In the framework of our attempts to develop cyclometalated Pt­(II) complexes toward bifunctional targeting inhibitors or agents for photodynamic therapy, diagnostics, and bioimaging, a series of bis-cyclometalated Pt­(II) complexes [Pt­(CNC)­(L)] (L = DMSO, MeCN) containing various (CNC)2– ligands based on 2,6-diphenylpyridine were synthesized and characterized analytically and spectroscopically, focusing on their electrochemical, luminescence, and antiproliferative properties. Electrochemical experiments and UV–vis absorption spectroscopy suggest ligand-centered LUMOs and metal-centered HOMOs in line with DFT calculations. Extension of the ancillary phenyl to naphthyl cores and a central 4-phenylpyridine group instead of pyridine results in bathochromic shifts of the long-wavelength absorption bands ranging from 420 to 440 nm, with the latter shift being more pronounced. The complexes of the fused CNC heterocyclic systems dba (H2dba = dibenzo­[c,h]­acridine), db­(ph)­a (H2db­(ph)­a = 7-phenyldibenzo­[c,h]­acridine), and bzqph (HbzqphH = 2-phenylbenzo­[h]­quinoline) absorb far more red-shifted in the range 500–530 nm. All complexes show reversible first electrochemical reductions and irreversible oxidations with an electrochemical gap of about 3 V, roughly in line with the absorption energies. While the 2,6-diphenylpyridine complexes [Pt­(CNC)­(DMSO)] show no luminescence at ambient temperature in solution, the fused dba, db­(ph)­a, and bzqph derivatives are efficient triplet emitters at ambient temperature with emission wavelengths in the region 575–600 nm and quantum yields ranging from 7 to 23%. Vibrationally resolved emission spectra calculated in the framework of DFT faithfully reproduce the experimental data. TD-DFT calculations at the excited-state T1 geometry reveal intraligand π–π*/MLCT character of the emission for all three investigated complexes. Antiproliferative tests on selected complexes gave very different toxicities, ranging from lower than 1 μM to virtually nontoxic. The data allowed drawing some structure–activity relationships (SAR), even though variations in solubility could also significantly account for the different toxicities.

ABSTRACT

The 5-aryl-3-(2′-pyridiyl)-1,2,4-triazine ligands under study L 1 ; 5-(3-methoxyphenyl)-(PyTZ3Me-OPh) L 2 ; 5-(4-methoxyphenyl)-(PyTZ4MeOPh) L 3 ; 5-(4-trifluoromethylphenyl)-(PyTZ4CF 3 Ph) L 4 ; 5-(4-fluorophenyl)-(PyTZ4FPh) L 5 ; and tris-3,5,6-(2′-pyridyl)-1,2,4-triazine (Py 3 TZ) L 6 ] react with [(COD)PdCl 2 ] (COD = 1,5-cyclooctadiene) to form complexes [(L 1-6 )PdCl 2 ] with N,N bidentate binding ligands, also including the potentially N,N,N tridentate ligand L 6 . This was concluded from an in-depth NMR spectroscopic study of the new complexes and from comparison with Pd-terpy complexes [(R′ terpy)PdCl]Cl [R′terpy = 4′-R′-2,2′:6′,2′′-terpyridine; R′ = H or SMe], showing definite tridentate N,N,N coordination, and with the Pd-bpy complexes [(bpy)Pd(Mes)Cl] and [(bpy)PdCl 2 ], show-Eur.

The five coordinate organoplatinum complex [Pt(bpy)(cod)(Me)][SbF6] (cod = 1,5-cyclooctadiene, bpy = 2,2'-bipyridine) was obtained reacting [Pt(cod)(Me)Cl] with Ag[SbF6] and bpy and characterized by multiple spectroscopy (IR and NMR) and single crystal XRD. Although the application of the τ values for the discrimination between trigonal bipyramidal vs. square pyramidal coordination fails, the molecular structure can be unequivocally described as basally-distorted trigonal bipyramidal. Detailed multinuclear NMR spectroscopy in solution at ambient temperature gives strong evidence for the same structure; corresponding low-temperature measurements down to −70 °C revealed no marked dynamic processes.

The redox chemistry of the N-nitrosamine complexes [IrCl5(RN(H)N[double bond, length as m-dash]O)]2- (R = benzyl or n-butyl) was studied in detail. One-electron oxidations at around 200 mV vs. ferrocene/ferrocenium were reversible in cyclic voltammograms. UV-vis spectroelectrochemistry reveals spectra characteristic of IrIV species but also partial decomposition of the oxidised species [IrIVCl5(RN(H)N[double bond, length as m-dash]O)]˙- on this timescale (minutes). Detailed studies on chemically oxidised solutions of the parent IrIII complexes gave evidence for paramagnetic IrIV from NMR spectra. Final products of the decomposition were the corresponding alcohols and presumably [IrIIICl5(L)]2- (L = N2, solvent, amine) complexes. Similar decomposition reactions of acidic DMSO solutions of [IrCl5(RN(H)N[double bond, length as m-dash]O)]2-revealed that this combination produces the so-called "activated" DMSO (Me2S+-O- or Me2S+-OE, with "E" being an electrophile) which oxidises the parent IrIII complexes. Finally, with the very reactive purple IrIV compound (PPh4)[IrCl5(BnN(H)N[double bond, length as m-dash]O)], the first primary N-nitrosamine coordinated to [IrIVCl5]- was isolated and characterised by UV-vis absorption, FTIR, NMR spectroscopy, ultra-high resolution electrospray mass spectrometry (UHR-ESI-MS) and iridium L3 X-ray absorption near-edge spectroscopy (XANES).