Doublet Ground State in a Vanadium(II) Complex: Redox and Coordinative Noninnocence of Tripodal Ligand Architecture.

We report on the geometric and electronic structures of a series of V2+/3+ tren-bridged iminopyridine complexes [tren = tris(2-aminoethyl)amine] that enable the observation of an unexpected doublet ground state for a nominally 3d3 species. Tren undergoes condensation reactions with picolinaldehyde or methyl-6-formylnictonate to form the respective tripodal ligand sets of (py)3tren and (5-CO2Mepy)3tren. The (py)3tren ligand is coordinated to V2+ and V3+ metal centers to form complex salts [1-H](OTf)2 and [1-H](OTf)3, respectively (OTf- = CF3SO3-). For [1-H]2+, strong metal-ligand π-covalency with respect to the V2+ (3d3) and iminopyridine ligands weakens its interelectronic repulsion. For [1-H]3+, the bridgehead nitrogen of the tren scaffold forms a seventh coordinate covalent bond with a V3+ (3d2) metal center. The coordination of (5-CO2Mepy)3tren to a V2+ metal center results in the redox noninnocent and heptacoordinate compound [1-CO2Me](OTf)2 with a doublet (S = 1/2) ground state that we support with magnetic susceptibility and spectroscopy measurements. The complexes are uniformly characterized experimentally with single-crystal X-ray diffraction, electronic absorbance, and electrochemistry, and electronic structures are corroborated by computational techniques. We present a new computational procedure that we term the spin-optimized approximate pair (SOAP) method that enables the visualization and quantification of electron-electron interactions.

Isotopomeric Elucidation of the Mechanism of Temperature Sensitivity in 59Co NMR Molecular Thermometers.

Understanding the mechanisms governing temperature-dependent magnetic resonance properties is essential for enabling thermometry via magnetic resonance imaging. Herein we harness a new molecular design strategy for thermometry─that of effective mass engineering via deuteration in the first coordination shell─to reveal the mechanistic origin of 59Co chemical shift thermometry. Exposure of [Co(en)3]3+ (1; en = ethylenediamine) and [Co(diNOsar)]3+ (2; diNOsar = dinitrosarcophagine) to mixtures of H2O and D2O produces distributions of [Co(en)3]3+-dn (n = 0-12) and [Co(diNOsar)]3+-dn (n = 0-6) isotopomers all resolvable by 59Co NMR. Variable-temperature 59Co NMR analyses reveal a temperature dependence of the 59Co chemical shift, Δδ/ΔT, on deuteration of the N-donor atoms. For 1, deuteration amplifies Δδ/ΔT by 0.07 ppm/°C. Increasing degrees of deuteration yield an opposing influence on 2, diminishing Δδ/ΔT by -0.07 ppm/°C. Solution-phase Raman spectroscopy in the low-frequency 200-600 cm-1 regime reveals a red shift of Raman-active Co-N6 vibrational modes by deuteration. Analysis of the normal vibrational modes shows that Raman modes produce the largest variation in 59Co δ. Finally, partition function analysis of the Raman-active modes shows that increased populations of Raman modes predict greater Δδ/ΔT, representing new experimental insight into the thermometry mechanism.

Electronic Structures of Cr(III) and V(II) Polypyridyl Systems: Undertones in an Isoelectronic Analogy.

A recently reported description of the photophysical properties of V2+ polypyridyl systems has highlighted several distinctions between isoelectronic, d3, Cr3+, and V2+ tris-homoleptic polypyridyl complexes of 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen). Here, we combine theory and experimental data to elucidate the differences in electronic structures. We provide the first crystallographic structures of the V2+ complexes [V(bpy)3](BPh4)2 (V-1B) and [V(phen)3](OTf)2 (V2) and observe pronounced trigonal distortion relative to analogous Cr3+ complexes. We use electronic absorption spectroscopy in tandem with TD-DFT computations to assign metal-ligand charge transfer (MLCT) properties of V-1B and V2 that are unique from the intraligand transitions, 4(3IL), solely observed in Cr3+ analogues. Our newly developed natural transition spin density (NTρα,ß) plots characterize both the Cr3+ and V2+ absorbance properties. A multideterminant approach to DFT assigns the energy of the 2E state of V-1B as stabilized through electron delocalization. We find that the profound differences in excited state lifetimes for Cr3+ and V2+ polypyridyls arise from differences in the characters of their lowest doublet states and pathways for intersystem crossing, both of which stem from trigonal structural distortion and metal-ligand π-covalency.

Hydrogen Bond Enhanced Halogen Bonds: A Synergistic Interaction in Chemistry and Biochemistry.

The halogen bond (XB) has become an important tool for molecular design in all areas of chemistry, including crystal and materials engineering and medicinal chemistry. Its similarity to the hydrogen bond (HB) makes the relationship between these interactions complex, at times competing against and other times orthogonal to each other. Recently, our two laboratories have independently reported and characterized a synergistic relationship, in which the XB is enhanced through direct intramolecular HBing to the electron-rich belt of the halogen. In one study, intramolecular HBing from an amine polarizes the iodopyridinium XB donors of a bidentate anion receptor. The resulting HB enhanced XB (or HBeXB) preorganizes and further augments the XB donors. Consequently, the affinity of the receptor for halogen anions was significantly increased. In a parallel study, a meta-chlorotyrosine was engineered into T4 lysozyme, resulting in a HBeXB that increased the thermal stability and activity of the enzyme at elevated temperatures. The crystal structure showed that the chlorine of the noncanonical amino acid formed a XB to the protein backbone, which augmented the HB of the wild-type enzyme. Calorimetric analysis resulted in an enthalpic contribution of this Cl-XB to the stability of the protein that was an order of magnitude greater than previously determined in biomolecules. Quantum mechanical (QM) calculations showed that rotating the hydroxyl group of the tyrosine to point toward rather than away from the halogen greatly increased its potential to serve as a XB donor, equivalent to what was observed experimentally. In sum, the two systems described here show that the HBeXB concept extends the range of interaction energies and geometries to be significantly greater than that of the XB alone. Additionally, surveys of structural databases indicate that the components for this interaction are already present in many existing molecular systems. The confluence of the independent studies from our two laboratories demonstrates the reach of the HBeXB across both chemistry and biochemistry and that intentional engineering of this enhanced interaction will extend the applications of XBs beyond these two initial examples.

Long-Lived Mixed 2MLCT/MC States in Antiferromagnetically Coupled d3 Vanadium(II) Bipyridine and Phenanthroline Complexes.

Exploration of [V(bpy)3]2+ and [V(phen)3]2+ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4MLCT ← 4GS (ground-state) absorption band (ε400-700 nm = 900-8000 M-1 cm-1), the 4MLCT state rapidly (τisc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)3]2+ and [V(phen)3]2+, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)3]3+, which rapidly forms a long-lived doublet metal-centered (2MC) state following photoexcitation and lacks strong visible GS absorption character. 2MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)3]3+, where the GS and 2MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2MLCT state at lower energy than the 4MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.

Protobranching as repulsion-induced attraction: a prototype for geminal stabilization.

Noncovalent interactions are traditionally defined within the context of their attractive components, such as electrostatics and dispersion. Sources of molecular strain are derived through the destabilization of Coulombic and exchange repulsion. Due to this binary designation, the underlying origin of geminal stability with respect to alkanes (referred to as protobranching) has been an active subject for debate between these competing perspectives. We recast this stabilization as a complementary (Gestalt) interaction between dispersion and exchange repulsion, each impacting the other. We use triplet hydrogen and argon dimer as foundational van der Waals adducts to develop a procedure for the visualization and quantification of both exchange repulsion, ΔρSCF, and medium-range correlation, ΔΔρ, as perturbations in electron density. We use the framework of the DFT-D3 correction to reproduce the shape of the dispersion potential at medium range and successfully model the trend in stability for the eighteen isomers of octane with a diverse series of functionals: BLYP, B3LYP, BP86, PBE, and PBE0. Collectively, our findings show that protobranching is a manifestation of steric repulsion-reduction in vibrational enthalpy and medium-range electron correlation.

Evidence for Reagent-Induced Spin-State Switching in Tripodal Fe(II) Iminopyridine Complexes.

We present evidence of a spin-state change that accompanies desilylation reactions performed on two related Fe(II) iminopyridine coordination complexes. To probe these systems, we performed titrations with CsF in solution and analyzed the speciation with in situ magnetometry, electrochemistry, and mass spectrometry techniques. We find that pendant tert-butyldimethylsilyl groups are readily cleaved under these conditions, and the resulting desilylated complexes exhibit overall decreased solution magnetic susceptibility values. Density functional theory and ab initio computations probe the impact of substituent identity (prior to- and post-desilylation) on the metal-ligand σ-donor and π-acceptor bonding properties. We attribute the observed spin-state changes to the decrease in entropy associated with the conformational freedom of the silylated high-spin complex, resulting in a more favored low-spin state upon desilylation.

The Mechanism of C-H Bond Oxidation by Aqueous Permanganate.

The permanganate ion (MnO4-) has been widely used as a reagent for water treatment for over a century. It is a strong enough oxidant to activate carbon-hydrogen bonds, one of the most important reactions in biological and chemical systems. Our current textbook understanding of the oxidation mechanism in aqueous solution involves an initial, rate-limiting hydride abstraction by permanganate followed by reaction of the carbocation with bulk water to form an alcohol. This mechanism fits well into the classic oxidation sequence of alkane → alcohol → aldehyde → carboxylate, the central paradigm for both abiotic and biotic alkane oxidation in aqueous environments. In this study, we provide three lines of evidence through (1) a broken-symmetry density functional theory approach, (2) isotope labeling experiments, and (3) kinetic network modeling to demonstrate that aqueous permanganate can circumvent prior alcohol formation and produce aldehydes directly via a reaction path that bifurcates after the initial transition state. In contrast to classic transition state theory, the rate-limiting step is found to not determine product distribution, bearing critical implications for pathway and rate predictions. This complex reaction network provides new insights into the oxidation mechanisms of organic compounds involving transition metal complexes as well as enzyme or metal oxide surface active sites.

Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States.

Halogen substitution at the 6 and 6″ positions of terpyridine (6,6″-Cl2-2,2:6',2″-terpyridine = dctpy) is used to produce a room-temperature high-spin iron(II) complex [Fe(dctpy)2](BF4)2. Using UV-vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state ((5)MLCT) can be accessed via visible light absorption and that the thermalized (5,7)MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the (1,3)MLCT within species such as [Fe(bpy)3](2+). This result opens a new strategy for extending iron(II) MLCT lifetimes for potential use in photoredox processes.

Uncovering the Roles of Oxygen in Cr(III) Photoredox Catalysis.

A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels-Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3](3+), where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr(3+)* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 µs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels-Alder cycloadduct radical cation to the final product and reforms oxygen. We compare the results of these studies with those from cycloadditions mediated by related Ru(II)-containing complexes and find that the distinct reaction pathways are likely part of a unified mechanistic framework where the photophysical and photochemical properties of the catalyst species lead to oxygen-mediated photocatalysis for the Cr-containing complex but radical chain initiation for the Ru congener. These results provide insight into how oxygen can participate as a sustainable reagent in photocatalysis.

Syntheses and photophysical investigations of Cr(III) hexadentate iminopyridine complexes and their tris(bidentate) analogues.

We report the preparation, photophysical characterization, and computed excited state energies for a family of Cr(III) complexes based on iminopyridine (impy) Schiff base ligands: compounds 1 and 2 feature hexadentate ligands where tren (tris-(2-aminoethyl)amine) caps three impy groups; compounds 3 and 4 are tris(bidentate) analogues of 1 and 2; compounds 2 and 4 contain methyl ester substituents to alter ligand donation properties relative to 1 and 3, respectively. Cyclic voltammograms exhibit multiple reversible ligand-based reductions; the hexadentate and tris(bidentate) analogues have almost identical reduction potentials, and the addition of ester substituents shifts reduction potentials by +200 mV. The absorption spectra of the hexadentate complexes show improved absorption of visible light compared to the tris(bidentate) analogues. Over periods of several hours to days, the complexes undergo ligand-substitution-based decomposition in 1 M HCl((aq)) and acetonitrile. For freshly prepared sample solutions in CH(3)CN, time-resolved emission and transient absorption measurements for 4 show a doublet excited state with 17-19 µs lifetime at room temperature, while no emission or transient absorption signals from the doublet states are observed for the hexadentate analogue 2 under the same conditions. The electronic structure contributions to the differences in observed photophysical properties are compared by extensive computational analyses (UB3LYP MD-DFT and TD-DFT-NTO). These studies indicate that the presence of nonligated bridgehead nitrogen atoms for 1 and 2 significantly reduce excited state doublet, quartet, and sextet energies and change the character of the low lying doublet states in comparison to species that show population of doublet excited states.

Dicyanamide Anion Reports on Water Induced Local Structural and Dynamic Heterogeneity in Ionic Liquid Mixtures.

The effects of limited amounts (under 21.6% χWater) of water on 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and 1-butyl-3-methylimidazolium dicyanamide (BmimDCA) room-temperature ionic liquid (RTIL) mixtures were characterized by tracking changes in the linear and two-dimensional infrared (2D IR) vibrational features of the dicyanamide anion (DCA). Peak shifts with increasing water suggest the formation of water-associated and nonwater-associated DCA populations. Further results showed clear differences in the dynamic behavior of these different populations of DCA at low (defined here as below 2.5% χWater), mid (defined here as between 2.5% χWater and 9.6% χWater), and high (defined here as between 11.6% χWater and 21.6% χWater) range water concentrations. Vibrational relaxation is accelerated with increasing water content for water-associated populations of DCA, indicating water facilitates population relaxation, possibly through the provision of additional bath modes. Conversely, spectral diffusion of water-associated populations slowed dramatically with increasing water, suggesting that water drives the formation of distinct and noninterchangeable or very slowly interchangeable local solvent environments.

Structural and electronic comparison of 1st row transition metal complexes of a tripodal iminopyridine ligand.

We report the preparation and characterization of a series of divalent 3d transition metal complexes (Cr to Zn, 1-7), featuring the multidentate, tripodal iminopyridine Schiff-base ligand trimethyl 6,6',6″-((1E,1'E,1″E)-((nitrilotris(ethane-2,1-diyl))tris(azanylylidene))tris-(methanylylidene))trinicotinate (L(5-OOMe)). X-ray structural studies carried out on 1-5 and 7 reveal complex geometries ranging from local octahedral coordination to significant distortion toward trigonal prismatic geometry to heptacoordinate environments. Regardless of coordination mode, magnetic and spectroscopic studies show the ligand to provide moderately strong ligand fields: the Fe complex is low-spin, while the Co and Mn complexes are high-spin at all temperatures probed. Cyclic voltammograms exhibit multiple reversible ligand-based reductions, which are relatively consistent throughout the series; however, the electrochemical behavior of the Cr complex 1 is fundamentally different from those of the other complexes. Time-dependent (TD) density functional theory (DFT) and natural transition orbital (NTO) computational analyses are presented for the ligand, its anion, and complexes 1-7: the computed spectra reproduce the major differential features of the observed visible absorption spectra, and NTOs provide viable interpretations for the observed features. The combined studies indicate that all complexes contain neutral ligands bound to M(II) ions, except for the Cr complex 1, which is best described as a Cr(III) species bound to a radical anionic ligand.

Topological and electronic influences on magnetic exchange coupling in Fe(III) ethynylbenzene dendritic building blocks.

Significant variance in the magnitude of reported exchange coupling parameters (both experimental and computed) for paramagnetic transition metal-ethynylbenzene complexes suggests that nuances of the magnetostructural relationship in this class of compounds remain to be understood and controlled, toward maximizing the stability of high-spin ground states. We report the preparation, electrochemical behavior, magnetic properties, and results of computational investigations of a series of iron ethynylbenzene complexes with coordination environments suitable for metallodendrimer assembly: [(dmpe)(2)FeCl(C(2)Ph)](OTf) (1), [(dmpe)(4)Fe(2)Cl(2)(µ-p-DEB)](BAr(F)(4))(2) (2), [(dmpe)(6)Fe(3)Cl(3)(TEB)] (3), [(dmpe)(6)Fe(3)Cl(3)(µ(3)-TEB)](OTf)(3) (4), and [(dmpe)(4)Fe(2)Cl(2)(µ-m-DEB)](BAr(F)(4))(2) (5) [dmpe = 1,2-bis(dimethylphosphino)ethane; p-H(2)DEB = 1,4-diethynylbenzene; BAr(F)(4) = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; H(3)TEB = 1,3,5-triethynylbenzene; m-H(2)DEB = 1,3-diethynylbenzene]. As expected, the ligand topology drives the antiferromagnetic coupling in 2 (J = -134 cm(-1) using the H = -2JS(1)·S(2) convention) and the ferromagnetic coupling in 4 and 5 (J = +37 cm(-1), J' = +5 cm(-1) for 4; J = +11 cm(-1) for 5); the coupling is comparable to but deviates significantly from values reported for related Cp*-containing species (Cp* = η(5)-C(5)Me(5)). The origins of these differences are explored computationally: a density functional theory (DFT) approach for treating the coupling of three spin centers as a linear combination of single-determinantal descriptions is developed and described, and the results of these computations can be generalized to other paramagnetic systems. Unrestricted B3LYP hybrid DFT calculations performed on rotamers of 4 and 5 and related complexes, as well as Cp* analogues, provide J values that correlate with the experimental values. We find that geometric considerations dominate the magnetism of the Cp* complexes, while topology and alkynyl ligand electronics combine more subtly to drive the magnetism of the new complexes reported here. These calculations imply that substantial magnetic exchange parameters, with accompanying well-isolated high-spin ground states, are achievable for ethynylbenzene-bridged paramagnetic metallodendrimers.

A Reduced Generalized Force Field for Biological Halogen Bonds.

The halogen bond (or X-bond) is a noncovalent interaction that is increasingly recognized as an important design tool for engineering protein-ligand interactions and controlling the structures of proteins and nucleic acids. In the past decade, there have been significant efforts to characterize the structure-energy relationships of this interaction in macromolecules. Progress in the computational modeling of X-bonds in biological molecules, however, has lagged behind these experimental studies, with most molecular mechanics/dynamics-based simulation methods not properly treating the properties of the X-bond. We had previously derived a force field for biological X-bonds (ffBXB) based on a set of potential energy functions that describe the anisotropic electrostatic and shape properties of halogens participating in X-bonds. Although fairly accurate for reproducing the energies within biomolecular systems, including X-bonds engineered into a DNA junction, the ffBXB with its seven variable parameters was considered to be too unwieldy for general applications. In the current study, we have generalized the ffBXB by reducing the number of variables to just one for each halogen type and show that this remaining electrostatic variable can be estimated for any new halogenated molecule through a standard restricted electrostatic potential calculation of atomic charges. In addition, we have generalized the ffBXB for both nucleic acids and proteins. As a proof of principle, we have parameterized this reduced and more general ffBXB against the AMBER force field. The resulting parameter set was shown to accurately recapitulate the quantum mechanical landscape and experimental interaction energies of X-bonds incorporated into DNA junction and T4 lysozyme model systems. Thus, this reduced and generalized ffBXB is more readily adaptable for incorporation into classical molecular mechanics/dynamics algorithms, including those commonly used to design inhibitors against therapeutic targets in medicinal chemistry and materials in biomolecular engineering.

Experimental evidence for magnetic exchange in di- and trinuclear uranium(IV) ethynylbenzene complexes.

We report the preparation and magnetic property investigations of a structurally related family of mono-, di-, and trinuclear U(IV) aryl acetylide complexes. The reaction between [(NN'(3))UCl] and lithiated aryl acetylides leads to the formation of the hexacoordinate complexes [(NN'(3))U(CCPh)(2)(Li.THF)] (1) and [(NN'(3))(2)U(2)(p-DEB)(THF)] (2) as red-brown and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN'(3))U] (bit-NN'(3) = [N(CH(2)CH(2)NSi(t)BuMe(2))(2)(CH(2)CH(2)Si(t)BuMeCH(2)]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN'(3))U(CCPh)] (3), [(NN'(3))(2)U(2)(m-DEB)] (4), [(NN'(3))(2)U(2)(p-DEB)] (5), and [(NN'(3))(3)U(3)(TEB)] (6), where NN'(3) = [N(CH(2)CH(2)NSi(t)BuMe(2))(3)]. The measured magnetic susceptibilities for 1-6 trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds 4-6 appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data, using the spin Hamiltonian H = -2J(S(i) x S(j)). These results are consistent with weak ferromagnetic coupling for complexes 4-6 (J = 4.76, 2.75, and 1.11 cm(-1), respectively), while the fit for 2 is consistent with a near-negligible exchange interaction (J = -0.05 cm(-1)). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes of 3-5. The mononuclear complex shows a triplet ground state with singly occupied degenerate f orbitals. The meta- and para-bridged species are computed to show very weak ferro- and antiferromagnetic coupling, respectively. All three complexes show only small net spin density on the acetylide-containing ligands. The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at -1.02 V versus [Cp(2)Fe](+/0), assignable to an oxidation of U(IV) to U(V).

Unusual electronic effects imparted by bridging dinitrogen: an experimental and theoretical investigation.

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC(2)Cr(µ-N(2))CrC(2)R](n+) (R = Ph-, (i)Pr(3)Si-; n = 0, 1, 2) backbone: [(dmpe)(4)Cr(2)(C(2)Ph)(2)(µ-N(2))] (1), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))] (2), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))]BAr(F)(4) (3), and [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))](BAr(F)(4))(2) (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp(2)Co](+) and [Cp*(2)Fe](+), respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hund's rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.

Ligand Control of 59Co Nuclear Spin Relaxation Thermometry.

Studying the correlation between temperature-driven molecular structure and nuclear spin dynamics is essential to understanding fundamental design principles for thermometric nuclear magnetic resonance spin-based probes. Herein, we study the impact of progressively encapsulating ligands on temperature-dependent 59Co T 1 (spin-lattice) and T 2 (spin-spin) relaxation times in a set of Co(III) complexes: K3[Co(CN)6] (1); [Co(NH3)6]Cl3 (2); [Co(en)3]Cl3 (3), en = ethylenediamine); [Co(tn)3]Cl3 (4), tn = trimethylenediamine); [Co(tame)2]Cl3 (5), tame = triaminomethylethane); and [Co(dinosar)]Cl3 (6), dinosar = dinitrosarcophagine). Measurements indicate that 59Co T 1 and T 2 increase with temperature for 1-6 between 10 and 60 °C, with the greatest ΔT 1/ΔT and ΔT 2/ΔT temperature sensitivities found for 4 and 3, 5.3(3)%T 1/°C and 6(1)%T 2/°C, respectively. Temperature-dependent T 2* (dephasing time) analyses were also made, revealing the highest ΔT 2*/ΔT sensitivities in structures of greatest encapsulation, as high as 4.64%T 2*/°C for 6. Calculations of the temperature-dependent quadrupolar coupling parameter, Δe 2 qQ/ΔT, enable insight into the origins of the relative ΔT 1/ΔT values. These results suggest tunable quadrupolar coupling interactions as novel design principles for enhancing temperature sensitivity in nuclear spin-based probes.

EXAFS investigations of temperature-dependent structure in cobalt-59 molecular NMR thermometers.

Cobalt-59 nuclei are known for extremely thermally sensitive chemical shifts (δ), which in the long term could yield novel magnetic resonance thermometers for bioimaging applications. In this manuscript, we apply extended X-ray absorption fine structure (EXAFS) spectroscopy for the first time to probe the exact variations in physical structure that produce the exceptional thermal sensitivity of the 59Co NMR chemical shift. We apply this spectroscopic technique to five Co(iii) complexes: [Co(NH3)6]Cl3 (1), [Co(en)3]Cl3 (2) (en = ethylenediamine), [Co(tn)3]Cl3 (3) (tn = trimethylenediamine), [Co(tame)2]Cl3 (4) (tame = 1,1,1-tris(aminomethyl)ethane), and [Co(diNOsar)]Cl3 (5) (diNOsar = dinitrosarcophagine). The solution-phase EXAFS data reveal increasing Co-N bond distances for these aqueous complexes over a ∼50 °C temperature window, expanding by Δr(Co-N) = 0.0256(6) Å, 0.0020(5) Å, 0.0084(5) Å, 0.0006(5) Å, and 0.0075(6) Å for 1-5, respectively. Computational analyses of the structural changes reveal that increased connectivity between the donor atoms encourages complex structural variations. These results imply that rich temperature-dependent structural variations define 59Co NMR thermometry in macrocyclic complexes.

Anionic guest-dependent slow magnetic relaxation in Co(ii) tripodal iminopyridine complexes.

We report the syntheses and magnetic property characterizations of four mononuclear cobalt(ii) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL5-ONHtBu]X2 (X = Cl (1), Br (2), I (3) and ClO4 (4)). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co-N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1. Ab initio computational studies carried out on a 7-coordinate Co(ii) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter (D), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.