Electrochemical investigations of cationictrans-haloarylisocyanidebis(tertiaryphosphino)platinum (II) complexes

Summary The electrochemical behaviour of a series of cationic platinum(II) isocyanide complexes has been studied in acetonitrile. All the tested compounds are oxidized at a platinum electrode via a two-electron process and reduced at a platinum or mercury electrode via two successive one-electron steps. The anodic step involves the formation of platinum(IV) complexes. The main reduction product formed in correspondence to the first cathodic process is a stable dimer platinum(I) containing bridging isocyanide ligands. Platinum(0) species are formed in the subsequent reduction step.     

Formation of anionic σ-aryl complexes of platinum (IV) in the reaction of H2PtCl6 with aromatic compounds. The crystal and molecular structures of platinum (IV) complexes of naphthalene and ortho-nitrotoluene

The crystal and molecular structures of anionic platinum(IV) complexes of naphthalene (I) and ortho-nitrotoluene (II) have been determined by X-ray diffraction. The structures of both complexes are similar. The platinum atom is octahedrally coordinated with four chlorine atoms occupying the equatorial positions and σ-bonded aryl and neutral ammonia ligands situated in the axial positions.     

Synthesis and mesomorphic properties of some platinum(II) and oxovanadium(IV) complexes

The synthesis and mesomorphic properties of a homologous series of N-(2-hydroxy-4-n-alkoxybenzylidene)-4'-n-decylphenylanilines and their platinum(II) and oxovanadium(IV) complexes are reported. All the ligands and their metal chelates exhibit enantiotropic mesophases, predominantly smectic A and smectic C phases. The transition temperatures and enthalpies have been determined for most of the compounds. The platinum(II) complexes have higher melting points and mesophase thermal stabilities. However, the oxovanadium(IV) complexes have a wider thermal range for the mesophase. Both platinum(II) and oxovanadium(IV) complexes containing only a chain on the biphenyl moiety exhibit a nematic phase.     

Synthesis and characterization of luminescent bis-cyclometalated platinum(IV) complexes

Presented is the synthesis and characterization of a series of luminescent heteroleptic bis-cyclometalated platinum(IV) complexes. An oxidation-facilitated cyclometalation is employed to convert platinum(II) pendant species into bis-cyclometalated platinum(IV) dichlorides, which are transformed into the tris-chelated diimine complexes through ligand substitution. The structure-property relationship is probed by judiciously varying substituents on both the C(∧)N and the N(∧)N ligands resulting in a family of complexes exhibiting blue emission, long excited-state lifetimes, and highly efficient oxygen quenching. Excited-state properties are corroborated by static and time-dependent density-functional theory calculations of both the singlet and the triplet state.     

Concerted reductive coupling of an alkyl chloride at Pt(IV)

Oxidation of a doubly cyclometallated platinum(II) complex results in two isomeric platinum(IV) complexes. Whereas the trans isomer is robust, being manipulable in air at room temperature, the cis isomer decomposes at -20 °C and above. Reductive coupling of an alkyl chloride at the cis isomer gives a new species which can be reoxidised. The independence of this coupling on additional halide rules out the reverse of an S(N)2 reaction, leaving a concerted process as the only sensible reaction pathway.     

Synthesis,characterization and optical properties of a new cationic cyclometalated platinum(ii) complex

The cyclometalated platinum (II) complex, [Pt(ppy)(ppyH)₂] OTF, 2 , in which ppy and ppyH denote the cyclometalated and non‐cyclometalated 2‐phenylpyridine ligand respectively, was prepared from the reaction of the platinum(IV) complex [PtMe₃(OTF)], 1 , with 3 equiv 2‐phenylpyridine at room temperature. The cyclometalated complex 2 was characterized using ¹H NMR spectroscopy. The solid state structure of 2 was further identified by single crystal X‐ray structure determination. 2 displays a green emission in solution and in solid state at room temperature and TD‐DFT calculations is used to elucidate the origin of the electronic transitions in the UV–vis spectrum of 2 .     

Heteroleptic platinum(II) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position

A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO2 (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H (d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460-480 nm) and emit from fluid solution and in the solid state at room temperature at 600-630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20-30 micros in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group.     

Synthesis, experimental and theoretical characterization of palladium(II) and platinum(II) saccharinate complexes with 2-(2-pyridyl)benzimidazole

New palladium(II) and platinum(II) complexes of saccharinate (sac) with 2-(2-pyridyl)benzimidazole (pybim) have been synthesized and characterized by elemental analysis and spectroscopic techniques. From the experimental studies, these complexes were formulated as [Pd(pybim)(sac)2] (1), and [Pt(pybim)(sac)2]·4H2O (2). The ground-state geometries of both complexes were optimized using density functional theory (DFT) methods at the B3LYP level. A bidentate pybim ligand together with two N-coordinated sac ligands form the square-planar MN4 coordination geometry around the palladium(II) and platinum(II) ions. The calculated IR and UV-vis spectral data have been correlated to the experimental results. Thermal analysis data support the molecular structures of both complexes.     

XANES determination of the platinum oxidation state distribution in cancer cells treated with platinum(IV) anticancer agents

Here we describe the use of X-ray absorption near edge spectroscopy (XANES) to provide information about the relative proportions of platinum(II) and platinum(IV) complexes by analyzing the XANES edge height. The intracellular reduction of platinum(IV) complexes in cancer cells has been observed directly, and the proportion of reduction after 2 h was found to correlate with the reduction potentials of the complexes.     

Sorption recovery of platinum(II) and platinum(IV) chloro complexes with a heterochain sulfur-containing sorbent

The recovery of platinum(II) and platinum(IV) chloro complexes from hydrochloric acid and chloride solutions with a new heterochain S,N-containing sorbent, MITKhAT, was studied. The suggested mechanism of formation of platinum(II) mercapto-thio ether complexes in the course of sorption was confirmed by DFT calculations. The results of group and selective recovery of platinum metals with MITKhAT sorbent from simulated and real industrial solutions are reported.     

Reactivity of cyclometallated platinum complexes with chiral ligands

The reaction of [Pt₂Me₄(μ-SMe₂)₂] with 3-substituted iminic thiophenes and 2-phenylpyridine gives platinum (II) [C,N] cyclometallated complexes which contain a labile ligand (SMe₂ or CH₃CN). Several platinum (II) complexes have been synthesized by substitution reactions with phosphine or sulfoxide ligands to introduce, in most cases, a second chiral center. The new complexes’ reactions with methyl iodide were subsequently studied and showed results that are dependent on the steric and electronic effects of both the cyclometallated ligand and the ancillary phosphine or sulfoxide ligand. The structure of [PtMe((R)-C₁₀H₇CHMeNCHC₄H₂S)(CH₃CN)], a synthetic precursor, is also reported.     

X-ray structures of the first platinum complexes with Z configuration iminoether ligands: trans-dichlorobis(1-imino-1-methoxy-2,2'-dimethylpropane)platinum(II) and trans-tetrachlorobis(1-imino-1-methoxy-2,2'-dimethylpropane)platinum(IV)

Platinum complexes with Z configuration iminoether ligands (trans-[PtCl(2)(HN=C(OMe)Bu(t))(2)], 1, and trans-[PtCl(4)(HN=C(OMe)Bu(t))(2)], 2) have been structurally characterized for the first time. The nearly planar Pt-N-C-O-C chain, all atoms being in gauche conformation, brings the terminal Pt and C atoms very close to one another. The steric clash is released by considerably increasing the Pt-N-C, N-C-O, and C-O-C bond angles (133, 124, and 121 degrees for 1, respectively; 147, 129, and 127 degrees for 2, respectively), which are well above the expected values (120 degrees for Pt-N-C and N-C-O; less than 120 degrees for C-O-C owing to the repulsive effect exerted by the lone pair of electrons on the oxygen atom). In the platinum(II) case the smaller increase of bond angles is accompanied by a greater value of the Pt-N-C-O torsion angle (27.3 and 15.6 degrees for 1 and 2, respectively). The stabilization of the Z configuration, notwithstanding the steric clashes described above, has been achieved by a careful choice of the R substituent in the iminoether moiety (a bulky tert-butyl group). The reactions of the platinum(IV) species (2) in basic and acidic conditions and with triphenylphosphine have been investigated. Bases and acids both interact with the coordinated ligand in such a way to weaken the coordinative bond and promote the release of the iminoether ligands. The phosphine promotes a ready and complete reduction of the platinum(IV) complex to the corresponding platinum(II) species (1). Compound 1 reacts with a stoichiometric amount of phosphine (1:1 molar ratio) to form cis-[PtCl(2)(PPh(3))(Z-HN=C(OMe)Bu(t))] and with excess phosphine to form [PtCl(2)(PPh(3))(2)] and free iminoether. The latter two reactions leading to formation of a mixed phosphine/iminoether platinum species and to free iminoether, which can be used as a synthon for further organic transformations, can be of synthetic utility.     

Synthesis, photophysical and electrophosphorescent properties of fluorene-based platinum(II) complexes

A series of platinum(II) complexes bearing tridentate cyclometalated C^N^N (C^N^N=6-phenyl-2,2'-bipyridine and π-extended R-C^N^N=3-[6'-(naphthalen-2'-yl)pyridin-2'-yl]isoquinoline) ligands with fluorene units have been synthesised and their photophysical properties have been studied. The fluorene units are incorporated into the cyclometalated ligands by a Suzuki coupling reaction. An increase in the π-conjugation of the cyclometalated ligands confers favourable photophysical properties compared to the 6-phenyl-2,2'-bipyridine analogues. The fluorene-based platinum(II) complexes display vibronic-structured emission bands with λ(max)=558-601 nm, and high emission quantum yields up to 0.76 in degassed dichloromethane. Their emissions are tentatively assigned to excited states with mixed (3)IL/(3)MLCT parentage (IL=intraligand, MLCT=metal-to-ligand charge transfer). The crystal structures of these platinum(II) complexes reveal extensive Pt(II)···π and/or π-π interactions. The fluorene-based platinum(II) complexes are soluble in organic solvents, have high thermal stability with decomposition temperature >350 °C, and can be thermally vacuum-sublimed or solution-processed as phosphorescent dopants for the fabrication of organic light-emitting diodes (OLEDs). A monochromic OLED with 3d as dopant (2 wt%) fabricated by vacuum deposition gave a current efficiency of 14.7 cd A(-1) and maximum brightness of 27000 cd m(-2). A high current efficiency (9.2 cd A(-1)) has been achieved in a solution-processed OLED using complex 3f (5 wt%) doped in a PVK (poly(9-vinylcarbazole)) host.     

Harnessing chemoselective imine ligation for tethering bioactive molecules to platinum(IV) prodrugs

Platinum(II) anticancer drugs are among the most effective and often used chemotherapeutic drugs. In recent years, there has been increasing interest in exploiting inert platinum(IV) scaffolds as a prodrug strategy to mitigate the limitations of platinum(II) anticancer complexes. In this prodrug strategy, the axial ligands are released concomitantly upon intracellular reduction to the active platinum(II) congener, offering the possibility of conjugating bioactive co-drugs which may synergistically enhance cytotoxicity on cancer cells. Existing techniques of tethering bioactive molecules to the axial positions of platinum(IV) prodrugs suffer from limited scope, poor yields and low reliability. This report explores the applications of current chemoselective ligation chemistries to platinum(IV) anticancer complexes with the aim of addressing the aforementioned limitations. Here, we describe the synthesis of a platinum(IV) complex bearing an aromatic aldehyde functionality and explored the scope of imine ligation with various hydrazide and aminooxy functionalized substrates. As a proof of concept, we tethered a six sequence long peptide mimetic (AMVSEF) of the anti-inflammatory protein, ANXA1.     

Luminescent cyclometallated platinum(II) complexes with N-benzoyl thiourea derivatives as ancillary ligands

A series of cyclometallated 2-phenylpyridine Pt(II) complexes having N-benzoyl thiourea derivatives as ancillary ligands were prepared and characterised by elemental analysis, IR and UV–Vis spectroscopy, ¹H and ¹³C NMR spectroscopy as well as by X-ray diffraction on single-crystal. All complexes were obtained as a single isomer with N atom of the 2-phenylpyridine ligand and S atom of the N-benzoyl thiourea derivatives coordinated in trans positions to the platinum metal as evidenced by using X-ray crystallography and NMR spectroscopy. All Pt(II) complexes show good luminescence properties at room temperature, both in dichloromethane solution and in solid state.     

Special effects of ortho-isopropylphenyl groups. Diastereoisomerism in platinum(II) and palladium(II) complexes of helically chiral PAr3 ligands

The coordination chemistry of the four phosphines, P{C6H3(o-CH3)(p-Z)}3 where Z = H (1a) or OMe (1b) and P{C6H3(o-CHMe2)(p-Z)}3 Z = H (1c) or OMe (1d) with platinum(II) and palladium(II) is reported. Mononuclear complexes trans-[PdCl2L2](L = 1a,b) and trans-[PtCl2L2](L = 1a-c) have been prepared and the crystal structures of trans-[PdCl2(1b)2] and trans-[PtCl2(1c)2] as their dichloromethane solvates have been determined. The structures show that in these complexes, the ligands adopt g+ g+ a conformations. Examination of the Cambridge Structural Database confirms this to be one of only two conformer types that tri-o-tolylphosphines adopt and the only viable conformer in 4 and 6 coordinate complexes. The binuclear complexes trans-[Pd2Cl4L2](L = 1c,d) are formed even when an excess of the bulky 1c,d is used in the synthesis and the crystal structure of trans-[Pd2Cl4(1c)2] as its chloroform solvate is reported. Reaction of [PtCl2(NCBu(t))2] with 1b-d in refluxing toluene gave the cycloplatinated species [Pt2Cl2(L - H)2] where L - H is phosphine 1b-d deprotonated at one of the ortho-methyl carbon atoms. Variable temperature 31P and 1H NMR spectroscopy reveals that all the complexes reported are fluxional. The processes are analysed in terms of restricted P-C and P-M rotations that give rise to diastereoisomeric rotamers because of the helically chiral orientations of the aryl substituents. For the complexes of the bulky ligands 1c,d, rotation about the P-C bond is slow on the NMR timescale even up to 75 degrees C. The crystal structure of the cyclometallated complex [Pt2Cl2(1d - H)2] has been determined.     

Hydrogen bonding directs the H2O2 oxidation of platinum(II) to a cis-dihydroxo platinum(IV) complex

The use of ligands with proximate hydrogen bonding substituents in the oxidation of platinum(II) dimethyl complexes with H2O2 leads to the exclusive formation of an unusual cis-dihydroxo platinum(IV) complex, which can dehydrate to form a trinuclear metalla-azacrown complex.     

Electrochemical behaviour ofcis-dichloro(arylisocyanide)(tertiaryphosphine) platinum (II) and palladium(II) complexes in an aprotic solvent

Summary The electrochemical behaviour of neutral platinum(II) and palladium(II) isocyanide complexes has been investigated in an aprotic medium at platinum and mercury electrodes. Platinum(II) derivatives are reduced to platinum(0) species, Palladium(II) compounds give rise to palladium(0) species at room temperature, while at 0° it is possible to obtain palladium(I) compounds.     

Rhodium, palladium and platinum complexes of tris(pyridylalkyl)amine and tris(benzimidazolylmethyl)amine N4-tripodal ligands

To investigate the influence of a potentially N4-tripodal amine ligand on the structure and internal exchange processes of its complexes with late transition metals, five rhodium, six palladium and two platinum complexes have been prepared from seven alkyl-bridged N-heterocyclic amine tripodal ligands: tris(2-pyridylmethyl)amine, (2-(2-pyridylethyl))bis(2-pyridylmethyl)amine, bis(2-(2-pyridylethyl))-2-pyridylmethylamine, bis(2-(2-pyridylethyl))amine, ((6-(hydroxymethyl)-2-pyridyl)methyl)bis(2-pyridylmethyl)amine, tris(2-benzimidazolylmethyl)amine (tbima) and tris(3-ethyl-2-benzimidazolylmethyl)amine. Single-crystal X-ray diffraction studies were completed for ten complexes: the d6-rhodium(III) complexes are octahedral with kappa 4 N-bound ligands, whereas the d8-palladium(II) and d8-platinum(II) complexes are square planar, kappa 3 N-bound by the tripodal ligand with a dangling N-donor leg, except for the unusual [Pd2(tbima)2Cl2]Cl2 dimer in which each palladium(II) ion is square planar and bound by two benzimidazole legs from one tbima ligand, one leg from the other tbima ligand and a chloride ancillary ligand. Cation bilayers are a common structural motif in the crystal structures. Variable-temperature 1H NMR studies reveal exchange occurs between the coordinated and dangling N-donor legs in the palladium and platinum complexes. Exchange free energy (Delta G++ c) values have been calculated and some general rules governing the favoured complex structures and exchange pathways elucidated. The palladium(II) and platinum(II) complexes of a ligand with an pyridylethyl leg are unstable with respect to elimination of vinylpyridine.     

Steric Crowding and Redox Reactivity in Platinum(II) and Platinum(IV) Complexes Containing Substituted 1,10-Phenanthrolines

The effect of the phenanthroline substituents on the structure and reactivity of platinum(II) and platinum(IV) complexes has been investigated. The X-ray crystal structures of the compounds [PtI(2)(4,7-Ph(2)phen)].CHCl(3) (1dz.CHCl(3)), [PtI(4)(4,7-Ph(2)phen)].CHCl(3) (2dz.CHCl(3)), [PtI(2)(2,9-Me(2)-4,7-Ph(2)phen)] (1fz), and [PtI(4)(2,9-Me(2)-4,7-Ph(2)phen)].I(2) (2fz.I(2)) have shown that complexes 1fz and 2fz, containing ortho-substituted phenanthrolines, exhibit a remarkable displacement of the equatorial iodine atoms from the N-Pt-N' plane (average 0.477(2) and 0.199(2) ?, respectively), a bending of the phenanthroline [angle between outer rings of 19.9(7) and 14.2(7) degrees, respectively] and a rotation of the N-C-C'-N' plane with respect to the N-Pt-N' plane [32.3(10) and 26.5(9) degrees, respectively]. Comparison between the structures of 1fz and 2fz, both having the phenanthroline with methyl substituents in the ortho position, indicates that, in the latter case, because of the presence of the two axial iodine ligands, the displacements of the ligands from the equatorial plane are smaller and find a compensation in a narrowing of the I(1)-Pt-I(1') angle (5 degrees ) and a lengthening of the Pt-N bonds (0.07 ?). The electrochemical behavior of the four-coordinate platinum(II) complexes shows that compounds possessing regular planar geometry have access to the one-electron reduced species, whereas those with distorted coordination geometry are irreversibly reduced by collapsing of the complex geometry. This is in sharp contrast with the behavior of related nickel complexes for which the pseudo-tetrahedral coordination imposed by bulky 2,9-substituents of phenanthroline stabilizes the nickel(I) species. Spectroscopic results allow us to assign a significant Pt(I) character to [1d](-) monoanions. The electrogenerated, plus one electron, complexes are not indefinitely stable and, because of conjugation with the phen ligand, progressively restore the Pt(II) oxidation state by transferring the electron to the peripheral organic ligand. The latter process can involve multiple electron additions in the macroelectrolysis time scale. The related platinum(IV) complexes [PtX(4)(L)] undergo irreversible two-electron reduction accompanied by fast release of the axial ligands and formation of the corresponding platinum(II) species.