Paramagnetic NMR investigations of high-spin nickel(II) complexes. Controlled synthesis, structural, electronic, and magnetic properties of dinuclear vs. mononuclear species

Ni-Centered Coordination-Induced Spin-State Switching Triggered by Electrical Stimulation

We herein report the synthesis and magnetic properties of a Ni(II)-porphyrin tethered to an imidazole ligand through a flexible electron-responsive mechanical hinge. The latter is capable of undergoing a large amplitude and fully reversible folding motion under the effect of electrical stimulation. This redox-triggered movement is exploited to force the axial coordination of the appended imidazole ligand onto the square-planar Ni(II) center, resulting in a change in its spin state from low spin (S = 0) to high spin (S = 1) proceeding with an 80% switching efficiency. The driving force of this reversible folding motion is the π-dimerization between two electrogenerated viologen cation radicals. The folding motion and the associated spin state switching are demonstrated on the grounds of NMR, (spectro)electrochemical, and magnetic data supported by quantum calculations.

Solvents and Ligands Matter: Structurally Variable Palladium and Nickel Clusters Assembled by Tridentate Selenium- and Tellurium-Containing Schiff Bases

Structurally variable organochalcogen clusters containing palladium(II) and nickel(II) ions were assembled starting from the salicylidene-substituted dichalcogenides (Y-C₆H₄-N═CH-C₆H₄-OH)₂ ({HL^Y}₂, where Y = Se or Te), and palladium or nickel acetate. The tetrameric palladium clusters contain reduced chalcogenolato ligands {Y-C₆H₄-N═CH-C₆H₄-O)}^2- ({L'^Y}^2-, where Y = Se or Te), while the initially formed trimeric nickel clusters contain the intact, coordinated dichalcogenides. The palladium clusters have a general formula of [Pd₄(L'^Y)₄] and represent the first examples of palladium complexes where both a gyrobifastigial and a pseudocubane arrangement of the central Pd₄Y₄ unit could be established with the same ligand, only depending on the solvents used for crystallization. Reduced density gradient (RDG) considerations based on density functional theory calculations suggest that the commonly referred to stabilizing chalcogen-palladium or palladium-palladium interactions for the two geometric arrangements are weak van der Waals contacts resulting from the contact of two nonbinding lone pairs. In the case of the pseudocubane arrangement, a repulsive steric effect, which is indicated by RDG analysis, is clearly supported by the cuplike distortions detected in the solid-state structure of the compound. In contrast to the reactions with palladium acetate, where the dichalcogenides were cleaved, during similar reactions with nickel acetate, the dichalcogenides remained intact and trimeric clusters of the composition [Ni-μ²-κ²-(Ni{κ⁵-L^Y}₂)₂-μ²-(OAc)₂] (Y = Se, Te) were formed. Air oxidation and hydrolysis of [Ni-μ²-κ²-(Ni{κ⁵-L^Te}₂)₂-μ²-(OAc)₂] gave a rare example of a hexanuclear nickel cluster of the composition [Ni₂-κ⁵-(Ni₄-κ⁶-μ⁶-{(L'^Te₂O₃)(L'^TeO₂)₂}₂)-μ²-(H₂O)₂], which is composed of a well-defined framework consisting of tellurinic anhydride and tellurinate units, which proves the comparably higher oxidation sensitivity of the trinickel dichalcogenide complexes. Electron spray ionization mass spectrometry spectra of both the palladium and nickel clusters indicate that they show fluctional behavior with varying nuclearity in solution and can adopt multiple charge states especially because of the noninnocence of the chalcogen-based ligands. The complexes were fully characterized by spectroscopic methods, elemental analyses, and X-ray diffraction.

A series of nickel(II) thiocyanate complexes comprising various molar contents of isonicotinamide and water as ligands or co-crystallized moieties – An experimental and computational study

Seven complexes, [Ni(NCS)2(isn)2(H2O)2]⋅2H2O (1), [Ni(NCS)2(isn)2(H2O)2] (2), [Ni(NCS)2(isn)3(H2O)]·2.5H2O (3), [Ni(NCS)2(isn)3(H2O)]·3[Ni(NCS)2(isn)4]·9H2O (4), [Ni(NCS)2(isn)4]·3H2O (5), [Ni(NCS)2(isn)4]·2(isn) (6) and [Ni(NCS)2(isn)4]·1.25H2O (7), of Ni(II) thiocyanate with isonicotinamide (isn = pyridine-4-carboxamide) and water as ligands of...

Metal-Templated Oligomeric Macrocyclization via Coupling for Metal-Doped π-Systems

A method for the synthesis of metal-doped aromatic macrocycles has been developed. The method, i.e., metal-templated oligomeric macrocyclization via coupling, adopts Ni as the template and assembles five pyridine units via a Ni-mediated coupling reaction to form aryl-aryl linkages. A pentameric oligopyridyl macrocycle was selectively obtained in good yield, and the reaction was also applicable to a gram-scale synthesis. The pentameric oligopyridyl macrocycle captured d⁸-Ni(II) at the center to form a paramagnetic pentagonal-bipyramidal complex. The method was applied to the synthesis of a large π-molecule to afford a nanometer-sized, bowl-shaped molecule having a unique combination of 120π and 8d electrons.

Nitric oxide dioxygenation (NOD) reactions of CoIII-peroxo and NiIII-peroxo complexes: NODversusNO activation

A comparative study of “nitric oxide dioxygenationversusdioxygen or nitric oxide activation”.

Homogeneous photochemical water oxidation with metal salophen complexes in neutral media

The development of water oxidation catalysts based on Earth-abundant metals that can function at neutral pH remains a basic chemical challenge.

Toward inert paramagnetic Ni(ii)-based chemical exchange saturation transfer MRI agents

The Ni²⁺ complexes with hexadentate ligands containing two 6-methylpicolinamide groups linked by ethane-1,2-diamine (dedpam) or cyclohexane-1,2-diamine (chxdedpam) spacers were investigated as potential contrast agents in magnetic resonance imaging (MRI). The properties of the complexes were compared to that of the analogues containing 6-methylpicolinate units (dedpa^2- and chxdedpa^2-). The X-ray structure of the [Ni(dedpam)]²⁺ complex reveals a six-coordinated metal ion with a distorted octahedral environment. The protonation constants of the dedpa^2- and dedpam ligands and the stability constants of their Ni²⁺ complexes were determined using pH-potentiometry and spectrophotometric titrations (25 °C, 0.15 M NaCl). The [Ni(dedpa)] complex (log K_NiL = 20.88(1)) was found to be considerably more stable than the corresponding amide derivative [Ni(dedpam)]²⁺ (log K_NiL = 14.29(2)). However, the amide derivative [Ni(chxdedpam)]²⁺ was found to be considerably more inert with respect to proton-assisted dissociation than the carboxylate derivative [Ni(chxdedpa)]. A detailed ¹H NMR and DFT study was conducted to assign the ¹H NMR spectra of the [Ni(chxdedpa)] and [Ni(chxdedpam)]²⁺ complexes. The observed ¹H NMR paramagnetic shifts were found to be dominated by the Fermi contact contribution. The amide resonances of [Ni(chxdedpam)]²⁺ at 91.5 and 22.2 ppm were found to provide a sizeable chemical exchange saturation transfer effect, paving the way for the development of NiCEST agents based on these rigid non-macrocyclic platforms.

Bite-Angle-Regulated Coordination Geometries: Tetrahedral and Trigonal Bipyramidal in Ni(II) with Biphenyl-Appended (2-Pyridyl)alkylamine N,N'-Bidentate Ligands

Two simple biphenyl-appended (2-pyridyl)alkylamine N-bidentate ligands, L^e and L^m, having ethylene and methylene spacers between donor groups, with bite angles L^e ≈ 100° and L^m ≈ 80°, dictate pseudotetrahedral and trigonal-bipyramidal geometries in six high-spin Ni(II)-halide complexes, [Ni(L^e)X₂] and [Ni(L^m)₂X](ClO₄) (where X = Cl^-, Br^-, I^-), respectively. The structures in the solid state, determined using X-ray crystallography, and in solution, determined using spectroscopic methods (UV-vis-NIR and paramagnetic ¹H NMR), which complement each other, are described.

Seven-coordinate Co(II), Fe(II) and six-coordinate Ni(II) amide-appended macrocyclic complexes as ParaCEST agents in biological media

The solution chemistry and solid-state structures of the Co(II), Fe(II), and Ni(II) complexes of 7,13-bis(carbamoylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L) are reported as members of a new class of paramagnetic chemical exchange saturation transfer (paraCEST) MRI contrast agents that contain transition metal ions. Crystallographic data show that nitrogen and oxygen donor atoms of the macrocyclic ligand coordinate to the metal ions to generate complexes with distorted pentagonal bipyramidal geometry for [Co(L)]Cl2·2H2O or [Fe(L)](CF3SO3)2. The Ni(II) complex [Ni(L)](CF3SO3)2·H2O features a hexadentate ligand in a distorted octahedral geometry. The proton NMR spectra of all three complexes show highly dispersed and relatively sharp proton resonances. The complexes were further characterized by monitoring their dissociation under biologically relevant conditions including solutions containing phosphate and carbonate, ZnCl2, or acidic conditions. Solutions of the paraCEST agents in 20 mM N-(2-hydroxyethyl)piperazine-N'-ethanesulfonic acid (pH 7.4) and 100 mM NaCl showed highly shifted and intense CEST peaks at 59, 72, and 92 ppm away from bulk water for [Co(L)](2+), [Ni(L)](2+), and [Fe(L)](2+), respectively at 37 °C on a 11.7 T NMR spectrometer. CEST spectra with corresponding rate constants for proton exchange are reported in 4% agarose gel (w/w), rabbit serum, egg white, or buffered solutions. CEST phantoms of 4 mM complex in buffer, 4% agarose gel (w/w), or rabbit serum on a 4.7 T MRI scanner at 37 °C, are compared. The most substantial change was observed for the reactive [Ni(L)](2+), which showed reduced CEST contrast in rabbit serum and egg white. The complexes with the least highly shifted CEST peaks ([Co(L)](2+) and [Ni(L)](2+)) showed a reduction in CEST contrast in 4% agarose gel (w/w) compared to that in buffered solutions, while the CEST effect for [Fe(L)](2+) in 4% agarose gel (w/w) was not substantially different.

Rational design of a room temperature molecular spin switch. The light-driven coordination induced spin state switch (LD-CISSS) approach

A record player type molecule changes spin state reversibly upon irradiation with green and blue light in solution at room temperature without measureable fatigue.

Solid state 13C and 2H NMR investigations of paramagnetic [Ni(II)(acac)2L2] complexes

Nine structurally related paramagnetic acetylacetonato nickel(II) complexes: [Ni(acac)2] and trans-[Ni(acac)2(X)2]nH/D2O, X = H2O, D2O, NH3, MeOH, PMePh2, PMe2Ph, or [dppe]1/2, n = 0 or 1, dppe = 1,2-bis(diphenylphosphino)ethane, as well as cis-[Ni(F6-acac)2(D2O)2], F6-acac = hexafluoroacetylonato, have been characterized by solid state (13)C MAS NMR spectroscopy. (2)H MAS NMR was used to probe the local hydrogen bonding network in [Ni(acac)2(D2O)2]D2O and cis-[Ni(F6-acac)2(D2O)2]. The complexes serve to benchmark the paramagnetic shift, which can be associated with the resonances of atoms of the coordinated ligands. The methine (CH) and methyl (CH3) have characteristic combinations of the isotropic shift (δ) and anisotropy parameters (d, η). The size of the anisotropy (d), which is the sum of the chemical shift anisotropy (CSA) and the paramagnetic electron-nuclei dipolar coupling, is much more descriptive than the isotropic shift. Moreover, the CSA is found to constitute up to one-third of the total anisotropy and should be taken into consideration when (13)C anisotropies are used for structure determination of paramagnetic materials. The (13)C MAS NMR spectra of trans-[Ni(acac)2(PMe2Ph)2], trans-[Ni(acac)2(PMePh2)2], and the noncrystallographically characterized trans-[Ni(acac)2(dppe)]n were assigned using these correlations. The complexes with L = H2O, D2O, NH3, and MeOH can be prepared by a series of solid state desorption and sorption reactions. Crystal structures for trans-[Ni(acac)2(NH3)2] and trans-[Ni(acac)2(PMePh2)2] are reported.

The NiCEST approach: nickel(II) paraCEST MRI contrast agents

Paramagnetic Ni(II) complexes are shown here to form paraCEST MRI contrast agents (paraCEST = paramagnetic chemical exchange saturation transfer; NiCEST = Ni(II) based CEST agents). Three azamacrocycles with amide pendent groups bind Ni(II) to form stable NiCEST contrast agents including 1,4,7-tris(carbamoylmethyl)-1,4,7-triazacyclononane (L1), 1,4,8,11-tetrakis(carbamoylmethyl)-1,4,8,11-tetraazacyclotetradecane (L2), and 7,13-bis(carbamoylmethyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (L3). [Ni(L3)](2+), [Ni(L1)](2+), and [Ni(L2)](2+) have CEST peaks attributed to amide protons that are shifted 72, 76, and 76 ppm from the bulk water resonance, respectively. Both CEST MR images and CEST spectroscopy show that [Ni(L3)](2+) has the largest CEST effect in 100 mM NaCl, 20 mM HEPES pH 7.4 at 37 °C. This larger CEST effect is attributed to the sharper proton resonances of the complex which arise from a rigid structure and low relaxivity.

Preparation, spectroscopic and high field NMR relaxometry studies of gadolinium(III) complexes with the asymmetric tetraamine 1,4,7,11-tetraazaundecane

The reaction of Gd(III) with asymmetric tetramine 1,4,7,11-tetraazaundecane (2,2,3-tet, L(1)) ligand has been studied via NMR spectroscopy. The ligand proton longitudinal relaxation rates (R(1)) have been used to estimate the distances of these protons from the Gd(III) center, in Gd(III)-L(1) reaction solutions, in H(2)O/D(2)O 5/1 mixtures. Two Gd(III) complexes [Gd(III)(L(1))(NH(3))(H(2)O)(4)](CH(3)COO)(3)*2H(2)O (1) and [Gd(III)(L(1))(NH(3))(H(2)O)(2)]Cl(3)*EtOH (2) have been isolated and characterized by elemental analyses, TGA, IR, NMR and relaxometry measurements. The NMR relaxation measurements of 2 in aqueous solutions have been performed, under various temperature or concentration conditions, and compared with those of the commercial contrast agents Gd(III)-DTPA and Gd(III)-DTPA-BMA. It has also been studied the influence of (i) the Gd(III) inner-sphere water molecule number (q) alteration and (ii) the steric constraint enhancement on the metal site, over the relaxation rate values of the parent aqueous solution of Gd(III)-2,2,3-tet, and of the aqueous solutions of 2.

Synthesis and structure-activity relationships of amide derivatives of (4,4-difluoro-1,2,3,4-tetrahydro-5H-1-benzazepin-5-ylidene)acetic acid as selective arginine vasopressin V2 receptor agonists

A series of (4,4-difluoro-1,2,3,4-tetrahydro-5H-1-benzazepin-5-ylidene)acetamide derivatives was synthesized, and their structure-activity relationships were examined in order to identify potent and selective arginine vasopressin V(2) receptor agonists. Attempts to substitute other chemical groups in place of the 2-pyridilmethyl moiety of 1a led to the discovery that potent V(2) binding affinity could be obtained with a wide range of functional groups. This structural tolerance allowed for the manipulation of other attributes, such as selectivity against V(1a) receptor affinity or avoidance of the undesirable inhibition of cytochrome P450 (CYP), without losing potent affinity for the V(2) receptor. Some representative compounds obtained in this study were also found to decrease urine volume in awake rats.

Synthesis and characterization of ruthenium bis(beta-diketonato) pyridine-imidazole complexes for hydrogen atom transfer

Ruthenium bis(beta-diketonato) complexes have been prepared at both the RuII and RuIII oxidation levels and with protonated and deprotonated pyridine-imidazole ligands. RuII(acac)2(py-imH) (1), [RuIII(acac)2(py-imH)]OTf (2), RuIII(acac)2(py-im) (3), RuII(hfac)2(py-imH) (4), and [DBU-H][RuII(hfac)2(py-im)] (5) have been fully characterized, including X-ray crystal structures (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene). For the acac-imidazole complexes 1 and 2, cyclic voltammetry in MeCN shows the RuIII/II reduction potential (E1/2) to be -0.64 V versus Cp2Fe+/0. E1/2 for the deprotonated imidazolate complex 3 (-1.00 V) is 0.36 V more negative. The RuII bis-hfac analogues 4 and 5 show the same DeltaE1/2 = 0.36 V but are 0.93 V harder to oxidize than the acac derivatives (0.29 and -0.07 V). The difference in acidity between the acac and hfac derivatives is much smaller, with pKa values of 22.1 and 19.3 in MeCN for 1 and 4, respectively. From the E1/2 and pKa values, the bond dissociation free energies (BDFEs) of the N-H bonds in 1 and 4 are calculated to be 62.0 and 79.6 kcal mol(-1) in MeCN - a remarkable difference of 17.6 kcal mol(-1) for such structurally similar compounds. Consistent with these values, there is a facile net hydrogen atom transfer from 1 to TEMPO* (2,2,6,6-tetramethylpiperidine-1-oxyl radical) to give 3 and TEMPO-H. The DeltaG degrees for this reaction is -4.5 kcal mol(-1). 4 is not oxidized by TEMPO* (DeltaG degrees = +13.1 kcal mol(-1)), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im) (6). A RuII-imidazoline analogue of 1, RuII(acac)2(py-imnH) (7), reacts with 3 equiv of TEMPO* to give the imidazolate 3 and TEMPO-H, with dehydrogenation of the imidazoline ring.

Multifrequency EPR and Redox Reactivity Investigations of a Bis(μ-thiolato)-dicopper(II,II) Complex

From a new tripodal ligand [N2SS'H] with mixed N, S(thioether), and S(thiolate) donor set, the corresponding bis(mu-thiolato)dicopper(II) complex has been prepared and characterized. X-ray crystallographic analysis of the complex [Cu2(N2SS')2](ClO4)2.C4H10O (1) demonstrates that the two five-coordinated Cu atoms are bridged by two thiolates leading to a nearly planar Cu2S2 core with a Cu1...Cu1* distance of 3.418(8) A and a large bridging angle Cu1S1Cu1* of 94.92 degrees. X-band (10 GHz), Q-band (34 GHz), and F-Band (115 GHz) EPR spectra of 1 are consistent with a weakly coupled dicopper(II,II) center attributed to an S = 1 state. Simulations for the three frequencies are obtained with a unique set of electronic parameters. The mean values of the spin Hamiltonian parameters for 1 are D = 0.210(3) cm(-1), E = 0.0295(5) cm(-1), |E/D| = 0.140, gx = 2.030(2), gy = 2.032(2), gz = 2.128(2). The electrochemical one-electron reduction of 1 generates the mixed-valent CuIICuI species. EPR and UV-vis spectra are consistent with a type I localized mixed-valent species, while dinuclear CuA centers of native cytochrome c oxidase (CcO)1-3 or nitrous oxide reductase (N2OR)4 have a delocalized CuIICuI mixed-valent state. After reoxidation of the CuIICuI species, the initial complex 1 is regenerated through a reversible interconversion process.

Synthesis and1H NMR studies of paramagnetic nickel(ii) complexes containing bis(pyrazolyl)methane ligands with dendritic substituents

The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me2pz)2(R=SiMe3, CH2Ph, G1, G2, and G3; Gn=Fréchet-type dendritic wedges of generation n) have been prepared starting from H2C(3,5-Me2pz)2. Reaction of these didentate ligands with [NiBr2(DME)] is a straightforward procedure that allows the synthesis of the nickel(II) complexes [NiBr2{RCH(3,5-Me2pz)2}]. The molecular structure of compound (R=CH2Ph) has been determined by X-ray diffraction studies. The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and the metallacycle Ni(NN)2C adopts a boat conformation with the benzyl group in an axial position. 1H NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution. Valuable information about the disposition of the ligands and dendritic wedges in solution has been obtained thanks to the influence of the paramagnetic centre on the proton resonances.

Proton NMR Spectroscopy and Magnetic Properties of a Solution-Stable Dicopper(II) Complex Bearing a Single μ-Hydroxo Bridge

The reaction of copper(II) perchlorate with the macrocyclic ligand [22]py4pz in the presence of base leads to formation of a dinuclear complex [Cu(2)([22]py4pz)(mu-OH)](ClO(4))(3)xH(2)O, in which two copper ions are bridged by a single mu-hydroxo bridge. Each copper ion is further surrounded by four nitrogen atoms of the ligand. The mu-hydroxo bridge mediates a strong antiferromagnetic coupling (2J = -691(35) cm(-1)) between the metal centers, leading to relatively sharp and well-resolved resonances in the (1)H NMR spectrum of the complex in solution. We herein report the crystal structure, the magnetic properties, and the full assignment of the hyperfine-shifted resonances in the NMR spectrum of the complex, as well as the determination of the exchange coupling constant in solution through temperature-dependent NMR studies.

Synthesis and characterization of molybdenum oxo complexes of two tripodal ligands: reactivity studies of a functional model for molybdenum oxotransferases

Reaction of the tetradentate ligand N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (L-OH) with MoO2Cl2 in methanol in the presence of NaOMe and PF6- results in the formation of [MoO2(L-O)]PF6. Similarly, the reaction of N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (L-SH) with MoO2(acac)2 leads to the formation of [MoO2(L-S)]+. The dioxo-molybdenum complex [MoO2(L-O)]+ reacts with phosphines in methanol to afford phosphine oxides and an air-sensitive molybdenum complex, tentatively identified as [Mo(IV)O(L-O)(OCH3)]. The latter complex is capable of reducing biological oxygen donors such as DMSO or nitrate, thereby mimicking the activity of DMSO reductase and nitrate reductase. Reaction of [MoO2(L-O)]PF6 with PPh3 in other solvents than methanol leads to the formation of the Mo(V) dimer [(L-O)OMo(micro-O)MoO(L-O)](PF6)2. The crystal structures of [MoO2(L-O)]PF6 and the micro-oxo bridged dimer are presented.

NMR studies of mononuclear octahedral Ni(II) complexes supported by tris((2-pyridyl)methyl)amine-type ligands

The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand (6-Ph(2)TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (1), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))]ClO(4) (3), and [(6-Ph(2)TPA)Ni-Cl(CH(3)CN)]ClO(4) (4), and one Ni(II) complex of tris((2-pyridyl)methyl)amine, [(TPA)Ni(CH(3)CN)(H(2)O)](ClO(4))(2) (2), have been characterized in acetonitrile solution using conductance methods and NMR spectroscopy. In acetonitrile solution, 1-4 have monomeric cations that exhibit isotropically shifted (1)H NMR resonances. Full assignment of these resonances was achieved using one- and two-dimensional (1)H NMR techniques and (2)H NMR of analogues having deuteration of the supporting chelate ligand. COSY cross peaks were observed for pyridyl protons of the 6-Ph(2)TPA ligand in 1 and 3. This study lays the groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance to ARD.