9 research outputs found
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 cobaltĀ(II) cage complexes
with the largest (among the transition-metal complexes) axial anisotropy
of magnetic susceptibility, reaching as high as 12.6 Ć 10<sup>ā32</sup> m<sup>3</sup> 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 cobaltĀ(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-Me<sub>2</sub>āX-SMe<sub>2</sub>ā7,8-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>8</sub>]<sup>ā</sup> (X = 9 and 10)
Polyhedral
rearrangement of iridacarborane 1,2-Me<sub>2</sub>-3,3-(cod)-4-SMe<sub>2</sub>-3,1,2<i>-closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>1</b>) proceeds in a solution at RT and affords
a product of 1,2 ā 1,7 isomerization 1,8-Me<sub>2</sub>-2,2-(cod)-7-SMe<sub>2</sub>-2,1,8-<i>closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>2</b>). Rhodium derivative 1,2-Me<sub>2</sub>-3,3-(cod)-4-SMe<sub>2</sub>-3,1,2<i>-closo</i>-RhC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>3</b>) is significantly
more stable toward heating than iridium complex <b>1</b> and
isomerizes at 110 Ā°C by 1,2 ā 1,2 and 1,2 ā 1,7
reaction schemes with 1,2 ā 1,2 being the predominant route
forming 1,2-Me<sub>2</sub>-4,4-(cod)-8-SMe<sub>2</sub>-4,1,2-<i>closo-</i>RhC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>4</b>) and 1,8-Me<sub>2</sub>-2,2-(cod)-7-SMe<sub>2</sub>-2,1,8-<i>closo</i>-RhC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>5</b>). Complexes <b>4</b> and <b>5</b> were characterized
by <sup>11</sup>BĀ{<sup>1</sup>H}ā<sup>11</sup>BĀ{<sup>1</sup>H} COSY NMR spectrometry. A mechanism of 1,2 ā 1,2 isomerization
of <b>3</b> to <b>4</b> was proposed on the basis of DFT
calculations. Reaction of the thallium salt TlĀ[7,8-Me<sub>2</sub>-9-SMe<sub>2</sub>-7,8-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>8</sub>] (CarbTl) with [Cp*RuCl]<sub>4</sub> in THF furnishes new
ruthenacarborane 1,2-Me<sub>2</sub>-3-(Cp*)-4-SMe<sub>2</sub>-3,1,2<i>-closo</i>-RuC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>6</b>). Heating of <b>6</b> at 80 and 144 Ā°C leads
to the partial and complete decomposition, respectively. Interaction
of CarbTl with [Cp*IrCl<sub>2</sub>]<sub>2</sub> in the presence of
TlPF<sub>6</sub> provides two new IrĀ(III) complexes [1,2-Me<sub>2</sub>-3-(Cp*)-4-SMe<sub>2</sub>-3,1,2<i>-closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub>]ĀPF<sub>6</sub> (<b>7</b>PF<sub>6</sub>) and 1,2-Me<sub>2</sub>-3-(Cp*)-4-SMe-3,1,2<i>-closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>8</b>) both
stable upon heating in boiling tetrachloroethane (146 Ā°C). A
new 10-substituted charge-compensated carborane [7,8-Me<sub>2</sub>-10-SMe<sub>2</sub>-7,8-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>9</sub>] (<b>9</b>) was synthesized via an interaction
of dicarbollide dianion [7,8-Me<sub>2</sub>-7,8-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>9</sub>]<sup>2ā</sup> with
dimethyl sulfide and acetaldehyde in acidic media. Thallium salt of <b>9</b> TlĀ[7,8-Me<sub>2</sub>-10-SMe<sub>2</sub>-7,8-<i>nido</i>-C<sub>2</sub>B<sub>9</sub>H<sub>8</sub>] (<b>10</b>) reacts
with [(cod)ĀIrCl]<sub>2</sub> furnishing positional isomer of <b>1</b> with a symmetrical emplacement of SMe<sub>2</sub> substituent
complex 1,2-Me<sub>2</sub>-3,3-(cod)-8-SMe<sub>2</sub>-3,1,2-<i>closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>11</b>). Iridacarborane <b>11</b> requires elevated temperatures
to undergo cage isomerization and converts upon heating at 110 Ā°C
to two new compounds 1,8-Me<sub>2</sub>-2,2-(cod)-11-SMe<sub>2</sub>-2,1,8-<i>closo</i>-IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>12</b>) and 1,2-Me<sub>2</sub>-4,4-(cod)-9-SMe<sub>2</sub>-4,1,2-<i>closo-</i>IrC<sub>2</sub>B<sub>9</sub>H<sub>8</sub> (<b>13</b>) as a result of 1,2 ā 1,7 and 1,2 ā
1,2 rearrangements respectively with <b>13</b> being the minor
product. The structures of <b>2</b>, <b>6</b>, <b>7</b>PF<sub>6</sub>, <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 cobaltĀ(II) clathrochelates, which do not change the
molecular volume upon SCO, we showed that even weak (1.2 kcal/mol)
ĻĀ·Ā·Ā·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 āchemical
pressureā 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 cobaltĀ(II) cage complex with a trigonal prism
geometry showing single ion magnet behavior with very high Orbach
relaxation barrier of 152 cm<sup>ā1</sup>. 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 DimethylamineāBorane Dehydrogenation Catalyzed by an Iridium(III) PCP-Pincer Complex
The
title complex (<sup><i>t</i>Bu</sup>PCP)ĀIrHĀ(Cl) (<b>1</b>; <sup><i>t</i>Bu</sup>PCP = Īŗ<sup>3</sup>-2,6-(CH<sub>2</sub>P<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) appeared to be moderately active in NHMe<sub>2</sub>Ā·BH<sub>3</sub> (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 (<sup>tBu</sup>PCP)ĀIrHClĀ(Ī·<sup>1</sup>-BH<sub>3</sub>Ā·NHMe<sub>2</sub>) (<b>3</b>) stabilized
by a NHĀ·Ā·Ā·Cl hydrogen bond is shown experimentally
at the first reaction step. This activates both BāH and IrāCl
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, (<sup>tBu</sup>PCP)ĀIrH<sub>2</sub>(Ī·<sup>1</sup>-BH<sub>3</sub>Ā·NHMe<sub>2</sub>)
complex (<b>6</b>). In these complexes, DMAB is coordinated
trans to the ipso carbon, allowing the steric repulsion between the
amineāborane and <i>tert</i>-butyl groups at the
phosphorus atoms to be overcome. Under catalytic conditions (2ā5
mol % of <b>1</b>) the hydride complex (<sup>tBu</sup>PCP)ĀIrHĀ(Ī¼<sup>2</sup>-H<sub>2</sub>BH<sub>2</sub>) (<b>5</b>) was identified,
which is not a dormant catalytic species but the steady-state intermediate
formed as a result of the BāN bond breaking. DMAB dehydrogenation
yields the borazane monomer H<sub>2</sub>Bī»NMe<sub>2</sub> (detected
by <sup>11</sup>B NMR); dimerization of this species gives the final
product [H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub> and (<sup>tBu</sup>PCP)ĀIrH<sub>4</sub> as the catalyst resting state. The scenario of
BāN bond cleavage in DMAB leading to byproducts of dehydrogenation
such as bisĀ(dimethylamino)Āhydroborane and (<sup>tBu</sup>PCP)ĀIrHĀ(Ī¼<sup>2</sup>-H<sub>2</sub>BH<sub>2</sub>) (<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(N<sub>2</sub>)<sub>2</sub>(dppe)<sub>2</sub>
The
interaction of <i>trans-</i>WĀ(N<sub>2</sub>)<sub>2</sub>(dppe)<sub>2</sub> (<b>1</b>; dppe = 1,2-bisĀ(diphenylphosphino)Āethane)
with relatively weak acids (<i>p</i>-nitrophenol, fluorinated
alcohols, CF<sub>3</sub>COOH) 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, WāNī¼NĀ·Ā·Ā·HāO,
that is preferred over H-bonding to the metal atom, WĀ·Ā·Ā·HāO,
despite the higher proton affinity of the latter. Protonation of the
core metalīøthe undesirable side step in the conversion of N<sub>2</sub> to NH<sub>3</sub>īøcan 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 metalĀ(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
NāHĀ·Ā·Ā·Cl hydrogen bonds that stabilize their
pseudomacrobicyclic frameworks. The MN<sub>6</sub> coordination polyhedra
possess a distorted trigonal prismatic geometry, with the distortion
angles Ļ between their nonequivalent N<sub>3</sub> bases of
approximately 0Ā°. Temperature dependences of the effective magnetic
moment for the paramagnetic complexes showed the encapsulated metalĀ(II)
ions to be in a high-spin state in the temperature range of 2ā300
K. In the case of the ironĀ(II) pseudoclathrochelate, density functional
theory (DFT) and time-dependent DFT calculations were used to assess
its spin state as well as the <sup>57</sup>Fe MoĢssbauer and
UVāvisāNIR 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,Ā 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
cobaltĀ(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 metalĀ(I) species
by perfluoroaryl ribbed groups, resulting in an unprecedented long-term
stability of the metalĀ(I) complexes even in acidic solutions