Tripodal Tetraamine Ligands Containing Three Pyridine Units: The other Polypyridyl Ligands

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.

Modulating the electronic properties of divalent lanthanoid complexes with subtle ligand tuning

Five new compounds of formula [Ln^II(Me_ntpa)₂](BPh₄)₂ (Ln = Eu, n = 0 (1-Eu), n = 2 (2-Eu) and n = 3 (3-Eu); Ln = Yb, n = 0 (1-Yb) and n = 2 (2-Yb); tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to successive methylation of the 6-position of the pyridine rings of Me_ntpa) have been synthesized and their structural, photophysical and electrochemical properties investigated. The Ln^II ions in the five complexes possess cubic coordination geometry and exhibit only small structural differences, due to the lengthening of the Ln-N bonds to accommodate the additional steric bulk associated with increasing methylation of the Me_ntpa ligands. Photophysical studies indicate moderate shifts in absorbance, emission and excitation bands associated with the 4f⁷ ↔ 4f⁶5d¹ (Eu^II) and 4f¹⁴ ↔ 4f¹³5d¹ (Yb^II) transitions, while electrochemistry reveals modulation of the redox potential of the Ln^II to Ln^III oxidation. There is a strong correlation between Ln-N bond lengths and both the photophysical transition energies and metal redox-potentials, revealing how subtle ligand changes and ligand field effects can be used to modulate the electronic properties of complexes of divalent lanthanoid ions. Utilization of these insights may ultimately afford design and property tuning strategies for future functional molecular complexes based on divalent lanthanoid metals.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

The chemistry of NHCcore pincer ligands of LX₂ type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R₃P⁺-CR₂^-) or by a heteroatom through amide (R₂N^-), oxide (RO^-), or thio(seleno)oxide (RS^-, RSe^-) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species.

Oxidative C-N bond cleavage of (2-pyridylmethyl)amine-based tetradentate supporting ligands in ternary cobalt(ii)-carboxylate complexes

Three mononuclear cobalt(ii)-carboxylate complexes, [(TPA)Co^II(benzilate)]⁺ (1), [(TPA)Co^II(benzoate)]⁺ (2) and [(iso-BPMEN)Co^II(benzoate)]⁺ (3), of N4 ligands (TPA = tris(2-pyridylmethyl)amine and iso-BPMEN = N¹,N¹-dimethyl-N²,N²-bis((pyridin-2-yl)methyl)ethane-1,2-diamine) were isolated to investigate their reactivity toward dioxygen. Monodentate (η¹) binding of the carboxylates to the metal centre favours the five-coordinate cobalt(ii) complexes (1-3) for dioxygen activation. Complex 1 slowly reacts with dioxygen to enable the oxidative decarboxylation of the coordinated α-hydroxy acid (benzilate). Prolonged exposure of the reaction solution of 2 to dioxygen results in the formation of [(DPA)Co^III(picolinate)(benzoate)]⁺ (4) and [Co^III(BPCA)₂]⁺ (5) (DPA = di(2-picolyl)amine and HBPCA = bis(2-pyridylcarbonyl)amide), whereas only [(DPEA)Co^III(picolinate)(benzoate)]⁺ (6) (DPEA = N¹,N¹-dimethyl-N²-(pyridine-2-ylmethyl)-ethane-1,2-diamine) is isolated from the final oxidised solution of 3. The modified ligand DPA (or DPEA) is formed via the oxidative C-N bond cleavage of the supporting ligands. Further oxidation of the -CH₂- moiety to -C([double bond, length as m-dash]O)- takes place in the transformation of DPA to HBPCA on the cobalt(ii) centre. Labelling experiments with ¹⁸O₂ confirm the incorporation of oxygen atoms from molecular oxygen into the oxidised products. Mixed labelling studies with ¹⁶O₂ and H₂O¹⁸ strongly support the involvement of water in the C-N bond cleavage pathway. A comparison of the dioxygen reactivity of the cobalt complexes (1-3) with those of several other five-coordinate mononuclear complexes [(TPA)Co^II(X)]^+/2+ (X = Cl, CH₃CN, acetate, benzoylformate, salicylate and phenylpyruvate) establishes the role of the carboxylate co-ligands in the activation of dioxygen and subsequent oxidative cleavage of the supporting ligands by a metal-oxygen oxidant.

Synthesis and structures of titanium complexes bearing tetradentate tripodal [O2XC] ligands (X = C, P)

We report the synthesis and structures of titanium complexes supported by tripodal mixed-donor [O₂CC] and [O₂PC] ligands. Stepwise cyclometallation of a chelating bis(phenoxide) complex led to tetradentate binding of the [O₂CC] ligand to the titanium center. Phosphination of a Ti-C bond of the [O₂CC] complex afforded the tripodal [O₂PC] ligand.

Click chemistry as a route to the synthesis of structurally new and magnetically interesting coordination clusters: a {Ni} complex with a trapezoidal prismatic topology

The synthesis of a new {Ni₈} cluster bearing tetrazolate- and azido-bridging ligands, and supported by chelating α-methyl-2-pyridine-methanol (mpmH) groups, is described herein. The reported compound has a unique trapezoidal prismatic topology, resulting from an unexpected in situ click reaction between the MeCN reaction solvent and the N₃^- ions under mild, room-temperature conditions. Such a click chemistry approach to the preparation of 0-D compounds is relatively unexplored and represents a fruitful strategy for the synthesis of new coordination clusters and molecule-based magnetic materials.

Tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl metal complexes, [TismPriBenz]M: a new class of metallacarbatranes, isomerization to a tris(N-heterocyclic carbene) derivative, and evidence for an inverted ligand field

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPriBenz]Li, [TismPriBenz]MgMe, [TismPriBenz]Cu and [TismPriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ3-coordination, namely [κ3-TismPriBenz]ZnMe, has also been obtained. Furthermore, the [TismPriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPriBenz]Cu to [κ4-C4-TismPriBenz*]Cu. The transannular M-C bond lengths in the atrane compounds are 0.19-0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu-Catrane interactions in [TismPriBenz]Cu and [κ4-C4-TismPriBenz*]Cu are characterized by an "inverted ligand field", in which the occupied antibonding orbitals are localized more on carbon than on copper.

Cadmium Compounds with an [N3C] Atrane Motif: Evidence for the Generation of a Cadmium Hydride Species

Tris(2-pyridylthio)methane ([Tptm]H) has been employed to synthesize a series of cadmium carbatrane compounds that feature an [N3C] coordination environment. Specifically, [Tptm]H reacts with Cd[N(SiMe3)2]2 to afford [Tptm]CdN(SiMe3)2, which thereby provides access to other derivatives. For example, [Tptm]CdN(SiMe3)2 reacts with (i) CO2 to form {[Tptm]Cd(μ-NCO)}2 and (ii) Me3SiOH and Ph3SiOH to form {[κ3-Tptm]Cd(μ-OSiMe3)}2 and [Tptm]CdOSiPh3, respectively. The siloxide compound {[κ3-Tptm]Cd(μ-OSiMe3)}2 reacts with Me3SiX (X = Cl, Br, O2CMe) to give [Tptm]CdX, while the reaction with PhSiH3 in the presence of CO2 generates the formate complex, [Tptm]CdO2CH, thereby providing evidence for the generation of a proposed cadmium hydride intermediate, {[Tptm]CdH}.

Reversible magnetogenic cobalt complexes

A cobalt tris(2-pyridylmethyl)amine complex cycles between stable paramagnetic Co(ii) and diamagnetic Co(iii) forms with corresponding changes in the MRI contrast.

Versatile Coordination Mode of a New Pyridine-Based Ditopic Ligand with Transition Metals: From Regular Pyridine to Alkyne and Alkenyl Bindings and Indolizinium Formation

The new BPMPB ligand, namely, bis[1-bis(2-pyridylmethyl),1 (pyridyl)]butyne, can be very easily obtained as a side product in the known reaction of picolyl chloride and sodium acetylide (which major product is the known terminal alkyne-substituted tripod). This symmetrical ligand contains two identical coordination sites with two methylenepyridines and one pyridyl group on each side, linked by an alkyne function providing a semirigid segment. Together with the molecular structure of the ligand which is reported, we describe the preparation of complexes with Fe(II)Cl2, Co(II)Cl2, Ni(II)Cl2, Cu(I)Cl, and Zn(II)Cl2 salts. All complexes have been characterized by X-ray diffraction studies as well as by standard spectroscopic techniques. The striking point in this work is the diversity of the structures that are obtained. Co(II) and Zn(II) provide isostructural dinuclear complexes in which both coordination sites are occupied within a tetrahedral symmetry. The Cu(I) complex is also a dinuclear compound, but in that case, the copper atom is coordinated to the alkyne moiety, two pyridines, and a bridging chloride. The (13)C NMR spectrum of the copper complex confirms that the metal center is coordinated to the alkyne in solution. The coordination of Ni(II) results in the formation of a mononuclear complex in which a pyridine has fused with the alkyne moiety to generate an indolizinium group; the structure of the corresponding alkenyl complex is reported. Finally, the addition of FeCl2 to the ligand results in the formation of a mononuclear complex with a free, noncoordinated indolizinium. The sequence developed in the present work illustrates the possibility for the metal centers to adopt various coordination modes which may be relevant to the conversion of an alkyne and a pyridyl unit into indolizinium.

Introducing [Mn(CO)3(tpa-κ(3)N)](+) as a novel photoactivatable CO-releasing molecule with well-defined iCORM intermediates - synthesis, spectroscopy, and antibacterial activity

[Mn(CO)3(tpa-κ(3)N)]Br was prepared as a novel photoactivatable CO-releasing molecule (PhotoCORM) from [MnBr(CO)5] and tris(2-pyridylmethyl)amine (tpa) for the delivery of carbon monoxide to biological systems, with the κ(3)N binding mode of the tetradentate tpa ligand demonstrated by X-ray crystallography. The title compound is a CORM prodrug stable in solution in the dark for up to 16 h. However, photoactivation at 365 nm leads to CO release from the metal coordination sphere and transfer to haem proteins, as demonstrated by the standard myoglobin assay. Different iCORM intermediates could be detected with solution IR spectroscopy and assigned using DFT vibrational calculations. The antibacterial activity of the complex was studied on Escherichia coli. No effects were observed when the cultures were either kept in the dark in the presence of PhotoCORM or illuminated in the absence of metal complex. However, photoactivation of [Mn(CO)3(tpa-κ(3)N)]Br at 365 nm led to the appearance of the spectral signatures of CO-coordinated haems in the terminal oxidases of the bacterial electron transport chain in whole-cell UV/Vis absorption spectra. Significant internalization of the PhotoCORM was demonstrated by ICP-MS measurement of the intracellular manganese concentration. In particular when using medium with succinate as the sole carbon source, a very pronounced and concentration-dependent decrease in the E. coli growth rate could be observed upon illumination in the presence of metal complex, which is attributed to the constrained energy metabolism under these conditions and a strong indicator of terminal oxidase inhibition by carbon monoxide delivered from the PhotoCORM.

A robust and efficient cobalt molecular catalyst for CO2 reduction

A robust and efficient cobalt catalyst for CO₂ reduction.

Dinuclear cobalt(ii) and cobalt(iii) complexes of bis-bidentate napthoquinone ligands

Seven dinuclear cobalt complexes with bridging bis-bidentate naphthoquinone ligands reveal the steric influence of the ancillary capping ligand on the redox potential of the cobalt centres, with metal-catalysed derivatisation of the bridging ligand also observed.

Unexpected metal ion-assisted transformations leading to unexplored bridging ligands in NiII coordination chemistry: the case of PO3F2− group

The bridging potency of the O-rich PO₃F²⁻ group in Ni^II/oximate chemistry has been studied leading to interesting coordination compounds and revealing an unprecedented PO₃F²⁻ to P₂O₅F₂²⁻in situ transformation.

Insights into water coordination associated with the CuII/CuI electron transfer at a biomimetic Cu centre

The coordination chemistry of an aqua Cu complex was investigated in non-coordinating solvents and in ionic liquids.

Tri(pyridylmethyl)phosphine: the elusive congener of TPA shows surprisingly different coordination behavior

Tri(pyridylmethyl)phosphine (TPPh), the remarkably elusive congener of tri(pyridylmethyl)amine (TPA), has been prepared, as well as the relative tri(N-methyl-pyridylamino)phosphine (TPAMP). The coordination properties of these new ligands have been evaluated for chromium(III), iron(II), and ruthenium(II) complexes and compared with the related TPA complexes. In all cases, a different coordination behavior has been observed whereby TPPh and TPAMP always act as tridentate ligands. A chromium(III) complex [Cr(TPPh)Cl3] has been prepared, which has shown low ethylene oligomerization activity. Octahedral low spin iron(II) complexes [Fe(TPPh)2](2+) and [Fe(TPAMP)2](2+) were obtained with two ligands bound to the metal center. Ruthenium(II) chloro complexes of TPA and TPPh undergo ligand exchange reactions in acetonitrile, and the ruthenium(II) complex [Ru(MeCN)2(TPA)](2+) can be oxidized by m-CPBA in acetonitrile to give a transient ruthenium(IV) oxo complex [Ru(O)(MeCN)(TPA)](2+). Attempts to generate high valent ruthenium(IV) oxo TPPh or TPAMP complexes could not be achieved, probably due to insufficient stabilization by these strong field ligands.