Oxidatively induced metal-carbon bond cleavage reactions in iridium dimethyl complexes: Formation of cationic pyridine and nitrosyl iridium(III) alkyl derivatives

Article abstract of DOI:10.1016/S0022-328X(97)00761-4

Reaction of [Ir(Me)₂Cp*(L)] complexes (Cp=η⁵-C₅Me₅; L=PPh₃, PMePh₂, PMe₂Ph, PMe₃) with equimolar amounts of [Fe Cp₂]⁺ in pyridine gives methane a

Full text of DOI:10.1016/S0022-328X(97)00761-4

Journal of Organometallic Chemistry 555 (1998) 135–139
Preliminary communication
Oxidatively induced metal–carbon bond cleavage reactions in iridium
dimethyl complexes: formation of cationic pyridine and nitrosyl
iridium(III) alkyl derivatives
Pietro Diversi a,*, Valentina Ermini a, Giovanni Ingrosso a, Antonio Lucherini a,
Calogero Pinzino b, Franco Simoncini a
a Dipartimento di Chimica e Chimica Industriale dell’Uni6ersita` di Pisa, Via Risorgimento 35, 56126, Pisa, Italy
b Istituto di Chimica Quantistica ed Energetica Molecolare del CNR, Via Risorgimento 35, 56126, Pisa, Italy
Received 8 September 1997; received in revised form 21 November 1997
Abstract
Reaction of [Ir(Me)2Cp*(L)] complexes (Cp*=p5-C5Me5; L=PPh3, PMePh2, PMe2Ph, PMe3) with equimolar amounts of [Fe
Cp2]+ in pyridine gives methane and the corresponding cationic derivatives [Ir(Me)Cp*(L) (pyridine)]+. Reaction with an excess
of NOBF4 yields methane and [Ir(Me)Cp*(L)(NO)]2+ (L=PMe3, PMe2Ph, PMePh2) or [Ir(Me)2Cp*(NO)]+ and
[Ir(Me)Cp*(L)2]+ (L=PPh3) via the formation of the primary oxidation products [Ir(Me)2Cp*(L)]+, which have been detected
by EPR spectroscopy. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Iridium–carbon bond cleavage; Electron transfer behaviour; EPR spectroscopy
gives the unstable species 1a+ [1](b), whereas the chem-
We have recently reported [1] that the complexes
[Ir(Me)2Cp*(L)] (Cp*=p5-C5Me5; L=PPh3 (1a),
PMePh2 (1b), PMe2Ph (1c), PMe3 (1d)) react with a
variety of substituted arenes (including the phenyl sub-
stituents of the phosphine ligands) in the presence of
catalytic amounts of one-electron oxidants (ETC catal-
ysis) to give methane and the corresponding methyl aryl
complexes [Ir(Me)(Ar)Cp*(L)] (Scheme 1).
ical oxidation of 1a in a polar solvent (acetonitrile)
yields [Ir(Me)Cp*(PPh3)(MeCN)]+ [3]. Such a different
In all cases methane is formed by C–H bond activa-
tion of the Cp* methyl group with subsequent H ab-
straction from the solvent.
These ‘|-metathesis reactions’ [2] of Ir–Me and C–H
aromatic bonds are entirely unexpected since when
1a–1d are oxidized under different conditions no C–H
activation products are observed. For instance, the
electrochemical one-electron oxidation of 1a in CH2Cl2
* Corresponding author. Tel.: +39 50 918225; fax: +39 50
918260; e-mail: div@dcci.unipi.it
Scheme 1.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00761-4

136
P. Di6ersi et al. / Journal of Organometallic Chemistry 555 (1998) 135–139
[6], and by subsequent reaction with AgPF6 in pyridine
(Scheme 2).
It is interesting to note that, when the oxidation of
1a–1d is carried out in deuteriated pyridine, only CH4
(l 0.10) is evolved as monitored by 1H-NMR spec-
troscopy. Therefore, hydrogen is abstracted from one
of the ligands in the coordination sphere of iridium,
most probably from the Cp* to give the same ‘tucked-
in’ intermediate [2](c) (Scheme 2) already proposed for
explaining the formation of CH4 in addition to CH3D
in the activation of aromatic C–D bonds by 1a–1d
under ETC catalysis. Then this intermediate may ab-
stract D from the solvent to restore the p5-C5Me5
ligand.
1a–1d have been also reacted with NOBF4. The
reaction can be conveniently studied by EPR spec-
troscopy, carrying out the reaction in the cavity of the
spectrometer [7]. The resulting one-electron oxidation
products 1a+ –1d+ have been detected at low tempera-
ture (183 K) where single line spectra at g=1.931
(1a+), 1.939 (1b+), 1.942 (1c+) and 1.943 (1d+) are
observed. Lowering the temperature to 113–123 K, the
spectra appear as structured signals, as shown in Fig. 1,
where the spectrum of 1d+ is reported as an example.
1a+ –1d+ exhibit anisotropic behaviour, with well
separated components of a recognizably axial spectrum
implying metal-centred character. Hyperfine splitting of
the perpendicular absorption into a quartet is at-
tributable to spin-coupling to the iridium nucleus (191Ir,
37%; 193Ir, 63%: both I=3/2). However, computer
simulations by spectral optimization software [8] show
that the spectra are better interpreted as corresponding
to rhombic EPR symmetry (g1"g2"g3; A1"A2"A3;
all tensor axes coincident) where g1 and g2, because of
the linewidths of the resonances, are overlapping (see
Table 1 and Fig. 1). Although in principle the associ-
ated geometry is in contrast with that one could have
Scheme 2.
reactivity illustrates quite well the unusual sensitivity of
the oxidation reactions of 1a–1d to the experimental
conditions [1](b); [4], and has stimulated further re-
search.
In the course of this study we met with some oxida-
tively induced stoichiometric iridium–carbon cleavage
reactions, which allow further insight in the above C–H
activation reactions, and provide a route to new
pyridine and nitrosyl cationic derivatives of iridiu-
m(III).
On reacting 1a–1d with pyridine in the presence of
[FeCp2]PF6, no catalytic activation of pyridine C–H
bonds is observed, instead the cationic methyl deriva-
tives [Ir(Me)Cp*(L)(pyridine)]+ (L=PPh3 2a, PMePh2
2b, PMe2Ph 2c, PMe3 2d) [5] are produced together
with methane (Scheme 2).
By using equimolar amounts of oxidant 2c and 2d
are formed in almost quantitative yields and have been
isolated, while 2a and 2b have been only detected by
1H-NMR spectroscopy as the pyridine-d5 derivatives.
The reaction has been found to require 1–75 h by
1H-NMR monitoring, depending on the nature of the
phosphine ligand (see ref. [5]). Structural assignment
was done on the basis of elemental analysis (2c and 2d),
1H-NMR spectroscopy, and confirmed by the synthesis
of the hexafluorophosphate salts by treatment of 1a–1d
with equimolar amounts of anilinium chloride to give
the corresponding complexes [Ir(Me)(Cl)Cp*(L)] 3a–3d
Fig. 1. EPR spectrum of the species deriving from the reaction of
Ir(Me)2Cp*(PMe3) (1d) with NOBF4 in CH2Cl2. (a) Observed spec-
trum; (b) simulated spectrum.

P. Di6ersi et al. / Journal of Organometallic Chemistry 555 (1998) 135–139
137
Table 1
X-band EPR parameters of the 1a+–1d+ species generated by oxidation of 1a–1d with NOBF4a
Species
g1
g2
g3
Žg
A1(G)
A2(G)
A3(G)
1a+
1b+
1c+
1d+
2.067
2.011
2.009
2.013
2.033
1.975
1.977
1.979
1.879
1.830
1.830
1.831
1.993
1.939
1.939
1.941
23.46
19.09
20.10
18.08
30.77
30.71
29.60
30.42
26.07
24.01
24.75
22.42
a Spectra recorded at 113 K in the case of 1a+ e 1d+; at 123 K in the case of 1b+ e 1c+
.
expected, small distortions could be sufficient to make
all of the axes of g and A coincident.
By reaction of 1a–1d in deuteriated solvents, CH4 is
formed exclusively (l 0.20 in dichloromethane-d2, l
0.14 in benzene-d6, l 0.16 in nitromethane-d3) as in the
case of the oxidation reactions with [FeCp2]+ in
pyridine. Moreover, the 2H-NMR spectrum of 6d
shows the presence of deuterium on the Cp* ligand
(nitromethane-d3, l 2.08). This clearly shows that the
loss of methane occurs intramolecularly and that the
resulting intermediate [2](c) on its turn abstracts D
from the solvent.
As for the origin of the nitrosyl derivatives, EPR
spectroscopic evidences are consistent in all cases with
the hypothesis of the formation of the primary oxida-
tion products 1a+ –1d+ as the first step. Then there are
two possible reaction patterns, depending on the nature
of the phosphine: the loss of triphenylphosphine (in the
case of 1a+) or the loss of methane followed by coordi-
nation of NO. According to this picture, the 1a“4
reaction should involve the phosphine substitution by
NO in the transient oxidation product 1a+, rather than
On raising the temperature, the EPR signals decrease
in intensity disappearing in the −30–0°C range (de-
pending on the nature of the phosphine). By carrying
out the reaction on a preparative scale at room temper-
ature, by using a NO+/Ir ratio=2, methane is evolved,
and a mixture of the cationic nitrosyl derivative
[Ir(Me)2Cp*(NO)]+ (4) and [Ir(Me)Cp*(PPh3)2]+ (5a)
is formed from 1a, while the dicationic nitrosyl deriva-
tives [Ir(Me)Cp*(L)(NO)]2+
(L=PMePh2 (6b),
PMe2Ph (6c), PMe3 (6d)) are formed from 1b, 1c and 1d
respectively (Scheme 3) [9]
The IR spectra show for all the nitrosyl complexes
strong NO absorptions at very high frequencies (1909–
1843 cm−1), pointing to a linear Ir–NO+ arrangement.
In the case of 1b–1d the reaction is quite sensitive to
the NO+/Ir ratio: actually when a 1/1 ratio is used, the
dications are not obtained, instead complex reaction
1
mixtures are obtained which contain 4 (by H-NMR).
Scheme 3.

138
P. Di6ersi et al. / Journal of Organometallic Chemistry 555 (1998) 135–139
the more common phosphine substitution by NO+ in
1a (as suggested in the literature [10] for similar cases).
The above oxidatively induced cleavage reactions of
the Ir–Me bonds of 1a–1d result in the formation of
methane via the C–H activation of the Cp* group with
H, d, PMe2Ph), 1.72 (3 H, d, PMe2Ph), 7.30–7.60 (7 H, m,
m+PMe2Ph), 7.88 (1 H, ddt, JHH=7.6 Hz, Hp), 8.40 (2 H,
dd, JHH=6.2 Hz, Ho). 2d: Analysis Found: C, 45.62; H, 6.21; N,
2.41. C19H32IrNP Calc.: C, 45.77; H, 6.47; N, 2.81%. 1H-NMR
(C5D5N): l 0.51 (3 H, d, JHP=7.3 Hz, IrMe), 1.26 (9 H, d,
H
JHP=10.0 Hz, PMe3), 1.39 (15 H, d, JHP=1.8 Hz, C5Me5).
1H-NMR (CD2Cl2): l 0.60 (3 H, d, IrMe), 1.40 (9 H, d, PMe3),
1.58 (15 H, d, C5Me5), 7.41 (2 H, ddd, Hm), 7.90 (1 H, ddt, Hp),
8.35 (2 H, dd, Ho). 1H-NMR (CD3COCD3): l 0.67 (3 H, d,
IrMe), 1.52 (9 H, d, PMe3), 1.66 (15 H, d, C5Me5), 7.55 (2 H,
ddd, Hm), 8.08 (1 H, ddt, Hp), 8.65 (2 H, dd, Ho). 31P-NMR
(CD2Cl2): l −32.12.
subsequent
H abstraction from the solvent, the
organometallic fragment being trapped by the nitrogen
ligand (pyridine or NO+). Then such reactions appear
to mimic the early mechanistic steps which are at the
basis of the activation of arenes by 1a–1d under elec-
tron transfer catalysis, where, in the absence of strong
coordinating agents as pyridine or NO, the aryl radicals
deriving from the aromatic solvent enter the coordina-
tion sphere of the metal [1].
Finally the reaction of 1a–1d with NOBF4 opens the
route to a new class of alkyl nitrosyl complexes, a topic
which has recently received much attention in the litera-
ture in the last 10 years [11].
[6] (a) 3a [6c] 3b [6c] and 3d [6b] were already reported in the
literature, 3c is a new compound which has been prepared by a
similar procedure. 3c: Analysis found: C, 44.02; H, 5.21.
C19H29IrClP Calc.: C, 44.17; H, 5.66%; 1H-NMR (C6D6): l 1.21
(3 H, d, JHP=6.4 Hz, IrMe), 1.29 (15 H, d, JHP=1.9 Hz,
C5Me5), 1.32 (3 H, partially obscured d, PMe2Ph), 1.64 (3 H, d,
J
HP=10.5 Hz, PMe2Ph), 7.00–7.12 (3 H, bm, Ph), 7.52–7.64 (2
1
H, bm, Ph). H-NMR (CD3COCD3): l 0.75 (3 H, d, IrMe), 1.43
(15 H, d, C5Me5), 1.66 (3 H, d, JHP=10.6 Hz, PMe2Ph), 1.70 (3
H, d, PMe2Ph), 7.40–7.50 (3 H, bm, Ph), 7.72–7.84 (2 H, bm,
Ph). (b) J.M. Buchanan, J.M. Stryker, R.G. Bergman, J. Am.
Chem. Soc. 108 (1986) 1537. (c) D.S. Glueck, R.G. Bergman,
Organometallics 10 (1991) 1479.
Acknowledgements
[7] EPR spectra were obtained by using an EPR Varian E 112
instrument equipped with an Oxford EPR 900 for temperature
control. The spectrometer was interfaced to a PC 486/100 MHz
by means of an acquisition board.
[8] (a) The software was provided by the Illinois EPR Research
Center, NIH Division of research, Resonances Grant. No
RR01811. (b) K.J. Mattson, R.B. Clarkson, R.L. Belford, 11th
International EPR Symposium, 30th Rocky Mountains Confer-
ence, Denver, Colorado, August 1988.
This work was supported by CNR (Rome). We
thank Dr Andrea Raffaelli for measuring IS-MS spec-
tra.
References
[9] 1a (0.05 g, 0.081 mmol) was dissolved in CH2Cl2 (5 ml) and
added with NOBF4 (0.010 g, 0.081 mmol). Methane (l 0.20,
dichloromethane-d2) evolved and the colour changed from light
yellow to golden yellow. After stirring for 3 h, the solvent was
evaporated to dryness; the residue was repeteadly washed with
benzene and dried under vacuum to give a mixture (0.029 g) of
4 (64%) and 5a (36%). By fractional crystallization from acetone,
pure 4 and 5a were obtained. 4: Analysis found: C, 30.05; H,
4.40. C12H21BF4IrNO Calc.: C, 30.31; H, 4.45%; IR (Nujol,
cm−1) 1843 (s, NO), 1054 (s, BF); 1H-NMR (CD3COCD3): l
1.81 (6 H, s, IrMe), 2.25 (15 H, s, C5Me5). 1H-NMR (CD2Cl2):
l 1.71 (6 H, s, IrMe), 2.14 (15 H, s, C5Me5); IS-MS (MeOH),
m/e: 386 [M]+, 370 [M–MeH]+, 340 [M–MeH–NO]+, 326
[IrC5Me5]+, 87 [BF4]−. 5a: Analysis found: C, 58.91; H; 4.91.
[1] (a) P. Diversi, S. Iacoponi, G. Ingrosso, F. Laschi, A. Lucherini,
P. Zanello, J. Chem. Soc., Dalton Commun. (1993) 351. (b) P.
Diversi, S. Iacoponi, G. Ingrosso, F. Laschi, A. Lucherini, C.
Pinzino, G. Uccello-Barretta, P. Zanello, Organometallics 14
(1995) 3275. (c) F. de Biani Fabrizi, P. Diversi, A. Ferrarini, G.
Ingrosso, F. Laschi, A. Lucherini, C. Pinzino, G. Uccello-Bar-
retta, P. Zanello, Gazz. Chim. It. 126 (1996) 391.
[2] (a) P.L. Watson, G.W. Parshall, Acc. Chem. Res. 18 (1985) 51.
(b) C.M. Fendrick, T.J. Marks, J. Am. Chem. Soc. 108 (1986)
425. (c) M.E. Thompson, S.M. Baxter, A.R. Bulls, B.J. Burger,
M.C. Nolan, B.D. Santarsiero, W.P. Schaefer, J.E. Bercaw, J.
Am. Chem. Soc. 109 (1987) 203. (d) I.P. Rothwell, in: C.L. Hill
(Ed.), Activation and functionalization of alkanes, Wiley, New
York, 1989, p. 151.
[3] A. Pedersen, M. Tilset, Organometallics 13 (1994) 4887.
[4] J.K. Kochi, Angew. Chem. Int. Ed. Engl. 27 (1988) 1227.
[5] General procedure for the preparation of 2a–2d: To a solution
of 1 in pyridine an equimolar amount of [FeCp2]PF6 was added
to give a red/brown solution. After the reaction was over (2a, 75
h; 2b, 70 h; 2c, 24 h; 2d, 1 h) the solvent was removed under
reduced pressure, and the residue was purified by precipitation
from dichloromethane/diethyl ether to give 2 as a beige solid. 2a:
1H-NMR (C5D5N): l 0.89 (3 H, d, JHP=6.4 Hz, IrMe), 1.23 (15
H, d, JHP=1.8 Hz, C5Me5), 7.00–7.50 (15 H, m, PPh3). 2b:
1H-NMR (C5D5N): l 0.73 (3 H, d, JHP=6.3 Hz, IrMe), 1.33 (15
H, d, JHP=1.8 Hz, C5Me5), 1.94 (3 H, d, JHP =9.3 Hz,
PMePh2), 7.15–7.50 (15 H, m, PMePh2). 2c: Analysis Found: C,
51.25; H, 6.02; N, 2.35. C24H34IrNP Calc.: C, 51.41; H, 6.12; N,
2.50%. 1H-NMR (C5D5N): l 0.65 (3 H, d, JHP=6.9 Hz, IrMe),
1.29 (15 H, d, JHP=1.9 Hz, C5Me5), 1.49 (3 H, d, JHP=9.9 Hz,
PMe2Ph), 1.62 (3 H, d, JHP=9.9 Hz, PMe2Ph). 1H-NMR
(CD2Cl2): l 0.70 (3 H, d, IrMe), 1.43 (15 H, d, C5Me5), 1.54 (3
C47H48BF4IrP2 Calc.: C, 59.10; H, 5.07%; IR (Nujol, cm−1
)
1062 (BF4); 1H-NMR (CD3COCD3): l 1.26 (15 H, t, JHP=2.2
Hz, C5Me5), 1.56 (3 H, t, JHP=4.9 Hz, IrMe). 1H-NMR
(CD2Cl2): 1.19 (15 H, t, C5Me5), 1.50 (3 H, t, IrMe); IS-MS
(MeOH), m/e: 866 [M]+, 603 [M–PPh3]+, 587 [M–PPh3–
CH4]+. 5a was also prepared by reaction in a NMR tube of
[Ir(Me)(Cl)Cp*(PPh3)] with AgBF4 in the presence of PPh3 in
CD2Cl2, its 1H-NMR spectrum being identical to that of 5a
prepared as above. 6b: 1b (0.1 g, 0.18 mmol) in CH2Cl2 (5 ml)
was added with NOBF4 (0.042 g, 0.36 mmol). The reaction
mixture was stirred at room temperature for 3 h. Methane was
evolved and a deep orange solid was formed, which was washed
repeteadly with CH2Cl2 and C6H6, and then dried under vac-
uum. 6b was obtained as a maroon hygroscopic solid (0.071 g,
53%). Analysis found: C, 36.98; H, 1.71; N, 3.83.
C24H31B2F8IrNOP Calc.: C, 38.54; H, 1.87; N, 4.18%; IR (Nujol,
cm−1) 1900 (s, NO), 1059 (BF); 1H-NMR (CD3COCD3): l 2.25
(3 H, d, JHP=4.8 Hz, IrMe), 2.30 (15 H, d, JHP=1.9 Hz,
C5Me5), 2.96 (3H, d, JHP=11.6 Hz, PMe), 7.7–8.2 (10 H, bm,
Ph); 1H-NMR (CD3CN): l 2.02 (3 H, d, IrMe), 2.09 (15 H, d,

P. Di6ersi et al. / Journal of Organometallic Chemistry 555 (1998) 135–139
139
C5Me5), 2.55 (3H, d, PMe), 7.3–7.65 (10 H, bm, Ph). IS-MS,
H+
87 [BF4]−
[10] N.G. Connelly, W.E. Geiger, Chem. Rev. 96 (1996) 877.
] ] ,
+, 417 [M–NO+ +, 401 [M–NO+ –MeH]+, 224 [M]2+
m/e: 585 [M–NO+ +MeCN]+, 543 [M–NO+
]
+, 527 [M–
.
NO+ –MeH]+, 286 [M]2+, 263 [M–MeNO]2+, 87 [BF4]−. 6c:
maroon hygroscopic solid prepared as above in 56% yield.
Analysis found: C, 32.87; H, 4.56; N, 1.18. C19H29B2F8IrNOP
Calc.: C, 33.28; H, 4.27; N, 2.04%.; IR (Nujol, cm−1) 1899 (s,
[11] (a) R.P. Stewart, C.P. Casey, M.A. Andrews, D.R. McAlister,
J.E. Rinz, J. Am. Chem. Soc. 102 (1980) 1927. (b) W.P. Weiner,
M.A. White, R.G. Bergman, J. Am. Chem. Soc. 103 (1981) 3612.
(c) M.D. Seidler, R.G. Bergman, Organometallics 2 (1983) 1897.
(d) W.P. Weiner, R.G. Bergman, J. Am. Chem. Soc. 105 (1983)
3922. (e) M.D. Seidler, R.G. Bergman, J. Am. Chem. Soc. 106
(1984) 6110. (f) B.N. Diel, J. Organomet. Chem. 284 (1985) 257.
(g) P. Legzdins, B. Wassink, F.W.B. Einstein, A.C. Willis, J.
Am. Chem. Soc. 108 (1986) 317. (h) J. Chang, R.G. Bergman, J.
Am. Chem. Soc. 109 (1987) 4298. (i) M.M. Holl, G.L. Hillhouse,
K. Folting, J.C. Huffmann, Organometallics 6 (1987) 1522. (l) P.
Legzdins, S.J. Rettig, L. Sa´nchez, Organometallics 7 (1988) 2394.
(m) J. Chang, M.D. Seidler, R.G. Bergman, J. Am. Chem. Soc.
111 (1989) 3258. (n) G.B. Richter-Addo, P. Legzdins, Metal
nitrosyls, Oxford University Press, New York, 1992. (o) P.
Legzdins, J.E. Veltheer, Acc. Chem. Res. 26 (1993) 41.
NO), 1057 (BF); 1H-NMR (CD3COCD3): 2.28 (15 H, d, JHP
=
2.1 Hz, C5Me5), 2.36 (3 H, d, JHP=5.3 Hz, IrMe), 2.56 (3H, d,
HP=12.5 Hz, PMe), 2.54 (3 H, d, JHP=11.9 Hz, PMe); IS-MS
J
+
(MeCN), m/e: 520 [M–NO+ +MeCN]+, 479 [M–NO+
] ,
463 [M–NO+ –MeH]+, 273 [M–MeNO+2MeCN]2+, 254.7
[M]2+, 87 [BF4]−. 6d: deep orange solid, prepared as above in
60% yield. Analysis found: C, 26.6; H, 4.5; N, 2.2.
C14H27B2F8IrNOP Calc.: C, 26.0; H, 4.4; N, 2.2%. IR (Nujol,
cm−1) 1903 (s, NO), 1056 (s, BF); 1H-NMR (CD3COCD3): l
2.28 (9 H, d, JHP=12.6 Hz, PMe), 2.31 (3 H, d, JHP=6.2 Hz,
IrMe), 2.53 (15 H, d, JHP=1.9 Hz, C5Me5); 1H-NMR (CD3CN):
l 1.97 (9 H, d, PMe), 2.05 (3 H, d, IrMe), 2.28 (15 H, d, C5Me5);
IS-MS (MeCN), m/e: 460 [M–NO+ +MeCN]+
, 446 [M–
.