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Dinuclear metal complexes of Cd(II), Zn(II) and Fe(II) with triple-helical structure and predetermined chirality
ELSEV ! E R
lnorganica Chimica Acta 271 (1998) 36-39
Dinuclear metal complexes of Cd(ll), Zn(ll) and Fe(ll) with triple-helical structure and predetermined chirality Hansruedi MUmer ~, Alex von Zelewsky ~'*, G~rard Hopfgartner b "Institute of Im~rganic Chemisto'. Univer.~ily. f Fn'bourg. P(~rolle.~.CH-1700 Fribourg. Switzerland "F. H.~munn.L, Reu'he Ltd.. Pharma Divisim~. Deparlment ~f Drag Met, b~.lism,nd Kinetics. Bi~m~dyd('.! Sectim~. CH.4(~)2 Ba.~('l.S.'it:('rlmul ' (~dilbmh~ In.~li,m. ~pfTechmdr~gy. Divisi~m¢~t'Chemi.~tO",nd Cheneica!E~gineering. M, il.C¢~I~. 127.72. P(~,~'(~(h'm~,('A 91125. USA Received 21) May 1997
Al~r~'t
The complex formationof membersof the so-calledchiragen iigand family (stereoseh.,ctivelinked bis-! 4,5 ]-pineno-2,2'-bipyridines) with Cd(Ii), Zn( II ) and Fe(II) was investigated. Using Sl~.'ctrophotomelri¢titr'ationsand electrospray MS it was determined that the major species formed in solution are complexes ~ith an M:L ~ 2:3 stoichiometry, Based on circular dichroism (CD) and NMR measurements, a triplehelical ~ructure with enantiomericallypare homochiral configuration at Ihe metal centers is pr~posed for these dinuclear complexes. This is one of the first examples of a spontaneous ~il:as.~mbly process leading to a dinuclear triple-helicate with pronounced preference for the tbrmalion of one of the possible stereoi~mer~. ¢~ 1998 El~vier Science S.A. Ke~'.'.r,h: Heli¢,l¢ complexes; S¢ll~.~embl),: C~idmium complexes: Zinc ~'omplexv~: Iron complexc~
I, I n t ~ u e t l o n
Several examples of spontaneously assembled tripleoheli. col structures with dinucleur complexes have bee. repor|ed re~ntly, Iron(il) complexes of" the type IFe~,L,I"' are tbrmed with ,~veral ligands 11=61, A series of well characterized complexes with Co(II) were reported by Williams and coworkers [ 7,8l. In most cases reporled so far, racemic forms of the helices were prepaY. Enemark and Stack 19 I de~ribed the stereo,~lective formation of a dinuclear triplehelical Ga(III) complex, The compound was well characteri ~ in ~lution and in the solid state, Shanzer and coworke~ [ IOI sm:ceeded in the preparation of a chiral triple-helical iron(Ill) complex with tripod.peptide ligands, Williams and coworkers I I 1,12l resolved a racemate of a triple-helical dinuclear col~lt(ill) complex, Lchn and coworkers I 131 and Raymond and coworkers 114] observed spontaneous ,~paration of racemic trinuclear triple-helical Ni(I!) and Fe( Iii ) complexes, res~tively, upon crystallization, A fascinating circular structure of tire iron( II ),~'enters and live hexadentate ligands was recently published by Lehn and coworkers i 15 i, The enantiomers of this compound have been partially
~p,arated, C~resl~q~vdmg ~,~hor. Tel.: +41 ~ .~X) 8732: I'~lx: +41 26 3(~) 9738. fN)~-159M~//$19.W) ~ I~F#8El.~v~er .~ie~.'e S.A. All fights re.~r~.ed.
We reporl here on one ol'the lirst stereoselective formations ol°dinuclear triple-helical complexes with predetermined ohio r|dity at Cd(ll)-. Zn(ll)o, and Fe( ll)ocenters with chiral derivatives of 2.2'obipyridine .s ligands. As shown betbre, the chiragen ligands ( FiB. I ) can coordinate stereos~cifically as tetradentate chelates at one metal center 1 16=.18I. They can, however, also act as bis-bidentate ligands, bridging two metal center, if complexes of the stoichiometry M~L~ are Ibrmed, two possibilities exist for the arrangement of the ligands. Either a mono,bridged ( Fig. 2A)
N
:
N
O-1 lip. I. ( - )-CGI nl-xyl I: ( - )-I, a member of the left, dentate ¢hiragcn ligand family. Each molecule consists of two .~tereo..~eleclivelylinked 4.5pineno-2.2'.bipyridinc subunits. The ligand.~ are systematized dccording to the nature of the bridge and abbreviated as CG I X I.
H. bliirner et al. / bu~rganh'a Chbnh'a Acre 271 t 199,~t 36--39
!
~LI
l"~
A
c Fig. 2. Schematic representation of possible arrangements in dimlclear complexes I M.,LaI"' with tetradentate ligands: A. mono-hridged: B. triply bridged, hetemchiral ~A.A }; C. triply bridged, homochiral I A.A or A.A }.
or a triply bridged (Fig. 2B,C) structure is possible. Since the chiragen ligands predetermine unmnbiguously the chirality at an OC-6 center if bound as tetradentate chelates, structure A must be homochiral with respect to the two metal centers. This requires C.,-symmetry for structure A. in the triply bridged case, in principle one heterochiral ( A,A structure B) and two homochiral (A,A and A,A structure C) arrangements are possible. The Ibrmer has Ca-symmetry, whereas the latter have symmetry l)~ and they are not an enantiomeric pair owing to the chirality of the ligands. 2. E x p e r i m e n t a l
2. !. Gem, ral it!formation All solvents and chemicals were o1' reagent o r spectral grade quality, purchased comnlercially and used without I'urther purilication. The NMR studies ( ~H and '~C NMR) were performed on a Bruker Avance DRX500 instrument using solvent as the internal standard. Chemical shifts are reported in ppm on the scale. Electronic spectra were measured using a Perk(nElmer Lambda 5 UV-Vis spectrophotometer. Circular dichroism ¢CD) spectra were measured on a Jobin-Yvon autodichrograph mark V. The electrospray spectra in positive mode were recorded on an AP! 300 triple quadrupole mass spectrometer by infusion of the complex dissolved in acetonitrile or methanol at a flow rate o1" approxinmtely I0 p,I rain ~ ' under low declustering energy conditions I 19 I.
2.2. Synthesis
o.I'(+ )- and ( - ~-CGIm-xyll
The synthesis of the xylene-bridged chiragcn ligands ( - )C G [ m - xyll ( ( - ) - I ) and ( + )-CGIm-xyl] (( + )-I) has been described elsewhere I 17 I.
2.3. Synthesis o.f the comph, xes Ethanolic solutions of ( - )-CGI m-xyl] ( 3 equiv, ) were mixed with 2 equiv. M-" in water (Fe. FeISO4),,-
37
(NH4),," 6H,O: Zn, ZnCI,,: Cd, CdCI_,. H,O). In the case of iron ( ! ! ), immediately alter mixing the characteristic red color of the I Fe( bpy )~1-"+ chromophore appeared. After stirring at room temperature overnight, the complexes can be precipitated by addition of NH4PFt,.
2.3. !. A./t-IZne(( - )-I ) d(PF, O,~ 'H NMR (d~,-DMSO, 100°C, 300 MHz): 68.9-8.7 ( 12H, broad m); 8.42 ( 6H, broad m ): 8.32 ( 6H. broad s): 7.79 (6H, broad m): 7.56 (3H, t, J = 7.8 Hz): 7.52 (3H, s): 7.43 ( 6H, d, J = 7.4 Hz): 3.72 ( 6H, d, J = ! 4.2 Hz): 3.66 ( 6H, d, J = ! 0.6 Hz); 3.19 ( 6H, dd, J = 5.0. 5.0 Hz); 3.05 ( 6H, dd, J = I 1.7, ! 1.5 Hz): 2.82 ( 6H, dt, J = 10.6, 5.6 Hz): 2.24 ( 6H, broad m); 1.62 (6H, d, J = 1 1 . 8 . 4 Hz), 1.60 (18H, s): 0.82 {18H, s). ES-MS: 1114.6 (20%, { [ Z n . , ( I - ) I)~]~PF,,):} -'+ ), 862.6 (60%), 694.5 (95c~, {IZn:(( - )I)~](PE,)} a+ ), 634.9 ( I00%, [Zn(( - )-1):! -'+ ), 603.3 180c/c, { I - )-I ) + H } ' ), 574.9 ( 60c~ ), 484.9 ( 40%. {IZn:(f - )-I)~1 }'~+ }, 423.9 ~25c//), 302.1 (!0%, {~ - )I} ÷2H} -'+ ). UV-Vis {E t O H / H : O I + I,c2.45 × I0- ~M): 303 { 96 O00), 314 (85 O00). CD ( E t O H / H : O I + i. c 2.45× IO -~ M): 309 ( - 5 6 ) , 319 ~ - 149). 2.3.2. A,A-ICd:(( - )-l )dtPFr,)4 ~H N M R (d~,-acetone. 25°C, 500 MHz): g 8.66 {6H, broad s ): 8.60 ( 6H, broad d. J = 7.4 Hz ): 8.53 ( 6H, broad m ); 8.17 ( 6H. broad m ): 8.13 t 6H, broad s ): 7.60 (6H, broad m }: 7.32 (3H, broad s): 7.30 (3H, t, ,I = 7.4 Hz); 7.20 (6H, d, .I = 7.4 Hz ): 3.54 ( 6H, broad d, J - 7.2 Hz ): 3.48 16H, m ): 2.86 {6H, m ): 2.77 ( 6H, broad d, ,I = I 1.0 Hz): 2.57 ( 6H. tn ); 1.99 (6H, m)' 1.40 ~6H, broad d, J ~ 7.4 Hz}: 1.28 { 18H, broad s J: 0.55 ( 18H, broad s }. '~C NMR {dt,.acetone. 25°C, 5 0 0 M H z ) ,8 150.2: 146.1' 140.8: 131.(): 129.4: 128.3: 126.8: 123.6: 122.6: 45.6: 44.2: 4 3 . 3 : 4 1 . 5 {q)' 39.7: 28.1; 26.3; 21.1. ES-MS: 1453.2 (It3, I I C d I { = } I)~I(PF,,)}' ), 1161.9(3~, {ICd_~(( = ) - I ) ~ I ( P F , ) : } :~ }, 751.3 (55c/~), 726.1 (30c,~. {lCd:(( =)-I),](PE,)} ~' ). 659.4 ( 90~. l ICd( ( = )-I ): l }: ' ). 603.3 ( 100~,~. { c = )I ):,+ H} ~' ). 508.3 ( 25rk. ICd.,( ( - )-I ),I "s'~). 302.1 ( 50~. {(-)-I)+2H} -'+), UV-Vis (EtOH/H:O I+I. c 1.18x I0 '~ M): 304 (90 000), 315 {69 000). CD (EtOH/ H,O i + I . c 1 . 1 8 × I0 -s M): 309 ( - 3 5 ) , 319 ~ = 192). 2.3.3. A , A - I F e 2 t t - J.IJ,lfPl~)~ ES-MS: 630.4 ( I(}()c~. [Fe(I - )-1)2] ~' ). 480.0 1319~. IFe:l( - ) - I ) , ] 4' ). 329.2 (3c,~. [Fef~ - } - I ) ] : ' }. U V Vis ( E t O H / H , O I + I. c 1.192x IO ~ M): 531 (23000). 485 ~ 17 000). 361 ( 17 000 ). 305 ( 137 000 ~. 290 ( 81 000 ). CD ( E t O H / H : O I + I) c 1.50x IO -4 M (250-350 nm): 295 (243). 315 ( - 529). c 4.00 x i 0 4 M {350-650 nm ): 395 (14). 477 ( i 2 ), 526 • - 8 ). 606 ( - 3 ). 2.3.4. A.A-/Fe:tt.+ )-I):ItPF;,)4 CD ( E t O H / H : O I + I ) c 1.50 x IO- "~M (250-350 ran) ): 295 ( - 236). 315 (566). c 4.00 x 10 4 M ( 350-650 nm ): 395 ( - 12).477 ( - i I). 526 (8). 606 (4).
38
if. Me'lineret al. I Inorganica Chimiea Acta 271 (1998136-39
3. RuulU and discussion Titration studies of chiragen[ m-xyl] ligands with Cd-"+, Zn:" and Fe: + in 50% aqueous ethanol show only the formation of complexes with M:L = 2:3 stoichiometry. Since the UV-Vis spectra of the free ligand and the Cd(!I) and Zn(11) complexes overl~ strongly, ~ titrations were monitored by C D spectroscopy only. In the case of Fe(11), the titrations wen: followed by CD and UV-Vis s~troscopy in the region of both charge-transfer and ligand centered transitions. As an example, the resul~ of the titration ofa I × !0 -'~ M Fe(I[) ~lution with an equally concentrated solution of ( - ) - 1 monitored by CD and UV-Vis spectroscopy in the charge transfer region are presented in Fig. 3. All other titration data can be found in the supplementary material. The M : L - 2 : 3 stoichiometry was confirmed by electrospray MS measurements. The elec~rospray spectra were recorded at relatively low concentration and the presence of
(a)
i' (b)
-
00
"10
..................... ...i_. .................... 2
:
3
=
other species in solution ( l ML l" + and [ ML_,]" + in all three complexes, mono- and diprotonated free ligand with the Cd(Ii) and Zn(II) species) is mainly due to partial decomplexation of the complexes (for details see Section 2). A product ion spectrum with argon as collision gas was recorded for Fe,L~ at m l z = 4 8 0 (Fig. 4). Two major fragments recorded correspond to [ FeL2] -"+ and [ FeL ]-"+, supporting the assignment of the ion at m / z = 480 to [ Fe,_.L3] "~+. Structural information can be obtained from CD and NMR measurements. The CD spectra (Fig. 5) show clearly that the heterochiral arrangement B (Fig. 2) of the triply bridged complex can be excluded in all three cases, since no CD activity is to be expected for this type of structure (the free ligands show no CD activity at wavelengths greater than 250 nm). Using the exciton coupling model for the assignment [20,211, the sign of the CD band at approximately 300 nm indicates a A,A absolute configuration for all three complexes [M,( ( - )-l).~! 4+ (M =Cd -'+, Zn -'+ and Fe -'+ ). The amount of coupling between two transition moments considered in exciton theory (in this case ligand centered 'n'-'n-*) is strongly dependent on their orientation with respect to each other. The significantly lower amplitude of the CD band centered at 303 nm of the Cd( i l) and Zn (It) complexes is therefore due possibly to distortions from an ideal octahedral coordination geometry. The IH NMR (Fig. 6) and I~C NMR data of the A,A[Cd,( ( - )-1 )~] ( PF~,L, complex indicate a complete equivalence of the three ligands and of the two halves of each ligand. These observations are in agreement only with the highly symmetrical (D~) arrangement of the triple helix in an ¢nantiomerically pure homochiral dinuclear structure ( Fig. 2, structureC). The same conclusion can he drawn for the Zn(II) and the Fo( [I) complexes. However, in the case of the Fe( I[) complex, no high resolution N M R spectra at room temperature and at 243 K and 373 K can he obtained. This ispossibly due to a relativelysmall ligand lield,giving rise to a near spincrossover situationfor the metal center as observed in similar
1 2 3 4 IIg~nd molNulee I Fe (#) atom
2.1
Fig, 3, Titrath~n of an Fe(il) ~olution ( I x 1 0 ~ M ) in ethanol:water I:1 with ~ ~lulion el" ~ = )-~X]lm~xyll ( I × It) ~ ~ M ) in elhanol:w,lcr I:1
monlt~w~l hy !a) UV=Vis spectroscopy, (h) CD spectr~)scopy. [Fe((.~S)l=° m,~ 14
I
/i
/
',u
o
m.~ [r~((.}.S)~l"
I~eoumorIon [~((.).S)~l~" lklO,Q
x 20
W
<1.400
i 350
~
~0
6~
wavelength [nm] rnlz Fi~ 4, Pr¢',duc-t~wl spect~m ofihe talc ~=480, I fragment (A,~-I Fe:( ( - ). I ) ~I ~" ;) wieh ~ , o n as collisinn g~s.
Fig. 5. CD sl~'Clra of the A.~-[ M.( ( - )-CG[ m-xyl ] ), I ~ + complexes in ethanol:water I:1. M--Cd"' ( - - ) , M=Zn-" (---), M=Fe:' (.... lb, Al~we 350 nm the ,signals of the iron(il) complex were scaled by a factor of 20,
ILl. Mih'ner et aL I bmrgani(',~ Chimh'a A "ta 271 ¢ 199,'q~ 3 6 - 3 9
39
5. Supplementary material
Ill, 9
8
Electrospray MS spectra of A , A - I M 2 ( ( - ) - I ) ~ I 4. ( M = Cd 2+, Zn -~÷ and Fe 2+ ). diagrams of the spectroscopic titrations followed by UV-Vis and CD spectroscopy (9 pages) are available from the authors. Ordering information is given on any current masthead page.
i 7
6
5
4 ppm
3
2
I
0
Fig, 6, I H NMR spectrum ( 50(1MHz ) of ,X,..X- I Cd2( I - )-CG I m-xyl I I a I (PF,,)4 in acetone-d,, at roonl temperature.
dinuclear Fe-lanthanide complexes [221. This tentative explanation is corroborated by the low energy of the first CD band at 606 am. Ferguson et al. 1231 attribute the opposite signs lbr the CD signals in the charge transfer region of A-IFe(bpy)al -'÷ compared with A-IRu(bpy)~! ~ and AI Os(bpy)a !-"+ by mixing of energetically comparable d-d and d-'rr* transitions in the Fe( !! ) case. Owing to the larger differences in the energy of these two transitions in Os( !! ) and Ru(!!). the mixing process cannot take place. The CD bands of A.A-IFe.~(( + )-I)~I ~' in the charge transfer region reseluble closely those of A-I Ru(bpy).~l; ~' and A108(bpy).~ 12 +. We conclude that the CGI m-xyl ] ligand exerts a weaker ligand lield than 2.2'-bipyridine. separating the d=d and d-'rr, transitions energetically to inhibit the mixing of the two types of transitions. Experiments on the tempenlture dependence o1" the magnetic susceptibility o f .,X...X-IFe,(( = )-I )~ i ~' are phuuled to v:didate this prelinl-
inary assignnlent. X - r a y d a t a w o u l d p r o v e tile stilled Iriple..helJcal Ililture o1'
Ihe dinuclear complexes
r e p o r l e d in the s o l i d slate, R e g r e t -
fully, w e w e r e nol a b l e to o b t a i n c r y s t a l s o f sul'licient q u a l i t y Ibr a s t r u c t u r e a n a l y s i s w i t h v a r i o u s c o u n t e r i o n s a n d u n d e r
different conditions.
4. Conclusions
We have proven earlier that chiragen ligands coordinate slereoselectively as tetradentale chelates at o,le metal center 116--181. The present study shows that these ligands also predetermine the cMndity at OC-6 ce,lters atld thereby the chirality of the emerging triple-helical structure if bound in a bis-bidentate nlanner to two labile octahedral centers. With Cd(!!). Zn(!!) and Fe(!I). dinucle:w compounds with an M,L~ stoicMometry and delined cMrality at the metal centers are fanned. The ligand geotnetry, as delined in tile chiral centers of Ihe pinene-moiety, controls stereoselectively the self assembly of such dinuclear triple-helical compounds.
Acknowledgements H.-R.M. and A.v.Z. thank the Swiss National Science Foundation for financial support. We thank Felix Fehr for carrying out the 500 MHz NMR experiments and Marco Ziegler and Nick Fletcher for valuable discussions. References I II B.R. Serr, K.A. Andersen. C.M. Elliott and O.P. Anderson, lnorg. Chem.. 27 ( I t)88 ) 44q9--4505. 121 CM. I-Iliott. D.L Derr. S. Ferrere. M.D. Ne~,,ton and Y.-P. Liu. J. Am. Chem. S.c.. 118 ( 19961 5221-5228. 131 D. Zurita. P. Baret and J.-L. Pierre. New J. Chem.. 18 ( 1994t 11431146. 141 M.-T. Youinou. R. Ziessel and J.-M, Lehn. Inorg. Chem.. 31) t I()ql ) 2144-2148. 151 J,A. Nachbaur. Thesi,~. [ T, ~versity of Fribourg. Switzerland. 1993, I¢~1 M. Albrecht and C. Riether. Chem. Bet,. 120 ( Iq%~ 829~832. 171 C. Piguet. G. Bemardinelli. B. Bocquet. (). Schaad and A.F. Williams. Inorg. Chem.. 33 I Iq941 4112--4121. I Sl C. I)iguet. It. Bernardinelli, B, Bocquet, A, Quattropani and A,F, Williams. J, Am. Chem, So¢,, 114 ( Iqq2 ) 74411~7451, I tj I I-,.I, Encnlark mid T.I), Stack, Angcw, ('hem,, Int, Ed, Engl,, 34 ( 10~)5 I I)~,)fl-OQS, Illl ,I, I.ibn|~tn. Y. Tar and A, Sllatl/er, J r Am. Chem, Sot:,, Illt} 11087) 5880~58t4 I, I I I I.,,I, Charl'~uutli~r~.', G, Ik, rn.rdir,.,lli, (', I~it~tu.'t, A,M, Sm'~e,,.u ~md A,I'*, Will.m,,, ,I. ('hem, S.c,, ('hem, ('t.nmun,, I I~)f,)41 14 Iq= 14211, I-~1 I.,J, ('harl~muiere. M,oI:, Gih.,I, K, Iterrtml~r and A,! :, Wiliam~, J, ('horn. Soc., Chem, Commurl., ( 10,)61 30~11. 131 R, Krtimor, ,I.-M, I.dm, A. !)~' ('iau mid J, I:i,,,,¢h~r, Atl~e~,, Cl'Dem,, Ill5 I II)()3l 764~767. 14] R.C, Scl|rrow. i).1., White ~illd K,N, Raymond, J, Am, Chem. So¢,, 107 (1985) t~5411-054(~. 151 M, Hasenknoiff. J.-M. Lehn, B,O, Kneisel, G, llaunl ~lttd D. I:en,,k¢, Angew. ('hem.. Int. Fd. Engl,. 35 ¢ IqOfi) 1838= 1840. I 61 P. Hayoz. A. van Zelewsky :rod H. Stoeckli- F.v:m,,. J, Am, Chem, S~.',. 115 I Iq93) 5111-5114. 171 H,-R, Mlirner. H. Stoeckli-Fvans and A. ~,on Zelew,,k~*, hmrg, Chem,, 35 ( It)()61 3t131=,~(135. 1141 H,-R, Miirller, P. I]elser :llld A. ~,on Zelew,~ky. J, Am. Chem. Soq.',. I 18 ( I OtJ6 ) 7t)Sq-7tJtM, It) l G. Hopfgartner, C. Piguet and J, Hellion. J. Am. Sac, Ma,,~. Speclr.m,. 5 (It}t)4) 748--75fi. 201 S,F. Mason and B,J, Nortnan, Inorg. Nucl, Chem. Ixt(,, 3 ( 10671 285288. 211 B. Bosnich. Inorg. Chem.. 7 (It)68) 237(I-2386. 221 C. I)iguet, F. Riv~m~-Mintcn, (;. Bernardmelli, J..C.CL Bthlzli and (i, Hopfgarlner. J. Chem, Sac,. Dalton Tran,~,. ( IO~)7) 421--433, 1231 J. I:erguson. I.. Herren aind G.M, Mcl.aughlin. Chem, Phys. lxtt,. 89 ( 1 9 8 2 ) 376-380.
lnorganica Chimica Acta 271 (1998) 36-39
Dinuclear metal complexes of Cd(ll), Zn(ll) and Fe(ll) with triple-helical structure and predetermined chirality Hansruedi MUmer ~, Alex von Zelewsky ~'*, G~rard Hopfgartner b "Institute of Im~rganic Chemisto'. Univer.~ily. f Fn'bourg. P(~rolle.~.CH-1700 Fribourg. Switzerland "F. H.~munn.L, Reu'he Ltd.. Pharma Divisim~. Deparlment ~f Drag Met, b~.lism,nd Kinetics. Bi~m~dyd('.! Sectim~. CH.4(~)2 Ba.~('l.S.'it:('rlmul ' (~dilbmh~ In.~li,m. ~pfTechmdr~gy. Divisi~m¢~t'Chemi.~tO",nd Cheneica!E~gineering. M, il.C¢~I~. 127.72. P(~,~'(~(h'm~,('A 91125. USA Received 21) May 1997
Al~r~'t
The complex formationof membersof the so-calledchiragen iigand family (stereoseh.,ctivelinked bis-! 4,5 ]-pineno-2,2'-bipyridines) with Cd(Ii), Zn( II ) and Fe(II) was investigated. Using Sl~.'ctrophotomelri¢titr'ationsand electrospray MS it was determined that the major species formed in solution are complexes ~ith an M:L ~ 2:3 stoichiometry, Based on circular dichroism (CD) and NMR measurements, a triplehelical ~ructure with enantiomericallypare homochiral configuration at Ihe metal centers is pr~posed for these dinuclear complexes. This is one of the first examples of a spontaneous ~il:as.~mbly process leading to a dinuclear triple-helicate with pronounced preference for the tbrmalion of one of the possible stereoi~mer~. ¢~ 1998 El~vier Science S.A. Ke~'.'.r,h: Heli¢,l¢ complexes; S¢ll~.~embl),: C~idmium complexes: Zinc ~'omplexv~: Iron complexc~
I, I n t ~ u e t l o n
Several examples of spontaneously assembled tripleoheli. col structures with dinucleur complexes have bee. repor|ed re~ntly, Iron(il) complexes of" the type IFe~,L,I"' are tbrmed with ,~veral ligands 11=61, A series of well characterized complexes with Co(II) were reported by Williams and coworkers [ 7,8l. In most cases reporled so far, racemic forms of the helices were prepaY. Enemark and Stack 19 I de~ribed the stereo,~lective formation of a dinuclear triplehelical Ga(III) complex, The compound was well characteri ~ in ~lution and in the solid state, Shanzer and coworke~ [ IOI sm:ceeded in the preparation of a chiral triple-helical iron(Ill) complex with tripod.peptide ligands, Williams and coworkers I I 1,12l resolved a racemate of a triple-helical dinuclear col~lt(ill) complex, Lchn and coworkers I 131 and Raymond and coworkers 114] observed spontaneous ,~paration of racemic trinuclear triple-helical Ni(I!) and Fe( Iii ) complexes, res~tively, upon crystallization, A fascinating circular structure of tire iron( II ),~'enters and live hexadentate ligands was recently published by Lehn and coworkers i 15 i, The enantiomers of this compound have been partially
~p,arated, C~resl~q~vdmg ~,~hor. Tel.: +41 ~ .~X) 8732: I'~lx: +41 26 3(~) 9738. fN)~-159M~//$19.W) ~ I~F#8El.~v~er .~ie~.'e S.A. All fights re.~r~.ed.
We reporl here on one ol'the lirst stereoselective formations ol°dinuclear triple-helical complexes with predetermined ohio r|dity at Cd(ll)-. Zn(ll)o, and Fe( ll)ocenters with chiral derivatives of 2.2'obipyridine .s ligands. As shown betbre, the chiragen ligands ( FiB. I ) can coordinate stereos~cifically as tetradentate chelates at one metal center 1 16=.18I. They can, however, also act as bis-bidentate ligands, bridging two metal center, if complexes of the stoichiometry M~L~ are Ibrmed, two possibilities exist for the arrangement of the ligands. Either a mono,bridged ( Fig. 2A)
N
:
N
O-1 lip. I. ( - )-CGI nl-xyl I: ( - )-I, a member of the left, dentate ¢hiragcn ligand family. Each molecule consists of two .~tereo..~eleclivelylinked 4.5pineno-2.2'.bipyridinc subunits. The ligand.~ are systematized dccording to the nature of the bridge and abbreviated as CG I X I.
H. bliirner et al. / bu~rganh'a Chbnh'a Acre 271 t 199,~t 36--39
!
~LI
l"~
A
c Fig. 2. Schematic representation of possible arrangements in dimlclear complexes I M.,LaI"' with tetradentate ligands: A. mono-hridged: B. triply bridged, hetemchiral ~A.A }; C. triply bridged, homochiral I A.A or A.A }.
or a triply bridged (Fig. 2B,C) structure is possible. Since the chiragen ligands predetermine unmnbiguously the chirality at an OC-6 center if bound as tetradentate chelates, structure A must be homochiral with respect to the two metal centers. This requires C.,-symmetry for structure A. in the triply bridged case, in principle one heterochiral ( A,A structure B) and two homochiral (A,A and A,A structure C) arrangements are possible. The Ibrmer has Ca-symmetry, whereas the latter have symmetry l)~ and they are not an enantiomeric pair owing to the chirality of the ligands. 2. E x p e r i m e n t a l
2. !. Gem, ral it!formation All solvents and chemicals were o1' reagent o r spectral grade quality, purchased comnlercially and used without I'urther purilication. The NMR studies ( ~H and '~C NMR) were performed on a Bruker Avance DRX500 instrument using solvent as the internal standard. Chemical shifts are reported in ppm on the scale. Electronic spectra were measured using a Perk(nElmer Lambda 5 UV-Vis spectrophotometer. Circular dichroism ¢CD) spectra were measured on a Jobin-Yvon autodichrograph mark V. The electrospray spectra in positive mode were recorded on an AP! 300 triple quadrupole mass spectrometer by infusion of the complex dissolved in acetonitrile or methanol at a flow rate o1" approxinmtely I0 p,I rain ~ ' under low declustering energy conditions I 19 I.
2.2. Synthesis
o.I'(+ )- and ( - ~-CGIm-xyll
The synthesis of the xylene-bridged chiragcn ligands ( - )C G [ m - xyll ( ( - ) - I ) and ( + )-CGIm-xyl] (( + )-I) has been described elsewhere I 17 I.
2.3. Synthesis o.f the comph, xes Ethanolic solutions of ( - )-CGI m-xyl] ( 3 equiv, ) were mixed with 2 equiv. M-" in water (Fe. FeISO4),,-
37
(NH4),," 6H,O: Zn, ZnCI,,: Cd, CdCI_,. H,O). In the case of iron ( ! ! ), immediately alter mixing the characteristic red color of the I Fe( bpy )~1-"+ chromophore appeared. After stirring at room temperature overnight, the complexes can be precipitated by addition of NH4PFt,.
2.3. !. A./t-IZne(( - )-I ) d(PF, O,~ 'H NMR (d~,-DMSO, 100°C, 300 MHz): 68.9-8.7 ( 12H, broad m); 8.42 ( 6H, broad m ): 8.32 ( 6H. broad s): 7.79 (6H, broad m): 7.56 (3H, t, J = 7.8 Hz): 7.52 (3H, s): 7.43 ( 6H, d, J = 7.4 Hz): 3.72 ( 6H, d, J = ! 4.2 Hz): 3.66 ( 6H, d, J = ! 0.6 Hz); 3.19 ( 6H, dd, J = 5.0. 5.0 Hz); 3.05 ( 6H, dd, J = I 1.7, ! 1.5 Hz): 2.82 ( 6H, dt, J = 10.6, 5.6 Hz): 2.24 ( 6H, broad m); 1.62 (6H, d, J = 1 1 . 8 . 4 Hz), 1.60 (18H, s): 0.82 {18H, s). ES-MS: 1114.6 (20%, { [ Z n . , ( I - ) I)~]~PF,,):} -'+ ), 862.6 (60%), 694.5 (95c~, {IZn:(( - )I)~](PE,)} a+ ), 634.9 ( I00%, [Zn(( - )-1):! -'+ ), 603.3 180c/c, { I - )-I ) + H } ' ), 574.9 ( 60c~ ), 484.9 ( 40%. {IZn:(f - )-I)~1 }'~+ }, 423.9 ~25c//), 302.1 (!0%, {~ - )I} ÷2H} -'+ ). UV-Vis {E t O H / H : O I + I,c2.45 × I0- ~M): 303 { 96 O00), 314 (85 O00). CD ( E t O H / H : O I + i. c 2.45× IO -~ M): 309 ( - 5 6 ) , 319 ~ - 149). 2.3.2. A,A-ICd:(( - )-l )dtPFr,)4 ~H N M R (d~,-acetone. 25°C, 500 MHz): g 8.66 {6H, broad s ): 8.60 ( 6H, broad d. J = 7.4 Hz ): 8.53 ( 6H, broad m ); 8.17 ( 6H. broad m ): 8.13 t 6H, broad s ): 7.60 (6H, broad m }: 7.32 (3H, broad s): 7.30 (3H, t, ,I = 7.4 Hz); 7.20 (6H, d, .I = 7.4 Hz ): 3.54 ( 6H, broad d, J - 7.2 Hz ): 3.48 16H, m ): 2.86 {6H, m ): 2.77 ( 6H, broad d, ,I = I 1.0 Hz): 2.57 ( 6H. tn ); 1.99 (6H, m)' 1.40 ~6H, broad d, J ~ 7.4 Hz}: 1.28 { 18H, broad s J: 0.55 ( 18H, broad s }. '~C NMR {dt,.acetone. 25°C, 5 0 0 M H z ) ,8 150.2: 146.1' 140.8: 131.(): 129.4: 128.3: 126.8: 123.6: 122.6: 45.6: 44.2: 4 3 . 3 : 4 1 . 5 {q)' 39.7: 28.1; 26.3; 21.1. ES-MS: 1453.2 (It3, I I C d I { = } I)~I(PF,,)}' ), 1161.9(3~, {ICd_~(( = ) - I ) ~ I ( P F , ) : } :~ }, 751.3 (55c/~), 726.1 (30c,~. {lCd:(( =)-I),](PE,)} ~' ). 659.4 ( 90~. l ICd( ( = )-I ): l }: ' ). 603.3 ( 100~,~. { c = )I ):,+ H} ~' ). 508.3 ( 25rk. ICd.,( ( - )-I ),I "s'~). 302.1 ( 50~. {(-)-I)+2H} -'+), UV-Vis (EtOH/H:O I+I. c 1.18x I0 '~ M): 304 (90 000), 315 {69 000). CD (EtOH/ H,O i + I . c 1 . 1 8 × I0 -s M): 309 ( - 3 5 ) , 319 ~ = 192). 2.3.3. A , A - I F e 2 t t - J.IJ,lfPl~)~ ES-MS: 630.4 ( I(}()c~. [Fe(I - )-1)2] ~' ). 480.0 1319~. IFe:l( - ) - I ) , ] 4' ). 329.2 (3c,~. [Fef~ - } - I ) ] : ' }. U V Vis ( E t O H / H , O I + I. c 1.192x IO ~ M): 531 (23000). 485 ~ 17 000). 361 ( 17 000 ). 305 ( 137 000 ~. 290 ( 81 000 ). CD ( E t O H / H : O I + I) c 1.50x IO -4 M (250-350 nm): 295 (243). 315 ( - 529). c 4.00 x i 0 4 M {350-650 nm ): 395 (14). 477 ( i 2 ), 526 • - 8 ). 606 ( - 3 ). 2.3.4. A.A-/Fe:tt.+ )-I):ItPF;,)4 CD ( E t O H / H : O I + I ) c 1.50 x IO- "~M (250-350 ran) ): 295 ( - 236). 315 (566). c 4.00 x 10 4 M ( 350-650 nm ): 395 ( - 12).477 ( - i I). 526 (8). 606 (4).
38
if. Me'lineret al. I Inorganica Chimiea Acta 271 (1998136-39
3. RuulU and discussion Titration studies of chiragen[ m-xyl] ligands with Cd-"+, Zn:" and Fe: + in 50% aqueous ethanol show only the formation of complexes with M:L = 2:3 stoichiometry. Since the UV-Vis spectra of the free ligand and the Cd(!I) and Zn(11) complexes overl~ strongly, ~ titrations were monitored by C D spectroscopy only. In the case of Fe(11), the titrations wen: followed by CD and UV-Vis s~troscopy in the region of both charge-transfer and ligand centered transitions. As an example, the resul~ of the titration ofa I × !0 -'~ M Fe(I[) ~lution with an equally concentrated solution of ( - ) - 1 monitored by CD and UV-Vis spectroscopy in the charge transfer region are presented in Fig. 3. All other titration data can be found in the supplementary material. The M : L - 2 : 3 stoichiometry was confirmed by electrospray MS measurements. The elec~rospray spectra were recorded at relatively low concentration and the presence of
(a)
i' (b)
-
00
"10
..................... ...i_. .................... 2
:
3
=
other species in solution ( l ML l" + and [ ML_,]" + in all three complexes, mono- and diprotonated free ligand with the Cd(Ii) and Zn(II) species) is mainly due to partial decomplexation of the complexes (for details see Section 2). A product ion spectrum with argon as collision gas was recorded for Fe,L~ at m l z = 4 8 0 (Fig. 4). Two major fragments recorded correspond to [ FeL2] -"+ and [ FeL ]-"+, supporting the assignment of the ion at m / z = 480 to [ Fe,_.L3] "~+. Structural information can be obtained from CD and NMR measurements. The CD spectra (Fig. 5) show clearly that the heterochiral arrangement B (Fig. 2) of the triply bridged complex can be excluded in all three cases, since no CD activity is to be expected for this type of structure (the free ligands show no CD activity at wavelengths greater than 250 nm). Using the exciton coupling model for the assignment [20,211, the sign of the CD band at approximately 300 nm indicates a A,A absolute configuration for all three complexes [M,( ( - )-l).~! 4+ (M =Cd -'+, Zn -'+ and Fe -'+ ). The amount of coupling between two transition moments considered in exciton theory (in this case ligand centered 'n'-'n-*) is strongly dependent on their orientation with respect to each other. The significantly lower amplitude of the CD band centered at 303 nm of the Cd( i l) and Zn (It) complexes is therefore due possibly to distortions from an ideal octahedral coordination geometry. The IH NMR (Fig. 6) and I~C NMR data of the A,A[Cd,( ( - )-1 )~] ( PF~,L, complex indicate a complete equivalence of the three ligands and of the two halves of each ligand. These observations are in agreement only with the highly symmetrical (D~) arrangement of the triple helix in an ¢nantiomerically pure homochiral dinuclear structure ( Fig. 2, structureC). The same conclusion can he drawn for the Zn(II) and the Fo( [I) complexes. However, in the case of the Fe( I[) complex, no high resolution N M R spectra at room temperature and at 243 K and 373 K can he obtained. This ispossibly due to a relativelysmall ligand lield,giving rise to a near spincrossover situationfor the metal center as observed in similar
1 2 3 4 IIg~nd molNulee I Fe (#) atom
2.1
Fig, 3, Titrath~n of an Fe(il) ~olution ( I x 1 0 ~ M ) in ethanol:water I:1 with ~ ~lulion el" ~ = )-~X]lm~xyll ( I × It) ~ ~ M ) in elhanol:w,lcr I:1
monlt~w~l hy !a) UV=Vis spectroscopy, (h) CD spectr~)scopy. [Fe((.~S)l=° m,~ 14
I
/i
/
',u
o
m.~ [r~((.}.S)~l"
I~eoumorIon [~((.).S)~l~" lklO,Q
x 20
W
<1.400
i 350
~
~0
6~
wavelength [nm] rnlz Fi~ 4, Pr¢',duc-t~wl spect~m ofihe talc ~=480, I fragment (A,~-I Fe:( ( - ). I ) ~I ~" ;) wieh ~ , o n as collisinn g~s.
Fig. 5. CD sl~'Clra of the A.~-[ M.( ( - )-CG[ m-xyl ] ), I ~ + complexes in ethanol:water I:1. M--Cd"' ( - - ) , M=Zn-" (---), M=Fe:' (.... lb, Al~we 350 nm the ,signals of the iron(il) complex were scaled by a factor of 20,
ILl. Mih'ner et aL I bmrgani(',~ Chimh'a A "ta 271 ¢ 199,'q~ 3 6 - 3 9
39
5. Supplementary material
Ill, 9
8
Electrospray MS spectra of A , A - I M 2 ( ( - ) - I ) ~ I 4. ( M = Cd 2+, Zn -~÷ and Fe 2+ ). diagrams of the spectroscopic titrations followed by UV-Vis and CD spectroscopy (9 pages) are available from the authors. Ordering information is given on any current masthead page.
i 7
6
5
4 ppm
3
2
I
0
Fig, 6, I H NMR spectrum ( 50(1MHz ) of ,X,..X- I Cd2( I - )-CG I m-xyl I I a I (PF,,)4 in acetone-d,, at roonl temperature.
dinuclear Fe-lanthanide complexes [221. This tentative explanation is corroborated by the low energy of the first CD band at 606 am. Ferguson et al. 1231 attribute the opposite signs lbr the CD signals in the charge transfer region of A-IFe(bpy)al -'÷ compared with A-IRu(bpy)~! ~ and AI Os(bpy)a !-"+ by mixing of energetically comparable d-d and d-'rr* transitions in the Fe( !! ) case. Owing to the larger differences in the energy of these two transitions in Os( !! ) and Ru(!!). the mixing process cannot take place. The CD bands of A.A-IFe.~(( + )-I)~I ~' in the charge transfer region reseluble closely those of A-I Ru(bpy).~l; ~' and A108(bpy).~ 12 +. We conclude that the CGI m-xyl ] ligand exerts a weaker ligand lield than 2.2'-bipyridine. separating the d=d and d-'rr, transitions energetically to inhibit the mixing of the two types of transitions. Experiments on the tempenlture dependence o1" the magnetic susceptibility o f .,X...X-IFe,(( = )-I )~ i ~' are phuuled to v:didate this prelinl-
inary assignnlent. X - r a y d a t a w o u l d p r o v e tile stilled Iriple..helJcal Ililture o1'
Ihe dinuclear complexes
r e p o r l e d in the s o l i d slate, R e g r e t -
fully, w e w e r e nol a b l e to o b t a i n c r y s t a l s o f sul'licient q u a l i t y Ibr a s t r u c t u r e a n a l y s i s w i t h v a r i o u s c o u n t e r i o n s a n d u n d e r
different conditions.
4. Conclusions
We have proven earlier that chiragen ligands coordinate slereoselectively as tetradentale chelates at o,le metal center 116--181. The present study shows that these ligands also predetermine the cMndity at OC-6 ce,lters atld thereby the chirality of the emerging triple-helical structure if bound in a bis-bidentate nlanner to two labile octahedral centers. With Cd(!!). Zn(!!) and Fe(!I). dinucle:w compounds with an M,L~ stoicMometry and delined cMrality at the metal centers are fanned. The ligand geotnetry, as delined in tile chiral centers of Ihe pinene-moiety, controls stereoselectively the self assembly of such dinuclear triple-helical compounds.
Acknowledgements H.-R.M. and A.v.Z. thank the Swiss National Science Foundation for financial support. We thank Felix Fehr for carrying out the 500 MHz NMR experiments and Marco Ziegler and Nick Fletcher for valuable discussions. References I II B.R. Serr, K.A. Andersen. C.M. Elliott and O.P. Anderson, lnorg. Chem.. 27 ( I t)88 ) 44q9--4505. 121 CM. I-Iliott. D.L Derr. S. Ferrere. M.D. Ne~,,ton and Y.-P. Liu. J. Am. Chem. S.c.. 118 ( 19961 5221-5228. 131 D. Zurita. P. Baret and J.-L. Pierre. New J. Chem.. 18 ( 1994t 11431146. 141 M.-T. Youinou. R. Ziessel and J.-M, Lehn. Inorg. Chem.. 31) t I()ql ) 2144-2148. 151 J,A. Nachbaur. Thesi,~. [ T, ~versity of Fribourg. Switzerland. 1993, I¢~1 M. Albrecht and C. Riether. Chem. Bet,. 120 ( Iq%~ 829~832. 171 C. Piguet. G. Bemardinelli. B. Bocquet. (). Schaad and A.F. Williams. Inorg. Chem.. 33 I Iq941 4112--4121. I Sl C. I)iguet. It. Bernardinelli, B, Bocquet, A, Quattropani and A,F, Williams. J, Am. Chem, So¢,, 114 ( Iqq2 ) 74411~7451, I tj I I-,.I, Encnlark mid T.I), Stack, Angcw, ('hem,, Int, Ed, Engl,, 34 ( 10~)5 I I)~,)fl-OQS, Illl ,I, I.ibn|~tn. Y. Tar and A, Sllatl/er, J r Am. Chem, Sot:,, Illt} 11087) 5880~58t4 I, I I I I.,,I, Charl'~uutli~r~.', G, Ik, rn.rdir,.,lli, (', I~it~tu.'t, A,M, Sm'~e,,.u ~md A,I'*, Will.m,,, ,I. ('hem, S.c,, ('hem, ('t.nmun,, I I~)f,)41 14 Iq= 14211, I-~1 I.,J, ('harl~muiere. M,oI:, Gih.,I, K, Iterrtml~r and A,! :, Wiliam~, J, ('horn. Soc., Chem, Commurl., ( 10,)61 30~11. 131 R, Krtimor, ,I.-M, I.dm, A. !)~' ('iau mid J, I:i,,,,¢h~r, Atl~e~,, Cl'Dem,, Ill5 I II)()3l 764~767. 14] R.C, Scl|rrow. i).1., White ~illd K,N, Raymond, J, Am, Chem. So¢,, 107 (1985) t~5411-054(~. 151 M, Hasenknoiff. J.-M. Lehn, B,O, Kneisel, G, llaunl ~lttd D. I:en,,k¢, Angew. ('hem.. Int. Fd. Engl,. 35 ¢ IqOfi) 1838= 1840. I 61 P. Hayoz. A. van Zelewsky :rod H. Stoeckli- F.v:m,,. J, Am, Chem, S~.',. 115 I Iq93) 5111-5114. 171 H,-R, Mlirner. H. Stoeckli-Fvans and A. ~,on Zelew,,k~*, hmrg, Chem,, 35 ( It)()61 3t131=,~(135. 1141 H,-R, Miirller, P. I]elser :llld A. ~,on Zelew,~ky. J, Am. Chem. Soq.',. I 18 ( I OtJ6 ) 7t)Sq-7tJtM, It) l G. Hopfgartner, C. Piguet and J, Hellion. J. Am. Sac, Ma,,~. Speclr.m,. 5 (It}t)4) 748--75fi. 201 S,F. Mason and B,J, Nortnan, Inorg. Nucl, Chem. Ixt(,, 3 ( 10671 285288. 211 B. Bosnich. Inorg. Chem.. 7 (It)68) 237(I-2386. 221 C. I)iguet, F. Riv~m~-Mintcn, (;. Bernardmelli, J..C.CL Bthlzli and (i, Hopfgarlner. J. Chem, Sac,. Dalton Tran,~,. ( IO~)7) 421--433, 1231 J. I:erguson. I.. Herren aind G.M, Mcl.aughlin. Chem, Phys. lxtt,. 89 ( 1 9 8 2 ) 376-380.