Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

Savita A. Kundaikar; Sudesh M. Morajkar; Wolfgang Bensch; Bikshandarkoil R. Srinivasan

doi:10.1515/znb-2021-0183

Publicly Available Published by De Gruyter January 5, 2022

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ₃-bridging amino alcohol

Savita A. Kundaikar , Sudesh M. Morajkar , Wolfgang Bensch and Bikshandarkoil R. Srinivasan

From the journal Zeitschrift für Naturforschung B

https://doi.org/10.1515/znb-2021-0183

Abstract

The reaction of Na₂MoO₄·2H₂O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) in water at room temperature results in the formation of the heterometallic coordination polymer [Mo₂O₆L₂(Na₂(H₂O)₄)]·2H₂O 1 (L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato). The structure of 1 consists of a neutral (Mo₂O₆) unit located on an inversion center. The Mo atoms exhibit hexa-coordination and are bonded to two terminal and two bridging oxido ligands, an alkoxide oxygen and the amine N atoms of an anionic ligand L^– resulting in the formation of an edge-sharing {Mo₂O₈N₂} bioctahedron. The Na⁺ cations of a centrosymmetric bis(μ₂-aqua)-bridged (Na₂(H₂O)₄)²⁺ unit are penta-coordinated and bonded to two symmetry related L^– ligands via the oxygen atoms of their OH groups. The µ₃-bridging tetradentate binding mode of L^– results in the formation of a two-dimensional heterometallic coordination polymer. The constituents of 1 viz. (Mo₂O₆), (L)^–, (Na₂(H₂O)₄)²⁺ and lattice water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions. The corresponding tungstate reported recently has been obtained through a similar synthetic protocol and is isostructural.

Keywords: amino-alkoxide ligand; molybdate; sodium hydrate clusters; tungstate; two-dimensional coordination polymer

1 Introduction

The chemistry of molybdenum oxido compounds mainly termed as polyoxomolybdates (POMo) is described in all standard chemistry text books [1, 2] and is a frontier area of research in materials science. The interest in these compounds is due to the structural diversity and wide-ranging applications in several areas of science [3], [4], [5], [6], [7]. POMos include a large variety of clusters with varying nuclearity, and compounds containing as many as 368 Mo atoms were reported [8]. The study of oxoanions of Mo dates back to the days of Berzelius when a POMo containing 12 Mo atoms having the formula (NH₄)₃PMo₁₂O₄₀ was first prepared in acidic medium [9]. However, enormous developments in the area of POMo chemistry have been achieved in the last few decades, especially the structural characterization of high nuclearity POMos [3], [4], [5], [6], [7], [8, 10], [11], [12], [13], [14], [15], [16], [17], [18]. The synthesis of POMos is generally accomplished by acidification of an aqueous tetraoxidomolybdate [MoO₄]^2– solution in the presence of appropriate counter cations.

Aqueous reaction mixtures of acidified molybdate solutions containing organic amines are known to result in the formation of hepta- or octamolybdates as the favored product [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In our research we have shown that the reaction of molybdic acid or its anhydrate MoO₃ with organic amines or inorganic bases is a convenient method for the facile synthesis of heptamolybdates [31, 33], [34], [35], [36], [37], [38]. For example, the reaction of MoO₃ with n-butylamine afforded a product containing both heptamolybdate and monomolybdate clusters [31]. However, weaker bases behave differently. The non-toxic compound 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH) also referred to as tris(hydroxymethyl)aminomethane or Tris is used extensively in biological studies as a low cost component in buffer solutions and has a pKa of ≈8.5 and a pH range of 7–9 in buffer solutions which fit in the physiological pH range of most living organisms [39], [40], [41], [42]. LH has been used in polyoxometalate (POM) chemistry to obtain a variety of POM products with interesting crystal structures [43], [44], [45], [46], [47], [48]. In an earlier study we have shown that attempts to exchange the ammonium cations of (NH₄)₆(Mo₇O₂₄)·4H₂O by (LH₂)⁺ by reaction with LH result in the formation of a mixed cationic monomolybdate (NH₄)(LH₂)(MoO₄) [49]. In a recent paper Gumerova et al. [50] have reported that a condensation reaction of the orthotungstate anion (WO₄)^2– acidified with HCl and buffered at pH = 7.5 with (LH₂Cl) namely 1,3-dihydroxy-2-(hydroxymethyl)propan-2-aminium chloride results in the formation of a discrete polytungstate anion (POT) containing the neutral dinuclear [W₂O₆] unit. In the present study we report that a hitherto unknown compound [Mo₂O₆L₂Na₂(H₂O)₄]·2H₂O 1 as well as its corresponding W analog 2 reported recently [50] can be synthesized in good yields by the reaction of sodium molybdate dihydrate (or sodium tungstate dihydrate) with 2-amino-2-(hydroxymethyl)propane-1,3-diol in water. The synthesis, spectral characterization and crystal structures of the isostructural compounds 1 and 2 are described in this report.

2 Experimental

2.1 Materials and methods

All chemicals were used as received from commercial sources without any further purification. Infrared (IR) spectra of the solid samples diluted with KBr were recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer in the range 4000–400 cm⁻¹ at a resolution of 4 cm⁻¹. Raman spectra of the solids and their aqueous solutions were measured by using an Agiltron PeakSeeker Pro Raman instrument with 785 nm laser irradiation and a laser power of 100 mW. Isothermal mass loss studies of 1 and 2 were performed in a temperature controlled electric furnace. Simultaneous thermogravimetry (TG) and differential thermal analyses (DTA) of powdered samples of 1 and 2 were performed in alumina crucibles in the temperature range of 30–600 °C, using a Netzsch STA-409 PC thermal analyzer. X-ray intensity data for 1 and 2 were collected on a STOE- Image Plate diffraction System (IPDS-2) and a Bruker D8 Quest Eco X-ray diffractometer, respectively, using graphite-monochromated Mo-Kα radiation. The structures were solved with Direct Methods using Shelxs-97 [51] and refinement was carried out against F² using Shelxl-2016 [51]. All non-hydrogen atoms were refined anisotropically. The C–H and N–H hydrogen atoms were located in difference maps but were positioned with idealized geometry and refined isotropically with U_iso(H) = 1.2 U_eq(C,N) using a riding model. The O–H hydrogen atoms were located in difference maps, their bond lengths were set to ideal values and finally they were refined isotropically with U_iso(H) = 1.5 U_eq(O) using a riding model. Technical details of data acquisition and selected refinement results for 1 and 2 are given in Table 1.

Table 1:

Crystal data and structure refinement for 1 and 2.

Refinement result	Compound 1	Compound 2
Empirical formula	C₈H₃₂Mo₂N₂Na₂O₁₈	C₈H₃₂W₂N₂Na₂O₁₈
Formula weight (g mol⁻¹)	682.21	858.03
Temperature (K)	170(2)K	100(2)
Wavelength (Å)	0.71073	0.71073
Crystal system	Triclinic	Triclinic
Space group	P 1 ‾	P 1 ‾
Unit cell dimensions
a (Å)	7.1728(4)	7.1802(5)
b (Å)	9.0692(5)	9.0454(7)
c (Å)	10.0146(6)	9.9898(8)
α (°)	64.636(4)	64.673(2)
β (°)	70.885(5)°	71.352(2)
Γ (°)	88.878(5)	89.525(2)
Volume (Å³)	550.62(6)	549.37(7)
Z	1	1
D_calc (g/cm³)	2.06	2.59
Absorption coefficient (mm⁻¹)	1.3	10.6
F(000)	344	408
Crystal size (mm³)	0.05 × 0.09 × 0.17	0.31 × 0.23 × 0.14
θ range for data collection (°)	2.406 to 29.003	3.615 to 28.312
Limiting indices	−9 ≤ h ≤ 9, −12 ≤ k ≤ 12, −13 ≤ l ≤ 13	−9 ≤ h ≤ 9, −12 ≤ k ≤ 12, −13 ≤ l ≤ 13
Reflections collected/unique	10615/2925 [R(int) = 0.0514]	10094/2724 [R(int) = 0.0233]
Completeness θ = 25.242°	100.0%	99.50%
Refinement method	Full- matrix least-squares on F²	Full-matrix least-squares on F²
Data/restraints/parameters	2925/0/146	2724/0/177
Goodness of fit on F²	1.056	1.179
Final R indices [I > 2σ(I)]	R1 = 0.0259, wR2 = 0.0665	R1 = 0.0132, wR2 = 0.0341
R indices (all data)	R1 = 0.0282, wR2 = 0.0671	R1 = 0.0134, wR2 = 0.0343
Largest diff. peak and hole (e Å⁻³)	0.81 and −0.85	0.78 and −1.28
CCDC deposition no	2128432	2128433

2.2 Synthesis of [M₂O₆(L)₂(Na₂(H₂O)₄)]·2H₂O (M = Mo 1; M = W 2)

To a solution of sodium molybdate dihydrate (Na₂MoO₄·2H₂O) (2.42 g, 10 mmol) dissolved in ∼10 mL distilled water, 2-amino-2-(hydroxymethyl)propane-1,3-diol (2.42 g, 20 mmol) taken in ∼10 mL of water was added. The reaction mixture was stirred well and the pH was found to be 11.97. Subsequently the clear solution was left undisturbed. After a few days, the crystals that precipitated were isolated by filtration to obtain 3.03 g of 1. The use of Na₂WO₄·2H₂O (3.298 g, 10 mmol) instead of Na₂MoO₄·2H₂O under identical reaction conditions resulted in the formation of 3.64 g of 2.

Anal. Found (Calcd.) for (1) (%): C, 14.08 (13.95); H, 4.60 (4.73); N, 4.05 (4.11); IR data (cm⁻¹): 3550–2950(br), 2261(br), 1761(m), 1652(m), 1563(m), 1280(m), 1081(ν_C–O), 1044(ν_C–O), 881(w), 855(s), 760(s), 618, 486. Raman data (cm⁻¹): 3575, 2791, 2554, 1446, 1052, 883(vs), 764, 321; DTA (°C): 132(endo), 456(exo), 534(exo).

Anal. Found (Calcd.) for (2) (%): C, 11.02 (11.20); H, 3.14 (3.80); N, 3.11 (3.26); IR data (cm⁻¹): 3619–3181(br), 2875(m), 2723(w), 2610(w), 2227(br), 1570(s), 1485(s), 1276(m), 1209(m), 1085(ν_C–O), 1076(ν_C–O), 1039(ν_C–O), 902(s), 848(s), 713(w), 684(w), 412(w). Raman data (cm⁻¹): 3029, 2790, 1466, 1274, 1067, 896(vs), 839, 609; DTA (°C): 128(endo), 209(endo), 320(exo), 491(exo), 584(exo).

3 Results and discussion

3.1 Description of the crystal structures of compounds 1 and 2

The compounds [M₂O₆L₂(Na₂(H₂O)₄)]·2H₂O (M = Mo 1; M = W 2; L = 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato) are isostructural and crystallize in the centrosymmetric triclinic space group P 1 ‾ . The crystal structures consist of a centrosymmetric neutral [M₂O₆] unit and a bis(μ₂-aqua) bridged (Na₂(H₂O)₄)²⁺ dication, a crystallographically independent 2-amino-3-hydroxy-2-(hydroxymethyl)propan-1-olato ligand abbreviated as L^– and a lattice water molecule (Figure 1, Figure S1). The midpoint of the line connecting Mo1 and its symmetry generated counterpart Mo1ⁱ (for symmetry code see Table 2) is the center of inversion. The geometric parameters of the amino alcohol ligand are in normal range (Table S1).

Figure 1:

The asymmetric unit of 1 showing hexa- and penta-coordination around the Mo and Na atoms, respectively. Displacement ellipsoids are drawn at 50% probability level for the non-hydrogen atoms. Intramolecular hydrogen bonding is shown by broken lines. For symmetry code See Table 2 (For the structure of the isotypic compound 2 see Figure S1).

Table 2:

Selected bond lengths and bond angles (Å, °) for 1 and 2. Estimated standard deviations are given in parentheses.

Compound 1		Compound 2
Mo1-O3	1.7469(16)	W1-O3	1.7612(18)
Mo1-O2	1.7469(16)	W1-O2	1.7708(19)
Mo1-O1	1.8239(15)	W1-O1	1.8621(18)
Mo1-O11	1.9888(14)	W1-O11	1.9727(18)
Mo1-O1ⁱ	2.2329(15)	W1-O1ⁱ	2.1653(18)
Mo1-N1	2.3418(17)	W1-N1	2.338(2)
Na1-O4ⁱⁱ	2.3150(18)	Na1-O4	2.312(2)
Na1-O13	2.3368(18)	Na1-O12ⁱⁱ	2.344(2)
Na1-O5	2.3492(18)	Na-O13	2.350(2)
Na1-O12ⁱⁱⁱ	2.3581(18)	Na1-O5	2.352(2)
Na1-O4	2.3581(18)	Na1-O4ⁱⁱⁱ	2.362(2)
Na···Na	7.145(21)	Na···Na	3.565(2)
Na···Na	3.538(15)	Na···Na	7.159(10)
O3-Mo1-O2	104.16(8)	O3-W1-O2	103.25(9)
O3-Mo1-O1	101.07(7)	O3-W1-O1	99.25(8)
O2-Mo1-O1	104.78(7)	O2-W1-O1	104.02(9)
O3-Mo1-O11	98.62(7)	O3-W1-O11	99.13(8)
O2-Mo1-O11	90.77(7)	O2-W1-O11	91.44(8)
O1-Mo1-O11	150.92(7)	O1-W1-O11	152.49(8)
O3-Mo1-O1ⁱ	166.12(7)	O3-W1-O1ⁱ	165.18(8)
O2-Mo1-O1ⁱ	89.71(7)	O2-W1-O1ⁱ	91.49(8)
O1-Mo1-O1ⁱ	75.12(7)	O1-W1-O1ⁱ	75.31(8)
O11-Mo1-O1ⁱ	80.64(6)	O11-W1-O1ⁱ	81.77(7)
O3-Mo1-N1	88.94(7)	O3-W1-N1	88.29(8)
O2-Mo1-N1	160.79(7)	O2- W1-N1	162.44(8)
O1-Mo1-N1	85.98(6)	O1-W1-N1	86.80(8)
O11-Mo1-N1	73.16(6)	O11-W1-N1	73.48(8)
O1ⁱ-Mo1-N1	77.55(6)	O1ⁱ-W1-N1	77.74(8)
O4ⁱⁱ-Na1-O13	93.59(6)	O4-Na1-O13	171.12(9)
O4ⁱⁱ-Na1-O5	91.38(6)	O4-Na1-O5	91.42
O13-Na1-O5	116.14(7)	O13-Na1-O5	89.29(8)
O4ⁱⁱ-Na1-O12ⁱⁱⁱ	172.44(7)	O4-Na1-O12ⁱⁱ	94.58(8)
O13-Na1-O12ⁱⁱⁱ	93.28(6)	O13-Na1-O12ⁱⁱ	92.67(8)
O5-Na1-O12ⁱⁱⁱ	88.41(6)	O5-Na1-O12ⁱⁱ	117.55(8)
O4ⁱⁱ-Na1-O4	81.60(6)	O4-Na1-O4ⁱⁱⁱ	80.59(8)
O13-Na1-O4	115.33(7)	O13-Na1-O4ⁱⁱⁱ	92.25(8)
O5-Na1-O4	128.37(7)	O5-Na1-O4ⁱⁱⁱ	130.73(9)
O12ⁱⁱⁱ-Na1-O4	92.61(6)	O12ⁱⁱ-Na1-O4ⁱⁱⁱ	111.56(8)

Symmetry transformations used to generate equivalent atoms: Symmetry code for compound 1 i) −x + 2, −y + 1, −z; ii) −x + 1, −y + 2, −z; iii) 3 −x + 1, −y + 1, −z + 1. Compound 2 i) −x + 1, −y + 1, −z; ii) −x, −y + 1, −z + 1; iii) −x, −y, −z + 2.

In the neutral dinuclear (M₂O₆) units the M⁶⁺ cation (Mo or W) is bonded to two terminal oxido ligands at shorter bond distances (Mo1-O3 = 1.7469(16) and Mo1-O2 = 1.7469(16) Å; W1-O3 = 1.7612(18) and W1-O2 = 1.7708(19) Å), and to two μ₂-bridging O atoms at slightly longer distances (Mo1-O1 = 1.8239(15) and 2.2329(15) Å; W1-O1 = 1.8621(18) and 1.9727(18) Å) (Table 2). The linking of the (M₂O₆) units with two symmetry related L ligands via the alkoxide oxygen atom (O11) at an intermediate distance (Mo1-O11 = 1.9888(14) Å and W1-O11 = 1.9727(8) Å) and an amino nitrogen atom (N1) at the longest distance (Mo1-N1 = 2.3418 Å; W1-N1 = 2.338(2) Å) completes the hexa-coordination around M. The M–O/N bond angles deviate from the ideal values of 90° (75.12–104.88(7)° in 1; 73.48(7) to 104.02(9)° in 2) and 180° (150.92(7) to 166.12(7)° in 1; 152.49(6) to 165.18(8)° in 2) indicating a distortion of the {MO₅N} octahedra (Table 2). The M-M distances are 3.226(12) and 3.194(6) Å in 1 and 2, respectively. The bridging nature of O1 and the coordination of L^– result in the formation of edge-sharing {M₂O₈N₂} bioctahedra with the anions L^– acting as handles (Figure S2). In addition, each anionic ligand forms bridges to two symmetry related Na⁺ cations (through its atoms O12 and O13) with a Na···Na separation of 7.155(21) Å in 1 (7.159(10) Å in 2) (Figure 2, Figure S3) and is thus a μ₃-bridging tetradentate ligand.

Figure 2:

The μ₃-bridging tetradentate ligand anion L^– in the structure of 1 with a Na···Na separation of 7.155(21) Å (left); the μ₂-bridging aqua ligand O4 in 1 with a Na···Na separation of 3.538(15) Å (right). For symmetry code See Table 2.

The Na⁺ cations in 1 (or 2) are in a distorted trigonal bipyramidal geometry (Figure S4) and are bonded to a terminal aqua ligand (O5), a pair of bridging aqua (O4) ligands, and two symmetry related L^– anions via the hydroxy groups of O12 and O13 (Figure 2). Although the Na–O distances scatter in a small range between 2.3150(8) and 2.3581(18) Å in 1 (2.312(2) to 2.362(2) in 2), the O–Na–O angles of the {NaO₅} polyhedron deviate considerably from the ideal values expected for a trigonal bipyramid (Table 2). An edge-sharing {Na₂O₈} polyhedron is generated by two symmetry related aqua ligands (O4) linking neighboring Na⁺ cations and a Na···Na separation of 3.538(15) and 3.565(7) Å is observed in 1 and 2, respectively (Figure 2).

The bridging modes of the aqua ligand O4 and L^– organize the Na⁺ ions into an infinite zig-zag chain extending along the c axis, with alternating Na···Na separations of 3.538(15) and 7.155(21) Å in 1 and 3.565(7) and 7.159(10) Å in 2 (Figure 3). Each Na⁺ cation in the chain is bonded to a terminal aqua ligand O5 and alternating pairs of Na⁺ are bridged by a pair of aqua ligands (O4) and a pair of organic ligands (Figure 3).

Figure 3:

View of the zig-zag chains of the Na⁺ cations with alternating Na···Na separations in 1 (top); the same arrangement with terminal and bridging aqua ligands and –OH groups of L to show the full chains. For clarity the H atoms attached to C and N are not shown (bottom).

The ligand L functions as a handle attached on either side of the {Na₂O₈} polyhedra. Thus the chain in 1 (or 2) consists of alternating bis(μ-aqua) bridged (Na₂(H₂O)₄)²⁺ units and L^– anions (Figure 4). The parallel chains are interlinked due to the binding of the alkoxide O11 atom and the amine N1 atom of L to the centrosymmetric {M₂O₆} moieties extending the connectivity along the a axis and resulting in a two-dimensional heterometallic polymer (Figure 5, Figure S5).

Figure 4:

Parallel chains of Na⁺ ions showing alternating {Na₂O₈} polyhedra and bridging ligand anions L^–.

Figure 5:

The binding of L⁻ in the chains with (M₂O₆) clusters results in 2D connectivity in crystals of 1. For the polyhedral representation of the (M₂O₈N₂) bioctahedra and {Na₂O₈} polyhedra and the 2D connectivity in 2 see Figure S5.

The constituents in the crystal structure of 1 (or 2) viz. (M₂O₆), L^–, (Na₂(H₂O)₄)²⁺ and water molecules are interlinked with the aid of three varieties of hydrogen bonding interactions (Table S2). A search of the Cambridge Structural Database (CSD) [52] reveals many examples of structurally characterized compounds containing the amino alcohol ligand LH (Table 3). In these compounds LH exhibits different bridging modes viz. μ₈-, μ₇-, μ₅-, μ₄-, which are classified under five categories, namely Type A, Type B, Type C, Type D and Type E. The μ₃-bridging tetradentate binding mode is observed only in the (M₂O₆) containing compounds 1 and 2.

Table 3:

Binding modes of the amino alcohol (LH) ligand listed in the CSD.

No.	Compound	Space group	Refcode
Type A: binding mode of LH is μ₈-ƞ³: ƞ³: ƞ³: ƞ¹; decadentate (O, N donor)
1	[TBA]₅[Mo₆O₁₈(L)MnMo₆O₁₈(LH)]	P 1 ‾	DUWSEV
2	{Ag₃[MnMo₆O₁₈(LH)₂(DMSO)₅]·3(DMSO)}_n	P2₁/n	ODERAR
3	{Ag₃[MnMo₆O₁₈{LH}₂(DMSO)₆(CH₃CN)₂]·DMSO}_n	P2₁/n	ODEREV
Type B: binding mode of LH is μ₇-ƞ³: ƞ³: ƞ³; nonadentate (O donor)
4	[TMA]₂[GaMo₆O₁₈(OH)₃(LH₂)]·7H₂O	P 1 ‾	YOSMOK
5	[TBA]₄{[(LH₂)CrMo₆O₁₈(OH)₄]₂·4[TBA]Br₂[NH₄]Br·15H₂O	P2 ₁	ZABKAR
6	Na₃[MnMo₆O₁₈(LH)₂]·3DMF·4H₂O	P 1 ‾	RAKFOB
7	Na₃[MnMo₆O₁₈(LH)₂]·4DMF·5H₂O	P 1 ‾	VEHSOS
8	Na₃[MnMo₆O₁₈(LH)₂]·10DMF	P 1 ‾	VEHSUY
9	(TMA)Na₅[MnMo₆O₁₈(LH)₂]·14DMF	P 1 ‾	RAKFER
10	(TMA)₃[MnMo₆O₁₈(LH)₂]·3DMF	C2/c	RAKGES
11	(TEA)₃[MnMo₆O₁₈(LH)₂]·DMF	C2/m	RAKGAO
12	(TEA)₃[MnMo₆O₁₈(LH)₂]·2DMF	C2/c	RAKFUH
13	[TBA]₃[(LH)CrMo₆O₁₈(OH)₃][TBA]Br·2H₂O	P2₁	ROQVOK02
14	[TBA]₂[CoMo₆O₁₈{(LH) }₂]·DMA	C2/c	TEPJEG
15	[TBA]₃[(LH)₂CrMo₆O₁₈(OH)₃]·6H₂O	P2₁/n	ROQTAU
16	[TBA]₃[MnMo₆O₁₈(LH)₂]	Pcmn	OJEHEQ
17	[TBA]₃[FeMo₆O₁₈(LH)₂]	C2/m	OJEHIU
18	[TBA]₄ {(LH)CrMo₆O₁₈(OH)₃}·Br·2H₂O	P2₁	ROQVOK01
19	[TBA]₄{(LH)MnMo₆O₁₈ (OH)₃}·Br	P2₁	FUDDUF02
20	[TBA]₆{[C₂H₅C(CH₂O)₃]CrMo₆O₁₈(LH)}₂·3DMF	P2₁2₁2₁	FUDFER02
21	[TBA]₆{[C₂H₅C(CH₂O)₃]MnMo₆O₁₈(LH)}₂·4DMF	P2₁2₁2₁	FUDFIV02
22	[TBA]₆{[C₂H₅C(CH₂O)₃]AlMo₆O₁₈(LH)}₂·3DMF	P2₁2₁2₁	FUDFOB
23	(NH₄)₄{[LH]₂CuMo₆O₁₈}	P4₂/n	DIRTOQ
Type C: binding mode of LH is μ₅- ƞ²:ƞ³: ƞ³ octadentate (O donor)
24	Na[TMA]₂[FeMo₆O₁₈(OH)₃(LH₂)](OH)·6H₂O	P 1 ‾	YOSMUQ
25	γ (TBA)₃[(LH)₂CrMo₆O₁₈(OH)₄] Br ·5DMF	P1	TUFQOC
26	[TBA]₂[H] [(LH)CrMo₆O₁₈(OH)₃]·3DMF·2H₂O	P 1 ‾	TUFPUH01
Type D: binding mode of LH is μ₄-ƞ³: ƞ²: ƞ²; heptadentate (O donor)
27	[TBA]₄{[(LH₂)]CrMo₆O₁₈(OH)₄}₂·4[TBA]Br·2[NH₄]Br·14H₂O	P2₁	ZUZVOH
28	β-{Cr(LH)₂Mo₆O₁₈}	C2/c	BAYTED
29	[NH₄] β-{(LH₂)₂MnMo₆O₁₈}	C2/c	TIWMEU
Type E: Binding mode of LH is μ₃-ƞ¹: ƞ¹: ƞ¹: ƞ¹; tetradentate (O, N donor)
30	[M₂O₆(L)₂Na₂(H₂O)₄]·2H₂O	P 1 ‾	This work
31	[W₂O₆(L)₂Na₂(H₂O)₄]·2H₂O	P 1 ‾	OWIYIF This work

TBA = tetrabutylammonium; DMSO = dimethylsulfoxide; TMA = tetramethylammonium; DMF = dimethylformamide; TEA = tetraethylammonium; DMA = dimethylacetamide.

3.2 Synthetic aspects, spectral characteristics and thermal studies of 1 and 2

The reaction of an aqueous solution of Na₂MoO₄·2H₂O (or Na₂WO₄·2H₂O) with LH in 1:2 M ratio at room temperature (RT) results in the facile formation of 1 (or 2) containing the neutral dinuclear (M₂O₆) unit. The conversion of two (MO₄)^2– units to a (M₂O₆) moiety requires four H⁺ ions to remove the oxido ligands as water (Eq. (1)), which are provided by the deprotonation of one of the –OH groups of LH by the alkaline (MO₄)^2– solution as shown below. The L^– anion thus formed binds to both M⁶⁺ and Na⁺ cations to afford 1 or 2.

(1) 2 ( MO 4 ) 2 − + 4 H + → ( M 2 O 6 ) + 2 H 2 O

Equation (1) can explain the requirement of two moles of LH per Mo. The use of less LH (equimolar ratio) also results in product formation but with reduced yields. The compounds thus prepared exhibit satisfactory elemental analytical data for C, H and N. A comparison of the X-ray powder pattern of 1 with that of the pattern calculated from single crystal data confirms the composition of the bulk material (Figure S6). In the case of 2, the unit cell data are in agreement with the data reported by Gumerova et al. [50].

An interesting aspect of the synthesis of 1 (or 2) is that the condensation of (MO₄)^2– (M = Mo or W) units occurs in alkaline solution to form products comprising a dinuclear unit. Aqueous solutions of Na₂MoO₄·2H₂O (or Na₂WO₄·2H₂O) and LH in 1:2 ratio are alkaline in nature and the pH value is ∼12.0. The Raman spectrum of the solution exhibits a strong signal at 897 cm⁻¹ for Mo (928 cm⁻¹ for W) which can be assigned to the symmetric stretching vibration (υ₁) of the tetrahedral (MO₄)²⁻ species (Figure 6). Slow evaporation resulted in the formation of crystalline 1 (or 2) which exhibits an intense Raman signal at 883 cm⁻¹ for 1 (896 cm⁻¹ for 2) due to Mo-O (or W-O) vibrations of 1 (or 2) (Figure 6, Figure S7). The absence of the characteristic υ₁ signals of (MO₄)²⁻ in the Raman spectra reveals the formation of new products. However, when compound 1 (or 2) is dissolved in water the pH is 8.73 (or 8.12). The Raman spectra exhibit a strong band at 897 cm⁻¹ (928 cm⁻¹) indicating that in solution 1 (or 2) dissociate to form the tetrahedral (MO₄)^2– species (Figure 6). A similar dissociation of 2 to orthotungstate (WO₄)^2– has been recently reported at a pH of 7.5 [50].

Figure 6:

Raman spectra of a solid sample of 1 (in blue); compound 1 in water (red); an aqueous mixture containing Na₂MoO₄·2H₂O and LH (black).

In recent work [49] we have shown that the reaction of LH with in situ generated (NH₄)₂MoO₄ (MoO₃ dissolved in excess ammonia) leads to crystallization of a mixed-cation monomolybdate (NH₄)(LH₂)(MoO₄). In contrast, the use of Na₂MoO₄ containing the oxophilic Na⁺ cation leads to the formation of the two-dimensional coordination polymer 1 in which (L)^– functions as a µ₃-bridging tetradentate ligand.

A survey of the literature reveals that several compounds (Table 4) containing the (Mo₂O₆) unit have been synthesized and structurally characterized [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65]. Most of these syntheses were performed either under hydrothermal conditions or at high temperatures except for 1 and 2, (TBA)₂[Mo₂O₆(pic)₂] and (nHex₄N)₂[Mo₂O₆(picOH)₂] (entry nos. 6 and 7 in Table 4) which are conducted at RT. A range of Mo containing reagents other than Na₂MoO₄ have been employed as the starting materials including MoO₃, (NH₄)₂MoO₄, (NH₄)₆Mo₇O₂₄·4H₂O, [MoO₃(2,2′-bipy)], (TBA)₄Mo₈O₂₆, and [MoO₂Cl₂(pypzEA)]. In the present study, we have shown that the isostructural W compound 2 can also be synthesized at RT, while in previous work it was obtained by heating an orthotungstate (WO₄)^2– solution containing LH acidified with HCl to 90 °C, and adjusting the pH of the reaction mixture to 7.5 after cooling to RT [50].

Table 4:

Reaction conditions for synthesis of (Mo₂O₆) containing compounds.

No	Compound	Reaction temperature/Mo reagent	Ref
1	[Mo₂O₆(imi)₆]	Hydrothermal, 150 °C/(NH₄)₂MoO₄	[53]
2	[Mo₂O₆(di-tBu-bpy)₂]	Hydrothermal,160 °C/MoO₃	[54]
3	(Pr₂NH₂)₄[Mo₂O₆(C₂O₄)₂]	Heated at 60 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[55]
4	(ⁱPr₂NH₂)₄[Mo₂O₆(C₂O₄)₂]	Heated at 60 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[55]
5	(HDabco)₄[Mo₂O₆(C₂O₄)₂]·2H₂O	Heated at 60 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[55]
6	(TBA)₂[Mo₂O₆(pic)₂]	RT/(TBA)₄Mo₈O₂₆	[57]
7	(n-Hex₄N)₂[Mo₂O₆(picOH)₂]	RT/Na₂MoO₄·2H₂O	[60]
8	[Mo₂O₆(2,2′-bipy)]	Solid-state synthesis at 300 °C/[MoO₃(2,2′-bipy)]	[58]
9	[Mo₂O₆(HpypzA)]	Hydrothermal, 100 °C/[MoO₂Cl₂(pypzEA)]	[56]
10	[Mo₂O₆(tr₂pr)]	Hydrothermal, 180 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[59]
11	[Mo₂O₆(tr₂cy)]	Hydrothermal,180 °C/MoO₃/Co²⁺Catalyst	[59]
12	[Mo₂O₆(tr₂ad)]·6H₂O	Hydrothermal, 180 °C/MoO₃/Co²⁺Catalyst	[59]
13	[Mo₂O₆(4,9-tr₂dia)]·0.5H₂O	Hydrothermal, 180 °C/Na₂MoO₄·2H₂O/Cu²⁺ catalyst	[59]
14	[Mo₂O₆(trethbz)₂]·H₂O	Hydrothermal/(NH₄)₆Mo₇O₂₄·4H₂O	[61]
15	[Mo₂O₆(trpzH)(H₂O)₂] H₂O	Reflux in water (100 °C)/MoO₃	[62]
16	[Mo₂O₆(m-trtzH)(H₂O)₂] H₂O	Hydrothermal, 140 °C/MoO₃	[62]
17	[Mo₂O₆(p-trtzH)] H₂O	Hydrothermal, 140 °C/MoO₃	[62]
18	[Mo₂O₆(tr₂ad)]·H₂O	Hydrothermal, 170 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[62]
19	[Mo₂O₆(p-tr₂Ph)] H₂O	Hydrothermal, 140 °C/(NH₄)₆Mo₇O₂₄·4H₂O	[62]
20	Cu(nic)(nicH)(H₂O)₂[Mo₂O₆]_n	Hydrothermal, 170 °C/MoO₃	[63]
21	[Mo₂O₆(pyrazine)]	Hydrothermal, 160 °C/MoO₃	[64]
22	[Mo₂O₆(4,4′-bipy)]	Hydrothermal, 150 °C/MoO₃	[65]
23	[Mo₂O₆(1,2,4-triazole)]	Hydrothermal, 200 °C/Na₂MoO₄	[65]
24	[Mo₂O₆(L)₂Na₂(H₂O)₄]·2H₂O 1	RT/Na₂MO₄·2H₂O	This work
25	[W₂O₆(L)₂Na₂(H₂O)₄]·2H₂O 2	RT/Na₂WO₄·2H₂O	This work
26	[W₂O₆(L)₂Na₂(H₂O)₄]·2H₂O	Heated at 90 °C/Na₂WO₄·2H₂O	[50]

imi = imidazole; di-tBu-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine; Pr₂NH = dipropylamine; ⁱPr₂NH = diisopropylamine; Dabco = 1,4-diazabicyclo[2.2.2]octane; TBA = tetrabutylammonium; pic = picolinate; n-Hex₄N; tetrahexylammonium; HpicOH = 3-hydroxypicolinic acid; 2,2′-bipy = 2,2′-bipyridine; HpypzA = [3-(pyridinium-2-yl)-1H-pyrazol-1-yl]- acetate; pypzEA = ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate; tr₂pr = bis(1,2,4-triazol-4-yl)1,3-propylene; tr₂cy = 1,4-bis(1,2,4-triazol-4-yl)cyclohexane; tr₂ad = 1,3-bis-(1,2,4-triazol-4-yl)adamantine; 4,9-tr₂dia = 9-bis(1,2,4-triazol-4-yl)diamantine; trethbz = (S)-4-(1-phenylpropyl)-1,2,4-triazole; trpzH = 5-dimethylpyrazol-4-yl]-4H-1,2,4-triazole; m-trtzH = m-[4-3’-(tetrazol-5-yl)phenyl]-4H-1,2,4-triazole; p-trtzH = p-[4-3’-(tetrazol-5-yl)phenyl]-4H-1,2,4-triazole; p-tr₂Ph = [1,4-bis-(1,2,4-triazol-4-yl)benzene; nic = nicotinate; 4,4′-bipy = 4,4′-bipyridine.

The infrared spectra of solid 1 (or 2) exhibit several peaks in the mid IR region indicating the presence of L (Figure S8). The broad absorption in the 3500 cm⁻¹ region in 1 can be attributed to the O–H stretching vibration of water molecules. The intense bands at around 1081 and 1044 cm⁻¹ can be assigned to the vibrations of C–O groups while the absorptions at ≈881, 855, 760 cm⁻¹, are caused by the vibrations of the dinuclear (Mo₂O₆) moiety.

The thermal studies of 1 (or 2) revealed that both compounds lose crystal water molecules accompanied by an initial endothermic event at 132 °C (or 128 °C) (Figure S9). Increasing the temperature, exothermic events are observed assignable to the decomposition of L resulting in the formation of 45.0% (62.5) residual mass. The IR spectra of the residues indicate the absence of water and organic components. Although the absence of mass spectral analysis of the emitted fragments precludes a detailed discussion of the exact nature of the decomposition processes, based on isothermal mass loss studies and featureless IR spectra the composition of the residue can be assigned as M₂Na₂O₇. The thermal behavior of 2 was also reported in [50] but without a complementary MS experiment the exact decomposition pathway cannot be described.

4 Conclusions

In this study, we describe a convenient synthetic protocol for the room temperature synthesis of the isostructural heterometallic coordination polymers [M₂O₆(μ₃-L)₂(Na₂(H₂O)₄)]·2H₂O (M = Mo 1; M = W 2) by an aqueous reaction of Na₂MO₄·2H₂O with 2-amino-2-(hydroxymethyl)propane-1,3-diol (LH). The condensation of (MO₄)²⁻ units to form the dinuclear moiety (M₂O₆) occurs in alkaline medium due to deprotonation of LH. The anionic amino alkoxide ligand L^– exhibits a μ_3-tetradentate bridging mode and functions as an organic handle for the centrosymmetric neutral (M₂O₆) unit and bis(μ₂-aqua) bridged [Na₂(H₂O)₄]²⁺ cations and organizes the Na⁺ cations into a chain with alternating Na···Na separations. The chains are interlinked by the binding of the (M₂O₆) units with the organic ligand resulting in 2D connectivity. Compound 1 is a new example of a dinuclear Mo compound based on the (Mo₂O₆) moiety. The synthesis of the previously reported W analogue 2 has now been performed at room temperature at a suitably adjusted pH value.

5 Supporting information

Deposition numbers CCDC 2128432 (1) CCDC 2128433 (2), contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Supplementary Data (Figures S1–S9) and Tables (Tables S1 and S2) associated with this article are available in electronic form.

Dedicated to Professor Christian Näther on the occasion of his 60th birthday.

Corresponding author: Bikshandarkoil R. Srinivasan, School of Chemical Sciences, Goa University, Goa 403206, India, E-mail: srini@unigoa.ac.in

Funding source: State of Schleswig-Holstein

Funding source: University Grants Commission, New Delhi

Acknowledgments

Financial assistance to the School of Chemical Sciences (formerly Department of Chemistry), Goa University at the level of DSA-I under the Special Assistance Programme (SAP) by the University Grants Commission, New Delhi is gratefully acknowledged. WB acknowledges financial support by the State of Schleswig-Holstein.

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: This work was funded by the University Grants Commission, New Delhi and the State of Schleswig-Holstein.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2021-0183).

Received: 2021-12-15

Accepted: 2021-12-22

Published Online: 2022-01-05

Published in Print: 2022-05-25

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ₃-bridging amino alcohol

Abstract

1 Introduction

2 Experimental

2.1 Materials and methods

2.2 Synthesis of [M₂O₆(L)₂(Na₂(H₂O)₄)]·2H₂O (M = Mo 1; M = W 2)

3 Results and discussion

3.1 Description of the crystal structures of compounds 1 and 2

3.2 Synthetic aspects, spectral characteristics and thermal studies of 1 and 2

4 Conclusions

5 Supporting information

Acknowledgments

References

Supplementary Material

Journal and Issue

Articles in the same Issue

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ3-bridging amino alcohol

Abstract

1 Introduction

2 Experimental

2.1 Materials and methods

2.2 Synthesis of [M2O6(L)2(Na2(H2O)4)]·2H2O (M = Mo 1; M = W 2)

3 Results and discussion

3.1 Description of the crystal structures of compounds 1 and 2

3.2 Synthetic aspects, spectral characteristics and thermal studies of 1 and 2

4 Conclusions

5 Supporting information

Acknowledgments

References

Supplementary Material

Journal and Issue

Articles in the same Issue

Synthesis and structural characterization of a new heterometallicmolybdate coordination polymer based on a µ₃-bridging amino alcohol

2.2 Synthesis of [M₂O₆(L)₂(Na₂(H₂O)₄)]·2H₂O (M = Mo 1; M = W 2)