Metal complexes of diamines

Metal complexes of diamines refers to coordination complexs of diamine ligands. The most common complexes are those of ethylenediamine. Complexes of en and related diamines have been thoroughly studied for their fundamental properties. ^[1] In a practical sense, diamines are mainly used to make polyamides such as nylon 66, not coordination complexes. This class of compounds are closely related to metal ammine complexes.

Ligand properties of diamines

Diamines have properties expected for two amines, ^[2] i.e. they are dibasic and binds well to hard Lewis acids, such as metal cations.

The coordination chemistry of diamines emphasizes 1,2- and 1,3-diamines, which form 5- and 6-chelate rings. Both enthalpic and entropic factors favor their formation 1,4- and longer diamines are floppy, forming polymers vs chelate rings. ^[3] For ethylenediamine complexes, the five-membered MN₂C₂ chelate ring is nonplanar, but exist in two rapidly interconverting conformations, referred to as δ and λ.

Of the ditertiary diamines, tetramethylethylenediamine (TMEDA) is most commonly encountered. Being bulky and lacking N-H bonds, it is popular as a ligand in main group chemistry. ^[4]

Complexes

Structure of the Δ-(lel)₃ (or Δ-(λλλ)) isomer of [Co(en)₃] . One of the three C₂ symmetry axes is shown in red.

Octahedral complexes

Ethylenediamine forms many homoleptic octahedral complexes of the formula [M(en)₃]ⁿ⁺. ^[5] Representative octahedral complexes are M = V²⁺, ^[6] Cr³⁺, Mn³⁺, Fe³⁺, Ru²⁺, Co²⁺ and Co³⁺, Rh³⁺, Ir³⁺, Ni²⁺, Pt⁴⁺ and Zn²⁺. ^[7] These complexes are chiral, and many have been resolved. Particularly famous is [Co(en)₃]³⁺.

More common than homoleptic [M(en)₃]ⁿ⁺ complexes are "mixed ligand" derivatives, e.g., of the type [M(en)L₄]ⁿ⁺ and [M(en)₂L₂]ⁿ⁺. One example is cis-dichlorobis(ethylenediamine)cobalt(III) cation.

Square planar complexes

Square planar complexes of the formula [M(en)₂](n⁺) are also well known. Representative square planar complexes are M = Pd²⁺, Pt²⁺, Cu²⁺, and Au³⁺.

Related diamine complexes

1,2-Propylenediamine, abbreviated pn, is chiral. It forms five-membered chelate rings analogous to en. The methyl substituent prefers the equatorial position on the MN2C2 ring. Octahedral complexes of one l-pn, i.e., [M(l⁻pn)₃](n⁺)) exist as two diastereomers. One diastereomer with C3 symmetry, has three methyl groups sharing one face. The other diastereomer has only C1 symmetry.

1,3-Propylenediamine, abbreviated tn, forms six-membered MN₂C₃ chelate rings. Octahedral complexes of type [M(tn)₃](n⁺) exist as two enantiomers.

Numerous 1,2-diamines are known, including trans-1,2-diaminocyclohexane and stilbenediamine. EDTA and many aminopolycarboxylates have 1,2-diamine cores. They are commercial chelating agents.

Reactions

Diamine ligands are often inert spectator ligands. One example is [Co(en)₂(PO₄)]. ^[8]

Reactions of ethylenediamine generally involve or are initiated at the N-H bonds

Their N-H groups are somewhat acidic as revealed by their easy exchange with D₂O: ^[1]

[Ru(H₂NCH₂CH₂NH₂)₃]³⁺ + 12 D₂O ⇌ [Ru(D₂NCH₂CH₂ND₂)₃]³⁺ + 12 HDO

In some redox-active metals, en can undergo dehydrogenation to give diimine complexes: ^[9]

[Ru(H₂NCH₂CH₂NH₂)₃]²⁺ + O₂ ⇌ [Ru(H₂NCH₂CH₂NH₂)₂(HN=CHCH=NH)]²⁺ + 2 H₂O

Tris(ethylenediamine)cobalt(III) and some related complexes condense with mixtures of formaldehyde and ammonia to give make clathrochelates  : ^[10]

[Co(H₂NCH₂CH₂NH₂)₃]³⁺ + 6 CH₂O + 2 NH₃ → [Co[N(CH₂HNCH₂CH₂NHCH₂)₃N]³⁺ + 6 H₂O

References

1. Beattie, James K. (1971). "Conformational analysis of tris(ethylenediamine) complexes". Accounts of Chemical Research. 4 (7): 253–259. doi:10.1021/ar50043a004.
2. Lucet, Denis; Le Gall, Thierry; Mioskowski, Charles (1998). "The Chemistry of Vicinal Diamines". Angewandte Chemie International Edition. 37 (19): 2580–2627. doi:10.1002/(SICI)1521-3773(19981016)37:19<2580::AID-ANIE2580>3.0.CO;2-L. PMID   29711625.
3. Paoletti, P. (1984). "Formation of metal complexes with ethylenediamine: A Critical Survey of equilibrium constants, enthalpy and entropy values". Pure and Applied Chemistry. 56 (4): 491–522. doi:10.1351/pac198456040491.
4. Haynes, R. K.; Vonwiller, S. C.; Luderer, M. R. (2006). "N,N,N′,N′-Tetramethylethylenediamine". In Paquette, L. (ed.). N,N,N′,N′-Tetramethylethylenediamine. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rt064.pub2. ISBN   0471936235.
5. Pham, Duyen N. K.; Roy, Mrittika; Golen, James A.; Manke, David R. (2017). "The first-row transition-metal series of tris(ethylenediamine) diacetate complexes [M (En)₃](OAc)₂ ( M is Mn, Fe, Co, Ni, Cu, and Zn)". Acta Crystallographica Section C Structural Chemistry. 73 (6): 442–446. doi:10.1107/S2053229617006738. PMID   28579564.
6. Daniels, Lee M.; Murillo, Carlos A.; Rodríguez, Kattia G. (1995). "Preparation of anhydrous vanadium(II) sulfate compounds from aqueous solutions: The synthesis and characterization of [V(en)3]SO4, V(bpy)2SO4 and V(py)4SO4". Inorganica Chimica Acta. 229 (1–2): 27–32. doi:10.1016/0020-1693(94)04222-H.
7. Muralikrishna, C.; Mahadevan, C.; Sastry, S.; Seshasayee, M.; Subramanian, S. (1983). "Structure of tris(ethylenediamine)zinc(II) chloride dihydrate, [Zn(C2H8N2)3]Cl2.2H2O". Acta Crystallographica Section C Crystal Structure Communications. 39 (12): 1630–1632. Bibcode:1983AcCrC..39.1630M. doi:10.1107/S0108270183009543.
8. Anderson, Bryan; Milburn, Ronald M.; Harrowfield, John M.; Robertson, Glen B.; Sargeson, Alan M. (1977). "Cobalt(III)-Promoted Hydrolysis of a Phosphate Ester". Journal of the American Chemical Society. 99 (8): 2652–2661. Bibcode:1977JAChS..99.2652A. doi:10.1021/ja00450a042. PMID   850030.
9. Diamond, Steven E.; Tom, Glenn M.; Taube, Henry (1975). "Ruthenium promoted oxidation of amines". Journal of the American Chemical Society. 97 (10): 2661–2664. Bibcode:1975JAChS..97.2661D. doi:10.1021/ja00843a012.
10. Gahan, Lawrence R.; Harrowfield, Jack M. (2015). "Sepulchrate: Four decades on". Polyhedron. 94: 1–51. doi:10.1016/j.poly.2015.03.036.