Transition metal carboxamide complex
From Wikipedia, the free encyclopedia
Transition metal carboxamide complexes are coordination complexes containing one or more amide ligands (RC(O)NH2 being the simplest members) bound to a transition metal.[1] Many amides are known, proteins for example. Amides are generally at least weakly basic, so the inventory of their coordination complexes is large. Amide complexation is an important structural motif in bioinorganic chemistry. This binding is also relevant to catalysis, since metal-amide complexes are intermediates in the metal-catalyzed hydrolysis of amides to carboxylic acids.[2]
Nomenclature
Several principles and trends are illustrated by the case of complexes of dimethylformamide (DMF), a very common amide ligand. Amides bind to metals through oxygen, which is the basic site of amides. Amides are thus L ligands according to the covalent bond classification method, i.e. charge-neutral 2e donors. With respect to HSAB theory, amides are classified as hard ligands.
The M-O=C(NH2)H entity is planar in complexes of formamide. Similarly, the M-O=C(NC2)H entity is planar in complexes of DMF. Two geometrically distinct bonding modes are possible depending on the relative positions of the metal ion and the N-substituent on the amide. For simple unidentate amides, like DMF, the M and N are transoid.
Often amide refers to carboxamide, and often amide refers to the anions R2N- and their derivatives. Tetrakis(dimethylamido)titanium (Ti(N(CH3)2)4) illustrates the ambiguity of the terminology.
Homoleptic complexes
Being a compact ligand, DMF forms homoleptic complexes with several metal cations. Some of those characterized by X-ray crystallography are listed below.
- [Mn(DMF)6](BPh4)2[4]
- [Fe(DMF)6](B(CN)4)2[5]
- [Co(DMF)6]I2[6]
- [Ni(DMF)6](BPh4)2[7]
- [Zn(DMF)6](BPh4)2[8]
- [Ru(DMF)6](O3SCF3)2[3]
- [Ru(DMF)6](O3SCF3)3[3]
- [Cd(DMF)6]B12H12[9]
By contrast with DMF, homoleptic complexes with formamide and methylformamide are rare.
Chelating amide ligands
Diacetamide (HN(C(O)CH3)2) and glycinamide (H2NC(O)CH2NH2) are two of many examples of chelating amide ligands. They respectively form the complexes [Co((HN(COCH3)2(SCN)2}}[10] and ([Co(H2NCOCH2NH2)(H2NCH2CH2NH2)2]3+.
Proteins and peptides
Some prominent examples of transition metal complexes of carboxamido (deprotonated carboxamide) ligands: bleomycin (Fe), Nickel superoxide dismutase (Ni), and nitrile hydratase (Co).[11]
