Synthesis and crystal structures of trimethylindium adducts with bidentate and macrocyclic tertiary amines

Abstract

The structures of trimethylindium (Me₃In) adducts with the bidentate tertiary amine N,N,N ′,N ′-tetramethyl-4,4′-methylenebis(aniline)[MBDA] and the macrocyclic amines 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane [N₄-aza crown] and 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane [N₆-aza crown] have been determined by single crystal X-ray diffraction. The adducts have the following stoichiometries, Me₃In·2MBDA 1, 2Me₃In·MBDA 2, 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4. Compound 1 contains five-coordinate Me₃In in a trigonal bipyramidal configuration and displays the longest In–N bond lengths yet reported (2.720(4), 2.865 Å); 2–4 contain four-coordinate Me₃In molecules in a tetrahedral configuration. Compounds 1–4 can be used as intermediate adducts in the adduct purification of Me₃In for use in the MOVPE (metal organic vapour phase epitaxy) of InP and related III–V semiconductors.

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1 Introduction

Tertiary amine adducts of trimethylindium (Me₃In) have been known for many years,¹ but relatively little structural information exists for these compounds. Recently there has been revived interest in trimethylindium–trialkylamine adducts as they are useful both as precursors in the metal organic vapour phase epitaxy (MOVPE) of InP and related III–V semiconductors,^2,3 and in the purification of Me₃In for subsequent use in MOVPE.⁴ For instance, the volatile adducts Me₃In·NR₃ (R = Me or Et) have been used as precursors for the MOVPE of InP,^2,3 whilst the less volatile adduct 2Me₃In· MBDA (MBDA = N,N,N ′,N ′-tetramethyl-4,4′-methylenebis(aniline)) has been used for purification of Me₃In by mild thermal dissociation of the adduct.^4,5

The amine ligand used in adduct purification must form a relatively weak donor–acceptor bond with the metal alkyl, allowing thermal dissociation of the adduct at relatively low temperatures (<150 °C) and must also be significantly less volatile than the liberated organometallic compound to allow easy separation.⁶ There are relatively few nitrogen donors which fulfil these criteria, for example the adducts Me₃In·NMe₃, Me₃In·NEt₃ ^2,3 and Me₃In·N(CH₂CH₂)₃N ⁷ all volatilise without appreciable dissociation. However, quantitative yields of high purity Me₃In have been obtained by thermal dissociation of the 2Me₃In·MBDA adduct.^4,5

The rapid growth of MOVPE as a production technology for optoelectronic devices based on AlGaAs, AlGaInAs and AlGaInP is placing increasingly stringent requirements on the purity of the Group III metal organic precursor. Trace oxygen is a particular problem in Al-containing III–V devices, seriously degrading luminescence efficiency,⁸ and diethyl ether (a common solvent in organometallic synthesis) has been identified as a major source of oxygen contamination in III–V materials grown by MOCVD.⁹ It is therefore necessary completely to eliminate diethyl ether from the metal organic synthesis route. Recently we have shown that ultra high purity Me₃In, essentially free from oxygen impurities, can be obtained by synthesis of the compound in a hydrocarbon solvent, followed by adduct purification using MBDA.¹⁰ Device data have been reported for a high quality low-oxygen content InAlGaAs GRINSCH laser structure grown by MOVPE using Me₃In purified by thermal dissociation of Me₃In·2MBDA. In this paper full structural details are given for the adducts Me₃In·2MBDA 1 and 2Me₃In·MBDA 2.

We have previously shown that the macrocyclic amines 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclooctadecane (N₄-aza crown) and 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane (N₆-aza crown) (Fig. 1) form adducts which dissociate at low/moderate temperatures with triethylgallium,¹¹ triisopropylgallium,¹¹ and dimethylzinc.¹² In this paper we report the synthesis and full structural characterisation of the analogous trimethylindium compounds, 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4, and show that 1–4 also dissociate at relatively low temperature to liberate base-free Me₃In. Although adducts between R₃Ga and R₃Al and macrocyclic amines have been reported,^13–15 very few analogous Me₃In adducts have been reported ¹⁶ and structural data are scarce.

Fig. 1 Bidentate and macrocyclic tertiary amines used in this study: (a) MBDA, (b) N₄-aza crown and (c) N₆-aza crown.

2 Experimental

General techniques

Trimethylindium was supplied by Epichem Limited. The nitrogen donor ligands used were MBDA, N₄-aza crown and N₆-aza crown. All were purchased from Aldrich Chemical Co., dried by distillation off sodium wire and deoxygenated before use.

¹H NMR spectroscopy was carried out using a Bruker 300 spectrometer and infrared spectroscopy on a Perkin-Elmer 1000 infrared Fourier Transform spectrometer using Nujol mulls between NaCl plates. Elemental microanalyses were carried out by the Chemistry Department service at Liverpool University.

Adduct synthesis

The same general synthesis method was used for compounds 1–4. This involved addition of the amine [MBDA (0.5 mol equivalent, 2 mol equivalent), N₄-aza crown (0.25 mol equivalent) or N₆-aza crown (0.17 mol equivalent)] to a stirred suspension of Me₃In (1 mol equivalent) in toluene. Removal of the toluene in vacuo gave 1–4 as crystalline solids which were characterised by ¹H NMR, IR spectroscopy, elemental microanalysis and single crystal X-ray diffraction.

Me₃In·2MBDA 1. Colourless crystalline solid. mp 71–72 °C. ¹H NMR (C₆D₆, 300 MHz, 20 °C): δ −0.19[s, 9H, InCH₃], 2.37[s, 24H, NCH₃], 3.75[s, 4H, CCH₂C] and 6.80[m, 16H, C₆H₄]. IR (cm⁻¹): 1613s, 1514s, 1347m, 1272w, 1221w, 1164m, 1142s, 947w, 920m, 852m, 810m, 795m and 677s. Found: C, 66.7; H, 8.10, N, 8.6%. Calc. for C₃₇H₅₃InN₄: C, 66.45; H, 8.00; N, 8.38%.

2Me₃In·MBDA 2. Colourless crystalline solid. mp 79–80 °C. ¹H NMR (C₆D₆, 300 MHz, 20 °C): δ −0.19[s, 18H, InCH₃], 2.37[s, 12H, NCH₃], 3.75[s, 2H, CCH₂C] and 6.8[m, 8H, C₆H₄]. IR (cm⁻¹): 1613s, 1514s, 1347m, 1272w, 1221w, 1164m, 1142s, 947w, 920m, 852m, 810m, 795m and 677s. Found: C, 47.77; H, 7.07; N, 4.79%. Calc. for C₂₃H₄₀In₂N₂: C, 48.09; H, 7.03; N, 4.88%.

4Me₃In·N₄-aza crown 3. Colourless crystalline solid. mp 181–182 °C. ¹H NMR (C₆D₆, 300 MHz, 20 °C): δ 0.00[s, 36H, InCH₃], 1.25[quintet, 4H, CH₂CH₂CH₂], 1.95[s, 4H, NCH₃] and 2.20[broad s, 16H, NCH₂CH₂CH₂N, NCH₂CH₂N]. IR (cm⁻¹): 2267w, 1608w, 1388m, 1329w, 1285w, 1215m, 1183m, 1148s, 1138m, 1089m, 1036s, 1027s, 1022m, 948w, 883w, 832w and 682s. Found: C, 34.73; H, 7.47; N, 5.80%. Calc. for C₁₃H₃₄In₂N₂: C, 34.84; H, 7.66; N, 6.25%.

6Me₃In·N₆-aza crown 4. Colourless crystalline solid. mp 103–104 °C. ¹H NMR (C₆D₆, 300 MHz, 20 °C): δ −0.12[s, 36H, InCH₃], 1.95[s, 18H, NCH₃] and 2.51[broad s, 24H, NCH₂CH₂N]. IR (cm⁻¹): 2274w, 1648w, 1302m, 1285w, 1210m, 1142m, 1121w, 1091m, 1067w, 1022m, 992m, 947w, 917m, 858w, 787w and 685s. Found: C, 33.30; H, 7.50; N, 6.7%. Calc. for C₆H₁₆InN: C, 33.19; H, 7.44; N, 6.45%.

Single crystal X-ray diffraction

Single crystals of compounds 1–4 suitable for X-ray diffraction were obtained by slow recrystallisation of each adduct from toluene. Crystallographic data were recorded on a STOE-IPDS diffractometer using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å), T = 200 K. Structures were solved by Direct Methods and refined by full-matrix least squares against F ² using all data.¹⁷ Crystal and data collection parameters are given in Table 1.

Table 1 Crystal and data collection parameters for Me₃In·2MBDA 1, 2Me₃In·MBDA 2, 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4

	1	2	3	4
Empirical formula	C37H53InN4	C23H40In2N2	C26H64In4N4	C36H96In6N6
Formula weight	668.76	574.29	892.22	1302.30
Crystal system	Triclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P1̄	P21/c	P21/n	P21/c
a/Å	10.641(5)	18.113(3)	7.8656(11)	10.6624(16)
b/Å	10.763(4)	8.3668(7)	18.871(3)	22.856(3)
c/Å	16.634(4)	18.058(3)	12.6821(18)	11.6461(17)
α/°	108.27(3)
β/°	91.13(4)	106.182(18)	96.714(17)	108.184(17)
γ/°	105.45(4)
V/Å^3	1732.5(11)	2628.3(6)	1869.5(5)	2696.5(7)
Z	2	4	4	2
μ/mm^−1	0.637	1.621	2.266	2.355
Reflections collected	9330	16126	10131	16905
Reflections unique (Rint)	4310 (0.0813)	4152 (0.0506)	2531 (0.0581)	4251 (0.0443)
R1 [I > 2σ(I )]	0.0370	0.0317	0.0640	0.0256
wR2 (all data)	0.0809	0.0805	0.1780	0.0580

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CCDC reference number 186/2258.

See http://www.rsc.org/suppdata/dt/b0/b008600m/ for crystallographic files in .cif format.

3 Results and discussion

Structures of Me₃In·2MBDA 1 and 2Me₃In·MBDA 2

The crystal structures of compounds 1 and 2 are shown in Figs. 2 and 3, whilst selected bond lengths and angles are given in Table 2.

Table 2 Selected bond lengths (Å) and angles (°) for Me₃In·2MBDA 1 and 2Me₃In·MBDA 2

1		2
In(1)–C(3)	2.157(5)	In(1)–C(1)	2.167(4)
In(1)–C(1)	2.164(5)	In(1)–C(2)	2.155(5)
In(1)–C(2)	2.189(5)	In(1)–C(3)	2.172(4)
In(1)–N(1)	2.720(4)	In(2)–C(4)	2.168(5)
In(1)–N(3)	2.865(4)	In(2)–C(5)	2.165(5)
		In(2)–C(6)	2.171(6)
		In(1)–N(1)	2.446(3)
		In(2)–N(2)	2.462(4)
C(3)–In(1)–C(1)	121.1(2)	C(2)–In(1)–C(3)	120.17(19)
C(3)–In(1)–C(2)	121.5(2)	C(2)–In(1)–C(1)	116.2(2)
C(2)–In(1)–C(2)	117.3(2)	C(3)–In(1)–C(1)	116.35(18)
C(3)–In(1)–N(1)	90.99(16)	C(4)–In(2)–C(5)	117.4(3)
C(1)–In(1)–N(1)	90.82(16)	C(4)–In(2)–C(6)	118.5(2)
C(2)–In(1)–N(1)	92.03(16)	C(5)–In(2)–C(6)	117.2(3)
C(3)–In(1)–N(3)	89.51(16)	N(1)–In(1)–C(1)	101.42(14)
C(1)–In(1)–N(3)	88.72(16)	N(1)–In(1)–C(2)	98.69(15)
C(2)–In(1)–N(3)	87.91(16)	N(1)–In(1)–C(3)	97.07(14)
N(1)–In(1)–N(3)	179.45(12)	N(2)–In(2)–C(4)	100.13(18)
		N(2)–In(2)–C(5)	97.0(2)
		N(2)–In(2)–C(6)	99.26(18)

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Fig. 2 Crystal structure of Me₃In·2MBDA 1.

Fig. 3 Crystal structure of 2Me₃In·MBDA 2.

The structure of compound 1 is noteworthy in that the Me₃In moiety is five-coordinate with the longest In–N bond lengths (2.720(4), 2.865(4) Å) reported to date. Only a few five-coordinate alkylindium adducts have been reported, and these generally involve chelating donor ligands (e.g. Me₃In·Me₂NCH₂NMe₂ ¹⁸ and Me₂InCl·2,2′-bipy ¹⁹) and structural data are scarce. The In–N bond lengths are significantly greater than those in the adducts Me₃In·[upper bond 1 start]NHCMe₂(CH₂)₃C[upper bond 1 end]Me₂ and Me₃In·NHMe(CH₂)₂NHMe·InMe₃ which are 2.502(5) and 2.369(7) Å, respectively.⁷ In 1 the Me₃In moiety is in a trigonal bipyramidal configuration with a linear N–In–N bond angle of 179.45(12)°, similar to the Me₃In configuration in the polymeric complex [Me₃In·N(CH₂CH₂)₃N]_x, which also displays very long In–N bond lengths (2.62 Å).⁷ The C–In–C bond angles in 1 of close to 120° are very close to those in pure unadducted Me₃In, in which the Me₃In exists in the trigonal planar configuration, providing further evidence of the very weak In–N interactions in 1.

The long In–N bond lengths in five-coordinate trigonal bipyramidal adducts such as 1 and [Me₃In·(NCH₂CH₂)₃N]_x have been attributed ⁷ to the In being sp³d hybridised (i.e. donation of nitrogen lone pairs into a combination of 5p_z and 5d_z² orbitals on In). These will be weaker interactions than donation into a single vacant sp³ hybrid orbital on In, as occurs in four-coordinate Me₃In·L adducts.

In compound 2 the In–N lengths of 2.446(3) and 2.462(4) Å are significantly shorter than in 1, indicating a stronger In–N interaction. The Me₃In is four-coordinate and exists in a distorted tetrahedral environment with N–In–C bond angles ranging from 97.0(2)° to 101.42(14)°, similar to those in Me₃In·NHMe(CH₂)₂NHMe·InMe₃ (98.8(4)–101.4(3)°) and Me₃In·[upper bond 1 start]NHCMe₂(CH₂)₃C[upper bond 1 end]Me₂ (92.8(2)–109.3(2)°). The C–In–C bond angles in 2 (≈116°) are similar to those in Me₃In· NHMe(CH₂)₂NHMe·InMe₃ (≈116°) and Me₃In·NHCMe₂(CH₂)₃CMe₂ (≈117.6°),⁷ which also display comparable In–N bond distances (2.37–2.50 Å).

The similarity in chemical shifts in the ¹NMR data for 1 and 2 suggests that both compounds may be present in mixtures of Me₃In and MBDA in solution. This may be similar to the case of mixtures of Et₃Ga and N₆-aza crown in which two separate species, 4Et₃Ga·N₆-aza crown and 6Et₃Ga·N₆-aza crown, were isolated from the same solution and characterised by X-ray diffraction.¹¹

Structures of 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4

The crystal structures of compounds 3 and 4 are shown in Figs. 4 and 5, respectively and selected bond distances and angles are given in Table 3. Each adduct displays the expected 4∶1 and 6∶1 Me₃In∶ligand stoichiometries. In the case of 4 this contrasts with N₆-aza crown complexes involving more bulky R₃Ga moieties, such as 4PrⁱGa·N₆-aza crown where steric constraints limit the stoichiometry at 4∶1.¹¹

Table 3 Selected bond lengths (Å) and angles (°) in 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4

3		4
In(1)–C(1)	2.169(16)	In(1)–C(1)	2.166(4)
In(1)–C(2)	2.158(17)	In(1)–C(2)	2.169(4)
In(1)–C(3)	2.175(16)	In(1)–C(3)	2.170(5)
In(2)–C(4)	2.170(17)	In(2)–C(4)	2.168(5)
In(2)–C(5)	2.162(18)	In(2)–C(5)	2.166(5)
In(2)–C(6)	2.180(17)	In(2)–C(6)	2.169(6)
In(1)–N(1)	2.433(12)	In(1)–N(1)	2.490(3)
In(2)–N(2)	2.427(11)	In(2)–N(2)	2.462(3)
		In(3)–N(3)	2.449(3)
C(1)–In(1)–C(2)	118.1(7)	C(1)–In(1)–C(2)	118.7(2)
C(1)–In(1)–C(3)	121.3(7)	C(1)–In(1)–C(3)	113.97(19)
C(2)–In(1)–C(3)	113.8(8)	C(2)–In(1)–C(3)	122.16(18)
C(6)–In(2)–C(5)	120.8(8)	C(7)–In(3)–C(8)	120.4(2)
C(6)–In(2)–C(4)	120.9(7)	C(7)–In(3)–C(9)	119.5(2)
C(4)–In(2)–C(5)	111.7(8)	C(8)–In(3)–C(9)	113.1(2)
N(1)–In(1)–C(1)	95.9(6)	N(1)–In(1)–C(1)	99.79(15)
N(1)–In(1)–C(2)	104.2(5)	N(1)–In(1)–C(2)	97.86(14)
N(1)–In(1)–C(3)	96.4(6)	N(1)–In(1)–C(3)	95.27(14)
N(2)–In(2)–C(4)	101.1(5)	N(2)–In(2)–C(4)	101.85(17)
N(2)–In(2)–C(5)	99.3(6)	N(2)–In(2)–C(5)	100.44(15)
N(2)–In(2)–C(6)	95.6(6)	N(2)–In(2)–C(6)	97.87(16)

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Fig. 4 Crystal structure of 4Me₃In·N₄-aza crown 3.

Fig. 5 Crystal structure of 6Me₃In·N₆-aza crown 4.

In compounds 3 and 4 the Me₃In molecules exhibit distorted tetrahedral geometry, with C–In–N bond angles ranging from 95.6(6) to 104.2(5)° in 3 and 95.27(14)–101.85(17)° in 4, significantly less than the tetrahedral angle of 109°. These are entirely typical of C–In–N bond angles in tetrahedrally coordinated Me₃In adducts, as shown by the comparative data for a range of Me₃In–tertiary amine adducts given in Table 4. The In–N bond lengths in 3 and 4 are close to the In–N bond length in 2, and fairly typical for In–N bond lengths in an sp³ hybridised tetrahedral configuration (see Table 4). The In–N bond lengths in 2, 3 and 4 are significantly shorter than the In–N length in the sp³d hybridised complex 1. The In–N bond distance in 4 is very slightly longer than that in 3, presumably due to a slight increase in steric hindrance in the 6Me₃In·N₆-aza crown complex.

Table 4 Bond lengths (Å) and angles (°) for some Me₃In–tertiary amine adducts

Compound	In–N	In–C	C–In–C	C–In–N	Reference
Me3In·2MBDA 1	2.720(4), 2.865(4)	2.157(5)–2.189(5)	117.3(2)–121.5(2)	87.91(16)–92.03(16)	This work
2Me3In·MBDA 2	2.446(3), 2.462(4)	2.165(5)–2.172(4)	116.2(2)–120.17(19)	97.0(2)–101.42(14)	This work
4Me3In·N4-aza crown 3	2.427(11)–2.433(12)	2.158(17)–2.180(17)	111.7(8)–121.3(7)	95.6(6)–104.2(5)	This work
6Me3In·N6-aza crown 4	2.449(3)–2.490(3)	2.166(4)–2.170(5)	113.1(2)–122.6(18)	95.27(14)–103.13(16)	This work
Me3In·[upper bond 1 start]NHCMe2(CH2)3C[upper bond 1 end]Me2	2.502(5)	2.170(5)–2.174(6)	109.3(3)–118.9(3)	92.8(2)–109.3(2)	7
Me3In·NHMe(CH2)2NHMe·InMe3	2.369(7), 2.393(7)	2.147(10)–2.183(9)	113.0(4)–120.2(4)	96.3(3)–101.4(3)	7
[Me3In·N(CH2CH2)3N]x	2.62	2.172(7)–2.176(5)	119.4(2)	—	7

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The Me₃Al and Me₃Ga analogues to 2 have been structurally characterised,^13,14 and this allows us to make a direct comparison of M–N, M–C bond lengths, and various intramolecular angles in the R₃M molecule (see Table 5). The most notable feature is that the M–N adduct bond length decreases in the order In > Ga > Al, in line with increasing Lewis acidity of the metal centre.²⁰ The M–C bond lengths show a similar trend, decreasing in the order In > Ga ≈ Al, broadly consistent with increasing strength of the metal–carbon bond. The M–C bond lengths in the 4Me₃Ga·N₄-aza crown and 4Me₃In·N₄-aza crown adducts are close to those in the unadducted compounds, Me₃Ga(g) (M–C: 1.967(2) Å) ²¹ and Me₃In(s) (M–C: 2.16(4)–2.25(6) Å).^22,23 However, the Al–C bond lengths of 1.960(5)–1.966(6) Å in 4Me₃Al·N₄-aza crown are slightly shorter than the terminal Al–C distances in the Me₃Al dimer (1.99 Å).²⁴ The C–M–N bond angle decreases in the order Al > Ga > In, with an increasing deviation from the tetrahedral angle of 109°.

Table 5 Bond lengths (Å) and angles (°) for Me₃M·N₄-aza crown adducts (M = Al, Ga or In)

Compound	M–N	M–C	C–M–C	C–M–N	Reference
4Me3Al·N4-aza crown	2.093(3), 2.102(3)	1.960(5)–1.966(6)	—	102.8(2)–107.1(2)	13
4Me3Ga·N4-aza crown	2.182(4), 2.202(4)	1.970(6)–1.973(6)	113.0(3)–118.1(3)	101.0(2)–105.9(2)	14
4Me3In·N4-aza crown	2.433(12), 2.427(11)	2.158(17)–2.180(17)	111.7(8)–121.3(7)	95.6(6)–104.2(5)	This work

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Dissociation of Me₃In–aza crown adducts

Complex 2 has previously been shown to liberate base free Me₃In when heated at 80–130 °C in vacuo.⁴ Complexes 1, 3 and 4 were also found to liberate base-free Me₃In (confirmed by ¹H NMR spectroscopy) when heated at relatively low temperatures 80–115 °C under vacuum. A sample of crude Me₃In containing high levels of MeI (18000 ppm, deriving from the Grignard-based synthesis route) was purified by thermal dissociation of the adducts 1–4 and it was shown that, in each case, the iodide impurity level was reduced to <10 ppm. Such a dramatic reduction can be attributed to formation of the involatile quaternary ammonium salt, R₄N⁺I⁻, resulting from reaction between the tertiary amine and MeI.²⁵ It has previously been shown that MBDA is highly effective in removing trace metal impurities (e.g. Si, Zn, Mg) from Me₃In,⁴ but removal of the organic impurity MeI is a useful additional advantage of tertiary amines in adduct purification processes.

4 Conclusion

The crystal structures of Me₃In·2MBDA 1, 2Me₃In·MBDA 2, 4Me₃In·N₄-aza crown 3 and 6Me₃In·N₆-aza crown 4 have been determined. Compound 1 is one of only two reported adducts containing five-coordinate Me₃In, whereas 2–4 contain four-coordinate Me₃In in a distorted tetrahedral configuration. The adducts 1–4 all liberate base-free Me₃In at low/moderate temperatures and are suitable for adduct purification of Me₃In for electronic applications.

Acknowledgements

We are very grateful to Epichem Limited for funding the work and for providing samples of Me₃In. We also thank Mr S. A. Apter (University of Liverpool, UK) for provision of the microanalytical data.

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