Organometallics 2010, 29, 2430â2445 - Chemistry - University of ...

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Article Organometallics, Vol. 29, No. 11, 2010 2439 Figure 13. (a) ORTEP drawing of Ni(dippe)(1,2-η 2 -1,4-dicyanonaphthalene) (G3). Ellipsoids are shown at the 50% probability level. Selected bond lengths (A˚ ): Ni(1)-C(1), 1.9794(11); Ni(1)-C(2), 1.9765(12); Ni(1)-P(1), 2.1866(4); Ni(1)-P(2), 2.1807(4); C(1)- C(11), 1.4404(16); C(11)-N(1), 1.1557(16); C(1)-C(2), 1.4556(16); C(2)-C(3), 1.4221(17); C(3)-C(4), 1.3704(18); C(4)-C(12), 1.4336(17); C(12)-N(2), 1.1522(17). Selected angles (deg): P(1)-Ni(1)-P(2), 92.092(15); C(2)-Ni(1)-C(1), 43.18(5); C(1)-C(11)- N(1), 178.43(14); C(4)-C(12)-N(2), 178.85(15). (b) ORTEP drawing of Ni(dippe)(CN)(4-CN-naphthyl) (G13). Ellipsoids are shown at the 50% probability level. Selected bond lengths (A˚ ): Ni(1)-C(1), 1.933(3); Ni(1)-C(12), 1.877(3); Ni(1)-P(1), 2.1847(8); Ni(1)- P(2), 2.1915(9); C(12)-N(2), 1.137(4). Selected angles (deg): P(1)-Ni(1)-P(2), 88.32(3); C(12)-Ni(1)-C(1), 88.95(12); N(2)-C(12)- Ni(1), 177.9(4); C(4)-C(11)-N(1), 178.6(4). Figure 14. Energetics of C-C bond activation of 1,4-dicyanonaphthalene by [Ni(dmpe)] (free energies in kcal mol -1 ) relative to the total energies of fragments ([Ni(dmpe)] and 1,4-dicyanonaphthalene) (PCM corrected in THF). The former has two doublets in the 31 P{ 1 H} NMR spectrum at δ 61.7 and 72.5 with J P-P = 68 Hz. The latter shows a pair of doublets at δ 55.6 and 62.6 with smaller coupling constant (J P-P = 58 Hz). In the corresponding 1 H NMR spectrum, the resonances for the η 2 -arene species are shifted upfield compared to the η 2 -nitrile species. When the temperature was increased to 30 °C, the η 2 -nitrile complex I1 completely converted into the η 2 -arene complex I2. A single-crystal X-ray structure for the η 2 -arene complex was obtained (Figure 17), which showed nickel coordinating to the CdC double bond next to the cyano substituent. Upon heating to 120 °C for 1 week in THF, no C-CN cleavage was observed. Upon irradiation for 30 min, the oxidative addition product I18 appeared in the 31 P{ 1 H} NMR spectrum, with two sharp doublets at δ 65.8 and 75.7 with J P-P = 21 Hz. After irradiation for 36 h, only about 27% of I18 was formed, and upon heating at 120 °C for 1 h, it was completely converted back to I2. Results from DFT calculations are summarized in Figure 18. The η 2 -nitrile species SI1 is about 1.4 kcal/mol higher in energy than the η 2 -arene species SI2. While both adducts could be observed at low temperature, upon warming, SI2 is calculated to be preferred thermodyanically, consistent with the experimental observations. All other possible η 2 -arene
2440 Organometallics, Vol. 29, No. 11, 2010 Li et al. Scheme 2. Reactions of [Ni(dippe)H] 2 with 9-Cyanoanthracene and 9-Cyanophenanthrene Figure 15. ORTEP drawing of C 29 H 41 NNiP 2 (H4). Ellipsoids are shown at the 50% probability level. Selected bond lengths (A˚ ): Ni(1)-C(1), 2.0285(16); Ni(1)-C(2), 1.9634(16); Ni(1)- P(1), 2.1631(5); Ni(1)-P(2), 2.1697(5); C(1)-C(2), 1.449(3); C(2)-C(3), 1.428(3); C(3)-C(4), 1.362(3); C(4)-C(12), 1.449(2); C(1)-C(11), 1.446(2); C(11)-C(12), 1.449(2). Selected angles (deg): P(1)-Ni(1)-P(2), 92.10(2); C(2)-Ni(1)- C(1), 42.52(7); N(1)-C(15)-C(9), 178.8(2). complexes (SI3-17) are much higher in energy than SI2 and would not be expected to be observed in the NMR spectrum. As for the 9-cyanoanthracene reaction, TSI218 for C-CN cleavage is also at high energy, and dissociation may occur in preference to C-CN activation. In reactions of the Ni(dippe) fragment with polycyclic aromatic nitriles, the η 2 -arene complexes with nickel coordinated via the CdC double bond containing a cyano substituent were in every case the intermediates leading to C-CN bond cleavage. Among all possible structures, only certain η 2 -arenes were stable enough to be observed, andsomewereevenmorestablethantheη 2 -nitrile species. The fluxional processes result from a [1,3] migration around the end of the fused polycyclic aromatic ring via an η 4 transition state. This same preference for the formation of η 2 -complexes in which the loss of resonance energy is minimized was seen in studies of Cp*Rh(PMe 3 )- (polycyclic arene) complexes 17 and other P 2 Ni(arene) complexes. 18 Conclusion The reaction of [Ni(dippe)H] 2 with cyanobenzenes, cyanonapthalene, and derivatives at low temperature leads to the formation of both η 2 -nitrile and η 2 -arene complexes. Both converted to C-CN activation products upon heating. Due to the high polarity of the C-CN bond cleavage products, solvent effects were taken into consideration in all DFT calculations in terms of the PCM correction, and the results show excellent agreement with the experimental observations. The transition state for the C-CN cleavage was located on the potential energy surface and found to connect with the η 2 -arene complex with nickel coordinated to the CdC double bond next to the cyano substituent. In the case of 9-cyanoanthracene and 9-cyanophenanthrene, η 2 -arene species were more stable than η 2 -nitrile complexes. No C-CN cleavage was observed in the reaction with 9-cyanoanthracene, ascribed to the inability to coordinate to the tetrasubstituted double bond adjacent to the nitrile. The fluxionalities of the η 2 -arene complexes were due to lowenergy barriers for migration of the [Ni(dippe)] fragment around the aromatic ring via a series of η 3 -orη 4 -coordinated transition states. Experimental Section General Procedures. All reactions were carried out using standard Schlenk and glovebox techniques under nitrogen. Solvents were dried and distilled before use from sodium/ benzophenone ketyl or directly taken from an Innovative Technologies MD-6 solvent purification system. Deuterated solvents (Cambridge Isotope Laboratories) for NMR experiments were dried over sodium/benzophenone ketyl and distilled under vacuum. All other chemicals and filter aids were reagent grade and were used as received. 1 Hand 31 P{ 1 H} NMR spectra were determined on AVANCE400 spectrometers in THF-d 8 or toluene-d 8 ; chemical shifts (δ) are relative to the deuterated solvent residual protons, and 31 P{ 1 H} NMR spectra are relative to external 85% H 3 PO 4 . The synthesis of [Ni(dippe)H] 2 was carried out using the previously reported procedure. 19 1,2-, 1,3-, and 1,4-Dicyanobenzene, 1- and 2-cyanonaphthalene, 9-cyanoanthracene, and 9-cyanophenanthrene were purchased from Aldrich. 1,4-Dicyanonapthalene was obtained from Alfa Aesar, and 1-cyano-4-methylnaphthalene was obtained from Matrix Scientific. All were dried under vacuum before use. (17) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones, W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685. (18) Brauer, D. J.; Kr€uger, C. Inorg. Chem. 1979, 16, 884. (19) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1997, 119, 10855.
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