Highly selective CO2 photoreduction to CO on MOF-derived TiO2.

Metal-Organic Framework (MOF)-derived TiO2, synthesised through the calcination of MIL-125-NH2, is investigated for its potential as a CO2 photoreduction catalyst. The effect of the reaction parameters: irradiance, temperature and partial pressure of water was investigated. Using a two-level design of experiments, we were able to evaluate the influence of each parameter and their potential interactions on the reaction products, specifically the production of CO and CH4. It was found that, for the explored range, the only statistically significant parameter is temperature, with an increase in temperature being correlated to enhanced production of both CO and CH4. Over the range of experimental settings explored, the MOF-derived TiO2 displays high selectivity towards CO (98%), with only a small amount of CH4 (2%) being produced. This is notable when compared to other state-of-the-art TiO2 based CO2 photoreduction catalysts, which often showcase lower selectivity. The MOF-derived TiO2 was found to have a peak production rate of 8.9 × 10-4 µmol cm-2 h-1 (2.6 µmol g-1 h-1) and 2.6 × 10-5 µmol cm-2 h-1 (0.10 µmol g-1 h-1) for CO and CH4, respectively. A comparison is made to commercial TiO2, P25 (Degussa), which was shown to have a similar activity towards CO production, 3.4 × 10-3 µmol cm-2 h-1 (5.9 µmol g-1 h-1), but a lower selectivity preference for CO (3 : 1 CH4 : CO) than the MOF-derived TiO2 material developed here. This paper showcases the potential for MIL-125-NH2 derived TiO2 to be further developed as a highly selective CO2 photoreduction catalyst for CO production.

Amino acid-derived bisphenolate palladium complexes as C-C coupling catalysts.

A series of amine bisphenol (ABP) pro-ligands featuring amino acid ester pendant arms were prepared. Optimisation of the synthetic method allowed the facile incorporation of naturally occurring, chiral amino acids into the ABP scaffold with minimal racemisation. Reaction of the pro-ligands (LH2) with Pd(OAc)2, in the presence of amines, led to the formation of complexes with an unprecedented pincer-like O,N,O coordination mode around the PdII centre. The complexations in the presence of trialkylamines (NR3) afforded a mixture of LPdNR3 and LPdNHR2 species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc)2(NHR2)2 intermediates. Using pyridine as base eliminated this dealkylation and resulted in the exclusive formation of LPd(py) complexes in high yields. In total, seven novel PdII ABP complexes were prepared, exhibiting distorted square-planar geometries with the asymmetric ligand moieties orientated towards the metal centre. The air- and moisture-stable LPd(py) complexes were successfully employed as catalysts in two types of C-C coupling reactions. The Suzuki-Miyaura coupling of 4'-bromoacetophenone and phenylboronic acid reached high yields (up to 81%), while a scope of further alkyl bromides was also efficiently converted using low catalyst loadings (1 mol%) and mild temperatures (40 °C). Furthermore, a Pd-pyridine complex achieved high activity in the Mizoroki-Heck coupling of styrene and 4'-bromoacetophenone.

Application of Discrete First-Row Transition-Metal Complexes as Photosensitisers.

This Minireview summarises and critically evaluates recent advances in the utilisation of discrete first-row transition-metal (TM) complexes as photosensitisers. Whilst many compounds absorb light, TM complexes are generally more desirable for photochemical applications, as they usually exhibit strong absorption of visible light, making them ideally suited to exploiting the sun as a freely available light source. Due to their outstanding activities, precious metals, such as iridium and ruthenium, are currently still at the forefront of photochemistry research. However, they also bear disadvantages with respect to abundance, cost and toxicity. Therefore, it is desirable to move to more abundant and less expensive systems that retain good photosensitising abilities. This Minireview will focus on first-row transition-metals, specifically titanium, copper, iron, and zinc, which have become the focus of increased attention over recent years as potential replacements for noble metals as photosensitisers. Their structure - activity relationships are explored and challenges in designing the ligands and complexes are discussed.

Discrete Ti-O-Ti Complexes: Visible-Light-Activated, Homogeneous Alternative to TiO2 Photosensitisers.

A series of novel bimetallic TiIV amine bis(phenolate) complexes was synthesised and fully characterised. X-ray crystallography studies revealed distorted octahedral geometries around the Ti centres with single or double oxo-bridges connecting the two metals. These robust, air- and moisture-stable complexes were employed as photosensitisers generating singlet oxygen following irradiation with visible light (420 nm) LED module in a commercial flow reactor. All five complexes showed high activity in the photo-oxygenation of α-terpinene and achieved complete conversion to ascaridole in four hours at ambient temperature. The excellent selectivity of these photosensitisers towards ascaridole (vs. transformation to p-cymene) was demonstrated with control experiments using a traditional TiO2 catalyst. Further comparative studies employing the free pro-ligands as well as a monometallic analogue highlighted the importance of the 'TiO2 -like' moiety in the polymetallic catalysts. Computational studies were used to determine the nature of the ligand to metal charge transfer (LMCT) states and singlet-triplet gaps for each complex, the calculated trends in the UV-vis absorption spectra across the series agreed well with the experimental results.

Reduction of Dinitrogen to Ammonia and Hydrazine on Low-Valent Ruthenium Complexes.

The ruthenium(0) dinitrogen complexes [Ru(N2)(PP3R)] [PP3R = P(CH2CH2PR2)3; R = iPr or Cy] react with triflic acid and other strong acids to afford mixtures of ammonia and hydrazine. In this reaction, Ru(0) is oxidized to Ru(II), and depending on the solvent, Ru(II) benzene or triflate complexes are isolated and characterized from the reactions with triflic acid as the final metal-containing products from the reaction. The Ru(II) products are isolated and reduced back to Ru(0) dinitrogen complexes providing a cycle for the reduction of coordinated dinitrogen.

Aggregated initiators: defining their role in the ROP of rac-lactide.

Reported examples of aggregated initiators for the ring-opening polymerisation (ROP) of lactide often lack detailed investigations as to the nature of the active species, making it difficult to reconcile ligand design with performance. Here, we offer additional stability to the polynuclear titanium complexes, TiL(OiPr) (L = 9-14), through a bridging carboxylate anchored to the supporting amine bis(phenolate) ligands. An in-depth study of solution-state behaviour determined the process of assembly was driven by interactions between the carboxylate and a vacant site on a neighbouring titanium centre. Furthermore, we establish that mononuclear units form dynamic mixtures of polynuclear aggregates, with a clear relationship between nuclearity of the aggregates and the steric bulk on the ligand. Smaller aggregates displayed increased activity towards the ROP of rac-lactide. Furthermore, addition of a chiral centre, on the ligand framework, was investigated as a route to influence the selectivity of the polymerisation via easily-accessible initiators.

Well-defined Ti4 pre-catalysts for the ring-opening polymerisation of lactide.

The synthesis and full characterisation of four discrete tetrametallic titanium complexes is reported. These well-defined compounds are isostructural in the solid state and share the same general formula: Ti4(µ-O)2L4 (L = 1, 2, 3 or 4). Using a combination of NMR techniques the complexes are found to be stable in solution, even at elevated temperatures. Further studies show that the carboxylate moieties of the supporting amine bis(phenolate) ligands can be displaced by a more strongly coordinating solvent. This reversible process causes the coordinatively saturated Ti4(µ-O)2L4 complexes to separate into two Ti2(µ-O)L2 subunits which we envisaged would be catalytically active. Proof-of-concept experiments establish that all four of these complexes display catalytic activity in the ring-opening polymerisation of rac-lactide. These aggregates can therefore be viewed as air and moisture stable pre-catalysts for a range of reactions.

Flexible macrocycles as versatile supports for catalytically active metal clusters.

Here we present three structurally diverse clusters stabilised by the same macrocyclic polyphenol; t-butylcalix[8]arene. This work demonstrates the range of conformations the flexible ligand is capable of adopting, highlighting its versatility in metal coordination. In addition, a Ti complex displays activity for the ring-opening polymerisation of lactide.

Discovering the pivotal role of carbonate in the formation of a bis-phenolate supported Co15 cluster.

Unexpected panelling by µ6-CO3(2-) anions has been observed to support the formation of a new and unusual Co15 cluster. The arrangement is further stabilised by bis-phenolate ligands which assemble in a manner akin to tetrahomodioxacalix[4]arene. The reaction proceeds in very low yield as µ6-CO3(2-) anions are formed in situ, but targeted synthesis by addition of carbonate improves synthetic yield and highlights the pivotal role of this anion in the overall assembly process.

Oxacalix[4]arene-supported di-, tetra- and undecanuclear copper(II) clusters.

Oxacalix[4]arenes containing either one or two oxa-bridges afford di and undecanuclear Cu(II) clusters respectively upon reaction with cupric nitrate under facile conditions. Variation in reaction conditions results in formation of a tetranuclear Cu(II) cluster with mono-oxacalix[4]arene, representing a structural expansion of the dinuclear assembly.

Complementary ligands direct the formation of a calix[8]arene-supported ferromagnetic Mn(IV)Mn(III) dimer.

A combination of complementary ligands, p-tert-butylcalix[8]arene (TBC[8]) and phenyl salicylaldoxime (Ph-saoH2) have been utilised in the facile synthesis of a Mn(III)Mn(IV) dimer. Magnetic measurements reveal ferromagnetic exchange between the two metal ions.

Influence of the metal (Al, Cr, and Co) and substituents of the porphyrin in controlling reactions involved in copolymerization of propylene oxide and carbon dioxide by porphyrin metal(III) complexes. 3. Cobalt chemistry.

A series of cobalt(III) complexes LCoX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyirn (OEP) and X = Cl or acetate, has been investigated for homopolymerization of propylene oxide (PO) and copolymerization of PO and CO2 to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC) or propylene carbonate (PC), respectively. These reactions were carried out both with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or PPN(+)Cl(-) (bis(triphenylphosphine)iminium chloride). The PO/CO2 copolymerization process is notably faster than PO homopolymerization. With ionic PPN(+)Cl(-) cocatalyst the TPPCoOAc catalyst system grows two chains per Co center and the presence of excess [Cl(-)] facilitates formation of PC by two different backbiting mechanisms during copolymerization. Formation of PPC is dependent on both [Cl(-)] and the CO2 pressure employed (1-50 bar). TPPCoCl and PO react to form TPPCo(II) and ClCH2CH(Me)OH, while with DMAP, TPPCoCl yields TPPCo(DMAP)2(+)Cl(-). The reactions and their polymers and other products have been monitored by various methods including react-IR, FT-IR, GPC, ESI, MALDI TOF, EXAFS, and NMR ((1)H, (13)C{(1)H}) spectroscopy. Notable differences are seen in these reactions with previous studies of (porphyrin)M(III) complexes (M = Al, Cr) and of the (salen)M(III) complexes where M = Cr, Co.

Base-induced dehydrogenation of ruthenium hydrazine complexes.

Treatment of [RuCl(PP(3)(iPr))](+)Cl(-) (PP(3)(iPr) = P(CH(2)CH(2)P(i)Pr(2))(3)) with hydrazine, phenylhydrazine, and methylhydrazine afforded side-on bound hydrazine complexes [RuCl(η(2)-H(2)N-NH(2))(η(3)-PP(3)(iPr))](+), [RuCl(η(2)-H(2)N-NHPh)(η(3)-PP(3)(iPr))](+), and [RuCl(η(2)-H(2)N-NHMe)(η(3)-PP(3)(iPr))](+). The analogous reactions of [RuCl(2)(PP(3)(Ph))] (PP(3)(Ph) = P(CH(2)CH(2)PPh(2))(3)) with hydrazine, phenylhydrazine, and methylhydrazine afforded end-on bound hydrazine complexes [RuCl(η(1)-H(2)N-NH(2))(PP(3)(Ph))](+), [RuCl(η(1)-H(2)N-NHPh)(PP(3)(Ph))](+), and [RuCl(η(1)-H(2)N-NHMe)(PP(3)(Ph))](+). Treatment of parent hydrazine complex [RuCl(N(2)H(4))(PP(3)(iPr))](+) with strong base afforded the dinitrogen and dihydride complexes [Ru(N(2))(PP(3)(iPr))] and [RuH(2)(PP(3)(iPr))]. Treatment of phenylhydrazine complex [RuCl(NH(2)NHPh)(PP(3)(iPr))](+) with strong base afforded the hydrido ruthenaindazole complex [RuH(η(2)-NH═NC(6)H(4))(η(3)-PP(3)(iPr))] while similar treatment of methylhydrazine complex [RuCl(NH(2)NHMe)(PP(3)(iPr))](+) afforded the hydrido methylenehydrazide complex [RuH(NHN═CH(2))(PP(3)(iPr))]. Treatment of the hydrazine complexes [RuCl(NH(2)NHR)(PP(3)(Ph))](+) (R = H, Ph, Me) with strong base afforded the dinitrogen complex [Ru(N(2))(PP(3)(Ph))].

p-tert-Butylcalix[8]arene: an extremely versatile platform for cluster formation.

p-tert-Butylcalix[4]arene is a bowl-shaped molecule capable of forming a range of polynuclear metal clusters under different experimental conditions. p-tert-Butylcalix[8]arene (TBC[8]) is a significantly more flexible analogue that has previously been shown to form mono- and binuclear lanthanide (Ln) metal complexes. The latter (cluster) motif is commonly observed and involves the calixarene adopting a near double-cone conformation, features of which suggested that it may be exploited as a type of assembly node in the formation of larger polynuclear lanthanide clusters. Variation in the experimental conditions employed for this system provides access to Ln(1), Ln(2), Ln(4), Ln(5), Ln(6), Ln(7) and Ln(8) complexes, with all polymetallic clusters containing the common binuclear lanthanide fragment. Closer inspection of the structures of the polymetallic clusters reveals that all but one (Ln(8)) are in fact based on metal octahedra or the building blocks of octahedra, with the identity and size of the final product dependent upon the basicity of the solution and the deprotonation level of the TBC[8] ligand. This demonstrates both the versatility of the ligand towards incorporation of additional metal centres, and the associated implications for tailoring the magnetic properties of the resulting assemblies in which lanthanide centres may be interchanged.

Calixarene-supported clusters: employment of complementary cluster ligands for the construction of a ferromagnetic [Mn5] cage.

A combination of complementary cluster ligands results in the formation of a new calixarene-supported ferromagnetic [Mn(5)] cage that displays the characteristic bonding modes of each support.

Oxacalix[3]arene-supported supertetrahedron.

The initial use of oxacalix[3]arene in manganese chemistry affords an unusual [Mn(10)] supertetrahedron with an even more unusual oxidation state distribution.

Calixarene-supported rare-earth clusters: heteroatom bridge influences cluster composition.

Rare-earth octahedra have been previously synthesised using p-tert-butylcalix[4]arene as a cluster support. Introduction of a heteroatom bridge in the calixarene framework, with concomitant alteration to the nature of the metal binding pocket, influences lanthanide composition in the cluster.

Side-on bound complexes of phenyl- and methyl-diazene.

Treatment of trans-[FeCl(2)(dmpe)(2)] with phenylhydrazine and 1 equiv of base afforded the side-on bound phenylhydrazido complex cis-[Fe(η(2)-NH(2)NPh)(dmpe)(2)](+). Further deprotonation of the phenylhydrazido complex afforded the side-on bound phenyldiazene complex cis-[Fe(η(2)-HN═NPh)(dmpe)(2)] as a mixture of diastereomers. Treatment of cis-[RuCl(2)(dmpe)(2)] with phenylhydrazine or methylhydrazine afforded the end-on bound phenylhydrazine or methylhydrazine complexes cis-[RuCl(η(1)-NH(2)NHR)(dmpe)(2)](+) (R = Ph, Me). Treatment of the substituted hydrazine complexes with base afforded the side-on bound phenylhydrazido complex cis-[Ru(η(2)-NH(2)NPh)(dmpe)(2)](+) as well as the phenyldiazene and methyldiazene complexes cis-[Ru(η(2)-HN═NR)(dmpe)(2)] (R = Ph, Me). cis-[RuCl(η(1)-NH(2)NHR)(dmpe)(2)](+) (R = Ph, Me), cis-[M(η(2)-NH(2)NPh)(dmpe)(2)](+) (M = Fe, Ru) and cis-[Ru(η(2)-HN═NPh)(dmpe)(2)] were characterized structurally by X-ray crystallography. cis-[Ru(η(2)-HN═NPh)(dmpe)(2)] is the first side-on bound phenyldiazene complex to be structurally characterized. In the structure of cis-[Ru(η(2)-HN═NPh)(dmpe)(2)], the geometry of the coordinated diazene fragment is significantly nonplanar (CNNH angle 137°) suggesting that the complex is probably better described as a Ru(II) metallodiaziridine than a Ru(0) diazene π-complex.

Calix[4]arene-supported rare earth octahedra.

A series of calix[4]arene-supported Ln(III)(6) clusters have been synthesised under facile bench top conditions. The magnetic and structural properties of these clusters are reported, the latter suggesting that the Ln(III)-calix[4]arene moiety may be used for the construction of other assemblies in a manner akin to that for the Mn(III)-calix[4]arene analogue.

p-tert-Butylcalix[8]arene: a support for sodium and sodium-manganese clusters that exhibit interesting self-assembly properties.

A series of new sodium and mixed sodium/manganese clusters have been formed using p-tert-butylcalix[8]arene as a support. In all cases the calixarene adopts the pleated-loop conformation, and the resulting complexes show interesting self-assembly properties depending on the ligated solvent molecules around individual cluster cores.