Unusual Coordination Mode Research Articles

Combining electron transfer, spin crossover, and redox properties in metal-organic frameworks

Hofmann coordination polymers (CPs) that couple the well-studied spin transition of the FeII central ion with electron-responsive ligands provide an innovative strategy toward multifunctional metal-organic frameworks (MOFs). Here, we developed a 2D planar network consisting of metal-cyanide-metal sheets in an unusual coordination mode, brought about by infinitely π-stacked redox-active bipyridinium derivatives as axial ligands. The obtained family of materials show vivid thermochromism attributed to electron transfer and/or electronic spin state change processes that can occur either independently or concomitantly. Importantly, the redox activity of the ligands within the structure leads to the quasi-reversible electrochemical reduction reaction on a spin-crossover complex at solid state. These observations have been confirmed via temperature-dependent single-crystal X-ray diffraction, magnetic measurements, Mössbauer, EPR, optical and vibrational spectroscopies as well as quantum chemical calculations.

An overview on copper based heterometallic complexes in combination with d10 metal ions derived from N2O2 donor di-Schiff base ligands

A good number of heterometallic complexes based on various Cu(II) bound “metalloligands” of N2O2 donor di-Schiff bases and some of their reduced counterpart along with group 12 metal ions have been synthesized and characterized by different research groups in the literature. This review focuses on an overview of the synthetic strategies, X-ray crystallographic structures and a few important properties of these complexes. These have been discussed in three major sub-classes: Cu(II)-Zn(II), Cu(II)-Cd(II) and Cu(II)-Hg(II) heterometallic complexes. Judicious combinations of carbonyls and di-amines in an easy and effective manner gives 14 selected di-Schiff base ligands (symmetrical and unsymmetrical) with particular shape, geometry and denticity that direct the synthesis of these metal complexes totally or preferentially towards di-, tri- or poly-nuclear entities. Moreover, owing to their ability to assemble three metal ions in close proximity, these resulting oxido-bridged flexible coordination clusters adapted to various other ligand functionalities with particular shape and sometimes unusual coordination modes with certain degree of control. Such unique structural aspect of this system is immensely important in inorganic synthesis. The inherent flexibility of the di-oxido bridged trinuclear coordination cluster led to produce isomeric linear and bent structures that usually depend upon various factors e.g. the nature of the Schiff base, anionic co-ligands, temperature, solvent etc. The structural aspects of resulting discrete and polymeric self-assemblies have enriched the coordination library that extended to develop numerous heterometallic systems with significant applications and new scopes

Enoxacin fluoroquinolone antibacterial agent complexes with Pt(IV), Ru(III) and Ir(III) ions: Synthesis, spectroscopic, physicochemical characterizations and biological studies

The synthesis, physicochemical characterizations, and biological investigations on the chelation of the antibiotic medication enoxacin (EXN) with certain vital metal ions, such as platinum(IV), ruthenium(III), and iridium(III) are discussed in this study. These compounds structures were interpreted based on elemental analysis and spectral measurements (FTIR, 1H-NMR, UV-Vis electronic, and XRD). The findings show that there is same coordination pathway for the chelation behavior of enoxacin complexes: a monodentate manner. The Pt(IV), Ru(III), and Ir(III) complexes are coordinated through the N atom of the piperazine ring, which is an unusual mode of coordination for this class of compounds. The dissolved complexes in DMSO were found to have non-electrolyte nature based on their molar conductance measurements. The formula for these complexes is [Pt(EXN)3(H2O)2Cl] (1), [Ru(EXN)3(H2O)3] (2), and [Ir(EXN)3(H2O)3] (3). Using scanning electron microscope (SEM) analysis and X-ray powder diffraction (XRD), the nano-scale range of the Pt(IV), Ru(III), and Ir(III) complexes has been determined. Results of in vitro cytotoxicity were examined using MCF-7 cells (human breast cancer cell line) and HepG-2 cells (human hepatocellular carcinoma) that had been grown. The cytotoxic activity of the metal complexes demonstrated a cytotoxic effect against the growth of human breast and liver cancer cell lines.
 KEY WORDS: Enoxacin, Platinum(IV), Complexity, Spectral analyses, Biological assessments
 Bull. Chem. Soc. Ethiop. 2024, 38(3), 685-699. 
 DOI: https://dx.doi.org/10.4314/bcse.v38i3.11

Intensity and lifetime ratiometric luminescent thermometer based on a Tb(III) coordination polymer.

A three-dimensional terbium(III) coordination polymer of formula [Tb(bttb)0.5(2,5-pzdc)0.5]n (1) [H4bttb = 1,2,4,5-tetrakis(4'-carboxyphenyl)benzene and H2-2,5-pzdc = 2,5-pyrazinedicarboxylic acid] was obtained under hydrothermal conditions. The bttb4- tetraanion in 1 adopts the bridging and chelating-bridging pseudo-oxo coordination modes while the 2,5-pzdc2- dianion exhibits a rather unusual bis-bidentate bridging pseudo-oxo coordination mode, both ligands being responsible for the stiffness of the resulting 3D structure. Solid-state photoluminescent measurements illustrate that 1 exhibits remarkable green luminescence emission, the most intense band occurring in the region of 550 nm (5D4 → 7F5) with lifetimes at the millisecond scale. Thermometric performances of 1 reveal a maximum relative sensitivity (Sm) of 0.76% K-1 at 295 K (δT = 0.05 K), constituting a TbIII ratiometric solid luminescent thermometer over the physiological temperature range. Variable-temperature static (dc) magnetic susceptibility measurements for 1 in the temperature range 2.0-300 K show the expected behavior for the depopulation of the splitted mJ levels of the 7F7 ground state of the magnetically anisotropic terbium(III) ion plus a weak antiferromagnetic interaction through the carboxylate bridges. No significant out-of-phase magnetic susceptibility signals were observed for 1 in the temperature range 2.0-10.0 K, either in the absence or presence of a static dc magnetic field.

Syntheses, Structures, and Electronic Properties of Mono‐ and Bimetallic Thiolato Complexes Containing Unusual Coordination Modes of Thiolato Ligands

AbstractSynthesis, bonding and chemistry of mono‐ and bimetallic complexes supported by chelating thiolato ligands have been established. Treatment of [Cp*VCl2]3 (1) with [LiBH4 ⋅ THF] followed by the addition of ethane‐1,2‐dithiol led to the formation of an EPR active bimetallic vanadium thiolato complex [(Cp*V){μ‐(SCH2CH2S)‐κ2S,S′)2{V(SCH2CH2S‐SH)}] (2). In complex 2, two ethane‐1,2‐dithiolato ligands are symmetrically coordinated to two vanadium atoms through μ‐S atoms. Interestingly, when similar reactions were carried out with heavier group 5 metal precursors, such as [Cp*NbCl4] (3 a), it afforded monometallic thiolato complex [Cp*Nb(SCH2CH2S)(SCH2CH2S−CH2S)] (4 a). On the other hand, the Ta‐analogue [Cp*TaCl4] (3 b) yielded thiolato species [Cp*Ta(SCH2CH2S)(SCH2CH2S−CH2S)] (4 b) and [Cp*Ta(SCH2CH2S) (SCH2CH2S−S)] (5). In complexes 4 a and 4 b, one ethane‐1,2‐dithiolato and one trithiolato ligand are coordinated to Nb and Ta centers, respectively. Whereas, in complex 5, one ethane‐1,2‐dithiolato and one 2‐disulfanylethanethiolato is coordinated to the Ta center. Moreover, the photolytic reaction of 5 with [Mo(CO)5 ⋅ THF] yielded heterobimetallic thiolato complex [(Cp*Ta){μ‐(SCH2CH2S)‐κ2S,S′}{μ‐(SCH2CH2S−CH2(CH3)S)κ2S′′ : κ1S‐′′′′ : κ1S′′′′′}{Mo(CO)3}] (6). All the complexes have been characterized by multinuclear NMR spectroscopy and single crystal X‐ray diffraction studies. Further, computational analyses were performed to provide an insight into the bonding of these complexes.

Unusual coordination mode for 1,3-diphosphete ligands towards a Cr-Cr quintuple bond complex.

The reaction of LCr5̲CrL (L = N2C25H29, 1) with the phosphaalkynes R-C≡P (R = tBu, Me, Ad) yields the neutral dimerisation compounds [L2Cr2(μ,η1:η1:η2:η2-P2C2R2)] (R = tBu (2), Me (3)) and the tetrahedrane complex [L2Cr2(μ,η2:η2-PCAd)] (4). The 1,3-diphosphete ligands in complexes 2 and 3 are the first to possess this structural feature spanned over a metal-metal multiple bond, while the slightly bigger adamantyl phosphaalkyne remains a monomer in 4 with a side-on coordination mode.

Complexing properties of 2-pyridylphoshonate and 2-pyridylsulfonate ligands for Zn2+ and Ag+ central atoms

The Zn2+/Ag+-pypo and pysu (pypo = 2-pyridylphosphonate, pysu = 2-pyridylsulfonate) complexes speciation in aqueous solution at I = 0.1 M (KNO3) and 25 °C is described in detail under our experimental synthetic conditions. Protonation and stability constants were determined, and distribution diagrams show protonated and deprotonated complexes formation in the case of Zn2+/Ag+ pypo/pysu binary system, respectively. Moreover, the new complex, {[Zn(H2pypo)2(H2O)2]}n(ClO4)2n·2nH2O, was isolated and its spectral and structural properties were determined. An unusual pypo bidentate coordination mode is described and compared with published ones.

Cytosine as a root to a nonconventional layered hydroxide nanostructure

A self-assembled layered compound of formula {[Cu5(OH)8Cyt2]·2ClO4} n (1) was obtained from the direct reaction between copper perchlorate and cytosine. Thanks to its peculiar structural features, this coordination compound can be classified as an intermediate nonconventional analog of Layered Simple Hydroxides (LSHs) and Layered Double Hydroxides (LDHs), two fascinating classes of inorganic materials with variable chemical composition and structures. Like LDHs, 1 consists of positively charged layers of metal hydroxides and non-framework charge-balancing anions located between the layers. However, the hydroxide layers, similar to LSHs, contain only divalent copper(II) metal ions, supported by cytosine molecules with long contacts. Cytosine coordinates to the metal centers adopting an unusual µ3 coordination mode, the exocyclic oxygen atom simultaneously binding to three adjacent copper(II) centers. This coordination mode has not been observed with transition metal ions. This work further demonstrates that nucleobases and their derivatives, despite presenting a limited pool of molecules, can potentially afford a significantly expanded set of compounds with diverse nanostructures, owing to their multiple metal binding sites of different coordination properties and their ability to direct assembly of highly ordered structures. Their potential as building blocks should be further explored for the preparation of new bio-inorganic hybrid materials.

Synthesis, chemical and physical properties of lanthanide(III) (Nd, Gd, Tb) complexes derived from (E)-ethyl 4-(2-hydroxybenzylideneamino)benzoate

The 1:2 reaction between Ln(NO3)3·6H2O (1) (Ln = Nd (a), Gd (b) or Tb (c)) and ((E)-ethyl 4-((2-hydroxybenzylidene)amino)benzoate) (LN,OH) in ethanol has provided access to the complexes [LnIII(LNH,O)2(NO3)3(H2O)] (2a–c) in good yield. The isomorphous structures of 2a–c have been determined by single-crystal X-ray crystallography. The LnIII ions in 2a–c are coordinated by 9 oxygen atoms: two from the phenolate oxygen atoms of the Schiff bases, six from three chelating nitrato groups and one aqua ligand. The ligands LN,OH behave as monodentate phenolate O–donors, whereby an additional hydrogen atom is bonded to the imino nitrogen atom, resulting in a formally neutral ligand that adopts an exceptionally unusual coordination mode engaging in the zwitterionic form. The nine donor atoms that define the coordination polyhedra surrounding the LnIII ion are best described as capped square antiprisms. The crystal structures are based on a variety of intermolecular interactions. Hirshfeld surface analysis was used to assess the degree of interactions between the molecules. Susceptibility measurements on 2a–2c confirm the occurrence of the free ion ground 2S+1LJ state at room temperature. However, the magnetization measurements at 2 K revealed a lack of saturation at the highest applied field of 70,000 Oe for both 2a and 2c, indicating the presence of substantial magnetic anisotropy in these compounds. On the other hand, 2b exhibits perfect paramagnetic behavior with magnetic saturation at high fields, indicating the absence of significant magnetic anisotropy in this compound. Solid-state IR and UV/VIS data are discussed in terms of the structural features. Solid 2a–c emit in the visible region upon UV-Excitation with the emission exhibiting dopant characteristics.

Redox-active BIAN-based Diimine Ligands in Metal-Catalyzed Small Molecule Syntheses.

α‐Diimine ligands have significantly shaped the coordination chemistry of most transition metal complexes. Among them, bis(imino)acenaphthene ligands (BIANs) have recently been matured to great versatility and applicability to catalytic reactions. Besides variations of the ligand periphery, the great versatility of BIAN ligands resides within their ability to undergo facile electronic manipulations. This review highlights key aspects of BIAN ligands in metal complexes and summarizes recent contributions of metal‐BIAN catalysts to syntheses of small and functionalized organic molecules.

A tryptophan-based copper(ii) coordination polymer: catalytic activity towards Suzuki–Miyaura cross-coupling reactions

An unusual tryptophan coordination mode with copper(ii) wherein l-tryp is coordinated through the carboxylate and amine groups. A heterogeneous catalyst for the Suzuki cross-coupling reaction with ∼98% yield under normal reaction conditions.

Unusual Coordination Pattern of the Lithium Center in a Chiral α-Lithiated Silicon Compound

An understanding of how stereogenic centers can influence the construction of a new metalated chirality center requires profound investigations concerning coordination and aggregation effects. A racemic silicon-stereogenic aminomethoxysilane (2) was chosen as the starting compound, which is additionally equipped with a (trimethylsilyl)methylene bridge and an aminomethyl function. Metalation of 2 with tert-butyllithium led to the inversion-symmetric dimer (u-3)2, which shows an unusual coordination mode with intramolecular agostic C–H···Li interactions capping the three-coordinate trigonal-planar lithium centers. Compound 3 was characterized by X-ray crystallography and NMR spectroscopy in solution. Reaction of the lithiated intermediate 3 with chlorotrimethylstannane provided the first insight into the configurational stability of the metalated stereogenic center.

Role of Metals and Thiolate Ligands in the Structures and Electronic Properties of Group 5 Bimetallic-Thiolate Complexes.

Syntheses, structures, and electronic properties of group 5 metal-thiolate complexes that exhibit unusual coordination modes of thiolate ligands have been established. Room-temperature reaction of [Cp*VCl2]3 (Cp* = η5-C5Me5) with Na5[B(SCH2S)4] led to the formation of [Cp*VO{(SCH2)2S}] (1). The solid-state X-ray structure of 1 shows the formation of six-membered l,3,5-trithia-2-vanadacyclohexane that adopted a chair conformation. In a similar fashion, reactions of heavier group 5 precursors [Cp*MCl4] (M = Nb or Ta) with Na5[B(SCH2S)4] yielded bimetallic thiolate complexes [(Cp*M)2(μ-S){μ-C(H)S3-κ2S:κ2S',S″}{μ-SC(H)S-κ2C:κ2S‴,S''''}] (3a: M = Nb and 3b: M = Ta). One of the key features of molecules 3a and 3b is the presence of square-pyramidal carbon, which is quite unusual. The reactions also yielded bimetallic methanedithiolate complexes [(Cp*Nb)2(μ-S)(μ-SCH2S-κ2S,S')(μ,η2:η2-BH3S)] (2) and [(Cp*Ta)2(μ-O)(μ-SCH2S-κ2S,S')(μ-H){μ-S2C(H)SCH2S-κ2S″:κ2S‴,S''''}] (4). Complex 2 contains a methanedithiolate ligand that stabilizes the unsaturated niobaborane species. On the other hand, one ((mercaptomethyl)thio)methanedithiolate ligand {C2H4S3} is present in 4, which is coordinated to metal centers and exhibits the {μ-κ2S″:κ2S‴,S''''} bonding mode. Along with the formation of 3b and 4, the reaction of [Cp*TaCl4] with Na5[B(SCH2S)4] yielded [(Cp*Ta)2(μ-S){μ-(SBS)S(CH2S)2(BH2S)-κ2B:κ2S:κ4S',S″,S‴,S''''}] (5) containing a trithiaborate unit (BS3). Complex 5 consists of pentacoordinate boron that resides in a square-pyramidal environment. All the complexes have been characterized by multinuclear NMR, UV-vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies.

Front Cover: Dynamic Gold(I) Complexes of Hexa‐tert‐butyl‐octaphosphane (Eur. J. Inorg. Chem. 9/2020)

The Front Cover shows a huge octopus wrecking a ship. Be on your guard! Like a shadow from the depths of the ocean arises the monstrous OCTAPhosphane from the almost forgotten realm of phosphorus-rich compounds. An ancient molecule with eight directly connected phosphorus atoms and an unquenchable thirst for coordination that devours gold(I) atoms like the octopus an unsuspecting ship. But do not grieve, from the merciless attack emerge magnificent complexes which show an unusual coordination mode of the gold(I) atoms that, in combination with the flexibility of the ligand, leads to a fascinating dynamic behavior in solution (cover design adapted from original public domain image https://hu.wikipedia.org/wiki/F%C3%A1jl:Pierre_Denys_de_Montfort_ske6.jpg). More information can be found in the Communication by T. Grell and E. Hey-Hawkins. For more on the story behind the cover research, see the Cover Profile.

A heterodinuclear, formal Au+IPt0 complex with weakly bound alkene ligands.

[(Ph3P)AuPt(nbe)3][BAr4F] (nbe = norbornene) constitutes the first olefin-containing, formal Au+IPt0 complex. The unusual coordination mode and the electronic properties have been analyzed spectroscopically and by calculations. The low binding energy of the nbe ligands make this complex a valuable precursor for formal Au+IPt0 complexes and a candidate for heterobimetallic catalysis.

Water-soluble titanium-polyoxomolybdate with external μ3 bridging oxygen coordination on a lacunary Keggin structure.

A titanium-polyoxomolybdate K6(H3O)8[{PMo9O34TiO}2]·32H2O (1) was successfully synthesized and structurally characterized. It is an unprecedented instance that the titanium-containing polyoxomolybdate is water-soluble and was synthesized by using a conventional aqueous solution method. The Ti4+ ions exist in an unusual coordination mode. Besides, compound 1 as a wide band gap semiconductor showed evident photocatalytic activity for the oxidation of benzaldehyde into benzoic acid.

Unusual coordination mode of aroyl/acyl thiourea ligands and their π‐arene ruthenium (II) piano‐stool complexes: Synthesis, molecular geometry, theoretical studies and biological applications

Two mononuclear ruthenium complexes (1 and 2) with aroyl/acylthiourea as an ancillary ligand of type, [(η6‐p‐cymene)RuCl(L‐N,S)], where [L1 = 2,4‐dichloro‐N‐(o‐tolylcarbamothioyl)benzamide] and L2 = N‐(phenylcarbamothioyl)cyclohexanecarboxamide] were synthesized and well characterized. The single crystal X‐ray diffraction studies revealed the coordination mode and the geometry of the complexes. The two complexes adopted general piano‐stool (three‐legged) geometry with a novel coordination mode of aroyl/acylthiourea through amide N (anionic) and thiocarbonyl S (neutral). This type of monobasic bidentate coordination of the aroyl/acylthiourea ligand was witnessed the first time around the metal ion. The coordination of the complexes was well explained through geometric parameters and frontier molecular orbital parameter values computed at the B3LYP/SDD level. The synthesized complexes were also screened for their antibacterial, antifungal, antioxidant and in vitro antiproliferative activities. Complexes exhibited good antimicrobial agents against various pathogens. The antioxidant activity of the complex 2 has shown most potent activity with IC50 value of 48.55 ± 1.7 μM compared to the reference drug. In addition, the in vitro antiproliferative activity of the complex 2 showed excellent activity against HepG‐2 cell line with the IC50 value of 24.30 ± 1.20 μM which is close to Doxorubicin standard drug.

Emissive Pd(II) thiosemicarbazones bearing anthracene: New complexes with unusual coordination mode

New emissive Pd(II) complexes with anthracene-based thiosemicarbazones have been readily prepared in high yields. X-ray crystallographic analysis reveals that N(4)-substituted groups of thiosemicarbazone ligands exert marked effect on coordination mode of Pd(II) central ion. Rare asymmetric chelation based on monothiosemicarbazone is found with small N(4)-substituted groups such as methyl or ethyl. TD-DFT calculations partly support the preferential formation of this unusual arrangement. The complexes display ligand-centered fluorescence emission which is independent of the coordination modes of Pd(II).

Unusual Bridging Coordination Modes of THF and Piv Anions in the Hexanuclear Nickel(II) Carboxylate Complex

The recrystallization of complex [Ni9(OH)6(Piv)12(HPiv)4] (Piv– is anion of trimethylacetic acid) from a THF–MeCN (6 : 1) mixture gives a new hexanuclear compound of nickel(II): [Ni6(µ3-OH)2(Piv)10(THF)4]. Its structure (CIF file CCDC no. 1579976) exhibits rare examples of coordination of the µ2-THF and µ4-Piv ligands.

Novel Topology in Semiconducting Tetrathiafulvalene Lanthanide Metal‐Organic Frameworks

AbstractTetrathiafulvalene tetrabenzoate (TTFTB) and several lanthanide ions self‐assemble into metal‐organic frameworks (MOFs) that exhibit a novel topology, a (3,3,3,6,6)‐coordinated net, which features an unusual ligand coordination mode and stacking motif. The Yb and Lu MOFs are electrically conductive, with pellet conductivity values of 9(7) × 10−7 and 3(2) × 10−7 S/cm, respectively. The crystallographically‐determined bond lengths indicate partial oxidation of the ligand, with close S ⋅ ⋅ ⋅ S contacts between ligands providing likely charge transport pathways in the material. Magnetometry reveals temperature‐independent paramagnetism, consistent with the presence of ligand‐based radicals, as well as weak antiferromagnetic coupling between Yb3+ centers. These results illustrate the diversity of MOF structures and properties that are accessible with the TTFTB ligand owing to its electroactive nature, propensity for intermolecular interactions, and conformational flexibility.