Gas phase germylation of simple aromatics by Me3Ge+ ions

ELSEVIER

Journal ofOrganometallic Chemistry 545-546 (l 997) 53-59

Gas phase germylation of simple aromatics by Me 3Ge+ ions Barbara Chiavarino, Maria Elisa Crestoni, Simonetta Fornarini • Dipartimento di Studt di Chimica e Tecnologia delle Sostanze Biologicamente Attire, Unirersita di Roma •La Sapienza', Piazzale AldoMora 5, I·OOlfi5, Roma, Italy

Received 28 October 1996; received in revised form 22 April 1997

Abstract The gennylation ofsimple aromatic hydrocarbons has been studied by two different techniques, The Fourier Transform Ion Cyclotron Resonance (FTICR) technique allowed the study ofMe 3Ge-!- transfer equilibria between aromatics and H20 at 300 K. The ion-molecule reaction of [MejGe "- arene] ions with selected neutrals are accounted for by a rr-cornplex species, retaining the primary ipso structure, where the germyl group and the former aromatic hydrogen are bound t<> the same tetrahedral carbon, The free energies of binding of Me 3Ge + to the selected aromatic compounds correlate linearly with the values of the corresponding Me.,Si + reaction with a slope of 0,93. The combined information from the FTICR and radiolytic technique has revealed a mechanistic pattern for the eJectrophilic aromatic substitution by Me-Ge + that closely reproduces the corresponding Me 3Si + reaction, the major difference lying in the greater ease ofdetachment ofMe 3Ge+ by nucleophilic assistance. © 1997 Elsevier Science SA Keywords: Trirnethylgermyl cation; Gas phase; Ion cyclotron resonance; Radiolysis

1. Introduction

In continuation of our research on the gas-phase trimethylsilylation reactions by Me3Si+ ions [1-6], we have undertaken a stuoy on the reactivity of the neighbor group IVA analogue, Me3Ge+, towards (mostly) aromatic compour.ds aimed to compare the reactivity behavior of the two ions. Although early reports have shown that MeJGe+ may form stable adducts with a variety of orga-nic molecules [7], this ion has never received a comparably widespread attention as Me3Si-!-. One practical reason may be bound to the complex isotopic pattern of germanium CaGe, n Ge, 73Ge, 7 Ge, 76 Ge, with relative abundances of 20%, 27%, 8%, 37%, 8%, respectively) which allows ready mass spectrometric identification of germylated ions but at the same time spreads the overall intensity of a single ionic species and enhances the likelihood of overlapping signals. We have used an approach based on two techniques. The first one is a mass spectrometric technique, Fourier Transform Ion Cyclotron Resonance (FrICR) [8J. endowed with the powerful capability of isolating an ion of interest within a complex mixture. This ion • Corre~ponding author. Tel.: + 3!}-6-499135 10; fax: +39-649913602, oo22-328Xj97/$17.oo © 1997 Elsevier Science S0022-328X(97)00318-5

eu

SA All rights reserved.

may be a single isotopic species or the whole isotopic cluster corresponding to a given composition. In the presence of one or more neutrals in the IeR cell, at typical pressures of W- H Torr, ion-molecule reactions can be recorded and equilibria can set up at the cell room temperature of 300 K. The second technique exploited, the radiolytic method [9]. performs ionmolecule reactions at the pressure of ca. I atm using ionic species produced by the action of '}'-radiation and yielding mechanistic information from the analysis of neutral products formed in the selected range of conditions. The combination of the twotechniques has yielded a comprehensive picture of the major features of the Me3Ge + reaction with aromatic compounds.

2. Experimental

Research grade gases with stated purity in excess of 99.9% were purchased from Matheson. All otherchemicals were obtained from commercial sources, including Me4Ge from Alfa Products. They were analyzed by GC and purified by preparative GC, whenever necessary. Gaseous samples for the radiolytic experiments were prepared by standard vacuum line techniques, as previously described [9]. The gaseous mixtures were irradi-

54

B. Chiatarino et al.] Journal of Organometallic Chemistry 545-546 (1997) 53-59

ated in a 220 Gammacell (Nuclear Canada) at 40°C, to total doses ranging from 0.5 X 104 Gy to 1 X l0 4 Gy, at the dose rate of I X 104 Gy h- I. The radiolytic product mixture was recovered under airtight conditions as cyclohexane solution and analyzed by GC-MS using a Hewlett-Packard 5890 gas chromatograph equipped with a 50-m long, O.20-mm i.d. fused silica column coated with a O.SO-1J.m cross-linked methyl silicone film (PONA column from Hewlett-Packard) in line with a Hewlett-Packard mass selective detector model 5970B. The FrICR experiments were performed using a Bruker Spectrospin Apex TM47espectrometer equipped with an external ion source and a cylindrical (Iz-cm length, l2-cm diameter) 'infinity' cell situated between the poles of a 4.7 Torr superconducting magnet. This instrumental configuration allows thegeneration of ionic species in the external ion source where a suitable chemical ionization (Cl) reagent gas is introduced. For example a CH4/Me4Ge (97:3 0101%) mixture at the pressure of I X 10-4 Torrwasused to generate Me3Ge+ ions which were allowed to react with a selected substrate (S =: aromatic hydrocarbon, H20 , MeOSiMe 3 ) introduced into the ion source by a separate inlet. The resulting ion mixture was transmitted into the FTICR cell at 300 K, where one or two different neutrals were present at stationary concentration. The ion of interest, typically Me3GeS +, was isolated from the plasma by sequential broad band ejection and 'single shots' [lO] and was allowed to react. Ion intensity vs. time profiles were obtained by recording mass spectra at increasing reaction time. The concentration of the neutrals was determined from the pressure reading at the ionization gauge located in the high vacuum lineof the FrICR cell housing, corrected by using appropriate calibration factors [11].

80 1(%)

60 40 20

H

time

21 (s)

2e

35

Fig. l. Normalized ion intensities of McC 6H~GeMe j
in

adduct ion was isolated from other ions which entered or were formed in the cell by resonance ejection techniques and was allowed to react with the mixture of the two neutrals for increasing reaction times (Eq. (I »: Me3GeX+ + y

~

Me3GeY++ X.

(I)

Plots of relative ion intensities vs. time were thus obtained, showing the attainment of equilibrium when the ion intensity ratio reaches a constant value. An example of such plots is shown in Fig. 1 where X =' MeC 6H s and Y =' H2 0 . The equilibrium constant at the cell temperature of 300 K is calculated from Eq. (2) (where i are ion intensities and P are partial pressures) and yielded a aG~oo value (kcal mol- I ) according to Eq. (3): i(Mc3GeY +) X P(X) K- i(Me3GeX +) X P(Y) , aG~oo

= -RT In K =' -0.60In K.

(~

(3)

The attainment of a true equilibrium is verified by the constancy of the right-hand ratio of Eq. (2) for

3. Results

3.1. FTlCR mass spectrometry FrICR has been used in the first place to measure Me3Ge + transfer equilibria between aromatics or between an aromatic and H20 (Eq. (J». The experiments were performed as follows. Me3Ge + ions from the CI of CH4/Mc4Ge iil the external ion source were allowed to react with an aromatic compound (X) introduced into the same ion source at 0.1 % relative concentration. Abundant adduct ions (MejGeX +) were formed, These were transferred into the FrICR cell where known concentrations of the same aromatic compound and of a reaction partner (another aromatic or H2 0 ) were present. An alternative route to Me3GeX + ions by reaction of Me3Ge + with an aromatic compound in the ICR cell failed to give significant abundances of adduct ions due to the inefficient collisional stabilization in the low pressure environment (10- 8-10- 7 TOlT). The primary

Fig. 2. Measured free energy changes, AGjoo. for Me.Ge " transfer between aromatics and H20. The direction of the preferred transfer (I1G' negative) is in the upward direction. Comparison of overlapping andsuccessive steps on the ladder show an internal consistency of about ±2 kcal mol ~ I . Reproducibility of I1Go values was better than ± I kca! mol- I.

B. Chiacarinoet al./ Journal of Organometollic Chemist/}· 545-546 ( 19971 53-59 100-...".-----------,

Table I Freeenerg)' changes for the process Me)Ge+ +X -= MeJGeX + X

- L\~~o (kcal mol I)

1,3.5-Me JC f,H J 1,3.MezCc> H4 MeJ Si Cc> H5 MeC(>H 5

20.2 18.8 17.1 16.5 13.7 18.3h

c.n, H 20

55

1(%)

10

1 -kh-.-,,-,--~.,..............,.............,-,_r_"]rr_..___._j o 4 6 a 10 2 lime (s)

"Avenlge error ± O.I kcal mol- I. h Rcf. [12].

different ratios of the partial pressures of the t\\ 0 neutrals. The aG~oo ladder obtained in this way is d.isplayed in Fig. 2 and was used to derive the free energies of binding of Me3Ge+ to the various compounds (reaction 4) from the known thermodynamic data relative to the Me3Ge + association to H20 [12l. Me3Ge++ X ~ Me3GeX+ . (4) The so obtained aG~.j) values :re. listed in Ta~le J. . The reaction of Me3GeC ijD6 IOns, formed III the IOn source by CH.j/Me 4Ge-CI of C(,D6 , with Et3N introduced into cell was also studied by FrICR, yielding the temporal profile of ion intensities shown in Fig. 3. The exponential decay of the reactant ion yields a bimolecular rate constant of 3.7 X 10- 9 em' S-I, corresponding exactly to the collisional rate constant [13]. This is indicative that the Me3GeC 6 ion reacts with the strong base on every collision. Among the ionic products, besides the trimethylgermylated amine at mlz 220 C4 Ge isotope), the ion at mlz 103 is significant. It corresponds to the Et3 ND + and shows that the reactant ion can transfer not only Me3Ge + but also a deuteron to the base. The reaction of MeO(SiMe 3)GeMe;' ions, obtained by Me3Ge+ addition to MeOSiMe 3 , with (MeO)3PO, a powerful oxygen nucleophile, has been performed to check as to what relative extent Me3Ge+ transfer vs. Me3 Si+ transfer would occur. The temporal profiles of

D:

Fig. 4. Normalized ion intensities of MeO(SiMe 3)GeMe .t ("'), (McO),POGeMe.i (t), (MeO»)POSiMcj (D). MeJGe + (e), and Me ,Si ~ (0) as a function of time. The pressure of (MeO)JPO was 1.2X10- 8 Torr.

ion intensities plotted in Fig. 4 show thatthe heterolytic cleavage of the Ge-O bond occurs preferentially to give both the free Me]Ge+ ion and a higher fraction (ca. 3:)) of the (MeO)3POGeMet product ions with respect to the silicon counterpart, (MeO)3POSiMej. A greater ease of Me 3Ge + transfer to oxygen nucleonhiles apparently emerges also from the reaction of Me3SiC 6 HsGeMet ions, illustrated in Fi~. 5, from CH4/Me4Ge-(,f of Me3SiC6H s• with H20 In the ICR cell at 5.9 X 10- 9 Torr. The products ions, corresponding to Me3GeOHi and Me3GeOH(SiMe3)+, are suggested both to follow from the primary product ion-neutral complex, [Me3GeOH r · Me3SiC6H sl, by direct dissociation or further reaction within the complex (Scheme I). Noteworthy, no Me3SiOHr is formed. 3.2. Radiolytic reactions

Radiolytic germylation reactions have been performed at 40°C in a 97:3 mol% mixture of CH4/Mc4Ge at 620 Torr, where ionization is effected by the secondary electrons following the interaction of ')I-radiation with the materials (mainly the vessel glass walls) of the radiolyzed sample. Table 2 summarizes the results fr?m the germylation of toluene and of benzene as competing

100
100_=-------------,

1(%) I ('Yo}

10

; ;

10 /

o

I

4

I

B

....-~+---<>

-, -

12

16

time (5)

Fig. 3. Normalized ion intensities of Me.,GeC(>D6+ (D), ~eJGeNEt ; (0). EtJNO -!- Lcd, [EIJN-H]+/EtJN+ (.), EIJNH ( ...). and Me,Gc+ (e) as a function of time, The pressure of EI,N was 1.7 X10- 9 Torr.

o

5

time (5)

10

15

Fig. S. Normalized ion intensities of Me.,SiC6H5Ge~ej ( ...). MeJGeOHr (e). Me.,Ge-!- (0). and Me3GeOH(Sj~:3) (0) as a function oftimc. The pressure of H20 was 5.9X 10 Torr.

B. Chiauarino et a/./ Journal of Organometallic Chelllistry 545-546 (1997) 53-59

S6

5..,------------, o

o 2

o o

o

o

o

[Me3GeOH· Me3Si~Htl

0+----,-....,--.,.----.--,---1

Me3GeDH(SiMe3)+ + C6H6

-

o

2

3

Scheme I.

4

5

6

P(Et3N)

Fig. 6. Overall radiochemical yield (GM' in units of IO - J fLmmol J- I ) of the germylation products of benzene and toluene ploued vs. P(Et;N) (Torr).

substrate. All systems contained a radical scavenger, 02' at 10 Torr, whose presence allows to neglect con-

ceivable radical pathways to the observed products. In order to obtain germylation products from the aromatic hydrocarbons, the presence of a strong nitrogen base. such as Et3 N. with proton affinity (PA) equal to 232 kcal mol -I [14]. was found to be a necessary condition. With no such additive present or when (MeO)3PO (PA = 212 kcal mol ") [14] is used in the place of Et3N , no aromatic gerrnylation takes place. The isomer distribution of the toluene products, MeC6H4GeMe3' is characterized by the absence of the ortho isomer and by the distinct preference of parawith respect to meta-substitution. The series of experiments run at nearly constant amounts of benzene and toluene and varying concentrations of Et3N yield both C6HsGeMe3 and MeC 6H4GeMe 3, whose combined yield are plotted against the Et3N partial pressure in Fig. 6. Their relative yields can be analyzed by the standard equation for competing reactions (Eq, (5), where GM are radiochemical yields in units of 10- 3 /Lmol J" I (note b of Table 2) to give apparent relative

reactivity ratios that are found to be dependent on Et3N concentration (Fig. 7). kr

k ll

=

GM(MeC6H 4GeMe3 ) XP(C nH 6 ) GM(C 6H sGeMe3 ) X P(MeC 6H 5 )

4. Discussion 4.J, The reagent ion

Me3Ge + is the only initial ion resulting from the reaction of Me4Ge with the major reagent ions CH; and C2 I-!; formed from methane under CI [15,16] and radiolytic conditions [17]. The exothermicities of the formation processes (Eqs, (6) and (7») were obtained from thermochemical data in Ref. [14], using IiH~ (Me 3Ge +) = 167 kcal mol -I [18] and 6.Hr (Me 4Ge) = - 24.5 kcal mClI- 1 [I 9].

CHt

+ Me4Ge -) Me 3Ge+ + 2CH 4 •

where Ii H(~) = - 68 kcal mol -

System composition (Torr)"

Product yields (GM )h

C6H 6

C6H5GeMe;

MeC 6H 4GeMc; (o/m/p)

IS

2.5

2.9 :U 3.2 2.9 3.3 3.3 3.3

Et;N

2.3 1.9 2.6 0.94 1.1 1.2 0.92 1.2 1.2 1.1

I,

(7)

where Ii Ht7J = - 37 keal mol - t • At the pressure of 10- 4 Torr in the CI ion source conditions of the FrICR instrument, Me3Ge+ is the 14.,.-----------,

c

1.0 1.6 0040 0.89 1.5 2.2 2.9 4.0 5.1

(6)

C2 Ht + Me 4Ge-.. Me3Ge ++C 2 H4+CH 4 ,

Table 2 Aromatic substitution by gaseous MeJGe + ions MeC 6H 5

(5)

•

12

0.20 0.36 1.0 1.0 1.0 0.82 0.59

5.2(0/18/82) 0.77(0/24/76) 1.5(0/IS/E2) 3.5(0/17/83) 2.4 (0/21/79) 2.1 (0/19/81) 1.3 (0/23/77) 0.73(0/18/82)

"The major gaseous components are CH4 (600 Torr), Me4Ge (20 Torr), and 0 1 (10 Torr). bAbsolute yields are in units of 10-; fLmol J " I • C(M~O)lPO was present, in place of EllN.

II

10

II

II

(kT'k n )npp

8

II

6

II II

4

2 -t----,---r---r-----r---r-----J

o

2

3

4

5

6

P(Et3N)

Fig. 7. Apparent relative reactivity ratios from the competitive germylation of benzene and toluene ploued vs, P(Et;N) (Torr). Values are obtained using Eq. (5).

B. Chiararino et 01./ Journal o!Org(l/Iollletallic Chemistry 545-546 (1997) 53-59 22.,.-------------,

Me3GeC 6H s is missing from the scant and scattered thermochemical data available for germanium compounds, !:J.H~ (Me3GeC6H s) was estimated as 8 kcal mol- I by the group increment method using thermochemical data from Ref. [22]. In this way, a PA value of ca. 212 kcal mol-I is derived for Me3GeC 6H s'

C

H20

16 14

benzene

4.3. Mechanism of the germylation reaction and structure of [Me3 Ge ';" arene] complexes

12 +---,--,..--.,..--,.--,----1

12

57

14

16

18

20

22

24

6.GO(Mc,Si?)

Fig.8. Thecorrelation between LlG~1J1J for theassociation of MeJGe + and LlGJIJO for theassociation of Mc3Si + 10 the selected compounds.

only germylating species. At the much higher Me4Ge pressure of the radiolytic experiments, an association equilibrium is expected to set up (Eq. (8» yielding Me7Gei" ions [12,]8] which may act as germylating species by Me4Ge displacement. Me3Ge+ + Me4Ge ~ Me7Gei.

(8)

Me3Ge + is expected to behave exclusively as Lewis acid in view of the rather high PA of its conjugate base (PA of Me2Ge=CH 2 eqrals to 206 kcal mol-I [20]). Accordingly, proton transfer from Me3Ge + to toluene is calculated to be endothermic by ]6 kcal mol- I (PA of toluene equals to 190 kcal mol- I [14]). The exclusive Lewis acid character of Me3Ge+ toward the selected substrates makes the apparent rate constant ratios k T / k B of Fig. 7 a direct measure of the corresponding reaction channels.

4.2. The association of Me3Ge + to aromatic hydrocarbons The free energies of binding of Me3Ge+ to aromatic hydrocarbons show an increasing trend in the series from benzene to mesitylene, corresponding to the increasing electron density available at the aromatic ring from the electron donating effect of the methyl substituentls), The same order corresponds to increasing PA [14] and increasing binding energy to Me3Si + [2IJ. The values of AG~oo for the binding of the two Me3X+ ions(X = Si, Gel to the aromatic basesare veryclose to eachotherand fit the linear relationship drawn in Fig. 8. The slope of 0.93 indicates that the association free energies of Mc3Ge+ spread over a narrower range, hence they are slightly less sensitive to the structural features of the aromatic base. Within the approximation that the close values of AG~oo for the association of Me3Ge+ and Me3Srr 10 benzene (13.7 and 13.5 kcal mol", respectively) are paralleled by similarly close Aft' values (23.9 kcal mor ') [21], a thermochemical cycle can be constructed, allowing the PA of Me)GeC6H s to be evaluated. It is unfortunately not surprising that the required value of AH~ for

The closely similar thermodynamic data for the association of Me3X+ (X = Ge, Si) to simple aromatics suggests for the [Me3Ge+' arene] adduct the same acomplex structure which several experimental [I -6] and theoretical [23] studies ascribed to gaseous [Me 3Si +. arene] complexes. This structural assignment is supported by the reactivity of the [Me 3Ge +. arene] adducts. Me3GeC6 Ht reacts with H20 by Me3Ge+ transfer, a process that may be compatible with either a 1T-complex (Ia) or an ipso-cr-complex (Ib) structure of the reactant ion.

Ib

la

However, the Me)GeC 6D: reaction with Et)N occurs by D+ transfer to an appreciable extent and this is strongly indicative of covalent CbenlP-ne-Ge bond formation. The overall reactivity pattern fits into a mechanistic scheme (Scheme 2 for C6H o as representative arene) that accounted for the salient features of the corresponding reaction of Mc3Si + with aromatic hydrocarbons []-6]. At variance with rr-complex Ia, expected to react exclusively by MejGe" transfer, the rr-complex Ib may undergo competitive H+ and Mc3Ge + transfer, depending on the features of the neutral reagent. The low PA of H20 (166.5 kcal mol- I) [I 4] prevents H+ transfer from Ib whereas Me3Ge+ transfer is driven by the intrinsically higher bond energy of Ge-O with respect to Ge-C, The same factor may account for the larger binding free energy of Me3Ge+ to H 20 with respect to CoH6 , in the reverse order of their PAs (166.5 and

Nu

Ib

_J

ll. Scheme 2.

B. C"iavarino et al./ Journal of Organometallic Chemisrry 545-546 (1997) 53-59

58

181.3 kcal mol'", respectively) [14]. The use of Et3N, a strong though sterically hindered nitrogen base, enables deprotonation of lb . The proposed mechanism is supported by the product pattern of the radiolytic germylation where the recovery of neutral germylation products unambiguously proves the formation of a MC3Ge-C covalent bond through a o-ccmplex intermediate. The presence of an appropriate base, Et3N, is required to ensure formation of germylated products. In its absence, traces of oxygenated nucleophiles, unavoidably present, either as adventitious impurities (e.g., H20 ) or as radiolytic products (e.g., H20 , MeOH, CH20 , etc.) react with Ib exclusively by Me3Ge+ transfer. Accordingly, no aromatic germylated products are formed. However, when Et3N is added to the gaseous mixtures, two opposing effects on the product yield can be envisaged. On the one hand, Et3N allows deprotonation of Ib favoring formation of the germylated products. On the other, the amine effectively intercepts the CHt IC 2 H; and Me3Ge + percursors, thus depressing the yields of the intermediates Ib and therefore of the ensuing neutral products. This dual behavior accounts for the dependence of the absolute yields of the germylated products on the partial pressure of Et3N shown in Fig. 6, which is characterized by a maximum at the Et3N pressure of ca. I.5 Torr. It is worth noting that a highly basic additive such as (MeO)3PO as well is not effective in yielding germylation products. This result supports an ipso-type structure for the germylated arenium intermediate since any isomeric ion obtained by 1,2-H+ shift would he deprotonated by (MeO)3PO leading to observable products (Scheme 3 for MeC 6Hs as representative arene), The unwillingness of ion Ib to undergo isomerization by 1,2-H+ shifts, which contrast with the behavior of alkylated arenium ions [24,25], is traced to the higher stability conferred to Ib by the f3 relationship of the formal positive charge on the arenium ring with respect to the Caryl-Oe bond with an orbital alignment allowing hyperconjugative interaction [26-28]' In conclusion, [Me3Ge +' arene] complexes undergo Me3Ge + transfer to oxygen nucleophiles and deprotonation by strong nitrogen bases. With a second arene molecule, they engage in reversible Me3Ge+ transfer till equilibrium is reached, as can be observed directly from the ICR kinetic plots. Direct evidence for Me3Ge+ transfer from Me3GeC 6 Ht to toluene is also obtained from the apparent krlk B ratios from the radiolytic

H~H

(M",>e.

(MeO)3POH+

+ MeC6""OcMe3

M:

GeMc3

Scheme 3.

reactions, which increase as the EtJN partial pressure is decreased (Fig. 7). This is accounted for by a rather indiscriminate primary attack of Me3Ge + at the two substrates to yield an ion population which subsequently tends to shift toward the equilibrium ratio. The Me3Ge + transfer to toluene competes with deprotonation by Et3N. Decreasing the Et3N partial pressure favors the approach to equilibrium by increasing the lifetime of the [Me 3Ge +. arene] ions, which cannot, however, be increased indefinitely due to the presence of trace oxygen nucleophiles, The highest apparent krlk B ratio of 12 in Fig. 7 is still far from the equilibrium value of [MeC 6 H4GeMetl/[Me 3GeC 6Ht] = 35, expected under the operative reaction conditions.

4.4. Comparison of the gas phase reactivity of MeJGe + and MeJSi+ Several times in the previous sections, the gas phase behavior of Me3Ge + towards aromatic hydrocarbons has been found to conform to the model Me3Si + reaction. The similar behavior of the two electrophiles is confirmed also by the positional selectivity of the attack at the ring positions of toluene. In both cases substitution at the ortho position is avoided due to steric hindrance, and the para/meta ratiois ca. 80/20, showing fair selectivity towards the more activated site. However, a noticeable difference emerges from the absolute radiochemical yields of the two reactions. The gcrmylated products are formed with a smaller yield, typically by a factor of ten, with respect to silylation products under strictly comparable reaction conditions. Whereas the similar binding free energies do not lend support to the hypothesis of a higher barrier for eletrophilic attack by Me3Ge+ with respect to Me3Si+, an explanation can be found in the greater ease of displacement of Me 3Ge+ from the arenium complex by oxygen nucleophiles. Such a feature was recently exploited to perform a silylation reaction via Me3Ge + displacement from Me3GeC6Hs. thus obtaining for the first time silylated products without the assistance of a nitrogen base [6]. The high mobility of Me3Ge + is shown in the reported reactions of Me3SiC 6H sGeMe j with H 20 and MeO(SiMe3)GeMe; with (MeO)3PO. Although the heterolytic cleavage and transfer of Me.Ge+ is not strictly comparable to the MejSi+ reaction as they yield different ionic and neutral products, still it appears significant that the former reaction prevails by a noticeable extent.

5. Conclusions The gas-phase behavior of Me3Ge + ions toward aromatic hydrocarbons has shown striking similarities to the corresponding Me3Si+ reaction. This conclusion

B. Chiararillo ct (//. j Joumal of OrgunometallicChemistry545-546 (1997i 53-59

emerges primarily from t~e close values of the free energies of binding of Me3 X'" (X = Ge, to the same aromatic compound, the only difference lying in the barely detectable smaller sensitivity of the Me3Ge + association to the structural features of the compounds in the series benzeneytciuene/m-xyleueymesltylene. The mechanistic pattern leading to aromatic substitution products was elucidated by the combined information from the FrICR and radiolytic techniques. The major features can be summarized as follows: (j) Me3Ge+ reacts at the aromatic ring giving a o-complex intermediate; (ii) this primary intermediate retains the hydrogen on the gerrnylated carbon, showing no tendency to isomerize by 1,2-H+ shifts; and (iii) the ipso-type o-complex may evolve eitherby deprotonation by strong nitrogen bases or by degermylation by oxygen nucleophiles, the latter an obviously blind reaction channel. All gathered evidence point to this last step as responsible for the major differences observed between the Me3Ge+ and Me3Si+ reaction. In fact, heterolytic cleavage of the C-Ge bond assisted by oxygen nucleophiles appears to occur with greater ease than the corresponding C-Si bond cleavage and the Me3Ge + group appears endowed with greater mobility than its silicon analogue. In the absence of pertinent thermodynamic and kinetic data, reasons accounting for this different behavior. including the role of the more expanded d orbitals on germanium, can only be guessed.

so

Acknowledgements Financial support by the Italian Ministero dell' Universita edella Ricerca Scientifica e Tecnologica is gratefully acknowledged. The authors thank Prof. F. Cacace for helpful discussions.

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