ELSEVIER Inorganica Chimica Acta 251 (1996) 5-7

Preliminary Communication

Organornetallic silicon-based dendrimers with peripheral Si-cyclopentadienyl, Si--Co and Si-Fe o'-bonds

Isabel Cuadrado *, Mois~s Morkn *, Angel Moya, Carmen M. Casado, Mario Barranco, Beatriz Alonso

Departamento de Qufraica Inorg~nica, Oniversidad Autdnoma de Madrid. Cantoblanco. 28049-Madrid. Spain

Received 16 April 1996

Abstract

The new cyclopentadienyl functionalized dendrimer Si [ (CH2)3SiMe2(CsHs) ]4 (1) has been prepared and used to It-coordinate Co(CO) 2 moieties, affording the tetranuclear Si[(CH2)3SiMe2(~-CsH4)Co(CO)2]4 (2). The Si-H and Si-Ci functionalized dendrimers Si[(CH2)3SiMe2H],t (G1HI) and Si[(CH2)3SiMe2CI]4 (G1CI0 have been reacted with C02(CO)s and Na + [(~-CsHs)Fe(CO)2]-, respectively, to yield the corresponding organometallic dendritic molecules Si[(CH2)3SiMe2Co(CO)4]4 (3) and Si[ (CH2)3SiMe2(~'f- CsHs)Fe(CO)2] 4 (4), containing peripheral Si--Co and Si-Fe o'-bonds.

Keywords: Organometallic macromolecules; Silicon denddmers; Cyclopentadienyl derivatives; Carbonyl complexes

The design and synthesis of dendrimers, that is, highly branched tree-like functionalized macromolecules con- structed by iterative reaction sequences, is an area of research which is undergoing dramatic growth [ 1 ]. The incorporation of metal complexes [2] and organometallic moieties [3,4] into dendritic structures, provides new valuable opportunities to modify the advantageous properties of dendrimers. We recently reported the preparation of several families ofredox- active fe~ocenyl silicon-based dendrimers [ 3,5 ], which have been used successfully in the modification of electrode sur- faces [ 6]. In addition, a series of silicon-containingdendritic molecules in which peripheral arene rings are ~/6-coordinated to Cr(CO)3 moieties have been prepared [7].

Within organometalfic chemistry cyclopentadiene is per- haps the most important and widely used ligand [8]. Since cyclopentadienyl derivatives, or- or u-bonded to the central atom, are known for all transition metals, we envisaged that a synthetic challenge could be the incorporation of the cyclo- pentadienyl ligand as a surface functionality into well defined dendritic structures, which would offer many opportunities to cover the surface of dendrimers with useful organometallic fragments. Thus, following our quest of new organometallic dendritic structures, we now wish to report some early gen- erations of silicon-based dendrimers in which cyclopenta-

* Corresponding authors.

0020-1693/96/$15.00 © 1996 Elsevier Science S.A. All fights reserved PII S 0020-169 3 ( 96 ) 05 347- 9

dienyl, carbonykobalt and carbonyliron functionalities ate attached at the dendritic surface.

The reaction of alkaline cyclopentadienides, either NaCp in TI-IF or LiCp in diethyl ether (Cp= CsHs), ~ t h the four- directional dendrimer GICII [3,5], was successfully per- formed, affording the desired cyclopentadienyl-fuuctionali- zed organosilicon dendrimer 1 1 (Scheme 1). The ~H HMR spectrum of this first generation dendritic cyclopentadienyI derivative shows the pattern of resonances characteristic of the cyclopentadiene a-bonded to a silicon atom at 6.58 and 6.47 ppm, together with the resonances corresponding to the methyl and methylene protons in the organosilicon dendritic core, in the expected integrated ratio, denoting the successful peripheral tetrafunctionalization of the startingdendrimer, To our knowledge, this is the first example of denddmers possessing reactive cyclopentadienyl Iigands as surface functionalities.

The coordinating ability of these surface located cyclopen- tadienyl ligands was assessed via the reaction with octacar- bonyldicobalt. Treatment of I with C02(CO)s in CH2CI2 at reflux temperature, and in the presence of 1,3-cyclohexadi- ene, afforded after appropriate work-up, the desired brown-

' All new compounds gave salisfactot0, spcclroscopic and anaIydcal l: selected NMR data (CDCI3): ~H HMR: 66.58, 6.47 (1~, 16H. CsHs), 2.96 (4H, CsHD, !.33 (m, 8H. CH2CH2CI-Iz ), 0.58 (m, 16H. C//~L'H2CH2), -0.09 (s, 24H, SiCH3); 13C('H} N'MR: 8133.06,129.94 (CfJ'Is), 29.31- 17.36 (CH2), - 1.95 (SiCH3).

6 !. Cuadrado et al./htorganica Chimica Acta 251 (1996) 5-7

H I

Me -S I -Me

H --Sl / ~--~_sl__Me

: N*-SI°Me !

H G 1 H|

h e x l ~

OC CO oc~ .'.,co

Co I

OG Me-SI-Me

^ . - ~ - - . , , . . . c o u~ / s,_co ~.co

Me \ Me-SI-Me CO

I Co

ac co 3

I ~ Me -SI-Me

/ I ~ S I I ~ _ _ .l~

Me.SI) Me Me ~CO I

eli Me-SI-Me

CI-SL / ~.~-s~,-~.,.. f s,-c,

Mo-SI-Me I~ CI

G1CI 1

Scheme 1.

- s I - M e

[~H¥' "

? "-,s:, 1 Me-'/-~.

Co

Me -Sl- Me oc,, , ~ , ~ f

~Co l~b'-Sl. ( oc" "~ ~.-',-~i-,_,.u. ,~ ,,,co

J Me ~ 'co Me - SI- Me

Co / \ OC CO

ish-red tetrametallic derivative 2 2. Support of the complete c/S-coordination of the cyclopentadienyl ligands to the Co(CO): moieties was provided by tH and ~3C NMR data and FAB-MS, as well as by the presence in the IR spectrum of 2, of the two typical v(CO) bands at 2022 and 1961 cm- 5 which are in accord with the proposed structure. In particular, in the tH NMR spectrum of 2 the protons of the "r/5-CsH4 ring show the expected upfield shift (to 5.2 and 4.8 ppm) con- firming the metal coordination to the four cyclopentadienyl ligands.

On the other hand, the reactivity of octacarbonyldicobalt towards Si-H bonds also offered an easy synthetic access to a different modification of the periphery of den&imers. Co2(CO) s reacts at 0°C in n-hexane with the Si-H function- alized dendrimer Gll-l~ [ 3 ], in a process that involves hydro- gon elimination accompanied by the cleavage of the metal- metal bond, to afford the tetrametallic dendritic macromole- cule 3 a, which bears the organometallic moieties directly attached to the dendritic framework through cobalt-:ilicon o-bonds. In the IR spectrum, the total absence of the u(SiH) band at 2113 cm - ', and the observation of the typical v (CO) stretchhlg vibrations characteristic of the newly introduced Co-(CO), units gave evidence of the complete silicon- cobalt attachment.

: 2: selected NMR data ( CDCI 3): )H NMR: 85.20, 4.8"/(br, 16H, CsH4), !.36 (m, 8H, CHzCH2CHz), 0.75 (m, 8H, CsH,SiCHz), 0.58 (m, 8H, SiCHz), O. 19 (s, 24H. SiCH~); i'~C{ IH } NMR: 6205.46 (CO), 88.77, 87.48 (C~H4) 23.66-17.16 (CH2), -3.96 (SiCHD. Mass spectrum (FAB): m/z ! 144 (M + ).

3 3: tH NMR (C,D,s): 6 2.17, 1.85, 0.88 (m, 24H, CH2), 0.59 (CH3). Mass spectrum (FAB): mlz 1112 (M+). IR: v(CO) 2090, 2028, 1994 cm- I.

Additional evaluation of the reactivity of the surface of the SiCl-functionalized dendrimer G1Cla was provided by treatment with a THF solution of the carbonyl anion, Na + [ (CsHs) Fe (CO) 2 ] - , affording the desired tetran uclear dendrimer 4 4, which contains iron-silicon a-bonds at the periphery. As for the preparation of the tetrametallic 2 and 3, the attachment of the iron carbonyl-containing fragment can be easily monitored by IR spectroscopy, in particular, for the disappearance of the v(CO) l~ands at 1880, 1863, 1808 and 1779 cm- ~ due to the starting carbonyl anion and the appear- ance of new carbonyl bands at 1989 and 1931 cm-t. In the ~H NMR spectrum, evidence for the complete functionali- zation of the organosilicon dendritic surface is provided by the ratio of integration of the cyclopentadienyl and methyl and methylene protons. The 29Si NMR spectrum shows the two expected signals corresponding to the two different sili- con atoms, and interestingly, the outermost silicon, directly bound to the iron atom, appears low-field shifted by about 40 ppm.

In view of the high reactivity and widespread use of the cyclopentadienyl ligand in organometallic chemistry, periph- eral cyclopentadienyl tings in dendritic structures, as in 1, are promising as they would allow the surface of dendrimers to be covered with many useful organometallic entities via or- coordination. This opens new possibilities in catalysis and multi-electron redox and photochemical processes. Work is

4 4: selected NMR data (CDCI3): 1H NMR: 64.69 (s, 20H, C~Hs), IAO (m, 8H, CH2CH2CH2), 0.91 (m, 8H, Fe-SiCH2), 0.60 (m, 8H, SiCH2), 0.35 (s, 241t, SiCH3); I~C{ 'H} NMR: 6215.54 (CO), 83.27 (CsH D, 20.52- 17.19 (CH2), -4.00 (SiCH3); 2vSi{IH} NMR: 8 42.7t (Si-F¢). Mass spectrum (FAB): mlz ! 136 (M +). IR: v(CO) 1992, 1936 cm- i

!. Cuadrado et al. / inorganica Chimica Acta 25 i (1996) 5-7 7

in progress in order to advance all the above described syn-

theses to higher generations.

Acknowledgements

We thank the Direcci6n General de Investigaci6n Cientf-

fica y T~cnica (Proyect PB 93-0287) for financial support.

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