A novel dimer of oxo-di(acetato)-bridged manganese(III) dimers complex of potential biological...

Ž .Inorganic Chemistry Communications 3 2000 361–367www.elsevier.nlrlocaterinoche

ž / ž /A novel dimer of oxo-di acetato -bridged manganese III dimers complexof potential biological significance

Rafael Ruiz a, Claudio Sangregorio a, Andrea Caneschi a,), Patrizia Rossi b, Ana B. Gaspar c,Jose A. Real c,1, M. Carmen Munoz d˜

a Dipartimento di Chimica, UniÕersita di Firenze, 50144 Florence, Italy`b Dipartimento di Energetica, UniÕersita di Firenze, 50139 Florence, Italy`

c Departament de Quımica Inorganica, UniÕersitat de Valencia, 46100 Burjassot, Valencia, Spain´ ` `d Departamento de Fısica Aplicada, UniÕersidad Politecnica de Valencia, 46071 Valencia, Spain´ ´

Received 5 May 2000

Abstract

Ž . w Ž . Ž . xŽ . Ž .Assembly of the tetranuclear oxomanganese III acetato cluster Mn O O CMe phen BF from the dinuclear oxo-di acetato -4 2 2 7 2 4Ž . w Ž . Ž . Ž . xŽ .bridged manganese III species Mn O O CMe H O phen BF P3H O in aqueousracetic acid MeOH solution occurs via the2 2 2 2 2 2 4 2 2

III w Ž . Ž .Ž . xŽ . w Ž . Žnew ‘dimer of dimers’ Mn complex Mn O O CMe H O phen BF PMeOH possesing an unprecedent Mn m-O m-2 2 3 2 2 4 4 2. Ž . . . . xO CMe m-OH O CMe core. q 2000 Elsevier Science S.A. All rights reserved.2 4 2 2 2

Keywords: Carboxylato complexes; Cluster compounds; Crystal structures; Manganese; N ligands; O ligands

The bioinorganic chemistry of manganese has been oflarge interest for the structural and functional modeling of

w xthe active sites of a diverse class of Mn redox enzymes 1 .Prominent among these enzymes are those involved in theproduction of dioxygen from its various reduced deriva-

Ž .tives, such as the various Mn catalases Cat in bacteria,that catalyze the disproportionation of hydrogen peroxideŽ . w xa two electron process 2 , or the O -evolving center2Ž . Ž .OEC of Photosystem II PSII in green plants and algae,

Žwhich catalyzes the photosyntetic oxidation of water a. w xfour-electron process 3 . Interestingly, the OEC of PS II

is also capable of undergoing a catalase-type reactionw xwhen O evolution is blocked 4 . Beyond these intriguing2

functional interrelationships, some stricking structural sim-ilarities exist between the dinuclear Mn site in Cats and theproposed tetranuclear Mn site in the OEC of PSII, in spiteof their different nuclearities.

In the oxidised form of Mn Cat, whose definitive X-rayw xcrystal structure has been only very recently solved 5 , the

) Corresponding author. Caneschi: Tel.: q39-055-354-841; fax: q39-055-354-845.

Ž .E-mail addresses: [email protected] A. Caneschi ,Ž [email protected] J.A. Real .

1 Corresponding author. Tel.: q34-96-386-4300; fax: q34-96-386-4322.

Ž .Mn active site assembly consists of two manganese III2

ions bridged by one carboxylate from a glutamate proteinresidue, one oxo group and one water molecule. Oneadditional water, two carboxylates from glutamate proteinresidues and two imidazoles from histidine residues com-plete the first coordination spheres of the Mn ions. The

w Ž .Ž .ŽMn–Mn distance within this Mn m-O m-OH m-2˚. xO CR Mn dimer core is 3.14 A. By contrast, in the2

Ž .‘superoxidised’ mixed-valence dimanganese III,IV form˚of the enzyme the Mn–Mn distance of 2.7 A, estimated

w xfrom X-ray absorption spectroscopic studies 6 , is consis-w Ž . Ž . xtent with a Mn m-O m-O CR Mn core.2 2

Although no crystallographic data are yet avalaible forPSII, structural characterization of the OEC has, neverthe-less, relied on a number of spectroscopic and biophysicalmethods. Based on X-ray absorption spectroscopic studiesof the S and S states of the OEC, the Mn cluster have1 2 4

Ž .been proposed to be an asymmetric dimer of di oxo -bridged manganese dimers joined by a oxo-carboxylato-

˚bridge, with Mn–Mn distances of 2.7 and 3.3 A and withvarious combinations of high valent MnIII and MnIV oxida-

w xtion states 7–11 . The coordination environment of thew Ž . Ž .Žpostulated dimer-of-dimers Mn m-O Mn m-O m-2

. Ž . xO CR Mn m-O Mn core complex of the OEC is com-2 n 2

pleted by oxygen donor atoms of carboxylate groups fromw xglutamato or aspartato protein residues 12–14 and at least

1387-7003r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S1387-7003 00 00099-X

( )R. Ruiz et al.r Inorganic Chemistry Communications 3 2000 361–367362

w xtwo water or hydroxo groups 15,16 , but also one imida-w xzole nitrogen donor atom from a histidine residue 17,18 .

Further, one of these carboxylate ligands has been assignedas being hydrogen bonded to a coordinated H O molecule2w x19 .

A major contribution to the elucidation of the structureof the Mn cluster of the OEC and, hence, to gain addi-4

tional understanding of the assembly process of the highvalent Mn cluster in PSII that occurs during photoactiva-

w xtion 20,21 has been provided by biomimetic studies.Thus, assembly of small high-valent oxomanganese clus-ters relevant to the OEC has been achieved with the use of

X Ž .N-donor diimine-type ligands like 2,2 -bipyridine bipyŽ . w xand 1,10-phenanthroline phen 22–25 . Further efforts in

this area have led to attempts to modelize the carboxylate-rich Mn environment actually found in the Mn active site4

of the OEC, but also in the Mn active site of the various2w xMn Cats 26 . It has been shown by the groups of Girerd

and Christou that dinuclear andror tetranuclear oxoman-Ž . Žganese III carboxylate complexes Scheme 1, forms A

.and C, respectively are easely obtained by the compropor-tionation reaction between MnII and MnOy with RCOOH4

Ž w xin the appropiate solvent aqueous MeOH 27 or non-w x.aqueous MeCN 28 and in the presence of bipy as

terminal ligand. The tetranuclear species can also be pre-pared by the reaction of preisolated trinuclear oxoman-

Ž . w x Žganese III carboxylato complexes 29 Scheme 1, form. w xB with bipy in MeCN media 30 . In the past, these small

Ž .oxomanganese III carboxylato clusters were proposed asw xsuitable structural and functional models for Mn Cats 31

w xand the OEC 32 . We report herein our results with therelated phen ligand in order to give further insight into the

Scheme 1.

aggregation process in this family of biologically relevantoxo-carboxylato-bridged manganese clusters.

Ž . wReaction of ‘manganese III acetate’ prepared in situŽ .. xfrom Mn MeCO 4H O and KMnO with phen in aque-2 2 2 4

Ž .ousracetic acid MeOH solution Girerd’s procedure inthe presence of NaBF affords the polynuclear manga-4

Ž . w Ž . Ž .nese III complexes of formulae Mn O O CMe H O2 2 2 2 2Ž . xŽ . . Ž . w Ž . Ž .phen BF 3H O 1 , Mn O O CMe H O2 4 2 2 2 2 3 2Ž . xŽ . Ž . w Ž . Ž . xphen BF MeOH 2 and Mn O O CMe phen2 4 4 2 2 7 2Ž . Ž . 2BF 3 , whose structures have been solved by single-4

Ž . 3crystal X-ray diffraction see Table 1 .Compounds 1 and 3 are analogous to the previously

w Ž . Ž . Ž . xreported complexes Mn O O CMe H O bipy2 2 2 2 2 2Ž . w x w Ž . Ž . xŽ .ClO 28 and Mn O O CMe bipy ClO P3H O4 2 4 2 2 7 2 4 2w x Ž .30 , respectively. Thus, 1 is a dinuclear manganese III

w Ž .Ž . x2qcomplex containing the Mn m-O m-O CMe core2 2 2Ž .depicted in Scheme 1 form A , with a Mn–Mn separation

˚Ž .of 3.153 2 A which is very close to that reported for theoxidised form of Mn Cat. Chelating phen acting as termi-nal bidentate ligand and one water molecule complets thetetragonally distorted octahedral coordination sphere at

Ž . Ž .each Mn atom Fig. 1 . 3 is a tetranuclear manganese IIIw Žcomplex possesing the common butterfly-type Mn m -4 3

. Ž . xq Ž .O m-O CMe core shown in Scheme 1 form C ,2 2 7

2 Ž .Synthesis and selected data for 1–3: to a solution of Mn MeCO P2 2Ž . Ž 3. Ž4H O 0.50 g, 2 mmol in MeOH 20 cm charged with MeCO H 32 2

3. Žcm and cooled to 08C in an ice-bath was added aqueous KMnO 0.143.M; 5.0 cm under stirring. To the resulting deep red–brown solution

Ž . Ž .phen 0.50 g, 2.5 mmol and NaBF 0.33 g, 3 mmol were added in4

solid, and the reaction mixture was further stirred for 15 min at 08C. Slowevaporation of the dark-brown solution in air at room temperature af-forded, after a few days, a small quantity of well-shaped large black–brown prismatic crystals of 2 which were filtered on paper and air driedŽ .5% . A second crop of black–green hexagonal crystals of 1 appeared ina large amount from the filtered dark-brown solution over night. They

Ž .were filtered on paper and air dried 35% . Upon standing in air at roomtemperature for several weeks, the initial dark brown solution becamebrown–reddish while small dark red rhombic crystals of 3 were depositedfrom the final deep red concentrated solution. They were also filtered on

Ž . Ž .paper and air dried 10% . Satisfactory chemical analyses C, H, N werey1 Ž . Ž .obtained for 1–3. n rcm KBr pellets : 3400s OH from H O,max 2

Ž . Ž . y1572vs CO asymmetric and 1386s CO symmetric from MeCO ,2 2 2Ž . Ž .and 757m MnO from Mn O for 1; 3427m OH from H O, 1584vs,2 2

Ž . Ž . Ž .1574vs CO asymmetric and 1410vs, 1390 sh CO symmetric from2 2y Ž .MeCO , and 700m MnO from Mn O for 2; 1620vs, 1576s, 1544s2 2

Ž . Ž . Ž . yCO asymmetric and 1392vs, 1365 sh CO symmetric from MeCO ,2 2 2Ž .and 662s MnO from Mn O for 3.4 2

3 Crystal data for 1–3: The crystal data for 1, 2 and 3 are summarizedin Table 1. Data were collected using a Enraf-Nonius CAD-4 diffractome-

Ž .ter with graphite-monochromated Mo K a radiation ls0.71069 for 1and 2, and using a Siemens P-4 diffractometer with graphite-monochro-

Ž .mated Cu K a radiation ls1.54180 for 3. Lorentz and polarizationeffects and absorption correction based on c scan were carried out. Thestructures were solved by direct methods and refined by least-squares onF 2. All non-hydrogen atoms were refined anisotropically. The hydrogenatoms were calculated at fixed distances and refined with an overallisotropic thermal parameter.

( )R. Ruiz et al.r Inorganic Chemistry Communications 3 2000 361–367 363

Table 1Crystallographic data for 1, 2 and 3

1 2 3

Formula C H B F Mn N O C H BF Mn N O C H BF Mn N O28 32 2 8 2 4 10 31 31 4 2 4 9 38 37 4 4 4 16

M 868.06 800.29 1112.29Crystal system monoclinic triclinic triclinicSpace group P2 rn P-1 P-11

˚Ž . Ž . Ž . Ž .a A 10.175 1 11.741 2 9.429 5˚Ž . Ž . Ž . Ž .b A 17.960 3 11.938 3 13.011 5˚Ž . Ž . Ž . Ž .c A 19.837 2 12.827 2 19.447 5Ž . Ž . Ž .a 8 – 81.73 2 70.930 5Ž . Ž . Ž . Ž .b 8 95.474 8 78.84 1 88.610 5Ž . Ž . Ž .g 8 – 66.10 2 81.390 5

3˚Ž . Ž . Ž . Ž .U A 3608.3 8 1608.3 5 2228.7 16y3Ž .d g cm 1.598 1.653 1.657

Z 4 2 2Ž .F 000 1752 816 1124

y1Ž .m mm 0.799 0.870 0.982Ž .T K 293 293 293

Unique refl. 4709 5650 7061w Ž .xObs. refl. I)2s I 4661 5579 4792

Parameters 489 550 614w Ž .xR I)2s I 0.081 0.050 0.061

w Ž .xR I)2s I 0.189 0.118 0.141w

with terminal chelating phen ligands completing the tetrag-onally distorted octahedral coordination sphere at each

Ž . Ž . Ž .‘wing-tip’ Mn 1 and Mn 2 atoms Fig. 2 . Alternatively,the Mn O ‘butterfly’ core of 3 can be viewed as resulting4 2

w Ž .Ž . x2qfrom the fusion of two Mn m-O m-O CMe units2 2 2

Ž .through two oxo linkages between the ‘hinge’ Mn 3 andŽ .Mn 4 atoms and three additional acetato bridging groups

w Ž . Ž .xqleading to the Mn m-O m-O CMe central core and2 2 2w Ž .Ž .x2qthe two Mn m-O m-O CMe peripheral ones. The2 2

Mn–Mn separations distances fall into two groups differ-

ŽFig. 1. Perspective view of the cationic dinuclear unit of 1 with the atom-numbering scheme thermal ellipsoids are drawn at the 30% probability level and˚. Ž . Ž . Ž . Ž . Ž .hydrogen atoms have been omitted for clarity . Selected bond distances A and angles 8 with standard deviations in parentheses: Mn 1 –O 1 1.799 7 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 1 –O 3 2.192 7 , Mn 1 –O 5 1.946 7 , Mn 1 –O 6 2.287 8 , Mn 1 –N 1 2.083 8 , Mn 1 –N 2 2.067 8 , Mn 2 –O 1 1.784 6 , Mn 2 –O 2Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .1.947 7 , Mn 2 –O 4 2.162 7 , Mn 2 –O 7 2.271 8 , Mn 2 –N 3 2.064 8 , Mn 2 –N 4 2.091 8 ; O 1 –Mn 1 –O 3 95.3 3 , O 1 –Mn 1 –O 5Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .99.2 3 , O 1 –Mn 1 –O 6 91.6 3 , O 1 –Mn 1 –N 1 169.1 3 , O 1 –Mn 1 –N 2 91.3 3 , O 3 –Mn 1 –O 5 89.9 3 , O 3 –Mn 1 –O 6 172.8 3 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 3 –Mn 1 –N 1 90.4 3 , O 3 –Mn 1 –N 2 91.4 3 , O 5 –Mn 1 –O 6 86.8 3 , O 5 –Mn 1 –N 1 90.1 3 , O 5 –Mn 1 –N 2 169.3 3 , O 6 –Mn 1 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 1 83.2 3 , O 6 –Mn 1 –N 2 90.7 3 , N 1 –Mn 1 –N 2 79.2 3 , O 1 –Mn 2 –O 2 99.8 3 , O 1 –Mn 2 –O 4 93.3 3 , O 1 –Mn 2 –O 7 94.1 3 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 1 –Mn 2 –N 3 170.7 3 , O 1 –Mn 2 –N 4 91.3 3 , O 2 –Mn 2 –O 4 92.0 3 , O 2 –Mn 2 –O 7 83.4 3 , O 2 –Mn 2 –N 3 89.3 3 , O 2 –Mn 2 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 4 167.6 3 , O 4 –Mn 2 –O 7 171.8 3 , O 4 –Mn 2 –N 3 88.4 3 , O 4 –Mn 2 –N 4 92.9 3 , O 7 –Mn 2 –N 3 84.8 3 , O 7 –Mn 2 –N 4 90.4 3 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 3 –Mn 2 –N 4 79.5 3 , Mn 1 –O 1 –Mn 2 123.3 4 .


ŽFig. 2. Perspective view of the cationic tetranuclear unit of 3 with the atom-numbering scheme thermal ellipsoids are drawn at the 30% probability level˚. Ž . Ž . Ž . Ž .and hydrogen atoms have bee omitted for clarity . Selected bond distances A and angles 8 with standard deviations in parentheses: Mn 1 –O 1

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .2.186 5 , Mn 1 –O 10 2.142 5 , Mn 1 –O 12 1.927 5 , Mn 1 –O 16 1.824 4 , Mn 1 –N 1 2.105 6 , Mn 1 –N 2 2.061 6 , Mn 2 –O 4 2.148 5 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 2 –O 6 1.917 4 , Mn 2 –O 7 2.192 5 , Mn 2 –O 15 1.846 4 , Mn 2 –N 3 2.053 5 , Mn 2 –N 4 2.081 5 , Mn 3 –O 2 1.971 4 , Mn 3 –O 3Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .1.946 5 , Mn 3 –O 5 2.194 5 , Mn 3 –O 13 2.131 5 , Mn 3 –O 15 1.890 4 , Mn 3 –O 16 1.910 4 , Mn 4 –O 8 1.941 4 , Mn 4 –O 9 1.933 5 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 4 –O 11 2.246 5 , Mn 4 –O 14 2.154 5 , Mn 4 –O 15 1.908 4 , Mn 4 –O 16 1.906 4 ; O 16 –Mn 1 –O 12 98.5 2 , O 16 –Mn 1 –N 2Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .173.2 2 , O 12 –Mn 1 –N 2 88.2 2 , O 16 –Mn 1 –N 1 94.2 2 , O 12 –Mn 1 –N 1 166.8 2 , N 2 –Mn 1 –N 1 79.2 2 , O 16 –Mn 1 –O 10Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .93.5 2 , O 12 –Mn 1 –O 10 92.9 2 , N 2 –Mn 1 –O 10 84.8 2 , N 1 –Mn 1 –O 10 90.0 2 , O 16 –Mn 1 –O 1 92.4 2 , O 12 –Mn 1 –O 1 94.8 2 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 2 –Mn 1 –O 1 88.3 2 , N 1 –Mn 1 –O 1 81.0 2 , O 10 –Mn 1 –O 1 169.5 2 , O 15 –Mn 2 –O 6 98.0 2 , O 15 –Mn 2 –N 3 174.1 2 , O 6 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 2 –N 3 87.8 2 , O 15 –Mn 2 –N 4 94.4 2 , O 6 –Mn 2 –N 4 167.3 2 , N 3 –Mn 2 –N 4 79.7 2 , O 15 –Mn 2 –O 4 89.9 2 , O 6 –Mn 2 –O 4Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .97.5 2 , N 3 –Mn 2 –O 4 89.8 2 , N 4 –Mn 2 –O 4 84.7 2 , O 15 –Mn 2 –O 7 92.7 2 , O 6 –Mn 2 –O 7 97.0 2 , N 3 –Mn 2 –O 7 86.0 2 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .N 4 –Mn 2 –O 7 80.0 2 , O 4 –Mn 2 –O 7 164.7 2 , O 15 –Mn 3 –O 16 81.8 2 , O 15 –Mn 3 –O 3 95.1 2 , O 16 –Mn 3 –O 3 171.8 2 , O 15 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 3 –O 2 172.4 2 , O 16 –Mn 3 –O 2 93.9 2 , O 3 –Mn 3 –O 2 90.0 2 , O 15 –Mn 3 –O 13 93.9 2 , O 16 –Mn 3 –O 13 86.1 2 , O 3 –Mn 3 –

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 13 86.6 2 , O 2 –Mn 3 –O 13 92.0 2 , O 15 –Mn 3 –O 5 91.3 2 , O 16 –Mn 3 –O 5 98.7 2 , O 3 –Mn 3 –O 5 88.9 2 , O 2 –Mn 3 –O 5Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .83.1 2 , O 13 –Mn 3 –O 5 173.4 2 , O 16 –Mn 4 –O 15 81.5 2 , O 16 –Mn 4 –O 9 97.6 2 , O 15 –Mn 4 –O 9 176.3 2 , O 16 –Mn 4 –O 8Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .173.7 2 , O 15 –Mn 4 –O 8 94.5 2 , O 9 –Mn 4 –O 8 86.8 2 , O 16 –Mn 4 –O 14 90.1 2 , O 15 –Mn 4 –O 14 88.8 2 , O 9 –Mn 4 –O 14Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .87.6 2 , O 8 –Mn 4 –O 14 94.6 2 , O 16 –Mn 4 –O 11 88.1 2 , O 15 –Mn 4 –O 11 99.3 2 , O 9 –Mn 4 –O 11 84.2 2 , O 8 –Mn 4 –O 11Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .87.9 2 , O 14 –Mn 4 –O 11 171.3 2 , Mn 2 –O 15 –Mn 3 123.9 2 , Mn 2 –O 15 –Mn 4 126.3 2 , Mn 3 –O 15 –Mn 4 97.4 2 , Mn 1 –O 16 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 4 124.5 2 , Mn 1 –O 16 –Mn 3 130.3 2 , Mn 4 –O 16 –Mn 3 96.8 2 .

˚ ˚Ž .ing by almost 0.5 A, i.e., 2.853 2 A for the Mn O planar2 2˚Ž . Ž .diamond core and in the range 3.296 2 –3.388 2 A for the

various Mn O bent cores, a similar situation to that pro-2

posed for the OEC.Compound 2 constitutes a novel structural type in oxo-

w xmanganese carboxylato chemistry 25,26 . The structure ofŽ .2 consists of cationic dinuclear manganese III units,

w Ž . Ž .Ž . xq w Ž .xMn O O CMe H O phen Fig. 3 a , tetrafluorob-2 2 3 2 2

orate anions and methanol molecules of crystallization.Ž . ŽThe asymmetric cationic unit contains a m-oxo di m-

. Ž .acetato dimanganese III bridging core with a differentligand environment at the two crystallographically inde-pendent manganese atoms. One manganese is coordinatedto a monodentate acetato group and the other to a watermolecule; the distorted octahedral coordination sphere at

each metal ion is completed by a terminal bidentate phenligand, one oxo and two symmetric acetato bridges. TheJahn–Teller distortion takes the form of an elongation

Ž . Ž . Ž . Ž . Ž . Ž .along the N 2 –Mn 1 –O 5 and O 2 –Mn 2 –O 7 axes,Ž . Ž .thus the Mn 2 –O 7 bond distance from coordinated wa-

Ž . Ž .ter is clearly larger that the Mn 1 –O 6 one from mon-˚w Ž . Ž . xodentate acetato 2.224 3 and 1.990 3 A, respectively .

For complex 2, unlike the related symmetrical species 1,the terminal acetato and water groups are cis to the Mn O2

w Ž . Ž . Ž . Ž .plane the O 6 –Mn 1 –Mn 2 –O 7 torsional angle isŽ . x7.3 2 8 due to the occurrence of a very weak hydrogen

w Ž .... Ž . Ž .bond between them the O 7 O 8 separation is 3.121 7˚ x Ž .A . Otherwise, within the Mn O O CMe bridging unit2 2 2

of this asymmetric complex, the Mn–O bond distances˚w Ž . Ž . x w Ž . x1.801 3 and 1.805 3 A , the Mn–O–Mn angle 121.5 2 8


Ž . ŽFig. 3. a Perspective view of the cationic dinuclear unit of 2 with the atom-numbering scheme thermal ellipsoids are drawn at the 30% probability level˚. Ž . Ž . Ž . Ž .and hydrogen atoms have bee omitted for clarity . Selected bond distances A and angles 8 with standard deviations in parentheses: Mn 1 –O 1

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .1.801 3 , Mn 1 –O 3 2.028 3 , Mn 1 –O 5 2.115 3 , Mn 1 –O 6 1.990 3 , Mn 1 –N 1 2.115 3 , Mn 1 –N 2 2.251 3 , Mn 2 –O 1 1.805 3 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 2 –O 2 2.144 3 , Mn 2 –O 4 1.983 3 , Mn 2 –O 7 2.224 3 , Mn 2 –N 3 2.091 3 , Mn 2 –N 4 2.120 3 ; O 1 –Mn 1 –O 3 92.97 12 , O 1 –Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .Mn 1 –O 5 100.59 12 , O 1 –Mn 1 –O 6 94.26 13 , O 1 –Mn 1 –N 1 173.33 12 , O 1 –Mn 1 –N 2 97.30 12 , O 3 –Mn 1 –O 5 88.48 13 ,

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 3 –Mn 1 –O 6 171.91 12 , O 3 –Mn 1 –N 1 87.63 12 , O 3 –Mn 1 –N 2 88.97 12 , O 5 –Mn 1 –O 6 93.76 13 , O 5 –Mn 1 –N 1 86.06 12 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 5 –Mn 1 –N 2 162.03 12 , O 6 –Mn 1 –N 1 84.78 13 , O 6 –Mn 1 –N 2 86.51 12 , N 1 –Mn 1 –N 2 76.06 13 , O 1 –Mn 2 –O 2 91.86 12 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 1 –Mn 2 –O 4 101.63 13 , O 1 –Mn 2 –O 7 88.46 12 , O 1 –Mn 2 –N 3 170.62 13 , O 1 –Mn 2 –N 4 92.75 13 , O 2 –Mn 2 –O 4 91.74 13 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 2 –Mn 2 –O 7 179.42 12 , O 2 –Mn 2 –N 3 90.27 12 , O 2 –Mn 2 –N 4 93.85 13 , O 4 –Mn 2 –O 7 87.72 12 , O 4 –Mn 2 –N 3 87.44 13 ,Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .O 4 –Mn 2 –N 4 164.39 12 , O 7 –Mn 2 –N 3 89.50 12 , O 7 –Mn 2 –N 4 86.62 12 , N 3 –Mn 2 –N 4 77.98 13 , Mn 1 –O 1 –Mn 2 121.5 2 .

Ž . Ž .b Schematic view of the bis-binuclear entity in 2 showing the hydrogen-bonding network dashed lines .

and, particularly, the intramolecular Mn...Mn separation˚w Ž . x Ž .3.147 10 A are similar within the 3s criterion to those

of the symmetric analogue.In the crystal lattice, two cationic binuclear units of 2

are related through an inversion center giving rise to aw Ž .xremarkable bis-binuclear entity Fig. 3 b . The uncoordi-

nated oxygen atom of the monodentate acetato group fromone Mn unit is hydrogen-bonded to the manganese-bound2

wwater molecule from another Mn unit, and vice-versa the2

... I ˚Ž . Ž . Ž . x Ž .O 7 O 8 separation is 2.670 2 A Isyx,yy,1yz .Thus, from a structural point of view 2 is best described as

Ž .... Ž I .a ‘dimer-of-dimers’ complex. The Mn 1 Mn 2 ,Ž .... Ž I . Ž .... Ž I .Mn 1 Mn 1 and Mn 2 Mn 2 separations within this

w Ž . Ž . Žu n p reced en ted M n m -O m -O C M e m -4 2 2 4... . x2q Ž . Ž .OH O CMe bridging core are 7.784 2 , 9.404 3 and2 2 2

˚Ž .7.250 2 A, respectively. Finally, the methanol solventmolecule is hydrogen-bonded to the bridging oxo group

w Ž .... Ž .and the coordinated water molecule the O 9 O 1 and


... ˚Ž . Ž . Ž . Ž .O 9 O 7 separations are 2.890 8 and 2.973 15 A, re-xspectively .

In summary, we have found an unusual example inŽ .which carboxylato oxomanganese III clusters of different

nuclearities, complexes 1–3, can be isolated from the samereaction mixture owing to their different solubility proper-

w xties 33 . Interestingly, compound 2 represents a novelŽ . Ž .example of a dimer of m-oxo di m-carboxylato man-

Žganese dimers joined by an unprecedented di m-.aquoracetato bridging network. As a matter of fact, there

are few examples of ‘dimer-of-dimers’ complexes inw xbiomimetic manganese chemistry 34–36 that could be

Ž .relevant to the dimer of di m-oxo manganese dimersŽmodel proposed for the OEC of PSII, where a m-

. Ž .oxo di m-carboxylato bridge has also been invoked.In addition, the isolation of 2 provides a direct evidence

that dinuclear Mn O species like 1 are intermediate in the2

formation of tetranuclear Mn O products such as 3, as4 2w xpostulated earlier by Christou et al. 28 for the bipy-con-

taining analogues. Although the detailed mechanism ac-counting for the formation of 3 from 1 via the intermediate2 is not yet fully understood, we propose the mechanisticpathway depicted in Scheme 2 based on our own observa-tions and previous studies on assembly of oxomanganese

w xclusters 25,37 . The initial two steps are waterracetateligand substitution equilibrium favoured by the knownlability of MnIII ion, followed by carboxylate-assysted

Ž .dehydration to form a tetrameric complex non-isolated , aprocess that is already well known in Mn carboxylatechemistry. The next step is protonation of ligated phen inexcess acetic acid media with acetato ligand replacingphen and concomitant formation of m -oxo linkages be-3

tween the coordinatively non-saturated five-coordinateMnIII ions, thus yielding 3 as final product. Indeed, undersimilar reaction conditions but with related bipy as ligandwe have been able to isolate and structurally characte-

Ž . wrize the tetranuclear manganese III complex Mn O -4 2Ž . Ž . xŽ ..O CMe bipy ClO HbipyClO , where bipy is also2 7 2 4 4

Scheme 2.


w xpresent in its protonated non-coordinated form 38 . Fur-ther studies are planned in order to clarify the exact role ofpH on the ligand protonationrdeprotonation equilibria andthe effect of other factors like ligand concentration,waterrmethanol solvent ratio and even counter-ion in thisassembly process.

1. Supplementary material

Tables of atomic coordinates, bond lengths and angles,and thermal parameters for 1–3 are avalaible from theauthors on request.

Acknowledgements

This work was supported by the DGICYT, MinisterioŽ .de Educacion y Ciencia Spain through project PB97-1397.´

ŽR. Ruiz thanks the 3MD-EU Network contract no. ERB.4061, PL97-0197 for a grant.

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A novel dimer of oxo-di(acetato)-bridged manganese(III) dimers complex of potential biological...

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