Competing Pathways in the Photochemistry of Ru(H)2(CO)(PPh3)3

The photochemistry of Ru(H)2(CO)(PPh3)3 (1) has been reinvestigated employing laser and conventional light sources in conjunction with NMR spectroscopy and IR spectroscopy. The sensitivity of NMR experiments was enhanced by use of p-H2-induced polarization (PHIP), and a series of unexpected reactions were observed. The photoinduced reductive elimination of H2 was demonstrated (a) via NMR spectroscopy by the observation of hyperpolarized 1 on pulsed laser photolysis in the presence of p-H2 and (b) via nanosecond time-resolved infrared (TRIR) spectroscopy studies of the transient [Ru(CO)(PPh3)3]. Elimination of H2 competes with photoinduced loss of PPh3, as demonstrated by formation of dihydrogen, triphenylarsine, and pyridine substitution products which are detected by NMR spectroscopy. The corresponding coordinatively unsaturated 16-electron intermediate [Ru(H)2(CO)(PPh3)2] exists in two isomeric forms according to TRIR spectroscopy that react with H2 and with pyridine on a nanosecond time scale. These two pathways, reductive elimination of H2 and PPh3 loss, are shown to occur with approximately equal quantum yields upon 355 nm irradiation. Low-temperature photolysis in the presence of H2 reveals the formation of the dihydrogen complex Ru(H)2(η2-H2)(CO)(PPh3)2, which is detected by NMR and IR spectroscopy. This complex reacts further within seconds at room temperature, and its behavior provides a rationale to explain the PHIP results. Furthermore, photolysis in the presence of AsPh3 and H2 generates Ru(H)2(AsPh3)(CO)(PPh3)2. Two isomers of Ru(H)2(CO)(PPh3)2(pyridine) are formed according to NMR spectroscopy on initial photolysis of 1 in the presence of pyridine under H2. Two further isomers are formed as minor products; the configuration of each isomer was identified by NMR spectroscopy. Laser pump-NMR probe spectroscopy was used to observe coherent oscillations in the magnetization of one of the isomers of the pyridine complex; the oscillation frequency corresponds to the difference in chemical shift between the hydride resonances. Pyridine substitution products were also detected by TRIR spectroscopy

Photochemical dihydrogen production using an analogue of the active site of [NiFe] hydrogenase

The photoproduction of dihydrogen (H2) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by the reduction of the [NiFe2] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)3(bpy)] or [Ru(bpy)3][PF6]2). Reductive quenching of the 3MLCT excited state of the photosensitiser by NEt3 or N(CH2CH2OH)3 (TEOA) generates PS•−, and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H2 production from [HTEOA][BF4], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a•− and 1b•−) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH3CN and DMF (dimethylformamide), while only 1a•− was detected in CH2Cl2. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a•− is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b•− two of the carbonyl groups bridge the Fe centres, consistent with the peak observed at 1714 cm−1 in the FTIR spectrum for 1b•− in CH3CN, assigned to a ν(CO) stretching vibration. The formation of 1a•− and 1b•− and the production of H2 was studied in CH3CN, DMF and CH2Cl2. Although the more catalytically active species (1a•− or 1b•−) could not be determined, photocatalysis was observed only in CH3CN and DMF

Rhodium- and iridium-catalyzed double hydroalkoxylation of alkynes, an efficient method for the synthesis of O,O-acetals: catalytic and mechanistic studies

An efficient method for the synthesis of O,O-acetals via metal-catalyzed double hydroalkoxylation of alkynes was developed using the Ir(I) and Rh(I) complexes [Ir(PyP)(CO)2]BPh4 (1) and [Rh(bim)(CO) 2]BPh4 (2), where PyP = 1-[2-(diphenylphosphino)ethyl] pyrazole and bim = bis(N-methylimidazol-2-yl)-methane, as catalysts for the consecutive addition of two alcohol functional groups to terminal and nonterminal alkynes to form O,O-acetals. When the catalyzed cyclization of alkynols was performed in the presence of an excess amount of methanol as a cosolvent, a molecule of methanol was incorporated into the acetal product. The catalyzed cyclization of alkynols in the absence of an alcoholic solvent led to cyclization with incorporation of a second molecule of substrate in the final acetal product. Complexes 1 and 2 were also effective as catalysts for the cyclization of alkyne diols to form bicyclic O,O-acetals. The iridium complex 1 was more efficient than the rhodium complex 2 in promoting the reactions of aliphatic alkyne diols. On the other hand, the rhodium complex 2 was more effective for promoting the reactions of aromatic substrates. Mechanistic investigation using low-temperature NMR spectroscopy showed that the catalytic cycle proceeded via π coordination of the alkyne of the substrate to the metal center followed by the sequential addition of two hydroxyl groups to form O,O-acetals. Deuteration studies and analysis of reaction intermediates supported the proposed mechanism

Synthesis of spiroketals by iridium-catalyzed double hydroalkoxylation

A highly efficient approach to the synthesis of spiroketals involves the double cyclization of alkynyl diols using transition-metal catalysts. The iridium complex [Ir(PyP)(CO)₂]BPh₄ where PyP = 1-[(2-diphenylphosphino)ethyl]pyrazole is an effective catalyst for promoting the formation of spiroketals via this double hydroalkoxylation reaction. The complex promotes the formation of a series of spiroketal products from alkynyl diol starting materials such as 3-ethynylpentane-1,5-diol and 2-(4-hydroxybut-1-ynyl)benzyl alcohol. Stereoselective cyclization occurs for 3-ethynylpentane-1,5-diol, 3-ethynylhexane-1,6-diol. The cycloadditions occur in all but one case with quantitative conversion in under 24h at 120 °C.6 page(s

Nanozymes for Environmental Pollutant Monitoring and Remediation

Nanozymes are advanced nanomaterials which mimic natural enzymes by exhibiting enzyme-like properties. As nanozymes offer better structural stability over their respective natural enzymes, they are ideal candidates for real-time and/or remote environmental pollutant monitoring and remediation. In this review, we classify nanozymes into four types depending on their enzyme-mimicking behaviour (active metal centre mimic, functional mimic, nanocomposite or 3D structural mimic) and offer mechanistic insights into the nature of their catalytic activity. Following this, we discuss the current environmental translation of nanozymes into a powerful sensing or remediation tool through inventive nano-architectural design of nanozymes and their transduction methodologies. Here, we focus on recent developments in nanozymes for the detection of heavy metal ions, pesticides and other organic pollutants, emphasising optical methods and a few electrochemical techniques. Strategies to remediate persistent organic pollutants such as pesticides, phenols, antibiotics and textile dyes are included. We conclude with a discussion on the practical deployment of these nanozymes in terms of their effectiveness, reusability, real-time in-field application, commercial production and regulatory considerations

Reactivity of Rh(I) homobimetallic complexes & the synthesis & reactivity of heterobimetallic complexes containing Rh(I), Ir(I), Ir(III), Au(I), Ru(II)

Bimetallic catalysts, both homo- and heterobimetallic, can exhibit unique cooperative properties such as enhanced reaction rates and selectivities in comparison to their monometallic counterparts. The degree of intermetallic cooperativity achieved using bimetallic complexes can be dependent on the distance between the metal pairs. Heterobimetallic complexes, which contain two different metal centres, are of particular interest due to their ability to promote two or more different transformations in one pot. We have been able to use the principle of bimetallic cooperativity to enhance the efficiency of Rh(I) bis(pyrazol-1-yl)methane (bpm) catalysts for the dihydroalkoxylation of alkyne diols. To determine the optimal spatial relationship between the two metal centres we synthesised a series of homobimetallic Rh(I) bpm complexes supported on rigid, semi-rigid and flexible bridging scaffolds and tested them as catalysts for the dihydroalkoxylation reaction. We were also interested in the catalysis of two step reactions using heterobimetallic complexes, and synthesised a series of heteroditopic ligands that contain two distinct binding pockets including the bidentate bis(pyrazol-1-yl)methane (bpm) ligand as well as a monodentate N-heterocyclic carbene (NHC) donor. The coordination chemistry of the bitopic ligands with Au(I), Rh(I), Ir(I) and Ir(III) is described. The derivitisation of the monodentate NHC motif of the bitopic ligand with benzyl triazole was also investigated and subsequent complexation with Ru(II), Rh(I) and Ir(I) is also described. The complexes were investigated as catalysts for sequential transformations such as dihydroalkoxylation and hydroamination/hydrosilylation reactions.1 page(s

New rhodium(I) and iridium(I) complexes containing mixed pyrazolyl-1,2,3-triazolyl ligands as catalysts for hydroamination

Two new bidentate pyrazolyl–triazolyl donor ligands, 4-((1H-pyrazol-1-yl)methyl)-1-benzyl-1H-1,2,3-triazole, PyT (1a), and 4-((1H-pyrazol-1-yl)methyl)-1-phenyl-1H-1,2,3-triazole, PyS (1b), were synthesized using the copper(I)-catalyzed “click” reaction between 1-propargylpyrazole and benzyl azide or phenyl azide, respectively. Cationic rhodium(I) and iridium(I) complexes containing the new N–N′ ligands of the general formulas [M(N–N′)(COD)]X (M = Rh or Ir, N–N′ = PyT or PyS, and X = BPh4– or BArF4– (tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) 2a–c for Rh and 3a–c for Ir) and [M(N–N′)(CO)2]X (M = Rh or Ir, N–N′ = PyT or PyS, and X = BPh4– or BArF4–4a–d for Rh and 5a–d for Ir) were successfully prepared and fully characterized. The solid-state structures of eight of these complexes were determined using single-crystal X-ray diffraction and show that the triazolyl moiety coordinates to the metal center via the N3′ atom, forming six-membered metallacycles. These metallacycles adopted a distorted boat conformation in all of the structures determined. The M–N(triazole) bonds were found to be slightly shorter than the M–N(pyrazole) bonds, illustrating the stronger donating capacity of the triazolyl donor in comparison to that of the pyrazolyl donor. All of the cationic rhodium and iridium complexes reported here are catalytically active for the intramolecular hydroamination of 4-pentyn-1-amine (6) to 2-methyl-1-pyrroline (7) at 60 °C, with TOFs > 400 h–1 in many cases. The dicarbonyl complexes [M(N–N′)(CO)2]BArF4 (M = Rh or Ir) 4c,d and 5c,d were efficient as catalysts for the intramolecular cyclization of nonterminal alkynamines (8a,b) to cyclic imines (9a,b) via the hydroamination reaction and the cyclization of alkenamines (10a–c) to their corresponding cyclic amines (11a–c) also via the hydroamination reaction

Intramolecular hydroamination with rhodium(I) and iridium(I) complexes containing a phosphine-N-heterocyclic carbene ligand

Cationic Rh(I) and Ir(I) complexes of the form [M(PC)(COD)]BPh4 (M = Rh (4), Ir (5); PC = 3-[2-(diphenylphosphino)ethyl]-1-methylimidazol-2-ylidene) were synthesized by the addition of 3-[2-(diphenylphosphino)ethyl]-1-methylimidazolium (3) to [M(μ-OEt)(COD)]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) in the presence of base. COD was successfully displaced from [Rh(PC)(COD)]BPh4 (4) by addition of carbon monoxide to a methanol/hexane suspension to form [Rh(PC)(CO)2]BPh4 (6). The analogous addition of CO to the iridium compound 5 resulted in the formation of the five-coordinate Ir(I) complex [Ir(PC)(COD)(CO)]BPh4 (7). The single-crystal X-ray structures of 4, 5, and 7 were determined. The metal centers of 4 and 5 are square planar, and the metal center of 7 is a distorted trigonal bipyramid. Complexes 4−7 are effective as catalysts for the intramolecular hydroamination of 4-pentyn-1-amine to 2-methyl-1-pyrroline. Complete conversion (>97%) of 4-pentyn-1-amine was observed using complexes 4−7 as catalysts, in both chloroform-d and tetrahydrofuran-d8. Reactions in chloroform-d in general exhibited higher turnover rates than reactions in tetrahydrofuran-d8.10 page(s

Rhodium(I) and iridium(I) complexes containing bidentate phosphine-imidazolyl donor ligands as catalysts for the hydroamination and hydrothiolation of alkynes

A series of novel cationic and neutral rhodium and iridium complexes containing bidentate phosphine-imidazolyl donor ligands of the general formulae [M(ImP)(COD)]BPh4 (M = Rh, ImP = ImP2, 3; ImP1a, 4a; ImP1b, 4b and M = Ir, ImP = ImP2, 5; ImP1a, 6a and ImP1b, 6b), [Ir(ImP)(CO)₂]BPh₄ (ImP = ImP2, 7; ImP1a, 8a and ImP1b, 8b), [Rh(ImP1b)(CO)₂]BPh₄(10b) and [M(ImP)(CO)Cl] (M = Rh, ImP = ImP2, 11; ImP1b, 12 and M = Ir, ImP = ImP2, 13; ImP1b, 14) where COD = 1,5-cyclooctadiene, ImP2 = 1-methyl-2-[(2-(diphenylphosphino)ethyl]imidazole, 1; ImP1a = 1-methyl-2-[(diphenylphosphino)methyl]imidazole, 2a and ImP1b = 2-[(diisopropylphosphino)methyl]-1-methylimidazole, 2b were successfully synthesised. The solid state structures of 3, 6a, 11 and 12 were determined by single crystal X-ray diffraction analysis. A number of these complexes are effective as catalysts for the intramolecular hydroamination of 4-pentyn-1-amine to 2-methyl-1-pyrroline. The cationic complexes are significantly more effective than analogous neutral complexes. The cationic iridium complex 8b, containing the phosphine-imidazolyl ligand with the bulky isopropyl groups on the phosphorus donor, is more efficient than analogous complexes with the phenyl substituents on the phosphorus donor atom, 7 and 8a. The complexes 7-8b are also moderately effective in catalysing the addition of thiophenol to a range of terminal alkynes. In contrast to the hydroamination reaction, placement of the isopropyl group on the phosphorus donor leads to a decrease in the reactivity of the resulting metal complexes as catalysts for the hydrothiolation reaction.16 page(s