Complexes of the heavier alkaline earth metals containing β-diketiminato and iodide ligand sets

Polyhedron 25 (2006) 224–234 www.elsevier.com/locate/poly

Complexes of the heavier alkaline earth metals containing b-diketiminato and iodide ligand sets Hani M. El-Kaderi, Mary Jane Heeg, Charles H. Winter

*

Department of Chemistry, Wayne State University, 5101 Cass Avenue, Detroit, MI 48202, USA Received 5 April 2005; accepted 30 June 2005 Available online 11 August 2005

Abstract 0

Treatment of ½ð2; 6-iPr2 C6 H3 ÞNCðtBuÞCH2 CðtBuÞNð2; 6-iPr2 C6 H3 Þ
ðLAr HÞ with potassium hydride in tetrahydrofuran afforded 0 0 Kðg1 -LAr ÞðTHFÞ3 (89%) as a yellow crystalline solid after workup. The crystal structure of Kðg1 -LAr ÞðTHFÞ3 revealed an open monomeric molecule in which the potassium ion is bonded to three tetrahydrofuran ligands and one g1-b-diketiminato ligand 0 through one of the nitrogen atoms. Treatment of Kðg1 -LAr ÞðTHFÞ3 with equimolar amounts of MI2 (M = Ca, Ba) afforded Ar0 Ar0 5 2 ½Caðg -L Þðl-IÞðTHFÞ
2 (62%) and ½Baðg -L Þðl-IÞ
2 (27%) as yellow crystalline solids. Treatment of M(g5-LtBu)2 (M = Ca, Sr, Ba, LtBu = tBuNC(CH3)CHC(CH3)NtBu) with the corresponding metal iodide in tetrahydrofuran afforded [Ca(g5-LtBu)(l-I)(THF)]2 (68%), [Sr(g5-LtBu)(l-I)(THF)]2 (53%), and [(g5-LtBu)Ba(l-I)(l-g2:g5-LtBu)Ba(g5-LtBu)]2(24%). Structural assignments for these complexes were based on spectral and analytical data as well as X-ray crystallography. [Ca(g5-LtBu)(l-I)(THF)]2 and [Sr(g5-LtBu)(l-I)(THF)]2 are dimers that are held together by bridging iodide ligands, while [Ba(g5-LtBu)(l-I)(l-g2:g5-LtBu)Ba(g5-LtBu)]2 is a tetramer containing two symmetrical dimeric units that are linked by two bridging iodide ligands. In benzene0 0 d6 solution, ½Caðg5 -LAr Þðl-IÞðTHFÞ
2 and ½Baðg2 -LAr Þðl-IÞ
2 are stable toward ligand redistribution. [Ca(g5-LtBu)(l-I)(THF)]2 exists as mixtures of three species in toluene-d8, but the mixed ligand complex is strongly favored under the conditions studied. At 31.3
C, the equilibrium constant for ligand redistribution of [Sr(g5-LtBu)(l-I)(THF)]2 to Sr(g5-LtBu)2 and SrI2(THF)2 in toluene-d8 was 5.3 · 105 M.  2005 Elsevier Ltd. All rights reserved. Keywords: N-ligands; Group 2 elements; Calcium; Strontium; Barium

1. Introduction Recent efforts in the coordination chemistry of calcium, strontium, and barium have been driven by applications in film growth by chemical vapor deposition (CVD) and related techniques [1], as well as organic synthesis and polymerization catalysis [2–4]. In polymerization reactions, unsymmetrical LMX complexes are the desired precatalysts, since X possesses the needed nucleophilic properties to initiate catalysis *

Corresponding author. Tel.: +1 313 577 5224; fax: +1 313 577 8289. E-mail address: [email protected] (C.H. Winter). 0277-5387/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.06.042

and the steric properties of L can be designed to promote stereoselectivity in the growing polymer [2–4]. As part of the development of group 2-based polymerization catalysis, there is a growing need to understand the redistribution equilibria of LMX, since ML2 is unlikely to initiate catalysis efficiently and MX2 may not promote stereoselective polymerization. Most calcium, strontium, and barium complexes containing mixed ligand sets are of the formula [M(C5R5)(X)(L)n]y, where C5R5 is a bulky substituted cyclopentadienyl ligand, X is a halide ligand, and L is a neutral oxygen donor ligand [5–10]. Bulky cyclopentadienyl ligands appear to stabilize these mixed ligand complexes toward ligand redistribution through steric inhibition of dimerization

H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234

[11]. Strongly coordinating oxygen ligands such as tetrahydrofuran also contribute to the stability of the mixed ligand complexes toward ligand redistribution [11a]. However, complexes of the formula [M(C5R5)(X)(L)n]y are difficult to handle in aromatic solvents in the absence of coordinating ligands, due to the appearance of facile redistribution equilibria (e.g., 2M(C5R5)(X) M M(C5R5)2 + MX2) [10,11a]. Furthermore, replacement of the halide ligand in [M(C5R5)(X)(L)n]y by salt metathesis routes can be problematic. For example, treatment of MN(SiMe3)2 (M = Li, Na, K) with (C5H2(SiMe3)3)CaI(THF) resulted in the formation of M(C5H2(SiMe3)3) and Ca(N(SiMe3)2)(I)(THF)n [6]. Recently we [12] and others [3,4] have been exploring the use of b-diketiminato ligands as ancillary ligands for the stabilization of the heavier group 2 metal complexes. The b-diketiminato ligand is isoelectronic with cyclopentadienyl and b-diketonato ligands, and its steric profile is easily adjusted at both the core nitrogen and carbon atoms [13]. Such steric manipulations can have a profound effect on the bonding modes of the b-diketiminato ligands, especially with barium [12b,14,15]. For example, the complex Ba2(LCy)3(N(SiMe3)2) (LCy = C6H11NC(CH3)CHC(CH3)N(C6H11)) contains g5-, l-g2:g5-, and l-g2:g3-b-diketiminato ligands [14], Ba2(LiPr)4 (LiPr = iPrNC(CH3)CHC(CH3)NiPr) features g5-, l-g5:g5-, and l-g1:g1-b-diketiminato ligands [12b], and Ba(LAr)2 (LAr = (2,6-iPr2C6H3)NC(CH3)CHC(CH3)N(2,6-iPr2C6H3)) possesses g2-b-diketiminato ligands [15]. We have recently reported that homoleptic calcium, strontium, and barium complexes containing g5-b-diketiminato ligands have many similarities to the analogous metallocenes [12b]. In view of the increasing importance of mixed ligand group 2 complexes in polymerization reactions [3,4], we set out to explore the synthesis and ligand redistribution reactions of calcium, strontium, and barium complexes that contain both halide and b-diketiminato ligands. Herein, we describe the synthesis, structure, and properties of a series of calcium, strontium, and barium 0 0 complexes that contain LAr ðLAr ¼ ð2; 6-iPr2 C6 H3 Þ NCðCðCH3 Þ3 ÞCHCðCðCH3 Þ3 ÞNð2; 6-iPr2 C6 H3 Þ Þ or tBu tBu  L (L = tBuNC(CH3)CH-C(CH3)NtBu ) and iodide ligand sets. Depending on the nature of the b-diketiminato ligand nitrogen atom substituents and the group 2 ion, iodide-bridged tetrahydrofuran-coordinated dimers, iodide-bridged solvent-free dimers, and a unique tetramer are obtained. In addition, the tendency of these complexes to undergo redistribution reactions 0 in solution0 is described. ½Caðg5 -LAr Þðl-IÞðTHFÞ
2 and ½Baðg2 -LAr Þðl-IÞ
2 show no evidence for redistribution equilibria in benzene, while [Ca(g5-LtBu)(l-I)(THF)]2 and [Sr(g5-LtBu)(l-I)(THF)]2 exist as equilibrium mixtures of two or more species. Estimated equilibrium constants, however, suggest that the mixed ligand species are strongly favored at ambient temperature.

225

2. Results 2.1. Synthetic aspects 0

KH (1 equiv.) in tetrahydroTreatment of LAr H with 0 furan afforded Kðg1 -LAr ÞðTHFÞ3 (1, 89%) as a yellow crystalline solid after workup (Eq. (1)). Complex 1 is soluble in toluene, easily crystallized, can be stored for several months under argon without decomposition, and is thus a very useful reagent. Treatment of 1 with MI2 (M = Ca, Ba) (1 equiv.) in anhydrous tetrahydrofuran 0 afforded0 ½Caðg5 -LAr Þðl-IÞðTHFÞ
2 (2, 62%), and ½Baðg2 -LAr Þðl-IÞ
2 (3, 27%) as yellow crystalline solids after workup (Eq. (1)). The structural assignments for 1–3 were based upon spectral and analytical data as well as X-ray structure determinations. In the solid state, 1 exists as a monomeric molecule in which the b-diketiminato ligand is bonded to the potassium center through one nitrogen atom with an g1-coordination mode. The potassium ion is also coordinated to three tetrahydrofuran ligands. The structural determination for 1 was of low precision, and is not discussed further herein. The solid state structures of 2 and 3 revealed dimeric molecules that are held together by bridging iodide ligands. Further details of the ligand bonding modes are described below. Several attempts were made to prepare the analogous strontium complex, however, this complex did not crystallize well and was very difficult to isolate in a pure state in good yields. In one case, a few crystals of the strontium complex were obtained, and analysis of the diffraction data revealed a molecular structure similar to that of 3. tBu

Ar N H

Ar

KH THF, 18 h

tBu Ar

N

N

tBu Ar Ar = 2,6-iPr2C6H3

Ar tBu

N

N

K(THF)3 tBu

1, 89%

O Ar tBu I N Ca Ca N tBu O I Ar tBu 2, 62%

N

Ar

Ar N Ba Ba I N N Ar Ar 3, 27% Ar

N

tBu

MI2, THF 25 ˚C, 18 h -KI

tBu

tBu

I

tBu

ð1Þ

The 1H NMR spectrum of 1 in toluene-d8 at 25
C showed one type of 2,6-diisopropylphenyl group, one type of tert-butyl group, and one type of tetrahydrofuran ligand. The b-CH resonance of the b-diketiminato ligand appeared as a singlet at d 4.89. These resonances are inconsistent with the solid state structure, where the potassium ion is coordinated to only one of the

226

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nitrogen atoms. It is very likely that the potassium ion exchanges with the two nitrogen atoms very rapidly on the NMR timescale, and thus an averaged spectrum is obtained. Alternatively, both nitrogen atoms could be coordinated to the potassium ion in solution, thus affording a species with a plane of symmetry. There were two doublets for the isopropyl methyl groups (d 0.96, 1.33), and one septet for the isopropyl methine groups at d 3.27. The observation of only one methine septet demonstrates that the isopropyl methyl groups are diastereotopic. There were no major changes in the 1H NMR spectra of 1 between 60 and +80
C. The lability of the tetrahydrofuran ligands in 1 was probed by drying a sample for 18 h at 0.1 Torr in a drying tube heated with refluxing acetone (56
C). A 1 H NMR spectrum of the dried material showed free 0 LAr H and unidentified resonances, demonstrating facile loss of tetrahydrofuran and concomitant decomposition. The 1H NMR spectrum of 2 in benzene-d6 at 25
C showed a single species, with the b-diketiminato ligand b-CH resonance at d 5.52, isopropyl methyl resonances at d 1.34 and 1.17, and a tert-butyl resonance at d 1.24. The isopropyl methine resonance (d 3.2) overlapped with the a-CH2 resonance of the tetrahydrofuran ligand. The 1H NMR spectrum of 2 at 75
C was very similar, except for small chemical shift changes. The 1H NMR spectrum of 3 in benzene-d6 at 25
C shows a single species. The b-diketiminato ligand b-CH proton resonance appeared at d 5.24, which is d 0.28 upfield of the same resonance in 2. The difference in the positions of this resonance for 2 and 3 may be related to the g5-b-diketiminato coordination mode in 2 and the g2-b-diketiminato coordination mode in 3. The isopropyl methyl group resonances appeared at d 1.24 and 1.21, while the isopropyl methine resonance was observed at d 2.99. The latter resonance is upfield compared to that of 2 (d 3.27). The tert-butyl protons resonated at d 1.19. The synthesis of mixed ligand complexes containing LtBu proceeded by a different route than was used for 2 and 3. Treatment of M(g5-LtBu)2 (M = Ca, Sr, Ba) [12b] and MI2 in refluxing tetrahydrofuran as indicated in Eq. (2) afforded [Ca(g5-LtBu)(l-I)(THF)]2 (4, 68%), [Sr(g5-LtBu)(l-I)(THF)]2 (5, 53%), and [Ba(g5-LtBu)(l-I)(l-g2:g5-LtBu)Ba(g5-LtBu)]2 (6, 24%) as colorless (4, 5) or yellow crystals (6) after workup. The structural assignments for 4–6 were based upon spectral and analytical data, as well as X-ray structure determinations as described below. In the solid state, 4 and 5 exist as iodide-bridged dimers with molecular structures analogous to that of 2. In contrast to 2–5, the molecule of 6 is a tetramer which consists of two dimeric units that are bridged by two iodide ligands. Further details of the ligand bonding modes are described below.

tBu

N

N

H3C

tBu

N

N

CH3

tBu H C 3 M CH3 tBu N

THF reflux, 3 h

CH3 N

+ n MI2 tBu

O tBu CH3 I M M N CH3 O I tBu

CH3 N

tBu

4, M= Ca, n = 1, 68% 5, M= Sr, n = 1, 53% tBu

N

H3C

N

tBu H C 3 Ba N CH3 tBu

CH3 tBu N Ba N H3C tBu

I

tBu

N

tBu CH3

I tBu N

H3C

N

tBu N

CH3 H3C Ba Ba N CH3 tBu N tBu H3C 6, n = 1/3, 24%

CH3 N

tBu

ð2Þ The 1H NMR spectrum of 4 in toluene-d8 at 25
C showed three b-diketiminato ligand b-CH resonances at d 4.45, 4.30, and 4.21 in a 26:61:13 ratio. The resonance at d 4.21 corresponds exactly to the resonance for Ca(g5-LtBu)2 [12b]. Presumably, one of the other two resonances corresponds to 4. However, the presence of three b-diketiminato ligand b-CH resonances suggests that 4 is engaged in multiple equilibria in solution. Unlike 4, the 1H NMR spectrum of 5 in toluene-d8 at 25
C showed two b-diketiminato ligand b-CH resonances at d 4.34 and 4.21 in a 94:6 ratio. The resonance at d 4.21 corresponds exactly to the resonance for Sr(g5-LtBu)2 [12b]. The observation of a major species with resonances consistent with 5, along with Sr(g5-LtBu)2, suggests the equilibrium process that is outlined in Eq. (3). The 1H NMR spectra of 5 were recorded between 30 and 85
C in toluene-d8 to assess how the equilibrium was affected by temperature. Above 60
C, the resonances for Sr(g5-LtBu)2 disappeared, and only one species was observed. Cooling the sample of 5 to 50
C did not change the ratio of the major species to Sr(g5-LtBu)2, within experimental error. Due to the small amount of Sr(g5-LtBu)2 and the inherent errors associated with measuring the concentrations by integration, we did not carry out a van
t Hoff analysis to obtain the equilibrium thermodynamic parameters. However, at 31.4
C, Keq was calculated to be 5.3 · 105 M for a 0.0134 M solution of 5 in toluene-d8. Thus, the equilibrium strongly favors the mixed ligand species 5 at all of the temperatures that we examined. We note that the assignment of SrI2(THF)2 is uncertain, and is offered for convenience to allow correct stoichiometry. SrI2(THF)5 has been crystallographically characterized,

H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234

but rapidly loses tetrahydrofuran in the solid state [16]. The number of coordinated tetrahydrofuran ligands in solution is thus unclear.

tBu

O tBu CH3 I N Sr Sr N CH3 O I tBu

N

CH3

tBu

5 tBu H C 3 N Sr CH3 tBu N

N

H3C

CH3 N

Keq toluene 25 ˚C

tBu

CH3 +

N 5

Keq = [Sr(η -L

SrI2(THF)2

tBu

tBu

)2][SrI2(THF)2]/[5]

ð3Þ

227

Table 2 ˚ ) and bond angles (
) for 2a Selected bond lengths (A Ca–I(1) Ca–I(1) 0 Ca(1)–N(1) Ca(1)–N(2) Ca(1)–O(1) Ca–C(13) Ca–C(14) Ca–C(15) N(1)–C(13) N(2)–C(15) C(13)–C(14) C(14)–C(15)

3.210(1) 3.273(1) 2.416(2) 2.386(2) 2.407(2) 2.955(3) 2.748(3) 2.723(3) 1.297(3) 1.351(3) 1.453(4) 1.399(4)

Ca–I(1)–Ca 0 N(1)–Ca–N(2) C(13)–C(14)–C(15) a

104.62(1) 79.16(7) 130.9(2)

Primes indicate symmetry equivalent positions.

1

The H NMR spectrum of 6 in toluene-d8 at 25
C consisted of very broad singlets centered at d 4.26, 2.07, and 1.39 for the b-diketiminato ligand b-CH, backbone methyl, and tert-butyl resonances, respectively. Further studies were hampered by the low solubility of 6 in toluene. The broad resonances suggest that the b-diketiminato ligands undergo site exchange in solution. 2.2. Structural aspects The X-ray crystal structures of 2–6 were determined to establish the geometries about the metal centers and the bonding modes of the b-diketiminato ligands. Experimental crystallographic data are summarized in Table 1, selected bond lengths and angles are given in Tables 2–6, and perspective views are presented in Figs. 1–5.

Table 3 ˚ ) and bond angles (
) for 3a Selected bond lengths (A Ba–I(2) Ba–I(2) 0 Ba–N(1) Ba–N(2) Ba–C(4) Ba–C(5) Ba–C(9) Ba–C(24) Ba–C(25) Ba–C(29) N(1)–C(1) N(2)–C(3) C(1)–C(2) C(2)–C(3)

3.362(2) 3.523(2) 2.680(7) 2.695(7) 3.029(8) 3.656(8) 3.367(8) 2.940(8) 3.436(8) 3.311(8) 1.339(10) 1.332(10) 1.404(12) 1.398(12)

N(2)–Ba–N(1) I(2)–Ba–I(2) 0 C(1)–C(2)–C(3) a

66.0(2) 81.97(3) 131.6(8)

Primes indicate symmetry equivalent positions.

Table 1 Crystal data and data collection parameters for 2–6 2 Formula C84H136Ca2I2N4O2 FW 1567.93 Space group P 1 ˚) a (A 10.2916(10) ˚) b (A 13.7305(15) ˚) c (A 16.3599(18) a (
) 82.157(2) b (
) 78.079(2) c (
) 70.133(2) ˚ 3) V (A 2121.8(4) Z 1 T (K) 295(2) ˚) k (A 0.71073 Density calculated (g cm3) 1.227 l (mm1) 0.907 R(F)a (%) 4.01 Rw(F)b (%) 9.25 P P a RðF Þ ¼ jjF o j  jF c jj= jF o j. P P b 2 2 2 Rw ðF Þ ¼ ½ wðF o  F c Þ = wðF 2o Þ2
1=2 .

3

4

5

6

C70H106Ba2I2N4 1532.07 P 1 9.778(5) 13.174(7) 15.984(9) 96.488(11) 106.886(12) 108.736(12) 1816.8(16) 1 295(2) 0.71073 1.400 1.965 7.62 14.85

C34H66Ca2I2N4O2 896.87 C2/c 23.314(4) 13.413(2) 13.695(2)

C34H66Sr2I2N4O2 991.95 C2/c 24.39(2) 13.428(13) 13.813(14)

97.303(4)

97.681(17)

4248.0(11) 4 212(2) 0.71073 1.402 1.753 3.43 7.74

4483.0(8) 4 295(2) 0.71073 1.470 3.786 3.33 6.69

C78H150Ba4I2N12 2059.26 P 1 10.7357(13) 15.581(2) 15.801(2) 106.921(3) 104.216(3) 100.502(3) 2357.68 1 295(2) 0.71073 1.450 2.345 6.00 11.95

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H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234

Table 4 ˚ ) and bond angles (
) for 4a Selected bond lengths (A Ca–I(1) Ca–I(2) Ca–O(1) Ca–N(1) Ca–N(2) Ca–C(1) Ca–C(3) Ca–C(4) N(1)–C(1) N(2)–C(4) C(1)–C(3) C(3)–C(4) Ca–I(1)–Ca 0 Ca–I(2)–Ca 0 I(1)–Ca–I(2) N(1)–Ca(1)–N(2) a

Table 6 ˚ ) and bond angles (
) for 6a Selected bond lengths (A 3.131(1) 3.157(1) 2.390(2) 2.332(2) 2.333(3) 2.779(3) 2.695(3) 2.759(3) 1.306(4) 1.309(4) 1.427(4) 1.428(4) 90.91(3) 89.96(2) 89.56(2) 76.75(9)

Primes indicate symmetry equivalent positions.

Table 5 ˚ ) and bond angles (
) for 5a Selected bond lengths (A Sr–I(1) Sr–I(2) Sr–O(1) Sr–N(1) Sr–N(2) Sr–C(1) Sr–C(3) Sr–C(4) N(1)–C(1) N(2)–C(4) C(1)–C(3) C(3)–C(4) Sr–I(1)–Sr 0 Sr–I(2)–Sr 0 I(1)–Sr–I(2) N(1)–Sr–N(2) a

3.275(2) 3.289(2) 2.538(3) 2.473(3) 2.466(3) 2.921(4) 2.845(4) 2.916(4) 1.287(4) 1.307(5) 1.427(5) 1.422(5)

Ba(1)–I(1) Ba(2)–I(1) 0 Ba(1)–N(1) Ba(1)–N(2) Ba(1)–N(3) Ba(1)–N(4) Ba(2)–N(3) Ba(2)–N(4) Ba(2)–N(5) Ba(2)–N(6) Ba(1)–C(1) Ba(1)–C(3) Ba(1)–C(4) Ba(1)–C(14) Ba(1)–C(16) Ba(1)–C(17) Ba(2)–C(14) Ba(2)–C(16) Ba(2)–C(17) Ba(2)–C(27) Ba(2)–C(29) Ba(2)–C(30) Ba(1)–Ba(2) Ba(1)–I(1)–Ba(2) 0 Ba(2)–Ba(1)–I(1) Ba(1)–Ba(2)–I(1) 0 N(1)–Ba(1)–N(2) N(3)–Ba(1)–N(4) N(3)–Ba(2)–N(4) N(5)–Ba(2)–N(6) a

3.362(1) 3.454(1) 2.644(8) 2.680(8) 3.070(8) 3.032(7) 2.837(7) 2.860(8) 2.638(8) 2.648(7) 3.105(11) 2.989(10) 3.088(11) 3.185(10) 3.139(11) 3.181(10) 3.56(1) 3.78(1) 3.54(1) 3.118(11) 3.039(10) 3.114(10) 4.7725(10) 173.88(4) 75.64(2) 99.49(2) 68.5(3) 61.5(2) 66.4(2) 67.7(2)

Primes indicate symmetry equivalent positions.

91.45(7) 90.97(7) 88.79(7) 72.79(11)

Primes indicate symmetry equivalent positions.

A perspective view of 2 is shown in Fig. 1. The overall molecular structure consists of a dimer connected by two bridging iodide ligands, with one tetrahydrofuran 0 and one g5 -LAr ligand bonded to each calcium ion. The calcium–iodine bond lengths are 3.210(1) ˚ . The calcium–iodine–calcium angle is and 3.273(1) A 104.62(1)
. The calcium–oxygen bond distance is ˚ . The calcium–nitrogen bond lengths are 2.407(2) A ˚ , while the calcium–carbon dis2.386(2) and 2.416(2) A ˚ for C(14) and C(15) tances are 2.748(3) and 2.723(3) A ˚ for C(13). The longer Ca–C(13) bond and 2.955(3) A length appears to arise from steric interactions associated with accommodation of the tetrahydrofuran ligand on the same side of the calcium ion as C(13). For comparison, the calcium–nitrogen and calcium–carbon bond lengths in Ca(g5-LtBu)2 range from 2.34 to 2.36 and 2.84 ˚ , respectively [12b]. The calcium–carbon disto 2.90 A ˚ [17]. The nitrogen– tance in Ca(C5(CH3)5)2 is 2.64(2) A calcium–nitrogen bite angle is 79.16(7)
.

Fig. 1. Perspective view of 2 with probability ellipsoids at the 50% level. The isopropyl and tert-butyl groups have been removed for clarity.

A perspective view of 3 is shown in Fig. 2. The overall structure of 3 consists of two barium ions that are bridged by two iodide ligands. Each barium ion is also 0 0 coordinated to one0 LAr ligand. Unlike the g5 -LAr Ar ligands in 2, the L ligand core in 3 is bonded to the barium atoms with the g2-coordination mode. The bar˚ , which ium–iodine distances are 3.362(2) and 3.523(2) A indicate some asymmetry in the Ba2I2 core. The barium– ˚. nitrogen bond distances are 2.680(7) and 2.695(7) A

H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234

229

A perspective view of 4 is shown in Fig. 3. The molecular structure of 4 is similar overall to that of 2. The cal˚, cium–iodine bond lengths are 3.131(1) and 3.157(1) A and the angles associated with the Ca2I2 core are very close to 90
. The calcium–oxygen bond length is ˚ , which is identical to that of 2 within experi2.390(2) A mental error. The calcium–nitrogen bond distances are ˚ , while the calcium–carbon dis2.332(2) and 2.333(3) A ˚ . The calcium– tances range from 2.695(3) to 2.779(3) A nitrogen bond distances in 4 are shorter than those of 2, possibly due to accommodation of the very bulky aryl groups in 2. The calcium–carbon distances are similar in 2 and 4, except that there is no asymmetry in the bond distances to LtBu in 4. The lack of asymmetry in the calcium–carbon distances is probably due to the lower steric profile of LtBu compared to LAr . The nitrogen– calcium–nitrogen bite angle is 76.75(9)
. A perspective view of 5 is shown in Fig. 4. The molecular structure of 5 consists of a dimer that is held together by two bridging iodide ligands. Each strontium ion is additionally coordinated to one g5-LtBu and one tetrahydrofuran ligand. The overall molecular structure is similar to those of 2 and 4. The strontium–iodine ˚ . The angles bond distances are 3.275(2) and 3.289(2) A associated with the Sr2I2 core are close to 90
, and the equivalent strontium–iodine bond lengths make this core nearly a perfect square. The strontium–nitrogen ˚ , while the bond distances are 2.466(3) and 2.473(3) A strontium–carbon distances range from 2.845(4) to ˚ . For comparison, the strontium–nitrogen 2.921(4) A and strontium–carbon bond lengths in Sr(g5-LtBu)2 ˚ , respectively range from 2.49 to 2.52 and 2.96 to 3.02 A [12b]. Thus, the strontium–nitrogen and strontium–carbon bond lengths in 5 are slightly shorter than those of Sr(g5-LtBu)2, and support the assignment of g5-LtBu ligands in 5. Unlike in 2, there is only a small bond length asymmetry on the side of the LtBu ligand that is closest to the tetrahydrofuran ligand. The larger size of the 0

Fig. 2. Perspective view of 3 with probability ellipsoids at the 50% level. The isopropyl and tert-butyl groups have been removed for clarity.

˚ , and The Ba–C(1,2,3) distances range from 3.79–4.14 A 2 are consistent with the g -b-diketiminato ligand coordination mode. Several of the aryl group carbon atoms have short contacts to the barium ion. In particular, the Ba–C(4) and Ba–C(24) distances are 3.029(8) and ˚ , respectively. Distances from barium to the 2.940(8) A ortho-carbon atoms of the aryl rings range from 3.31 ˚ . For comparison, the barium– carbon disto 3.66 A ˚ [12b], tances in Ba(g5-LtBu)2 range from 3.06 to 3.16 A while the barium–carbon distances in Ba(C5(CH3)5)2 ˚ [16]. Thus the barium–carbon distances are 2.98–2.99 A to the ipso-carbon atoms of the aryl groups are comparable to those of metallocenes, and must constitute strong interactions. The fact that 3 is isolated as a solvent free complex is probably related to the saturation of the coordination sphere about the barium atoms by bonding to the bulky aryl groups.

Fig. 3. Perspective view of 4 with probability ellipsoids at the 50% level.

Fig. 4. Perspective view of 5 with probability ellipsoids at the 50% level.

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strontium ion, compared to the calcium ion, lessens the steric interactions between the b-diketiminato and tetrahydrofuran ligands. The nitrogen–strontium–nitrogen bite angle is 72.79(11)
. Perspective views of 6 are shown in Fig. 5. Fig. 5(a) shows one half of the tetrameric structure, while Fig. 5(b) presents the complete tetrameric molecule. Complex 6 is located on an inversion center, which makes the two halves of the molecule identical by symmetry. The overall structure consists of two Ba2(LtBu)3I units that are connected through two essentially linear bridging iodide ligands. Each barium ion within the Ba2(LtBu)3I unit bonds to one iodide and one terminal g5-LtBu ligand, and is connected to an adjacent barium ion through a l-g2:g5-b-diketiminato ligand. The bar˚, ium–iodine bond lengths are 3.362(1) and 3.454(1) A and are thus slightly asymmetric. These bond lengths and degree of asymmetry are very similar to those of 3. The barium–nitrogen and barium–carbon distances associated with the terminal g5-LtBu ligands range from ˚, 2.638(8) to 2.680(8) and 2.989(10) to 3.114(10) A respectively. These barium–nitrogen distances are very similar to those of 3. These values for 6 can also be compared to those of Ba(g5-LtBu)2, for which the barium

Fig. 5. Perspective views of 6 with probability ellipsoids at the 50% level: (a) one-half molecule with atom labels, (b) overall tetrameric molecule, with tert-butyl groups removed for clarity.

nitrogen and barium–carbon distances were 2.62–2.68 ˚ , respectively [12b]. These bond lengths and 3.08–3.16 A 5 tBu for Ba(g -L )2 are essentially identical to the values for the terminal g5-LtBu ligands in 6. The barium–nitrogen distances associated with the l-g2:g5-LtBu ligands ˚ for the g5-interaction with are 3.032(7) and 3.070(8) A ˚ for the g2-interaction Ba(1) and 2.837(7) and 2.860(8) A with Ba(2). These values are longer than those of the terminal g5-LtBu ligands, the related values in 3, and those of Ba(g5-LtBu)2 [12b], as expected for a bridging b-diketiminato ligand. The barium–carbon distances for the bridging g5-interaction with Ba(1) range from 3.14 to ˚ , which are slightly longer than those of the termi3.18 A nal g5-LtBu ligands in 6 and 3 and are comparable to the related distances in Ba(g5-LtBu)2 [12b]. The barium–car˚ , supbon distances to Ba(2) ranged from 3.54 to 3.76 A 2 porting the assignment of an g -interaction to the nitrogen atoms. The bite angles of the terminal g5-LtBu ligands were 67.7(2)
and 68.5(3)
, while the values for the l-g2:g5-LtBu ligand were 66.4(2)
and 61.5(2)
.

3. Discussion 3.1. Characterization of the g5-bonding The g5-coordination of the b-diketiminato ligands is a prominent structural feature of 2 and 4–6. We have recently reported that group 2 complexes adopt sandwich-like structures with g5-LtBu ligands [12]. In our paper on M(g5-LtBu)2 (M = Ca, Sr, Ba) [12b], the g5-LtBu bonding was analyzed in terms of the planes incorporating the atoms MN2 (a), N2(Ca)2 (b), and (Ca)2Cb (c). For these complexes, the a/b angles were between 70
and 72
, the b/c angles were between 25
and 26
, and these metrical parameters were used to support the g5-coordination mode and differentiate it from the more common g2-form (which has much smaller a/b and b/c values [12b,13]). The corresponding metrical parameters for 2–5 are as follows: 2, a/b = 74.0(1)
and b/c = 30.0(2)
; 3, a/b = 172.0(4)
and b/c = 4.6(1.4)
; 4, a/b = 77.7(1)
and b/c = 28.4(2)
; 5, a/b = 75.5(1)
and 28.3(3)
. For 2, 4 and 5, these values are close to those of M(g5-LtBu)2, and are consistent with similar g5-bonding of the LtBu ligands. For 6, the terminal g5-LtBu ligands have a/b values of 73.3(3)
and 75.3(4)
and b/c values of 31(2)
and 31.5(8)
, which are similar to those of M(g5-LtBu)2, 2, 4, and 5. For the l-g2:g5-b-diketiminato ligand, the side with the g5-interaction has a/b = 89.4(4)
and b/c = 5(2)
, while the side with the g2-interaction has a/b = 54.2(5)
and b/c = 5(2)
. The a/b angle associated with g5-interaction in 6 is larger than the related angles in M(g5-LtBu)2, 2, 4, 5, and the g5-LtBu terminal ligands in 6, possibly to maximize bonding to C(16) (which is essentially coplanar with the other l-g2:g5-b-diketiminato ligand core atoms). In 3, the assignment of the

H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234

g2-coordination mode is clearly warranted by the large a/b and small b/c angles. 3.2. Comparison with previously reported group 2 b-diketiminato complexes All of the other heavier group 2 b-diketiminato complexes described to date feature 2,6-diisopropylphenyl [3,4b,4c,4d,15], 2-methoxyphenyl [4a], or cyclohexyl [14] moieties as the nitrogen atom substituents and methyl groups on the a-carbon atoms of the ligand core. Calcium, strontium, and barium complexes containing LAr exhibit the g2-coordination mode, through the nitrogen atoms of LAr. Adoption of the g2-coordination mode in complexes containing LAr is presumably a steric effect that is driven by the bulky aromatic isopropyl substituents. Use of the N-cyclohexyl-substituted diketiminato ligand LCy afforded the dimeric barium complex Ba2(LCy)3(N(SiMe3)2), which contains g5-, l-g2: g5-, and l-g2:g3b-diketiminato ligands [14]. The various coordination modes of b-diketiminato ligands have been recently reviewed [13]. The terminal g5-b-diketiminato ligand has been structurally documented in a few main group, lanthanide, and early transition metal complexes, but still remains rare [12b,13,18]. In the present work, 2 and 4–6 possess the terminal g5-b-diketiminato ligand coordination mode. In particular,0 2 is the first group 2 complex that contains an g5 -LAr ligand. In 3, the g2-coordination mode appears to be stabilized through several close Ba–Caryl contacts. With the smaller calcium ion in 2, such contacts may be too long to afford much stabilization, and instead tetrahydrofuran and 0 g5 -LAr coordination ensue. The present results, our previous report on M(g5-LtBu)2 [12b], and Ba2(LCy)3(N(SiMe3)2) [14] suggest that the g5-b-diketiminato coordination mode should be common in the heavier group 2 complexes when alkyl groups or less bulky aromatic rings are present on the nitrogen atoms. Adoption0 of the g5-b-diketiminato coordination mode with LAr and related ligands containing bulky aryl groups will depend upon the size of the group 2 metal ion, the degree of coordinative unsaturation at the metal center, and the possibility of aryl group interactions with the metal center. 3.3. Redistribution equilibria in solution The redistribution equilibria of 2–6 are of relevance to recent reports of lactide polymerization catalysis by unsymmetrical group 2 complexes (LMX) containing b-diketiminato ligands (L) [3,4]. A recent report by Hill has explored the stability of b-diketiminato complexes of the formula LArM(N(SiMe3)2)(THF) toward ligand redistribution in solution [3c]. Interestingly, LArCa(N(SiMe3)2)(THF) was found to be inert toward ligand redistribution in toluene-d8 solution at ambient temper-

231

ature, while LArBa(N(SiMe3)2)(THF) was in equilibrium with Ba(LAr)2 and Ba(N(SiMe3)2)(THF)x under the same conditions. It was proposed that the larger size and lower Lewis acidity of the barium ion makes it more susceptible toward ligand redistribution, compared to LArCa(N(SiMe3)2)(THF) [3c]. In the present work, 2 and 3 did not show any evidence for ligand redistribution in solution. The lack of ligand redistribution in 3 is probably related to the coordination of both aryl ligands to the barium ion, which creates a more rigid, very large ligand. Harder has reported the synthesis and structure of Ba(LAr)2, which contains no Ba–Caryl interactions and only g2-LAr ligands [15]. The Ar0 L ligand employed in the present work is bulkier than LAr, due to the presence of the tert-butyl groups on the a-carbon atoms of the ligand core. These tert-butyl groups 0 may provide enough steric bulk to destabilize BaðLAr Þ2 , and thus remove low energy ligand redistribution paths for 3. The lack of redistribution equilibria for 0 2 may also be related to destabilization of CaðLAr Þ2 . In solution, 4 showed multiple equilibria, and as a result, it was not possible to carry out equilibrium constant determinations. However, the amount of products other than the unsymmetrical starting materials was small and the equilibrium constants are thus small. For 5, NMR spectra suggested the equilibrium process depicted in Eq. (3), and an equilibrium constant of 5.3 · 105 M was determined at 31.3
C. Thus, the equilibrium lies far toward the unsymmetrical starting material 5. These experiments demonstrate that the unsymmetrical complexes 2–5 are strongly favored thermodynamically in solution. There are few other equilibrium constant data with which the behavior of 2–5 can be compared. Cp*CaI(THF)2 is reported to dissociate in tetrahydrofuran-d8 to afford Cp2 CaðTHFÞ2 and CaI2(THF)2 with Keq = 4 [5]. (C5HiPr4)CaI(OEt2)n dissociates in diethyl ether to Ca(C5HiPr4)2 and CaI2(OEt2)n with Keq = 2 [6]. Mixtures of Ba(CH2C6H5)2(THF)2 and Ba(C5Me4SiMe2C6H5)2 favor the heteroleptic complex Ba(C5Me4SiMe2C6H5)(CH2C6 H5)(THF) with Keq values that are much greater than one [19]. The complex [Ba(C5HiPr4)(I)(THF)2]2 resists ligand redistribution in tetrahydrofuran, but does scramble ligands in less coordinating solvents such as benzene [8]. We have recently reported that [Sr(l-g5:g2-tBu2pz)(g5-LtBu)]2 exists in equilibrium with Sr(g5-LtBu)2 and Sr4(tBu2pz)8 (tBu2pz = 3,5-di-tert-butylpyrazolato) [12c]. A van
t Hoff analysis of this equilibrium between 46 and 104
C afforded DH0 = 22.2 ± 0.7 kcal/mol, DS0 = 43.7 ± 1.9 cal/mol Æ K, and DG0(298K) = 9.2 ± 0.9 kcal/ mol. The equilibrium constant at 46.0
C was 2.17 · 106 M. Thus, the mixed ligand complex [Sr(l-g5:g2-tBu2pz)(g5-LtBu)]2 is strongly favored thermodynamically. Redistribution equilibria are also important for the heavier group 2 Grignard analogs [20].

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3.4. Conclusions We have prepared and characterized a series of mixed ligand complexes containing b-diketiminato and iodide ligands. Observation of tetrahydrofuran coordination depended on the nature of the b-diketiminato ligand and 0 the metal ion. ½Caðg5 -LAr Þðl-IÞðTHFÞ
2 , [M(g5-LtBu)(l-I)(THF)]2 (M = Ca, Sr), and Ba4(LtBu)6(I)2 contain terminal g5-b-diketiminato ligands, which is a novel coordination mode for this class of ligands. In addition, 2 5 Ba4(LtBu)6(I)2 contains novel l-g :g -b-diketiminato Ar0 2 ligands. The complex ½Baðg -L Þðl-IÞ
2 contains g2-bdiketiminato ligands, and this coordination mode appears to be stabilized by close Ba–Caryl contacts to the ipso- and ortho-carbon atoms of the aryl rings. [M(g5LtBu)(l-I)(THF)]2 (M = Ca, Sr) undergo ligand redistribution in solution, although the position of the equilibrium strongly favors the mixed ligand complexes Ar0 5 under the conditions studied. ½Caðg -L Þðl-IÞðTHFÞ
2 0 and ½Baðg2 -LAr Þðl-IÞ
2 show no evidence for ligand redistribution equilbria in solution. The combined results sug0 gest that LAr and LtBu are strongly coordinating ligands for group 2 metal ions, and are effective at stabilizing mixed ligand complexes toward redistribution equilibria.

4. Experimental 4.1. General considerations All reactions were performed under an inert atmosphere of argon using either glove box or Schlenk line techniques. Tetrahydrofuran and toluene were distilled from sodium. M(g5-LtBu)2 [12b], LtBuH [21], and Ar0 L H [22] were prepared according to literature methods. Anhydrous metal iodides were purchased from Aldrich Chemical and were used as received. 1H and 13C {1H} NMR were obtained at 300 and 75 MHz in the indicated solvents. Infrared spectra were obtained using Nujol as the medium. Elemental analyses were performed by Midwest Microlab, Indianapolis, IN. Melting points were obtained on a Haake-Buchler HBI digital melting point apparatus and are uncorrected. 0

4.2. Preparation of K(LAr )(THF )3 (1) 0

A 100-mL Schlenk flask was charged with LAr H (5.00 g, 9.96 mmol) and tetrahydrofuran (60 mL). To this solution was added potassium hydride (0.80 g, 19.9 mmol), and the resultant solution was stirred at ambient temperature for 24 h. During this time a deepyellow solution was observed. The resulting solution was filtered through a 2-cm pad of Celite on a coarse glass frit to afford a clear yellow solution. The volume of the filtrate was reduced to 20 mL, and then this solu-

tion was layered with hexane (40 mL). Equilibration at ambient temperature for 48 h gave yellow crystals. Removal of the solvent with a cannula, followed by vacuum drying, afforded 1 as a yellow crystalline solid (6.70 g, 89%): m.p. 332–334
C (dec); IR (Nujol, cm1) 1547 (w), 1311 (s), 1254 (s), 1165 (s), 1082 (s), 1018 (s), 921 (w), 798 (s); 1H NMR (toluene-d8, 23
C, d) 6.77– 6.99 (m, 6H, aryl-H), 4.89 (s, 1 H, b-CH), 3.54 (m, 12H, OCH2CH2), 3.27 (septet, J = 6.3 Hz, 4H, CH(CH3)2), 1.46 (m, 12H, OCH2CH2), 1.34 (s, 18H, C(CH3)3), 1.33 (d, J = 6.3 Hz, 12H, CH(CH3)2), 0.96 (d, J = 6.3 Hz, 12H, CH(CH3)2); 13C {1H} NMR (toluene-d8, 23
C, ppm) 166.13 (s, CC(CH3)3), 151.69 (s, Cipso), 135.96 (s, Cortho), 123.5 (s, Cpara), 118.43 (s, Cmeta), 89.83 (s, b-CH), 67.57 (s, OCH2CH2), 44.22 (s, C(CH3)3), 32.74 (s, C(CH3)3), 27.21 (s, CH(CH3)2), 25.67 (s, OCH2CH2), 25.00 (s, CH(CH3)2), 23.47 (s, CH(CH3)2). Anal. Calc. for C47H77KN2O3: C, 74.55; H, 10.25; N, 3.70. Found: C, 75.46; H, 10.32; N, 4.06%. 0

4.3. Preparation of [Ca(LAr )(THF )(l-I)]2 (2) A 100-mL Schlenk flask was charged with 1 (0.553 g, 0.731 mmol), CaI2(0.215 g, 0.731 mmol), and tetrahydrofuran (40 mL). The resultant solution was stirred for 18 h at ambient temperature to afford a yellow solution. The volatile components were removed under reduced pressure and the residue was extracted with toluene (40 mL). This mixture was filtered through a 2-cm pad of Celite on a medium glass frit to afford a clear yellow solution. The filtrate was concentrated to a volume of 15 mL, hexane (10 mL) was mixed with the toluene solution, and this solution was stored at 25
C for 7 days to afford 2 as yellow crystals (0.35 g, 62%): m.p. 340–347
C (dec); IR (Nujol, cm1) 1546 (s), 1399 (s), 1259 (s), 1211 (w), 1185 (w), 1097 (w), 1024 (s), 930 (w), 871 (s), 798 (s), 687 (w); 1H NMR (benzene-d6, 23
C, d) 7.00–7.06 (m, 12H, arylH), 5.52 (s, 2H, b-CH), 3.24 (septet, J = 7.2 Hz, 8H, CH(CH3)2), 3.12 (m, 8H, OCH2CH2), 1.34 (d, J = 7.2 Hz, 24H, CH(CH3)2), 1.24 (s, 36H, C(CH3)3), 1.17 (d, J = 7.2 Hz, 24H, CH(CH3)2), 0.89 (m, 8H, OCH2CH2); 13 C {1H} NMR (benzene-d6, 23
C, ppm) 174.63 (s, CC(CH3)3), 145.69 (s, Cipso), 139.25 (s, Cortho), 123.64 (s, Cmeta), 123.33 (s, Cpara), 90.25 (s, b-CH), 69.65 (s, OCH2CH2), 44.77 (s, C(CH3)3), 32.46 (s, CH(CH3)3), 28.63 (s, CH(CH3)2), 26.44 (s, CH(CH3)2), 24.52 (s, OCH2CH2), 23.46 (s, CH(CH3)2). Anal. Calc. for C78H122Ca2I2N4O4: C, 63.22; H, 8.30; N, 3.78. Found: C, 62.98; H, 8.15; N; 3.31%. The X-ray crystal structure was solved as 2 Æ (C6H14)1.0, but the crystals lose hexane rapidly at ambient temperature and the bulk material was isolated as hexane-free crystals (Anal. Calc. for C78H122Ca2I2N4O4 Æ C6H14: C, 64.35; H, 8.74; N, 3.57%).

H.M. El-Kaderi et al. / Polyhedron 25 (2006) 224–234 0

4.4. Preparation of [(LAr )Ba(l-I)]2 (3) In a fashion similar to the preparation of 2, treatment of 1 (0.553 g, 0.731 mmol) with BaI2 (0.288 g, 0.731 mmol) afforded 3 as yellow crystals (0.150 g, 27%): m.p. 310–314
C (dec); IR (Nujol, cm1) 1614 (w), 1544 (s), 1404 (s), 1260 (s), 1214 (w), 1139 (w), 1092 (s), 1020 (m), 872 (w), 798 (s); 1H NMR (benzene-d6, 23
C, d) 7.03–7.17 (m, 12H, aryl-H), 5.24 (s, 2H, b-CH), 2.99 (septet, J = 6.9 Hz, 8H, CH(CH3)2), 1.24 (d, J = 6.9 Hz, 24H, CH(CH3)2), 1.21 (d, J = 6.9 Hz, 24H, CH(CH3)2), 1.19 (s, 36 H, C(CH3)3); 13 C {1H} NMR (benzene-d6, 23
C, ppm) 169.77 (s, CC(CH3)3), 146.68 (s, Cipso), 137.67 (s, Cortho), 126.43 (s, Cmeta), 123.76 (s, Cpara), 94.55 (s, b-CH), 43.83 (s, C(CH3)3), 32.41 (s, C(CH3)3), 28.19 (s, CH(CH3)2), 24.47 (s, CH(CH3)2), 23.37 (s, CH(CH3)2). Anal. Calc. for C70H106Ba2I2N4: C, 54.88; H, 6.97; N, 3.66. Found: C, 53.86; H, 6.83; N, 3.54%. 4.5. Preparation of [Ca(LtBu)(l-I)(THF)]2 (4) A 100-mL Schlenk flask was charged with Ca(g5-LtBu)2 (0.40 g, 0.87 mmol), CaI2 (0.255 g, 0.87 mmol), a stir bar, and tetrahydrofuran (30 mL). The resultant solution was refluxed for 3 h, and then the volatile components were removed under reduced pressure. The residue was dissolved in hot toluene (40 mL) and the resultant solution was filtered through a 2-cm pad of Celite on a medium glass frit. The clear, colorless solution was then concentrated to a volume of 30 mL and was stored at 25
C for 7 days to afford 4 as a colorless solid (0.53 g, 68%): m.p. 202–204
C (dec); IR (Nujol, cm1) 1631 (w), 1548 (s), 1519 (m), 1496 (s), 1359 (s), 1256 (s), 1210 (s), 1093 (w), 1032 (s), 1002 (m), 975 (w), 917 (w), 879 (m), 799 (m), 725 (m); 1H NMR (toluene-d8, 23
C, d) 4.30 (s, 2H, b-CH), 3.85 (m, 8H, OCH2CH2), 2.05 (s, 12H, C-CH3), 1.47 (s, 8H + 36H, overlapped OCH2CH2 and C(CH3)3); 13C {1H} NMR (toluene-d8, 23
C, ppm) 161.18 (s, C-CH3), 90.07 (s, b-CH), 69.66 (OCH2CH2), 54.38 (s, C(CH3)3), 32.89 (s, C(CH3)3), 25.39 (OCH2CH2), 24.81 (s, C-CH3). NMR data are reported only for the major isomer (61% of total integration). In addition, Ca(g5-LtBu)2 (13% of total integration) and an unidentified species (26% of total integration) were observed. Anal. Calc. for C34H66Ca2I2N4O2: C, 45.53; H, 7.42; N, 6.25. Found: C, 46.18; H, 7.45; N, 5.88%. 4.6. Preparation of [Sr(LtBu)(l-I)(THF)]2 (5) In a fashion similar to the preparation of 4, treatment of Sr(g5-LtBu)2 (0.80 g, 1.58 mmol) with SrI2 (0.60 g, 1.58 mmol) in tetrahydrofuran afforded 5 as colorless crystals (0.83 g, 53%): m.p. 250–255
C (dec); IR (Nujol, cm1) 1630 (w), 1548 (m), 1459 (s), 1379 (s), 1358 (s),

233

1260 (s), 1210 (m), 1093 (m), 1025 (m), 878 (w), 800 (s); 1H NMR (toluene-d8, 23
C, d) 4.32 (s, 2H, b-CH), 3.69 (m, 8H, OCH2CH2), 2.05 (s, 12H, C-CH3), 1.46 (s, 8H + 36H, overlapped OCH2CH2 and C(CH3)3); 13 C {1H} NMR (toluene-d8, 23
C, ppm) 160.55 (s, C-CH3), 89.73 (s, b-CH), 68.74 (OCH2CH2), 54.49 (s, C(CH3)3), 32.98 (s, C(CH3)3), 25.60 (OCH2CH2), 24.79 (s, C-CH3). Anal. Calc. for C34H66Sr2I2N4O2: C, 41.17; H, 6.71; N, 5.65. Found: C, 39.79; H, 6.63; N, 5.63%. 4.7. Preparation of Ba4(LtBu)6(I)2 (6) In a fashion similar to the preparation of 4, treatment of Ba(g5-LtBu)2 (0.20 g, 0.359 mmol) with BaI2 (0.047 g, 0.112 mmol) in tetrahydrofuran afforded 6 as yellow crystals (0.06 g, 24%): m.p. 244–248
C (dec); IR (Nujol, cm1) 1529 (s), 1406 (s), 1248 (s), 1092 (w), 1023 (s), 996 (w), 963 (m), 799 (m), 773 (w), 751 (w), 693 (w); 1H NMR (toluene-d8, 23
C, d) 4.31 (broad s, 6H, b-CH), 1.99 (broad s, 36H, C-CH3), 1.39 (broad s, 108H, C(CH3)3); the low solubility in toluene-d8 precluded collection of the 13C {1H} NMR spectrum. Anal. Calc. for C78H150Ba4I2N12: C, 45.49; H, 7.34; N, 8.16. Found: C, 46.84; H, 7.36; N, 7.45%. 4.8. X-ray crystallographic structure determinations for 2–6 Diffraction data for 2–6 were collected on a Bruker P4/CCD diffractometer equipped with Mo radiation and a graphite monochromator. Complexes 2, 3, 5, and 6 were mounted in thin walled glass capillaries. Data collections were conducted at ambient temperature. The data collection for 4 was carried out at 61
C. In all cases, a sphere of data was measured at 10–20 s/frames and 0.2
between frames. The frame data were indexed and integrated with the manufacturer
s SMART, SAINT, and SADABS software [23]. The models were refined and reported using Sheldrick
s SHELX-97 software [24]. Complex 2 crystallized as yellow flat fragments. The hydrogen atoms were placed in calculated positions. One equivalent of hexane solvate was disordered near an inversion center and was modeled with partial atoms and refined isotropically. All other non-hydrogen atoms were refined anisotropically. There is a tetrahydrofuran ligand on the calcium atom that exhibits unreliable bond lengths and angles due to disorder. The overall molecule occupies a crystallographic inversion center. Complex 3 crystallized as yellow rods. Hydrogen atoms were placed in calculated positions. All nonhydrogen atoms were refined anisotropically. The molecule occupies a crystallographic inversion center with no solvent present. Complex 4 crystallized as colorless flat rods. The dimer occupies a crystallographic 2-fold rotation axis with

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both bridging iodine atoms on the 2-fold axis. Hydrogen atoms were placed in observed or calculated positions. Partial occupancy positions were used to describe the disorder in the tert-butyl group on C(10) and the disordered tetrahydrofuran carbon atoms. All non-hydrogen atoms were nevertheless refined anisotropically. Complex 5 crystallized as colorless cubes, is isostructural with 4, and shows similar disorder. Complex 6 crystallized as yellow rhomboids. The molecule is a tetramer occupying a crystallographic inversion center. Hydrogen atoms were placed in calculated positions, and all non-hydrogen atoms were refined anisotropically.

[4]

[5] [6]

5. Supplementary material Crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, quoting No. 267402 for 2, No. 267403 for 3, No. 267404 for 4, No. 267405 for 5, and No. 267406 for 6. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1233 336 033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).

Acknowledgments We thank the Army Research Office (grants no. DAAD19-01-1-0575 and W911NF-04-1-0332) for generous support of this research.

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