Multiple C-H bond activation. Threefold-deprotonated 6-phenyl-2,2′-bipyridine as a bridging ligand in dinuclear platinum(II) derivatives

Article abstract of DOI:10.1021/om010913h

The role of a threefold-deprotonated 6-Phenyl-2,2′-bipyridine as a bridging ligand in dinuclear platinum(II) derivatives was analyzed. Activation of C-H bonds was invoked to explain the formation of Ar-H in a process of thermal rearrangement of (Ar)₂Pt(bipy) complexes. The H NMR spectra provided evidence for the formation of one isomer only.

Full text of DOI:10.1021/om010913h

Organometallics 2002, 21, 783-785
783
Mu ltip le C-H Bon d Activa tion . Th r eefold -Dep r oton a ted
6-P h en yl-2,2′-bip yr id in e a s a Br id gin g Liga n d in
Din u clea r P la tin u m (II) Der iva tives
Antonio Zucca,*,† Angelino Doppiu,†,‡ Maria Agostina Cinellu,† Sergio Stoccoro,†
Giovanni Minghetti,† and Mario Manassero*,§
Dipartimento di Chimica, Universita` di Sassari, via Vienna 2, I-07100 Sassari, Italy, and
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita` di Milano,
Centro CNR, via Venezian 21, I-20133 Milano, Italy
Received October 19, 2001
Summary: Unusual platinum(II) trimetalated deriva-
tives have been synthesized and characterized by reaction
of cis-[Pt(R)2(DMSO)2] (R ) Me, Ph) and 6-phenyl-2,2′-
bipyridine. The behavior of the substituted 2,2′-bipyri-
dine as a threefold-deprotonated ligand is unprec-
edented.
The so-called “rollover” metalation of 2,2′-bipyridine,
which entails cleavage of a C-H bond of a pyridine ring,
was first observed several years ago in an iridium(III)
complex.1 Since then, examples of cyclometalated N′,C(3)
species have been described by Wimmer and co-workers
in the case of N-substituted 2,2′-bipyridines.2 Activation
of C-H bonds has been invoked to explain the formation
of Ar-H in a process of thermal rearrangement of
(Ar)2Pt(bipy) complexes:3 no simple mononuclear species
1
was isolated, but the metalation was confirmed by the
X-ray structure of a more complex dinuclear species.
Quite recently, we have reported palladium(II) and
platinum(II) derivatives where 6-alkyl-substituted-2,2′-
bipyridines, LH, are N′- and C(3)-coordinated to give
five-membered cycles.4 The complexes [Pd(L)Cl]2 (HL
) 6-CHMe2-2,2′-bipy) and [Pt(L)Cl(SMe2)] (HL ) 6-CMe3-
2,2′-bipy) have been characterized also in the solid state
by X-ray diffraction.
The H NMR spectra provide evidence for the forma-
tion of one isomer only. The reactions imply elimination
of methane or benzene, respectively. An AB system due
to H(4) and H(5) accompanied by satellites (e.g., for
compound 1: δ 7.65, H(5), 4J Pt-H ) 25 Hz; δ 8.07, H(4),
3J Pt-H ) 52 Hz) confirms coordination of platinum with
the C(3) atom. The Pt-H coupling constants of the
methyls in the coordinated DMSO, equal to 18.3 Hz (1)
and 17.6 Hz (2), are consistent with DMSO trans to a
carbon atom.6
Here we report the unusual behavior of one of the
6-substituted 2,2′-bipyridines currently investigated by
us.
(5) Synthesis of [Pt(L)(R)(DMSO)] (R ) Me, 1; R ) Ph, 2). A
suspension of 6-Ph-2,2′-bipy (HL; 0.100 mmol) and cis-[Pt(R)2(DMSO)2]
(i, R ) Me; ii, R ) Ph; 0.100 mmol) in toluene (5.0 mL) was stirred
under argon at 90 °C for 2 h (i) or 8 h (ii). The mixture was cooled to
room temperature and evaporated to dryness. The solid obtained was
dissolved in CH2Cl2 (10 mL): the resulting solution was filtered over
Celite and then concentrated to a small volume and treated with
diethyl ether. The yellow precipitate formed was filtered and washed
with diethyl ether to give the analytical sample. Compound 1: yield
93%; mp 174-176 °C dec; 1H NMR (299.9 MHz, CDCl3) δ 9.71 (d, 1H,
H(6′), 3J H-H ) 5.6 Hz, J Pt-H ) 19.5 Hz), 8.50 (d, 1H, H(3′), 3J H-H ) 7.3
Reaction of cis-[(Me)2Pt(DMSO)2] and cis-[(Ph)2Pt-
(DMSO)2] with 6-Ph-2,2′-bipy in a 1/1 HL/Pt molar ratio
(reaction i) gives compounds 1 and 2,5 respectively,
which are new examples of rollover cyclometalation.
* To whom correspondence should be addressed. A.Z.: e-mail,
zucca@ssmain.uniss.it; fax, +39 079 229559. M.M.: e-mail,
m.manassero@csmtbo.mi.cnr.it.
3
3
Hz), 8.14 (d, 2H, Ho Ph, J H-H ) 7.8 Hz), 8.07 (d, 1H, H(4), J H-H
)
† Universita` di Sassari.
8.1 Hz, 3J Pt-H ) 52 Hz), 7.97 (dt, 1H, H(4′), J H-H ) 1 Hz, J H-H ) 7.8
3 4
4
3
‡ Present address: Institut fu¨r Anorganische Chemie, RWTH
Aachen, Professor-Pirlet-Strasse 1, 52074 Aachen, Germany.
§ Universita` di Milano.
Hz), 7.65 (d, 1H, H(5), J H-H ) 8.1 Hz, J Pt-H ) 25 Hz), 7.54-7.32 (m,
3
4H), 3.26 (s, 6H, CH3 (DMSO), J Pt-H ) 18.3 Hz), 0.75 (s, 3H, CH3-
Pt, 2J Pt-H ) 81.8 Hz); 13C NMR (75.4 MHz, [D6]DMSO) δ 164.0 (s, J Pt-C
≈ 25 Hz), 161.6 (s, J Pt-C ) 56.4 Hz), 151.4 (s, J Pt-C ) 214.7 Hz), 149.9
(s), 145.5 (s), 140.8(s, J Pt-C ) 90.2 Hz), 139.3 (s), 138.9 (s, J Pt-C ) 90.6
Hz), 128.7 (s), 128.5 (s), 125.9 (s), 125.1 (s), 121.1 (s, J Pt-C ≈ 20 Hz),
120.1 (s, J Pt-C ) 60.0 Hz), -13.8 (s, CH3-Pt, J Pt-C ) 772.5 Hz).
Compound 2: yield 51%; mp 240 °C; 1H NMR (299.9 MHz, CDCl3) δ
(1) (a) Nord, G.; Hazell, A. C.; Hazell R. G.; Farver, O. Inorg. Chem.
1983, 22, 3429. (b) Spellane, P. J .; Watts, R. J .; Curtis, C. J . Inorg.
Chem. 1983, 22, 4060. (c) Braterman, P. S.; Heat, G. H.; Mackenzie,
A. J .; Noble, B. C.; Peacock, R. D.; Yellowlees, L. J . Inorg. Chem. 1984,
23, 3425.
(2) Dholakia, S.; Gillard, R. D.; Wimmer, F. L. Inorg. Chim. Acta
1983, 69, 179.
(3) Skapski, A. C.; Sutcliffe, V. F.; Young, G. B. Chem. Commun.
1985, 609.
(4) Minghetti, G.; Doppiu, A.; Zucca, A.; Stoccoro, S.; Cinellu, M.
A.; Manassero, M.; Sansoni, M. Chem. Heterocycl. Compd. (N.Y.) 1999,
35(8), 992.
2.98 (s, 6H, CH3(DMSO), 3J Pt-H ) 17.6 Hz); 6.94 (d, 1H, H(4), 3J H-H
)
3
7.8 Hz), 7.03-7.47 (m, 8H, aromatics), 7.51 (d, 2H, Ho (Ph-Pt), J H-H
3
3
) 6.8 Hz, J Pt-H 65.0 Hz), 7.98 (t, 1H, H(4′), J H-H ) 7.7 Hz), 8.02 (d,
3 3
2H, Ho Ph bipy, J H-H ) 7.8 Hz), 9.63 (d, 1H, H(6′), J H-H ) 5.4 Hz).
(6) (a) Romeo, R.; Monsu` Scolaro, L.; Nastasi, N.; Mann, B. E.;
Bruno, G.; Nicolo`, F. Inorg. Chem. 1996, 35, 7691. (b) Eaborn, C.;
Kundu, K.; Pidcock, A. J . Chem. Soc., Dalton Trans. 1981, 933.
10.1021/om010913h CCC: $22.00 © 2002 American Chemical Society
Publication on Web 01/31/2002

784 Organometallics, Vol. 21, No. 5, 2002
Communications
Compounds 1 and 2 react further, in a separate step,
with [(Me)2Pt(DMSO)2] or [(Ph)2Pt(DMSO)2] in excess,
to give 3 and 4.7
Compounds 3 and 4 can be obtained also in an one-
pot reaction with a 1/2.5 HL/Pt molar ratio. Compounds
3 and 4 show a remarkable thermal stability: mp >250
°C for 3 and >240 °C for 4). They have been fully
characterized by elemental analyses, by 1H and 13C{1H}
NMR and FAB-MS spectroscopy, and, in the case of 4,
by X-ray diffraction.8-10
An ORTEP view of the complex molecule is shown in
Figure 1, with selected interatomic distances and angles
given in the figure caption.
F igu r e 1. ORTEP view of 4. Ellipsoids are drawn at the
30% probability level. Selected interatomic distances (Å)
and angles (deg): Pt(1)-S(1) ) 2.281(1), Pt(1)-N(2) )
2.166(4), Pt(1)-C(3) ) 2.018(5), Pt(1)-C(18) ) 2.031(6),
Pt(2)-S(2) ) 2.187(2), Pt(2)-N(1) ) 1.996(4), Pt(2)-C(8)
) 2.105(5), Pt(2)-C(13) ) 2.079(6); S(1)-Pt(1)-N(2) )
97.7(1), S(1)-Pt(1)-C(3) ) 178.4(1), S(1)-Pt(1)-C(18) )
91.1(2), N(2)-Pt(1)-C(3) ) 81.4(2), N(2)-Pt(1)-C(18) )
171.1(2), C(3)-Pt(1)-C(18) ) 89.8(2), S(2)-Pt(2)-N(1) )
178.0(1), S(2)-Pt(2)-C(8) ) 97.8(2), S(2)-Pt(2)-C(13) )
101.9(2), N(1)-Pt(2)-C(8) ) 80.8(2), N(1)-Pt(2)-C(13) )
79.5(2), C(8)-Pt(2)-C(13) ) 160.2(2).
The two platinum atoms are in a slightly distorted
square planar coordination, the main distortion being
that due to the bites of the chelating ligands, whereas
no loss of planarity is observed. The dihedral angle
between the best planes of the two PtSNC2 moieties is
1.7(6)°, so that the whole molecule can be described as
approximately planar, with the obvious exceptions of the
methyl and oxygen moieties and ortho and meta phenyl
C-H groups. All the metric parameters are as expected
if the trans influence of the three coordinated carbon
atoms is taken into account.4,11,12 The phenyl ligand is
nearly perpendicular to the Pt(1) coordination plane,
with a dihedral angle of 94.3(1)°.
(7) Compounds
3 and 4 were obtained by following the same
procedure described for 1 and 2, with the exception that a 2.5/1 Pt/HL
molar ratio was used. Compound 3:. yield 62%; mp >250 °C; 1H NMR
4
3
3
(CD2Cl2) δ 9.00 (dd, 1H, H(6′), J H-H ) 1.2 Hz, J H-H ) 5.6 Hz, J Pt-H
4
3
3
) 15 Hz), 8.17 (dd, 1H, H(4′), J H-H ) 1.5 Hz, J H-H ) 7.6 Hz, J Pt-H
3
3
) 21 Hz), 7.86 (d, 1H, H(4), J H-H ) 8.1 Hz, J Pt-H ) 46.6 Hz), 7.53
3
4
3
(dd, 1H, H(3′′), J H-H ) 7.3 Hz, J H-H ≈ 1 Hz, J Pt-H ) 25.6 Hz), 7.45
(dd, 1H, H(6′′), 4J H-H ≈ 1 Hz, 3J H-H ) 7.3 Hz), 7.21 (d, 1H, H(5), 3J H-H
) 8.1 Hz partially overlapping), 7.19 (dd, 1H, H(4′′)), 7.12 (m, 1H, H(5′)
partially overlapping), 7.09 (m, 1H, H(5′′) partially overlapping), 3.61
(s, 6H, CH3 (DMSO), 3J Pt-H ) 26.1 Hz), 3.19 (s, 6H, CH3 (DMSO), 3J Pt-H
) 20.0 Hz), 0.72 (s, 3H, CH3-Pt, 2J Pt-H ) 83.0 Hz). Compound 4: yield
63%; mp >265 °C; 1H NMR (299.9 MHz, CDCl3) 2.93 (s, 6H, CH3
In the case of 3, 2D H-H COSY and NOESY NMR
spectra allowed the assignment of all the signals in the
spectra.
3
3
(DMSO) J Pt-H ) 19.5 Hz), 3.63 (s, 6H, CH3 (DMSO), J Pt-H ) 26.1
Hz), 6.71 (d, 1H, H(4)), 6.90 (d, 1H, H(5)), 7.00-8.24 (m, 12H,
aromatics), 8.94 (d, 1H, H(6′)).
(8) Crystal data for 4:
space group P21/c (No. 14), a ) 11.973(1) Å, b ) 8.571(1) Å, c )
25.141(3) Å, â ) 99.36(1)°, U ) 2545.6(6) Å3, Z ) 4, Dc ) 2.225 g cm-3
C26H26N2O2Pt2S, Mr ) 852.8, monoclinic,
In complexes 3 and 4, 6-Ph-2,2′-bipy displays an
unprecedented behavior, acting as a threefold-deproto-
nated ligand. All three Pt-C bonds originate from
activation of C(sp2)-H bonds. The resulting compounds
3 and 4 are unique in several aspects. (i) The two
platinum(II) ions are bridged through a ten-electron
donor. (ii) Three five-membered rings are assembled in
the same molecule. (iii) The tridentate dianionic system
has a C,N,C sequence which is not unprecedented but
,
µ ) 112.8 cm-1, F(000) ) 1600, T ) 223 K, 39 758 reflections measured,
7576 independent reflections with Rint ) 0.048, empirical absorption
correction, SADABS9 (Tmax ) 1.00, Tmin ) 0.404), final R1(F2, all
reflections) ) 0.055, wR2 ) 0.097, conventional R1 ) 0.036 for 307
parameters, SMART CCD area detector, Mo KR radiation (λ ) 0.710 73
Å), ω scan mode, θmin ) 3°, θmax ) 26°. The structure was solved by
direct methods (SHELXS 8610) and refined by full-matrix least squares
with anisotropic thermal parameters for all non-hydrogen atoms;
hydrogen atoms were placed in calculated positions. The program used
was Personal SDP on a Pentium III computer. Atomic coordinates,
bond lengths and angles, and thermal parameters have been deposited
at the Cambridge Crystallographic Data Base.
(11) Peris, E.; Loch, J . A.; Mata, J .; Crabtree, R. H. Chem. Commun.
2001, 201.
(12) (a) Cave, G. W. V.; Fanizzi, F. P.; Deeth, R. J .; Errington, W.;
Rourke, J . P. Organometallics 2000, 19, 1355. (b) Cave, G. W. V.;
Alcock, N. W.; Rourke, J . P. Organometallics 1999, 18, 1801.
(9) Sheldrick, G. M. SADABS: Empirical Absorption Program;
University of Go¨ttingen, 1996.
(10) Sheldrick, G. M. SHELXS86: Program for the Solution of
Crystal Structures; University of Go¨ttingen, 1985.

Communications
Organometallics, Vol. 21, No. 5, 2002 785
still very rare. Furthermore, most of the reported species
do not arise from C-H activation: thus, platinum(II)
species have been obtained from lithiated 2,6-diphen-
ylpyridine,13 palladium(II) derivatives from 2,6-(CH-
(COOR)2)-functionalized pyridines able to generate, in
the presence of alkali metal, a carbanion,14 and gold(III)
complexes by transmetalation with mercury intermedi-
ates.15 Only quite recently has a C,N,C dicyclometalated
platinum(II) complex been synthesized in good yields
directly from K2PtCl4 and 2,6-diphenylpyridine.12,16 The
second metalation is induced by water. In addition, (iv)
the neutral ligands (DMSO) are trans to atoms of very
different trans effect and trans influence.
complex [Pt(L)Cl]. No transfer of hydrogen from the
bipyridine rings was observed. It seems therefore that
HL shows the behavior described here only toward the
dialkyl and diaryl derivatives.
Quite recently in a paper of ours18 we reported the
unprecedented behavior of 2,2′:6′-2′′-terpyridine as a
twofold-deprotonated N,C∧C,N ligand bridging two [Pt-
(Me)(L)] units (L ) neutral ligand). The present results
confirm that, in the case of heterocyclic nitrogen ligands,
metal-promoted intramolecular C-H activation is still
a topic worthy of investigation.
Studies on the photophysical and photochemical
properties of these new platinum species are ongoing.
For comparison it is worth noting that reaction of
6-phenyl-2,2′-bipyridine with trans-[PtCl(Me)(SMe)2]
recently studied by us17 only affords the adduct [PtCl-
(Me)(HL)] or the well-known N,N,C cyclometalated
Ack n ow led gm en t. Financial support from the Uni-
versita` di Sassari is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
structural information on compound 4 and text and figures
giving analytical and spectroscopic data of compounds 1-4 and
NMR spectra of complex 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) Cornioley-Deutschel, C.; Ward, T.; von Zelewsky, A. Helv. Chim.
Acta 1988, 71, 130.
(14) Newkome, G. R.; Kawato, T.; Kohli, D. K.; Puckett, W. E.;
Olivier, B. D.; Chiari, G.; Fronczek, F. R.; Deutsch, W. A. J . Am. Chem.
Soc. 1981, 103, 3423.
(15) Wong, K. H.; Cheung, K. K.; Chan M. C. W.; Che, C. M.
Organometallics 1998, 17, 3505.
(16) Lu, W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Organome-
tallics 2001, 12, 2477.
OM010913H
(17) Doppiu, A.; Cinellu, M. A.; Minghetti, G.; Stoccoro, S.; Zucca,
A.; Manassero, M.; Sansoni, M. Eur. J . Inorg. Chem. 2000, 2555.
(18) Doppiu, A.; Minghetti, G.; Cinellu, M. A.; Stoccoro, S.; Zucca,
A.; Manassero, M. Organometallics 2001, 20, 1148.