Peripheral Methyl Activation in η4‑1,2,3,4-Tetramethylcyclobutadienylcobalt Complexes: Template Synthesis and Subsequent Reactivity of Triphosphamacrocycles

The cationic complex (η4-1,2,3,4-tetramethylcyclobutadienyl)­cobalt­(trisacetonitrile), [(η4-C4Me4)­Co­(NCCH3)3]+ (1), allows the stepwise introduction of suitable phosphine precursors to the [(η4-C4Me4)­Co]+ fragment by replacement of the labile acetonitrile ligands. These reactions give rise to the piano-stool complexes [(η4-C4Me4)­Co­(dppe)­(NCCH3)]+ (2), [(η4-C4Me4)­Co­(dppe)­(PH2Ph)]+ (3), [(η4-C4Me4)­Co­(dfppb)­(NCCH3)]+ (4), and [(η4-C4Me4)­Co­(dfppb)­(PH2Ph)]+ (5), where dfppb = 1,2-bis­{di­(2-fluorophenyl)­phosphino}­benzene and dppe = 1,2-bis­(diphenylphosphino)­ethane. Complex 5 is a template for the synthesis of the P3 macrocycle complex [(η4-C4Me4)­Co­{1,4-bis­(2-fluorophenyl)-7-phenyl­[b,e,h]­tribenzo-1,4,7-triphosphacyclononane}]+ (6), through base-promoted intramolecular macrocyclization. The hydrogens of two of the ring methyls of the tetramethylcyclobutadienyl ligand in the macrocycle complex 6 are sufficiently acidic to undergo deprotonation by KOtBu, promoting nucleophilic attack at the fluorine-bearing ortho-carbons of the 2-fluoroaryl groups on two of the phosphorus donors in 6. The resultant hemi-incarcerand complex [{η4,κP,κP,κP-Me2C4-[1,4-bis­(2-CH2C6H4)-7-C6H5-[b,e,h]­tribenzo-1,4,7-triphosphacyclononane]-1,2}­Co]+ (cis-7) contains a hybrid phosphorus/carbon donor ligand where the P3 macrocycle is connected to the cyclobutadienyl function through two cis-2-methylphenyl links. The new complexes have been characterized fully by spectroscopic and analytical techniques including single-crystal X-ray structure determinations of 2, 3, 4, 5, 6, and cis-7.