Isocyanide ligands carbynes

General behaviors are summarized in Table 1. For ligands such as carbon monoxide, isocyanides (CNR), carbynes, nitrosyl NO+ and, although to a smaller extent, nitriles (NCR), which are not only a donors but also good jr acceptors, the net electron-donor ability is modest or poor and the Pl parameter has a high value. However, ligands that are not involved in jt-bonding, such as ammonia (NH3) and pyridine, exhibit lower Pl values that, nevertheless, are usually not... 

The first synthesis of a complex containing a metal-carbon triple bond was reported in 1973 [1]. Since then, numerous carbyne complexes have been prepared. In recent years, the study of the reactivity of these complexes has attracted considerable interest [2]. E.g. carbyne complexes have extensively been used as building blocks in the synthesis of transition metal clusters [3]. The coupling of carbyne ligands with CO or isocyanide ligands has also been studied in detail [4]. However, the number of reports on the use of carbyne complexes in synthetic organic chemistry is rather limit in contrast to carbene complexes which have found many applications in the synthesis of carbo- and heterocycles [5]. 

The route described herein consists mainly of a 3-electrophilic attack at an unsaturated alkyne-derived or isocyanide ligand when activated by an electron-rich group 6 or 7 heavy metal phosphinic centre. Metal ligating carbynes of the types =CCH2R, =CNH2, =CNHR, =CN(AlEt )R or =CN(M)R (M = transition metal Lewis acid) nave thus been obtained and are the object of this discussion. 

These reactions provided the first examples of electrophilic attack at an isocyanide ligand and constituted a novel route for carbyne complexes. 

In agreement with this interpretation, the dialkylaminocarbyne complexes trans-[M(CNER) (CNR) (dppe)9] (j6 E=Me or Et R=Me or Bu ) are also unstable in solution, undergoing a trans to cis isomerization [31], thus converting into species in which the ligand trans to the carbyne is a phosphine rather than the stronger 7r-acceptor isocyanide ligand. 

The nitrogen atoms of the isocyanides ligands CNR in M2(CO)3(CNR)Cp2 ( M = Fe, Ru) has been proved to be very susceptible to alkylation reactions [12]. The cationic products contain the CN(R )R group invariably located in the bridging position. The carbyne character of these ligands has been evidenced from their p-C nucleophilic addition reactions of H" and CN" v/hich may form either bridging or terminal aminocarbene derivatives. The results of our studies, collected in Scheme 3, can be summarized as follows ... 

Other ligands that have been successfully converted to coordinated carbynes include thiocarbonyls, isocyanides, and vinyl groups. The... 

Isocyanides also display over a wide range of Pi values (Table 2), from benzoyl isocyanide Pi = +0.20 V) [15] with a net electron-acceptance similar to that of the carbyne ligands, down to the metallo-cyanide adduct C=N-PdCl2(PPh3) Pi = -0.61 V) [27]. Between these limiting cases, one finds the following types ordered according to their net electron-acceptance (Tables 2... 

Protonation of a ligand normally leads to a significant increase in its net electron-withdrawing ability (or decrease of its net electron-donor character), as observed for the following pairs (Pi increase of 0.3-0.5 V) (Table 2) carbynes CCH2R versus vinylidenes C=CHR [14], aminocarbyne CNH2 versus isocyanide CNR [17],... 

The lability of the N2 ligand towards alkyl and aryl isocyanides has been demonstrated by equation (2) which occurs readily with both alkyl and aryl isocyanides.40 The resulting complexes are electron rich , the alkyl isocyanide derivatives ReCl(CNR)(dppe) (R = Me or Bul) undergoing electrophilic attack upon treatment with HBF4 to form the carbyne complexes trcms-[ReCl(CNHR)(dppe)2]BF4.41 Other noteworthy features of the series of complexes ReCl(CNR)(dppe)2 and ReCl(CNAr)(dppe)2, are the low value of v(CN), the presence of a bent M—CNR coordination mode, and their ease of oxidation (as measured by cyclic voltammetry) to the corresponding rhenium(II) monocations.40... 

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

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