Position Assignment and Oxidation State Recognition of Fe and Co

Article pubs.acs.org/IC

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 Craig M. Lieberman,† Matthew C. Barry,† Zheng Wei,† Andrey Yu. Rogachev,‡ Xiaoping Wang,§ Jun-Liang Liu,∥,⊥,# Rodolphe Clérac,∥,⊥ Yu-Sheng Chen,g Alexander S. Filatov,*,h and Evgeny V. Dikarev*,† †

Department of Chemistry, University at Albany, Albany, New York 12222, United States Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States § Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ CNRS, CRPP, UPR 8641, F-33600 Pessac, France ⊥ Univ. Bordeaux, UPR 8641, F-33600 Pessac, France # MOE Key Lab of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China g ChemMatCARS, Center for Advanced Radiation Sources, The University of Chicago, Argonne, Illinois 60439, United States h Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States ‡

S Supporting Information *

ABSTRACT: A series of mixed-valent, heterometallic (mixed-transition metal) diketonates that can be utilized as prospective volatile singlesource precursors for the low-temperature preparation of MxM′3−xO4 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 [FeIII(acac)3][CoII(hfac)2] (1), [Co II (hfac) 2 ][Fe III (acac) 3 ][Co II (hfac) 2 ] (2), and [Fe II (hfac) 2 ][FeIII(acac)3][CoII(hfac)2] (3). In the above heteroleptic complexes, the Lewis acidic, coordinatively unsaturated CoII/FeII centers chelated by two hexafluoroacetylacetonate (hfac) ligands maintain bridging interactions with oxygen atoms of acetylacetonate (acac) groups that chelate the neighboring FeIII 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 FeIII- and CoII-based fragments in the gas phase upon evaporation of precursors 1 and 2 as well as of FeIII, FeII, and CoII species for complex 3. Theoretical investigation of two possible “valent isomers”, [FeIII(acac)3][CoII(hfac)2] (1) and [CoIII(acac)3][FeII(hfac)2] (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 FeIII and CoII 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 FexCo3−xO4 known as important materials for diverse energy-related applications.

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INTRODUCTION

ular, spinel-type oxides of the general formula AxB3−xO4 (where A and B are the first-row transition metals Cr−Zn) have attracted attention of researchers as promising materials for various applications that include: catalysts,6 electronics,7 super

Experimental and theoretical studies of transition-metal oxides have a long distinguished history throughout physics and chemistry.1 Because of their remarkable electrochemical properties, mixed-transition metal oxides play a significant role in development of the low-cost and environmentally friendly energy storage/conversion technologies.2−5 In partic© 2017 American Chemical Society

Received: April 25, 2017 Published: July 31, 2017 9574

DOI: 10.1021/acs.inorgchem.7b01032 Inorg. Chem. 2017, 56, 9574−9584

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Inorganic Chemistry capacitors,8 metal−air batteries,9 and fuel cells.10 The investigation of mixed-metal spinels incorporating Fe and Co has been a topic of great interest for a long time.11 In particular, the cobalt-rich spinel FeCo 2O4 has been regarded as prospective electrocatalyst in the splitting of water,12 as well as electrode material for lithium ion batteries.13 More recently, FeCo2O4 nanoflakes on a nickel foam electrode have been successfully employed in symmetrical supercapacitors and in anodes for high-performance rechargeable batteries.14 The structure and properties of iron-rich spinel Fe2CoO4 have also been widely studied.15 The cobalt ferrite Fe2CoO4 has been proven useful in catalytic processes,16 as well as in biomolecular tagging, imaging, and drug delivery.17 Iron−cobalt oxides are typically obtained by conventional high-temperature ceramic methods. To the best of our knowledge, the use of single-source precursors (SSP) for the preparation of Fe−Co spinels is limited to just a single example. Hot injection thermolysis of an oxo-carboxylate complex [Fe2CoO(O2CtBu)6·(HO2CtBu)3] has resulted in the formation of iron-rich spinel Fe2CoO4 nanoparticles18 that were shown to increase the catalytic activity of oxide material due to a larger specific surface area.19 The major goal of our study was to design heterometallic single-source precursors that (i) contain Fe and Co in different specific ratios; (ii) have well-defined discrete molecular structures; (iii) are highly volatile; (iv) exhibit clean, lowtemperature decomposition leading to the target FexCo3−xO4 oxide materials. We employed a combination of mixed-valent and mixed-ligand approaches to design heterometallic diketonate complexes of three different Fe/Co stoichiometries that contain metal ions in oxidation states of +2 and +3. Characterization of heterometallic mixed-valent precursors that contain elements with close atomic numbers and very similar coordination properties/environments represents a great challenge. An extensive search of the Cambridge Structural Database (CSD, 2017, v 5.38) revealed more than 900 structurally characterized heterometallic coordination compounds simultaneously containing Fe and Co metals. Approximately 120 of those are mixed-valent species, almost equally divided between FeIII/CoII and CoIII/FeII combinations. The major group among those is the Fe−Co cyanometallatebased compounds with molecular or extended structures.20 For these compounds, the differences in oxidation and spin states of Fe and Co centers have been revealed by examining the crystal structures, magnetic measurements, as well as IR and Mössbauer spectra. Similar techniques have been applied for analyzing the positions and oxidation states of metal ions in another class of above mixed-valent compounds: Fe−Co oxocarboxylates and their derivatives.21 Herein, we report the synthesis and characterization of a new series of heteroleptic mixed-valent transition-metal β-diketonates with three different Fe/Co ratios: [FeCo(acac)3(hfac)2] (1), [FeCo2(acac)3(hfac)4] (2), and [Fe2Co(acac)3(hfac)4] (3), where acac = acetylacetonate and hfac = hexafluoroacetylacetonate. Preliminary assignment of Fe and Co positions/ oxidation states derived from X-ray structural investigation was unambiguously confirmed by a number of complementary techniques such as single-crystal resonant synchrotron diffraction and neutron diffraction (for location of Fe and Co sites in the molecules), direct analysis in real time (DART) mass spectrometry, theoretical calculations, and magnetic susceptibility measurements (for assignment of oxidation states and spin states of the metal ions). Thermal decomposition of

heterometallic precursors to yield target mixed-transition metal spinel oxides FexCo3−xO4 was also investigated.

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RESULTS AND DISCUSSION 1. Synthesis and Properties of Heterometallic Precursors. A number of synthetic routes were explored (Scheme 1) to isolate mixed-valent heteroleptic iron−cobalt diketonates Scheme 1. Synthetic Routes for Complexes 1−3

[Fe(acac)3][Co(hfac)2] (1), [Co(hfac)2 ][Fe(acac)3][Co(hfac)2] (2), and [Fe(hfac)2][Fe(acac)3][Co(hfac)2] (3) with three different Fe/Co ratios. Heterometallic precursors were originally prepared by stoichiometric reactions in the solid state between FeIII(acac)3 and unsolvated CoII(hfac)2/FeII(hfac)2 in evacuated glass ampules (see Supporting Information, Table S1): Fe(acac)3 + Co(hfac)2 → [Fe(acac)3 ][Co(hfac)2 ] (1) (1)

Fe(acac)3 + 2Co(hfac)2 → [Co(hfac)2 ][Fe(acac)3 ][Co(hfac)2 ] (2)

(2)

Fe(acac)3 + Co(hfac)2 + Fe(hfac)2 → [Fe(hfac)2 ][Fe(acac)3 ][Co(hfac)2 ] (3)

(3)

Heterometallic complexes 1−3 were gathered in the form of red crystals from the cold zones of the ampules with nearly quantitative yields. The product purity was confirmed by the Le Bail fit of X-ray powder diffraction patterns using single-crystal data (Supporting Information, Figures S2−S5 and Table S3). Changing the oxidation state of starting reagents in reaction 1 resulted in an instant ligand exchange between Fe and Co centers giving the product that is identical to 1 (vide infra): Co(acac)3 + Fe(hfac)2 → [Fe(acac)3 ][Co(hfac)2 ] (1) (4)

When the ligands on metal atoms in reaction 1 were switched, the interaction between stoichiometric amounts of Fe(hfac)3 and Co(acac)2 led to yet another trinuclear heterometallic diketonate [Co(hfac)2][Fe(acac)2(hfac)][Co(hfac)2] (2.1). Homometallic Fe(acac)2(hfac) complex was identified as the second product of the reaction that proceeds according to the equation: 2Fe(hfac)3 + 2Co(acac)2 → Fe(acac)2 (hfac) + [Co(hfac)2 ][Fe(acac)2 (hfac)][Co(hfac)2 ]

(5)

Furthermore, heterometallic diketonate 1 can be readily and quantitatively transformed to precursor 2 on addition of Co(hfac)2 and vice versa, on addition of Fe(acac)3: 9575


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Inorganic Chemistry [Fe(acac)3 ][Co(hfac)2 ] + Co(hfac)2 → [Co(hfac)2 ][Fe(acac)3 ][Co(hfac)2 ] (2)

(6)

[Co(hfac)2 ][Fe(acac)3 ][Co(hfac)2 ] + Fe(acac)3 → 2[Fe(acac)3 ][Co(hfac)2 ] (1)

(7)

Similarly, the complex 1 can be readily converted to 3 upon addition of Fe(hfac)2: [Fe(acac)3 ][Co(hfac)2 ] + Fe(hfac)2 → [Fe(hfac)2 ][Fe(acac)3 ][Co(hfac)2 ] (3)

(8) Figure 2. Molecular structure of mixed-valent heterometallic βdiketonate [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2). Atoms are represented by spheres of arbitrary radii. Bridging Co−O interactions are shown as dashed lines.

The procedures represented by eqs 1−8 can be effectively scaled up by running the reactions in hexanes or dichloromethane solutions at room temperature. Heterometallic complexes 1−3 can be isolated in nearly quantitative yields either by cooling the saturated hexane solutions or by sublimation of the solid residue obtained by evaporation of dichloromethane under vacuum (Supporting Information, Table S2). The products do not contain detectable impurities and correspond to those obtained by the solid-state approach. Compounds 1−3 are highly volatile (above 60 °C) and can be quantitatively resublimed at 70 (1), 90 (2), and 85 °C (3). Evaporation starts at 60 (1), 75 (2), and 70 °C (3), and the decomposition under vacuum takes place above 80 (1), 95 (2), and 90 °C (3). Products are readily soluble in noncoordinating (CH2Cl2, CHCl3, C6H14, C6H6) and coordinating (Me2CO, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO)) solvents. Precursors 1−3 are not moisture-sensitive and can be operated in open air for a reasonable period of time. 2. Assignment of Positions and Oxidation States of Iron and Cobalt Metal Ions in the Molecules of Heterometallic Precursors. 2.1. X-ray Structural Analysis. On the basis of single-crystal X-ray structural investigation, the mixed-valent iron−cobalt diketonates were identified as dinuclear [Fe III(acac) 3 ][Co II(hfac) 2] (1) and trinuclear [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2) and [FeII(hfac)2][FeIII(acac)3][CoII(hfac)2] (3) molecules (Figures 1−3). Inductively coupled plasma mass spectrometry (ICP-MS) analysis confirmed the Fe/Co metal ratios as 1:1, 1:2, and 2:1 for precursors 1, 2, and 3, respectively.

Figure 3. Molecular structure of mixed-valent heterometallic βdiketonate [FeII(hfac)2][FeIII(acac)3][CoII(hfac)2] (3). Atoms are represented by spheres of arbitrary radii. Bridging Fe−O and Co−O interactions are shown as dashed lines. Only one orientation of trinuclear molecule is depicted.

Compound [Fe(acac)3][Co(hfac)2] (1) crystallizes in three polymorph modifications (1a−c, Supporting Information, Table S5) upon application of various crystal growth techniques (Supporting Information, page S13). The metal−oxygen distances are essentially the same in all three polytypes (Supporting Information, Table S17) that are different by the packing of bimetallic molecules (Supporting Information, Figure S15). Complex 1 consists of typical edgesharing bioctahedral molecules (Figure 1) containing tris-acac and bis-hfac chelated metal ions. On the one hand, metal− oxygen bond distances for the tris-chelated metal center in 1 (Table 1) show striking similarity to those for the FeIII ion in the structures of [FeIII(acac)3]22 and analogous homometallic [FeIII(acac)3][FeII(hfac)2]23 complex. On the other hand, the M−O distances to chelating acac ligands in 1 are significantly longer than those for the low-spin trivalent cobalt in [Co I II (acac) 3 ] 2 4 and in isomorphous homometallic [CoIII(acac)3][CoII(hfac)2] complex (Table 1) that was specifically obtained for comparison (Supporting Information, Figure S6 and Tables S4 and S8). This tris-acac-chelated atom was identified as trivalent iron. The second metal ion in the dinuclear complex 1 is coordinated by two hfac ligands and has two additional cisbridging interactions with O atoms of iron-chelating acac

Figure 1. Molecular structure of mixed-valent heterometallic βdiketonate [FeIII(acac)3][CoII(hfac)2] (1). Atoms are represented by spheres of arbitrary radii. Bridging Co−O interactions are shown as dashed lines. The full views of the structures drawn with thermal ellipsoids can be found in Supporting Information, Figures S8−S12. 9576


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Inorganic Chemistry Table 1. Average M−O Distancesa (Å) in Heterometallic Diketonate [FeIII(acac)3][CoII(hfac)2] (1) and in Related Compounds complex [FeIII(acac)3] [FeII(hfac)2] [FeIII(acac)3] [CoIII(acac)3] [CoII(hfac)2] [CoIII(acac)3] [FeIII(acac)3] [CoII(hfac)2] (1a) [FeIII(acac)3] [CoII(hfac)2] (1b) [FeIII(acac)3] [CoII(hfac)2] (1c)

ref

Table 2. Average M−O Distancesa (Å) in Trinuclear Heterometallic Diketonates [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2) and [FeII(hfac)2][FeIII(acac)3][CoII(hfac)2] (3) and in Related Compounds

MIII−Ocb MIII−Oc,bc MII−Ocb MII−Obd

23

1.98

22 this work 24 this work this work this work

1.99 1.88

2.04

2.07

2.18

complex II

1.91

2.05

[Co (hfac)2][Fe (acac)3] [CoII(hfac)2] (Λ,Λ,Λ-2) [CoII(hfac)2][FeIII(acac)3] [CoII(hfac)2] (Δ,Δ,Δ-2) [FeII(hfac)2][FeIII(acac)3] [FeII(hfac)2]e [CoII(hfac)2][CoIII(acac)3] [CoII(hfac)2] [FeII(hfac)2][FeIII(acac)3] [CoII(hfac)2] (3)

2.15

1.88 1.98

2.03

2.05

2.15

1.98

2.04

2.05

2.15

1.98

2.03

2.05

2.16

III

MIII−Ocb

MIII−Oc,bc MII−Ocb

1.96

2.03

2.03

1.96

2.03

2.03

1.96

2.03

2.05

1.90

1.89

2.03

1.96

2.03

2.04

MII−Obd 2.16, 2.19 2.16, 2.19 2.19, 2.25 2.16, 2.17 2.17, 2.22

a

For a full list of distances, see Supporting Information, Tables S8 and S10−S12. bChelating. cChelating−bridging. dBridging.

a

groups. While both chelating and bridging M−O distances for this center are slightly (0.02−0.03 Å) shorter than the corresponding bonds for the FeII site in homometallic [FeIII(acac)3][FeII(hfac)2] analogue,23 they are exactly the same as CoII−O distances in a mixed-valent dicobalt counterpart (Table 1). On the basis of the above geometrical considerations as well as on reaction yields and elemental analysis data, this center was identified as divalent cobalt rendering the complex 1 as [FeIII(acac)3][CoII(hfac)2]. It is worth mentioning that the refinement of the crystal structure of 1 as [CoIII(acac)3][FeII(hfac)2] (1′) does not result in increase of the R-value (0.0381 vs 0.0341 for 1) or alternations in M−O bond lengths (0.000−0.001 Å) but leads to a significant change of thermal parameters for the MIII and MII sites (0.0188/0.0128 vs 0.0154/0.0161 Å2, respectively, for 1). It is also important to note that the reaction 4 between equimolar amounts of FeII(hfac)2 and CoIII(acac)3 reagents results in exactly the same product [FeIII(acac)3][CoII(hfac)2] (1). Evidently, the redox process takes place even at low temperatures and is accompanied by a full ligand exchange between two metal centers. This observation is in accord with the standard redox potentials of trivalent iron and cobalt as well as with the tendency of diketonate ligands with electronwithdrawing groups to be located on more electron-rich transition-metal centers. In trinuclear complex 2, the central [M(acac)3] fragment offers four acac oxygens for bridging interactions with two [M(hfac)2] units (Figure 2). While one of the acac ligands remains purely chelating, the other two groups use both of their oxygens for bridging of the end metal units. The M−O bonds within the central tris-chelated unit (Table 2) are essentially identical to the corresponding interations in complex 1 (Table 1) as well as to those in isomorphous homometallic trinuclear iron counterpart.23 At the same time, it is clearly different from the central Co(acac)3 unit in the homometallic trinuclear complex [Co(hfac)2][Co(acac)3][Co(hfac)2] (Supporting Information, Tables S4 and S9). On the basis of above considerations, this metal site was identified as trivalent Fe. Two [M(hfac)2] fragments flanking the central tris-chelated unit are slightly different from those in homometallic iron analogue. They exhibit somewhat shorter both bridging (2.16− 2.19 Å) and chelating (2.03 Å) M−O distances. Concurrently, those are similar to the corresponding bonds in homometallic

trinuclear Co complex. Taking into account the synthetic observations, especially the quantitative formation of 2 by the reaction of 1 with Co(hfac)2, and the results of elemental analysis, these atoms were assigned as divalent Co giving the formulation of heterometallic complex 2 as [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2]. Trinuclear complex 2.1, identified as a main product of the reaction described by the eq 5, conforms to the above formulation as [CoII(hfac)2][FeIII(acac)2(hfac)][CoII(hfac)2]. Molecular structure of heterometallic diketonate 2.1 (Supporting Information, Figure S11 and Table S15) is very similar to that of 2 with an exception of having one hfac ligand on the central iron atom. Notably, this ligand with electron-withdrawing groups remains purely chelating and does not provide its oxygens for bridging to CoII centers on both ends of the molecule. The structure of trinuclear complex 3 (Figure 3) is isomorphous to that of 2. While the central atom can be clearly labeled as FeIII (Table 2), the M−O distances for the metal atoms on both ends of trinuclear molecule are between those for the Fe−O and Co−O in analogous complexes. Taking into account the composition of the complex as Fe/Co = 2:1, we conclude that heterometallic molecules [FeII(hfac)2][FeIII(acac)2(hfac)][CoII(hfac)2] (3) are randomly oriented within the structure, resulting in a mixed Fe/Co occupancy for both divalent metal positions. In all heteroleptic complexes (1, 2, 2.1, and 3), the oxygen atoms of electron-donating acac ligands that chelate FeIII atom provide bridging interaction to Lewis acidic, coordinatively unsaturated CoII/FeII centers coordinated by two electronwithdrawing hfac groups. The combination of two diketonate ligands with such different substituents appears to be crucial for holding together the title heterometallic assemblies. Heterometallic Fe−Co diketonate precursors consist of two (1) or three (2, 2.1, and 3) octahedral units, respectively, in which metals are chelated by three or two (cis) ligands. All such metal units are chiral and may appear as Δ or Λ enantiomers. Dinuclear complex [Fe(acac)3][Co(hfac)2] (1) that crystallizes in centrosymmetric P1̅ space group features a pair (or two, in the case of 1c) of Δ,Δ and Λ,Λ enantiomers in the unit cell (Supporting Information, Figure S13). On the contrary, trinuclear complexes [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2) and [Fe(hfac)2][Fe(acac)3][Co(hfac)2] (3) crystallize in the Sohncke space group P21 and were determined to contain either

For a full list of distances, see Supporting Information, Tables S9 and S13−S16. bChelating. cChelating-bridging. dBridging. eRef 23.

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Inorganic Chemistry Λ,Λ,Λ or Δ,Δ,Δ enantiomers (Δ,Δ,Δ-2, Supporting Information, Figure S14) that, as expected, have the same unit-cell parameters (Supporting Information, Table S6). Interestingly, two pairs of Δ,Δ,Δ and Λ,Λ,Λ enantiomers appear in the unit cell of trinuclear complex [Co(hfac)2][Fe(acac)2(hfac)][Co(hfac)2] (2.1) as it conforms to centrosymmetric C2/c space group (Supporting Information, Table S7). Notably, no diastereomers were found for all heterometallic molecules upon careful analysis of X-ray powder patterns as well as after checking a number of single crystals. 2.2. Resonant Diffraction. The crystal structure of heterometallic complex [Fe(acac)3][Co(hfac)2] (1c) was investigated by X-ray anomalous diffraction studies that in recent years proved to be effective25 for analyzing the precise mixing of metals at the independent atomic sites. This method allows one to recognize the elements with close atomic numbers and can be readily applied for distinction of iron/ cobalt metal sites due to significant differences in the anomalous dispersion factors of the elements at their absorption edges.26 The observed contrast is also aided by the fact that dispersion factors for other elements present in the structure remain essentially constant at the above edges. Thus, the anomalous X-ray scattering method can be effectively employed as a site-specific elemental analysis technique to elucidate the composition of a particular atomic site. With a synchrotron source, a series of wavelength combinations can be readily selected to cover the particular K-edges of the metals discriminating their precise crystallographic positions. A total of seven data sets at different wavelengths (three at or near Fe Kedge, three at or near Co K-edge, and one away from the above absorption edges) were collected. Analysis of anomalous difference Fourier electron density maps provides a useful visualization picture of the metal site occupation pattern. Data sets measured at the metal edges show deep electron density holes for the respective crystallographic metal positions (Figure 4). Additional data sets measured near the two metal edges clearly demonstrate how the electron density at the metal sites is changing in respect to the largest absorption observed at each particular edge (Supporting Information, Figures S16 and S17). The structure of the heterobimetallic complex [Fe(acac)3][Co(hfac)2] (1c) measured at λ = 0.41328 Å is essentially identical (Supporting Information, Tables S19−S21) to that determined by the standard in-house X-ray diffraction (λ = 0.710 73 Å). While the electron density maps are visually consistent with fully occupied metal positions, the least-squares refinement of the anomalous diffraction data sets conducted with GSAS-II crystallographic software package further proved the absence of metal mixing over the atomic positions. Simultaneous data refinement of a common crystallographic model against several data sets collected at different wavelengths near-edge of the anomalous scatterers provides a quantitative estimation of occupancies along with the relative errors associated with those. For the two crystallographically independent molecules in the structure of 1c, the least-squares refinement of the bis-chelated Co/Fe sites converged at 0.993(5)/0.007(5) and 0.998(5)/0.002(5) and for the trischelated Fe/Co positions at 0.972(5)/0.028(5) and 0.988(5)/ 0.012(5) values. These results are in excellent agreement with the initial assignment of iron and cobalt positions in heterometallic complex 1 made with the standard in-house Xray diffraction data. 2.3. Single-Crystal Neutron Diffraction. The location of metal atoms in the [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2)

Figure 4. Difference Fourier electron density map for the data measured away from absorption edges at 30 keV (top) and anomalous difference Fourier electron density maps at the Co (middle) and Fe (bottom) K-edges for the [Fe(acac)3][Co(hfac)2] molecule in the structure of 1c (grid spacing is 0.05 Å; plane center: 1.057 × a, 0.367 × b, 0.137 × c).

molecule was confirmed by neutron diffraction experiment using the TOPAZ single-crystal neutron diffractometer at the ORNL Spallation Neutron Source. The TOPAZ instrument utilizes the neutron wavelength-resolved Laue technique for three-dimensional reciprocal space mapping and is capable of measuring a structure at the subatomic resolution. Neutron diffraction provides an opportunity to differentiate between neighboring elements in the periodic table and to correctly assign their atomic positions in the crystal structure. Furthermore, it allows the refinement of element ratios in the case of atoms mutually occupying the same local site. Such discrimination is possible due to the differences in elastic scattering amplitudes (scattering lengths) as neutrons scatter from the nuclei of the atoms, which do not change in a smooth progression like electron density of the elements. In heterometallic molecule 2, iron has significantly greater bound coherent scattering length (9.45) than cobalt (2.49) providing a sufficient contrast between two metals (Fe/Co ratio of scattering lengths is 3.8). The neutron diffraction data collected on a single crystal of 2 with dimensions of 1.10 × 1.00 × 0.90 mm3 unequivocally confirmed the structure assignment as [Co(hfac)2][Fe(acac)3][Co(hfac)2] (Figure 5). Refinement of site occupancies revealed the absence of metal mixing in all atomic sites that are fully occupied by the corresponding metals in a full agreement with position assignments made with X-ray diffraction data. It is important to note that the absolute 9578


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Figure 5. Molecular structure of heterometallic diketonate [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2) refined using neutron singlecrystal diffraction data. All atoms are represented by thermal ellipsoids at the 40% probability level. Only one orientation of disordered CH3 and CF3 groups is shown. Only metal and oxygen atoms are labeled. Bridging Co−O interactions to oxygen atoms of acac ligands are drawn by dashed lines.

Figure 6. Fragments of the positive-ion DART mass spectra of solid homo- and heterometallic diketonates showing characteristic MIII and MII (M = Fe, Co) mononuclear ions.

with electron-withdrawing substituents. Negative peaks [CoII(hfac)2+O2]−, [CoII(hfac)2(acac)]−, and [CoII(hfac)3]− can be found in the mass spectra of 1, 2, and homometallic Co analogues (Supporting Information, Tables S32 and S33), while the spectrum of 3 contains the corresponding FeII-based anions (Supporting Information, Table S36). 2.5. Theoretical Investigation. To provide further support for the assignment of metal positions and oxidation states, a theoretical study of possible “valent isomers” [FeIII(acac)3][CoII(hfac)2] (1) and [CoIII(acac)3][FeII(hfac)2] (1′) was performed at the B2PLYP-D328/TZVP level of theory.29 Relativistic effects were accounted explicitly through the ZORA approximation.30 Previously, we found that this approach results in excellent agreement with experimental data for heterometallic diketonate systems.23 Importantly, for both isomers 1 and 1′ all possible combinations of high- and low-spin electronic configurations of Fe and Co metal centers were tested (Table 3).

configuration of the [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2) molecule cannot be unambiguously determined using neutron diffraction experiment. Therefore, the crystal investigated on TOPAZ diffractometer was recollected on a singlecrystal X-ray instrument to confirm it to be the Δ,Δ,Δ enantiomer (Supporting Information, Tables S22−S24). 2.4. Mass Spectrometry. DART mass spectrometry was recently shown23,27 to work well in detecting complex heterometallic ions upon analyzing of volatile precursors. In the positive-mode spectrum of solid [Fe(acac)3][Co(hfac)2] (1), the peak corresponding to [M−L]+ (L = hfac; meas/calcd = 618.9856/618.9901) ion appears with characteristic isotope distribution pattern (Supporting Information, Figure S23 and Tables S28 and S29) confirming the presence of heterometallic species in the gas phase. While the application of mass spectrometry for analysis of heterometallic diketonates has notoriously been limited by severe fragmentation, these same splinters appeared to be the most informative in analysis of oxidation states of iron and cobalt atoms in the structures of heterometallic precursors 1−3 (Supporting Information, Figures S23−S25 and Tables S28, S30, and S31). Positive mode mass spectra of solid [FeIII(acac)3][CoII(hfac)2] (1) and [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2) feature several homonuclear ions (Figure 6) that correspond to trivalent iron ([Fe I I I (acac) 2 ] + , [FeIII(acac)3+H]+, and [FeIII(acac)2(hfac)+H]+) and divalent cobalt ([CoII(acac)2+H]+ and [CoII(acac)(hfac)+H]+), whereas any FeII and CoIII-based peaks are absent. In the spectrum of [FeII(hfac)2][FeIII(acac)3][CoII(hfac)2] (3), in addition to the above fragments, the divalent iron peaks analogous to the corresponding cobalt species ([Fe II (acac) 2 +H] + and [FeII(acac)(hfac)+H]+) are clearly seen. At the same time, the CoIII peaks instantly appear in the spectra of homometallic [CoIII(acac)3][CoII(hfac)2] and [CoII(hfac)2][CoIII(acac)3][CoII(hfac)2] analogues of 1−3 (Supporting Information, Figures S21 and S22 and Tables S25 and S27). While the positive mode spectra feature both MIII and MII cations with predominantly acac ligands having electrondonating groups, the negative mode mass spectra (Supporting Information, Figures S26−S31 and Tables S32−S36) reveal anionic MII-based fragments mostly coordinated by hfac ligands

Table 3. Relative Energies (Erel, kcal/mol) Calculated for Different Spin States of the Metal Ions in [FeIII(acac)3][CoII(hfac)2] (1) and [CoIII(acac)3][FeII(hfac)2] (1′) Isomers (B2PLYP-D3/ TZVP/ZORA) molecule III

II

[Fe (acac)3][Co (hfac)2] [FeIII(acac)3][CoII(hfac)2] [FeIII(acac)3][CoII(hfac)2] [FeIII(acac)3][CoII(hfac)2] [CoIII(acac)3][FeII(hfac)2] [CoIII(acac)3][FeII(hfac)2] [CoIII(acac)3][FeII(hfac)2]

(1-LS/LS) (1-LS/HS) (1-HS/LS) (1-HS/HS) (1′-LS/LS) (1′-LS/HS) (1′-HS/HS)

SFe

SCo

Erel

1/2 1/2 5/2 5/2 0 4/2 4/2

1/2 3/2 1/2 3/2 0 0 4/2

+49.67 +120.12 +29.06 0.00 +34.98 +6.48 +39.88

All systems under consideration were found to be local minima on the corresponding potential energy surfaces. The [FeIII(acac)3][CoII(hfac)2] (1-HS/HS) system, which has both metal centers in their high-spin (HS) states (SFe = 5/2 and SCo = 3/2), was found to be the lowest minimum. The high-spin configurations of metals were further confirmed by analysis of singly occupied natural orbitals (Figure 7), which unambiguously indicate the localization of 9579


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magnetic anisotropy of the Co(II) site and the magnetic interaction between high-spin FeIII and CoII magnetic centers, the experimental magnetic properties (χT vs T and M vs H data shown in Figure 8) of 1 were fitted with the theoretical susceptibilty calculated31 from the following Hamiltonian: ̂ ·SFe ̂ ) + DCoSẑ ,Co 2 + μ H⃗ [g Sẑ ,Co Ĥ = −2J(SCo B z ,Co ̂ ] + gxy ,Co(Sx̂ ,Co + Sŷ ,Co) + gFeSFe

As shown in Figure 8, the theory/experiment agreement is remarkably good with the following set of parameters: gCo,xy = 1.06, gCo,z = 3.84, gFe = 2.00, DCo/kB = 20.0 K, and J/kB = −0.78 K. The magnetic coupling between the S = 5/2 iron spin and the strongly anisotropic cobalt magnetic center through the (μO)2 bridges is thus weak and of anti-ferromagnetic nature. In the case of the trinuclear complex [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2), the χT product at room temperature of 10.1 cm3·K/mol also supports the presence of high-spin metal ions (Figure 9) with gCo ≈ 2.4 and gFe ≈ 2

Figure 7. Singly occupied natural orbitals for the molecule [FeIII(acac)3][CoII(hfac)2] (1-HS/HS), localized on FeIII-center (a) and on CoII-center (b) (B2PLYP-D3/TZVP/ZORA).

five unpaired electrons on the FeIII-center and three electrons on the CoII-center along with characteristic spin density distribution (Supporting Information, Figure S33). Other electronic configurations of 1, in which one or both metal centers are in low-spin state, were found to be dramatically (>29 kcal/mol) higher in energy. This observation is in excellent agreement with findings from other methods, especially with the results of magnetic measurements (vide infra). Alternative assignment of metal centers, namely, [CoIII(acac)3][FeII(hfac)2] (1′-LS/HS (LS = low-spin); see Supporting Information, Figures S35−S37 for details of electronic structure), was calculated to be at least 6.48 kcal/mol higher than the 1-HS/HS model, which implies the formation of such a “valent isomer” as thermodynamically unfavorable in accord with experimental observations. 2.6. Magnetic Properties. The results of magnetic susceptibility measurements for heterometallic compounds [Fe(acac)3][Co(hfac)2] (1) and [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2) fully support the assignment of the iron and cobalt oxidation/spin states made based on the structural data and theoretical calculations. At room temperature, the χT products for dinuclear complexes [CoIII(acac)3][CoII(hfac)2] and [FeIII(acac)3][CoII(hfac)2] (1) (2.9 and 6.8 cm3·K/mol, respectively) are in good agreement with the presence of high-spin magnetic centers (Figure 8 and Supporting Information, Figure S38) with gCo ≈ 2.4−2.5 and gFe ≈ 2 values. Considering the intrinsic

Figure 9. Temperature dependence of the χT product at 0.1 T for [CoII(hfac)2][FeIII(acac)3][CoII(hfac)2] (2) (χ is defined as magnetic susceptibility equal to M/H per mole of 2). (inset) Field dependence of the magnetization below 8 K for 2. The solid lines are the best fit of the experimental data with magnetic model described in the text.

values. Using the approach adopted for complex 1, the experimental χT versus T and M versus H data (shown in Figure 9) were simultaneously fitted with the theoretical susceptibilty calculated32 from the following Hamiltonian: ̂ · ∑ SCo( ̂ i)) + DCo Ĥ = −2J(SFe

∑

i = 1,2

i = 1,2

̂ + + μB H⃗ [gFeSFe

∑

Sẑ ,Co(i)

2

gz ,Co(i)Sẑ ,Co(i) + gxy ,Co(i)

i = 1,2

(Sx̂ ,Co(i) + Sŷ ,Co(i))]

As in the case of molecule 1, the magnetic model is able to reproduce the experimental data remarkably well with the following set of parameters: gCo,xy = 0.85, gCo,z = 3.99, gFe = 2.19, DCo/kB = 30.6 K, and J/kB = −1.0 K. Similarly to the complex 1, the couplings between high-spin iron and cobalt magnetic centers through the (μ-O)2 bridges are weak and antiferromagnetic. 3. Thermal Decomposition of Heterometallic Precursors. Thermal decomposition of mixed-transition metal diketonate precursors was demonstrated before23 to result in M2O3 type of oxide for the Fe−Mn precursor and in MO-type oxide for the Fe−Ni one. Thermolysis of mixed-valent iron− cobalt complexes was investigated. According to our observa-

Figure 8. Temperature dependence of the χT product at 0.1 T for [FeIII(acac)3][CoII(hfac)2] (1) (χ is defined as magnetic susceptibility equal to M/H per mole of 1). (inset) Field dependence of the magnetization below 8 K for 1. The solid lines are the best fit of the experimental data with magnetic model described in the text. 9580


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of heterotrimetallic precursors of the type [Co(hfac)2][Fe(acac)3][M(hfac)2], where M is divalent metal.

tions, degradation of compounds 1−3 begins at relatively low temperatures of ∼100 °C. Thermal decomposition of heterometallic precursors 1−3 in air below 300 °C yields amorphous phases. Increasing decomposition temperature to 400 °C results in crystalline residues that were identified by Xray powder diffraction as M3O4 spinel-type oxides (Figure 10).

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

General Procedures. All of the manipulations were performed in a dry, oxygen-free argon atmosphere by employing standard Schlenk and glovebox techniques. The attenuated total reflection (ATR) spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer. Mass spectra were acquired using a DART-SVP ion source (IonSense, Saugus, MA) coupled to a JEOL AccuTOF time-offlight mass spectrometer (JEOL USA, Peabody, MA) in positive and negative ion modes. Metal ratios were determined using ICP analysis performed on a PerkinElmer Optima 3300 DV ICP instrument. Thermal decomposition of heterometallic precursors was studied in air at ambient pressure. The solid sample (ca. 40 mg) was placed into a 20 mL Coors high-alumina crucible (Aldrich) and heated at a rate of ca. 35 °C/min in a muffle furnace (Lindberg Blue M). The decomposition residues were analyzed by X-ray powder diffraction, which were collected on a Bruker D8 Advance diffractometer (Cu Kα radiation, focusing Göbel Mirror, LynxEye one-dimensional detector, step of 0.02° 2θ, 20 °C). Magnetic susceptibility measurements were performed on a Quantum Design SQUID magnetometer MPMS-XL housed at the Centre de Recherche Paul Pascal at temperatures between 1.8 and 300 K and direct-current (dc) magnetic fields ranging from −7 to +7 T. Single-crystal X-ray diffraction data for all compounds were measured on a Bruker D8 VENTURE with PHOTON 100 CMOS shutterless mode detector system equipped with a Mo-target X-ray tube (λ = 0.710 73 Å). Resonant diffraction experiments were executed using Bruker D8 diffractometer, integrated with an APEX-II CCD detector modified for synchrotron use at the Chem-MatCARS 15-ID-B beamline at the Advanced Photon Source (Argonne National Laboratory). Diffraction data were collected at seven different energies with 0.5 s frames using ϕ scans while manually attenuating the beam to minimize overages of individual pixels. Highresolution single-crystal neutron diffraction measurements were performed with TOPAZ neutron time-of-flight single-crystal Laue diffractometer located at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. More details on characterization techniques can be found in Supporting Information. Synthesis. Homometallic [Co(acac)3][Co(hfac)2] and [Co(hfac)2][Co(acac)3][Co(hfac)2] and heterometallic [Fe(acac)3][Co(hfac)2] (1), [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2), [Co(hfac)2][Fe(acac)2(hfac)][Co(hfac)2] (2.1), and [Fe(hfac)2][Fe(acac)3][Co(hfac)2] (3) diketonates were obtained using two synthetic techniques. For the solid-state/gas phase method, a mixture of two powdered reagents was sealed in an evacuated glass ampule and placed in an electric furnace having a temperature gradient along the length of the tube. The ampule was kept at constant temperature for several days to allow crystals of the products to be deposited in the cold section of the container, where the temperature was set ∼5 °C lower. The yield was calculated in each case based on the amount of crystals collected. For a solution approach, a mixture of two powdered reagents was loaded into a 50 mL Schlenk flask with 15 mL of oxygen-free solvent and stirred under argon flow for 2−3 h. The reaction mixture was concentrated by evaporating approximately half of the solvent volume under reduced pressure, sealed in glass tube, and placed in a freezer. Alternatively, solution in the reaction flask was completely removed under vacuum, and the resulting solid was dried at room temperature for 24 h, followed by sublimation in a sealed, evacuated ampule. The experimental conditions for different synthetic methods and starting reagents are summarized in the Supporting Information, Tables S1 and S2.

Figure 10. X-ray powder patterns of residues obtained by thermal decomposition of heterometallic precursors [Co(hfac)2][Fe(acac)3][Co(hfac)2] (2, red), [Fe(acac)3][Co(hfac)2] (1, blue), and [Fe(hfac)2][Fe(acac)3][Co(hfac)2] (3, black) in air. Theoretical peak positions for Fe3O434 (pink) and Co3O435 (green) spinels are shown at the bottom.

Raising decomposition temperature and/or annealing time leads to improved crystallinity of the target spinel materials (Supporting Information, Figure S39). LeBail fit of powder diffraction patterns (Supporting Information, Figures S40− S42) confirms the expected increase of the unit-cell size on going from the Co-rich residue obtained by thermolysis of precursor 2 to more Fe-rich samples derived from 1 and 3 (Supporting Information, Table S45). The former oxide residue corresponds well to the reported11c FeCo2O4 spinel, while the latter residues fell closer to that of Fe2CoO433 and are best described in the cubic space group Fd3̅m.

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CONCLUSIONS Mixed-transition metal volatile precursors with three different Fe/Co stoichiometries have been designed using a combination of mixed-valent and mixed-ligand synthetic approaches. A number of characterization techniques can be effectively utilized for analysis of heterometallic complexes containing transition metals with close atomic numbers and similar coordination properties, in the case when X-ray crystallography does not provide a definite answer on positions and oxidation states of the metal ions. Single-crystal neutron diffraction (differences in elastic scattering amplitudes) and synchrotron resonant diffraction (differences in the anomalous dispersion factors at absorption edges) techniques are ideal for unambiguous metal position assignment and refinement of site occupancy factors in the case of mutual occupancy. DART mass spectrometry (analysis of mononuclear ions resulting from fragmentation) and magnetic susceptibility measurements (analysis of even weak anti-ferromagnetic coupling between metal centers) can be employed for assignment of oxidation/ spin states. Theoretical calculations at the high levels of theory allow one to convincingly predict the correct “valent isomer”. The above approach can be extended to design and characterization of other precursor systems that contain two or even more different transition metals. The synthetic technique employed in this work to obtain the complexes 2 and 3 by an addition of homometallic reagents (Co(hfac)2 and Fe(hfac)2, respectively) to heterometallic complex [Fe(acac)3][Co(hfac)2] (1) can be successfully utilized for the preparation

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01032. 9581


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Full synthetic and characterization details; MS and IR spectra; X-ray powder diffraction patterns; full details on interatomic distances and angles in the structures of homo- and heterometallic diketonates; additional information on theoretical calculations; phase analysis of thermal decomposition products (PDF) Accession Codes

CCDC 1539930−1539940 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: afi[email protected]. (A.S.F.) *E-mail: [email protected]. (E.V.D.) ORCID

Evgeny V. Dikarev: 0000-0001-8979-7914 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation (CHE-1152441 and CHE-1429329 (DART mass spectrometer)) and the ACS-PRF (52674-ND10) is gratefully acknowledged (E.V.D.). R.C. and J.-L.L. thank the CNRS, the Univ. of Bordeaux, the Conseil Regional d’Aquitaine, and the GdR MCM-2. A.Y.R. is thankful for financial support from Illinois Institute of Technology (start-up funds) and partial support from 381688-FSU/Chemring/DOD-DOTC. Neutron singlecrystal diffraction measurements were performed at the Spallation Neutron Source, a Department of Energy (DOE) Office of Science User Facility operated by the Oak Ridge National Laboratory, under Contract No. DE-AC0500OR22725 with UT-Battelle, LLC. Resonant single-crystal diffraction measurements were performed at the Advanced Photon Source, Sector 15. ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under Grant No. NSF/CHE-1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

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REFERENCES

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Inorganic Chemistry

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Position Assignment and Oxidation State Recognition of Fe and Co

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