Are M–N bonds indeed inherently weaker when N is a tertiary rather than a primary or secondary nitrogen atom?

https://doi.org/10.1016/S0010-8545(98)00258-6Get rights and content

Abstract

N-alkylation of amine ligands decreases the stability constants of their metal ion complexes, stabilizes low valent complexes, increases the acidity of the central cation and turns it into a harder acid. The major source of these effects is the decrease in the outer sphere solvation energy induced by the N-alkylation.

Introduction

It is commonly accepted that metal–nitrogen bonds are inherently weaker when the nitrogen is a tertiary rather than a primary or a secondary nitrogen atom [1]. This notion is based on three main observations:

  • 1.

    The stability constants of complexes with tertiary amines are considerably lower than those with the analogous primary or secondary amines [2], [3], [4], [5], [6], [7], [8], [9], [10].

  • 2.

    The ligand field induced by tertiary amines, as deduced from the energies of the d→d transitions of their complexes, is considerably lower than that induced by the analogous primary or secondary amines [4], [6], [10], [11], [12], [13], [14]1. This phenomenon is usually significant only when several amines are ligated to the same central cation [2], [3].

  • 3.

    N-alkylation of amine ligands shifts the redox potentials of the couples Mn+1/nLm anodically relative to those of the corresponding non-alkylated complexes [4], [5], [6], [10], [11], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27].

These observations are surprising as alkyl substituents are electron donors, and are therefore expected to increase the σ donating properties of the N atom. Usually this discrepancy is attributed to steric hindrance imposed by the tertiary amines which causes an elongation of the M–N bond and or affects the angles in the coordination sphere [17], [18], [19]. This argument clearly does not explain the observation that β2 is smaller for Ag(N(CH3)3)2+ than for Ag(NH3)2+ or Ag(NH2(CH3))2+ [28]. It should be noted that tertiary amine ligands are weaker bases than the analogous primary and tertiary amines. This phenomenon is commonly attributed to the loss of solvation stabilization via hydrogen bonds to the solvent, induced by the N-alkylation. Therefore, it seemed of interest to check whether the properties of tertiary amines as ligands have a similar origin.

Section snippets

Discussion

For this purpose the properties of first row transition metal complexes with the following ligands were studied recently:

the results point out the following general conclusions.

1. N-methylation decreases the stability constants of the corresponding complexes, thus for all systems studied Kn(L2n−1)>Kn(L2n) is observed [2], [3], [4], [6], [7], [8], [9], [10], [16], [29], [30], [31]; where KjLi=[MjLi]/[Mjaq][Li] and j is the oxidation state of the central metal cation M.

2. The effect of N

Concluding remarks

The results obtained in recent years point out that N-alkylation of amine ligands affects the properties of their metal ion complexes via several, often opposing, effects:

1. It decreases the outer sphere solvation energy of the complexes.

2. It eliminates the M–N–H⋯O hydrogen bonds, thus turning the nitrogen a poorer σ donor.

3. It elongates the M–N bonds due to steric hindrance.

4. The alkyl substituents are electron donating groups which turn the nitrogen into a better σ donor.

5. It often induces

Note added in proof

Recently (H. Deng, P. Kebarbe, J. Am. Chem. Soc. 120 (1998) 2925) reported that ΔG°393 for the reaction CuL2+⇄Cu++2L in the gas phase equals 82.7, 93.1 and 98.7 kcal/mol for L=NH3; NH2–C3H7 and N(n-C4H9)3, respectively, i.e. N-alkylation stabilizes the complexes in the gas phase.

Acknowledgements

I am indebted to my many colleagues whose work is cited in this manuscript, without their hard work it could not have been written. I wish to thank The Alexander von Humboldt Stiftung for a research prize which enabled my stay at the University of Erlangen-Nurnberg, during which this manuscript was written. This study was supported in part by grants from The Israel Science Foundation administered by The Israel Academy of Sciences and Humanities and by grants from the Budgeting and Planning

References (35)

  • G. Golub et al.

    Inorg. Chim. Acta

    (1997)
  • N. Jubran et al.

    Inorg. Chim. Acta

    (1986)
  • N. Jubran et al.

    Inorg. Chim. Acta

    (1986)
  • D. Guldi et al.

    Inorg. Chim. Acta

    (1991)
  • D. Guldi et al.

    Inorg. Chim. Acta

    (1992)
  • I. Bertini et al.

    Inorg. Chim. Acta

    (1995)
  • I. Zilbermann et al.

    Inorg. Chim. Acta

    (1995)
  • C. Bazzicalupi et al.

    Inorg. Chim. Acta

    (1998)
  • F.A. Cotton et al.
  • D.A. Baldwin, E.A. Betterton, J.M. Pratt, J. Chem. Soc. Dalton Trans. (1983)...
  • P.R. Norris, P.L.S. Harper, J.M. Pratt, J. Chem. Soc. Dalton Trans. (1997)...
  • G. Golub et al.

    J. Am. Chem. Soc.

    (1995)
  • G. Golub et al.

    Supramol. Chem.

    (1996)
  • G. Golub, H. Cohen, P. Paoletti, A. Bencini, D. Meyerstein, J. Chem. Soc. Dalton Trans. (1996)...
  • C. Bazzicalupi, A. Bencini, H. Cohen, C. Giorgi, G. Golub, D. Meyerstein, N. Navon, P. Paoletti, B. Valtancoli, J....
  • L. Lindoy

    Pure Appl. Chem.

    (1997)
  • N. Navon, G. Golub, H. Cohen, P. Paoletti, B. Valtancoli, A. Bencini, D. Meyerstein, submitted for...
  • Cited by (98)

    • Thermodynamics of complex formation in dimethylsulfoxide: The case of Co(II)complexes with nitrogen donor ligands and their O<inf>2</inf> adducts

      2019, Inorganica Chimica Acta
      Citation Excerpt :

      The N-alkylation in polyamines stabilizes the the low oxidation states of metal cations as a result of different effects [137]. Methyl groups increase the σ-donating properties of the N atoms in gas phase, however, when in solution the solvation energy of the complex is decreased due to the higher complex size and absence of hydrogen bonding with the surrounding solvent molecules [137]. In addition, methyl groups induce steric strain due to the non-bonded interactions between alkyl groups which elongate the M−N bonds [137].

    • Bio-inspired oxidation chemistry of a Cu(II)-fluoride cryptate with C3-symmetry

      2018, Inorganica Chimica Acta
      Citation Excerpt :

      They are, however, limited in the area of biomimetic oxidation chemistry because in the presence of metals, the secondary amines are easily oxidized to form imines complicating the reactions with several by-products [17–19]. N-Methylation is an easy solution to reduce side reactions under oxidative conditions but the additional steric constraint on the cryptand prevents encapsulation of transition metals with anionic hosts, [20,21] with encapsulation defined here as coordination of the five donor atoms of Tren. Using cryptand LTEA (Fig. 1A), we here present Cu(II)-halide cryptates and their reactivity with basic hydrogen peroxide (H2O2/Et3N).

    • Bioinspired superoxide-dismutase mimics: The effects of functionalization with cationic polyarginine peptides

      2016, Journal of Inorganic Biochemistry
      Citation Excerpt :

      For wave b′ the potential was stationary but its size increased with increasing pH. Taken together, the wave a/b couple (the quasi-reversible wave observed at high scan rate) was assigned to the proton coupled electron transfer of the solvated [MnIIL′-Gly1(H2O)]+/[MnIIIL′-Gly1(OH)]+ couple which has been previously observed for [MnL]2 +[40]. N-alkylation of the ligand shifted anodically the E1/2 relative to the parent compound, a behaviour which has been observed in other metal complexes containing amine ligands [41,42]. Wave b′ was assigned to the reduction of a chemically generated product of [MnIIIL′-Gly(OH)]+, most likely dimeric species, which are known to be favoured at higher pH [43].

    • Oxygen atom transfer mediated by an iron(IV)/iron(II) macrocyclic complex containing pyridine and tertiary amine donors

      2012, Journal of Inorganic Biochemistry
      Citation Excerpt :

      It also elongates the Fe–N bonds and decreases the thermodynamic stability of iron(II) and iron(III) complexes [95]. Often, N-alkylation results in the higher FeIII/FeII redox potentials [95]. Ligand stability under oxidative conditions allowed us to generate iron-peroxo and –oxo intermediates (3a and 3b, respectively) that mimic proposed intermediates in the catalytic cycles of non-heme iron oxidative enzymes.

    View all citing articles on Scopus
    View full text