Transition metal imidazole complex

:File:PicketFenceGenericRevised.png. This synthetic complex mimics aspects of myoglobin.]]

Only the imine nitrogen (HC=N-CH) of imidazole is basic, and it is this nitrogen that binds to metal ions. Imidazole is a pure sigma-donor ligand. The pK_a of protonated imidazolium cation is about 6.95, which indicates that the basicity of imidazole is intermediate between pyridine (pK_a of pyridinium = 5.23) and ammonia (pK_a = 9,24 of ammonium). The donor properties of imidazole can also inferred from the redox properties of its complexes.{{cite journal |doi=10.1021/ja00809a011|title=Nitrogen-Bound and Carbon-Bound Imidazole Complexes of Ruthenium Ammines|year=1974|last1=Sundberg|first1=Richard J.|last2=Bryan|first2=Robert F.|last3=Taylor|first3=Ivan F.|last4=Taube|first4=Henry|journal=Journal of the American Chemical Society|volume=96|issue=2|pages=381–392}} It is classified as an L ligand in the Covalent bond classification method. In the usual electron counting method, it is a two-electron ligand.

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]⁺.{{cite journal |doi=10.1016/S0010-8545(99)00128-9|title=Basic Aqueous Chemistry of [M(OH₂)₃(CO)₃]⁺ (M=Re, Tc) Directed Towards Radiopharmaceutical Application|year=1999|last1=Alberto|first1=Roger|last2=Schibli|first2=Roger|last3=Waibel|first3=Robert|last4=Abram|first4=Ulrich|last5=Schubiger|first5=August P.|journal=Coordination Chemistry Reviews|volume=190-192|pages=901–919}}

Imidazole is a compact, flat ligand. Six imidazole ligands fit comfortably around octahedral metal centers, e.g., [Fe(imidazole)₆]²⁺.{{cite journal |doi=10.1021/ic034110t|title=Spectroscopic and Structural Characterization of the [Fe(imidazole)₆]²⁺ Cation|year=2003|last1=Carver|first1=Graham|last2=Tregenna-Piggott|first2=Philip L. W.|last3=Barra|first3=Anne-Laure|last4=Neels|first4=Antonia|last5=Stride|first5=John A.|journal=Inorganic Chemistry|volume=42|issue=18|pages=5771–5777|pmid=12950228}} 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.{{cite journal |doi=10.1021/ic00287a027|title=EPR Reinvestigation of the Copper(II)-Imidazole System|year=1988|last1=Bonomo|first1=Raffaele P.|last2=Riggi|first2=Francesco|last3=Di Bilio|first3=Angel J.|journal=Inorganic Chemistry|volume=27|issue=14|pages=2510–2512}} Zn²⁺, although crystallized as the hexakis complex, more typically forms a tetrahedral complex.{{cite journal |doi=10.1021/ja00487a014|title=Nitrogen-14 Nuclear Quadrupole Resonance Spectra of Coordinated Imidazole|year=1978|last1=Ashby|first1=Carol I. H.|last2=Cheng|first2=C. P.|last3=Brown|first3=Theodore L.|journal=Journal of the American Chemical Society|volume=100|issue=19|pages=6057–6063}}

file:Cobalamin skeletal.svg, illustrating the dimethylbenzimidazole ligand]]

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 pK_a near 10 (M = Co, Rh, Ir):

:[M(NH₃)₅(N₂C₃H₄)]³⁺ {{Eqm}} [M(NH₃)₅(N₂C₃H₃)]²⁺ + H⁺

The d⁵ complex [Ru(NH₃)₅(N₂C₃H₄)]³⁺ is more acidic, with a pK_a 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:

:[Ru(NH₃)₅(N₂C₃H₄)]²⁺ → [Ru(NH₃)₅(C(NH)₂(CH)₂)]²⁺

File:(Cu3(Im)2(ImH)8)4+.svg

The pK_a of imidazole (to give imidazolate) is 14,{{cite journal |doi=10.1021/ic00181a015|title=Influence of the Metal Centers on the p Ka of the Pyrrole Hydrogen of Imidazole Complexes of (NH₃)₅³⁺, M(III) = Co(III), Rh(III), Ir(III), Ru(III)|year=1984|last1=Hoq|first1=M. Fazlul|last2=Shepherd|first2=Rex E.|journal=Inorganic Chemistry|volume=23|issue=13|pages=1851–1858}} thus it is easier to deprotonate than many other amines or imines. 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).{{cite journal |doi=10.1021/ja809459e|title=Control of Pore Size and Functionality in Isoreticular Zeolitic Imidazolate Frameworks and their Carbon Dioxide Selective Capture Properties|year=2009|last1=Banerjee|first1=Rahul|last2=Furukawa|first2=Hiroyasu|last3=Britt|first3=David|last4=Knobler|first4=Carolyn|last5=o'Keeffe|first5=Michael|last6=Yaghi|first6=Omar M.|journal=Journal of the American Chemical Society|volume=131|issue=11|pages=3875–3877|pmid=19292488}}

{{Coordination complexes}}

Category:Transition metal compounds