Alkaline-Earth Metal Alkylaluminate Chemistry Revisited

Organometallics 2009, 28, 4783–4790 DOI: 10.1021/om900342h

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Alkaline-Earth Metal Alkylaluminate Chemistry Revisited Olaf Michel,† Christian Meermann,† Karl W. T€ ornroos,† and Reiner Anwander*,†,‡ †

Department of Chemistry, University of Bergen, All egaten 41, 5007 Bergen, Norway, and ‡Institut f€ ur Anorganische Chemie, Universit€ at T€ ubingen, Auf der Morgenstelle 18, 72076 T€ ubingen, Germany Received April 30, 2009

Heterobimetallic peralkylated calcium complexes [Ca(AlR4)2]n (R=Me, Et) were obtained from {Ca[N(SiMe3)2]2}2 and an excess AlR3 in almost quantitative yield. A similar silylamide elimination protocol was previously applied for the synthesis of the corresponding ytterbium and samarium complexes. The solid-state structure of polymeric [Ca(AlEt4)2]n revealed agostically linked righthanded helices featuring a pitch of ∼14 A˚ and a repeat length of ∼27 A˚ with the latter composed of nine alkyl-bridged [Ca(AlEt4)3]- and [Ca(AlEt4)]þ building units. Reaction of [Ca(AlR4)2]n (R = Me, Et) with equimolar amounts of donor molecules gave X-ray structurally authenticated hexacoordinate monomeric adducts Ca(AlMe4)2Phen (Phen = 1,10-phenanthroline) and Ca(AlEt4)2(thf)2. Treatment of [Ca(AlEt4)2]n, [Yb(AlEt4)2]n, and Mg(AlMe4)2 with excess thf resulted in the formation of solvent-separated ion pairs of composition [M(II)(thf)6][AlR4]2. Heating barely soluble [Ca(AlMe4)2]n in a pressure tube produced vinyloxide-bridged complex [{Ca(μ-OCHdCH2)(thf )4}2][AlMe4]2 via thf degradation. Furthermore, treatment of MgMe2 with thf yielded quantitatively the donor adduct [Mg(Me)(μ-Me)(thf)]2.

Introduction Karl Zieglers epoch-making organoaluminum chemistry addressed combinations of aluminum alkyls and compounds of d-transition metals, which led to the phenomenal Ziegler catalysts.1 His initial work on mixed magnesium/aluminum alkyls was further triggered by the thermal stability of the alkali metal derivatives MAlEt4 (M=Li, Na).2 The homoleptic compound Mg(AlEt4)2 and partially complexed EtMg(AlEt4) were obtained from the reaction of AlEt3 with MgEt2 (or more favorably with EtMgCl) and subsequent donor (diethyl ether)-induced cleavage (eqs 1 and 2).3 The thermal behavior of the corresponding methyl congeners Mg(AlMe4)2 was described as quite peculiar, yielding partially complexed MeMg(AlMe4) upon distillation and solidification (mp 88.5 °C).3 Reinvestigation of the latter reaction by Atwood and Stucky produced homoleptic Mg(AlMe4)2 as a sublimable compound (mp 39 °C), the solid-state structure of which was determined by X-ray diffraction.4 The mixed alkali metal/organoaluminum chemistry performed in M€ ulheim revealed also the beneficial effect of alkoxide ligands for the generation of pure MAlEt4 (M=Li, K). These reactions exploit AlEt3 as a strong Lewis acid to displace dialkylaluminum alkoxides in complexes [MAlEt3(OR)]

(OR = OEt, OC3H7, OnBu).5 The same reaction protocol was later applied by Ziegler’s former co-worker H. Lehmkuhl to synthesize analytically pure alkaline-earth metal alkylaluminates M(AlEt4)2 (M (mp)=Ca (41 °C), Sr (109 °C), Ba (138 °C); eqs 3 and 4).6 The calcium and strontium derivatives show enhanced thermal stability and could be purified by distillation under high vacuum. Moreover, it was found that magnesium behaves differently in such alkoxide-promoted reactions, producing a mixed ethyl/ ethoxide complex upon distillation at 120-150 °C and 10-3 Torr (eq 5).6 Lehmkuhl also described the molecular structure of [Ca(AlEt4)2] and its thf adduct by equilibria between ethyl-bridged dimers and (solvent) dissociated ion pairs (eqs 6 and 7), based on NMR spectroscopic data.6 The molecular structures of solvent-separated derivatives [Ca(thf)6][AlMe4-xPhx]2 (x = 0, 1, 2, 3, 4) were recently reported by Westerhausen and co-workers.7,8 MgEt2 þ 2AlEt3 f MgðAlEt4 Þ2

ð1Þ

MgðAlEt4 Þ2 þ exc OEt2 f EtMgðAlEt4 Þ þ AlEt3 ðOEt2 Þ ð2Þ MðORÞ2 þ 2AlEt3 f MðORÞ2 ðAlEt3 Þ2

ð3Þ

*Corresponding author. E-mail: [email protected]. (1) (a) Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. 1955, 67, 541–547. (b) Ziegler, K. Angew. Chem. 1964, 76, 545–553. (c) Wilke, G. Liebigs Ann. Chem. 1975, 804–833. (2) (a) Hurd, D. T. J. Org. Chem. 1948, 13, 711–713. (b) Baker, E. B.; Sisler, H. H. J. Am. Chem. Soc. 1953, 75, 5193–5195. (c) Gerteis, R. L.; Dickerson, R. E.; Brown, T. L. Inorg. Chem. 1964, 3, 872–875. (3) Ziegler, K.; Holzkamp, E. Liebigs Ann. Chem. 1957, 605, 93–97. (4) Atwood, J. L.; Stucky, G. D. J. Am. Chem. Soc. 1969, 91, 2538– 2543.

(5) (a) Lehmkuhl, H. Angew. Chem., Int. Ed. Engl. 1964, 3, 107–114. (b) Lehmkuhl, H.; Eisenbach, W. Angew. Chem., Int. Ed. Engl. 1962, 1, 590–591. (6) Lehmkuhl, H.; Eisenbach, W. Liebigs Ann. Chem. 1967, 705, 42– 53. (7) Krieck, S.; G€ orls, H.; Westerhausen, M. Organometallics 2008, 27, 5052–5057. (8) Krieck, S.; G€ orls, H.; Westerhausen, M. Inorg. Chem. Commun. 2008, 11, 911–913.

r 2009 American Chemical Society

Published on Web 07/23/2009

pubs.acs.org/Organometallics

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Organometallics, Vol. 28, No. 16, 2009

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MðORÞ2 ðAlEt3 Þ2 þ 2AlEt3 f MðAlEt4 Þ2 þ 2AlðORÞEt2 ðM ¼ Ca, Sr, Ba; OR ¼ OEt, OC3 H7 , OnBuÞ

ð4Þ

MgðOEtÞ2 þ 2AlEt3 f EtMgðOEtÞ þ AlEt3 þ AlðOEtÞEt2

ð5Þ Et2 Alðμ-EtÞ2 Caðμ-EtÞ2 AlEt2 / Ca2þ þ 2AlEt4 -

ð6Þ

Et2 Alðμ-EtÞ2 CaðthfÞ2 ðμ-EtÞ2 AlEt2 / Ca2þ ðthfÞ2 þ 2AlEt4 -

ð7Þ

During the past decade we have been investigating synthesis approaches toward homoleptic rare-earth metal alkylaluminate complexes Ln(III)(AlMe4)3 and Ln(II)(AlR4)2 as well as their reactivity toward donor molecules, clearly revealing reactivity patterns similar to those shown in eqs 1-7.9-12 Given the cation size-implied parallels in rareearth(II) and alkaline-earth metal structural chemistry as well as similar reactivity behavior we thought it worthwhile to re-examine alkylaluminate derivatives of the alkalineearth metals.13 Herein we describe an alternative synthesis pathway toward [Ca(AlR4)2]n (R=Me, Et) (including the X-ray structure analysis of [Ca(AlEt4)2]n) and the reactivity of alkaline-earth alkylaluminates toward donor molecules.

Results and Discussion Synthesis and Characterization of [M(II)Al2(alkyl)8]n. We have previously reported that heterobimetallic peralkylated rare-earth metal complexes [Ln(AlR4)2]n (Ln=Sm, Yb; R= Me, Et; herein referred as aluminates) are readily accessible by the silylamide elimination reaction protocol.10,11 Similarly, the calcium derivatives [Ca(AlR4)2]n (1, R=Me; 2, R= Et) could be obtained from {Ca[N(SiMe3)2]2}2 and an excess of AlR3 in almost quantitative yield (Scheme 1).14 Performing the reaction in hexane, permethylated [Ca(AlMe4)2]n (1) precipitated as a white solid, which is also insoluble in aromatic solvents. This solubility behavior is consistent with that observed for the ytterbium and samarium derivatives. Solid-state NMR spectroscopic investigations of 1 show a broad 1H resonance centered at ∼0.1 ppm (9) (a) Evans, W. J.; Anwander, R.; Ziller, J. W. Organometallics 1995, 14, 1107–1109. (b) Zimmermann, M.; Frøystein, N. A˚.; Fischbach, A.; Sirsch, P.; Dietrich, H. M.; T€ornroos, K. W.; Herdtweck, E.; Anwander, R. Chem.;Eur. J. 2007, 13, 8784–8800. (10) (a) Klimpel, M. G.; Anwander, R.; Tafipolsky, M.; Scherer, W. Organometallics 2001, 20, 3983–3992. (b) Sommerfeldt, H.-M.; Meermann, C.; Schrems, M. G.; T€ ornroos, K. W.; Froeystein, N. A˚.; Miller, R. J.; Scheidt, E.-W.; Scherer, W.; Anwander, R. Dalton Trans. 2008, 1899–1907. (11) (a) Schrems, M. G.; Dietrich, H. M.; T€ ornroos, K. W.; Anwander, R. Chem. Commun. 2005, 5922–5924. (b) We would like to use the term “aluminate” and formula [M(AlR4)2] synonymous to [MAl2R8] despite the distinct reactivity toward Do molecules compared to aluminates Ln(III)(AlR4)3. (12) (a) Dietrich, H. M.; Raudaschl-Sieber, G.; Anwander, R. Angew. Chem., Int. Ed. 2005, 44, 5303–5306. (b) Dietrich, H. M.; Meermann, C.; T€ ornroos, K. W.; Anwander, R. Organometallics 2006, 25, 4316–4321. (13) Harder, S. Angew. Chem. Int. Ed. 2004, 43, 2714–2718. (14) The reaction of Ca[N(SiMe3)2]2 with Al(CH2SiMe3)3 does not yield the peralkylated derivative but comes to a halt at the partly alkylated compound {[μ-N(SiMe3)2]Ca[(μ-CH2SiMe3)2Al(CH2SiMe3)2]}2 probably due to steric restrictions. See: Westerhausen, M.; Birg, C.; N€ oth, H.; Knizek, J.; Seifert, T. Eur. J. Inorg. Chem. 1999, 2209–2214.

Figure 1. 1H (A) and (AlMe4)2]n (1).

13

C CP MAS NMR spectra (B) of [Ca-

Scheme 1. Synthesis of Homoleptic Calcium(II) Tetraalkylaluminates (R = Me (1), R = Et (2))

(Figure 1A). The 13C MAS NMR spectrum reveals dominant peaks at 4.3 and -2.1 ppm indicative of two distinct methyl environments (Figure 1B). Signals at 71.6, 28.4, and 6.3 ppm can be ascribed to the presence of Ca(AlMe4)2(OEt2)2. For comparison, the 13C MAS NMR spectrum of [Yb(AlMe4)2]n shows two overlapping resonances at 13.2 and 4.1 ppm for the bridging methyl groups.10b In contrast, the perethylated compound [Ca(AlEt4)2]n (2) is soluble in hexane and can be obtained as colorless single crystals by fractional crystallization at -35 °C. Complete silylamide elimination and the absence of byproduct [Et2AlN(SiMe3)2]2 was further proven by 1H and 13C NMR spectroscopy. As pointed out by Lehmkuhl in his original work6 compound 2 shows a low melting point of 56.6 °C (Lehmkuhls work: 41 °C) and can be purified by distillation at ca. 80 °C/10-4 Torr. This is in agreement with a compound featuring weakly interacting molecules only, in strong contrast to the permethylated derivative 1, which decomposes above 322 °C. Compound 2 crystallizes from hexane in the trigonal (in hexagonal setting) space group P32, being isotypic to the polymeric network structure of [Ln(AlEt4)2]n (Ln = Yb (2a), Sm (2b)). The solid-state structure of 2 can be described as agostically linked righthanded helices featuring a pitch of ∼14 A˚ and a repeat length of ∼27 A˚, with the latter composed of nine alkyl-bridged [Ca(AlEt4)3]- and [Ca(AlEt4)]þ formal building units (Figures 2 and 3). Important bond distances and angles of isostructural complexes [M(II)(AlEt4)2]n (M = Ca, Yb) are compiled in Table 1. A detailed discussion of the solid-state structure of the rare-earth metal complexes 2a and 2b was presented previously.10 The permethylated magnesium derivative Mg(AlMe4)2 was initially synthesized from a 2:1 mixture of neat trimethylaluminum and dimethylmagnesium.4 We performed the same reaction in hexane, and complete conversion of hexane-insoluble MgMe2 was indicated by a clearing of the suspension. Due to the absence of any byproduct, singlecrystalline Mg(AlMe4)2 could be obtained in high yield from the original hexane solution. In contrast, the reaction of MgMe2 with 2 equiv of GaMe3 in hexane did not afford the putative tetramethylgallate species Mg(GaMe4)2 (3), but dimethylmagnesium dissolved in neat GaMe3 (eq 8).

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Table 1. Comparison of Selected Bond Distances and Angles for Isostructural [Ca(AlEt4)2]n (2) and [Yb(AlEt4)2]n (2a)10a 2 (M = Ca)

2a (M = Yb)

Bond Distances [A˚] M1-C3 M1-C5 M1-C7 M1-C15 M2-C11 M2-C13 M2-C21 M2-C23 M2-C25 M2-C27 M1-Al1 M2-Al2 M2-Al3 M2-Al4

2.776(3) 2.718(3) 2.770(4) 2.816(2) 2.656(3) 2.659(3) 2.673(3) 2.657(3) 2.659(3) 2.628(3) 2.774(1) 3.1682(9) 3.195(2) 3.1683(9)

2.824(5) 2.744(6) 2.79(1) 2.818(5) 2.683(7) 2.666(5) 2.700(7) 2.677(5) 2.676(6) 2.649(5) 2.809(2) 3.188(2) 3.218(2) 3.193(2)

Angles [deg]

Figure 2. Molecular structure of [Ca(AlEt4)2]n (2) showing the formally anionic and cationic units [Ca(AlEt4)3]- (A) and [Ca(AlEt4)]þ (B), respectively. Atomic displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

Figure 3. Helical repetition of 2 depicted as ellipsoids (A) and spacefills (B).

Applying vacuum to the latter reaction mixture in order to remove excessive GaMe3 regenerated MgMe2, indicating an extremely weak tetramethylgallate coordination in Mg(GaMe4)2 (3) according to eq 8. MgMe2 þ 2GaMe3 / MgðGaMe4 Þ2

ð8Þ

[M(II)Al2(alkyl)8]n. Donor Addition and Donor-Induced Ion Separation. We have previously described the reaction of divalent homoleptic tetraalkylaluminates [Ln(AlR4)2]n

M1-C3-C4 M1-C5-C6 M1-C15-C16 C3-M1-C5 C3-M1-C15 M2-C11-C12 M2-C13-C14 M2-C21-C22 M2-C23-C24 M2-C25-C26 M2-C27-C28 C11-M2-C13 C21-M2-C23 C25-M2-C27 C11-M2-C21 C11-M2-C23 C11-M2-C25 C11-M2-C27

124.9(2) 115.7(3) 100.0(2) 72.4(2) 163.7(1) 168.8(2) 169.4(2) 168.9(3) 168.4(2) 170.6(2) 169.2(2) 80.10(8) 79.4(1) 80.05(8) 94.2(2) 93.7(1) 93.46(9) 170.10(9)

123.4(5) 115.5(6) 97.3(3) 72.0(2) 163.4(2) 169.3(4) 169.6(4) 170.2(5) 167.7(5) 169.9(4) 168.6(4) 79.7(2) 78.7(2) 79.8(2) 93.9(2) 94.2(3) 93.3(2) 169.6(2)

(Ln = Sm, Yb; R = Me, Et) with donor molecules such as thf, pyridine, and 1,10-phenanthroline (Phen).10,11 Equimolar reactions yielded the structurally characterized adducts Ln(II)(AlR4)2(Do)x (R=Me, Et; x=2, Do=thf; x=1, Do= Phen).10,11 It is noteworthy that an excess donor or subsequent treatment of Ln(II)(AlMe4)2(Do)2 with donor molecules did not yield putative [Ln(II)Me2]n, analog to the formation of [Ln(III)Me3]n,12a or any other isolable species. Actually, formation of insoluble material was observed, which was not further characterized.11a While treatment of a suspension of [Ca(AlMe4)2]n (1) in hexane with 2 equivs of thf did not yield any soluble species, the corresponding reaction of hexane-soluble [Ca(AlEt4)2]n (2) gave the bis(thf) adduct Ca(AlEt4)2(thf)2 (4; mp: 75.8 °C (Lehmkuhls work: 57-60 °C)). Complex 4 could be isolated and fully characterized. Instead, permethylated [Ca(AlMe4)2]n (1) could be partly dissolved in a hexane-toluene mixture by addition of an equimolar amount of the chelating donor Phen to afford monomeric complex Ca(AlMe4)2(Phen) (5) (Scheme 2). Complex 4 is isostructural to the corresponding ytterbium (4a) and samarium complexes Ln(AlEt4)2(thf)2 (Figure 4, Table 2).11 The Ca-C1/3 bond lengths of 2.632(2) and 2.655(3) A˚, respectively, match the average Yb-C(CH2) bond distance of 2.663 A˚ and the average Ca-C σ-bond distance of six-coordinate complex Ca[Zn(CH2SiMe3)3]2(thf )2 (2.655(2) and 2.717(2) A˚).15 (15) Westerhausen, M.; G€ uckel, C.; Piotrowski, H.; Vogt, M. Z. Anorg. Allg. Chem. 2002, 628, 735–740.

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Figure 4. Molecular structure of Ca(AlEt4)2(thf)2 (4). Hydrogen atoms are omitted for clarity. Heavy atoms are represented by atomic displacement ellipsoids at the 50% level. Scheme 2. Conversion of 1 and 2 into the Donor Adduct Complexes 4 and 5

Figure 5. Molecular structure of Ca(AlMe4)2(Phen) (5). Hydrogen atoms are omitted for clarity. Heavy atoms are represented by atomic displacement ellipsoids at the 50% level. Table 3. Comparison of Selected Bond Distances and Angles for Isostructural Ca(AlMe4)2(Phen) (5) and Yb(AlMe4)2(Phen) (5a)10b 5 (M = Ca)

5a (M = Yb)

Bond Distances [A˚]

Table 2. Comparison of Selected Bond Distances and Angles for Isostructural Ca(AlEt4)2(thf)2 (4) and Yb(AlEt4)2(thf)2 (4a)11 4 (M = Ca)

4a (M = Yb)

Bond Distances [A˚] M1-O1 M1-C1 M1-C3 M1-Al1

2.344(2) 2.632(2) 2.655(3) 3.1979(8)

2.395(1) 2.652(2) 2.673(3) 3.2139(5)

Angles [deg] O1-M1-O10 O1-M1-C1 O1-M1-C3 C1-M1-C3 M1-C1-C2 M1-C3-C4 Al1-C1-M1 Al1-C3-M1 C1-Al1-C3

87.3(1) 92.79(8) 91.44(7) 79.85(8) 166.7(2) 169.9(2) 85.07(9) 84.29(9) 110.8(2)

87.23(6) 92.39(5) 91.31(5) 79.42(5) 166.7(1) 169.8(1) 85.06(6) 84.35(5) 111.15(7)

Colorless single crystals of Ca(AlMe4)2(Phen) (5) could be harvested in minimal yield, however, sufficient for carrying out an X-ray structure analysis (Figure 5, Table 3). Complex 5 crystallizes in the monoclinic space group Cc and adopts the same coordination geometry as found in Yb(AlMe4)2(Phen) (5a) and Yb(AlEt4)2(Phen).10b Due to severe disorder in the Phen moiety, we refrain from a detailed discussion of the structural parameters. Compared to Yb(AlMe4)2(Phen) (5a) the M-C and M-N bond distances of 5 appear slightly shorter, which would be in accordance with the marginally smaller ion size of the Ca(II) cation. However, the bonding situation in ytterbium complexes, bearing chelating

M1-N1 M1-N2 M1-C1 M1-C2 M1-C5 M1-C6 M1-Al1 M1-Al2

2.498(9) 2.478(7) 2.610(7) 2.582(7) 2.574(7) 2.571(7) 3.118(3) 3.104(3)

2.494(4) 2.496(4) 2.621(6) 2.615(5) 2.600(5) 2.606(6) 3.149(3) 3.126(2)

Angles [deg] C1-M1-C2 C6-M1-C5 M1-C6-Al2 M1-C5-Al2 M1-C1-Al1 M1-C2-Al1 N1-M1-N2 N1-M1-C1 N1-M1-C2 N2-M1-C6 N2-M1-C5

82.0(2) 81.5(2) 83.8(3) 83.7(2) 83.1(2) 83.4(2) 67.0(2) 87.3(4) 107.1(2) 94.6(3) 93.7(2)

81.4(2) 81.6(2) 83.4(2) 83.3(2) 83.6(2) 83.6(2) 67.0(2) 89.8(2) 98.0(2) 90.4(2) 102.5(2)

N-heterocyclic aromatic donor ligands (N-donors) such as Phen or Bipy (2,20 -bipyridyl), is also affected by the donation of electron density from the metal to the N-ligand. This was studied in detail for the intensely colored complexes (C5Me5)2Yb(N-ligand) and described by the occurrence of two discrete electron tautomers, (C5Me5)2Yb(II)(N-ligand0) and (C5Me5)2Yb(III)(N-ligand-).16a In contrast to Yb(AlMe4)2(Phen) (5a, dark green), complex Ca(AlMe4)2(Phen) (5) crystallized as colorless plates from a purple solution. For comparison, complex (C5Me5)2Ca(Bipy) was obtained as an orange-red precipitate from a red-purple solution.16b Given the distinct solid-state structures and concomitant solubility behavior of Mg(AlMe4)2 (monomeric, soluble in hexane) and [Ca(AlMe4)2]n (polymeric, insoluble in thf ), we wondered whether controlled donor addition to Mg(AlMe4)2 would result in any isolable product. Accordingly, (16) (a) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R. A. Organometallics 2002, 21, 460–472. (b) Burns, C. J.; Andersen, R. A. J. Organomet. Chem. 1987, 325, 31–37.

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Figure 6. Molecular structure of [Mg(thf)6][AlMe4]2 (6). Hydrogen atoms are omitted for clarity. Heavy atoms are represented by atomic displacement ellipsoids at the 50% level. Co-crystallized toluene (two molecules per unit cell) is not shown.

Figure 7. Molecular structure of [Mg(Me)(μ-Me)(thf)]2 (7). Hydrogen atoms, except for the CH3 moieties, are omitted for clarity. Heavy atoms are represented by atomic displacement ellipsoids at the 50% level.

Table 4. Selected Bond Distances and Angles for [Mg(thf)6][AlMe4]2 (6)

Scheme 3. Synthesis Pathways for Compounds 6, 9, 10, and 10a

Bond Distances [A˚] Mg1-O1 Mg1-O2 Mg1-O3 Mg1-O4 Mg1-O5 Mg1-O6

2.113(2) 2.112(2) 2.094(2) 2.098(2) 2.090(2) 2.093(2)

Angles [deg] O1-Mg1-O2 O1-Mg1-O3 O1-Mg1-C4 O1-Mg1-C5 O1-Mg1-C6

179.6(2) 89.61(8) 90.48(8) 89.08(8) 90.53(8)

treatment of Mg(AlMe4)2 with 2 equiv of thf in hexane yielded instantly an insoluble white precipitate. Visually, this reaction behavior was reminiscent of the donor-induced aluminate cleavage of Y(AlMe4)3 to [YMe3]n.12a The second cleavage product AlMe3(thf) could be detected in the supernatants by NMR spectroscopy, being indicative of an initial reaction as shown in eq 2. The carbon elemental analysis of the white precipitate of the Mg(AlMe4)2-2thf reaction matched that of MeMg(AlMe4). The precipitate dissolved readily in thf, affording compound 6, the X-ray structure analysis of which revealed a solvent-separated ion pair of composition [Mg(thf)6][AlMe4]2 (Figure 6, Table 4).17,18 The cationic fragment [Mg(thf)6]2þ features an octahedral MgO6 core geometry with longer bond distances for the apical oxygen atoms (2.113 versus 2.094 A˚). Overall, the [Mg(thf)6]2þ unit is a commonly observed structural motif in organomagnesium chemistry, and complexes containing such solvated alkalineearth cations shielded by bulky organic anions were previously compared to a “metal-in-a-box”.19,20 Not surprisingly, the yield of complex 6 could be drastically increased when Mg(AlMe4)2 was treated with an excess of thf (6 equiv) in hexane (Scheme 3). (17) For alkylaluminate separation in an alkali metal complex, see: Craig, F. J.; Kennedy, A. R.; Mulvey, R. E.; Spicer, M. D. Chem. Commun. 1996, 1951–1952. (18) For alkylaluminate separation in rare-earth metal complexes, see: (a) Nakamura, H.; Nakayama, Y.; Yasuda, H.; Maruo, T.; Kanehisa, N.; Kai, Y. Organometallics 2000, 19, 5392–5399. (b) Arndt, S.; Spaniol, T. P.; Okuda, J. Angew. Chem. Int. Ed. 2003, 42, 5075–5079. (19) Bond, A. D.; Layfield, R. A.; MacAllister, J. A.; Rawson, J. M.; Wright, D. S.; McPartlin, M. Chem. Commun. 2001, 1956–1957. (20) Harder, S.; Feil, F.; Repo, T. Chem.;Eur. J. 2002, 8, 1991–1999. (21) Saheki, Y.; Sasada, K.; Satoh, N.; Kawaichi, N.; Negoro, K. Chem. Lett. 1987, 2299–2300.

Table 5. Selected Bond Distances and Angles for [Mg(Me)(μ-Me)(thf)]2 (7) Bond Distances [A˚] Mg1-O1 Mg1-C5 Mg1-C50 Mg1-C6

2.045(2) 2.262(2) 2.263(2) 2.121(2)

Angles [deg] C6-Mg1-C5 C6-Mg1-C50 O1-Mg1-C5 O1-Mg1-C50 O1-Mg1-C6 C5-Mg1-C50

119.32(6) 118.10(6) 100.64(5) 100.62(5) 110.04(5) 104.82(5)

For the compilation of other potential products of such donor addition reactions, we separately treated [MgMe2]n21 with thf. This reaction yielded quantitatively the donor adduct [Mg(Me)(μ-Me)(thf)]2 (7), which could be crystallized from a saturated thf solution at -35 °C. The presence of coordinated thf was confirmed by 1H and 13C NMR spectroscopy. The molecular structure of 7 was unequivocally proven by X-ray diffraction analysis (monoclinic space group I2/a) showing a methyl-bridged dimer with fourcoordinate magnesium centers (Figure 7 and Table 5). The terminal Mg-C6 and Mg-O bond distances of 2.121(2) and 2.045(1) A˚, respectively, compare well with the corresponding bond lengths in complex D (Chart 1; av Mg-C, 2.13; av Mg-O, 2.071 A˚).22 It is noteworthy that only monomeric N-donor complexes of dimethylmagnesium have been structurally characterized so far (Chart 1; A, B, C),23-25 except for the mixed donor

(22) Yousef, R. I.; Walfort, B.; R€ uffer, T.; Wagner, C.; Schmidt, H.; Herzog, R.; Steinborn, D. J. Organomet. Chem. 2005, 690, 1178–1191. (23) Viebrock, H.; Weiss, E. J. Organomet. Chem. 1994, 464, 121–126. (24) Toney, J.; Stucky, G. D. J. Organomet. Chem. 1970, 22, 241–249. (25) Greiser, T.; Kopf, J.; Thoennes, D.; Weiss, E. J. Organomet. Chem. 1980, 191, 1–6.

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Michel et al. Scheme 4. Formation of [{Ca(μ-OCHdCH2)(thf)4}2][AlMe4]2 (11) from Compound 9 via Thf Degradation

Table 6. Crystallographic Parameters for Compounds 2, 4, 5, 6, 7, and 11

Figure 8. Molecular structure of [{Ca(μ-OCHdCH2)(thf)4}2][AlMe4]2 (11). Selected bond lengths (A˚) and angles (deg) for 11: Ca1-O1: 2.259(7), Ca1-Ca10 : 3.574(4), O1-C1: 1.310(14), C1-C2: 1.297(17), O1-Ca1-O10 : 76.4(3), O1-C1-C2: 131.1(16), Ca1-O1-Ca10 : 103.6(3). Chart 1. Structurally Characterized MgMe2 Complexes with N-Donors

complex D, which was obtained from the reaction of MgMe2 with 1,4-diazabicyclo[2,2,2]octane (dabco) in thf.22 In addition, treatment of a suspension of MgMe2 in hexane with 2 equiv of AlEt3 followed by exposure to an excess of thf yielded also a white precipitate. On the basis of the NMR data we propose the formation of a solventseparated ion pair, [Mg(thf)6][AlEt3Me]2 (8). When a suspension of [Ca(AlMe4)2]n (1) in hexane was treated with an excess of thf (Scheme 3), bulky white solid 9 formed, comparable to the formation of compound 6. After several hours of stirring, the precipitate was separated and thf added. Contrary to the respective magnesium species 6, the calcium-containing precipitate 9 displayed only poor solubility in thf and the supernatant did not contain any isolable calcium complexes. Nevertheless, on the basis of the

chemical formula Mr cryst system space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z F(000) T/K Fcalcd/g cm-3 μ/mm-1 R1 (obsd)a wR2 (all)b Sc

2

4

5

C32H80Al4Ca2 653.04 hexagonal P32 11.6998(12) 11.6998(12) 27.105(3) 90.00 90.00 120.00 3213.2(6) 3 1092 123(2) 1.012 0.365 0.0427 0.1067 1.106

C24H56Al2CaO2 470.73 monoclinic C2/c 14.7336(8) 14.5164(8) 14.3334(8) 90.00 98.076(1) 90.00 3035.2(3) 4 1048 123(2) 1.030 0.280 0.0510 0.1130 1.114

C20H32Al2CaN2 394.52 monoclinic Cc 19.9737(9) 11.1587(5) 12.2837(6) 90.00 122.767(1) 90.00 2302.15(18) 4 848 123(2) 1.138 0.354 0.0422 0.0928 1.035

6

7

11

chemical formula C46H88Al2MgO C12H28Mg2O2 C44H94Al2Ca2O10 Mr 815.43 252.96 917.31 cryst syst orthorhombic monoclinic triclinic I2/a P1 space group P212121 a/A˚ 14.1714(5) 12.9530(6) 8.8759(9) b/A˚ 18.3361(6) 10.0259(5) 12.5718(13) c/A˚ 19.4765(7) 13.5302(6) 12.6784(13) R/deg 90.00 90.00 107.515(2)° β/deg 90.00 114.1420(10) 90.132(2)° γ/deg 90.00 90.00 97.781(2)° 5060.9(3) 1603.42(13) 1335.3(2) V/A˚3 Z 4 4 1 F(000) 1800 560 504 T/K 100(2) 123(2) 103(2) 1.070 1.048 1.141 Fcalcd/g cm-3 -1 0.111 0.137 0.294 μ/mm 0.0474 0.0495 0.1500 R1 (obsd)a 0.1236 0.1504 0.3705 wR2 (all)b 1.083 1.139 1.200 Sc P P P a b (|Fo| - |FP |Fo|, Fo > 4σ(Fo). wR2 = { [w(F2o - F2c )2]/ c|)/ P R1 2=2 1/2 c 1/2 2 2 2 [w(Fo) ]} . S = [ w(Fo - Fc ) /(no - np)] .

elemental analysis and the NMR spectra we propose a chemical composition of [Ca(thf )6][AlMe4]2 for 9, which is consistent with 6. Similar solvent-separated alkylaluminate complexes, e.g., [Ca(thf )6][AlMe4-xPhx]2, were described previously by Westerhausen and co-workers.7,8 Additionally, the six-coordinate species [Ca(thf )6]2þ is a well-known

Article


structural motif in organocalcium chemistry.20,26-28 Correspondingly, the ion-separated complexes [Ca(thf)6][AlEt4]2 (10) and [Yb(thf )6][AlEt4]2 (10a) could be isolated from the reaction of [Ca(AlEt4)2]n (2) and [Yb(AlEt4)2]n (2a), respectively, with thf (Scheme 3). The formation of complexes 10 was supported by 1H and 13C NMR spectroscopic investigations as well as elemental analyses. Heating a suspension of 9 in thf in a pressure tube at 60 °C for approximately four days led to the formation of the vinyloxide-bridged complex [{Ca(μ-OCHdCH2)(thf)4}2][AlMe4]2 (11), as evidenced by X-ray structure analysis (Scheme 4 and Figure 8). We assume that the bridging vinyloxide ligands originate from thf cleavage, a degradation pathway that has been previously observed in the presence of oxophilic metal centers29 and in particular organocalcium compounds.30,31 The quality of the crystals of 11 allowed only for the assignment of the molecular connectivity, but precluded a detailed discussion of metrical parameters.

Conclusion Not surprisingly, silylamide complex [Ca[N(SiMe3)2]2]2 reacts with an excess of AlR3 via silylamide elimination to give heterobimetallic peralkylated complexes [Ca(AlR4)2]n (R = Me, Et), a reaction behavior that was previously observed for divalent lanthanides Yb(II) and Sm(II). X-ray structure analyses of polymeric [M(II)(AlEt4)2]n (M(II) = Ca, Yb, Sm) corroborate cation size-implied parallels in rare-earth metal and alkaline-earth metal structural chemistry. Treatment of [Ca(AlR4)2]n (R = Me (1), Et (2)) with neutral hard donor molecules confirms major reactivity pathways that have been proposed by Lehmkuhl et al. more than 50 years ago on the basis of NMR data. Accordingly, the interaction of [Ca(AlR4)2]n with thf follows a reaction sequence with initial donor addition and subsequent ion separation, as evidenced by the isolation of Ca(AlEt4)2(thf)2 (4) and [Ca(thf)6][AlEt4]2 (10). This is in sharp contrast to trivalent alkylaluminate chemistry with donor-induced alkylaluminate cleavage of Ln(AlMe4)3 and formation of [LnMe3]n being the predominant reaction pathway. Also, the magnesium derivatives follow an adduct formation-alkylaluminate separation sequence. We are currently investigating the reactivity of alkaline-earth alkylaluminates according to protonolysis and salt metathesis reaction protocols. Preliminary results show that both half-sandwich complexes of type [Cp*Ca(thf )4][AlMe4] and calcocene derivatives Cp2*Ca(thf )2 can be readily obtained.

Experimental Section General Remarks: Materials and Methods. All reactions were performed under a dry argon atmosphere using standard Schlenk (26) Fedushkin, I. L.; Lukoyanov, A. N.; Dechert, S.; Schumann, H. Eur. J. Inorg. Chem. 2004, 2421–2424. (27) Harder, S.; M€ uller, S.; H€ ubner, E. Organometallics 2004, 23, 178–183. (28) Perruchas, S.; Simon, F.; Uriel, S.; Avarvari, N.; Boubekeur, K.; Batail, P. J. Organomet. Chem. 2002, 643-644, 301–306. (29) For similar thf fragmentation reactions in organolanthanide chemistry, see: (a) Thiele, K.-H.; Unverhau, K.; Geitner, M.; Jacob, K. Z. Anorg. Allg. Chem. 1987, 548, 175–179. (b) Evans, W. J.; Dominguez, R.; Hanusa, T. P. Organometallics 1986, 5, 1291–1296. (c) Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2003, 22, 4705–4714. (d) Daniel, S. D.; Lehn, J.-S. M.; Korp, J. D.; Hoffman, D. M. Polyhedron 2006, 25, 205–210. (30) G€ artner, M.; G€ orls, H.; Westerhausen, M. J. Organomet. Chem. 2008, 693, 221–227. (31) Krieck, S.; G€ orls, H.; Westerhausen, M. J. Organomet. Chem. 2009, 694, 2204-2209.

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and glovebox techniques (MBraun MB150B-G-II;

Alkaline-Earth Metal Alkylaluminate Chemistry Revisited

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