Aryldiazenido ligands

The third area is the synthesis and characterization of aryldiazenido complexes of transition metals. In 1964 King and Bisnette isolated the first metal complex with an aryldiazenido ligand. The interest of organometallic chemists was concentrated mainly on the isolation and characterization of stable aryldiazenido complexes and not on potential metastable intermediates involved in metal-catalyzed dediazonia-tions. The situation is different, however, for metal complexes with alkyl-diazenido ligands. Complexes with aryl- and alkyldiazenido ligands are the subject of Chapter 10 in the forthcoming second book (Zollinger, 1995). 

Diiodo ligands, in platinum(II) complexes, 8, 504 Diiridium complexes, with bridging aryldiazenido ligands,... 

The first reported diazenido metal complex was obtained by King and Bisnette (1964) by ligand exchange, as shown in (10-5) one of the three carbonyl groups in the molybdenum complex 10.20 was replaced by an aryldiazenido ligand (10.21) in tetrahydrofuran. The 18-electron Mo configuration requires the aryldiazenido ligand to be a three-electron donor the anionic complex 10.20 becomes a neutral product (10.21) by this substitution. 

Many attempts to replace a carbonyl by an aryldiazenido ligand in transition metal complexes containing only carbonyl ligands, failed, however, as King and Bisnette made attempts with [Y(CO)6] , [Mn(CO)5] , and [Co(CO)4] in 1964, and Schrauzer (1961), Clark and Cookson (1962) earlier. The latter two investigations, indeed, represent the first reactions of organometallic compounds with arenedi-azonium ions, but only vigorous evolution of N2 and CO was observed. 

Zinc is considered to be a borderline transition metal. ZnCl2 is used as an additive to arenediazonium chlorides because it increases the inertness ( kinetic stability ) towards explosive dediazoniation. Three X-ray investigations of such so-called double salts (ArNi )2ZnCl4 (see Zollinger, 1994, p. 24) reveal that the two diazonium ions are not coordinated to the metal atom (Mostad and Romming, 1968). These compounds are not, therefore, complexes containing aryldiazenido ligands. 

The doubly bent aryldiazenido group (10.4) is characteristic for contributing one electron to a two-electron, two-center o-bond with the metal. Formally, therefore, such a one-electron donor ligand is an arenediazo anion (ArNi" in 10-1 A), but the synthesis is still based on arenediazonium ions. They attack, however, an electron-rich metal atom or, in other words, the metal atom undergoes a formal one-electron oxidation (10-1B). 

Organometaiiic chemistry started some forty years ago and developed rapidly, particularly that part involving transition metals. This can be illustrated by the facts that, in the period 1981-1992, not less than 1319 stable organometaiiic compounds containing rhodium, a relatively rare member of the platinum group, were described (Sharp, 1995), or that stable complexes containing diazenido ligands with at least 19 transition metals are known (Sutton, 1993). On the other hand, no aryldiazenido complex of copper has been described, in spite of the fact that such coordination compounds may be formed in the Sandmeyer, Pschorr, and Meerwein reactions. The latter have been known, in part, for more than a century We are aware, of course, that the search for the structure of catalysts is methodically very different from that for stable compounds. This is likely to be the reason that in the majority of review papers either structures of stable compounds or catalysts are discussed but correlations between these areas of interest are not. ... 

Equation (13) shows the use of a diazonium salt for the preparation of iridium-tetraazadiene complexes, e.g. (65), which are formed in addition to an aryldiazenido complex, e.g. (69). The mechanism of formation of a coordinated tetraazadiene ligand in equations (10) and (11) has been discussed extensively. " " In Scheme "7 the proposed routes and intermediates have been summarized. One proposal (route a) involves initial formation of a monodentate, metal-nitrene species (70) to which an organic azide coordinates (72), followed by a 3-N(azide)-l-... 

Through comparative studies for a range of [Tp M(CO)3] analogues with different substituents in the pyrazolyl ring, Lalor demonstrated that the oxidation by arenediazonium cations occurred in response to the steric rather than the electronic effect of the 3-methyl substituents. However, further steric crowding in either the hydrotris(pyrazolyl)borate ligand or the diazonium cation promoted a reversion to the carbonyl-substitution pathway, producing aryldiazenido complexes Tp M(NNAr)(CO)2, which are also the products observed for the... 

---