Polycarbene

Among the polydentate carbene ligands, particular interest has recently been placed on cyclic polycarbenes. ImidazoUum precursors like 23 [89] or 24 [90, 91], which upon C2 deprotonation would lead to tetradentate or even hexadentate double-pincer NHC ligands, have been prepared. Their interesting coordination chemistry will be discussed in Sect. 4. Finally, Arnold et al. developed and reviewed NHC ligands which are functionalized with additional anionic (alkoxide or amido) donor groups [92]. 

Linear and 2D branched poly(diarylcarbene) systems95 were much pursued as prototypical potential magnetic systems (Fig. 8). For example, Iwamura and coworkers developed synthetic methodologies to put many diazo sites (photochemical carbene precursor sites) into proper connectivity to photogenerate all or most of the carbene sites. Itoh and coworkers developed methods to obtain and analyze HS ESR spectra of polycarbenes and polycarbene ions.96 Although... 

Fig. 8 Experimentally investigated polycarbene high spin organic molecules.

Iwamura et al.[93 have reported the preparation of a branched-chain nonacarbene possessing a nonadecet ground state with the goal of constructing superparamagnetic polycarbenes. The preparation of three-directional, diradicals was reported by Bock et alJ94J and a tetrahedral, tetraradical has been synthesized by Kirste et al. The synthesis, characterization, and physical properties of the perchloro-2,6-bis(diphenylmethyl)-pyridine-a,a -ylene biradical have also been reported. 

The polymer polyyne, -(C=C) , also known as polycarbene, and shown in Fig. 1.4(c), is formed with sp hybrids in this way. Its structure may be either an alternation of single and triple bonds or a sequence of double bonds, depending on the terminal groups - see Fig. 1.5. 

Fig. 1.5 The alternative structures of polymers formed with. -hybridised orbitals, (a) polyyne and (b) polycarbene. Overlap of the rc-electron orbitals, shown by the off-axis lines, gives an interatomic 7u-bond. The cr-bonds are shown by on-axis lines.

Nakamura, N., Inoue, K. and Iwamura, H. (1993). A branched-chain nonacarbene with a nonadecet ground state a step nearer to superparamagnetic polycarbenes. Angew. Chem. Int. Ed. 32, 872-874... 

Figure 11-6 Dependence of the magnetization of various paramagnetic species on the external magnetic field strength (H) divided by temperature (7) at 2.1 ( ), 4.8 ( ) and 10.0 K (a). The increase in the magnetization with H/T is at first linear (region 1), then slows down (region 2), and finally levels off (region 3). See text for explanation. The top two curves representing S = 9 and 6 are for polycarbenes 48 and 47, respectively.

From High-spin Polycarbenes To Carbene-based Magnets... 

The chemistry of high-spin polycarbenes started in 1967 when two groups led by K. Itoh [1] and E. Wasserman [2] reported independently their findings that OT-phenylene-dicarbene Ic generated by the photolysis of diazo compound Id has a ground quintet state. Prior to this report, A. M. Trozzolo and coworkers... 

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