Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals

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Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements Olaf Michel,†,‡ H. Martin Dietrich,† Rannveig Litlabø,‡ Karl W. Törnroos,‡ Cac̈ ilia Maichle-Mössmer,† and Reiner Anwander*,† †

Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18A, D-72076 Tübingen, Germany Department of Chemistry, University of Bergen, Allégaten 41, N-5007 Bergen, Norway

‡

S Supporting Information *

ABSTRACT: A series of 3,5-substituted tris(pyrazolyl)borate (TpR,Me; R = Me, Ph, tBu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR2 (R = AlMe4, CH3) react with KTpR,Me to form the heteroleptic complexes (TpMe,Me)Mg(CH3), (TptBu,Me)Mg(CH3), and (TpMe,Me)Mg(AlMe4). The latter tetramethylaluminate complex can also be obtained by treatment of TpMe,Me Mg(CH 3 ) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C5Me5)Mg(Me)(thf)2 is synthesized from MeMgBr and 1 equiv of K(C5Me5). Equimolar reactions of the tetraethylaluminates [M(AlEt4)2]n of the heavier alkaline-earth metals calcium and barium with KTpR,Me give the homoleptic complexes of Ca(TpR,Ph)2 and Ba(TpR,Me)2. Heterotrimetallic [BaK(AlEt4)3]n is identified as a ligand rearrangement product and can be independently obtained by adding [K(AlEt4)]n to [Ba(AlEt4)2]n. Treatment of Ba[N(SiMe3)2]2(thf)2 with KTpMe,Me generates the heteroleptic complex (TpMe,Me)Ba[N(SiMe3)2](thf)2. All complexes are fully characterized including X-ray structure analyses.

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INTRODUCTION In 1929 when Wilhelm Schlenk and his son Wilhelm Jr. discovered the equilibrium between organomagnesium halide compounds RMgX and their corresponding homoleptic species2 RMgX ↔ R2Mg + MgX2the so-called “Schlenk equilibrium”,1 they could certainly not foresee its impact on and omnipresence in the organometallic chemistry of the alkalineearth and rare-earth elements. Schlenk-type rearrangement reactions depend on a multitude of factors such as the metal− ligand(R) bond (e.g., for the alkaline-earth metals it basically weakens with increasing metal size: Mg > Ca > Sr > Ba),2 the sterical bulk of the ligand R, the solvent, the temperature, and the concentration of the “Grignard”(-type) reagent. Particularly for catalytic reactions, suppression of the generalized Schlenk rearrangement (also known as ligand redistributions) is crucial, since the heteroleptic reagent LMR (L = ancillary ligand; R = active ligand) as a rule displays the active catalyst envisioned.3 Hanusa and co-workers have comprehensively studied the synthesis of kinetically stabilized heteroleptic alkaline-earth metal compounds by using bulky cyclopentadienyl (Cp) ligands.4 It was reported that the “encapsulating” cyclopentadienyl ligand 1,2,3,4-C5(iPr)4H (Cp4i) can stabilize (Cp4i)CaI(thf)n, while for Sr and Ba the same ligand engages in the Schlenk rearrangement, forming the homoleptic complexes (Cp4i)2M(thf)n (M = Sr, Ba). However, protected © 2012 American Chemical Society

mono(cyclopentadienyl) alkaline-earth metal complexes of Ca, Sr, and Ba are accessible by employing the sterically more demanding tris(trimethylsilyl)cyclopentadienyl (=Cp3Si) ligand, producing dimeric [(Cp3Si)MI(thf)n]2 (M = Ca, n = 1; M = Sr, n = 2) and a polymeric zigzag chain of [(Cp3Si)BaI(thf)2]n in the solid state.4a In addition, the heteroleptic phosphoniumbridged dienyl amide complexes Me2P(2-Me-4-tBu-C5H2)2Ba[N(SiMe3)2]5 and Me(tBu)P(C5Me4)2Ca[N(SiMe3)2]6 were reported. Substituted tris(pyrazolyl)borato ligands such as the monoanionic tris(3-R-5-R′-pyrazolyl)borate ligand set (TpR,R′, Trofimenko’s scorpionates) also display a unique ancillary ligand environment, particularly, the superbulky TptBu,Me ligand (cone angle: 243°).7 Chart 1 summarizes heteroleptic monoalkaline-earth metal Tp complexes, which were analyzed by X-ray crystallography.8−10 Clearly, organometallic derivatives are known only for the smallest alkaline-earth metal magnesium, and derivatives of type TpMgR (R = alkyl) have been studied in detail by Parkin et al. The Sella and Takats groups have recently described the synthesis of stable heteroleptic hydrocarbyl complexes of the divalent lanthanides Sm(II) and Yb(II), (TptBu,Me)Ln[CH(SiMe3)2] and (TptBu,Me)Received: January 16, 2012 Published: March 21, 2012 3119

dx.doi.org/10.1021/om300032c | Organometallics 2012, 31, 3119−3127

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Chart 1. X-ray Structurally Authenticated Heteroleptic Monoalkaline-Earth Metal (Mg, Ca, Ba) Tris(pyrazolyl)borate Complexes8−10

Scheme 1. Reaction Pathways for Compounds 1, 2, 3, 5, 8, and 9a

Ln(CH2SiMe3)(thf), employing heteroleptic (TptBu,Me)LnI(thf)n as precursors.11 Herein, we report on salt metathesis protocols utilizing alkylaluminate compounds Mg(AlMe4)2 and [M(AlEt4)2]n (M = Ca, Ba) or dimethyl magnesium and KTp. Especially for the heavier alkaline-earth metals Ca and Ba Schlenk equilibria prevail (producing unprecedented heterotrimetallic alkylaluminate [KBa(AlEt4)3]n), while derivatives (R′)Mg(TpR,Me) (R′ = Me, AlMe4) prove kinetically stable.

a

(a) Mg(AlMe4)2, KTpMe,Me, toluene, 2 h, ambient temperature; [MgMe2], KTpMe,Me, toluene, 16 h, ambient temperature; [MgMe2], KTptBu,Me, toluene, 16 h, ambient temperature; [Ba(AlEt4)2]n, KTpMe,Me, toluene, 2 h, ambient temperature; [Ca(AlEt4)2]n, KTpPh,Me, toluene, 2 h, ambient temperature; Ba[N(SiMe3)2]2(thf)2, KTpMe,Me, toluene, thf, 5 h, 100 °C.

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RESULTS AND DISCUSSION Having in mind the feasibility of heteroleptic organometallic tris(pyrazolyl)borate complexes of magnesium (Chart 1), we examined the reaction of Mg(AlMe4)2 with KTpMe,Me according to a salt metathesis protocol (Scheme 1).12 Reaction of the permethylated magnesium aluminate with 1 equiv of KTpMe,Me in toluene at ambient temperature straightforwardly yielded the heteroleptic complex (TpMe,Me)Mg(AlMe4) (1) (Scheme 1, reaction a). After centrifugation and separation from KAlMe4, the supernatant solution was concentrated under vacuum. Cubic single crystals of 1 could be harvested at −40 °C in good yield. The solid-state structure of 1 (Figure 1, Table 1) revealed two distinct molecules per asymmetric unit, with the tetramethylaluminato ligands bonded in either a η2 or a η3 fashion to the respective magnesium center. As a consequence, the Mg−C bond lengths range between 2.212(2) and 2.856(3) Å. The Mg···Al distance of 2.6752(9) Å of the η3-coordinated [AlMe4] unit is considerably shorter than that of the η2-coordinated [AlMe4], which is listed at 2.8988(9) Å. Five-coordinate [PhCH(Me)NCH2CH2N CHPh]Mg(AlMe4)(AlMe3) is the only other heteroleptic magnesium tetramethylaluminate complex which has been Xray structurally authenticated (M−Caluminate = 2.265(2) and 2.422(2) Å).13 Complex 1 shows only one set of NMR signals at ambient temperature for both the [TpMe,Me] and the [AlMe4] ligands, indicating fluxional behavior in solution.

(b) (c) (d) (e) (f)

Similarly, treatment of a suspension of MgMe2 in toluene with 1 equiv of KTpMe,Me at ambient temperature yielded (TpMe,Me)Mg(Me) (2) (Scheme 1, reaction b).8 Compound 2

Figure 1. Molecular structure of complex (TpMe,Me)Mg(AlMe4) (1) with two independent molecules in the asymmetric unit. Cocrystallized toluene (one molecule per asymmetric unit) is not shown. Non-hydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. 3120


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Table 1. Selected Bond Lengths and Angles for (TpMe,Me)Mg(AlMe4) (1) molecule 1 Mg1−N1 Mg1−N3 Mg1−N5 Mg1−C1 Mg1−C2 Mg1---Al1

N1−Mg1−N3 N1−Mg1−N5 N1−Mg1−C1 N1−Mg1−C2 N3−Mg1−C2 C1−Al1−C2 Mg1−C1−Al1 Mg1−C2−Al1

Table 2. Selected Bond Lengths and Angles for (TpMe,Me)Mg(Me) (2), (TptBu,Me)Mg(Me) (3), and (C5Me5)Mg(Me)(thf)2 (4)

molecule 2 Bond Lengths [Å] 2.079(2) Mg2−N7 2.183(2) Mg2−N9 2.089(2) Mg2−N11 2.212(2) Mg2−C27 2.646(2) Mg2−C40 2.8985(6) Mg2−C48 Mg2---Al2 Angles [deg] 87.10(4) N7−Mg2−N9 95.69(5) N7−Mg2−N11 132.79(6) N7−Mg2−C27 89.93(5) N7−Mg2−C40 142.15(3) N7−Mg2−C48 111.48(6) C27−Al2−C40 84.70(6) C27−Al2−C48 75.41(5) Mg2−C27−Al2 Mg2−C40−Al2 Mg2−C48−Al2

2

2.115(2) 2.112(2) 2.115(2) 2.714(2) 2.434(2) 2.853(2) 2.6755(6) 89.18(5) 91.05(5) 93.89(5) 166.16(5) 93.28(5) 104.7(1) 110.35(5) 67.00(5) 72.72(5) 64.11(5)

3

Mg1−N12 Mg1−N22 Mg1−C1

2.084(2) 2.084(4) 2.095(5)

N12−Mg1− N12′ N12−Mg1− N22

90.0(1)

N12−Mg1− C1

125.8(1)

N22−Mg1− C1

123.7(2)

90.4(1)

Bond Lengths [Å] Mg1−N1 2.129(1) Mg1−C1 2.119(3) Angles [deg] N1−Mg1− 91.51(6) N1′ N1−Mg1− 124.19(4) C1

4 Mg1−Ct Mg1−C1 Mg1−O1

2.145 2.197(4) 2.095(3)

Ct−Mg1− C1 O1− Mg1− O2 O1− Mg1− C1

126.11 90.1(1)

100.0(1)

resonances of the methyl ligands appear at −0.13 ppm (2) and 0.18 ppm (1) in the 1H NMR spectra (Figure S1; Supporting Information). The formation of 1 was also confirmed by the elemental analysis of the isolated compound. We could not observe the formation of putative (TptBu,Me)Mg(AlMe4) from the corresponding reaction of 3 with AlMe3. We ascribe this primarily to the sterical shielding of the tert-butyl substituents of this superbulky Tp ligand. Moreover, we targeted the transformation of complex 1 into 2 via ether cleavage.14 It is known that [Mg(AlMe4)2] gets partly cleaved by ethereal or N-donor molecules, forming [Mg(Me)(AlMe4)] species.14 Accordingly, exposure of 1 to a slight excess of Et2O and subsequent removal of volatile AlMe3·(Et2O)x gave a product showing a broad peak at 0.0 ppm with an integral of 8 in the 1H NMR spectrum. This is consistent with an incomplete alkylaluminate cleavage. Applying high vacuum for several hours yielded complex 2 upon removal of coordinated AlMe3 (Scheme 2) and undefined degradation products (1H NMR spectra are displayed in Figure S1; Supporting Information). This reaction behavior of the magnesium derivatives is reminiscent of the corresponding rareearth metal(III) chemistry15 and in contrast to that of the divalent (TptBu,Me)Yb(AlR4)2 (R = Me, Et) compounds, which did not indicate any alkylaluminate cleavage when exposed to Et2O or thf.16 In order to investigate any alkaline-earth metal size effect, barium tetraethylaluminate, [Ba(AlEt4)2]n, was treated with 1 equiv of the 3,5-methyl-substituted tris(pyrazolyl)borate salt

crystallized from a saturated toluene solution at −40 °C (orthorhombic space group Pnma, Figure 2, Table 2). In the solid state, the magnesium center adopts a distorted tetrahedral geometry, with the Tp ligand coordinating in a κ3(N,N,N) fashion (av Mg−N, 2.084 Å) and the methyl ligand exhibiting a Mg−C bond length of 2.097(4) Å. Complex (TptBu,Me)Mg(Me) (3) was obtained analogously from KTptBu,Me and MgMe2 (Scheme 1, reaction c). Suitable single crystals of 3 were obtained from a saturated toluene solution at ambient temperature in moderate yield (cubic space gorup I4̅3d, Figure 2, Table 2). Complex 3 shows the same structural features as complex 2, although the enhanced steric encumbrance causes slightly elongated Mg−C (2.119(3) Å) and Mg−N (2.129(1) Å) bond lengths (cf., Table 3).8 For comparison, the formally six-coordinate (C5Me5)Mg(Me)(thf)2 (4), obtained via reaction of MeMgBr with 1 equiv of K(C5Me5) in thf at ambient temperature, displays a Mg−C(CH3) bond length of 2.197(3) Å. Alternatively, complex (TpMe,Me)Mg(AlMe4) (1) could be obtained by addition of 1 equiv of AlMe3 to complex (TpMe,Me)Mg(Me) (2) (Scheme 2). The reaction was monitored by NMR spectroscopy, showing complete [Mg− (CH3)] → [Mg−(AlMe4)] transformation: the sharp singlet

Figure 2. Molecular structures of (TpMe,Me)Mg(Me) (2) (left), (TptBu,Me)Mg(Me) (3) (middle), and (C5Me5)Mg(Me)(thf)2 (4) (right). Nonhydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. 3121


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Table 3. Comparison of Mg−C and Mg−N Bond Lengths (Å) in Complexes TpR,R′Mg(alkyl) compound

Mg−C

Mg−N

ref

(TptBu,H)Mg(CH3) (PhTptBu,H)Mg(CH3) (TpMe,Me)Mg(CH3) (2) (TptBu,Me)Mg(CH3) (3) (PhTptBu,H)Mg(CH2CH3) (TpPh,H)Mg(CH2CH3)(THF) (TptBu,H)Mg[CH(CH3)2] (TpMe,Me)Mg[CH2Si(CH3)3] (TpMe,Me)Mg(AlMe4) (1)

2.118(11) 2.136(2) 2.095(5) 2.119(3) 2.163(2) 2.155(3) 2.182(8) 2.096(9) 2.212(2)−2.853(2)

2.13(1)−2.137(7) 2.103(1)−2.148(1) 2.084(2)−2.084(4) 2.129(1) 2.114(1)−2.170(1) 2.182(3)−2.286(1) 2.157(6)−2.170(4) 2.012(6)−2.119(6) 2.079(2)−2.183(2)

8a,d 8e this work this work 8e 8f 8b,d 8c,d this work

coordination mode,19 complex 6 features Ba and K centers that are six-coordinate by [AlEt4] ligands interconnecting in a μ4η1:η1:η1:η1 fashion. The average Ba−CH2 bond length of 3.173 Å in 6 is consistent with that in [Ba(AlEt4)2]n (Ba−C 3.125 Å), while the average K−CH2 bond length is 3.258 Å. The Ba− CH2−Al−CH2−K linkages build up fused eight-membered rings (Figure 3). Compound 6 was further analyzed by

Scheme 2. Reversible Formation of 1 by 2 via AlMe3 Addition and Ether Cleavage

KTpMe,Me in toluene (Scheme 1, reaction d). The colorless suspension was stirred for 2 h, resulting in a clear solution. Its volume was reduced, and single crystals of Ba(TpMe,Me)2 (5) could be grown at ambient temperature. Complex 5 was previously synthesized independently by Chisholm and Carmona according to a salt metathesis reaction utilizing BaI2 and 2 equiv of KTpMe,Me, and crystal structure reports were given.10b,17 Presumably because of the Schlenk equilibrium, we were not able to isolate heteroleptic complex (TpMe,Me)Ba(AlEt4) (Scheme 3).18 The supernatant solution of compound 5 was Scheme 3. Proposed Mechanism for the Formation of Compounds 5 and 6

Figure 3. Section of the polymeric network of [BaK(AlEt4)3]n (6). Non-hydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ba1−C11 3.134(3), Ba1−C13 3.211(3), K1−C15 3.240(5), K1−C17 3.275(3); C11− Al1−C13 106.5(1), C11−Al1−C15 108.9(1), C11−Al1−C17 108.4(1).

elemental analysis and NMR spectroscopy. The 1H NMR spectrum of 6 in C6D6 displays a triplet at 1.56 ppm and a quartet at −0.02 ppm for the CH3 and CH2 protons, respectively (Figure S2; Supporting Information; DRIFT spectra are displayed in Figure S3). With the solid-state structures of [Li(AlEt4)]n and [Na(AlEt4)]n already known, we also got interested in that of the potassium congener. [K(AlEt4)]n (7) could be easily obtained by an amide elimination reaction utilizing K[N(SiMe3)2] and AlEt3, while single crystals formed from a saturated toluene solution at ambient temperature (trigonal space group P3121). Similar to 6, complex 7 forms a polymeric network of [AlEt4] anions and potassium cations with all of the ethyl groups engaging in metal coordination, that is, the aluminato moieties displaying a μ4-η1:η1:η1:η1-coordination mode (Figure 4). Each potassium center is four-coordinate, and the K−CH2−Al linkages expand into eight-membered rings. In contrast to 7,

isolated and stored at ambient temperature for several days, producing another batch of morphologically different single crystals. X-ray diffraction analysis revealed a molecular composition of [BaK(AlEt4)3]n (6), a proposed mechanism of formation of which is shown in Scheme 3. In a control experiment, an equimolar mixture of [K(AlEt4)]n (7) and [Ba(AlEt4)2]n was reacted in toluene at ambient temperature, yielding compound 6 in quantitative yield. The solid-state structure of 6 revealed a polymeric network with each [AlEt4] anion being coordinated to two Ba and two K centers in a η1-coordination mode. In contrast to the previously presented structure of homoleptic [Ba(AlEt4)2]n, which shows six-coordinate Ba(II) centers and [AlEt4] units in a μ3-η1:η1:η13122


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Figure 4. Section of the polymeric network of [K(AlEt4)]n (7). Nonhydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): K1−C11 3.226(2), K1−C15 3.102(1); C11−K1−C11 89.51(3), C11−K1−C15 155.46(4), C15− K1−C15 96.81(4).

Figure 5. Molecular structure of Ca(TpPh,Me)2 (8). Non-hydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Ca1−N6 2.500(3), Ca1−N4 2.506(3), Ca1−N1 2.497(3); N6−Ca1−N4 83.03(11), N1−Ca1−N6 78.81(9), N1− Ca1−N4′ 103.96(4).

the solid-state structure of polymeric lithium tetraethylaluminate, [Li(AlEt4)]n, features a linear chain with μ2-η2:η2 [AlEt4] moieties interconnecting the lithium atoms,20 while the sodium tetraethylaluminate of [Na(AlEt4)]n is composed of a threedimensional network with two distinct Na sites, which are 4fold coordinated by the alkylaluminato moieties in a μ4η1:η1:η1:η1 fashion (monoclinic space group P21/c).21 The 1H NMR spectrum of 7 in C6D6 displays a CH2 signal shifted to higher field relative to 6 (−0.14 versus −0.02 ppm), while the CH3 signal appears at lower field (1.66 versus 1.56 ppm) (Figure S2; Supporting Information). In a further approach we used the sterically even more demanding tris(pyrazolyl)borate ligand KTpPh,Me, which comprises phenyl and methyl groups in 5- and 3-positions of the pyrazol ring, respectively, and calcium tetraethylaluminate, [Ca(AlEt4)2]n, featuring a smaller-sized metal center (Scheme 1, reaction e). All we could identify from the reaction mixture upon applying a routine workup procedure and crystallization was homoleptic Ca(TpPh,Me)2 (8), and not the putative (TpPh,Me)Ca(AlEt4). The molecular structure of 8 was unequivocally proven by X-ray diffraction analysis (monoclinic space group C2/c; Figure 5). The generation of a heteroleptic barium complex was finally successful when using Ba[N(SiMe3)2]2(thf)2 as a precursor instead of [Ba(AlEt4)2]n. Since a reaction between the barium silylamide and KTpMe,Me in toluene did not occur at ambient temperature, the reaction mixture was heated in a pressure tube to 100 °C (Scheme 1, reaction f). Subsequent filtration and addition of a few drops of thf were beneficial for the crystallization of (TpMe,Me)Ba[N(SiMe3)2](thf)2 (9) from a saturated solution at −40 °C. Complex 9 was analyzed by NMR and DRIFT spectroscopy and X-ray diffraction analysis (Figure 6, triclinic space group P1̅). Selected bond lengths and angles are listed in Table 4. Complex 9 is 6-fold coordinated, showing the favored κ3(N,N,N) coordination mode of the Tp ligand with an average Ba−N bond length of 2.806 Å. The coordination sphere of the barium center is completed by one silylamido ligand (Ba−N 2.645(5) Å) and two molecules of thf. The 1H NMR spectrum of 9 shows singlets at 0.42, 2.19, and 2.38 ppm with a ratio of 2:1:1, belonging to the methyl groups of the silylamido and the methyl substituents of the tris(pyrazolyl)borato ligand.

Figure 6. Molecular structure of (TpMe,Me)Ba[N(SiMe3)2](thf)2 (9). Non-hydrogen atoms are represented by atomic displacement ellipsoids at the 50% level. Hydrogen atoms are omitted for clarity.

Table 4. Selected Bond Lengths and Angles for (TpMe,Me)Ba[N(SiMe3)2](thf)2 (9) bond lengths [Å] Ba1−N12 Ba1−N22 Ba1−N32 Ba1−N10 Ba1−O301 Ba1−O401

2.789(5) 2.787(4) 2.839(4) 2.645(5) 2.773(4) 2.786(4)

angles [deg] N12−Ba1−N22 N12−Ba1−N32 N12−Ba1−N10 O301−Ba1−O401 Si10−N10−Si11 Ba1−N10−Si10 Ba1−N10−Si11

69.2(1) 70.8(1) 122.8(2) 129.7(2) 130.3(3) 116.3(2) 113.3(2)

For comparison, the calcium center in (TptBu,H)Ca[N(SiMe3)2](thf) is five-coordinate (Chart 1),10c while in (TpC*)Ca[N(SiMe3)2](thf), featuring hemilabile O-donor groups (Tp C * = tris[3-(2-methoxy-1,1-dimethylethyl)pyrazolyl]hydroborate), the alkaline-earth metal center is sixcoordinate due to one dangling pyrazolyl moiety.10e Other heteroleptic group 2 complexes containing the bis(trimethylsilyl)amido ligand were described by Hill et al. for formally four-coordinate LM[N(SiMe3)2](thf) (L = β-diketiminato; M = Ca, Sr, Ba),22 revealing a Ba−N(silylamide) bond distance of 2.593(2) Å, being slightly shorter compared to that 3123


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detected but not listed. 13C NMR (100 MHz, C6D6, 25 °C): δ 150.2 (3-pz-C), 145.0 (5-pz-C), 106.4 (4-pz-C), 13.7 (pz-CH3), 12.6 (pzCH3), −4.3 (Al-CH3) ppm; toluene signals of minor intensity were detected but not listed. Anal. Calcd for C19H34AlBMgN6·0.5C7H8 (454.68): C, 59.44; H, 8.42; N, 18.48. Found: C, 58.99; H, 8.06; N, 18.68. Alternative Synthesis of 1. To a stirred solution of complex 2 (85 mg, 0.25 mmol) in 5 mL of toluene was added 1.5 equiv of AlMe3 (27 mg, 0.38 mmol). The clear solution was stirred for 1 h and filtrated, and its solvent removed under vacuum until constant weight. A white powder of compound 1 was received in high yield (99%, 0.25 mmol). (TpMe,Me) Mg(Me) (2). A suspension of MgMe2 (30 mg, 0.55 mmol) in toluene was added to a suspension of KTpMe,Me (169 mg, 0.50 mmol) in toluene. The reaction mixture was allowed to stir for 16 h at ambient temperature. The supernatant solution was collected by centrifugation, and the solid residue was extracted with 3 × 3 mL of toluene. The combined toluene solutions were filtrated and concentrated under reduced pressure. Crystallization at −40 °C afforded single crystals of complex 2 (80%, 0.44 mmol). IR (KBr cm−1): ν 1974 m, 2953 m, 2922 m, 2859 m, 2814 w, 2780 w, 2524 m, 1541 vs, 1445 s, 1386 m, 1346 m, 1180 s, 1114 w, 1063 s, 979 m, 845 m, 792 s, 690 m, 648 s, 508 s. 1H NMR (400 MHz, C6D6, 25 °C): δ 5.61 (s, 3H, 4-pz-CH), 4.72 (br, 1 H, BH), 2.34 (s, 9H, pz-CH3), 2.26 (s, 9H, pz-CH3), −0.13 (s, 3H, Mg-CH3) ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ 150.7 (3-pz-C), 146.3 (5-pz-C), 105.5 (4-pz-C), 12.5 (pz-CH3), 12.9 (pz-CH3), −17.2 (Mg-CH3) ppm. Anal. Calcd for C16H25BMgN6 (336.53): C, 57.10; H, 7.49; N, 24.97. Found: C, 57.49; H, 7.25; N, 24.55. (TptBu,Me)Mg(Me) (3). A suspension of MgMe2 (50 mg, 0.92 mmol) in toluene was added to a suspension of KTptBu,Me (425 mg, 0.92 mmol). The reaction mixture was allowed to stir for 16 h at ambient temperature. The supernatant solution was extracted by centrifugation, and the solid residue was extracted with 3 × 3 mL of toluene. The combined extracts were filtrated and concentrated under reduced pressure. Crystallization at ambient temperature afforded single crystals of complex 3 (45%, 0.41 mmol). IR (KBr cm−1): ν 2953 s, 2904 s, 2866 m, 2798 w, 2566 m, 1536 vs, 1466 m, 1431 s, 1360 s, 1344 s, 1236 m, 1194 vs, 1131 m, 1074 m, 1018 m, 981 m, 791 s, 765 s, 653 m, 538 m. 1H NMR (400 MHz, C6D6, 25 °C): δ 5.78 (s, 3H, 4pz-CH), 4.82 (s, 1H, BH), 2.26 (s, 9H, pz-CH3), 1.58 (s, 27H, pztBu), 0.08 (s, 3H, Mg-CH3) ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ 164.3 (3-pz-C), 144.5 (4-pz-C), 102.9 (4-pz-C), 31.7 (pz-tBu), 30.8 (pz-tBu), 12.6 (pz-CH3), −4.2 (Mg-CH3) ppm. Anal. Calcd for C25H43BMgN6 (462.77): C, 64.89; H, 9.37; N, 18.16. Found: C, 63.96; H, 8.97; N, 17.82. (Cp*)Mg(Me)(thf)2 (4). To a stirred solution of KCp* (260 mg, 1.50 mmol) in thf was added 0.5 mL of a 3 M solution of MeMgBr (1.50 mmol) in Et2O. The solution was allowed to stir for 16 h at ambient temperature. The supernatant orange solution was extracted by centrifugation, and its volume reduced under vacuum. Fractional crystallization at −40 °C afforded needle-like single crystals of complex 4 (60%, 0.90 mmol). IR (KBr cm−1): ν 2981 s, 2.957 s, 2.904 vs, 2850 vs, 2719 vw, 1449 m, 1248 w, 1182 m, 1030 s, 918 w, 871 s, 798 vw, 674 vw, 571 m, 491 m, 414 m. 1H NMR (400 MHz, C6D6, 25 °C): δ 3.50 (t, 8H, thf), 2.15 (s, 12H, Cp-CH3), 1.93 (s, 3H, Cp-CH3), 1.34 (t, 8H, thf), −1.28 (br, 3H, Mg-CH3) ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ 115.8 (C5Me5), 110.9 (C5Me5), 109.2 (C5Me5), 69.1 (thf), 25.8 (thf), 11.5 (Cp-CH3), 9.8 (Cp-CH3), −14.7 (Mg-CH3) ppm. Anal. Calcd for C19H34MgO2 (318.77): C, 71.59; H, 10.75. Found: C, 70.64; H, 10.63. Ba(TpMe,Me)2 (5). To a suspension of KTpMe,Me (75 mg, 0.22 mmol) in toluene was added a toluene solution of [Ba(AlEt4)2]n (95 mg, 0.22 mmol), and the reaction mixture stirred for 2 h at ambient temperature. The resulting clear solution was filtrated and concentrated under reduced pressure. Single crystals of compound 5 (32%, 0.07 mmol) suitable for X-ray diffraction analysis were grown from a saturated toluene solution at ambient temperature. IR (KBr cm−1): ν 3199 w, 2953 m, 2923 m, 2857 m, 2373 vw, 2537 w, 1541 vs, 1484 m, 1433 s, 1417 s, 1370 m, 1342 s, 1182 vs, 1076 m, 1032 ms, 981 m, 801 s, 700 m, 650 s. 1H NMR (400 MHz, C6D6, 25 °C): δ 5.83

in compound 9. Attempts to displace the silylamido ligand in 9 with tetraethylaluminate were not successful. Treatment of complex 9 with AlEt3 (>2 equiv) resulted in a complicated reaction mixture, escaping any conclusive identification.

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CONCLUSIONS Alkyl complexes of the small alkaline-earth metal magnesium such as [MgMe2] and Mg(AlMe4)2 can be readily employed according to salt metathesis protocols along with tris(pyrazolyl)borate potassium derivatives to access heteroleptic complexes of type (TpR,Me)Mg(R′) (R′ = Me, AlMe4). The interconversion of (Tp)Mg(Me) and (Tp)Mg(AlMe4) via addition of AlMe3 and donor-induced aluminate cleavage, respectively, is strongly dependent on the steric bulk of the Tp ancillary ligand. For example, alkylaluminate formation was not observed for (TptBu,Me)Mg(Me). Independently synthesized (C5Me5)Mg(Me)(thf)2, bearing two thf ligands, nicely underlines the increased steric bulk of Trofimenko's scorpionate ligands. The reactions of [Ca(AlEt4)2]n or [Ba(AlEt4)2]n with KTp proceed via Schlenk-type rearrangements and formation of the homoleptic complexes Ba(TpMe,Me)2 and Ca(TpPh,Me)2. Heterotrimetallic [BaK(AlEt 4) 3]n , representing the first MIMIIMIII−alkyl complex, was identified as the second rearrangement product of the latter barium reaction. The successful synthesis of heteroleptic (TpMe,Me)Ba[N(SiMe3)2] via a similar salt metathesis reaction employing Ba[N(SiMe3)2]2(thf)2 with KTpMe,Me might be enforced by the presence of either donor solvent molecules or the silylamido ligand.

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EXPERIMENTAL SECTION

General Considerations. All operations were performed with rigorous exclusion of water and air using standard Schlenk, highvacuum, and glovebox techniques (MBraun MB250B;

Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals

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