Synthesis and Self-Association of Organoplatinum (IV) Boronic Acids

Article pubs.acs.org/Organometallics

Synthesis and Self-Association of Organoplatinum(IV) Boronic Acids Muhieddine A. Safa,† Anwar Abo-Amer,†,‡ Aneta Borecki,† Benjamin F. T. Cooper,† and Richard J. Puddephatt*,† †

Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7 Department of Chemistry, Al al-Bayt University, Al-Mafraq, Jordan

‡

S Supporting Information *

ABSTRACT: Oxidative addition of ortho, meta, and para isomers of BrCH2C6H4B(OH)2 to dimethylplatinum(II) complexes gave the corresponding organoplatinum(IV) boronic acids [PtBrMe2{CH2C6H4B(OH)2}(NN)], with the bidentate ligand NN = 4,4′-bis(ethoxycarbonyl)-2,2′-bipyridine or 2,5-bis(2-pyridyl)-1,3,4-oxadiazole. The complexes undergo self-assembly in the solid state through hydrogen bonding to give dimers, polymers, or sheet structures.

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INTRODUCTION Within the field of supramolecular chemistry using hydrogen bonding for self-assembly,1 there is much current interest in using building blocks which incorporate organometallic boronic acids.2 These compounds often form dimers A, analogous to carboxylic acid dimers B (Chart 1) by complementary

groups by oxidative addition of precursors containing the hydrogen-bonding group as well as a bromomethyl group to allow oxidative addition to platinum(II), and both carboxylic acid and amide groups have been incorporated in this way.5 The oxidative addition occurs more rapidly than the alternative reaction involving protonolysis of methylplatinum bonds by the carbo xy lic acid; thus, F react s select ively with BrCH2C6H5CO2H to give G, which can then undergo selfassembly to form the dimer H (Scheme 1).5 Introduction of

Chart 1. Self-Assembly of Boronic Acids and Related Groups Containing R22(8) Rings in A−C

Scheme 1. Introduction of Functional Groups by Oxidative Addition

hydrogen-bonding interactions.2 However, they have an extra proton which, in a way similar to that in amide derivatives C, can give additional complementary hydrogen bonding to give more complex self-assembled materials as in dimers D and E (Chart 1) or polymers or to interact with other hydrogen bond acceptors.2,3 The development of the supramolecular chemistry of organotransition-metal complexes has been challenging, because the metal−carbon bonds are often reactive toward hydrogen bond donors.1,4 However, alkylplatinum(IV) complexes are inert toward protic reagents, and so it has been possible to develop their supramolecular chemistry by incorporation of hydrogen-bonding groups.4,5 A particularly useful method has been to introduce the hydrogen-bonding © 2012 American Chemical Society

two or more carboxylic acid groups can give polymers or network materials with unusual structures, such as polyrotaxanes or materials forming channels.5 Special Issue: F. Gordon A. Stone Commemorative Issue Received: October 1, 2011 Published: January 10, 2012 2675

dx.doi.org/10.1021/om2009217 | Organometallics 2012, 31, 2675−2681

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For example, the 1H NMR spectrum of complex 6 contained a methylplatinum resonance at δ 1.52 with 2J(PtH) = 71 Hz and a Pt−CH2 resonance at δ 3.36 with 2J(PtH) = 94 Hz. The product of cis oxidative addition would give two methylplatinum resonances and two resonances for the diastereotopic PtCHaHb protons.5 The complexes 6−8 (Scheme 2) were isolated as yellow-red, air-stable solids. The structure of complex 6 is shown in Figure 1. There are two independent, but similar, molecules, and the structure

This paper describes the extension of this chemistry to the synthesis of boronic acid derivatives of platinum(IV) and a study of the formation of supramolecular dimers or polymers by self-assembly. The reagents used are shown in Chart 2. The Chart 2. Organoplatinum and Organoboron Reagents

organoplatinum compounds contain additional functional groups which have the potential to act as hydrogen bond acceptors, while the organoboronic acid compounds contain the bromomethyl substituents in three different positions. Previously, boronic acid complexes of platinum(II) or platinum(IV) have been prepared by using 3-pyridyl- or 4pyridylboronic acid as a nitrogen-donor ligand or the protonated form as a cation with a platinum-containing anion, but no organoplatinum derivatives appear to have been reported.6 We note that Gordon Stone had an enduring interest in both organoplatinum and organoboron chemistry, as well as in their combination.7

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RESULTS AND DISCUSSION The reactions of complex 1 (Chart 2)8,9 with the three (bromomethyl)phenyl boronic acid derivatives 3−510 are shown in Scheme 2. In each case, the product of trans

Figure 1. Views of the structures of dimers formed by the two independent molecules of 6. Selected bond distances (Å): Pt(1)− N(1), 2.147(6); Pt(1)−N(2), 2.059(7); Pt(1)−C(1), 2.037(7); Pt(1)−C(21), 2.059(7); Pt(1)−C(18), 2.080(7); Pt(1)−Br(1), 2.579(1). Hydrogen-bond distances (Å): O(2)···O(5A), 2.935(7); O(4)···O(6A), 2.833(7). Symmetry equivalents x, y, z: (A) −x, 1 − y, −z.

Scheme 2. Formation of Isomeric Complexes 6−8

confirms the trans oxidative addition. Each molecule forms a dimer with its symmetry-equivalent molecule by hydrogen bonding of the type BOH···OC between the boronic acid groups and the carbonyl oxygens of the CO2Et substituents of the bipyridine ligand (I, Chart 3). This form of hydrogen bonding, and not the more common form A shown in Chart 1, was not predicted because ester groups are relatively weak hydrogen bond acceptors.11 Formation of the hydrogenbonded ring structure, described as R22(15) in graph theory, appears to be very favorable. Each molecule has P or M conformational helicity, and the dimers each contain a PM pair, with component molecules related by a center of inversion. In order to form the ring structure, the two ester groups on each bipyridine ligand are oriented with the carbonyl groups syn to one another (Figure 1, Chart 3). It should also be noted that the planes defined by the two hydrogen-bonded bipy(CO2)2BO2 units are exactly parallel to one another and are separated by 3.2−3.3 Å, in such a way that π-stacking forces will also contribute significantly to the self-association. Similar intramolecular and intermolecular π-stacking can be identified in all of the complexes described below.

oxidative addition of the C−Br bond was formed rapidly and selectively. The stereochemistry was clearly indicated by the 1H NMR spectra, which gave a single methylplatinum resonance and a singlet resonance for the Pt−CH2 protons in each case. 2676


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Chart 3. Hydrogen-Bonding Modes Observed in the New Complexes

Figure 3. View of the molecular structure of complex 8. Selected bond distances (Å): Pt(1)−N(1), 2.158(4); Pt(1)−N(2), 2.167(5); Pt(1)− C(17), 2.061(6); Pt(1)−C(25), 2.050(6); Pt(1)−C(18), 2.076(6); Pt(1)−Br(1), 2.598(1).

The structure of complex 7 is shown in Figure 2. The nature of the self-assembly is the same as in complex 6 (Figure 1), with

distance is in the region for a weak hydrogen bond,12 there are no other possible hydrogen bond acceptor atoms in the vicinity of O(6), and only a single broad OH stretch is observed in the IR; thus, it is likely that the bonding is of the type J rather than K (Chart 3). In either case, the self-assembly gives rise to a supramolecular hydrogen-bonded polymer (Figure 4). Each chain contains molecules with the same helicity, while a neighboring chain contains molecules related by an inversion center and so with opposite helicity. There is π-stacking of bipyridine groups between these neighboring chains. The reactions of the 2,5-bis(2-pyridyl)-1,3,4-oxadiazole complex 2 (Chart 2)13,14 with the three (bromomethyl)phenyl boronic acid derivatives 3−510 are shown in Scheme 3. In each case, a single product was formed rapidly and selectively. However, the product of either cis or trans oxidative addition would have only C1 symmetry; therefore, the stereochemistry is not immediately obvious from the NMR spectra. For example, the 1H NMR spectrum of the product of reaction of 2 with oC6H4(CH2Br)B(OH)2 gave two methylplatinum resonances at δ 1.57, with 2J(PtH) = 75 Hz, and δ 1.74, with 2J(PtH) = 76 Hz, and an [AX] multiplet for the PtCHaHx protons at δ 3.18 (2J(PtH) = 86 Hz, 2J(HH) = 9 Hz) and δ 3.61([2J(PtH) = 97 Hz, 2J(HH) = 9 Hz). The stereochemistry of the products 9− 11 was determined by X-ray structure determinations, as described below. The structure of complex 9 is shown in Figure 5 and confirms that the reaction occurs by trans oxidative addition. The complex was crystallized from acetonitrile, and one BOH group is hydrogen-bonded to a solvate molecule (O(2)−N(1S) = 2.90(1) Å). The second hydroxyl group forms a BOH···Br hydrogen bond to a neighboring molecule (O(3)···Br(1A) = 3.293(8) Å), and propagation of this intermolecular hydrogen bonding leads to formation of a supramolecular polymer, as shown in Figure 6. The overall hydrogen bonding is represented as L (Chart 3). The molecules are chiral (A, anticlockwise; C, clockwise), and both neighbors of a given molecule have the opposite chirality; therefore, the polymer chains are syndiotactic (ACAC...). The molecular structure of complex 10 is shown in Figure 7 and confirms the expected stereochemistry at platinum(IV). The molecules self-assemble through hydrogen bonding of type M (Chart 3), and this gives rise to a supramolecular sheet structure, shown in Figure 8. Each molecule is connected to

Figure 2. View of the dimer formed by self-assembly of molecules of complex 7. Selected bond lengths (Å): Pt(1)−N(1), 2.157(3); Pt(1)− N(2), 2.150(3); Pt(1)−C(1), 2.100(4); Pt(1)−C(25), 2.067(4); Pt(1)−C(24), 2.070(4); Pt(1)−Br(1), 2.5807(5). Hydrogen-bond distances (Å): O(1)···O(5A), 2.988(5); O(2)···O(3A), 2.832(5). Symmetry equivalents x, y, z: (A) −x, 1 − y, −z.

R22(15) rings formed by pairwise hydrogen bonding between the B(OH)2 groups and the carbonyl groups of pairs of molecules, which are related by an inversion center. The benzyl group is rotated in 7 compared to its position in 6 so that the B(OH)2 group can hydrogen bond to both carbonyl groups of the neighboring molecule in each case (Figures 1 and 2). The molecular structure of complex 8 is shown in Figure 3. Again the product is formed by trans oxidative addition and the molecular structure is similar to those of 6 and 7 (Figures 1 and 2). However, one significant difference is that the carbonyl groups are oriented anti to another and the complex undergoes self-assembly in a very different way, as shown in Figure 4. The primary hydrogen-bonding interaction is between one BOH group and the Pt−Br group of a neighboring molecule, with O(5)···Br(1A) = Br(1)···O(5B) = 3.29(1) Å. There is a second, longer interaction between the second BOH group and the Pt−Br group with O(6)···Br(1A) = Br(1)···O(6B) = 3.64(1) Å, indicated by a thinner dashed line in Figure 4. We have been unable to directly locate the hydrogen atoms of the B(OH)2 group; therefore, we cannot determine if this longer distance also represents a BOH···BrPt hydrogen bond. The 2677


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Scheme 3. Boronic Acid Derivatives of Complex 2

Figure 5. View of the molecular structure of the C enantiomer of complex 9. Selected bond distances (Å): Pt(1)−N(1), 2.209(7); Pt(1)−N(2), 2.153(8); Pt(1)−C(1), 2.026(10); Pt(1)−C(2), 2.027(9); Pt(1)−C(15), 2.100(9); Pt(1)−Br(1), 2.592(1).

Figure 4. Supramolecular polymer formed by BOH···Br hydrogen bonding between molecules of complex 8. Hydrogen bond distance (Å): O(5)···Br(1A) = Br(1)···O(5B) = 3.29(1). The distance Br(1)···O(6B) = 3.64(1) Å may represent a weak hydrogen bond. Symmetry equivalents x, y, z: (A) x, y + 1, z; (B) x, y − 1, z.

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CONCLUSIONS

The oxidative addition of the C−Br bond of bromomethylphenylboronic acid derivatives to platinum(II) gives an easy, high-yield route to the first organoplatinum(IV) derivatives of boronic acids. Boronic acids are commonly used in C−C coupling reactions; therefore, these compounds have potential for further use in organometallic synthesis,15a and the method could also be used to prepare functional boronic acids of other metals, with potential uses in neutron capture therapy or photonic materials.15 The new complexes undergo self-assembly in the solid state through hydrogen bonding, but not always in the anticipated manner. Only two of the six complexes, 10 and 11, give the common complementary self-association to give R22(8) rings of type A (Chart 1),2,3 and both of these also give hydrogen bonds of the form BOH···BrPt, with the bromoplatinum group preferred over other potential sites as the hydrogen bond acceptor (Figures 8 and 10). The 2,5-bis(2-pyridyl)-1,3,4oxadiazole ligand in complexes 9−11 has free oxygen and nitrogen donors which could participate in hydrogen bonding,

three others, each of which has the opposite chirality, through hydrogen bonding. Each B(OH)2 group forms a complementary head-to-tail dimer of the classic type A (Chart 1) with one neighbor, then the outwardly directed OH group acts as a hydrogen bond donor to a BrPt group of a second neighbor, and the PtBr group acts as a hydrogen bond acceptor for a similar BOH group of a third neighbor (M, Chart 3). Propagation of this pattern gives rise to the racemic sheet structure. The structure of complex 11 is shown in Figure 9. As in complex 10, the benzyl group is oriented above the oxadiazole ring, with no intramolecular hydrogen bonding, and the intermolecular hydrogen bonding occurs according to the general type M (Chart 3). Each molecule is connected to three others of opposite chirality in forming a racemic sheet structure (Figure 10). 2678


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Figure 6. Syndiotactic, supramolecular polymer structure of complex 9. Symmetry equivalents x, y, z: (A) x, 1/2 − y, −1/2 + z; (B) x, 1/2 − y, 1 /2 + z.

Figure 10. Hydrogen-bonded sheet structure of complex 11. Complexes with C and A chirality are shown in red and blue, respectively. Hydrogen bond distances (Å): O(2)···O(3A) = O(3)···O(2A), 2.80(1); O(2)···Br(1B) = Br(1)···O(2C), 3.34(1) Å. Symmetry equivalents x, y, z: (A) −x, −1 − y, 1 − z; (B) x, −1/2 − y, −1/2 + z; (C) x, −1/2 − y, 1/2 + z.


but hydrogen bonding to the PtBr group is preferred in all cases (Figures 6, 8, and 10). However, hydrogen bonding to carbonyl substituents of the 4,4′-bis(ethoxycarbonyl)-2,2′-bipyridine ligand is observed in complexes 6 and 7 (Figures 1 and 2) and only dimers are formed in these cases. Hydrogen bonding to the bromoplatinum(IV) group is preferred in complex 8. The platinum(IV) derivatives of o-BrCH2C6H4B(OH)2 give lower order structures (dimer in 6, 1-D polymer in 9), while the derivatives of p-BrCH2C6H4B(OH)2 give higher order structures (1-D polymer in 8, 2-D sheet in 11). Clearly, there is potential to design more complex structures as the understanding of design features improves.2,3

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EXPERIMENTAL SECTION

NMR spectra were recorded by using a Varian Mercury 400 or Varian Inova 400 or 600 NMR spectrometer. 1H and 13C chemical shifts are reported relative to tetramethylsilane (TMS). The ligands 4,4′bis(ethoxycarbonyl)-2,2′-bipyridine (bebipy) and 2,5-bis(2-pyridyl)1,3,4-oxadiazole (bpox) and their dimethylplatinum(II) complexes [PtMe2(bebipy)] (1) and [PtMe2(bpox)] (2) were prepared by methods described previously.8,9,13,14 [PtBrMe2{CH2-2-C6H4B(OH)2}(bebipy)] (6). To a solution of complex 1 (0.079 g, 0.150 mmol) in acetone (10 mL) was added a

Figure 8. Hydrogen-bonded sheet structure of complex 10. Complexes with C and A chirality are shown in red and blue, respectively. Hydrogen bond distances (Å): O(2)···O(3B), 2.77(1); O(3)···Br(1C) = Br(1)···O(3A), 3.36(1). Symmetry equivalents x, y, z: (A) x, 1/2 − y, 1/2 + z; (B) −x, −y, −z; (C) x, 1/2 − y, −1/2 + z. 2679


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solution of o-BrCH2C6H4B(OH)2 (0.032 g, 0.150 mmol) in acetone (1 mL), and the mixture was stirred at room temperature for 1 h. The solvent was evaporated to give the product as a yellow solid, which was washed with pentane and dried under vacuum. Yield: 88%. It was recrystallized from acetone/pentane. Anal. Calcd for C25H30BBrN2O6Pt: C, 40.56; H, 4.08; N, 3.78. Found: C, 40.55; H, 3.97; N, 3.72. IR: ν(OH) 3342 (br); ν(CO) = 1725, 1720 cm−1. ESI-MS: m/z 740 (M + 1); 1479 (2M + 1). NMR in acetone-d6: δ(1H) 1.44 (t, 6H, 3JHH = 7 Hz, MeC); 1.52 (s, 6H, 2JPtH = 71 Hz, MePt); 3.36 (s, 2H, 2JPtH = 94 Hz, PtCH2); 4.50 (q, 4H, 3JHH = 7 Hz, MeCH2); 6.62−6.93 (m, 4H, C6H4); 8.09 (d, 2H, 3JHH = 5 Hz, H5); 8.82 (s, 2H, H3); 8.90 (d, 2H, 3JHH = 5 Hz, 3JPtH = 19 Hz, H6). [PtBrMe2{CH2-3-C6H4B(OH)2}(bebipy)] (7). This was prepared similarly from complex 1 and m-BrCH2C6H4B(OH)2 and isolated as a yellow solid. Yield: 86%. It was recrystallized from acetone/pentane. Anal. Calcd for C25H30BBrN2O6Pt·(acetone): C, 42.12; H, 4.54; N, 3.51. Found: C, 41.81; H, 4.07; N, 3.60. IR: ν(OH) 3340 (br); ν(C O) 1710 cm−1. NMR in acetone-d6: δ(1H) 1.45 (t, 6H, 3JHH = 8 Hz, MeC); 1.52 (s, 6H, 2JPtH = 70 Hz, MePt); 2.80 (s, 2H, 2JPtH = 90 Hz, PtCH2); 4.50 (q, 4H, 3JHH = 8 Hz, MeCH2); 6.46−7.17 (m, 4H, C6H4); 8.15 (d, 2H, 3JHH = 6 Hz, H5); 8.81 (s, 2H, H3); 8.96 (d, 2H, 3 JHH = 6 Hz, 3JPtH = 19 Hz, H6). [PtBrMe2{CH2-4-C6H4B(OH)2}(bebipy)] (8). This was prepared similarly from complex 1 and p-BrCH2C6H4B(OH)2 and isolated as a red solid. Yield: 85%. It was recrystallized from acetone/pentane. Anal. Calcd for C25H30BBrN2O6Pt: C, 40.56; H, 4.08; N, 3.78. Found: C, 40.64; H, 4.05; N, 3.75. IR: ν(OH) 3422 (br); ν(CO) 1728 cm−1. NMR in acetone-d6: δ(1H) 1.44 (t, 6H, 3JHH = 7 Hz, MeC); 1.52 (s, 6H, 2JPtH = 70 Hz, MePt); 2.80 (s, 2H, 2JPtH = 93 Hz, PtCH2); 4.50 (q, 4H, 3JHH = 7 Hz, MeCH2); 6.31 (d, 2H, 3JHH = 8 Hz, 4JPt−H = 18 Hz, C6H4); 7.10 (d, 2H, 3JHH = 8 Hz, C6H4); 8.15 (d, 2H, 3JHH = 6 Hz, H5); 8.87 (s, 2H, H3); 8.96 (d, 2H, 3JHH = 6 Hz, 3JPtH = 19 Hz, H6). [PtBrMe2{CH2-2-C6H4B(OH)2}(bpox)] (9). This was prepared similarly from complex 2 and o-BrCH2C6H4B(OH)2 and isolated as a yellow solid. Yield: 82%. It was recrystallized from acetonitrile/ acetone. IR: ν(OH) 3405 (br) cm−1. ESI-MS: m/z 664 (M + 1). NMR in acetone-d6: δ(1H) 1.57 (s, 3H, 2JPtH = 75 Hz, MePt); 1.74 (s, 3H, 2 JPtH = 76 Hz, MePt); 3.18 (d, 1H, 2JHH = 9 Hz, 2JPtH = 86 Hz, PtCHaHx); 3.61 (d, 1H, 2JHH = 9 Hz, 2JPtH = 97 Hz, PtCHaHx); 6.58− 6.92 (m, 4H, C6H4); 7.70−8.87 (m, 8H, bpox). [PtBrMe2{CH2-3-C6H4B(OH)2}(bpox)] (10). This was prepared similarly from complex 2 and m-BrCH2C6H4B(OH)2 and isolated as a yellow solid. Yield: 86%. IR: ν(OH) 3385, 3325 cm−1. NMR in acetone-d6: δ(1H) 1.58 (s, 3H, 2JPtH = 75 Hz, MePt); 1.80 (s, 3H, 2JPtH = 74 Hz, MePt); 2.86 (d, 1H, 2JHH = 9 Hz, 2JPtH = 90 Hz, PtCHaHx); 3.03 (d, 1H, 2JHH = 9 Hz, 2JPtH = 87 Hz, PtCHaHx); 6.60−7.00 (m, 4H, C6H4); 7.50−8.85 (m, 8H, bpox). [PtBrMe2{CH2-4-C6H4B(OH)2}(bpox)] (11). This was prepared similarly from complex 2 and p-BrCH2C6H4B(OH)2 and isolated as a yellow solid. Yield: 85%. It was recrystallized from acetonitrile/ acetone. IR: ν(OH) 3400, 3305 cm−1. NMR in acetone-d6: δ(1H) 1.55 (s, 3H, 2JPtH = 74 Hz, MePt); 1.75 (s, 3H, 2JPtH = 76 Hz, MePt); 2.81 (d, 1H, 2JHH = 9 Hz, 2JPtH = 89 Hz, PtCHaHx); 3.00 (d, 1H, 2JHH = 9 Hz, 2JPtH = 92 Hz, PtCHaHx); 6.49 (d, 2H, 2JHH = 8 Hz, 4JPtH = 9 Hz, C6H4); 7.12 (d, 2H, 2JHH = 8 Hz, C6H4); 7.71−8.87 (m, 8H, bpox]. X-ray Structure Determinations. Details of the data collections and structure refinements are given in the CIF files and in Tables S1 and S2 in the Supporting Information. In a typical procedure, the subject crystal was mounted using a MiTeGen Micro Mount and placed in the dry N2 cold stream of the low-temperature Oxford Cryostream apparatus attached to the diffractometer. The data were collected at 150 K using the APEX2 software on a Bruker APEX-II CCD diffractometer with Mo Kα radiation (λ = 0.710 73 Å).16a Multiscan absorption correction was performed using SADABS.16b The structure was solved by direct methods and refined by full-matrix least squares on F2 with anisotropic displacement parameters for the nondisordered heavy atoms using the SHELXL-97 software package.16c,d

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2, giving crystal and refinement data for 6−11, and CIF files, giving details of the X-ray structure determinations for 6−11. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. DEDICATION Dedicated to the memory of Professor F. G. A. Stone, who made so many fundamental contributions to organometallic chemistry.

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REFERENCES

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Organometallics

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