Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE½ = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency

Close Encounters of the Weak Kind: Investigations of Electron-Electron Interactions between Dissimilar Spins in Hybrid Rotaxanes.

We report a family of hybrid [2]rotaxanes based on inorganic [Cr7NiF8(O2C t Bu)16]- ("{Cr7Ni}") rings templated about organic threads that are terminated at one end with pyridyl groups. These rotaxanes can be coordinated to [Cu(hfac)2] (where Hhfac = 1,1,1,5,5,5-hexafluoroacetylacetone), to give 1:1 or 1:2 Cu:{Cr7Ni} adducts: {[Cu(hfac)2](py-CH2NH2CH2CH2Ph)[Cr7NiF8(O2C t Bu)16]}, {[Cu(hfac)2][py-CH2NH2CH2CH3][Cr7NiF8(O2C t Bu)16]}, {[Cu(hfac)2]([py-CH2CH2NH2CH2C6H4SCH3][Cr7NiF8(O2C t Bu)16])2}, {[Cu(hfac)2]([py-C6H4-CH2NH2(CH2)4Ph][Cr7NiF8(O2C t Bu)16])2}, and {[Cu(hfac)2]([3-py-CH2CH2NH2(CH2)3SCH3][Cr7NiF8(O2C t Bu)16])2}, the structures of which have been determined by X-ray diffraction. The {Cr7Ni} rings and CuII ions both have electronic spin S = 1/2, but with very different g-values. Continuous-wave EPR spectroscopy reveals the exchange interactions between these dissimilar spins, and hence the communication between the different molecular components that comprise these supramolecular systems. The interactions are weak such that we observe AX or AX2 type spectra. The connectivity between the {Cr7Ni} ring and thread terminus is varied such that the magnitude of the exchange interaction J can be tuned. The coupling is shown to be dominated by through-bond rather than through-space mechanisms

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin‐Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [Fe L 2 ][BF 4 ] 2 ∙2H 2 O ( L = 2,6‐ bis ‐[5‐methyl‐1 H ‐pyrazol‐3‐yl]pyridine) affords an anhydrous material, which undergoes a spin‐transition at T ½ = 205 K with a 65 K thermal hysteresis loop. This occurs via a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [Cu L 2 ][BF 4 ] 2 ∙2H 2 O and [Zn L 2 ][BF 4 ] 2 ∙2H 2 O are not perfectly isostructural but, unlike the iron compound, they undergo single‐crystal‐to‐single‐crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase, but undergo a phase change near room temperature upon recooling. The low‐temperature phase of [Cu L 2 ][BF 4 ] 2 involves ordering of its Jahn‐Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low‐temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm the structural changes in the anhydrous zinc complex are reproduced in the high‐spin iron compound, before the onset of spin‐crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper‐doped [Fe 0.97 Cu 0.03 L 2 ][BF 4 ] 2 imply its low spin phase contains two distinct cation environments in a 2:1 ratio

Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes

The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi2(F)(O2CtBu)6]{(BH)[Cr7NiF8(O2CtBu)16]}3 (3) and [CrNi2(F)(O2CtBu)6(THF)]{(BH)[Cr7NiF8(O2CtBu)16]}2 (4), where B = py-CH2CH2NHCH2C6H4SCH3. The [4]-rotaxane 3 is an isosceles triangle of three [2]-rotaxanes bound to the central triangle while the [3]-rotaxane 4 contains only two [2]-rotaxanes bound to the central triangle. Studies of the behavior of 3 and 4 in solution by small-angle X-ray scattering and atomistic molecular dynamic simulations show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. Continuous wave and pulsed electron paramagnetic resonance spectroscopy was used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of conformations

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Structurally diverse mononuclear, dinuclear, and tetranuclear cobalt organophosphates and a three-dimensional framework based on a D4R cobalt phosphate are reported. The role of auxiliary ligands in determining the nuclearity of the phosphate clusters has further been established. Reaction of cobalt acetate tetrahydrate with 2,6-di-iso-propylphenylphosphate (dippH(2)) in methanol or DMSO in the presence of ancillary N-donor ligands leads to the formation of mononuclear octahedral cobalt phosphate [Co(dippH)(2)(py)(4)] (1), dinuclear octahedral cobalt phosphates [Co(dipp)(NN)(MeOH)(2)](2)center dot 2MeOH (NN = bpy 2; phen 3), tetrahedral cobalt phosphates [Co(dipp)(L)(2)](2)center dot 2(MeOH) (L = imz 4; dmpz 5) and symmetric and asymmetric tetranuclear tetrahedral cobalt phosphates [Co(dipp)(2-apy)](4) (6) and [Co-4(dipp)(4)(2-apy)(3)(DMSO)]center dot(DMSO)center dot(H2O) (7), in nearly quantitative yields. The use of a linear N-donor ditopic linker, 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (dptz), as the ancillary ligand leads to the formation of a robust three dimensional, four-fold interpenetrated network based on the D4R platform, {[Co(dipp) (dptz)(0.5)](4)}(n) (8), under ambient conditions. The new compounds have been characterized by analytical, thermo-analytical and spectroscopic techniques. Further, the molecular structures of compounds 1-8 have been established using single crystal X-ray diffraction studies. Compound 1 is a mononuclear complex in which the dippH ligands occupy trans-positions around the octahedral cobalt ion. The core structure of compounds 2-5, a Co2P2O4 ring, resembles the S4R (single-4-ring) SBU of zeolites, whereas the Co4P4O12 inorganic core found in compounds 6 and 7 resembles the D4R (double-4-ring) SBU. Cobalt organophosphate framework 8 shows significant CO2 adsorption at 273 K (7.73 wt% at 1 bar and 18.21 wt% at 15.5 bar) with high selectivity to CO2 uptake over N-2 and H-2 at 273 K. Magnetic studies of these symmetric complexes indicate the presence of weak antiferromagnetic interactions between the metal ions via the phosphate bridging moiety