridyl)methanesulfonate,
(dpms)-, and its methyl analogue (Me-dpms)- (Chart 1). These
Indeed, an 11.6 mM aqueous (H2O) solution of K(dpms)PtMe2
is alkaline (pH ) 10.1, 1.1% of (dpms)PtMe2H at equilibrium);
fast evaporation of this solution at 0 °C allows recovery of K(dpms)-
PtMe2 quantitatively. More convincing, LPtMe2H (5% yield, L )
dpms; 50% yield, L ) Me-dpms) was isolated from solutions of
K(L)PtMe2 in H2O by extracting them with dichloromethane, which
shifts the equilibrium (eq 2a) to neutral and more lipophilic
LPtMe2H. These observations suggest that platinum(IV) alkyl
hydrides can form in the Shilov-type systems as well as they can
undergo a reVersible proton loss in water (eqs 1b, 2a) to give
platinum(II) alkyls. On the basis of pD measured (eq 2a) we
estimated ∆Gp ≈ 10.5 kcal/mol and therefore the effective activation
Chart 1
ligands were designed to allow the sulfonate group to play an actiVe
solVent-dependent role in coordination to the metal (compare with
tris(pyrazolyl) methanesulfonates13). The donicity of the anionic
SO3- tail toward the metal and therefore the ability of the ligands
L to stabilize octahedral methane addition products LPtMe2H were
expected to be a function of the solvent polarity. Strong interactions
q
barrier for the reductive coupling, ∆Grc ≈ 12.3 kcal/mol.
In contrast to methanol-d4, D2O solutions of K(dpms)PtMe2
(2.3:1 endo-/exo-, respectively) slowly liberate methane (7.3(2) ×
10-7 M-1 s-1, τ1/2 ) 290 min, 296 K) to form cleanly the K(dpms)-
PtMe(OD) complex and imply an effective barrier for methane
-
of the polar SO3 group with a polar solvent such as water could
ultimately help break SO3-Pt bonds, switch L to an η2-coordination
mode, and form the unsaturated highly reactive transient (η2-L)-
PtMe2H. In weakly polar solvents such as dichloromethane not
q
reductive elimination of (dpms)PtMe2D, ∆Gre ≈ 15.1 kcal/mol.
In the presence of air, clean oxidation of K(L)PtMe2, but not
hydrolysis, occurred to produce (dpms)PtMe2(OH) (Figure 2a).
Thus, the ligand-enhanced basicity of [LPtMe2]- allows for the
formation of LPtMe2H at low concentrations in water and estimation
of the rate constants of very fast CH reductive coupling and
reductive elimination of these species at room temperature.19
The complexes LPtMe2H were also obtained in 35-55% yield
as white precipitates by protonation of aqueous solutions of K(L)-
-
capable of interacting strongly with the SO3 group, more robust
SO3-Pt bonds are formed and the kinetically more inert (η3-L)-
PtMe2H could be expected.
Ligand exchange with Pt2Me4(SMe2)2 in methanol gave the
14
anionic platinum(II) complexes K(L)PtMe2 that are stable in
methanolic solutions as a mixture of exo- and endo-conformers,15
7:1, respectively, for L ) dpms (established by NOE experiments):
9
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J. AM. CHEM. SOC. 2004, 126, 11160-11161
10.1021/ja046310p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
to 2 atm of 13CH4 in CH2Cl2 solution reacted to produce 13C-iso-
topomer (10% enrichment after 30 h). This is the first, to our
knowledge, published report of a reaction between methane and a
platinum complex leading to platinum(IV) methyl hydride.
In summary, we have shown that anionic hydrophilic ligands of
the di(2-pyridyl)methanesulfonate family open new opportunities
to control reactivity of LPtMe2H in aqueous and organic media.
Acknowledgment. We thank the University of Maryland for
financial support of this work.
Supporting Information Available: Experimental details, char-
acterization data, and CIF files for (dpms)PtMe2H and (dpms)PtMe2-
(OH). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 1. Energy of protonation of [LPtMe2]-, ∆Gp, the observed effective
activation energy of H/D exchange in PtMe groups, ∆Gexq, reductive
coupling, ∆Grcq, and reductive elimination, ∆Greq, of LPtMe2D in D2O
solution (L ) dpms), molarity scale.
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Figure 2. ORTEP drawings (50% probability ellipsoids) of (a) (dpms)-
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PtMe2 with triflic acid at 0 °C. Thus, a transition to a solid or liquid
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stronger coordination of SO3 group and PtIV as opposed to the
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ability of water to cause dissociation of the SO3- tail due to strong
solvation. Such coordination disfavors formation of low-coordinate
platinum species prone to facile methane elimination.20
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H. Organometallics 1997, 16, 2175-2182.
The LPtMe2H complexes synthesized are not thermodynamically
stable in water. When isolated LPtMe2H was stirred with H2O at
25 °C in the absence of organic solvent, it dissolved slowly to form
methane and the endo-LPtMe(OH2). The methane reductive elimi-
nation was preceded by very fast and reversible CH reductive cou-
pling. Thus, in H2O-D2O mixtures isotopomeric LPt(CHnD3-n)(OH2)
(n ) 0, 1, 2, 3) was observed, but no methyl group exchange was
detected when this solution was exposed to 2 atm of 13CH4 at room
temperature for one week.
(7) Prokopchuk, E. M.; Jenkins, H. A.; Puddephatt, R. J. Organometallics
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(11) A number of cationic platinum(IV) alkyl hydrides can exist in methanol
though they never were isolated from these solutions. See refs 7, 12.
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Similarly, the protonation of K(L)PtMe2 with triflic acid in CH2-
Cl2 at -80 °C yielded LPtMe2H (40-55%). LPtMe2H is stable in
the solid state but decomposes in dichloromethane in the course of
several days at room temperature, producing an unidentified white
(14) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643-1648.
(15) We do not consider isomeric [(N,O-η2-dpms)PtMe2]- as important
components of these solutions, assuming that the pyridyl fragments of
the ligand can coordinate stronger to the metal atom than the sulfonate
group. In the gas phase the exo-[(dpms)PtMe2]- is more stable than the
endo-conformer by 6.3 kcal/mol (DFT, see Supporting Information).
(16) Vedernikov, A. N.; Shamov, G. A.; Solomonov, B. N. Russ. J. Gen. Chem.
1999, 69, 1102-1114.
1
precipitate. According to H NMR data, (dpms)PtMe2H exists in
CH2Cl2 as a mixture of two isomers that slowly eliminate methane
at 296 K. The Pt-H resonance for the predominant species of Cs
symmetry is at -26.15 ppm (s, 1JPt-H ) 1813 Hz) with a hydrido
ligand trans to the sulfonate group. This geometry was also
established by XRD (Figure 2b). The position of the Pt-H
resonance at -20.30 ppm (1JPt-H ) 1428 Hz) for the other isomer
having C1 symmetry is consistent with a structure with the hydride
trans to one of the pyridine fragments.
In contrast to aqueous solutions, (dpms)PtMe2H is kinetically
more stable in weakly polar and weakly coordinating CH2Cl2,
whereas the corresponding solvent-complex LPtMe(solvent)21 is
much more reactive. The mixture of isomeric complexes reacts with
Et3SiH in the course of few days at room temperature, producing
cleanly a single isomer of (dpms)PtMe(SiEt3)H with the Pt-H reso-
nance at -18.76 ppm (s, 1JPt-H ) 1360 Hz). The reaction of (dpms)-
PtMe2H with benzene (30 % vol) dissolved in dichloromethane is
slower than its rate of decomposition. The methyl analogue (Me-
dpms)PtMe2H, however, reacts faster22 and produces methane and
a single isomer of LPtMe(Ph)H with the Pt-H resonance at -24.80
ppm (s, 1JPt-H ) 1816 Hz). Moreover, (Me-dpms)PtMe2H exposed
(17) Rostovtsev, V. V.; Labinger, J. A.; Bercaw, J. E.; Lasseter, T.; Goldberg,
K. I. Organometallics 1998, 17, 4530-4531.
(18) TpPtMe(H)2: Iron, M. A.; Lo, H. C.; Martin, J. M. L.; Keinan, E. J. Am.
Chem. Soc. 2002, 124, 7041-7054.
(19) Another protonation equilibrium besides eq 2a may lead to a complex
with the exo-sulfonate group and H2O trans to the hydride. Nevertheless,
-
the SO3 tail in the endo-conformer is closer to platinum and therefore
better stabilizes electrostatically the protonated metal, ∆Gp ≈ ∆Gp(endo)
< ∆Gp(exo). See also: Lucey, D. W.; Helfer, D. S.; Atwood, J. D.
Organometallics 2003, 22, 826-833. Assuming that alkane reductive
elimination from LPtMe2H proceeds via low-coordinate intermediate (see
-
ref 20), the closer proximity of the SO3 tail to cationic platinum and
thus stronger electrostatic interaction of these two makes the low-
coordinate transition states for CH reductive coupling and methane
elimination of the endo-conformer lower in energy than corresponding
transition states of the exo-conformer. Thus, data in Figure 1 reflect mainly
the reactivity of the endo-[LPtMe2]- and the related endo-LPtMe2H.
(20) (a) Hill, G. S.; Puddephatt, R. J. J. Am. Chem. Soc. 1996, 118, 8745-
8746. (b) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc.
2003, 125, 9442-9456.
(21) Butts, M. D.; Scott, B. L.; Kubas, G. J J. Am. Chem. Soc. 1996, 118,
11831-11843.
(22) (Me-dpms)PtMe2H is less stable kinetically than (dpms)PtMe2H, presum-
ably because of increased steric interactions between ligands.
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