Electron-rich transition metal dinitrogen complexes, such as Fe(depe)2N2, W(dppe)2(N2)2 or (TPB)Fe(N2)]− (K as counterion encapsulated by 18-crown-6 or [2.2.2]cryptand) and electrophilic trivalent uranium species, such as U(HMDS)3, U(OAr)3 (Ar = 2,6-ditertbutylphenyl) or U(C5Me4)3 were synthesized according to literature procedures using strictly air and moisture-free conditions. Two heterobimetallic N2-bridged complexes, (depe)2FeN2U(HMDS)3 and (depe)2FeN2U(OAr)3, displaying rare end-on N2 coordination to uranium, were synthesized and characterized by multinuclear NMR, EPR, elemental analysis and single crystal x-ray diffraction.
These complexes presented dynamic solution behaviour, dissociating into (depe)2FeN2 and the U(III) precursor even at low temperatures. Because of the reactive nature of the independent fragments, attempts to ascertain the reactivity of the Fe-N2-U species were unfruitful. Photolytic splitting into two nitrides did not work under the screened conditions. To address these shortcomings, the activation of CO2 using Fe(depe)2CO2 and U(III) species was investigated, leading to the formation of Fe(depe)2CO and terminal or bridged uranium oxo species. Furthermore, a family of transition metal-lanthanide end-on N2-bridged species was isolated, employing homoleptic HMDS derivatives of Ce(III), Sm(III), Dy(III), Tm(III) and Sm(II) and Yb(II), including the heterotrimetallic [Fe(depe)2N2]2Sm(HMDS)2.
An in depth computational investigation on the bonding of these unusual species has been carried out, focusing on the U-N2 and U-CO interactions, by means of open-shell Density Functional Theory, NBO, EDA-NOCV and QTAIM. In particular, the EDA-NOCV methodology allowed to decompose these interaction into the corresponding components.
Finally, in the return phase some of the key concepts of the TRUFA project (open-shell complexes, bimetallic cooperation, computational methods) were employed in iridium catalysts. The underlying mechanisms were studied in depth: homogeneous, gold-promoted bimetallic inhibition was strongly supported in hydrogenation catalysis by a combination of structural, spectroscopic and computational investigations, and a highly unusual monometallic Ir(II)/Ir(IV) redox cycle was proposed for olefin isomerization, further supported by Activation Strain Analysis (ASA).
