Synthesis and structural characterization of a fluorinated α-diimine platinum(II) complex

Article abstract of DOI:10.1021/om060897u

The preparation of an electron-deficient (N-N)PtCl₂ complex in which the diimine C is substituted by CF₃ is described. Thus, reactions between the diimine ligand 1 (R = CF₃, R′ = 2,6-dimethylphenyl) and PtCl₂ (thermal reaction) or KPtCl ₃(C₂H₄) (photochemically activated) furnishes the corresponding (N-N)PtCl₂ complex 2. The ligand 1 and its Pt complexes 2, as well as related (N-N)PtCl₂ complexes 3 (R = CO ₂CD₃, R′ = 2,6-dimethylphenyl; a methanolysis product) and 4 (R -H, R′ = 2,6-dimethylphenyl) have been characterized by X-ray crystallography.

Full text of DOI:10.1021/om060897u

Organometallics 2007, 26, 1581-1587
1581
Articles
Synthesis and Structural Characterization of a Fluorinated
r-Diimine Platinum(II) Complex
Martin Lersch,§ Alexander Krivokapic, and Mats Tilset*
Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
ReceiVed September 29, 2006
The preparation of an electron-deficient (N-N)PtCl2 complex in which the diimine C is substituted by
CF3 is described. Thus, reactions between the diimine ligand 1 (R ) CF3, R′ ) 2,6-dimethylphenyl) and
PtCl2 (thermal reaction) or KPtCl3(C2H4) (photochemically activated) furnishes the corresponding (N-
N)PtCl2 complex 2. The ligand 1 and its Pt complexes 2, as well as related (N-N)PtCl2 complexes 3 (R
) CO2CD3, R′ ) 2,6-dimethylphenyl; a methanolysis product) and 4 (R ) H, R′ ) 2,6-dimethylphenyl)
have been characterized by X-ray crystallography.
significant consequences for the mechanistic features and
inherent reactivities of diimine-Pt complexes in C-H activation
chemistry.7
Introduction
Metal complexes with 1,4-diaza-1,3-butadiene ligands1,2
(N-N ) R′NdC(R)C(R)dNR′; R′ ) alkyl, aryl), which belong
to the class of R-diimine ligands, have inspired considerable
research efforts. For example, such complexes have found
widespread use in efficient olefin polymerization catalysts3,4 and
in C-H bond activation reactions.5-7 The interesting photo-
chemical and photophysical properties of such compounds have
also been thoroughly investigated.8 The substituent R at the
imine carbon is most commonly a hydrogen atom or an alkyl
or aryl group. These R-diimines are usually prepared by
condensation of primary amines or anilines with 1,2-diones (R
) alkyl, aryl) or glyoxal (R ) H). The strong π-acceptor ability
that results from a rather low-lying π* LUMO is a characteristic
feature of the R-diimine systems.9 This property greatly influ-
ences the structure and reactivity of R-diimine metal coordina-
tion compounds.2 Semiempirical calculations have shown that
the introduction of R ) perfluoroalkyl, perfluoroaryl groups
should further lower the LUMO and further enhance the
π-acceptor ability of the ligands.10 This would render the metal
center of their complexes more electron deficient, with possibly
There have been a limited number of reports that describe
the incorporation of R ) perfluoroalkyl groups in R-diimines.
Diel et al.10 prepared R-diimines with R ) CF3, C6F5, R′ )
SiMe3 from perfluorobiacetyl and decafluorobenzil, respectively.
Uneyama and co-workers11,12 synthesized a variety of R-di-
imines with R ) CF3, C3F7, R′ ) 4- and 2,6-substituted phenyl
employing a CO-promoted Pd-catalyzed homocoupling of
fluorinated imidoyl iodides. An alternative approach was
presented by Sadighi et al.,13 starting from an analogous
fluorinated imidoyl iodide but using SmI2 as the reductive
coupling reagent (R ) CF3, R′ ) 3,5-(CF3)2C6H3). Finally, Diel
et al. recently described the reductive coupling of perfluoroalkyl
and perfluoroaryl cyanides with [Cp2TiCl]2 followed by pro-
tonolysis to liberate the formed R-diimines (R ) CF3, C6F5, R′
) H).14
Despite the aforementioned reports of R-diimines with
perfluorinated hydrocarbyl groups at the diimine “backbone”
positions, the only metal complex of such diimines known to
date is a species in which a Pd(PPh3)2 moiety is presumed to
be coordinated in an η2(C,N) fashion to one imine.13 This
complex was not isolated and thence not structurally character-
ized; its structure was proposed solely on the basis of 31P and
19F NMR spectra, which indicated nonequivalence of the halves
of the diimine ligand and two nonequivalent phosphines. Metal
chelate complexes of the fluorinated diimines have not yet been
described.
* To whom correspondence should be addressed. E-mail: mats.tilset@
kjemi.uio.no.
§ Present address: Borregaard Ind. Ltd., P.O.B. 162, N-1701 Sarpsborg,
Norway.
(1) tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch., B 1981,
36B, 823-832.
(2) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151-
In this contribution we present thermal and photochemical
pathways to (N-N)PtCl2 (2; N-N ) R′NdC(R)C(R)dNR′
with R ) CF3, R′ ) 2,6-Me2C6H3) together with X-ray
structures of the ligand 1, the complex 2, a methanolysis product
239.
(3) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203.
(4) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-315.
(5) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed.
1998, 37, 2181-2192.
(6) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259-320.
(7) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471-2526.
(8) van Slageren, J.; Klein, A.; Zalis, S. Coord. Chem. ReV. 2002, 230,
193-211.
(9) Reinhold, J.; Benedix, R.; Birner, P.; Hennig, H. Inorg. Chim. Acta
1979, 33, 209-213.
(10) Diel, B. N.; Deardorff, P. J.; Zelenski, C. M. Tetrahedron Lett. 1999,
40, 8523-8527.
(11) Amii, H.; Kohda, M.; Seo, M.; Uneyama, K. Chem. Commun. 2003,
1752-1753.
(12) Uneyama, Kenji, Amii, Hideki, and Koda, Mitsuhiro. Jpn. Patent
2001253861, 2001.
(13) Sadighi, J. P.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E.
Tetrahedron Lett. 2003, 44, 8073-8076.
(14) Diel, B. N.; Deardorff, P. J.; Zelenski, C. M.; Incarvito, C.; Liable-
Sands, L.; Rheingold, A. M. Inorg. Chim. Acta 2004, 357, 3902-3910.
10.1021/om060897u CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/16/2007

1582 Organometallics, Vol. 26, No. 7, 2007
Lersch et al.
Scheme 1a
3 (R ) CO2CD3) that is derived from 2, and the related Pt
complex 4 (R ) H, R′ ) 2,6-Me2C6H3), which is of interest
for comparison of certain structural features found in 2 and 3.
Results
1. Preparative Work. Attempts at metalating the R-diimine
ligand 1 to furnish 2 under thermal conditions with (SMe2)2-
PtCl2 and KPtCl3(C2H4) (Zeise’s salt) in toluene or methanol
were unsuccessful. This is in line with previous observations.13
a Legend: (a) KPtCl3(C2H4), CD3OD, UV light, then ambient
temperature in the dark; (b) (PhCN)2PtCl2, PhCl, 100 °C, 4 days; (c)
PtCl2, PhCl, 140 °C, 3 days.
to be facilitated by UV irradiation.20 It should also be noted
that some metallic Pt is deposited during the reaction, which
also opens the possibility for Pt(0)-mediated C-F activation.21-23
An NMR tube containing equimolar amounts of 1 and Zeise’s
salt in methanol-d4 which was exposed to 20 h of irradiation
from the chromatography UV lamp furnished 2 as dark crystals
in up to 48% yield, but only after standing in the dark for 15
weeks. We have not attempted a more elaborate study of the
photochemistry of the system with more sophisticated photo-
chemical equipment.
Zeise’s anion quantitatively yields the solvento complex
trans-PtCl2(C2H4)(CH3OH) in methanol solution.24 Our results
indicate that 1 does not thermally react with this species. On
the basis of the observation that 2 is formed by a photochemical
reaction and not by a thermal reaction from Zeise’s salt, we
speculate that photochemically generated16 cis-PtCl2(CH3OH)-
(L), where L ) CH3OH, C2H4, may be a crucial intermediate.
The primitive photochemistry experiments above established
that 2 is indeed isolable and stable. In a search for Pt precursors
thermally more robust than Zeise’s salt or (SMe2)2PtCl2, we
were intrigued by a report that (PhCN)2PtCl2 had been reacted
with unsymmetrical R-diimines in chloroform or toluene at their
reflux temperatures.25 Accordingly, when 1 was stirred with an
equimolar amount of (PhCN)2PtCl2 for 3 days at 100 °C in
chlorobenzene, ca. 25% conversion to the desired product 2 was
seen. Unfortunately, excess (PhCN)2PtCl2 could not be separated
from 2. Attempts to remove excess (PhCN)2PtCl2 by reaction
with 2,2′-bipyridyl resultedsnot too surprisinglysin complete
loss of the ligand 1 from 2, presumably with the formation of
(2,2′-bipyridyl)PtCl2.
1
At ambient temperature, no reactions were detected by H or
19F NMR; prolonged heating decomposed the Pt precursors to
Pt black. Similarly, attempts at using microwave heating and
medium pressures (<10 bar) failed, producing Pt black within
minutes. In water/methanol mixtures, K2PtCl4 and 1 were
soluble but decomposed to Pt black upon heating.
However, in one NMR sample of the ligand 1 and KPtCl3-
(C2H4) in methanol-d4 which had only been moderately heated
so that the starting materials remained mainly intact, dark
crystals had formed after 6 months. These were indeed shown
to be the desired product 2 by an X-ray diffraction analysis (vide
infra), and this was the first indication that this product is not
only accessible but also quite stable. A clue to what might have
mediated the reaction was provided by the fact that the NMR
sample had been exposed to indirect daylight.
Zeise’s salt, known since 1827, continues to fascinate
chemists.15 Its photochemistry has been well studied and differs
significantly from its thermal chemistry.16,17 Inspired by this,
we were gratified to find that attempts to facilitate the metalation
of 1 with UV light from an 8 W chromatography UV lamp
gave promising results. After 2 weeks of continuous irradiation
of a methanol-d4 solution of 1 and Zeise’s salt in an NMR tube,
crystals formed in the solution. However, these were identified
as 3 by X-ray crystallography (vide infra) and are likely derived
from 2 by methanolysis of the trifluoromethyl groups (Scheme
1). Characteristic 19F peaks appeared and later disappeared in
the 19F NMR spectra during the course of the reaction, signaling
the presence of unidentified intermediates. Hydrolysis of tri-
fluoromethyl groups has been described18,19 and is also known
Interestingly, the direct reaction of 1 with stoichiometric
quantities of PtCl2sironically, the simplest Pt(II) source con-
ceivable for this chemistrysfor 3 days in chlorobenzene at reflux
(20) Chaignon, P.; Cortial, S.; Guerineau, V.; Adeline, M. T.; Giannotti,
C.; Fan, G.; Ouazzani, J. Photochem. Photobiol. 2005, 81, 1539-1543.
(21) Colmenares, F.; Torrens, H. J. Phys. Chem. A 2005, 109, 10587-
10593.
(15) Benedetti, M.; Fanizzi, F. P.; Maresca, L.; Natile, G. Chem.
Commun. 2006, 1118-1120.
(22) Jasim, N. A.; Perutz, R. N.; Whitwood, A. C.; Braun, T.; Izundu,
J.; Neumann, B.; Rothfeld, S.; Stammler, H.-G. Organometallics 2004, 23,
6140-6149.
(16) Natarajan, P.; Adamson, A. W. J. Am. Chem. Soc. 1971, 93, 5599-
5605.
(23) Hofmann, P.; Unfried, G. Chem. Ber. 1992, 125, 659-661.
(24) Plutino, M. R.; Otto, S.; Roodt, A.; Elding, L. I. Inorg. Chem. 1999,
38, 1233-1238.
(17) Adamson, A. W., Fleischauer, P. D., Eds. Concepts of Inorganic
Photochemistry; Wiley: New York, 1975.
(18) Kimoto, H.; Cohen, L. A. J. Org. Chem. 1979, 44, 2902-2906.
(19) Kobayashi, Y.; Kumadaki, I. Acc. Chem. Res. 1978, 11, 197-204.
(25) Yang, K.; Lachicotte, R. J.; Eisenberg, R. Organometallics 1997,
16, 5234-5243.

A Fluorinated R-Diimine Pt(II) Complex
Organometallics, Vol. 26, No. 7, 2007 1583
Figure 2. UV-visible spectra for 2-4 and 6 in dichloromethane.
Table 1. 195Pt NMR Data for (N-N)PtCl2 Complexes
Figure 1. 19F NMR spectrum of 2 with coupling 4JPt-F ) 4.4 Hz.
complex
R
195Pt (δ)
under an atmosphere of N2 gave pure 2 in 11% yield (Scheme
1, reaction c). Excess ligand could easily be removed with
pentane, and excess PtCl2 was removed by centrifugation or
by filtration of a dichloromethane solution of the crude product
through Celite. The unreacted starting materials could be
recovered and resubmitted to the reaction conditions, giving
another crop of product for a combined 19% yield.
The reactivity of PtCl2 toward electron-deficient 1 came as a
surprise to us, despite previous reports of direct reactions with
less electron-deficient diimine ligands.26 The diimine ligand 1
and the product 2 show a remarkable stability, even after several
days in refluxing chlorobenzene.
2
3
4
6
CF3
COOCD3
H
-1183
-1559
-2100
-2179
CH3
2 shows a characteristic quartet from the CF3 group due to
coupling with fluorine (1JC-F ) 285 Hz). No Pt satellites were
observed in the 13C NMR spectrum, possibly due to increased
chemical shift anisotropy (CSA) at the high field used.27-31 The
imine C resonance at δ 161.7 is significantly broadened (half-
height peak width ca. 100 Hz), due to the proximity of spin-
active N, F, and Pt nuclei, but these couplings could not be
resolved.
Since 3 was furnished in H NMR silent methanol-d4, the
identity of this complex with its H NMR unobservable ester
group remained a mystery until the complex was structurally
characterized by X-ray diffraction. The ester functionality does,
however, show a characteristic IR absorption at ν(CO) 1753
2. Spectroscopic Characterization. Complete spectroscopic
and characterization data for all new compounds in this study
are provided in the Experimental Section. Only some notable
features are highlighted in the following.
1
1
The fluorinated ligand 1 displays two broad signals with
1
different intensities for the N-aryl methyl signals in the H (δ
cm-1
.
1.54 and 2.08; 1:0.83 intensity ratio) and for the CF3 groups in
the 19F (δ -66.1 and -68.4) NMR spectra. This may be caused
by the coexistence of E and Z configurations relative to the
CdN bonds (see Discussion). At elevated temperatures in
The 195Pt NMR spectra of the metal complexes 2-4 and 6
show a clear trend in their chemical shifts, which correlates with
the electron-withdrawing ability of the R groups at the diimine
ligand backbone (Table 1). More electron-withdrawing groups
result in a downfield shift. The colors of the metal complexes
in dichloromethane-d2 solutions are red-purple (2, 3) and orange-
red (4, 6), and these differences are manifested in the UV-
visible spectra (Figure 2).
3. Single-Crystal X-ray Crystallography. X-ray-quality
crystals of the fluorinated ligand 1, which is an oil at ambient
temperature, were obtained by storing the pure compound in a
freezer at -20 °C. X-ray-quality crystals of 2 and 3 appeared
in the NMR-tube reaction mixtures after 6 months and 2 weeks,
respectively (see Experimental Section for details). X-ray-quality
crystals of 4 were obtained by crystallization from a saturated
dichloromethane-d2 solution. Details of the structure determina-
tions are given in the Experimental Section. Table 2 gives
experimental and crystallographic data, selected bond distances
1
toluene-d8, the H NMR spectra of 1 underwent gradual and
reversible changes so that the two N-aryl methyl peaks
underwent coalescence at ca. 70 °C. Conversely, gradual cooling
to -80 °C resulted in sharpening, splitting, and further broaden-
ing of peaks, indicative of the intervention of several unidentified
dynamic processes. Interestingly, only 1 and the related R′ )
2,6-iPr2C6H3 substituted ligand system have been reported to
exhibit such behavior, whereas ligands with R′ ) 4-substituted
aryl groups show the expected NMR spectra that indicate simple,
symmetrical diimines with no discernible intervention of other
species.11 With the notable exception of 1, all other ligands and
metal complexes discussed in the following exhibit the expected
C2V symmetry by NMR.
The Pt complex 2 was characterized by 1H, 19F, and 13C NMR
(the last was assigned by HSQC and HMBC spectrassee the
Experimental Section for details) and elemental analysis. The
19F NMR spectrum of 2 is quite diagnostic (Figure 1), as the
singlet for CF3 at δ -60.9 in dichloromethane-d2 exhibits
coupling to 195Pt (4JPt-F ) 4.4 Hz). The 13C NMR spectrum of
(27) Lallemand, J. Y.; Soulie, J.; Chottard, J. C. J. Chem. Soc., Chem.
Commun. 1980, 436-438.
(28) Pregosin, P. S. Coord. Chem. ReV. 1982, 44, 247-291.
(29) Dechter, J. J.; Kowalewski, J. J. Magn. Reson. 1984, 59, 146-149.
(30) Anklin, C. G.; Pregosin, P. S. Magn. Reson. Chem. 1985, 23, 671-
675.
(26) Buffin, B. P.; Kundu, A. Inorg. Chem. Commun. 2003, 6, 680-
684.
(31) Skvortsov, A. N. Russ. J. Gen. Chem. 2000, 70, 1023-1027.

1584 Organometallics, Vol. 26, No. 7, 2007
Table 2. Crystal Data and Structure Refinement Details for 1-4
Lersch et al.
1
2
3
4
formula
formula wt
C20H18F6N2
400.36
C20H18Cl2N2F6Pt
666.36
C22H24Cl2N2O4Pt
646.42
C18H20Cl2N2Pt ‚CH2Cl2
615.30
cryst syst
color
orthorhombic
yellow
monoclinic
black
monoclinic
black
orthorhombic
red
space group
Pnna
P21/c
P21/n
Pnna
a/Å
b/Å
c/Å
17.3068(10)
13.2544(8)
8.1088(5)
90
90
90
8.3808(2)
12.9274(2)
19.9372(4)
90
100.8220(10)
90
7.1784(12)
18.192(3)
18.014(3)
90
93.953(3)
90
16.485(3)
18.169(4)
7.1720(14)
90
90
90
R/deg
â/deg
γ/deg
V/Å3
1860.09(19)
4
2121.62(7)
4
2346.8(7)
4
2148.1(7)
4
Z
T/K
105
105
105
105
F(000)
824
1272
1256
1184
radiation
0.710 73
2.35-28.29
15 957
2252
1262/0/127
1.1102
0.0434, 0.0478
0.0684, 0.0736
-0.23, 0.20
0.710 73
1.89-40.34
36 018
13 009
9914/0/280
1.0891
0.0229 , 0.0264
0.0332, 0.0395
-1.54, 3.26
0.710 73
1.59-28.35
22 632
5650
4679/0/280
1.0230
0.0189, 0.0132
0.0239, 0.0138
-1.40, 0.89
0.710 73
2.24-28.31
19 739
2716
2208/0/124
1.0897
0.0152, 0.0174
0.0200, 0.0224
-0.66, 1.12
θ range/deg
no. of rflns measd
no. of unique rflns
no. of data/restraints/params
goodness of fit, F
R1, wR2 (I > 3σ(I))
R1, wR2 (all data)
min, max diff peak/e Å-3
Table 3. Selected Bond Lengths with Esd’s (Å) and Angles (deg) from Crystal Structures of 1-4
1
2
3
4
Pt-N
1.9962(14)/1.9926(14)
2.2767(4)/2.2745(4)
1.307(2)/1.303(2)
1.449(2)/1.454(2)
1.469(2)
115.9/116.3
121.6/121.2
79.2
96.3/95.8
88.67
4.7
83.9/81.8
2.0047(13)/1.9984(11)
2.2784(5)/2.2832(5)
1.303(2)/1.298(2)
1.453(2)/1.454(2)
1.461(2)
115.8/116.1
121.9/124.2
78.9
96.1/95.8
89.2
2.8
87.9/86.5
2.0055(18)
2.2951(6)
1.292(3)
1.452(3)
1.456(4)
115.1
125.8
79.2
94.6
91.6
Pt-Cl
NdCimine
N-Cipso
Cimine-Cimine
Pt-NdCimine
Pt-N-Cipso
N-Pt-N
1.260(3)
1.421(3)
1.521(4)
N-Pt-Cl
Cl-Pt-Cl
NdCimine-CiminedN
54.9
61.5
0.0
88.4
a
CiminedN-Cipso-Cortho
a Torsion angle between mean planes of CiminedN-Cipso and the aryl group.
and angles for compounds 1-4 are given in Table 3, and Figure
nomenclature priorities of the CH3 and CF3 groups). Structural
features similar to those of 1 have been seen for related diimines
such as ArNdC(CF3)C(CF3)dNAr (Ar ) 4-ClC6H4,11 3,5-
(CF3)2C6H313) and ArNdC(Ph)C(Ph)dNAr (Ar ) 4-MeC6H433).
In solution, the observation of two sets of 1H and 19F signals of
unequal intensities suggests that at least two species with
different E/Z configurations coexist in solution at ambient
temperature (see also the description of even more complex low-
temperature spectra in the Results).
Despite the π-stacking and E configuration, Sadighi et al.
argue that the geometry of these diimines does not preclude
metalation to give κ2(N,N) diimine complexessfor example,
N,N′-diphenyl-9,10-phenanthrenequinonediimine is readily met-
alated by (PhCN)2PdCl2, despite the fact that the free ligand
has the “wrong geometry”.13 The observed problems of meta-
lating ligands with R ) CF3 was rather attributed to the inductive
effect of the CF3 group, which renders the imine N lone pair
poorly nucleophilic. This could well explain the great reluctance
with which 2 forms in this work.
3 gives ORTEP drawings of the structures.
Discussion
1. Structural Properties. When the crystal structure of 1 is
compared with the published structure32 of the analogue that is
substituted with CH3 rather than CF3 at the imine C’s (5), several
features are particularly noteworthy. What first strikes the eye
is that, whereas the structure of 1 is folded (Figure 3), the
previously reported structure of 5 is not (both are schematically
depicted in Chart 1). The methyl groups at the diimine backbone
of 5 are perfectly anti oriented with respect to the central C-C
bond, as demonstrated by the 180° N-C-C-N dihedral angle.
The aryl and methyl groups at each imine moiety in 5 are cis
disposed relative to the CdN bonds (E configuration). On the
other hand, in 1swith the somewhat larger CF3 groups at the
backbonesthe structure is folded, resulting in what more closely
resembles a syn relationship between the trifluoromethyl groups
at the backbone (N-C-C-N dihedral angle of 54.9°). The
folded structure of 1 leads to obvious π-stacking between the
two aryl rings, which is facilitated by the fact that the aryl and
CF3 groups are trans disposed relative to the CdN bonds (but
also here with an E configuration because of the different
The CdN bonds in 1 are shorter (by 0.015 Å) and the central
C-C bond is longer (by 0.017 Å) in the 1,4-diaza-1,3-butadiene
fragment than they are in 5. This is possibly a result of negative
hyperconjugation in 1,34,35 stabilizing the CdN bond. A
(33) Netland, K. A.; Krivokapic, A.; Tilset, M. Manuscript in preparation.
(34) Sleigh, J. H.; Stephens, R.; Tatlow, J. C. J. Chem. Soc., Chem.
Commun. 1979, 921-922.
(32) Kuhn, N.; Steimann, M.; Walker, I. Z. Kristallogr.: New Cryst.
Struct. 2001, 216, 319.

A Fluorinated R-Diimine Pt(II) Complex
Organometallics, Vol. 26, No. 7, 2007 1585
Figure 3. ORTEP drawings of 1-4. Ellipsoids are given at the 30% probability level for 1 and the 50% level for 2-4.
Chart 1
inductive effect of the CF3 group therefore appears to be
counteracted by the improved π-acceptor properties of the
ligand. This observation is contrasted by a comparison of
structural data for TpMe PtMe3 and Tp(CF ) PtMe3. Here, a
substantially weaker coordination of the fluorinated Tp ligand
is suggested by longer Pt-N bonds and slightly shorter Pt-
CH3 bonds;37 however, this comparison is less straightforward
to interpret, as the TpPt systems are Pt(IV) complexes.
2
3 2
The larger steric bulk of the substituents in the backbones of
2 and 3 (R ) CF3, CO2CD3) compared to that in 4 (R ) H) is
evidenced by the smaller Pt-N-C(ipso) angles in 2 and 3. The
effects of sterically demanding substituents in diimine-Ni(II)
complexes for ethene/acrylonitrile copolymerization has recently
been studied by Ziegler and co-workers by DFT methods.38
Substituents such as R ) CF3, t-Bu led to reduced monomer
complexation energies. A slight decrease in polymerization
activity was calculated for R ) CF3 compared with R ) CH3.
An increased activity was observed by introducing t-Bu.
2. Spectroscopic Properties. The 195Pt chemical shift is
comparison of the metric data for 1 and 2 shows that complex-
ation of the diazabutadiene moiety of 1 at PtCl2 leads to a
lengthening of the CdN bonds (by 0.043 and 0.047 Å) and a
shortening of the C-C bond (by 0.052 Å). These changes are
qualitatively expected, since Pt(II) acts as a good electron donor
toward the ligand π system; the ligand LUMO is CdN
antibonding and C-C bonding and is particularly prone to
accept electron density due to the lowering of the LUMO level
by the CF3 substituents.9,10
The changes in metric parameters that occur upon complex-
ation are far from equally pronounced for diimines that are
substituted by H or Me at the backbone. Unfortunately, structural
data are not available for the free ligand of 4 or of complex 6.
However, if we assume that the metric data for the backbone
of ArNdCHCHdNAr (Ar ) 2,4,6-Me3C6H2)36 will be similar
to those of the ligand of 4, it may be inferred that the CdN
bond lengthening will be ca. 0.019 Å and the C-C bond
shortening ca. 0.007 Å upon complexation of the free ligand of
4 at PtCl2. The fact that much greater C-C and CdN bond-
distance changes are seen upon complexation of 1 at PtCl2
underscores the important electronic effects that are introduced
by the backbone CF3 groups.
proportional to and largely determined by the paramagnetic
39,40
shielding term σpara
.
This term is proportional to the
reciprocal electronic excitation energy ∆E. We have recently
found that, for a number of neutral diimine Pt(II) complexes,
there is a good linear correlation between 195Pt chemical shifts
and the inverse of electronic excitation energies ∆E’s obtained
from UV-visible spectra or calculated HOMO-LUMO gaps.41
A similar correlation is seen between 195Pt chemical shifts of
the (N-N)PtCl2 complexes in this work and the respective λmax
values of the lowest energy UV-vis transitions (Figure 4). λmax
values for 2 and 3 were estimated from the broad shoulders.
As expected for an MLCT transition, the electron-withdrawing
groups lead to red-shifted λmax values. Smaller ∆E’s (greater λ
Thus, the bond alterations induced by the CF3 groups in 2
clearly emphasize the strong π-acceptor abilities of the diimine
ligand 1. The Pt-N bonds of 2 are slightly shorter than those
of 3 and 4, suggesting a stronger Pt-N interaction. The reduced
nucleophilicity of the imine N lone pair arising from the
(37) Fekl, U.; van Eldik, R.; Lovell, S.; Goldberg, K. I. Organometallics
2000, 19, 3535-3542.
(38) Szabo, M. J.; Galea, N. M.; Michalak, A.; Yang, S. Y.; Groux, L.
F.; Piers, W. E.; Ziegler, T. J. Am. Chem. Soc. 2005, 127, 14692-14703.
(39) Ramsey, N. F. Phys. ReV. 1950, 78, 699-703.
(40) Griffith, J. S.; Orgel, L. E. Trans. Faraday Soc. 1957, 53, 601-
606.
(35) Dawson, W. H.; Hunter, D. H.; Willis, C. J. J. Chem. Soc., Chem.
Commun. 1980, 874-875.
(36) Mu¨ller, T.; Schrecke, B.; Bolte, M. Acta Crystallogr., Sect. E 2003,
E59, o1820-o1821.
(41) Lersch, M.; Swang, O.; Tilset, M. Manuscript in preparation.

1586 Organometallics, Vol. 26, No. 7, 2007
Lersch et al.
recorded on a Perkin-Elmer One or a Nicolet Magna-IR 550
spectrometer. UV-visible spectra were recorded on an Agilent
8452A spectrophotometer and are reported as λmax/nm (ꢀ × 10-3
/
(M-1 cm-1)). λmax values estimated from shoulders are marked with
sh. Mass spectra were recorded on a ProSpecQ instrument with
EI. MS data are given as m/z (relative intensities, fragment).
Elemental analyses were performed by Ilse Beetz Mikroanalytisches
Laboratorium, Kronach, Germany. All reactions involving metal
complexes were conducted under an atmosphere of N2.
[ArNdC(CF3)C(CF3)dNAr]PtCl2 (2). Method A: Thermal,
Preparative Scale from PtCl2. PtCl2 (15.1 mg, 56.8 µmol) and 1
(20.0 mg, 50.0 µmol) in 1 mL of chlorobenzene were stirred at
reflux (140 °C) under nitrogen for 3 days. After evaporation of the
solvent, the residue was washed with pentane (3 × 3 mL) to remove
excess ligand (17.6 mg, 88% recovered). The residue was dissolved
in dichloromethane and sentrifuged. The precipitated PtCl2 (12.8
mg, 85% recovered) was washed twice with dichloromethane. The
organic phases were combined, 5% heptane was added to facilitate
better powder formation, and the solvents were removed in vacuo
to yield pure 2 as a black powder (3.6 mg, 11%). The recovered
ligand and PtCl2 were resubmitted to the aforementioned reaction
conditions, yielding more 2 after 3 days (2.7 mg), resulting in an
overall yield of 19% and a new portion of recovered, unreacted
starting materials. δH (200 MHz, CD2Cl2): 2.320 (12H, s, Ar CH3),
7.21-7.37 (6H, m, Ar H). δH (200 MHz, CD3OD): 2.356 (12H, s,
Ar CH3), 7.20-7.28 (6H, m, Ar H). δC (150 MHz, CD2Cl2): 18.0
(Ar CH3), 119.9 (q,1JC-F ) 285 Hz, NdCCF3), 128.0 (Cmeta), 129.6
(Cortho), 129.9 (Cpara), 146.1 (Cipso), 161.7 (br, NdCCF3). δF (188
MHz, CD2Cl2): -60.9 (4JPt-F ) 4.4 Hz). δF (188 MHz, CD3OD):
-60.5. δPt (107 MHz, CD2Cl2): -1183. λmax (CH2Cl2): 411 (6.3),
498 (2.6), 603 sh (1.4). m/z (EI): 630 (15, M+ - HCl), 593 (16,
M+ - 2HCl), 524 (49), 385 (63), 200 (100). m/z (EI): found for
C20H17N2F6Cl194Pt, 628.0603 (M+ - HCl); calcd, 628.0611 (1.2
ppm). Anal. Calcd for C20H18Cl2F6N2Pt: C, 36.05; H, 2.72; N 4.20.
Found: C, 35.98; H, 3.19; N, 4.13.
Figure 4. Plot of λmax vs 195Pt chemical shifts of 2-4 and 6.
values) should lead to increased chemical shifts,39,40 as observed.
When the backbone R group is changed from CF3 via CO2CD3
and H to CH3, a substantial 1000 ppm upfield shift is seen for
δ(195Pt). In comparison, chemical shifts change by less than 100
ppm when substituents at the N-aryl groups are changed from
4-CF3 to 4-CH3.41 Thus, electronic tuning of the diimine
properties is greatly facilitated by backbone substitution, due
to the closer proximity of these substituents to Pt.
Conclusion
The fluorinated R-diimine Pt(II) complex 2 can be prepared
by thermal and photochemical routes. The simplest procedure
involves heating the ligand 1 with PtCl2 in chlorobenzene at
reflux for 3 days. Conveniently, the starting materials can be
recovered and reused. The structural features of 2 corroborate
the notion that the metal complex is quite electron deficient.
195Pt chemical shifts of 2-4 and 6 span a range of nearly 1000
ppm and correlate well with λmax values from UV spectra.
Method B: Photochemical, NMR-Tube Scale from Zeise’s
Salt. Zeise’s salt (3.2 mg, 8.3 µmol) and 1 (2.0 mg, 5.0 µmol)
were dissolved in 0.7 mL of methanol-d4. After irradiation with
the chromatography UV lamp (20 h), the NMR tube was left to
stand in the dark. After 15 weeks, dark crystals of 2 had formed
(ca. 1.6 mg, 48%).
Experimental Section
[ArNd(CO2CD3)C(CO2CD3)dNAr]PtCl2 (3). Zeise’s salt (19.6
mg, 50.7 µmol) and 1 (5.6 mg, 14.0 µmol) were dissolved in 700
µL of methanol-d4 and transferred to a NMR tube. The reaction
mixture was irradiated by UV light, and the reaction was monitored
by NMR. After 2 weeks the ligand was consumed, the color of the
solution turned lighter, and thin, dark crystals appeared. Crystals
suitable for X-ray diffraction were collected. The solvent was
evaporated, and the residue was washed with pentane and ether,
dissolved in dichloromethane, and filtered through glass wool in a
pipet. Evaporation of the solvent yielded 3 as a dark purple powder
(4.0 mg, 44%). δH (200 MHz, CD2Cl2): 2.324 (12H, s, Ar CH3),
7.16-7.35 (6H, m, Ar H). δH (200 MHz, CD3OD): 2.321 (12H, s,
Ar CH3), 7.15-7.34 (6H, m, Ar H). δPt (107 MHz, CD2Cl2):
-1559. λmax (CH2Cl2): 400 (4.8), 475 (2.0), 566 sh (1.4). νmax(CH2-
Cl2)/cm-1: 1753 (CO).
General Considerations. In the following section, Ar is an
abbreviation for the 2,6-dimethylphenyl group. Zeise’s salt and PtCl2
were purchased from Strem Chemicals and Alfa Aesar, respectively.
All reagents were used as received. (PhCN)2PtCl2,42 the diimine
complex 6,43 and the ligands 111 and ArNdCHCHdNAr1 were
synthesized according to the published procedures. The UV light
source was of the type typically used to analyze TLC plates, with
an output of approximately 8 W and λ ∼355 nm (the glass in the
irradiated NMR tubes/vials effectively blocks radiation with λ <300
nm). Samples were situated approximately 15 cm from the lamp.
NMR spectra were recorded on Bruker DPX200, DRX500, and
1
AV II 600 instrumetns (200 and 500 MHz for H, 150 MHz for
13C, 188 MHz for 19F, 107 MHz for 195Pt). 1H and 19F NMR
chemical shifts (δ) are reported in ppm relative to TMS via CHDCl2
at 5.32 ppm, CD2Cl2 at 53.8 ppm, CHD2OD at 3.30 ppm, and CFCl3
at 0 ppm. 195Pt NMR chemical shifts are reported in ppm and
referenced relative to TMS using ¥Pt ) 21.496 784 as recommended
by IUPAC.44 All chemical shifts are reported such that lower
frequencies give more negative shifts. 195Pt NMR spectra were
acquired and processed as previously described.43 IR spectra were
[ArNdHCHdNAr]PtCl2 (4). Zeise’s salt (113.8 mg, 294.3
µmol) was dissolved in 2 mL of methanol at 0 °C, the free ligand
ArNdCHCHdNAr (85.6 mg, 323.7 µmol), dissolved in 4 mL of
methanol, was added, and the reaction was stirred at 0 °C for 30
min and another 3 h at room temperature. The precipitate was
isolated using a microfiltration kit and washed with methanol/ether.
The solid was then dissolved in 100 mL of dichloromethane and
the solution refluxed for 1 h. Cooling in a refrigerator produced
dark orange microcrystals of 4, and evaporation of the supernatant
produced 4 as a dark orange powder (65.7 and 24.3 mg, respec-
tively, 58% total yield). δH (200 MHz, CD2Cl2): 2.33 (12H, s, Ar
(42) Anderson, G. K.; Lin, M. Inorg. Synth. 1990, 28, 60-63.
(43) Wong-Foy, A. G.; Henling, L. M.; Day, M.; Labinger, J. A.; Bercaw,
J. E. J. Mol. Catal. A: Chem. 2002, 189, 3-16.
(44) Harris, R. K.; Becker, E. D.; Cabral De Menezes, S. M.; Goodfellow,
R.; Granger, P. Pure Appl. Chem. 2001, 73, 1795-1818.

A Fluorinated R-Diimine Pt(II) Complex
Organometallics, Vol. 26, No. 7, 2007 1587
CH3), 7.20-7.35 (6H, m, Ar H), 8.80 (2H, s, 3JPt-H ) 97 Hz, Cd
NH). δPt (107 MHz, DMSO-d6): -2100. Anal. Calcd for C18H20-
Cl2N2Pt: C, 40.76; H, 3.80; N, 5.28. Found: C, 40.11; H, 3.55; N,
5.14. λmax (CH2Cl2): 378 (5.7), 491 (1.9), 528 (2.0).
using the Sir9247 or Sir9748 programs and refined on F using the
program Crystals.49
The non-hydrogen atoms were refined with anisotropic thermal
parameters; the H atoms were all located in a difference map, but
those attached to carbon atoms were repositioned geometrically.
The H atoms were initially refined with soft restraints on the bond
lengths and angles to regularize their geometry (C-H in the range
0.93-0.98 Å) and isotropic thermal parameters (U(H) in the range
1.2-1.5 × Uequiv of the adjacent atom), after which they were
refined with riding constraints. Table 2 gives the experimental and
crystallographic data. Selected bond lengths, bond angles, and
torsion angles are given in Table 3. Crystallographic data for all
four compounds may be obtained as individual CIF files in the
Supporting Information.
[ArNdC(Me)C(Me)dNAr]PtCl2 (6). Complex 6 was synthe-
sized according to the published procedure from Zeise’s salt and
5.44
δPt (107 MHz, DMSO-d6): -2179. λmax (CH2Cl2): 362 (4.8),
460 (2.4), 493 (2.4).
X-ray Crystallographic Structure Determination of 1-4.
Storing 1 in the form of an orange oil in a freezer at -20 °C resulted
in the formation of crystals after a couple of weeks. Crystals of 2
were collected from an NMR tube initially containing ligand 1 and
Zeise’s salt in methanol-d4 after approximately 6 months on the
lab bench (i.e., method B above, only with ambient light).
Continuous irradiation of Zeise’s salt and 1 in methanol-d4 for 2
weeks produced crystals of 3. Crystals of 4 were grown by
crystallization from saturated dichloromethane. The crystals were
mounted on glass fiber with perfluoro polyether, and the data were
collected at 105 K on a Siemens 1K SMART CCD diffractometer
using graphite-monochromated Mo KR radiation. Data collection
method: ω-scan, range 0.3°, crystal to detector distance 5 cm. Data
reduction and cell determination were carried out with the SAINT
and XPREP45 programs. Absorption corrections were applied by
the use of the SADABS46 program. All of the structures were solved
Acknowledgment. We gratefully acknowledge the Norwe-
gian Research Council (NFR) for generous stipends to M.L.
and A.K., Osamu Sekiguchi for help with translation of a
Japanese patent12 and recording of MS spectra, Dirk Petersen
for recording 13C, HSQC and HMBC spectra of 2 on a Bruker
AV II 600 instrument financed by the NFR, and the NMR
steering committee for allowing the instrument to be used for
this study.
Supporting Information Available: CIF files giving crystal-
lographic data for 1-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM060897U
(45) SMART and SAINT Area-Detector Control and Integration Soft-
ware; Siemens Industrial Automation, Inc., Madison, WI, 1995.
(46) SADABS Area-Detector Absorption Correction; Siemens Industrial
Automation, Inc., Madison, WI, 1996.
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M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
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R. J. Appl. Crystallogr. 1999, 32, 115-119.
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