Photo- and triboluminescent pyridinophane Cu complexes: new organometallic tools for mechanoresponsive materials

The development of mechanoresponsive polymers has emerged as a new, attractive area of research in which changes at the molecular level exert macrolevel effects in the bulk material, and vice versa, as simple mechanical action on the bulk material exerts an effect on bonding at the microlevel. In many of these polymers, molecules known as mechanopores, which undergo chemical or conformational changes in response to mechanical action, are typically incorporated into the polymer chain. The field has been dominated by the use of organic mechanophores, and only recently has the use of organotransition metal complexes as tunable, dynamic mechanophores emerged as a promising research direction. Herein we will summarize recent developments towards the use of N-heterocyclic carbene (NHC) complexes of copper with dynamic, conformationally fluxional pyridinophane ligands as new organometallic mechanophores. We will discuss the interplay between the dynamic behaviour, steric bulk, and the photoluminescent and triboluminescent properties of these complexes, which enabled their use in the development of new, mechanoresponsive polymer materials.journal articl

Stable Nickel(I) Complexes with Electron-Rich, Sterically-Hindered, Innocent PNP Pincer Ligands

The electronic and steric properties of a new class of electron-rich and sterically hindered tertamethylated PNP pincer ligands (Me4PNPR = 2,6-bisRdialkylphosphino)propylipyridine with R = Pr-i, Bu-t) are discussed. Introducing the methyl groups on the pincer arm prevents dearomatization of the pincer framework and increases the bulkiness and electron-donating capacity of the ligand. Highly reactive Ni-I species are thus prevented from dimerizing and can be analyzed by a wide variety of spectroscopic methods. X-ray diffraction study shows that steric bulk has an important influence on the resulting geometric and spectroscopic properties of the Ni-I complexes. Complexes S and 6, which contain Pr-i groups on the phosphorus atoms, show a very rare seesaw geometry around the metal center, while Bu-t complexes 7 and 8 show a distorted square-planar geometry. Computational analysis reveals that the SOMO for all complexes has a d(x)(-y)(2)(2) character with the spin density mostly residing on the nickel

Dynamic Phosphorescent Probe for Facile and Reversible Stress Sensing

Dynamic phosphorescent copper complex incorporated into the main chain of polyurethanes produces a facile and reversible response to tensile stress. In contrast to common deformation sensors, the applied stress does not lead to bond scission, or alters the phosphor structure. The suppression of dynamics responsible for the nonradiative relaxation is found to be the major pathway governing stress response. As a result, the response of dynamic phosphor described in this work is stress specific. Compared to initial unloaded state, a nearly twofold increase of photoluminescence intensity occurs in response to a 5–35 MPa stress applied to pristine metalated polymers or their blends with various polyurethanes. Finally, the dynamic sensor proves useful for mapping stress distribution patterns and tracking dynamic phenomena in polyurethanes using simple optical imaging techniques

Heterometallic bond activation enabled by unsymmetrical ligand scaffolds: bridging the opposites

Heterobi- and multimetallic complexes providing close proximity between several metal centers serve as active species in artificial and enzymatic catalysis, and in model systems, showing unique modes of metal–metal cooperative bond activation. Through the rational design of well-defined, unsymmetrical ligand scaffolds, we create a convenient approach to support the assembly of heterometallic species in a well-defined and site-specific manner, preventing them from scrambling and dissociation. In this perspective, we will outline general strategies for the design of unsymmetrical ligands to support heterobi- and multimetallic complexes that show reactivity in various types of heterometallic cooperative bond activation

Nickel(II) Complexes with Electron-Rich, Sterically Hindered PNP Pincer Ligands Enable Uncommon Modes of Ligand Dearomatization

We report the reactivity and characterization of hydride, methyl, and bromo Ni-II complexes with a new class of electron-rich and sterically hindered PNP pincer ligands, Me₄PNPR (R = Pr-i, Bu-t), in which a classical metal-ligand cooperative mode of reactivity via CH2 arm deprotonation is blocked by methylation. This enables new, uncommon modes of PNP ligand dearomatization that involve reactivity in the para position of the pyridine ring. In particular, the reduction of [(Me₄PNPiPr) (NiMe)-Me-II]B(Ar-F)(4) with KC₈ leads to the formation of a new C-C bond via dimerization of two complexes through the para position. This reactivity stands in sharp contrast to the previously reported bromo or chloro complexes, where stable Ni-I halogen moieties are formed. Computational analysis showed a greater propensity for ligand-centered radical formation for the presumed intermediate one-electron-reduced species. UV-induced homolysis of the Ni-II-Me bond in [(Me₄PNPiPr) (NiMe)-Me-II]B(Ar-F)(4) leads to the formation of a Me radical detected by radical traps and Ni(I )intermediates that can be trapped in the presence of halide ions to give previously characterized, stable Ni-I halogen complexes. In addition, treatment of the bromo complexes [(Me₄PNPR)(NiBr)-Br-II]BPh₄ with a powerful hydride source, LiBEt₃H, leads to the reduction of the pyridine ring and the formation of Ni-II complexes with an anionic amide donor reduced pincer ligand, although aromatic Ni-II hydride complexes could also be obtained with a weaker hydride source. We have observed that steric bulk at the phosphine donors controls the reactivity of the resulting Ni(II)H( )complexes. While t-Bu-substituted [(Me₄PNPtBu) (NiH)-H-II]Y(Y=BPh₄, B(Ar-F)(4)) does not react with O-2, the less sterically hindered Pr-i-substituted [(Me₄PNPiPr)(NiH)-H-II]Y reacts instantaneously to give an unstable superoxide adduct that can be observed by EPR

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O2 or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent NiIII and structurally characterized mixed-valent NiII–NiIV intermediates and radical intermediates, resembling O2 activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF3)2 complexes resulting in the formation of a stable NiIII product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.journal articl

Cobalt Complexes of Bulky PNP Ligand: H2 Activation and Catalytic Two-Electron Reactivity in Hydrogenation of Alkenes and Alkynes

The reactivity of cobalt pincer complexes supported by the bulky tetramethylated PNP ligands Me4PNPR(R = iPr, tBu) has been investigated. In these ligands, the undesired H atom loss reactivity observed earlier in some classical CH2-arm PNP cobalt complexes is blocked, allowing them to be utilized for promoting two-electron catalytic transformations at the cobalt center. Accordingly, reaction of the formally CoIMe complex 3 with H2 under ambient pressure and temperature afforded the CoIII trihydride 4-H, in a reaction cascade reasoned to proceed by two-electron oxidative addition and reductive eliminations. This mechanistic proposal, alongside the observance of alkene insertion and ethane production upon sequential exposure of 3 to ethylene and H2, prompted an exploration into 3 as a catalyst for hydrogenation. Complex 4-H, formed in situ from 3 under H2, was found to be active in the catalytic hydrogenation of alkenes and alkynes. The proposed two-electron mechanism is reminiscent of the platinum group metals and demonstrates the utility of the bulky redox-innocent Me4PNPR ligand in the avoidance of one-electron reactivity, a concept that may show broad applicability in expanding the scope of earth-abundant first-row transition-metal catalysis.journal articl

Facile and reversible double dearomatization of pyridines in non-phosphine MnI complexes with N,S-donor pyridinophane ligand

Single and double dearomatization of pyridine rings was observed in Mn(I) complexes with an N2S2 pyridinophane ligand via deprotonation of one or two CH2 arms, respectively. In contrast to other N,S-donor pincer-like systems, the dearomatized (N2S2)Mn species were found to be stable, with the dearomatization being reversible

Transfer Hydrogenation of Carbonyl Groups, Imines and N‐Heterocycles Catalyzed by Simple, Bipyridine‐Based MnI Complexes

Utilization of hydroxy‐substituted bipyridine ligands in transition metal catalysis mimicking [Fe]‐hydrogenase has been shown to be a promising approach in developing new catalysts for hydrogenation. For example, MnI complexes with 6,6′‐dihydroxy‐2,2′‐bipyridine ligand have been previously shown to be active catalysts for CO2 hydrogenation. In this work, simple bipyridine‐based Mn catalysts were developed that act as active catalysts for transfer hydrogenation of ketones, aldehydes and imines. For the first time, Mn‐catalyzed transfer hydrogenation of N ‐heterocycles was reported. The highest catalytic activity among complexes with variously substituted ligands was observed for the complex bearing two OH groups in bipyridine. Deuterium labeling experiments suggest a monohydride pathway

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 ortho-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