Cyclometallation of the NNN pincer ligand in complexes of platinum

Abstract

Platinum complexes of the diimino/carbazolyl NNN pincer ligand have been prepared and characterized. Cyclometallation of the mesityl substituent was observed to occur unexpectedly easily in (NNN)PtOTf, which also catalyzed cyclometallation in (NNN)PtMe.

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Introduction

Pincer complexes have gained in popularity in part because they offer a robust and well-defined framework for study of elementary reactions at transition metal centers.¹ Our group, among many others, has been particularly attracted to pincer ligands based on the diarylamido central unit with ortho-positioned side donors (Chart 1).^2–5 They provide a semi-rigid framework for binding transition metals while the amido donor may display redox⁶ or acid/base^7,8 non-innocence. Carbazole can be regarded as a special diarylamine with greatly reduced basicity of the amido and increased rigidity of the underlying framework.⁹ We recently explored¹⁰ Pd complexes of the bis(imino)carbazolyl (NNN) pincer ligand pioneered by Gibson et al.¹¹ Here we report the synthesis of NNN complexes of Pt, and observations of unexpectedly facile cyclometallation.

Chart 1 Examples of diarylamido based pincer complexes.

Results and discussion

Thermolysis of (NNN)Me with (MeCN)₂PtCl₂ in mesitylene for 24 h at reflux led to the formation of (NNN)PtCl (1) that was isolated in 41% yield (Scheme 1). Although it required considerably more forcing conditions, this preparation mimicked the synthesis of (NNN)PdCl,¹⁰ and also the related syntheses of PNP pincer complexes of group 10 metals by N-Me cleavage.¹² Similarly to (NNN)PdCl, a quasi-reversible oxidation was observed for 1 by cyclic voltammetry. The formal potential for this primarily ligand-based^10,13 redox event for 1 (0.65 V vs. Fc/Fc⁺) is a few hundredths of a volt more negative than for (NNN)PdCl (0.69 V), as was seen for other Pd–Pt comparisons with diarylamido-based pincer ligands.¹³ The synthesis of (NNN)PtMe (2) was accomplished by treatment of 1 with MeMgCl in toluene, followed by heating the mixture at 80 °C for 20 h, with a 74% isolated yield. 1 and 2 displayed the expected C_2v symmetry in their ¹H and ¹³C NMR spectra. The Pt-bound Me group in 2 gave rise to singlet resonances in the ¹H NMR (δ 0.32 ppm) and ¹³C{¹H} NMR (−10.5 ppm) spectra with the expected satellite peaks from coupling to ¹⁹⁵Pt (²J_Pt–H = 70 Hz, ¹J_Pt–C = 772 Hz).

Scheme 1 Syntheses and reactivity of (NNN)Pt complexes.

A suspension of 2 in benzene rapidly reacted with one equiv. of triflic acid (in an ether solution) to lose methane and produce 3. When the reaction was performed under thoroughly dry conditions, 89% yield of pure 3 was isolated. It was noticed that in some reactions an impurity was present that was identified as the water adduct [(NNN)Pt–OH₂][OTf] 4. We previously noted an analogous water adduct formation in the synthesis of the Pd analog (NNN)PdOTf.¹⁰ However, unlike in the Pd case, molecular sieves or application of vacuum were not effective at removing water from 4 and (re)generating 3. A small sample of the pure water adduct 4 was isolated following passing crude 3 through silica gel. The ¹H NMR signal corresponding to the coordinated water molecule was observed at 4.86 ppm in C₆D₆. We were able to obtain two single crystals of 4 with different cocrystallized molecules: one contained a molecule of benzene in the asymmetric unit, while the other contained half of a disordered molecule of fluorobenzene, a molecule of triflic acid, and solvent-accesible voids modeled with SQUEEZE plausibly containing two water molecules each (Fig. 1 and ESI†). Triflic acid appears to be hydrogen bonded to the triflate anion of 4. The presence of HOTf was confirmed by its observation in the NMR spectrum of the dissolved crystals from this batch. The geometries of the central 4 cations in both structures are very similar. The corresponding Pt–N and Pt–O distances are statistically indistinguishable while the angles about Pt define a nearly perfect square planar environment with only 0–2° differences between the two structures. Similarly to the structure of (NNN)PdCl, the NNN ligand is nearly perfectly planar and pairwise π-stacking of the carbazole units is evident in the solid-state structures of 4.

Fig. 1 ORTEP drawing (50% probability ellipsoids) of 4. Omitted for clarity: H atoms and C6D6. Selected distance (Å) and angles (°) follow: Pt1–O1, 2.078(6); Pt1–N1, 1.933(6); Pt1–N2, 2.018(6); Pt1–N3, 2.023(6); N2–Pt1–N3, 177.1(2); N1–Pt1–O1, 175.7(2); N1–Pt1–N2, 91.0(3); N1–Pt1–N3, 90.5(3); N2–Pt1–O1, 87.9(2); N3–Pt1–O1, 90.8(2).

Treatment of 3/4 mixtures with molecular sieves or heating under vacuum led to the appearance of a new Pt product in the mixture, which was identified as cyclometallated (NNN)Pt–CH₂–C₆H₂(CH₃)₂ (5). 5 arises from cyclometallation of one of the mesityl substituents with a formal loss of triflic acid from 3. We were able to prepare pure 5 by treatment of 3 with Et₃N in benzene at ambient temperature. The NMR spectra of 5 are consistent with partial loss of symmetry associated with cyclometallation. The equivalent protons of the Pt-bound CH₂ group resonate at δ 2.35 ppm in C₆D₆. The ¹⁹⁵Pt NMR chemical shifts¹⁴ of 2 (−2483 ppm) and 5 (−2589 ppm) are quite similar, and distinct from those of 1 (−1849 ppm) and 3 (−1493 ppm). Interestingly, we found that 3 serves as the catalyst for cyclometallation of 2. The latter is indefinitely stable in C₆D₆ solutions at ambient temperature, but addition of 5% 3 led to the appearance of 15% of 5 and free methane within 5 min (and 80% conversion to 5 after 18 h). Since 3 loses HOTf under vacuum and to a weak base such as water, it seems plausible that 2 can serve as the base to promote cyclometallation of 3 to 5 by accepting the formal equivalent of HOTf, evolving methane and regenerating 3.

The solid-state structure of 5 was determined by X-ray diffraction methods (Fig. 2). The coordination environment about Pt is approximately square-planar and is defined by the now tetradentate cyclometallated NNN ligand. With the exception of the intact mesityl group, this molecule possesses a remarkably flat extended conjugated system about the Pt center. Not surprisingly, molecules of 5 in the solid state display extensive π-stacking (Fig. 2).

Fig. 2 ORTEP drawing (50% probability ellipsoids) of 5 with crystal packing and Pov-ray generated figure depicting side-view and shadow. Omitted for clarity: H atoms. Selected distance (Å) and angles (°) follow: Pt1–C23, 2.027(5); Pt1–N1, 2.007(4); Pt–N2, 2.011(4); Pt–N3, 2.012(4); N2–Pt1–N3, 178.37(15); N1–Pt1–C23, 176.43(16); N1–Pt1–N2, 92.89(16); N1–Pt1–N3, 88.70(16); N2–Pt1–C23, 83.59(18); N3–Pt1–C23, 94.82(18).

Mesityl group is a common substituent on a nitrogen atom in polydentate ligands. Cyclometallation of an N-mesityl moiety is well documented in the chemistry of early transition,¹⁵ lanthanide,¹⁶ and main group¹⁷ metals. However, it is quite rare for late metals¹⁸ and the existing examples have not been formed by electrophilic cyclometallation observed in 3. On the other hand, cyclometallation of the NNN ligand in this work can be related to the literature examples of intermolecular C–H activation by Pt^II–OTf complexes of amido-based pincer ligands (Scheme 2). In those examples from Peters,¹⁹ Liang,²⁰ and our own work,⁸ activation of aromatic solvents proceeded sluggishly at elevated temperatures in contrast to the rapid ambient-temperature C–H activation of 3. While the intramolecular nature of the reaction in 3 cannot be underestimated, it is worth pointing out that the examples in Scheme 2 were judged to proceed by a dissociative mechanism. This loss of triflate may be more facile in the NNN system as evidenced by the ready displacement of it by water (for both Pt and Pd), which is not observed for the corresponding PNP complexes. The steric bulk projected by the mesityl groups of the NNN ligand towards the non-pincer coordination site at the square planar metal center may be favouring coordination of smaller ligands (e.g., water). We hypothesize that evolution of the NNN ligand aimed at preventing cyclometallation while preserving strong steric pressure on the reactive site may lead to exciting intermolecular reactivity, and this will be a subject of future studies.

Scheme 2 C–H activation of benzene by (XNX)Pt–OTf pincer complexes.

Conclusions

In summary, a series of Pt complexes supported by a bis(imino)carbazolyl pincer ligand possessing mesityl sidearms has been synthesized. Comparison to the previously reported (NNN)PdX complexes¹⁰ demonstrates very similar metrics, redox potentials, and facile displacement of triflate by water at the metal center. This behavior is in contrast to that observed for analogous (PNP)MOTf complexes and is believed to result from the alternative steric profile and extended steric bulk of the mesityl groups. Surprisingly, weak bases including 2 in the presence of a catalytic amount of 3 were observed to consume an equivalent of HOTf to form the cyclometallated mesityl complex 5 through facile intramolecular C–H activation at room temperature.

Experimental section

General considerations and physical methods

Unless otherwise noted, all manipulations and reactions were performed under argon, using standard glovebox and Schlenk line techniques. Toluene, triethylamine, pentane, and C₆D₆ were dried over NaK/Ph₂CO/18-crown-6, distilled and stored over molecular sieves in an Ar-filled glovebox. Diethyl ether was dried and deoxygenated (by purging) using a solvent purification system and stored over molecular sieves in an Ar-filled glovebox. Fluorobenzene and CH₂Cl₂ were dried over CaH₂, distilled, and stored over molecular sieves in an Ar-filled glovebox.

NMR spectra were recorded on a Varian Inova 300 spectrometer (¹H NMR, 300.0 MHz, ¹³C NMR, 75.4 MHz, ¹⁹F NMR, 282.2 MHz, ³¹P NMR, 121.4 MHz), a Varian Inova 400 spectrometer (¹H NMR, 399.5 MHz, ³¹P NMR, 161.7 MHz, ¹³C NMR, 100.5 MHz, ¹⁹⁵Pt NMR, 85.5 MHz) and a Varian Inova 500 spectrometer (¹H NMR, 499.7 MHz, ¹³C NMR, 125.7 MHz, ¹⁹F NMR, 470.1 MHz, ³¹P NMR, 202.3 MHz) in noted solvents. Chemical shifts are given in δ (ppm). ¹⁹F NMR spectra were referenced externally using CF₃COOH at δ −78.5 ppm. ³¹P NMR spectra were referenced externally with 85% phosphoric acid at δ 0. ¹⁹⁵Pt NMR spectra were referenced externally with K₂PtCl₄ aqueous solution at δ −1620. ¹H NMR and ¹³C NMR spectra were referenced using the solvent signals. Elemental analyses were performed by CALI, Inc. (Parsippany, NJ).

Synthesis of Pt complexes

(NNN)PtCl (1). (NNN)Me¹⁰ (3.12 g, 6.25 mmol) was dissolved in approximately 150 mL mesitylene. (MeCN)₂PtCl₂ ²¹ (2.17 g, 6.23 mmol) was added to the 250 mL Schlenk flask. Reaction was put under reflux in an oil bath at 180 °C for 24 h. Reaction changed color from yellow to brown-orange after approximately an hour sitting in the oil bath. Volatiles were removed by vacuum. The resulting oily brown residue was filtered through Celite and silica gel using toluene. Volatiles were removed under vacuum which yielded an orange powder. Solid was filtered off and washed with cold pentane (3 × 10 mL). Yield: 1.8 g, 41%. ¹H NMR (300 MHz, C₆D₆): δ 8.16 (s, 2H, Ar–H), 8.15 (s, 2H, N=CH), 7.10 (s, 2H, Ar–H), 6.84 (s, 4H, Ar–H), 2.45 (s, 6H, Ar–CH₃), 2.32 (s, 12H, Ar–CH₃), 2.18 (s, 6H, Ar–CH₃). ¹³C{¹H} NMR (75.4 MHz, CDCl₃): δ 157.8 (C=N), 151.8 (CAr), 136.2 (CAr), 135.7 (CAr), 131.3 (CAr), 130.9 (CAr), 128.4 (CAr), 128.0 (CAr), 125.6 (CAr), 117.1 (CAr), 21.5 (Ar–CH₃), 21.0 (Ar–CH₃), 18.5 (Ar–CH₃). ¹⁹⁵Pt NMR (85.7 MHz, CDCl₃): δ −1849. Elem. An. Found (Calculated) for C₃₄H₃₄N₃PtCl: C, 57.07 (57.10); H, 4.73 (4.79)%.

(NNN)PtMe (2). 1 (0.196 g, 0.274 mmol) was dissolved in approximately 30 mL toluene. MeMgCl (0.90 mL, 2.7 mmol) was added to flask via syringe. Flask was heated in oil bath at 80 °C for 20 h. The product was filtered through Celite and silica gel using toluene. Volatiles were removed under vacuum yielding a red powder. The solid was collected without further purification. Yield: 141 mg, 73.6%. ¹H NMR (300 MHz, C₆D₆): δ 8.50 (s, 2H, N=CH), 8.33 (s, 2H, Ar–H), 7.26 (s, 2H, Ar–H), 6.83 (s, 4H, Ar–H), 2.53 (s, 6H, Ar–CH₃), 2.24 (s, 12H, Ar–CH₃), 2.19 (s, 6H, Ar–CH₃), 0.32 (s, 3H, J_Pt–H = 69.6 Hz, Pt–CH₃). ¹H NMR (300 MHz, CD₂Cl₂): δ 8.79 (s, 2H, J_Pt–H = 98 Hz, N=CH), 8.56 (s, 2H, Ar–H), 7.73 (s, 2H, Ar–H), 6.93 (s, 4H, Ar–H), 2.72 (s, 6H, Ar–CH₃), 2.32 (s, 6H, Ar–CH₃), 2.13 (s, 12H, Ar–CH₃), −0.39 (s, 3H, J_Pt–H = 70.5 Hz, Pt–CH₃). ¹³C{¹H} NMR (100.5 MHz, CDCl₃): 158.0 (C=N), 151.2 (CAr), 138.77 (CAr), 135.5 (CAr), 131.0 (CAr), 130.8 (CAr), 128.4 (CAr), 127.4 (CAr), 127.0 (CAr), 117.6 (CAr), 21.7 (Ar–CH₃), 20.9 (Ar–CH₃), 18.1 (Ar–CH₃), −10.5 (s, J_Pt–C = 772 Hz, Pt–Me). ¹⁹⁵Pt NMR (85.7 MHz, CD₂Cl₂): δ −2483. Elem. An. Found (Calculated) for C₃₅H₃₇N₃Pt: C, 60.53 (60.51); H, 5.46 (5.37)%.

(NNN)PtOTf (3). 2 (64 mg, 0.092 mmol) was dissolved in C₆D₆ in a J. Young NMR tube forming a red solution with some solid remaining undissolved due to low solubility. HOTf (34 μL of 2.7 M solution in Et₂O, 0.092 mmol) was added via syringe and solution immediately became yellow-orange. In situ¹H NMR analysis at 15 min showed the presence of free methane. Volatiles were removed yielding an orange solid. Solid was washed with cold pentane (3 × 1 mL) and dried under vacuum. Yield: 68 mg, 89%. ¹H NMR (300 MHz, C₆D₆): δ 8.04 (s, 2H, Ar–H), 7.83 (s, 2H, N=CH), 7.00 (s, 2H, Ar–H), 6.92 (s, 4H, Ar–H), 2.50 (s, 12H, Ar–CH₃), 2.41 (s, 6H, Ar–CH₃), 2.22 (s, 6H, Ar–CH₃). ¹³C{¹H} NMR (75.4 MHz, C₆D₆): 160.7, 149.4, 136.7, 135.9, 132.4, 131.8, 129.4, 125.5, 117.0, 21.1, 21.0, 19.5, –CF₃ signal not resolved. ¹⁹F NMR (282.2 MHz, C₆D₆): δ −77.78. ¹⁹⁵Pt NMR (85.7 MHz, C₆D₆): δ −1493. Elem. An. Found (Calculated) for C₃₅H₃₄F₃N₃O₃SPt: C, 50.57 (50.72); H, 4.18 (4.14)%.

[(NNN)PtOH₂]OTf (4). 2 (0.14 g, 0.20 mmol) was dissolved in toluene in a 25 mL Schlenk flask. HOTf (0.10 mL, 0.20 mmol) was added via syringe. After 20 h at room temperature, volatiles were removed under vacuum yielding an orange solid. The solid was redissolved in fluorobenzene and filtered over Celite and silica gel using fluorobenzene. Orange crystals of 5 were obtained from a solution of fluorobenzene layered with pentane. The supernatant was decanted from the crystals and the volatiles were removed generating a yellow-orange powder found to be 4. ¹H NMR (300 MHz, C₆D₆): δ 8.07 (s, 2H, Ar–H), 7.73 (s, 2H, N=CH), 7.12 (s, 2H, Ar–H), 7.07 (s, 4H, Ar–H), 4.86 (br, 2H, Pt–OH₂), 2.45 (s, 6H, Ar–CH₃), 2.35 (s, 6H, Ar–CH₃), 2.20 (s, 12H, Ar–CH₃). ¹³C{¹H} NMR (75.4 MHz, C₆D₆): 158.6 (C=N), 145.9 (C_Ar), 138.8 (C_Ar), 135.8 (C_Ar), 132.0 (C_Ar), 130.8 (C_Ar), 129.4 (C_Ar), 128.8 (C_Ar), 125.9 (C_Ar), 117.1 (C_Ar), 21.12 (Ar–CH₃), 21.0 (Ar–CH₃), 18.0 (Ar–CH₃), CF₃ signal not resolved. ¹⁹F NMR (282.2 MHz, C₆D₆): δ −78.70. Elem. An. Found (Calculated) for C₃₅H₃₆F₃N₃O₄SPt: C, 49.43 (49.64); H, 4.44 (4.29)%.

Cyclometallated (NNN)Pt (5). 3 (22 mg, 0.027 mol) was transferred to a J. Young NMR tube. C₆D₆ (0.6 mL) was added, giving an orange solution but not all of the solid was dissolved. Upon addition of NEt₃ (4 μL, 0.03 mmol), the solution immediately became red. After mixing for 30 min on an NMR tube spinner, ¹H NMR analysis showed complete conversion to cyclometallated product. The solution was filtered over a pad of silica gel and Celite. Volatiles were removed, yielding an orange-red solid. Solid was washed with cold pentane (3 × 1 mL) and dried under vacuum. Yield: 11 mg, 59%. ¹H NMR (500 MHz, C₆D₆): δ 9.29 (s, 1H, J_Pt–H = 93 Hz, N=CH), 8.43 (s, 1H, J_Pt–H = 90 Hz, N=CH), 8.37 (s, 1H, Ar–H), 8.31 (s, 1H, Ar–H), 7.50 (s, 1H, Ar–H), 7.34 (s, 1H, Ar–H), 7.00 (s, 1H, Ar–H), 6.96 (s, 2H, Ar–H), 6.52 (s, 1H, Ar–H), 2.95 (s, 2H, J_Pt–H = 66 Hz, Pt–CH₂–Ar), 2.56 (s, 3H, Ar–CH₃), 2.53 (s, 3H, Ar–CH₃), 2.35 (s, 3H, Ar–CH₃), 2.32 (s, 3H, Ar–CH₃), 2.28 (s, 6H, Ar–CH₃), 2.09 (d, 3H, Ar–CH₃). ¹H NMR (300 MHz, CD₂Cl₂): δ 9.68 (s, 1H, J_Pt–H = 90 Hz, N=CH), 8.77 (s, 1H, J_Pt–H = 90 Hz, N=CH), 8.58 (s, 1H, Ar–H), 8.53 (s, 1H, Ar–H), 7.90 (s, 1H, Ar–H), 7.78 (s, 1H, Ar–H), 7.06 (s, 2H, Ar–H), 6.93 (s, 1H, Ar–H), 6.76 (s, 1H, Ar–H), 2.85 (s, 3H, Ar–CH₃), 2.76 (s, 3H, Ar–CH₃), 2.74 (s, 3H, Ar–CH₃), 2.46 (s, 3H, Ar–CH₃), 2.35 (s, 2H, coupling to Pt obscured, Pt–CH₂–Ar), 2.27 (s, 3H, Ar–CH₃), 2.19 (d, 6H, Ar–CH₃). ¹³C{¹H} NMR (75.4 MHz, CD₂Cl₂): δ 157.3, 155.1, 152.4, 151.4, 151.2, 138.2, 138.0, 136.2, 135.9, 131.9, 131.0, 130.9, 130.4, 129.6, 128.6, 128.0, 127.8, 127.7, 127.7, 126.7, 126.3, 118.8, 118.1, 24.5, 21.7, 21.0, 20.4, 17.9, 16.4. ¹⁹⁵Pt NMR (85.7 MHz, C₆D₆): δ −2589. Elem. An. Found (Calculated) for C₃₄H₃₃N₃Pt: C, 60.28 (60.17); H, 4.87 (4.90)%.

Reactivity studies

Attempt to remove H₂O from 4. A sample of 4 as a yellow powder was transferred to a 25 mL Schlenk flask and heated at 130 °C under vacuum for 48 h. After 48 h, the discolored solid was redissolved in C₆D₆ forming a red solution. ¹H NMR analysis showed a mixture of 53% 3 and 47% 5 was formed.

Reaction of 2 with 5 mol% 3. 2 (18 mg, 0.026 mmol) was dissolved in 1.8 mL of C₆D₆. 3 was then added (55 μL of 0.024 M soln in C₆D₆, 0.0013 mmol). The reaction was monitored by ¹H NMR spectroscopy. After five minutes, free methane was observed in the ¹H NMR spectrum along with 15% conversion of 2 to 5. After 18 h, 80% of the 2 was converted to the Pt(II) cyclometallated species. 5% of the 3 along with free methane were also observed in the ¹H NMR spectrum.

Acknowledgements

We are thankful to the US National Science Foundation (grant CHE-1062840 for the REU site at TAMU and grant CHE-1300299 to O.V.O.) and the Welch Foundation (grant A-1717 to O.V.O.) for support of this research. We are also grateful to Ms. Linda Redd for editorial assistance, to Allen Lunsford for assistance with cyclic voltammetry measurements, and to Prof. Andrei Vedernikov for insightful discussions.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: NMR spectra, X-ray crystallographic data. CCDC 1405023 (4·HOTf·C₆H₅F), 1405024 (4·C₆D₆), and 1405025 (5). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qi00102a

‡ J.R.D. was an NSF-REU participant from Hampden-Sydney College in 2013. Current address: Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USA.

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