Ruthenium-Locked Helical Chirality: A Barrier of Inversion and Formation of an Asymmetric Macrocycle

Upon coordination to metal centers, tetradentate ligands based on the 6,6'-bis(2 ''-aminopyridyl)-2,2'-bipyridine (bapbpy) structure form helical chiral complexes due to the steric clash between the terminal pyridines of the ligand. For octahedral ruthenium(II) complexes, the two additional axial ligands bound to the metal center, when different, generate diastereotopic aromatic protons that can be distinguished by NMR. Based on these geometrical features, the inversion barrier of helical [Ru-II(L)(RR'SO)Cl](+) complexes, where L is a sterically hindered bapbpy derivative and RR'SO is a chiral or achiral sulfoxide ligand, was studied by variable-temperature H-1 NMR The coalescence energies for the inversion of the helical chirality of [Ru(bapbpy)(DMSO)(Cl)]Cl and [Ru(bapbpy)(MTSO)(Cl)]Cl (where MTSO is (R)-methyl p-tolylsulfoxide) were found to be 43 and 44 kJ/mol, respectively. By contrast, in [Ru(biqbpy)(DMSO)(Cl)]Cl (biqbpy = 6,6'-bis(aminoquinolyl)-2,2'-bipyridine increased strain caused by the larger terminal quinoline groups resulted in a coalescence temperature higher than 376 K, which pointed to an absence of helical chirality inversion at room temperature. Further increasing the steric strain by introducing methoxy groups ortho to the nitrogen atoms of the terminal pyridyl groups in bapbpy resulted in the serendipitous discovery of a ring-closing reaction that took place upon trying to make [Ru(OMe-bapbpy)(DMSO)Cl](+) (OMe-bapbpy = 6,6'-bis(6-methoxy-aminopyridyl)2,2'-bipyridine). This reaction generated, in excellent yields, a chiral complex [Ru(L '')(DMSO)Cl]Cl, where L '' is an asymmetric tetrapyridyl macrocycle. This unexpected transformation appears to be specific to ruthenium(II) as macrocyclization did not occur upon coordination of the same ligand to palladium(II) or rhodium(III).Macromolecular Biochemistr

Shorter Alkyl Chains Enhance Molecular Diffusion and Electron Transfer Kinetics between Photosensitisers and Catalysts in CO2 -Reducing Photocatalytic Liposomes.

Funder: Nederlandse Organisatie voor Wetenschappelijk Onderzoek; Id: http://dx.doi.org/10.13039/501100003246Covalent functionalisation with alkyl tails is a common method for supporting molecular catalysts and photosensitisers onto lipid bilayers, but the influence of the alkyl chain length on the photocatalytic performances of the resulting liposomes is not well understood. In this work, we first prepared a series of rhenium-based CO2 -reduction catalysts [Re(4,4'-(Cn H2n+1 )2 -bpy)(CO)3 Cl] (ReCn ; 4,4'-(Cn H2n+1 )2 -bpy=4,4'-dialkyl-2,2'-bipyridine) and ruthenium-based photosensitisers [Ru(bpy)2 (4,4'-(Cn H2n+1 )2 -bpy)](PF6 )2 (RuCn ) with different alkyl chain lengths (n=0, 9, 12, 15, 17, and 19). We then prepared a series of PEGylated DPPC liposomes containing RuCn and ReCn , hereafter noted Cn , to perform photocatalytic CO2 reduction in the presence of sodium ascorbate. The photocatalytic performance of the Cn liposomes was found to depend on the alkyl tail length, as the turnover number for CO (TON) was inversely correlated to the alkyl chain length, with a more than fivefold higher CO production (TON=14.5) for the C9 liposomes, compared to C19 (TON=2.8). Based on immobilisation efficiency quantification, diffusion kinetics, and time-resolved spectroscopy, we identified the main reason for this trend: two types of membrane-bound RuCn species can be found in the membrane, either deeply buried in the bilayer and diffusing slowly, or less buried with much faster diffusion kinetics. Our data suggest that the higher photocatalytic performance of the C9 system is due to the higher fraction of the more mobile and less buried molecular species, which leads to enhanced electron transfer kinetics between RuC9 and ReC9

Speciation of Ferric Phenoxide Intermediates during the Reduction of Iron(III)-mu-Oxo Dimers by Hydroquinone

The aqueous speciation of iron(III)-tris(pyridylmethyl)amine (TPA) complexes was determined from potentiometric titration data, and the overall formation constants (beta) for relevant species were calculated. At pH \u3c 3 the mononuclear complex [Fe(TPA)](+3)(aq) predominates (log beta = 10.75(15). Above pH 3 Fe(3+)-OH2 hydrolysis produces the mu-oxo dimer [Fe2(mu-O)(TPA)2(H2O)2](+4) (1a; log beta = 19.91(12)). This species is a diprotic acid with the conjugate bases [Fe2(mu-O)(TPA)2(H2O)(OH)](+3) (1b; log beta = 15.53(6)) and [Fe2(mu-O)(TPA)2(OH)2](+2) (1c; log beta = 10.27(7)). The pKas of 1a are 4.38(14) and 5.26(9). Compounds 1a-c quantitatively oxidize hydroquinone to benzoquinone with concomitant formation of 2 equiv of Fe(II). Kinetic and spectroscopic data at pH 5.6 are consistent with rapid equilibrium formation of a diiron(III)-phenoxide intermediate followed by rate-controlling electron transfer. The equilibrium constant for the formation of the intermediate complex is 25(3) M(-1), and the rate constant for its decomposition is 0.56(9) s(-1). A kinetic isotope effect of kH/kD = 1.5 was determined from proton inventory experiments in mixed H/D media. The mu-oxo-diiron(III) phenoxide intermediate is hydrolyzed in a pH dependent process to form a mononuclear iron(III)-phenoxide, which complicates the kinetics by introducing a fractional dependence on total iron(III) concentration in the pH range 4.1-5.2. The pH-dependent cleavage of mu-oxo-diiron(III)-phenoxides was investigated with phenol, a redox-inert proxy for hydroquinone. The addition of phenol to 1 facilitates acidic cleavage of the mu-oxo dimer to form [Fe(TPA)(OPh)(H2O)](+2), which becomes the dominant iron(III)-phenoxide as the pH decreases to 4. The 2-naphtholate analogue of this intermediate, [Fe(TPA)(2-naphtholate)(OCH3)]ClO4 (6), was characterized by single-crystal X-ray diffraction (C29H28FeN4O2,ClO4; P21; a = 13.2646(2) A, b = 15.2234(3) A, c = 13.7942(3) A; Z = 4)

Enhanced Stability of the Fe(II)/Mn(II) State in a Synthetic Model of Heterobimetallic Cofactor Assembly

Heterobimetallic Mn/Fe cofactors are found in the R2 subunit of class Ic ribonucleotide reductases (R2c) and R2-like ligand binding oxidases (R2lox). Selective cofactor assembly is due at least in part to the thermodynamics of M(II) binding to the apoprotein. We report here equilibrium studies of Fe(II)/Mn(II) discrimination in the biomimetic model system H5(F-HXTA) (5-fluoro-2-hydroxy-1,3-xylene-alpha,alpha\u27-diamine-N,N,N\u27,N\u27-tetraacetic acid). The homobimetallic F-HXTA complexes [Fe(H2O)6][1]2·14H2O and [Mn(H2O)6][2]2·14H2O (1 = [Fe(II)2(F-HXTA)(H2O)4](-); 2 = [Mn(II)2(F-HXTA)(H2O)4](-)) were characterized by single crystal X-ray diffraction. NMR data show that 1 retains its structure in solution (2 is NMR silent). Metal exchange is facile, and the heterobimetallic complex [Fe(II)Mn(II)(F-HXTA)(H2O)4](-) (3) is formed from mixtures of 1 and 2. (19)F NMR was used to quantify 1 and 3 in the presence of excess M(II)(aq) at various metal ratios, and equilibrium constants for Fe(II)/Mn(II) discrimination were calculated from these data. Fe(II) is preferred over Mn(II) with K1 = 182 ± 13 for complete replacement (2 ⇌ 1). This relatively modest preference is attributed to a hard-soft acid-base mismatch between the divalent cations and the polycarboxylate ligand. The stepwise constants for replacement are K2 = 20.1 ± 1.3 (2 ⇌ 3) and K3 = 9.1 ± 1.1 (3 ⇌ 1). K2 \u3e K3 demonstrates enhanced stability of the heterobimetallic state beyond what is expected for simple Mn(II) Fe(II) replacement. The relevance to Fe(II)/Mn(II) discrimination in R2c and R2lox proteins is discussed

Dinuclear Nickel Complexes of Thiolate-Functionalized Carbene Ligands and Their Electrochemical Properties

Four dimeric nickel­(II) complexes [Ni₂Cl₂(BnC₂S)₂] [<b>1</b>], [Ni₂Cl₂(BnC₃S)₂] [<b>2</b>], [Ni₂(PyC₂S)₂]­Br₂ [<b>3</b>]­Br₂, and [Ni₂(PyC₃S)₂]­Br₂ [<b>4</b>]­Br₂ of four different thiolate-functionalized N-heterocyclic carbene (NHC) ligands were synthesized, and their structures have been determined by single-crystal X-ray crystallography. The four ligands differ by the alkyl chain length between the thiolate group and the benzimidazole nitrogen (two −C₂– or three −C₃– carbon atoms) and the second functionality at the NHC being a benzyl (Bn) or a pyridylmethyl (Py) group. The nickel­(II) ions are coordinated to the NHC carbon atom and the pendent thiolate group, which bridges to the second nickel­(II) ion creating the dinuclear structure. Additionally, in compounds [<b>1</b>] and [<b>2</b>], the fourth coordination position of the square-planar Ni­(II) centers is occupied by the halide ions, whereas in [<b>3</b>]²⁺ and [<b>4</b>]²⁺, the additional pendant pyridylmethyl groups complete the coordination spheres of the nickel ions. The electrochemical properties of the four complexes were studied using cyclic voltammetry and controlled-potential coulometry methods. The thiolate-functionalized carbene complexes [<b>1</b>] and [<b>2</b>] appear to be poor electrocatalysts for the hydrogen evolution reaction; the complexes [<b>3</b>]­Br₂ and [<b>4</b>]­Br₂, bearing an extra pyridylmethyl group, show higher catalytic activity in proton reduction, indicating that the pyridine group plays an important role in the catalytic cycle