An iron(II) dihydrogen hydrido complex containing the tripodal tetraphosphine ligand P(CH₂CH₂PMe₂)₃

The dihydrogen hydrido complex [FeH(H₂)(PP₃)] ⁺ 1 (PP₃ = P(CH₂CH₂PMe ₂)₃ 2) was formed by the protonation of the dihydrido complex FeH₂(PP₃) 3 with methanol or ethanol. The observation of H-D coupling in partially deuterated isotopomers of 1 and measurement of T₁ relaxation times for the hydrido and dihydrogen resonances of 1 confirmed the presence of the η²-dihydrogen ligand. Complex 1 shows dynamic NMR behaviour in both the ³¹P and H NMR spectra with facile exchange between the protons in the η²- dihydrogen ligand and the η¹-hydrido ligand. The dihydrogen ligand of 1 is easily displaced by both anionic and neutral ligands to afford the corresponding hydrido complexes [FeHX(PP₃)]⁺ (X = CO 11, X = PPh₃ 12) or FeHX(PP₃) (X = Cl 13, X = Br 14, X = 115, X = N₃ 116). Small quantities of the alkoxy hydrido complexes FeH(OR)(PP₃) (R = Me 4; R = Et 5) are observed in methanol and ethanol solutions containing 1. In methanol solution, FeH(OMe)(PP₃) 4 reacts to form the carbonyl hydrido complex [FeH(CO)(PP₃)] ^- 11 and isotopic labelling confirms that the carbonyl ligand of 11 is derived from the methanol solvent. The mechanism of methanol oxidation presumably proceeds through β-hydride elimination from FeH(OMe)(PP ₃) to produce formaldehyde as an intermediate which is further dehydrogenated to form the carbonyl ligand. [FeH(H₂)(PP ₃)]⁺ 1 and FeHC1(PP₃) 13 react rapidly with paraformaldehyde to also form [FeMe(CO)(PP₃)]⁺ 11. Complex 11 also decarbonylates acetaldehyde to afford the methyl carbonyl complex [FeMe(CO)(PP₃)]⁺ 17. The structure of 17 was confirmed by X-ray crystallography.