Synthesis and Photophysical Properties of Lanthanide Pyridinylphosphonic Tacn and Pyclen Derivatives: From Mononuclear Complexes to Supramolecular Heteronuclear Assemblies.

Synthetic methodologies were developed to achieve the preparation of ligands L1 and L2 consisting of tacn- and pyclen-based chelators decorated with pyridinylphosphonic pendant arms combined with ethylpicolinamide or acetate coordinating functions, respectively. Phosphonate functions have been selected for their high affinity toward Ln3+ ions compared to their carboxylated counterparts and for their steric hindrance that favors the formation of less-hydrated complexes. Thanks to regiospecific N-functionalization of the macrocyclic backbones, the two ligands were isolated with good yields and implicated in a comprehensive photophysical study for the complexation of Eu3+, Tb3+, and Yb3+. The coordination behavior of L1 and L2 with these cations has been first investigated by means of a combination of UV-vis absorption spectroscopy, steady-state and time-resolved luminescence spectroscopy, and 1H and 31P NMR titration experiments. Structural characterization in solution was assessed by NMR spectroscopy, corroborated by theoretical calculations. Spectroscopic characterization of the Ln3+ complexes of L1 and L2 was done in water and D2O and showed the effective sensitization of the lanthanide metal-centered emission spectra, each exhibiting typical lanthanide emission bands. The results obtained for the phosphonated ligands were compared with those reported previously for the corresponding carboxylated analogues.

Effect of Magnetic Anisotropy on the 1H NMR Paramagnetic Shifts and Relaxation Rates of Small Dysprosium(III) Complexes.

We present a detailed analysis of the 1H NMR chemical shifts and transverse relaxation rates of three small Dy(III) complexes having different symmetries (C3, D2 or C2). The complexes show sizeable emission in the visible region due to 4F9/2 → 6HJ transitions (J = 15/2 to 11/2). Additionally, NIR emission is observed at ca. 850 (4F9/2 → 6H7/2), 930 (4F9/2 → 6H5/2), 1010 (4F9/2 → 6F9/2), and 1175 nm (4F9/2 → 6F7/2). Emission quantum yields of 1-2% were determined in aqueous solutions. The emission lifetimes indicate that no water molecules are present in the inner coordination sphere of Dy(III), which in the case of [Dy(CB-TE2PA)]+ was confirmed through the X-ray crystal structure. The 1H NMR paramagnetic shifts induced by Dy(III) were found to be dominated by the pseudocontact mechanism, though, for some protons, contact shifts are not negligible. The analysis of the pseudocontact shifts provided the magnetic susceptibility tensors of the three complexes, which were also investigated using CASSCF calculations. The transverse 1H relaxation data follow a good linear correlation with 1/r6, where r is the distance between the Dy(III) ion and the observed proton. This indicates that magnetic anisotropy is not significantly affecting the relaxation of 1H nuclei in the family of complexes investigated here.

Rigidified Derivative of the Non-macrocyclic Ligand H4OCTAPA for Stable Lanthanide(III) Complexation.

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA4-, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log KLaL = 17.82(1) and log KYbL = 19.65(1)]. Luminescence studies on the Eu3+ and Tb3+ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H2O and D2O solutions indicate the presence of a water molecule coordinated to the metal ion. 1H nuclear magnetic relaxation dispersion profiles and 17O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (kex298 = 1.58 × 106 s-1) and relatively high relaxivities of 5.6 and 4.5 mM-1 s-1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb3+ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu2+. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu3+ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

Imine Palladacycles: Synthesis, Structural Analysis and Application in Suzuki-Miyaura Cross Coupling in Semi-Aqueous Media.

Treatment of the imines a-c with palladium(II) acetate in acetic acid yielded the µ-acetate dinuclear complexes 1a-c, which readily reacted with sodium chloride or bromide to provide µ-halide analogues. The reaction of the latter with nitrogen, phosphorus and oxygen donor nucleophiles yielded new imine palladacycles following the cleavage of the Pd2X2 unit. The complexes were fully characterized by microanalysis, 1H, 13C and 31P NMR spectroscopies, as appropriate. The compounds were applied as catalysts in the Suzuki-Miyaura coupling reaction in aqueous and semi-aqueous media.

A Highly Effective Strategy for Encapsulating Potassium Cations in Small Crown Ether Rings on a Dinuclear Palladium Complex.

The potential of 15-crown-5 ethers to link large cations, such as potassium, is limited by the quasi-parallel arrangement of two oxygen donor moieties upon appropriate orientation of the corresponding ether-ring-containing molecules. Substrates bearing the two crown ethers that are capable of achieving such coordination are hitherto unknown. The synthesis and isolation of a tailor-made dinuclear palladacycle bearing 15-crown-5 ether rings on the metallated phenyls offers such a possibility, providing the adequate environment for the formation of the sandwiched [K(metallacycle-15-crown-5)2 ] moiety. This synthetic strategy also culminates in the isolation of the first palladacycle able to entrap a potassium cation through bonding to two 15-crown-5 ether rings in a single molecule.

Unravelling the 6sp ← 6s absorption spectra of Bi(III) complexes.

We report a spectroscopic and computational study that investigates the absorption spectra of Bi(III) complexes, which often show an absorption band in the UV region (∼270-350 nm) due to 6sp ← 6s transitions. We investigated the spectra of three simple complexes, [BiCl5]2-, [BiCl6]3- and [Bi(DMSO)8]3+, which show absorption maxima at 334, 326 and 279 nm due to 3P1 ← 1S0 transitions. Theoretical calculations based on quasi-degenerate N-electron valence perturbation theory to second order (QD-NEVPT2) provide an accurate description of the absorption spectra when employing CAS(2,9) wave functions. We next investigated the absorption spectra of the [Bi(NOTA)] complex (H3NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid), which forms ternary complexes [Bi(NOTA)X]- (X = Cl, Br or I) in the presence of excess halide in aqueous solutions. Halide binding has an important impact on the position of the 3P1 ← 1S0 transition, which shifts progressively to longer wavelengths from 282 nm ([Bi(NOTA)]) to 298 nm (X = Cl), 305 nm (X = Br) and 325 nm (X = I). Subsequent QD-NEVPT2 calculations indicate that this effect is related to the progressive stabilization of the spin-orbit free states associated with the 6s16p1 configuration on increasing the covalent character of the metal-ligand(s) bonds, rather than with significant differences in spin-orbit coupling (SOC). These studies provide valuable insight into the coordination chemistry of Bi(III), an ion with increasing interest in targeted alpha therapy due to the possible application of bismuth isotopes bismuth-212 (212Bi, t1/2 = 60.6 min) and bismuth-213 (213Bi, t1/2 = 45.6 min).

Rigid H4OCTAPA derivatives as model chelators for the development of Bi(III)-based radiopharmaceuticals.

Octadentate ligands containing ethyl (H4OCTAPA), cyclohexyl (H4CHXOCTAPA) or cyclopentyl (H4CpOCTAPA) spacers were assessed as chelators for Bi(III)-based radiopharmaceuticals. The H4CHXOCTAPA chelator displays excellent properties, including 205/206Bi-nuclide radiolabelling under mild conditions, excellent stability in serum and in the presence of competing cations or H5DTPA. The poor performance of H4CpOCTAPA appears to be related to the stereochemical activity of the Bi(III) lone pair.

Cyclometallated Platinum(II) Complexes with Small Crown Ether Rings: Appropriate Choice of the Bridging Diphosphane to Coordinate Potassium Cations.

This account reports the synthesis and structural characterization of the first cyclometallated platinum(II) complex that coordinates a potassium cation in a sandwich arrangement via two 15-crown-5 ether rings within the same molecule. The cooperation of the two small crown ether moieties allows the entrapment of the non-ideal potassium ion. The reaction of the parent thiosemicarbazone ligand 3,4-(C8H16O5)C6H3C(Me)=NN-(H)C(=S)NHMe, 1, containing the crown ether ring, with K2[PtCl4], or alternatively with PtCl2(DMSO)2, and subsequent treatment with the diphosphanes Ph2PCH2PPh2 (dppm) and Ph2PC(=CH2)PPh2 (vdpp) produced the double nuclear platinacycles 3a, 3b, and 4, probably via formation of the 2a and 2b intermediates. Complex 3a with the K+ cation in a sandwich coordination was slightly mixed with 3b lacking any K+. Alternatively, reaction of 1 with K2[PtCl4] or with PtCl2(DMSO)2 followed by the diphosphane Ph2PC(=CH2)PPh2 (vdpp) only gave the dinuclear phosphane-bridged compound 4; this highlights the importance of choosing the right diphosphane ligand. Density functional theory calculations (B3LYP-D3/LANL2DZ-ECP-6.311++G**) revealed similar affinities for both dppm and vdpp derivatives to coordinate potassium cations. Crystal structure analysis was performed for compounds 3a and 4.

From Chemical Serendipity to Translational Chemistry: New Findings in the Reactivity of Palladacycles.

In the world of science, in particular the section concerning the field of chemistry, when the results encountered during the experiment do not meet our expectations, our shrewdness may play an important role to open up new unexplored fields that could be much more interesting than what we were seeking. In those cases, our research undergoes an unforeseen shift, delivering novel and challenging results that may altogether alter our point of view and our future work. We have then struck serendipity. Specifically, in our investigation linked to palladacycles we have found that the new trends in their reactivity, as well as in their structure, have been, in many cases, related to this experience, broadening our research scope within this field. Herein, we describe our most relevant findings, which have shed new light upon the reactivity of palladacycles, thus opening new routes that lead to novel unexpected structures.

The chelate-to-bridging shift of phosphane dipalladacycles: convenient synthesis of double A-frame tetranuclear complexes.

Palladacycles of the type [Pd2(Ph2PCH2PPh2-P,P)2(C,N:C,N)] (C,N:C,N = bis(N-2,3,4-trimethoxybenzylidene)-4,4'-sulfonyldianiline or -4,4'-oxydianiline) can undergo a spontaneous slow chelate-to-bridging diphosphane coordination shift in solution. Following this strategy a tailor-made synthetic procedure was devised that culminates in isolation of double A-frame tetranuclear palladium complexes.

Palladium iminophosphorane complexes: the pre-cursors to the missing link in triphenylphosphane chalcogenide metallacycles.

Herein we report on the synthesis, characterization and the ensuing chemistry of iminophosphorane palladacycles. Treatment of Ph3P[double bond, length as m-dash]N-(2-OHC6H4), 1, with sodium tetrachloropalladate gives 2 with the ligand as terdentate [C,N,O] allowing for only one µ-Cl ligand bonding the metal centers, resulting in a dinuclear complex. Treatment of 2 with PPh3 gives the mononuclear complex 3, whereas the reaction of 2 with diphosphanes Ph2P(CH2)nPPh2 in 1 : 2 ratio gives mixtures of 4 and 5 (n = 2) and 6 and 7 (n = 3). From them, the mononuclear complexes 4 and 6, and the dinuclear compounds, 5 and 7, were obtained with the parent ligand as bidentate [C,N]. The former two are of zwitterionic nature void of any counterion, with the phosphane ligand in the chelating mode. In a remarkable case of chemical serendipity, a solution of 2 left to stand produced crystals of complex 8: this is the missing link in the series of triphenylphosphane chalcogenide metallacycles. The experiment is repeatable; however, direct metallation of triphenylphosphane oxide was not possible.

Palladacycle catalysis: an innovation to the Suzuki-Miyaura cross-coupling reaction.

Herein we report a Suzuki-Miyaura type cross-coupling between an aryl halide and a functionalized boronic acid palladacycle in the absence of an external catalyst. This reaction is an unprecedented case of catalysis in palladium metallacycle chemistry.