zyxwvu
zyxwvuts
zyxw
zyxw
zyxw
zyxw
Organometallics 1987, 6 , 2548-2550
2548
Cationic Intermediates in Oxidative Addition Reactions of Alkyl
Halides to de Complexes: Evidence for the SN2 Mechanism
Margarita Crespo and Richard J. Puddephatt"
Department of Chemistty, University of Western Ontario, London, Ontario, Canada N6A 587
Received April 28, 1987
The oxidative addition reactions of RX to [PtMezLz](1) to give [PtXMe2RL2](3) (L, = 2,2'-bipyridine,
RX = MeI, PhCH,Br; L = PMe2Ph,RX = MeI) are shown to occur by way of an ionic intermediate,
[PtSMe2RL2]+X-(2), when the solvent (S) = CD3CN and, for RX = Me1 and Lz = 2,2'-bipyridine, when
S = CD30D or acetone4 The intermediates 2 were detected and characterized by low-temperature *H
and, when L = PMezPh,Fs;PNMR studies. A t temperatures above -20 "C the ligand substitution of Xfor S occurred rapidly and hence the intermediateswere detected only for the most reactive systems, which
underwent oxidative addition at temperatures below -20 "C. Less reactive systems, for example, Et1 with
[PtMez(bpy)]or Me1 with [IrC1(CO)(PPh3),],failed to give detectable ionic intermediates. The observation
of ionic intermediates provides strong evidence for the sN2 mechanism of oxidative addition.
during oxintermediate, fuc-[PtMe3(SMe2),(CD3CN)]+I-,
idative addition of Me1 to c i ~ - [ P t M e ~ ( S M ein~ CD3CN
)~]
to give fuc-[PtMe31(SMe2),]was therefore surprising7and
led us to make a more general study of the conditions
under which such cationic intermediates could be detected.
The results are reported below.
Introduction
The sN2 mechanism of oxidative addition of alkyl halides was first proposed on the bases of the kinetic order
and the similarity of the activation parameters, for reaction
of Me1 with tr~ns-[IrCl(CO)(PPh~)~],
to those for the
Menschutkin reaction.' An sN2 mechanism should lead
to inversion of configuration for chiral alkyl halides, and
this has been confirmed in some cases.2 However, free
radical chain and non-chain mechanisms are also important in many oxidative additions, and there have been
difficulties in distinguishing between the possible mechanisms in some case^.^^^
The oxidative addition of Me1 to square-planar da complexes is of especial interest because of its role in the
Monsanto process for acetic acid.4 In some cases the
reaction is accelerated in the presence of iodide, due to a
preequilibrium involving formation of a five-coordinate
18-electron complex (eq 1,phen = 1,lO-phenathroline, cod
= 1,5-~yclooctadiene).~
I-,
=
C
Results and Discussion
The scope of the reaction of eq 2 was investigated.
Me,
,Pt<
hl e
L
L
+RX
-
Me,
CD3CN
I
N
*,L
i
K
CIr(phenNcod)I+
IrI(phen)tcod)l
-1-
I
1
hi Me1
R2 M c I .
C I r ( p hen) (cod ) M e 1 2 + I -
fPSt
-I-
(1)
C I r I M e ( p h e n ) t c o d )If
Both [Ir(phen)(cod)]+and [IrI(phen)(cod)]reacted by
the SN2mechanism, but the ratio k z / k l = 7. The proposed
intermediate [Ir(phen)(c~d)Me]~+
was not identified presumably because iodide addition to give the product [IrIMe(phen)(cod)]+was very fast.5 The observation of such
cationic intermediates would be good evidence for the sN2
mechanism of oxidative addition, since they are not expected in free radical or concerted mechanisms of reaction.,% However, numerous attempts to identify such
species from 16-electron precursors have been unsuccessfu1.2-6 The observation by low-temperature NMR of an
(1) Chock, P. B.; Halpern, J. J . Am. Chem. Soc. 1966,88, 3511.
(2) For excellent recent reviews, see: Stille, J. K. In The Chemistry
of the Metal-Carbon Bond; Hartley, F. R.; Patai, S., Eds.; Wiley: New
York, 1985; Chapter 9. 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, 1987;Chapter 5.
(3) Lappert, M. F.;Lednor, P. W. Adu. Organomet. Chem. 1976, 14,
345.
(4) Forster, D. J . Chem. SOC.,Dalton Trans. 1979, 1639. Hickey, C.
E.; Maitlis, P. M. J . Chem. Soc., Chem. Commun. 1984, 1609.
(5) de Waal, D. J. A,; Gerber, T. I. A.; Louw, W. J. Inorg. Chem. 1982,
21, 1259.
(6) Cationic intermediates are often observed in oxidative addition of
18-electron complexes such as [Ir(q-C,H,)(CO)(PPh,)]. Hart-Davis, A.
J.; Graham, W. A. G. Inorg. Chem. 1970,9, 2658; 1971, 10, 1653.
The reactions were carried out at low temperature in an
NMR tube using CD3CNsolvent, monitoring the progress
of the reaction at -40, -20, and 0 "C. When RX = Me1
and L2 = 2,2'-bipyridine, the spectrum at -40 "C contained
peaks due to both 2a and 3a (but none due to la). On
further warming peaks due to 2a decayed and pure 3a was
p r e ~ e n t . ~The
. ~ initial oxidative addition was shown to
be trans by use of CD31as reagent. Initially, neither the
intermediate 2a' nor the product 3a' showed resonances
at 6 0.47 or 0.58 for the methylplatinum resonance trans
to CD3CN or I, respectively (Figure 1). However, on
further reaction 2a' decayed to give 3a' and scrambling of
the CH3 and CD3 groups occurred, this scrambling occurring faster for 2a' than for 3a' (Figure 1). The intermediate 2a is characterized by a high value of 2J(PtCH,)
= 78 Hz for the methyl group trans to CD3CN, which has
a low trans i n f l ~ e n c e . ~
Very similar results were obtained with cis-[PtMe,(PMe2Ph)z](lb) though it reacts less rapidly than la. In
this case reaction occurred at -20 "C to give 2b, and then,
on warming to 0 "C, complex 3b was formed. This reaction
zyxwvuts
zyxw
(7) Puddephatt, R. J.; Scott, J. D. Organometallics 1985, 4 , 1221.
(8) Clegg, D. E.; Hall, J. R.; Swile, G. A. J . Organomet. Chem. 1972,
38, 403.
(9) Jawad, J. K.; Puddephatt, R. J. J. Chem. SOC.,Dalton Trans. 1977,
1466.
(10)Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Reu.
1973, 10, 335.
(11) Ruddick, J. D.; Shaw, B. L. J . Chem. SOC.A 1969, 2801.
0276-733318712306-2548$0l.50/0 0 1987 American Chemical Society
zyxw
zy
zy
zyxwvutsr
zyxwvutsrqpo
zyxwvut
Organometallics, Vol. 6, No. 12, 1987 2549
Oxidative Addition Reactions of Alkyl Halides
Table I. 'H and 31PNMR Dataa
R trans to X
Me trans to N
[PtMeAbpy)1 ( 1 4
[PtMe31(bpy)l ( 3 4
[PtMe.EtIlbDv)l
(3d)
*
.
.
.",>
(JP~H) (JHH)
0.58 (76), Me
1.39 (72) (7.6). CHa
0.01 (68) (7.6), CH;
2.75 (921, CHzPh
6.3 (m)-6.6 (m), Ph
2.80 (93), CH2Ph
6.3 (m)-6.6 (m), Ph
0.47 (78), Me
0.54 (80), Me
0.64 (80), Me
0.60 (801, Me
2.84 (96), CHzPh
[PtMezBzCl(bpy)] (3e)
[PtMezBzBr(bpy)] (3c)
[PtMe3(CD&N)(bpy)1I ( 2 4
[PtMe3(CD3OD)(bpy)1IWb
[PtMe3(CH3I)(bpy)1I(2fY
[PtMe3(CD3COCD3)(bpy)]I(2eY
[PtMezBz(CD3CN)(bpy)]Br(2c)
MePt6(H) (JPtH) (JPH)
[Pt(CD3)z(PMe2Ph)z1
(lb')
[Pt(CD3)zMeI(PMeziPh)2]
(3b')
0.63 (70) (7.6)
[Pt(CD3)zMe(CD3CN)(PMezPh)z]I
(2b')
0.59 (75) (8.6)
bpy resonances
6(H3), OL), W M ,
8.22, 8.19, 7.59, 9.17
8.66, 8.17, 7.71, 8.95
8.55, 8.29, 7.82, 9.04
1.32 (70)
8.20, 8.04, 7.55, 8.66
1.40 (70)
8.17, 8.01, 7.51, 8.63
1.14 (68)
1.13 (66)
1.01 (71)
1.17 (68)
1.30 (67)
8.52, 8.27, 7.82, 8.87
Solvent CD3CN unless otherwise stated. J values in hertz.
PMe2Ph6 ( W (JP~H) (JPH)
1.41 (20) (7.9)
1.63 (12) (9)
1.70 (11) (5.3)
1.51 (11) (5.3)
1.56 (11) (5.7)
W) (JPtP)
-12.29 (1842)
-36.28 (1200)
-46.51 (1192)
6(H) (JPH)
0.98 (5.7)
1.14 (5.3)
[IrClI(CH3)(CO)(PPh3121
[I~Iz(CH~)(CO)(PP~~)ZI
a
6(H) (JPtH)
0.89 (86)
1.36 (70)
1.35 (71)
zyxwvutsrq
WU
Solvent CD30D. "Solvent (CD3)&0.
1
2a.S :CD3CN
30
lb, L:PMe2Ph
2b,S:CD3CN
3b
2a
a)
2b
lb
I
,&
-
Figure 1. 'H NMR spectra (200 MHz) during the reaction of
CDBIwith [PtMez(bpy)] in CD3CN: (a) before addition of CD31;
(b) spectrum a t -40 "C; (c) spectrum a t -20 "C; (d) spectrum a t
0 "C.
Figure 2. 31P(1H)NMR spectra (121.5 MHz) during the reaction
(a) a t -40 "C; (b)
of Me1 with c i ~ - [ P t M e ~ ( P M e ~ Pinh )CD,CN:
~]
a t -20 "C; (c) at 0 "C; (d) a t 20 "C.
was monitored by both 'H and 31P NMR spectroscopy
in CD3CN were also unsuccessful. Again reaction occurred
(Figure 2). The value of lJptp
fell from 1842 Hz for l b
at 0 OC to give the final products 3d and 3e. Qualitatively,
to 1200 Hz for 2b and 1192 Hz for 3b, the changes being
it seems that the ionic intermediates can be detected only
typical of those expected on oxidation of platinum(I1) to
when oxidative addition occurs at -20 "C or lower. Above
platinum(IV).12 Oxidative addition was again trans, as
this temperature, the reaction of 2 to give 3 occurs rapidly,
shown by reaction of Me1 with ~~S-[P~(CD~),(PM~~P~)~],
and hence 2 cannot be detected. There is good evidence
and no scrambling of CH3 and CD3 groups occurred at
that the reactions of l a with Et1 and PhCHzCl occur by
room temperature in this system.
the SN2mechanism,14 and so the ionic intermediate 2 is
No ionic intermediate analogous to 2 could be detected
expected to be formed.
on reaction of trans-[IrC1(CO)(PPh3),]with Me1 in CD3Now, knowing that only the most reactive alkyl halides
in combination with the most reactive transition-metal
CN. No reaction was observed at temperatures up to -20
complexes were likely to give detectable ionic intermediOC, and at 0 OC reaction occurred to give [IrClIMeThis complex reacted further at room
(CO)(PPh3),]
ates, the reaction of PhCHzBr with la was studied.15 The
ionic intermediate 2c was detected at -40 "C (Table I), and
temperature to give [IrIzMe(CO)(PPh3)2],which was
prepared independently from [IrC1(CO)(PPh3),]with Me1
further reaction to give 3c was complete at 0 "C. Only
trans addition was observed in both 2c and 3c.
in the presence of NaI. Attempts to detect ionic intermediates in reactions of Et1 or PhCHzCl to [PtMe,(bpy)]
.12313
(12) Pidcock, A. Adu. Chem. Ser. 1982, No. 196, 1.
(13) Stang, P. J.; Schiavelli, M. D.; Chenault, H. K.; Breidegam, J. L.
n . n n n n m n + n l l ; n o lClP.4
9 11'2'2
(14) Monaghan, P. K.; Puddephatt, R. J. Inorg. Chim. Acta 1983, 76,
T
nnn
LLJI.
(15) Datta, D.; Sharma, G . T. Inorg. Chem. 1987,26,329. Schrauzer,
G. N.; Deutsch, E. J . Am. Chem. SOC.1969, 91, 3341.
zyxwvutsr
zyxwvuts
zyxwvutsrq
2550 Organometallics, Vol. 6, No. 12, 1987
Crespo a n d P u d d e p h a t t
Solvents Which Give Ionic Intermediates. A survey
of some polar solvents was made to determine which gave
detectable ionic intermediates in eq 3.
2a
2d
a
2'
-
Our attempts to identify ionic intermediates in oxidative
addition to Vaska's iridium(1) complex have been unsuccessful. This is due to a slower rate of oxidative addition
for the iridium(1) complex and, most probably, a higher
rate of ligand substitution for the iridium(II1) cationic
intermediate.'P2J6 Nevertheless, the direct observation of
ionic intermediates in oxidative addition to platinum(II),
together with the strong similarity in solvent effects on
rates and in activation parameters for oxidative addition
of methyl iodide to iridium(1) and platinum(I1) systems,
strongly supports the s N 2 mechanism in both cases for
reactions involving methyl iodide. 12 9 13,17
zyxwvutsrqponmlk
zyxwvutsrq
S CD3Ch
5 CD3CD
8 lCD3ZCC
S ' e1
3c
-
The intermediate was most easily detected when S =
CD3CN. However, when S = CD30D, the ionic intermediate 2d could be detected at -80 OC along with 3a. On
warming, 2d reacted further to give 3a. When S = (CD&CO, two intermediates were detected at -80 "C and
are thought to be 2e and 2f. The relative abundances of
2e and 2f were found to change as the concentration of
Me1 was changed, 2f being more abundant at higher concentration of MeI. This situation presumably arises because Me1 is a better ligand for platinum than is acetone.
Both intermediates had disappeared at -20 O C , and only
3a was present. NMR data are given in Table I.
Conclusions
This work has shown that ionic intermediates can be
detected by low-temperature NMR spectroscopy during
oxidative addition of alkyl halides to platinum(I1) complexes, provided the oxidative addition occurs at about -20
O C or lower. It is essential that the oxidative addition step
should be faster than the subsequent substitution of halide
for solvent in the ionic intermediate (eq 2 and 3), if the
intermediate is to be detectable. It is then clear that the
cationic intermediate is formed as a result of kinetic control, providing good evidence for the s N 2 mechanism of
oxidative addition. We consider it likely that the solvent
coordination (eq 2 and 3) occurs synchronously with the
oxidative addition step, thus contributing to the large
negative entropies of activation observedgand reducing the
incipient positive charge buildup on the platinum center.
The other experimental evidence, for example the trans
stereochemistryof addition, is consistent with but does not
prove this hypothesis.
For octahedral platinum(1V) complexes the ligand substitution step is expected to occur by a dissociative (D or
Id) me~hanism.'~Normally such substitutions in platinum(IV) complexes are very slow, but in this case it should
be accelerated due to the high trans influence of the methyl
group trans to solvent (eq 2 and 3) and the poor coordinating ability of the solvent molecules. As expected, the
ligand substitution occurs more rapidly for the oxygen
donor solvents, S = CD30D or acetone-d,, than for the
nitrogen donor, S = CD3CN.
I
zyxw
zyxw
1
9
Experimental Section
NMR spectra were recorded by using Varian XL200 (lH) or
XL300 (31P)NMR spectrometers, and chemical shifts are referenced to Mel& ('H) or (Me0)3P0 (31P). The complexes
[PtMe2(bpy)]and c i ~ - [ P t M e ~ ( P M e ~ Pwere
h ) ~ ]prepared by literature methods."J8 Most of the final products of oxidative
addition were known complexes and were identified by comparison
of the NMR spectra (Table I) with those of authentic Samp l e ~ . ~ , ~ , The
~ , ~NMR
, ~ ~ spectral
, ~ ~ , ~ studies
~
showed that the
products were formed in essentially quantitative yields (e.g.
Figures 1 and 2), and they were isolated after crystallization in
yields of a t least 70%. The following new complexes were prepared.
[PtC1Me2(C€12Ph)(bpy)].To a solution of [PtMez(bpy)](10
mg) in acetone (10 mL) was added PhCH2Cl (10 NL). After 15
min the product was isolated by evaporation of the solvents and
purified by recrystallization from CH2Clz/pentane;yield 75%.
Anal. Calcd for C,9H21C1N2Pt:C, 44.9; H, 4.2; N, 5.5. Found:
C, 44.5; H, 4.4; N, 5.0%.
[PtBrMez(CH2Ph)(bpy)]was prepared in a similar way; yield
78%. Anal. Calcd for C19H21BrN2Pt:C, 41.3; H, 3.8; N, 5.1.
Found: C, 41.2; H, 3.9; N, 5.3.
Detection of Intermediates. Typically a solution of
[PtMe2(bpy)](5 mg) in CD3CN (0.5 mL) in an NMR tube was
cooled to -45 "C, and CD31(3 fiL) was added. The tube was then
placed in the cooled probe of the NMR spectrometer, and spectra
were recorded at -40, -20,0, and 20 "C. Results are given in Table
I and Figures 1 and 2.
Acknowledgment. We thank NSERC (Canada) for
financial support.
Registry No. la, 52594-52-2; lb, 24917-48-4; lb', 69721-14-8;
2a, 11063829-4;2a', 110638-30-7;2b, 110638-31-8;2b', 110638-32-9;
2c, 110661-47-7;2d, 110638-33-0;2e, 110638-34-1;2f, 110638-35-2;
3a, 38194-05-7; 3a', 110638-36-3;3b, 24833-69-0; 3b', 110661-48-8;
3c, 62343-16-2; 3d, 62342-98-7; 3e, 110638-37-4; trans-[IrCl(CO)(PPh,),], 15318-31-7; IrClIMe(CO) (PPh,),, 24315-50-2;
Ir12Me(CO)(PPh3)z,
110716-19-3.
(16) Langford, C. H.; Gray, H. B. Ligand Substitution Processes; W.
A. Benjamin: New York, 1965.
(17) Jawad, J. K.; Puddephatt, R. J. J.Organomet. Chem. 1976,117,
297.
(18) Monaghan,P. K.; Puddephatt, R. J. Organometallics 1984,3,444.