Synthesis and Characterization of Pt(II) Complexes with Pyridyl Ligands: Elongated Octahedral Ion Pairs and Other Factors Influencing 1H NMR Spectra

Catalyzing Singlet Fission by Transition Metals: Second versus Third Row Effects

The synthesis and characterization of platinum(II) and palladium(II) complexes bearing two (dimers Pt(Lpc)2Cl2 and Pd(Lpc)2Cl2), one (monomers Pt(Lpc)(Lref)Cl2 and Pd(Lpc)(Lref)Cl2), or no (reference compounds Pt(Lref)2Cl2 and Pd(Lref)2Cl2) pentacene-based pyridyl ligands are presented. Photophysical properties of the dimers are probed by means of steady-state and time-resolved transient absorption measurements in comparison to the monomer and model compounds. Our results document that despite enhanced spin-orbit coupling from the presence of heavy atoms, intramolecular singlet fission (iSF) is not challenged by intersystem crossing. iSF thus yields correlated triplet pairs and even uncorrelated triplet excited states upon decoherence. Importantly, significant separation of the two pentacenyl groups facilitates decoupling of the two chromophores. Furthermore, the mechanism of iSF is altered depending on the respective metal center, that is, Pt(II) versus Pd(II). The dimer based on Pt(II), Pt(Lpc)2Cl2, exhibits a direct pathway for the iSF and forms a correlated triplet pair with singlet-quintet spin-mixing within 10 ns in variable solvents. On the other hand, the dimer based on Pd(II), Pd(Lpc)2Cl2, leads to charge transfer mixing during the population of the correlated triplet pair that is dependent on solvent polarity. Moreover, Pd(Lpc)2Cl2 gives rise to a stable equilibrium between singlet and quintet correlated triplet pairs with lifetimes of up to 170 ns. Inherent differences in the size and polarizability, when contrasting platinum(II) with palladium(II), are the most likely rationale for the underlying trends.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

Enhanced catalytic performance derived from coordination-driven structural switching between homometallic complexes and heterometallic polymeric materials

A bifunctional ligand 4,4-dimethyl-1-(pyridin-4-yl)pentane-1,3-dione (HL) able to provide two distinct coordination sites, i.e. anionic β-diketonate (after deprotonation) and neutral pyridine, has been used in the synthesis of Ag(I), Pd(II) and Pt(II) complexes that then have been applied as metalloligands for the construction of new heterometallic polymeric materials. The ambidentate nature of L- enables switching between different modes of coordination within mononuclear complexes or their conversion into polymeric species in a fully controllable way. The coordination-driven processes can be triggered by various stimuli, i.e. a metal salt addition or acid-base equilibria, and presents an efficient strategy for the generation of metallosupramolecular materials. As a consequence of self-assembly, new multimetallic coordination aggregates have been synthesized and characterized in depth in solution (¹H NMR, ESI-MS) as well as in the solid state (XPS, SEM-EDS, FTIR, pXRD, TGA). Furthermore, the Pd-based assemblies have been found to be efficient catalyst precursors in the Heck cross-coupling reaction, demonstrating a direct impact of compositional and morphological differences on their catalytic activity.

Pd(II) Complexes with Pyridine Ligands: Substituent Effects on the NMR Data, Crystal Structures, and Catalytic Activity

A wide range of functionalized pyridine ligands have been employed to synthesize a variety of Pd(II) complexes of the general formulas [PdL₄](NO₃)₂ and [PdL₂Y₂], where L = 4-X-py and Y = Cl^– or NO₃^–. Their structures have been unambiguously established via analytical and spectroscopic methods in solution (NMR spectroscopy and mass spectrometry) as well as in the solid state (X-ray diffraction). This in-depth characterization has shown that the functionalization of ligand molecules with groups of either electron-withdrawing or -donating nature (EWG and EDG) results in significant changes in the physicochemical properties of the desired coordination compounds. Downfield shifts of signals in the ¹H NMR spectra were observed upon coordination within and across the complex families, clearly indicating the relationship between NMR chemical shifts and the ligand basicity as estimated from pK_a values. A detailed crystallographic study has revealed the operation of a variety of weak interactions, which may be factors explaining aspects of the solution chemistry of the complexes. The Pd(II) complexes have been found to be efficient and versatile precatalysts in Suzuki–Miyaura and Heck cross-coupling reactions within a scope of structurally distinct substrates, and factors have been identified that have contributed to efficiency improvement in both processes.

A wide range of pyridine derivatives have been employed to synthesize a variety of di- and tetrasubstituted Pd(II) complexes of square-planar geometry. This in-depth characterization has shown that the functionalization of ligand molecules with groups of either electron-withdrawing or -donating nature results in significant changes in the physicochemical properties of the coordination compounds. Moreover, the complexes have been found to be of practical utility as efficient precatalysts for both Suzuki−Miyaura and Heck cross-coupling reactions.

Linear Bidentate Ligands (L) with Two Terminal Pyridyl N-Donor Groups Forming Pt(II)LCl2 Complexes with Rare Eight-Membered Chelate Rings

NMR and X-ray diffraction studies were conducted on Pt(II)LCl₂ complexes prepared with the new N-donor ligands N(SO₂R)Me _ndpa (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- d₇, but in DMSO- d₆ the N(SO₂R)6,6'-Me₂dpa ligands dissociate completely. In DMSO- d₆, 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- d₆ to form the [Pt( N(SO₂R)3,3',5,5'-Me₄dpa)(DMSO- d₆)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.