New cyanido-bridged iron(II) spin crossover coordination polymers with an unusual ladder-like topology: an alternative to Hofmann clathrates.

Two new cyanido-bridged {FeIIMII} double chains were obtained by reacting cyanido anions [M(CN)4]2- with complex cations [FeII(tptz)]2+ (preformed in situ by mixing a hydrated tetrafluoroborate salt of iron(II) and a tptz ligand, tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) having the general formula [FeII(tptz)MII(CN)4]·2H2O·CH3CN, where M = Pd (1) or Pt (2). Additionally, two molecular complexes formulated as [FeII(tptz)2][MII(CN)4]·4.25H2O, where M = Pd (3) or Pt (4), were subsequently obtained from the same reaction, as secondary products. Single crystal X-ray analysis revealed that 1 and 2 are isostructural and crystallize in the P-1 triclinic space group. Their structure consists of a double-chain with a ladder-like topology, in which cyanido-based [M(CN)4]2- metalloligands coordinate, through three CN- ligands and three [FeII(tptz)]2+ complex cations. Compounds 3 and 4 are also isostructural and crystallize in the P1̄ triclinic space group, and the X-ray structural data show the formation of [FeII(tptz)2]2+ and [MII(CN)4]2- ionic units interconnected through H-bonds and π⋯π stacking supramolecular interactions. The static DC magnetic measurements recorded in the temperature range of 2-300 K showed that 1 and 2 exhibit incomplete spin transition on cooling, which is also confirmed by single crystal XRD analysis and Mössbauer spectroscopy. Compounds 3 and 4 are diamagnetic, most likely due to the encapsulation of Fe(II) in a tight pocket formed by two tptz ligands that preserve the low-spin state in the temperature range of 2-400 K.

Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand).

The synthetic availability of mol-ecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of di-ammonium {µ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexa-onato}ferrate(IV) acetic acid tris-olvate, (NH4)2[FeIV(C12H12N12O6)]·3CH3COOH or (NH4)2[FeIV(L-6H)]·3CH3COOH is reported. The FeIV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping tri-aza-cyclo-hexane fragments making three five- and six six-membered alternating chelate rings with the central FeIV ion. The local coord-ination environment of FeIV is formed by six deprotonated hydrazide nitro-gen atoms, which stabilize the unusual oxidation state. The FeIV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is inter-mediate between a trigonal prism (distortion angle φ = 0°) and an anti-prism (φ = 60°) with φ = 31.1°. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) Å, as expected for tetra-valent iron. Structure analysis revealed that three acetic acid mol-ecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid mol-ecules are inter-connected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.

Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues.

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Cooperative Spin Crossover above Room Temperature in the Iron(II) Cyanoborohydride-Pyrazine Complex.

Hysteretic spin crossover in coordination complexes of 3d-metal ions represents one of the most spectacular phenomena of molecular bistability. In this paper we describe a self-assembly of pyrazine (pz) and Fe(BH3CN)2 that afforded the new 2D coordination polymer [Fe(pz)2(BH3CN)2]∞. It undergoes an abrupt, hysteretic spin crossover (SCO) with a T1/2 of 338 K (heating) and 326 K (cooling) according to magnetic susceptibility measurements. Mössbauer spectroscopy revealed a complete transition between the low-spin (LS) and the high-spin (HS) states of the iron centers. This LS-to-HS transition induced an increase of the unit cell volume by 10.6%. Meanwhile, a modulation of multiple [C-Hδ+···Hδ--B] dihydrogen bonds stimulates a contraction in direction c (2.2%). The simplicity of the synthesis, mild temperatures of transition, a pronounced thermochromism, stability upon thermal cycling, a striking volume expansion upon SCO, and an easy processability to composite films make this new complex an attractive material for switchable components of diverse applications.

Water Oxidation by Pentapyridyl Base Metal Complexes? A Case Study.

The design of molecular water oxidation catalysts (WOCs) requires a rational approach that considers the intermediate steps of the catalytic cycle, including water binding, deprotonation, storage of oxidizing equivalents, O-O bond formation, and O2 release. We investigated several of these properties for a series of base metal complexes (M = Mn, Fe, Co, Ni) bearing two variants of a pentapyridyl ligand framework, of which some were reported previously to be active WOCs. We found that only [Fe(Py5OMe)Cl]+ (Py5OMe = pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane]) showed an appreciable catalytic activity with a turnover number (TON) = 130 in light-driven experiments using the [Ru(bpy)3]2+/S2O82- system at pH 8.0, but that activity is demonstrated to arise from the rapid degradation in the buffered solution leading to the formation of catalytically active amorphous iron oxide/hydroxide (FeOOH), which subsequently lost the catalytic activity by forming more extensive and structured FeOOH species. The detailed analysis of the redox and water-binding properties employing electrochemistry, X-ray absorption spectroscopy (XAS), UV-vis spectroscopy, and density-functional theory (DFT) showed that all complexes were able to undergo the MIII/MII oxidation, but none was able to yield a detectable amount of a MIV state in our potential window (up to +2 V vs SHE). This inability was traced to (i) the preference for binding Cl- or acetonitrile instead of water-derived species in the apical position, which excludes redox leveling via proton coupled electron transfer, and (ii) the lack of sigma donor ligands that would stabilize oxidation states beyond MIII. On that basis, design features for next-generation molecular WOCs are suggested.

Four-Step Spin Crossover in a New Cyano-Bridged Iron-Silver Coordination Polymer.

Spin-crossover complexes with multistep transitions attract much attention due to their potential applications as multi-switches and for data storage. A four-step spin crossover is observed in the new iron(II)-based cyanometallic guest-free framework compound Fe(2-ethoxypyrazine)2 {Ag(CN)2 }2 during the transition from the low-spin to the high-spin state. A reverse process occurs in three steps. Crystallographic studies reveal an associated stepwise evolution of the crystal structures. Multiple transitions in the reported complex originate from distinct FeII sites which exist due to the packing of the ligand with a bulky substituent.

1D iron(II)-1,2,4-triazolic chains with spin crossover assembled from discrete trinuclear complexes.

We report on a molecular cationic iron(II) complex with a 4-amino-1,2,4-triazole ligand and a tetraiodomercurate anion exhibiting an incomplete spin crossover (SCO). The complex exhibits an unusual disordered structure with a linear arrangement of ligand and water molecules that can potentially accommodate up to four iron atoms, but both terminal metal positions have half chemical occupancies, while occupancies of all ligands are full. This corresponds to the crystallisation of disordered trinuclear complexes arranged into 1D supramolecular chains. Iron cations have different N6 or N3O3 coordination environments, leading to the thermally induced SCO in two thirds of the metal centres. This SCO behaviour was characterised by magnetic susceptibility measurements and Mössbauer spectroscopy.

Two-Step Spin Crossover in Hofmann-Type Coordination Polymers [Fe(2-phenylpyrazine)2{M(CN)2}2] (M = Ag, Au).

Two 2D Hofmann-type complexes of the composition [Fe(Phpz)2{M(CN)2}2] (where Phpz = 2-phenylpyrazine; M = Ag, Au) have been synthesized, and their spin-crossover (SCO) behavior has been thoroughly characterized. Single-crystal X-ray analysis reveals that these complexes contain a crystallographically unique Fe(II) center surrounded by two axial Phpz ligands and four equatorial cyanide [M(CN)2]- bridges. It is shown that, using of a ligand with two aromatic rings, an advanced system of weak supramolecular interactions (metal-metal, C-H···M, and π···π stacking contacts) is realized. This ensures additional stabilization of the structures and the absence of solvent-accessible voids due to dense packing. Both complexes are characterized by a highly reproducible two-step SCO behavior, as revealed by different techniques (superconducting quantum interference device magnetometry, optical microscopy, etc.). Research shows the exceptional role of the presence of various supramolecular interactions in the structure and the influence of the bulky substituent in the ligand on SCO behavior. Moreover, the perspective of substituted pyrazines for the design of new switchable materials is supported by this work.

Expanding manganese(IV) aqueous chemistry: unusually stable water-soluble hexahydrazide clathrochelate complexes.

Mn cage complexes are rare, and the ones successfully isolated in the solid state are not stable in water and organic solvents. Herein, we present the first report of mononuclear Mn clathrochelates, in which the encapsulated metal exists in the oxidation state +4. The complexes are extremely stable in the crystalline state and in solutions and show rich redox chemistry.

Spin crossover in iron(II) Hofmann clathrates analogues with 1,2,3-triazole.

Hofmann-like cyanometallic complexes represent one of the biggest and well-known classes of FeII spin-crossover compounds. In this paper, we report on the first FeII Hofmann clathrate analogues with unsubstituted 1,2,3-triazole, which exhibit temperature induced spin transition. Two new coordination polymers with the general formula [FeII(1,2,3-triazole)2MII(CN)4] (M = Pt, Pd) undergo abrupt hysteretic spin crossover in the range of 190-225 K as revealed by magnetic susceptibility measurements. Two compounds are isostructural and are built of infinite cyanometallic layers which are supported by 1,2,3-triazole ligands. The thermal hysteresis loop is very stable at different scan rates from 0.5 to 10 K min-1. The compounds display strong thermochromic effect, changing their colour from pink in the low-spin state to white in the high-spin state. Our findings show that 1,2,3-triazole is suitable for elaboration of spin-crossover Hofmann clathrate analogues, and its use instead of more classical azines can advantageously expand this family of complexes.

Hybrid compound based on diethylenetriaminecopper(ii) cations and scarce V-monosubstituted ß-octamolybdate as water oxidation catalyst.

Herein, we report on a new hybrid compound (NH4){[Cu(dien)(H2O)2]2[ß-VMo7O26]}·1.5H2O (1), where dien = diethylenetriamine, containing an extremely rare mixed-metal pseudo-octamolybdate cluster. An ex situ EPR spectroscopy provided insights into the formation of paramagnetic species in reaction mixture and in solution of 1. The magneto-structural correlations revealed weak antiferromagnetic exchange interactions between the [Cu(dien)]2+ cations transmitted by intermolecular pathways. The cyclic voltammetry showed the one-electron process associated with the Cu3+/Cu2+ oxidation followed by the multi-electron catalytic wave due to water oxidation with a faradaic yield of 86%. The title compound was thus employed in homogeneous water oxidation catalysis using tris(bipyridine)ruthenium photosensitizer. At pH 8.0, efficiency of the catalytic system attained 0.19 turnovers per second supported by the relatively mild water oxidation overpotential of 0.54 V.

Electronic and geometric structure effects on one-electron oxidation of first-row transition metals in the same ligand framework.

Developing new transition metal catalysts requires understanding of how both metal and ligand properties determine reactivity. Since metal complexes bearing ligands of the Py5 family (2,6-bis-[(2-pyridyl)methyl]pyridine) have been employed in many fields in the past 20 years, we set out here to understand their redox properties by studying a series of base metal ions (M = Mn, Fe, Co, and Ni) within the Py5OH (pyridine-2,6-diylbis[di-(pyridin-2-yl)methanol]) variant. Both reduced (MII) and the one-electron oxidized (MIII) species were carefully characterized using a combination of X-ray crystallography, X-ray absorption spectroscopy, cyclic voltammetry, and density-functional theory calculations. The observed metal-ligand interactions and electrochemical properties do not always follow consistent trends along the periodic table. We demonstrate that this observation cannot be explained by only considering orbital and geometric relaxation, and that spin multiplicity changes needed to be included into the DFT calculations to reproduce and understand these trends. In addition, exchange reactions of the sixth ligand coordinated to the metal, were analysed. Finally, by including published data of the extensively characterised Py5OMe (pyridine-2,6-diylbis[di-(pyridin-2-yl)methoxymethane])complexes, the special characteristics of the less common Py5OH ligand were extracted. This comparison highlights the non-innocent effect of the distal OH functionalization on the geometry, and consequently on the electronic structure of the metal complexes. Together, this gives a complete analysis of metal and ligand degrees of freedom for these base metal complexes, while also providing general insights into how to control electrochemical processes of transition metal complexes.

Hofmann-Like Frameworks Fe(2-methylpyrazine)n[M(CN)2]2 (M = Au, Ag): Spin-Crossover Defined by the Precious Metal.

Hofmann-like cyanometalates constitute a large class of spin-crossover iron(II) complexes with variable switching properties. However, it is not yet clearly understood how the temperature and cooperativity of a spin transition are influenced by their structure. In this paper, we report the synthesis and crystal structures of the metal-organic coordination polymers {FeII(Mepz)[AuI(CN)2]2} ([Au]) and {FeII(Mepz)2[AgI(CN)2]2} ([Ag]), where Mepz = 2-methylpyrazine, along with characterization of their spin-state behavior by variable-temperature SQUID magnetometry and Mössbauer spectroscopy. The compounds are built of cyanoheterometallic layers, which are pillared by the bridging Mepz ligands in [Au] but separated in [Ag]. The complex [Au] exhibits an incomplete stepped spin transition as a function of the temperature with TSCO1 = 170 K and TSCO2 = 308 K for the two subsequent steps. In contrast, the complex [Ag] attains the high-spin state over the whole temperature range. In the crystal structure of [Ag], weak interlayer contacts (Ag-π, Me-π, and Ag-N) are found that may be responsible for an unusual axial elongation of the FeN6 polyhedra. We propose that this structural distortion contributes to the trapping of iron in its high-spin state.

Spin transition in a ferrous chloride complex supported by a pentapyridine ligand.

Ferrous chloride complexes [FeIILxCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T1/2 = 80 K in [FeII(Py5OH)Cl]+ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.