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

Abstract

Reaction of Ph 3 P-[Co] - ([Co] = Co(dmgH) 2 ; dmgH 2 = dimethylglyoxime), prepared by reduction of Ph 3 P-[Co]-Cl with NaBH 4 in methanolic NaOH, with BrCH 2 CH 2 CH 2 F resulted in formation of Ph 3 P-[Co]-CH 2 CH 2 CH 2 F (3). Inneutral 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 BrCH 2 CHF 2 , yielding the 2,2-difluoroethyl complexes py*-[Col-CH 2 CHF 2 (py* = py (4a), 4-(t-Bu)py (4b), 3-Fpy (4c)). Reactions of XCH 2 CH 2 F (X = Br, TfO; TfO = triflate) with reduced cobaloximes in alkaline and neutral methanolic solutions resulted in formation of the 2-methoxyethyl complexes py*-[Co]-CH 2 -CH 2 OMe (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 TfCCH 2 CMe 2 F, yielding py*-[Co]-CH=CMe 2 (py* = py (6a), 3-Fpy (6b)) and H 2 C=CMe 2 . All these reactions indicate the formation of the (unseen) intermediate py*-[Col-CH 2 CR 2 F (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]-C 6 H 1 0 F (py* = py (7a), 4-(t-Bu)py (7b)) and py*-[Co]-C 5 H 8 F (py* = py (10a), 4-(t-Bu)py (10b)). All these complexes were fully characterized by 1 H, 1 3 C, and 1 9 F NMR spectroscopic investigations. Molecular structures of the cobaloximes 3, 4a, 7b, and 7b.CH 2 CL were obtained by single-crystal X-ray diffraction analyses, exhibiting complexes with an equatorial pseudomacrocyclic (dmgH) 2 ligand as well as axial base (PPh 3 , 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 1 H and 1 3 C NMR spectra. Further proof for this came from DFT calculations of py-[Co]-C 6 H 1 0 F (11) and, for comparison, of the cyclohexyl complex py-[Co]-C 6 H 1 1 (12). The conformational energy diagrams of 11 and 12 are discussed.