Tripodal Tetradentate Ligands Research Articles

Electrocatalysis of the Oxygen Reduction Reaction by Copper Complexes with Tetradentate Tripodal Ligands.

The tetradentate tripodal ligand scaffold is capable of supporting the expected geometries of the copper ion during the oxygen reduction reaction (ORR) catalysis. As such, we probed the reactivity of copper complexes with these types of ligands by electronically and structurally tweaking the tris(pyridin 2-ylmethyl)amine (tmpa) scaffold by progressively replacing the terminal pyridines with carboxylate donors. This work shows that systems with one carboxylato donor (bpg = bis(pyridin-2-ylmethyl)glycine), (bpp = (3-(bis(pyridin-2-ylmethyl)amino)propanoic acid)) are active in electrocatalyzing the homogeneous ORR under circumneutral aqueous conditions. Turnover frequencies in the range from 105 to 106 s-1, on par with that for Cu-tmpa under identical conditions, were obtained. It is noteworthy that the CuII/CuI redox potentials for the Cu-bpg, Cu-bpp, and Cu-tmpa systems in phosphate-buffered water (pH 7, under Ar) are similar at -0.409, -0.375, and -0.401 V vs Ag/AgCl, respectively. This is rationalized by the influence of the Lewis acidity of the copper ions on the water coligand. Corroborating this are pKa values for [Cu(tmpa)(H2O)]2+, Cu(bpg)(H2O)]+, and [Cu(bpp)(H2O)]+ of 6.6, 8.8, and 10.2, respectively. Thus, the overall charge of the solution species for all three complexes will be +1 at pH 7 and this will be an important determinant for the redox potentials and, in turn, the catalytic overpotentials, which are also similar. A cis carboxylato donor offers H-bonding possibilities for exogenous resting state water and intermediate hydroperoxo coligands. This is reflected by the higher pKa values for Cu-bpp and Cu-bpg compared with that for Cu-tmpa, with the Cu-bpp system furnishing the least strained H-bonding.

Assembly of an iron-based complex into a metal-organic framework: a space confinement strategy for isolation of mono-iron complexes to protect from dimerization.

Non-heme mononuclear iron complexes, especially when supported by tripodal tetradentate ligands, show promising C-H bond activation efficiency in catalytic reactions. Nevertheless, they intrinsically decay readily to their dinuclear form, and the dimerization process is inevitable in homogenous solution, which dramatically hinders their further application. Hence, we demonstrate that the mononuclear iron complex [(TPA)FeII-2L]2+ (L = labile ligands, mainly solvent molecules) was successfully encapsulated in a highly robust metal-organic framework UiO-66 via a two-step "ship-in-a-bottle" strategy. The nearly perfect size matching of the octahedral cages of the host UiO-66 provides ideal space confinement for the guest complex to protect from dimerization and dramatically increases the mono-nuclear complex stability compared to its un-confined state. The successful encapsulation of [(TPA)FeII-2L]2+ in UiO-66 was verified thoroughly by spectroscopy, microscopy, N2 adsorption, and electrochemistry characterization techniques. This work shows that encapsulating an unstable molecular complex in MOFs via a two-step "ship-in-a-bottle" strategy highlights opportunities for extending the heterogenization of homogeneous complexes.

Mononuclear nickel(ii)-flavonolate complexes of tetradentate tripodal 4N ligands as structural and functional models for quercetin 2,4-dioxygenase: structures, spectra, redox and dioxygenase activity.

Three new nickel(ii)-flavonolate complexes of the type [Ni(L)(fla)](ClO4) 1-3, where L is the tripodal 4N ligand tris(pyrid-2-ylmethyl)amine (tpa, L1) or (pyrid-2-ylmethyl)bis(6-methylpyrid-2-ylmethyl)amine (6-Me2-tpa, L2) or tris(N-Et-benzimidazol-2-ylmethyl)amine (Et-ntb, L3), have been isolated as functional models for Ni(ii)-containing quercetin 2,4-dioxygenase. Single crystal X-ray structures of 1 and 3 reveal that Ni(ii) is involved in π-back bonding with flavonolate (fla-), as evident from enhancement in C[double bond, length as m-dash]O bond length upon coordination [H(fla), 1.232(3); 1, 1.245(7); 3, 1.262(8) Å]. More asymmetric chelation of fla- in 3 than in 1 [Δd = (Ni-Ocarbonyl - Ni-Oenolate): 1, 0.126; 3, 0.182 Å] corresponds to lower π-delocalization in 3 with electron-releasing N-Et substituent. The optimized structures of 1-3 and their geometrical isomers have been computed by DFT methods. The HOMO and LUMO, both localized on Ni(ii)-bound fla-, are highly conjugated bonding π- and antibonding π*-orbitals respectively. They are located higher in energy than the Ni(ii)-based MOs (HOMO-1, dx2-y2; HOMO-2/6, dz2), revealing that the Ni(ii)-bound fla- rather than Ni(ii) would undergo oxidation upon exposure to dioxygen. The results of computational studies, in combination with spectral and electrochemical studies, support the involvement of redox-inactive Ni(ii) in π-back bonding with fla-, tuning the π-delocalization in fla- and hence its activation. Upon exposure to dioxygen, all the flavonolate adducts in DMF solution decompose to produce CO and depside, which then is hydrolyzed to give the corresponding acids at 70 °C. The highest rate of dioxygenase reactivity of 3 (kO2: 3 (29.10 ± 0.16) > 1 (16.67 ± 0.70) > 2 (1.81 ± 0.04 × 10-1 M-1 s-1)), determined by monitoring the disappearance of the LMCT band in the range 440-450 nm, is ascribed to the electron-releasing N-Et substituent on bzim ring, which decreases the π-delocalization in fla- and enhances its activation.

Experimental and theoretical magnetostructural studies on discrete heterometallic cyanide-bridged dinuclear FeIIIMnII and tetranuclear FeIII2CuII2 complexes bearing tripodal pyrazolyl borate and tetradentate phenolate-based ligands.

Two new complexes [FeIII(Tp)(CN)2(μ-CN)MnIICl(HL1)]·3DMF (1) and {[FeIII(Tp)(CN)(μ2-NC)2CuII(HL2)](ClO4)}2·6DMF (2) (HL1 = 2-((((1-methylbenzimidazol-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenol and HL2 = 2-(((pyridin-2-ylmethyl)(quinolin-2-ylmethyl)-amino)methyl)phenol) have been synthesized and characterized by elemental analysis and IR and UV/vis spectroscopy. Structural analysis revealed that 1 is a discrete dinuclear coordination complex and 2 is a discrete tetranuclear coordination complex. In complex 1, each MnII is in a distorted octahedral MN4OCl environment where coordination is satisfied by three nitrogen atoms and one oxygen atom of the ligand, and a chloride group and one nitrogen atom from cyanide. In complex 2, each Cu is in a distorted octahedral MN5O environment where coordination is satisfied by three nitrogen atoms and one oxygen atom of the ligand, and two nitrogen atoms from two cyanides. Direct current (dc) variable-temperature magnetic susceptibility measurements on polycrystalline samples of 1 and 2 were carried out in the temperature range of 1.8-300 K. Investigation of the magnetic properties reveals the occurrence of weak antiferromagnetic coupling between the low-spin FeIII (S = 1/2) ions and high-spin MnII (S = 5/2) ions in 1, while 2 exhibits ferro- and antiferromagnetic coupling between the metal ions in the tetranuclear CuII2FeIII2 unit. DFT calculations show ferromagnetic coupling in both complexes, although this appears to be weak in the case of complex 1. In addition, magnetostructural correlations reveal the magnetic behavior against Mn-N-C and Fe-C-N angles in 1 and Cu-N-C and Fe-C-N angles in 2.

Reactivity of Copper Complexes with Tripodal Tetradentate Ligands based on Camphoric Acid towards Dioxygen

AbstractThe reaction of dioxygen with copper(I) complexes containing camphor‐derived ligands was investigated. Stopped‐flow measurements revealed the formation of bis(μ‐oxido) copper complexes at low temperatures. However, these intermediates were not stable enough to be isolated and decomposed quickly. Sterically more demanding alkyl groups slowed the formation of the bis(μ‐oxido) copper complexes. A kinetic analysis was performed and showed ‐ in line with previous reports ‐ that the rate‐determining step could be assigned to forming a mononuclear superoxido copper complex. For one of the reactions investigated, a product could be structurally characterized and turned out to be a copper(II) complex with an additional hydroxide as a ligand (most likely caused by a C−H abstraction from the solvent acetone). One of the complexes oxidized thioanisole to the corresponding sulfoxide (conversion of 34 % according to GC‐MS) with no byproducts. Chiral GC gave an enantiomeric excess of 14 %.

Synthesis and preclinical evaluation of rhenium and technetium-99m “4 + 1” mixed-ligand complexes bearing quinazoline derivatives as potential EGFR imaging agents

Epidermal growth factor receptors (EGFR) of tyrosine kinase (TK) have shown high expression levels in most cancers and are considered a promising target for cancer diagnosis and therapy. Expanding the investigation for novel targeted radiopharmaceuticals, an EGFR inhibitor such as 4-aminoquinazoline derivatives along with a radionuclide such as technetium-99m (99mTc) could be ideal. Thus, we report herein the synthesis, characterization, and biological evaluation of new “4 + 1” mixed-ligand ReIII- and 99mTcIII-complexes of the general formula [99mTc][Tc(NS3)(CN-R)] bearing tris(2-mercaptoethyl)-amine (NS3) as the tetradentate tripodal ligand and a series of isocyanide derivatives (CN-R) of tyrosine kinase inhibitor (3-bromophenyl)quinazoline-4,6-diamine as the monodentate ligand. The quinazoline isocyanide derivatives 4a-d were prepared in two steps and reacted with the [Re(NS3)PMe2Ph] precursor leading to the final complexes 5a-d in high yield. All compounds were characterized by elemental analysis, IR, and NMR spectroscopies. In vitro studies, for their potency to inhibit the cell growth, using intact A431 cells indicate that the quinazoline derivatives 4a-d and the Re complexes 5a-d significantly inhibit the A431 cell growth. In addition, the EGFR autophosphorylation study of complex 5b shows an IC50 value in the nanomolar range.The corresponding “4 + 1” 99mTc-complexes 6a-d were prepared by employing the [99mTc]TcEDTA intermediate and the appropriate monodentate 4a-d in a two-step synthetic procedure with a radiochemical yield (RCY) from 63 to 77 % and a radiochemical purity (RCP) > 99 % after HPLC purification. Their structures have been established by HPLC comparative studies using the well-characterized Re-complexes 5a-d as reference. All 99mTc-complexes remain stable for at least 6 h, and their logD7.4 values confirmed their anticipated lipophilic character. Biodistribution studies in healthy Swiss albino mice of 99mTc-complexes showed hepatobiliary excretion and initial fast blood clearance. Complex 6b was also tested in Albino SCID mice bearing A431 tumors and showed rapid tumor uptake at 5 min (2.80 % ID/g) with a moderate tumor/muscle ratio (2.06) at 4 h p.i. The results encourage further investigation for this type of 99mTc-complexes as single-photon emission computed tomography (SPECT) radio agents for imaging tumors overexpressing EGFR.

Self-assembled discrete and polymeric cobalt(II) complexes of a carboxylate appended tripodal tetradentate ligand: reactivity with aerial dioxygen or aqueous hydrogen peroxide

Self-assembled discrete and polymeric cobalt(II) complexes of a carboxylate appended tripodal tetradentate ligand: reactivity with aerial dioxygen or aqueous hydrogen peroxide

Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels-Alder and 1,3-dipolar cycloadditions.

An improved synthesis of the racemic rhodium compound [RhCl2(κ4 C,N,N',P-L1)] (1) containing an achiral tripodal tetradentate ligand is reported. Their derived solvate complexes [Rh(κ4 C,N,N',P-L1)(Solv)2][SbF6]2 (Solv = NCMe, 2; H2O, 3) are resolved into their two enantiomers. Complexes 2 and 3 catalyze the Diels-Alder (DA) reaction between methacrolein and cyclopentadiene and the 1,3-dipolar cycloaddition reaction between methacrolein and the nitrone N-benzylidenphenylamine-N-oxide. When enantiopure (A Rh,R N)-2 was employed as the catalyst, enantiomeric ratios >99/1, in the R at C2 adduct, and up to 94/6, in the 3,5-endo isomer, were achieved in the DA reaction and in the 1,3-dipolar cycloaddition reaction, respectively. A plausible catalytic cycle that accounts for the origin of the observed enantioselectivity is proposed.

Heteroleptic Lanthanide Complexes Coordinated by Tripodal Tetradentate Ligand: Synthesis, Structure, and Magnetic and Photoluminescent Properties

We report the synthesis, structures, magnetic and luminescent properties of a series of heteroleptic lanthanide complexes based on a tripodal tetradentate ligand [Ln(Tpma)(NO3)3]•nMeCN (Ln = Eu (1)...

A Dinucleating Ligand System with Varying Terminal Donors to Mimic Diiron Active Sites

AbstractTo mimic dinuclear active sites of metalloproteins, we have developed a dinucleating ligand system consisting of two tetradentate tripodal ligand compartments with varying terminal donors (carboxylates, phenolates, and pyridines). These ligands provide access to a series of μ‐oxo‐bridged diferric complexes. The spectroscopic study allows to investigate the molecular structures even in solution, e. g. depending on protonation/deprotonation of coordinated OH− and H2O ligands or to observe a reversible pH‐dependent carboxylate‐shift between terminal and bridging binding mode. The electrochemical behavior is strongly influenced by the exogenous ligands, e. g. OH− facilitates oxidation to FeIV by 690 mV relative to Cl−. Using the terminal carboxylates and a {FeIII(μ‐O)2FeIII} core even allows oxidation with O2 to a high‐valent species with FeIV (S=2). The implications of this study for further generation of high‐valent or peroxo species and their utilization in catalysis is discussed.

Efficient alkane hydroxylation catalysis of nickel(ii) complexes with oxazoline donor containing tripodal tetradentate ligands.

Tris(oxazolynylmethyl)amine TOAR (where R denotes the substituent groups on the fourth position of the oxazoline rings) complexes of nickel(ii) have been synthesized as catalyst precursors for alkane oxidation with meta-chloroperoxybenzoic acid (m-CPBA). The molecular structures of acetato, nitrato, meta-chlorobenzoato and chlorido complexes with TOAMe2 have been determined using X-ray crystallography. The bulkiness of the substituent groups R affects the coordination environment of the nickel(ii) centers, as has been demonstrated by comparison of the molecular structures of chlorido complexes with TOAMe2 and TOAtBu. The nickel(ii)-acetato complex with TOAMe2 is an efficient catalyst precursor compared with the tris(pyridylmethyl)amine (TPA) analogue. Oxazolynyl donors' strong σ-electron donating ability will enhance the catalytic activity. Catalytic reaction rates and substrate oxidizing position selectivity are controlled by the structural properties of the R of TOAR. Reaction of the acetato complex with TOAMe2 and m-CPBA yields the corresponding acylperoxido species, which can be detected using spectroscopy. Kinetic studies of the decay process of the acylperoxido species suggest that the acylperoxido species is a precursor of an active species for alkane oxidation.

Synthesis and properties of dimethylpalladium complex with new PS3-type tripodal tetradentate ligand

Dimethylpalladium complex bearing PS3-type tripodal tetradentate ligand, [PdMe2{P(C6H4-2-S-i-Pr)3}], was synthesized by the reaction of [PdCl{P(C6H4-2-S-i-Pr)3}]X (X = Cl, BF4) with methyllithium (Br free). The dimethylpalladium complex reacted with an equimolar amounts of HCl to give the corresponding chloromethylpalladium complex, [PdClMe{P(C6H4-2-S-i-Pr)3}], while the similar reaction using 2 M amounts of HCl afforded the dichloropalladium complex, [PdCl{P(C6H4-2-S-i-Pr)3}]Cl. The detailed structures of the complexes were revealed by X-ray crystallography and variable temperature 1H NMR studies.

Unprecedented direct cupric-superoxo conversion to a bis-μ-oxo dicopper(III) complex and resulting oxidative activity

Investigations of small molecule copper-dioxygen chemistry can and have provided fundamental insights into enzymatic processes (e.g., copper metalloenzyme dioxygen binding geometries and their associated spectroscopy and substrate reactivity). Strategically designing copper-binding ligands has allowed for insight into properties that favor specific (di)copper-dioxygen species. Herein, the tetradentate tripodal TMPA-based ligand (TMPA = tris((2-pyridyl)methyl)amine) possessing a methoxy moiety in the 6-pyridyl position on one arm (OCH3TMPA) was investigated. This system allows for a trigonal bipyramidal copper(II) geometry as shown by the UV–vis and EPR spectra of the cupric complex [(OCH3TMPA)CuII(OH2)](ClO4)2. Cyclic voltammetry experiments determined the reduction potential of this copper(II) species to be −0.35 V vs. Fc+/0 in acetonitrile, similar to other TMPA-derivatives bearing sterically bulky 6-pyridyl substituents. The copper-dioxygen reactivity is also analogous to these TMPA-derivatives, affording a bis-μ-oxo dicopper(III) complex, [{(OCH3TMPA)CuIII}2(O2−)2]2+, upon oxygenation of the copper(I) complex [(OCH3TMPA)CuI](B(C6F5)4) at cryogenic temperatures in 2-methyltetrahydrofuran. This highly reactive intermediate is capable of oxidizing phenolic substrates through a net hydrogen atom abstraction. However, after bubbling of the precursor copper(I) complex with dioxygen at very low temperatures (−135 °C), a cupric superoxide species, [(OCH3TMPA)CuII(O2−)]+, is initially formed before slowly converting to [{(OCH3TMPA)CuIII}2(O2−)2]2+. This appears to be the first instance of the direct conversion of a cupric superoxide to a bis-μ-oxo dicopper(III) species in copper(I)-dioxygen chemistry.

En Route to Chiral-at-Metal Ruthenium Complexes Containing Tripodal Tetradentate Ligands

Synthetic routes for ruthenium(II) complexes bearing the {Ru(κ4C,N,N′,P-L)} fragment (L = tripodal tetradentate ligand) are explored. Tripodal ligands LH1–LH3 diastereoselectively react with the dimer [{(Cy)RuCl}2(μ-Cl)2] (Cy = η6-p-MeC6H4iPr), affording the p-cymene compounds [(Cy)Ru(κ3N,N′,P-LH)][SbF6] (1–3). Refluxing complexes 1–3 in MeCN gives [Ru(κ3N,N′,P-LH)(NCMe)3][SbF6]2 (4a–6a). In refluxing MeOH, isomers 4a–6a evolve to the corresponding mer complexes 4c–6c. Reaction of [RuCl2(PPh3)3] with LH1 affords [RuCl2(PPh3)(κ3N,N′,P-LH1)] (7) as only one isomer. [RuClH(CO)(PPh3)3] reacts with ligands LH1–LH3 to give [RuH(CO)(PPh3)(κ3N,N′,P-LH)]Cl (8–10). Complexes 8–10 were isolated as a mixture of only two diastereomers as pairs of enantiomers. Refluxing suspensions of complexes 8–10 in 2-ethoxyethanol afforded the corresponding metalated complexes [RuCl(CO)(κ4C,N,N′,P-L)] (11–13), in which the tripodal ligands L1–L3 act as tetradentate ligands. Only one stereoisomer was detected for complexes 11 and 13...

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non‐Heme Oxoiron(IV) Complex that Makes it a Better Oxidant

AbstractOxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non‐heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N‐methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [FeIV(O)(L)]2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe–N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k2′ values for O‐ and H‐atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N‐Fe=O arrangement by 10°.

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non-Heme Oxoiron(IV) Complex that Makes it a Better Oxidant.

Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [FeIV (O)(L)]2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe-N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k2 ' values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.

Tripodal N,P Mixed-Donor Ligands and Their Cobalt Complexes: Efficient Catalysts for Acceptorless Dehydrogenation of Secondary Alcohols.

A new tetradentate tripodal ligand, iPrPPPNHPyMe, and the cobalt complexes were synthesized and characterized. The well-defined cobalt complexes efficiently catalyzed acceptorless dehydrogenation of secondary alcohols into ketones.

Metal as Source of Chirality in Octahedral Complexes with Tripodal Tetradentate Ligands.

The challenging control of the absolute configuration of chiral-at-metal complexes is efficiently achieved using the tripodal tetradentate ligand L. The optical resolution of rac-[RhCl2(κ4C,N,N',P-L)] mediated by (S)-α-phenylglycine provides access to enantiopure complexes of general formula [Rh(κ4C,N,N',P-L)A(Solv)][SbF6]n that enantioselectively catalyze the Diels-Alder reaction between methacrolein and HCp with enantiomeric ratio of up to >99/1. The nature of the active species, the origin of the enantioselectivity and mechanistic details are disclosed by means of NMR spectroscopy and DFT studies.

Structures of cis-dioxomolybdenum(VI) with chiral tetradentate tripodal ligands

Structures of <i>cis</i>-dioxomolybdenum(VI) with chiral tetradentate tripodal ligands

Solution Behaviour and Catalytic Potential towards Oxidation of Dopamine by Oxidovanadium(V) Complexes of Tripodal Tetradentate Ligands

The reaction of dibasic tripodal tetradentate ligands H2L1–4 (I–IV) (prepared by the reaction of 4‐(2‐aminoethyl)morpholine and 2,4‐disubstituted phenols) with [VIVO(acac)2] or VIVOSO4·5H2O in a 1:1 molar ratio in ethanol gave the corresponding mononuclear oxidoethoxidovanadium(V) complexes [VVO(OEt)(L1–4)] (1–4) in good yields. Similar reactions with H2L2 (II) and H2L4 (IV) in acetonitrile resulted in the formation of the corresponding dinuclear complexes [{VVO(L2)}2µ‐O] (5) and [{VVO(L4)}2µ‐O] (6). Single‐crystal X‐ray diffraction analysis of 1, 2 and 4 confirmed the formulation of the mononuclear compounds as VVO complexes, with the ligands behaving in a tetradentate manner, overall yielding a distorted octahedral geometry. The molecular structures of 1, 2 and 5 were calculated by DFT, as well as the corresponding 51V NMR chemical shifts (in methanol and chloroform) and IR frequencies. The V compounds depict time‐dependent solution behaviour, which was studied and rationalised through the combined interpretation of data from spectroscopic techniques (mainly UV/Vis and 51V NMR), mass spectrometry and DFT calculations. They exhibit distinct speciation in different solvents, and it was confirmed that in acetonitrile, they form µ‐O oxido dinuclear species. The alkoxido ligands of complexes [VVO(OR)(L)] in CDCl3 are exchanged upon addition of alcohols; namely the 51V NMR chemical‐shift values of [VVO(OR)(L2)] depict an increasing shielding effect of the OR moiety in the order: OMe &lt; OEt &lt; OiPr &lt; OtBu. Compounds 1–4 were employed as catalyst precursors in the oxidation of dopamine to aminochrome in MeOH using dioxygen (air) as the primary oxidant.