Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

Abstract

Dedicated to Professor Cecilia SarasolaSince the fortuitous discovery [1] of ferrocene, [(C 5 H 5 ) 2 Fe], the cyclopentadienyl anion has emerged as the most versatile aromatic ring for the synthesis of sandwich-like complexes. Indeed, it has been estimated that more than 80 % of all known organometallic complexes of transition metals contain a substituted or unsubstituted cyclopentadienyl ligand. [2] Nevertheless, new aromatic compounds have recently been identified whose incorporation into sandwich complexes opens new routes for chemical creativity. In particular, we have predicted [3] that the all-metal [Al 4 ] 2À aromatic ring [4] can complex Ti to form [Al 4 TiAl 4 ]2À . The tetraazacyclobutadiene dianion, N 4 2À , also fulfills the aromaticity requirements.[5] It has a square-planar structure with six delocalized p electrons, four of which occupy the nonbonding p highest occupied molecular orbitals (HOMOs); the other two p electrons participate effectively in the aromatic stabilization, in agreement with the (4 n+2) rule. The nucleus-independent chemical shift (NICS) [6] at 1.0 above the molecular plane is À4.09 ppm, [7] confirming further the p-aromatic character of the ring. Consequently, N 4 2À can, in principle, be incorporated into sandwich-like complexes although, as far as we know, no such structures have been reported to date.We have therefore theoretically characterized stable sandwich-type complexes of the N 4 2À aromatic ring with early (Ti, V, and Cr) and late (Fe, Co, and Ni) transition-metal centers, with charges 2À (Ti, Fe), 1À(V, Co), and 0 (Cr, Ni). The optimized structure of the Ti sandwich-like complex has D 4h symmetry; the V and Cr analogues have C 2v symmetry, although they can also be viewed as bent D 4h structures (Figure 1). The optimum geometries of the late-transitionmetal complexes have D 4d symmetry. Consequently, for the complexes of the early-transition metals the nitrogen atoms are eclipsed whereas they are staggered for those of the late transition metals.The calculations were carried out with the B3LYP functional, as implemented in Gaussian 98, [8] with the standard 6-311G basis set.

Sandwich Complexes Based on the “All‐Metal” Al42− Aromatic Ring

“…À sandwich complex, [32] a structure with Li + interacting with the titanium was found. The geometrical and NICS data obtained for the {Li/Na/K}A C H T U N G T R E N N U N G [Al 4 …”

A Theoretical Study on Structures, Bonding Energies and Aromaticity of Two New Series of Dinuclear Phosphametallocenes: (η5‐P5)MM′(η5‐P5) and (η5‐C5H5)MM′(η5‐P5) (M, M′ = Zn, Cd)

“…[49,50] Aromaticity, antiaromaticity, and multiple fold aromaticity, [51] presented in molecules that possess more than one independent delocalized bonding system, either σ-type or π-type, have also been found in the all-metal systems [52,53] and the sandwich-like complexes. [33,38,43,[54][55][56][57] Recently, we extended NICS to P 5 -and [Ti(P 5 ) 2 ] 2-, [33] and dinuclear metallocene [CpZn 2 Cp]. [58] Negative NICSs denote aromaticity, positive NICSs denote antiaromaticity, and zero NICS means nonaromaticity.…”

[(η6‐B6X)2M] (X=C, N; M=Mn, Fe, Co, Ni): A New Class of Transition‐Metal Sandwich‐Type Complexes

“…[9][10][11] In these sandwich-type structures, the transition-metal center, M, is coordinated between two aromatic h n -E n monocycles (n = 4-6), each of which has 6-p electrons. The p-d interactions between the delocalized p molecular orbitals (MOs) of the ligands and the partially occupied d orbitals of the transitionmetal center play a crucial role in stabilizing the systems.…”