TMPDO (2.58 g, 69%, m.p. 498 K (493 K 1221) was prepared by treatment of 2,3,5,6-tetramethylpyrazine (2.95 g, Aldrich) in CH,CI, (50 mL) with potassium peroxomonosulfate (66.6 g, 5 equiv; OXONE, Aldrich) in H,O (400 mL) with vigorous stirring at 298 K for 24 h. TCNE (Aldrich) was purified by multiple sublimation (373 K, 0.05 mmHg) through activated charcoal. Conditions for the BenesiHildebrand analysis: [TMPDO] =1.90m~; [TCNE] =18.7, 34.0, 51.1, 66.4, 75.7, and 90.6m~. Purple crystals of (TMPDO),TCNE were prepared by cooling a mixture of TMPDO (40m~) and TCNE (20m~) in CH,CN to 253 K for 2-5 h. Red prismatic crystals of (TMPDO),TCNE were prepared analogously, after 2-3 d crystallization. Crystal X-ray diffraction was performed on a Siemens SMART diffractometer with CCD area detection at 293 (purple (111) and 173 K (red 1131) with monochromatic Mo,.radiation (A = 0.71073 8,). Solution and refinement (against were performed with SHELXTL. No absorption correction was employed. Hydrogen atoms were placed in calculated positions (0.95 8, from the methyl carbon atom with tetrahedral angles). They were given torsional freedom and their thermal parameters were set to 1.2 x Oc, of the bonded carbon atom. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-100438. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12Union Road, Cambridge CB21EZ, UK (fax: int.