Regiospecific Coordination of Ambidentate Tetrazoles to Cobalt Oximes

Article abstract of DOI:10.1021/ja00529a017

A series of complexes of the type (n-Bu3P)Co(DH)2(5R-tetrazolate) (DH is the monoanion of dimethylglyoxime; R = CF3, CH3, C6H5, C6H5CH2, (CH3)2N, 4-FC6H4, and 3-FC6H4) have been prepared and characterized by conductance studies, elemental analyses, and 1H, 13C<1H>, 31P<1H>, and 19F NMR spectroscopy.Quantum-mechanical calculations (MINDO/3) indicate that the N₁ nitrogens of the aromatic tetrazolate anion are slightly more nucleophilic than the N₂, yet in each complex the ambidentate tetrazolate anion is coordinated to cobalt via the N₂ nitrogen, showing that regiospecific coordination is sterically induced.This is in marked contrast to tetrazole complexes of platinum and palladium wherein both N₁ and N₂ bound tetrazoles are found in approximately equal abundances.These cobalt complexes react with alkyl halides such as CH3I and C6H5CH2Br to produce exclusively 1,5-disubstituted tetrazoles.None of the isomeric 2,5-disubstituted tetrazole is detected in these reactions in marked contrast to the reactions of sodium tetrazolates or gold, palladium, or platinum tetrazolate complexes with alkyl halides which produce mixtures of the 1,5- and 2,5-disubstituted tetrazoles.The crystal structure of n-Bu3PCo(DH)2(5-CF3-tetrazolate) was determined using three-dimensional X-ray diffraction techniques.The molecule crystallizes in the orthorhombic space group Pbca in a unit cell of dimensions a = 12.040(2) Angstroem, b = 21.531(3) Angstroem, c = 23.536(4) Angstroem, ρ_calcd = 1.368 g/cm³, ρ_obsd = 1.374 g/cm³.Refinement converged to 5.4percent with 2525 independent reflections.The tetrazolato ring is coordinated to cobalt via N₂ and is planar.The Co-P bond (2.263 Angstroem) is one of the shortest observed in LCo(DH)2X structures.The acute dihedral angle between the planes of the two glyoximato groups (10.4 deg) is among the largest dihedral angles observed in structures of this type.Surprisingly, the tetrazolato ring does not lie in the fold of the two glyoximato rings but is nearly normal to it.Thus, the regiospecificity of the alkylations of the coordinated tetrazolate with alkyl halides is sterically promoted.