Synthesis, Characterization, and Reactivity of (Fluoroalkyl)- and (Fluorocycloalkyl)cobaloximes:  Molecular Structure of a (2-Fluorocyclohexyl)cobaloxime Complex and Hindered Rotation of 2-Fluorocycloalkyl Ligands

Reaction of Ph₃P−[Co]^- ([Co] = Co(dmgH)₂; dmgH₂ = dimethylglyoxime), prepared by reduction of Ph₃P−[Co]−Cl with NaBH₄ in methanolic NaOH, with BrCH₂CH₂CH₂F resulted in formation of Ph₃P−[Co]−CH₂CH₂CH₂F (3). In neutral methanolic solutions py*−[Co]−H/py*−[Co]^- (py* = py, 4-(t-Bu)py, 3-Fpy; 4-(t-Bu)py = 4-tert-butylpyridine, 3-Fpy = 3-fluoropyridine) were found to react with BrCH₂CHF₂, yielding the 2,2-difluoroethyl complexes py*−[Co]−CH₂CHF₂ (py* = py (4a), 4-(t-Bu)py (4b), 3-Fpy (4c)). Reactions of XCH₂CH₂F (X = Br, TfO; TfO = triflate) with reduced cobaloximes in alkaline and neutral methanolic solutions resulted in formation of the 2-methoxyethyl complexes py*−[Co]−CH₂CH₂OMe (py* = py (5a), 4-(t-Bu)py (5b), 3-Fpy (5c)) with ethylene as side product. py*−[Co]−H/py*−[Co]^- (py* = py, 3-Fpy) was found to react with TfOCH₂CMe₂F, yielding py*−[Co]−CHCMe₂ (py* = py (6a), 3-Fpy (6b)) and H₂CCMe₂. All these reactions indicate the formation of the (unseen) intermediate py*−[Co]−CH₂CR₂F (R = H, Me), which decomposes via nucleophilic substitution (F → OMe), heterolytic fragmentation, yielding olefins, and HF elimination, yielding vinyl complexes, respectively. Analogous reactions of reduced cobaloximes with trans-1-bromo-2-fluorocyclohexane and trans-1-bromo-2-fluorocyclopentane resulted in the formation of (2-fluorocyclohexyl)- and (2-fluorocyclopentyl)cobaloximes, py*−[Co]−C₆H₁₀F (py* = py (7a), 4-(t-Bu)py (7b)) and py*−[Co]−C₅H₈F (py* = py (10a), 4-(t-Bu)py (10b)). All these complexes were fully characterized by ¹H, ¹³C, and ¹⁹F NMR spectroscopic investigations. Molecular structures of the cobaloximes 3, 4a, 7b, and 7b·CH₂Cl₂ were obtained by single-crystal X-ray diffraction analyses, exhibiting complexes with an equatorial pseudomacrocyclic (dmgH)₂ ligand as well as axial base (PPh₃, py*) and organo ligand in mutually trans positions. The cycloalkyl complexes are the ae isomers, having the sterically demanding Co(dmgH)₂ moieties as equatorial substituents. The axially oriented fluoro substituents give rise to hindered rotation of 2-fluorocycloalkyl ligands, as indicated by two distinct sets of signals for dmgH ligands in the ¹H and ¹³C NMR spectra. Further proof for this came from DFT calculations of py−[Co]−C₆H₁₀F (11) and, for comparison, of the cyclohexyl complex py−[Co]−C₆H₁₁ (12). The conformational energy diagrams of 11 and 12 are discussed.