Probing Spin Crossover in a Solution by Paramagnetic NMR Spectroscopy

Spin transitions in spin-crossover compounds are now routinely studied in the solid state by magnetometry; however, only a few methods exist for studies in solution. The currently used Evans method, which relies on NMR spectroscopy to measure the magnetic susceptibility, requires the availability of a very pure sample of the paramagnetic compound and its exact concentration. To overcome these limitations, we propose an alternative NMR-based technique for evaluating spin-state populations by only using the chemical shifts of a spin-crossover compound; those can be routinely obtained for a solution that contains unknown impurities and paramagnetic admixtures or is contaminated otherwise

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt­(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10^–32 m³ at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt­(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance

Polyhedral Rearrangements in the Complexes of Rhodium and Iridium with Isomeric Carborane Anions [7,8-Me₂‑X-SMe₂‑7,8-<i>nido</i>-C₂B₉H₈]⁻ (X = 9 and 10)

Polyhedral rearrangement of iridacarborane 1,2-Me₂-3,3-(cod)-4-SMe₂-3,1,2<i>-closo</i>-IrC₂B₉H₈ (<b>1</b>) proceeds in a solution at RT and affords a product of 1,2 → 1,7 isomerization 1,8-Me₂-2,2-(cod)-7-SMe₂-2,1,8-<i>closo</i>-IrC₂B₉H₈ (<b>2</b>). Rhodium derivative 1,2-Me₂-3,3-(cod)-4-SMe₂-3,1,2<i>-closo</i>-RhC₂B₉H₈ (<b>3</b>) is significantly more stable toward heating than iridium complex <b>1</b> and isomerizes at 110 °C by 1,2 → 1,2 and 1,2 → 1,7 reaction schemes with 1,2 → 1,2 being the predominant route forming 1,2-Me₂-4,4-(cod)-8-SMe₂-4,1,2-<i>closo-</i>RhC₂B₉H₈ (<b>4</b>) and 1,8-Me₂-2,2-(cod)-7-SMe₂-2,1,8-<i>closo</i>-RhC₂B₉H₈ (<b>5</b>). Complexes <b>4</b> and <b>5</b> were characterized by ¹¹B­{¹H}–¹¹B­{¹H} COSY NMR spectrometry. A mechanism of 1,2 → 1,2 isomerization of <b>3</b> to <b>4</b> was proposed on the basis of DFT calculations. Reaction of the thallium salt Tl­[7,8-Me₂-9-SMe₂-7,8-<i>nido</i>-C₂B₉H₈] (CarbTl) with [Cp*RuCl]₄ in THF furnishes new ruthenacarborane 1,2-Me₂-3-(Cp*)-4-SMe₂-3,1,2<i>-closo</i>-RuC₂B₉H₈ (<b>6</b>). Heating of <b>6</b> at 80 and 144 °C leads to the partial and complete decomposition, respectively. Interaction of CarbTl with [Cp*IrCl₂]₂ in the presence of TlPF₆ provides two new Ir­(III) complexes [1,2-Me₂-3-(Cp*)-4-SMe₂-3,1,2<i>-closo</i>-IrC₂B₉H₈]­PF₆ (<b>7</b>PF₆) and 1,2-Me₂-3-(Cp*)-4-SMe-3,1,2<i>-closo</i>-IrC₂B₉H₈ (<b>8</b>) both stable upon heating in boiling tetrachloroethane (146 °C). A new 10-substituted charge-compensated carborane [7,8-Me₂-10-SMe₂-7,8-<i>nido</i>-C₂B₉H₉] (<b>9</b>) was synthesized via an interaction of dicarbollide dianion [7,8-Me₂-7,8-<i>nido</i>-C₂B₉H₉]^2– with dimethyl sulfide and acetaldehyde in acidic media. Thallium salt of <b>9</b> Tl­[7,8-Me₂-10-SMe₂-7,8-<i>nido</i>-C₂B₉H₈] (<b>10</b>) reacts with [(cod)­IrCl]₂ furnishing positional isomer of <b>1</b> with a symmetrical emplacement of SMe₂ substituent complex 1,2-Me₂-3,3-(cod)-8-SMe₂-3,1,2-<i>closo</i>-IrC₂B₉H₈ (<b>11</b>). Iridacarborane <b>11</b> requires elevated temperatures to undergo cage isomerization and converts upon heating at 110 °C to two new compounds 1,8-Me₂-2,2-(cod)-11-SMe₂-2,1,8-<i>closo</i>-IrC₂B₉H₈ (<b>12</b>) and 1,2-Me₂-4,4-(cod)-9-SMe₂-4,1,2-<i>closo-</i>IrC₂B₉H₈ (<b>13</b>) as a result of 1,2 → 1,7 and 1,2 → 1,2 rearrangements respectively with <b>13</b> being the minor product. The structures of <b>2</b>, <b>6</b>, <b>7</b>PF₆, <b>8</b>, <b>10</b>, <b>12</b>, and <b>13</b> were determined by single-crystal X-ray diffraction

Spin-Crossover Anticooperativity Induced by Weak Intermolecular Interactions

As a rule, rational design of cooperative spin-crossover (SCO) molecular switches is largely based on consideration of sizes and structures of individual building blocks, whereas a meticulous analysis of crystal packing, including the weakest intermolecular interactions, is often assumed to play a secondary role or is even fully neglected. By investigating cobalt­(II) clathrochelates, which do not change the molecular volume upon SCO, we showed that even weak (1.2 kcal/mol) π···Cl intermolecular interactions can cause a pronounced anticooperativity of SCO, being more gradual in the solid state than in solution. Our results clearly demonstrate that the “chemical pressure” concept is not as general as it is thought to be, and the successful design of molecular switches requires in-depth analysis of intermolecular interactions, however weak they seem

A Trigonal Prismatic Mononuclear Cobalt(II) Complex Showing Single-Molecule Magnet Behavior

Single-molecule magnets (SMMs) with one transition-metal ion often rely on unusual geometry as a source of magnetically anisotropic ground state. Here we report a cobalt­(II) cage complex with a trigonal prism geometry showing single ion magnet behavior with very high Orbach relaxation barrier of 152 cm^–1. This, to our knowledge, is the largest reported relaxation barrier for a cobalt-based mononuclear SMM. The trigonal prismatic coordination provided by the macrocyclic ligand gives intrinsically more stable molecular species than previously reported SMMs, thus making this type of cage complexes more amendable to possible functionalization that will boost their magnetic anisotropy even further

Mechanism of Dimethylamine–Borane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex

The title complex (^<i>t</i>BuPCP)­IrH­(Cl) (<b>1</b>; ^<i>t</i>BuPCP = κ³-2,6-(CH₂P<i>t</i>Bu₂)₂C₆H₃) appeared to be moderately active in NHMe₂·BH₃ (DMAB) dehydrogenation, allowing the systematic spectroscopic (variable-temperature NMR and IR) investigation of the reaction intermediates and products, under both stoichiometric and catalytic regimes, combined with DFT/M06 calculations. The formation of the hexacoordinate complex (^tBuPCP)­IrHCl­(η¹-BH₃·NHMe₂) (<b>3</b>) stabilized by a NH···Cl hydrogen bond is shown experimentally at the first reaction step. This activates both B–H and Ir–Cl bonds, initiating the precatalyst activation and very first DMAB dehydrogenation cycle. The same geometry is suggested by the DFT calculations for the key intermediate of the catalytic cycle, (^tBuPCP)­IrH₂(η¹-BH₃·NHMe₂) complex (<b>6</b>). In these complexes, DMAB is coordinated trans to the ipso carbon, allowing the steric repulsion between the amine–borane and <i>tert</i>-butyl groups at the phosphorus atoms to be overcome. Under catalytic conditions (2–5 mol % of <b>1</b>) the hydride complex (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) was identified, which is not a dormant catalytic species but the steady-state intermediate formed as a result of the B–N bond breaking. DMAB dehydrogenation yields the borazane monomer H₂BNMe₂ (detected by ¹¹B NMR); dimerization of this species gives the final product [H₂BNMe₂]₂ and (^tBuPCP)­IrH₄ as the catalyst resting state. The scenario of B–N bond cleavage in DMAB leading to byproducts of dehydrogenation such as bis­(dimethylamino)­hydroborane and (^tBuPCP)­IrH­(μ²-H₂BH₂) (<b>5</b>) is proposed. The results obtained allow us to suggest the mechanism of catalytic DMAB dehydrocoupling that could be generalized to other processes

Steric and Acidity Control in Hydrogen Bonding and Proton Transfer to <i>trans-</i>W(N₂)₂(dppe)₂

The interaction of <i>trans-</i>W­(N₂)₂(dppe)₂ (<b>1</b>; dppe = 1,2-bis­(diphenylphosphino)­ethane) with relatively weak acids (<i>p</i>-nitrophenol, fluorinated alcohols, CF₃COOH) was studied by means of variable temperature IR and NMR spectroscopy and complemented by DFT/B3PW91-D3 calculations. The results show, for the first time, the formation of a hydrogen bond to the coordinated dinitrogen, W–NN···H–O, that is preferred over H-bonding to the metal atom, W···H–O, despite the higher proton affinity of the latter. Protonation of the core metalthe undesirable side step in the conversion of N₂ to NH₃can be avoided by using weaker and, more importantly, bulkier acids

Chloride Ion-Aided Self-Assembly of Pseudoclathrochelate Metal Tris-pyrazoloximates

Chloride ion-aided one-pot template self-assembly of a mixed pyrazoloxime ligand with phenylboronic acid on a corresponding metal­(II) ion as a matrix afforded the first boron-capped zinc, cobalt, iron, and manganese pseudoclathrochelate tris-pyrazoloximates. The presence of a pseudocross-linking hydrogen-bonded chloride ion is critical for their formation, as the same chloride-capped complexes were isolated even in the presence of large excesses of bromide and iodide ions. As revealed by X-ray diffraction, all complexes are capped with a chloride ion via three N–H···Cl hydrogen bonds that stabilize their pseudomacrobicyclic frameworks. The MN₆ coordination polyhedra possess a distorted trigonal prismatic geometry, with the distortion angles φ between their nonequivalent N₃ bases of approximately 0°. Temperature dependences of the effective magnetic moment for the paramagnetic complexes showed the encapsulated metal­(II) ions to be in a high-spin state in the temperature range of 2–300 K. In the case of the iron­(II) pseudoclathrochelate, density functional theory (DFT) and time-dependent DFT calculations were used to assess its spin state as well as the ⁵⁷Fe Mössbauer and UV–vis–NIR parameters. Cyclic voltammetry studies performed for these pseudomacrobicyclic complexes showed them to undergo irreversible or quasi-reversible metal-localized oxidations and reductions. As no changes are observed in the presence of a substantial excess of bromide ion, no anion-exchange reaction occurs, and thus the pseudoclathrochelates have a high affinity toward chloride anions in solution

A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents

The study tackles one of the challenges in developing platinum-free molecular electrocatalysts for hydrogen evolution, which is to seek for new possibilities to ensure large turnover numbers by stabilizing electrocatalytic intermediates. These species are often much more reactive than the initial electrocatalysts, and if not properly stabilized by a suitable choice of functionalizing substituents, they have a limited long-time activity. Here, we describe new iron and cobalt­(II) cage complexes (clathrochelates) that in contrast to many previously reported complexes of this type do not act as electrocatalysts for hydrogen evolution. We argue that the most probable reason for this behavior is an excessive stabilization of the metal­(I) species by perfluoroaryl ribbed groups, resulting in an unprecedented long-term stability of the metal­(I) complexes even in acidic solutions