Ancillary Ligand Dependent Shifts in Charge Distribution for CobaltЦQuinone Complexes.код для вставкиСкачать
COMMUNICATIONS  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.  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.  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 COMMUNICATIONS 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  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.  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  C. G. Pierpont. 0.-S Jung. h r g . Chrm 1995. 34. 42x1  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.  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.