Cyclooctadiene Research Articles

A stereodynamic phosphoramidite ligand derived from 3,3'-functionalized ortho-biphenol and its rhodium(I) complex.

Stereodynamic ligands and complexes bearing functional groups to attach chiral or achiral binding sites and auxiliaries are highly attractive due to the interesting opportunities for controlling the stereochemical outcome of enantioselective transformations. In this study we report the preparation of a 3,3'-functionalized biphenol (BIPOL) phosphoramidite ligand (PAm ) bearing 3,5-dichlorobenzoyl (3,5-DCB) amide binding sites for noncovalent interactions. Upon coordination to [Rh(COD)2 ]BF4 this substitution pattern directs one of the 3,5-DCB binding sites in close proximity of the metal center resulting in liberation of both COD ligands and the formation of a [Rh(PAm )2 ]BF4 complex. Coordination of the amide carbonyl unit was found to be reversible, since the complex acted as an active catalyst in the hydrogenation of dehydroamino acid derivatives. X-ray crystallographic investigation revealed that the second 3,5-DCB unit is capable of forming noncovalent π-π interactions connecting both phosphoramidite ligands.

2-(Methylamido)pyridine-Borane: A Tripod κ(3)-N,H,H Ligand in Trigonal Bipyramidal Rhodium(I) and Iridium(I) Complexes with an Asymmetric Coordination of Its BH3 Group.

The complexes [M(κ(3)-N,H,H-mapyBH3)(cod)] (M = Rh, Ir; HmapyBH3 = 2-(methylamino)pyridine-borane; cod = 1,5-cyclooctadiene), which contain a novel anionic tripod ligand coordinated to the metal atom through the amido N atom and through two H atoms of the BH3 group, were prepared by treating the corresponding [M2(μ-Cl)2(cod)2] (M = Rh, Ir) precursor with K[mapyBH3]. X-ray diffraction studies and a theoretical Quantum Theory of Atoms in Molecules analysis of their electron density confirmed that the metal atoms of both complexes are in a very distorted trigonal bipyramidal coordination environment, in which two equatorial sites are asymmetrically spanned by the H-B-H fragment. While both 3c-2e BH-M interactions are more κ(1)-H (terminal σ coordination of the B-H bond) than κ(2)-H,B (agostic-type coordination of the B-H bond), one BH-M interaction is more agostic than the other, and this difference is more marked in the iridium complex than in the rhodium one. This asymmetry is not evident in solution, where the cod ligand and the BH3 group of these molecules participate in two concurrent dynamic processes of low activation energies (variable-temperature NMR and density functional theory studies), namely, a rotation of the cod ligand that interchanges its two alkene fragments (through a square pyramidal transition state) and a rotation of the BH3 group about the B-N bond that equilibrates the three B-H bonds (through a square planar transition state). While the cod rotation has similar activation energy in 2 and 3, the barrier to the BH3 group rotation is higher in the iridium complex than in the rhodium one.

Synthesis and characterisation of organo-platinum(II) complexes of the N,O-donor ligands hippuric acid (N-benzoylglycine) and N-phenylanthranilic acid

Reaction of [PtCl2(cod)] (cod=cyclo-octa-1,5-diene) with either hippuric acid [PhC(O)NHCH2COOH] or N-phenylanthranilic acid (ortho-PhNHC6H4COOH) in refluxing dichloromethane in the presence of silver(I) oxide gave the new organoplatinum derivatives [Pt{N(COPh)CH2COO}(cod)] and [Pt{N(Ph)C6H4COO}(cod)] respectively. Ligand substitution reactions of the cod ligand in [Pt{N(COPh)CH2COO}(cod)] provided a facile route to a selection of phosphine-substituted analogues [Pt{N(COPh)CH2COO}L2] [L=phosphatriazaadamantane (pta), PPh3, or L2=Ph2PCH2CH2PPh2 or Fe(η5-C5H4PPh2)2] via displacement of the labile cod ligand. The complexes were characterised using NMR spectroscopy, IR spectroscopy, and ESI mass spectrometry. The X-ray structure of [Pt{N(COPh)CH2COO}(cod)] is also reported.

Aqueous NMR Signal Enhancement by Reversible Exchange in a Single Step Using Water-Soluble Catalysts.

Two synthetic strategies are investigated for the preparation of water-soluble iridium-based catalysts for NMR signal amplification by reversible exchange (SABRE). In one approach, PEGylation of a variant N-heterocyclic carbene provided a novel catalyst with excellent water solubility. However, while SABRE-active in ethanol solutions, the catalyst lost activity in >50% water. In a second approach, synthesis of a novel di-iridium complex precursor where the cyclooctadiene (COD) rings have been replaced by CODDA (1,2-dihydroxy-3,7-cyclooctadiene) leads to the creation of a catalyst [IrCl(CODDA)IMes] that can be dissolved and activated in water—enabling aqueous SABRE in a single step, without need for either an organic cosolvent or solvent removal followed by aqueous reconstitution. The potential utility of the CODDA catalyst for aqueous SABRE is demonstrated with the ∼(−)32-fold enhancement of 1H signals of pyridine in water with only 1 atm of parahydrogen.

Versatile deprotonated NHC: C,N-bridged dinuclear iridium and rhodium complexes.

Bearing the versatility of N-heterocyclic carbene (NHC) ligands, here density functional theory (DFT) calculations unravel the capacity of coordination of a deprotonated NHC ligand (pNHC) to generate a doubly C2,N3-bridged dinuclear complex. Here, in particular the discussion is based on the combination of the deprotonated 1-arylimidazol (aryl = mesityl (Mes)) with [M(cod)(μ-Cl)] (M = Ir, Rh) generated two geometrical isomers of complex [M(cod){µ-C3H2N2(Mes)-κC2,κN3}]2). The latter two isomers display conformations head-to-head (H-H) and head-to-tail (H-T) of CS and C2 symmetry, respectively. The isomerization from the H-H to the H-T conformation is feasible, whereas next substitutions of the cod ligand by CO first, and PMe3 later confirm the H-T coordination as the thermodynamically preferred. It is envisaged the exchange of the metal, from iridium to rhodium, confirming here the innocence of the nature of the metal for such arrangements of the bridging ligands.

Ru(0)触媒による共役ジエンとスチレンのクロスカップリング反応における置換基効果の理論的解明

Mechanistic studies for linear cross-dimerization between 2,3-dimethylbuta-1,3-diene and para-substituted styrenes by a Ru(0) complex, Ru(η6-naphthalene) (η4–1,5-COD), were performed computationally. The COD ligand is actually not a simple spectator ligand, it engages to assist the hydrogen migration steps. The computational studies verified the oxidative coupling step to govern the overall reaction as a rate-determining step. The reaction rate was found to be correlated well with the electronegativity of styrenes.

Crystal structure of μ-fluorido-bis-{(η(4)-cyclo-octa-diene)[hexa-fluorido-anti-monato(V)]platinum(II)} hexa-fluorido-anti-monate(V) hydrogen fluoride 0.75-solvate.

In the complex cation of the binuclear solvated title salt, [Pt2F(SbF6)2(C8H12)2]SbF6·0.75HF, an F atom bridges the two platinum(II) atoms with a bond angle of 123.3 (2)°. The corresponding Pt-F bond lengths are in the range of other fluorine-bridged binuclear platinum(II) complexes. Two of the three SbF6 (-) anions each coordinate with one F atom to one platinum(II) atom. Including the η(4)-bound cyclo-octa-diene (COD) ligands, the overall coordination sphere of each platinum(II) atom is square-planar. The third SbF6 (-) anion is not bound to the complex. Hydrogen fluoride is present in the crystal structure as a solvent disordered over three positions, each with an occupancy of 0.25. F⋯F distances of 2.5512 (7), 2.6076 (8) and 3.2215 (10) Å to surrounding SbF6 (-) anions are indicative of F-H⋯F hydrogen-bonding inter-actions although no H atoms could be localized for the disordered solvent mol-ecules. The resulting hydrogen-bonded network is three-dimensional.

Preparation, structure, and dynamic and electrochemical behaviors of dinuclear rhodium(I) complexes with bridging formamidinato ligands

Dinuclear rhodium(I) complexes, [Rh(4-Me-pf) (cod)]2 (1), [Rh(3,5-Me2-pf) (cod)]2 (2), [Rh(4-Me-pf) (nbd)]2 (3), [Rh(3,5-Me2-pf) (nbd)]2 (4), and [Rh(2,6-F2-pf) (nbd)]2 (5), have been synthesized and characterized by X-ray structure analysis, 1H, 13C, and 19F NMR, UV–vis, ESI-TOF-MS, and elemental analysis. In these complexes, two rhodium atoms are bridged by two formamidinato ligands and each rhodium atom is coordinated by one chelating cod or nbd ligand to form an approximately square planar coordination structure with two nitrogen atoms and two double bonds. The Rh··Rh distances are in the range of 3.2668 to 2.9726 Å, suggesting a direct bonding interaction between two rhodium atoms. Variable temperature NMR studies in CD2Cl2 solution have revealed that 1–5 exhibit a novel dynamic behavior, that is, an interconversion between two enantiomers. The activation parameters for racemization have been determined by the line shape analyses of the 1H and 19F NMR spectra taken at various temperatures. Variable temperature NMR studies have also revealed that the rotation rates of the four aryl groups around N–C(aryl) bonds are extremely different in each complex. The cyclic voltammetry study has shown that the oxidation potentials corresponding to Rh23+/Rh2+ are 0.41 V in the cod complexes (1 and 2) while those in the nbd complexes (3–5) have shown negative shift by ca. 0.2 V. The reasons for the difference in dynamic behaviors and redox properties among these complexes have been discussed.

Synthesis and characterisation of a novel mixed donor P,O,P' nixantphos ligand and its metal complex

The complex [(NixC8OH)Ir(cod)Cl] 4 has been synthesized and structurally characterized by NMR, IR and single crystal X-ray diffraction. The synthesis and characterisation of the novel ligand NixC8OH is also presented. The coordination around Ir is trigonal bipyramidal with both P groups of the NixC8OH ligand bound in a bis-equatorial mode. The bis-chelating cod (C8H12) ligand occupies the remaining equatorial position and an axial position. This mode of bonding has resulted in a large bite angle (P1–Ir–P2) of 102.92(12)° for the title complex 4. The IR and NMR data further support the elucidated structure. Thermal analyses of 4 indicate that it is thermally stable up to a decomposition temperature of >400 °C.

Iridium complexes containing N-donor-functionalized η1-fluorenyl ligands

Iridium complexes of two different nitrogen-donor-functionalized fluorenyl ligands have been synthesized and characterized, L1Ir(COD) and L2Ir(COD), where L1−=[9-(2-dimethylamino)ethyl]fluorenyl anion, L2−=[9-(2-ortho-pyridyl)ethyl]fluorenyl anion, and COD=1,5-cyclooctadiene. The structures of both ligands (in their protonated form) and of both complexes were determined by single-crystal X-ray crystallography. Structural consequences of the nature of the nitrogen ligand (dimethylamino substituent versus ortho-pyridyl substituent) are significantly different Ir–N distances (shorter for L2Ir(COD)) and indirect cis-influence on the COD ligand resulting in a shorter Ir–olefin bond cis to the nitrogen donor in L2Ir(COD). Both complexes act as precatalysts for arene hydrogenation.

Use of a Bidentate Ligand Featuring an N-Heterocyclic Phosphenium Cation (NHP(+)) to Systematically Explore the Bonding of NHP(+) Ligands with Nickel.

A novel bidentate ligand featuring an N-heterocyclic phosphenium cation (NHP(+)) linked to a phosphine side arm is used to explore the coordination chemistry of NHP(+) ligands with nickel. Direct P-Cl bond cleavage from a chlorophosphine precursor [PP]-Cl (1) by Ni(COD)2 affords the asymmetric bimetallic complex [Cl2Ni(μ-PP)2Ni] (2) via a nonoxidative process. Abstraction of the halide with either NaBPh4 or K[B(C6F5)4] prior to metal coordination to form the free phosphenium ligand [PP](+) in situ, followed by coordination to Ni(COD)2, afforded the halide-free Ni(0) complexes [(PP)Ni(COD)] [B(C6F5)4] (4) and [(PP)Ni(COD)][BPh4] (5). Chloride abstraction from 1 is problematic in the presence of a PF6(-) counterion, however, as evident by the formation of [(PP)Ni(PP-F)][PF6] (3). The COD ligand in 5 can be readily displaced with PMe3 or PPh3 to afford [(PP)NiL2][BPh4] (L = PMe3 (6), PPh3 (7)). Complexes 2-7 feature planar geometries about the NHP(+) phosphorus atom and unusually short Ni-P distances, indicative of multiple bonding resulting from both P → Ni σ donation and Ni → P π backbonding. This bonding description is supported by theoretical studies using natural bond orbital analysis.

Room-temperature C–H activation of the phosphino-ketone Ph2PCH2C(O)Ph leading to an iridium(III) complex with a hybrid phosphino-enolate ligand

The reaction of [Ir(cod)( μ -Cl)] 2 (cod = 1,5-cyclooctadiene) with 2 equiv of the ketophosphine Ph 2 PCH 2 C(O)Ph in the presence of TlPF 6 afforded the hydrido, phosphino-enolate Ir(III) complex [IrH(cod){Ph 2 PCH ··· C( ··· O)Ph, κP , κO }{Ph 2 PCH 2 C(O)Ph, κP }]PF 6 ( 4 ), which results from the room temperature activation of a C–H bond from the PCH 2 moiety. The distorted octahedral coordination environment around the metal centre in 4 contains the cod ligand, the P atom of the monodentate ketophosphine and the P,O donor atoms of a chelating phosphino-enolate ligand acting as a 3-electron donor. The hydride ligand was located on the difference Fourier map obtained by single-crystal X-ray diffraction studies and is trans to the enolate oxygen and cis to the two, mutually cis P atoms. The reaction of this complex with NaH in THF led to the isolation of the Ir(I) complex [Ir(cod){Ph 2 PCH ··· C( ··· O)Ph, κP , κO }{Ph 2 PCH 2 C(O)Ph, κP }] ( 5 ). The penta-coordination environment around the metal centre in 5 includes the cod ligand, one 3-electron donor P,O chelating phosphino-enolate ligand and a P-bound Ph 2 PCH 2 C(O)Ph ligand containing an uncoordinated ketone function. The structures of 4 ·CH 2 Cl 2 and 5 ·C 7 H 8 have been determined by X-ray diffraction analysis.

Dinuclear iridium and rhodium complexes with bridging arylimidazolide-N(3),C(2) ligands: synthetic, structural, reactivity, electrochemical and spectroscopic studies.

Deprotonation of 1-arylimidazoles (aryl = mesityl (Mes), 2,6-diisopropylphenyl (Dipp)), with n-butyl lithium afforded the corresponding derivatives (1-aryl-1H-imidazol-2-yl)lithium (1a, Ar = Mes; 1b, Ar = Dipp) in good yield. Reaction of 1a with 0.5 equiv. of [Ir(cod)(μ-Cl)]2 yielded two geometrical isomers of a doubly C2,N3-bridged dinuclear complex [Ir(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (3), 3H-H, a head-to-head (H-H) isomer of CS symmetry, and 3H-T, the thermodynamically preferred head-to-tail (H-T) isomer of C2 symmetry. The metallated carbon of the 4 electron donor anionic bridging ligands has some carbene character, reminiscent of the situation in N-metallated protic NHC complexes. Displacement of cod ligands from 3H-H and 3H-T afforded the tetracarbonyl complexes [Ir(CO)2{μ-C3H2N2(Mes)-κC2,κN3}]24H-H and 4H-T, respectively. The reaction with PMe3, which gave only one complex, [Ir(CO)(PMe3){μ-C3H2N2(Mes)-κC2,κN3}]2 (5), demonstrates that the isomerization of the central core Ir[μ-C3H2N2(Mes)-κC2,κN3]2Ir from H-H to H-T on going from 4H-H to 5 is readily triggered by phosphine substitution under mild conditions. Oxidative-addition of MeI to 5 afforded the formally metal-metal bonded d(7)-d(7) complex [Ir2(CO)2(PMe3)2(Me)I{μ-C3H2N2(Mes)-κC2,κN3}2] (6). The blue [Ir(C2H4)2{μ-C3H2N2(Mes)-κC2,κN3}]2 (7) and purple [Rh(C2H4)2{μ-C3H2N2(Dipp)-κC2,κN3}]2 (9) tetraethylene complexes were also obtained with only a H-T arrangement of the bridging ligands. Although only modestly efficient in alkane dehydrogenation, complex 7 was found to be a more active pre-catalyst than 3H-T, 4H-T and 5, probably because of the favorable lability of the ethylene ligands. From cyclic voltammetry, exhaustive coulometry and spectroelectrochemistry studies, it was concluded that 3H-T undergoes a metal-based one electron oxidation to generate the mixed-valent Ir(i)/Ir(ii) system. The energy of the intervalence band for the orange dirhodium complex [Rh(cod){μ-C3H2N2(Mes)-κC2,κN3}]2 (8) is shifted toward lower energies in comparison with 3H-T, reflecting the decrease of the energy with the intermetallic distance. It was concluded from the EPR study that the Ir and Rh centres contribute substantially to the experimental magnetic anisotropy and thus to the singly occupied molecular orbital (SOMO) in the mixed-valent Ir(i)/Ir(ii) and Rh(i)/Rh(ii) systems. The molecular structures of 3H-H, 3H-T, 8 and 9 have been determined by X-ray diffraction.

Three-way cooperativity in d8 metal complexes with ligands displaying chemical and redox non-innocence.

Reversible proton- and electron-transfer steps are crucial for various chemical transformations. The electron-reservoir behavior of redox non-innocent ligands and the proton-reservoir behavior of chemically non-innocent ligands can be cooperatively utilized for substrate bond activation. Although site-decoupled proton- and electron-transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod-H)M(μ-L(2-)) M(cod-H)] (M = Pt(II), [1] or Pd(II), [2], cod = 1,5-cyclooctadiene, H2L = 2,5-di-[2,6-(diisopropyl)anilino]-1,4-benzoquinone), in which cod acts as a proton reservoir, and L(2-) as an electron reservoir. Protonation of [2] leads to an unusual tetranuclear complex. However, [1] can be stepwise reversibly protonated with up to two protons on the cod-H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L(2-) ligand. The doubly protonated form of [1] is also shown to react with OMe(-) leading to an activation of the cod ligands. The site-decoupled proton and electron reservoir sources work in tandem in a three-way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton- and electron-transfer reactions by using metal complexes of non-innocent ligands.

Platinum complexes with the SC6F4H-4 ligand – Synthesis, structures and spectroscopy

The reaction of HSC6F4H-4 and base with suitable platinum(II) chlorido precursor complexes leads to mononuclear complexes containing diimine ligands [(N^N)Pt(SC6F4H-4)2], or [(N^N)Pt(Me)(SC6F4H-4)] (N^N=2,2′-bipyridine (bpy), dipyrido[3,2-a:2′,3′-c]phenazine (dppz) or 11-trifluoromethyl-dipyrido[3,2-a:2′,3′-c]phenazine (CF3dppz)), cycloocta-1,5-diene (COD) complexes [(COD)Pt(SC6F4H-4)2] or [(COD)Pt(R)(SC6F4H-4)] (R=Me, Bn (benzyl) or C6F5), or phosphine complexes cis-[(PPh3)2Pt(SC6F4H-4)2], [(dppe)Pt(SC6F4H-4)2] (dppe=1,2-bis(diphenylphosphino)ethane) and [(dppb)Pt(C6F5)(SC6F4H-4)] (dppb=1,4-bis(diphenylphosphino)butane). Reaction of cis-[(DMSO)2PtCl2] lead to the labile cis- and trans-[(DMSO)2Pt(SC6F4H-4)2] which rapidly lose DMSO to yield polymeric [Pt(SC6F4H-4)2]m. Reaction of HSC6F4H-4 and KOtBu with K2PtCl4 gave K2[Pt(SC6F4H-4)4]. The new compounds were analysed and characterised by single crystal XRD, 1H, 19F and 195Pt NMR and IR spectroscopy. Both single crystal XRD (in the solid) and NMR (in solution) reveal that the ligand strength of the thiolate ligand −SC6F4H-4 strongly depends on the further ligands in the complexes. The crystal structures exhibit various inter- and intra molecular π stacking interactions of the SC6F4H-4 coligands. The absorption spectroscopy and electrochemistry of the diimine complexes [(N^N)Pt(SC6F4H-4)2] (N^N=bpy, dppz and CF3dppz) have been investigated and reveal that the Pt complexes are superior to the recently reported Pd derivatives in view of their application in photoactive materials.

μ-3,3'-Diisopropyl-1,1'-(propane-1,3-di-yl)bis-(1,3-diazinan-2-yl-idene)]bis-[bromido-(η(4)-cyclo-octa-1,5-diene)rhodium(I)

The title compound, [Rh2Br2(C8H12)2(C17H32N4)], was obtained by the reaction of 3,3′-(propane-1,3-di­yl)bis­(1-isopropyl-3,4,5,6-tetra­hydro­pyrimidin-1-ium) bromide and [{Rh(cod)Cl}2] (cod is cyclo­octa-1,5-diene) in tetra­hydro­furan. The two RhI atoms each have a distorted square-planar coordination environment, defined by a bidentate cod ligand, a bromide anion and one C atom of the bridging bidentate bis-N-heterocyclcic carbene (NHC) ligand. The average Rh—CNHC distance is 2.038 (7) Å, suggesting that the bond has a major σ contribution with very little back donation. The distances between the cod ligands and the RhI atoms vary between 2.104 (4) and 2.210 (4) Å.

Ruthenium-Catalyzed Ortho-Selective C–H Alkenylation of Aromatic Compounds with Alkenyl Esters and Ethers

A direct, regioselective alkenylation of aromatic C–H bonds of aryl- and heteroarylpyridines and related compounds with alkenyl esters was developed using Ru(cod)(cot) as a catalyst. Aromatic compounds bearing electronically diverse substituents and various heterocyclic directing groups are reacted with alkenyl acetates bearing mono- and disubstituted alkenyl groups with aliphatic and aromatic substituents to give ortho-alkenylation products in high yields. The results of deuterium-labeling experiments and competition reactions using different ratios of the E and Z isomers of β-styryl acetate suggested that the C–H alkenylation proceeded via a ruthenium–alkene intermediate and the C–O bond of the alkenyl acetate was cleaved by β-acetoxy elimination. Two types of catalytically relevant species were identified, and the reactivities of these species, combined with the results of the kinetic studies, suggest that the rate-limiting step is the exchange of the COD ligand with the alkenyl ester. On the basis of the elucidated mechanism, the first catalytic coupling reaction using aromatic C–H bonds with C–O bonds of ethers was also developed.

New Supramolecular Structural Motif Coupling a Ruthenium(II) Polyazine Light Absorber to a Rhodium(I) Center

Two new complexes, [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 and [(Me2bpy)2Ru(dpp)Rh(I)(COD)](PF6)2(BF4) (bpy = 2,2'-bipyridine, Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine, and COD = 1,5-cyclooctadiene), representing a new Ru(II),Rh(I) structural motif, have been prepared and characterized by mass spectrometry, (1)H NMR spectroscopy, electrochemistry, electronic absorption spectroscopy, and emission spectroscopy. These two complexes represent a new type of supramolecular complex with a [(TL)2Ru(dpp)](2+) (TL = terminal ligand) light absorber (LA) coupled to a Rh(I) center and are models for Ru(II),Rh(I) intermediates in the photochemical reduction of water using dpp-bridged Ru(II),Rh(III) photocatalysts. Electrochemical study reveals overlapping reversible Ru(II/III) and irreversible Rh(I/II/III) oxidations and a quasi-reversible dpp(0/-) reduction, demonstrating that the lowest unoccupied molecular orbital (LUMO) is dpp(π*) based. The COD ligand is sterically bulky, displaying steric repulsions between hydrogen atoms on the alkene of COD and dpp about the square planar Rh(I) center. An interesting reactivity occurs in coordinating solvents such as CH3CN, where Rh(I) substitution leads to an equilibrium between the Ru(II),Rh(I) bimetallic and [(TL)2Ru(dpp)](2+) and [Rh(I)(COD)(solvent)2](+) monometallic species. The electronic absorption spectra of both complexes feature transitions at ca. 500 nm attributed to a Ru(dπ) → dpp(π*) metal-to-ligand charge transfer (MLCT) transition that is slightly red-shifted from the Ru synthon upon Rh(I) complexation. The methylation of TL on the Ru impacts the electrochemical and optical properties in a minor but predictable manner. The photophysical studies, by comparison with the model complex [{Ru(bpy)2}2(dpp)](PF6)4 and related Rh(III) complex [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3, reveal the expected absence of a Ru(dπ) → Rh(dσ*) (3)MMCT state (metal-to-metal charge transfer) in the title complexes, which is present in Rh(III) systems. The absence of this (3)MMCT state in Ru(II),Rh(I) complexes results in a longer lifetime and higher emission quantum yield for the Ru(dπ) → dpp(π*) (3)MLCT state than [(bpy)2Ru(dpp)Rh(III)Cl2(phen)](PF6)3. Both complexes display photocatalytic hydrogen production activity in the presence of water and a sacrificial electron donor, with the [(bpy)2Ru(dpp)Rh(I)(COD)](PF6)3 possessing a higher catalytic activity than the methyl analogue. Both display low activities, hypothesized to occur due to steric crowding about the Rh(I) site.

Modes of Activation of Organometallic Iridium Complexes for Catalytic Water and C–H Oxidation

Sodium periodate (NaIO4) is added to Cp*Ir(III) (Cp* = C5Me5(-)) or (cod)Ir(I) (cod = cyclooctadiene) complexes, which are water and C-H oxidation catalyst precursors, and the resulting aqueous reaction is investigated from milliseconds to seconds using desorption electrospray ionization, electrosonic spray ionization, and cryogenic ion vibrational predissociation spectroscopy. Extensive oxidation of the Cp* ligand is observed, likely beginning with electrophilic C-H hydroxylation of a Cp* methyl group followed by nonselective pathways of further oxidative degradation. Evidence is presented that the supporting chelate ligand in Cp*Ir(chelate) precursors influences the course of oxidation and is neither eliminated from the coordination sphere nor oxidatively transformed. Isomeric products of initial Cp* oxidation are identified and structurally characterized by vibrational spectroscopy in conjunction with density functional theory (DFT) modeling. Less extensive but more rapid oxidation of the cod ligand is also observed in the (cod)Ir(I) complexes. The observations are consistent with the proposed role of Cp* and cod as sacrificial placeholder ligands that are oxidatively removed from the precursor complexes under catalytic conditions.

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N-heterocyclic carbene ligands 1 M. Rhodium(I) complexes 6 M-9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni(II) , Zn(II) , Mn(III) , Al(III) , 2H). The electronic properties of the corresponding N-heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [(1 M)Rh(cod)Cl] and [(1 M)Rh(CO)2 Cl] complexes (cod=1,5-cyclooctadiene). Porphyrin-NHC ligands 1 M with a trivalent metal cation such as Mn(III) and Al(III) are overall poorer electron donors than porphyrin-NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni(II) and Zn(II) . Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring-opening polymerization of L-lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.