BackgroundRe(I) tricarbonyl complexes exhibit immense potential as fluorescence imaging agents. However, only a handful of rhenium complexes have been utilized in biological imaging. The present study describes the synthesis of four novel rhenium complexes, their characterization and preliminary biological studies to assess their potential as biological imaging agents.ResultsFour facial rhenium tricarbonyl complexes containing a pyridyl triazine core, (L1 = 5,5′(3-(2-pyridyl)-1,2,4-triazine-5,6-diyl)-bis-2-furansulfonic acid disodium salt and L2 = (3-(2- pyridyl)-5,6-diphenyl-1,2,4-triazine-4′,4′′-disulfonic acid sodium salt) have been synthesized by utililzing two different Re metal precursors, Re(CO)5Br and [Re(CO)3(H2O)3]OTf in an organic solvent mixture and water, respectively. The rhenium complexes [Re(CO)3(H2O)L1]+ (1), Re(CO)3L1Br (2), [Re(CO)3(H2O)L2]+ (3), and Re(CO)3L2Br (4), were obtained in 70–85% yield and characterized by 1H NMR, IR, UV, and luminescence spectroscopy. In both H2O and acetonitrile, complexes display a weak absorption band in the visible region which can be assigned to a metal to ligand charge transfer excitation and fluorescent emission lying in the 650–710 nm range. Cytotoxicity assays of complexes 1, 3, and 4 were carried out for rat peritoneal cells. Both plant cells (Allium cepa bulb cells) and rat peritoneal cells were stained using the maximum non-toxic concentration levels of the compounds, 20.00 mg ml−1 for 1 and 3 and 5.00 mg ml−1 for 4 to observe under the epifluorescence microscope. In both cell lines, compound concentrated specifically in the nuclei region. Hence, nuclei showed red fluorescence upon excitation at 550 nm.ConclusionsFour novel rhenium complexes have been synthesized and characterized. Remarkable enhancement of fluorescence upon binding with cells and visible range excitability demonstrates the possibility of using the new complexes in biological applications.Graphical abstractMicrograph of rat peritoneal cells incubated with novel rhenium complex under epifluorescence microscope. Electronic supplementary materialThe online version of this article (doi:10.1186/s13065-016-0218-4) contains supplementary material, which is available to authorized users.

[Pt(N(R)-1,1'-Medma)Cl]Cl complexes with tridentate ligands (bis(1-methyl-2-methylimidazolyl)amine, R = H; N-(methyl)bis(1-methyl-2-methylimidazolyl)amine, R = Me) were prepared in order to investigate Pt(N(R)-1,1'-Medma)G adducts (G = monodentate N9-substituted guanine or hypoxanthine derivative). Solution NMR spectroscopy is the primary tool for studying metal complexes of nucleosides and nucleotides because such adducts rarely crystallize. However, [Pt(N(H)-1,1'-Medma)(3'-GMPH)]NO·5HO (5) was crystallized, allowing, to our knowledge, the first crystallographic molecular structure determination for a 3'-GMP platinum complex. The structure is one of only a very few structures of a 3'-GMP complex with any metal. Complex 5 has the syn rotamer conformation, with 3'-GMP bound by N7. All Pt(N(R)-1,1'-Medma)G adducts exhibit two new downfield-shifted G H8 signals, consistent with G bound to platinum by N7 and a syn/anti rotamer mixture. Anticancer-active monofunctional platinum(II) complexes have bulky carrier ligands that cause DNA adducts to be distorted. Hence, understanding carrier-ligand steric effects is key in designing new platinum drugs. Ligand bulk can be correlated with the degree of impeded rotation of the G nucleobase about the Pt-N7 bond, as assessed by the observation of rotamers. The signals of syn and anti rotamers are connected by EXSY cross-peaks in 2D ROESY spectra of Pt(N(H)-1,1'-Medma)G adducts but not in spectra of Pt(N(H)dpa)G adducts (N(H)dpa = bis(2-picolyl)amine), indicating that rotamer interchange is more facile and carrier-ligand bulk is lower in Pt(N(H)-1,1'-Medma)G than in Pt(N(H)dpa)G adducts. The lower steric hindrance is a direct consequence of the greater distance of the G nucleobase from the H4/4' protons in the N(R)-1,1'-Medma carrier ligand in comparison to that from the H6/6' protons in the N(H)dpa carrier ligand. Although in 5 the nucleotide is 3'-GMP (not the usual 5'-GMP) and the N(H)-1,1'-Medma carrier ligand is very different from those typically present in structurally characterized Pt(II) G complexes, the rocking and canting angles in 5 adhere to long-recognized trends.

NMR and X-ray diffraction studies were conducted on Pt­(II)­LCl2 complexes prepared with the new N-donor ligands N(SO2R)­Me n dpa (R = Me, Tol; n = 2, 4). These ligands differ from N(H)­dpa (di-2-picolylamine) in having the central N within a tertiary sulfonamide group instead of a secondary amine group and having Me groups at the 6,6′-positions (n = 2) or 3,3′,5,5′-positions (n = 4) of the pyridyl rings. The N(SO2R)­3,3′,5,5′-Me4dpa ligands are coordinated in a bidentate fashion in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes, forming a rare eight-membered chelate ring. The sulfonamide N atom did not bind to Pt­(II), consistent with indications in the literature that tertiary sulfonamides are unlikely to anchor two meridionally coordinated five-membered chelate rings in solutions of coordinating solvents. The N(SO2R)­6,6′-Me2dpa ligands coordinate in a monodentate fashion to form the binuclear complexes [trans-Pt­(DMSO)­Cl2]2(N(SO2R)­6,6′-Me2dpa). The monodentate instead of bidentate N(SO2R)­6,6′-Me2dpa coordination is attributed to 6,6′-Me steric bulk. These binuclear complexes are indefinitely stable in DMF-d 7, but in DMSO-d 6 the N(SO2R)­6,6′-Me2dpa ligands dissociate completely. In DMSO-d 6, the bidentate ligands in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes also dissociate, but incompletely; these complexes provide rare examples of association–dissociation equilibria of N,N bidentate ligands in Pt­(II) chemistry. Like typical cis-PtLCl2 complexes, the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes undergo monosolvolysis in DMSO-d 6 to form the [Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­(DMSO-d 6)­Cl]+ cations. However, unlike typical cis-PtLCl2 complexes, the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes surprisingly do not react readily with the excellent N-donor bioligand guanosine. A comparison of the structural features of over 50 known relevant Pt­(II) complexes having smaller chelate rings with those of the very few relevant Pt­(II) complexes having eight-membered chelate rings indicates that the pyridyl rings in Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes are well positioned to form strong Pt–N bonds. Therefore, the dissociation of the bidentate ligand and the poor biomolecule reactivity of the Pt­(N(SO2R)­3,3′,5,5′-Me4dpa)­Cl2 complexes arise from steric consequences imposed by the −CH2–N­(SO2R)–CH2– chain in the eight-membered chelate ring.

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