![Structure of the histidine complex [Ni(k -histidinate)2] . RAGZAD.png](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/RAGZAD.png/220px-RAGZAD.png)
A transition metal imidazole complex is a coordination complex that has one or more imidazole ligands. Complexes of imidazole itself are of little practical importance. In contrast, imidazole derivatives, especially histidine, are pervasive ligands in biology where they bind metal cofactors. [2]
Only the imine nitrogen (HC=N-CH) of imidazole is basic, and it is this nitrogen that binds to metal ions. The pyrrole-like nitrogen ((HC-NH-CH) projects away from the metal. The pKa of protonated imidazolium cation is about 6.95, which indicates that the basicity of imidazole is intermediate between pyridine (pKa of pyridinium = 5.23) and ammonia (pKa = 9,24 of ammonium). The donor properties of imidazole are also indicated by the redox properties of its complexes.
Imidazole is a pure sigma-donor ligand. There is no evidence for pi backbonding in metal-imidazole complexes, a property that can be attributed to the presence of the pi-donor pyrrole-like NH center. [3] For this reason, imidazole can be classified as hard ligand. Nonetheless, complexes between low-valent metals and imidazole are well known, e.g., [Re(imidazole)3(CO)3]+. [4]
Imidazole is a compact, flat ligand. Six imidazole ligands fit comfortably around octahedral metal centers, e.g., [Fe(imidazole)6]2+. [5] The M-N(imidazole) bond is freely rotating.
Homoleptic octahedral complexes have been characterized by X-ray crystallography for the following dications: Fe2+, Co2+, Ni2+, Zn2+, Cd2+. Hexakis complexes of both Ru2+ and Ru3+ are also known. Cu2+, Pd2+, and Pt2+ form homoleptic square planar complexes. [6] Zn2+, although crystallized as the hexakis complex, more typically forms a tetrahedral complex. [7]
N-methylimidazole is slightly more basic than imidazole but is otherwise similar, if more lipophilic. Many salts of [M(imidazole-1-R)6]2+ are known (R = alkyl, vinyl, etc.). 2-Methylimidazoles are somewhat bulky ligands owing to the steric clash between the 2-methyl group and other ligands in octahedral complexes.
A modified benzimidazole ligand is found in all versions of vitamin B12.
Histidine complexes comprise an important subset of transition metal amino acid complexes. In common with other 3-substituted imidazoles, histidine can coordinate to metals via either of two nonequivalent tautomers. The free amino acid can coordinate through the imidazole and either or both of the carboxylate and amine.
The imidazole side chain of histidine residues in proteins are common binding sites for metal ions. Unlike the free amino acid, the histidine residue (i.e., as a component of a peptide or protein), coordinates solely via the imidazole substituent. Examples include myoglobin (Fe), carbonic anhydrase (Zn), azurin (Cu), and alpha-ketoglutarate-dependent hydroxylases (Fe). Polyhistidine-tag ("his tag") is an amino acid motif in proteins consisting of several histidine (His) residues that is attached to proteins to facilitate purification. The concept relies on the affinity of the imidazole side chain for metal cations.
Especially in cationic imidazole complexes, the N-H center is acidified. For tricationic d6 pentammines, deprotonation of the imidazole ligand gives imidazolate complexes with pKa near 10 (M = Co, Rh, Ir): [8]
The d5 complex [Ru(NH3)5(N2C3H4)]3+ is more acidic, with a pKa of 8.9. Thus, complexation to tricationic complexes acidify the pyrrolic NH center by at least 10,000.
Imidazole ligands are isomers of N-heterocyclic carbenes. This conversion has been observed: [3]
The pKa of imidazole (to give imidazolate) is 14, [8] thus it is easy to deprotonate? high pKa - low Ka - hard deprotonation. Many metal complexes feature imidazolate as a bridging ligand. One example of an imidazolate complex from biochemistry is found at the active site of copper-containing superoxide dismutase.
The M2(μ-imidazolate) motif underpins materials comprising zeolitic imidazolate frameworks ("ZIF"s). [10]
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the deprotonated –COO− form under biological conditions), and an imidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiological pH. Initially thought essential only for infants, it has now been shown in longer-term studies to be essential for adults also. It is encoded by the codons CAU and CAC.
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins contain zinc-binding protein domains although there may be up to 3000 human zinc metalloproteins.
Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-adjacent nitrogen atoms in meta-substitution.
In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.
Tm is an abbreviation for anionic tridentate ligand based on three imidazole-2-thioketone groups bonded to a borohydride center. They are examples of scorpionate ligands. Various ligands in this family are known, differing in what substituents are on the imidazoles. The most common is TmMe, which has a methyl group on the nitrogen. It is easily prepared by the reaction of molten methimazole (1-methylimidazole-2-thione) with sodium borohydride, giving the sodium salt of the ligand. Salts of the TmMe anion are known also for lithium and potassium. Other alkyl- and aryl-group variations are likewise named TmR according to those groups.
Nitrite reductase refers to any of several classes of enzymes that catalyze the reduction of nitrite. There are two classes of NIR's. A multi haem enzyme reduces NO2− to a variety of products. Copper containing enzymes carry out a single electron transfer to produce nitric oxide.
Metal nitrosyl complexes are complexes that contain nitric oxide, NO, bonded to a transition metal. Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.
1-Methylimidazole or N-methylimidazole is an aromatic heterocyclic organic compound with the formula CH3C3H3N2. It is a colourless liquid that is used as a specialty solvent, a base, and as a precursor to some ionic liquids. It is a fundamental nitrogen heterocycle and as such mimics for various nucleoside bases as well as histidine and histamine.
Zinc compounds are chemical compounds containing the element zinc which is a member of the group 12 of the periodic table. The oxidation state of zinc in most compounds is the group oxidation state of +2. Zinc may be classified as a post-transition main group element with zinc(II). Zinc compounds are noteworthy for their nondescript appearance and behavior: they are generally colorless, do not readily engage in redox reactions, and generally adopt symmetrical structures.
In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand, but of course many complexes are known to consist of a mix of aquo and other ligands.
Transition metal thiolate complexes are metal complexes containing thiolate ligands. Thiolates are ligands that can be classified as soft Lewis bases. Therefore, thiolate ligands coordinate most strongly to metals that behave as soft Lewis acids as opposed to those that behave as hard Lewis acids. Most complexes contain other ligands in addition to thiolate, but many homoleptic complexes are known with only thiolate ligands. The amino acid cysteine has a thiol functional group, consequently many cofactors in proteins and enzymes feature cysteinate-metal cofactors.
Imidazolate (C3H3N−
2) is the conjugate base of imidazole. It is a nucleophile and a strong base. The free anion has C2v symmetry. Imidazole has a pKa of 14.05, so the deprotonation of imidazole (C3H3N2H) requires a strong base.
Transition metal amino acid complexes are a large family of coordination complexes containing the conjugate bases of the amino acids, the 2-aminocarboxylates. Amino acids are prevalent in nature, and all of them function as ligands toward the transition metals. Not included in this article are complexes of the amides and ester derivatives of amino acids. Also excluded are the polyamino acids including the chelating agents EDTA and NTA.
Transition metal isocyanide complexes are coordination compounds containing isocyanide ligands. Because isocyanides are relatively basic, but also good pi-acceptors, a wide range of complexes are known. Some isocyanide complexes are used in medical imaging.
In organometallic chemistry, transition metal complexes of nitrite describes families of coordination complexes containing one or more nitrite ligands. Although the synthetic derivatives are only of scholarly interest, metal-nitrite complexes occur in several enzymes that participate in the nitrogen cycle.
A transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds.
Transition metal azide complexes are coordination complexes containing one or more azide (N3−) ligands.
Transition metal complexes of 2,2'-bipyridine are coordination complexes containing one or more 2,2'-bipyridine ligands. Complexes have been described for all of the transition metals. Although few have any practical value, these complexes have been influential. 2,2'-Bipyridine is classified as a diimine ligand. Unlike the structures of pyridine complexes, the two rings in bipy are coplanar, which facilitates electron delocalization. As a consequence of this delocalization, bipy complexes often exhibit distinctive optical and redox properties.
Transition metal complexes of pyridine-N-oxides encompass coordination complexes that contain pyridine-N-oxides as ligands. Particularly common are the octahedral homoleptic complexes of the type [M(ONC5H5)6]2+ where M = Mn(II), Fe(II), Co(II), Ni(II). Many variations of pyridine N-oxide are known, such as the dioxides of 2,2'- and 4,4'-2,2'-bipyridine. Complexes derived from the trioxide of terpyridine have been crystallized as well.
Transition metal complexes of thiocyanate describes coordination complexes containing one or more thiocyanate (SCN-) ligands. The topic also includes transition metal complexes of isothiocyanate. These complexes have few applications but played significant role in the development of coordination chemistry.
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