C. John McAdam

‘Click’ to functionalise: synthesis, characterisation and enhancement of the physical properties of a series of exo- and endo-functionalised Pd2L4 nanocages

The synthesis of self-assembled metallosupramolecular architectures has been of steadily growing interest in recent years due to their diverse applications. Appending additional functionality to the ligands of these architectures has been limited as this often involves incorporation of coordinating groups that can potentially disrupt formation of the desired structure. Herein we report the use of the facile, functional group tolerant and high yielding CuAAC ‘click’ reaction to attach a variety of functional moieties to a tripyridyl ligand system. Despite the presence of the potentially coordinating 1,2,3-triazole rings, self-assembly of quadruply-stranded dipalladium(II) cage architectures in the presence of Pd(II) ions was almost universally observed for the functionalised “click” ligands. The only system which did not assemble into the expected cage featured a 2-(1,2,3-triazol-4-yl)pyridine binding pocket which sequestered the Pd(II) ions. Blocking this chelating pocket with an inert [Re(CO)3Cl] moiety restored the ability of the ligand to self-assemble into the desired quadruply-stranded dipalladium(II) cage, generating a heterometallic cage architecture. All ligands and cage architectures have been characterised using 1H, 13C and DOSY NMR, IR, UV-Vis and emission spectroscopies, mass spectrometry and in some cases by X-ray crystallography. Whilst the parent cage system is devoid of useful physical properties and displays a limited range of solubility, the CuAAC methodology provides a facile method to enhance the cages properties. A variety of fluorescent, redox active and biologically relevant species have been appended to the external surface of these cages. These groups enabled the generation of a series of aqueous soluble, fluorescent and electrochemically active Pd2L4 cages in a modular fashion.

Synthesis and fluorescence properties of new enaminenaphthalimides

The first 4-enamine-N-methyl-1,8-naphthalimides (amine = piperidine, pyrrolidine, morpholine, Prn2NH, Pri2NH) are reported. The X-ray crystal structure of the piperidine derivative confirmed the E-stereochemistry and showed strong Π–Π stacking in the unit cell. The enamines are strongly fluorescent and quantum yields do not vary with the pKb of the amine. The solvent dependence of λ abs and λ flu is consistent with significant charge separation in the excited state.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)]n+ Complexes in Aqueous Solution

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Luminescent Cages: Pendant Emissive Units on [Pd2L4]4+ “Click” Cages

The photophysics of a family of exo-functionalized [Pd2L4](4+) metallo-supramolecular cage architectures constructed from a tripyridyl 1,2,3-triazole backbone are reported. Several spectroscopic techniques are employed including both electronic (steady-state and transient absorption and emission) and vibrational (resonant and nonresonant Raman) methods. These experimental results are interpreted alongside simulated results from density functional theory calculations of the systems vibrational and electronic properties. The ligands and cages are shown to be essentially insulated from the exo-functionalization. They exhibit electronic transitions in the UV region and excited-state properties that are little affected by formation of the cage. Upon functionalization, characteristic Raman bands, electronic transitions, and emission bands associated with, and confined to, the substituent are observed.

Synthesis and redox chemistry of 1,1′-bis(diphenylphosphino)ferrocene derivatives of R2C2Co2(CO)6 (R MeO2C, CF3)

Abstract Fe(C5H4PPh2)2 (dppf) undergoes facile thermal substitution reactions with R2C2Co2(CO)6 (R  MeO2C, CF3) to yield a variety of products. When R  MeO2C, initial coordination gives μ2-(MeO2C2)2C2Co2(CO)5(η1-dppf); its crystal and molecular structure, monoclinic, P2 1 /c, a = 8.954(3), b = 20.211(2) c = 23.836(5) A , β = 100.55(2)°, Z = 4, R1 = 0.0345 for 5764 reflections 1 > 2σ(1), confirms the monodentate coordination mode for dppf. At low dppf/alkyne-complex ratios, oligomeric products with dppf ligands linking up to five (μ2-MeO2C)2C2Co2 modules have been characterised but, as the proportion of the phosphine ligand increases. unstable products, which include a η1-μ-η1 dppf configuration, are obtained as well. In contrast, for R  CF3, only [(CF3)2C2]Co2(CO)4(η2-dppf) is found in significant yield. A molecule with two different reduction centres, [μ2-(MeO2C)2C2Co2(CO)5](μ-dppf)[Co3(μ3-CPh)(CO)8], was also characterised. Electrochemistry of the dppf complexes was characterised by fast ligand dissociation upon reduction of the (μ-alkyne)Co2 redox centre and oxidation of coordinated dppf. There was no evidence for communication between redox centres.

Linear electrochemical actuators with very large strains using carbon nanotube-redox gel composites

The synthesis and performance of novel redox-controlled gel actuators are described. The gels are constructed with quinone groups that can be reversibly oxidised or reduced, producing a marked change in the swelling of the gel. The use of highly sterically hindered quinones means that they can be polymerised directly without protection. Incorporation of carbon nanotubes produces good electrical conductivity in the gel and even swelling. By using a mechanical biasing element such as a spring doubling as the electrode, one-dimensional linear actuation can be realised, with strains over 40%. The device is robust over many cycles, and is capable of producing a force of 30 kPa at potentials less than 1 V.

Preparation and redox properties of phosphite derivatives of R2C2Co2(CO)6−n[P(OMe)3]n (R=CF3, MeO2C)

Abstract A series of phosphite complexes R 2 C 2 Co 2 (CO) 6− n [P(OMe) 3 ] n (R=CF 3 , MeO 2 C), n =1–4, have been prepared and characterised. Cyclic and square-wave voltammetry shows that the kinetic stability of the radical cations increases with substitution and when R=CF 3 . The radical cation {(CF 3 ) 2 C 2 Co 2 (CO) 2 [P(OMe) 3 ] 4 }PF 6 4a + has been characterised and its crystal structure compared with that of the neutral parent. Analysis of the anisotropic ESR spectrum of 4a + is consistent with an unpaired electron in a SOMO with little d z 2 character.

Probing CH-π(alkyne) interactions in a series of ethynylferrocenes

A neoteric correlation between the cyclopentadienyl (Cp)-aryl dihedral angle and crystal packing arrangement is confirmed in the structural determination of three ethynylferrocene substituted naphthalenes. Two of the naphthalenes have approximately orthogonal Cp and aryl ring systems and feature an inversion dimer motif with CH⋯π(alkyne) short contacts. This motif is found in other arylethynylferrocenes. DFT calculations suggest the difference in energy between different dihedral conformations is of the same order as a weak hydrogen bond. Focussing on weak hydrogen bonds that involve the ethynyl group, other comparisons are made with a new polymorph of diferrocenylbutadiyne and the trans-isomer of 1,4-diferrocenylbut-1-ene-3-yne. Although the intermolecular forces described for the subject ethynylferrocenes are weak, they nonetheless give rise to distinctive structural motifs that may be exploitable in future supramolecular design.

6-Hy­droxy-5,7,8-trimethyl­chroman-2-one

The title compound, C(12)H(14)O(3), consists of a chromanone unit with an -OH substituent at the 4-position and methyl substituents on the remaining C atoms of the aromatic ring. The fused pyran-one ring adopts a distorted envelope conformation with the methyl-ene group adjacent to the carbonyl carbon as the flap atom. The crystal structure is stabilized by classical O-H⋯O hydrogen bonds and weak C-H⋯O and C-H⋯π inter-actions, generating a three-dimensional network.

Switchable cycloplatinated ferrocenylamine derivatives of acridone, naphthalimide and anthraquinone

Acridone, naphthalimide and anthraquinone derivatives of the cycloplatinated ferrocenylamine {Pt[(η5-CpFe(σ,η5-C5H3CH2NMe2)](dmso)CC–} have been prepared and the X-ray structure for the anthraquinone derivative has been determined. Spectroelectrochemistry has been used to probe the ground and excited states of these molecules. Irrespective of whether the fluorophore is bound to the ethynylPt(II) link via a nodal nitrogen (acridone) or to the aromatic ring (naphthalimide and anthraquinone) fluorophore emission is quenched in the ground state and partially restored in the oxidised species. Low energy donor-acceptor charge-transfer bands of the oxidised compounds are characteristic.