Competitive activation of a methyl C-H bond of dimethylformamide at an iridium center

Article abstract of DOI:10.1021/om9002413

During the synthesis of [AsPh₄][Ir(CO)₂I ₃Me] by refluxing IrCl₃·3H₂O in DMF (DMF = dimethylformamide) in the presence of aqueous HCl, followed by sequential treatment with [AsPh₄]Cl, NaI,

Full text of DOI:10.1021/om9002413

Organometallics 2009, 28, 4229–4230 4229
DOI: 10.1021/om9002413
Competitive Activation of a Methyl C-H Bond of Dimethylformamide at
an Iridium Center
Valerie J. Scott, Lawrence M. Henling, Michael W. Day, John E. Bercaw,* and
Jay A. Labinger*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received March 30, 2009
Summary: During the synthesis of [AsPh4][Ir(CO)2I3Me]
by refluxing IrCl3 3H2O in DMF (DMF = dimethylforma-
sites in aldehydes would rarely be expected to be competitive.
Indeed, we have found only one example: reaction of Rh(ttp)
Cl (ttp = tetrakis(4-tolyl)porphyrinato) with p-methylben-
zaldehyde or p-tert-butylbenzaldehyde gives (initially) a
few percent of (ttp)Rh(CH2C6H5CHO) or (ttp)Rh(CH2-
CMe2C6H5CHO), respectively, along with the major
product, (ttp)Rh(COAr).10 To our knowledge there have
been no reports of C-H activation of a methyl group of
DMF or any closely related species. We report here the
structure of an unusual iridium complex resulting from such
a reaction.
3
mide) in the presence of aqueous HCl, followed by sequential
treatment with [AsPh4]Cl, NaI, and methyl iodide and finally
recrystallization from methylene chloride/pentane, three crys-
talline byproducts were obtained: [AsPh4]2[Ir(CO)I5],
[AsPh4]2[trans-Ir(CO)I4Cl], and [AsPh4][Ir(CO)(κ2O,
C-CH2NMeCHO)Cl2I]. The last of these, whose structure
(along with the others) was determined by X-ray diffraction,
results from activation of a methyl C-H bond of dimethylfor-
mamide, rather than the normally much more reactive aldehy-
dic C-H bond.
Studies in our laboratories required the synthesis of the
iridium carbonyl complexes [AsPh4][Ir(CO)2I2] (1) and
[AsPh4][Ir(CO)2I3Me] (2). Kalck and co-workers reported
that decarbonylation of DMF by IrCl3 initially formed
[Ir(CO)(DMF)Cl2]- (3), which underwent further transfor-
mation to [Ir(CO)2Cl2]-, which was isolated as the [AsPh4]+
salt (4).1 Halide exchange of the latter with NaI or KI in
methanol gave 1 in quantitative yields, as monitored by
infrared spectroscopy (Scheme 1). In a separate paper, 1
(synthesized by a different route: refluxing IrCl3 and NaI in a
mixture of 2-methoxyethanol and water while bubbling CO
through the solution) was found to react with methyl iodide
to give 2.9
In our hands, the reaction of 1 (synthesized via decarbo-
nylation of DMF as in Scheme 1) with MeI gave mainly the
expected product 2, according to infrared spectroscopy.
However, on attempted recrystallization of the crude pro-
duct from a mixture of CH2Cl2 and pentane two different
species were obtained: red needles and a smaller amount
of yellow crystals. Both were examined by X-ray crystallo-
graphy.
A red needle from one such reaction was determined to be
a known complex, [AsPh4]2[Ir(CO)I5] (5a), whose structure
has been reported.11,12 Another needle from a duplicate
reaction, surprisingly, turned out to have a different (but
closely related) structure: [AsPh4]2[trans-Ir(CO)I4Cl] (5b).
More interesting is the minor yellow product, which was
crystallographically determined to be [AsPh4][Ir(CO)(κ2O,
C-CH2NMeCHO)Cl2I] (6), the product of C-H activ-
ation of a methyl group from DMF (Figure 1). As noted
Dimethylformamide (DMF) is a common and convenient
carbonylation reagent in organometallic chemistry.1-5 The
mechanism of these carbonylation reactions has not often
been studied in detail, but activation of the aldehydic C-H
bond to give a carbamoyl intermediate, followed by C-N
bond cleavage (the reverse of either migratory insertion or
nucleophilic addition), seems a likely route. Structures re-
sulting from DMF activation at the aldehydic C-H bond
have previously been reported. Hidai and co-workers found
that cis-[WH(η2-CONMe2)(dpe)2] (dpe=(diphenylphosphi-
no)ethane) can be obtained from DMF and trans-
[W(N2)2(dpe)2] after 20 min; on longer reaction it converts
to [W(CO)(DMF)(dpe)2].6 Another decarbonylation inter-
mediate, Ru(H)2(Cl)(η2-CONMe2)(PiPr3)2, was generated
by reaction of Ru(H)(H2)Cl(PiPr3)2 with DMF.7
The activation of aldehydic C-H bonds at transition
metal centers is well-known, most prominently in the
catalytic decarbonylation of aldehydes.8 Because this pro-
cess appears to be relatively facile, reaction at other C-H
*To whom correspondence should be addressed. E-mail: bercaw@
caltech.edu (J.E.B.); jal@caltech.edu (J.A.L.).
(1) Serp, P.; Hernandez, M.; Richard, B.; Kalck, P. Eur. J. Inorg.
Chem. 2001, 2327.
(2) Mizobe, Y.; Ishida, T.; Egawa, Y.; Ochi, K.; Tanase, T.; Hidai, M.
J. Coord. Chem. 1991, 23, 57.
ꢀ
(3) Rusina, A.; Vlcek, A. A. Nature 1965, 206, 295.
(4) Collman, J. P.; Sears, J. T.Jr.; Kubota, M. Inorg. Synth. 1969, 11,
101.
(5) Basu, A.; Kasar, T. G.; Sapre, N. Y. Inorg. Chem. 1988, 27, 4539.
(6) Ishida, T.; Mizobe, Y.; Tanase, T.; Hidai, M. Chem. Lett. 1988,
441. Ishida, T.; Mizobe, Y.; Tanase, T.; Hidai, M. J. Organomet. Chem.
1991, 409, 355.
(7) Coalter, J. N.III; Huffman, J. C.; Caulton, K. C. Organometallics
2000, 19, 3569.
(8) See: Fristrup, P.; Kreis, M.; Palmelund, A.; Norrby, P.-O.;
Madsen, R. J. Am. Chem. Soc. 2008, 130, 5206–5215. Alaimo, P. J.;
Arndtsen, B. A.; Bergman, R. G. Organometallics 2000, 19, 2130–2143.
(9) Chan, K. S.; Lau, C. M. Organometallics 2006, 25, 260.
(10) Haynes, A.; Maitlis, P. M.; Morris, G. E.; Sunley, G. J.; Adams,
H.; Badger, P. W.; Bowers, C. M.; Cook, D. B.; Elliott, P. I. P.; Ghaffar,
T.; Green, H.; Griffin, T. R.; Payne, M.; Pearson, J. M.; Taylor, M. J.;
Vickers, P. W.; Watt, R. J. J. Am. Chem. Soc. 2004, 126, 2847.
(11) Malatesta, L.; Naldini, L.; Cariati, F. J. Chem. Soc. 1964, 961.
(12) Dahm, D. J.; Forster, D. Inorg. Nucl. Chem. Lett. 1970, 6, 15.
r
2009 American Chemical Society
Published on Web 05/28/2009
pubs.acs.org/Organometallics

4230 Organometallics, Vol. 28, No. 14, 2009
Scott et al.
Scheme 1
Experimental Section
General Considerations. All chemicals and solvents were used
as received, without further purification. IrCl3 3H2O was
3
purchased from Alfa Aesar; [AsPh4]Cl, MeI, CH2Cl2, hexanes,
and DMF were all purchased from Sigma-Aldrich. Infrared
spectra were recorded on a Nicolet 6700 by ThermoFisher
Scientific using OMNIC software. X-ray diffraction experi-
ments were carried out in the Beckman Institute Crystallo-
graphic Facility on
diffractometer.
a Bruker KAPPA APEXII X-ray
[AsPh4][Ir(CO)2I2] (1). [AsPh4][Ir(CO)2Cl2] (372.0 mg,
0.5296 mmol) was dissolved in methanol and stirred with
5 equiv of NaI (952.6 mg, 6.355 mmol) for 30 min. Volatiles
were removed in vacuo. The product was extracted with
CH2Cl2, and volatiles were removed again in vacuo to give the
desired product in virtually quantitative yield.
Figure 1. Anisotropically refined thermal ellipsoid representa-
tion of [Ir(CO)(κ2-CH2N(CH3)CHO)Cl2I]-, the anion of 6. The
[AsPh4]+ cation is not shown, and phenyl and methyl hydro-
gen atoms have been removed for clarity. Selected bond dis-
tances (A) and angles (deg): Ir-C1=1.8221(12), Ir-C3=2.0651
(11), Ir-I = 2.76746(12); C1-Ir-C3 = 95.23(5), C3-Ir-I =
171.68(3).
˚
[AsPh4][Ir(CO)(K2O,C-CH2NMeCHO)Cl2I] (6) and [As-
Ph4]2[Ir(CO)I4X] (5; X =Cl, I). 1 (200 mg, mmol), prepared
using the method of decarbonylation of DMF,1 was reacted
with neat MeI (2 mL) for 1 h. Volatiles were removed in vacuo
to give a red-orange solid, whose infrared spectrum matched
above, this appears to be an unprecedented transformation
for DMF; the structure demonstrates that the aldehydic
C-H bond has remained intact during the methyl C-H
activation.
the
literature
spectrium
for
[AsPh4][Ir(CO)2I3Me]
(νCO(CH2Cl2) 2098, 2046 cm-1).9 The solid was dissolved in
approximately 7 mL of CH2Cl2. This solution was layered
with hexanes and stored at -36 °C overnight to give red
needles (5) and trace yellow crystals (6) suitable for X-ray
diffraction (80 mg total). The infrared spectrum of these
crystals displayed only one CO stretch at 2046 cm-1, corre-
sponding to 5a.11 Crystallographic data have been deposited
at the CCDC; the deposition number for 6 is 679443, and that
for 5b is 725088.
The metalated methyl group in 6 is located trans to the
˚
iodide ligand, with an Ir-C3 bond length of 2.0651(11) A.
The aldehydic oxygen is also coordinated to Ir, forming a
˚
five-membered ring; the Ir-O2 bond distance is 2.0875(9) A,
˚
while the O2-C2 distance is 1.2704(15) A, somewhat longer
than that of unbound, liquid-phase DMF (1.24 A).13 Other-
˚
wise, the complex displays fairly typical octahedral geo-
˚
Structural Data for 6. Details are as follows: crystals
metry, with normal carbonyl Ir-C1 (1.8221(12) A) and
from CH2Cl2/hexanes; [C24H20As]+[C4H5NO2Cl2IIr]-, Mr =
˚
C1-O1 (1.1455(15) A) bond lengths and very small devia-
˚
872.41; triclinic, space group P1; a = 10.0906(3) A, b =
tions from 90° angles, except for those constrained by the
five-membered ring (C1-Ir-C3 = 95.23(5)o; C3-Ir-I =
171.68(3)o).
˚
˚
10.4936(3) A, c=14.0657(4) A; R=90.0840(10)°, β=107.5460
(10)°, γ=94.2860(10)°; U=1415.63(7) A ; dcalcd = 2.047 g/cm3;
3
˚
T = 100(2) K; maximum 2θ x°; Mo KR radiation
The origin of 6 is not clear: although there was no evidence
of either free or bound DMF in the sample of precursor 1 by
IR spectroscopy, trace amounts must have been present.
(Formation of 6 was quite reproducible with different
batches of the starting material 1.) One obvious candidate
is intermediate 3, which contains DMF; reaction with MeI
could conceivably initiate the observed C-H activation.
However, authentic samples of 3 do not react with MeI at
all. While the exact mechanism for formation of 6, specifi-
cally the timing of the methyl C-H activation step, remains
uncertain, the fact that the latter can compete at all with
activation of the much more reactive aldehydic C-H bond is
unexpected and intriguing.
˚
(λ=0.710 73 A); 24 422 unique reflections were obtained and
19 699 with I > 2.5σ(I) were used in the refinement; data were
collected on a Bruker KAPPA APEXII X-ray diffractometer;
for significant reflections merging R value 0.0277; residuals
Rf=0.0448, Rw=0.0408 (significant reflections); GOF 1.404.
Acknowledgment. We wish to thank Dr. Nilay Hazari
for useful discussions. This work was supported by BP
through the MC2 program. The Bruker KAPPA APEXII
X-ray diffractometer was purchased via an NSF CRIF:
MU award to the California Institute of Technology (No.
CHE-0639094).
Supporting Information Available: A CIF file giving crystal-
lographic data for 6. This material is available free of charge via
the Internet at http://pubs.acs.org.
(13) Ohtaki, H.; Itoh, S.; Yamaguchi, T.; Ishiguro, S.-i.; Rode, B. M.
Bull. Chem. Soc. Jpn. 1983, 56, 3406.