Substituents' Effect on the Photophysics of Trinuclear Copper(I) and Silver(I) Pyrazolate-Phosphine Cages

A series of structurally similar trinuclear macrocyclic copper(I) and silver(I) pyrazolate complexes bearing various short-bite diphosphine R2PCH(R')PR2 ligands are reported. Upon diphosphine coordination, the planar geometry of the initial complexes undergoes bending along the line between two metal atoms coordinated to the phosphorus moieties. The complexes based on dcpm ligands (R = cyclohexyl, R' = H, Ph) do not exhibit dynamic behavior in solution at room temperature on the 31P NMR time scale as it was previously observed for similar trinuclear copper complexes bearing the dppm (R = Ph, R' = H) scaffold. All copper(I) complexes exhibit thermally activated delayed fluorescence (TADF) behavior in the solid state. Importantly, the use of aliphatic substituents on the phosphorus atoms instead of aromatic ones leads to an almost double increase in the quantum efficiency (ΦPL) of photoluminescence by eliminating nonradiative decay from the 3LCPh states of the dppm aromatic rings. The higher donating ability of the substituents in the pyrazolate ligand (CF3 vs CH3) lowers the energy of the metal-centered excited state, allowing for a significant metal impact on the T1 state. Finally, the Ag(I) complex displays an emission efficiency of approximately 14%, being the highest among known trinuclear silver(I) pyrazolate homometallic derivatives.

Synthesis and structural characterization of stable coinage metal (Cu, Ag, Au) cyclopentadienyl complexes

The electron withdrawing and oxidatively stable perfluorinated Cp* ligand [C5(CF3)5]- allowed for the isolation of rare and unusually stable coinage metal complexes [M(C5(CF3)5)(PtBu3)] (M = Cu, Ag, Au), representing the first complete and structurally comparable series of group 11 Cp coordination compounds. Full characterization and structure analysis revealed distinct and partly unknown coordination motifs with hapticities ranging from η1, η3/η1 and η3/η2 for gold, silver and copper, respectively. Quantum-chemical studies using DFT methods confirm these findings and connect them to the unique electronic structure of the given ligand system.

Synthesis and Investigations of the Antitumor Effects of First-Row Transition Metal(II) Complexes Supported by Two Fluorinated and Non-Fluorinated β-Diketonates

The 3d transition metal (Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)) complexes, supported by anions of sterically demanding β-diketones, 1,3-dimesitylpropane-1,3-dione (HLMes) and 1,3-bis(3,5-bis(trifluoromethyl)phenyl)-3-hydroxyprop-2-en-1-one (HLCF3), were synthesized and evaluated for their antitumor activity. To assess the biological effects of substituents on phenyl moieties, we also synthesized and investigated the analogous metal(II) complexes of the anion of the less bulky 1,3-diphenylpropane-1,3-dione (HLPh) ligand. The compounds [Cu(LCF3)2], [Cu(LMes)2] and ([Zn(LMes)2]) were characterized by X-ray crystallography. The [Cu(LCF3)2] crystallizes with an apical molecule of solvent (THF) and features a rare square pyramidal geometry at the Cu(II) center. The copper(II) and zinc(II) complexes of diketonate ligands, derived from the deprotonated 1,3-dimesitylpropane-1,3-dione (HLMes), adopt a square planar or a tetrahedral geometry at the metal, respectively. We evaluated the antitumor properties of the newly synthesized (Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II)) complexes against a series of human tumor cell lines derived from different solid tumors. Except for iron derivatives, cellular studies revealed noteworthy antitumor properties, even towards cancer cells endowed with poor sensitivity to the reference drug cisplatin.

Synthesis and cytotoxicity studies of Cu(I) and Ag(I) complexes based on sterically hindered β-diketonates with different degrees of fluorination

Design, synthesis, and in vitro antitumor properties of Cu(I) and Ag(I) phosphane complexes supported by the anions of sterically hindered β-diketone ligands, 1,3-dimesitylpropane-1,3-dione (HLMes) and 1,3-bis(3,5-bis(trifluoromethyl)phenyl)-3-hydroxyprop-2-en-1-one (HLCF3) featuring trifluoromethyl or methyl groups on the phenyl moieties have been reported. In order to compare the biological effects of substituents on the phenyl moieties, the analogous copper(I) and silver(I) complexes of the anion of the parent 1,3-diphenylpropane-1,3-dione (HLPh) ligand were also synthesized and included in the study. In the syntheses of the Cu(I) and Ag(I) complexes, the phosphane coligands triphenylphosphine (PPh3) and 1,3,5-triaza-7-phosphaadamantane (PTA) were used to stabilize silver and copper in the +1 oxidation state, preventing the metal ion reduction to Ag(0) or oxidation to Cu(II), respectively. X-ray crystal structures of HLCF3 and the metal adducts [Cu(LCF3)(PPh3)2] and [Ag(LPh)(PPh3)2] are also presented. The antitumor properties of both classes of metal complexes were evaluated against a series of human tumor cell lines derived from different solid tumors, by means of both 2D and 3D cell viability studies. They display noteworthy antitumor properties and are more potent than cisplatin in inhibiting cancer cell growth.

Terminal and Internal Alkyne Complexes and Azide-Alkyne Cycloaddition Chemistry of Copper(I) Supported by a Fluorinated Bis(pyrazolyl)borate

Copper plays an important role in alkyne coordination chemistry and transformations. This report describes the isolation and full characterization of a thermally stable, copper(I) acetylene complex using a highly fluorinated bis(pyrazolyl)borate ligand support. Details of the related copper(I) complex of HC≡CSiMe₃ are also reported. They are three-coordinate copper complexes featuring η²-bound alkynes. Raman data show significant red-shifts in C≡C stretch of [H₂B(3,5-(CF₃)₂Pz)₂]Cu(HC≡CH) and [H₂B(3,5-(CF₃)₂Pz)₂]Cu(HC≡CSiMe₃) relative to those of the corresponding alkynes. Computational analysis using DFT indicates that the Cu(I) alkyne interaction in these molecules is primarily of the electrostatic character. The π-backbonding is the larger component of the orbital contribution to the interaction. The dinuclear complexes such as Cu₂(μ-[3,5-(CF₃)₂Pz])₂(HC≡CH)₂ display similar Cu-alkyne bonding features. The mononuclear [H₂B(3,5-(CF₃)₂Pz)₂]Cu(NCMe) complex catalyzes [3 + 2] cycloadditions between tolyl azide and a variety of alkynes including acetylene. It is comparatively less effective than the related trinuclear copper catalyst {μ-[3,5-(CF₃)₂Pz]Cu}₃ involving bridging pyrazolates.

Tetranuclear and trinuclear copper(I) pyrazolates as catalysts in copper mediated azide-alkyne cycloadditions (CuAAC)

Homoleptic, tetranuclear copper(I) pyrazolates {[3,5-(t-Bu)₂Pz]Cu}₄, {[3-(CF₃)-5-(t-Bu)Pz]Cu}₄, and {[4-Br-3,5-(i-Pr)₂Pz]Cu}₄ are excellent stand-alone catalysts for azide-alkyne cycloaddition reactions (CuAAC). This work demonstrates that a range of pyrazolates, including those with electron donating and electron-withdrawing groups to sterically demanding substituents on the pyrazolyl backbones, can serve as effective ligand supports on tetranuclear copper catalysts. However, in contrast to the tetramers and also highly fluorinated {[3,5-(CF₃)₂Pz]Cu}₃, trinuclear copper(I) complexes such as {[3,5-(i-Pr)₂Pz]Cu}₃ and {[3-(CF₃)-5-(CH₃)Pz]Cu}₃ supported by relatively electron rich pyrazolates display poor catalytic activity in CuAAC. The behavior and degree of aggregation of several of these copper(I) pyrazolates in solution were examined using vapor pressure osmometry. Copper(I) complexes such as {[3,5-(CF₃)₂Pz]Cu}₃ and {[3-(CF₃)-5-(t-Bu)Pz]Cu}₄ with electron withdrawing pyrazolates were found to break up in solution to different degrees producing smaller aggregates while those such as {[3,5-(i-Pr)₂Pz]Cu}₃ and {[3,5-(t-Bu)₂Pz]Cu}₄ with electron rich pyrazolates remain intact. In addition, kinetic experiments were performed to understand the unusual activity of tetranuclear copper(I) pyrazolate systems.

A Computational Study of Metallacycles Formed by Pyrazolate Ligands and the Coinage Metals M = Cu(I), Ag(I) and Au(I): (pzM)n for n = 2, 3, 4, 5 and 6. Comparison with Structures Reported in the Cambridge Crystallographic Data Center (CCDC)

The structures reported in the Cambridge Structural Database (CSD) for neutral metallacycles formed by coinage metals in their valence (I) (cations) and pyrazolate anions were examined. Depending on the metal, dimers and trimers are the most common but some larger rings have also been reported, although some of the larger structures are not devoid of ambiguity. M06-2x calculations were carried out on simplified structures (without C-substituents on the pyrazolate rings) in order to facilitate a comparison with the reported X-ray structures (geometries and energies). The problems of stability of the different ring sizes were also analyzed.