ARTICLE Received 4 Jan 2013 | Accepted 28 Jun 2013 | Published 26 Jul 2013 DOI: 10.1038/ncomms3223 Probing the nature of gold–carbon bonding in gold–alkynyl complexes Hong-Tao Liu1,*, Xiao-Gen Xiong2,*, Phuong Diem Dau1, Yi-Lei Wang2, Dao-Ling Huang1, Jun Li2 & Lai-Sheng Wang1 Homogeneous catalysis by gold involves organogold complexes as precatalysts and reaction intermediates. Fundamental knowledge of the gold–carbon bonding is critical to understanding the catalytic mechanisms. However, limited spectroscopic information is available about organogolds that are relevant to gold catalysts. Here we report an investi- gation of the gold–carbon bonding in gold(I)–alkynyl complexes using photoelectron spec- troscopy and theoretical calculations. We find that the gold–carbon bond in the ClAu–CCH À complex represents one of the strongest gold–ligand bonds—even stronger than the known gold–carbon multiple bonds, revealing an inverse correlation between bond strength and bond order. The gold–carbon bond in LAuCCH À is found to depend on the ancillary ligands and becomes stronger for more electronegative ligands. The strong gold–carbon bond underlies the catalytic aptness of gold complexes for the facile formation of terminal alkynyl–gold intermediates and activation of the carbon–carbon triple bond. 1 Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA. 2 Department of Chemistry & Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to L.S.W. (email: [email protected]). NATURE COMMUNICATIONS | 4:2223 | DOI: 10.1038/ncomms3223 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3223 omogeneous catalysis by gold complexes has been a – undergoing a remarkable development over the past ClAuCCH D Hdecade, in particular, for alkyne activation1–7. Numerous XA C reaction intermediates have been proposed, but in general it is a challenge to directly observe and characterize these transient B species. Clearly, the nature of the bonding between the Au centre and its substrates is critical to the understanding of the E mechanisms of homogeneous gold catalysis. Well-defined gold complexes can serve as model systems to probe the Au-substrate bonding. However, although it is generally known that the strong relativistic effects8 give rise to the many unique chemical 9–11 b properties for gold , there is a scarcity of experimental – studies of the Au–C bonding in organogold complexes that are IAuCCH E most relevant to understanding the viability of catalytic 12,13 D intermediates and mechanisms . As a powerful experimental A B technique to probe the electronic structure and chemical bonding X of molecules, photoelectron spectroscopy has been used to study a C series of Au–CN complexes recently14,15 but has not been applied to any organogold complexes. Here we present the first photoelectron spectroscopy investiga- Relative electron intensity Relative electron intensity tion of a series of Au(I)–alkynyl complexes, LAuCCH À (L ¼ Cl, I, CCH), accompanied by ab initio calculations, to probe the c – XA nature of the Au–C bonding. Vibrationally resolved photoelec- Au(CCH)2 tron spectra suggest that the Au–C bonding in LAuCCH À is extremely strong. Theoretical calculations show that the Au–C À bond in ClAu–CCH is one of the strongest known gold bond C with a dissociation energy of 5.01 eV. Further theoretical studies B À find that the Au–C single bond in ClAuCCH is even stronger D than the known Au ¼ C double bond in ClAu ¼ CH2 and the AuC triple bond in ClAuC, uncovering an unprecedented inverse correlation between bond strength and bond orders for Relative electron intensity À gold. Such strong LAu–CCH bonding implies that the 345 6 7 formation of terminal alkynyl–gold is thermodynamically favour- Binding energy (eV) able, which is known to have important roles in alkyne activation. Figure 1 | Photoelectron spectroscopy. Measurements performed at 157 nm and room temperature for (a) ClAuCCH–,(b) IAuCCH– and Results (c) Au(CCH)2. Photoelectron spectroscopy. We produced the LAuCCH À complexes (L ¼ Cl, I, CCH) by electrospray ionization of an the X and A bands overlap and the D band is weak and broad. For À acetonitrile solution dissolved with HCC–MgCl and gold Au(CCH)2 at 20 K, spectra are taken at both 266 nm (Fig. 2c) iodide. The photoelectron spectra were obtained for these Au(I) and 245 nm (Fig. 2d). A vibrational progression with a frequency À 1 complexes at various temperatures and several detachment laser of 480 cm is observed for the X band, corresponding to the wavelengths. Figure 1 presents the room temperature spectra of Au–C stretching mode in the neutral Au(CCH)2 ground state. LAuCCH À (L ¼ Cl, I, CCH) measured at 157 nm. Higher-reso- Two vibrational progressions are observed for the A band À 1 lution spectra for the low-binding energy features were obtained (Fig. 2d) with frequencies of 450 and 1,850 cm , respectively. at lower photon energies (245 and 266 nm) for vibrationally cold The lower frequency mode should be due to the Au–C stretching anions at 20 K, as shown in Fig. 2. and the higher-frequency mode due to the CC stretching in À The 157-nm spectrum of ClAuCCH displays six major the first excited state of neutral Au(CCH)2. The spectral features À detachment bands labelled with letters (X, A–E; Fig. 1a). The X and vibrational structures for Au(CCH)2 are similar to those for À and A bands are vibrationally resolved in the 20 K spectrum at ClAuCCH . 245 nm (Fig. 2a). The observed vibrational frequency of the X The adiabatic detachment energies (ADEs), vertical detach- band (380 cm À 1) corresponds to the ClAu stretching mode in the ment energies (VDEs) and the resolved vibrational frequencies of neutral ground state of ClAuCCH. Two vibrational progressions the X and A bands for all three complexes are summarized in are resolved in the A band: a low-frequency mode of 360 cm À 1 Table 1, where they are compared with the theoretical calcula- mainly corresponding to ClAu stretching and a high-frequency tions. The VDEs and assignments for all the observed features are mode of 1960 cm À 1, which should be due to the CC stretching. given in Supplementary Table S1. We note that the Au–C À À The 157-nm spectrum of IAuCCH also exhibits six major vibrational frequency observed in the spectra of Au(CCH)2 is detachment bands (Fig. 1b). For the 245-nm spectrum of larger than the Au–C frequency observed in the photoelectron À À À 1 14 IAuCCH at 20 K (Fig. 2b), only a very short progression of spectra of Au(CN)2 reported previously (400 cm ) . This the CC stretching mode with a frequency of 1,860 cm À 1 is observation is surprising, suggesting that that Au–CCH bond discernible for the X and A bands. The relatively broad peak may be even stronger than the Au–CN bond in their widths of the X and A bands suggest that there are unresolved corresponding neutral ground states. low-frequency vibrational excitations, most probably because of the low-frequency I–Au stretching, similar to the ClAu stretching observed in the ClAuCCH À spectrum. The 157-nm spectrum of Quantum chemical calculations and spectral interpretation. À Au(CCH)2 reveals five major detachment bands (Fig. 1c), where High-level theoretical calculations were performed for the three 2 NATURE COMMUNICATIONS | 4:2223 | DOI: 10.1038/ncomms3223 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3223 ARTICLE a A c X – ClAuCCH– Au(CCH)2 X A Relative electron intensity Relative electron intensity bdA A – IAuCCH– Au(CCH)2 X X Relative electron intensity Relative electron intensity 4 5 45 Binding energy (eV) – – – – Figure 2 | Photoelectron spectroscopy at 20 K. (a) ClAuCCH at 245 nm. (b) IAuCCH at 245 nm. (c) Au(CCH)2 at 266 nm. (d) Au(CCH)2 at 245 nm. The vertical lines indicate the resolved vibrational structures. Table 1 | Detachment energies and vibrational frequencies. Feature Final state ADE (eV) VDE (eV) Experimental* Theoreticalw Experimental* Theoreticalz Vibrational frequency (cm À 1)y – 2 ClAuCCH X P3/2 4.585 (15) 4.76 4.632 (15) 4.63 (4.90) 380 2 A P1/2 4.786 (15) 4.76 360, 1960 – 2 IAuCCH X P3/2 4.27 (3) 4.48 4.32 (3) 4.32 (4.59) 1860 2 A P1/2 4.72 (3) 4.68 1860 – 2 Au(CCH)2 X P3/2 g 4.504 (15) 4.61 4.504 (15) 4.50 (4.77) 480 2 A P1/2 g 4.604 (15) 4.58 450, 1850 Observed and calculated vertical (VDE) and adiabatic (ADE) detachment energies and vibrational frequencies for the first two detachment features of LAuCCH– (L ¼Cl, I, CCH). *The numbers in parentheses represent the experimental uncertainties in the last digits. Because of the elimination of vibrational hot bands at low temperatures and the resolution of vibrational – – – – structures, the ADEs for ClAuCCH and Au(CCH)2 are directly measured from the first vibrational peaks (Fig. 2a for ClAuCCH and Fig. 2c for Au(CCH)2), which correspond to the transitions from the ground vibrational level of the anions to those of the corresponding neutrals. The VDEs are measured from the strongest vibrational peaks in the ground-state transitions in each case. Because of unresolved low-frequency I–Au stretching vibrations in the case of IAuCCH– (Fig.