Tuning of the Properties of Transition-Metal Bispidine Complexes by Variation of the Basicity of the Aromatic Donor Groups

Mn2+ Bispidine Complex Combining Exceptional Stability, Inertness, and MRI Efficiency

As an essential metal ion and an efficient relaxation agent, Mn²⁺ holds a great promise to replace Gd³⁺ in magnetic resonance imaging (MRI) contrast agent applications, if its stable and inert complexation can be achieved. Toward this goal, four pyridine and one carboxylate pendants have been introduced in coordinating positions on the bispidine platform to yield ligand L³. Thanks to its rigid and preorganized structure and perfect size match for Mn²⁺, L³ provides remarkably high thermodynamic stability (log K_MnL = 19.47), selectivity over the major biological competitor Zn²⁺ (log(K_MnL/K_ZnL) = 4.4), and kinetic inertness. Solid-state X-ray data show that [MnL³(MeOH)](OTf)₂ has an unusual eight-coordinate structure with a coordinated solvent molecule, in contrast to the six-coordinate structure of [ZnL³](OTf), underlining that the coordination cavity is perfectly adapted for Mn²⁺, while it is too large for Zn²⁺. In aqueous solution, ¹⁷O NMR data evidence one inner sphere water and dissociatively activated water exchange (k_ex²⁹⁸ = 13.5 × 10⁷ s^-1) for MnL³. Its water proton relaxivity (r₁ = 4.44 mM^-1 s^-1 at 25 °C, 20 MHz) is about 30% higher than values for typical monohydrated Mn²⁺ complexes, which is related to its larger molecular size; its relaxation efficiency is similar to that of clinically used Gd³⁺-based agents. In vivo MRI experiments realized in control mice at 0.02 mmol/kg injected dose indicate good signal enhancement in the kidneys and fast renal clearance. Taken together, MnL³ is the first chelate that combines such excellent stability, selectivity, inertness and relaxation properties, all of primary importance for MRI use.

A Biophysical Study of Ru(II) Polypyridyl Complex, Properties and its Interaction with DNA

Mononuclear Ru(II)Polypyridyl complexes of type [Ru(A)₂BPIIP] (ClO₄)₂.2H₂O, where BPIIP = 2-(3-(4-bromophenyl)isoxazole-5-yl)-1 H-imidazo [4,5-f] [1, 10] phenanthroline and A = bpy = bipyridyl (1), phen = 1,10 Phenanthroline (2), dmb = 4, 4' -dimethyl 2, 2'- bipyridine (3) & dmp = 4,4'-dimethyl-1,10 -Ortho Phenanthroline (4), were synthesized and their antibacterial activity were examined. The synthesized complexes were characterized and their interaction with DNA was studied using Computational and Biophysical methods (Absorption, emission methods, and viscosity). Molecular modelling studies were carried out for molecular geometry and electronic properties (Frontier molecular orbital HOMO-LUMO). The electrostatic potential surface contours for the complexes were analysed to give their nucleophilic level of sensitivity. The study reveals that the Ru(II) Polypyridyl complexes bind to DNA preponderantly by intercalation. The results recommend that the phen and dmp complex have more effective binding ability than the bpy and dmb, indicating the role of the ancillary ligand in determining their specificity for DNA binding. Further molecular docking studies suggested an octahedral geometry and bind to DNA by preferential binding to Guanine. The docking study additionally sustains the binding constant data acquired with the absorption and emission techniques.The results reveal that the nature of the ancillary Ligand plays a considerable role for the intercalation of the Ru(II) polypyridyl complex to DNA, which subsequently influences the antibacterial activity. Biological studies conducted on Gram-Negative (E.coli and K.pneumonia) and Gram-Positive (S. aureus and E. faecalis) bacteria establish that complex 1 and 2 were considerably active against S. aureus and E. coli.

Metal-ligand bonding in bispidine chelate complexes for radiopharmaceutical applications

The complexes of selected radionuclides relevant for nuclear medicine (InIII, BiIII, LuIII, AcIII and in addition LaIII for comparative purposes) with the octadentate (6,6′-((9-hydroxy-1,5-bis(methoxycarbonyl)-2,4-di(pyridin-2-yl)-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(methylene))dipicolinic acid) ligand, H2bispa2, have been studied by density functional theory calculations modelling both isolated and aqueous solution conditions. The properties in focus are the encapsulation efficiency of the ligand for the different-size metals (M), the differences in bonding with the various MIII ions analysed using Bader’s atoms in molecules theory and the possibility and characteristics of nona- and decacoordination by H2O ligands. The computed results confirmed strong steric effects in the case of the In complex excluding higher than octacoordination. The studied properties depend strongly on the interplay of the sizes and electronic structures of the MIII ions. The computations support high stability of the complexes in aqueous solution, where also the solvation energies of the MIII ions (as dissociation products) play a significant role.

Non-Heme-Iron-Mediated Selective Halogenation of Unactivated Carbon-Hydrogen Bonds

Oxidation of the iron(II) precursor [(L¹)Fe^IICl₂], where L¹ is a tetradentate bispidine, with soluble iodosylbenzene (^sPhIO) leads to the extremely reactive ferryl oxidant [(L¹)(Cl)Fe^IV=O]⁺ with a cis disposition of the chlorido and oxido coligands, as observed in non‐heme halogenase enzymes. Experimental data indicate that, with cyclohexane as substrate, there is selective formation of chlorocyclohexane, the halogenation being initiated by C−H abstraction and the result of a rebound of the ensuing radical to an iron‐bound Cl⁻. The time‐resolved formation of the halogenation product indicates that this primarily results from ^sPhIO oxidation of an initially formed oxido‐bridged diiron(III) resting state. The high yield of up to >70 % (stoichiometric reaction) as well as the differing reactivities of free Fe²⁺ and Fe³⁺ in comparison with [(L¹)Fe^IICl₂] indicate a high complex stability of the bispidine‐iron complexes. DFT analysis shows that, due to a large driving force and small triplet‐quintet gap, [(L¹)(Cl)Fe^IV=O]⁺ is the most reactive small‐molecule halogenase model, that the Fe^III/radical rebound intermediate has a relatively long lifetime (as supported by experimentally observed cage escape), and that this intermediate has, as observed experimentally, a lower energy barrier to the halogenation than the hydroxylation product; this is shown to primarily be due to steric effects.

The Stronger the Better: Donor Substituents Push Catalytic Activity of Molecular Chromium Olefin Polymerization Catalysts

The donor strength of bifunctional pyridine-cyclopentadienyl ligands was altered systematically by the introduction of donating groups in the para-position of the pyridine. In the resulting chromium complexes an almost linear correlation between donor strength and the nitrogen-chromium distance as well as the electronic absorption maximum is experimentally observed. The connection of electron-donating groups in the ligand backbone leads to an efficient transfer of the electronic influences to the catalytically active metal centre without restricting it through steric effects. Therefore, catalytic olefin polymerization activity, which is already very high for the previously studied catalysts, increase considerably by attaching para-amino groups to the chelating pyridine or quinoline, respectively. Combining electron-rich indenyl ligands with para-amino substituted pyridines lead to the highest catalytic activities observed so far for this class of organo chromium olefin polymerisation catalysts. The resulting polymers are of ultra-high molecular weight and the ability of the catalysts to incorporate co-monomers is also very high.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

Spin state and reactivity of iron(iv)oxido complexes with tetradentate bispidine ligands

The iron(iv)oxido complex [(bispidine)Fe^IVO(Cl)]⁺is shown by experiment and high-level DLPNO-CCSD(T) quantum-chemical calculations to be an extremely short-lived and very reactive intermediate-spin (S= 1) species.

Supramolecular Organogels Based on N-Benzyl, N'-Acylbispidinols

The acylation of unsymmetrical N-benzylbispidinols in aromatic solvents without an external base led to the formation of supramolecular gels, which possess different thicknesses and degrees of stability depending on the substituents in para-positions of the benzylic group as well as on the nature of the acylating agent and of the solvent used. Structural features of the native gels as well as of their dried forms were studied by complementary techniques including Fourier-transform infrared (FTIR) and attenuated total reflection (ATR) spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and small-angle X-ray scattering and diffraction (SAXS). Structures of the key crystalline compounds were established by X-ray diffraction. An analysis of the obtained data allowed speculation on the crucial structural and condition factors that governed the gel formation. The most important factors were as follows: (i) absence of base, either external or internal; (ii) presence of HCl; (iii) presence of carbonyl and hydroxyl groups to allow hydrogen bonding; and (iv) presence of two (hetero)aromatic rings at both sides of the molecule. The hydrogen bonding involving amide carbonyl, hydroxyl at position 9, and, very probably, ammonium N-H⁺ and Cl- anion appears to be responsible for the formation of infinite molecular chains required for the first step of gel formation. Subsequent lateral cooperation of molecular chains into fibers occurred, presumably, due to the aromatic π-π-stacking interactions. Supercritical carbon dioxide drying of the organogels gave rise to aerogels with morphologies different from that of air-dried samples.

2,4-Substituted bispidines as rigid hosts for versatile applications: from κ-opioid receptor to metal coordination

Bispidines and their applications in medicine, catalysis, magnetism and medical imaging: what do we know about the influence of substituents?

Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

Three new tetradentate imine ligands, HLHBr, HLClH and HLBrH (HLR1R2) were synthesised by 2 : 1 condensation of the appropriately n-halo substituted pyridine-2-carboxaldehyde (5-bromo-4a, 6-bromo-4b or 6-chloro-4c) with 1,3-diaminopropan-2-ol (5). Reactions of each of these three ligands with one equivalent of cobalt(ii) tetrafluoroborate resulted in the formation of three N4O2 coordinated cobalt(ii) complexes: the anticipated mononuclear complex [CoII(HLHBr)(MeOH)2](BF4)2 (1), and two unexpected dinuclear complexes, [CoII2(LBrH-BF2OMe)]2(BF4)2 (2) and [CoII2(LClH-BF2OMe)]2(BF4)2 (3). Dinuclear 2 and 3 result from complexation of cobalt(ii) to the ligands derived from the sterically demanding 6-halo substituted pyridine-2-carboxaldehydes (4b and 4c) undergoing rearrangement, reacting with MeOH and a BF4 anion, resulting in a pair of borate ester bridges between the two cobalt(ii) centres. A similar type of rearrangement is proposed for the PF6 analogues. Cyclic voltammetry in acetonitrile reveals that cobalt(ii) complexes 1-3 undergo a quasi-reversible oxidation: Em = 0.57, 0.38 and 0.29 V vs. 0.01 AgNO3/Ag, respectively. The observed Em value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest Em value being observed for the corresponding cobalt complex, 3, rather than for either of the complexes of the n-bromo-substituted ligands (n = 6 or 5), 2 and 1.

Synthesis and Characterization of the Most Active Copper ATRP Catalyst Based on Tris[(4-dimethylaminopyridyl)methyl]amine

The tris[(4-dimethylaminopyridyl)methyl]amine (TPMA^NMe2) as a ligand for copper-catalyzed atom transfer radical polymerization (ATRP) is reported. In solution, the [Cu^I(TPMA^NMe2)Br] complex shows fluxionality by variable-temperature NMR, indicating rapid ligand exchange. In the solid state, the [Cu^II(TPMA^NMe2)Br][Br] complex exhibits a slightly distorted trigonal bipyramidal geometry (τ = 0.89). The UV-vis spectrum of [Cu^II(TPMA^NMe2)Br]⁺ salts is similar to those of other pyridine-based ATRP catalysts. Electrochemical studies of [Cu(TPMA^NMe2)]²⁺ and [Cu(TPMA^NMe2)Br]⁺ showed highly negative redox potentials (E_1/2 = -302 and -554 mV vs SCE, respectively), suggesting unprecedented ATRP catalytic activity. Cyclic voltammetry (CV) in the presence of methyl 2-bromopropionate (MBrP; acrylate mimic) was used to determine activation rate constant k_a = 1.1 × 10⁶ M^-1 s^-1, confirming the extremely high catalyst reactivity. In the presence of the more active ethyl α-bromoisobutyrate (EBiB; methacrylate mimic), total catalysis was observed and an activation rate constant k_a = 7.2 × 10⁶ M^-1 s^-1 was calculated with values of K_ATRP ≈ 1. ATRP of methyl acrylate showed a well-controlled polymerization using as little as 10 ppm of catalyst relative to monomer, while side reactions such as Cu^I-catalyzed radical termination (CRT) could be suppressed due to the low concentration of L/Cu^I at a steady state.

Bispidines for radiopharmaceuticals

Radiometal based radiopharmaceuticals for imaging and therapy require selective ligands (bifunctional chelators, BFCs) that form metal complexes, which are inert against trans-chelation under physiological conditions, linked to a biological vector, directing them to the targeted tissue. Bispidine ligands with a very rigid backbone and widely variable donor sets are reviewed as an ideal class of BFCs, and recent applications are discussed.

Bifunctional bispidine derivatives for copper-64 labelling and positron emission tomography

A bispidine cage coordinates ⁶⁴Cu²⁺ rapidly and quantitatively at room temperature, and biotin and maleimide functions allow for targeted PET imaging.

Copper-Bispidine Complexes: Synthesis and Complex Stability Study

A new series of dicarboxylic derivatives of bispidines have been synthesized to develop novel copper(II) complexes suitable as imaging agents for positron emission tomography. For characterization purposes, copper complexes of bispidines were synthesized in the pure form and in quantitative yields by neutralization of ligands with malachite. The formation of complexes and their stoichiometries were studied by potentiometric titration, cyclic voltammetry, and spectroscopic methods. The stability constants were found to be fairly suitable for copper cation fixation inside dianionic chelate molecules.

A Bispidine Iron(IV)-Oxo Complex in the Entatic State

For a series of Fe(IV) =O complexes with tetra- and pentadentate bispidine ligands, the correlation of their redox potentials with reactivity, involving a variety of substrates for alkane hydroxylation (HAT), alkene epoxidation, and phosphine and thioether oxidation (OAT) are reported. The redox potentials span approximately 350 mV and the reaction rates over 8 orders of magnitude. From the experimental data and in comparison with published studies it emerges that electron transfer and the driving force are of major importance, and this is also supported by the DFT-based computational analysis. The striking difference of reactivity of two isomeric systems with pentadentate bispidines is found to be due to a destabilization of the S=1 ground state of one of the ferryl isomers, and this is supported by the experimentally determined redox potentials and published stability constants with a series of first-row transition metal ions with these two isomeric ligands.

Improving the stability and inertness of Cu(ii) and Cu(i) complexes with methylthiazolyl ligands by tuning the macrocyclic structure

A tacn based ligand bearing two methylthiazolyl arms (no2th) was synthesized with the aim to find ligands forming very stable and inert complexes with Cu(ii) and Cu(i) in aqueous medium for radiopharmaceutical applications. The no2th ligand was efficiently prepared following the orthoamide intermediate synthesis. The complexes with Cu(2+) and Zn(2+) were obtained and analyzed by X-ray diffraction. The [Cu(no2th)](2+) complex presents a pentacoordinated distorted square pyramidal coordination geometry, while the metal ion in [Zn(no2th)](2+) adopts a hexacoordinated distorted trigonal prismatic geometry involving the coordination of a perchlorate counter ion. The acid-base properties of no2th have been studied using potentiometric titrations, and the stability constants of Cu(2+) and Zn(2+) complexes were determined by potentiometric and UV-vis titrations using H4edta as a competitor ligand. The stability constant determined for the Cu(2+) complex is rather high (log KCuL = 20.77 and pCu = 17.15), and moreover no2th exhibits a high selectivity for copper(ii) in relation to zinc(ii). The kinetics of the copper(ii) complexation process is very fast even in acidic medium. In addition, the [Cu(no2th)](2+) complex was found to be inert under rather harsh conditions (up to 2 M HCl and 60 °C), displaying a very high half-life time of about 15 days in 2 M HCl at 90 °C. The electrochemical reduction of the copper(ii) complex in water leads to the reversible formation of a stable copper(i) species. Spectroscopic studies performed by NMR, UV-vis and EPR, assisted by theoretical calculations, show that the [Cu(no2th)](2+) complex presents a structure in solution similar to that observed in the solid state. When compared to its cyclam di-N-methylthiazolyl counterpart, the results reported in this paper unambiguously show that replacing the cyclam unit by a tacn moiety improves the stability and inertness of its Cu(ii) and Cu(i) complexes.

Spectroscopic, Structural, and Computational Characterization of Three Bispidinone Derivatives, as Ligands for Enantioselective Metal Catalyzed Reactions

Three chiral derivatives of the alkaloid sparteine (bispidines), characterized by the 3,7-diazabicyclo[3.3.1]nonane moiety, were designed as efficient ligands in a number of enantioselective reactions due to their metal coordination properties. A full evaluation of the 3D properties of the compounds was carried out, as the geometrical features of the bicyclic framework are strictly related to the efficiency of the ligands in the asymmetric catalysis. The selected molecules have different molecular complexity for investigating the effects of different chiral groups on the bicycle conformation. We report here a thorough analysis of their molecular arrangement, by NMR spectroscopy, single crystal X-ray crystallography, and computational techniques, which put in evidence their conformational preferences and the parameters needed for the design of more efficient ligands in asymmetric synthetic routes. The results confirmed the high molecular flexibility of the compounds, and indicated how to achieve a control of the chair-chair/boat-chair conformational ratio, by adjusting the relative size of the substituents on the piperidine nitrogens.

C-H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight

A family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized, and employed as C-H oxidation catalysts. Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe(OTf)2 in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (L1)2Fe(OTf)(ClO4) and (L3)2Fe(OTf)2 are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. (1)H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (LX)2Fe(OTf)2 efficiently catalyze the oxidation of alkyl C-H bonds employing H2O2 as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C-H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C-H bonds. Isotopic labeling analyses show that H2O2 is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O-O cleavage, without the assistance of a cis-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H2O2 at (LX)2Fe(OTf)2 differs from that occurring in biomimetic iron catalysts described to date.

Influence of functionalized pyridine ligands on the radio/chemical behavior of [M(I)(CO)3](+) (M = Re and (99m)Tc) 2 + 1 complexes

While a number of chelate strategies have been developed for the organometallic precursor fac-[M(I)(OH2)3(CO)3](+) (M = Re, (99m)Tc), a unique challenge has been to improve the overall function and performance of these complexes for in vivo and in vitro applications. Since its discovery, fac-[M(I)(OH2)3(CO)3](+) has served as an essential scaffold for the development of new targeted (99m)Tc based radiopharmaceuticals due to its labile aquo ligands. However, the lipophilic nature of the fac-[M(I)(CO)3](+) core can influence the in vivo pharmacokinetics and biodistribution of the complexes. In an effort to understand and improve this behavior, monosubstituted pyridine ligands were used to assess the impact of donor nitrogen basicity on binding strength and stability of fac-[M(I)(CO)3](+) in a 2 + 1 labeling strategy. A series of Re and (99m)Tc complexes were synthesized with picolinic acid as a bidentate ligand and 4-substituted pyridine ligands. These complexes were designed to probe the effect of pKa from the monodentate pyridine ligand both at the macro scale and radiochemical concentrations. Comparison of X-ray structural data and radiochemical solution experiments clearly indicate an increase in overall yield and stability as pyridine basicity increased.

Oxidation of methane by an N-bridged high-valent diiron–oxo species: electronic structure implications on the reactivity

Methane activation by dinuclear high-valent iron–oxo species: do we need two metals to activate such inert bonds? Our theoretical study using DFT methods where electronic structure details and mechanistic aspects are established answers this intriguing question.

Synthesis and transition metal coordination chemistry of a novel hexadentate bispidine ligand

With the hexadentate bispidine ligand, the Cu^II and Fe^II (red) complexes are hexacoordinated with a semi-coordinated py4, the Mn^II complex (green) is heptacoordinated.