Indian Journal o f Chemistry Vol. 45A, November 2006, pp. 2369-2380

Nucleophilicity/electrophilicity excess in analyzing molecular electronics

D R Roy", V Subramanianb. * & P K Chattaraj"' *

"Department of C he mistry, Indian Institute of Technology, Kharagpur 721302, India bChemical Laboratory, Central Leather Research In stitute, Adyar, Chennai 600020, India

Email: pkc@chem.iitkgp.ernet.in. subuchem @hotmai l.com

Received 9 Jun e 2006; revised 7 Septe/llber 2006

Intramolecular e lec tron transfer capability of all metal aromatic and anti-aromatic aluminum cluster compounds is reported here in te rms of density functional theory based global and local reacti vi ty descriptors. Associated intermolecular reac ti vity pattern among the related molecules of va rious categories is also studied. A new local reactivity descriptor, nuc!eo(elec tro)philic ity excess is designed for thi s purpose. The findings wi ll provide important inputs towards the fabri cati on of the mate rial required for molecular electronics.

The concept of aromaticity has been extended to the all-metal molecules in the very recent past l-9. The area of this specialization is termed as metalla­aromatici ty l-9. Investigation on aromaticity of the various ionic units of AI, Ga, In, Hg, Si, etc. and their neutral and ionic complexes is one of the most interesting research topicsl-t). Both , the experimental technique like laser vaporization using photoelectron spectroscopy 1.2 and ab initio or densi ty functional theory (DFT)lo.11 based analysis are performed to characterize the nature of those ionic units and their complexes . All-metal aromatic compounds, viz. MAI4- (M = Li, Na, K and Cu) have been synthesized by Li el 01.1 for the first time. The square planar geometry and the presence of two delocali zed 7I­electrons in the AI / - dianion makes it aromatic by obeying Hlickel's (4n+2) 7I-electron mle. The transformation of a nonaromatic AI4C14(N H3)4 molecule into a 7I-aromatic Na2AI4C14(NH3)4 molecule have also been shown by theoretical investigation3

.

Chattaraj et 01.4 have proposed two new aromaticity indices based on the polarizability (a) and hardness (11) from the electronic stmcture principles of DFT I2

-22

. While the global reactivity analysis was reported there4, the corresponding local analysis is being reported in the present work .

All-metal antiaromatic molecule Li3A14- has been synthesized as reported by Kuznetsov et 01. for the first time2. The presence of four 7r-electrons obeying Hi.ickel's 4n rule and the rectangular structure of its A14

4- tetraanion unit provides antiaromatic nature of that molecule. It is shown that A14

4- is overall

antiaromatic through the electron localization function

(ELF) analysi s5. On the other hand, these molecul es have been shown be to net aromatic6

.7 according to the

magnetic criterion of aromaticity like nucleus independent chemical shift6 and magnetic field induced current densiti analyses because a­aromaticity overwhelms its 7r-antiaromaticity. Thi s controvers/3 is still a point of interest of the recent li terature 1-9.

The important insights into the reactivity and electronic properties24

-3o of these multi-metallic

clusters are obtained through various aspects of all oy formation 24-3o. For the last two decades, application of the aluminum alloys is spread over the electronic, mechanical and optical devices, corrosion protection, aerospace engineering, etc. Recent trends on the development and application of the metallurgy of aluminum powders and alloys are of great interest24 . Application of Al alloy brazing sheet in automobile heat exchangers is also very useful25 . The Al alloys are used as a cathode in corrosion protection26. Kim et of. 27 fabricated and characterized double-layer-type electroluminescent devices with the structure of tris(S-hydroxy quinoline)aluminium (Alq\ The aluminium-copper-lithium alloys have a wide application in the field of aerospace engineering28

.

Park and Sagawa29 have experimentally studied the charge transfer effect in aluminium-magnesium alloy formation. They showed that charge transfer from Mg to Al takes place on alloying. Chang et 01. 30 have investigated the effect · of charge transfer and hybridization in Ni3Al and Ni3Ga alloys with the X­ray absorption spectroscopy and theoretical calCulations . They found that Al loses some p-orbital

2370 INDIAN J CHEM, SEC A, NOVEMBER 2006

charge whereas Ni gains some charge to form Ni 3Al. As a consequence, the study of the intra and inter cluster reacti vity of the all-metal aromatic and anti­aromatic molecules in terms the DFT based local reactivity descriptors, viz. Fukui function (FF) 10 and philicit/ 7 has gained importance as no such work in detail has been done on thi s issue.

DFT-based lo.11 g lobal reactivity descriptors (e .g.,

I . . J7 13 h' I . 111 e ectronegatlvlty -. , c emlca potentia .,

I d 14 15 d I I ' 1" 16) . 1ar ness' an ' e ectrop 11 IClty are qUite successful in unraveling chemical reactivity. Fukui function (FF)IO is one of the widely used loca l density functional descriptors to model che mical reactivity and site selectivity. The atom with highes t FF is highly reactive compared to the other atoms in the molecule. The FF is defined as the derivative of the electron den sity per) with respect to the total number

of electrons N in the sys tem , at constant external potential vCr) acting on an electron due to all the

I .. h 10 nuc el 111 t e system

f (r) = [8,u/8v(r) lv =[ap(r)/aN 1(;') (1)

where ,u is the chemical potenti a l of the system.

Parr et a/. 16 defined the elecrophilicity index (w), which measures the stabilization in energy when a sys tem acquires an additional electron ic charge from the environment:

w = ,u 2 /21] = ,u 2 S ... (2)

where fl, J7 and S are the chemical potential , hardness and softness, respectively .

The generalized concept of philicity was proposed by Chattaraj et al. 17 which contains almost all information about hitherto known different global and local reactivity and selectivity descriptors, in addition to the information regarding electrophilic/nucleophilic power of a given atomic site in a molecule. It is possible to define a local quantity called philicity associated with a site k in a molecule with the aid of the corresponding condensed-to-atom variants of

F k . f . ~a 17 U UI unctIOn.J k as

a 2 a 2 fa fa w k =,u Sk =,u S. k = (J) k . .. (3)

where (ex = +, - and 0) represents local philic quantities describing nucleophilic, electrophilic and radical attacks. Relation (3) predicts that the most electrophilic site in a mo lecule is the one providing

the maximum value of CDk+' Thi s si te al so coincides with the softest site in a molecule. When two mol ecules react, which one will act as an electrophile (nucleophil e) will depend on one which has a higher (lower) e lectrophilicity index . Thi s g lobal trend originates from the local beh avior o f the molecules or preci sely the atomic site(s) that is(are) prone to electrophilic (nucleophilic) attack. Chattaraj et at. established a generali zed treatment of both global and local electrophilicity, as well as nucl eophilici ty. In principle, any normali zed-to-one quantity like shape function , a(r) = p(r)/ N , may be used in place of FF

for defining philicity . However, FF may be a bette r descriptor because of the explicit information of electron addition/removal in it.

The group concept of philicity is very useful in unraveling reactivity of various molecular systems lS

.

The condensed philicity summed over a group of relevant atoms is defined as the "group philicity". It can be expressed as:

Il

wa = L wa g k=l k

(4)

where n is the nLlll1ber of atoms coordinated to the

reactive atom, W; is the local electrophilicity of the

atom k, and cot is the group philicity obtained by adding the local philicity of the nearby bonded atoms,

where (ex= +, 0) represents nucleophilic , electrophilic and radical attacks respectively. It is allowed owing to the additivity of the Fukui function

('LIt =1) . k

The purpose of the present study is to investigate the intramolecular reactivity of the aromatic AI/ ­dian ion and anti-aromatic A14

4- tetraanion units

Fig . l---Optimi zed structures of various isomers of AI.!>'

ROY et 01.: NUCLEOPHlLICITY/ELECTROPHILlCITY EXCESS IN ANALYZING MOLECULAR ELECTRONICS 2371

M=Li 1-2=2.56 ; 2-3=2.44; 3-4=2.43; 4-5=2.62 M=Na: 1-2=2.87; 2-3=2.45; 3-4=2.42; 4-5=2.61 M=K 1-2=3.29; 2-3=2.46; 3-4=2.43; 4-5=2.60 M=Cu: 1-2=2.33; 2-3=2.41 ; 3-4=2.41 ; 4-5=2.61

MAI4- [Coov]

M=Li: 1-2=2.86; 2-3=2.60 M=Na: 1-2=3.16; 2-3=2.61 M=K: 1-2=3.53; 2-3=2.60 M=Cu 1-2=2.59; 2-3=2.66

MAI4- [C4v]

M=Li 1-2=2.62; 2-3=2.51 ; 3-4=2.56; 2-5=2.64 M=Na: 1-2=2.96; 2-3=2.52; 3-4=2.58; 2-5=2.66 M=K 1-2=3.35; 2-3=2.53; 3-4=2.58; 2-5=2.64 M=Cu: 1-2=2.40; 2-3=2.50; 3-4=2.57; 2-5=2.74

MAI4- [C2v]

M*=Li : 2-4=2.72; 3-10=2.76; 8-11=2.80; 10-11=2.75 M*=Na: 2-4=3.03; 3-10=3.07; 8-11=2.82; 10-11=2.76 M*=K: 2-4=3.48; 3-10=3.53; 8-11 =2.81; 10-11 =2.78

M*2(AI4TiAI4)

Fi g. 2--Gptimized structures of various isomers o f MAI4- (M =0 Li , Na , K, Cu) and aromatic sand wich complexes M* 2(A14 Ti AI.j) (M*=oLi , Na, K).

associated with various all-metal complexes and also the intermolecular reactivity of those units among the molecules of different categories. The stable isomers of aromatic Al/- and MAI4- (M=Li, Na, K and Cu) (Figs 1 and 2) and anti-aromatic AI4

4-, Li3A14- and Li4Al4 isomers4 (Figs 3-5) are chosen in the present study. Also, the sandwich complexes based on aromatic molecules8 (Fig. 2) and anti -aromatic molecules9 (Fig. 6) are selected.

Theoretical The quantitative definitions for chemical potential

(11) and electronegativity (X)1 2.13 for an N-electron system with total energy E can respectively be given as:

[dE] J.1= dN vcr)

(5)

and

. , . (6)

where v(r) is the external potential. Chemical hardness (ll) has been identified as an

useful global reactivity index in atoms, molecules and clusters 14,15. The theoretical definition of chemical hardness has been provided by DFT as the second derivative of electronic energy with respect to the

2372 IND IAN J CHEM, SEC A, NOVEMI3ER 2006

number of electrons (N), for a' constant external potential vCr), viz"

." (7)

Using a finite difference method the working equations for the calculation of chemical potential, electronegativity and chemical hardness can be given by:

J1= IP+EA

2 x

IP+EA

2 17

IP-EA

2 , . . (8)

where IP and EA are ioni zation potential and electron affinity of the system, respectively,

Using the ~SCF finite difference approach the IP and EA can be calculated for the N-electron system as follows:

IP", E(N-I) - E(N); EA '" £(N)-E(N+I) ". (9)

where E (N) is the electronic energy for the N electron system.

Depending on the electron transfer, three types of FF 10 are defined as:

F(r)=PN+l(r)-PNCn for nucleophilic attack

." (lOa)

F(r)=PN (F)-PN-1 (r) for electrophilic attack

." (lOb)

for radical attack

". (1 Oc)

The condensed FF are calculated using the procedure proposed by Yang and Mortier:!:! based on a finite difference method

f/ =qk (N + l)-qk (N) for nucleophilic attack

OJ a)

ftc- =qk (N)-qk (N -I) for electrophilic attack

(11 b)

for radical attack

".(llc)

where qk is the electronic population of atom k in a

mol ecule.

The electric dipole polarizability is a measure of the linear response of the electron density in the presence of an intinites imal electric tield F and it represents a second order variation in energy

ex - ( ,PE) b {I,b -- fJF" fJ0, a, = x, y, Z ". (12)

The polarizability a is calculated as the mean value as

given in the following equation

." (13)

Two new molecular descriptors are defined in the next section for analyzing the molecular electronics aspects of these all-metal aromatic compounds.

Nucleophilicity/electrophilicity excess (Net nucleophilicity/ eiectrophilicity)

A new local reactivity descriptor, nucleophilicity

excess (~w:), can be defined along the line of dual

descriptol.3 1 as :

" . (14)

where w,~ ( == ~w; J and w; ( == ~W; J are the group

philicities of the nucleophile in the molecule due to electrophilic and nucleophilic attacks, respectively. It

is expected that the nucleophilicity excess (~w,; ) for

a nucleophile should always be positive whereas it will provide a negative value for an electrophile in a molecule. Essentially, this nucleophil icity excess is the net nucleophilicity of a given group32. It can be equivalently defined for individual atomic centres, which on summation will provide the corresponding group quantity,

Ai1 electrophile in a molecule should possess i110re group philicity due to nucl eophilic attack over the electrophilic attack on it. The electrophilicity excess

(~w~ ) for an electrophile can be expressed as: 8

". (IS)

ROY el a/.: NUCLEOPH lLICITY/ELECTROPHILICITY EXCESS IN ANALYZING MOLECULA R ELECTRONICS 2373

where w,; and w; are the group philicities of the

electrophile in the molecule due to nucleophilic and electrophilic attacks , respectively. It is ,expected that

the electrophilicity excess (t,.w~ ) for an electrophile

shou ld always be positive whereas it will provide a negative value for a nucleophile in a molecule. These nucleophilicity/electrophilicity excess quantities are better intermolecular descriptors than the corresponding dual descriptors owing to the lack of global information in the latter.

For a molecular sys tem with only two distinct units ,

the nucleophilicity excess (t,.w,: ) of the nucleophile

should be equal to the electrophilicity excess (t,.w,~)

of the electrophile, as expected from the conservation of FF and philicity , i.e.

t,.w; (nucleophile) = t,.w: (electrophile) .. . (16)

In the present paper we have analyzed several molecules (with various symmetries) using the group philicity excess. We believe that the potential of a pOltion of a molecule to behave as a molecular cathode/anode would depend on its electrophilic power over the nucleophilic power (net electrophilic power) and vice versa and not a quantity measured relative to zero. It may be noted that f g- would be a poor

intermolecular descriptor than cvg- since the former IS

devoid of any global information.

Computational details All the all-metal aromatic and anti-aromatic

molecules, viz. Al/-, MAI4- (M=Li, Na, K and Cu) , AI4

4-, Li 3AI4- , Li4Al4 are minimized in the B3LYP method with the 6-311 +G* basis set. For aromatic sandwich complexess M2(AI4TiAI4) (M=Li, Na and K) B3L YP/6-311 G level of calculation is followed as already reporteds. Also, for anti-aromatic sandwich complexes\! Li4AI4Fe(COh, (Li4AI4)2Ni and bis(Li4Al4 nickel(ll) chloride) single point calculation with the same level of theory, B3LYP/6-311G* *, is used with the geometry as reported alI·ead/. All the structures correspond to minima on the potential energy surface with the number of imaginary frequency (NIMAG) to be zero but for KAI/- (Cx>v) with NIMAG=2. Several orientations (different point groups) of the metal complexes were considered and only the structures with zero NIMAG value are reported. Some of these structures were already reported4. For calcu lation of (N+I) and (N-I) systems, same geometry of the N­electron system is used . Using t,.SCF method, IP and EA are calculated using Eq. (9) and 11, 11 and a are calculated using relations 8 and 13. The electrophilicity index (w ) is calculated using Eq. (2). The Fukui function (FF) and philicities are calculated

Table I- Energy (E), polarizability (a), hardncss (q), chcmical potcntial (~l) and thc eleetrophilicity (m) va lues of differcnt isomcrs of AL/- and MAI4- and sandwich co mplexcs

Molecule PG Energy (E) ex 17 ~l (V

11//- isolllers AI/- D ::t"Jh -969.69800 675.819 1.5 12 3.106 3.191

D J h -969.70234 665.292 1.741 3.415 3.349 D4h -969.74053 525.790 1.965 3.625 3.344

MA/~- isolllers

Cr/ J V -977.27812 686.377 1.273 -0.183 0.013 LiAI 4- C2v -977.35661 414.451 1.765 -0.155 0.007

C4 v -977.36180 375 .031 3.622 1.633 0.368 C (I)V - I 132.06276 719.792 1.272 -0.192 0.014

NaA I4- C2v - 11 32. 13482 458.250 1.671 -0. 178 0.009 C4v -1132.14400 404.265 2.672 0.810 0.123 C~V -1569.69609 922.935 1.090 -0.1 8 1 0.015

KAI 4- C2v - 1569.76827 574.811 1.438 -0.201 0.014 C4 v -1569.77716 471.425 1.545 -0.164 0.009 C·fo V -26 10.28544 500.603 1.662 -0.1 22 0.004

CuA I4- C2v -2610.36263 343.957 2.370 0.145 0.004 C4v -2610.37140 33 1.243 2.933 -0.426 0.031

Arolllatic salldwich clJlllp/exes Li 2(A14 TiAI4) -2804.00780 594.629 1.577 -3.422 3.713 Na2(A I4TiAI4) -3113.57218 654.603 1.519 -3.266 3.510 K2(A I4TiAI4) -3988.85016 738.307 1.438 -2.917 2.959

2374 INDIAN J CHEM, SEC A, NOVEMBER 2006

Table 2- Fuku i functi on (j+,f ) and phili ci ty (w +,w ) values for nucleophi lic and elcctrophilic attacks respectively for the

aluminum atoms of different isomers of A142

-

Isomers Atom f+ f - W+ W AI 0.4853 0.3589 1.5483 1.1451

AI/- AI 0.0149 0.1403 0.0476 0.4476 (Dych) AI 0.015 1 0.1397 0.0480 0.4458

AI 0.4848 0.3611 1.5467 1.1520 AI 0.2258 0.3655 0.7564 1.2243

AI/ AI 0.0904 -0.0730 0.3027 -0.2430 (D,h) AI 0.4570 0.3376 1.5306 1. 1307

AI 0.2268 0.3695 0.7598 1.2375 AI 0.2498 0.2496 0.8352 0.8346

AI.j 2- AI 0.2510 0.2502 0. 8392 0.8366 (D.jh) AI 0.2499 0.2506 0.8357 0.8379

AI 0.2493 0.2496 0.8337 0.8347

using relations 3, 4 and 11 through the Mulliken population analysis scheme.

Results and Discussion A carefu l study on the electronic structure, property

and reactivity of all-metal aromatic compounds, viz. AI/-, MA I4- (M=Li, Na, K and Cu) and aromatic sandwich complexes M2(AI4TiAl~) (M=Li, Na, K) and anti-aromatic compounds , viz. A 1/-, Li 3AI4-, Li4Al4 and anti-aromatic sandwich complexes Li~AI4Fe(COh, (Li4AI4)2Ni and bis(Li4Al4 nickel(II) chloride) has been made. It can be noticed that in all the molecules, four membered aluminium unit AI4 is present and it may be considered as a superatom" . This super unit can easily take part in charge transfer process with the M (== Li, Na, K, Cu, Fe, Ni , Ti) atom in those all-metal aromatic and anti-aromatic complexes. The point group symmetries adopted here (see below) are obtained within a slightly tolerated option in so me cases.

Electronic propcrties and reactivity of all-mctal aromatic compounds

Table I presents global electronic properties, e.g. energy (E), polari zabi lity (a), chemical hardness (11), chemical potential (11) and the eJcctrophilicity (CD) of all-metal aromatic compounds, viz. AI/-, MAI4-(M=Li, Na, K and Cu) and aromatic sandwich complexes M2(A I ~TiAI4 ) (M=Li, Na, K) (Figs I, 2). The D4h isomer of A14

2- (Fig. 1) and C4v isomer of MAI4- (Fig. 2) are energeticall y most stable l

,4 . Also, the energetically most stable isomer of aromatic molecules is found to be the hardest and the least polarizable4

.

Table 2 shows the Fukui funct ion (FF) and philicity v~lues at each atomic site of A14

2- isomers. It is found

that in the case of D4h isomer, the FF (j+,fl as well as philicity (w +, W -) values for (nucleo/electro) philic attack at each aluminium site are almost equal , as expected in this sq uare planar structure. For the other two stable isomers (Dooh and D3h), the FF and philicity values at each atomic site are not equal due to the absence of symmetric electron local ization.

Tables 3-6 present the group Fukui function (j/, .(g-) and group philicity (w/' wg- ) values of the ALt nucleophile and M+ (M=Li, Na, K, Cu) electrophile in the MA I4- isomers. It is found that in all MAI~­

isomers, the nucleophilicity of the A142- aromatic

super atom'3 overwhelms its electrophilic trend (i.e.

ri >-- 1..: and Wg >-- W; ) and therefore 6.W,: is

positive, whereas the electrophilicity of M+ dominates

over its nucleophilicity (i .e. f;>--1..; and w;>--Wg) and therefore LlW.: is negative as expected. It tS

important to note that LlW: of AI/- is maximum In

the case of most stable C4v isomer of the MAl~­

molecule. The order of the LlW,: values of AI/-

nucleophile in MAI4-, viz., C4v >-- C2v >-- Cool" is

identica l with the order of their stability, i.e. stabilization of an MAI4- isomer (except in KAI -I-) increases its nucleophilicity and accordingly can be used as a better molecu lar cathode. It may be noted that this parallelism between stability and nucleophilicity is not obvious and cannot be guaranteed a priori. The presence of thi s property in this class of molecules makes them candidates for better molecular cathodes. The group Fukui function and group philicity values of the aromatic sandwich complexes M2(A I4TiAI4) are reported in Table 7. It may be noted that the group nucleophilicity of the AI/ - unit in those complexes dominates over its

group electrophilicity, i.e. Ma,: is positive as

expected. It is also important to note that the nucleophilicity of the Al/- unit in MAI4- (C-Iv) increases as K -< Cu -< Na -< Li whereas the order

gets reversed for M2(AI4TiAI4): M=Li, Na, K sandwich complexes as dictated by the respective nucleophilicity excess values. This fact provides important insights into the design of new molecular electronics materials. For example LiAI-I- (C4y) would be by far the best choice among MAI 4- (CooY, C2y, C-Iv): M=Li, Na, K, Cu whereas K2 (AI4TiAI4) would be the

ROY et of. : NUCLEOPI-IILlCITY/ELECTROPHILICITY EXCESS IN ANALYZING MOLECULAR ELECTRONI CS 2375

Table 3--Group Fukui fun ction (fK+ , fK~ ) and group philicity (w; , w; ) valu<!s for nuc leoph ilic and

electrophili c attacks respectively for the ion ic units o f different isomers o f LiAI.j~

Isomcrs Ionic f~+ IK- !'J..f,+ w+ w; f.,cv+

unit .' g

LiAI.j~ AI}~ 0.5262 0.7172 0.1909 0.0070 0.0095 0 .0025 (c.h l ) Li + 0.4738 0.2828 -0.1909 0.0063 0 .0037 -0.0025 LiA I.j- AI/~ 0.0019 0.8089 0.8070 I.3E-05 0.0055 0.0055 (C2v) Li+ 0.998 1 0.191 I -0.8070 0.0068 0.001 3 -0.0055 LiAL,~ AI/~ -0.1011 0.8052 0.9062 -0.0372 0.2965 0.3338

(C.jv) Li+ 1.1 0 11 0.1948 -0.9062 0.4055 0.0718 -0.3338

Table 4-Group Fukui fu nct ion ( .( , /~- ) and group philicity (w; , w; ) values for nuc leophili c and

elect rophili c attacks respec ti ve ly for the io nic units o f different isomers of NaA I .j~ .

Iso mers Ioni c .c I,- !'J..f,+ w+ wK f.,(rJ+ unit

g g

NaAV AI/~ 0.4864 0.7099 0.2235 0 .0070 0.0102 0.0032

(C."v) Na+ 0.5 136 0.2901 -0.2235 0.0074 0.0042 -0.0032 NaAI.j- A I .j 2 ~ -0.0 128 0.8227 0 .8356 -0.0001 0.0078 0.0079 (C2J Na+ 1.0 128 0.1773 -0.8356 0.0096 0.0017 -0.0079

NaA I.j- A I/~ -0.0592 0. 8339 0.8930 -00073 0.1024 0.1097

(C.jv) Na+ 1.0592 0.1 66 1 -0 .8930 0 .1 301 0.0204 -0.1097

Table 5--Group Fuk ui function (.C ,.C ) and group philicity (w~ , cv; ) val ues for nucleophili c and

e lectrophilic attacks respect ive ly fo r the ionic units o f d ilTcrent isomers o f KAI.j-

Isomcrs Ionic .c IK~ f.,j~+ w+ (0, f.,w+ Unit

., .,

KAI.,~ A I/~ 0.2953 0.6365 0.3413 0.0044 0.0095 0.0051 (Cv) K+ 0.7047 0.3635 -0.3413 0.0 106 0.0054 -0.005 I KA I.j~ A I/~ 0.16 14 0.7198 0.5584 0.0023 0.0 10 1 0.0078 (C2v) K+ 0.8386 0.2802 -0.5584 0.011 8 0.0039 -0.0078 KAI .j~ AI/~ 0.0982 0.7597 0.6615 0.0008 0.0066 0.0057

(C.j ,.) K+ 0.90 18 0.2403 -0.6615 0.0078 0.002 1 -0.0057

Table 6-Group Fukui function (.I~+ , .C) and group phili c ity (w; , w~) va lues for nuc leophili c and

e lectrophili c attacks respecti vely for the ioni c units o f different isome rs o f CuA I .j~

Iso mers Ion ic .1/ C !'J..f,+ w+ w., f.,w+ unit

g g

CuA I .j~ A I/~ 0.6823 0.8044 0. 122 1 0.0031 0.0036 0.0006 (CI) Cu+ 0.3 177 0.1955 -0.1 221 0.00 14 0.0009 -0.0006 CuA I .j~ AI}~ -0.1 552 0.91 66 1.0718 0.0036 0.0036 0.0048 (C2v) Cu+ 1.1 55 18 0.0834 -1.0718 0.0008 0.0008 -0.0048

CuA I.j~ A I/~ 0.5752 1.0738 0.4986 0.0 178 0.0332 0.0 154

(C'v) C u+ 0.4248 -0.0738 -0.4986 0.0 13 1 -0.0023 -0.0 154

Tab le 7--Group Fukui fun ction ( f ,+ , .I~- ) and group philicity (w;, w~) values for nucleophi li c and

e lectrophili c attacks respective ly for the AI/~ unit of variou s sandwich co mplexes based on all -metal aromatic clusters

Aromat ic Ionic IK+ C f.,t~+ w+ w

K f.,(O~

Sandwich Unit g

Li 2(AL,TiA I.j) 2A I/~ 0.8326 0.8422 0.0096 3.0913 3.1 269 0.0356 Na2(AI .jTiAI.j) 2A I .j2~ 0.7523 0.7841 0.03 18 2.6409 2.7526 0.111 6 K2(A I.jTiAI.j) 2AI/- 0.7024 0 .7425 0 .040 1 2.0784 2.1970 0.11 86

2376 INDIAN J C HEM, SEC A, NOVEMI3ER 2006

Table 8-Energy (E) , polarizab ility (a), hardncss (11), chcmi cal potcntial (~l) and the clectrophili cit y (w) va lucs of diffcrent isomers of AI4

4- , Li .1A I4- and Li4A I4 and sandwich complexes

Molec ule PG Encrgy (E)

AI/- isoll/ers A14

4- D',oh -969.26643 (S ing lct) D21l -969 .26 169

AI/- D",11 -969.27504 (Tripl et) D21l -969.25921

LilA/~- isolllers Singlet Cs -992.43349 Trip let Cs -992.43389

Fork Cs -992.43452 Hood C2 -992.43033

Scooter C 1 -992.43083 Rabbit C:!v -992.41938

Li~A/~ isolllers C21l -999 .933 10 C2v -999.91382

D21l -999.93242

Allti-arolllatic sml(li vic/r cOlI/plexes Li4AI4Fc(CO).1 -2603.863 15

(Li4A14hNi -3508.30 169 bis(Li4A 14n ickel(J 1)

chloride) -6857.96 187

Fig . 3 - Optimized structures o f isomers o f AI/- (singlet).

best among M2(A l ~TiAI4): M=Li, Na, K. In this work, it is tacitly assu med that a stronger nucleophil e is a better e lec tron donor.

Electronic properties and reactivity of all-metal anti-aromatic compounds

Table 8 presents the g lobal electronic properties, viz., energy (E), polari zability (a), hardness (11) , chemical potential (~l) and the electrophilicity (w) of the various isomers of all-metal anti-aromatic compounds, viz. AI/- (singlet and triplet) , Li3AI4-,

Li~A l~ (Figs 3-5) and their sandwich complexes Li~A l ~ Fe(CO)J, (L i ~ AI~)2 Ni and bi s(LiAI~ nickel(II) chloride) (Fig. 6). The linear AL1

4- is found to be

sli ghtly more s table~ compared to its cyclic counterpart in both the singlet and triplet states. The Cs (Fork)

ex 7J ~l (jJ

1915.578 0.954 8.062 34.066 1910.644 1.1 94 8.703 31.714 2149 .025 1.206 8.719 31.521 2276.394 1.187 8.055 27.321

564.935 1.388 0.073 0.002 530.666 1.460 0.003 3E-06 522.497 LSI6 -0030 2E-04 602.844 1.393 0.046 8E-04 618.850 1.356 0.062 0.00 1 726.708 1.440 0.1 6 1 0.009

392.79 1 1.998 -2 .868 2.058 364.974 1. 822 -3.125 2.679 452.724 1.920 -2.579 1.732

370.024 2.436 -3 .334 2.282 844.603 1.694 -2.671 2.1 07

660.01 7 1.784 -3.749 3.940

isomer of Li3AI4 - is found to be energetically the most stable, the least polarizable and the hardest}· 4. The C2h

isomer of Li4AI4 is the most stable.

Tables 9 and 10 present the Fukui functi on and philicity values at each atomic site of AI/- isomers (Fig. 3) of both singlet and triplet states . The unequal Fukui function if +, f -) and philicity (0./ , w-) values may be viewed as an effect of locali zed elec trons . Two Al atoms behave differently than the remaining two vindicating the rectangular (not square) geometry of Al/- ion in comparison to AI/ - ion (singlet, D4h, Table 2). Higher g lobal e lectrophili ci ty value of AI/­than that of AI/- is in conformity with the fact that the former would love to have two more electrons (total six) to attain the (4n+2) 7r-e lectron (aromatic) configuration as in AI/- with two 7r-electrons.

Table 11 presents the group Fukui fu nction (f/, .f~-) and group phili c ity (w/, wg- ) values of all the Li 3 AI~­isomers (Fig. 4) for nucleophilic and electrophilic

attacks, respecti ve ly. The positive I1w.~ values of the

A144

- unit in a ll Li 3A14- isomers provide the nucleophilic nature of AI/ - unit in those compounds.

Also negative I1w.~ values of Li/+ uni t in all isomers

of Li 3AI~- imply its electrophilic nature over its nucleophilic trend.

ROY el III.: NUCLEOPHILlCITY/ELECTROPHILlCITY EXCESS IN ANALYZING MOLECULAR ELECTRONICS 2377

CS (Singlet) CS (Triplet) CS ("Fork")

C2 ("Hood") C1 ("Scooter") C2v ("Rabbit")

Fig. 4--Optim izcd structures of isomers Li, AI4- .

C2v

Fig. 5--Optimizeel structures of isomers of Li 4A14 .

Table 9-Fuku i func tion (j'+,f-) anel phi lici ty Cw +, w ) va lues for nucleophil ic anel e lectrophilic attacks respecti vely for the aluminum atoms of diffe rent isomers

of AI.j4-Csingl et)

Isomers Atom r r w+ w

AI 0.7034 0.6926 23.9627 23.5935 A14

4 - AI -0.2030 -0. 1930 -6:9299 -6 .5606 (D",h) AI -0.2030 -0.1930 -6.9299 -6.5606

A I 0.7034 0.6926 23 .9627 23.5935 AI . 0.39 13 0.46 17 12.408 1 14.6425

AI/- AI 0.1074 0.0387 3.4070 1.2272 (021,) AI 0.3938 0.4609 12.489 1 14.6 168

AI 0.1075 0.0387 ' "3-.4096 1.2274

2378 INDIAN J CHEM. SEC A. NOVEMBER 2006

bis(Li<jA14 nickel(lI) chloride)

Fig. 6 - Optimi zed structures of the sandwich complexes of Li4AI4.

Tab le IO-Fukui function 1J+,f ) and philicity (w+,w ' ) va lues for nucleophilic and electrophilic attacks rcspectively fo r the alu minum atoms of different isomers of AI/-(tri plet)

Isomers Atom r r w+ w

AI 0.7344 0.6628 20.0640 IS.1080 AI:' AI -0.2340 -0 .1 628 -6.4030 -4.4470 (D,nh) AI -0.2340 -0.1628 -6.4030 -4.4470

AI 0.7344 0.6628 20.0640 IS. IOSO AI 0.2492 0.2708 7.8549 8.5349

AI/ AI 0.2508 0.2292 7.9056 7.2257 (D'h) AI 0.251 6 0.2285 7.9300 7.201 3

AI 0.2484 0.2715 7.8306 8.5593

Table II--Group Fuku i function (1/ , Ix-) and group ph il ici ty (w; , w;) va lues for nucleoph ilic and electrophilic attacks

respectivcly for the ionic units of different isomcrs of Li)AI,'

Li )A I,' Isomers Ionic r Ix' Nx+ w+ w; L'lw; Unit x "

Li)AV AI/- 0.0974 0.5105 0.4131 0.0002 0.0010 O.OOOS (Cs Singlet) Li/+ 0.9026 0.4895 -0.41 3 1 0.0017 0.0009 -O.OOOS

LiJAV AI: - 0. 1316 0.4560 0.3244 4.2E-07 1.5E-06 IE-06 (Cs Triplet) Li/+ 0.S684 0.5440 -0.3244 2.8E-06 l.7E-06 - IE-06

Li )AI ,' AI/- 0.0654 0.609 1 0.5437 I.4E-05 0.0001 0.0001 (Cs Fork) L' J+ I ) 0.9346 0.3909 -0.5437 0.0002 8.6E-05 -0.0001

Li .,AI'- AI:' 0.0632 0.4963 0.433 1 4.9E-05 0.0004 0.0003 (C, Hood) L' J+

I ) 0.9368 0.5037 -0.433 I 0 .0007 0.0004 -0.0003 Li )AI.- AV' -0.2498 0.47 15 0.7213 -0.0004 0.0007 0.0010

(C , Scooter) Li:'l3+ 1.249S 0.5285 -0.721 3 0.00 18 0.0008 -0.0010 LiJAV AI: ' 0.0146 0.5695 0.5549 0.0001 0.0051 0.0050

(C" Rabb it) Li ,J+ 0.9854 0.4305 -0.5549 0.0088 0.0039 -0.0050

ROY el of.: NUCLEOPH ILlCITY/ELECTROPHTLlCITY EXCESS IN ANALYZING MOLECULAR ELECTRONICS 2379

Table 12-Group Fukui function (f/ ' f"- ) and group phi licity (w; , w;) values for nucleophilic and e lec trophilic attacks respecti vely

for the ionic units of different isomers of Li4AI4

Li4AI4 Ionic f ,' f"- t::,f,+ w' w, !::'w" .,

Isomers Unit ,

Li4AI4 AI.j4- 0.3053 0.5018 0.1965 0.6283 1.0328 0.4044 (C2h) L' 4+ 14 0.6947 0.4982 -0.1965 1.4298 1.0254 -0.4044

Li4AI4 A144- 0.3420 0.6032 0.2612 1.4791 1.0016 0.7000

(C2v) L' 4+ 14 0.6580 0.3968 -0.2612 1.2003 1.6777 -0.7000 Li4 AI4 A14

4- -0.0220 0.7314 0.7532 -0.0377 1.2669 1.3045 (02h) L' 4+ 14 1.0217 0.2686 -0.7532 1.7()97 0.4652 -1 .3045

Table l3-Group Fukui function (f/ ' f,- ) and group philicity (w; , w; ) values for nucleophilic and electrophilic attacks respectively

for the AI/- unit of va ri ous sandwich complexes based on all-metal anti-aromatic clusters

Anti-aromatic Sandwich

Li4AI4Fe(CO)3 (Li4AI4)2Ni bi s(Li4 Al4 nickel(II) chloride)

Ionic Unit

AI/-2A1 4

4-

2AI/-

f/

0.2400 0.2648 0.8425

Table 12 shows the group Fukui function (fg+, f g-)

and group philicity (cv/, cvg- ) values of all the Li4AI4 isomers (Fig. 5) for nucleophilic and electrophilic

attacks, respectively. The positive L}W; values of the

A144- unit in all Li4Al4 isomers provide the nucleophilic nature of A14

4- unit in those compounds .

Also negative L}W; values of Li44+ unit in all isomers

of Li4Al4 imply its electrophilic nature over nucleophilic trend.

We have also investigated the nucleophilicity of the antiaromatic A14

4- unit in the anti-aromatic sandwich complexes9 Li4AI4Fe(COh, (Li4AI4)2Ni and bis(Li4Al4 nickel(II) chloride), as shown in Fig. 6. The positive

L}fg+ and L}W; values A144- units in those sandwich

complexes provide its nucleophilic nature. After we completed our work, we have come across a related paper34. In this paper34 the local reactivity trends of AI4Li-, AI4Na-, Al4Li4 and AI4N~ have been analyzed using group Fukui functions. A preliminary report of our work is presented elsewhere35 .

Conclusions It has been demonstrated through the analysis of

nucleophilicity/electrophilicity excess values of all­metal aromatic and anti-aromatic cluster compounds and their alkali metal and sandwich complexes that the Al/-/ AI/- unit behaves as a nucleophile in all cases wherein the electrophiles will prefer to attack. Important insights into the associated molecular

0.3820 0.4810 0.8471

!::'f,+

0.1420 0.2162 0.0046

w' ,

0.5476 0.5578 3.3193

w" !::'w" "

0.8717 0.3241 1.0133 0.4555 3.3375 0.0183

electronics can be obtained through this systematic study of electron localization pattern in the AI4 group by changing the attached metal ion .

Acknowledgement We thank Board of Research in Nuclear Sciences

(BRNS), Mumbai for financial assistance, Mr M Elango for helpful discussion and Mr S Giri for going through the manuscript.

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