Carmen Esther Castillo

[Fe^II(TSC)₂] complexes selectively catalyze oxidation of styrene to benzaldehyde with H₂O₂. These iron complexes, the free TSC ligands, and [Zn(Dp44mT)₂] also catalyze sulfoxidation of thioanisole, but only the iron complexes catalyze styrene oxidation.

Two oxoiron(IV) isomers (^R2a and ^R2b) of general formula [Fe^IV(O)(^RPyNMe₃)(CH₃CN)]²⁺ are obtained by reaction of their iron(II) precursor with NBu₄IO₄. The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between ^R2a and ^R2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that ^R2a reacts one order of magnitude faster than ^R2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in ^R2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

Two oxoiron(IV) isomers (^R2a and ^R2b) of general formula [Fe^IV(O)(^RPyNMe₃)(CH₃CN)]²⁺ are obtained by reaction of their iron(II) precursor with NBu₄IO₄. The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between ^R2a and ^R2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that ^R2a reacts one order of magnitude faster than ^R2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in ^R2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

The novel [W₃S₄H₃(edpp)₃]⁺ (edpp=(2‐aminoethyl)diphenylphosphine) (1⁺) cluster hydride with an acidic −NH₂ functionality has been synthetized and studied. Its crystal structure shows the characteristic incomplete W₃S₄ cubane core with the outer positions occupied by the P and N atoms of the edpp ligands. Although no signal due to the hydride ligands is observed in the ¹H NMR spectrum, hydride assignment is supported by ¹H‐¹⁵N HSQC techniques, the changes in the ³¹P{¹H} NMR chemical shift, and FT‐IR spectra in the W−H region of the deuterated [W₃S₄D₂H(edpp)₃]⁺ (1⁺‐d₂) samples. Moreover, all NMR evidences suggest that one of the hydrogen atoms of the NH₂ group in 1⁺ is rapidly exchanging with the hydride. The reaction of 1⁺ with acids (HCl, HBr and DCl) features complex polyphasic kinetics with zero‐order dependence with respect to the acid concentration, being also independent of the solvent nature. This behavior differs from that of their diphosphino analogues, suggesting a different mechanism.

Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non‐heme iron‐dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [Fe^II(CH₃CN)₂L](SbF₆)₂ ([1](SbF₆)₂ and [2](SbF₆)₂) and [Fe^II(CF₃SO₃)₂L] ([1](OTf)₂ and [2](OTf)₂ (1, L=^Me,HPytacn; 2, L=^nP,HPytacn; ^R,R′Pytacn=1‐[(6‐R′‐2‐pyridyl)methyl]‐4,7‐ di‐R‐1,4,7‐triazacyclononane) with Bu₄NIO₄ to form the corresponding [Fe^IV(O)(CH₃CN)L]²⁺ (3, L=^Me,HPytacn; 4, L=^nP,HPytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl⁻ as entering ligand, which indicates that formation of [Fe^IV(O)(CH₃CN)L]²⁺ is kinetically controlled by substitution in the starting complex to form [Fe^II(IO₄)(CH₃CN)L]⁺ intermediates that are converted rapidly to oxo complexes 3 and 4. The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin‐crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high‐valent iron oxo species.

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The transition metal catalysed formation and cleavage of C−C bonds is of utmost importance in synthetic chemistry. While most of the existing homogeneous catalysts are mononuclear, knowledge of the behaviour of polynuclear species is much more limited. By using computational methods, here we shed light into the mechanistic details of the thermally‐induced isomerization of Cp*₃Ru₃(μ‐H)₂(μ₃‐η²‐pentyne)(μ₃‐pentylidyne) (2) into Cp*₃Ru₃(μ‐H)₂(μ₃‐η²‐octyne)(μ₃‐ethylidyne) (3), a process that involves the migration of a C₃ fragment between the hydrocarbyl ligands and across the plane formed by the three Ru centres. Our results show this to be a complex transformation that comprises of five individual rearrangements in an A→B→A→B→A order. Each so‐called rearrangement A consists of the CH migration from the μ₃‐η²‐alkyne into the μ₃‐alkylidine ligand in the other side of the Ru₃ plane. This process is facilitated by the cluster's ability to adopt open‐core structures in which one Ru−Ru bond is broken and a new C−C bond is formed. In contrast, rearrangements B do not involve the formation or cleavage of C−C bonds, nor do they require the opening of the cluster core. Instead, they consist of the isomerization of the μ₃‐η²‐alkyne and μ₃‐alkylidyne ligands on each side of the triruthenium plane into μ₃‐alkylidyne and μ₃‐η²‐alkyne, respectively. Such transformation implies the migration of three H atoms within the hydrocarbyl ligands, and in this case, it is aided by the cluster's ability to behave as a H reservoir. All in all, this study highlights the plasticity of these Ru₃ clusters, whereby Ru−Ru, Ru−C, Ru−H, C−C, and C−H bonds are formed and broken with surprising ease.

We previously reported that the tetraazamacrocyclic Schiff base complex [Co^III(CR14)(X)₂]ⁿ⁺ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, X = Cl (n = 1) (1-Cl₂) or H₂O (n = 3) (1-(H₂O)₂)) is a very efficient H₂-evolving catalyst (HEC) in fully aqueous solutions at pH 4.0–4.5 when used in a photocatalytic system including a photosensitizer and ascorbate as sacrificial electron donor. The excellent H₂-evolving activity of this complex, compared to other cobalt and rhodium catalysts studied in the same photocatalytic conditions, can be related to the high stability of its two-electron reduced form, the putative “Co(I)” state. These very interesting results led us to investigate the H₂-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [Co^II(CR15)(H₂O)₂]Cl₂ complex (2, CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), which comprises a larger macrocycle with five nitrogen atoms instead of four. Electrochemical as well as spectroscopic investigations in CH₃CN coupled to density functional theory (DFT) calculations point to decoordination of one of the amine upon reduction of Co(II) to the low-valent “Co(I)” form. The resulting unchelated amine could potentially act as a proton relay promoting the H₂ formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [Fe^II(CR15)(X)₂]ⁿ⁺ (X = Cl (n = 0) or H₂O (n = 2)) (3), [Mn^II(CR15)(CH₃CN)₂](PF₆)₂ (4), and {[Zn^II(CR15)Cl](PF₆)}_n (5) were also synthesized and investigated. The photocatalytic activity of 2–5 toward proton reduction was then evaluated in a tricomponent system containing the [Ru^II(bpy)₃]Cl₂ photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of 2 was also compared with that of 1 in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for 2 are slightly lower than that for 1, suggesting that if the amine released upon reduction of 2 plays a role in promoting the H₂-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.

We previously reported that the [Rh^III(dmbpy)₂Cl₂]⁺ (dmbpy = 4,4′-dimethyl-2,2′-bipyridine) complex is an efficient H₂-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [Ru^II(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [Rh^I(dmbpy)₂]⁺, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H₂ following different pathways. To improve the current knowledge of these key intermediate species for H₂ production, we performed herein a detailed electrochemical investigation of the [Rh^III(dmbpy)₂Cl₂]⁺ and [Rh^III(dtBubpy)₂Cl₂]⁺ (dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine) complexes in CH₃CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [Rh^I(Rbpy)₂]⁺ and, more importantly, the hydride [Rh^III(Rbpy)₂(H)Cl]⁺ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF₃CO₃H), owing to the fast reaction of the electrogenerated [Rh^I(Rbpy)₂]⁺ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310–340 mV. Besides, 0.5 equiv of H₂ is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H₂ evolution process.