Exploring the Electrochemistry of Iron Dithiolene and Its Potential for Electrochemical Homogeneous Carbon Dioxide Reduction.

In this work, the dithiolene complex iron(III) bis-maleonitriledithiolene [Fe(mnt)2] is characterised and evaluated as a homogeneous CO2 reduction catalyst. Electrochemically the Fe(mnt)2 is reduced twice to the trianionic Fe(mnt)2 3- state, which is correspondingly found to be active towards CO2. Interestingly, the first reduction event appears to comprise overlapping reversible couples, attributed to the presence of both a dimeric and monomeric form of the dithiolene complex. In acetonitrile Fe(mnt)2 demonstrates a catalytic response to CO2 yielding typical two-electron reduction products: H2, CO and CHOOH. The product distribution and yield were governed by the proton source. Operating with H2O as the proton source gave only H2 and CO as products, whereas using 2,2,2-trifluoroethanol gave 38 % CHOOH faradaic efficiency with H2 and CO as minor products.

Dithiolene Complexes of First-Row Transition Metals for Symmetric Nonaqueous Redox Flow Batteries.

Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt2- ) with M=V, Fe, Co, Ni, Cu were studied as redox-active materials for nonaqueous redox flow batteries (RFBs). All five complexes exhibit at least two redox processes, making them applicable to symmetric RFBs as single-species electrolytes, that is, as both negolyte and posolyte. Charge-discharge cycling in a small-scale RFB gave modest performances for [(tea)2 Vmnt ], [(tea)2 Comnt ], and [(tea)2 Cumnt ] whereas [(tea)Femnt ] and [(tea)2 Nimnt ] (tea=tetraethylammonium) failed to hold any significant capacity, indicating poor stability. Independent negolyte- and posolyte-only battery cycling of a single redox couple, as well as UV/Vis spectroscopy, showed that for [(tea)2 Vmnt ] the negolyte is stable whereas the posolyte is unstable over multiple charge-discharge cycles; for [(tea)2 Comnt ], [(tea)2 Nimnt ], and [(tea)2 Cumnt ], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize Vmnt 3-/2- as a negolyte, and Comnt 2-/1- , Nimnt 2-/1- , and Cumnt 2-/1- as posolytes could lead to their use in asymmetric RFBs.

Dinuclear helicate and tetranuclear cage assembly using appropriately designed ditopic triazole-azine ligands.

Four new, symmetrical, bis-bidentate ditopic Rdpt-type, Rat (R azine-triazole), ligands have been prepared, Lnpym-meta/para (n = 2 or 4), which contain bidentate n-pyrimidine/triazole binding pockets connected through an appropriate aromatic spacer, meta/para-phenyl, to enable assembly into dinuclear helicates or tetranuclear tetrahedral cages, respectively. The 3 : 2 self assembly reactions of each Lnpym-meta/para ligand with iron(ii) tetrafluoroborate gave the desired complexes, as shown by X-ray crystal structure determinations of the pair of helicates [FeII2(Lnpym-meta)3(BF4)4]·6CH3CN, with n = 2 (1·6CH3CN) or 4 (2·6CH3CN), and the pair of Td cages [FeII4(Lnpym-para)6(BF4)8]·xsolvent, with n = 2 (3·xsolvent) or 4 (4·xsolvent). Reversible FeII/III processes at Em = 0.95 ± 0.05 V vs. 0.01 M AgNO3/Ag in MeCN are a feature of 1-4, with little variation in the redox potential as a function of nuclearity, architecture or choice of n-pyrimidine isomer. In all four complexes the iron(ii) centres are low spin, despite having employed the weakest field diazines, the 2- and 4-pyrimidines, in these ditopic Rat ligands. Nevertheless this is an exciting proof of principle that Rdpt-type ligands (previously used to generate about 50 spin crossover-active complexes) can be extended into ditopic forms that are suitable for supramolecular assembly of helicates and cages.

Predictable Substituent Control of CoIII/II Redox Potential and Spin Crossover in Bis(dipyridylpyrrolide)cobalt Complexes.

A family of five easily prepared tridentate monoanionic 2,5-dipyridyl-3-(R1)-4-(R2)-pyrrolide anions (dppR1,R2)-, varying in the nature of the R1 and R2 substituents [R1, R2 = CN, Ph; CO2Et, CO2Et; CO2Me, 4-Py; CO2Me, Me; Me, Me], has been used to generate the analogous family of neutral [CoII(dppR1,R2)2] complexes, two of which are structurally characterized at both 100 and 298 K. Both the oxidation and spin states of these complexes can be switched in response to appropriate external stimuli. All complexes, except [CoII(dppMe,Me)2], exhibit gradual spin crossover (SCO) in the solid state, and SCO activity is observed for three complexes in CDCl3 solution. The cobalt(II) centers in the low spin (LS) complexes are Jahn-Teller tetragonally compressed along the pyrrolide-Co-pyrrolide axis. The complexes in their high spin (HS) states are more distorted than in the LS states, as is also usually the case for SCO active iron(II) complexes. The reversible CoIII/II redox potentials are predictably tuned by choice of substituents R1 and R2, from -0.95 (Me,Me) to -0.45 (CN,Ph) V vs Fc+/Fc, with a linear correlation observed between E1/2(CoIII/II) and the Swain-Lupton parameters of the pyrrolide substituents.

Spin crossover in discrete polynuclear iron(ii) complexes.

Iron(ii) spin crossover (SCO) materials have been widely studied as molecular switches with a wide variety of potential applications, including as displays, sensors, actuators or memory components. Most SCO materials have been either monometallic or polymeric, and it is only relatively recently that chemists have really started to focus on linking multiple metal centres together within the one, discrete, molecule in an effort to enhance the SCO properties, such as abrupt, hysteretic, and multistep switching, as well as the potential for quantum cellular automata, whilst still being readily amenable to characterisation. Here we present a review of the ligand designs of the last two decades that have led to self assembly of discrete di- to poly-nuclear iron(ii) complexes of helicate, cage, cube, and other supramolecular architectures with rich SCO activity, and to an increased focus on host-guest interactions. Analysis of selected octahedral distortion parameters (Σ, CShM) reveals interesting differences between these structural types, for example that the iron(ii) centres in grids are generally significantly more distorted than those in squares or cages, yet are still SCO-active. Of the 127 complexes reviewed (79 published 2012-Feb. 2018), 54% are dinuclear, 10% trinuclear, 31% tetranuclear, and the remaining 5% are penta, hexa and octanuclear. Of the 93 designer ligands utilised in these polynuclear architectures: 60 feature azoles; 55 provide all donors to the Fe(ii) centres (no co-ligands coordinated) and form exclusively 5-membered chelate rings via either bidentate azole-imine/pyridine or tridentate heterocycle-imine/amine/thioether/pyridine-heterocycle binding pockets.

A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution.

Seventeen cobalt complexes-eleven dinuclear cobalt(II) complexes and three tetranuclear cobalt complexes (two mixed valent) of ditopic ligands, with varying N-donor aromatic bridging moieties and pendant pyridine side arms, as well as three mononuclear cobalt(II) complexes of Schiff base macrocyclic ligands-have been screened for photocatalytic hydrogen evolution reaction (HER) activity. All 17 complexes are active catalysts for the HER, in both DMF and aqueous solution, in tandem with the [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) photosensitiser. All are benchmarked to the literature standard [CoIII (dmgH)2 (py)Cl] (dmg=dimethylglyoxime, py=pyridine) under identical conditions. Two families of dinuclear cobalt(II) complexes of bis-tetradentate ligands that provide a triazole bridging moiety and mononuclear cobalt(II) complexes of tetradentate Schiff base macrocycles were found to be the most active catalysts, outperforming [CoIII (dmgH)2 (py)Cl] by two- to three-fold. Within these two families, the use of shorter alkyl linkers between the N donors, and hence, smaller chelate ring sizes, was found to significantly enhance catalytic performance, whereas the variation of peripheral functional groups was found to have little effect. This last point will be convenient for subsequent surface immobilisation studies.

Manipulating and quantifying spin states in solution as a function of pressure and temperature.

Monitoring the spin states of species in solution is a crucial aspect of understanding magnetic properties as well as spin-labile sensing, supramolecular, catalytic and biochemical processes. Herein, we describe the first quantitative variable-pressure and variable-temperature method of determining spin states in solution, demonstrate that it is accurate, and identify a simultaneous T and P sensor system.

Self-Assembly of Cyclohelicate [M3 L3 ] Triangles Over [M4 L4 ] Squares, Despite Near-Linear Bis-terdentate L and Octahedral M.

Self-assembly of 1:1:2 MII (BF4 )2 (M=Zn or Fe), pyrazine-2,5-dicarbaldehyde (1) and 2-(2-aminoethyl)pyridine gave trimetallic triangle architectures rather than the anticipated tetrametallic [2×2] squares. Options for the nontrivial synthesis of 1 are considered, and synthetic details provided for both preferred routes. Rare cyclohelicate triangle architectures are observed for the pair of structurally characterized yellow-brown [Zn3 L3 ](BF4 )6 and dark green [Fe3 L3 ](BF4 )6 complexes of the neutral bis-terdentate Schiff base L. In order to form these pyrazine-edged triangles, the octahedral metal ions-with all 6 N-donors provided by the terdentate binding pockets of two L-are located 0.4-0.5 Šout of the plane of the bridging pyrazines, towards the center of the triangle. Density functional theory calculations confirm that simple particle counting entropic arguments, which predict triangles over squares, are correct here, with the triangles shown to be energetically favored over the corresponding squares. However, importantly, DFT analysis of these and related triangle versus square systems also show that vibrational contributions to entropy dominate and may significantly influence the preferred architecture, such that simple particle counting cannot in general be reliably employed to predict the observed architecture.

Spin Crossover in Dinuclear N4S2 Iron(II) Thioether-Triazole Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States.

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PS(i)BuT, PS(t-Bu)PhT, and PS(Me)PhT, with R = Ph, (i)Bu, (t-Bu)Ph, and (Me)Ph, respectively. Three dinuclear colorless to pale green iron(II) complexes, [Fe(II)2(PSRT)2](BF4)4·solvent, featuring N4S2 donor sets, were prepared. Single-crystal structure determinations on [Fe(II)2(PSPhT)2](BF4)4·2MeCN·H2O, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2MeCN·(1)/2H2O·THF, [Fe(II)2(PS(Me)PhT)2](BF4)4·2MeCN, and [Fe(II)2(PS(i)BuT)2](BF4)4·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe(II)2(PS(Me)PhT)2](BF4)4·1(1)/2MeCN·2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe(II)2(PS(i)BuT)2](BF4)4·MeCN·H2O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe(II)2(PSPhT)2](BF4)4·(1)/2CHCl3·2(1)/2H2O (solvatomorph A) reveals a trace of LS character (572 nm band (1)A1g → (1)T1g). Evans' (1)H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.

Hysteretic spin crossover in iron(II) complexes of a new pyridine-triazole-pyrazine ligand is tuned by choice of NCE co-ligand.

A family of three new mononuclear complexes of the general form [Fe(L(pz))2(NCE)2] has been prepared (L(pz) = 4-p-tolyl-3-(2-pyrazinyl)-5-(2-pyridyl)-1,2,4-triazole; E = S, Se, BH3). All three exhibit spin crossover, in two cases with hysteresis, with T1/2 being predictably tuned by varying the coordinated anion.