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The pioneer in clarification of the substitution reactions between platinum(II), palladium(II) and gold(III) complexes with biomolecules such as guanosine-5'-monophosphate (5'-GMP), glutathione (GSH) and L-methionine (L-Met) was Professor Dr. Živadin D. Bugarčić and his research group. In this minireview we highlight the important results of his research in the field of studying mechanisms of the substitution reactions of various transition metal complexes with square-planar geometry. Knowledge of the kinetic coordination chemistry of these transition metal ions helps to clarify the mechanism of action of metallodrugs for development of complexes with potent antitumor activity. The results are presented comparatively to aid in the future development of novel antitumor agents with unique mechanisms of action. [ABSTRACT FROM AUTHOR]

In this work, Pt(II) complexes of previously synthesized steroidal thiosemicarbazones were synthesized and characterized. The ligands and their metal complexes were studied by analytical and spectroscopic data (elemental analysis, IR, 1D-NMR and 2D-NMR, HSQC, HMBC, NOESY, COSY), the analysis of which enabled complete 1H and 13C assignments of each compound including E and Z isomers. All the synthesized ligands and complexes were screened for their cytotoxic and antimicrobial activity. The results demonstrate that the new steroidal thiosemicarbazone complexes were significantly less cytotoxic than the corresponding steroidal thiosemicarbazones. In addition, complexes showed lower antimicrobial activity than the standard drugs, similar to the activity of the starting thiosemicarbazones.

Abstract A ditopic bridging ligand, N , N ′- bis (pyridine)-1,6,7,12- tetrakis -(4-methoxyphenoxy)perylene diimide (1) was synthesized. Metallosupramolecular self-assembly of 1 with 2,2′:6′,2″-terpyridyl-platinum(II) and/or -palladium(II) complex ions afforded to new triads [(terpy)M(II)-(1)-M(II)(terpy)](NO 3) 4 where, M(II) = Pt(II) (2), and Pd(II) (3), respectively. These isolated triads 2 and 3 and also free ligand 1 were fully characterized by FT-IR, 1H NMR, 13C DEPT NMR, 1H-1H COSY NMR, MALDI TOF and HRMS (MALDI-TOF/TOF-MS) mass techniques and UV/Vis spectroscopy. DFT studies revealed that the molecular structure of 1 is highly distorted and deviated from the planarity by bulky methoxyphenyl groups at bay positions. The calculated HOMO and LUMO are located on the PDI core and phenyl groups, and the band gap was found to be 2.27, 2.23 and 2.24 eV in DMSO for 1 , 2 and 3 , respectively. The calculated UV/Vis spectra are in good agreement with those of experimental results. Electrochemical, in situ spectroelectrochemical and in situ electrocolorimetric measurements in solution suggested that redox processes of the compounds are accompanied by favourable electron transport properties with remarkably narrow HOMO-LUMO gaps and intense light absorption over the visible range. The association of usually reversible and fast electron transfer processes with the net colour changes in solution pointed out the applicability of the compounds as an electrochromic material. The electrochromic performance measurements with the films of the compounds on indium tin oxide glass provided additional support for their usability as a colour changing material in electrochromic devices. Surface morphologies of the films were characterized by scanning electron microscopy (SEM). The metal complexes of 1 , especially the platinum(II) complex (2), behaved as a good neutral and cathodic colouring electrochromic material with fast response time, long-term stability, and long-term write erase efficiency. The photovoltaic performance of the compounds as electron acceptor material was investigated by varying their relative ratio in the bulk heterojunction solar cells. It was found that the photovoltaic conversion efficiency of the devices strongly depends on the blend ratio. The optimized performance for the devices was obtained with the P3HT: PDIs (weight ratio 1.0: 1.5) blended films. For this blend ratio, the device showed a maximum conversion efficiency of 3.91%, 8.17% and 5.18% for 1 , 2 and 3 , respectively. The results obtained reveal that employing 2 as an acceptor material has great potential for the development of highly efficient nonfullerene bulk heterojunction photovoltaic devices. Graphical abstract Image 1 Highlights • Triads consisting of square planar platinum(II)- and palladium(II)-2,2':6′,2″-terpyridyl chromophores were prepared. • These chromophores coordinated to a ditopic twisted bis(pyridine)-perylene diimide. • These hybrid materials behaved as a colouring electrochromic material. • The device showed a maximum photovoltaic conversion efficiency of 8.17% for the platinum-triad. [ABSTRACT FROM AUTHOR]

The four‐coordinate Pt0 complex [Pt(PPh3)(Eind2‐BPEP)] (Eind=1,1,3,3,5,5,7,7‐octaethyl‐1,2,3,5,6,7‐hexahydro‐s‐indacen‐4‐yl; BPEP=2,6‐bis(1‐phenyl‐2‐phosphaethenyl)pyridine), which bears a PNP‐pincer‐type phosphaalkene ligand (Eind2‐BPEP; PNP=N,N‐bis(diphenylphosphine)‐2,6‐diaminopyridine), were found to adopt a square‐planar configuration around the Pt center (τ4=0.11). This coordination geometry is very uncommon for formal d10 complexes. In this study, a series of ligands with different electronic properties (i.e. DMAP, 2,6‐lutidine, PMe3, tBuNC, and CO) were introduced in place of PPh3, and their effects on the coordination geometry were examined. X‐ray diffraction analysis revealed that all complexes adopted a square‐planar configuration (τ4=0.20–0.27). In contrast, DFT calculations indicated that the geometrical stability towards distortion around Pt varied with the ligand. The complexes with pyridine‐based ligands had rigid planar structures, whereas those with π‐accepting ligands, such as CO, were relatively flexible towards distortion. The electronic effects of the ligands were reflected in the spectroscopic properties of the complexes, which showed a large color change in the near‐infrared region. π‐Accepting ligands cause geometrical distortion: The four‐coordinate Pt0 complexes [Pt(L)(Eind2‐BPEP)] (Eind2‐BPEP; Eind=1,1,3,3,5,5,7,7‐octaethyl‐1,2,3,5,6,7‐hexahydro‐s‐indacen‐4‐yl; BPEP=2,6‐bis(1‐phenyl‐2‐phosphaethenyl)pyridine) that bear a pincer‐type phosphaalkene ligand adopt a square‐planar configuration around Pt; this coordination geometry is very uncommon for formal d10 complexes. These complexes have extremely narrow HOMO–LUMO gaps and show strong absorption in the near‐infrared region. [ABSTRACT FROM AUTHOR]

[MCl(H2L)(OH2)]·1.5H2O (M = Pd(II) ( 1) and Pt(II) ( 2)) and [Ru(H2L)2(OH2)2]·3H2O ( 3) (H3L: N-phenyl, N`-(3-triazolyl)thiourea) were synthesized, characterized and tested for their antibacterial activities against Staphylococcus aureus and Escherichia coli bacteria. The thiourea derivative is coordinated to Mn+ ions as a mono-negatively N,S-bidentate ligand via the enolization of C = S group and triazole N center. The density functional theory calculations reveal that presence of a water molecule in a trans position to triazole ring increased the stability of d8 metal ions complexes via the formation of strong Cl...NH intramolecular H-bond. The cis-Ru(II)-isomer with two isoenergetically H2L− molecules are more stable than the trans-analog. Coordination of H3L to Ru(II) ion did not alter the toxicity of the free ligand, while the interaction with the d8 metal ions gave rise to inactive compounds. [ABSTRACT FROM AUTHOR]

The results of geometric analysis of 588 crystal structures of square-planar nickel complexes containing short intermolecular Ni...X contacts (X is a non-metal atom) are reported. The frequency of occurrence of Ni...X interactions of a particular type was evaluated depending on the nature of the X atom and geometric characteristics. The promolecular functions RDG and sign(λ)•ρ( r) were used to analyze the character of Ni...X interactions in attractive/repulsive terms. The relative strength of these interactions was determined as the function of the nature of the X atom and the direction of charge transfer. [ABSTRACT FROM AUTHOR]

[Display omitted] • DMSO and metal ion have roles in regulating the triazolate isomers ratio of the iClick reaction. • A non-equivalent mixture of N1 and N2 triazolate isomers in the case of symmetrical alkyne. • The nature of the alkyne affects the ratio of the formed triazolate Pd(II) isomers. At the ambient temperature and under catalyst-free conditions, reactions between [MN 3 (LET)]PF 6 (M = Pd(II) and Pt(II); LET = 2,6-bis(1-ethylbenzimidazol-2′-yl)pyridine) and substituted alkynes (dimethyl acetylene-dicarboxylate and 4,4,4-trifluoro-2‑butynoic acid ethyl ester) afforded the corresponding triazolate complexes. These reactions produced a mixture of triazolate bound isomers. The N1 triazolate isomer of the Pt(II) complexes, which first appeared in the [3 + 2] coupling, eventually isomerized to the N2-analogue during the incubation in DMSO. The reactions of [MN 3 (LET)]PF 6 with asymmetric alkyne, 4,4,4-trifluoro-2‑butynoic acid ethyl ester, produced a mixture of the three anticipated triazolate isomers, according to 19F NMR spectroscopy. The stability of the triazolate complexes was investigated, along with the cycloaddition reaction's mechanism and the nature of the intermediates, using NMR spectroscopy. [ABSTRACT FROM AUTHOR]

The Pt0 complex [Pt(PPh3)(Eind2-BPEP)] with a pyridine-based PNP-pincer-type phosphaalkene ligand (Eind2-BPEP) has a highly planar geometry around Pt with ∑(Pt)=358.6°. This coordination geometry is very uncommon for formal d10 complexes, and the Pd and Ni homologues with the same ligands adopt distorted tetrahedral geometries. DFT calculations reveal that both the Pt and Pd complexes are M0 species with nearly ten valence electrons on the metals whereas their atomic orbital occupancies are evidently different from one another. The Pt complex has a higher occupancy of the atomic 6s orbital because of strong s-d hybridization due to relativistic effects, thereby adopting a highly planar geometry reflecting the shape and orientation of the partially unoccupied [ABSTRACT FROM AUTHOR]

Complex 1 [( Ph I 2 P 2− ) AlCl] was widely applied in advanced materials due to its interesting structure. Two Square-planar (SP) complexes 2 [( Ph I 2 P 2− ) GaCl] and 3 [( Ph I 2 P 2− ) NiCl] together with SP complex 1 have been investigated by density functional theory. The results indicate that the interaction between M (M = Al, Ga, Ni) and L ( Ph I 2 P 2− ) are not typical true two-electron three-center interactions but a mixture of electronic and covalent. The presence of covalent bond character between M and L is supported by the localized orbital locator, electron localization function and energy decomposition analysis. Further, we also predicted the absorption spectrum of complexes 1 – 3 by time-dependant density functional theory. The results of absorption spectrum shows a red-shift trend from complexes 1 , 2 to 3 . Investigation of the bond interaction at the molecular level can benefit the design and preparation of such SP complexes in chemistry and materials science. [ABSTRACT FROM AUTHOR]

6,6′′-Bis(2,4,6-trimethylanilido)terpyridine (H2TpyNMes) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H2TpyNMes)copper(I)-halide (Cl, Br and I) complexes is dictated by the strength of the NH-halide hydrogen bond. The CuICl and CuIICl complexes are nearly isostructural, the former presenting a highly unusual square-planar geometry about CuI. The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid CuI/CuII electron-transfer self-exchange rates. Cu(H2TpyNMes)Cl shows similar fast electron transfer (≈105 m−1 s−1) which is the same order of magnitude as biological systems. [ABSTRACT FROM AUTHOR]

The B3LYP/6-31G** method was used for the study of nucleophilic substitution reaction in platinum and palladium square-planar cis-diaminate complexes. The processes under consideration take place in two steps through pentacoordinated transition states. Activation barriers for both steps of these processes in gaseous phase and aqueous solution were calculated. A major role in the increase in the activation energies is played by the formation of the intermediate electrostatic Van der Waals' complexes. For the complex cis-[Pd(NH)Cl], the activation barriers in the nucleophilic substitution reaction are considerably lower than those for the complex cis-[Pt(NH)Cl]. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of three new ester functionalised bis(phosphino)amines Ph2PN{R}PPh2 [R = C6H4(3-CO2Me) 1a; C6H3(3,5-CO2Me)2 1b; C6H4(4-CO2Me) 1c] upon stoichiometric reaction of Ph2PCl and the appropriate H2N{R} in Et2O is described. Reaction of 1 equiv. of 1a−c with MCl2(cod) (M = Pt, Pd) in CH2Cl2 afforded the dichlorometal(II) complexes PtCl2(1a) 2a, PtCl2(1b) 2b, PtCl2(1c) 2c and PdCl2(1b) 2''b respectively. The corresponding dibromo (and diiodo) platinum(II) complexes 3b (and 4b) were synthesised, in >80% isolated yields, from PtBr2(cod) or PtI2(cod). When a suspension of 2b in MeOH was stirred at r.t. for ca. 16 h the mixed complex cis-PtCl2[Ph2PNH{R}](Ph2POMe) 5a [R = C6H3(3,5-CO2Me)2] was cleanly generated. Metathesis of 5a using excess NaBr or NaI in MeOH/acetone afforded cis-PtBr2[Ph2PNH{R}](Ph2POMe) 5b or cis-PtI2[Ph2PNH{R}](Ph2POMe) 5c. Methanolysis of PtCl2(1a) 2a at ambient temperature afforded, in low yield (18%), the regiospecific P,C-orthometallated complex cis-PtCl[Ph2PNH{C6H3(3-CO2Me)}](Ph2POMe) 6. We speculate the C−H activated complex 6 is obtained via initial formation of cis-PtCl2[Ph2PNH{C6H4(3-CO2Me)}](Ph2POMe) 7. A similar observation was also found using 2c whereupon examination of the isolated solid, by 31P{1H} NMR spectroscopy, revealed formation of three complexes namely orthometallated cis-PtCl[Ph2PNH{C6H3(4-CO2Me)}](Ph2POMe) 9, cis-PtCl[Ph2PNH{C6H4(4-CO2Me)}](Ph2POMe) 10 and cis-PtCl2(Ph2POMe)2 8. All new compounds reported here have been characterised by multinuclear NMR and IR spectroscopy, microanalysis and in six cases by single crystal X-ray crystallography. The X-ray structure of cis-PtCl[Ph2PNH{C6H3(3-CO2Me)}](Ph2POMe) 6 revealed selective C−H cycloplatination at the 6-position (as opposed to the 2-position) of the N-arene ring. [Copyright &y& Elsevier]

Abstract: Pyridines of Cs symmetry coordinated to [Pd(bipy)(pyr)2]2+ cations (bipy = 2,2′-bipyridyl; pyr = substituted pyridines) lead to syn and anti isomers due to their perpendicular orientation. Rate constants of syn–anti isomerizations of [Pd(bipy)(2pic)2]2+ (2pic = 2-methyl-pyridine) have been measured by quantitative 2D-EXSY NMR. Results suggest a dissociative (D) pathway involving an unsaturated 14-electrons PdII intermediate stabilized by γ-agostic interactions. Kinetic isotope effect, activation parameters, and crystal structure of the anti isomer support the D mechanism. [Copyright &y& Elsevier]

Abstract: A DFT study on the Au–N interaction for some groups of N-donor heterocycles L {L=pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AuX3L complexes {AuX3 =AuBr3, AuCl3, trans-AuCl(CN)2, Au(CN)3} is reported. Linear relationships between the AuX3 Mulliken charge in AuX3L and the computed proton affinity (PA) of the heterocycle were found for all the considered ligands. The different slopes found on changing the N-donor species represent a measure of the π-acidity of these nitrogen ligands once coordinated to the metal centre, by the consequence a π-acceptor ability scale has been derived. The π-acceptor ability of the 5-membered N-donor ligands resulted in all the cases greater than that of the 6-membered N-heterocycles. The proton affinity average value corresponding to a zero charge of the AuX3 and L fragments in the AuX3L species has been estimated. This parameter represents the minimum PA value for the formation of a bond between the N-heterocycles and gold(III) and it does not depend on the electronic features of the coordinated ligands. The sensitivity of the AuX3 fragments towards ligands PA variations follows the order Au(CN)3 < trans-AuCl(CN)2

Abstract: A computational DFT approach for the comparison of the π-acceptor character of some N-donor heterocycles L {L=pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} in neutral AuCl3L complexes is reported. The electronic properties of these ligands have been tuned by adding methyl and/or trifluoromethyl groups in various positions. Linear relationships between the Mulliken charge of the AuCl3 fragment in AuCl3L and the computed proton affinity (PA) of the heterocycle were obtained for all the considered ligands. The different slopes found on changing the N-donor type represent a measure of the π-acidity of these nitrogen ligands once coordinated to AuCl3, and as a consequence the π-acceptor ability scale pyridines≈pyrimidines

Abstract: The synthesis and characterization of a range of chiral β-diimine ligands and their complexes with palladium(II) has been investigated. The introduction of chirality can be easily achieved through a combination of both achiral and chiral building blocks. The absolute configuration of the stereochemical centers has been determined. In addition, representative X-ray structures of both ligands and complexes have been determined. [Copyright &y& Elsevier]

The effects of hydrostatic pressure on the luminescence spectra of tetragonal transition metal complexes with nondegenerate electronic ground states are analyzed quantitatively by means of models based on potential energy surfaces defined along normal coordinates. Pressure-induced changes of intensity distributions within vibronic progressions, band maxima, electronic origins and relaxation rates are discussed for metal-oxo complexes of rhenium(V) and molybdenum(IV) (d 2 electron configuration) and for square-planar complexes of palladium(II) and platinum(II) (d 8 electron configuration). [ABSTRACT FROM AUTHOR]

Four new square-planar Ni(II) and Cu(II) complexes with [N2O2] binding system were synthesized by metal-directed condensation of 2,6-diformyl-4-methylphenol with benzoyl or acetylacetone mono-S-methylisothiosemicarbazone. Only mononuclear “one-armed” complexes were obtained as a consequence of the different reactivity of the two carbonyl groups in the hydroxydialdehyde. The complexes were characterized by elemental analysis, EI MS and UV–Vis spectroscopy. The structural assignment was confirmed by X-ray diffraction analysis and NMR spectroscopy, for the Ni(II) complexes, and by ESR spectroscopy and magnetic measurements, for the Cu(II) complexes. [Copyright &y& Elsevier]

A neutral four-coordinate CuIIN3O complex with Shiff base ligand, product of template assembling of two S-methylisothiosemicarbazide units with benzoylacetone and nitromalondialdehyde, was synthesized by two steps procedure via CuIIN2O2 chelate precursor and was fully characterized by elemental analysis, IR, UV–Vis, ESR spectra and magnetochemistry. X-ray analysis confirms its structural assignment. The assembling ligand assumes an acyclic structure of ‘4+1’ type where a non-bonding interaction between the metal and the amino group of one S-methylisothiosemicarbazide unit takes place. [Copyright &y& Elsevier]

Two new square-planar Cu(II) complexes with [N3O] chelate tetradentate ligands were synthesized and characterized by elemental analysis, EI MS, UV–vis and ESR spectroscopy, and magnetic measurements. They were obtained by template condensation of mono-S-methylisothiosemicarbazone of acetylacetone or 1-phenyl-butane-1,3-dione with anthranil aldehyde. One of two title complexes crystallizes in two forms, monoclinic and orthorhombic. Both of them were analysed by X-ray diffraction and significant differences concerning the distortion from planarity of the coordination environment were observed. Their structure was also compared with that of the Ni complex having the same tetradentate ligands, using previously reported data. Small but significant differences between the coordination complexes of the two metals were observed in the bond distances and angles of the central core. [Copyright &y& Elsevier]

Synthesis, structure and physico-chemical investigation of four new nickel(II) complexes with [N3O] chelate tetradentate ligands are described. They were obtained by template condensation of mono-S-methylisothiosemicarbazone of acetylacetone or 1-phenyl-butane1,3-dione with bifunctional carbonyl compounds such as anthranil aldehyde or 4-amino-3-penten-2-one. Square–planar complexes were obtained as only products with good yield and were fully characterised by EI MS, NMR (1H, 13C) spectroscopy, elemental analysis, and, in the case of three complexes, by X-ray investigation. A Cambridge Structural Database surveys on related square–planar Ni(II) complexes was also included, showing a small but significant correlation between Ni&z.sbnd;O distances and O&z.sbnd;Ni&z.sbnd;N angles. [Copyright &y& Elsevier]

This article describes the synthesis, characterization, and biological activity of novel square-planar cationic platinum(II) complexes containing glucoconjugated triazole ligands and a comparison with the results obtained from the corresponding five-coordinate complexes bearing the same triazole ligands. Stability in solution, reactivity with DNA and small molecules of the new compounds were evaluated by NMR, fluorescence, and UV–vis absorption spectroscopy, together with their cytotoxic action against pairs of immortalized and tumorigenic cell lines. The results show that the square-planar species exhibit greater stability than the corresponding five-coordinate ones. Furthermore, although the square-planar complexes are less cytotoxic than the latter ones, they exhibit a certain selectivity. These results simultaneously demonstrate that overall stability is a fundamental prerequisite for preserving the performance of the agents and that coordinative saturation constitutes a point in favor of their biological action. [ABSTRACT FROM AUTHOR]