LED-induced Ru-photoredox Pd-catalyzed C-H arylation of (6-phenylpyridin-2-yl)pyrimidines and heteroaryl counterparts

N-heterocycles are essential building blocks and scaffolds in medicinal chemistry. A Pd-catalyzed, Ru-photoredox-mediated C-H arylation is applied herein, for converting a series of functionality-inclusive (6-phenylpyridin-2-yl)pyrimidines to single arylated derivatives, using phenyldiazonium tetrafluoroborate as aryl source. This green chemistry-compliant transformation is induced by LED light. The drug-like modular substrates are constructed via combination of Biginelli multi-component condensation and Suzuki C-C cross-coupling, in order to strategically install, adjacent to the Ph-ring intended to undergo C-H arylation, a (6-pyridin-2-yl)pyrimidine that plays the role of a chelating directing moiety for the C-H arylation catalyst. The scope has been demonstrated on a series of 26 substrates, comprising diverse Ph-ring substituents and substitution patterns, as well as with 13 different aryl donors. Substrates in which the Ph-ring (arylation acceptor) was replaced by an electron-rich heteroaryl counterpart (2-/3-thiophene or -benzofuran) have also been examined and found to undergo arylation regioselectively. End-product conformations afford interesting motifs for occupying 3D chemical space, as implied by single-crystal X-ray diffraction, which has allowed the elucidation of six structures of aryl derivatives and one of an unprecedented pyrimidine-pyridine-benzofuran carbopalladated complex, believed to be a C-H activation derivative.

Rational design of fluorescent chemosensor for Pd2+ based on the formation of cyclopalladated complex

According to the assumption that the formation of C-Pd bond becomes a cyclopalladated complex (CPC), we designed and synthesized two C-N-N pincer ligands of BODIPY appended 2,2'-bipyridine derivatives (BP and BPB). It has been confirmed that the C-Pd bond does exist and plays a crucial role in "on-off" fluorescence behavior. Based on it, a coordination-induced fluorescence quenching sensor for Pd²⁺ was constructed. The results indicated that BP possessed high sensitivity and specificity for Pd²⁺ in solution. The limit of detection (LOD) of BP is determined to be 0.97 nM within a linear range between 1.0 and 50.0 nM, meanwhile, the platinum-group ions demonstrate no interference. The bio-imaging application of BP was investigated and it exhibited a promising vitro test for fluorescent imaging of Pd²⁺ ions in MCF-7 cells. Meanwhile, BPB coated sensor label for Pd²⁺ was set up. The visible color variation was displayed under UV light with increasing concentrations of Pd²⁺. Briefly speaking, fluorescence probes of BP and BPB offer new approaches for Pd²⁺ detection in a lab and on-site test, as well as the vivo imaging. Then, with the aid of (TD)DFT calculation, the internal reason for the optical difference between the two ligands was disclosed. This concept of CPC containing a Pd-C covalent bond provides a promising perspective of coordination fluorescence sensors.

Diruthenium and triruthenium compounds of the potential redox active non-chelated η1-N,η1-N-benzothiadiazole bridge

In the present study, a series of non-chelated BTD (2,1,3-benzothiadiazole)-bridged diruthenium(II) ([{(CH₃CN)(acac)₂Ru^II}₂(μ-BTD)] 1, [{CH₃CN(acac)₂Ru^II}(μ-BTD){Ru^II(acac)₂(η¹-N-BTD)}] 2, [{(η¹-N-BTD)(acac)₂Ru^II}₂(μ-BTD)] 3), and triruthenium ([{(acac)₂Ru^II}₃(μ-BTD)₂(η¹-N-BTD)₂] 4) complexes with varying ratios of η¹-N and μ-bis-η¹-N,η¹-N modes of BTD were studied. Complexes 1-4 (S = 0) were obtained via the one-pot reaction of electron-rich Ru(acac)₂(CH₃CN)₂ and electron-deficient BTD in refluxing acetone. The relatively low Ru(II)/Ru(III) potential of 1-4 (0.08-0.44 V versus SCE) further facilitated the isolation of the corresponding mixed valent Ru^IIRu^III (S = 1/2) and Ru^IIRu^IIRu^III (S = 1/2)/Ru^IIRu^IIIRu^III (S = 1) forms [1]ClO₄-[3]ClO₄ and [4]ClO₄/[4](ClO₄)₂, respectively. The single-crystal X-ray structures of the representative mixed valent [1]ClO₄ and [3]ClO₄ established (i) Ru⋯Ru distances of 6.227 Å and 6.256 Å (molecule A)/6.184 Å (molecule B), respectively, (ii) a significant variation of the N-S distance of BTD in [3]ClO₄ as a function of its binding mode μ versus η¹ and (iii) similar Ru-N (μ-BTD) distances in each case corresponding to a valence delocalised situation. The mixed valent diruthenium (1+-3+) and triruthenium (4+/42+) complexes exhibited metal-based anisotropic electron paramagnetic resonance (EPR) and moderately intense low-energy intervalence charge-transfer (IVCT) transitions in the near-infrared region of 1730-1890 nm. Analysis of the IVCT band using the Hush treatment revealed a valence delocalised class III mixed valent state with the electronic coupling V_ab of ≈2640-2890 cm^-1, as also corroborated by the K_c values of 10⁵-10⁸, solvent independency of the IVCT band and uniform spin distribution between the metal ions in the singly occupied state(s). Furthermore, the involvement of the BTD (η¹ and μ)-based orbitals in the reduction processes was evident by its free radical EPR feature.

Structural and electronic forms of doubly oxido/Pz and triply oxido/(Pz)2 bridged mixed valent and isovalent diruthenium complexes (Pz = pyrazolate)

The article reported on the diastereomeric dinuclear mixed-valent complexes [(acac)₂Ru(III)(μ-O)(μ-Pz^R)Ru(IV)(acac)₂] (R = H, Me, meso: ΔΛ, 1a-1c; rac: ΔΔ/ΛΛ, 2a-2c) and rac-[(acac)₂Ru(III)(μ-O)(μ-Iz)Ru(IV)(acac)₂], (2d) (HPz = pyrazole, HIz = indazole, acac = acetylacetonate). Moreover, the diruthenium(II,II) complexes [(HPz)₃Ru(II)(μ-O)(μ-Pz)₂ Ru(II)(HPz)₃] (3a) and [(HIz)₃Ru(II)(μ-O)(μ-Iz)₂Ru(II)(HIz)₃] (3d) were presented. The analogous form of 3a, i.e., [(HPz)₂(Pz)Ru(III)(μ-O)(μ-Pz)₂Ru(III)(Pz)(HPz)₂], was previously reported. Single crystal X-ray structures of 1a-1c/2a-2d and representative 3a showed their molecular forms, including the diastereomeric nature of the former. The Ru-O-Ru angle decreased appreciably on switching from doubly bridged 1 and 2 (128-135°) to triply bridged 3a (114°). Both series of complexes displayed rhombic symmetry in their EPR spectra, with g₁ and g₂ being very similar for 1a-1c with an almost axial look. The mixed-valence complex with a Ru(III)Ru(IV) (S = 1/2) state of 1 and 2 would lead to iso-valence complexes of Ru(III)Ru(III) and Ru(IV)Ru(IV) with an EPR inactive state by one electron redox reaction. On the other hand, metal based {Ru(II)Ru(II)/Ru(II)Ru(III), 3a/3a⁺} and terminal ligand (HPz/HPz^-, 3a/3a^-) based redox processes displayed anisotropic and free radical EPR, respectively. An IVCT (intervalence charge transfer) band was found for the delocalised mixed valent 1 and 2 {Ru(III)Ru(IV)} or 3a⁺ {Ru(II)Ru(III)} in the NIR region. The intense metal-to-ligand charge transfer (MLCT) transitions of 1-3 in the visible region varied systematically as a function of the metal oxidation state.

Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap)₂(abbt)]ⁿ (1ⁿ) and [Ru(bpy)₂(abbt)]ⁿ (2ⁿ), where pap = 2-phenylazopyridine and bpy = 2,2'-bipyridine. In this regard, the complexes [Ru^II(pap)₂(abbt^•-)]ClO₄ ([1]ClO₄), [Ru^II(pap)₂(abbt⁰)](ClO₄)₂ ([1](ClO₄)₂), [Ru^II(bpy)₂(abbt⁰)](ClO₄)₂ ([2](ClO₄)₂), and [Ru^II(bpy)₂(abbt^•-)]ClO₄ ([2]ClO₄) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1⁺/2⁺ and 1²⁺/2²⁺, respectively, were made primarily based on their redox-sensitive azo (N═N) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt^•- was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1⁺, the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt⁰ in 2²⁺, along with the radical form in 2⁺ as a minor (<10%) component. The oxidized abbt⁰-derived [1](ClO₄)₂ was, however, obtained via the chemical oxidation of [1]ClO₄. Both 1⁺ and 2²⁺ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1-R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1⁺ and bpy-derived 2²⁺ as well as facile reduction of pap and abbt in 1⁺ and 2⁺, respectively.

Noninnocence of the deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine bridge in diruthenium frameworks - a function of co-ligands

The article deals with the sensitive electronic forms in accessible redox states of structurally and spectroscopically authenticated deprotonated 1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (H2LR, R = H) or 1,2-bis((3,5-dimethyl-1H-pyrrol-2-yl)methylene)hydrazine (H2LR, R = Me), a BODIPY analogue bridged diruthenium complex as a function of varying ancillary ligands. It involved rac-(acac)2RuIII(μ-LR 2-)RuIII(acac)21a, R = H; 1b, R = Me (S = 1, acac = acetylacetonate), rac-[(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2 [2](ClO4)2 (S = 0, bpy = 2,2'-bipyridine) and diastereomeric [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2meso-[3a](ClO4)2/rac-[3b](ClO4)2 (S = 0, pap = phenylazopyridine). The crystal structure established the linkage of the conjugated -C5[double bond, length as m-dash]N2-N3[double bond, length as m-dash]C6- central unit with the two terminal deprotonated pyrrole units of coordinated L2-. The bridging L2- in 1a, 1b, [2](ClO4)2, [3b](ClO4)2 and [3a](ClO4)2 was slightly twisted and planar with torsional angles of 41.54°, 42.91°, 37.38°, 35.33° and 0°, respectively, with regard to the central N2-N3 bond. The extent of twisting of the bridge followed an inverse relationship with the RuRu separation: 4.935/4.934 Å 1a/1b < 5.141 Å [2](ClO4)2 < 5.201 Å [3b](ClO4)2 < 5.351 Å [3a](ClO4)2. This is also attributed to the intermolecular ππ/CHπ interactions between the nearby aromatic rings of L and bpy or pap in [2](ClO4)2 or [3](ClO4)2, respectively. The multiple redox steps of the complexes varied appreciably based on the σ-donating (acac) and π-acidic (bpy, pap) characteristics of the ancillary ligands. Experimental (structure, EPR) and theoretical (DFT) evaluation pertaining to the electronic forms of 1n, 2n and 3n demonstrated the preferential involvement of L based frontier orbitals in electron transfer processes even in combination with the redox facile ruthenium ion. This in turn highlighted its redox non-innocent feature as in the case of well-documented metal coordinated quinonoid, formazanate, diimine (bpy), azo (pap) and β-diketiminate functions.