Periodic Reporting for period 2 - TRUFA (Transition metal-Uranium FLPs for the cooperative Activation of dinitrogen)
Reporting period: 2024-01-01 to 2024-12-31
The conversion of atmospheric dinitrogen into ammonia consumes around 1% of the global energy demand. Milder efficient nitrogen fixation methodologies remain one of the greatest challenges in modern chemistry. Frustrated Lewis Pairs (FLPs) excel to activate small molecules, yet their involvement in N2 chemistry remains an unmet challenge. Inspired by the independent 2017 reports of Szymczak and Simonneau on the activation of transition metal dinitrogen complexes by Lewis acids, and taking into consideration the rich chemistry of low-valent uranium towards N2, this project envisioned the cooperative activation and functionalization of N2 by electron rich transition metals and electrophilic U(III) complexes. Importantly, at the time of the project, there was only one report displaying cooperative N2 activation by a transition metal and uranium (Cummins, 1998). Besides this, end-on coordination of N2 to uranium was only known for a couple of examples (Evans 2003, Liddle 2019).
In addition, transition metal-catalysis is a key area of industrial interest: exploring new avenues to improve the performance of catalysts remains a pivotal aspect to enable more efficient and sustainable transformations.
In addition, transition metal-catalysis is a key area of industrial interest: exploring new avenues to improve the performance of catalysts remains a pivotal aspect to enable more efficient and sustainable transformations.
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).
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).
The successful isolation of Fe-N2-U complexes holds promise for the cooperative functionalization of dinitrogen by transition metals and f-elements. Modulation of the metal of choice, charge of the fragments, and the stereoelectronics of the ligand frameworks can potentially modify the degree of N2 activation, enable direct N−N cleavage, impart increased stability towards dissociation or reactivity towards N2 reduction.
The related family of heterobimetallic lanthanide complexes are also unique, encompassing the first isolated end-on bound dinitrogen complex of Ce, as well as N2 binding by Yb(II) and the binding of two N2 molecules by a Sm(II) complex. In addition to potential magnetic applications based on communication between distinct metal centers, the isolation of an N2-bound Sm(II) species with a vacant site at Sm provides an interesting framework to study the role of intramolecular PCET in N2 reduction upon ligand tuning.
The selectivity of iridium catalysts in hydrogenation reactions and their activity in olefin isomerization were improved. Bimetallic inhibition, well known in heterogeneous catalysis, remains comparatively underexplored for homogeneous systems. Furthermore, it was shown that unusual oxidation states, largely underexplored for platinum-group metals, can induce large increases in activity.
The related family of heterobimetallic lanthanide complexes are also unique, encompassing the first isolated end-on bound dinitrogen complex of Ce, as well as N2 binding by Yb(II) and the binding of two N2 molecules by a Sm(II) complex. In addition to potential magnetic applications based on communication between distinct metal centers, the isolation of an N2-bound Sm(II) species with a vacant site at Sm provides an interesting framework to study the role of intramolecular PCET in N2 reduction upon ligand tuning.
The selectivity of iridium catalysts in hydrogenation reactions and their activity in olefin isomerization were improved. Bimetallic inhibition, well known in heterogeneous catalysis, remains comparatively underexplored for homogeneous systems. Furthermore, it was shown that unusual oxidation states, largely underexplored for platinum-group metals, can induce large increases in activity.