Acidic Dicationic Iron(II) Dihydrogen Complexes and Compounds Related by H2 Substitution

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Abstract
[trans-Fe(H2)(CO)(dppe)2]2+ (3) (dppe = 1,2-bis(diphenylphosphino)ethane) was generated by protonation of [trans-FeH(CO)(dppe)2]+ in CD2Cl2. [trans-Fe(H2)(CO)(depe)2]2+ (6) (depe = 1,2-bis(diethylphosphino)ethane) was generated by the treatment of [trans-FeCl(CO)(depe)2]+ in CD2Cl2 with AgSbF6 under 1 atm of H2. Complex 3 is more acidic than trifluoromethanesulfonic acid (HOTf) in CD2Cl2, while 6 is suspected to be less acidic than [Et2OH]+. 3[OTf]2 is stable to H2 loss under reduced pressure for several hours, an indication of strong three-center (Fe−H2), two-electron σ-bonding. Both complexes 3 and 6 undergo H2 substitution reactions. There is evidence of the formation of [trans-Fe(H2O)(CO)(dppe)2]2+ and [trans-Fe(OTf)(CO)(dppe)2]+, although these complexes could not be isolated. [trans-FeY(CO)(depe)2]Y complexes (Y- = [BF4]-, 7[BF4]; Y- = [OTf]-, 8[OTf]) were isolated from the corresponding reactions of [trans-FeH(CO)(depe)2]Y with [Et2OH][BF4] or HOTf. 7[BF4] was structurally characterized by single-crystal X-ray diffraction. Attempts to grow crystals of 8[OTf] yielded salts containing the complex [trans-Fe(H2O)(CO)(depe)2]2+ (9), which were structurally characterized by single-crystal X-ray diffraction. Coordination of [BF4]- in 7[BF4] was demonstrated, by variable-temperature 31P{1H} NMR spectroscopy, to be dynamic. Dissolving 7[BF4] in methanol results in nucleophilic substitution at B to yield the new complex [trans-FeF(CO)(depe)2]+ (10). An attempt to grow crystals of 10[BF4] from the reaction mixture resulted in crystals of [H2(depe)][BF4], which were structurally characterized by single-crystal X-ray diffraction.

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