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Preparation of isocyano polymer supports and their complexes with catalytically relevant transition metal centers
Journat of Molecular
Catalysis,
53 (1989)
111 - 128
PREPARATION OF ISOCYANO POLYMER SUPPORTS AND THEIR COMPLEXES WITH ~ATAL~ICALLY RELEVA~ TR~SITION METAL CENTERS R. ARSHADY* Department
of Chemistry,
M. BASATO*,
College of Science,
B. CORAIN*,
Kashan (Iran)
M. RONCATO, M. ZECCA
Centro di Studio sulla Stabilita’e Reattivita’dei Composti di Coordinazione, Ripartimento di Chimica Inorganica, Metallorganica ed Analitica, University I-35131 Padua (Italy)
C.N.R. of Padua,
L. DELLA GIUSTINA ENEL - Compart~mento di Venezia, Sezione I-301 70 Porto Marghera (Venice) (Italy)
Chimica - c/o G.I.T. Marghera,
S. LORA Istituto di Fotochimica e Radiazioni I-35020 Legnaro (Padua} (italy}
d’Alta Energia, C.N.R. - Sezione
di Legnaro,
and G. PALMA Dipartimento
di Chimica Fisica, University
(Received July 15,1988;
of Padua, I-35131 Padua (Italy)
accepted November 10,1988)
Suspension or mass polymerization of vinyl comonomers with 3formamidopropyl acrylate affords widely solvent compatible, medium crosslinked polymers, which can be transformed into isocyano-containing resins. These materials (0.2 - 1.3 meq of isocyano function~ity per g resin), exhibit a strong coordinating ability towards a variety of metal centers including catalytically relevant metal complexes. The combination of IR spectroscopy and other analytical data makes it possible to reliably speculate on the actual composition and structure of the respective polymer-supported complexes. X-ray microprobe analysis shows that the metal distribution is homogeneous throughout the polymeric beads. Thermal analyses reveal that both resins and polymer-supported complexes are thermally stable up to 300 “C. Severe leaching tests (dichloroethane or toluene at 100 “C) show that the polyme~metal complexes are characterized by high formation constants. Under certain conditions, the involvement of one polymer-bound -NC group per metal atom is achieved, so that the anchoring of catalytically active species to the support produces only a slight modification of the coordination sphere of the metal centers. *Authors to whom correspondence should be addressed. 0304-5102/89/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
112
Introduction Nearly two decades ago, Haag and Whitehurst published the first reports [ 11 concerning a conceptual breakthrough in industrial catalysis [2]. The basic idea was the ‘robust’ anchoring of catalytically active metal complexes to insoluble supports, by means of covalent or ionic [3] bonds between the potential catalytic site and the support. In a large majority of cases [ 4,5], the supporting materials consisted of either crosslinked polystyrene or silica, and the main thrust of the idea was the achievement of a delicate balance between the stability of the metal-support bond and the survival of the catalytic activity of the metal center. However, none of these hybrid [2] catalysts has yet become the basis of an industrial process, for a number of reasons [6]. Certainly, reduced activity and leaching of the metal are among the most important reasons for the lack of industrial utilization. Quite surprisingly, little effort appears to have been devoted to employ polymer supports bearing ‘soft’ ligands other than phosphines. Isocyanides are ligands which are very versatile in their ability to form strong bonds with many transition metal centers, thus giving rise to homoand heteroleptic complexes [ 71. Isocyanides preferably bind low-valent metal ions, but they are also known to form stable complexes with medium to high valent metal centers (e.g. Rh(III), Ir(II1) and Co(II1) [S]). For all these reasons, isocyano polymer supports appear very interesting for the preparation of novel hybrid catalysts and also as convenient precursors for the preparation of highly dispersed supported metal catalysts. Styrene-divinylbenzene copolymers bearing the isocyano function were described by Skoma and Ugi in 1978 [9], and the first example of metal coordination of insoluble isocyano polymers was reported by Howell and Berry in 1980 [lo]. In 1982, Arshady and Ugi reported [ll] a new type of isocyano polymer support with relatively general solvent and substrate compatibility. At that time we initiated our work on the development of these isocyano resins for the preparation of polymer-supported transition metal complexes. Since then our preliminary results have been published in 1983 [12a], in 1986 [12b] and in 1987 [ 12~1. During this period, a number of other reports concerning related work on insoluble [13] and soluble [ 14 - 161 isocyano polymers have appeared in the literature. The present paper describes full details of the first phase of our work on the synthesis and coordination chemistry of tailored isocyano polymer supports. Results and discussion Synthesis and characterization of the isocyano polymer supports The structure of the isocyano polymer supports, @--NC, is illustrated by Scheme 1, and the quantitative details of the resins are given in Table 1.
113 TABLE 1 Quantitative data on isocyano resins Resin
-NC content (meq g-l)
Crosslinker (mol%)
Remarks
Pla Plb Plc Pld Ple Plf P2a P2b
0.30 0.49 0.10 0.58 1.28 0.23 0.36 0.31
8 8 8 8 8 8
-
1 6
a=88;c=lla a=84;c=10a
*Mel%, see Scheme 1 for the meanings of coefficients a and c.
V P2a P2b
0CH2CH20H OCH2CH20H
z NH
n CH2NH
0 CH2CH20
Pla, Plc
Plb, I Ple
Pld,Plf
1 6
Scheme 1. Schematic drawing of polymers employed.
Resin samples Pl were prepared by peroxide-initiated suspension polymerization according to [17,18]. These materials were obtained in bead form (average diameter about 50 pm). The isocyano functionality on the resins was titrated as described in [19]. Polymer samples P2 were obtained in spherical form (diameter - 1 mm) or as bulky cylinders (about 5 cm3) by y-ray initiated ‘bulk’ polymerization in ice matrix at -78 “C [203. In the latter case, the material was ground to small fragments of -1 - 5 mm. All the polymer samples swell in water and in dimethylformamide, methanol, dichloromethane, dichloroethane, acetic acid and to a small extent in toluene. Good shrinking media are diethyl ether and isobutanol. Typical SEM pictures of the swollen polymer types Pl and P2 are shown in Fig. 1. It can be seen that both resin types exhibit a sponge-like structure, indicating easy diffusion of reagents inside the bulk of the materials.
of monomer composition in type P2 resins The y-ray initiated polymerization is carried out to about 80% consumption of comonomers. Under these conditions, the overall compositions of the formamide precursors of P2a and P2b are in fair and excellent, respectively, agreement with the expected composition. The dehydration procedure employed is expected to also give tosylation of the OH groups present in the resins. The analytical comparison between the expected and the observed C, H, N, 0 and S values is therefore based on the assumption of complete esterification of the OH groups. The agreement is again excellent in the case of P2b and fair in the case of P2a. Both P2a and P2b show the expected medium-intensity YcN band at 2150 cm-’ and the titration shows 0.36 and 0.31 meq g-’ of isocyano groups respectively. Control
Evaluation
of the dehydration
procedure
The nitrogen content of polymer P2b stems only from the 3-formamidopropylacrylate (3FAPA) content. Therefore, a comparison between the nitrogen contents determined by elemental analysis and -NC titration [19a] makes it possible to evaluate the efficiency of both dehydration and titration procedures. In fact, titration values for P2b correspond to a recovery of 80% of the total nitrogen (0.43% vs. 0.53%). This result may be due either to incomplete dehydration or to some inaccuracy of the titration. In order to shed light onto this, we prepared a polymer (P3, Scheme 2), which is structurally related to P2b, but does not contain hydroxyl groups and with nitrogen present only in the 3FAPA unit. The agreement between the nitrogen percentage values obtained from the microanalysis and the titration for P3 was quite good (0.59% vs. 0.62%). This supports the efficiency of dehydration and titration procedures.
115
a ~83 isonitrile
;
bs4
;
charger0.44
c =,3
hOI.
xi
meg/Q
Scheme 2. Sketch of polymer P3.
Thermal stability of type PI and P2 polymers In view of the possible utilization of the crosslinked polymers herein described as supports for heterogeneous catalysts, their thermal behaviour was investigated under argon by means of combined TG-DTA measurements (Table 2). In all cases no endo-exothermal event was observed in the indicated temperature ranges. The loss of weight is observed only during the first heating half-cycle; the second heat treatment gave no weight loss. This suggests that the first weight loss is probably due to removal of traces of solvent. SEM pictures after thermal treatment reveal the perfect maintenance of the original morphology of the resins. Coordination chemistry of the isocyano resins The polymeric ligands Pl and P2, after swelling in a suitable solvent, reacted rapidly with concentrated solutions of all of the complexes listed in Table 3. Colour change of the resin and partial or complete bleaching of the solution of the reacting complex were taken as an immediately TABLE 2 Thermal bebaviour of polymers Pl and P2 under argon (heating rate 5 “C min-‘) Polymer
Stability range
Weight loss during 1st cycle (see text) (%)
Pld Ple Plf P2a
r.t. r.t. r.t. r.t.
6 12 2 0
- 250 - 320 - 200 - 180
“C “C “C “C
TABLE 3. Complexes of macromolecular isocyanides with various metal centers. Entry
Polymer (RNC, meq/g)
Reacting species
Reactiona (metal%)
Prevalent species formed
Solventh
Remarks
1
Pla (0.30)
Pd( acetate)2
lOO(3.2)
@-[(NC)Pd(acetate)z]
DCM
2
Plb (0.49)
Pd( acac),
30(1.5)
@-[(NC)Pd(acac)2
3
Plb (0.49)
PdC12(PhCN)2
.89(4.3)
DCM
4
Plf (0.23)
PdC12(PhCN)2
93(2.2)
5
Plf (0.23)
Pd(acetate)Q
90(2.1)
@-[(NC)PdCl,] or @--[WW’d’TP-C~)12. @--[ (NC)PdC12 ] or @-[(NCWCWC1)12 @-[ (NC)Pd(acetate)z]
6
P2a (0.36)
PdC12(PhCN)2
82( 3.0)
DCM
7
P2b (0.31)
PdC12(PhCN)2
92( 2.9)
8
Pld (0.58)
RuCls(PPhs)s
83(3.3)
@-[ (NC)PdC12 ] or @--[(NC)PdCWCI)lz @-[ (NC)PdClz ] or @-[(NC)PdCl(fi-C1)]2 @-[(NC)RuCl,(PPhs)s]
9
Pla (0.30)
RuC&(PPhs)s
92( 2.2)
@-[(NC)RuC12(PPhs)s]
DCM
10
Ple (1.28)
Ru&(PPhs)a
64(4.6)
@-[(NC)RuC12(PPhs)3]
DCM
11
Pld (0.58)
‘RuCls ! b
92(1.3)C
@--[(NC)RuC13(CH,),]
BTH
12
Ple (1.28)
‘RuC13’ b
57(6.3)
@-t(NC)2RuCls(CH2)1
BTH
2227(w, +76); no detectable free NC 2205(s, +58), 217O(w, + 23); no detectable free NC 2241(s, +94); no detectable free NC 2241(m, +91); no detectable free NC 2237(w, + 87); no detectable free NC 2249(m, +99); 215O(v.w, free NC) 2258(m, +108); 215O(v.w, free NC) 2135(s, -19); 2175(w, +24); no free detectable NC 2135(s, -19); 2175(w, +24); no free detectable NC 2135(s, -19); 2175(w, +24); no free detectable NC 2167(s, +17); 215O(w, free NC); initial RNC/Ru = 4.2 2185(s, +36); 2165(s, +16); no detectable free NC
DCM
I
DCM DCM
DCM DCM
CiS
13 14
Pld-(NC)RuC13(0H2)2 (entry 11) Plc (0.1)
RhCI(PPh3)3
15
Ple (1.28)
RhCl(PPh3)3
‘CoC12’
b
98(2.5)d e
/[WRuWOH2)21 ‘](NC)CoCWH,)l @-[(NC)RhCW’h3h
60(4.6)
AN
@
@--[(NC)2RhCI(PPh3)2]
1
DCM DCM
2228(w, +80); 2167(s, +19) no detectable free NC 2159(s, + 11); no detectable free NC 2182(s, +32); 2161(m, +ll); 2150(m, free NC)
16
Plb (0.49)
Rh(acac)(CO)z
55(2.6)
DCM
17
Pld (0.53)
Rh(acac)(CO)a
72(3.9)
DCM
18
Ple (1.28)
Rh(acac)(CO)z
56(6.3)
@-[(NWWacacWO)l
DCM
19
Pld (0.58)
‘RhCls ’ b
46(2.6)
@-[tNC)zRhWOH2)1
MET
20
Pld (0.58)
‘RhCls’ b
98(1.0)
21
Ple (1.28)
‘RhCla’ b
52(6.0)
22
Pld (0.58)
Cu( acac)z
20(0.7)
trans @-[UWzRhW0Hz)l trans @--[WhRh%(0H2)1 trans ~~(NC~u(acac)2
23
Pld (0.58)
Cu(acac)z
14(0.5)
@--[W2Wacac)2
24
Pld (0.58)
Cu(acac)2
20(0.7)
@-ltNCWtacach1
DCM
25
Ple (1.23)
CuCl
36(2.8)
uncertain
H2O
26
Pld (0.58)
‘CoCl*’ h
@-ItNC)CoCMOH,)I
AN
27 23
Pld (0.58) Plf (0.23)
‘CoC12’
29
Pld~NC~u(acac)~ (entry 24)
[WOWa 1sI’* b
‘Cocl2 ’
b
lOO(3.3)
15(0.5) 99(1.3) loo(2.7)a
MET MET
1
DCM
1
DCM
uncertain G+-[(NC)CoC12(OH2)] /[(NC)Cu(acac)z 3 @, [(CN)CoCla(CH)2 I
H2O
DCM/AN
AN
2188(s, +41); 1996(s, v(CO)~; no detectable free NC 2188(s, +38); 1996(s,~(CO)f; no detectable free NC 2186(s, +39); 1996(s, v(COd; no detectable free NC 2243(m, +93); RNC/Rh = 1; no detectable free NC 2243(m, +93); 2182(w, +32); 2150(m, free NC); RNC/Rh = 6 2243(m, +96); no detectable free NC 2195(s, +47); 215O(w, free NC); Cu/RNC = 1 spectral data as above; Cu/RNC =2 spectral data as above; CufRNC =3 2185(s, +36); no detectable free NC 2228(s, +80); 217O(w, + 20); 2135(w, -15); no detectable free NC 222O(w, +70); 2150(m, free NC) 2225(w, +76); 2149(w, free NC) 2228(w, +80); 2195(s, +47); no detectable free NC
*Reaction percentage estimated from the experimental metal content (in parentheses) and the hypothesis of a 1 to 1 (M:CNR) adduct formation.bSee experimental for the meaning of this notation. cO.47 meq g-l RNC are expected to be available for further coordination. dCalculated on the basis of the estimate made at point c. eNot determined. f~(CO) at 2065 and 2012 cm-l in the reacting complex. sCalculated on the basis of the number of equivalents of NC functions estimated in the material referring to entry 24. “DCM = dichloromethane; ETH = ethanol; AN = acetonitrile; MET = methanol.
118
apparent test for resin reactivity. IR spectroscopy of the polymer-metal complexes provided, on the other hand, an ideal analytical tool for the qualitative estimation and structural characterization of metal coordination on the resins. In order to simplify the interpretation of the results, the molar ratio --NC:metal was usually chosen equal to one, and only in certain experiments (see later) was this ratio higher than one. The combination of this ‘imposed’ 1:l stoichiometry (uide in@) with the quantitative evaluation of the meqs of metal per gram of resin and the analysis of the IR spectra of the polymer-metal complexes provide an effective insight into the nature of the metal complexes attached to the polymer chains. It is worth pointing out that, in the case of the presently described polymer supports, IR spectra are far more useful than those of the more usually employed phosphine-bearing polymer-metal complexes. Under the 1:l ratio conditions between isocyano groups and the metal centers, four distinct cases can be recognized: (i) 90 to 100% reaction, indicating the involvement of usually one -NC group per metal atom, even though the resin is highly swollen, the polymer-bound -NC groups are thought to be highly flexible; (ii) cu. 50% reaction, with direct IR or analytical indications suggesting the utilization of two -NC groups per metal atom; (iii) degrees of metal incorporation indicating that a substantial fraction of the available -NC groups, detected by IR spectroscopy of the supported metal complexes, may not have reacted (entries 6, 11, 15, 22 - 24, 27, 28, Table 3); (iv) cases in which metal incorporation is lower than expected, but unreacted -NC groups are not detected in the respective IR spectra (entries 2, 10, 17, 19, 25, Table 3). The fate of the isocyano groups in this category is not clear at present. Coordination of Pd(II) Pd(acetate), (l),[Pd(acac)*] (2) and [PdCl,(PhCN),] (3) undergo facile coordination with the -NC groups of the resins. Reaction percentages greater than 80% are invariably found for 1 and 3, both of which are catalyst precursors in important metal-catalyzed organic syntheses such as oxidation of alkenes to ketones, carbonylation of olefins, rearrangements, etc. [21]. The appearance of a single, symmetric v(CN) band (at the expected wavenumbers [22a]) in the IR spectra of the respective polymer-l and polymer-3 complexes, together with an almost 100% metal incorporation under 1:l stoichiometry suggests the formation of the species depicted in Scheme 3. The data available for a polymer-PdClz adduct do not allow an unambiguous choice between possibilities (a) and (b). In fact, although complexes related to possibility (a) are known [22b], their formation would imply the involvement of two isocyano ligands for two adjacent palladium atoms throughout the bulk of the polymeric matrix, which seems to be a rather unlikely circumstance. The relatively low amount of PdClz in
119 entries
3,4&l
entries 1,5
7 $ T I:: C
C I
X -.
I
Pd-X ,/‘”
I
I:: C
\x
I
X-Pd-X I
(b)
C III N
x=cr
i
(a) Scheme 3. Suggested structures for Pd(I1) complexes (entries 1, 3 - 7, Table 3).
the respective resins makes the use of IR spectroscopy unreliable for choosing between the two possibilities. Indeed, chemical reactivity of the anchored palladium centers may be useful for this purpose. Coordination
of Ru(II)
and Ru(III)
[RuC12(PPhs)s] (4), a well-known hydrogenation catalyst [21], and ‘RuCls’ (5), a convenient reagent to prepare 4, undergo ready coordination by all isocyano-bearing polymers employed. A careful mass-balance control for entry 8 (Table 3) showed that the sum of reacted and unreacted 4 (which was recovered analytically pure after complete removal of the solvent) was identical to the total amount used. This observation and the IR data (predominant single v(CN) band) provide strong clues as to the nature of the molecular species formed (see Scheme 4). The bathochromic shift of the coordinated isocyano v(CN) band is known for such ligands bound to Ru(I1) in conventional octahedral complexes [23]. ‘RuCls’ (5) is also quite reactive with Pld and Ple (see Table 3). Under 1: 1 ratio conditions, ca. 50% of the available Ru(II1) undergoes coordination by the polymeric ligands. The IR spectra of the complexes clearly display two v(CN) bands (with comparable intensity), which strongly suggests the formation of cis-bis(isocyano) complexes [24]. In view of the known tendency of Ru(II1) to six-coordination, we propose the formation of the species depicted in Scheme 4. When a 4.2:1 ratio -NC:metal is used, the amount of incorporated Ru(II1) corresponds to the engagement of 92% of the employed metal in the formation of a mono-isocyano complex (single v(CN) band at 2167 cm-‘). This assumption is confirmed by the fact that reaction of the still
120 entries
entry 13
entry 12
840
T
T
’
‘/L X-Ru-X
7,X
/x
X-Ru I L
f C
1;1
C
C
L’
m It
l!i
-CSN
X’l
X-Ru
,co\\x
-I
L’
L=PPh3;
I
--L’
X’i
L’= H20 ; X&I
L’
L’
x
-
Scheme 4. Suggested structures of Ru(I1) and Ru(II1) complexes (entries 8 - 13, Table 3).
16-16
.entries
entry
~~?yy‘ivylvuL
II L--h-O
0
n
14
7
aJ o=
X-Rh
19-21
1y‘
L =
4-c_.
-I.
L
entries
i
ru
I
15
entry
X’
I L
8C8C-
Scheme 5. Suggested structures of Rh( I) and Rh(II1) complexes (entries 14 - 21, Table 3 ).
available isocyano groups in the resin with Co& under 1:l ratio conditions affords the quantitative formation of a mono-isocyano cobalt(I1) complex, as shown in Scheme 4. Heterometallic polymer-bound metal complexes are already known, but they are usually supported by phosphine-bearing polymers [4, 51. Coordination of Rh(I) and Rh(III) [RhCI(PPh,),] (6), [Rh(acac)(CO)z] (7) and ‘RhClJ’ (8) react with polymers Pl according to various incorporation patterns. Thus, 8 undergoes quantitative coordination by Pld when a 6:l excess of -NC functions is employed. Under 1:l conditions, a 46% metal incorporation occurs, which is in accord with the formation of a bis(isocyano) complex. The single v(CN) band suggests that in both cases a presumably trans six-coordinate octahedral species [25a, b] is formed, as depicted in Scheme 5.
121
Both Rh(1) species, 6 and 7, undergo non-quantitative coordination, under 1:l stoichiometry. The reaction of 7 leads to CO evolution, and the IR spectrum of the adduct is in agreement with the presence of one CO and one -NC ligand coordinated to Rh(1) in the molecular planar fourcoordinate complex produced. The formation of a truns-bis(isocyano) complex is ruled out on the basis of the observed incorporation percentages, which are invariably much greater than 50%. Coordination of Cu(II), Cu(I) and Co(U) Bis(2,4-pentanedionate)copper(II) (9), an effective and selective catalyst for Michael syntheses [26], is known to exhibit an appreciable tendency to give five-coordinate adducts with nitrogen bases [27]. 9 reacts with polymer Pld, as shown in Table 3. The polymer-9 adduct is formed in 15 - 20% yield. Several attempts to involve the unreacted fraction of isocyano ligands in incorporating more Cu(acac), were unsuccessful. However, the residual -NC groups turned out to be quite reactive towards anhydrous CoClz, thus giving rise to a heterobimetallic polymer-metal complex (see footnotes in Table 3). The combination of the IR datum (single v(CN) band at 2195 cm-’ [27]) with the known tendency of Cu(acac)z to five-coordination suggests the formation of the adduct sketched in Scheme 6. Nominally, anhydrous CoClz (10) reacts rapidly and almost quantitatively with Pld, Plf and even with the residual groups present in Pld-9 (see above). The molecular species formed exhibits a single strong to weak v(CN) band at cu. 2225 cm-’ (see Table 3) and the visible spectrum displays a broad and composite band centered at cu. 650 nm. The combination of analytical and spectral data indicates that the molecular species formed must be a tetrahedral [28] mono-isocyano complex. In view of the not strictly anhydrous conditions employed, it seems likely that the molecular species can be formulated as @--[ (NC)CoC12(OH,)] (see Scheme 6). The weak bands at 2170 and 2135 cm-’ observed in the case of reaction with Pld may be attributed to the formation of traces of a bis(isocyano) cobalt(I) species [ 281. entries
26,26
entries 22-24
T
T=y==T= N III
N III F
x L’=H20
Scheme
\
0’
0-O 6. Possible
28, 29, Table 3).
)
0 = acac-
structures
29
N III
III C
O\I”,O (
/=px
i
entry
L
/“e\’ xx
&)
x=c1of copper(I1)
and cobalt(I1)
complexes
(entries
22
- 24, 26,
122
A dramatic effect due to the nature of the preexisting metal coordination sphere is observed when Pld is reacted with [Co(OH2)J2+ (11). In this case, the metal incorporation is definitely low and the nature of the molecular metal species formed is quite uncertain. CuCl is also found to react with Ple. The presence of a single strong v(CN) band rules out the formation of a tris(isocyanide) chlorocopper(1) species, which could have been expected [29], and seems to support the formation of a mono-isocyanide chlorocopper(1) linear complex. Metal distribution inside the polymeric beads or granules In view of the often quantitative involvement of all of the isocyano functions under 1:l ratio conditions and the presumably uniform distribution of -NC groups in the resins, it is expected that metal distribution is also homogeneous throughout the beads. This fact appears of great importance in order to employ these materials as precursors of polymersupported metal catalysts. To this end, we examined @-[(NC)Cu(acacM, @-[(NC)RUC~,(OH~)~] (P = Pld) and @-[(NC)2RhC13(0H2)] (P = Pie), by X-ray microprobe. In all cases, the resulting scanning pictures indicated an almost perfectly homogeneous metal distribution, with a slight tendency to ‘zonation’ in certain samples. This confirms the expected uniform distribution of isocyano functions throughout the polymer matrix. Leaching tests in dichloroethane and tolupe on @-[(NC)2RhC13(lXf~] (p = Pld, Ple), @- [(NC)PdCI,] (P = Plfi The resins were treated under severe and dynamic conditions at 100 “C under argon, and were subsequently examined by IR spectroscopy and metal elemental analysis. The results are shown graphically in Fig. 2. (a) @--[(NC),RhC1,(OH2)] in dichloroethane (residual metal %: time (h) in parentheses): 2.6 (0); 2.4 (10); 2.3 (20); 2.0 (30); 2.0 (40); 1.9 (50). smetal 6.0 [
-4-a
5.0
ub
4.0
--o-c
~
Fig. 2. Residual metal content conditions.
of the examined polymer-metal
complexes
under test
123
No change occurs in intensity and wavenumber of the 2243(s) cm-’ u(CN) band for 40 h. After 50 h it appears at 2237(s) cm-‘. (b) @-[(NC),RhCl,(OH,)] in dichloroethane: 6.0 (0); 6.2 (10); 6.2 (20); 6.0 (30); 6.3 (40); 6.0 (50). No change in intensity and wavenumber of the 2243(s) cm-’ band. (c) @-[(CN)PdCl,] in toluene: 2.2 (0); 1.9 (10); 2.0 (20); 2.1 (30). Disappearance of the v(CN) band immediately after the first cycle. It is seen that only in case (a) does the metal content gradually decrease. The erratic behaviour of the analytical data in cases (b) and (c) is probably related to different metal contents among different groups of beads. The SEM technique reveals, in fact, that the beads of Pld and Ple are not isolated (single) (see Fig. 3), but rather grouped in clusters and this circumstance may cause a macroscopic non-equivalence among groups of clustered beads. In the case of @-[(NC)PdCl,], the leaching of the metal is very small (if any), but the treatment leads to thermal degradation of the Pd(I1) complex, with presumably formation of highly dispersed metallic palladium within the resin matrix. Stability of @--[(NC),RhCZ,(OH,)] under dihydrogen and dioxygen in polar solvents The title resin was chosen as a model of precursor of hydrogenation and oxygenation catalysts, and was tested in dichloroethane and 2-methoxyethanol. The main results are collected in Table 4. It is seen that the resin is quite stable under Hz and O2 in dichloroethane at 100 “C. Conclusions Functionalized polymer supports containing from 0.2 to 1.3 meq gg’ of isocyano functions appear to possess strong coordinating ability towards
Fig. 3. SEM micrograph of a dry sample of @--[(NC)2RhC13(0H2)].
124 TABLE 4 Behaviour of @-[ (NC)2RhC1a (OH*) ] under Hz and 02 Gi3.5
Solventa
Temperature (“C)
Remarks
02
DCE DCE 2ME 2ME
100 100 100 100
no change in % metal and IR spectrumb analytical data as above analytical data as above no change in % metal; the 2240 cm-’ band disappears and a weak band develops at 2220 cm-i
HZ 02 HZ
aDCE = 1,2-dichloroethane; 2ME = 2-methoxyethanoi. bWith respect to their behaviour under argon (see above). Contact vigorous stirring.
time 10 h, under
many transition metal species. Relatively low charged resins appear to be suitable for the accommodation of one metal atom per isocyano ligand, and hence the possibility of preparing metal complexes with unusual coordination spheres. This type of matrix isolation is somewhat surprising, considering that in polystyrene-type resins site isolation is usually difficult to achieve [4] and that the presently described resins have a matrix structure considerably more flexible (see Fig. 1 and [ 301) than polystyrene-based resins. Whatever the explanation, however, the implications of the present results are clear: both the number of isocyano ligands per metal atom and the lower overall coordination number are likely to give the metal center novel reactivity features. Moreover, these moderately crosslinked, tailor-made supports appear to combine interesting physical and chemical features in view of their utilization for the preparation of polymer-supported transition metal catalysts. Three basic lines of application can be envisaged: (i) hybrid catalysis especially concerned with dioxygen chemistry; (ii) highly dispersed heterogeneous catalysis with a wide choice of metal species and operating temperatures; (iii) synthesis of heteroleptic @-NC-ML, metal complexes exhibiting unprecedented chemical behaviour. Research lines (i) and (ii) are currently being followed in these and other laboratories 1311 and the satisfactory maintenance of the catalytic activity of polymer-supported [Cu(acac)z] in Michael synthesis has been already reported [ 261. Experimental Analytical methods and materials IR spectra were recorded on a Perkin Elmer ‘781 spectrophotometer. UV-VIS spectra were recorded on a Perkin Elmer X-5 spectrophotometer.
I
125
DTA-TG data were acquired on a Netzsch DTA-TG 429/3/6 apparatus. Elemental analyses were performed on a Carlo Erba elemental analyzer mod. 2000. Atomic absorption meas~emen~ were performed on a Perkin Elmer 3030 atomic absorption spectrophotometer. Commercial chemicals (reagent grade) were used as purchased if not otherwise stated. RhCl(PPh& and Rh(acac)(CO)z were prepared according to literature methods [32,33]. The procedure employed for the Wilkinson’s complex was also used for the synthesis of the Ru(I1) analog, RuCl~~PPh~)~. In Table 3 and in the discussion section, rhodium trichloride and ruthenium trichloride are indicated as ‘RhCIJ’ and ‘RuCl,‘. The commercial products are labelled as trihydrated species. The nature of the rhodium salt is uncertain, whilst the ~thenium compound contains small amounts of polynuclear complexes of Ru(IV) [ 341. For this reason the notation ‘RhC13’ and ‘RuC13’ is used. Literature procedures were followed for the titration of the isocyano functionality on the resins [19a] and for the synthesis of 1,6diacrylamidohexane [ 351. Synthesis of 3-formamidopropyl acryiate 110 cm3 of methyl formate were added dropwise, under vigorous stirring and at room temperature, to a solution of 37.8 g (0.5 mol) of 3Gino-l-prop~ol in 200 cm3 of methanol and 10 cm3 of triethylamine. After standing overnight, most of the solvent was removed in a rotatory evaporator and the intermediate product, N-(3-hydroxypropyl)formamide, was distilled under vacuum. The recovered product was diluted with 180 cm3 of dichloromethane and 78 cm3 of triethylamine, and 0.8 g of 4-N,Ndimethylaminopy~dine were added. Into this solution was dropped a mixture of a stoichiometric amount of acryloyl chloride in ea. 40 cm3 of dichloromethane, under vigorous stirring at 0 “C. The reaction mixture was stirred overnight at room temperature, then filtered and washed with aqueous sodium hydrogen carbonate, hydrochloric acid and water. The organic layer was dried over ma~esium sulphate and the solvent was removed in a rotatory evaporator. The product obtained was finally distilled under vacuum (141 “C, 2 mmHg), in presence of 2,6-d&t-butyl-4-methylphenol. ~epara~~~ of type PI beaded resins by suspension po~yrner~a~~on The following procedure was used for the synthesis of Pld: 150 cm3 of 1,2dichloroethane and 3.0 g of cellulose acetate butyrate were introduced into a previously described suspension polymerization apparatus [18] at 50 “C. The mixture was stirred until the solid was completely dissolved; the solution was then deaereated by flushing with argon for at least 5 min. Meanwhile, a mixture of 8.0 g of N,Ndimethylacrylamide, 1.2 g of 20 cm3 3formamidopropyl aerylate, 1.8 g of 1,6diacrylamidohexane, of water and 10 cm3 of d~ethy~o~amide was prepared, The stirrer speed was set to 500 - 600 rpm and 0.6 g of ~monium peroxodisulphate were added to the reactant mixture, which was subsequently introduced into
126
the polymerization reactor through the side arm. The copolymerization was allowed to proceed for 5 h under a slow nitrogen flux, and the volume of the mixture was kept constant by means of further additions of the solvent. The mixture was transferred into a beaker and 250 cm3 of acetone were added. The copolymer beads were allowed to settle and the liquid was then decanted. The same procedure was repeated four times using water and acetone. Finally, the beads were washed with diethyl ether and dried overnight under vacuum in the presence of phosphorus pentoxide. Preparation ‘of type P2 resins by radiation-promoted polymerization The following procedure was used for the synthesis of P2b: the monomers, 2-hydroxyethyl methacrylate (1 g), 3-formamidopropyl acrylate (0.15 g), diacrylamidomethane (0.08 g) and water (1.2 g) were introduced into a 5 cm3 Pyrex vial. The solution was deaerated by means of three freeze-thaw cycles. The vial was flame-sealed under vacuum, cooled to -78 “C (solid carbon dioxide-acetone) and exposed to a 6oCo source (dose ca. 10 kGy = 1 MRad). The copolymer was recovered, cut in thin slices and repeatedly washed with water, methanol and diethyl ether. Finally, the polymer was dried under vacuum at room temperature. Generation of the isocyano groups on the resins The following procedure is a slight modification of a previously described one [19] : the polymer was allowed to swell overnight in anhydrous pyridine (8 - 10 cm3 g-l), and p-toluenesulphonyl chloride was added to the vigorously stirred suspension. After - 1 h, an excess of acetone was added, the polymer was recovered by filtration and washed on the filter as follows: acetone (2X), methanol (3X), water (3X), aqueous 7% potassium hydrogen carbonate and methanol (2X), water (2X), acetone (3X) and diethyl ether (3X). The polymer was finally dried under vacuum in the presence of phosphorus pentoxide. Synthesis of the polymer-supported complexes The resin and the metal compound to react were transferred into a specially built two-cell reactor. The amount of the reacting metal complex was calculated on the basis of the -NC meq on the resin and the desired initial -NC:metal ratio (usually 1:l). The resin was swollen in the minimum volume, while a saturated metal solution was prepared in the same solvent. The reactants were then mixed and the reactor was vigorously shaked by hand for some minutes. The reaction mixture was afterwards kept under gentle mechanical stirring for 2 h. The obtained adduct was filtered, carefully washed with fresh solvent and diethyl ether, and finally dried under vacuum in the presence of phosphorus pentoxide. The unreacted metal complex (if any) was recovered for mass-balance. Determination of the metal content of thepolymermetal complexes 15 mg of the polymer-metal sample were introduced to a 50 cm3 round-bottom flask and 5 cm3 of a 1:l mixture of concentrated nitric acid
127
and sulphuric acid were added. The suspension was refluxed for -2 h; the obtained solution was poured into a lOO-cm3 volumetric flask and the volume was adjusted with distilled water. The solution was then analyzed by atomic absorption spectroscopy. Leach ing tests Ca. 250 mg of the sample were introduced to a 50 cm3 glass reactor together with 15 cm3 of 1,2dichloroethane. The mixture was deaerated by flushing with an inert gas. The reactor was immersed into a thermostatted bath set at 100 “C and kept under gentle stirring. After 10 h, the polymer was filtered, washed with fresh solvent (4 X 5 cm3) and diethyl ether, and dried under vacuum. A small amount was used to record the IR spectrum and to determine the metal content, while the remainder of the sample was used for the subsequent cycles. The cycles were repeated three, four or five times, so that each complete test lasted 30 - 50 h. An almost identical procedure was used to verify the stability of the polymer-metal complexes under reactive gases at various temperatures, except that the initial suspension was deaerated with the desired gas (H,, O,), the temperature was set to the chosen value and each test consisted only of one 10 h cycle.
Acknowledgements This work stems from our discussion and preliminary experiments at the Technical University of Munich in Garching during 1981. We would like to express our gratitude to our hosts Professors I. Ugi (to R.A.) and H. F. Klein (to B.C.) and to the Alexander von Humboldt Foundation for a grant (to B.C.). We are also grateful to Mr. A. Ravazzolo for his skillful technical assistance (to R.A. and B.C.).
References (1969); Ger. Pat. 1800380 (1969); 1 Ger. Pat. 1 800371 (1969); Ger. Pat. 1800379 Brit. Pat. 721 686 (1969) to W. 0. Haag and D. D. Whitehurst. 2 G. W. Parshall, Homogeneous Catalysis, Wiley, New York, 1980. 3 (a) G. G. Allan and A. N. Neogi, J. Catal., 90 (1970) 256; (b) S. C. Tang, T. E. Paxson and L. Kim, J. Mol. Catal., 9 (1980) 313. Organometallic Chemistry, Pergamon, Oxford, 4 C. U. Pittman Jr., Comprehensive 1982. Catalysis with Metal Phosphine Complexes, Plenum, New 5 N. L. Holy, Homogeneous York, 1983. 6 F. R. Hartley, Supported Metal Catalysts, Reidel, Dordecht, 1985. 7 E. Singleton and H. E. Oosthuizen, Adv. Organometall. Chem., 22 (1984) 209. 8 P. M. Treichel, Adv. Organometall. Chem., 11 (1979) 21 and references therein. 9 G. Skorna and I. Ugi, Chem. Ber., 111 (1978) 3965. 10 J. A. S. Howell and M. Berry, J. Chem. Sot., Chem. Commun., (1980) 1039.
128 11 R. Arshady and I. Ugi, Angew. Chem., 94 (1982) 367; ibid., Angew. Chem. Suppl., (1982) 761; ibid., Angew. Chem. Int. Ed. Engl., 21 (1982) 374. 12 (a) B. Corain and R. Arshady, Transition Met. Chem., 8 (1983) 182; (b) R. Arshady, M. Basato, B. Corain, R. De Pieri and S. Sitran, presented at the 5th Int. Symp. Relations between Homogeneous and Heterogeneous Catalysis, Novosibirsk, 1986; (c) B. Corain, M. Basato, P. Main, M. Zecca, R. De Pieri and R. Arshady, J. Organometall. Chem., 326 (1987) C43. 13 H. Menzel, W. D. Fehlhammer and W. Beck, 2. Naturforsch., 37b (1982) 201. 14 J. A. S. Howell, M. Berry and R. K. Champaneria, J. Mol. Catal., 37 (1986) 243. 15 C. G. Francis, D. Klein and P. D. Morand, J. Chem. Sot., Chem. Commun., (1985) 1142. 16 C. G. Francis, P. D. Morand and P. P. Radford, J. Chem. Sot., Chem. Commun., (1986) 211. 17 R. Arshady, Polymer, 23 (1982) 1099. 18 R. Arshady and A. Ledwith, Reactive Polymers, I (1983) 159. 19 R. Arshady and I. Ugi, Talanta, 31 (1984) 842. 20 B. Corain, M. Basato, M. Zecca, S. Lora and G. Palma, Makromol. Chem., Rapid Commun., 7 (1986) 651 and references therein. 21 (a) P. Challoner, Handbook of Coordination Catalysis in Organic Chemistry, Buttarworths, London, 1985; (b) S. T. Davies, Organotransition Metal Chemistry -Applications to Organic Synthesis, Pergamon, London, 1982. 22 (a) J. Browning, P. L. Gogging and R. J. Goodfellow, J. Chem. Res. (S), (1978) 328; (b) T. Boschi, B. Crociani, M. Nicolini and U. Belluco, Znorg. Chim. Acta, 12 (1975) 39. 23 B. E. Prater, J. Organometall. Chem., 34 (1972) 379. 24 (a) G. Mercati and F. Morazzoni, Gazz. Chim. Ital., 109 (1979) 161; (b) T. Gusson, G. Mercati and F. Morazzoni, op. cit., 545. 25 (a) R. V. Parish and P. G. Simms, J. Chem. Sot., Dalton Trans., (1972) 809; (b) A. L. Balch and J. Miller, J. Organometall. Chem., 32 (1971) 263. 26 M. Basato, B. Corain, P. DeRoni, G. Favero and R. Jaforte, J. Mol. Catal., 42 (1987) 115. 27 D. P. Graddon, Coord. Chem. Rev., 4 (1969) 1. 28 P. M. Boorman, P. J. Craig and T. W. Swaddle, Can. J. Chem., 48 (1970) 838. 29 (a) R. W. Stephany and W. Drenth, Reel. Trau. Chim. Pays-Bas, 89 (1970) 305; (b) A. Bell, R. A. Walton, D. A. Edwards and M. A. Poulter, Inorg. Chim. Acta, 104 (1985) 171. 30 R. Arshady, Reactive Polymers, (1989) in press. 31 G. Braca and A. M. Raspolli Galletti, unpublished work. 32 J. A. Osborne, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chem. Sot. (A), (1966) 1711. 33 Y. S. Varshavskii and T. G. Cherkasova, Russ. J. Znorg. Chem. (Eng. Transl.), 12 (1967) 899. 34 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn., Wiley, New York, 1980. 35 J. A. Halpern and J. T. Sparrow, Synth. Commun., 10 (1980) 569.
Catalysis,
53 (1989)
111 - 128
PREPARATION OF ISOCYANO POLYMER SUPPORTS AND THEIR COMPLEXES WITH ~ATAL~ICALLY RELEVA~ TR~SITION METAL CENTERS R. ARSHADY* Department
of Chemistry,
M. BASATO*,
College of Science,
B. CORAIN*,
Kashan (Iran)
M. RONCATO, M. ZECCA
Centro di Studio sulla Stabilita’e Reattivita’dei Composti di Coordinazione, Ripartimento di Chimica Inorganica, Metallorganica ed Analitica, University I-35131 Padua (Italy)
C.N.R. of Padua,
L. DELLA GIUSTINA ENEL - Compart~mento di Venezia, Sezione I-301 70 Porto Marghera (Venice) (Italy)
Chimica - c/o G.I.T. Marghera,
S. LORA Istituto di Fotochimica e Radiazioni I-35020 Legnaro (Padua} (italy}
d’Alta Energia, C.N.R. - Sezione
di Legnaro,
and G. PALMA Dipartimento
di Chimica Fisica, University
(Received July 15,1988;
of Padua, I-35131 Padua (Italy)
accepted November 10,1988)
Suspension or mass polymerization of vinyl comonomers with 3formamidopropyl acrylate affords widely solvent compatible, medium crosslinked polymers, which can be transformed into isocyano-containing resins. These materials (0.2 - 1.3 meq of isocyano function~ity per g resin), exhibit a strong coordinating ability towards a variety of metal centers including catalytically relevant metal complexes. The combination of IR spectroscopy and other analytical data makes it possible to reliably speculate on the actual composition and structure of the respective polymer-supported complexes. X-ray microprobe analysis shows that the metal distribution is homogeneous throughout the polymeric beads. Thermal analyses reveal that both resins and polymer-supported complexes are thermally stable up to 300 “C. Severe leaching tests (dichloroethane or toluene at 100 “C) show that the polyme~metal complexes are characterized by high formation constants. Under certain conditions, the involvement of one polymer-bound -NC group per metal atom is achieved, so that the anchoring of catalytically active species to the support produces only a slight modification of the coordination sphere of the metal centers. *Authors to whom correspondence should be addressed. 0304-5102/89/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
112
Introduction Nearly two decades ago, Haag and Whitehurst published the first reports [ 11 concerning a conceptual breakthrough in industrial catalysis [2]. The basic idea was the ‘robust’ anchoring of catalytically active metal complexes to insoluble supports, by means of covalent or ionic [3] bonds between the potential catalytic site and the support. In a large majority of cases [ 4,5], the supporting materials consisted of either crosslinked polystyrene or silica, and the main thrust of the idea was the achievement of a delicate balance between the stability of the metal-support bond and the survival of the catalytic activity of the metal center. However, none of these hybrid [2] catalysts has yet become the basis of an industrial process, for a number of reasons [6]. Certainly, reduced activity and leaching of the metal are among the most important reasons for the lack of industrial utilization. Quite surprisingly, little effort appears to have been devoted to employ polymer supports bearing ‘soft’ ligands other than phosphines. Isocyanides are ligands which are very versatile in their ability to form strong bonds with many transition metal centers, thus giving rise to homoand heteroleptic complexes [ 71. Isocyanides preferably bind low-valent metal ions, but they are also known to form stable complexes with medium to high valent metal centers (e.g. Rh(III), Ir(II1) and Co(II1) [S]). For all these reasons, isocyano polymer supports appear very interesting for the preparation of novel hybrid catalysts and also as convenient precursors for the preparation of highly dispersed supported metal catalysts. Styrene-divinylbenzene copolymers bearing the isocyano function were described by Skoma and Ugi in 1978 [9], and the first example of metal coordination of insoluble isocyano polymers was reported by Howell and Berry in 1980 [lo]. In 1982, Arshady and Ugi reported [ll] a new type of isocyano polymer support with relatively general solvent and substrate compatibility. At that time we initiated our work on the development of these isocyano resins for the preparation of polymer-supported transition metal complexes. Since then our preliminary results have been published in 1983 [12a], in 1986 [12b] and in 1987 [ 12~1. During this period, a number of other reports concerning related work on insoluble [13] and soluble [ 14 - 161 isocyano polymers have appeared in the literature. The present paper describes full details of the first phase of our work on the synthesis and coordination chemistry of tailored isocyano polymer supports. Results and discussion Synthesis and characterization of the isocyano polymer supports The structure of the isocyano polymer supports, @--NC, is illustrated by Scheme 1, and the quantitative details of the resins are given in Table 1.
113 TABLE 1 Quantitative data on isocyano resins Resin
-NC content (meq g-l)
Crosslinker (mol%)
Remarks
Pla Plb Plc Pld Ple Plf P2a P2b
0.30 0.49 0.10 0.58 1.28 0.23 0.36 0.31
8 8 8 8 8 8
-
1 6
a=88;c=lla a=84;c=10a
*Mel%, see Scheme 1 for the meanings of coefficients a and c.
V P2a P2b
0CH2CH20H OCH2CH20H
z NH
n CH2NH
0 CH2CH20
Pla, Plc
Plb, I Ple
Pld,Plf
1 6
Scheme 1. Schematic drawing of polymers employed.
Resin samples Pl were prepared by peroxide-initiated suspension polymerization according to [17,18]. These materials were obtained in bead form (average diameter about 50 pm). The isocyano functionality on the resins was titrated as described in [19]. Polymer samples P2 were obtained in spherical form (diameter - 1 mm) or as bulky cylinders (about 5 cm3) by y-ray initiated ‘bulk’ polymerization in ice matrix at -78 “C [203. In the latter case, the material was ground to small fragments of -1 - 5 mm. All the polymer samples swell in water and in dimethylformamide, methanol, dichloromethane, dichloroethane, acetic acid and to a small extent in toluene. Good shrinking media are diethyl ether and isobutanol. Typical SEM pictures of the swollen polymer types Pl and P2 are shown in Fig. 1. It can be seen that both resin types exhibit a sponge-like structure, indicating easy diffusion of reagents inside the bulk of the materials.
of monomer composition in type P2 resins The y-ray initiated polymerization is carried out to about 80% consumption of comonomers. Under these conditions, the overall compositions of the formamide precursors of P2a and P2b are in fair and excellent, respectively, agreement with the expected composition. The dehydration procedure employed is expected to also give tosylation of the OH groups present in the resins. The analytical comparison between the expected and the observed C, H, N, 0 and S values is therefore based on the assumption of complete esterification of the OH groups. The agreement is again excellent in the case of P2b and fair in the case of P2a. Both P2a and P2b show the expected medium-intensity YcN band at 2150 cm-’ and the titration shows 0.36 and 0.31 meq g-’ of isocyano groups respectively. Control
Evaluation
of the dehydration
procedure
The nitrogen content of polymer P2b stems only from the 3-formamidopropylacrylate (3FAPA) content. Therefore, a comparison between the nitrogen contents determined by elemental analysis and -NC titration [19a] makes it possible to evaluate the efficiency of both dehydration and titration procedures. In fact, titration values for P2b correspond to a recovery of 80% of the total nitrogen (0.43% vs. 0.53%). This result may be due either to incomplete dehydration or to some inaccuracy of the titration. In order to shed light onto this, we prepared a polymer (P3, Scheme 2), which is structurally related to P2b, but does not contain hydroxyl groups and with nitrogen present only in the 3FAPA unit. The agreement between the nitrogen percentage values obtained from the microanalysis and the titration for P3 was quite good (0.59% vs. 0.62%). This supports the efficiency of dehydration and titration procedures.
115
a ~83 isonitrile
;
bs4
;
charger0.44
c =,3
hOI.
xi
meg/Q
Scheme 2. Sketch of polymer P3.
Thermal stability of type PI and P2 polymers In view of the possible utilization of the crosslinked polymers herein described as supports for heterogeneous catalysts, their thermal behaviour was investigated under argon by means of combined TG-DTA measurements (Table 2). In all cases no endo-exothermal event was observed in the indicated temperature ranges. The loss of weight is observed only during the first heating half-cycle; the second heat treatment gave no weight loss. This suggests that the first weight loss is probably due to removal of traces of solvent. SEM pictures after thermal treatment reveal the perfect maintenance of the original morphology of the resins. Coordination chemistry of the isocyano resins The polymeric ligands Pl and P2, after swelling in a suitable solvent, reacted rapidly with concentrated solutions of all of the complexes listed in Table 3. Colour change of the resin and partial or complete bleaching of the solution of the reacting complex were taken as an immediately TABLE 2 Thermal bebaviour of polymers Pl and P2 under argon (heating rate 5 “C min-‘) Polymer
Stability range
Weight loss during 1st cycle (see text) (%)
Pld Ple Plf P2a
r.t. r.t. r.t. r.t.
6 12 2 0
- 250 - 320 - 200 - 180
“C “C “C “C
TABLE 3. Complexes of macromolecular isocyanides with various metal centers. Entry
Polymer (RNC, meq/g)
Reacting species
Reactiona (metal%)
Prevalent species formed
Solventh
Remarks
1
Pla (0.30)
Pd( acetate)2
lOO(3.2)
@-[(NC)Pd(acetate)z]
DCM
2
Plb (0.49)
Pd( acac),
30(1.5)
@-[(NC)Pd(acac)2
3
Plb (0.49)
PdC12(PhCN)2
.89(4.3)
DCM
4
Plf (0.23)
PdC12(PhCN)2
93(2.2)
5
Plf (0.23)
Pd(acetate)Q
90(2.1)
@-[(NC)PdCl,] or @--[WW’d’TP-C~)12. @--[ (NC)PdC12 ] or @-[(NCWCWC1)12 @-[ (NC)Pd(acetate)z]
6
P2a (0.36)
PdC12(PhCN)2
82( 3.0)
DCM
7
P2b (0.31)
PdC12(PhCN)2
92( 2.9)
8
Pld (0.58)
RuCls(PPhs)s
83(3.3)
@-[ (NC)PdC12 ] or @--[(NC)PdCWCI)lz @-[ (NC)PdClz ] or @-[(NC)PdCl(fi-C1)]2 @-[(NC)RuCl,(PPhs)s]
9
Pla (0.30)
RuC&(PPhs)s
92( 2.2)
@-[(NC)RuC12(PPhs)s]
DCM
10
Ple (1.28)
Ru&(PPhs)a
64(4.6)
@-[(NC)RuC12(PPhs)3]
DCM
11
Pld (0.58)
‘RuCls ! b
92(1.3)C
@--[(NC)RuC13(CH,),]
BTH
12
Ple (1.28)
‘RuC13’ b
57(6.3)
@-t(NC)2RuCls(CH2)1
BTH
2227(w, +76); no detectable free NC 2205(s, +58), 217O(w, + 23); no detectable free NC 2241(s, +94); no detectable free NC 2241(m, +91); no detectable free NC 2237(w, + 87); no detectable free NC 2249(m, +99); 215O(v.w, free NC) 2258(m, +108); 215O(v.w, free NC) 2135(s, -19); 2175(w, +24); no free detectable NC 2135(s, -19); 2175(w, +24); no free detectable NC 2135(s, -19); 2175(w, +24); no free detectable NC 2167(s, +17); 215O(w, free NC); initial RNC/Ru = 4.2 2185(s, +36); 2165(s, +16); no detectable free NC
DCM
I
DCM DCM
DCM DCM
CiS
13 14
Pld-(NC)RuC13(0H2)2 (entry 11) Plc (0.1)
RhCI(PPh3)3
15
Ple (1.28)
RhCl(PPh3)3
‘CoC12’
b
98(2.5)d e
/[WRuWOH2)21 ‘](NC)CoCWH,)l @-[(NC)RhCW’h3h
60(4.6)
AN
@
@--[(NC)2RhCI(PPh3)2]
1
DCM DCM
2228(w, +80); 2167(s, +19) no detectable free NC 2159(s, + 11); no detectable free NC 2182(s, +32); 2161(m, +ll); 2150(m, free NC)
16
Plb (0.49)
Rh(acac)(CO)z
55(2.6)
DCM
17
Pld (0.53)
Rh(acac)(CO)a
72(3.9)
DCM
18
Ple (1.28)
Rh(acac)(CO)z
56(6.3)
@-[(NWWacacWO)l
DCM
19
Pld (0.58)
‘RhCls ’ b
46(2.6)
@-[tNC)zRhWOH2)1
MET
20
Pld (0.58)
‘RhCls’ b
98(1.0)
21
Ple (1.28)
‘RhCla’ b
52(6.0)
22
Pld (0.58)
Cu( acac)z
20(0.7)
trans @-[UWzRhW0Hz)l trans @--[WhRh%(0H2)1 trans ~~(NC~u(acac)2
23
Pld (0.58)
Cu(acac)z
14(0.5)
@--[W2Wacac)2
24
Pld (0.58)
Cu(acac)2
20(0.7)
@-ltNCWtacach1
DCM
25
Ple (1.23)
CuCl
36(2.8)
uncertain
H2O
26
Pld (0.58)
‘CoCl*’ h
@-ItNC)CoCMOH,)I
AN
27 23
Pld (0.58) Plf (0.23)
‘CoC12’
29
Pld~NC~u(acac)~ (entry 24)
[WOWa 1sI’* b
‘Cocl2 ’
b
lOO(3.3)
15(0.5) 99(1.3) loo(2.7)a
MET MET
1
DCM
1
DCM
uncertain G+-[(NC)CoC12(OH2)] /[(NC)Cu(acac)z 3 @, [(CN)CoCla(CH)2 I
H2O
DCM/AN
AN
2188(s, +41); 1996(s, v(CO)~; no detectable free NC 2188(s, +38); 1996(s,~(CO)f; no detectable free NC 2186(s, +39); 1996(s, v(COd; no detectable free NC 2243(m, +93); RNC/Rh = 1; no detectable free NC 2243(m, +93); 2182(w, +32); 2150(m, free NC); RNC/Rh = 6 2243(m, +96); no detectable free NC 2195(s, +47); 215O(w, free NC); Cu/RNC = 1 spectral data as above; Cu/RNC =2 spectral data as above; CufRNC =3 2185(s, +36); no detectable free NC 2228(s, +80); 217O(w, + 20); 2135(w, -15); no detectable free NC 222O(w, +70); 2150(m, free NC) 2225(w, +76); 2149(w, free NC) 2228(w, +80); 2195(s, +47); no detectable free NC
*Reaction percentage estimated from the experimental metal content (in parentheses) and the hypothesis of a 1 to 1 (M:CNR) adduct formation.bSee experimental for the meaning of this notation. cO.47 meq g-l RNC are expected to be available for further coordination. dCalculated on the basis of the estimate made at point c. eNot determined. f~(CO) at 2065 and 2012 cm-l in the reacting complex. sCalculated on the basis of the number of equivalents of NC functions estimated in the material referring to entry 24. “DCM = dichloromethane; ETH = ethanol; AN = acetonitrile; MET = methanol.
118
apparent test for resin reactivity. IR spectroscopy of the polymer-metal complexes provided, on the other hand, an ideal analytical tool for the qualitative estimation and structural characterization of metal coordination on the resins. In order to simplify the interpretation of the results, the molar ratio --NC:metal was usually chosen equal to one, and only in certain experiments (see later) was this ratio higher than one. The combination of this ‘imposed’ 1:l stoichiometry (uide in@) with the quantitative evaluation of the meqs of metal per gram of resin and the analysis of the IR spectra of the polymer-metal complexes provide an effective insight into the nature of the metal complexes attached to the polymer chains. It is worth pointing out that, in the case of the presently described polymer supports, IR spectra are far more useful than those of the more usually employed phosphine-bearing polymer-metal complexes. Under the 1:l ratio conditions between isocyano groups and the metal centers, four distinct cases can be recognized: (i) 90 to 100% reaction, indicating the involvement of usually one -NC group per metal atom, even though the resin is highly swollen, the polymer-bound -NC groups are thought to be highly flexible; (ii) cu. 50% reaction, with direct IR or analytical indications suggesting the utilization of two -NC groups per metal atom; (iii) degrees of metal incorporation indicating that a substantial fraction of the available -NC groups, detected by IR spectroscopy of the supported metal complexes, may not have reacted (entries 6, 11, 15, 22 - 24, 27, 28, Table 3); (iv) cases in which metal incorporation is lower than expected, but unreacted -NC groups are not detected in the respective IR spectra (entries 2, 10, 17, 19, 25, Table 3). The fate of the isocyano groups in this category is not clear at present. Coordination of Pd(II) Pd(acetate), (l),[Pd(acac)*] (2) and [PdCl,(PhCN),] (3) undergo facile coordination with the -NC groups of the resins. Reaction percentages greater than 80% are invariably found for 1 and 3, both of which are catalyst precursors in important metal-catalyzed organic syntheses such as oxidation of alkenes to ketones, carbonylation of olefins, rearrangements, etc. [21]. The appearance of a single, symmetric v(CN) band (at the expected wavenumbers [22a]) in the IR spectra of the respective polymer-l and polymer-3 complexes, together with an almost 100% metal incorporation under 1:l stoichiometry suggests the formation of the species depicted in Scheme 3. The data available for a polymer-PdClz adduct do not allow an unambiguous choice between possibilities (a) and (b). In fact, although complexes related to possibility (a) are known [22b], their formation would imply the involvement of two isocyano ligands for two adjacent palladium atoms throughout the bulk of the polymeric matrix, which seems to be a rather unlikely circumstance. The relatively low amount of PdClz in
119 entries
3,4&l
entries 1,5
7 $ T I:: C
C I
X -.
I
Pd-X ,/‘”
I
I:: C
\x
I
X-Pd-X I
(b)
C III N
x=cr
i
(a) Scheme 3. Suggested structures for Pd(I1) complexes (entries 1, 3 - 7, Table 3).
the respective resins makes the use of IR spectroscopy unreliable for choosing between the two possibilities. Indeed, chemical reactivity of the anchored palladium centers may be useful for this purpose. Coordination
of Ru(II)
and Ru(III)
[RuC12(PPhs)s] (4), a well-known hydrogenation catalyst [21], and ‘RuCls’ (5), a convenient reagent to prepare 4, undergo ready coordination by all isocyano-bearing polymers employed. A careful mass-balance control for entry 8 (Table 3) showed that the sum of reacted and unreacted 4 (which was recovered analytically pure after complete removal of the solvent) was identical to the total amount used. This observation and the IR data (predominant single v(CN) band) provide strong clues as to the nature of the molecular species formed (see Scheme 4). The bathochromic shift of the coordinated isocyano v(CN) band is known for such ligands bound to Ru(I1) in conventional octahedral complexes [23]. ‘RuCls’ (5) is also quite reactive with Pld and Ple (see Table 3). Under 1: 1 ratio conditions, ca. 50% of the available Ru(II1) undergoes coordination by the polymeric ligands. The IR spectra of the complexes clearly display two v(CN) bands (with comparable intensity), which strongly suggests the formation of cis-bis(isocyano) complexes [24]. In view of the known tendency of Ru(II1) to six-coordination, we propose the formation of the species depicted in Scheme 4. When a 4.2:1 ratio -NC:metal is used, the amount of incorporated Ru(II1) corresponds to the engagement of 92% of the employed metal in the formation of a mono-isocyano complex (single v(CN) band at 2167 cm-‘). This assumption is confirmed by the fact that reaction of the still
120 entries
entry 13
entry 12
840
T
T
’
‘/L X-Ru-X
7,X
/x
X-Ru I L
f C
1;1
C
C
L’
m It
l!i
-CSN
X’l
X-Ru
,co\\x
-I
L’
L=PPh3;
I
--L’
X’i
L’= H20 ; X&I
L’
L’
x
-
Scheme 4. Suggested structures of Ru(I1) and Ru(II1) complexes (entries 8 - 13, Table 3).
16-16
.entries
entry
~~?yy‘ivylvuL
II L--h-O
0
n
14
7
aJ o=
X-Rh
19-21
1y‘
L =
4-c_.
-I.
L
entries
i
ru
I
15
entry
X’
I L
8C8C-
Scheme 5. Suggested structures of Rh( I) and Rh(II1) complexes (entries 14 - 21, Table 3 ).
available isocyano groups in the resin with Co& under 1:l ratio conditions affords the quantitative formation of a mono-isocyano cobalt(I1) complex, as shown in Scheme 4. Heterometallic polymer-bound metal complexes are already known, but they are usually supported by phosphine-bearing polymers [4, 51. Coordination of Rh(I) and Rh(III) [RhCI(PPh,),] (6), [Rh(acac)(CO)z] (7) and ‘RhClJ’ (8) react with polymers Pl according to various incorporation patterns. Thus, 8 undergoes quantitative coordination by Pld when a 6:l excess of -NC functions is employed. Under 1:l conditions, a 46% metal incorporation occurs, which is in accord with the formation of a bis(isocyano) complex. The single v(CN) band suggests that in both cases a presumably trans six-coordinate octahedral species [25a, b] is formed, as depicted in Scheme 5.
121
Both Rh(1) species, 6 and 7, undergo non-quantitative coordination, under 1:l stoichiometry. The reaction of 7 leads to CO evolution, and the IR spectrum of the adduct is in agreement with the presence of one CO and one -NC ligand coordinated to Rh(1) in the molecular planar fourcoordinate complex produced. The formation of a truns-bis(isocyano) complex is ruled out on the basis of the observed incorporation percentages, which are invariably much greater than 50%. Coordination of Cu(II), Cu(I) and Co(U) Bis(2,4-pentanedionate)copper(II) (9), an effective and selective catalyst for Michael syntheses [26], is known to exhibit an appreciable tendency to give five-coordinate adducts with nitrogen bases [27]. 9 reacts with polymer Pld, as shown in Table 3. The polymer-9 adduct is formed in 15 - 20% yield. Several attempts to involve the unreacted fraction of isocyano ligands in incorporating more Cu(acac), were unsuccessful. However, the residual -NC groups turned out to be quite reactive towards anhydrous CoClz, thus giving rise to a heterobimetallic polymer-metal complex (see footnotes in Table 3). The combination of the IR datum (single v(CN) band at 2195 cm-’ [27]) with the known tendency of Cu(acac)z to five-coordination suggests the formation of the adduct sketched in Scheme 6. Nominally, anhydrous CoClz (10) reacts rapidly and almost quantitatively with Pld, Plf and even with the residual groups present in Pld-9 (see above). The molecular species formed exhibits a single strong to weak v(CN) band at cu. 2225 cm-’ (see Table 3) and the visible spectrum displays a broad and composite band centered at cu. 650 nm. The combination of analytical and spectral data indicates that the molecular species formed must be a tetrahedral [28] mono-isocyano complex. In view of the not strictly anhydrous conditions employed, it seems likely that the molecular species can be formulated as @--[ (NC)CoC12(OH,)] (see Scheme 6). The weak bands at 2170 and 2135 cm-’ observed in the case of reaction with Pld may be attributed to the formation of traces of a bis(isocyano) cobalt(I) species [ 281. entries
26,26
entries 22-24
T
T=y==T= N III
N III F
x L’=H20
Scheme
\
0’
0-O 6. Possible
28, 29, Table 3).
)
0 = acac-
structures
29
N III
III C
O\I”,O (
/=px
i
entry
L
/“e\’ xx
&)
x=c1of copper(I1)
and cobalt(I1)
complexes
(entries
22
- 24, 26,
122
A dramatic effect due to the nature of the preexisting metal coordination sphere is observed when Pld is reacted with [Co(OH2)J2+ (11). In this case, the metal incorporation is definitely low and the nature of the molecular metal species formed is quite uncertain. CuCl is also found to react with Ple. The presence of a single strong v(CN) band rules out the formation of a tris(isocyanide) chlorocopper(1) species, which could have been expected [29], and seems to support the formation of a mono-isocyanide chlorocopper(1) linear complex. Metal distribution inside the polymeric beads or granules In view of the often quantitative involvement of all of the isocyano functions under 1:l ratio conditions and the presumably uniform distribution of -NC groups in the resins, it is expected that metal distribution is also homogeneous throughout the beads. This fact appears of great importance in order to employ these materials as precursors of polymersupported metal catalysts. To this end, we examined @-[(NC)Cu(acacM, @-[(NC)RUC~,(OH~)~] (P = Pld) and @-[(NC)2RhC13(0H2)] (P = Pie), by X-ray microprobe. In all cases, the resulting scanning pictures indicated an almost perfectly homogeneous metal distribution, with a slight tendency to ‘zonation’ in certain samples. This confirms the expected uniform distribution of isocyano functions throughout the polymer matrix. Leaching tests in dichloroethane and tolupe on @-[(NC)2RhC13(lXf~] (p = Pld, Ple), @- [(NC)PdCI,] (P = Plfi The resins were treated under severe and dynamic conditions at 100 “C under argon, and were subsequently examined by IR spectroscopy and metal elemental analysis. The results are shown graphically in Fig. 2. (a) @--[(NC),RhC1,(OH2)] in dichloroethane (residual metal %: time (h) in parentheses): 2.6 (0); 2.4 (10); 2.3 (20); 2.0 (30); 2.0 (40); 1.9 (50). smetal 6.0 [
-4-a
5.0
ub
4.0
--o-c
~
Fig. 2. Residual metal content conditions.
of the examined polymer-metal
complexes
under test
123
No change occurs in intensity and wavenumber of the 2243(s) cm-’ u(CN) band for 40 h. After 50 h it appears at 2237(s) cm-‘. (b) @-[(NC),RhCl,(OH,)] in dichloroethane: 6.0 (0); 6.2 (10); 6.2 (20); 6.0 (30); 6.3 (40); 6.0 (50). No change in intensity and wavenumber of the 2243(s) cm-’ band. (c) @-[(CN)PdCl,] in toluene: 2.2 (0); 1.9 (10); 2.0 (20); 2.1 (30). Disappearance of the v(CN) band immediately after the first cycle. It is seen that only in case (a) does the metal content gradually decrease. The erratic behaviour of the analytical data in cases (b) and (c) is probably related to different metal contents among different groups of beads. The SEM technique reveals, in fact, that the beads of Pld and Ple are not isolated (single) (see Fig. 3), but rather grouped in clusters and this circumstance may cause a macroscopic non-equivalence among groups of clustered beads. In the case of @-[(NC)PdCl,], the leaching of the metal is very small (if any), but the treatment leads to thermal degradation of the Pd(I1) complex, with presumably formation of highly dispersed metallic palladium within the resin matrix. Stability of @--[(NC),RhCZ,(OH,)] under dihydrogen and dioxygen in polar solvents The title resin was chosen as a model of precursor of hydrogenation and oxygenation catalysts, and was tested in dichloroethane and 2-methoxyethanol. The main results are collected in Table 4. It is seen that the resin is quite stable under Hz and O2 in dichloroethane at 100 “C. Conclusions Functionalized polymer supports containing from 0.2 to 1.3 meq gg’ of isocyano functions appear to possess strong coordinating ability towards
Fig. 3. SEM micrograph of a dry sample of @--[(NC)2RhC13(0H2)].
124 TABLE 4 Behaviour of @-[ (NC)2RhC1a (OH*) ] under Hz and 02 Gi3.5
Solventa
Temperature (“C)
Remarks
02
DCE DCE 2ME 2ME
100 100 100 100
no change in % metal and IR spectrumb analytical data as above analytical data as above no change in % metal; the 2240 cm-’ band disappears and a weak band develops at 2220 cm-i
HZ 02 HZ
aDCE = 1,2-dichloroethane; 2ME = 2-methoxyethanoi. bWith respect to their behaviour under argon (see above). Contact vigorous stirring.
time 10 h, under
many transition metal species. Relatively low charged resins appear to be suitable for the accommodation of one metal atom per isocyano ligand, and hence the possibility of preparing metal complexes with unusual coordination spheres. This type of matrix isolation is somewhat surprising, considering that in polystyrene-type resins site isolation is usually difficult to achieve [4] and that the presently described resins have a matrix structure considerably more flexible (see Fig. 1 and [ 301) than polystyrene-based resins. Whatever the explanation, however, the implications of the present results are clear: both the number of isocyano ligands per metal atom and the lower overall coordination number are likely to give the metal center novel reactivity features. Moreover, these moderately crosslinked, tailor-made supports appear to combine interesting physical and chemical features in view of their utilization for the preparation of polymer-supported transition metal catalysts. Three basic lines of application can be envisaged: (i) hybrid catalysis especially concerned with dioxygen chemistry; (ii) highly dispersed heterogeneous catalysis with a wide choice of metal species and operating temperatures; (iii) synthesis of heteroleptic @-NC-ML, metal complexes exhibiting unprecedented chemical behaviour. Research lines (i) and (ii) are currently being followed in these and other laboratories 1311 and the satisfactory maintenance of the catalytic activity of polymer-supported [Cu(acac)z] in Michael synthesis has been already reported [ 261. Experimental Analytical methods and materials IR spectra were recorded on a Perkin Elmer ‘781 spectrophotometer. UV-VIS spectra were recorded on a Perkin Elmer X-5 spectrophotometer.
I
125
DTA-TG data were acquired on a Netzsch DTA-TG 429/3/6 apparatus. Elemental analyses were performed on a Carlo Erba elemental analyzer mod. 2000. Atomic absorption meas~emen~ were performed on a Perkin Elmer 3030 atomic absorption spectrophotometer. Commercial chemicals (reagent grade) were used as purchased if not otherwise stated. RhCl(PPh& and Rh(acac)(CO)z were prepared according to literature methods [32,33]. The procedure employed for the Wilkinson’s complex was also used for the synthesis of the Ru(I1) analog, RuCl~~PPh~)~. In Table 3 and in the discussion section, rhodium trichloride and ruthenium trichloride are indicated as ‘RhCIJ’ and ‘RuCl,‘. The commercial products are labelled as trihydrated species. The nature of the rhodium salt is uncertain, whilst the ~thenium compound contains small amounts of polynuclear complexes of Ru(IV) [ 341. For this reason the notation ‘RhC13’ and ‘RuC13’ is used. Literature procedures were followed for the titration of the isocyano functionality on the resins [19a] and for the synthesis of 1,6diacrylamidohexane [ 351. Synthesis of 3-formamidopropyl acryiate 110 cm3 of methyl formate were added dropwise, under vigorous stirring and at room temperature, to a solution of 37.8 g (0.5 mol) of 3Gino-l-prop~ol in 200 cm3 of methanol and 10 cm3 of triethylamine. After standing overnight, most of the solvent was removed in a rotatory evaporator and the intermediate product, N-(3-hydroxypropyl)formamide, was distilled under vacuum. The recovered product was diluted with 180 cm3 of dichloromethane and 78 cm3 of triethylamine, and 0.8 g of 4-N,Ndimethylaminopy~dine were added. Into this solution was dropped a mixture of a stoichiometric amount of acryloyl chloride in ea. 40 cm3 of dichloromethane, under vigorous stirring at 0 “C. The reaction mixture was stirred overnight at room temperature, then filtered and washed with aqueous sodium hydrogen carbonate, hydrochloric acid and water. The organic layer was dried over ma~esium sulphate and the solvent was removed in a rotatory evaporator. The product obtained was finally distilled under vacuum (141 “C, 2 mmHg), in presence of 2,6-d&t-butyl-4-methylphenol. ~epara~~~ of type PI beaded resins by suspension po~yrner~a~~on The following procedure was used for the synthesis of Pld: 150 cm3 of 1,2dichloroethane and 3.0 g of cellulose acetate butyrate were introduced into a previously described suspension polymerization apparatus [18] at 50 “C. The mixture was stirred until the solid was completely dissolved; the solution was then deaereated by flushing with argon for at least 5 min. Meanwhile, a mixture of 8.0 g of N,Ndimethylacrylamide, 1.2 g of 20 cm3 3formamidopropyl aerylate, 1.8 g of 1,6diacrylamidohexane, of water and 10 cm3 of d~ethy~o~amide was prepared, The stirrer speed was set to 500 - 600 rpm and 0.6 g of ~monium peroxodisulphate were added to the reactant mixture, which was subsequently introduced into
126
the polymerization reactor through the side arm. The copolymerization was allowed to proceed for 5 h under a slow nitrogen flux, and the volume of the mixture was kept constant by means of further additions of the solvent. The mixture was transferred into a beaker and 250 cm3 of acetone were added. The copolymer beads were allowed to settle and the liquid was then decanted. The same procedure was repeated four times using water and acetone. Finally, the beads were washed with diethyl ether and dried overnight under vacuum in the presence of phosphorus pentoxide. Preparation ‘of type P2 resins by radiation-promoted polymerization The following procedure was used for the synthesis of P2b: the monomers, 2-hydroxyethyl methacrylate (1 g), 3-formamidopropyl acrylate (0.15 g), diacrylamidomethane (0.08 g) and water (1.2 g) were introduced into a 5 cm3 Pyrex vial. The solution was deaerated by means of three freeze-thaw cycles. The vial was flame-sealed under vacuum, cooled to -78 “C (solid carbon dioxide-acetone) and exposed to a 6oCo source (dose ca. 10 kGy = 1 MRad). The copolymer was recovered, cut in thin slices and repeatedly washed with water, methanol and diethyl ether. Finally, the polymer was dried under vacuum at room temperature. Generation of the isocyano groups on the resins The following procedure is a slight modification of a previously described one [19] : the polymer was allowed to swell overnight in anhydrous pyridine (8 - 10 cm3 g-l), and p-toluenesulphonyl chloride was added to the vigorously stirred suspension. After - 1 h, an excess of acetone was added, the polymer was recovered by filtration and washed on the filter as follows: acetone (2X), methanol (3X), water (3X), aqueous 7% potassium hydrogen carbonate and methanol (2X), water (2X), acetone (3X) and diethyl ether (3X). The polymer was finally dried under vacuum in the presence of phosphorus pentoxide. Synthesis of the polymer-supported complexes The resin and the metal compound to react were transferred into a specially built two-cell reactor. The amount of the reacting metal complex was calculated on the basis of the -NC meq on the resin and the desired initial -NC:metal ratio (usually 1:l). The resin was swollen in the minimum volume, while a saturated metal solution was prepared in the same solvent. The reactants were then mixed and the reactor was vigorously shaked by hand for some minutes. The reaction mixture was afterwards kept under gentle mechanical stirring for 2 h. The obtained adduct was filtered, carefully washed with fresh solvent and diethyl ether, and finally dried under vacuum in the presence of phosphorus pentoxide. The unreacted metal complex (if any) was recovered for mass-balance. Determination of the metal content of thepolymermetal complexes 15 mg of the polymer-metal sample were introduced to a 50 cm3 round-bottom flask and 5 cm3 of a 1:l mixture of concentrated nitric acid
127
and sulphuric acid were added. The suspension was refluxed for -2 h; the obtained solution was poured into a lOO-cm3 volumetric flask and the volume was adjusted with distilled water. The solution was then analyzed by atomic absorption spectroscopy. Leach ing tests Ca. 250 mg of the sample were introduced to a 50 cm3 glass reactor together with 15 cm3 of 1,2dichloroethane. The mixture was deaerated by flushing with an inert gas. The reactor was immersed into a thermostatted bath set at 100 “C and kept under gentle stirring. After 10 h, the polymer was filtered, washed with fresh solvent (4 X 5 cm3) and diethyl ether, and dried under vacuum. A small amount was used to record the IR spectrum and to determine the metal content, while the remainder of the sample was used for the subsequent cycles. The cycles were repeated three, four or five times, so that each complete test lasted 30 - 50 h. An almost identical procedure was used to verify the stability of the polymer-metal complexes under reactive gases at various temperatures, except that the initial suspension was deaerated with the desired gas (H,, O,), the temperature was set to the chosen value and each test consisted only of one 10 h cycle.
Acknowledgements This work stems from our discussion and preliminary experiments at the Technical University of Munich in Garching during 1981. We would like to express our gratitude to our hosts Professors I. Ugi (to R.A.) and H. F. Klein (to B.C.) and to the Alexander von Humboldt Foundation for a grant (to B.C.). We are also grateful to Mr. A. Ravazzolo for his skillful technical assistance (to R.A. and B.C.).
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