With Sharpless' and Meldal's discovery of the immensely supportive effect that metal catalysis has on Huisgen's classical 1, 3-dipolar cycloaddition, azides (RN3) – long underappreciated in organic synthesis – suddenly got in the focus of attention as most crucial players in sensational ‘click chemistry'. Less noisy though with the same commitment and even a much broader scope of scientific topics and objectives, the inorganic azide chemistry has made just as great strides in the last few decades. This review (Part I) gives an introductory survey of the most important results, and informs about modern developments and general trends. Particular emphasis is placed on the recent successful approaches to highly unstable homoleptic azido metal complexes of the main group and early transition elements, as well as on the enormous structural versatility caused by the ‘flexidentate' N3– ligand with its unsurpassed bridging capacities. The presentation in this paper of selected compounds and reactions is meant, in a way, as a prelude to the [3+2]-cycloadditions of metal azides and related species which will be covered in-depths in Part II. A large part of the comments finally deals with applications in fields such as catalysis, high explosive performance or magnetism of metal compounds containing azide, today certainly one of the most attractive research areas world-wide.

Azide Chemistry – An Inorganic Perspective, Part I Metal ­Azides: Overview, General Trends and Recent Developments

In whatever state of bonding – whether covalent to an organic residue or a heteroatom, or polar to ionic in contact with a metal – the azide moiety N3 is characterized by its high potential of reactivity which essentially manifests itself in two basic processes: the elimination of dinitrogen and the entry into 1, 3-dipolar cycloadditions with suitable dipolarophiles, the latter of which clearly predominates the chemistry of azide, also that of its metal compounds. In a preceding review entitled “Part I – Metal Azides: Overview, General Trends and Recent Developments” which was meant to lay the foundations for the present paper, these and other reactions have already been touched upon. The present review – Part II – now focusses in great detail on the formation of five-membered heterocycles – tetrazol(at)es, triazol(at)es, triazolin(at)es, thiatriazol(at)es, etc. as well as various consecutive products – from azide and nitriles, isocyanides, alkynes, alkenes and heteroallenes (CS2, RN=C=S) in the ligand sphere of the metal. Generally, these [3+2]-cycloadditions are found to proceed under much milder conditions in comparison with the strictly organic case whose triumphant progress since the 1960s is intimately bound up with the name of Huisgen. Mechanistic considerations on the matter are presented. A secondary aspect still occupying quite a part of the discussion is concerned with the role of metals in [3+2]-cycloadditions particularly of the highly topical “click”-type, e.g. (CuAAC), (RuAAC). Likewise, a short chapter deals with the question of pentazol(at)e (N5–) which according to numerous theoretical studies could well be stabilized and isolated in combination with metals, e.g., in the form of azametallocenes. A last chapter is devoted to a cursory survey of related systems, in particular fulminato complexes, metallo nitrile ylides and metallo nitrile imines, in which the metal acts as a substituent on the 1, 3-dipole (metallo-1, 3-dipole). Other systems with a metal substituent on the dipolarophile (metallo-dipolarophile), or, with metal itself in the three- (two-) atom arrangement constituting the dipole (dipolarophile) [metalla-1, 3-dipole, metalla-dipolarophile] are only quoted by way of example.

Azide Chemistry – An Inorganic Perspective, Part II[‡] [3+2]-Cycloaddition Reactions of Metal Azides and Related Systems†

Antimycobacterial and antitumor activities of palladium(II) complexes containing isonicotinamide (isn): X-ray structure of trans-[Pd(N3)2(isn)(2)].

3,5-Dimethyl-1-thiocarbamoylpyrazole and its Pd(II) complexes: Synthesis, spectral studies and antitumor activity

Synthesis and biological evaluation of ternary silver compounds bearing N,N-chelating ligands and thiourea: X-ray structure of [{Ag(bpy)(μ-tu)}2](NO3)2 (bpy = 2,2′-bipyridine; tu = thiourea)

The compound [Pd(bzan)(μ-N3)]2 1, bzan = benzylideneaniline, was prepared from [Pd(bzan) (μ-OOCCH3)]2 by an anion exchange reaction. The 1,3-dipolar cycloaddition of carbon disulfide to the bridged coordinated azide in the cyclometallated compound 1 was investigated. The species resulting from this reaction, di(μ-N,S-1,2,3,4-thiatriazol-5-thiolate)bis [(benzylidene-aniline)palladium(II)] 2, was characterized by IR spectroscopy and X-ray diffraction. The compound 2 is a dimer containing two [Pd(benzylideneaniline)] moieties connected by two vicinal bridging N,S-1,2,3,4-thiatriazole-5-thiolate anions in a square-planar coordination geometry for the palladium atoms.

1,3-dipolar cycloaddition of CS2 to the coordinated azide in the cyclopalladated [Pd(bzan)(mu-N-3)](2). Crystal and molecular structure of di(mu-N,S-1,2,3,4-thiatriazole-5-thiolate)bis[(benzylideneaniline-C-2,N)palladium(II)]

Mononuclear pyrazolyl Pd(II) complexes of the type [PdX 2(phmPz)2] (X = Cl−, N− 3) have been prepared. The 1-phenyl-3-methylpyrazole displaces acetonitrile from [PdCl2(CH3CN)2] to form [PdCl2(phmPz)2] (phmPz=1-phenyl-3-methylpyrazole) (1). [Pd(N3)2(phmPz)2] (2) could be obtained by metathesis from [PdCl2(CH3CN)2] or by substitution of the chloride in (1) by the azide ion. Both complexes were characterized by elemental analysis, infrared spectroscopy, 1H and 13C NMR and by single crystal X-ray diffraction. The coordination geometry around Pd(II) in these complexes is nearly square-planar, with the ligands in a trans configuration.

Synthesis and structural characterization of dichlorobis (1-phenyl-3-methylpyrazole) palladium(ii) and diazidobis (1-phenyl-3-methylpyrazole) palladium(ii)

J. R. Zoia

Papers

1,3-dipolar cycloaddition of CS2 to the coordinated azide in the cyclopalladated [Pd(bzan)(mu-N-3)](2). Crystal and molecular structure of di(mu-N,S-1,2,3,4-thiatriazole-5-thiolate)bis[(benzylideneaniline-C-2,N)palladium(II)]

Synthesis and structural characterization of dichlorobis (1-phenyl-3-methylpyrazole) palladium(ii) and diazidobis (1-phenyl-3-methylpyrazole) palladium(ii)