Protonation of platinum(II) dialkyl complexes containing ligands with proximate H-bonding substituents

Article abstract of DOI:10.1021/om060036l

The possibility of activating C-H bonds at a platinum(II) center that contains a ligand with a proximate H-bonding functionality, for example a NH₂ substituent, has been investigated. A series of platinum(II) dimethyl complexes [Pt(L)Me₂], where L is an unsymmetrically substituted bipyridine ligand, has been prepared. Protonation reactions in acetonitrile with 1 equiv of a strong acid generate cationic complexes of the type [Pt(L)Me(CH₃CN)]₊. The selectivity of the protonation reactions appears to be governed by steric effects rather than H-bonding effects. This is believed to be the result of the low basicity of the amino subsituent in 2-amino pyridines.

Full text of DOI:10.1021/om060036l

2074
Organometallics 2006, 25, 2074-2079
Protonation of Platinum(II) Dialkyl Complexes Containing Ligands
with Proximate H-Bonding Substituents
George J. P. Britovsek,*,† Russell A. Taylor,† Glenn J. Sunley,‡ David J. Law,‡ and
Andrew J. P. White†
Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AY, U.K., and
Hull Research and Technology Centre, BP Chemicals Ltd., Saltend, Hull, HU12 8DS, U.K.
ReceiVed January 11, 2006
The possibility of activating C-H bonds at a platinum(II) center that contains a ligand with a proximate
H-bonding functionality, for example a NH2 substituent, has been investigated. A series of platinum(II)
dimethyl complexes [Pt(L)Me2], where L is an unsymmetrically substituted bipyridine ligand, has been
prepared. Protonation reactions in acetonitrile with 1 equiv of a strong acid generate cationic complexes
of the type [Pt(L)Me(CH3CN)]+. The selectivity of the protonation reactions appears to be governed by
steric effects rather than H-bonding effects. This is believed to be the result of the low basicity of the
amino subsituent in 2-amino pyridines.
Introduction
Alkanes are notoriously difficult to functionalize selectively
and are therefore currently underused as a chemical feedstock.
With the advent of large-scale gas to liquid (GTL) processing,1
higher alkanes are likely to become available in ever increasing
amounts. The future efficient utilization of this carbon resource
will depend on the development of highly active, selective, and
stable catalysts that can convert these unreactive hydrocarbons
into useful intermediates or products.2-5 To this end, the
activation of a C-H bond at a transition metal center is generally
regarded as the first step toward alkane functionalization and
has therefore received a great deal of attention during the last
20 years.6-12
Figure 1. Various C-H activation modes.
Several modes of C-H activation have been identified and
are summarized in Figure 1. Oxidative addition of a C-H bond
(A) and σ-bond metathesis (B, where X is a carbon-based
fragment) are classic examples of C-H activation at a single
metal center. Electrophilic activation (B, where X is, for
example, chloride) has attracted a great deal of interest in recent
years because this C-H activation process constitutes the first
step in the Shilov reaction and also in related reactions.4,11,13
Other approaches such as the oxidative addition of a C-H bond
across two metal centers (C)14,15 or the addition across a metal-
heteroatom multiple bond MdY, where Y ) CR2 or NR (D),
have also been explored.16-18 In the case of Y ) O in method
D, C-H activation by high-valent metal oxo complexes has
been investigated,19,20 including studies involving non-heme iron
systems.21,22
* To whom correspondence should be addressed. Tel: +44-(0)20-
75945863. E-mail: g.britovsek@imperial.ac.uk.
† Imperial College London.
‡ BP Chemicals Ltd.
(1) Wilhelm, D. J.; Simbeck, D. R.; Karp, A. D.; Dickenson, R. L. Fuel
Proc. Technol. 2001, 71, 139-148.
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E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen,
K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.;
Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer,
L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R. A.; Que
Jr., L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.;
Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. ReV. 2001,
101, 953-996.
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1998, 37, 2180-2192.
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Linn, K.; Spaniol, T.; Raabe, G.; Kru¨ger, C. Organometallics 1994, 13,
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Y.; Mironov, O.; Jones, C. J.; Ziatdinov, V. R. J. Mol. Catal. 2004, 220,
7-25.
(5) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer:
Dordrecht, 2000.
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10.1021/om060036l CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/16/2006

Protonation of Platinum(II) Dialkyl Complexes
Organometallics, Vol. 25, No. 8, 2006 2075
Several reports have shown that the binding of H2 to a
transition metal center containing a basic functionality in the
proximity can lead to heterolytic cleavage of the H-H bond.
Examples of such reactivity are the 2-aminobenzoquinolate
iridium system developed by Crabtree23 and a nickel PNP
system developed by DuBois.24 We were intrigued whether this
reactivity could also be applied to alkanes, as it is known that
the pKa of an alkane (and of H2) decreases dramatically upon
coordination to a metal center,8 and this could provide another
C-H activation mode, schematically depicted as E in Figure
1. In this mode, which can also be viewed as a base-assisted
electrophilic activation process, a basic H-bonding functionality
is held in close proximity to the metal center, such that it cannot
coordinate to the metal center, but it can interact with a hydrogen
atom from a coordinated alkane. After C-H bond cleavage,
the proton binds to the basic functionality and the metal will
be left with the alkyl moiety (eq 1). An important advantage of
this activation process is that only one coordination site at the
metal center is occupied, leaving space for further reactivity.
tionality may also favor the protonation of the methyl group
closest to the basic group, as shown schematically in eq 2.
To distinguish between a H-bonding effect or an electronic
or steric effect due to the 6-amino substituent, we also
investigated variations such as 2, 3, and 4 (Figure 2). The
phenanthroline ligand 4 was chosen to prevent the possibility
of “rollover” cyclometalation as observed by Minghetti and co-
workers for Pt(II) complexes containing a 6-alkyl-substituted
bipyridine ligand such as 3.31,32
Figure 2. Functionalized ligands 1-4.
The basic H-bonding functionality, for example a nitrogen
base (C-H‚‚‚N hydrogen bonding is well established),25 should
be more basic than the metal center, and the possibility of direct
coordination to the metal center must be prevented. Energeti-
cally, this process is highly favorable, as the cost of cleaving a
C-H bond (440 kJ/mol) is more than compensated by the
formation of a Pt-CH3 (251-268 kJ/mol) and a N-H+ (343-
533 kJ/mol) bond (cf. Pt-H bond enthalpies: 306-314 kJ/
mol).26,27 Very recently, the possibility of activating a C-H bond
at a Pt(II) center using a basic donor (pyridine) near the metal
center was elegantly demonstrated by Caulton and co-workers.
An equilibrium was observed between a N-PtIV-H (the classic
oxidative addition activation process A) and a N-H‚‚‚PtII
species (the base-assisted process E),28,29 the latter being favored
by electron-withdrawing ligands on the Pt center.
We started our investigations into the activation of alkanes
via mode E with the preparation of platinum(II) complexes
containing the 6-amino-substituted bipyridine ligand 1. Surpris-
ingly, no metal complexes containing this ligand have been
reported. To determine whether the NH2 substituent can act as
a basic H-bonding functionality, we decided to investigate
initially the microscopic reverse reaction, the protonation of a
platinum(II) dimethyl complex.9,12 The metal center is normally
the kinetic site for protonation of platinum dimethyl com-
plexes,30 and in the case of a symmetrical bipyridine ligand there
would be no preference for the subsequent reaction with either
methyl group. However, if a basic H-bonding functionality is
to assist in C-H bond cleavage, this basic H-bonding func-
Results and Discussion
Synthesis of Ligands and Complexes. Previously reported
procedures for the synthesis of 6-aminobipyridine 1 by amina-
tion of 6-bromobipyridine gave only low and unreliable yields
in our hands.33,34 We therefore developed a new procedure,
based on the method described by Lang et al., using 6-bromo-
bipyridine, ammonia, and a catalytic amount of Cu2O.35
6-Dimethylaminobipyridine 5 was prepared via a similar
procedure using dimethylamine.
The reaction of 1, 2, and 4 with [PtMe2(SMe2)]2 in toluene
resulted in the clean formation of the platinum(II) dimethyl
complexes [Pt(1)Me2], [Pt(2)Me2], and [Pt(4)Me2] (eqs 3-5).
The 1H NMR spectra (in acetone-d6) show the Pt-Me signals at
0.94/1.02 ppm for [Pt(1)Me2], 0.78/0.85 ppm for [Pt(2)Me2],
(23) Lee, D.-H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R. H.
Chem. Commun. 1999, 297-298.
(24) Curtis, C. J.; Miedaner, A.; Ciancanelli, R.; Ellis, W. W.; Noll, B.
C.; Rakowski; DuBois, M.; DuBois, D. L. Inorg. Chem. 2003, 42, 216-
227.
(25) Mascal, M. Chem. Commun. 1998, 303-304.
(26) Martinho Simo˜es, J. A.; Beauchamp, J. L. Chem. ReV. 1990, 90,
629-688.
(27) Liu, W.-Z.; Bordwell, F. G. J. Org. Chem. 1996, 61, 4778-4783.
(28) Vedernikov, A. N.; Caulton, K. G. Chem. Commun. 2003, 368-
369.
(29) Vedernikov, A. N.; Pink, M.; Caulton, K. G. Inorg. Chem. 2004,
43, 3642-3646.
(30) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124,
12116-12117.

2076 Organometallics, Vol. 25, No. 8, 2006
BritoVsek et al.
Figure 3. Molecular structure of [Pt(1)Me2].
and 1.16/1.20 ppm for [Pt(4)Me2] together with their charac-
teristic Pt(II) satellites (JPt-H ≈ 85-90 Hz).
The reaction of 6-methylbipyridine 3 with [PtMe2(SMe2)]2
resulted in rollover cyclometalation, as previously observed by
Minghetti for similar complexes (eq 6). In an attempt to increase
the basicity of the 6-amino substituent, we also prepared the
6-dimethylamino derivative 5. However, the reaction of this
ligand with the platinum(II) precursor did not give the expected
dimethyl platinum complex [Pt(5)Me2] but the cyclometalated
monomethyl product [Pt(5′)Me(SMe2)] (eq 7). The increased
size of the dimethylamino group compared to a simple NH2
group apparently does not allow direct coordination, and a
rotation followed by C-H activation and elimination of CH4
occurs instead. The conformation of [Pt(5′)Me(SMe2)] with the
methyl group trans to the pyridine donor was confirmed by
NOESY spectroscopy (see Figure S3).
Figure 4. Molecular structure of [Pt(1)Me2]. The N-H‚‚‚Pt
hydrogen bond (c) has N‚‚‚Pt ) 3.406(5) Å, H‚‚‚Pt ) 2.52 Å, and
N-H‚‚‚Pt ) 170°.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
[Pt(1)Me2].
Pt-N(1)
Pt-C(13)
2.180(4)
2.025(6)
Pt-N(12)
Pt-C(14)
2.092(4)
2.055(6)
N(1)-Pt-N(12)
N(1)-Pt-C(14)
N(12)-Pt-C(14)
77.34(16)
104.0(2)
178.4(2)
N(1)-Pt-C(13)
N(12)-Pt-C(13)
C(13)-Pt-C(14)
171.3(2)
94.1(2)
84.6(3)
that to N(12) [2.092(4) Å]. Since the bond lengths within the
two pyridine rings are comparable, this is likely to be a steric
rather than an electronic effect; the intramolecular N(2)‚‚‚C(14)
separation is only 3.026(9) Å. The two Pt-C distances are the
same within statistical significance. Other than this lengthening
of the Pt-N(1) bond, the coordination distances are comparable
to those seen in the closely related complex [Pt(bipy)Me2].36
Likewise, the C(py)-NH2 bond length [1.351(8) Å] is comparable
to those seen in other 2-aminopyridine compounds.37 The NH2
protons were located in the structure (see Experimental Section)
and found to be coplanar with the adjacent pyridyl ring, giving
a trigonal planar arrangement around the parent nitrogen, which
suggests a significant degree of delocalization of the amine
nitrogen lone pair into the aromatic ring.
The amino group is also involved in an intermolecular
hydrogen-bonding interaction to the platinum center of a
neighboring molecule (see Figure 4), with N‚‚‚Pt and H‚‚Pt
distances of 3.406(5) and 2.52 Å and an N-H‚‚‚Pt angle of
170°; the H‚‚‚Pt vector is inclined by ca. 71° to the platinum
coordination plane. This interaction is probably an indication
of the basic character of the platinum center.
Solid-State Structure. The molecular structure of complex
[Pt(1)Me2], as determined by X-ray crystallography, is depicted
in Figure 3. The geometry of the complex was shown to be
only slightly distorted from planarity with N(2) and C(13) ca.
+0.14 and -0.14 Å out of the plane of the remaining non-
hydrogen atoms, which are coplanar to within ca. 0.06 Å. The
cis angles range from 77.34(16)° to 104.0(2)°, the most acute
angle being associated with the bite of the bipyridyl ligand; the
trans angles are 171.3(2)° and 178.4(2)° (Table 1). It is notable
that the Pt-N distance to the N(1) pyridyl ring [2.180(4) Å]
(the one that bears the NH2 substituent) is ca. 0.1 Å longer than
Protonation Studies. The reaction between [Pt(1)Me2] and
1 equiv of the acid [H(Et2O)2][BArF] (where BArF ) B(3,5-
(CF3)2C6H3)4) was carried out at -20 °C in acetonitrile solution.
After warming to room temperature, the product was isolated
(31) Minghetti, G.; Stoccoro, S.; Cinellu, M. A.; Soro, B.; Zucca, A.
Organometallics 2003, 22, 4770-4777.
(32) Minghetti, G.; Doppiu, A.; Zucca, A.; Stoccoro, S.; Cinellu, M. A.;
Manassero, M.; Sansoni, M. Chem. Heterocycl. Compd. (N.Y.) 1999, 35,
992-1000.
(36) (a) Achar, S.; Catalano, V. J. Polyhedron 1997, 16, 1555-1561.
(b) Klein, A.; van Slageren, J.; Zalis, S. Eur. J. Inorg. Chem. 2003, 1917-
1938.
(37) (a) Kvick, A.; Backeus, M. Acta Crystallogr., Sect. B 1974, 30,
474-480. (b) Chao, M.; Schempp, E.; Rosenstein, R. D. Acta Crystallogr.,
Sect. B 1975, 31, 2922-2924.
(33) Burstall, F. J. Chem. Soc. 1938, 1662-1672.
(34) Araki, K.; Mutai, T.; Shigemitsu, Y.; Yamada, M.; Nakajima, T.;
Kuroda, S.; Shimao, I. J. Chem. Soc., Perkin Trans. 2 1996, 613-617.
(35) Lang, F.; Zewge, D.; Houpis, I. N.; Volante, R. P. Tetrahedron Lett.
2001, 42, 3251-3254.

Protonation of Platinum(II) Dialkyl Complexes
Organometallics, Vol. 25, No. 8, 2006 2077
and characterized by 1H and 13C NMR spectroscopy. The
protonation reaction resulted in the immediate loss of methane
and the formation of a single product, [Pt(1)Me(CH3CN)][BArF],
containing a coordinated acetonitrile ligand (eq 8). The coor-
dination of the acetonitrile ligand cis to the aminopyridine
moiety was unambiguously confirmed by NOESY spectroscopy
(see Figure S4). A strong interaction between the amine and
acetonitrile protons, as well as between the Pt-methyl and the
proton in the 6′-position of the unsubstituted pyridine ring, was
observed.
effect. In addition, the 4-amino substitution favors the proto-
nation of the trans methyl group, resulting in an excess of the
opposite isomer as observed in the case of [Pt(1)Me2]. This is
most likely due to the stronger trans effect of the substituted
pyridine donor weakening this particular Pt-methyl bond.
The possibility of sterics governing the protonation of
complex [Pt(1)Me2] was investigated by substituting the NH2
for a CH3 group. Protonation of complex [Pt(4)Me2] under the
same conditions as above afforded a single product, [Pt(4)Me-
(CH3CN)][BArF], which was isolated and confirmed by NOESY
spectroscopy (Figure S6) to be the isomer with a coordinated
acetonitrile ligand cis to the methyl-substituted side of the ligand
(eq 11).
Attempts to activate C-H bonds were made, by reacting 20
equiv of benzene-d6 with the complex [Pt(1)Me(CH3CN)][BArF]
in pentafluoropyridine for one week at 70 °C, but no reaction
was observed (eq 9). C-H activation reactions at cationic Pt-
(II) complexes generally require a labile ligand in the coordina-
tion sphere of the platinum center.9 Presumably the acetonitrile
ligand is bound too strongly and inhibits substitution by the
arene. Further attempts to generate cationic complexes of the
type [Pt(1)Me(L)][BArF], where L is a more weakly coordinat-
ing ligand such as pentafluoropyridine or trifluoroethanol, were
unsuccessful. In the case of pentafluoropyridine the resulting
cationic complex was found to be unstable and rapidly
decomposed (probably due to steric congestion), whereas in the
case of trifluoroethanol, an immediate reaction occurred upon
dissolution of [Pt(1)Me2] in trifluoroethanol, resulting in several
unidentifiable species.
From these studies we can conclude that the protonation of
complex [Pt(4)Me2] and therefore also of [Pt(1)Me2] is pre-
dominantly governed by steric effects. The clash between the
amino or methyl group in the 6-position of the pyridine donor
and the Pt-methyl group in cis position weakens this Pt-N bond,
consistent with the bond distances observed in the molecular
structure of complex [Pt(1)Me2].
Protonation of the 2-amino substituent in [Pt(1)Me2] was
never observed, even at low temperature. In addition, the
protonation of [Pt(1)Cl2] at room temperature led to decomposi-
tion, and again no protonation on the amino substituent was
observed, probably due to the reduced basicity. The lone pair
on the amino nitrogen atom in 2-aminopyridines such as 1 is
tied up in the delocalized π-system, resulting in a planar sp2-
hybridized nitrogen atom as seen in the solid-state structure of
[Pt(1)Me2], and this lone pair is therefore not available for
protonation. Indeed, the pKa value of the conjugate acid of
2-aminopyridinium is only -7.6.38 It thus appears that our initial
choice of ligand containing a basic functionality in the proximity
of the metal center was not suitable, and we are currently
investigating other ligand types. However, the valuable lessons
learned and described here may inspire new directions in the
development of C-H activating systems.
The remarkable selectivity of the protonation reaction (eq 8)
could be due to an electronic or steric effect (or both) or perhaps,
and which is the purpose of these studies, a H-bonding effect
as shown in eq 2. To distinguish between these three possible
effects, we have carried out similar protonation reactions with
complexes [Pt(2)Me2] and [Pt(4)Me2]. The 4-amino substituent
in complex [Pt(2)Me2] should provide an electronic effect on
the pyridine nitrogen donor similar to that of a 2-amino
substituent, and therefore a similar selectivity would be expected
if the protonation reaction was simply governed by an electronic
effect due to the amino substituent. Protonation of [Pt(2)Me2]
under the same conditions as for [Pt(1)Me2] resulted in a mixture
of two isomeric products, cis- and trans-[Pt(2)Me(CH3CN)]-
[BArF], in a 40/60 ratio (eq 10). The identity of each complex
was unambiguously confirmed by NMR, including NOESY
(Figure S5), but the two isomers could not be separated and
characterized individually.
The 40/60 ratio observed is close to the 50/50 ratio expected
for the protonation of [Pt(bipy)Me2], which indicates that the
contribution of an electronic effect due to an amino substituent
in the 4-position is small. A similar weak electronic effect can
be expected from the 2-amino substituent in [Pt(1)Me2], and
the selectivity seen in eq 8 is therefore more likely due to another
(38) Wall, M.; Linkletter, B.; Williams, D.; Lebuis, A.-M.; Hynes, R.
C.; Chin, J. J. Am. Chem. Soc. 1999, 121, 4710-4711.

2078 Organometallics, Vol. 25, No. 8, 2006
BritoVsek et al.
6-Amino-2,2′-bipyridineplatinum(II) Dichloride [Pt(1)Cl2]. A
25 mL round-bottom flask was loaded with Pt(SMe2)2Cl2 (0.390
g, 1 mmol) and 6-amino-2,2′-bipyridine (1) (0.171 g 1 mmol), after
which 10 mL of CHCl3 was added. Once dissolution was complete,
the clear yellow solution was refluxed for 3 h, during which time
a yellow solid had formed. The solid was filtered and washed with
3 × 5 mL of cold CHCl3 and subsequently dried under vacuum.
Yield: 0.372 g (0.85 mmol, 85%). 1H NMR ((CD3)2SO): 6.80 (d,
1H, J ) 1.2, 8.5 Hz), 7.60-7.68 (m, 2H), 7.74 (t, 1H), 8.26 (t,
1H), 8.33 (d, 1H, J ) 7.1 Hz), 8.42 (br, NH2), 9.65 (d, 1H). 13C
NMR ((CD3)2SO): 112.0, 116.7, 123.2, 126.0, 138.3, 140.0, 147.8,
153.4, 158.7, 162.9.
Experimental Section
All moisture-sensitive compounds were manipulated using
standard vacuum line, Schlenk, or cannula techniques or in a
conventional nitrogen-filled glovebox. NMR spectra were recorded
on either a Bruker AC-250 or a DRX-400 spectrometer; chemical
shifts for H and 13C NMR are referenced to the residual protio
1
impurity and to the 13C NMR signal of the deuterated solvent. Mass
spectra were recorded on either a VG Autospec or a VG Platform
II spectrometer. Elemental analyses were performed by the Science
Technical Support Unit at the London Metropolitan University.
Solvents and Reagents. Toluene and pentane were dried by
passing through a column, filled with commercially available Q-5
reagent (13 wt % CuO on alumina) and activated alumina (pellets,
3 mm). Diethyl ether and tetrahydrofuran were dried over potassium
metal with a benzophenone ketyl indicator, whereas dichlo-
romethane and acetonitrile were dried over CaH2. The following
compounds were prepared according to literature procedures:
6-bromo-2,2′-bipyridine,39 4-amino-2,2′-bipyridine (2),40 2-methyl-
1,10-phenanthroline (4),41 [PtCl2(SMe2)2],42 [PtMe2(SMe2)]2,42 and
6-Amino-2,2′-bipyridineplatinum(II) Dimethyl [Pt(1)Me2]. A
50 mL Schlenk tube was loaded with [PtMe2(SMe2)]2 (0.239 g 0.41
mmol) and 6-amino-2,2′-bipyridine (1) (0.142 g, 0.82 mmol), to
which was added 10 mL of dry toluene. The toluene immediately
turned yellow, and an orange precipitate began to form. The
suspension was allowed to stir for 24 h, after which time the toluene
solution was heavy with an orange precipitate. The volume was
reduced to approximately 2 mL and the resulting suspension washed
with 3 × 15 mL of dry pentane. The product was dried under
[H(Et2O)2][BArF ]43 (where BArF4 ) B(3,5-(CF3)2C6H3)4). 4-Amino-
4
1
2,2′-bipyridine (2) was purified by sublimation (140 °C, 0.5 mbar),
as was 2-methyl-1,10-phenanthroline (4) (135 °C, 0.1 mbar).
Pentafluoropyridine and trifluoroethanol were purchased from
Apollo and distilled over 4 Å molecular sieves prior to use.
vacuum, yielding a yellow solid. Yield: 0.23 g, (69%). H NMR
2
((CD3)2CO): 0.94 (s, 3H, JPtH ) 87.1 Hz, PtMe), 1.02 (s, 3H,
2JPtH ) 85.3 Hz, PtMe), 6.62 (br, NH2), 6.91 (t, 1H, Ar-H), 7.51-
7.60 (m, 2H, Ar-H), 8.15-8.25 (m, 2H, Ar-H), 9.08 (d, 1H, 3JPtH
) 25.5 Hz, Ar-H). 13C NMR ((CD3)2CO): -22.1 (Pt-C), -12.8
(Pt-C), 111.9, 113.2, 123.8, 126.7, 137.1, 138.1 146.7, 155.8,
158.8, 161.3. MS (FAB, m/z (%)): 396 (50), 381 (60), 366 (55).
Anal. Calcd for C12H15N3Pt: C, 36.36; H, 3.81; N, 10.60. Found:
C, 36.16; H, 3.77; N, 10.66. Crystals of [Pt(1)Me2] were grown by
cooling a saturated acetone solution to -30 °C for 1 week. Crystal
data for [Pt(1)Me2]: C12H15N3Pt, M ) 396.36, orthorhombic, Pbca
(no. 61), a ) 16.8836(5) Å, b ) 7.7972(3) Å, c ) 17.9120(6) Å,
V ) 2358.02(14) Å3, Z ) 8, Dc ) 2.233 g cm-3, µ(Mo KR) )
11.876 mm-1, T ) 173 K, yellow plates, Oxford Diffraction
Xcalibur 3 diffractometer; 4052 independent measured reflections,
F2 refinement, R1 ) 0.047, wR2 ) 0.123, 3987 independent
observed absorption-corrected reflections [|Fo| > 4σ(|Fo|), 2θmax
) 66°], 155 parameters. The NH2 protons were located from a ∆F
map and refined subject to a distance constraint. CCDC 288812.
Synthesis of Ligands and Complexes. 6-Amino-2,2′-bipyridine
(1). An autoclave was loaded with 0.52 g (2.2 mmols) of 6-bromo-
2,2′-bipyridine and dissolved in 15 mL of ethylene glycol, to which
2.5 mg of copper(I) oxide was added. This was cooled to -40 °C,
and 10 mL of liquid ammonia was added to the solid mixture. The
autoclave was sealed and allowed to warm to room temperature
and then heated to 110 °C for 16 h. After the autoclave had cooled
and vented, the resultant red solution was poured from the autoclave
into a beaker and the autoclave then flushed with 15 mL of water,
which was added to the red solution. A further 20 mL of water
was added to the mixture such that it turned brown. The basic
aqueous solution was then extracted with 3 × 25 mL portions of
dichloromethane. The combined extracts were washed with 2 ×
15 mL potions of water and saturated brine solution, dried over
magnesium sulfate, and evaporated under reduced pressure. The
resultant brown solid was recrystallized from hot hexane. Yield:
0.24 g (63%). 1H NMR (CDCl3): 4.49 (br, 2H, NH2), 6.55 (d, 1H,
J ) 8.0 Hz, Ar-H), 7.26 (m, 1H, Ar-H), 7.57 (t, 1H, J ) 7.9 Hz,
Ar-H), 7.75 (m, 2H, Ar-H), 8.26 (d, 1H, J ) 8.0 Hz, Ar-H),
8.66 (d, 1H, J ) 4.7 Hz, Ar-H). 13C NMR (CDCl3): 158.0, 156.3,
154.5, 149.1, 138.6, 136.7, 123.3, 121.0, 111.6, 109.0. MS (CI,
m/z (%)): 172 (100).
4-Amino-2,2′-bipyridineplatinum(II) Dimethyl [Pt(2)Me2].
This was prepared by the same procedure as for 1a as a light yellow
1
solid. Yield: 0.27 g, (72%). H NMR ((CD3)2CO): 0.78 (s, 3H,
2JPtH ) 86.1 Hz, PtMe), 0.85 (s, 3H, 2JPtH ) 86.4 Hz, PtMe), 6.18
(br, NH2), 6.86 (dd, 1H, J ) 2.4, 6.4 Hz, Ar-H), 7.56 (m, 2H,
Ar-H), 8.09 (d, 1H, J ) 7.9 Hz, Ar-H), 8.23 (td, 1H, J ) 1.5,
7.6 Hz, Ar-H), 8.55 (d, 1H, 3JPtH ) 21.1 Hz, J ) 6.4 Hz, Ar-H),
9.11 (d, 1H, 3JPtH ) 27.3 Hz, J ) 5.5 Hz, Ar-H). 13C NMR ((CD3)2-
6-(Dimethylamino)-2,2′-bipyridine (5). 6-(Dimethylamino)-2,2′-
bipy was prepared by the same procedure as for 6-aminobipyridine,
from 6-bromo-2,2′-bipy and dimethylamine (33% in EtOH), and
the crude product was purified by vacuum distillation (100 °C, 0.1
1
1
CO): -17.0 (Pt-C, JPtC ) 828 Hz), -16.8 (Pt-C, JPtC ) 826
Hz), 108.3, 111.5, 123.2, 127.5, 136.9, 146.9, 147.2, 155.9, 157.5,
157.9. MS (FAB, m/z (%)): 396 (40), 381 (100), 366 (80). Anal.
Calcd for C12H15N3Pt: C, 36.36; H, 3.81; N, 10.60. Found: C,
36.29; H, 3.70; N, 10.75.
1
mbar) to give a yellow oil. Yield: 55%. H NMR (CDCl3): 3.16
(s, 6H, N(CH3)2), 6.56 (dd, 1H, J ) 0.8, 8.2 Hz, Ar-H), 7.24 (ddd,
1H, J ) 1.2, 4.8, 7.4 Hz, Ar-H), 7.63 (m, 2H, Ar-H), 7.77 (td,
1H, J ) 1.8, 7.6 Hz, Ar-H), 8.42 (dt, 1H, J ) 1.0, 8.0 Hz, Ar-
H), 8.64 (ddd, 1H, J ) 0.8, 1.8, 4.8 Hz, Ar-H). 13C NMR
(CDCl3): 158.8, 157.1, 153.7, 148.8, 138.0, 136.6, 123.1, 120.9,
108.7, 106.1, 38.0. MS (CI, m/z (%)): 200 (100).
2-Methylphenanthrolineplatinum(II) Dimethyl [Pt(4)Me2]. A
50 mL Schlenk tube was loaded with [PtMe2(SMe2)]2 (0.162 g,
0.28 mmol) and 2-methyl-1,10-phenanthroline (4) (0.099 g, 0.510
mmol) to which was added 10 mL of dry toluene to form a red
solution. After several minutes of stirring, an orange precipitate
began to form. The suspension was allowed to stir overnight. The
red toluene solution was filtered, and the resulting orange-yellow
solid was washed with two further portions of toluene (15 mL in
total). The solid was washed with three portions of pentane (60
(39) Uenishi, J.; Tanaka, T.; Wakabayashi, S.; Oae, S. Tetrahedron Lett.
1990, 31, 4625-4628.
(40) Nguyen, T. Q.; Qu, F.; Huang, X.; Janzen, A. F. Can. J. Chem.
1992, 70, 2089-2093.
1
(41) Belser, P.; Bernhard, S.; Guerig, U. Tetrahedron 1996, 52, 2937-
mL in total) and dried under vacuum. Yield: 0.11 g (52%). H
2
2944.
NMR ((CD3)2CO): 1.16 (s, 3H, JPtH ) 87 Hz, PtMe), 1.20 (s,
(42) Hill, G. S.; Irwin, M. J.; Levy, C. J.; Rendina, L. M.; Puddephatt,
R. J. Inorg. Synth. 1998, 32, 149-153.
(43) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920-3922.
2
3H, JPtH ) 89 Hz, PtMe), 3.03 (s, 3H, C-CH3), 7.90 (m, 2H,
Ar-H), 8.02 (s, 2H, Ar-H), 8.65 (d, 1H, J ) 8.24 Hz, Ar-H),
8.85 (dd, 1H, J ) 1.5, 8.2 Hz, Ar-H), 9.50 (dd, 1H, 3JPtH ) 23.8

Protonation of Platinum(II) Dialkyl Complexes
Organometallics, Vol. 25, No. 8, 2006 2079
2
4
Hz, Ar-H). 13C NMR ((CD3)2CO): -17.9 (Pt-C, 1JPtC ) 832 Hz),
CO): 1.03 (s, 3H, JPtH ) 77.3 Hz PtMe), 2.82 (s, 3H, JPtH )
1
-14.2 (Pt-C, JPtC ) 858 Hz), 27.5 (C-CH3), 126.2 (JPtC ) 21
14.9 Hz CH3CN), 6.08 (br, NH2), 7.19 (d, 1H, Ar-H), 7.71-7.80
(m, 2H, Ar-H), 7.89 (t, 1H, Ar-H), 8.34 (td, 1H, Ar-H), 8.41
Hz), 126.9, 128.2, 128.5, 129.3, 131.9, 136.3, 137.1, 146.5 (JPtC
)
3
34 Hz), 148.6, 149.3, 164.4 (Ar C, JPtC ) 31 Hz). MS (FAB, m/z
(%)): 419 (50), 404 (65), 388 (100). Anal. Calcd for C15H16N2Pt:
C, 42.96; H, 3.85; N, 6.68. Found: C, 42.89; H, 3.87; N, 6.58.
(6-Dimethylaminobipyridine)(dimethyl sulfide)platinum Meth-
yl [Pt(5′)Me(SMe2)]. A 50 mL Schlenk tube was loaded with
[PtMe2(SMe2)]2 (0.239 g, 0.41 mmol) to which was added 10 mL
of dry diethyl ether. A 25 mL Schlenk was loaded with 6-(dim-
ethylamino)-2,2′-bipyridine (5) which was dissolved in 10 mL of
dry diethyl ether and added via cannula to the suspension of [PtMe2-
(SMe2)]2. Upon addition, the ether suspension turned opaque red
and was allowed to stir for 10 min. Subsequently, all volatiles were
removed, leaving a sticky red solid. The solid was redissolved in
10 mL of diethyl ether and filtered, and subsequently pentane was
added to precipitate a yellow solid. The solid was filtered and
washed with two further portions of pentane, then dried in vacuo.
Yield: 0.22 g (75%). 1H NMR ((CD3)2CO): 0.87 (s, 3H, 2JPt-H
(td, 1H, Ar-H), 8.91 (d, 1H, JPtH ) 61.2 Hz, Ar-H). 13C NMR
((CD3)2CO): -10.4 (Pt-C), 4.7 (NCCH3), 113.3, 115.0, 118.0 (Bp-
ArF), 119.0, 123.8, 125 (q, BAr-CF3, JCF ) 271 Hz), 126.8, 129.3
(q, Bm-ArF, JCF ) 31 Hz), 131.0, 135.2 (Bo-ArF), 140.4, 149.0,
151.5, 159.4, 160.2, 162.2 (q, B-C, JCB ) 50 Hz).
[(4-Amino-2,2′-bipyridine)(acetonitrile)platinum(II)methyl]-
[BArF] [Pt(2)Me(CH3CN)][BArF]. This was prepared by the same
procedure as for [Pt(1)Me(CH3CN)][BArF] as a mixture of two
isomers. H NMR ((CD3)2CO): 0.83 (s, JPtH ) 75.1 Hz, PtMe),
0.86 (s, 2JPtH ) 76.9 Hz, PtMe), 2.81 (s, 4JPtH ) 14.6 Hz, CH3CN),
2.84 (s, 4JPtH ) 14.6 Hz, CH3CN), 6.69 (br, NH2) 6.90-6.99 (m),
8.25-8.39 (m), 8.93-9.00 (m). Further assignment of isomers was
by NOESY spectroscopy (see Figure S5).
[(2-Methylphenanthroline)(acetonitrile)platinum(II)methyl]-
[BArF] [Pt(4)Me(CH3CN)][BArF]. This was prepared by the same
procedure as for [Pt(1)Me(CH3CN)][BArF]. 1H NMR ((CD3)2CO):
1.23 (s, 3H, 2JPtH ) 76.6 Hz, PtMe), 2.93 (s, 3H, 4JPtH ) 15.3 Hz,
CH3CN), 3.10 (s, 3H, C-CH3), 7.67 (s, 4H, BArf), 7.78 (s, 8H,
BArf), 8.10 (m, 2H, Ar-H), 8.21 (m, 2H, Ar-H), 8.82 (d, 1H, J
) 8.5 Hz, Ar-H), 9.00 (dd, 1H, J ) 8.2, 1.2 Hz, Ar-H), 9.34
(dd, 1H J ) 5.5, 1.2 Hz, 3JPtH ) 62.9 Hz, Ar-H). 13C NMR ((CD3)2-
CO): -11.8 (Pt-C), 4.0 (NCCH3), 26.9 (2-CH3), 118.4 (Bp-ArF),
125 (q, BAr-CF3, JCF ) 272 Hz), 126.5, 127.6 129.1, 129.7, 130.0
(q, Bm-ArF, JCF ) 47 Hz), 132.1, 135.5 (Bo-ArF), 140.2, 141.2,
150.3, 162.5 (q, B-C, JCB ) 50 Hz), 164.0.
1
2
3
) 84.0 Hz, Pt-CH3), 2.47 (s, 6H, JPt-H ) 26.7 Hz, S(CH3)2),
4
3.06 (s, 6H, N(CH3)2) 6.60 (d, 1H, J ) 8.3 Hz, JPt-H ) 15.8
3
Hz), 7.40 (m, 1H), 7.76 (d, 1H, J ) 8.4 Hz, JPt-H ) 55.2 Hz),
8.03 (td, 1H, J ) 1.6, 7.5 Hz), 8.16 (d, 1H, J ) 7.9 Hz), 8.85 (d,
1H, J ) 5.3 Hz, 3JPt-H ) 16.4 Hz). 13C NMR ((CD3)2CO): -15.52
(Pt-CH3), 19.87 (S(CH3)2), 38.28 (N(CH3)2), 108.3, 121.7, 124.9,
135.2, 138.6 141.8, 146.2, 155.5, 157.5, 165.0.
[(6-Amino-2,2′-bipyridine)(acetonitrile)platinum(II)methyl]-
[BArF] [Pt(1)Me(CH3CN)][BArF]. Two Schlenk flasks were
loaded, one with [Pt(1)Me2] (21.3 mg, 0.054 mmol) and the other
with [H(Et2O)2][BArF ] (53.7 mg, 0.053 mmol). Subsequently, ca.
4
Acknowledgment. We are grateful to BP Chemicals Ltd.
and EPSRC for financial support and to Johnson Matthey for a
generous loan of K2PtCl4. Mr. Dick Sheppard and Mr. Peter
Haycock are thanked for NMR experiments.
20 mL of dry acetonitrile was added to [Pt(1)Me2] and the
suspension stirred until as much of the solid had dissolved as
possible. [H(Et2O)2][BArF ] was dissolved in approximately 10 mL
4
of dry acetonitrile. The yellow suspension of [Pt(1)Me2] was then
cooled to -20 °C, and the acetonitrile solution of [H(Et2O)2][BArF ]
4
Supporting Information Available: Crystallographic informa-
tion as well as an ORTEP diagram for [Pt(1)Me2] and NOESY
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
was added dropwise over the course of an hour, via cannula, over
which time the yellow suspension turned to an orange solution.
The solution was allowed to warm to room temperature, during
which time no further changes were apparent. All volatiles were
1
subsequently removed to yield a sticky solid. H NMR ((CD3)2-
OM060036L