Position Assignment and Oxidation State Recognition of Fe and Co Centers in Heterometallic Mixed-Valent Molecular Precursors for the Low-Temperature Preparation of Target Spinel Oxide Materials

Abstract

A series of mixed-valent, heterometallic (mixed-transition metal) diketonates that can be utilized as prospective volatile single-source precursors for the low-temperature preparation of MM'O spinel oxide materials is reported. Three iron-cobalt complexes with Fe/Co ratios of 1:1, 1:2, and 2:1 were synthesized by several methods using both solid-state and solution reactions. On the basis of nearly quantitative reaction yields, elemental analyses, and comparison of metal-oxygen bonds with those in homometallic analogues, heterometallic compounds were formulated as [Fe(acac)][Co(hfac)] (1), [Co(hfac)][Fe(acac)][Co(hfac)] (2), and [Fe(hfac)][Fe(acac)][Co(hfac)] (3). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated Co/Fe centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring Fe metal ion. Preliminary assignment of Fe and Co positions/oxidation states in 1-3 drawn from X-ray structural investigation was corroborated by a number of complementary techniques. Single-crystal resonant synchrotron diffraction and neutron diffraction experiments unambiguously confirmed the location of Fe and Co sites in the molecules of dinuclear (1) and trinuclear (2) complexes, respectively. Direct analysis in real time mass spectrometry revealed the presence of Fe- and Co-based fragments in the gas phase upon evaporation of precursors 1 and 2 as well as of Fe, Fe, and Co species for complex 3. Theoretical investigation of two possible "valent isomers", [Fe(acac)][Co(hfac)] (1) and [Co(acac)][Fe(hfac)] (1'), provided an additional support for the metal site/oxidation state assignment giving a preference of 6.48 kcal/mol for the experimentally observed molecule 1. Magnetic susceptibility measurements data are in agreement with the presence of high-spin Fe and Co magnetic centers with weak anti-ferromagnetic coupling between those in molecules of 1 and 2. Highly volatile heterometallic complexes 1-3 were found to act as effective single-source precursors for the low-temperature preparation of iron-cobalt spinel oxides FeCoO known as important materials for diverse energy-related applications.

Heterotrimetallic Mixed‐Valent Molecular Precursors Containing Periodic Table Neighbors: Assignment of Metal Positions and Oxidation States

“…The latter indicates an apparent redox process accompanied by ligand exchange (Supporting Information, Scheme S1 and detailed synthetic procedure therein). This finding is in line with previous observations that diketonate ligands support the Fe 3+ /Co 2+ couple in the corresponding heterometallic complexes. Furthermore, it confirms our initial prediction that metal oxidation state and its coordination environment in heterometallic molecules are coupled with each other, forcing electron‐donating ligands to chelate electron‐poorer metal centers and vice‐versa.…”

“…We first tested the feasibility of site‐specific substitution at the trivalent metal position. Trivalent iron was selected to replace Co III in the model molecule 1 (Figure a) based on our previous experience with mixed‐valent Fe‐Co diketonate complexes . Modified Reaction (1) readily yields the hetero tri metallic complex [NaFeCo(acac) 3 (hfac) 3 ] ( 2 ) that was found to be isomorphous to the model structure 1 (Figure a; Supporting Information, Table S: …”

Heterotrimetallic Mixed‐Valent Molecular Precursors Containing Periodic Table Neighbors: Assignment of Metal Positions and Oxidation States

“…We first tested the feasibility of site‐specific substitution at the trivalent metal position. Trivalent iron was selected to replace Co III in the model molecule 1 (Figure a) based on our previous experience with mixed‐valent Fe‐Co diketonate complexes . Modified Reaction (1) readily yields the hetero tri metallic complex [NaFeCo(acac) 3 (hfac) 3 ] ( 2 ) that was found to be isomorphous to the model structure 1 (Figure a; Supporting Information, Table S: …”

A three body problem: a genuine heterotrimetallic molecule vs. a mixture of two parent heterobimetallic molecules

“…The situation becomes even more complicated in the case when there is an extra symmetry within the molecule, and the metal atoms in question are not crystallographically independent. Apparently, other diffraction methods such as neutron and resonant diffraction that we have successfully applied before 49 for analysis of site occupancy by 3d transition metal atoms cannot resolve the above problem, other than to confirm mixed-occupancy and to provide “elemental analysis” data for a particular single crystal. Thus we turned to mass spectrometry in order to analyse the composition of heterometallic molecules based on the rich isotope distribution patterns of accessible mother ions.…”

“…DART (Direct Analysis in Real Time) mass spectrometry has already been established as a powerful tool for investigating the gas phase behaviour of volatile heterometallic compounds 42 , 43 , 47 as well as for confirmation of transition metal oxidation states. 49 In this work, we extended the application scope of the technique for analysis of hetero tri metallic compounds primarily based on their isotope distribution patterns. In accord with their heterocyclic tetranuclear structure, the title molecules exhibit high volatility along with a sufficiently large temperature window between sublimation and decomposition, as well as sound structural stability upon retaining the heterometallic assembly in the gas phase.…”