Scope of the work
Due to the spontaneous nature of the reaction, all our efforts to isolate the intermediate were unsuccessful. As found experimentally, the Grignard reagent preferentially adds over the more electronegative carbonyl oxygen than the imine nitrogen, which was confirmed by the shorter Mg---O bond compared to the Mg----N bond in the intermediate suggested by computational studies . However, due to the unsymmetrical nature of the ligand and the presence of two coordination sites (oxygen and nitrogen atoms), we were motivated to explore the possibility of intramolecular chelation involving magnesium from the Grignard reagent.
13 would also play an important role in the course of the reaction due to the rigid three-membered ring and bulky aryl substituent on the imine nitrogen.
Computational Methodology
- Computational Study
In the well-known Kohn and Sham equation, the potential possessed by an electron was expressed as a function of the electron density 19. The scientist Walter Kohn received the Nobel Prize in Chemistry in 1998 for his contribution to the development of the theory of density functions. The exchange energy is also calculated using approximations, including taking into account the percent exchange from HF theory.
QST looks superficially similar to the Linear Synchronous Transit (LST) algorithm which looks for a maximum along the linear path between reactants and products. However LST often leads to a structure with two or more negative frequencies, so QST which looks for the maximum on the parabola connecting reactants and products is an improvement on LST 23. Unless otherwise specified, energies reported include the energy of a B3LYP point with zero. point energy corrections from the HF method, since this method compared to the experimental results of the so-called G2 group of molecules has a reasonable value of the maximum absolute deviation.
Since several attempts to experimentally isolate the Grignard reaction intermediate with the Ar-BIAO class of ligands were unsuccessful, the gas-phase reaction of CH3MgBr with 2 and 2' was investigated to suggest a likely intermediate and mechanism. In order to gain better insight into the role of steric constraints in the reaction pathway, the model system 2' was chosen to contain the α-imino carbonyl functionality common to all Ar-BIAO ligands, but a methyl substituent instead of an aryl group at the nitrogen atom and the presence of a simple cyclopentanone ring relieves 2' of most geometric constraints. CH3MgBr coordinating to ligand 2 and 2' is considered as the precomplex R and R' for the real and model systems (Figure 3) and all energies were calculated relative to the precomplex. 16 The pre-complex (R & R') and post-complex (P and P') were freely optimized without any geometrical constraints and there was a decrease in the Mg1-N1 bond distances in the optimized figures compared to the input structures, indicating simultaneous coordination nitrogen and oxygen with magnesium.
Initially, our attempt to optimize the predicted input structure to the transition state was unsuccessful, so we examined the carbonyl carbon (C1) and the methyl carbon of the Grignard reagent bond (C2) without any symmetry constraints on HF/3-21G. (d) level to perform a series of energy minimizations where the C1-C2 bond length was varied at regular intervals and the bounds were the C1-C2 bond lengths in the post- and pre-complex optimized structures. A likely high-energy transition state was evident from the plots obtained, and the TS energies were calculated using the above-mentioned combined method.
Results and Discussion
- Experimental Study
- Chelation vs Non-Chelation Coordination
- Model System
- Real System
The transition states were characterized by a single imaginary frequency, which in the visualization was the vibration between the forming C-C bond which is the C1-C2 bond. In one of the optimized geometries, the imine nitrogen coordinated with Mg (Mg1-N1=2.22 Å) and in the other situation there was a negligible interaction between these two atoms (Mg1-N1=4.63 Å) (Figure 5). The post-complex (P') was stabilized by -23.99 kcal/mol compared to the pre-complex (R'). The carbonyl carbon and methyl carbon of the Grignard reagent are at a distance of 4.36 Å (C1- C2) and 1.54 Å (C1-C2) in the pre-complex (R') and post-complex (P'), respectively. Thus for localization of the possible TS structure, the distance of carbonyl carbonyl and methyl carbon was scanned between this interval leading to an energy transition Structure between C1-C2 distances of 2.76 Å and 1.96 Å.
20 For further investigation, this interval was scanned in 0.1 Å intervals, and energetically a structure was maximized at C1-C2=2.36 Å from the scan plot, with the corresponding Mg1-N1 bond distance being 2, 17 Å (Figure 7). To gain further insight into the transition structure, we scanned our model system 2' by varying the C1-C2 and Mg1-N1 distance, which resulted in a maximum in the potential energy surface at the C1-C2 bond distances and Mg1-N1 being 2.36 Å and 2.21 Å respectively. The transition state obtained from QST2 calculations (18.75 kcal/mol above R') was in close agreement with TS' both energetically and structurally.
However, when performing scan and QST2 calculations using B3LYP/6-311+G(2d,p) we observed that both methods predict virtually the same transition state, located at 17.62 kcal/mol above the pre-complex . Even the structural parameters show negligible deviations of 0.03 Å and 0.01 Å for the C1-C2 and Mg1-N1 bond lengths, respectively. 21 Table 1: Selected bond lengths (Å) and angles (°) for model system transition states obtained by different methodologies.
23 As can be seen from the studies of the model system, both the HF/B3LYP and B3LYP/B3LYP methods give almost similar results. The C1-C2 bond lengths of 2.42 Å and 2.41 Å and the Mg1-N1 bond lengths of 2.21 Å and 2.20 Å (Table 2) in the transition states obtained by scanning the C1-C2 bond and the QST2- the calculations revealed structural similarities between the transition states obtained by the two methods mentioned above. 25 The energy difference of approx. 3kcal/mol can be attributed to the deviation of the Mg1-C1 bond by 0.04 Å in the transition states obtained by the two methods.
The deviation, although apparently small, has a significant impact on the energy as the Mg1-C1 bond is broken in the course of the reaction to form the C1-C2 bond.
Conclusion
The Gibbs free energy in the above reaction (1) is defined as the basicity of the gas phase. Therefore, here we are interested in studying the basicity of the synthesized ligand 1 (1,3-di-tert-butyl-N-(diphenylphosphino)-1H-imidazole-2(3H)-imine) and comparing it with the model systems 1 ′ (1,3-di-tert-butyl-N(diphenylphosphino)imidazolin-2-imine) and 1″(1,3-di-tert-butyl-1-3-dimethyl-2(diphenylphosphino)guanidine using density functional calculations The enhanced basicity and nucleophilicity of imidazoline-2-imine systems due to the negative charge on the exocyclic nitrogen atom; the introduction of P-N bonding to the ligands favors the delocalization of the nitrogen lone pairs to the vacant d-orbital of the phosphorus atom .
Thus, it helps us to infer the effect of the imidazoline ring on the basicity of a system by comparing it with the two model systems. We calculated the protonation energy14 of the ligands and the Natural Bond Order (NBO) charge on the nitrogen atom for a better understanding of entity reactivity. Computational analyzes have also been performed on the chalcogenides of ligand 1 with the aim of making a comparative study of the basicity of ligands 1(a), 1(b) and 1(c) with the changing electronegativity of the chalcogens.
Upon protonation of the ligands, exocyclic nitrogen is more likely to pick up the proton than phosphorus, with the protonation energy of 1 being 248.05 compared to 252.88 kcal/mol (Scheme 2). From the calculated protonation energies we can deduce that they are not decisive when comparing the basicity of the three systems. 38 The higher negative charge on exocyclic nitrogen of 1 can be attributed to the stabilization of the zwitterionic form by the imidazoline ring.
From Table 3 it is evident that there is a decrease in the negative charge on the exocyclic nitrogen and -1.095 for 1(a), 1(b) and 1(c) respectively as we go from oxygen to selenium, so there is a decrease. in the propensity of the exocyclic nitrogen to accept a proton, as the electronegativity of the phosphorus-bound chalcogenide increases. Also, the electronic structure around the exocyclic nitrogen has been investigated with various phosphorus-bound chalcogenides and it has been observed that there is a decrease in basicity with increasing electronegativity of the chalcogenides.
Scope of the work
A study of ligand basicity can help predict ligand reactivity for stabilizing electron-deficient transition metal and lanthanide complexes. Investigating the protonation energies and NBO charges would help us gain more insight not only into the basicity of the ligand, but also to assess the stabilization of the positive charge by the imidazoline ring.
Methodology
Results and Discussion
33 with the experimental structure (108.52° and 138.02°). The optimized structures 1 and 1' are geometrically close, while 1" shows clear differences in bond angles and lengths compared to 1 and 1". A comparison of all calculated structural parameters with experimental data for ligand 1 is shown in Table 1. The calculated geometries of 4 and 5 (Figure 4) with B3LYP/6-311+G(2d,p) were in excellent agreement with the geometry of ligand 1. structural parameters , obtained by X-ray diffraction.
Most other bond lengths and angles are similar for the three chalcogenides, except for the C1-N1-P1 angle. The energies were calculated exclusively using the B3LYP method, without thermal correction; the difference between these two methods is approximately 10 kcal/mol, and there is no change in the trend toward protonation. Furthermore, when calculating the NBO charges on ligand 1, the nitrogen atoms on the imidazole ring have charges of -0.0458 and -0.469; the exocyclic nitrogen has a charge of -1.011, while the phosphorus has a charge of +1.024.
Thus, the exocyclic nitrogen that has the highest negative charge is more likely to be protonated. Therefore, the NBO charges clearly indicate that ligand 1 is more basic compared to model systems 1′ and 1′. For the sulfur and selenide derivative, the chalcogens have a charge of -0.642 and -0.591 respectively and thus the exocyclic nitrogen having the highest negative charge is more likely to be protonated and we have calculated the corresponding protonation energies (by the combined method) which were 244.91 kcal/mol and 243.97 kcal/mol, respectively (Scheme 4).
Protonation at oxygen (Scheme 5) leads to a protonation energy (243.34 kcal/mol) that is about 0.6 kcal/mol higher than the protonated form where the exocyclic nitrogen carries the H+. While the protonation energy does not always give a clear picture, NBO analysis clearly shows that for 1(a) the more electronegative oxygen is likely to be protonated.
![Table 1: Selected bond lengths [Å] and angles[ o ] of ligand 1 by different methodologies Bond](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10552073.0/33.918.106.733.387.1058/table-selected-lengths-angles-ligand-different-methodologies-bond.webp)
Conclusion
