Article pubs.acs.org/Organometallics
Reaction of 2‑Methylanisole with TpMe2Ir(C6H5)2(N2): A Comprehensive Set of Activations Laura L. Santos,* Kurt Mereiter, and Margarita Paneque* Instituto de Investigaciones Quı ́micas (IIQ) and Departamento de Quı ́mica Inorgánica, Consejo Superior de Investigaciones Cientı ́ficas (CSIC) and Universidad de Sevilla, Avda. Américo Vespucio 49, 41092 Sevilla, Spain S Supporting Information *
ABSTRACT: The thermal activation of 2-methylanisole (60 °C) by the Ir(III) complex Tp Me2 Ir(C 6 H 5 ) 2 (N 2 ) (1; Tp Me2 = hydrotris(3,5dimethylpyrazolyl)borate) yielded a mixture of hydride complexes derived from each of the possible activation processes that might be anticipated in view of the previously investigated reactions of anisole and 2,6-dimethylanisole. Four isomeric species resulted from this reaction, namely hydride−carbene 6, emanating from two C(sp3)H and one C(sp2)H bond activation, hydride−alkylidene 7, whose formation requires in addition cleavage of the O−CH3 bond and the formation of a new C−C bond, and hydride−alkene derivative 4 and its hydride−alkylidene tautomer 3-Me, generated through somewhat related rearrangements. The appearance of these products may be rationalized on the basis of the reactivity previously ascertained for anisole and 2,6-dimethylanisole.
A
Scheme 1. Reactions of Complex 1 with (A) Anisole and (B) Phenetolea
s a continuation of independent and almost simultaneous reports from Werner and co-workers1 and from our own group,2 a number of studies on the C−H bond activation reactions of ethers promoted by reactive organometallic species has been provided by different researchers in the last 20 years.3−16 Recently, a timely overview of this chemistry has appeared.17 Our own studies focused on an assortment of ethers and utilized as metal precursors various iridium complexes containing ancillary hydrotris(pyrazolyl)borate ligands,18 particularly the TpMe2 ligand (i.e., that based on the 3,5dimethylpyrazolyl unit). A variety of products stemmed from these reactions, including inter alia tautomeric hydride−alkene and hydride−alkylidene structures that were demonstrated to exist in equilibrium under thermal, readily accessible conditions.19 Pertinent to the chemical reactivity discussed in this contribution were the reactions of the Ir(III) complex TpMe2Ir(C6H5)2N2 (1)20 with anisole (methyl phenyl ether) and phenetole (ethyl phenyl ether), whose results are summarized in parts A and B of Scheme 1, respectively. The first gave rise to two complexes, hydride−carbene 2-H and hydride−alkylidene 3-H, of which 2-H required the activation of three C−H bonds of a molecule of anisole, whereas 3-H involved in addition cleavage of the O−CH3 bond of the ether and the formation of a C−C bond.21 In turn, the phenetole reaction provided the related hydride−carbene and hydride− alkylidene complexes 2-Me and 3-Me (in the numbering scheme employed for these complexes we use H or Me for carbenes that contain a H atom or a methyl group as substituents at the carbene carbon atom), together with small amounts of the hydride−alkene derivative 4 (90%). However, complex 7 could not be isolated in a pure form due to its high reactivity toward oxygen and water.
reaction in very high yield and was characterized by NMR, exhibiting diagnostic δ values for the alkylidene unit at 15.21 (1H) and 251.0 ppm (13C{1H}). With regard to the formation of all these products, we had previously proposed a reasonable mechanistic pathway for the production of hydrides 2-H and 2-Me of Scheme 1, closely related to 6. On the basis of these precedents,21b 6 would form through intermediates A and B in Scheme 2 (top). The formation of 4 can also be easily explained by invoking the analogous reaction of 1 and 2,6-dimethylanisole22 (eq 1), in a process starting by the C−H activation of the Me group in the aryl ring of intermediate A (Scheme 2, bottom left). As mentioned above, the known derivative 3-Me had been obtained by the activation of phenetole (Scheme 1), but for the formation of 3-Me from 2-methylanisole a different pathway is required. Thus, we propose that 3-Me is generated from 4, as a result of the thermally induced prototropic tautomerization already reported.23 Hydride−alkylidene 7, which possesses a chelating aryloxide−carbene ligand, is suggested to form through a key step that consists of the α-aryloxide elimination connecting intermediates A and D (Scheme 2, bottom right). A second C−H activation step would then lead to the methylidene species E, which would render the reaction product 7 by means of migratory insertion of the aryl terminus into the IrCH2 bond and α-H elimination. The same scheme would be responsible for the formation of products 3-H and 3-Me upon activation of anisole and phenetole, respectively (see Scheme 1). A different α-OAr elimination could be suggested as an alternative way for the generation of intermediate E from B, but this can be discarded, since the formation of none of them (6 or 7) from the other has ever been observed (even under forcing conditions, 100−120 °C, in C6H6 or C6H12 solutions). In summary, close inspection of the reaction of the dinitrogen complex TpMe2Ir(C6H5)2(N2) (1) and 2-methylanisole revealed the formation of four isomeric products, namely complexes 3-Me, 4, 6, and 7 (eq 2), the first two of which exist in equilibrium and are interconnected by a reversible tautomeric exchange.23 Similarly to related C−H 567
dx.doi.org/10.1021/om301033c | Organometallics 2013, 32, 565−569
Organometallics
Article
Scheme 2. Proposed Mechanism for the Formation of Compounds 3-Me, 4, 6, and 7
1.630 g cm−3, T = 297 K, 67451 reflections collected (θmax = 30.04°) and merged to 7236 independent data (Rint = 0.023), final R indices (all data) R1 = 0.0293 and wR2 = 0.0612, 300 parameters.
1 H NMR (CD2Cl2, 25 °C): δ 15.21 (s, 1 H, IrCH), 7.17, 7.14, 6.55 (d, d, dd, 1 H each, 3JHH ≈ 7.5 Hz, 3 CHar), 6.00, 5.97, 5.64 (s, 1 H each, 3 CHpz), 2.54, 2.47, 2.46, 2.39, 2.37, 2.36, 1.29 (s, 3 H each, 6 Mepz + CqMe), −20.58 (s, 1 H, IrH). 13C{1H} NMR (CD2Cl2, 25 °C): δ 251.0 (IrC), 192.0 (CqO), 153.4, 153.0, 152.8, 152.6, 152.2, 145.1, 145.0, 144.2 (Cqpz + CqMe + CqCH), 137.8, 128.3, 114.9 (CHar), 107.0, 106.8, 106.6 (CHpz), 16.2, 15.8, 14.1, 13.1, 12.4, 12.3, 11.5 (Mepz + CqMe). 11B NMR (CDCl3, 25 °C): δ −8.8. IR (Nujol): ν(Ir− H) 2141, ν(B−H) 2524 cm−1. HRMS (FAB): m/z calcd for C23H30BN6OIr 609.2125, found 609.2104. X-ray Structure Determination for Compound 6. X-ray data were collected on a Bruker Smart APEX CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) and ω scans. Corrections for absorption were applied using the multiscan method and the program SADABS.26 Structure solution with direct methods and refinement on F2 was carried out with SHELXTL.26 Non-hydrogen atoms were refined anisotropically. All C- and B-bonded H atoms were placed in calculated positions and thereafter treated as riding. The Ir-bonded hydride H atom was freely refined in x,y,z and Uiso and gave a satisfactory result. A solvent-accessible void of ca. 57 Å3 volume centered at x,y,z = 0,1/2,0 was found unoccupied. Crystallographic data of 6 are as follows: C23H30BIrN6O, Mr = 609.54, yellow prism from CH2Cl2/hexane, 0.28 × 0.20 × 0.10 mm, monoclinic, space group P21/n (No. 14), a = 11.4749(7) Å, b = 13.7394(9) Å, c = 15.8998(10) Å, β = 97.678(1)°, V = 2484.3(3) Å3, Z = 4, μ = 5.40 mm−1, dexptl =
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ASSOCIATED CONTENT
S Supporting Information *
CIF file giving crystallographic data for 6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (L.L.S.);
[email protected] (M.P.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Financial support (FEDER contribution) from the Spanish MINECO (grants CTQ2007-62814 and Consolider Ingenio 2010 CSD 2007-0006) and the Junta de Andalucı ́a (FQM119, P09-FQM-4832) is acknowledged. 568
dx.doi.org/10.1021/om301033c | Organometallics 2013, 32, 565−569
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