Thermal and Photochemical Ring-Bromination in Naphthyl

Apr 23, 2015 - UV–vis spectra were recorded on a Cary 50 or Hewlett-Packard ..... Harvey , R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH: New Yo...
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Thermal and Photochemical Ring-Bromination in Naphthyl‑, Naphthdiyl‑, and Dicarboximideperyl-Platinum Complexes Alice Raphael Karikachery, Mehdi Masjedi, and Paul R. Sharp* 125 Chemistry, University of Missouri, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: Brominated polycyclic aromatic compounds are important synthons, but their synthesis can be difficult. Herein, we report that Pt(IV) centers σ-bonded to naphthalene and a dicarboximideperylene activate the ring systems to selective thermal and photochemical bromination. Thus, trans-Pt(PEt3)2(Br)3(4-bromo-1-naphthyl) and Br2 give trans-Pt(PEt3)2(Br)3(7,4-dibromo-1-naphthyl). Introduction of a second Pt(IV) center is achieved by double oxidative addition of 1,4-dibromonaphthalene to 2Pt(PEt3)4. Bromination of [trans-Pt(PEt3)2Br]2(1,4-naphthdiyl) yields [trans-Pt(PEt3)2(Br)3]2(1,4-naphthdiyl), which further brominates on the ring to give [transPt(PEt3)2(Br)3]2(6,7-dibromo-1,4-naphthdiyl). Photoreduction of the Pt(IV) centers with 1-hexene gives first mixed-valent [trans-Pt(PEt3)2(Br)3][trans-Pt(PEt3)2(Br)](6,7-dibromo-1,4-naphthdiyl) and then [trans-Pt(PEt3)2Br]2(6,7-dibromo-1,4-naphthdiyl). Photoreduction of trans-Pt(PEt3)2(Br)3(PMI) (PMI = N-(2,5-di-tert-butylphenyl)perylen-3-yl-9,10-dicarboximide) without 1-hexene slowly gives ring-bromination at the PMI 12-position. HOTf treatment cleaves the Pt−PMI bond to give 12bromo-N-(2,5-di-tert-butylphenyl)perylene-9,10-dicarboximide. The reaction chemistry indicates that the Pt(IV) center is equivalent to a bulky, electron-donating group for the naphthalene and PMI ring systems.

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INTRODUCTION Polycyclic aromatic hydrocarbons are useful synthons but must be functionalized for use. However, selective functionalization can be a difficult task.1 Selective monohalogenation is often possible, but polyhalogenation frequently gives mixtures.2−4 It has long been known that σ-bonding of a late transition metal center to an aryl group can activate the aryl ring to selective monohalogenation.5 This has been extended to chelating aryl6−12 and polycyclic aromatic systems,13,14 and we recently reported the selective mono-, di-, tri-, tetra-, and pentabromination of anthracene σ-bonded to a Pt(II) center.15 Herein, we report thermal bromination of naphthalene bonded to one and two Pt centers and photobromination of a perylene monoimide bonded to a single Pt center. In one case, we demonstrate that the brominated polycyclic hydrocarbon can be easily removed from the Pt center.

position 7 gives 3 (second reaction in Scheme 1). The bromination site in 3 is established by an X-ray crystal structure determination (Figure 1). Metrical parameters and other details for the structure determination are provided in the SI. A second platinum(II) center can be introduced onto the naphthyl ring by adding 2 equiv of Pt(PEt3)4 to 1,4dibromonaphthalene (Scheme 2), instead of 1 equiv as used in one of the syntheses of 1.16 (For other examples of

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RESULTS Syntheses. Previously, we reported the synthesis of naphthyl Pt(II) complex 1 and its bromination to Pt(IV) complex 2 (Scheme 1).16 Now we find that with excess bromine selective bromination of the naphthalene ring at Scheme 1. Synthesis of 2 and 3 Figure 1. XSeed/POV-Ray drawing of 3. Hydrogen atoms are omitted. Atoms are drawn as 50% probability ellipsoids. Received: December 23, 2014

© XXXX American Chemical Society

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Organometallics Scheme 2. Synthesis of 4, 5, and 6

polyplatinated polycyclic aromatic compounds see refs 17−23.) Resulting white, diplatinum(II) complex 4 is isolated in good yield (64%). Crystals for a single-crystal structure determination were obtained, and a drawing of the solid-state structure is shown in Figure 2. The expected arrangement of the two platinum(II) centers on the naphthalene ring is observed. Details of the structure determination may be found in the SI. Figure 3. XSeed/POV-Ray drawing of 6. Hydrogen atoms are omitted. Atoms are drawn as 50% probability ellipsoids.

Scheme 3. Photosynthesis of 7 and 8 from 6

Figure 2. XSeed/POV-Ray drawing of 4. Hydrogen atoms are omitted. Atoms are drawn as 50% probability ellipsoids.

As with 1,16 bromination of 4 occurs first at the Pt(II) centers to give the diplatinum(IV) complex 5. Again, 4 follows 1 and further bromination with excess bromine selectively brominates the ring, but now in two positions to give 6. Bromination at positions 6 and 7 is confirmed by an X-ray crystal structure determination (Figure 3). Ring-brominated 6 is photochemically active, and photolysis (313 nm) in the presence of 1-hexene (bromine trap16) yields first the mixed platinum(II)/platinum(IV) complex 7, through loss of 1 equiv of bromine, and then the diplatinum(II) complex 8 (Scheme 3), which is the ring-brominated analogue of 4. Complex 8 can be isolated pure, as it is the only species at the end of the photolysis. However, 7 is always present with either 6 or 8, or both, and was not isolated. Its mixed-valent platinum(II)/platinum(IV) nature is apparent from the 31P NMR spectrum (see below). An X-ray structure determination on crystals of 8 (Figure 4) confirms the photoreduction of both platinum(IV) centers in 6. In a previous publication16 we reported bromine photoreductive elimination from Pt(IV) PMI complex 9 with 1hexene to trap the eliminated bromine (Scheme 4) (PMI = N(2,5-di-tert-butylphenyl)perylen-3-yl-9,10-dicarboximide; the more common numbering is N-(2,5-di-tert-butylphenyl)perylen-9-yl-3,4-dicarboximide;24−27 we use the current numbering to maintain compatibility with previous work16). We

Figure 4. XSeed/POV-Ray drawing of 8. Hydrogen atoms are omitted. Atoms are drawn as 50% probability ellipsoids. Disorder in the P2- and P4-ethyl groups is not shown.

now find that if the alkene is omitted, a much slower photochemical reaction occurs, giving a 10:1 mixture of the Pt(II) complexes 10 and 11 (Scheme 4). Complex 1127 is the same product obtained in the presence of the alkene16 and is the expected product of bromine elimination from 9. Complex 11 presumably arises from bromine trapping by solvent or impurities in the reaction mixture. Complex 10 can be considered a self-trapping product where the eliminated bromine has reacted with the PMI ligand, eliminating HBr.26 PMI ligand bromination in 10 is evident from the 1H NMR spectrum, which shows the appearance of a new singlet in the B

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signal (δ 9.58) with weak four-bond coupling (2 Hz) to H6. H6 is strongly coupled (9 Hz) to H5, and H5 surprisingly shows weak coupling (3 Hz) to the platinum center. 1 H NMR spectra for the symmetric diplatinum complexes 4, 5, 6, and 8 show complex coupling patterns for H2 and H3 (Figure 5) and even H5 and H8 for 8. Although these protons

Scheme 4. Photolysis of 9 with and without 1-Hexene

aromatic region (δ 8.87) attributed to the now isolated proton at position 11 (Scheme 4). Bromine addition to the photogenerated mixture of 10 and 11 results in bromination at the Pt centers to give 9 and the ring-brominated analogue of 9, complex 12 (see Scheme 5). Figure 5. 1H NMR signal for H2 and H3 of 5 (500 MHz, CDCl3). Red lines are an approximate line representation of the experimental spectrum (* = impurity).

Scheme 5. Photosynthesis and Protonation of 12

are symmetry equivalent, the presence of 34% platinum-195 results in isotopomers with different spectral properties, including one with reduced magnetic symmetry where H2 and H3 are magnetically inequivalent. (Similar examples have been reported for other diplatinum complexes and ditungsten complexes.29−31) This is illustrated in Figure 6, which shows

The bromination and the photolysis can be combined into a single step, yielding clean and complete conversion of 11 into 12 simply by photolysis of a mixture of 11 and three or more equivalents of bromine (Scheme 5). Cleavage of the brominated PMI ring from the Pt center can then be achieved by triflic acid addition to 12. 12-Bromo-N-(2,5-di-tertbutylphenyl)perylene-9,10-dicarboximide (13) was isolated in an 80% crude yield at ∼80% purity. No attempt was made to optimize the conditions or to purify 13. Compound 13 is readily identified by NMR and mass spectroscopy. Finally, 12 can be quantitatively converted to 10 by photolysis (470 nm) in the presence of 1-hexene as a bromine trap. NMR Data. As mentioned above and as described in previous publications, 31P NMR spectroscopy readily distinguishes between platinum(II) and platinum(IV) centers in this class of complexes.16,28 Thus, platinum(IV) complexes 5, 6, 9, and 12 and the platinum(IV) center in 7 show singlets with platinum-195 satellites at δ −8.1 to −9.3 and coupling constants (JPt−P) between 1594 and 1635 Hz. On the other hand, the platinum(II) complexes 4, 8, and 10 and the platinum(II) center in 7 show shifts at δ 11.6 to 12.0 and JPt−P in the 2653 to 2783 Hz range. The bromination sites in naphthyl complexes 3 and 6 are revealed by the X-ray crystal structure determinations and the aromatic 1H NMR signals for these complexes, and their derivatives are then readily assigned. Thus, in 3, H2 is identified by strong coupling to platinum-195 (47.8 Hz) and to H3. H8, an isolated peri-hydrogen, appears as the lowest field proton

Figure 6. Isotopomers for diplatinum 1,4-naphthdiyl complexes with simulated 1H NMR signals for H2 and H3 (parameters from 5 at 500 MHz).

the three possible isotopomers and the simulated 1H NMR signals for H2 and H3 using parameters established for 5. Isotopomer A contains only platinum isotopes without spin, and so H2 and H3 are magnetically equivalent, do not couple to platinum, and give rise to a singlet. In contrast, isotopomer B contains one platinum-195 center and another platinum center without spin. As a result of the two magnetically different platinum centers, H2 and H3 are magnetically inequivalent and, although still having essentially identical chemical shifts, couple C

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Scheme 6. Bromination Chemistry of trans-Pt(PEt3)2Br(9anthracenyl)

to one another. The simulated spectrum is non-first-order, but to a first-order approximation has a central doublet for H2, which is only coupled to H3, and then a doublet-of-doublets for H3, which is coupled to H2 and the platinum-195 center. Finally, isotopomer C contains two identical platinum-195 centers. H2 and H3 are magnetically equivalent, and each couples to their adjacent platinum-195 center, giving rise to a doublet. The actual spectrum is a combination of each isotopomer spectrum weighted according to their abundance. With 34% platinum-195, the A:B:C ratio is 4:4:1. The H2 and H3 isotopomer signals for 4, 5, 6, and 8 (and H5 and H8) were simulated, and the resulting parameters are listed in the NMR data for the complexes in the Experimental Section. 1 H NMR and COSY 1H−1H NMR spectra of 10 and 12 reveal proton signal identities and the bromination sites. The aryl ring proton signals, H13, H14, and H15, are typically found upfield from the peryl unit proton signals,16,25−27 and this is the case in 10 and 12. Signals from H4, H5, and H6 are readily identified as a coupled doublet−triplet−doublet pattern with the peri-proton H4 typically found at lower field. Proton H2, “ortho” to the platinum center, displays 195Pt satellites in both 10 and 12, and this allows ready identification of coupled H1. The low-field shift of H1 indicates proximity to the bromine atom at position 12, and a low-field singlet is assigned to isolated H11. A pair of remaining coupled doublets is then assigned to H7 and H8, with bay-region H7 giving the lower field signal. An interesting feature of the 1H NMR spectrum of the platinum(II) complexes 1, 4, 8, 10, and 11 is the resolution of the diastereotopic methylene protons of the phosphine ethyl groups. Provided that there is no rotation around the Pt−C bond and fast rotation around the Pt−P bonds on the NMR time scale, all of the complexes should display two signals for the phosphine ethyl group protons due to the asymmetry of the platinum-PMI or platinum-naphthyl unit. Curiously, this is only observed for the platinum(II) complexes where the diastereotopic proton signals are separated into two identical multiplets with a δ 0.14 ± 0.02 shift difference. Why this should also not be observed for the platinum(IV) complexes is not clear.

Scheme 7. Resonance Forms for Naphthalene Devivative Ring-Brominations

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trans-Pt(PEt3)2Br3) are invoked instead along with steric considerations. The positive charge on the carbon center in resonance forms D−J′ is offset by the polarized Pt−C bond instead of by back-donation for the Pt center. (Again, a radical mechanism can be invoked using equivalent resonance forms.) Resonance forms E (if a Br atom were not already present) and F for 2 and J and J′ for 5 show activation at positions that would give 3 and 6. Forms D, G, H, H′, I, and I′ indicate activation at other positions, but these are sterically hindered by the presence of the very large, six-coordinate platinum center(s) (peri and ortho-like positions) or a bromine atom (2). The bromination chemistry of 2 and 5 can be compared with that of alkoxy-substituted naphthalenes, which are activated by electron donation from the alkoxy groups and do not present the same steric hindrance as the platinum(IV) center. Thus, bromination of 1-methoxynaphthalene (K, Scheme 8) gives L from bromination at the 2 and 4 positions,33−35 as expected from resonance forms M and N (Scheme 8, X = OMe), which are equivalent to resonance forms D and E but with the 2 position now accessible with the small methoxy group. Similarly, 1,4-dialkoxynaphthalene O (Scheme 8) undergoes facile bromination36 to give P, consistent with resonance forms Q and Q′ (Scheme 8, X = OC6H13), which are equivalent to J and J′ but without steric blocking.

DISCUSSION Previously we showed that platinum(II) centers σ-bonded to an anthracene ring at the 9-position, and thereby protected from oxidation by the peri-hydrogen atoms, activate the anthracene to electrophilic bromination (Scheme 6).15 (Photoinduced oxygen oxidation of a platinum-bonded anthracene ring has also been reported.32) The bromination pattern in the product complexes are consistent with resonance forms with an electron-donating platinum(II) center (box in Scheme 6), where electrophilic Br2 attacks the sites of high charge density. Equivalent resonance forms where the negative charge of the electron pair is replaced by a single electron can be invoked for a radical mechanism where Br2 functions as a single-electron oxidant generating a bromine radical, bromide, and a radical cation complex. Br radical than adds at the anthracenyl sites where radical electron density is highest, and a proton is lost, giving the final bromination product and HBr. Similarly, complexes 2 and 5 give bromination patterns suggesting electron-donating ring activation by the platinum(IV) centers. However, while back-donation and Pt−C double bond formation is not unreasonable for Pt(II), it seems less likely for Pt(IV), and the resonance forms in Scheme 7 (X = D

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Organometallics Scheme 8. Alkoxy Naphthalene Brominations

Scheme 11. Ring-Bromination of 9 from an Excited State

Although we did not attempt it, it should be possible to cleave the Pt−C bonds of 3, 6, and 7 with acid (cf. 12) to give dibrominated naphthalenes R and S (Scheme 9). While S is

brominated 10. In the alternative mechanism in Scheme 11, occasional ring-bromination occurs from an excited state that more commonly decays back to the ground state or eliminates Br2, which then recombines to give ground state 9. Both processes may contribute to the ring-bromination.

Scheme 9. Available Di- and Tribrominated Naphthalenes

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CONCLUSIONS The Pt(IV) fragment −PtX3L2 (X = a halide, L = a phosphine) behaves as a bulky, electron-donating, activating group on aromatic rings, in both thermal and photolytic ring-bromination reactions. Unlike the Pt(II) fragment −PtXL2, which also behaves as an electron-donating, activating group,15 the Pt(IV) center does not need to be sterically protected from bromination. The activation and directing ability of the Pt(IV) fragment can be rationalized through resonance forms with positive charge on the Pt-bonded carbon atom stabilized by the δ+−δ− polarization of the Pt−C bond. Acid cleavage of the Pt− C bond in the brominated complexes promises access to unusual bromination patterns on polycyclic aromatic compounds.

available by a variety of other methods,37−40 only one synthesis41 of R has been reported. Other polybrominated naphthalenes (T−W) are available as major products by bromination of naphthalene and 1-bromonaphthalene under various conditions3,42 The photobromination of 9 also appears to show electrondonor activation. In free PMI the carbodiimide group directs bromination to the 12 and 7 positions after initial bromination at position 3.24−26 With a platinum at position 3, resonance forms place negative charge at positions 2, 5, 7, and 12. Cooperative interaction of the carbodiimide and platinum groups should therefore favor positions 7 and 12, with the position 12 evidently more strongly favored by the proximity of the platinum center. Two possible schemes for the photobromination of the PMI ligand can be envisioned. In a previous publication we presented evidence that the photochemistry of 9 and related complexes involves the release of Br2 but also the generation of an excited state that can brominate alkenes.16 Scheme 10 and Scheme 11 are based on these two possibilities. In Scheme 10, Br2 is photoeliminated from 9 but most of the time recombines with coproduct 11 to regenerate 9. Occasionally, the Br2 attacks the ring, generating HBr and ring-

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EXPERIMENTAL SECTION

Materials and General Procedures. Pt(PEt3)4,43 1,4-dibromonaphthalene,3 trans-Pt(PEt3)2(4-bromo-1-naphthyl)Br (1),16 and trans-Pt(PEt3)2(Br)3(PMI) (9)16,27 were prepared by reported procedures. Reagents were purchased from commercial sources (Aldrich or Acros) and used as received. Experiments were performed in air unless otherwise indicated. Solvents for dinitrogen atmosphere experiments were dried, degassed, and stored under dinitrogen over 4 Å molecular sieves or sodium metal. NMR spectra were recorded on Bruker AMX-250, -300, -500, or -600 spectrometers at ambient probe temperatures except as noted. NMR shifts are given in δ with positive values downfield of TMS (1H) or external H3PO4 (31P). Peak assignments were assisted by COSY experiments (see SI). The nmrdb.org Web-based NMR simulator44 was used for the NMR spectra simulations. UV−vis spectra were recorded on a Cary 50 or Hewlett-Packard 8452 diode array spectrophotometer in quartz cells. Photolyses were performed in quartz or borosilicate glass vessels using a Philips PL-S 9W/01, 9 W lamp (313 nm emission), or LEDs of the indicated wavelength.

Scheme 10. Ring-Bromination of 9 from Photoeliminated Br2

trans-Pt(PEt3)2(Br)3(7,4-dibromo-1-naphthyl) (3). Br2 (0.06 mL, 200 mg, 1.2 mmol) was added dropwise with stirring to 6.9 mg of 1 (9.6 μmol) in CDCl3 (0.7 mL). The resulting brownish-red solution was stirred another 15 min. The volatiles were then removed E

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(s with complex satellites, JPtH = 44 Hz, JHH = 7 Hz, 2H, H2 and H3), 7.30 (A2B2, 2H, H6 and H7), 2.343 (m, 24H, CH2CH3), 1.05 (app quintet, J = 7.5 Hz, 36H, CH2CH3).

in vacuo to yield 11.4 mg (quantitative) of brownish-orange solid 3. Orange crystals for the X-ray analysis were obtained by slow evaporation of a concentrated CH2Cl2/C6D6 (50:50) solution of 3. Anal. Calcd (found) for C22H35Br5P2Pt: C, 27.64 (27.57); H, 3.69 (3.58). Complex 3 was also obtained along with 6 from the following reaction. Approximately 6 g (38 mmol) of Br2 was added dropwise with agitation to 54 mg of a 61:39 mixture of 1 (33 μmol) and 4 (26 μmol) in CDCl3 (∼2 mL) (amounts from 31P NMR integration). This yielded an opaque, brownish-maroon mixture. The volatiles were removed in vacuo to yield a red, oily residue. This was dissolved in CH2Cl2 (∼2 mL) and layered with Et2O (∼2 mL) to yield red and orange crystals. The red and orange crystals were separated manually to yield 15 mg (47%) of orange 3 and 26 mg (62%) of red 6. 31 P NMR (101 MHz, C6D6): −9.2 (JPtP = 1599 Hz). 1H NMR (250 MHz, C6D6): 9.58 (d, JHH = 1.9 Hz, 1H, H8), 8.30 (d with satellites, JHH = 8.3, JPtH = 47.8 Hz, 1H, H2), 8.09 (d with satellites, JHH = 9.0, JPtH = 3 Hz, 1H, H5), 7.33 (dd, JHH = 9.0 Hz, JHH = 1.9 Hz, 1H, H6), 7.27 (d, JHH = 8.4, 1H, H3), 2.17 (m, 12H, PCH2CH3), 0.75 (app quintet, J = 7.9 Hz, 18H, PCH2CH3).

[trans-Pt(PEt3)2(Br)3]2(6,7-dibromo-1,4-naphthdiyl) (6). Br2 (0.11 mL, 340 mg, 2.1 mmol) was added dropwise with stirring to 7.4 mg of 4 (6.4 μmoles) in CDCl3 (0.7 mL). The resulting brownishred solution was further stirred for 15 min. The volatiles were then removed in vacuo to yield 13.3 mg (quantitative) of reddish-orange solid 6. Red crystals for the X-ray analysis were obtained by slow evaporation of a concentrated C6D6 solution of 6. Complex 6 was also obtained along with 3 (see second preparation of 3). An analysis sample was obtained by crystallization from CH2Cl2. Anal. Calcd (found) for C34H64Br8P4Pt2·CH2Cl2: C, 24.57 (24.42, 24.29); H, 3.89 (3.80, 3.66). (The presence of CH2Cl2 was confirmed by 1H NMR.) 31 P NMR (101 MHz, C6D6): −9.3 (JPtP = 1613 Hz). In CDCl3: −8.6 (JPtP = 1607 Hz). 1H NMR (250 MHz, C6D6): 9.60 (s, 2H, H5 and H8), 8.14 (s with complex satellites, JPtH = 46 Hz, JHH = 8 Hz, 2H, H2 and H3), 2.29 (m, 12H, PCH2CH3), 0.87 (app quintet, J = 7.9 Hz, 18H, PCH2CH3). 1H NMR (250 MHz, CDCl3): 9.14 (s, 2H, H8 and H5), 7.81 (s with complex satellites, JPtH = 46 Hz, JHH = 8 Hz, 2H, H2 and H3), 2.34 (m, 12H, PCH2CH3), 1.06 (app quintet, J = 7.8 Hz, 18H, PCH2CH3).

[trans-Pt(PEt3)2(Br)]2(1,4-naphthdiyl) (4). This reaction was conducted under an N2 atmosphere. A clear orange solution of Pt(PEt3)4 (19.5 mg, 29.2 μmol) in THF (∼0.6 mL) was added to a clear pale yellow solution of 1,4-dibromonaphthalene (4.1 mg, 14 μmol) dissolved in THF (∼0.6 mL). The resulting clear orange solution was stirred overnight at 128 °C, during which time it turned pale yellow. The mixture was cooled to room temperature, and the volatiles were removed in vacuo to give a pale yellow solid. The solid was washed with 2 × ∼0.5 mL of cold hexane and dried in vacuo to yield 10.5 mg (64%) of off-white solid 4. Anal. Calcd (found) for C34H66Br2P4Pt2: C, 35.55 (35.77); H, 5.79 (5.84). Colorless crystals for the X-ray analysis were obtained by slow evaporation of a concentrated hexane solution of 4. The UV−visible absorption spectrum is available in the SI. 31 1 P{ H} NMR (101 MHz, CDCl3): 11.6 (s with satellites, JPtP = 2778 Hz). In THF: 12.0 (JPtP = 2783 Hz). In C6D6: 11.7 (JPtP = 2781 Hz). 1H NMR (250 MHz, C6D6): 8.86 (A2B2, 2H, H5 and H8), 7.34 (s with complex satellites, JPtH = 71 Hz, JHH = 7 Hz, 2H, H2 and H3), 7.32 (A2B2, 2H, H6 and H7), 1.64 (m, 12H, CH2CH3), 1.51 (m, 12H, CH2CH3), 0.94 (app quintet, J = 7.9 Hz, 36H, CH2CH3).

Di-trans-[Pt(PEt3)2(Br)3][Pt(PEt3)2Br](6,7-dibromo-1,4-naphthyl) (7) and di-trans-Pt(PEt3)2Br(6,7-dibromo-1,4-naphthyl) (8) or Photolysis of [trans-Pt(PEt3)2(Br)3]2(6,7-dibromo-1-naphthyl) (6) with 1-Hexene at 313 nm in CDCl3. Under an N2 atmosphere, a 5 mm quartz NMR tube was charged with a solution of 6 (2.7 mg, 1.7 μmol) in 0.5 mL of CDCl3. The NMR tube was capped and sealed with wax film. The sample was then irradiated in air with 313 nm light. The progress of the photolysis was monitored by 31P and 1 H NMR spectroscopy. After 10 min irradiation, a 51% yield of 7 was observed with 49% of 6 remaining. Another 10 min irradiation showed 63% 7 and 25% 8 with 12% of 6 remaining. A further 20 min irradiation showed 63% 7 and 37% 8 with no remaining 6. After another 105 min irradiation all 7 converted to 8. Total irradiation time was 145 min. 1H NMR spectroscopy indicated the formation of the bromine addition product 1,2-dibromohexane (88%). The NMR sample was then transferred to a vial and the solvent removed in vacuo to yield 2.6 mg (quantitative) of light brown solid 8. Pale yellow crystals for the X-ray analysis were obtained by slow evaporation of a concentrated CH2Cl2/hexane (50:50) solution of 8. NMR Data for 7: 31P NMR (101 MHz, CDCl3): 12.0 (s with satellites, JPtP = 2682 Hz, Pt(II)-P), −9.3 (s with satellites, JPtP = 1635 Hz, Pt(IV)-P). NMR Data for 8: 31P NMR (101 MHz, CDCl3): 12.0 (JPtP = 2718 Hz). 1H NMR (250 MHz, CD2Cl2): 8.96 (s with complex satellites, JPtH = 9 Hz, JHH = 2 Hz, 2H, H8 and H5), 7.20 (s with complex satellites, JPtH = 71 Hz, JHH = 7 Hz, 2H, H2 and H3), 1.65 (m, 12H, CH2CH3), 1.49 (m, 12H, CH2CH3), 1.03 (app quintet, J = 7.8 Hz, 36H, CH2CH3).

[trans-Pt(PEt3)2(Br)3]2(1,4-naphthyl) (5). To a clear colorless solution of 9.8 mg of 4 (8.5 μmol) in CDCl3 (∼2 mL) was added dropwise with stirring 0.8 mL of 0.2 M bromine (10 μL, 30 mg, 0.2 mmol) in CDCl3. The resulting clear orange solution was further stirred for 15 min. The volatiles were removed in vacuo to give 13.9 mg (quantitative) of red solid 5. Anal. Calcd (found) for C34H66Br6P4Pt2: C, 27.81 (27.99); H, 4.53 (4.51). The UV−visible absorption spectrum is available in the SI. 31 P NMR (101 MHz, CDCl3): −8.50 (s with satellites, JPtP = 1631 Hz). 1H NMR (500 MHz, CDCl3): 8.79 (A2B2, 2H, H5 and H8), 7.76 F

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Organometallics

CH2CH3). 31P{1H} NMR (CDCl3): δ −8.1 (s with satellites, JPPt = 1594 Hz). See Scheme 1 for atom numbering.

trans-Pt(PEt3)2(Br)(PMI-12-Br) (10). (a) Under an N2 atmosphere, a 5 mm NMR tube was charged with a C6H6 (0.8 mL) solution of trans-Pt(PEt3)2(Br)3(PMI) (9) (6 mg, 0.005 mmol) and exposed to light from a 470 nm LED reactor (containing 32 LEDs) for 5 h. A 31P NMR spectrum indicated complete conversion to products. The volatiles were removed in vacuo to give 5.3 mg of a mixture of transPt(PEt3)2(Br)(PMI-12-Br) (10) and trans-Pt(PEt3)2(Br)(PMI) (11) in a 10:1 ratio, respectively (by 1H NMR). Purple crystals of transPt(PEt3)2(Br)(PMI-12-Br) (10) were obtained by crystallization from methylene chloride/hexane at room temperature. Yield: 4.8 mg (86%). (b) A C6H6 (0.8 mL) solution of trans-Pt(PEt3)2(Br)3(PMI-12-Br) (12) (8 mg, 0.0063 mmol) and 1-hexene (20 mg, 0.24 mmol) in a 5 mm NMR tube was photolyzed at 470 nm for 2 h. A 31P NMR spectrum indicated complete conversion to a single phosphoruscontaining product. The volatiles were removed in vacuo, and the residue was dissolved in minimum CH2Cl2 and precipitated with excess methanol to give 6.5 mg (93%) of purple solid transPt(PEt3)2(Br)(PMI-12-Br) (10). The elemental analysis sample was obtained through this procedure. Anal. Calcd (found) for C48H59Br2NO2P2Pt: C, 52.47 (52.49); H, 5.41 (5.85). 1 H NMR (500 MHz, CDCl3): δ 9.22 (d, J = 8.5 Hz, H1), 8.96 (d, J = 8.0 Hz, H4), 8.87 (s, H11), 8.65 (d, J = 8.0 Hz, H7), 8.47 (d, J = 8.0 Hz, H8), 8.43 (d, J = 7.5 Hz, H6), 7.89 (d with satellites, J = 8.0, 75 Hz, H2), 7.64 (t, J = 8.0 Hz, H5), 7.59 (d, J = 8.5 Hz, H13), 7.46 (dd, J = 8.5 Hz, J = 2 Hz, H14), 7.02 (d, J = 2 Hz, H15), 1.73 (m, 6H, CH2CH3), 1.59 (m, 6H, CH2CH3), 1.33 (s, 9H, tBu), 1.28 (s, 9H, t Bu), 1.06 (app septet, J = 7.8 Hz, 18H, CH2CH3). 31P{1H} NMR (CDCl3): δ 12.0 (s with satellites, JPPt = 2653 Hz).

PMI-12-Br (13). Under an N2 atmosphere, about 7 mg of triflic acid was added to a solution of trans-Pt(PEt3)2(Br)3(PMI-12-Br) (12) (8 mg, 0.0063 mmol) in 3 mL of C6H6, and the mixture was stirred for 5 min. A blue precipitate formed upon standing the solution at room temperature. The precipitate was removed by filtration and discarded. The volatiles were removed from the filtrate in vacuo to give crude PMI-12-Br (13) as a red solid. Residual HOTf was removed by dissolving the crude product in minimum CH2Cl2 and precipitating with excess methanol. Yield: 3.4 mg (80%). 1 H NMR (500 MHz, CDCl3): δ 9.60 (d, J = 8.0 Hz, 1H, H1), 8.92 (s, 1H, H11), 8.67 (d, J = 8.0 Hz, 1H, H8), 8.47 (d, J = 7.5 Hz, 2H, H6 and H7), 8.00 (d, J = 9.0 Hz, 2H, H3 and H4), 7.69 (t, J = 8.0 Hz, 2H, H2 and H5), 7.59 (d, J = 8.5 Hz, 1H, H13), 7.45 (dd, J = 8.5 Hz, J = 2 Hz, 1H, H14), 6.99 (d, J = 2 Hz, 1H, H15), 1.32 (s, 9H, tBu), 1.29 (s, 9H, tBu). Exact mass (major isotopomers) found (calcd): m/z [M + H] + 588.15328 (588.153 27); 590.151 23 (590.151 22); [M] + 587.145 54 (587.145 44), 589.143 51 (589.143 40). (See SI for spectrum.) Dibrominated PMI was also detected: m/z [M + H]+ 670.059 96 (670.059 69); 668.062 23 (668.061 73), 666.064 21 (666.063 78); [M]+ 669.052 00 (669.051 86), 667.053 84 (667.053 91), 665.055 98 (665.055 95). (See SI for spectrum.)

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

CIF files, 1H NMR spectra, mass spectral data, and UV−vis absorption spectra (4 and 5). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support was provided by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-88ER13880). We thank Dr. Charles Barns for X-ray data collection and processing, Dr. Wei Wycoff for assistance with the NMR measurements, and Dr. Nathan Leigh for mass spectral analysis.

trans-Pt(PEt3)2(Br)3(PMI-12-Br) (12). A stock solution of 0.50 M Br2 (10 mL) in CH2Cl2 was prepared by dissolving 0.26 mL of Br2 (MW = 160 g/mol, d = 3.11 g/mL) in 10 mL of CH2Cl2. An aliquot of the stock Br2 solution (35 μL, 0.0175 mmol Br2) was added to a solution of trans-Pt(PEt3)2(Br)(PMI) (11) (6 mg, 0.0058 mmol) in 0.8 mL of C6H6 in a 5 mm NMR tube. The mixture was exposed to light from a 470 nm LED reactor (containing 32 LEDs) overnight. A 31 P NMR spectrum indicated complete conversion to a single product. The volatiles were removed in vacuo to give 7.1 mg (96%) of red solid trans-Pt(PEt3)2(Br)3(PMI-12-Br) (12). 1 H NMR (500 MHz, CDCl3): δ 9.04 (d, J = 8.5 Hz, H4), 8.99 (d, J = 9.0 Hz, H1), 8.82 (s, H11), 8.61 (d with satellites, J = 8.5 Hz, 48 Hz, H2), 8.60 (d, J = 8.0 Hz, H7), 8.38 (d, J = 7.5 Hz, H6), 8.37 (d, J = 8.0 Hz, H8), 7.55 (t, J = 8.0 Hz, H5), 7.52 (d, J = 8.5 Hz, H13), 7.39 (dd, J = 8.5 Hz, J = 2 Hz, H14), 6.94 (d, J = 2 Hz, H15), 2.45−2.28 (m, 12H, CH2CH3), 1.26 (s, 9H, tBu), 1.22 (s, 9H, tBu), 1.10−0.95 (m, 18H,

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REFERENCES

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DOI: 10.1021/om501322f Organometallics XXXX, XXX, XXX−XXX