A. Crystal growth

Single crystals of [(CH₃)₂NH₂]Mn(HCOO)₃ as well as [(CH₃)₂NH₂]Mn_0.9M _0.1(HCOO)₃ (M = Zn, Ni, and Co) were grown by solvothermal reaction followed by slow evaporation quite similar to the growth of other MOFs (Mączka et al., Reference Mączka, Gągor, Macalik, Pikul, Ptak and Hanuza2013). Commercially available precursor materials were used without further purification. In a typical crystal growth of Mn-MOF, 5 mM of MnCl₂·4H₂O was dissolved in 40 ml dimethylformamide and 40 ml deionized water stirred vigorously for a minimum of 30 min. In the case of 10 at.% Zn, Co, and Ni substituted Mn-MOFs, crystals were grown by mixing 4.5 mM MnCl₂·4H₂O with 0.5 mM of respective metal chlorides. The solution was transferred to a Teflon-lined stainless steel autoclave of 100 ml capacity and sealed well. The solvothermal reaction was carried out for 3 days at 140°C and was allowed to furnace cool. The supernatant solution was transferred to a beaker and left undisturbed for 7 days at room temperature for crystal growth. Grown crystals were harvested by filtering and washed using ethanol several times, dried at room temperature, and the dried crystals kept in a vacuum desiccator.

B. Composition analysis

The Mn-transition metal ion ratios of the crystals were analyzed using energy-dispersive X-ray analysis (EDAX) technique which is available in scanning electron microscopy (SEM) instrument (SEM-Carl Zeiss FE-SEM model, Supra 55; EDAX – Oxford instruments). The crushed crystals were spread on the carbon tape and placed on the SEM sample holder. When high-energy (~20 kV) electrons hit on the sample surface, characteristic X-ray of the elements emitted, which was recorded as EDAX spectra. From these spectra, the at.% and wt.% of Mn, Zn, Ni, and Co compositions were determined.

C. Laue diffraction

Laue diffraction on selected crystals were carried out using Mo target in the back-scattered geometry using an imaging plate. Because of smaller size, crystals were glued to a sharp tip of Kapton foil and mounted on to a goniometer. Care was taken that the collimated X-ray beam (0.5 mm) was incident perpendicular to the surface of the crystals. The exposed imaging plate was off-line scanned using an imaging plate reader. The Laue diffraction image was converted to digital imaging and communications in medicine (DICOM) format. The DICOM image file was processed using the freely available software ImageJ and stored in JPG format (Schneider et al., Reference Schneider, Rasband and Eliceiri2012).

D. Powder X-ray diffraction

Room temperature PXRD measurements were carried out on well-crushed single crystals of respective MOFs. Diffraction patterns were recorded using a STOE X-ray diffractometer (STOE, Darmstadt, GmbH) operated in Bragg–Brentano geometry and in θ–θ mode. The diffraction patterns were recorded using CuKα radiation and data were collected at a step size of 0.05° 2theta and with a counting time of 7 s step^–1. The diffracted X-ray was detected using a point detector attached along with a secondary monochromator. The detector made up of NaI:Tl scintillation crystal connected to a photomultiplier tube was used.

A. EDAX results

The EDAX results showed the presence of C, O, N, Mn and the transition metals. But for calculating the Mn–transition metals ratios, the presence of C, O, N atoms is ignored and only Mn and substituted transition metals are considered. From this analysis, it was found that the ratio of Mn to transition metals has maintained nearly 0.96–0.11 closely matching with nominal composition. The obtained results are given in Table I. In the Rietveld refinement, occupancy factors were refined and it varied between 0.9 and 0.873 for Mn and the other transition elements varied from 0.1 to 0.127 in the refinement.

Table I.

Metallic ratios in the MOFs obtained from EDAX analysis.

B. Optical micrograph

The optical micrographs of harvested MOF crystals are recorded by keeping them in a 1 mm graph paper [Figures 2(a)–2(d)]. The crystals exhibit well-defined facets with cubic and cuboidal morphologies. The size of Mn-MOF crystal is nearly 4 mm × 4 mm, whereas the sizes of the substituted crystals are of the order of 250–750 µm. Even though bigger size crystals are there in the substituted crystals, whatever brought inside the square of the graph sheet is captured and depicted. All the crystals are transparent though they differ in their color. The intrinsic color is because of the crystal field splitting of the d-orbitals of transition metal ions occupying the octahedral site. While Mn- and 10 at.% Zn substituted MOF crystals are colorless, 10 at.% substituted Ni- and Co-MOFs exhibit green and pink colors, respectively.

Figure 2.

As grown crystals of (a) Mn-MOF, (b) Mn_0.9Zn_0.1-MOF, (c) Mn_0.9Co_0.1-MOF, and (d) Mn_0.9Ni_0.1-MOF crystals. The inset in “a” is in a 5 mm × 5 mm square size graph sheet, whereas in “b”, “c”, and “d” are in a 1 mm × 1 mm square graph sheet.

C. Laue diffraction

The Laue diffraction patterns of the Mn-MOF, Mn_0.9Zn_0.1-MOF, Mn_0.9Co_0.1-MOF, and Mn_0.9Ni_0.1-MOF crystals were recorded [Figures 3(a)–3(d)]. The diffraction patterns exhibit sharp dots, indicating high crystalline nature of the crystals. Also, the Laue patterns have same symmetry indicating same crystal symmetry of the MOFs.

Figure 3.

The back-scattered Laue diffraction pattern of as grown crystals of (a) Mn-MOF, (b) Mn_0.9Zn_0.1-MOF, (c) Mn_0.9Co_0.1-MOF, and (d) Mn_0.9Ni_0.1-MOF crystals.

D. Structural analysis

The powder XRD patterns of Mn-, Mn_0.9Zn_0.1-, Mn_0.9Co_0.1-, and Mn_0.9Ni_0.1-MOFs were recorded. Mn-MOF crystallizes in the rhombohedral structure with $\; R\bar{3}c$ space group (S.G. No. 167). The standard data of Mn-MOF from ICDD file No. 01-083-6748 were used as a model data for Rietveld refinement and tabulated in Table II (Figure 4). This pattern provides a reasonable fit to the measured data of Mn-MOF and hence used for the refinement of all the four data sets. Peak positions match well, but there is some difference in intensity ratio. No additional peaks could be discerned indicating phase purity of the sample. No change in the diffraction pattern of the substituted MOFs could be observed except for the shift in the Bragg reflections. This signifies the absence of structural transition upon 10 at.% substitution of Zn, Co, and Ni in Mn-MOF and is in accordance with Laue diffraction studies. Rietveld refinement (Rietveld, Reference Rietveld1969) of the patterns was carried out using the GSAS software (Larson and Von Dreele, Reference Larson and Von Dreele2000) along with the graphic user interface software EXPGUI (Toby, Reference Toby2001). Refined pattern along with experimental data is shown in Figure 5. The Rietveld refined lattice parameters of the MOFs are given in Table III. The lattice parameters of Mn-MOF, a = 8.3281(03) Å, c = 22.8874(16) Å, and V = 1374.72(01) Å³, compare well with the earlier report on single crystal (Mączka, Reference Mączka, Gągor, Macalik, Pikul, Ptak and Hanuza2014). The model data are taken from the single-crystal diffraction result, whereas here the crystal is subjected to some pressure while crushing for powder XRD measurement. The differences in intensities observed and calculated may be because of various factors such as the stacking fault introduced while crushing the crystals and poor scattering factors for C, H, and N atoms present in the MOFs.

Figure 4.

The rhombohedral crystal structure of dimethylammonium manganese formate with the general formula [(CH₃)₂Mn](HCOO)₃ in hexagonal coordinate system.

Figure 5.

The Rietveld refined XRD patterns of powdered crystals of (a) Mn-MOF, (b) Mn_0.9Zn_0.1-MOF, (c) Mn_0.9Co_0.1-MOF, and (d) Mn_0.9Ni_0.1-MOF.

Table II.

Crystallographic details of dimethylammonium manganese formate.

Table III.

Crystallographic parameters of [(CH₃)₂NH₂]Mn_0.9M_0.1(HCOO)₃ (M = Zn, Co, and Ni).

The lattice parameters and unit-cell volume are found to decrease with substituent in the order Zn, Ni, Co (shown in Figure 6). The reason for such a variation can be understood based on the Shannon ionic radii (Shannon, Reference Shannon1976). The ionic radius (IR), even for a given valance state, depends also on the occupation of orbital by the unpaired electron, i.e. the spin configuration of the ions. There are two spin states, high and low for Mn²⁺ and Co²⁺, whereas Zn²⁺ and Ni²⁺ have single spin configuration. Zn²⁺ being d ¹⁰ ion diamagnetic, it carries no net magnetic moment and has no spin state. The IR of these four elements is given in Table IV. Shannon IR of Zn²⁺ is smaller than that of Mn²⁺. Hence both the lattice parameters a and c decrease. This implies anisotropic bonding of Zn²⁺ with surrounding ligand anions O²⁻. Both the lattice parameters of Co and Ni substituted MOF are smaller than that of Mn-MOF. This is in accordance with their smaller IR compared with that of Mn²⁺.

Figure 6.

Variation of lattice parameter unit-cell parameters with a substituent for the MOFs: (a) a, (b) c, and (c) unit-cell volume.

Table IV.

High-spin and low-spin Shannon ionic radii of Mn, Zn, Co, and Ni elements.

Using the lattice parameters obtained from the Rietveld refinement, the observed peak positions (in 2θ°) were given as input to the NBS*AIDS83 program which is the up-graded version of NBS*Aids80 (Mighell, et. al Reference Mighell, Hubbard and Stalick.1981,), without the Miller indices, and the output of this program was used to generate the standard ICDD database files. The Rietveld refined values as well as the NBS*AIDS83 results of these materials all agree with each other and are given in Table III.