Article pubs.acs.org/IC
Synthesis, Characterization, and Luminescence Studies of Gold(I) Complexes with PNP- and PNB-Based Ligand Systems Shiv Pal, Neha Kathewad, Rakesh Pant, and Shabana Khan* Department of Chemistry, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune 411 008, India S Supporting Information *
ABSTRACT: The synthesis, X-ray crystal structures, and spectroscopic studies of a series of PPh2N(2,6-iPr2C6H3)PPh2 (PNP) and PPh2N(2,6iPr2C6H3)BCy2 (PNB; Cy = cyclohexyl) based gold(I) complexes are presented herein. The gold(I) chloride complexes 2 and 6 were treated with AgSbF6 to yield the corresponding dimeric dinuclear AuI cation (3) and dimeric mononuclear AuI cation (7) with PNP and PNB systems, respectively. The molecular structure of 3 revealed the presence of a strong intramolecular aurophilic interaction with a Au···Au bond distance of 2.7944(19) Å, one of the shortest aurophilic interactions known in the literature. However, complex 7 displays no aurophilic interaction. The reaction of 3 with diphenyl disulfide was performed, which led to a multinuclear tetragold(I) complex (4), keeping the aurophilic interaction intact. The effect of an aurophilic interaction is also illustrated through the study of the luminescent properties of these gold(I) complexes. Complexes 3 and 4 exhibit luminescence in solution as well as in the solid state, whereas the other gold(I) complexes remain nonluminescent.
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INTRODUCTION Aurophilic interaction, a noncovalent interaction between the two closed-shell Au atoms, has become the subject of intense current research. Such interaction between the two Au atoms often led to remarkable photophysical properties displayed by a majority of multinuclear gold complexes and thereby reckoned as potential candidates for sensors and optical devices.1−3 Aurophilic interaction ranges roughly from 2.5 to 3.5 Å and was thereby exploited as a tool for the synthesis of a number of unusual multinuclear gold complexes, which exhibit intriguing electronic absorption and luminescence properties.4 Many gold(I) complexes are found to be luminescent at room temperature in the solution state, and some of them are emissive in the solid state. The luminescent properties of these complexes heavily rely on the nature and distance of Au···Au centers. Recent years have witnessed interesting developments starting from solvoluminescence to on−off switching of luminescence arising from the switching of intra- to intermolecular Au···Au interactions (Chart 1).5 These AuIbased luminescent materials, especially the blue luminescent materials, can be of great use for the development of certain organic light-emitting diodes.6 To date, various P-, N-, S-, and C-based fully supported, semisupported, and unsupported gold(I) systems have been used to study the luminescence properties.1−5 In order to explore the influence of intramolecular aurophilic interaction on the color of the luminescence properties, we synthesized Ph2PN(2,6-iPr2C6H3)PPh2 (PNP) and Ph2PN(2,6-iPr2C6H3)BCy2 (PNB; Cy = cyclohexyl) based gold(I) complexes and studied their © XXXX American Chemical Society
structural and luminescent properties. Herein we report on the preparation, structure, and luminescence studies of PNPand PNB-based gold(I) complexes.
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RESULTS AND DISCUSSION Synthesis and Structural Characterization of [AuCl{(2,6-iPr 2 C 6 H 3 N)(PPh 2 ) 2 }] (2), [Au(2,6-iPr 2 C 6 H 3 N)(PPh 2 ) 2 ] 2 [SbF 6 ] 2 (3), and [Au 2 (SPh)(2,6-iPr 2 C 6 H 3 N)(PPh2)2]2[SbF6]2 (4). The complex 2 was prepared by treating ligand 2,6-iPr2C6H3N(PPh2)2 (1) and [AuCl(Me2S)] in CH2Cl2 in a 1:1 molar ratio (Scheme 1). Compound 2 was crystallized from a CH2Cl2/n-pentane (1:1) mixture, and crystals were used to record the NMR data. The appearance of two signals at 67.67 and 20.09 ppm in the 31P NMR spectrum confirmed the presence of two chemically nonequivalent P atoms in the ligand and can be assigned to “P−Au” and “P”, respectively. Both signals are shifted to low field compared to the parent compound 1 (−6.40 ppm). The lowfield shift for an uncoordinated P atom can be attributed to the deshielding effect due to P → Au donation. There is no similar example of a diphosphinegold complex, to the best of our knowledge, where one P atom is coordinated to the AuCl moiety and another is free. However, there are reports where both P atoms are coordinated to the AuCl moiety, e.g., [(AuCl)2Ph2PN(p-BrC6H4)PPh2]7a and [(AuCl)2(dmbpaip)]7b (dmbpaip = dimethyl 5-[N,N-bis(diphenylphosphanyl)amino]Received: May 10, 2015
A
DOI: 10.1021/acs.inorgchem.5b01046 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Chart 1. Selected Luminescent Gold(I) Complexes
Scheme 1. Synthesis of Complex 2
isophthalate), respectively, and resonates downfield (89.4 and 87.8 ppm, respectively) if compared to the parent compounds [Ph2PN(p-BrC6H4)PPh2 (70.8 ppm) and dmbpaip (69.1 ppm), respectively].7 The crystals suitable for single-crystal X-ray diffraction studies were grown from a CH2Cl2/n-pentane (1:1) mixture. The molecular structure of 2 revealed that one P atom is coordinated to the Au center, while the other remains intact (Figure 1). The geometry of the two-coordinate AuI center is nearly linear with the P−Au−Cl angle of 175.57(4)° and the Au−P bond length of 2.2313(12) Å, which is typical for the P → Au coordination bond and closely matches with the reported two-coordinate complexes [(AuCl)2Ph2PN(p-BrC6H4)PPh2] [Au−P = 2.233(2) Å and P−Au−Cl = 177.99(7)°] and [(AuCl)2(dmbpaip)] [Au−P = 2.227(2) Å and P−Au−Cl = 174.07(10)°], respectively.2−4,7,8 It is apparent that abstraction of the Cl atom from 2 will result in cationic gold complexes. The stoichiometric addition of AgSbF6 to 2 led to dimeric dinuclear cation 3 (Scheme 2). The 31P NMR spectrum reveals a singlet at 100 ppm, which
Figure 1. Molecular structure of complex 2. Thermal ellipsoids are shown at the 50% probability level. H atoms are not shown for clarity.
matches well with the similar dinuclear cationic complex [(Au)2(dmbpaip)][ClO4]2 (97.9 ppm).7b The appearance of only one singlet at 100 ppm in the 31P NMR spectrum suggested a symmetrical environment in complex 3, and the substantial downfield shift indicates the formation of a cationic complex, which is further confirmed by single-crystal X-ray diffraction studies. Single crystals suitable for X-ray analysis B
DOI: 10.1021/acs.inorgchem.5b01046 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 2. Preparation of AuI Cation 3 and Its Reaction with Ph2S2
Table 1. Selected Bond Lengths for 2−4, 6, and 7 Au−P (Å)
Au−Cl (Å)
2 3 4
Au1−P1 Au1−P1 Au1−P1
2.2313(12) 2.330(5) 2.269(6)
6 7
Au2−P2 Au3−P3 Au4−P4 Au1−P1 Au1−P1
2.274(6) 2.289(6) 2.293(7) 2.2451(11) 2.3345(6)
Au1−Cl1
Au···Au (Å) Au1−Au1 Au1−Au2 Au3−Au4
Au1−Cl1
Au−S (Å)
2.280(12) 2.7944(19) 2.9256(13) 2.9229(13)
Au1−S1
2.307(6)
Au2−S2 Au3−S1 Au4−S2
2.365(5) 2.332(6) 2.359(7)
2.2944(11)
Table 2. Selected Bond Angles for 2−4, 6, and 7 P−Au−Cl (deg) 2 3 4
P1−Au1−Cl1
6 7
P1−Au1−Cl1
P−Au−P (deg)
P−Au−S (deg)
Au−S−Au (deg)
175.57(4) P1−Au1−P1
170.9(2) P1−Au1−S1 P2−Au2−S2 P3−Au3−S1 P4−Au4−S2
176.2(2) 166.5(2) 165.2(2) 169.6(2)
Au1−S1−Au3 Au4−S2−Au2
95.6(2) 91.3(2)
179.72(5) P1−Au1−P1
180.0
Figure 2. (a) Molecular structure of the cation of complex 3. Thermal ellipsoids are shown at the 30% probability level. H atoms and phenyl groups are not shown for clarity. (b) Folding mode of two arms.
were grown from a CH2Cl2/n-pentane (1:1) mixture. Crystallographic and data collection parameters are summarized in Table S1 (Supporting Information), and selected bond lengths and angles are given in Tables 1 and 2, respectively. The solid-state
structure of 3 revealed formation of the anticipated [Au(2,6iPr2C6H3N)(PPh2)2]22+ along with two noncoordinated SbF6− anions in the framework (Figure 2a). The molecular structure of 3 discloses the presence of C2 symmetry in the molecule. C
DOI: 10.1021/acs.inorgchem.5b01046 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Molecular structure of the cation of complex 4. Thermal ellipsoids are shown at the 30% probability level. H atoms and phenyl groups on the S and P atoms are not shown for clarity.
However, the most significant feature of 3 is the Au···Au interaction of 2.7944(19) Å. The Au(1)···Au(2) distance falls within the range of aurophilic interaction, and to the best of our knowledge, this is one of the shortest distances (e.g., 2.798,7b 2.858,8a 2.862,8b and 2.799 Å8c) known for such aurophilic interactions.8 The coordination geometry at the AuI atoms is distorted from linearity with P1−Au1−P1 bond angle of 170.9(2)° like [(Au) 2 (dmbpaip)][ClO 4 ] 2 [P1−Au1−P2 169.0(2)°].7b The backbone of 3 contains an eight-membered ring, bridging by an Au···Au interaction, and is folded by 57.14° with respect to the Au···Au axis (taking P1 → Au1···Au1 ← P1 into the account; Figure 2b). All four P → Au bond distances [Au1−P1 2.330(5) Å] are identical and in good agreement with the literature values.7,8 Aurophilic interaction has also been used to elaborate the structural dimensionality as well as complexity. This is illustrated when the reaction of 3 with Ph2S2 resulted in the tetragold(I) complex 4 (Scheme 2). The mechanism of the reaction is not well-defined because the complex was isolated as a major product along with some unidentified products, but it can be assumed that “AuSPh” was formed in the initial step. In the subsequent step, the AuSPh unit is inserted into the Au−P bonds, leading to the formation of 4. In the structure of 4, there is some residual electron density around Au the atoms, which is common for the structures containing heavy metals. Therefore, the absolute values for the bond lengths might not be as reliable as their standard deviations indicate and should be considered cautiously (vide infra). However, the most significant feature is that the Au···Au interactions remain intact during the reaction (Figure 3). In 4, small increases in the Au···Au bond distances [Au1···Au2 2.9256(13) Å and Au3···Au4 2.9229(13) Å] compared to those in 3 are observed. The four Au atoms are arranged in a zigzag fashion within a six-membered Au4S2 ring with a very weak [Au2···Au3 3.459(1) Å] interaction (Figure 4).9 The average P−Au−S bond angle is 170.25°, which suggests distorted linearity at the AuI center, while average Au− S−Au bond angle is 93.45°, giving the structure a curvature at the S center. Formation and Characterization of [AuCl(2,6iPr2C6H3N)(BCy2)(PPh2)] (6) and [Au(2,6iPr2C6H3N)2(BCy2)2(PPh2)2][SbF6] (7). In the previous cases, we have demonstrated intramolecular aurophilic interaction with a PNP-based system. In order to facilitate intermolecular
Figure 4. Zigzag Au4 core of 4.
aurophilic interaction, it was thought to deliberately design a ligand system where intramolecular Au···Au interaction can be avoided. Hence, a PNB-based ligand system (5) was designed in which a B atom is present at one arm, and being electron deficient in nature, it cannot donate any electron density to AuI center. With this view, ligand 5 was reacted with [AuCl(Me2S)] in CH2Cl2 into a 1:1 molar ratio to afford 6 (Scheme 3). Routine NMR analysis suggested the formation of a single product. The constitution of 6 was unequivocally characterized by single-crystal X-ray diffraction studies. Figure 5 depicts a molecular view of 6, which reveals coordination of the P atom to the AuI center with a Au−P bond distance of 2.2451(11) Å, while the B atom is kept from coordination. The geometry at the two-coordinate AuI atom is almost linear with a P−Au−Cl bond angle of 179.72(5)°. There is no inter- or intramolecular Au···Au interaction in the crystal structure. The shortest distance present between Au atoms in 6 is given in Table 3. To generate dimeric cation, 6 was treated with AgSbF6 in a 1:1 molar ratio in CH 2 Cl 2 , which provided dimeric mononuclear AuI cation 7 (Scheme 3). X-ray data confirmed the formation of a dimeric mononuclear AuI cation, as expected in the absence of another donor site. It is also apparent from the crystal packing that there is no interaction between the cationic Au center and anion SbF6−. Moreover, there is no intermolecular Au···Au interaction present in the crystal packing of 7, and the reason for the absence of an intermolecular Au···Au interaction can be attributed to the presence of bulky substituents on the N and P atoms, which makes them unavailable for further interactions (the shortest D
DOI: 10.1021/acs.inorgchem.5b01046 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 3. Synthesis of Complex 6 and Cation 7
Figure 6. Molecular structure of the cation of complex 7. Thermal ellipsoids are shown at the 50% probability level. H atoms are not shown for clarity.
Figure 5. Molecular structure of complex 6. Thermal ellipsoids are shown at the 50% probability level. H atoms are not shown for clarity.
nm (ε = ∼1.79 × 104 L mol−1 cm−1), which is tentatively related to the mixture of ligand-to-metal charge-transfer (LMCT) and intraligand charge-transfer (ILCT) transitions.10 The emission spectrum of 3 in a CH2Cl2 solution shows the appearance of a band at 417 nm with a Stokes shift of 87 nm. The short luminescence lifetimes (Table 4) and the relatively small Stokes shift between the absorption and emission bands suggest that the emission is fluorescent between singlet states. Upon a comparison with similar systems from the literature, it was found that most of such systems have higher lifetimes and are phosphorescent in nature, while our system is fluorescent (Table 5).11−17 This is presumably due to the presence of more electron-rich amino substituent instead of prototypical alkyl groups, which might lead to an increase in the energy of the highest occupied molecular orbital (HOMO) levels. Perhaps the narrower HOMO−lowest unoccupied molecular orbital (LUMO) gap could account for a faster deactivation of the excited states.18 The quantum yield of 3 is found to be 0.15, which is more than those of [Au2(dcpm)2](SO3CF3)2]12 and [Au2(dcpm)2](ClO4)2]12 but comparable to that of the [Au2(dppm)2(SO3CF3)2]13 system (Table 5). The emission spectrum of 3 was recorded in the solid state at ambient temperature and shows an emission band centered at 418 nm (λex = 330 nm; Figure 7C). A negligible shift in the emission energy is observed between the solid and solution spectra, which suggests very little or no ILCT transition effect on the excited state.10,19 Compound 1 (free ligand) has shown no emission in the visible range when excited at λex = 330 nm
Table 3. Nearest Au···Au Distances for 6 and 7 compound
nearest Au···Au distance (Å)
6 7
8.156 10.964
distance present between Au atoms in 7 is given in Table 3). The AuI center in 7 adopts a linear geometry, with two P → Au coordination bonds displaying an Au−P bond distance and a P−Au−P bond angle of 2.3345(6) Å and 180°, respectively (Figure 6). It is evident from the literature that the AuI···AuI interaction is the reason behind various intriguing electronic transitions in gold(I) complexes, and therefore luminescence properties have become a characteristic measurement for the aurophilicity. In fact, the aurophilicity and luminescence properties are complementary to each other. To investigate further, fluorescence studies of 3 and 4 were performed. Luminescence Studies of 3. Blue luminescence was observed from colorless crystals of 3 under irradiation of UV light. The absorption spectrum was recorded at ambient temperature, while emission spectra were recorded at ambient temperature and 77 K (Figure 7). The absorption spectrum of 3 was recorded in a dichloromethane solution, and the main spectral features for the gold complexes include (a) an absorption band around 260−280 nm (related to the aromatic rings of the phosphine ligands); (b) a low-energy band at 330 E
DOI: 10.1021/acs.inorgchem.5b01046 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 7. (A) Normalized absorption−emission spectra of 3 (2.5 × 10−5 M, CH2Cl2; λex = 330 nm) at ambient temperature: (a) solution of 3 under normal light; (b) solid 3 under UV light; (c) solution of 3 under UV light. (B) Absorption−emission spectra of 3 in different solvents at ambient temperature. (C) Emission spectrum of 3 in the solid state at ambient temperature. (D) Excitation and emission spectra of 3 at 77 K. (E) Normalized absorption−emission spectra of 1 (in CH2Cl2) at ambient temperature.
Table 4. Photophysical Data for 3 and 4 emission (nm)
complex
CH2Cl2
3 4
417 405, 430, 460 (sh)
room temperature (solid state) 418 400 (sh), 466, 510 (sh)
Table 5. Selected Gold(I) Complexes and Their Photophysical Data
lifetime (ns)
compound
emission (nm)
quantum yield
77 K
quantum yield
solution
solid
[Au2(dmpm)2][(ClO4)2]10
455, 555
420 499
0.15 0.17
9.66 6.65
48.99 8.13
[Au2(dcpm)2][(ClO4)2]11
370, 510
0.048 ± 0.007
[Au2(dcpm)2][(SO3CF3)2]11
370, 500
0.027 ± 0.002
[Au2(dppm)2][(SO3CF3)2]12 [Au2{S2P(OMe)2}2]13
565 415, 456, 560 400 442, 516, 547, 593 467, 590
0.23
(Figure 7D). This behavior is in accordance with our interpretation on the UV−vis absorption measurements, where the absorption shoulder at ca. 330 nm was assigned mainly to the LMCT excitations. On the basis of the emission spectra of the free ligand, it can be proposed that the ILCT transitions have no great effect on the emission spectra of 3. Therefore, the blue emission at 417 nm for complex 3 can tentatively be assigned to the LMCT excitations, which is in well accordance with the time-dependent density functional theory (TDDFT) interpretation (discussed in a later part). Upon recording of the fluorescence spectra of 4 in different solvents, e.g., CH2Cl2, CH3CN, tetrahydrofuran (THF), and N,N-dimethylformamide (DMF), variation in the original band was observed (Figure 7b). The THF solution did not show any significant change in the emission, but the DMF solution shows a broadening of the band with no change in the color of
[Au2(dppm) (SPh)2]14 [Au2{(4,4′-S2-1,1′-biphenyl) (PPh3)2}]15 [Au2{Ph2PN(C6H4OMe-4) PPh2} (SC6H4NHCONHPh)2]16
lifetime 1.2 μs, 2.8 μs 0.74 μs, 4.96 μs 0.83 μs, 7.74 μs 21 μs 20 ns, 2.2 μs 1.1 μs 12 μs, 54 s
