Structure and Polymorphism of M (thd) 3 ( M = Al, Cr, Mn, Fe, Co, Ga, and In)

ARTICLE

DOI: 10.1002/zaac.201200478

Structure and Polymorphism of M(thd)3 (M = Al, Cr, Mn, Fe, Co, Ga, and In)

Mohammed A. K. Ahmed,[a] Helmer Fjellvåg,[a] Arne Kjekshus,*[a] and David S. Wragg[a]

Keywords: Metal(III) β-diketone complexes; Formation; Crystal structures; Polymorphism; Transition between polymorphs

Abstract. Formation, crystal structure, polymorphism, and transitionbetween polymorphs are reported for M(thd)3, (M = Al, Cr, Mn, Fe,Co, Ga, and In) [(thd)– = anion of H(thd) = C11H20O2 = 2,2,6,6-tetramethylheptane-3,5-dione]. Fresh crystal-structure data are pro-vided for monoclinic polymorphs of Al(thd)3, Ga(thd)3, and In(thd)3.Apart from adjustment of the M–Ok bond length, the structural charac-teristics of M(thd)3 complexes remain essentially unaffected by changeof M. Analysis of the M–Ok, Ok–Ck, and Ck–Ck distances support thenotion that the M–Ok–Ck–Ck–Ck–Ok– ring forms a heterocyclic unitwith σ and π contributions to the bonds. Tentative assessments accord-ing to the bond-valence or bond-order scheme suggest that thestrengths of the σ bonds are approximately equal for the M–Ok,Ok–Ck, and Ck–Ck bonds, whereas the π component of the M–Ok bondsis small compared with those for the Ok–Ck, and Ck–Ck bonds. The

1 Introduction

Metal (M) β-diketone chelates have attracted considerableattention from basic as well as applied research throughoutmore than half a century.[1] The basic interests have from thevery beginning been focused largely on structural features de-rived by single-crystal X-ray diffraction (SXD). More recently,the access to supplementary information from gas-phase elec-tron diffraction (GED),[2–4] infrared (IR),[5] and Raman[6] spec-troscopy, and density-functional-theory (DFT) computations[4–6]

have greatly stimulated the interest, which, in turn, has led toa growing activity in the charting of all sorts of propertiesof such materials. The relationships, which are established[7]

between polymorphs of tris 2,2,4,4-tetramethylhepatane-2,4-dione chromium(III) [Cr(thd)3] with regard to conditions dur-ing formation of a given polymorph and its thermal stabilitycan serve as examples of findings, which have tickled the curi-osity. Based on structural information collected with high-(HT) and low-temperature (LT) PXD (P = powder) and SXDand differential scanning calorimetry (DSC) we advanced[7]

the working hypothesis that rotational disorder increases with

* Prof. Dr. A. KjekshusFax: +47-22855441E-Mail: [email protected]

[a] Centre for Material Science and NanotechnologyDepartment of ChemistryUniversityof OsloP. O. Box 1033 Blindern0315 Oslo, NorwaySupporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/zaac.201200478 or from the au-thor.

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2013, 639, (5), 770–778770

contours of a pattern for the occurrence of M(thd)3 polymorphs suggestthat polymorphs with structures of orthorhombic or higher symmetryare favored on crystallization from the vapor phase (viz. sublimation).Monoclinic polymorphs prefer crystallization from solution at tem-peratures closer to ambient. Each of the M(thd)3 complexes subject tothis study exhibits three or more polymorphs (further variants arelikely to emerge consequent on systematic exploration of the crystalli-zation conditions). High-temperature powder X-ray diffraction showsthat the monoclinic polymorphs convert irreversibly to the correspond-ing rotational disordered orthorhombic variant above some 100–150 °C (depending on M). The orthorhombic variant is in turn trans-formed into polymorphs of tetragonal and cubic symmetry before en-tering the molten state. These findings are discussed in light of thecurrent conceptions of rotational disorder in molecular crystals.

lattice symmetry as the temperature is increased. It is the ac-cumulated amount of rotational disorder together with the ther-mal motions of atoms and groups of atoms that decides be-tween sublimation, melting, and thermal decomposition. Oneof the aims of the present contribution has been to test thishypothesis on a broader selection of main group (Al, Ga, In)and transition metal (Cr, Mn, Fe, Co) M(thd)3 complexes. Aparticular object has been to procure sufficiently accuratestructural information for a thorough comparison of the maingroup M(thd)3 complexes with those comprising transitionmetals.

The growing attention from applied research has been di-rected toward utilization of β-diketone complexes for variouspurification purposes,[8,9] and for use as precursors in materialsyntheses. The preparation of the β-diketone complexes is nor-mally relatively straight forward,[9–12] but careful characteriza-tion is recommended to ensure that the final product has thedesired purity. There are essentially three options for purifica-tion of M(thd)3 complexes, (1) recrystallization from solution,in which the selected solvent (or mixture of solvents) andcrystallization temperature are prime variables; (2) growthfrom the vapor phase (viz. sublimation), where the growth pro-cess is principally controlled by pressure and temperatures ofvaporization and deposition; (3) zone melting, which presup-poses that the compound in question does not decompose priorto melting. Among the M(thd)3 complexes, which have beentested hitherto only few have exposed properties which makethem unsuitable for purification according to these methods.However, comprehensive and systemized information is ingreat demand.

Structure and Polymorphism of M(thd)3 (M = Al, Cr, Mn, Fe, Co, Ga, In)

A main reason for the growing interest in applied researchon M(thd)3 complexes stems from their suitability as precur-sors for preparation of thin films.[13–15] It is the high volatilityaccompanied by thermal stability at moderate temperatures andeasy synthesis, purification, and handling that make β-diketonecomplexes to excellent precursors[13–15] for chemical vapor de-position [CVD; in particular to atomic layer (AL) CVD].Among such complexes, those based on thd appear to top thelist with regard to desired properties for ALCVD [e.g., higherthermal decomposition temperatures than the chemically sim-pler acetylacetonate (acac) complexes].

Thin polycrystalline coatings of aluminum, gallium, and in-dium oxides have, over the last couple of decades, attractedconsiderable attention for probable use in oxygen sensors.However, surprisingly little information was available, onAl(thd)3, Ga(thd)3, and In(thd)3,[2,8,16] when the presented pro-ject was commenced. Very recently there appeared a compre-hensive paper[4] on the crystal and molecular structures ofAl(thd)3 by SXD, GED, and DFT methods. The findings arepartly complentary to the presented results, and a joint dis-cussion is given in Section 3.

2 Experimental Section

2.1 Reactants and Solvents

The following chemicals were used (without further purification) asreactants for the syntheses of the M(thd)3 complexes studied.AlCl3·6H2O (Fluka, p.a., � 99.0%), GaCl3 (Aldrich, anhydrous,99.99%), InCl3 (Aldrich, anhydrous, 98%), MnCl2·4H2O (Fluka,� 99%), KMnO4 (Merck, purum, 99%), FeCl3·6H2O (Merck, 99%),CoCl3·6H2O (Fluka, p.a., � 98 %), H(thd) (Aldrich, purum, � 98%),CH3COONa·3H2O (Merck, p.a., � 99.5%), H2O2 [Prolabo (BDH), ca.33%], NH4OH (Merck, p.a., � 25%), and NaOH (Merck, p.a.,� 99.0%). Hexane (Fluka, p.a., � 99.5%), methanol (Merck, p.a.),abs. ethanol (Arcus, prima), and 1-propanol (Fluka, p.a., � 99.5%)were used as solvents. The alcohols were purified by heating ca. 50 mLROH with magnesium and iodine until first the iodine disappears andsubsequently all magnesium is consumed. Afterwards, ca. 500 mL ofthe alcohol was added, the resulting mixture was refluxed for 3 h,distilled into 3A molecular sieves (10% w/V), and allowed to standfor at least 24 h prior to use.

2.2 Syntheses

All syntheses were performed in round-bottomed flasks equipped withreflux condenser and magnetic stirrer. The reaction mixtures were keptwhilst stirring at temperatures between r.t. and b.t. of the solution fordifferent reaction periods. The solid products were filtered through asintered-glass funnel, washed, dried under vacuum, and finally sub-jected to purification by recrystallization and/or sublimation. Thephase purity of all final products was checked by PXD.

The syntheses of M(thd)3 complexes followed the standard procedureof Hammond et al. [11] with necessary modifications enforced by thestarting materials. The preparation of Al(thd)3 may serve as an exam-ple. H(thd) (0.12 mol) dissolved in abs. ethanol (50 mL) was added toa solution containing AlCl3·6H2O (0.04 mol) in distilled water(120 mL). Afterwards, a solution of NaOH (0.12 mol) in distilled water

Z. Anorg. Allg. Chem. 2013, 770–778 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 771

(15 mL) was added dropwise under vigorous stirring. The stirring ofthe reaction mixture was continued for 2 d at room temp. and then thesolid particles were left to settle for a few hours. The latter “trick”facilitates filtration, which otherwise may turn out to be quite cumber-some. The white solid product was washed with distilled water, driedunder vacuum at 60–65 °C for 1 d and further purified by sublimationat 95 °C.

2.3 X-ray Diffraction (XD)

All samples were characterized by PXD at 22 °C with a Bruker D5000diffractometer (capillary or flat-plate reflection geometry) using mono-chromatic Cu-Kα1 radiation (λ = 1.540598 Å) from an incident-beamgermanium monochromator and a position-sensitive detector. Si (a =5.431065 Å) served as internal standard. The diffraction patterns werecollected over the 2θ range 3–90° and analyzed using the programTOPAS academic.[17] The variable-temperature PXD data were col-lected on the same instrument. Samples were packed between plugs ofsilica wool in 0.7 mm internal diameter silica-glass capillaries. Thecapillaries were mounted in a flow cell,[18] and nitrogen was flowedover the sample during the measurements. The temperature at the posi-tion of the sample was calibrated using silver as external standard.

SXD data were collected for different crystals at various temperaturesbetween 100 and 295 K (Bruker D8 Apex II diffractometer; Mo-Kα

radiation (λ = 1.540598 Å). Crystals were mounted on thin glass fib-bers glued to brass pins. The data were integrated with SAINT[19] andcorrected for absorption using SADABS.[20] The crystal structureswere solved by direct methods with the program SIR2004[21] and re-fined using full-matrix least-squares against |F|2 with SHELXL-97[22]

as implemented in Farrugia’s WinGX suite.[23] Twinning was exploredusing the program TWINROMAT.[24] All non-hydrogen atoms wererefined with anisotropic displacement parameters (introduced for Malready from the beginning and for carbon and oxygen atoms at thepenultimate stage before the hydrogen atoms were added). Hydrogenatoms were placed in idealized positions and refined with isotropicthermal parameters proportional to the parameter for the atoms, towhich they are attached (riding model). Information on the crystals,data collection, and reliabilities factors for the reported structures isrecorded in Table 1 together with unit-cell dimensions and space-groupassignments.

Full crystallographic data have been deposited on the crystallographyopen database (www.crystallography.net). Copies of the data can beobtained free of charge.

2.4 Elemental Analysis

Elemental analyses were performed by the standard combustion tech-nique at Ilse Beetz or Birmingham University (UK). The content ofmetal in M(thd)3 (M = Al, Mn, Fe, Ga, and In) was determined onconverting the complexes into Al2O3, MnO2, Fe2O3, Ga2O3, and In2O3,respectively. Care must be exercised during the incipient oxidationstages to avoid weight loss owing to sublimation of volatile intermedi-ates. Elemental analysis data and melting temperatures (m.t.) for thepresent complexes are summarized in Table 2, the findings forCr(thd)3 and Co(thd)3 being quoted from references.[7,12]

2.5 Thermogravimetric Analysis

Thermogravimetric (TG) analysis was performed with a Perkin-ElmerTGA7 system in a nitrogen atmosphere. Silica-glass containers were

A. Kjekshus et al.ARTICLETable 1. Single-crystal data and relevant parameters used in the refinements of the crystal structures of Al(thd)3, Ga(thd)3, and In(thd)3. Estimatedstandard deviations are given in parentheses.

Al(thd)3 Ga(thd)3 In(thd)3

Empirical formula C33H57O6Al C33H57O6Ga C33H57O6InFormula mass 576.77 619.51 664.61Crystal system monoclinic monoclinic monoclinica /Å 28.618(3) 9.928(7) 20.221(3)b /Å 18.809(2) 17.883(12) 17.576(3)c /Å 21.706(3) 21.514(15) 23.583(3)α /° 90.00 90.00 90.00β /° 96.888(1) 95.708(8) 112.102(2)γ /° 90.00 90.00 90.00Unit-cell volume /Å3 11599(2) 3801(5) 7765(2)Temperature /K 293(2) 293(2) 293(2)Space group C2/c C2/c C2/cNo, of formula units per unit cell, Z 12 4 8Max / min θ /° 2.22, 23.13 2.28, 27.82 1.59, 24.56No. of reflections measured 34277 11506 26292No. of independent reflections 8265 4274 6516No. of refined parameters 543 183 364Rint 0.0214 0.0416 0.0374Final R1 values [I � 2σ(I)] 0.0876 0.0976 0.0571Final wR(F2) values [I � 2σ(I)] 0.2976 0–3335 0.1682Final R1 values (all data) 0.1105 0.1258 0.0818Final wR(F2) values (all data) 0.3367 0.3540 0.1963Goodness of fit 1.383 1.093 1.017Max / min residual electron density 1.418, –0.495 1.911, –0.594 0.362, –0.267

Table 2. Analytical and theoretical (in italics) composition (in wt-%)of M(thd)3 complexes. Melting temperatures (m.t. in °C) are given inthe right-hand column with corresponding literature data (in italics).

M C H O M m.t.

Al 68.98 10.03 16.57 4.46 264–265a)

68.72 9.96 16.64 4.68 257[8], 264–265[10], 265–266[11],266[9]

Cr 65.88 9.48 15.88 8.64 233–235b)

65.86 9.55 15.87 8.64 229[10], 236[9]

Mn 65.88 9.62 15.56 8.87 168–169c)

65.54 9.50 15.87 9.09 163–171[10], 164–165[9], 165[9]

Fe 65.30 9.31 16.01 9.38 165d)

65.44 9.48 15.86 9.22 164–165[9,10], decomp.[9]e)

Co 65.58 9.52 15.46 9.48 248–250f)

65.11 9.44 15.77 9.68 240[10], 245[9]

Ga 64.14 9.14 15.46 10.94 228–231a)

63.98 9.27 15.49 11.25 220[8]

In 59.59 8.39 14.38 17.57 170a)

59.64 8.64 14.44 17.28 204–209[8]

a) Colorless in solid and liquid state. b) Dark purple solid, black inliquid state. c) Dark brown almost black solid, turns darker upon melt-ing. d) Orange solid, becomes black upon melting. e) One of the papersunder Ref. [9] reports that Fe(thd)3 is subject to decomposition uponmelting; the other states that Fe(thd)3 does not exist (see Section 3.3).f) Dark green, almost black solid, turns darker upon melting.

used as sample holders. The heating rate was 5 and/or 10 K·min–1, thetemperature interval covered was 30–700 °C, and the sample mass was15–30 mg.

Supporting Information (see footnote on the first page of this article):High-temperature X-ray diffraction data.

www.zaac.wiley-vch.de © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2013, 770–778772

3 Results and Discussion

3.1 Prototype Structures and Polymorphs of M(thd)3

A strategy for the presented project on M(thd)3 complexeswas to test thoroughly all batches of crystals, which appearsuitable for SXD. This has not only provided structural data,but has also given valuable insight into how and why crystalimperfections that cause trouble in the refinement process de-velop.

The literature on M(thd)3 and derivatives thereof containsmany examples of structure determinations, for which high Rfactors can be traced to insufficient account for disorder. Forthe orthorhombic high-temperature variants of Cr(thd)3

[7] andCo(thd)3

[12] the amount of defects and their diversity are soextensive that the SXD data could not be resolved to a sensiblestructure model for refinement. However, we have also ob-tained more perfect crystal specimens of the orthorhombicform in other batches of Co(thd)3, and the crystal structure[12]

could accordingly be resolved. As to whether strongly defec-tive (o*) or more defect-free (o) orthorhombic variants of theM(thd)3 complexes are formed is primarily a question of thetemperature during the crystallization (see also Sections 3.2and 3.3). (Note that strongly defective is a qualitative descrip-tive term, which does not refer to a distinct lattice symmetry.)The presented results suggest that defective and regular ortho-rhombic variants appear for all M(thd)3 complexes. For SXDof Fe(thd)3, which was selected as the next test case, the pro-gress has been slow owing to the extensive disordering of thehitherto obtained o/o* Fe(thd)3 crystals.

The crystal structures of monoclinic forms of Al(thd)3,Ga(thd)3, and In(thd)3 were determined (Table 1, Table 3, and


Table 3. Range and average of bond lengths (dmin. to dmax. and dav. in Å) and bond angles (φmin. to φmax. and φav. in °) together with individual bond valences(vi = exp[(Di–di)/bj]; Di and b from reference[25]) and bond-valence sums (V = ∑vi) for m3-Al(thd)3, m1-Ga(thd)3, and m2-In(thd)3. Intra- and interligandOk–M–Ok bond angles are differentiated and data for the latter category appear in square brackets. Bonds outside the chelate rings are not included.

Al(thd)3, m3-Co(thd)3 typeDomain of Al1

Al1–Ok Ok–Al1–OkRange: 1.862(2)–1.866(3) Range: 89.16(11)–90.33(12) [89.16(11)–91.23]Average: 1.864 Average: 89.91 [90.03]vAl1–O = 0.56; VAl1 = 6·0.56 = 3.36Ok–Ck Al1–Ok–CkRange: 1.258(4)–1.267(4) Range: 129.7(2)–130.1(2)Average: 1.260 Average: 130.0vO–C = 1.42; VO = vO–Al1 + vO–C = 1.98Ck–Ck Ok–Ck–CkRange: 1.378(5) –1.380(5) Range: 122.4(3) –123.2(3)Average: 1.379 Average: 122.9

Ck–Ck–CkRange: 122.4(2)–123.8(5)Average: 122

Al(thd)3; m3-Co(thd)3 typeDomain of Al2

Al2–Ok Ok–Al2–OkRange : 1.857(3)–1.871(3) Range: 89.73(13)–90.43(12) [88.81(12)–92.13(12)]Average: 1.864 Average: 90.10 [89.97]vAl2–O = 0.56; VAl2 = 6·0.56 = 3.36Ok–Ck Al2–Ok–CkRange: 1.241(5)–1.279(4) Range: 128.9(2)–130.5(3)Average: 1.261 Average: 130.0vO–C = 1.42; VO = vO–Al2 + vO–C = 1.98Ck–Ck Ok–Ck–CkRange: 1.369(5) –1.388(5) Range: 122.2(3)–124.1(4)Average: 1.379 Average: 122.7

Ck–Ck–CkRange: 123.0(4)–124.5(3)Average: 123.9

Ga(thd)3; m1-Mn(thd)3 type

Ga–Ok Ok–Ga–OkRange: 1.939(6)–1.951(5) Range: 89.7(2)–91.1(2) [86.6(2)–93.5(2)]Average: 1.946 Average: 90.6 [90.0]vGa–O = 0.56; VGa = 6·0.56 = 3.36Ok–Ck Ga–Ok–CkRange: 1.248(10)–1.268(8) Range: 126.0(5)–128.7(5)Average: 1.258 Average: 126.9vO–C = 1.43; VO = vO–Ga + vO–C = 1.99Ck–Ck Ok–Ck–CkRange: 1.367(11) –1.399(11) Range: 124.1(6)–124.1(8)Average: 1.386 Average: 124.1


In(thd)3; m2-Fe(thd)3 typeDomain of In1

In1–Ok Ok–In1–OkRange : 2.116(3)–2.125(4) Range: 84.63(15)–92.77(13) [83.40(13)–94.65(14)]Average: 2.120 Average: 90.06 [89.31]vIn1–O = 0.56; VIn1 = 6·0.56 = 3.36Ok–Ck In1–Ok–CkRange: 1.267(6)–1.280(6) Range: 123.7(3) –129.4(4)Average: 1.279 Average: 127.7vO–C = 1.35; VO = vO–In1 + vO–C = 1.91Ck–Ck Ok–Ck–CkRange: 1.385(9) –1.395(7) Range: 124.6(4) –124.8(6)Average: 1.389 Average: 124.7


In(thd)3; m2-Fe(thd)3 typeDomain of In2

In2–Ok Ok–In2–OkRange : 2.121(4)–2.125(4) Range: 83.46(14)–92.08(11) [83.46(14)–95.13(14)]Average: 2.124 Average:86.53 [90.10]vIn2–O = 0.55; VIn2 = 6. vIn2–O = 6·0.55 = 3.30Ok–Ck In2–Ok–CkRange: 1.261(6)–1.282(6) Range: 123.4(3)–129.8(3)Average: 1.274 Average: 126.0vO–C = 1.37; VO = vO–In2 + vO–C = 1.92Ck–Ck Ok–Ck–CkRange: 1.372(9) –1.389(6) Range: 123.3(5) –124.7(5)Average: 1.384 Average: 124.2



A. Kjekshus et al.ARTICLE

Figure 1. X-ray crystal structures of Al(thd)3 [m3-Co(thd)3 type], Ga(thd)3, [m1-Mn(thd)3 type], and In(thd)3 [m2-Fe(thd)3 type] at 293(2) K.Each structure is shown: (top) as a thermal ellipsoid plot of the asymmetric unit (50% ellipsoid probability) with the molecules in real orienta-tions, hydrogen atoms omitted; (middle) as a sketch to show the atom labeling clearly; and, (bottom) as a packing diagram (molecules furtherback indicated by thinner lines). The tert-butyl groups are omitted in the schematic and packing illustrations.

Figure 1). Whereas molecular structures of the M(thd)3 com-plexes are virtually identical in all M(thd)3 variants their ar-rangement in the crystal lattice may differ. This is the casefor the crystal structures of monoclinic Al(thd)3, Ga(thd)3, andIn(thd)3. However, the crystal lattices of the entire M(thd)3

family are closely related.These and similar findings demonstrate the need for a label-

ing scheme that exposes distinctions with regard to composi-tion as well as structure characteristics. (The traditional label-ing scheme with α, β, γ etc. for high-/low-temperature poly-morphs does not distinguish between phases with differentcomposition.)

A simple scheme for the present purpose is to mark eachstructure with its chemical formula, introduce a prefix, whichgives the lattice symmetry, and, if appropriate, complete thelabel with a number that specifies to the particular polymorph.Abbreviations used for lattice symmetry in this paper are a, m,o, h, t, and c for triclinic (a for anorthic is used herein assynonym to avoid confusion with tetragonal), monoclinic, or-thorhombic, hexagonal (trigonal), tetragonal, and cubic,respectively. The numbering should be unambiguous but isotherwise according to taste. In this paper, polymorphs are


numbered according to increasing unit-cell volume per formulaunit. But to avoid renumbering when a future detected poly-morphs break this system, we intend to convert to progressiverandom numbering. This gives the type designations m3-Co(thd)3, m1-Mn(thd)3, and m2-Fe(thd)3 for the presently re-ported crystal structures of Al(thd)3, Ga(thd)3, and In(thd)3,respectively.

The triclinic symmetry of the a-Al(thd)3 structure appearsto make this prototype special within the M(thd)3 family. Thebond lengths and angles reported[4] for a-Al(thd)3 concur withthe present and earlier findings for the M(thd)3 and M(acac)3

(Table 4). However, despite numerous attempts to grow crys-tals of a-Al(thd)3 by sublimation or from solution, crystals ofthis variant have hitherto never been obtained (reference[4]

does not give any information on synthesis and crystallization).A slightly disturbing feature of the report on a-Al(thd)3 is alsothat it specifies the unit-cell content as one formula unit. Thisis obviously wrong, two formula units per unit cell (Z = 2) areneeded to bring a-Al(thd)3 in line structurally with the rest ofM(thd)3 family. It may be difficult to visualize the relationshipbetween the a-Al(thd)3 crystal structure and that of the m3-Al(thd)3. However, if one instead compares the unit cells of a-


Table 4. Juxtapositioned bond lengths and angles (in Å and deg., respectively; averaged over data for known polymorphs) of M(thd)3 andM(acac)3 (M = Al, Cr, Mn, Fe, Co, Ga, and In). Bond assessments according to the bond-valence (for M–Ok and Ok–Ck; parameters fromreference[25]) or bond-order (for Ck–Ck; auxiliary data from reference[26]) scheme are printed in italics and listed directly after the correspondingbond length. Literature consulted in the preparation of the table is listed in the reference column.

Complex M–Ok Ok–Ck Ck–Ck Ok–M–Ok M–Ok–Ck Ok–Ck–Ck Ck–Ck–Ck References

Al(thd)3; SXD 1.869; 0.55 1.260; 1.42 1.379; 1.72 90.1 130.0 122.9 122.9 [4] presentAl(thd)3; DFT 1.897; 0.51 1.280; 1.36 1.411; 1.55 89.8 130.5 123.1 123.2 [4]

Al(acac)3; SXD 1.880; 0.54 1.260; 1.42 1.383; 1.69 90.5 128.4 123.6 124.2 [5]

Al(acac)3; DFT 1.916; 0.49 1.278; 1.36 1.403; 1.58 90.0 128.5 124.6 122.8 [5]

Cr(thd)3; SXD 1.944; 0.54 1.256; 1.44 1.399; 1.60 90.3 127.8 124.6 124.0 [7] presentCr(acac)3; SXD 1.953; 0.54 1.260; 1.42 1.403; 1.59 91.1 126.7 125.0 125.0 [5]

Cr(acac)3; DFT 1.979; 0.50 1.276; 1.36 1.404; 1.58 90.7 127.1 125.5 124.1 [5]

Mn(thd)3; SXD 1.988; 0.54 1.280; 1.35 1.369; 1.75 90.0 126.9 123.7 124.0 [27] presentMn(acac)3; SXD 1.993; 0.53 1.268; 1.39 1.384; 1.68 89.2 127.0 124.8 125.8 [5,28]

Mn(acac)3; DFT 2.010; 0.51 1.276; 1.36 1.405; 1.58 89.2 127.7 125.6 124.2 [5]

Fe(thd)3; SXD 1.995; 0.53 1.278; 1.38 1.405; 1.58 84.7 128.2 124.2 122.3 [29] presentFe(acac)3; SXD 1.992; 0.53 1.262; 1.42 1.384; 1.68 96.6 129.1 124.5 124.8 [5]

Fe(acac)3; DFT 2.011; 0.51 1.275; 1.36 1.405; 1.58 96.0 129.8 124.1 123.5 [5]

Co(thd)3; SXD 1.871; 0.63 1.268; 1.39 1.392; 1.65 90.1 126.9 124.2 122.3 [12] presentCo(acac)3; SXD 1.885; 0.61 1.269; 1.39 1.382; 1.70 96.6 123.5 125.3 125.5 [5]

Co(acac)3; DFT 1.899; 0.60 1.274; 1.37 1.402; 1.59 96.0 124.1 126.0 123.8 [5]

Ga(thd)3; SXD 1.946; 0.58 1.258; 1.43 1.386; 1.68 90.1 126.9 125.9 124.9 presentGa(acac)3; SXD 1.952; 0.55 1.261; 1.42 1.390; 1.66 90.0 126.6 125.2 124.3 [30]

In(thd)3; SXD 2.122; 0.54 1.277; 1.36 1.387; 1.67 88.3 126.9 124.5 127.0 presentIn(acac)3; SXD 2.132; 0.54 1.261; 1.42 1.386; 1.68 90.0 127.2 126.2 126.9 [31]

Average n.a.; 0.54 1.269; 1.39 1.393; 1.64 90.9 127.5 124.7 124.3

and o/o*-Al(thd)3 the axes of the former immediately emergesas halved face diagonals of the latter. The relationship betweenthe o- and m3-Co(thd)3 prototypes is outlined in reference.[7]

Incidentally, ongoing work has provided strong indications thata variant(s) of M(thd)3 complexes may appear as kinetic stabi-lized intermediate(s) on rapid quenching of specimens heattreated near m.t.

The unit-cell dimensions reported for In(thd)3 by Lisoivanand Gromilov[16] are in excellent agreement with thepresented data for m2-In(thd)3, and the type specification ism2-Fe(thd)3.

3.2 Bonding and Structure

It follows from the crystal and molecular structure data com-piled in Table 4 that, apart from the M–Ok bond lengths, themolecular geometry remains essentially invariant throughoutthe M(thd)3 and M(acac)3 series. The experimentally estab-lished M–Ok bond lengths exhibit a very nearly linear depen-dence on the size of M. (As an adequate measure for the sizedemands of M, we used the bond-valence parameter for M–O,which by definition equals the length of one single M–Obond.[25] Specific electronegativity corrections are not neededsince these are implicit in the M–O parameters.)

The findings further support our suggestion that all effectsof the exchange of M are taken care of by adjustments withinthe MO6 octahedral co-ordination sphere. Moreover, sincethere appears to be no significant distinction between main-group and transition-metal M(thd)3 complexes in this respect,it seems safe to conclude that d orbitals are not significantlyinvolved in bonding.

The six-membered M–Ok–Ck–Ck–Ck–Ok– rings in β-dike-tone complexes are very nearly planar and the bond lengths


and angles are in accordance with the expectation for such aheterocyclic σ–π bonding system. However, time is probablyripe for an attempt to go beyond such a qualitative descriptionof the bonding situation. The first step in a quantification pro-cess is to find an adequate measure for the total σ and π contri-butions involved. The semi-theoretical bond-valence versusbond length correlation[25] gives valence per bond for theM–Ok and Ok–Ck bonds. This approach cannot be used for theCk–Ck bonds, which instead were assessed from the corre-sponding relation between bond order and bond length.[26] Thethus derived total σ plus π valences involved in the M–Ok,Ok–Ck, and Ck–Ok bonds, fall within the ranges (Table 4;averages in parentheses) 0.49–0.63 (0.54), 1.35–1.44 (1.39),and 1.55–1.75 (1.64), respectively. These σ–π valence com-posites are tentatively resolved on the assumptions that: (i)Equal bonding capacity is attached to σ and π electrons. (ii)Total σ–π refers to the plain sum of the σ and π contributions.(iii) A single bond is associated with an electron pair, and thebond-valence and bond-order approaches are presupposed tobe compatible. (iv) According to traditional valence considera-tions MIII, Ok, and Ck contribute 1, 2 �2, and 3� 3 electrons,respectively, to the σ–π valence-electron pool (total 14 elec-trons) of each M–Ok–Ck–Ck–Ck–Ok– ring system. (v) The ex-perimental based bond valences/orders require 0.54�2 �2 +1.39� 2�2 + 1.65 �2� 2 = 2.16 + 5.56 + 6.60 = 14.32 elec-trons per ring. (vi) Reserving 12 electrons per ring for the σ-bond skeleton 2.32 electrons per ring would be left for π bond-ing. The survey in point (iv) shows that only 14 electrons perring should be available for the entire σ and π bonding. Thediscrepancy is quite small and certainly not alarming in rela-tion to the way of thinking.

The simplicity of the above train of ideas is attractive, butthorough checking of the concepts against data from

A. Kjekshus et al.ARTICLEother systems is required. The fact that the trend inthe π contribution from different parts of the chelate ring(M–Ok � Ok–Ck � Ck–Ck) comply with findings of the core-bonding-energy study[32] on Al(thd)3 and Al(acac)3 lends somesupport to our inferences.

Incidentally, the very nature of a mixed σ–π cyclic bondconfiguration appears to exclude the possibility of bonding in-teraction across the so-called O–O bite.

3.3 Origin of Polymorphism of M(thd)3

Polymorphism is a commonly observed feature, but the exis-tence of (at least) five polymorphs of Cr(thd)3

[7] is unusual. Aclaim of polymorphism must be supported by structural docu-mentation. This requirement is usually well fulfilled, whereasinformation on syntheses, transitions, stability limits etc. sel-dom is reported in papers on polymorphism. This deficiencymay make it difficult for followers to reproduce sample prepa-ration and obscures verification of polymorphism.

Polymorphism appears as a result of differing conditionsduring synthesis or subsequent treatments. A rational account-ing for the polymorphism among the M(thd)3 complexes usesthe stages where the terminating crystallizations take place tospecify conditions of formation. Polymorphs of M(thd)3 haveappeared as single-crystal deposits from solutions or the vaporphase. These specimens have provided the most detailed struc-tural information (Table 1, Table 3, and Table 4).

The common feature of M(thd)3 molecules in dissolved andvaporized state is that they behave as essentially non-inter-acting individual molecules.[2–4] When the complex precipi-tates, the particular polymorph obtained depends on tempera-ture and solvent as well as M. A systematic mapping of allimplications of changes in these variables is a rather formida-ble task and the present study should only be looked upon asan introductory survey, covering a couple of temperatureswithin the range from r. t.. to b.t. of the solvents considered.

The clear trend in the findings is that crystallization fromsolution favors the m-type variants (Table 5). We expect thata systematic exploration will uncover other m variants withinthe M(thd)3 family as a whole and more m polymorphs associ-ated with each family member. It is easy to imagine that thepresence of the solvent can have a significant influence onthe crystal-growth process as well as the resulting structure byreinforcing some of the weak (van-der-Waals type) intermo-lecular interactions and suppressing others. Crystal growthfrom the vapor phase was performed at 50–100 °C accordingto the customary sublimation procedure. The temperaturerange between 100 °C and the m.t. for M(thd)3 was examinedby HTPXD. The results show that o- and o*-type variants oc-cur for all M(thd)3 subject to this study. (As mentioned beforeit is likely that o* does not satisfy the formal requirements forregistration as a separate prototype. However, the notation isnevertheless retained as a convenient short-hand for highly dis-ordered o.) Literally speaking the o–o* combination holds acentral position among the prototypes of the M(thd)3 family,and always with o* on the HT side of o. In the same tempera-ture range as o is converted to o* the various m forms are


converted to o*. When these processes are completed abovesome 150 °C, o* and its higher symmetric successors t* andc* (Table 5) rule the temperature range up to m.t. of M(thd)3.Note that t* and/or c* may be skipped in some cases, viz. thecomplex may enter the melt phase directly from the o* stage.Under certain conditions (crystal growth by LT sublimationbut otherwise only superficially uncovered terms) metastablebatches of the t polymorph were obtained and we were able tofollow the gradual time-dependent conversion t to o to m bySXD.[7] (The latter step was observed after more than a yearstorage at room temp.) Although c variants were not observedin these LT experiments it seems probable that the initiallyformed crystals belong to the c polymorphs. We interpret thesefindings as tracks left behind from early stages of the crystalgrowth of M(thd)3 by sublimation but the information is toofragmentary to discuss kinetic aspects of the initial crystalli-zation process.

Table 5. Stable and/or metastable polymorphs of M(thd)3 complexes(specified by prototype) at room temp. and elevated temperatures.Transition temperatures /°C) refer to the appearance of the polymorphconcerned in HTPXD (see text for comments on uncertainty of thenumerical data).

M Prototype o/o* t* c* melt

Al a-Al(thd)3a); m3-Co(thd)3 120 235 250 265

Cr m1-Mn(thd)3; m3-Co(thd)3 155 – 200 235Mn m1-Mn(thd)3 110 – 155 165Fe m2-Fe(thd)3 110 – – 165Co m3-Co(thd)3; h-Co(thd)3; 135 190 225 250

o-Co(thd)3

Ga m1-Mn(thd)3 115 150 200 230In m2-Fe(thd)3 105 125 – 170

a) From reference[4].

A brief comment about m.t. for Fe(thd)3 is appropriate.Yoshida et al.[9] reported first that Fe(thd)3 decomposes priorto melting. In a later paper the same authors claim thatFe(thd)3 does not exist, and that the traditional preparation pro-cedure (which uses EtOH as solvent) leads to [Fe(thd)2(OEt)]2

formation. Hua et al.[10] established that both complexes doexist and provided an m.t. for Fe(thd)3 in line with the presentfindings (Table 1 and Table 5). There are indications of onsetof decomposition of Fe(thd)3 at m.t. This assertion (based onvisual-observations) is open for verification, an undertakingconveniently combined with a more extensive examination ofthe properties of Fe(thd)3 and [Fe(thd)2(OEt)]2. These com-plexes are expected to exhibit an abundance of interestingchemistry; probably a matching diversity to Cr(thd)3

[7] and[Cr(thd)2(OEt)]2.[33]

A comparison of the crystal growth of M(thd)3 by the solu-tion and sublimation methods shows that: (i) The growth rateis best controlled by the solution method. This is of particularimportance since moderate growth rate appears to provide thebest crystal perfection [for growth of M(thd)3 the best condi-tions appear to be slow evaporation of solvent fairly near roomtemp.]. The reproducibility of the solution method is also bet-ter than for the sublimation method, a feature, which is ap-preciated in dealing with compounds with many closely relatedpolymorphs. (ii) We speculate that the improved crystal quality


in samples prepared from solution is due to the lower impactof molecules joining the crystal surface from solution com-pared to vapor. The smoothening influence of the surroundingsolvent molecules makes the difference. (iii) The most impor-tant source to structural differences between crystals grownfrom solution and vapor phase is the perturbation of the inter-molecular forces caused by the presence of the solvent. Sincethe interactions between M(thd)3 molecules alone is weak,even small changes in the intermolecular forces can havemajor consequences for the crystal structure.

Eight prototypes (Table 5) were established with certaintywithin the M(thd)3 family. The crystal structures are indeedclosely related and vectorial equations and illustrations relatingthe various unit cells are given in reference.[7] A structuralrelation between two prototypes does not imply that a corre-sponding polymorph pair exists and even so transition may nottake place, e.g., a conversion m1-Cr(thd)3 to m3-Cr(thd)3 orvice versa, does apparently not occur, whereas all m forms areconverted to o* on heating to 100–150 °C.

Note that numerical data for structure variables and transi-tion temperatures for polymorph series like M(thd)3 may ap-pear as more exact and mutually compatible than there is basisfor. One thing is the temperature calibration, which always rep-resents a challenge when scanning instruments are used forcontinuous recording of data. For exploration of the M(thd)3

polymorphs the heating/cooling rates have proved to representa bottle neck. It is progressive o to o* disordering, and thefurther conversion to t* and c*, which impede a detailed com-parison between different M(thd)3 polymorphs. When theamount of disorder exceeds a certain level (which may differwithin the M(thd)3 series) there are, moreover, indications ofordering of the defects.

Transition temperature is a composition [M of M(thd)3] andstructure (kinds and amounts of defects, thermal displacementmodes, etc.) dependent property. A quick glance at m.t. for theM(thd)3 family in Table 2 may lead one to infer that it variesquite randomly. However, Figure 2 shows that melting occursat nearly the same unit-cell volume per formula unit.[1150�1250 Å3 (f.u.)–1] for all the M(thd)3 complexes studiedin this work.

The individual M(thd)3 molecules have from their very for-mation an approximately spherical shape, and with somesmoothing of uneven charge distribution, ccp, hcp, and bccarrangements could have been imagined for the M(thd)3 crystalstructures even at low temperatures. Creation of rotational dis-order (pseudo rotation when the disorder has a pronounceddynamic character) is a well-known means[34] to improve theapproximation to spherical shape. In general, the rotational dis-order increases with increasing temperature, because the ther-mal motions of atoms, groups of atoms or the molecule aswhole increase. A particular librational/rocking mode, e.g.,may be supplied with sufficient energy to cross the barrier intoa corresponding local-energy-minimum site. The ability to ac-commodate rotational disorder also increases with increasinglattice symmetry. Hence, the rotational disorder is expected toincrease strongly on going through the transition sequence o*to t* to c* to melt. The rapid increase in unit-cell volume on


Figure 2. Unit-cell volume per formula unit vs. temperature for poly-morphs of M(thd)3. Some curves are smoothed. Temperatures for tran-sitions between polymorphs are given in Table 5.

the approach to the melt stage (Figure 2) can be taken as asign that the lattice is preparing for the melting process byproviding space for the thermal movements of atoms andgroups of atoms. The composite thermal movement patternthus gradually adopts characteristic features of the melt state.At m.t. the distinction between such a virtual melt and an ac-tual melt has vanished.

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Received: October 30, 2012Published Online: March 14, 2013

Documents

Structure and Polymorphism of M (thd) 3 ( M = Al, Cr, Mn, Fe, Co, Ga, and In)