Mononuclear Metal Complexes Research Articles

Nonaqueous capillary gel electrophoretic analysis of metal nanoclusters in polymeric-DMSO-Li+ systems.

In this study, we investigated a combination of nonaqueous CE with capillary gel electrophoresis to achieve highly efficient analysis of metal nanoclusters. In the nonaqueous capillary gel electrophoresis (NACGE), PVA and hydroxypropyl methylcellulose were dissolved in DMSO. In addition, to enhance the entanglement of the polymer chains, Li+ ions were also added. By employing the PVA-DMSO-Li+ solution, we studied the effects of the molecular weight, the degree of hydrolysis, and the concentration of the polymers and Li+ on the separation. As a result, good separations of standard mononuclear metal complexes and tetrairon nanoclusters were achieved by NACGE.

Ruthenium, rhodium and iridium complexes containing pyrimidine based thienyl pyrazoles: Synthesis and antibacterial studies

The reaction of pyrimidine based electron-rich heterocyclic thiophene pyrazoles and halide bridged arene d6 metal precursors yielded a series of mononuclear and dinuclear half sandwich d6 metal complexes. Mononuclear and dinuclear complexes formed by the ratio-based reaction between ligand and metal precursor. All these cationic complexes have been characterized by IR, UV–Vis, 1H NMR, 13C NMR spectroscopic techniques. Complex 5 has been established by single-crystal analysis. X-ray diffraction studies revealed the formation of mononuclear and dinuclear complexes and suggest that the vicinity around the metal atom is distorted octahedral. An in vitro study to screen the antibacterial potential of these complexes against pathogenic bacteria, S. aureus, K. pneumoniae, and E. coli was addressed. All the complexes display a better zone of inhibitions for both Gram-positive (S. aureus) and Gram-negative strains (K. pneumoniae, and E. coli). The minimum inhibitory concentrations (MICs) for the most active complex ranged from 0.125 to 0.25 mg/ml for S. aureus and Klebsiella Pneumoniae and 0.25–0.5 mg/ml for E. coli.

Crystal structures and Hirshfeld surface analysis of trans-bis-(thio-cyanato-κN)bis-{2,4,6-trimethyl-N-[(pyridin-2-yl)methyl-idene]aniline-κ2 N,N'}manganese(II) and trans-bis-(thio-cyanato-κN)bis-{2,4,6-trimethyl-N-[(pyridin-2-yl)methyl-idene]aniline-κ 2 N,N'}nickel(II)).

Two new mononuclear metal complexes involving the bidentate Schiff base ligand 2,4,6-trimethyl-N-[(pyridin-2-yl)methyl-idene]aniline (C15H16N2 or PM-TMA), [Mn(NCS)2(PM-TMA)2] (I) and [Ni(NCS)2(PM-TMA)2] (II), were synthesized and their structures determined by single-crystal X-ray diffraction. Although the title compounds crystallize in different crystal systems [triclinic for (I) and monoclinic for (II)], both asymmetric units consist of one-half of the complex mol-ecule, i.e. one metal(II) cation, one PM-TMA ligand, and one N-bound thio-cyanate anion. In both complexes, the metal(II) cation is located on a centre of inversion and adopts a distorted octa-hedral coordination environment defined by four N atoms from two symmetry-related PM-TMA ligands in the equatorial plane and two N atoms from two symmetry-related NCS- anions in a trans axial arrangement. The tri-methyl-benzene and pyridine rings of the PM-TMA ligand are oriented at dihedral angles of 74.18 (7) and 77.70 (12)° for (I) and (II), respectively. The subtle change in size of the central metal cations leads to a different crystal packing arrangement for (I) and (II) that is dominated by weak C-H⋯S, C-H⋯π, and π-π inter-actions. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to qu-antify these inter-molecular contacts, and indicate that the most significant contacts in packing are H⋯H [48.1% for (I) and 54.9% for (II)], followed by H⋯C/C⋯H [24.1% for (I) and 15.7% for (II)], and H⋯S/S⋯H [21.1% for (I) and 21.1% for (II)].

Photophysics and photochemistry with Earth-abundant metals - fundamentals and concepts.

Recent exciting developments in the area of mononuclear photoactive complexes with Earth-abundant metal ions (Cu, Zr, Fe, Cr) for potential eco-friendly applications in (phosphorescent) organic light emitting diodes, in imaging and sensing systems, in dye-sensitized solar cells and as photocatalysts are presented. Challenges, in particular the extension of excited state lifetimes, and recent conceptual breakthroughs in substituting precious and rare-Earth metal ions (e.g. Ru, Ir, Pt, Au, Eu) in these applications by abundant ions are outlined with selected examples. Relevant fundamentals of photophysics and photochemistry are discussed first, followed by conceptual and instructive case studies.

Novel Cr(III), Co(II), Zn(II) and Cd(II) Complexes of Schiff Base Ligand Derived from Salicyaldehyde and 3,4-Diaminotoluene: Synthesis, Characterization and Biological Screening

A new Schiff base ligand (L) [2,2′-((1Z,1′Z)-((4-methyl-1,2- phenylene)bis(azanylidene))bis(methanylylidene))diphenol], derived from condensation of salicyaldehyde and 3,4-diamino toluene. The synthesized ligand was used for complexation with different metal ions Cr(III), Co(II), Zn(II) and Cd(II) using a molar ratio of metal:ligand (1:1). The synthesized ligand and its metal complexes were characterized by TLC, NMR (1H & 13C), UV-visible, mass, FT-IR spectroscopies, elemental analysis, magnetic moment and conductivity measurement. On the basis of above studies the proposed structure of synthesized mononuclear metal complexes have been found to possess tetrahedral geometry while Cr(III) and Co(II) ions possess octahedral geometry. The spectral studies revealed that the synthesized ligand was acting as tetradentate chelating agent and coordinated to metal centre via deprotonated phenolate oxygen and azomethine-N atom. The biological activity of ligand and its metal complexes were screened in vitro against Gram-negative bacteria (Salmonella typhimurium and E. coli), Gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis) and fungus (Aspergillus fumigatus) to ascertain their antibacterial and antifungal properties.

Synthesis of Two Mononuclear Schiff Base Metal (M = Fe, Cu) Complexes: MOF Structure, Dye Degradation, H2O2 Sensing, and DNA Binding Property.

O-Vanillin and ethylenediamine were used for the synthesis of N,N′-bis(O-vanillinidene)ethylenediamine (O-VEDH2) Schiff base ligand, which was characterized by UV–visible and 1H NMR spectroscopy. The Schiff base ligand O-VEDH2 has been employed to synthesize novel [C18H22FeN2O6]NO3 (1) and C18H16CuN2O5 (2) complexes by the simple slow evaporation method. The single crystals were characterized by X-ray crystallography, as well as Fourier-transform infrared and UV–visible spectroscopy techniques. Complexes 1 and 2 crystallize in monoclinic and orthorhombic space groups with P121/n1 (14) and Pnma (62) point groups, respectively, and both show metal–organic frameworks like structures. Complexes 1 and 2 have optical band gaps of 4.1 and 2.9 eV, respectively, indicating their semiconducting properties. The dye degradation activity and H2O2 sensing of the complexes 1 and 2 were determined in different conditions. The photocatalytic test was performed in the presence of sunlight, methylene blue (MB), and complexes 1 and 2. An interesting result was obtained that complex 2 has degradation ability against MB and the rate constant (K) is 5.46 × 10–5 s–1, whereas complex 1 has H2O2 sensing properties. Both complexes are bound to Calf-thymus DNA by intercalation binding mode, and binding constants (Kb) of complexes 1 and 2 are 3.77 × 103 and 1.49 × 104 M–1, respectively.

Exploring the High-Temperature Frontier in Molecular Nanomagnets: From Lanthanides to Actinides

Molecular nanomagnets based on mononuclear metal complexes, also known as single-ion magnets (SIMs), are crossing challenging boundaries in molecular magnetism. From an experimental point of view, this class of magnetic molecules has expanded from lanthanoid complexes to both d-transition metal and actinoid complexes. From a theoretical point of view, more and more improved models have been developed, and we are now able not only to calculate the electronic structure of these systems on the basis of their molecular structures but also to unveil the role of vibrations in the magnetic relaxation processes, at least for lanthanoid and d-transitionmetal SIMs. This knowledge has allowed us to optimize the behavior of dysprosocenium-based SIMs until reaching magnetic hysteresis above liquid-nitrogen temperature. In this contribution, we offer a brief perspective of the progress of theoretical modeling in this field. We start by reviewing the developed methodologies to investigate the electronic structures of these systems and then move on focus to the open problem of understanding and optimizing the vibrationally induced spin relaxation, especially in uranium-based molecular nanomagnets. Finally, we discuss the differences in the design strategies for 4f and 5f SIMs, including an analysis of the metallocenium family.

Crystal structure, DNA interactions, antioxidant and antitumor activity of thermally stable Cu(II), Ni(II) and Co(III) complexes of an N,O donor Schiff base ligand

Three mononuclear binary metal complexes of the general formula [M(L)2] and [M1(L)3] (M = Cu, Ni and M1 = Co, LH = 2-(((furan-2-yl)methylimino)methyl)-4-methylphenol) were synthesized. The obtained compounds could be crystallized and were successfully characterized using spectroscopic techniques, together with elemental analysis. The spectroscopic elucidation indicates a monobasic bidentate nature of the ligand, coordinated via a deprotonated phenolic oxygen atom and an azomethine nitrogen atom, which suggests a square planar geometry around the copper and nickel ions, and an octahedral geometry around the cobalt ion. The absorption spectral titrations of the complexes with DNA impart that the binding affinity (Kb) of the complexes vary in the order 3 > 1 > 2, with hypochromism percentages of 12.44–10.47. The interactions of the complexes with pBR322 DNA were followed by electrophoretic mobility spectroscopic studies and the results distinctly show that the complexes exhibit good cleavage properties. The antioxidant activities of the synthesized compounds were investigated with DPPH and the complexes have superior radical scavenging activity. The cytotoxic activities of all the compounds were evaluated towards the human breast adenocarcinoma cell line (MCF-7) and the oral carcinoma cell line (KB3) using an MTT assay with Etoposide as a standard and the results show the IC50 values of the complexes vary from 40.29 ± 0.87 to 75.07 ± 0.83. The complexes and the ligand were screened for their in vitro antibacterial activity against bacterial strains; the complexes are more active than the ligand against the tested microbial strains.

Joshua S. Figueroa

Joshua S. Figueroa

Indole-derived water-soluble N, O bi-dentate ligand-based mononuclear transition metal complexes: in silico and in vitro biological screening, molecular docking and macromolecule interaction studies

A novel tryptophan-derived Schiff base ligand (potassium (E)-2-((4-chloro-3-nitrobenzylidene)amino)-3-(1H-indol-3-yl)propanoate) and a series of its transition metal complexes of the types [ML2] and [ML(1,10-phen)2]Cl where M = Cu(II), Co(II), Ni(II) and Zn(II) were prepared. They were analyzed by various spectral and physicochemical studies. The XRD data were also used to determine the average lattice parameters and crystalline size of the compounds. All the synthesized compounds were tested against a series of five bacterial and fungal strains. The obtained results showed that the biological activity of free ligand was increased on complexation. PASS online software predicts the various biological activities of ligand such as enzyme inhibitor, antiviral, analgesic and antituberculosis. The in silico theoretical prediction of synthesized compounds is also deliberated by Swiss ADME predictor which gives the properties of molecular hydrophobicity (log P), topological polar surface area (TPSA) and oral bioavailability score. The binding energy of the docked molecule with macromolecules 1BNA and 3EQM is also determined by using Hex 8.0 software. The ligand has the least binding energy score which signifies that the potential of binding is greater in the receptor. Moreover, the interactions of complexes with DNA have been explored by electronic absorption titration, fluorescence emission titration, viscosity measurements and gel electrophoresis.HighlightsSynthesis and characterization of novel indole-derived compounds.X-ray diffraction studies demonstrate average crystalline size of the compounds.Metal complexes act as good metallointercalators.Metal complexes show higher antimicrobial activity compared to ligand.Prediction of biological activities of the ligand by PASS online software.Drug-like nature and bioavailability of synthesized compounds predicted by Swiss ADME predictorDocking of the synthesized compounds with 1BNA and 3EQM using HEX 8.0 software.Communicated by Ramaswamy H. Sarma

(Invited) Atomically Dispersed Eletrocatalysts for High-Efficiency Ambient N2 Fixation

NH3 synthesis by the electrocatalytic N2 reduction reaction (NRR) under ambient conditions is regarded as an appealing alternative to the industrial method that requires high temperature and pressure. Finding a material that can efficiently catalyze electrochemical N2 fixation at ambient condition is a subject of considerable current interest. Atomically dispersed catalysts with mononuclear metal complexes or single metal atoms anchored on supports would be a promising candidate, because of their maximum atom efficiency, unique catalytic performances, and the similarity of the metal coordination environment to the ligand fields in molecular catalysts. A series of cost-effective and optimized atomically-dispersed eletrocatalysts for NRR will be reported. Our results show that the catalysts are featured with high density of single metal atoms supported on hierarchically porous carbon frameworks. They exhibited remarkable selectivity of NH3 formation and high NH3 yield rate at low applied potentials at room temperature. Moreover, the catalysts show negligible activity decay in an NRR electrolysis as long as 50,000 s. On the basis of our results and the previously reported work, a possible mechanism for NRR on the catalyst will be proposed to provide a fundamental insight into the high-efficiency NRR.

Hydrogen and Oxygen Evolution Reactions Catalyzed By Metal Porphyrins and Corroles

Catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are key steps involved in artificial photosynthesis for solar-energy conversion for the production of clean, carbon-free, and renewable energy sources, and thus they are considered to be appealing ways to eliminate the energy and environmental problems caused by burning fossil fuels. Extensive efforts have been made to develop efficient and robust catalysts for these two processes. Research on mononuclear metal complexes has attracted increasing attention recently because not only they are capable of catalyzing HER and OER as efficiently as multinuclear metal complexes, but also there are several advantages in synthesis and mechanism studies by using single site catalysts. In this talk, we will introduce hydrogen and oxygen evolution reactions catalyzed by single site metal porphyrins and corroles.

Probing through-space and through-bond magnetic exchange couplings in a new benzotriazinyl radical and its metal complexes.

The synthesis and characterization of a new chelating benzotriazinyl radical (Rad2) are described. Crystallographic studies coupled with SQUID magnetometry on Rad2 reveal the presence of discrete radical pairs which are antiferromagnetically coupled. The reaction of Rad2 with the 3d transition metal complexes M(hfac)2·xH2O (hfac- = hexafluoroacetylacetonate) led to mononuclear metal complexes of general formula M(hfac)2(Rad2) [M = Zn(ii) (1); Ni(ii) (2) and Co(ii) (3)] whose structures have been determined by single crystal X-ray diffraction. Compounds 1-3 are isostructural and crystallize in the monoclinic space group P21/n with two molecules in the asymmetric unit. In the case of the Zn(ii) complex (1) through-space intermolecular radicalradical antiferromagnetic exchange interactions viaπ*π* contacts are observed, whereas strong intramolecular through-bond metal-radical ferromagnetic interactions [J = +59.3(9) cm-1] are observed for the Ni(ii) complex (2). For the Co(ii) complex (3), computational and magnetic studies reveal substantial zero field splitting and ferromagnetic metal-radical interactions.

Some divalent metal(II) complexes of salicylaldehyde‐derived Schiff bases: Synthesis, spectroscopic characterization, antimicrobial and in vitro anticancer studies

Mononuclear transition metal(II) complexes of the type M(L)2⋅2H2O (where M = Co, Ni, Cu, Zn) have been synthesized from uninegative Schiff base ligands (HL1–HL4) designed by condensation of 4‐fluorobenzylamine with 2‐hydroxy‐1‐naphthaldehyde/3,5‐dichlorosalicylaldehyde/3,5‐dibromosalicylaldehyde/3‐bromo‐5‐chlorosalicylaldehyde. The compounds were successfully characterized using spectroscopic and physiochemical methods together with elemental analysis. Spectroscopic elucidation indicates a monobasic bidentate nature of ligands coordinated via deprotonated phenolic oxygen and azomethine nitrogen atom which suggests an octahedral geometry around the central metal ions. The complexes and ligands were screened for their in vitro antimicrobial activity against bacterial and fungal strains, the zinc(II) complexes being more active against the tested microbial strains. Further, the metal complexes were found to be more active than the uncomplexed ligands due to chelation process and, moreover, the complexes were more active against fungal strains than bacterial strains. Cytotoxic activities of all compounds were evaluated towards human alveolar adenocarcinoma epithelial cell line (A549), human breast adenocarcinoma cell line (MCF7), human prostate cancer cell line (DU145) and one normal human lung cell line (MRC‐5) using MTT colorimetric assay with doxorubicin as a standard. The zinc complexes were most active against the cancer cell lines and also found to be less toxic against MRC‐5 normal cell line than standard doxorubicin.

Mononuclear Co(III), Ni(II) and Cu(II) complexes of 2‐(2,4‐dichlorobenzamido)‐N'‐(3,5‐di‐tert‐butyl‐2‐hydroxybenzylidene)benzohydrazide: Structural insight and biological assay

A series of mononuclear metal complexes of Co(III), Ni(II) and Cu(II) with 2‐(2,4‐dichlorobenzamido)‐N′‐(3,5‐di‐tert‐butyl‐2‐hydroxybenzylidene)benzohydrazide (LH3) have been synthesized and characterized using various physico‐chemical, spectroscopic and single crystal X‐ray diffraction techniques. Structural studies of [Co(LH)(LH2)]·H2O (4) revealed the presence of both amido and imidol tautomeric forms of LH3, resulting in a distorted octahedral geometry around the Co(III) ion. [Ni(LH)(H2O)]·H2O (5) and [Cu(LH)(H2O)]·H2O (6) are isomorphous structures and crystallize in the monoclinic P21/c space group. The crystal structures of 4, 5 and 6 are stabilized by hydrogen bonds formed by the enclathrated water molecules, C‐H···π and π···π interactions. Complexes along with the ligand (LH3) were screened for their in vivo anti‐inflammatory activity (carrageenan‐induced rat paw edema method) and in vitro antioxidant activity (DPPH free radical scavenging assay). Metal complexes have shown significant anti‐inflammatory and antioxidant potential.

Biomimetics of mononuclear and dinuclear Cu(II) and Fe(III) complexes of a newly synthesized piperazyl Mannich base with or without thiocyanate towards catechol

This paper presents the synthesis and characterization of Cu(II) and Fe(III) complexes of a newly synthesized Mannich base N-[4-hydroxy-3-[[4-(pyridin-2-yl)piperazin-1-yl]methyl]phenyl]acetamide. The spectro-analytical techniques employed include 1H, 13C NMR, IR, UV spectroscopy, conductivity measurements, and elemental analysis. Both mononuclear and dinuclear metal complexes were encountered in the study. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, catecholase activity of all four complexes was evaluated with the detailed kinetic studies reported. The nature of the bonding of the thiocyanate is discussed based on the observation of νCN (2150–2050 cm−1). All the metal complexes are catalytically active with turn over numbers ranging from 9.47 to 31.86 h−1 in DMF.

Studies on the interaction of mononuclear metal(II) complexes of amino‑naphthoquinone with bio-macromolecules

Three metal(II) complexes [CoLCl2], [CuLCl2] and [ZnL2Cl2] {L = 2‑chloro‑3‑((3‑dimethylamino)propylamino)naphthalene‑1,4‑dione} have been synthesized and characterized using analytical, thermal and spectral techniques (FT-IR, UV–Vis, ESR and ESI-MS). The structure of the L has been confirmed by single crystal XRD study. The complexes show good binding propensity to bovine serum albumin (BSA) having relatively higher binding constant values (104 M−1) than the ligand. Fluorescence spectral studies indicate that [CoLCl2] binds relatively stronger with CT DNA through intercalative mode, exhibiting higher binding constant (2.22 × 105 M−1). Agarose gel electrophoresis run on plasmid DNA (pUC18) prove that all the complexes showed efficient DNA cleavage via hydroxyl radical mechanism. The complexes were identified as potent anticancer agents against two human cancer cell lines (MCF7 and A549) by comparing with cisplatin. Co(II) complex demonstrated greater cytotoxicity against MCF7 and A549 cells with IC50 values at 19 and 22 μM, respectively.

In vitro cytotoxic activity of a novel Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde and its mononuclear metal complexes

Mononuclear Cu(II), Ni(II) and V(IV) complexes of a bidentate Schiff base ligand derived from 2-hydroxy-1-naphthaldehyde and 2-methoxyethylamine were synthesized and determined by spectroscopic methods. The crystal structures of HL ligand and Cu(II) complex was determined using single crystal X-ray diffraction method. HL Schiff base behaves as a bidentate ligand and coordinates to the metal ions via the nitrogen and oxygen. The obtained data revealed that the Cu(II) center in the complex (1) is coordinated by oxygens and nitrogens of two ligands in a regular square-planar fashion. The electrochemical behavior of the complexes was studied using cyclic voltammetry. The inhibitory activity of ligand and all the obtained complexes were tested in vitro against MKN-45 cancer cell line. It was found that among synthesized complexes, Ni(II) Schiff base complex namely (2), showed strong inhibitory activity against cancer cells, with IC50 = 6.654 ± 0.449 μg/mL.

Hydrogen and Oxygen Evolution Reactions Catalyzed By Single Site Metal Porphyrins and Corroles

Catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are key steps involved in artificial photosynthesis for solar-energy conversion for the production of clean, carbon-free, and renewable energy sources, and thus they are considered to be appealing ways to eliminate the energy and environmental problems caused by burning fossil fuels. Extensive efforts have been made to develop efficient and robust catalysts for these two processes. Research on mononuclear metal complexes has attracted increasing attention recently because not only they are capable of catalyzing HER and OER as efficiently as multinuclear metal complexes, but also there are several advantages in synthesis and mechanism studies by using single site catalysts. In this talk, we will introduce hydrogen and oxygen evolution reactions catalyzed by single site metal porphyrins and corroles.1-6 References (1) Zhang, W.; Lai, W. Z.; Cao, R. Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems. Chem. Rev. 2017, 117, 3717-3797. (2) Han, Y. Z.; Fang, H. Y.; Jing, H. Z.; Sun, H. L.; Lei, H. T.; Lai, W. Z.; Cao, R. Singly Versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 5457-5462. (3) Lei, H. T.; Fang, H. Y.; Han, Y. Z.; Lai, W. Z.; Fu, X. F.; Cao, R. Reactivity and Mechanism Studies of Hydrogen Evolution Catalyzed by Copper Corroles. ACS Catal. 2015, 5, 5145-5153. (4) Lei, H. T.; Liu, C. Y.; Wang, Z. J.; Zhang, Z. Y.; Zhang, M. N.; Chang, X. M.; Zhang, W.; Cao, R. Noncovalent Immobilization of a Pyrene-Modified Cobalt Corrole on Carbon Supports for Enhanced Electrocatalytic Oxygen Reduction and Oxygen Evolution in Aqueous Solutions. ACS Catal. 2016, 6, 6429-6437. (5) Han, Y. Z.; Wu, Y. Z.; Lai, W. Z.; Cao, R. Electrocatalytic Water Oxidation by a Water-Soluble Nickel Porphyrin Complex at Neutral pH with Low Overpotential. Inorg. Chem. 2015, 54, 5604-5613. (6) Sun, H. L.; Han, Y. Z.; Lei, H. T.; Chen, M. X.; Cao, R. Cobalt Corroles with Phosphonic Acid Pendants as Catalysts for Oxygen and Hydrogen Evolution from Neutral Aqueous Solution. Chem. Commun. 2017, 53, 6195-6198. Figure 1

Synthesis, structures and magnetic properties of two iron(ii) tris(pyridyl)phosphine sulfide complexes

Two iron(ii) mononuclear metal complexes with tris(pyridyl)-phosphine sulfides and Fe(ClO4)2·6H2O have been synthesized and characterized. Detailed structural analyses and magnetic susceptibility measurements confirm no spin transition from low-spin to the high-spin state between 2 and 300 K in these complexes.