Introduction
Homonuclear metal-metal bonds has always attracted chemists due to their enormous applications.1 One such landmark advancement in this area was triggered by the discovery of first Zn(I) – Zn(I) σ bonded complex [(η 5-Cp*)2-Zn2] (Cp* = C5Me5) by Carmona and coworkers.2 This seminal discovery has paved the way for isolation of various Zn(I) – Zn(I) bonded complexes involving various ligands.3 Recent years has also witnessed examples of multiply bonded Zn-Zn complexes.4,5
Bare Zn2 dimer, characterized by UV spectroscopy, appears to be a van der Waals complex with a Zn-Zn distance of 4.19 Å.6 In principle, the nature of bond in Zn2 and Be2 is almost similar as Be2 has also been characterized in gas phase and described as van der Waals complex with Be-Be distance of 2.45 Å.7-9 The close analogy between the Zn2 and Be2 dimer is their similarity in valence electronic configuration (Be = 2s22p0; Zn = 3d104s24p0) and exhibit typical Morse-van der Waals type potential.10,11 Thus, it appears that Zn will prefer similar chemistry as that of Be to a large extent. Brea and coworkers investigated an interesting phenomenon of single electron intramolecular Be-Be bond in 1,8-diBeX-naphthalene radical anion derivatives (Scheme 1).12 Their calculations reveal that the extra electron forms the Be-Be bond with dramatic shortening of the Be-Be bond with respect to the neutral molecule. Both –BeX and –ZnX are acidic in nature and there is similarity in valence electron configuration of Be and Zn, we therefore, expect that Zn will exhibit similar intramolecular one electron bond formation. It should be noted that the distance between the H atoms of naphthalene attached to C1 and C8 atoms is almost similar (2.47 Å) to the distance between the H atoms attached to C1 and C2 atoms of benzene (Scheme 1). The choice of C1 and C8 disubstituted BeX in the work of Brea and coworkers12 is obvious as these two positions are chemically most reactive.13However, all positions in benzene are chemically equivalent. Thus, one can envision that similar intramolecular one electron Be-Be and Zn-Zn bonds formation will be possible in substituted benzene derivatives. We therefore, have undertaken a detailed theoretical study on the possible existence of intramolecular one-electron Be-Be and Zn-Zn bonds in 1,2-diBeX-benzene and 1,2-diZnX-benzene radical anion derivatives (Scheme 1). The probable questions that are likely to be answered in the contribution are – How strong can this one electron bond be? What will be the effect of X group? Are these radical anions experimentally realizable?