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Ancillary Ligand Dependent Shifts in Charge Distribution for CobaltЦQuinone Complexes.

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[21] Cf a ) R Azerad. B i t / / . .So<. Chrtii. Ft-. 1995. /32. 17: h ) E. Ssntaniello. P.
Feri-aboschi. P Grisenti. A. Mnnzocchi. CIinil. Rrr. 1992. Y7. 1071 : c ) M.
Ohno. M . Otsuka. O y . R w ( r . 1989. 37. 1 Maximum rnantioselectivity is
obtained when the reaction IS conducted in 25 "A, aqueous DMSO at 35 C d ) M A. C. Andrade. F. A. C. Andrude. R. S. Phillips. Bioorp Mml. C h f .
Lrri 1991, I . 373.
[22] Determined by comparison of the 'H NMR spectra o l the aniide, obt;iined
Amidc forfrom raceinic and scalemic 17 and ( S ) - (- )-~-niethylbenzyI~iniine.
mation was best carried out with the Mukaiyama reagent (231.
1231 T. Mukaiyama. M. tisui. E. Shimada. K. Saigo. C h i . Lert. 1975. 1045 The
DCC procedure gave poor rcsults.
[24] eiir-5 is readily available from 17 by reaction witli excess Et2NLi (inethyl
ester diethylamide exchange) and esterification (CHIN3. 63% over the two
steps). followed by DIBAL reduction (100%). Biologically inactiw c w r - 1 wis
obtained rrom erir-5 as described above. confirming thc absolute coniiguration
of 5.
tant in defining T, through both enthalpic and entropic contributions, and the properties of ancillary ligands may be used to
tune transition temperature with great sensitivity.['] Herein we
describe an unexpectedly large change in T, associated with a
minor change in coligand structure.
In an earlier study we described the properties of
[Co"'(tmeda)(3,6-dbsq)(3,6-dbcat)] (tmeda = N,N,N',N'-tetramethylethylenediamine) .[21 The hard-donor coligand used in
this case provides stabilization for the Co"' isomer and is responsible for the relatively high transition temperature of 310 K
in solution (toluene). The T, value for this complex is 35 K
higher than that of the related bipyridine complex, even though
the Co-N bond lengths are 0.1 A longer with tmeda. Temperature-dependent changes in magnetic moment that accompany
shifts in equilibrium are shown in Figure 1 for complexes prepared with N,N.N',N'-tetramethylmethylenediamine
(tmmda).
Ancillary Ligand Dependent Shifts in Charge
Distribution for Cobalt -Quinone Complexes**
Ok-Sang J u g , * Du Hwan Jo, Young-A Lee,
Youn So0 Sohn, and Cortlandt G. Pierpont*
Transition metal complexes containing chelated o-quinone
ligands have metal- and quinone-localized electronic levels that
are unusually close in energy."] As a consequence, intramolecular charge transfer between metal and ligand occurs at low energy, and, in some cases, under conditions of thermal equilibri31 Structural changes that accompany shifts in metal
oxidation and spin states have been applied to materials that
exhibit photomechanical properties and optical switching eff e c t ~ . [51~ The
.
most extensively studied equilibria of this type
occur for complexes of cobalt with general formula [Co(NN)(dbq),], where dbq is the semiquinonate (dbsq) or catecholate (dbcat) form of either 3,5- or 3,6-di-tert-butyl-l,2-benzoquinone and N -N is a bidentate nitrogen-donor ancillary
ligand.c2] Temperature-dependent equilibria [Eq. (a)] may be
[Co"'(N -N)(dbsq)(dbcat)]
e [Co"(N-N)(dbsq)J
100
/
150
200
-
250
TI K
300
350
400
Fig. I Temperature dependence ofthe magnetic moments of complexes of the type
]Co/Me2N(CH,),NMe,l(3.6-dbq)ii. I I = 1-3.
(a)
observed in the solid state as well as in solution by monitoring
changes in the optical spectrum or in the magnetism for the
shift from the low-spin Co"' ion to the high-spin Co" ion.[']
The Co"'/Co" transition temperature (T,) is dependent upon the
nature of the nitrogen ~ o l i g a n d . [ By
~ ] defining the transition
temperature as the point where concentrations of redox isomers
are equal, T, is equal to the ratio of enthalpic and entropic
changes associated with the equilibrium.[61The enthalpy change
is primarily associated with changes in bond energy that occur
with the shift in metal charge and spin state. Low-frequency
vibrational shifts that accompany the transition to the high-spin
Co" ion provide the greatest contribution to AS, and both effects are associated with the change in population of the metal
d, (e,) orbitals. Donor effects of the coligands are clearly impor[*] Prof. 0:s. Jung. Dr. D. H. Jo. Prof. Y.-A. Lee. Dr Y S. Sohn
Inorganic Chemistry Laboratory
Korea Institute of Science and Technology
Cheoiigryang. Seoul 136-791 (Korea)
I**]
f
t 4l
Prof. C. G . Pierpont
Department of Chemistry and Biochemistry
University of Colorado, Boulder. CO 80309 (USA)
Fax. Int code +(303)492-5894
Support for rcseorch carried out at KIST was provided as part of the €-project
under grant 2NI3694. Research at the tiniversit) of Colorado was supported
by tlie Natioiial Science Foundkition through grant C H E 9023636.
tmeda, and N,N,N',N'-tetramethylpropylenediamine
(tmpda).
The Co"' redox isomer has a single unpaired electron localized
on the sq ligand. Metal-radical magnetic exchange that occurs
for the Co" isomer is responsible for values in magnetic moment
that generally range between 4.5 and 5.5 pB. The solid-state T,
for [Co(tmeda)(3,6-dbsq)(3,6-dbcat)] lies above 400 K, and
magnetic measurements made on [Co(tmmda)(3,6-dbsq)(3,6dbcat)] indicate that it behaves similarly. In marked contrast,
the related complex prepared with a propylene bridge between
nitrogen donors has a solid-state T, of 178 K. more than 200 K
below the value for the corresponding complex with tmeda ligands. Solid-state equilibria have been monitored spectroscopically by observing intensity changes for a low-energy charge
transfer transition that occurs characteristically for the Co"'
redox isomers.[21This band appears at 2500 nm for [Co(tmpda)(3,6-dbsq)(3,6-dbcat)I1and temperature-dependent changes
in intensity are shown in Figure 2. Structural features of
[Co"(tmpda)(3,6-db~q)~]
in the solid state are shown in Figure 3.[91 In contrast to the features of [Co"'(tmeda)(3,6-dbsq)(3.6-dbcat)]. Co-N and C o - 0 bond lengths are typical of
high-spin Co" ions, and dimensions of the quinone ligands are
those of a semiquinone. The tmpda ligand is disordered in the
structure by rotation about the Co-N2 bond. Crystallographic
data collected at room temperature failed to provide clear resolution of the disordered carbon atoms, but data collected at
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I
A
-
2475
1750
hlnm
3200
Fig. 2. Changes i n the intensity of the transition at 2500 nm i n [Co"'(tmpda)(3.6dbsq)(3.6-dbc;it)] with the equilibrium shift to [Coii(tmpda)(3.6-dbsq),]at higher
temperature i n the solid state. Spectra were recorded on a sample prepared as a KBr
disk. .4 = abwrption.
large, but in agreement with values obtained for metal redox
processes that place charge in M - L antibonding d, orbitals.["]
A value of 98 J mol - K - was obtained from a similar analysis
on [Co(bpy)(3,5-dbq),] (bpy = 22-bipyridine) in the solid
state. The enthalpy change for [Co(tmpda)(3,6-dbq),] is roughly
half the value of 32 kJmol-' obtained for the bipyridine complex, but within the range of values associated with Co"/Co"'
electron transfer reactions." 21 A detailed thermodynamic analysis has not been carried out for [Co(tmeda)(3.6-dbsq)(3.6-dbcat)] due to its high T,, but the difference between complexes
containing tmeda and tmpda ancillary ligands seems clearly related to the difference in chelate ring size. The six-membered
chelate ring of tmpda permits greater vibrational flexibility and
a N-Co-N bond angle that is more conducive to high-spin Co"'
ions.
E.yperin2enrcil Procedure
[Co~Me,N(CH,),NMe2i(3.6-dbq)J,
ii = I - 3: Synthetic procedures used to prepare complexe, with tmmda (11
hynthests of the related tmeda
tmpda ( 1 1 = 3 ) colif"nd\ were similar to the
2) complex described prebiousiy .!?I
= 1 ) and
(11
=
Received- January 15. 19Y6 [28719IE]
German version A i i , w i t . Chivti 1996. /OX. 1796 1797
Keywords: cobalt compounds . complexes with quinone ligands
- electron transfer . isomerism - redox systems
Fig. 3. View of the sti-ucture of [Coit(tmpda)(3.6-dbsq)].Carbon atoms bonded to
N? are shown in one oftwo disordered locations Selected average bond lengths [A]:
Cu 0 2.019(6). C o N Z.lXI(I0). C - 0 (quinone ligands) l.29(1).
150 K gave locations for two sets of carbon atoms bonded to
N2. Furthermore, data collected at 110 K has given clear resolution of the disorder and provided structural evidence for the
transition to the Co"' isomer at low temperature."'] The results
of these structural studies will be described in a separate publication. but they show no evidence for an axially elongated lowspin Co" intermediate that might appear with the high-spin
Co"/Iow-spin Co"' transition.
Energy changes that influence the equilibrium shown in
Equation (a) provide fundamental insights on intramolecular
metal - ligand electron transfer, information that is difficult to
obtain directly for systems studied under conditions that are
often dominated by solvation effects." '. 12] Solid-state equilibria are of particular interest since the molecular environment
can be precisely defined. Structural characterization on both
[Co1'(tmpda)(3.6-dbsq),l and [Co"'(tmeda)(3,6-dbsq)(3,6-dbcat)]
has shown that there are no close intermolecular contacts in the
solid state. Temperature-dependent changes in both magnetism
and the intensity of the band a t 2500 nm in the spectrum have
been used to calculate relative concentrations of Co" and Co"'
isomers present in the solid state, and from this data thermodynamic parameters have been obtained for the equilibrium.[61
The enthalpy change has been determined to be 14.2 kJmol- ',
and the entropy change is 80 J m o l - ' K - ' . The value for A S is
[I]C. G. Pierpont. C. b! Lange, Prog hiorg. Ciioii 1993. 41. 381
[2] 0 -S. Jung. C. G. Pierpont, /itor$ Chriii. 1994. 33. 2227
[ 3 ] A. S. A t t m C. G. Picrpont. /itor,?. Chm. 1995, 34. 1171.
[4] 0:s. Jung. C. G. Pierpont, J A m . CIicim S m 1994. / 1 6 . 2279
[ S ] 0.2.Jung. D. H. Jo. Y.-A Lee. B. J. Conklin. C. G Pierpont. .-liigc,ii. Client.
submitted
[6] C. G. Pierpont. 0.-S Jung. h r g . Chrm 1995. 34. 42x1
[7] Transition temperature (T,) for the equilibrtum described in Equation (a) IS
defined as the temperature at which concentrations o f t h e Co"' and Co" redox
isomers are equal.
181 0:s. Jung. C. G. Picrpont, J Ani. C/imi. Soc 1994. 1 / 6 . I127
191 Dark blue-green prismaticcrystals or[Co(tmpda)(3.6-dbsq)?lwere obtained by
slow evaporation of a saturated solution in toluene Monoclinic crystal system.
space group P 2 , c. ~ = 1 6 . 9 8 7 ( 2 ) . h=11.771(2).
=19.723(3)& /$=
111.82(1). V=3661.1(9)A3. Z = 4 . T = 2 4 C. At T'= -125 C: ( I =
16.804(2). h = I I .691(2). = 19.415(3)
/j = 1 11.52( 1) . 1' = 3548.3(9) A3.
Data were collected on a Siemens P3F diffractometer i i t both temperatures.
Calculations were carried out by using the SHELXTL library of crystallographic programs. The structure was solved by using ;I \harpencd Patterson
map. Refinement with data collected at 24 C converged n i t h R = 0 OX0 and
RII.= 0 100 for 7242 observed (I>2a(l)) i-ellections. Data collected at
- 125 C converged with R = 0.074 and Rii. = 0.105 for 1753 observed reflections. Crystallographic data (excluding structure factors) Ibr the structure(s)
reported in this paper have been deposited uith the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-I 79-40. Copies ofthe
data can he obtained free ofcharge on application to The Director. CCDC. 1 2
Union Road. Cambridge CB2 IEZ, UK ( f a x . Int. codi. +(1223) 336-033:
e-mail. teched,cr cherncrys.cani.ac.uk).
[lo] 0:s. Jung. D. H. Jo. Y.-A Lee. C. G. Pierpont. I i i ~ r g C / i < v ~ isubmitted.
[ I 11 a ) D. E Richardson. P. Sharpe. Iiiorg. Clioii 1991.30. 1417: b) ;hid 1993.32.
A.
1809.
[12] a ) P. W. Crawford. F.A. Schultz. Inorg. Chrfii 1994. 33. 4344: b) Y.-D. Gao.
K. B Lipkowitz. F A Schultz. J. Ain. Chew So<. 1995. 117. 11932.
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distributions, cobaltцquinone, dependence, complexes, ancillary, ligand, shifts, charge
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