Nitronyl nitroxide structure

Let us end this section with the synthesis of three relevant molecules TCNQ, Qf-nitronyl nitroxides and CuPc. The synthesis route that efficiently leads to the preparation of the strong acceptor TCNQ is depicted in Fig. 2.10. It is based on the ready condensation of malonitrile with 1,4-cyclohexadione. Further treatment with C5H5N and Br2 gives TCNQ in excellent yields (Acker Herder, 1962). The electronic band structure of neutral TCNQ will be discussed in Section 6.1. 

The first experimental confirmation of Glauber s prediction was shown with the material Co(hfac)2(NITPhOMe), based on a nitronyl nitroxide radical (Caneschi et al, 2001). Co(hfac)2(NITPhOMe) consists of alternating Co(hfac)2 and radical moieties arranged in ID arrays with a helical structure, and crystallizes in the P3i space group with a = b = 1.129 nm and c = 2.057 nm. A detail of the chain structure is depicted in Fig. 6.21. 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

Figure 9.2 The radical structure of nitronyl nitroxide radical, imino nitroxide radical, and semo-quinonato.

Figure 9.7 The structure of NITR radical and the schematic view of the crystal structure of RE(hfac)3NITEt [76], (Reprinted from A. Caneschi, D. Gatteschi, and R. Sessoli, Magnetic properties of a layered molecular material comprising manganese hexafluoroacetylacetonate and nitronyl nitroxide radicals, Inorganic Chemistry 32, 4612 616, 1993. 1993 American Chemical Society.)...

Luneau, D. and Rey, P. (1995) New manganese(II) complexes of nitronyl nitroxide radicals. Synthesis, structure and magnetic properties. Molecular Crystals and Liquid Crystals Science and Technology Section A, 273, 81-87. 

In the crystal structures of nitronyl nitroxide 27 and iminyl nitroxide 28 (Figure 6), magnetic linear chains along the r-axis have been observed when R is H. These solids exhibit strong antiferromagnetism. In contrast, Cl substitutions in both radicals result in a less favorable geometry for intermolecular spin interactions <1997JOC8854>. 

Figure 1. Chemical structure of a nitronyl nitroxide radical, where R is the substituent that differentiates among members of this family (representative cases range from a simple H atom up to substituted six-membered rings).

In the following sections, our first-principles bottom-up methodology will be described from a physical perspective (a full description on mathematical terms can be found in Ref. [7]). Then, the use of this methodology will be illustrated on the a-2-hydro nitronyl nitroxide [8] (hereafter called a-HNN), the simplest member of the nitronyl nitroxide family (R = H in Figure 1). The macroscopic magnetic properties and crystal structure of this compound are well known (its crystal structure is stored in the Cambridge Crystallographic Database [9] with refcode name TOLKEK). 

Figure 3. (a) Valence bond representation of the electronic structure of the (HON)-CH-(NOH) radical, a prototype of nitronyl nitroxide. (b) Potential energy curve for the interaction of two (HON)-CH-(NOH) radicals placed one on top of the other (the coordinates differing by a displacement along the vertical coordinate z). Each fragment is a doublet state, and the curve was computed for the triplet state at the UB3LYP/6-31+G(d) level. 

In a laser flash-photolysis study, 2-phenyladamantene was generated in benzene at room temperature from 3-noradamantyl(phenyl)diazomethane. This strained cycloalkene decays with second-order kinetics to give a dimer, and reacts much faster with O2 and Bu3SnH than with methanol, thus revealing a substantial radical character. Diphenyldiazomethanes possessing stable /er -butylaminoxyl and Ullman s nitronyl nitroxide radicals, e.g. (25), have been prepared by photolysis of the parent diazomethanes. Analysis of ESR fine structures showed that the carbene and radical centres couple ferromagnetically in these molecules, as expected. 

As mentioned above, intermolecular interactions are the interactions that a stable molecule experiences in the presence of other molecules (not necessarily of the same type) [1], We consider as stable molecules any aggregate of atoms having a long enough stability and lifetime, on a chemical scale. This definition also considers as stable molecules, open-shell radicals whose chemical structure makes them long-living, like those found in molecular magnets (for instance the family of the nitronyl nitroxide radicals). 

Fig. 4 General chemical structure of the family of a-nitronyl nitroxides and a representation of their singly occupied molecular orbitals (SOMO). Reprinted from ref 55 [AmabUino DB, Cirujeda 1, Veciana J, Crystal architectures of organic molecular-based magnets , (1999) Phil Trans R Soc Lond A 357 2873] by permission of The Royal Society, Copyright 1999...

Organometallic and Coordination Compounds. - Crystallization of ferrocene and ruthenocene substituted in the 1- and T-positions by two nitronyl nitroxide radicals gave the new crystal phases p-1 (besides the known phase a-1, a-2, and a-2 whose structures were determined by X-ray analysis and were investigated by C and H NMR spectroscopy with MAS NMR. The solid state Sn CP MAS NMR spectra of a series of triaryltin chlorides of the form ArsSnCl have been acquired. The indirect spin-spin coupling constants (J( Sn- C1)), quadrupolar-dipolar shifts (6( Sn- Cl)), and the Sn chemical shift tensors were extracted. " Powders of the zinc and copper(II) dimethyl-(MDtc), diethyl-(EDtc), and morpholinedithiocarbamate (MfDtc) complexes quantitatively absorb hexamethyleneimine (Hmi) to produce the adducts... 

Free radical chemistry is another field of application of the Sn reactions. Lithium salts of nitronyl nitroxide radicals proved to be appropriate nucleophilic species to modify the structure of a number of azaaromatic compounds. This reaction provides an access to a new family of free radical compounds (Scheme 44) [148-150]. 

Figure 25 Schematic view of three ID magnetic structure obtained with nitronyl-nitroxide radicals and...

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