meso‐(2‐Pyridyl)‐boron(III)‐subporphyrin: Perimeter Iridium(III) Coordination

Peripherally metalated porphyrinoids are promising functional π‐systems displaying characteristic optical, electronic, and catalytic properties. In this work, 5‐(2‐pyridyl)‐ and 5,10,15‐tri(2‐pyridyl)‐B^III‐subporphyrins were prepared and used to produce cyclometalated subporphyrins by reactions with [Cp*IrCl₂]₂, which proceeded through an efficient C?H activation to give the corresponding mono‐ and tri‐Ir^III complexes, respectively. While the mono‐Ir^III complex was obtained as a diastereomeric mixture, a C₃‐symmetric tri‐Ir^III complex with the three Cp*‐units all at the concave side was predominantly obtained in a high yield of 90 %, which displays weak NIR phosphorescence even at room temperature in degassed CH₂Cl₂, differently from the mono‐Ir^III complexes.     

Reactions of Pyridyl‐Functionalized,Chelating λ3‐Phosphinines in the Coordination Environment of RhIII and IrIII

Rh^III and Ir^III complexes based on the λ³‐P,N hybrid ligand 2‐(2′‐pyridyl)‐4,6‐diphenylphosphinine ( 1 ) react selectively at the P?C double bond to chiral coordination compounds of the type [( 1 H ? OH)Cp*MCl]Cl ( 2 , 3 ), which can be deprotonated with triethylamine to eliminate HCl. By using different bases, the pK_a value of the P? OH group could be estimated. Whereas [( 1 H ? O)Cp*IrCl] ( 4 ) is formed quantitatively upon treatment with NEt₃, the corresponding rhodium compound [( 1 H ? O)Cp*RhCl] ( 5 ) undergoes tautomerization upon formation of the λ⁵σ⁴‐phosphinine rhodium(III) complex [( 1? OH)Cp*RhCl] ( 6 ) as confirmed by single‐crystal X‐ray diffraction. Blocking the acidic P? OH functionality in 3 by introducing a P? OCH₃ substituent leads directly to the λ⁵σ⁴‐phosphinine iridium(III) complex ( 8 ) upon elimination of HCl. These new transformations in the coordination environment of Rh^III and Ir^III provide an easy and general access to new transition‐metal complexes containing λ⁵σ⁴‐phosphinine ligands.     

2‐(2′‐Pyridyl)‐4,6‐diphenylphosphinine versus 2‐(2′‐Pyridyl)‐4,6‐diphenylpyridine: Synthesis,Characterization, and Reactivity of Cationic RhIII and IrIII Complexes Based on Aromatic Phosphorus Heterocycles

The bidentate P,N hybrid ligand 1 allows access for the first time to novel cationic phosphinine‐based Rh^III and Ir^III complexes, broadening significantly the scope of low‐coordinate aromatic phosphorus heterocycles for potential applications. The coordination chemistry of 1 towards Rh^III and Ir^III was investigated and compared with the analogous 2,2′‐bipyridine derivative, 2‐(2′‐pyridyl)‐4,6‐diphenylpyridine ( 2 ), which showed significant differences. The molecular structures of [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl (Cp*=pentamethylcyclopentadienyl) were determined by means of X‐ray diffraction and confirm the mononuclear nature of the λ³‐phosphinine–Rh^III and Ir^III complexes. In contrast, a different reactivity and coordination behavior was found for the nitrogen analogue 2 , especially towards Rh^III as a bimetallic ion pair [RhCl(Cp*)( 2 )]⁺[RhCl₃(Cp*)]^? is formed rather than a mononuclear coordination compound. [RhCl(Cp*)( 1 )]Cl and [IrCl(Cp*)( 1 )]Cl react with water regio‐ and diastereoselectively at the external P?C double bond, leading exclusively to the anti‐addition products [MCl(Cp*)( 1 H ? OH)]Cl as confirmed by X‐ray crystal‐structure determination.     

Redox Non‐Innocent Behavior of a Terminal Iridium Hydrazido(2−) Triple Bond

The synthesis of the first terminal Group 9 hydrazido(2‐) complex, Cp*IrN(TMP) ( 6 ) (TMP=2,2,6,6‐tetramethylpiperidine) is reported. Electronic structure and X‐ray diffraction analysis indicate that this complex contains an Ir?N triple bond, similar to Bergman's seminal Cp*Ir(N^tBu) imido complex. However, in sharp contrast to Bergman's imido, 6 displays remarkable redox non‐innocent reactivity owing to the presence of the N_β lone pair. Treatment of 6 with MeI results in electron transfer from N_β to Ir prior to oxidative addition of MeI to the iridium center. This behavior opens the possibility of carrying out facile oxidative reactions at a formally Ir^III metal center through a hydrazido(2?)/isodiazene valence tautomerization.     

Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η5‐Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one‐dimensional (1D) hydrogen‐bonded (H‐bonded) rhodium(II)–η⁵‐semiquinone complex, [Cp*Rh(η⁵‐p‐HSQ‐Me₄)]PF₆ ([ 1 ]PF₆; Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl; HSQ=semiquinone) exhibits a paraelectric–antiferroelectric second‐order phase transition at 237.1 K. Neutron and X‐ray crystal structure analyses reveal that the H‐bonded proton is disordered over two sites in the room‐temperature (RT) phase. The phase transition would arise from this proton disorder together with rotation or libration of the Cp* ring and PF₆^? ion. The relative permittivity ε_b′ along the H‐bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of ¹³C CP/MAS NMR spectra demonstrates that the proton is dynamically disordered in the RT phase and that the proton exchange has already occurred in the low‐temperature (LT) phase. Rate constants for the proton exchange are estimated to be 10^?4–10^?6 s in the temperature range of 240–270 K. DFT calculations predict that the protonation/deprotonation of [ 1 ]⁺ leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the π‐bonded rhodium fragment, producing the stable η⁶‐hydroquinone complex, [Cp*Rh³⁺(η⁶‐p‐H₂Q‐Me₄)]²⁺ ([ 2 ]²⁺), and η⁴‐benzoquinone complex, [Cp*Rh⁺(η⁴‐p‐BQ‐Me₄)] ([ 3 ]), respectively. Possible mechanisms leading to the dielectric response are discussed on the basis of the migration of the protonic solitons comprising of [ 2 ]²⁺ and [ 3 ], which would be generated in the H‐bonded chain.     

Reactions of 2‐Substituted Pyridines with Titanocenes and Zirconocenes: Coupling versus Dearomatisation

Reactions of group 4 metallocene sources with 2‐substituted pyridines were investigated to evaluate their coordination type between innocent and reductive dearomatisation as well as to probe the possibility for couplings. A dependence on the cyclopentadienyl ligands (Cp, Cp*), the metals (Ti, Zr), and the substrates (2‐phenyl‐, 2‐acetyl‐, and 2‐iminopyridine) was observed. While 2‐phenylpyridine is barely reactive, 2‐acetylpyridine reacts vigorously with the Cp‐substituted complexes and selectively with their Cp* analogues. With 2‐iminopyridine, in all cases selective reactions were observed. In the isolated [Cp₂Ti], [Cp₂Zr], and [Cp*₂Zr] compounds the substrate coordinates by its pyridyl ring and the unsaturated side‐chain. Subsequently, the pyridine was dearomatised, which is most pronounced in the [Cp*₂Zr] compounds. Using [Cp*₂Ti] leads to the unexpected paramagnetic complexes [Cp*₂Ti^III(N,O‐acpy)] and [Cp*₂Ti^III(N,N′‐impy)]. This highlights the non‐innocent character of the pyridyl substrates.     

Koordination von Rhodium(III), Iridium(III) und Kupfer(II) mit dem potentiell vierzähnigen Akzeptorliganden Bis(1‐methylimidazol‐2‐yl)glyoxal (big)

Coordination of Rhodium(III), Iridium(III), and Copper(II) with the Potentially Tetradentate Acceptor Ligand Bis(1‐methylimidazol‐2‐yl)glyoxal (big) Bis(1‐methylimidazol‐2‐yl)glyoxal (big) which has hitherto not been used in coordination chemistry crystallizes to form two perpendicular 1‐methylimidazol‐2‐yl‐carbonyl molecular halves. Out of the various possibilities for mono‐ and bis‐chelate coordination the N,N′‐alternative with a seven‐membered chelate ring is realized in [Cp*Cl(big)Rh](PF₆) as evident from crystal structure analysis. The iridium analogue reacts under hydration of big and elimination of HCl to form a complex cation [Cp*(bigOH)Ir]⁺ which dimerizes in the crystal through hydrogen bonding and contains one five‐ and one six‐membered chelate ring involving the alcoholate‐O. Cu(ClO₄)₂ and the ligand big yield a complex ion [Cu(big)₂]²⁺ with an ESR spectrum that suggests the coordination of the central metal by four N atoms in an approximately planar setting.     

Synthesis of planar chiral iridacycles by cationic metal π-coordination: facial selectivity, and conformational and stereochemical consequences

Facial selectivity during the π-coordination of pseudo-tetrahedral iridacycles by neutral (Cr(CO)(3)), monocationic (Cp*Ru(+)), and biscationic (Cp*Ir(2+)) metal centers was directly influenced by the coulombic imbalance in the coordination sphere of the chelated Ir center. We also showed by using theoretical calculations that the feasibility of the related metallacycles that displayed metallocenic planar chirality was dependent to the presence of an electron-donating group, such as NMe(2), which contributed to the overall stability of the complexes. When the π-bonded moiety was the strongly electron-withdrawing Cp*Ir(2+) group, the electron donation from NMe(2) resulted in major conformational changes, with a barrier to rotation of about 17?kcal mol(-1) for this group that became spectroscopically diastereotopic (high-field (1)H?NMR spectroscopy). This peculiar property is proposed as a means to introduce a new type of constitutional chirality at the nitrogen center: planar chirality at tertiary aromatic amines.     

μ3‐η2:η2:η2‐Coordination of Primary Silane on a Triruthenium Plane

A μ₃‐η²:η²:η²‐silane complex, [(Cp*Ru)₃(μ₃‐η²:η²:η²‐H₃SitBu)(μ‐H)₃] ( 2 a ; Cp*=η⁵‐C₅Me₅), was synthesized from the reaction of [{Cp*Ru(μ‐H)}₃(μ₃‐H)₂] ( 1 ) with tBuSiH₃. Complex 2 a is the first example of a silane ligand adopting a μ₃‐η²:η²:η² coordination mode. This unprecedented coordination mode was established by NMR and IR spectroscopy as well as X‐ray diffraction analysis and supported by a density functional study. Variable‐temperature NMR analysis implied that 2 a equilibrates with a tautomeric μ₃‐silyl complex ( 3 a ). Although 3 a was not isolated, the corresponding μ₃‐silyl complex, [(Cp*Ru)₃(μ₃‐η²:η²‐H₂SiPh)(H)(μ‐H)₃] ( 3 b ), was obtained from the reaction of 1 with PhSiH₃. Treatment of 2 a with PhSiH₃ resulted in a silane exchange reaction, leading to the formation of 3 b accompanied by the elimination of tBuSiH₃. This result indicates that the μ₃‐silane complex can be regarded as an “arrested” intermediate for the oxidative addition/reductive elimination of a primary silane to a trinuclear site.     

Na4IrO4: Square‐Planar Coordination of a Transition Metal in d5 Configuration due to Weak On‐Site Coulomb Interactions

Local environments and valence electron counts primarily determine the electronic states and physical properties of transition‐metal complexes. For example, square‐planar coordination geometries found in transition‐metal oxometalates such as cuprates are usually associated with the d⁸ or d⁹ electron configuration. In this work, we address an unusual square‐planar single oxoanionic [IrO₄]^4? species, as observed in Na₄IrO₄ in which Ir^IV has a d⁵ configuration, and characterize the chemical bonding through experiments and by ab initio calculations. We find that the Ir^IV center in ground‐state Na₄IrO₄ has square‐planar coordination geometry because of the weak Coulomb repulsion of the Ir‐5d electrons. In contrast, in its 3d counterpart Na₄CoO₄, the Co^IV center is tetrahedrally coordinated because of strong electron correlation. Na₄IrO₄ may thus serve as a simple yet important example to study the ramifications of Hubbard‐type Coulomb interactions on local geometries.     

Enantiopure Ferrocene‐Based Planar‐Chiral Iridacycles: Stereospecific Control of Iridium‐Centred Chirality

Reaction of [IrCp*Cl₂]₂ with ferrocenylimines (Fc=NAr, Ar=Ph, p‐MeOC₆H₄) results in ferrocene C?H activation and the diastereoselective synthesis of half‐sandwich iridacycles of relative configuration S_p*,R_Ir*. Extension to (S)‐2‐ferrocenyl‐4‐(1‐methylethyl)oxazoline gave highly diastereoselective control over the new elements of planar chirality and metal‐based pseudo‐tetrahedral chirality, to give both neutral and cationic half‐sandwich iridacycles of absolute configuration S_c,S_p,R_Ir. Substitution reactions proceed with retention of configuration, with the planar chirality controlling the metal‐centred chirality through an iron–iridium interaction in the coordinatively unsaturated cationic intermediate.     

Molekül‐ und Kristallstruktur von Bis[chloro(μ‐phenylimido)(η5‐pentamethylcyclopentadienyl)tantal(IV)](Ta–Ta), [{TaCl(μ‐NPh)Cp*}2]

Molecular and Crystal Structure of Bis[chloro(μ‐phenylimido)(η⁵‐pentamethylcyclopentadienyl)tantalum(IV)](Ta–Ta), [{TaCl(μ‐NPh)Cp*}₂] Despite the steric hindrance of the central atom in [TaCl₂(NPh)Cp*] (Ph = C₆H₅, Cp* = η⁵‐C₅(CH₃)₅), caused by the Cp* ligand, the imido‐ligand takes a change in bond structure when this educt is reduced to the binuclear complex [{TaCl(μ‐NPh)Cp*}₂] in which tantalum is stabilized in the unusual oxidation state +4.     

Theoretical prediction on the synthesis of 2,3‐dihydropyridines through Co(III)‐catalysed reaction of unsaturated oximes with alkenes

Cobalt‐based catalysts can replace the homologous group‐9 rhodium‐based ones. Herein, we used density functional theory (DFT) calculations to predict the synthesis of 2,3‐dihydropyridines using α,β‐unsaturated oxime pivalates and alkenes catalysed by [Cp*CoOAc]⁺ instead of [Cp*RhOAc]⁺. The catalytic cycle involves reversible acetate‐assisted metalation‐deprotonation, migratory insertion of alkenes, and reductive elimination/N‐O cleavage. The migratory insertion of alkenes was determined to be the rate‐determining step, and the reaction is irreversible due to the strongly exergonic reductive elimination/N? O cleavage. When using the CF₃‐substituted Cp*Co(III) catalyst, the apparent activation energy indicates that the title reaction can proceed at higher temperatures. Electron‐withdrawing substituent groups on Cp* facilitate the reaction. In contrast, substituting phenyl with the electron‐deficient p‐CF₃‐phenyl at the 2‐position of α,β‐unsaturated oxime pivalate hinders the reaction, and so does the use of polarized alkenes with electron‐withdrawing substituent groups     

Dual Metal Active Sites in an Ir1/FeOx Single‐Atom Catalyst: A Redox Mechanism for the Water‐Gas Shift Reaction

Herein, we report a theoretical and experimental study of the water‐gas shift (WGS) reaction on Ir₁/FeO_x single‐atom catalysts. Water dissociates to OH* on the Ir₁ single atom and H* on the first‐neighbour O atom bonded with a Fe site. The adsorbed CO on Ir₁ reacts with another adjacent O atom to produce CO₂, yielding an oxygen vacancy (O_vac). Then, the formation of H₂ becomes feasible due to migration of H from adsorbed OH* toward Ir₁ and its subsequent reaction with another H*. The interaction of Ir₁ and the second‐neighbouring Fe species demonstrates a new WGS pathway featured by electron transfer at the active site from Fe³⁺?O???Ir²⁺?O_vac to Fe²⁺?O_vac???Ir³⁺?O with the involvement of O_vac. The redox mechanism for WGS reaction through a dual metal active site (DMAS) is different from the conventional associative mechanism with the formation of formate or carboxyl intermediates. The proposed new reaction mechanism is corroborated by the experimental results with Ir₁/FeO_x for sequential production of CO₂ and H₂.     

Dreiecksdodekaedrische Heterotri‐ und ‐tetrametall‐Eisen–Ruthenium‐Cluster mit CpR‐ und Pn‐Liganden (n = 5, 4)

Triangulated Dodecahedral Heterotrimetallic‐ and ‐tetrametallic Iron–Ruthenium Clusters with Cp^R and P_n Ligands (n = 5, 4) The cothermolysis of [Cp*Fe(η⁵‐P₅)] ( 1 ) and [{Cp″(OC)₂Ru}₂](Ru–Ru) ( 2 ), Cp″ = C₅H₃But₂‐1,3, affords low yields of [Cp″Ru(η⁵‐P₅)] ( 3 ) and [{Cp″Ru}₂P₄] ( 4 ) as well as the triangulated dodecahedral hetero‐ and homotrimetallic clusters [{Cp″Ru}₂{Cp*Fe}P₅] ( 5 ), [{Cp″Ru}₃P₅] ( 6 ), [{Cp*Fe}₂{Cp″Ru}P₅] ( 7 ) and the tetranuclear compound [{Cp″Ru}₃{Cp*Fe}P₄] ( 8 ). X‐ray crystallographic studies show that the P₅ ligand in the distorted M₂M′P₅‐triangulated dodecahedra of 5 and 7 offers an unusual novel coordination mode derived from the educt 1 .     

Self‐Assembly through Coordination and π‐Stacking: Controlled Switching of Circularly Polarized Luminescence

Multiple noncovalent interactions can drive self‐assembly through different pathways. Here, by coordination‐assisted changes in π‐stacking modes between chromophores in pyrene‐conjugated histidine (PyHis), a self‐assembly system with reversible and inversed switching of supramolecular chirality, as well as circularly polarized luminescence (CPL) is described. It was found that l ‐PyHis self‐assembled into nanofibers showing P‐chirality and right‐handed CPL. Upon Zn^II coordination, the nanofibers changed into nanospheres with M‐chirality, as well as left‐handed CPL. The process is reversible and the M‐chirality can change to P‐chirality by removing the Zn^II ions. Experimental and theoretical models unequivocally revealed that the cooperation of metal coordination and π‐stacking modes are responsible the reversible switching of supramolecular chirality. This work not only provides insight into how multiple noncovalent interactions regulate self‐assembly pathways.     

The Versatile Coordination Modes of Monophosphine‐o‐Carborane in the Formation of Iridium and Rhodium Complexes: Synthesis,Reactivity, and Characterization

Monophosphine‐o‐carborane has four competitive coordination modes when it coordinates to metal centers. To explore the structural transitions driven by these competitive coordination modes, a series of monophosphine‐o‐carborane Ir,Rh complexes were synthesized and characterized. [Cp*M(Cl)₂{1‐(PPh₂)‐1,2‐C₂B₁₀H₁₁}] (M=Ir ( 1 a ), Rh ( 1 b ); Cp*=η⁵‐C₅Me₅), [Cp*Ir(H){7‐(PPh₂)‐7,8‐C₂B₉H₁₁}] ( 2 a ), and [1‐(PPh₂)‐3‐(η⁵‐Cp*)‐3,1,2‐MC₂B₉H₁₀] (M=Ir ( 3 a ), Rh ( 3 b )) can be all prepared directly by the reaction of 1‐(PPh₂)‐1,2‐C₂B₁₀H₁₁ with dimeric complexes [(Cp*MCl₂)₂] (M=Ir, Rh) under different conditions. Compound 3 b was treated with AgOTf (OTf=CF₃SO₃^?) to afford the tetranuclear metallacarborane [Ag₂(thf)₂(OTf)₂{1‐(PPh₂)‐3‐(η⁵‐Cp*)‐3,1,2‐RhC₂B₉H₁₀}₂] ( 4 b ). The arylphosphine group in 3 a and 3 b was functionalized by elemental sulfur (1 equiv) in the presence of Et₃N to afford [1‐{(S)PPh₂}‐3‐(η⁵‐Cp*)‐3,1,2‐MC₂B₉H₁₀] (M=Ir ( 5 a ), Rh ( 5 b )). Additionally, the 1‐(PPh₂)‐1,2‐C₂B₁₀H₁₁ ligand was functionalized by elemental sulfur (2 equiv) and then treated with [(Cp*IrCl₂)₂], thus resulting in two 16‐electron complexes [Cp*Ir(7‐{(S)PPh₂}‐8‐S‐7,8‐C₂B₉H₉)] ( 6 a ) and [Cp*Ir(7‐{(S)PPh₂}‐8‐S‐9‐OCH₃‐7,8‐C₂B₉H₉)] ( 7 a ). Compound 6 a further reacted with nBuPPh₂, thereby leading to 18‐electron complex [Cp*Ir(nBuPPh₂)(7‐{(S)PPh₂}‐8‐S‐7,8‐C₂B₉H₁₀)] ( 8 a ). The influences of other factors on structural transitions or the formation of targeted compounds, including reaction temperature and solvent, were also explored.     

Functionalization of a cyclo‐P5 Ligand by Main‐Group Element Nucleophiles

Unprecedented functionalized products with an η⁴‐P₅ ring are obtained by the reaction of [Cp*Fe(η⁵‐P₅)] ( 1 ; Cp*=η⁵‐C₅Me₅) with different nucleophiles. With LiCH₂SiMe₃ and LiNMe₂, the monoanionic products [Cp*Fe(η⁴‐P₅CH₂SiMe₃)]^? and [Cp*Fe(η⁴‐P₅NMe₂)]^?, respectively, are formed. The reaction of 1 with NaNH₂ leads to the formation of the trianionic compound [{Cp*Fe(η⁴‐P₅)}₂N]^3?, whereas the reaction with LiPH₂ yields [Cp*Fe(η⁴‐P₅PH₂)]^? as the main product, with {[Cp*Fe(η⁴‐P₅)]₂PH}^2? as a byproduct. The calculated energy profile of the reactions provides a rationale for the formation of the different products.     

Metalloradicals Supported by a meta‐Carborane Ligand

In this work, a pincer‐type complex [Cp*Ir‐(SNPh)(SNHPh)(C₂B₁₀H₉)] ( 2 ) was synthesized and its reactivity studied in detail. Interestingly, molecular hydrogen can induce the transformation between the metalloradical [Cp*Ir‐(SNPh)₂(C₂B₁₀H₉)] ( 5 ^.) and 2 . A mixed‐valence complex, [(Cp*Ir)₂‐(SNPh)₂(C₂B₁₀H₈)] ( 7 ^.+), was also synthesized by one‐electron oxidation. Studies show that 7 ^.+ is fully delocalized, possessing a four‐centered‐one‐electron (S‐Ir‐Ir‐S) bonding interaction. DFT calculations were also in good agreement with the experimental results.     

η2(π)‐Coordination of a Bis‐Phosphonio‐Benzophospholide Cation: A Novel Complexation Mode for Unsaturated Phosphorus Heterocycles

The bis‐phosphonio‐benzo[c]phospholide tetraphenylborate 4 [BPh₄] reacts with CpCo(C₂H₄)₂ to form a chelate complex [Co(η⁵–Cp)(κ²P,η²(P=C) –4 )][BPh₄] ( 6 [BPh₄]) which was characterized by means of spectroscopic techniques and a single crystal X‐ray diffraction study. The observed η²(π)‐coordination of the benzophospholide moiety in the cation 6 is highly unusual for aromatic phosphorus heterocycles. The structural data suggest a pronounced coordination‐induced localization of π‐electrons in the condensed ring system.