Heterometallic Molecular and Ionic Isomers

Numerous descriptions of structural isomerism in metal complexes do not list any molecular vs ionic isomers. At the same time, one of the most striking examples of structural isomerism in organic chemistry is molecular urea, which has the same atomic composition as the chemically distinct ionic ammonium cyanate. This iconic organic couple now meets its inorganic heterometallic counterpart. We introduce a new class of structural isomers, molecular vs ionic, that can be consummated in complex and coordinatively unsaturated polynuclear/heterometallic compounds. We report inorganic molecular and ionic isomers of the composition [NaCrFe (acac)3(hfac)3] (acac = acetylacetonate; hfac = hexafluoroacetylacetonate). Heterometallic molecular [CrIII(acac)3-Na-FeII(hfac)3] (1m) and ionic {[CrIII(acac)3-Na-CrIII(acac)3]+[FeII(hfac)3-Na-FeII(hfac)3]-} (1i) isomers have been isolated in pure form and characterized. While both ions are heterobimetallic trinuclear entities, the neutral counterpart is a heterotrimetallic trinuclear molecule. The two isomers exhibit distinctly different characteristics in terms of solubility, volatility, mass spectrometry ionization, and thermal behavior. Unambiguous assignment of the positions and oxidation/spin states of the Periodic Table neighbors, Fe and Cr, in both isomers have been made by a combination of characterization techniques that include synchrotron X-ray resonant diffraction, synchrotron X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and DART mass spectrometry. The transformation between the two isomers that does take place in solutions of noncoordinating solvents has also been tested.

From cubane-assembled Mn-oxo clusters to monodispersed manganese oxide colloidal nanocrystals

The assemblies of [M4O4] (M = metal) cubanes represent a fascinating class of materials for a variety of application fields. Although such a structural characteristic is relatively common in small molecules and in extended bulk solids, high nuclearity clusters composed of multiple [M4O4] units as their backbones are rare. In this work, we report two new Mn-oxo clusters, MnII 8MnIII 10O10(OOCMe)12(OMe)14(py)2 ([Mn18-Ac]) and MnII 4MnIII 14O14(OOCCMe3)8(OMe)14(MeOH)5(py) ([Mn18-Piv]), whose core structures are assemblies of either 6- or 7-cubanes in different packing patterns, which have been unambiguously revealed by single crystal X-ray diffraction technique. The cubane-assembled structural features can be deemed as the embryonic structures of the bulk manganese oxide. Herein, this report demonstrates the first case study of utilizing Mn-oxo clusters as precursors for the preparation of manganese oxide nanocrystals, which has never been explored before. Through a simple colloidal synthetic approach, high-quality, monodisperse Mn3O4 nanocrystals can be readily prepared by employing both precursors, while their morphologies were found to be quite different. This work confirms that the structural similarity between precursors and nanomaterials is instrumental in affording more kinetically efficient pathways for materials formation, and the structure of the precursor has a significant impact on the morphology of final nanocrystal products.

Trickier than It Looks: Isomerization between Five- and Six-Coordinated Zinc in Heterometallic Li2Zn2 Molecule

This report describes the synthesis and characterization of two heterobimetallic Li-Zn coordination isomers [Li2Zn2(tbaoac)6] (tbaoac = tert-butyl acetoacetato) that have been isolated separately by the same stoichiometric reaction run in different organic solvents. The 6-coordinated zinc isomer (6-Zn) was synthesized in acetone with high yield, while the 5-coordinated one (5-Zn) was readily obtained from ethanol. The 5-Zn isomer has a low solubility in organic solvents such as alkanes and haloalkanes, while its 6-Zn counterpart exhibits a good solubility in almost all common solvents. Two isomeric molecules feature similar centrosymmetric tetranuclear cyclic assemblies, which are different in their arrangement of tbaoac ligands. While all ligands act as μ2-type in the structure of 5-Zn, the two tbaoac groups chelating Li appear as μ3-type in 6-Zn, thus providing an additional coordination for Zn ions. However, the real structural transformation between these isomers was shown to be more complex than simply making or breaking a couple of Zn-O bonds. X-ray single-crystal structure analysis, powder X-ray diffraction, multinuclear NMR, DART mass spectrometry, ICP-OES analysis, and TGA have been employed for the characterization of the isomers. The combination of powder X-ray diffraction and 1H NMR investigation revealed that 6-Zn isomer can be quantitatively transformed to 5-Zn in ethanol, while the reverse conversion instantly takes place in acetone.

Multi-functional Single-Source Molecular Precursors for Carbon-Coated Mixed-Metal Phosphates

We introduce a new synthetic concept that can be broadly adopted for the low-temperature preparation of mixed-metal energy storage materials, such as phosphates, silicates, fluorides, fluorophosphates, and fluorosulfates that exhibit intrinsic low electronic conductivity and thus require a carbon modulation. The development of novel low-temperature approaches for assembling energy-related materials with a complex core-shell microstructure is of great importance for expanding their application scope. The traditional definition of single-source precursors refers to their ability to yield a phase-pure material upon thermal decomposition. We have developed a new way for the utilization of heterometallic molecular precursors in synthesis that goes beyond its common delineation as a single-phase maker. The utility of this approach has been demonstrated upon the low-temperature synthesis of lithium-iron phosphate@C, which represents a celebrated cathode material for Li-ion batteries. The first atomically precise carbonaceous molecular precursors featuring a desired Li:Fe:P ratio of 1:1:1, divalent iron, and sufficient oxygen content for the target LiFeIIPO4 phosphate were shown to enable a spontaneous formation of both the olivine core and conductive carbon shell, yielding a carbon-coated mixed-metal phosphate.

Crystal structure and Hirshfeld surface analysis of bis-(μ-4-tert-but-oxy-4-oxobut-2-en-2-olato)bis-[(4-tert-but-oxy-4-oxobut-2-en-2-olato)ethano-lzinc(II)]

The mol-ecular and crystal structure of the title binuclear Zn²⁺ complex, [Zn₂(C₈H₁₃O₃)₄(C₂H₅OH)₂], with enolated anionic tert-butyl-aceto-acetate and ethanol was analysed. The coordination polyhedra of the Zn atoms are distorted octa-hedra formed by six oxygen atoms that belong to three ligand mol-ecules and a coordinated ethanol mol-ecule. In the crystal phase, alternating layers can be distinguished parallel to the ac plane. A Hirshfeld surface analysis showed that there are no strong inter-molecular inter-actions in the structure. The most significant contributions to the overall crystal packing are from H⋯H inter-molecular contacts.

Pentanuclear clusters resembling the cubane-dangler connectivity in the native oxygen-evolving center of photosystem II

A series of pentametallic "cubane-plus-dangler" complexes have been target synthesized. Among them, the [Fe3Ni2] aggregate strongly resembled the native oxygen-evolving center by mimicking the "cubane-plus-dangler" skeleton, the aqua binding site, and the connectivity between the pendent ion and the parent cubane. Our synthetic strategy that uses tri-substituted methanol as the "cubane-generator" and carboxylate as the pendant ligand provides a feasible approach for accessing model compounds of biological catalyst systems.

The use of mixed-metal single source precursors for the synthesis of complex metal oxides

This Feature Article highlights the use of mixed-metal single source precursors to directly access useful complex metal oxide materials.

Three to tango requires a site-specific substitution: heterotrimetallic molecular precursors for high-voltage rechargeable batteries

Design of heterotrimetallic molecules, especially those containing at least two different metals with close atomic numbers, radii, and the same coordination number/environment is a challenging task. This quest is greatly facilitated by having a heterobimetallic parent molecule that features multiple metal sites with only some of those displaying substitutional flexibility. Recently, a unique heterobimetallic complex LiMn2(thd)5 (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate) has been introduced as a single-source precursor for the preparation of a popular spinel cathode material, LiMn2O4. Theoretical calculations convincingly predict that in the above trinuclear molecule only one of the Mn sites is sufficiently flexible to be substituted with another 3d transition metal. Following those predictions, two heterotrimetallic complexes, LiMn2-x Co x (thd)5 (x = 1 (1a) and 0.5 (1b)), that represent full and partial substitution, respectively, of Co for Mn in the parent molecule, have been synthesized. X-ray structural elucidation clearly showed that only one transition metal position in the trinuclear molecule contains Co, while the other site remains fully occupied by Mn. A number of techniques have been employed for deciphering the structure and composition of heterotrimetallic compounds. Synchrotron resonant diffraction experiments unambiguously assigned 3d transition metal positions as well as provided a precise "site-specific Mn/Co elemental analysis" in a single crystal, even in an extremely difficult case of severely disordered structure formed by the superposition of two enantiomers. DART mass spectrometry and magnetic measurements clearly confirmed the presence of heterotrimetallic species LiMnCo(thd)5 rather than a statistical mixture of two heterobimetallic LiMn2(thd)5 and LiCo2(thd)5 molecules. Heterometallic precursors 1a and 1b were found to exhibit a clean decomposition yielding phase-pure LiMnCoO4 and LiMn1.5Co0.5O4 spinels, respectively, at the relatively low temperature of 400 °C. The latter oxide represents an important "5V spinel" cathode material for the lithium ion batteries. Transmission electron microscopy confirmed a homogeneous distribution of transition metals in quaternary oxides obtained by pyrolysis of single-source precursors.

Dinuclear lanthanide–lithium complexes based on fluorinated β-diketonate with acetal group: magnetism and effect of crystal packing on mechanoluminescence

A convenient and straightforward synthesis of a novel type of Ln–Li β-diketonates with a discrete molecular structure is presented.

From lithium to sodium: design of heterometallic molecular precursors for the NaMO2 cathode materials

Based on the Li₂M₂L₆ model structure, an addition of extra neutral donor ligand enables the formation of Na₂M₂L₆(THF)₂ molecular precursors for the NaMO₂ oxide cathode materials.

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

Tetranuclear molecular precursor Li₂CoNiL₆ for oxide cathode materials has been shown to contain heterotrimetallic and both parent heterobimetallic molecules.

This work raises a fundamental question about the “real” structure of molecular compounds containing three different metals: whether they consist of genuine heterotrimetallic species or of a mixture of parent heterobimetallic species. Heterotrimetallic complex Li₂CoNi(tbaoac)₆ (1, tbaoac = tert-butyl acetoacetate) has been designed based on the model tetranuclear structure featuring two transition metal sites in order to be utilized as a molecular precursor for the low-temperature preparation of the LiCo_0.5Ni_0.5O₂ battery cathode material. An investigation of the structure of 1 appeared to be very challenging, since the Co and Ni atoms have very similar atomic numbers, monoisotopic masses, and radii as well as the same oxidation state and coordination number/environment. Using a statistical analysis of heavily overlaid isotope distribution patterns of the [Li₂MM′L₅]⁺ (M/M′ = Co₂, Ni₂, and CoNi) ions in DART mass spectra, it was concluded that the reaction product 1 contains both heterotrimetallic and bimetallic species. A structural analogue approach has been applied to obtain Li₂MMg(tbaoac)₆ (M = Co (2) and Ni (3)) complexes that contain lighter, diamagnetic magnesium in the place of one of the 3d transition metals. X-ray crystallography, mass spectrometry, and NMR spectroscopy unambiguously confirmed the presence of three types of molecules in the reaction mixture that reaches an equilibrium, Li₂M₂L₆ + Li₂Mg₂L₆ ↔ 2Li₂MMgL₆, upon prolonged reflux in solution. The equilibrium mixture was shown to have a nearly statistical distribution of the three molecules, and this is fully supported by the results of theoretical calculations revealing that the stabilization energies of heterotrimetallic assemblies fall exactly in between those for the parent heterobimetallic species. The LiCo_0.5Ni_0.5O₂ quaternary oxide has been obtained in its phase-pure form by thermal decomposition of heterometallic precursor 1 at temperatures as low as 450 °C. Its chemical composition, structure, morphology, and transition metal distribution have been studied by X-ray and electron diffraction techniques and compositional energy-dispersive X-ray mapping with nanometer resolution. The work clearly illustrates the advantages of heterometallic single-source precursors over the corresponding multi-source precursors.

Heterometallic molecular precursors for a lithium-iron oxide material: synthesis, solid state structure, solution and gas-phase behaviour, and thermal decomposition

Three heterometallic single-source precursors with a Li : Fe = 1 : 1 ratio for a LiFeO₂ oxide material are reported. Heterometallic compounds LiFeL₃ (L = tbaoac (1), ptac (2), and acac(3)) have been obtained on a large scale, in nearly quantitative yields by one-step reactions that employ readily available reagents. The heterometallic precursor LiFe(acac)₃ (3) with small, symmetric substituents on the ligand (acac = pentane-2,4-dionate), maintains a 1D polymeric structure in the solid state that limits its volatility and prevents solubility in non-coordinating solvents. The application of the unsymmetrical ligands, tbaoac (tert-butyl acetoacetate) and ptac (1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionate), that exhibit different bridging properties at the two ends of the ligand, allowed us to change the connectivity pattern within the heterometallic assembly. The latter was demonstrated by structural characterization of heterometallic complexes LiFe(tbaoac)₃ (1) and LiFe(ptac)₃ (2) that consist of discrete heterocyclic tetranuclear molecules Li₂Fe₂L₆. The compounds are highly volatile and exhibit a congruent sublimation character. DART mass spectrometric investigation revealed the presence of heterometallic molecules in the gas phase. The positive mode spectra are dominated by the presence of [M - L]⁺ peaks (M = Li₂Fe₂L₆). In accord with their discrete molecular structure, complexes 1 and 2 are highly soluble in nearly all common solvents. In order to test the retention of the heterometallic structure in solution, the diamagnetic analog of 1, LiMg(tbaoac)₃ (4), has been isolated. Its tetranuclear molecular structure was found to be isomorphous to that of the iron counterpart. ¹H and ⁷Li NMR spectroscopy unambiguously confirmed the presence of heterometallic molecules in solutions of non-coordinating solvents. The heterometallic precursor 1 was shown to exhibit clean thermal decomposition in air that results in phase-pure α-modification of layered oxide LiFeO₂, the prospective cathode material for lithium ion batteries.

A versatile single molecular precursor for the synthesis of layered oxide cathode materials for Li-ion batteries

A versatile single molecular precursor, LiTM(acac)₃, featuring a 1D chain structure, was developed to achieve the layered oxides LiTMO₂(TM = Ni/Mn/Co, NMC) for Li-ion batteries.

A heterobimetallic single-source precursor enabled layered oxide cathode for sodium-ion batteries

A heterobimetallic single-source precursor, NaCo(acac)₃, featuring a 1D chain structure was for the first time designed to achieve layered oxide P2-Na_xCoO₂ with superior rate capability for Na-ion batteries (SIBs). It opens a new avenue to synthesize high-performance layered oxide cathode materials for SIBs.

Multifunctional SA-PProDOT Binder for Lithium Ion Batteries

An environmentally benign, highly conductive, and mechanically strong binder system can overcome the dilemma of low conductivity and insufficient mechanical stability of the electrodes to achieve high performance lithium ion batteries (LIBs) at a low cost and in a sustainable way. In this work, the naturally occurring binder sodium alginate (SA) is functionalized with 3,4-propylenedioxythiophene-2,5-dicarboxylic acid (ProDOT) via a one-step esterification reaction in a cyclohexane/dodecyl benzenesulfonic acid (DBSA)/water microemulsion system, resulting in a multifunctional polymer binder, that is, SA-PProDOT. With the synergetic effects of the functional groups (e.g., carboxyl, hydroxyl, and ester groups), the resultant SA-PProDOT polymer not only maintains the outstanding binding capabilities of sodium alginate but also enhances the mechanical integrity and lithium ion diffusion coefficient in the LiFePO4 (LFP) electrode during the operation of the batteries. Because of the conjugated network of the PProDOT and the lithium doping under the battery environment, the SA-PProDOT becomes conductive and matches the conductivity needed for LiFePO4 LIBs. Without the need of conductive additives such as carbon black, the resultant batteries have achieved the theoretical specific capacity of LiFePO4 cathode (ca. 170 mAh/g) at C/10 and ca. 120 mAh/g at 1C for more than 400 cycles.

Mixed-valent, heteroleptic homometallic diketonates as templates for the design of volatile heterometallic precursors

A novel series of mixed-valent, heteroleptic transition metal diketonates that can be utilized as prospective single-source precursors for the low-temperature preparation of oxide materials are reported. The first mixed-valent iron β-diketonates with different FeIII/FeII ratios have been synthesized by applying the mixed-ligand approach. Based on nearly quantitative reaction yields and analysis of iron-oxygen bonds, these compounds were formulated as [FeIII(acac)3][FeII(hfac)2] (1) and [FeII(hfac)2][FeIII(acac)3][FeII(hfac)2] (2). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated FeII centers chelated by two hfac (hexafluoroacetylacetonate) ligands with electron-withdrawing substituents maintain bridging interactions with oxygen atoms of electron-donating acac (acetylacetonate) groups that chelate the neighboring FeIII atoms. Switching the ligands on FeIII and FeII atoms in starting reagents resulted in the instant ligand exchange between iron centers and in yet another polynuclear homometallic diketonate [FeII(hfac)2][FeIII(acac)2(hfac)][FeII(hfac)2] (3) that adheres to the same bonding pattern as in complexes 1 and 2. The proposed synthetic methodology has been extended to design heterometallic diketonates with different M : M' ratios. Homometallic parent molecules have been used as templates to obtain heterometallic mixed-valent [FeIII(acac)3][MnII(hfac)2] (4) and [NiII(hfac)2][FeIII(acac)3][NiII(hfac)2] (5) complexes. The combination of two different diketonate ligands with electron-donating and electron-withdrawing substituents was found to be crucial for maintaining the above mixed-valent heterometallic assemblies. Theoretical investigation of two possible "isomers", [FeIII(acac)3][MnII(hfac)2] (4) and [MnIII(acac)3][FeII(hfac)2] (4') provided an additional support for the metal site assignment giving a preference of 9.78 kcal mol-1 for the molecule 4. Heterometallic complexes obtained in the course of this study have been found to act as effective single-source precursors for the synthesis of mixed-transition metal oxide materials M x M'2-xO3 and M x M'1-xO. The title highly volatile precursors can be used for the low-temperature preparation of both amorphous and crystalline heterometallic oxides in the form of thin films or nanosized particles that are known to operate as efficient catalysts in oxygen evolution reaction.