Aryldiazenido complexes

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

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). 

Aryldiazene complexes can be formed from arenediazonium ions and metal hydrides, and subsequent deprotonation leads to aryldiazenido complexes. Mechanistic details of ArN insertion into Rh—H and Pt—H bonds have been recently studied (equations 124-126).453 454... 

The reaction of metal hydrides with jV-tosyl-iV-methylnitrosamine is a useful synthetic method for preparing nitrosyl complexes, but this is not true for aryldiazenido complexes. The analogous precursor, l-phenyl-3-tosyl-3-methyltriazene, can be prepared but is thermally unstable much above... 

There are fewer documented examples of these N2-elimination reactions than might be expected. Aryldiazenido complexes are known to react with molecular hydrogen to yield arylhydrazine complexes via aryldiazene intermediates (equations 158 and 159). 

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).187,194... 

SECM feedback measurements have been applied, together with cyclic voltammetry and controlled potential electrolysis (30), to the reduction of the rhenium aryldiazenido complex [Cp Re(CO)2(/7-N2CsH4OMe)] [BF4] (III(BF4) Cp = T -C5Me5). A one-electron reduction results in the 19-elec-tron complex, IV [Eq. (40)], which then decomposes to give products [Eq. (41)]. 

On the basis of this introduction, we will now discuss the structures found for aryldiazenido complexes. For an organic chemist, the most surprising observation in metal complex formation of arenediazonium ions is the fact that the nitrogen atoms have the function of electron donors and not electrophiles as in additions to organic and (nonmetallic) inorganic compounds. We shall return to this phenomenon later in this section. 

Rattray and Sutton s binuclear complex 10.42 contains Pd -atoms. Palladium(O) complexes resulted, however, in spontaneous elimination of N2 to give arylpalladi-um complexes (Kikukawa and Matsuda, 1977 Rattray and Sutton, 1978). Yet, Mat-suda s group (Yamashita et al., 1980) was able to obtain (aryldiazenido)palladium(O) complexes by adding two equivalents of an arenediazonium tetrafluoroborate or hexafluorophosphate to a suspension of [[P(C6H5)3)4Pd j in dichloromethane at — 78°C and allowing the mixture to warm to room temperature. Scheme (10-17) demonstrates that a mixture of the aryldiazenido and the aryl complex of Pd is formed. The aryldiazenido complex is subject to dediazoniation at room temperature. UV-irradiation facilitates this dediazoniation. Only in the case of 4-methoxy-benzenediazonium hexafluorophosphate was it possible to isolate the aryldiazenido complex and to identify it by elemental analysis, NMR, and IR spectroscopy. 

In the next section, we will discuss several alkyldiazenido complexes which were obtained by alkylation of dinitrogen complexes with strongly nucleophilic alkylation reagents. At least one such case is also known for aryldiazenido complexes. Sellmann and Weiss (1978) showed that a (dinitrogen)manganese complex reacts with phenyllithium (see Sect. 3.3). 

In Section 10.2, Sutton (1993) was quoted because all attempts to synthesize aryldiazenido complexes of nickel were without success. Nevertheless, alkyldiazenido complexes are known, as seen below. 

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... 

Cationic dihydrogen complexes of the type [TpRu(L)(L )(H2)] (L = P(OEt)3, PPh(OEt)2, L = PPhs, L = L = P(OEt)3) have been utilized to prepare various RuTp diazenido compounds. Treatment of [TpRu(L)(L )2(H2)] with aryldiazonium salts give aryldiazenido complexes [TpRu(L)(L )(NNAr)]. Analogous reactions were performed with hydrazines. With monohydrido complexes TpRuH(L)(L ) and aryldiazonium salts, aryldiazene complexes [TpRu(L)(L )2(NH=NAr)]" were obtained. 

SECM feedback measurements have been applied, together with cyclic voltammetry and controlled potential electrolysis, to the reduction of the rhenium aryldiazenido complex... 

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