Synthesis and properties of (η²-C,X) chelate arylcarbene complexes [Fe(C₅Me₅)(L){η²-C(OMe)C₆H ₄-o-X}][OTf] (L = CO, PMe₃; X = OMe, Cl)

The methoxycarbene complexes [Fe(C₅Me₅)(CO)₂{=C(OMe)C₆H ₄-o-X}][OTf] (2a, X = OMe; 2b, X = Cl) are good precursors of the corresponding (η²-C,X) chelate carbene complexes [Fe(C₅Me₅)(CO){η²-C(OMe)C₆H ₄-o-X}][OTf] (OTf = CF₃SO₃) (5a,b). The η₂-chloro derivative 5b has been characterized by X-ray diffraction, confirming the formation of a five-membered ring metallacycle. All the new carbene complexes, including the nonheteroatom-stabilized carbene complex [Fe(C₅Me₅)(CO)₂{=C(H)C₆H ₄-O-OMe}][OTf] (4a), have been fully characterized by ¹H and ¹³C NMR spectrocopy. The lability of the chelating o-substituent is chemically demonstrated by the formation of the corresponding neutral iodo carbene complexes [Fe(C₅Me₅)(CO)(I){η¹-C(OMe)C ₆H₄-o-X}] (6a,b), the competitive O-demethylation process being thus inhibited. Selective ligand exchange reactions of 5a,b afford various substituted complexes such as [Fe(C₅Me₅)(CO)_n(PMe₃){η ^x-C(OMe)C₆H₄-o-OMe}][OTf] (11, n = 1, x = 1; 12, n = 0 x = 2); the mono- and bis(acetonitrile) complexes [Fe(C₅Me₅)(L)(CH₃CN){η¹ C(OMe)C₆H₄-o-Cl}][OTf] (14, L = CH₃CN; 15, L = CO) have been also synthesized. The reactivity of 5a toward NaBH₄ is highly dependent on the solvent. Specific hydride addition occurs in 9:1 THF-MeOH to give the expected complex [Fe(C₅Me₅)(CO){η²-CH(OMe)C ₆H₄-o-OMe}] (9) as a single diastereoisomer, while reduction in pure THF affords the organoborohydride complex [Fe(C₅Me₅)(CO){η²-H₂BHCH ₂C₆H₄-o-OMe}] (10). The latter reaction involves a formal insertion of BH₃ into a Fe-C bond, promoted by the potential vacant coordination site. The carbene ligand is easily displaced and recovered as free carbonylcontaining organic substrates (aldehyde or ester) from both types of carbene complexes upon bubbling of O₂.