J. CHEM. SOC. DALTON TRANS. 1995 707Properties and Reactivity of Unusual Diiron Complexes of aLinear Tetradentate Ligand. Crystal Structures of Diiron-(11)and -(III) Complexes tRita HazeW Kenneth B. Jensen! Christine J. McKenzie *.b and Hans Toftlundba Department of Chemistry, Aarhus University, OK-8000 hhus C, DenmarkDepartment of Chemistry, Odense University, OK-5230 Odense M, DenmarkDinuclear iron complexes containing the tetradentate N, ligand NN-dimethyl-N,N'- bis(2-pyridyl-methyl)-ethane-l,2-diamine (L) have been prepared. The diiron(I1) complex [LFe(O,CMe),FeL]-[CIO,], 1 has been characterized by X-ray crystallography: triclinic, space group PT, with a = 13.756(3),b = 15.033(3), c = 13.637(3) A, a = 97.82(1), p = 108.95(1), y = 118.07(1)* and Z = 2.The structurerefined to a final R value of 0.046 for 3556 reflections. The complex contains two iron(r1) ions bridged bytwo acetate groups, Fe Fe 4.382(2) A. Each acetate bridges the iron atoms in an 0.0' mode using thesyn lone pair of one carboxylate oxygen atom and the anti lone pair of the other. The ligand shows a cis-aconformation about the iron(I1) centres. Complex 1 is readily oxidized in solution by air and from thesesolutions a diiron (111) complex containing a p-0x0-p-acetato core, [ LFeO( 0,CMe) FeL] [CIO,],, wasrecovered. Another diiron(lt1) complex, prepared in the absence of other possible bridging groups, isformulated as [LFeO(OH)FeL] [CIO4],-2H,O, 2. The spectroscopic data for it are consistent with theunusual formulation of a doubly bridged p-0x0-p- hydroxo-diiron (111) core, only the second reportedexample of a small molecule with this core structure.In solution, 2 is sensitive towards carbondioxide, reacting to give [LFe0(CO,)FeL][CIO4],~2H,O, 3. The structure of 3 has been solved byX-ray crystallography and shows a dibridged p-0x0-p-carbonato-0,0' core: monoclinic, space groupC2/c, a = 19.565(8), b = 13.427(2), c = 13.722(9) A, p = 94.31 (3)" and Z = 8. The structure refinedto a final R value of 0.066 for 4575 reflections. The average Fe-(p-0) is 1.804(5) A, Fe-0-Fe is126.1 (3)" and Fe... Fe 3.216(1) A. Again L shows a cis-a conformation. Complex 2 was used toprepare other complexes containing the [LFeOFeLI4+ core: [LFeO(O,CMe)FeL] [CIO,],, [LFeO(O,CH)-FeL] [CIO,], and [LFeO(SO,)FeL] [CIO,],, by addition of acetic acid, formic acid and sodiumhydrogensulfate respectively; and [L(CI) FeOFe(C1) L] [CIO,], by addition of chloride ions.In recent years substantial progress has been made in modellingthe diiron centres in haemerythrin, ribonucleotide reductaseand methane monooxidase.' Conversely, few examples ofstable diiron(I1) and iron(rI)iron(nr) complexes have appearedin the literature.Low-oxidation-state diiron complexes have thepotential to simulate some of the reactivity towards dioxygenshown by these metalloenzymes in their active reduced states.Of the few examples reported, some are based on terminalcapping ligands,2 and others are complexes of dinucleatingligands containing a hingeing alkoxo oxygen atom.3 We areinterested in the preparation of the former class of complexes.Dinuclear complexes of metal ions with a preference foroctahedral co-ordination using the archetypal tripodal tetra-dentate capping ligand tris(2-pyridylmethy1)amine (tpa) havebeen reported showing that both mono- and di-bridged M(p-oxo)M core structures (M = V, Cr, Mn or Fe) are accessible.,The dibridged diiron complexes are model compounds for theactive metallocentre in the fully oxidized B2 subunit ofEscherichia coli ribonucleotide reductase which shows adibridged (p-oxo)(p-acetato) core in its recently reportedstructure.' Que and co-workers 4d-h have employed tpa in someof their work on diiron(rr1) complexes. This has proved a flexiblesystem in which the p-0x0 atom in the diiron(1rr) systems can besupported by a variety of bidentate bridging ligands with a rangeof 'bite' distances [i.e.generating Fe 9 9 Fe distances in therange 3.196(2)-3.402(2) 8, and Fe-0-Fe 125.4(3)-143.4(3)].t Supplementary data available: see Instructions for Authors, J. Chem.Soc., Dalton Trans., 1995, Issue 1 , pp. xxv-xxx.Non-SI unit employed: pB x 9.27 x J T-'.Part of the motivation for the work presented here was tosynthesize complexes of N, ligands which are less capable thantpa in stabilizing higher oxidation states, due to their weakerligand-field strengths. Dimeric iron(rr) complexes of theseligands may therefore be more accessible.We have investigated the diiron complexes of a lineartetradentate N, capping aminopyridyl ligand based on N,N'-bis(2-pyridylmethy1)ethane- 1 '2-diamine (bpen).Our previouswork with iron(r1) spin cross-over systems of the type Fe-(N,)(NCS);! showed that the N-alkylated linear N4 compoundsformed high-spin iron@) complexes below 70 K whereas theiron(@ complex of tpa and bpen remains low spin.6 Theevidently weaker ligand-field strengths of the N-alkylatedbpen ligands suggested to us their suitability in the preparationof low-oxidation-state diiron complexes. In the present studywe use the methyl-subs tituted ligand N, N',dimet hyl-N,N'-bis(2-pyridylmethy1)ethane-1 ,Zdiamine (L). Replacement of the NHgroups of bpen with NMe groups offers the further advantageof eliminating the possibility of formation of very stable low-spin iron(@ diimine complexes via the oxidative dehydrogen-ation of the co-ordinated bpen.We suspected this type ofreaction during our work on the iron(@ complexes of bpen6bwhere the formation of deep purple oxidation products wasobserved on exposure of the solutions to air. Raleigh andMartell ' have observed analogous reactions of amine ligandsin related cobalt(rr) systems.Iron,6b manganese and cobalt complexes of lineartetradentate N4 chelating ligands based on bpen apparentlyhave a high preference for a cis-a co-ordination geometry. Thisgeometry is ideal for the construction of singly and doublybridged diiron core units. We have recently prepared a stabl708 J . CHEM. soc. DALTON TRANS. 1995dibridged (p-oxo)( p-acetato)dimanganese(m) complex of Lwhich lends support to the feasibility of obtaining stable low-oxidation-state complexes of this ligand.l oThe crystal structure of methaemerythrin shows, as well asthe (p-oxo)bis(p-acetato)diiron(iII) core and terminal co-ordination by five histidines, an available site for exogenousions. Under physiological conditions, molecular oxygen bindsat this site as H02-." The structure of fully oxidized R2subunit of ribonucleotide reductase shows one co-ordinatedwater molecule at each iron atom.', The recently reportedstructure of the apoprotein form of this reductase saturatedwith Mn2+ shows a water ligand bound to one of themanganese centres.' The very recent crystal structure ofmethane monooxidase l 4 shows likewise a terminal watermolecule bound to one of the iron(n1) atoms.Besides this anacetate ligand (originating from the crystallization buffermedia) bridging the two iron(rn) atoms indicates the availabilityof exchangeable sites on each iron atom. The vacant co-ordination site and substitution labile aqua ligands in thestructures of these diiron enzymes are presumably of funda-mental importance for their biological activities. A secondobjective of the present work, which may be particularlyrelevant in modelling naturally occurring metalloenzymes, hasbeen the synthesis of dinuclear iron complexes containing labilewater ligands, in attempts to produce compounds withexchangeable and/or reactive sites. The discovery of complexeswhich show reactivity towards dioxygen and/or catalyseoxidation reactions is a desirable outcome of these efforts.Wehave previously isolated p-oxo-diiron(u1) complexes of tripodaltetradentate ligands with additional terminal water ligands l 5and with a hydrogen-bonded aqua-hydroxo (H302 -) bridg-ing group. ' " The complexes [(bpp)(H,O)FeOFe(H,O)(bpp)]-[CIO,], { Hbpp = 3-[bis(2-pyridylmethyl)amino]propionicacid) and [(tpa)FeO(H,0,)Fe(tpa)][ClO4], have been struc-turally characterized by X-ray crystallography; the latter showsinteresting reactivity in that it is capable of promoting thehydrolysis of acetonitrile and triphenyl phosphate. ' 5 bExperiment a1Infrared spectra were measured as KBr discs using a Hitachi270-30 spectrometer, NMR spectra with a Bruker AC 250 FTspectrometer, and UV/VIS absorption spectra on a ShimadzuUV-3 100 spectrophotometer.Electrospray (ES) mass spectrawere obtained using a Finnigan TSQ 710 spectrometer witha combined electrospray and atmospheric pressure chemicalionization source. Samples were 0. I mmol dm-, in acetonitrile.Elemental analyses were carried out at the microanalyticallaboratory of the H.C. Orsted Institute, Copenhagen.CAUTION: The following compounds were isolated asperchlorate salts and were treated as potential explosives.N,N'-Dimethyl-N,N'-bis(2-pyridylmethyl)et hane- I ,2-dia-mine dihydroperchlorate ( L02HC10,) was prepared accordingto the method of Toftlund et a1.6bDi-p-acetato-bis{ [N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-ethane- 1 ,2-diamine] iron( 11) 3 [ LFe( 0 CM e) , -FeL][C104]2 1.-The synthesis was carried out under Arusing standard Schlenk techniques.A mixture of L*2HC104(0.680 g, 1.45 mmol) and triethylamine (0.330 g, 3.27 mmol)dissolved in dry methanol (10 cm3) was added to a stirredsuspension of freshly prepared Fe(O,CMe), (0.250 g, 1.45mmol) in dry methanol (5 cm3). On warming, the Fe(O,CMe),dissolved to give a yellow solution from which a yellowprecipitate was deposited within minutes. The precipitate wasseparated and dissolved immediately in dry acetonitrile.Diffusion of diethyl ether into the acetonitrile solution gaveyellow block-shaped crystals of complex 1. These were collectedand washed with a mixture of acetonitrile and diethyl ether,yield 0.456 g, 65% (Found: C, 44.35; H, 5.10; Cl, 7.45; N, 11.50.C36H5~C12Fe2N80,2 requires C, 44.60; H, 5.20; Cl, 7.30; N,Perchlorate,11.55%).h,,,/nm (MeCN) 274 (&/dm3 mol-' cm-' 14 600), 325(1770), and 383 (2700). m/z 385.2 (M2', 100%); other speciesobserved in the mass spectra are described in the Results andDiscussion section.pHydroxo-p-0x0-bis{ [N,N'-dimethyl-N ,N'-bis(2-pyridyf-methy1)ethane- I ,2-diarnine]iron(111)) Perchlorate, [LFeO(OH)-FeL][CI0,13-2H,0 2.-A mixture of L.2HClO4 (0.376 g,0.8 mmol) and sodium hydroxide (0.094 g, 2.35 mmol)dissolved in ethanol-water (1 : 1, 10 cm3) was added toFe(ClO,),~6H20 (0.529 g, 1.1 mmol) dissolved in ethanol (2cm3). The colour changed to a deep maroon within seconds. Theproduct crystallized as maroon crystals over several days. Thecrystals were collected and washed with ethanol, yield 0.133 g,34% (Found: C, 37.45; H, 4.65; C1, 10.70; N, 10.90.1 1 .OO%).The ES mass spectrum is described in the Results andDiscussion section.C,,H,,CI~Fe2N8Ol6 requires c, 37.70; H, 4.85; CI, 10.40; N,p-Carbonato-p-0x0-bis{ p,N'-dimethyl-N,N'-bis( 2-pyridyl-methyl )ethane- 1,2-diarnine]iron(111)) Perchlorate, [ LFeO(C0,)-FeL][C104]2*2H20 3.-A mixture of L-2HC104 (1.024 g, 2.2mmol) and sodium hydroxide (0.276 g, 6.9 mmol) dissolved inethanol-water (1 : 1, 15 cm3) was added to Fe(C1O4),.6H,O(1.589 g, 3.4 mmol) dissolved in ethanol (1 5 cm3). Over a fewdays green crystals of the product were deposited along with aflocculent precipitate of iron oxide. The bulk of the latter wasremoved by decanting.The green crystals were dissolved inacetonitrile, and the remaining undissolved iron oxide wasfiltered off. The acetonitrile solution slowly evaporated toproduce complex 3 as green needles which were collected andwashed with acetonitrile, yield 0.120 g, 1 I % (Found: C, 4 I .05; H,4.90; c1,7.25; N, 11.65. C,,H48C12Fe2N80,, requires c, 41.15;H, 5.00; Cl, 7.35; N, 11.65%). m/z 364.2 ( M 2 + , 100) and 827.1([A4 + CIO,]', 1.4%). The crystals were suitable for X-raydiffraction studies. Crystal data and details of the structuredetermination are collected in Table 2. A full description of thestructure refinement will not be given here since the crystalstructure was also reported by Arulsamy et ~ 1 . ' ~ during thepreparation of this manuscript.The authenticity of this complex was checked also by itspreparation uia standard self-assembly procedures: a mixture ofL*2HCI04 (0.302 g, 0.60 mmol) and sodium carbonate (0.289 g,2.7 mmol) dissolved in methanol-water (1 : 1, 15 cm3) wasadded to Fe(CI0,),-6H20 (0.308 g, 0.66 mmol) dissolved inmethanol (4 cm3).The resulting green solution was filtered toremove traces of iron oxide and allowed slowly to evaporate.Green crystals of 3 were deposited over a few days. They werecollected and washed with methanol, yield 0.071 g, 12%. Thespectral characteristics of this product were identical to thoseabove.p- Acetato-p-0x0-bis{ [N,N'-dimethyl-N,N'-bis(2-pyridyl-methy1)ethane- 1,2-diamine]iron(111) ] Perchlorate, [LFeO(O,C-Me)FeL][CIO,], 4.-Complex 4 can be prepared by threedifferent methods: (i) the air oxidation of 1 in solution; (ii) thereaction of 2 with acetate ions; (iii) the standard self-assemblyreaction described.The spectral characteristics of the productsobtained by these different methods are identical.A mixture of L.2HClO4 (0.5291 g, 1.1 mmol) and sodiumacetate (0.2352 g, 2.86 mmol) dissolved in methanol (3 cm3)and water (1 cm3) was added to Fe(C10,),-6H20 (0.6086 g,1.3 mmol) dissolved in methanol (2 cm3). The colour changedto green and the product precipitated as fine olive-green/brown crystals. After standing for several hours complex 4was collected and washed with methanol. It was recrystallizedfrom either acetonitrile or acetone-water, yield 0.330 g, 57%(Found: C, 39.80; H, 4.65; C1, 10.30; N, 11.00.C34H47-C13Fe2N8015 requires C, 39.80; H, 4.60; CI, 10.35; N,10.90%). m/z 242.5 ( M 3 + , 100) and 413.1 ( [ M + C10,]2+,18%)J. CHEM. SOC. DALTON TRANS. 1995 709p-Formato-p-0x0-bis{ [N,N'-dimethyl-N,N'-bis(2-pyridyl-methyl )ethane- 1 ,2-diamine] iron(rr1)) Perchlorate, [LFe0(02C-H)FeL][C10,]3 5-Formic acid (0.015 g, 0.32 mmol) inacetonitrile (1 cm3) was added to [LFe0(OH)FeL][C104]3 2(0.04 g, 0.04 mmol) dissolved in acetonitrile (5 cm3) Theresulting green solution was heated at 50°C for 1 h. Theproduct was deposited as a brown microcrystalline product,collected and washed with cold methanol, yield 0.026 g, 64%(Found: C, 38.50; H, 4.15; C1, 10.25; N, 11.05. C33H45-C13Fe,N,015 requires C, 39.45; H, 4.55; CI, 10.65; N,11.20%).m/z 237.7 (M3', 100%) and 406.3 ( [ M + C104]2+,1 8%)-p-Oxo-psulJh.to-bis{ [N,N'-dimethyl-N,N'-bis( 2-pyridyl-methyl )ethane- 1 ,2-diamine]iron(111)} Perchlorate, [LFeO(SO,)-FeL][C104]2 6.-An aqueous solution of sodium sulfate (0.036g, 0.31 mmol) was added to [LFeO(OH)FeL][CIO,], 2 (0.1 g,0.1 mmol) dissolved in water (5 cm3). The resulting greensolution was filtered and left to stand. The product wasdeposited as green plate-like crystals which were collected andwashed with cold methanol, yield 0.068 g, 69% (Found: C, 39.20;H, 4.50; C1, 7.20; N, 11.40; S, 3.25. C3,H,,C12Fe2N80,3Srequires C, 39.90; H, 4.60; C1, 7.35; N, 11.65; S, 3.35%). m/z382.2 ( M 3 + , 100) and 863.1 ([M + C1O4I2+, 0.75).p- 0x0- bis{ chloro p, N'-dimethyl-N, N '-bis( 2-pyridylme t hy1)-ethane- 1,2-diamine]iron(rrr)} Perchlorate, [L(CI)FeOFe(Cl)L]-[C10,]2*0.5H,0 7.-An aqueous solution of sodium chloride(0.2 cm3 of 2 mol dm-3) was added to [LFeO(OH)FeL][C104],3 (0.1 g, 0.1 mmol) dissolved in water (5 cm3).The resulting redsolution was filtered and left to stand. The product wasdeposited as red crystals over 2 d. The crystals were collectedand washed with cold methanol, yield 0.040 g, 43% (Found: C,40.60; H, 4.70; C1, 14.85; N, 11.65. C,2H,5C1,Fe,N809+5requiresC,40.60;H,4.80;Cl, 15.00;N, 1 1.85%).m/z369.2(M2+,100) and 837.1 ( [ M + C104]+, 2%).Dichloro~,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diumine]iron(111) Perchlorate, [FeCl, L]C104-3 H ,O 8.-Toa mixture of L-2HC1O4 (0.196 g, 0.42 mmol) and FeC13*6H20(0.1 17 g, 0.43 mmol) dissolved in water (2 cm3) was addedtriethylamine (ca.0.06 g, 0.6 mmol) to bring the solution to pHca. 4. The product separated as a bright yellow microcrystallineproduct and was collected and washed with water, yield 0.075 g,34% (Found: C, 35.10; H, 4.10; C1, 19.50; N, 10.05. c16-H2,C1,FeN,0, requires C, 34.90; H, 5.10; C1,19.30; N, 10.15%).h,,,/nm (MeCN) 294 (&/dm3 mol-' cm-' 4007) and 367 (3325).Magnetic Studies.-Magnetic susceptibility measurementswere performed by the Faraday method in the temperaturerange 6300 K at a field strength of 1.3 T using instrumentationdescribed elsewhere. ' The variation of susceptibility withtemperature can be described by the equations derived from theHeisenberg-Dirac-Van Vleck model for isotropic binuclearmagnetic exchange interactions (H = - WS,42).1 The molarsusceptibility was corrected for underlying diamagnetism by theuse of Pascal's constants and fitted by a least-squares method,in the case of iron(I11) dimers, by use of equation (1) where2e2x + 10e6" + 28eI2" + 6 0 e 2 O x + 1 IOe3O"I + 3e2X + 5e6x + 7elZx + 9e2OX + 1 1 ~ ~ ~ " +35P - + t.i.p.( I )8Tx = J/kT, p = mole fraction of paramagnetic impurities, andt.i.p. = temperature-independent paramagnetism. For theiron(rr) dimer 1 an analogous equation, taking account of thedifferent spin multiplicity, was used.Resonance- Raman Studies. -Raman spectra were recordedon a Czerny-Turner scanning spectrometer.The samples wereprepared as discs containing 300 mg of KBr, 20 mg of thecomplex and 20 mg of K2S04 for use as internal standard[referenced to v,(SO,~-) 983 cm-'1. Spectra were recorded atthree excitation wavelengths, 457.9,487.9 and 5 14.5 nm.X-Ray Crystallography.-A crystal of complex 1 ofdimensions 0.2 x 0.2 x 0.1 mm was used for the collection ofX-ray diffraction data. Crystal data and details of the structuredeterminations of 1 and 3 are collected in Table 2. Data werecorrected for dead-time losses, background, Lorentz, polariz-ation, and absorption effects (minimum and maximumtransmission 0.91 and 0.94). The structure was solved by directmethods,' and refined by the full-matrix least-squarestechnique.20 Refinement on F with weights w = { [ o ( F 2 ) +1.03F2]* - F}-2.All non-hydrogen atoms were refined aniso-tropically. The hydrogen atoms were included in calculatedpositions after verification by means of a Fourier-differencesynthesis. The positions as well as thermal parameters wererefined for hydrogen atoms of the bridging acetate group; theother hydrogen atoms were constrained to have one commonthermal parameter. The perchlorate groups show large thermalmovement and other signs of disorder. Attempts at refining twoorientations of each perchlorate with idealized geometry and acommon TLX model for the thermal motion resulted insignificantly higher R values. No extinction effects were found.The final shift/e.s.d. was 0.24. Scattering factors includinganomalous dispersion for Fe were taken from ref.21.Fractional atomic coordinates are listed in Table 3, and theco-ordination geometry 22 in Table 4.Additional material available from the Cambridge Crystallo-graphic Data Centre comprises H-atom coordinates, thermalparameters and remaining bond lengths and angles.Results and DiscussionThe syntheses of an unusual diiron(r1) and six new p-0x0-di-iron(r~~)complexesof N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-ethane-I ,Zdiamine (L) are described in the Experimentalsection. Spectroscopic and magnetic properties for the diiron-(111) complexes are listed in Table 1. The integrities of all thediiron complexes were verified using electrospray ionization(ES) mass spectrometry. The poxo-diiron(rr1) complexes 3-7all show clean mass spectra with a peak for the triply or doublycharged cationic complex [LFeO(X)FeL]" + as the mostintense.A peak for the doubly or singly charged ion pair{ [LFeO(X)FeL](ClO,)}" is also observed as a much less-intense peak. Molecular ion peaks are listed in the Experimentalsection.p-Acetato-bridged Diiron-(11) and -(III) Complexes of L.-Anunusual di-p-acetato-diiron(I1) complex [LFe(O,CMe),FeL]-[ClO,], 1 was isolated as yellow crystals from the reactionof L with iron(I1) acetate. The structure solved by X-raycrystallography shows a doubly bridged pacetato-0,O' core,see below. The variation of the magnetic susceptibility versustemperature shows the two iron(1r) centres to be very weaklyantiferromagnetically coupled.The room-temperature mag-netic moment per dimer is 7.5 pB, which drops to 3.8 p, at 5 K.Fitting of the data by the Heisenberg-Dirac-Van Vleck modelfor isotropic binuclear magnetic exchange interactions whereS , = S2 = 2 and, without taking zero-field splitting intoaccount, leads to a - J value of 1.5 cm-'.Complex 1 is readily oxidized in solution: to an 0x0-bridgediron(r1r) complex, which can be observed in the UV/VIS and ESmass spectra. The UV/VIS spectrum of 1 (Experimentalsection) is relatively featureless, however if the acetonitrilesolution is allowed to stand it becomes green and features above400 nm appear in the spectrum. These features are characteristicfor a diiron(rI1) complex containing a bent Fe-O-Fe unit.'" Th710 J.CHEM. SOC. DALTON TRANS. 1995Table 1 Properties of p-0x0-diiron(1n) complexes [LFeO(X)FeL][ClO,],2 3 4 5 6 7X = OH- CO,, - MeCO, - HC0,- so4* - 2c1-Electronic Spectra'LaJnm 339 (5995)( E dm3 mol-' cm-') 374 (5085)438(sh) (1821)460(sh) (993)490(sh) (619)539 (290)562 (601)800(sh) (97)337(sh) (7588) 340 (9780)368(sh) (6070) 367(sh) (7180)437 (1 270) 422(sh) (1 100)464 (1310)505 (662) 495(sh) (860)512 (800)578(sh) (1 13) 543(sh) (230)667 (1 40) 670 (1 60)340 (7500) 319 (10 852) 330(sh) (8963)372(sh) (4900) 375 (7584)428(sh) (800) 427 (1 124)467 (1000) 493 (484)495(sh) (720) 504 (453) 505(sh) (1 93)5 1 4 (600)545(sh) (200) 530 (190)670 (1 50) 629 (159) 535(sh) (164)v(Fe0Fe) Data (cm-')"asym 728 740 743 740Vsym 593 495 487 48 7 474 335Magnetic properties- J/cm-' 42 104 112 104 93 70p/mol fraction 0.004 2 0.002 4 0.006 0 0.003 7 0.005 6t.i.p./cm3 mol-' 0.000 900 0.000 140 0.000 080 0.000 000 0.000 060 0.000 000g 2.00 2.01 2.01 2.10 1.99 2.010.025 0' In MeCN solution.b ~ , s y m from IR spectrum, identified by comparison to the 'fingerprint' pattern of the IR spectrum of the mononuclear iron(Ir1)complex of L, [FeCl,L]ClO, 8; not assigned for complexes 2 and 7; vSym from the resonance-Raman spectrum.10080.h s.- 560.2vacac .-.- c40,a20.0363.7 \LFe'; 'Fe'L IL/"To 1393.1 ,2+m/rFig. 1 Electrospray ionization mass spectrum of complex 1 in acetonitrile together with a structural assignment for the major peaksES mass spectrum, part of which is shown in Fig.1, clearlyshows a peak corresponding to the molecular ion, [LFe(O,-CMe,),FeL],+, as the most intense feature. Howeversignificant peaks are assigned to the various fully and partiallyoxidized derivatives of 1 with the core structures as illustrated.An oxidized derivative of complex 1, the dibridged [LFeO-(O,CMe)FeL][ClO,], 4, was isolated, as green crystals, fromsolutions of 1 exposed to air. Its identity was verified bypreparation by the simpler self-assembly method starting withFe3+, L and sodium acetate, described in the Experimentalsection. The UV/VIS spectral characteristics are very similar tothose reported for the acetato-0x0-bridged tpa analogue ofcomplex 4. The value of 487 cm-' for vsym in the Ramanspectrum is consistent with a Fe-O-Fe angle close to 130°,typical for acetato-0x0-bridged diiron(m) complexes.2Synthesis, Spectroscopic and Magnetic Characterization of theComplex [ LFeO( OH) Fe L] [CIO,] , -2H ,O 2 .-The react ionof L with Fe3+, in the absence of other possible supportingbridging ligands, was carried out with varying amounts of base,in attempts to prepare a complex based on the [LFe-GFeLI4+unit and comprising substitution labile terminal ligands.Theprocedure described in the Experimental section gives marooncrystals which we have formulated as [LFeO(OH)FeL][ClO,],J. CHEM. SOC. DALTON TRANS. 1995 71 12H,O 2. Although unable to determine the crystal structure ofthis complex due to the lack of suitable crystals, we propose thatit contains the unique doubly bridged Fe"',(p-O)(p-OH) corefor the reasons presented below.At the outset of the present work with the linear N, ligand Lwe had anticipated that the preparation of a p-0x0-diiron(m)complex in aqueous solutions in the absence of possiblebridging ligands would lead to the formation of species ofthe type [ L( H , 0)Fe-0-Fe( H ,0)Ll4 + or [ L( H,O)Fe-0-Fe(OH)L] + , similar to those we have characterized previouslywith different terminal tetradentate ligands.' The formulationof 2 as a p-0x0-diiron(rr1) complex of the type [L(H,O)Fe-0-Fe(H,0)LJ4+ could be eliminated on the basis of the elemeptalanalysis which is consistent with three perchlorate counteranions. The presence of a hydroxide was necessary to balancethe charge, however the spectroscopic characteristics suggestedthat the complex, although composed chemically similarlyto [(tpa)(H20)FeOFe(OH)(tpa)][C10,],, contains a fund-amentally different core structure; the Fe-O-Fe unit isapparently much more bent. Hence the formulation of aFe"'(p-O)(p-OH)Fe''' core for 2.The isolation of a complex in which two iron(1rr) atoms arebridged by an 0x0 and a hydroxo group, although postulated iniron(rrr)-oxo-hydroxo aggregation, is extremely rare in di-iron(w) chemistry.This is only the second example reported.The first structurally characterized (X-ray crystallography andextended X-ray absorption fine structure, EXAFS) example of acomplex containing this core was reported during the writingof this manuscript by Zang et al.24 for the structure of[( tmpa)Fe(p-O)(p-OH)Fe( tmpa)] [ClO,] 24 { tmpa = trisC(6-methyl-2-pyridy1)methyl)amine). Their structure gives us con-fidence that our assignment of this structure is valid.Relatedfour-atom, edge-shared structures are known in iron(rr1)chemistry.' For example, dinuclear complexes with the Fe(p-OR),Fe core (R = H, alkyl or aryl) have been reported.lbOther edge-shared structures contain oxygen atoms bridgingmore than two iron atoms and form the skeleton framework ofpolynuclear complexes.2 Since none of these core structurescontains an unsubstituted p-0x0 atom, the magnetic andspectroscopic characteristics of complexes containing thesecore structures do not aid in the interpretation of the spectraland magnetic properties of 2.The M(p-O)(p-0H)M moiety (M = transition-metal ion)is, in fact, surprisingly rare. It has been found in dichromium(rr1)complexes, ' 7b*'6 with only one structurally characterizedexample.26 There are, however, several structurally character-ized examples of the conjugate acid and base of this core, M(pz-OH),M and M(p-O),M respectively, in transition-metalchemistry.The [M(p-OH)2M]4+ core is well known inchromium(Ir1) and cobalt(rr1) chemistry and several complexescontaining the [Fe(p-OH),Fe14+ core have been reported loand two such cores structurally characterized by Thich etal.25S and Borer et aLZ7 in neutral complexes of anionicterminal ligands. The so-called aqua dimer [(H20),Fe(p-OH),Fe(H,0),]4+ has been proposed also to contain a di-p-hydroxo core.' The acid-base equilibria involving the Fe(p-OH),Fe unit and its mono- and di-deprotonated counterpartsFe(p-O)(p-0H)Fe and Fe(p-O),Fe have not been studied.Nostable small molecule containing a Fe(p-O),Fe is so farknown and may be difficult to obtain due to the propensityof Fe"' to form higher-nuclearity 0x0 aggregates and finallyrust in basic media. The M(p-OH),M core is well known inearly transition-metal chemistry. Linear N, ligands based onbpen have been utilized previously to prepare dimanganesecomplexes containing Mn'''(p-0)2Mn1V cores. * The kinetics ofthe protonation reaction of one of the p-0x0 bridges of aMn(p-O),Mn complex has been studied and found to beA diiron(rI1) complex in which a p-0x0 atom is supported byanother one-atom bridge represents an extreme case for the Fe-(p-0)-Fe angles in 0x0-bridged complexes.By comparison to aS ~ O W.dichromium(m) complex containing a [Cr(p-0)(p-OH)Crl3 + core,26 a Fe-(p-0)-Fe angle close to 100.6(2)" is expected for 2.This is ca. 20° more acute than in any previously reported p-oxo-diiron(rr1) complex, la apart from the very recently reportedstructure of [(tmpa)Fe(p-O)(p-OH)Fe( tmpa)] [C104]3 by Zanget al.,,, where, although the p-0x0 and p-hydroxo bridgeswere not distinguished in the crystal structure and theFe-O-Fe and Fe-OH-Fe angles averaged to 98.7(6) A, theirresults using EXAFS inferred a Fe-(p-0)-Fe angle of 106".Sanders-Loehr et al.have used resonance-Raman spectroscopyto predict the Fe-O-Fe angle in a series of p-oxo-diiron(Ir1)complexes containing none, one and two supporting bridginggroups. As the Fe-O-Fe angle decreases in the mono- throughto di- and tri-bridged complexes, the frequency of the symmetricstretch, observable in the Raman spectrum, increases. A linearrelationship between the square of this frequency and the cosineof the Fe-0-Fe angle has been predicted from theory.30 Theresonance-Raman spectrum of 2 shows a strong band at 593cm-',* which, in the absence of other appropriate bands above300 cm-', we assign to the symmetric stretch, v,,,(Fe-O-Fe).This band occurs at a higher frequency than the v,,,(Fe-0-Fe)for any other p-0x0-diiron(rrr) complexes reported.23 In fact, ifthe plot of Fe-O-Fe angle uersus v,,,(Fe-O-Fe) reported bySanders-Loehr et al.,, is extrapolated to vsyY(Fe-O-Fe) 593cm-' a Fe-O-Fe angle of around 100" is predicted.Hence theresonance-Raman spectrum for 2 is consistent with a Fe(p-0)-(p-0H)Fe core structure. In contrast, the symmetric Fe-0-Festretch recorded for the chemically similar [(tpa)(H,O)FeO-Fe(OH)(tpa)][C10,]3,15b 453 cm-', falls in the range recordedby Sanders-Loehr et al. 23 appropriate for the Fe-0-Fe angle of138.9(4)" found in its crystal structure (the Fe-0-Fe unit isforced to bend as a consequence of a hydrogen-bondinginteraction between the cis aqua and hydroxo groups).The magnetic moment recorded for complex 2 decreases withtemperature, showing antiferromagnetic coupling as is usuallythe case in p-0x0 diiron(m) complexes.The fit by an isotropicHeisenberg-Dirac-Van Vleck model, although not completelysatisfactory, gives thecouplingconstant, - J = 42cm-', which isvery low compared to - Jfor other p-0x0-diiron(r1r)complexes. laIn all of these complexes it is the p-0x0 bridge that isaniticipated to provide the major magnetic exchange path-way.31 A p-hydroxo bridge is a poor mediator of electroniccoupling3' and is not expected to contribute a significantpathway between the metal centres in the putative Fe(p-O)(p-0H)Fe core of 2.In general, p-0x0-bridged iron(m) dimers have - J values ofaround 100 cm-' and this coupling constant is found not to becorrelated with the Fe-0-Fe angle.32-33 Although diiron(rr1)complexes containing an Fe-0-Fe angle as acute as thatpostulated for 2, are not included in these studies, we believe thelow value for - J of 42 cm-' is not inconsistent with this lack ofcorrelation (i.e. that the strong antiferromagnetic couplingusually found in p-0x0-diiron(rr1) complexes is not found for 2).A very acute Fe-0-Fe angle is likely to effect a decrease in theK bonding between the iron atoms resulting in a reducedmagnetic exchange coupling. An analogy can be drawn from the0x0-bridged dichromium(w) complexes: the linear 0x0-bridgedbasic rhodo ion [(NH3),CrOCr(NH3),]4+ shows a very highcoupling constant, - J, of 225 cm-'. 34 However the couplingconstant for [(bipy),Cr(p-0)(p-0H)Cr(bipy),][C10,],~4H20(bipy = 2,2'-bipyridyl) is much less, 30 cm-'.' 7 bThe electronic spectrum of complex 2 in acetonitrile showsdistinctive differences in comparison to the other p-0x0-diiron(rr1) complexes of L, >7 (Table l), and of other capping* The intensity of this band is enhanced ten times by the h = 457.9 nmlaser line as compared to the h = 514.5 nm line. The enhancementprofile for 2 is the reverse of the resonance enhancement found for theother complexes in this series which show the most enhancement fromthe lowest-energy laser line (A = 514.5 nm) of the three used712ligands l a and lends further support for the structural proposal.Previously bands in the 550-700 nm region of the electronicspectra of multiply bridged p-0x0-diiron(xr1) complexes (wherethe supporting bridges are 0x0 acids, i.e.are three-atombridging groups) have been attributed to an 0x0-to-iron charge-transfer transition, the energy of which depends on the Fe-O-Fe angle. These bands are expected to be blue shifted as the Fe-0-Fe angle increase^.^^,^^ The blue shift can be attributed to anincreased II bonding between the iron atoms and the p-0x0atom as the Fe-0-Fe angle approaches 180°, thereby decreas-ing the basicity of the 0x0 bridge and the Lewis acidity of theiron centres. Consequently the gap between the ligand donorand metal acceptor orbitals widens. We assign a shoulder at 800nm in the spectrum of an acetonitrile solution of 2 to this oxo-to-iron charge-transfer transition. The considerable red shift ofthis band, compared to the h,,, assigned for this transition forthe other p-0x0-diiron(ir1) complexes in Table 1 (667 nm for thep-0-p-C0,-bridged 3, 670 nm for the p-0-p-0,CR-bridged 4and 5; 629 nm for the p-0-p-SO,-bridged 6; 535 nm for the p-0x0-bridged 7) is consistent with a much more acute Fe-0-Feangle.Of particular note is a band at 562 nm ( E 601 dm3 mol-'cm-') in the spectrum of 2. We believe this is assignable to oneof the bands usually observed in the 400-550 nm region ofthe electronic spectra of multiply bridged p-0x0-diiron(rr1)complexes. These features are blue shifted and/or lose intensityas the Fe-O-Fe angle increases. The red shift to above 550 nmof this h,,,, and a gain in intensity, is, therefore, consistent witha decrease of the Fe-O-Fe angle.As with other p-oxo-diiron(m)complexes, there is a band in the 400-550 nm region of mediumintensity at 490 nm. This has been assigned to a 6Al --+ (,E,,A,) transition, which is independent of the Fe-O-Feangle.35Reactivity of [LFeO(OH)FeL][ClO,],.2H,O 2 iowardsNucieophi1es.-The ES mass spectrum of complex 2 supportsthe structural proposal for a Fe(p-O)(p-0H)Fe core. Aspectrum is shown in Fig. 2. Assignments are indicated. Thepeak at m/z 228.3 is assigned to the molecular ion, [LFeO-(OH)FeLI3+ and that at m/z 392.2 to the ion pair ([LFeO-(OH)FeL]C10,}2+. The peak at m/z 342.2 is assigned to the100-80 -A$ Y s c *g 60-a, 3+0 228.3LFe<O> FeL ]H 1J. CHEM. SOC. DALTON TRANS. 1995deprotonated derivative of 2, namely [LFe(p-O)2FeL]2 +.Thefacile removal of one proton suggests a unique environment forthat proton, i.e. that the hydrogen atom of the postulated p-OHbridge has been lost (removal of a ligand proton under the mildconditions of the experiment is extremely unlikely). In fact,in other spectra, the peak for the doubly charged [LFe-(p-0)2FeL]2+ was usually more intense than for that for thetriply charged [LFeO(OH)FeLI3 + ion. However the relativeintensities of the peaks in the mass spectra do not reflect theconcentration of the species in an equilibrium mixture, andlower charged species usually give more intense peaks. Perhapsmost notable is the 100% peak at m/z 242.2 which we assign tothe acetamide-bridged complex, [LFeO(HNOCMe)FeLI3 + .Although we have not isolated this complex in the solid state wehave precedence for its formation by hydrolysis of the solventacetonitrile promoted by complex 2: we have previouslyobserved this type of reaction with [(tpa)(H,O)FeOFe(OH)-(tpa)][C10,]3.1 5b The assignment is supported by the absenceof this peak in the spectrum of 2 recorded in methanol.Thisresult is an indication of the reactivity of complex 2 insolution.In order to record the spectrum in Fig. 2 it was necessary toprepare the sample under Ar. If this precaution was not takenthe spectra usually showed an intense peak at m/z 364.0 whichwe assign to the product of the reaction between complex 2 andcarbon dioxide. The sensitivity of 2 towards carbon dioxide,evident in the mass spectra, had been forecasted by attemptedrecrystallizations of 2.Products, which proved to be mixtures,showing bands in their IR spectra recognized to be carbonate-derived were usually isolated from attempted recrystallizations.On some occasions green crystals of the pure p-oxo-p-carbonato bridged complex [LFeO(C03)FeL][C10,]2~2Hz03 were recovered. It appears that 2 reacts readily with carbondioxide to give 3. The formulation of the latter was verified byits synthesis by the self-assembly reaction of L, Fe3+ andC03'- which gives a product with identical spectral features tothose of the product derived from the reaction of 2 with carbondioxide. The ES mass spectrum of 3 is clean and corroboratesthe assignment of a peak due to contamination by [LFeO-(C03)FeLIZ + in the spectrum of 2.The structure of 3 has been2\ 342.2 I 392.2 I /160 200Fig. 2 Electrospray ionization mass spectrum of complex 2 in acetonitrile together with a structural assignment for the major peakJ. CHEM. soc. DALTON TRANS. 1995 713"400 600 800WnmFig. 3 The UV/VIS spectra of a solution of complex 2 and 1equivalent of acetic acid in MeCN. Successive spectra were recordedwith 8 min intervals except for the final spectrum which was recordedafter 3 h. The spectra on the right have been magnified ten times. Initialand final parameters for 2 and 4 are given in Table 1solved by X-ray crystallography and shows a doubly bridgedp-0x0-p-carbonato-O,O' core, see below.Substitution of the Bridging p-OH in [LFeO(OH)FeLJ-[C1O4],*2H2O 2.-One objective of the work here in syn-thesizing diiron complexes containing water and/or hydroxoligands is to provide exchangeable sites in preformed Fe-O-Fecores.Hence, we utilized complex 2 as a starting material forother p-0x0-diiron(r1r) complexes of L. We have succeeded insubstituting the p-hydroxo group to give both mono- and di-bridged complexes in the present study.The addition of acetic or formic acid to a solution of complex2 in acetonitrile gives the acetato-bridged [LFeO(O,CMe)-FeL][ClO,],, 4, or the formato-bridged [LFeO(O,CH)FeL]-[ClO,], 5. The addition of sodium hydrogensulfate to asolution of 2 gives the sulfato-bridged complex, [LFeO-(SO,)FeL][ClO,], 6. The substitution reactions were moni-tored in solution by the changes in the UV/VIS spectrum.Theacetate-substitution reaction in acetonitrile over 3 h, is shown inFig. 3. Interestingly, the reaction of 2 with acetic acid is slowcompared to that of [(tpa)(H20)FeOFe(OH)(tpa)][C10,], togive [(tpa)FeO(O2CMe)Fe(tpa)][C1O4], which occurs withinseconds.'5b The differences in the rates of the acetate-substitu-tion reactions of 2 and the two complexes probably reflectthe structural differences in their diiron cores. A plausiblemechanism for the acetate-substitution reaction is illustrated inScheme 1. The p-hydroxo bridge in 2 is required to undergo abridge-cleavage step a --+ b, to generate the labile speciesrepresented by b, which rapidly eliminates water to give theacetate-bridged complex c.The bridge-cleavage step may firstinvolve a protonation of the bridging hydroxide group whichends up as a terminal water ligand. An alternative pathway,particularly in aqueous solvent, may be uia the transientformation of a solvated species d. This is probably not thecase in the reaction followed spectrophotometrically in Fig. 3in which the concentration of water is negligible and theco-ordination of acetate ion is apparently favoured over theco-ordination of acetonitrile. The formation of the acetamide-bridged complex is much slower than the formation of theacetate-bridged complex, as no sign of the acetamido-complexis apparent from electronic spectra of solutions of 2 inacetonitrile after 3 h.The formation of an aqua-hydroxo species d in Scheme 1,solv = H20, is probably the reason that solutions of complex 2in acetonitrile lose their characteristic UV/VIS spectrum whenwater is added and the characteristic features above 500 nmdisappear.This effect is consistent with a disruption of thep-hydroxo bridge by the co-ordination of a water ligand to oneasolv OHFe-0-FeI I0 OH2 I IFe-0-FebMeO A O I \Fe, ,Fe0CdScheme 1yOH in complex 2 by p-MeCO,-. soh = Solvent. ( i ) MeC02HProposed reaction mechanism for the substitution of theof the iron atoms and the hydroxide ligand becoming a terminalligand on the adjacent iron atom, i.e. the Fe-0-Fe unitapproaches linearity resulting in a less detailed electronicspectrum. Clearly this type of aquahydroxo species is accessiblein p-0x0-diiron(1n) complexes since we have isolated andstructurally characterized one in the tpa series.'5b The fact thatwe are unable to isolate this core structure using L as thecapping ligand under the same preparative conditions for whichit can be isolated with tpa, suggests that the different stericrequirements of the capping ligand is an important factor indetermining which core structure is formed.An influencingfactor evident in a space-filling model for 2 is that a four-atomcore may be particularly favoured over a linear p-0x0 core dueto x-stacking interactions of the pyridyl groups of adjacentligands in the cis-a conformation. The pyridyl groups are forcedinto a severe and repulsive interaction if the p-OH is cleavedand the Fe-O-Fe angle is opened up.A time-consumingrearrangement of the linear N4 ligands may be enforced in orderto accommodate a terminal ligand on each iron centre. The factthat the aqua and hydroxo groups in [(tpa)(H,O)FeOFe-(OH)(tpa)][ClO,], are cis to each other and strongly hydrogenbonded is probably necessary for the stabilization of theterminal hydroxide ligand, which until the characterizationof [(tpa)(H2O)FeOFe(0H)(tpa)][ClO4], was unknown iniron(n1) chemistry. The cis arrangement of the aqua andhydroxide ligands with core structure d, solv = H20, using Las a capping ligand may be unfavourable due to the stericrequirements of the ligand.We can envisage several factors to explain a slow bridge-cleavage reaction.From space-filling models of complex 2,assuming a cis-a-conformation for both ligands {similar to theconformation found for most complexes of bpen-derivedligands * *' and notably for [LMn(p-O), MnL] [C104] 3, 36 whichcontains a four-atom Mn202 core unit] it can be seen that anattack by an incoming ligand will be severely impeded by thesterically demanding N-methyl groups of each ligand. A similarsteric hindrance by bulky N-methyl groups has been noted byWieghardt et aZ.37 in a study of hydroxo-bridge cleavage in thetriply bridged dicobalt trio1 complexes of 1,4,7-triazacyclo-nonane (tacn) and its 1,4,7-trimethylated derivative. Apparentlycleavage of one of the p-OH bridges to give the correspondingdiaquabis(p-hydroxo) species is possible only when unsubsti-tuted tacn is the capping ligand.Complex 2 can also be used to prepare a complex containingan unsupported p-0x0 bridge.The addition of chloride ions tosolutions containing 2 gives [L(Cl)FeOFe(Cl)L] [ClO,], 7,which was isolated as a red crystalline solid. The spectroscopicevidence suggests a linear p-0x0-bridged core structure for 7.The ES mass spectrum shows that, in solution, the chloride ionsare co-ordinated, rather than as counter anions, consistent withterminal chloro ligation, similar to the structure found for thecorresponding tpa complex, [(tpa)ClFeOFeCl(tpa)] [C104]2for which we have determined the crystal structure.'5714 J. CHEM. SOC. DALTON TRANS. 1995There are distinctive differences in the UV/VIS and Ramanspectra of singly and doubly bridged p-oxo-diiron(II1) com-plexes.As discussed above with reference to complex 2, theposition of v,,,(Fe-0-Fe) in the Raman spectra gives anindication of the Fe-0-Fe angle. The band assigned to v,,,(Fe-0-Fe) of 7 occurs at the lowest frequency of the complexeslisted in Table 1, consistent with an expected Fe-0-Fe angleclose to 180". The UV/VIS spectrum of 7, in comparison tothose for the dibridged complexes 2-6, is relatively featureless,consistent also with an almost linear Fe-0-Fe unit.The magnetic exchange coupling constant, - J = 70 cm-', islower than expected for a linear 'p-0x0 bridge. In the absence ofa crystal structure determination for complex 7 we do not wishto comment extensively on this result.However intuition tells usthat the source of the reduced coupling may be due to the strainevident in space-filling models of a linear 0x0-bridged complex,not only with regard to the possible clashes between adjacentpyridyl groups, but also particularly close repulsive interactionsbetween the bulky cis chloro and N-methyl groups at each ironatom. A possible outcome of these steric demands may be anelongation of the Fe-(p-0) bonds, consequently reducing the 7coverlap between the 7c-bonding orbitals of the p-0x0 and ironatoms. This should effect a reduction in the magnetic exchangeinteraction. A lower bond order and consequently lower forceconstants for each Fe-(p-0) bond is consistent also with thelow vsym assigned from the Raman spectrum of 7.The value of335 cm-' is lower than for any of the mono 0x0-bridgedcomplexes reported by Sanders-Loehr et al. 23Crystal and Molecular Structure of [LFe(p-O,CMe),FeL]-[ClO,], 1.-The crystal structure of the complex consists of[LFe(pO,CMe),FeL]Z + cations and perchlorate anions. Aview of the cation is shown in Fig. 4. The iron atoms, linkedby two acetato-O,O' bridges, are approximately octahedrallyco-ordinated to the four nitrogen atoms of L and two acetatooxygen atoms. Important interatomic distances and angles aregiven in Table 4. The ligand shows the cis-Cr configuration aboutboth iron centres, in which the axial ligation is provided by thepyridine groups and the equatorial ligation by the aminenitrogens and the bridging acetato oxygen atoms.The com-pound is isomorphous to [LMn(p-OZCMe)zMnL][C104]zthough described in a different cell.38 Each acetate groupbridges the iron atoms in an 0,O' mode using the syn lone pairon one carboxylate oxygen and the anti lone pair on the other.The Fe" - Fe" distance is 4.382(2) A. A similar arrangementfor the p-acetato bridges was observed in a diiron(I1) complex oftpa.2g Interestingly, a structural hypothesis for the diiron centreof reduced B2 from E. coli ribonucleotide reductase has beenproposed recently, based on multifield saturation magnetiz-ation measurements, to contain a di(p-carboxylato) core unitsimilar to that in l.39 The recent structural determination of thecorresponding manganese(I1)-substituted reductase shows adoubly bridged dimanganese(1r) core where the two bridgingcarboxylates are derived from Glu-238 and Glu-1 1 5.13 Theshort Mn".Mn" separation of 3.6 compared to theFe". Fe" distance in 1 probably indicates a syn-synbidentate bridging by the bridging carboxylate groups in theMn-substituted apoprotein.Crystal and Molecular Structure of [LFeO(CO,)FeL]-[C1O4],~2H,O 3.-The crystal structure of the complexconsists of [LFe(p-O)(p-CO,)FeL]'+ cations, perchlorateanions and solvent water. A view of the cation is shown in Fig.5. The iron atoms, linked by the p-0x0 and p-carbonato-O,O'bridging groups, are octahedrally co-ordinated to the fournitrogen atoms of L, a carbonate oxygen atom, and the bridgingoxygen atom. Important interatomic distances and angles aregiven in Table 5.Again the ligand shows the cis-a conformationabout both iron centres, in which the axial ligation is providedby the pyridine groups and the equatorial ligation by theamine nitrogens and the bridging 0x0 and acetato groups. AUC(43)Fig. 4scheme. Hydrogen atoms are omitted for clarityDrawing of [LFe(O2CMe),FeLl2+ 1 showing the numberingFig. 5scheme. Hydrogen atoms are omitted for clarityDrawing of [LFeO(CO,)FeL]'+ 3 showing the numberingcomparison by means of half-normal probability plots 40 to therecently published crystal structure determination of the samecompound'6 shows that the bond lengths found in the twoinvestigations agree as could be expected within the standarddeviations given. However, the coordinates agree less well,the y values being systematically 0.0045 different.The otherdifferences seem random but about three times as large asexpected from the standard deviations. The structure is built oflayers normal to the c direction with water between, andchanges in water content may possibly cause the layers to shiftslightly.ConclusionPerhaps the most significant result of this work is the successfulisolation and characterization of a simple diiron(Ir1) complexcontaining a [Fe(p-O)(p-OH)Fel3 + core. This core mayrepresent an important link in the formative process of largeiron oxohydroxo aggregates such as rust and the core off e r ~ - i t i n . ~ ~ ~ . ~ ' The present work with the linear N, ligand Ltogether with our earlier work with other tetradentate cappinJ.CHEM. SOC. DALTON TRANS. 1995 715Table 2 Crystal data and details of structure determinations for [LFe(p-O,CMe),FeL][CIO4], 1 and [LFe(p-O)(p-C03)FeLJ[C104J2~2H20 3FormulaMCrystal systemSpace groupQlAb/A 4a/OPI”Y/”U ~ A 3ZDJg cm-3Crystal dimensions/mmDiffractometerF(OO0)h, k, 1 rangesp/cm-’Scan type20m*xl”ReflectionsRR’1C3,H soC12Fe2N8Ol 2969.475TriclinicPI-13.756(3)15.033(s)13.637(3)97.82( 1)108.95( I )118.07(1)2208.6(5)21.460.2 x 0.2 x 0.1HUBER four-circle10080-16, - 14 to 12, - 16 to 148.400- 2046.061 573556 [I > 3o(I)]0.0460.052* Details in common: 295 K; Mo-Ka radiation (h = 0.710 69 A); graphite monochromator.3963.403Monoclinic1 9.565( 8)13.427(2)32.722(9)C33H48C1,Fe2N801 4a / c94.31(3)8571(5)81.350.2 x 0.2 x 0.3Enraf-Nonius CAD-440008.656.097560.0660.069- 24 to 24,0-14,042w4575 [I > 20(1)]Table 3 Atomic coordinates for [LFe(p-02CMe),FeL][C10,1, 1X-0.741 63(8)- 0.476 22(8)- 0.583 O(4)-0.763 l(4)- 0.692 5(6)-0.740 9(10)- 0.572 4(4)- 0.508 l(4)- 0.520 2(6)-0.461 6(14)- 0.608 8(5)- 0.668 7(6)-0.751 8(7)-0.773 6(7)-0.712 5(7)- 0.630 5(6)-0.559 5(7)-0.321 7(5)-0.319 6(7)- 0.220 8(8)-0.120 8(8)-0.119 6(6)- 0.220 5(6)- 0.227 7(6)- 0.832 O(4)- 0.873 4(6)- 0.934 4(6)-0.956 8(6)-0.916 6(7)-0.851 2(6)Y-0.749 06(7)- 0.739 02(7)-0.859 O(3)-0.882 3(3)-0.914 2(5)- 1.028 6(7)-0.610 9(3)-0.618 7(3)- 0.570 3(5)-0.450 4(7)-0.808 3(4)-0.766 9(5)- 0.8 17 O(8)-0.91 1 8(8)- 0.954 5(6)- 0.900 9(6)- 0.943 O(5)-0.665 O(5)-0.701 6(6)- 0.647 O(8)-0.552 3(7)-0.514 6(6)-0.571 9(5)-0.534 O(5)- 0.7 18 3(4)-0.773 6(5)-0.755 l(6)-0.676 5(7)-0.619 O(6)-0.639 3(5)-0.484 lO(7)- 0.190 64(7)- 0.343 3(4)-0.438 3(4)-0.414 9(5)- 0.476 3( 10)-0.369 7(4)- 0.200 5(4)- 0.266 7(6)- 0.21 5 8(9)- 0.124 2(4)- 0.097 4(6)- 0.056 3(6)- 0.043 O(6)- 0.068 6(6)-0.108 2(6)-0.138 4(6)-0.230 9(5)-0.325 l(7)-0.348 l(7)- 0.269 4(9)-0.173 2(8)- 0.153 8(6)-0.050 3(6)- 0.394 4(4)-0.333 3(6)-0.280 O(6)-0.292 2(6)-0.357 l(6)-0.404 l(5)X-0.795 6(6)-0.663 5(4)-0.544 2(6)- 0.497 9(7)-0.577 6(8)-0.700 3(7)-0.741 2(6)-0.871 6(6)-0.789 7(5)- 0.700 O(6)-0.914 O(6)- 0.926 6(6)- 0.907 3(4)- 1.018 l(6)-0.31 1 5(5)- 0.340 4( 7)- 0.257 4(6)-0.354 2(6)-0.389 7(7)- 1.016 9(2)- 0.892 6(5)- 1.091 7(6)- 1.032 5(7)- 1.045 4(7)-0.787 3(2)-0.799 3(12)-0.890 6(7)-0.688 4(8)- 0.789 3(9)-0.440 5(5)Y-0.575 O(5)-0.781 9(4)-0.722 l(5)-0.745 l(6)-0.833 2(7)-0.895 5(6)-0.867 8(5)-0.934 3(5)- 0.638 6(4)- 0.569 6(6)- 0.709 O(6)- 0.805 6(5)- 0.868 6(4)-0.937 3(5)- 0.623 2(4)- 0.58 1 O(6)- 0.684 4(6)-0.795 8(6)-0.855 6(4)-0.901 O(6)-0.784 9(2)-0.713 4(4)-0.801 5(6)-0.877 8(6)- 0.735 4(8)-0.287 8(2)-0.302 3(14)-0.368 9(7)-0.287 O( 10)- 0.202 8(7)-0.469 O(6)- 0.586 2(4)- 0.563 6(6)-0.631 l(7)-0.727 6(7)- 0.750 6(6)-0.680 O(5)-0.698 2(5)-0.555 9(5)-0.590 6(6)- 0.650 8(6)-0.717 5(6)-0.648 5(4)- 0.636 2(6)- 0.026 4(4)0.056 6(6)0.01 1 9(6)-0.005 3(6)-0.124 l(4)-0.179 4(7)-0.01 1 9(2)0.064 3(6)0.040 2(5)- 0.060 O(9)- 0.086 7(7)-0.320 O(2)-0.418 8(8)-0.316 l(10)- 0.256 5(7)-0.275 8(10)ligands l 5 show that the core arrangements of p-oxo-iron(II1)complexes containing terminal hydroxo and water ligands canbe tuned by an appropriate choice of capping ligand, and this inturn controls/prevents the growth of larger polyoxohydroxo-iron(II1) species.The differences in the reactivities of the chemically similarcomplexes [LFeO(OH)FeL][ClO,], 2 and C(tpa)(H,O)FeO-Fe(OH)(tpa)][ClO,],’ 5b is noteworthy.Both complexes areable to promote the hydration of acetonitrile to give acetamide-bridged complexes, however the latter shows no sensitivitytowards carbon dioxide and undergoes rapid acetate sub-stitution. In contrast, complex 2 reacts readily with carbondioxide and undergoes relatively slow substitution reactionswith carboxylic acids. These differences, along with spectro-scopic differences, lead us to the assignment of fundamentallydifferent core structures in these two otherwise chemicallysimilar complexes. An outcome of these different corearrangements will be that the Lewis acidity of the iron atoms aswell as the Lewis basicity of the p-0x0 atom will be greater inthe p-0-p-OH complex 2, compared to the p-0-p-HOHO716 J.CHEM. SOC. DALTON TRANS. 1995Table 4 Important distances (A) and angles (") for [LFe(p-O,CMe), LFe] [C104] 1singly bridged p-0x0-diiron(rI1) complexes was prepared also bysubstitution of the p-hydroxo bridge of 2.Fe( 1) - - - Fe(2)Fe( 1 )-O( 1 ) 2.061(4) Fe(2)-O(2) 2.095(4)Fe( 1)-0(3) 2.064(4) Fe(2)-O(4) 2.092(4)Fe( 1 t N ( 1 1 2.185(5) Fe(2)-N(3) 2.152(5)Fe( 1 tN(2) 2.1 81(6) Fe(2)-N(4) 2.150(5)Fe( 1 )-N( 5) 2.261(5) Fe(2)-N(7) 2.283(5)Fe( 1 )-N( 6) 2.256(5) Fe(2)-N(8) 2.277( 5)4.382(2)O( 1 )-Fe( 1)-0(3)O( 1 )-Fe( 1 )-N( 1 )O( 1 )-Fe( 1)-N(2)O( 1 t F e ( 1)-N(5)O( 1 )-Fe( 1)-N(6)O( 3tFe( 1 )-N( 1 )O( 3)-Fe( 1)-N(2)O( 3)-Fe( 1 )-N( 5)O(3)-Fe( 1)-N(6)N( 1 )-Fe( 1)-N(2)N(2)-Fe( 1)-N(5)N( 2)-Fe( 1)-N(6)N( 1 )-Fe( 1 )-N( 5)N( 1 )-Fe( 1 )-N(6)N(5)-Fe( 1)-N(6)108.4(2)9 6 .O( 2)90.7(2)159.4(2)86.9(2)90.8(2)93.2( 2)87.8(2)1 60.7( 2)170.7(2)75.4(2)98.4( 2)96.4(2)75.6(2)80.3(2)0(2kFe(2)-0(4)O( 2)-Fe( 2)-N( 3)0(2tFe(2)-N(4)0(2)-Fe(2)-N(7)0(2)-Fe(2tN(8)0(4)-Fe(2)-N( 3)0 ( 4 t W 2 t N ( 4 )0(4)-Fe( 2)-N( 7)0(4)-Fe(2)-N(8)N(3)-Fe(2)-N(4)N( 3)-Fe(2)-N(7)N( 3)-Fe( 2)-N( 8)N(4)-Fe(2)-N(7)N(4)-Fe(2)-N(8)N(7)-Fe(2)-N(8)109.5(2)93.1(2)89.3( 2)160.3(2)86.8(2)90.7(2)92.5(2)87.5(2)1 60.1 (2)175.1(2)76.3( 2)1 00.1 (2)1 00.2( 2)75.8( 2)78.9(2)Table 5 Important distances (A) and angles (") for [LFe(p-O)(p-C0,)FeL][CI04],~2H,0 3Fe( 1) - - Fe(2) 3.2 1 6( 1) Fe(2)-0( I ) 1.807(5)Fe( 1 )-O( 1 ) 1.800(5) Fe(2)-O(3) 1.929( 5)Fe( 1)-0(2) 1.932(5) Fe(2)-N(5) 2.149(6)Fe( 1 t N ( 1 ) 2.154(6) Fe(2)-N(6) 2.21 5(6)Fe( 1 )-N( 2) 2.224(5) Fe(2)-N(7) 2.279(6)Fe( 1 tN(3) 2.255( 7) Fe( 2)-N( 8) 2.165(7)Fe( 1 W ( 4 ) 2.160(7)126.1(3)102.4(2)97.1 (2)96.5(2)168.6(2)96.1(2)90.9(2)157.9(2)84.0(2)95.0( 2)75.4(2)92.2(2)164.0(3)79.3(2)94.2( 3)73.8(2)1 02.7( 2)96.6(2)95.4( 2)168.1(2)96.4(2)91.8(2)159.3(2)8 5.5(2)92.8(2)76.0(2)91.7(2)164.9(2)78.3(2)95.1(2)74.4(2)complex [(tpa)(H2O)Fe0Fe(OH)(tpa)lCC10,1,.This will beparticularly important to their respective reactivities. Theapparent reactivity of 2 towards carbon dioxide illustrates thispoint. The mechanism of this reaction may involve either anattack by the very basic pox0 atom on the carbon atom ofcarbon dioxide or nucleophilic attack by an oxygen atom ofcarbon dioxide at one of the Lewis-acidic iron centres.Thesteric constraints of the capping ligands have apparentlyprevented the further nucleation to give species with an oxygenbridging between three iron(n1) atoms, but on the other handhave allowed a reaction with carbon dioxide to occur.Peaks that can be assigned to various species formed from theoxidation of [LFe11(p-02CMe)2Fe11L]2 + 1 in solution havebeen identified by ES mass spectrometry. These include onemixed-valence 0x0-bridged Fe"Fe"' complex. However it wasonly possible to isolate the p-0x0-p-acetato-diiron(m) complex4 from these solutions.Complex 4 as well as other doubly andAcknowledgementsThis work was supported by a grant from the Danish researchcouncil (1 1-9227 to H. T. and C. J. 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