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Synthesis of new dimolybdenum(II,II) di-μ-carboxylato dicationic and dianionic complexes, and their use as polymerization catalysts
0277-5387/91 f3.00+.00 0 1991 PergamonPressplc
.“o/ykedron Vol. IO,No. 19, pp. 2283-2291, 1991 Printedin Great Britain
SYNTHESIS OF NEW DIMOLYBDENUM(II,II) DEpCARBOXYLAT0 DICATIONIC AND DIANIONIC COMPLEXES, AND THEIR USE AS POLYMERIZATION CATALYSTS MALACHY
MCCANN? and PAULINE
GUINAN
Chemistry Department, St. Patrick’s College, Maynooth, Co. Kildare, Ireland (Received 29 April 1991; accepted 21 June 1991) Abstract-The dicationic complex salt cis-[MO,@-02CCH3),(MeCN),][BF& (l), reacts with an excess of unsaturated carboxylic acid (RC02H) to give a family of new dicationic *xHzO [R = HC=CH2 (2), (CH3)C=CHz (4), salts, [Mo2(~-02CR)2(MeCN),I[BF,I2 H2CCHC=CH2 (6), (CH3)HCH2CHC=CH2 (7), C7H9 (8); x = 1 or 21. Complex 1 also reacts with Et,NBr to give the dianionic salt (Et4N)2[Mo2(p-OzCCH3)2Br4] (9). Complexes 1, 4 and 9 catalyse the room temperature polymerization of bicyclo[2.2.1]hept-2-ene. EtAlC12 was employed as a co-catalyst in these reactions. The dicatiomc salts 1, 2 and 4 also catalyse the room temperature polymerization of cyclopentadiene, but the dianionic salt 9 is inactive towards this monomer.
A large number of dimolybdenum(II,II) complexes of the general formula [Mo2(~-02CR).J have been made using saturated alkyl and aryl carboxylate ligands. ’ We have recently described the synthesis of a range of MoZ4t, CuZ4+ and RuZSf tetra-pcarboxylato complexes containing unsaturated alkyl carboxylate ligands, and demonstrated how the pendant C=C functions of the complexed ligands could react with cyclopentadiene to form the respective Diels-Alder cycloaddition adducts. In this paper we describe the synthesis of some dimolybdenum(II,II) dicationic complexes containing two unsaturated carboxylate ligands, and the preparation of a new di-p-acetato dianionic complex. The use of these complexes as catalysts for the polymerization of bicyclo[2.2.l]hept-Zene and cyclopentadiene is also reported. RESULTS
AND DISCUSSION
Two different research groups have independently prepared and structurally characterized the red dimolybdenum(II,II) salt cis-[MO& O,CCH,),(MeCN),][BF& (l), the complex dication having the structure shown be10w.“~ Earlier,
t Author to whom correspondence
should be addressed.
Drago and Telser3’ reported the synthesis of the closely related salts [Mo2(,u-02CCH3)z(MeCN)4] and W3S0312 ~02(@2CCH3)2(M~N)51 [BF,OH],, which contain four and five MeCN ligands, respectively. 2+ CH3
L=MeCN
L
L
We have employed 1 as the starting material for the preparation of a series of similar dicationic complexes containing two unsaturated carboxylate ligands. Solutions of 1 in MeCN were reacted with an excess of the unsaturated carboxylic acid so as to replace the two acetate ligands with unsaturated carboxylate ligands (Scheme 1). Two successive metathesis cycles were usually required for complete substitution of the acetate by the new carboxylate ligand. The dications of the new salts have the common formula [Mo2(p-carboxylate), and given that the structural data for WeCN)412’,
2283
M. MCCANN
2284
and P. GUINAN [Ms(~~-~C(~~)CZ~[BF~Z~ZO
[Moz~-Oz~H,)~~[BFd,.2H20 y
\
[Mo2(rr-02~C~32L4][BF~2~*0
f
[M~C~~~C~~~C=.O.~~O
Q
(2)
+ [M~@O$HC=CH&l
(4)
(8)
k
Pfozclr-~~H&jlPF&
(1) &
[M~~-~C(~HH~z~C~~~l~F~~H*O
.0.5H20 (3) 2%
[Moz~--q~2~C~~2L~~F~2~~0
(7)
(6)
@t&Fl~~-OzC(3H3~l
(9)
Scheme 1. L = MeCN, (i) CH+ZHCO,H, (ii) CH2--_C(CH3)C02H,(iii) CH+ZHCH,CO,H, CH,--CHCH,CH(CH,)C02H, (v) C7H9C02H, (vi) Et.,NBr, (vii) cyclopentadiene.
1 show the four MO-N (equatorial) distances to be considerably shorter than the two MO-N (axial) distances, 3a,bit is reasonable to assume that the four MeCN ligands in the present complexes are bonded equatorially to the bimetallic core. During the synthesis of the di-p-propenoato and di-p-2-methylpropenoato salts 2 and 4 small amounts of the respective neutral tetra-p-propenoato and tetra-p-2-methylpropenoato complexes (3 and 5) also formed. Pure samples of 3 and 5 have previously been obtained in high yield by reacting [Mo~(~-O~CCH~)~] with an excess of the respective unsaturated acid.’ It is interesting to note that when [Mo2(p-02CCHJ4] was reacted with the longer chain unsaturated acids 3-butenoic acid and 2-methylpent-4-enoic acid, the tetrasubstituted complexes [Mo2(p-02CH2CHC=CH&] and [Mo*(p-02C(CH3)HCH2CHC=CHJ4] did not form, and only Mo-oxo complexes were recovered.4 However, when the same two long-chain unsaturated acids were reacted with salt 1, the metathesis products 6 and 7 were recovered in high yield and, again, none of the corresponding neutral tetra-p-carboxylato complexes formed. The di-pbicyclo[2.2.l]hept-5-ene-2-carboxylato complex 8 was synthesized by reacting the propenoato complex 2 with a large excess of cyclopentadiene (cpd). Some polycyclopentadiene (pcpd) also formed during the reaction (discussed later). The conversion of 2 to 8 occurs via a Diels-Alder cycloaddition reaction of the type mentioned previously.’ Complex 8 was also prepared by reacting 1 directly with endo-bicyclo[2.2.l]hept-5-ene-2-carboxylic acid (C, H&O*H). Finally, when a solution of 1 in MeCN was reacted with Et,NBr, the red salt 9 formed, which contains the complex dianion ‘$402(~-02 CCH&Br4]*-.
(iv)
The IR spectra of the new di-p-carboxylato salts have vasym(C02)and v,,,(CO,) bands at frequencies consistent with those expected for bridging carboxylates, and a v(C=C) band was also evident for the complexes (except 8) containing unsaturated carboxylate ligands (Table 1). Weak bands attributable to the MeCN ligands were present at ca 2280 cm-‘. In the spectrum of the dianionic complex 9 the acetate v(C02) bands were at frequencies similar to those found in the spectrum of the structurally characterized diacetate 1, and a strong v(Mo-Br) band5*6 was observed at 268 cm- ‘. Although the ‘H NMR spectra of the new salts (Table 1) did show peaks attributable to the different hydrogen atoms of the ligands not much structural information on the complexes was forthcoming due to the complexity of the observed splitting patterns. The spectrum of a DMSO-d6 solution of the dianionic complex 9 contained two distinct methyl (CH,CO,) resonances at 1.93 and 2.00 ppm in a ratio of ca 9 : 1. The presence of two signals suggests that in solution there are two different species present. This type of behaviour was similar to that observed for CDC13 solutions of the neutral complex [Mo2(,u-01CCH3)C1,(PMe3),]* 0.75THF.’ An acetonitrile solution of complex 9 was prepared under anaerobic conditions and its electronic absorption spectrum immediately recorded (Fig. 1, t = 0). A single visible peak corresponding to the metal-metal 6 + 6* transition’ was present at 540 nm (E = 838 dm3 mall ’ cm- ‘). This spectrum was similar to that reported for methanol solutions of the closely related dianions [Mo2(p-03SCH3)2Br4]2(&,,,, = 522 nm)” and [Mo~(~-O~CCH~)~C~~]*- (n,,, = 495 nm).9 A solution of 9 was allowed to slowly air-oxidize and the spectrum was recorded at intervals up to t = 24 h,
(9”
(8)
1.98(s) CH+ZN; 1.55(d) CH,; 2.5-3.0(m) CH,CH; 3.4(tq) CH,CH; 5.1-5.4(m) CH-CH; 5.7-6.3(m) CH,--CH
1640
1425 1422 1438
1500 1490 1515
9
1.98(s) CH,CN; 3.94.1(m) 6&6.6(m) CH,--CH
1640
1420
1505
1.42.5(m), 2.84.1(m);
i 1.93(s) ; 2.0(s) CH,
1.98(s) CH,CN;
6.&6.9(m) CH=CH
CH2; 5.2-5.6(m) CH,--CH;
5.9(m), 6.4(m) CH,--C
2.25(dd) CH,;
1.98(s) CH,CN;
1620
1412
1472
6.&7.3(m) CH,--CH
1.98(s) CH,CN;
1635
1435
2.9(s) CH,
1490
1.98(s) CH,CN;
1400
‘H NMR
1500
@==C)
v,,,(&)
va,,nl(CW
“KBr disc. bValues in cm- ‘. ‘CD&N solutions : values in ppm. d(s) singlet, (d) doublet, (dd) doublet of doublets, (tq) triplet of quartets, (m) multiplet. eL = MeCN. /See ref. 5. ~v(C=C) not visible. hv(Mo-Br) 268 cm- ‘. ‘Solvent = DMSO-d,.
(Et,N)2[Mo2(~-02CCH,)zBr,l
[MoXy02CC,H9)*Ld[BF4~.2HzO
Complex
Table 1. IR”,b and ‘H NMRc,d data for the complexes’
s El ‘0 s 5
E. c) $ a E E. 0 0e.
Et: B E
3 r t 8
-c E
E
n .!! B
2 Y B kl 5
M. MCCANN and P. GUINAN
2286
550
603
6%
700
Nanometers
Fig. 1.
R
at which time the 540 nm peak had completely disappeared and three new absorption peaks had grown (481, 419 and 382 nm). The disappearance of the 540 nm peak signifies the rupture of the MO-MO 6 bond. In conclusion, it is thought that in the solid state the new dicationic complexes are essentially isostructural with 1 [Fig. 2(a)]. It should be mentioned that these complexes could also have a trans configuration [Fig. 2(b)]. The dianion, [MO&U02CCH3)2Br4]2-, is thought to be structurally similar to the known dianions [Mo~(~-O~SCH~)~X~]~(X = Cl, Br, I)6 and [Mo~(I.L-O~CCH~)~C~~].*-~ The X-ray crystal structure of the latter dianion showed the two acetates to be trans. Possible cis and tram structures for [Mo2(p-02CCH3)2Br4]2- are illustrated in Figs l(c) and (d).
2+
A
i,okR
L/MTMo
I
i
I
i
L=MeCN
(4
I
(W
a3
(4
R
2-
CH3
A 7 ,o&fH3 BY”TMO I
I
Br
Br
Cc)
Fig. 2. (a) and (b) Possible structures for the new dications [Mo~(~-O$R)~LJ*+ [R = HC=CH2 (2), (CH&=CHz (4), H2CCHC==CHz (6), R = (CHS)HCH2CHC=CH2(7), C,H, (8); L = MeCN]. (c) and (d) Possible structures for the dianion, [Mo,(~-O~CCH~~B~J-.
Dimolybdenum(II,II)
Polymerization catalysis
di-p-carboxylato
dicationic and dianionic complexes
2287
pcpd + (8)
(1) Bicyclo[2.2.l]hept-2-ene(norbornene). In 198 1 Lamotte et al.” reacted aluminium isopropoxide t (2) with [Mo&J-O&CH~)~] to give [MO&-O2 @y 9”“” the CCH,)&-(OCH(CH&),Al(OCH(CH,),}& X-ray crystal structure of which showed tram acetates and tram aluminium isopropoxides. Later, 0\ Diefenbach reported that dimolybdenum(II,II) carboxylato complexes in the presence of aluminium, DW-WWl/ ‘y titanium or zirconium alkoxides were good catalysts for the metathesis of alkynes. ’ ’ Furthermore, no reaction he prepared and characterized complexes of the no reaction type [Mo2(0FR)2{~@-OCgH4C1)4}21 P = CH3, Scheme 2. pcpd = polycyclopentadiene. C(Me)3, CFJ, and subsequently showed that they also catalysed alkyne metathesis reactions. Earlier, we had shown that the neutral and anionic dimolybdenum(II,II) complexes [Mo&-O~CR)~] (R = nificant change in the stereochemistry of the double CH3, CF,) and K4[Mo2Cl,] are good metathesis bonds in the polymer chain. catalysts for ring-opening polymerization reac(2) Cyclopentadiene. We have recently described tions. “3 ’ 3 For example, each of the catalyst/cohow some neutral dimolybdenum(II,II), dicatalyst systems [MO&-02CCH,),]/EtAlCl, and copper(II,II) and diruthenium(II,III) complexes K4[Mo2Cls]/EtA1C12 polymerized bicyclo[2.2.1] containing four unsaturated carboxylate ligands hept-2-ene and 1-methylbicyclo[2.2.l]hept-2-ene at react with excess cyclopentadiene to give the respec2o”C, and [Mo&-O&CF~)~] polymerized the tive Diels-Alder cycloaddition adducts. 2 No polysame monomers even in the absence of a co-catalyst. cyclopentadiene (pcpd) formed during these reacThe dicationic complexes 1 and 4 and the tions. The results of reactions of some of the present dianionic complex 9, when used in the presence of complex salts with excess cyclopentadiene in EtA1C12 as a co-catalyst, catalysed (0.28 mol % acetonitrile at room temperature are outlined in catalyst) the room temperature ring-opening polyScheme 2. A 0.09 mol % solution of 4 rapidly catamerization of bicyclo[2.2.l]hept-2-ene to give lysed the addition polymerization of cyclopentapoly( 1,3-cyclopentylene-vinylene) (pcpv). When diene, with the pcpd precipitating as a white solid the reaction solvent was changed from toluene to (yield: 35%). Unchanged 4 was subsequently reacetonitrile no polymerization occurred.‘4 From the covered from the reaction filtrate. Although 1 also 13C NMR spectra of the polymers the fraction of catalysed the formation of pcpd (yield : 8%), it was double bonds in the polymer chain with cis characnot as active as 4. Complex 2 was even less active ter (a=) was calculated. 12x13The close similarity in than 1 and only a very small amount of pcpd was obtained. Furthermore, the Diels-Alder cycloaddition adduct 8 was isolated in high yield from the filtrate of the latter reaction (Scheme 1). The dianionic complex 9 failed to react with cyclopentadiene, and unchanged 9 was recovered. In addition, no pcpd formed when the neutral complex [Mo~(~-O~CCH~)~] was reacted (either as a suspension in acetonitrile or as a solution in hot meth(R = H, Me) anol) with cyclopentadiene. Although there are many examples known where non-transition metal Lewis acids have been used as of cycloof the polymerization catalysts pentadiene, ’ 5 there are only a few cases in which transition metal complexes [other than those of titanium(IV)] have been employed,16 and it is generally thought that all of these catalysts cause the values of a, (ca 0.36) found for the three pcpv the polymerization to proceed via a cationic mechsamples indicates that changing the ligands on the anism. “-I9 Normally, the pcpd obtained from these dimolybdenum(II,II) core does not cause a sig- literature reactions is soluble2’ in CHC13 and from
’
I
2288
M. MCCANN
and P. GUINAN
their NMR spectra ’ 5~2 ’ it was deduced that these addition polymers contain both 1,4- and 1,2-structural units. Furthermore, it has been shown that the 1,4- and 1,Zcontent of the polymers is generally dependent on the type of catalyst used, solvent and temperature. ’ 7 Unfortunately, the insolubility of the pcpd obtained using the present complex salts prohibited detailed structural characterization by NMR spectroscopy. However, the IR spectra of these insoluble polymers were identical to those illustrated in the literature’ 5 for soluble poly-1,4-/ 1,2_cyclopentadiene.
towards cyclopentadiene. The inactivity of salt 9 is probably attributable to the fact that the dianion [Mo2(p-02CCH3)2Br4]2- can be viewed as a Lewis base and, as such, would effectively repel the electron-rich cyclopentadiene molecule. The neutral complex [Mo2(p-O&CH,),] also failed to catalyse the polymerization of cyclopentadiene, suggesting that it too is not a strong enough Lewis acid for diene activation.
0
All reactions were carried out under nitrogen using dry solvents. Standard Schlenk techniques were employed during the preparation of the molybdenum complexes. Although all of the complex salts were air-sensitive the dicationic salts appeared to be considerably more sensitive than the dianionic salt. Literature methods were used to prepare cis-[Mo2(p-02CCH3),(MeCN)J[BFJ2 (l),3b en&-bicyclo[2.2. l] [Mo2(~-02CCJ&h1*~ and hept-5-ene-2-carboxylic acid.25 Ethylaluminiumdichloride was used as a 25% solution in hexane. IR spectra were recorded in the region -200 cm-’ on a Perkin-Elmer 783 grating spectrometer, and electronic spectra were run on a Milton Roy Spectronic 3000 Array. ‘H NMR spectra of solutions of the complexes (in CDC13 or DMSO-de, and under nitrogen) and 13C NMR of bicycle [2.2.l]hept-2-ene polymers (in CDC13) were recorded using a Bruker AC 80 spectrometer. Elemental analyses were performed by the Microanalytical Laboratory, University College, Cork, Ireland.
\
catalyst I
1,4-
(cis and mans)
1,2-
(cis and tram)
Assuming that the present polymers are comprised of 1,4- and 1,Zunits a possible cationic addition mechanism for their formation can be visualized (Scheme 3). ‘H and 13C NMR studies on complex 13bhave shown that the MeCN ligands are labile, and we have further demonstrated here that they are readily replaced by bromide ions (formation of 9). Thus, a first step in the polymerization process is likely to be the displacement of one or more MeCN ligands by cyclopentadiene to give species (a). The cyclopentadiene molecule may bond to either one or both (i.e. bridging) molybdenum atoms of the bimetallic core to give the carbocation (b), which itself may isomerize to (c).” Addition of cyclopentadiene molecules to (b) and (c) will give rise to separate 1,2-pcpd and 1,4-pcpd, respectively. A single polymer molecule containing a mixture of 1,2- and 1,Cunits in the chain can form if there is isomerization of either (b) or (c) during polymerization (Scheme 4). Given the close similarity in both the structural and electronic properties of the complex dications of salts 1, 2 and 4 it is difficult to understand the large differences observed in their catalytic activity
* Microanalytical and IR data for complexes 3 and 5 corresponded with that reported in ref. 2 for these complexes.
EXPERIMENTAL
[Mo2(p-02CCH=CH2)2(MeCN)4][BF4]2*H20 *0.5H20 (3)*
(2)
and [Mo~(~-O~CHC=CH~)~]
A solution of complex 1 (0.36 g, 0.49 mmol) and propenoic acid (0.5 g, 6.9 mmol) in acetonitrile (20 cm’) was stirred at room temperature for 12 h, and the resulting light red solution was then reduced under high vacuum (at 20°C) to ca 1 cm3. Addition of toluene (40 cm”) induced the formation of a dense red oil. The upper yellow liquid was decanted off and fresh toluene (20 cm’) and acetonitrile (3 cm3) was added to the red oil. The mixture was stirred vigorously and the crude product 2 eventually precipiated as a red solid. The solid was filtered off, washed with toluene (2 x 20 cm’) and dried in vucuo. The solid was then redissolved in a fresh solution of propenoic acid (0.5 g, 6.9 mmol) in acetonitrile (20 cm’) and stirred at room temperature for a further 3 h. The red complex was then reprecipitated from the oil form as described above ; yield : 0.2 g
Dimolybdenum(II,II)
di-p-carboxylato
dicationic and dianionic complexes
0) MO-MO]+
2289
(c) mo-MO]+
I
etc.
I
etc.
Scheme 3. [Mo-MO]
= [Mo2(p-02CR),(MeCN),],
R = CHI,HC=&H2,
(CH&kCH2,
x <
6.
(59%). The combined reaction filtrates (yellow liquid and toluene washings) were concentrated to low volume under high vacuum (at 20°C) and the yellow solid 3 precipitated ; yield : 0.05 g (21%). (2) : Found: C, 24.5; H, 2.8; N, 7.7. Calc.: C, 24.4; H, 2.9; N, 8.1%.
[Mo,(~-O,C(CH,)C---CH~3,(MeCN)4l[BF4l~.H~0 (4) and[MoZ(~-OIC(CH$Z=CHJ4]*0.5H20
(5)*
Using 2-methylpropenoic acid these complexes (red and yellow coloured, respectively) were prepared and recovered by the same method as described for 2 and 3 ; yield (4) : (76%). Found : C, 26.2; H, 3.6; N, 7.9. Calc.: C, 26.8; H, 3.4; N, 7.8%.
Wo - MO]+
I 1
1
2
3
I
4
+
Using
prepared
was and recovered by the same method as
3-butenoic
acid
this
red
complex
Scheme 4.
1
M. MCCANN
2290
described for 2; yield : (80%). Found : C, 26.9 ; H, 3.6; N, 7.4. Calc.: C, 26.8; H, 3.4; N, 7.8%. [Mo&-02C(CH3)HCH,CHC=CH,),(MeCN),] [BF& *Hz0 (7) Using 2-methylpent-4-enoic acid this red complex was prepared and recovered by the same method as described for 2 ; yield : (93%). Found : C, 31.3; H, 4.1; N, 7.7. Calc.: C, 31.0; H, 4.2; N, 7.2%.
[Mo2(~-02CC7H9)2(MeCN)41[BF412.2H20 (8) Method(a). Using endo-bicyclo[2.2.l]hept-5-ene2-carboxylic acid this red complex was prepared and recovered by the same method as described for 2; yield : (68%). Method(b). Complex 2 (0.17 g, 0.25 mmol) and cyclopentadiene (0.4 g, 6.1 mmol) were stirred together in acetonitrile (20 cm’) at room temperature for 24 h. A small amount of white pcpd solid was filtered off and complex (8) was subsequently recovered from the filtrate using the method outlined for the isolation of 2 ; yield : 0.14 g (70%). Found : C,34.3;H,4.1;N,7.1.Calc.:C,34.3;H,4.1;N, 6.7%. (EtqN)2[Mo2(~-02CCH3)2Br41 (9) To a solution of 1 (0.33 g, 0.45 mmol) in acetonitrile (20 cm’) was added tetraethylammonium bromide (0.57 g, 2.1 mmol), and this resulted in an immediate red to dark purple colour change. The solution was stirred for 0.5 h and the solvent was then removed using high vacuum. Ethanol (30 cm3) was added and the resulting suspension was stirred for 1.5 h. The red product was removed by filtration and then resuspended in fresh ethanol ( 10 cm ‘) and stirred for another 10 min. The solid was filtered off, washed with ethanol (3 x 5 cm’) and then dried in vacua; yield : 0.23 g (57%). Found : C, 26.9 ; H, 5.4; N, 3.2; Br, 36.2. Calc.: C, 27.0; H, 5.2; N, 3.1; Br, 35.9%. Complex 9 gives an orange solution in MeOH and DMSO, and a purple solution in MeCN. Polymerization
reactions
All polymerization reactions were performed at room temperature and under nitrogen. Typical polymerization procedures are outlined below. Polymerization
of bicyclo[2.2.l]hept-Zene
Bicyclo[2.2.l]hept-2-ene (0.5 g, 5.3 mmol) was dissolved in dry toluene (1 cm3) and the dimolyb-
and P. GUINAN denum(II,II) salt (0.015 mmol) was added. Ethylaluminiumdichloride solution (0.5 cm3, 0.33 mmol) was then added and this resulted in rapid polymer formation (as indicated by an increase in solution viscosity). The mixture was then allowed to stand at room temperature for 1 h. Ethanol (ca 25 cm’) was added and the precipitated polymer (pcpv) was washed several times with small volumes of ethanol. The wet solid was then dissolved in chloroform (ca 75 cm3), and the resulting polymer solution was dripped into ethanol (300 cm’) containing concentrated hydrochloric acid (0.2 cm3) causing the polymer to reprecipitate as a fine white solid. This solid was filtered off, washed with ethanol and dried in uucuo for 1 h at room temperature. A solution of the polymer in CDC13 was then characterized using 13C NMR spectroscopy. For example, using complex 4 as catalyst, 40 mg of pcpv was obtained. Polymer o, values (in parentheses) obtained using the various catalysts were as follows: 1 (0.36), 4 (0.36), 9 (0.35). Polymerization
of cyclopentadiene
A solution of the dimolybdenum(II,II) complex salt (0.036 mmol) and cyclopentadiene (2.6 g, 39.4 mmol) in acetonitrile (5 cm’) was stirred for 72 h during which time the white pcpd gradually precipitated. The solid was filtered off, washed with acetonitrile and dried in vucuo. For example, using complex 4 as catalyst pcpd was formed in 35% yield. The pcpd formed in these reactions was insoluble: in all common organic solvents. The polymers discoloured slightly on standing.
REFERENCES 1. F. A. Cotton and R. A. Walton, Multiple Bonds Between Metal Atoms, Ch. 8.3. Wiley, New York (1982). 2. M. McCann, A. Carvill, P. Guinan, P. Higgins, J. Campbell, H. Ryan, M. Walsh, G. Ferguson and J. Gallagher, Polyhedron 1991, 10, 2273. 3. (a) F. A. Cotton, A. H. Reid Jr and W. Schwotzer, Znorg. Chem. 1985, 24, 3965; (b) W. Clegg, G. Pimblett and C. D. Gamer, Polyhedron 1986, 5, 31; (c) J. Telser and R. S. Drago, Znorg. Chem. 1984,23, 198. 4. When [MO,@-O,CCH&] was reacted with 3butenoic acid a brown Mo-0x0 solid formed (see ref. 2). Under similar experimental conditions 2-methylpent-4-enoic acid also produced a brown Mo-oxo complex. Analytical data : Found : C, 23.32; H, 3.60. The two brown solids had similar IR spectra. 5. E. Hochberg and E. H. Abbott, Znorg. Chem. 1978, 17, 506.
Dimolybdenum(II,II)
di-p-carboxylato
6. J. San Filippo, H. J. Sniadoch and R. L. Grayson, Znorg. Chem. 1974, 13,212l. 7. P. A. Bates, A. J. Nielson and J. M. Waters, Polyhedron 1987,6,2111. 8. Although the authors of references 3(a) and 3(b) reported the synthesis and X-ray crystal structure of complex 1 they did not report the IR band positions for the carboxylate groups or the NMR peak positions for the ligands. As an aid to structural assignment for the complexes described in this work we report some relevant IR and NMR data for 1 in Table 1. 9. W. Clegg, C. D. Garner, S. Parkes and I. B. Walton, Znorg. Chem. 1979, 18, 2250. 10. J. Lamotte, 0. Oideberg, L. DuPont and P. Durbut, Cryst. Struct. Comm. 1981, 10, 59. 11. S. P. Diefenbach, U.S. Patent 4,704,377 (1987). 12. J. G. Hamilton, K. J. Ivin, M. McCann and J. J. Rooney, Makromol. Chem. 1985,186,1477. 13. K. J. Ivin, D. T. Laverty and J. J. Rooney, Makromol. Chem. 1977,178, 1545. 14. Diefenbach reported that alkyne metathesis did not occur when acetonitrile was used as the reaction solvent (see ref. 11). 15. C. Aso, T. Kunitake and Y. Ishimoto, .Z.Polym. Sci. A-l 1968,6, 1163. 16. R. V. Honeychuck, P. V. Bonnesen, J. Farahi and W. H. Hersh, J. Org. Chem. 1987,562,5293 and refs therein.
dicationic and dianionic complexes
2291
17. C. Aso, T. Kunitake and Y. Ishimoto, J. Polym. Sci. A-l 1968,6, 1175. 18. C. Aso, T. Kunitake, K. Ito and Y. Ishimoto, J. Polym. Sci. Polym. Lett. 1966, 4, 701. 19. J.-P. Vairon and P. Sigwalt, Bull. Sot. Chim. Fr. 1971,559. 20. When [(Me,P)(CO),(NO)W-FSbF,] was used as a catalyst to polymerize a solution of cyclopentadiene in ether a soluble pcpd product was obtained. However, when dichloromethane was used as the reaction solvent an insoluble form of pcpd precipitated (see ref. 16). Similarly, [WCI,(OCH(CH&l),-Al(C,H,) Cl-THF] polymerizes cyclopentadiene in THF and in toluene to give soluble and insoluble pcpd, respectively (see ref. 21). 21. A. V. Orlov, Yu. B. Korshak, M. A. Tlenkopachev and E. A. Robas, Dokl. Akad. Nauk SSSR 1988, 300, 138. 22. It is not possible to estimate the relative ratio of 1,4- : 1,2-units in the polymer from the IR spectrum (see ref. 15). 23. Initially, species (c) would be expected to be more stable than (b) as the two positive charges in the latter are closer together, but as the polymer chain grows this repulsive feature becomes less important as the two charges get further apart. 24. T. A. Stephenson, E. Bannister and G. Wilkinson, J. Chem. Sot. 1964,2538. 25. K. Alder and G. Stein, Ann. 1934,514, 197.
.“o/ykedron Vol. IO,No. 19, pp. 2283-2291, 1991 Printedin Great Britain
SYNTHESIS OF NEW DIMOLYBDENUM(II,II) DEpCARBOXYLAT0 DICATIONIC AND DIANIONIC COMPLEXES, AND THEIR USE AS POLYMERIZATION CATALYSTS MALACHY
MCCANN? and PAULINE
GUINAN
Chemistry Department, St. Patrick’s College, Maynooth, Co. Kildare, Ireland (Received 29 April 1991; accepted 21 June 1991) Abstract-The dicationic complex salt cis-[MO,@-02CCH3),(MeCN),][BF& (l), reacts with an excess of unsaturated carboxylic acid (RC02H) to give a family of new dicationic *xHzO [R = HC=CH2 (2), (CH3)C=CHz (4), salts, [Mo2(~-02CR)2(MeCN),I[BF,I2 H2CCHC=CH2 (6), (CH3)HCH2CHC=CH2 (7), C7H9 (8); x = 1 or 21. Complex 1 also reacts with Et,NBr to give the dianionic salt (Et4N)2[Mo2(p-OzCCH3)2Br4] (9). Complexes 1, 4 and 9 catalyse the room temperature polymerization of bicyclo[2.2.1]hept-2-ene. EtAlC12 was employed as a co-catalyst in these reactions. The dicatiomc salts 1, 2 and 4 also catalyse the room temperature polymerization of cyclopentadiene, but the dianionic salt 9 is inactive towards this monomer.
A large number of dimolybdenum(II,II) complexes of the general formula [Mo2(~-02CR).J have been made using saturated alkyl and aryl carboxylate ligands. ’ We have recently described the synthesis of a range of MoZ4t, CuZ4+ and RuZSf tetra-pcarboxylato complexes containing unsaturated alkyl carboxylate ligands, and demonstrated how the pendant C=C functions of the complexed ligands could react with cyclopentadiene to form the respective Diels-Alder cycloaddition adducts. In this paper we describe the synthesis of some dimolybdenum(II,II) dicationic complexes containing two unsaturated carboxylate ligands, and the preparation of a new di-p-acetato dianionic complex. The use of these complexes as catalysts for the polymerization of bicyclo[2.2.l]hept-Zene and cyclopentadiene is also reported. RESULTS
AND DISCUSSION
Two different research groups have independently prepared and structurally characterized the red dimolybdenum(II,II) salt cis-[MO& O,CCH,),(MeCN),][BF& (l), the complex dication having the structure shown be10w.“~ Earlier,
t Author to whom correspondence
should be addressed.
Drago and Telser3’ reported the synthesis of the closely related salts [Mo2(,u-02CCH3)z(MeCN)4] and W3S0312 ~02(@2CCH3)2(M~N)51 [BF,OH],, which contain four and five MeCN ligands, respectively. 2+ CH3
L=MeCN
L
L
We have employed 1 as the starting material for the preparation of a series of similar dicationic complexes containing two unsaturated carboxylate ligands. Solutions of 1 in MeCN were reacted with an excess of the unsaturated carboxylic acid so as to replace the two acetate ligands with unsaturated carboxylate ligands (Scheme 1). Two successive metathesis cycles were usually required for complete substitution of the acetate by the new carboxylate ligand. The dications of the new salts have the common formula [Mo2(p-carboxylate), and given that the structural data for WeCN)412’,
2283
M. MCCANN
2284
and P. GUINAN [Ms(~~-~C(~~)CZ~[BF~Z~ZO
[Moz~-Oz~H,)~~[BFd,.2H20 y
\
[Mo2(rr-02~C~32L4][BF~2~*0
f
[M~C~~~C~~~C=.O.~~O
Q
(2)
+ [M~@O$HC=CH&l
(4)
(8)
k
Pfozclr-~~H&jlPF&
(1) &
[M~~-~C(~HH~z~C~~~l~F~~H*O
.0.5H20 (3) 2%
[Moz~--q~2~C~~2L~~F~2~~0
(7)
(6)
@t&Fl~~-OzC(3H3~l
(9)
Scheme 1. L = MeCN, (i) CH+ZHCO,H, (ii) CH2--_C(CH3)C02H,(iii) CH+ZHCH,CO,H, CH,--CHCH,CH(CH,)C02H, (v) C7H9C02H, (vi) Et.,NBr, (vii) cyclopentadiene.
1 show the four MO-N (equatorial) distances to be considerably shorter than the two MO-N (axial) distances, 3a,bit is reasonable to assume that the four MeCN ligands in the present complexes are bonded equatorially to the bimetallic core. During the synthesis of the di-p-propenoato and di-p-2-methylpropenoato salts 2 and 4 small amounts of the respective neutral tetra-p-propenoato and tetra-p-2-methylpropenoato complexes (3 and 5) also formed. Pure samples of 3 and 5 have previously been obtained in high yield by reacting [Mo~(~-O~CCH~)~] with an excess of the respective unsaturated acid.’ It is interesting to note that when [Mo2(p-02CCHJ4] was reacted with the longer chain unsaturated acids 3-butenoic acid and 2-methylpent-4-enoic acid, the tetrasubstituted complexes [Mo2(p-02CH2CHC=CH&] and [Mo*(p-02C(CH3)HCH2CHC=CHJ4] did not form, and only Mo-oxo complexes were recovered.4 However, when the same two long-chain unsaturated acids were reacted with salt 1, the metathesis products 6 and 7 were recovered in high yield and, again, none of the corresponding neutral tetra-p-carboxylato complexes formed. The di-pbicyclo[2.2.l]hept-5-ene-2-carboxylato complex 8 was synthesized by reacting the propenoato complex 2 with a large excess of cyclopentadiene (cpd). Some polycyclopentadiene (pcpd) also formed during the reaction (discussed later). The conversion of 2 to 8 occurs via a Diels-Alder cycloaddition reaction of the type mentioned previously.’ Complex 8 was also prepared by reacting 1 directly with endo-bicyclo[2.2.l]hept-5-ene-2-carboxylic acid (C, H&O*H). Finally, when a solution of 1 in MeCN was reacted with Et,NBr, the red salt 9 formed, which contains the complex dianion ‘$402(~-02 CCH&Br4]*-.
(iv)
The IR spectra of the new di-p-carboxylato salts have vasym(C02)and v,,,(CO,) bands at frequencies consistent with those expected for bridging carboxylates, and a v(C=C) band was also evident for the complexes (except 8) containing unsaturated carboxylate ligands (Table 1). Weak bands attributable to the MeCN ligands were present at ca 2280 cm-‘. In the spectrum of the dianionic complex 9 the acetate v(C02) bands were at frequencies similar to those found in the spectrum of the structurally characterized diacetate 1, and a strong v(Mo-Br) band5*6 was observed at 268 cm- ‘. Although the ‘H NMR spectra of the new salts (Table 1) did show peaks attributable to the different hydrogen atoms of the ligands not much structural information on the complexes was forthcoming due to the complexity of the observed splitting patterns. The spectrum of a DMSO-d6 solution of the dianionic complex 9 contained two distinct methyl (CH,CO,) resonances at 1.93 and 2.00 ppm in a ratio of ca 9 : 1. The presence of two signals suggests that in solution there are two different species present. This type of behaviour was similar to that observed for CDC13 solutions of the neutral complex [Mo2(,u-01CCH3)C1,(PMe3),]* 0.75THF.’ An acetonitrile solution of complex 9 was prepared under anaerobic conditions and its electronic absorption spectrum immediately recorded (Fig. 1, t = 0). A single visible peak corresponding to the metal-metal 6 + 6* transition’ was present at 540 nm (E = 838 dm3 mall ’ cm- ‘). This spectrum was similar to that reported for methanol solutions of the closely related dianions [Mo2(p-03SCH3)2Br4]2(&,,,, = 522 nm)” and [Mo~(~-O~CCH~)~C~~]*- (n,,, = 495 nm).9 A solution of 9 was allowed to slowly air-oxidize and the spectrum was recorded at intervals up to t = 24 h,
(9”
(8)
1.98(s) CH+ZN; 1.55(d) CH,; 2.5-3.0(m) CH,CH; 3.4(tq) CH,CH; 5.1-5.4(m) CH-CH; 5.7-6.3(m) CH,--CH
1640
1425 1422 1438
1500 1490 1515
9
1.98(s) CH,CN; 3.94.1(m) 6&6.6(m) CH,--CH
1640
1420
1505
1.42.5(m), 2.84.1(m);
i 1.93(s) ; 2.0(s) CH,
1.98(s) CH,CN;
6.&6.9(m) CH=CH
CH2; 5.2-5.6(m) CH,--CH;
5.9(m), 6.4(m) CH,--C
2.25(dd) CH,;
1.98(s) CH,CN;
1620
1412
1472
6.&7.3(m) CH,--CH
1.98(s) CH,CN;
1635
1435
2.9(s) CH,
1490
1.98(s) CH,CN;
1400
‘H NMR
1500
@==C)
v,,,(&)
va,,nl(CW
“KBr disc. bValues in cm- ‘. ‘CD&N solutions : values in ppm. d(s) singlet, (d) doublet, (dd) doublet of doublets, (tq) triplet of quartets, (m) multiplet. eL = MeCN. /See ref. 5. ~v(C=C) not visible. hv(Mo-Br) 268 cm- ‘. ‘Solvent = DMSO-d,.
(Et,N)2[Mo2(~-02CCH,)zBr,l
[MoXy02CC,H9)*Ld[BF4~.2HzO
Complex
Table 1. IR”,b and ‘H NMRc,d data for the complexes’
s El ‘0 s 5
E. c) $ a E E. 0 0e.
Et: B E
3 r t 8
-c E
E
n .!! B
2 Y B kl 5
M. MCCANN and P. GUINAN
2286
550
603
6%
700
Nanometers
Fig. 1.
R
at which time the 540 nm peak had completely disappeared and three new absorption peaks had grown (481, 419 and 382 nm). The disappearance of the 540 nm peak signifies the rupture of the MO-MO 6 bond. In conclusion, it is thought that in the solid state the new dicationic complexes are essentially isostructural with 1 [Fig. 2(a)]. It should be mentioned that these complexes could also have a trans configuration [Fig. 2(b)]. The dianion, [MO&U02CCH3)2Br4]2-, is thought to be structurally similar to the known dianions [Mo~(~-O~SCH~)~X~]~(X = Cl, Br, I)6 and [Mo~(I.L-O~CCH~)~C~~].*-~ The X-ray crystal structure of the latter dianion showed the two acetates to be trans. Possible cis and tram structures for [Mo2(p-02CCH3)2Br4]2- are illustrated in Figs l(c) and (d).
2+
A
i,okR
L/MTMo
I
i
I
i
L=MeCN
(4
I
(W
a3
(4
R
2-
CH3
A 7 ,o&fH3 BY”TMO I
I
Br
Br
Cc)
Fig. 2. (a) and (b) Possible structures for the new dications [Mo~(~-O$R)~LJ*+ [R = HC=CH2 (2), (CH&=CHz (4), H2CCHC==CHz (6), R = (CHS)HCH2CHC=CH2(7), C,H, (8); L = MeCN]. (c) and (d) Possible structures for the dianion, [Mo,(~-O~CCH~~B~J-.
Dimolybdenum(II,II)
Polymerization catalysis
di-p-carboxylato
dicationic and dianionic complexes
2287
pcpd + (8)
(1) Bicyclo[2.2.l]hept-2-ene(norbornene). In 198 1 Lamotte et al.” reacted aluminium isopropoxide t (2) with [Mo&J-O&CH~)~] to give [MO&-O2 @y 9”“” the CCH,)&-(OCH(CH&),Al(OCH(CH,),}& X-ray crystal structure of which showed tram acetates and tram aluminium isopropoxides. Later, 0\ Diefenbach reported that dimolybdenum(II,II) carboxylato complexes in the presence of aluminium, DW-WWl/ ‘y titanium or zirconium alkoxides were good catalysts for the metathesis of alkynes. ’ ’ Furthermore, no reaction he prepared and characterized complexes of the no reaction type [Mo2(0FR)2{~@-OCgH4C1)4}21 P = CH3, Scheme 2. pcpd = polycyclopentadiene. C(Me)3, CFJ, and subsequently showed that they also catalysed alkyne metathesis reactions. Earlier, we had shown that the neutral and anionic dimolybdenum(II,II) complexes [Mo&-O~CR)~] (R = nificant change in the stereochemistry of the double CH3, CF,) and K4[Mo2Cl,] are good metathesis bonds in the polymer chain. catalysts for ring-opening polymerization reac(2) Cyclopentadiene. We have recently described tions. “3 ’ 3 For example, each of the catalyst/cohow some neutral dimolybdenum(II,II), dicatalyst systems [MO&-02CCH,),]/EtAlCl, and copper(II,II) and diruthenium(II,III) complexes K4[Mo2Cls]/EtA1C12 polymerized bicyclo[2.2.1] containing four unsaturated carboxylate ligands hept-2-ene and 1-methylbicyclo[2.2.l]hept-2-ene at react with excess cyclopentadiene to give the respec2o”C, and [Mo&-O&CF~)~] polymerized the tive Diels-Alder cycloaddition adducts. 2 No polysame monomers even in the absence of a co-catalyst. cyclopentadiene (pcpd) formed during these reacThe dicationic complexes 1 and 4 and the tions. The results of reactions of some of the present dianionic complex 9, when used in the presence of complex salts with excess cyclopentadiene in EtA1C12 as a co-catalyst, catalysed (0.28 mol % acetonitrile at room temperature are outlined in catalyst) the room temperature ring-opening polyScheme 2. A 0.09 mol % solution of 4 rapidly catamerization of bicyclo[2.2.l]hept-2-ene to give lysed the addition polymerization of cyclopentapoly( 1,3-cyclopentylene-vinylene) (pcpv). When diene, with the pcpd precipitating as a white solid the reaction solvent was changed from toluene to (yield: 35%). Unchanged 4 was subsequently reacetonitrile no polymerization occurred.‘4 From the covered from the reaction filtrate. Although 1 also 13C NMR spectra of the polymers the fraction of catalysed the formation of pcpd (yield : 8%), it was double bonds in the polymer chain with cis characnot as active as 4. Complex 2 was even less active ter (a=) was calculated. 12x13The close similarity in than 1 and only a very small amount of pcpd was obtained. Furthermore, the Diels-Alder cycloaddition adduct 8 was isolated in high yield from the filtrate of the latter reaction (Scheme 1). The dianionic complex 9 failed to react with cyclopentadiene, and unchanged 9 was recovered. In addition, no pcpd formed when the neutral complex [Mo~(~-O~CCH~)~] was reacted (either as a suspension in acetonitrile or as a solution in hot meth(R = H, Me) anol) with cyclopentadiene. Although there are many examples known where non-transition metal Lewis acids have been used as of cycloof the polymerization catalysts pentadiene, ’ 5 there are only a few cases in which transition metal complexes [other than those of titanium(IV)] have been employed,16 and it is generally thought that all of these catalysts cause the values of a, (ca 0.36) found for the three pcpv the polymerization to proceed via a cationic mechsamples indicates that changing the ligands on the anism. “-I9 Normally, the pcpd obtained from these dimolybdenum(II,II) core does not cause a sig- literature reactions is soluble2’ in CHC13 and from
’
I
2288
M. MCCANN
and P. GUINAN
their NMR spectra ’ 5~2 ’ it was deduced that these addition polymers contain both 1,4- and 1,2-structural units. Furthermore, it has been shown that the 1,4- and 1,Zcontent of the polymers is generally dependent on the type of catalyst used, solvent and temperature. ’ 7 Unfortunately, the insolubility of the pcpd obtained using the present complex salts prohibited detailed structural characterization by NMR spectroscopy. However, the IR spectra of these insoluble polymers were identical to those illustrated in the literature’ 5 for soluble poly-1,4-/ 1,2_cyclopentadiene.
towards cyclopentadiene. The inactivity of salt 9 is probably attributable to the fact that the dianion [Mo2(p-02CCH3)2Br4]2- can be viewed as a Lewis base and, as such, would effectively repel the electron-rich cyclopentadiene molecule. The neutral complex [Mo2(p-O&CH,),] also failed to catalyse the polymerization of cyclopentadiene, suggesting that it too is not a strong enough Lewis acid for diene activation.
0
All reactions were carried out under nitrogen using dry solvents. Standard Schlenk techniques were employed during the preparation of the molybdenum complexes. Although all of the complex salts were air-sensitive the dicationic salts appeared to be considerably more sensitive than the dianionic salt. Literature methods were used to prepare cis-[Mo2(p-02CCH3),(MeCN)J[BFJ2 (l),3b en&-bicyclo[2.2. l] [Mo2(~-02CCJ&h1*~ and hept-5-ene-2-carboxylic acid.25 Ethylaluminiumdichloride was used as a 25% solution in hexane. IR spectra were recorded in the region -200 cm-’ on a Perkin-Elmer 783 grating spectrometer, and electronic spectra were run on a Milton Roy Spectronic 3000 Array. ‘H NMR spectra of solutions of the complexes (in CDC13 or DMSO-de, and under nitrogen) and 13C NMR of bicycle [2.2.l]hept-2-ene polymers (in CDC13) were recorded using a Bruker AC 80 spectrometer. Elemental analyses were performed by the Microanalytical Laboratory, University College, Cork, Ireland.
\
catalyst I
1,4-
(cis and mans)
1,2-
(cis and tram)
Assuming that the present polymers are comprised of 1,4- and 1,Zunits a possible cationic addition mechanism for their formation can be visualized (Scheme 3). ‘H and 13C NMR studies on complex 13bhave shown that the MeCN ligands are labile, and we have further demonstrated here that they are readily replaced by bromide ions (formation of 9). Thus, a first step in the polymerization process is likely to be the displacement of one or more MeCN ligands by cyclopentadiene to give species (a). The cyclopentadiene molecule may bond to either one or both (i.e. bridging) molybdenum atoms of the bimetallic core to give the carbocation (b), which itself may isomerize to (c).” Addition of cyclopentadiene molecules to (b) and (c) will give rise to separate 1,2-pcpd and 1,4-pcpd, respectively. A single polymer molecule containing a mixture of 1,2- and 1,Cunits in the chain can form if there is isomerization of either (b) or (c) during polymerization (Scheme 4). Given the close similarity in both the structural and electronic properties of the complex dications of salts 1, 2 and 4 it is difficult to understand the large differences observed in their catalytic activity
* Microanalytical and IR data for complexes 3 and 5 corresponded with that reported in ref. 2 for these complexes.
EXPERIMENTAL
[Mo2(p-02CCH=CH2)2(MeCN)4][BF4]2*H20 *0.5H20 (3)*
(2)
and [Mo~(~-O~CHC=CH~)~]
A solution of complex 1 (0.36 g, 0.49 mmol) and propenoic acid (0.5 g, 6.9 mmol) in acetonitrile (20 cm’) was stirred at room temperature for 12 h, and the resulting light red solution was then reduced under high vacuum (at 20°C) to ca 1 cm3. Addition of toluene (40 cm”) induced the formation of a dense red oil. The upper yellow liquid was decanted off and fresh toluene (20 cm’) and acetonitrile (3 cm3) was added to the red oil. The mixture was stirred vigorously and the crude product 2 eventually precipiated as a red solid. The solid was filtered off, washed with toluene (2 x 20 cm’) and dried in vucuo. The solid was then redissolved in a fresh solution of propenoic acid (0.5 g, 6.9 mmol) in acetonitrile (20 cm’) and stirred at room temperature for a further 3 h. The red complex was then reprecipitated from the oil form as described above ; yield : 0.2 g
Dimolybdenum(II,II)
di-p-carboxylato
dicationic and dianionic complexes
0) MO-MO]+
2289
(c) mo-MO]+
I
etc.
I
etc.
Scheme 3. [Mo-MO]
= [Mo2(p-02CR),(MeCN),],
R = CHI,HC=&H2,
(CH&kCH2,
x <
6.
(59%). The combined reaction filtrates (yellow liquid and toluene washings) were concentrated to low volume under high vacuum (at 20°C) and the yellow solid 3 precipitated ; yield : 0.05 g (21%). (2) : Found: C, 24.5; H, 2.8; N, 7.7. Calc.: C, 24.4; H, 2.9; N, 8.1%.
[Mo,(~-O,C(CH,)C---CH~3,(MeCN)4l[BF4l~.H~0 (4) and[MoZ(~-OIC(CH$Z=CHJ4]*0.5H20
(5)*
Using 2-methylpropenoic acid these complexes (red and yellow coloured, respectively) were prepared and recovered by the same method as described for 2 and 3 ; yield (4) : (76%). Found : C, 26.2; H, 3.6; N, 7.9. Calc.: C, 26.8; H, 3.4; N, 7.8%.
Wo - MO]+
I 1
1
2
3
I
4
+
Using
prepared
was and recovered by the same method as
3-butenoic
acid
this
red
complex
Scheme 4.
1
M. MCCANN
2290
described for 2; yield : (80%). Found : C, 26.9 ; H, 3.6; N, 7.4. Calc.: C, 26.8; H, 3.4; N, 7.8%. [Mo&-02C(CH3)HCH,CHC=CH,),(MeCN),] [BF& *Hz0 (7) Using 2-methylpent-4-enoic acid this red complex was prepared and recovered by the same method as described for 2 ; yield : (93%). Found : C, 31.3; H, 4.1; N, 7.7. Calc.: C, 31.0; H, 4.2; N, 7.2%.
[Mo2(~-02CC7H9)2(MeCN)41[BF412.2H20 (8) Method(a). Using endo-bicyclo[2.2.l]hept-5-ene2-carboxylic acid this red complex was prepared and recovered by the same method as described for 2; yield : (68%). Method(b). Complex 2 (0.17 g, 0.25 mmol) and cyclopentadiene (0.4 g, 6.1 mmol) were stirred together in acetonitrile (20 cm’) at room temperature for 24 h. A small amount of white pcpd solid was filtered off and complex (8) was subsequently recovered from the filtrate using the method outlined for the isolation of 2 ; yield : 0.14 g (70%). Found : C,34.3;H,4.1;N,7.1.Calc.:C,34.3;H,4.1;N, 6.7%. (EtqN)2[Mo2(~-02CCH3)2Br41 (9) To a solution of 1 (0.33 g, 0.45 mmol) in acetonitrile (20 cm’) was added tetraethylammonium bromide (0.57 g, 2.1 mmol), and this resulted in an immediate red to dark purple colour change. The solution was stirred for 0.5 h and the solvent was then removed using high vacuum. Ethanol (30 cm3) was added and the resulting suspension was stirred for 1.5 h. The red product was removed by filtration and then resuspended in fresh ethanol ( 10 cm ‘) and stirred for another 10 min. The solid was filtered off, washed with ethanol (3 x 5 cm’) and then dried in vacua; yield : 0.23 g (57%). Found : C, 26.9 ; H, 5.4; N, 3.2; Br, 36.2. Calc.: C, 27.0; H, 5.2; N, 3.1; Br, 35.9%. Complex 9 gives an orange solution in MeOH and DMSO, and a purple solution in MeCN. Polymerization
reactions
All polymerization reactions were performed at room temperature and under nitrogen. Typical polymerization procedures are outlined below. Polymerization
of bicyclo[2.2.l]hept-Zene
Bicyclo[2.2.l]hept-2-ene (0.5 g, 5.3 mmol) was dissolved in dry toluene (1 cm3) and the dimolyb-
and P. GUINAN denum(II,II) salt (0.015 mmol) was added. Ethylaluminiumdichloride solution (0.5 cm3, 0.33 mmol) was then added and this resulted in rapid polymer formation (as indicated by an increase in solution viscosity). The mixture was then allowed to stand at room temperature for 1 h. Ethanol (ca 25 cm’) was added and the precipitated polymer (pcpv) was washed several times with small volumes of ethanol. The wet solid was then dissolved in chloroform (ca 75 cm3), and the resulting polymer solution was dripped into ethanol (300 cm’) containing concentrated hydrochloric acid (0.2 cm3) causing the polymer to reprecipitate as a fine white solid. This solid was filtered off, washed with ethanol and dried in uucuo for 1 h at room temperature. A solution of the polymer in CDC13 was then characterized using 13C NMR spectroscopy. For example, using complex 4 as catalyst, 40 mg of pcpv was obtained. Polymer o, values (in parentheses) obtained using the various catalysts were as follows: 1 (0.36), 4 (0.36), 9 (0.35). Polymerization
of cyclopentadiene
A solution of the dimolybdenum(II,II) complex salt (0.036 mmol) and cyclopentadiene (2.6 g, 39.4 mmol) in acetonitrile (5 cm’) was stirred for 72 h during which time the white pcpd gradually precipitated. The solid was filtered off, washed with acetonitrile and dried in vucuo. For example, using complex 4 as catalyst pcpd was formed in 35% yield. The pcpd formed in these reactions was insoluble: in all common organic solvents. The polymers discoloured slightly on standing.
REFERENCES 1. F. A. Cotton and R. A. Walton, Multiple Bonds Between Metal Atoms, Ch. 8.3. Wiley, New York (1982). 2. M. McCann, A. Carvill, P. Guinan, P. Higgins, J. Campbell, H. Ryan, M. Walsh, G. Ferguson and J. Gallagher, Polyhedron 1991, 10, 2273. 3. (a) F. A. Cotton, A. H. Reid Jr and W. Schwotzer, Znorg. Chem. 1985, 24, 3965; (b) W. Clegg, G. Pimblett and C. D. Gamer, Polyhedron 1986, 5, 31; (c) J. Telser and R. S. Drago, Znorg. Chem. 1984,23, 198. 4. When [MO,@-O,CCH&] was reacted with 3butenoic acid a brown Mo-0x0 solid formed (see ref. 2). Under similar experimental conditions 2-methylpent-4-enoic acid also produced a brown Mo-oxo complex. Analytical data : Found : C, 23.32; H, 3.60. The two brown solids had similar IR spectra. 5. E. Hochberg and E. H. Abbott, Znorg. Chem. 1978, 17, 506.
Dimolybdenum(II,II)
di-p-carboxylato
6. J. San Filippo, H. J. Sniadoch and R. L. Grayson, Znorg. Chem. 1974, 13,212l. 7. P. A. Bates, A. J. Nielson and J. M. Waters, Polyhedron 1987,6,2111. 8. Although the authors of references 3(a) and 3(b) reported the synthesis and X-ray crystal structure of complex 1 they did not report the IR band positions for the carboxylate groups or the NMR peak positions for the ligands. As an aid to structural assignment for the complexes described in this work we report some relevant IR and NMR data for 1 in Table 1. 9. W. Clegg, C. D. Garner, S. Parkes and I. B. Walton, Znorg. Chem. 1979, 18, 2250. 10. J. Lamotte, 0. Oideberg, L. DuPont and P. Durbut, Cryst. Struct. Comm. 1981, 10, 59. 11. S. P. Diefenbach, U.S. Patent 4,704,377 (1987). 12. J. G. Hamilton, K. J. Ivin, M. McCann and J. J. Rooney, Makromol. Chem. 1985,186,1477. 13. K. J. Ivin, D. T. Laverty and J. J. Rooney, Makromol. Chem. 1977,178, 1545. 14. Diefenbach reported that alkyne metathesis did not occur when acetonitrile was used as the reaction solvent (see ref. 11). 15. C. Aso, T. Kunitake and Y. Ishimoto, .Z.Polym. Sci. A-l 1968,6, 1163. 16. R. V. Honeychuck, P. V. Bonnesen, J. Farahi and W. H. Hersh, J. Org. Chem. 1987,562,5293 and refs therein.
dicationic and dianionic complexes
2291
17. C. Aso, T. Kunitake and Y. Ishimoto, J. Polym. Sci. A-l 1968,6, 1175. 18. C. Aso, T. Kunitake, K. Ito and Y. Ishimoto, J. Polym. Sci. Polym. Lett. 1966, 4, 701. 19. J.-P. Vairon and P. Sigwalt, Bull. Sot. Chim. Fr. 1971,559. 20. When [(Me,P)(CO),(NO)W-FSbF,] was used as a catalyst to polymerize a solution of cyclopentadiene in ether a soluble pcpd product was obtained. However, when dichloromethane was used as the reaction solvent an insoluble form of pcpd precipitated (see ref. 16). Similarly, [WCI,(OCH(CH&l),-Al(C,H,) Cl-THF] polymerizes cyclopentadiene in THF and in toluene to give soluble and insoluble pcpd, respectively (see ref. 21). 21. A. V. Orlov, Yu. B. Korshak, M. A. Tlenkopachev and E. A. Robas, Dokl. Akad. Nauk SSSR 1988, 300, 138. 22. It is not possible to estimate the relative ratio of 1,4- : 1,2-units in the polymer from the IR spectrum (see ref. 15). 23. Initially, species (c) would be expected to be more stable than (b) as the two positive charges in the latter are closer together, but as the polymer chain grows this repulsive feature becomes less important as the two charges get further apart. 24. T. A. Stephenson, E. Bannister and G. Wilkinson, J. Chem. Sot. 1964,2538. 25. K. Alder and G. Stein, Ann. 1934,514, 197.