The Tetracyanopyrazinide Dimer Dianion, [TCNP]22−. 2-Electron 8

Published on Web 06/04/2009

The Tetracyanopyrazinide Dimer Dianion, [TCNP]22-. 2-Electron 8-Center Bonding Juan J. Novoa,*,† Peter W. Stephens,*,‡ Mahika Weerasekare,§ William W. Shum,§ and Joel S. Miller*,§ Department of Physics & Astronomy, Stony Brook UniVersity, Stony Brook, New York 11794-3800, Departament de Quı´mica Fı´sica and IQTCUB, Facultat de Quı´mica, UniVersitat de Barcelona, AV. Diagonal 647, 08028-Barcelona, Spain, and Department of Chemistry, 315 S. 1400 E. RM 2124, UniVersity of Utah, Salt Lake City, Utah 84112-0850 Received April 7, 2009; E-mail: [email protected]; [email protected]; [email protected]

Abstract: The reaction of Cr(C6H6)2 and 1,2,4,5-tetracyanopyrazine (TCNP) forms [Cr(C6H6)2][TCNP], and TCNP is reduced and forms the eclipsed π-[TCNP]22- dimer. Diamagnetic [TCNP]22- has an intradimer separation of 3.14(2) Å. The intradimer C · · · C and N · · · N separations are 3.29(2) and 3.42(2) Å, respectively, and increase with the distance from the center of the molecule, due to nitriles bending away from the plane of the molecule by 5 ( 1°. [TCNP]22- is best described by an atoms-in-molecules analysis as having a 2e-/8c C-C bond involving the four C atoms from each six-member ring. The results of B3LYP/6-31+G(d)computed interactions indicate that the [TCNP]•- · · · [TCNP]•- interactions in an isolated [TCNP]22- are repulsive by 58.9 kcal/mol, and that the stability of [TCNP]22- primarily originates from [TCNP]•- · · · cation+ electrostatic interactions, whose sum (-209.8 kcal/mol) exceeds the sum of the repulsive [TCNP]•- · · · [TCNP]•- and cation+ · · · cation+ interactions (140.3 kcal/mol).

Introduction

Electron poor cyanocarbons form numerous Mulliken donor-acceptor complexes,1 and frequently form radical anions that are stabilized via charge and spin delocalization. This is the basis for numerous stable electron transfer salts that have been characterized to exhibit technologically important properties such as magnetic ordering,2 metal-like electrical conductivity3 and even superconductivity.4 In the former case, for example,thereactionofV(CO)6 withTCNE(tetracyanoethylene),5,6 TCNQ (7,7,8,8-tetracyano-p-quinodimethane),7 TCNB (1,2,4,5tetracyanobenzene),8a and TCNP (1,2,4,5-tetracyanopyrazine),9 forms organic-based magnets with magnetic ordering temperatures exceeding room temperature. †

Universitat de Barcelona. Stony Brook University. University of Utah. (1) E.g.: (a) Mulliken, R. S.; Person, W. B. Molecular Complexes: A Lecture and Reprint Volume; Wiley: New York, 1969. (2) E.g.: (a) Ovcharenko, V. I.; Sagdeev, R. Z. Russ. Chem. ReV. 1999, 68, 345. Kinoshita, M. Philos. Trans. R. Soc. London, A 1999, 357, 2855. (b) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. 1999, 33, 385. Miller, J. S.; Epstein, A. J. Chem. Eng. News 1995, 73 (40), 30. (c) Blundell, S J.; Pratt, F, P. J. Phys.: Condens. Matter 2004, 16, R771. Crayson, J. A.; Devine, J. N.; Walton, J. C. Tetrahedron 2000, 56, 7829. (3) E.g.: Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R., Eds. Handbook of conducting polymers, 2nd ed.; M. Dekker: New York, 1998. Ferraro, J. R.; Williams, J. M. Introduction to synthetic electrical conductors; Academic Press: Orlando, 1987. (4) E.g.: Williams, J. M. Organic superconductors (including fullerenes): synthesis, structure, properties, and theory; Prentice Hall: Englewood Cliffs, NJ, 1992. Ishiguro, T.; Yamaji, K.; Saito, S. Organic superconductors; Springer: New York, 1998. (5) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508; Angew. Chem. 2006, 117, 2570. ‡ §

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Some of these electron transfer salts, however, do not exhibit collective electronic properties, as they form closed-shell singlet (diamagnetic) dimeric dianions with exceptionally long, in(6) (a) Manriquez, J. M.; Yee, G. T.; McLean, R. S.; Epstein, A. J.; Miller, J. S. Science 1991, 252, 1415. Miller, J. S.; Yee, G. T.; Manriquez, J. M.; Epstein, A. J. In Proceedings of Nobel Symposium #NS-81 Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure; Salaneck, W. R., Lundstro¨m, I., Rånby, B., Eds.; Oxford Univ. Press: Oxford, 1993; p 461. (b) Zhang, J.; Zhou, P.; Brinckerhoff, W. B.; Epstein, A. J.; Vazquez, C.; McLean, R. S.; Miller, J. S. ACS Symp. Ser. 1996, 644, 311. (7) Vickers, E. B.; Selby, T. D.; Thorum, M. D.; Taliaferro, M. L.; Miller, J. S. Inorg. Chem. 2004, 43, 6414. (8) (a) Taliaferro, M. L.; Thorum, M. S.; Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 5326. (b) Bagnato, J. D.; Shum, W. W.; Strohmeier, M.; Grant, D. M.; Arif, A. M.; Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 5322. (9) Vickers, E. B.; Selby, T. D.; Miller, J. S. J. Am. Chem. Soc. 2004, 126, 3716. (10) (a) Del Sesto, R. E.; Miller, J. S.; Novoa, J. J.; Lafuente, P. Chem.sEur. J. 2002, 8, 4894. (b) Miller, J. S.; Novoa, J. J. Acc. Chem. Res. 2007, 40, 189. (c) Garcia-Yoldi, I.; Miller, J. S.; Novoa, J. J. Phys. Chem. Chem. Phys. 2008, 10, 4106. (d) Garcia-Yoldi, I.; Miller, J. S.; Novoa, J. J. J. Phys. Chem. A 2009, 113, 484. (e) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.; Shiomi, D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang, J.; Nakasuji, K. J. Am. Chem. Soc. 1999, 121, 1619. 10.1021/ja902790q CCC: $40.75  2009 American Chemical Society

Tetracyanopyrazinide Dimer Dianion

Figure 1. MO diagram arising from the overlap of the SOMO on each A•- to form [A]22- (The same diagram can be used for A•+ or A•.)

ARTICLES

electron reductions at the substantially more negative potentials of -0.64 and -0.31 V, respectively.13 As a consequence, examples of reduced forms of TCNB and TCNP were sought to study the intradimer bonding. While reduced TCNB and TCNP are present in V-based magnets8a,9 and Ru-based coordination compounds,13 except for the unusual [TCNB]32trimeric dianion,8b they have not been structurally characterized. [TCNE]22- 10a and [TTF]22+ 10c form eclipsed dimers; however, benzene-based [cyanil]22- 10b and [TCNQ]22- 12b dimers have a slipped arrangement. The latter examples can be understood as arising from a compromise between the most attractive SOMO-SOMO intradimer bonding allowed by the symmetry and the electrostatic repulsion of an eclipsed conformation, which provides for good overlap for the slipped arrangement for these species without the steric and electrostatic interactions present for an eclipsed dimer. The au symmetry of the SOMO of [TCNP]•-, however, does not support a slipped arrangement for [TCNP]22-; hence, unlike [cyanil]22-, it should be eclipsed. Herein, we report the synthesis, as well as structural, magnetic, and electronic characterization of [TCNP]22-. Experimental Section

Figure 2. High-resolution synchrotron powder diffraction data (blue) and Rietveld fit of the data (red) for [Cr(C6H6)2]2[TCNP]2. The lower trace is the difference, measured - calculated, plotted to the same vertical scale (black).

tradimer C-C bonds. Long, 2.9 Å, multicenter C-C bonds have been established for eclipsed cofacial dimers of [TCNE]•-, and diamagnetic [TCNE]22- is best described as having a 2-electron 4-center (2e-/4c) C-C bond that arises from the overlap of the b2g singly occupied MO (SOMO) on each moiety to form b2u and b1g bonding and antibonding [TCNE]22- orbitals.10a In addition, dispersion contributes to the bonding.11 Likewise, long, multicenter bonding has been reported for [cyanil]22- (cyanil ) tetracyano-1,4-benzoquinone),10b as well as cationic [TTF]22+ (TTF ) tetrathiafulvalene),10c and neutral [2,5,8-tri-tbutylphenalenyl•]210d among other examples including [TCNQ]22-.12 The generic MO diagram for these multicenter bonded dimers is illustrated for A22- in Figure 1. TCNE {E° [vs SCE(MeCN)] ) 0.14 V} and TCNQ [E° ) 0.17 V] are easily reversibly reduced, but TCNB and TCNP are more difficult to reduce, as they undergo reversible one

Cr(C6H6)2 was used as purchased, and TCNP was prepared by a literature route.14 THF was distilled under nitrogen over sodium and benzophenone. All syntheses were performed in an oxygenfree (