Au(I) and Au(III) Complexes bearing N-Heterocyclic Carbenes: Synthesis, Crystal Structures and Photochemistry J. Hölbling 1, M. List 2, M. Zabel 3, U. Monkowius 1 References Introduction 1 Institut für Anorganische Chemie, Johannes Kepler Universität Linz, A-4040 Linz, Austria 2 Institut für Chemische Technologie Organischer Stoffe, Johannes Kepler Universität Linz, 3 Universität Regensburg, Zentrale Analytik, Röntgenstrukturanalyse, D Regensburg, Germany Summary N-Heterocyclic carbenes (NHC) are established ligands for Au(I) and Au(III). [1] Their synthesis and crystal structures were reported frequently. By using NHC ligands, both oxidation states +I and +III of the gold atom are well stabilized. Thus, the “switching” between Au(I) and Au(III) becomes feasible [2] which is a prerequisite for two-electron (photo)catalysis. Therefore, we synthesized complexes of the type (NHC)AuBr 3 (A) and [(NHC) 2 AuBr 2 ]Br (B) and explored their photochemical behavior in solution. Just recently, Teets and Nocera reported the halogen photoreductive elimination from similar Phosphine-Au(III)-X 3 (X = Cl, Br) complexes. [3] Synthesis of mono-substituted Au(I) and Au(III) complexes Crystal structure of (BnMeIm)AuI 2 Br Photochemical reactivity of [(Bn 2 Im) 2 AuBr 2 ]Br [1] R. Jothibasu, H.V. Huynh, L.L. Koh, J. Organomet. Chem. 693 (2008) 374; H.G. Raubenheimer, P.J. Olivier, L. Lindeque, M. Desmet, J. Hrušak, G.J. Kruger, J. Organomet. Chem. 544 (1997) 91. [2] P. Kühlkamp, H.G. Raubenheimer, J.S. Field, M. Desmet, J. Organomet. Chem. 552 (1998) 69. [3] T. S. Teets, D. G. Nocera, J. Am. Chem. Soc. 131 (2009) A B Synthesis of bis-substituted Au(I) and Au(III) complexes Bond lenghts [Å]Bond angles [°] Au1-C12.012(8)C1-Au1-I (5) Au1-Br (9)C1-Au1-I (2) Au1-I (7)C1-Au1-Br (2) Au1-I (8) Formula: C 11 H 12 N 2 AuBrI 2 Crystal system: triklinic,, Z = 2 a = (10) Å, b = (10) Å, c = (14) Å  = (14)°,  = (14)°,  = (13)° Formula: C 22 H 24 N 4 AuBr 3 Crystal system: monoklinic, P2 1 /c, Z = 4 a = (18) Å, b = (8) Å, c = (16) Å  = 90°,  = (12)°,  = 90° Crystal structure of [(BnMeIm) 2 AuBr 2 ]Br Crystal structure of (BnMeIm)AuBr Formula: C 11 H 12 N 2 AuBr Crystal system: orthorhombic, P , Z = 4 a = 6.631(4) Å, b = (6) Å, c = (11) Å Bond lenghts [Å]Bond angles [°] Au1-C11.993(7)C1-Au1-Br1176.8(2) Au1-Br12.384(1) Bond lenghts [Å]Bond angles [°] Au1-C12.018(5)C1-Au1-C (2) Au1-C (5)C1-Au1-Br189.6(1) Au1-Br (6)C1-Au1-Br289.5(1) Au1-Br (6) ESI-Mass-Spectrometry UV-Vis-Spectroscopy Irradiation of a 3 mM methanolic solution of [(Bn 2 Im) 2 AuBr 2 ]Br with = 280 nm. Before irradiation: After 18 min irradiation: Z-Isomer E-Isomer A series of mono- and bis-NHC-gold-complexes (RR’Im)AuBr, (RR’Im)AuBr 3, [(RR’Im) 2 Au]Br, [RR’Im) 2 AuBr 2 ]Br have been synthesized with R, R’ = Me, Et, Bn. Convenient synthesis of mono-substituted (NHC)Au(I)Br complexes is possible via silver salt method starting from (NHC)AgBr and (tht)AuBr. Reaction between the free carbene and (R 2 S)AuX (R 2 S = Me 2 S or tetrahydrothiophene, X = Cl, Br) yields the bis-substituted [(NHC) 2 Au]Br complexes in good yields and purities. These compounds are not accessible via the silver salt method. Direct synthesis of NHC-Au(III) complexes is not possible from a Au(III) precursor. Instead, the NHC-Au(I)-complexes are smoothly oxidized with Br 2 to yield (NHC)AuBr 3 or [(NHC) 2 AuBr 2 ]Br. Upon irradiation with ultraviolet light (λ = nm), both the mono-substituted (NHC)AuBr 3 and the bis-substituted [(NHC) 2 AuBr 2 ]Br show photoreductive elimination of Br 2 in acetonitrile or methanolic solution. The synthesis of Au-complexes bearing unsymmetrical substituted NHC ligands yields a mixture of E/Z-isomers. No changes of the intensities of the 1 H-NMR signals at 50 °C indicate a high rotational barrier of the Au(III) compounds. In the UV-vis-spectrum of [(Bn 2 Im) 2 AuBr 2 ]Br in MeOH, the low energy band at 316 nm is assigned to a Br→ Au ligand-to-metal-charge-transfer (LMCT). The high energy signals at 261 nm and below are due to intraligand transitions. Upon irradiation at = 280 nm, the LMCT is decreasing and the absorption spectrum of the homologues [(Bn 2 Im) 2 Au]Br occurs. The isosbestic points at 239 and 278 nm are indicative for a clean photo-reaction. No plasmon resonance at  520 nm is detectable. There are no spectral variations of the not-irradiated solution. After 18 min irradiation, no [(Bn 2 Im) 2 AuBr 2 ] + -cation is detected by ESI-MS.  Photoreductive elimination of Br 2 and formation of [(Bn 2 Im) 2 Au]Br.