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Transition metal carboxamide complex

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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]

Ligand properties

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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.

Nomenclature

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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

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Structure of the dication in the salt [Ru(dmf)6]2+(-O3SCF3)2. Selected distances: Ru-O, 2.089, C-O, 1.22, C-N, 1.32 Å. Color code: red = O, blue = N, gray = C. The structure of the cation is very similar for the related Ru(III) salt, except that the Ru-O distance is 2.02 Å.[3]

Being a compact ligand, DMF forms homoleptic complexes with several metal cations. Some of those characterized by X-ray crystallography are listed below.

By contrast with DMF, homoleptic complexes with formamide and methylformamide are rare.

Chelating amide ligands

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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

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Some prominent examples of transition metal complexes of carboxamido (deprotonated carboxamide) ligands: bleomycin (Fe), Nickel superoxide dismutase (Ni), and nitrile hydratase (Co).[11]

Reactions

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The amide ligand in cationic complexes is prone toward hydrolysis:[12]

[Co(NH3)5(OCH(NMe2)]3+ + OH− → [Co(NH3)5(O2CH]2+ + HNMe2 (Me = CH3)

The N-H bonds in amide ligands are acidified relative to the free ligand. Consequently, amide complexes are susceptible to deprotonation. This conversion is often accompanied by isomerization to the N-bonded form. This form of linkage isomerism is manifested in glycinamide complexes.[13]

Acid-base reaction of a dicationic glycinamide complex (L = arbitrary ligand).

Ureas

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Urea (O=C(NH2)2) is more basic at oxygen than simple amides owing to the combined pi-donation from the two amino groups. One consequence is that the inventory of urea complexes is large, including many homoleptic derivatives. Urea forms a broader range of complexes, reflected by the existence of [M(urea)6](ClO4)3 (M = Ti, Mn).[14][15] As for other complexes of carboxamide ligands, the MOC(NH2)2 core of urea is planar with a bent M-O-C angle.

Biuret (H2NC(O)N(H)C(O)NH2) is a derivative of urea but with two amido groups. Biuret forms a variety of metal complexes, e.g. [Cu(NH2CONHCONH2+)2]2.[16] In addition to the parent urea and biuret, many derivatives are known where N-H is replaced by alkyl or aryl.

References

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  1. ^ Clement, O.; Rapko, B.M; Hay, B.P. (1998). "Structural aspects of metal–amide complexes". Coordination Chemistry Reviews. 170: 203–243. doi:10.1016/S0010-8545(98)00066-6.
  2. ^ Hegg, Eric L.; Burstyn, Judith N. (1998). "Toward the development of metal-based synthetic nucleases and peptidases: A rationale and progress report in applying the principles of coordination chemistry". Coordination Chemistry Reviews. 173: 133–165. doi:10.1016/s0010-8545(98)00157-x.
  3. ^ a b c Judd, Robert J.; Cao, Renhai; Biner, Margret; Armbruster, Thomas; Buergi, Hans-Beat; Merbach, Andre E.; Ludi, Andreas (1995). "Syntheses and Crystal and Molecular Structures of the Hexakis(N,N-dimethylformamide) Complexes of Ruthenium(II) and Ruthenium(III)". Inorganic Chemistry. 34 (20): 5080–5083. doi:10.1021/ic00124a026.
  4. ^ Suzuki, Ryo; Chiba, Yukako; Yamaguchi, Ryo; Yoshioka, Daisuke; Mikuriya, Masahiro; Sakiyama, Hiroshi (2013). "Synthesis and Crystal Structure of a Trigonally Compressed Hexakis-DMF Manganese(II) Complex". X-Ray Structure Analysis Online. 29: 11–12. Bibcode:2013XRAO...29...11S. doi:10.2116/xraystruct.29.11.
  5. ^ Nitschke, Christian; Köckerling, Martin (2011). "Iron Salts with the Tetracyanidoborate Anion: [FeIII(H2O)6][B(CN)4]3, Coordination Polymer [FeII(H2O)2{κ2N[B(CN)4]}2], and [FeII(DMF)6][B(CN)4]2". Inorganic Chemistry. 50 (10): 4313–4321. doi:10.1021/ic102278z. PMID 21488670.
  6. ^ Uflyand, Igor E.; Tkachev, Valerii V.; Zhinzhilo, Vladimir A.; Dzhardimalieva, Gulzhian I. (2021). "Study of the products of the reaction of cobalt(II) acetate with 2-iodoterephthalic acid and 1,10-phenanthroline". Journal of Coordination Chemistry. 74 (4–6): 649–662. doi:10.1080/00958972.2021.1881067.
  7. ^ Yamaguchi, Ryo; Yamasaki, Mikio; Sakiyama, Hiroshi (2011). "Synthesis and Crystal Structure of a Hexa-DMF Nickel(II) Complex that Belongs to an S6 Point Group". X-Ray Structure Analysis Online. 27: 71–72. Bibcode:2011XRAO...27...71Y. doi:10.2116/xraystruct.27.71.
  8. ^ Ito, Misaki; Mitsuhashi, Ryoji; Mikuriya, Masahiro; Sakiyama, Hiroshi (2016). "Crystal Structure of a Trigonally Compressed Hexakis-DMF Zinc(II) Complex". X-Ray Structure Analysis Online. 32: 21–22. Bibcode:2016XRAO...32...21I. doi:10.2116/xraystruct.32.21.
  9. ^ Korolenko, S. E.; Goeva, L. V.; Kubasov, A. S.; Avdeeva, V. V.; Malinina, E. A.; Kuznetsov, N. T. (2020). "Synthesis, Structures, and Properties of Zinc(II) and Cadmium(II) Complexes with Boron Cluster Anions [M(solv)6][BnHn] (M = Zn(II), Cd(II); solv = DMF, DMSO; n = 10, 12)". Russian Journal of Inorganic Chemistry. 65 (6): 846–853. doi:10.1134/S0036023620060091.
  10. ^ Khodashova, T. S.; Nikolaev, V. P.; Porai-Koshits, M. A.; Butman, L. A.; Tabidze, E. I.; Tsintsadze, G. V. (November 1986). "Structure of complex compounds of cobalt with acetylacetamide". Koord.Khim. (in Russian). 12 (1): 128. OSTI 6476419. Retrieved April 25, 2025.{{cite journal}}: CS1 maint: date and year (link)
  11. ^ Panda, Chakadola; Sarkar, Aniruddha; Sen Gupta, Sayam (2020). "Coordination chemistry of carboxamide 'Nx' ligands to metal ions for bio-inspired catalysis". Coordination Chemistry Reviews. 417. doi:10.1016/j.ccr.2020.213314.
  12. ^ Buckingham, D. A.; Harrowfield, J. Macb.; Sargeson, A. M. (1974). "Metal ion activation in the base hydrolysis of amides. Hydrolysis of the dimethylformamidepentaamminecobalt(III) ion". Journal of the American Chemical Society. 96 (6): 1726–1729. Bibcode:1974JAChS..96.1726B. doi:10.1021/ja00813a013.
  13. ^ Sigel, Helmut; Martin, R. Bruce (1982). "Coordinating properties of the amide bond. Stability and structure of metal ion complexes of peptides and related ligands". Chemical Reviews. 82 (4): 385–426. doi:10.1021/cr00050a003.
  14. ^ Figgis, B.N.; Wadley, L.G.B. (1972). "The Structure of Ti(urea)6(ClO4)3 at 90 K". Australian Journal of Chemistry. 25 (10): 2233. doi:10.1071/CH9722233.
  15. ^ Aghabozorg, Hossein; Palenik, Gus J.; Stoufer, R. Carl; Summers, J. (1982). "Dynamic Jahn-Teller Effect in a Manganese(III) Complex. Synthesis and Structure of Hexakis(urea)manganese(III) Perchlorate". Inorganic Chemistry. 21 (11): 3903–3907. doi:10.1021/ic00141a009.
  16. ^ Freeman, H. C.; Smith, J. E. W. L. (1966). "Crystallographic Studies of the Biuret Reaction. II. Structure of Bis-Biuret-Copper(II) Dichloride, Cu(NH2CONHCONH2)2C12". Acta Crystallographica. 20 (2): 153–159. Bibcode:1966AcCry..20..153F. doi:10.1107/S0365110X66000343.
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