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

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Polyoxometalate-Assisted, One-Pot Synthesis of a Pentakis[(triphenylphosphane)gold]ammonium(2+) Cation Containing Regular Trigonal-Bipyramidal Geometries of Five Bonds to Nitrogen Kenji Nomiya,*,† Kohei Endo,† Yuichi Murata,† Shinya Sato,† Sho Shimazaki,† Shogo Horie,† Eri Nagashima,† Yuta Yasuda,† Takuya Yoshida,‡ Satoshi Matsunaga,† and Toshiaki Matsubara*,† †

Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan Research Center for Gold Chemistry and Department of Applied Chemistry, Tokyo Metropolitan University, Minami-osawa 1−1, Hachioji, Tokyo 192−0397, Japan

‡

S Supporting Information *

ABSTRACT: Novel intercluster compounds consisting of pentakis[(triphenylphosphane)gold]ammonium(2+) cation (1) and Keggin polyoxometalate (POM) anions, i.e., {[Au(PPh3)]5(μ5-N)}3[α-PM12O40]2 (1-PW for M = W; 1-PMo for M = Mo), were synthesized in 30−36% yield by one-pot reaction of the protonic acid form of the Keggin POMs, H3[α-PM12O40]· nH2O (n = 13 for M = W; n = 15 for M = Mo) with monomeric (triphenylphosphane)gold(I) carboxylate [Au(RS-pyrrld)(PPh3)] [RS-Hpyrrld = (RS)-2-pyrrolidone-5-carboxylic acid] in the presence of aqueous NH3 at a molar ratio of 2:15:x (x = 3 for 1-PW; x = 7.5 for 1-PMo). These compounds resulted from the nitrogen-centered phosphanegold(I) clusterization of in situ generated monomeric phosphanegold(I) units, [Au(PPh3)]+ or [Au(L)(PPh3)]+ (L = NH3 or solvent), during the carboxylate elimination of [Au(RS-pyrrld)(PPh3)] in the presence of the Keggin POMs and aqueous NH3. The products 1-PW and 1-PMo were characterized by elemental analysis, Fourier transform infrared, thermogravimetric and differential thermal analyses (TGA/DTA), X-ray crystallography, and solid-state crosspolarization magic-angle-spinning (CPMAS) (31P and 15N) and solution (31P{1H} and 1H) NMR spectroscopy. The lattice contained three independent {[Au(PPh3)]5(μ5-N)}2+ cations, of which two took regular trigonal-bipyramidal (TBP) geometries and the third took a distorted, square-pyramidal (SP) geometry. These geometries are in contrast to those reported by Schmidbaur’s group for {[Au(PPh3)]5(μ5-N)}2+ cations as BF4 salts. Density functional theory and ONIOM calculations for {[(L3P)Au]nN}(n−3)+ (L = H or Ph; n = 4−6) showed that the pentacoordinate cluster is energetically most stable and its TBP structure is only 1.6 kcal mol−1 more stable than its SP structure, in accordance with the experimental facts.

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INTRODUCTION

was generated during the carboxylate elimination of [Au(RSpyrrld)(PPh3)] [RS-Hpyrrld = (RS)-2-pyrrolidone-5-carboxylic acid]25 in dichloromethane (CH2Cl2), in the presence of the protonic acid form of the α-Keggin POM, H3[α-PW12O40]· 7H2O in ethanol (EtOH)/water (H2O) (5:1, v/v). The tetrakis[phosphanegold(I)]oxonium(2+) cation with Td symmetry, in {[Au(PR3)]4(μ4-O)}(BF4)2 (R = o-tolyl, phenyl), was originally reported by Schmidbaur’s group.26 We also discovered that the reaction of [Au(RS-pyrrld)(PPh3)] in CH2Cl2 with the sodium salt of the α-Keggin POM, Na3[α-PW12O40]·9H2O, in EtOH/H2O (2:1, v/v) afforded the heptakis[(triphenylphosphane)gold(I)]dioxonium cation {[[Au(PPh3)]4(μ4-O)][[Au(PPh3)]3(μ3-O)]}3+.19 The forma-

Polyoxometalates (POMs) are discrete soluble metal oxide clusters with applications in catalysis, medicine, and materials science.1−8 The preparation of POM-based materials is therefore an active field of research. In particular, combinations of POMs with cluster cations or macrocations have yielded various intercluster compounds that are of interest in connection with ionic crystals, crystal growth, crystal engineering, structure, sorption properties, catalysis for organic transformation, and so on.9−17 Recently, we discovered that POM-mediated clusterization of in situ generated monomeric [Au(PR3)]+ or [Au(L)(PR3)]+ species (L = solvent) resulted in the formation of an intercluster compound composed of tetra[phosphanegold(I)]oxonium cluster cations and Keggin POMs, i.e., {[Au(PPh3)]4(μ4-O)}3[α-PW12O40]2·4EtOH.18−24 This compound © XXXX American Chemical Society

Received: November 9, 2017

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DOI: 10.1021/acs.inorgchem.7b02795 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Keggin POMs play an important role in the one-pot synthesis and in determining the molecular structure of the pentacoordinate, two-electron-deficient bonding species of the ammonium(2+) cation, 1. Herein, we report full details of the one-pot synthesis of 1PW and 1-PMo, as well as their characterization by elemental analysis, Fourier transform infrared, thermogravimetric and differential thermal analyses (TGA/DTA), and solid-state cross-polarization magic-angle-spinning (CPMAS) 31P NMR and solution (31P{1H} and 1H) NMR spectroscopy. An X-ray crystallographic study was also carried out, and solid-state CPMAS 15N NMR was measured using a freshly prepared, 15Nenriched sample (1-PW-15N). The density functional theory (DFT) and ONIOM calculations were further performed to examine the stability and structure of the tetra-, penta-, and hexacoordinate gold(I) clusters.

tion of the heptakis[phosphanegold(I)]dioxonium cluster cation occurred only in the presence of POM anions. Phosphane ligands in the carboxylatophosphanegold(I) precursors were found to have a significant influence on the POM-mediated clusterization of monomeric [Au(PR3)]+ or [Au(L)(PR3)]+ species (L = solvent). For example, compounds in the {[(Au[P(p-RPh)3])2(μ-OH)]2}3[α-PM12O40]2 series (R = Me, M = W; R = Me, M = Mo; R = F, M = Mo) have been synthesized by the reaction of [Au(RS-pyrrld){P(p-RPh)3}] in CH2Cl2 with H3[α-PM12O40]·nH2O (M = W, n = 7; M = Mo, n = 14) in EtOH/H2O.20,21 In each cluster cation, the OHbridging dimer of the diphosphanegold(I) units, {(Au[P(pRPh)3])2(μ-OH)}+, was formed in a crossed-edge arrangement for R = Me but in a parallel-edge arrangement for R = F, depending upon the substituent on the aryl group of the triarylphosphanes. Novel intercluster compound {[Au(PPh3)]4(μ4-O)}{αXW12O40[Au(PPh3)]3}·3EtOH (X = Al, B) has also been prepared by the reaction of [Au(RS-pyrrld)(PPh3)] in CH2Cl2 with the protonic acid form of the highly negatively charged Keggin POM, H5[α-XW12O40]·nH2O (X = Al, n = 12; X = B, n = 14) in EtOH/H2O at room temperature, at an Au/POM molar ratio of 7:1.22 Hence, the POM-mediated clusterization of [Au(PR3)]+ or [Au(L)(PR3)]+ units provides an effective synthetic route for the preparation of novel phosphanegold(I) cluster cations.8 Element (E)-centered gold(I) clusters [E(AuL)n]m+ have been studied extensively by Schmidbaur27−30,33−39,42 and Laguna,31,40,41 and various hypercoordinated species, such as [(AuPPh3)3(μ3-S)]+,32,33 [(AuPPh3)5(μ5-C)]+,34 [(AuPPh 3 ) 6 (μ 6 -C)] 2 + , 3 5 [(AuPPh 3 ) 5 (μ 5 -N)] 2 + , 3 6 , 3 7 [(AuPPh 3 ) 5 (μ 5 -P)] 2+ , 38 [(AuL) 6 (μ 6 -P)] 3+ (L = P t Bu 3 , P i Pr 3 ), 39 [(AuPPh 3 ) 4 (μ 4 -S)] 2+ , 40,41 and [(AuPPh 3 ) 4 (μ 4 As)]+,42 have been reported. Schmidbaur’s group has reported two solvated species of a two-electron-deficient bonding, pentakis[(triphenylphosphane)gold]ammonium(2+) cation in the form of BF4− salts, i.e., {[Au(PPh3)]5(μ5N)} 3 (BF 4 ) 6 (CH 2 Cl 2 ) 4 3 7 and {[Au(PPh 3 )] 5 (μ 5 -N)}(BF4)2(THF)2.36 These compounds were obtained by the reaction of separately prepared {[Au(PPh3)]4(μ4-N)}BF4 with in situ generated, monomeric [Au(PPh3)]BF4 in hexamethylphosphoric triamide at 20 °C, followed by crystallization. The structure of the {[Au(PPh3)]5(μ5-N)}2+ cation was dependent upon the solvating species; i.e., the geometries of the three independent structures found in the CH2Cl2-solvated species were intermediate between those of trigonal-bipyramidal (TBP) and square-pyramidal (SP),37 whereas that of the tetrahydrofuran (THF)-solvated species was close to that of TBP.36 In contrast, in this work, we unexpectedly obtained a pentakis[(triphenylphosphane)gold]ammonium(2+) cation (1), {[Au(PPh3)]5(μ5-N)}2+, as a salt of Keggin POM counteranions, {[Au(PPh3)]5(μ5-N)}3[α-PM12O40]2 (1-PW for M = W; 1-PMo for M = Mo), by the one-pot reaction of H3[α-PM12O40]·nH2O (n = 13 for M = W; n = 15 for M = Mo) in methanol (MeOH) with monomeric phosphanegold(I) carboxylate, [Au(RS-pyrrld)(PPh3)] in MeOH/CH2Cl2, in the presence of aqueous NH3 at a molar ratio of 2:15:x (x = 3 for M = W; x = 7.5 for M = Mo). The lattice contained three independent {[Au(PPh3)]5(μ5-N)}2+ cations (units A−C for 1), of which two (units A and B) took regular TBP geometries and the other (unit C) took a distorted, SP geometry. The

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EXPERIMENTAL SECTION

Materials. The following reactants were used as received: H[AuCl4]·4H2O, a 25% NH3 aqueous solution, methanol (MeOH), diethyl ether (Et2O), dimethyl sulfoxide (DMSO), dichloromethane (CH2Cl2) (all from Wako); DMSO-d6 and 15NH4Cl (Isotec); a 2 M MeOH solution of 15NH3 (98% enriched) and triphenylphosphane (PPh3) (Aldrich); (RS)-2-pyrrolidone-5-carboxylic acid (RS-Hpyrrld) (TCI). As for the Keggin POM precursors, Na3[α-PW12O40]·14H2O was prepared according to the literature,43 H3[α-PW12O40]·13H2O was derived from the sodium salt using an H+-form cation-exchange column, and H3[α-PMo12O40]·15H2O was prepared according to the so-called ether extraction method.44 Their identities were all confirmed by means of FTIR, TGA/DTA, and solution 31P NMR spectroscopy. The (triphenylphosphane)gold(I) carboxylate, [Au(RS-pyrrld)(PPh3)], was synthesized according to the literature25 with some modifications, and its identity was confirmed by means of CHN elemental analysis, FTIR, TGA/DTA, and solution (1H, 13C, and 31 1 P{ H}) NMR spectroscopy. Instrumentation/Analytical Procedures. CHN elemental analyses were carried out with a PerkinElmer 2400 CHNS elemental analyzer II (Kanagawa University). IR spectra were recorded on a Jasco 4100 FTIR spectrometer in KBr disks at room temperature. TGA/DTA were conducted on a Rigaku Thermo Plus 2 series TGA/ DTA TG 8120 instrument. 1 H (399.65 MHz) and 31P{1H} (161.70 MHz) NMR spectra in a DMSO-d6 solution were recorded in 5-mm-outer-diameter tubes on a JEOL JNM-ECS 400 FT-NMR spectrometer with a JEOL ECS-400 NMR data-processing system. 1H (399.65 MHz) and 31P{1H} (161.70 MHz) NMR spectra in a DMSO-d6 solution were also recorded in 5mm-outer-diameter tubes on a JEOL JNM-ECA 400 FT-NMR spectrometer with a JEOL ECA-400 NMR data-processing system. The 1H NMR spectra were referenced to tetramethylsilane as an internal standard. The 31P NMR spectra were referenced to 25% phosphoric acid (H3PO4) in H2O in a sealed capillary as an external standard. 31P NMR data with the usual 85% H3PO4 reference are shifted by +0.544 ppm from our data. Solid-state CPMAS 31P (121.00 MHz) NMR spectra were recorded in 6-mm-outer-diameter rotors on a JEOL JNM-ECP 300 FT-NMR spectrometer with a JEOL ECP-300 NMR data-processing system and referenced to (NH4)2HPO4 as an external standard (δ 1.60). Solidstate CPMAS 15N (50.17 MHz) NMR spectra were recorded at 26 °C in a JEOL RESONANCE JNM-ECZ 500R spectrometer using a 3.2 mm HXMAS probe with a JEOL JNM-ECZ 500R data-processing system. The 15N NMR spectrum was referenced to CH3NO2 (δ 0.0) and 15NH4Cl (δ −341.168) as external standards. Preparation of {[Au(PPh3)]5(μ5-N)}3[α-PW12O40]2 (1-PW) by the Liquid−Liquid Diffusion Method from H3[α-PW12O40]· 13H2O, [Au(RS-pyrrld)(PPh3)], and Aqueous NH3 (Molar Ratio of 2:15:3). [Au(RS-pyrrld)(PPh3)] (0.110 g, 0.188 mmol) was dissolved in 10 mL of MeOH/CH2Cl2 (1:7, v/v), to which 37.5 μL (0.038 mmol) of a 1 M NH3 aqueous solution was added. A solution B


Article

Inorganic Chemistry

and dried first by suction and then in vacuo for 2 h. Yield: 0.040 g (30.2%). The crystals were soluble in DMSO but insoluble in H2O, CH3CN, MeOH, and Et2O. Anal. Found: C, 30.69; H, 1.63; N, 0.43. Calcd for C270H225N3O80P17Au15Mo24 or 1-PMo: C, 30.67; H, 2.14; N, 0.40. TGA/DTA under atmospheric conditions: no weight loss was observed below 200.0 °C. IR (KBr, cm−1): 1479(w), 1436(m), 1183(vw), 1162(vw), 1101(w), 1061(m), 998(vw), 954(s), 878(m), 808(vs), 744(m), 711(m), 691(s), 538(s), 502(m). 31P{1H} NMR (21.6 °C, DMSO-d6): δ −3.23 ([α-PMo12O40]3−), 25.41 ({[Au(PPh3)]5(μ5-N)}2+), 32.04 ({[Au(PPh3)]2(μ-NH2)}+ or [Au(NH3)(PPh3)]+),45 43.69 ([Au(PPh3)2]+). 1H NMR (23.1 °C, DMSO-d6): δ 7.25−7.59 (m, Ph). Solid-state CPMAS 31P NMR: δ −3.16 ([αPMo12O40]3−), 23.77, 27.30 ({[Au(PPh3)]5(μ5-N)}2+). Control Experiments: Preparation of 1-PW by the Liquid− Liquid Diffusion Method from Na3[α-PW12O40]·14H2O, [Au(RSpyrrld)(PPh3)], and Aqueous NH3 (Molar Ratio of 2:15:6). [Au(RS-pyrrld)(PPh3)] (0.088 g, 0.15 mmol) was dissolved in 8 mL of MeOH/CH2Cl2 (1:7, v/v), to which 60 μL (0.06 mmol) of a 1 M NH3 aqueous solution was added. A solution of Na3[α-PW12O40]· 14H2O (0.064 g, 0.020 mmol) dissolved in 6 mL of MeOH was slowly added along the interior wall of a round-bottomed flask containing a clear, colorless solution of the gold(I) complex. The round-bottomed flask containing the gold(I) complex solution in the lower layer and the POM solution in the upper layer was sealed and left in the dark at room temperature. After 2 days, clear, colorless plate crystals formed around the interface of the two layers. These were collected on a membrane filter (JG 0.2 μm), washed with MeOH (20 mL × 2), Et2O (20 mL × 2), and CH2Cl2 (20 mL × 2), and dried first by suction and then in vacuo for 2 h. Yield: 0.011 g (8.7%). The crystalline samples were soluble in DMSO but insoluble in H2O, MeOH, and Et2O. Anal. Found: C, 25.32; H, 1.30; N, 0.18. Calcd for C270H225N3O80P17Au15W24 or 1-PW-15N: C, 25.57; H, 1.79; N, 0.33. TGA/DTA under atmospheric conditions: no weight loss was observed below 200.0 °C. IR (KBr, cm−1): 1479(vw), 1436(w), 1185(vw), 1163(vw), 1101(w), 1078(m), 1028(vw), 976(s), 895(m), 817(vs), 743(m), 711(m), 691(s), 538(s), 506(m). 31P{1H} NMR (21.6 °C, DMSO-d6): δ −14.87 ([α-PW12O40]3−), 25.30 ({[Au(PPh3)]5(μ5-N)}2+), 30.86, 31.91 ({[Au(PPh3)]2(μ-NH2)}+ or [Au(NH3)(PPh3)]+).45 1H NMR (21.0 °C, DMSO-d6): δ 7.25−7.52 (m, Ph). Solid-state CPMAS 31P NMR: δ −14.64 ([α-PW12O40]3−), 23.87, 27.21 ({[Au(PPh3)]5(μ5-N)}2+). X-ray Crystallography. A clear, colorless plate crystal of 1-PW (0.13 × 0.12 × 0.02 mm3) was mounted on a cryoloop using liquid paraffin (paratone-N) and cooled under a stream of cold N2 gas. Data collection was performed on a Rigaku VariMax with a Saturn CCD diffractometer at 120 K. Intensity data were automatically collected to capture Lorentz and polarization effects during integration. The structure was solved by direct methods (program SHELXS-97),46 followed by difference Fourier calculations, and refined by a full-matrix least-squares procedure on F2 (program SHELXL-97).47 Most atoms in the main part of the structure were refined anisotropically, while disordered Ph groups were refined isotropically. The contribution of the highly disordered solvent electron density was removed by using the SQUEEZE routine in PLATON.48,49 All solvent molecules in the crystal were eliminated by drying in vacuo before characterization by CHN elemental analysis, TGA/DTA, FTIR, and NMR, while the crystal used for X-ray crystallography was taken straight from the mother liquor. Thus, the content of solvent molecules would have differed between the samples used for X-ray analysis and other characterizations. The composition and formula were determined by CHN elemental analysis, TGA/DTA, and 1H NMR. Crystal Data for 1-PW: C272H219N3O80P17Cl4W24Au15, M = 12844.69, triclinic, space group P1̅, a = 18.23260(10) Å, b = 29.6164(2) Å, c = 31.5447(2) Å, α = 83.0916(4)°, β = 89.4057(5)°, γ = 72.6295(5)°, V = 16132.92(17) Å3, Z = 2, Dc = 2.644 Mg m−3, μ(Mo Kα) = 15.490 mm−1, GOF = 1.019, R1 = 0.0532, wR2 = 0.1015 (for all data), R1 = 0.0393, wR2 = 0.0953 [I > 2σ(I)]. The CIF data of CCDC 1577172 contains three {[Au(PPh3)]5(μ5-N)}2+ cations, two [α-PW12O40]3− anions, and two CH2Cl2 molecules and can be

of H3[α-PW12O40]·13H2O (0.079 g, 0.025 mmol) dissolved in 7.5 mL of MeOH was slowly added along the interior wall of a roundbottomed flask containing the colorless clear solution of the gold(I) complex. The flask containing the gold(I) complex solution in the lower layer and the POM solution in the upper layer was sealed and left in the dark at room temperature. After 2 days, clear, colorless plate crystals formed around the interface of the two layers. These were collected on a membrane filter (JG 0.2 μm), washed with MeOH (20 mL × 2), Et2O (20 mL × 2), and CH2Cl2 (20 mL × 2), and dried first by suction and then in vacuo for 2 h. Yield: 0.060 g (32.3%). The crystals were soluble in DMSO but insoluble in H2O, acetonitrile (CH3CN), MeOH, and Et2O. Anal. Found: C, 25.47; H, 1.21; N, 0.32. Calcd for C270H225N3O80P17Au15W24 or 1-PW: C, 25.57; H, 1.79; N, 0.33. TGA/DTA under atmospheric conditions: no weight loss was observed below 200.0 °C. IR (KBr, cm−1): 1479(w), 1436(m), 1183(vw), 1161(vw), 1101(m), 1078(s), 976(s), 895(s), 817(vs), 743(s), 711(m), 691(s), 538(s), 503(m). 31P{1H} NMR (21.3 °C, DMSO-d6): δ −14.70 ([α-PW12O40]3−), 25.44 ({[Au(PPh3)]5(μ5N)}2+), 31.01, 32.07 ({[Au(PPh3)]2(μ-NH2)}+ or [Au(NH3)(PPh3)]+),45 43.51 ([Au(PPh3)2]+). 1H NMR (21.3 °C, DMSO-d6): δ 7.25−7.52 (m, Ph). Solid-state CPMAS 31P NMR: δ −14.62 ([αPW12O40]3−), 24.26, 27.01 ({[Au(PPh3)]5(μ5-N)}2+). Preparation of 15N-Enriched {[Au(PPh3)]5(μ5-N)}3[α-PW12O40]2 (1-PW-15N). In this synthesis, a 2 M MeOH solution of 15NH3 (98% enriched; Aldrich) was used. [Au(RS-pyrrld)(PPh3)] (0.660 g, 1.12 mmol) was dissolved in 60 mL of MeOH/CH2Cl2 (1:7, v/v), to which 225 μL (0.225 mmol) of a 2 M MeOH solution of 15NH3 was added. A clear, colorless solution of H3[α-PW12O40]·13H2O (0.467 g, 0.150 mmol) dissolved in 45 mL of MeOH was slowly added along the interior wall of a round-bottomed flask containing a colorless clear solution of the gold(I) complex. The round-bottomed flask containing the gold(I) complex solution in the lower layer and the POM solution in the upper layer was sealed and left in the dark at room temperature. After 2 days, clear, colorless plate crystals formed around the interface of the two layers. These were collected on a membrane filter (JG 0.2 μm), washed with MeOH (20 mL × 2), CH2Cl2 (20 mL x 2), and Et2O (20 mL × 2), and dried first by suction and then in vacuo for 2 h. Yield: 0.285 g (38.7%). Anal. Found: C, 25.27; H, 1.62; N, 0.33. Calcd for C270H225N3O80P17Au15W24 or 1-PW: C, 25.57; H, 1.79; N, 0.33. TGA/DTA under atmospheric conditions: no weight loss was observed below 200.0 °C. IR (KBr, cm−1): 1480(w), 1436(m), 1417(w), 1184(w), 1161(w), 1101(m), 1079(m), 976(s), 895(m), 818(vs), 744(m), 711(m), 691(m), 538(m), 510(w). Solid-state CPMAS 15N NMR [−341.17 ppm (standard 15NH4Cl) and 0 ppm (15N reference CH315NO2)]: δ −271.48 (Δ1/2 124 Hz). Minor peaks due to NH and NH2 species, assignable to {[Au(PPh3)]2(μ-NH2)}+, [Au(NH3)(PPh3)]+, and so on, were observed at δ −328.43 (Δ1/2 391 Hz), −369.96 (Δ1/2 510 Hz). 1H NMR (21.3 °C, DMSO-d6): δ 7.25− 7.52 (m, Ph). 31P{1H} NMR (21.5 °C, DMSO-d6): δ −14.71 ([αPW12O40]3−), 25.46 (d, J 30.7 Hz, due to coupling with a 15N nucleus), {[Au(PPh3)]5(μ5-N)}2+), 32.08 (d, J 37.19 Hz, due to coupling with a 15 N nucleus) ({[Au(PPh3)]2(μ-NH2)}+ or [Au(NH3)(PPh3)]+).45 The 15N−31P coupling was readily observed in the solution 31P{1H} NMR, but it was difficult to find in the solid-state CPMAS 15N NMR. Preparation of {[Au(PPh3)]5(μ5-N)}3[α-PMo12O40]2 (1-PMo) by the Liquid−Liquid Diffusion Method from H3[α-PMo12O40]· 15H2O, [Au(RS-pyrrld)(PPh3)], and Aqueous NH3 (Molar Ratio of 2:15:7.5). [Au(RS-pyrrld)(PPh3)] (0.110 g, 0.188 mmol) was dissolved in 10 mL of MeOH/CH2Cl2 (1:7, v/v), to which 93.75 μL (0.094 mmol) of a 1 M NH3 aqueous solution was added. A solution of H3[α-PMo12O40]·15H2O (0.052 g, 0.025 mmol) dissolved in 7.5 mL of MeOH was slowly added along the interior wall of a roundbottomed flask containing a colorless clear solution of the gold(I) complex. The round-bottomed flask containing the gold(I) complex solution in the lower layer and the POM solution in the upper layer was sealed and left in the dark at room temperature. After 3 days, clear, yellow plate crystals formed around the interface of the two layers. These were collected on a membrane filter (JG 0.2 μm), washed with MeOH (20 mL × 2), Et2O (20 mL × 2), and CH2Cl2 (20 mL × 2), C


Article

Inorganic Chemistry

Figure 1. FTIR spectra in KBr disks of (a) 1-PW and (b) 1-PMo. obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. We adopted model clusters {[(H3P)Au]nN}(n−3)+ (n = 4−6) in addition to the real clusters {[(Ph3P)Au]nN}(n−3)+ (n = 4−6) to examine the stability and structure of the tetra-, penta-, and hexacoordinate gold clusters. We use the labels 1−3 to indicate the tetra-, penta-, and hexacoordinate clusters, respectively, and added m and r as suffixes to distinguish the model and real clusters, respectively. All quantum-mechanical (QM) calculations were performed using the Gaussian 09 program.50 For the model clusters 1m−3m, we used the hybrid density functional method M06-2X51 with the basis set called basis set I (BSI) in this paper, which includes for the Au atom an effective core potential replacing core electrons up to 4f and double-ζ valence basis functions (7s6p3d)/[3s3p2d] according to Hay and Wadt52 augmented by the single set of f orbitals with an exponent of 1.05053 and, for the other atoms, 6-31+G*. For the real clusters 1r−3r, we used the ONIOM method,54 where the inner part is {[(H3P)Au]nN}(n−3)+, in which the Ph groups are replaced by H atoms and the outer part is the Ph groups; calculation was done at the ONIOM(M06-2X/BSI:PM6) level. In the geometry optimizations, we fixed the free rotation of the phosphane ligands PL3 and adopted the Td, D3h, and Oh symmetries for the core parts {[(P)Au]nN}(n−3)+ without the H atoms of 1m−3m, respectively, and C1 symmetry for 1r−3r. All of the structures were confirmed to be the equilibrium structures by means of frequency calculations, although some structures still showed imaginary frequencies only for PL3 rotation.

W, Mo) should be kept separate, in order to avoid precipitation of the insoluble NH4+ salt of the Keggin POM. The CHN elemental analysis, TGA/DTA, FTIR, and solidstate CPMAS (31P and 15N) NMR and solution (31P{1H} and 1 H) NMR in DMSO-d6 of the intercluster compounds 1-PM (M = W, Mo) prepared here were all consistent with the molecular formulas. The result of X-ray crystallography of 1PW was also consistent with the molecular formula. However, although 1-PMo was also crystallized, the crystal data were of low quality and the structure was poorly refined. The formation of 1-PM (M = W, Mo) can be represented by eq 1. 2H3[α ‐PM12O40 ] (M = W, Mo) + 15[Au(RS ‐pyrrld)(PPh3)] + 3NH3 → {[Au(PPh3)]5 (μ5 ‐N)}3[α ‐PM12O40 ]2 (1‐PM) + 15Hpyrrld

(1)

The thoroughly dried samples of 1-PM (M = W, Mo) were confirmed to be free of solvating species by CHN elemental analysis, 1H NMR, and TGA/DTA measurements under atmospheric conditions. On the other hand, the crystal used for X-ray crystallography was not dried and incorporated two CH2Cl2 molecules of solvation (see the X-ray Crystallography section). The solid-state FTIR spectra (Figure 1) showed prominent vibrational bands due to the saturated Keggin tungsto-POMs (1078, 976, 895, and 817 cm−1) for 1-PW and the Keggin molybdo-POM (1061, 954, 878, and 808 cm−1) for 1-PMo, both with heteroatom P,55−57 in addition to the characteristic vibrational bands based on the coordinating PPh3 ligands. Upon formation of 1-PM (M = W, Mo), the carbonyl vibrational bands of the anionic RS-pyrrld ligand in [Au(RSpyrrld)(PPh3)], which were observed at 1696 and 1632 cm−1, disappeared, indicating that the carboxylate ligand had been eliminated. This was also confirmed by the 1H NMR spectra in DMSO-d6. Thus, the carboxylate serves only as a leaving group.

■

RESULTS AND DISCUSSION Synthesis, Compositional Characterization, and Formation of 1-PM (M = W, Mo). Crystalline samples of the intercluster compounds 1-PW and 1-PMo were readily prepared by the reaction of the protonic acid form of the Keggin POMs, H3[α-PM12O40]·nH2O (n = 13 for M = W; n = 15 for M = Mo) with monomeric (triphenylphosphane)gold(I) carboxylate [Au(RS-pyrrld)(PPh3)] [RS-Hpyrrld = (RS)-2pyrrolidone-5-carboxylic acid] in the presence of aqueous NH3 in one pot using the liquid−liquid diffusion method at room temperature. A critical point in this liquid−liquid diffusion methodology is that NH3 and H3[α-PM12O40] (M = D


Article

Inorganic Chemistry In order to measure the solid-state CPMAS 15N NMR of 1PW, we prepared clear, colorless plate crystals of 1-PW-15N, which showed a 31P{1H} NMR doublet signal in DMSO-d6 at 25.46 ppm (J 30.7 Hz) due to coupling with a 15N nucleus. In our previous studies on the reactions of [Au(RSpyrrld)(PPh3)] in CH2Cl2, the tetrakis[(triphenylphosphane)gold(I)]oxonium cation, {[Au(PPh3)]4(μ4-O)}2+, was formed using H 3 [α-PW12 O 40 ]·7H 2 O in EtOH/H 2 O, while the heptakis[(triphenylphosphane)gold(I)]dioxonium cluster cation, {[[Au(PPh3)]4(μ4-O)][[Au(PPh3)]3(μ3-O)]}3+ was generated using Na3[α-PW12O40]·9H2O in EtOH/H2O.18,19 In order to examine the effect of the acidity of the Keggin POM in the synthesis of 1-PW, we performed control liquid−liquid diffusion experiments at a 2:15:6 molar ratio of [Au(RSpyrrld)(PPh3)], Na3[α-PW12O40]·14H2O, and NH3. Characterization of the crystalline product showed that it was 1-PW, although the yield was greatly reduced. Thus, the acidity of POM did not significantly influence formation of the μ5-Nbridging pentakis[phosphanegold(I)] cation (1). The low yield in this synthesis of 1-PW will be attributable to the ease of carboxylate elimination in [Au(RS-pyrrld)(PPh3)] by the sodium salt of POM, in comparison with the protonic acid form of POM. X-ray crystallography of 1-PW showed the formation of a discrete intercluster compound between the pentakis[(triphenylphosphane)gold]ammonium(2+) cation (1) and the saturated Keggin POM (see the Molecular Structure of 1 section). Such a {[Au(PPh3)]5(μ5-N)}2+ cation [pentacoordinate nitrogen cation(2+)] was first prepared as the BF4 salt by Schmidbaur’s group, by treatment of the separately prepared {[Au(PPh3)]4(μ4-N)}+BF4− with 1 equiv of a freshly prepared solution of [(PPh3)Au]+BF4−.36,37 In contrast, in this work, the {[Au(PPh3)]5(μ5-N)}2+ cation was directly formed during the course of carboxylate elimination of the monomeric O−AuI−P bonding complex [Au(RS-pyrrld)(PPh3)]. The reaction of [Au(RS-pyrrld)(PPh3)] with the POMs in the presence of aqueous NH3 readily provides the precursor [Au(NH3)(PPh3)]+ and/or the reactive species [Au(PPh3)]+, and stepwise substitution of the protons of [Au(NH3)(PPh3)]+ by [Au(PPh3)]+ occurs, as shown in Scheme 1. In support of this scheme, precursors such as [Au(NH3)(PPh3)]+ and {[Au(PPh3)]2(μ-NH2)}+ were detected by solidstate CPMAS 15N NMR and solution (31P{1H}) NMR (see the Solid-State CPMAS (31P, 15N) NMR and Solution (31P{1H}, 1 H) NMR Characterization section). The present reaction strongly depends upon the POMs. It seems likely that the bulkiness and large anionic charge of the POMs significantly contribute to the stepwise reaction of [Au(PPh3)]+, resulting in a one-pot formation of the {[Au(PPh3)]5N}2+ cation. Molecular Structure of 1. Single-crystal X-ray analysis revealed that 1-PW crystallizes in the triclinic P1̅ space group and the unit cell is composed of three countercations {[Au(PPh 3 )] 5 (μ 5 -N)} 2+ , two Keggin POM anions [αPW12O40]3−, and two CH2Cl2 molecules of crystallization (Figure 2). All of the cations had pentanuclear phosphanegold(I) cores consisting of five Au atoms and one μ5-bridged N atom. These pentakis[phosphanegold(I)]ammonium(2+) cations existed as three crystallographically independent units (denoted as units A−C). Selected bond lengths and angles for these units are shown in Tables 1 and 2. Units A and B both took a regular

Scheme 1. Formation of the {[Au(PPh3)]5(μ5-N)}2+ Cation by Stepwise Substitution of the Protons in [Au(NH3)(PPh3)]+ by [Au(PPh3)]+ in the Reaction of an MeOH/CH2Cl2 Solution of [Au(RS-pyrrld)(PPh3)] Containing Aqueous NH3 with H3[α-PW12O40] in MeOH

TBP structure. The angles of the principal axis (Au−N−Au) for units A and B were 179.7(4)° and 179.0(3)°, respectively, indicating that the Au−N−Au bonds were almost linear. These angles were quite different from those [168.2(4)°, 161.9(3)°, and 172.8(3)°] of the three NAu5 cluster cations reported by Schmidbaur et al.37 The Au−N−Au angles within the equatorial positions were close to 120° (i.e., 119.4(3)°, 119.8(3)°, and 120.7(3)° for unit A and 122.7(3)°, 117.9(3)°, and 118.9(3)° for unit B), and the lengths between the central N atom and the equatorial Au atoms were close to 2.06 Å (i.e., 2.060−2.062 Å for unit A and 2.047−2.068 Å for unit B). The angles between the N−Au bond in the axial position and the triangular plane consisting of the three equatorial Au atoms were approximately 90° (i.e., 88.6−91.4° for unit A and 86.8−92.9° for unit B). Thus, units A and B both show D3h symmetry. In the TBP structures, we can find six aurophilic interactions between the equatorial and axial Au atoms (average 2.946 Å for unit A and average 2.950 Å for unit B; Table 2). These lengths were longer than the metallic Au− Au length (2.88 Å)58 but shorter than the sum of the van der Waals radii for gold (3.32 Å).59 However, within the equatorial triangle, the Au−Au distances were in the range of 3.518−3.621 Å, suggesting that there is no aurophilic interaction. On the other hand, the structure of unit C was a distorted SP, composed of the apical atom (Au11) and the basal plane (Au12, Au13, Au14, and Au15 atoms). In the basal plane, the sum of the four Au−N−Au angles was 362.4°, showing that the N atom is located above the basal plane but not within the plane. This idea was also supported by the Au13−N3−Au15 [151.2(4)°] and Au14−N3−Au12 [172.2(4)°] angles, both of which significantly deviated from 180°. Two of the four angles between the apical Au−N bond and the four basal atoms were greater than 90° (i.e., 102.9−105.0°), but the remaining two angles were smaller than 90° (i.e., 86.8−88.4°). Two Ph groups in unit C were disordered. We observed aurophilic interactions at seven Au···Au distances in unit C, except for Au11···Au13 E


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Inorganic Chemistry

Figure 2. (a) Packing diagram of 1-PW in the crystal and (b) molecular structures of units A−C of 1.

Table 1. Selected Bond Lengths (Å) and Angles (deg) in Units A−C of 1 unit A

unit B

unit C

Lengths N1−Au1 N1−Au2 N1−Au3 N1−Au4 N1−Au5

2.060(6) 2.062(6) 2.061(6) 2.117(6) 2.095(6)

N2−Au6 N2−Au7 N2−Au8 N2−Au9 N2−Au10

Au1−N1−Au2 Au2−N1−Au3 Au3−N1−Au1 Au4−N1−Au1 Au4−N1−Au2 Au4−N1−Au3 Au5−N1−Au1 Au5−N1−Au2 Au5−N1−Au3 Au5−N1−Au4

119.4(3) 119.8(3) 120.7(3) 88.8(2) 89.1(2) 88.6(2) 91.4(2) 90.7(2) 91.3(2) 179.7(4)


2.068(6) 2.058(6) 2.047(5) 2.136(7) 2.092(7)

N3−Au11 N3−Au12 N3−Au13 N3−Au14 N3−Au15

2.092(7) 2.130(7) 2.094(8) 2.070(7) 2.037(8)

122.7(3) 117.9(3) 118.9(3) 87.9(2) 86.8(2) 89.0(2) 91.5(2) 92.9(2) 92.0(2) 179.0(3)


88.4(3) 105.0(3) 86.8(3) 102.9(3) 84.4(3) 90.9(3) 97.7(3) 89.4(3) 151.2(4) 172.2(4)

Angles

The pentakis[phosphanegold(I)]ammonium(2+) cations (units A and B) formed by POM-mediated clusterization exhibited regular TBP structure (D3h symmetry), despite the presence of two solvating CH2Cl2 molecules in the crystal, in marked contrast to the previously reported dication with CH2Cl2 solvation.37 These differences can be attributed to the large size and high negative charge of the POM couteranion. On the other hand, the molecular structure of the saturated αKeggin POM, [α-PW12O40]3−, as a counterion in 1-PW, was

(distance, 3.322 Å). This structural model of unit C corresponds to that previously reported by Schmidbaur et al. for the pentakis[phosphanegold(I)]ammonium(2+) cation with a dichloromethane solvate, {[Au(PPh3)]5(μ5-N)}(BF4)2· 2CH2Cl2.37 In relation to the SP geometry of unit C, similar structures of the two-electron-deficient species of μ4-O, μ4-S, and μ4-As cations with C4v symmetry, {[AuP(p-ClPh)3]4(μ4O)}2+,24 [(AuPPh3)4(μ4-S)]2+,40,41 and [(AuPPh3)4(μ4-As)]+,42 respectively, have been reported. F


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Inorganic Chemistry

identical with that of previously reported POMs.2 The W−O bond lengths of the saturated α-Keggin units were in the normal range:2 W−Ot (terminal) [average 1.696 Å], W−Oc (corner-sharing) [average 1.903 Å], W−Oe (edge-sharing) [average 1.910 Å], and W−Oa (coordinating to the heteroatom) [average 2.32 Å]. There is no direct bonding between the{[Au(PPh3)]5(μ5N)}2+ cation and the Keggin POM anion in 1-PW. Thus, intercluster compound 1-PW is formed by the ionic interaction between them. As a matter of fact, the POM anions in 1-PW were exchanged with BF4− anions using an anion-exchange resin in the BF4− form (see the Supporting Information). The BF4− salt (1-BF; CCDC 1583072) of the {[Au(PPh3)]5(μ5N)}2+ cation was obtained as a crystalline sample containing one Et2O and one CH3CN solvated molecules. Its molecular structure was determined as a slightly distorted TBP geometry, in which the angle (Au5−N1−Au4) of the principal axis was 177.17(16)° and the three Au−N−Au angles within the equatorial positions were 127.17(15)°, 113.36(13)°, and

Table 2. Au···Au Distances (Å) in Units A−C of 1 Unit A Au1···Au4 Au2···Au4 Au3···Au4

2.923 2.932 2.919

Au6···Au9 Au7···Au9 Au8···Au9

2.918 2.882 2.932

Au11···Au12 Au11···Au13 Au11···Au14 Au11···Au15

2.943 3.322 2.858 3.228

Au1···Au5 Au2···Au5 Au3···Au5 average

2.974 2.958 2.971 2.946

Au6···Au10 Au7···Au10 Au8···Au10 average

2.979 3.009 2.977 2.950

Unit B

Unit C Au12···Au13 Au13···Au14 Au14···Au15 Au(15)···Au(12) average

2.837 2.968 3.093 2.931 3.023

Figure 3. (a) Solid-state CPMAS 31P NMR of 1-PW. The signals denoted by daggers are from spinning side bands, and the signal by double asterisk is from the [α-PW12O40]3− anion. (b) Solid-state CPMAS 15N NMR of 1-PW-15N. The signal denoted by an open circle is from the standard 15 NH4Cl. G


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Inorganic Chemistry

Figure 4. Solution 31P{1H} NMR in DMSO-d6 of (a) 1-PW and (b) 1-PMo. The signals denoted by asterisks are from [Au(PPh3)2]+, those by double asterisks are from [α-PM12O40]3− (M = W, Mo), and those by double daggers are from the external standard of 25% aqueous H3PO4.

Solid-State CPMAS (31P, 15N) and Solution (31P{1H} and H) NMR Characterization. The solid-state CPMAS 31P NMR spectrum of 1-PW (Figure 3a) shows broad signals at 24.26 and 27.01 ppm originating from the nonequivalent phosphane groups, together with an explicit peak at −14.62 ppm assignable to the heteroatom P of [α-PW12O40]3−. The broad signals at 24.26 and 27.01 ppm (relative 1:2 ratio) can be assigned to the phosphane groups of {[Au(PPh3)]5(μ5-N)}2+ cations with distorted SP and regular TBP structures, respectively. The solid-state CPMAS 31P NMR spectrum of 1-PMo shows broad signals at 23.77 and 27.30 ppm due to the {[Au(PPh3)]5(μ5-N)}2+ cations and an explicit peak at −3.16 ppm assignable to the heteroatom P of [α-PMo12O40]3−. The solid-state CPMAS 15N NMR spectrum of the 15Nenriched sample 1-PW-15N (Figure 3b) shows three peaks at −271.5 ppm (Δ1/2 124 Hz), −328.4 ppm (Δ1/2 391 Hz), and −370.0 ppm (Δ1/2 510 Hz), together with a small, sharp peak at −341.2 ppm due to the nitrogen resonance of 15NH4Cl used as a reference. The main peak at −271.5 ppm is due to the

119.37(14)° (Figure S1 and Table S1). The sum of the three equatorial Au−N−Au angles was 359.9°, indicating coplanarity with the N atom. These data are largely different from those of {[Au(PPh3)]5(μ5-N)}3(BF4)6(CH2Cl2)4 reported by Schmidbaur’s group, the latter of which took the strongly distorted geometries. Schmidbaur’s group refer to the fact that the very minor changes in the crystalline environment (CH2Cl2 vs THF solvate) cause major distortions.37 This may be the present case for the different solvate systems of 1-BF with one Et2O and one CH3CN. As mentioned in the last section, the DFT and ONIOM calculations suggested that the pentacoordinate {[Au(PPh3)]5(μ5-N)}2+ is much more stable in energy than the tetracoordinate {[Au(PPh3)]4(μ4-N)}+ and the hexacoordinate {[Au(PPh 3 )] 6 (μ 6 -N)} 3+ , and the SP geometry of the pentacoordinate {[Au(PPh3)]5(μ5-N)}2+ is only 1.6 kcal mol−1 less stable than the regular TBP geometry of the pentacoordinate {[Au(PPh3)]5(μ5-N)}2+.

1

H


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Inorganic Chemistry

Figure 5. Optimized structures of the nitrogen-centered gold clusters, {[(H3P)Au]4N}+ (1m), {[(H3P)Au]5N}2+ (2m), and {[(H3P)Au]6N}3+ (3m) at the M06-2X/BS1 level and {[(Ph3P)Au]4N}+ (1r), {[(Ph3P)Au]5N}2+ (2r), and {[(Ph3P)Au]6N}3+ (3r) at the ONIOM(M06-2X/BS1:PM6) level.

nitrogen resonance of the {[Au(PPh3)]5(μ5-N)}2+ cation, whereas the minor peaks at −328.4 and −370.0 ppm are due to protonated nitrogen resonances, such as NH-, NH2-, etc. These assignments are based on CPMAS 15N NMR measurements of 1-PW-15N by changing the contact time (1−8 ms; Figure S2). The signals at −328.4 and −370.0 ppm were less influenced by the contact time, suggesting that the N atoms interact strongly with 1H. Thus, the minor signals are assignable to species such as [Au(NH3)(PPh3)]+ and {[Au(PPh3)]2(μNH2)}+. However, the main signal at −271.5 ppm was significantly dependent on the contact time, indicating a weak interaction of nitrogen with 1H. Thus, the main signal is assigned to the resonance of nitrogen without a 1H nucleus, i.e., to the nitrogen resonance of the μ5-N Au5 core. Because the main nitrogen signal is observed as a single peak, the environments around the N atoms in the regular TBP (units A and B) and distorted SP (unit C) structures cannot be discriminated on the NMR time scale. The solution 31P{1H} NMR spectra of 1-PM (M = W, Mo) in DMSO-d6 (Figure 4) show single, sharp signals at −14.70 ppm for 1-PW and at −3.23 ppm for 1-PMo due to the heteroatoms P of [α-PW12O40]3− and [α-PMo12O40]3−, respectively. These signals correspond to the solid-state CPMAS 31P NMR signals (Figure 3a). The main peaks at 25.44 ppm for 1-PW and at 25.41 ppm for 1-PMo are assignable to the phosphane groups of the {[Au(PPh3)]5(μ5N)}2+ cations. Because single peaks were observed as the main signals in solution 31P{1H} NMR of 1-PM (M = W, Mo), there is a rapid exchange of PPh3 groups or interconversion between the regular TBP and distorted SP structures, e.g., through the Berry pseudorotation pathway.37 On the other hand, the minor 31 1 P{ H} NMR peaks at 31.01 and 32.07 ppm for 1-PW and at 32.04 ppm for 1-PMo are assignable to the phosphane groups of the [Au(NH3)(PPh3)]+ and/or {[Au(PPh3)]2(μ-NH2)}+ cations.45 Two compounds, 1-PW and 1-PMo, are unstable in DMSO, and minor peaks around 44 ppm due to [Au(PPh3)2]+45 are observed, resulting from decomposition in a DMSO-d6 solution.

Compound 1-PW readily decomposed in a DMSO solution to give [Au(PPh3)2]+, and metallic gold was deposited on the wall of the vial. The solution 31P{1H} NMR in DMSO-d6 of 1PW showed that the main signal due to the {[Au(PPh3)]5(μ5N)}2+ cation 1 rapidly decreased upon heating, but the minor signals due to [Au(NH3)(PPh3)]+ and/or {[Au(PPh3)]2(μNH2)}+ were unchanged. Thus, the [Au(NH3)(PPh3)]+ and/or {[Au(PPh3)]2(μ-NH2)}+ species are not generated by the decomposition of 1 but are contained in the original samples. These facts and the solid-state CPMAS 15N NMR measured under DMSO-free conditions suggest that [Au(NH3)(PPh3)]+ and/or {[Au(PPh3)]2(μ-NH2)}+ are precursors of 1. The formation of 1 should occur via stepwise substitution of the protons of the [Au(NH3)(PPh3)]+ species by the [Au(PPh3)]+ cation, as shown in Scheme 1. In the previous work by Schmidbaur’s group,36,37 formation of the {[Au(PPh3)]5(μ5-N)}2+ cation by the reaction of {[Au(PPh3)]4(μ4-N)}+ with separately prepared monomeric [Au(PPh3)]+ species was a key process (step 6 in Scheme 1). On the other hand, in our work, steps 1−6 readily proceed because sufficient amounts of the monomeric [Au(PPh3)]+ species are generated by the reaction of [Au(RS-pyrrld)(PPh3)] with POM in the presence of NH3 (Scheme 1). QM Calculations. We also performed theoretical calculations for model and real clusters, {[(L3P)Au]nN}(n−3)+ (L = H, Ph; n = 4−6), by means of DFT and ONIOM methods in order to examine the structure and stability of the tetra-, penta-, and hexacoordinate clusters. The calculations showed that all of them exist as equilibrium structures (the optimized structures are shown in Figure 5). In the model clusters with L = H, the tetra-, penta-, and hexacoordinate clusters have tetrahedral, TBP, and octahedral structures, respectively, in accordance with previous calculations.60−62 In the tetracoordinate cluster (1m), the N atom at the center of the cluster interacts with each Au atom by electron donation mainly from the p-type orbital of the N atom to the s-type orbital of the Au atom. The N−Au distance is 2.037 Å, and its population is 0.068 e in 1m, as presented in Table 3. On the other hand, the interaction I


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Inorganic Chemistry

view. These small differences in the energy and geometry between the two structures would be the reason why both structures were observed in our peculiar experimental conditions. Although the hexacoordinate cluster (3m) is formed by the coordination of one more [(H3P)Au]+ to 2m, 3m is 40.4 kcal mol−1 less stable than 1m, in contrast to the case of 2m. This destabilization of 3m can be readily understood from the geometrical point of view as follows. The additional [(H3P)Au]+ coordinates to the pentacoordinate cluster with the SP structure, but, in principle, charge-transfer interaction between the centered N atom and the four surrounding Au atoms in the SP structure is impossible, even though it is possible between the centered N atom and the three surrounding Au atoms in the TBP structure. The population also no longer shows interactions between the N and Au atoms (Table 3), and instead weak interactions between the Au atoms are generated. The N−Au distances in 3m are further stretched compared to those in 2m because of both geometrical congestion and the disappearance of the N−Au interaction. To examine the effects of the Ph groups of the phosphane ligands of the Au atoms, we examined the real clusters with L = Ph by the ONIOM method, with the Ph groups included in the outer part. The tetrahedral, TBP, and octahedral structures of 1m−3m did not change upon replacement of the H atoms with Ph groups. However, the N−Au distances were elongated because of the steric and electronic effects of the Ph groups. The order of stability of the clusters also did not change, i.e., 2r > 1r > 3r. The reason why destabilization of the hexacoordinate cluster decreases by about 11 kcal mol−1 in the case of the real cluster (Table 4) is presumably the formation of CH/π interactions between the congested Ph groups in 3r. This was supported by stabilization of the outer part (the Ph groups) by 8.7 kcal mol−1 in 3r. Thus, the computational finding that the pentacoordinate cluster is energetically most stable is in good agreement with the experimental result.

between the Au atoms is very weak, as shown by the population of only 0.030 e for Au−Au, i.e., about half that of N−Au. Table 3. Selected Distances (Å) and Population of the Nitrogen-Centered Gold Clusters, {[(L3P)Au]nN}(n−3)+ (L = H, Ph; n = 4−6) distancea (Å) +

populationb (e) +

{[(H3P)Au]4N} (1m)/{[(Ph3P)Au]4N} (1r) 2.037/2.060−2.062 0.068 3.326/3.359−3.375 0.030 2.292/2.261−2.270 0.105 {[(H3P)Au]5N}2+ (2m)/{[(Ph3P)Au]5N}2+ (2r) N−Au(ax) 2.194/2.208−2.214 0.125 N−Au(eq) 2.113/2.134−2.142 0.083 Au(ax)−Au(eq) 3.046/3.022−3.101 −0.006 Au(eq)−Au(eq) 3.659/3.625−3.773 0.020 Au(ax)−P 2.292/2.271−2.277 0.172 Au(eq)-P 2.301/2.280−2.285 0.115 {[(H3P)Au]6N}3+ (3m)/{[(Ph3P)Au]6N}3+ (3r) N−Au 2.230/2.235−2.270 −0.829 Au−Au 3.154/3.106−3.272 0.085 Au−P 2.303/2.278−2.293 −0.067 N−Au Au−Au Au−P

a

Numbers to the left of the slash are for the model clusters with L = H, 1m−3m, and those in italics to the right of the slash are for the real clusters with L = Ph, 1r−3r. bNumbers are for the model clusters with L = H, 1m−3m.

The pentacoordinate cluster 2m was 10.7 kcal mol−1 more stable than the tetracoordinate cluster (1m), as presented in Table 4. Although 1m becomes more unstable by 11.1 kcal Table 4. Calculated Relative Energies (kcal mol−1) for the Nitrogen-Centered Gold Clusters, {[(L3P)Au]nN}(n−3)+ (L = H, Ph; n = 4−6) L

method

n=4

n=5

n=6

H Ph

QM ONIOM inner outer

0.0 0.0 0.0 0.0

−10.7 −9.5 −10.6 1.1

40.4 29.2 38.0 −8.7

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CONCLUSION In the presence of the protonic acid form of Keggin POMs such as H3[α-PM12O40]·nH2O (n = 13 for M = W; n = 15 for Mo), aqueous NH3 and the monomeric (triphenylphosphane)gold(I) complex, [Au(RS-pyrrld)(PPh3)] [RS-Hpyrrld = (RS)-2pyrrolidone-5-carboxylic acid], directly formed the pentakis{(triphenylphosphane)gold}ammonium(2+) cation 1, as a countercation of the POM anions in reasonable yield. These intercluster compounds, 1-PW and 1-PMo, have been unequivocally characterized in the solid state and solution. The availability of a one-pot synthesis of 1 significantly depends upon the POMs: the bulkiness and large anionic charge of the POMs contribute to the direct clusterization of [Au(PPh3)]+ species to the {[Au(PPh3)]5(μ5-N)}2+ cation after removal of the RS-pyrrld− ligands. The POMs readily generate the [Au(PPh3)]+ species from [Au(RS-pyrrld)(PPh3)] and promote the stepwise substitution of [Au(NH3)(PPh3)]+ (steps 1−6 in Scheme 1). The two regular TBP geometries (units A and B) and the one distorted, SP geometry (unit C) found in 1 were also significantly related to the role of POMs as counteranions, in contrast to the {[Au(PPh3)]5(μ5-N)}2+ cations previously reported by Schmidbaur et al. as BF4 salts.36,37 The POM anions in 1-PW were exchanged with BF4− anions to form the BF4 salt (1-BF), indicating that it comprises ionic crystals (see the Supporting Information). The stability of the two geometries (TBP and SP) of the two-

mol−1 when the tetrahedral structure is deformed to the TBP one, 2m would be markedly stabilized by the coordination of an additional [(H3P)Au]+ to the axial site. In fact, the population of N−Au(ax) in 2m is twice that of N−Au in 1m (Table 3), suggesting that stabilization of the pentacoordinate cluster originates from the interaction of N with Au on the axial axis. This seems to be characteristic of the gold cluster because the NH52+ with the TBP structure was less stable by 102.1 kcal mol−1 than the NH4+ with the tetrahedral structure in the absence of stabilization due to the interaction of N with H on the axial axis. On the other hand, the population of N−Au(eq) does not increase so much compared to that of N−Au in 1m. The population of Au−Au is also small in 2m, indicating that the contribution of the Au−Au interaction to the formation of the cluster is small. The N−Au distances in 2m are stretched by 0.076−0.157 Å compared to those in 1m due to the geometrical congestion. The pentacoordinate cluster with the SP structure was only 1.6 kcal mol−1 less stable than that with the TBP structure, although it is the transition state of the Berry pseudorotation63 between two TBP structures. Also, the SP structure is close to the TBP one from the geometrical point of J


Article

Inorganic Chemistry

(6) Long, D.-L.; Tsunashima, R.; Cronin, L. Polyoxometalates: Building Blocks for Functional Nanoscale Systems. Angew. Chem., Int. Ed. 2010, 49, 1736−1758. (7) Nomiya, K.; Sakai, Y.; Matsunaga, S. Chemistry of Group IV Metal Ion-Containing Polyoxometalates. Eur. J. Inorg. Chem. 2011, 2011, 179−196. (8) Nomiya, K.; Yoshida, T.; Matsunaga, S. Formation of Phosphanegold(I) Cluster Cations Mediated by Polyoxometalates, Molecular Structures and Catalysis for Organic Transformations. In Polyoxometalates: Properties, Structure, and Synthesis; Roberts, A. P., Ed.; Nova Science Publishers: New York, 2016; pp 213−242. (9) Schulz-Dobrick, M.; Jansen, M. Supramolecular Intercluster Compounds Consisting of Gold Clusters and Keggin Anions. Eur. J. Inorg. Chem. 2006, 2006, 4498−4502. (10) Schulz-Dobrick, M.; Jansen, M. Structure-Directing Effects in the Supramolecular Intercluster Compound [Au9(PPh3)8]2[V10O28H3]2: Long-Range versus Short-Range Bonding Interactions. Inorg. Chem. 2007, 46, 4380−4382. (11) Schulz-Dobrick, M.; Jansen, M. Characterization of Gold Clusters by Crystallization with Polyoxometalates: the Intercluster Compounds [Au9(dpph)4][Mo8O26], [Au9(dpph)4][PW12O40] and [Au11(PPh3)8Cl2]2[W6O19]. Z. Anorg. Allg. Chem. 2007, 633, 2326− 2331. (12) Zhai, Q. G.; Wu, X. Y.; Chen, S. M.; Zhao, Z. G.; Lu, C. Z. Construction of Ag/1,2,4-Triazole/Polyoxometalates Hybrid Family Varying from Diverse Supramolecular Assemblies to 3-D Rod-Packing Framework. Inorg. Chem. 2007, 46, 5046−5058. (13) Streb, C.; Tsunashima, R.; MacLaren, D. A.; McGlone, T.; Akutagawa, T.; Nakamura, T.; Scandurra, A.; Pignataro, B.; Gadegaard, N.; Cronin, L. Supramolecular Silver Polyoxometalate Architectures Direct the Growth of Composite Semiconducting Nanostructures. Angew. Chem., Int. Ed. 2009, 48, 6490−6493. (14) Song, Y. F.; Abbas, H.; Ritchie, C.; McMillian, N.; Long, D. L.; Gadegaard, N.; Cronin, L. From Polyoxometalate Building Blocks to Polymers and Materials: the Silver Connection. J. Mater. Chem. 2007, 17, 1903−1908. (15) Mizuno, N.; Uchida, S. Structures and Sorption Properties of Ionic Crystals of Polyoxometalates with Macrocation. Chem. Lett. 2006, 35, 688−693. (16) Uchida, S.; Mizuno, N. Design and Syntheses of NanoStructured Ionic Crystals with Selective Sorption Properties. Coord. Chem. Rev. 2007, 251, 2537−2546. (17) Arai, H.; Yoshida, T.; Nagashima, E.; Hatayama, A.; Horie, S.; Matsunaga, S.; Nomiya, K. Silver- and Acid-Free Catalysis by Polyoxometalate-Assisted Phosphanegold(I) Species for Hydration of Diphenylacetylene. Organometallics 2016, 35, 1658−1666. (18) Nomiya, K.; Yoshida, T.; Sakai, Y.; Nanba, A.; Tsuruta, S. Intercluster Compound between a Tetrakis{triphenylphosphinegold(I)}oxonium Cation and a Keggin Polyoxometalate (POM): Formation during the Course of Carboxylate Elimination of a Monomeric Triphenylphosphinegold(I) Carboxylate in the Presence of POMs. Inorg. Chem. 2010, 49, 8247−8254. (19) Yoshida, T.; Nomiya, K.; Matsunaga, S. Novel Intercluster Compound between a Heptakis{triphenylphosphinegold(I)}dioxonium Cation and an α-Keggin Polyoxometalate Anion. Dalton Trans. 2012, 41, 10085−10090. (20) Yoshida, T.; Matsunaga, S.; Nomiya, K. Two Types of Tetranuclear Phosphanegold(I) Cations as Dimers of Dinuclear Units, [{(Au{P(p-RPh)3})2(μ-OH)}2]2+ (R = Me, F), Synthesized by Polyoxometalate-Mediated Clusterization. Dalton Trans. 2013, 42, 11418−11425. (21) Yoshida, T.; Yasuda, Y.; Nagashima, E.; Arai, H.; Matsunaga, S.; Nomiya, K. Various Oxygen-Centered Phosphanegold(I) Cluster Cations Formed by Polyoxometalate (POM)-Mediated Clusterization: Effects of POMs and Phosphanes. Inorganics 2014, 2, 660−673. (22) Yoshida, T.; Matsunaga, S.; Nomiya, K. Novel Intercluster Compounds Composed of a Tetra{phosphanegold(I)}oxonium Cation and an α-Keggin Polyoxometalate Anion Linked by Three

electron-deficient species of the ammonium(2+) cation 1 was also supported by the results of DFT and ONIOM calculations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02795. Preparation of 1-BF by anion exchange of 1-PW with BF4− using the batch method, molecular structure of the {[Au(PPh3)]5(μ5-N)}2+ cation in 1-BF (Figure S1), CPMAS 15N NMR measurements of 1-PW-15N by changing the contact time (1−8 ms; Figure S2), selected bond lengths (Å) and angles (deg) in the {[Au(PPh3)]5(μ5-N)}2+ cation in 1-BF (Table S1), and Cartesian coordinates (Å) of the optimized structures of the model (1m−3m) and real (1r−3r) clusters, {[(L3P)Au]nN}(n−3)+ (L = H, Ph; n = 4−6; Table S2) (PDF) Accession Codes

CCDC 1577172 and 1583072 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.N.). Phone: 81-463-594111. Fax: 81-463-58-9684. *E-mail: [email protected] (T.M.). Phone: +81463-59-4111. ORCID

Kenji Nomiya: 0000-0003-0225-877X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funds from the Strategic Research Base Development Program for Private Universities (Ministry of Education, Culture, Sports, Science and Technology of Japan). The computations were performed in part at the Research Center for Computational Science, Okazaki, Japan.

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