J. Phys. Chem. 1995,99, 14641-14646
14641
Infrared Spectroscopic Investigation of Adducts of MezCd with Various Lewis Base Donor Molecules in Frozen Solid Films at -196 “Ct Matthew J. Almond,* Sharon A. Cooke, David A. Rice, and Liam A. Sheridan Department of Chemistry, University of Reading, Whiteknights, Reading, RG6 2AD, U.K. Received: March IO, 1995@
An infrared spectroscopic study of the formation of adducts of Me2Cd in cryogenic thin films at - 196 “C is reported. The formation of adducts has been monitored by measuring shifts in the position of bands arising from p(CH3) and v,(C-Cd-C) vibrations of the MezCd moiety and by activation of vSym(C-Cd-C), a vibration which is infrared inactive in free Me2Cd. In the first series of experiments a number of potential chelating donor ligands were studied. Thus it is shown that MezNCHzCH2NMe2 (TMEDA) coordinates strongly to Me2Cd under these conditions. The adduct formed in the cryogenic film almost certainly has the same structure as in the gas phase, Le., where the TMEDA ligand is chelating. Similar strong interactions with Me2Cd in cryogenic films are observed with the ligand Me2NCH2CH2N(Me)CH2CH2NMe2 (PMDETA) and for the oxygen species MeOCH2CH20Me (DME) and MeOCH2CH20CH2CH20Me (“diglyme”). By contrast the P-donor Me2PCH2CH2PMe2 (TMDPE) gives only a weak interaction as do the cyclic molecules l,Cdioxane, 1,4-thioxane, and 1,Cdithiane. The sulfur donors diethyl sulfide (EtZS), ethylene sulfide (EES), thiirane), and propylene sulfide (PES, methyl thiirane) show some interaction to MezCd. By contrast there is no evidence for any interaction between carbonyl sulfide (OCS) and Me2Cd under the same conditions. It is likely that all adducts detected have a 1:l stoichiometry. The strength of adduct formation to MezCd shown by these donor molecules is discussed in terms of the proton affinity of the Lewis base.
Introduction Metal organic chemical vapor deposition (MOCVD) is an established method for the growth of thin layers of the wide band gap semiconductor CdS.’V2 The most commonly used precursors are MezCd and H2S as the metal and sulfur source, re~pectively.~ A problem which plagues this process is that of “prereaction” in which the precursors react before the heated substrate is r e a ~ h e d . This ~ wastes valuable precursor compounds and may lead to the formation of an inferior, nonepitaxial product. Two possible ways have been suggested to reduce prereaction. The first is to form Lewis acid-base adducts of Me2Cd and so lower the reactivity of the cadmium center in MOCVD. This approach appears to have been successful in reducing the prereaction encountered when thin layers of ZnS are produced by MOCVD from MezZn and H2S as precursor^^-^ but is much less successful in reducing the prereaction in the analogous cadmium system.8 Probably the most useful Lewis base donors in this respect are the chelating bidentate ligands which have a strong interaction with MenCd. Thus it is possible to make an adduct, Me2CdTMEDA (TMEDA = tetramethylethylenediamine, M ~ z N C H ~ C H ~ Nwhich M ~ ~will ) , sublime undissociated.E However, we have shown that use of Me2CdsTMEDA in MOCVD reduces but does not eliminate prereaction. The number of adducts of MezCd that have been studied with chelating donors is small, and so we have chosen to study adduct formation with a range of ligands in frozen cryogenic films at -196 “C. Such an approach is an established method to monitor adduct formation; it has been used inter alia to investigate the formation of adducts of MezZn with various hydrides, e.g., NH3, PH3, and HzSl2 and with group 15 and 16 alkyl$ e.g., (CH3)2S and (CH3)3N.I3 +This work was presented, in part, at the International Conference on Low Temperature Chemistry, Moscow, Russia, September 1994. Abstract published in Advance ACS Abstracrs, September 1, 1995. @
0022-365419512099-14641$09.0010
Two series of donors have been studied: (i) potentially strong chelating ligands and (ii) S-donor monodentate ligands. In the first set of experiments we began with the ligand Me2NCH2CH2NMe2 (TMEDA) since the structure of MezCd-TMEDA has been determined in the gas phase by electron diffraction measurementsEand shown to contain a chelating ligand. Gas phase infrared data are also available for the compound,Efor comparison with the thin film data obtained here. We then proceeded to examine the phosphorus analogue of TMEDA, Le., Me2PCH2CH2PMe2 (tetramethyldiphosphinoethane,TMDPE) thus obtaining data for analogous “hard” and “soft” bases. A similar hard soft base study was undertaken with some group 16 donors. One of the simplest ligands capable of chelation is the ether MeOCH2CH20Me (dimethoxyethane, DME). It has been studied and its reactivity compared to the cyclic donors 1,4-dioxane, 1A-thioxane, and 1,Cdithiane. The reaction of 1,4-dioxane with Me2Cd has been studied in detail.I4 Infrared spectroscopy has shown that there is no interaction between 1,Cdioxane and Me2Cd in the gas phase. However, 1,Cdioxane and MezCd form a crystalline solid that contains polymeric chains of formula -(Mez)Cd- 1,4-dioxane-(Me~)Cd- 1,4-dioxane-.I4 Mass spectrometric studies on the solid suggest that the adduct dissociates upon vaporization. Having obtained infrared spectra for adducts in cryogenic films whose spectra have already been recorded in the gas phase, conclusions regarding the possible structures of the adducts in the film may be drawn. This starting point allows us to move to other species where gas phase spectroscopic data are not available, e.g., because the adduct dissociates in the gas phase and to search for weak interactions between Me2Cd and the S-donor molecules. In recent experiment^'^^'^ we have demonstrated the usefulness of propylene sulfide as a sulfur source in the growth of CdS thin layers by MOCVD. Preliminary experiments suggest that OCS might also be useful in this role.” We therefore wished to determine whether or not the effectiveness of these two compounds as the sulfur sources in MOCVD 0 1995 American Chemical Society
Almond et al.
14642 J. Phys. Chem., Vol. 99, No. 40,1995 depends on any extent to the ability of the molecule to form an interaction with MezCd. Our experiments have already shown that the molecules, Et2S, ethylene sulfide (EES, thiirane), propylene sulfide (PES, methyl thiirane), and carbonyl sulfide (OCS) give no evidence for the existence of adducts with MezCd in the gas phase. However, the method of cocondensation to produce adducts in cryogenic thin films is known to be a good way to monitor weak adduct f o r m a t i ~ n . ' ~ . ' ~Accord.'~ ingly, we hoped that by adopting the approach outlined in this paper we might obtain evidence for the existence of weak adducts between Me2Cd and the sulfur donor molecules listed. Experimental Section Dimethyl cadmium was prepared following the method of Krause which has been detailed e l s e ~ h e r e . ' ~The . ~ ~compound was purified by the formation of the crystalline adduct Me2C&2,2'-bipyridyl, and subsequent cracking of this adduct at ca. 67 "C in vacuo to afford pure Me2Cd.20-22 All other reagents were used as supplied (ex. Aldrich, except for PMDETA which was supplied by Lancaster synthesis) following drying either with 4A molecular sieves or sodium wire. The dryness of reagents was assessed by recording infrared and 'H NMR spectra. Reagents were deposited upon a CsI window held at a temperature of - 196 "C by means of a liquid nitrogen reservoir and located inside a glass vacuum shroud.23 Infrared spectra were recorded on a Perkin-Elmer 1720 Fourier transform infrared spectrometer. Blank spectra of all reactants were recorded prior to codeposition experiments. Mixtures were codeposited by the twin jet method, i.e., a separate inlet to the cell was used for each reagent. The samples of MezCd and ligand were held at temperatures such that each had a vapor pressure of 5 Torr. For experiments with Me2Cd and OCS a mixture consisting of 20 Torr of each reactant was placed in a 1-L bulb. Reagents were deposited onto the cold window under a dynamic vacuum; the cell was sealed by means of two Young's greaseless taps and removed from the pumping system prior to recording infrared spectra. Where appropriate, annealing experiments were carried out by allowing the CsI window to warm, under a static vacuum, to a temperature at which the solid was judged to be appreciably softened. It was found that the maximum annealing temperature of a mixture was where one-or-other of the reagents had a vapor pressure approximately equal to 0.1 Torr. Annealing was carried out for a period of 5 min. The window was recooled to -196 "C, and the cell was evacuated on the vacuum line before spectra were rerecorded. An alternative approach would have been to carry out isotopic enrichment with I3C or 2H. However, the process was deemed unnecessary as enrichment in 2H would give little information about the structure of the Me2Cd moiety in any adduct. While the measurement of the shift in va,(C-Cd-C) upon substitution with I3C may give information about the C-Cd-C bond angle and hence, indirectly, the strength of adduct binding, this method is no more accurate than the altemative procedure which we have employed, Le., to measure the integrated intensity ratio of the infrared bands arising from vas and v,(C-Cd-C) for the range of adducts studied. No structure on any of the bands, due to the presence of the isotopes of Cd in their natural abundance, was observable due to the broadness of the spectral bands. Therefore, we could not carry out normal coordinate analysis of the adducts studied especially as for our systems, isotopic data, if available, would be of little value. Results The strongest adduct would appear to be that formed between MezCd and TMEDA (1). In Table 1 are listed the infrared
n
am-
w b
Id onJol
A
i5 5
A a 5 a a A
Ib 4
a a
A d m o
D-1
Figure 1. Infrared absorption spectra of thin films: (i) TMEDA at -196 "C; (ii) MezCd at -196 "C; (iii) a 1:l mixture of TMEDA and Me2Cd condensed at -196 "C, annealed at -50 "C, and recooled to
-196 "C. spectroscopic features in the region below 1600 cm-I (above this wavenumber only bands arising from v(C-H) vibrations were seen which were of little diagnostic value) observed for 1 alongside the features seen for the parent TMEDA ligand. It may be seen that there are several changes in the infrared spectrum of the TMEDA moiety upon coordination in this strong adduct. However, the bands in the region 700- 1600 cm-' are difficult to assign to specific molecular vibrations with absolute certainty and coupling between vibrational modes such as p(CH,), d(CH,), v(C-N), and v(C-C) will almost certainly occur for a complex molecule with low symmetry such as 1. Welldefined spectral bands of 1 arise from the vibrations p(CH3) and Ysym and vas(C-Cd-C) of the Me2Cd moiety; these occur in the region 400-700 cm-I, and it is upon these features that the arguments given for 1 and for the other adducts mentioned in this paper will concentrate. Such a procedure is wellestablished in the interpretation of adduct infrared s p e ~ t r a . ' ~ , ' ~ , ' ~ In Figure 1 are shown infrared spectra (400-800 cm-I) illustrating the formation of 1 upon cocondensing a mixture of the two reagents at -196 OC and annealing the condensate at ca. -50 "C. Here vas(C-Cd-C) shifts some 41 cm-' from its position in "free" Me2Cd at 520 to 479 cm-I. Moreover, Ysym(C-Cd-C) becomes infrared active and gives rise to a band at 418 cm-I. The integrated intensity ratio Isym/Zasis approximately 0.46. As for solid MezCd in the system under study, p(CH3) gives rise to a doublet. These features occur at 651 and 638 cm-I, representing a shift of some 50 cm-' from the position of the corresponding features for free MezCd. The C-Cd-C bond angles in gaseous8 and solid24 1 are 132(11) and 157(8)", respectively. Because of its relatively low volatility, some bands seen in the infrared spectrum of gaseous 1 are necessarily weak in intensity. However, Vas and Vsym(C-CdC) are relatively strong and can be assigned to bands at 485 and 425 cm-' with confidence.8,21 The similarity in the positions of these bands when compared to those of the solid adduct on the cold-cell window leads us to conclude that the structure of the adduct is similar under the two different sets of conditions, i.e., that the TMEDA ligand in 1 formed by cocondensation of the reagents on the cold-cell window has chelating coordination. With mixtures of the potentially tridentate ligand PMDETA [pentamethyldiethylenetriamine,Me2NCH2CH2N(Me)CHzCH2NMe2] and MezCd, condensed at -196 "C and allowed to anneal at -50 "C, the infrared bands of the MezCd moiety occur at a very similar position to the corresponding bands of 1 under similar conditions. We thus conclude that PMDETA chelates to Me2Cd in the cryogenic film. However, the potentially chelating "soft" phosphorus base TMDPE does not.
IR Spectroscopic Investigation of Adducts of MezCd
J. Phys. Cheni., Vol. 99, No. 40, 1995 14643
TABLE 1: Infrared Absorption Wavenumbers for MezCd, TMEDA, and for the Adduct Me2CdoTMEDA in Cryogenic Thin Films at -196 "C TMEDA," approximate MezCd-TMEDA (1): vkm-'
vkm-'
1461 s 1407 w 1337 m 1297 ms 1257 ms 1180m 1160m 1133 s 1121 ms 1094 ms 1075 s 1046 ms 1036 vs 952 vs 935 w 825 m(sh) 789 vs 773 m 702 m 692 m 652 s 640 s 578 w 520 m
1467 s 1405 w 1341 m 1271 s 1255 s 1163 s 1143 ms
description n
DO
V
1101 ms 1046 s 1031 s 840 ms 797 m
n
499 wm 479 ms 455 w 418 wm f 2 cm-'. moiety.
449 w Probably arising from skeletal vibrations of the TMEDA
Figure 4. Infrared absorption spectra of thin films: (i) diglyme at -196 "C; (ii) MezCd at -1!>6 "C; (iii) a 1:l mixture of diglyme and MezCd condensed at -196 "C; (iv) the same mixture as in (iii) but following annealing to -75 "C and recooling to -196 "C; (v) the same mixture as in (iii) and (iv) but following subsequent annealing to -60 "C and recooling to - 196 "C". The growth of features due to the adduct MeZCd-diglyme upon progressive annealing may clearly be seen.
Figure 2. Infrared absorption spectra of thin films: (i) TMDPE at -196 "C; (ii) MezCd at -196 "C; (iii) a 1:l mixture of TMDPE and Me2Cd condensed at -196 "C, annealed at -50 "C, and recooled to -196 "C.
A study of mixtures of 1,Zdimethoxyethane (DME) and Me2Cd revealed evidence for the formation of an adduct DMEMe2Cd (2) in which the infrared spectrum of the Me2Cd moiety is very similar to the spectrum of the corresponding part of the adduct 1. The bands arising from p(CH3), vas, and vsym are seen at 666,5 12, and 464 cm-I, respectively, when 2 is formed by cocondensing the reagents at - 196 "C followed by annealing the mixture to -50 "C (Table 1). The formation of 2 is illustrated by the spectra given in Figure 3. The integrated intensity ratio Isymllas is ca. 0.20. Evidence has previously been obtained for the formation of 2 in the gas phase.2' Gaseous 2 shows p(CH3) at 687, vasat 525 cm-I, and a very weak band which may arise from v,,,(C-Cd-C) at ca. 490 cm-1.21 Gaseous MezCd shows bands arising from p(CH3) and v&Cd-C) centered at 708 and 537 cm-l, respectively. The shifs in band position from the positions of the corresponding bands
of "free" Me2Cd in the s , m e phase as the adduct, i.e., solid or gas respectively, are thus very similar. Although the structure of 2 remains unknown, the results suggest that the adduct adopts the same structure in gills and solid phases. The somewhat smaller shifts when conipared with 1 may indicate that the coordination of the DME ligand in 2 is somewhat weaker than the coordination of the W E D A ligand in 1. Very similar results are obtained for the solid adduct formed between Me2Cd and the potentially tridentate ligand "diglyme" [CH30CH2CH20CH2CH20CH31 (3). For 3 bands arising from p(CH3), v,,(C-CdC), and Vsym(C-Cd-C) are seen at 660 (average position of a doublet at 663 and 557 cni-I), 505, and 453 cm-I, respectively. The integrated intensity ratio Zs,m/las is ca. 0.29. It is most likely that DME and diglyme skiow chelating bidentate coordination to MezCd under these conditions. It should be noted that single crystal X-ray diffraction studies have demonstrated that the ligands d i g l ~ m eand ~ ~tei rpyridy126form tridentate complexes with the molecule Cd[Mn (CO)5]2.27 It is therefore not possible to make an unequivocal decision between bidentate and tridentate coordination of diglyme or of PMDETA to MezCd in the experiments reported in tliis work. There is, to our knowledge, no precedent, however, for a tridentate adduct of MezCd. Spectra showing the gralilual formation of the adduct 3 upon progressive annealing of at frozen mixture of Me2Cd and diglyme (initially held at -196 "C, first to -75 "C,and then to -60 "C) are illustrated in Figure 4. A quite different situation pertains when the cyclic compounds lP-dioxane, 1,4. thioxane, and 1P-dithiane are consid-
Almond et al.
14644 J. Phys. Chem., Vol. 99, No. 40, 1995
TABLE 3: Summary of Selected Vibrational Modes of the MezCd Moiety of Adducts of MezCd with Various Monodentate Donor Ligand9
dioxane thioxane dithiane Et2S EES PES
ne1
Figure 5. Infrared absorption spectra ,of thin films: (i) 1,4-dioxane at -196 "C; (ii) MeZCd at -196 "C; (iii) a 1:l mixture of 1,4-dioxane and MezCd at - 196 "C; (iv) the same mixture as in (iii) but following annealing to -70 "C and recooling to -196 "C; (v) the same mixture as in (iii) and (iv) but following subsequent annealing to -35 "C and recooling to -196 "C.
p(CHj)," A,b cm-' cm-' cm-' 51 479 646d 697d 0 518 29 505 668 662 35 512
Ab cn1-I
51.1 2 15
8
vsymY A?,c cm-' cm-' ISym/Id 418 52 0.46
19
518 520
2
f f 2 520 f 29 503 17 454 16 0.28 3 465 5 0.14 17 517 27 508 12 454 16 0.23 ocs 0 520 0 f &2 cm-I. A = shift in position of band from position of corresponding band in "free" MezCd: f 4 cm-l. Position of v,,(CCd-C) for "free" MezCd estimated from Raman spectroscopic mea~urements.~~ Average position of a doublet of bands presumably caused by a solid state effect (see text). e Very broad band; f 5 cm-I. f Inactive or too weak to be observed. g Integrated intensity values.kAll spectra recorded on frozen solid films at - 196 "C. MI MI
TABLE 2: Summary of Selected Vibrational Modes of the MezCd Moiety of Adducts of MezC'dwith Various Potentially Chelating Donor Liganclsg ligand TMEDA TMDPE diglyme DME
678d 690d 695d 668 680e 670e 697d
7
0 0
-
I1 H- 1I .
-
n
e
453 464
17 6
0.29 0.20
A = shift in positiorr of band from position of a f 2 cm-'. corresponding band in "free" Me2Cd: i4cm-I. Position of vSym(CCd-C) for "free" MezCd estimated from Raman spectroscopic meas~rements.~~ Average position of a pair of bands, presumably caused by solid state effects (see text). Inactive or too weak to be observed. f Integrated intensity values. g All spectra recorded on frozen solid films at -196 "C.
ered. The structure of the solid crystalline adduct Me~Cd.1,4dioxane (4) where the dioxane moieties adopt a bridging bidentate mode of coordination has been mentioned earlier.I4 The C-Cd-C bond angle in 4 is wide [173.0(8)"]. The structures of the 1P-thioxane (5) and! lA-dithiane (6) adducts of Me2Cd are unknown, although crystals of 5 are isomorphous to those of 4,21and the two compounds are probably isostructural. Solid single crystals of 4 show infrared bands arising from p(CH3) and vas at 675 and 490 cm-I, respectively. By contrast, solid 4, formed by freeicing the two constituent compounds onto a CsI window at - 196 "C and annealing the mixture to -20 "C, shows corresponding bands at 675 and 518 cm-I. There is no sign in any of these spectra of an infrared band arising from the vsym(C-Cd-C) vibration in 4 reflecting the large C-Cd-C angle. Data relatmg to 4,5, and 6 are given in Table 3. In Figure 5 are illustreated spectra showing the formation of the adduct 4 upon progressive annealing of a mixture of MezCd and 1,Cdioxane firozen onto a CsI window. Although from the spectral changes (Le., the shifts in bands arising from p(CH3) of the Me2Cd moiety and from the ring bend mode of the dioxane fragment) lit is clear that if an adduct has been formed, the small shift in v,,,,(C-Cd-C) suggests that coordination is weak. The strength of coordination of 1,4thioxane to MezCd is apparently (on the basis of the infrared spectra) similar (see Table 3). The: difference between the spectra obtained from cryogenic films of Me2Cddioxane and MezCdthioxane mixtures to those of the related solid-state structures may be attributable to the kinetic trapping of monodentate dioxan and thioxan in the cryogenic films. These "cold-cell" studies have been amplific: d by gas phase measure-
m
Figure 6. Infrared absorption spectra of thin films: (i) Et2S at - 196 "C; (ii) MeZCd at -196 "C; (iii) a 1:l mixture of Et$ and Me2Cd following annealing at -120 "C and recooling to -196 "C.
l - - m- - - m- m~ ~
u,lm
.7
'
m m u m u m ' u' uI m m m'
. I UI I
.I I ~ d, ~
OH
Figure 7. Infrared absorption spectra of thin films: (i) PES at - 196 "C; (ii) Me2Cd at -196 "C; (iii) a 1:l mixture of PES and MezCd following annealing at -120 "C and recooling to -196 "C. ments which show that all of these adducts are completely dissociated in the gas phase. The spectral data listed in Table 3 show that Et2S forms a relatively strong adduct with MezCd (7) [see Figure 61. The shift in v,,(C-Cd-C) and integrated intensity ratio ZsYm/Zas are approximately the same as for 3. It is also clear that both ethylene sulfide (EES) and propylene sulfide (PES) form adducts. There are definite changes to the spectra upon coordination of these molecules (Figure 7) and activation of the mode v,,,(C-Cd-C) gives a clear indication of adduct formation. The broadness of the adduct bands makes it difficult to give absolute values for the shifts of the bands from the positions of corresponding bands of MezCd. However, it would appear that while both EES and PES behave similarly with the
IR Spectroscopic Investigation of Adducts of MezCd
J. Phys. Chem., Vol. 99, No. 40, 1995 14645 TABLE 4: Comparison of the Shift in Y,(C-Cd-C) and p(CH3) (A) for Selected Lewis Base Adducts of MezCd with the Proton Affinities of the Lewis BasG9 proton affinity, A(vas): A(P(CH~)),~ base Idmol-’ z9 cm-1 cm-’ TMEDA diglyme Et# DME PES EES
dioxane
ocs a
coordination of PES causing larger spectral changes, carbonyl sulfide, by contrast, gives no evidence of coordination to MezCd (see Figure 8 and Table 3). Unfortunately, due to the volatility of OCS, it was not possible to carry out annealing experiments on this mixture. We have previously studied the formation of adducts between MezCd and E ~ Z TEtzTe ~ . ~shows ~ an interaction comparable in strength to that of Et2S. Studies of gaseous mixtures of MezCd with EtzS, Et2Te, EES, PES, or OCS show that all of these adducts are fully dissociated in the gas phase. Discussion For 1, where the structure of the gaseous adduct is known,8 it is clear from the infrared spectra that a similar species has been formed upon the cold cell window as is formed in the gas phase with the TMEDA ligand chelating to the Cd center of MezCd. It would appear that PMDETA, DME, and diglyme also form adducts in cryogenic films where the ligands are chelating with the DME adduct (2) adopting the same structure in the frozen solid as in the gaseous phase. A definitive decision between bidentate and tridentate coordination for diglyme and PMDETA is not possible on the basis of our results alone. The cyclic molecules 1,4-dioxane, 1,4-thioxane, and 1,4dithiane show evidence for only weak coordination with Me2Cd at low temperature. Dioxane and thioxane are known to form crystalline adducts with a polymeric structure, where the ligands bridge two metal centers, upon mixing of the two constituents in their liquid phase^.'^,^' However, it is perhaps not too surprising that such adducts are not formed upon the cold cell window. Here the reagents are presumably frozen too rapidly for the adduct to adopt its polymeric form. Even upon annealing at temperatures to -20 OC (above -20 “C evaporation of one or both of the reagents from the cold window began to occur) so there was no sign of the formation of any adduct with the structure determined by single crystal methods. Thus any interaction between MezCd and these ligands on the cold-cell window remains weak: probably any coordination of 1,4dioxane or its sulfur-containing congeners is monodentate. It is not absolutely certain whether 1:1 or 1:2 Me2Cd-Lx(x = 1 or 2) adducts have been formed with the various monodenate Lewis base molecules studied. However, a body of evidence points toward a 1:l stoichiometry. For the mixture Me2Cd EtzTe, a wide range of molar ratios of reactants has been studied in previous work in this laboratory.28 This study demonstrated that only a single adduct is formed (Le., there is only one set of product bands). This result is in line with the findings of Bai and AultI3 who, for the system Me2Zn Me2E (E = 0, S or Se), observed only a single product over quite a wide concentra-
+
+
41 15 17 8 12 3 2 0
989 918 858 857 839 814 81 1 632
51 29 29 35 27 17 19 0
k 4 cm-’
tion range. It seems unlikely that a 1:2 adduct could be formed without the intermediacy of a 1:l species. Thus, if only one product is seen, this is most likely to be the 1:l adduct. Moreover, the fact that all of our experiments reported in this paper were carried out at a 1:l molar ratio argues strongly in favor of a 1:l stoichiometry. Further evidence in favor of a 1:l formulation comes from the results of experiments where the mixture of reagents on the cold cell window is annealed at progressively higher temperatures. Such an experiment is illustrated for the mixture MezCd dioxane by the spectra given in Figure 5. As the annealing temperature is raised, a simple 1:1 mixture of reagents is gradually converted into an adduct. Thus we suggest a 1:l adduct with a monodentate ligand is first formed that transforms to the 1:l adduct with bridging ligand being formed after annealing at the higher temperatures. Again there is spectral evidence for only one adduct which must therefore be assumed to be the 1:l species. Similar results may be obtained for the other adducts studied. When considering the strength of coordination of these adducts there are a number of factors to be considered. First, as emphasized by Ault and his c o - w o r k e r ~ , ’is~ ~ the~ ~ proton ~’~ affinity of the base.29 This is clearly the ovemding effect-not surprisingly so since the proton affinity is a measure of the availability of the lone pair(s) on the base for bonding. In Table 4 is listed, for a selection of MezCd adducts, the shift in vas(C-Cd-C) and p(CH3) [a measure of adduct strength] and the proton affinity of the base. It may be seen that a strong correlation exists. Thus, by far the strongest adduct is 1, where the proton affinity of the base (TMEDA) is the highest. A clear trend between adduct strength and proton affinity is seen for the sulfur donors E@, EES, PES, and OCS. The last of these compounds has, by a large margin, the lowest proton affinity of the bases studied, and it gives no detectable sign of adduct formation with Me2Cd. It is noteworthy that the rather complex range of adducts studied by us shows a clear correlation between adduct strength and proton affinity. In order to establish a definite correlation between adduct strength and “A” and “B” or hard and soft character of the Lewis acid30 it would be necessary to study a much wider range of adducts. However, the “harder” bidentate ligands TMEDA and DME certainly form stronger adducts under these conditions than does the “softer” TMDPE. A second factor to be considered is the stoichiometry of the adduct. As discussed earlier, it appears that all of the adducts studied by us have a 1:l stoichiometry. However, in those adducts where the hard ligand is capable of chelating to the Cd center it does. It has been reported previously’2 that larger spectral shifts are observed for Me2Zn adducts with group 15 and 16 hydrides when the metal center accepts two pairs of electrons (i.e., 1:2 adducts) than when it accepts only one pair of electrons (1:l adducts). Thus it is not surprising to find, in our studies, very large shifts for the adducts of MezCd with
+
14646 J. Phys. Chem., Vol. 99, No. 40, 1995
TMEDA and PMDETA (Table 2) where the ligands are almost certainly chelating. Our results show that only for the more strongly coordinating chelating ligands is it possible to obtain spectral evidence for the existence of gaseous adducts. An investigation of frozen solid films therefore allows us to investigate the formation of weaker adducts which are fully dissociated in the gas phase so allowing an interesting comparison, for example, between the three S-donor ligands OCS, PES, and EtZS. None of these molecules shows any gas phase interaction with MezCd. However, our low temperature work shows clearly that the interaction of PES and EtzS is significantly stronger than that of OCS. The fact that preliminary experiments suggest that OCS and MezCd make a useful pair of precursors in CdS layer growth by MOCVD]’ emphasizes the point made elsewhere that adduct formation plays little if any role in many MOCVD processes forming 11-VI semiconductor^.^^ Clearly of much more importance are surface reactions upon the heated substrate. Acknowledgment. We thank the University of Reading for providing studentships to S.A.C. and L.A.S. We are grateful to Dr. C. A. Yates who carried out preliminary experiments on the MezCd EtzTe system.
+
References and Notes (1) Halsall, M. P.; Davies, J. J.; Nicholls, J. E.; Cockayne, B.; Wright, P. J.; Russell, G. J. J . C y s t . Growth, 1988, 91, 135. (2) Halsall, M. P.; Nicholls, J. E.; Davies, J. J.; Cockayne, B.; Wright, P. J.; Cullis, A. G. Semicond. Sci. Technol. 1988, 3, 1126. (3) Stutius, W. Appl. Phys. Left. 1978, 33, 656. (4) Cockayne, B.; Wright, P. J.; Armstrong, A. J.; Jones, A. C.; Orell, E. D. J . Cryst. Growth 1988, 91, 57. (5) Wright, P. J.; Cockayne, B.; Williams, A. J.; Jones, A. C.; Orrell, E. D. J . Cryst. Growth 1987, 84, 552. (6) Wright, P. J.; Parbrook, P. J.; Cockayne, B.; Jones, A. C.; Orrell, E. D.; O’Donnell, K. P.; Henderson, B. J. Cryst. Growth 1989, 94, 441. (7) Khan, 0. F. Z.; O’Brien, P.; Hamilton, P. A.; Walsh, J. R.; Jones, A. C. Chemtronics 1989, 4, 244. (8) Almond, M. J.; Beer, M. P.; Hagen, K.; Rice, D. A,; Wright, P. J. J . Mater. Chem. 1991, 1, 1065.
Almond et al. (9) Takata, S.; Minami, T.; Minata, T.; Nanto, H. J . Cryst. Growth 1988, 86, 257.
(10) Kuznetzov, P. I.; Shemet, V. V.; Odin, I. N.; Novoseleva, A. V. Izu. Akad. Nauk. SSSR, Neorgan. Mater. 1981, 17, 791. (11) Wright, P. J.; Griffiths, R. J. M.; Cockayne, B. J . Cryst. Growth 1984, 66, 26. (12) Bai, H.; Ault, B. S. J . Phys. Chem. 1994, 98, 6082. (13) Bai, H.; Ault, B. S. J . Phys. Chem. 1994, 98, 10001. (14) Almond, M. J.; Beer, M. P.; Drew, M. G. B.; Rice, D. A. J. Organomet. Chem. 1991, 421, 129. (15) Almond, M. J.; Beer, M. P.; Cooke, S. A,; Rice, D. A,; Yates, H. J . Mater. Chem. 1995, 5, 853. (16) Almond, M. J.; Cockayne, B.; Cooke, S. A.; Rice, D. A,; Smith, P. C.; Wright, P. J. In press. (17) Almond, M. J.; Cooke, S. A,; Rice, D. A. Unpublished results. (18) Auk, B. S. Rev. Chem. lntermed. 1988, 9, 233. (19) Krause, E. Ber. 1917, 50, 1813. (20) Jacobs, P. R.; Orrell, E. D.; Mullin, J. B.; Cole-Hamilton, D. J. Chemtronics 1986, 1, 15. (21) Beer, M. P. Ph.D. Thesis, University of Reading, 1991. (22) Almond, M. J.; Beer, M. P.; Drew, M. G. B.; Rice, D. A. Organometallics 1991, 10, 2072. (23) Ebsworth; E. A. V.; Rankin, D. W. H.; Cradock, S. Structural Methods in Inorganic Chemistry, 2nd ed.; Pub. Blackwell: Oxford, p 184. (24) O’Brien, P.; Hursthouse, M. B.; Motevalli, M.; Walsh, J. R.; Jones, A. C. J . Organomet. Chem. 1993, 449, 1. (25) Clegg, W.; Wheatley, P. J. J . Chem. SOC.,Dalton Trans. 1971,424. (26) Clegg, W.; Wheatley, P. J. J . Chem. Soc., Dalton Trans. 1972,90. (27) The fragments Mn(C0)s and CH3 are related by the isolobal relationship since the approximate energies and shapes of their frontier orbitals are the same. Thus similarities may be expected in the chemistry of the molecules Cd[Mn(COs)]z and (CH3)zCd. See: Hoffman, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 71 1. (28) Almond, M. J.; Jenkins, C. E.; Rice, D. A.; Yates, C. A. J . Mol. Struct. 1990, 222, 219. (29) Lias, S. G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1984, 13, 695. (30) Pearson, R. G. Acc. Chem. Res. 1993, 26, 250. (31) O’Brien, P. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; J. Wiley: Chichester, 1992; p 512. (32) Butler, I. S.; Newbury, M. L. Spectrochim. Acta, Part A 1977,33, 669. JP950692R
