Emissive Biphenyl Cyclometalated Gold(III) Diethyl Dithiocarbamate

Article pubs.acs.org/Organometallics

Emissive Biphenyl Cyclometalated Gold(III) Diethyl Dithiocarbamate Complexes Lakshmi Nilakantan,† David R. McMillin,*,§ and Paul R. Sharp*,† †

125 Chemistry, University of Missouri, Columbia, Missouri 65211, United States Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States

§

S Supporting Information *

ABSTRACT: We report here a series of emissive biphenyl cyclometalated gold(III) diethyl dithiocarbamate complexes having H, CF3, OMe, and tBu substitutions on the biphenyl moiety. Synthesis of these complexes was accomplished by a single-step reaction of the appropriate dilithio-biphenyl reagent with Au(dtc)Cl2 (dtc = diethyl dithiocarbamate). All four complexes exhibit weak room-temperature phosphorescence in solution and much more intense phosphorescence in the solid state and in lowtemperature glasses with lifetimes in the microseconds. From experimental data and computational modeling, the emission originates mainly from a metal-perturbed 3(π−π*) state of the biphenyl moiety with a minor contribution from ligand-to-ligand charge transfer. Weak solution emission is attributed to deactivation via a distorted charge-transfer state that is less accessible in the solid state or in a low-temperature glass.

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INTRODUCTION Gold metal has a very long history, and the chemical properties of gold have fascinated chemists for many years.1−3 Gold chemistry is dominated by the Au(I) and Au(III) oxidation states. There exists a vast literature on gold(I) chemistry,4−9 but reports on gold(III) compounds are fewer,1,10−12 although in recent years there has been an increasing interest in Au(III) compounds owing to their application in photoactive materials, as potential drugs for arthritis and cancer treatment,1,13 and as photoluminescent probes in biological systems.14 In the area of emissive cyclometalated compounds a major challenge with Au(III) complexes is that the majority of them do not exhibit room-temperature emission. This has been attributed to lowlying, nonemissive d−d states.1 To circumvent this issue, Yam and co-workers suggested the use of strong σ-donating ligands, which would raise the energy of the d−d states and reduce their thermal population.10 Thus, room-temperature emission in Au(III) complexes has been achieved with strong ligand fields, typically using cyclometalating ligands.1,12 Cyclometalated gold(III) compounds are typically synthesized from organolithuim reagents, Grignards, organotin compounds, or organomercury compounds.12,15,16 However, the transfer of the organic group from these reagents to the gold center has been extensively studied only for κ-C,N-donor ligands,15 with only limited studies for κ-C,C-donor ligands. The main difficulty associated with the synthesis of Au(III) κC,C-donor ligand complexes is the common use of gold(III) starting materials such as AuCl3·THT (THT = tetrahydrothiofuran). These starting materials are readily reduced to gold metal by κ-C,C-donor ligand lithium or Grignard reagents but are otherwise successful with the κ-C,N-donor ligand reagents. In fact, there are only a few literature reports on cyclometalated κ-C,C-donor ligand gold(III) complexes.15−20 © XXXX American Chemical Society

Here we report a one-step synthesis of a series of cyclometalated gold(III) compounds (3a−d) bearing κ-C,Cdonor ligands from the dilithio reagents (1a−d) and gold(III) complex AuCl2(dtc) (2, dtc = diethyldithiocarbamate) as starting materials. The gold(III) center in 2 is stabilized by the chelating dithiocarbamate ligand, which prevents the gold(III) center from being reduced to metallic gold. Photophysical studies of these room-temperature emissive complexes are also detailed here with supporting DFT calculations.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of the title biphenyl cyclometalated gold(III) diethyl dithiocarbamate complexes 3a−d was achieved in a single step from the dilithio reagents 1a−d and AuCl2(dtc) 2 in moderate to good yields (Scheme 1). Previously reported procedures for the synthesis of 3a and 3d (SI Scheme 1) involve multiple steps, low yields, and long reaction times. The new synthesis in Scheme 1 is an improvement and relies on the stabilization of the Au(III) center by dtc in AuCl3(dtc), allowing the use of strongly reducing lithium reagents. This does, however, limit the synthesis to auracycles with a dtc coligand. Complexes 3a−d were characterized by 1H, 13C{1H}, and 19 1 F{ H} (3c) NMR spectroscopy, elemental analysis (3b, 3c), and single-crystal X-ray diffraction (3a−c). Drawings of the solid state structures are given in Figure 1, and important bond lengths and bond angles are given in Table 1. In all complexes, the gold center is coordinated by the biphenyl ligand and a bidentate dithiocarbamate ligand and possesses a square-planar geometry typical for Au(III) complexes.12 Received: April 7, 2016

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DOI: 10.1021/acs.organomet.6b00275 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Syntheses of 3a−d

Table 1. Selected Experimental (X-ray) and Calculated (DFT) Distances (Å) and Angles (deg) for 3a−c distance or angle Au−C

Au−S

S−C

C−Ca C−Au−C C−Au−S (trans)

C−Au−S (cis)

S−Au−S S−C−S a

3a (X-ray) 2.028(3), 2.030(3) 2.033(3), 2.034(3)b 2.3851(7), 2.3911(7) 2.3877(8), 2.3909(7)b 1.732(3), 1.735(3) 1.730(3), 1.731(3)b 1.484(4) 1.474(4)b 81.26(11) 81.39(11)b 176.88(8), 176.26(8) 176.02(8), 175.96(8)b 101.86(8), 102.42(8) 101.89(8), 102.17(8)b 74.46(3) 74.47(3)b 112.92(15) 113.33(16)b

3a (DFT)

3b (X-ray)

3c (X-ray)

2.040

2.030(4), 2.033(4) 2.023(4)b

2.026(2)

2.502

2.3802(11), 2.3833(12) 2.3862(12)b

2.3968(5)

1.724

1.724(5), 1.734(5) 1.732(4)b

1.7349(18)

1.463

1.478(6) 1.474(9)b 81.48(18) 80.9(3)b 176.65(12), 176.40(14) 176.01(13)b

1.485(4)

80.66 175.88

80.80(12) 176.86(6)

103.46

102.03(13), 101.71(14) 102.19(13)b

102.32(6)

72.42

74.80(4) 74.79(6)b 113.6(3) 113.6(4)b

74.56(3)

118.05

113.61(18)

Biphenyl unit ring-to-ring distance. bSecond independent molecule.

molecular packing, and this is described below, as it could be important in the solid-state luminescence. Crystals of 3a are built from two approximately planar, independent molecules lying side-by-side in a head-to-tail fashion. Inversion centers above each molecule generate another set, forming independent side-by-side stacks of inversion-related molecules (Figure 2). The two stacks differ

Figure 1. X-ray structures of 3a−c (50% probability ellipsoids, hydrogen atoms omitted for clarity). One of two independent molecules for 3a and 3b. Atoms of the same name in 3c are related by a 2-fold axis passing through the Au center and the dtc ligand.

Figure 2. Dimer stacks of 3a.

A comparison of X-ray bond lengths and angles of 3a−c and literature-reported 3d16 shows no significant differences in common bond lengths and angles. For example, the Au−C and Au−S bond lengths of 3a−c (2.026−2.034 and 2.380−2.397 Å) are comparable to those of the two independent molecules of 3d (2.027−2.039 and 2.380−2.408 Å).16 Similarly, the S−Au− S bond angles of 3d (74.3° and 74.6°) are near or in the range of 74.5−74.8° for 3a−c, and the C−Au−C angles (81.3° and 81.5°) of 3d are close to those for 3a−c (80.8−81.3°). However, significant differences are observed in the crystal

in their alignment. In one the two gold atoms are directly above each other, giving a Au−Au distance of 3.51 Å. In the second stack the molecules are displaced from each other along the long axis (a-axis direction) such that the Au centers lie above the inversion-related Au−dtc ring, giving a Au−Au distance of 3.94 Å (Figure 2). The stacks then extend along the long molecular axis (a-axis direction), creating two-molecule-wide, double ribbons that are surrounded by other double ribbons approximately perpendicular to the first. B


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Organometallics

Figure 3. Infinite stacks of 3b.

Complex 3b packing consists of infinite stacks containing two independent molecules where the stacking axis passes approximately through the center of the Au−dtc ring (Figure 3). The roughly planar molecules are tilted by ∼15° from being perpendicular to the stacking axis. Each molecule moving up the stack is rotated by ∼120° from the one below, giving a pseudo-3-fold screw axis. The Au−Au distances alternate from 3.90 Å to 4.63 Å. The molecular plane distances, as measured by the Au−dtc ring centroid distances, are 3.77 and 3.89 Å. In contrast to 3a and 3b, there is only one independent molecule (located on a 2-fold axis) in crystals of 3c. The molecules are arranged in a head-to-tail alignment, giving single ribbons along the c-axis. Adjacent ribbons are aligned at the edge, displaced by 0.5 molecule and tilted by ∼90°, forming corrugated sheets. The sheets stack on each other to form the crystal. This gives a zigzag or herringbone pattern down the caxis, little to no overlap of the molecules in the stacked sheets, and long Au−Au distances (shortest = 7.741 Å). Photophysical Studies. UV−vis absorption spectral data for 3a−d are presented in Figure 4 and Table S1. An intense high-energy absorption band around 252 nm and structured bands from 287 to 346 nm are observed for all complexes. The absorption spectra of 3b and 3d are fairly similar to that of 3a, which shows that CF3 or tBu substitution on the biphenyl does not have a significant impact on the absorption spectrum. However, in 3c the absorptions are slightly red-shifted (∼3 nm) and there is an increase in molar absorption coefficients, likely due to the electron-donating character of the methoxy substituent. We assign the transitions in the high-energy region (200−300 nm) to biphenyl intraligand π−π* transitions (ε = 106−105 M−1 cm−1) and the lower-energy transitions ranging

Figure 4. Absorption spectra of 3a−d at room temperature in CH2Cl2 (3a, 3c, 3d) or cyclohexane (3b).

from 300 to 400 nm to metal-perturbed ligand-to-ligand charge transfers (LL′CT). These assignments are consistent with reported Pt(bph)L2 compounds (L = CO, pyridine, and CH3CN),21−23 Pt(II) complexes containing ligands such as bipyridine, phenanthroline, and alkylated derivatives, and anionic complexes of 2-phenylpyridine and 2-(2-thienylpyridine).24 Complexes 3a−d are weakly photoluminescent in solution and intensely photoluminescent in the solid and in 2C


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Organometallics

for 3a (Figure S16),3 indicating little to no charge-transfer character to the transition. Solvent-independent emission spectra are also observed for other cyclometalated gold(III) complexes. Other than the expected blue shift from solvent immobilization, the emission spectra do not change significantly from room temperature to 77 K for 3a, 3b, and 3d (Figure S14). However, that for 3c shows additional structure (Figure 6).

methyltetrahydrofuran (2-MeTHF) glass at 77 K (Figures S12 and S13). Data for the complexes are tabulated in Table 2. Table 2. Photoemission Data for 3a−da complex 3a

3b

3c

3d

medium, T (K) CH2Cl2, 298 solid, 298 MeTHF, 77 cyclohexane, 298 solid, 298 MeTHF, 77 CH2Cl2, 298 solid, 298 glass, 77 CH2Cl2, 298 solid, 298 MeTHF, 77

λmax (nm) 473, 477, 468, 471,

505, 508, 501, 503,

539 544 534 537

473, 508, 547 465, 498, 528 505, 540, 588 507, 548, 594 462, 480, 493, 514, 553 484, 521, 559 482, 517, 551 478, 513, 541

τo (μs)b

ϕc 1.7(1) × 10−3

64 ± 5 306 ± 11 33 ± 5 333 ± 2

2.7(2) × 10−3

0.32(9) × 10−3

54 ± 9 249 ± 8 5.0(7) × 10−3 40 ± 4 333 ± 8

λex (nm) = 337 (3a), 334 (3b), 336 (3c), 346 (3d). bSingleexponential decay of integrated total intensity. cEmission quantum yield, quinine sulfate standard. a

Figure 6. Photoemission spectrum of 3c in 2-MeTHF at 77 K (λex = 336 nm).

Similar structured emission spectra in the 450−600 nm range are observed for all four complexes in solution (Figure 5), in

Emission from an impurity can be eliminated, as all peaks decay with the same lifetime. The additional structure is therefore assigned to supplemental vibronic coupling, possibly from the C−O vibration of the OMe groups. Excitation spectra of the complexes (Figure 7 for 3a and Figures S17−S19 for 3b−d) are

Figure 7. Excitation and UV−visible absorption spectra of 3a (CH2Cl2).

independent of the emission wavelength (473 and 505 nm), consistent with emission from a single emitting state. In comparison to the UV−visible absorption spectrum, the relative intensity of the excitation spectrum increases at lower wavelengths. Finally, solution photoemission intensity of the complexes decreased by about one-half in aerated solutions (Figure S20). DFT Studies. For a better understanding of the UV−vis and luminescence spectra, DFT calculations using the M06 functional25 (Gaussian 09 suite26) were performed. Model complexes 3a′−d′ were constructed starting from the x, y, z coordinates of the X-ray structure of 3a. For ease of calculation the NEt2 group was replaced with an NH2 group, and the geometries were optimized. Table 1 compares bond lengths and angles from the X-ray and DFT studies for 3a and 3a′ (Table S2 for 3b′−d′). The bond lengths and angles of the

Figure 5. Photoemission spectra of 3a−d at room temperature in CH2Cl2 (3a, 3c, 3d) or cyclohexane (3b). (λex (nm) = 337 (3a), 334 (3b), 336 (3c), 346 (3d).)

the solid (Figure S15), and in the 2-MeTHF glass (S14). Complexes 3c and 3d, possessing electron-donating OMe and t Bu groups, show ∼30 and ∼20 nm red-shifted emissions, respectively, when compared with 3a. However, the electronwithdrawing CF3 groups in 3b do not cause significant shifts from 3a, as the spectra of 3a and 3b are nearly identical. The vibrational progression in the emission spectra (solution: 1340, 1249 (3a), 1351, 1259 (3b), 1284, 1512 (3c), and 1467, 1305 (3d) cm−1) indicate vibronic coupling typical of aromatic molecules such as biphenyl (1055 and 1274 cm−1).16 The shape and position of the emission spectra are solvent independent D


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Organometallics optimized structures agree with those from the crystal structures to within a few percent. Gaussview drawings of the DFT frontier orbitals for 3a′ are given in Figure 8. Those for

Table 3. TDDFT (Gas-Phase) Low-Energy Singlet and Triplet Vertical Transitions for 3a′a multiplicity singlet

triplet

λ (nm)

oscillator strength

388 337

0.0003 0.0231

313 303

0.0006 0

299

0.0001

297

0.0125

296

0

292

0.0132

457 407

a

contribution (>6%)

description

H→L (96%) H→L+1 (86%), H→L +2 (7%) H-1→L (95%) H-3→L (87%), H-4→ L+1 (10%) H-2→L (95%)

π(bph)-σ*(Au-L) π(bph)-π*(dtc)

H-1→L+1 (71%), H2→L+1 (18%) H-4→L+1 (85%), H3→L (11%) H→L+2 (76%), H→L +1 (8%) H→L+2 (58%), H→L +1 (24%) H→L (96%)

π(dtc)-σ*(Au-L) π(bph)-σ*(Au-L) π(bph/dtc)σ*(Au-L) π(bph/dtc)π*(dtc) σ(Au-L)-π*(dtc) π−π*(bph) π−π*(bph)/ π(bph)-π*(dtc) π(bph)-σ*(Au-L)

H = HOMO, L = LUMO.

dark (oscillator strength 280 nm). The lowest-energy, singlet-to-singlet transition (388 nm, 3.195 eV) is dark, involves the HOMO and LUMO, and may be described as a π(bph)-to-σ*(Au-L) transition. The next lowest at 337 nm, with an oscillator strength of 0.0231, is mainly HOMO → LUMO+1 (84%) in character and can be considered a metalperturbed transition from a π orbital of the biphenyl ligand to a π* orbital of the dithiocarbamate ligand (ligand-to-ligand′ charge transfer, LL′CT). The next three transitions are dark, are between the HOMO−1, HOMO−2, or HOMO−3 and the LUMO, and may be described as combinations of π(bph)-toσ*(Au-L) and π(dtc)-to-σ*(Au-L) transitions. The next two (one dark) mostly involve transitions to the LUMO+1 from the HOMO−1 and the HOMO−4, leading to descriptions as π(bph/dtc)-to-π*(dtc) and σ(Au-L)-to-π*(dtc) transitions. Finally, at 292 nm is a transition that is predominately π-toπ*(bph) since it involves (76%) the HOMO and LUMO+2. Although of more mixed character the HOMO-to-LUMO+2 transition (58%) is the lowest (457 nm) singlet-to-triplet transition and is followed by the HOMO-to-LUMO (the lowest singlet-to-singlet transition) at 407 nm. This change in order most likely arises from the larger singlet−triplet gap for the πto-π*(bph) transition and is expected for a transition with greater spatial overlap of the involved orbitals.27 Thus, the TDDFT results are generally consistent with the experimental observation of emission from a triplet π-to-π*(bph) excited state. Table 4 compares the calculated lowest singlet-to-triplet transition with those of the experimental emission energies for all the complexes. The TDDFT transitions are at higher energy than the emissions, as would be expected since the vibrational components are not included. In order to study the nature of the emitting triplet states and the structural changes from the corresponding ground states of the Au(III) complexes, an unrestricted Kohn−Sham approach (UM06) was used to optimize the lowest-lying triplet states of the model complexes 3a′−d′. A surprising number of energy minima were located. Their energies relative to the ground-

Figure 8. DFT frontier orbitals of 3a′.

3b′ and 3d′ (Figures S29, S31) are similar to those for 3a′, while the orbitals below the HOMO in 3c′ (Figure S30) have greater bph π character due to antibonding interactions of the methoxy group with the bph π system. The HOMO in all the model complexes is dominated by the bph π system, with minor contribution from a d orbital of gold. The HOMO−1 in 3a′, 3b′, and 3d′ has major contributions from sulfur p-type orbitals and small contributions from a gold d orbital and the biphenyl π system. That for 3c′ is dominated by the bph π system, as is the HOMO−2. The HOMO−2 in 3a′, 3b′, and 3d′ has a minor contribution from sulfur p-type orbitals and no significant gold orbital contribution. The LUMO for all model complexes is essentially the gold-ligand σ-antibonding orbital of Au dx2−y2 character. The π* orbitals of the CS and CN bonds dominate the LUMO+1, and a π* orbital of biphenyl has a major contribution to the LUMO+2 in addition to a minor contribution from the π* orbitals of the CS2 unit and no significant orbital contribution from gold. TDDFT calculations (gas phase) were employed to obtain the electronic transitions. Table 3 lists the resulting λ values and the MO contribution levels for the low-energy (λ > 290 nm), singlet-to-singlet and the first two singlet-to-triplet transitions for 3a′ (Tables S3−S5 for 3b′−d′). The transitions are similar for all complexes, and only those for 3a′ will be discussed. Five out of the eight singlet-to-singlet transitions are predicted to be E


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contrast, only one of the model DFT complexes (3b′) optimized to a local minimum triplet structure (T13b′) with π-to-π* character. The others gave two or more distorted, twisted triplet structures with CT character, and similar structures were also located for 3b′. All of the triplet complexes are at similar energies, as are transition states connecting the structures. For 3a′, the analogous planar structure is a transition state (TS22a′) (Figure S42) between the two enantiomers of a mildly twisted triplet structure T23a′. The energy of the transition state above T23a′ is, however, a negligible 2.9 kcal/ mol. The energy gap between T13b′ and TS22a′ and their corresponding ground-state singlet structures matches reasonably well with the emission energies (Table 4). Transition states between the other triplets are also not significantly above the triplet local minima. Thus, the DFT modeling indicates that the triplet surface is flat, and the triplet structures should be rather dynamic in unhindered environments. The excitation spectra suggest that population of the emitting triplet state may be more efficient from the lower-energy singlet excited states than from the higher-energy singlet excited states or, probably more likely, direct excitation into the emitting triplet excited state at the lower wavelengths. However, it is worth mentioning that lowest-energy absorption has contributions mainly from HOMO-L+1 (LLCT), in contrast with the lowest-energy emission from HOMO-L+2 (π−π*). The difference in nature of molecular orbitals involved in the emitting state from that of the absorbing state is explained by the Jablonski diagram in Figure 9.

Table 4. Comparison of Experimental Emission and Computed Vertical Transition Energies (nm/eV) for 3a−d complex

exptl

TD-DFTa

ΔEb

3a or 3a′ 3b or 3b′ 3c or 3c′ 3d or 3d′

473/2.62 471/2.63 505/2.45 484/2.56

457/2.71 451/2.75 443/2.80 469/2.64

466/2.63c 484/2.56d

a Singlet-to-triplet vertical transition at singlet geometry. bΔE = E(triplet) − E(singlet) using thermal and electronic energies of optimized structures. cTriplet = transition state TS22a′. dTriplet = T1 3b′.

state structures are given in Table 5, and the structures with their description are given in detail in the Supporting Information (SI Figures S32−S41). Table 5. Triplet Free Energies (kcal/mol, Gas Phase) Relative to Optimized Ground Singlets x a b c d

T1

3x′

T2

T3

58.1

57.7

56.1

56.8 55.6

3x′

3x′

57.7

T4

3x′

55.9 56.2 56.5 56.4

Only one of the complexes (3b′) yielded a triplet minimum consistent with the bph π−π* (HOMO → LUMO+2) character indicated by the emission data. The other structures are mildly distorted (T23a′, T23d′, T33a′, T33b′, T33d′) to highly distorted (T43a′−T43d′). The T43a′−T43d′ states are so distorted that they could come close to crossing the ground state. Such a crossing would represent a potential triplet thermal deactivation pathway. This possibility was investigated in two ways. First, the TDDFT singlet-to-triplet vertical transitions were calculated for the singlets with structures T4 3a′−T43d′. The first transition occurs at negative wavelengths for structures T43a′ (−2323 nm, −12.3 kcal/mol), T43b′ (−2573 nm or −11.1 kcal/mol), and T43d′ (−2598 nm or −11.0 kcal/mol), indicating that the crossing has already occurred and that the singlet is higher in energy than the triplet. For structure T43c′ a positive wavelength of 4105 nm is calculated, indicating a gap of 7.0 kcal/mol remains between the two surfaces. The second approach utilized the MECP (minimum energy crossing point) method of Harvey et al.28 to optimize the singlet/triplet structure to the crossing point (i.e., to where the singlet and triplet energies of the structure are identical). The resulting structures (Figure S47) are almost identical to T43a′−T43d′ and are only 0.2 to 0.8 kcal/mol higher in energy, indicating low energetic barriers to triplet− singlet crossing for T43a′−T43d′. In essence, the DFT studies on triplet surfaces support the dominance of thermal deactivation occurring in solution, thereby explaining the lowintensity luminescence in solution in comparison with the solidstate emission. Understanding the Photophysical Properties from DFT Results. The experimental data strongly indicate emission from a bph 3(π-to-π*) state of 3a−d. The vibronic structure and solvent independence of the emission are both supportive of this assignment, and analogous assignments have been made for related emissive complexes.3,16 The TDDFT calculations support this assignment in that the lowest-energy triplet absorption has majority π-to-π* character for all complexes. In

Figure 9. Jablonski diagram for 3a. Triplet- and singlet-state energies shown versus GS at 0.0 eV. LLCT, ligand-to-ligand charge transfer; LMCT, ligand-to-metal charge transfer; ISC, intersystem crossing; GS, ground state; P, phosphorescence; E, energy.

The 1LLCT states of 3a are readily populated by electronic transitions, which we see in the UV−vis spectrum. However, the emission arises from 3π−π* of the biphenyl, it being lower in energy than the 3LMCT state (0.3 eV). A large singlet− triplet splitting is common among π−π states in most aromatic compounds.29

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CONCLUSION We report here on the syntheses, X-ray structures, and photophysical properties of a series of emissive gold(III) complexes. While substitution on the biphenyl moiety by electron-donating OMe and tBu groups induces red shifts in the emission band when compared with the hydrogen counterpart, the electron-withdrawing CF3 groups have little effect. On the basis of the experimental observations of vibronically structured emission,3 large Stokes shifts, solvent-independent emission spectra, oxygen-induced quenching, as well as TD-DFT calculations and literature precedents,30 we assign the emission F


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Organometallics as metal-perturbed 3π−π*transitions of the biphenyl moiety. More detailed DFT calculations indicate a shallow triplet potential energy surface with low barriers connecting emissive 3 π−π* excited states and distorted LMCT states. The most twisted of the latter lies very near the crossing point with the ground state. Although the theory may underestimate the relative energies of the 3LMCT states, it does nicely account for the fact that the emission is much more intense and longer lived in the solid state or a 77 K glass, where the barrier to distortion is much higher. Other authors have identified lowenergy 3LMCT states in gold(III) halide complexes.31,32

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decanted, leaving the yellow solid product, which was washed with ether and vacuum-dried in a desiccator with P4O10. Yield: 150 mg (70%). 1H NMR (300 MHz, DMSO): 3.763 (q, 4H), 1.295 (t, 6H). The NMR signals match the literature data.43 Synthesis of (2,2′-Biphendiyl)(κ2-Sdiethyldithiocarbamate)gold(III) (3a). A solution of dilithio reagent 1a (30 mg, 0.075 mmol) in THF (4 mL) was cannula transferred into a precooled suspension of 2 (31 mg, 0.075 mmol) in THF (3 mL) at −30 °C. The reaction mixture was maintained at −30 °C for 45 min, slowly warmed to room temperature, and stirred overnight. The reaction mixture was then filtered, and the volatiles were removed from the filtrate in vacuo. The resulting residue was dissolved in CH2Cl2, and the solution was filtered using diatomaceous earth to remove LiCl. The volatiles were then removed from the filtrate in vacuo, leaving crude solid 3a. Recrystallization from benzene/ ether (1:1), layered with an equal volume of hexane, afforded 3a as offwhite crystals. Yield: 30 mg (73%). 1H NMR (500 MHz, CD2Cl2): 7.45 (dd, 7.7, 1.4 Hz, 2H), 7.21 (dd, 7.6, 1.2 Hz, 4H), 6.99 (dt, 7.5, 1.5 Hz, 2H), 3.83 (q, 7 Hz, 4H), 1.40 (t, 7.2 Hz, 6H). 13C{1H} NMR (CD2Cl2): 203.2, 153.9, 153.7, 131.8, 127.8, 127.7, 122.5, 47.2, 12.7. MS (EI) (C17H18AuS2N) m/z: mass required 497 mass observed (M + H+) 498. IR (KBr): 1519 cm−1 (C−N), 998 cm−1 (C−S). Synthesis of [(4,4′-Bis(trifluoromethyl)-2,2′-biphendiyl)](κ2S-diethyldithiocarbamate)gold(III) (3b). n-BuLi (2.5 M in hexane, 0.40 mL, 0.90 mmol) was added slowly to a solution of 2,2′-dibromo5,5′-bis(trifluoromethyl)biphenyl (20 mg, 0.45 mmol) in ether (15 mL). The reaction mixture was heated to reflux for 2 h. The resulting solution of dilithio reagent 1b was cooled to 0 °C and then cannula transferred into a suspension of 2 (187 mg, 0.45 mmol) in ether (6 mL) at the same temperature. The reaction mixture was stirred at 0 °C for 30 min, slowly (20 min) warmed to room temperature, and then heated at reflux for 2 h. The reaction mixture was filtered, and all volatiles were removed from the filtrate in vacuo. The residue was dissolved in CH2Cl2, and the solution was filtered using diatomaceous earth to remove LiCl. The volatiles were then removed from the filtrate in vacuo, leaving crude solid 3b. Recrystallization from benzene/ether (1:1) layered with an equal volume of hexane afforded 3b as off-white crystals. Yield: 174 mg (60%). 1H NMR (500 MHz, CD3COCD3): 8.0 (s, 2H), 7.38 (s, 2H), 7.36 (s, 2H), 3.92 (q, 7 Hz, 4H), 1.41 (t, 7 Hz, 6H). 13C{1H} NMR (CD3COCD3): 200.6, 156.5, 153.1, 132.0, 129.5 (q, J = 32.3 HZ), 124.8 (q, J = 3.1 Hz), 124.5 (q, J = 268 Hz), 119.2 (q, J = 3.1 Hz), 47.1, 12.0. 19F{1H} NMR (CD3COCD3) δ −63.5. Anal. Calcd for C19H16AuF6NS2 (%): C, 36.03; H, 2.55; N, 2.21. Found: C, 36.28; H, 2.70; N, 2.14. Synthesis of [(4,4′-Bis(dimethoxy)-2,2′-biphendiyl)](κ2-Sdiethyldithiocarbamate)gold(III) (3c). n-BuLi (2.5 M in hexane, 0.50 mL, 0.6 mmol) was added slowly to a solution of 2,2′-dibromo5,5′-dimethoxybiphenyl (20 mg, 0.45 mmol) in THF (9 mL) at −78 °C. The resulting solution of dilithio reagent 1c at −78 °C was cannula transferred into a precooled suspension of 2 (250 mg, 0.6 mmol) in THF (9 mL) at the same temperature. The reaction mixture was maintained at −78 °C for 45 min, slowly warmed to room temperature, and stirred overnight. The reaction mixture was filtered, and the volatiles were removed from the filtrate in vacuo. The residue was dissolved in CH2Cl2, and the solution was filtered using diatomaceous earth to remove LiCl. The volatiles were then removed from the filtrate in vacuo, leaving crude solid 3c. Purification of the residue by preparative TLC (hexane/ethyl acetate) gave 3c as a pale yellow solid. Recrystallization from toluene/ether (1:1) and layering with an equal volume of hexane afforded off-white X-ray quality crystals of 3c. Yield: 75 mg (30%). 1H NMR (300 MHz, CD2Cl2): 7.11 (d, 2H, 8 Hz), 6.97 (d, 2H, 3 Hz), 6.55 (dd, 2H, 8 Hz, 3 Hz), 3.81 (s, 6H, OMe) 3.80 (q, 4H, 7 Hz), 1.38 (t, 6H, 7 Hz). 13C{1H} NMR (CD2Cl2): 202.7, 159.2, 153.4, 144.5, 131.5, 111.6, 108.0, 55.1, 46.4, 11.9. Complex 3c analyzed as hemihydrate. Anal. Calcd for C19H23AuNO2.5S2 (%): C, 40.29; H, 4.09; N, 2.47. Found: C, 40.14; H, 4.22; N, 2.47. Synthesis of [(5,5′-Bis(tert-butyl)-2,2′-biphendiyl)](κ2-Sdiethyldithiocarbamate)gold(III) (3d). n-BuLi (2.5 M in hexane, 0.18 mL, 0.46 mmol) was added slowly to a solution of 2,2′-dibromo-

EXPERIMENTAL SECTION

Unless otherwise stated, all syntheses were carried out in a glovebox or by standard Schlenk line techniques. All solvents were dried before use with a solvent purification system provided by Pure Process Technology. For fluorescence measurements solvents were subjected to three freeze−pump−thaw cycles prior to use. NMR spectra were recorded on a Bruker DRX-500 or 300 MHz spectrometer. 1H and 13 C{1H} NMR spectra are referenced to Me4Si at δ 0, and 19F{1H} NMR are referenced to (trifluoromethyl)benzene. Infrared (IR) spectra were recorded on a Thermo Nicolet 670 Fourier transform spectrophotometer using KBr pellets with values quoted in wavenumbers (cm−1). UV−vis measurements were carried out on a Cary Bio 50 UV/vis spectrophotometer, and the absorption spectra recorded using 10 mm path length Spectrocell quartz cuvettes. Emission spectra were acquired on a Varian Cary Eclipse fluorimeter. Solid-state emission spectra for 3a−d were obtained by dissolving a ∼4 mg of compound in a minimum amount of solvent, and then the solution was added dropwise onto a cuvette wall (Fischer model 14385914B triangular-based cell) about one-third of the way from the bottom. The solvent was then allowed to evaporate at room temperature for 24 h, leaving a deposit of the complex for recording of the emission data. Emission spectra at 77 K for 3a−d were obtained in 2-MeTHF in Willmad 707-SQ-250 M 4 mm thin wall quartz EPR tubes placed in a quartz Dewar flask containing liquid nitrogen. The excitation wavelength for the emission measurements (77 K) was 337 nm. Solution photophysical data were obtained by dissolving 3a, 3c, and 3d in CH2Cl2 and 3b in cyclohexane at concentrations of approximately 10−5 M. The quantum yield measurements were done using the method of Parker and Rees33 at 25 °C with quinine sulfate as a standard. Lifetime measurements were done at 298 and 77 K by integrating the entire emission spectrum at different time delays and fitting the data to a single-exponential decay.34 A satisfactory fit was obtained in all cases. 2,2′-Dibromo-5,5′-bis(trifluoromethyl)biphenyl35 and 2,2′-dibromo-5,5′-dimethoxybiphenyl36 were prepared according to the literature procedure. Dilithio complexes 1a−d16,35−37 were prepared by literature procedures. AuCl2(dtc) (2) was prepared by adapting the procedure for the dimethyl analogues described below.13 DFT Calculations. Gaussian 0926 with the M0625 (X = Cl) or B3LYP38 (X = Br) functional was used for all calculations (gas phase). Basis sets employed were LANL2TZ(f)39,40 for Au, LANL2DZ39−41 augmented with d (0.496) and p (0.0347) functions for S, and 631G(d) for all other atoms. Initial structures were derived from crystal coordinates and were modified with Gaussview.42 All geometries were optimized without symmetry constraints. Analytical frequency calculations gave no imaginary frequencies for the complexes except for the transition states. Free energies were calculated at 298.15 K and 1 atm. The identity of the transition states was confirmed by inspection of the imaginary vibrational mode and by optimizing structures after small displacements in the directions of the vibrational mode. Synthesis of Dichlorodiethyldithiocarbamate Gold(III) (2). A solution of diethyldithiocarbamate sodium salt (120 mg, 0.53 mM) in water (3 mL) was added dropwise to an aqueous solution (2 mL) of potassium tetrachloroaurate (200 mg, 0.53 mM) under continuous stirring. A yellow precipitate formed, and the mixture was stirred for an hour. The stirring was stopped, and the mixture was carefully G


Article

Organometallics 4,4′-dibutylbiphenyl (100 mg, 0.23 mmol) in ether (9 mL) at −78 °C. The resulting solution of dilithio reagent 1d was cannula transferred into a precooled suspension of 2 (31 mg, 0.075 mmol) in ether (3 mL) at −78 °C. The reaction mixture was maintained at −78 °C for 45 min, slowly warmed to room temperature, and stirred overnight. The reaction mixture was filtered, and the volatiles were removed from the filtrate in vacuo. The residue was dissolved in CH2Cl2, and the solution was filtered using dictomacious earth to remove LiCl. The volatiles were then removed from the filtrate in vacuo, leaving crude solid 3d. Recrystallization from benzene/ether (1:1) and layering with an equal volume of hexane afforded 3d as off-white crystals. Yield: 26 mg (57%). The identity of 3d was confirmed by comparing the NMR data with the literature.16

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(12) Henderson, W. In Advances in Organometallic Chemistry; Robert, W., Anthony, F. H., Eds.; Academic Press, 2006; Vol. 54, pp 207−265. (13) Ronconi, L.; Giovagnini, L.; Marzano, C.; Bettìo, F.; Graziani, R.; Pilloni, G.; Fregona, D. Inorg. Chem. 2005, 44, 1867−1881. (14) Mansour, M. A.; Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. Inorg. Chem. 1998, 37, 4625−4632. (15) Garg, J. A.; Blacque, O.; Fox, T.; Venkatesan, K. Inorg. Chem. 2010, 49, 11463−11472. (16) David, B.; Monkowius, U.; Rust, J.; Lehmann, C. W.; Hyzak, L.; Mohr, F. Dalton Trans. 2014, 43, 11059−11066. (17) Usón, R.; Vicente, J.; Cirac, J. A.; Chicote, M. T. J. Organomet. Chem. 1980, 198, 105−112. (18) Dyadchenko, V. P.; Krasik, P. E.; Grandberg, K. I.; Kuz’mina, L. G.; Dvortsova, N. V.; Porai-Koshits, M. A.; Perevalova, E. G. Metalloorg. Khim. 1990, 3, 1260−1265. (19) Wu, C.; Horibe, T.; Jacobsen, C. B.; Toste, F. D. Nature 2015, 517, 449−454. (20) Joost, M.; Estévez, L.; Miqueu, K.; Amgoune, A.; Bourissou, D. Angew. Chem., Int. Ed. 2015, 54, 5236−5240. (21) Blanton, C. B.; Rillema, D. P. Inorg. Chim. Acta 1990, 168, 145− 147. (22) Chen, Y.; Merkert, J. W.; Murtaza, Z.; Woods, C.; Rillema, D. P. Inorg. Chim. Acta 1995, 240, 41−47. (23) Blanton, C. B.; Murtaza, Z.; Shaver, R. J.; Rillema, D. P. Inorg. Chem. 1992, 31, 3230−3235. (24) Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1994, 33, 2913− 2923. (25) Zhao, Y.; Truhlar, D. Acc. Chem. Res. 2008, 41, 157−167. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian09; Gaussian, Inc.: Wallingford, CT, 2009. (27) Lam, W. H.; Lam, E. S.; Yam, V. W. J. Am. Chem. Soc. 2013, 135, 15135−15143. (28) Harvey, J. N.; Aschi, M. Phys. Chem. Chem. Phys. 1999, 1, 5555− 5563. (29) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. J. Am. Chem. Soc. 2012, 134, 14706−14709. (30) Zehnder, T. N.; Blacque, O.; Venkatesan, K. Dalton Trans. 2014, 43, 11959−11972. (31) Li, W.-L.; Li, Y.; Xu, C.-Q.; Wang, X.-B.; Vorpagel, E.; Li, J. Inorg. Chem. 2015, 54, 11157−11167. (32) Hirtenlehner, C.; Krims, C.; Holbling, J.; List, M.; Zabel, M.; Fleck, M.; Berger, R. J. F.; Schoefberger, W.; Monkowius, U. Dalton Trans. 2011, 40, 9899−9910. (33) Parker, C. A.; Rees, W. T. Analyst 1960, 85, 587−600. (34) Golkowski, R. T.; Settineri, N. S.; Zhao, X.; McMillin, D. R. J. Phys. Chem. A 2015, 119, 11650−11658. (35) Van Kalkeren, H. A.; Leenders, S. H. A. M.; Hommersom, C. R. A.; Rutjes, F. P. J. T.; Van Delft, F. L. Chem. - Eur. J. 2011, 17, 11290− 11295. (36) Ito, M.; Kubo, H.; Itani, I.; Morimoto, K.; Dohi, T.; Kita, Y. J. Am. Chem. Soc. 2013, 135, 14078−14081. (37) Neugebauer, W.; Kos, A. J.; Von Ragué Schleyer, P. J. Organomet. Chem. 1982, 228, 107−118. (38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (39) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00275. Crystallographic data (CIF) Experimental and DFT details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by Office of Research, University of Missouri, Columbia. We thank Prof. Mahdi M. Abu-Omar (Purdue University) and Chemistry Department, Purdue University, for providing the lab, office space, and research facilities for L.N. to carry out research toward her Ph.D. We also thank Dr. Charles Barnes of University of Missouri, Columbia, for his assistance with the X-ray crystallography.

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REFERENCES

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Emissive Biphenyl Cyclometalated Gold(III) Diethyl Dithiocarbamate

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