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Stepwise substituting magnetic centres in a giant cluster host system
ELSEVIER
lnorganica Chimica Acta 271 t 1998 ) 9-I 2
Stepwise substituting magnetic centres in a giant cluster host system Achim Miiller ~"*, Winfried Plass ", Erich Krickemeyer ", Roberta Sessoli b, Dante Gatteschi b, Jochen Meyer ", Hartmut B6gge ~, Monika Kr6ckel ~, Alfred X. Trautwein ~ Fakultat fiir Chemie der Universitiit. la,hrstuhl fiir Anor~anische Chemie L Postfach 100131. D-33501 Bielefeld. Genmmv "Diparlimento di Chimica. Universita di Firenze. Via Aho'ag/iano 75/77. 1-50144 Florence. hair • b~slilt*l,l?h"Physik. Medizinisvhe Universitiil LiibevL Ralzel)m:~erAll(,,, 160. D-23538 L(ibeck. Germ~my Received 28 No~ember 1996
Abstract in the polyoxomolyhdate shell of the cluster compound (NH4).~tl{V'VO(H~O)J,{Mo(p.-H20).,(P.-OH)Mo}dMo,dMoNO):" Osa(HaO) a}~] ( lV,I >. {Feill(H~O) 2} and {Fell( H20)2 } moieties can be incorporated at the positions of the {vet H:O) }groups. On adding Feels. 6H20 it) excess to a solution froth which pure lV, i ca, be (sol,tied, tile cluster compound (NH.~)t~[ {Felii( 820)2 },,{Me( IX-H20)2(Ix-OH) Me }~{Mot st MoNO),O~(H.,O) 2}~I ([Fe,]) is formed. By adding varying amounts of FeCl.,.4H.,O or FeCl~.6H.,O to such a solution, a series el' compounds can be isolated with different ratios of oxovanadium( IV ). iron( ill ). and iron( lit centres. The experimental conditions allow tuning of these ratios in the prepared compounds, (V4Fe2). (V.~Ye,O.(V2Fe.t) and (VFe.~) with varying iron( lilt/iron( ll ) ratio (depending on the degree el' oxidation of iron(!!) centres) and (VFem,q). These fornmlae correspond to those of [V,] and [Fe, i with the relevant intermediate NH~' content. The synthesized compounds were characterized by elemental analyses, different spectroscopic methods, and X-ray crystal structure analyses as well as ~TFe-M6Bbauerspectroscopy which allowed determination of the iron( Ill )/iron( li) ratio. The magnetic properties of all cluster compounds were carefully investigated. For oneof the mixed systems at) interesting ferrimagnetic h~haviour is observed and can only he exphlined if tile dil'ferettt tuugttetic Celttt'es occur within a single cluster sllell and are i1otseparated in different clusters. ~) 1998 Elsevier Science S.A. Ke~ woril, w (?llister ¢on|l~ound,~: IMl~,,o.xoil~olybd[it¢ ,/o1111~OUlltIN;M¢ignetic l~ropei'tie.~
I. Introduction Magnetic properties of large spin clusters or magnetic nanoclusters ale attracting increasing interest. Several different chemical approaches for the synthesis of such systems have been described, it is possible to embed the particles in protecting media, such as protein shells or micelles, polymers or resins, but in these cases it is difficult to achieve monodispersed and easily characterized particles. Perhaps the most challenging approach is that of making well defined nanoscaled molecular species which can I~ characterized with the usual approaches of ;nolecular chemistry [ I ]. The structurally characterized cluster compounds [V6I [ 21 and IFe,,i 131 (NH4)211 {VIVO(H:O) h,{ Me(Ix-H,~O) ~
(tbrmula¢ without crystal water) can formally ~ regarded as inorganic equivalents ot' small proteins, in which the basic diamagnetic moiety has a structural function, providing the shape of the cluster and leaving space available for different paramagnetic active sites. However. they also have a more important role. by transmitting magnetic interaction between the 'active paramagnetic centres', which in the case of iV,,] is tbund to be rather effective [41. Here we demonstrate a fitscinating aspect in that we can substitute step by step diflerent paramagnetic centres in the same cluster species, generating different ant(ferromagnetic and even feet(magnetic molecular systems.
2. Experimental
X {Mols( MoNO)~.Oss(H:O).. ),l IVol (NH4) ,si{ Fc"t( H:O): ),{ Mo(I*-H20) 2(I,-OH) Mo ), x {Mo,~(MoNO)20~,(H,O); }~1 [Fed * Corresponding author. {102{)-1693/98/$19.(10 {~> 1998 Elsevier Science S.A. All rights reserved. Pll S 0 0 2 0 - 1 t")93 ( 97 ~()5885-4
2. !. Syntheses The solution employed in the synthesis of the different compounds was prepared as described previously in the synthesis of [Vo] 121. By adding varying amounts of
A. Miiller et ,I. / heorg, nh', Chimica Act, 271 (1998) 9-12
tO
! hltrate+lOgFeCl!
oH:(}
_
. IFe, l
r~flux 1 20 mm / argon
7,46 g Na:MoO, 2 H:O ,.,9 g Nl~hVO, 12.83 g NH:OX HC1 in 200 ml H..O and 9 5 ml 3.5 % HC1 ! Mtxtu~ keW for 20h at ~O"C (4~ h in cas~ of <~T-¢~>) on a I~anng plait w i l h ~ sltmng
filtrate + 2 g FcCh 6 H:O reflux ! 20 nun I ar~m
•
-
I Filtration in air
2 ~onng of tl~ filtrate for [ t i.4 ~¢¢ks at Y'C under i argon for crystalltzanon
1
filuate + 28.2 g F~CI: 4 I I ,-O, . ~ ,,, ~. 95°C / 3 hlalr
filtrate • 2 3 5 g FcCI: 4 H:O reflux / ~ m m I ~ g o ~
•
. ]
filtrate + 14.1 g F¢CI: 4 H:O neflux I ~ mm I argon
•
2. F~lwauon in air filffab: • 4.7 ~ FcCI: 4 H;O reflux 120 tmn / a~g~m
,
~4m-m~ o ! the |lltrale |:04" l -.~ "~,'¢¢k~at ~Olll t¢lllp~r:_|tUl~ 111mr tot cr)'~,lalll/~lwfl
. IV, I
Scheme I. Tabl~ I X:ray co~alh,$raphic, analytical and M61~aucr data
( V.Ne.,0
Fc 2.8 (~.';.4)
I%. 0.86 ( ~ 1.7)
I:e 1.5 (~2.9)
Fe 1.9 (~3.7)
Fe 2.4 (---4.6)
Fe 2.5 (-4.8)
M~uer duma I~{ III ) { ~ )
I(X)
~0± ~
50 :L ~
6fi :1:5
I(X)
F~( III ) /~ (mm s ' ) ~iE~ (ram s t)
0.51 1.16
0.4.'; I.II
0.50 1.24
0.51 1.10
0.52 1.17
I..I.~ ,I,22
1,3~ 3,14
1.33 3,12
Composition M~lal analysis Found I t~) (No~ Fe atoms (talc.))
IV,J
IF~I
tI:,,,N:,Mo~70:~:V.
H:~TN:~Fe.Mos~():,.,
V 2.9
Fe{ II ) {ram ~ ') A~:~ (ram s o)
t# !pm ) {: t pit|) V! I I P p m t )
=~lh~,h! ,i) ~7(~),~(4) l,ll7fit,I)
=~.171,0(7 ) .~7,11,~{ II,) 13,11~(7)
~,~¢~, I{ 4; ~711(L3( I01 I,ll;~
~,if;,~,,h.~) 27,{1),~ Iq) It, ,,:8 ~
2olh5,I( 3 ) ,~7,.8,8( ~ ) 132 I q
2,1(;7,{~( 31
...... (
I.~Z%
13033
No, u~iqu~t ~ f v e d t~t~tltms I F > 4,¢(I,') ) No. variables R ~ ~I IF,,I = IF, II l~I/~,,l
2?30
2433
403~
.I~08
3(~14
278fi
26.14
29{t ().{~2
287 o.1(~
2~) o.o78
293 o.l~l
29o o.o79
288 o.06.~
288 o.(177
F~:I,.4HaO or FeCI~. OHiO in presence of air to this solution, a series of compounds can ~ isolated with different ratios of oxova,adium( IV ), iron(III), and imnl !i ) centres within the polyoxomolybdate cluster shell, The experimental details for the preparation of the compounds, , , , and are summarized in Scheme I ( see Table I ),
2,2, Phy,~iccd merlr~l,~' Atomic absorption s ~ t m s c o p y was used fi)r analysis of" the iron content in the compounds, The electronic spectra were measmx,d using u Beckman Acta MIV instrument ( UVVis, ~lid state ~flectance with cellulo~ as calibration stan~ : NIR, KBr pellets in transmission), Vibrational spu~tra
were recorded using a Braker IFS66 FT-IR spectrometer which was equipped with an additional Braker FRAI06 Raman unit (A,,= 1064 nm). The ~Fe-M6Bbauer spectra were measured at 77 K and were analysed using one or it" necessary two quadrupolar doublets to account for iron( il ) and iron( III ), Isomer shiits are reported versus ot-Fe ut room temperature. The magnetic susceptibilities were measured using a Metmnique Ingenierie SQUID magnetometer with an applied tield of 10 kOe, The experimental data were c o l retted for diamagnetic contributions and temperature-independent paramagnetism according to Rel; 14l. The crystal structures were determined from single-crystal diffraction data collected with a Siemens R3m/V diffractometer. Crystal data and details concerning the structure relinement are given in Table I. The unit cell parameters were
A. Miiller et aL / hmrganh'. Chhnh'a Acia 271 f 19981!t-12
obtained from the angular settings of about twenty reflections in the 20 range of 20 to 30 ° . A n empirical absorption correction was applied to all compounds and the data corrected for Lorentz and polarization effects. Structures were solved by direct methods ( SHELXTL PLUS program package) and the subsequent least-squares refinement (SHELXL-931 15 !.
II
,,j
3. R e s u l t s a n d d i s c u s s i o n
The structures of the cluster compounds IVy,] and [Fee>] have been reported previously [2,3 ] and are characterized by three Molt ligands, {MoVll.~(MoNO)_~O.~8(H.,O)~} 2°- , which are connected by six octahedrally coordinated metal centres (oxovanadium( IV ) or iron( ill ) ) and three formally cationic { Me( p,-H,,O).,(p,-OH) Me }" + moieties. A representation of the cluster anion structures is depicted in Fig. I(d) and (e). From the magnetic point of view the interesting fact concerning these giant cluster systems is the possibility of embedding paramagnetic centres in a shell of large diamagnetic polyoxomolybdate n~;i~i,s (ligands) which are transmitting the magnetic interaction between the six paramagnetic centres [ 4 I. From the solution I'rom which the [Vf,] cluster compound crystallizes, a series of cluster compounds with corresponding structural core, but statistical occupation of the six octahedral metal sites ( oxovanadium( IV ), iron( il ), and iron( ill ) tenires within the cluster anions ), can be synthesized by adding different anlounts of FeCI.~.4H~O or FeCI.~.rH.~O (see Scheme I ). Therefore, variation el' the experimental condilions allows tuning el' the ratio o1' vanadium to iron centres on the six positions which I'ornl li Irilonal prism within die lhinl cluster sySlelll I see Fig. I ). I1' lUl ironl III) source is used, only snilill anlounis are necessary lo incorporale iron hi the giant cluster sysielns, wherelis in the case ot' an iron( II ) source a large excess is needed tO achieve a sinlilar vanadiilln to iron ratio. This can be attributed to the higher aftinity ot' the more electrophilic iron( ill ) cenlres to the strongly basic coordination environment set up by the cluster shell. Consequently, upon addition of an excess of FeCl.~.6H.~O, [Fe¢,] can be isolated. The prepared compounds with both iron( il ) and iron(Ill) centres (V4Fel), (V~Fe,~), (V1Fe,i), and (VFe,~) are labelled according to the clo:iest integral vanadium to iron ratio, whereas the mixed cluster compound with only iron( I!1 ) is labelled as (VFenls). Both the total amount of iron incorporated and the relative iron( ll )/iron( Ill ) ratio are influenced to some extent by the exposure of the reaction solution to air. The compositions of the cluster compounds were proven by elmnm|tal analyses, magnetic measurements, ~TFeMiiBbauer spectroscopy and X-ray crystal structure analyses (see "Fable I, the fiirmula corresponds to that o1' IVy,] and lFe~,] with the relevant interlnediate NH,~+ content). The crystal structure an:ilyses prove the presence of {VO(H:O) } and {Fe(H,~O),) groups at the same crystallographic sites (Fig. I) and the MiiBbauer spectra that of iron(ll) and
Fig, I. Repre..,entation of the positions of the p~zranzaLzneli¢metal centre,,, in the ociahcdral IMO.} site,~ ( M ~ V , l'e) in IFe, I (a), IV,,I (hi as~d the mixed Fc/V clusters Icl (with the excepliiitl of (VFe.4) the two nlelal i~llsilion,s could he reslllved fronl differeni:e Fourier nllil~S and wt.'i'¢ r¢lilll:d with lippropriille o¢¢uplllion factors i. The iron alOlil,~ arc situated approxinllilel), in Ihl~ ¢enlre of Ihe ociahl.'dra I liverllgl.' di.,,lant:¢.~ l:i~-(Ott~ I 199.1,1 and 209.9 pill ) wlil.'reas the vanadium atonls are shiticd Iowards Olle verlex (iiverti~e distances V--O 163.3 and V - I O H : I 242.7 pni). The Iril, ttnalpl'i.,dilali¢ arrmigelnelil of llie six rlliranilignelit: t.:i.'lilres elnhedded in Ih¢ anionic cluster siruclui'e is shown ill ( d ) I view along one {':°axis ) and in (el i view along the C~-.'ixis I. The Iril-,onal I~risni i.,, ellii'lhiisi#.l.'d hy ihh:k lines. In .~Oliie crystals of IV,J mid ¢onlpoullds with a low iroli ¢o111¢ni six addilional .,,ylnliielr), t.'quivllleni exlr:i electron deliSil), pl.,iik~ silulilt:d wilhin ii letrahedral enviiolllilt'iil of ox),[~¢ll iilOlll.S ( three t'rt~ill Ih¢ clu.sler anion lilid Oli¢ lerniiliol oxo [~l'oup ) could be d¢lecled. The ptl.,,iliOll.,, el ° Ilire¢ llf Ilie,,,e peliks are shown by arrows. They weri~ r¢lilied as IlOl fully ~ceupied Mt~i VI I sites with occuplition flitters ranging fronl I0.() hi 5(Y)~. sl~e also I -'!-I.
12
A. Mi#ler vt .L I Im.'gm~h,a Chimh'a Acre 271 f l~k'~/ 9-12
0.~5 0.990
[
0.985[
,
0,980
-4
,
,
,
,
i
-2
,
0
,
2
4
Velocity [ram/s] Fi~. 2. t~ttlmuer sp:¢Irum oI' (VFes) showing Ihe pre.~nce of iron(II) I ~ 1.33 rams '..tEu~3.12 mm.~ ~) and iron(Ill) (~=O.51 mm~ '. A l f ~ , I . I O m m s ').
++ [S~l ,¢IV~g@'v'S>
++
|
IIIIIIII
~.
_i_I
ill[ I
•
•
II
_-
•
•
•
•
II •
•
II
i (V~Fe~)
-- -
Acknowledgements I lv+1
0
loo
ferrimagnetic behaviour becomes less pronounced, in the limiting case of a low iron content, an antifermmagnetic behaviour like that of IVy,! is observed as for instance in the case of <(V4Fe,). It is interesting to note that the incorporation of iron(ll) is increasing the fendmagnetic character of the magnetic behaviour, which is evident from the comparison of the xTversus Tcurves of {VFes) and (VFems). Although the xTcurve of
2O0
TEMPEI~TU¢~E(K) Fi+t, ?,, +tit+,'s,Tcurv+sfor I%1 ~A), ( ~ ) , i + i, ..,~ 11%11+ i
iron( III ) cemre,~(¢t\ Fig,, 2 ), Moreover, the MOBb.uer SlOeIra prove that the ,~ametype of site with an ocl.hedral crow° dination is occupied by iron centre,~ with d i l l ~ n l oxidation
~tute, The magnetic behaviour of the mixed compounds is dependent on both the total iron(III) content and the relative iron! II ) / i r o n ( I I I ) ratio, With increasing content of iron the x T values increase as expected (see Fig, 3), Remarkably, for the species with mixed iron(II) and iron(Ill) content
((V~Fe~), (V,~Fe,~), (VaFe~), and (VFes)), X7" goes through a maximum upon going to lower temperatures, which is more pronounced for clusters with higher iron content, Although a complete analysis of the magnetic parameterstbr the~ systems is not feasible, this temperature dependence ol"
xTis indicative ofa t'erdmagnetic t'
The financial support of the Deutsche Forschungsgemeinschafl, the Fonds der Chemischen lndustrie, MURST, the Herbert Quandt.Slit'lung der VARTA AG (W.P.), and the Council of the European Community (Grant ERBCHR.
XCT92(H)80) is 81~:atlyappreciated. References I I I A. MOiler. H. reuler mid S. I)illi,ser. A,gew. Chem.. h,. Ed. E,pl.. 34 ! I~.~) 232X; A. M|lller a,d C. Beut,'holl. N~.tur¢. 383 (I~%) 2%: A. MOiler, A, Dress and F. VOglle (ed~,i, From Simplicity to Complexity in Chemi.~lryand Beyoud, P~rl I, Vieweg, Wiesbaden, 19%. J21 A, MOiler. E. Kriekemeyer. S. Dilling.er. H. B{Igg.e.W. Plu~s. A. Pmu~l. L. DIm:s.ik. C. Menkc. J. Meyer and R. Rohlling. Z. Anor~. AIIg. Chem.. I.~1 A. M011¢r. W. Pla.~. F+. Krickemeyer..% Dilli,ger. H. Bi~ge. A. Armalut~e. A. Prou,~l, (', Beu~holl and U, Bergmann. Angew. Chem.. Int. Ed. Engl.. 33 ( 19941 849: A. M011er. H. B(~gge. E. Krickemeyer a,d S. Dillinger. Bull. Pol. Aead. Sci.. 42 ! IL~41 2tll. 141 D. Gatle.~chi. R. Ses....li. W. I%.......A. MOIler. E. Krickem~:yer. J. Meyer. D..S~ller and P. Adler. I,m'g. Cilem...~.~ ( Iq% t 192h. I .~I SHELXTI° PLUS. ~:~°~tall.gmphic ~)..tem. Siemen~ A.alylicul X-ray h~slrum~nt.. Inc.. Madly... Wl. 1~89: G.M Sheldrick. SHELXL-~L~. t!,i~cr~il) of (;iittiugc,. (h:rma,~'. I~)t).~.
lnorganica Chimica Acta 271 t 1998 ) 9-I 2
Stepwise substituting magnetic centres in a giant cluster host system Achim Miiller ~"*, Winfried Plass ", Erich Krickemeyer ", Roberta Sessoli b, Dante Gatteschi b, Jochen Meyer ", Hartmut B6gge ~, Monika Kr6ckel ~, Alfred X. Trautwein ~ Fakultat fiir Chemie der Universitiit. la,hrstuhl fiir Anor~anische Chemie L Postfach 100131. D-33501 Bielefeld. Genmmv "Diparlimento di Chimica. Universita di Firenze. Via Aho'ag/iano 75/77. 1-50144 Florence. hair • b~slilt*l,l?h"Physik. Medizinisvhe Universitiil LiibevL Ralzel)m:~erAll(,,, 160. D-23538 L(ibeck. Germ~my Received 28 No~ember 1996
Abstract in the polyoxomolyhdate shell of the cluster compound (NH4).~tl{V'VO(H~O)J,{Mo(p.-H20).,(P.-OH)Mo}dMo,dMoNO):" Osa(HaO) a}~] ( lV,I >. {Feill(H~O) 2} and {Fell( H20)2 } moieties can be incorporated at the positions of the {vet H:O) }groups. On adding Feels. 6H20 it) excess to a solution froth which pure lV, i ca, be (sol,tied, tile cluster compound (NH.~)t~[ {Felii( 820)2 },,{Me( IX-H20)2(Ix-OH) Me }~{Mot st MoNO),O~(H.,O) 2}~I ([Fe,]) is formed. By adding varying amounts of FeCl.,.4H.,O or FeCl~.6H.,O to such a solution, a series el' compounds can be isolated with different ratios of oxovanadium( IV ). iron( ill ). and iron( lit centres. The experimental conditions allow tuning of these ratios in the prepared compounds, (V4Fe2). (V.~Ye,O.(V2Fe.t) and (VFe.~) with varying iron( lilt/iron( ll ) ratio (depending on the degree el' oxidation of iron(!!) centres) and (VFem,q). These fornmlae correspond to those of [V,] and [Fe, i with the relevant intermediate NH~' content. The synthesized compounds were characterized by elemental analyses, different spectroscopic methods, and X-ray crystal structure analyses as well as ~TFe-M6Bbauerspectroscopy which allowed determination of the iron( Ill )/iron( li) ratio. The magnetic properties of all cluster compounds were carefully investigated. For oneof the mixed systems at) interesting ferrimagnetic h~haviour is observed and can only he exphlined if tile dil'ferettt tuugttetic Celttt'es occur within a single cluster sllell and are i1otseparated in different clusters. ~) 1998 Elsevier Science S.A. Ke~ woril, w (?llister ¢on|l~ound,~: IMl~,,o.xoil~olybd[it¢ ,/o1111~OUlltIN;M¢ignetic l~ropei'tie.~
I. Introduction Magnetic properties of large spin clusters or magnetic nanoclusters ale attracting increasing interest. Several different chemical approaches for the synthesis of such systems have been described, it is possible to embed the particles in protecting media, such as protein shells or micelles, polymers or resins, but in these cases it is difficult to achieve monodispersed and easily characterized particles. Perhaps the most challenging approach is that of making well defined nanoscaled molecular species which can I~ characterized with the usual approaches of ;nolecular chemistry [ I ]. The structurally characterized cluster compounds [V6I [ 21 and IFe,,i 131 (NH4)211 {VIVO(H:O) h,{ Me(Ix-H,~O) ~
(tbrmula¢ without crystal water) can formally ~ regarded as inorganic equivalents ot' small proteins, in which the basic diamagnetic moiety has a structural function, providing the shape of the cluster and leaving space available for different paramagnetic active sites. However. they also have a more important role. by transmitting magnetic interaction between the 'active paramagnetic centres', which in the case of iV,,] is tbund to be rather effective [41. Here we demonstrate a fitscinating aspect in that we can substitute step by step diflerent paramagnetic centres in the same cluster species, generating different ant(ferromagnetic and even feet(magnetic molecular systems.
2. Experimental
X {Mols( MoNO)~.Oss(H:O).. ),l IVol (NH4) ,si{ Fc"t( H:O): ),{ Mo(I*-H20) 2(I,-OH) Mo ), x {Mo,~(MoNO)20~,(H,O); }~1 [Fed * Corresponding author. {102{)-1693/98/$19.(10 {~> 1998 Elsevier Science S.A. All rights reserved. Pll S 0 0 2 0 - 1 t")93 ( 97 ~()5885-4
2. !. Syntheses The solution employed in the synthesis of the different compounds was prepared as described previously in the synthesis of [Vo] 121. By adding varying amounts of
A. Miiller et ,I. / heorg, nh', Chimica Act, 271 (1998) 9-12
tO
! hltrate+lOgFeCl!
oH:(}
_
. IFe, l
r~flux 1 20 mm / argon
7,46 g Na:MoO, 2 H:O ,.,9 g Nl~hVO, 12.83 g NH:OX HC1 in 200 ml H..O and 9 5 ml 3.5 % HC1 ! Mtxtu~ keW for 20h at ~O"C (4~ h in cas~ of <~T-¢~>) on a I~anng plait w i l h ~ sltmng
filtrate + 2 g FcCh 6 H:O reflux ! 20 nun I ar~m
•
-
I Filtration in air
2 ~onng of tl~ filtrate for [ t i.4 ~¢¢ks at Y'C under i argon for crystalltzanon
1
filuate + 28.2 g F~CI: 4 I I ,-O, . ~ ,,, ~. 95°C / 3 hlalr
filtrate • 2 3 5 g FcCI: 4 H:O reflux / ~ m m I ~ g o ~
•
. ]
filtrate + 14.1 g F¢CI: 4 H:O neflux I ~ mm I argon
•
2. F~lwauon in air filffab: • 4.7 ~ FcCI: 4 H;O reflux 120 tmn / a~g~m
,
~4m-m~ o ! the |lltrale |:04" l -.~ "~,'¢¢k~at ~Olll t¢lllp~r:_|tUl~ 111mr tot cr)'~,lalll/~lwfl
. IV, I
Scheme I. Tabl~ I X:ray co~alh,$raphic, analytical and M61~aucr data
( V.Ne.,0
Fc 2.8 (~.';.4)
I%. 0.86 ( ~ 1.7)
I:e 1.5 (~2.9)
Fe 1.9 (~3.7)
Fe 2.4 (---4.6)
Fe 2.5 (-4.8)
M~uer duma I~{ III ) { ~ )
I(X)
~0± ~
50 :L ~
6fi :1:5
I(X)
F~( III ) /~ (mm s ' ) ~iE~ (ram s t)
0.51 1.16
0.4.'; I.II
0.50 1.24
0.51 1.10
0.52 1.17
I..I.~ ,I,22
1,3~ 3,14
1.33 3,12
Composition M~lal analysis Found I t~) (No~ Fe atoms (talc.))
IV,J
IF~I
tI:,,,N:,Mo~70:~:V.
H:~TN:~Fe.Mos~():,.,
V 2.9
Fe{ II ) {ram ~ ') A~:~ (ram s o)
t# !pm ) {: t pit|) V! I I P p m t )
=~lh~,h! ,i) ~7(~),~(4) l,ll7fit,I)
=~.171,0(7 ) .~7,11,~{ II,) 13,11~(7)
~,~¢~, I{ 4; ~711(L3( I01 I,ll;~
~,if;,~,,h.~) 27,{1),~ Iq) It, ,,:8 ~
2olh5,I( 3 ) ,~7,.8,8( ~ ) 132 I q
2,1(;7,{~( 31
...... (
I.~Z%
13033
No, u~iqu~t ~ f v e d t~t~tltms I F > 4,¢(I,') ) No. variables R ~ ~I IF,,I = IF, II l~I/~,,l
2?30
2433
403~
.I~08
3(~14
278fi
26.14
29{t ().{~2
287 o.1(~
2~) o.o78
293 o.l~l
29o o.o79
288 o.06.~
288 o.(177
F~:I,.4HaO or FeCI~. OHiO in presence of air to this solution, a series of compounds can ~ isolated with different ratios of oxova,adium( IV ), iron(III), and imnl !i ) centres within the polyoxomolybdate cluster shell, The experimental details for the preparation of the compounds
2,2, Phy,~iccd merlr~l,~' Atomic absorption s ~ t m s c o p y was used fi)r analysis of" the iron content in the compounds, The electronic spectra were measmx,d using u Beckman Acta MIV instrument ( UVVis, ~lid state ~flectance with cellulo~ as calibration stan~ : NIR, KBr pellets in transmission), Vibrational spu~tra
were recorded using a Braker IFS66 FT-IR spectrometer which was equipped with an additional Braker FRAI06 Raman unit (A,,= 1064 nm). The ~Fe-M6Bbauer spectra were measured at 77 K and were analysed using one or it" necessary two quadrupolar doublets to account for iron( il ) and iron( III ), Isomer shiits are reported versus ot-Fe ut room temperature. The magnetic susceptibilities were measured using a Metmnique Ingenierie SQUID magnetometer with an applied tield of 10 kOe, The experimental data were c o l retted for diamagnetic contributions and temperature-independent paramagnetism according to Rel; 14l. The crystal structures were determined from single-crystal diffraction data collected with a Siemens R3m/V diffractometer. Crystal data and details concerning the structure relinement are given in Table I. The unit cell parameters were
A. Miiller et aL / hmrganh'. Chhnh'a Acia 271 f 19981!t-12
obtained from the angular settings of about twenty reflections in the 20 range of 20 to 30 ° . A n empirical absorption correction was applied to all compounds and the data corrected for Lorentz and polarization effects. Structures were solved by direct methods ( SHELXTL PLUS program package) and the subsequent least-squares refinement (SHELXL-931 15 !.
II
,,j
3. R e s u l t s a n d d i s c u s s i o n
The structures of the cluster compounds IVy,] and [Fee>] have been reported previously [2,3 ] and are characterized by three Molt ligands, {MoVll.~(MoNO)_~O.~8(H.,O)~} 2°- , which are connected by six octahedrally coordinated metal centres (oxovanadium( IV ) or iron( ill ) ) and three formally cationic { Me( p,-H,,O).,(p,-OH) Me }" + moieties. A representation of the cluster anion structures is depicted in Fig. I(d) and (e). From the magnetic point of view the interesting fact concerning these giant cluster systems is the possibility of embedding paramagnetic centres in a shell of large diamagnetic polyoxomolybdate n~;i~i,s (ligands) which are transmitting the magnetic interaction between the six paramagnetic centres [ 4 I. From the solution I'rom which the [Vf,] cluster compound crystallizes, a series of cluster compounds with corresponding structural core, but statistical occupation of the six octahedral metal sites ( oxovanadium( IV ), iron( il ), and iron( ill ) tenires within the cluster anions ), can be synthesized by adding different anlounts of FeCI.~.4H~O or FeCI.~.rH.~O (see Scheme I ). Therefore, variation el' the experimental condilions allows tuning el' the ratio o1' vanadium to iron centres on the six positions which I'ornl li Irilonal prism within die lhinl cluster sySlelll I see Fig. I ). I1' lUl ironl III) source is used, only snilill anlounis are necessary lo incorporale iron hi the giant cluster sysielns, wherelis in the case ot' an iron( II ) source a large excess is needed tO achieve a sinlilar vanadiilln to iron ratio. This can be attributed to the higher aftinity ot' the more electrophilic iron( ill ) cenlres to the strongly basic coordination environment set up by the cluster shell. Consequently, upon addition of an excess of FeCl.~.6H.~O, [Fe¢,] can be isolated. The prepared compounds with both iron( il ) and iron(Ill) centres (V4Fel), (V~Fe,~), (V1Fe,i), and (VFe,~) are labelled according to the clo:iest integral vanadium to iron ratio, whereas the mixed cluster compound with only iron( I!1 ) is labelled as (VFenls). Both the total amount of iron incorporated and the relative iron( ll )/iron( Ill ) ratio are influenced to some extent by the exposure of the reaction solution to air. The compositions of the cluster compounds were proven by elmnm|tal analyses, magnetic measurements, ~TFeMiiBbauer spectroscopy and X-ray crystal structure analyses (see "Fable I, the fiirmula corresponds to that o1' IVy,] and lFe~,] with the relevant interlnediate NH,~+ content). The crystal structure an:ilyses prove the presence of {VO(H:O) } and {Fe(H,~O),) groups at the same crystallographic sites (Fig. I) and the MiiBbauer spectra that of iron(ll) and
Fig, I. Repre..,entation of the positions of the p~zranzaLzneli¢metal centre,,, in the ociahcdral IMO.} site,~ ( M ~ V , l'e) in IFe, I (a), IV,,I (hi as~d the mixed Fc/V clusters Icl (with the excepliiitl of (VFe.4) the two nlelal i~llsilion,s could he reslllved fronl differeni:e Fourier nllil~S and wt.'i'¢ r¢lilll:d with lippropriille o¢¢uplllion factors i. The iron alOlil,~ arc situated approxinllilel), in Ihl~ ¢enlre of Ihe ociahl.'dra I liverllgl.' di.,,lant:¢.~ l:i~-(Ott~ I 199.1,1 and 209.9 pill ) wlil.'reas the vanadium atonls are shiticd Iowards Olle verlex (iiverti~e distances V--O 163.3 and V - I O H : I 242.7 pni). The Iril, ttnalpl'i.,dilali¢ arrmigelnelil of llie six rlliranilignelit: t.:i.'lilres elnhedded in Ih¢ anionic cluster siruclui'e is shown ill ( d ) I view along one {':°axis ) and in (el i view along the C~-.'ixis I. The Iril-,onal I~risni i.,, ellii'lhiisi#.l.'d hy ihh:k lines. In .~Oliie crystals of IV,J mid ¢onlpoullds with a low iroli ¢o111¢ni six addilional .,,ylnliielr), t.'quivllleni exlr:i electron deliSil), pl.,iik~ silulilt:d wilhin ii letrahedral enviiolllilt'iil of ox),[~¢ll iilOlll.S ( three t'rt~ill Ih¢ clu.sler anion lilid Oli¢ lerniiliol oxo [~l'oup ) could be d¢lecled. The ptl.,,iliOll.,, el ° Ilire¢ llf Ilie,,,e peliks are shown by arrows. They weri~ r¢lilied as IlOl fully ~ceupied Mt~i VI I sites with occuplition flitters ranging fronl I0.() hi 5(Y)~. sl~e also I -'!-I.
12
A. Mi#ler vt .L I Im.'gm~h,a Chimh'a Acre 271 f l~k'~/ 9-12
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0.985[
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0,980
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,
,
,
,
i
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,
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,
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4
Velocity [ram/s] Fi~. 2. t~ttlmuer sp:¢Irum oI' (VFes) showing Ihe pre.~nce of iron(II) I ~ 1.33 rams '..tEu~3.12 mm.~ ~) and iron(Ill) (~=O.51 mm~ '. A l f ~ , I . I O m m s ').
++ [S~l ,¢IV~g@'v'S>
++
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ill[ I
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Acknowledgements I lv+1
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ferrimagnetic behaviour becomes less pronounced, in the limiting case of a low iron content, an antifermmagnetic behaviour like that of IVy,! is observed as for instance in the case of <(V4Fe,). It is interesting to note that the incorporation of iron(ll) is increasing the fendmagnetic character of the magnetic behaviour, which is evident from the comparison of the xTversus Tcurves of {VFes) and (VFems). Although the xTcurve of
2O0
TEMPEI~TU¢~E(K) Fi+t, ?,, +tit+,'s,Tcurv+sfor I%1 ~A),
iron( III ) cemre,~(¢t\ Fig,, 2 ), Moreover, the MOBb.uer SlOeIra prove that the ,~ametype of site with an ocl.hedral crow° dination is occupied by iron centre,~ with d i l l ~ n l oxidation
~tute, The magnetic behaviour of the mixed compounds is dependent on both the total iron(III) content and the relative iron! II ) / i r o n ( I I I ) ratio, With increasing content of iron the x T values increase as expected (see Fig, 3), Remarkably, for the species with mixed iron(II) and iron(Ill) content
((V~Fe~), (V,~Fe,~), (VaFe~), and (VFes)), X7" goes through a maximum upon going to lower temperatures, which is more pronounced for clusters with higher iron content, Although a complete analysis of the magnetic parameterstbr the~ systems is not feasible, this temperature dependence ol"
xTis indicative ofa t'erdmagnetic t'
The financial support of the Deutsche Forschungsgemeinschafl, the Fonds der Chemischen lndustrie, MURST, the Herbert Quandt.Slit'lung der VARTA AG (W.P.), and the Council of the European Community (Grant ERBCHR.
XCT92(H)80) is 81~:atlyappreciated. References I I I A. MOiler. H. reuler mid S. I)illi,ser. A,gew. Chem.. h,. Ed. E,pl.. 34 ! I~.~) 232X; A. M|lller a,d C. Beut,'holl. N~.tur¢. 383 (I~%) 2%: A. MOiler, A, Dress and F. VOglle (ed~,i, From Simplicity to Complexity in Chemi.~lryand Beyoud, P~rl I, Vieweg, Wiesbaden, 19%. J21 A, MOiler. E. Kriekemeyer. S. Dilling.er. H. B{Igg.e.W. Plu~s. A. Pmu~l. L. DIm:s.ik. C. Menkc. J. Meyer and R. Rohlling. Z. Anor~. AIIg. Chem.. I.~1 A. M011¢r. W. Pla.~. F+. Krickemeyer..% Dilli,ger. H. Bi~ge. A. Armalut~e. A. Prou,~l, (', Beu~holl and U, Bergmann. Angew. Chem.. Int. Ed. Engl.. 33 ( 19941 849: A. M011er. H. B(~gge. E. Krickemeyer a,d S. Dillinger. Bull. Pol. Aead. Sci.. 42 ! IL~41 2tll. 141 D. Gatle.~chi. R. Ses....li. W. I%.......A. MOIler. E. Krickem~:yer. J. Meyer. D..S~ller and P. Adler. I,m'g. Cilem...~.~ ( Iq% t 192h. I .~I SHELXTI° PLUS. ~:~°~tall.gmphic ~)..tem. Siemen~ A.alylicul X-ray h~slrum~nt.. Inc.. Madly... Wl. 1~89: G.M Sheldrick. SHELXL-~L~. t!,i~cr~il) of (;iittiugc,. (h:rma,~'. I~)t).~.