Supramolecular Polymer and Sheet and a Double Cubane Structure in Platinum(IV) Iodide Chemistry: Solution of a Longstanding Puzzle.

The complexes [PtMe2(L)], L = 2-C5H4NCH2NH-x-C6H4OH (x = 2, 3, or 4), react with iodine to form [PtI2Me2(L)], by trans oxidative addition, when x = 3 or 4, and they are shown to have polymeric or sheet structures formed through NH···I hydrogen bonding. However, ligand dissociation occurs when x = 2 to give [(PtI2Me2) n ] and, with methyl group transfer, the complex [(PtIMe3·PtI2Me2)2]. This tetraplatinum cluster complex is shown to have a double cubane structure, thus solving a longstanding puzzle.

Introduction

Trimethylplatinum iodide was the first alkyl complex of a transition metal to be reported, so it is important in the development of organometallic chemistry.[1] Many related complexes have since been reported and they have the classic cubane structure [(PtXMe3)4], A, Chart , typically with X– = halide, OH–, SR–, in which each platinum(IV) center is octahedral and each anion is triply bridging.[2−4] There are also dialkylplatinum dihalides [(PtX2R2)], including those with R = Me and R2 = C3H6 (the first metallacyclobutane complex), and mixed complexes [(PtXMe3·PtX2Me2)].[4−6] On the basis of vibrational spectroscopy and mass spectrometry, as well as by analogy to the established structure A, the complexes [(PtXMe3·PtX2Me2)] and [(PtX2R2)] were assigned cubane structures such as B or C, respectively, with two (B) or four (C) terminal Pt–X bonds as well as the four Pt3(μ3-X) groups.[4,5] These halide complexes are insoluble in noncoordinating solvents, and it was not possible to grow crystals to confirm the proposed structures crystallographically. More recently, the complexes [{Pt(OH)Me3·Pt(OH)2Me2}2] and [{Pt(OH)Me3·Pt(OMe)2Me2}2] were prepared and shown to have the double cubane structure D, X = OH or OMe, which contain no terminal Pt–X bonds.[7] It was suggested that the halide complexes [(PtXMe3·PtX2Me2)] might also have this structure instead of B and that the complexes [(PtX2R2)] might have a polymeric structure E instead of C (Chart ), but no proof was possible at that time.[7] This paper describes the synthesis and structures of some new platinum(IV) iodide complexes and their supramolecular chemistry. In this study, crystals of the Hall cluster [(PtIMe3·PtI2Me2)2] were obtained,[4] and it is finally proved after 47 years to have the double cubane structure D.

Chart 1

Structures and Proposed Structures of Methylplatinum Halide Complexes: A, the Established Structure of [(PtXMe3)4]; B and C, the Originally Proposed Structures of [(PtXMe3·PtX2Me2)] and [(PtX2Me2)]; D and E, the Structures Proposed in This Work

Results and Discussion

As part of a study on the effect of ligands with hydrogen bonding substituents on reactivity and mechanism in oxidative addition reactions of dimethylplatinum(II) complexes,[8] the aminopyridine ligands L1–L3, which contain phenol substituents at the amine donor, were studied (Scheme ).[9−11] The ligands contain both an NH group and an OH group, which might act as hydrogen bond donors. Ligand L1 has been used often,[9] but L2 and L3 are not so well-known and are incompletely characterized.[10,11] The reaction between [Pt2Me4(μ-SMe2)2], 1,[12] and these ligands gave an equilibrium mixture, which contained cis-[PtMe2(SMe2)2], 2,[12] the expected product [PtMe2(L)], 3–5, and free ligands L and SMe2, as monitored by 1H NMR spectroscopy in acetone-d6 solution (Scheme , Figures S5–S7), and they were not isolated but were prepared and used in solution for subsequent studies. As an example, in the 1H NMR spectrum for complex 3 in acetone-d6, two methyl platinum resonances were observed at δ 0.24 and 0.65, with satellites with coupling constants 2J(PtH) = 86 and 90 Hz, respectively, in the range expected for methylplatinum(II) complexes.[8] The complexes have no symmetry, and therefore, the CH2 group has nonequivalent CHaHb protons.

Scheme 1

Ligands L1–L3 and the Synthesis of the Dimethylplatinum(II) Complexes 3–5

The reactions of complexes 4 and 5 with iodine occurred in the expected way to give the corresponding platinum(IV) complexes 6 and 7 by trans oxidative addition (Scheme ).[13] The 1H NMR spectra of 6 contained two methylplatinum resonances at δ = 1.75, 2J(PtH) = 74 Hz, and δ = 2.40, 2J(PtH) = 75 Hz, and the lower values of the coupling constants indicate the formation of a platinum(IV) complex.[8,13] The methylene group gave two sharp CHaHb doublets of doublet peaks at δ(1H) = 5.08 [2J(HaHb) = 15 Hz; 3J(CH2–NH) = 4 Hz] and 5.53 [2J(HaHb) = 15 Hz; 3J(CH2–NH) = 12 Hz] (Figure S8). The NMR data are consistent with the proposed structure (Scheme ) but do not define the stereochemistry. However, the structures were finally proved by structure determinations (Figure ). Each platinum center contains two cis methyl groups, the chelate ligand L3 or L4, and two trans iodide ligands, as expected for the trans oxidative addition reaction. Complex 6 is not solvated, but the lattice of complex 7 contains an acetone solvate molecule, which is hydrogen-bonded to the phenol proton (Figure ).

Scheme 2

Synthesis of Complexes 6 and 7

Figure 1

Molecular structures of complexes 6 (right) and 7 (left). Selected bond distances: 6, Pt(1)C(1) 2.211(8), Pt(1)C(2) 2.061(9), Pt(1)N(1) 2.186(8), Pt(1)N(2) 2.252(7), Pt(1)I(1) 2.6593(19), Pt(1)I(2) 2.6157(19); 7, Pt(1)C(1) 2.086(6), Pt(1)C(2) 2.064(7), Pt(1)N(1) 2.173(5), Pt(1)N(2) 2.257(5), Pt(1)I(2) 2.6402(7), Pt(1)I(1) 2.6405(7) Å.

The supramolecular structures of complexes 6 and 7, formed by intermolecular NH···I and/or OH···I hydrogen bonding, are shown in Figures and 3. For complex 7, the phenol proton is hydrogen-bonded to a solvate molecule of acetone (Figure ), so there is only NH···I intermolecular hydrogen bonding and a one-dimensional polymer is formed. Each molecule is chiral at the amine nitrogen, with R or S configurations, and the polymer chains are syndiotactic, with alternating RSRS configurations of neighboring molecules (Figure ). For complex 6, there is both intermolecular NH···I and OH···I hydrogen bonding, so each molecule forms four hydrogen bonds to four different neighbors, two as donors and two as acceptors. Two of these neighbors have the same configuration and two have the opposite configuration, so each sheet is racemic (Figure ).

Figure 2

Supramolecular polymeric structure of complex 7. Intermolecular distances: N(2)···I(2B) = I(2)···N(2A) = 3.66(1) Å. Symmetry-related atoms: x, y, z; x, 3/2 – y, −1/2 + z; x, 3/2 – y, 1/2 + z.

Figure 3

Supramolecular sheet structure of complex 6. Intermolecular distances: N(2)···I(1A) = I(1)···N(2C) = 3.67(1); O(1)···I(2B) = I(2)···O(1D) = 3.58(1) Å. Symmetry-related atoms: x, y, z; −1/2 + x, y, 1/2 – z (A); −1/2 + x, 1/2 – y, −z (B); 1/2 + x, y, 1/2 – z (C); 1/2 + x, 1/2 – y, −z (D).

The reaction of the equilibrium mixture of complexes 2 and 3 in acetonitrile solution occurred in a different way to give [(PtIMe3·PtI2Me2)2], 8, and [(PtI2Me2)], 9, as shown in Scheme , with loss of the ligand L1. The reaction immediately gave an insoluble brown precipitate, identified as mostly the known complex 9.[4] Filtration of the mixture, followed by slow diffusion of ether into the acetonitrile solution, then gave orange crystals identified as complex 8.[4] The formation of 8 requires a methyl-transfer reaction to form the trimethylplatinum(IV) groups and should also give a monomethylplatinum complex which was not identified. Such methyl-transfer reactions are well-known and may occur either by direct methyl for halogen exchange or by a redox mechanism involving methyl iodide transfer from platinum(IV) to a dimethylplatinum(II) complex.[14] The formation of 8 and 9 also requires dissociation of the ligands L1 and dimethylsulfide on the oxidation to platinum(IV), a reaction which is common with dimethylsulfide complexes but infrequent with chelate ligands.[7] Steric effects of the ortho hydroxyl group in L1 probably account for the difference in the reaction of complex 3 with iodine compared to the simple oxidative addition observed for 4 and 5 (Scheme ). This steric effect is not important in forming the platinum(II) complex 3 and indeed may be helpful in forming a Pt···HO hydrogen bond in 3(8) but is expected to be more important in an octahedral platinum(IV) complex analogous to 6 or 7. The structure of complex 9 shown in Scheme is tentative, but the structure of 8 was determined and is shown in Figure . It finally confirms the prediction that the complex [(PtIMe3·PtI2Me2)2], 8, has the face-bridged double cubane framework with two vertices missing and not the originally proposed simple cubane structure B (Chart ).[4,7] Each platinum(IV) center has octahedral stereochemistry. The two trimethylplatinum(IV) groups [Me3Pt(1) and Me3Pt(1A)] are at either end of the cluster, and the two dimethylplatinum(IV) units [Me2Pt(2) and Me2Pt(2A)] are at the center. Two of the iodide groups are triply bridging, μ3-I [I(2) and I(2A)], and the other four are doubly bridging, μ2-I ligands [I(1), I(1a), I(3), I(3a)]. The triply bridging μ3-I ligands [I(2) and I(2A)] are trans to carbon, and the Pt–I distances fall in the range 2.812(1)–2.814(1) Å. However, the doubly bridging, μ2-I groups [I(1), I(1a), I(3), I(3a)] are trans to both carbon and iodine, and the distances Pt(1)–I(3) = 2.814(1) and Pt(1)–I(1) = 2.808(1) Å trans to methyl are longer than the distances Pt(2)–I(3) = 2.644(1) and Pt(2)–I(1) = 2.634(1) Å.

Scheme 3

Synthesis of Complexes 8 and 9

Figure 4

Structure of [(PtIMe3·PtI2Me2)2], 8. Selected bond distances: Pt(1)C(1) 2.083(6), Pt(1)C(2) 2.067(6), Pt(1)C(3) 2.074(6), Pt(1)I(1) 2.8078(12), Pt(1)I(2) 2.8117(12), Pt(1)I(3A) 2.8138(10), Pt(2)C(5) 2.065(6), Pt(2)C(4) 2.071(6), Pt(2)I(1) 2.6336(12), Pt(2)I(3) 2.6438(12), Pt(2)I(2) 2.8238(12), Pt(2)I(2A) 2.8254(10) Å. Symmetry equivalent atoms: x, y, z; 1 – x, 1 – y, 1 – z.

Conclusions

The aminomethylpyridine ligands L1–L3 do not coordinate strongly to the dimethylplatinum(II) center, as shown by the inability to completely displace the dimethylsulfide ligands from [PtMe2(SMe2)2] (Scheme ). Qualitatively, the equilibrium constants for the formation of the new dimethylplatinum(II) complexes follow the sequence L3 > L1 > L2 (Scheme and Figures S5–S7). However, L2 and L3 bind relatively more strongly to platinum(IV), compared to L1, and selectively form [PtI2Me2(L)] by trans oxidative addition of iodine. These complexes form a supramolecular polymer (L3) or sheet (L2) by intermolecular hydrogen bonding. The ligand L1 evidently binds less strongly and the complex [PtMe2(L1)] reacts with iodine with loss of the ligand to form the bridging iodide complexes [(PtIMe3·PtI2Me2)2], 8, and [(PtI2Me2)], 9.[4] Complex 8 was prepared much earlier,[4] but it is essentially insoluble in most organic solvents and so could not be crystallized. It was assigned a simple cubane structure B (Chart ),[4] but this is now shown to be incorrect and the face-bridged double cubane structure, with two missing vertices, is established crystallographically (Figure ). It is very likely that other complexes with stoichiometry [(PtXR3·PtX2R2)] also have this structure. The structure containing μ2-I and μ3-I units is evidently preferred over the simple cubane structure, which contains terminal iodide and μ3-I units. The M4X6 dicubane structure is known, with X = complex oxygen donor ligands, in complexes of nickel and cobalt and in some mixed metal clusters.[15] However, complex 8 is the simplest example and so can be considered as a paradigm for the unusual structure.[7,15]

Experimental Section

NMR spectra were recorded using Bruker 400, Inova 400, and Inova 600 NMR spectrometers. Spectra are referenced to tetramethylsilane and assignments are given according to the labeling scheme in Chart . Single-crystal X-ray diffraction measurements were made using a Bruker APEX-II CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. Single crystals of the complexes were immersed in paraffin oil and mounted on MiteGen micromounts. The structures were solved using direct methods and refined by the full-matrix least-squares procedure of SHELXTL. Crystallographic data are given in the CIF files (CCDC 1834513–1834515). The ligand L1 and complex [Pt2Me4(μ-SMe2)2] were synthesized according to literature procedures.[9,12]

Chart 2

NMR Labeling Scheme

2-C5H4NCH2NH-3-C6H4OH, L2

A solution of 3-aminophenol (2.20 g, 20.20 mmol), 2-chloromethylpyridine hydrochloride (3.31 g, 20.20 mmol), and triethylamine (5.60 mL, 40.4 mmol) in ethanol (150 mL) was heated under reflux in ethanol for 8 h. The solvent was evaporated, the residue was dissolved in CH2Cl2 (100 mL), the solution was washed with water to remove salts, and then the solvent was evaporated. The product was purified by chromatography on a silica gel using ethyl acetate as the eluent. The first yellow fraction from the silica gel column was collected, the solvent was evaporated, and then yellow powder was dried in vacuo. Yield: 2.83 g, 70%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.50 (d, 1H, 3J(HH) = 5 Hz, H6a), 7.77 (dd, 1H, 3J(HH) = 9, 8 Hz, H4a), 7.48 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.29 (dd, 1H, 3J(HH) = 5, 9 Hz, H5a), 6.90 (dd, 1H, 3J(HH) = 8, 8 Hz, H5), 6.14 (d, 1H, 3J(HH) = 8 Hz, H4), 6.10 (d, 1H, 3J(HH) = 8 Hz, H6), 6.06 (s, 1H, H2), 4.41 (s, 2H, CH2); δ(13C) 161.6, 159.4, 151.2, 149.7, 138.9, 130.9, 123.6, 123.1, 106.2, 105.6, 100.9, 50.1 (CH2).

2-C5H4NCH2NH-4-C6H4OH, L3

A mixture of 4-aminophenol (4.00 g, 36.65 mmol) and 2-pyridinecarboxaldehyde (3.5 mL, 36.65 mmol) in tetrahydrofuran (60 mL) was stirred for 8 h. Solid NaBH4 (1.85 g, 49 mmol) was added to the resulting mixture in portions. The solvent was evaporated, the residue was dissolved in CH2Cl2 (100 mL), the solution was washed with water to remove salts and dried over MgSO4, and then the solvent was evaporated to give the product as a yellow powder. Yield: 5.36 g, 73%. NMR in CD3OD (400 MHz, 25 °C): δ(1H) 8.46 (d, 1H, 3J(HH) = 5 Hz, H6a), 7.72 (dd, 1H, 3J(HH) = 9, 8 Hz, H4a), 7.45 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.25 (dd, 1H, 3J(HH) = 5, 9 Hz, H5a), 6.60 (d, 1H, 3J(HH) = 8 Hz, H2,6), 6.51 (d, 1H, 3J(HH) = 8 Hz, H3,5), 4.35 (s, 2H, CH2); δ(13C) 161.5, 150.5, 149.6, 142.8, 138.8, 123.6, 123.4, 117.0, 115.9, 51.3 (CH2).

Formation of [PtMe2(L1)], 3

To a stirred solution of [Pt2Me4(μSMe2)2] (0.057 g, 0.100 mmol) in acetone (5 mL) was added a solution of ligand L1 (0.040 g, 0.200 mmol) in acetone (5 mL). The solution was stirred for 2 h under inert atmosphere, and the solvent was evaporated under vacuum to give the product in equilibrium with [PtMe2(SMe2)2] and free ligand. NMR in acetone-d6: δ(1H) 8.82 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.08 (t, 1H, 3J(HH) = 8 Hz, H4a), 7.59 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.45 (dd, 1H, 3J(HH) = 5, 8 Hz, H5a), 7.21 (d, 1H, 3J(HH) = 8 Hz, H3), 7.06 (dd, 1H, 3J(HH) = 7, 8 Hz, H5), 6.89 (d, 1H, 3J(HH) = 8 Hz, H6), 6.82 (dd, 1H, 3J(HH) = 8, 7 Hz, H4), 6.60 (br, 1H, NH), 4.65 (d, 1H, 2J(HH) = 16 Hz, 3J(HH) = 5 Hz, Ha), 4.34 (d, 1H, 2J(HH) = 16 Hz, 3J(HH) = 9 Hz, Hb), 0.65 (s, 3H, 2J(PtH) = 90 Hz, Mea), 0.24 (s, 3H, 2J(PtH) = 86 Hz, Meb).

Complexes 4 and 5 were prepared similarly. NMR in acetone-d6: 4, δ(1H) 8.76 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.08 (dd, 1H, 3J(HH) = 9, 8 Hz, H4a), 7.62 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.38 (dd, 1H, 3J(HH) = 5, 9 Hz, H5a), 6.99 (t, 1H, 3J(HH) = 8 Hz, H5), 6.65 (br, 1H, NH), 6.46 (d, 1H, 3J(HH) = 8 Hz, H4), 6.45 (s, 1H, H2), 6.44 (d, 1H, 3J(HH) = 8 Hz, H6), 4.68 (dd, 1H, 2J(HH) = 16 Hz, 3J(NHH) = 6 Hz, Ha), 4.46 (dd, 2J(HH) = 16 Hz, 3J(NHH) = 4 Hz, Hb), 0.58 (s, 2J(PtH) = 92 Hz, Mea), 0.41 (s, 2J(PtH) = 87 Hz, Meb). 5, δ(1H) 8.78 (d, 1H, 3J(HH) = 5 Hz, H6a), 8.06 (dd, 1H, 3J(HH) = 8, 7 Hz, H4a), 7.56 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.35 (dd, 1H, 3J(HH) = 5, 7 Hz, H5a), 6.96 (d, 2H, 3J(HH) = 8 Hz, H2,6), 6.68 (d, 2H, 3J(HH) = 8 Hz, H3,5), 6.60 (br, 1H, NH), 4.67 (d, 1H, 2J(HH) = 16 Hz, 3J(NHH) = 6 Hz, Ha), 4.38 (d, 2J(HH) = 16 Hz, 3J(NHH) = 5 Hz, Hb), 0.57 (s, 3H, 2J(PtH) = 90 Hz, Mea), 0.35 (s, 3H, 2J(PtH) = 84 Hz, Meb).

[PtMe2I2(L2)], 6

To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.054 g, 0.094 mmol) in acetone (10 mL) was added a solution of ligand L2 (0.038 g, 0.189 mmol) in acetone (10 mL), followed by the addition of I2 (0.048 g, 0.189 mmol). The solution was stirred for 6 h. The mixture was filtered, and the filtrate was layered with pentane to give orange plate crystals after 2 days, which were collected, washed with pentane, and dried in vacuo. Yield: 0.09 g, 70%. Anal. Calcd for C14H18I2N2OPt: C, 24.76; H, 2.67; N, 4.12. Found: C, 25.01; H, 2.65; N, 4.04%. NMR in (CD3)2CO: δ(1H) 8.75 (d, 1H, 3J(HH) = 6 Hz, H6a), 8.09 (dd, 1H, 3J(HH) = 9, 8 Hz, H4a), 7.85 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.67 (dd, 1H, 3J(HH) = 6, 9 Hz, H5a), 7.23 (t, 1H, 3J(HH) = 8 Hz, H5), 7.02 (s, 1H, H2), 7.00 (d, 1H, 3J(HH) = 8 Hz, H4), 6.68 (d, 1H, 3J(HH) = 8 Hz, H6), 6.48 (br, 1H, NH), 5.53 (d, 1H, 2J(HH) = 15 Hz, 3J(CH2–NH) = 12 Hz, Ha), 5.08 (d, 2J(HH) = 15 Hz, 3J(CH2–NH) = 4 Hz, Hb), 2.40 (s, 2J(PtH) = 75 Hz, Mea), 1.75 (s, 2J(PtH) = 74 Hz, Meb); δ(13C) 160.3, 158.7, 148.5, 145.7, 140.5, 130.5, 126.0, 124.4, 113.4, 112.1, 108.3, 60.2 (CH2), −14.1 (Me), −17.0 (Me).

[PtMe2I2(L3)], 7

This was prepared similarly but using ligand L3. Yellow crystals of 7·Me2CO were obtained in 66% yield. Anal. Calcd for C14H18I2N2OPt·(CH3)2CO: C, 27.69; H, 3.28; N, 3.80. Found: C, 28.03; H, 3.15; N, 3.80%. NMR in acetone-d6: δ(1H) 8.74 (d, 1H, 3J(HH) = 7 Hz, H6a), 8.07 (t, 1H, 3J(HH) = 8 Hz, H4a), 7.83 (d, 1H, 3J(HH) = 8 Hz, H3a), 7.65 (dd, 1H, 3J(HH) = 7, 8 Hz, H5a), 7.40 (d, 2H, 3J(HH) = 9 Hz, H2,6), 6.88 (d, 2H, 3J(HH) = 9 Hz, H3,5), 6.40 (br, 1H, NH), 5.48 (d, 1H, 2J(HH) = 15 Hz, 3J(CH2–NH) = 12 Hz, Ha), 5.05 (d, 2J(HH) = 15 Hz, 3J(CH2–NH) = 4 Hz, Hb), 2.35 (s, 2J(PtH) = 74 Hz, Mea); 1.73 (s, 2J(PtH) = 73 Hz, Meb); δ(13C) 160.2, 155.9, 148.4, 140.4, 136.5, 126.0, 124.2, 122.1, 116.2, 61.2 (CH2), −14.3 (Me), −17.3 (Me).

[Pt4I6Me10], 8

To a stirred solution of [Pt2Me4(μ-SMe2)2] (0.022 g, 0.039 mmol) in MeCN (5 mL) was added a solution of ligand L1 (0.015 g, 0.077 mmol) in MeCN (10 mL), followed by the addition of I2 (0.029 g). The mixture was stirred for 5 h to give a brown suspension. The mixture was filtered to remove [(PtI2Me2)], 9,[4] and the filtrate was layered with diethyl ether (50 mL). After 2 weeks, the orange block crystals of 8 which formed were collected, washed with pentane, and dried in vacuo. Yield: 0.013 g, 10%. Once formed, the crystals of 8 were insoluble in common organic solvents.[4]