E L S E V I E R lnorganica Chimica Acta 274 (t998) 224 228

Note

Synthesis and crystal structure of a seven-coordinate dinuclear iron(III) complex containing only bidentate ligands

Hideaki Matsushima, Kaku Iwasawa, Kouji Ide, Md. Yeamin Reza. Masayuki Koikawa, Tadashi Tokii *

Del)arlntent <~[ CJwmistrv and Applied Chemi.wrv. Fat,try ~![ Sciem'e and En~,ipwerin~. 5a~,a Untversity. Saga 840. Jal)an

Receixed 30 June 1997'. accepled 29 Seplember 1997

Abstract

A novel seven-coordinate dinuclear iron{ III ) complex, [ Fe2( O ) (N()~) 4( bpy )::] ( 1 ) ( bpy = 2,2'-bipyridine ), with a linear Fe O-Fe bridge has been prepared and characterized by X-ray crystallography and magnetic susceptibilities. Each iron atom of 1 has a distorted pentagonal bipyramidal geometry in which four oxygen atoms of two bidentate nitrato ligands and one nitrogen atom of bpy form the pentagonal plane. The other nitrogen atom of bpy and the bridging oxo ion occupy the axial positions. The iron-iron and iron-oxo distances are 3.550( 3 ) and 1.775( I ) A, respectively. The - J value of 104 cm J for 1 indicates the presence of an antiferromagnetic interaction due to superexchange coupling between two iron(lll) ions via an oxo-bridge. © 1998 Elsevier Science S.A. All rights reserved.

Kevu'ordw Seven coordinate complexes; h'on complexes: ('ryslal struclures: BJdentatc ligand complexes: I)inuclear complexes

1. Introduct ion

Several dinuclear i ron(III) complexes with the F e - O - F e core have so far been isolated, and a large number of spectro- scopic, magnetic, and structural studies have been carried out with biological implications for the active site in certain i ron- protein systems I1] . The coordination number of each iron( Ill ) atom in those complexes is usually five or six. The stereochemistry of seven-coordination is still rare in both mononuclear [2 ] and dinuclear 13 ] i ron(I l I ) complexes. To the best of our knowledge only one example of seven-coo> dinate dinuclear i ron(I l l ) complexes with a quinquedentate ligand is known [ 3 ]. In this paper we report on the prepara- tion and the structural characterization of the first example of a seven-coordinate dinuclear i ron(I l l ) complex with only bidentate ligands.

2. Experimental

2. I. Synthesis q[ /Fe_,(O)(NO~)4(bpy)2] (1)

A methanol solution ( 1 0 m l ) of i ron(I l l ) nitrate ennea- hydrate ( 1.2[ g, 3 mmol) was added to a methanol solution

* Corresponding author.

0020-1693/98/$19.00 ¢') 1998 Elsevier Science S.A. All rights reserved, PII S0020- 1693 { 97 ) 06038-6

( 10 ml) of bpy (0.47 g, 3 retool). The earlier precipitates were filtered off, and the filtrate was allowed to stand over- night at room temperature. Dark red crystals were collected by filtration, washed with cold methanol, and dried in vacuo over P,Os. Yield: 0.602 g (58.3%). Anal. Found: C, 35.20; H, 2.44: N, 16.08: Fe, 15.83%. Calc. for CeoH~(,F%NsOi3: C, 34.91: H, 2.34; N, 16.28: Fe. 16.23~/,. IR: t,(NO3) , 1512, 1278, 1264cm ~ 14,5]; u , ,~(Fe-O-Fe) ,861 cm ~16].

2.2. Physical tneasuremetzts

Carbon, hydrogen, and nitrogen analyses were carried out at the Service Center of Elemental Analysis, Kyushu Uni- versity. Iron analysis was made using a titrimetric method. IR spectra were measured with a Perkin-Elmer 2000 FT-IR spectrometer in the region of4000-650 cm ~ on Nujol mulls. The magnetic susceptibilities in the temperature range 80- 300 K were determined by the Faraday method. The suscep- tibilities were corrected for the diamagnetism of the consti- tuent atoms by the use of Pascal 's constants 171. The effective magnetic moments were calculated from Eq. ( I )

#,,,, = 2 .83¢x~,T 1 )

where XA is the atomic magnetic susceptibility'.

H. Matsushima et al. / lnorganica Chimica Acta 274 (19981 224-22,~ 225

Table I Crystallographic data for i Fe=(O) ( NO3)4( bpy )21 ( l )

Formu I a C2~H ~ ~Fe2NsO ~ Formula weight 688.13 Crystal dimensions ( mm I 0.30 × 0.15 x 0.45 Crystal system monoclinic Space group P2,/;1 a (A) 9.312(5) b (A) 12.98114) c (A) 11.020(4) /3 (o) 96.74(3) V ( .~.31 1322.9(8) Z 2 D~,,k (g cm 3) 1.728 Dm (gem ~) 1.71 F(000) 716 /x(MoKoe) (cm ~) 11.79 Temperature ( °C ) 23 Radiation Mo Kc/ ( A = 0.71069 A. ) 2 0,,,~ (~') 45 No. collected data 1957 No. unique data 1828 No. observations ( 1 > 3.00o- (1) ) 978 No. variables 194 R ~ 0.048 R~ h 0.052 Largest peak (e ,~ ~) 0.53

~, R = EIIF,,I - IF~II/F_.IF,,I. b R~ = { F,~*( I f . I - Ib, I )~/~wl f , , l" I n w : 4F,,2/~r2(F,,) e.

2.3. X - r a y c r y s t a l s t r u c t u r e d e t e r m i n a t i o n

The diffraction data were measured on a Rigaku A F C 5 S

automated four-circle dif f ractometer with graphi te-mono-

chromated Mo Kc~ (A = 0.71069 A) radiation. The unit cell

parameters of the crystal were obtained f rom a least-squares

ref inement based on 25 high-angle reflections. The crystal

data are shown in Table 1. The intensities o f three represen-

tative reflections, which were measured after every 150

reflections, remained constant throughout data collection,

indicating crystal and electronic stabilities (no decay correc-

tion was appl ied) . An empir ical absorption correction, based

on azimuthal scans of several reflections, was applied, which

resulted in transmission factors ranging from 0.84 to 1.00.

The data were corrected for Lorentz and polarization effects.

The structure was solved by direct methods [ 81. The non-

hydrogen atoms were refined anisotropically. Ref inements

were carried out by the ful l -matr ix least-squares method

Some of the hydrogen atoms were located from the subse-

quent difference Four ier maps and included in the refinement.

All calculat ions were per formed using the T E X S A N [9]

crystal lographic software package of Molecula r Structure

Corporation. The final posit ional and thermal parameters

along wilh their est imated standard deviat ions are given in Table 2.

Function minimized: *w(IF,,I - IF,.I)2, where w=41:./(re(F~,2). Standard deviation of an observation of unit weight: [~2w(IF, A - I F~. I ) e / (N,, - N, ) 1 ~"=, where N~, = number of observations, N, = number of variables.

Table 2 Positional and equivalent thermal parameters of non-hydrogen atoms for [ Fe:(O)(NOO4(bpy)=] (11

Atom a v : Be,i

Fe 0.8465l 11 0.081511 ) 0.9732( I J 2.38(5) O( 1 ) I 0 1 3 . 0 ( 4 ) 0(2) 0.9633(6~ 0.2184(51 0.9303(6i 3.4(3) 0(3) 0.9411(7~ 0 . 1 9 3 5 1 5 1 1.1198(61 3.8(3t O(4) 1.0721(81 (l.3203( 6} 1.0680(71 5.8(4) O(5) 0.8285(6) 0.0846(6) 0.7782i51 3.6(3) O~6) 0.7179(7! -0.034115) (I.8617(61 4.3(4) 0(7) 0.7253(8) 0.0385(6) (I.6645(71 5.8(4) Ni l ) 0.6557(7) 0.1781(51 0.9766(6t 2.7(4) N(21 0.7451(7) 0.0273(5) 1.1237(61 2.4(31 N(31 0.995218) 0.2469(71 1.0413(81 3.5(4) N(41 0.7547[91 0.0014(7! 0.7632(8) 3.5(4) C(I) 0.609(I) 0.2484( 8 i 0.8921(81 3.6(5) C(2) 0.495(I) 0.3137(8! 0.905(I) 4.6(6) CI 3) 0.428( II 0.3063( 81 1.008( 1 ) 3.8(5) Ci4) 0.474(I) 0.2334(81 1.0959(91 3.3(5) Ci5) 0.587(11 0.1696(7i 1.0789(81 2.5(4) C(61 0.6434(8) 0.0883(7) 1.1644(71 2.5(4) Ct7) 0.5983(9) 0 . 0 7 7 0 { 8 1 1.2801(8) 3.0(4) (7!8) I).651(l) -0.0047{ 8i 1.3516(91 3.9(5 ) C(91 0.748(l) -0.0701(81 1.308( I ) 4.4(5) C([0) 0.794( I1 -0.0521 (71 1.196( I ) 3.7(5)

3 . R e s u l t s a n d d i s c u s s i o n

3.1. X - r a y s t r u c t u r e a n a l y s i s

The molecu la r structure o f 1 is shown in Fig. 1. The

selected bond distances and angles are listed in Table 3. The

molecule consists of two seven-coordinated i r o n ( l l l ) ions

bridged by an oxo ion, and has a center of symmetry at the

oxo ion. Thus, the oxygen atom was taken to be at the special

position ( 1, 0, 1 ) as shown in 'Fable 2. The F e - O - F e bond

angle o f 180 ° is due to the requirement of the cent rosymmetry .

The configurat ion about the iron atom is a distorted pentag-

onal b ipyramid in which the pentagonal plane is composed

of four oxygen atoms ( 0 ( 2 ) , 0 { 3 ) , O ( 5 ) , and 0 ( 6 ) ) of

two bidentate nitrato l igands and one ni trogen atom (N(21) )

of bpy. The axial posit ions are occupied by another nitrogen

atom ( N ( I ) ) o f b p y and the bridging oxo ion ( O ( 1 1 ) . The

Fe . . .Fe and F e - O ..... distances are 3 .550(3) and 1.775( 1 ) ]~,

respectively. The F e - O ..... distance falls in the range 1.73-

1.82 A, observed for usual dinuclear i r o n ( I l l ) complexes

with a single oxygen bridge [1 ,3 ,10-17] . The m a x i m u m

deviat ion of a contributing atom from the O4N least-squares

plane is 0.09 A. The iron atom is deviated by 0.19 i t f rom

the equatorial plane toward the oxo ion. The F e - O distances

in the pentagonal plane are in the range of 2 . 1 3 6 ( 6 ) -

2 .273(61 ,~, comparable with those found for I Ph4As]-

[Fe (NO3)4] [18] and I F e ( T P P ) ( N O 3 ) ] [5] in w h i c h e a c h

central iron (III) ion has been coordinated by nitrato l igands

with a bidentate chelate fashion. The differences between

each pair of the F e - O distance in the nitrate ions are only

226 H. MaLsl¢.s'h#na el al. /Inor,~,anica ('himica kc ta 274 (1998) 224--228

((± ! 0 ( 4 )

it • i ct;7 ) ~ U ' " -

< ~ , , , ~ . , ~ a).lJ / .~N ]q , ,~ )o(3) C(1) C(2)

\ i r o(,) ~ ! g . ,o)L.'. . .,,..c(a) r

~ ; ) o(~ ',~& J ~ c 7 i "" " ,,%;! : > "

C(~ 0) < /~-<: } /0 (8)

cf [ " C(9}

Fig, I, ORTEP drawillg of I wilb 5()5; probability Ihermal ellipsoids.

Table 3 Selected bond distances and angles of { Fed O)( NO~Jd bpy) , [ ( 1 I

Distances ( A ) Fe-()i I )

Fe-()( 2 ) Fc-O( ', ) Fe-O( 51 Fc O( 6 Fc N ( I ) Fe-N( 2 )

Angles ( ' ) O( I )-Fe O(2) O(1~ Fe-O(3) ()(1) {:e-O(51 Oi 1 ) - F c - O ( 6 ) O i l ) Fc-N( I ) O ( I ) Ve N(2) (){21-Fc O(3) O(2J } 'e-O(5) ( ) (2i }:e-t)(6) 0 ( 2 ) I:e-NI1) O ( 2 ) - F e N(2 i OI 3 )--I:e O(5) Oi3 ) Fe -O(6 ) O { 3 l - F e N ( I ) O(31 Fc -N(2 ) 0 ( 5 ) Fc 0 ( 6 )

1.775[ I ) ()(2) N{3} 2.16416) ()(31 N(3! 2.273(6) O(41--N( 3t 2.136(61 0 ( 5 ) N(4~ 2.202(6) O ( 6 ) - N ( 4 i 2.179(7) O ( 7 ) - N t 4 ) 2.123{7) Fe. "Fe "

1.279(9) 1.259(9) 1.21( [,28( 1,261 1,21i 3.550(3)

96.3(2) O(5 ) - F e - N ( I ) 92.212) 91.5(2} O(5J Fe -N(2 ) 143.4(3 / 98.412i O ( 6 ) - F e N ( I ) 9{).6(3~ 93.5(2) O ( 6 ) - F e - N ( 2 ) 87.0(3! 69.3(2i N( I ) - F e - N ( 2 ) 74.9(3 i 95.5(2) Fe O(2) N(31 95.5(5) 57.8(2) Fe-O(3 ) -N(3 ~ 91.015) 75.3(3) Fe-O(5 )-N( 4 ) 95.3(51 33.8(3) F e - O ( 6 ) - N ( 4 ! 92.7t5) 87.9(3) 0 ( 2 ) N(3! O(31 115.5(8) 36.4(3) 0 ( 2 ) N ( 3 1 - O ( 4 ) 121.7(9) 32.9(3) O ( 3 ) - N ( 3 i -O (4 ) 122.8~9)

166.5(3) O ( 5 ) - N ( 4 ) - O ( 6 ) l l 3 .1 (8 i 82.4(2) 0 ( 5 ) N ( 4 ) - O i 7 ) 123II) 80,1(2) O ( 6 ) - N ( 4 ) Oi7 ) 124(1) 58.6(3) Fe O ( I ) - F c " IXO

a SylTllllCtly operalor 2 ~. -. x, 2 2.

0.07 and 0.11 ,~, indicating that both nitrate ions act as a practically symmetric bidentate chelate. The Fe-N( I ) axial bond distance (2 .179(7) ,~ ) is considerably longer than the Fe-Nt 2 ) equatorial bond (2 .123(7) ,~ ) owing to the trans influence of the oxo ion. Although the five angles subtended at the Fe(Ill) ion by pairs of atoms in the pentagonal plane are belween 57.8(2 } and 87.0( 3 ) o with much deviation from the ideal pentagonal angle of 72< the sum for the/ive angles of 358.8 ° reflects the coplanarity of the FeOaN atoms in the equatorial plane. Of the ten cis angles involving O ( I ) and N( l) , the N( l )-Fe N(2) angle is decreased by 15.1 °from 90 ° owing to a concomitant of a narrow bite angle of bpy. The oilier nine cis angles are all within 8.4 ° a r o u n d 90 °.

3.2. Magnetic properties

The magnetic data for 1 are shown in Fig. 2 as plots of molar nmgnetic susceptibility (XA) and magnetic moment

(/x~n) versus temperature. The magnetic moments of 1.84 (291.8 K) and 0.66 (81.3 K) BM indicate the existence of an antiferromagnetic interaction between the two high-spin iron( Ill ) ions. The magnetic susceptibility data for 1 are well represented by Eq. (2) [ t ]

[Nfi2g2~[ 55 + 3 0 x ' " + 1 4 x ' S + 5x 24 + x -'s

x,, = t .t--77-)~ l l + !5.,.'"+ 7 + 5 7 7 3 .÷ ~ + .----T W"

{35N[32 V2~ ( l - , , ) J , (2>

where x represents exp( - J/kT), developed under the isotro- pic exchange Hamiltonian ( ~ = - 2JSt "$2 for S~ = $2 = 5 /2 ) . The fraction of paramagnetic impurity is given by p and was treated as high-spin Fe(l l l ); other symbols have their usual meanings. The best-fit parameters of - ,I = 104 cm i, g =: 2.00, and p = 0.0042 were obtained by a nonlinear least-squares fitting procedure. Gorun and Lip- pard [19] reported that a quantitative relationship exists between the antiferromagnetic exchange coupling ( - J ) and the length (P) of tile shortest superexchange pathway for 36 di- and trinuclear iron( II1 ) complexes with multiple bridges; the relationship was , l=Aexp(BP) , where A = 8 . 7 6 3 × 10 ~l and B = 12.663 are numerical constants. Applica- tion ol' this equation to complex I produced a much greater value of - J = 152 crn i than the experimentally determined value ( 104 cm '). Gatteschi et al. 120] recently proposed a modified equation, - , / = 1.625 × I0 ~ e x p ( - 9 . 3 P ) , esti- mated on the basis of 50 compounds: the calculated - J value of 110 cm ~ using this modified equation is 6°A greater than that determined experimentally. However, the antiferromag- netic interaction is closely related with the shortest Fe-O .... distance of dinuclear iron( III ) complexes with a linear Fe- ()-Fe bridge [ 1,10,11,19 I. Plots of the shortest Fe-O .... dis- tance versus - , / for some singly oxo-bridged diiron(lll) complexes, which are summarized in Table 4, a re shown in

Fig. 3. A linear correlation has been found between the short- est Fe-O ..... distance, and - , I for these complexes. These results suggest that Ihe antilen'omagnetic interaction in the seven-coordinate complex of 1 is consistent with those

2 0 0 0 4 0

1500 4 3 3.0

~°E 1000 2 .0

x

5 0 0 1.0

0 i l , i , i 0 .0

0 100 2 0 0 300

T / K Fig. 2. Temperature dependence of magnetic susceptibilities ( () i and effec- live magnetic moments ( 0 ) lor I.

H. Matsushima et al. /lnorganica Chimica Acta 274 (1998) 224-228

Table 4 A comparison of the structural and magnetic data for the singly oxo-bridged dinuclear iron( Ill ) complexes

227

No. Complex Fe O 1 A) Fe-O-Fe (°) - J ( cm ~ ) CN ~' Re(.

I [Fez(O) (NO3)4(bpy)2] 1.77511 ) 2 i Fe2(O) (H20) 2(pa12,2,2) 2 ] (CIO4)4 h 1.8 3 [ Fe~ (O) ( bbimae ) ~Cl 2 ] ( NO; ) z ~ 1.7816( 7 ) 4 [ Fe2(O) ( H20).~ (Cl-dipic), ] • 4H~O " 1.772 ( 31. 1.773 ( 3 5 { Fe2(O) (TPC)2] ~' 1,747(51, 1.763(5 6 [Fe2(O) (phen)~( H20)2 ] ( NO04" 5H:O ~ 1.78315 ). 1.787( 5 7 (H,en) [Fe2(O) /HEDTAt21 .6H:O ,~ 1.7911). 1.80( I ) 8 [ Fe2(O) ( 3-'Busaltmen ) : ] h 1.77414 ). 1.783 ( 4 9 [ Fe~(O) (FF) ] • H20' 2C,,HsCH~ ' 1.77416 ), 1.80016 ) 10 [ Fe2(O) (N5)CI~ ] C1.2C,HsOH i 1.75114). 1.78214)

181) 1114 7 180 100 7 180 103 6 180 1117 6 180 129 5 155.114) 110 6 165.0( 8 ) 95 6 173.4( 3 ) 100 5 161.1 (4) 108 5 149.8(3) 125 4,6

this work

131 II01 Illl 112] 113l 114]

116] 1171

" CN = coordination number. t' pa12,2,2 : 2,13-dimethyl-3,6,9,12,18 pentaazabicyclo I 12.3.1 ] ocmdeca- I ( 18 ),2.12,14,16-pentane. ' bbimae = 2- l bis(benzimidazol-2-ylmethyl )aminoethanol. ,t H~[ Cl-dipic ] = 4-chloropyridine-2,6-dicarboxylic acid. ' H,TPC = 7,8-dihydro-5,10,15,20-tetraphenylporphyrin. ~ phen= 1,10-phenanthroline.

en = ethylenediamine. H~HEDTA = N-hydroxyethylethylenediaminetriacetic acid. ~' H,I 3-'Busaltlnen } = 2,3-dimethyl-2,3-bis( 3-tert-butylsalicylideneamino)butane. H4FF = N,N-bis] 5- ( o-phenyl ) - 10,15,20-triphenylporphyrin ] urea. N5 = N,N,N '-tris[ ( 2-benzimidazolyl ) methyl ] -N '- ( 2-hydroxyethyl )- 1,2-diaminoethane.

200 Acknowledgements

150

100 ?

50

0 1.72

I I I I I I I I

1.74 1.76 1.78 1.80 1.82

F e - O ~ i~oM) / A

Fig. 3. A magneto-structural correlation between the shortest Fe-O,,~,, dis- tances and the - J values in the singly oxo-bridged dinuclear iron(lIl) complexes. The numbers correspond to those of complexes in Table 4.

observed in six-coordinate dinuclear iron(lIl) complexes, although the usual energy level diagrams for pentagonal- bipyramidal geometry [ 21 ] must be different from those for octahedral geometry.

4. Supplementary material

Additional material lor the structure, available from the Cambridge Crystallographic Data Center, Union Road, Cam- bridge CB2 IEZ, UK, comprises H atoms parameters, thermal parameters, and a complete listing of bonds, angles, and least- squares planes.

This work was partially supported by a Grant-in-Aid for Scientific Research (No. 08454212) from the Ministry of Education, Science and Culture of Japan.

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