J Inorg Organomet Polym (2010) 20:457–467 DOI 10.1007/s10904-010-9357-6
New Pt(II)(dithiolate) Compounds Possessing an Energetically Accessible Diphosphine-Based LUMO: Syntheses, Redox Properties, and Solid-State Structures of PtCl2(pbpcd), Pt(tdt)(pbpcd), and Pt(tdt)(bpcd) Sean W. Hunt • Li Yang • Xiaoping Wang • Vladimir Nesterov • Michael G. Richmond
Received: 5 February 2010 / Accepted: 29 March 2010 / Published online: 17 April 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The new ligand 2-(pyren-1-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (pbpcd) has been synthesized from the Knoevenagel condensation using 1-pyrenecarboxaldehyde with 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd). Displacement of the cod ligand in PtCl2(cod) by pbpcd furnishes PtCl2 (pbpcd) (2) in near quantitative yield. Treatment of 2 with the dipotassium salt of toluene-3,4-dithiol (K2tdt) affords the dithiolate compound Pt(tdt)(pbpcd) (3) as a 1:1 mixture of diastereomers. An alternative synthesis of 3 from Pt(tdt)(bpcd) (5) and 1-pyrenecarboxaldehyde also affords 3 in 23% yield. The pbpcd ligand and all new diphosphinesubstituted compounds have been isolated and fully characterized in solution by IR and NMR spectroscopies, and the solid-state structures of 2CH2Cl2, 3toluene, and 5CH2Cl2 established by X-ray diffraction analyses. The solid-state structure of each product exhibits a square-planar architecture at the metal center. The redox properties of the pbpcd ligand and the tdt-substituted compound 3 have been explored by cyclic and differential-pulse voltammetry, and these data are discussed with respect to extended Hu¨ckel MO calculations and the nature of the HOMO and LUMO levels in each compound.
Dedicated to Professor Charles U. Pittman, Jr. (aka ‘‘CUP’’) on the occasion of his retirement and those memorable ‘‘Bama’’ days past. S. W. Hunt L. Yang V. Nesterov M. G. Richmond (&) Department of Chemistry, University of North Texas, Denton, TX 76203, USA e-mail:
[email protected] X. Wang Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Keywords Platinum compounds Diphosphine ligand Dithiolate ligand Redox chemistry MO calculations X-ray crystallography
1 Introduction The insatiable global consumption of fossil fuels continues to dominant the geopolitical landscape [1]. In terms of alternative fuels and energy production, new materials capable of producing sustainable energy offer the promise of energy independence and remain under active investigation in corporate, government, and academic laboratories [2–5]. The creation of photosynthetic mimics that can split water inexpensively and efficiently to hydrogen and oxygen has the potential to solve our long-term energy needs. The utilization of water as a fuel is attractive due to the fact that hydrogen is clean burning and does not produce deleterious greenhouse by-products. The splitting of water into hydrogen and oxygen has been demonstrated by several groups using a variety of inorganic and organometallic compounds containing light-harvesting chromophores [5–8]. Appropriately constructed nanoscaled devices possessing light-harvesting antennas have the ability to generate long-lived, charge-separated states, which in turn are a prerequisite if the full chemical potential from the absorption of visible light is to be realized in any watersplitting scheme [9–12]. Water-splitting aspects notwithstanding, many of the same inorganic and organometallic compounds are multiversatile since their electron-transfer behavior renders them as ideal components for the construction of molecular wires and directional charge-transfer devices [13, 14]. Square-planar dithiolate-based platinum(II) compounds account for a majority of such devices [15–19]. Here the
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ability to ‘‘tune’’ the physicochemical properties of platinum(II) compounds through the controlled manipulation of the HOMO and LUMO levels is critical if visible light is to be harnessed and the excited-state properties successfully exploited for device fabrication and commercialization [12]. Our groups have employed the diphosphine ligand 4,5bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) as a redox-active auxiliary in the preparation of different mono- and polynuclear compounds. The presence of a lowlying p* orbital allows this diphosphine to function as an superb electron reservoir in chemical and electrochemical reactions involving electron transfer [20–23]. Currently, we have turned our attention towards the modification of the bpcd platform in an effort to prepare new, more highly conjugated diphosphine ligands. Functionalization of the bpcd ligand is readily accomplished through the Knoevenagel condensation reaction, with the diphosphine ligand serving as the active hydrogen compound given the presence of the 1,3-dione moiety. We have successfully coupled different aldehydes to the bpcd ligand to produce the secondgeneration ligands depicted in Scheme 1 [24–28]. These dyad arrays display rich redox behavior as a result of extended conjugation between the dione ring and the C-2 fused organic moiety. To date, the p*-based LUMO remains localized on the dione platform in these second-generation ligands, and it is the HOMO that is easily tuned via the chosen aldehyde. For example, the ferrocene condensation product, 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fbpcd), reveals an iron-based HOMO and a 0/1? redox couple displaying an E1/2 value of 0.63 V [27]. Functionalization of bpcd with 4-(dimethylamino)benzaldeyde gives the dbpcd ligand, whose HOMO is localized over the dione and Me2N-substituted benzylidene ligand and which may be switched ‘‘on and off’’ at the 4-Me2N moiety through protonation or quaternization [28]. Wishing to prepare new ligands possessing chromophoric appendages based on highly conjugated aromatic units, we have embarked on the synthesis of new pyrenesubstituted phosphine ligands. For a recent report describing the synthesis of pyrene-substituted phosphine ligands,
O
see [29]. The union of a pyrene moiety with bpcd has the potential to afford a luminescent diphosphine ligand having a compositionally different LUMO vis-a´-vis the parent phosphine. Herein we report the synthesis of the new diphosphine ligand 2-(pyren-1-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (pbpcd) and its use as a ligand in the preparation of the new platinum(II) compound Pt(tdt)(pbpcd). The redox properties of the free pbpcd ligand and Pt(tdt)(pbpcd) have been examined, and the electrochemical data are discussed relative to MO calculations performed at the extended Hu¨ckel level.
2 Experimental Section 2.1 Materials and Equipment PtCl2(cod) was prepared from chloroplatinic acid and 1,5-cyclooctadiene (cod) [30], while the 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) was synthesized from 4,5-dichloro-4-cyclopenten-1,3-dione and Ph2PSiMe3 [31, 32]. The chemicals 1-pyrenecarboxaldehdye, toluene3,4-dithiol, and potassium tert-butoxide were purchased from Aldrich Chemical Co. and used as received. The latter chemical, which is extremely hygroscopic, was stored in the dry box when not in use. All reaction solvents were distilled from an appropriate drying agent under inert atmosphere or purified by an Innovative Technology solvent delivery system. All distilled/purified solvents were handled via inertatmosphere techniques, and when not in use these solvents were stored in Schlenk vessels equipped with Teflon stopcocks [33]. The tetra-n-butylammonium perchlorate (TBAP) electrolyte was purchased from Johnson Matthey Electronics and recrystallized from a 1:1 mixture of hexane/ ethyl acetate and dried under vacuum for at least 30 h prior to use. All quoted percent yields are relative to the limiting reagent(s) employed in each reaction. The IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer in amalgamated NaCl cells capable of handling air-sensitive samples. The quoted 1H and 31P NMR spectral data were recorded at 500 and 201 MHz,
O
O
Ph 2 P
Ph 2 P
Ph 2 P
Ph 2 P O
S
Ph 2 P
Ph 2 P
O
O
NMe 2
Fe tbpcd
fbpcd
Scheme 1 Selected second-generation diphosphine ligands
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dbpcd
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respectively, on a Varian VXR-500 spectrometer. The 31P NMR spectra were collected in the proton-decoupled mode and the reported chemical shifts referenced to external H3PO4 (85%), taken to have d = 0. The ESI-APCI mass spectral data were recorded at the UC San Diego mass spectrometry facility in the positive ionization mode, using MeOH as the sample matrix.
J = 9.5 Hz), 8.84 (s, 1H, alkenyl), 9.18 (d, 1H, J = 8.5 Hz). 31P NMR (CDCl3): d 27.02 (d, JP–P = 1.6 Hz, JPt–P = 3695 Hz), 28.65 (d, JP–P = 1.6 Hz, JPt–P = 3695 Hz).
2.2 Synthesis of 2-(pyren-1-ylidene)-4,5bis(diphenylphosphino)-4-cyclopenten-1,3-dione (pbpcd)
To a stirred solution containing 25 mg (0.16 mmol) of toluene-3,4-dithiol in a round-bottom flask and 50 mL of MeOH under argon was added 18 mg (0.32 mmol) of KOH at room temperature. After ca. 30 min, 0.15 g (0.48 mmol) of 2 in 30 mL of THF was added dropwise to the in situ generated K2tdt solution, and the solution color gradually turned purple-red with the formation of 3. TLC analysis using CH2Cl2 revealed the presence of a new spot (Rf = 0.31), which was subsequently isolated by flashcolumn chromatography using CH2Cl2. Yield: 0.14 g (84%). Single crystals of 3 for X-ray diffraction analysis were grown from the slow diffusion of hexane into a toluene solution containing 3. IR (CH2Cl2): m(CO) 1720 (vw, sym dione), 1685 (m, antisym dione) cm-1. 1H NMR (CDCl3): d 2.29 and 2.31 (s, Me), 7.10–7.61 (m, 15H, phenyl and tdt), 8.01–8.10 (m, 10H), 8.15 (d, 1H J = 8.5 Hz), 8.18–8.28 (m, 4H), 8.49 (d, H-10, J = 9.5 Hz), 8.83 (s, 1H, alkenyl), 9.09 (d, 1H, J = 8.5 Hz). 31P NMR (CDCl3): d 33.36, 34.80 (AB quartet, JP–P = 4.4 Hz; JPt–P = 2790 Hz), 33.43, 34.87 (AB quartet, JP–P = 4.4 Hz; JPt–P = 2790 Hz). ESI-MS: m/z 1048.26 (calcd for [M ? Na]?: C53H36NaO2P2PtS2 1048.12).
To a medium Schlenk flask under argon was charged 70 mg (0.15 mmol) of bpcd, followed by 50 mL of methanol. The solution was then cooled to ca. 0 °C with an external ice bath, after which 17 mg (0.15 mmol) of KOBut was added in one portion. The solution was stirred for 30 min at 0 °C with the color changing from yellow to dark orange. At this point, 35 mg (0.15 mmol) of 1-pyrenecarboxaldehyde was added and stirring continued with warming to room temperature, after which the solution was then gently refluxed for ca. 2 h. TLC analysis of the reaction solution using CH2Cl2/hexane (2:1) as the eluent revealed the near complete consumption of the bpcd and the presence of a new orange-red spot (Rf = 0.72) attributed to the desired product. The solvent was removed under vacuum and the crude residue purified by column chromatography using the aforementioned mobile phase. Yield: 92 mg (90%) of pbpcd. IR (CH2Cl2): m(CO) 1720 (w, sym dione), 1677 (s, antisym dione) cm-1. 1H NMR (CDCl3): d 7.20–7.44 (m, 20H), 8.02 (m, 2H), 8.07 (d, 1H, J = 8.5 Hz), 8.14 (m, 2H), 8.22 (m, 2H), 8.45 (d, H-10, J = 9.5 Hz), 8.62 (s, 1H, alkenyl), 9.04 (d, 1H, J = 8.5 Hz). 31P NMR (CDCl3): d -21.02 (AB quartet, 3 JP–P = 63 Hz). 2.3 Synthesis of PtCl2(pbpcd) (2) from PtCl2(cod) (1) and pbpcd To 68 mg (0.10 mmol) of pbpcd in 50 mL of CH2Cl2 in a Schlenk vessel was added 41 mg (0.11 mmol) of PtCl2(cod) under argon flush. The slurry was stirred at room temperature with monitoring by TLC, and the volatiles were removed under reduced pressure once the diphosphine ligand had been consumed (ca. 30 min). Compound 2 was isolated as a red solid by column chromatography over silica gel using CH2Cl2. Yield: 90 mg (96%). Single crystals of 2 suitable for diffraction analysis were grown from a CH2Cl2 solution containing 2 that had been layered with hexane. IR (CH2Cl2): m(CO) 1729 (vw, sym dione), 1687 (m, antisym dione) cm-1. 1H NMR (CDCl3): d 7.48–7.61 (m, 12H), 8.00–8.10 (m, 10H), 8.14 (d, 1H J = 8.5 Hz), 8.22–8.32 (m, 4H), 8.48 (d, H-10,
2.4 Synthesis of Pt(tdt)(pbpcd) (3) from 2 and K2tdt
2.5 Synthesis of Pt(tdt)(bpcd) (5) from PtCl2(bpcd) (4) and K2tdt To a 50 mL MeOH solution containing 25 mg (0.16 mmol) of toluene-3,4-dithiol under argon at 0 °C was added with 18 mg (0.32 mmol) of KOH and stirring continued for ca. 30 min, after which 0.11 g (0.16 mmol) of PtCl2(bpcd) in 25 mL of THF was added dropwise over a period of 45 min. TLC analysis using CH2Cl2 revealed the presence of the desired product as a yellow-green spot (Rf = 0.56), which was purified by chromatography over silica gel. Single crystals suitable for diffraction analysis were obtained from a CH2Cl2 solution containing 5 that was layered with hexane. Yield: 0.10 g (80%). IR (CH2Cl2): m(CO) 1751 (vw, sym dione), 1719 (m, antisym dione) cm-1. 1H NMR (CDCl3): d 2.19 (s, Me), 6.58 (1H, d, J = 8.5 Hz, tdt), 7.27 (s, 1H, tdt), 7.33 (d, 1H, J = 8.5 Hz, tdt), 7.48 (m, 12H, para and meta phenyl), 7.95 (m, 8H, ortho phenyl). 31P NMR (CDCl3): d 30.89 (s, JPt–P = 2779 Hz). ESI-MS: m/z 814.27 (calcd for [M ? H]?: C36H29O2P2PtS2 814.07).
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2.6 Synthesis of 3 from 5 and 1-pyrenecarboxaldeyde To 50 mg (0.061 mmol) of 5 in 50 mL of MeOH was added 14 mg (0.061 mmol) of 1-pyrenecarboxaldehyde, followed by 6.9 mg (0.061 mmol) of KOBut. The solution was stirred overnight at room temperature and then examined by TLC, which confirmed the presence of 3 and considerable material that remained at the origin of the plate. The solvent was removed under vacuum and the residue purified by column chromatography, as described earlier in the preparation of 3. Yield: 14 mg (23%). 2.7 Electrochemical Studies The quoted echem data were recorded on a PAR Model 273 potentiostat/galvanostat, equipped with positive feedback circuitry to compensate for iR drop. The cyclic and differential pulse voltammograms were recorded under oxygen- and moisture-free conditions in a homemade three-electrode cell. A platinum disk was utilized as the working and auxiliary electrode, and the reference electrode utilized a silver wire as a quasi-reference electrode, with the reported potential data standardized against the formal potential of Cp*2Fe/Cp*2Fe? redox couple (external), taken to have E1/2 = -0.20 V [34]. 2.8 Extended Hu¨ckel MO Calculations The extended Hu¨ckel data reported here were carried out using the original program developed by Hoffmann [35, 36], as modified by Mealli and Proserpio [37]. The weighted Hij’s contained in the program were used in the calculations, and the input Z-matrices for the model compounds pbpcd-H4 and Pt(dtd)(pbpcd-H4) were constructed by using bond distances and angles from the available X-ray diffraction data from 3toluene. The phenyl groups associated with the phosphorus atoms were replaced with ˚ in our hydrogens, using a P–H bond distance of 1.41 A calculations [38]. 2.9 X-ray Crystallographic Data The X-ray data for the compounds 2CH2Cl2, 3toluene, and 5CH2Cl2 were collected on an APEX II CCD-based diffractometer at 100(2) K. The frames were integrated with the available APEX2 software package using a narrow-frame algorithm [39], and the structures were solved and refined using the SHELXTL program package [40]. Each molecular structure was checked using PLATON [41], and all non-hydrogen atoms were refined anisotropically. Absorption corrections were applied to all three compounds using SADABS [42]. The large peaks of residual electron density located near the platinum center in
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˚ from Pt) and 3toluene the structures of 2CH2Cl2 (0.87 A ˚ from Pt) do not reflect the presence of real atoms, (0.61 A but are assumed to be artifacts associated with the imperfect absorption corrections one encounters during the refinement of heavy-metal atom structures. The disorder found in the solvent and one of the phenyl rings in 2CH2Cl2 and 3toluene, respectively, and the disorder found in the tdt ligand and the CH2Cl2 solvent in 5CH2Cl2 were refined accordingly with distance and similarity restraints. All hydrogen atoms were assigned calculated positions and allowed to ride on the attached carbon atom. The refinement for 2CH2Cl2 converged at R = 0.0387 and wR2 = 0.0800 for 8299 independent reflections with I [ 2r(I), with 3toluene yielding convergence values of R = 0.0369 and wR2 = 0.0890 for 10,351 independent reflections with I [ 2r(I). 5CH2Cl2 afforded convergence values of R = 0.0257 and wR2 = 0.0515 for 6925 independent reflections with I [ 2r(I). The X-ray data and processing parameters for the three platinum(II) compounds are reported in Table 1, with selected bond distances and angles quoted in Table 2.
3 Results and Discussion 3.1 Synthesis and Spectroscopic Properties of 2-(pyren-1-ylidene)-4,5-bis(diphenylphosphino)-4cyclopenten-1,3-dione (pbpcd) Knoevenagel condensation of bpcd with 1-pyrenecarboxaldehyde takes place in MeOH solvent and KOBut to furnish the new diphosphine ligand 2-(pyren-1-ylidene)4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (pbpcd), as depicted in Eq. 1. The pbpcd ligand was isolated by column chromatography over silica gel as an orange-red solid in high yield and characterized in solution by IR and NMR spectroscopy. The vibrationally coupled dione carbonyl groups exhibit m(CO) bands at 1720 and 1677 cm-1, whose frequencies are in good agreement with the IR data of related derivatives prepared by us [24–28]. The 1H NMR spectrum of pbpcd recorded in CDCl3 confirmed the twenty phenyl hydrogens as a closely spaced set of overlapping resonances from d 7.20–7.44, with the remaining exocyclic vinyl and the nine pyrene hydrogens appearing in the range of d 8.02–9.04. A 1H COSY experiment established the resonance at d 8.62 as the lone alkenyl hydrogen that derives from the coupling of the bpcd ligand to the 1-pyrenecarboxaldehyde. A subsequent NOESY experiment revealed a strong off-diagonal response involving the alkenyl hydrogen and the resonance at d 8.45, establishing the latter as that of H-10 on the pyrene ring. Complete assignment of the other pyrene hydrogens could not be made with certainty due to the extensive overlap of
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Table 1 X-ray crystallographic data and processing parameters for the platinum(II) compounds 2CH2Cl2, 3toluene, and 5CH2Cl2 Compound
2CH2Cl2
3toluene
5CH2Cl2
CCDC entry
737454
744124
729791
Cryst system
Monoclinic
Triclinic
Monoclinic
Space group ˚) a (A
P21/n
P-1
P21/n
8.738(1)
11.7984(7)
11.2170(8)
˚) b (A ˚) c (A
21.308(3)
14.6344(9)
23.215(2)
21.954(3)
14.6778(9)
13.392(1)
a (°)
90
110.239(1)
90
b (°)
96.454(2)
95.634(1)
94.095(1)
c (°) ˚ 3) V (A
90
95.637(1)
90
4061.6(9)
2342.5(2)
3478.3(4)
Mol formula
C47H32Cl4O2P2Pt
C60H44O2P2PtS2
C37H30Cl2O2P2PtS2
fw
1027.56
1118.10
898.66
formula units per cell (Z)
4
2
4
Dcalcd (Mg/m3) ˚) k (Mo Ka) (A
1.680 0.71073
1.585 0.71073
1.716 0.71073
l (mm-1)
3.837
3.199
4.433
Absorption correction
Numerical
Numerical
Semi-empirical from equivalents
F(000)
2024
1120
1768
Crystal size (mm)
0.11 9 0.06 9 0.05
0.17 9 0.09 9 0.06
0.16 9 0.13 9 0.04
Abs corr factor
0.8403/0.6865
0.8239/0.6123
0.8461/0.5355
Total reflections
47,303
29,244
39,022
Independent reflections
8299
10,351
6925
Data/res/parameters
8299/1/511
10,351/9/642
6925/61/467
R1a (I C 2r(I)]
0.0387
0.0369
0.0257
wR2b
0.0800
0.0890
0.0515
GOF on F2
1.025
1.015
1.041
˚ 3) Dq(max), Dq(min) (e/A
1.475, -1.280
2.240, -1.073
0.700, -0.868
a
b
R1 ¼ RjjFo j jFc jj=RjFo j 2 . h 2 2 i1=2 R w Fo R2 ¼ R w F2o F2c
remaining hydrogens. The 31P NMR spectrum displayed a classic AB quartet at d -21.02 with a J/Dm value of 0.15 for the inequivalent PPh2 moieties in agreement with the formulated structure [43].
3.2 Synthesis, Spectroscopic Data, and X-ray Diffraction Structure of PtCl2(pbpcd) (2) Treatment of PtCl2(cod) (1) with pbpcd furnished the desired compound 2 in near quantitative yield after
O
O
OHC
Ph2P
-H2O
Ph2P (1)
Ph2P
Ph2P
O bpcd
O 1-pyrenecarboxaldehyde
pbpcd ligand
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˚ ) and angles (°) for the platiTable 2 Selected bond distances (A num(II) compounds 2CH2Cl2, 3toluene, and 5CH2Cl2 Bond distances
Bond angles
Compound 2CH2Cl2 Pt(1)–P(1)
2.210(1)
P(1)–Pt(1)–P(2)
89.94(5)
Pt(1)–Cl(1)
2.347(1)
P(2)–Pt(1)–Cl(1)
176.03(5)
C(1)–C(2)
1.482(7)
P(2)–Pt(1)–Cl(2)
89.84(5)
C(2)–C(6)
1.353(7)
C(4)–P(1)–Pt(1)
105.4(2)
C(3)–C(4)
1.507(7)
C(2)–C(6)–C(7)
131.3(5)
C(6)–C(7) Pt(1)–P(2)
1.438(7) 2.218(1)
P(1)–Pt(1)–Cl(1) P(1)–Pt(1)–Cl(2)
87.21(5) 178.41(5)
Pt(1)–Cl(2)
2.351(1)
Cl(1)–Pt(1)–Cl(2)
93.09(5)
C(1)–C(5)
1.524(7)
C(5)–P(2)–Pt(1)
105.2(2)
C(2)–C(3)
1.472(7)
C(4)–C(5)
1.323(7) 88.69(4)
Compound 3toluene Pt(1)–P(1)
2.250(1)
P(1)–Pt(1)–P(2)
Pt(1)–S(2)
2.301(1)
P(2)–Pt(1)–S(2)
91.50(4)
C(1)–C(5)
1.334(6)
P(2)–Pt(1)–S(1)
178.98(4)
C(2)–C(3)
1.471(7)
C(23)–S(1)–Pt(1)
103.6(2)
C(3)–C(4)
1.493(7)
C(1)–P(1)–Pt(1)
105.7(2)
C(6)–C(7)
1.438(7)
C(3)–C(6)–C(7)
132.4(5)
Pt(1)–P(2)
2.250(1)
P(1)–Pt(1)–S(2)
177.19(4)
Pt(1)–S(1)
2.303(1)
P(1)–Pt(1)–S(1)
90.48(4)
C(1)–C(2) C(3)–C(6)
1.511(6) 1.364(7)
S(2)–Pt(1)–S(1) C(28)–S(2)–Pt(1)
89.36(4) 103.8(2)
C(4)–C(5)
1.508(7)
C(5)–P(2)–Pt(1)
105.8(2)
Compound 5CH2Cl2 Pt(1)–S(2)
2.237(5)
S(2)–Pt(1)–P(2)
174.60(9)
Pt(1)–P(2)
2.247(1)
P(2)–Pt(1)–P(1)
88.51(4)
S(1)–C(1)
1.785(5)
P(2)–Pt(1)–S(1)
85.21(9)
C(8)–C(9)
1.328(6)
C(9)–P(1)–Pt(1)
106.1(1)
C(9)–C(10)
1.507(6)
C(1)–S(1)–Pt(1)
101.7(2)
C(11)–C(12)
1.508(7)
S(2)–Pt(1)–P(1)
96.78(9)
Pt(1)–S(1)
2.355(4)
S(2)–Pt(1)–S(1)
89.6(1)
Pt(1)–P(1)
2.252(1)
P(1)–Pt(1)–S(1)
173.14(9)
S(2)–C(2)
1.740(5)
C(8)–P(2)–Pt(1)
105.9(1)
C(8)–C(12)
1.501(6)
C(2)–S(2)–Pt(1)
105.2(2)
C(10)–C(11)
1.506(7)
chromatographic purification. 2 was characterized by spectroscopic methods and X-ray diffraction analysis. The IR spectrum reveals the signature dione m(CO) bands at 1729 and 1687 cm-1 that are shifted ca. 10 cm-1 to higher energy relative to free ligand, in accord with the coordination of a Pt(II) center. The 1H NMR data are consistent with the formulated structure, with the assignment of the alkenyl (d 8.84) and the H-10 (d 8.48) hydrogens receiving support through COSY and NOESY experiments. The two
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Fig. 1 Thermal ellipsoid plot of 2CH2Cl2 at the 50% probability level with the hydrogen atoms shown as small spheres of arbitrary radii. The CH2Cl2 solvent has omitted for clarity
31
P doublets recorded at d 27.02 and 28.65 and their accompanying 195Pt satellites of 3695 Hz are consistent with the proposed structure and in concert with other platinum(II) compounds containing a chelating diphosphine and two chlorine ligands [44, 45]. Figure 1 shows the thermal ellipsoid plot of the molecular structure of PtCl2(pbpcd)CH2Cl2 and confirms the coordination of the pbpcd ligand to the PtCl2 moiety. The Pt–P and ˚, Pt–Cl bonds display a mean distance of 2.214 and 2.349 A respectively, and agree with those distances reported by us for PtCl2(bpcd), PtCl2(fbpcd), and PtCl2(abpcd) [26, 28, 46]. The C–C bond distances associated with the pyrene moiety ˚ [C(18)–C(19)] to 1.450(7) A ˚ range from 1.335(8) A ˚ . The [C(19)–C(20)] and exhibit a mean distance of 1.407 A P(1)-Pt(1)-P(2) bond angle of 89.94(5)° is in excellent agreement with the P–Pt–P angle exhibited by the aforementioned bpcd and bpcd-substituted Pt(II) compounds structurally characterized by us and the related diphosphine-substituted compounds PtCl2(dppe) and PtCl2[(Z)Ph2PCH=CHPPh2] [47–49]. The pyrene ring is tipped slightly out of the plane defined by the dione platform based on an interplanar angle of ca. 32.6(2)°, so as to obviate unfavorable van der Waals’ interactions between the O(1)C(8), O(1)H(8A), and H(16A)H(19A) atoms in the dione and pyrene rings. Finally, the packing diagram for 2CH2Cl2 (not shown) displays shifted stacks of pyrene rings ˚ , confirming the with distances on the order of 3.500 A absence of any significant intermolecular p–p* interactions in the solid state.
J Inorg Organomet Polym (2010) 20:457–467 Ph2 P
Cl
463
O
Pt
SK P Ph2
Cl
O
SK
PtCl2(pbpcd) S
Ph2 P
O
P Ph2
O
Pt OHC
S
Pt(tdt)(pbpcd) S
Ph2 P
O
P Ph2
O
Pt S
Pt(tdt)(bpcd)
Scheme 2 Synthetic routes to Pt(tdt)(pbpcd)
3.3 Syntheses, Spectroscopic Data, and X-ray Diffraction Structure of Pt(tdt)(pbpcd) (3) Scheme 2 illustrates the two routes employed in the synthesis of the dithiolate-substituted compound 3. The more direct of the two routes involves the displacement of the chloride groups in 2 using K2tdt, which yields the desired the dithiolate compound 3 in 84% yield after flash-column chromatography. 3 was isolated as an air-stable, purple-red solid that exhibits a greatly reduced solubility in common organic solvents relative to the free ligand and compound 2. 3 was characterized by a combination of IR and NMR spectroscopy, mass spectrometry, and X-ray crystallography. Particularly informative was the 31P NMR spectrum which revealed the presence of two closely spaced AB quartets (d 33.36 and 34.80; d 33.43, 34.87) for the pbpcd ligand. This 1:1 mixture of diastereomers results from proximal and distal orientations of the pyrene moiety relative to the methyl group of the tdt residue. The JPt–P couplings of 2790 Hz underscore the greater trans influence of the dithiolate ligand vis-a´-vis the chlorine ligand in 2 [50]. The ESI mass spectrum of 3 displayed an intense m/z peak at 1048.26 consistent with the sodiated species [3 ? Na]?. Unfortunately, 3 did not show any visible luminescence in fluid solution at room temperature. The molecular structure of the proximal diastereomer of 3,1 as the toluene solvate (Fig. 2), was crystallographically established. The compound contains pbpcd and tdt ligands
coordinated to a square planar platinum(II) center. The ˚ for the Pt–P and mean bond distance of 2.250 and 2.302 A Pt–S vectors, respectively, are unremarkable in comparison to those distances in related Pt(dithiolate)P2 compounds (P2 = mono- and bidentate phosphines) [51–55]. The S(1)–C(23) and S(2)–C(28) bond distances of 1.770(5) and ˚ , respectively, are consistent with the presence 1.765(5) A of closed-shell dianionic [tdt]2- ligand. Bond-length alterations involving these distances have been shown by Wieghardt to be very sensitive indicators for the metal oxidation state and electronic structure [56]. The pyrene ring is tipped slight out of the plane defined by the phosphine atoms and the dione and tdt rings. Here the observed angle of 21.6(3)° formed by the above planes minimizes unfavorable van der Waals’ contacts between the O(1)C(8) and O(1)H(8A) atoms, and the alkenyl H(16A) and the pyrene H(19A) atoms. The packing diagram for 3toluene (Fig. 3) displays moderately strong intermolecular p stacking between the pyrene rings of adjacent molecules, along with weaker intermolecular contacts between the phenyl rings of different molecules and the phenyl groups and toluene solvent. The shifted stacks of pyrene rings exhibit several short intermolecular ˚, CC contacts on the order of 3.326(7) to 3.358(7) A whose distances are less than the sum of the van der Waals radii of the interacting carbon atoms [57]. Compound 3 may also by synthesized via the known compound PtCl2(bpcd) (4) [46]. Treatment of 4 with K2tdt gives the corresponding thiolate compound Pt(tdt)(bpcd)
1
Here the relationship between the crystallographically defined C-8 pyrene atom (alternatively referred to as C(2) within the IUPAC formalism for naming polynuclear aromatic pyrene-based rings) and the tdt methyl group are used in the proximal and distal definitions. In
Footnote 1 continued the case of 3toluene, these two groups are oriented in a syn or proximal fashion.
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Fig. 2 Thermal ellipsoid plot of 3toluene at the 50% probability level with the hydrogen atoms shown as small spheres of arbitrary radii. The toluene solvent has omitted for clarity
Fig. 3 Packing diagram for 3toluene showing the intermolecular pyrenepyrene, PhPh, and Phtoluene interactions along the a axis
(5) in good yield as a yellow-green solid. While the IR and 1 H NMR spectral data for 5 are consistent with the formulated structure, the 31P NMR spectrum showed a singlet at d 30.89 despite the presence of inequivalent phosphines groups. The absence of the expected 31P AB pattern
Fig. 4 Thermal ellipsoid plot of 5CH2Cl2 (left) and the zigzag one-dimensional chain of molecules formed along the crystallographic a-direction (right). Both plots are draw at the 30% probability
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presumably derives from an accidental degeneracy of the two 31P groups [58]. The ESI mass spectrum exhibits a strong m/z peak at 814.27 for [M ? H]?. The unequivocal identity of 5 was established by X-ray crystallography, with Fig. 4 showing the thermal ellipsoid plot of the molecular structure of 5CH2Cl2. The structure consists of a squareplanar Pt(II) center containing chelating tdt and bpcd ligands, where close intermolecular contacts are observed between the tdt methyl group and a phenyl ligand on an adjacent molecule. These CH3p interactions, which are depicted in the right-hand portion of Fig. 4, display inter˚ for the C(7)C(23A) and molecular distances of 3.178(9) A ˚ for C(7)C(24A) contacts that are significantly 3.141(8) A shorter than the van der Waals’ contact distances. The dihedral angle between the plane defined by the tdt ligand and the phenyl ring is nearly orthogonal at ca. 103°. The CH3p interactions found in 5CH2Cl2 are not unlike those reported in tdt-substituted compounds [Zn(tdt)(1, 10-phen)]21,4-diphenyl-1,3-1,3-butadiene and [AuCu8(ldppm)3(tdt)5]? [59, 60]. 5CH2Cl2 does not contain any unusual or unexpected bond lengths or angles in comparison to 3.
level with the hydrogen atoms shown as small spheres of arbitrary radii. The CH2Cl2 solvent has omitted for clarity
J Inorg Organomet Polym (2010) 20:457–467
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Finally, Knoevenagel condensation of 5 with 1-pyrenecarboxaldehyde was found to give 3 in the presence of KOBut. Here the isolated yields of 3 were low, typically on the order of 20–25%. While no attempts were made to maximize the production of 3 through this route, we have observed lower reaction yields in the Knoevenagel reaction when the bpcd ligand is pre-coordinated to a metal center as opposed to the reaction employing the free ligand. 3.4 Electrochemical Properties and MO Calculations on pbpcd and Pt(tdt)(pbpcd-H4) The redox properties of the pbpcd ligand and 3 were next examined by cyclic voltammetry (CV) in CH2Cl2 containing 0.25 M TBAP as the supporting electrolyte. The CV of pbpcd ligand (not shown) recorded over the potential range of 1.0 to -1.2 V and at a scan rate of 250 mV/s revealed a diffusion-controlled, quasi-reversible wave at E1/2 = -0.97 V, which has been assigned to the 0/1redox couple associated with the dione platform. Electrochemical reversibility and electron stoichiometry were determined via the usual methods, see [61]. The locus of the reduction was verified by extended Hu¨ckel MO calculations on the model compound pbpcd-H4, which
PPh2
Me
of 1.0 to -1.0 V showed two reversible waves at E1/2 = 0.69 and -0.51 V that are ascribed to the 0/1? and 0/1- redox couples, respectively. The one-electron nature of these redox couples was ascertained by calibration of the CV wave heights against the one-electron standard ferrocene and differential pulse voltammetry. The nature of the HOMO and LUMO in the model compound Pt(tdt)(pbpcdH4) was established by extended Hu¨ckel calculations. The LUMO occurs at -10.37 eV and is identical in all respects to that found in the free ligand. The HOMO exhibits a pp–dp interaction between the tdt ligand and the platinum(II) center, whose parentage derives from the out-ofphase union of the pz and dzy orbitals of the [tdt]2- and [Pt(pbpcd)]2? fragments, respectively. For some references dealing with the frontier orbitals of dithiolate and Pt(II) fragments, see [63–65]. A similar dithiolate-Pt(II) interaction has also been found by Eisenberg in the compounds Pt(mnt)(bpy), Pt(mnt)(1,2-ethylenediamine), and related platinum derivatives [66]. The MO calculations on Pt(tdt)(pbpcd-H4) show that the HOMO occurs at -11.49 eV in excellent agreement with the computed energies of -11.48 and -11.44 eV for the HOMO levels in Pt(mnt)(bpy) and Pt(mnt)(1,2-ethylenediamine), respectively [66].2
O z y
PPh2
x
O
Pt(tdt)(pbpcd-H4) HOMO (-11.49 eV)
revealed a LUMO at -10.37 eV. The p* LUMO displayed by this ligand is localized exclusively on the dione moiety ([98%), as depicted below, and is unremarkable when compared to the LUMO in the parent ligand bpcd and other C-2 functionalized ligands based on bpcd [20–24, 26–28, 46]. The orbital composition of the LUMO resembles the W4 LUMO exhibited by the six-p-electron systems maleic anhydride and hexatriene [62].
4 Conclusions The new diphosphine ligand 2-(pyren-1-ylidene)-4,5bis(diphenylphosphino)-4-cyclopenten-1,3-dione (pbpcd) has been synthesized and used as a ligand in the preparation of the new platinum(II) compound Pt(tdt)(pbpcd). The redox properties of pbpcd and Pt(tdt)(pbpcd) have been investigated by electrochemical methods and the data discussed relative to MO calculations performed at the
H2 P pbpcd-H4 LUMO (-10.37 eV) H2P
The CV of Pt(tdt)(pbpcd) recorded at 250 mV/s in CH2Cl2 containing 0.25 M TBAP over the potential range
2 The redox and MO properties of compound 5 were also studied and the data were similar to those displayed by 3. Here E1/2 values of 0.66 and -0.56 V for the 0/1? and 0/1- redox couples were recorded by cyclic voltammetry, and the computed HOMO (-11.57 eV) and LUMO (-10.41) levels were found to be compositionally similar to those levels in Pt(tdt)(pbpcd-H4).
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extended Hu¨ckel level. Future studies will concentrate on the synthesis and photophysical examination of other suitably functionalized diphosphine ligands that can be used in light-harvesting applications.
5 Supplementary Material Crystallographic data for PtCl2(pbpcd)CH2Cl2 (737454), Pt(tdt)(pbpcd)toluene (744124), and Pt(tdt)(bpcd)CH2Cl2 (729791), in CIF format, have been deposited with the Cambridge Crystallographic Data Centre (CCDC). Copies of these latter data may be obtained free of charge via www.ccdc.cam.ac.uk/data_request/Cif, by emailing data_
[email protected], or by contacting the CCDC, 12 Union Road, Cambridge CB2 IEZ, UK; Fax: ?44 1223 336033. Acknowledgments Financial support from the Robert A. Welch Foundation (Grant B-1093-MGR) is greatly appreciated, and X. Wang acknowledges the support by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC05-00OR22725 managed by UT Battelle, LLC. Dr. Yongxuan Su is thanked for recording the ESI mass spectra of the platinum compounds 3 and 5 reported herein.
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