HIGHLIGHTS

[lo] Optically iictive products can also be formed with racemic metal catalysts. With :I chiral additive one of the enantiomeric forms of the catalyst is "poisoned" in \ i tt i . a ) K Maruoka. H Yamamoto.J. Af77 Clim? SJC 1989. I l l . 789: b) 1. M. Brown. P. 1. Maddox. C'hii-dif?, 1991. 3. 345: c) J. W. Faller. J. Parr. J ,4177.

c'h~wi Sor 1993, 1 / 5 . 804, d) J. W Faller. M . Tokunaga. 7hrahnlrofi Lrtr 1993. 34. 73s'). e ) J. W. Faller. D. W. I Sams. X . Liu. J A m c'limi .So(. 1996. 118. 1217

[ I I] F. C. Frank. Birwhfi7i. ~kIp/i i..\. Acro 1953. 11. 459. (121 a ) _I L. Bada. .Vurwr, 1995, 374, 594: b) W. A. Bonner. To/>. S r e r e o h i i i . 1988.

18. I : c ) W 1. Metring. Nrtr i i re 1987. 32Y. 71 2 : d) P. Decker. N d i r . Climi. Zdi. L ~ J 1975. 23. 167; e) S. Mason. Chew Sw. Rei. 1988. 17. 347: f l W. A. Bon- ncr. (Yioni /id 1992. 640; g) S. Mason. N L I ~ Y P 1985. 314. 400.

[I31 Noiia\yiiiiiictric molecular replication and autocntalyses: a ) L. E. Orgel. No- r m ' 1992. 3iX. 203: h) E. A. Wintner. M. M Conn. J. Rebek. Jr.. Ace. C/iwi. Rm. 1994. 2'. 198. c) .I A m . Chrw7. S o . . 1994. 116. 8877: d) G von Kiedrowski.

J Helbing. B. Wlotzka. S. Jordan. M. Mathen. T. Achilles. D. Severs. A. Terfort. B C. Kahrs. ,Vd i i - Clirwi. E d ? . Lrih 1992. 411. j ? X . c ) T. Achilles. G. von Kiedrowski. A t i p i i . . Climi. 1993. 105. 1225: A~,FPII U i m 7 . h i . €r/. Eiigl. 1993. 32. 1 198.

[14] Formation of homochiral crystals froin solutions of opticallq inacti\e com- pounds: a ) J Jacques. A. Collet. S H. Wilen. € n ~ i f i t i ~ f i w i , Rocrwiorn. u i i d

Rr.so/iitroni. Wiley. New York. 1981: b) D. K. Kondcpudi. R. J. Kaufmman. N. Singh. Scicwe 1990. ZW, 975: c ) J. M. McBrtde. R. L. ('nrter. A n , p . Cl im i 1991. 103. 298: A i i p . . C l i e m / f i r €d. €rig/. 1991. 311. 3 . 3 . and references therein.

[15] K. Soai. S. Niwa. H. Hori, J. Chrvii. Soc C'hein Coiiiiniiii 1990. 983 [16] a ) K . Soai. T. Hayase. C. Shimada. K Isobe. Prrnhetli-oii A.s i~ f i i i i ~e / i~ i~ 1994. 5.

789: b) K Soai. T. Hayase, K Takai. ihid. 1995. 6. 637: c ) C Bolm. G. Schlin- gloff. K . Harms. Clirni. Ber. 1992. 125. 1191 : d ) S. Li. Y. Jmnp. A Mi. G Ymp. J CIi~n7. So.. Perkin E-ms I 1993. 885.

Organocyanide Acceptor Molecules as Novel Ligands

Kim R. Dunbar"

Pioneering research carried out at Dupont in the 1950s and 1960s established a wealth of interesting chemistry for conjugat- ed organic molecules with cyanide functionalities.['] Forty years later, research involving organocyanide molecules continues to flourish, owing to their promise as precursors for molecule- based materials. Among the organocyanide materials demon- strated to exhibit unusual properties is a class of ionic materials that consists of paramagnetic transition metal metallocene cations and radical anions of tetracyanoethylene (TCNE, Scheme la).['' The compounds [M(Cp),*][TCNEJ (M =

Mn, Fe) crystallize in columns of donors and acceptors,

A primary motivation for coassembling metal centers and organic radicals in this manner is to achieve new pathways for electronic coupling through p,-d, overlap in addition to the usual p, overlap of the organic acceptors. With the proper ener- gy match of metal and organic orbitals it may be possible to achieve an interplay between superexchange and charge-trans- port pathways, perhaps leading to a synergistic state wherein superconductivity and ferromagnetism coexist. At the very least i t appears that this strategy holds promise for the design of highly conducting organometallic polymers, as evidenced by the work of Hunig and co-workers, who synthesized a new family of organic acceptors known as dicyanoquinodiimines (DCNQIs, Scheme 1). These DCNQIs form crystalline network solids with copper that exhibit extraordinarily high conductivities that per- sist to the lowest temperature^.^^] The structures of [Cu(2,5- Me,-DCNQI)], (Fig. 1. 2,5-Me2-DCNQI = DM-DCNQI)

TCNE DM-DCNQI TCNQ

Scheme 1. Important organocyanide acceptor molecules.

D + A - D + A - . (D' = [M(Cp*),]+; A - = TCNE-) and are re- markable in that they order ferromagnetically at Curie temper- atures 7; of 4.8 K (Fe) and 8.8 K (Mn). These results are quite surprising in the context of classical magnets, considering that the materials are not three-dimensional and that the properties are based on spins of organic molecules.

Solids containing transition metals cations n-bonded to the nitrile groups of polycyano anions are an entirely different cat- egory of organocyanide materials than the ionic, stacked sys-

[*I Prof K R Dunbai- Department of Chemistry and The Center lbr Fundamental Materials Research Michigan State University East Lansing. MI 48824 (USA) Fax, Int . code t(S17)353-1793 e-mail . dunbarro cemvax.cem rnsu.edu

Fig. 1. Pluto representation of a portion of the extended framework structure of [Cu(DM-DCNQI),]

HIGHLIGHTS

consist of an infinite array of tetrahedrally ligated Cu cores bridged by four independent DCNQI ligands, which stack in 1 -D columns. The unusual charge-transport properties of these compounds are attributed to the existence of an isotropic 3-D conduction pathway from Robin- Day class 111, mixed-valent behavior for the Cu"" ions bridged by DCNQI in addition to the usual 1-D pathway through stacks of DCNQI radicals.[4b1

In addition to the DCNQI conductors, the most remarkable discovery in the context of polycyano radical chemistry in recent years is that a coordination compound of TCNE formulated as [V(TCNE),].yCH,Cl, behaves as a bulk ferromagnet with a T, above room temperature.c5l Unfortunately, no structural infor- mation is available for this fascinating binary TCNE com- pound. In fact, although they date back several decades,[61 the only structurally characterized example of a binary metal/ TCNX (X = E or Q) compound is [Ag(p,-TCNQ)], (TCNQ =

tetracyanoquinodimethane) .I7] In the absence of structural in- formation on the simple phases, which are insoluble and there- fore quite difficult to crystallize, researchers have turned to mixed-ligand model compounds, whose structures are more eas- ily determined.[3b, 8 * 91 In this way, researchers are building the fundamental structure-property relationships that are essen- tial, not only for the full understanding of the physics behind the properties, but for the rational design of new materials with predictable behavior. Among the crystallographically determined TCNE and TCNQ coordination compounds that have been reported in the past few years are novel 1-D and 3-D polymeric structures incorporating p2-TCNX and p4-

In addition to the well-known acceptors depicted in Scheme 1, a number of more exotic organocyanide molecules are also being used as novel ligands for transition metals. Sever- al of these are decomposition products unearthed in the course of investigating the complexation chemistry of TCNE: for ex- ample, the unprecedented reductive coupling of TCNE mole- cules on a metal center to give two pyrrolizinato ligands (L =

[CllN,H2]-) (Scheme 2a).[''"] The resulting [ML,] compounds (M = Fe, Ni, Cu, Zn) are of considerable interest in materials applications owing to their similarities to the metal phthalocya nines. The free ligand (LH) was also prepared and crystal-

TCNX.[~, 91

lized,['Obl as was the free anion, which exists in two tautomeric forms 1 and 2 [Scheme 2b).['0'1

In another report, the new organocyanide {C[=C(CN),]- CPh=C(CN),) was synthesized from an insertion reaction of TCNE into the acetylide group in [Ru($-C,Me,)- CI(C-CPh)(CNR)] (Scheme 3).[' 'I The ligand is bound only through Ru- C bonds, but there are four uncomplexed nitrile groups that are open for possible coordination to other metal centers.

Scheme3 Insertion of TCNE into the acetylide group in [Ru(qb-C,Me,)- CI(C=CPh)(CNR)].

The cyanocarbon ligand [C,,N,]- (1,1,2,4.5,5-hexacyano-3- azapenta-l,4-dienide) and its 1 : 1 complex with Ag' were first reported in 1958['] but were only recently subjected to X-ray studies by Pala et al., who determined the structures of [Et,N][CloN7] and the intriguing coordination polymer [(Ag(C,,N,)},].['21 In the latter compound, [C,,N,]- behaves as a tetradentate bridging ligand to two distinctly different types of silver ions, namely with tetrahedral and with a highly unusual square-planar geometry. Figure 2 depicts a single plane in which

C d -

NF CN -\

:N

"$"

""XN

NC CN\

NC CN .

Fig. 2. Schematic representation of polymeric [{Ag(C,,N,)},] in the (001) plane emphasizing the square-planar Ag sites.

CN CN the [C,,N,]- ions are bonded to square-planar Ag' ions, which stack along the (001) direction at a distance of 3.15 A and are linked by the tetrahedral Ag' ions located between the layers.

[(Ag(Cl0N7))J reveals that coordination to a metal center does not significantly alter the geometry of the ligand. Electrochemi- cal studies indicate that [C,,N,]- exhibits two reversible, one- electron reduction Processes, the first Of which corresponds to the radical species [C,,N,]2-, as verified by EPR spectroscopy.

% b)

* N*

N*

" ' - k N CN NC-kNin3 A comparison of the anion unit in [Et,N][C,,N,] and

*

H 2 N WN,

H 1

Scheme 2. a) Reductive coupling of TCNE molecules on a metal center to give two pyrrolizinato ligands (L = [C,,N,H,]-). b) Tautomeric forms 1 and 2 of the free anion [C,,N,HJ.

1660 C; VCH Verlugsgesellschufr mbH, 0-69451 Weinheim. 1996 0570-0833 96;3515-1660 $ 15.00+ -2510 Angen. Chem. Int. Ed. Engl. 1996. 35, No. I S

HIGHLIGHTS

A recent attempt to prepare [Mn'"(CN),]*- from [Mn"'(CN)J3 led Miller and co-workers to the serendipitous discovery of the novel cyanocarbon {(1,1,2,2-tetracyano- 1,2-ethanediyl)bis[imino(cyanomethylene)]} bis[cyanamide] ion [C, ,N *I2-, which was characterized by a single crystal X-ray study.L' 31 The dianion is centrosymmetric about the central C-C bond and is planar except for the C3 and C3' cyano groups (Scheme 4). The authors reason that the oxidative decomposi-

G & I\ N N

Scheme 4. The new cyanocar- [C12NIJ2- bon [C,2N,2]z-.

tion of [Mn"'(CN),l3- leads to CN- and CN', which react in a complicated series of reactions that most likely involve cyanogen (CN), . Cyanide is known to dimerize to cyanogen, which reacts with CN- to form several products, among them the anion [C,N,] -, which can undergo electropolymerization reactions with cyanogen to yield low molecular weight polycyanogen [C,N,,]. Clearly there are many possibilities for the preparation of new cyanocarbons from reactions of cyanogen and CN- that lead to various combinations of C-C, C-N, and N-N bonds.

The aforementioned results hint at a wealth of untapped potential for the use of organocyanides as ligands in new ' ' complexes and materials. Noncen- trosymmetric molecules are of particu- lar interest owing to their potential for exhibiting extensive second-order non- linear optical effects, as was recently found for the tricyanoguanylidine dian- ion svnthesized bv Rasmussen and co-

NEC --N -

- N P" 'c $

ICd'J,12- Scheme 5. Sketch Of the new cyanocdrbon tri- cyanoguanylidine d e ~ i c t - workers (Scheme 5 ) . ed as one of the three pos- sible resonance forms.

German version: Angen Chem 1996, 108, 1769-1771

Keywords: complexes with nitrogen ligands - cyanides

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[9] Polymeric compounds with coordinated TCNE: a) J. S. Miller. J. C. Calabrese, R. S. McLean, A. J. Epstein, Adv. Muter. 1992, 4, 498; b) A. G Bunn, P. J. Carroll, B. B. Wayland. Inorg. Chem. 1992, 31, 1297; c) M. M. Olrnstead, G. Speier. L. SzdbO. 1 Chem. Sor. Chem. Commun. 1994. 541 : d) J. S. Miller, C. Vazquez, J. C. Calabrese, R. S . McLean, A. J. Epstejn, Ah. Muter. 1994, 6. 217; e) F. A Cotton. Y Kim, J. Lu, Znorg. Chim. Arra 1994, 221. 1: f) F. A. Cotton, Y. Kim, J. Am. Chem. Soc. 1993, 115, 8511

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Angenc Chen?. In [ . Ed. EngI. 1996, 35, No. 15 VCH Verlagsgesellschafr mhH. 0-69451 Weinheim, 1996 ~S70-0833/96/35/5-1661 $ 15.00+ .25/0 1661