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Synthesis and characterization of calcium b-diketonate complexes. X-Raycrystal and molecular structures of: [{Ca(tmhd)2}2(18-crown-6)],[Ca(dpp)2(thf)2] and [Ca(dpp)2(triglyme)]†

Naida El Habra,*a,b Franco Benetollo,a Maurizio Casarin,b Marco Bolzan,a Andrea Sartoria andGilberto Rossettoa

Received 27th January 2010, Accepted 28th May 2010First published as an Advance Article on the web 14th July 2010DOI: 10.1039/c001854f

Reactions of 2,2,6,6-tetramethyl-3,5-heptanedione (Htmhd), 1,1,1,5,5,5-hexafluoro-2,4-pentanedione(Hhfa) or 1,3-diphenyl-1,3-propanedione (Hdpp) with calcium methoxide in hexane or toluene affordthe corresponding known oligomeric b-diketonates: [Ca3(tmhd)6], [{Ca(hfa)2}n] and [{Ca(dpp)2}n]. Thecomplexes react with tetrahydrofuran, 2,5,8,11-tetraoxadodecane (triglyme) or 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) leading to the formation of the new mononuclear[Ca(dpp)2(thf)2], [Ca(dpp)2(triglyme)] and dinuclear [{Ca(dpp)2}2(18-crown-6)] and [{Ca(tmhd)2}2-(18-crown-6)] adducts. The obtained complexes were characterized by elemental analyses, IR, 1H and13C NMR spectroscopies; moreover, single crystal X-ray diffraction measurements were also carried outfor: [{Ca(tmhd)2}2(18-crown-6)], [Ca(dpp)2(thf)2] and [Ca(dpp)2(triglyme)].

Introduction

Molecular structures and properties of alkaline-earth metal b-diketonates are of great interest in both experimental and the-oretical aspects and are receiving growing attention as a conse-quence of their extensive applications as molecular precursorsfor sol–gel and CVD (e.g. MOCVD, LI-MOCVD and ALD)1

processes for the preparation of technologically interesting high-quality thin films (e.g. electroceramic materials,1 superconductorsand luminescent materials).2–5 A significant advantage of theCVD techniques in comparison with the physical depositionprocesses (e.g. molecular beam epitaxy, laser ablation, sputteringor evaporation) is their capability of coating complex large areashapes with high rates of deposition as well as their potentialuse for large scale processing.6 The quality and the successof the synthetic approaches, ranging from vapor- to liquid-phase routes, critically depends on the properties of molecularprecursors since both their nature and frameworks drive thefinal quality of materials.7 It is well understood that, in order toachieve the best precursor delivery properties (volatility and/orsolubility), the oligomerization of the precursor molecules isundesirable. In this regard, it is noteworthy that the Lewis acidityof calcium acetylacetonate complexes1 determines their tendencyto generate polymeric derivatives characterized by low solubilityand volatility. A precursor improvement can be obtained by thepresence of small Lewis base molecules that favors the formationof oligomeric species,1 as in the case of [{Ca(hfa)2(OH2)2}2],which exists in dimeric form as shown by its X-ray structure.8

Intermolecularly associated clusters of Ca b-diketonates mayalso be obtained by using sterically hindered ligands, such as -

aICIS–CNR, Corso Stati Uniti 4, 35127 - Padova, Italy. E-mail: elhabra@icis.cnr.it; Fax: +39 049 8295951; Tel: +39 049 8295901bDipartimento di Scienze Chimiche, Universita di Padova, Via Marzolo 1,35131, - Padova, Italy† CCDC reference numbers 755292–755294. For crystallographic data inCIF or other electronic format see DOI: 10.1039/c001854f

tmhd or -dpp, as demonstrated by the crystal structures of thetrimeric [Ca3(tmhd)6],9 dimeric [Ca2(tmhd)4(C2H5OH)2]9 and thetetrameric [{Ca2(dpp)4(C2H5OH)}2]10 species.

Nevertheless, calcium monomeric species are highly desirablebecause, at least in principle, they should ensure higher volatilityand solubility. In this regard, it deserves to be emphasized thatoligomerization is usually avoided by adding ancillary coordi-nating Lewis base molecules through the use of poly-oxygenand/or poly-nitrogen donor ligands.1,11 In fact, the coordinativesaturation of Ca2+ ions determined by these species favors thegeneration of simpler molecular species with higher solubility andvolatility. More specifically, adducts such as [Ca(hfa)2(triglyme)],12

[Ca(tmhd)2(triglyme)],13 [Ca(hfa)2(tetraglyme)]13,14 (tetraglyme =2,5,8,11,14-pentaoxapentadecane), have been synthesized, and allof them have shown higher volatility and, as a consequence, betterperformances than analogous free compounds as MOCVD orLI-MOCVD11,14–17 sources. Besides the cleavage of the oligomericensemble and the consequent formation of mononuclear speciesin the solid state is well documented in the literature (see X-raydata pertaining to [Ca(tmhd)2(triglyme)], [Ca(hfa)2(tetraglyme)],[Ba(tmhd)2(triglyme)]13) and herein confirmed (see below).

In this paper, we focused on the synthesis of new [{Ca(b-diket)2}n(L)] complexes. To achieve coordinative saturation ofcalcium, both sterically hindered b-diketonates (tmhd and dpp)and triglyme and 18-crown-6 as ancillary coordinating Lewisbases (L), were investigated. Literature data seem to indicatethat, when dealing with calcium, the acyclic triglyme ethershould represent a better choice than other open poly-ethers.1

As far as 18-crown-6 is concerned, the macrocyclic compact-ness enables to coordinate all its oxygen atoms to the metalcenter and, to our knowledge, only one example of its usehas been so far reported ([Ca(hfa)2(18-crown-6)]).18 The use ofthese ligands favors the reduction of the calcium b-diketonatesoligomerization, leading to the formation of the new mononuclear[Ca(dpp)2(triglyme)] and dinuclear [{Ca(dpp)2}2(18-crown-6)] and[{Ca(tmhd)2}2(18-crown-6)] adducts. Besides we demonstrate that

8064 | Dalton Trans., 2010, 39, 8064–8070 This journal is © The Royal Society of Chemistry 2010

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an appropriate choice of the ligand made possible to preparethe monomeric complex [Ca(dpp)2(thf)2]. Finally, an alternativesynthesis of [Ca(hfa)2(18-crown-6)] is reported and X-ray crystalstructures of [{Ca(tmhd)2}2(18-crown-6)], [Ca(dpp)2(thf)2], and[Ca(dpp)2(triglyme)] are described.

Results and discussion

Syntheses and spectral data

Alkaline-earth metal b-diketonate derivatives can be synthesizedby adopting different routes such as the protolysis of Ca(OH)2

with the desired b-diketone19 or through the metathesis reactionbetween hydrated Ca halide and a b-diketonate salt.20 In bothcases undesirable hydrated clusters are obtained. Anhydrous[{Ca(hfa)2}n] was prepared by Purdy et al.21 by direct reactionof calcium with b-diketonate excess; however, the product wasobtained with difficulty and low yield. Moreover, we verified thatthis reaction was unsuccessful with Htmhd and Hdpp.

The obtainment in good yield (~ 85%) of oxo- and water-free products can be achieved by adopting a synthetic strat-egy similar to the one proposed by Arunasalam et al.9 for[Ca3(tmhd)6]. More specifically, the homoleptic b-diketonate com-plexes [Ca3(tmhd)6], [{Ca(hfa)2}n], and [{Ca(dpp)2}n] have beenreadily obtained by reacting [{Ca(OCH3)2}n] with the correspond-ing b-diketone in n-hexane. Their formation can be schematized asfollows:

[{Ca(OCH3)2}n] + 2n H(b-diket) → [{Ca(b-diket)2}n]+ 2n CH3(OH)

with b-diket = tmhd, hfa and dpp.The compound [{Ca(dpp)2}n] is a pale yellow powder in-

soluble in aliphatic and aromatic hydrocarbons, chloroform,dichloromethane and acetonitrile, indicative of its oligomericnature. However, it is soluble in solvents containing oxygen donoratoms, such as acetone, tetrahydrofuran and dimethyl sulfoxidethanks to the formation of adducts that minimize its molecularoligomerization.

The 1H-NMR spectrum for [{Ca(dpp)2}n] exhibits a sharpsinglet at dH = 6.64 due to the methine (C–H) proton that doesnot shift appreciably from resonances of the methine proton in theHdpp sodium salts (lit.,22 dH = 6.30), and three sets of signals inthe aromatic region assigned to the phenyl hydrogen resonance.These signals appear at slightly lower field with respect to theprotons of benzene (Dd ~0.5) and therefore, as expected, theelectron-withdrawing power of the carbonyl group deshields theprotons directly attached to the phenyl ring. The 13C{1H}-NMRspectrum shows the same pattern reported for Na(dpp)22 with thecarbonyl carbon that displays sizeable high-field shift relative tothe analogous carbon of the neutral Hdpp (Dd = 2),22,23 while alow-field shift is observed for the ipso carbon of the phenyl ringdue to the deshielding contribution of the electron-withdrawingof the carbonyl group.

The reactions of the preformed calcium b-diketonates with O-donor ligands give new air stable calcium b-diketonate complexesas shown in the following equation:

x/n [{Ca(b-diket)2}n] + yL → [{Ca(b-diket)2}xLy]

with b-diket = dpp, L = thf (x = 1, y = 2); b-diket = dpp, L =triglyme (x = 1, y = 1); b-diket = dpp and tmhd, L = 18-crown-6(x = 2, y = 1); b-diket = hfa and L = 18-crown-6 (x = 1, y =1). This method represents a convenient and easy way contraryto the reported synthesis of [Ca(hfa)2(18-crown-6)], for whichthe reaction among calcium hydride, 18-crown-6 and Hhfa in a1 : 1 : 2 molar ratio allows the obtainment of purified product onlyafter a series of repeated sublimations in severe conditions (150–200 ◦C/10-3 Torr).18

The formation of these new calcium complexes is confirmedby their IR, 1H- and 13C-NMR spectra, elemental analyses andin several cases by single-crystal X-ray structure determinations(vide infra). The difference between found and calculated values ofelemental analysis of some compounds is probably due to moistureabsorption during measurement procedure.

The FT-IR spectra of complexes show strong absorptionbands in the 1660–1482 cm-1 region assignable toand bands of the b-diketonates.13,24–26 The presence ofpolyether ligands is indicated by low intensity C–O–C stretches,due to the crown ether,27 in the regions: 1176–1082 cm-1

for [{Ca(dpp)2}2(18-crown-6)], 1113–1044 cm-1 for [Ca(hfa)2(18-crown-6)] and 1110–1040 cm-1 for [{Ca(tmhd)2}2(18-crown-6)].Two medium bands at 1347 and 1120 cm-1 are assigned totriglyme18 for [Ca(dpp)2(triglyme)]. The coordination of THF for[Ca(dpp)2(thf)2] is confirmed by the presence of absorption bands28

at 1035 and 902 cm-1. The pale yellow crystalline [Ca(dpp)2(thf)2]and [Ca(dpp)2(triglyme)] are soluble in toluene, CH2Cl2 andcoordinative solvents as tetrahydrofuran and acetone, but insol-uble in aliphatic hydrocarbons. The compound [Ca(dpp)2(thf)2]loses partially the coordinated thf for prolonged pumping underdynamic vacuum at room temperature, while [Ca(dpp)2(triglyme)]and [{Ca(dpp)2}2(18-crown-6)] are stable toward the loss ofcoordinated ligands. The 1H-NMR patterns of [{Ca(dpp)2}n·Lx]in CD2Cl2 exhibit characteristic signals for dpp (the methineproton resonance is observed as a singlet in the range 6.91–6.40 ppm) and the donor ligands (18-crown-6, triglyme and thf),and their integration confirms the proposed formulation and thenumber of coordinated molecules. Similarly to 1H-NMR spectra,13C{1H}-NMR spectra reveal the dpp and the respective ligand(18-crown-6, triglyme and thf) resonances; moreover, signals donot show significant changes of chemical-shifts when movingfrom the free compound [{Ca(dpp)2}n] to the coordinated one,except for the resonances of the methine and carbonyl carbons of[Ca(dpp)2(thf)2] that result shifted low-field (Dd ~ 3).

The reaction of the preformed [Ca3(tmhd)6] with 18-crown-6ligand gives the complex [{Ca(tmhd)2}2(18-crown-6)]. The newcalcium complex is air stable and quite soluble in both aliphaticand aromatic hydrocarbons and in coordinating solvents, suchas ethers. In its 1H-NMR spectrum, at room temperature, thesignal forms suggest the presence of two types of b-diketonateenvironments and one type of 18-crown-6 ligand environment.The integration ratio of the b-diketonate to 18-crown-6 resonanceswas found to be 4 : 1 and the spectral profiles in toluene-d8

solution are temperature dependent. In the explored tempera-ture range between -60 to 80 ◦C the spectra show a steadyprogression towards one time-average tmhd environment andsingle resonances, in the tert-butyl and CH ring regions for theprotons of the tmhd ligands, occur only at temperature above thecoalescence temperature (ca. 30 ◦C). Two separate tert-butyl and

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CH resonances can be observed below this temperature and atleast four tmhd environments at -60 ◦C. These results indicatethat at temperatures above 30 ◦C the system is highly fluxional,while at lower temperatures the fluxionality is eliminated.

A sublimation experiment showed that the complex decomposesat 100 ◦C/10-3 Torr with loss of the coordinated ligand as shown by1H-NMR of the obtained sublimate. According to the literature,1

it is known that some [{M(b-diket)2}n(L)] derivates lose some orall their polydentate ligands on sublimation to give sublimates ofcomposition [{M(b-diket)2}n]. However, it is well demonstratedthat these compounds are still useful sources of volatile [{M(b-diket)2}n] species.29,30

Differently from [{Ca(hfa)2}n], where the metal with 18-crown-6 gives [Ca(hfa)2(18-crown-6)] of 1/1 stoichiometric ratio, thereaction of [Ca3(tmhd)6] and [{Ca(dpp)2}n] with the cyclic ligandgives complexes with 2/1 stoichiometric ratio. Therefore, as theX-ray diffraction shows, the central metals of [{Ca(tmhd)2}2(18-crown-6)] cannot behave like the central calcium of [Ca(hfa)2(18-crown-6)] that can be accommodated inside the ring of themacrocyclic ligand as to form a stable complex31 and that canbe sublimed at 90 ◦C/10-3 Torr. This different thermal behaviorbetween [{Ca(tmhd)2}2(18-crown-6)] and [Ca(hfa)2(18-crown-6)]is probably due to the stronger basicity of tmhd ligand than hfa.In fact, as basicity of the b-diketonate ligands increases, the bondsbetween the metal and ether oxygen atoms become weaker, leadingto the possible complex decomposition at high temperature.

X-Ray diffraction structure of [{Ca(tmhd)2}2(18-crown-6)]†

The crystal structure of [{Ca(tmhd)2}2(18-crown-6)] is shown inFig. 1. The calcium ions are related by a crystallographic inversioncenter and are approximately halfway between the crown etherand two b-diketonate moieties. The coordination geometry of thecalcium is almost a capped trigonal prism, with the O(2) oxygenof the crown ether as the capping atom (Fig. 2), while four oxygenatoms of the chelated tmhd ligands and two oxygen atoms of the18-crown-6 ether complete the hepta-coordination around eachcalcium atom.

The carbons of tert-buthyl (C(26), C(27) and C(28)) aredisordered in two positions with an occupation factor of 0.6 and0.4 which is in line with the solution spectra data.

Fig. 1 Solid-state molecular structure of [{Ca(tmhd)2}2(18-crown-6)]with thermal ellipsoids at 30% probability level (¢ at 1 - x, 1 - y, 1 -z). Only one position of the disordered tert-butyl (C(26), C(27), C(28)) isreported, hydrogen atoms are omitted for clarity.

Fig. 2 Coordination polyhedron of [{Ca(tmhd)2}2(18-crown-6)] withthermal ellipsoids at 40% probability level.

It is worthwhile noticing that, in the 1 : 1 guest–host complexof [Ca(hfa)2(18-crown-6)], the calcium atom is inserted in themacrocycle and the bidentate ligands are in trans position withrespect to the plane of the six coordinated crown ether with a tencoordination of the calcium atom.31

The Ca–O bond lengths (Table 1) fall into two distinct categorieswhich can be correlated to the nature of the oxygen linkage:thus, the shorter Ca–O distances of 2.273(2)–2.360(2) A (average2.313 A) are associated with the chelating tmhd ligands, whilst thelarger ones of 2.540(2)–2.604(2) A (average 2.581 A) belong to thecoordinated 18-crown-6.

The same effect was observed in the [Ca(hfa)2(18-crown-6)]31

and [Ba(hfa)2(18-crown-6)]32 where the M–Odiketone distances areshorter than the M–Ocrown. The calcium ion is apart from the planedefined by the crown oxygen atoms by 1.870(1) A compared withthe value of 1.678 A found in the 1 : 2 host : guest complexes ofdicyclohexano-18-crown-6 with potassium phenoxide.33

Table 1 Relevant bond lengths (A) and angles (◦) for [{Ca(tmhd)2}2(18-crown-6)]

Ca–O(1) 2.602(2) Ca–O(5) 2.360(2)Ca–O(2) 2.540(2) Ca–O(6) 2.273(2)Ca–O(3) 2.604(2) Ca–O(7) 2.292(2)Ca–O(4) 2.330(2) O(4)–C(7) 1.248(2)O(1)–C(1) 1.431(3) O(5)–C(9) 1.260(2)O(2)–C(2) 1.424(3) O(6)–C(18) 1.253(2)O(2)–C(3) 1.429(3) O(7)–C(20) 1.255(2)O(3)–C(4) 1.418(3) C(5)–O(3) 1.426(3)O(1)¢–C(6) 1.436(2)O(1)–Ca–O(2) 64.5(1) O(3)–Ca–O(4) 73.1(1)O(2)–Ca–O(3) 63.2(1) O(3)–Ca–O(5) 132.6(1)O(1)–Ca–O(3) 99.1(1) O(3)–Ca–O(6) 84.7(1)O(1)–Ca–O(4) 134.9(1) O(3)–Ca–O(7) 143.7(1)O(1)–Ca–O(5) 82.5(1) O(4)–Ca–O(5) 73.1(1)O(1)–Ca–O(6) 139.1(1) O(4)–Ca–O(6) 85.4(1)O(1)–Ca–O(7) 77.8(1) O(4)–Ca–O(7) 133.9(1)O(2)–Ca–O(4) 135.3(1) O(5)–Ca–O(6) 124.2(1)O(2)–Ca–O(5) 146.4(1) O(5)–Ca–O(7) 83.4(1)O(2)–Ca–O(6) 81.9(1) O(6)–Ca–O(7) 75.8(1)O(2)–Ca–O(7) 83.7(1)

¢ 1 - x, 1 - y, 1 - z.

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X-Ray diffraction structure of [Ca(dpp)2(thf)2]†

As reported in Fig. 3, the crystal structure of [Ca(dpp)2(thf)2]consists of discrete isolated centrosymmetric molecules and thetwo crystallographically equivalent thf and dpp groups are coordi-nated to the calcium atom [O(1)–Ca–O(2) 77.2(1)◦, O(1)–Ca–O(3)89.2(1)◦, O(2)–Ca–O(3) 86.4(1)◦] in an octahedral coordinationgeometry.

Fig. 3 Crystal structure of [Ca(dpp)2(thf)2] with thermal ellipsoids at 30%probability level (¢ at -x, -y, -z). Selected bond lengths (A) and angles(◦): Ca–O(1) 2.259(2), Ca–O(2) 2.292(2), Ca–O(3) 2.371(2), C(1)–O(1)1.264(3), C(3)–O(2) 1.259(2), C(1)–C(2) 1.384(3), C(2)–C(3) 1.399(3),O(1)–Ca–O(2) 77.21(6), O(1)–Ca–O(3) 89.2(1), O(2)–Ca–O(3) 93.6(1),Ca–O(1)–C(1) 131.1(1), Ca–O(2)–C(3) 130.3(1), O(1)–C(1)–C(2) 125.1(2),O(2)–C(3)–C(2) 124.1(2), C(1)–C(2)–C(3) 126.8(2).

The Ca–O bond lengths of the b-ketoenolate O atoms [Ca–O(1)2.260(2) and Ca–O(2) 2.292(2)] are in good agreement with thosereported for a similar complex.34 The Ca–O(thf) bond distancesare larger [2.371(2) A] as found in [Ca(o-TolForm)2(thf)2]35 and intrans-bi-aqua-tetrakis(tetrahydrofuran-O)-calcium di-iodide.36

The dpp groups chelate the calcium almost symmetrically. Thetwo endocyclic C–O [O(1)–C(1) and O(2)–C(3)] and C–C [C(1)–C(2) and C(2)–C(3)] bond distances are nearly equal, withinexperimental error, with values in between those for single anddouble bonds, indicating complete p-electron delocalization overthe b-ketoenolate system.

The coordinated thf molecules, which occupy mutually transpositions, are exactly coplanar, like the two equivalent b-ketoenolatecalcium heterocycles due to the presence of a crystallo-graphic inversion center of the molecule. The two phenyl rings aretwisted with respect to the ketoenolatecalcium moiety by 23.4(1)and 30.6(1)◦, respectively.

X-Ray diffraction structure of [Ca(dpp)2(triglyme)]†

The calcium atom is coordinated to the eight available oxygenatoms, with the triglyme ligand partially encapsulating the metalion in the equatorial belt, with the b-diketonate ligands in oppositepositions as reported in Fig. 4.

The coordination polyhedron can be described as bicappedtrigonal prism with O(1) oxygen of the b-diketonate and O(6)oxygen of the triglyme ligands as the capping atoms (Fig. 5).

Fig. 4 Crystal structure of [Ca(dpp)2(triglyme)] with thermal ellipsoidsat 30% probability level. Selected bond lengths (A) and angles (◦):Ca–O(1) 2.354(4), Ca–O(2) 2.367(4), Ca–O(3) 2.406(4), Ca–O(4) 2.357(4),Ca–O(5) 2.492(5), Ca–O(6) 2.582(5), Ca–O(7) 2.605(5), Ca–O(8) 2.436(4),O(1)–Ca–O(2) 72.6(1), O(3)–Ca–O(4) 71.7(1), O(5)–Ca–O(6) 64.1(2),O(6)–Ca–O(7) 63.7(2), O(7)–Ca–O(8) 64.3(2).

Fig. 5 Coordination polyhedron of [Ca(dpp)2(triglyme)] with thermalellipsoids at 40% probability level.

The b-diketonate ligands are asymmetrically chelated to themetal centre with Ca–O(1), Ca–O(2) and Ca–O(3), Ca–O(4)bond distances of 2.354(4), 2.367(5) and 2.406(4), 2.357(4) A,respectively.

The triglyme ligand chelates the calcium metal centre with Ca–O(5), Ca–O(6), CaO(7) and CaO(8) with distances of 2.492(5),2.582(5), 2.605(5) and 2.436(5) A, respectively. One may note thatthe two glyme oxygen atoms O(5) and O(8) bind closer to thecalcium than O(6) and O(7) being at the degree flexibility withrespect to O(6) and O(7). All these distances are in agreement withthose found for analogues derivative [Ca(tmhd)2(triglyme)].18

Experimental

All reactions were carried out in N2 atmosphere-dry-box. Allsolvents (n-hexane, toluene and tetrahydrofuran) were dried andpurified by distillation from sodium benzophenone ketyl before

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their use. Methanol was dried with CaH2 and distilled, whilecalcium metal, Htmhd, Hhfa and Hdpp were purchased fromAldrich and used as received. Calcium methoxide was obtainedby reaction of methanol with calcium in neat.

C and H elemental analyses were performed using a Fisons1108 elemental analyzer. IR spectra were recorded as nujol mullsbetween KBr plates and measurements were carried out by meansof a Mattson Galaxy Series FT-IR 3000 spectrometer operatingin the range 4000–400 cm-1.

1H- and 13C-NMR spectra were recorded by using a BrukerAMX 300 (300 MHz) spectrometer and proton spectra werereferenced to the used deuterated solvents (the protio impurities ofthe deuterated solvent were used as reference for 1H-NMR, whilethe 13C-NMR resonance of the solvent as a reference for 13C-NMRspectra; the obtained chemical shifts were computed respect toTMS). The assignments were secured by HMQC experiments.

Synthesis of [Ca3(tmhd)6]

Htmhd (0.8 mL, 4 mmol) was added to a white suspension of[{Ca(OCH3)2}n]37 (0.204 g, 2 mmol) in hexane (40 mL) and thereaction mixture was allowed to stir overnight giving a clearcolorless solution. The solvent and the released methanol wereremoved under reduced pressure to yield a white solid, whoseelemental analysis, 1H- and 13C-NMR signals are indicative of[Ca3(tmhd)6] according to literature data.9

Synthesis of [{Ca(hfa)2}n]

The synthesis was done by reaction between [{Ca(OCH3)2}n]37

(0.511 g, 5.0 mmol) with Hhfa (1.4 mL, 10 mmol) in a similarmanner as above described for [Ca3(tmhd)6]. The product wascharacterized by elemental analysis and by comparison of its 1H-NMR spectrum according to literature data.21

Synthesis of [{Ca(dpp)2}n]

Hdpp (2.600 g, 11.6 mmol) was added to a suspension of[{Ca(OCH3)2}n]37 (0.592 g, 5.8 mmol) in toluene (30 mL) andthe reaction mixture was stirred overnight at room temperaturewith the formation of a yellow solid. The product was collected,washed with hexane and held under vacuo for 1 h. Anal. (Found:C, 74.27; H, 4.57. Calc. for C30H22CaO4: C, 74.05; H, 4.56%). IR(Nujol), n/cm-1: 1597 s, 1553 m, 1515 m, 1480 s, 1406 m, 1307 w,1276 w, 1220 w, 1068 w, 1021 w, 941 w, 825 w, 748 w, 686 w, 618w, 608 w, 521 w. 1H-NMR (300 MHz, DMSO-d6), d (ppm): 7.97(8H, br d, ortho-H of Ph), 7.41 (12H, br m, para-H and meta-H ofPh), 6.64 (2H, s, CH). 13C{1H}-NMR (75.47 MHz, DMSO-d6), d(ppm): 183.05 (CO), 142.82 (C-CO-CH), 129.95 (para-C of Ph),128.01 (meta-C of Ph), 126.94 (ortho-C of Ph), 92.49 (CH).

Synthesis of [Ca(dpp)2(thf)2]

[{Ca(dpp)2}n] (1.500 g, 3.1 mmol) was dissolved in 30 mL oftetrahydrofuran and the complete solubilization of the productrequired about 10 min. Suitable crystals for X-ray analysis weregrown for slow evaporation of the solvent. Anal. (Found: C, 71.48;H, 5.93. Calc. for C38H38CaO6: C, 72.35; H, 6.07%). IR (Nujol),n/cm-1: 1602 s, 1548 s, 1515 s, 1482 s, 1410 s, 1300 m, 1265 m,1222 m, 1179 w, 1154 w, 1064 w, 1059 w, 1035 m, 1019 m, 902 w,

779 w, 738 s, 684 m, 606 w, 505 m. 1H-NMR (300 MHz, CD2Cl2),d (ppm): 7.72 (8H, br d, ortho-H of Ph), 7.24 (12H, br m, para-Hand meta-H of Ph), 6.40 (2H, br s, CH), 3.63 (8H, m, CH2 of thf),1.77 (8H, m, CH2 of thf). 13C{1H}-NMR (75.47 MHz, CD2Cl2),d (ppm): 186.27 (CO), 141.77 (C-CO-CH), 130.57 (para-C of Ph),128.44 (meta-C of Ph), 127.81 (ortho-C of Ph), 96.05 (CH), 68.22and 25.87 (CH2 of thf).

Synthesis of [Ca(dpp)2(triglyme)]

Triglyme (0.180 g, 1 mmol) was added to a solution of[Ca(dpp)2·(thf)2] (0.630 g, 1 mmol) in toluene (30 mL). After30 min the solvent was removed under vacuum giving a paleyellow powder which was dissolved in dichloromethane. Afterslowly evaporation of the solvent, suitable crystals for X-rayanalysis were obtained. Anal. (Found: C, 68.68; H, 7.70. Calc.for C38H40CaO8: C, 68.65; H, 6.06%). IR (Nujol), n/cm-1: 1604 s,1566 m, 1510 s, 1421 s, 1408 s, 1347 m, 1305 m, 1268 w, 1248w, 1216 m, 1120 m, 1091 m, 1073 m, 1053 w, 1021 m, 949 w,874 w, 854 w, 825 w, 779 w, 731 m, 691 m, 617 w, 608 m, 515 w.1H-NMR (300 MHz, CD2Cl2), d (ppm): 7.96 (8H, br d, ortho-Hof Ph), 7.37 (12H, br m, para-H and meta-H of Ph), 6.58 (2H, s,CH), 3.99 (4H, br s, CH3OCH2CH2OCH2CH2OCH2CH2OCH3),3.72 (4H, br m, CH3OCH2CH2OCH2CH2OCH2CH2OCH3),3.50 (4H, s, CH3OCH2CH2OCH2CH2OCH2CH2OCH3), 3.33(6H, s, CH3OCH2CH2OCH2CH2OCH2CH2OCH3). 13C{1H}-NMR (75.47 MHz, CD2Cl2), d (ppm): 183.89 (CO), 143.14(C-CO-CH), 129.93 (para-C of Ph), 128.24 (meta-C of Ph),127.53 (ortho-C of Ph), 92.77 (CH), 71.83 (CH3OCH2CH2OCH2-CH2OCH2CH2OCH3), 70.44 and 70.00 (CH3OCH2CH2-OCH2CH2OCH2CH2OCH3 and CH3OCH2CH2OCH2CH2-OCH2CH2OCH3), 59.60 (CH3OCH2CH2OCH2CH2OCH2-CH2OCH3).

Synthesis of [{Ca(dpp)2}2(18-crown-6)]

An excess of 18-crown-6 (0.260 g, 1 mmol) was added to a solutionof [Ca(dpp)2(thf)2] (0.630 g, 1 mmol) in toluene (30 mL). Afterone night, a pale yellow solid was formed, it was filtered, washedseveral times with hexane and dried under reduced pressure. Anal.(Found: C, 70.21; H, 5.27. Calc. for C36H34CaO7: C, 69.88; H,5.55%). IR (Nujol), n/cm-1: 1600 s, 1557 s, 1510 s, 1425 s, 1410 s,1297 m, 1272 m, 1216 m, 1176 w, 1109 w, 1082 m, 1066 m, 1022 w,939 w, 869 w, 833 w, 781 w, 740 m, 692 m, 610 m, 517 w. 1H-NMR(300 MHz, CD2Cl2), d (ppm): 7.99 (16H, br d, ortho-H of Ph),7.52 (24H, br m, para-H and meta-H of Ph), 6.91 (4H, br s, CH),3.59 (24H, br s, OCH2). 13C{1H}-NMR (75.47 MHz, CD2Cl2), d(ppm): 184.73 (CO), 142.97 (C-CO-CH), 129.70 (para-C of Ph),129.07 (meta-C of Ph), 127.88 (ortho-C of Ph), 94.34 (CH), 68.81(OCH2).

Synthesis of [Ca(hfa)2(18-crown-6)]

[{Ca(hfa)2}n] (0.681 g, 1.5 mmol) was suspended in hexane (30 mL)and 18-crown-6 (0.395 g, 1.5 mmol) was added directly to thesolution. The reaction mixture was stirred for a night, after whichit was filtered and washed several times with cold hexane andthen dried under reduced pressure. Anal. (Found: C, 36.99; H,4.10. Calc. for C22H26CaF12O10: C, 36.78; H, 3.65%). IR (Nujol),n/cm-1: 1674 s, 1663 s, 1531 s, 1358 m, 1348 w, 1303 w, 1295 w,

8068 | Dalton Trans., 2010, 39, 8064–8070 This journal is © The Royal Society of Chemistry 2010

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Table 2 Crystallographic data of [{Ca(tmhd)2}2(18-crown-6)], [Ca(dpp)2(thf)2] and [Ca(dpp)2(triglyme)]

Compound [{Ca(tmhd)2}2(18-crown-6)] [Ca(dpp)2(thf)2] [Ca(dpp)2(triglyme)]·0.25CH2Cl2

Chemical formula C56H100Ca2O14 C38H38CaO6 C38.25H40.5CaCl0.5O8

Formula weight 1077.52 630.76 686.01Crystal system Triclinic Monoclinic MonoclinicSpace group P1 P21/c P21/ca/A 10.558(2) 9.939(2) 12.321(3)b/A 10.574(2) 15.574(3) 26.791(3)c/A 15.776(3) 10.861(2) 12.451(3)a/◦ 72.84(3)b/◦ 80.19(3) 104.10(3) 111.06(3)g /◦ 72.33(3)V/A3 1597.1(5) 1630.5(5) 3835(1)Z 1 2 4Dc/g cm-3 1.120 1.285 1.187F(000) 588 668 1450m(Mo-Ka)/mm-1 0.234 0.239 0.246Reflections collected 7307 3733 7043Reflections observed [I ≥ 2s (I)] 6301 2002 3286Final R1, wR2 0.050; 0.119 0.055; 0.110 0.089; 0.195

R1 = ∑‖F o| - |F c‖/∑

|F o|; wR2 = {∑

[w(F o2 - F c

2)2]/∑

[w(F o2)2]} 1

2 .

1255 s, 1194 s, 1137 s, 1113 s, 1095 s, 1082 s, 1044 m, 960 s, 950m, 841 m, 789 m, 755 m, 659 m, 578 m, 525 w. Moreover, theproduct was characterized by comparison of its 1H-NMR and13C{1H}-NMR spectra as previously reported in the literature.18

Synthesis of [{Ca(tmhd)2}2(18-crown-6)]

[Ca3(tmhd)6] (0.600 g, 0.5 mmol) was dissolved in hexane (30 mL)and 18-crown-6 (0.198 g, 0.75 mmol) was added directly to thesolution and stirred overnight. The solvent was removed undervacuum giving a white powder. A small part of the obtainedresidue was dissolved at room temperature in a minimum amountof n-hexane and well-formed single crystals suitable for X-raydiffraction were obtained by slow evaporation of the solvent. Anal.(Found: C, 63.02; H, 9.93. Calc. for C56H100Ca2O14: C, 62.47; H,9.36%). IR (Nujol), n/cm-1: 1590 s, 1577 m, 1536 m, 1505 m,1354 s, 1269 w, 1235 w, 1184 w, 1135 m, 1110 w, 1088 m, 1072 m,1040 w, 1024 w, 940 m, 925 w, 866 m, 848 w, 821 w, 790 w, 753 w,734 m, 597 w, 572 w, 507 w, 472 m. 1H-NMR (300 MHz, C7D8),d (ppm): 5.73 (4H, br s, CH), 3.54 (24H, s, CH2), 1.27 (72H,br s, CH3).13C{1H}-NMR (75.47 MHz, C6D6), d (ppm): 199.09(CO), 89.66 (CH), 70.78 (OCH2), 40.94 (C(CH3)3), 28.87 (CH3).The complex does not sublime and its dissociation takes place at100 ◦C/10-3 Torr with the evaporation of 18-crown-6 ligand.

Crystal structure determination of [{Ca(tmhd)2}2(18-crown-6)],[Ca(dpp)2(thf)2] and [Ca(dpp)2(triglyme)]

Crystals were lodged in Lindemann glass capillaries and centeredon a four circle Philips PW1100 diffractometer using graphitemonochromated Mo-Ka radiation (0.71073 A), at room tem-perature, following the standard procedures. The diffraction datawere corrected for Lorentz-Polarization effects and for absorption,as described by North et al.38 The structures were solved bystandard direct methods39 and subsequently completed by Fouriersyntheses. Non-hydrogen atoms were refined anisotropically. Thehydrogen atoms were introduced at the calculated positions withfixed isotropic thermal parameters (1.2 U equiv of the parent carbon

atom). Structure refinement and final geometrical calculationswere carried out with SHELXL-9740 program, implemented inthe WinGX package,41 drawings were produced using ORTEP-3.42 The data collection and structure solutions parameters arelisted in Table 2.

Conclusion

In this paper we were able to demonstrate that, despite the calciumb-diketonates tendency to generate polymeric derivatives, the useof bulky b-diketonates (dpp and tmhd) and Lewis bases suchas thf, triglyme and 18-crown-6 favors the achievement of lesscomplex adducts with unusually low coordination numbers. As awhole the obtained results demonstrate, on one hand, the dpp andtmhd capacity of providing sufficient steric hindrance to determinelow coordination numbers; on the other hand, the capabilityof ancillary coordinating molecules to reduce oligomerizationprocesses. Nevertheless, it has to be stressed that the mononuclearcomplex with an unusually low Ca2+ coordination number ob-tained with dpp and thf is not stable under sublimation conditions(it loses the coordinated thf molecules and it probably formsnon-volatile oligomers). A similar behavior is shown by the twoadducts obtained with cyclic polyether, [{Ca(dpp)2}2(18-crown-6)] and [{Ca(tmhd)2}2(18-crown-6)], that dissociate by heating,thus indicating that the intermolecular bridging is favored overintramolecular chelation.

Acknowledgements

Skillful cooperation of Mr Valerio Corrado and Mr Luigi Rizzois gratefully acknowledged. FISR-MIUR Project “Inorganic andhybrid nanosystems for the development and innovation of fuelcells” is gratefully acknowledged for financial assistance.

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