This work continues a series of studies of saturated and overheated vapors over Schiff base complexes [1â. 3]. Its purpose is to determine the composition of the.
ISSN 00360236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 3, pp. 421–426. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.V. Tverdova, G.V. Girichev, S.A. Shlykov, V.V. Rybkin, O.V. Kotova, N.P. Kuz’mina, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 3, pp. 438–445.
PHYSICAL METHODS OF INVESTIGATION
Mass Spectrometric Study of the Overheated Vapor over Nickel(II), Copper(II), and Zinc(II) N,N'Ethylenebis(salicylaldiminato) Complexes N. V. Tverdovaa, G. V. Giricheva, S. A. Shlykova, V. V. Rybkina, O. V. Kotovab, and N. P. Kuz’minab a
Ivanovo State University of Chemistry and Technology, pr. Engelsa 7, Ivanovo, 153460 Russia b Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia Received December 29, 2008
Abstract—A mass spectrometric study of the overheated vapor over the nickel(II), copper(II), and zinc(II) N,N'ethylenebis(salicylaldiminato) complexes between 300 and 865°C has been carried out. Throughout this temperature range, the overheated vapor over all of the complexes contains no ions heavier than the molecular ion [MO2N2C16H14]+. At ~600°C, Cu(salen) and Zn(salen) interact with the structural material of the doublechamber twotemperature effusion cell (Kh18N10T steel). The complexes are thermally very stable. The fragmentation pattern of the chelates under electronimpact ionization is metaldependent. DOI: 10.1134/S0036023610030204
This work continues a series of studies of saturated and overheated vapors over Schiff base complexes [1– 3]. Its purpose is to determine the composition of the vapor over nickel(II), copper(II), and zinc(II) N,N' ethylenebis(salicylaldiminato) complexes (M(salen), where H2salen = N,N'ethylenebis(salicylaldimine), C16H16N2O2; Fig. 1a) at various degrees of overheating. Interest in the volatility properties of these com pounds is mainly due to their usability in the synthesis of volatile heteronuclear complexes with the general formula [LnX3M(SB)] (M(SB) = Schiff base com plex). These heterometallic compounds, possessing a variety of practically valuable properties, are promis ing precursors for the deposition of thin, nanosized oxide films and coatings with a strictly preset compo sition from the gas phase (MO CVD technique). Therefore, determining the composition of the gas phase over various Schiff base complexes as at various temperatures is of both applied and theoretical signif icance. Mass spectrometric studies of complexes of some 3d metals with H2salen and similar tetradentate Schiff bases, namely, benzaldehyde and salicylaldehyde 1
derivatives—M(salen), M(salibn), and M(oaben) [4–7]—demonstrated that these chelates show similar metaldependent fragmentation patterns. There have been reports on the evaporation thermodynamics and gas phase composition for Ni(salen) and Ni(acacen) complex [1, 2, 8] (H2acacen = N,N'ethyle nebis(acetylacetonimine)), which have structurally 1 H salibn 2
= isobutylbis(salicylaldimine); H2oaben = ethyle nebis(oaminobenzoylaldimine).
similar coordination polyhedra, but the ligand of the latter contain no aromatic fragments. The saturated vapor over Ni(salen) and Ni(acacen) consists only of monomer molecules, and Ni(acacen) is more volatile than Ni(salen). The lower volatility of Ni(salen) was attributed [2] to the presence of [Ni(salen)]2 dimers in its crystal structure [9] (Fig. 1b), which result from the stacking interaction between the aromatic moieties of the monomer molecules. The evaporation of Ni(salen) monomers implies the breaking of these dimers. The crystal structure of Ni(acacen) is built from mononuclear molecules [10]. Note that the mass spectra of M(salen) (M = Co, Ni, Cu) [5] indicate the presence of ions containing two metal atoms— [M2(O2N2C16H14)]+. The appear ance of these dinuclear fragments in the gas phase was explained by the presence of dimers in the crystal structure of Cu(salen) and Ni(salen) [9, 11]. However, this explanation seems implausible because this frag mentation pattern is unlikely for the dimers resulting from stacking interaction, the more so as the intensity of the dinuclear ion [M2(O2N2C16H14)]+ relative to the most intensive, mononuclear ion [M(O2N2C16H14)]+ is 1.1, 1.3, and 1.7% for M = Co, Ni, and Cu, respec tively. In addition, these measurements were taken below 200°С. Although the mass spectra of M(salen) (M = Ni, Cu, Zn) were described in the literature [1, 4–8], there have been no reports on the overheated vapor over these complexes. Gas phase composition data for Schiff base complexes at various degrees of overheat ing are essential for estimating the thermal stability of these compounds in the gasphase and may be of use
421
422
TVERDOVA et al. (а) H
H C
H H
H C
H C H
C
C
H
N
C
O
H C
M
C
C
N
O
C
C C
C H
C
H
H
C H
(b)
Fig. 1. (a) Model of the M(salen) molecule and (b) dinuclear fragment of the Ni(salen) crystal structure [9] constructed using the ConQuest structural database, version 1.10, issued by the Cambridge Crystallographic Data Centre, 2008.
in carrying out gas phase transport processes in thin film deposition. EXPERIMENTAL The complexes Ni(salen), Cu(salen), and Zn(salen) were synthesized via a procedure described in [12]. The synthesis products were identified by ele mental analysis for the metals, IR spectroscopy, and mass spectrometry. For Ni(salen) (C16H14N2O2Ni) anal. calcd. (%): C, 59.13; H, 4.34; N, 8.62. Found (%): C, 59.51; H, 4.40; N, 8.69. IR, ν, cm–1: 3014, 2938, 2926 (C–H); 1622, N+C O); 1532, 1464 (C C). 1596 (C For Cu(salen) (C16H14N2O2Cu) anal. calcd. (%): C, 58.19; H, 4.24; N, 8.48. Found (%): C, 58.46; H, 4.32; N, 8.52. IR, ν, cm–1: 3020, 2954, 2926 (C–H); 1626 (C N+C O); 1530, 1468, 1450 (C C). For Zn(salen) (C16H14N2O2Zn) anal. calcd. (%): C, 57.9; H, 4.2; N, 8.5. Found (%): C, 58.5; H, 4.7; N, 8.5. IR, ν, cm–1: 2898, 2846 (C–H); 1530, 1540 C); (C N+C O); 1436, 946, 860, 734 (C 1184, 1130, 1125, 1090 (C–O).
The mass spectra of the Schiff base complexes were recorded using an updated APDM1 monopolar mass analyzer (mass range of 1–2500 a.m.u., ionizing electron energy of 50 eV) [13–15]. Vapor was over heated using a doublechamber twotemperature effu sion cell (Fig. 2). The chambers, fitted with VR5/20 thermocouples, were heated with separate resistance furnaces. The sample was placed in the lowtempera ture (nickel) chamber, which communicated with the hightemperature (Kh18N10T steel) chamber via a nickel capillary. The hightemperature chamber was filled with Kh18N10T steel chips in order to prevent the free flyby of the molecules arriving from the low temperature chamber. The sample chamber was main tained at 364(5)°С for Ni(salen), 302(5)°С for Cu(salen), and 383(5)°С for Zn(salen). The maximum temperature of the upper chamber was 817°С for Ni(salen), 865°С for Cu(salen), and 700°С for Zn(salen). At high temperatures, Cu(salen) and Zn(salen) interacted with the structural material of the upper chamber (stainless steel Kh18N10T) of the two temperature effusion cell.
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8
3
4
T2
423
1
T1
2 5 5
7
7
Fig. 2. Doublechamber twotemperature effusion cell: (1) lowtemperature effusion chamber (constant temperature), (2) solid sample, (3) hightemperature effusion chamber (variable temperature), (4) capillary, (5) heaters, (6) metal chips, (7) thermocou ples, and (8) molecular beam.
RESULTS AND DISCUSSION The most abundant ions in the vapor over all of the complexes at equal upper and lower chamber temperatures are [M(salen)]+, [M–1/2(salen)]+, [M⎯1/2(salen)–14]+, and [1/2(salen)]+ (table). The formation of the m/e = 133 and M + 133 ions, whose
composition is [1/2(salen)]+ and [M–1/2(salen)]+, respectively, is likely due to the breaking of the –СH2– СH2– bond in the fivemembered cyclic fragment ⎯ N–CH2–CH2–N–. The presence of the [MONC9H9]+ ion (m/e = M + 147) in the vapor over all of the three complexes is evidence of the breaking
Mass spectra of the vapors over the M(salen) complexes (M = Zn, Cu, Ni) at various temperatures Relative ion intensity, %** Ion*
Mass number
Ni(salen)
Cu(salen)
436°C
681°C
382°C
595°C
368°C
595°C
3
12
5
9
6
7
3
2
14
9
13
7
13
6
3
23
7
3
14
7
5
17
3
3
12
6
3
6
133
4
5
17
19
10
19
147 or 149
3
3
11
3
2
3
161 or 163
7
3
4
5
2
2
M + 119
7
10
1
3
6
3
M + 133
6
3
3
4
2
2
M + 147
2
2
2
2
2
[C4H4]+
52
[N2C2H4]+ +
56
[M]
64
[NC4H7]+ [NC5H5]+ or [N2C4H6]+ [OC6H4]+ or [N2C5H8]+ [N2OC5H4]+ [ONC7H5]+ or [O2C7H5]+ [ONC8H7]+ ([1/2(salen)]+) [ONC9H9]+ or [O2NC8H7]+ [ON2C9H9]+ or [O2NC9H9]+ [MNOC7H5]+ [MONC8H7]+ ([M–1/2(salen)]+) [MONC9H9]+ [MON2C9H9]+ or [MO2NC9H9]+ [MO2N2C14H12]+ + ***
69
2
3
79 or 82
3
5
12
92 or 96
3
5
108
2
119 or 121
[MO2N2C16H14]+
M +266
[Cr(salen)]
([M(salen)]+)
Zn(salen)
7
13
M + 161 or M + 163
1
M + 240
1
1
Cr + 266
1
1
17
12
5
5
3
5
Notes: * The following atomic weights (a.m.u.) were accepted in the interpretation of the mass spectra: Zn, 65; Cu, 64; Ni, 59. M = metal atom. ** Normalized to the total ion intensity. The ion intensities below 1% are omitted. *** Ion appearing at high temperatures as a result of the interaction between the sample and the structural material of the effusion cell. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
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TVERDOVA et al. Ii/Itot 0.3
[C4H4]+ [NC5H5]+ or [N2C4H6]+ [1/2(salen)]+ [Ni–1/2(salen)]+ [Ni(salen)]+
Ni(salen)
0.2
0.1
0 400
450
500
550
600
650
700
750
800
850 T, °C
Fig. 3. Temperature dependence of the relative intensities of the most abundant ions in the overheated vapor over Ni(salen). Here and in Figs. 4 and 5, Ii is the intensity of the ith ion and Itot is the total ion intensity.
of the C–N bond in the cyclic fragment –N–CH2– CH2–N– (Fig. 1a). Note that, unlike Gilbert et al. [5], we did not observed ions heavier than [M(salen)]+ for any com plex. Therefore, there are no dinuclear species in the vapor over Ni(salen), Cu(salen), and Zn(salen). This verifies the assumption that the sublimation of these compounds is preceded by the breaking of the dimer fragments in their crystals. The doubly charged ion [M(salen)]2+ was not detected either. The three complexes are very similar in terms of the ion composition of their saturated and overheated vapors (table). Therefore, similar processes take place in the dissociative electronimpact ionization of the M(salen) monomers. However, there is a welldefined correlation between the relative intensity of the most abundant ions— [M(salen)]+, [M–1/2(salen)]+, and [1/2(salen)]+ —and the metal atom (Figs. 3–5). Sim ilar intensity ratios of these ions were observed in a mass spectrometric study of the overheated vapors over M(acacen) (M = Ni, Cu, Zn) complexes [3]. For example the metalfree ions are the most intense in the mass spectra of the zinc compounds and the metal containing ions dominate in the vapor over the nickel compounds. The similar fragmentation patterns of the zinc and nickel complexes with different Schiff bases under electronimpact ionization likely reflects the specific features of the electronic structure of the cen tral metal atom, which were considered in our earlier publication [3]. At the same time, a comparative ana
lysis of the dissociative ionization of Cu(acacen) and Cu(salen) revealed essential differences between their mass spectra (Fig. 6). These differences possibly indi cate the occurrence of additional, parallel processes specific to Cu(salen). In an earlier work [6], the frag mentation patterns of Schiff base complexes were con sidered in terms of the central atom oxidation num ber–ion charge concept. Presumably, there can be charge transfer between the central atom and the ligand. The possibility of this charge transfer depends on the capability of the metal to accept one electron (or more) from the ligand (to decrease its oxidation number) or donate electrons to the ligand (to increase its oxidation number). The probability of these elec tron transfer events is governed by the stability of dif ferent oxidation states of the given metal. A mass spec 2
trometric study of deuterated Cu(salen) [6] revealed the following two steps in the fragmentation of this complex, which account for the high intensity of the [1/2(salen)]+: (1) C–C bond breaking in the five membered cycle Cu−N−CH2−CH2−N, yielding [1/2(salen)]+ and [Cu(II)−1/2(salen)]+; (2) subsequent Cu(I)H elimination from the [Cu(II)−1/2(salen)]+ ion, yielding an ion with m/e = 133. For the zinc com plexes, similar processes accompanied by a change in the oxidation state of zinc are unlikely. The electronic structure of the zinc atom is more favorable for ligand −СH2−СH2− bridge of Cu(salen) was labeled with deute rium (Fig. 1a).
2 The
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Ii/Itot Cu(salen) [1/2(salen)]+ [Cu–1/2(salen)]+ [Cr(salen)]+ [Cu(salen)]+
0.2
0.1
0 350
400
450
500
550
600
650
700
750
800
850
900 T, °C
Fig. 4. Temperature dependence of the relative intensities of the most abundant ions in the overheated vapor over Cu(salen).
Ii/Itot 0.3
[C4H4]+ [NC5H5]+ or [N2C4H6]+ [1/2(salen)]+ [Zn–1/2(salen)]+ [Zn(salen)]+
Zn(salen)
0.2
0.1
0 350
400
450
500
550
600
650
700
750 T, °C
Fig. 5. Temperature dependence of the relative intensities of the most abundant ions in the overheated vapor over Zn(salen).
abstraction, and this can be another cause of the high intensity of the metalfree ion [1/2(salen)]+. There fore, the similarity of the mass spectra of the zinc and copper complexes are explicable in terms of different mechanisms. RUSSIAN JOURNAL OF INORGANIC CHEMISTRY
It was found by vapor overheating experiments that, as the temperature is raised, the ion currents due to metalcontaining species decrease (Figs. 3–5). The fact that the intensities of the metalcontaining ions of the zinc and nickel complexes gradually decrease with Vol. 55
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I, % 20
[1/2(salen)] Cu(salen) T = 495°C 79
10
64
108
[CuNOC7H5]+
15
119 147 52
92
5
[Cu(salen)]+ [Cu–1/2(salen)]+
161
211
0 I, % 35 30
50
100
150
200
250
300
350 m/e
[Cu–1/2(acacen)]+ Cu(acacen) T = 498°C
25 20 [1/2(acacen)]+
15
[Cu(acacen)]+
10 5 0 I, % 20
68 56
147
96 83
161 189
100
50
150
200
300
350 m/e
15 [ZnNOC7H5]+ [Zn–1/2(salen)]+
108
10
92
5
0
52
50
69
119
147 161
100
150
[Zn(salen)]+
305 212
200
ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project no. 070300656a. REFERENCES
250
Zn(salen) [1/2(salen)] T = 488°C
79
structural elements of the doublechamber twotem perature effusion cell. As is demonstrated in Fig. 4, the replacement of the central atom by Cr begins at ~600°С. The temperaturedependent ion current data for the metalcontaining ions of Zn(salen), as distinct from those for Cu(salen), do not indicate any appre ciable degree of replacement of the central atom by chromium. The only evidence in favor of the hypo thetic interaction between the sample and the cell material is that an m/e = 318 ion current exceeding the instrument background appears at the same tempera ture as in the case of the copper complex. Unfortu nately, because of the interaction of Cu(salen) and Zn(salen) with the structural material of the effusion cell, it was impossible to determine the onset temper ature of the thermal decomposition of these com plexes. We observed no interaction between the nickel complex and the structural material of the effusion cell. The onset temperature of the thermal decompo sition of Ni(salen), indicated by a sharp increase in the intensities of metalfree ions, is ~700°С (Fig. 3). The disappearance temperatures of the metalcontaining ions—approximately 770°С for Ni(salen) and 700°С for Cu(salen) and Zn(salen)—indicate that these complexes, like M(acacen), are thermally very stable.
250
300
350 m/e
Fig. 6. Mass spectra of Cu(salen) (495°C), Cu(acacen) (498°C), and Zn(salen) (488°C). The total intensity of all ion currents is taken to be 100%.
increasing temperature can be explained by the tem perature variation of the mass spectra. The compli cated temperature dependence of the relative ion cur rents for the copper complex (Fig. 4) indicates pro nounced interaction between the compound and the
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