Intramolecular CH/OH Bond Cleavage with Water and Alcohol Using a Phosphine‐Free Ruthenium Carbene NCN Pincer Complex

Transition metal complexes that exhibit metal–ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine‐free ruthenium NCN pincer complexes with a central N‐heterocyclic carbene donor and methylpyridyl N‐donors. Reaction with base generates a neutral, dearomatized alkoxo–amido complex, which has been structurally and spectroscopically characterized. The tert‐butoxide ligand facilitates regioselective, intramolecular proton transfer through a C?H/O?H bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo–amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine‐free metal–ligand cooperative water splitting.     

Ruthenium(II) Complexes Containing Lutidine‐Derived Pincer CNC Ligands: Synthesis,Structure, and Catalytic Hydrogenation of CN bonds

A series of Ru complexes containing lutidine‐derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver‐carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N‐heterocyclic carbene fragments. In the presence of tBuOK, the Ru‐CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand‐assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru‐CNC complex 5 e (BF₄) is able to add aldimines to the metal–ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer‐sphere stepwise hydrogen transfer to the C?N bond assisted either by the pincer ligand or a second coordinated H₂ molecule.     

A tridentate CNO‐donor palladium(II) complex as efficient catalyst for direct C―H arylation: Application in preparation of imidazole‐based push–pull chromophores

A series of imidazolium chlorides for the formation of tridentate CNO‐donor palladium(II) complexes featuring N‐heterocyclic carbene moieties have been developed from cheap and readily available starting materials with high yields. Their palladium complexes were prepared by reactions between the ligand precursors and PdCl₂ using K₂CO₃ as base in pyridine with reasonable yields. These air‐stable metal complexes were characterized using ¹H NMR and ¹³C{¹H} NMR spectroscopy and elemental analyses. Heteronuclear multiple bond correlation experiments were performed to identify key NMR signals of these compounds. The structures of two of the complexes were also established by single‐crystal X‐ray diffraction analysis. One of these complexes was successfully applied in the direct C―H functionalization reactions between heterocyclic compounds and aryl bromides, producing excellent yields of coupled products. The coupling reactions were scalable, allowing grams of coupled products to be obtained with a mere 2 mol% of Pd loading. The catalyst system developed allowed the large‐scale preparation of several push–pull chromophores straightforwardly. Photophysical properties based on UV–visible and fluorescence spectroscopy for these chromophores were investigated. Deep blue photoluminescence with moderate quantum efficiency and twisted intramolecular charge transfer excited state were observed for these chromophores. Density functional theory (DFT) and time‐dependent DFT calculations were performed to support the experimental results.     

Charged Behaviour from Neutral Ligands: Synthesis and Properties of N‐Heterocyclic Pseudo‐amides

Deprotonation of the 1‐isopropyl‐3‐(phenylamino)pyridin‐1‐ium iodide gives the corresponding neutral betaine, which is formalised as a pyridinium‐amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo‐amide and is a strong donor akin to alkyl phosphines and N‐heterocyclic carbenes. Furthermore, rotation about both N substituent C? N bonds occurs, which is in contrast to the two alternative pyridinium positional isomers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norharman were prepared, which contain an additional C? C bond supporting conjugation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norharman‐derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to dioxygen and water.     

Exploring Coordination Modes: Late Transition Metal Complexes with a Methylene‐bridged Macrocyclic Tetra‐NHC Ligand

A tetranuclear silver(I) N‐heterocyclic carbene (NHC) complex bearing a macrocyclic, exclusively methylene‐bridged, tetracarbene ligand was synthesized and employed as transmetalation agent for the synthesis of nickel(II), palladium(II), platinum(II), and gold(I) derivatives. The transition metal complexes exhibit different coordination geometries, the coinage metals being bound in a linear fashion forming molecular box‐type complexes, whereas the group 10 metals adapt an almost ideal square planar coordination geometry within the ligand's cavity, resulting in saddle‐shaped complexes. Both the Ag^I and the Au^I complexes show ligand‐induced metal–metal contacts, causing photoluminescence in the blue region for the gold complex. Distinct metal‐dependent differences of the coordination behavior between the group 10 transition metals were elucidated by low‐temperature NMR spectroscopy and DFT calculations.     

A Metal‐Containing N‐Heterocyclic Germylene Based on an Oxalamidine Framework

A metal‐containing N‐heterocyclic germylene based on a N‐mesityl (Mes)‐substituted oxalamidine framework is reported. The precursor (MesN=)₂C–C(–N(H)Mes)₂ ( 1 H₂) was converted into its rhodium complex [Rh(κ²N‐ 1 H₂)(cod)][OTf] ( 2 ) (cod = 1,5‐cyclooctadiene; OTf = triflate) in 62 % isolated yield. Subsequent reaction of 2 with Ge{N(SiMe₃)₂}₂ gave the crystalline N‐heterocyclic germylene [Rh(cod)(μ‐ 1 )Ge][OTf] ( 3 ) in 50 % yield. The compounds under study were fully characterized by various methods, also including X‐ray crystallographic studies on single crystals of 2 and 3 . Density functional theory (DFT) calculations revealed that π conjugation in the bridging oxalamidine framework is increased and n(N)–p(Ge) π bonding is decreased upon κ²N metal coordination; a further weakening of the Ge–N bond occurs through triflate coordination to the Ge^II atom. Nevertheless, preliminary coordination studies revealed that 3 behaves as 2‐electron (L ‐type) germylene donor ligand. Treatment of 3 with [Ir(cod)Cl]₂ furnished the heterobimetallic complex [Rh(cod)(μ‐ 1 )Ge‐Ir(cod)Cl][OTf] ( 4 ), as evidenced by NMR spectroscopic investigations and DFT calculations.     

Hypervalent Silicon Compounds by Coordination of Diphosphine–Silanes to Gold

Coordination of ambiphilic diphosphine–silane ligands [o‐(iPr₂P)C₆H₄]₂Si(R)F (R=F, Ph, Me) to AuCl affords pentacoordinate neutral silicon compounds in which the metal atom acts as a Lewis base. X‐ray diffraction analyses, NMR spectroscopy, and DFT calculations substantiate the presence of Au→Si interactions in these complexes, which result in trigonal‐bipyramidal geometries around silicon. The presence of a single electron‐withdrawing fluorine atom is sufficient to observe coordination of the silane as a σ‐acceptor ligand, provided it is positioned trans to gold. The nature of the second substituent at silicon (R=F, Ph, Me) has very little influence on the magnitude of the Au→Si interaction, in marked contrast to N→Si adducts. According to variable‐temperature and 2D EXSY NMR experiments, the apical/equatorial positions around silicon exchange in the slow regime of the NMR timescale. The two forms, with the fluorine atom in trans or cis position to gold, were characterized spectroscopically and the activation barrier for their interconversion was estimated. The bonding and relative stability of the two isomeric structures were assessed by DFT calculations.     

Mechanistic insights into the cis-trans isomerization of ruthenium complexes relevant to catalysis of olefin metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single‐step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by ¹H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square‐pyramidal trans isomer, with the Ru?CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high‐energy structure collapses into the cis isomer. Then, the influence of the N‐heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square‐pyramidal metallacycle with the N‐heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis–trans isomerization of these Ru complexes.     

Enhanced π‐back‐donation resulting in the trans labilization of a pyridine ligand in an N‐heterocyclic carbene (NHC) PdII precatalyst: a case study

The molecular structure of the benzimidazol‐2‐ylidene–PdCl₂–pyridine‐type PEPPSI (pyridine‐enhanced precatalyst, preparation, stabilization and initiation) complex {1,3‐bis[2‐(diisopropylamino)ethyl]benzimidazol‐2‐ylidene‐κC²}dichlorido(pyridine‐κN)palladium(II), [PdCl₂(C₅H₅N)(C₂₃H₄₀N₄)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N‐heterocyclic carbene (NHC) complexes. The presence of weak C—H…Cl‐type hydrogen‐bond and π–π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol‐2‐ylidene (Bimy), two chlorides, pyridine (Py) and the Pd^II ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back‐donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ‐donor with a considerable amount of π‐back‐donation from Pd to C_carbene. The electron‐poor character of Pd^II is supported by π‐back‐donation from the Pd centre and the weakness of the Pd—N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ‐donating ability than imidazol‐2‐ylidene ligands in PEPPSI complexes.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Electrochemical Synthesis and Characterization of Zinc(II) Complexes with [(4‐Methylphenyl)Sulfonyl]‐1H‐Amido‐2‐Phenyl‐2‐Oxazolines

A series of [(4‐methylphenyl)sulfonyl]‐1H‐amido‐2‐phenyl‐2‐oxazoline ligands, HTs‐ROz, has been synthesized by the reaction of substituted 2‐(2‐aminophenyl)oxazolines and p‐toluensulfonyl chloride. The electrochemical oxidation of a sacrificial zinc anode in an acetonitrile solution of the corresponding ligand gave compounds of general formula [Zn(Ts‐ROz)₂]. All complexes have been characterized by microanalysis, IR and ¹H NMR spectroscopy and single‐crystal X‐ray diffraction. In all cases, the metal atom is coordinated by the nitrogen atoms of two monoanionic ligands.     

[N‐(2‐Hydroxy­ethyl)ethyl­enediamine‐κ3N,N′,O]‐cis‐bis(isothio­cyanato‐κN)copper(II)

In the novel transition metal isothio­cyanate complex of N‐(2‐hydroxy­ethyl)ethyl­enediamine (hydet‐en) with copper, [Cu(NCS)₂(C₄H₁₂N₂O)], the Cu atom lies in a distorted square‐pyramidal environment, coordinated by four N atoms in the basal plane and an apical O atom. The hydet‐en ligand is N,N,O‐tridentate, in contrast to the disposition in previously studied complexes, while the isothio­cyanate ions act as N‐atom donor ligands. The monomeric units are linked to one another by hydrogen bonds.     

1‐Ferrocenylmethyl‐3‐(2,4,6‐trimethylbenzyl)‐1H‐imidazolidin‐3‐ium iodide and trans‐bis(3‐benzyl‐1‐ferrocenylmethyl‐1H‐imidazolidin‐2‐ylidene)diiodidopalladium(II)

Owing to increasing interest in the use of N‐heterocyclic carbenes (NHCs) based on imidazolidinium ions as ligands in the design of highly efficient transition‐metal‐based homogeneous catalysts, the characterizations of the 1‐ferrocenylmethyl‐3‐(2,4,6‐trimethylbenzyl)imidazolidin‐3‐ium iodide salt, [Fe(C₅H₅)(C₁₉H₂₄N₂)]I, (I), and the palladium complex trans‐bis(3‐benzyl‐1‐ferrocenylmethyl‐1H‐imidazolidin‐2‐ylidene)diiodidopalladium(II), [Fe₂Pd(C₅H₅)₂(C₁₆H₁₇N₂)₂I₂], (II), are reported. Compound (I) has two iodide anions and two imidazolidinium cations within the asymmetric unit (Z′ = 2). The two cations have distinctly different conformations, with the ferrocene groups orientated exo and endo with respect to the N‐heterocyclic carbene. Weak C—H donor hydrogen bonds to both the iodide anions and the π system of the mesitylene group combine to form two‐dimensional layers perpendicular to the crystallographic c direction. Only one of the formally charged imidazolidinium rings forms a near‐linear hydrogen bond with an iodide anion. Complex (II) shows square‐planar coordination around the Pd^II metal, which is located on an inversion centre (Z′ = 0.5). The ferrocene and benzyl substituents are in a trans–anti arrangement. The Pd—C bond distance between the N‐heterocyclic carbene ligands and the metal atom is 2.036 (7) Å. A survey of related structures shows that the lengthening of the N—C bonds and the closure of the N—C—N angle seen here on metal complexation is typical of similar NHCs and their complexes.     

Coordination Chemistry of N‐Heterocyclic Nitrenium‐Based Ligands

Comprehensive studies on the coordination properties of tridentate nitrenium‐based ligands are presented. N‐heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh^I, Rh^III, Mo⁰, Ru⁰, Ru^II, Pd^II, Pt^II, Pt^IV, and Ag^I complexes based on these unusual ligands. Formation of nitrenium–metal bonds is unambiguously confirmed both in solution by selective ¹⁵N‐labeling experiments and in the solid state by X‐ray crystallography. The generality of N‐heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second‐row transition and post‐transition metals (Y–Cd) in terms of the corresponding bond‐dissociation energies.     

Lanthanide(III) Complexes of 4,10‐Bis(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecane‐1,7‐diacetic acid (trans‐H6do2a2p) in Solution and in the Solid State: Structural Studies Along the Series

Complexes of 4,10‐bis(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecane‐1,7‐diacetic acid (trans‐H₆do2a2p, H₆ L ) with transition metal and lanthanide(III) ions were investigated. The stability constant values of the divalent and trivalent metal‐ion complexes are between the corresponding values of H₄dota and H₈dotp complexes, as a consequence of the ligand basicity. The solid‐state structures of the ligand and of nine lanthanide(III) complexes were determined by X‐ray diffraction. All the complexes are present as twisted‐square‐antiprismatic isomers and their structures can be divided into two series. The first one involves nona‐coordinated complexes of the large lanthanide(III) ions (Ce, Nd, Sm) with a coordinated water molecule. In the series of Sm, Eu, Tb, Dy, Er, Yb, the complexes are octa‐coordinated only by the ligand donor atoms and their coordination cages are more irregular. The formation kinetics and the acid‐assisted dissociation of several Ln^III–H₆ L complexes were investigated at different temperatures and compared with analogous data for complexes of other dota‐like ligands. The [Ce( L )(H₂O)]^3? complex is the most kinetically inert among complexes of the investigated lanthanide(III) ions (Ce, Eu, Gd, Yb). Among mixed phosphonate–acetate dota analogues, kinetic inertness of the cerium(III) complexes is increased with a higher number of phosphonate arms in the ligand, whereas the opposite is true for europium(III) complexes. According to the ¹H NMR spectroscopic pseudo‐contact shifts for the Ce–Eu and Tb–Yb series, the solution structures of the complexes reflect the structures of the [Ce(H L )(H₂O)]^2? and [Yb(H L )]^2? anions, respectively, found in the solid state. However, these solution NMR spectroscopic studies showed that there is no unambiguous relation between ³¹P/¹H lanthanide‐induced shift (LIS) values and coordination of water in the complexes; the values rather express a relative position of the central ions between the N₄ and O₄ planes.     

Anticancer Activity of Hydrogen‐Bond‐Stabilized Half‐Sandwich RuII Complexes with Heterocycles

Neutral half‐sandwich organometallic ruthenium(II) complexes of the type [(η⁶‐cymene)RuCl₂(L)] ( H1 – H10 ), where L represents a heterocyclic ligand, have been synthesized and characterized spectroscopically. The structures of five complexes were also established by single‐crystal X‐ray diffraction confirming a piano‐stool geometry with η⁶ coordination of the arene ligand. Hydrogen bonding between the N? H group of the heterocycle and a chlorine atom attached to Ru stabilizes the metal–ligand interaction. Complexes coordinated to a mercaptobenzothiazole framework ( H1 ) or mercaptobenzoxazole ( H6 ) showed high cytotoxicity against several cancer cells but not against normal cells. In vitro studies have shown that the inhibition of cancer cell growth involves primarily G1‐phase arrest as well as the generation of reactive oxygen species (ROS). The complexes are found to bind DNA in a non‐intercalative fashion and cause unwinding of plasmid DNA in a cell‐free medium. Surprisingly, the cytotoxic complexes H1 and H6 differ in their interaction with DNA, as observed by biophysical studies, they either cause a biphasic melting of the DNA or the inhibition of topoisomerase IIα activity, respectively. Substitution of the aromatic ring of the heterocycle or adding a second hydrogen‐bond donor on the heterocycle reduces the cytotoxicity.     

Metal Derivatives of Heterocyclic‐2‐thiones: Crystal Structures of [Bis(diphenylphosphanyl)propane][bis(pyrimidine‐2‐thiolato)]ruthenium(II) and [Bis(diphenylphosphanyl)methane][bis(pyridine‐2‐thiolato)]ruthenium(II)

Ruthenium(II) Complexes containing pyrimidine‐2‐thiolate (pymS^–) and bis(diphenylphosphanyl)alkanes [Ph₂P–(CH₂)_m–PPh₂, m = 1, dppm; m = 2, dppe; m = 3, dppp; m = 4, dppb] are described. Reactions of [RuCl₂L₂] (L = dppm, dppp) and [Ru₂Cl₄L₃] (L = dppb) with pyrimidine‐2‐thione (pymSH) in 1:2 molar ratio in dry benzene in the presence of Et₃N base yielded the [Ru(pymS)₂L] complexes (pymS^– = pyrimidine‐2‐thiolate; L = dppm ( 1 ); dppp ( 3 ); dppb ( 4 )). The complex [Ru(pymS)₂(dppe)] ( 2 ) was indirectly prepared by the reaction of [Ru(pymS)₂(PPh₃)₂] with dppe. These complexes were characterized using analytical data, IR, ¹H, ¹³C, ³¹P NMR spectroscopy, and X‐ray crystallography (complex 3 ). The crystal structure of the analogous complex [Ru(pyS)₂(dppm)] ( 5 ) with the ligand pyridine‐2‐thiolate (pyS^–) was also described. X‐ray crystallographic investigation of complex 3 has shown two four‐membered chelate rings (N, S donors) and one six‐membered ring (P, P donors) around the metal atom. Compound 5 provides the first example in which Ru^II has three four‐membered chelate rings: two made up by N, S donor ligands and one made up by P, P donor ligand. The arrangement around the metal atoms in each complex is distorted octahedral with cis:cis:trans:P, P:N, N:S, S dispositions of the donor atoms. The ³¹P NMR spectroscopic data revealed that the complexes are static in solution, except 2 , which showed the presence of more than one species.     

New Donor‐Functionalized Cp Ligands: Synthesis and Complexation Behaviour of Quinoxalyl and Benzothiadiazolyl Systems

Sodium cyclopentadienide reacts as nucleophile with 4,7‐dibromo‐2,1,3‐benzothiadiazole (BTZ) and leads to the new donor‐functionalized ligand Cp^BTZ. Related quinoxalyl Cp systems have been prepared using Pd‐catalyzed coupling with zincated Cp‐metal complexes. The new ligands comprise two N‐donor atoms; one of them is located in a distal position relative to the metal centre so that it cannotcoordinate in a chelating manner. With Cp^BTZ ligand derivatives severalmetal complexes have been synthesized. The new chromium(III) complex Cp^BTZCrCl₂ ( 12 ) becomes upon activation an active catalyst for the polymerization of ethylene. Relying on DFT calculations and analysis of spin‐density distribution combined with paramagnetic NMR data a chelating coordination of the Cp^BTZ ligand is feasible in 12 .     

Nitridorhenium(V) Complexes with 1,3,4‐Triphenyl‐1,2,4‐triazol‐5‐ylidene

[ReNCl₂(PPh₃)₂] and [ReNCl₂(PMe₂Ph)₃] react with the N‐heterocyclic carbene (NHC) 1,3,4‐triphenyl‐1,2,4‐triazol‐5‐ylidene (HL^Ph) under formation of the stable rhenium(V) nitrido complex [ReNCl(HL^Ph)(L^Ph)], which contains one of the two NHC ligands with an additional orthometallation. The rhenium atom in the product is five‐coordinate with a distorted square‐pyramidal coordination sphere. The position trans to the nitrido ligand is blocked by one phenyl ring of the monodentate HL^Ph ligand. The Re–C(carbene) bond lengths of 2.072(6) and 2.074(6) Å are comparably long and indicate mainly σ‐bonding between the NHC ligand and the electron deficient d² metal atom. The chloro ligand in [ReNCl(HL^Ph)(L^Ph)] is labile and can be replaced by ligands such as pseudohalides or monoanionic thiolates such as diphenyldithiophosphinate (Ph₂PS₂^?) or pyridine‐2‐thiolate (pyS^?). X‐ray structure analyses of [ReN(CN)(HL^Ph)(L^Ph)] and [ReN(pyS)(HL^Ph)(L^Ph)] show that the bonding situation of the NHC ligands (Re–C(carbene) distances between 2.086(3) and 2.130(3) Å) in the product is not significantly influenced by the ligand exchange. The potentially bidentate pyS^? ligand is solely coordinated via its thiolato functionality. Hydrogen atoms of each one of the phenyl rings come close to the unoccupied sixth coordination positions of the rhenium atoms in the solid state structures of all complexes. Re–H distances between 2.620 and 2.712Å do not allow to discuss bonding, but with respect to the strong trans labilising influence of “N^3?”, weak interactions are indicated.     

Strongly Absorbing π–π* States in Heteroleptic Dipyrrin/2,2′‐Bipyridine Ruthenium Complexes: Excited‐State Dynamics from Resonance Raman Spectroscopy

A recently reported new class of ruthenium complexes containing 2,2′‐bipyridine and a dipyrrin ligand in the coordination sphere exhibit both strong metal‐to‐ligand charge‐transfer (MLCT) and π–π* transitions. Quantitative analysis of the resonance Raman scattering intensities and absorption spectra reveals only weak electronic interactions between these states despite direct coordination of the bipyridyl and dipyrrin ligands to the central ruthenium atom. On the basis of DFT calculations and time‐dependent DFT (TD‐DFT), we propose that the electronic excited states closely resemble “pure” MLCT and π–π* states. Resonance Raman intensity analysis demonstrates that a large amplitude transannular torsional motion provides a mechanism for relaxation on the π–π* excited‐state surface. We assert that this result is generally applicable to a range of dipyrrin complexes such as boron–dipyrrin and metallodipyrrin systems. Despite the large torsional distortion between the phenyl ring and the dipyrromethene plane, π–π* excitation extends out onto the phenyl ring which may have important consequences in solar‐energy‐conversion applications of ruthenium–dipyrrin complexes.