Building Manganese Halide Hybrid Materials with 0D, 1D, and 2D Dimensionalities

Abstract

In recent years, metal-halide hybrid materials have attracted considerable attention because materials, such as lead-iodide perovskites, can have excellent properties as photovoltaics, light-emitting devices, and photodetectors. These materials can be obtained in different dimensionalities (1D, 2D, and 3D), which directly affects their properties. In this article, we built 0D, 1D, and 2D manganese halide materials with 3-aminopyridine (3AP) or 4-ethylpyridine (4EtP). Two isomorphic complexes with 3AP and manganese chloride ([MnCl₂(3AP)₄]) or manganese bromide ([MnBr₂(3AP)₄]) were obtained with the amino group in 3AP assisting in the formation of 0D structures via hydrogen bonding. By modifying the reaction conditions, 3AP can also be used to build a 2D coordination polymer with manganese chloride ([MnCl₃3AP]⁻ [3APH]⁺). Unlike 3AP, 4EtP does not provide the opportunity for hydrogen bonding, leading to the formation of two additional isomorphic compounds built of individual 1D chains with manganese chloride ({MnCl₃(4EtP)₂}_n) and manganese bromide ({MnBr₂(4EtP)₂}_n). In the visible region, the 0D and 1D manganese halide compounds have similar photoluminescence properties; however, 0D and 1D have different near-IR emissions. In conclusion, hydrogen-bonding groups can play a role in the formation of discrete manganese-halide units, 1D halide chains, or 2D polymeric sheets.

1. Introduction

Hybrid metal-halide perovskites (MHPs) have recently attracted attention due to their interesting properties that make them candidates for different applications, including, but not limited to, photovoltaics [1,2,3,4], transistors [5,6], light-emitting diodes (LEDs) [7,8], and memory devices [9,10]. These materials mostly contain three main components: a divalent cation, which is frequently lead, a nitrogen-based cation, and halide anions. Since lead is a toxic element, research has been conducted to replace this element with different options [11,12]. In this paper, we use manganese halide compounds to build materials emitting in the near-infrared (NIR) region. Unlike Pb²⁺ and other M²⁺ cations, manganese can be tetracoordinated, pentacoordinated, hexacoordinated, or even, in a few examples, produce higher coordination numbers [13,14,15]. Therefore, predicting the desired product of manganese halides is more challenging than other metal ions. While extensive studies have been performed on tetrahedral 0D manganese halides, in part due to their excellent emission properties [16,17], less studies have been performed on octahedral 1D or 2D materials. Additionally, the dimensionality of manganese halide materials affects the type of emission they produce [16]. For example, molecular 0D tetrahedral complexes emit a yellow or green color, and as the material increases the number of manganese ions coordinated within the same framework [18], a red shift is observed, leading to an NIR emission in 1D materials [19]. In this article, we present the synthesis of hybrid manganese-halide materials comprised of ligands 3-aminopyridine (3AP) or 4-ethyl-pyridine (4EtP) and chloride or bromide. Our results show that a change in the substituents on a pyridine ligand can lead to materials with different dimensionalities. Changing the dimensionality of a manganese halide can be achieved in a variety of ways. First, this can be performed by changing the coordination sphere of manganese, in addition to varying the reaction conditions (pH or solvent) and the ratios of the components [20]. Second, it is also possible to modify or add different functional groups to the organic moiety. Finally, a variation in the synthetic routes used in this work is the coordination of a nitrogen base to a manganese ion. For example, 3-aminopyridine (3AP) can lead to 0D complexes or 2D coordination polymers when used as ligand/or cation, respectively.

2. Results and Discussion

While attempting to construct 1D coordination polymers with 3AP and manganese(II) chloride or manganese(II) bromide, two isomorphic complexes with the triclinic Pī space group (

Table 1. Selected crystallographic data.

Abbreviation	[MnCl2(3AP)4]	[MnBr2(3AP)4]	[MnCl33AP]− [3APH]+	{MnCl2(4EtP)2}n	{MnBr2(4EtP)2}n
Empirical formula	C20H24Cl2MnN8	C20H24Br2MnN8	C44H44Cl12Cu2Mn4N28O2	C14H18Cl2MnN2	C14H18Br2MnN2
Formula weight	502.31	591.23	350.53	340.14	429.06
Temperature (K)	100	100	100	100	100
Crystal system	Triclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic
Space group	Pī	Pī	P21/c	C2/c	C2/c
a (Å)	7.974(5)	8.154(1)	11.749(3)	24.06(3)	23.671(2)
b (Å)	8.823(6)	8.856(1)	10.478(3)	3.875(5)	3.9513(4)
c (Å)	10.072(6)	10.134(1)	14.676(4)	17.91(2)	18.167(2)
α (°)	97.60(2)	98.018(4)	90	90	90
β (°)	91.84(2)	91.186(3)	107.711(7)	114.11(4)	112.722(2)
γ (°)	113.93(2)	114.446(3)	90	90	90
Volume (Å3)	639.0(7)	657.2(2)	1721.1(8)	1524(3)	1567.3(3)
Z	1	1	4	4	4
ρcalc (g/mL)	1.305	1.494	1.353	1.483	1.818
μ (mm−1)	0.748	3.564	1.221	1.204	5.929
F(000)	259	295	708	700	844
Data collection range (°)	4.1 to 53.9	4 to 58	3.6 to 53.5	3.7 to 50.6	3.7 to 51.3
Reflections collected	7961	16035	19781	7526	1483
Independent reflections	2773	3446	3648	1378	1483
Goof on F2	1.086	1.041	1.067	1.050	1.085
R1 [I ≥ 2σ (I)]	0.0243	0.0212	0.0269	0.0292	0.0296
wR2 all reflections	0.0685	0.0539	0.0717	0.0578	0.0685
H-atom treatment	Mixed	Mixed	Mixed	Constrained	Constrained

) were obtained. These complexes had the formula [MnX₂(3AP)₄], and to simplify the description, only [MnCl₂(3AP)₄] was discussed with the respective distances for the bromine analog. In the asymmetric unit, the manganese ion is located in an inversion center and, as a result, the asymmetric unit contains only one halide atom and two 3AP ligands. Both complexes have the same trans-octahedral coordination, with four 3AP ligands coordinated in the equatorial position and two halide atoms coordinated in the axial position (Figure 1a). An overlay of [MnX₂(3AP)₄] showed minimal differences with the rms of 0.0746 (Figure 1b). The coordination is best described as an elongated octahedron because the Mn-Cl and Mn-Br distances (2.561(2) Å and 2.7304(3) Å, respectively) are longer than the average Mn-N distances in both complexes (2.31 Å). It should be noted that three other structures similar to [MnX₂(3AP)₄] were found in the CSD database with MnCl₂ and pyridine [21], MnCl₂ and nicotinic acid [22], and MnBr₂ and 3-cyanopyridine [23]. Additionally, two isomorphs with cadmium [24] ([CdCl₂(3AP)₄]) and cobalt [25] ([CoCl₂(3AP)₄]) were found in the CSD database. Only the pyridine nitrogen atom was coordinated to the manganese ion, while the amino atom assisted crystal packing by hydrogen bonding (Figure 2). The three weak hydrogen bonds observed in both isomorphic structures were longer than the sum of the van der Waals radii of nitrogen and the halide (N-3A…X1, N-3B…X1, and N-4A…X1;

Table 2. Hydrogen bond details of [MnCl₂(3AP)₄] and [MnBr₂(3AP)₄].

D-H⸳⸳⸳A	D-H Distance (Å)	H⸳⸳⸳A Distance (Å)	D-A Distance (Å)	D-H⸳⸳⸳A Angle (°)
[MnII(3AP)4Cl2]
N3-H3A⸳⸳⸳Cl1	0.892(13)	2.639(14)	3.523(3)	171.5(15)
N3-H3B⸳⸳⸳Cl1	0.878(14)	2.522(14)	3.397(2)	175.0(16)
N4-H4A⸳⸳⸳Cl1	0.887(15)	2.549(16)	3.414(2)	165(2)
[MnII(3AP)4Br2]
N3-H3A⸳⸳⸳Br1	0.845(15)	2.803(17)	3.616(2)	162(2)
N3-H3B⸳⸳⸳Br1	0.849(15)	2.661(16)	3.507(2)	175(2)
N4-H4A⸳⸳⸳Br1	0.857(16)	2.672(18)	3.509(2)	166(2)

). These weak hydrogen bonds are the same as those observed in [CdCl₂(3AP)₄] and [CoCl₂(3AP)₄], which exemplify the importance of weak interactions in the crystal lattice. Potentially, another isomorphic structure of [MX₂(3AP)₄] can be obtained with Fe²⁺. One tool that can be used to compare hydrogen bonds from N-H groups is infrared spectroscopy (IR) [26]. Our results show near-identical IR spectra for [MnX₂(3AP)₄] (Figure S1), with the main difference being a shift of symmetric N-H stretching at 3372 cm⁻¹ in the chloride complex as compared to 3384 cm⁻¹ in bromide. In summary, IR indicates the same molecular framework in both structures with slightly different hydrogen bonding results due to the molecular difference between chlorine and bromine.

Using 3AP with the addition of an equimolar amount of hydrochloric acid during the reaction allows the synthesis of a layered 2D structure with the formula [MnCl₃3AP]⁻[3APH]⁺ (Figure 3a). This structure has two independent layers (Figure 3b). The first layer is a 2D coordination polymer with the asymmetric unit containing one manganese(II) ion, one 3AP ligand, and three chlorine ions ([MnCl₃3AP]⁻). The second layer has 3APH⁺ acting only as a cation to balance the overall charge. The coordination sphere around the manganese(II) ion has four chlorine and two nitrogen atoms. The first nitrogen atom comes from the pyridine ring in 3AP (N1), and the second one from an amino group (N2) in a different 3AP molecule (Figure 2a). One of the chlorine ions (Cl2) is duplicated by an inversion center and acts as a bridge to a second manganese ion (Figure 2a). Unlike the 0D unit, 3AP bridges different manganese ions resulting in a 2D coordination polymer. In summary, the 2D layer is constructed by a manganese trichloride dimer that is bridged by 3AP ligands, as it can be seen on top of this layer (Figure S4). A similar coordination sphere was previously discovered with 2-aminopyridine [27], although this compound crystallized in 1D chains. To the best of our knowledge, [MnCl₃3AP]⁻[3APH]⁺ represents the first 2D coordination polymer reported with manganese(II) chloride and a nitrogen-based ligand. Finally, the synthesis of the bromine analog [MnBr₃3AP]⁻[3APH]⁺ was not possible in a number of different reaction conditions.

Surprisingly, using 4-ethylpyridine (4EtP) in the same reaction conditions as 3AP led to the formation of 1D chain structures with the formulas {MnCl₂(4EtP)₂}_n and {MnBr₂(4EtP)₂}_n (Figure 4). These 1D chain structures were isomorphic with the C2/c space group (Table 1). To simplify the description, only {MnCl₂-(4EtP)₂}_n was discussed with the respective distances for the bromine analog. A trans-octahedral coordination around the manganese ion was observed, with two 4EtP ligands coordinated in axial positions, and four halide atoms coordinated in the equatorial plane, each of them bridging a separate manganese ion in an edge-sharing fashion (Figure 4). The coordination polyhedron is best described as a elongated octahedron as the Mn-N distance is shorter than the Mn-X distance. In the asymmetric unit, the manganese ion is located in a C₂ axis that reproduces 4EtP by symmetry. On the other hand, the halide is located near an inversion center, and it is reproduced by the C₂ axis and inversion center. As a result, the asymmetric unit contains three components: one 4EtP ligand, one manganese center, and one halide. Despite the highly symmetrical nature of these 1D chain structures, only three other structures with similar 1D chains were found in the literature: benzotriazole [28], benzo-2,1,3-selenenadiazole [29], and 3-cyanopyridine [23]. As expected, similar IR and Raman spectra were observed in these 1D chains (Figures S2 and S3), indicating the same framework.

In the solid state, the 0D and 1D materials studied had very similar broad photoluminescence (PL) values from 400 to 550 nm (Figure 5a), with the maximum emission at 468 nm, and near-identical peak patterns. Unexpectedly, the 2D material [MnCl₃3AP]⁻[3APH]⁺ lacked any relevant PL when compared to the rest of the materials. The main differences between the 0D and 1D materials were observed in the NIR region. There was an additional emission band at 760 nm observed in the 1D chains that was absent from the 0D units (Figure 5b).

3. Conclusions

Different functional groups on pyridine ligands can be used to construct manganese halide materials with different dimensionalities. In the solid state, 0D or 1D materials with similar PL values were obtained, with key differences in the NIR emission peaks. Hydrogen bonding and changing the ligand ratio, cation, and metal ion are important factors that affect the dimensionality of the materials created. Under similar conditions, the halide atoms, chlorine, or bromine resulted in isomorphic structures, which were primarily supported by weak hydrogen bonds. Further studies will allow us to identify the main factors that affect the solid-state near-IR PL values of these types of materials.

4. Methods and Materials

All samples were phase pure, as shown by the powder X-ray diffraction (PXRD; Figures S5–S8). PXRD was performed using a Rigaku Ultima IV with Cu Kα radiation. Solid-state PL was collected using an RF-5301 PC Spectrofluorometer from Shimadzu. The solid was loaded in a special holder covered by a quartz slide and [MnCl₂(3AP)₄] was analyzed with an excitation wavelength of 300 nm, [MnBr₂(3AP)₄] and {MnCl₂(4EtP)₂}_n with 340 nm, and finally {MnCl₂(4EtP)₂}_n with 350 nm. A thermogravimetric analysis was performed on an STA7200 from Hitachi. IR spectra were collected using Spectrum Two from PerkinElmer. Raman spectra were collected from solid samples using a DXR3 Smart Raman from Thermo Scientific. Solvents were obtained from commercial sources and used without further purification. The following reagents were purchased from commercial sources and used without further purification: manganese(II) chloride tetrahydrate (99%, Apolo Scientific, Chesire, UK), manganese(II) bromide tetrahydride (98%, Thermo Scientific, Ward Hill, USA), 4-ethylpyridine (98%, Sigma-Aldrich, St. Louis, USA), and 3-aminopyridine (99%, TCI America, Portland, USA).

[MnCl₂(3AP)₄]. In a 125 mL Erlenmeyer flask, MnCl₂⸳4H₂O (5.1 mmol, 1.01 g) was dissolved in 15 mL of ethanol using gentle heating, and then allowed to cool to room temperature. In a separate flask, 3-aminopyridine (20.4 mmol, 1.9 g) was also dissolved in 15 mL of ethanol using a gentle heating, and then allowed to cool to room temperature. Then, the 3-aminopyridine solution was added dropwise into the flask containing the solution of MnCl₂. Finally, the solution mixture was capped and left to crystalize for four days, resulting in light-brown crystals that were filtered and rinsed with cold isopropanol. The yield of dry product after filtration was 77%. IR ν (cm⁻¹): 3461(m), 3372(m), 3319(s), 3216(m), 3086(w), 3057(w), 3018(w), 2978(w), 2963(w), 1623(m), 1582(s), 1488(m), 1439(s), 1302(m), 1262(m), 1195(m), 1130(m), 1089(m), 1047(m), 1019(m), 968(w), 912(w), 898(w), 877(w), 849(m), 799(s), 700(s), 639(s), 543(m), 520(m), 487(w), 419(w), and 410(m).

[MnBr₂(3AP)₄]. In a 125 mL Erlenmeyer flask, MnBr₂⸳4H₂O (7.3 mmol, 1.6 g) was dissolved in 15 mL of ethanol using gentle heating, and then allowed to cool to room temperature. In a separate flask, 3-aminopyridine (29 mmol, 2.7 g) was also dissolved in 15 mL of ethanol using gentle heating, and then allowed to cool to room temperature. Then, the 3-aminopyridine solution was added dropwise into the flask containing the solution of MnBr₂. Finally, the solution mixture was capped and left to crystalize for four days, resulting in brown crystals that were filtered and rinsed with cold acetone. The yield of dry product after filtration was 44%. IR ν (cm⁻¹): 3454(w), 3414(w), 3370(w), 3316(s), 3215(m), 3086(w), 3054(w), 2977(w), 2963(w), 1622(m), 1591(w), 1581(s), 1487(s), 1442(s), 1351(w), 1302(m), 1281(w), 1263(m), 1198(m), 1139(m), 1089(m), 1049(m), 1020(m), 968(w), 943(w), 907(m), 878(w), 852(m), 829(w), 798(s), 700(s), 639(s), 543(m), 515(m), and 411(m).

{MnCl₂(4EtP)₂}_n. In a 125 mL Erlenmeyer flask, MnCl₂⸳4H₂O (5.1 mmol, 1.01 g) was dissolved with NaCl (2.6 mmol, 0.15 g) in 20 mL of isopropanol using gentle heating, and then allowed to cool to room temperature. Then, 4-ethylpyradine (10.2 mmol, 1.09 g) was added dropwise via a syringe into the flask containing the solution of MnCl₂. Finally, the solution mixture was capped and left to crystalize for a week, resulting in white crystals that were filtered and rinsed with cold isopropanol. The yield of dry product after filtration was 89%. IR ν (cm⁻¹): 3377(br), 3127(w), 3087(w), 3066(w), 3040(m), 3013(w), 2976(m), 2945(m), 2918(w), 2883(w), 1693(w), 1615(s), 1558(m), 1505(m), 1464(w), 1450(m), 1436(w), 1425(s), 1379(w), 1362(m), 1317(m), 1270(w), 1258(w), 1226(s), 12,059(w), 1108(w), 1079(s), 10,449(w), 1016(s), 994(w), 973(w), 869(m), 822(s), 780(s), 771(w), 721(m), 670(w), 600(w), 562(s), 487(s), and 422(w). Raman ν (cm⁻¹): 78(s), 118(s), 197(m), 239(w), 287(w), 400(w), 492(m), 566(w), 668(m), 784(m), 874(m), 1018(s), 1042(w), 1075(m), 1208(m), 1238(m), 1261(m), 1320(w), 1361(w), 1388(w), 1443(w), 1453(w), 1464(w), 1502(w), 1558(w), 1617(m), 2920(m), 2947(m), 2985(m), and 3070(s).

{MnBr₂(4EtP)₂}_n. In a 125 mL Erlenmeyer flask, MnBr₂⸳4H₂O (5 mmol, 1.06 g) was dissolved with NaBr (1.5 mmol, 0.15 g) in 20 mL of isopropanol by heating and allowed to cool to room temperature. Then, 4-ethylpyradine (10 mmol, 1.06 g) was added dropwise via a syringe into the flask containing the solution of MnBr₂. Finally, the solution mixture was capped and left to crystalize for 5 days, resulting in white crystals that were filtered and rinsed with cold acetone. The yield of dry product after filtration was 57%. IR ν (cm⁻¹): 3432(br), 3128(w), 3104(w), 3086(w), 3065(w), 3038(m), 3012(w), 2975(s), 2938(m), 2914(w), 2876(m), 1696(w), 1614(s), 1556(m), 1504(m), 1448(w), 1425(s), 1374(w), 1361(m), 1315(m), 1266(w), 1227(s), 1156(w), 1117(w), 1074(s), 1045(w), 1015(s), 992(w), 973(w), 953(w), 862(w), 823(s), 782(s), 715(m), 672(w), 639(w), 596(w), 560(m), 485(s), 426(w), and 404(w). Raman ν (cm⁻¹): 78(s), 406(w), 489(w), 562(w), 668(m), 784(m), 979(w), 1017(s), 1044(w), 1072(w), 1202(w), 1237(w), 1262(w), 1361(w), 1436(w), 1503(w), 1557(w), 1618(m), 2908(m), 2941(m), and 3069(s).

[MnCl₃3AP]⁻[3APH]⁺. A sample of 3-aminopyridine (3AP) (20.3 mmol, 1882.2 mg) was dissolved in ethanol and slowly added into a flask containing a solution of MnCl₂.4H₂O (20.3 mmol, 4017.4 mg) and 13M HCl (1.6 mL) in ethanol. The molar ratio of 4AP to HCl to MnCl₂.4H₂O was 1:1:1, respectively. The solution mixture was dried in vacuo resulting in a pink solid that was then crystallized in methanol (using a slow evaporation technique) to obtain pale-pink crystals. After two weeks, the crystals were filtered out. The yield of dry product after filtration was 27%. IR ν (cm⁻¹): 3322(m), 3216(w), 3095(w), 2974(w), 1649(w), 1585(w), 1528(w), 1405(w), 1196(w), 997(w), and 760(w). Raman ν (cm⁻¹): 3098(m), 1659(w), 1532(w), 1238(w), 1047(m), 1000(s), 849(s), 826(w), 645(w), 528(w), and 410(w).