Reversible photo-induced isomerization of cis-[M(L-κS,O)2] (M=PdII, PtII) complexes with N,N-dialkyl-N’-acyl(aroyl)thiourea ligands (HL): Key to the isolation of novel geometric trans-[M(L-κS,O)2] and trans-[M(L-κS,N)2] isomers

Nkabyo, Henry Ane (2018-03)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The reaction of a series of selected N,N-dialkyl-N’-acyl(aroyl)thioureas to PtII or PdII formed exclusively cis-[M(Ln -κS,O)2] complexes which were fully characterized by melting point determination, 1H, 13C{1H} NMR, FT-IR and single-crystal X-ray diffraction analyses. In acetonitrile solutions and under polychromatic light irradiation, the cis-[M(Ln -κS,O)2] (M = Pt(II) or Pd(II)) complexes undergo photo-induced isomerization to their respective trans- [M(Ln -κS,O)2] counterparts. The cis→trans isomerization was monitored by 1H NMR and RPHPLC due to differences in chemical shifts and retention times of the two geometric isomers respectively. In the photo-irradiated acetonitrile solution, the trans-[M(Ln -κS,O)2] complexes were less soluble than their thermodynamically stable cis-[M(Ln -κS,O)2] counterparts. Consequently, either controlled vapour diffusion-induced crystallization or slow evaporation of irradiated cis-[Pd(Ln -κS,O)2] complexes in acetonitrile led to isolation of novel trans- [Pd(Ln -κS,O)2] isomers in high yields > 60%. The trans-κS,O configuration in the resulting complexes was confirmed by single-crystal X-ray diffraction. Also, significant differences in melting points and 1H NMR chemical shifts were observed between the cis-trans isomers. For cis-bis(N,N-diethyl-N’-1-naphthoylthioureato-κ 2 S,O)-palladium(II) cis-[Pd(L7 -κS,O)2], an unprecedented photo-induced isomerization occurred, yielding a novel four-membered trans- [Pd(L7 -κS,N)2] isomer in addition to trans-[Pd(L7 -κS,O)2]. This unusual trans-[Pd(L7 -κS,N)2] complex was further characterized by RP-HPLC, LC-MS and represents the first example of a trans-κS,N coordination complex of Pd with the N,N-dialkyl-N’-acyl(aroyl)thioureas. In general, the trans-[Pd(Ln -κS,O)2] and trans-[Pd(Ln -κS,N)2] complexes were found to have significantly higher melting points compared to their cis-[Pd(Ln -κS,O)2] isomers, in addition to the relative upfield shift of their 1H NMR resonances. Also, the Pd-S bond distances in the trans complexes were longer, while the Pd-O bonds experienced significant shortening compared to that of the cis complexes. The addition of Pt(II) or Pd(II) solutions to asymmetrically substituted N,N-dialkyl-N’- acyl(aroyl)thioureas generated cis-[M(Ln -κS,O)2] complexes which were found to exist in chloroform as cis-[M(ZZ-Ln -κS,O)2], cis-[M(EE-Ln -κS,O)2] and cis-[M(EZ-Ln -κS,O)2] isomers. Assignment of these configurational isomers was carried out by 1H, 13C, HMBC and 1D NOESY NMR spectroscopy. Although significant overlap was observed between the cisEZ and cis-EE isomers, for cis-[Pt(Ln -κS,O)2] complexes all the ZZ, EZ and EE configurational isomers were represented by three well-resolved 195Pt{1H} resonances in the chemical shift range -2650 to -2750 ppm. The chemical shift and relative distribution of the E and Z isomers in chloroform were found to be dependent on the nature of ligand substituents. The presence of long chain N-alkyl or N-phenyl groups favoured a higher relative distribution of the cis- [Pt(ZZ-L-κS,O)2] isomer compared to complexes of ligands with smaller N-alkyl groups. In chloroform, cis-[Pt(EE-L8 -κS,O)2] complexes of PtII and PdII were isolated for HL8 = Nmethyl,N-ethyl, N’-benzoylthiourea, while a cis-[Pt(ZZ-L15 -κS,O)2] structure crystallized for HL15 = N-4-methoxy, N-isopropyl-N’-(2,2-dimethyl-pronanoyl)thiourea as revealed by singlecrystal X-ray diffraction studies. Photo-induced isomerization of a mixture of cis-[Pt(EE-Ln - κS,O)2], cis-[Pt(EZ-Ln -κS,O)2] and cis-[Pt(ZZ-Ln -κS,O)2] isomers resulted in trans-[Pt(EE-Ln - κS,O)2], trans-[Pt(EZ-Ln -κS,O)2] and trans-[Pt(ZZ-Ln -κS,O)2] isomers in chloroform as represented by appearance of three additional 195Pt{1H} resonances after light irradiation. The 195Pt resonances corresponding to the trans configurational isomers were significantly shifted downfield relative to their cis-counterparts by ca 750 ppm. By slow evaporation and under polychromatic light irradiation of an acetonitrile solution of cis-[Pd(L8 -κS,O)2] (HL8 = Nmethyl, N-ethyl, N’-benzoythiourea), a trans-[Pd(L8 -κS,O)2] complex was isolated as a mixture of trans-[Pd(ZZ-L8 -κS,O)2] and trans-[Pd(EZ/EE-L8 -κS,O)2] configurational isomers. In the absence of light, all isolated trans-[Pd(Ln -κS,O)2] complexes were found to undergo a spontaneous trans→cis isomerization in chloroform. The presence of different ligand substituents showed no significant influence on the relative rates of trans→cis isomerization. Moreover, no consistent trend in rates of isomerization based on electronic effect of ligand substituents was observed in acetonitrile and chloroform. The relative rates of trans→cis isomerization were significantly higher in acetonitrile compared to chloroform indicating the role of coordinating acetonitrile solvent in assisting the reverse isomerization process. Addition of trace amounts of free N,N-dialkyl-N’-acylthioureas catalysed the trans→cis isomerization leading to a rapid dissapearance of trans 1H NMR resonances and RP-HPLC peaks. In contrast, the presence of N,N,N’,N’-tetramethyl-1,8-naphthalene diamine appeared to remove free ligands in solution leading to a decrease in the rate of trans→cis isomerization. When the concentration of added ligand in solution was increased, a first-order increase in the rate of trans→cis isomerization was observed. The ligand exchange mediated trans→cis isomerization resulted in the formation of mixed-ligand complexes of the type cis-[Pd(La,b - κS,O)2] evident from RP-HPLC chromatograms. Further support of an associative ligand exchange process during trans→cis isomerization was provided by temperature dependence in the range 25-55°C in chloroform during which large negative activation entropy and positive activation enthalpy values were obtained.

AFRIKAANSE OPSOMMING: Die reaksie van ’n reeks geselekteerde N,N-dialkiel-N’-asiel(aroïel)tioüreums met PtII, PdII vorm uitsluitlik cis-[M(Ln -κS,O)2] komplekse, wat ten volle gekarakteriseer is deur middel van smeltpuntmetings, 1H, 13C{1H} KMR, FT-IR en enkel kristal X-straal diffraksie analise. In asetonitriel oplossings, onder bestraling met polichromatiese wit lig, ondergaan die cis-[M(Ln - κS,O)2] komplekse ’n foto-geïndusseerde isomerisasie na hul onderskeie trans-[M(Ln -κS,O)2] vorme. Die cis→trans isomerisasie is gemonitor deur 1H KMR en RP-HPLC danksy verskille in die onderskeidelik chemiese verskuiwings en vloeitye van die twee geometriese isomere. In die bestraalde asetonitriel oplossings is die trans-[M(Ln -κS,O)2] komplekse minder oplosbaar as hul meer termodinamies stabiele cis-[M(Ln -κS,O)2] isomere. Gevolglik lei beheerde dampdiffusie of die stadige verdamping van bestraalde cis-[Pd(Ln -κS,O)2] komplekse in asetonitriel tot die isolasie van nuwe trans-[Pd(Ln -κS,O)2] isomere, met hoë opbrengs > 60%. Die transκS,O konfigurasie in hierdie produkte is bevestig deur enkel kristal X-straal diffraksie. Verder is aansienlike verskille in die smeltpunte en 1H KMR chemiese verskuiwings waargeneem tussen die cis-trans isomere. Vir cis-bis(N,N-diëtiel-N’-1-naftoïeltioüreato-κ 2 S,O)- palladium(II) cis-[Pd(L7 -κS,O)2] het ’n ongekende foto-geïndusseerde isomerisasie plaasgevind wat gelei het tot nie net trans-[Pd(L7 -κS,O)2], maar ook ’n nuwe vier-lid trans- [Pd(L7 -κS,N)2] isomeer. Hierdie ongewone trans-[Pd(L7 -κS,N)2] kompleks is verder gekarakteriseer deur RP-HPLC, LC-MS en verteenwoordig die eerste voorbeeld van ’n transκS,N-gekoördineerde kompleks van Pd met N,N-dialkiel-N’-asiel(aroïel)tioüreums. Oor die algemeen is daar gevind dat trans-[Pd(Ln -κS,O)2] en trans-[Pd(Ln -κS,N)2] aansienlik hoër smeltpunte as hul cis-[Pd(Ln -S,O)2] isomere het, benewens hul relatiewe “upfield”, of laer frekwensie, posisies van hul 1H KMR seine. Verder is gevind dat die Pd-S bindingslengtes in hierdie trans-komplekse langer, terwyl die Pt-O bindings aansienlik korter is as dié van die ciskomplekse. Die toevoeging van Pt(II), Pd(II) oplossings tot asimmetriese-gesubstitueerde N,Ndialkiel-N’-asiel(aroïel)tioüreums genereer cis-[M(Ln -κS,O)2] komplekse en is ontdek bestaan in chloroform as cis-[M(ZZ-L n -κS,O)2], cis-[M(EE-L n -κS,O)2], cis-[M(EZ-L n -κS,O)2] isomere. Toekenning van hierdie konfigurasionele isomere is uitgevoer deur 1H, 13C, HMBC en 1D NOESY KMR spektroskopie. Alhoewel aansienlike oorvleuling van seine waargeneem is vir die cis-EZ en cis-EE isomere, vir die cis-[Pt(Ln -κS,O)2] komplekse is die ZZ, EZ, EE konfigurasionele isomere voorgestel deur drie duidelike individuele 195Pt{1H} seine, met chemise verskuiwings in die gebied 2650-2750 ppm. Die chemise verskuiwing en relatiewe verspreiding van E,Z isomere in chloroform is gevind om tot ’n groot mate afhanklik te wees van die aard van ligand substituente. Die teenwoordigheid van lang-ketting N-alkiel of Nfenielgroepe bevorder ’n relatiewe hoër teenwoordigheid van cis-[Pt(ZZ-L-κS,O)2], in vergelyking met komplekse waarvan die ligande oor kleiner N-alkiel substituente beskik. In chloroform is cis-[Pt(EE-L 8 -κS,O)2] komplekse van PtII, PdII geïsoleer vir HL8 = N-metiel,Netiel, N’-bensoïeltioüreum, terwyl ’n cis-[Pt(ZZ-L 15 -κS,O)2] struktuur gekristalliseer het vir HL15 = N-4-metoksie, N-isopropiel-N’-(2,2-dimetiel-pronanoïel)tioüreum, soos getoon deur Xstraal diffraksie studies. Foto-geïndusseerde isomerisasie van ’n mengsel van cis-[Pt(EE-L n - κS,O)2], cis-[Pt(EZ-L n -κS,O)2] en cis-[Pt(ZZ-L n -κS,O)2] isomere lei tot trans-[Pt(EE-L n κS,O)2], trans-[Pt(EZ-L n -κS,O)2], trans-[Pt(ZZ-L n -κS,O)2] isomere in chloroform, soos aangedui deur die verskyning van drie addisionele 195Pt{1H} seine, na bestraling met lig. Die 195Pt resonansie seine wat ooreenstem met die trans konfigurasionele isomere kom aansienlik ‘downfield’ (hoër frekwensies) voor in vergelyking met hul cis-ekwiwalente, met ca. 750 ppm. Deur stadige verdamping van cis-[Pd(L8 -κS,O)2] (HL8 = N-metiel, N-etielbensoïeltioüreum) in ’n asetonitrieloplossing terwyl onder bestraling met polychromatiese wit lig, is ’n trans-[Pd(L8 - κS,O)2] kompleks geïsoleer as ’n mengsel van trans-[Pd(ZZ-L 8 -κS,O)2], trans-[Pd(EZ/EE-L 8 - κS,O)2] konfigurasionele isomere. Daar is gevind dat in die afwesigheid van lig, alle geïsoleerde trans-[Pd(Ln -κS,O)2] komplekse ’n spontane trans→cis isomerisasie ondergaan in chloroform. Die teenwoordigheid van verskillende ligandsubstituente toon geen beduidende invloed op die relatiewe tempo van trans→cis isomerisasie. Verder is geen konsekwent tendense in isomerisasietempo’s, gebasseer op elektroniese effekte van ligandsubstituente, waargeneem in asetoon en chloroform. Die relatiewe tempo’s van trans→cis isomerisasie is aansienlik hoër in asetonitriel in vergelyking met chloroform, wat dui op die rol van die koördinerende asetonitriel oplosmiddel in die terugwaardse isomerisasie proses. Toevoeging van klein hoeveelhede vry N,N-dialkiel-N’-asieltioüreums kataliseer die trans→cis isomerisasie en lei tot die vinnige verdwyning van die trans 1H NMR resonansie seine en RP-HPLC pieke. In kontras blyk dit dat die teenwoordigheid van N,N,N’,N’-tetrametiel-1,8-naftaleen diamine vrye ligande uit die oplossing verwyder wat lei tot ’n afname in die tempo van trans→cis isomerisasie. Wanneer die konsentrasie van vrye ligande in oplossing verhoog word, word ’n eerste-orde toename in die trans→cis isomerisasietempo waargeneem. Die ligand-uitruil-bemiddelde trans→cis isomerisasie lei tot die vorming van gemengde-ligand komplekse van die tipe cis-[Pd(La,b-S,O)2], duidelik sigbaar in die RP-HPLC chromatogramme. Verdere ondersteuning vir ’n assosiatiewe ligand-uitruil proses gedurende trans→cis isomerisasie is verkry van die temperatuur afhanklikheid, in die gebied 25-55°C in chloroform, waartydens groot negatiewe aktiverings entropie en positiewe aktiverings entalpie waardes gevind is.

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