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 CoIII 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]–.