Review
NO and NO2 as non-innocent ligands: A comparison

Dedicated to Professor Goutam K. Lahiri for his Outstanding Achievements in Coordination Chemistry and Catalysis on the Occasion of his 60th Birthday.
https://doi.org/10.1016/j.ccr.2019.213114Get rights and content

Highlights

  • Redox systems (NO)n and (NO2)n are presented to reveal parallels and differences.

  • The suitability of (NO)n and (NO2)n for metal coordination is contrasted.

  • The scarcity of metal complexes with (NO2)n ligands (n = 0,+) is discussed.

  • Strategies to stabilize an NO2radical dot ligand are pointed out.

  • The systems {O-NO}n and {M-NO}n are compared.

Abstract

The potential of environmentally and technically relevant NO2 for non-innocent ligand behavior in coordination compounds is being evaluated in comparison with the well-studied metal complexes of nitrosyl, NOn (n = +,0,-,2-). A ruthenium semiquinone (Qradical dot) radical complex platform [Ru(Q)(L)(NOx)] (x = 1 or 2, L = mer tridentate ligand) can serve to contrast the ligand characteristics of (NO)n and (NO2)n. Strategies to stabilize metal coordinated nitrogen dioxide (as “nitro” ligand NO2radical dot) versus the well established nitrite NO2 are suggested.

Section snippets

Introduction: The NOn ligands

The chemistry of the NOx molecules (x = 1, 2) has been increasingly researched because of their analytical, industrial, environmental and physiological significance [1]. In many cases the NOx forms are associated with metal partners such as on metal particles or surfaces, in metal complexes or in metalloenzymes. In connection with metal centers M, the non-innocence of the “nitrosyl” species NOn in coordination compounds Mm(NOn) has been recognized early, defined in a pioneering article in this

The potential for NO2 ligand non-innocence

In contrast to the plethora of examples for well studied Mm(NOn) compounds and their redox series [6], [7], [8], [9] there is a surprising lack of similar information available on the formally related series of coordination compounds Mm(NO2n). Neither the linear nitronium cation NO2+ nor the stable radical NO2radical dot from Fig. 3 have been identified as ligands in well characterized complexes [16], [17]; only the nitrite ion (n = −1) with its ambidentate ligand characteristics (Fig. 4 [18]) has been

A ruthenium example for NOn and NO2n coordination

Employing ruthenium as the much more inertly binding homologue of iron [27] a corresponding platform has been developed to investigate the redox activities of both coordinated (NO)n and (NO2)n. The platform contains mer tridentate chelate ligands such as terpy (2,2′:6,2″-terpyridine) or tppz (2,3,5,6-tetrakis(2-pyridyl)pyrazine, Fig. 6) [28], and a non-innocent bidentate ligand based on the o-amidophenolato/o-iminobenzosemiquinonato/o-iminobenzoquinone redox system ([29], Fig. 7). The latter

Conclusion

The case study from Chapter 3 thus used a metal complex framework containing

  • (i)

    one tridentate terpy or tppz as largely innocent spectator ligand (in this context),

  • (ii)

    one non-innocent unsymmetrical bidentate ligand belonging to the popular [29] o-iminobenzoquinone/o-iminobenzosemiquinone/o-amidophenolate redox series, and one monodentate ligand (Fig. 12):

    • 1.

      innocent chloride,

    • 2.

      potentially non-innocent nitrite/nitro (NO2/NO2radical dot), or

    • 3.

      highly [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14],

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Support from the Land Baden-Württemberg, Germany, is gratefully acknowledged. We wish to thank the collaborators mentioned in the citations of our own work and Angela Winkelmann for her contributions to this article.

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