An Entry to Stable Mixed Phosphine-Osmium-NHC Polyhydrides

Ligand Regulation Strategy to Modulate ROS Nature in a Rhodamine-Iridium(III) Hybrid System for Phototherapy

The efficacy of photodynamic therapy (PDT) is highly dependent on the photosensitizer features. The reactive oxygen species (ROS) generated by photosensitizers is proven to be associated with immunotherapy by triggering immunogenic cell death (ICD) as well. In this work, we establish a rhodamine-iridium(III) hybrid model functioning as a photosensitizer to comprehensively understand its performance and potential applications in photodynamic immunotherapy. Especially, the correlation between the ROS generation efficiency and the energy level of the Ir(III)-based excited state (T1'), modulated by the cyclometalating (C∧N) ligand, is systematically investigated and correlated. We prove that in addition to the direct population of the rhodamine triplet state (T1) formed through the intersystem crossing process with the assistance of a heavy Ir(III) metal center, the fine-tuned T1' state could act as a relay to provide an additional pathway for promoting the cascade energy transfer process that leads to enhanced ROS generation ability. Moreover, type I ROS can be effectively produced by introducing sulfur-containing thiophene units in C∧N ligands, providing a stronger M1 macrophage-activation efficiency under hypoxia to evoke in vivo antitumor immunity. Overall, our work provides a fundamental guideline for the molecular design and exploration of advanced transition-metal-based photosensitizers for biomedical applications.

Reactions of an Osmium-Hexahydride Complex with 2-Butyne and 3-Hexyne and Their Performance in the Migratory Hydroboration of Aliphatic Internal Alkynes

Reactions of the hexahydride OsH₆(PⁱPr₃)₂ (1) with 2-butyne and 3-hexyne and the behavior of the resulting species toward pinacolborane (pinBH) have been investigated in the search for new hydroboration processes. Complex 1 reacts with 2-butyne to give 1-butene and the osmacyclopropene OsH₂(η²-C₂Me₂)(PⁱPr₃)₂ (2). In toluene, at 80 °C, the coordinated hydrocarbon isomerizes into a η⁴-butenediyl form to afford OsH₂(η⁴-CH₂CHCHCH₂)(PⁱPr₃)₂ (3). Isotopic labeling experiments indicate that the isomerization involves Me-to-C_Os hydrogen 1,2-shifts, which take place through the metal. The reaction of 1 with 3-hexyne gives 1-hexene and OsH₂(η²-C₂Et₂)(PⁱPr₃)₂ (4). Similarly to 2, complex 4 evolves to η⁴-butenediyl derivatives OsH₂(η⁴-CH₂CHCHCHEt)(PⁱPr₃)₂ (5) and OsH₂(η⁴-MeCHCHCHCHMe)(PⁱPr₃)₂ (6). In the presence of pinBH, complex 2 generates 2-pinacolboryl-1-butene and OsH{κ²-H,H-(H₂Bpin)}(η²-HBpin)(PⁱPr₃)₂ (7). According to the formation of the borylated olefin, complex 2 is a catalyst precursor for the migratory hydroboration of 2-butyne and 3-hexyne to 2-pinacolboryl-1-butene and 4-pinacolboryl-1-hexene. During the hydroboration, complex 7 is the main osmium species. The hexahydride 1 also acts as a catalyst precursor, but it requires an induction period that causes the loss of 2 equiv of alkyne per equiv of osmium.

Dissimilarity in the Chemical Behavior of Osmaoxazolium Salts and Osmaoxazoles: Two Different Aromatic Metalladiheterocycles

The preparation of aromatic hydride-osmaoxazolium and hydride-oxazole compounds is reported and their reactivity toward phenylacetylene investigated. Complex [OsH(OH)(≡CPh)(IPr)(PiPr3)]OTf (1; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF3SO3) reacts with acetonitrile and benzonitrile to give [OsH{κ2-C,O-[C(Ph)NHC(R)O]}(NCR)(IPr)(PiPr3)]OTf (R = Me (2), Ph (3)) via amidate intermediates, which are generated by addition of the hydroxide ligand to the nitrile. In agreement with this, the addition of 2-phenylacetamide to acetonitrile solutions of 1 gives [OsH{κ2-C,O-[C(Ph)NHC(CH2Ph)O]}(NCCH3)(IPr)(PiPr3)]OTf (4). The deprotonation of the osmaoxazolium ring of 2 and 4 leads to the oxazole derivatives OsH{κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (5), CH2Ph (6)). Complexes 2 and 4 add their Os-H and Os-C bonds to the C-C triple bond of phenylacetylene to afford [Os{η3-C 3 ,κ1-O-[CH2C(Ph)C(Ph)NHC(R)O]}(NCCH3)2(IPr)]OTf (R = Me (7), CH2Ph (8)), bearing a tridentate amide-N-functionalized allyl ligand, while complexes 5 and 6 undergo a vicarious nucleophilic substitution of the hydride at the metal center with the alkyne, via the compressed dihydride adduct intermediates OsH2(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (9), CH2Ph (10)), which reductively eliminate H2 to yield the acetylide-osmaoxazoles Os(C≡CPh){κ2-C,O-[C(Ph)NC(R)O]}(IPr)(PiPr3) (R = Me (11), CH2Ph (12)).

Coordination Complexes as a New Generation Photosensitizer for Photodynamic Anticancer Therapy

Photodynamic therapy (PDT) has become an alternative to standard cancer treatment methods such as surgery, chemotherapy and radiotherapy. The uniqueness of this method relies on the possibility of using various photosensitizers (PS) that absorb and convert light emission in radical oxygen-derived species (ROS). They can be present alone or in the presence of other compounds such as metal organic frameworks (MOFs), non-tubules or polymers. The interaction between DNA and metal-based complexes plays a key role in the development of new anti-cancer drugs. The use of coordination compounds in PDT has a significant impact on the amount ROS generated, quantum emission efficiency (Φem) and phototoxic index (PI). In this review, we will attempt to systematically review the recent literature and analyze the coordination complexes used as PS in PDT. Finally, we compared the anticancer activities of individual coordination complexes and discuss future perspectives. So far, only a few articles link so many transition metal ion coordination complexes of varying degrees of oxidation, which is why this review is needed by the scientific community to further expand this field worldwide. Additionally, it serves as a convenient collection of important, up-to-date information.

Cyclometalated Osmium Compounds and beyond: Synthesis, Properties, Applications

The synthesis of cyclometalated osmium complexes is usually more complicated than of other transition metals such as Ni, Pd, Pt, Rh, where cyclometalation reactions readily occur via direct activation of C–H bonds. It differs also from their ruthenium analogs. Cyclometalation for osmium usually occurs under more severe conditions, in polar solvents, using specific precursors, stronger acids, or bases. Such requirements expand reaction mechanisms to electrophilic activation, transmetalation, and oxidative addition, often involving C–H bond activations. Osmacycles exhibit specific applications in homogeneous catalysis, photophysics, bioelectrocatalysis and are studied as anticancer agents. This review describes major synthetic pathways to osmacycles and related compounds and discusses their practical applications.

Reduction of Benzonitriles via Osmium-Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers

The reduction of the N≡C bond of benzonitriles promoted by OsH₆(PⁱPr₃)₂ (1) has been studied. Complex 1 releases a H₂ molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH₄{κ¹- N-(N≡CC₆H₃Me₂)}(PⁱPr₃)₂ (2), which is thermally stable toward the insertion of the nitrile into one of the Os-H bonds. In contrast to 2,6-dimethylbenzonitrile, benzonitrile and 2-methylbenzonitrile undergo insertion, via Os(η²-N≡CR) intermediates, to give the azavinylidene derivatives OsH₃(═N═CC₆H₄R)(PⁱPr₃)₂ [R = H (3) or Me (4)]. The analysis by means of computational tools (EDA-NOCV) of the bonding situation in these compounds suggests that the donor-acceptor nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor-acceptor and pure electron-sharing bonding modes. The N atom is strongly nucleophilic, whereas one of the hydrides is electrophilic. In spite of the different nature of these centers, the migration of the latter to the N atom is kinetically prevented. However, the use of water as a proton shuttle allows hydride migration, as a consequence of a significant decrease in the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH₃{κ²- N, C-(NH═CHC₆H₃R)}(PⁱPr₃)₂ [R = H (5) or Me (6)]. The presence of electrophilic and nucleophilic centers in 3 confers upon it the ability to activate σ-bonds, including H₂ and pinacolborane (HBpin). The reaction with the latter gives OsH₃{κ²- N, C-[N(Bpin)═CHC₆H₄]}(PⁱPr₃)₂ (7).