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Organocyanide Acceptor Molecules as Novel Ligands.

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[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
\itti. 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/ii..\.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. Nrtriire 1987. 32Y. 71 2 : d ) P. Decker. N d i r . Climi. Zdi.
L ~ 1975.
J 23. 167; e) S. Mason. Chew Sw.Rei. 1988. 17. 347: f l W. A. Bonncr. (Yioni /id 1992. 640; g) S. Mason. N L I ~ Y
P 314. 400.
[I31 Noiia\yiiiiiictric molecular replication and autocntalyses: a ) L. E. Orgel. Nor m ' 1992. 3iX. 203: h) E. A. Wintner. M. M Conn. J. Rebek. Jr.. Ace. C/iwi.
R m . 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. , V d 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 comi,
pounds: a ) J Jacques. A. Collet. S H. Wilen. € n ~ i f i t i ~ f i wRocrwiorn. 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 . C l i m i
1991. 103. 298: A i i p . . C l i e m / f i r €d. €rig/. 1991. 311. 3 . 3 . and references
[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 /1994.
i ~ i ~ 5.
789: b) K Soai. T. Hayase, K Takai. ihid. 1995. 6. 637: c ) C Bolm. G. Schlingloff. 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 conjugated organic molecules with cyanide functionalities.['] Forty years
later, research involving organocyanide molecules continues to
flourish, owing to their promise as precursors for moleculebased materials. Among the organocyanide materials demonstrated 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 energy match of metal and organic orbitals it may be possible to
achieve an interplay between superexchange and charge-transport 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 persist to the lowest temperature^.^^] The structures of [Cu(2,5Me,-DCNQI)], (Fig. 1. 2,5-Me2-DCNQI = DM-DCNQI)
Scheme 1. Important organocyanide acceptor molecules.
D + A - D + A - . (D' = [M(Cp*),]+; A - = TCNE-) and are remarkable in that they order ferromagnetically at Curie temperatures 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 category of organocyanide materials than the ionic, stacked sys[*I Prof K R DunbaiDepartment 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
Fig. 1. Pluto representation of a portion of the extended framework structure of
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 information is available for this fascinating binary TCNE compound. In fact, although they date back several decades,[61the
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 information on the simple phases, which are insoluble and therefore quite difficult to crystallize, researchers have turned to
mixed-ligand model compounds, whose structures are more easily determined.[3b,8 * 91 In this way, researchers are building the
fundamental structure-property relationships that are essential, 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 p4TCNX.[~,
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. Several of these are decomposition products unearthed in the course
of investigating the complexation chemistry of TCNE: for example, the unprecedented reductive coupling of TCNE molecules 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-
lized,['Oblas 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
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-3azapenta-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
Fig. 2. Schematic representation of polymeric [{Ag(C,,N,)},] in the (001) plane
emphasizing the square-planar Ag sites.
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.
" ' - k N
A comparison of the anion unit in [Et,N][C,,N,]
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.
VCH Verlugsgesellschufr mbH, 0-69451 Weinheim. 1996
[(Ag(Cl0N7))Jreveals that coordination to a metal center does
not significantly alter the geometry of the ligand. Electrochemical studies indicate that [C,,N,]- exhibits two reversible, oneelectron reduction Processes, the first Of which corresponds to
the radical species [C,,N,]2-, as verified by EPR spectroscopy.
0570-0833 96;3515-1660 $ 15.00+ -2510
Angen. Chem. Int. Ed. Engl. 1996. 35, No. I S
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-tetracyano1,2-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-
Scheme 4. The new cyanocarbon [C,2N,2]z-.
tion of [Mn"'(CN),l3- leads to C N - 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- N
trosymmetric molecules are of particular
interest owing to their potential for
exhibiting extensive second-order nonlinear optical effects, as was recently
ICd'J,12found for the tricyanoguanylidine dianScheme 5. Sketch Of the
ion svnthesized bv Rasmussen and co-
new cyanocdrbon tricyanoguanylidine d e ~ i c t ed as one of the three possible resonance forms.
workers (Scheme 5 ) .
German version:
Angen Chem 1996, 108, 1769-1771
Angenc Chen?. I n [ . Ed. EngI. 1996, 35, No. 15
Keywords: complexes with nitrogen ligands
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acceptor, molecules, novem, ligand, organocyanide
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