Superalkalis as Building Block for Ecient Lewis Base

In quest for efficient Lewis base an in-silico investigation on NH 3 and its structurally similar different molecules have been carried out. It has been observed that the ionization energy of the groups attached to N-centre play an important role in determining their stability and reactivity. Among different groups, superalkali ligands make better Lewis base. Several conceptual DFT descriptors like electrophilicity, nucleophilicity, dual descriptors have been used to analyse the stability and reactivity of proposed Lewis base. Calculated charge transfer descriptor describes the efficiency of the designed Lewis base to form an adduct with Lewis acid. nc-2e (n=1,2) AdNDP calculation lend additional support to the bonding of the studied molecules.


Introduction
Lewis base [1,2], a well-known chemical species that donates electron pairs because of the presence of lone pair. The typical example of Lewis bases are NH3, PH3, N(Me)3, N(tBu)3, etc. Lewis bases play an important role in different catalytic reactions in modern organic synthesis. Nowadays sterically hindered Lewis base and acids are also used to design efficient frustrated Lewis pairs (FLP) [3][4][5][6][7][8][9][10][11] which has immense application in catalysis and small molecule activation. It is well known fact that with increasing the electron releasing (+I effect) power of substituents on the group 15 elements can increase the basicity or nucleophilicity of the Lewis base in the gas phase. So, the trend of Lewis basicity for nitrogen series will be NH3<N(Me)3 <N(Et)3<N(iPr)3<N(tBu)3 according to their electron-donating power of attaching substituents on nitrogen centre. Very recently a new super Lewis basic Tris(dialkylamino)-substituted terpyridines (TYP) was synthesize by kleoff et.al, where NMe2 which is an electron donating group increases the basicity of TPY [12]. So, it is expected that the efficiency of the Lewis base can be tuned by introducing more powerful electron releasing species. In this regard we can think about a special kind of molecule, superalkali. Superalkali [13][14][15] is the common term for the last three decades in chemistry, that can easily release electrons with very low ionization energy even lower than the alkali metals. There are many different types of inorganic binuclear, polynuclear, non-metallic, and aromatic superalkalis reported in the literature [16][17][18][19]. Not only inorganic, but organic heterocyclic molecules also show superalkali behaviour [20]. Very recently Rohrbach et.al [20] synthesize organic heterocyclic superalkali type neutral super electron donor (SED), which have an efficient catalytic activity in a chemical reaction. Looking at this fact, In this study, we try to design more effective Lewis bases by introducing organic superalkalis at the N-centre. Very recently Reddy et.al [22] proposed superhalogens as a building block of super Lewis acids, which can accept the electron pair efficiently. So, it can be assumed that super alkali will work in a reverse manner. To serve this purpose different organic heterocyclic based superalkali ligands have been used to make the system more nucleophilic with better Lewis base character.

Theory and computational details
Conceptual DFT based global and local reactivity descriptors play an important role in describing the stability and reactivity of molecules. One of the most important global reactivity descriptor is hardness (η) [23,24] which is represented as, (1) Where is the ionisation energy and is the electron affinity of the system. In ΔSCF method, the ionization energy (I) and electron affinity (A) of the system can be calculated as (2) (3) Where, , , represents the energy of neutral, anion and cation system respectively.
Electrophilicity [25] is also considered to be an important global reactivity descriptor which can be define as- Where η is the hardness and μ is the chemical potential. The chemical potential can be expressed as Based on the frontier orbital energy and inverse relation between electrophilicity and nucleophilicity, there exist four different expressions for the calculation of nucleophilicity index (N), [26][27][28] which are as follow Fukui function for nucleophilic attack, ƒk where ρN, ρN-1, ρN+1 are the population of neutral, cation and anionic system on the atom k.
Dual descriptor, (Δƒ) which is defined as an intramolecular local reactivity descriptor, can be calculated from the densities of frontier molecular orbitals as (9) To account for the intermolecular reactivity, multiphilic descriptor has been calculated by using the following expression, The ground state geometries of all the studied molecules are obtained by using wB97XD [35] as hybrid functional and 6-311+G (d, p) in the form of basis set without imposing any symmetry constrain. Vibrational frequency analysis has been performed at same level of theory to know their existence on the minimum of the potential energy surface. For all the cases we obtained zero imaginary frequency. All the optimizations have been performed by using Gaussian 09 program [36].

Results and discussion
As discussed earlier, our primary goal of this work is to design efficient Lewis base, we have initially taken different N centred molecules with known aliphatic/aromatic groups which are structurally similar to NH3. At first, variation in properties have been analysed with different groups attached to the N. The Lewis basicity of these molecules have analysed by looking at the electrophilicity and nucleophilicity values.

N-centre attached with known organic aliphatic ligand
A total six different molecules analogous to NH3 have been taken whose ground state geometries are shown in figure1. From the optimized geometries, it is evident that all of them are analogous to NH3. It can be observed from figure 1 that the groups are attached to N center are -H, -OCH3, -CH3, -C2H5, -CH(CH3)2, and -t Bu. So, it is expected that -t Bu being more powerful electron releasing group, the nucleophilicity of N( t Bu)3 will be more than the others. To analyse their properties, we have calculated the vertical ionization energy (VIE), hardness (η), electrophilicity (ω) and four different nucleophilicity indices (N). The corresponding values are given in Table 1. From the Table1, it is observed that VIE is gradually decreasing when hydrogens of NH3 are replaced by more electron-donating groups.
The vertical ionization energy values for molecules a to f change from 10.79 eV to 7.139 eV.
Among the molecules, f has lowest VIE values of 7.139 eV which is expected. This indicates that f can release electron more easily and will be more efficient Lewis base than other molecules.  , except a and b, molecules c-

N-centre attached with known organic aromatic ligand
As for aliphatic groups, we have a correlation between VIE and nucleophilicity index, we wanted to see whether the same is valid for aromatic groups. To serve this purpose, we have taken four different groups namely benzene, imidazole, N-linked imidazole tripodal and Clinked imidazole tripodal [37].
The ground state geometries and their different properties are given in figure 2 and table 2 respectively. The values provided in table 2 reveal that, like aliphatic groups, aromatic groups are also capable of making good Lewis base. As VIE decreases, nucleophilicity of these molecules are also increasing.

N-centre attached with aromatic heterocyclic superalkali ligand
From the above results, we can see that, molecules having more electron-donating group make efficient Lewis base. In this aspect, superalkali will be the best candidate for the making of efficient Lewis base as they possess very low VIE values even lower than alkali metals.  [20]. The ground state geometries (kn) given in figure 3 portray similar structural features like NH3, N(Ph)3, etc. To know the stability and reactivity of these molecules, we have calculated η, ω, N alongwith VIE. The corresponding values are tabulated in table 3. We have also calculated the VIE of individual groups which are provided in first bracket in table 3. The calculated VIE suggests that the groups as well as the molecules are superalkali in nature. In fact, these molecules are having less VIE than the individual groups which are superalkalis. So, it is expected that their nucleophilicity will also be high. Looking at the electrophilicity values it reveal that these molecules have very less tendency to accept electrons. The nucleophilicity values are quite high in comparison to other groups which suggest that these systems can act as better Lewis base. For further checking, we have calculated all the parameter in MP2 level, which is shown in supporting information table S1-S3. Although the calculated values are not same for both level of theories, the qualitative trend remains same. To analyse the reactivity of these molecules, especially the local reactivity, we have calculated the local reactivity descriptors like Fukui functions for electrophilic/nucleophilic attack, dual descriptor [33], philicity [38] and multiphilic descriptor [34] for the N-centre. The values are given in Table 4. From the Table 4, the dual descriptor Δƒ and multiphilic descriptor (Δωk ± ) of nitrogen atom in the design complexes have negative values, which represent that all the systems seem to be nucleophilic character and prefer for electrophilic attack. The dual descriptor plots which has been calculated from the electron densities of the frontier molecular orbitals (figure 4) also tells the same findings. descriptor (Δωk ± ) at N centre for molecules k-n.
Lewis Base The Blue colour which corresponds to nucleophilic region suggests that N center is electron rich. This indicates that electron donation can be possible like a Lewis base to an electron deficient center like Lewis acid through N center.

nc-2e AdNDP Analysis
To study the bonding features of the newly designed Lewis base system, we analyze the 2c-2e bond to ensure that the bonding between N and carbon centre of the superalkalis ligands is covalent in nature. The 2c-2e bond was calculated using the AdNDP method in Multiwfn software [39,40].  and their corresponding occupation number is 1.98 |e|.

Conclusion
From this study, we conclude that there is an inverse relation between ligand ionization energy with nucleophilicity of N-centre amine. Having lower ionisation energy, superalkali ligand makes better Lewis base with very high nucleophilicity. Among all the studied molecules, tetramethyl imidazolium-based superalkali ligand makes an efficient Lewis base.
Local reactivity descriptors like dual and multiphilic descriptors tell us N center reactivity which favours the electrophilic attack. From the charge transfer descriptor, it proves that design Lewis bases can from the adduct with Lewis acid. As the size of the heterocyclic ligands are big, it is expected that, these molecules can capable of creating frustrated Lewis pair with high reactivity.