versus Six-Membered Chelate Rings - ACS Publications - American

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Photophysical Properties of Pt(II) Polypyridines with Five- versus SixMembered Chelate Rings: Trade-Offs in Angle Strain Sean N. Natoli, Lauren M. Hight, Matthias Zeller, and David R. McMillin* H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: This report describes the synthesis and characterization of a series of eight [Pt(NNN)X]+ complexes where the tridentate NNN ligand is (2,2′-bipyrid-6-yl)(pyrid2-yl)sulfide (btp) or methyl(2,2′-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) and X is OMe, Cl, phenylethynyl (C2Ph), or cyclohexylethynyl (C2Cy). The expectation was that inserting a heteroatom into the backbone of 2,2′:6′,2″-terpyridine (trpy) would expand the overall intraligand bite angle, introduce ILCT character into the excited states, and improve the photophysical properties. Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp) Cl]+ reveal that atom insertion into the trpy backbone successfully expands the bite angle of the ligand by 8−10°. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C2Ph]+ and [Pt(btp) C2Ph]+ complexes display appreciable emission in fluid solution, and they exhibit shorter emission lifetimes than [Pt(trpy)C2Ph]+. One reason is that the bond angle preferences of platinum and the inserted heteroatom induce the sixmembered rings to deviate from planarity and adopt a boat-like conformation, impairing charge delocalization within the ligand. In addition, angle strain induces the donor atoms about platinum to assume a pseudotetrahedral arrangement, which offsets any benefit due to the increase in overall bite angle by promoting deactivation via d−d excited states. The results reveal that, in order to improve the luminescence of a [Pt(NNN)X]+ system, one must take care to avoid trading one kind of angle strain for another.

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INTRODUCTION Developing tridentate polypyridine ligands that complex with platinum(II) and support the observation of long-lived photoluminescence signals in fluid solution has been an ongoing effort.1−3 Applications for photoluminescent platinum(II) complexes include ion sensing,4,5 vapor sensing,6 organic light-emitting diodes,7 intracellular imaging,8 and phototherapy,9 to name a few. The orbital parentage of the active excited state is an important consideration, and a platinumcentered, or d−d excited state, in particular, is undesirable due to a low radiative rate constant and a very efficient nonradiative decay pathway that potentially involves distortion of the platinum(II) complex to a tetrahedral geometry.10,11 On the other hand, complexes incorporating the tridentate 2,2′:6′,2″terpyridine (trpy) ligand have intraligand (IL) π−π* and/or metal-to-ligand charge-transfer (MLCT) excited states that are potentially emissive. The trpy framework is also attractive because it reinforces a planar coordination geometry and disfavors tetrahedral distortions.12,13 Nevertheless, d−d states remain thermally accessible,12 at least in part because the overall bite angle of the trpy ligand is ca. 160° and less than the ideal value of 180°.3,12 As a consequence, the [Pt(trpy)Cl]+ complex itself is nonemissive in fluid solution. However, ligand variants yield platinum(II) complexes capable of exhibiting long-lived emission in fluid solution;14−17 see Chart 1 for a selection of NNN polypyridine ligands employed to date. One strategy for driving the deactivating d−d states to higher energies focuses on ligand design, for example, replacing a ring nitrogen with a CH group, which becomes an extremely strong© XXXX American Chemical Society

Chart 1. Polypyridine Frameworks

field donor center upon deprotonation.18,19 Alternatively, one can replace the chloride coligand with a strong-field donor such as an acetylide or cyanide.4,20 Yet another way to raise the energy of the d−d states involves expanding the trpy framework and increasing the effective bite angle.17,21,22 A different approach to enhancing the barrier to radiationless decay entails stabilizing the emitting state vis-à-vis the deactivating state.14,15 Received: March 10, 2018

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DOI: 10.1021/acs.inorgchem.8b00636 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(6.56). FT-IR (cm−1): 3102, 2250, 830. 1H NMR (CD3CN): δ 9.60 (d, 1H, J = 8.0 Hz), 9.54 (ddd, 1H, J = 7.8, 6.0, 1.5 Hz), 9.48 (d, 1H, J = 7.6 Hz). 8.40−8.248 (m, 3H), 8.14−8.07 (m, 3H), 7.80 (ddd, 1H, J = 7.8, 6.0, 1.8 Hz), 7.54 (ddd, 1H, J = 6.0, 4.2, 1.8 Hz). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 356 (9, 520), 341 sh, 278 (27, 050), 269 (29, 190). Preparation of [Pt(bmap)Cl]PF6. The synthetic procedure is analogous to that used for the btp analogue using bmap instead of btp. Yield: 120 mg (0.19 mmol) (47% based on Pt). Data for [Pt(bmap)Cl]PF6: ESI-MS m/z (MeOH): 493-[Pt(bmap)Cl]+. Elem. Anal. Found (calcd) for C16H14ClF6N4PPt: C, 29.94 (30.13); H, 2.16 (2.21); N, 8.83 (8.78). FT-IR (cm−1): 3137, 825. 1H NMR (300 MHz, CD3CN): δ 9.52 (d, 1H, J = 6 Hz), 8.90 (d, 1H, J = 4.2 Hz), 8.58 (d, 1H, J = 3.3 Hz), 8.37−8.00 (m, 3H), 7.75 (q, 1H, J = 5.4, 3.9 Hz), 7.65−7.51 (m, 2H), 7.38 (t, 1H, J = 7.5 Hz), 7.23 (t, 1H, J = 6.3 Hz), 1.91 (s, 3H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 401sh, 380sh, 360 (9, 910), 345 (9, 210), 321 (9, 700), 302 (9, 610). Preparation of [Pt(btp)OMe]PF6. A round-bottom flask contains 100 mg (0.16 mmol) of [Pt(btp)Cl]PF6 dissolved in 20 mL of MeOH. The addition of a mixture of 0.2 mL (1.4 mmol) of Et3N and 5 mL of MeCN initiated the reaction. After stirring for 48 h under ambient conditions, the addition of Et2O (100 mL), and overnight storage in a freezer (at ca. −20 °C), a red precipitate becomes evident. The next steps involve the collection of the precipitate by filtration, a Et2O rinse, and dissolution in MeCN. After reintroduction of Et2O, crystalline [Pt(btp)OMe]PF6 forms in the freezer. Yield: 64 mg (0.10 mmol) (64% based on [Pt(btp)Cl]PF6). Data for [Pt(btp)OMe]PF6: ESI-MS (MeOH): 491-[Pt(btp)OMe]+. Elem. Anal. Found (calcd) for C16H14F6N3OPPtS: C, 30.93 (30.20); H, 2.33 (2.22); N, 6.52 (6.60). FT-IR (cm−1): 3130, 1100, 833. 1H NMR (CD3CN): δ 9.76 (d, 1H, J = 6.3 Hz), 9.35−9.11 (m, 2H), 8.92−8.67 (m, 3H), 8.38 (d, 1H, J = 8.7 Hz), 8.11−7.94 (m, 2H), 7.82 (d, 1H, J = 8.4 Hz), 7.63 (d, 1H, J = 8.1 Hz), 3.95 (s, 3H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 450 (1, 000), 364sh, 341 (11, 600), 328 (10, 030), 310 (8, 500). Preparation of [Pt(bmap)OMe]PF6. An analogous procedure using [Pt(bmap)Cl]PF6 instead of [Pt(btp)Cl]PF6 yields [Pt(bmap)OMe] PF6, except in this case the crystalline material is yellow. Yield: 54 mg (0.08 mmol) (54% based on [Pt(bmap)Cl]PF6). Data for [Pt(bmap) OMe]PF6: ESI-MS (MeOH): 488-[Pt(bmap)OMe]+. Elem. Anal. Found (calcd) for C17H17F6N4OPPt: C, 31.97 (32.24); H, 2.60 (2.71); N, 8.80 (8.85). FT-IR (cm−1): 3117, 2906, 1056, 833. 1H NMR (CD3CN): δ 9.34 (d, 1H, J = 5.7 Hz), 8.97 (d, 1H, J = 5.7 Hz), 8.16− 7.37 (m, 7H), 7.15 (t, 1H, J = 7.2 Hz), 7.03 (t, 1H, J = 6.6 Hz), 3.59 (s, 3H), 1.92 (s, 3H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 390sh, 362 (7, 900), 345 (7, 500), 317 (8, 700). Preparation of [Pt(btp)C2Ph]PF6. The Schlenk flask contains 350 mg (0.54 mmol) of [Pt(btp)Cl]PF6, 20 mL of DMF, 0.07 mL (0.64 mmol) of HC2Ph, and 0.2 mL (1.4 mmol) of Et3N. Aluminum foil is used to protect the reaction solution from room light. After deoxygenating, the introduction of a catalytic amount of CuI (2 mol %) initiates the reaction. After 48 h of reaction, the next steps were solvent removal in vacuo and dissolution in DCM, followed by extraction with NaCl(aq) (50 mL × 3) solution. After the organic layer dried over Na2SO4, addition of hexane leads to crystallization of [Pt(btp)C2Ph]PF6 as a yellow solid. Yield: 166 mg (0.23 mmol) (43% based on [Pt(btp)Cl]PF6). Data for [Pt(btp)C2Ph]PF6: ESI-MS (MeOH): 561-[Pt(btp)C2Ph]+. Elem. Anal. Found (calcd) for C23H16F6N3PPtS: C, 38.89 (39.10); H, 2.22 (2.28); N, 5.68 (5.95). FT-IR (cm−1): 3071, 2118 (CC), 1071, 840. 1H NMR (CD3CN): δ 10.10 (d, 1H, J = 7.8 Hz), 9.88 (d, 1H, J = 6.9 Hz), 9.48 (t, 1H, J = 6.7 Hz), 8.57 (d, 1H, J = 7.2 Hz), 8.33−8.03 (m, 5H), 7.87−7.66 (m, 3H), 7.54−7.22 (m, 4H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 450sh, 407 (5, 800), 354 (9, 220). Preparation of [Pt(bmap)C2Ph]PF6. Prior to reaction, introduce 350 mg (0.55 mmol) of [Pt(bmap)Cl]PF6, 20 mL of DMF, 0.07 mL (0.64 mmol) of HC2Ph, and 0.2 mL (1.4 mmol) of Et3N in a Schlenk flask wrapped with aluminum foil. After deoxygenating, introduction of a catalytic amount of CuI (2 mol %) initiates the reaction. After 48 h of reaction, the next steps were solvent removal in vacuo and

For example, the emitting state takes on intraligand chargetransfer (ILCT) character and moves to lower energy when one installs an electron-donating dimethylamino group at the 4′ position of trpy. The [Pt(dma-T)CN]+ complex that results emits from a state that is more ligand centered and has an impressive lifetime of 20 μs in deoxygenated dichloromethane solution.20 Both strategies come into play with the (2,2′-bipyrid-6yl)(pyrid-2-yl)sulfide (btp) and methyl(2,2′-bipyrid-6-yl)(pyrid-2-yl)amine (bmap) ligands used herein (Chart 1). More specifically, the insertion of a heteroatom into the trpy backbone provides for an expansion of the overall bite angle of both ligands. In the case of bmap, the heteroatom is electron donating and potentially capable of introducing ILCT character as well. Unexpectedly, however, both platforms in combination with multiple coligands yield platinum(II) complexes that are weakly or nonluminescent in fluid solution, due to confounding effects of intraligand strain.

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EXPERIMENTAL SECTION

General Procedures. Used as received reagents phenylacetylene, cyclohexylacetylene, K2PtCl4, 1,5-cyclooctadiene (cod), 6-bromo-2,2′bipyridine (Br-bpy), 2-(methylamino)pyridine, bis(dibenzylideneacetone)palladium(0) (Pd(dba)2), pyridine-2-thiol (HS-py), Pt(cod)Cl2, and 1,1′-ferrocenediyl-bis(diphenylphosphine) (DPPF) came from Sigma-Aldrich. A literature procedure afforded the btp ligand.23 Except as noted, reaction conditions involved standard Schlenk equipment and a dry N2 atmosphere. Electrochemical studies involved a glassy carbon working electrode (diameter = 2 mm) along with a Pt-wire auxiliary electrode in combination with a Ag/Ag+ reference electrode. The concentration of analyte was always 1.0 mM in 4 mL of dry DMF under an Ar atmosphere. Separate scans permitted the use of ferrocene as an external reference. The methods used for measuring excited state lifetimes appear in the literature.24 Alantic Microlab, Norcross, GA, and Midwest Microlab, Indianapolis, IN, returned elemental analyses (EA). Preparation of bmap. The first step is charging a Schlenk flask fitted with a condenser with 23 mg (0.04 mmol) of Pd(dba)2, 22 mg (0.04 mmol) of DPPF, 235 mg (1.0 mmol) of Br-bpy, and 135 mg of KOtBu. To that one adds 20 mL of dry toluene and 0.15 mL (1.5 mmol) of 2-(methylamino)pyridine. Cross-coupling occurs upon refluxing for 2 h.25 Addition of EtOAc and H2O yields a separable combined organic phase. A yellow oil is left after washing with saturated aqueous NaCl(aq) (3 × 30 mL) and solvent removal. Separation is possible on an Al2O3 column during elution with a DCM/hexane solution gradient (1:20 to 1:5). After solvent removal and recrystallization from a THF/MeOH solution, one obtains bmap as a white solid. Yield: 240 mg (0.91 mmol) (91% based on 6-bromo2,2′-bipyrdine). Data for bmap: Rf = 0.52 (DCM/hexane 1:3). ESI-MS m/z (DCM): 263 (M + H). FT-IR (cm−1): 3056, 2998, 1851. 1H NMR (300 MHz, CDCl3): δ 8.72 (dt, 2H, J = 5.1, 1), 8.66 (m, 2H), 8.48 (dd, 1H, J = 7.8, 1.5 Hz), 8.01 (t, 2H, J = 8.1, Hz), 7.89 (td, 2H, J = 7.8, 1.8 Hz), 7.35 (m, 2H), 1.56 (s, 3H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 287 (20, 800), 304 (16, 300), 318sh, 340sh. Preparation of [Pt(btp)Cl]PF6. The reaction begins with the suspension of 200 mg (0.53 mmol) of Pt(cod)Cl2 in a round-bottom flask containing 30 mL of H2O under atmospheric conditions. To this solution one adds 170 mg (0.64 mmol) of btp dissolved in 3 mL of THF before heating at 60 °C for 2 h.23 After cooling to room temperature, pour in an aqueous solution of NaPF6 and cool the mixture overnight in a refrigerator at ca. 15 °C. Filtration affords [Pt(btp)Cl]PF6 as a light-yellow powder that one can rinse with H2O (50 mL) and dry by rinsing with Et2O (50 mL). Salt metathesis with NaOTf facilitated growing crystals suitable for X-ray analysis. Yield: 202 mg (0.32 mmol) (59% based on Pt). Data for [Pt(btp)Cl]PF6: ESI-MS m/z (MeOH): 495-[Pt(btp)Cl]+. Elem. Anal. Found (calcd) for C15H11ClF6N3PPtS: C, 27.72 (28.11); H, 1.86 (1.73); N, 6.46 B

DOI: 10.1021/acs.inorgchem.8b00636 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry dissolution in DCM, followed by extraction with NaCl(aq) (50 mL × 3) solution. After drying the organic layer over Na2SO4, the addition of hexane leads to crystallization of [Pt(bmap)C2Ph]PF6 as a yellow solid. Yield: 216 mg (0.30 mmol) (56% based on [Pt(bmap)Cl]PF6). Data for [Pt(bmap)C2Ph]PF6: ESI-MS (MeOH): 558-[Pt(bmap) C2Ph]+. Elem. Anal. Found (calcd) for C24H19F6N4PPt: C, 40.87 (40.98); H, 2.77 (2.72); N, 7.91 (7.96). FT-IR (cm−1): 3106, 2919, 2126 (CC), 831. 1H NMR (CD3CN): δ 9.67 (dd, 1H, J = 8.1, 1.5 Hz), 9.50 (d, 1H, J = 5.7 Hz), 8.11−8.04 (m, 3H), 7.91 (t, 1H, J = 8.7 Hz), 7.85 (d, 1H, J = 8.1 Hz), 7.49−7.09 (m, 8H), 6.95 (t, 1H, J = 6.3 Hz), 1.94 (s, 3H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 425sh, 360 (9, 200), 350 (8, 770). Preparation of [Pt(btp)C2Cy]PF6. A Schlenk flask prepared as before contains 200 mg (0.31 mmol) of [Pt(btp)Cl]PF6, 20 mL of DMF, 0.05 mL (0.38 mmol) of HC2Cy, and 0.2 mL (1.4 mmol) of Et3N. Reaction occurs with the addition of a catalytic amount of CuI (2 mol %) while stirring under N2 for 48 h. Preliminary purification involves removing the solvent in vacuo, dissolution into DCM, and washing with a NaCl(aq) (5 mL × 3) solution. After exposing the organic layer to Na2SO4, one can isolate [Pt(btp)C2Cy]PF6 as a yellow solid by crystallization induced via the addition of hexane. Yield: 116 mg (0.16 mmol) (52% based on [Pt(btp)Cl]PF6). Data for [Pt(btp)C2Cy]PF6: ESI-MS (MeOH): 567-[Pt(btp)C2Cy]+. Elem. Anal. Found (calcd) for C23H24F6N3OPPtS: C, 37.64 (37.81); H, 3.43 (3.31); N, 5.64 (5.75). FT-IR (cm−1): 3107, 2923, 2851, 2147 (C C), 834. 1H NMR (CD3CN): δ 10.20 (d, 1H, J = 7.9 Hz), 9.93 (ddd, 1H, J = 7.6, 5.4, 1.4 Hz), 8.327 (m, 3H), 8.05 (m, 4H), 7.42 (ddd, 1H, J = 7.6, 5.4, 1.3 Hz), 7.46 (ddd, 1H, J = 6.0, 4.2, 1.8 Hz), 2.15−1.94 (m, 11H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 410 (2, 150), 370sh, 350 (7, 470). Preparation of [Pt(bmap)C2Cy]PF6. A Schlenk flask prepared as before contains 150 mg (0.24 mmol) of [Pt(bmap)Cl]PF6, 20 mL of DMF, 0.4 mL (0.30 mmol) of HC2Cy, and 0.2 mL (1.4 mmol) of Et3N. Reaction occurs with the addition of a catalytic amount of CuI (2 mol %) while stirring under N2 for 48 h. Purification of [Pt(bmap) C2Cy]PF6 as a yellow solid followed as before. Yield: 118 mg (0.17 mmol) (71% based on [Pt(bmap)Cl]PF6). Data for [Pt(bmap)C2Cy] PF6: ESI-MS (MeOH): 564-[Pt(bmap)C2Cy]+. Elem. Anal. Found (calcd) for C24H29F6N4O2PPt: C, 38.72 (38.66); H, 3.73 (3.92); N, 7.75 (7.51). FT-IR (cm−1): 3083, 2926, 2852, 2154 (CC), 835. 1H NMR (CD3CN): δ 9.89 (dd, 1H, J = 6, 1.8 Hz), 9.77 (d, 1H, J = 5.7 Hz), 8.33−7.65 (m, 7H), 7.54 (d, 1H, J = 8.4 Hz), 7.13 (t, 1H, J = 6.9 Hz), 2.77−1.84 (m, 14H). UV−vis (DCM): λmax (εmax, L mol−1cm−1) 425sh, 400sh, 368 (8, 360), 351(8, 770), 335 (8, 020). Equipment. The spectrometer used for UV−vis spectra was a Jasco V-670 and that used for FT-IR measurements was a Jasco FT/IR-6300 spectrometer equipped with an ATR accessory. A Varian Cary Eclipse spectrophotometer yielded emission data, while 1H NMR spectra came from a Varian Mercury 300 NMR spectrometer; all chemical shifts (δ) are referenced to the residual solvent signal (CH3OH, CHCl3, or CH3CN). A Waters 600 LC/MS Electrospray ionization mass spectrometer yielded (ESI-MS) data, and a CHI620A voltammetric analyzer recorded voltammograms. X-ray Data Collection, Processing, Structure Analysis, and Refinement for Crystals. The Rigaku Rapid II curved image plate diffractometer was equipped with a Cu Kα X-ray microsource (λ = 1.54178 Å) with a laterally graded multilayer (Goebel) mirror for monochromatization. Single crystals were mounted on Mitegen microloop mounts using a trace of mineral oil. Direct methods solved the structures with Shelxl software used for refinements by full matrix least-squares against F2 against all reflections.26 See the Supporting Information for additional details about data collection, refinement, and disorder or twinning. H atoms attached were positioned geometrically and constrained to ride on their parent atoms, with carbon hydrogen bond distances of 0.95 Å for aromatic C−H and 0.98 Å for CH3 moieties, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. Uiso(H) values were set to a multiple of Ueq(C) with 1.5 for CH3 and 1.2 for C−H units, respectively. See Table 1 for a brief description of the crystallographic details. Complete sets of crystallographic data,

Table 1. Crystal Data for Compounds [Pt(btp)Cl]OTf and [Pt(bmap)C2Ph]PF6 sum formula fw, g mol−1 space group a, Å b, Å c, Å α, ° β, ° γ, ° V, Å3 Z ρcalcd, g cm−3 T, K final R indices (I > 2σ(I)) GOF on F2

[Pt(btp)Cl]OTf

[Pt(bmap)C2Ph]PF6

[C15H11ClN3PtS]·CF3O3S 644.94 P1̅ 7.4898(6) 10.4215(5) 13.1514(7) 73.710(4) 79.533(5) 82.179(6) 964.98(11) 2 2.220 150 0.0385 0.1079 1.109

[C24H19N4Pt]·PF6·C2H3N 744.54 Pbca 6.8873(2) 25.4139(9) 29.0162(9) 90 90 90 5078.8(3) 8 1.947 100 0.0437 0.1196 1.065

deposited in CIF format, are available. The Cambridge Crystallographic Data Centre archives the supplementary crystallographic data as entries CCDC 1824533 ([Pt(btp)Cl]OTf) and 1824534 ([Pt(bmap)C2Ph]PF6) which one can obtain free of charge via www.ccdc.cam.ac.uk/structures/.

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RESULTS Syntheses. Attempts to synthesize methyl(2,2′-bipyrid-6yl)(pyrid-2-yl)amine (bmap) by heating a mixture of 6-bromo2,2′-bipyridine and 2-(methylamino)pyridine produced little product, but the cross-coupling of 6-bromo-2,2′-bipyridine and 2-(methylamino)pyridine is possible in the presence of a palladium catalyst under Hartwig amination conditions.25,27 Complexation of bmap and btp with platinum yielded [Pt(btp) Cl]PF6 and [Pt(bmap)Cl]PF6 which provide access to a variety of alkynyl and alkoxy complexes in moderate yields (Scheme 1).23 The methoxy complexes [Pt(btp)OMe]PF6 (64%) and [Pt(bmap)OMe]PF6 (54%) formed during the reaction of the parent halide compounds with methanol in the presence of Et3N. Pt−alkynyl complexes were accessible from halide precursors in the presence of the transmetalation catalyst CuI under Sonogashira type coupling conditions.24,28−30 Isolates of the phenyl acetylide derivatives [Pt(btp)C2Ph]PF6 (43%) and [Pt(bmap)C2Ph]PF6 (56%) as well as the cyclohexyl acetylides [Pt(btp)C 2 Cy]PF 6 (52%) and [Pt(bmap)C2Cy]PF6 (71%) are obtained in moderate yields as crystalline yellow solids. All of the reported complexes appear to be stable diamagnetic compounds, authenticated by ESI-MS, 1H NMR, and elemental analysis as well as single crystal X-ray diffraction analysis in the cases of the [Pt(btp) Cl]+ and [Pt(bmap)C2Ph]+ cations. Molecular Structures. Slow evaporation of a concentrated MeCN solution of either [Pt(btp)Cl]OTf or [Pt(bmap)C2Ph] PF6 yielded single crystals suitable for X-ray diffraction. Figures 1 and 2 present views of the complex ions [Pt(btp)Cl]+ and [Pt(bmap)C2Ph]+, respectively. In each case, the platinum(II) center adopts a pseudosquare-planar geometry, with the coligand trans to the N2 nitrogen of the NNN chelating ligand. All Pt−N, Pt−Cl, and Pt−C bond distances are within ranges found for other platinum terpyridine complexes.13,31 Table 2 reports selected bond lengths and angles. C

DOI: 10.1021/acs.inorgchem.8b00636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Platinum(II) Complexesa

a

Conditions: (i) MeOH, Et3N, r.t., 48 h; (ii) 1.2 equiv of HC2R, Et3N, CuI, DMF, r.t., 48 h.

for [Pt(trpy)Cl]+.13 The Pt···Pt distance of 4.182(1) Å is too long for significant interaction. In contrast to the btp structure, there is evidence of head-to-tail packing in the [Pt(bmap) C2Ph]+ structure (Figure 3). However, the intermolecular Pt···

Figure 1. Perspective view of [Pt(btp)Cl]+ at the 30% probablility level. Hydrogen atoms and the counterion are omitted for clarity.

Figure 2. Perspective view of [Pt(bmap)C2Ph]+ at the 30% probablility level. Hydrogen atoms and the counterion are omitted for clarity.

Figure 3. Crystal packing diagram for [Pt(bmap)C2Ph]+. Hydrogen atoms and counterions are omitted for clarity.

Table 2. Selected Bond Lengths (Å) and Bond Angles (o) for [Pt(btp)Cl]OTf and [Pt(bmap)C2Ph]PF6

Pt distance (3.474(1) Å) is still too long for a significant metal−metal interaction, consistent with the yellow color.32,33 The [Pt(bmap)C2Ph]+ molecule is closer to ideal square planar geometry with Pt−N bonds nearly equivalent in length (2.018(5)−2.027(5) Å) and increased internal N−Pt−N bond angles (N(1)−Pt(1)−N(2), 92.8(2)°; N(2)−Pt(1)− N(3), 80.6(2)°; N(1)−Pt(1)−N(3), 170.8(2)°). The phenyl ring of the acetylide ligand tilts to make a dihedral angle of 17.8(1)° relative to the platinum-bipyridine moiety, while the CC and Pt−C1 bond distances of 1.202(9) and 1.966(6) Å, respectively, are comparable with corresponding distances found in related platinum(II) acetylide complexes.13,31 The departure from a planar coordination geometry is also quantifiable, vide infra. The method used involves first calculating the least-plane defined by platinum and the four donor atoms and then adjusting the atom displacements so that they are relative to the parallel plane that passes through platinum. Electrochemistry. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) obtained for compounds [Pt(btp)Cl]PF6 and [Pt(bmap)Cl]PF6 appear in Figure 4, and Table 3 includes a compilation of related electrochemical data. The [Pt(bmap)Cl]+ ion exhibits a reversible reduction centered at −1.39 V and two chemically less reversible waves at −1.97 V and ca. −2.57 V, respectively. In part, the loss of the chloride coligand may complicate the observed process

[Pt(btp)Cl]+ Pt1−N1 Pt1−N2 Pt1−N3 Pt1−Cl1 C5−S1 C6−S1

N1−Pt1−N2 N2−Pt1−N3 N1−Pt1−N3 N1−Pt1−Cl1 N2−Pt1−Cl1 N3−Pt1−Cl1 C5−S1−C6

[Pt(bmap)C2Ph]+ 2.040(5) 1.992(5) 2.013(5) 2.296(2) 1.771(6) 1.757(6)

96.7(2) 80.7(2) 168.0(2) 90.6(2) 170.3(2) 93.4(2) 107.7(3)

Pt1−N1 Pt1−N2 Pt1−N3 Pt1−C1 C13−N4 C14−N4 C15−N4 C1−C2 N1−Pt1−N2 N2−Pt1−N3 N1−Pt1−N3 N1−Pt1−C1 N2−Pt1−C1 N3−Pt1−C1 Pt1−C1−C2 C13−N4−C15 C14−N4−C15 C13−N4−C14

2.025(5) 2.027(5) 2.018(5) 1.966(6) 1.398(7) 1.482(7) 1.393(7) 1.202(9) 92.8(2) 80.6(2) 170.8(2) 94.5(2) 171.7(2) 92.6(2) 172.9(5) 127.8(5) 116.1(5) 115.4(5)

As anticipated, the N(1)−Pt(1)−N(3) angle of 168.0(2)o in [Pt(btp)Cl]+ is larger than the 161.8° value previously reported D

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Figure 4. Cyclic voltammograms (CV; black) and differential pulse voltammograms (DPV; gray) recorded for compounds [Pt(btp)Cl] PF6 and [Pt(bmap)Cl]PF6 in a 0.10 M DMF solution of Bu4NPF6 at a scan rate of 0.10 V s−1 for CV and a pulse width of 0.05 s for DPV.

because, as noted by Gray and co-workers,34 the analogous [Pt(bipy)(py)2]+ complex exhibits three reversible reductions. Chemical irreversibility is also evident in the electrochemistry of the [Pt(btp)Cl]+ ion which exhibits a reversible wave at −1.27 V (A) and a quasi-reversible one at −1.91 V (B) followed by additional irreversible reductions. Measurements for [Pt(btp)Cl]+ obtained at varying scan rates appear in Figure S29 which reveals that the peak current (ip) increases linearly with the square root of the scan rate ν (V s−1), while the peak-to-peak separation increases. Furthermore, no detectable electrochemical features appear after rinsing and then transferring the working electrode to a fresh electrolyte solution. The latter results indicate that no strongly absorbed surface material is present and that the analyte solution remains homogeneous during voltammetry experiments.36 The second reduction (B) of [Pt(btp)Cl]+ is therefore likely the result of a reversible electron transfer followed by a chemical reaction, involving the loss of the coligand, or a conformational change of the ligand framework, vide infra. At scan rates approaching 0.5 V s−1, the second reduction of [Pt(btp)Cl]+ becomes chemically reversible (Figure S29). Absorbance and Emission. The btp and bmap complexes exhibit intense absorptions below 300 nm that are attributable to intraligand transitions typical of polypyridine complexes.3,4,12,37 At slightly lower energies, another series of mainly intraligand π−π* transitions maximize at wavelengths around 360 nm with intermediate intensities of εmax ≈ 5000− 10 000 M−1cm−1. See Figures 5 and 6 and Table 4. The longwavelength absorption maximum at 460 nm in the spectrum of [Pt(btp)OMe]+ accounts for the red color of the solid and plausibly corresponds to an MLCT band also found in the spectrum of [Pt(trpy)OMe]+.38 If so, it is puzzling that there is no counterpart in the spectrum of the bmap analogue. The absorption spectra of both phenylacetylide complexes exhibit

Figure 5. Absorbance spectra of btp complexes in room-temperature dichloromethane solution.

shoulders at wavelengths beyond 400 nm to which one can ascribe to ligand-to-ligand charge-transfer (LLCT) orbital parentage.31 The latter two complexes are the only ones in the series that exhibit appreciable emission in solution. Table 5 includes a summary of their photophysical properties, and Figure 7 portrays the emission spectra.

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DISCUSSION In a dilute glass, [Pt(trpy)Cl]+ emits from a state with mainly 3 IL character, but it probably includes an admixture of 3MLCT parentage as well.39 The accepted explanation for the fact that the complex is nonemissive in fluid solution is that a thermally accessible d−d excited state promotes efficient nonradiative decay.4,12 In principle, it should be possible to drive d−d excited states to higher energy and improve the emission efficiency by increasing the overall bite angle of the trpy ligand. Indeed, results reported herein reveal that inserting a heteroatom between the 6′ and 2″ carbons of the trpy ligand, to make one of the chelate rings six-membered, successfully increases the overall bite angle of the ligand to 168.0(2)° and 170.8(2)° in the [Pt(btp)Cl] + and [Pt(bmap)C 2 Ph] + structures, respectively. However, crystal structures reveal that

Table 3. Redox Potentials (V, vs Fc/Fc+) of [Pt(btp)Cl]PF6 and [Pt(bmap)Cl]PF6 complex +

[Pt(btp)Cl] [Pt(bmap)Cl]+ [Pt(trpy)Cl]+34 [Pt(dma-T)Cl]+14 [Pt(PhQ)Cl]+21 [Pt(bipy)(py)Cl]+35 a

E1/2/V (A) (Ep,a − Ep,c, ic/ia)

E1/2/V (B) (Ep,a − Ep,c, ic/ia)

−1.27 (59, 0.80) −1.39 (60, 0.81) −1.25 −1.53 −1.23 −1.38

−1.91 −1.97 (50, 0.83) −1.81 −1.97 −1.81 −2.07

E1/2/V (C)

a

−2.57a −2.6

Irreversible electrochemical potential. E

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Figure 7. Uncorrected emission spectra of [Pt(btp)C2Ph]+ and [Pt(bmap)C2Ph]+ measured at room temperature in deoxygenated dichloromethane solution. Figure 6. Absorbance spectra of bmap complexes in room-temperature dichloromethane solution.

severe in the case of the btp ligand because divalent sulfur heteroatom normally prefers a bond angle of about 90° as well. As listed in Table 5, the sums of the interior angles of the sixmembered rings in [Pt(btp)Cl]+ and [Pt(bmap)C2Ph]+ are 699.8(1.0)° and 711.4(1.0)°, respectively, and are both less than the ideal sum of 720°. As a consequence, the sixmembered chelate rings adopt a conformation distorted toward half-boat, in agreement with results obtained with other systems.21,40,41 With the mean plane defined by the platinum atom and the four donor centers as the reference, the sulfur atom of the btp complex exhibits the largest out-of-plane displacement at 1.226(6) Å. By comparison, the bridging N(4) nitrogen of the bmap structure is only 0.524(7) Å out of the coordination plane. Even then, torsion angles reveal the distortion is large enough to inhibit conjugation of the lone pair of the bridging nitrogen with the remainder of the bmap ligand. Thus, the C(14)−N(4)−C(15)−N(2) torsion angle is −162.4(5)°, and the C14−N4−C13−N1 angle is 152.6(5)° rather than the idealized value of ±180°. Emission Trends. Expanding the chelate ring size nevertheless improves the photophysical properties, as data in Table 5 reveal that the emission lifetime increases as the overall bite angle of the tridentate ligand approaches 180°. However, the lifetime obtained for [Pt(bmap)C2Ph]+ is not as impressive as some obtained in previous reports. For example, Huo and coworkers have obtained lifetimes approaching 10 μs in solution,

Table 4. Long Wavelength Absorbance Data in Dichloromethane Solution X

λmax, nm (ε, M−1cm−1)

Cl OMe C2Cy C2Ph Cl OMe C2Cy C2Ph

345 (9210), 360 (9910), 380sh, 401sh 345 (7500), 362 (7900), 390sh 351 (8770), 368 (8360), 400sh, 425sh 350 (8770), 360 (9200), 425sh 341sh, 356 (9520) 341 (11 600), 364sh, 450 (1000) 350 (7470), 370sh, 410 (2150) 354 (9220), 407 (5800), 450sh 287 (20 800), 304 (16 300), 318sh 240 (17 500), 285 (13 600), 308sh

complex [Pt(bmap)X]

+

[Pt(btp)X]+

bmap btpa a

Ref 23, in MeOH.

another type of strain sets in. In particular, the six-membered chelate ring is nonplanar in both complexes which ensures some disruption of π conjugation within the ligand polypyridine framework. An ideal six-sided figure like benzene is planar because each equal interior angle measures exactly 120°, so strain in the chelate ring is inevitable because Pt(II) prefers a bond angle of only 90°. The problem is even more

Table 5. Physical Data Including Emission Yields (ϕ’s) and Excited State Lifetimes (τ, s) in Deoxygenated Dichloromethane at Room Temperature complex +

[Pt(btp)Cl] [Pt(bmap)C2Ph]+ [Pt(PhQ)Cl]+ [Pt(QpQ)Cl]+ a

ϕ

τ, ns

Σφi, dega

0.0014 0.014 0.002 0.036

38 170 310 16 000

699.8(1.0) 711.4(1.0) 716.2(1.3)

(N(1)−Pt(1)−N(3)), deg 170.8(2) 174.8(2)21 178.822

kr,b s−1 3.7 8.2 6.5 2.2

× × × ×

104 104 103 103

Sum of the interior angles of the six-membered chelate ring. bRadiative rate constant calculated as ϕ/τ. F

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radiationless decay because a tetrahedral distortion favors the formation of d−d excited states.10,12,39,42 Such a distortion may account for the fact that the btp complex emits at a longer wavelength and with a shorter excited state lifetime than the bmap analogue in that the btp complex is likely to suffer from greater angle strain. Reber and co-workers have reported that in-plane bending distortions influence lifetimes in the solid state, and they could conceivably play a role in solution as well.43

though they employed carbometalating CNN ligands which are sure to drive the d−d states to even higher energies.17 Williams and co-workers reported an even longer lifetime of 16 μs in solution in their studies involving the QpQ, an NNN ligand that supports the formation of two six-membered chelate rings (Chart 1).22 That study did not include an X-ray structure, but DFT calculations predicted an overall bite angle of 178.8°. Evidently, there is reduced strain in complexes of quinolylcontaining ligands because the sum of the interior angles of the six-membered chelate ring is very close to 720° (Table 5). Another motivation for introducing an electron-rich CH3N in the interior of the ligand was that the emissive state of bmap complexes would drop to lower energy by virtue of taking on ILCT character, as is the case with the [Pt(dma-T)Cl]+ system. However, a comparison of the spectra of [Pt(bmap)Cl]+ and [Pt(btp)Cl]+ provides no evidence of enhanced absorption intensity in the bmap complex.14,20 One explanation for the absence of ILCT character may be impaired conjugation within the π system of the bmap ligand due to nonplanarity. The meta relationship between the NCH3 group and the opposite pyridine of the bpy moiety probably inhibits charge migration as well. Insight into the orbital parentage of the emitting state comes from comparisons of the radiative rate constants of the complexes listed in Table 5. Note that the kr calculated value for the [Pt(bmap)C2Ph]+ system is relatively high at 8.2 × 104 s−1, consistent with greater heavy metal participation, i.e., greater MLCT character. On the other hand, the lower rate constants exhibited by the [Pt(PhQ)Cl]+ and [Pt(QpQ)Cl]+ systems, at 6.5 × 103 and 2.2 × 103 s−1, respectively, are consistent with greater intraligand character. The difference in orbital parentage may also explain why the [Pt(PhQ)Cl]+ and [Pt(QpQ)Cl]+ complexes exhibit vibronically structured emissions, whereas the btp and bmap complexes exhibit comparatively broad and unstructured emission bands. Yet another factor to consider in assessing emission lifetimes is the susceptibility of the platinum complex to distortion. Figure 8 reveals the extent to which the complexes, especially the btp complex, exhibit tetrahedral distortions in the electronic ground state. Further distortion along that coordinate in the electronic excited state would doubtlessly enhance the rate of

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CONCLUSIONS Crystal structures of [Pt(bmap)C2Ph]+ and [Pt(btp)Cl]+ reveal that inserting a heteroatom between the 6′ and 2″ carbons of the trpy backbone makes one of the chelate rings six-membered and expands the overall bite angle of the ligand by 8−10° in platinum(II) complexes. However, the impact on the photophysics is minimal. Indeed, of the eight systems investigated, only the [Pt(bmap)C2Ph]+ and [Pt(btp)C2Ph]+ complexes display appreciable emission in fluid solution and even then with shorter lifetimes than [Pt(trpy)C2Ph]+.31 Impaired conjugation within the π system of the bmap ligand may account for the lack of any sign of intraligand chargetransfer character in either the absorption or emission spectra. Like [Pt(trpy)OMe]+,38 [Pt(btp)OMe]+ exhibits enhanced charge-transfer absorption in the visible region, but the btp complex is less active because it fails to exhibit an emission signal in fluid solution, probably because of facile deactivation via a low energy d−d state. In the bmap and btp complexes, the six-membered rings deviate from planarity and adopt a boat-like distorted conformation due to the preference of the metal center to bind adjacent chelating nitrogen centers at an angle of 90°. Angle strain in [Pt(bmap)C2Ph]+ and especially [Pt(btp) C2Ph]+ also induces the four donor atoms to assume a pseudotetrahedral arrangement, likely to promote formation of d−d excited states and offset any benefit due to the increase in overall bite angle of the NNN ligand. Viewed as a whole, the results suggest that the emission signals are weak because substituting trpy with bmap or btp replaces one type of angle strain for another, at the same time impairing charge delocalization within the ligand and allowing structural reorganization to occur in the electronic excited state. All are obviously important factors to consider in designing polypyridine ligands for transition metal ions. Finally, the analysis implies that the QpQ ligand employed by Williams and coworkers is worthy of further investigation because it appears uniquely capable of incorporating six-membered chelate rings without introducing unfavorable distortions,22 while yielding a platinum derivative that emits from an excited state of mainly intraligand character.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00636. Additional figures and complete characterizations of the prepared compounds (PDF) Accession Codes

CCDC 1824533−1824534 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The

Figure 8. Atom displacements from the mean coordination planes of the btp and bmap complexes. In each case, the atoms used for the least-squares fit of the mean plane are the platinum center and all four donor atoms. Shifts reported relative to platinum; see text. G

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Substituted Phenyl Terpyridines and Crystal-Structure of [Pt(terpy)Cl][CF3SO3. J. Chem. Soc., Dalton Trans. 1993, 2933−2938. (14) Crites, D. K.; Cunningham, C. T.; McMillin, D. R. Remarkable substituent effects on the photophysics of Pt(4 ′-X-trpy)Cl+ systems (trpy = 2,2 ′; 6 ′,2 ″-terpyridine). Inorg. Chim. Acta 1998, 273, 346− 353. (15) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.; Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Long-lived emissions from 4 ′-substituted Pt(trpy)Cl+ complexes bearing aryl groups. Influence of orbital parentage. Inorg. Chem. 2001, 40, 2193− 2200. (16) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.; Wilson, C. An Alternative Route to Highly Luminescent Platinum(II) Complexes: Cyclometalation with N∧C∧N-Coordinating Dipyridylbenzene Ligands. Inorg. Chem. 2003, 42, 8609−8611. (17) Ravindranathan, D.; Vezzu, D. A. K.; Bartolotti, L.; Boyle, P. D.; Huo, S. Q. Improvement in Phosphorescence Efficiency through Tuning of Coordination Geometry of Tridentate Cyclometalated Platinum(II) Complexes. Inorg. Chem. 2010, 49, 8922−8928. (18) Lai, S. W.; Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C. M. Probing d8-d8 Interactions in luminescent mono- and binuclear cyclometalated platinum(II) complexes of 6-phenyl-2,2 ′-bipyridines. Inorg. Chem. 1999, 38, 4046−4055. (19) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Zhu, N. Y.; Lee, S. T.; Che, C. M. [(C∧N∧N)Pt(CC)nR] (HC∧N∧N) = 6-aryl-2,2 ′-bipyridine, n = 1−4, R = aryl, SiMe3) as a new class of light-emitting materials and their applications in electrophosphorescent devices. Chem. Commun. 2002, 206−207. (20) Wilson, M. H.; Ledwaba, L. P.; Field, J. S.; McMillin, D. R. Push-pull effects and emission from ternary complexes of platinum(II), substituted terpyridines, and the strong-field cyanide ion. Dalton Trans. 2005, 2754−2759. (21) Hu, Y. Z.; Wilson, M. H.; Zong, R. F.; Bonnefous, C.; McMillin, D. R.; Thummel, R. P. A luminescent Pt(II) complex with a terpyridine-like ligand involving a six-membered chelate ring. Dalton Trans. 2005, 354−358. (22) Garner, K. L.; Parkes, L. F.; Piper, J. D.; Williams, J. A. G. Luminescent Platinum Complexes with Terdentate Ligands Forming 6-Membered Chelate Rings: Advantageous and Deleterious Effects in N N N and N C N-Coordinated Complexes. Inorg. Chem. 2010, 49, 476−487. (23) Hirotsu, M.; Tsukahara, Y.; Kinoshita, I. Manganese(II), Nickel(II), and Palladium(II) Complexes of a Terpyridine-Like Ligand Containing a Sulfur Linkage, and an Analogous NCN Pincer Palladium(II) Complex: Synthesis, Characterization, and Pd-Catalyzed Reactions. Bull. Chem. Soc. Jpn. 2010, 83, 1058−1066. (24) Hight, L. M.; McGuire, M. C.; Zhang, Y.; Bork, M. A.; Fanwick, P. E.; Wasserman, A.; McMillin, D. R. π Donation and Its Effects on the Excited-State Lifetimes of Luminescent Platinum(II) Terpyridine Complexes in Solution. Inorg. Chem. 2013, 52, 8476−8482. (25) Driver, M. S.; Hartwig, J. F. A second-generation catalyst for aryl halide amination: Mixed secondary amines from aryl halides and primary amines catalyzed by (DPPF)PdCl2. J. Am. Chem. Soc. 1996, 118, 7217−7218. (26) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (27) Hamann, B. C.; Hartwig, J. F. Sterically hindered chelating alkyl phosphines provide large rate accelerations in palladium-catalyzed amination of aryl iodides, bromides, and chlorides, and the first amination of aryl tosylates. J. Am. Chem. Soc. 1998, 120, 7369−7370. (28) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray, H. B. Electronic Spectroscopy of Chloro(Terpyridine)Platinum(II). Inorg. Chem. 1995, 34, 4591−4599. (29) Leung, S. Y. L.; Tam, A. Y. Y.; Tao, C. H.; Chow, H. S.; Yam, V. W. W. Single-Turn Helix-Coil Strands Stabilized by Metal ··· Metal and π-π Interactions of the Alkynylplatinum(II) Terpyridyl Moieties in meta-Phenylene Ethynylene Foldamers. J. Am. Chem. Soc. 2012, 134, 1047−1056.

Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David R. McMillin: 0000-0002-6025-0189 Funding

We gratefully acknowledge support from National Science Foundation (CHE 0847229) and Purdue Graduate Student Research funds. S.N.N acknowledges Fellowship support from the Alfred P. Sloan Foundation (MPHD) and Purdue University. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Tong Ren (synthetic equipment, spectroscopic equipment, and electrochemical equipment), Dr. Patricia Bishop (ESI-MS), Dr. Phillip E. Fanwick for assistance in Xray crystallography, and Prof. John F. Hartwig for funding support for S.N.N. during the completion of this manuscript.

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