Stepwise solid-phase synthesis of di- and trinuclear metal complexes

Article abstract of DOI:10.1002/anie.200351849

Step by step: Oligonuclear organometallic complexes are constructed in a stepwise manner on insoluble polymeric matrices and directly characterized on the support (see scheme). Liberation under mild conditions yields the corresponding soluble oligonuclear complexes.

Full text of DOI:10.1002/anie.200351849

Angewandte
Chemie
Polymer-Bound Mo Complexes
Stepwise Solid-Phase Synthesis of Di- and
Trinuclear Metal Complexes**
Katja Heinze* and Juan D. Bueno Toro
Monodisperse oligomers play a central role in understanding
the chemistry and the physics of polymeric organic and
inorganic materials.[1] The synthesis of monodisperse oligom-
ers can be achieved by self-organization processes, random
synthesis followed by separation processes, or by a step-by-
step synthesis. All these strategies present their own advan-
tages and disadvantages. Full control over chain length, end
groups, and building-block sequence can be achieved using a
stepwise approach. While this strategy is conceptually
straightforward, it becomes increasingly tedious for higher
oligomers owing to the purification steps needed. A particular
elegant method for a repeated stepwise synthesis is the solid-
phase synthesis methodology which allows the purification of
all intermediates by simple filtration. This strategy has been
successfully applied to the synthesis of organic oligomeric
compounds[2] while organometallic oligonuclear complexes
have not been prepared in a stepwise manner by solid-phase
synthesis. Mononuclear metal complexes have been prepared
by solid-phase synthesis[3–12] for example, for biolabeling
purposes or oligonucleotide-DNA/RNA binding studies and
a dinuclear complex[13] has been synthesized by simultaneous
double metalation of a peptide bound to a solid phase.
We have already designed a system (polystyrene (PS)/
divinyl benzene–silyl ether linker–bidentate Schiff base
ligand; 1b in Scheme 1) which allows standard reactions of
coordination and organometallic chemistry to be performed
under solid-phase reaction conditions and the release of the
final products from the insoluble matrix by fluoridolysis of the
silyl ether linker.[14,15] The first application of this solid-phase
reaction system to the synthesis of di- and trimetallic
complexes is reported herein.
The molybdenum carbonyl complexes reported herein
possess several properties which make them suitable for solid-
phase synthesis approach, these are: 1) substitutionally inert
metal–ligand bonds (molybdenum–carbonyl and molybde-
num–isonitrile bonds), 2) carbonyl and isonitrile ligands
which serve as sensitive analytical probes for IR spectroscopic
analysis of the solid-phase system,[16,17] 3) highly colored
complexes which permits UV/Vis spectroscopic analysis of
[*] Dr. K. Heinze, Dipl.-Chem. J. D. B. Toro
Anorganisch-Chemisches Institut
Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax: (+49)6221-545-707
E-mail: katja.heinze@urz.uni-heidelberg.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. The generous support of
Prof. Dr. G. Huttner is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 4533 –4536
DOI: 10.1002/anie.200351849
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4533

Communications
Scheme 1. Synthesis of oligonuclear molybdenum carbonyl complexes by solution phase (1a–5a; X=H) and solid-phase (1b–5b; X=PS-Si(iPr)2)
ꢀ
methods. 1) [(CH3CN)3Mo(CO)3]; 2) CN (N\N’); 3) [(CH3CN)2Mo(CO)4].
the solid-phase system), and 4) thermally releasable carbonyl
ligands which can be detected by thermogravimetric (TG)
analysis of the solid-phase system.
The reactions performed are summarized in Scheme 1.
Stirring the functionalized resin 1b[15] with an excess
of [(CH3CN)3Mo(CO)3] yields
a reactive, immobilized
[(diimine)Mo(CO)3(CH3CN)] complex which, after filtration
and washing, can then be treated with an excess of the
[18]
ꢀ
bridging isonitrile Schiff base ligand CN (N\N’) to give
the green, immobilized tricarbonyl isonitrile complex 2b
ꢀ
(Scheme 1) (the replacement of the CO ligands by CN
(N\N’) does not occur under these conditions[15]). This
complex possesses a potential chelating diimine moiety. To
this diimine unit a {Mo(CO)4} end cap is attached by treating
the resin with an excess of [(CH3CN)2Mo(CO)4] to give the
black dinuclear complex 4b (Scheme 1). Alternatively, the
chain-growth steps can be repeated by adding
Figure 1. IR spectra of resins 2b and 3b.
conveniently be observed by these methods. Figure 1 shows
the IR spectra of polymers 2b and 3b. The relative intensities
ꢀ1
ꢀ
~
~
[(CH3CN)3Mo(CO)3] and CN (N\N’) to form the green
of the nCO band (n = 1923, 1856, 1828 cm ) and nCN band (n =
dinuclear complex 3b (Scheme 1). End capping with
[(CH3CN)2Mo(CO)4] furnishes the immobilized trinuclear
complex 5b.
2080 cmꢀ1) approximately double when
a
second
ꢀ
{Mo(CO)3CN (N\N’)} fragment is added to resin 2b to
give 3b (compared to the nCH bands of the polystyrene
backbone as internal standard). The same observation holds
Thermogravimetric analysis of the metal-complex-func-
tionalized resins 2b–5b shows that upon heating to 2008C
carbon monoxide is released. This result is confirmed by IR
spectroscopic analysis of the gas phase and by thermogravi-
metric analysis of the free complex 2a (see Supporting
Information). The amount of CO released at 2008C corre-
sponds well to the expected CO loss.[19] This finding indicates
the expected number of molybdenum–carbonyl units are
present in the polymers 2b–5b.
As IR and UV/Vis spectra of these oligonuclear com-
plexes are superpositions of spectra of corresponding mono-
nuclear complexes (see the Supporting Information and refs.
[18,20,21]) the complex growth on the solid support can
for resins 4b and 5b which additionally display the character-
ꢀ1
~
istic nCO absorption bands (n = 2013 cm ) of the {Mo(CO)4}
fragments (Figure 2).
The colors of the complex-functionalized resins arise from
several absorption bands in the visible region: p–p* absorp-
tions of the diimine chromophore at around 400 nm, metal-to-
ligand charge transfer (MLCT) bands of the [(diimine)Mo-
(CO)3(CN-R)] chromophore at around 680 nm and MLCT
bands of the [(diimine)Mo(CO)4] chromophore at around
600 nm.[15,18,20,21] Consequently, the UV/Vis spectra of the
immobilized di- and trinuclear complexes display absorption
bands corresponding to the presence of the incorporated
4534
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Angewandte
Chemie
tedious separations. In contrast the solid-phase approach
enables the use of simple purification techniques that are
identical for all intermediates and optimization of each step is
not needed. Moreover, regents can be used in excess to drive
the reactions to completion and thus avoiding premature
chain truncation. These advantages outweigh the additional
steps needed for solid-phase synthesis (functionalization of
the polymer and cleavage) especially for higher oligomers.
Experimental Section
ꢀ
1a, 1b and (4-isocyanophenyl)pyridine-2-ylmethylene amine CN
(N\N’) were prepared by published procedures.[15,18] All the poly-
styrene resins used were cross-linked with 2% divinyl benzene.
Ligand loading of resin 1b was 0.50 mmolgꢀ1. All manipulations were
carried out under argon. For the solid-phase reactions a flask with a
nitrogen inlet and a coarse-porosity fritted glass filter that allows
addition and removal of solvents and reagents without exposure to
the atmosphere was used. Spectroscopic measurements were carried
out as described elsewhere.[15]
Figure 2. IR spectra of resins 4b and 5b.
chromophores: resins 2b and 3b exhibit absorption bands at
400 and 680 nm, while resins 4b and 5b additionally absorb at
600 nm. Thus resins 2b and 3b with only [(diimine)Mo-
(CO)3(CN-R)] chromophores appear green and resins 4b and
5b with additional [(diimine)Mo(CO)4] chromophores
appear black.
Cleavage of the immobilized complexes 2b, 3b, 4b, and
5b with tetra-n-butylammonium fluoride and protonation of
the soluble anionic complexes with acetic acid (Scheme 2) is
2b: [(CH3CN)3Mo(CO)3] (115 mg, 0.38 mmol) dissolved in
CH3CN (4 mL) was added to resin 1b (500 mg, 0.25 mmol) swollen
in toluene (5 mL). The suspension was stirred for 20 min and the resin
turned from yellow to blue. The solvents were removed by filtration
and the polymer was washed with toluene/CH3CN (1:1) until IR
spectroscopy indicated complete removal of the excess
[(CH3CN)3Mo(CO)3]. Owing to the sensitivity of the immobilized
complex, the resin swollen in toluene was immediately treated with
ꢀ
the ligand CN (N\N’) (79 mg, 0.38 mmol) dissolved in THF(5 mL).
The polymer turned green immediately and after 30 min the resin was
washed with THFuntil IR spectroscopy indicated complete removal
ꢀ
of the excess CN (N\N’). The resin 2b was dried in vacuo giving a
Scheme 2. Cleavage of the complexes 2a–5a from the support.
green powder or used directly for further reactions. No soluble
[(diimine)Mo(CO)x] complexes were detected by IR spectroscopy at
easily monitored by IR spectroscopy of the solution. The
appearance of nCO absorption bands in the IR spectra of the
solutions indicates the release of the carbonyl complexes from
the solid support into solution, protonation of the anionic
complexes shifts all the nCO absorption bands to higher energy
as expected (see Experimental Section). Removing the
insoluble support by filtration gives complexes 2a, 3a, 4a,
and trinuclear complex 5a, respectively (Scheme 2). The
soluble complexes 2a–5a were characterized by standard
spectroscopic techniques. The 1H NMR spectra of solutions of
2a–5a display several resonance signals in the aromatic
region. Significant shifts of signals arising from the protons of
~
any stage of the reaction. IR (CsI): n = 2084 (m, CN), 1921 (vs, CO),
1857 (s, CO), 1826 cmꢀ1 (s, CO). UV/Vis: lmax = 660 nm. TG: Dm/m =
4.4% at 2008C.
3b: prepared by the same procedure as 2b but starting from 2b
IR (CsI): n = 2083 (m, CN), 1923 (vs, CO), 1856 (s, CO), 1830 cmꢀ1 (s,
~
CO). UV/Vis: lmax = 660 nm. TG: Dm/m = 7.7% at 2008C.
4b: [(CH3CN)2Mo(CO)4] (110 mg, 0.38 mmol) dissolved in THF
(4 mL) was added to resin 2b (obtained from 500 mg 1b) swollen in
toluene (5 mL). After stirring for 2 h the resin was washed with THF
until IR spectroscopy indicated complete removal of the excess
[(CH3CN)2Mo(CO)4]. The resin 4b was dried in vacuo giving a black
powder. No soluble [(diimine)Mo(CO)x] complexes were observed by
~
IR spectroscopy at any stage of the reaction. IR (CsI): n = 2086 (m,
CN), 2013 (m, CO), 1919 (vs, CO), 1851 (s, CO), 1826 cmꢀ1 (s, CO).
UV/Vis: lmax = 710 nm. TG: Dm/m = 8.7% at 2008C.
ꢀ
the dangling CN (N\N’) ligand in 2a and 3a are observed
upon coordination of the {Mo(CO)4} unit to give 4a and 5a,
respectively: the signal of the a proton of the pyridine part of
the dangling ligand is shifted downfield by 0.6 ppm, the signal
of the d proton is shifted to upfield by 0.3 ppm, and the signal
of the imine proton is shifted to higher field by 0.1 ppm as a
result of coordination to the metal and the transoid/cisoid
rearrangement of the ligand (Scheme 1, see Supporting
Information).
5b: was obtained by the same procedure as for 3b but by starting
~
from 3b. IR (CsI): n = 2084 (m, CN), 2014 (m, CO), 1922 (vs, CO),
1853 (s, CO), 1833 cmꢀ1 (s, CO). UV/Vis: lmax = 720 nm. TG: Dm/m =
11.2% at 2008C.
Cleavage of the resin bound complexes: tetra-n-butylammonium
fluoride (TBAF·3H2O; 158 mg, 0.5 mmol) in THF(5 mL) was added
to the appropriate resin 2b–5b (starting from 500 mg of 1b; 0.25
mmol) swollen in THF(3 mL). The suspension was stirred slowly for
6 h. The colored solution was collected by filtration and the resin was
washed thoroughly with THFuntil the washings were colorless. The
combined filtrates were carefully acidified with degassed acetic acid
(0.05 mL) to give intensely colored solutions of the respective
complexes. Characteristic IR absorption bands before acidification:
For comparison purposes all these complexes have also
been prepared by standard solution synthesis (Scheme 1, X =
H; see Supporting Information). In the solution-phase syn-
thesis fewer synthetic steps are needed for the synthesis of
oligonuclear complexes. However, purification of intermedi-
ates is much more difficult. Additionally, in solution, exact
stoichiometric conditions have to be complied with to avoid
ꢀ
ꢀ1
~
2a : n =ꢀ2093 (m, CN), 1914 (vs, CO), 1840 (s, CO), 1810 cm (s,
ꢀ1
~
ꢀ
CO); 3a : n = 2091 (m, CN), 1914 (vs, CO), 1839 (s, CO), 1812 cm
~
(s, CO); 4a : n = 2094 (m, CN), 2009 (m, CO), 1912 (vs, CO), 1897 (vs,
CO), 1835 (s, CO), 1808 cmꢀ1 (s, CO); 5a : n = 2094 (m, CN), 2009
ꢀ
~
Angew. Chem. Int. Ed. 2003, 42, 4533 –4536
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ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
(m, CO), 1911 (vs, CO), 1900 (vs, sh, CO), 1835 (s, CO), 1808 cmꢀ1 (s,
CO). The complexes were precipitated by adding diethyl ether/
hexanes. Overall yields (solid-phase synthesis, cleavage, and acid-
ification) based on the loading of 1b were between 40 and 85%
(depending on the solubility of the complex). After cleavage, mass
spectrometry of the remaining silyl fluoride resins indicated the
complete removal of the complexes.[14,15] The complexes were
characterized by IR, UV/Vis, and 1H NMR spectroscopy and the
obtained data correspond to those obtained from the solution-phase
synthesized complexes 2a–5a (see Supporting information).
Received: May 9, 2003 [Z51849]
Keywords: bridging ligands · molybdenum · polymer-bound
.
complexes · polynuclear complexes · solid-phase synthesis
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[19] The theoretical amount of CO loss is calculated according to the
following formula: Dm/m = 100(nCO ꢀ MCO)/(Mresin + Mcomplex
)
with nCO = 3, 6, 7, 10, MCO = 28 gmolꢀ1, Mresin = 1500 gmolꢀ1
(resin loading 0.5 mmolgꢀ1) and Mcomplex = 387, 774, 595, 982
gmolꢀ1 for resins 2b, 3b, 4b,and 5b, respectively, giving Dm/m =
4.45, 7.39, 9.36, 11.28% for resins 2b, 3b, 4b, and 5b,
respectively.
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4536
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Angew. Chem. Int. Ed. 2003, 42, 4533 –4536