Hafnocene-based Bicyclo[2.1.1]hexene Germylenes â Formation

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Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity and Structural Flexibility Zhaowen Dong, Katja Bedbur, Marc Schmidtmann, and Thomas Müller J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13536 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Journal of the American Chemical Society

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity and Structural Flexibility Zhaowen Dong, Katja Bedbur, Marc Schmidtmann, Thomas Müller* Institute of Chemistry, Carl von Ossietzky University of Oldenburg, Carl von Ossietzky-Str. 9-11, D-26129 Oldenburg, Federal Republic of Germany, European Union KEYWORDS Germanium, Carbene Analogues, Transition Metal Complexes, NMR spectroscopy, X-ray diffraction

ABSTRACT: 2,5-Disilylsubstituted germole dianions 1 react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]hexene germylenes (BCHGe’s) 3. Their formation proceeds via hafnocene-germylene complexes 2 that were identified by NMR and UV-spectroscopy. Germylenes 3 are stabilized by homoconjugation between the empty 4p(Ge) orbital and the π-bond of the innercyclic C2=C3 double bond. This interaction can be understood as σ2, π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e- hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0) and Au(I) and in reaction products with N-heterocyclic carbenes (NHCs) showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group.

I INTRODUCTION Germylenes, the germanium analogues to carbenes, have found renewed interest on one hand due to their basic ability to mimic transition metal complexes in bond activation reactions and small molecule activation.1 In this respect, the amphiphilic character of germylenes is important. It allows fixation and activation of the substrate molecule through an acceptor orbital and subsequent nucleophilic attack by a donor orbital. In this context, the recently synthesized germylenes A – D are important examples for the creative use of substituent effects to modify and control the reactivity of germylenes (Chart 1).2 On the other hand, silylenes and germylenes have been used as ligands in transition metal complexes.3 Compared to the widespread use of carbenes as substituents in organometallic compounds this chemistry is still in its infancy. There are however promising results showing that silylenes and bis-silylenes might serve as valuable steering ligands for catalysis.4 For these applications the strong σ-donating ability of germylenes is favorable and their π-accepting properties allow the needed fine tuning of the electronic situation of the transition metal complex. Recently, we reported on the synthesis of a novel type of germylene, the hafnocene-based bicyclo[2.1.1]hexene germylene (BCHGe) 3a.5 Germylene 3a is stabilized by homoconjugation between the disubstituted germanium atom and the remote C=C bond. In dependance on the well-established terminology for the bonding modes of the butadiene ligand, this description is analogous to a σ2, π−coordination of a butadiene ligand to the germanium atom.6 Our first explorative reactivity studies indicated that germylene 3a serves predominately as nucleophile and in

complexes with low-valent early transition metals as strong σdonor.5 A particular interesting feature of hafnocene-based BCHGe 3a is that with the hafnocene group a second electron deficient center is present in the molecule apart from the lowcoordinated germanium atom. This arrangement of two electron-deficient parts around the electron-donating butadiene ligand opens the possibility that the structure of the germylene ligand changes readily as a function of the electronic situation at the ligating germanium atom. Here we would like to disclose a detailed study on the synthesis of BCHGe’s 3 and its reactivity towards carbenes and late transition metal complexes. The molecular structures of the isolated products indicate the large structural and electronic variability of the germylene ligand that is enabled by the interplay between the topological flexible unsaturated germanium ring and the hafnocene group.

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Chart 1. Bicyclo[2.1.1]hexene germylenes (BCHGe’s) 3 and recently published germylenes A – D. Ge

R

Me Hf

Me

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99.6° indicating the close approach of Ge atom to the C2=C3 double bond and α(Hf) is 122.9° which suggests no interaction. For related hafnocene butadiene complexes such as 6 for which a classical σ2, π-coordination of the butadiene to the electron deficient hafnocene is discussed, the hafnium flap angle is markedly smaller (α(Hf) = 116.6°, Chart 2).6a

R 3a: R = SiMe3 3b: R = SiMe2tBu

Scheme 1. Synthesis of BCHGe 3a and 3b Me

Me 3

Dipp

Dipp

Ge N N

Ge N

t

W CO OC CO Dipp

PR 2 PR2 =

A

Mes

Ph Si(SiMe3)3 Ph

Mes C

Me

Ge

R

Ge

Me

THF -80°C

+ Cp2HfCl 2

1 2

- KCl Me

SiMe2

P

K

K

Cl HfCp2

4

R

K2[1a]: R=SiMe3 K2[1b]: R=SiMe2tBu

Bu

Ge

3

N

t

N

1

4

R

Bu

N

B iPr Si 3

Ge

Dipp = 2,6-diiso-propylphenyl

P

R

2

K

K[2a]: R=SiMe 3 K[2b]: R=SiMe2tBu Ge

ZnL∗ L∗ =

Ph Ph

Dipp

N

Me3Si

Me

T = r.t.

B N

R

4

-KCl

Dipp

1

2

Me

N

HfCp2 3

R

D

3a: R=SiMe3 3b: R=SiMe2tBu

II RESULTS AND DISCUSSION Synthesis of BCHGe’s. As reported previously, BCHGe 3a can be readily synthesized by salt metathesis reaction between dipotassium germole dianion K2[1a] and hafnocene dichloride in THF at temperatures between T = -80 °C and r.t..5 The general methodology can be extended for the synthesis of BCHGe 3b from the reaction of the more bulky germole dianion 1b with hafnocene dichloride (Scheme 1). Interestingly, germole dianion 1b did not react with decamethyl hafnocene dichloride even at elevated temperatures, indicating the onset of severe steric hindrance between the bulky dianion and the permethylated hafnocene. BCHGe 3b was isolated after recrystallization from pentane as orange crystals in 40% yield. It was fully characterized by NMR, UV spectroscopy, mass spectrometry and by the results of an X-ray diffraction (XRD) analysis. Most significant for the formation of germylene 3b is the high-field shift of the 13C NMR resonance of the carbon atoms C1 and C4 compared with the germole dianion 1b (δ13C(C1/4) = 109.7 (3b), 150.8 (1b) Table 2). In general, the NMR spectroscopic parameters are very similar to that reported for 3a (see Table 2 and SI material).5 An absorption at λmax = 372 nm in the UV spectrum of BCHGe 3b in pentane arises from the HOMO-LUMO transition and is responsible for its yellow color. The molecular structure of BCHGe 3b is shown in Figure 1. As expected, there is a close agreement in all relevant structural parameters between germylenes 3a,b (See Table 1). At this point we would like to emphasize the peculiar situation that in germylenes 3a,b as well as in the related silylene 47 and stannylene 5,8 the structural parameters clearly indicate the interaction of the dicoordinated tetrylene center with the remote C2=C3 double bond while the hafnocene group shows no sign of coordination to this bond despite its electron deficiency. The structural parameters which demonstrate this situation most clearly are the two significantly different flap angles, α(Ge) and α(Hf). These angles are defined by the Ge (Hf) atom, the midpoint of the C1 / C4 distance and the midpoint of the C2 – C3 bond (Figure 2). For BCHGe 3b α(Ge) is

Figure 1. Molecular structure of BCHGe 3b in the crystal. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected atom distances [pm] and angles [°]: Ge1–C1 207.7(2), Ge1–C2 228.3(2), Ge1–C3 228.0(2), Ge1–C4 207.7(2), Ge1–Hf1 300.4(4), C1–C2 148.0(3), C2–C3 143.5(4), C3–C4 148.4(3), C1–Hf1 221.2(2), C2–Hf1 274.1(22), C3–Hf1 274.7(22), C4–Hf1 221.0(2), α (Ge) 99.6, α (Hf) 122.9 (For the definition of the flap angles, see Figure 2 and Table 1). Ge α(Ge)

1 2

3

4

HfCp2 α(Hf)

Figure 2. Basic molecular structure of hafnocene-based bicyclohexene-type germylene and definition of the flap angles α(Ge) and α(Hf).


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electronic transition of the Ge=Hf double bond of the germylene complex 2a by comparison with literature data and is responsible for the green color of the solution.9 The reaction in THF was also monitored by NMR spectroscopy and showed the smooth transformation of anionic germylene hafnocene 2a into BCHGe 3a (see SI Material). Particularly indicative for the intermediate 2a are the 13C NMR resonances of the germole ring (δ13C(C1/4) = 151.5, δ13C(C2/3) = 145.9) that are significantly different from those of the starting germole dianion 1a and the product BCHGe 3a (1a: δ13C(C1/4) = 156.2, In analogy to the results for related silicon and tin comδ13C(C2/3) = 130.8, 3a: δ13C(C1/4) = 111.5, δ13C(C2/3) = 131.4, 7-8 pounds, the formation of BCHGe’s is thought to proceed via see Table 2). As stabilizing donor we suggest in the case of donor stabilized hafnocene germylene complexes 2, which intermediate 2a a chloride ion on the basis of the observation undergo rearrangement reactions to yield the BCHGe’s 3 that removal of chloride by precipitation of KCl has a rate(Scheme 1). For the reaction of potassio germolediide K2[1a] accelerating influence on formation of BCHGe 3a. Sekiguchi with hafnocene dichloride in THF, we noticed at low temperaand coworkers as well as the group of Marschner demonstrattures an immediate color change from red-brown to green and ed recently that disilylgermylene hafnocene complexes can be at ambient temperature a slow color change from green to stabilized and isolated by using strongly donating phosyellow during days. This last color change occurred immediphanes.9 In our case, the addition of PMe3 resulted in the apately when the solvent THF was replaced by less polar hydropearance of a new set of NMR resonances, which are in carbon solvents such as hexane, pentane, benzene or toluene.5 agreement with the formation of a phosphane stabilized These solvents promote the precipitation of KCl and increase germylene hafnocene (see Table 2 and SI material). The stabithe rate of the reaction. For the intermediate we suggest the lization by coordination of PMe3 to the hafnium atom was structure of the anionic hafnium germylene complex 2a. UVhowever not sufficient to allow the isolation of the neutral vis spectra of the intermediate green solution showed three complex 2a(PMe3). The competing rearrangement to form absorption bands (λmax = 251, 349, 608 nm). The longest BCHGe 3a was still the dominant process (for details, see the SI material). wavelength band at λmax = 608 nm is attributed to the π – π* Table 1. Structural parameters of BCHGe’s 3 and derivatives 7, 14, 15, 16 and 24, that are relevant to the discussion. Chart 2. Bicyclohexene-type silylene 4 and stannylene 5, and the related σ2, π-coordinated butadiene hafnocene complex 6.

3aa

3b

7

[iPr2Me2NHC (H)]14

15a M = Fe

16 M = Ni

24 M = Au

M–Ge1b

–

–

–

–

231.2(3)

223.2(2)

243.9(4)

C1–C2

148.3(8)

148.0(3)

144.5(8)

146.7(12)

149.1(2)

148.6(2)

147.5(4)

142.5(8)

143.5(4)

140.7(9)

140.2(12)

141.4(2)

140.9(2)

139.4(4)

2

3

3

4

C –C C –C

148.2(8)

148.4(3)

143.4(9)

145.7(12)

148.1(2)

147.2(2)

147.9(4)

C1–Ge1

210.0(7)

207.7(2)

195.4(8)

210.1(8)

201.3(17)

204.4(14)

203.7(3)

C2–Ge1

226.7(6)

228.3(2)

282.5(52)

281.6(9)

232.0(16)

247.7(15)

265.6(27)

C3–Ge1

227.0(6)

228.0(2)

281.7(62)

281.2(8)

232.2(15)

250.8(15)

265.1(27)

C4–Ge1

208.8(6)

207.7(2)

196.1(5)

208.2(8)

201.0(15)

205.8(14)

203.1(3)

C1–Hf1

220.3(7)

221.2(2)

249.6(2)

224.4(8)

225.0(16)

223.6(14)

228.7(3) 254.4(3)

2

1

274.7(62)

274.1(22)

244.8(5)

255.2(8)

281.0(17)

270.0(14)

3

1

C –Hf

274.1(58)

274.7(22)

250.9(5)

256.0(8)

280.3(16)

268.0(14)

255.8(3)

C4–Hf1

220.1(6)

221.0(2)

254.2(5)

226.2(8)

224.4(15)

223.9(15)

229.7(3)

Ge1–Hf1

301.0(9)

300.4(4)

291.8(5)

277.1(5)

291.8(5)

291.1(4)

303.9(4)

C4–Ge1–C1

85.0(2)

85.8(9)

84.9(2)

82.6(3)

87.9(6)

86.0(3)

84.2(13)

α (Hf1)c

124.8

122.9

83.4

105.0

123.3

115.8

100.5

97.9

99.6

169.4

143.7

106.1

116.8

129.6

C –Hf

1 d

α (Ge ) a

Reference 5. bSum of covalent radii for M–Ge single bonds: M = Ni: 231; M = Au: 245; M = Fe: 237 pm. Sum of covalent radii for M=Ge double bonds: M = Ni: 212; M = Au: 232; M = Fe: 220 pm. cFlap angle α(Hf) is defined as angle between the Hf atom, the midpoint between atoms C1 and C4 and the midpoint of the C2 / C3 bond. dFlap angle α(Ge) is defined as angle between the Ge atom, the midpoint between atoms C1 and C4 and the midpoint of the C2 / C3 bond. eSum of the covalent radii expected for Hf−E single bonds: E = C: 227 pm, E = Ge: 273 pm. fSum of the covalent radii expected for C−E single bonds: E = C: 150 pm, E = Ge: 196 pm. Covalent radii are taken from ref 19.



Table 2. Selected 13C NMR chemical shift data of BCHGe’s 3 and related compounds. K2[1a]ab K2[1b]a K[2a]a 2a(PMe3)c 3abc 3bc 7c 14d 15bc 16c 23c 24c

δ13C (C1/4)

δ13C (C2/3)

δ13C (Cp)

156.2 150.8 151.5 167.2 111.5 109.7 174.9/158.5 105.5 101.3 101.9 140.5 80.1

130.8 129.6 145.9 159.6 131.4 131.6 156.9/139.5 123.1 136.9 130.6 141.9 132.5

— — 104.6/111.9 — 101.7/101.8 102.4/102.8 112.0 99.2 102.2/103.2 101.9/102.1 113.1 103.6/107.7/117.7

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isolation of chloro complex 7 provides important insights into the formation mechanism of BCHGe’s 3 as it suggests the presence of the anionic chloro-hafnocene germylene 2a as an intermediate. In addition, it shows also the flexibility of the germole group in the ligand sphere of the hafnium atom switching between a chelating σ-bonding mode in the BCHGe 3a to a η5, π-bonding ligand in complex 7. Scheme 2. The reaction of hafnium-germylene 2a with Me4 NHC N

SiMe3 Me

1 2

Ge

3

T = -30 °C

HfCp 2

-CpK

4

Me

N

N

Cl

SiMe3

Ge 3

Me

K

SiMe3

1

2

Me

N

Hf

Cl

K[2a]

4

SiMe3

7

a

Data were recorded in THF/D2O. bReference 5. cData were recorded in C6D6. dData were recorded in d8-THF.

Addition of the even stronger σ-donating tetramethylimidazolidyne Me4NHC to a freshly prepared THF solution of the anionic germylene hafnocene complex 2a resulted in the substitution of one Cp ligand and the unusual η5-germolediide hafnium complex 7 was isolated in high yields (70 %, Scheme 2) and as by-product CpK isolated (1H NMR = 5.70, 13C NMR = 104.0).10 The neutral NHC complex 7 was characterized by multinuclear NMR, by UV spectroscopy and by mass spectrometry. In addition, XRD analysis of dark green single crystals that were obtained from diethyl ether solution provided structural information. Signals for six magnetically nonequivalent methyl groups and for two diastereotopic trimethylsilyl groups in the 1H NMR spectrum of compound 7 indicated its non-symmetric structure in solution at room temperature. This is further supported by the detection of two 15N NMR signals in the 1H/15N HMBC NMR spectrum and two 29 Si resonances in the 29Si{1H} NMR spectrum (δ15N = 176.4, 179.4; δ29Si = -8.3, -9.9). The coordination of the NHC is shown by the characteristic 13C NMR resonance of the carbene carbon atom at δ13C = 197.0.11 Four 13C NMR signals in the low-field region from δ13C = 174.9 - 139.5 for the ring carbon atoms of the germolediide ligand indicate its non-symmetric η5-coordination. A non-perfect η5-coordination of the germolediide ligand to the Hf center in complex 7 is also apparent from its molecular structure (Figure 3). The GeC4 ring is slightly bent with the germanium atom 26 pm placed below the least square plane spanned by the carbon atoms C1 – C4. The short Ge – C bonds (195.4, 196.1 pm) and the nearly equidistant inner cyclic C – C bonds (140.7 – 144.5 pm) indicate the delocalized structure of the germole ligand. Complex 7 is one of the rare examples for a hafnium complex with a germolyl ligand in a η5-coordination mode. Interestingly, it differs from previous examples 8 – 10 from the Tilley group (Chart 3) by its free coordination side at the germanium atom which is not occupied by a main group ligand (as in 8) or by a metal complex fragment (as in 9, 10).12 This demonstrates the spatially shielding influence of the silyl groups and the NHC ligand in complex 7 in agreement with its rigid structure that was already indicated by the NMR spectroscopic results. The

Figure 3. Molecular structure of complex 7 in the crystal. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected atom distances [pm] and angles [°]: Ge1–C1 195.4(5), Ge1–C4 196.1(5), Ge1–Hf1 291.8(5), C1–C2 144.5(8), C2–C3 140.7(9), C3–C4 143.4(9), C1–Hf1 249.6(2), C2–Hf1 244.8(5), C3–Hf1 250.9(5), C4–Hf1 254.2(5), Cl1–Hf1 244.5(18), C18–Hf1 236.5(5), C18–N1 135.0(8), C18–N2 135.8(7), Ge*–Hf1 218.3(2), Cp*–Hf1 220.8(3), Ge*–Hf1 –Cp* 136.1, C1–Ge1–C4 84.9 (Ge* and Cp* is defined as the centroids of the germole and Cp rings, respectively). α (Ge) 169.4, α (Hf) 83.4 (For the definition of the flap angles see Table 1 and Figure 2).

Chart 3. Hafnium complexes with a η5 bonded germolyl ligand 8-10 and η4 bonded germole ligand 11.12 Cl

Me Cl

Hf Me

Ge

Me

Me Hf

Me SiMe3

Me

Me

Ge

Me 9 Me

Me Hf

Li Ge

Ge

THF

Me Me

Hf

Li

Me

Hf

Me

Me Ge

Me

Me

THF 10


Rh(dmpe)2

Me

8

Me

Me

11

Et

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Selected Reactions of BCHGe 3a. As demonstrated above, the addition of an NHC had significant influence on formation of BCHGe’s 3. In addition, we were intrigued by the observation that related [2.2.1] bicyclic systems with a silicon atom in the bridgehead position, such as 12, react with NHCs to yield NHC-stabilized low-coordinated silicon species.11 This idea was further fueled by our observation that the related metalfree BCHGe 13 was not stable under ambient conditions (Chart 4).13 BCHGe 13 eliminates elemental germanium and the corresponding silole was isolated. Recently, several groups provided intriguing examples of NHC stabilized mono- and digermanium species.14 These observations prompted us to test the reactivity of germylenes 3 versus NHCs as possible transfer reagents for elemental germanium. Chart 4. Bicyclic precursors for low coordinated group 14 element compounds (Ter: 2,4-bis-(2,4,6-tri-iso-propylphenyl)phenyl).

Initially, the reaction of 1 equiv of BCHGe 3a with 1 equiv of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene(iPr2Me2NHC) was tested (Scheme 3). However, no germanium atom transfer took place but one of the Cp-ligands of the hafnocene group was deprotonated by the NHC and the novel germyl anion 14 paired with the formed imidazolium cation [iPr2Me2NHC(H)]+ was isolated (Scheme 3). Compound [iPr2Me2NHC(H)]14 is soluble in THF but insoluble in hexane, toluene, benzene and it is stable at ambient temperature in THF-d8 solution for three months. The 1H NMR spectrum of the yellow compound [iPr2Me2NHC(H)]14 shows characteristic resonances for the η5Cp ligand (δ1H = 4.86), for the germanium bonded cyclopentadiene ligand (δ1H = 6.58–6.59, 6.24–6.25) and for the NCH hydrogen atom (δ1H = 8.01). The well-separated positions of the 13C NMR resonances of the two magnetically different kinds of carbon atoms of the GeC4 cycle at relative high field are typical for butadienes coordinated in a σ2, π-fashion to an early transition metal (δ13C(C1/4) = 105.5 and δ13C(C2/3) = 123.1, see Table 2). A side process in this reaction is the cleavage of one of the cyclopentadiene substituents from the hafnocene with formation of the imidazolium cyclopentadienide [iPr2Me2NHC(H)][Cp] as evidenced by NMR spectroscopy (see SI material). This side reaction became more important when excess of NHC was applied. Although the reaction of germylene 3a with 2 equivalents of the smaller tetramethylsubstituted NHC, Me4NHC, gave the imidazolium salt [Me4NHC(H)]14 (see SI material) as main product, several unidentified byproducts were formed. The cleavage of one Cpligand from the hafnocene fragment is shown by NMR spectroscopy and by the results of an XRD analysis of crystals obtained from the reaction mixture after work-up. These crystals consisted of a 1 : 1 mixture of the imidazolium salts [Me4NHC(H)]14 and [Me4NHC(H)]Cp (see SI material).

Scheme 3. Reaction of BCHGe 3a with iPr2Me2NHC

Yellow single crystals of the imidazolium salt [iPr2Me2NHC(H)]14 that were suitable for X-ray diffraction analysis were obtained from a concentrated THF solution at T = -30°C. The asymmetric unit of [iPr2Me2NHC(H)]14 is shown in Figure 4. The imidazolium ion is well separated from the germanium anion (Ge / C23 distance: 386 pm), although the C23 – H23 bond vector points directly to the germanium atom suggesting the onset of ion-pairing through a C – H – Ge hydrogen bond. The strongly pyramidalized germanium atom (sum of the bond angles around the germanium atom, Σα(Ge) = 265.5°) is bonded to three carbon atoms (Ge1 – C1/4 = 210.1, 208.2 pm and Ge1 – C13 = 202.7 pm, see Table 1). This pyramidalization of the germanium atom is typical for germanates(II) such as the [GeCl3]- anion.15 The formation of the third Ge – C13 bond is accompanied by a significant pyramidalization of C13 as indicated by the bending of the Ge – C13 bond by 29° out of the plane of the Cp – ligand.16 Both non-equivalent cyclopentadienyl ligands are η5-bonded to the hafnium atom as indicated by the planar arrangement of all five carbon atoms and nearly equidistant C – C bonds (140 – 143 pm). Nevertheless, the coordination of the Cp-ligand bonded to the germanium atom is less symmetric with Hf – C(Cp) distances varying from 232 pm to 275 pm due to the formation of the new Ge – C13 linkage.17 As expected for the electronically saturated germanium atom, there are no structural indications for an interaction with the C2 = C3 double bond in the structure of anion 14. Instead the electron deficient Cp2Hf fragment is coordinated to the C2 = C3 double bond (see Figure 4, in particular the small C2/3 / Hf separations and the short C1/4 – Hf bonds in comparison to the regular Hf – C bond length of 227 pm and the small flap angle α(Hf) = 105°). Focusing on the interaction of the hafnocene fragment with the GeC4 perimeter, germanate 14 is a typical example for σ2, π-coordination of the butadiene part of the germole ring to the hafnium atom with no significant Ge / Hf interaction.6



iPr Me

Figure 4. Molecular structure of ion pair [ 2 2NHC(H)]14 in the crystal. Hydrogen atoms, methyl groups of Me3Si-substituents and solvent molecules are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected atom distances [pm] and angles [°]: Ge1–C1 210.1(8), Ge1–C2 281.6(9), Ge1–C3 281.2(8), Ge1–C4 208.2(8), Ge1–C13 202.7(9), C1–C2 146.7(12), C2–C3 140.2(12), C3–C4 145.7(12), Hf1–C1 224.4(8), Hf1–C2 255.2(8), Hf1–C3 256.0(8), Hf1–C4 226.2(8), Ge1–Hf1 277.1(5), C13–C14 143.6(13), C13–C17 143.3(13), C14–C15 141.1(14), C15–C16 140.1(15), C16–C17 141.4(13), α (Ge) 143.7, α(Hf) 105.0.

Chart 5. Bicyclo[2.1.1]hexene germylene iron complex 15.

that the relative ratio of nickel complex 16 increased with lower temperature (see Figure 5 for VT 29Si{1H} INEPT NMR measurements). A quantitative evaluation of the VT 1H NMR measurements in benzene-d6 allowed the determination of the thermodynamic parameters of the equilibrium shown in Scheme 4. (∆H = -29.6 kJ mol-1 and ∆S = -116 J mol-1K-1, see SI material). At ambient conditions the formation of the bisgermylene complex 16 according to Scheme 4 is slightly endergonic (∆G(T = 303K) = 5.5 kJ mol-1). There is no noticeable decomposition of the NMR sample when stored at room temperature for one week. Black single crystals that were suitable for XRD analysis were isolated from toluene solution at T = -30°C. In the solid state, nickel complex 16 is stable at room temperature for at least six months. In contrast, when dissolved in benzene, it underwent a fast decomposition reaction. 1H NMR spectroscopic investigation of the decomposition reaction support the mechanism shown in Scheme 5 with BCHGe 3a, Ni(COD)2 and Ni metal as final products (see SI material). Scheme 4. The reversible reaction of BCHGe 3a with Ni(COD)2

Me

2

Cp Hf

SiMe3

Cp

SiMe3

C 6D6 Hf

Me

CO Fe

Cp

Ge

+ Ni(COD) 2

Me3Si Ge

Ge

Me3Si

CO OC

Page 6 of 11

SiMe3

3a

Cp Hf SiMe3 + COD

Ni COD

16

CO △ #

Ge

10°C

SiMe3

Me

20°C

HfCp2

Me Me3Si

30°C

15

The described reactivity pattern of BCHGe’s 3 versus Lewis-basic NHCs are in line with our preliminary investigations on the reactivity of germylene 3a which revealed its predominately nucleophilic character.5 These studies revealed that germylene 3a behaves in complexes with transition metals as strong σ-donor with only small π-accepting abilities. For example, in complex 15 with low valent iron (Chart 5), the Fe – Ge bond is in the range of typical single bonds and the structure of germylene 3a is widely conserved and is dominated by the Ge / C2 = C3 interaction.5 Here, we extended our study to late transition metals such as low valent nickel and gold. In their low oxidation states both metals found widespread use as catalysts in organic synthesis. Therefore, possible modifications of their electronic structure and, in consequence, of their reactivity by ligation with BCHGe’s 3 are of principle interest.

40°C

50°C

60°C

0

-1

-2

-3

-4

-5

-6

-7

Figure 5. VT 29Si{1H}INEPT NMR spectra (from δ29Si = 1.0 to -7.5) of the reaction of BCHGe 3a with Ni(COD)2 to form nickel complex 16 (# nickel complex 16, △ BCHGe 3a).

The reaction of two equivalents of BCHGe 3a with Ni(COD)2 (COD: cyclooctadiene) at room temperature was evidenced by a color change from orange to brown-red. The NMR spectra of the reaction mixture indicated the formation of the expected bis-germylene nickel complex 16 (Scheme 4) but also an incomplete reaction even after a reaction time of 24 h. At room temperature, the finally obtained ratio of product 16 to starting germylene 3a was 0.28 : 1 (for details, see SI material). Variable temperature NMR measurements showed


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Scheme 5. Decomposition reaction of nickel complex 16 in benzene solution. Cp

Cp Hf

Cp

SiMe3

Cp

Ge

Hf

Me3Si

SiMe3

Me

C 6H 6

SiMe3

2

Ge Ge SiMe3 Ni COD

205.8(14), C1–C2 148.6(2), C2–C3 140.9(2), C3–C4 147.2(2), Hf1–C1 223.6(14), Hf1–C2 270.0(14), Hf1–C3 268.0(14), Hf1– C4 223.9(15), Ge1–Hf1 291.1(4), Ge1–Ni1–Ge2 115.4(10), α(Ge1) 116.8, α(Hf1) 115.8.

Hf

Me

i Pr

+ NiCOD

Hf

Me

N t

Pr 4

i SiMe3 + 1/2 Ni(COD) 2 + 1/2 Ni 0

Ge - Ni: 224 pm

Me3Si

Ni

Ge

Ni

N

Ge

2

N Ge

3a

Me

BuCH2

N

Me3Si

16

t

BuCH2

4

Ge - Ni: 223-224 pm

17

18

3a

The molecular structure of complex 16 is shown in Figure 6. The nickel atom adopts a distorted tetrahedral coordination environment. The Ni – C(COD) distances (209.9-214.5 pm) in complex 16 are close to those in Ni(COD)2 (211-213 pm).18 The two germylene units are coordinated to the nickel center and enclose a relatively wide Ge1 – Ni1 – Ge2 angle of 115.4°. Similar to iron complex 15, the germanium atoms in complex 16 adopt nearly ideal trigonal planar structures (Σα(Ge1) = 355.2°, Σα(Ge2) = 352.6°). The Ni – Ge bonds are 223 - 224 pm, in between of the sum of theoretically predicted covalent radii for Ni – Ge single bonds (231 pm) and Ge = Ni double bonds (212 pm).19 For homoleptic Ni complexes 17 and 18 with N-heterocyclic germylenes (NHGe’s) similar Ni – Ge bond lengths have been reported (Figure 7).20 The short Ni – Ge bond in 19 indicates the presence of Ni – Ge 3d – 4p – πbonding,21 while the long Ni – Ge bond in the anionic germanate(II) nickel complex 20 provides a typical example for a Ni – Ge(II) single bond (Figure 7).22 Based on this short literature survey the Ni – Ge bond lengths in complex 16 indicates some degree of 3d(Ni) – 4p(Ge) – π-backbonding, which competes with the electron donation from the C2C3 π-bond into the 4p(Ge) orbital. As a result the flap angles α(Ge) increase significantly (α(Ge) = 113.2°, 116.8°) which indicates less homoconjugative stabilization of the Ge(II) center. Consequently, the electron deficient hafnocene group approaches now the C2 = C3 bond as shown by the smaller flap angle for Hf (α(Hf) = 114.0, 115.8°, see Figure 6) suggesting electron donation from the C2 = C3 bond to the hafnium center and onset of a σ2, π-coordination.

Figure 6. Molecular structure of nickel complex 16 in the crystal. Hydrogen atoms, methyl groups of Me3Si-substituents and solvent molecules are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected atom distances [pm] and angles [°]: Ge1–Ni1 223.2(2), Ge2–Ni1 223.6(3), Ge1–C1 204.4(14), Ge1–C2 247.7(15), Ge1–C3 250.8(15), Ge1–C4

FMes

PPh 3 Ge Ni

F

PPh 3

Mes

Ge - Ni: 218 pm 19

F5C2 F5C2 Ge Ni(CO)3 F5C2

Ge - Ni: 231 pm 20

Figure 7. Selected Ge–Ni bond lengths of germylene-nickel complexes and related germanate-nickel complexes (FMes: 2,4,6tris-(trifluormethyl)-phenyl.

Less clear was the outcome of the reaction of BCHGe 3a with the Au(I) complex, [Ph3PAuCl]. From the reaction of germylene 3a with [Ph3PAuCl] in THF, two major products were isolated and separated due to their different solubility in hexane (Scheme 6). We propose as the first intermediate in this reaction the cationic gold germylene complex 21. Complex 21 undergoes a valence isomerization to form the η5germole complex 22. Subsequent addition of chloride and substitution of one cyclopentadienide group gives the η5germole hafnium complex 23. The formation of the complex 23 is a slow process as the formed Cp- anion is able to serve as a scavenger reagent for either of the cationic complexes 21 or 22 to form the germyl complex 24 in 23 % isolated yield. Our structure proposal for the η5-germoledianion hafnocene complex 23 is based on analysis of the NMR spectra. The 1H NMR resonances at δ1H = 7.47, 7.02 and 6.79 indicate the presence of three phenyl groups and one Cp substituent. Two additional sharp signals at δ1H = 2.81 and 0.58 in the typical region for methyl- and trimethylsilyl groups of the germol ring were detected. The relative low field resonance of the methyl groups indicate deshielding due to an aromatic ring current and suggest a η5-coordination of the germol ring to the hafnium atom. This is further supported by low-field-shifted 13C NMR resonances for the two magnetically different germole ring carbon atoms at δ13C = 140.5 (C1/4) and 141.9 (C2/3) (Table 2). Similar NMR parameters have been reported for the related hafnium complex 8 (δ1H(Me) = 2.25, δ13C = 135.8, 146.0).12a The 31P NMR signal of 23 is not visible at room temperature in C6D6 but appears at T = -70 °C in C7D8 (δ31P = 50.7).23 The second product of the reaction was identified as the gold germolyl complex 24 by NMR spectroscopy and by an XRD analysis of red crystals grown from hexane solution at -30°C. The well separated 13C NMR signals of the carbon atoms of the germole ring at relative high field (δ13C = 80.1 (C1/4) and 132.5 (C2/3), Table 2) suggested its formulation as a η4-germole complex as their positions are



characteristic for hafnium complexes in which a diene is bound in a σ2, π-metallocyclopentene fashion to the hafnium center. Similar 13C NMR chemical shifts are found for the germanate 14 (Table 2) and are reported for the hafnium complex 11 (δ13C = 97.2, 130.0).12b Three singlets of equal intensity in the 1H NMR spectra at δ1H = 6.46, 6.24, 5.36 suggest the presence of three different Cp rings along with the phenyl groups of the triphenylphosphane ligand (two multiplets at δ1H = 7.44 and 7.01). The presence of the PPh3 ligand is indicated by its typical 31P NMR resonance at δ31P = 47.7.23b These assignments are confirmed by the results of the XRD analysis that revealed the molecular structure of a gold(I) complex with linear coordination of a germolyl and a phosphane ligand (α(Ge-Au-P = 177.3°, Figure 8). One additional cyclopentadienyl substituent is bonded to the germanium atom in a η1fashion and completes its coordination sphere. The new formed Ge – C23 bond (Ge – C23 = 213.3 pm) is significantly longer than regular Ge – C single bonds involving tetracoordinate germanium and carbon atoms (Ge – C = 196 pm).19 As a result of the tetracoordination of the germanium atom, its interaction with the C2 = C3 double bond is canceled and as already exemplified for germanate 14, the germole ring is now η4-bonded to the hafnium atom. The structural parameters of complex 24 clearly indicate the σ2, π-interaction between the butadiene part and the hafnium center with short Hf / C2/3 separations and regular single bonds between Hf and the bridgehead atoms C1 and C4 (see Figure 8 and Table 1). The Ge – Au bond in complex 24 is 243.9 pm, nearly the value expected from the tabulated covalent single bond radii (Ge – Au = 245 pm)19 and from comparison with compounds with germanate Ge(II) − Au(I) single bonds (i.e. Ge – Au = 241.2 pm in [Au{GeCl(N(SiMe3)2)2}{Ge(N(SiMe3)2)2} or Ge – Au = 242.8 pm in Ph3PAu{Ge(CN)(Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2)].24 Not unexpected, it is longer than the Ge(II)−Au(I) bonds in previously reported germylene-gold complexes (Ge – Au = 233 – 237 pm).25 Scheme 6. Reaction of BCHGe 3a with [Ph3PAuCl] PPh 3

PPh 3

Au

3a + Ph3PAuCl

Me 3

Ge

1 SiMe3

2

4

Me

Cl Cl Hf SiMe3 1 Me 2 Ge Au PPh 3 Me 3 4 SiMe

Au

Ge

Me3Si

1

Me3Si

2

3

Cl

-

-Cp -

Me

Me

21[Cl]

+ Cl-

Hf

4

Hf

SiMe3

Cl

3

-

22[Cl]

23

Hf + Cp- Cl-

1

Me 2 Me 3

4

SiMe3

Ge SiMe 3

24

Au

PPh 3

Page 8 of 11

Figure 8. Molecular structure of gold complex 24 in the crystal. Hydrogen atoms, methyl groups of Me3Si-substituents and solvent molecules are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected atom distances [pm] and angles [°]: Ge1–Au1 243.9(4), Au1–P1 231.2(8), Ge1–C23 213.3(3), Ge1–C1 203.7(3), Ge1–C2 265.6(27), Ge1–C3 265.1(27), Ge1–C4 203.1(3), C1–C2 147.5(4), C2–C3 139.4(4), C3–C4 147.9(4), Hf1–C1 228.7(3), Hf1–C2 254.4(3), Hf1–C3 255.8(3), Hf1–C4 229.7(3), Ge1–Hf1 303.9(4), C23–C24 145.1(5), C23–C27 146.0(5), C24–C25 136.9(5), C25–C26 142.0(5), C26–C27 135.7(5), α (Ge) 129.6, α (Hf) 100.5.

III CONCLUSIONS Hafnocene-based bicyclo[2.1.1]hexene-type germylenes 3 can be prepared by reaction of aromatic germole dianions 1 and hafnocene dichloride (Scheme 1). We investigated the previously reported formation of hafnocene based bicyclo[2.1.1]hexene-type germylenes 3 from aromatic germole dianions 1 and hafnocene dichloride in detail. Our NMR and UV results provide ample evidence for hafnocene germylene intermediates 2 in agreement with previous theoretical models.5 The germylene complexes 2 are stabilized by an additional chloride ion as donor for the electron-deficient hafnium center. Our attempts to substitute the chloro-ligand by the Nheterocyclic carbene Me4NHC resulted in the replacement of one cyclopentadiene ring by the NHC and isolation of the hafnium NHC complex 7. Interestingly, the germole ring acts in complex 7 as an aromatic η5-germole dianion ligand which provides six electrons to the hafnium center. The transformation of the germylene complex 2a to η5-germole complex 7 points already to the intriguing structural flexibility of the germole ligand. The instability of BCHGe 13 towards loss of elemental germanium suggested to us that the hafnocene based BCHGe’s 3 might be an effective Ge(0) transfer reagent. Surprisingly, in reactions with NHCs the C1/4 – Ge linkage of BCHGe 3a remained untouched. Achilles’ heel proofed to be the C – H bonds of the cyclopentadiene rings. An unprecedented deprotonation of the hafnocene moiety by the NHC occurred to yield the unusual triorganogermanate(II) 14. In anion 14, the germanium atom is electronically saturated and the diene part of the germole ring forms with the hafnocene group a σ2, π-metallocyclopentene structure, which is typical for a η4-coordination of the germole ring to the hafnium. As we reported previously, BCHGe 3a forms stable metal com-


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plexes with low valent tungsten and iron.5 The bonding in these complexes is controlled by σ-donation from the germylene to the metal and the homoconjugative interaction between the C2=C3 bond and the germanium atom remains for the germylene ligand structurally dominant. The reactivity of BCHGe 3a versus more electron rich late transition metals proved to be more complex. With the Ni(0) precursor, Ni(COD)2, the reversible formation of the bis-germylene nickel complex 16 was observed. We determined for this reaction a slightly positive change in the Gibbs energy by NMR methods (∆G = 5.5 kJ mol -1, at T = 303 K), which indicates the lability of the germylene nickel complex 16. Its structural parameters suggest the onset of 3d(Ni) – 4p(Ge) πbackbonding that competes with the homoconjugative interaction between the 4p(Ge) and the remote C2=C3 double bond. This is verified by the structure of the germylene ligand in nickel complex 16 which shows a reduced interaction between the germanium atom and the C2=C3 bond and the onset of a σ2, π-coordination for the hafnocene part of the molecule. In the reaction with the Au(I) reagent [Ph3PAuCl] two different germole gold complexes were obtained. After elimination of one cyclopentadiene ligand the η5 germole gold complex 23 was obtained, which shows close similarities to the NHC hafnium complex 7. In addition, the gold germanate(II) complex 24 was isolated, which is structurally closely related to anion 14.

ing electron back-donation from the substituent at germanium, homoconjugation is pushed back and the coordination of the hafnocene moiety to the C2=C3 double bond is enhanced. In this respect, the nickel complex 16 takes up an intermediate position (α(Ge) = 113, 117°) but the molecular structures of anion 14 and the germanium ligand in gold complex 24 are determined by the σ2, π-coordination of the hafnocene (α(Hf) = 105, 101°) and the tri-coordinated germanium atom. In cases where the electron deficiency of the hafnium is overwhelming, i.e. after elimination of a cyclopentadiene ring, the germanium atom is integrated into the diene π-system and the germole ring serves as an aromatic η5-coordinated ligand to the hafnium center as demonstrated for complex 7.

ASSOCIATED CONTENT Supporting Information. Experimental, NMR spectra and details of the structure solution of compound K[2a], 3b, 7, 14, 16, 23, 24. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is dedicated to Robert West in occasion of his 90th birthday and was supported by the Carl von Ossietzky University Oldenburg and by the Lower Saxony State by a Lichtenberg Fellowship to Z.D..

REFERENCES

Figure 9. Comparison between structural parameters of germylene 3a, germylene complexes 15, 16 σ2, π-metallocyclopentene 5 complex 14, 24 and η , π-bonding complex 7.

In all these compounds the germole ring reveals an intriguing structural flexibility which is shown by the variations of the flap angles α(Ge) and α(Hf) (Figure 9). Homoconjugation between the germanium atom and the C2=C3 double bond is structure determining for the free BCHGe’s 3 and for iron carbonyl complex 15. The central structural motif can be described as σ2, π-coordination of the diene part to the germanium(II) center as shown by small angles α(Ge) (α(Ge) = 98 – 106°, Figure 9). The hafnocene part is bonded by two σbonds, which closes the bicyclohexene structure and leaves the hafnium atom electronically unsaturated (16e). With increas-

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Homoconjugative interaction between Ge atom and C2=C3 increases

(Ge) Ge Flexible 1 2 3

HfCp2

4

(Hf) Coordination number between Hf atom and germole ring increases

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Hafnocene-based Bicyclo[2.1.1]hexene Germylenes â Formation

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