Photocatalytic Hydrogen Production from Water Using Heterogeneous Two-dimensional Rhodium Coordination Polymer [Rh2(p-BDC)2]n

Coordination polymers (CPs) have been widely studied because of their diverse and adjustable topologies and wide-ranging applications in luminescence, chemical sensors, magnetism, photocatalysis, gas adsorption and separation. In the present work, two coordination polymers, namely poly[(μ5-benzene-1,3,5-tricarboxylato-κ6O1:O1':O3:O3:O5,O5'){μ3-1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene-κ3N:N':N''}di-μ3-hydroxido-dicobalt(II)], [Co2(C9H3O6)(OH)(C12H12N6)]n or [Co2(btc)(OH)(mtrb)]n, (1), and poly[[diaquabis(μ3-benzene-1,3,5-tricarboxylato-κ3O1:O3:O5)bis{μ3-1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene-κ3N:N':N''}tetra-μ3-hydroxido-tetracopper(II)] monohydrate], {[Cu4(C9H3O6)2(OH)2(C12H12N6)2(H2O)2]·H2O}n or {[Cu4(btc)2(OH)2(mtrb)2(H2O)2]·H2O}n, (2), were synthesized by the hydrothermal method using 1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene (mtrb) and benzene-1,3,5-tricarboxylate (btc3-). CP (1) exhibits a (3,8)-coordinated three-dimensional (3D) network of the 3,8T38 topological type, with a point symbol of {4,5,6}2{42·56·616·72·82}, based on the tetranuclear hydroxide cobalt(II) cluster [Co4(μ3-OH)2]. CP (2) shows a (3,8)-coordinated tfz-d topology, with a point symbol of {43}2{46·618·84}, based on the tetranuclear hydroxide copper(II) cluster [Cu4(μ3-OH)2]. The different (3,8)-coordinated 3D networks based on tetranuclear hydroxide-metal clusters of (1) and (2) are controlled by the different central metal ions [CoII for (1) and CuII for (2)]. The thermal stabilities and solid-state optical diffuse-reflection spectra were measured. The energy band gaps (Eg) obtained for (1) and (2) were 2.72 and 2.29 eV, respectively. CPs (1) and (2) exhibit good photocatalytic degradation of the organic dyes methylene blue (MB) and rhodamine B (RhB) under visible-light irradiation.

A new ligand H2L with pyridine and salen moieties and its coordination polymers (CPs) [Mn(L)Cl]·DMF (1) and [Fe(L)Cl]·DMF (2) were synthesized and their photocatalytic activity for the conversion of CO2 into CO under visible-light irradiation was investigated. This is the first instance of pyridyl-salen-ligand based CPs for photocatalyzing CO2 reduction.

The work presented in this thesis entails the synthesis and characterisation of new iron(II) tris(dioxime) clathrochelate complexes functionalised with apical capping groups suitable for supramolecular applications, and their subsequent incorporation into self-assembled structures. These complexes possess a number of desirable characteristics as supramolecular building blocks, such as their synthetic accessibility and broad scope for structural and functional modification, which are highlighted throughout. Chapter 1 introduces certain key concepts and background literature relevant to this thesis. Clathrochelate-based 4,4'-bipyridyl metalloligands are presented in Chapter 2. Simple complexes bearing 4-pyridyl capping groups were prepared from commercially available starting materials and their potential as building blocks for supramolecular architectures demonstrated through the preparation of a discrete molecular square and a 3D coordination polymer (CP). Furthermore, bent and extended linear 4,4'-bipyridyl metalloligands up to 5.4 nm in length were prepared by two distinct homologation strategies. Clathrochelate-based 3,3'-bipyridyl metalloligands are presented in Chapter 3. Simple complexes were prepared from commercially available starting materials and subsequently combined with Pd(II) ions to give unprecedented [Pd6L12]12+ octahedral cage structures by coordination-driven self-assembly. These assemblies were shown to bind the large, non-coordinating anion BPh4- in aqueous solvent mixtures. Furthermore, linear and bent bis(clathrochelate) complexes bearing 3-pyridyl apical capping groups were also prepared by a one-pot approach. Clathrochelate complexes bearing carboxylic acid-functionalised capping groups are presented in Chapter 4. Ditopic and tetratopic metalloligands were synthesised in good yields. A heterometallic 2D CP was prepared through combination of Zn(II) ions with a clathrochelate complex functionalised with phenyl-4-carboxylic acid groups and the structure of this material determined by single crystal X-ray diffraction. Clathrochelates bearing functional groups suitable for imine-based self-assembly are presented in Chapter 5. Complexes functionalised with amine or aldehyde moieties as well as complexes functionalised with 3-pyridine-4-carboxaldehyde groups, were prepared in either one or two steps. Preliminary investigations into the preparation of metal-organic cage architectures through a well-established subcomponent self-assembly approach combining imine-based DCC and coordination-driven self-assembly gave promising results, including the synthesis of an [Fe4L6]8+ cage, which was characterised by single crystal X-ray crystallography.

Two novel Zn(II) coordination polymers constructed by the same dicarboxylate and different bis-imidazole as co-ligand: Syntheses, crystal structures and properties

A novel dopant-free TiO(2) photocatalyst (V(o)(.)-TiO(2)), which is self-modified by a large number of paramagnetic (single-electron-trapped) oxygen vacancies, was prepared by calcining a mixture of a porous amorphous TiO(2) precursor, imidazole, and hydrochloric acid at elevated temperature (450 °C) in air. Control experiments demonstrate that the porous TiO(2) precursor, imidazole, and hydrochloric acid are all necessary for the formation of V(o)(.)-TiO(2). Although the synthesis of V(o)(.)-TiO(2) originates from such a multicomponent system, this synthetic approach is facile, controllable, and reproducible. X-ray diffraction, XPS, and EPR spectroscopy reveal that the V(o)(.)-TiO(2) material with a high crystallinity embodies a mass of paramagnetic oxygen vacancies, and is free of other dopant species such as nitrogen and carbon. UV/Vis diffuse-reflectance spectroscopy and photoelectrochemical measurement demonstrate that V(o)(.)-TiO(2) is a stable visible-light-responsive material with photogenerated charge separation efficiency higher than N-TiO(2) and P25 under visible-light irradiation. The V(o)(.)-TiO(2) material exhibits not only satisfactory thermal- and photostability, but also superior photocatalytic activity for H(2) evolution (115 μmol h(-1) g(-1)) from water with methanol as sacrificial reagent under visible light (λ>400 nm) irradiation. Furthermore, the effects of reaction temperature, ratio of starting materials (imidazole:TiO(2) precursor) and calcination time on the photocatalytic activity and the microstructure of V(o)(.)-TiO(2) were elucidated.

The photocatalytic conversion of CO2 to fuels by light is of research interest owing to its relevance to environment and energy concerns. In this work, a hybrid H-TiO2–rGO–Re(bpy)(CO)3Cl composite consisting of a rhenium 2,2′-bipyridine compound, Re(bpy)(CO)3Cl, which was covalently grafted on reduced graphene oxide (rGO) and hydrogenated TiO2 (H-TiO2) was prepared. H-TiO2–rGO–Re(bpy)(CO)3Cl demonstrated high efficiency for visible light photo-conversion of CO2 into CO in triethanolamine/N,N-dimethylformamide (V:V = 1:9) solution with total turnover numbers larger than 580. In addition, the composite showed high stability with no leveling-off tendency after four catalytic experimental runs. The experimental results indicate that the suitable assembly of the Re(bpy)(CO)3Cl, rGO, and H-TiO2 components in the H-TiO2–rGO–Re(bpy)(CO)3Cl composite is necessary for the observed high performance of the catalyst for CO2 reduction under visible light irradiation.

Shape-controlled synthesis of novel self-assembled BiVO4 hierarchical structures with enhanced visible light photocatalytic performances

To improve the photocatalytic properties of coordination polymers under irradiation in the visible-light region, coordination polymer nanobelts (CPNB) were loaded on functional carbon fiber (FCF) through the use of a simple colloidal blending process. The resulting coordination polymer nanobelt loaded functional carbon fiber composite material (CPNB/FCF) exhibited dramatically improved photocatalytic activity for the degradation of rhodamine B (RhB) under visible-light irradiation. Optical and electrochemical methods illustrated the enhanced photocatalytic activity of CPNB/FCF originated from high separation efficiency of photogenerated electrons and holes on the interface of CPNB and FCF, which was produced by the synergy effect between them. In the composite material, the role of FCF could be described as photosensitizer and good electron transporter. For FCF, the number of functional groups on its surface has a significant influence on the photocatalytic performance of the resulting CPNB/FCF composite material, and an ideal FCF carrier was obtained as a highly efficient CPNB/FCF photocatalyst. CPNB/FCF showed outstanding stability during the degradation of rhodamine B (RhB); thus, the material is suitable for use as a photocatalyst in the treatment of organic dyes in water.

In this thesis we combine surface chemistry and surface physics to architecture molecular layers in a bottom-up approach. The formation of self-assembled molecular structures at surfaces on the basis of dipole-dipole interactions, H-bonding, metal coordination and covalent bonding is studied. The molecules of different structure and with specific functional groups are investigated on selected substrates, namely Au(111), Ag(111), Cu(111), Cu(100) or Bi reconstructed Cu(100). A number of model cases for controlling on-surface architectures and their properties has been found and is reported about in this thesis: 1) Architecture control of a coordination polymer, comprised from chiral and flexible molecular building blocks, by tuning of the intermolecular bonding motif; 2) Chirality transfer in a 1D coordination polymer formed from chiral molecules; 3) Dimensionality (0D, 1D, 2D) control via selection of the transition metal adatom, which modifies a ligand and participates in an on-surface coordination complex; 4) Demonstration of a 2D molecular ‘sponge’, created on the basis of a borylene derived covalent link with angular flexibility; 5) Investigation of confined 2D electron states in quantum boxes of different size and shape; 6) Self -sorting of bi-molecular system in a 2D array by the coulomb interaction of the surface dipole which depends on band-alignment, charge transfer and the screening in the substrate. Specifically, we show that chiral and flexible [7]helicene molecules with cyano-groups, covalently attached in symmetric positions, give rise to a 1D arrangement. The intrinsically chiral species imprints its chirality onto the weakly H-bonded assembly, which occurs if molecules are deposited on samples held at low (~90 K) temperatures. This imprint vanishes under the influence of stronger metal-coordination bonds formed after providing metal coordination centers to the H-bonded assembly. The flexibility of the helicene as well as the competition between intermolecular and molecule-surface interactions allow the coordinated chains to assemble in structures with the mirror symmetry apparently being reduced. The next important issue, addressed in this thesis, is the on-surface modification of the ligand as an approach to control the dimensionality of the resulting on-surface polymer. We present a novel metal-specific reaction of amino- /imino- functionalized perylene derivatives. This precursor is modified upon addition of Co, Fe or Ni at room-temperature into an endo-ligand. In contrast, the presence of Cu adatoms in conjunction with thermal activation leads to the formation of an exo-ligand. Thus the type of metal ligand defines whether a 1D or 2D coordinated polymer can be formed. We show that borylene-functionalized molecules react upon thermal activation with trimesic acid in a novel on-surface reaction. Moreover, the covalent connection, formed in this reaction, exhibits a high degree of flexibility and allows for the formation of the differently sized pores. The resulting molecular ‘sponge’, created this way, serves as a template confining the surface state electrons. We investigate the effect of size and shape of the pores on this quantum phenomenon. Furthermore, we present a new way of creating highly-ordered bimolecular self-sorted chessboard arrays. The bi-component mixture of Mn-phthalocyanine (MnPc) and Cu-phthalocyanine (CuPc) on Bi/Cu(100) self-assembles without participation of any chemical bonding or molecular functionalization but only on the basis of the lateral 2D Coulomb interactions. We resolve charge-transfer channels of two types, directing the supramolecular self-assembly: one oriented perpendicular to the substrate surface, the other oriented in-plane. These investigations are performed in ultra-high vacuum conditions (UHV) with the use of variable temperature Scanning Tunneling Microscopy / Spectroscopy (STM/STS), X-Ray Photoelectron Spectroscopy (XPS) and synchrotron-based Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. The experimental results are supported by Density Functional Theory (DFT) calculations, performed by research partners.

Five novel silver(I) coordination polymers with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3- thienyl)ethene (cis-dbe) were synthesized and are characterized in this paper. Treatment of AgCF(3)SO(3) or AgCF(3)CO(2) with cis-dbe afforded [Ag(2)(cis-dbe)(CF(3)SO(3))(2)] (1) and [Ag(2)(cis-dbe)(CF(3)CO(2))(2)] (2), and both complexes exhibit a 1-D infinite chain structure with two cyano groups and two thienyl groups of the ligand bridging four metal ions. Reaction of AgC(n)()F(2)(n)(+1)CO(2) with cis-dbe gave rise to an unprecedented cocrystallization of a 2-D sheet structure, [Ag(2)(cis-dbe)(C(n)F(2)(n)(+1)CO(2))(2)], where n = 2 (3), 3 (4), and 4 (5). Upon irradiation with 450 nm light, these five silver(I) complexes turned orange or red from yellow, and the color reverted to yellow on exposure to 560 nm light, indicative of the reversible cyclization/ring-opening reaction occurring in the crystalline phase. Furthermore, different anions gave not only the different structural dimensions but also the different photoresponsive patterns. The correlation between the crystal structures and the photochromic reactivity is discussed.

A facile strategy for fabricating C@Cu2O/CuO composite for efficient photochemical hydrogen production with high external quantum efficiency

A new class of six mono- (1; 3-Cl-, 2; 5-Cl-, 3; 6-Cl-) and di-(4; 3,6-Cl, 5; 5,6-Cl-, 6; 3,5-Cl-) chloro-substituted pyrazin-2-amine ligands (1–6) form complexes with copper (I) bromide, to give 1D and 2D coordination polymers through a combination of halogen and hydrogen bonding that were characterized by X-ray diffraction analysis. These Cu(I) complexes were prepared indirectly from the ligands and CuBr2 via an in situ redox process in moderate to high yields. Four of the pyrazine ligands, 1, 4–6 were found to favor a monodentate mode of coordination to one CuI ion. The absence of a C6-chloro substituent in ligands 1, 2 and 6 supported N1–Cu coordination over the alternative N4–Cu coordination mode evidenced for ligands 4 and 5. These monodentate systems afforded predominantly hydrogen bond (HB) networks containing a catenated (μ3-bromo)-CuI ‘staircase’ motif, with a network of ‘cooperative’ halogen bonds (XB), leading to infinite polymeric structures. Alternatively, ligands 2 and 3 preferred a μ2-N,N’ bridging mode leading to three different polymeric structures. These adopt the (μ3-bromo)-CuI ‘staircase’ motif observed in the monodentate ligands, a unique single (μ2-bromo)-CuI chain, or a discrete Cu2Br2 rhomboid (μ2-bromo)-CuI dimer. Two main HB patterns afforded by self-complimentary dimerization of the amino pyrazines described by the graph set notation R22(8) and non-cyclic intermolecular N–H∙∙∙N’ or N–H∙∙∙Br–Cu leading to infinite polymeric structures are discussed. The cooperative halogen bonding between C–Cl∙∙∙Cl–C and the C–Cl∙∙∙Br–Cu XB contacts are less than the sum of the van der Waals radii of participating atoms, with the latter ranging from 3.4178(14) to 3.582(15) Å. In all cases, the mode of coordination and pyrazine ring substituents affect the pattern of HBs and XBs in these supramolecular structures.

The solvothermal reactions of 2-S-HBim and Cd(NO3)2·4H2O in combination with H2ip, or 3-Hpyc or 4-Hpyc afforded three new coordination polymers, namely [Cd(ip)(2-S-HBim)2] n ·n(H2O) (1), [Cd(3-pyc)2(2-S-HBim)] n ·n(H2O) (2), and [Cd(4-pyc)(2-S-HBim)(Br)(H2O)] n (3) (2-S-HBim = 2-mercaptobenzimidazole, H2ip = isophthalic acid, 3-Hpyc = nicotinic acid, 4-Hpyc = isonicotinic acid). X-ray diffraction analyses revealed that compound 1 features a 1D infinite chain structure, which was further hydrogen-bonded into a 3D supramolecular framework. Compound 2 features a 2D layered structure with 4-connected sql plane topology, which further stacked into a 3D supramolecular framework. Compound 3 also shows a 1D infinite chain structure with dinuclear [Cd2(μ 2-H2O)2] subunits, which was further connected into a 3D supramolecular via intermolecular hydrogen bonds. Furthermore, the thermal stabilities and photoluminescent properties of 1–3 have been investigated.

Herein, we report the use of a molecular-defined rhodium(II) coordination polymer (Rh-CP) as a heterogeneous, recyclable catalyst in carbene transfer reactions. We showcase the application of this heterogeneous catalyst in a range of carbene transfer reactions and conclude with the functionalization of natural products and drug molecules.

Three-dimensional porous N-doped graphitic carbon framework with embedded CoO for photocatalytic CO2 reduction