Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis

The E‒H bond activation chemistry of <i>tris</i>-phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex (<b>TPB</b>)Fe(N₂) heterolytically activates H‒H and the C‒H bonds of formaldehyde and arylacetylenes across an Fe‒B bond. In particular, H‒H bond cleavage at (<b>TPB</b>)Fe(N₂) is reversible and affords the iron-hydride-borohydride complex (<b>TPB</b>)(μ‒H)Fe(L)(H) (L = H₂, N₂). (<b>TPB</b>)(μ‒H)Fe(L)(H) and (<b>TPB</b>)Fe(N₂) are competent olefin and arylacetylene hydrogenation catalysts. Stoichiometric studies indicate that the B‒H unit is capable of acting as a hydride shuttle in the hydrogenation of olefin and arylacetylene substrates. The heterolytic cleavage of H₂ by the (<b>TPB</b>)Fe system is distinct from the previously reported (<b>TPB</b>)Co(H₂) complex, where H₂ coordinates as a non-classical H₂ adduct based on X-ray, spectroscopic, and reactivity data. The non-classical H₂ ligand in (<b>TPB</b>)Co(H₂) is confirmed in this work by single crystal neutron diffraction, which unequivocally shows an intact H‒H bond of 0.83 Å in the solid state. The neutron structure also shows that the H₂ ligand is localized at two orientations on cobalt <i>trans</i> to the boron. This localization in the solid state contrasts with the results from ENDOR spectroscopy that show that the H₂ ligand freely rotates about the Co‒H₂ axis in frozen solution. Finally, the (<b>TPB</b>)Fe system, as well as related <i>tris</i>-phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in (<b>TPB</b>)Fe, were studied for CO₂ hydrogenation chemistry. The (<b>TPB</b>)Fe system is not catalytically competent, while the silicon, borate, carbon variants, (<b>SiP^R₃</b>)Fe, (<b>PhBP^<i>i</i>Pr₃</b>)Fe, and (<b>CP^<i>i</i>Pr₃</b>)Fe, respectively, are catalysts for the hydrogenation of CO₂ to formate and methylformate. The hydricity of the CO₂ reactive species in the silatrane system (<b>SiP^<i>i</i>Pr₃</b>)Fe(N₂)(H) has been experimentally estimated.