Binuclear Dimethylplatinum(II) Complexes: Reactivity and Mechanism in Protonolysis and Oxidative Addition Reactions

Article abstract of DOI:10.1021/om030650m

The reaction of [Pt₂Me₄(μ-SMe₂) ₂] with the ligands 1,3- and 1,4-C₆H ₄(CH=NCH₂CH₂NMe₂)₂, 1,3-BAIB, and 1,4-BAIB, respectively, gave the corresponding complexes [Pt ₂Me₄(1,3-BAIB)], 1, and [Pt₂Me ₄(1,4-BAIB)], 2. Complex 1 reacted with 2 equiv of HCl to give [(PtClMe)₂(1,3-BAIB)], 3, in which the chloride ligands are trans to the imine nitrogen atoms. Complex 1 undergoes oxidative addition with methyl triflate to give, after recrystallization from moist solvent, the diplatinum(IV) aqua complex [{PtMe₃(OH₂)}₂(1,3-BAIB)](CF ₃SO₃)₂,4, which has the anti stereochemistry of the two aqua ligands. Complex 4 forms a dimer of dimers structure in the solid state through hydrogen bonding between aqua ligands and triflate anions. The reactions of 1 and 2 with HCl are shown to proceed through oxidative addition/reductive elimination sequences, and intermediate hydridoplatinum(IV) intermediates have been identified at low temperature by their ¹H NMR spectra. For example, in reactions with complex 1, binuclear hydride complexes of the types [(PtMe₂)(PtHClMe₂)(1,3-BAIB)], [(PtHClMe₂)₂(1,3-BAIB)], and [(PtClMe)(PtHClMe ₂)(1,3-BAIB)] were identified. The initial oxidative addition reactions occur with trans stereochemistry, to give hydride trans to chloride, but isomerization was observed in some cases to give complexes with hydride trans to a nitrogen donor of the 1,3-BAIB ligand. The reactions with DCl or CF₃SO₃D in CD₃OD or CD₃OD/CD ₃CN solvent systems gave all isotopomers of methane CH _nD_4-n, indicating that there is an easy equilibrium between hydrido(methyl)platinum(IV) and (methane)platinum(II) complexes and easy hydrogen-deuterium exchange between hydridoplatinum groups and -OD groups of the solvent.

Full text of DOI:10.1021/om030650m

1396
Organometallics 2004, 23, 1396-1404
Bin u clea r Dim eth ylp la tin u m (II) Com p lexes: Rea ctivity
a n d Mech a n ism in P r oton olysis a n d Oxid a tive Ad d ition
Rea ction s
Fenbao Zhang, Michael C. J ennings, and Richard J . Puddephatt*
Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7
Received October 31, 2003
The reaction of [Pt2Me4(µ-SMe2)2] with the ligands 1,3- and 1,4-C6H4(CHdNCH2CH2NMe2)2,
1,3-BAIB, and 1,4-BAIB, respectively, gave the corresponding complexes [Pt2Me4(1,3-BAIB)],
1, and [Pt2Me4(1,4-BAIB)], 2. Complex 1 reacted with 2 equiv of HCl to give [(PtClMe)2(1,3-
BAIB)], 3, in which the chloride ligands are trans to the imine nitrogen atoms. Complex 1
undergoes oxidative addition with methyl triflate to give, after recrystallization from moist
solvent, the diplatinum(IV) aqua complex [{PtMe3(OH2)}2(1,3-BAIB)](CF3SO3)2, 4, which has
the anti stereochemistry of the two aqua ligands. Complex 4 forms a dimer of dimers structure
in the solid state through hydrogen bonding between aqua ligands and triflate anions. The
reactions of 1 and 2 with HCl are shown to proceed through oxidative addition/reductive
elimination sequences, and intermediate hydridoplatinum(IV) intermediates have been
identified at low temperature by their 1H NMR spectra. For example, in reactions with
complex 1, binuclear hydride complexes of the types [(PtMe2)(PtHClMe2)(1,3-BAIB)],
[(PtHClMe2)2(1,3-BAIB)], and [(PtClMe)(PtHClMe2)(1,3-BAIB)] were identified. The initial
oxidative addition reactions occur with trans stereochemistry, to give hydride trans to
chloride, but isomerization was observed in some cases to give complexes with hydride trans
to a nitrogen donor of the 1,3-BAIB ligand. The reactions with DCl or CF3SO3D in CD3OD
or CD3OD/CD3CN solvent systems gave all isotopomers of methane CHnD4-n, indicating that
there is an easy equilibrium between hydrido(methyl)platinum(IV) and (methane)platinum-
(II) complexes and easy hydrogen-deuterium exchange between hydridoplatinum groups
and -OD groups of the solvent.
In tr od u ction
ligands, since these are useful both in alkane activation
and in related mechanistic studies.1-12 In particular, it
has been possible to form hydrido(methyl)platinum(IV)
complexes, similar to those proposed as reaction inter-
The selective, efficient activation of alkanes remains
one of the great challenges of contemporary chemistry,
and there have been several major reviews of the
progress made and the problems that remain to be
solved.1 Within this broad field, the activation of C-H
bonds of alkanes by platinum(II) complexes has been
particularly important. Platinum complexes gave the
first true homogeneous metal complex activation of
methane and still maintain a central role in the de-
velopment of useful catalysis and in studies of the de-
tailed mechanisms of catalysis.1-12 The emphasis in
most previous studies has been on mononuclear plati-
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10.1021/om030650m CCC: $27.50 © 2004 American Chemical Society
Publication on Web 02/18/2004

Binuclear Dimethylplatinum(II) Complexes
Organometallics, Vol. 23, No. 6, 2004 1397
Ch a r t 1
F igu r e 1. View of the structure of complex 1.
mediates in methane activation, by protonation of the
corresponding methylplatinum(II) complexes. It was of
interest to extend these studies of monoplatinum com-
plexes by studying complexes containing two or three
platinum atoms. In such compounds, there is the po-
tential for interesting cooperative effects between ad-
jacent metal centers that can give rise to reaction path-
ways or products that are not possible in the mono-
nuclear analogues.13 For example, oxidative addition of
alkyl halides to bridged binuclear complexes has been
studied previously and can yield products formed by
single or double oxidative addition.14 To model inter-
mediates in C-H bond activation of methane, binuclear
bis[dimethylplatinum(II)] complexes were required, and
the two compounds shown in Chart 1 were designed.
These complexes 1 and 2 each contain amine-imine
donors, based on N,N-dimethylethylenediamine, with a
phenylene spacer group but with meta and para sub-
stitution, respectively. Through-bond electronic com-
munication between platinum centers is possible in
these complexes with arene spacer groups, but the dis-
tance between the two dimethylplatinum units is ex-
pected to be greater in complex 2 than in 1.
F igu r e 2. Views of the structure of complex 2: (a) the
molecular structure and (b) with the hydrogen-bonded
chloroform solvate molecules.
2
of equal intensity at δ ) 0.41, J (PtH) ) 84 Hz, and at
δ ) -0.06, 2J (PtH) ) 90 Hz, corresponding to the
nonequivalent methyl groups trans to imine and amine
donors. The magnitudes of the coupling constants
2J (PtH) are typical for methylplatinum(II) complexes in
which the methyl group is trans to nitrogen.2-12 There
was a single imine resonance at δ ) 9.03 with coupling
constant 3J (PtH) ) 45 Hz. The Me2N protons gave a
single resonance, indicating that the complex has ef-
fective C2v symmetry, with a mirror plane containing
the C6H4(CHdNCCN)2 atoms (note that the effective
symmetry arises from fluxionality between conformers
with lower C2 symmetry, which would give rise to two
MeN resonances).
Resu lts a n d Discu ssion
The dimethylplatinum(II) complexes 1 and 2 were
prepared by reaction of the appropriate ligand with the
complex [Pt2Me4(µ-SMe2)2]14 and were readily charac-
terized by their 1H NMR spectra, by comparison with
related mononuclear and binuclear dimethylplatinum-
(II) complexes.1-12 For example, complex 1 gave two
The structures of complexes 1 and 2 were determined
crystallographically, and views of the complexes are
shown in Figures 1 and 2, with selected bond distances
and angles listed in Table 1. A comparison of the
distances between the two platinum atoms and of the
angles between the square planes of the two platinum
atoms in the complexes is given in Table 2.
In complex 1 (Figure 1), the dimethylplatinum units
are skewed on either side of the central arene ring and
oriented syn to one another. The square planes of the
two syn-oriented PtC2N2 units are close to parallel
(angle θ between coordination planes ) 6°). In this
conformation, the platinum atoms are relatively close
with d(Pt1‚‚‚Pt2) ) 4.70 Å. The deviation of the imine
groups away from coplanarity with the aryl group in 1
is given by the dihedral angles N6-C7-C8-C13 ) 42°
and N15-C14-C12-C13 ) 20°. An idealized structure
with the imine coplanar with the aryl group would not
1
methylplatinum resonances in the H NMR spectrum
(11) (a) Hill, G. S.; Rendina, L. M.; Puddephatt, R. J . Organome-
tallics 1995, 14, 4966. (b) Hill, G. S.; Puddephatt, R. J . J . Am. Chem.
Soc. 1996, 118, 8745. (c) Prokopchuk, E. M.; J enkins, H. A.; Pud-
dephatt, R. J . Organometallics 1999, 18, 2861. (d) Hinman, J . G.; Baar,
C. R.; J ennings, M. C.; Puddephatt, R. J . Organometallics 2000, 19,
563. (e) Puddephatt, R. J . Coord. Chem. Rev. 2001, 219/221, 157. (f)
J enkins, H. A.; Klempner, M. J .; Prokopchuk, E. M.; Puddephatt, R.
J . Inorg. Chim. Acta 2003, 352C, 72. (g) Prokopchuk, E. M.; Pud-
dephatt, R. J . Organometallics 2003, 22, 787. (h) Puddephatt, R. J .
Angew. Chem., Int. Ed. 2002, 41, 261.
(12) De Felice, V.; De Renzi, A.; Panunzi, A.; Tesauro, D. J .
Organomet. Chem. 1995, 488, C13.
(13) (a) Trepanier, S. J .; McDonald, R.; Cowie, M. Organometallics
2003, 22, 2638. (b) Tejel, C.; Bordonaba, M.; Ciriano, M. A.; Edwards,
A. J .; Clegg, W.; Lahoz, F. J .; Oro, L. A. Inorg. Chem. 1999, 38, 1108.
(14) (a) Scott, J . D.; Puddephatt, R. J . Organometallics 1986, 5, 1538.
(b) Scott, J . D.; Puddephatt, R. J . Organometallics 1986, 5, 2522. (c)
Song, D.; Sliwowski, K.; Pang, J .; Wang, S. Organometallics 2002, 21,
4978.

1398 Organometallics, Vol. 23, No. 6, 2004
Zhang et al.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for th e Bin u clea r Dim eth ylp la tin u m (II)
Com p lexes
Complex 1
Pt1-C22
2.05(1)
2.10(1)
2.02(2)
2.08(1)
86.2(7)
Pt1-C21
2.05(1)
2.17(1)
2.04(1)
2.19(1)
81.4(4)
Pt1-N6
Pt1-N3
Pt2-C24
Pt2-N15
C22-Pt1-C21
Pt2-C23
Pt2-N18
N6-Pt1-N3
Complex 2
Pt1-C1
2.036(8)
2.128(6)
88.0(3)
Pt1-C2
2.043(9)
2.204(6)
81.8(2)
Pt1-N8
Pt1-N5
C1-Pt1-C2
N8-Pt1-N5
Complex 3
Pt-C2
2.111(6)
2.006(6)
86.9(2)
Pt-Cl1
2.288(2)
2.172(6)
83.0(2)
F igu r e 3. View of the structure of complex 3.
Pt-N8
Pt-N5
C2-Pt-Cl1
N8-Pt-N5
be separated, but the reaction of complex 1 with 2 equiv
of HCl occurred to give the complex [Pt2Cl2Me2(1,3-
BAIB)], 3, as the major product (eq 1). Complex 3 could
Ta ble 2. Dista n ce [d (P t‚‚‚P t), Å] betw een P la tin u m
Atom s, An gle betw een th e Coor d in a tion P la n es of
th e Tw o P la tin u m Atom s (θ, d eg), a n d Dih ed r a l
An gles Defin in g th e Tw ist of th e Im in e Gr ou p s
Rela tive to th e Ar yl Gr ou p [Φ(NdCCC), d eg] in
Com p lexes 1-4
complex
d(Pt‚‚‚Pt)
θ
Φ
1
2
3
4
4.70
9.54
4.71
8.23
6
0
1
20, 42
28
32
65
38, 126
be possible since there would be major steric repulsions
between the methylplatinum groups. There is no crys-
tallographically imposed symmetry, but the symmetry
approximates to C2. The NMR data described above
indicate effective C2v symmetry, and so there must be
easy rotation about the aryl-imine C-C bonds in solu-
tion.
be crystallized in pure form from the reaction mixture.
In the 1H NMR spectrum, complex 3 gave a methyl-
2
platinum resonance at δ ) 0.64 with coupling J (PtH)
) 80 Hz and a single Me2N resonance at δ ) 2.78. The
imine resonance appeared at δ ) 8.99, and the coupling
constant 3J (PtH) ) 136 Hz was much larger than in
complex 1 (45 Hz). This difference arises because the
trans chloride ligand in 3 has a much lower trans-
influence than the trans methyl group in 1.
In complex 2 (Figure 2), in which there is a crystal-
lographically imposed inversion center, the platinum
atoms are further apart [d(Pt1‚‚‚Pt1A) ) 9.54 Å] than
in complex 1 and the dimethylplatinum units are
oriented anti to one another. The angle between the
coordination planes of the anti-oriented platinum cen-
ters is 157°, and the twisting of the imine out of the
aryl plane is defined by the dihedral angle N8-C9-
C10-C11 ) 28°.
The structure of complex 3 is shown in Figure 3, with
selected bond parameters in Tables 1 and 2. The overall
structure is similar to that of complex 1 with mutually
syn PtClMe groups in which the methyl group is trans
to NMe2 and the chloro ligand trans to imine. The
complex has crystallographically imposed C2 symmetry
in the solid state, but rotation about the aryl-imine C-C
bond leads to effective C2v symmetry in solution. The
Pt-N (imine) bond is shorter in complex 3 than in 1
(Table 1), as expected from the lower trans-influence of
chloride compared to methyl.
The minor products formed on reaction of complex 1
with HCl were not identified but are likely to include
the two isomers of complex 3 with formula [(PtClMe)2-
(1,3-BAIB)]. It is established that, in complexes with cis-
PtMe2N2 coordination, the first methylplatinum bond
is cleaved more easily than the second, but the degree
of regioselectivity of cleavage of the methylplatinum
group trans to imine or amine in complex 1 is not easily
predicted. Complex 3 is estimated, on the basis of
integration of the 1H NMR spectra of reaction mixtures,
to be formed with about 75% selectivity in the reaction
of complex 1 with 2 equiv of HCl, but the analogous
reaction with complex 2 appears less selective and the
complex product mixture could not be separated.
1
The observation of a single NMe resonance in the H
NMR spectrum of 2 shows that there must be easy
rotation about the aryl-imine C-C bonds, as in complex
1, giving effective C2h symmetry in solution compared
to the Ci symmetry in the solid state. The structural
data clearly confirm that the two dimethylplatinum(II)
centers are further apart in complex 2, and so would
be expected to react more independently of each other,
than in complex 1. The crystals of complex 2 were grown
from chloroform solution, and an interesting feature of
the structure is that there are two chloroform molecules
associated with each dimethylplatinum(II) center, with
close contacts Pt‚‚‚H21A ) 2.85 Å and Pt‚‚‚H31A ) 2.55
Å, presumably with the electron-rich dimethylplatinum-
(II) center forming a hydrogen bond to each chloroform
molecule.
R ea ct ion w it h H Cl a n d t h e St r u ct u r e of
[P t2Cl2Me2(1,3-BAIB)]. The reactions of complexes 1
and 2 with HCl were studied in order to determine the
selectivity of methyl-platinum bond cleavage. Most of
these reactions gave mixtures of products that could not

Binuclear Dimethylplatinum(II) Complexes
Organometallics, Vol. 23, No. 6, 2004 1399
F igu r e 4. View of the structure of the dicationic com-
plex 4.
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for th e Bin u clea r
Tr im eth yl(a qu a )p la tin u m (IV) Com p lex 4
Pt1-C2
Pt1-C1
Pt1-N16
Pt2-C4
Pt2-C6
Pt2-N26
2.020(7)
2.044(7)
2.218(5)
2.030(7)
2.047(7)
2.201(5)
Pt1-C3
Pt1-O1
Pt1-N11
Pt2-C5
Pt2-O2
Pt2-N21
2.040(6)
2.204(4)
2.263(6)
2.044(6)
2.212(5)
2.255(6)
F igu r e 5. Structure of complex 4, showing two dications
bridged by triflate anions to give a “dimer of dimers”
assembly.
Sch em e 1
C2-Pt1-C3
C3-Pt1-C1
C1-Pt1-O1
O1-Pt1-N11
C4-Pt2-C5
C5-Pt2-C6
C5-Pt2-O2
O2-Pt2-N21
87.4(3)
85.2(3)
91.2(3)
87.7(2)
86.4(3)
87.3(3)
94.2(2)
84.5(2)
C2-Pt1-C1
C2-Pt1-O1
O1-Pt1-N1
N16-Pt1-N11
C4-Pt2-C6
C4-Pt2-O2
N26-Pt2-O2
N26-Pt2-N21
85.2(3)
90.2(2)
691.2(2)
80.1(2)
91.3(3)
88.3(2)
86.0(2)
80.3(2)
Oxid a tive Ad d ition of Meth yl Tr ifla te to Com -
p lex 1. The protonolysis reactions are thought to occur
by oxidative addition of HX followed by reductive
elimination of methane, and so model oxidative addition
reactions were carried out using methyl iodide or methyl
triflate with complexes 1 and 2. The reactions were sur-
prisingly complex, and only in the reaction of complex
1 with methyl triflate was a pure compound crystallized.
Thus, treatment of complex 1 with 2 equiv of methyl
triflate, followed by recrystallization from dichloro-
methane/hexane, gave the triflate salt of a dicationic
bis[trimethylplatinum(IV)] aqua complex [Pt2Me6(OH2)2-
(1,3-BAIB)](CF3SO3)2, 4. The structure of the cationic
complex 4 is shown in Figure 4, with selected bond
distances and angles listed in Tables 2 and 3.
The structure of the dication shows the presence of
two fac-trimethylplatinum(IV) centers with the octahe-
dral coordination at each platinum(IV) atom completed
by two nitrogen donors of the bridging 1,3-BAIB ligand
and an aqua ligand. The ligand geometry is quite
different from that seen in the platinum(II) complexes
1 and 3, in which the orientation of the platinum centers
was syn, since the conformation in 4 is anti. This is most
easily demonstrated by the dihedral angles N16-C17-
C32-C31 ) 38° and N26-C27-C36-C31 ) 126°. The
change in orientation of the platinum atoms in 4 leads
to a longer Pt‚‚‚Pt separation (8.23 Å) than in complexes
1 and 3. This conformational change is clearly necessary
to minimize steric hindrance between the octahedral
platinum(IV) centers in 4, which are bulkier than the
square-planar platinum(II) centers in 1 and 3. Each
platinum(IV) center is chiral, and within a given di-
cation, they have the same chirality, C,C or A,A. This
stereochemistry leads to the aqua ligands being on the
same side of the aryl spacer group. However, the product
is formed from 1 by approach of the two methyl triflate
reagents from opposite sides of the arene spacer group,
as illustrated in Scheme 1. The highest possible sym-
metry of the dication 4, through rotation about the aryl-
imine C-C bond in solution, is then expected to be C2.
The diplatinum(IV) cations associate pairwise in the
crystal lattice by hydrogen bonding between the aqua
ligands and triflate anions, as illustrated in Figure 5.
Each “dimer of dimers” contains two dications 4 and four
triflate anions. There are two bridging triflate anions,
each of which hydrogen bonds to three aqua ligands (two
of one dication and one of the other) and two triflates
that hydrogen bond to just one aqua ligand. There is a
center of symmetry at the center of each “dimer of
dimers” such that the two units of the dication 4 have
opposite chirality, CC and AA.
1
The characterization of complex 4 by H NMR was
not easy. The complex is sparingly soluble in CD2Cl2

1400 Organometallics, Vol. 23, No. 6, 2004
Zhang et al.
Sch em e 2
Sch em e 3
and the 1H NMR spectrum was broad and poorly
1
resolved. In D2O solution, the H NMR spectrum was
consistent with the structure of Figure 4, with easy
rotation about the arene-imine C-C bond to give
effective C2 symmetry. Thus there were three methyl-
platinum resonances at δ ) 0.23, 2J (PtH) ) 70 Hz; 0.67,
2J (PtH) ) 67 Hz for methyl groups trans to nitrogen
Ta ble 4. Selected 1H NMR Da ta [δ (J P tH, Hz)] for
th e Com p lexes of Sch em e 4
2
and at δ ) 0.75, J (PtH) ) 79 Hz for the methyl group
1
8a
8b
9
trans to oxygen. However, the imine groups were easily
hydrolyzed in water and the spectrum changed over the
course of several days as sequential hydrolysis to give
the new complexes 5 and 6 and free 1,3-C6H4(CHdO)2
occurred (Scheme 2).
Pt(IV)-H
Pt(II)-Me
-21.28 (1620) -21.74 (1423)
-0.06 (90) -0.15 (85)
0.01 (80)
0.38 (86)
-0.16 (86)
0.34 (80)
0.64 (80)
0.41 (84)
0.36 (90)
Pt(IV)-Me
Pt(II)NdCH
Pt(IV)NdCH
0.51 (68)
0.90 (68)
9.00 (42)
0.70 (68)
0.86 (68)
9.04 (42)
9.03 (45)
8.99 (44)
8.92 (121)
Complex 4 dissolved in CD3CN intact, but the aqua
ligands were slowly replaced by CD3CN to give complex
7 (Scheme 3). The 1H NMR spectrum of complex 7
indicated that there were equal amounts of the racemic
and meso isomers present. The spectrum was decep-
tively simple since only one imine resonance and one
set of aryl group resonances were present. However,
there were four methylplatinum resonances in a 2:2:
1:1 ratio [δ ) 0.24, 2J (PtH) ) 70 Hz; 0.68, 2J (PtH) ) 70
Hz; 0.60, 2J (PtH) ) 75 Hz; 0.61, 2J (PtH) ) 75 Hz,
respectively]. This spectrum cannot be rationalized in
terms of a single isomer, but is consistent with the
presence of an equimolar mixture of racemic and meso
isomers, 7a and 7b, respectively, in which only the
methylplatinum groups trans to CD3CN give separate
resonances.
8.93 (33)
8.69 (32)
1H NMR spectra are shown in Figure 6, and the data
are interpreted in terms of Scheme 4.
The reaction of complex 1 with 1 equiv of HCl at -78
°C gave complex 8a as major product. Complex 8a is
characterized by a hydride resonance at δ ) -21.28,
with satellite peaks due to the coupling 1J (PtH) ) 1620
Hz. This coupling constant indicates that the hydride
is trans to chloride, as expected for trans oxidative
addition.3-5,11 There were two methylplatinum(II) and
two methylplatinum(IV) resonances, assigned on the
basis of the associated coupling constants 2J (PtH) as
given in Table 4, and two imine resonances. This
complex remained as the major one present at temper-
atures up to -10 °C. However, above this temperature,
isomerization to complex 8b and reductive elimination
of methane to give complex 9 occurred (Scheme 4). At
room temperature, complex 8b was the only hydrido-
platinum(IV) complex detected, and reductive elimina-
tion to give 9 and methane was complete in about 30
Detection of In ter m ed ia tes in P r oton olysis Re-
a ction s. Complex 1 in solution in CD2Cl2 was treated
with 1 equiv of HCl at -78 °C, and the course of the
reaction was monitored by 1H NMR as the temperature
was raised slowly to room temperature. Selected data
are given in Table 4, the hydride regions of some typical

Binuclear Dimethylplatinum(II) Complexes
Organometallics, Vol. 23, No. 6, 2004 1401
F igu r e 6. Variable-temperature 1H NMR spectra (400
MHz) in the hydride region for the protonation of complex
1 in CD2Cl2 with 1 equiv of HCl at (a) -30 °C; (b) -20 °C;
(c) -10 °C; and (d) 0 °C. The peaks labeled * are assigned
to 8a , and those labeled + are assigned to 8b.
F igu r e 7. Variable-temperature 1H NMR spectra (400
MHz) in the hydride region for the protonation of complex
1 in CD2Cl2 with 2 equiv of HCl at (a) -78 °C; (b) -30 °C;
(c) -20 °C; (d) -10 °C; and (e) 0 °C. Labeled peaks are
assigned to the complexes as follows: * and ** to 10a and
10b; + to 11a ; ++ to 11b; # to 8a .
Sch em e 4
Sch em e 5
min at this temperature. Complex 8b is characterized
by a hydride resonance at δ ) -21.74, with coupling
1
constant J (PtH) ) 1423 Hz, corresponding to a hydri-
doplatinum(IV) complex with the hydride trans to a
nitrogen-donor ligand.6 Complex 8b also gives four
methylplatinum and two imine resonances, similar to
those of 8a (Table 4). In complex 9, the chloride ligand
is shown to be trans to the imine group by the large
value of the coupling constant, 3J (PtH) ) 121 Hz, which
is similar to the value in complex 3 (136 Hz). The minor
products present at intermediate stages of reaction were
the starting complex 1 and the complexes of formula
[(PtHClMe2)2(1,3-BAIB)], discussed below, and the final
product 9 was accompanied by minor amounts of 1 and
3, which were not easily separated. Evidently, the first
HCl oxidative addition deactivates the second dimeth-
ylplatinum(II) center, but not by a large enough factor
to give very high selectivity.
1
new hydride resonance grew in at δ ) -21.6, J (PtH)
) 1616 Hz, which is assigned to complex 11a . Finally,
in the range -20 to -10 °C, a hydride resonance at δ )
-21.75, J (PtH) ) 1400 Hz, indicative of hydride trans
1
to a nitrogen donor, was observed and tentatively
assigned to complex 11b. At 0 °C, no hydride complexes
remained and the final product was complex 3. The
reaction was similar if 3 equiv of HCl was used.
From these reactions it is clear that the oxidative
addition of HCl to the first platinum(II) center of
complex 1 deactivates the second one. In addition, the
Pt(IV)Pt(IV) complexes 10a and 10b undergo reductive
elimination of methane more easily than does the
analogous Pt(II)Pt(IV) complex 8a .
The reaction of complex 1 with 2 equiv of HCl at -78
°C gave a mixture of two hydride complexes, formed in
about equal yield (Figure 7). These were characterized
1
by hydride resonances at δ ) -22.2, J (PtH) ) 1610
Hz, and at δ ) -21.8, 1J (PtH) ) 1605 Hz. The coupling
constants indicate hydride trans to chloride in both
cases, so they are assigned arbitrarily to the products
of syn and anti oxidative addition, 10a and 10b (Scheme
5). At -30 °C, the elimination of methane began and a
The reactions of complex 2 were similar to those of
complex 1. For example, the reaction with 1 equiv of

1402 Organometallics, Vol. 23, No. 6, 2004
Zhang et al.
Sch em e 6
observed, and at 0 °C, at which temperature the reac-
tions were effectively complete. No further isotopic ex-
change into the methane occurred after this point.
Table 6 and Figure 8 show that all isotopomers of
methane CH4, CH3D, CH2D2, and CHD3 (and presum-
ably CD4) were formed under all conditions studied. The
simple deuteriolysis would give only CH3D, and the
other products are formed by exchange, as reported
previously for several mononuclear dimethylplatinum-
(II) complexes.1-4,6,11 Methane, CH4, can be formed by
addition of a single D+ ion followed by intramolecular
isotopic exchange within the hydrido(dimethyl)plati-
num(IV) and methyl(methane)platinum(II) complexes,
but the formation of CH2D2 and CHD3 also requires
reversibility of the protonation/deuteration step (shown
in simplified form in Scheme 7). For reactions with DCl,
Table 6 and Figure 8 show that, relative to CH3D, the
amount of CH4 produced increases as the concentration
of CD3OD in the CD3OD/CD3CN solvent mixture de-
creases. However, the amounts of CH2D2 and CHD3,
relative to CH3D, vary relatively little over the range
of solvent compositions studied and are highest at
intermediate solvent compositions. There was even less
variation in reactions with CF3SO3D, though the forma-
tion of CH2D2 and CHD3 was consistently less than in
the analogous reactions with DCl (Table 6). Since the
reactions with complexes 1 and 2 gave similar results,
it is unlikely that binuclear mechanisms play a signifi-
cant role in these reactions, and so it is reasonable to
discuss the chemistry in terms of two independent
dimethylplatinum(II) centers. There are too many un-
knowns in Scheme 7 to allow a quantitative kinetic
model for the isotopic distribution to be developed. Most
of the key reactions are proposed to occur from isotopi-
cally substituted derivatives of the five-coordinate plati-
num(IV) complex [PtHMe2(NN)]+ in easy equilibrium
with the methane complex of platinum(II) [PtMe-
(CH4)(NN)]+.3-11 The complex [PtHMe2(NN)]+ can be
trapped reversibly as the unreactive six-coordinate
complexes [PtHXMe2(NN)] and [PtH(S)Me2(NN)]+, where
S represents the solvent methanol or acetonitrile (of
which acetonitrile is expected to be the stronger ligand).
The solvent and anion may also play a role in the
reversible protonation of the dimethylplatinum(II) com-
plex and in the displacement of methane from the
complex [PtMe(CH4)(NN)]+. Two recent articles have
provided evidence for associative displacement of meth-
ane or benzene by solvent molecules in related mono-
nuclear systems.3,6 If this mechanism of displacement
operated in the present system, it might be predicted
that the yield of CH3D would increase as the concentra-
tion of the better ligand CD3CN increased in the solvent
mixture CD3OD/CD3CN, or for the more strongly ligat-
ing X- ) Cl- over X- ) CF3SO3-, but neither of these
trends was observed (Table 6). However, since the
protonation/deprotonation steps are also likely to be
strongly solvent dependent, the observed trends do not
constitute definitive evidence for a dissociative mecha-
nism of methane loss.
HCl occurred according to Scheme 6, with data sum-
marized in Table 5. Complex 12a is formed at -78 °C,
then isomerizes to 12b in the temperature range -30
to -10 °C and reductively eliminates methane to give
13. The aromatic protons, H2 and H3, are equivalent in
complex 2 but not in the unsymmetrical complexes 12
and 13, and so they appear as apparent pairs of doublets
in these complexes. With 2 equiv of HCl at -78 °C, the
reaction with complex 2 yields complexes 14a and 14b,
which have very similar overlapping resonances in the
1H NMR spectrum. For example the hydride resonances
1
occurred at δ ) -22.0 and -22.2, each with J (PtH) )
1620 Hz. These complexes reacted further over the
temperature range -30 to 0 °C by elimination of
methane. The complex [(PtClMe)(PtHClMe2)(1,4-BAIB)],
with stereochemistry analogous to 11b, was formed in
low concentration and was identified only by its hydride
1
resonance at δ ) -21.9 with J (PtH) ) 1400 Hz, but it
was the most stable complex with respect to loss of
methane and could be observed for about 1 h at 20 °C.
H/D Exch a n ge Rea ction s. The protonolysis of com-
plexes 1 and 2 with DCl or CF3SO3D in CD3OD/CD3CN
solvent was carried out in order to determine if isotopic
exchange occurred in the methane produced, under
conditions with excess labile deuterium available.1-4,6,11
The acid addition was made at -20 °C, and then the
1
reactions were monitored by H NMR spectroscopy as
the temperature was raised to room temperature.
Spectra at lower temperatures were unsatisfactory as
a result of poor solubility and the freezing out of
acetonitrile (mp -45 °C). Some typical data for reactions
with complex 1 are listed in Table 6, and typical spectra
are in Figure 8; similar data were obtained with complex
2. Data are given in Table 6 from spectra at -20 °C, at
which temperature some intermediates could still be
Overall, there is no evidence for binuclear cooperative
mechanisms of reaction in the protonolysis of complexes
1 and 2. However, the reactions of complex 1 were more
selective than those of 2. The differences appear to be
mostly steric in origin. The platinum centers in complex

Binuclear Dimethylplatinum(II) Complexes
Organometallics, Vol. 23, No. 6, 2004 1403
Ta ble 5. Selected 1H NMR Da ta [δ (J P tH, Hz)] for th e Com p lexes of Sch em e 6
2
12a
12b
13
14a /b
Pt(IV)-H
Pt(II)-Me
-22.0 (1616)
-21.7 (1400)
-22.0 (1620)
-22.1 (1620)
0.02 (90)
0.46 (84)
-0.11 (84)
0.36 (84)
0.01 (85)
0.38 (86)
-0.09 (90)
0.37 (85)
0.38 (85)
Pt(IV)-Me
0.54 (69)
0.98 (69)
9.01 (50)
0.75 (68)
0.94 (68)
9.04 (42)
0.50 (68)
0.96 (68)
Pt(II)NdCH
8.97 (45)
8.34
8.99 (44)
8.96 (121)
Pt(IV)NdCH
8.75 (32)
8.04, 8.37
8.75 (30)
8.12, 8.32
8.82 (32)
8.11
CH2,4
8.26, 8.34
Ta ble 6. Com p osition of Meth a n e Isotop om er s (%)
on Deu ter olysis of Com p lex 1 in CD3OD/CD3CN
Sch em e 7
DCl at -20 °Cb
DCl at 0 °Cb
CD3ODa CH4 CH3D CH2D2 CHD3 CH4 CH3D CH2D2 CHD3
100
93
67
50
33
7
10
8
10
13
16
24
43
47
33
35
38
45
20
26
30
27
19
17
27
19
28
26
26
14
7
5
7
10
10
19
35
34
25
26
36
37
42
44
43
42
37
36
16
17
24
23
17
8
CF3SO3D at -20 °Cb
CF3SO3D at 0 °Cb
CD3ODa CH4 CH3D CH2D2 CHD3 CH4 CH3D CH2D2 CHD3
100
93
67
50
33
7
17
18
21
17
19
32
43
49
48
48
47
47
29
26
20
27
24
16
11
8
11
9
10
5
18
17
17
16
18
29
48
48
47
45
46
45
26
25
24
28
25
20
8
10
12
11
11
6
a
The % by volume of CD3OD in the CD3OD/CD3CN solvent
b
mixture. The molar % of each H-containing isotoper of methane,
ignoring CD4, which was not analyzed.
between the two platinum centers does lead to enhanced
selectivity in the reactions of complex 1 compared to 2.
Exp er im en ta l Section
All reactions were carried out under an N2 atmosphere using
standard Schlenk techniques or using a drybox. Solvents were
dried and distilled under N2 immediately before use. [Pt2Me4-
(µ-SMe2)2] and 1,3-BAIB were prepared as described in the
literature.15 NMR spectra were recorded by using a Varian
Mercury 400 MHz, Varian Inova 400 MHz, or Varian Inova
600 MHz spectrometer. Analytical data were obtained from
Guelph Chemical Labs.
[P t2Me4(1,3-BAIB)], 1. To a solution of [Pt2Me4(µ-SMe2)2]
(0.230 g) in diethyl ether (20 mL) was added 1,3-BAIB (0.110
g) in diethyl ether (20 mL). The solution immediately turned
yellow, and the product 2 precipitated as a bright yellow solid
over 40 min. The solvent was removed under vacuum until
there was 2 mL of solvent, then 40 mL of pentane was added
to complete the precipitation of the product, which was
collected by filtration, washed with ether (3 × 2 mL) and
pentane (3 × 8 mL), and dried under vacuum. Yield: 91%;
mp 205 °C. Anal. Calcd for C20H38N4Pt2: C, 33.1; H, 5.3; N,
7.7. Found: C, 32.8; H, 5.3; N, 7.8. 1H NMR (CD2Cl2): δ -0.06
[s, 6H, 2J (PtH) ) 90 Hz, Pt-Me]; 0.41 [s, 6H, 2J (PtH) ) 84 Hz,
Pt-Me]; 2.63 [m, 4H, 3J (PtH) ) 4 Hz, CH2]; 2.75 [s, 12H,
F igu r e 8. 1H NMR spectra (600 MHz) in the methane
region for the protonation of complex 1 with HCl: (a) in
CD3OD; (b) in CD3OD/CD3CN, 50:50; and (c) in CD3OD/
CD3CN, 7:93.
2 are well separated and so can react essentially in-
dependently of one another, whereas the platinum cen-
ters in 1 are much closer. The separation between the
platinum(II) centers in 1 is short enough to allow a
single atom bridge to span between them, but no evi-
dence for this has been found. Instead, the oxidation to
octahedral platinum(IV) is accompanied by conforma-
tional change to allow a larger separation between the
two platinum centers. Nevertheless, the interaction
3
3J (PtH) ) 19 Hz, N-Me]; 3.99 [m, 4H, J (PtH) ) 4 Hz, CH2];
7.35 [t, 1H, J (HH) ) 8 Hz, H5]; 8.41 [d, 2H, J (HH) ) 8 Hz,
3
H4,6]; 9.03 [s, 2H, J (PtH) ) 45 Hz, NdCH]; 9.37 [s, 1H, H2].
13C NMR (CDCl3): δ -24.8 [1J (PtC) ) 800 Hz, PtMe]; -18.5
(15) (a) Scott, J . D.; Puddephatt, R. J . Organometallics 1983, 2, 1643.
(b) Yamami, Y.; Tanaka, M.; Sakiyama, H.; Koga, T.; Kobayashi, K.;
Miyasaka, H.; Ohba, M.; Okawa, H. J . Chem. Soc., Dalton Trans. 1997,
4595.

1404 Organometallics, Vol. 23, No. 6, 2004
Ta ble 7. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for th e Com p lexes
Zhang et al.
1‚C2H4Cl2
2‚4CHCl3
3‚CH2Cl2
4
formula
C
22H42Cl2N4Pt2
C
24H42Cl12N4Pt2
C
19H32Cl4N4Pt2
C24H48F6N4O8Pt2S2
fw
temp/K
823.68
200(2)
1202.20
200(2)
848.47
200(2)
1088.96
150(2)
λ(Mo KR)/Å
cryst syst
space group
cell dimens a/Å
b/Å
0.71073
triclinic
P1h
0.71073
monoclinic
P21/c
14.3712(4)
11.9440(2)
11.8166(2)
90
104.887(1)
90
1960.23(7)
2
0.71073
monoclinic
C2/c
12.7928(3)
10.9447(4)
19.3462(6)
90
101.317(2)
90
2656.06(1)
4
0.71073
triclinic
P1h
8.8641(2)
11.4676(3)
14.2479(5)
88.549(1)
87.412(1)
76.852(2)
1408.73(7)
2
11.2493(4)
12.0187(3)
15.1904(5)
111.180(1)
98.152(1)
101.929(2)
1820.2(1)
2
c/Å
R/deg
â/deg
γ/deg
volume/Å3
Z
d(calc)/Mg m-3
abs coeff/mm-1
F(000)
1.942
10.125
784
2.037
7.97
1148
2.122
10.938
1592
1.987
7.869
1052
no. of reflns/ind reflns
abs corr
no. of data/restr/params
R1, wR2 [I > 2σ(I)]
16 510/8255
integration
8255/0/272
0.072, 0.176
19 908/4477
integration
4477/0/191
0.049, 0.123
11 461, 3037
integration
3037/2/137
0.036, 0.081
19 272, 7233
integration
7233/16/483
0.039, 0.073
[1J (PtC) ) 840 Hz, PtMe]; 49.7 [NMe2]; 65.5, 66.3 [CH2]; 128,
129, 132, 134 [aromatic C].
N-Me]; 2.80, 2.88, 3.92, 4.01 [m, 4H, CH2N]; 7.61 [t, 1H, 3J (HH)
) 8 Hz, CH5]; 7.93 [s, 1H, CH2]; 7.72, 7.98 [d, each 1H, 3J (HH)
3
[P t2Me4(1,4-BAIB)], 2. This was prepared similarly from
[Pt2Me4(µ-SMe2)2] (0.0576 g) and 1,4-BAIB (0.0285 g). Yield:
88%; mp 195 °C. Anal. Calcd for C20H38N4Pt2: C, 33.1; H, 5.3;
N, 7.7. Found: C, 33.6; H, 5.7; N, 7.5. 1H NMR (CD2Cl2): δ
) 8 Hz, CH4,6]; 9.17 [s, 1H, J (PtH) ) 35 Hz, NdCH]; 9.86 [s,
2
1H, CHdO]. 6: δ 0.47 [s, 3H, J (PtH) ) 70 Hz, Pt-Me]; 0.70
[s, 3H, 2J (PtH) ) 70 Hz, Pt-Me]; 0.71 [s, 3H, 2J (PtH) ) 79 Hz,
Pt-Me]; 2.33 [s, 3H, N-Me]; 2.46 [s, 3H, N-Me]; 2.58, 2.59, 2.90,
2.94 [m, 4H, CH2N].
2
2
0.02 [s, 6H, J (PtH) ) 90 Hz, Pt-Me]; 0.46 [s, 6H, J (PtH) )
84 Hz, Pt-Me]; 2.63 [m, 4H, 3J (PtH) ) 5 Hz, CH2]; 2.76 [s,
12H, 3J (PtH) ) 18 Hz, NMe]; 3.98 [m, 4H, 3J (PtH) ) 6 Hz,
CH2]; 8.34 [s, 4H, H2,3,5,6]; 8.97 [s, 2H,3J (PtH) ) 36 Hz,
NdCH].
Ch a r a ct er iza t ion of H yd r id o(m et h yl)p la t in u m (IV)
Com p lexes. In a typical experiment, a solution of HCl (0.01
mmol) in CD2Cl2 (0.25 mL) (produced by reaction of Me3SiCl
with H2O) was added by syringe to a solution of complex 1
(7.2 mg, 0.01 mmol) in CD2Cl2 (0.30 mL) in a 5 mm NMR tube
at -78 °C. The tube was shaken to ensure mixing, then
immediately placed in the precooled NMR probe of the Varian
[P t2Cl2Me2(1,3-BAIB)], 3. To a solution of complex 1 (0.044
g) in CH2Cl2 (20 mL) was added H2O (3.3 µL) and Me3SiCl
(23 µL) to form HCl in situ. The product formed as a yellow
precipitate, which was isolated by filtration, washed with
CH2Cl2 and pentane, and dried under vacuum. Yield: 63%.
Anal. Calcd for C18H32Cl2N4Pt2: C, 28.2; H, 4.2; N, 7.3.
1
Inova 400 MHz spectrometer. H NMR data were recorded at
-78 °C, and then at -40, -30, -20, -10, 0, and 20 °C as the
sample was warmed in steps to room temperature. Complex
9 was characterized by its NMR spectrum: δ(1Η) -0.06 [s,
3H, 2J (PtH) ) 92 Hz, Pt-Me]; 0.05 [s, 3H, 2J (PtH) ) 86 Hz,
1
Found: C, 28.6; H, 4.7; N, 7.4%. H NMR (DMSO-d6): δ 0.64
[s, 6H, 2J (PtH) ) 80 Hz, Pt-Me]; 2.62 [m, 4H, CH2]; 2.78 [s,
12H, NMe]; 3.99 [m, 4H, CH2]; 7.37 [t, 1H, H5]; 8.42 [d, 2H,
Pt-Me]; 0.41 [s, 3H, J (PtH) ) 82 Hz, Pt-Me].
2
3
H4,6]; 8.99 [s, 2H, J (PtH) ) 136 Hz, NdCH]; 9.41 [s, 1H, H2].
Other experiments were performed in a similar way, and
data are listed in Tables 4 and 5.
[P t2Me6(OH2)2(1,3-BAIB)](CF 3SO3)2, 4. To a solution of
complex 1 (0.15 g) in CH2Cl2 (30 mL) was added CF3SO3Me
(0.12 mL). The solution slowly turned pale yellow in color.
After 3 days the solvent was evaporated under vacuum to yield
the product, which was recrystallized from CH2Cl2/hex-
ane. Yield: 86%. Anal. Calcd for C24H48F6N4O8Pt2S2: C, 26.5;
H, 4.4; N, 5.1. Found: C, 26.6; H, 4.8; N, 4.8%. 1H NMR
(CD3CN): δ 0.24 [s, 6H, 2J (PtH) ) 70 Hz, Pt-Me]; 0.60 [s, 3H,
2J (PtH) ) 75 Hz, Pt-Me]; 0.61 [s, 3H, 2J (PtH) ) 75 Hz,
Pt-Me]; 0.68 [s, 6H, 2J (PtH) ) 70 Hz, Pt-Me]; 2.35 [s, 6H,
H/D Exch a n ge Exp er im en ts. In a typical experiment, a
sample of complex 1 (0.01 mmol) in a 5 mm NMR tube (loaded
in a drybox to ensure anhydrous) was dissolved in CD3OD/
CD3CN (1.0 mL, 1:1 v/v), and the solution was cooled to below
-20 °C. Acetyl chloride (1.5 µL, 0.02 mmol) was then added
to form DCl. The tube was shaken to give good mixing, then
placed in the NMR probe precooled to -20 °C. Spectra were
recorded at -20, -10, 0, and 20 °C, as the solution was
warmed slowly to room temperature. Other experiments were
carried out in a similar way, and selected data are listed in
Table 6.
3
3J (PtH) ) 16 Hz, N-Me]; 2.69 [s, 6H, J (PtH) ) 9 Hz, N-Me];
2.79 [m, 2H, CHN]; 2.85 [m, 2H, CHN]; 3.92 [m, 2H, CHN];
3
3.95 [m, 2H, CHN]; 7.45 [d, 2H, J (HH) ) 8 Hz, CH4,6]; 7.53
Xr a y Str u ctu r e Deter m in a tion s. A suitable crystal was
mounted on a glass fiber, and data were collected by using a
Nonius Kappa-CCD diffractometer. Details of the data collec-
tions and structure refinements are given in Table 7.
[t, 1H, 3J (HH) ) 8 Hz, CH5]; 7.54 [s, 1H, CH2]; 9.05 [s,
2H, 3J (PtH) ) 35 Hz, NdCH]. 1H NMR (D2O): δ 0.23 [s,
6H, 2J (PtH) ) 70 Hz, Pt-Me]; 0.67 [s, 6H, 2J (PtH) ) 67 Hz,
Pt-Me]; 0.75 [s, 6H, 2J (PtH) ) 79 Hz, Pt-Me]; 2.55 [s, 6H,
N-Me]; 2.80 [s, 6H, N-Me]; 2.80, 2.88, 3.91, 4.00 [m, 8H, CH2N];
Ack n ow led gm en t. We thank the NSERC (Canada)
for financial support. R.J .P. thanks the Government of
Canada for a Canada Research Chair.
3
7.48 [s, 1H, CH2]; 7.57 [t, 1H, J (HH) ) 8 Hz, CH5]; 7.71 [d,
2H, 3J (HH) ) 8 Hz, CH4,6]; 9.13 [s, 2H, 3J (PtH) ) 34 Hz,
NdCH]. Hydrolysis of 4 led to formation of 5 and 6, identified
Su p p or tin g In for m a tion Ava ila ble: This material is
available free of charge via the Internet at http://pubs.acs.org.
2
by their 1H NMR spectra. 5: δ 0.12 [s, 3H, J (PtH) ) 70 Hz,
Pt-Me]; 0.66 [s, 3H, 2J (PtH) ) 67 Hz, Pt-Me]; 0.78 [s, 3H,
2J (PtH) ) 80 Hz, Pt-Me]; 2.41 [s, 3H, N-Me]; 2.55 [s, 3H,
OM030650M