Cyclic Trinuclear Gold(I) Clusters with N,N and Unusual C,C Mixed

Nov 7, 2016 - Assemblies 1 and 2 are the first gold(I) trinuclear clusters featuring mixed-ligand bridges from different N,N and C,C donors; 3 is a pr...
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Cyclic Trinuclear Gold(I) Clusters with N,N and Unusual C,C MixedLigand Bridges Doris Y. Melgarejo, Gina M. Chiarella, and John P. Fackler, Jr.* Department of Chemistry, P.O. Box 3012, Texas A&M University, College Station, Texas 77842-3012, United States S Supporting Information *

ABSTRACT: Three crystalline trinuclear gold(I) clusters, [Au3f2y] (1), [Au3fy2] (2), and [Au3y3] (3), where f = N,N′-bis(2,6-dimethylphenyl)methanimidamidate and y = dimethylendiphenylphosphinate, exhibit bridges from the N,N-formamidinate and/or from the ylide anion ligand whose P-methylene groups chelate in an unusual fashion, where the chelate CPC unit is perpendicular to the trigonal plane of the metal atoms. Assemblies 1 and 2 are the first gold(I) trinuclear clusters featuring mixed-ligand bridges from different N,N and C,C donors; 3 is a previously unknown homoleptic ylide anion cyclic trinuclear assembly. Formamidinate bridges in 1 and 2 connect gold(I) atoms at aurophilic distances of 3.084(2) and 3.0543(4) Å, whereas an out-ofplane (perpendicular) P-ylide anion bite produces AuI−AuI distances of as large as 3.900(2) Å in 3. The crystal space groups for 1 and 2 are triclinic P1̅ and that for 3 is monoclinic P21/c, with Z = 2 for 1 and 2 and Z = 4 for 3. Compounds are synthesized under Schlenk conditions at −20 °C in toluene by reacting the proper ratios of the gold(I) formamidinate [Au2f2] with the phosphorus ylide [Hy] under basic conditions (KOH), followed by extraction with ether. This synthesis also produces a dinuclear cation, [Au2f(Hy)2]+, previously reported by our group. A neutral mixed-ligand dinuclear complex, [Au2fy], was not observed. Under UV light, 1 and 2 display a bright-green luminescence at room temperature and in frozen methyltetrahydrofuran solutions under liquid nitrogen, with microsecond lifetimes. All three complexes 1−3 are characterized by their X-ray crystal structures, 1H NMR, IR, UV−visible, and luminescence spectroscopies, and elemental analysis.

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dithiocarbamate acts a monodentate ligand.11 These complexes all have AuI−AuI distances of around 3.0 Å. Short Au−Au distances (2.62−2.67 Å) are found12 in a trinuclear AuI2Au0 complex discovered by Sadighi et al., with monodentate carbene ligands. The addition of an oxo atom bridging the three gold atoms in a similar carbene complex increases the metal−metal distances from 2.6324(8)−2.6706(8) Å to 3.1669(3)− 3.3149(3) Å.13 Since the 1970s, planar gold(I) trinuclear clusters have achieved attention because of their chemical, structural, and photophysical properties.1 Studies showed that optoelectronic properties of the solids arise from a supramolecular organization influenced by their planarity and intermolecular metal−metal interactions14,15 and also from the NC ligand character of the highest occupied molecular orbitals available for optical excitation.16,17 This prompted us to evaluate the luminescent properties of these new cyclic trinuclear gold(I) clusters, which show no intermolecular metal−metal interactions. Luminescence18 in gold(I) complexes is of practical use in the construction of optical devices and sensors.19 Some studies have correlated luminescence to aurophilic interactions with d → p transitions.20 Also some luminescence in gold(I) compounds has been assigned to metal-centered pz → dσ transitions as well.21,22

INTRODUCTION Gold chemistry has advanced considerably over the past 40 years, stimulated largely by the observation of interesting new structural and optical properties.1 Dinuclear gold(I) complexes with various bridging ligands have led to dinuclear oxidative addition and the formation of metal−metal-bonded gold(II) species.2 While phosphine, thiolate, ylide, and amidinate ligands have been extensively studied,1 relatively few reports on mixedligand coordination exist. While the structural and oxidativeaddition properties are similar, the luminescence properties of dinuclear gold(I) amidinates, [Au2f2], are considerably different2 from those dinuclear gold(I) ylides, [Au2y2], where f = N,N′bis(2,6-dimethylphenyl)methanimidamidate and y = dimethylenediphenylphosphinate. Here we expand on our earlier work3 with amidinates and ylides. The synthetic procedures reported here lead to the formation of three new cyclic trinuclear gold(I) complexes. In the cyclic dinuclear gold(I) complexes [Au2f2] and [Au2y2], the gold(I) centers are ca. 3.00 Å apart. Various bifunctional bridging ligands have produced nonstrained nine-membered planar rings with Au−Au distances considerably larger than 3.0 Å.4−6 The most common combination is {N-,C-} as in imidazolate,7 isocyanide,8 iminoacyl, imidoyl, or carbeniate9 ligands. A {P-sp2-N} coordination is found for the diphenyl(2-pyridyl)phosphane ligand10 with a large bite angle that produces a distorted trinuclear gold(I) cluster. Dithiocarbamates have produced trinuclear gold(I) complexes, including one in which the © XXXX American Chemical Society

Received: August 12, 2016

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DOI: 10.1021/acs.inorgchem.6b01801 Inorg. Chem. XXXX, XXX, XXX−XXX

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establishes a bridge to another gold(I) (Scheme 2B). The formamidinate f released in the process also takes up protons, as observed in the 1H NMR spectrum and by the formation of Hf crystals. After vacuum pumping to remove toluene, compounds 1−3 were extracted with small amounts of ether and carefully layered with hexanes. This produced crystals suitable for X-ray analysis in 2−3 weeks. Colorless crystals of 1 were thin planes with a triangular shape. Colorless crystals of 2 were bulky blocks. Crystals of 3 resembled those of compound 2. The yields were low as follows: 1, 50%; 2, 35−40%; 3, 40%. The reactions that produced 1 and 2 also led to the dinuclear complex [Au2f(Hy)2]NO3 in ∼30% yield, which was relatively insoluble in ether but extractable in toluene. When reacted with [AuCl(THT)] and KOH (to remove protons of Hy), it produced compound 2 in a 30% yield. While the neutral ylide, Hy, along with KOH and y also may be involved in deprotonation, added amounts of Hy led to the formation of homobridged [Au2y2] (as observed by crystal analysis and as seen in the spectroscopy25) and no trinuclear compound. While KOH was added as the base to form y, KOH also is known to react with the ylide anion ligand at P, releasing phenyls, as reported in the syntheses of open dinuclear complexes.3 The product, benzene, in that study required a hydrogen-bonding solvent such as ethanol, and although protons are available in the reactions reported here, benzene was not observed. Admittedly, it would have been difficult to detect if it was present in small amounts. While it seems that dinuclear mixed-ligand complex [Au2fy] might have been a plausible complex formed in this synthetic study, it was not detected in the syntheses used here for these trinuclear complexes or in our earlier studies forming open dinuclear products.3 Both procedures used [Auf2] and the same P-ylide ligand. The short bite distance of the formamidinate compared with that of the ylide may make formation of the neutral dinuclear mixed-ligand complex structurally unfavorable, although other factors may prevent this product from forming. The Au−Au distances in the cyclic trinuclear complexes are expanded to almost 3.90 Å from the Au−Au distances in the homobridged dinuclear complexes, which have separations in the range of 2.5−2.7 Å. Isolation of the trinuclear complex 3 rather than the well-known [Au2y2] complex demonstrates that specif ic synthetic conditions play important roles in governing product formation. Structural Characteristics. The structures of 1−3 with thermal ellipsoids are shown in Figure 1a−c. The formamidinate f is a methanimidamide (NCHN) moiety N,N′-substituted with 2,6 dimethylphenyl rings with two nitrogen donors; the ylide anion y is an onium phosphine with two C,C-coordinating methylenes and two phenyls. The gold(I) trinuclear complexes have nonplanar structures with three bridging ligands f and/or y, the latter in an unusual coordination of the CPC unit of y perpendicular to the metal atom plane. Compounds 1 and 2 crystallize in the space group P1̅, and compound 3 crystallizes in the space group P21/c. In compound 1, there are two twisted noncoplanar formamidinate bridges with Au−Au distances of 3.138(2) and 3.084(2) Å. At the shared gold atom, the N2−Au2−N3 angle is strained away from linearity up to 167.3(6)°. The Au1 and Au3 atoms are bridged by the y methylene groups with C1−Au1 and C2−Au3 coordination, in which the out-of-plane Au−C bonds diverge from the metal atom plane by nearly 90°. The Au1−Au3 separation is 3.900(2) Å. This generates a pseudo-C2 axis, which passes through the y P1 and Au2 atoms. The C−Au−N angles are almost linear, 175.5(7)° and 172.4(7)° (Table 1).

The formamidinate, f (N,N donor), and ylide anion, y (C,C donor), ligands used in this study generally produce the homobridged cyclic dinuclear complexes [Au2f2] and [Au2y2]. Under the synthetic protocol followed in this paper, the reaction of the ylide with [Au2f2] gives rise to two new luminescent nonplanar cyclic trinuclear clusters, [Au3f2y] (1) and [Au3fy2] (2). The ylide anion bridges in these compounds are coordinated with the CPC unit almost perpendicular to the trinuclear gold(I) plane, not in the nearly parallel manner observed in the dinuclear complexes. This bridging feature also appears in the assembly of the nonluminescent trinuclear ylide anion complex [Au3y3] (3; Scheme 1). Mixed N−Au−C coordination to gold(I) in clusters Scheme 1. Synthetic Conditions Used To Produce Trinuclear Gold(I) Complexes 1−3

is rare and has been reported previously in the cubal array3b [Au8f4(η5-C-(y-H)2] and in three open dinuclear clusters with a bridging formamidinate and a monocoordinating ylide, Hy.3

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RESULTS AND DISCUSSION Synthetic Procedures. The reaction of [Au2f2] with the ylide Hy in toluene under Schlenk conditions at −20 °C, with dry KOH added and with avoidance of light, leads to the formation of 1−3 (Scheme 1). Three factors appear to play a role in the syntheses of these trinuclear complexes: (1) the relative stoichiometric amounts of starting materials; (2) the use of KOH as a base in a nonpolar (water free) solvent; (3) the low temperature and solvent (toluene), which also appear to contribute to the formation of the observed products. The second factor presumably leads to the formation of Au−C-ylide bonds and the rupture of the Au−N bonds. The base also may deprotonate a coordinated ylide (Scheme 2A), which then Scheme 2. Proposed (A) KOH-Assisted Deprotonation of the Au−C-Bonded Ylide Anion and (B) Formation of a New Au− C−C−Au Linkage

B

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Figure 1. Structures of trinuclear gold(I) clusters (a) 1, (b) 2, and (c) 3, with displacement ellipsoids at 30% probability and hydrogen atoms omitted.

Table 1. Selected Bond Distances and Angles for Cyclic Trinuclear Gold(I) Mixed-Ligand Complexes compound

Au−Au (Å)

Au−N (Å)

Au3f2y (1)

3.138(2), 3.084(2) , 3.900(2) 3.0543(4), 3.6730(4), 3.8899(3) 3.3578(7), 3.8953(7), 3.9315(8)

2.057(14), 2.031(16), 2.038(15), 2.069(15) 2.063(4), 2.059(4)

Au3fy2 (2) Au3y3 (3)

C−Au−N (deg)

Au−C (Å) 2.048(19), 2.035(19) 2.077(6), 2.084(6), 2.047(5), 2.056(5) 2.069(12), 2.068(12), 2.081(13), 2.083(12), 2.070(11), 2.073(12)

In compound 2, there is a nontwisted formamidinate bridge with the phenyl groups well aligned as in [Au2f2], but the Au2− Au3 distance is considerably longer [3.054(4) Å] than that in the homobridged dinuclear formamidinate2 [2.711(3) Å]. Trans to each Au−N bond sit methylene CH2−Au bonds from the two

175.5(7), 172.4(7) 177.2(2), 174.5(2)

−Au− angles (deg) 167.3(6) for N−Au−N 175.6(2) for C−Au−C 175.0(5), 176.2(5), and 178.6(5) for C−Au−C

bridging y ligands positioned on the same side of the Au2− NCN−Au3 plane, resulting in ylide anion methylene (P-CH2) groups that connect out of the formamidinate plane to the third Au1 atom. This generates a pseudo mirror plane sitting through the formamidinate C29 and Au1 atoms. There are no aurophilic C

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signals are due to methyls (CH3) on the f phenyls and to Pmethylenes on the y. The CH2 signals are split because of 31P−1H coupling. The presence of three distinctive linkages in the structures, N− Au−N, C−Au−C, and the mixed N−Au−C, is reflected in the shifts of the amidinate methyl protons. The Au−N phenylmethyl signals are to a higher field and the y methylene proton signals (on the Au−CH2 linkage) are to a lower field compared to those in the homoligand dinuclear complexes, which have only N− Au−N and C−Au−C bonds. This follows a trend already noted in open dinuclear gold(I) compounds with these ligands.3 The shift is more pronounced on the y methylene protons closer to the gold centers than on the f phenylmethyl groups. In 1, the phenylmethyls on the N−Au2−N linkage produce methyl signals at 2.38 ppm, a slightly higher field than that for the methyls in [Au2f2], 2.43 ppm. Methyls on phenyls at the Ph−N− Au−C linkage resonate at a 2.32 ppm because the trans σ-carbon donor y methylenes influence the N−Au bonds. This influence also is revealed in a shift of the split y methylene signal to a lower field (1.74−1.70 ppm) compared with the signal from [Au2y2] (1.42−1.38 ppm). In 2, with one C−Au3−C and two N−Au−C linkages, a shift to lower field is observed for the y methylene signal (1.74−1.70 ppm) as well as a slight shift to a higher field for the f phenylmethyls (2.31 ppm). The trans coordination of the y methylenes around Au3 is reflected in a phosphorus-coupled split signal (at 1.35−1.31 ppm) compared to the methylene signal in [Au2y2] (1.38−1.42 ppm). The influence of a strong σ-carbon donor and a weaker σnitrogen-donor amidinate, as was also observed in open binuclear compounds,3 leads to the observed shift in the 1H NMR spectrum to intermediate strength fields for the f methyls and y methylene protons. This shift is especially dramatic for the y methylenes, which sit close to the N−Au−C linkage. These signals characteristically appear at 1.70−1.74 ppm in 1 and 2 as well as in the open dinuclear complexes3 [Au2fy2]NO3, [Au2fy(ph)], and [Au2fyCl]. In 3, methylene signals (CH2−P) from y ligands appear at 1.35−1.39 ppm compared to the dinuclear [Au2y2] signals at 1.38−1.42. The 1H NMR signals for C−Au−CH2 in 2 also are observed in this range. Emission Spectra. Polycrystalline samples of 1 and 2 showed emission with UV-wavelength excitation. With short wavelengths (Ex1, 311 nm; Ex2, 306 nm), the emissions occur in the near-UV (Em1, 356 nm; Em2, 345 nm; Figure 3). The small

contacts from Au1 to either Au2 [3.8899(3) Å] or Au3 [3.6730(4) Å]. At the three C1−Au1−C15, N1−Au2−C2, and N2−Au3−C16 linkages, the angles are close to linear, 175.6(2)°, 177.2(2)°, and 174.5(2)°, respectively. The opposing Au−N and Au−C bond distances are similar to the average of the Au−N and Au−C values in nonmixed dinuclear [Au2f2] and [Au2y2] complexes, an observation also made in the open dinuclear clusters with similar N−Au−C bonds.3 The influence of the strong σ-carbon donor y on the trans Au−N bonds is to lengthen the Au−N bond a small amount. The Au−N bond distance in [Au2f2] is 2.0365(4) Å, while in 1, it is 2.069(15) Å. In 2, the Au−N bond distances are 2.057(14), 2.059(4), and 2.063(4) Å. The Au−C bonds appear shortened a bit from 2.078(5) Å in [Au2y2] to 2.048(19) and 2.035(19) Å in 1 and 2.056(5) and 2.047(5) Å in 2. In 3, the Au− C bond distances range from 2.068 to 2.083 Å. The P−C bond lengths of the ylide ligands in 3 range from 1.742(11) to 1.772(13) Å, not statistically different25 from the P−C values in Au2y2. In 2, the difference between the distances Au1−Au2 and Au1− Au3, which is greater than 0.2 Å, is explained by the crystal packing caused by the orientation of the phenyl rings. The phenyl ring from the y at P2 and one of the phenyl rings from an f lie almost perpendicular to each other [the angle between these rings is 78.1(1)°]. The other phenyl ring on f lies parallel to a phenyl ring from an adjacent molecule (Figure 2). This leads to the asymmetric structure observed.

Figure 2. Space-filling drawing of 2 showing the packing of neighboring molecules.

The structure of compound 3 is similar to that of 2 displaying a nontwisted y bridge with nearly coplanar coordination of the P1CH2 groups to the Au3−Au2−P1 plane. This resembles the Au2−NC29N−Au3 ligation of the formamidinate in 2. The Au2 and Au3 atoms in 3 are separated by 3.358 Å. As in 2, the P2- and P3-ylide anion ligands lead to the presence of a pseudo mirror plane dividing the molecule through the P1−Au1 atoms. The CH2 groups from P2 and P3 connect to the gold centers Au1, Au2, and Au3 in a manner that enlarges the Au1−Au2 and Au1− Au3 distances to 3.895(7) and 3.931(8) Å, respectively. As a result, the angles at the three C−Au−C linkages are close to linear, 178°, 176°, and 174°. 1 H NMR Spectra. 1H NMR spectra were taken in benzene-d from crystals isolated in grease and rinsed with hexane. In the aromatic region (6−8 ppm), there is an overlap of signals due to phenyl protons from both the formamidinate and P-ylidic ligands. The characteristic signal for the N−CH−N amidinate is located at ∼7.4 ppm. In the aliphatic region (3−1 ppm), the

Figure 3. High-energy emission spectra of 1 and 2 with excitation spectra in frozen 2-MeTHF at 77 K and in the solid. D

DOI: 10.1021/acs.inorgchem.6b01801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Stokes shift and microsecond lifetimes, 14−16 μs (Supporting Information), point to ligand-to-metal charge-transfer (LMCT) fluorescence, presumably arising from the amidinate ligands. The UV absorption spectra (Supporting Information) of the two complexes exhibit relatively weak bands, ε = ca. 2000 L M−1 cm−1 at 291 and 293 nm, respectively, which may be forbidden metalto-ligand charge transfer involving the amidinate π system. Even weaker absorption bands occur near 340 nm, ε = ca. 300 L M−1 cm−1, close to the energy of the long-wavelength excitations. These weak absorption and emission spectra presumably involve the π system of the amidinate perturbed by the gold(I). With the longer-wavelength excitation (Ex near 350 nm) at room temperature (RT), the crystalline samples of 1 and 2 show a green glow, with an emission maxima centered around 485 nm. Solutions of these compounds in tetrahydrofuran were screened for emission under a UV lamp. They showed emission only when frozen at 77 K (liquid nitrogen; Figure 4). A 2-methyltetrahy-

distilled after refluxing for several hours over Mg/I2 under nitrogen, was used. The phosphorus ylide [Ph2P(CH2)CH3] was prepared by stirring a 1:1 mmol mixture of [Ph2P(CH3)2]NO3 (0.277 g) and KH (0.05 g from a 25% slurry in mineral oil) in 10 mL of THF (at −20 °C, in the dark) under nitrogen, in a Schlenk flask capped with a ground-glass tip with a stopcock. After 6 h, the white slurry turned yellow and the THF was removed by vacuum pumping, with the flask always kept at −20 °C. Then toluene (10−20 mL) was added, and the yellow solution was stored at −20 °C in the freezer as described.25 The potassium formamidinate was prepared by refluxing at 60 °C the formamidine [(2,6-Me2)PhNCHN(2,6-Me2)Ph] with KOH at a 1:1 ratio in ethanol. Slow evaporation of the solvent in air produced crystals that were rinsed with hexanes. Gold(I) formamidinate was prepared as indicated in the synthesis of gold(II) formamidinate dinitrate. The ethanol solution was evaporated under vacuum, leaving white solid [Au2f2]. Elemental analyses were performed by Robertson Microlit Laboratories, Inc. (Madison, NJ). 1H NMR spectra were recorded on a Mercury-300 NMR spectrometer. IR spectra were recorded on a PerkinElmer 16PC FT-IR spectrophotometer using KBr pellets. Emission and excitation spectra were recorded on a Photon Technology International (PTI) spectrofluorometer model QuantaMaster 4/ 2006SE equipped with a xenon lamp. Synthesis of [Au3f2y] (1). The gold(I) formamidinate [Au2f2] (0.30 mmol, 0.268 g) was put under nitrogen into a tall Schlenk flask capped with a ground-glass stopcock. This flask was connected via cannula to one containing the ylide [Ph2P(CH2)CH3] (0.20 mmol) dissolved in 10 mL of toluene (at −20 °C, in the dark, under nitrogen). The flask with the formamidinate complex was chilled to around −20 °C and protected from light. A slight vacuum was applied to this flask to transfer the ylide solution through the cannula. Then 0.1 mmol of dry KOH (0.006 g) was added under a nitrogen stream. The reaction flask was closed, and the yellow suspension was stirred 4 h at −20 °C, until the cloudiness cleared (turning colorless), and it was kept overnight in the freezer. After removal of the toluene solvent by reduced pressure, the residue was extracted with 10 mL of ether. The ether solution was layered with 30 mL of hexane and was closed with a ground-glass cap under nitrogen. Thin colorless triangular crystals of 1 grew in 3 weeks. Yield: 50%. Anal. Calcd for C48H52N4PAu3 (MW 1306.8): C, 44.11; N, 4.29; H, 4.01. Found: C, 44.58; N, 4.00; H, 3.91. IR: 3160 (y, m), 2950 (s), 2889 (y, s), 1610 (s), 1650 (phenyl, s) 1565 (s), 1465 (s), 1440 (s), 1373 (m), 1345 (m) 1290 (m), 1310 (phenyl, m), 15 (m), 1199 (m), 1179 (phenyl, m), 1160 (w), 1150 (y, m), 1090 (m), 1129 (y, m), 1028 (m), 980 (m), 930 (y, m), 920 (m), 850 (phenyl, m), 829 (y, w), 750 (s), 7 (m), 689 (m), 653 (y, m), 600 (w), 470 (m), 4 (m). 1H NMR (C6D6): δ 7.4 (NCHN), 2.38 (CH3-Ph at N−Au−N), 2.32 (CH3-Ph at the N− Au−C linkage), 1.73−1.69 (CH2-y at the N−Au−C linkage). Synthesis of [Au3fy2] (2). The procedure follows steps similar to those used for 1. A tall Schlenk flask (capped with a ground-glass stopcock) containing the gold(I) formamidinate [Au2f2] (0.15 mmol, 0.134 g) was attached via a cannula to another Schlenk flask containing the ylide [Ph2P(CH2)CH3] (0.20 mmol) dissolved in 10 mL of toluene (at −20 °C, in the dark, under nitrogen). The flask with the formamidinate was cooled to −20 °C and protected from light. Vacuum was applied to this flask to transfer the ylide solution through the cannula. Then 0.1 mmol of dry KOH (0.006 g) was added under a nitrogen flow. The reaction flask was closed, and the yellow suspension was stirred for 4 h until discoloration and left overnight in the freezer. The toluene solvent was removed under vacuum, and the residue was extracted with 10 mL of ether. The ether solution was layered with 30 mL of hexane, and the flask was closed with a ground-glass cap under nitrogen. Bulky colorless crystals of 2 grew in 3 weeks. Yield: 40%. Anal. Calcd for C45H47N2P2Au3 (MW 1268.22): C, 42.58; N, 2.21; H, 3.73. Found: C, 42.18; N, 2.23; H, 3.54. IR: 3169 (y, m), 2959 (s), 2890 (y, s), 1610 (s), 1650 (phenyl, s) 1565 (s), 1465 (s), 1440 (s), 1373 (m), 1345 (m), 1290 (m), 1310 (phenyl, m), 1265 (m), 1200 (m), 1182 (phenyl, m), 1165 (w), 1150 (y, m), 1099 (m), 1140 (y, m), 1028 (m), 980 (m), 930 (y, m), 930 (m), 853 (phenyl, m), 829 (y, w), 750 (s), 720 (m), 690 (m), 683 (ylide, m), 600 (w), 470 (m), 427 (m). 1H NMR (C6D6): δ 7.4

Figure 4. Low-energy emission with 350 nm excitation (maxima listed) in a frozen 2-MeTHF solution at 77 K: 1 in purple and 2 in green.

drofuran (2-MeTHF) solvent was selected because it turns glassy at 77 K. The emissions of frozen solutions excited at 344 and 343 nm, which correlate with the UV absorptions of solutions in 2MeTHF at RT of 340 and 339 nm for 1 and 2, respectively, were observed as a bright, almost white, greenish glow. The emission maxima were found at 485 nm for 1 and at 482 nm for 2, followed by a smooth decrease as the wavelength increased until near 600 nm. This low-energy, low-intensity emission was not observed from the solid. This low-intensity emission explains the almost white glow of the emission from RT solutions. It also may reflect an influence of the solvent on the emission, although this has not been tested with other solvents. The luminescent behavior of the mixed-ligand trinuclear complexes 1 and 2 is not observed in the homogeneous ligand, solid dinuclear complex [Au2f2] or [Au2y2], which emit with a dull, reddish glow at 77 K and do not emit in a RT solution. The emission band of solid [Au2f2] centered at 430 nm has been assigned to LMCT.23 [Au2y2] emits weakly at 483 nm.24 A weak, dull, visible-region emission is noticed (Supporting Information) in the trinuclear complex 3, similar to [Au2y2], taken under conditions similar to those of 1 and 2.

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EXPERIMENTAL SECTION

Syntheses were carried out under an inert atmosphere using standard Schlenk techniques unless otherwise noted. Solvents [tetrahydrofuran (THF), toluene, ether, and hexane] were dried using a Glass Contour solvent system and stored over sodium, under nitrogen. Ethanol, freshly E

DOI: 10.1021/acs.inorgchem.6b01801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (NCHN), 2.31 (CH3-Ph at N−Au−C), 1.74−1.70 (CH2-y at N−Au− C), 1.35−1.31 (CH2-y at the C−Au−C linkage). Alternate Synthesis of 2 from [Au2f(Hy)2]NO3. After ether extraction in the synthesis of 1 or 2, the residue was extracted with toluene and set for vapor diffusion under hexane in a closed vial. Bundles of crystals of [Au2f(Hy)2]NO33a with a leaf shape grew in 1 week. A mixture of [Au2f(Hy)2]NO3 (0. mmol, 0.2838 g), [Au(THF)Cl] (0. mmol, 0.0800 g), and 10 mL of toluene was placed into a Schlenk flask under nitrogen, to which 0.5 mmol of KOH (0.028 g) was added. After stirring in the dark for over 3 h, the toluene solvent was removed by vacuum and the residue was extracted with ether, from which crystals of 2 grew by layering with hexane under nitrogen, with a yield of 35%. Synthesis of [Au3y3] (3). The gold(I) formamidinate [Au2f2] (0.30 mmol, 0.268 g), under nitrogen, was put into a tall Schlenk flask (capped with a ground-glass stopcock). The flask was coupled via cannula to one containing the ylide [Ph2P(CH2)CH3] (0.60 mmol) solution in 10 mL of toluene (at −20 C, in the dark). The flask with the formamidinate complex, covered from the light, was cooled to around −20 °C, and then a slight vacuum was applied to transfer the ylide solution through the cannula. Next 0.1 mmol of dry KOH (0.006 g) was added under a nitrogen stream. The reaction flask was closed, and the yellow suspension was stirred for 4 h until it turned colorless. It was left overnight in the freezer. The solvent was removed under vacuum. The residue was extracted with 10 mL of ether and layered with 30−40 mL of hexane under nitrogen. Bulky colorless crystals of 3 grew in 3 weeks. Yield: 40%. Anal. Calcd for C42H42P3Au3 (MW 1230.60): C, 40.99; H, 3.44. Found: C, 40.18; H, 3.54. IR: 3167 (y, m), 2964 (s), 2892 (y, s), 1605 (s), 1650 (phenyl, s) 1571 (s), 1440 (s), 1345 (m), 1290 (m), 1310 (phenyl, m), 1295 (m), 1179 (phenyl, m), 1160 (w), 1150 (y, m), 1090 (m), 1117 (y, m), 1031 (m), 943 (y, m), 848 (phenyl, m), 819 (y, w), 755 (s) 720 (m), 691 (m), 663 (ylide, m), 618 (w), 475 (m), 427 (m). 1 H NMR (C6D6): δ 8.79 (Ph), 1.39−1.35 (CH2-y at the C−Au−C linkage). X-ray Structure Determination. Data for 1−3 were collected on a Bruker APEX-II 100026 CCD area detector system using ω scans of 0.3°/frame and 30 s/frame (2400 frames, 23 h 8 min) for 1, 0.5°/frame and 10 s/frame (1440 frames, 6 h 24 min) for 2, and 0.5°/frame and 4 s/ frame (1440 frames, 1 h 41 min) for 3. For all structures, the cell parameters were determined using the program APEX. Data reduction and integration were performed with the software package SAINT,27 which corrects for Lorentz and polarization effects, while absorption corrections were applied using the program SADABS.28 See the Supporting Information for the crystallographic parameters obtained. The positions of the gold atoms were found via direct methods using the program SHELXTL.29 Subsequent cycles of least-squares refinement followed by difference Fourier syntheses revealed the positions of the remaining non-hydrogen atoms. Hydrogen atoms were added in idealized positions and included in the calculation of the structure factors. All non-hydrogen atoms were refined with anisotropic displacement parameters. Compounds 1 and 2 crystallized in the space group P1̅, and 3 crystallized in the space group P21/c. Absorption corrections were applied.30 Crystallographic information for 1−3 are given in Supporting Information. Emission Studies of Trinuclear Au3f2y, Au3fy2, and Au3y3 Clusters. Spectra were obtained in the Texas A&M University spectroscopy laboratory on a Jobin-Yvon Horiba Fluorolog 3-22 Tau3 spectrofluorimeter. The spectra were corrected for instrumental response using Felix Software provided by PTI. Solid-state and frozen glassy solution low-temperature measurements were made using a cryogenic Dewar coldfinger. Liquid nitrogen was used to obtain the 77 K measurements using a 2-MeTHF solvent. Other Spectroscopic Results. NMR and IR spectra of all three complexes are presented in the Supporting Information.

Their structures show an unusual coordination of the y ligand with the CPC plane of y, perpendicular to the trinuclear gold plane. This coordination also is found in 3, the first cyclic trinuclear gold(I) ylide anion complex to be reported. A similar Au3f3 trinuclear assembly has not been observed to date. The solid complexes 1 and 2 emit under UV light and in 2-MeTHF frozen solutions at 77 K, displaying a bright-green phosphorescence, which appears to be associated with the f ligands and previously assigned2,3 to a heavy-metal-influenced LMCT. A dull emission of compound 3 is observed under the same conditions and is similar to the known behavior of [Au2f2] and [Au2y2].

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01801. Spectroscopic data (PDF) X-ray crystallographic data for 1 in CIF format (CIF) X-ray crystallographic data for 2 in CIF format (CIF) X-ray crystallographic data for 3 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation of Houston, TX (Grant A-0960), and Texas A&M University. REFERENCES

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CONCLUSIONS Trinuclear gold(I) clusters 1 and 2, with mixed-ligand coordination from two well-studied bridging ligands, N,N′bis(2,6-dimethylphenyl)methanimidamidate and dimethylenediphenylphosphinate, have been synthesized and characterized. F

DOI: 10.1021/acs.inorgchem.6b01801 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b01801 Inorg. Chem. XXXX, XXX, XXX−XXX