Contents lists available at ScienceDirect Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche Short communication Photo-isomerization of palladium(II) N,N-di-substituted acylthioureas: The role of free ligands and formation of mixed-ligand complexes Henry A. Nkabyo ⁎ , Robert C. Luckay, Klaus R. Koch Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1 Matieland 7602, South Africa GRAPHICALABSTRACT ARTICLEINFO Keywords: Palladium acylthioureas Photo-isomerization RP-HPLC Mixed-ligand complexes ABSTRACT Photo-irradiation of the cis-[Pd(L 1-4 -κS,O) 2 ] complexes with visible light leads to formation of trans-[Pd(L 1-4 - κS,O) 2 ] isomers in acetonitrile, as determined by reverse-phase HPLC (RP-HPLC). In the absence of light, the trans-[Pd(L 1-4 -κS,O) 2 ] isomers spontaneously revert to cis-[Pd(L 1-4 -κS,O) 2 ]. The trans-[Pd(L 1-4 -κS,O) 2 ]→ cis-[Pd (L 1-4 -κS,O) 2 ] reversion was found to be catalyzed by the presence of free HL ligands, and results in the formation of mixed-ligand complexes of the type cis-[Pd(L 1 -κS,O)(L 2 -κS,O)] as confirmed by RP-HPLC with ESI-MS de- tection. The mixed-ligand cis-[Pd(L 1 -κS,O)(L 2 -κS,O)] complex also undergoes photo-induced isomerism to form trans-[Pd(L 1 -κS,O)(L 2 -κS,O)] isomers in solution with visible light irradiation. 1. Introduction The presence of carbonyl (CO) and thioamidic moieties in the N,N- dialkyl-N’-acyl(aroyl)-thioureas (HL) enables coordination to transition metal ions most commonly through monobasic bidentate (L-κS,O), neutral mono-dentate (HL-κS), and neutral bidentate (HL-κS,O) modes. The predominant (L-κS,O) coordination mode results in the formation of the thermodynamically stable cis-[M II (L-κS,O) 2 ] complexes [1–15], with the loss of a proton. Potential uses of the (L-κS,O) complexes include inter alia antitumor [2,3], antibacterial [16], chemotherapeutic [17,18] agents, and for obtaining luminescent materials [19]. A re- versible protonation of the cis-[Pt II (L-κS,O) 2 ] complexes occurs in so- lutions of HX (X = Cl - ,Br - ) yielding mixtures of trans-or cis-[Pt(H 2 L- κS) 2 X 2 ] complexes, in which Pt(II) coordinates to HL through the sulfur atom in a monodentate manner [20,21]. Such (M-κS) coordination modes are usually observed in complexes of N-mono-substituted acyl- thiourea derivatives (H 2 L), and ascribed to intramolecular hydrogen bonding which causes ‘locking’ of the -C(O)NHC(S)NHR moiety thereby https://doi.org/10.1016/j.inoche.2020.108035 Received 22 May 2020; Received in revised form 14 June 2020; Accepted 14 June 2020 ⁎ Corresponding author. E-mail address: nkabyoh@sun.ac.za (H.A. Nkabyo). Inorganic Chemistry Communications 119 (2020) 108035 Available online 18 June 2020 1387-7003/ © 2020 Elsevier B.V. All rights reserved. T