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Organic Tri- and Tetraradicals with High-Spin or Low-Spin States.

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HIGHLIGHTS
Organic Tri- and Tetraradicals with High-Spin or Low-Spin States
Werner M. Nau*
When carbon, nitrogen, or oxygen atoms carrying an unpaired electron ("spin carriers") are linked through a coupling
unit (CU) and incorporated into an organic molecular framework, polyradicals result. After the appearance of the most recent reviews on polyradicals and related structures with unpaired electrons, for example polycarbenes,['] the design of
novel tri- and tetraradicals, the lowest homologues, has been
actively continued.[2- ''I The fascination arises not only from
the idea of using them as building blocks for larger polyradicals
with potential practical applications['. 71 and the interest of ESR
spectroscopists in such model systems, but also from the structural and synthetic challenges as well as the aesthetic appeal they
present. The possible topologies of tri- and tetraradicals are
shown in Scheme 1. The introduction of three or four spin carriers into a molecular framework allows two- or three-dimensional spin-spin interactions, which are required for desirable
bulk properties such as ferromagnetism.[']
Scheme 1 represents the spin coupler's toolbox, since the
building blocks A --F contain all the relevant topological information for constructing higher polyradicals. Although the jury
is still out as to which of the candidates A-F will first be a
practically applicable magnetic material, examples for all but
one type are known to date: Triradicals 1['"]and 2[21correspond
to type A, triradicals 3-513.4a.
51 to type B, tetraradicals 6 1 1 [ 6 , 7 , 8 a * 9 - to
1 ' 1type C, and tetraradicals 12["] and 13['"l to
types E and F, respectively. No example has been reported for
D, the type of lowest symmetry, but it is contained as a substructure in higher polyradicals.l'O1Interestingly, the simplest
derivative of type F, the tetramethylenemethane tetraradical
C(R,C'), , has remained elusive.
Types A and C are chain-like homologues. Open chains lead
to linear structures such as 6-9, whereas linking the ends of the
chain results in cyclic structures (for example 10 and l l ) , which
display distinct
In contrast, B, D, and F are
derived from branching at a coupling unit. In tetraradicals there
is also the possibility for branching at a spin carrier (E) rather
than at a coupling unit (D and F). Valence considerations for
type E restrict the maximum connectivity at a carbon or nitrogen radical center to three (one unpaired electron and three free
valences for branching), whereas no branching is possible for an
oxygen atom as the spin carrier. However, the use of complexing
metal centers as coupling units, akin to that in compound 11,
could be employed to bypass such valence limitations.[4b."1 On
the other hand, the connectivity at the coupling units is not
restricted to structures like B and F but limited only by the
fantasy of the synthetically inclined polyradical engineer, who
may employ higher conjugated cyclic polyenes or even fullerenes. Types B, E, and F constitute the core of "dendritic"
polyradicals.["l
[*] Dr. W M. Nau
Institut fur PhysikaIische Chemie der Universitat
Klingelbergstrasse 80, CH-4056 Base1 (Switzerland)
Fax: Int. code +(61)267-3855
e-mail: nau(Q ubaclu.unibas.ch
Angew Chem. In!. Ed. Engl. 1997, 36, No. 22
0 WILEY-VCH
typical dimension of spin-spin interaction
1D
OD
2D
-
3D
I
I
fn
a
$@
c
2
@%
,$:;
a
9";
coupling unit
QX
F
Scheme 1. Possible topologies of tri- and tetraradicals.
Generally speaking, the formation of structures A-F requires
homolytic cleavage of three or four chemical bonds, or three or
four one-electron oxidation or reduction steps. Experimentally,
there appear to be four basic strategies for achieving this goal,
the choice of which depends on the type of spin carrier and the
coupling unit. First, oxidation of carbanions,[2.'* I'
or
aminesf3][Eqs. (la) and (lb)] was used to access structures 2,3,
iAr
Ar
Ar
I
Ar /N\nr
12,180OC
orp-chloranil
thianthrenium
perchlorate
Verlag GmbH, D-69451Weinheim, 1997
CH2C12
Ar
Ar
Ar
I
Ar
0570-0833197/3622-2445 $17.50+ SO10
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HIGHLIGHTS
Ar\
Ar 2 = O
3
2
5
11
10
Ph
Ph
9
0
0
a
Ar
13
12
5,10, and 12; the required intermediate carbanions are obtained
by deprotonation of triarylrnethane~,[~]
or by reduction of triarylmethyl ethers with lithium in tetrahydrofuran.[’* lo] Since
this procedure appears to be restricted to triaryl-substituted spin
carriers, the second method, which employs photolysis of bisazoalkanes, is complementary [Eq. ( 2 ) ] ; the resulting hydrocarbon tetraradicals 6-9,[6
however, are persistent only in
low-temperature glasses. Third, reduction of aromatic ketones
to produce ketyl radical anions [Eq. (3)] can be extended to
generate tetraradicals such as 11.“ Of course, the well-established synthetic sequences for preparing “stable free radical”
units such as nitroxide and galvinoxyl [Eq. (4)] can be used
~
2446
0 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1997
Li or K
THF
*‘\
,-0-
(3)
Ar
analogously for introducing more than one oxygen spin carrier (structures 1,4, and 13).[’.41The
resulting “stable free tri- and tetraradicals”,[lhl
like some of the Gomberg-type derivatives with
triaryl-substituted spin sites,[’. 3*5 , are chemically stable at ambient temperature and relatively insensitive towards oxygen, an important aspect for potential practical use.
Coupling units may promote parallel or antiparallel spin alignment by a variety of electronic mechanisms (orthogonal orbitals, superexchange, and spin-polarization) .[*I A parallel
alignment leads to “ferromagnetic” or “highspin” coupling, whereas an antiparallel orientation results in “antiferromagnetic” or “lowspin” behavior. From the application point of
view, high-spin polyradicals could be useful as
organic ferromagnets,[’] and most of the tri- and
tetraradicals 1- 13 have been studied in this context. Conversely, some low-spin polyradicals, for
example those derived from the singlet tetraradical 14, have recently been suggested to support
electric conductivity.[’1
In open-chain tri- or tetraradicals the exclusive
use of ferromagnetic coupling units gives rise to
quartet ( S = 3/2) or quintet spin states (S = 2) as
ground states, as is the case for 1 and 2 (type A)
or 6 and 9 (type C).In contrast, not all coupling
units need to be antiferromagnetic to produce
low-spin states. For example, to construct singlet
( S = 0) tetraradicals of type C (three coupling
units), it is sufficient to confer antiferromagnetic
character either to the two outer coupling units
(14)[8b1or only to the central unit (15).[”
Analogously, in triradicals of type A already a
single antiferromagnetic coupling unit favors low-spin doublet
states ( S = 1/2). When all three electron spins in triradicals of
the cyclic type A (or of type B) are antiferromagnetically coupled, an interesting situation arises. At least two electrons must
be aligned in a parallel fashion-despite the antiferromagnetic
interaction between them- such that a “spin-frustrated’’ system with two virtually degenerate low-spin (doublet) ground
states may result. This exceptional phenomenon was recently
described for structure 4.r4a1
Experimental and theoretical studies[’. 21 demonstrated that
the ferromagnetic or antiferromagnetic properties of various
coupling units do not always follow intuitive or readily pre-
0570-0833j97j3622-2446 S 17.50+.50/0
Angew. Chem. Int. Ed. Engi. 1997, 36, No. 22
HIGHLIGHTS
dictable trends. The methylene group is the simplest coupling
unit and, depending on the conformation, may give rise to ferromagnetic or antiferromagnetic coupling;[sb1difluoro substitution of the methylene group promotes the latter.['2b1 The
m-phenylene coupling unit has been most extensively employed
(see 1- 12) due to its robust ferromagnetic coupling properties,
synthetic compatibility, and the possibility of 2D spin-spin interaction through the 1,3,5-substitution pattern (Mataga polymer) .['I Moreover, the ferromagnetic coupling of m-phenylene
can be modified by conformational constraints,[8c1aryl substituents,fAa.' and the use of heteroaromatic analogues or
ionic derivatives.[8a1Such magnetic fine-tuning promises some
interesting twists in the design of more sophisticated structures.
While the selection of the coupling unit is crucial for controlling
spin multiplicity in tri- and tetraradicals, the type of spin
carrier ( C ,N', 0') appears to be less important, at least for
m-phenylene as the coupling unit. Hence, even the hetero-substituted triradical cation 3 and the tetraradical anion 11 exhibit
high-spin ground states, just as their neutral analogues 5 and 10.
Tri- and tetraradicals can be conveniently observed and identified by ESR spectroscopy, with the exception of ESR-silent
singlet tetraradicals such as 14 and 15 (here, the absence of ESR
signals was taken as evidence for their singlet multiplicity).[7.8b1
antiferromagnetic
antiferromagnetic
ferromagnetic
00 0@
s=o
14
magnetic
rately predicting the resulting ESR features.16-- 'I The ESR
spectra produced upon photolysis of bisazoalkanesC6- are
particularly instructive, because stepwise nitrogen extrusionthat is, pairwise formation of spin carriers-produces first the
triplet diradical ESR spectra [Eq. (S)], unless the diradicals have
5
singlet ground states.['* 8b1 Discernible spectral changes are observed upon prolonged irradiation, which can be convincingly
assigned to the quintet tetraradicals; ideally, all of the intermediate triplet diradicals are converted into the tetraradicals, leaving behind the pure quintet ESR spectra.
Magnetization studies offer another means of characterizing
the high-spin character of tri- and tetraradicals, and constitute
the greatest hurdle for any polyradical as far as potential applications as an organic ferromagnet are concerned. Depending on
the applied magnetic field, the amount and magnetic moment of
paramagnetic molecules, and the temperature, the spins in a
sample align and the degree of alignment can be measured as
magnetization or magnetic susceptibility."' Plotting the data
according to theoretical functional relationships allows one to
extract the spin multiplicity of a tri-, tetra-, or higher polyradical. Deviations from the expected magnetization functions were
interpreted in terms of 1) molecuIar defects, 2) thermal equilibration between different spin states, and 3) concomitant intermolecular coupling between the paramagnetic molecules.['. 'I
This last effect is the stumbling block for many organic tri- or
tetraradicals, since intermolecular coupling is often antiferromagnetic in nature,[la. '1 which makes their practical use as
organic ferromagnets questionable. This limitation necessitates
the design of higher and truly 3D polyradicals, or, as alternative,
warrants suitable crystal packing[''* * 31 and metal-coordinating
"1 to ensure ferromagnetic coupling even in the
absence of covalent molecular connectivity.
15
German version: Angew. Chem. 1997. 109.2551 -2554
Quartet and quintet states produce very distinct ESR spectra,
which yield the zero-field splitting parameters characteristic of
tri- and tetraradicals, and provide a sensitive probe for their
detection. In addition, the temperature dependence of the intensity of the ESR signal (Curie plot) can be employed to establish
the quartet or quintet state as the ground state, and, in some
cases, to estimate the energy gaps to the lower spin states (that
is, the doublet of a triradical or the triplet and singlet of a
tetraradical) .I3, 8b]
The assignment of ESR spectra to high-spin tri- and tetraradicals is nowadays facilitated by computer simulations. Conversely, experimental high-spin ESR spectra were crucial for
refining theoretical models of spin- spin interactions and accuAngen.. Chem. In!. Ed. Engl 1997,36, No. 22
0 WILEY-VCH
Keywords: EPR spectroscopy
cals - radicals spin states
*
magnetic properties * polyradi-
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EVENTS
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Angew. Chem. Int. Ed. Engl. 1997, 36, No. 22
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