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Design and performance of rigid nanosize multimetallic cartwheel pincer compounds as lewis-acid catalysts


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Dijkstra, HP, Meijer, MD, Patel, J, Kreiter, R, van Klink, GPM, Lutz, M, Spek, AL, Canty, AJ and van Koten, G 2001 , 'Design and performance of rigid nanosize multimetallic cartwheel pincer compounds as lewis-acid catalysts' , Organometallics, vol. 20, no. 14 , pp. 3159-3168 , doi: 10.1021/om0101689.

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Novel strategies for the preparation of rigid cartwheel pincer metal complexes have been
developed. The aromatic backbone of these materials ensures a high rigidity, which is
expected to be important for a high retention when these multimetallic nanosize complexes
are applied as homogeneous catalysts in a nanomembrane reactor. The ligand precursors
C6[C6H3(CH2Y)2-3,5]6 (10, Y = NMe2; 11, Y = SPh; 12, Y = PPh2; 13, Y = pz = pyrazol-1-yl)
have been prepared in high yields from the key intermediate C6[C6H3(CH2Br)2-3,5]6 (9). The
hexakis(pincer) palladium(II) complexes C6[(PdX)-4-C6H2(CH2Y)2-3,5]6 (14, Y = SPh, L =
Cl; 15, Y = PPh2, L = Cl; 16, Y = pyrazol-1-yl, L = OAc; 17, Y = pyrazol-1-yl, L = Cl) have
been prepared via direct electrophilic palladation of the corresponding ligands. The
(tris)pincer ligand C6H3[Br-4-C6H3(CH2NMe2)2-3,5]3-1,3,5 (20) was prepared via a triplecondensation
reaction of 4-bromo-3,5-bis[(dimethylamino)methyl]acetophenone (19). Reaction
of 20 with Pd(dba)2 yielded the tripalladium complex C6H3[(PdBr)-4-C6H3(CH2NMe2)2-3,5]3-
1,3,5 (21). The crystal structure of 21 shows a propeller-like structure with D3 symmetry
and a fixed bromine-bromine distance of 17.4573(4) Å, approximately forming a triangle
with a height of 15.2 Å. These nanosize cartwheel pincer metal complexes based on tridentate
Y,C,Y' pincer ligands have been used as homogeneous Lewis-acid catalysts. Moreover, the
influence of the donor substituent Y on the catalytic activity of cationic mono-Y,C,Y' PdII
complexes as Lewis-acid catalysts in the double Michael reaction between methyl vinyl ketone
and ethyl alpha-cyanoacetate, as a model reaction, has been investigated. It was found that
cationic N,C,N'-type pincer complexes (1a, Y = NMe2; 1b, Y = pz; 1c, Y = pz* =
3,5-dimethylpyrazol-1-yl; 23) were superior to the P,C,P'- and S,C,S'-pincer complexes (1d,
Y = PPh2; 1e, Y = SPh). The nanosize cationic tri-N,C,N' PdII complex 23 was found to have
a catalytic activity per catalytic site in the double Michael reaction of the same order of
magnitude as the monopincer analogue 1a (k = 279 x 10-6 s-1 for 1a vs k = 232 x 10-6 s-1
for 23). The combination of the nanosize dimensions, the catalytic activity, and the high
thermal and air stability makes these complexes excellent candidates for application in a
continuous process in a nanomembrane reactor.

Item Type: Article
Authors/Creators:Dijkstra, HP and Meijer, MD and Patel, J and Kreiter, R and van Klink, GPM and Lutz, M and Spek, AL and Canty, AJ and van Koten, G
Journal or Publication Title: Organometallics
DOI / ID Number: 10.1021/om0101689
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