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Polyhedron 25 (2006) 2691–2697

Synthesis and structure of the diglyme-bridged leadhexafluoroacetylacetonate complex [Pb(hfac)2(l-g3:g1-diglyme)]2

William J. Evans *, Daniel B. Rego, Joseph W. Ziller

Department of Chemistry, University of California, Irvine, CA, 92697-2025, United States

Received 29 November 2005; accepted 20 March 2006Available online 3 April 2006

Abstract

The hexafluoroacetylacetonate (hfac) chemistry of lead was investigated to make comparisons with the similarly sized lanthanides thatform air stable Ln(hfac)3(diglyme) complexes. [Pb(hfac)2(l-g3:g1-diglyme)]2 was obtained from PbO, (MeOCH2CH2)2O, and H(hfac)and was found to exist in the solid state as chiral pairs linked via diglyme. The metal is eight coordinate, but has sufficient vacant spacein the coordination sphere to be considered to have an hemi-directed coordination environment. In contrast to the Ln(hfac)3(digyme)/Kreactions that generate [LnF(hfac)3K(diglyme)]2 fluoride complexes, the potassium reduction of the lead complex gave only[(18-crown-6)K(hfac)]n in pure form, a complex that displays bridging CF3 linkages in the solid state.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Lead; Hexafluoroacetylacetonate; Volatility; 18-Crown-6; Diglyme

1. Introduction

The hexafluoroacetylacetonate (hfac) ligand can bereadily attached to lanthanide ions starting from lantha-nide oxides to make volatile air stable complexes of theformula, Ln(hfac)3(diglyme) [1–3], Eq. (1). The ease of syn-thesis and formation of an anhydrous product underaerobic conditions makes this an important system synthet-ically for f elements. In addition, the volatility imparted bythe hfac ligand [4–8] offers opportunities in separation [9]and metal vapor deposition [6,10].

Ln2O3 + 2diglyme + 6Hhfac! 2Ln(hfac)3(diglyme) + 3H2O

ð1ÞTo learn more about the hfac ligand with metals similar

in size to the lanthanides [11], the hfac chemistry of leadwas investigated. Pb(II) has a 1.29 A 8-coordinate Shannonionic radius that is the same as the radius of Nd(II) [11],but it is electronically much different [4,12,13]. We reporthere that a lead hfac complex can be easily made from

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.03.011

* Corresponding author.E-mail address: wevans@uci.edu (W.J. Evans).

the oxide in the presence of diglyme in a synthetic routethat is even more facile than that used to make the lantha-nide complexes. The lead complex has an unusual solidstate structure that is reported along with structural dataon an hfac derivative of potassium that was obtained inthe course of studying the reactivity of the lead complex.

2. Experimental

2.1. General considerations and materials

Lead(II) oxide, hexafluoroacetylacetone, and 18-crown-6were purchased from Aldrich. 18-crown-6 was placed undervacuum overnight prior to use. Potassium was purchasedfrom Strem and washed with dry hexane under N2 andfreshly cut prior to use. All other chemicals were used asreceived. Reactions involving potassium were kept free fromoxygen and moisture using Schlenk, vacuum line, and glovebox techniques. The toluene used in the glovebox was driedusing GlassContour� columns.1 Deuterated solvents were

1 http://www.glasscontour.com.

Table 1X-ray data collection parameters for [Pb(hfac)2(l-g3:g1-diglyme)]2 (1) and[(18-crown-6)K(hfac)]n (2)

Empirical formula C16H16F12O7Pb (1) C17H25F6O8K (2)Formula weight 755.48 510.47Temperature (K) 163(2) 163(2)Crystal system triclinic triclinicSpace group P�1 P�1a (A) 10.0794(10) 8.7662(16)b (A) 10.7790(11) 9.2344(17)c (A) 11.9284(12) 14.376(3)a (�) 74.665(2) 71.996(3)b (�) 70.677(2) 89.864(3)c (�) 85.778(2) 87.148(3)Volume (A3) 1179.3(2) 1105.3(4)Z 2 2qcalc (Mg/m3) 2.128 1.534l (mm�1) 7.282 0.331R1 [I > 2.0r(I)] 0.0275 0.0694wR2 (all data) 0.0783 0.1850

2692 W.J. Evans et al. / Polyhedron 25 (2006) 2691–2697

dried over NaK and vacuum transferred prior to use. 1H,13C, 19F, 207Pb NMR spectra were recorded with BrukerDRX 400 MHz and Omega 500 MHz spectrometers. 19FNMR spectra were referenced to FCCl3 in C6D6 (d = 0).207Pb NMR spectra were referenced to Pb(NO3)2 1 M inH2O (d = �2961.2) [14]. Infrared spectra were recorded asthin films from benzene using a Perkin–Elmer 2000 FTIRspectrometer. Elemental analyses were performed by DesertAnalytics (Tucson, AZ). MS (ES) analyses were recorded ona Micromass LCT spectrometer.

2.2. Synthesis of Pb(hfac)2(diglyme) (1)

Hexafluoroacetylacetone (493 mg, 2.37 mmol) in ca15 mL of toluene was added dropwise to a stirred suspen-sion of PbO (210 mg, 0.94 mmol) in diglyme (148 mg,1.10 mmol). The solution rapidly became clear and lightyellow within 30 min. Solvent was removed in vacuo togive a yellow powder. Crystals of 1 (458 mg, 0.60 mmol,64%) suitable for X-ray diffraction were obtained fromdiethyl ether at room temperature; m.p.: 69 �C. Complex1 sublimes at 35 �C at 2 · 10�5 Torr. Anal. Calc. forC16H16F12O7Pb: C, 25.43; H; 2.12; F, 30.16; Pb, 27.43.Found: C, 25.43; H, 2.62; F, 30.26; Pb, 26.82%. 1HNMR (C6D6): d 2.91 (8 H, OCH2CH2O), 3.06 (6 H,OCH3), 6.13 (2 H, OCCHCO); 13C (C6D6): d 58.76(OCH3), 70.15, 71.15 (OCH2CH2O), 91.78 (OCCHCO),121.23 (q, CF3), 176.62 (q, CCOCF3); 19F NMR (C6D6):d �79.52. 207Pb NMR (C6D6): d �2097.9. IR 3138 w,3002 w, 2933 s, 2846 m, 1952 w, 1913 w, 1729 w, 1673 s,1641 s, 1594 m, 1552 s, 1532 s, 1492 s, 1463 s, 1381 w,1354 w, 1340 m, 1319 m, 1255 vs, 1200 vs, 1145 vs, 1088 s,1010 m, 944 m, 857 m, 830 m, 798 s, 768 m, 740 m, 662 s,580 s, 523 m cm�1. MS (ES) 415 [Pb(hfac)]+, 549[Pb(hfac)(diglyme)]+.

2.3. Synthesis of [(18-crown-6)K(hfac)]n (2)

K (4 mg, 0.10 mmol) was added to ca 5 mL of toluenecontaining 18-crown-6 (28 g, 0.11 mmol) and stirred for2 h. The resulting mixture was added dropwise toPb(hfac)2(diglyme) (84 g, 0.11 mmol) in 5 mL of toluene.The solution immediately formed a gray-black precipitatewhich turned drab green within an hour and was allowedto stir overnight. The resulting orange solution wasseparated from gray-black insolubles by centrifuga-tion. Removal of solvent in vacuo left an orange powder.Crystals of [(18-crown-6)K(hfac)]n were obtained from atoluene solution of the orange powder at �35 �C.

2.4. X-ray data collection, structure solution and refinement

A colorless crystal of 1 of approximate dimensions0.28 · 0.30 · 0.32 mm and a yellow crystal of 2 of approxi-mate dimensions 0.15 · 0.28 · 0.29 mm were each mountedon a glass fiber and transferred to a Bruker CCD platformdiffractometer. The SMART [15] program package was used

to determine the unit-cell parameters and for data collection(25 sec/frame scan time for a sphere of diffraction data).The raw frame data were processed using SAINT [16] andSADABS [17] to yield the reflection data file. Subsequent cal-culations were carried out using the SHELXTL [18] program.There were no systematic absences nor any diffraction sym-metry other than the Friedel condition. The centrosymmet-ric triclinic space group P�1 was assigned for both 1 and 2

and was later determined to be correct (see Table 1).The structures were solved by direct methods and

refined on F2 by full-matrix least-squares techniques. Theanalytical scattering factors [19] for neutral atoms wereused throughout the analysis. Hydrogen atoms wereincluded using a riding model. In 2, the fluorine atomsF(4), F(5) and F(6) were disordered and included usingmultiple components with partial site-occupancy-factors.

3. Results and discussion

3.1. Synthesis of Pb(hfac)2(diglyme) (1)

A hexafluoroacetylacetonate complex of lead can bereadily synthesized in air by reacting hexafluoroacetylace-tone [H(hfac)] with lead(II) oxide and diglyme in toluene,Eq. (2). This synthesis is analogous to the preparation ofLn(hfac)3(diglyme) [1–3], Eq. (1). The solution turns yellowalmost immediately as the suspension of PbO is rapidlyconsumed leaving a clear solution. The reaction reachescompletion typically within an hour at room temperature.In contrast, the Ln(hfac)3(diglyme) syntheses requireseveral hours at reflux [1–3], which is typical of synthesesof other metal hfac complexes [1–4,20,21]. Crystallizationfrom diethyl ether of the yellow powder obtained fromthe reaction mixture gives crystals of [Pb(hfac)2(l-g3:g1-diglyme)]2 (1) suitable for X-ray analysis, Fig. 1

PbO + diglyme + 2Hhfac !Pb(hfac)2(diglyme) + H2O

ð2Þ

Fig. 1. Thermal ellipsoid plot of the [Pb(hfac)2(l-g3:g1-diglyme)]2 dimer (1) with hydrogen atoms omitted for clarity and ellipsoids drawn at the 50%probability level.

W.J. Evans et al. / Polyhedron 25 (2006) 2691–2697 2693

Elemental analysis as well as NMR spectroscopy wereconsistent with the composition Pb(hfac)2(diglyme). The1H and 13C NMR spectra of 1 in C6D6 contained a singleset of signals for the diglyme protons (methylene hydrogensat d 2.91 ppm and the methyl hydrogens at d 3.06 ppm) andcarbons (methyl carbon at d 58.76 ppm and methylene car-bons at d 70.15 and 71.15 ppm). In an HMQC-NMRexperiment, both of the methylene carbons in the 13CNMR spectrum correlated with the single peak in the 1HNMR. The 19F and 207Pb NMR spectra each containedsingle signals at d �79.52 and �2098 ppm, respectively.Complex 1 has a 207Pb resonance consistent with an oxygencoordinated ligand system [14]. The closest divalent leadcomplex with a fluorinated oxygen bound ligand [14]is Pb(O2CC6F5)2, which has a 207Pb resonance at d�1044 ppm [22]. The more negative value for 1 suggeststhat Pb(hfac)2(diglyme) has a coordination number greaterthan four [14], thus indicating at least some coordination ofthe diglyme to the lead in solution.

3.2. Structure of [Pb(hfac)2(l-g3:g1-diglyme)]2 (1)

In the solid state, 1 exists as a racemic pair of enantio-mers in which one diglyme oxygen of one monomeric unitis oriented toward the lead of the other, Figs. 1 and 2.Selected bond distances and angles are given in Table 2.

Each lead in 1 is coordinated by two hfac ligands withPb–O distances in the range 2.417(3)–2.497(3) A. Compar-ison of these distances with two other lead(II) b-diketonates shows a wide range of Pb–O lengths in thesecompounds. The Pb–O distances in 1 are up to 0.2 Alonger than those in bis(2,2,6,6,-tetramethylheptane-3,5-

dionato)lead, Pb(Me3CCOCH2COCMe3)2, which hasPb–O distances of 2.287(9)–2.303(8) A [20]. Bis(2,2-dimethyl-6,6,7,7,8,8,8-heptafluorooctane-3,5-dionato)lead,Pb(CF3CF2CF2COCH2COCMe3)2, has a range of Pb–Odistances, 2.272(6)–2.410(5) A [20], that overlaps those inboth complexes. The Pb–O distances are shorter than theLn–O distances of Ln(hfac)3(diglyme) (Ln = La, Sm, Eu,Gd, Tb) complexes [1–3] after correcting for variations inatomic radii [11]. For example, Sm(hfac)3(diglyme) hasSm–O distances of 2.386(1)–2.437(1) A. Since the 1.079 Aeight coordinate radius of Sm is 0.21 A shorter than thatof Pb2+ [11], the comparable lead length in 1 would be2.596–2.647 A.

All of the Pb–O(diglyme) distances are longer than thePb–O(hfac) distances. The Pb–O distances of 2.706(3)and 2.759(3) A for the diglyme oxygen atoms, O(5) andO(6), respectively, are distinctly shorter than that of thethird diglyme oxygen, O(7), which is found 2.851(3) A fromPb(1). However, each O(7) is also oriented towards theother lead center, Pb(1 0), at a distance of 3.021(3) A.

If the long bridging Pb–O linkages are included in thecoordination sphere of each metal, the formal coordinationnumber is eight. These eight donor atoms do not describeone of the typical eight coordinate geometries [23], a resultthat may be due to the effect of a stereoactive lone pair.Lead complexes with this effect are described as ‘‘hemi-directed’’ as opposed to ‘‘holo-directed’’ [4] when no voidor perturbation is found in the sphere around the metal.The most open area in the coordination sphere of 1 isshown in the view in Fig. 3. It can be seen from this orien-tation that part of the coordination sphere contains nodonor atoms.

Table 2Selected bond distances (A) and angles (�) for [Pb(hfac)2(l-g3:g1-diglyme)]2 (1)

Compound 1

Pb(1)–O(1) 2.417(3)Pb(1)–O(2) 2.497(3)Pb(1)–O(3) 2.476(3)Pb(1)–O(4) 2.444(3)Pb(1)–O(5) 2.706(3)Pb(1)–O(6) 2.759(3)Pb(1)–O(7) 2.851(3)Pb(1)–O(7 0) 3.021(3)

O(6)–Pb(1)–O(7 0) 101.68(9)O(1)–Pb(1)–O(2) 72.54(11)O(4)–Pb(1)–O(3) 72.56(11)O(5)–Pb(1)–O(6) 61.32(11)O(6)–Pb(1)–O(7) 60.83(10)O(7)–Pb(1)–O(7 0) 83.94(9)Pb(1)–O(7)–Pb(1 0) 96.07

Fig. 2. The two enantiomeric units of Pb(hfac)2(diglyme) (1) shown as mirror images.

Fig. 3. Comparison of monomer units of [Pb(hfac)2(l-g3:g1-diglyme)]2 (1, lediglyme oxygen in the top center. The barium complex had its ligands orientespace in the upper right quadrant.

2694 W.J. Evans et al. / Polyhedron 25 (2006) 2691–2697

The unsymmetrical nature of the coordination sphere of1 can best be evaluated by comparing it to the structure ofthe closely related [Ba(thd)2(diglyme)]2 (3) [24] Fig. 3(thd = tBuC(O)CHC(O)tBu). The barium complex notonly has the same composition, but the diglyme ligandbridges in a similar g3:g1 manner. Fig. 3 presents a viewof 3 analogous to that of 1, which shows how the full coor-dination sphere of a metal can be filled by this ligand set.

The differences in the orientation of diglyme in 1 versusthat in 3 can be seen in several structural features. TheO(6)–Pb(1)–O(7 0) in 1 is 101.68(9)� compared to the analo-gous angle of 89.4(2)� in 3. Similarly, the 83.94(9)� O(7)–Pb(1)–O(7 0) angle in 1 is wider than the corresponding64.4(2)� angle between the bridging oxygens and the metalin 3. These angles show that even though lead is the smallermetal, the diglyme oxygens in 1 bend away from this partof the coordination sphere. In addition, the 2.851(3) APb(1)–O(7) distance of the bridging diglyme oxygen in 1

ft) and [Ba(thd)2(diglyme)]2 (3, right). Both views show the coordinatedd more globally around the metal than the lead complex, which has open

Fig. 4. Thermal ellipsoid plot of the monomer of [(18-crown-6)K(hfac)]n(2) with hydrogen atoms omitted for clarity and ellipsoids drawn at the50% probability level.

W.J. Evans et al. / Polyhedron 25 (2006) 2691–2697 2695

is much longer than either Pb(1)–O(6) or Pb(1)–O(5) at2.759(3) and 2.706(3) A, respectively. In contrast, the Ba–O(diglyme) distances in 3 are in the narrow range of2.857(6)–2.888(6) A.

3.3. Stability and volatility of Pb(hfac)2(diglyme) (1)

The addition of diglyme to the coordination sphereof Pb(hfac)2 appears to make 1 both more thermallystable and more resistant to coordination of water. Onlya simple recrystallization is necessary to obtain a water-free diglyme complex, whereas Pb(hfac)2 will coordinatewater, even in the solid state [20]. In addition, crystalsof 1 are stable at ambient temperatures for monthscompared to Pb(hfac)2 which has a more limited lifetime[20].

Complex 1 shows a melting point of 69 �C, which is sig-nificantly lower than the 156 �C value of Pb(hfac)2 [9,20].Consistent with this, the volatility of complex 1 is high: itsublimes at 35 �C (ca 2 · 10�5 Torr). This combination ofstability and volatility is not always found with metal hfaccomplexes [5,6,20,21], although it is consistent with the sta-bilizing presence of diglyme as a ligand [1–3,25], as in theLn(hfac)3(diglyme) series.

3.4. Reactivity of 1 with potassium

The addition of potassium to Ln(hfac)3(diglyme) com-plexes containing both redox active (Eu, Sm, Nd) and tra-ditionally redox inactive (Gd) metals has previously been

Fig. 5. Ball and stick representation of the po

found to lead to C–F bond activation and cleavage [1] withthe formation of fluoride ‘‘ate’’ salts, Eq. (3).

2Ln(hfac)3(diglyme) + 2K! [LnF(hfac)3K(diglyme)]2

ð3ÞIn order to compare this reactivity with that of the

lead complex, 1 was treated with K and 18-crown-6 intoluene. The reaction generates a series of color changes

lymer chain of [(18-crown-6)K(hfac)]n (2).

Table 3Selected bond distances (A) and angles (�) for [(18-crown-6)K(hfac)]n (2)

Compound 2

K(1)–O(1) 2.802(3)K(1)–O(2) 2.898(3)K(1)–O(3) 2.953(3)K(1)–O(4) 2.876(3)K(1)–O(5) 2.864(3)K(1)–O(6) 2.941(3)K(1)–O(7) 2.690(3)K(1)–O(8) 2.770(3)K(1)–F(5B0)a 2.934(7)K(1)–F(50) 3.081(6)

O(7)–K(1)–O(8) 65.10(8)C(17)–F(5)–K(1 0) 147.0(4)C(17)–F(5B)–K(1 0)a 155.4(5)

a F5B is one of the disordered positions of F5.

2696 W.J. Evans et al. / Polyhedron 25 (2006) 2691–2697

as well as a complex 1H NMR spectrum that suggestthat this is a complicated reaction system. The onlyproduct definitively identified from the mixture was[(18-crown-6)K(hfac)]n (2) which was characterized byX-ray crystallography, Fig. 4. Since K(hfac) is not read-ily soluble in toluene, the crown ether evidently broughtthis ligand into solution.

Only one other solid state structure of an 18-crown-6adduct of a potassium b-diketonate is in the literature:(18-crown-6)K[Me3C(O)CHCO2Et] [26] (4). The K–O(crown ether) distances in 2 and 4 are similar, as arethe K–O(acetylacetonate) lengths: 2.690(3) and2.770(3) A for K(1)–O(7) and K(1)–O(8), respectively, in2, and 2.650 and 2.732 A in 4. Complex 2 is different from4 in that it contains a short F(5B)–K(1 0) distance,2.934(7) A, that is comparable to the K(1)–O(3) andK(1)–O(6) crown ether distances of 2.953(3) and2.941(3) A, respectively. This produces the chain structureshown in Fig. 5. No short intermolecular contacts werefound in 4 (see Table 3).

4. Conclusion

An hfac diglyme complex of lead can be prepared fromPbO and Hhfac more easily than the analogous synthesesof Ln(hfac)3(diglyme) complexes. The [Pb(hfac)2(l-g3:g1-diglyme)]2 complex exists in the solid state as a dimer ofchiral enantiomers connected with the third diglyme oxy-gen of one unit oriented toward the Pb of the other.Despite the dimeric structure, the complex is quite vola-tile. Treatment of [Pb(hfac)2(l-g3:g1-diglyme)]2 with K/18-crown-6 forms [(18-crown-6)K(hfac)]n that exists inthe solid state as a linear polymeric chain via CF3–Kinteractions.

Acknowledgements

We thank the Chemical Sciences, Geosciences, and Bio-sciences Division of the Office of Basic Energy Sciences of

the Department of Energy for support and Dr. Philip R.Dennison for assistance with the 207Pb NMR spectra.

Appendix A. Supplementary material

Crystallographic data for 1 (CCDC No. 290349) and 2(CCDC No. 290350) have been deposited with the Cam-bridge Crystallographic Data Centre. These data can beobtained free of charge via www.ccdc.cam.ac.uk, or bycontacting The Cambridge Crystallographic Data Centre,12 Union Road, Cambridge CB2 1EZ, UK; fax: +441223 336033 (e-mail: deposit@ccdc.cam.ac.uk). Supple-mentary data associated with this article can be found, inthe online version, at doi:10.1016/j.poly.2006.03.011.

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