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An Easy Route to Homo- and Heteroleptic Biscycloplatinated Complexes without Recourse to Organolithium Compounds.

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Addition of azobenzene, after filtering off AgCI, triggered a reaction which occurred readily at 25 -50 "C. The main products are
cis complexes 1 a and 2a. When using 4,4'-azotoluene. the corresponding complexes l b and 2b were isolated. The 3Jp,,H
coupling constants clearly show that cis species are formed under
these conditions.[9a1Such complexes usually have cis geometry,[31but trans compounds are also known.['] Complexes 1 and,
properties
in
' H N M R ( ~ O O M H ~ , C D C I , : ~ = ~ . ~ ~ ( ~ , ~ H ) , ~ . ~ ~ ( S , particular,
I H ) , ~ . ~2~display
(~,~H
) . ~ . O ~typical of biscycloplatinated
(~,3H).394(~.6H),3.00(brt.J=6.2Hz,2H),2.88(brt,J=6.2Hz,2H), compound^.^^] Their UVjVIS spectra contain many bands, and
1.94-2.02 (m. 2H). 1.85-1.92 (in,2 H ) ; "CNMR (CDCI,): 6 = 185.3, 183.4.
in addition, 2 a shows fluorescence in CH,CI, at room tempera154.4. 153.0. 151.6. 151.4. 139.4, 137.7, 132.0, 131.7, 126.8. 125.5. 124.9, 118.1.
ture with a maximum at 359 nm when excited with light with
118.0. 110.2, 106.4, 56.8, 56.7. 56.3, 27.1, 25.0, 22.2, 21.9; H R M S (FAB) mi;
= 254 nm.
wavelength i.
( M i ) cdlcd 404.1498. found 404.1515. 4: R, = 0.50 (silica, 10% methanol in
calcd 408.1811. found 408.1825. 10: R, = 0.65 (silica, diethyl ether); IR (CDCl,):~,,,[cm-']=I621; 'HNMR(400MHz.CDC13):b = 8.56(s, 1H),8.27
(s. 1 H ) , 6.80 (d. J = 8.4H2, 1H), 6.76 (d, J = 8.4 Hz, 1 H). 6.72 (s, 1 H ) , 4.09
(s. 3 H ) . 4.08 (s, 3H), 4.04 (s, 3H), 3.02 (hr t, J = 6 . 2 H z , 2H). 2.92 (br t,
J = 6.2 Hz.ZH), 1.94-2.01 (m,2H), 1.85-1.91 (m.2H):l3CNMR(CDC1,):
d =159.2. 155.1. 152.4, 149.5, 143.5. 141.2. 140.9, 130.0, 128.0. 126.1, 125.3.
115.8. 111 5. 1039, 103.8, 103.7, 94.5, 56.7. 55.8, 55.5, 27.1, 25.8, 22.6. 22.1;
MS: fii/z ( M i ) cdlcd 390.1705, found 390.1715. 3: R, = 0.50 (silica, 1 0 %
methanol in ethyl acetate): I R (CDCI,): i.,.,[cm~'] =1682 (quinone);
ethyl acetate); IR (CDCI,): t,,,.[cm-'] =1680 (C=O); ' H N M R (400 MHz;
CDCI,): 6 = 9.14(s. 1H).9.06(s, 1 H ) . 7 . 1 8 ( ~ , 2 H ) , 4 . 1 1(s, 3H),3.97(s. 3H).
3.96(s. 3H),3.15(brt, J = 6.0 Hz,ZH),2,84(brt. J = 6.0 Hz.2H).2.24-2.33
(m, 2 H ) ; "CNMR (CDCI,): d = 200.5. 184.9, 183.0, 156.7. 153.1. 151.7.
139.9, 139.5. 134.9, 132.1, 129.7, 128.4, 126.9, 125.3, 118.3. 118.2, 110.2, 56.9,
56.8. 56.5. 40.9. 28.0, 22.6; H R M S (FAB) ml; (hi'+) calcd 418.1291, found
41 X.1308.
Sdlv
An Easy Route to Homo- and Heteroleptic
Biscycloplatinated Complexes without Recourse
to Organolithium Compounds**
z
QN
N
z*
Qz
Alexander D. Ryabov* and Rudi van Eldik
Cycloplatination reactions are characterized by a diversity of
mechanisms for C-H bond cleavage['] on one hand, and low
yields of platinacycles formed on the other. Moreover, certain
types of complexes, such as cis-[Pt(C N)J, have not yet been
prepared by metalation of C-H bonds. These homoleptic species were obtained by lithiation of organic ligands followed by
interaction with the appropriate platinum(r1) derivatives.[31The
synthesis of heteroleptic species of the type cis-[Pt(C N)
(C* N)] is even more laborious.[4]Such homo- and heteroleptic complexes exhibit remarkable stereo~hemistry,[~]
photochemistry, and chemical reactivity,[31 but the synthetic difficulties limit their wide use.
We report here on facile cycloplatinations through C-H bond
cleavage leading to both homo- and heteroleptic complexes, as
well as on novel exchange of cycloplatinated ligands, similar to
that observed in palladium(I1) complexes.r61The platination of
C-H bonds is expected to be easier when promoted by abstraction of the chloro ligands by AgNO,."] We have found that it
is even more advantageous to use AgBF, instead of AgNO,. The
role of the silver salts is obvious: they create coordinative unsaturation at platinum(i1) and thus increase the electrophilicity of
the metal center. Naturally, higher efficiency is anticipated with
a poorly coordinating anion, BF,, as Ag counterion. The principal results are shown in Scheme 1 . A solution of the chlorobridged diplatinum complex [Pt(C,H,CH,NMe,)CI],r81 in
CH,CI, or C,H, was treated with AgBF, dissolved in acetone.
-
-
-
1
'
I Z
Q
Q
Z
Z
H (a), Me (b)
2 Z = 2' = H (a)
Z = Z' = M e (b)
3 Z=H,Z'=Me
Scheme 1. A new route to homo- and heteroleptic monomeric hiscyclopldtinum
complexes.
=
The data in Figure 1 suggest that complex 1 a is formed first
and is then converted into 2a, a novel example for the exchange
of cyclometalated ligands in platinum(I1) complexes. It illustrates
an easy way to the heteroleptic biscycloplatinated compounds.
since the primary product 1 a, if treated with a ligand other than
azobenzene, could afford the heteroleptic complex after dissoci-
50
+
40
4
i
30
A [%I
20
[*] Prof. A. D. Ryabov,[+' Prof. R. van Eldik
['I
[**I
lnstitut fur Anorganische Chemie der Universitit Witten/Herdecke
Stockumer Strasse 10, D-58453 Witten (FRG)
Permanent address:
Division of Chemistry. G. V. Plekhanov Russian Economic Academy
Stremyanny per. 28. 113054, Moscow (Russia)
We are gratelul to Prof. Paul S . Pregosin for his persistent advice to test the use
of silver salts in these systems and to R. Wimmer for experiments with proton
scavengers. A stipend (1991 -1993) from the Alexander von Humboldt Foundation for A D R is gratefully acknowledged.
Aflyc31t..( ' l r t w l i l t .
Ed. EwI. 1994, 33. No. 7
(3
10
0
0
50
100
150
t [min]
200
250
-+
Fig. 1. The course of the formation of l a (A) and Z a (0) from [Pt(C,H,CH,NMe,)(Solv),]BF, and azobenzene in C,H,/acetone (1 : 1 v/v) at 50 'C
monitored by UVjVIS spectroscopy; [Pt(n)] = [azobenzene] = 1.09 x l o - * M.
VCH V L ~ l u ~ s ~ r s e N . mbH,
~ c h u ~0-69451
~
Weinhrim, 1994
+
0570-0833~94/0707-0783R 10.00 .25/0
783
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ation of N,N-dimethylbenzylamine. We attempted to carry out
this transformation by treating 1 a with 4,4'-azotoluene under
exactly the same conditions: in CH,Cl,/acetone or C,H,/acetone mixtures at 50 "C. Surprisingly, there was no exchange.
Assuming that there was a catalytic process, we tried unsuccessfully to induce the reaction by addition of AgBF,, [Pt(C,H,CH,NMe,)CI],,
or both to the reaction mixture. What
was then responsible for the exchange? The analogy with the
palladium(1r) reactions helped to solve the problem.r6]The first
cycloplatination to form 1 from the solvento complex (Scheme 1)
must generate a strong acid, HBF,, which in turn is able to
cleave the more basic dimethylaminomethylbenzenido ligand of
1, as does acetic acid as cosolvent in related palladium(r1) complexes.["' Since the acid anion is noncoordinating, the product
[Pt(C,H,N=NPh)(Solv>,]BF, is able to cyclometalate the next
azobenzene molecule to form 2.
If this rationalization is correct, the following two processes
must occur: Firstly, strong acids with noncoordinating anions
should induce the conversion of 1 into 2 and, secondly, heteroleptic compounds could be generated under the same conditions as in Scheme 1 from the cycloplatinated azobenzene dimer
[Pt(C,H,N=NPh)CI],.~"l We were, in fact, able to carry out
both these reactions. The exchange, achieved by addition of
stoichiometric amounts of F,CSO,H to the mixture of 1 a and
4,4'-azotoluene, and direct cycloplatination of the same ligand
by the solvento complex derived from [Pt(C,H,N=NPh)CI],
gave the identical heteroleptic complex 3.
One of the referees of this paper raised the question whether it
is possible to preclude the formation of 2 by addition of a "proton sponge" to the reaction mixture. Therefore, I &bis(dimethylamino)naphthalene or 2,6-di-rerr-butylpyridine was added to
the solvento species to bind the acid liberated in subsequent reaction with azobenzene. However, these nitrogen ligands themselves
react instantaneously with [Pt(C,H,CH,NMe,)(Solv),]BF, affording dark brownish complexes. No attempt was made to
characterize these species.
The formation of homoleptic complexes 2 from the N,Ndimethylaminomethylbenzenide complex (Scheme 1) is a unique
example of "suicidal" cyclometalation, since the activation of
C-H bonds by electrophilic substitution generates a strong acid
which then "kills" the more basic ligand of the primary product
1 . Azobenzenes are less basic than tertiary amines, and this
makes complexes 2 and 3 more resistant to small concentrations
of strong acids. The exchange reaction resembles processes that
are well-known in palladium(ii) chemistry.r61The principal difference is that the exchange reactions of the platinum complexes
involve monomeric biscycloplatinum complexes of the type
[Pt(C N)(C* N)]. In the palladium(Ir) case, however, exclusively monometalated dimeric complexes of the type [Pd(C
N)X], in which only organic ligand is bound to the metal center, participate. Related monometalated platinum(n) dimers are
insufficiently labile to undergo the proton-induced exchange
and react with azobenzenes in a different
The Pt-C bond
of one platinacycle is needed to increase the lability of the other
cycloplatinated leaving ligand toward proton attack. We have
recently shown that the kinetic labilization brought about by
orthoplatination (that is, labilization by the Pt-C bond trans
to the leaving ligand) increases the reactivity by four orders
of magnitude, with the result that the anation rate of
platinum(I1) complexes reaches that of analogous [Pd(dien)]
complexes." 3 1 In the present case, the Pt -C bond of azobenzene
labilizes the frans, Pt-N bond of the leaving dimethylaminomethylbenzenido ligand. The corresponding Pt- C bond
becomes less stabilized by chelation and is easily cleaved by the
acid.
-
-
.
784
J
VCH V e r l r i ~ ~ ~ e s ~ l l smbH
c l ~ a f0-694.51
t
Wcmhemi 1994
-
Experimental Procedure
The typlcal procedure was as follows: the solution of [Pt(C,H,CH,NMe,)Cl],
(0.080 mmol) in CH,CI, (1.5 mL) was mixed with AgBF, (0.164 mmol) dissolved in
acetone (1 mL); AgCl was removed, and azobenzene (0.170 mmol) was added. The
mixture was kept for 18 min at 3 0 T , then cooled in ice, and the products were
separated by preparative TLC (SiO,, C,H,). If the goal is the isolation of 1, as in
this example, TLC operations must be carried out fast. Azobenzene is eluted first.
then 2 a (brown hand) followed by 1a (orange). The products were washed off with
CHCI, and recrystallized from EtOH/H,O after removal of the solvent. A brownish
red material that was difficult to elute from SiO, was always present. Its ' H N M R
spectrum showed a mixture of compounds which was not investigated further.
Yields: l a 53%, 2 a 7 % . Satisfactory C,H,Pt analyses were obtained;
l a : ' H NMR (CDCI,): 6 = 2.41 (s, 'J(Pt,H) =16.1 Hz, NCH,), 3.85 (s.
'J(Pt,H) = 20.4 Hz, NCH,). 7.06 (t, H4 ("dimethyibenzyiamine", dmba)), 7 12 (d,
H3 (dmba)), 7.20 (t. H5 (dmba)), 7.21 (td, H4 ("azobenzene", azb)), 7.34 (td, H5
(azb)). 7.45 (t, H4(azb)), 7.51 (d, H2',6(azb)), 7.56 (t, H3',5' (azb)), 8.00 (d,
'J(Pt.H) = 53.0 Hz. H6(azb)). 8.02 (d, 'J(Pt, H) = 53.5 Hz. H6 (dmba)), 8.02 (dd,
H3 (azb)); IR (KBr)- i. = 699, 769 (monosubstituted azb ring), 721 (1.2-disubstituted a r b ring), 742 c m - ' (1.2-disuhstituted dmba ring)[12]; UVWIS (CH,CI,): A,,,
(r:[M-'cm-']) = 474 (5300). 394 (4250), 334 (SSOO) nm. - 2a: 'H N M R : b =7.01
(t. H3'.5'). 7.11 (t. H 4 ) , 7.31 (t. H4), 7.49(d, H2,6).7.50(t, H5),8.13 (d, H3),8.21
(d, '4Pt.H) = 47.3 Hz. H6); I R : C = 691, 762 (monosubstituted ring), 718 cm-'
(1.2-disubstituted ring); UVjVlS I,,, ( 6 ) = 600sh. 513 (6900), 377 (16000). 337
(15100), 317 (14700), 284 (19400), 251 (19700) nm. - I b : 'H N M R : 6 = 2.41 (s,
CCH,), 2.43 (s, NCH,). 2.45(s. CCH,). 3.85 (s, 'J(Pt,H) = 20.05 Hz, NCH,), 6 99
(d. H4 ("azotoluene". azt)). 7.06 (t. H4 (dmba)), 7.12 (d, H3 (dmba)). 7.22 (t, H5
(dmba)), 7.32 (d. H3',5'(azt)), 7.40 (d, H2'.6'(azt)), 7.78 (s, 'J(Pt,H) = 54.4 Hz, H6
(art)), 7.87 (d, H3 (azt)), 8.01 (d, ,J(Pt,H) = 45.39 Hz, H6(dmba)); IR: i. =741
(1.2-disuhstituted dmba ring), 804, 820, 844 cin-'(azt); UVjVIS: A,, ( E ) = 454
(SlOO), 342 (97001, 281 (12000) nm. - 2b: ' H N M R : 6 = 2.26 (s, CCH,), 2.52 (s,
CCH,), 6.78(d, H3'.5'), 7.10 (d, H4). 7.32 (d, H2',6), 7.97 (d. H3). 7.97 (s,
'J(Pt.H) = 48.6 Hz, H6); IR: i. = 805, 820 c m - ' ; UV/VlS: I,,, ( E ) = 597 (2350),
510 (7800), 393 (21000), 359 (18600), 325 (22900). 289 (23200), 259 (22200)nm.
- 3: 'H NMR: 6 = 2.24 (s, CCH,), 2.52 (s, CCH,), 6.76 (d, H3',5' (azt)), 7.00 (t.
H3',S(azb)),7.lO(d,H4(azt)),7 11 (t,H4'(azb)),7.31 (t,HS(azb)).7.34(d.H2',6'
(azt)), 7.46 (d, H2',6 (azb)), 7.51 (td, H4 (azb)), 7.98 (s; 'J(Pt,H) = 49.1 Hz, H6
(azt)), 7.98 (d, H3 (art)). 8.14 (dd, H3 (azb)), 8 20 (d, 'J(Pt.H) = 46.6 Hz, H6
(azb)): IR: i. = 692, 766 (monosubstituted azh ring), 718 (1.2-disubstituted azb
ring), 823 cm-'(azt);UV,'VIS:i.,,,(E) = 590sh.512(3600),387(8700),359(8500),
322 (90001, 286 (lOSOO), 257 (10100) nm.
The data shown in Figure 1 were obtained froin samples (0.10 mL) withdrawn
intermittantly from the reaction mixture and purified on Merck silica gel 5 x 10 cm
aluminium sheets for TLC (eluted with benzene). The brown and orange bands of
2 a and l a , respectively, were cut out, the products washed off with CHCI,, and
their concentrations determined by UV/VIS spectroscopy.
Received: May 8, 1993
Revised version: December 1, 1993 [Z 6070 IE]
German version: Anger. Chem. 1994, 106, 798
[l] A. D. Ryabov, C h m . Rev. 1990, 90, 403-424.
[2] J. Dehand, M. Pfeffer. Coord. Chem. Rev. 1976, 18, 327-352; M. I. Bruce,
Angew. Chem. 1977,89,75-84; Angew. Chem. I n t . Ed. Engl. 1977,16, 73-86;
I. Omae, Chem. Reu. 1979, 79, 287-331.
[3] Review: M. Maestri, V. Balzani, C. Deuschel-Cornioley, A. voii Zelewsky, Adv.
Photorhem. 1992. 17, 1-68.
[4] C. Deuschel-Cornioley, R. Luond, A. von Zelewsky. Helv. Chrm. Aria 1989.
72. 377-382.
[5] C. Deuschel-Cornioley, H. Stoeckli-Evans. A. von Zelewsky, J. Chem. Soc.
Chem. Commun. 1990, 121-122.
[6] Review: A. D. Ryabov in Perspecfives in Coordrnution Chemrs/r,c-(Eds.: A. F.
Williams, C. Floriani, A. E. Merbach), Verlag Helvetica Chimica Acta, Basel
VCH, Weinheim, 1992, pp. 271 -292.
[7] P. S. Pregosin. F. Wombacher. A. Albinat~.F Lianza, .IOrganomet. Chem.
1991, 418, 249-267.
[S] A. C. Cope, E. C. Friedrich, 1 Am. Chem. Sac. 1968, 90, 909-913.
191 a) G. Longoni, P. Fantucci, P. Chini, F. Canziani, J. Orgunornet. Chem. 1972,
39,413-425; b) H:P. Abicht, K. Issleib. J. Organomer. Chem. 1985.28Y. 201 213.
[lo] A. D. Ryahov, A. K. Yatsimirsky, H.-P. Abicht, Polyhedron 1987, 6. 16191620.
[ l l ] A. C. Cope, R. W. Siekman, J. Am. Chem. Sue. 1965, 87, 3272-3273.
[12] A. D. Ryabov, L. G . Kuz'mina, N. V. Dvortsova. D . Stufkens, R. van Eldik,
Inurg. Chem. 1993.32, 3166-3174.
[13] M . Schmiilling, A. D. Ryabov, R. van Eldik, J. Chem. Soc. Chem. Cornmun.
1992,1609-1611.
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