The Role of the Amino Protecting Group during Parahydrogenation of Protected Dehydroamino Acids

A series of dehydroamino acids endowed with different protective groups at the amino and carboxylate moieties and with different substituents at the double bond have been reacted with parahydrogen. The observed ParaHydrogen Induced Polarization (PHIP) effects in the ¹H NMR spectra are strongly dependent on the amino protecting group. DFT calculations allowed us to establish a relationship between the structures of the reaction intermediates (whose energies depend on the amido substitution) and the observed PHIP patterns

Bio-Inspired Mn(I) Complexes for the Hydrogenation of CO₂ to Formate and Formamide

Developing new, efficient catalysts that contain Earth-abundant metals and simple, robust ligands for CO₂ hydrogenation is important to create cost-effective processes of CO₂ utilization. Inspired by nature, which utilizes an <i>ortho</i>-OH-substituted pyridine motif in Fe-containing hydrogenases, we developed a Mn complex with a simple N-donor ligand, 6,6′-dihydroxy-2,2′-bipyridine, that acts as an efficient catalyst for CO₂ hydrogenation. Turnover numbers of 6250 for hydrogenation of CO₂ to formate in the presence of DBU were achieved. Moreover, hydrogenation of CO₂ to formamide was achieved in the presence of a secondary amine

Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides

An integrated experimental–theoretical approach for the solid-state NMR investigation of a series of hydrogen-storage materials is illustrated. Seven experimental room-temperature structures of groups I and II metal hydrides and borohydrides, namely, NaH, LiH, NaBH₄, MgH₂, CaH₂, Ca­(BH₄)₂, and LiBH₄, were computationally optimized. Periodic lattice calculations were performed by means of the plane-wave method adopting the density functional theory (DFT) generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Quantum ESPRESSO package. Projector augmented wave (PAW), including the gauge-including projected augmented-wave (GIPAW), methods for solid-state NMR calculations were used adopting both Rappe–Rabe–Kaxiras–Joannopoulos (RRKJ) ultrasoft pseudopotentials and new developed pseudopotentials. Computed GIPAW chemical shifts were critically compared with the experimental ones. A good agreement between experimental and computed multinuclear chemical shifts was obtained

A Single Organoiridium Complex Generating Highly Active Catalysts for both Water Oxidation and NAD⁺/NADH Transformations

The complex [Cp*Ir­(pica)­Cl] (<b>1</b>; pica = picolinamidate = κ²-pyridine-2-carboxamide) was found to be an effective catalyst for both water oxidation to molecular oxygen and NAD⁺/NADH transformations, which are the key reactions of light-dependent natural photosynthesis. In particular, <b>1</b> exhibits high activity in water oxidation driven by CAN and NaIO₄. With the former, the initial TOF exceeds that of [Cp*Ir­(pic)­Cl] (<b>2</b>; pic = picolinate = κ²-pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO₄ it compares well with the best catalysts. <b>1</b> exhibits top performances also in the hydrogenation of NAD⁺ with HCOOK, leading to the regiospecific formation of 1,4-NADH (pH 7) with TOF = 143 h^–1, which is about 3 times higher than the previous highest value (54 h^–1) reported for [Cp*Ir­(4-(1<i>H</i>-pyrazol-1-yl-κ<i>N</i>²)­benzoic acid)­(H₂O)]­SO₄ (<b>3</b>). The activity seems to be critically affected by the presence of the NH functionality, as indicated by its drop of about 1 order of magnitude when <b>2</b> (TOF = 17 h^–1) was used as the catalyst instead of <b>1</b>. <b>1</b> is also able to mediate the dehydrogenation of β-NADH, under slightly acidic conditions, as determined by NMR and GC measurements. Furthermore, an in-depth investigation carried out combining 1D, 2D, and diffusion NMR techniques indicate a remarkable speciation of β-NADH leading not only to the expected β-NAD⁺ but also to α-NAD⁺, nicotinamide (NA), and 1,2,5,6-tetrahydronicotinamide (NAH₃). The formation of NAH₃ has been identified as the cause of the low TON values obtained with <b>1</b> and <b>3</b>, because it consumes part of the produced H₂

A Single Organoiridium Complex Generating Highly Active Catalysts for both Water Oxidation and NAD⁺/NADH Transformations

The complex [Cp*Ir­(pica)­Cl] (<b>1</b>; pica = picolinamidate = κ²-pyridine-2-carboxamide) was found to be an effective catalyst for both water oxidation to molecular oxygen and NAD⁺/NADH transformations, which are the key reactions of light-dependent natural photosynthesis. In particular, <b>1</b> exhibits high activity in water oxidation driven by CAN and NaIO₄. With the former, the initial TOF exceeds that of [Cp*Ir­(pic)­Cl] (<b>2</b>; pic = picolinate = κ²-pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO₄ it compares well with the best catalysts. <b>1</b> exhibits top performances also in the hydrogenation of NAD⁺ with HCOOK, leading to the regiospecific formation of 1,4-NADH (pH 7) with TOF = 143 h^–1, which is about 3 times higher than the previous highest value (54 h^–1) reported for [Cp*Ir­(4-(1<i>H</i>-pyrazol-1-yl-κ<i>N</i>²)­benzoic acid)­(H₂O)]­SO₄ (<b>3</b>). The activity seems to be critically affected by the presence of the NH functionality, as indicated by its drop of about 1 order of magnitude when <b>2</b> (TOF = 17 h^–1) was used as the catalyst instead of <b>1</b>. <b>1</b> is also able to mediate the dehydrogenation of β-NADH, under slightly acidic conditions, as determined by NMR and GC measurements. Furthermore, an in-depth investigation carried out combining 1D, 2D, and diffusion NMR techniques indicate a remarkable speciation of β-NADH leading not only to the expected β-NAD⁺ but also to α-NAD⁺, nicotinamide (NA), and 1,2,5,6-tetrahydronicotinamide (NAH₃). The formation of NAH₃ has been identified as the cause of the low TON values obtained with <b>1</b> and <b>3</b>, because it consumes part of the produced H₂

Probing Hydrogen Bond Networks in Half-Sandwich Ru(II) Building Blocks by a Combined ¹H DQ CRAMPS Solid-State NMR, XRPD, and DFT Approach

The hydrogen bond network of three polymorphs (<b>1α</b>, <b>1β</b>, and <b>1γ</b>) and one solvate form (<b>1·H</b>_<b>2</b><b>O</b>) arising from the hydration–dehydration process of the Ru­(II) complex [(<i>p</i>-cymene)­Ru­(κN-INA)­Cl₂] (where INA is isonicotinic acid), has been ascertained by means of one-dimensional (1D) and two-dimensional (2D) double quantum ¹H CRAMPS (Combined Rotation and Multiple Pulses Sequences) and ¹³C CPMAS solid-state NMR experiments. The resolution improvement provided by homonuclear decoupling pulse sequences, with respect to fast MAS experiments, has been highlighted. The solid-state structure of <b>1γ</b> has been fully characterized by combining X-ray powder diffraction (XRPD), solid-state NMR, and periodic plane-wave first-principles calculations. None of the forms show the expected supramolecular cyclic dimerization of the carboxylic functions of INA, because of the presence of Cl atoms as strong hydrogen bond (HB) acceptors. The hydration–dehydration process of the complex has been discussed in terms of structure and HB rearrangements

Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO₂ Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn0–Mn0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn0–Mn0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications

Probing Hydrogen Bond Networks in Half-Sandwich Ru(II) Building Blocks by a Combined ¹H DQ CRAMPS Solid-State NMR, XRPD, and DFT Approach

The hydrogen bond network of three polymorphs (<b>1α</b>, <b>1β</b>, and <b>1γ</b>) and one solvate form (<b>1·H</b>_<b>2</b><b>O</b>) arising from the hydration–dehydration process of the Ru­(II) complex [(<i>p</i>-cymene)­Ru­(κN-INA)­Cl₂] (where INA is isonicotinic acid), has been ascertained by means of one-dimensional (1D) and two-dimensional (2D) double quantum ¹H CRAMPS (Combined Rotation and Multiple Pulses Sequences) and ¹³C CPMAS solid-state NMR experiments. The resolution improvement provided by homonuclear decoupling pulse sequences, with respect to fast MAS experiments, has been highlighted. The solid-state structure of <b>1γ</b> has been fully characterized by combining X-ray powder diffraction (XRPD), solid-state NMR, and periodic plane-wave first-principles calculations. None of the forms show the expected supramolecular cyclic dimerization of the carboxylic functions of INA, because of the presence of Cl atoms as strong hydrogen bond (HB) acceptors. The hydration–dehydration process of the complex has been discussed in terms of structure and HB rearrangements

[MnBrL(CO)₄] (L = Amidinatogermylene): Reductive Dimerization, Carbonyl Substitution, and Hydrolysis Reactions

The manganese­(I) carbonyl complex [MnBr­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₄] (<b>1</b>; ^<i>i</i>Pr₂bzam = 1,3-di­(isopropyl)­benzamidinate), which contains an amidinatogermylene ligand, reacts with LiPh or Li^<i>t</i>Bu at room temperature undergoing a reductive dimerization process that leads to the manganese(0) dimer [Mn₂­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}₂­(CO)₈]. This complex and the monosubstituted derivative [Mn₂­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₉] have also been prepared by reacting [Mn₂(CO)₁₀] with Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu at high temperature (110 °C). These binuclear complexes contain their germylene ligands in axial positions (<i>trans</i> to the Mn–Mn bond). The large volume of the germylene ligand clearly affects the reactivity of complex <b>1</b> with neutral two-electron donor reagents, since for bulky reagents, the CO substitution occurs <i>trans</i> to the germylene ligand, as in <i>trans-mer</i>-[MnBrL­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₃] (L = Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu, PMe₃), whereas for small reagents, the CO substitution occurs <i>cis</i> to the germylene ligand, as in <i>fac</i>-[MnBr­(CN^<i>t</i>Bu)­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₃]. The IR spectra (ν_CO) of these complexes have confirmed that the germylene Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu is a very strong electron-donating ligand, even stronger than the most basic trialkylphosphanes and N-heterocyclic carbenes. The hydrolysis of complex <b>1</b> leads to the salt [^<i>i</i>Pr₂bzamH₂]­[MnBr­{Ge­(OH)₂^<i>t</i>Bu}­(CO)₄], the anion of which contains an unprecedented germanato­(II) ligand, [Ge­(OH)₂^<i>t</i>Bu]⁻, in <i>cis</i> to the Br atom. This hydrolysis product and its precursor <b>1</b> have been tested as catalyst precursors for the electrolytic reduction of CO₂, showing no significant activity

[MnBrL(CO)₄] (L = Amidinatogermylene): Reductive Dimerization, Carbonyl Substitution, and Hydrolysis Reactions

The manganese­(I) carbonyl complex [MnBr­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₄] (<b>1</b>; ^<i>i</i>Pr₂bzam = 1,3-di­(isopropyl)­benzamidinate), which contains an amidinatogermylene ligand, reacts with LiPh or Li^<i>t</i>Bu at room temperature undergoing a reductive dimerization process that leads to the manganese(0) dimer [Mn₂­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}₂­(CO)₈]. This complex and the monosubstituted derivative [Mn₂­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₉] have also been prepared by reacting [Mn₂(CO)₁₀] with Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu at high temperature (110 °C). These binuclear complexes contain their germylene ligands in axial positions (<i>trans</i> to the Mn–Mn bond). The large volume of the germylene ligand clearly affects the reactivity of complex <b>1</b> with neutral two-electron donor reagents, since for bulky reagents, the CO substitution occurs <i>trans</i> to the germylene ligand, as in <i>trans-mer</i>-[MnBrL­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₃] (L = Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu, PMe₃), whereas for small reagents, the CO substitution occurs <i>cis</i> to the germylene ligand, as in <i>fac</i>-[MnBr­(CN^<i>t</i>Bu)­{Ge­(^<i>i</i>Pr₂bzam)­^<i>t</i>Bu}­(CO)₃]. The IR spectra (ν_CO) of these complexes have confirmed that the germylene Ge­(^<i>i</i>Pr₂bzam)^<i>t</i>Bu is a very strong electron-donating ligand, even stronger than the most basic trialkylphosphanes and N-heterocyclic carbenes. The hydrolysis of complex <b>1</b> leads to the salt [^<i>i</i>Pr₂bzamH₂]­[MnBr­{Ge­(OH)₂^<i>t</i>Bu}­(CO)₄], the anion of which contains an unprecedented germanato­(II) ligand, [Ge­(OH)₂^<i>t</i>Bu]⁻, in <i>cis</i> to the Br atom. This hydrolysis product and its precursor <b>1</b> have been tested as catalyst precursors for the electrolytic reduction of CO₂, showing no significant activity