Diorgano-Gallium and -Indium Complexes Derived from Benzoazole

Alan Kwun-Wa Chan , Elizabeth Suk-Hang Lam , Anthony Yiu-Yan Tam , Daniel Ping-Kuen Tsang , Wai Han Lam , Mei-Yee Chan , Wing-Tak Wong , Vivian ...
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Diorgano-Gallium and -Indium Complexes Derived from Benzoazole Ligands: Synthesis, Characterization, Photoluminescence, and Computational Studies Manoj K. Pal,† Nisha Kushwah,† Debashree Manna,‡ Amey Wadawale,† V. Sudarsan,† Tapan K. Ghanty,‡ and Vimal K. Jain†,* †

Chemistry Division and ‡Theoretical Chemistry Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India S Supporting Information *

ABSTRACT: The reactions of triorgano-gallium and -indium etherate with benzoazole ligands 2-(2′-hydroxyphenyl)benzoxazole (Hhbo) (1a), 2-(2′-hydroxyphenyl)benzothiazole (Hhbt) (1b), and 2-(2′-hydroxyphenyl)benzimidazole (Hhbi) (1c) in benzene yielded complexes of the type [R2ML]n with n = 1 for gallium and 2 for indium (where R = Me, Et; M = Ga, In; L = hbo, hbt, and hbi) in nearly quantitative yields. These complexes have been characterized by elemental analysis, IR, UV−vis, and NMR (1H and 13C{1H}) spectroscopy. Photoluminescence studies of these complexes showed that the quantum yield is always higher than that of the corresponding ligands due to reduced intermolecular interactions in complexes as compared to free ligands. The molecular structures of [Me2Ga(hbo)] (2a) and [Me2In(μ-hbt)]2 (3b) were established by X-ray crystallography. The analyses revealed that the gallium complex exists as a four-coordinated monomer, whereas the indium complex forms a dimer comprising five-coordinated indium atoms. Density functional calculations have been carried out for computing the structural and energetic parameters of the complexes, which indicate that the formation of the monomer is favorable with the Ga3+ ion, whereas the dimeric structure is preferred with the In3+ ion.

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an electron transport host or a green emitter.14−16 However, relatively low photoluminescence quantum yield and electron mobility of AlQ3 have led to exploration of luminescent group 13 metal complexes.17 Gallium complexes and classical18−20 and organometallic derivatives5,6,21 exhibit bright luminescence and in several instances even show higher efficiency than AlQ3.22 Accordingly, gallium complexes are projected as promising candidates to replace aluminum derivatives.20 The remarkable high intensity of fluorescence emission and large Stokes shift in ESIPT (excited state intramolecular proton transfer) molecules have drawn significant recent interest.23,24 These molecules with basic skeleton as depicted in Scheme 1 contain one acidic proton (−OH, −NH2) and a basic center (−N, −CO) in close proximity within the molecule. These molecules, however, often show very low fluorescence quantum yield, and their emission properties are concentration and environment sensitive.23 A wide range of molecules showing ESIPT are reported in the literature. These include

INTRODUCTION The chemistry of organo-gallium and -indium complexes has witnessed a rapid development during the last two decades or so.1 The sustained interest in these complexes stems from several reasons including their rich structural diversity1−3 and polymorphism,4 their interesting photophysical3−6 and antitumor properties,7 and their relevance in catalysis8 and materials science as molecular precursors.9,10 Compounds of composition R2ML (R = alkyl, M = Ga or In; L = anionic ligands), with the metal coordination number varying between three and six, have been isolated as mono- (e.g., [But2Ga(OCPh3)]),11 di- [e.g., [But2Ga(μ-OR)]2 (R = Me, Et, Pri)],12 and trimeric (e.g., [Me2Ga(μ-OMe)]3)13 species. Structural preferences in these complexes are influenced by the nature of R, M, and L, with indium tending to acquire higher coordination number.1 Since the pioneering work of Tang and van Slyke on tris(8hydroxyquinolinate)aluminum(III) (AlQ3),14 organic lightemitting diodes (OLEDs) have attracted considerable attention due to their application in electron display devices. AlQ3, a sublimable stable metal tris chelate, is widely used in OLEDs as © 2012 American Chemical Society

Received: September 5, 2012 Published: December 31, 2012 104

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Scheme 1

Scheme 3. Synthesis of Diorgano-Gallium and -Indium Complexes

salicylidene anilines, quinolines, oxazolines, benzoazoles, triazoles, etc. In the metal complexes derived from these molecules, not only is the fluorescence intensity enhanced dramatically,25 but also the quantum yield is increased significantly.4 Among ESIPT fluorophores, 2-(2′-hydroxyphenyl)benzoazoles (Scheme 2) represent an interesting family of Scheme 2.

a

a

E = O (2-(2′-hydroxyphenyl)benzoxazole (Hhbo)) (1a), S (2-(2′hydroxyphenyl)benzothiazole (Hhbt)) (1b), NH (2-(2′hydroxyphenyl)benzimidazole (Hhbi)) (1c).

ligands23,24 that have been shown to bind several metal ions (e.g., Zn, Ni, Cd, Co).25,26 Classical complexes of gallium, [Ga(hbo)2(OAc)],27 and indium, [In(hbo)3],28 have been reported. In view of the above and in pursuance of our interest in photophysical properties of organo-gallium and -indium complexes,3−5 we have synthesized diorgano-gallium and -indium complexes with 2-(2′-hydroxyphenyl)benzoazoles (Scheme 2) and studied their luminescence properties and also performed calculations using density functional theory (DFT). Results of this work are reported herein.

derivatives display a characteristic high-field singlet in the region −0.32 to −0.16 ppm. The 13C{1H} NMR spectra exhibited resonances due to alkyl metal carbons in the expected regions. The chemical shifts of C-2′, C-1′, C-2, C-8, and C-9 carbons are expected to be influenced by deprotonation of phenolic OH and on coordination of the CN nitrogen to the metal atom. Accordingly their shifts have been evaluated. The C-2′ resonance in the 13C NMR spectra of complexes is significantly shifted downfield (5.7−9.7 ppm) with respect to the resonance for the corresponding ligand,26,30 whereas C-8 and C-9 resonances are shielded, which is more pronounced in the gallium derivatives. The C-2 resonance in the complexes derived from hbo and hbt is also shifted downfield (2.0−7.3 ppm); however this resonance in the hbi complexes is slightly shielded (0.6 ± 0.2 ppm). The C-1′ showed a small change (0− 2.1 ppm) in the complexes (shielding in hbo and hbi derivatives, while deshielding in hbt complexes). Crystal Structures of 2a and 3b. The molecular structures of [Me2Ga(hbo)] (2a) and [Me2In(μ-hbt)]2 (3b) have been elucidated by single-crystal X-ray diffraction analyses. Perspective drawings are depicited in Figures 1 and 2, and selected interatomic parameters are given in Tables 1 and 2. Complex 2a is a monomer with a four-coordinated gallium atom. The metal atom is coordinated to two methyl carbon atoms, a phenolate oxygen, and a benzoxazole N atom. The sixmembered “O1−C11−C10−C9−N1−Ga1” chelate ring is nonplanar, while the individual phenyl and benzoxazole rings are planar. The planes defined by the “Ga1−N1−C9” ring and O1−C11 phenyl rings are at an angle of 8.9°. The phenolate oxygen atom is 0.326 Å below the Ga−N1−C9 ring plane. The Ga−C,4,5,31 Ga−O,4,27,32 and Ga−N3,4,27,32 distances are in conformity with the values reported in the literature. Complex 3b is a centrosymmetric dimer. It consists of two Me2In units bridged by two phenolate oxygen atoms forming a planar four-membered In2O2 rectangle. The four-membered

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. Reactions of trialkyl-gallium and -indium etherate with benzoazoles, viz., 2-(2′-hydroxyphenyl)benzoxazole (Hhbo) (1a), 2-(2′hydroxyphenyl)benzothiazole (Hhbt) (1b), and 2-(2′hydroxyphenyl)benzimidazole (Hhbi) (1c), in benzene at room temperature yielded complexes of the type [R2ML]n with n = 1 for gallium and 2 for indium (where R = Me, Et; M = Ga, In; L = hbo, hbt, and hbi) in 94−98% yield (Scheme 3). These complexes could be recrystallized from toluene−hexane or dichloromethane−hexane mixtures as a cream or light yellow colored crystalline solid in 75−80% yield. In the IR spectra of the complexes containing hbo and hbt ligands, the CN stretching modes are shifted to lower wave numbers (15−30 cm−1) with reference to the corresponding absorption for the free ligands, indicating coordination of C N nitrogen to the metal atom.27 The absorptions in the region 505−550 cm−1, absent in the IR spectra of free ligands, have been assigned to the M−C stretching vibrations.10,29 The OH proton resonances of the ligand [Hhbo (11.20 ppm), Hhbt (11.61 ppm), and Hhbi (13.24 ppm)] are absent in the 1H NMR spectra of the complexes due to deprotonation of the ligands. The 1H NMR spectra of the methyl metal 105

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Table 2. Selected Geometric Parameters (Å/deg) for [Me2In(μ-hbt)]2 (3b) C1−In1 O1−In1 N1−In1 C2−In1−C1 C1−In1−N1 O1−In1−C2 In1−O1−In1′ O1−In1′−C1 C9−N1−In1 C11−O1−In1

Figure 2. Crystal structure of [Me2In(μ-hbt)]2 (3b) with 50% ellipsoid probability (inset top view).

Table 1. Selected Geometric Parameters (Å/deg) for [Me2Ga(hbo)] (2a) 1.949 (5) 1.891 (4) 1.309 (5) 124.1 (3) 105.59 (19) 110.4 (2) 122.5 (3) 129.9 (3)

Ga1−C2 Ga1−N1 N1−C3 O1−Ga1−C1 O1−Ga1−C2 O1−Ga1−N1 C3−N1−Ga1

1.947 2.041 1.399 111.5 109.8 90.23 130.6

C2−In1 O1′−In1′

2.143 (3) 2.553 (2)

O1−In1−C1 O1−In1−N1 C2−In1−N1 O1−In1′−C2 C3−N1−In1 C11−O1−In1 O1−In1−O1′

108.13 (13) 81.13 (9) 96.02 (12) 91.50 (12) 121.4 (2) 134.13 (19) 76.99 (8)

bonds (e.g., [Me2In(O2C(C5H4N)]2,5 [But2In(OEt)2]2,33 and [Ph2In(OSiMe3)]234) or a rectangle (e.g., [Me2In(OC6H4OxMe2)]210 and [Me2In(OCH2dmpz)]231). Each ligand forms a puckered six-membered chelate ring involving phenolate oxygen and benzothiazole nitrogen. The coordination geometry around the indium atom is defined by C2NO2 donor atoms and can be described as square pyramidal (τ = 0.13) based on τ index.35 The O−In−O angle (∼77°) is within the range (74− 79°) reported for dimeric indium complexes. The In−C,5,10 In−O,10 and In−N5,10 distances are well in agreement with the reported values. Luminescence Properties. The emission and excitation spectra of ligands 1a, 1b, and 1c are shown in Figure 3. Wavelengths corresponding to emission and excitation maxima are quite different, indicating large Stokes shifts associated with these molecules. On the basis of the previous luminescence studies23,28,36 on such compounds, excitation and emission bands have been attributed to the transitions between π and π* levels of enol (high HOMO−LUMO gap) and keto (low HOMO−LUMO gap) forms, respectively. The significantly Stokes shifted emission arises from the keto form, which is formed through excited-state intramolecular proton transfer from the enol form of the ligand molecule.23 Generally the presence of electron-withdrawing groups in the ligand leads to bathochromic shift (red shift) in the emission maximum, while the electron-donating groups result in hypsochromic shift (blue shift) in the emission spectra.37 As the pKb values of benzimidazole, benzothiazole, and benzoxazoles are 8.52, 11.56, and 13.52, respectively, the emission from the benzimidazole moiety will be blue-shifted compared with the other two ligands due to its highest extent of electron-donating nature (Table 3). The ligand with a benzothiazole moiety is expected to show emission maxima at higher wavelengths as compared to benzoxazole due to the lower electronegativity of sulfur compared to oxygen and associated decrease in the HOMO−LUMO gap. Figure 4 shows the emission spectra from representative gallium complexes formed with the above-mentioned ligands. Wavelengths corresponding to emission maxima are blueshifted as compared to free ligands26a (Figure 4 and Table 3). Generally one would expect a red shift in the emission maximum when a ligand is coordinated to a d10 metal ion. The observed difference can be attributed to the fact that once the ligand is deprotonated and binds with metal ions, an excitedstate intramolecular proton transfer process is not possible; consequently the keto form of the ligand with low HOMO− LUMO gap is not formed. The excited states corresponding to the emission from these complexes have been found to decay single exponentially with a lifetime in the range 2−3 ns, which

Figure 1. Crystal structure of [Me2Ga(hbo)] (2a) with 50% ellipsoid probability (inset top view).

Ga1−C1 Ga1−O1 N1−C9 C1−Ga1−C2 N1−Ga1−C1 N1−Ga1−C2 C9−N1−Ga1 C11−O1−Ga1

2.144 (3) 2.141 (2) 2.434 (3) 142.12 (15) 97.04 (13) 108.90 (12) 103.01 (8) 89.17 (13) 127.8 (2) 122.85 (18)

(5) (4) (6) (2) (2) (15) (3)

In2O2 ring in binuclear indium complexes has been reported to exist either as a square due to the equivalence of the In−O 106

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Figure 3. Emission and excitation spectra (with inset) from ligands (a) 1a, (b) 1b, and (c) 1c in dichloromethane. The excitation wavelengths are 354, 375, and 353 nm for ligands 1a, 1b, and 1c, respectively.

Table 3. UV−Vis Absorption, Excitation, and Emission Data of Ligands and Their Diorgano-Gallium and -Indium Complexes in Dichloromethane compound Hhbo (1a) Hhbt (1b) Hhbi (1c) [Me2Ga(hbo)] (2a) [Me2Ga(hbt)] (2b) [Me2Ga(hbi)] (2c) [Et2Ga(hbo)] (2d) [Et2Ga(hbt)] (2e) [Et2Ga(hbi)] (2f) [Me2In(μ-hbo)]2 (3a) [Me2In(μ-hbt)]2 (3b) [Et2In(μ-hbo)]2 (3c)

UV−vis absorption, λ in nm 293, 288, 275, 293, 298, 248, 292, 291, 250, 293, 288, 249,

320, 334, 287, 303, 309, 292, 304, 304, 293, 302, 334, 293,

333 344 293, 322, 324, 300, 340, 381 300, 320, 347, 303,

excitation λ in nm

emission λ in nm

Stokes shift

quantum yield (η) in %

354 375 353 312 360 309 316 341 312 345 370 346

490 528 460 450 458 410 440 467 456 437 467 440

136 153 107 118 98 101 124 126 144 92 97 94

2 3 28 3 19 28

319, 334 334, 373 349, 392 357 377 362 333, 376 395 321, 334, 379

4 22

Figure 4. Emission spectra from complexes (a) 2a, (b) 2b, and (c) 2c in dichloromethane. The excitation wavelength is 312, 360, and 309 nm for 2a, 2b, and 2c complexes, respectively.

compared to the corresponding ligands (Table 3). This could be due to the reduced vectorial interactions between ligand moieties in complexes as compared to that of pure ligands in

is in conformity with the values reported for complexes derived from such ligands.28 A representative decay curve is shown in Figure 5. The complexes showed higher quantum yields as 107

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Figure 5. Decay curve corresponding to emission from [Me2In(μhbt)]2 (3b). The emission and excitation wavelengths are 467 and 370 nm, respectively.

solution. Increased quantum yield in complexes derived from a benzimidazole moiety may be due to its higher electrondonating tendency. Emissions from the corresponding binuclear indium complexes were more or less similar; however the quantum yields were slightly higher in the former (Table 3). Computational Studies. Complexation behavior of three different benzoazole ligands with Ga3+ and In3+ metal ions has been considered in this work. The optimized structures of the free ligands are given in the Supporting Information. Monomer and dimer structures of diorgano-gallium/-indium complexes involving these ligands have been optimized, and the corresponding structures are given in Figures 6 and 7. Figure 7. Optimized structure of (a) Ga-dimer and (b) In-dimer complexes, where X = O/S/NH.

donor centers are provided in Tables 4 and 5, respectively. All the calculated structural and energetic parameters clearly indicate that the formation of monomer is favorable with Ga3+ ions, whereas dimeric structure is preferred with the In3+ ions.

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CONCLUSION Diorgano-gallium and -indium complexes derived from benzoazoles have been synthesized. The monomeric structures are energetically more favorable with gallium, while indium tends to form dimeric structures. The complexes are emissive in solution at room temperature. The emission peaks are blueshifted with respect to the peak for the corresponding free ligand. The quantum yields of the complexes are always higher than that for the corresponding ligands.

Figure 6. Optimized structure of M-monomer complexes, where M = Ga3+/In3+ and X = O/S/NH.

Monomer complexes involving In3+ are less stable as compared to the corresponding Ga3+ complexes. In the case of the Ga3+ system complexation energy values corresponding to the dimer are less negative as compared to twice the monomer complexation energies. It is worth noting that the negative value of complexation energy for a particular complex has been considered as a stable complex. On the other hand, in the case of In3+ there is some extra stability gain for the dimer complexes as compared to twice the monomer complexation energies. Consequently, formation of dimer is favorable with indium and less probable with gallium. The unrealistic bridging bond distance values, viz., 4.001, 5.023, and 4.984 Å for the [Me2Ga(μ-hbo)]2, [Me2Ga(μ-hbt)]2, and [Me2Ga(μ-hbi)]2 complexes, respectively, further confirm less probability of formation of the dimeric gallium complexes. For any particular metal−ligand bond, the bond distance is larger in the dimer as compared to the same in the monomer. The complexation energy values and the charge distributions on the metal and the

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All experiments involving organo-gallium/-indium compounds were carried out in anhydrous conditions under a nitrogen atmosphere using Schlenk techniques. Solvents were dried using standard methods. The R3Ga·OEt2 (R = Me, Et) were prepared using gallium−magnesium alloy and alkyl iodide in diethyl ether, while R3In·OEt2 (R = Me or Et) were obtained by a reaction between anhydrous InCl3 and RMgI in diethyl ether.4 Ether contents in each preparation were evaluated by 1 H NMR integration. The ligands 2-(2′-hydroxyphenyl)benzoxazole (Hhbo) (1a), 2-(2′-hydroxyphenyl)benzothiazole (Hhbt) (1b), and 2(2′-hydroxyphenyl)benzimidazole (Hhbi) (1c) were purchased from commercial sources. Infrared spectra were recorded as KBr plates on a Jasco FT-IR 6100 spectrometer. The NMR spectra were recorded on a Bruker Avance-II 108

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Table 4. Calculated M−L Bond Distances (Å) and Complexation Energies (eV) for In and Ga Complexes Using the def2TZVP/BP86 Method complex [Me2Ga(hbo)] [Me2In(hbo)] [Me2Ga(hbt)] [Me2In(hbt)] [Me2Ga(hbi)] [Me2Ga(μ-hbo)]2 [Me2In(μ-hbo)]2 [Me2Ga(μ-hbt)]2 [Me2In(μ-hbt)]2 [Me2Ga(μ-hbi)]2

dM−S/M−O (Bridging)

dM−O/M−S

dM−C

dM−N

complexation energy (in eV)

4.001 2.607 5.023 2.612 4.984

1.932 2.144 1.193 2.137 1.932 1.994 2.220 1.925 2.202 1.931

1.988 2.165 1.986 2.166 1.987 1.985 2.162 1.986 2.163 1.986

2.077 2.301 2.083 2.311 2.059 2.089 2.395 2.087 2.411 2.061

−65.42 −59.88 −65.51 −59.94 −65.64 −128.70 −119.93 −130.99 −120.02 −131.26

Table 5. Calculated Atomic Charges on Metal and Donor Centers of In and Ga Complexes Using the def2-TZVP/BP86 Method complex

qM

qS/O/N(Non bonded)

qC

qO

qN

[Me2Ga(hbo)] [Me2In(hbo)] [Me2Ga(hbt)] [Me2In(hbt)] [Me2Ga(hbi)] [Me2Ga(μ-hbo)]2 [Me2In(μ-hbo)]2 [Me2Ga(μ-hbt)]2 [Me2In(μ-hbt)]2 [Me2Ga(μ-hbi)]2

1.589 1.535 1.590 1.529 1.589 1.593 1.534 1.585 1.530 1.590

−0.353 −0.359 0.420 0.408 −0.470 −0.351 −0.364 0.401 0.394 −0.471

−1.191 −1.136 −1.192 −1.136 −1.190 −1.194 −1.142 −1.193 −1.142 −1.192

−0.731 −0.731 −0.729 −0.729 −0.733 −0.727 −0.749 −0.725 −0.750 −0.729

−0.545 −0.530 −0.528 −0.508 −0.540 −0.541 −0.496 −0.526 −0.473 −0.539

lium etherate (530 mg, containing 260 mg (2.27 mmol) Me3Ga) was added a solution of (2-(2′-hydroxyphenyl)benzoxazole (479 mg, 2.27 mmol) with stirring, which continued for 5 h. The solvent was evaporated under reduced pressure to give an off-white crystalline solid (690 mg, 98% yield), which was recrystallized from toluene−hexane as a colorless crystalline solid, mp 115 °C. Anal. Calcd for C15H14GaNO2: C, 58.12; H, 4.55; N, 4.52. Found: C, 58.45; H, 5.09; N, 4.58. IR (ν in cm−1): 1615 (CN); 550 (Ga−C). 1H NMR (dmso-d6) δ: −0.23 (s, Me2Ga); 6.77 (t, 3JHH = 7.4 Hz, 1H); 6.85(d, 3JHH = 8.7 Hz, 1H); 7.39 (t, 3JHH = 7.1 Hz); 7.51 (m, 1H); 7.69 (m, 1H); 7.89 (d,d, 3JHH = 1.5, 8.1 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: −5.4 (s, Me2Ga); 109.2, 112.1, 116.6, 116.8, 122.9, 126.8, 127.0, 128.7, 135.7, 136.4, 148.4, 163.9, 166.8. Mass (m/e): 310 (m+); 294 (m-Me). All the diorgano-gallium and -indium complexes were prepared in nearly quantitative yields (95−98%) by the method similar to 2a and recyrstallized from a toluene−hexane or dichloromethane−hexane mixture in 75−80% yield. [Me2Ga(hbt)] (2b): mp 145 °C. Anal. Calcd for C15H14GaNOS: C, 55.25; H, 4.32; N, 4.29; S, 9.83. Found: C, 55.16; H, 4.11; N, 4.45; S, 9.74. IR (ν in cm−1): 1607 (CN); 540 (Ga−C). 1H NMR (dmsod6) δ: −0.22 (s, Me2Ga); 6.76 (t, 3JHH = 7.5 Hz, 1H); 6.85 (d, 3JHH = 8.7 Hz, 1H); 7.34−7.61 (m); 7.82 (d, 3JHH = 7.8 Hz, 1H); 7.85 (d, 3 JHH = 8.1 Hz, 1H); 8.19 (d, 3JHH = 7.8 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: −5.3 (s, Me2Ga); 117.4, 119.7, 120.2, 122.8, 123.2, 126.4, 128.0, 130.3, 131.9, 135.2, 148.9, 163.8, 171.6. [Me2Ga(hbi)] (2c): mp 213 °C. Anal, Calcd for C15H15GaN2O: C, 58.30; H, 4.89; N, 9.06. Found: C, 57.82; H, 4. 79; N, 9.27. IR (ν in cm−1): 1603 (CN); 542 (Ga−C). 1H NMR (dmso-d6) δ: −0.29 (s, Me2Ga); 3.41 (br, NH); 6.77 (t, 3JHH = 7.3 Hz, 1H); 6.84 (d, 3JHH = 8.1 Hz, 1H); 7.32 (d, 3JHH = 7.8 Hz, 1H); 7.39−7.43 (m, 2H); 7.57 (d, 3 JHH = 7.2 Hz, 1H); 7.67 (d, 3JHH = 7.2 Hz, 1H); 7.91 (d, 3JHH = 7.8 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: −6.0 (s, Me2Ga); 111.9, 112.7, 115.3, 116.4, 123.1, 124.3, 124.8, 128.8, 132.9, 134.0, 137.6, 151.3, 164.7. Mass (m/e): 309 (m+); 293 (m-Me). [Et2Ga(hbo)] (2d): mp 75 °C. Anal. Calcd for C17H18GaNO2: C, 60.40; H, 5.37; N, 4.14. Found: C, 60.72; H, 5.26; N, 3.83. IR (ν in cm−1): 1615 (CN); 525 (Ga−C). 1H NMR (dmso-d6) δ: 0.45 (q, 3 JHH = 7.7 Hz, −CH2Ga); 0.92 (t, 3JHH = 8.1 Hz, CH3CH2Ga); 6.73 (t,

300 spectrometer in 5 mm tubes as dmso-d6 solutions. Chemical shifts were referenced to internal dimethyl sulfoxide peak. Electronic spectra were recorded in dichloromethane on a UV−vis Jasco V-630 spectrophotometer. Mass spectra of a few representative complexes were recorded on a Varian MS-500 ion trap mass spectrometer at Sophisticated Analytical Instrumentation Facility (SAIF), IIT-Bombay, Mumbai. All luminescence measurements were carried out at room temperature on an Edinburgh Instruments FLSP 920 system, having a 450 W Xe lamp and a nanosecond hydrogen flash lamp as excitation sources for steady-state and lifetime measurements, respectively. Redsensitive PMT was used as the detector. Quantum yields were measured using an integrating sphere coated with BaSO4. All emission spectra were corrected for the detector response and excitation spectra for the lamp profile. Emission measurements were carried out with a resolution of 5 nm. X-ray Crystallography. Intensity data for [Me2Ga(hbo)] (2a) crystallized from toluene−hexane and [Me2In(μ-hbt)]2 (3b), crystallized from dichloromethane−hexane, were collected at room temperature on a Rigaku AFC 7S diffractometer using graphite-monochromated Mo Kα radiation (0.71069 Å). The structures were solved using direct methods and refined by the full matrix least-squares method38 on F2 using data corrected for absorption effects using empirical procedures.39 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in their geometrically idealized position with coordinate and thermal parameters riding on host atoms. The molecular structures are drawn using ORTEP.40 Crystallographic and structural determination data are given in the Supporting Information. Computational Methods. Geometries of all the bare monoanionic ligands and their resulting organo-gallium/-indium complexes have been fully optimized using the Turbomole 6.0 program41 within the framework of density functional theory. BP86 functional42,43 and standard def2-TZVP basis sets have been used for all the atoms at the all-electron level except for indium. For the indium atom 28 core electrons have been considered using an effective core potential, and the remaining electrons have been treated explicitly. Synthesis of Diorgano-Gallium and -Indium Complexes. [Me2Ga(hbo)] (2a). To a benzene solution (25 mL) of trimethylgal109

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JHH = 7.5 Hz, 1H); 6.87 (d, 3JHH = 8.4 Hz, 1H); 7.39 (t, 3JHH = 6.9 Hz, 1H); 7.52 (m, 1H); 7.66 (m, 1H); 7.88 (br, 2H). 13C{1H} NMR (dmso-d6) δ: 4.0 (s, −CH2Ga); 10.0 (s, CH3CH2Ga); 108.9, 112.1, 116.5, 116.7, 122.8, 126.9, 127.0, 128.6, 135.9, 136.5, 148.3, 164.3, 167.4. [Et2Ga(hbt)] (2e): mp 60 °C. Anal. Calcd for C17H18GaNOS: C, 57.66; H, 5.12; N, 3.95; S, 9.05. Found: C, 57.45; H, 5.42; N, 4.14; S, 9.08. IR (ν in cm−1): 1605 (CN); 512 (Ga−C). 1H NMR (dmsod6) δ: 0.47 (q, 3JHH = 7.3 Hz, −CH2Ga); 0.91 (t, 3JHH = 7.8 Hz, CH3CH2Ga); 6.73 (t, 3JHH = 7.4 Hz, 1H); 6.86 (d, 3JHH = 8.4 Hz, 1H); 7.36 (t, 3JHH = 7.4 Hz, 1H); 7.54 (t, 3JHH = 7.7 Hz, 1H); 7.65 (t, 3JHH = 7.8 Hz, 1H); 7.74 (d, 3JHH = 7.5 Hz, 1H); 7.83 (d, 3JHH = 8.1 Hz, 1H); 8.21 (d, 3JHH = 7.8 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: 4.2 (s, −CH2Ga); 10.0 (s, CH3CH2Ga); 116.6, 117.3, 119.1, 123.0, 123.4, 126.7, 128.4, 130.6, 131.2, 135.6, 148.7, 164.6, 172.8. [Et2Ga(hbi)] (2f): mp 166 °C. Anal. Calcd for C17H19GaN2O·(H2O)0.5: C, 58.99; H, 5.82; N, 8.09. Found: C, 58.81; H, 5.57; N, 8.20. IR (ν in cm−1): 1605 (CN); 522 (Ga−C)). 1 H NMR (dmso-d6) δ: 0.39 (q, 3JHH = 7.1 Hz, −CH2Ga); 0.90 (t, 3JHH = 7.2 Hz, CH3CH2Ga); 3.45 (br, NH); 6.72 (t, 3JHH = 7.1 Hz, 1H); 6.84 (d, 3JHH = 8.1 Hz, 1H); 7.27 (t, 3JHH = 7.5 Hz, 1H); 7.37 (br, 2H); 7.53 (d, 3JHH = 6.0 Hz, 1H); 7.64 (d, 3JHH = 5.7 Hz, 1H); 7.89 (d, 3JHH = 7.5 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: 3.5 (s, −CH2Ga); 10.2 (s, CH3CH2Ga); 111.9, 112.7, 115.2, 116.1, 122.9, 124.3, 124.7, 128.0, 132.8, 133.9, 138.0, 151.8, 165.5. [Me 2 In(μ-hbo)] 2 (3a): mp 205 °C dec. Anal. Calcd for C30H28In2N2O4: C, 50.74; H, 3.97; N, 3.94. Found: C, 50.72; H, 3.86; N, 3.74. IR (ν in cm−1): 1615 (CN); 529 (In−C). 1H NMR (dmso-d6) δ: −0.32 (s, Me2In); 6.61 (t, 3JHH = 7.1 Hz, 1H); 6.79 (d, 3 JHH = 8.1 Hz, 1H); 7.27−7.35 (m, 1H); 7.40−7.43 (br, m, 2H); 7.61 (br, 1H); 7.76−7.78 (m, 1H); 7.86 (d, 3JHH = 7.8 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: −3.8 (s, Me2In); 110.8, 111.3, 114.8, 117.5, 123.8, 125.8, 125.9, 129.1, 134.7, 138.8, 148.4, 163.9, 169.3. [Me 2 In(μ-hbt)] 2 (3b): mp 244 °C dec. Anal. Calcd for C30H28In2N2O2S2: C, 48.54; H, 3.80; N, 3.77; S, 8.64. Found: C, 48.25; H, 3.75; N, 2.88 S, 8.63. IR (ν in cm−1): 1601 (CN); 526 (In−C). 1H NMR (dmso-d6) δ: −0.16 (s, Me2In); 6.61 (t, 3JHH = 6.3 Hz, 1H); 6.83 (d, 3JHH = 7.8 Hz, 1H); 7.21 (t, 3JHH = 7.8 Hz, 1H); 7.34 (br, 1H); 7.46 (t, 3JHH = 7.2 Hz, 1H); 7.88 (d, 3JHH = 7.5 Hz, 1H); 8.04 (br, 2H). 13C{1H} NMR (dmso-d6) δ: −2.0 (s, Me2In); 114.9, 120.6, 121.3, 122.0, 122.1, 124.6, 126.4, 129.1, 132.6, 134.7, 151.9, 166.5, 167.8. [Et 2 In(μ-hbo)] 2 (3 c): mp 155 °C. Anal. Calcd for C34H36In2N2O4·2H2O: C, 48.71; H, 5.29; N, 3.34. Found: C, 48.85; H, 5.04; N, 3.56. IR (ν in cm−1): 1614 (CN); 506 (In−C). 1H NMR (dmso-d6) δ: 0.50 (q, 3JHH = 7.7 Hz, −CH2In); 0.98 (t, 3JHH = 7.8 Hz, CH3CH2In); 6.56 (t, 3JHH = 7.4 Hz, 1H); 6.78 (d, 3JHH = 6.9 Hz, 1H); 7.26 (br); 7.40 (br, 2H); 7.63 (br, 1H); 7.75 (br, 1H); 7.84 (d, 3JHH = 7.4 Hz, 1H). 13C{1H} NMR (dmso-d6) δ: 9.0 (s, −CH2In); 12.1 (s, CH3CH2In); 110.6, 111.2, 114.3, 117.5, 123.6, 125.7, 125.8, 129.1, 134.6, 139.3, 148.3, 164.3, 170.0.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. T. Mukherjee and D. Das for encouragement of this work.

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ASSOCIATED CONTENT

S Supporting Information *

CCDC-Nos. 895022 and 895023 contain the supplementary crystallographic data for [Me2Ga(hbo)] (2a) and [Me2In(μhbt)]2 (3b), respectively, for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/ retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK [fax: + 441223/336-033; e-mail: [email protected]]. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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