Iridium Cyclooctene Complex Forms a Hyperpolarization Transfer Catalyst Before Converting to a Binuclear C-H Bond Activation Product Responsible for Hydrogen Isotope Exchange

[IrCl(COE)2]2 ( 1 ) reacts with pyridine and H2 to form crystallo-graphically characterized IrCl(H)2(COE)(py)2 ( 2 ). 2 undergoes pyridine loss to form 16-electron IrCl(H)2(COE)(py) (3) with equivalent hydride ligands. When this reaction is studied with parahydrogen, 1 efficiently achieves the hyperpolarization of free pyridine (and nicotinamide, nicotine, 5-aminopyrimidine and 3,5-lutudine) via signal amplification by reversible exchange (SABRE) and hence reflects a simple and readily available precatayst for this process. 2 reacts further over 48 hrs at 298 K to form crystallographically characterized (Cl)(H)(py)(μ-Cl)(μ-H)(κ-μ-NC5H4)Ir(H)(py)2 (4). This dimer is shown to be active in the hydrogen isotope exchange process that is used in radiophar-maceutical preparations. Furthermore, while [Ir(H)2(COE)(py)3]PF6 ( 6 ) forms on addition of AgPF6 to 2 , its stability precludes its efficient involvement in SABRE

Effect of Secondary Interactions on the Fundamental Properties of Small Molecule Models of the Diiron Hydrogenase Active Site

The unique active site of [FeFe]-hydrogenase has inspired over 300 small molecule models derived from the classical organometallic complex, (μ-SRS-)[Fe(CO)3]2. However, no model complex has yet reproduced the hydrogen production capabilities of the enzyme. One reason for this is that the model complexes are not subject to the large number of second coordination sphere interactions that are present in the active site. This work represents two approaches to explore the importance of non-covalent interactions on the properties of small molecule models, (μ-SRS)[Fe(CO)3]2 and (μ-SRS[Fe(CO)2L]2, of the enzyme active site. First, a series of diiron models with additional steric bulk built into the dithiolate linker that connects the two irons were synthesized. While the electron donating ability of the dithiolate is not drastically altered, as evidenced by infrared spectroscopy, variable temperature - NMR studies show that the energy barrier for certain intramolecular dynamic processes, important in modeling the enzyme active site, is significantly lowered. Electrochemical studies on the all-CO derivatives showed no significant differences in the redox properties of the sterically bulky complexes compared to complexes without added steric bulk. For substituted complexes in which CO is replaced by strong donor ligands greater electrochemical changes were observed, with some events being more accessible by up to 230 mV. One electron oxidation of the disubstituted complexes has produced a series of rare mixed-valent FeIFeII complexes. An X-ray crystal structure of one of these complexes, (μ-SCH2C(CH3)2CH2S-)[Fe(CO)2PMe3]2PF6 shows both a semi-bridging carbonyl and an open site similar to the 2-Fe subsite in the Hox state of the enzyme active site. Another method for studying secondary interactions on the model complexes used a host-guest approach to provide an artificial protein environment. Reaction of an aryl sulfonate-containing diiron complex with natural β-cyclodextrin results in encapsulation of the model. The X-ray crystal structure of the inclusion complex, Na (μ-SCH2N(C6H4SO3-)CH2S-)[Fe(CO)3]2·2 β-cyclodextrin shows complete enclosure of the diiron model within two cyclodextrin units. Solution studies support the formation of an inclusion complex and show that the cyclodextrin is capable of producing significant redox changes to the model complex in H2O. This work has provided a new highly modifiable method for affecting change in the properties of model complexes through intermolecular interactions

Ligand effects on bioinspired iron complexes

The synthesis of diiron thiolate complexes was carried out using two ligands that were expected to furnish improved catalytic activity, solubility in water, and stability to the metal complexes. The water-soluble phosphine 1,3,5-triaza-7- phosphaadamantane, PTA, coordinates to the Fe centers forming the disubstituted complex (m-pdt)[Fe(CO)2PTA]2, which presents one PTA in each iron in a transoid arrangement. Substitution of one CO ligand in the (m-pdt)[Fe(CO)3]2 parent complex forms the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2PTA]. Enhanced water solubility was achieved through reactions with electrophiles, H+ and CH3 +, which reacted with the N on the PTA ligand forming the protonated and methylated derivatives, respectively. The 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IMes, was reacted with (m-pdt)[Fe(CO)3]2 yielding the asymmetric (m-pdt)[Fe(CO)3][Fe(CO)2IMes], an electron rich, air stable complex that does not show reactivity with H+. Electrocatalytic production of hydrogen was studied for the all-CO, bis-PMe3, mono- and di-PTA FeIFeI complexes, as well as the PTA-protonated and -methylated derivatives. The all-CO species produce H2, in the presence of the weak HOAc, at their second reduction event, FeIFe0 ?? Fe0Fe0, that occurs at ca. ??1.9 V, through an EECC mechanism. The mono- and di-substituted phosphine complexes present electrocatalytic production of H2 from the Fe0FeI redox state; this reduction takes place at ??1.54 V for (m-pdt)[Fe(CO)3][Fe(CO)2PTA], and at ca. ??1.8 for the disubstituted PMe3 and PTA derivatives. A positive charge on the starting complex does not have an effect on the production of H2. It was found that the protonated and methylated derivatives are not the catalytic species for H2 production. At their first reduction event the neutral precursor forms, and catalysis occurs from the FeIFeI complex in all cases. The possibility of enhanced catalytic activity in the presence of H2 O was explored by conducting electrochemical experiments in the mixed CH3CN:H2O solvent system for the PTA-substituted complexes. The reduction potential of the catalytic peak is shifted to more positive values by the presence of H2 O. The cyclic voltammogram of {(m-pdt)[Fe(CO)2(PTA?? H)]2}2+ in CH3CN:H2O 3:1 shows the reduction of a more easily reduced species in the return scan. This curve-crossing event provides evidence for the (h2-H2)FeII intermediate proposed in the ECCE mechanism

Reactivity of [Cp*Fe(η⁵ ‐As₅)] towards Carbenes, Silylenes and Germylenes

The reaction behavior of [Cp*Fe(η5-As5)] (I) (Cp*=C5Me5) towards carbenes and their heavier analogs was investigated. The reaction of I with NHCs (NHCs=N-heterocyclic carbenes) results in the first substitution products of polyarsenic ligand complexes by NHCs [Cp*Fe(η4-As5NHC)] (1 a: NHC=IMe=1,3,4,5-tetramethylimidazolin-2-ylidene, 1 b: NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene). In contrast, the reaction of I with EtCAAC (EtCAAC=2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) leads to a fragmentation and the formation of an unprecedented As6-sawhorse-type compound [As2(AsEtCAAC)4] (2). The reaction of (LE)2 (L=PhC(NtBu)2; E=Si, Ge) with I resulted in a rearrangement and an insertion of LE fragments, forming unique silicon- (4: [Cp*Fe(η4-As4SiL)], 5 a: [Cp*Fe(η4-As6SiL)) and germanium-containing (5 b: [Cp*Fe(η4-As6GeL)) cyclic polyarsenic ligand complexes

A role for low concentration reaction intermediates in the signal amplification by reversible exchange process revealed by theory and experiment

A route to monitor the involvement of less abundant species during the catalytic transfer of hyperpolarisation from parahydrogen into a substrate is detailed. It involves probing how the degree of hyperpolarisation transfer catalysis is affected by the magnetic field experienced by the catalyst during this process as a function of temperature. The resulting data allows the ready differentiation of the roles played by hard to detect and highly reactive complexes, such as [Ir(H)2(NHC)(substrate)2(methanol)]Cl, from dominant species such as [Ir(H)2(NHC)(substrate)3]Cl. The difference in behaviour results from changes in the interligand spin-spin coupling network within the active SABRE catalysts

Immobilized metallodithiolate ligand supports for construction of bioinorganic model complexes

The A-cluster active site in acetyl coA synthase exploits a Ni(CGC)2- metallopeptide as a bidentate ligand to chelate the catalytically active square-planar nickel center used to produce acetyl coA. As Nature utilizes polypeptides to isolate and stabilize the active sites, we have set out to immobilize biomimetic complexes to polyethylene-glycol (PEG) rich polystyrene polymer beads (TentaGel). The PEG rich resin-beads serve to imitate the peptidic superstructure of enzyme active sites as well as to protect the resin-bound models from O2 decomposition. As a model of the NiN2S2 ligand observed in the A-cluster of acetyl coA synthase, the CGC tripeptide was constructed on resins using Merrifield solid phase peptide synthesis and then metallated with NiII to produce bright orange beads. Derivatization with M(CO)x (M = Rh, W) provided qualitative identification of Ο-Ni(CGC)M(CO)x n- via ATR-FTIR. Additionally, Neutron Activation Analysis (NAA) and UV-vis studies have determined the concentration of Ni and CGC, and qualitatively identify Ο-Ni(CGC)2-. Furthermore, infrared studies and NAA experiments have been used to identify and quantify Ο- Ni(CGC)Rh(CO)2 1-. The S-based reactivity of Ni(ema)2-, a good model of Ni(CGC)2-, toward oxygenation and alkylation has been pursued and compared to neutral NiN2S2 complexes. The spectroscopic, electrochemical and structural effects of these modifications will be discussed and supported using DFT computations and electrostatic potential maps of the resulting Ni(ema)*O2 2- and Ni(ema)*(CH2)3 complexes. Having firmly established the synthesis, characterization and reactivity of NiN2S2 2- systems in solution and resin-bound, CuIIN2S2 analogues were explored. The synthesis and identification of solution complexes, Cu(ema)2-, Cu(emi)2-, and Cu(CGC)2- via UV-Vis, EPR, and –ESI-MS will be discussed in addition to their S-based reactivity with Rh(CO)2 + . Furthermore, the resin-bound Cu(CGC)2- complex has been produced and characterized by EPR and its Rh(CO)2 adduct identified by ATR-FTIR and compared to the analogous NiN2S2 2- systems. As the active site of [FeFe] Hydrogenase utilizes a unique peptide-bound propane dithiolate bridge to support the FeFe organometallic unit, [FeFe]Hydrogenase models have been covalently anchored to the resin-beads via similar carboxylic acid functionalities. The characterization (ATR-FTIR, EPR, Neutron Activation Analysis), stability and reactivity of the immobilized models complexes are discussed as well as work toward establishing the microenvironment of resin-bound complexes

Synthesis and Reactions of a Tungsten Dioxo Complex

Metal dioxo complexes are largely known for their ability to epoxidize olefins. The importance of these reactions is reflected in the patents for the ARCO and Halcon processes from the late 1960\u27s. It is very difficult to find literature that shows other indications for metal dioxo complexes. Catalytic olefin dimerization has become significant in the realm of green chemistry for atom economical reactions. Many transition metals have been examined as catalysts for olefin dimerization, but the largest volume of work has been performed using group 8 transition metals, particularly nickel, palladium and platinum. This is especially true for the specific case of styrene dimerization. Herein, we wish to report the synthesis and X-ray crystal structure for Cp*W(=O)2CH3 (1), as well as the X-ray crystal structure for Cp*W(CO)3CH3 (2). The tungsten dioxo complex (1) was found to have high selectivity for the dimerization of some para-substituted styrenes, yielding the desired trans-1,3-diaryl-1-butene products. We also wish to report the X-ray crystal structure of trans-1,3-di-(4-bromophenyl)-1-butene which helped to confirm the structure of our products

Reaktivität von [Cp*Fe(η5‐As5)] gegenüber Carbenen, Silylenen und Germylenen

Das Reaktionsverhalten von [Cp*Fe(η5-As5)] (I) (Cp*=C5Me5) gegenüber Carbenen und ihren schwereren Analoga wurde untersucht. Die Reaktion von I mit NHCs (NHCs=N-heterocyclische Carbene) gibt die ersten Substitutionsprodukte von Polyarsen-Ligandkomplexen mittels NHCs [Cp*Fe(η4-As5NHC)] (1 a: NHC=IMe=1,3,4,5-Tetramethylimidazolin-2-yliden, 1 b: NHC=IMes=1,3-Bis(2,4,6-trimethylphenyl)-imidazolin-2-yliden). Im Gegensatz dazu führt die Reaktion von I mit EtCAAC (EtCAAC=2,6-Diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-yliden) zu einer Fragmentierung und Bildung einer neuartigen As6-sägebockartigen Verbindung [As2(AsEtCAAC)4] (2). Die Reaktion von (LE)2 (L=PhC(NtBu)2; E=Si, Ge) mit I resultiert in einer Umlagerung und einer Insertion von LE-Fragmenten, was zur Bildung einzigartiger silizium- (4: [Cp*Fe(η4-As4SiL)], 5 a: [Cp*Fe(η4-As6SiL)) und germanium-haltiger (5 b: [Cp*Fe(η4-As6GeL)) cyclischer Polyarsen-Ligandkomplexe führt