Stability of metal-metal interactions in transmetallation intermediates based on electronics of bridging arene ligands determined through pyridine titrations

In this contribution, we prepare the dinuclear complex [(CNCF)(PPh3)Pt-Au(PPh3)]+ (2-F) supported by an electron deficient derivative of 2,6-diphenylpyridine (CNC), 2,6-di(4-fluorophenyl)pyridine (CNCF). Solution state spectroscopic data and solid-state structural data reveals formation of the desired dinuclear complex occurs and that it remains intact in solution. The solid state structure of 2-F, compared to [(CNC)(PPh3)Pt-Au(PPh3)]+ (2), reveals a substantial change in the C-Au-P bond angle. We postulated that this change in bond angle arises due to a weaker interaction between [(PPh3)Au]+ and (CNCF)Pt(PPh3) (1-F) vs. (CNC)Pt(PPh3) (1). Through pyridine titration experiments, we demonstrate that the interaction is indeed weaker between [(PPh3)Au]+ and 1-Fvs. 1. Cyclic voltammetry (CV) experiments confirm that 1-F is less electron rich than 1. DFT calculations demonstrate that the HOMO of 1 and 1-F is not dz2, helping explain the differences in electrochemical behavior of 1 and 1-F and bonding between 1 and 1-F with [(PPh3)Au]+.

Influence of the Steric/Electronic Properties of N-Aryl Substituents in Cycloplatinated Guanidinate(1-) Complexes on the Formation of Discrete Pt → Ag Complexes and One-Dimensional Coordination Polymer

The reactions of cycloplatinated guanidinate(1-) complexes 1-6 with AgTFA (TFA = OC(O)CF₃) in 1:1 and 1:2 Pt^II/Ag^I molar ratios afforded complexes containing three types of Pt₂Ag₂ skeletons (7-10, 11, and 13), one 1D CP containing Pt₂Ag₃ skeleton (16), and a Pt₂Ag₄ complex (17) in 91-95% (method 1), 73-82% (method 2) (7-10), and 54-79% (11, 13, 16, and 17) yields. The reactions of 11 with 2,6-XylNC (2,6-Xyl = 2,6-Me₂C₆H₃) and 4-DMAP (4-dimethylaminopyridine) gave a neutral complex 18 and an ionic complex 19, respectively. Molecular structures of 12 complexes were unambiguously determined by single-crystal X-ray diffraction. Ten complexes contain unprecedented PtAg skeletons and are shown to contain multiple number of dative Pt → Ag bonds supported by the TFA ligand, platinated carbon of the TAG ligand, or both these ligands in conjunction with the Ag-Ag contacts stabilized by argentophilic interaction. Further, the new complexes were characterized by analytical, IR, and multinuclear NMR (¹⁹F, ¹H, ¹³C{¹H}, and ¹⁹⁵Pt) spectroscopies, powder X-ray diffraction, and TGA/DTA. In solution, 9 and 19 exist in more than one form as identified by multinuclear NMR, and this behavior is ascribed to the restricted (N₂)C-N(H)Ar single-bond rotation of the TAG ligand. The photophysical properties of 9 and 11 are reported.

Luminescent Anionic Cyclometalated Organoplatinum (II) Complexes with Terminal and Bridging Cyanide Ligand: Structural and Photophysical Properties

We present the synthesis and characterization of two series of mononuclear heteroleptic anionic cycloplatinated(II) complexes featuring terminal cyanide ligand Q⁺[Pt(C^N)(p-MeC₆H₄)(CN)]^- [C^N = benzoquinolate (bzq), Q⁺ = K⁺ 1 and NBu₄⁺ 4; 2-phenylpyridinate (ppy), Q⁺ = K⁺ 2 and NBu₄⁺ 5 and 2-(2,4- difluorophenyl)pyridinate (dfppy), Q⁺ = K⁺ 3 and NBu₄⁺ 6] and a series of symmetrical binuclear complexes (NBu₄)[Pt₂(C^N)₂(p-MeC₆H₄)₂(μ-CN)] (C^N = bzq 7, ppy 8, dfppy 9). Compounds 5, 6, and 7-9 were further determined by single-crystal X-ray diffraction. There are no apparent intermolecular Pt···Pt interactions owing to the presence of bulky NBu₄⁺ counterion. Slow crystallization of K[Pt(ppy)(p-MeC₆H₄)(CN)] 2 in acetone/hexane evolves with formation of yellow crystals, which were identified by single-crystal X-ray diffraction methods as the salt complex {[Pt(ppy)(p-MeC₆H₄)(CN)]₂K₃(OCMe₂)₄(μ-OCMe₂)₂}[Pt(ppy)(p-MeC₆H₄)(μ-CN)Pt(ppy)(p-MeC₆H₄)]·2acetone (10), featuring the binuclear anionic unit 8^- neutralized by an hybrid inorganic-organometallic coordination polymer {[Pt(ppy)(p-MeC₆H₄)(CN)]₂K₃(OCMe₂)₄(μ-OCMe₂)₂}⁺. The photophysical properties of all compounds were recorded in powder, polystyrene film, and solution states with a quantum yield up to 21% for 9 in the solid state. All complexes displayed bright emission in rigid media, and for the interpretation of their absorption and emission properties, density functional theory (DFT) and time-dependent DFT calculations were applied.

Transition Metal Chain Complexes Supported by Soft Donor Assembling Ligands

The chemistry of discrete molecular chains constituted by metals in low oxidation states, displaying metal-metal proximity and stabilized by suitable metal-bridging, assembling ligands comprising at least one soft donor atom is comprehensively reviewed; complexes with a single (hard or soft) bridging atom (e.g., μ-halide, μ-sulfide, or μ-PR₂ etc.) as well as "closed" metal arrays (that fall in the realm of cluster chemistry) are excluded. The focus is on transition metal-based systems, with few excursions to cases combining transition and post-transition elements. Most relevant supporting ligands have neutral C, P, O, or S donor (mainly, N-heterocyclic carbene, phosphine, ether, thioether) or anionic donor (mainly phenyl, ylide, silyl, phosphide, thiolate) groups. A supporting-ligand-based classification of the metal chains is introduced, using as the classifying parameter the number of "bites" (i.e., ligand bridges) subtending each intermetallic separation. The ligands are further grouped according to the number of donor atoms interacting with the metal chain (called denticity in the following) and the column of the Periodic Table to which the set of donor atoms belongs (in ascending order). A complementary metal-based compilation of the complexes discussed is also provided in a concise tabular form.

Reactions of Cycloplatinated Guanidine Complexes with Hg(OC(O)CF3)2: Formation of a One-Dimensional Coordination Polymer Containing a Pt2Hg(μ2-S(O)Me2-S,O) Repeating Unit versus a Discrete Pt2Hg2 Complex

Separate reactions of cycloplatinated 2-tolyl- and 2-anisylguanidine complexes, [Pt{κ²(C,N)}(OC(O)CF₃)(S(O)Me₂)] (1 and 2), with Hg(OC(O)CF₃)₂ in 1:0.5 and 1:1 molar ratios afforded the one-dimensional coordination polymer (1D CP) {[Pt^III{κ²(C,N)}(OC(O)CF₃)₂]₂Hg⁰}(μ₂-S(O)Me₂-S,O)·C₇H₈ (3·C₇H₈) as bright red crystals and the discrete tetrametallic complex [Pt^II{κ²(C,N)}(μ₂-OC(O)CF₃)₂Hg^I-]₂ (4) as yellow crystals in good yields. The two different products obtained in the aforementioned reactions are ascribed to the subtle differences in the N substituent of the guanidinate(1-) ligands in 1 and 2. The plausible mechanisms of formation of 3 and 4 are outlined. Complexes 3 and 4 were characterized by elemental analyses and IR and multinuclear NMR (¹H, ¹³C{¹H}, ¹⁹F, and ¹⁹⁵Pt) spectroscopy. Complex 4 was also characterized by ¹⁹⁹Hg NMR spectroscopy. The molecular structures of 3·C₇H₈ and 4 were determined by single-crystal X-ray diffraction studies. 1D CP 3·C₇H₈ contains a Pt(III)-Hg(0)-Pt(III)(μ₂-S(O)Me₂-S,O) repeating unit with a pair of unsupported Pt-Hg covalent bonds, while 4 contains a Pt(II)-Hg(I)-Hg(I)-Pt(II) chain with a pair of trifluoroacetate ligand supported Pt→Hg coordinate bonds. 1D CP 3·C₇H₈ falls apart into a mixture of three species, namely 6-8 and 9-11 in C₆D₆ and CDCl₃, respectively, as revealed by multinuclear NMR spectroscopy. In CDCl₃, 4 partially isomerizes to [Pt^II(OC(O)CF₃)(μ₂-OC(O)CF₃){κ³μ₂(C,N,O)}Hg^I-]₂ (12), wherein each Pt→Hg coordinate bond is supported by one μ₂-bridging trifluoroacetate ligand and one chelating bridging guanidinate(1-) ligand, as inferred from variable-temperature ¹H and ¹⁹F NMR spectroscopy. Complex 12 is the major species and 4 is the minor species in CDCl₃, while opposite situation prevails in C₆D₆. The observance of a mixture of two solution species for 4 is ascribed to a rapid "carboxylate shift" process induced by the oxygen atom of the ═N(C₆H₄(OMe)-2) unit of the guanidinate(1-) ligand through neighboring-group participation. UV-visible absorption and emission spectra of 4 were measured in CHCl₃, and from the outcome of the investigation, the possible existence of [Cl₂(H)C-Cl···Pt^II(OC(O)CF₃)(μ₂-OC(O)CF₃){κ³μ₂(C,N,O)}Hg^I-]₂ (12″) was suggested, which is likely to have a pair of Pt-Hg covalent bonds made possible by CHCl₃ coordination on the sixth site of the Pt(II) atom.

On the Heterogeneous Nature of Cisplatin-1-Methyluracil Complexes: Coexistence of Different Aggregation Modes and Partial Loss of NH3 Ligands as Likely Explanation

The conversion of the 1 : 1-complex of Cisplatin with 1-methyluracil (1MeUH), cis-[Pt(NH3 )2 (1MeU-N3)Cl] (1 a) to the aqua species cis-[Pt(NH3 )2 (1MeU-N3)(OH2 )]+ (1 b), achieved by reaction of 1 a with AgNO3 in water, affords a mixture of compounds, the composition of which strongly depends on sample history. The complexity stems from variations in condensation patterns and partial loss of NH3 ligands. In dilute aqueous solution, 1 a, and dinuclear compounds cis-[(NH3 )2 (1MeU-N3)Pt(μ-OH)Pt(1MeU-N3)(NH3 )2 ]+ (3) as well as head-tail cis-[Pt2 (NH3 )4 (μ-1MeU-N3,O4)2 ]2+ (4) represent the major components. In addition, there are numerous other species present in minor quantities, which differ in metal nuclearity, stoichiometry, stereoisomerism, and Pt oxidation state, as revealed by a combination of 1 H NMR and ESI-MS spectroscopy. Their composition appears not to be the consequence of a unique and repeating coordination pattern of the 1MeU ligand in oligomers but rather the coexistence of distinctly different condensation patterns, which include μ-OH, μ-1MeU, and μ-NH2 bridging and combinations thereof. Consequently, the products obtained should, in total, be defined as a heterogeneous mixture rather than a mixture of oligomers of different sizes. In addition, a N2 complex, [Pt(NH3 )(1MeU)(N2 )]+ appears to be formed in gas phase during the ESI-MS experiment. In the presence of Na+ ions, multimers n of 1 a with n=2, 3, 4 are formed that represent analogues of non-metalated uracil quartets found in tetrastranded RNA.

From homonuclear to heteronuclear: a viable strategy to promote and modulate phosphorescence

The use of weakly emissive homonuclear Pt(ii) precursors to fabricate Pt–M heteronuclear complexes with intermetallic contacts paves a unique avenue to achieve highly efficient phosphorescence as well as modulate emission energy.

Coordination Isomers of Trinuclear Pt2Hg Complex That Differ in Type of Metal-Metal Bond

New type isomers that differ in metal-metal bond type are isolated. One is the Pt-Hg-Pt complex, which has two metal-metal covalent bonds, Pt-Hg = 2.5459(8) and 2.5483(7) Å, and the other is the Pt→Hg-Pt complex, which has one metal-metal covalent bond, Pt-Hg = 2.5744(6) Å, and one metal-metal dative bond, Pt→Hg = 2.6635(7) Å.

New Pt→M (M = Ag or Tl) complexes based on anionic cyclometalated Pt(ii) complexes

Anionic cyclometalated complexes (NBu₄)[Pt(CNC)X] (X = Cl (1), CN (2), or S-2py (pyridine-2-thiolate) (3); -CNC- = 2,6-di(phen-2-ide)-pyridine) have been used as precursors in the synthesis of heteropolynuclear Pt-Ag or Pt-Tl complexes containing donor-acceptor metal-metal bonds. Their reaction with AgClO₄ or [Ag(OClO₃)(PPh₃)] produces complexes in which the nuclearity and structure seem to be determined by the ability of the ligand X to form bridges between the metals. Thus, the characteristic linear bridging of the cyano ligand leads to the formation of an octanuclear [{Pt(CNC)(μ-CN)}₄Ag₄] (4) or tetranuclear [{Pt(CNC)(μ-CN)}₂{Ag(PPh₃)}₂] (6) complex, with CN bridges between different "Pt-Ag" units. However, the S-2py ligand can act as a bridge between Pt and Ag of the same "Pt-Ag" unit, giving rise to the complex [{Pt(CNC)(S-2py)}₂Ag₂]·CH₂Cl₂ (5). On the other hand, the reaction of 1-3 with TlPF₆ yields the complexes [PtTl(CNC)Cl] (7), [PtTl(CNC)(CN)] (8), and (NBu₄)[{Pt(CNC)(S-2py)}₂Tl] (9), while with [Tl(S-2py)], [PtTl(CNC)(S-2py)] (10) is obtained. The structures of all these Pt-Tl complexes show great variation, with several geometric arrangements which sometime co-exist in the same crystal structure: discrete Pt-Tl, Pt-Tl-Pt and Pt-Tl-Pt-Tl units, as well as infinite Pt-Tl-Pt-Tl chains. This variability could be due to the lability of the Pt-Tl bonds and the ability of the thallium center to establish secondary interactions with donor atoms or aromatic π electron density from neighboring moieties.

Cyclometalated Platinum(II) Cyanometallates: Luminescent Blocks for Coordination Self-Assembly

A family of cyanide-bridged heterometallic aggregates has been constructed of the chromophoric cycloplatinated metalloligands and coordinatively unsaturated d¹⁰ fragments {M(PPh₃)_n}. The tetranuclear complexes of general composition [Pt(C^N)(CN)₂M(PPh₃)₂]₂ [C^N = ppy, M = Cu (1), Ag (2); C^N = tolpy (Htolpy = 2-(4-tolyl)-pyridine), M = Cu (4), Ag (5); C^N = F₂ppy (HF₂ppy = 2-(4, 6-difluorophenyl)-pyridine), M = Cu (7), Ag (8)] demonstrate a squarelike arrangement of the molecular frameworks, which is achieved due to favorable coordination geometries of the bridging ligands and the metal ions. Variation of the amount of the ancillary phosphine (for M = Ag) afforded compounds [Pt(C^N)(CN)₂Ag(PPh₃)]₂ (C^N = ppy, 3; C^N = tolpy, 6); for the latter one an alternative cluster topology, stabilized by the Pt-Ag metallophilic and η¹-C_ipso(C^N)-Ag bonding, was observed. The solid-state structures of all of the title species 1-8 were determined crystallographically. The complexes exhibit moderately strong room-temperature phosphorescence as crystalline powders (Φ_em = 16-34%, λ_em = 470-511 nm). The luminescence studies and time-dependent density functional theory computational analysis indicate that the photophysical behavior is dominated by the ³π-π* electronic transitions localized on the cyclometalated fragment and mixed with M_PtLCT contribution, while the d¹⁰-phosphine motifs have a negligible contribution into the frontier orbitals and therefore show a little influence on the emission performance of the described compounds.

Reversible formation and cleavage of Pt→Ag dative bonds in a pre-organized cavity of a luminescent heteropolynuclear platinum(ii) complex

A heteropolynuclear complex can reversibly capture an Ag(i) ion and release it from the pre-organized cavity by the concerted formation and cleavage of Pt→Ag dative bonds and Ag–C(ipso) bonds, respectively.

Unusual Metal-Metal Bonding in a Dinuclear Pt-Au Complex: Snapshot of a Transmetalation Process

The dinuclear Pt-Au complex [(CNC)(PPh3 )Pt Au(PPh3 )](ClO4 ) (2) (CNC=2,6-diphenylpyridinate) was prepared. Its crystal structure shows a rare metal-metal bonding situation, with very short Pt-Au and Au-Cipso (CNC) distances and dissimilar Pt-Cipso (CNC) bonds. Multinuclear NMR spectra of 2 show the persistence of the Pt-Au bond in solution and the occurrence of unusual fluxional behavior involving the [Pt(II) ] and [Au(I) ] metal fragments. The [Pt(II) ]⋅⋅⋅ [Au(I) ] interaction has been thoroughly studied by means of DFT calculations. The observed bonding situation in 2 can be regarded as a model for an intermediate in a transmetalation process.

An Extended Chain and Trinuclear Complexes Based on Pt(II)-M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical Behavior

The syntheses and structural characterizations of a Pt-Tl chain [{Pt(bzq)(C6F5)2}Tl(Me2CO)]n 1 and two trinuclear Pt2M clusters (NBu4)[{Pt(bzq)(C6F5)2}2Tl] 2 and [{Pt(bzq)(C6F5)2}2Pb] 3 (bzq = 7,8-benzoquinolinyl), stabilized by donor-acceptor Pt → M bonds, are reported. The one-dimensional heterometallic chain 1 is formed by alternate "Pt(bzq)(C6F5)2" and "Tl(Me2CO)" fragments, with Pt-Tl bond separations in the range of 2.961(1)-3.067(1) Å. The isoelectronic trinuclear complexes 2 (which crystallizes in three forms, namely, 2a, 2b, and 2c) and 3 present a sandwich structure in which the Tl(I) or Pb(II) is located between two "Pt(bzq)(C6F5)2" subunits. NMR studies suggest equilibria in solution implying cleavage and reformation of Pt-M bonds. The lowest-lying absorption band in the UV-vis spectra in CH2Cl2 and tetrahydrofuran (THF) of 1, associated with (1)MLCT/(1)L'LCT (1)[5dπ(Pt) → π*(bzq)]/(1)[(C6F5) → bzq], displays a blue shift in relation to the precursor, suggesting the cleavage of the chain maintaining bimetallic Pt-Tl fragments in solution, also supported by NMR spectroscopy. In 2 and 3, it shows a blue shift in THF and a red shift in CH2Cl2, supporting a more extensive cleavage of the Pt-M bonds in THF solutions than in CH2Cl2, where the trinuclear entities are predominant. The Pt-Tl chain 1 displays in solid state a bright orange-red emission ascribed to (3)MM'CT (M' = Tl). It exhibits remarkable and fast reversible vapochromic and vapoluminescent response to donor vapors (THF and Et2O), related to the coordination/decoordination of the guest molecule to the Tl(I) ion, and mechanochromic behavior, associated with the shortening of the intermetallic Pt-Tl separations in the chain induced by grinding. In frozen solutions (THF, acetone, and CH2Cl2) 1 shows interesting luminescence thermochromism with emissions strongly dependent on the solvent, concentration, and excitation wavelengths. The Pt2Tl complex 2 shows an emission close to 1, ascribed to charge transfer from the platinum fragment to the thallium [(3)(L+L')MM'CT]. 2 also shows vapoluminescent behavior in the presence of vapors of Me2CO, THF, and Et2O, although smaller and slower than those of 1. The trinuclear neutral complex Pt2Pb 3 displays a blue-shift emission band, tentatively assigned to admixture of (3)MM'CT (3)[Pt(d) → Pb(sp)] with some metal-mediated intraligand ((3)ππ/(3)ILCT) contribution. In contrast to 1 and 2, 3 does not show vapoluminescent behavior.

Luminescent pentafluorophenyl-cycloplatinated complexes: synthesis, characterization, photophysics, cytotoxicity and cellular imaging

Pentafluorophenyl cycloplatinated complexes have been synthesized and their photophysical properties studied and investigated by DFT, TD-DFT calculations. The cellular localization and the cytotoxicity of [Pt(C^N)(C₆F₅)(DMSO)] complexes towards two different cell lines has been assessed.