Phosphaquinomethane and Phosphathienoquinomethanes, and Their Anion Radicals Fumiki Murakami, Shigeru Sasaki, and Masaaki Yoshifuji* Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba, Sendai 980-8578, Japan Received April 8, 2005; E-mail: yoshifj@mail.tains.tohoku.ac.jp Only a few phosphaquinoid compounds, a hybrid of low- coordinated phosphorus compounds with quinoid skeletons, have been isolated to date despite continuing synthetic effort on the low- coordinated phosphorus compounds for the last two decades 1 as well as enormous research on quinoid molecules (Scheme 1). Sterically protected diphosphaquinone 1 2 was synthesized by Ma ¨rkl and co-workers and was isolated as chromium(0) complexes (Mes* ) 2,4,6-tri-tert-butylphenyl). We synthesized phosphaquinone 2 3 and diphosphathienoquinones 3 and 4 4 and revealed the structural similarity to that of the conventional quinoid compounds. The phosphaquinoid compounds 2 and 3 are two-step redox systems, but anion radicals obtained by alkaline metal reduction of them, phosphasemiquinone radicals, have significant unpaired electron density on the phosphorus atoms and formulated as phosphinyl radicals. Herein, we report synthesis, structure, and redox properties of phosphaquinomethane 5 and phosphathienoquinomethanes 6, which behave as quinoid molecules not only in the neutral but also in the anion radical state. Phosphaquinoid compounds 1, 2, 3, and 4 were synthesized by 1,6-elimination of the corresponding aromatic precursors. As a route to phosphaquinomethanes, dehydration from the precursors, 1-hy- droxymethyl-4-phosphinobenzene 9 and 2-hydroxymethyl-5-phos- phinothiophene 10, was employed (Scheme 2). Carbinol 9 was prepared by the halogen-metal exchange of bromobenzene 7 4 followed by reaction with benzophenone. Dehydration of 9 catalyzed by trifluoroacetic acid in benzene with MS4A afforded 5. 5 On the other hand, reaction of the thienyllithium derived from 8 with benzophenones afforded not only the carbinols 10 but also the phosphathienoquinomethanes 6. Therefore, the mixture was subjected to complete conversion without isolation of 10, and refluxing with anhydrous CuSO 4 in benzene 6 afforded 6 in excellent yields as sterically favored Z-isomers. 5 Phosphaquinomethane 5 and phosphathienoquinomethane (Z)- 6a show a 31 P NMR signal at δ 244.4 (s) and 201.4 (s), respectively, supporting formation of a PdC double bond. More shielded value of (Z)-6a is ascribed to electron donation by the sulfur atom similarly to diphosphathienoquinones. 41 H and 13 C NMR spectra of 5 reflect the phosphaquinoid structure, where a vinyl proton cis to Mes* shows more upfield shift with a smaller J PH value (δ 5.48, J PH ) 9.6 Hz) than that cis to the lone pair (δ 7.11, J PH ) 13.8 Hz). 7 The carbon adjacent to the phosphorus appears in a low field (δ 170.35, d, J ) 29.1 Hz) similarly to phosphaalkenes. 1 The exocyclic carbon (δ 134.68, d, J ) 28.5 Hz) has considerable coupling with the 31 P nucleus owing to an extended π-conjugated system. 1 H and 13 C NMR spectra of (Z)-6a are also consistent with its formulation as the phosphathienoquinomethane. Deshielded chemical shift and considerably large J PH values of the proton adjacent to the phosphinidene moiety (δ 6.96, J PH ) 8.1 Hz) reflect the Z-configuration. Phosphaquinomethane 5 and (Z)-6a have a deep orange color (λ max (log ǫ/hexanes) 440 (4.49) (5); 446 (4.49) ((Z)- 6a) nm) which is similar to phosphaquinone 2 (λ max (log ǫ/hexanes) 372 (4.34) nm) 3 due to the low-lying excited state. Phosphathieno- quinomethanes (Z)-6b and (Z)-6c display 1 H, 13 C, and 31 P NMR and UV-vis data similar to those of (Z)-6a, reflecting little influence of substituents at the 4-positions of the aryl groups on the phosphaquinoid skeleton. Molecular structure of (Z)-6b obtained by X-ray crystallography is shown in Figure 1. 8 The PdC bond length (1.704(2) Å) lies in the longest category in the reported values (1.61-1.71 Å), 1 but is similar to those of the phosphaquinoid compounds (1.705(2) (2), 3 1.712(2), 1.714(2) (4) 4 Å). The C-Pd C angle reflects the absence of severe steric interaction among substituents and takes a small value (98.48(6)°) as low-coordinated phosphorus compounds carrying a Mes* group. The bond lengths within the phosphathienoquinomethane skeleton show apparent bond alternation, where C1-C2 (1.450(2) Å) and C3-C4 (1.441- (2) Å) are longer than C2-C3 (1.349(2) Å) and C4-C5 (1.365(2) Å). The whole phosphathienoquinomethane skeleton, except for the PdC double bond, has bond lengths similar to those reported for thienoquinomethanes. 9 The phosphathienoquinomethane is planar with the maximum deviation from the least squares plane (S1, P1, C1-C6, C13, C20) as 0.0634(13) Å (C3). Cyclic voltammetries of 5 and (Z)-6 consist of two-step irrevers- ible waves, which do not become reversible even when scanned within the range of one wave (Figure 2). The first reduction Scheme 1. Phosphaquinoid Compounds Scheme 2. Synthesis of Phosphaquinomethanes Figure 1. ORTEP drawing of (Z)-6b with 50% thermal ellipsoids. Published on Web 06/07/2005 8926 9 J. AM. CHEM. SOC. 2005, 127, 8926-8927 10.1021/ja052271l CCC: $30.25 © 2005 American Chemical Society