New metal-organic precursors for MOCVD applications: Synthesis, characterization, crystal structure and thermal properties of mixed-ligand Mg(II) complexes

Journal of Molecular Structure 1035 (2013) 416–420

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New metal-organic precursors for MOCVD applications: Synthesis, characterization, crystal structure and thermal properties of mixed-ligand Mg(II) complexes Sanjaya Brahma a,d, M. Srinidhi b, S.A. Shivashankar a,⇑, T. Narasimhamurthy a, R.S. Rathore c,⇑ a

Materials Research Centre, Indian Institute of Science, Bangalore 560 012, Karnataka, India Poorna Prajna Institute for Scientific Research, Bangalore 560 064, Karnataka, India Bioinformatics Infrastructure Facility, School of Life Sciences, University of Hyderabad, Hyderabad 500 046, Andhra Pradesh, India d Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan b c

a r t i c l e

i n f o

Article history: Received 18 August 2012 Received in revised form 18 November 2012 Accepted 19 November 2012 Available online 24 November 2012 Keywords: MOCVD precursor Crystal structure Metal-organic complex Acetylacetonate 1,10-Phenanthroline 2,20 -Bipyridine

a b s t r a c t In an effort to develop new MOCVD precursors, mixed-ligand metal-organic complexes, bis (acetylacetonato-k2O,O0 ) (2,20 -bipyridine-k2N,N0 ) magnesium(II), and bis (acetylacetonato-k2O,O0 ) (1,10-phenanthroline-k2N,N0 ) magnesium(II) were synthesized. Spectroscopic characterization and crystal structures confirmed them to be monomeric and stable complexes. Crystal structure analysis suggests in each of the magnesium(II) complexes, the metal center has a distorted octahedral coordination geometry. Thermo-gravimetric analysis (TGA/DTA) suggests that these complexes are volatile and thermally stable. The thermal characteristics of newly designed complexes make them attractive precursors for MOCVD applications. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The metal-organic chemical vapor deposition (MOCVD) process, which employs metal-organic compounds as precursors, has attracted considerable interest because of its versatility in producing uniform conformal films of a wide variety of materials [1–4]. An essential requirement in a MOCVD process is the availability of suitable precursors, possessing characteristic thermal properties i.e., high volatility, preferably at low temperature, long-term stability and high thermal decomposition temperature [5,6]. This has led to precursor engineering, aiming to tailor and fine-tune the thermal properties of precursors [7,8]. An important requirement for precursor engineering is the knowledge of relationship between structures of metal complexes and their physical and thermal properties. Towards this objective, several alkaline, rare-earth and transition metal precursors with ligands such as b-diketonates (acetylacetonate; acac), alkoxides, chlorides, nitrates have been synthesized, studied and employed for obtaining metal-oxide films by MOCVD process [9,10,8,11]. These ligands are able to coordinate ⇑ Corresponding authors. Tel.: +91 9632149187; fax: +91 40 23010120 (R.S. Rathore). E-mail addresses: [email protected] (S.A. Shivashankar), ravindranath_ [email protected] (R.S. Rathore). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.11.038

with various metals and form stable chelates, and among metal coordination compounds, they are the most volatile [10,12]. With the purpose of developing new precursors, we have synthesized metal–organic complexes (Fig. 1) of magnesium(II) with acac, 1,10-phenanthroline (phen) and 2,20 -bipyridine (bipy) ligands. Herein, we report the synthesis, characterization and structural investigation of the new complexes, bis (acetylacetonato-k2O,O0 ) (2,20 -bipyridine-k2N,N0 ) magnesium(II), (I), and bis (acetylacetonato-k2O,O0 ) (1,10-phenanthroline-k2N,N0 ) magnesium(II), (II), and discuss their thermal characteristics. 2. Experimental 2.1. Synthesis of Mg(II) complexes Mg complexes were synthesized in high yields using a method described by Brahma et al. [13] for synthesis of Zn complexes. (I) was prepared from its precursor dihydrate complex i.e., Mg(acac)2 dihydrate. A methanolic solution of 2,20 -bipyridine (5 mmol, 0.78 g in 20 ml of methanol) was added to the methanolic solution of Mg(acac)22H2O, (5 mmol, 1.29 g in 40 ml of methanol) and the mixture was stirred at atmospheric temperature and pressure. Although a white precipitate started to form after 4–5 h of stirring, stirring was continued overnight to allow for the completion of the

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Fig. 1. Chemical scheme: (a) (I) and (b) (II).

Table 1 Crystal data. No

Crystal data

Mg(acac)2bipy (I)

Mg(acac)2phen (II)

(i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x) (xi) (xii) (xiii) (xiv)

Chemical formula Molecular weight Crystal size (mm) Morphology Crystal system Space group Unit cell parameters, a (Å), b (Å), c (Å) Volume (Å3) Z/Z0 Cell measuring reflections h-range (°) l (mm1) absorption correction F (0 0 0) Dx (calculated) (g cm3)

[Mg(C5H7O2)2(C10H8N2)] 378.71 0.21  0.12  0.11 Needle, colorless Orthorhombic P212121 8.1298(11), 15.362(2), 15.604(2) 1948.8(5) 4/1 2138 2.26–30.4 0.119/multi-scan 800 1.29

[Mg(C5H7O2)2(C12H8N2)] 402.73 0.30  0.14  0.12 Needle, colorless Orthorhombic Pbcn 15.5879(9), 10.2447(6), 12.5893(6) 2010.43(19) 4/0.5 450 0.98–27.99 0.120/multi-scan 848 1.331

Data collection (xv) (xvi) (xvii) (xviii)

Radiation (Å) Temperature (°K) h Range (°) Indices

0.71073 (MoKa) 292 2.93–25.0 h = 9 ? 9 k = 16 ? 18 l = 16 ? 18

0.71073 (MoKa) 292 2.88–26.0 h = 19 ? 18 k = 12 ? 11 l = 15 ? 14

(xix) (xx) (xxi)

Scan type Independent reflections Observed reflections [I > 2r(I)]

/ and x scans 3281 2608

/ and x scans 1976 1377

Refinement (xxii) (xxiii) (xxiv) (xxv) (xxvi)

Final indices Goodness of fit (S) (D/r)max Dqmax and Dqmin (e Å3) Data/restraints/parameter

R = 0.0769, wR = 0.197 1.033 0.000 0.475/0.543 3281/0/248

R = 0.0548, wR = 0.1509 1.067 0.000 0.279/0.365 1976/0/134

w = 1/[r2(F 20 ) + (aP)2 + bP] where P = (F 20 þ 2F 2c )/3, parameters a and b are: 0.1261, 0.0 (I) and 0.0958, 0.0869 (II), respectively.

reaction. The white precipitate formed was filtered, washed thoroughly with distilled water and dried in vacuum. A similar procedure was followed to prepare (II). A methanolic solution of 1,10-phenanthroline (5 mmol 0.99 g in 20 ml of methanol) was added to the methanolic solution of Mg(acac)22H2O (5 mmol, 1.29 g in 40 ml of methanol) and the resulting mixture was stirred. After stirring at room temperature for 8 h, the adducted compound precipitated out. The precipitates thus formed were filtered, washed repeatedly with water and dried in vacuum.

2.2. Spectroscopic characterization The microanalysis was performed on a Thermo Finnigan FLASH EA 1112 CHNS analyzer model: 1106. FT-IR spectra were recorded by using a Perkin–Elmer Spectrometer-SPECTRUM GX, (resolution 4 cm1, KBr disks) in the wave number range of 400–4000 cm1.

LCAMS (liquid chromatography–mass spectrometry) data were obtained using a Micromass Q-TOF mass spectrometer. 2.3. X-ray crystallography Suitable single crystals of (I) and (II) for X-ray diffraction studies were obtained by slow evaporation of their methanolic solutions at room temperature. Data were collected on a singlecrystal Bruker SMART APEX-2 diffractometer. Cell parameters were refined and data were processed using SMART and SAINT-Plus [14]. The structure was solved by applying the direct phase-determination technique using SHELXS-97, and anisotropically refined by full-matrix least-square on F2 using SHELXL-97 [15]. All structure calculations were performed with WinGX suit of programs [16]; Version 1.80.05). Hydrogens were placed in the geometrically expected positions and refined with the riding options. The torsion angles for the methyl group hydrogens were set with reference

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S. Brahma et al. / Journal of Molecular Structure 1035 (2013) 416–420 100 3068

90

2921

Mg (acac)2 bipy

654 438 1186 1256 917 548

Transmittance (%)

80

1011 768

70 1516

60

1605 14671411

50

645 444 1194 1255 735 544 1022 850 920

Mg (acac)2 phen 3063 2919

140 130

1601 1514

120

1463

110 4000

1408

3500

3000

2500

2000

1500

1000

500

-1

Wave Number (cm ) Fig. 2. FT-IR spectra of (I) (spectrum on top) and (II) (spectrum at the bottom).

to a local difference Fourier calculation. The0 distances with hydro0 gen atoms are: aromatic/Csp2 CAH = 0.93 Å A, methyl CAH = 0.96 Å A, and Uiso = 1.2Ueq(parent)/1.5Ueq(Cmethyl). Essential crystal data are listed in Table 1. Crystallographic data have been deposited at Cambridge Crystallographic Data Center with entry numbers, CCDC840255 (I) and CCDC714080 (II). 2.4. Database search Search in Cambridge Structural Database (CSD; Version 5.32, [17] was performed using Conquest (Version 1.12). 2.5. Thermal analysis (TGA/DTA) Thermal analysis of the metal-organic complexes was carried out using a SDTQ 600 simultaneous TG/DT analyzer (TA Instruments, USA), under 99.99% purity nitrogen (300 ml/min) gas stream. About 10 mg of finely ground sample was placed in a platinum crucible, which was attached to a sensitive microbalance. The platinum crucible was heated from ambient to 500 °C at a heating rate of 10 °C/min. 3. Results and discussion 3.1. Characterization of the complexes 3.1.1. Microanalysis The results of the elemental analysis are within ±0.4% of the calculated values. Calc. for [MgII(C20H22N2O4)](I): C 69.77, H 5.32, N

7.39, found: C 69.65, H 5.25, N 7.21; calc. for [MgII(C22H22N2O4)](II): C 65.61, H 5.50, N 7.39, found: C 65.57, H 5.43, N 7.18. 3.1.2. FT-IR The IR spectrum of (I) and (II) are shown in Fig. 2. Both complexes (I) and (II) show the presence of bands in the chelate carbonyl region and in the MAO bond region, indicating the formation of the complex. The band observed in the range 1605– 1411 cm1 in (I), and 1601–1408 cm1 in (II) are due to the C@O and C@C vibrations in the acetylacetonate (acac) ligand. In the intermediate region from 1260 to 600 cm1, there are several bands corresponding to the various bending, stretching, and ring deformations of the coordinated complex. Different band assignments are listed in the Supplementary Table S1. The band in the low wave number region [438 cm1 in (I), 444 cm1 in (II)] is due to the stretching vibrations of the MgAO bond. The feature at 444 cm1 is of low intensity. The vibrations at 548 cm1 and 544 cm1 correspond to MgAO bending vibration, in (I) and (II), respectively. 3.1.3. Mass spectrum The mass spectra of (I) and (II) are shown in Fig. 3, which are similar to spectra reported earlier for similar lanthanide complexes [18]. The spectrum consists of peaks corresponding to Mg(acac)2bipy [m/z = 279], indicating the formation of the complex. The spectrum also shows that the bipyridyl moiety does not break up from the molecule during the evaporation stage, a desirable attribute for the complex to be thermally stable. Peak at [m/ z = 179] corresponds to the loss of one ‘‘acac’’ moiety from the metal complex. All other peaks in the mass spectrum correspond to the fragments produced during the ESI process. The spectrum of (II) consists of peaks at m/z = 303 and 181. The former corresponds to the Mg (acac)phen after the loss of one ‘‘acac’’ moiety, and the latter is attributed to the phenanthroline moiety. The peak at m/z = 303, indicates that the phenanthroline moiety does not break up in the initial stage, confirming the thermal stability of the compound. The peak at [m/z = 203] corresponds to the loss of another ‘‘acac’’ moiety in Mg (acac)2phen. 3.2. X-ray diffraction The atomic details of the synthesized complexes are shown in Fig. 4. Both molecules (I) and (II) possess C2 point-group symmetry. However, the molecular symmetry is retained in the crystals of (II), containing the rigid phenanthroline group with MgII atom located on the 2-fold axis. The bipyridyl ring in (I) is substantially distorted, leading to non-retention of molecular symmetry in the crystal. The topic of molecular vs. crystallographic symmetry has

Fig. 3. Mass spectrum of (a) (I) and (b) (II).

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(a)

(b)

C19

C15

C17

C16

C9

C13

O2

O2

O3 C20

C11

C12

C18

C8

Mg1

Mg1 C7

O4

C11

N1

N1

N2

C10

C2

C5

C5

C6

C3

C3

C7

C8

C1

C1 C2

C9

C10

O1

C14

O1

C4

C4 C6

Fig. 4. A view of coordination complex of: (a) Mg(acac)2bipy (I) and (b) Mg(acac)2phen (II). Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radii.

0.5

(a) 100

(b)

Mg (acac) 2 bipy

60

40

20

-0.5

-1.0

-1.5

90

80

70

60 -2.0

0

50 100

200

300

400

500

0.00

-0.05

o

Weight loss (%)

o

0.0

Temperature difference ( C/mg)

Temperature difference ( C/mg)

80

Weight loss (%)

Mg (acac) 2 phen

100

o

145 C

0

100

o

200

300

400

-0.10

-0.15

-0.20

-0.25 500

o

Temperature ( C)

Temperature ( C)

(c) 110 100 90 80

Mg (acac) phen

Weight (%)

70 60 50 40

Mg (acac) bipy

30 20 10 0 0

100

200

300

400

500

o

Temperature ( C) Fig. 5. TG/DTA pattern of (a) Mg(acac)2bipy (I). (b) Mg(acac)2phen (II). In each of the figures, the upper curve is TGA pattern and overlapped lower curve is DTA pattern and (c) An overlay of TGA graph of (I) and (II).

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been discussed earlier [19]. It is shown that C2 point group symmetry is conserved in about 60% of the reported cases [20]. The metal ions possess a distorted octahedral coordination geometry in (I) and (II) as a result of differences in O. . .O/N. . .N bite distances, and OAMgAO/NAMgAN bite angles. The bite distances and angles for (I) and (II), respectively, are as follows: O. . .O = 2.806(5)/2.813(4) Å and 2.802(2)Å; N. . .N = 2.661(4) Å and 2.710(3) Å; OAMgAO = 87.74(14)/87.22(14)° and 87.06(7)°; NAMgAN = 72.85(12)° and 74.54(10)°. A search in CSD for M[(acac)2bipy] and M[(acac)2phen] (where M stands for any metal) retrieved 19 and 14 fragments respectively. Refcodes and geometric details (bite distances, bite angles and MAO/MAN distances) are given in the Supplementary Table S2. The distorted octahedral geometry is predominantly observed in these six-coordinate metal complexes formed by bidentate mixed-ligands. Exceptions are POCMUQ, POCMUQ01/QEYSIX; DODMUM, POCNAX, QEYSET trigonal-prismatic configuration in [Mn(acac)2(bipy)] [21]. The bite distances and bite angles have been observed to vary in the following ranges in these complexes. O. . .O, 2.72–2.99 Å (bipy)/2.71–3.06 Å (phen), N. . .N, 2.56–2.75 Å (bipy)/2.65–2.78 Å (phen), OAMAO, 68.9–88.8° (bipy)/66.9–89.7° (phen), NAMAN, 56.4–80.7° (bipy)/60.4–79.1° (phen). The six-member chelate rings formed by the MgII and O atoms in (I) is planar. Mg1/O1/O2/C11-C13; 0.05(1), Mg1/O3/O4/ C16AC18, 0.007(3); Mg1/N1/N2/C5/C6, 0.12(1). The five-membered chelate ring of MgII and bipyridyl N atoms is considerably non-planar. In (II), the rings are slightly puckered and the dihedral angle between the Mg1/O1/O2 and O1/O2/C7AC9 planes, describing the amount of puckering, is 13.8(1)°. The flanking pyridyl rings are inclined at a dihedral angle of 13.1(1)°; the N1AC59AC6AN2 torsion angle is 13.3(6)°. In (I), the bipyridyl system is twisted to accommodate a C1AH1. . .O2 intramolecular hydrogen bond [H. . .A = 2.60 Å, D. . .A = 3.136(6) Å, DAH. . .A = 117°]. No intramolecular interactions were observed in (II) due to the rigidity of the phen ring, and partly due to the crystallographically imposed symmetry. The crystal packing in (I) and (II) is stabilized by van der Waal interactions only. In (I) a short contact associated with methyl group, C20AH20C. . .O1i [Symmetry code (i): 1 + x, y, z, H. . .A = 2.44 Å, D. . .A = 3.387(6) Å DAH. . .A = 170°] was observed.

3.3. Thermal analysis (TG/DTA studies) The TG/DTA pattern of complex (I) is shown in Fig. 5a. The pattern shows two regions of weight loss at 145 °C and 250 °C. The data show a monotonic decrease in weight of the material starting at a temperature 145 °C, which continues till 220 °C. The second step of weight loss starts at 250 °C and continues up to 300 °C. The complex starts to sublime at 145 °C. The corresponding DTA pattern (Fig. 5a) shows two different endothermic peaks at 215 °C and 280 °C. The weight loss in the complex amounts to 3% at 170 °C, implying that the complex can be used as precursor around this temperature in a practical CVD process. This is because a 3% weight loss of the precursor is empirically found to be sufficient to provide adequate precursor flux in a practical CVD process. Fig. 5b shows a simultaneous TG/DTA pattern of (II). In contrast with Fig. 5a, the graph shows a single stage sublimation of the material without any intermediate state. The weight loss begins at 240 °C. The DTA pattern suggests one sharp endothermic peak at 303 °C, which is in good agreement with the TGA pattern. Weight loss amounts to 3% at 277 °C. The two-step weight loss in (I) as compared to (II) may be attributed to the differences in the molecular structures (Fig. 4). In (I), one of the two bipyridyl moieties is dissociated from the complex at about 145 °C, followed by the sublimation of the remainder of the complex at about 250 °C.

On the other hand in (II), the entire complex adducted with phenanthroline sublimes, beginning about 240 °C. Thermal analysis of these complexes thus confirms the suitability of the present metal-organic complexes as CVD precursors, as they are sufficiently volatile at temperatures significantly lower than their decomposition temperatures. An overlay of the TGA patterns of (I) and (II) is shown in Fig. 5c. The graph indicates that complex (I) containing flexible bipy ligand is more volatile than (II) and hence would be a preferred precursor for the MOCVD process. 4. Conclusion In an effort to develop MOCVD precursors, we have synthesized MgII acetylacetonate complexes adducted (‘‘mixed’’) with 1,10-phenanthroline and 2,20 -bipyridine ligands. Spectroscopic characterization and crystal structures confirmed the monomeric and stable complex formation of these complexes. In the crystal, the metal center possesses a distorted octahedral coordination geometry. To examine the suitability of the complexes as MOCVD precursors, thermal analyses were carried out. The new adducts of MgII are volatile and thermally stable. Their thermal characteristics at atmospheric pressures make them attractive candidate precursors for MOCVD processes. Acknowledgements The use of the CCD facility (under the IRHPA–DST program) at IISc., Bangalore, is acknowledged. S.B. thanks CSIR, Government of India, for CSIR research associateship and R.S.R. thanks CSIR for financial assistance. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.11. 038. References [1] H.O. Pierson, Handbook of Chemical Vapor Deposition (CVD) – Principles, Technology and Applications, second ed., William Andrew Publishing/Noyes, 1999. [2] J.O. Williams, Angew. Chem. Int. Ed. Engl. 28 (1989) 1110–1120. [3] D.C. Bradley, Chem. Rev. 89 (1989) 1317–1322. [4] G. Malandrino, R.G. Toro, R.L. Nigrob, I.L. Fragalà, ECS Trans. 25 (2009) 125– 134. [5] F. Maury, Chem. Vap. Deposition 2 (1996) 113–116. [6] U. Patil, PhD Thesis. Ruhr-University, Bochum, Germany, 2005, reference therein. [7] G. Malandrho, R. Licata, F. Castelli, I.L. Fragalh, Inorg. Chem. 34 (1995) 6233– 6234. [8] A.C. Jones, J. Mater. Chem. 12 (2002) 2576–2590. [9] E. Weiss, Angew. Chem. 32 (1993) 1501–1670. [10] D.J. Otway, W.S. Rees Jr., Coord. Chem. Rev. 210 (2000) 279–328. [11] G.M. Neelgund, S.A. Shivashankar, T. Narasimhamurthy, R.S. Rathore, Acta Cryst. C63 (2007) m74–m76. references therein. [12] A.B.P. Lever, Comprehensive Coordination Chemistry II, vol. 1, Elsevier, Oxford, 2003. [13] S. Brahma, H.P. Sachin, S.A. Shivashankar, T. Narasimhamurthy, R.S. Rathore, Acta Cryst. C64 (2008) m140–m143. [14] Bruker, SMART (Version 5.628) and SAINT (Version 6.45a). SADABS (Version 2004/1) Bruker AXS Inc., Madison, Wisconsin, USA, 2004. [15] G.M. Sheldrick, Acta Cryst. A64 (2008) 112–122. [16] L.J. Farrugia, J. Appl. Crystallogr. 30 (1999) 565. [17] F.H. Allen, Acta Crystallogr., Sect. B: Struct. Sci. 58 (2002) 380–388. [18] K. Shalini, PhD Thesis. Materials Research Centre, Indian Institute of Science, Bangalore, 560012, India, 2002, p. 149. (Chapter 6). [19] .R.S. Narasegowda, S.M. Malathy Sony, S. Mondal, B. Nagaraj, H.S. Yathirajan, T. Narasimhamurthy, P. Charles, M.N. Ponnuswamy, M. Nethaji, R.S. Rathore, Acta Cryst. E61 (2005) o843–o845. references therein. [20] E. Pidcock, W.D.S. Motherwell, J.C. Cole, Acta Cryst. B59 (2003) 634–640. [21] R. van Gorkum, F. Buda, H. Kooijman, A.L. Spek, E. Bouwman, J. Reedijk, Eur. J. Inorg. Chem. (2005) 2255–2261.