Control of axial–equatorial interactions in (dimethylglyoximato)cobalt(III) complexes. Part 1. Counter cation effect on the strength of the intermolecular hydrogen bond, OH ⋯ O–, of trans-bis(dimethylglyoximato)dinitrocobaltate(III) anions and crystal structure of the tetramethylammonium salt

Abstract

The infrared frequencies observed for the particular functional group of both the axial and the equatorial ligands of the complex anion [Co(Hdmg)₂(NO₂)₂](H₂dmg = dimethylglyoxime) have been found to be widely different upon changing the counter cations M⁺(M = Li, Na, K, Rb or Cs) or NR₄⁺(R = H, Me, Et or Bu). In particular, the O–H stretching frequency of the intramolecular hydrogen bond, O–H ⋯ O^–, changes markedly depending on the kind of counter cation. Alkali-metal counter cations, M⁺, would exert a co-ordination effect upon the donor atom in the complex anion, while tetraalkylammonium counter cations would give a predominantly long-range potential effect only. In order to investigate the difference in the counter cation effects on the strength of the in-plane intramolecular hydrogen bond, crystal and the molecular structures of [NMe₄][Co(Hdmg)₂(NO₂)₂] and Na[Co(Hdmg)₂(NO₂)₂]·2H₂O were compared. An unusual conformation around the Co–NO^–₂ moiety was observed in [NMe₄][Co(Hdmg)₂(NO₂)₂]. The orientation of the axial ligands, NO₂^–, is restricted in such a way that they are located over the five-membered chelate ring formed by the Hdmg^– moiety and the Co^III atom. On the other hand, Na[Co(Hdmg)₂(NO₂)₂]·2H₂O has a usual conformation which would lead to minimum non-bonded interaction between the Hdmg^– moietry and the two axial ligands, NO₂^–. In this conformation, two NO₂^– are positioned over the pseudo-six-membered chelate rings. A further important structural feature in [NMe₄][Co(Hdmg)₂(NO₂)₂] is a very long Co–NO^–₂ distance [1.974(3), as compared with 1.949(3) and 1.943(3)Å in the sodium salt]. This difference is ascribed to the counter cation effect. The stretching frequencies of O–H ⋯ O^– and CN in [NMe₄][Co(Hdmg)₂(NO₂)₂] are quite different from those in Na[Co(Hdmg)₂(NO₂)₂]·2H₂O. These facts suggest that the counter cations play an important role in controlling the axial–equatorial interactions in [Co(Hdmg)₂(NO₂)₂]^–.