Effects of the halogenido ligands on the Kumada-coupling catalytic activity of [Ni{tBuN(PPh2)2-κ2P}X2], X = Cl, Br, I, complexes

Novel nickel(ii) complexes bearing (tbutyl)bis(diphenylphosphanyl)amine and different halogenido ligands, [Ni(P,P)X2] = [Ni{tBuN(PPh2)2-κ2P}X2], (X = Cl, Br, I) are prepared, characterized by IR and NMR spectroscopy, mass spectrometry and X-ray crystallography, and tested as catalysts in the Kumada cross-coupling reaction of model substituted iodobenzenes and p-tolylmagnesium bromide. The data obtained together with DFT calculations indicate that these new catalysts operate in the Ni(i)–Ni(iii) mode. The highest catalytic activity and selectivity are exhibited by [Ni(P,P)Cl2], which is most easily reduced by the used Grignard reagent to the Ni(i) state. This process is much more energy demanding in the case of the bromido and iodido complexes, causing the appearance of the induction period. [Ni(P,P)Cl2] is also very active in the cross-couplings of substrates with iodine atoms sterically shielded by ortho substituents. The data obtained are in good accordance with the described positive effect of the increased electron-releasing power of N-substituents R′ on the overall catalytic performance of [Ni{R′N(PPh2)2-κ2P}X2] complexes.

reaction. The latter catalyst has also been anchored onto mesoporous molecular sieves, thus providing an active heterogenized catalyst. 23 In homogeneous catalytic reactions, both catalysts bearing R 0 ¼ (S)-CHMePh) showed a substrate conversion (68% for X ¼ Cl and 63% for X ¼ Br), 35 signicantly lower compared to that of the catalyst with R 0 ¼ (CH 2 ) 3 Si(OMe) 3 and X ¼ Cl (79%). 23 These results suggested that the increased electronegativity of coordinated halogenido ligands and the increased electron-donating power of the R 0 moiety have positive effects on the catalytic efficiency of this type of nickel(II) complexes. In the work presented herein, the effects exerted by the identity of halogenido ligands X À and the R 0 moiety on catalytic activity and selectivity were further assessed by exploring three novel complexes, [Ni{ t BuN(PPh 2 ) 2 -k 2 P}X 2 ], X ¼ Cl, Br, I, henceforth referred to as [Ni(P,P)X 2 ], bearing the strongly electron-releasing t butyl ( t Bu) group as R 0 .

Results and discussion
Synthesis, spectroscopic and mass spectrometric characterization of nickel(II) complexes Complexes [Ni(P,P)X 2 ], X ¼ Cl, Br were prepared by mixing equimolar amounts of t BuN(PPh 2 ) 2 ligand 48 with a dichloromethane solution of [Ni(PPh 3 ) 2 Cl 2 ] or [Ni(DME)Br 2 ] (DME ¼ 1,2dimethoxyethane), respectively (see Scheme 2). Complex [Ni(P,P)I 2 ] was prepared by adding a four-fold stoichiometric excess of NaI to complex [Ni(P,P)Cl 2 ], in acetone, to replace the chlorido ligand by iodide (Scheme 2). The three complexes were characterized by IR, UV/vis and 1 H, 13 C and 31 P NMR spectroscopy, mass spectrometry, as well as by UV/vis spectroscopy (see spectra in ESI †).
The 31 P NMR spectra of complexes [Ni(P,P)X 2 ], X ¼ Cl, Br, I, each exhibits a singlet at d 46.6, 53.0 and 63.7 ppm, respectively, due to their coordinated (P,P) ¼ t BuN(PPh 2 ) 2 ligand. The signal of the free ligand at d 57.8 ppm 48 is absent in all spectra. These observations demonstrate that, in all complexes, the ligand is coordinated to nickel(II) via both Phosphorus atoms that are equivalent to each other, in agreement with recently reported data on the analogous [Pd(P,P)X 2 ] complexes. 14 In addition, the spectra monitored during the six-month storage of samples did not show any signicant change, which conrmed the longterm stability of all three complexes when exposed to air at room temperature. 13 C NMR spectra are affected by the presence of paramagnetic impurities formed in the presence of the air and moisture. It should be stressed that the amount of impurities affected only the 13 C NMR line shape.
The broadening of 13 C NMR lines remained constant for at least six months, conrming the sufficient stability of all complexes in the environment.
Based on the comparison with similar [Ni{R 0 N(PPh 2 ) 2 -k 2 P} X 2 ] complexes reported in the literature,  as well as with the corresponding [Pd(P,P)X 2 ] complexes, 14 the IR bands of [Ni(P,P) X 2 ], occurring at 1437, 1433 and 1437 cm À1 for X ¼ Cl, Br and I, respectively, should be assigned to the n(P-Ph) stretching modes, whereas the band occurring uniformly at 868 cm À1 is assigned to the n(P-N-P) stretching mode. These data indicate nearly identical conformation of the P-N-P backbone of the ligand in all three complexes, as conrmed by X-ray crystallography analysis (vide infra).
The LDI-MS spectra of the [Ni(P,P)X 2 ] samples (see ESI †) contain molecular peaks at 592.09, 679.98, and 775.95 Da (for X Scheme 2 Synthesis of complexes [Ni(P,P)X 2 ], DME ¼ 1,2dimethoxyethane.  (12) 117.68 (5) ¼ Cl, Br, I, respectively) that correspond to the structures ionized with Na + ion. The respective masses observed at 534.13, 578.07, and 626.05 Da correspond to the charged molecular fragments formed by a loss of one halogenido ligand during the ionization process.

X-ray crystallography
Single crystals suitable for X-ray crystallographic studies of [Ni(P,P)X 2 ] were obtained by the layering method employing a CH 2 Cl 2 /n-hexane (1 : 3 by vol.) solvent system. Selected bond lengths and angles are listed in Table 1. As can be seen in Fig. 1, the chlorido complex crystallized in the P2 1 2 1 2 1 space group as disolvate [Ni(P,P)Cl 2 ]$2CH 2 Cl 2 (Fig. 1A), while the bromido and iodido complexes crystallized as pure substances in the P2 1 cn and P2 1 /c space group, respectively. The structure of the [Ni(P,P) X 2 ], X ¼ Cl, Br, complexes is shown in Fig. S13 (ESI †). The overlapped crystal-structure representations of molecules of the three complexes indicate a high structural similarity (Fig. 1B).
Individual molecules of all three complexes exhibit a nearly square planar geometry of their NiP 2 X 2 core units (with the Ni atom in the center) (Fig. 1). The NiP 2 X 2 core shows a mean deviation from the best plane dened by the Ni, P and X atoms equal to 0.0598Å for X ¼ Cl, 0.1086Å for X ¼ Br, and 0.0311Å for X ¼ I, with the P1 atom exhibiting the largest deviation (À0.0809Å for X ¼ Cl and À0.1408Å for X ¼ Br), and the P2 atom (À0.0417Å) for X ¼ I. The NiP 2 X 2 core is slightly tetrahedrally distorted, with the X1-Ni-X2 and P1-Ni-P2 planes exhibiting a dihedral angle of 6.06 , 9.59 , and 2.67 for X ¼ Cl, Br, and I, respectively. 49 The nickel(II) center is coordinated to the (P,P) chelating ligand, forming a four-membered Ni-P-N-P ring. The mean deviation from the best plane dened by all four atoms is 0.0187Å, 0.0065Å and 0.0314Å for X ¼ Cl, Br and I, respectively.
The Ni-P and Ni-X bond lengths, as well as the P-Ni-P endocyclic angles of [Ni(P,P)X 2 ], X ¼ Cl, Br, I complexes (73.32 , 73.29 and 73.73 , respectively, Table 1) are typical of [Ni {R 0 N(PPh 2 ) 2 -k 2 P}X 2 ] complexes reported in the literature.  The narrow range of the observed endocyclic bite P-Ni-P angles is attributed to the formation of the four-membered Ni-P-N-P ring. The P1-N-P2 angles of the three complexes are also similar (Table 1), conrming the proposed similar conformation of their P-N-P backbone based on the conserved n(P-N-P) IR transitions of the three complexes (868 cm À1 , vide supra). As expected, the Ni-X bond lengths of [Ni(P,P)X 2 ] increase with the increasing size of the halogen atom X, whereas the respective Ni-P bond lengths remain rather conserved (Table 1).

Catalytic activity of [Ni(P,P)X 2 ] in Kumada cross-coupling
The catalytic activity of new complexes in Kumada coupling was tested using 4-( t butyl)-1-iodobenzene (4-t Bu) as the substrate and p-tolylmagnesium bromide (T) as the reagent (Scheme 3). Grignard reagent T was used in a stoichiometric excess of 20% to compensate for the decay of T due to its homo-coupling caused by traces of oxygen. (Up to 12% homocoupling of the product: 4,4 0 -dimethylbiphenyl, was detected even in the supplied reagents).
Experimental data indicating the effect of halogenido ligand on the catalytic activity of [Ni(P,P)X 2 ] complexes are presented in Fig. 4. As can be seen, [Ni(P,P)Cl 2 ] showed not only clearly the  highest initial and overall (nal) yield of the cross-coupling product, Y CROSS , but also high product selectivity. Further, it is seen that the bromido and iodido complexes showed a very low initial activity (Y CROSS of 1%), but a reasonably high nal yield (Y CROSS of 60-65%), albeit signicantly lower compared to [Ni(P,P)Cl 2 ] (90%). Such data indicate a difficult start of propagation (the presence of an induction period) but a reasonably long lifetime of the active species (vide infra).
A nearly constant yield of the substrate homo-coupling product, Y HOMO , up to 2% occurred in all experiments. This side product most likely resulted from the Ullmann type reaction that took place during degassing of the mixed solutions of substrate and catalyst before adding the Grignard reagent T. Nickel compounds are well known to catalyze Ullmann-type reactions. [50][51][52] The observed difference in the catalytic activity [Ni(P,P)Cl 2 ] and its bromido and iodido counterparts underscores the role of the halogenido ligands in the catalytic activity of nickel(II) complexes. Therefore, the catalytically most active complex [Ni(P,P)Cl 2 ] was in addition tested in couplings of T with substrates in which the reacting iodine atom is sterically hindered by one or two methyl groups in ortho positions (see Scheme 3). These experiments were aimed to assess how this shielding affects the catalytic activity and product selectivity of the catalyst. As can be seen in Fig. 3, the increase in iodine atom shielding signicantly but not substantially decreased the cross-coupling conversion (Y CROSS ) and increased the substrate homo-coupling conversion (Y HOMO ). Interestingly, the nal total substrate conversion was nearly the same (Y CROSS + Y HOMO ¼ 90 AE 2%) for all three substrates, regardless of shielding (see also Tables S1 and S2 in the ESI †).
Useful information for understanding the mechanism of the catalytic action of Ni(PNP) complexes provides the comparison of the data shown in Fig. 2 with literature data (see Table 2). The literature data show that [Ni(P,P)X 2 ] complexes bearing R' ¼ t Bu exhibit a signicantly higher catalytic efficiency and selectivity in the Kumada cross-coupling as compared to the earlier re- 3 showed an overall activity (Y CROSS z 64%) comparable to that of [Ni(P,P)X 2 ], X ¼ Br, I, but signicantly lower cross-coupling selectivity (Y HOMO z 13% vs. 2%, respectively). The X ¼ Cl, Br complexes with R 0 ¼ (S)-CHMePh showed lower activity (Y CROSS of 48 and 42%, respectively) as well as selectivity (Y HOMO z 20%). These data pointed to the important role of the Nsubstituent (R 0 group) of the (P,P) ligand. The electron-donating power of the N-substituents decreases in the sequence: t Bu > (CH 2 ) 3 Si(OMe) 3 > (S)-CHMePh. The data presented in Fig. 2 and  3 and Table 2 show that the catalytic activity and selectivity decrease in the same order. Hence it is clear that the increased electron-releasing power of the N-substituent in [Ni(P,P)Cl 2 ] increases its catalytic activity and selectivity in the Kumada cross-coupling reactions of aromatic compounds. The reason for this observation might be that electron density released by the t Bu group towards the nickel(II) center facilitates the oxidative addition step of the overall reaction mechanism.

Mechanistic considerations
In an effort to probe the operative mechanism, the following experiments were carried out. Specically, to a THF solution of [Ni(P,P)Cl 2 ], the reagents 4-( t butyl)-1-iodobenzene (4-t Bu) and ptolylmagnesium bromide (T) were added in this order, with concomitant recording of UV-vis spectra (Fig. 4A). In a second experiment, the above order of adding the two reagents was reversed (Fig. 4B).    %   60  64  14  48 20 42 20 The spectra showed in Fig. 4 provide evidence that the addition of the Grignard reagent (T) may lead to the reduction of the nickel(II) pristine catalyst to catalytically active nickel(I) species, as previously described in the literature. 53,54 The reaction mixture shown in Fig. 4A was also studied by 1 H and 31 P NMR spectroscopy (Fig. 5 and 6, respectively).
Two multiplets around 8.4 and 7.6 ppm are assigned to the [Ni(P,P)Cl 2 ] complex, whereas those at 7.59 and 7.18 ppm are ascribed to 4-t Bu. Upon the addition of 4-t Bu, the catalyst's multiplet at 8.4 ppm persists unchanged. Aer the addition of T, a reaction is taking place, observable in the region 7.6-6.9 ppm. Conversely, when T was added rst to the solution of [Ni(P,P)Cl 2 ], the latter changes were not observed.
The NMR spectra for the reaction mixture of Fig. 4B (i.e. rst addition of T and then of 4-t Bu), are presented in the ESI ( Fig. S17 and S18, † respectively). The 1 H NMR signals ascribed to the catalyst disappeared in both cases, irrespective of the order of the reagents' addition. The 31 P NMR signal was shied from 46.6 ppm to 33 ppm upon addition of 4-t Bu rst, and disappeared for the reversed order of reagents' addition. The signal disappearance is most probably caused by the reduction of the diamagnetic nickel(II) precatalyst to the paramagnetic nickel(I) species, and hence broadening of the corresponding peaks. The observed 31 P NMR signal shi (when 4-t Bu was added rst) is probably related to a species resulting from the pristine complex (vide infra), but the identity of this species cannot be conclusively determined. Moreover, we do not have a plausible explanation for the difference in behavior upon the reverse order of the reagents' addition (mixture of Fig. 4B). Based on the above experimental evidence, the catalytic cycle may be operative via the Ni(I)-Ni(III) mode (vide infra), however, the Ni(0)-Ni(II) mode cannot be entirely ruled out. Additional investigations would be needed for such an endeavor, but this is beyond the scope of the current work. [55][56][57][58] DFT calculations X-ray crystallographic and DFT-calculated structures of complexes [Ni(P,P)X 2 ] are depicted in Fig. 7. The signicant structural similarity indicates an adequate accuracy of the performed DFT calculations.
The DFT-calculated HOMO and LUMO orbitals of [Ni(P,P) X 2 ], X ¼ Cl, Br, I, are depicted in Fig. S16 (ESI †). As can be seen, the LUMOs are similar for all three complexes and exhibit antibonding character. However, the HOMOs differ signicantly. The HOMOs of the bromido and iodido complexes dominantly comprise non-bonding orbitals located on the halogenido ligands (lone pairs), whereas the HOMO of [Ni(P,P)Cl 2 ] is signicantly contributed with the electrons of d orbitals of nickel(II). In turn, the opposite trend is observed in HOMO-2 orbitals of [Ni(P,P)Cl 2 ], with almost no contribution of d orbitals of nickel(II).
It is known that in the palladium-catalyzed C-C bondforming couplings (including Kumada), palladium alternates exclusively between oxidation states Pd(0) (when entering the oxidative addition) and Pd(II) (at reductive elimination). In contrast, the nickel-based catalysts are known to operate either in the Ni(0)-Ni(II) or the Ni(I)-Ni(III) mode. [53][54][55][56][57][58] Consequently, the nickel(II) precatalyst (pristine nickel complex) undergoes reduction to the nickel(I) state (initiation), before entering to the propagation cycle operating in the Ni(I)-Ni(III) mode. The reduction occurs by reaction with the Grignard reagent. Based on literature screening and our experimental observation, we presume that initiation provides nickel(I)-based catalytic active species, which, subsequently, operate in the Ni(I)-Ni(III) mode, as depicted in Fig. 8.
In order to shed more light on the halogenido effect, we investigate the transmetalation step of the catalytic cycle in detail, Fig. 9. First, the interaction of [Ni(P,P)X] intermediate with Grignard reagent leads to transient species (TS), with   signicantly lower DG values. These transient species can be regarded as true transition states with inherent imaginary frequency corresponding to the reaction coordinate. Then, the respective MgBrX molecule is released and an aryl derivative [Ni(P,P)Ar] enters the oxidative addition step.
It can be presumed that the initiation step, being a radical process, is rate-determining, however, it was computationally very time-consuming to locate the corresponding transition state. Nevertheless, as shown in Fig. 9, the corresponding DG values are systematically lower for the chlorido intermediates with respect to bromido and iodido intermediates. Building on the fact that the initiation step comprises interaction with the Grignard reagent, it can be presumed that this step will follow a similar trend as shown for the transmetalation step.
Consequently, the DFT calculations on the nickel(II) precatalysts [Ni(P,P)X 2 ] revealed differences in the electronic structure of the chlorido complex compared to the bromido and iodido complexes. These differences, together with the aforementioned mechanistic considerations (vide supra), probably explain the differences in the catalytic activity as well as in the observed induction periods for the bromido and iodido complexes. The full set of the DFT-optimized structures investigated in this study is gathered in the ESI2 as a text le. † Noteworthy, magnesium halides (reaction byproducts) optimized in the bulk THF showed bent structures (linear in the gasphase), an effect that was observed for water microsolvation of magnesium chloride. 59

Conclusions
The work presented herein demonstrates that the catalytic activity and product selectivity of [Ni{R 0 N(PPh 2 ) 2 -k 2 P}X 2 ] complexes in Kumada cross-coupling reactions depend on two tunable factors: (i) the identity of the halogenido ligand, and (ii) the nature of the N-substituent R 0 moiety of the (P,P) ligand. All three [Ni(P,P)X 2 ] complexes showed excellent product selectivity in the Kumada cross-coupling of Grignard reagent T with 4-t Bu substrate bearing the spatially unshielded reacting iodine atom. Iodine atom shielding by ortho-substituents signicantly decreases the cross-coupling selectivity of [Ni(P,P)Cl 2 ]. The yield of the Kumada cross-coupling catalyzed by [Ni(P,P)X 2 ] complexes decreases in line Cl > Br > I. The comparison of the activity of [Ni(P,P)X 2 ] with that of [Ni{R 0 N(PPh 2 ) 2 -k 2 P}X 2 ] complexes bearing different R 0 moieties indicates that increasing the electron-releasing power of R 0 increases both the catalytic activity and the cross-coupling selectivity of this family of nickel(II) catalysts. We outline the transformation of the precatalyst complex to a nickel(I) active species via reduction with Grignard reagent accompanied by a one halogenido ligand loss. DFT calculations reveal that [Ni(P,P)X] active species formation is thermodynamically preferred for chlorido complex, as well as, the transmetalation step of the catalytic cycle is smoother for this species than for bromido and iodidio counterparts. Consequently, calculated results explain the difference in the catalytic activity of the chlorido complex compared to the bromido and iodido complexes.

Experimental section
Syntheses were performed under argon atmosphere using standard Schlenk techniques. Solvents were dried and distilled according to procedures reported in the literature. 60 The t BuN(Ph 2 P) 2 ligand was synthesized according to the literature procedure. 48 Determination of C, H, and N contents: FlashSmart™ Elemental Analyzer (Thermo Fisher Scientic) was used for the concurrent determination of C, H and N content. Approximately 1.5 mg of the sample was weighed to a tin capsule using the precision weighing balance Sartorius SE 2-OCE. The capsule was sealed and inserted into the  autosampler of the Analyzer and the standard operational procedures were employed. 1 H and 31 P NMR spectra were monitored on a Varian NMR Systems 300 MHz spectrometer at 300 MHz for 1 H NMR spectra referenced to the solvent signal d ¼ 7.25 ppm and Varian UNITY Inova 400 MHz spectrometer for 31 P NMR spectra at 162 MHz using 1 H decoupling, referenced to the external standard 85% H 3 PO 4 d ¼ 0.00 ppm, in CHCl 3 -d at 25 C. First-order analysis was used for spectra deciphering. All NMR spectra are shown in the ESI. † 13 C NMR spectra were recorded on BRUKER Avance 600 MHz (151 MHz, CDCl 3 ) and Varian Unity Inova 400 MHz (101 MHz, CDCl 3 ). FTIR spectra were obtained on a Shimadzu IR Affinity-1 instrument using KBr pellets in transmission mode. All FTIR spectra are shown in the ESI. † The samples for Laser Desorption Ionization -Time Of Flight Mass spectrometry (LDI-TOF MS) were prepared using a modied dried droplet method: the sample with sodium triuoroacetate as ionization agent was dissolved in CHCl 3 . The solution, without any matrix, was deposited on the target and le to dry before measurement. Mass spectra were recorded on an ultraeXtreme ToF-ToF mass spectrometer (Bruker Daltonics) equipped with a 2000 Hz smartbeam-II laser (355 nm) using the positive-ion reectron mode and panoramic pulsed ion extraction, aer external calibration with poly(ethyleneglycol). 61 The MS spectra are shown in the ESI in Fig. S14. † UV-vis spectra in THF solution are presented in ESI in Fig. S15. †

Synthesis of complexes
Dichlorido[( t butyl)bis(diphenylphosphanyl-k 2 P)amine]nickel(II), [Ni(P,P)Cl 2 ]. Weighed amounts of [Ni(PPh 3 ) 2 Cl 2 ] (0.120 g, 0.183 mmol) and t BuN(PPh 2 ) 2 (0.081 g, 0.183 mmol) were dissolved in CH 2 Cl 2 (10 mL), affording an orange solution, which was allowed to react under stirring at room temperature for 24 h. Then, the solution was concentrated to 2 mL, adding n-hexane (5 mL), which afforded an orange powder. This powder was ltered and dried under vacuum (yield 0.100 g, 96%). Selected IR bands (KBr, cm À1 , see Dibromido[( t butyl)bis(diphenylphosphanyl-k 2 P)amine]nickel(II), [Ni(P,P)Br 2 ]. Weighed amounts of [Ni(DME)Br 2 ] (0.077 g, 0.260 mmol) and t BuN(PPh 2 ) 2 (0.110 g, 0.250 mmol) were dissolved in CH 2 Cl 2 (10 mL), affording a red solution, which was allowed to react under stirring at room temperature for 24 h. Then, the solution was concentrated to 4 mL, adding n-hexane (10 mL), which afforded a red powder. This powder was ltered and dried under vacuum (yield 0.130 g, 79%). Selected IR bands (KBr, cm À1 , see  General experimental procedure for the Kumada coupling Catalytic Kumada coupling reactions were performed using a modied literature procedure. 35 In a Schlenk ask, equipped with rubber septum, 1-iodoaryl derivative ((a) 4-iodo-t butylbenzene, (b) 2-iodotoluene, (c) 2-iodo-1,3-dimethylbenzene (1.0 mmol)), 10 mmol of complex [Ni(P,P)X 2 ] and 0.5 mmol of mesitylene were dissolved in 3 mL of freshly distilled THF. The mixture was cooled down with liquid nitrogen (10 min). Schlenk ask was connected to the vacuum-argon manifold and evacuated (0.5 mbar). The sidearm stopcock of the Schlenk ask was closed, removing the cooling, and mixture was le standing to reach room temperature. The Schlenk ask was ushed with argon in the nal degassing cycle stage. The degassing cycle was repeated three times. Finally, a hexane solution p-tolylMgBr (1.2 mmol) was carefully added under argon. The mixture was le to react at room temperature. Pick outs (50 mL) were performed under argon at 20, 90, 1080 min. Withdrawn samples were diluted in 2 mL of pure methanol (quenching of the reaction). Each sample was analyzed on a Shimadzu GC-2010 chromatograph, equipped with an FID detector and a 30 m, 0.25 mm (0.25 mm lm of anchored phase) Zebron DB-5 capillary column. Spectral mechanistic study. 5 mg of [Ni(P,P)Cl 2 ] was dissolved in 4 mL of THF-d 8 . Stock solutions of 4-iodo-t butylbenzene and p-tolylMgBr in THF-d 8 were prepared (approximately 0.1 M each). Five drops of each solution were stepwise added to the complex solution in the appropriate order (vide supra). 1 H NMR, 31 P NMR, and UV-vis spectra were recorded at each stage.
Computational details. All structures employed here were optimized using the M06 functional 66 in conjunction with the LanL2DZ 67-69 (for the nickel center and halogens) and cc-pVDZ 70 (other atoms) basis sets, as implemented in the Gaussian 09 program package. 71 Geometry optimizations for precatalysts were started from the experimentally determined solid-state structures. Frequency analyses at the same level of theory were performed in all the stationary geometries in order to assign them as genuine minima (no imaginary frequency) or transition states (only one imaginary frequency corresponding to the reaction coordinate) on the potential energy surface as well as to evaluate the thermochemical corrections at 298.15 K. The solvent effects were estimated through the SCRF single point calculations in tetrahydrofuran as the continuum dielectric (3 ¼ 7.4257) at the optimized geometries in the gas phase using an SMD model. 72 The change in the number of moles (n) in the reaction was accounted for by a correction of 1.9Dn kcal mol À1 (the effect of the change associated with moving from a standard state pressure to standard state concentration).