Cooperative Ge–N Bond Activation in Hydrogallation Products of

Article pubs.acs.org/Organometallics

Cooperative Ge−N Bond Activation in Hydrogallation Products of Alkynyl(diethylamino)germanes (Et2N)nGe(CCtBu)4−n

Werner Uhl,*,† Jens Tannert,† Marcus Layh,† Alexander Hepp,† Stefan Grimme,‡ and Tobias Risthaus‡,§ †

Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, D-48149 Münster, Germany Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstraße 4, D-53115 Bonn, Germany § International NRW Graduate School of Chemistry, Wilhelm-Klemm-Straße 10, D-48149 Münster, Germany ‡

S Supporting Information *

ABSTRACT: Treatment of the alkynyl(diethylamino)germanes Et2NGe(CCtBu)3 (1) and (Et2N)2Ge(CCtBu)2 (2) with dialkylelement hydrides tBu2MH (M = Al, Ga) afforded in high yields the hydrometalation products (tBuCC)2(Et2N)Ge[C(MtBu2)C(H)tBu] (3), (tBuCC)(Et2N)Ge[C(MtBu2)C(H)tBu]2 (4) and (tBuCC)(Et2N)2Ge[C(GatBu2)C(H)tBu] (6). The Lewis acidic aluminum and gallium atoms showed a close contact to the nitrogen atoms of the amino groups attached to germanium, which resulted in relatively long Ge−N bonds and short Al−N or Ga−N distances. The structures of these molecules and the strengths of the interactions were investigated by dispersion-corrected density functional theory. This activation of the Ge−N bonds caused an unprecedented reactivity of compounds 4b and 6. 4b reacted with PhCCH under mild conditions and elimination of HNEt2 to give the mixed dialkynyl compound (tBuCC)(PhCC)Ge[C(GatBu2)C(H)tBu]2 (5), while facile insertion of RNCX into a Ge−N bond of 6 led to the formation of the six-membered Ge−C−Ga−X−C−N heterocycles 7 (R = Ph, Et; X = O, S).

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INTRODUCTION The activation of molecular substrates by suitably functionalized main-group-element compounds is in the focus of current research activities. Particularly encouraging for a wider application are results obtained with frustrated Lewis pairs (FLPs), in which the expected formation of the Lewis adduct by combination of a Lewis acidic and a Lewis basic center is prevented or at least partially hindered due to steric reasons.1 Due to their specific functionality, these compounds are extraordinarily well suited for the effective dipolar activation of small molecules such as H2, NO, N2O, CO2, etc.1 They are therefore thought to be a viable alternative or addition to traditional transitionmetal-based catalysts.1 Initial research has focused mainly on bimolecular ArF3B/PR3 or comparable monomolecular systems, but recently the inherently more Lewis acidic heavier group 13 elements Al and, to a lesser extent, Ga were also investigated and found to be promising candidates for the generation of efficient FLPs. The Al/P-based FLP Ph(H)CC(AltBu2)PMes2 and bimolecular AlX3/PR3 systems were shown to activate or coordinate terminal alkynes,2,3 CO2,2,4 and alkali-metal hydrides5 or to catalyze the dehydrogenation of amine− boranes,6,7 while Al−N-based systems were found to activate PhCCH8 and (c-hex)NCN(c-hex)8 and to catalyze © 2013 American Chemical Society

the oligomerization of cyanamides with the formation of acyclic oligomers.9 In recent studies we investigated the generation and chemical reactivity of functionalized silicon and germanium compounds.10−12 Hydrometalation of alkynylsilanes and -germanes carrying one to four alkynyl substituents with aluminum or gallium hydrides yielded the corresponding hydrometalation products with up to three metal atoms by varying the stoichiometric ratio of the starting materials.10−12 Products with free alkyne substituents obtained by incomplete hydrometalation of the oligoalkynes showed a remarkable interaction of the α-C atoms of the remaining alkynes, bearing a relatively high partially negative charge with the coordinatively unsaturated, Lewis acidic Al or Ga atoms, as was evident from X-ray structure analysis and a lowering of the stretching vibration νCC in the IR spectrum.10,11 The close proximity of α-C and metal atoms resulted in an activation of the corresponding Si−C and Ge−C bonds, which facilitated the Special Issue: Applications of Electrophilic Main Group Organometallic Molecules Received: June 13, 2013 Published: July 26, 2013 6770

dx.doi.org/10.1021/om400543v | Organometallics 2013, 32, 6770−6779

Organometallics

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synthesis of sila- and germacyclobutenes10,11 and of a unique spirogermane12 by thermally induced rearrangement and 1,1organometalation. The germacyclobutene derivatives with exocyclic coordinatively unsaturated dialkylaluminum or -gallium groups show promising photophysical properties.13 Bismetalated derivatives were found to be excellent chelating Lewis acids.11 In a continuation of this work we tried to generate amino-substituted alkynylgermanes. Hydrometalation may afford highly functionalized molecules with Lewis acidic metal (Al, Ga) and Lewis basic nitrogen atoms in close proximity. The lone pair at the nitrogen atoms may favor interactions between the nitrogen and aluminum or gallium atoms stronger than those observed between the metal and the α-carbon atoms of the alkynes. This may considerably activate the Ge−N bonds and facilitate unusual insertion reactions. Taking this concept to the extreme, cleavage of the Ge−N bond may result in the formation of germyl cations, examples of which have so far been isolated only in very few instances.14 In this paper we present the synthesis of a series of dialkylaminogermylalkynes (Et2N)nGe(CCBut)4−n (n = 1, 2) and their hydrometalation with 1 (for n = 1, 2) or 2 equiv (for n = 1) of t Bu2AlH or tBu2GaH. The hydrometalation products were found to show an unprecedented reactivity toward PhCCH and RNCX.

The most interesting features of the molecular structures of 3a,b and 4b are the strong intramolecular metal−nitrogen interactions between the geminally arranged tBu2M substituents and the Ge atoms, which resulted in GeCAlN and GeCGaN heterocycles (Figures 1 and 2). These heterocycles are essentially

Figure 1. Molecular structure and atomic numbering scheme of compound 3b. Displacement ellipsoids are drawn at the 40% level. The incorporated solvent molecule C6F5H and hydrogen atoms (except H32, arbitrary radius) have been omitted for clarity. An isotypic structure was found for 3a.

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RESULTS AND DISCUSSION The alkynyl(diethylamino)germanes Et2NGe(CCtBu)3 (1) and (Et2N)2Ge(CCtBu)2 (2) were obtained conveniently in high yields by treating GeCl4 first with 1 or 2 equiv of LiNEt2 followed by consecutive addition of stoichiometric quantities of the in situ prepared lithium tert-butylalkynide. Subsequent reaction of the (diethylamino)tris(alkynyl)germane 1 with 1 or 2 equiv of the metal hydrides tBu2MH (M = Al, Ga) yielded the respective cis-addition products 3 and 4 (eq 1) regio- and

Figure 2. Molecular structure and atomic numbering scheme of compound 4b. Displacement ellipsoids are drawn at the 40% level. Methyl groups (GatBu) and hydrogen atoms (except H12 and H32; arbitrary radius) have been omitted for clarity.

planar with a maximum deviation from the average plane of only about 0.1 Å. The endocyclic Ge−N−M and N−Ge−C angles (Table 1) deviate only slightly from 90°; smaller angles are observed at the metal atoms (M = Al, Ga; C−M−N = 82.0° (average)) and larger angles at carbon (96.4° (average)). The Ge−N distances (1.961 Å (average)) are slightly longer than those usually observed for tetracoordinate Ge atoms.16 The Al−N distance of 3a is shorter than the Ga−N distances of 3b and 4b (2.104(1) vs 2.236 Å (average)); all values are in the upper range of dative M−N bonds.8,16 The difference may essentially be attributed to the higher Lewis acidity of Al and

stereoselectively in good yields (cis is defined with respect to the relative orientation of Al or Ga to H). The observed regiospecificity is a result of the higher negative partial charge on the Ge-bound C atom of the triple bond due to the large difference in electronegativity between Ge and the sp-hybridized C atom that strongly favors the addition of the electropositive metal atom on this side. This effect is well documented for the addition of group 13 metal hydrides to alkynylsilanes and -germanes.15 6771

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Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds 3a,b, 4b, 5, and 6a 3a param

4b

5

6

QC

exptl

QC

exptl

QC

exptl

QC

exptl

QC

b

M− Bu (av) M−C(C)b

2.023(1) 2.003(1)

2.036 2.004

2.029(3) 2.016(3)

2.054 2.024

2.034 2.031 2.021

2.032(2) 2.015(2)

2.055 2.028

2.104(1)

2.095

2.246(3)

2.257

2.052 2.025 2.031 2.196 2.582

2.006(3) 2.012(3) 1.994(3)

M−Nb Ga···C(C)

2.033(4) 2.014(2) 2.022(2) 2.226(2) 2.81

2.213(2)

2.209

2.74 2.74

2.514 2.558

Ge−N(bridge) Ge−C(C)

1.968(1) 1.889(1)

1.992 1.930

1.943(2) 1.917(3)

1.957 1.928

1.946 1.952

1.968(2) 1.917(2)

1.982 1.925

83.83(4) 89.96(4) 89.90(4) 94.52(5)

85.19 89.43 90.05 93.91

81.0(1) 88.74(9) 91.9(1) 96.9(2)

81.24 88.38 91.93 96.32

1.997 1.946 1.957 81.81 90.62 89.31 97.43 103.48

1.934(3) 1.949(3)

C−M−N Ge−N−M N−Ge−C Ge−C−M

1.972(2) 1.937(2) 1.952(2) 81.22(8) 90.17(8) 90.02(9) 97.8(1) 109.2(1)

82.06(7) 89.18(7) 91.38(8) 96.77(9)

82.14 89.43 90.97 96.63

t

a

3b

exptl

106.0(1) 106.8(1)

102.45 100.45

QC denotes quantum chemical calculations; TPSS-D3/def2-TZVP. bM = Al (3a) or Ga (3b, 4b, 5, 6).

not the size of the metal atoms, as is evident from the very similar Al−C and Ga−C distances (Table 1). Compound 4b (Figure 2) shows a significant interaction between the Ga and Cα atoms of the CC triple bond (Ga···Cα = 2.81 Å, Table 1), leading to the formation of an approximately planar GeCGaC heterocycle with a spiro configuration at the Ge atom and interplanar angles between the two heterocycles of 88°. The Ga···Cα distance is slightly longer than that observed in the related alkynylgermanes R2(R′CC)Ge[C(GaBut2)C(H)R′] (2.61−2.73 Å)10−12 and has little influence on the linearity of the alkynyl moieties or the CC distances, which do not significantly deviate from values in free alkynes or alkynides. In all compounds the metal atom is cis to the vinylic hydrogen atom, a result of the concerted nature of the addition process.17 A rearrangement to the thermodynamically favored trans isomer was not observed, probably due to the coordinative saturation of the group 13 metal atom by interaction with the NEt2 or an adjacent alkynide substituent (3) that may prevent isomerization via a bimolecular process.15 The third alkynido group was not accessible to hydrometalation, and an excess of the hydride resulted only in decomposition with a hydride− amine exchange. Local potential energy minimum structures (not shown) for the products of triple addition have been found in silico at the TPSS-D3/TZ level of theory; therefore, the assumption that the third addition is kinetically hindered is reasonable. We hypothesized that the interaction of the Lewis acidic Al or Ga atoms with the N atoms of the amino groups and the formation of Ga−N−Ge bridges could result in an activation of the Ge−N bond. We therefore treated compound 4b in a preliminary experiment with the terminal alkyne PhCCH, which has been used previously to verify the reactivity of frustrated Lewis pairs by C−H bond activation.1−3 At 60 °C we observed cleavage of the Ge−N bond by release of diethylamine and formation of the heteroleptic alkynide 5 that features two different alkynide substituents at Ge (eq 2, Figure 3). The more basic Et2N− anion is thereby substituted by the less basic PhCC− anion. Despite the different ethynyl substituents, the bonding parameters of the GeCCR groups are almost identical. Both Ga atoms show an interaction to an α-C atom of an ethynyl group with Ga−C distances of 2.74 Å. They are 0.26 and 0.29 Å above the plane of the three directly bound C atoms, and two planar GaC2Ge heterocycles result with a

Figure 3. Molecular structure and atomic numbering scheme of compound 5. Displacement ellipsoids are drawn at the 40% level. Methyl groups (GatBu) and hydrogen atoms (except H12 and H32; arbitrary radius) are omitted for clarity.

maximum deviation of an atom from the average plane of 0.08 Å (torsion angles C−Ge−C−Ga in the rings of 8.6(2) and 9.5(2)°). 6772

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No reaction was observed when the tris(alkynide) 1 was stirred with PhCCH at room temperature or heated for more than 30 h at 80 °C. It has been reported that the related reaction of (R2N)nGeR′4−n with alkyl- or arylacetylenes R″CCH to yield R′4−nGe(CCR″)n (n = 0, 1) required stoichiometric amounts of Lewis acidic zinc dihalides for activation.18 The unique reaction and the unexpectedly high reactivity of 4b is a clear indication of the cooperative activation of the Ge−N bond by the Lewis acidic GatBu2 group. Despite the prolonged heating of the reactants at 60 °C during the synthesis of compound 5, no indication for a cis/trans isomerization of the vinyl groups was found. Density functional theory calculations at the PW6B95-D3/QZ//TPSS-D3/TZ level with rovibrational and solvent corrections confirmed that the formation of alkynide 5 and diethylamine is favored by a ΔG(298 K)solv of −10 kcal/mol over 4b and phenylethyne. A geometry optimization starting with PhCCH reasonably far from the GatBu2 residue (alkyne-H−Ga distance = 2.6 Å) immediately yielded a new bond between the Ga atom and the terminal C atom of the alkyne. In this species, the alkyne hydrogen was oriented toward the amine group. A similar situation has been observed in the molecular structure of a phenylethyne adduct of an Al−N-based Lewis pair.8 Since no transition state could be located for the postulated proton transfer, we believe this barrier to be very small and the final elimination/substitution step affording the transfer of the phenylalkynide to the Ge center to be the rate-determining step of this reaction. The bis(alkynide) (Et2N)2Ge(CCtBu)2 (2) behaved similarly to the tris(alkynide) 1 and yielded after treatment with 1 equiv of tBu2GaH in a straightforward reaction the monohydrogallation product 6 (eq 3). Treatment of 2 with 2 equiv of

Figure 4. Molecular structure and atomic numbering scheme of compound 6. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12; arbitrary radius) have been omitted for clarity.

is perpendicular to the Ge(1)−N(1) bond (torsion angles C−N(2)−Ge(1)−N(1) of −94.4(2) and 93.8(2)°). This observation may indicate some hyperconjugation with a strengthening of the Ge(1)−N(2) and a further weakening of the Ge(1)−N(1) bond. The widely differing Ge−N distances with a surprisingly short Ge−N bond to the terminal amino group seem to support a further activation by hyperconjugation of the Ge−N bond in the GeCGaN heterocycle. We therefore treated 6 with heterocumulenes RNCX (R = Ph, Et; X = O, S). With phenyl isocyanate the selective insertion of the heteroallene into a Ge−N bond was observed within a few minutes at room temperature. The reaction of PhNCS under these conditions was slow and required for completion heating to 90 °C for 18 h. The simultaneous breakage of the Ga−N and formation of new Ga−X bonds afforded the six-membered GeCGaXCN heterocycles 7 in high yields (>75%, eq 3, for 7c: NMR experiment only). Similar insertion reactions of heteroallenes such as CS2, CH2CS, and PhNCX into GeIV−P or GeIV−N bonds have previously been observed for heterocyclic compounds such as germaphospholanes,19 2-germa1,3-diazolidines,20 and 2-germa-1,3-oxazolidines.21 Ring expansion resulted in the formation of seven-membered heterocycles such as Me2Gea N(Ph)C(S)P(Ph)(CH2)2CbH2(Gea−Cb) and Et2GeaN(Ph)C(O)N(Me)(CH2)2Ob(Gea−Ob). The noncyclic trialkylaminogermanes R3GeNR2 were found to give analogous insertion reactions only under more drastic reaction conditions in the absence of a solvent and at elevated temperature.22 More recent papers have focused on GeII or SnII compounds. E[N(SiMe3)2]2 (E = Ge, Sn) was described to undergo metathetical exchange with CO2,23 while CX2 (X = O, S) reacted with (Me2N)2Sn to give [Me2NCX2]2Sn24 and Sn[Ph2PNPPh2]2 reversibly yielded a CO2 adduct.25 The insertion products 7a,b feature in their backbone unprecedented six-membered GeCGaXCN (X = O, S) heterocycles in twist and twist-boat conformations, respectively (Figures 5 and 6), as is evident from the sequence of torsion angles in the ring (cf. ref 26). Bond lengths and endocyclic angles of the heterocycles are very similar (Table 2), with the exception of those involving the chalcogen atom. Ga−S and C−S distances are, not surprisingly, much longer than their respective oxygen

the hydride did not yield the product of dual hydrometalation but resulted in partial decomposition by amine−hydride exchange. Reaction with the aluminum hydride tBu2Al−H in an equimolar ratio gave a mixture of unknown products. Compound 6 has a planar GeCGaN heterocycle with a NEt2 group in a bridging position between the Ge and Ga atoms and a second terminal NEt2 substituent (Figure 4). The Ge−N distances differ considerably. A relatively long bond length to the bridging nitrogen atom (1.968 Å) was observed. It is similar to the values of 3a,b and 4b and slightly longer than those usually observed for tetracoordinate germanium atoms.16 In contrast, the Ge(1)−N(2) distance to the terminal amino group is very short (1.810(2) Å). N(2) has a planar coordination sphere (sum of the angles 359.5°) and the resulting plane 6773

dx.doi.org/10.1021/om400543v | Organometallics 2013, 32, 6770−6779

Organometallics

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds 7a,ba 7a

Figure 5. Molecular structure and atomic numbering scheme of compound 7a. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12; arbitrary radius) have been omitted for clarity.

a b

7b

param

exptl

QC

exptl

QC

Ga−tBu (av) Ga−C(C) Ga−Xb Ge−N(ring) Ge−N(term) Ge−C(C) C2−N(ring) C2−N(term) C2−Xb C−Ga−Xb Ge−N−C N(ring)−Ge−C Ge−C−Ga N(ring)−C−Xb C−X−Gab

2.012(1) 1.996(1) 2.0018(9) 1.939(1) 1.807(1) 1.923(1) 1.371(2) 1.345(2) 1.263(1) 97.21(5) 117.50(8) 107.63(5) 114.45(6) 121.31(1) 133.21(8)

2.035 2.019 2.005 1.975 1.840 1.925 1.364 1.369 1.272 97.49 123.34 107.84 116.59 121.71 134.17

2.025(1) 2.000(1) 2.4197(3) 1.9669(9) 1.8021(9) 1.925(1) 1.358(2) 1.349(1) 1.719(1) 97.19(3) 120.35(7) 111.08(4) 116.04(5) 120.83(9) 110.57(4)

2.044 2.026 2.420 1.985 1.844 1.928 1.361 1.365 1.713 97.37 122.90 109.08 119.21 124.07 111.17

QC denotes quantum chemical calculations; TPSS-D3/def2-TZVP. X = O (7a), S (7b).

The chemical shifts of the different tert-butyl groups differ characteristically in the 1H NMR spectrum with MtBu (δ 1.30− 1.58) found downfield in comparison to CCtBu (δ 1.07− 1.39) and CCtBu (δ = 0.93−1.21), this sequence being observed strictly for all compounds except 4a. A similar sequence is found in the 13C NMR spectrum for the quaternary C atom of the tBu groups with CCCMe3 (δ 37.9−39.4) shifted downfield in comparison to CCCMe3 (δ 28.2−29.6) and MCMe3 (δ 17.8−30.8), respectively. The strong polarization of the alkynides that has been mentioned above is evident in the difference ΔδGeC − δCC > 37 of the carbon atoms constituting the triple bond. Large Δ values are typical for Siand Ge-centered alkynes and were linked to the polarity of the triple bond29 and more recently to the reactivity of alkynylgermanes similar to 3 and 4 in intramolecular cyclization reactions.11,12 While the Δ value generally is no unambiguous indicator if an alkynyl group is not coordinated (“free”) to gallium (1−3, 6, 7) or interacts with a Lewis acidic metal center (4, 5), there is at least for this series of closely related compounds a clearly visible trend for the tBuCC substituent. The chemical shift of the Cα atom (δ ca. 84) in metalcoordinated CCtBu groups is found about 5 ppm downfield in comparison to a “free” alkynide (δ ca. 78). A more widely applicable indicator is the stretching frequency νCC in the IR spectrum, which is observed at ca. 50 cm−1 lower wavenumber than in the “free” alkynides.10−12 DFT calculations at the rovibrationally and solvent-corrected PW6B95-D3/QZ//TPSSD3/TZ level confirm the hypothesis that compounds 3 are essentially “free” alkynes in solution that are not coordinated with their α-carbon atoms to Al or Ga atoms. The ΔG(298 K)solv value between an alkyne-coordinated and an amine-coordinated species is always in favor of the latter (9.0 for 3a, 1.0 for 3b, 18.8 for 4a, 11.7 for 4b; in kcal/mol). These values reflect the different dynamic behaviors of compounds 4a (Al) and 4b (Ga) in solution with a relatively high activation barrier of the exchange of the amino groups in compound 4a (see below). The 1H and 13C NMR spectra of 1 and 2 and the monoaddition products 3 are comparatively simple with magnetically equivalent NEt2 (except NCH2 of 3a), MtBu2, and CCtBu groups. In contrast, compounds 6 and especially 4, 5, and 7 give

Figure 6. Molecular structure and atomic numbering scheme of compound 7b. Displacement ellipsoids are drawn at the 40% level. Hydrogen atoms (except H12; arbitrary radius) have been omitted for clarity.

counterparts (Table 2). The much larger C−O−Ga angle (133.21(8)°) in comparison to the analogous C−S−Ga angle (110.57(4)°) may reflect the larger covalent radius of sulfur and the higher degree of ionic bonding in the Ga−O contact. The bond lengths of the urea fragment in 7a are indicative of a significant amount of delocalization of electron density among N1, C2, N2, and O1, as is typical in this class of compounds.27 The relevant bond lengths of compound 7a fit well into the data obtained from a statistical analysis of CSD structures that shows an inverse correlation between the lengths of the C−O and C−N bonds and observes a shortening of C−N(alkyl) in comparison to C−N(aryl) distances due to the greater inductive effect of the alkyl group.27 The C−S distance in 7b (X = S; 1.72 Å) approaches the standard bond lengths of C(sp2)−S single bonds (1.75 Å).28 The NMR spectroscopic characterization of compounds 1−7 shows the following trends. The vinylic protons are found in the 1H NMR spectrum in the typical region between δ 7.4 and 7.2. 6774

dx.doi.org/10.1021/om400543v | Organometallics 2013, 32, 6770−6779

Organometallics

Article

NMR spectra of high complexity with all individual tBu and Et(N) groups becoming inequivalent as a result of the chirality of the Ge atoms and hindered or at least partially hindered rotation at the virtually spiro-configured Ge atoms. The influence of the metal atom is apparent from a comparison of compounds 4a,b. The NEt2 group of 4a (M = Al) shows at room temperature two triplets for the NCH2Me protons and four doublets of quartets for each of the NCH2 protons in the 1 H NMR spectrum, while the analogous Ga derivative 4b is dynamic at this temperature and shows only one triplet and two broad singlets, respectively, which resolve only at 220 K. On the basis of a coalescence temperature of 290 K for the NCH2 protons a free activation enthalpy ΔG⧧ of 63.8 kJ/mol was estimated. The difference between Ga and Al may be attributed to the higher Lewis acidity of Al and the resulting stronger and shorter contacts between Al and N or CαC, respectively (cf. structure discussion and refs 10−12). Molecular structures and Wiberg bond indices (WBIs; a KS wave function based measure for the strength of an interaction where 0.9 roughly corresponds to a C−C bond, 1.8 to a CC bond, and 2.1 to a CC bond in the TZ basis set used here) were obtained at the TPSS-D3/TZ level. The WBIs show a much stronger M···N or M···C interaction for the aluminum derivatives than for the gallium compounds. The WBI of the Al−N interaction is 0.78, while the corresponding WBI of the Ga−N interaction is only 0.49. A similar trend is observed for the Al/Ga−Cα-alkyne interactions (0.42 vs 0.16). The same trend is observed when comparing the calculated ΔG(298 K)solv values mentioned above, which show larger interaction differences for the Al derivatives. The unique coordination of the bridging amino group by Al or Ga atoms results in a concomitant weakening and activation of the corresponding Ge−N bond (WBI: tris(alkyne) 1, 1.08; hydrometalation products 3a,b, 0.68 and 0.79; bis(alkyne) 2, 1.06 and 1.03; hydrogallation product 6, 1.01 (terminal) and 0.83 (bridging); 1.02 vs 0.71 of the unisolated aluminum species). The mass spectra of all compounds are typically characterized by fragments corresponding to [M − tBu]+ and [M − NEt2]+.

Et2O and toluene with Na; pentafluorobenzene with molecular sieves). NMR spectra were recorded in C6D6 or C7D8 at ambient probe temperature (except compound 4b) using following Bruker instruments Avance I (1H, 400.13; 13C, 100.62 MHz) and Avance III (1H, 400.03; 13C, 100.59 MHz) and referenced internally to residual solvent resonances (chemical shift data in δ). 13C NMR spectra were all proton decoupled. IR spectra were recorded as Nujol mulls between CsI plates on a Shimadzu Prestige 21 spectrometer. (Me3C)2AlH30 and (Me3C)2GaH30 were obtained according to literature procedures. The assignment of NMR spectra is based on HMBC, H,H-ROESY, HSQC, and DEPT135 data. Synthesis of Et2NGeCl3. This compound was obtained previously by another route.31 LiNEt2 was synthesized in situ by adding a solution of n-BuLi (30 mL, 48.0 mmol, 1.6 M in n-hexane) at −30 °C dropwise to a solution of HNEt2 (3.53 g, 48.0 mmol) in Et2O (50 mL) and stirring the reaction mixture for 1 h at the same temperature. The mixture was warmed to room temperature and stirred for 30 min. The solution was then added at −78 °C to a solution of GeCl4 (10.25 g, 47.8 mmol) in Et2O (50 mL). The reaction mixture was slowly warmed to room temperature and stirred for 5 h. The solvent was removed in vacuo and the product isolated from the residue by vacuum distillation (80 °C, 10−3 mbar), yielding a colorless, mobile liquid (10.37 g, 86%). 1H NMR (C6D6, 300 K): δ 0.82 (t, 3JHH = 7.1 Hz, 6 H, NCH2Me), 2.82 (q, 3JHH = 7.1 Hz, 4H, NCH2) ppm. Synthesis of (Et2N)2GeCl2. LiNEt2 was synthesized in situ by adding a solution of n-BuLi (30 mL, 48.0 mmol, 1.6 M in n-hexane) at −30 °C dropwise to a solution of HNEt2 (3.53 g, 48.0 mmol) in Et2O (50 mL). After 1 h the mixture was warmed to room temperature and stirred for 30 min. The solution was then added to a cooled (−78 °C) solution of GeCl4 (5.15 g, 24.0 mmol) in Et2O (50 mL). The mixture was slowly warmed to room temperature and stirred overnight. After filtration the solvent was removed in vacuo and the product isolated from the residue by vacuum distillation (65 °C, 10−2 mbar) to yield a colorless liquid (5.18 g, 75%). 1H NMR (C6D6, 300 K): δ 0.96 (t, 3JHH = 7.0 Hz, 12H, NCH2CH3), 2.97 (q, 3JHH = 7.0 Hz, 8H, NCH2CH3) ppm. 13C NMR (C6D6, 300 K): δ 15.1 (NCH2CH3), 41.5 (NCH2CH3) ppm. IR (CsI plates, paraffin, cm−1): 1462 vs, 1379 vs (paraffin); 1344 m, 1292 s (δ(CH3)); 1194 s, 1167 s, 1099 m, 1059 m, 1015 s, 903 s, 785 s (ν(CC), ν(CN)), 721 m (paraffin); 619 m, 604 m, 478 m (ν(GeN), ν(GeCl)). MS (EI, 20 eV, 300 K): m/z 288 (0.1% [M]+), 273 (0.3% [M − CH3]+), 253 (0.3% [M − Cl]+). Synthesis of (tBuCC)3GeNEt2 (1). n-BuLi (38.8 mL, 62.1 mmol; 1.6 M in n-hexane) was added dropwise to a solution of Me3CCCH (5.10 g, 62.0 mmol) in Et2O (75 mL) at −78 °C. The reaction mixture was stirred for 1 h and was then slowly warmed to room temperature and stirred for 3 h. This solution was then added over a period of 2.5 h to a solution of Et2NGeCl3 (5.19 g, 20.7 mmol) in Et2O (50 mL) at −78 °C. The mixture was warmed to room temperature overnight, the solvent was distilled at room temperature into a trap cooled with liquid nitrogen, and the residue was then dried overnight in vacuo at 50 °C and extracted with n-hexane (100 mL). The suspension was filtered and the solvent removed in vacuo at 50 °C to give compound 1 as a colorless, waxy solid (6.55 g, 82%). Mp (argon, sealed capillary): 39 °C. 1H NMR (C6D6, 300 K): δ 1.08 (s, 27H, CMe3), 1.29 (t, 3JHH = 7.0 Hz, 6H, NCH2Me), 3.31 (q, 3JHH = 7.0 Hz, 4H, NCH2) ppm. 13C NMR (C6D6, 300 K): δ 15.8 (NCH2Me), 28.3 (CMe3) 30.6 (CMe3), 43.0 (NCH2), 77.3 (GeCC), 114.5 (GeCC) ppm. IR (CsI plates, paraffin, cm−1): 2188 vs, 2153 vs (ν(CC)); 1943 vw, 1506 w; 1455 vs, 1375 vs (paraffin); 1362 vs, 1343 s, 1293 s, 1252 vs (δ(CH3)); 1203 vs, 1176 vs, 1100 s, 1074 s, 1058 s, 1017 vs, 922 vs, 896 vs, 787 vs, 752 vs (ν(CC), ν(CN)); 682 m, 606 s, 553 m, 491 vs (ν(GeC), ν(GeN)). MS (EI, 20 eV, 398 K): m/z 389 (5.8%, [M]+), 374 (100.0%, [M − CH3]+), 317 (49.3%, M − Et2N]+). Anal. Calcd for C22H37GeN: C, 68.07; H, 9.6; N, 3.6. Found: C, 68.10; H, 9.6; N, 3.0. Synthesis of (tBuCC)2Ge(Et2N)2 (2). n-BuLi (20.0 mL, 33.0 mmol; 1.6 M in n-hexane) was added dropwise to a solution of Me3CCCH (2.75 g, 33.0 mmol) in Et2O (50 mL) at −78 °C. The mixture was stirred for 1 h and then slowly warmed to room temperature and stirred for 3 h. The solution was then added to a solution of Cl2Ge(NEt2)2 (4.2 mL, 16 mmol) in Et2O (50 mL) at −78 °C. After 1 h the mixture

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CONCLUSION Hydroalumination of alkynylaminogermanes with ditert-butylaluminum and -gallium hydrides afforded highly functionalized molecules with Lewis acidic, coordinatively unsaturated aluminum and gallium atoms and Lewis basic nitrogen atoms in close proximity. Intramolecular adduct formation between the opposite functionalities resulted in four-membered GeCMN heterocycles (M = Al, Ga) and an activation of the Ge−N bonds. Preliminary experiments showed the exceptional reactivity of these species under mild conditions and their high potential for a wide application as active Lewis pairs in further secondary reactions. A terminal alkyne led by C−H bond cleavage and replacement of an amino group with a mixed bis(alkynide) and phenyl isocyanate or isothiocyanate reacted by insertion into the activated Ge−N bond. Of particular interest is the modification of the substitution pattern of these germanium compounds with the aim of optimizing their applicability as active Lewis pairs or facilitating an intramolecular rearrangement with migration of the amido group to aluminum or gallium and the formation of a germyl cation.

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EXPERIMENTAL SECTION

General Considerations. All procedures were carried out under an atmosphere of purified argon in dried solvents (n-hexane with LiAlH4; 6775

dx.doi.org/10.1021/om400543v | Organometallics 2013, 32, 6770−6779

Organometallics

Article 3

JHH = 7.3 Hz, 3H, NCH2Me), 1.02 (overlapping t, 3JHH = 7.3 Hz, 3H, NCH2Me), 1.07 (s, 9H, CCtBu), 1.13 (s, 9H, CCtBu), 1.14 (s, 9H, CCtBu), 1.30, 1.32, 1.35, 1.36 (s, 9H, AltBu), 3.11, 3.19, 3.61, 3.62 (overlapping m, 1H, NCH2), 6.88, 7.14 (s, 1H, CCH) ppm. 13 C NMR (C6D6, 300 K): δ 12.3, 14.1 (NCH2Me), 17.5, 18.8, 19.6, 20.2 (AlCMe3), 29.2 (CCCMe3), 29.6 (CCCMe3), 30.2 (CCCMe3), 30.5 (CCCMe3), 31.8, 32.4, 33.0, 33.4 (AlCMe3), 38.0 39.1 (CCCMe3), 42.1 (NCH2), 85.8 (GeCC), 130.8 (GeCC), 143.8 (CC GeCAlC heterocycle), 151.7 (br CC GeCAlN heterocycle), 166.4 (GeNAlCC), 167.3 (GeCAlCC) ppm. IR (CsI plates, paraffin, cm−1): 2149 s, 2108 s (ν(CC)); 1660 vw, 1599 s, 1551 m, 1536 m (ν(CC)); 1463 vs, 1380 vs, 1362 vs (paraffin); 1287 vw, 1248 vs (δ(CH3)); 1199 s, 1175 s, 1129 s, 1108 s, 1075 vw, 1043 m, 1028 m, 1002 vs, 933 s, 892 s, 851 m, 808 vs, 781 sh s (ν(CC), (ν(CN)); 719 vs (paraffin); 637 vw, 586 s, 567 s, 507 w, 469 m, 428 s (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 398 K): m/z 616 (100%, [M − Bu]+), 545 (14.8%, [M − Bu − NEt2]+). Anal. Calcd for C38H75Al2GeN: C, 67.86; H, 11.24; N, 2.08. Found: C, 67.25; H, 11.17; N, 1.94. Synthesis of (tBuCC)(Et2N)Ge[C(GatBu2)C(H)tBu]2 (4b). A solution of Et2NGe(CC-tBu)3 (1; 0.519 g, 1.34 mmol) in toluene (10 mL) was added at room temperature to a solution of tBu2GaH (0.594 g, 3.21 mmol) in toluene (25 mL), and the mixture was stirred for 2 days. The solvent was removed in vacuo, and the pale yellow residue was recrystallized from pentafluorobenzene (1.5 mL, −30 °C) to give compound 4b as a colorless solid (0.536 g, 53%). Mp (argon, sealed capillary): 159 °C. 1H NMR (C7D8, 275 K): δ 1.05 (t, 3JHH = 6.9 Hz, 6H, NCH2Me), 1.12 (s, 9H, CCtBu), 1.15 (s, 9H, CCtBu), 1.18 (s, 9H, CCtBu), 1.32, 1.34, 1.39, 1.41 (s, 9H, GatBu), 3.19 (dq, 3 JHH = 6.9 Hz, 2JHH = 13.7 Hz, 2H, NCH2), 3.64 (dq, 3JHH = 7.1 Hz, 2 JHH = 14.2 Hz, 2H, NCH2), 6.41, 6.79 (s, 1H, CCH) ppm. 13C NMR (C7D8, 275 K): δ 13.7 (very br, NMe), 25.6, 26.9 (GaCMe3), 28.9 (CCCMe3), 29.7 (CCCMe3), 30.0 (GaCMe3), 30.2 (C CCMe3), 30.7 (CCCMe3), 30.8 (GaCMe3), 31.5, 32.0, 33.0, 33.1 (GaCMe3), 37.9, 39.0 (CCCMe3), 42.7 (NCH2), 84.8 (GeCC), 122.4 (GeCC), 146.9 (CC), 153.8 (CC), 162.1, 163.0 (CC) ppm. IR (CsI plates, paraffin, cm−1): 2152 w, 2127 m (ν(CC)); 1680 vw, 1597 m, 1560 m (ν(CC)); 1449 vs, 1377 vs (paraffin); 1362 vs, 1283 vw, 1250 s (δ(CH3)); 1200 s, 1169 w, 1134 m, 1109 w, 1005 m, 937 w, 897 w, 880 vw, 853 vw, 808 s, 743 m (ν(CC), (ν(CN)); 721 m (paraffin); 700 s, 590 s, 538 w, 500 w, 476 w, 453 w (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 373 K): m/z 699 (48.4%, [M − HBu]+), 676 (24.7%, [M − CCtBu]+), 644 (73.1%, [M − Bu − H2CCMe2]+), 628 (66.9% [M − 2 × Bu − Me]+). Anal. Calcd for C38H75Ga2GeN: C, 60.21; H, 9.97; N, 1.85. Found: C, 59.69; H, 9.89; N, 1.75. Synthesis of (tBuCC)(PhCC)Ge[C(GatBu2)C(H)tBu]2 (5). A solution of 4b (0.249 g, 0.33 mmol) in n-hexane (10 mL) was treated with PhCCH (0.034 g, 0.33 mmol). The reaction mixture was then heated for 30 h at 60 °C. The volatiles were removed in vacuo, and the residue was recrystallized from pentafluorobenzene (0.5 mL, 2 °C) to give compound 5 as a colorless solid (0.144 mg, 51%). Mp (argon, sealed capillary): 136 °C. 1H NMR (C6D6, 300 K): δ 1.21 (s, 9H, GeCCCMe3), 1.25 (s, 18H, CCHtBu), 1.40, 1.45 (s, 9H, GaCMe3), 6.55 (s, 2H, CCH), 6.89 (overlapping m, 3H, m-, p-Ph), 7.55 (m, 2H, o-Ph) ppm. 13C NMR (C6D6, 300 K): δ 28.8 (CCCMe3), 29.2 (GaCMe3), 30.2 (CCCMe3), 31.0 (GeCCCMe3), 31.4, 31.6 (GaCMe3), 38.4 (CCCMe3), 83.9 (GeCCCMe3), 96.1 (GeCCPh), 108.8 (GeCCPh), 120.3 (GeCCCMe3), 122.3 (ipso-C), 128.7 (m-C), 129.6 (p-C), 132.5 (o-C), 147.6 (CCH), 160.6 (CCH) ppm. IR (CsI plates, paraffin, cm−1): 2183 sh vw, 2155 m, 2132 m (ν(CC)); 1654 w, 1600 m, 1570 m, 1563 m, 1532 s, 1510 m (ν(CC)); 1487 sh m, 1459 vs, 1377 vs (paraffin); 1364 s, 1304 w, 1284 w, 1251 m (δ(CH3)); 1201 m, 1178 m, 1071 m, 1027 m, 1014 m, 979 sh w, 955 m, 940 m, 912 w, 900 m, 877 w, 834 w, 809 m, 794 m, 769 w, 754 s, 739 sh s (ν(CC), (ν(CN)); 720 s (paraffin); 698 m, 587 s, 565 m, 551 m, 533 m, 499 w, 410 m (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 298 K): m/z 728 (9.7%, [M − HtBu]+), 545 (4.6%,

was warmed to room temperature overnight. The solvent was removed in vacuo, and the residue was extracted with n-hexane (50 mL). 2 was obtained after filtration and removal of the solvent from the filtrate in vacuo as a colorless, viscous liquid (5.20 g, 86%). 1H NMR (C6D6, 300 K): δ 1.11 (s, 18H, CMe3), 1.21 (t, 3JH,H = 7.0 Hz, 12H, NCH2CH3), 3.23 (q, 3JH,H = 7.0 Hz, 8H, NCH2CH3) ppm. 13C NMR (C6D6, 300 K): δ 16.0 (NCH2CH3), 28.4 (CMe3), 30.8 (CMe3), 41.8 (NCH2CH3), 77.6 (CC), 115.2 (CC) ppm. IR (CsI plates, cm−1): 2187 vs, 2151 vs (ν(CC)); 2083 vw; 1458 vs, 1375 vs (paraffin); 1344 s, 1292 s, 1252 vs (δ(CH3)); 1173 vs, 1099 vs, 1057 vs, 1022 vs, 1011 vs, 918 vs, 893 vs, 785 vs, 750 vs (ν(CC), ν(CN)); 723 vw (paraffin); 692 vw, 667 vw, 606 s, 552 w, 478 vs, 422 w (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 300 K): m/z 380 (2%, [M]+), 307 (100% [M+ − N(CH2CH3)2 + H]+). Synthesis of (tBuCC)2(Et2N)Ge[C(AltBu2)C(H)tBu] (3a). A solution of Et2NGe(CCtBu)3 (1; 1.067 g, 2.75 mmol) in toluene (10 mL) was added to a solution of tBu2AlH (0.391 g, 2.75 mmol) in toluene (50 mL) at 0 °C. The mixture was warmed to room temperature and stirred for 16 h. The solvent was removed in vacuo and the pale yellow residue recrystallized from pentafluorobenzene (1 mL, −20 °C) to give 3a as a colorless crystalline solid (1.18 g, 81%). Mp (argon, sealed capillary): 64 °C. 1H NMR (C6D6, 300 K): δ 1.06 (t overlapped by tBu, 6H, NCH2Me), 1.06 (s, 18H, GeCCCMe3), 1.28 (s, 9H, CCCMe3), 1.36 (s, 18H, AlCMe3), 3.26, 3.31 (m, 2H, NCH2), 7.19 (s, 1H, CCH) ppm. 13C NMR (C6D6, 300 K): δ 12.6 (NCH2Me), 17.8 (br, AlCMe3), 28.5 (GeCCCMe3) 29.9 (CCHCMe3), 30.5 (GeCCCMe3), 32.5 (AlCMe3), 39.4 (CCCMe3), 41.8 (NCH2Me), 77.9 (GeCC), 119.0 (GeCC), 144.1 (CCH), 167.8 (CCH) ppm. IR (CsI plates, paraffin, cm−1): 2183 vs, 2149 vs (ν(CC)); 1682 w, 1643 vw, 1605 vs, 1553 m, 1533 m, 1508 w (ν(CC)); 1452 vs, 1379 vs (paraffin); 1362 vs, 1329 sh w, 1288 w, 1252 vs (δ(CH3)); 1202 vs, 1175 s, 1132 s, 1109 vs, 1072 w, 1043 s, 1028 m, 1003 vs, 957 vw, 922 s, 903 s, 854 m, 808 vs, 779 s, 750 vs (ν(CC), ν(CN)); 718 vs (paraffin); 600 vs, 577 vs, 500 s, 471 s, 428 s (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 398 K): m/z 474 (100%, [M − Bu]+), 403 (72.3%, [M − H2CCMe2 − NEt2]+. Anal. Calcd for C30H56AlGeN: C, 67.93; H, 10.64; N, 2.64. Found: C, 66.89; H, 10.57; N, 2.31. Synthesis of (tBuCC)2(Et2N)Ge[C(GatBu2)C(H)tBu] (3b). A solution of Et2NGe(CCtBu)3 (1; 0.668 g, 1.72 mmol) in toluene (10 mL) was added to a solution of tBu2GaH (0.350 g, 1.89 mmol) in toluene (25 mL) at room temperature, and the mixture was stirred for 16 h. The solvent was removed in vacuo and the pale yellow residue recrystallized from pentafluorobenzene (2 mL, −20 °C) to give 3b as a colorless crystalline solid (1.28 g, 74%). Mp (argon, sealed capillary): 56 °C. 1H NMR (C6D6, 300 K): δ 1.07 (t, overlapped by tBu, 6H, NCH2Me), 1.08 (s, 18H, GeCCCMe3), 1.32 (s, 9H, CCCMe3), 1.42 (s, 18H, GaCMe3), 3.30 (q, 3JHH = 6.9 Hz, 4H, NCH2Me), 6.97 (s, 1H, CCH) ppm. 13C NMR (C6D6, 300 K): δ 12.8 (NCH2Me), 26.1 (GaCMe3), 28.5 (GeCCCMe3) 30.2 (CCCMe3), 30.5 (GeC CCMe3), 32.4 (GaCMe3), 39.2 (CCCMe3), 42.4 (NCH2Me), 78.3 (GeCC), 118.5 (GeCC), 147.0 (CCH), 165.1 (CCH) ppm. IR (CsI plates, paraffin, cm−1): 2186 vs, 2152 vs (ν(CC)); 1685 vw, 1697 vs, 1559 w (ν(CC)); 1458 vs, 1445 vs, 1377 vs (paraffin); 1362 vs, 1305 w, 1287 w, 1253 vs (δ(CH3)); 1202 s, 1174 w, 1137 s, 1112 m, 1074 vw, 1045 m, 1030 w, 1004 vs, 921 m, 905 m, 807 vs, 782 s, 750 vs (ν(CC), ν(CN)); 722 s (paraffin); 703 vs, 602 vs, 587 s, 538 vs, 500 s, 454m, 407 s (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 398 K): m/z = 516 (100%, [M − Bu]+), 461 (7.6%, [M − Bu − H2CCMe2]+), 445 (68%, [M − H2CCMe2 − NEt2]+, 389 (2.5% [Et2NGe(C CtBu)3]+. Anal. Calcd for C30H56GaGeN: C, 62.87; H, 9.85; N, 2.44. Found: C, 62.84; H, 9.85; N, 2.41. Synthesis of (tBuCC)(Et2N)Ge[C(AltBu2)C(H)tBu]2 (4a). A solution of Et2NGe(CC-tBu)3 (1; 0.883 g, 2.27 mmol) in toluene (10 mL) was added at room temperature to a solution of tBu2AlH (0.712 g, 5.00 mmol) in toluene (50 mL), and the mixture was stirred for 14 h. The solvent was removed in vacuo, and the pale yellow residue was recrystallized from pentafluorobenzene (10 mL, −20 °C) to give compound 4a as a colorless solid (1.031 g, 67%). Mp (argon, sealed capillary): 156 °C. 1H NMR (C6D6, 300 K): δ 1.00 (overlapping t, 6776

dx.doi.org/10.1021/om400543v | Organometallics 2013, 32, 6770−6779

Organometallics

Article

[M − HtBu − HCCtBu − CCPh]+). Anal. Calcd for C42H70Ga2Ge: C, 64.09; H, 8.96. Found: C, 63.31; H, 8.63. Synthesis of (Et2N)2(tBuCC)Ge[C(GatBu2)C(H)tBu] (6). (Et2N)2Ge(CCtBu)2 (0.921 g, 2.43 mmol) was added at room temperature to a solution of tBu2GaH (0.483 g, 2.61 mmol) in toluene (25 mL), and the reaction mixture was stirred for 16 h. The solvent was removed in vacuo, and the residue was recrystallized from pentafluorobenzene (2.5 mL, −30 °C) to yield compound 6 as a colorless crystalline solid (1.317 g, 96%). Mp (argon, sealed capillary): 67 °C. 1H NMR (C6D6, 300 K): δ 0.99, 1.06 (t, br, not resolved, 12H, NCH2Me), 1.11 (s, 9H, CCtBu), 1.21 (s, 9H, CCtBu), 1.40, 1.42 (s, 9H, GatBu), 3.07 (overlapping m, br, 2H, NCH2), 3.09 (overlapping m, br, 4H, NCH2), 3.20 (overlapping m, br, 2H, NCH2), 7.00 (s, 1H, CCH) ppm. 13C NMR (C6D6, 300 K): δ 12.9, 14.8 (br, NMe), 25.2, 26.5 (GaCMe3), 28.4 (CCCMe3), 29.3 (CCCMe3), 30.4 (CCCMe3), 32.5, 32.6 (GaCMe3), 39.4 (CCCMe3), 41.4, 42.5 (br NCH2), 78.5 (GeCC), 118.4 (GeCC), 145.3 (CC), 168.8 (CC) ppm. IR (CsI plates, paraffin, cm−1): 2185 s, 2151 s, 2119 w sh (ν(CC)); 1679 vw, 1664 vw, 1604 s, 1557 w, 1533 m, 1510 w (ν(CC)); 1464 s, 1455 s, 1376 s (paraffin); 1362 s, 1346 sh m, 1290 m, 1253 m δ(CH3); 1196 m, 1182 m, 1137 m, 1112 m, 1071 m, 1048 m, 936 w, 920 vw, 901 w, 857 w, 808 m, 791 m, 782 m, 749 s (ν(CC), (ν(CN)); 718 m (paraffin); 703 s, 610 s, 594 s, 540 m, 498 m, 482 m, 407 m (ν(GeC), (ν(GeN), δ(CC)). MS (EI, 20 eV, 298 K): m/z 507 (51.5%, [M − tBu]+), 436 (100%, [M − tBu − NEt2]+). Anal. Calcd for C28H57GaGeN2: C, 59.63; H, 10.19; N, 4.97. Found: C, 58.96; H, 10.05; N, 4.65. Synthesis of Compound 7a. PhNCO (0.073 mL, 0.61 mmol) was added at room temperature to a solution of compound 6 (0.343 g, 0.61 mmol) in n-hexane (25 mL). Removing the solvent in vacuo and recrystallizing the obtained residue from pentafluorobenzene (1 mL, −20 °C) yielded compound 7a as a colorless crystalline solid (0.322 g, 77%). Mp (argon, sealed capillary): 173 °C. 1H NMR (C6D6, 275 K): δ 0.65 (t, 3JHH = 7.0 Hz, 6H, CNCH2Me), 0.93 (s, 9H, CCtBu), 1.08 (t, 3JHH = 7.0 Hz, 6H, GeNCH2Me), 1.35 (s, 9H, CCtBu), 1.49, 1.55 (s, 9H, tBt), 2.66 (dq, 3JHH = 6.9 Hz, 2JHH = 13.9 Hz, 1H, CNCH2Me), 3.00 (overlapping m, 1H, CNCH2Me), 3.09 (overlapping m, 1H, GeNCH2Me), 3.30 (dq, 3JHH = 7.0 Hz, 2JHH = 14.1 Hz, 1H, GeNCH2Me), 6.90 (t, 3JHH = 7.5 Hz, 1H, p-Ph), 7.04 (s, CCH), 7.08 (pseudo t, 3JHH = 7.8 Hz, 2H, m-Ph), 7.32 (d, 3JHH = 7.8 Hz, 2H, o-Ph) ppm. 13C NMR (C6D6, 300 K): δ 12.6 (CNCH2Me), 15.4 (GeNCH2Me), 23.0, 23.4 (GaCMe3), 28.2 (CCCMe3), 30.2 (CCCMe3), 30.4 (CCCMe3), 32.0, 32.2 (GaCMe 3 ), 39.2 (CCCMe 3 ), 42.5 (GeNCH 2 Me), 43.4 (CNCH2Me), 80.0 (GeCC), 119.5 (GeCC), 125.0 (p-C), 128.5 (m-C), 128.9 (o-C), 141.5 (CC), 145.5 (ipso-C), 166.2 (CC), 167.0 (CN) ppm. IR (CsI plates, paraffin, cm−1): 2164 s, 2152 s (ν(CC)); 1930 w, 1849 vw, 1748 vw, 1720 vw (phenyl); 1648 vw, 1601 vw, 1626 w, 1595 m, 1584 m, 1565 s, 1538 vs (ν(CN), ν(CO, ν(CC)); 1470 vs, 1462 vs, 1370 vs (paraffin); 1315 s, 1292 s, 1281 m, 1253 s (δ(CH3)); 1182 vs, 1154 sh m, 1101 m, 1074 m, 1059 w, 1051 w, 983 m, 923 s, 938 s, 934 s, 896 s, 863 m, 812 vs, 803 sh vs, 783 s, 752 vs (ν(CC), (ν(CN)); 726 m (paraffin); 712 m, 699 vs (phenyl); 678 m, 633 m, 585 m, 551 w, 515 m, 477 w, 454 w, 412 vw (ν(GeC), ν(GeN), ν(GaO), δ(CC)). MS (EI, 20 eV, 298 K): m/z 626 (100%, [M − tBu]+), 555 (24.3%, [M − tBu − NEt2]+). Anal. Calcd for C35H62GaGeN3O: C, 61.53; H, 9.15; N, 6.15. Found: C, 61.52; H, 8.98; N, 6.12. Synthesis of Compound 7b. PhNCS (0.100 mL, 0.74 mmol) was added at room temperature to a solution of compound 6 (0.382 g, 0.74 mmol) in toluene (10 mL), and the reaction mixture was stirred for 18 h at 90 °C. The solvent was removed in vacuo and the residue recrystallized from 1,2-difluorobenzene (0.5 mL, 2 °C) to yield compound 7b as a colorless crystalline solid (0.36 g, 75%). Mp (argon, sealed capillary): 125 °C. 1H NMR (C6D6, 300 K): δ 0.55 (t, 3 JHH = 7.1 Hz, 6H, CNCH2Me), 0.97 (s, 9H, CCtBu), 1.18 (t, 3JHH = 7.0 Hz, 6H, GeNCH2Me), 1.39 (s, 9H, CCtBu), 1.51, 1.57 (s, GatBu), 2.99 (dq, 3JHH = 7.0 Hz, 2JHH = 14.0 Hz, 1H, CNCH2Me), 3.19 (dq, 3JHH = 6.90 Hz, 2JHH = 13.9 Hz, 2H, CNCH2Me, GeNCH2Me), 3.38 (dq, 3JHH = 7.1 Hz, 2JHH = 14.2 Hz, 1H,

GeNCH2Me), 6.89 (t, 3JHH = 7.4 Hz, 1H, p-Ph), 7.03 (pseudo t, 3 JHH = 7.8 Hz, 2H, m-Ph), 7.12 (s, CCH) 7.34 (d, 3JHH = 7.6 Hz, 2H, o-Ph) ppm. 13C NMR (C6D6, 300 K): δ 11.8 (CNCH2Me), 14.8 (GeNCH2Me), 23.8, 24.5 (GaCMe3), 28.3 (CCCMe3), 30.1 (C CCMe3), 30.4 (CCCMe3), 32.8, 33.2 (GaCMe3), 39.0 (C CCMe3), 42.0 (GeNCH2Me), 47.7 (CNCH2Me), 82.5 (GeCC), 119.8 (GeCC), 125.4 (p-C), 128.3 (m-C), 129.5 (o-C), 142.6 (CC), 147.4 (ipso-C), 166.2 (CC), 189.6 (CN) ppm. IR (CsI plates, paraffin, cm−1): 2181 s, 2148 (ν(CC)); 1939 w, 1794 vw, 1711 vw, 1682 vw, 1587 m, 1553 s (phenyl, ν(CN), ν(CC)); 1448 vs, 1397 vs (paraffin); 1341 sh, 1295 sh, 1256 vs (δ(CH3)); 1182 s, 1119 s, 1098 s, 1077 s, 1058 m, 1201 vs, 977 sh, 951 s, 912 s, 899 s, 857 s 808 vs, 799 sh, 767 vs, 747 s (ν(CC), (ν(CN)); 719 vs (paraffin); 697 vs (phenyl); 660 w, 607 s, 583 s, 555 w, 517 s, 492 s, 455 s (ν(GeC), ν(GeN), δ(CC)). MS (EI, 20 eV, 298 K): m/z 642 (11.2%, [M − tBu]+). Anal. Calcd for C35H62GaGeN3S: C, 60.11; H, 8.94; N 6.01. Found: C, 60.20; H, 8.77; N, 5.94. Synthesis of Compound 7c (NMR Experiment). 1H NMR (C6D6, 300 K): δ 0.89 (t, 3JHH = 7.1 Hz, 6H, CNCH2Me), 1.12 (t, 3JHH = 7.0 Hz, 6H, GeNCH2Me), 1.16 (s, 9H, CCtBu), 1.26 (t, 3JHH = 7.0 Hz, 3H, CNCH2Me), 1.36 (s, 9H, CCtBu), 1.38, 1.42 (s, 9H, GaCMe3), 3.04 (m, 2H, CNCH2Me), 3.09 (m, 2H, GeNCH2Me), 3.18 (m, 2H, GeCH2Me), 3.36 (m, 2H, CNCH2Me), 3.38 (m, 1H, CNCH2Me) 3.71 (dq, 3JHH = 6.9 Hz, 2JHH = 13.8 Hz, 1H, CNCH2Me), 7.10 (s, CCH) ppm. 13C NMR (C6D6, 300 K): δ 13.1 [GeN(CH2Me)2], 14.8 (GeNCH2Me), 17.5 (CNCH2Me), 23.8, 24.1 (GaCMe3), 28.5 (CCCMe3), 30.5 (CCCMe3), 30.7 (CCHCMe3), 33.0, 33.2 (GaCMe3), 38.5 (CCHCMe3), 41.9 (GeNCH2Me), 46.5 (CNCH2Me), 49.0 (CNCH2Me), 82.8 (GeCC), 116.7 (GeCC), 142.8 (CCH), 167.1 (CCH), 191.5 (CN) ppm. X-ray Crystallography. Crystals suitable for X-ray crystallography were obtained by recrystallization from 1,2-difluorobenzene (3a, 7b) or pentafluorobenzene (3b, 4b, 5, 6, 7a). The handling of single crystals of compound 5 is extremely difficult; in most cases they fall apart spontaneously to give an amorphous powder even at low temperature. Intensity data were collected on Bruker APEX II, IPDSII, and D8-Venture diffractometers with monochromated Mo Kα radiation. The collection method involved ω scans. Data reduction was carried out using the program SAINT+.32 The crystal structures were solved by direct Methods using SHELXTL.33 Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full-matrix least-squares calculation based on F2 using SHELXTL.33 Hydrogen atoms were positioned geometrically and allowed to ride on their respective parent atoms. 3a crystallizes with half a molecule of difluorobenzene and 3b and 5 with half a molecule of pentafluorobenzene per formula unit; the solvent molecules are disordered across the inversion center. One tBu group of 3a, three tBu groups of 5, and one Me group of 7a (GeNEt2) were disordered and refined in split positions (0.74:0.26; 0.30:0.70, 0.53:0.47, 0.52:0.48; 0.31:0.69). Further details of the crystal structure determinations are available from the Cambridge Crystallographic Data Center on quoting the depository numbers CCDC 943917 (3a), 943922 (3b), 943921 (4b), 943918 (5), 943916 (6), 943919 (7a), and 943920 (7b). Computational Details. DFT calculations were carried out using Turbomole 6.4.34 The TPSS meta-GGA functional35 was used in conjunction with the def2-TZVP (TZ) basis set36 and our standard dispersion correction with the Becke−Johnson damping scheme37 (denoted as -D3) to optimize the geometries of the species referenced. In some cases, PW6B95 meta-hybrid functional38 single-point calculations in conjunction with the def2-QZVP (QZ) basis set39 and the mentioned dispersion correction were carried out on the TPSS-D3/TZ geometries. Auxiliary basis sets40 were used for the RIJ and RIJK approximation as appropriate. The ΔG(298 K)solv values refer to rovibrationally corrected41a (using PM6 as implemented in MOPAC201242) and solvent-corrected (using COSMO-RS43 based on BP86/def-TZVP44) free enthalpies. Where ΔG(298 K)solv values are reported, the PM6- and TPSS-optimized structures involved have been confirmed to be the same conformer by inspection. For discussion and more details on this computational scheme, see ref 41. 6777

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Organometallics

Article

Wiberg bond indices (WBI)45 were used in some cases to characterize the strength of an interaction.

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving Cartesian coordinates and CIF files giving crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for W.U.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Deutsche Forschungsgemeinschaft for generous financial support. T.R. thanks the International NRW Graduate School of Chemistry (based at the WWU Münster) for financial support in the form of a Ph.D. scholarship.

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