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Cite this: DOI: 10.1039/c4dt03666b

Received 29th November 2014,Accepted 16th January 2015

DOI: 10.1039/c4dt03666b

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Synthesis and first use of pyridine-2,6-diylbis-(pyrazine-2-ylmethanone) in metal clusterchemistry: a {MnIII

3Na2} complex with an idealtrigonal bipyramidal geometry†

Dimosthenis P. Giannopoulos,a Cody Wilson-Konderka,a Kevin J. Gagnon,b

Simon J. Teat,b Albert Escuer,c Costa Metallinos*a and Theocharis C. Stamatatos*a

The successful organic synthesis of a new dipyrazole/pyridine-dicarbonyl organic molecule, namely pyri-

dine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO(pz)], followed by its employment in Mn coordi-

nation chemistry has yielded the neutral cluster compound [Mn3Na2O(N3)3(L)3] (1), where L2− is the (pz)C-

(CH2COCH3)(O−)(py)C(CH2COCH3)(O

−)(pz) dianion. The latter group was formed in situ, presumably by

the nucleophilic attack of the carbanion −CH2COCH3 to the carbonyl carbon atoms of (pz)CO(py)CO-

(pz), in the presence of Mnn+ ions under basic conditions and in solvent Me2CO. Complex 1 possesses an

almost ideal trigonal bipyramidal topology, with the two NaI ions occupying the apical positions and the

three MnIII ions residing in the equatorial trigonal plane. The bridging ligation about the metal ions is pro-

vided by a μ3-O2− ion and six μ-OR− groups from the L2− ligand, while peripheral ligation is completed by

three terminal azido groups and the pyridine N and carbonyl O atoms of L2−. Magnetic susceptibility

studies revealed the presence of predominant antiferromagnetic exchange interactions between the para-

magnetic MnIII centres; the use of an anisotropic, equilateral MnIII3 triangle model allowed us to fit the

magnetic data and obtain the best-fit parameters: J = −10.8 cm−1, D = −5.3 cm−1, and g = 1.99. The com-

bined results demonstrate the rich chemical reactivity of carbonyl groups and the ability of poly-ketone

ligands to stabilize cluster compounds with unprecedented structural motifs and interesting architectures.

Introduction

Since the establishment of di-2-pyridyl ketone [(py)2CO,Scheme 1] as one of the most versatile and multidentate che-lating/bridging ligands in metal cluster chemistry,1 new syn-thetic routes to polymetallic compounds have been sought,most of which were aimed at the development of similarligands with enhanced coordination capabilities.2 The particu-lar interest in the (py)2CO-based precursor arises from thereactivity of its carbonyl group, which can undergo metal-assisted hydrolysis or alcoholysis (ROH; R = Me, Et) formingthe first class of anionic (py)2CO2

2− and (py)2C(OR)O− ligands

derived from (py)2CO.3 These anions have shown a rich coordi-

nation affinity to 3d-metal ions leading to a multitude numberof clusters with beautiful topologies, unusual nuclearities andfascinating physical properties.1,3 Recent progress in the reac-tivity chemistry of the (py)2CO ligand involves the attack bynucleophiles other than H2O and alcohols (i.e., MeCN, Me2CO,

Scheme 1 Structural formulae and abbreviations of the ligands dis-cussed in the text.

†Electronic supplementary information (ESI) available: Crystallographic data(CIF format) of complex 1, and synthetic details and 1H/13C NMR spectra of thereported organic compounds. CCDC 1036374. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/c4dt03666b

aDepartment of Chemistry, Brock University, St. Catharines, Ontario, L2S3A1,

Canada. E-mail: tstamatatos@brocku.ca; Tel: (+1)-905-688-5550, Ext. 3400bAdvanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,

Berkeley, CA 94720, USAcDepartament de Quimica Inorganica, Universitat de Barcelona, Diagonal 645,

08028 Barcelona, Spain

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etc.) on its carbonyl C atom in the presence of metal ions,which introduces a second class of ligands derived from(py)2CO.

4 These new derivatives of (py)2CO have led to mole-cular species with different structural and physicochemicalproperties, further illustrating the flexibility of pyridyl ketonesin adopting a wide variety of binding forms.

A reasonable leap forward in the synthesis of novel 3d-metal clusters with carbonyl-based functionalities was theexploration of the coordination and reactivity chemistry of 2,6-di-(2-pyridylcarbonyl)pyridine [(py)CO(py)CO(py), Scheme 1],which contains two carbonyl groups directly bonded to two2-pyridyl groups, each in a way similar to that found in(py)2CO. Boudalis and coworkers have shown that (py)CO(py)-CO(py) is also an excellent bridging ligand but with increasedbinding properties due to the increased number of coordi-nation sites (N,N,N,O,O), thus fostering the formation of aplethora of new clusters with totally different topologies andmagnetic properties compared to the ones reported with(py)2CO-based ligands.5

Inspired by the rich chemistry of (py)2CO-based ligands, wehave decided to slightly “hybridize” the (py)CO(py)CO(py)derivative by replacing the 2,6-di-(2-pyridylcarbonyl) functionalgroups with the 2-pyrazinecarbonyl ones, hence altering theelectronic properties of the compound and introducingadditional donors for binding to metal ions. Note that pyra-zines are very π-deficient heteroaromatics that can potentiallyyield metal complexes with unusual ligand transformations andstructural topologies. We have thus started a program aiming (i)at the organic synthesis of the novel (pz)CO(py)CO(pz) ligand(Scheme 1) and (ii) its employment originally in Mn coordi-nation chemistry, as a means of obtaining new cluster com-pounds with unprecedented structural and magneticcharacteristics, not seen before with (py)2CO or any of its deriva-tives. The interest in polynuclear Mn complexes with moderateoxidation states (II, III, IV, or mixed-valence) mainly stems fromtheir relevance to various research disciplines such as bioinor-ganic chemistry and molecular magnetism. In the bioinorganicfield, a {Mn3CaO4} distorted cubane linked to a fourth, “dan-gling” Mn atom has been located at the active site of theoxygen-evolving complex on the donor side of photosystem II(PS II).6 In the molecular magnetism arena, it is now estab-lished that MnIII-containing clusters often possess high-spin (S)ground states and easy-axis-type magnetic anisotropy that giverise to a significant energy barrier to magnetization reversal atlow temperatures;7 such nanoscale molecular magnetic par-ticles are known as single-molecule magnets (SMMs) and canpotentially find applications in information storage, spintronicsand quantum computation.8

We here report the synthesis, structural and magneticcharacterization of an unusual heterometallic {MnIII

3Na2}cluster with an ideal trigonal bipyramidal topology resultingfrom a chemically interesting, metal-ion assisted crossed-aldolreaction of acetone solvent with the bis(ketone)-type (pz)CO-(py)CO(pz) molecule. The reported compound has no struc-tural similarities with any of the Mn/(py)2CO

1b or Mn/(py)CO-(py)CO(py)5g clusters reported to date, and presages a new,

promising route to polynuclear molecular magnetic materialswith poly-carbonyl functional groups.

ExperimentalSynthetic procedures

For the synthesis of (pz)CO(py)CO(pz) ligand, all reagentswere purchased from commercial sources and used as receivedunless otherwise noted. Tetrahydrofuran (THF) was distilledand dried over sodium/benzophenone ketyl under a nitrogenatmosphere. All alkyllithium reagents were titrated againstN-benzylbenzamide to a blue endpoint.9 All organomagnesiumand magnesium amide reagents [i-PrMgCl·LiCl,10

TMPMgCl·LiCl11 and (TMP)2Mg·2LiCl12] were prepared accord-ing to literature procedures and titrated using the Paquettemethod to a violet endpoint.13 Lithium chloride was driedunder high vacuum at 140 °C for 4 hours before use. In con-trast, all manipulations for the synthesis of cluster compound1 were performed under aerobic conditions using chemicalsand solvents as received. WARNING: Metal azide and perchlor-ate salts are potentially explosive; such compounds should be syn-thesized and used in small quantities, and treated with utmostcare at all times.

6-(Hydroxyl(pyrazine-2-yl)methyl)picolinaldehyde (L-1) and(rac/meso)-pyridine-2,6-diylbis(pyrazine-2-ylmethanol) (L-2). Asolution of pyrazine (1.23 g, 15.3 mmol) in THF (50 mL) atroom temperature under argon was exposed to TMPMgCl·LiCl(28.0 mL, 0.60 M, 16.8 mmol). The mixture was stirred for15 min, during which time a color change from olive-green/brown to black was observed. In a second flask, a solution of2,6-pyridinedicarboxaldehyde21 (1.14 g, 8.42 mmol) in THF(60 mL) at 0 °C was treated with BF3·OEt2 (1.15 mL,9.34 mmol), and the solution was stirred at 0 °C for 15 min7

before cooling to −40 °C. After 15 min, the solution of meta-lated pyrazine was transferred dropwise by cannula to the solu-tion of BF3-protected 2,6-pyridinedicarboxaldehyde, and thereaction mixture was allowed to warm slowly from −40 °C toroom temperature (approx. 2 h). A solution of saturatedaqueous NH4Cl (5 mL) was added and the organic solventswere removed under reduced pressure to afford a black oil thatwas taken up in water (30 mL). The crude product was isolatedby 24 hour continuous extraction into CH2Cl2 (50 mL) contain-ing a small amount of MeOH (5 mL). The organic phase wasdried over anhydrous Na2SO4 and concentrated under reducedpressure. Flash column chromatography (silica gel, 92 : 6 : 2 : 1CH2Cl2–MeOH–acetone–NH4OH, Rf = 0.38) gave, sequentially,aldehyde L-1 (325 mg, 10%) as a dark brown oil, and diol L-2(330 mg, 14%, Rf = 0.18) as a dark brown oil.

L-1: IR (neat) νmax 3220, 1708, 1402, 1054, 1017 cm−1; 1HNMR (300 MHz, CDCl3) δ 10.11 (s, 1H), 8.98–8.99 (d, 1H, J =0.6 Hz), 8.53 (s, 2H), 7.91–7.89 (t, 2H, J = 3 Hz), 7.84–7.81 (m,1H), 6.08 (s, 1H); 13C NMR (75.5 MHz, CDCl3) δ 192.7, 155.7,151.4, 146.3, 143.9, 143.7, 143, 138.2, 125.6, 120.9, 73.5; EI-MS[m/z (%)] 215 (M+, 36), 198 (64), 136 (100), 106 (26), 79 (29);HR-MS (EI) calcd for C11H9N3O2: 215.0695; found 215.0701.

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L-2: IR (neat) νmax 3245, 1401, 1064, 1017 cm−1; 1H NMR(300 MHz, CDCl3): δ 8.80 (s, 2H), 8.47 (s, 4H), 7.71–7.66 (t, 1H,J = 7.7 Hz), 7.47–7.45 (d, 2H, J = 7.7 Hz), 5.97 (s, 1H), 5.34 (bs,2H); 13C NMR (75.5 MHz, CDCl3): δ 158.7, 155.8, 143.8, 143.7,143.0, 138.7, 120.7, 73.6; EI-MS [m/z (%)] 295 (M+, 60), 291(60), 277 (52), 216 (86), 214 (92), 198 (100), 184 (92), 170 (45),136 (46), 109 (24), 107 (35) 79 (99); HR-MS (EI) calcd forC15H13N5O2: 295.1069, found 295.1066.

Pyridine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO-(pz)]. A solution of diol L-2 (500 mg, 1.70 mmol) and SeO2

(208 mg, 1.87 mmol) in dioxane (10 mL) was heated at refluxfor 30 min in the open air.21 After cooling to room tempera-ture, the solvent was removed under reduced pressure. Thereaction mixture was re-dissolved in CH2Cl2 (15 mL) andpassed through a pad of silica gel to remove inorganic impuri-ties. After evaporation of the solvent, flash column chromato-graphy (silica gel, EtOAc, Rf = 0.15) gave dione (pz)CO(py)CO-(pz) as a brown oil (420 mg, 85%) that was recrystallized fromacetone–hexane (50 : 50) in two batches to afford (pz)CO(py)-CO(pz) as small brown needles (395 mg, 80%); mp 155–156 °C(acetone–hexane); νmax 1685, 1324, 1154 cm−1; 1H NMR(300 MHz, CDCl3) δ 9.35 (s, 2H), 8.77–8.73 (m, 4H), 8.44–8.41(d, 2H, J = 8.1 Hz), 8.23–8.18 (t, 1H, J = 7.8 Hz); 13C NMR(75.5 MHz, CDCl3) δ 190.3, 152.3, 148.7, 146.9, 146.8, 144.1,138.6, 127.8; EI-MS [m/z (%)] 291 (M+, 95), 184 (100), 156 (13),107 (28), 79 (64); HR-MS (EI) calcd for C15H9N5O2: 291.0750,found 291.0756.

[Mn3Na2O(N3)3(L)3] (1). To a stirred solution of (pz)CO(py)-CO(pz) (0.03 g, 0.1 mmol) and nBu3N (0.07 mL, 0.3 mmol) inMe2CO (10 mL) was added solid Mn(ClO4)2·6H2O (0.04 g,0.1 mmol). The resulting orange solution was kept under mag-netic stirring at room temperature for about 20 min, followedby the consecutive addition of solid NaN3 (0.01 g, 0.1 mmol).The resulting suspension was stirred for 4 h, during whichtime all the solids dissolved and the color of the solutionchanged to dark red. The solution was filtered and left to evap-orate slowly at room temperature. After 7 days, X-ray qualitydark-blue plate-like crystals of 1·Me2CO·xH2O had appearedand were collected by filtration, washed with cold Me2CO (2 ×5 mL) and dried in air; the yield was 30% (based on the totalavailable Mn). The crystalline solid was analyzed as solvent-free 1. Found: C, 48.06; H, 3.42; N, 21.58%. Calcd forC63H57Mn3Na2N24O13: C, 48.23; H, 3.66; N, 21.42. Selected ATRdata (cm−1): 2033 (vs.), 1704 (s), 1591 (m), 1572 (m), 1523 (w),1458 (m), 1397 (m), 1362 (m), 1283 (m), 1157 (s), 1076 (vs.),1045 (s), 1017 (s), 820 (m), 774 (mw), 700 (s), 629 (sb), 545 (sb).

X-ray crystallography and solution of structure

A clear dark-blue plate-like crystal (0.10 × 0.10 × 0.02 mm3) of1·Me2CO·xH2O was mounted on a MiTeGen kapton loop in the100(2) K nitrogen cold stream provided by an Oxford Cryo-systems Cryostream 700 Plus apparatus. The crystal was trans-ferred to the goniometer head of a Bruker D8 diffractometerequipped with an ApexII CCD detector on beamline 11.3.1 atthe Advanced Light Source in Berkeley National Laboratory.Diffraction data were collected in synchrotron radiation, mono-

chromated using silicon(111) to a wavelength of 0.7749(1) Å.An approximate full sphere of data to 2θmax = 64.34° was col-lected using 0.3° ω scans. A total of 26 944 reflections were col-lected, of which 3473 were unique (Rint = 0.0498) and 2816were observed [I > 2σ(I)]. The structure was solved by intrinsicphasing and refined by full-matrix least-squares on F2

(SHELXL-2014/614) using 175 parameters and 15 restraints.The lattice Me2CO and H2O solvate molecules were severelydisordered and could not be modeled properly; thus, they wereremoved by utilizing the SQUEEZE algorithm in the PLATONsoftware package.15 The coordinated azide groups were alsodisordered and split across the mirror plane. The restraintsused are to model the disordered azides. The hydrogen atomson carbon atoms were generated geometrically and refined asriding atoms with C–H = 0.95–0.99 Å and Uiso(H) = 1.2 × Ueq(C)for CH and CH2 groups. The programs used for moleculargraphics were MERCURY16 and DIAMOND.17 Unit cell para-meters and structure solution and refinement data are listedin Table 1. Further crystallographic details can be found in thecorresponding CIF file provided in the ESI.† Crystallographicdata for the structure reported in this work have been de-posited to the Cambridge Crystallographic Data Centre (CCDC)as supplementary publication numbers: CCDC–1036374 (1).

Physical measurements and other studies

All organic reactions were performed under atmospheres ofargon or nitrogen in flame- or oven-dried glassware usingsyringe-septum cap techniques, unless otherwise indicated.Column chromatography was performed on silica gel 60(70–230 mesh). NMR spectra were obtained on a BrukerAvance DPX-300 MHz instrument and are referenced to the

Table 1 Crystallographic data for complex 1

Formulaa C63H57Mn3Na2N24O13

M/g mol−1 a 1569.12Crystal system HexagonalSpace group P63/ma/Å 13.3404(4)b/Å 13.3404(4)c/Å 24.3913(9)α/° 90β/° 90γ/° 120V/Å3 3759.3(3)Z 2T/K 100(2)λ/Åb 0.7749ρcalc/g cm−3 1.386μ/mm−1 0.731Measd/independent (Rint) reflns 26 944/3473Obsd reflns [I > 2σ(I)] 2816R1

c,d 0.0531wR2

e 0.1700GOF on F2 1.070(Δρ)max,min/e Å

−3 0.707, −0.494

a Excluding the SQUEEZED solvate molecules. b Synchrotron, ‘ALSbeamline 11.3.1’, ‘silicon 111’ monochromator. c I > 2σ(I). d R1 =∑(||Fo| − |Fc||)/∑|Fo|

ewR2 = [∑[w(Fo2 − Fc

2)2]/∑[w(Fo2)2]]1/2, w =

1/[σ2(Fo2) + [(ap)2 + bp], where p = [max(Fo

2, 0) + 2Fc2]/3.

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residual proton signal of the deuterated solvent for 1H spectra,and to the carbon multiplet of the deuterated solvent for the13C spectra, according to published values. FT-IR spectra wereobtained on a Bruker ALPHA Platinum ATR system on com-pounds in their pure state. Mass spectral data were obtainedon a MSI/Kratos Concept 1S Double Focusing Spectrometer.Melting points were obtained on recrystallized material unlessotherwise stated using a Kofler hot-stage apparatus, and areuncorrected. Elemental analyses (C, H, and N) were performedon a Perkin-Elmer 2400 Series II Analyzer. Magnetic suscepti-bility studies were carried out on a polycrystalline sample ofcomplex 1 using a MPMS5 Quantum Design susceptometer,working in the temperature range 30–300 K under an externalmagnetic field of 0.3 T and under a field of 0.03 T in the30–2 K range to avoid saturation effects. Pascal’s constantswere used to estimate the diamagnetic correction, which wassubtracted from the experimental susceptibility to give themolar paramagnetic susceptibility (χM).

18

Results and discussionSynthesis of (pz)CO(py)CO(pz)

The synthesis of pyridine-2,6-diylbis(pyrazine-2-ylmethanone)-[(pz)CO(py)CO(pz)] requires a different route than that used toprepare tripyridyl analogue [(py)CO(py)CO(py)].19 The latterligand was made by metalation of [1,2,3]triazolo[1,5-a]pyridineaccording to Scheme S1,† in which the second pyridyl ring isrevealed after hydrolysis with H2SO4/SeO2. A direct synthesis of(pz)CO(py)CO(pz) was envisioned involving metalation ofunsubstituted pyrazine, followed by its double addition to 2,6-pyridinedicarboxaldehyde as the first step. While reports ofthe metalation of unsubstitited pyrazine are sparse, Quéguinerand coworkers showed in 1995 that metalation of thisdiazine20 with lithium tetramethylpiperidide (LiTMP) providedalcohol X-1 after electrophile quench with benzaldehyde(Scheme 2).21 The yield of this reaction was found to vary con-siderably depending on lithiation times and base concen-tration. More recently, Knochel and coworkers demonstratedthat salt-modified magnesium amide bases such as(TMP)2Mg·2LiCl and TMPMgCl·LiCl metalate pyrazine12 or 2,3-dichloropyrazine.22 The corresponding adducts X-1 and X-2were isolated after quench with benzaldehyde or MeSO2SMe,respectively. All of the preceding methods seemed directlyapplicable to the synthesis of (pz)CO(py)CO(pz).

Preliminary experiments focused on adapting Quéguiner’sprocedure. While the synthesis of carbinol X-1 was reproduci-ble, attempts to employ 2,6-pyridinedicarboxaldehyde as theelectrophile in place of benzaldehyde failed to give anyaddition products. The use of TMEDA·ZnCl2 in the LiTMPdeprotonation, as reported by Mongin for the synthesis ofiodopyrazine,23 did not change the outcome of the reaction. Atfirst, identical results were observed with Knochel’s mag-nesium amide bases, (TMP)2Mg·2LiCl/ZnCl2 andTMPMgCl·LiCl, in that addition to 2,6-pyridinedicarboxalde-hyde was not observed. Reasoning that the pyridyl nitrogen

was somehow interfering with carbanion quench, the electro-phile was complexed with BF3·OEt2

24 before exposure to themetalated intermediate. Consequently, magnesiation of pyra-zine with TMPMgCl·LiCl under Knochel’s conditions,22 fol-lowed by addition of the carbanion to a solution of 2,6-pyridinedicarboxaldehyde·BF3 at −40 °C, afforded mono-addition product L-1 (10%) along with the desired diol L-2(14%) as a 1 : 1 mixture of rac and meso stereoisomers. Thehigh polarity and water solubility of L-2 required its isolationby continuous extraction before careful purification by columnchromatography. The yield of L-2 from the magnesiation reac-tion was consistent up to a scale of two grams in pyrazine. Inany case, subsequent oxidation of L-2 with SeO2 in dioxane25

furnished the desired (pz)CO(py)CO(pz) ligand in 80% yieldas a light brown crystalline compound.

Synthesis of complex 1

Once the first objective of this research, namely the synthesisof the novel ligand (pz)CO(py)CO(pz) was accomplished,various reactions were systematically explored with differingMnII salts, reagent ratios, reaction solvents, organic bases, andother conditions, in order to isolate polynuclear Mn clusters inmoderate-to-high oxidation states. However, all such reactionsbut one failed to give any crystalline material, the remainderproducing oily products and amorphous precipitates that we

Scheme 2 Metalation of pyrazine and synthesis of (pz)CO(py)CO(pz).

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were not able to further characterize. Most likely thesematerials are polymeric and/or mixtures of various products.The reaction though of Mn(ClO4)2·6H2O and NaN3 with (pz)-CO(py)CO(pz) and nBu3N in a 1 : 1 : 1 : 3 molar ratio in Me2COgave, upon stirring for a prolonged time at room temperature,a dark red solution from which we were able to successfullygrow well-formed, dark-blue crystals of [MnIII

3Na2O(N3)3(L)3](1) in 30% yield.

The self-assembly synthesis of heterometallic 1 is associ-ated with some notable features which deserve further discus-sion. First, the oxidation of MnII to MnIII is almost certainly byatmospheric O2 under the prevailing basic conditions. Inaddition, the use of Me2CO as reaction solvent is crucial forboth the clean and crystalline product formation and themetal-ion promoted (or assisted) generation of the unusualligand L2−, which is the dianion (pz)C(CH2COCH3)(O

−)(py)-C(CH2COCH3)(O

−)(pz) (vide infra). Finally, the role of nBu3N asproton acceptor is also important for the in situ generation ofdeprotonated L2− groups, and consequently for the stabiliz-ation of polynuclear complex 1. When base was omitted fromthe reaction mixture, only pale yellow solutions indicative ofMnII products were obtained, likely containing neutral deriva-tives of the (pz)CO(py)CO(pz) ligand. Omission of NaN3 fromthe reaction that led to 1 resulted in the isolation of solid,non-crystalline materials that had poor solubilities in allcommon organic solvents. Reactions with higher Mn:(pz)CO-(py)CO(pz) ratios, to foster the formation of higher nuclearityMn clusters, did not give any new products other than 1 inlower yields (5–10%).

A simplified mechanism for the formation of L2− is pro-posed in Scheme 3. The moderately strong base nBu3N (pKa ∼11)26 abstracts an α-hydrogen from acetone (pKa ∼ 20) which isin equilibrium with the corresponding resonance-stabilizedcarbanion.5g,27 It is also possible that the acidity of acetone isincreased by its prior coordination to Mn, which therebyassists its deprotonation. Once the carbanion CH3COCH2

− is

formed, it may attack the electrophilic carbonyl carbons of(pz)CO(py)CO(pz). As the nucleophilic addition forms newC–C bonds, the π electrons of the two carbonyl groups of (pz)-CO(py)CO(pz) are transferred completely to the correspondingoxygen atoms, forming two alkoxide ions. The reaction thatleads to (pz)C(CH2COCH3)(O

−)(py)C(CH2COCH3)(O−)(pz) is

not an aldol condensation, because the product is not furtherdehydrated to form a carbon–carbon double bond.5g,27 Poss-ible reasons for resistance of (pz)C(CH2COCH3)(O

−)(py)-C(CH2COCH3)(O

−)(pz) to loss of water are the strongly basicmedium that does not permit the existence of neutral alcoholfunctions and/or its prior stabilization by the Mnn+ centresmentioned previously. We have failed to date to synthesize thefree, neutral LH2 ligand (this compound is not known) by thereaction of (pz)CO(py)CO(pz) and acetone in the presence ofnBu3N or any other strong base under several reaction con-ditions. This further emphasizes the importance of Mn ions inthe metal-assisted formation of L2−.

Structural description of [Mn3Na2O(N3)3(L)3] (1)

A partially labelled representation of complex 1 is shown inFig. 1. Selected interatomic distances and angles are listed inTable 2. The compound crystallizes in the hexagonal spacegroup P63/m and its crystal structure consists of pentanuclear[Mn3Na2O(N3)3(L)3] molecules and solvate Me2CO and H2Omolecules; all lattice solvents are severely disordered and willnot be further discussed.

Compound 1 consists of three Mn and two Na atomsarranged in an almost perfect trigonal bipyramidal topologywith an ideal D3h point-group symmetry (Fig. 2, right). The

Scheme 3 The proposed simplified mechanism that leads to L2− whichis present as chelating/bridging ligand in complex 1; the Mnn+ ionsstabilize the resonance-stabilized enolate intermediate and the finalketone/alkoxido product.

Fig. 1 Partially labelled plot of complex 1. H atoms have been omittedfor clarity. Colour scheme: MnIII, blue; Na, yellow; O, red; N, green;C, grey. Symmetry operations: a = x, y, 1.5 − z; b = − x + y, 1 − x, z; c =1 − y, 1 + x − y, z; d = − x + y, 1 − x, 1.5 − z; e = 1 − y, 1 + x − y, 1.5 − z.

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apical positions are occupied by the two NaI ions, whereas thethree MnIII ions reside in the equatorial trigonal plane. Theoxidation states for the MnIII ions were confirmed by bondvalence sum (BVS) calculations,28 which gave a value of 3.12(the three Mn atoms are symmetry-related). TheMn⋯Mn⋯Mn, Mn⋯Mn⋯Na, and Mn⋯Na⋯Mn angles are60°, 63.4(3)°, and 53.2(4)°, respectively, while the anglebetween Na⋯Mn⋯Na is 117.7(4)°, deviating only slightly fromthe ideal 120° value of a trigonal bipyramidal geometry. Thethree MnIII ions are solely bridged by a μ3-O2− ion (BVS = 2.07)to form an equilateral triangle (Mn⋯Mn separations of3.208 Å) without additional bridging ligation provided on anyMn⋯Mn edge. The crystallographic C3 axis is passing from thecentral O2− ion. To our knowledge, this class of ‘edge-naked’equilateral triangles is unprecedented in metal cluster chem-istry. The central oxido ion, O3, lies exactly in the Mn3 plane(Mn–O3–Mn = 120°), while the MnIII–O3 bonds (1.852(5) Å) arethe shortest and strongest in compound, consistent with the

HSAB principle. Each MnIII ion is linked to the two apical NaI

ions by two monoatomic RO− bridges of the same L2− ligand,thus stabilizing a complete [Mn3Na2(μ3-O)(μ-OR)6]3+ core(Fig. 2, left).

Each of the Mn atoms is coordinated to the pyridine Natom and two alkoxido O atoms from one L2− ligand, as wellas a terminal N3

− group, to complete a MnN2O3 chromophore.The coordination spheres of Na atoms is each completed bythree ketone O atoms belonging to three different L2− ligands,thus giving an overall NaO6 chromophore. Consequently, Mnatoms are five-coordinate with distorted square pyramidal geo-metries (τ = 0.20,29 where τ is 0 and 1 for perfect square pyra-midal and trigonal bipyramidal geometries, respectively); theterminally-ligated N3

− ions occupy the apical positions. In con-trast, the Na atoms are six-coordinate with very distorted octa-hedral geometries. That was confirmed by the ContinuousShape Measure (CShM) approach which essentially allows oneto numerically evaluate by how much a particular structuredeviates from an ideal shape.30 The best fit was obtained forthe octahedron (Fig. 3, left) with CShM value of 5.03. Thecorresponding CShM value for a trigonal prismatic geometrywas 6.06, close to the octahedral one, hence justifying the verydistorted coordination geometries of Na atoms. Values ofCShM between 0.1 and 3 usually correspond to a not negligiblebut still small distortion from ideal geometry.31 Finally, eachof the potentially nonadentate L2− ions practically acts as a η1 :η2 : η1 : η2 : η1 : μ3 pentadentate ligand, with both pyrazinefunctionalities remaining unbound (Fig. 3, right). This is notsurprising given the fact that pyrazine N atoms are expected tobe much poorer donors than the corresponding pyridine one(pKa (pz) ≪ pKa (py)). Distinguishable C–O single [C4–O1 =1.410(3) Å] and CvO double bonds [C6–O2 = 1.206(4) Å] areevident in L2−, confirming two single and two double carbon–oxygen bonds. From a supramolecular viewpoint, there are nosignificant intermolecular interactions other than some weakπ–π stacking interactions between the pyrazine groups ofneighbouring molecules and H-bonding interactions that pre-sumably derive from the interactions of dangling N atoms(from both pyrazine and azido groups) with –CH3 groups(from L2− and likely acetone lattice solvents).

Complex 1 is the first molecular species stabilized by theorganic ligand L2− and the first coordination cluster bearing

Table 2 Selected interatomic distances (Å) and angles (°) for 1a

Mn1–O1 1.880(2) Na1–O1 2.423(2)Mn1–O1a 1.880(2) Na1–O1b 2.423(2)Mn1–O3 1.852(5) Na1–O1c 2.423(2)Mn1–N1 1.922(3) Na1–O2 2.480(2)Mn1–N4 2.319(3) Na1–O2b 2.480(2)Mn…Mn 3.208(2) Na1–O2c 2.480(2)Mn…Na 3.580(1) Na…Na 6.128(2)

Mn1–O3–Mn1b 120.0(3) O3–Mn1–N1 174.08(9)Mn1–O3–Mn1c 120.0(3) O3–Mn1–N4 90.76(8)Mn1b–O3–Mn1c 120.0(3) N1–Mn1–N4 95.15(11)Mn1–O1–Na1b 112.0(8) O1–Na1–O1b 97.4(6)O1–Mn1–O1a 162.0(11) O1–Na1–O2 72.8 (6)O1–Mn1–O3 97.40(5) O1–Na1–O2b 154.1(6)O1–Mn1–N1 82.11(5) O1–Na1–O2c 107.6(6)O1–Mn1–N4 95.00(5) O2–Na1–O2b 87.04(7)

a Symmetry codes as appear in caption of Fig. 1. b All angles are relatedby the symmetry operations.

Fig. 2 (left) PovRay representation of the complete [Mn3Na2(μ3-O)-(μ-OR)6]

3+ core of 1. (right) The {Mn3Na2} trigonal bipyramidal topologyof 1. Colour scheme and symmetry operations as in Fig. 1.

Fig. 3 (left) Distorted octahedral coordination geometry of NaI atomsin 1; the points connected by the black lines define the vertices of theideal polyhedron. (right) The coordination mode of the dianion of L2− incomplex 1. Colour scheme as in Fig. 1; H atoms are herein illustratedwith a purple coloration.

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any form of the (pz)CO(py)CO(pz) ligand. Further, 1 has a3Mn:2Na stoichiometry not previously observed for any mole-cular system.

Solid-state magnetic susceptibility studies on complex 1

With respect to the magnetic properties, complex 1 comprisesa {MnIII

3(μ3-O2−)}7+ triangle with a C3 symmetry. Althoughthere are many triangular motifs reported to date, most ofthem either exhibit a lower symmetry than that of 1 or includeadditional bridges across the Mn⋯Mn edges. To that end,complex 1 is a unique MnIII

3 triangle. High-symmetry, oxido-bridged MnIII

3 triangles, albeit with additional oximate,32

alkoxido33 or carboxylate34 bridges, are of precedence. Thesecompounds can be either ferro- or antiferromagnetic, with thetorsion and Mn–OR–Mn angles, oxido displacement from theMn3 plane, and orientation of Jahn–Teller axes contributingthe most to the large differences in magnetic behavior.35 In aperfectly symmetric, equilateral MnIII

3 triangle (pure 1-Jmodel) there are two possible ground state spin values, S = 0or S = 6 for antiferromagnetic or ferromagnetic interactions,respectively. However, an isosceles triangle model (2-J model)is almost always applied in such complexes because, even ifthese compounds are crystallographically equilateral or equi-lateral within the usual 3σ criterion, they are known toundergo magnetic Jahn–Teller distortion,36 resulting in an iso-sceles situation.

Variable-temperature magnetic susceptibility measure-ments were performed on a powdered polycrystalline sampleof complex 1 in the temperature range 2.0–300 K; a dc (directcurrent) field of 0.3 T was applied from 20 to 300 K and aweaker dc field of 0.03 T was applied from 2 to 20 K to avoidsaturation effects. The data are shown as χMT versus T plot inFig. 4. The χMT product shows a room temperature value of7.24 cm3 mol−1 K, lower than the value of 9.00 cm3 mol−1 Kexpected for three non-interacting MnIII atoms (g = 2.00). Oncooling, the χMT value steadily decreases down to a value of∼3 cm3 mol−1 K at 19 K, then slightly increases to a value of

∼3.2 cm3 mol−1 K at 5 K before further decreasing to aminimum value of 2.96 cm3 mol−1 K at 2 K. The shape of theχMT versus T curve clearly indicates the presence of predomi-nant antiferromagnetic exchange interactions between thethree MnIII atoms with a resulting non-zero, but most likely anS = 2 ground state spin value; the spin-only (g = 2) value for acomplex with an S = 2 ground state is 3.00 cm3 mol−1 K. Thedecrease in the χMT value at the lowest temperatures (<5 K) isprobably due to zero-field splitting (ZFS) and any weak inter-molecular antiferromagnetic exchange interactions.32–34 Mag-netization (M) versus field (H) studies at 2 K (inset of Fig. 4)show a nearly saturated value equivalent to 2.51 electronsunder a maximum field of 5 T.

Isotropic and equilateral homometallic triangles can showcompetitive antiferromagnetic interactions that lead to an S =0 ground state. For isosceles MnIII

3 triangles, several groundstates can be populated as a function of the J1/J2 ratio (i.e., S =2 for J1/J2 < 1/3 or J1/J2 > 2.0, S = 1 for 1/3 < J1/J2 < 2/3 or 3/2 <J1/J2 < 2.0 and S = 0 for 2/3 < J1/J2 < 3/2), with various frustra-tion points at J1/J2 = 1/3, 2/3, 3/2 and 2.0.32–36 However, such adistribution of ground states seems incompatible with thestructural data of complex 1, which shows an imposed C3 sym-metry associated with an S = 0 ground state. Deviations fromthe expected magnetic response could be attributed to twomain factors: asymmetrization of the equilateral triangle dueto structural distortions below the temperature at which thecrystal structure was determined and/or oversimplification ofthe magnetic model due to the assumption of a purely isotro-pic system.37 Investigation of the magnetic response of pre-viously reported equilateral, or near equilateral, planartriangles, with Mn–O–Mn bond angles spanning the range 120± 2°, revealed that only in few cases the magnetic data weresuccessfully reproduced with the equilateral, isotropicmodel.38 In contrast, for the majority of such compounds thefitting of the experimental data required the employment ofthe isosceles model with two coupling constants (2-Jmodel).32–35 As a result, S = 139 or S = 2 ground states40 wereestablished, even for the case of imposed C3 crystallographicsymmetry.40 A very precise explanation to this fact was pro-vided by Hendrickson and coworkers;37,40c,41 in particular, thein-depth analysis of the energy levels of equilateral MnIII

3 tri-angles possessing D values of the same order of magnitude asthe J coupling constant(s) revealed a strong mixing of spinlevels which were better fit to an apparently isoscelessystem.37,40c,41

In light of these findings, we have initially simulated theχMT versus T and M versus H data of 1 using the equilateral tri-angle model with J = −10 cm−1, g = 2.00, and variable D valuesspanning the range 0–10 cm−1 (Fig. 5); the spin Hamiltonianemployed was H = –J (Ŝ1·Ŝ2 + Ŝ2·Ŝ3 + Ŝ1·Ŝ3) and the fittingprogram was PHI.42 Three important features emerged fromthese plots: (i) as a function of the D parameter, the χMT dataand shape of plots reach the isotropic limit with S = 1 or2 ground states, (ii) the χMT data are almost insensitive to D atT > 50 K, thus providing reliable J values at that temperatureregion, and (iii) M tends to an unusual value close to 2.5

Fig. 4 Plot of χMT versus T for complex 1. The solid red line is the fit ofthe data; see the text for the fit parameters. (inset) Plots of magnetiza-tion (M) versus field (H) at 2 K. The solid black lines show the calculatedmagnetization plots for an isosceles or scalene triangle model; see thetext for details.

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electrons, deviating from the expected 2 or 4 electrons for an S= 1 or 2 ground state, respectively. Therefore, we decided toperform simultaneous fits of the χMT and M data using theanisotropic equilateral triangle model; good fits were obtainedfor both types of measurements. The best-fit parameters were:J = −10.8 cm−1, D = −5.3 cm−1, and g = 1.99 (red line in Fig. 4).Alternative fits using either the isosceles or scalene trianglemodel were discarded because, despite the fact that they canfit the χMT data until low temperatures, they give unreasonableJ values (−14.1/−2.5 cm−1 for the isosceles model and −12.2/−4.7/−1.3 cm−1 for the scalene model). These values mustimply significant deviations of the Mn–O–Mn bond anglesfrom 120°, opposed to the case of 1, and are further unable toreproduce the shape and values of the magnetization data(Fig. 4, inset).

The cores of all the previously reported, planar oxido-centered MnIII triangles are further supported by either carbox-ylate or oximate bridges, or both; the corresponding J valuesare strongly dependent on the nature of the carboxylate groupand the Mn–O–N–Mn torsion angles.32–35 Complex 1 is thefirst Mn triangle in which the superexchange interactions aresolely propagated by the central μ3-O2− ion, and therefore strictcomparisons with other Mn3 triangles may lead to superficialconclusions. Theoretical and DFT calculations are underway inorder to understand the mechanism of magnetic super-exchange in 1 and evaluate the degree of contribution of theoxido group to the overall antiferromagnetic response.

Conclusions

In summary, we have been able to accomplish both researchobjectives namely the synthesis of a new organic chelating/bridging ligand and its first use in transition metal clusterchemistry. From these initial research endeavours, it has beenshown that the synthesized (pz)CO(py)CO(pz) ligand iscapable of undergoing a unique, Mn-assisted reactivity inacetone under basic conditions, due to the presence of acidicα-hydrogens in the solvent and to the δ+ character of the carbo-nyl groups of the ligand. The resulting dianion, L2− of the bis-(β-hydroxy)ketone that is formed bridges three MnIII and two

NaI ions through its alkoxido groups facilitating the formationof a highly symmetric [Mn3Na2(μ3-O)(μ-OR)6]3+ core with analmost ideal trigonal bipyramidal geometry. The magnetic sus-ceptibility data of the equilateral MnIII

3 triangle have beentreated with an anisotropic model that includes zero-field split-ting and serves to quantify the antiferromagnetic interactionsbetween the metal centres. Finally, the reported cluster com-pound has also some implications in the bioinorganic chem-istry field with respect to modelling the structure andproperties of the active site of PS II. Complex 1 contains thesame total number of metal atoms as the OEC and currentefforts are oriented toward the deliberate replacement of NaI

ions with Ca2+ without affecting dramatically the structuralidentity of the parent compound. We are also investigating (i)the incorporation of additional bridging groups, such as car-boxylates, in the general Mnn+/(pz)CO(py)CO(pz) reactionsystem, (ii) the effect of the solvent and base on the structuralchemistry of cluster compounds, and (iii) the isolation ofmany new 3d- and/or 4f-metal clusters that will contain anaccessible form of the (pz)CO(py)CO(pz) ligand and will pre-ferably possess large spin ground states and SMM behavior.

Acknowledgements

This work was supported by Brock University and NSERC Dis-covery Grant (Th.C.S and C.M), the CICYT (project CTQ2009-07264) and Excellence in Research ICREA-Academia Award (toA.E). The Advance Light Source is supported by The Director,Office of Science, Office of Basic Energy Sciences of the U.S.Department of Energy under contract no. DE-AC02-05CH11231. C.W.-K. would like to thank Prof. Tomas Hudlickyfor assistance with the purification of L-2 and (pz)CO(py)CO(pz).

Notes and references

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Fig. 5 Simulations of the χMT product (left) and magnetization (right)for a symmetric MnIII

3 triangle with a fixed J = −10.0 cm−1 and variableDion values; the external field used for the susceptibility fits is 0.01 T.

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