Metallaboratrane Facilitated E‒H Bond Activation and Hydrogenation Catalysis

The E‒H bond activation chemistry of tris-phosophino-iron and -cobalt metallaboratranes is discussed. The ferraboratrane complex (TPB)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 (TPB)Fe(N2) is reversible and affords the iron-hydride-borohydride complex (TPB)(μ‒H)Fe(L)(H) (L = H2, N2). (TPB)(μ‒H)Fe(L)(H) and (TPB)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 (TPB)Fe system is distinct from the previously reported (TPB)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 (TPB)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 trans 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 (TPB)Fe system, as well as related tris-phosphino-iron complexes that contain a different apical ligand unit (Si, PhB, C, and N) in place of the boron in (TPB)Fe, were studied for CO2 hydrogenation chemistry. The (TPB)Fe system is not catalytically competent, while the silicon, borate, carbon variants, (SiPR3)Fe, (PhBPiPr3)Fe, and (CPiPr3)Fe, respectively, are catalysts for the hydrogenation of CO2 to formate and methylformate. The hydricity of the CO2 reactive species in the silatrane system (SiPiPr3)Fe(N2)(H) has been experimentally estimated