Cyanil. Synthesis and characterization of the strongest isolated electron acceptor and its reduced forms
作者:Carlos Vazquez、Joseph C. Calabrese、David A. Dixon、Joel S. Miller
DOI:10.1021/jo00053a017
日期:1993.1
The strong acceptor tetracyano-1,4-benzoquinone, cyanil, Q[nu(C=N) at 2210 w and 2241 w cm-1; nu(C=O) at 1697 s cm-1 (CH2Cl2); nu(C=C) 1616 s cm-1 (Raman)], as well as tetracyanohydrobenzoquinone, QH2(2241 m and 2263 m cm-1), the radical anion, [Q].-[nu(C=N) approximately 2216 m cm-1 (MeCN)], anion [QH]-, dianions [Q]2- (2185 s cm-1), [H]+[Q]2-, and the dimer dianion [Q]2(2-)(2217 m and 2226 sh cm-1) have been prepared. QH2 was prepared from NaCN and p-bromanil, and Q was prepared from the bromine oxidation of the disilver salt of QH2, QAg2. QAg2 is a versatile synthon for the preparation of Q, [QH]-, [H]+[Q]2-, and [Q]2-. The single-crystal X-ray structures of QH2, Na+[H]+[Q]2-, [Me4N]+[H]+[Q]2-, [Et4N]+[QH]-, [n-Bu4N]+[QH]-, [Et4N]+}2[Q]2(2-), [(Ph3P)2N]+}2[Q]2-, Q, [Fe(C5Me5)2].+[H]+[Q]2-, and [Fe(C5Me5)2].+}2[Q]2(2-) have been determined. The average CO (OC-CCN, and NC-CN) distances are 1.204 (1.488, and 1.338), 1.233 (1.463, and 1.378),1.268 (1.426, and 1.414), and 1.322 (1.413, and 1.372) angstrom for Q, [Q]-, [Q]2-, and QH2, respectively. Several salts exhibit very strong intermolecular hydrogen bonding with O...O separations less than 2.5 angstrom observed for the [Et4N]+[QH]- (2.38 angstrom), [Me4N]+[H]+[Q]2- (2.43 angstrom), and Na+[H]+[Q]2- (2.48 angstrom) salts. (The former is the shortest reported for an an organic system with intermolecular hydrogen bonding.) The degree of protonation varies with counterion with examples of [QH]- and [H]+[Q]2- being observed and strongly influences the electronic and nu(C=N) absorptions. The radical anion was structurally characterized as its diamagnetic dimer dianion, [Q]2(2-), with an intradimer separation of 2.88 angstrom. The EPR of [Q].- in dilute CH2Cl2/MeCN displays hyperfine interaction with four equivalent nitrogens (a(CN) = 0.32 G) and two types of C-13 (2.58 and 5.25 G). Cyanil is the strongest acceptor that has been isolated in the neutral form and has three quasi-reversible one-electron reductions at 0.90,0.09, and -1.81 V vs SCE in MeCN. It is unstable in aqueous media. The reduction potential of cyanil is 0.30 V or more greater than those of DDQ, n-C4(CN)6, and perfluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQF4). Q dehydrogenates 2,3-dichloro-5,6-dicyanohydroquinone (H2DDQ); thus it is a stronger hydrogen abstraction reagent than conventionally used DDQ. Molecular orbital (ab initio) calculations are in general agreement with the above results, and optimized geometries were used to understand the complex crystal structures.