Methyl(hydrido)platinum(IV) complexes that are resistant to reductive elimination, including the first (μ-hydrido)diplatinum(IV) complex

Full text of DOI:10.1021/ja961404n

J. Am. Chem. Soc. 1996, 118, 8745-8746
8745
Scheme 1
Methyl(hydrido)platinum(IV) Complexes That Are
Resistant to Reductive Elimination, Including the
First (µ-Hydrido)diplatinum(IV) Complex
Geoffrey S. Hill and Richard J. Puddephatt*
Department of Chemistry
The UniVersity of Western Ontario
London, Ontario, Canada N6A 5B7
ReceiVed April 29, 1996
Scheme 2
The first examples of methyl(hydrido)platinum(IV) com-
plexes, which are proposed intermediates in the protonolysis
of the methylplatinum(II) bond and in methane activation by
platinum(II), were reported very recently (Scheme 1).1 In most
cases, these complexes [PtHXMe2(LL)], where LL is a bidentate
nitrogen-donor ligand and X ) halide, SO3CF3, or O2CCF3,
are characterized only in solution at low-temperature since, at
room temperature, they decompose rapidly. The mechanism
of decomposition is proposed to involve dissociation of the
ligand X trans to the hydride, to form a five-coordinate
intermediate which then undergoes easy reductive elimination
of methane (Scheme 1).1b-d
If this mechanism is correct, a complex [PtHXMe2(LL)]
should be stable to reductive elimination of methane if the group
X cannot easily undergo dissociation from platinum. This
suggested that a complex [PtHMe3(LL)], in which all ligands
are strongly bound, would be stable to reductive elimination.
In addition, this complex would have mutually trans hydrido
and methyl ligands (both with large trans effects) and so should,
by analogy with the related tetramethylplatinum(IV) complex
[PtMe4(NN)] (NN ) 2,2′-bipyridine),2 give a rich reaction
chemistry. Thus the plan was to prepare fac-[PtHMe3(bu2bpy)]
(2) (bu2bpy ) 4,4′-di-tert-butyl-2,2′-bipyridine) from NaBH4
and fac-[PtMe3(O3SCF3)(bu2bpy)], with the O3SCF3 ligand
acting as a good leaving group (Scheme 2).3 Although this
reaction did occur if a large excess of hydride was present, it
proceeded through the unexpected cationic intermediate, [Pt2(µ-
H)Me6(bu2bpy)2]+ (1), which is the first example of a (µ-
hydrido)diplatinum(IV) complex (Scheme 2).
1
The H NMR spectrum (300 MHz, CD2Cl2) of complex 1
shows the expected three aromatic resonances and one tert-butyl
resonance due to the two equivalent pyridine moieties of the
bu2bpy ligand. These data rule out the alternative isomer with
µ-H trans to nitrogen, which would have nonequivalent pyridyl
groups. There were two methylplatinum resonances in a 2:1
intensity ratio due to the methylplatinum groups trans to bu2-
2
bpy [δ ) 0.47, J(PtH) ) 69.6 Hz] and trans to hydride [δ )
2
0.13, J(PtH) ) 65.9 Hz], respectively. Both peaks showed a
small coupling with the hydrido ligand. The most convincing
evidence for a bridging hydrido ligand comes from the low-
1
frequency Pt-H resonance at δ ) -11.7 with J(PtH) ) 442
Hz (Figure 1). The resonance appears as a 1:8:18:8:1 multiplet
due to coupling to 195Pt, thus proving the presence of a Pt2(µ-
H) group.6 This Pt-H resonance is absent in the H NMR
1
Treatment of fac-[PtMe3(O3SCF3)(bu2bpy)]4 with a stoichio-
metric amount of NaBH4 in THF solution affords the cationic
spectrum of [Pt2(µ-D)Me6(bu2bpy)2]+ (1*), prepared using
NaBD4. The 2H{1H} NMR spectrum (30.70 MHz, CH2Cl2) of
complex 1* shows only the expected resonance at δ ) -11.7
with 1J(PtD) ) 68.4 Hz.7 The 1H-coupled 195Pt NMR spectrum
(42.92 MHz, THF-d8) of 1 contains a doublet at δ ) -1238
(from K2[PtCl4] in D2O) due to coupling with the bridging
hydrido ligand [1J(PtH) ) 440 Hz], thus proving the presence
of only a single µ-H ligand. The 195Pt NMR spectrum (CD2-
Cl2) of [Pt2(µ-D)Me6(bu2bpy)2]+ (1*) shows only a broad singlet
at δ ) -1240, since the line width (∆ν1/2 ) ca. 160 Hz) is
-
[Pt2(µ-H)Me6(bu2bpy)2]+ (1) which, as the BPh4 salt, can be
isolated as a yellow powder in 65% yield. Complex 1 is
indefinitely stable at room temperature, both as a solid and in
1
solution, which allowed full characterization by H and 195Pt
NMR spectroscopies.5
(1) (a) De Felice, V.; De Renzi, A.; Panunzi, A.; Tesauro, D. J.
Organomet. Chem. 1995, 488, C13. (b) Hill, G. S.; Rendina, L. M.;
Puddephatt, R. J. Organometallics 1995, 14, 4966. (c) Stahl, S. S.; Labinger,
J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1995, 117, 9371. (d) Stahl, S. S.;
Labinger, J. A.; Bercaw, J. E. Personal communication. (e) Canty, A. J.
Personal communication.
1
greater than the coupling J(PtD).
Complex 1 is stable in solution in common organic solvents.
The presence of a large excess of NaBH4 results in formation
of an equilibrium mixture of 1 and 2 equiv of [PtHMe3(bu2-
bpy)] (2) (Scheme 2).8 Under these strongly basic conditions,
the complexes decompose slowly with precipitation of metallic
(2) (a) Hux, J. E.; Puddephatt, R. J. Inorg. Chim. Acta. 1985, 100, 1.
(b) Hux, J. E.; Puddephatt, R. J. J. Organomet. Chem. 1988, 346, C31.
(3) Note that this route is formally H- + Pt(IV) f Pt(IV)-H, in contrast
to the route H+ + Pt(II) f Pt(IV)-H used previously.1
(4) (a) [PtMe3(O3SCF3)(bu2bpy)] is conveniently prepared by reaction
of [PtMe2(bu2bpy)]4b with MeO3SCF3 in ether at room temperature. NMR
1
2
in CDCl3: δ(1H) ) 0.66 [s, 3H, J(PtH) ) 84 Hz, PtMe trans to triflate];
platinum, but 2 was readily characterized by H NMR spec-
1.26 [s, 6H, 2J(PtH) ) 67 Hz, PtMe trans to bu2bpy]. (b) Achar, S.; Scott,
J. D.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1993, 12, 4592. Note
that [PtMe2(bu2bpy)] is stable in the presence of NaBH4.
(6) (a) Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. J.
Chem. Soc., Dalton Trans. 1978, 516. (b) Brown, M. P.; Cooper, S. J.;
Frew, A. A.; Manojlovic-Muir, Lj.; Muir, K. W.; Puddephatt, R. J.;
Thompson, M. A. J. Chem. Soc., Dalton Trans. 1982, 299.
-
(5) Spectroscopic data for 1 as the BPh4 salt. NMR (300 MHz) in
CD2Cl2: δ(1H) ) 8.21 [d, 4H, 3J(H6H5) ) 6.3 Hz, 3J(PtH) ) 14.0 Hz,
4
4
1
H6]; 8.09 [d, 4H, J(H3H5) ) 2.0 Hz, H3]; 7.51 [dd, 4H, J(H5H3) ) 1.9
(7) γH/γD ) 6.51 ≈ J(PtH)/1J(PtD) ) 6.47.
Hz, J(H5H6) ) 6.2 Hz, H5]; 1.49 [s, 36H, bu]; 0.47 [s, 12H, J(PtH) )
(8) Spectroscopic data for 2. NMR (300 MHz) in acetone-d6: δ(1H) )
8.58 [d, 2H, 3J(H6H5) ) 6.0 Hz, H6]; 8.51 [d, 2H, 4J(H3H5) ) 2.0 Hz, H3];
3
t
2
69.6 Hz, 3J(HH) ) ca. 1.0 Hz, Pt-Me (trans to bu2bpy)]; 0.13 [s, 6H,
2J(PtH) ) 65.9 Hz, J(HH) ) ca. 1.0 Hz, Pt-Me (trans to H)]; -11.7 [s,
7.85 [dd, 2H, J(H5H5) ) 2.0 Hz, J(H5H6) ) 6.0 Hz, H5]; 1.45 [s, 18H,
tbu]; 0.75 [s, 6H, 2J(PtH) ) 66.0 Hz, Pt-Me (trans to bu2bpy)]; -0.79 [s,
3H, 2J(PtH) ) 43.0 Hz, Pt-Me (trans to H)]; -7.0 [s, 1H, 1J(PtH) ) 805
Hz, Pt-H]. Complex 2 is also formed on reaction of 1 with PPh3 in
refluxing acetone and is stable under these conditions.
3
4
3
1H, 1J(PtH) ) 442 Hz, Pt-H]. 195Pt NMR (42.92 MHz) in THF-d8: δ )
-1238 [d, 1J(PtH) ) 440 Hz]. We draw 1 with a linear PtHPt bond though
M2(µ-H) groups are usually bent. The degree of bending must be small in
this case due to steric effects between substituents on platinum.
S0002-7863(96)01404-7 CCC: $12.00 © 1996 American Chemical Society

8746 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Communications to the Editor
hydridic in nature than in the latter complexes. It is presumably
this hydridic character that leads to reaction of 2 with [PtMe3(O3-
SCF3)(bu2bpy)] with displacement of triflate to give the stable
complex 1.9
In a very recent study, it was shown that the protonolysis of
the Pt-C bond by DCl in [PtMe2(tmeda)] (tmeda ) N,N,N′,N′-
tetramethylethylenediamine) proceeds through the detectable
Pt(IV)-deuteride, [PtD(Cl)Me2(tmeda)] and that deuterium
incorporation into the methylplatinum groups of [PtD(Cl)Me2-
(tmeda)] occurs faster than the reductive elimination of
methane.1c,d It was proposed that this occurred by dissociation
of the chloride ligand to form a five-coordinate complex as in
Scheme 1, followed by easy, reversible formation of a Pt(CH3D)
σ-complex.1d Since neither [PtDMe3(bu2bpy)] nor [Pt2(µ-D)-
Me6(bu2bpy)2]+ can readily undergo ligand dissociation to form
the required five-coordinate intermediate, no isotopic H-D
exchange within PtDMe groups would be expected. This
1
Figure 1. Low-frequency region of the H NMR spectrum of 1 in
CD2Cl2 at 25 °C. The 1:8:18:8:1 intensity ratio in the multiplet due to
195Pt1H coupling is characteristic of a doubly bridging hydride.
troscopy (acetone-d6) and appears stable to reductive elimination
of methane, which would give the stable complex [PtMe2(bu2-
bpy)].4b Again, the two equivalent bipyridine moieties of the
bu2bpy give rise to three aromatic and one tert-butyl resonance.
The methylplatinum resonances appear in a 2:1 ratio at δ )
0.75 (trans to bu2bpy) and -0.79 (trans to H) with J(PtH) )
66.0 and 43.0 Hz, respectively. Note that the methyl group
trans to the hydrido ligand has a very low value of J(PtH),
which is significantly smaller than that of the methylplatinum
ligand trans to the bridging hydride in complex 1 [2J(PtH) )
65.9 Hz] but is similar to that of the mutually trans methyl-
platinum ligands in [PtMe4(bpy)] [2J(PtH) ) 44 Hz].2 Thus
the terminal hydrido ligand in complex 2 has a stronger trans
influence than the bridging hydride in 1. The Pt-H ligand
resonates at δ ) -7.0 with 1J(PtH) ) 805 Hz. This resonance
appears as a 1:4:1 multiplet due to coupling to 195Pt, thus proving
the presence of a terminal Pt-H group. This coupling constant
has approximately twice the magnitude of that found in 1, which
is reasonable since the s-electron density of the hydride is shared
between two platinum centers in 1. Nevertheless, the value of
1J(PtH) for complex 2 is still significantly smaller than that of
[PtHMe2X(bu2bpy)] (X ) Cl, Br, I; 1J(PtH) ) 1589.7, 1630.5,
1655.5, respectively) again illustrating the effect of the trans
methyl ligand.1b The hydride ligand in 2 is trans to the strong
σ-donor methyl group and so is expected to be much more
1
2
prediction was upheld; for example, both the H and H{1H}
NMR spectra of 1* showed the absence of deuterium incorpora-
tion into the MePt groups or of H into the PtD groups, even
after several days in solution.
2
In conclusion, the new methyl(hydrido)platinum(IV) com-
plexes [PtHMe3(bu2bpy)] (2) and [Pt2(µ-H)Me6(bu2bpy)2]+ (1),
which have no ligand which can easily dissociate, are thermally
stable to reductive elimination of methane and to isotopic
exchange within PtD(CH3) groups, thus giving strong support
to the theory that both reactions occur within a five-coordinate
intermediate [PtHMe2(bu2bpy)]+.1 It is the strongly hydridic
nature of 2 which leads to formation of the unique (µ-hydrido)-
diplatinum(IV) complex 1. The remarkable thermal stability
of 1 is probably due to three factors, namely, the absence of an
easily dissociable ligand (see above discussion), the less hydridic
nature compared to 2, and steric protection of the hydride ligand.
2
Acknowledgment. We thank the NSERC (Canada) for financial
support to R.J.P. and a postgraduate scholarship to G.S.H.
JA961404N
(9) The ligating ability of terminal hydrides in the synthesis of µ-hydrido
complexes has been exploited before. Venanzi, L. M. Coord. Chem. ReV.
1982, 43, 251.