Published: March 11, 2011 r2011 American Chemical Society 3234 dx.doi.org/10.1021/ic101658a | Inorg. Chem. 2011, 50, 3234–3246 ARTICLE pubs.acs.org/IC Interplay among Aromaticity, Magnetism, and Nonlinear Optical Response in All-Metal Aromatic Systems Satadal Paul and Anirban Misra* Department of Chemistry, University of North Bengal, Darjeeling, PIN 734 013, West Bengal, India b S Supporting Information ’ INTRODUCTION The isolation and characterization of MAl 4 2 (M = Na, K, Cu) by Li et al. is a breakthrough in the concept of aromatic compounds. 1a This series of bimetallic clusters is found to have square-planar Al 4 2 , which has been confirmed to have aroma- ticity through photoelectron spectroscopic investigation and electronic structure analysis by ab initio calculations. This finding sparked interest among researchers to investigate the domain of all-metal aromatic systems (AMASs). There are numerous reports on aromaticity in all-metal clusters, whose stabilities are verified through experimental or theoretical studies. 113 These systems include XAl 3  (X = Si, Ge, Sn, Pb), 2 M 4 2 (M = Ga, In, Tl, Sb, Bi), 3 T 5 6 (T = Ge, Sn, Pb), 4 M 4 2þ (M = Se, Te), 5 M 3 (M = Al, Ga), 6 Al 6 2 , 7 Hg 4 , 5,8 M 5  (M = Sb, Bi), 9 Au 5 Zn þ , 10 Cu 3 3þ , 11 Cu 4 2 , 12 [Fe(X 5 )] þ (X = Sb, Bi), 13 and so on. A detailed explanation of the stability and reactivity of a wide range of all-metal aromatic and antiaromatic systems has been given by Chattaraj and co-workers on the basis of density functional theory calculations. 14 Aromatic systems have a usual inclination to form coordination bonds with metals through their dispersed electron cloud. Mercero, Ugalde, and co-workers theoretically verified the possibility of such complexes with AMASs and demonstrated that the all-metal aromatic Al 4 2 deck can be used to sandwich transition-metal atoms. 15 Another possibility of such a complex is explored by sandwiching transition metals between aromatic As 4 2 decks. 16 These works, in fact, invoke the synth- esis of novel all-metal metallocenes. Yang et al. further extended this idea of such sandwich complexes with main-group metals. 17 The origin of aromaticity in such metal clusters can be explained through the H€ uckel (4n þ 2) π electron rule. 115 Other than this simple electron count rule, the aromatic character of these metal cluster ions has also been diagnosed through their response toward the magnetic field. The ability to sustain a diamagnetic ring current induced by a perpendicular magnetic field has been considered as the magnetic criterion of aromaticity. 18 Because “diatropicity” is synonymous with “aro- maticity”, relying on a similar argument, it has been suggested by many authors that “paratropicity” implies “antiaromaticity”. 19 The diatropic (paratropic) ring current may be maintained by circulation of either π- or σ-bonded electrons, and the system is termed as π-aromatic (π-antiaromatic) or σ-aromatic Received: August 15, 2010 ABSTRACT: All-metal aromatic molecules are the latest inclusion in the family of aromatic systems. Two different classes of all-metal aromatic clusters are primarily identified: one is aromatic only in the low spin state, and the other shows aromaticity even in high-spin situations. This observation prompts us to investigate the effect of spin multiplicity on aromaticity, taking Al 4 2 , Te 2 As 2 2 , and their copper complexes as reference systems. Among these clusters, it has been found that the molecules that are aromatic only in their singlet state manifest antiaromaticity in their triplet state. The aromaticity in the singlet state is characterized by the diatropic ring current circulated through the bonds, which are cleaved to generate excess spin density on the atoms in the antiaromatic triplet state. Hence, in such systems, an antagonistic relationship between aromaticity and high-spin situations emerges. On the other hand, in the case of triplet aromatic molecules, the magnetic orbitals and the orbitals maintaining aromaticity are different; hence, aromaticity is not depleted in the high-spin state. The nonlinear optical (NLO) behavior of the same set of clusters in different spin states has also been addressed. We correlate the second hyperpolarizability and spin density in order to judge the effect of spin multiplicity on third-order NLO response. This correlation reveals a high degree of NLO behavior in systems with excess spin density. The variance of aromaticity and NLO response with spin multiplicity is found to stem from a single aspect, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and eventually the interplay among aromaticity, magnetism, and NLO response in such materials is established. Hence, the HOMOLUMO energy gap becomes the cornerstone for tuning the interplay. This correlation among the said properties is not system-specific and thus can be envisaged even beyond the periphery of all-metal aromatic clusters. Such interplay is of crucial importance in tailoring novel paradigm of multifunctional materials.