Formamidinate Clusters - TAMU Chemistry - Texas A&M University

Journal of Cluster Science, Vol. 14, No. 3, September 2003 (© 2003)

Gold(I) Formamidinate Clusters: The Structure, Luminescence, and Electrochemistry of the Tetranuclear, Base-Free [Au 4 (ArNC(H)NAr)4 ] Ahmed A. Mohamed, 1 Hanan E. Abdou, 1 Michael D. Irwin, 1 José M. López-de-Luzuriaga, 2 and John P. Fackler, Jr. 1,3 Received May 1, 2003 The structures of the tetragold(I) formamidinate cluster complexes, [Au 4 (ArNC(H)NAr)4 ], Ar=C 6 H 4 -4-OMe (1), C 6 H 3 -3,5-Cl (2), C 6 H 4 -4-Me (3), have been characterized by x-ray crystallography. The range of Au · · · Au distances is 2.8–3.0 Å. The angles at Au · · · Au · · · Au are acute and obtuse 70 and 109°, 88 and 91°, and 63 and 116° in 1, 2, and 3, respectively. The four gold atoms are located at the corner of a rhomboid with the formamidinate ligands bridged above and below the near plane of the four Au(I) atoms. The tetranuclear gold(I) complexes show a bright blue-green luminescence under UV light, with an emission at ’ 490 nm and a weak emission at ’ 530 nm in the solid state, at room temp and 77 K. The oxidation of the formamidinate cluster, 1, has been studied electrochemically in 0.1 M Bu 4 NPF6 /CH 2 Cl 2 at a Pt working electrode with different scan rates. Three waves were obtained, 0.75, 0.95, and 1.09 V vs. Ag/AgCl at a scan rate of 500 mV/s, the three waves are reversible. The potentials are independent of the scan rate in the range 50 mV/s to 3 V/s. The current at the third wave is larger than those at the first two. KEY WORDS: Gold; formamidinates; tetrameric; luminescence; electrochemistry.

This manuscript is dedicated to Professor Achim Müller, a superb scientist and friend, in recognition of his 65th birthday. 1 Laboratory for Molecular Structure and Bonding, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255. 2 Departamento de Química, Grupo de Síntesis Química de La Rioja, UA-CSIC, Universidad de la Rioja, Madre de Dios 51, E-26006 Logron˜o, Spain. 3 To whom correspondence should be addressed. 253 1040-7278/03/0900-0253/0 © 2003 Plenum Publishing Corporation

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Mohamed, Abdou, Irwin, López-de-Luzuriaga, and Fackler, Jr.

INTRODUCTION Formamidinates, ArNHC(H)NAr, are versatile ligands, capable of forming flexible coordination modes which lead to various molecular arrangements [1]. Among the best known examples are the paddle wheels [2]. Transition metal complexes of formamidinates display novel electronic properties and recently show an extraordinary ability to stabilize high oxidation states [2]. Amidines are isoelectronic with carboxylic acids. Current interest in amidines arises from their ability to bridge between ions, facilitating short metal–metal bonding. Unsymmetrically substituted amidino groups are known to produce chiral complexes [1]. Pyrazolates are hard donor ligands, similar to formamidinate, and have been studied extensively in our laboratory (Sketch 1). Typically pyrazolate complexes of group 11 elements are trinuclear. Previous work [3] has led to the structural characterization of the homogenous series [M(m-3,5-Ph 2 Pz)]3 , M=Cu(I), Ag(I), Au(I). The hexanuclear gold complex [Au(m-3,5-Ph 2 Pz]6 also was obtained, although in poor yield [4]. Recently we reported the structure and luminescence of the base stabilized tetragold(I) cluster [(dppm)2 Au 4 (m-3,5-Ph 2 pz)2 ](NO 3 )2 [5]. During exploratory studies of the chemistry of group (11) metal pyrazolates as materials for the synthesis and study polynuclear Ag(I) complexes, the silver m-3,5-diphenylpyrazolate dimers of the trimer [Ag 3 (m-3,5-Ph 2 pz)3 ]2 has been synthesized [6]. Gold(I) cluster complexes have attracted much attention due to their intriguing structural diversity and often show short metal–metal bonding interactions. This is particularly obvious in tetragold(I) complexes, where short Au · · · Au contacts are often observed as a result of the relativistic effects. An understanding of the aurophilic bonding interactions between closed shell d 10 gold centers has attracted much attention due to the ability of a number of them to exhibit rich luminescence properties [7]. Previous studies have shown that formamidinates form dimers silver(I) and copper(I) [8–10]. The steric bulk of alkyl substituents on the NCN moiety is expected to contribute significantly to determining the most stable structural arrangement. C-alkyl functionalized formamidinates form tetrameric and trimeric structural motifs with silver(I) [11]. R

R

Ph

Ph -

N

-

N

N

Sketch 1. Formamidinate and Pyrazolate ligands.

N

Gold(I) Formamidinate Clusters

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This paper reports tetranuclear gold(I) formamidinate clusters. Reaction of Au(THT)Cl (THT=tetrahydrothiophene) with sodium formamidinates produces the tetragold(I) clusters [Au(ArNC(H)NAr)]4 that are structurally and spectroscopically described here. These complexes exhibit aurophilic Au · · · Au bonding commonly found in many gold complexes. Recently we were also able to isolate a trinuclear gold(I) formamidinate species. The complexes reported here exhibit luminescence under UV excitation in the solid state at RT and 77 K. The complexes are the first tetragold(I) cluster species from group 11 elements that show fluorescence at room temperature. Preliminary electrochemical results indicate reversible oxidation of 1. EXPERIMENTAL Synthesis of Formamidine Ligands All formamidine ligands were synthesized by modified literature procedures [11]. An example synthesis is described here: Triethyl orthoformamate (orthoester) (0.4 mol) and the aniline derivative (0.8 mol) were mixed and the reaction mixture was heated to 140–160° in a reflux vessel for 1–2 hr. The reaction mixture was distilled at the same temperature to remove ethanol and left at room temperature to solidify. The solid was extracted with warm toluene and left overnight at room temp or in the refrigerator to give white blocks in yields of 70–85%. All formamidine ligands were characterized by 1 H NMR. Synthesis of Gold Formamidinate Complexes, 1 4-methoxyaniline (256 mg, 1 mmol) was stirred with (40 mg, 1 mmol) of NaOH in THF for 1 hr. Gradually the colorless solution turned yellow. Au(THT)Cl (320 mg, 1 mmol) was added and stirring continued for additional 12 hr. The solution was filtered over Celite 545 bed after treatment with Darco. The volume was decreased under vacuum to 5 ml and ether was added to form an off-white precipitate. The product was filtered and washed with cold ether (3 × 10 ml) to give the tetragold cluster product. The product is air stable in the solid state and in a THF solution. Gold(I) N,N-4-methoxydiphenylamidinate, 1 Yield=225 mg, 50%. 1 H NMR (CDCl 3 ): d 3.70(s, 24H, CH 3 ), 6.61 (d, 16H, CH phenyl), 7.05 (br, 16H, CH phenyl), 8.19 (s, 4H, CH amidinate).

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13

C NMR: d 25.8 (CH 3 ), 122.6 (phenyl), 129.2 (phenyl), 133.1 (phenyl), 148.2 (phenyl), and 165 (amidinate C). IR (Nujol): 1611, 1577, 1295, 1109, 825, 770 cm −1.

Gold(I) N,N-3,5-dichlorodiphenylamidinate, 2 Yield=334 mg, 62%. 1 H NMR (CDCl 3 ): d 6.93 (s, 8H, CH phenyl), 7.12 (s, 8H, CH phenyl), 7.29 (s, 8H, CH phenyl), 8.23 (s, 4H, CH amidinate). IR (Nujol): 1607, 1580, 1573, 1567, 1555, 1295, 1240, 1170, 1030, 822, 720 cm −1.

Gold(I) N,N-4-methyldiphenylamidinate, 3 Yield=240 mg, 57%. 1 H NMR (CDCl 3 ): d 2.22(s, 24H, CH 3 ), 6.86 (d, 16H, CH phenyl), 7.02 (br, 16H, CH phenyl), 8.25 (s, 4H, CH amidinate). 13 C NMR: d 20.8 (CH 3 ), 122.6 (phenyl), 129.2 (phenyl), 133.1 (phenyl), 148.2 (phenyl), and 165 (amidinate C). IR (Nujol): 1613, 1579, 1573, 1210, 818 cm −1.

Structure Determination Data were collected using a Siemens (Bruker) SMART CCD (charge coupled device) based diffractometer equipped with a LT-2 low-temperature apparatus operating at 110 K. A suitable crystal was chosen and mounted on a glass fiber using cryogenic grease. Data were measured using omega scans of 0.3° per frame for 60 seconds, such that a hemisphere was collected. The first 50 frames were recollected at the end of data collection as a monitor for decay. No decay was detected. Cell parameters were retrieved using SMART [12] software and refined using SAINT [13] on all observed reflections. Data reductions were performed using SAINT software, which corrects for Lp and decay. Absorption corrections were applied using SADABS [14]. The structures were solved by direct methods using SHELXS-97 [15] and refined by least squares on F 2, with SHELXL97 incorporated in SHELXTL-PC V 5.03 [16]. The structures were determined in the space groups P2 1 /c, P2/c, C2/c by analysis of systematic absences. Hydrogen atom positions were calculated by geometrical methods and refined as a riding model. The crystallographic details for 1–3 are given in Table I. Selected bond distances and angles are presented in Tables II–IV.


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Table I. Crystal Data, Data Collection, and Structure Refinement for 1–3 Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group a (Å); b (Å); (°) c (Å); Volume (Å 3 ) Z Density (cal.) (Mg/m 3 ) Abs. coefficient (mm −1 ) Crystal size (mm 3 ) Theta range (°) Reflections collected Data/restraints/parameters GOF on F 2 R1, wR2 [I > 2s(I)]

C 60 H 60 Au 4 N 8 O 8 1809.03 110 (2) 0.71073 Monoclinic P2 1 /c 14.906(3) 20.501(4), 95.602(4) 22.226(5) 6759(3) 4 1.778 8.708 0.25 × 0.15 × 0.09 1.35 to 28.29 70562 15733/0/729 0.995 0.0714, 0.215

C 54 H 34 Au 4 N 8 Cl 16 O 2157.65 110 (2) 0.71073 Monoclinic P2/c 22.793(5) 12.962(3), 94.73(3) 23.236(5) 6842(2) 4 2.120 9.220 0.20 × 0.10 × 0.09 0.9 to 23.66 48479 9843/0/487 1.003 0.0825, 0.207

Table II. Bond Distances (Å) and Angles (°) for 1 Bond lengths (Å) Au(1) · · · Au(2) Au(2) · · · Au(3) Au(3) · · · Au(4) Au(1) · · · Au(4) Au(1)–N(1) Au(1)–N(8) Au(2)–N(7) Au(2)–N(6) C(1)–N(1) C(1)–N(2)

2.9016(8) 3.0057(9) 2.9265(9) 2.9447(10) 2.022(11) 2.047(11) 2.029(11) 2.057(11) 1.322(17) 1.306(17) Bond angles (°)

Au(1) · · · Au(4) · · · Au(3) Au(2) · · · Au(3) · · · Au(4) Au(3) · · · Au(2) · · · Au(1) Au(4) · · · Au(1) · · · Au(2) N(2) · · · Au(4) · · · Au(3) N(4) · · · Au(3) · · · Au(2) N(5) · · · Au(3) · · · Au(2) N(4) · · · Au(3) · · · Au(4) N(3)–C(16)–N(4) N(2)–C(2)–N(1) N(7)–Au(4)–N(6) N(5)–Au(3)–N(4) N(7)–Au(2)–N(6) N(1)–Au(1)–N(8)

109.62(2) 70.273(18) 108.62(2) 71.473(18) 100.8(3) 104.0(3) 79.8(3) 82.0(3) 127.2(12) 126.5(13) 173.6(5) 175.2(4) 173.3(4) 175.8(4)

C 60 H 60 Au 4 N 8 1681.03 110 (2) 0.71073 Monoclinic C2/c 19.75(2) 11.281(12), 90.92(2) 24.10(2) 5367(9) 4 2.080 10.948 0.35 × 0.20 × 0.10 1.69 to 23.37 16599 3869/0/330 0.876 0.0376, 0.0783

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Mohamed, Abdou, Irwin, López-de-Luzuriaga, and Fackler, Jr. Table III. Bond Distances (Å) and Angles (°) for 2 Bond lengths (Å) Au(1) · · · Au(2A) Au(1) · · · Au(2) Au(1)–N(1) Au(1)–N(2) Au(2)–N(3) Au(2)–N(4) C(1)–N(3) C(2)–N(3) C(1)–N(2)

2.8662(14) 2.9699(14) 2.021(15) 2.049(17) 2.051(15) 2.035(16) 1.31(3) 1.44(2) 1.43(3)

Bond angles (°) Au(2A) · · · Au(1) · · · Au(2) Au(1A) · · · Au(2) · · · Au(1) N(1)–Au(1)–N(2) N(4)–Au(2)–N(3) N(2)–Au(1) · · · Au(2A) N(2)–Au(1) · · · Au(2) N(1)–Au(1) · · · Au(2) N(1)–Au(1) · · · Au(2A) N(4)–Au(2) · · · Au(1A) N(3)–Au(2) · · · Au(1) N(3)–Au(2) · · · Au(1A) N(4)–Au(2) · · · Au(1) N(1)–C(14)–N(4A) N(3)–C(1)–N(2)

91.53(4) 88.30(4) 176.2(7) 177.9(7) 100.9(5) 84.4(5) 98.6(5) 81.4(5) 82.7(5) 80.3(4) 95.4(5) 100.6(6) 129(2) 126(2)

Electrochemical Studies Cyclic voltammetry experiments were conducted using a Bioanalytical Systems Inc. electrochemical analyzer, Model 100 under computer control. CV measurements were performed in methylene chloride with 0.1 M Bu 4 NPF6 as supporting electrolyte. Fresh solutions containing supporting electrolyte (10 ml) were prepared prior to each CV experiment. Each solution was deoxygenated by purging with nitrogen for 2–5 minutes. Background CV’s were acquired before the addition of the gold complexes. A three-electrode system was used, comprised of a platinum (1.6 mm diameter) working electrode, a platinum wire auxiliary electrode, and a silver/silver chloride (Ag/AgCl) reference electrode. The working electrode was wiped prior to each experiment with fine sand paper and rinsed. Potentials are reported vs. Ag/AgCl at room temperature and are not corrected for junction potentials. Each CV experiment was repeated a number of times at different scan rates.


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Table IV. Bond Distances (Å) and Angles (°) for 3 Bond lengths (Å) Au(1) · · · Au(2) Au(2) · · · Au(3) Au(3) · · · Au(2A) Au(1) · · · Au(2A) Au(1)–N(1) N(3)–C(2) C(1)–N(3) C(1)–N(4)

2.968(2) 3.103(2) 3.103(2) 2.968(2) 2.019(7) 1.452(11) 1.308(11) 1.329(11)

Bond angles (°) N(1)–Au(1)–N(1A) N(1A)–Au(1) · · · Au(2A) N(1A)–Au(1) · · · Au(2) N(3)–Au(2)–N(2) N(3)–Au(2) · · · Au(1) N(2)–Au(2) · · · Au(1) N(3)–Au(2) · · · Au(3) N(2)–Au(2) · · · Au(3) Au(2A) · · · Au(1) · · · Au(2) Au(1) · · · Au(2) · · · Au(3) Au(2) · · · Au(3) · · · Au(2A) N(2)–C(16)–N(1A) N(4A)–Au(3)–N(4) N(4A)–Au(3) · · · Au(2) N(4)–Au(3) · · · Au(2) N(3)–C(1)–N(4)

174.8(4) 102.4(2) 80.2(2) 172.0(3) 106.6(2) 81.0(2) 79.6(2) 106.6(2) 120.52(7) 63.59(7) 112.30(7) 125.8(8) 168.4(4) 107.9(2) 78.8(2) 128.0(9)

Photoluminescence Studies Emission and excitation spectra were recorded on a SLM AMINCO, Model 8100 spectrofluorometer equipped with a xenon lamp. Spectra were corrected for instrumental response. Solid-state low-temperature measurements were made using a cryogenic sample holder of local design. Powder samples were attached to the holder with a mixture of copper powder, Cryogen oil (used for mounting crystals for x-ray structures) and collodion (a ether and alcohol soluble transparent nitrocellulose). The glue was scanned for a baseline subtraction. Liquid nitrogen was used to obtain the 77 K measurements. RESULTS AND DISCUSSION Molecular Structures The tetranuclear gold(I) clusters were synthesized in ’ 55% yield by the metathesis of sodium formamidinate with gold(I) chloride in THF.

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1

H NMR at room temperature shows a singlet at 8.2 ppm for methine protons, suggesting that all the fomamidinate ligands are symmetry related. The tetranuclear Au(I) formamidinate complex [Au 4 (ArNC(H)NAr)4 ], Ar=C 6 H 4 -4-OMe, 1, crystallizes as colorless needles in the monoclinic space group P2 1 /c. The molecular structure of 1 is shown in Fig. 1. The four gold atoms are located at the corner of a rhomboid with the formamidinate ligands bridged above and below the near plane of the four Au(I) atoms. The average Au · · · Au distance is 2.94 Å, typical of Au(I) · · · Au(I) interactions. It appears that in all tetragold species, a short aurophilic bonding supports their stability. The angles at Au · · · Au · · · Au are acute (70.26(2)–71.47(2)°) and obtuse (108.64(2)–109.62(2)°). The N–Au–N angles of 175° (av.) show a deviation from linearity consistent with Au · · · Au interactions. The four gold atoms in 1 lie in a near plane with a small torsion angle of 0.7° at Au(1)Au(2)Au(3)Au(4). The rhomboidal geometry of the Au4 units seems to be attributable to steric factors. Eclipsed phenyls and interplanar separations probably destabilize the structure. The NCN linkage is slanted by weak crossing aurophilic interaction between Au(2) and Au(4) (3.417 Å). As a result, a torsion angle emerged at Au(3)N(5)N(6)Au(4) of 11.5°. The bite angle at NCN increased from 124.70° in the ligand, which crystallizes as dimers linked through

Fig. 1. View of 1 showing atomic numbering. Thermal ellipsoid drawing at the 50% level.


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NH · · · N hydrogen bonds, to 126.05° in the complex [17]. There is a slight twist in the aryl rings out of the formamidinate plane. The molecular structure of the protonated ligand is dinuclear through H-bonding. Replacing the methoxy group by the chloride substituents, 2, resulted in a nearly perfect square with equal Au · · · Au distances (2.91 Å) and angles close to ’ 90° (Fig. 2). The N–Au–N angles average to ’ 178°. The methyl group, 3, resulted in a significant distortion in the Au 4 square to Au · · · Au of 2.96 and 3.10 Å. The N–Au–N decreased from 178° in 1 to 168–174° in 3 (Fig. 3). The angles at Au · · · Au · · · Au in 3 are acute 63.59° and obtuse 112.30 and 120.52°. In all complexes, the NC bond length in NCN is 1.3 Å indicating that delocalization across the amidinate bridge is retained. Table V gives Au · · · Au distances with Au · · · Au · · · Au and ligand X–Au–Y angles for several tetranuclear Au(I) complexes. Similar structural arrangements have been found in the tetrameric 1,3-diphenyltriazenidogold(I) complex, [Au(PhNNNPh)]4 (Au · · · Au=2.85 Å) [19], [Au 4 (CH 3 CS 2 )4 ] (Au · · · Au=3.01 Å [20] and [(dppm)2 Au 4 (3,5-Ph 2 pz)2 ] (NO 3 )2 (Au · · · Au=3.28 Å), (Ph 2 pz=m-3,5-diphenylpyrazolate) [5]. The gold(I) atoms in the tetranuclear gold pyrazolate complex [Au 4 (m-4- t Bu-pz)4 ] form rhomboid with Au · · · Au distances of 3.1155(7)–3.1886(7) Å [21]. The Au(I) atoms bridged by the more flexible formamidinate ligands show shorter Au · · · Au distances than those bridged by the rigid pyrazolate ligands (i.e., 2.9 Å vs. 3.1 Å) [11].


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Silver centers in the C-alkyl functionalized formamidinate tetranuclear silver(I) cluster have a planar rhombic arrangement with bridging amidinate ligands above and below the Ag 4 plane, typical of the structures reported here. The Ag · · · Ag distances within the range 2.87–2.98 Å.

Table V. Bond Distances (Å) and Angles (°) for Selected Neutral Tetranuclear Gold(I) Complexes

a b

Complex

Au · · · Au (av)

Au(1) · · · Au(2) · · · Au(3) (av)

[AuCl(pip)]4 [Au(PhNNNPh)]4 [Au (CH 3 CS 2 )]4 [Au(m-4- t Bu-pz)]4 [Au(N(SiMe 3 )2 ]4 [Au(form)]4

3.30 2.85 3.01 3.11 3.01 2.86–3.01

88.3 89.9 89.9

Too small to permit analysis. See text and tables.

X–Au–Y 176.0 176.0 167.6 175.0

90 b

b

Yield (%) a

25 47 18.9 ’ 55

Ref. 18 19 20 21 22 This work


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1.2

Absorbance (a.u.)

1 0.8 0.6 0.4 0.2 0 230

280

330

380

430

Wavelength (nm)

Fig. 4. UV-vis of 1 (1.8 × 10 −5 M) in CH 2 Cl 2 .

Photoluminescence

Intensity, arb units

The absorption spectra of the ligand (Hform) show a high-energy (HE) peak centered at 290 nm and a low energy (LE) shoulder at 330 nm in CH 2 Cl 2 and CH 3 CN (Fig. 4). These two bands in the spectrum of the ligand are red shifted in the cluster, 1, to 315 nm (Emax =28,000 L/M-cm) and 360 nm (Emax =12,800 L/M-cm) in CH 2 Cl 2 (300 and 375 nm in CH 3 CN) with a third band in CH 2 Cl 2 at 260 nm (Emax =50,000 L/M-cm). The excitation spectrum is consistent with the low energy absorption spectrum, near 375 nm. The absorption spectra of sodium formamidinate in CH 2 Cl 2 and CH 3 CN show profiles typical of formamidine (Hform) with a slight red shift. The tetranuclear gold(I) complexes show a bright blue-green luminescence under UV light, with an emission at ’ 490 nm and a weak emission at ’ 530 nm in the solid state, at room temp and 77 K (Fig. 5). Solutions

250

290

330

370

410

450

490

530

570

610

Emis s ion, nm

Fig. 5. Excitation and Emission of 3 at 77 K.

650

690

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of 1–3 in CH 2 Cl 2 , THF, and acetone are non-emissive at room temperature but emit when frozen. Evaporation of the solvent forms a film that emits at room temperature. The small Stokes shift and symmetric band profiles between the excitation and emission spectra suggest the high energy emission to be fluorescence. Preliminary lifetime data for 1 at 77 K suggests the presence of two lifetimes, one with a 2.28 ns lifetime and another (a lower energy emission) with a 5 ms lifetime upon excitation at 350 nm. The intensity of the emission at ’ 490 nm is enhanced and sharpened by lowering the temp to 77 K. The low energy emission (shoulder in Fig. 5) may be associated with a triplet excited state. It dominates the emission when a 400 nm excitation is used. Emission spectra of [Cu 2 (PhNNNPh)2 ] [23] and Ag 2 (form)2 [8] were assigned to a fluorescence originate using a p–p* intraligand/MLCT model based on the vibronic structure that is observed. In silver(I) formamidinate the small Stokes shift between excitation and emission spectra (the separation between the maxima, 2210 cm −1), and the emission life-time ([ 1 ns) support fluorescence. However, the [Au 4 Cl 4 (piperidine)4 ] emits at 700 nm and the rhomboidal [Au 4 (dithioacetate)4 ] emits at 743 nm with an emission assigned to a metal-centered 5d-6s transition which is modified by the metal–metal interaction in the Au 4 unit [24]. Electrochemistry The oxidation of the formamidinate cluster, 1, has been studied electrochemically in 0.1 M Bu 4 NPF6 /CH 2 Cl 2 at a Pt working electrode with different scan rates. Three waves were obtained at 0.75, 0.95, and 1.09 V vs.

Current (µA)

2µ A

1.3

1.2

1.1

1

0.9

0.8

0.7

0.6

0.5

Potential (mv) Vs Ag/AgCl

Fig. 6. Cyclic voltammogram of 1 mmol of 1 using Pt working and auxiliary electrodes and Ag/AgCl reference in 0.1 M Bu 4 NPF6 /CH 2 Cl 2 at 500 mV/s.


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Ag/AgCl with a scan rate of 500 mV/s, the three waves are reversible (Fig. 6). The potentials are independent of the scan rate in the range 50 mV/s to 3 V/s. The current for the third wave is larger than those at the first two waves. ACKNOWLEDGMENT We are grateful to the Robert A. Welch Foundation of Houston, Texas for financial support of this work. SUPPLEMENTAL MATERIALS AVAILABLE Crystallographic data for the structural analysis of 1, 2, and 3 has been deposited with the Cambridge Crystallographic Data Center, CCDC, No. 209567–209569. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, United Kingdom, by quoting the publication citation and the deposition number. [Fax: (int. code) +44 (1223) 336–033, E-mail: [email protected], URL: http://www.ccdc.cam.ac.uk]. SYNOPSIS A novel tetragold(I) formamidinate complex showed room temperature luminescence is presented. The tetranuclear gold(I) complexes show a bright blue-green luminescence under UV light, with an emission at ’ 490 nm and a weak emission at ’ 530 nm in the solid state, at room temp and 77 K. The average of Au · · · Au distances is ’ 2.9 Å.

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