Journal of Chemical Crystallography

, Volume 39, Issue 4, pp 229–240

Classification of Structural Motifs in Porphyrinic Coordination Polymers Assembled from Porphyrin Building Units, 5,10,15,20-Tetrapyridylporphyrin and Its Derivatives

Authors

  • Lucas D. DeVries
    • Department of Chemistry, Nebraska Center for Materials and NanoscienceUniversity of Nebraska-Lincoln
    • Department of Chemistry, Nebraska Center for Materials and NanoscienceUniversity of Nebraska-Lincoln
Review Paper

DOI: 10.1007/s10870-008-9474-z

Cite this article as:
DeVries, L.D. & Choe, W. J Chem Crystallogr (2009) 39: 229. doi:10.1007/s10870-008-9474-z

Abstract

In this review, we classify 1D, 2D, and 3D structural motifs found in porphyrinic coordination polymers assembled from 5,10,15,20-tetrapyridylporphyrin (TPyP) and its derivatives. The classifications are based on dimensionality, metal-to-porphyrin linkage, porphyrin type, and metal-to-porphyrin ratio. 1D porphyrin polymers often share the same connectivity (or structural motifs) with analogous 2D and 3D polymers. We identify interrelationships among 1D, 2D, and 3D coordination polymers and examine the connectivity of such interrelated structures. We also discuss the broad similarities and differences of the synthetic methods of all structures presented here.

Graphical Abstract

We classify 1D, 2D, and 3D structural motifs found in porphyrinic coordination polymers assembled from 5,10,15,20-tetrapyridylporphyrin (TPyP) and its derivatives. The classifications are based on dimensionality, metal-to-porphyrin linkage, porphyrin type, and metal-to-porphyrin ratio. We identify interrelationships among 1D, 2D, and 3D coordination polymers and examine the connectivity of such interrelated structures.
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Keywords

PorphyrinCoordination polymersStructural motifTopological control

Introduction

Topological control of self-assembled coordination polymers is an important theme in materials chemistry, but remains a significant challenge for chemists seeking to synthesize new materials with tailored properties [13]. Although a priori predictions of solid state organization remain difficult, it is possible to generate a wide variety of self-assembled coordination polymers by the careful selection of organic and inorganic components [3, 4]. For example, the ditopic ligand (i.e., a ligand with two coordination sites) 4,4′-bipyridine shows various structural motifs when combined with transition metals, including 1D ladder, 1D chain, 1D railroad, 2D square grid, 2D hexagonal grid, 2D double-layer, and 3D undulated grid [58]. Switching to a tritopic ligand (with three coordination sites) such as 1,3,5-tris[4-pyridyl(ethenyl)]benzene or 2,4,6-tris(4-pyridyl)1,3,5-trazine has produced various 3D nets [4].

Compared with ditopic and tritopic ligands, tetratopic ligands (with four coordination sites) are relatively rarely used as molecular building blocks. Among the available tetratopic ligands [911], we and others are particularly interested in the tetraarylporphyrins and their use in coordination polymers [1118]. Porphyrins are important materials because of their exceptional photochemical and catalytic properties, which enable their use in sensors, catalysts, and other applications [12]. These macrocycles often possess approximate D4h symmetry, a point group rarely encountered in organic chemistry. Therefore, such porphyrins are exotic ligands for building frameworks with unprecedented topology, which might prove inaccessible using other types of ligands with different connectivity and symmetry [1118].

During the past decade, numerous porphyrinic coordination polymer have been reported [1118]; however, the field of porphyrinic coordination polymers is still in its infancy compared with the rich porphyrin chemistry developed in recent decades. In this review, we focus on coordination polymers constructed from the pyridyl-based porphyrins, specifically 5,10,15,20-tetrapyridylporphyrin (H2TPyP) and its less symmetrical derivatives 5-pyridyl-10,15,20-triphenylporphyrin (H2MPyP), 5,10-dipyridyl-15,20-diphenylporphyrin (cis-H2DPyP), and 5,15-dipyridyl-10,20-diphenylporphyrin (trans-H2DPyP), and their respective metalloporphyrins (Fig. 1).
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Fig. 1

Four pyridyl-based porphyrin derivatives used as building units in the coordination polymers discussed in this review

Based on 44 analyzed porphyrin structures, we construct 17 classifications according to dimensionality, connectivity, and topology. We also discuss the structural trends found in porphyrinic coordination polymers and their interrelationships. Table 1 summarizes the compounds analyzed in this review, in which the structural reference number used for each structure is listed next to the stoichiometric formula. For previous reviews of porphyrin coordination solids [1317] and porphyrin supermolecules [1921], see the works of Suslick [13, 14], Goldberg [15, 16], Proserpio [17], Kobuke [19], Alessio [20], and Hupp [21].
Table 1

Pyridyl-based porphyrinic coordination polymers

Refcode

Composition

Structural motif

Ref.

BAKJOM

[(HgI2)2(H2TPyP)] · 2(TCE), 1

1D tape

[22]

BAKNOQ

[(HgI2)2(Zn0.5 TPyP)] · 4(TCE), 2

1D tape

[22]

BAKKED

[(HgI2)2(Zn0.3 TPyP)] · 4(TCE)

1D tape

[22]

BAKPOS

[(HgBr2)2(H2TPyP)] · 6(TCE)

1D tape

[22]

BAKREK

[(HgI2)2(ZnTPyP)] · 4(TCE)

1D tape

[22]

QODHEW

[(HgI2)2(H2TPyP)] · 2(TCE)

1D tape

[23]

QODHEW1

[(HgI2)2(H2TPyP)] · 2(TCE)

1D tape

[23]

UFOMEH

[(HgI2)2(H2TPyP)] · C70(TCE) · (MeOH), 3

1D tape

[24]

HAZSOQ

[(HgBr2)2(H2TPyP)] · 2(CHCl3)

1D tape

[25]

TUSHAQ

[Ag(H2TPyP)] · (NO3), 4

1D rod

[26]

QARNOM

[Ag(H2TPyP)](PF6) · 1.5(TCE) · (MeOH) · (H2O)

1D rod

[27]

GETLAS

[(ZnMPyP)], 5

1D single zig-zag chain

[28]

NAZZUK

[(cis-ZnDPyP)] · (CH3OH), 6

1D single zig-zag chain

[29]

YOVTEI

[(ZnTPyP)] · (C6H7 N)

1D single zig-zag chain

[30]

YOVTAE

[(ZnTPyP)], 7

1D single zig-zag chain

[30]

SAWJIK

[(ZnTPyP)] · 1.33(CHCl3), 8

1D double zig-zag chain

[31]

BIJSAP

[(trans-CoDPyP)3] · 4(C3H7NO), 9

1D double zig-zag chain

[32]

BABCEN

[Zn2(TPyP)2] · 5(C6H5NO2), 10

1D ladder

[33]

IGOHOB

[FeTPyP], 11

2D grid

[34]

IGOHOB01

[FeTPyP]

2D grid

[34]

BAKKIH

[(PbI2)(H2TPyP)] · 4(TCE), 12a

2D sheet-I

[22]

BAKNEG

[(CdI2)(H2TPyP)] · 4(TCE)

2D sheet-I

[22]

UFOMAD

[Pb(NO3)2 · (H2TPyP)] · C60 · 6(TCE), 12b

2D sheet-I

[24]

FEWGEU

[Ag2(H2TPyP)] · (m-C6H4NH2Cl)4 · 2(C7H7O3S), 13

2D sheet-II

[17]

FEWGOE

[Ag2(ZnTPyP)] · (CH3C6H5SO3)2 · (DMA)], 14

2D sheet-II

[17]

TUSGIX

[Ag4(H2TPyP)3] · 4(NO3), 15

2D sheet-III

[26]

FEWGAQ

[Ag2(H2TPyP)] · 2(CF3SO3), 16

2D undulated tape

[17]

FEWGIY

[Ag2(ZnTPyP)2] · (m-C6H4NH2) · 2(CF3SO3), 17

2D hybrid net

[17]

PIZJEN

[Cu(CuTPyP)] · (BF4), 18

3D net-I (pts)

[18]

YOVTOS

[(ZnTPyP)] · 3(H2O), 19

3D net-II (nbo)

[30]

YOVTIM

[(ZnTPyP)] · (MeOH) · 2(H2O)

3D net-II (nbo)

[30]

SAZQEQ

[(ZnTPyP)] · 1.33(CHCl3)

3D net-II (nbo)

[31]

CAYSOK

[(MnTPyP)] · 10(H2O)

3D net-II (nbo)

[36]

CAYSIE

[(CoTPyP)] · 2(CH3COOH) · 2(H2O)

3D net-II (nbo)

[36]

CAYYEG

[(MnTPyP)] · 2(EtOH) · 4(H2O)

3D net-II (nbo)

[36]

QANSEE

[(ZnTPyP)] · 1.6(C2H4O2)

3D net-II (nbo)

[37]

CATDEH

[(trans-ZnDPyP)] · 0.33(MeOH)

3D net-II (nbo)

[38]

CATDIL

[(trans-ZnDPyP)] · 0.33(C6H12)

3D net-II (nbo)

[38]

CAYRIE

[(trans-ZnDPyP)] · 0.33(C2H6O)

3D net-II (nbo)

[38]

CAYROK

[(trans-ZnDPyP)]

3D net-II (nbo)

[38]

CAZGOZ

[Fe(FeTPyP)3] · 2(Mo6O19) · 38(H2O), 20

3D net-III (pcu)

[39]

TUSGUJ

[Ag8(ZnTPyP)7] · 8(NO3), 21

3D net-IV

[26]

TUSGOD

[Ag2(H2TPyP)] · (NO3), 22

3D net-V

[26]

SOBTUY

[Cd2(PdTPyP)] · 2(NO3) · (py) · 8.6(H2O), 23

3D net-VI

[11]

TCE tetrachloroethane, DMA N,N′-dimethylacetamide, Py pyridine

Classification

A classification system is implemented (Table 2) to better understand the coordination polymers reviewed in this study. Porphyrinic coordination polymers are first divided into three broad categories based on dimensionality. Structural motifs are then classified by examining three main parameters: (1) geometry of the metal-to-porphyrin linkage, (2) porphyrin type, and (3) porphyrin-to-metal ratio.
Table 2

Classification of porphyrinic coordination polymers

Classification

Metal nodea

Porphyrin nodeb

M:Pc

Porphyrin unitsd

# Netse

1D tape

2 (bent)

0

2:1

H2TPyP, MTPyP

9

1D rod

2 (linear)

0

1:1

H2TPyP

2

1D single zig-zag chain

0

1

1:1

MPyP, cis-DPyP, H2TPyP

4

1D double zig-zag chain

0

1, 2

1:1

trans-DPyP, H2TPyP

2

1D ladder

0

1, 2

1:1

H2TPyP

1

2D grid

0

2

1:1

H2TPyP

2

2D sheet-I

4 (square planar)

0

1:1

H2TPyP, MTPyP

3

2D sheet-II

2 (linear)

0

2:1

H2TPyP, MTPyP

2

2D sheet-III

3 (T-shape)

0

4:3

H2TPyP

1

2D undulated tape

2 (bent)

0

2:1

H2TPyP

1

2D hybrid net

2 (bent), 4 (seesaw)

1

2:1

ZnTPyP

1

3D net-I (pts)

4 (tetrahedral)

0

1:1

CuTPyP

1

3D net-II (nbo)

0

2

1:1

MTPyP, trans-MDPyP

11

3D net-III (pcu)

6 (octahedral)

2

4:3

FeTPyP

1

3D-net-IV

3 (T-shape)

1, 0

12:7

ZnTPyP

1

3D net-V

2 (linear)

0

2:1

H2TPyP

1

3D net-VI

2 (bent), 2 (linear)

0

2:1

H2TPyP

1

aNumber of connections from metal to pyridyl-based porphyrin, and their geometry

bNumber of axial connections in porphyrins (coordinating anions and solvents are not considered)

cMetal-to-porphyrin ratio (only metals coordinated to porphyrins are considered)

dType of porphyrin that formed the specified structure

eNumber of coordination polymers for each classification. The three-letter net classifications shown here, pts, nbo, and pcu, are PtS, NbO, and primitive cubic nets, respectively

Table 2 details all structural classifications used in this review, as well as the four parameters used to differentiate the coordination polymers into 17 different classes. The Metal node column in Table 2 describes the coordination number, and the geometry around the metal. For example, in the “1D tape” row, the coordination geometry around the metal node for 1 is actually tetrahedral (because of the attached iodide ions); however, it only links two porphyrins, and therefore it is recorded as a 2 (bent) geometry. The Porphyrin node column lists the number of axial connections in metallated porphyrins. Coordinating solvent molecules or anions are not considered here. The metal-to-porphyrin ratio plays a critical role in determining the type of structure formed. Metals in the center of a porphyrin are considered in this ratio only when the adjacent porphyrins are connected to these metal centers.

1D Porphyrin Coordination Arrays

1D Tapes (1,2,3)

The 1D tape motif resembles a length of “tape” extending infinitely in one dimension. The tape consists of porphyrins linked together by metal nodes. In the case of 1 (Fig. 2a), the porphyrins are of the H2TPyP type, and the metal nodes are mercury atoms with iodide ions attached at tetrahedral positions. The resulting 1D planar array can be partially metallated to produce 2 (Fig. 2b). The general formula of this 1D tape series is [(HgX2)2MyTPyP] · n(TCE), where X = Br, I; M = Zn, or H2; y = 1, 0.5; and n = 2,4,6 (TCE = tetrachloroethane) [22, 23]. These 1D tape structures can hold various amounts of TCE molecules between the tapes [22, 23], and even accommodate C70 molecules, as evident in 3 (Fig. 2c) [24]. Pan et al. reported an identical 1D tape motif using CHCl3 as a solvent [25]. The 1D tape motif is interesting because it is observed in multiple 2D variants [17, 22].
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Fig. 2

Variations of the 1D tape structural motif in a1, [(HgI2)2(H2TPyP)] · 2(TCE), b2, [(HgI2)2(Zn0.5TPyP)] · 4(TCE), and c3, [(HgI2)2(H2TPyP)] · C70(TCE) · (MeOH)

1D Rod (4)

The 1D rod motif is so named because a single “rod” of metal nodes can be visualized as the backbone of the porphyrin structure (Fig. 3). In 4, only two pyridyl arms of each H2TPyP molecule are coordinated to a silver metal node [26]. The uncoordinated nature of the “extra” pyridyl arms is puzzling until the synthesis conditions are examined. Given that the silver-to-porphyrin ratio is 1:1, there exist insufficient silver atoms to fully coordinate all of the pyridyl arms [27]. Given sufficient silver atoms, a 2D structure would be possible. This type of porphyrin rod formation is unusual, and has only been demonstrated by porphyrin structures containing silver salts, such as Ag(NO3) [26] and AgPF6 [27].
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Fig. 3

1D rod in 4, [Ag(H2TPyP)](NO3)

1D Single Zig-Zag Chains (5,6,7)

The 1D single zig-zag chain 5 is a common structural motif found in 1D porphyrin coordination solids (Fig. 4) [28]. This motif requires a metallated porphyrin because the only connection point is the metal node in the center of the porphyrin. It is also noteworthy that Zn atoms in these porphyrins are all five-coordinated. Zinc-metallated MPyPs are linked through their zinc metal centers to one of the pyridyl arms of a neighboring ZnMPyPs [28], as illustrated in 5. The resulting geometry is the “zig-zag” pattern. Other pyridyl-based porphyrins such as cis-ZnDPyP (6) and ZnTPyP (7) also form similar zig-zag patterns [29, 30], despite additional uncoordinated pyridyl arms. A plausible explanation for this is the competition for pyridyl coordination sites from solvent molecules hydrogen-bonded to the vacant pyridyl arms [29].
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Fig. 4

1D single zig-zag chain in 7, [(ZnTPyP)]

1D Double Zig-Zag Chains (8,9)

The 1D double zig-zag chain 8 is closely related to the 1D zig-zag chains described above (Fig. 5a) [31]. A metallated porphyrin is again needed to provide a metal node; however, in this topology there are fewer uncoordinated pyridyl arms. Unlike the single chain, in this double chain motif there are two distinct metalloporphyrin units: five- and six-coordinate zinc (II) ions in a 2:1 ratio (Fig. 5a). As shown in Fig. 5b, Pan et al. reported the identical 1D motif of 9, but the two structures display a notable difference in stability [31, 32]. The structure 8 is highly sensitive to solvent loss, and rapidly becomes amorphous upon solvent removal [31]. In contrast, the structure 9 retains its crystallinity in the absence of solvent molecules [32]. This is a good example of the same topology displaying significantly different physical properties.
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Fig. 5

1D double zig-zag chains in a8, [(ZnTPyP)] · 1.33(CHCl3), and b9, [(trans-CoDPyP)3] · 4(C3H7NO)

1D Ladder (10)

The 1D ladder structure of 10 is illustrated in Fig. 6 [33]. The ladder configuration is related to both the 1D single zig-zag chain and the 1D double zig-zag chain. A major difference between this ladder and the 1D double zig-zag chain is the ratio between five- and six-coordinate ZnTPyP, being 2:1 for 1D double zig-zag chain and 1:1 for 1D ladder. Therefore, this ratio is an important variable in controlling the topology of 1D porphyrin arrays. The formation of this 1D ladder rather than a 1D single zig-zag chain or a 1D double zig-zag chain is possibly influenced by the use of nitrobenzene as a solvent [33]. Diskin-Posner et al. suggested that the rigidity and polarity of nitrobenzene are influential in the formation of 10 [33].
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Fig. 6

1D ladder motif in 10, [Zn2(TPyP)2] · 5(C6H5NO2)

2D Porphyrinic Coordination Polymers

2D Grid (11)

The 2D grid of 11 is shown in Fig. 7 [34]. An individual FeTPyP porphyrin is connected to four neighboring ones: two through its metal center and the other two through its pyridyl arms. This coordination mode leaves two pyridyl arms uncoordinated. The resulting 2D grid pattern shows two different stacking sequences: AB or ABCD. This 2D structure is exceptionally robust and thermally stable up to 550 °C under nitrogen [34]. In this motif, all the Fe(II) ions inside the porphyrin ring are octahedrally coordinated. With a changing ratio between the five- and six-coordination metal centers inside the porphyrin from 100% to 67%, 50%, and finally 0%, the respective structural motifs vary from a 1D single zig-zag chain to a 1D double zig-zag chain, 1D ladder, and finally a 2D grid, respectively. With one exception, the synthesis of 11 is significantly different from the synthesis methods used for the 1D single zig-zag chain, 1D double zig-zag chain, and 1D ladder [2834]. The compound 11 was synthesized under a solvothermal condition, and the 1D double zig-zag chain compound 9 uses cobalt in dimethylformamide (DMF) and a similar solvothermal process to that for 11 [32, 34]. The similarities do not end at the synthesis techniques, because iron and cobalt ions both prefer octahedral coordination.
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Fig. 7

2D grid-I motif in 11, (FeTPyP)

2D Sheets, I (12), II (13, 14), and III (15)

2D sheet-I is a variation of the 1D tape motif described earlier (Fig. 8a, b). The tape motif extends in two perpendicular directions, while the metal node changes from tetragonal HgI2 to octahedral PbI2 or CdI2 [22]. The connectivity of the metal node to the porphyrin sheet is square planar, as seen in 12a and 12b [22, 24]. Structure 12a consists of H2TPyP units linked by PbI2 (see Fig. 8a), connecting four neighboring H2TPyP units [22]. This 2D sheet-I pattern (12b) can accommodate C60 molecules in the space between the 2D sheets, as illustrated in Fig. 8b [24]. The 2D sheet-II pattern of 13 (Fig. 9a) is a variation of the previously discussed 1D rod, with a greater stoichiometric ratio of silver atoms to porphyrin units: 2:1 versus 1:1 for the 1D rod [17, 26]. A metallated version of this 2D sheet-II 14 (see Fig. 9b) has also been reported [17]. Recently, Ohmura et al. reported a similar 2D sheet pattern using CuTPyP units and Cu paddle-wheel clusters [35]. The 2D sheet-III shown by 15 (Fig. 10) also uses silver nodes to connect the porphyrin units in a T-shaped geometry [26]. The metal-to-porphyrin ratio is again a major variable in forming a structure that is topologically different from 4, 13, and 14. As stated above, the 2D sheet-II (13) has a silver-to-porphyrin ratio of 2:1; however, this ratio is 4:3 in 15 [17, 26]. Other than this ratio, the synthetic conditions are essentially identical [17, 26]. The effective coordination number of the metal connector plays an important role in the 2D sheets shown here: the 2D sheet pattern varies as the effective coordination number changes from 4 (2D sheet-I) to 3 (2D sheet-III) and to 2 (2D sheet-II).
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Fig. 8

a 2D sheet-I in 12a, [(PbI2)(H2TPyP)] · 4(TCE), b 2D sheet-I with C60 molecules found in 12b, [(Pb(N2O6)(H2TPyP)] · C60 · 6(TCE)

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Fig. 9

a 2D sheet-II in 13, [Ag2(H2TPyP)] · (m-C6H4NH2Cl)4 · 2(C7H7SO3), b 2D sheet-II with ZnTPyP, 14, [Ag2(ZnTPyP)] · (CH3C6H5SO3)2 · (DMA)

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Fig. 10

2D sheet-III in 15, [Ag4(H2TPyP)3] · 4(NO3)

2D Undulated Tape (16)

The 2D undulated tape 16 in Fig. 11 possesses the same connectivity as 13, but has a significantly different topology [17]. It also has the same silver-to-porphyrin ratio as 13 (2:1) [17]. The solvents used in each synthesis are possibly a factor in differentiating these two structures. For 16, two m-chloroaniline molecules are coordinated to each silver atom along with two pyridyl arms [17]. These solvent molecules may block the pyridyl arms and inhibit the formation of a possible competing phase 13. Structure 16 can also be viewed as a 2D variation of the 1D tape motif [17]. Similarly, the metal node connection geometry of 16 is similar to that of 1D tape (1); however, instead of a planar array, the metal nodes connect to porphyrins above and below the plane of the 1D tape.
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Fig. 11

2D undulated tape motif in 16, [Ag2(H2TPyP)] · 2(CF3SO3)

2D Hybrid Net (17)

The 2D hybrid net of 17 shows the combination of a 1D tape layer and a 1D single zig-zag chain layer (Fig. 12a, b) [17]. As seen in Fig. 12a, 17 can be visualized as a 1D single zig-zag chain layer sandwiched between two 1D tape layers. The structure propagates in a “stair-step” pattern, with the 1D tape motif as the base and the 1D zig-zag layers as the upright (Fig. 13a, b). Another remarkable feature of this structure is that it contains three different coordination environments and two different metal nodes [17]. The two metal nodes are the zinc atoms in the center of the porphyrins and the silver atoms that make up the connectivity of the 1D tape layers. All of the porphyrins (i.e., those in the 1D single zig-zag chain layer and those in the 1D tape layer) are metallated with zinc. These zinc atoms in the porphyrin are all five-coordinate and are axially connected to the pyridyl arm of a neighboring porphyrin. The silver nodes display two different coordination geometries, depending on their position in the structure. The silver nodes at the junction of each “stair-step” section are four-connected (see Fig. 13a), while those at the non-shared edges of the “stairs” are two-connected (Fig. 13b). Finally, 17 is notable because it is a bimetallic framework using porphyrin units.
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Fig. 12

a Visualization of 1D tape and 1D single zig-zag motifs present in 17. b Assembled “stair-step” 2D hybrid net in 17, [Ag2(ZnTPyP)2] · (m-C6H4NH2) · 2(CF3SO3)

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Fig. 13

a Alternate view of the propagation of 17, emphasizing the four-coordinate silver connections (orange arrows) and showing the “stair” units of Fig. 12 in a different view. b Assembled view showing the four-coordinate silver atoms connected and the two-coordinate silver connections (green arrows)

3D Porphyrin Coordination Frameworks

3D Net-I (pts, 18)

Figure 14 shows the pts (or PtS structure type) net 18, as synthesized by Robson and coworkers [18]. In this structure, pyridyl groups of CuTPyP molecules coordinate to Cu(I) ions, forming a tetrahedral geometry. This structure contains an equal number of tetrahedral copper atoms and square planar CuTPyP. These two SBUs are connected alternatively and form an open channel structure of a pts net [4], demonstrating that ca. 70% of the cell volume is void [18]; however, this framework is not robust and the channels collapse upon the loss of solvent molecules [18].
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Fig. 14

3D net-I (pts) in 18, [Cu(CuTPyP)] · (BF4)

3D Net-II (nbo, 19)

The 3D nbo (NbO structure type) net of 19 was first reported by Goldberg [30], and the same pattern has been studied extensively [30, 31, 3638]. As seen in Fig. 15, this net is significantly distorted from its ideal nbo topology [4], creating large, open hexagonal channels. This structure can accommodate various solvent molecules (e.g., water, methanol, acetic acid, and cyclohexane) inside the hexagonal channel [30, 31, 3638]. Using trans-DPyP molecular building units, Hosseini and coworkers reported a single-crystal-to-single-crystal transformation upon solvent exchange [37], thereby demonstrating the robustness of this framework.
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Fig. 15

3D net-II (nbo) in 19, [(ZnTPyP)] · 3(H2O)

3D Net-III (pcu, 20)

The 3D net-III structure 20 contains two different six-coordinate Fe centers: the Fe center in the FeTPyP unit and the Fe node joining six surrounding porphyrins [39]. This forms a primitive cubic (pcu) structure (Fig. 16a) that shares the octahedral connectivity of the 2D grid (11) shown earlier in Fig. 7; however, the 2D “layers” of the 3D net-III consist of FeTPyP units and iron nodes, whereas 11 only contains FeTPyP units [34, 39]. The connectivity of structure 20 can be viewed as a single 2D pattern translated by (½ ½ ½) to form the observed AB packing (Fig. 16b). In this example, the 2D layers are essentially identical: only the orientation of the layers changes (Fig. 16c). The syntheses of 11 and 20 both use the solvothermal method and metallate the porphyrin in situ [34, 39]; however, this is where the similarities end. The 2D structure (11) uses DMF as a solvent, whereas the 3D structure (20) uses water [34, 39]. The metal sources also differ: ferrocene for 11 and FeCl2 · 4(H2O) for 20 [34, 39]. Perhaps the most significant difference is the addition of MoO3 to the synthesis of 20 [39], which is noteworthy because hexamolybdate [Mo6O19]2− clusters are formed during synthesis, and the structure becomes cationic [Fe8(TPyP)6]8+ [39]. The clusters are centered in some of the large voids of the 3D FeTPyP structure; the remaining adjacent voids contain water molecules [39]. These water-filled voids are arranged in an octahedral orientation around each hexamolybdate-filled void [39]. The presence of the hexamolybdate ion prevents any potential interpenetration, which is otherwise common in frameworks with such large voids [4].
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Fig. 16

a 3D net-III (pcu) in 20, [Fe(FeTPyP)3] · 2(Mo6O19) · 38(H2O). b View of the 2D layers related by a (½ ½ ½) translation to form the AB packing of 20. The structure has been separated into individual layers to emphasize the (½ ½ ½) translation. c “A” and “B” layers in 20

3D Net-IV (21)

The 3D net-IV (21) is shown in Fig. 17 [26]. The vertical (colored red in the figure) and horizontal (blue) layers connect through the axial coordination of the ZnTPyP molecules of each layer to a “rod” (green) of porphyrins running down the center of the channel. The vertical and horizontal layers are analogous to a zinc metallated version of the 2D sheet-III (15) mentioned previously (Fig. 18), and the “rod” is made of stacked ZnTPyP molecules. In addition, the vertical and horizontal 2D layers are interpenetrated through the large void space of the other 2D layers (Fig. 19) [26]. The red section in Fig. 18 corresponds to both the vertical red layers in Fig. 17 and the vertical red layers in Fig. 19. This interpenetration is not limited to one set of vertical and horizontal layers: a second set of parallel 2D layers runs along each vertical and horizontal sheet, also interpenetrating the other 2D layers (Fig. 19). This second set of 2D layers is connected to the neighboring porphyrin “rods,” and each 2D layer is connected to three other layers, as shown in Fig. 20 [26]. It should also be noted that not all of the ZnTPyP porphyrins are structurally coordinated. The stacked ZnTPyP porphyrins that make up each “rod” are not coordinated, and neither are the porphyrins located where the 2D sheets intersect (see the central part of Fig. 20).
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Fig. 17

3D net-IV in 21, [Ag8(ZnTPyP)7] · 8(NO3)

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Fig. 18

2D layer of 21, a variation of 2D Sheet-III 15. The highlighted red section is a “front” view of all of the red layers present in Figs. 17, 19, and 20

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Fig. 19

Interpenetration of the 2D layers of 21 through the large voids in each layer

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Fig. 20

Alternate view of 21, showing the intersection of the 2D layers at the voids where they interpenetrate, and the pattern in which the porphyrin “rods” (green) connect to the 2D layers (red and blue)

The synthesis conditions of 15 and 21 are remarkably similar, differing in only two respects. The two structures were synthesized using the solvent layering method [26]. They share the same solvent metal layer (AgNO3 dissolved in dimethylamine), and the respective porphyrins used for each structure were dissolved in a 3:1 mixture of TCE and MeOH [26]. The differences that separate 15 and 21 are the temperature of synthesis (−20 °C for 21 versus room temperature for 15) and the use of freebase H2TPyP or ZnTPyP [26]. The framework pattern of 21 would be impossible without metallated porphyrins to connect the 2D layers.

3D Net-V (22)

This 3D structure (22) is a 3D variation of the 2D sheet-II [26]. The main difference is that three coordinate silver atoms are used for the junctions instead of the two coordinate silver atoms in the 2D sheets. The layers are joined by the NO3 ions to form the 3D structure of 22 (Fig. 21).
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Fig. 21

3D net-V in 22, [Ag2(H2TPyP)] · (NO3)

3D Net-VI (23)

Robson and coworkers synthesized 23 [11]. In this structure, Cd atoms are used as two different types of connectors: linear (N–Cd–N, 180°) and bent L-shaped connectors (N–Cd–N, 103°). The porphyrin linkers (PdTPyPs) are joined by these Cd atoms to form 3D frameworks in which the porphyrin molecules are all stacked along the [001] direction (Fig. 22).
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Fig. 22

3D net-VI in 23, [Cd2(PdTPyP)] · 2(NO3) · (py) · 8.6(H2O)

Synthetic Techniques

This study has considered a total of 44 porphyrinic coordination polymers with various topologies, yet the synthetic methods employed in making these structures are not as varied as might be expected. Upon examination, three main synthetic methods are used to synthesize the structures detailed in this paper: solvent layering, evaporation, and the solvothermal method. The solvent layering method involves the addition of starting materials to two or more solvents. The solvents are layered on top of each other to either reduce the solubility of the product in a single layer to promote crystallization, or to slow diffusion between starting materials. The choice of the two (or more) solvents is important (based on polarity, viscosity, solubility, etc.). The 2D sheet-I in Fig. 7b was made by layering the metal source, dissolved in one solvent, over C60 and H2TPyP dissolved in another solvent [24]. The evaporation method involves simply allowing the reaction mixture to sit undisturbed for an extended period (days to weeks) while the solvent evaporates. The 1D ladder in Fig. 5 was formed using the evaporation method [33]. The solvothermal method uses a pressure vessel to contain the reaction mixture while it is heated, under autogeneous pressure, to a certain temperature (usually greater than 100 °C) for a set time. The thermal ramping, reaction temperature, and duration can all be altered. The 2D grid in Fig. 6 was the product of solvothermal reaction between ferrocene and H2TPyP at 150 °C for 5 days in a Teflon-lined autoclave [34].

Concluding Remarks

Numerous structural motifs are self-assembled from pyridyl-based porphyrin building units. Due to the intrinsic properties of porphyrin, these multi-dimensional porphyrinic coordination polymers and/or subsequent solids have potential as useful materials for practical applications in catalytic chemistry, optoelectronics, gas storage, and molecular magnets [12]. Topological control is important in providing the properties required for each of these applications. It is therefore essential to understand the underlying building principle of these porphyrin coordination solids. The metal coordination, porphyrin type, and metal-to-porphyrins ratio can all be used to produce varied structural motifs. A solid working knowledge of the effects of changing any one (or more) parameter(s) is the key to producing a coordination polymer with the desired topology. Many structures discussed in this paper can be thought of as being made of two distinct units: metalloligands and metal nodes. Kitagawa demonstrated that the physical properties of materials using metalloligands can be altered by using a different organic linker [40]. If a correlation could be found between the properties and structure of such metalloligand-based frameworks, a material with tailored properties could be produced by combining different structural motifs. A structural understanding can be further utilized to engineer new types of functional 3D porous porphyrin frameworks.

Acknowledgements

The authors gratefully acknowledge financial support from the University of Nebraska-Lincoln.

Copyright information

© Springer Science+Business Media, LLC 2008