Mixed-Ligand Approach to Changing the Metal Ratio in Bismuth–Transition Metal Heterometallic Precursors

A new series of heteroleptic bismuth–transition metal β-diketonates [BiM­(hfac)₃(thd)₂] (M = Mn (<b>1</b>), Co (<b>2</b>), and Ni (<b>3</b>); hfac = hexafluoroacetylacetonate, thd = tetramethylheptanedionate) with Bi:M = 1:1 ratio have been synthesized by stoichiometric reactions between homometallic reagents [Bi^III(hfac)₃] and [M^II(thd)₂]. On the basis of analysis of the metal–ligand interactions in heterometallic structures, the title compounds were formulated as ion-pair {[Bi^III(thd)₂]⁺­[M^II(hfac)₃]⁻} complexes. The direct reaction between homometallic reagents proceeds with a full ligand exchange between main group and transition metal centers, yielding dinuclear heterometallic molecules. In heteroleptic molecules <b>1</b>–<b>3</b>, the Lewis acidic, coordinatively unsaturated Bi^III centers are chelated by two bulky, electron-donating thd ligands and maintain bridging interactions with three oxygen atoms of small, electron-withdrawing hfac groups that chelate the neighboring divalent transition metals. Application of the mixed-ligand approach allows one to change the connectivity pattern within the heterometallic assembly and to isolate highly volatile precursors with the proper Bi:M = 1:1 ratio. The mixed-ligand approach employed in this work opens broad opportunities for the synthesis of heterometallic (main group–transition metal) molecular precursors with specific M:M′ ratio in the case when homoleptic counterparts either do not exist or afford products with an incorrect metal:metal ratio for the target materials. Heteroleptic complexes obtained in the course of this study represent prospective single-source precursors for the low-temperature preparation of multiferroic perovskite-type oxides

Mixed-Ligand Approach to Design of Heterometallic Single-Source Precursors with Discrete Molecular Structure

Heterometallic single-source precursors for the Pb/Fe = 1:1 oxide materials, PbFe­(β-dik)₄ (β-dik = hexafluoroacetylacetonate (hfac, <b>1</b>), acetylacetonate (acac, <b>2</b>), and trifluoroacetylacetonate (tfac, <b>4</b>)), have been isolated by three different solid-state synthetic methods. The crystal structures of heterometallic diketonates <b>1</b>, <b>2</b>, and <b>4</b> were found to contain polymeric chains built on alternating [Fe­(β-dik)₂] and [Pb­(β-dik)₂] units that are held together by bridging M–O interactions. Heterometallic precursors are highly volatile, but soluble only in coordinating solvents, in which they dissociate into solvated homometallic fragments. In order to design the heterometallic precursor with a proper metal/metal ratio and with a discrete molecular structure, we used a combination of two different diketonate ligands. Heteroleptic complex Pb₂Fe₂­(hfac)₆­(acac)₂ (<b>5</b>) has been obtained by optimized stoichiometric reaction of an addition of homo-Fe­(acac)₂ to heterometallic Pb₂Fe­(hfac)₆ (<b>3</b>) diketonate that can be run in solution on a high scale. The combination of two ligands with electron-withdrawing and electron-donating groups allows changing the connectivity pattern within the heterometallic assembly and yields the precursor with a discrete tetranuclear structure. In accord with its molecular structure, heteroleptic complex <b>5</b> is soluble even in noncoordinating solvents and was found to retain its heterometallic structure in solution. Thermal decomposition of heterometallic precursors in air at 750 °C resulted in the target Pb₂­Fe₂O₅ oxide, a prospective multiferroic material. Prolonging the annealing time or increasing the decomposition temperature leads to another phase-pure lead–iron oxide Pb­Fe₁₂O₁₉ that is a representative of the important family of magnetic hexaferrites

Expanding the Structural Motif Landscape of Heterometallic β‑Diketonates: Congruently Melting Ionic Solids

The first example of ionic β-diketonates in which both the cation and anion are octahedral coordinatively saturated metal diketonate moieties are reported. Heterometallic tin–transition-metal heteroleptic diketonates were obtained through solid-state redox reactions and are formulated as {[Sn^IV(thd)₃]⁺[M^II(hfac)₃]⁻} (M^II = Mn (<b>1</b>), Fe (<b>2</b>), Co (<b>3</b>); thd = 2,2,6,6-tetramethyl-3,5-heptanedionate, hfac = hexafluoroacetylacetonate). X-ray single-crystal structural investigations along with DART mass spectrometry, multinuclear NMR, and magnetic susceptibility measurements have been used to confirm an assignment of metal oxidation states in compounds <b>1</b>–<b>3</b>. Ionic compounds were found to melt congruently at temperatures below the decomposition point. As such, they represent prospective materials that can be utilized as ionic liquids as well as reagents for the soft transfer of diketonate ligands. An unexpected volatility of ionic compounds <b>1</b>–<b>3</b> was proposed to occur through a transport reaction, in which the transport agent is one of the products of their partial decomposition in the gas or condensed phase

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

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 M_<i>x</i>M′_3–<i>x</i>O₄ 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^III(acac)₃]­[Co^II(hfac)₂] (<b>1</b>), [Co^II(hfac)₂]­[Fe^III(acac)₃]­[Co^II(hfac)₂] (<b>2</b>), and [Fe^II(hfac)₂]­[Fe^III(acac)₃]­[Co^II(hfac)₂] (<b>3</b>). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated Co^II/Fe^II centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring Fe^III metal ion. Preliminary assignment of Fe and Co positions/oxidation states in <b>1</b>–<b>3</b> 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 (<b>1</b>) and trinuclear (<b>2</b>) complexes, respectively. Direct analysis in real time mass spectrometry revealed the presence of Fe^III- and Co^II-based fragments in the gas phase upon evaporation of precursors <b>1</b> and <b>2</b> as well as of Fe^III, Fe^II, and Co^II species for complex <b>3</b>. Theoretical investigation of two possible “valent isomers”, [Fe^III(acac)₃]­[Co^II(hfac)₂] (<b>1</b>) and [Co^III(acac)₃]­[Fe^II(hfac)₂] (<b>1′</b>), provided an additional support for the metal site/oxidation state assignment giving a preference of 6.48 kcal/mol for the experimentally observed molecule <b>1</b>. Magnetic susceptibility measurements data are in agreement with the presence of high-spin Fe^III and Co^II magnetic centers with weak anti-ferromagnetic coupling between those in molecules of <b>1</b> and <b>2</b>. Highly volatile heterometallic complexes <b>1</b>–<b>3</b> were found to act as effective single-source precursors for the low-temperature preparation of iron–cobalt spinel oxides Fe_<i>x</i>Co_3–<i>x</i>O₄ known as important materials for diverse energy-related applications