Volatility Enhancement in Calcium, Strontium, and ... - ACS Publications

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Organometallics 2009, 28, 1032–1038

Volatility Enhancement in Calcium, Strontium, and Barium Complexes Containing β-Diketiminate Ligands with Dimethylamino Groups on the Ligand Core Nitrogen Atoms Baburam Sedai, Mary Jane Heeg, and Charles H. Winter* Department of Chemistry, Wayne State UniVersity, 5101 Cass AVenue, Detroit, Michigan 48202 ReceiVed July 16, 2008

Treatment of M(N(SiMe3)2)2(THF)2 with 2 equiv of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3penten-2-one (LNMe2H) in toluene at ambient temperature afforded Ca(η2-LNMe2)2 (88%), [Sr(η5-LNMe2)(µ-η1:η5-LNMe2)]2 (85%), and [Ba(η5-LNMe2)(µ-η2:η3-LNMe2)]2 (83%) as colorless or pale yellow crystalline solids. The formulations of the new complexes were assigned from spectral and analytical data and by X-ray crystal structure determinations. In the solid state, Ca(η2-LNMe2)2 exists as a tetrahedral monomer. [Sr(η5-LNMe2)(µ-η1:η5-LNMe2)]2 is a dimer that contains a terminal η5-LNMe2 ligand on each strontium ion. The dimer is held together by two µ-η1:η5-LNMe2 ligands, in which one dimethylamino moiety per LNMe2 ligand acts as a donor ligand to the adjacent strontium ion. [Ba(η5-LNMe2)(µ-η2:η3-LNMe2)]2 crystallizes as a dimer that contains a terminal η5-LNMe2 ligand on each barium ion. The two bridging LNMe2 ligands adopt a µ-η2:η3-coordination mode, where each LNMe2 ligand is bonded to one barium ion through the in-plane nitrogen atom lone pairs and to the other barium ion through the out-of-plane π-system of one nitrogen and two carbon atoms. Ca(η2-LNMe2)2, [Sr(η5-LNMe2)(µ-η1:η5-LNMe2)]2, and [Ba(η5-LNMe2)(µ-η2: η3-LNMe2)]2 sublime at 90, 115, and 145 °C, respectively, at 0.05 Torr. These sublimation temperatures are 25 to 55 °C lower than those of previously reported analogous calcium, strontium, and barium complexes that contain isopropyl groups on the β-diketiminate ligand core nitrogen atoms. The increased volatility of complexes containing LNMe2 ligands is attributed to the lattice energy-lowering effects of the dimethylamino group lone pairs of electrons, much like what is observed in group 2 β-diketonate complexes containing fluorinated alkyl groups. The relevance to film growth precursor design is discussed. Introduction Calcium, strontium, and barium complexes containing β-diketiminate ligands are of significant interest with respect to lactide polymerization1 and organic synthesis,2 as well as precursors for the growth of thin films.3-8 Precursor molecules used in chemical vapor deposition3 (CVD) or atomic layer deposition4 (ALD) techniques should have the highest possible volatilities to allow efficient vapor transport. In addition, ALD * Corresponding author. E-mail: [email protected]. (1) For leading references, see: Chisholm, M. H.; Zhou, Z. J. Mater. Chem. 2004, 14, 3081. Chisholm, M. H.; Galucci, J. C.; Phomphrai, K. Inorg. Chem. 2004, 43, 6717. Chisholm, M. H.; Galucci, J. C.; Phomphrai, K. Chem. Commun. 2003, 48. (2) Buch, F.; Harder, S. Z. Naturforsch. 2008, 63b, 169. Spielmann, J.; Harder, S. Eur. J. Inorg. Chem. 2008, 1480. Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Organometallics 2007, 26, 2953. Spielmann, J.; Harder, S. Chem.-Eur. J. 2007, 13, 8928. Ruspic, C.; Harder, S. Inorg. Chem. 2007, 46, 10426. Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Procopiou, P. A. Angew. Chem., Int. Ed. 2007, 46, 6339. Crimmin, M. R.; Barrett, A. G. M.; Hill, M. S.; Procopiou, P. A. Org. Lett. 2007, 9, 331. Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005, 127, 2042. (3) Selected reviews: Matthews, J. S.; Rees, W. S., Jr. AdV. Inorg. Chem. 2000, 50, 173. Wojtczak, W. A.; Fleig, P. F.; Hampden-Smith, M. J. AdV. Organomet. Chem. 1996, 40, 215. Marks, T. J. Pure Appl. Chem. 1995, 67, 313. (4) Recent reviews: Putkonen, M.; Niinisto¨, L. Top. Organomet. Chem. 2005, 9, 125. Niinisto¨, L.; Païva¨saari, J.; Niinisto¨, J.; Putkonen, M.; Nieminen, M. Phys. Stat. Sol. A 2004, 201, 1443. Leskela¨, M.; Ritala, M. Angew. Chem., Int. Ed. 2003, 42, 5548. (5) For selected examples, see: Drake, S. R.; Hursthouse, M. B.; Malik, K. M. A.; Otway, D. J. J. Chem. Soc., Dalton Trans. 1993, 2883. Rossetto, G.; Polo, A.; Benetollo, F.; Porcha, M.; Zanella, P. Polyhedron 1992, 11, 979. Drozdov, A. A.; Trojanov, S. I. Polyhedron 1992, 11, 2877.

precursors must be thermally stable at the film growth temperature to avoid loss of surface-limited growth through precursor decomposition.4 A major challenge in the development of highly volatile heavier group 2 precursors is the small charge to size ratios for these ions, which leads to poor Lewis acidity and attendant low thermal decomposition temperatures. Furthermore, the large sizes of the heavier group 2 ions frequently afford oligomeric complexes that possess low volatilities. The most widely employed precursors, Sr(thd)2 and Ba(thd)2 (thd ) 2,2,6,6-tetramethylheptane-3,6-dionato), are oligomeric in the solid state and have very low vapor pressures.5 They are also moisture and air sensitive at typical temperatures used for vapor transport. Classes of precursors with improved properties relative to Sr(thd)2 and Ba(thd)2 include metallocenes with bulky alkyl substituents,6 fluorinated β-diketonate complexes containing neutral polydentate ether ligands,7 and complexes containing β-ketoiminato ligands with polyether substituents appended from (6) Leading references: Hatanpaä¨, T.; Ritala, M.; Leskela¨, M. J. Organomet. Chem. 2007, 692, 5256. Vehkama¨ki, M.; Hatanpaä¨, T.; Ritala, M.; Leskela¨, M.; Vaÿrynen, S.; Rauhala, E. Chem. Vap. Deposition 2007, 13, 239. Kukli, K.; Ritala, M.; Sajavaara, T.; Ha¨nninen, T.; Leskela¨, M. Thin Solid Films 2006, 500, 322. Hatanpaä¨, T.; Vehkama¨ki, M.; Mutikainen, I.; Kansikas, J.; Ritala, M.; Leskela¨, M. Dalton Trans. 2004, 1181. (7) Teren, A. R.; Belot, J. A.; Edleman, N. L.; Marks, T. J.; Wessels, B. W. Chem. Vap. Deposition 2000, 6, 175. Belot, J. A.; Neumayer, D. A.; Reedy, C. J.; Studebaker, D. B.; Hinds, B. J.; Stern, C. L.; Marks, T. J. Chem. Mater. 1997, 9, 1638. Marks, T. J.; Belot, J. A.; Reedy, C. J.; McNeely, R. J.; Studebaker, D. B.; Neumayer, D. A.; Stern, C. L. J. Alloys Compd. 1997, 251, 243. Studebaker, D. B.; Stauf, G. T.; Baum, T. H.; Marks, T. J.; Zhou, H.; Wong, G. K. Appl. Phys. Lett. 1997, 70, 565. (8) Studebaker, D. B.; Neumayer, D. A.; Hinds, B. J.; Stern, C. L.; Marks, T. J. Inorg. Chem. 2000, 39, 3148. Schulz, D. L.; Hinds, B. J.; Neumayer, D. A.; Stern, C. L.; Marks, T. J. Chem. Mater. 1993, 5, 1605.

10.1021/om8006739 CCC: $40.75  2009 American Chemical Society Publication on Web 01/22/2009

Volatility Enhancement in Ca, Sr, and Ba Complexes

the ligand core nitrogen atoms.8 Ba(C5H2tBu3)2 and related metallocenes have been demonstrated as useful ALD precursors at substrate temperatures of up to 350 °C, but are too reactive for use in CVD.6 Fluorinated β-diketonate complexes containing neutral polydentate ether ligands sublime well at reasonable temperatures (120-160 °C) and have been successfully employed in CVD thin film growth.7 However, the presence of fluorine is a liability, since this element is easily incorporated into the thin films and can lead to changes in materials properties. Complexes containing β-ketoiminato ligands with appended polyether substituents do not contain fluorine, but decompose in the solid state concurrently with sublimation.8 As such, calcium, strontium, and barium precursors with optimized properties for use in ALD and CVD remain research goals. We have recently reported the synthesis, structure, and properties of a series of calcium, strontium, and barium complexes containing β-diketiminato ligands with isopropyl (LiPr) or tert-butyl (LtBu) groups on the core nitrogen atoms.9 For M(LtBu)2 and Sr(LiPr)2, sandwich complexes with η5-βdiketiminato ligands were obtained. By contrast, Ca(LiPr)2 contained η2-β-diketiminato ligands, and Ba2(LiPr)4 adopted a dinuclear structure with η5-, µ-η5:η5-, and µ-η1:η1-β-diketiminato ligands. Significantly, these complexes are volatile and sublime between 110 and 170 °C at 0.05 Torr. In view of the good volatility of these complexes, we sought to examine synthetic modifications to these structures that might lead to improved precursor properties. In fluorinated β-diketonate complexes containing neutral polydentate ether ligands, increased volatility presumably occurs through reduction of lattice energy arising from intermolecular fluorine atom lone pair/lone pair repulsions. In seeking to extend this effect to nonfluorinated ligands, we envisioned that incorporation of dialkylamino groups within a β-diketiminato ligand might mimic the lattice energy-lowering effects of fluoroalkyl groups without introducing deleterious elements such as fluorine. The β-diketimine 4-(2,2-dimethylhydrazino)dimethylhydrazone-3-penten-2-one (LNMe2H), which contains dimethylamino groups on the core nitrogen atoms, can be prepared by a simple synthetic procedure.10 Herein, we describe the synthesis, structure, properties, and volatility characteristics of calcium, strontium, and barium complexes containing the LNMe2 ligand.

Results Synthesis of New Complexes. Treatment of M(N(SiMe3)2)2(THF)211 with 2 equiv of LNMe2H in toluene afforded Ca(η2LNMe2)2 (1, 88%), [Sr(η5-LNMe2)(µ-η1:η5-LNMe2)]2 (2, 85%), and [Ba(η5-LNMe2)(µ-η2:η3-LNMe2)]2 (3, 83%), respectively, as colorless (1, 2) or pale yellow (3) solids after workup as described in the Experimental Section (eq 1). Complexes 1-3 are slightly soluble in hexane and readily soluble in toluene. It was not possible to grow high-quality single crystals of 1-3 from solution, despite many attempts under a variety of conditions. However, sublimation, as described below, afforded single crystals of 1-3 that were acceptable for microanalyses and single-crystal X-ray diffraction experiments. Complexes 1-3 are air and moisture sensitive, and the sensitivity increases from calcium to barium, presumably due to the increasing size of (9) El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. Organometallics 2004, 23, 4995. (10) Domnin, N. A.; Yakimovich, S. I. Zh. Org. Khim. 1965, 1, 658. (11) Vaartstra, B. A.; Huffman, J. C.; Streib, W. E.; Caulton, K. G. Inorg. Chem. 1991, 30, 121. Westerhausen, M. Inorg. Chem. 1991, 30, 96.

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the metal ions, increasingly facile access of water and dioxygen to the metal centers, and increasing ionic character.

The structural assignments of 1-3 were based on spectral and analytical data, as well as X-ray crystal structures. In the solid state, 1 exists as a monomer, whereas 2 and 3 are dimeric. The structures are described in detail below. The 1H NMR spectra of 1-3 in benzene-d6 at 23 °C showed sharp (1) or slightly broad (2, 3) singlets for the β-CH fragment of the β-diketiminato ligand at δ 4.15, 3.97, and 4.00, respectively, while the 13C{1H} NMR spectra revealed resonances for the associated carbon atom at 84.87, 80.84, and 81.30 ppm, respectively. Since slightly broadened resonances for the β-CH fragment of the LNMe2 ligands in 2 and 3 were observed at 23 °C, variable-temperature 1H NMR spectra were recorded to afford insight into the solution structures. In toluene-d8 at 23 °C, a solution of 2 exhibited a slightly broad singlet for the β-CH fragment of the LNMe2 ligand at δ 3.94. At temperatures below -20 °C, the NMR spectra became complex and exhibited many β-CH fragment resonances. For example, at -40 °C, 14 singlets were observed in the β-CH fragment region between δ 3.87 and 4.88. Among these species, a broad singlet at δ 4.00 represented the major species (∼50% of the total integration) and corresponded to the species that gave rise to the δ 3.94 resonance at 23 °C. Resonances at δ 4.47, 4.45, 4.11, 4.09, and 3.87 accounted for about 45% of the remaining total integration. The number of resonances and chemical shifts of these resonances changed additionally upon cooling from -40 to -80 °C, where nine β-CH fragment resonances were observed. Among these, the resonance observed at 23 °C (δ 3.94) had shifted to δ 4.10 and accounted for 22% of the integrated intensity for the β-CH fragment resonances. Complex 3 also exhibited complex, temperature-dependent 1H NMR spectra. At 23 °C, the β-CH fragment of the LNMe2 ligand in 3 appeared as a broad singlet at δ 3.94 in toluene-d8. Upon cooling to -30 °C, the broad singlet split into new broad resonances at δ 4.01, 3.96, and 3.89 (94% of integrated intensity), and new minor resonances appeared at δ 4.41, 4.37, and 4.35 (6% of integrated intensity). Upon further cooling to -55 °C, major resonances were observed at δ 4.14, 4.06, 4.01, 3.98, 3.96, and 3.90 (91% of integrated intensity), and minor resonances were present at δ 4.46 and 4.38 (9% of integrated intensity). Both 2 and 3

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Sedai et al.

Table 1. Melting Points, Sublimation Data, and Decomposition Temperatures for 1-6 sublimation decomposition group 2 temperature sublimation sublimation temperature in complex mp/°C °C/0.05 Torr recovery % residue % capillary/°C 1 2 3 4 5 6

114 163 165 90 93 155

90 115 145 145 150 170

90 85 79 72 80 45

7 11 14 15 17 50

180 210 215 250 183 188

exhibit more complex solution behavior at lower temperatures than would be expected for the dimeric solid state structures, consistent with the presence of multiple species. Volatility Assessment and Thermal Behavior. To determine their initial viability as ALD and CVD precursors, 1-3 were evaluated for their volatility and thermal stability. Table 1 summarizes the sublimation and decomposition point data for 1-3 as well as the structurally related complexes 4-6. Analytically pure solids of 1-3 were loaded into a horizontal sublimation apparatus. Complexes 1-3 sublimed at 90, 115, and 145 °C, respectively, at 0.05 Torr. The percent recovery after sublimation was 90%, 85%, and 79% for 1-3, respectively, with nonvolatile residues of 7%, 11%, and 14% at the end of the sublimations. These residues suggest some thermal decomposition during the sublimation experiments. 1H NMR spectra of the sublimed materials were identical to those prior to sublimation, indicating that no reactions occurred upon vapor transport. The decomposition temperatures of 1-3 were determined by carefully monitoring sealed melting point tubes containing a few milligrams of sample. Complexes 1-3 exhibited melting points at 114, 163, and 165 °C, respectively, and showed no evidence of discoloration upon melting. Upon further heating, rapid decomposition of 1-3 was observed at 180, 210, and 215 °C, respectively, as indicated by darkening of the samples at these temperatures. For comparison, sublimation and decomposition temperatures were also obtained for the analogous, previously reported complexes Ca(η2-LiPr)2 (4), Sr(η5LiPr)2 (5), and [(η5-LiPr)Ba(µ-η5:η5-LiPr)(µ-η1:η1-LiPr)Ba(η5-LiPr)] (6).9 Complexes 4 and 5 sublimed at 145 and 150 °C at 0.05 Torr, respectively, to afford 72% and 80% recoveries. At the end of the sublimations, there were 15% and 17% nonvolatile residues for 4 and 5, respectively. Complex 6 sublimed poorly at 170 °C/0.05 Torr, with a 45% sublimed recovery and a 50% nonvolatile residue at the end of the sublimation. Complexes 4-6 exhibited melting points of 90, 93, and 155 °C, respectively, and decomposition temperatures of 250, 183, and 188 °C in sealed melting point capillary tubes. Thermogravimetric analyses were not carried out for 1-6, since the required brief exposure to ambient atmosphere led to extensive decomposition due to the extreme air sensitivity. X-ray Crystal Structures of 1-3. The X-ray crystal structures of 1-3 were determined to establish the geometry about the metal centers and to assess the various bonding modes of the β-diketiminato ligands. Experimental crystallographic data are summarized in Table 2, selected bond lengths and angles are given in Tables 3-5, and perspective views are shown in Figures 1-3. Complex 1 crystallizes as a monomer, with two η2-βdiketiminato ligands bonded through the nitrogen atoms to a calcium ion exhibiting distorted tetrahedral geometry. The calcium-nitrogen bond lengths range from 2.355(1) to 2.370(1) Å, with an average of 2.362 Å. The nitrogen-carbon and carbon-carbon bond lengths associated with the β-diketiminato ligand backbone range from 1.324(1) to 1.332(1) Å and 1.410(1)

Table 2. Crystal Data and Data Collection Parameters for 1-3 empirical formula fw space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (A3) Z temp (K) λ (Å) density calcd (g/cm3) µ (mm-1) R(F) (%)a Rw(F) (%)b

1

2

3

C18H38CaN8 406.64 P21/c 17.6343(5) 8.5167(2) 15.6458(4)

C36H76Sr2N16 908.37 P21/n 9.4146(5) 14.0852(7) 17.6111(9)

97.7400(10)

95.272(3)

2328.37(10) 4 100(2) 0.71073 1.160 0.288 3.57 9.08

2325.5(2) 2 100(2) 0.71073 1.297 2.337 2.66 5.90

C36H76BaN16 1007.81 P1j 9.8051(6) 10.7146(6) 12.7221(8) 72.444(2) 69.361(2) 89.532(2) 1185.28(12) 1 100(2) 0.71073 1.412 1.694 1.89 4.50

R(F) ) ∑|Fo| - |Fc|/∑|Fo| for I > 2σ(I). a

b

Rw(F)2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2

Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1 Ca-N(1) Ca-N(2) Ca-N(5) Ca-N(6) N(1)-Ca-N(2) N(1)-Ca-N(5) N(1)-Ca-N(6) N(2)-Ca-N(5) N(2)-Ca-N(6) N(5)-Ca-N(6)

2.355(1) 2.364(1) 2.370(1) 2.355(1) 79.53(3) 123.84(3) 117.35(3) 138.82(3) 120.32(3) 81.38(3)

Table 4. Selected Bond Lengths (Å) and Angles (deg) for 2 Sr-N(2) Sr-N(3) Sr-N(6) Sr-N(7) Sr-N(8)′ Sr-C(1) Sr-C(2) Sr-C(3) Sr-C(10) Sr-C(11) Sr-C(12) Sr-C(17)′ N(2)-Sr-N(3) N(2)-Sr-N(6) N(2)-Sr-N(7) N(3)-Sr-N(6) N(3)-Sr-N(7) N(6)-Sr-N(7) N(2)-Sr-N(8)′ N(3)-Sr-N(8)′ N(6)-Sr-N(8)′ N(7)-Sr-N(8)′

2.544(1) 2.544(1) 2.562(1) 2.603(1) 2.911(1) 3.294(1) 3.422(1) 3.264(1) 3.102(1) 3.071(1) 3.091(1) 3.307(1) 75.23(3) 109.11(3) 128.03(3) 101.34(3) 156.68(3) 71.51(3) 102.29(3) 93.32(3) 147.78(3) 83.69(3)

to 1.416(1) Å, respectively. These narrow ranges reflect delocalized bonding in the β-diketiminato ligand core. The nitrogen-calcium-nitrogen bite angles for the η2-β-diketiminato ligands are 79.53(3)° and 81.38(3)°, while the interligand nitrogen-calcium-nitrogen angles lie between 117.35(3)° and 138.82(3)°. The dihedral angle between the N(1)-Ca-N(2) and N(5)-Ca-N(6) planes is 86.18(3)°, consistent with a distorted tetrahedral geometry about the calcium ion. Complex 2 crystallizes as a dimer, with two terminal η5-βdiketiminato ligands and two µ-η1:η5-β-diketiminato ligands bound to the strontium centers. The terminal η5-ligands are characterized by strontium-nitrogen distances of 2.544(1) Å and strontium-carbon distances of 3.294(1) (to C(1)), 3.422(1) (to C(2)), and 3.264(1) Å (to C(3)). This ligand exhibits strongly distorted η5-bonding, apparently due to intradimer steric interac-



Table 5. Selected Bond Lengths (Å) and Angles (deg) for 3 Ba-N(2) Ba-N(3) Ba-N(6) Ba-N(7) Ba′-N(6) Ba′-N(7) Ba-C(1) Ba-C(2) Ba-C(3) Ba′-C(10) Ba′-C(11) Ba′-C(12) Ba-Ba′ N(2)-Ba-N(3) N(2)-Ba-N(6) N(2)-Ba-N(7) N(2)-Ba-N(6)′ N(2)-Ba-N(7)′ N(3)-Ba-N(6) N(3)-Ba-N(7) N(3)-Ba-N(6)′ N(3)-Ba-N(7)′ N(6)-Ba-N(7) N(6)-Ba-N(6)′ N(6)-Ba-N(7)′ N(7)-Ba-N(7)′ N(6)′-Ba-N(7)′

2.679(1) 2.764(1) 2.888(1) 2.826(1) 3.266(1) 3.976(1) 3.144(1) 3.063(1) 3.127(1) 3.176(1) 3.046(1) 3.845(1) 4.2482(2) 66.32(3) 98.06(3) 108.51(3) 168.51(3) 132.94(3) 162.76(3) 124.81(3) 102.34(3) 117.63(3) 65.61(3) 92.92(3) 66.97(2) 104.81(2) 49.44(2)

tions between C(1)-C(3) and the coordinated dimethylamino group of another LNMe2 ligand. The other unique β-diketiminato ligand exhibits a µ-η1:η5-coordination mode, in which one dimethylamino moiety acts as a donor ligand to the adjacent strontium ion. The strontium-nitrogen distance for this interaction is 2.911(1) Å, which is very long and suggests weak bonding. The η5-interaction within the µ-η1:η5-β-diketiminato ligands is characterized by strontium-nitrogen distances of 2.562(1) and 2.603(1) Å and strontium-carbon distances of 3.071(1), 3.091(1), and 3.102(1) Å. In addition to the two strontium-nitrogen bridges within each dimer, each bridging β-diketiminato ligand has three agostic strontium-hydrogen interactions. Two hydrogen atoms bonded to C(17) have strontium-hydrogen distances of 3.171 and 3.186 Å, while a hydrogen atom bonded to C(16) has a strontium-hydrogen distance of 2.892 Å. These agostic interactions, while undoubtedly weak, must help to stabilize the dimeric structure. The nitrogen-carbon lengths and carbon-carbon bond lengths in this β-diketiminato ligand backbone range from 1.310(1) to

Figure 1. Perspective view of 1 with thermal ellipsoids at the 50% probability level.



1.330(1) Å and from 1.419(1) to 1.430(2) Å, respectively. These values are similar to those observed in 1 and are consistent with delocalized bonding. Complex 3 crystallizes as a dimer, with two terminal η5-βdiketiminato ligands and two µ-η2:η3-β-diketiminato ligands bound to the barium centers. The barium-nitrogen bond distances associated with the terminal η5-β-diketiminato ligands are 2.679(1) and 2.764(1) Å, while the barium-carbon distances for the same ligands are 3.063(1) (to C(2)), 3.127(1) (to C(3)), and 3.144(1) Å (to C(1)). The barium-nitrogen bond distances for the µ-η2:η3-β-diketiminato ligands are 2.826(1) and 2.888(1) Å in the η2-interaction and 3.266(1) Å in the out-of-plane η3interaction. The barium-carbon distances within the η3-interaction are 3.046(1) (to C(11)′) and 3.176(1) Å (to C(10)′). By contrast, the Ba-C(12)′ and Ba-N(7)′ distances are 3.556(1) and 3.976(1) Å, which are much longer than the other related distances for this ligand and support the proposed η3-interaction. There are agostic interactions involving one hydrogen atom on C(13) (Ba-H ) 2.885 Å) and two hydrogen atoms on C(18) (Ba-H ) 2.884, 2.975 Å). These agostic interactions are probably engendered by the relatively low coordination number at the barium ion. The nitrogen-carbon and carbon-carbon bond lengths in the β-diketiminato ligand cores range from 1.320(2) to 1.326(2) Å and from 1.420(2) to 1.427(2) Å, respectively. There are no statistically significant differences

1036 Organometallics, Vol. 28, No. 4, 2009

in these values between the two types of ligands. Also, the carbon-nitrogen bond length of the fragment containing N(7) and C(12) within the µ-η2:η3-β-diketiminato ligand is identical within experimental error to the C-N fragment containing N(6) and C(10). The nitrogen-carbon and carbon-carbon bond lengths within the β-diketiminato ligand cores are similar to those in 1 and 2 and reflect similar delocalized bonding. We have recently described a careful evaluation of the distortions observed in calcium, strontium, and barium complexes containing η5-β-diketiminato ligands and compared them with distortions commonly seen in the more common η2coordination mode.9 To assess the bonding, the angles between the planes incorporating the atoms MN2 (a), N2(CR)2 (b), and (CR)2Cβ (c) were calculated. The a/b angle assesses the position of the metal ions over the π-face of the β-diketiminato ligand, while the b/c angle measures the distortion of Cβ toward the metal atom due to possible bonding interactions. For 1, the average dihedral angles are a/b ) 30.3° and b/c ) 14.1°. In 2, the terminal η5-β-diketiminato ligands have a/b ) 50.8° and b/c ) 17.3°, while the η5-interactions within the µ-η1:η5-βdiketiminato ligands have a/b ) 70.1° and b/c ) 27.0°. For 3, the terminal η5-β-diketiminato ligands have a/b ) 77.4° and b/c ) 26.0°, the η2-interactions within the µ-η2:η3-β-diketiminato ligands have a/b ) 15.3°, and the η3-interactions within the µ-η2: η3-β-diketiminato ligands have a/b ) 69.9° and b/c ) 16.2°. For comparison, the η2-β-diketiminato ligands in 4 have a/b angles between 20° and 29° and b/c values between 3.8° and 6.1°.9 These values are similar to those found in 1. The η5-βdiketiminato ligands in 5 have an a/b angle of 69.11(11)° and a b/c angle of 22.1(3)°.10 These values are in the range of those found in the η5-ligands of 2 and 3.

Discussion The lowering of the sublimation temperatures of 1-3, relative to 4-6, ranged from 55 °C for monomeric 1 to 35 and 25 °C, respectively, for dimeric 2 and 3. This translates into a 4-11 °C lowering of the sublimation temperatures per dimethylamino substituent in each M(LNMe2)2 unit, relative to complexes containing M(LiPr)2 units. For comparison, the structurally similar, monomeric complexes Mg(thd)2(TMEDA)12 and Mg(hfac)2(TMEDA)13 (hfac ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, TMEDA ) N,N,N′,N′-tetramethylethylenediamine) have been synthesized and structurally characterized, and their sublimation temperatures have been determined. Mg(thd)2(TMEDA) is reported to sublime at 100 °C/10-4 Torr12a and 120-130 °C/0.20 mbar,12b while Mg(hfac)2(TMEDA) sublimes between 40 and 60 °C at 0.0013 Torr.13 Using the data reported by Marks and co-workers12a,13 leads to a reduction of 40 to 60 °C in sublimation temperature for Mg(hfac)2(TMEDA), relative to the nonfluorinated analogue Mg(thd)2(TMEDA). This sublimation temperature reduction ranges from 3.3 to 5.0 °C per fluorine atom. While many assumptions go into comparing the sublimation temperature reductions in 1-3 and Mg(hfac)2(TMEDA), it does appear that the effect of the dimethylamino group substitution is similar to or possibly greater than analogous fluorine substitution. Fluorinated magnesium β-diketonate complexes exhibit melting points that are 100 to 150 °C lower than those observed in (12) (a) Babcock, J. R.; Benson, D. D.; Wang, A.; Edleman, N. L.; Belot, J. A.; Metz, M. V.; Marks, T. J. Chem. Vap. Deposition 2000, 6, 180. (b) Hatanpaä¨, T.; Kansikas, J.; Mutikainen, I.; Leskela¨, M. Inorg. Chem. 2001, 40, 788. (13) Wang, L.; Yang, Y.; Ni, J.; Stern, C. L.; Marks, T. J. Chem. Mater. 2005, 17, 5697.

Sedai et al.

nonfluorinated complexes.12 Low melting points are desirable for film growth precursors, since the precursors can be conveniently manipulated as solids at ambient temperatures but undergo vapor transport as liquids. Liquid precursors have a constant surface area, which leads to constant vapor transport. By contrast, the surface area of solid precursors can change in the precursor bubbler over time, leading to changes in the concentration of precursors in the gas phase. In the present work, there are no obvious trends in the melting points of 1-3 (1, 112-114 °C; 2, 161-163 °C; 3, 164-165 °C), relative to the isopropyl-substituted analogues 4-6 (4, 92-94 °C; 5, 87-89 °C; 6, 176-178 °C). The thermal decomposition temperatures of 1-3 (1, 180 °C; 2, 210 °C; 3, 215 °C) increase slightly upon going from calcium to barium and are roughly similar to those of 4-6 (4, 250 °C; 5, 183 °C; 6, 188 °C). The low decomposition temperatures of 1-6 suggest that these complexes will not be useful ALD precursors, since a large temperature difference between vapor transport and precursor decomposition is desirable to avoid precursor condensation.4 In addition, precursor decomposition leads to loss of the surface-limited growth in ALD. Complexes 1-6 may serve as useful CVD precursors, since they sublime with low residues and should be reactive toward oxygen sources. The present work allows comparison of 3 with the structurally characterized dimeric barium complexes 6-8,10,14,15 which also contain β-diketiminate ligands (Chart 1). The bond lengths from barium to the core atoms of the terminal η5-β-diketiminate ligands in 3 (Ba-N 2.679(1), 2.764(1) Å; Ba-C 3.063(1), 3.127(1), 3.144(1) Å), 6 (Ba-N 2.685(2), 2.722(2) Å; Ba-C 3.144(2), 3.168(2), 3.180(2) Å), 7 (Ba-N 2.635(4), 2.689(6) Å; Ba-C 3.161(5), 3.202(5), 3.244(5) Å), and 8 (Ba-N 2.633(6), 2.674(1) Å; Ba-C 3.110(8), 3.121(8), 3.176(8) Å) are comparable and suggest similar bonding to these ligand cores. The small differences in barium-nitrogen and bariumcarbon distances probably reflect various steric interactions between the β-diketiminate ligand nitrogen atom substituents and the bridging ligand substituents. The major structural differences between 3, 6, and 7 relate to the bonding modes of the bridging β-diketiminate ligands. In 3, the bridging LNMe2 ligands adopt a µ-η2:η3-coordination mode, in which one barium ion forms bonds to two nitrogen atoms and lies approximately in the C3N2 ligand plane, and the other barium ion forms an η3-interaction with the out-of-plane π-orbitals on one nitrogen atom and two carbon atoms of the C3N2 ligand core. Apparently, the barium ions are not large enough to adopt the µ-η2:η5bridging coordination mode, and instead the µ-η2:η3-mode is observed. In 6, the bridging ligands consist of one U-shaped µ-η5:η5-LiPr ligand and one W-shaped µ-η1:η1-LiPr ligand. It is likely that the W-shaped µ-η1:η1-LiPr ligand results because adoption of two µ-η5:η5-LiPr ligands would be energetically prohibitive due to steric interactions between the isopropyl groups. The bridging ligands in 7 consist of one µ-η2:η5-ligand and one µ1:η3-ligand, which bear some resemblance to the bridging ligands in 6, in that one W-shaped and one U-shaped ligand are present. Unlike 6, however, the U-shaped ligand is canted toward one barium ion to afford the µ-η2:η5-coordination mode, and the W-shaped ligand contains η1- and η3-interactions. It is surprising that there is so much structural diversity among 3, 6, and 7, since the shapes and steric profiles of the dimethylamino, isopropyl, and cyclohexyl substituents are (14) Clegg, W.; Coles, S. J.; Cope, E. K.; Mair, F. S. Angew. Chem., Int. Ed. 1998, 37, 796. (15) El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. Eur. J. Inorg. Chem. 2005, 2081.



Chart 1. Selected Structurally Characterized Dimeric Barium Complexes Containing β-Diketiminate Ligands

similar, especially for dimethylamino and isopropyl groups. The exact nature of the bridging ligands in dimeric barium complexes containing β-diketiminate ligands appears to be exceptionally sensitive to the size and chemical nature of the nitrogen atom substituents.

Experimental Section General Considerations. All reactions were performed under argon using either glovebox or Schlenk line techniques. Tetrahydrofuran and diethyl ether were freshly distilled from purple solutions of sodium benzophenone ketyl. Toluene was distilled from sodium. Hexane was distilled from P2O5. Starting materials M(N(SiMe3)2)2(THF)2 (M ) Ca, Sr, Ba)11 were prepared according to the literature procedures. 1 H NMR spectra were obtained at 500 or 300 MHz, and 13C{1H} NMR spectra were obtained at 125 or 75 MHz in benzene-d6 or toluene-d8, as indicated. Infrared spectra were obtained using Nujol as the medium. Elemental analyses were performed by Midwest Microlab, Indianapolis, IN. V2O5 was added as a combustionenhancing agent for 2 and 3. The low carbon values are a common problem for the heavy alkaline earth complexes.16 Melting points were obtained on a Haake-Buchler HBI digital melting point apparatus and are uncorrected. X-ray crystal structure searches were conducted using version 5.29 (November 2007) of the Cambridge Crystallographic Database. For the sublimation experiments, 2.5 cm diameter, 30 cm long glass tubes were employed. One end of the tube was sealed and the other end was equipped with a 24/40 male glass joint. In an argon-filled glovebox, the compound to be sublimed was loaded into a 1.0 × 4.0 cm glass tube, and this tube was placed at the sealed end of the glass sublimation tube. The sublimation tube was fitted with a 24/40 vacuum adapter and then was inserted into a horizontal Buchi Kugelrohr oven such that about 15 cm of the tube was situated in the oven. A vacuum of 0.05 Torr was established, and the oven was heated to the indicated temperature. The compounds sublimed to the cool zone just outside of the oven. Preparation of 4-(2,2-Dimethylhydrazino)dimethylhydrazone3-penten-2-one (LNMe2H). A modification of the literature procedure10 was employed to produce LNMe2H. A 100 mL roundbottomed flask, equipped with a magnetic stir bar, was charged (16) Harder, S. Organometallics 2002, 21, 3782. Burkey, D. J.; Hanusa, T. P. Organometallics 1996, 15, 4971.

with 2,4-pentanedione (4.98 g, 49.77 mmol). To this stirred solution at 0 °C was added slowly N,N-dimethylhydrazine (7.47 g, 124.43 mmol), which afforded a pale yellow solution. The solution was stirred for 24 h. The crude product was purified by fractional distillation to afford LNMe2H as a pale yellow liquid (7.56 g, 82%) that consisted of a 46:31:23 mixture of three isomers, as judged by integration of the C-CH3 resonances in the 1H NMR spectrum at 23 °C in benzene-d6: bp 46-48 °C/0.05 Torr (lit. bp 77-78 °C/5 Torr10); IR (neat, cm-1) 3584 (w, νNH), 3429 (w, νNH), 3240 (w, νNH), 2984 (s), 2952 (s), 2895 (m), 2955 (s), 2817 (s), 2772 (m), 1623 (s), 1575 (m), 1554 (m), 1467 (s), 1449 (s), 1436 (s), 1360 (s), 1290 (m), 1229 (w), 1195 (m), 1154 (m), 1058 (m), 1021 (s), 972 (m), 959 (m), 920 (w), 909 (w), 733 (w); 1H NMR (C6D6, 23 °C, δ) 10.82 (s, NH), 4.35 (s, β-CH), 3.35 (s, CH2), 3.07 (s, N(CH3)2), 2.39 (s, N(CH3)2), 2.38 (s, N(CH3)2), 2.37 (s, N(CH3)2), 2.33 (s, N(CH3)2), 2.03 (s, C-CH3), 1.89 (s, C-CH3), 1.81 (s, C-CH3) 1.78 (s, C-CH3), 1.73 (s, C-CH3); 13C{1H} NMR (C6D6, 23 °C, ppm) 164.10 (s, C-CH3), 163.35 (s, C-CH3), 163.09 (s, C-CH3), 159.17 (s, C-CH3), 91.65, (β-CH), 48.73 (s, N(CH3)2), 48.67 (s, N(CH3)2), 47.44 (s, N(CH3)2), 47.10 (s, N(CH3)2), 47.01 (s, N(CH3)2), 46.91 (s, N(CH3)2), 40.81 (s, CH2), 22.87 (s, C-CH3), 18.74 (s, C-CH3), 16.86 (s, C-CH3), 16.29 (s, C-CH3); MS (EI) m/z (%): 185 ([M + H]+, 100); HRMS calcd for C9H20N4 m/z 184.1688, found 184.1679. Preparation of Ca(η2-LNMe2)2 (1). A 100 mL Schlenk flask, equipped with a magnetic stir bar and a rubber septum, was charged with Ca(N(SiMe3)2)2(THF)2 (0.801 g, 1.59 mmol), a stir bar, and toluene (50 mL). To this stirred solution at ambient temperature was slowly added LNMe2H (0.584 g, 3.17 mmol). The resultant mixture was stirred at ambient temperature for 12 h, and then the volatile components were removed under reduced pressure to afford 1 as a colorless solid (0.560 g, 88%). An analytical sample and single crystals suitable for X-ray crystallographic analysis were obtained by sublimation at 90 °C/0.05 Torr: mp 112-114 °C; IR (Nujol, cm-1) 1625 (m), 1554 (w), 1292 (w), 1154 (w), 1022 (w), 921 (w); 1H NMR (C6D6, 23 °C, δ) 4.14 (s, 2H, β-CH), 2.59 (s, 24H, N(CH3)2), 2.22 (s, 12H, C-CH3); 13C{1H} NMR (C6D6, 23 °C, ppm) 165.76 (s, R-C), 84.99 (s, β-C), 49.42 (s, N(CH3)2), 22.22 (s, C-CH3). Anal. Calcd for C18H38CaN8: C, 53.17; H, 9.42; N, 27.56. Found: C, 53.48; H, 9.69; N, 27.26. Preparation of [Sr(η5-LNMe2)(µ-η1:η5-LNMe2)]2 (2). In a fashion similar to the preparation of 1, treatment of Sr(N(SiMe3)2)2(THF)2 (0.801 g, 1.45 mmol) with LNMe2H (0.534 g, 2.89 mmol) afforded 2 as a colorless solid (0.559 g, 85%). An analytical sample and

1038 Organometallics, Vol. 28, No. 4, 2009 single crystals suitable for X-ray crystallographic analysis were obtained by sublimation at 115 °C/0.05 Torr: mp 161-163 °C; IR (Nujol, cm-1) 1518 (m), 1402 (s), 1262 (w), 1217 (w), 1147 (w), 1082 (w), 1018 (m), 916 (m), 838 (m); 1H NMR (C6D6, 23 °C, δ) 3.97 (s, 4H, β-CH), 2.51 (s, 48H, N(CH3)2), 2.20 (s, 24H, C-CH3); 13 C{1H} NMR (C6D6, 23 °C, ppm) 163.29 (s, R-C), 80.98 (s, β-C), 49.20 (s, N(CH3)2), 22.04 (s, C-CH3). Anal. Calcd for C36H76N16Sr2: C, 47.60; H, 8.43; N, 24.67. Found: C, 46.92; H, 8.54; N, 24.36. Preparation of [Ba(µ-η2:η3-LNMe2)(η5-LNMe2)]2 (3). In a fashion similar to the preparation of 1, treatment of Ba(N(SiMe3)2)2(THF)2 (1.01 g, 1.67 mmol) with LNMe2H (0.61 g, 3.35 mmol) afforded 3 as a pale yellow solid (0.69 g, 83%). An analytical sample and single crystals suitable for X-ray crystallographic analysis were obtained by sublimation at 145 °C/0.05 Torr: mp 164-165 °C; IR (Nujol, cm-1) 1520 (m), 1413 (m), 1262 (w), 1194 (s), 1221 (w), 1149 (w), 1088 (w), 1023 (s), 970 (m), 912 (m), 818 (m), 751 (w), 735 (m); 1H NMR (C6D6, 23 °C, δ) 4.01 (s, 4H, β-CH), 2.57 (s, 48 H, N(CH3)2), 2.24 (s, 24H, C-CH3); 13C{1H} NMR (C6D6, 23 °C, ppm) 161.49 (s, R-C), 81.36 (s, β-C), 48.56 (s, N(CH3)2), 22.18 (s, C-CH3). Anal. Calcd for C36H76Ba2N16: C, 42.91; H, 7.60; N, 22.24. Found: C, 39.84; H, 7.33; N, 21.79. X-ray Crystal Structure Determinations for 1-3. Diffraction data were measured on a Bruker X8 APEX-II kappa geometry diffractometer with Mo radiation and a graphite monochromator. Frames were collected at 100(2) K as a series of sweeps with the detector at 40 mm and 0.3° between each frame and were recorded for 5 or 10 s. APEX-II17 and SHELX18 software were used in the

Sedai et al. collection and refinement of the models. Crystals of 1 were obtained as colorless fragments; 60 936 reflections were measured, yielding 7351 unique data. Hydrogen atoms were placed in calculated positions. The structure is comprised of neutral monomeric molecules without solvent. Crystals of 2 grew as colorless fragments; 61 166 reflections were counted, which averaged to 9439 independent data. Hydrogen atoms were placed in observed positions. The complex crystallized as a dimer, which occupies a crystallographic inversion center. Crystals of 3 were obtained as pale yellow fragments; 30 028 data were integrated, which averaged to 14 761 independent data. Hydrogen atoms were placed in calculated positions. The complex crystallized as a dimer, which occupies a crystallographic inversion center.

Acknowledgment. We are grateful to the Army Research Office (Grant No. W911NF-07-0489) for support of this work. Supporting Information Available: Tables of crystallographic data and bond lengths and angles for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org. OM8006739 (17) APEX II collection and processing programs are distributed by the manufacturer; Bruker AXS Inc.: Madison, WI, 2005. (18) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found Crystallgr. 2008, 64, 112.

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