Redox chemistry of dimethylplatinum(II) diimine complexes. Oxidatively induced Pt-Me transfer between transient platinum(III) cation radicals

Article abstract of DOI:10.1021/om9801498

The redox chemistry of the series of Pt(II) diimine complexes L₂PtMe₂ (1; L₂ = Ar-N= CRCR=N-Ar, where Ar/R = 4-MeC₆H₄/H (a), 4-MeOC₆H₄/H (b), 4-MeC₆H₄/Me (c

Full text of DOI:10.1021/om9801498

Organometallics 1998, 17, 3957-3966
3957
Red ox Ch em istr y of Dim eth ylp la tin u m (II) Diim in e
Com p lexes. Oxid a tively In d u ced P t-Me Tr a n sfer
betw een Tr a n sien t P la tin u m (III) Ca tion Ra d ica ls
Lars J ohansson,1a Olav B. Ryan,1b Christian Rømming,1a and Mats Tilset*,1a,c
Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway,
and SINTEF Applied Chemistry, Department of Hydrocarbon Process Chemistry,
P.O. Box 124 Blindern, N-0314 Oslo, Norway
Received March 2, 1998
The redox chemistry of the series of Pt(II) diimine complexes L2PtMe2 (1; L2 ) Ar-Nd
CRCRdN-Ar, where Ar/R ) 4-MeC6H4/H (a), 4-MeOC6H4/H (b), 4-MeC6H4/Me (c), 4-MeOC6H4/
Me (d )), with particular emphasis on the oxidation processes, has been studied in detail.
As seen by cyclic voltammetry, 1a -d undergo two successive, reversible one-electron
reductions at the diimine ligands and an irreversible, metal-centered one-electron oxidation.
The oxidation of 1b has been investigated in some detail. Chemical oxidation of 1b with
-
Cp2Fe+PF6 in acetonitrile yields a near 1:1 ratio of the corresponding Pt(II) and Pt(IV)
cations L2Pt(NCMe)Me+ (2b) and fac-L2Pt(NCMe)Me3+ (3b). Controlled-potential electrolysis
of 1b yields mixtures of 2b and 3b in a 1:1 ratio, as well as the cis,cis (4b) and one cis,trans
(5b) isomer of the dicationic Pt(IV) complexes L2Pt(NCMe)2Me22+. The percentage of the
dications 4b and 5b depended on the electrode potential. A mechanism involving methyl
group transfer between two transient Pt(III) intermediates L2PtMe2•+ is proposed to account
for the generation of 2b and 3b, whereas further oxidation of the Pt(III) species at the
electrode eventually provides 4b and 5b. The X-ray crystal structures of 1b and 3b(OTf-)
have been determined. All Pt-Me bond distances in these two species are essentially
identical, averaging 2.057(1) Å.
In tr od u ction
unsaturated cationic metal centers for early7,8 (d0) as
well as late9 (d10) metals.
Cationic, coordinately unsaturated transition-metal
complexes are involved as highly reactive intermediates
in important catalytic and stoichiometric reactions. For
example, alkene polymerization is catalyzed by cationic,
highly electrophilic catalysts derived from group 4 d0
metallocene2 as well as group 10 d8 square-planar3
catalyst precursors. The selective ethene/CO copolym-
erization is accomplished with similar cationic species
derived from square-planar d8 precatalysts.4 Recently,
alkane C-H bond activation has been achieved at
cationic 16-electron Ir(III)5 and 14-electron Pt(II)6 metal
centers. The reactive cationic species are commonly
generated by electrophilic abstraction of anionic ligands
(halide, alkyl, hydride) from suitable neutral precursors.
Interestingly, it has been less commonly demonstrated
that one-electron oxidation processes can also generate
The one-electron oxidation of closed-shell dialkylmetal
and hydridoalkylmetal complexes can induce homolytic
(eq 1) or apparently concerted (eq 2) reductive elimina-
tion reactions. We have reported for some thoroughly
explored cases that the kinetics for the concerted-type
reaction suggest that solvent coordination does not take
place prior to or during the rate-limiting reductive
elimination.10,11 Consequently, these reactions appear
to generate transient 15-electron species (eq 2) as the
initially formed products. In the coordinating solvent
(1) (a) University of Oslo. (b) SINTEF. (c) E-mail: mats.tilset@
kjemi.uio.no.
(2) (a) Bochmann, M. J . Chem. Soc., Dalton Trans. 1996, 255. (b)
Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(3) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414.
(4) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663.
(5) Arndtsen, B. A.; Bergman, R. G. Science 1996, 270, 1970.
(6) Holtcamp, M. W.; Labinger, J . A.; Bercaw, J . E. J . Am. Chem.
Soc. 1997, 119, 848.
(7) J ordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett,
R. J . Am. Chem. Soc. 1987, 109, 4111.
(8) Burk, M. J .; Tumas, W.; Ward, M. D.; Wheeler, D. R. J . Am.
Chem. Soc. 1990, 112, 6133.
(9) Seligson, A. L.; Trogler, W. C. J . Am. Chem. Soc. 1992, 114, 7085.
(10) Pedersen, A.; Tilset, M. Organometallics 1993, 12, 56.
(11) Fooladi, E.; Tilset, M. Inorg. Chem. 1997, 36, 6021.
S0276-7333(98)00149-6 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/06/1998

3958 Organometallics, Vol. 17, No. 18, 1998
J ohansson et al.
Sch em e 1
Sch em e 2
acetonitrile, these are subsequently transformed to the
observed closed-shell products through additional oxida-
tion, ligand coordination, or ligand redistribution pro-
cesses, as shown by the examples10,11 in Scheme 1.12
A plethora of square-planar Pd(II) and Pt(II) dialkyl
complexes have been reported and vigorously studied,
and the reduction chemistry of complexes with diimine
and bipyridine or related ligands has been particularly
well explored.13 In contrast to this situation, their
electron-transfer oxidation chemistry has not been
investigated in much detail, perhaps due to the com-
monly observed irreversible electrode processes. The
one-electron oxidation of the dialkyl compounds (dmpe)-
9
PdMe2 and (dmpe)Pd(CH2SiMe3)2 was reported to
give an account of the synthesis, characterization, and
redox chemistry of a series of (diimine)PtMe2 complexes.
In particular, we describe for the first time the products
of the oxidatively induced reactions of the (diimine)-
PtMe2 complexes and show that they do not arise from
concerted reductive elimination reactions or homolytic
processes analogous to those described above. Rather,
the predominant reaction pathway is a methyl group
transfer from a transient Pt(III) intermediate, ulti-
mately producing Pt(II) and Pt(IV) cationic products.
induce the homolytic cleavage of a Pd-C bond to
produce the cationic Pd(II) species (dmpe)Pd(R)(NC-
Me)+. Chen and Kochi14 reported that the two-electron
oxidation of cis-L2PtR2 (R ) Me, Et; L ) PPh3, PMe2-
Ph) with IrCl62- in acetonitrile proceeded to give as Pt-
containing products the Pt(II) complexes L2Pt(R)(X) (X
) Cl, MeCN+), resulting from single Pt-R homolysis
(for R ) Et and in part for Me), and two-electron-
oxidized Pt(IV) products L2PtR2X2 (for R ) Me). Oxida-
tion of the platinacyclobutane (bpy)Pt(CH2CH2CH2) was
reported not to yield any C2 or C3 hydrocarbon gases;
the fate of the Pt center of this complex and of (PPh3)2-
Pt(CH2CH2CH2) after oxidation remains unclear.15 Ir-
reversible oxidative behavior of closely related Pt(II)
bis(hydrocarbyl) species with bidentate, nitrogen-based
chelating ligands such as substituted bipyridines or
diimines has been frequently noted,13,16-18 but the
products of these oxidations have not yet been reported.
Interestingly, some (diimine)Pt dimesityl complexes are
reported to undergo reversible, metal-based oxidations
to give Pt(III) cation radicals.13,19-22 This enhanced
stability has been attributed to steric shielding against
axially directed nucleophilic attack, by solvent or other
species, at the incipient Pt(III) cations. These species
have, however, not been stable enough to be isolated
nor have their decomposition products been identified.
We therefore wanted to explore the oxidation chem-
istry of these species in more detail. In this paper, we
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of (d iim in e)-
P tMe2 Com p lexes. Most of the complexes reported in
this paper are new, and the details of their synthesis
and characterization will be briefly given here. The
dimethylplatinum(II) complexes L2PtMe2 (1a -d ) were
conveniently prepared (Scheme 2) in high yields from
Pt2Me4(µ-SMe2)223,24 and the appropriate diimine ligand
12
L2 (L2 ) ArNdCRCRdNAr; Ar/R ) Tol/H (a ; (previ-
ously reported13,24); An/H, b; Tol/Me, c; An/Me, d ),
adapting the procedures used by others for the pre-
paration of related L2PtMe2 and (bpy)PtMe2 com-
plexes.13,16,24-26 The compounds were characterized by
1H and 13C{1H} NMR and UV-vis spectroscopy and by
elemental analyses. A summary of key spectroscopic
data for complexes prepared in this work is given in
Table 1. The NMR spectra of 1a -d clearly reflect
their molecular symmetry. Satellites due to coupling
(12) Abbreviations: Cp* ) (η5-C5Me5); Cn ) 1,4,7-trimethyl-1,4,7-
triazacyclononane; dmpe ) 1,4-bis(dimethylphosphino)ethane; Cy )
cyclohexyl; Tol ) p-tolyl; An ) p-anisyl.
(13) Kaim, W.; Klein, A.; Hasenzahl, S.; Stoll, H.; Za´lisˇ, S.; Fiedler,
J . Organometallics 1998, 17, 237 and references therein.
(14) Chen, J . Y.; Kochi, J . K. J . Am. Chem. Soc. 1977, 99, 1450.
(15) Klingler, R. J .; Huffman, J . C.; Kochi, J . K. J . Am. Chem. Soc.
1982, 104, 2147.
(16) Hasenzahl, S.; Hausen, H.-D.; Kaim, W. Chem. Eur. J . 1995,
1, 95.
(17) Braterman, P. S.; Song, J .-I.; Vogler, C.; Kaim, W. Inorg. Chem.
1992, 31, 222.
1
to 195Pt were observed in the H NMR spectra for the
Pt-Me groups (2J Pt-H ) ca. 87 Hz) as well as for the
chelate ring C-H resonance in 1a ,b. The absorptions
of the intensely colored compounds in their electronic
spectra at λ ca. 500-600 nm are assigned to metal-to-
ligand, d(Pt) f π*(diimine), charge-transfer absorptions,
in analogy with what has been established for related
(diimine)Pt and (bpy)Pt complexes.13 The MLCT ab-
sorptions exhibit the expected13,17 solvent dependence
(18) Vogler, C.; Schwederski, B.; Klein, A.; Kaim, W. J . Organomet.
Chem. 1992, 436, 367.
(19) Klein, A.; Kaim, W.; Waldho¨r, E.; Hausen, H.-D. J . Chem. Soc.,
Perkin Trans. 2 1995, 2121.
(20) Klein, A.; Kaim, W.; Hornung, F. M.; Fiedler, J .; Zalis, S. Inorg.
Chim. Acta 1997, 264, 269.
(21) Klein, A.; Hausen, H.-D.; Kaim, W. J . Organomet. Chem. 1992,
440, 207.
(22) Klein, A.; Kaim, W. Organometallics 1995, 14, 1176.
(23) Scott, J . D.; Puddephatt, R. J . Organometallics 1983, 2, 1643.
(24) van Asselt, R.; Rijnberg, E.; Elsevier, C. J . Organometallics
1994, 13, 706.
(25) Achar, S.; Scott, J . D.; Vittal, J . J .; Puddephatt, R. J . Organo-
metallics 1993, 12, 4592.
(26) Monaghan, P. K.; Puddephatt, R. J . Organometallics 1984, 3,
444.

Dimethylplatinum(II) Diimine Complexes
Organometallics, Vol. 17, No. 18, 1998 3959
Ta ble 1. Key Sp ectr oscop ic Da ta for New Com p lexes
1H NMR, δa
13C{1H} NMR,
δa Pt-Me
UV-vis λmax, nm (10-3ꢀ, M-1 cm-1
acetonitrile toluene
)
Pt-Me
Pt-NCMe diimine H(Me)
compd (2J (195Pt-H)) (4J (195Pt-H)) (3J (195Pt-H)) (2J (195Pt-H))
1a
1b
1c
1d
2a
1.67 (87.9)
1.63 (87.8)
0.92 (86.9)
0.95 (86.6)
1.08 (76.6)
9.33 (29.1)
9.30 (28.7)
1.43
-11.4 (809)
-11.5 (806)
-13.5 (803)
-13.5 (801)
377 (7.7), 550 sh (1.5), 595 (1.9) 397 (7.8), 600 sh (1.5), 651 (2.4)
394 (12.7), 560 sh (2.1), 595 (2.2) 411 (14.0), 605 (2.4), 649 (3.0)
372 (4.1), 510 sh (2.0), 541 (2.2) 453 (3.8), 595 (0.4)
1.45
369 (7.6), 510 sh (3.0), 543 (3.2) 378 (4.0), 550 (0.9), 597 (0.9)
2.44 (13.9)
2.46 (14.0)
2.11 (13.6)
2.17 (13.6)
8.94 (99.8)
8.98 (35.6)
8.87 (97.9)
8.89 (33.6)
2.05 (10.2)
2.13
-10.8 (674)b 329 (6.9), 400 (9.9)
-10.7 (675)b 322 (5.0), 432 (13.2)
2b
2c
2d
3b
1.11 (76.5)
0.62 (75.1)
0.65 (75.1)
-11.9d
-11.7d
314 (4.8)
2.06 (10.3)
2.15
306 (4.4), 393 (5.1)
438 (12.4)
0.53 (76.3) (ax)b not obsdb
0.94 (69.9) (eq)b
8.81 (26.6)b
-7.1 (ax)b,d
-2.9 (eq)b,d
4bc 1.59 (71.4)b
1.61 (61.2)b
2.61 (10.9)b
2.62 (12.4)b
2.66 (10.1)
8.75 (94.4)b
8.85 (30.4)b
9.05 (25.6)b
5bc 1.85 (62.7)b
a
b
Dichloromethane-d2 unless otherwise noted. Acetonitrile-d3. c Not isolated. Spectra of product mixture from exhaustive electrolysis.
d
Not resolved.
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 1b a n d 3b(OTf-)
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 1b a n d 3b(OTf-)
1b
3b(OTf-)
1b
3b(OTf-)
empirical formula
fw
temp, K
C
18H22N2O2Pt
C22H28F3N3O5PtS
698.62
150(2)
0.710 73
triclinic
a ) 10.310(1)
R ) 99.47(1)
b ) 12.027(1)
â ) 113.60(1)
c ) 12.253(1)
γ ) 99.08(1)
1330.60(6); 4
1.744
Pt(1)-C(11)
2.053(8)
2.060(8)
2.066(8)
2.160(6)
2.187(7)
2.194(7)
1.297(10)
1.458(9)
1.289(11)
1.441(9)
1.132(10)
1.470(11)
1.457(11)
89.6(3)
88.3(3)
84.6(3)
177.9(3)
91.4(3)
93.6(3)
91.9(3)
99.7(3)
175.8(3)
86.1(2)
91.1(3)
493.47
150(2)
0.710 73
orthorhombic
a ) 7.452(1)
Pt(1)-C(9A)
Pt(1)-C(9B)
2.061(4)
2.046(5)
Pt(1)-C(9A)
Pt(1)-C(9B)
Pt(1)-N(2)
wavelength, Å
cryst syst
Pt(1)-N(1A)
Pt(1)-N(1B)
N(1A)-C(8A)
N(1A)-C(1A)
N(1B)-C(8B)
N(1B)-C(1B)
2.138(4)
2.113(4)
1.300(8)
1.426(6)
1.301(6)
1.421(6)
Pt(1)-N(1A)
Pt(1)-N(1B)
N(1A)-C(8A)
N(1A)-C(1A)
N(1B)-C(8B)
N(1B)-C(1B)
N(2)-C(10)
unit cell dimensa
b ) 8.532(1)
c ) 26.301(2)
1672.03(3); 4
V, Å3; Z
C(8A)-C(8B)
1.442(10) C(8A)-C(8B)
C(10)-C(12)
density (ca.cd), Mg m-3 1.960
abs coeff, mm-1
F(000)
8.403
952
0.3 × 0.25 × 0.1
2.51-40.68
25 365
5.408
684
0.3 × 0.3 × 0.1
1.77-28.73
7746
C(11)-Pt(1)-C(9A)
C(11)-Pt(1)-C(9B)
C(9A)-Pt(1)-C(9B)
C(11)-Pt(1)-N(2)
C(9A)-Pt(1)-N(2)
C(9B)-Pt(1)-N(2)
C(11)-Pt(1)-N(1A)
C(9A)-Pt(1)-N(1A)
C(9B)-Pt(1)-N(1A)
N(2)-Pt(1)-N(1A)
C(11)-Pt(1)-N(1B)
C(9A)-Pt(1)-N(1B)
C(9B)-Pt(1)-N(1B)
N(2)-Pt(1)-N(1B)
N(1A)-Pt(1)-N(1B)
cryst size, mm
C(9A)-Pt(1)-C(9B)
85.6(2)
θ range for data, deg
no. of rflns collected
no. of indep rflns
refinement method
no. of data/params
goodness of fit on F2
final R indices
9530 (R(int) ) 0.042) 5569 (R(int) ) 0.050)
full-matrix least squares on F2
9526/210
1.095
5561/317
1.053
C(9A)-Pt(1)-N(1A)
C(9B)-Pt(1)-N(1A)
99.4(2)
173.1(2)
R1 ) 0.040
wR2 ) 0.086
R1 ) 0.048
wR2 ) 0.092
5.33
R1 ) 0.053
wR2 ) 0.122
R1 ) 0.068
wR2 ) 0.146
3.32
(I > 2σ(I))
R indices (all data)
C(9A)-Pt(1)-N(1B)
C(9B)-Pt(1)-N(1B)
176.1(2)
98.1(2)
175.2(3)
100.2(3)
87.8(2)
∆F(max), e Å-3
∆F(min), e Å-3
-3.65
-2.73
N(1A)-Pt(1)-N(1B)
C(8A)-N(1A)-C(1A) 116.4(5)
C(8A)-N(1A)-Pt(1)
C(1A)-N(1A)-Pt(1)
C(8B)-N(1B)-C(1B) 117.1(4)
C(8B)-N(1B)-Pt(1)
C(1B)-N(1B)-Pt(1)
77.1(2)
75.6(2)
a
C(8A)-N(1A)-C(1A) 117.9(7)
a, b, c in Å; R, â, γ in deg.
113.6(4)
129.9(3)
C(8A)-N(1A)-Pt(1)
C(1A)-N(1A)-Pt(1)
4.7(5)
126.9(5)
with absorptions occurring at considerably shorter
wavelengths for more polar solvents (acetonitrile vs
toluene).
C(8B)-N(1B)-C(1B) 118.2(7)
113.7(3)
129.1(3)
C(8B)-N(1B)-Pt(1)
C(1B)-N(1B)-Pt(1)
C(10)-N(2)-Pt(1)
113.5(5)
128.2(6)
169.8(7)
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a -
tion of 1b. Details about the structure determination
for 1b are given in the Experimental Section. Table 2
lists experimental and crystallographic data, and se-
lected bond distances and angles for 1b are given in
Table 3. Figure 1 shows an ORTEP drawing of the
X-ray crystal structure of 1b. The molecule assumes
the square-planar structure that is normally expected
for a four-coordinate d8 species. The average Pt-Me
bond distance is 2.054 Å, and the Pt-N average is 2.126
Å. The chelate ring is planar, with a mean deviation
from the least-squares plane of 0.026 Å. The planes of
the arene rings A and B are rotated relative to the
N(1A)-C(8A)-C(8B) 117.0(4)
N(1B)-C(8B)-C(8A) 118.3(4)
N(1A)-C(8A)-C(8B) 116.8(8)
N(1B)-C(8B)-C(8A) 119.3(7)
N(2)-C(10)-C(12)
178.9(9)
chelate ring plane by an average of 42.7° and are within
7.7° of being coplanar. The rotation of the aromatic
rings out of planarity with the chelate is presumably
caused by repulsions that would arise between the
arene-ring ortho hydrogens and Pt-methyl and C(ch-
elate)-hydrogen groups.
The main focus of the investigation was chosen to be
on complex 1b. The electrochemical and chemical
oxidation (vide infra) of 1b yielded mixtures of the four-
coordinate Pt(II) species L2Pt(NCMe)Me+ (2b) and the

3960 Organometallics, Vol. 17, No. 18, 1998
J ohansson et al.
Ta ble 4. 1H a n d 13C NMR Assign m en ts for 2ba
H or C
atom
1H NMR
(dichloromethane-d2)
13C NMR
(acetonitrile-d3)
F igu r e 1. ORTEP plot of the structure of 1b.
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
1.11 (2J Pt-H ) 76.5 Hz)
-10.7 (1J Pt-C ) 675 Hz)
not obsd
not obsd
n.a.
six-coordinate Pt(IV) species fac-L2Pt(NCMe)Me3+ (3b)
as the major products. These have been independently
prepared and characterized.
2.46 (4J Pt-H ) 14.0 Hz)
8.87 (3J Pt-H ) 97.9 Hz)
172.6
8.89 (3J Pt-H ) 33.6 Hz)
160.7 (2J Pt-C ) 57 Hz)
Syn th esis a n d Ch a r a cter iza tion of L2P t(NCMe)-
Me+ (2a -d ). When complexes 1a -d were treated with
a slight excess of HBF4‚Et2O in acetonitrile, good yields
n.a.
7.56
7.10
n.a.
3.91
n.a.
7.27
7.01
n.a.
3.87
140.9
125.9
115.9
163.0
56.5
141.4
of the complexes L2Pt(NCMe)Me+BF4 (2a -d ) were
-
obtained. These salts have been characterized by their
1H and 13C{1H} NMR spectra, UV-vis spectroscopy, and
elemental analyses. Key spectroscopic data are given
125.6 (3J Pt-C ) 20 Hz)
115.0
161.5
56.4
1
in Table 1. The H NMR spectra are consistent with
species of symmetry lower than the precursors 1a -d ,
a
and separate signals are observed from the halves of
Based on H-H COSY, C-H COSY, and NOESY spectra
2
provided as Supporting Information.
the diimine ligands. The J Pt-H couplings to the Pt-
Me groups are consistently somewhat smaller than for
1a -d (ca 76 Hz vs 87 Hz). Couplings between Pt and
coordinated MeCN were also seen; 4J (195Pt-H) ) ca. 14
Hz.
and two Pt-Me groups define an equatorial plane and
the last Pt-Me and the acetonitrile ligand occupy the
two apical positions, as shown in Scheme 2.
The 195Pt couplings to the C-H protons of the chelate
ring were observed for 2a ,b. One such coupling was
near 100 Hz and the other near 35 Hz. A complete
1
The H NMR spectrum of 3b in acetonitrile-d3 failed
to show a signal due to the coordinated acetonitrile.
However, a singlet due to free acetonitrile was clearly
seen adjacent to the solvent signal, with correct integra-
tion. We attribute this to rapid exchange of initially
coordinated acetonitrile and the solvent. In support of
a rapid exchange process, we note that Elsevier and co-
workers24 have reported that a L2PtMe3(NCMe)+OTf-
complex in dichloromethane-d2 solution exists as rapidly
interconverting [Pt]+-NCMe and [Pt-OTf] or five-
coordinate [Pt]+(OTf-) species ([Pt] ) L2PtMe3). We
have made similar observations that will be published
elsewhere.
1
assignment of the H and 13C NMR spectra of 2b was
carried out using a combination of H-H COSY, C-H
COSY, and NOESY techniques. These spectra and
details about the analysis are given in the Supporting
Information; the results are summarized in Table 4. An
inspection of the data shows that the signals assigned
to atoms in the “left half” of the molecule (which includes
the Pt-Me group), as drawn in Table 4, are slightly
upfield relative to the corresponding signals of the “right
half”, except for the carbon resonances of the chelate
ring and the arene ring ipso carbons. Simple consid-
erations of relative trans effects of ligands are often used
to rationalize trends in Pt-X couplings in square-planar
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a -
tion of fa c-(An NdCHCHdNAn )P t(NCMe)Me3+OTf-.
Details about the structure determination for 3b(OTf-)
are given in the Experimental Section. Table 2 lists
experimental and crystallographic data, and Table 3
lists selected bond distances and angles. Figure 2 shows
an ORTEP drawing of the X-ray crystal structure. The
molecule adopts a pseudo-octahedral geometry. The
apical and equatorial Pt-Me bond distances are statis-
tically indistinguishable, averaging 2.060 Å, which is
also essentially identical with the average Pt-Me
distance of 2.053 Å in 1b. The occurrence of identical
apical and equatorial Pt-Me bond lengths in 3b is in
contrast to the situation for the Pt(IV) complex (CyNd
CHCHdNCy)PtMe4, in which the equatorial and apical
Pt-Me distances were 2.045 and 2.140 Å, respectively.16
The significantly greater apical Pt-Me distance in the
latter case may be attributed to the greater trans effect
of the Me ligand relative to MeCN. The chelate ring is
planar, with a mean deviation from the least-squares
plane of 0.020 Å. The planes of the arene rings A and
3
complexes.27 In 2b, the J Pt-H(d) coupling (98 Hz) is
3
greater than the J Pt-H(e) coupling (34 Hz). This is as
might be anticipated since the methyl group, transoid
relative to position (e), has a greater trans effect than
the acetonitrile, which is transoid relative to position
(d).
Syn th esis a n d Ch a r a cter iza tion of fa c-(An Nd
CHCHdNAn )P t(NCMe)Me3+OTf- (3b(OTf-)). Treat-
ment of 1b with methyl triflate in acetonitrile at -13
°C produced 3b(OTf-) in high yield. The 1H NMR
spectrum of 3b exhibits two Pt-Me signals in a 2:1 ratio
2
with the diagnostic J Pt-H couplings, one signal due to
coordinated acetonitrile, and one set of signals due to
the diimine ligand. This is only consistent with a
pseudo-octahedral structure in which the diimine ligand
(27) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1989; p 70.

Dimethylplatinum(II) Diimine Complexes
Organometallics, Vol. 17, No. 18, 1998 3961
F igu r e 2. ORTEP plot of the structure of 3b(OTf-).
F igu r e 3. Cyclic voltammograms for the oxidation and
reduction of (AnNdCHCHdNAn)PtMe2 (1b). Conditions:
1.0 mM solution of 1b in acetonitrile/0.1 M Bu4N+PF6- at
20 °C (d ) 0.4 mm Pt disk electrode, voltage sweep rate ν
) 1.0 V/s). The oxidative and reductive scans are done
separately, with freshly conditioned electrode surface for
each.
Ta ble 5. Cyclic Volta m m etr y Da ta for (d iim in e)P t
Com p lexesa
compd
redn,b V vs Fc
oxidn,c V vs Fc
1a
1b
1c
1d
2a
2b
2c
2d
3b
-1.36, -2.11
-1.39, -2.15
-1.75, -2.55c
-1.76, -2.57c
-0.84, -1.65
-0.88, -1.70
-1.20, -1.92d
-1.21, -1.96d
-1.05, -1.74
0.6-0.7c,e
0.6-0.7c,e
0.7c,e
0.7c,e
independent of the voltage sweep rate, and a plot of Ip/
v1/2 was linear. These data are in accord with two near-
Nernstian one-electron-reduction processes. The oxi-
dation of 1b occurs irreversibly at ca. 0.7 V vs Fc.
Adsorption problems caused the position of this ir-
reversible peak to be quite variable. Peak potentials
ranged between ca. 0.6 and 0.7 V at a freshly polished
electrode and gradually underwent displacement to at
least 1.0 V vs Fc after multiple repeated scans without
intermittent electrode cleaning. Considerable signal
broadening occurred at higher sweep rates, but it was
evident that the oxidation was chemically irreversible
even at v ) 100 V/s, as evidenced by the lack of a
cathodic peak during the reverse scan. This situation
persisted when the voltammetry was performed in the
poorly coordinating solvent dichloromethane. When the
reductive scan after the oxidation of 1b was taken to
-2.4 V vs Fc, reduction peaks attributed to products
from the follow-up reaction of 1b•+ were observed at ca
-0.5 and -0.9 V. The peak at -0.5 V is assumed to
arise from the reduction of a dicationic product of the
oxidation (vide infra). The peak at -0.9 V closely
matches the reduction peak for 2b (vide infra), another
oxidation product; a contribution from 3b might be
hidden under this peak due to the rather poor electrode
behavior caused by the adsorption. These combined
findings suggest that the observed Pt-containing prod-
ucts are formed on the CV time scale.
a
Conditions: 1.0 mM substrate in acetonitrile/0.1 M Bu4NPF6,
b
20 °C, Pt-disk electrodes (d ) 0.4 mm), v ) 1.0 V/s. Chemically
reversible unless otherwise noted. c Peak potential for chemically
irreversible couple. Partially reversible couple. e Best response,
d
as seen immediately after electrode polish. Great uncertainty due
to adsorption problems.
B are rotated relative to the chelate ring plane by 48.9
and 50.5°, respectively, again presumably to relieve
intramolecular repulsions. In contrast to the situation
in 1b, however, the A and B rings are mutually almost
perpendicularly oriented (81.8°). The triflate anion is
unexceptional, and the structure displays no unusual
intermolecular contacts.
Cyclic Volta m m etr y Stu d ies of (d iim in e)P t Com -
p lexes. Transition-metal complexes containing bpy and
diimine ligands typically undergo ligand-centered re-
ductions to form reduced species that may be viewed
as metal centers bonded to bpy or diimine anion
radicals, as mentioned in the Introduction. In contrast,
the consequences of the irreversible oxidation chemistry
of (diimine)Pt dimethyl species remain poorly under-
stood. We report in the following some interesting
findings regarding the reductive and oxidative processes
of these new species. Table 5 summarizes cyclic volta-
mmetry data for most of the complexes that are reported
in this paper.
Figure 3 shows cyclic voltammograms for the reduc-
tion and oxidation of 1b (1.0 mM in acetonitrile/0.1 M
Bu4N+PF6-, 20 °C, d ) 0.4 mm Pt-disk electrode, voltage
sweep rate v ) 1.0 V/s). The complex undergoes two
reversible reductions at -1.39 and -2.15 V vs the Cp2-
Fe/Cp2Fe+ couple (Fc), with peak-to-peak separation of
ca. 60 and 80-90 mV, respectively (the latter depending
on electrode history), at v ) 1.0 V/s. In comparison, the
separation for Cp2Fe was 60-62 mV under identical
conditions. The reversible potentials E°, taken as the
midpoints between the anodic and cathodic peaks, were
Each of the compounds 1a ,c,d also exhibited poorly
reproducible, irreversible oxidation waves in acetoni-
trile.28 However, whereas two reduction waves were
also seen for 1b-d , the behavior of 1c,d differed from
that of 1a ,b in that the last reduction process was
chemically irreversible. When the reduction potential
data for 1a -d are compared, it is evident that the E°
values are very sensitive to the identity of the substitu-
ent at the carbon atoms of the chelate ring. The methyl-
substituted complexes (1c,d ) are more difficult to reduce
than the unsubstituted ones (1a ,b) by as much as 0.37-
(28) Kaim et al. have recently13 reported that 1a undergoes revers-
ible reductions at -1.36 and -2.12 V and an irreversible oxidation at
0.65 V vs Fc. These data are in good agreement with ours.

3962 Organometallics, Vol. 17, No. 18, 1998
J ohansson et al.
tures were thoroughly analyzed. For complexes 2a -d ,
it is again noteworthy that the chelate substituent has
a great influence on the reduction potentials (0.33-0.36
V more difficult for Me vs H for the first reduction
process), whereas the exchange of p-Me for p-MeO at
the aryl rings has essentially no effect.
For the reduction of 3b, a faint wave is seen on the
reverse scan at ca. -1.25 V vs Fc. This wave was absent
when the sweep was reversed after the first reduction.
The faint wave therefore must arise from a product that
is formed in the homogeneous reaction that follows after
the second electron transfer. No attempts have been
made at identifying the reduction product(s).
E lect r och em ica l Bu lk Oxid a t ion of (An Nd
CHCHdNAn )P tMe2 (1b). Bulk oxidation of 1b was
performed under various conditions. The oxidations
were performed at Pt- and Au-mesh electrodes and at
each of the electrodes at working potentials of 0.85 and
0.95 V vs Fc. The location of these potentials in relation
to the CV peak potential for 1b cannot be exactly
established, since the reproducibility of the CV oxidation
peak potential was rather poor, and cyclic voltammo-
grams at the large-mesh electrodes are rather ill-defined
in any case. The charge that passed through the
electrolysis cell was 1.1-1.6 faraday/mol, depending on
the exact reaction conditions. Although trends ex-
tracted from the results of a limited number of experi-
ments should be regarded with caution, the results
suggest that a greater charge is consumed at the higher
oxidation potential. The products of these electrolyses
F igu r e 4. Cyclic voltammogram for the reduction of
(AnNdCHCHdNAn)Pt(NCMe)Me+BF4 (2b (BF4-), bot-
-
tom)and (AnNdCHCHdNAn)Pt(NCMe)Me3+OTf-(3b(OTf-),
top). Conditions as for the caption to Figure 3.
1
were analyzed by H NMR spectroscopy. In all cases,
2b and 3b formed in a near 1:1 ratio (see Experimental
Section for details). In addition, most notably when the
higher electrode potential was used, dicationic species
that have been tentatively identified as cis,cis-[(AnNd
CHCHdNAn)PtMe2(NCMe)2]2+ (4b ) and cis,trans-
[(AnNdCHCHdNAn)PtMe2(NCMe)2]2+ (5b) were also
produced. The spectroscopic data are given in the
Experimental Section. The downfield chemical shifts
(δ ca. 1.6) of the Pt-Me groups are consistent with a
greater positive charge in these complexes than in 2b
and 3b. Double sets of signals for all ligands were seen
for 4b; this is consistent only with the cis,cis formula-
tion. For 5b, single sets of signals are seen; this
requires the presence of a mirror plane and could be
consistent with structures in which the methyl groups
are cis-disposed and occupy the equatorial plane while
the acetonitrile ligands are trans-disposed in the apical
positions, or vice versa. We have noticed that, in
0.39 V. On the other hand, a change of the substituent
at the aromatic ring has virtually no effect: the p-MeO-
substituted species (1b,d ) are more difficult to reduce
than the p-Me-substituted species by only 0.03 V or less.
This lack of sensitivity may be rationalized by the fact
that the arene rings are twisted out of conjugation with
the chelate ring, as found in the X-ray structure of 1b.
Thus, π interactions with the arene rings and their
substituents are insignificant. Similar substituent ef-
fects have been previously observed for other metal
complexes.29
Figure 4 shows cyclic voltammograms for the reduc-
tion of 2b(BF4-) (bottom) and 3b(OTf-) (top). The
cationic Pt(II) complex 2b undergoes two reversible one-
electron reduction processes at -0.88 and -1.70 V vs
Fc. Both processes are presumed to be ligand-centered
reductions. The first reduction occurs at a potential 0.51
V less negative than that for 1b, which is also Pt(II).
This difference reflects the fact that 2b has a true
positive charge, whereas 1b does not. In comparison,
the cationic Pt(IV) complex 3b is reduced in a reversible
wave at -1.05 V and an irreversible one at -1.74 V vs
Fc. It is quite interesting to note that for the first
reduction process, the Pt(IV) complex is more difficult
to reduce than the closely related Pt(II) complex! Kaim
and co-workers13,16 noted that (CyNdCHCHdNCy)-
PtMe2 (Pt(II)) and (CyNdCHCHdNCy)PtMe4 (Pt(IV))
exhibited essentially identical redox behavior as judged
by CV experiments, for the reductive as well as for the
oxidative processes, and the underlying electronic fea-
3
general, a J Pt-H coupling of the bridge hydrogens in
the diimine ligand is observed to be relatively small in
the cases where the hydrogens have a transoid Pt-Me
group (3J Pt-H: 1a , 29.1; 1b, 28.7; 2a , 35.6, 2b, 33.6; 3b,
26.6 Hz), whereas the corresponding value when the
hydrogens have a transoid Pt-NCMe group are much
greater (2a , 99.8; 2b, 97.9; [(AnNdCHCHdNAn)Pt-
(NCMe)2]2+, 97.7 Hz30). The J value for 5b, 25.6 Hz,
may therefore indicate that the methyl groups are
located in the equatorial plane and the acetonitrile
ligands in the apical positions, as suggested in Scheme
3.
(30) The dication can be straightforwardly prepared as its triflate
salt by treatment of 1b with 2 equiv of HOTf in dichloromethane,
followed by addition of acetonitrile: J ohansson, L.; Tilset, M. Unpub-
lished work.
(29) See for example: Rossenaar, B. D.; Hartl, F.; Stufkens, D. J .
Inorg. Chem. 1996, 35, 6194.

Dimethylplatinum(II) Diimine Complexes
Organometallics, Vol. 17, No. 18, 1998 3963
Sch em e 3
Sch em e 4
Sch em e 5
lecular methyl transfer processes must be of major
importance for these reactions, whether induced by
chemical or electrochemical oxidation. Two scenarios
are plausible for the bimolecular methyl transfer: a
cation/cation 1b•+/1b•+ reaction (Scheme 4), or a cation/
neutral substrate 1b•+/1b reaction (Scheme 5).
The cation/cation reaction in Scheme 4 is attractive
because it has been demonstrated, at least for closed-
shell 18-electron complexes,31 that one-electron oxida-
tive processes lead to decreased M-X bond dissociation
energies. Accordingly, Pt-Me cleavage should be more
In those experiments where 4b and 5b were produced,
4b was by far the dominant product of these two (4b:
5b ratio of 3:1 or greater). Attempts at preparing these
dicationic species by treatment of 2b with methyl
triflate were not successful; thus, their identification
•+
facile for L2PtMe2 than for L2PtMe2. According to
Scheme 4, the two major cationic Pt(II) and Pt(IV)
products will be formed in the observed 1:1 ratio
observed for 2b and 3b after acetonitrile coordination
at the immediate products of the methyl transfer.
Arguing against the first step in Scheme 4 is the fact
that, at least under conditions of chemical oxidation
with Cp2Fe+, the electron-transfer equilibrium concen-
tration of L2PtMe2•+ will be very low (Keq for the electron
transfer is ca. 10-12 if Ep ) 0.7 V vs Fc is taken as an
approximate E° value for the oxidation of L2PtMe2). This
significantly reduces the likelihood that two such species
will encounter each other to react, although it is not at
all impossible within the constraints of diffusion control.
As an alternative, in Scheme 5 we have indicated the
transfer of a methyl radical from L2PtMe2•+ to L2PtMe2
or vice versa; coordination of acetonitrile at the resulting
L2PtMe3+ obviously gives the Pt(IV) product 3b, whereas
oxidation of the incipient L2PtMe• leads to the Pt(II)
product 2b. Oxidation of L2PtMe• may well occur via
prior acetonitrile coordination. The radical that is
produced by acetonitrile addition, L2PtMe(NCMe)•, will
be readily oxidized under the reaction conditions (its
oxidation potential will be identical with the reduction
potential for L2PtMe(NCMe)+ (2b; -0.88 V vs Fc).
Kaim and co-workers have suggested that the reac-
tions of the Pt(III) intermediates L2PtR2•+ may involve
axially directed attack by solvent or other nucleo-
philes,13,17,21,22 although dimerization or aggregation
phenomena were not discounted.22 The indications for
this were (1) solvent had a significant effect on the
irreversible oxidation peak potentials for certain Pt(II)
substrates and (2) dimesityl complexes L2Pt(mesityl)2,
for which the approach of nucleophiles toward the axial
positions is sterically blocked by the mesityl groups,
exhibited reversible behavior by cyclic voltammetry.
This possibility is fully consistent with our results: the
acetonitrile solvent molecules may well coordinate at
1
rests solely on the H NMR data. A CV analysis of the
electrolyte after a partial electrolysis oxidation at +0.95
V vs Fc showed, in addition to signals attributable to
2b and 3b, an intense reduction wave at -0.5 V. This
value is ca. 0.5 V less negative than the reduction
potentials for the monocationic products 2b and 3b, and
therefore, we tentatively assign this wave to the reduc-
tion of one or both of the doubly charged species 4b and
5b.
Ch em ical Oxidation of (An NdCHCHdNAn )P tMe2
(1b). The oxidation of 1b was effected with Cp2Fe+PF6
.
-
The observed peak potential for the oxidation of 1b (ca.
+0.6 V vs Fc) dictates that the electron transfer from
1b to Cp2Fe+ is highly endergonic; however, the rapid
follow-up reaction caused the oxidation to take place
relatively quickly in acetonitrile-d3 at 55 °C. The 1H
NMR spectrum showed that a 56:44 mixture of 2b and
3b had been formed. Only small amounts of methane
were discernible in the spectrum. The product composi-
tion was independent of the reaction temperature (55
°C vs slow heating from -25 °C to ambient over 4 days)
as well as substrate concentration. Thus, chemical
oxidation of 1b leads to a near 1:1 mixture of 2b and
3b, in good agreement with the results of the electro-
chemical bulk oxidation.
Oxid a tion of 1b: Mech a n istic Con sid er a tion s.
The combination of poor reproducibility and very fast
follow-up homogeneous reaction causes the CV response
for the oxidation of 1b to be unsuitable for any detailed
mechanistic analysis of the nature of this follow-up
reaction. However, the product distribution under
different reaction conditions allows an educated analysis
of likely scenarios.
The essentially complete lack of methane or ethane
in the homogeneous oxidation experiments rules out
homolysis or reductive-elimination (eqs 1 and 2) pro-
cesses as significant reaction pathways. Instead, bimo-
(31) Tilset, M.; Hamon, J .-R.; Hamon, P. Chem. Commun., in press.

3964 Organometallics, Vol. 17, No. 18, 1998
J ohansson et al.
Sch em e 6
C-H activation and alkene polymerization reactions.
This opens the possibility that such catalysts may be
generated by electron-transfer initiation.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations involving orga-
nometallic compounds were carried out with use of vacuum
line, Schlenk, syringe, or drybox techniques. Acetonitrile was
distilled from P2O5, and acetonitrile-d3, dichloromethane, and
dichloromethane-d2 were distilled from CaH2. Acetonitrile and
dichloromethane containing the supporting electrolyte were
passed through a column of active neutral alumina prior to
use to remove water and protic impurities before electrochemi-
cal measurements. The electrolyte was freed of air by purging
with purified solvent-saturated argon, and all measurements
and electrolyses were carried out under a blanket of argon.
1H and 13C{1H} NMR spectra were recorded on Bruker
Advance DXP 200, 300, and 500 instruments. Chemical shifts
are reported in ppm relative to tetramethylsilane, with the
residual solvent proton resonance as internal standard. El-
emental analyses were performed by Ilse Beetz Mikroana-
lytisches Laboratorium, Kronach, Germany. UV-visible spec-
tra were recorded on a Shimadzu UV-260 spectrophotometer
and are reported as λmax (nm) and ꢀ × 10-3 (M-1 cm-1).
Electrochemical measurements were performed with an
EG&G-PAR Model 273 potentiostat/galvanostat driven by an
external HP 3314A sweep generator. The signals were fed to
a Nicolet 310 digital oscilloscope and processed by an on-line
personal computer. The working electrodes were Pt-disk
electrodes (d ) 0.4 mm), the counter electrode was a Pt wire,
and the Ag-wire reference electrode assembly was filled with
acetonitrile/0.01 M AgNO3/0.1 M Bu4N+PF6-. The reference
electrode was calibrated against Cp2Fe, which was also used
as the reference in this work. The positive-feedback iR
compensation circuitry of the potentiostat was employed; the
separation of anodic and cathodic peaks for the Cp2Fe oxida-
tion was 60-62 mV in acetonitrile.
•+
L2PtMe2 to give 17-electron pentacoordinate species
L2PtMe2(NCMe)•+ before methyl transfer takes place
according to Scheme 4 or 5, with the same options for
the nature of the crucial methyl group transfer. Our
finding that the oxidation of 1b was chemically ir-
reversible also in the poorly coordinating solvent dichlo-
romethane may call for alternative explanations. For
example, it is possible, and is consistent with experi-
mental data available, that the “mesityl effect” is caused
by a retardation of a bimolecular (Pt/Pt) reaction, as it
could be impossible for two such sterically congested Pt
centers to approach each other to within the distance
required for the transfer of an organic ligand. The
solvent effect on the oxidation peak potentials for the
sterically less encumbered species may well be caused
by changes in the heterogeneous electron-transfer rate
constant, rather than by kinetic potential shifts caused
by changes in the rates of the follow-up reactions.
We must next consider the results of the electrochemi-
cal oxidation reactions. It was established, at Pt as well
as Au working electrodes, that the dicationic products
4b and 5b were favored when the working electrode
potential was increased. Since the two dicationic prod-
ucts have retained both of the methyl groups that are
present in the substrate, it is highly unlikely that their
formation occurs via the one-electron oxidation products
2b and 3b. In support of this, the CV investigation also
established that 2b and 3b showed no oxidation waves
before the solvent/electrolyte discharge limit. Therefore,
the formation of 4b and 5b must occur by interception
of 1b•+ for further oxidation at the electrode surface
before the methyl group is transferred. This explains
why the yield of the dications is electrode potential
dependent and, by extension, why no dications were
produced by oxidation with the relatively mild oxidizing
agent Cp2Fe+. (The oxidizing power of Cp2Fe+ is at
least 0.8 V lower than the electrode potential employed
for the electrolysis experiments.)
23
Pt2Me4(µ-SMe2)2 and the diimine ligands ArNdCHCHd
NAr32 and ArNdCMeCMedNAr33 were prepared according to
reported methods.
Gen er a l P r oced u r e for th e P r ep a r a tion of Com p lexes
(Ar NdCRCRdNAr )P tMe2 (1a-d). The diimine ligand ArNd
CRCRdNAr was added to a solution of Pt2Me4(µ-SMe2)2 in
toluene. The solution was stirred at ambient temperature for
20 h. The solvent was removed, and the residue was recrys-
tallized from a dichloromethane/pentane mixture. The product
was obtained after drying in vacuo. For 1a , this procedure
resulted in a yield significantly improved over that reported
recently (65%).13
(TolNdCHCHdNTol)P tMe2 (1a ): from Pt2Me4(µ-SMe2)2
(600 mg, 1.044 mmol) and TolNdCHCHdNTol (494 mg, 2.090
mmol) in toluene (120 mL); dark green shimmering microc-
rystals (896 mg, 93%). 1H NMR (300 MHz, dichloromethane-
d2): δ 1.67 (s, 2J (195Pt-H) ) 87.9 Hz, 6 H, PtMe), 2.42 (s, 6 H,
Scheme 6 summarizes the principal mechanistic
features that have been discussed. The homogeneous
reactions lead to the upper-right products, whereas the
lower-right products arise from a secondary, heteroge-
neous electron-transfer reaction.
3
ArMe), 7.30 (m, 8 H, Ar H), 9.33 (s, J (195Pt-H) ) 29.1 Hz, 2
H, NCHCHN). 13C{1H} NMR (75 MHz, dichloromethane-d2):
δ -11.4 (1J (195Pt-C) ) 809 Hz, PtMe), 21.3 (ArMe), 123.3 (Ar
Co), 129.8 (Ar Cm), 139.0 (Ar Cp), 147.8 (2J (195Pt-C) ) 28.5
Hz, Ar Cipso), 161.6 (NCHCHN). UV-vis (acetonitrile): λmax
377 (7.7), 550 sh (1.5), 595 (1.9). UV-vis (toluene): 397 (7.8),
600 sh (2.0), 651 (2.4). Anal. Calcd for C18H22N2Pt: C, 46.85;
H, 4.81; N, 6.07; Pt, 42.28. Found: C, 45.98; H, 4.54; N, 6.17;
Pt, 42.05.
Con clu sion s
We have, for the first time, carried out a detailed
study of the irreversible oxidation chemistry of (di-
imine)PtMe2 complexes. Whereas previously investi-
gated square-planar Pt(II) dialkyl complexes appear
to14,15 undergo Pt-R homolysis upon oxidation, the
short-lived Pt(III) cation radicals L2PtMe2•+ studied by
us appear to undergo intermolecular methyl transfer
reactions to give the Pt(II) and Pt(IV) complexes L2Pt-
(An NdCHCHdNAn )P tMe2 (1b): from Pt2Me4(µ-SMe2)2 (600
mg, 1.044 mmol) and AnNdCHCHdNAn (560 mg, 2.089
mmol) in toluene (120 mL); dark green shimmering micro-
(NCMe)Me+ and fac-L2Pt(NCMe)Me3 as products. It
+
(32) Kliegman, J . M.; Barnes, R. K. J . Org. Chem. 1970, 35, 3140.
(33) tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. 1981,
36B, 823.
is possible that the former is preceded by the 14-electron
cation L2PtMe+, an analogue of the active species in

Dimethylplatinum(II) Diimine Complexes
Organometallics, Vol. 17, No. 18, 1998 3965
crystals (969 mg, 94%). 1H NMR (300 MHz, dichloromethane-
d2): δ 1.63 (s, 2J (195Pt-H) ) 87.8 Hz, 6 H, PtMe), 3.87 (s, 6 H,
OMe), 7.01 (“d”, 3J (HaHb) ) 8.9 Hz, 4 H, Ar Ha), 7.42 (“d”,
3J (HaHb) ) 8.9 Hz, 4 H, Ar Hb), 9.30 (s, 3J (195Pt-H) ) 28.7
Hz, 2 H, NCHCHN). 13C{1H} NMR (75 MHz, dichloromethane-
d2): δ -11.5 (1J (195Pt-C) ) 806 Hz, PtMe), 56.0 (OMe), 114.4
(Ar Cm), 124.9 (Ar Co), 143.6 (Ar Cipso), 160.4 (Ar Cp), 160.8
(NCHCHN). UV-vis (acetonitrile): λmax 394 (12.7), 564 (2.1),
595 (2.2). UV-vis (toluene): 411 (14.0), 605 (2.4), 649 (3.0).
Anal. Calcd for C18H22N2O2Pt: C, 43.81; H, 4.49; N, 5.68; Pt,
39.53. Found: C, 44.32; H, 4.70; N, 6.43; Pt, 39.64.
ambient temperature. The solvent was removed, and the solid
was washed with several portions of ether before it was
dissolved in a minimum amount of acetonitrile. Precipitation
by addition of toluene yielded the product as a red powder (222
mg, 90%). 1H NMR (300 MHz, dichloromethane-d2): δ 1.11
2
4
(s, J (195Pt-H) ) 76.5 Hz, 3 H, PtMe), 2.46 (s, J (195Pt-H) )
14.0 Hz, 3 H, PtNCMe), 3.87 and 3.91 (s, 3 H each, MeO, Me′O),
3
3
7.01 (“d”, J (HaHb) ) 9.0 Hz, 2 H, Ar Ha), 7.10 (“d”, J (HaHb)
) 9.0 Hz, 2 H, Ar Hb), 7.27 (“d”, J (HaHb) ) 9.0 Hz, 2 H, Ar′
3
Ha), 7.56 (“d”, 3J (HaHb) ) 9.0 Hz, 2 H, Ar′ Hb), 8.87 (s, 3J -
3
(
195Pt-H) ) 97.9 Hz, 1 H, NCHC′HN′), 8.89 (s, J (195Pt-H) )
33.6 Hz, 1 H, NCHC′HN′). 13C{1H} NMR (75 MHz, acetoni-
trile-d3): δ -10.7 (1J (195Pt-C) ) 675 Hz, PtMe), 56.4 and 56.5
(OMe and OMe′), 115.0 and 115.9 (Ar Cm and Ar′ Cm), 125.6
(3J (195Pt-C) ) 20.4 Hz, Ar′ Co), 125.9 (Ar Co), 140.9 and 141.4
(Ar Cipso and Ar′ Cipso), 161.5 and 163.0 (Ar Cp and Ar′ Cp),
160.7 (2J (195Pt-C) ) 56.8 Hz, NCHC′HN) and 172.6 (NCMeC-
′MeN) (for a complete assignment, see Table 4). UV-vis
(acetonitrile): λmax 322 (5.0), 432 (13.2). Anal. Calcd for
C19H22BF4N3O2Pt: C, 37.64; H, 3.66; N, 6.93. Found: C, 38.55;
H, 3.75; N, 6.65.
(TolNdCMeCMedNTol)P tMe2 (1c): From Pt2Me4(µ-SMe2)2
(500 mg, 0.870 mmol) and TolNdCMeCMedNTol (460 mg,
1.740 mmol) in toluene (100 mL); dark purple shimmering
microcrystals (716 mg, 84%). 1H NMR (200 MHz, dichlo-
2
romethane-d2): δ 0.92 (s, J (195Pt-H) ) 86.9 Hz, 6 H, PtMe),
1.43 (s, 6 H, NCMeCMeN), 2.43 (s, 6 H, ArMe), 6.89 (“d”,
3
3J (HaHb) ) 8.3 Hz, 4 H, Ar Ha), 7.29 (“d”, J (HaHb) ) 8.0 Hz,
4 H, Ar Hb). 13C{1H} NMR (75 MHz, dichloromethane-d2): δ
-13.5 (1J (195Pt-C) ) 803 Hz, PtMe), 21.1 (NCMeCMeN), 21.1
(ArMe), 122.0 (Ar Co), 129.5 (Ar Cm), 136.2 (Ar Cp), 145.6 (2J -
195Pt-C) ) 25.7 Hz, Ar Cipso), 171.2 (2J (195Pt-C) ) 16.7 Hz,
[(TolNdCMeCMedNTol)P tMe(NCMe)]+(BF4-) (2c(BF4-).
A solution of 1c (150 mg, 0.31 mmol) in acetonitrile (14 mL)
was cooled to -13 °C, whereupon a 54% solution of HBF4 in
ether (50 µL, 0.37 mmol) was added with a syringe. The
mixture was stirred for 1.5 h, while it was slowly warmed to
ambient temperature. Workup following the procedure for
2a +(BF4-) yielded the product as orange microcrystals (154
mg, 83%). 1H NMR (200 MHz, dichloromethane-d2): δ 0.62
(
NCMeCMeN). UV-vis (acetonitrile): λmax 372 (4.1), 510 sh
(2.0), 541 (2.2). UV-vis (toluene): 453 (3.8), 595 (0.4). Anal.
Calcd for C20H26N2Pt: C, 49.07; H, 5.35; N, 5.72; Pt, 39.85.
Found: C, 49.01; H, 5.29; N, 5.85; Pt, 40.03.
(An NdCMeCMeAn )P tMe2 (1d ): from Pt2Me4(µ-SMe2)2
(600 mg, 1.044 mmol) and AnNdCMeCMedNAn (619 mg,
2.089 mmol) in toluene (130 mL); dark purple shimmering
microcrystals (991 mg, 91%). 1H NMR (300 MHz, dichlo-
2
4
(s, J (195Pt-H) ) 75.1 Hz, 3 H, PtMe), 2.05 (s, J (195Pt-H) )
10.2 Hz, 3 H, NCMeC′MeN), 2.11 (s, J (195Pt-H) ) 13.6 Hz, 3
4
2
romethane-d2): δ 0.95 (s, J (195Pt-H) ) 86.6 Hz, 6 H, PtMe),
H, PtNCMe), 2.13 (s, 3 H, NCMeC′MeN), 2.42 and 2.44 (s, 3
1.45 (s, 6 H, NCMeCMeN), 3.86 (s, 6 H, OMe), 6.95 (“d”,
H each, ArMe and Ar′Me), 6.92 (“d”, J (HaHb) ) 8.3 Hz, 2 H,
3
3
3J (HaHb) ) 9.0 Hz, 4 H, Ar Ha), 7.01 (“d”, J (HaHb) ) 9.0 Hz,
Ar Ha), 7.08 (“d”, 3J (HaHb) ) 8.3 Hz, 2 H, Ar Hb), 7.33 (“d”,
4 H, Ar Hb). 13C{1H} NMR (75 MHz, dichloromethane-d2): δ
-13.5 (1J (195Pt-C) ) 801 Hz, PtMe), 21.1 (NCMeCMeN), 55.8
(OMe), 114.1 (Ar Cm), 123.4 (ArCo), 141.3 (2J (Pt-C) ) 26.0 Hz,
Ar Cipso), 158.2 (Ar Cp), 171.4 (2J (Pt-C) ) 18.2 Hz, NCMeC-
MeN). UV-vis (acetonitrile): λmax 369 (7.6), 510 sh (3.0), 543
(3.2). UV-vis (toluene): 378 (4.0), 550 (0.9), 597 (0.9). Anal.
Calcd for C20H26N2O2Pt: C, 46.06; H, 5.03; N, 5.37; Pt, 37.41.
Found: C, 46.58; H, 4.88; N, 5.76; Pt, 37.31.
3J (HaHb) ) 8.0 Hz, 2 H, Ar′ Ha) and 7.37 (“d”, J (HaHb) ) 8.1
3
Hz, 2 H, Ar′ Hb). 13C{1H} NMR (75 MHz, dichloromethane-
d2): δ -11.9 (PtMe), 3.5 (PtNCMe), 19.9 and 21.6 (NCMeC-
′MeN), 21.2 (ArMe and Ar′Me), 121.9 and 122.3 (Ar Co and Ar′
Co), 130.2 (Ar Cm and Ar′ Cm), 138.6 and 138.7 (Ar Cp and Ar′
Cp), 142.7 and 143.1 (Ar Cipso and Ar′ Cipso), 173.8 and 183.7
(NCMeC′MeN). UV-vis (acetonitrile): λmax 314 (4.8). Anal.
Calcd for C21H26BF4N3Pt: C, 41.87; H, 4.35; N, 6.98. Found:
C, 41.66; H, 4.27; N, 7.19.
[(TolNdCHCHdNTol)P tMe(NCMe)]+(BF 4-) (2a (BF 4-)).
A solution of 1a (100 mg, 0.217 mmol) in acetonitrile (7 mL)
was cooled to -13 °C, whereupon a 54% solution of HBF4 in
ether (31 µL, 0.23 mmol) was added with a syringe. The
mixture was stirred for 2 h, while it was slowly warmed to
ambient temperature. The solvent was removed, and the
residue was washed with several portions of ether. The solid
was dissolved in a minimum amount of dichloromethane. The
product was obtained as an orange powder (89 mg, 72%) by
the addition of pentane. 1H NMR (200 MHz, dichloromethane-
d2): δ 1.08 (s, 2J (195Pt-H) ) 76.6 Hz, 3 H, PtMe), 2.43 and
2.45 (s, 3 H each, ArMe, Ar′Me), 2.44 (s, 4J (195Pt-H) ) 13.9
[(An NdCMeCMedNAn )P tMe(NCMe)]+(BF4-) (2d(BF4-)).
A solution of 1d (100 mg, 0.19 mmol) in acetonitrile (10 mL)
was cooled to -13 °C, whereupon a 54% solution of HBF4 in
ether (26 µL, 0.19 mmol) was added with a syringe. The
mixture was stirred for 1 h, while it was slowly warmed to
ambient temperature. The product was obtained as orange
crystals after recrystallization from acetonitrile/toluene (107
mg, 88%). 1H NMR (200 MHz, dichloromethane-d2): δ 0.65
2
4
(s, J (195Pt-H) ) 75.1 Hz, 3 H, PtMe), 2.06 (s, J (195Pt-H) )
10.3 Hz, 3 H, NCMeC′MeN), 2.15 (s, 3 H, NCMeC′MeN), 2.17
4
(s, J (195Pt-H) ) 13.6 Hz, 3 H, PtNCMe), 3.86 and 3.87 (s, 3
3
Hz, 3 H, PtNCMe), 7.18 (“d”, J (HaHb) ) 8.3 Hz, 2 H, Ar Ha),
H each, OMe and OMe′), 6.97 (“d”, J (HaHb) ) 9.1 Hz, 2 H, Ar
Ha), 7.03, (“d”, J (HaHb) ) 9.1 Hz, 2 H, Ar Hb) 7.08 (“d”, J (HaHb)
) 9.1 Hz, 2 H, Ar′ Ha), 7.15 (“d”, J (HaHb) ) 9.1 Hz, 2 H, Ar′
Hb). 13C{1H} NMR (75 MHz, dichloromethane-d2): δ -11.7
(PtMe), 3.6 (PtNCMe), 20.0 and 21.7 (NCMeC′MeN), 55.9 and
56.1 (OMe and OMe′), 114.8 and 114.8 (Ar Cm and Ar′ Cm),
123.8 and 123.8 (ArCo and Ar′Co), 138.2 and 138.7 (Ar Cipso
and Ar′ Cipso), 159.4 and 159.8 (ArCp and Ar′Cp), 173.6 and
183.9 (NCMeC′MeN). UV-vis (acetonitrile): λmax 306 (4.4),
393 (5.1). Anal. Calcd for C21H26BF4N3O2Pt: C, 39.76; H, 4.13;
N, 6.62. Found: C, 40.55; H, 4.08; N, 7.32.
3
3
7.31 (“d”, J (HaHb) ) 8.4 Hz, 2 H, Ar Hb), 7.38 (“d”, J (HaHb)
) 8.9 Hz, 2 H, Ar′ Ha), 7.45 (“d”, J (HaHb) ) 8.8 Hz, 2 H, Ar′
3
Hb), 8.94 (s, J (195Pt-H) ) 99.8 Hz, 1 H, NCHC′HN′), 8.98 (s,
3
3J (195Pt-H) ) 35.6 Hz, 1 H, NCHC′HN′). 13C{1H} NMR (75
MHz, acetonitrile-d3): δ -10.8 (1J (195Pt-C) ) 674 Hz, PtMe),
21.1 and 21.3 (ArMe and Ar′Me), 123.8 (s, Ar Co), 123.8 (3J (195
-
Pt-C ) 19.0 Hz), Ar′ Co), 130.5 and 131.2 (Ar Cm and Ar′ Cm),
141.0 and 142.8 (Ar Cp and Ar′ Cp), 145.2 and 145.8 (Ar Cipso
and Ar′ Cipso), 162.5 (2J (195Pt-C) ) 57.0 Hz, NCHC′HN), 173.5
(NCHC′HN). UV-vis (acetonitrile): λmax 329 (6.9), 400 (9.9).
Anal. Calcd for C19H22BF4N3Pt: C, 39.74; H, 3.86; N, 7.32.
Found: C, 39.93; H, 3.86; N, 7.16.
[(An NdCHCHdNAn )P tMe(NCMe)]+(BF 4-) (2b(BF 4-)).
A solution of 1b (200 mg, 0.41 mmol) in acetonitrile (15 mL)
was cooled to -13 °C, after which a 54% solution of HBF4 in
ether (60 µL, 0.44 mmol) was added with a syringe. The
mixture was stirred for 2 h, while it was slowly warmed to
[(An NdCHCHdNAn )P tMe3(NCMe)]+(OTf-) (3b(OTf-)).
A solution of 1b (150 mg, 0.30 mmol) in acetonitrile (15 mL)
was cooled to -13 °C, whereupon methyl triflate (35 µL, 0.32
mmol) was added with a syringe. The mixture was stirred
for 4 h, during which it was slowly warmed to ambient
temperature. The solution was filtered, and the solvent was

3966 Organometallics, Vol. 17, No. 18, 1998
J ohansson et al.
removed. The yellow oily residue was washed repeatedly with
ether, leading to the precipitation of red-orange crystals.
Recrystallization from acetonitrile/toluene and drying in vacuo
gave the product as orange crystals (189 mg, 89%). 1H NMR
(b) E ) 0.85 V, Au Wor k in g Electr od e. The solution was
electrolyzed for 3930 s, at which point 1.31 faraday/mol had
been transferred. 1H NMR showed a product composition
consisting of 2b (33%), 3b (32%), 4b (26%), and 5b (8%).
2
(300 MHz, acetonitrile-d3): δ 0.53 (s, J (195Pt-H) ) 76.3 Hz,
(c) E ) 0.95 V, P t Wor k in g Electr od e. The solution was
electrolyzed for 3000 s, at which point 1.21 faraday/mol had
been transferred. 1H NMR showed a product composition
consisting of 2b (35%), 3b (31%), 4b (28%), and 5b (6%).
3H, PtMeax), 0.94 (s, 2J (195Pt-H) ) 69.9 Hz, 6 H, PtMeeq), 3.87
3
(s, 6 H, OMe), 7.10 (“d”, J (HaHb) ) 9.0 Hz, 4 H, Ar Ha), 7.30
(“d”, 3J (HaHb) ) 9.0 Hz, 4 H, Ar Hb), 8.81 (s, 3J (195Pt-H) )
26.6 Hz, 2 H, NCHCHN). 13C{1H} NMR (75 MHz, acetonitrile-
d3): δ -7.1 (PtMeax), -2.9 (PtMeeq), 56.5 (OMe), 115.7 (Ar Cm),
125.3 (Ar Co), 140.2 (Ar Cipso), 162.0 (Ar Cp), 165.8 (NCHCHN).
UV-vis (acetonitrile): λmax 438 (12.4). Anal. Calcd for
(d ) E ) 0.85 V, P t Wor k in g Electr od e. The solution was
electrolyzed for 4500 s, at which point 1.07 faraday/mol had
been transferred. However, at this point a considerable
current (>0.1 mA) still flowed. 1H NMR showed a product
composition consisting of 2b (50%) and 3b (50%).
C
22H28F3N3O5PtS: C, 37.82; H, 4.04; N, 6.01; Pt, 27.92.
Found: C, 37.98; H, 3.32; N, 6.52; Pt, 27.23.
Oxid a tion of 1b w ith Cp 2F e+P F 6- in Aceton itr ile. Two
NMR tubes equipped with ground-glass joints were each
loaded with 1b (5.4 mg, 0.011 mmol) and Cp2Fe+PF6- (3.4 mg,
0.010 mmol). Acetonitrile-d3 (0.7 mL) was added to each tube
by vacuum transfer, and the tubes were sealed under vacuum.
One tube was placed in a water bath at +55 °C. A rapid
Attem p ted P r ep a r a tion of Sa lts of [(An NdCHCHd
NAn )P tMe2(NCMe)2]2+ (4b a n d 5b). An NMR tube equipped
with a ground-glass joint was loaded with 2b(BF4-) (3.0 mg,
4.9 µmol) and MeOTf (0.6 µL, 5 µmol). Acetonitrile-d3 (0.7 mL)
was added by vacuum transfer, and the tube was sealed under
vacuum. 1H NMR analysis of the sample showed no reaction
at ambient temperature. When the tube was slowly heated
to 75 °C, 2b was completely consumed to give only uncharac-
terizable products. No traces of 4b or 5b were observed.
reaction ensued; the green suspension initially turned dark
1
red and then, within
/ min, changed to an orange-red
2
transparent solution. The 1H NMR spectrum showed the
products to be 2b (56%), 3b (44%), and traces of methane. The
composition remained constant for days at ambient temper-
ature. The reaction in the second tube was started at -25 °C
with a gradual temperature increase to ambient temperature
over a period of 4 days. These conditions resulted in a very
slow reaction, in which 2b had not completely dissolved until
the mixture was at ambient temperature. Nevertheless, the
final product composition, as revealed by 1H NMR, was
essentially identical with that in the first tube (57% of 2b and
43% of 3b).
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion of 1b
a n d 3b(OTf-). Crystals of 1b and 3b(OTf-) were obtained
by crystallization from dichloromethane and acetonitrile/
toluene, respectively. X-ray data were collected on a Siemens
SMART CCD diffractometer34 using graphite-monochromated
Mo KR radiation. Data collection method: ω-scan, range 0.6°,
crystal to detector distance 5 cm; further information given
in Table 2. Data reduction and cell determination were carried
out with the SAINT and XPREP programs.34 Absorption
corrections were applied by the use of the SADABS program.35
Con sta n t-P oten tia l Electr och em ica l Oxid a tion of 1b
in Aceton itr ile. All electrolysis experiments were performed
in an H-shaped cell, the compartments of which were sepa-
rated by a medium-frit glass junction. Gold- or platinum-
gauze working electrodes were used. In the experiments, 1b
(10 mg, 0.020 mmol) was suspended in acetonitrile/0.05 M
Me4N+BF4- (20 mL) in the electrolysis cell. The solutions were
electrolyzed at constant potentials until the current had
dropped to less than 0.1 mA. The electrolyzed solutions were
immediately transferred to round-bottomed flasks, and the
solvent was removed. The residue was extracted with dichlo-
romethane (leaving the electrolyte behind), and the extract was
filtered. Dichloromethane was removed from the filtrate, and
The structures were determined and refined using the
SHELXTL program package.36 The non-hydrogen atoms were
refined with anisotropic thermal parameters; hydrogen posi-
tions were calculated from geometrical criteria with isotropic
thermal parameters. Final figures of merit are included in
Table 2.
Positional and equivalent isotropic thermal parameters for
non-hydrogen atoms for 1b and 3b(OTf-) are listed in Table
3. Lists of thermal parameters, hydrogen parameters, and all
bond lengths, bond angles, and torsion angles may be obtained
as Supporting Information.
1
the residue was analyzed by H NMR spectroscopy.
Ack n ow led gm en t. We gratefully acknowledge gen-
erous support from the Norwegian Research Council,
NFR (stipends to L.J . and C.R.), and from Statoil under
the VISTA program, administered by the Norwegian
Academy of Science and Letters.
(a ) Electr olysis a t E ) 0.95 V vs F c, Au Wor k in g
Electr od e, 1.6 fa r a d a y/m ol of Ch a r ge. The 1H NMR
spectrum showed 2b (20%), 3b (18%), cis,cis-[(AnNdCHCHd
NAn)PtMe2(NCMe)2]2+ (4b, 53%), and cis,trans-[(AnNdCHCHd
NAn)PtMe2(NCMe)2]2+ (5b, 9%). 1H NMR of 4b (300 MHz,
2
acetonitrile-d3): δ 1.59 (s, J (195Pt-H) ) 71.4 Hz, 3 H, PtMe),
2
4
1.61 (s, J (195Pt-H) ) 61.2 Hz, 3 H, PtMe), 2.61 (s, J (195Pt-
Su p p or tin g In for m a tion Ava ila ble: Tables and figures
giving details of the full NMR characterization of 2b and tables
of thermal parameters, hydrogen parameters, and all bond
lengths, bond angles, and torsion angles and figures giving
additional views of 1b and 3b(OTf-) (22 pages). Ordering
information is given on any current masthead page.
H) ) 10.9 Hz, 3 H, PtNCMe), 2.62 (s, J (195Pt-H) ) 12.4 Hz,
4
3 H, PtNCMe), 3.88 (s, 3 H, MeO), 3.93 (s, 3 H, MeO), 7.12
(“d”, J (HaHb) ) 9.0 Hz, 2 H, Ar Ha), 7.21 (“d”, J (HaHb) ) 9.1
Hz, 2 H, Ar Hb), 7.29 (“d”, J (HaHb) ) 9.2 Hz, 2 H, Ar′ Ha), 7.63
3
(“d”, J (HaHb) ) 9.1 Hz, 2 H, Ar′ Hb), 8.75 (d, J (H-H) ) 1.0
3
Hz, 3J (195Pt-H) ) 94.4 Hz, 1 H, NCHCHN), 8.85 (d, J (H-H)
3
) 1.0 Hz, J (195Pt-H) ) 30.4 Hz, 1 H, NCHC′HN). 1H NMR
OM9801498
4
of 5b (300 MHz, dichloromethane-d2): δ 2.66 (s, J (195Pt-H)
1
) 10.1 Hz, 6 H, PtNCMe). H NMR of 5b (300 MHz, acetoni-
trile-d3): δ 1.85 (s, 2J (195Pt-H) ) 62.7 Hz, 6 H, PtMe), 3.89 (s,
6 H, MeO), 7.16 (“d”, J (HaHb) ) 9.1 Hz, 4 H, Ar Ha), 7.39 (“d”,
J (HaHb) ) 9.1 Hz, 4 H, Ar Hb), 9.05 (s, 3J (195Pt-H) ) 25.6 Hz,
2 H, NCHCHN).
(34) SMART and SAINT Area-detector Control and Integration
Software; Siemens Analytical X-ray Instruments Inc., Madison, WI.
(35) Sheldrick, G. M. Private communication, 1996.
(36) Sheldrick, G. M. SHELXTL, Version 5; Siemens Analytical
X-ray Instruments Inc., Madison, WI.