Diverse Functionalization of Ruthenium-Chelated 2-Picolylamines: Oxygenation, Dehydrogenation, Cyclization, and N-Dealkylation

Superior electrocatalytic hydrogen evolution activity of a triply bridged diruthenium(II) complex on a carbon cloth support

This article describes the structural authentication of a unique triply bridged [1](ClO4)2 and monomeric [2]ClO4/[3]ClO4. Electrochemical HER on a carbon cloth support demonstrated the superior performance of [1](ClO4)2 with high TON (>105) and its long-term stability. The primary kinetic isotope effect of [1](ClO4)2 revealed the involvement of PCET in the rate-determining step.

The isomer-sensitive electrochemical HER of ruthenium(II)-hydrido complexes involving redox-active azoheteroaromatics

The article deals with the development of isomeric ruthenium(II)-hydrido complexes [RuII(H)(L1)(PPh3)2(CO)]ClO4 ([1a]ClO4-[1b]ClO4)/[RuII(H)(L2)(PPh3)2(CO)]ClO4 ([2a]ClO4-[2b]ClO4) involving azo coupled L1 [L1: (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene]/L2 [L2: (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene], respectively. Structural evaluation of the complexes affirmed the syn conformation of the coordinated/uncoordinated pyrazole groups of L and its unperturbed neutral azo (NN) state. Isomeric forms in [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 differed with respect to the cis and trans orientations of the coordinated CO and N(azo) donor of L, respectively. It also demonstrated the formation of intermolecular hydrogen-bonded dimeric or 1D-polymeric chains in [1a]ClO4/[2b]ClO4 or [1b]ClO4, respectively. Successive two-electron reductions of the complexes varied to an appreciable extent as a function of the heterocycles connected to L. The involvement of the azo function of L towards the reductions ([NN]0 → [NN]˙- → [NN]2-) was supported by the DFT calculated MOs and Mulliken spin density at the paramagnetic state, which was further validated by the radical EPR profile of the first reduced (S = 1/2) state. Isomeric [1a]ClO4/[1b]ClO4 or [2a]ClO4/[2b]ClO4 immobilised on the carbon cloth support underwent various electrochemical acidic HERs (hydrogen evolution reactions) with TOF/10-1 s-1: [1a]ClO4 (0.83) > [1b]ClO4 (0.68) > [2a]ClO4 (0.50) > [2b]ClO4 (0.37).

On the Question of S-S Bond Cleavage of 2,2'-Dithiodipyridine on Selective Ru and Os Platforms. MLCT or Hydride or Solvent Mediated Event

This article deals with the S-S bond scission of the model substrate 2,2'-dithiodipyridine (DTDP) in the presence of a selective set of metal precursors: Ru^II(acac)₂, [Ru^IICl₂(PPh₃)₃], [Ru^IIHCl(CO)(PPh₃)₃], [Ru^II(H)₂(CO)(PPh₃)₃], [Ru^II(bpy)₂Cl₂], [Ru^II(pap)₂Cl₂], [Os^II(bpy)₂Cl₂], and [Os^II(pap)₂Cl₂] (acac, acetylacetonate; bpy, 2,2'-bipyridine; pap, 2-phenylazopyridine). This led to the eventual formation of the corresponding mononuclear complexes containing the cleaved pyridine-2-thiolate unit in 1-4/[5]ClO₄-[8]ClO₄. The formation of the complexes was ascertained by their single-crystal X-ray structures, which also established sterically constrained four-membered chelate (average N1-M-S1 angle of 67.89°) originated from the in situ-generated pyridine-2-thiolate unit. Ruthenium(III)-derived one-electron paramagnetic complexes 1-2 (S = 1/2, magnetic moment/B.M. = 1.82 (1)/1.81(2)) exhibited metal-based anisotropic electron paramagnetic resonance (EPR) (Δg: 1/2 = 0.64/0.93, ⟨g⟩: 1/2 = 2.173/2.189) and a broad ¹H nuclear magnetic resonance (NMR) signature due to the contact shift effect. The spectroelectrochemical and electronic structural aspects of the complexes were analyzed experimentally in combination with theoretical calculations of density functional theory (DFT and TD-DFT). The unperturbed feature of DTDP even in refluxing ethanol over a period of 10 h can be attributed to the active participation of the metal fragments in facilitating S-S bond cleavage in 1-4/[5]ClO₄-[8]ClO₄. It also revealed the following three probable pathways toward S-S bond cleavage of DTDP as a function of metal precursors: (i) the metal-to-ligand charge-transfer (MLCT) (Ru^II → σ* of DTDP)-driven metal oxidation (Ru^II → Ru^III) process in the case of relatively electron-rich metal fragments {Ru^II(acac)₂} or Ru^IICl₂ in 1 or 2, respectively; (ii) metal hydride-assisted formation of 3 or 4 with the concomitant generation of H₂; and (iii) S-S bond reduction with the simultaneous oxidation of the solvent benzyl alcohol to benzaldehyde.

Redox-Induced Intramolecular C-C Coupling of Acyclic Bis(2-pyridylmethylene)ethylenediamine on a Ru(acac)2 Platform

The paper documents redox-triggered C-C coupling of acyclic N,N'-bis(2-pyridylmethylene)ethylenediamine (BPE) to yield 2,3-bis(2-pyridyl)pyrazine (DPP) upon coordination to an electron-rich {Ru(acac)₂} (acac = acetylacetonate) unit. This led to DPP-bridged [{Ru(acac)₂}₂(DPP)]^0/+ (2 and [2]ClO₄) along with the unperturbed BPE-bridged [{Ru(acac)₂}₂(BPE)] (1). On the contrary, electron-poor {Ru(Cl)(H)(CO)(PPh₃)₃} yielded BPE-bridged [3](ClO₄)₂ as an exclusive product. Synergistic metal (Ru)-ligand (BPE) redox participation toward chemical noninnocence of the Schiff base ligand and DPP-mediated electronic communication in Ru^IIRu^III-derived [2]ClO₄ are addressed.

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.

Redox induced oxidative C-C coupling of non-innocent bis(heterocyclo)methanides

Redox driven C-C bond formation has gained recent attention over the traditional sequence of oxidative addition, insertion and reductive elimination reactions. In this regard, the transient radical mediated diverse reactivity profile of bis(heterocyclo)methanes (H-BHM: HL1-HL4) has been demonstrated as a function of varying metal ions and ligand backbones. It highlighted the following events: (a) redox induced homocoupling of deprotonated HL1 and HL4 on coordination to M(OAc)₂ precursors (M = Cu^II, Zn^II, Pd^II, Ag^I), including the effective role of molecular oxygen in the transformation process; (b) steric inhibition of C-C coupling of HL1 or HL4 on inserting the substituent at the bridged methylene centre (Ph in HL2 or CH₃ in HL3); (c) competitive C-C coupling versus oxygenation of free HL1 with varying concentrations of Pd^II(OAc)₂ as the ease of oxygenation over dimerisation of the deprotonated HL1 was corroborated by the DFT calculated lower activation barrier and greater thermodynamic stability of the former; and (d) redox non-innocence of BHMs on a coordinatively inert ruthenium platform, which in turn favored the involvement of a radical pathway for the aforestated coupling or oxygenation process. A combined structural, spectroscopic and DFT calculated transition state analysis demonstrated the mechanistic outline for the metal assisted oxidative coupling of BHMs.

Mechanistic investigation of the aerobic oxidation of 2-pyridylacetate coordinated to a Ru(II) polypyridyl complex

A new ruthenium polypyridyl complex, [Ru(bpy)₂(acpy)]⁺ (acpy = 2-pyridylacetate, bpy = 2,2'-bipyridine), was synthesized and fully characterized. Distinct from the previously reported analog, [Ru(bpy)₂(pic)]⁺ (pic = 2-pyridylcarboxylate), the new complex is unstable under aerobic conditions and undergoes oxidation to yield the corresponding α-keto-2-pyridyl-acetate (acpyoxi) coordinated to the Ru^II center. The reaction is one of the few examples of C-H activation at mild conditions using O₂ as the primary oxidant and can provide mechanistic insights with important implications for catalysis. Theoretical and experimental investigations of this aerobic oxidative transformation indicate that it takes place in two steps, first producing the α-hydroxo-2-pyridyl-acetate analog and then the final product. The observed rate constant for the first oxidation was in the order of 10^-2 h^-1. The reaction is hindered in the presence of coordinating solvents indicating the role of the metal center in the process. Theoretical calculations at the M06-L level of theory were performed for multiple reaction pathways in order to gain insights into the most probable mechanism. Our results indicate that O₂ binding to [Ru(bpy)₂(acpy)]⁺ is favored by the relative instability of the six-ring chelate formed by the acpy ligand and the resulting Ru^III-OO˙^- superoxo is stabilized by the carboxylate group in the coordination sphere. C-H activation by this species involves high activation free energies (ΔG^‡ = 41.1 kcal mol^-1), thus the formation of a diruthenium μ-peroxo intermediate, [(Ru^III(bpy)₂(O-acpy))₂O₂]²⁺via interaction of a second [Ru(bpy)₂(acpy)]⁺ was examined as an alternative pathway. The dimer yields two Ru^IVO centers with a low ΔG^‡ of 2.3 kcal mol^-1. The resulting Ru^IVO species can activate C-H bonds in acpy (ΔG^‡ = 23.1 kcal mol^-1) to produce the coordinated α-hydroxo-2-pyridylacetate. Further oxidation of this intermediate leads to the α-keto-2-pyridyl-acetate product. The findings provide new insights into the mechanism of C-H activation catalyzed by transition-metal complexes using O₂ as the sole oxygen source.

Redox induced S-S bond cleavage of 2,2'-dithiobisbenzothiazole - leading to a [2Ru-2S] core analogous to [2Fe-2S] cluster

Facile reduction of 2,2'-dithiobisbenzothiazole by the mediation of metal-to-ligand charge transfer or by internal reducing equivalent is demonstrated. It leads to various binding modes of thiolates (κ¹, κ², μ) in a series of mononuclear and dinuclear ruthenium complexes. The dinuclear complex exhibited electron transfer processes similar to a [2Fe-2S] cluster.

Osmium(II)-Coordination Induced C-C Bond Functionalization of Bis-lawsone

Metal-coordination-driven C-C bond functionalization without involvement of the traditional route of oxidative addition, insertion, and reductive elimination has gained immense importance. In this context, the present Communication highlights the facile ring contraction process of the deprotonated bis-lawsone (L^2-) to functionalized L1^2- upon coordination to {Os(bpy)₂} or isomeric {Os(pap)₂} (bpy = 2,2'-bipyridine and pap = 2-phenylazopyridine) in 1-3. Further, recognition of fractional redox noninnocence of L1 in 1⁺-3⁺ via experimental and theoretical events facilitated its inclusion in the redox noninnocent family.

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.

Ruthenium-Hydride Assisted Remarkable Diversity Towards Non-Spectator Feature of Benzodifuroxan

The present article demonstrates that ruthenium-hydride [Ru^II (H)(Cl)(CO)(PPh₃ )₃ ] mediated diverse functionalization modes of benzodifuroxan (BDF) encompassing two furoxan rings. Hydride transfer from the metal precursor facilitated multiple cascade reactions involving unsymmetrical cleavage of the furoxan rings of BDF, leading to the one-pot formation of a series of ruthenium (II) coordinated functionalized ligands exhibiting bidentate κ² -N,O, κ² -N,N' and bis-bidentate μ-bis(κ² -N,O) modes. Further, a moderately stable intermediate species was also encountered in the reaction sequence in which the transformed deoxygenated ligand coordinated to the metal ion via the rarely manifested furazan ring (κ² -N,N'' mode). The products were authenticated by their single-crystal X-ray structures and other spectroscopic/analytical techniques. Redox non-innocence of the functionalized ligands in the complexes was illustrated by spectroelectrochemistry (cyclic voltammmetry, UV-Vis. and EPR) in conjunction with DFT/TD-DFT calculations. Mechanistic outline for the facile ring opening processes of BDF including interconversions of complexes (e. g. reductive ring opening) were also addressed.