Optical Response of the Cu 2 S 2 Diamond Core in Cu II 2 (NGuaS) 2 Cl 2 Matthias Witte, [a] Benjamin Grimm-Lebsanft, [b] Arne Goos, [b] Stephan Binder, [b] Michael R€ ubhausen, [b] Martin Bernard, [c] Adam Neuba, [c] Serge Gorelsky, [d] Uwe Gerstmann, [e] Gerald Henkel, [c] Wolf Gero Schmidt, [e] and Sonja Herres-Pawlis* [a] Density functional theory (DFT) and time-dependent DFT cal- culations are presented for the dicopper thiolate complex Cu 2 (NGuaS) 2 Cl 2 [NGuaS52-(1,1,3,3-tetramethylguanidino) ben- zenethiolate] with a special focus on the bonding mechanism of the Cu 2 S 2 Cl 2 core and the spectroscopic response. This complex is relevant for the understanding of dicopper redox centers, for example, the Cu A center. Its UV/Vis absorption is theoretically studied and found to be similar to other struc- tural Cu A models. The spectrum can be roughly divided in the known regions of metal d-d absorptions and metal to ligand charge transfer regions. Nevertheless the chloride ions play an important role as electron donors, with the thiolate groups as electron acceptors. The bonding mechanism is dissected by means of charge decomposition analysis which reveals the large covalency of the Cu 2 S 2 diamond core mediated between Cu d z 2 and S-S p and p* orbitals forming Cu-S r bonds. Meas- ured resonant Raman spectra are shown for 360- and 720-nm excitation wavelength and interpreted using the calculated vibrational eigenmodes and frequencies. The calculations help to rationalize the varying resonant behavior at different optical excitations. Especially the phenylene rings are only resonant for 720 nm. V C 2016 Wiley Periodicals, Inc. DOI: 10.1002/jcc.24439 Introduction Dinuclear transition metal complexes have been, and still are, of vital interest due to their biological relevance and a lot of frontier work has been done to understand the bonding mechanism in these complexes. [1–5] In particular, the Cu A site as found in cytochrome c oxidase or the nitrous oxide reduc- tase is one of the important redox centers in biological charge transfer processes. [6–12] Hence, the dinuclear thiolate bridged copper core is a widely studied system in literature. Due to a lot of research effort its electronic as well as geometric struc- ture is well determined by extended X-ray absorption fine structure, [9,10] electron paramagnetic resonance (EPR) [13,14] (hence S 5 1/2), optical absorption, [15,16] low-temperature mag- netic circular dichroism, [17] resonance Raman spectroscopy, [18] and X-ray crystallography. [6–8,19] It turned out that is has a dinuclear mixed valent (MV) [Cu 1.51 ...Cu 1.51 ] core bridged by two S(Cys) ligands. The Cu...Cu distance is quite small and ranges from 2.3 to 2.6 A ˚ indicating a direct Cu–Cu interaction. The nature of the Cu thiolate bond is well known to be highly covalent as it is also occurring in the blue copper site, like being found in plastocyanin [20] which has been studied considerably. [21–25] As for the biological importance, a noteworthy amount of effort has been put into synthesizing functional Cu A model complexes. Tolman et al. succeeded first, creating a MV com- plex (Fig. 1c). [26] A theoretical description was given by Solo- mon and coworkers. [29,30] The molecule has a rather large Cu– Cu distance of 2.9 A ˚ (see Fig. 1c) excluding a direct Cu–Cu interaction. Further the complex is not reversibly reducible. A fully functional biomimetic model complex has been synthe- sized by Duboc and coworkers. [31] Although it also has a rather large Cu–Cu distance of 2.9 A ˚ , it shows a, compared to natural Cu A , slow but reversible reducibility. Henkel and coworkers synthesized the thiolate bridged dinuclear copper complex Cu 2 (NGuaS) 2 Cl 2 (with NGuaS:5o- SC 6 H 4 N 5 C(NMe 2 ) 2 ) with a diamond core which is shown in Figure 1a. [27] The complex is formed by conversion from the corresponding Cu(I) disulfide-guanidino complex. [32,33] Guani- dine ligands, being p and r donors to metal bonds, play an important role in the stabilization of copper complexes. [34–41] Contrary to Cu A , the diamond core of the Henkel complex is [a] M. Witte, S. Herres-Pawlis Lehrstuhl F€ ur Bioanorganische Chemie, Fachgruppe Chemie, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany E-mail: sonja.herres-pawlis@ac.rwth-aachen.de [b] B. Grimm-Lebsanft, A. Goos, S. Binder, M. R€ ubhausen Institut f € ur Nanostruktur- und Festk € orperphysik and Center for Free Electron Laser Science, University of Hamburg, Luruper Chaussee 149, 22761, Hamburg, Germany [c] M. Bernard, A. Neuba, G. Henkel Lehrstuhl F€ ur Anorganische Chemie, Universit € at Paderborn, Warburger Str. 100, Paderborn 33098, Germany [d] S. Gorelsky Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada [e] U. Gerstmann, W. G. Schmidt Lehrstuhl F€ ur Theoretische Physik, Universit € at Paderborn, Warburger Str. 100, Paderborn 33098, Germany Dedicated to Prof. Dr. Peter Kl € ufers on the occasion of his 65th birthday. Contract grant sponsor: Deutsche Forschungsgemeinschaft; Contract grant number: FOR1405 V C 2016 Wiley Periodicals, Inc. Journal of Computational Chemistry 2016, 37, 2181–2192 2181 FULL PAPER WWW.C-CHEM.ORG