Quinone methide generation based on a cis-(N,N) platinum complex

Article abstract of DOI:10.1021/om061178i

Quinone methides (QMs) are highly reactive compounds that play important roles in various chemical and biological processes. In this work, a stable imino-pyridine platinum(IV) complex that bears a silylprotected oxy-benzyl precursor of the quinone methide BHT - QM, derived from the food preservative 2,6-di-tert-butyl-4-metnylphenol (BHT), was prepared and fully characterized, including by X-ray structural characterization. De-silylation results in a rarely observed, fully characterized, zwitterionic intermediate η¹-methylene-p-phenoxy-Pt(IV), which undergoes de-aromatization to the unstable BHT - QM, which was trapped in solution.

Full text of DOI:10.1021/om061178i

2178
Organometallics 2007, 26, 2178-2182
Quinone Methide Generation Based on a cis-(N,N) Platinum
Complex
Elena Poverenov,† Linda J. W. Shimon,‡ and David Milstein*,†
Department of Organic Chemistry and Unit of Chemical Research Support, The Weizmann Institute of
Science, RehoVot 76100, Israel
ReceiVed December 26, 2006
Quinone methides (QMs) are highly reactive compounds that play important roles in various chemical
and biological processes. In this work, a stable imino-pyridine platinum(IV) complex that bears a silyl-
protected oxy-benzyl precursor of the quinone methide BHT-QM, derived from the food preservative
2,6-di-tert-butyl-4-methylphenol (BHT), was prepared and fully characterized, including by X-ray structural
characterization. De-silylation results in a rarely observed, fully characterized, zwitterionic intermediate
η1-methylene-p-phenoxy-Pt(IV), which undergoes de-aromatization to the unstable BHT-QM, which
was trapped in solution.
quinone methides.7 While release of the QM molecule was
demonstrated in these systems (by associative displacement),
we were interested in a more biologically compatible skeleton
than diphosphines and in the use of platinum as an anchor. Such
complexes may benefit from possible biological activity of the
platinum center8 and of the released QM moiety, potentially
providing enhanced effectiveness and selectivity.9
Here we report the preparation of a stable imino-
pyridine10-based platinum(IV) complex bearing a coordinated
benzyl precursor of BHT-QM, the quinone methide derived
from the food preservative 2,6-di-tert-butyl-4-methylphenol
(BHT). This Pt(IV) complex can serve as a QM source,
concomitantly generating a cis-Pt(II) complex. The generated
BHT-QM was directly observed and trapped in solution. The
η1-methylene-p-phenoxy intermediate was detected and fully
Introduction
Quinone methides (QMs, quinones in which one of the
oxygen atoms is replaced by an alkylidene group) are highly
reactive compounds that are involved as intermediates in various
chemical and biological processes.1 Transient p-QM intermedi-
ates are often utilized in organic synthesis, including in the
synthesis of natural products.2 Biosynthesis of the natural
polymers melanin and lignin involves p-QM intermediates.3
Several drugs, including clinically employed anthracyclins and
mitomycin C, are believed to generate a QM moiety as the active
form.4 However, isolation and reactivity studies of simple
quinone methides (those not having substituents at the exocyclic
methylene group) are difficult, since they are very unstable. They
react very rapidly with the medium or polymerize, the driving
force behind these rapid reactions being aromatization to the
zwitterionic compound, which is capable of reactions with both
electrophiles and nucleophiles. Recently, efforts have been
directed toward coordination of QMs to metal centers,5-7 for
the purpose of their stabilization, study, and controlled release.7
The first stable p-QM-metal complex incorporated the QM
moiety as part of a bis-chelating PCP ligand framework, and
consequently, the QM was very strongly coordinated and was
not suitable for release.6 Later, an approach to form one-site-
coordinated QM complexes was developed in our group, leading
to (Ph2PCH2CH2PPh2)Pd-QM complexes with η2-coordinated
(5) (a) Lev, D. A.; Grotjahn, D. B.; Amouri, H. Organometallics 2005,
24, 4232. (b) Fujisawa, S.; Ishihara, M.; Yokoe, I. Internet Electron. J.
Mol. Des. 2004, 3, 241. (c) Stokes, S. M. Jr.; Ding, F.; Smith, P. L.; Keane,
J. M.; Kopach, M. E.; Jervis, R.; Sabat, M.; Harman, W. D. Organometallics
2003, 22, 4170. (d) Amouri, H.; Vaissermann, J.; Rager, M. N.; Grotjahn,
D. B. Organometallics 2000, 19, 1740. (e) Amouri, H.; Le Bras, J. Acc.
Chem. Res. 2002, 35, 501. (f) Amouri, H.; Becace, Y.; Le Bras, J.;
Vaisserman, J. J. Am. Chem. Soc. 1998, 120, 6171. (g) Amouri, H.;
Vaissermann, J.; Rager, M. N.; Grotjahn, D. B. Organometallics 2000, 19,
5143.
(6) (a) Vigalok, A.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 7873. (b)
Vigalok, A.; Shimon, L. J. W.; Milstein, D. J. Am. Chem. Soc. 1998, 120,
477. Complexation of a metal center to the endocyclic double bond of a
o-QM has been reported: Kopach, M. E.; Harman, W. D. J. Am. Chem.
Soc. 1994, 116, 6581. An unobserved o-QM rhodium complex has been
proposed as intermediate in a carbonylation reaction: Heldeweg, R. F.;
Hogeveen, H. J. Am. Chem. Soc. 1976, 98, 6040.
(7) (a) Rabin, O.; Vigalok, A.; Milstein, D. J. Am. Chem. Soc. 1998,
120, 7119. (b) Rabin, O.; Vigalok, A.; Milstein, D. Chem. Eur. J. 2000, 6,
454.
(8) (a) Hall, M. D.; Hambley, T. W. Coord. Chem. ReV. 2002, 232,
49. (b) Barnes, K. R.; Lippard, S. J. Met. Ions Biol. Syst. 2004, 42,
143. (c) Fimiani, V.; Minniti, D. Anti-Cancer Drugs 1992, 3, 9. (d) Scolaro,
L. M.; Mazzaglia, A.; Romeo, A.; Romeo, R. J. Inorg. Biochem. 2002, 91,
237.
* Corresponding author. E-mail: david.milstein@weizmann.ac.il Fax:
972-8-9344142.
† Department of Organic Chemistry.
‡ Unit of Chemical Research Support.
(1) For reviews, see: (a) Wan, P.; Barker, B.; Diao, L.; Fischer, M.;
Shi, Y.; Yang, C. Can. J. Chem. 1996, 74, 465. (b) Peter, M. G. Angew.
Chem. 1989, 101, 572. (c) Volod’kin, A. A.; Ershov, V. V. Usp. Khim.
1988, 57, 595. (d) Diao, L.; Yang, C.; Wan, P. J. Am. Chem. Soc. 1995,
117, 5369, and references therein.
(2) Shevchenko, S. M.; Apushkinskii, A. G. Russ. Chem. ReV. 1992, 61,
105.
(3) (a) Prota, G. Progress in the Chemistry of Organic Natural Products;
Springer-Verlag: New York, 1995; Vol. 64, pp 93-148. (b) Bolon, D. A.
J. Org. Chem. 1970, 35, 3666. (c) Angle, S. R.; Louie, M. S. Tetrahedron
Lett. 1993, 34, 4751. (d) Marino, J. P.; Dax, S. L J. Org. Chem. 1984, 49,
3671.
(4) For reviews, see: (a) Thompson, D. C.; Tompson, J. A.; Sugumaran,
M.; Moldeus, P. Chem. Biol. Interact. 1993, 86, 129. (b) Moore, H. W.;
Czerniak, R.; Hamdan, A. Drugs Exp. Clin. Res. 1986, 12, 475. (c) Moore,
H. W.; Czerniak, R. Med. Res. ReV. 1981, 1, 249.
(9) For a recent review see: (a) Colangelo, D.; Osella, D. Curr. Med.
Chem. 2005, 12, 3091. (b) Kostova, I. Rec. Pat. Anti-Cancer Drug DiscoVery
2006, 1, 1. (c) Wong, E.; Giandomenico, C. M. Chem. ReV. 1999, 99, 2451.
(c) Ho, Y-P.; Au-Yeung, S. C. F.; To, K. K. W. Med. Res. ReV. 2003, 23,
633.
(10) (a) Nikolcheva, L. G.; Vogels, C. M.; Stefan, R. A.; Darwish, H.
A.; Duffy, S. J.; Ireland, R. J.; Decken, A.; Hudson, R. H. E.; Westcott, S.
A. Can. J. Chem. 2003, 81, 269. (b) Brunner, H.; Schoenenberger, H. Inorg.
Chim. Acta 1986, 123, 201.
10.1021/om061178i CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/23/2007

Quinone Methide Generation
Scheme 1
Organometallics, Vol. 26, No. 9, 2007 2179
Figure 1. ORTEP view of the crystal structure of 4 with the
thermal ellipsoids at the 50% probability level. Hydrogen atoms
are omitted for clarity.
Table 1. Selected Bond Distances and Angles of Complex 4
bond length (Å)
angle (deg)
Pt(5)-C(1)
2.051(3)
2.053(3)
2.13(3)
2.163(2)
2.178(3)
2.601(7)
1.286(4)
1.369(4)
1.462(4)
1.487(4)
C(1)-Pt(5)-C(2)
86.82(14)
87.03(13)
93.2(3)
174.80(11)
93.53(11)
173.88(11)
95.12(11)
76.22(10)
90.10(10)
176.99(8)
Pt(5)-C(2)
Pt(5)-C(11)
Pt(5)-N(3)
Pt(5)-N(10)
Pt(5)-Br(7)
N(3)-C(4)
C(5)-N(10)
C(5)-C(4)
C(11)-C(12)
C(1)-Pt(5)-C(11)
C(1)-Pt(5)-N(3)
C(2)-Pt(5)-N(3)
C(11)-Pt(5)-N(3)
N(1)-C(1)-C(2)
C(11)-Pt(5)-N(10)
N(3)-Pt(5)-N(10)
C(1)-Pt(5)-Br(7)
C(11)-Pt(5)-Br(7)
characterized spectroscopically, providing a very rare direct
observation of de-aromatization of a zwitterionic oxy-benzyl
compound to a quinone methide.
Results and Discussion
Orange prisms of 4 suitable for a single-crystal X-ray
diffraction study were obtained from a two-phase pentane/
benzene solution at room temperature (Figure 1). Complex 4
exhibits a slightly distorted octahedral structure, in which the
NN ligand, Pt atom, and two methyl groups are in the same
plane, whereas the coordinated benzyl group and the Br atom
are trans to each other and perpendicular to the plane. The N3-
C4 bond length (1.286(4) Å) and the C4-N3-C31 angle
(118.8(3)°) correspond to the expected sp2 configuration of the
imine group. The angle between the planes of the aromatic rings
of the ligand is 74.4°. Additional bond distances and angles of
4 are listed in Table 1.
Formation of Chelating (N,N)Pt(II) and Pt(IV) Complexes.
The imino-pyridine chelating NN ligand 1 was synthesized
according to a literature procedure11 and reacted with (norbor-
nadiene)PtMe2.12 Upon stirring of the two reagents in benzene
at room temperature for 12 h, the violet-purple complex (NN)-
PtMe2 (2) was formed in 83% yield (Scheme 1). Pure complex
1
2 was fully characterized by 195Pt, H, and 13C NMR spec-
troscopies, ES/MS, and elemental analysis. It gives rise to a
singlet at -3228.00 ppm in the 195Pt{1H} NMR spectrum, while
the Pt(Me)2 units are observed as two sets of singlets with Pt
satellites at 1.25 (JPt-H ) 87 Hz) and 0.88 ppm (JPt-H ) 89
1
Hz) in the H NMR spectrum and as two singlets with Pt
Complex 4 is exceptionally stable in various solvents,
including protic solvents such as water and MeOH. It is
thermally stable, and no reductive elimination or other decom-
position modes were observed upon heating to 100 °C in
toluene.
satellites at -14.83 (JPt-C ) 823 Hz) and -15.07 ppm (JPt-C
) 834 Hz) in the 13C{1H} NMR spectrum.
When the benzyl bromide 37b (the silylated and brominated
derivative of BHT) was added to a benzene solution of 2,
oxidative addition to yield the Pt(IV) complex (NN)Pt(Me)2-
(BHT)Br (4) took place (Scheme 1). Pure complex 4, obtained
in 85% yield, was characterized by multinuclear (195Pt, 13C, 1H)
NMR spectroscopies, ES/MS, elemental analysis, and X-ray
crystallography. The 195Pt{1H} NMR spectrum of 4 exhibits a
Reactivity of Complex 4. The reactivity of 4 with respect
to QM formation was investigated. Interestingly, when 4 was
treated with 1 equiv of the fluoride salt Bu4NF‚3H2O in C6D6,
immediate formation of complex 2 accompanied by a color
change from orange to violet-purple took place (Scheme 2). 1H
NMR measurement of the resulting solution, measured im-
mediately, exhibited formation of free BHT-QM.7b Formation
and release of the QM molecule in this reaction was also proven
by its trapping with methanol. This was possible since BHT-
QM can exist for a short time in solution, while the reaction
with fluoride is very fast. Thus, when a few drops of MeOH
were added following the addition of the fluoride reagent,
immediate reaction with free BHT-QM took place to give the
expected Michael addition product 2,6-di-tert-butyl-4-meth-
oxymethylphenol, which was detected by GC/MS. Correspond-
ing signals in the 2H NMR spectrum and in GC/MS were
obtained when the analogous reaction was performed with d4-
MeOH.
1
singlet at -2474.04 ppm. In the H NMR spectrum the two
methyl groups of Pt(CH3)2 give rise to signals at 1.51 ppm (s,
JPt-H ) 73 Hz) and at 1.12 ppm (s, JPt-H ) 72 Hz), while two
doublets are observed for the Pt-CH2-Ar protons at 2.70 ppm
(JPt-H ) 94 Hz, JH-H ) 10 Hz) and at 2.55 ppm (JPt-H ) 86
Hz, JH-H ) 10 Hz). The benzylic protons are not equivalent,
due to the lack of a symmetry plane. In the 13C{1H} NMR
spectrum, the benzyl carbon gives rise to a signal at 23.28 (s,
JPt-C ) 632 Hz) and the Pt-Me groups are observed as two
singlets at -2.40 (JPt-C ) 706 Hz) and -2.80 (JPt-C ) 690
Hz) ppm. The carbon atom of the ligand CdN group appears
as a singlet at 140.29 (JPt-C ) 16 Hz).
(11) Bianchini, C.; Lee, H. M.; Mantovani, G.; Meli, A.; Oberhauser,
W. New J. Chem. 2002, 26, 387.
(12) Appleton, T. G.; Hall, J. R.; Williams, M. A. J. Organomet. Chem.
1986, 303, 139.
Trapping of the Zwitterionic Intermediate. Interestingly,
when the reaction of the Pt(IV) complex 4 with Bu4NF‚3H2O

2180 Organometallics, Vol. 26, No. 9, 2007
PoVerenoV et al.
Scheme 2
(s, JPt-C ) 690 Hz) ppm. The signals of the trimethylsilyl
protecting group that were present in the spectra of complex 4
(at 0.37 and 4.29 ppm in 1H and 13C NMR, respectively)
disappeared and were replaced by signals of the generated Me3-
SiF (0.02 ppm in 1H and 1.81 ppm in 13C NMR).13 A significant
downfield shift was observed for the imine group carbon, which
gives rise to a singlet at 147.66 with Pt satellites, JPt-C ) 14
Hz. In complex 4 the analogous signal was found at 140.29
ppm with JPt-C ) 16 Hz. The NMR data suggest that the
positive charge of the zwitterionic form is mostly localized on
the Pt center and on the imine ligand, and not on the benzylic
moiety, since the main changes are observed for the (NN)Pt
unit, as a result of electron density decrease at the metal
center. Upon gradual temperature increase, slow conversion of
5 to 2 took place. When the sample was warmed to room
temperature and complexes 5 and 2 were in a 1:2.3 ratio,
correspondingly, an ES/MS measurement exhibited the
following signals at m/z+: 626.71 (M + 1 of complex 5), 430.50
(M + Na+ of complex 2), and 447.58 (M + K+ of 2).14
It is important to note that in the case of complex 4
ES/MS analysis exhibited a signal at 698.81, the difference
between 4 and 5 being due to the trimethylsilyl protecting
group. This observation confirms that in the case of complex 5
the protecting group was removed from the benzylic ring.
Moreover, no additional signals above 626.71 in the ES/MS
spectrum were observed (for example, signal of (Bu)4N+ salt
of the complex), supporting the assumption of a zwitterion
with a naked phenoxy (O-). Further confirmation that there is
no bromide coordination to Pt was obtained from parallel
experiments with AgBF4 under identical conditions at -30 °C.
When AgBF4 was added, the solution in which complex 5 was
formed, which contains Bu4Br, immediate precipitation of AgBr
was observed, while in the case of complex 4, in which the
bromide is coordinated to the Pt center, no reaction occurred
under these conditions. Overall, the multinuclear NMR spec-
troscopy data, together with the mass spectroscopic data and
the observed reactivity, clearly confirm the zwitterionic nature
of 5.15
Scheme 3
To the best of our knowledge, this is the first direct
observation of de-aromatization of a zwitterionic oxy-benzyl
compound to a quinone methide. The observation of this η1-
methylene-p-phenoxy intermediate (formed by removing the
Me3Si unit with fluoride and elimination of Bu4NBr) was
probably made possible due to charge stabilization by the polar
acetone medium, while in the case of a nonpolar solvent (such
as benzene) this intermediate is unstable and undergoes fast
charge transfer to give the QM-coordinated form, followed by
QM release by the (unobserved) pentacoordinated Pt(II)
complex.
was performed in a more polar (relative to benzene) d6-acetone
medium, the 195Pt{1H} NMR spectrum indicated formation of
the new zwitterionic benzyl complex 5 (see below), which gave
rise to a signal at -2419.65 ppm, in addition to complex 2,
with a ratio of 5:2 ) 1:4. Complex 5 was transformed to
complex 2 within 2 h, indicating that 5 was an intermediate in
the conversion of the neutral benzyl Pt(II) complex 4 to the
Pt(II) complex 2 (Scheme 3). Indeed, 5 was the major product
when the reaction was performed at low temperature. Thus,
when to a d6-acetone solution of complex 4, precooled to
-30 °C, was added a d6-acetone solution of Bu4NF‚3H2O,
precooled to -30 °C, a color change from orange to light purple
was observed, and 195Pt NMR (-30 °C) revealed formation of
complex 5 in 77% yield, accompanied by only a minor amount
of complex 2.
Summary
In summary, a stable, chelated imino-pyridine Pt(IV) complex
bearing a quinone methide benzylic precursor (complex 4) was
synthesized and fully characterized (including by X-ray structure
analysis). Removal of a silyl protecting group by fluoride
The zwitterionic complex 5 was characterized at -30 °C. It
gives rise to two sets of Pt-Me groups at 1.46 (s, JPt-H ) 69
Hz) and 1.08 (s, JPt-H ) 73 Hz) ppm and two doublets for the
Pt-benzyl protons at 2.61 (JPt-H ) 92 Hz, JH-H ) 9 Hz) and
(13) 1H NMR peak integration of the dissolved gaseous Me3SiF
was smaller than 9H probably because some of it remained in the gas
phase.
(14) Explanation for M + Na+ and M + K+: When ES/MS analysis is
performed under regular conditions (regular glass and MeOH solvent),
positive ionization may occur from H+ or Na+. In case of Na+ ionization
1
2.50 (JPt-H ) 86 Hz, JH-H ) 9 Hz) ppm in the H NMR
one more experiment with K+ is usually done, in order to confirm that the
spectrum. In the 13C{1H} NMR spectrum, the Pt-coordinated
benzyl carbon appears at 22.93 ppm (JPt-C ) 624 Hz), while
Pt-Me groups appear at -2.51 (s, JPt-C ) 708 Hz) and -2.86
+
mass increase by 23 is really due to Na
.
(15) Coordinated acetone is probably too loosely bound to be reflected
in the mass of the complex.

Quinone Methide Generation
Organometallics, Vol. 26, No. 9, 2007 2181
195Pt{1H} NMR (d6-acetone): -2474.04 (s). 1H NMR (d6-
attack led to the release and trapping in solution of the quinone
methide BHT-QM and formation of the Pt(II) complex cis-
(NN)PtMe2. A benzylic zwitterionic intermediate in this process,
η1-methylene-p-phenoxy Pt(IV), was characterized and directly
observed to release the quinone methide BHT-QM, with
concomitant reduction of the Pt(IV) metal center to Pt(II). The
high thermal stability of complex 4 and its stability in protic
media makes it, as well as analogous complexes, potentially
interesting in terms of selective delivery of quinone methides.
Further studies of complex 4 that involve its chemical modifica-
tions and investigation of its reactivity with biological substrates
are in progress.
acetone): 9.23 (s, JPt-H ) 29 Hz, 1H, H-CdN), 8.67 (d, JH-H
)
5 Hz, 1H, pyridine ring), 8.25 (d, JH-H ) 7 Hz, 1H, pyridine ring),
8.21 (td, JH-H ) 7 Hz, JH-H ) 1 Hz, 1H, pyridine ring), 7.73 (td,
JH-H ) 5 Hz, JH-H ) 1 Hz, 1H, pyridine ring), 7.50 (t, JH-H ) 6
Hz, 2H, Ar), 7.49 (d, JH-H ) 6 Hz, 1H, Ar), 7.38 (bd, JH-H ) 6
Hz, 2H, Ar), 6.62 (s, JPt-H ) 11 Hz, 2H, BHT), 2.70 (d, JPt-H
)
94 Hz, JH-H ) 10 Hz, 1H, Pt-CH2), 2.55 (d, JPt-H ) 86 Hz, JH-H
) 10 Hz, 1H, Pt-CH2), 1.51 (s, JPt-H ) 73 Hz, 3H, Pt-CH3),
1.25 (s, 18H, t-Bu), 1.12 (s, JPt-H ) 72 Hz, 3H, Pt-CH3), 0.37 (s,
9H, Si(CH3)3). 13C{1H} NMR (d6-acetone): 167.65 (s, pyridine
ring), 155.20 (s, pyridine ring), 150.75 (s, pyridine ring), 148.25
(s, JPt-C ) 8 Hz, pyridine ring), 140.29 (s, JPt-C ) 16 Hz, CdN),
139.93 (s, pyridine ring), 137.84 (s, BHT), 130.59 (s, Ar), 129.77
(s, Ar), 129.71 (s, Ar), 129.15 (s, JPt-C ) 13 Hz, Ar), 128.95 (s,
BHT), 125.92 (s, JPt-C ) 21 Hz, BHT), 123.47 (s, BHT), 35.26 (s,
C(CH3)3), 31.51 (s, C(CH3)3), 23.28 (s, JPt-C ) 632 Hz, Pt-CH2),
4.29 (s, Me3Si), -2.40 (s, JPt-C ) 706 Hz, Pt-CH3), -2.80 (s,
JPt-C ) 690 Hz, Pt-CH3). (Assignment of 13C{1H} NMR signals
was confirmed by 13C DEPT). ES-MS: m/z+ 698.81 (M - Br)
[calc 698.81], m/z- (Br). Anal. Found (calcd for C32H45N2-
PtOSiBr): C, 49.51 (49.35); H, 6.15 (6.08); N, 3.55 (3.60).
Experimental Section
General Procedures. All experiments with metal complexes
were carried out under an atmosphere of purified nitrogen in a
Vacuum Atmospheres glovebox equipped with an MO 40-2 inert
gas purifier or using standard Schlenk techniques. All solvents were
reagent grade or better. All nondeuterated solvents were refluxed
over sodium/benzophenone ketyl and distilled under an argon
atmosphere. Deuterated solvents were used as received. All the
solvents were degassed with argon and kept in a glovebox over 4
Å molecular sieves. Commercially available reagents were used
as received. The NMR spectra were recorded at 500 (1H), 126 (13C),
and 107 (195Pt) using a Bruker DPX 500 spectrometer. All
spectra were recorded at 23 °C (if not stated otherwise). 1H NMR
and 13C{1H} NMR chemical shifts are reported in ppm downfield
X-ray Structural Analysis of 4. Orange crystals of complex 4
were obtained from a benzene solution by layering it with pentane
at room temperature.
Crystal Data. C32H40BrN2OPtSi + 1/2(C6H6), prism, orange,
0.4 × 0.3 × 0.2 mm3, triclinic, P1h, a ) 10.598(2) Å, b ) 12.904-
(3) Å, c ) 13.385(6) Å, R ) 95.03(3)°, â ) 105.03(2)°, γ ) 91.20-
(3)° from 20 degrees of data, T ) 120(2) K, V ) 1759.2(7) Å3, Z
1
from tetramethylsilane. H NMR chemical shifts were referenced
to the residual hydrogen signal of the deuterated solvent (2.04
acetone). In 13C{1H} NMR measurements the signal of d6-acetone
(206.0 ppm) was used as a reference. In 195Pt{1H} NMR chemical
shifts are reported in ppm and referenced to an external solution
of K2PtCl4. Screw-cap 5 mm NMR tubes were used in the NMR
follow-up experiments. Abbreviations used in the description of
NMR data are as follows: s, singlet; d, doublet; t, triplet; m,
multiplet.
) 2, fw ) 810.80, Dc ) 1.531, Mg/m3, µ ) 5.185 mm-1
.
Data Collection and Treatment. Nonius KappaCCD diffrac-
tometer, Mo KR (g ) 0.71073 Å), graphite monochromator, 33 673
reflections collected, -13 < h < 13, -16 < k < 16, 0 < l < 17,
frame scan width 1.0°, scan speed 1° per 20 s, typical peak
mosaicity 0.53°, 8005 independent reflections (Rint ) 0.047). The
data were processed with Denzo-Scalepack.
Formation of (NN)PtMe2 (2). To a benzene solution of (NBD)-
PtMe2 (40 mg, 0.12 mmol) was added a benzene solution of ligand
1 (25 mg, 0.14 mmol), and the reaction mixture was stirred for 12
h at room temperature. The solvent was removed under vacuum,
and the violet-purple solid was washed with pentane (10 mL) and
ether (7 mL) and dissolved in C6H6. Removal of the solvent under
vacuum yielded 40 mg (0.10 mmol, 83% yield) of 2.
Solution and Refinement. The structure was solved by direct
methods with SHELXS. Full-matrix least-squares refinement based
on F2 with SHELXL-97; 379 parameters with 0 restraints, final
R1 ) 0.0264 (based on F2) for data with I > 2σ(I) and R1 ) 0.0296
on 7996 reflections, goodness-of-fit on F2 ) 1.068, largest electron
density peak ) 1.348 e/Å3.
Formation of [(NN)Pt(Me)2(BHT-O-)(d6-acetone)]+ (5). To
a precooled, -30 °C d6-acetone solution of complex 4 (25 mg,
0.03 mmol) was added a precooled, -30 °C d6-acetone solution of
Bu4NF‚3H2O (8.4 mg, 0.03 mmol), resulting in an immediate color
change from orange to light purple. The 195Pt NMR spectrum
revealed formation of complex 5 in 77% yield (according to 195Pt
NMR).
195Pt{1H} NMR (d6-acetone): -3228.00 (s). 1H NMR (d6-
acetone): 9.66 (s, JPt-H ) 32 Hz, 1H, H-CdN), 9.29 (d, JH-H
)
5 Hz, 1H, pyridine ring), 8.36 (t, JH-H ) 7 Hz, 1H, pyridine ring),
8.14 (d, JH-H ) 7 Hz, 1H, pyridine ring), 7.85 (t, JH-H ) 5 Hz,
1H, pyridine ring), 7.50 (t, JH-H ) 8 Hz, 2H, Ar), 7.36 (d, JH-H
)
8 Hz, 1H, Ar), 7.32 (dd, JH-H ) 8 Hz, 2H, Ar), 1.25 (s, JPt-H ) 87
Hz, 3 H, Pt-CH3), 0.88 (s, JPt-H ) 89 Hz, 3 H, Pt-CH3). 13C-
{1H} NMR (d6-acetone): 165.61 (s, pyridine ring), 158.06 (s,
pyridine ring), 150.11 (s, pyridine ring), 147.70 (s, JPt-C ) 35 Hz,
CdN), 138.00 (s, pyridine ring), 129.77 (s, JPt-C ) 14 Hz, pyridine
ring), 129.42 (s, Ar), 129.09 (s, JPt-C ) 8 Hz, Ar), 128. 32 (s, Ar),
123.95 (s, JPt-C ) 8 Hz, Ar), -14.83 (s, JPt-C ) 823 Hz,
Pt-CH3), -15.07 (s, JPt-C ) 834 Hz, Pt-CH3). ES-MS: m/z+
430.50 (M + Na+), 447.58 (M + K+) [calc 407.24]. Anal. Found
(calcd for C14H16N2Pt): C, 41.08 (41.26); H, 3.90 (3.96); N, 6.80
(6.88).
Complex 5 was characterized at -30 °C.
195Pt{1H} NMR (d6-acetone): -2419.65 (s). 1H NMR (d6-
acetone): 9.34 (s, JPt-H ) 29 Hz, 1H, H-CdN), 8.67 (d, JH-H
)
6 Hz, 1H, pyridine ring), 8.29 (s, 1H, pyridine ring), 8.27 (d, JH-H
) 6 Hz, 1H, pyridine ring), 7.90 (m, 1H, pyridine ring), 7.56-
7.35 (5H, Ar), 6.41 (s, JPt-H ) 10 Hz, 2H, BHT), 2.61 (d, JPt-H
)
92 Hz, JH-H ) 9 Hz, 1H, Pt-CH2), 2.50 (d, JPt-H ) 86 Hz, JH-H
) 9 Hz, 1H, Pt-CH2), 1.46 (s, JPt-H ) 69 Hz, 3H, Pt-CH3), 1.08
(s, JPt-H ) 73 Hz, 3 H, Pt-CH3). 13C{1H} NMR (d6-acetone):
167.71 (s, pyridine ring), 154.75 (s, pyridine ring), 147.98 (s, JPt-C
) 14 Hz, pyridine ring), 147.66 (s, JPt-C ) 14 Hz, CdN), 139.98
(s, pyridine ring), 138.74 (s, pyridine ring), 135.88 (s, BHT), 130.51
(s, Ar), 129.71 (s, Ar), 129.27 (s, Ar), 128.94 (s, Ar), 127.06 (s,
BHT), 125.47 (s, BHT), 124.35 (s, JPt-C ) 21 Hz, BHT), 34.76 (s,
Formation of (NN)Pt(Me)2(BHT-OSiMe3)Br (4). To a ben-
zene solution of complex 2 (30 mg, 0.07 mmol) was added a
benzene solution of the bromide 1 (30 mg, 0.08 mmol), and the
solution was stirred for12 h at rt. Solvent removal under vacuum
yielded complex 3, which was recrystallized from a mixture of
benzene (1 mL) and pentane (10 mL) after 12 h at -20 °C. The
orange solid was washed with pentane and dried under vacuum,
yielding 46 mg (0.06 mmol, 85% yield) of 4.
JPt-C ) 32 Hz, C(CH3)3), 30.50 (s, C(CH3)3), 22.93 (s, JPt-C
)
624 Hz, Pt-CH2), -2.51 (s, JPt-C ) 708 Hz, Pt-CH3), -2.86 (s,

2182 Organometallics, Vol. 26, No. 9, 2007
PoVerenoV et al.
JPt-C ) 690 Hz, Pt-CH3). (Assignment of 13C{1H} NMR signals
was confirmed by 13C DEPT and C-H correlation.) ES-MS: m/z+
626.71 (M + 1) [calc 626.73].
D.M. holds the Israel Matz Professorial Chair of Organic
Chemistry.
Supporting Information Available: CIF file for complex 4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This research was supported by the Israel
Science Foundation, by the Minerva Foundation, and by the
Helen and Martin Kimmel Center for Molecular Design.
OM061178I