(IV) Salts - ACS Publications - American Chemical Society

DOI: 10.1021/cg100325c

Pyridineboronic Acids as Useful Building Blocks in Combination with Perchloroplatinate(II) and -(IV) Salts: 1D, 2D, and 3D Hydrogen-Bonded Networks Containing X-H 3 3 3 Cl2Pt- (X = C, Nþ), B(OH)2 3 3 3 Cl2Pt-, and B(OH)2 3 3 3 (HO)2B Synthons

2010, Vol. 10 3182–3190

Jose J. Campos-Gaxiola,† Araceli Vega-Paz,† Perla Rom an-Bravo,† Herbert H€ opfl,*,† and Mario Sanchez-V azquez‡ †

Centro de Investigaciones Quı´micas Universidad Aut onoma del Estado de Morelos Av. Universidad 1001, on en Materiales Avanzados, S.C. Alianza C.P. 62209 Cuernavaca, M exico, and ‡Centro de Investigaci Norte 202, PIIT, Carretera Monterrey-Aeropuerto Km. 10, C.P. 66600 Apodaca NL, M exico Received March 12, 2010; Revised Manuscript Received June 2, 2010

ABSTRACT: Five metal complexes have been prepared by reacting 3- and 4-pyridineboronic acid (3- and 4-pba) with potassium tetrachloroplatinate (K2PtCl4) and hexachloroplatinic(IV) acid hydrate (H2PtCl6 3 aq): [4-pbaH]2[PtCl4], [3-pbaH]2[PtCl4], [4-pbaH][Pt(4-pba)Cl3], cis-[Pt(4-pba)2Cl2] 3 H2O, and cis-[PtIV(3-pba)2Cl4] 3 2H2O. All compounds have been characterized by single-crystal X-ray diffraction analysis, showing that the primary hydrogen bonding interactions of the resulting 1D, 2D, and 3D networks contain at least one of the following synthons: X-H 3 3 3 Cl2Pt- (X = C, Nþ), B(OH)2 3 3 3 Cl2Pt-, and B(OH)2 3 3 3 (HO)2B. The dimensions are enhanced further by secondary þN-H 3 3 3 ClPt, O-H 3 3 3 O, and O-H 3 3 3 ClPt hydrogen bonding interactions between donor and acceptor atoms located at the periphery of these synthons. Additional weak C-H 3 3 3 O, C-H 3 3 3 Cl, B 3 3 3 N, and π 3 3 3 π stacking interactions stabilize the crystal structures further.

1. Introduction During the past few years there has been an increasing interest in the controlled formation of high-dimensional molecular networks, and the research activities in this field have shown that 2D and 3D frameworks can be formed through (i) coordinate covalent bonds,1 (ii) hydrogen bonding interactions,1i,2 and (iii) covalent bonds.3 In the most straightforward synthetic approaches, either oligofunctional molecular building blocks are combined between each other or metal ions are reacted with di- or oligofunctional organic ligands. Depending on the nature of the principal bonding interactions between the building blocks, the resulting materials are denominated metal-organic frameworks (MOFs),1d covalent organic frameworks (COFs),3 or hydrogen-bonded organic frameworks (HOFs).4 Aside from oligofunctional organic ligands, hydrogen-bonded networks can also be created from metal complexes with ligands that act as hydrogen bonding donors or acceptors.5 An almost unemployed strategy consists in the use of building blocks with hydrogen bonding functions that form synthons capable of forming additional hydrogen bonds in the periphery of the previously created supermolecules.6 In analogy to di- and oligonuclear metal complexes used for the generation of highly organized MOFs, which are known as secondary building units (SBUs),7 such supermolecules might be considered as hydrogenbonded secondary building units (HSBUs). The reason for the limited exploration of this strategic principle can be attributed to the reduced number of hydrogen bonding functions available for this purpose, since control over the formation of both the primary and secondary synthons is required. Based on this principle, a reliable crystal engineering strategy for the formation of 2D and 3D hydrogen-bonded architectures would consist in *To whom correspondence should be addressed. Fax: (þ52) 77-73-29-79-97. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 06/18/2010

the controlled cross-linkage of previously formed low-dimensional aggregates to higher dimension through a second set of interactions. One advantage of this association process is an enhanced variability in the resulting products; however, it is indispensable that further weak noncovalent interactions are significantly less-dominant than the aforementioned primary and secondary interactions. To guarantee directionality in the crystal engineering process, double- or higher-bridged functions are required. One of the most common synthons in hydrogen-bonded architectures is the carboxylic acid dimer I (Scheme 1).8 However, this synthon lacks additional hydrogen atoms required for a selfcomplementary association to higher dimensions. Two synthons which fulfill this criterion are the carboxamide and the boronic acid dimers II and III, both of which have been employed for the purposes of crystal engineering.6,9,10 Aside from the homodimeric boronic acid synthon III, heterodimeric motifs are also known (IV-VII).11,12 One important difference between the homodimeric motifs II and III is that in the carboxamide dimer the hydrogen atoms are distributed in an asymmetric manner between the donor and acceptor functions. In the -B(OH)2 group the distribution is symmetric, which allows for the formation of additional synthons such as VIII and those including charge-assisted hydrogen bonding interactions (IX-XI, Scheme 1).11a,d,13,14 A search of the Cambridge Structural Database (CSD)15 revealed that boronic acids are indeed capable of forming higher-dimensional hydrogen-bonded assemblies. Of 140 entries for boronic acids, 83 (59%) contain the double-bridged synthon III. This occurrence seems to indicate a relatively high reliability for the B(OH)2 3 3 3 (HO)2B synthon, at least when compared to the carboxylic acid and carboxamide homodimers I and II (33 and 35%);16 however, a larger number of hits is required in order to consolidate this statement. The catemer analogue XII (Scheme 2) is significantly less frequent r 2010 American Chemical Society

Article Scheme 1. Hydrogen Bonding Motifs with Boronic Acids (III-VII) Are Comparable to the Well-known Carboxylic Acid and Carboxamide Dimers (I and II) but May Also Be Different Due to the Symmetric Distribution of the Donor and Acceptor Functions (e.g. VIII-XI)a

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lattice energy, which increases the robustness of the synthons. Orpen and co-workers have examined a series of crystal structures formed between pyridiniumboronic acids (3- and 4-pbaHþ) and [M(oxalate)2]2- as well as [M(dithiooxalate)2]2- (with M = Ni, Cu, Pt, and Pd), showing that the participating building blocks are linked mainly by B(OH) 3 3 3 O(H)B, B(OH)2 3 3 3 carboxylate, B(OH)2 3 3 3 thiocarboxylate, and þN-H 3 3 3 carboxylate interactions.13e Herein we report on the preparation and structural characterization of five metal complexes, which have been prepared by reacting 3- and 4-pyridineboronic acid (3- and 4-pba) with potassium tetrachloroplatinate (K2PtCl4) and hexachloroplatinic(IV) acid hydrate (H2PtCl6 3 aq). One of the most relevant findings in this report is the discovery of the robust chargeassisted heterodimeric synthon B(OH)2 3 3 3 Cl2M-. 2. Experimental Section

a The numbers in parentheses indicate the number of hits for the corresponding synthon in the CSD.15

(11 cases, 8%). Twenty-six cases have one of the neutral or charge-assisted double-bridged synthons shown in Scheme 1 (IV-X). So far, a total of 23 cocrystals have been reported, most of which are combinations of boronic acids with carboxylates or thiocarboxylates (IX).11d,13 In 36 of the 83 entries having the boronic acid dimer, the B(OH)2 3 3 3 (HO)2B synthons are cross-linked through two or four additional hydrogen bonds, thus giving one-dimensional chains, ribbons, or tubes (XIII, XV, XVII, XVIII)10c,f,g,11b,17 and two-dimensional layers (XVI and XIX).10f,11c,18 As previously shown, with either of these motifs, the dimension can be increased by using di- or oligoboronic acids instead of monoboronic acids, e.g. 1,4benzenediboronic acid,10b,c 2-methoxy-1,3-benzenediboronic acid,19 bis(2-(dihydroxyboryl)phenyl)ethyne,20 bis(2-(dihydroxyboryl)phenyl)ethyne,13d ferrocene-1,10 -diboronic acid,11a and tetrakis(4-(dihydroxyboryl)phenyl)methane.10a Water-expanded motifs such as XIV are known also, but are rare.21 Based on these observations and inspired by previous reports on charge-assisted hydrogen-bonded aggregations formed from pyridinium cations and perhalometalates,22,23 in this contribution we examined the possibility to form hydrogenbonded networks by combining tetrachloroplatinate with neutral and protonated pyridineboronic acids (3- and 4-pyridineboronic acid = 3- and 4-pba). Aside from the aforementioned hydrogen bonding capabilities of boronic acids, the use of pyridineboronic acids enhances the structural variety for the purposes of crystal engineering still further, since they can be linked also through coordinative bonds and þN-H 3 3 3 Cl2Minteractions. Furthermore, the use of ionic building blocks enhances the hydrogen bonding energy and thus the crystal

Instrumental. IR spectra have been recorded on a Bruker Vector 22 FT spectrophotometer. Elemental analyses have been carried out on an Elementar Vario ELIII instrument. Thermogravimetric analyses were performed under nitrogen (50 mL/Min) in the temperature range 50-400 C (5.0 C min-1) using a TA SDT Q600 apparatus. Preparative Part. Commercial starting materials and solvents were used. Preparation of [4-pbaH]Cl (1). Two drops of concentrated HCl (37%) were added to an aqueous solution (10 mL) of 4-pyridineboronic acid (0.100 g, 0.814 mmol) under stirring. After evaporation of the solvent at room temperature, colorless crystals had formed, which were suitable for X-ray diffraction analysis. Yield: 0.09 g (69%). C5H7BClNO2 (159.38) Calcd: C, 37.68; H, 4.43; N, 8.79. Found: C, 37.92; H, 4.03; N, 8.01. Preparation of [4-pbaH]2[PtCl4] (2). A solution of 4-pyridineboronic acid (0.100 g, 0.814 mmol) in water (10 mL) and concentrated HCl (37%, 3 mL) was added dropwise to a stirring solution of potassium tetrachloroplatinate (0.34 g, 0.814 mmol) in water (5 mL). The resulting mixture was stirred over a period of 5 h and then allowed to evaporate at room temperature. After one week, red crystals suitable for X-ray diffraction analysis had formed that were collected by filtration. Yield: 0.21 g (44%). IR (KBr): 3443(s), 3361(s), 3188(m), 3123(m), 3055(m), 2923(m), 2860(m), 1589(m), 1488(s), 1401(m), 1331(s), 1228(w), 1191(w), 1118(w), 1015(m), 801(m), 735(w), 650(m), 610(m) cm-1. C10H14B2Cl4N2O4Pt (584.74) Calcd: C, 20.54; H, 2.41; N, 4.79. Found: C, 20.94; H, 2.27; N, 4.52. Preparation of [3-pbaH]2[PtCl4] (3). Red crystals of compound 3 were obtained by the same procedure as described for 2. Yield: 0.22 g (46%). IR (KBr): 3426(br,s), 3215(m), 3170(m), 3118(m), 3074(s), 2916(m), 1602(m), 1547(m), 1462(s), 1408(s), 1352(s), 1139(w), 1002(m), 681(s), 433(m) cm-1. C10H14B2Cl4N2O4Pt (584.74) Calcd: C, 20.54; H, 2.41; N, 4.79. Found: C, 20.18; H, 2.49; N, 5.04. Preparation of [4-pbaH][Pt(4-pba)Cl3] (4). Potassium tetrachloroplatinate (0.34 g, 0.814 mmol) and 4-pyridineboronic acid (0.100, 0.814 mmol) were dissolved in 15 mL of water and 0.5 mL of concentrated HCl (37%) to a give a clear red solution, which was concentrated by heating. After cooling, red crystals of 4 had formed that were suitable for X-ray diffraction analysis. Yield: 0.16 g (36%). IR (KBr): 3445(s), 3188(m), 3124(m), 2924(w), 1589(w), 1489(w), 1407(m), 1334(s), 1228(w), 1192(w), 1120(w), 1016(m), 800(m), 736(w), 651(m), 558(w) cm-1. C10H13B2Cl3N2O4Pt (548.28) Calcd: C, 21.91; H, 2.39; N, 5.11. Found: C, 22.09; H, 2.30; N, 4.89. Preparation of cis-[Pt(4-pba)2Cl2] 3 H2O (5). Potassium tetrachloroplatinate (0.34 g, 0.814 mmol) was added to a stirring solution of 4-pyridineboronic acid (0.100 g, 0.814 mmol) in methanol/water (1:2, 15 mL). After reflux for 3 h, the solution was concentrated.

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Scheme 2. Homodimeric B(OH)2 3 3 3 (HO)2B Synthons Are Frequently Cross-Linked through Additional Secondary Hydrogen Bonding Interactionsa

a

Thus, the B(OH)2 3 3 3 (HO)2B homodimer can be considered as a hydrogen-bonded secondary building unit (HSBU).

After cooling and partial evaporation of the solvent mixture at room temperature, pale yellow crystals of the product had formed, which were suitable for X-ray diffraction analysis. Yield: 0.14 g (32%). IR (KBr): ν~ = 3452(br,s), 1625(m), 1497(w), 1423(s), 1382(s), 1339(s), 1216(m), 1173(m), 1050(m), 837(w), 748(m), 642(m) cm-1. C10H12B2Cl2N2O4Pt 3 H2O (529.84) Calcd: C, 22.67; H, 2.66; N, 5.29. Found: C, 22.00; H, 2.68; N, 4.81. Preparation of cis-[PtIV(3-pba)2Cl4] 3 2H2O (6). A solution of 3-pyridineboronic acid (0.050 g, 0.407 mmol) in water (10 mL) containing concentrated HCl (37%, 2 drops) was added dropwise to a stirring solution of hexachloroplatinic(IV) acid hydrate (0.083 g, 0.203 mmol) in water (5 mL). The resulting clear yellow solution was stirred for a period of 3 h and then allowed to evaporate at room temperature to give the product in the form of yellow crystals, which were suitable for X-ray diffraction analysis. Yield: 0.04 g (32%). IR (KBr): ν~ = 3552(s), 3455(m), 3239(m), 1606(m), 1430(s), 1386(s), 1214(w), 1061(w), 696(m) cm-1. C10H12B2Cl4N2O4Pt 3 2H2O (618.76) Calcd: C, 19.41; H, 2.61; N, 4.53. Found: C, 19.03; H, 2.82; N, 5.03. X-ray Crystallography. X-ray diffraction studies were performed on a Bruker-APEX diffractometer with a CCD area detector (λΜο KR = 0.71073 A˚, monochromator: graphite). Frames were collected via ω- and φ-rotation at 10 s per frame (SMART).24a The measured intensities were reduced to F2 and corrected for absorption with SADABS (SAINT-NT).24b Corrections were made for Lorentz and polarization effects. Structure solution, refinement, and data output were carried out with the SHELXTL-NT program package.24c,d Non-hydrogen atoms were refined anisotropically, while hydrogen atoms were placed in geometrically calculated positions using a riding model. O-H and N-H hydrogen atoms have been localized by difference Fourier maps and refined by fixing the bond lengths to 0.84 A˚; the isotropic temperature factors have been restrained to a value 1.5 times that of the corresponding oxygen/nitrogen atoms. In compounds 2 and 3 the [PtCl4]2- dianions are located on

crystallographic inversion centers, while in compound 6 the metal complex molecules, [PtIV(3-pba)2Cl4], are located on crystallographic C2-rotation axes. Water molecules are present in the crystal lattices of compounds 5 and 6. In compound 5 the hydrogen atoms of the -B(OH)2 groups and one of the hydrogen atoms of the water molecule are disordered over two positions (occ. 0.50). Figures were created with SHELXTL-NT and MERCURY.25 Hydrogen bonding interactions in the crystal lattice were calculated with the WINGX program package.26 Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no. CCDC-767863-767868. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (þ44)1223-336-033; e-mail, [email protected]. uk; www, http://www.ccdc.cam.ac.uk).

3. Results and Discussion Compounds 1-6 have been characterized by single-crystal X-ray diffraction analysis. The most relevant crystallographic data are summarized in Table 1. Selected bond lengths and bond angles are given in Tables 2 and 3. Hydrogen bonding geometries are listed in Table 4. In the crystal structure of [4-pbaH]Cl (1), each chloride ion is hydrogen-bonded to four [4-pbaH]þ cations via (B)OH 3 3 3 Cl-, þN-H 3 3 3 Cl-, and C-H 3 3 3 Cl- hydrogen bonding interactions. The fragment shown in Figure 1 is part of a supramolecular chain running parallel to the ac plane. Since the coordinating functions of the pyridiniumboronic acid molecules are directed alternately above and below the corresponding glide plane, an overall 3D hydrogen-bonded network is formed, which is further stabilized by C-H 3 3 3 O

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Table 1. Crystallographic Data for Compounds 1-6 a

crystal data

1

2

3

4

5

C5H7BClNO2 159.38 Pc 293(2) 6.1958(9) 5.3278(8) 11.357(2) 90 105.132(2) 90 361.88(9) 2 0.459 1.463 0.80 0.97 2472 1205 (0.02) 1175 100 0.033 0.081

6

C10H14B2Cl4N2O4Pt C10H14B2Cl4N2O4Pt C10H13B2Cl3N2O4Pt C10H14B2Cl2N2O5Pt 584.74 584.74 548.28 529.84 P1 P21/c Pbca P1 293(2) 293(2) 293(2) 100(2) 7.366(1) 6.9763(9) 7.2649(7) 19.016(2) 8.049(1) 8.146(1) 28.544(3) 8.3989(8) 8.186(1) 8.236(1) 7.9492(8) 19.303(2) 68.767(2) 70.567(2) 90 90 77.992(2) 79.375(2) 101.622(2) 90 85.515(2) 83.735(2) 90 90 442.5(1) 433.2(1) 1614.6(3) 3083.0(5) 1 1 4 8 8.547 8.729 9.201 9.470 2.194 2.241 2.255 2.283 0.29 0.22 0.14 0.05 0.60 0.27 0.28 0.09 4215 3289 7730 27702 1561 (0.04) 1522 (0.03) 2840 (0.03) 2707 (0.06) 1560 1522 2726 2615 115 115 214 217 0.032 0.032 0.029 0.042 0.079 0.080 0.060 0.080 P P P P a b c d e 2 2 2 λMo KR = 0.71073 A˚. Fo > 4σ(Fo). R = ||Fo|-|Fc||/ |Fo|. All data. Rw = [ w(Fo - Fc ) / w(Fo2)2]1/2.

formula MW (g mol-1) space group temp (K) a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z μ (mm-1) Fcalcd (g cm-3) Tmin Tmax no. of collected refl. no. of ind. refl. (Rint) no. of obs. refl.b no. of variables Rb,c Rwd,e

Table 2. Selected Bond Lengths (A˚) and Bond Angles (deg) for the Description of the Coordination Geometries of the Platinum Atoms in Compounds 2-6a

Table 3. Selected Bond Lengths (A˚) and Bond Angles (deg) for the Description of the Coordination Geometries of the Boron Atoms in Compounds 1-6

Pt(1)-Cl(1) Cl(1)-Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(2)#1

2 2.307(2) 90.95(7) 89.05(7)

Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(1)#1

2.304(2) 180.0

B(1)-O(1) B(1)-C(3) O(1)-B(1)-C(3)

Pt(1)-Cl(1) Cl(1)-Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(2)#1

3 2.297(2) 90.54(7) 89.46(7)

Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(1)#1

2.295(2) 180.0

B(1)-O(1) B(1)-C(1) O(1)-B(1)-C(1)

Pt(1)-N(1) Pt(1)-Cl(1) N(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(2) N(1)-Pt(1)-Cl(3)

4 2.010(4) 2.300(1) 90.3(1) 89.0(1) 176.9(1)

Pt(1)-Cl(2) Pt(1)-Cl(3) Cl(1)-Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(3) Cl(2)-Pt(1)-Cl(3)

2.292(1) 2.307(1) 177.13(5) 90.06(5) 90.75(5)

Pt(1)-N(1) Pt(1)-N(2) N(1)-Pt(1)-N(2) N(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(2)

5 2.007(7) 2.008(6) 89.4(3) 177.6(2) 88.5(2)

Pt(1)-Cl(1) Pt(1)-Cl(2) N(2)-Pt(1)-Cl(1) N(2)-Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(2)

2.296(2) 2.296(2) 88.5(2) 177.5(2) 93.65(7)

Pt(1)-N(1) Pt(1)-Cl(1) N(1)-Pt(1)-N(1)#2 N(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(1)#2 N(1)-Pt(1)-Cl(2)

6 2.062(4) 2.314(1) 90.0(2) 89.8(1) 178.9(1) 90.1(1)

Pt(1)-Cl(2) N(1)-Pt(1)-Cl(2)#2 Cl(1)-Pt(1)-Cl(1)#2 Cl(1)-Pt(1)-Cl(2) Cl(1)-Pt(1)-Cl(2)#2 Cl(2)-Pt(1)-Cl(2)#2

2.318(1) 90.2(1) 90.50(7) 88.74(5) 91.06(5) 179.73(7)

1

a Symmetry codes: (#1) -x þ 1, -y þ 1, -z þ 1; (#2) -x þ 1/2, -y þ /2, z.

interactions. The -B(OH)2 groups adopt the most preferred syn-anti conformation.12b The geometry of the two (B)OH 3 3 3 Cl- hydrogen bonds (H 3 3 3 Cl, 2.31 A˚; O 3 3 3 Cl, 3.103(2) and 3.141(2) A˚; 158 and 170) in 1 indicates relatively strong interactions.27 A revision of the CSD shows that there are only two entries for (B)O-H 3 3 3 Cl- interactions so far, both of which have similar geometries to that found in 1.15 The geometries of the remaining hydrogen bonding interactions are all within previously established limits (Table 4).27,28

C10H16B2Cl4N2O6Pt 618.76 Pccn 293(2) 9.506(1) 13.789(2) 14.276(2) 90 90 90 1871.2(4) 4 8.098 2.196 0.08 0.12 16416 1649 (0.03) 1601 129 0.029 0.052

B(1)-O(1) B(1)-C(1) O(1)-B(1)-C(1) B(1)-O(1) B(1)-C(3) B(2)-O(4) O(1)-B(1)-O(2) O(2)-B(1)-C(3) O(3)-B(2)-C(8) B(1)-O(1) B(1)-C(3) B(2)-O(4) O(1)-B(1)-O(2) O(2)-B(1)-C(3) O(3)-B(2)-C(8) B(1)-O(1) B(1)-C(2) O(1)-B(1)-C(2)

1 1.345(4) 1.602(5) 122.9(3) 2 1.35(1) 1.60(1) 116.7(7) 3 1.34(1) 1.58(1) 116.2(7) 4 1.347(8) 1.580(9) 1.357(8) 125.7(6) 116.7(6) 117.0(5) 5 1.33(1) 1.59(1) 1.34(1) 121.2(7) 121.7(7) 122.2(7) 6 1.345(7) 1.576(8) 121.6(5)

B(1)-O(2) O(1)-B(1)-O(2) O(2)-B(1)-C(3)

1.349(5) 121.3(3) 115.7(3)

B(1)-O(2) O(1)-B(1)-O(2) O(2)-B(1)-C(1)

1.34(1) 127.2(7) 116.1(7)

B(1)-O(2) O(1)-B(1)-O(2) O(2)-B(1)-C(1)

1.33(1) 127.4(8) 116.4(7)

B(1)-O(2) B(2)-O(3) B(2)-C(8) O(1)-B(1)-C(3) O(3)-B(2)-O(4) O(4)-B(2)-C(8)

1.345(8) 1.350(8) 1.582(8) 117.5(5) 126.7(6) 116.3(5)

B(1)-O(2) B(2)-O(3) B(2)-C(8) O(1)-B(1)-C(3) O(3)-B(2)-O(4) O(4)-B(2)-C(8)

1.36(1) 1.39(1) 1.58(1) 116.9(7) 119.7(8) 118.0(7)

B(1)-O(2) O(1)-B(1)-O(2) O(2)-B(1)-C(2)

1.351(7) 119.4(5) 118.9(5)

Compounds [4-pbaH]2[PtCl4] (2) and [3-pbaH]2[PtCl4] (3) are isostructural. The primary hydrogen bonding motifs (Figure 2) are ribbons with antiparallel orientation of the pyridiniumboronic acid cations that are linked by [PtCl4]2anions having an approximate perpendicular orientation to the mean planes of the pyridine rings (73.8 for 2; 74.7 for 3). Each [PtCl4]2- anion forms eight hydrogen bonds, two for each chlorine atom. In the 4-pyridiniumboronic acid salt 2, the [4-pbaH]þ cations and [PtCl4]2- anions are connected by þ N-H 3 3 3 Cl2Pt- and B(OH)2 3 3 3 Cl2Pt- interactions, while in the 3-pyridiniumboronic acid derivative 3 the þN-H 3 3 3 Cl2Pt- interactions are replaced by C-H 3 3 3 Cl2Pt- synthons

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Figure 1. Fragment of the crystal structure of [4-pbaH]Cl (1) showing (B)O-H 3 3 3 Cl-, þN-H 3 3 3 Cl-, and C-H 3 3 3 Cl- hydrogen bonds.

Scheme 3. [PtCl4]2- Dianions Form Charge-Assisted Hydrogen Bonding Motifs with Pyridinium Ions (XX) and Boronic Acids (XXI), Whose Conformations Can Range between the Planar and Folded Extremes a and ba


Figure 2. [4-pbaH]2[PtCl4] (2) (a) and [3-pbaH]2[PtCl4] (3) (b) have isostructural crystal structures. B(OH)2 3 3 3 Cl2Pt-, þN-H 3 3 3 Cl2Pt-, and C-H 3 3 3 Cl2Pt- hydrogen bonds are indicated.

a In the case of conformation b, bicyclic hydrogen bonding motifs become possible (XXII and XXIII).

(Figure 2). This phenomenon has been observed previously22e,23c and is responsible for the isostructural relationship of compounds 2 and 3. Albeit known, the folded conformation of the þN-H 3 3 3 Cl2Pt - synthon (XXb) is uncommon (Scheme 3). Generally, this synthon is planar or at least almost planar (XXa).22b,e Since the B(OH)2 3 3 3 Cl2Pt- synthon is also folded (XXIb), a bicyclic hydrogen bonding motif (XXII) becomes possible, leading to the formation of interesting cyclophane-type macrocyles having compositions of {[PtCl4]2[4-pbaH]2} (2) and {[PtCl4]2[3-pbaH]2} (3) (Figure 2). The geometries of the þN-H 3 3 3 Cl2Pt- (2: H 3 3 3 Cl, 2.46 and 2.90 A˚; N 3 3 3 Cl, 3.237(7) and 3.487(6) A˚; 128 and 155) and C-H 3 3 3 Cl2Pt- (3: H 3 3 3 Cl, 2.81 and 2.84 A˚; C 3 3 3 Cl, 3.608(8) and 3.640(7) A˚; 145) hydrogen bonds are within the ranges found for other pyridinium perhalometallates.22,23 Interestingly, the bifurcated þN-H 3 3 3 Cl2Pt- hydrogen bonds in 2 are significantly asymmetric, while the C-H 3 3 3 Cl2Pt- hydrogen bonds in 3 are symmetric (ΔdH 3 3 3 Cl = 0.03 A˚). On the contrary, the hydrogen bonds in the B(OH)2 3 3 3 Cl2Ptinteractions are almost symmetric in both compounds (H 3 3 3 Cl, 2.40-2.46 A˚; O 3 3 3 Cl, 3.230(6)-3.287(6) A˚; 170-174) but seem to be slightly weaker than those found for the B(OH)2 3 3 3 Cl- interactions in the hydrochloride salt 1 (H 3 3 3 Cl, 2.31 A˚; O 3 3 3 Cl, 3.103(2) and 3.141(2) A˚; 158 and 170). Within the ribbons, the perpendicular separations between the mean planes of the pyridinium rings are 3.61 and 3.51 A˚ for 2 and 3, respectively. The corresponding centroid 3 3 3 centroid distances are 3.96 and 3.77 A˚, thus

Figure 3. (a) In [4-pbaH]2[PtCl4] (2), neighboring ribbons are crosslinked through three different C-H 3 3 3 ClPt hydrogen bonds. (b) In [3-pbaH]2[PtCl4] (3), neighboring ribbons are cross-linked through two þN-H 3 3 3 Cl2Pt and one C-H 3 3 3 ClPt hydrogen bond. For clarity, only one layer of neighboring ribbons and the corresponding interactions are shown in each case.

indicating π-π stacking interactions.29 The Pt 3 3 3 Pt distances are 11.07 and 11.38 A˚ for 2 and 3, respectively. In the extended crystal structure, neighboring antiparallel running ribbons are linked through additional π-π stacking interactions between the pyridinium cations, thus forming a 2D π-π stacked layer of ribbons. In this case, the perpendicular separations are 3.96 and 3.53 A˚ for 2 and 3, respectively. The corresponding centroid 3 3 3 centroid distances are 4.14 and 4.48 A˚. The stacked arrangement of the pyridine moieties in compounds 2 and 3 resembles the structures of pyridine derivatives found in coordination polymers30 and other hydrogen-bonded systems.11f,34 In the third dimension, the ribbons in neighboring layers are cross-linked through three different C-H 3 3 3 ClPt interactions in compound 2 and through two þN-H 3 3 3 Cl2Pt and one C-H 3 3 3 ClPt interaction in compound 3 (Figure 3, Table 4).27,31 Thus, in both compounds, the pyridine moieties are involved in a total of five hydrogen bonds, showing that the nitrogen atom position has little effect on the overall 3D supramolecular structure. Compound 4 is a salt composed of [Pt(4-pba)Cl3]- anions and [4-pbaH]þ cations. In the crystal structure, the [Pt(4-pba)Cl3]- anions are linked through B(OH)2 3 3 3 Cl2Pt- hydrogen

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Table 4. Geometrical Parameters for the Hydrogen Bonding Interactions in the Crystal Structures of Compounds 1-6 compound 1

2

3

4

5

6

H-bond 0

O1-H1 3 3 3 Cl1 O2-H20 3 3 3 Cl1 N1-H10 3 3 3 Cl1 C5-H5 3 3 3 O1 C1-H1 3 3 3 O2 C2-H2 3 3 3 Cl1 C5-H5 3 3 3 Cl1 O1-H10 3 3 3 Cl2 O2-H20 3 3 3 Cl1 N1-H10 3 3 3 Cl1 N1-H10 3 3 3 Cl2 C2-H2 3 3 3 Cl2 C3-H3 3 3 3 Cl2 C5-H5 3 3 3 Cl1 O1-H10 3 3 3 Cl2 O2-H20 3 3 3 Cl1 N1-H10 3 3 3 Cl1 N1-H10 3 3 3 Cl2 C4-H4 3 3 3 Cl1 C4-H4 3 3 3 Cl2 C5-H5 3 3 3 Cl1 O1-H10 3 3 3 Cl3 O2-H20 3 3 3 Cl1 O3-H30 3 3 3 Cl3 O4-H40 3 3 3 Cl1 N2-H20 3 3 3 Cl2 C5-H5 3 3 3 O2 C9-H9 3 3 3 Cl3 C10-H10 3 3 3 Cl1 C10-H10 3 3 3 O3 O1-H10 3 3 3 O3 O2-H20 3 3 3 O4 O3-H30 3 3 3 O31 O4-H40 3 3 3 O2 O31-H31A 3 3 3 Cl1 O31-H31B 3 3 3 Cl2 C1-H1 3 3 3 Cl2 C5-H5 3 3 3 O1 C6-H6 3 3 3 Cl1 C7-H7 3 3 3 O2 C9-H9 3 3 3 Cl1 O1-H1A 3 3 3 O1 O1-H1B 3 3 3 O31 O2-H2A 3 3 3 O2 O2-H2B 3 3 3 O31 O31-H31A 3 3 3 Cl1 O31-H31B 3 3 3 O1 O31-H31C 3 3 3 O2 C10-H10 3 3 3 O2

D-H (A˚) 0.84 0.84 0.84 0.93 0.93 0.93 0.93 0.84 0.84 0.84 0.84 0.93 0.93 0.93 0.84 0.84 0.84 0.84 0.93 0.93 0.93 0.84 0.84 0.84 0.84 0.84 0.93 0.93 0.93 0.93 0.84 0.84 0.84 0.84 0.84 0.84 0.95 0.95 0.95 0.95 0.95 0.84 0.84 0.84 0.84 0.84 0.84 0.84 0.93

H 3 3 3 A (A˚) 2.31 2.31 2.30 2.56 2.44 2.89 2.84 2.45 2.41 2.46 2.90 2.99 2.91 2.91 2.46 2.40 2.85 2.45 2.84 2.81 2.84 2.69 2.33 2.30 2.49 2.39 2.31 2.92 2.84 2.55 1.98 1.95 1.89 2.01 2.49 2.43 2.84 2.40 2.87 2.60 2.92 2.04 1.97 1.88 1.94 2.48 1.91 1.92 2.51

bonds (synthon XXIb) to 1D zigzag chains, which are further connected by pairs of uncoordinated [4-pbaH]þ cations having antiparallel orientation (Figure 4). This pillared connectivity resembles the H-bonded structures described previously for guanidinium organodisulfonates.32 The {[Pt(4-pba)Cl3]2[4-pbaH]2} fragment is structurally related to the HSBU {[PtCl4]2[4-pba]2} found in the crystal structure of compound 2 (Pt 3 3 3 Pt, 11.36 A˚); however, an important difference is that compound 4 contains the planar bicyclic B(OH)2 3 3 3 Cl2Pt 3 3 3 (HO)2B motif XXIII, while compound 2 shows the þNH 3 3 3 Cl2Pt 3 3 3 (HO)2B motif XXII (Scheme 3). In contrast to compounds 2 and 3, the B(OH)2 3 3 3 Cl2Pt- interactions are now significantly asymmetric (ΔdH 3 3 3 Cl = 0.19 and 0.36 A˚; Table 4). The perpendicular and centroid 3 3 3 centroid distances between the pyridinium rings in compound 4 are 3.48 and 3.83 A˚, respectively. Furthermore, the mutual orientation of the [4-pbaH]þ cations indicates an interaction between the nitrogen and the boron atom (dB 3 3 3 N = 3.46 A˚). Since the boron atom is involved in pπ-pπ interactions with the oxygen atoms, thus enhancing its electron density, it can be assumed that this interaction is mainly of electrostatic nature.33

D 3 3 3 A (A˚) 3.103(2) 3.141(2) 3.107(3) 3.204(4) 3.153(5) 3.783(3) 3.599(4) 3.279(6) 3.243(6) 3.237(7) 3.487(6) 3.574(7) 3.58(1) 3.66(1) 3.287(6) 3.230(6) 3.365(7) 3.203(8) 3.640(7) 3.608(8) 3.66(1) 3.407(4) 3.159(5) 3.141(4) 3.305(4) 3.195(5) 3.147(8) 3.731(6) 3.492(6) 3.301(7) 2.747(8) 2.733(8) 2.636(8) 2.779(8) 3.168(6) 3.170(6) 3.530(8) 3.19(1) 3.589(8) 3.37(1) 3.664(8) 2.749(6) 2.730(7) 2.697(6) 2.741(7) 3.270(5) 2.730(7) 2.741(7) 3.383(7)

— DHA (deg)

symmetry code

158 170 161 127 133 161 139 170 174 155 128 122 130 138 171 172 122 150 145 145 149 145 171 178 164 160 150 146 128 139 151 155 147 151 138 147 131 141 133 139 137 141 150 161 160 157 164 164 156

þx, -y þ 2, þz - 1/2 þx, þy þ 1, þz þx - 1, -y þ 1, þz - 1/2 þx - 1, þy - 1, þz þx, -y þ 2, þz - 1/2 þx, -y þ 2, þz - 1/2 þx - 1, þy, þz -x þ 1, -y þ 2, -z þ 1 þx, þy þ 1, þz þx - 1, þy, þz þ 1 -x, -y þ 1, -z þ 2 þx, þy, þz þ 1 þx, þy, þz þ 1 -x, -y þ 1, -z þ 1 þx, þy - 1, þz -x þ 1, -y, -z þ 1 -x þ 1, -y þ 1, -z -x þ 1, -y þ 1, -z þx þ 1, þy, þz - 1 -x þ 2, -y þ 1, -z þx þ 1, þy, þz þx - 1, -y þ 1/2, þz - 1/2 þx - 1, -y þ 1/2, þz - 1/2 -x þ 1, -y þ 1, -z þ 1 -x þ 1, -y þ 1, -z þ 1 þx þ 1, þy, þz þx þ 1, þy, þz -x þ 2, -y þ 1, -z þ 1 þx þ 1, þy, þz - 1 þx þ 1, þy, þz þx, -y þ 1/2 þ 1, þz þ 1/2 -x þ 1/2, -y þ 1, þz þ 1/2 -x þ 1, -y þ 1, -z þ 1 þx, -y þ 1/2 þ 1, þz - 1/2 þx, þy þ 1, þz þx, þy þ 1, þz -x þ 1/2, þy þ 1/2, þz -x þ 1, þy - 1/2, -z þ 1/2 þ 1 -x þ 1/2, þy þ 1/2, þz -x þ 1/2, -y þ 1, þz - 1/2 -x þ 1, -y, -z þ 1 -x þ 1/2, -y - 1/2, þz þx, þy, þz -x þ 1/2, -y - 1/2, þz -x þ 1/2, þy, þz - 1/2 þx, þy, þz þx, þy, þz -x þ 1/2, þy, þz þ 1/2 þx, -y þ 1, þz - 1/2

The pillaring of the one-dimensional [Pt(4-pba)Cl3]nnchains by pairs of [4-pbaH]þ cations results in a 2D herringbone pattern (Figure 4b), and a closer view shows that the repeating motif in this layer consists of a macrocyclic hydrogen-bonded assembly, which is formed by six [Pt(4-pba)Cl3]anions and four [4-pbaH]þ cations (Figure 4a). The dimensions of this rectangular arrangement can be described by the distances between the platinum atoms located at the corners (9.97 21.28 A˚2). In the third dimension, neighboring 2D layers are cross-linked through a total of four crystallographically different C-H 3 3 3 O and C-H 3 3 3 Cl contacts (Table 4). In the neutral complex cis-[Pt(4-pba)2Cl2] 3 H2O (5), two chloride ions have been substituted by 4-pba. In previous studies, Orpen and Brammer have shown that pyridinium salts of [PtCl4]2- lose HCl after thermal treatment, thus allowing for the coordination of pyridine to the metal atom.34 That this is also true for the pyridiniumboronic acid salts employed herein has been evidenced by a thermogravimetric analysis of compounds 2-4. Compound 2 suffers a weight

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Figure 4. In the crystal structure of [4-pbaH][Pt(4-pba)Cl3] (4), 1D zigzag chains formed by the [Pt(4-pba)Cl3]- anions are pillared by antiparallel pairs of [4-pbaH]þ cations (a) to give a 2D herringbone pattern (b).

loss of 14.7% within the temperature range 190-230 C, which corresponds to the elimination of 2 equiv of HCl (calculated 12.5%). Similarly, compound 3 showed a mass loss of 13.6% between 170 and 210 C (calculated 12.5%). In the case of the monosubstituted complex 4, a mass loss of 6.9% corresponding to the elimination of 1 equiv of HCl was observed in the temperature range 180-240 C (calculated 6.7%). In the crystal structure, the primary hydrogen bonding interaction corresponds to the well-known homodimeric B(OH)2 3 3 3 (HO)2B synthon (III) to give a 1D zigzag chain with Pt 3 3 3 Pt distances of 16.56 A˚ (Figure 5a). Neighboring zigzag chains are connected by two of four possible B(OH) 3 3 3 O(H)B hydrogen bonds between the donor and acceptor atoms located at the periphery of synthon III. Due to the nonplanar conformation of the homodimeric B(OH)2 3 3 3 (HO)2B units, the 1D chains are stacked to give a 2D hydrogen-bonded layer. The hydrogen bonding motif resulting from these secondary hydrogen bonding interactions is shown in Figure 5b and corresponds to motif XVIII (Scheme 2). Within this motif, the Pt 3 3 3 Pt distances are 6.25 A˚. The water molecules located at the periphery of the 2D layers interact with the PtCl2 moieties of neighboring layers through PtCl2 3 3 3 H2O hydrogen bonding interactions to give a 3D hydrogen-bonded network (Figure 5c). Interestingly, the (B)O-H 3 3 3 Ow hydrogen bond (H 3 3 3 O, 1.89 A˚; O 3 3 3 O, 2.636(8) A˚; 147) is significantly shorter than that typically

Figure 5. In the crystal structure of cis-[Pt(4-pba)2Cl2] 3 H2O (5), 1D chains formed between the complex molecules (a) are linked through B(OH) 3 3 3 O(H)B interactions to give 2D layers (b). The central hydrogen bonding motif in the 2D layers contains water molecules at the periphery, which allow for the formation of a 3D network through PtCl2 3 3 3 H2O hydrogen bonding interactions (c).

Figure 6. Fragment of the crystal structure of cis-[PtIV(3-pba)2Cl4] 3 2H2O (6), showing the 2D layer resulting from the formation of motif XIV.

found in water clusters and ice (2.77-2.85 A˚).10b A total of five crystallographically different C-H 3 3 3 O and C-H 3 3 3 Cl contacts contribute to an increased stability of the crystal lattice (Table 4).

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Similarly to compound 5, in the crystal structure of the Pt(IV) complex cis-[PtIV(3-pba)2Cl4] 3 2H2O (6), the primary hydrogen bonding interactions are formed by synthon III, which connects the neutral complex molecules to 1D chains. As in the previous case, the B(OH)2 3 3 3 (HO)2B dimers are involved in additional secondary hydrogen bonding interactions with water molecules to give the 2D layer shown in Figure 6. The central hydrogen bonding motif corresponds to the water-expanded tape XIV described in Scheme 2. The O 3 3 3 O distances in the O-H 3 3 3 O hydrogen bonds range from 2.697(6) to 2.749(6) A˚. The Pt 3 3 3 Pt distances are 7.14 and 13.79 A˚. In the third dimension, only van der Waals contacts could be detected between these layers. 4. Conclusions The analysis of the crystal structures described herein has shown that pyridineboronic acids can be versatile ditopic building blocks for crystal engineering of higher-dimensional networks when combined with perhalometalates. This is because the pyridine moiety can form either coordinatecovalent NfM bonds or charge-assisted þN-H 3 3 3 Cl2Mhydrogen bonds, while the -B(OH)2 group participates simultaneously in B(OH)2 3 3 3 Cl2M- or B(OH)2 3 3 3 (HO)2B interactions. In the latter case, the dimension is increased further through secondary hydrogen bonding interactions, thus showing that the B(OH)2 3 3 3 (HO)2B homodimer is a reliable HSBU. Acknowledgment. This work was supported by Consejo Nacional de Ciencia y Tecnologia (CIAM-59213). Supporting Information Available: X-ray crystallographic information files (CIF) for compounds 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.

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