Interligand C–C Coupling between α-Methyl N-Heterocycles and bipy

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Interligand C−C Coupling between α‑Methyl N‑Heterocycles and bipy or phen at Rhenium Tricarbonyl Complexes Rebeca Arévalo,† Lucía Riera,‡ and Julio Pérez*,†,‡ †

Departamento de Química Orgánica e Inorgánica, Universidad de Oviedo, C/Julián Clavería 8, 33006 Oviedo, Spain Centro de Investigación en Nanomateriales y Nanotecnología (CINN), CSIC, Universidad de Oviedo, Principado de Asturias, Avenida de la Vega 4−6, 33940 El Entrego, Spain

‡

S Supporting Information *

triflate or [BArF4] [ArF = 3,5-bis(trifluoromethyl)phenyl] salts in good yields (e.g., 85% for 1) by reacting [Re(CO)3(N-N)(OTf)] precursors and the appropriate 2-MeHet ligand.6 Compounds 1−6 (Scheme 1) exhibited typical 1H and 13C NMR signals of Cs-

ABSTRACT: Intramolecular C−C coupling between Nbonded 1,2-dimethylimidazole, 2-methyloxazoline, or 2methylpyridine and either 2,2′-bipyridine (bipy) or 1,10phenanthroline (phen) ligands results from α-methyl group deprotonation in the coordination sphere of Re(CO) 3 fragments. The nucleophilic CH 2 group generated by the deprotonation attacks the 6 (bipy) or 2 (phen) positions of the diimines, dearomatizing the involved pyridine ring and generating new asymmetric, fac-capping tridentate ligands.

Scheme 1. Coupling between Rhenium-Coordinated bipy or phen and α-Methyl Heterocyclesa

C

urrent methodologies for the synthesis of metal complexes require first the synthesis of the ligands and then their coordination to the metal fragment. The synthesis of polydentate ligands, especially asymmetric ones, is often a tedious, multistep procedure. The development of new synthetic methods to expand the typology of available metal complexes is of obvious relevance for the advance of coordination chemistry. A metaltemplated, modular synthesis could be conceived in which simple, stable, easily available building blocks are first coordinated to the metal center and then coupled to directly afford the metal complex of the polydentate ligand. Because the free ligand would not need to be obtained, this methodology could allow access to complexes containing ligands of limited stability. As a proof of concept, we have chosen octahedral rhenium(I) tricarbonyl complexes, which are currently employed in a variety of research areas1 and, therefore, are attractive synthetic targets. Many of these complexes contain 2,2′bipyridine (bipy) or 1,10-phenanthroline (phen), which are widely employed in coordination chemistry and are conventionally regarded as very inert ligands.2 Deprotonation of α-methyl (α-CH3) groups has been used to derivatize N-heterocycles of different ring size and degree of aromaticity, such as α-picoline (2-MePy),3 1,2-dimethylimidazole (1,2-Me2Im),4 or 2-methyloxazoline (2-MeOx),5 which are the ones employed here. However, it has never been applied to postcoordination coupling between ligands. As discussed below, cationic rhenium tricarbonyl complexes containing both α-methyl-substituted Nheterocycles and either bipy or phen react with a strong base to produce neutral complexes containing tridentate ligands that are a result of interligand C−C coupling. Cationic [Re(N-N)(CO)3(2-MeHet)]+ complexes [N-N = phen (1−3) and bipy (4−6); 2-MeHet = 2-MeOx (1, 4), 1,2Me2Im (2, 5), or 2-MePy (3, 6)] were synthesized as either © 2017 American Chemical Society

a [Re] = Re(CO)3. The chelate is phen for 1−3 and 7−9 and bipy for 4−6 and 10−12. The IR νCO (cm−1) wavenumbers for phen complexes in THF solution are given in italics. Reaction conditions: (i) KN(SiMe3)2, THF, −78°C.

symmetric species with a coordinated heterocycle. Determination of the structure of 1 by X-ray diffraction confirmed the expected octahedral geometry of the complex (Figure 1) and κnitrogen coordination of the oxazoline. The addition of KN(SiMe3)2 to tetrahydrofuran (THF) solutions of 1−6 resulted in a color change from yellow to purple Received: January 17, 2017 Published: April 3, 2017 4249

DOI: 10.1021/acs.inorgchem.7b00078 Inorg. Chem. 2017, 56, 4249−4252

Communication

Inorganic Chemistry

Figure 1. Thermal ellipsoid plots (30% probability) of the cation in compound 1 and of complexes 7, 9, and 11.

and in a shift to lower wavenumbers of the IR νCO bands (see Scheme 1), consistent with the formation of neutral products. Both the cationic complexes and neutral products show three IR νCO bands of similar intensity, diagnostic of fac-tricarbonyl species. Products 7−9 and 11 were found to be stable at room temperature and could be isolated in good yield (e.g., 76% for 7) by filtration and crystallization and fully characterized by NMR.7 Similar key features can be found in the NMR spectra of all of the neutral products; therefore, only data of 7 in THF-d8 are used in the discussion below. The NMR spectra of 7−12 indicate C1symmetric molecules in which the mirror plane of the precursors 1−6 is lost. For instance, asymmetric units from bipy or phen display eight 1H NMR signals (1H each), and in 7, all of the oxazoline-ring hydrogen atoms are inequivalent [multiplets at 4.37 (1H), 4.10 (2H), and 3.78 (1H) ppm], in contrast to the two triplets of precursor 1. Upfield bipy and phen signals (between 6.56 and 4.94 ppm, 1H each in 1H NMR, and a signal at 56.5 ppm in 13C NMR) are diagnostic of a dearomatized ring. 1H NMR singlets assigned to the 2-methyl groups in the precursors have been replaced by two doublet of doublets (at 2.73 and 2.35 ppm, 1H each) assigned to two diasterotopic hydrogen atoms of a new CH2 unit and further confirmed by 13C NMR DEPT-135 spectra (signal at 32.8 ppm, opposite to the CH signals). These features suggest dearomatization of a bipy or phen pyridyl ring as a result of intramolecular nucleophilic attack by the deprotonated 2-methyl group. Single products were obtained in all of the coupling reactions, so that no product of nucleophilic attack to either the CO ligands or the rhenium center was formed. The hydrogen nuclei of the new CH2 group show a cross-peak (3JHH) with the 1H most shielded signal (4.96 ppm) of the dearomatized ring in the 2D COSY spectrum, showing that the methylene group is bonded to a phen or bipy CH group. Moreover, the 2D 1H−13C HMBC spectra showed a 2JCH correlation between one of the CH2 hydrogen atoms (2.73 ppm) and the most shielded carbon atom of the phen or bipy unit (56.5 ppm), identifying the latter as the attacked CH group (1H−13C HSQC and DEPT-135 spectra). Full 1H and 13C NMR assignments of the products (see the Supporting Information) indicate nucleophilic attack on a nitrogen-adjacent CH group of bipy (position 6) or phen (position 2). This attack yields new sixmembered rings, in contrast with the five-membered rings formed in related reactions involving the deprotonation of coordinated N-alkylimidazoles,8 SMe2,9 and PMe3.10,11 KN(SiMe3)2 was found to react as a nucleophile toward [Re(CO)3(bipy)(PMe3)]OTf (the addition to one of the CO ligands)10 and [Re(CO)3(phen)(PMe3)]OTf (the reversible addition to phen, detected at low temperature).11 In the

reactions reported here, it was found to act only as a base, a difference attributed to the more acidic character of the methyl groups of the α-methyl heterocycles compared with those of PMe3. The solid-state structures of 7, 9, and 11, determined by X-ray diffraction (Figure 1), confirm the presence of fac-{Re(CO)3} fragments N,N′,N″-bonded to ligands formed by coupling between the deprotonated methyl group of the monodentate heterocyclic ligand and the C6 atom of bipy or the C2 atom of phen, in accordance with the solution NMR data. The sums of the angles around N1 [351.4(3)° in 7] indicate slightly pyramidalized amido groups (resulting from nucleophilic attack on the imino groups of the precursor), and angles around C2/C6 close to 109° are consistent with sp3 hybridization for this carbon atom. The more stable phen derivatives 7−9 were reacted with PMe3 in THF at room temperature (see Scheme 2 for the reaction of 7), affording green solutions with IR νCO wavenumbers slightly higher than those of their precursors. Scheme 2. Reaction of 7 with PMe3a

[Re] = Re(CO)3. The IR νCO (cm−1) wavenumbers in THF solution are given in italics. a

In the products 13−15, PMe3 has substituted the oxazoline, picoline, and imidazole donors, respectively, as shown by 1D and 2D NMR spectroscopy. The reaction time varies from 5 min for 9 to 18 h for 7 and 2 days for 8.12 1H and 13C NMR spectra display patterns for dearomatized asymmetric phen ligands similar to those of 7−9, indicating that the N-heterocyclic donor remains bonded to phen. However, signals of the CH2 groups are downfield-shifted in the 1H spectrum (e.g., 1.80 and 2.47 ppm, 1H each, for 7 versus 3.12 ppm, 2H, for 13 both in toluene-d8). Further confirmation of substitution was provided by 1H−15N HMBC experiments of complexes 9 and 15, which show a shift of the picoline nitrogen (from 252.0 ppm in 9 to 320.0 ppm in 15, close to that of free α-picoline, 319.1 ppm, in toluene-d8) upon reaction with PMe3, while the imine and amido nitrogen atoms of the phen-derived ligand remain virtually unchanged. The PMe3 4250

DOI: 10.1021/acs.inorgchem.7b00078 Inorg. Chem. 2017, 56, 4249−4252

Inorganic Chemistry

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adducts 13−15 were found to be stable in a toluene solution at room temperature for several hours, showing that the loss of the six-membered chelate ring does not induce significant thermal instability. Attempts to carry out coupling reactions between the free diimine and monodentate heterocycle under the same conditions as those used for coupling in rhenium complexes were unsuccessful because the 1H NMR spectra of the reaction crudes showed only the unreacted reagents. Deprotonation of 2-methyl heterocycles by the reported procedures3−5 followed by the addition to either phen or bipy also failed to afford coupled products, except in the reaction of α-picolyllithium with phen, where the 1H NMR spectrum of the reaction crude featured signals of a diasterotopic methylene group and a dearomatized pyridyl ring (along with unknown impurities) similar to those found in complex 9.13 In the rhenium-templated intramolecular C−C coupling reaction, coordination to the cationic Re(CO)3 fragment would increase the acidity of the CH3 group and the electrophilic character of the pyridyl rings. The resulting enhancement in the reactivity14 allows the reactions to proceed instantaneously under very mild conditions. Coordination to the transition-metal center also preorganizes the reactant molecules, facilitating their subsequent C−C coupling reaction, as can be seen in a comparison of the geometries of 1 and 7 (see Figure 1), and probably helps to selectively obtain the product of the attack to the 6 position of bipy or the 2 position of phen. Note that a major drawback of intermolecular nucleophilic additions to pyridines is that they usually afford mixtures of the products of the addition to the ortho and para positions.15 In turn, the preorganization in the geometry that favors the intramolecular coupling can be traced to the tendency of the Re(CO)3 fragment to adopt a fac disposition. Free dihydropyridines resulting from nucleophilic attack to pyridines are unstable intermediates with a marked tendency to recover their aromaticity either by elimination of metal hydride or by reaction with external oxidants.16,17 The relative stability of the rhenium complexes containing a dearomatized pyridine ring can be attributed to delocalization of the amido lone pair provided by the metal carbonyl fragment,18 to the steric protection that the bulky metal fragment lends to the CH group from which formal hydride abstraction would need to occur to rearomatize the ring, and to the fact that either the C−C bond created as a result of deprotonation or a Re−N bond would need to be broken for the pyridyl ring to recover its aromaticity. In conclusion, C−C coupling between deprotonated 2-methyl groups of 2-methyloxazoline, 1,2-dimethylimidazole, and 2methylpyridine and o-carbon atoms of bipy and phen (C6 and C2, respectively) has been demonstrated to occur in cationic Re(CO)3 complexes. These reactions directly afford, under mild conditions, complexes of a previously unknown type, containing fac-capping ligands with one dearomatized pyridyl ring. The compounds obtained from phen precursors are stable, and their reaction with PMe3 results in substitution of the 2-methyl heterocycle donor by phosphane. Previous metal coordination of the reactant building blocks has been demonstrated to be an effective tool for the direct synthesis of metal complexes containing structurally complex, asymmetric ligands from easily available, nonfunctionalized reagents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00078. CIF data for compounds 1, 7, 9, and 11 (CIF) Experimental procedures and NMR characterization (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34985103465. Fax: +34985103446. ORCID

Julio Pérez: 0000-0002-4788-2296 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank MINECO and FEDER (Grant CTQ201570231-P) and Principado de Asturias (Grant FC-15-GRUPIN14-103) for funding and the Ministerio de Educación for an FPU predoctoral fellowship (to R.A.).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00078 Inorg. Chem. 2017, 56, 4249−4252