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Article

Synthesis and Structures of the Capped and Sandwiched Cobalt Heptamolybdate Polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O and Its Extended Structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}

by
Matthieu R. Spriet
1,2,
Matthew J. Polinski
3,
Brandon Q. Mercado
4 and
Eric M. Villa
1,*
1
Department of Chemistry and Biochemistry, Creighton University, 2500 California Plaza, Omaha, NE 68178, USA
2
Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, 42nd and Emile, Omaha, NE 68198, USA
3
Department of Chemistry and Biochemistry, Bloomsburg University of Pennsylvania, 400 E. 2nd Str., Bloomsburg, PA 17815, USA
4
Department of Chemistry, Yale University, New Haven, CT 06520, USA
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(9), 1201; https://doi.org/10.3390/cryst12091201
Submission received: 11 August 2022 / Revised: 20 August 2022 / Accepted: 21 August 2022 / Published: 26 August 2022
(This article belongs to the Section Inorganic Crystalline Materials)
Browse Figures
Versions Notes

Abstract

:
Two polyoxometalate-based compounds containing the heptamolybdate anion and cobalt(II) have been synthesized, structurally characterized via single-crystal X-ray diffraction and spectroscopically analyzed with solid-state UV–Vis–NIR. The first crystal structure presented is a capped- and sandwich-style polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24] [Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O (Co3Mo14-POM). This orange product crystallizes in the P-1 space group and has an interesting structure for polyoxometalates (POMs). Here, the cobalt(II) groups are functioning both as a bridge between two heptamolybdate units and also capping units at the top and the bottom of the polyoxometalate. The second structure, another product within the same reaction, is constructed on a similar bonding motif but now forms an extended structure: (NH4)6{[Co(H2O)2]3[Mo7O24]2} (Co3Mo14-ES). This compound also crystallizes in the space group P-1 but is easily separated due to its much darker red-violet color. These structures present themselves as a noteworthy compounds within POM-based crystal structures. Herein, the syntheses, crystal structures and characterization of these compounds are presented.

1. Introduction

Polyoxometalate ions (POMs) are distinct (usually anionic) clusters made up of oxygen and predominately early row transition metals with a wide variety of sizes and structure types [1,2,3,4,5,6,7,8,9,10]. Even with the first POMs being synthesized in the early 1800s, this constantly expanding field sees continuing interest due to their ever increasing number of valuable applications [11,12,13,14,15]. These applications include, but are not limited to, antiviral/antitumor activity, acid catalysis, water oxidation catalysis, MRI contrast agents, conducting polymers, luminescent and magnetic materials [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. This prevalent use of POMs throughout many different areas of chemistry is owed to the captivating structural diversity that they display.
In an effort to expand and enhance these applications, much work has been put into developing novel synthetic methods for the creation of both new classes and new types of POMs. As has been recently demonstrated, unique and interesting compounds can be formed when carefully controlling the synthesis with heptamolybdate [31,32,33]. Of particular interest are types of POMs that contain an extra (sometimes hydrated) metal group. This additional group can sometimes serve as a capping group [34,35,36,37,38,39,40,41] be present in the middle of the POM creating sandwich [42,43,44] or, in fact, in both positions [45]. The sandwich-style POMs have been of interest owing to their ability to perform water oxidation catalysis [23,46,47]. These compounds can prove difficult to isolate, as the POM may simply rearrange to form a POM where the metal is enclosed within the POM (structure type examples include the Anderson, Keggin, Wells-Dawson and others). In an effort to explore these different structure types, we have recently synthesized and structurally characterized a type of capped and sandwiched polyoxometalate ion (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O along with its corresponding condensed, extended structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}.

2. Materials and Methods

2.1. Synthesis and Crystallization

Note: The ammonium molybdate tetrahydrate [(NH4)6Mo7O24 · 4 H2O] (Fisher Scientific, Hampton, NH, USA, Certified ACS), cobalt acetate tetrahydrate (Co(CH3CO2)2 · 4 H2O Fisher Scientific) and 190 proof ethanol (Decon Lab., King of Prussia, PA, USA) were all used as received. The concentrated hydrochloric acid (HCl; Fisher Scientific, Certified ACS) was diluted to 1.00 M solution.
Both of the compounds presented here were isolated in the same reaction mixture. A 50 mL stock solution containing a 6:1 molar ratio of Mo6+:Co2+ was initially prepared by dissolving 2.648 g of (NH4)6Mo7O24 (2.143 mmols (NH4)6Mo7O24 or 0.0486 M Mo7O246− or 0.300 M Mo6+) and 0.6225 g of Co(CH3CO2)2 · 4 H2O (2.499 mmols of Co2+ or 0.0500 M Co2+) were dissolved into 50.0 mL of water, which yielded a deep red solution with a pH of approximately 5.4. A 2.00 mL aliquot of this stock solution was then placed into a PTFE liner. Next 50.0 μL of the 1.00 M HCl was added to the liner—no visual changes were observed to the solution. The PTFE autoclave liner was sealed into a stainless-steel autoclave and placed into the box furnace. The furnace’s temperature was increased to 180 °C over a 30 min period, held at 180 °C for 24 h under autogenous pressure, and then cooled at 5 °C/h. There was little to no precipitate observed in the bottom of the reaction vessel; therefore, the subsequent solution was transferred to a small vial and layered with ~1.0 mL of ethanol (the lid of the vial was left ajar to allow for evaporation). This solution was allowed to sit undisturbed for 5 days, after which the solid product was isolated by transferring it to a Petri dish and rinsed with ethanol to ensure proper distribution of the crystals present. Two types of crystals were present: the orange crystals of the molecular (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O (Co3Mo14-POM) and deep red crystals of its condensed, extended structure (NH4)6{[Co(H2O)2]3[Mo7O24]2} (Co3Mo14-ES). These crystals had to be manually separated for crystallographic and UV-vis studies.
This synthesis can be repeated with between 25 μL and 150 μL of 1.00 M HCl, but it gave the best yields with 50 μL and, to a lesser extent, 100 μL of 1.00 M HCl. This reaction can also be repeated without heating; however, the yield of crystals is much smaller, and a recrystallization of ammonium molybdate is a common contaminant. In all, we were never able to isolate pure (Co3Mo14-POM) or pure (Co3Mo14-ES), but we always obtained a mixture of the two. Co3Mo14-POM was always found in a smaller amount but could isolated in greater amounts by using slightly lower amounts of ethanol; conversely, more Co3Mo14-ES could be isolated by using greater amounts of ethanol.

2.2. Crystallographic Studies

Crystals were then mounted on MiTeGen Microloop with non-drying immersion oil and then optically aligned on the Rigaku SCX-Mini diffractometer (MoKα, λ = 0.71073 Å) using a digital camera. Initial matrix images were collected to determine the unit cell, validity and proper exposure time. Three hemispheres (where φ = 0.0, 120.0 and 240.0) of data were collected with each consisting of 180 images each with 1.00° widths and a 1.00° step. The structure was initially solved using SHELXT intrinsic phasing and refined using SHELXL [48,49]. Olex2 was used as a graphical interface [50]. Images of the above compound were made using CrystalMaker® for Windows, version 10.5.7 [51].
Refinement of Co3Mo14-POM proceeded without any incidents for the heavy atom positions and there was no need to model any disorder or twinning. There were initial difficulties in distinguishing the number of solvent/cation positions in the model. As such, the program SQUEEZE was used to generate a mask that was used to estimate the number of solvent/cation positions [52] which yielded ~12 sites per cell or ~6 per asymmetric unit. It was these primary positions that were then refined as either ammonium cations or solvent waters (Note: the final structure does not use SQUEEZE, but it was used to properly assign the peaks). The hydrogen atoms on the bound and free waters were calculated, but the hydrogen atoms on the ammonium cations were found in difference map. Here, the latter was refined using a distance restraint of 0.90 (4) Å for the N-H bond length. The 1,3 distances of the H···H interactions in ammonia were restrained to be similar within a standard uncertainty of 0.04 Å. All hydrogen atoms were refined as riding on their parent atoms with Uiso(H) = 1.5 Ueq(O,N). The maximum electron density peak of 1.53 is 0.72 Å from H2WB, which constitutes more disorder in the solvent molecules in the structure; a second peak of 1.15 is 1.35 Å from O5, which leads to nothing chemically reasonable. All other maximum peaks are at or below 1.00 e/Å3. See supplementary material for all atomic positions, bond lengths and bond angles.
Refinement of Co3Mo14-ES proceeded without any incidents and without any need for modelling disorder, twinning or restraints except for the hydrogen atoms. The hydrogen atoms on the bound waters were calculated, but the hydrogen atoms on the ammonium cations were found in difference map. All were refined as riding on their parent atoms with Uiso(H) = 1.5 Ueq(O,N). The same distances and similarity commands were used in this refinement as listed above. The highest remaining peak in the difference Fourier map is 4.04 electrons and is 1.47 Å from atom H3WB and 2.083 Å from O10, which is nothing reasonable. All other maximum peaks are around or under 1.000. Crystallographic information for both obtained phases is summarized in Table 1. Atomic coordinates and additional structural information are provided in the supplementary material (CIF’s).

2.3. UV–Vis–NIR Spectroscopy

Solid state UV–Vis–NIR data were collected on single crystals of Co3Mo14-POM and Co3Mo14-ES using a Craic Technologies 508 PV Microscope Spectrophotometer equipped with a 1.3 MP high sensitivity digital imaging system. Samples were mounted onto glass slides and data collected from 375–850 nm. Exposure time and sampling rate were auto-optimized using Craic’s Lambdafire Control and Analysis software (Version 1.2.69; CRAIC Technologies, Inc., San Dimas, CA, USA).

2.4. Scanning Electron Microscopy and Energy Dispersive X-ray

Scanning electron microscopy (SEM) images were acquired with a Thermo Fisher Scientific Phenom Pharos FEG-SEM, equipped with a back-scattered electron detector at an accelerating voltage of 15 kV. Single-point energy-dispersive X-ray spectra were acquired in situ using the same electron source and accelerating voltage. Sample spectra for both compounds presented here can be found in the supplementary material with both measured and theoretical weight concentrations.

3. Results and Discussion

3.1. Crystal Structure of Co3Mo14-POM

The first compound is a capped and sandwiched polyoxometalate ion species. It is based on two heptamolybdate building units with two different cobalt(II) groups both bridging and capping these units forming the compound (NH4)6{[Co(H2O)5] [Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O (Co3Mo14-POM). The orangish-brown blocks crystallized in the triclinic space group P-1 and were distinct from the extended structure phase, which were reddish-violet prisms. The Co3Mo14-POM consists of two heptamolybdate units linked together by a Co(H2O)2 unit via four μ2-bridging oxygen atoms—two from each heptamolybdate anion (Figure 1a). Each of heptamolybdate units is subsequently capped by a Co(H2O)5 unit, again, through a μ2-bridging oxygen atom. The asymmetric unit is shown in a thermal ellipsoid plot in Figure 1b.
In terms of the bonding in Co3Mo14-POM, the one crystallographically unique heptamolybdate unit is largely the same as previously published structures of ammonium salts of heptamolybdate by Evans, sometimes also called paramolybdate [54,55]. Likewise, it has some similarities to other cobalt–heptamolybdate compounds [37,39,41,56] and the other capped heptamolybdate compounds listed above; however, this species contains both a sandwich-style species with additional inorganic capping groups. The Mo-O bond lengths in Co3Mo14-POM can easily be compared to heptamolybdate anion using the Evans’ nomenclature for the oxygen positions (the crystallographic oxygen positions will be listed in parentheses behind each label as they are shown in Figure 1b). The terminal oxygen atoms Oa (O8, O11, O20, O23), Ob (O7, O12, O19, O24), Oc (O1, O15) and Od (O2, O16) have average bond lengths of 1.736(3) Å, 1.717(3) Å, 1.710(4) Å and 1.716(4) Å, respectively. The μ2-bridging oxygen of Oe (O3, O4, O13, O18), Of (O5, O17) and Og (O9, O22) have average bond lengths of 1.961(3) Å, 2.121(3) Å and 1.930(3) Å, respectively. Lastly, the μ3-bridging oxygen positions Oh (O10, O21) have an average bond length of 2.132(3) Å and the μ4-bridging oxygen positions Oi (O6, O14) have an average bond length of 2.184(3) Å. All of these average bond distances are within ±0.03 Å of the published ammonium heptamolybdate structure [55]. Moreover, there is no marked difference in the bond angles in the heptamolybdate unit, and bond valence sum calculations for these molybdenum positions arrive unsurprisingly at nearly 6.
There are two crystallographically unique cobalt positions in the structure. The first, Co1, acts as a capping group on the heptamolybdate unit. The two are linked through a μ2-bridging oxygen yielding a corner-sharing interaction between the cobalt and molybdenum polyhedra. The bond distance between the cobalt and this bridging oxygen (Co1-O24) is fairly long at 2.150(3) Å, which explains why not much deviation was seen in corresponding Mo3-O24 distance when compared to the bare anion. The rest of the positions in this slightly distorted octahedron are occupied by bound waters (O2W–O6W) with an average Co-O distance of 2.087(4) Å. The second cobalt position, Co2, is the linking unit between the two heptamolybdates and has corner-sharing interactions with four molybdenum polyhedra (two from each heptamolybdate). Here, the μ2-bridging oxygen of O8 and O11 have Co-O bond distances of 2.118(3) Å and 2.075(3) Å, respectively. The bound water O1W has a Co-O distance of 2.038(4) Å and completes the distorted octahedron of Co2. A complete listing of bond distances and angles can be found in the Supplementary Materials.
Lastly, an intricate network of hydrogen bonding interactions holds the anionic Co3Mo14-POM into a three-dimensional network (Table S2). Hydrogen bonds between the solvent waters and/or ammonium cations to the POM range from short interactions, such as 1.85 Å (O18···H3A-N3) and 1.90 Å (O18···H3WA-O3W), to longer interactions, such as 2.61 Å (O1···H2WB-O2W). According to bond valence sum calculations for the oxygen atoms in the structure, most are slightly under-coordinated, which supports the nearly 30 independent interactions between the Co3Mo14-POM and the hydrogen atoms from solvent waters and/or ammonium cations. There are also several interactions amongst the solvent waters and ammonium cations themselves that further contribute to the intricacy of the overall structure.

3.2. Structural Comparison of Cobalt-Capped Heptamolybdates

A comparison can be drawn to other molecular cobalt-capped heptamolybdate species (note: the structure and bonding of the heptamolybdate ions present in the complexes discussed below do not deviate much from the isolated, parent ion and will therefore not be examined in detail). The first compound is a Co(H2O)5Mo7O244− species, which is charge balanced by four 2-aminopyridinium cations (Hapy+) and synthesized hydrothermally [39]. Here, the cobalt(II) ion is bound to the heptamolybdate ion via a μ2-bridging oxygen atom, which can be referenced as the terminal Oc position in the Evan’s model described above; the other five positions of the distorted octahedron are occupied by bound water (Table 2 gives the average and range of bond lengths for the cobalt(II) in the complex). The second compound is another Co(H2O)5Mo7O244− species but is now charge balanced by sodium and guanidinium cations and was synthesized through an aqueous solution reflux [37]. While the coordination is the same, the location of the cobalt has moved from the “side” of the heptamolybdate ion to Oa located on the “bottom” of the heptamolybdate ion. Lastly, a third molecular species of {Co(3-ampy)(H2O)4}Mo7O24]4−, where 3-ampy is 3-aminopyridine, is charge balanced by four 3-aminopyridinium cations and was synthesized via a room temperature, aqueous reaction [41]. The bonding here is nearly the same as in the first compound discussed above, but now the cobalt(II) contains four bound waters and one bound 3-ampy. Table 2 compiles the data above with that of the Co3Mo14-POM presented here. By comparison the bridging oxygen for Co1 observed in Co3Mo14-POM is significantly longer than the other compounds, while its Co-O distance for the bound waters is very much in line with previous observations. Conversely, the Co-O distance for the bound waters attached to Co2 in Co3Mo14-POM are much shorter than in the previous species, which may be due to it having four bridging oxygen to the heptamolybdate ion instead of the normal one.

3.3. Crystal Structure of Co3Mo14-ES

The second structure to be isolated from the reaction mixture is the extended structure (NH4)6{[Co(H2O)2]3[Mo7O24]2} (Co3Mo14-ES), as shown in Figure 2, which appears to be a sort of compressing of the previously discussed Co3Mo14-POM. The crystals of Co3Mo14-ES are a darker red-purple color. These columns are easy to physically separate from the previous compound, and often would appear as the major product in the reaction. Like the POM above, this compound still contains two heptamolybdate subspecies and a Co(H2O)2 linking the two together (Figure 2b); however, instead of the second cobalt capping the POM, here a second Co(H2O)2 links together four heptamolybdate species, which creates a three-dimensional network.
In Co3Mo14-ES, the Mo-O bond lengths can, again, easily be compared using the Evans’ nomenclature with the crystallographic oxygen positions listed in parentheses behind each label. The terminal oxygen atoms Oa (O10, O13, O21, O23), Ob (O11, O14, O20, O24), Oc (O1, O17) and Od (O2, O18) have average bond lengths of 1.753(5) Å, 1.704(5) Å, 1.725(5) Å and 1.720(6) Å, respectively. The μ2-bridging oxygen of Oe (O3, O4, O15, O19), Of (O5, O7) and Og (O12, O22) have average bond lengths of 1.940(5) Å, 2.137(5) Å and 1.928(5) Å, respectively. Lastly, the μ3-bridging oxygen positions Oh (O6, O8) have an average bond length of 2.146(5) Å and the μ4-bridging oxygen positions Oi (O9, O16) have an average bond length of 2.184(5) Å. Like before, these are all very similar to other published bond distances of the heptamolybdate structure. Additionally, again, the bond valence sum calculations for these molybdenum positions are each nearly 6.
The Co3Mo14-ES structure contains two crystallographically unique cobalt positions (Figure 2a). The Co1 position links the two heptamolybdate species together. This slightly disordered octahedron has two bound waters and four μ2-briding oxygen atoms, two to each of the heptamolybdate species. The average Co-O bond distance for the bridging oxygen in Co1 is 2.089(5) Å and the distance for the bound waters is 2.072(6) Å. The second cobalt position Co2 caps the heptamolybdate dimer. Co2 is also has slightly disordered octahedral geometry and, again, has two bound waters and four μ2-briding oxygen atoms, but now these bridging oxygen atoms connects the cobalt to three other heptamolybdate units. Here, the μ2-bridging oxygen have an average Co-O bond distance of 2.134(5) Å, respectively. The bound waters O2W and O3W have an average Co-O distance of 2.046(6) Å and completes the distorted octahedron of Co2. A complete listing of bond distances, bond angles, and hydrogen bonding interactions can be found in the supplementary materials. Unsurprisingly, both cobalt centers have bond valance sum values of approximately two.
In the overall structure, the capped and sandwich cobalt heptamolybdate species only differs from the previously described polyoxometalate ion by the location of the capping group. In the extended structure, the capping group has moved from the Ob, group (observed in Co3Mo14-POM) to the Oc and Oa in the structure Co3Mo14-ES. This transition has the effect of transforming it into a three-dimensional network, as shown in Figure 3.

3.4. Solid-State UV–Vis–NIR

Solid-State UV–Vis–NIR spectra were recorded for each of the above compounds (Figure 4). The several peaks below 475 nm are likely due to the ligand-to-metal charge transfer in the heptamolybdate ion. When comparing the two regions, there is a red-shift of ~27 nm as one transitions from the Co3Mo14-POM to the Co3Mo14-ES. As compounds oligomerize, it is not uncommon to see a red-shift in the spectra. The other peaks higher in the region seem to correlate to the presence of octahedral cobalt(II) in the compounds, specifically the peak around 560 nm.

4. Conclusions

Two heptamolybdate-based POM crystal structures have been presented. The first structure of Co3Mo14-POM contains both sandwich and capping motifs and expands the structural information for these types of POM compounds. More experiments will be conducted on the separation, solubility, and reactivity of this species in solution. The second compound is the Co3Mo14-ES, which was manually isolated from the same reaction mixture. It is likely that the addition of more ethanol used in the layering technique, which by changing the polarity of the solution, encouraged the molecular fragments to condense into this extended structure. Further investigations are ongoing into how these two compounds can be independently prepared; moreover, future studies will also focus on the solubility and, more importantly, the stability of these compounds when redissolved in water, as it is not currently known if these species will maintain their structural integrity upon dissolution. The compounds presented here provide insights into the complexities within POM chemistry.

Supplementary Materials

The supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12091201/s1, Tables S1–S7: crystallographic information for Co3Mo14-POM; Figure S1: SEM-EDX data for Co3Mo14-POM; Tables S8–S13: crystallographic information for Co3Mo14-ES; Figure S2: SEM-EDX data for Co3Mo14-ES. Additionally, crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC 2067380-2067381. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.) +44-1223/336-033; e-mail: [email protected]].

Author Contributions

Conceptualization, E.M.V. and M.R.S.; methodology, E.M.V.; synthesis, E.M.V. and M.R.S.; crystallography, E.M.V. and B.Q.M.; UV–Vis–NIR, M.J.P.; SEM-EDX, E.M.V.; investigation, M.J.P., B.Q.M., E.M.V. and M.R.S.; writing—original draft preparation, E.M.V.; writing—review and editing, M.R.S., M.J.P., B.Q.M. and E.M.V.; funding acquisition, E.M.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was initially supported by Creighton University’s Summer Faculty Research Fund from the Center for Undergraduate Research and Scholarship (Summer 2016) and then funded by Nebraska EPSCoR First Award (EPS-1004094; January–September 2016).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The SEM-EDX was acquired through a Nebraska EPSCoR MRI award to Joel Destino (Creighton) and was wholly funded by Nebraska EPSCoR. The authors would like to thank Creighton University and the Creighton University Chemistry Department for continuing to support undergraduate research.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 01201 g001 550
Figure 1. (a) The polyoxometalate Co3Mo14-POM anion is shown as a ball-and-stick model with a central Co(H2O)2 group linking the two heptamolybdate units, which are capped by a Co(H2O)5 group. (b) The thermal ellipsoid plot (50% probability) is shown for the asymmetric unit of Co3Mo14-POM. In both, the molybdenum atoms are in mauve, the cobalt are in blue, the oxygen are in red and hydrogen are shown as light pink in (a) only. For clarity, the ammonium cation and solvent water positions as well as the hydrogen atoms from (b) have been omitted. Top-right: A picture of the orange product can be seen.
Figure 1. (a) The polyoxometalate Co3Mo14-POM anion is shown as a ball-and-stick model with a central Co(H2O)2 group linking the two heptamolybdate units, which are capped by a Co(H2O)5 group. (b) The thermal ellipsoid plot (50% probability) is shown for the asymmetric unit of Co3Mo14-POM. In both, the molybdenum atoms are in mauve, the cobalt are in blue, the oxygen are in red and hydrogen are shown as light pink in (a) only. For clarity, the ammonium cation and solvent water positions as well as the hydrogen atoms from (b) have been omitted. Top-right: A picture of the orange product can be seen.
Crystals 12 01201 g001
Crystals 12 01201 g002 550
Figure 2. (a) The building unit of Co3Mo14-ES is shown as a thermal ellipsoid plot (50% probability). In contrast to the POM discussed above, the capping cobalt(II) is now bound to Oc and Oa, instead of only Ob. (b) The ball-and-stick model of the [Co(H2O)2]3[Mo7O24]26− dimer structure found within the Co3Mo14-ES structure. Here, the same type of central Co(H2O)2 group linking the two heptamolybdate units can be observed as was found in Co3Mo14-POM. Each of the capping Co(H2O)2 bridges together four heptamolybdates into an extended structure and has two bound waters. Here, the molybdenum atoms are in mauve, the cobalt are in blue, the oxygen are in red and hydrogen are shown as light pink in (a) only. For clarity, the ammonium cation positions as well as the hydrogen atoms from (a) have been omitted. Top-middle: A picture of the red-purple column can be seen.
Figure 2. (a) The building unit of Co3Mo14-ES is shown as a thermal ellipsoid plot (50% probability). In contrast to the POM discussed above, the capping cobalt(II) is now bound to Oc and Oa, instead of only Ob. (b) The ball-and-stick model of the [Co(H2O)2]3[Mo7O24]26− dimer structure found within the Co3Mo14-ES structure. Here, the same type of central Co(H2O)2 group linking the two heptamolybdate units can be observed as was found in Co3Mo14-POM. Each of the capping Co(H2O)2 bridges together four heptamolybdates into an extended structure and has two bound waters. Here, the molybdenum atoms are in mauve, the cobalt are in blue, the oxygen are in red and hydrogen are shown as light pink in (a) only. For clarity, the ammonium cation positions as well as the hydrogen atoms from (a) have been omitted. Top-middle: A picture of the red-purple column can be seen.
Crystals 12 01201 g002
Crystals 12 01201 g003 550
Figure 3. The three-dimensional network of Co3Mo14-ES is shown in the [1 1 0] plane. Here, the {[Mo7O24][Co(H2O)2][Mo7O24]}10− dimer can be seen with the capping cobalt units. These capping units connect four different heptamolybdate species together, which connect them into a three-dimensional network. The charge balancing ammonium cations can be seen in elongated channels of the network. In both, the molybdenum polyhedra are in mauve, the cobalt octahedra are in blue and nitrogen are shown as light blue spheres. The hydrogen and oxygen atoms have been omitted for clarity.
Figure 3. The three-dimensional network of Co3Mo14-ES is shown in the [1 1 0] plane. Here, the {[Mo7O24][Co(H2O)2][Mo7O24]}10− dimer can be seen with the capping cobalt units. These capping units connect four different heptamolybdate species together, which connect them into a three-dimensional network. The charge balancing ammonium cations can be seen in elongated channels of the network. In both, the molybdenum polyhedra are in mauve, the cobalt octahedra are in blue and nitrogen are shown as light blue spheres. The hydrogen and oxygen atoms have been omitted for clarity.
Crystals 12 01201 g003
Crystals 12 01201 g004 550
Figure 4. Solid-state UV–Vis–NIR for both Co3Mo14-POM (shown in orange) and Co3Mo14-ES (shown in red), with an observable red-shift in the peaks corresponding to ligand-to-metal transfer from the heptamolybdate species. A collection of CoCl2 · 6 H2O is shown in grey and helps to assign the above 500 nm to the presence of cobalt(II) in the structure.
Figure 4. Solid-state UV–Vis–NIR for both Co3Mo14-POM (shown in orange) and Co3Mo14-ES (shown in red), with an observable red-shift in the peaks corresponding to ligand-to-metal transfer from the heptamolybdate species. A collection of CoCl2 · 6 H2O is shown in grey and helps to assign the above 500 nm to the presence of cobalt(II) in the structure.
Crystals 12 01201 g004
Table
Table 1. Table of crystallographic details.
Table 1. Table of crystallographic details.
Co3Mo14-POMCo3Mo14-ES
Chemical formula(NH4)6[Co3H24Mo14O60•6(H2O)](NH4)6[Co3H12Mo14O54]
Habit, ColorBlocks, Orange-BrownPrism, Red-Violet
Mr2720.492504.30
Crystal system,
Space group		Triclinic,
P-1		Triclinic,
P-1
Temperature (K)293293
a, b, c (Å)8.0045 (3), 11.2181 (4), 18.0591 (7)7.8945 (2), 9.9241 (3), 16.2957 (4)
α, β, γ (°)107.385 (4), 95.428 (3), 100.586 (3)96.765 (2), 94.010 (2), 110.520 (3)
V (Å3)1501.77 (10)1178.85 (6)
Z11
Radiation typeMo KαMo Kα
µ (mm−1)3.754.74
Crystal size (mm)0.09 × 0.07 × 0.050.07 × 0.04 × 0.04
DiffractometerDtrek-CrysAlis PRO-abstract goniometer imported rigaku-D*TREK imagesDtrek-CrysAlis PRO-abstract goniometer imported rigaku-D*TREK images
Absorption correctionMulti-scan
CrysAlis PRO 1.171.38.46 Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.		Multi-scan
CrysAlis PRO 1.171.40.53
Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.
Tmin, Tmax0.958, 1.0000.976, 1.000
No. of measured, independent and
observed [I > 2σ (I)] reflections		19,093, 9006, 6850 12,130, 5783, 4452
Rint0.0370.036
(sin θ/λ)max (Å−1)0.7140.667
R [F2 > 2σ(F2)], Wr (F2), S0.038, 0.091, 1.070.042, 0.102, 1.10
No. of reflections90065783
No. of parameters467388
No. of restraints5757
H-atom treatmentH atoms treated by a mixture of independent and constrained refinementH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)1.53, −1.264.04, −1.54
Computer programs: CrysAlis PRO 1.171.38.46 [53] ShelXT [48] SHELXL [49] and Olex2 [50].
Table
Table 2. A brief comparison of the average cobalt–oxygen bond lengths in anionic cobalt–heptamolybdate species; ranges of values are listed below the average in italics where appropriate.
Table 2. A brief comparison of the average cobalt–oxygen bond lengths in anionic cobalt–heptamolybdate species; ranges of values are listed below the average in italics where appropriate.
Compoundμ2-Bridging
Co-O		Avg. Terminal Co-O(Aqua)Co-N
(Hapy)4[Co(H2O)5Mo7O24]·9H2O [39]2.099(3) Å [a]2.079(5) ÅN/A
(2.051–2.101)
(CH6N3)7Na[CoMo7O24(H2O)5]2·8H2O [37]2.082(7) Å [b]2.099(10) ÅN/A
(2.081–2.121)
[3-ampH]4[{Co(3-ampy)(H2O)4}Mo7O24]·4H2O [41]2.073(2) Å [a]2.099(3) Å2.122(3) Å
(2.071–2.123)
This Work—Co12.150(3) Å [c]2.087(4) ÅN/A
(2.064–2.109)
This Work—Co22.097(3) Å [b]2.038(5) Å
(2.075–2.119)
[a] Bridging through terminal Oc on the heptamolybdate species; [b] Bridging through terminal Oa on the heptamolybdate species; [c] Bridging through terminal Ob on the heptamolybdate species.
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Spriet, M.R.; Polinski, M.J.; Mercado, B.Q.; Villa, E.M. Synthesis and Structures of the Capped and Sandwiched Cobalt Heptamolybdate Polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O and Its Extended Structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}. Crystals 2022, 12, 1201. https://doi.org/10.3390/cryst12091201

AMA Style

Spriet MR, Polinski MJ, Mercado BQ, Villa EM. Synthesis and Structures of the Capped and Sandwiched Cobalt Heptamolybdate Polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O and Its Extended Structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}. Crystals. 2022; 12(9):1201. https://doi.org/10.3390/cryst12091201

Chicago/Turabian Style

Spriet, Matthieu R., Matthew J. Polinski, Brandon Q. Mercado, and Eric M. Villa. 2022. "Synthesis and Structures of the Capped and Sandwiched Cobalt Heptamolybdate Polyoxometalate (NH4)6{[Co(H2O)5][Mo7O24][Co(H2O)2][Mo7O24][Co(H2O)5]} · 6 H2O and Its Extended Structure (NH4)6{[Co(H2O)2]3[Mo7O24]2}" Crystals 12, no. 9: 1201. https://doi.org/10.3390/cryst12091201

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