Transition Met. Chem., 20, 569-573 (1995) Pt a-acetylide complexes with Coz(CO)8 569

Reactions of platinum a-acetylide complexes with dicobalt octa- carbonyl: Conversion of an alkyne unit into cyclopentenone Jack Lewis*, Bing Lin and Paul R. Raithby University Chemical Laboratory, Lensfield Road, Cambridge CB21EW, UK

Summary

Treatment of platinum ~-acetylide complexes trans- Pt(PR3) / (C~CC~CR')2 (1) with two equivalents of Co2(CO)8 gave complexes trans-Pt(PR3)2(C~-CC 2- [Co2(CO)6]R')2 (3) and (4), where R = Et or n-Bu and R' = H or SiMe 3, in high yields. The unsubstituted plati- num-alkyne-cobalt complex (i.e. R ' = H) reacted with alkenes (norbornylene or cyclopentene) to produce new platinum acetylides containing 2-cyclopentenone groups. All new compounds were characterized by i.r., u.v.-vis. and n.m.r. (1H, alp, laC) spectroscopies.

Introduction

Transition metal acetylide complexes are under intensive study because of their potential application as catalysts and as building blocks in the design of novel materials. The alkyne ligand is known to coordinate in a variety of bonding modes in metal clusters ~). It may be anticipated that the combination of two functions, i.e. the C ~ C ligand and the metal centre, may exhibit unexpected chemical behaviour. For example, it has been shown that treatment of metal acetylide with organometallic reagent leads to the formation of homo- and heteronuclear metal clusters. This has been attributed to the metal-metal interaction brought about by the presence of the bridging alkyne ligand t2). Transition metal actylides are also in- volved in catalytic processes such as the coupling of acetylene with carbon monoxide (3). In addition, the recent discovery of novel physical properties of metal acetylides, e.9. liquid crystal behaviour (4) and non-linear optical response ~5), have prompted the investigation of these compounds as advanced molecular-based materials.

We have been interested in the polymeric metal polyyne complexes of the type [ - -M(L , ) - -C~C- -R- -C~-~- C - - ] , (M-= Mn, Fe, Ru, Os, Co, Rh, Ni, Pd or Pt; L = P P h a , Pn-Bu 3, DPPM or DPPE; R---C6H4, C6H4C6H4, C6HzMe2, C6H2F 2 or C4H4S) ~6). A wide range of conjugated acetylene ligands was synthesized during the studies and we have reported that several cobalt polyyne complexes have been derived from these compounds ~7). The C02(CO)6 units were found to coordi- nate with the C~-C bonds in an t/Z-mode, forming a C02C 2 tetrahedron, and the C02C 2 units were arranged along the polyyne backbone in a trans-fashion. The coor- dination of the cobalt carbonyl groups maintains the rigidity of the system and still allows for potential elec- tronic delocalization down the chain (7'8). In this publica- tion we describe the reactions of platinum a-acetylides with dicobalt octacarbonyl and the construction of the cyclopentenone framework using the resultant platinum- alkyne-cobalt complexes and alkene ligands. The n.m.r. (~H, 31p, 13C) characterization of these complexes is presented.

* Author to whom all correspondence should be directed.

Experimental

General

All reactions were carried out under N2 using standard Schlenk techniques. Solvents were dried by the usual methods and distilled under N 2 before use. Column chromatography was carried out on a column packed with silica gel 230-400 mesh, Merck. Thin layer chromatography (t.l.c.) was performed on glass plates (20 • 20cm) coated with ca 2mm silica gel (type 60 GF254, Merck). The reagents MeLi-LiBr (1.5 M in THF), n-Bu4NF (1.0 M in THF), cyclopentene and norbornylene were obtained from Aldrich and used as received. The following compounds were prepared by modified litera- ture methods; MeaSiC~CC~---CSiMe3 ~9) and trans- Pt(PR3)2CI2, where R = Et or n-Bu (1~

I.r. spectra were recorded as CH2C12 solutions in NaC1 cell (0.5 mm path length) on a Perkin-Elmer 1710 Fourier Transform spectrometer. U.v. spectra were measured in a CH2C12 solution on a PU 8730 u.v.-vis, spectro- photometer. FAB-MS were recorded on an AE1/Kratos MS 50 spectrometer. N.m.r. spectra were recorded on a Bruker WH 400 Fourier Transform spectrometers in appropriate solvents. The chemical shifts were referenced to residual protons in CDC13 (7.25p.p.m.) for IH, to CDC13 (77.0 p.p.m.) or C6D 6 (128.0 p.p.m.) for 13C and to external P(OMe)3 for 31p.

Preparation of trans-Pt(PR3)2(C~-CC~CSiMe3) 2 (1) and trans-Pt( P R3)2( C - ~ C C ~ C H )2 (2)

To a solution of M e a S i C ~ C C = C S i M e 3 (150mg, 0.77 mmol) in THF (20 cm 3) was added one equivalent of MeLi-LiBr (515 mml, 0.77 mmol) at - 78 ~ The mixture was warmed to 25 ~ and stirred for 3 h. The pale yellow solution (which turned brown when exposed to the light) was then cooled to - 7 8 ~ and a THF solution (10cm 3) of Pt(PEta)zC12 (250 rag, 0.5 mmol) was added dropwise. The mixture was warmed slowly to 25 ~ and stirred for another 1 h. The solvent was removed and the crude product was separated on t.l.c, plates using hexane- CH2C12 (1:1) as eluant. The desired product trans- Pt(PEta)z(C~-CC=---CSiMe3) 2 ( la) was obtained as yellow crystals after crystallization from hexane (272 mg, 81%). (Found: C, 46.2; H, 7.2, M +, 674; C26H48PESi2Pt calcd.: C, 46.3; H, 7.2%; M, 674.)

The complex trans-Pt(PBu3)z(C~-CC~-CSiMe3) 2 ( lb) was prepared similarly using Pt(PBu3)2C12 as described above. Yield 76%. (Found: C, 54.1; H, 8.5; M +, 843; C3aHTzPESizPt calcd.: C, 54.2; H, 8.6%; M, 842.)

Treatment of (Ia) or ( lb) with n-Bu4NF in THF at 25 ~ gave complex (2a) or (2b), respectively. The reac- tion was monitored by the appearence of v(C--H) peak at 3302 cm- 1 in the i.r. spectra and worked-up as described above.

0340-4285 �9 1995 Chapman & Hall

570 Lewis et al. Transition Met. Chem., 20, 569-573 (1995)

Preparation of Pt(PRa)2(C==CCz[Co2(CO)6]- SiMe3) 2 (3) and trans-Pt(PR3)2(C~--CC2- [Co2(CO)6]H)E (4)

A solution of Co2(CO) 8 (82mg, 0.24mmol) and trans- Pt(PEt3)z(C~CC~CSiMe3) 2 (la) (80 mg, 0.12 mmol) in hexane (75cm 3) was stirred at 0 ~ The i.r. spectrum showed that the bridging carbonyl bands from Co2(CO)s disappeared completely after 30rain. The solvent was removed and the residue was separated on t.l.c, plates eluted with hexane-CH2C12 (2:1). From the first band was isolated the desired product trans-Pt(PEt3)2- (C~CC2[ ,Co2(CO)6]SiMe3) 2 (3a) as deep red crystals (138rag, 92%). (Found: C, 36.4; H, 3.9; M +, 960; C3sH48012P2Si2Co~Pt calcd.: C, 36.6; H, 3.9%; M, 960.)

The complex trans-Pt(PBu3)z(C~-CCz[,Coz(CO)6]- SiM%)2 (3b) was prepared similarly using the pro- cedure described above. Yield 86%. (Found: C, 42.3; H, 5.2; M +, 1127; CsoHTzOlzPzSizCo4Pt calcd.: C, 42.5; H, 5.2%; M, 1128.)

The complex trans-Pt(PR3)z(C~CC2[Coz(CO)6]H)2 (4) was prepared by two methods: (1) treatment of (2) with Coz(CO) 8 in CH2C12; (2) treatment of (3) with KzCO 3 in MeOH. These reactions were monitored by the characteristic v(CO) bands in i.r. spectroscopy and worked-up as described above.

Pauson-Khand reaction between trans-Pt( PR3) 2 (C~CC2[,Co2(C0)63H)2 (4) and alkenes

Complex trans-Pt(PEt3)z(C~CC2[,Co2(CO)6]H)2 (4a) ( l l0mg, 0.1 mmol) and norbornylene (21 rag, 2.2mmol) were mixed in 15 cm 3 MeCN and boiled under reflux until the deep red colour disappeared (ca 4 h). The solution was allowed to cool and filtered through a bed of celite. The solvent was removed and the residue was separated on �9 a silica column using hexane-ethyl acetate (2:1) as eluant. The first band was collected and product (5a) was iso- lated as yellow crystals yield 79%, 62 mg. (Found: C, 57.0; H, 7.2; M +, 774 (M-43); C36H 52OzP2Pt'0.5 hexane calcd.: C, 57.3; H, 7.3%; M, 817.) Complex (6a) was prepared similarly by reacting (4a) and cyclopentene. Yield 72%. (Found: C, 56.5; H, 7.7; M +, 722 (M-86); C32H47- OzPzPt.hexane calcd.: C, 56.5; H, 7.8%; M, 808.)

Complexes (5b) and (6b) were prepared similarly by reacting (4b) with norbornylene or cyclopentene, respect- ively. Complex (5b): (Found: C, 57.7; H, 7.5; M +, 942 (M-85); C48H7602P2Pt'CHECI2 calcd.: C, 57.3; H, 7.7%; M, 1027.) Complex (6b): (Found: C, 55.1; H, 7.5;; M +, 889; C44H7aOzPzPt.CH2Clz calcd.: C, 55.4; H, 7.7%; M, 975.)

R e s u l t s a n d d i s c u s s i o n

Reaction of platinum a-acetylide complexes with dicobalt oetacarbonyl

The platinum a-acetylide complexes were prepared from trans-Pt(PR3)2C12 and H C ~ C R ' for Pt(PR3)z(C~ CR')2, where R = Et or n-Bu; R' = H or SiMe3 ~11), or from trans-Pt(PR3)2Cl 2 and L i C ~ C C ~ C S i M e 3 ~a2) for Pt(PR3)z(C~CC~---CSiMe3)2, where R = E t (la) or n-Bu (lb). The complexes P t ( PR3 ) z ( C~ CC~ CH) 2 (2) were obtained from (1) by desilylation using tetra- butylammonium fluoride (n-Bu~NF) ~13).

The initial reaction of monoacetylene ligands in trans- Pt(PR3)z(C~CR')2 with C02(CO)s resulted in decompo- sition. Trace amounts of the known cobalt r/Z-alkyne complexes R'CzI-Coz(CO)6]H and R'C2[,Coz(CO)6]- C2[,Co2(C0)6]R' , where R' = H or SiMe3, were detected in the residue by i.r. spectroscopy~14).We had previously isolated the latter compound from the reaction between M C ~ C P h (M = AuPPh 3 or AgPPh3) or Hg(C~CPh)2 with C02(CO)8 ~Ta). In contrast, the platinum butadiynyl complexes P t (PR3)z(C~CC~CR')2 reacted with C02(CO)s smoothly, forming complexes Pt(PR3) z (C~---~CC2[-Co2(C0)6]R')2, where R' = SiMe 3 (3); R' = H (4), respectively. Treatment of (3) with potas- sium carbonate in methanol produced (4) but with some decomposition. Both (3) and (4) are moderately air- stable as solids at room temperature; complex (4) is less stable than (3).

On the basis of steric effects the Coz(CO)6 groups were expected to coordinate with the terminal alkyne units in (3) and (4), this was supported by the spectroscopic data shown in Table 1. The difference in the carbonyl stretching frequencies of (3) and (4) reflects the different substituent R'(SiMe 3 or H) on the adjacent alkyne bonds. In the ~H-n.m.r. spectra the significant low-field proton resonance from R' groups [-0.38 p.p.m, for SiMe 3 in (3)

Table 1. Selective spectroscopic data for (I)-(6)

Entry I.r. ( c m - 1) ,)~max a 31p_n.m.r.b v(C=C) v(CO) (nm) ~ (p.p.m.)

R = Et (la) 2179,2126 - 331(17600) (2a) 2142 ~ - 320(16400) (3a) 2146 2082,2045,2016 (4a) 2144 2089,2050,2021 - (5a) 2099 1702 333(15300) (6a) 2098 1700 332(17300)

R = n-Bu (Ib) 2182,2128 - 329(16700) (2b) 2141 c 318(15500) (3b) 2143 2081,2044,2018 - (4b) 2144 2088,2049,2018 - (5b) 2098 1703 330(16400) (6b) 2099 1703 326(14000)

-- 128.5(2307) -- 128.6(2298) -- 129.1(2332) -- 129.0(2326) -- 130.2(2342) -- 130.2(2342)

-- 136.5(2285) -- 135.7(2293) -- 136.0(2324) -- 136.6(2322) -- 137.2(2326) -- 138.4(2327)

ae (mol- 1 dm 3 cm- 1) In parentheses; brecorded in CDCI 3 with J(Pt--P) (Hz) in parentheses; cv(C--H) at 3302cm 1.

Transition Met. Chem., 20, 569-573 (1995) Pt a-acetylide complexes with Co2(C0)8 571

and 6.10p.p.m. for H in (4)], compared with those in parent compound [0.15p.p.m. for SiMe 3 in (1) and 3.20p.p.m. for H in (2)], is characteristic of cobalt t/2- alkyne complexes (a2). On the other hand, the 31p-n.m.r. spectra of these complexes are quite similar before and after the complexation of cobalt carbonyl, suggesting little change in the platinum-containing moieties. The coor- dination of cobalt carbonyl to alkyne bonds remote from the a-bonded platinum centre has been observed in other similar systems. It has been shown that two equivalents of Co2(CO)6 add to the alkyne bonds between the silicon atoms in the polymeric [--SiMezC=---CC~CSiMe 2- C~CPt (PBu3)C~-C- - ] , species ~15~, whilst only the internal alkyne bonds in the dimeric PhPt(PEt3)2(C~C- C~CC~CC~C)Pt(PEt3)zPh complex is coordinated by the cobalt carbonyl group ~

Reaction of platinum-alkyne-cobalt complexes with alkenes

The cobalt ~/2-alkyne complex is known to react with alkenes and form cyclopentenone derivatives, the Pauson-Khand reaction(~7( It was found that (3) was inert to this reaction presumably because of the steric crowding around the alkyne bonds. The reaction with norbornylene in benzene left (4) unaffected but reaction in acetonitrile resulted in the expected adduct (5). This is consistent with the previous reports that acetonitrile is a preferred solution for the Pauson-Khand reaction (is). A similar reaction product (6) was isolated from the reaction between (4) and cyclopentene.

PR3 PR 3 I I

" I 2

PR 3 PR 3

(1) (2)

PR 3 C0(CO)3 pR 3 C0(CO)3 I f_i \ I /1 x f~t-( C'---CI3CQI C~S iMe3) Pt-( Ca---CIrC. ~- C~-H )2

/ "2 I \I/ PR3 PR 3

Co(CO)3 Co(CO)3

(3) (,#

O PR~ II

PR 3 3 ~ 2

(6)

The formation of the platinum-alkyne-cyclopen- tenone complexes (5) and (6) were supported by spectro- scopic analysis as summarized in Table 1. The carbonyl stretching frequencies in the i.r. for the cyclopentadene group appeared near 1700 cm- 1 for (5) and (6), while it is interesting to note that the v(C~--~C) stretching frequen- cies were much lower than the values for initial platinum acetylides [e.0., 2099cm -~ for (Sa) compared with 2142 cm-1 for (2a)]. The proposed structure was based on comparison of the spectra with the organic product from the reaction of the cobalt r/Z-alkyne complex with analogous alkene ligand ~ 7,19). In the 1H-n.m.r. spectra of (5) and (6), the coupling pattern (doublet) and the coupling constant [-3J(H--H) = 3 Hz] are characteristic of H(3) in 2-cyclopentenone compounds (z~

Electronic spectra and 13 C-n.m.r. characterization

The u.v.-vis, spectra of (I), (2), (5) and (6) were re- corded in dichloromethane solution and found to be similar to those of platinum a-acetylide complexes (9). The lowest maximum absorption is assigned as a charge trans- fer band. It is difficult to identify this band in the spectra of (3) and (4) which are dominated by the broad absorp- tions of the Co2(CO)6 moieties (21). The charge transfer bands in (5) and (6) shift ca 10nm to a longer wave number compared with the parent acetylide precursors (2a) and (2b), respectively, which is consistent with a better 7t-delocalization in the conjugated platinum- alkyne-cyclopentenone complex prepared by the Pauson-Khand reaction (2~

Compounds (1), (3), (5) and (6) were fully character- ized by 13C-n.m.r. spectroscopy (Tables 2 and 3). The platinum-alkyne-cobalt complex (3) exhibited a single CO signal at 201 p.p.m., while the platinum-alkyne- cyclopentenone complexes (5) and (6) showed carbonyl signals at 209 p.p.m. The chemical shifts of the carbon

Table 2. 1H- and 13C-n.m.r. data for complexes (1) and (3)

(la) (lb) (3a) (3b)

1H chem. shift" SiMe 3 0.16 0.14 0.38 0.39 PR3 0.85, 1.78 0.88, 1.35, 2.00 1.01, 1.85 0.95, 1.51, 2.05 laC chem. shift a SiMe3 0.36 0.33 0.51 0.69 PR3 8.6, 16.4 14.0, 22.4, 24.5, 26.4 8.0, 16.6 14.1, 23.6, 24.0, 26.6 C~ 102.7 (1268 b, 14.8 r 102.9 (1003 b, 14.8 r 128.8 (986 b, 14.9 c) 129.5 (1000 b, 14.8 r C a 93.6 (284 b) 93.3 (305 b) 105.3 (2818) 104.2 (281 b) C 7 94.1 (358) 94.1 91.4 91.5 C o 76.6 76.5 78.3 78.5 CO - 200.9 201.0

"Recorded in CDCI 3 for 1H and C6D 6 for 13C, in p.p.m, with coupling constants in Hz in parentheses; bJ(Pt--C); cJ(P--C).

572 Lewis et al.

Table 3. 1H- and 13C-n.m.r. data for complexes (5) and (6)

Transition Met. Chem., 20, 569-573 (1995)

(5a) (5b) (6a) (6b)

1H chem. shift a PR 3 1.15, 2.14 0.88, 1.24, 1.41 1.15, 2.16 0.89, 1.24, 1.42 Cyril0 0.92, 1.60, 2.37, 2.52 1.54, 2.12, 2.36, 2.53 - - CsH s - 1.61, 1.86, 2.66, 3.18 1.52, 2.10, 2.66, 3.18 = C H - - 7.16 (d, 2.8 b) 7.13 (d, 2.9 b) 7.19 (d, 2.6 b) 7.14 (d, 2.7 b) 13C chem. shift a PR 3 8.4, 16.1 13.8, 23.6, 24.3, 26.3 8.4, 1611 14.0, 23.6, 24.0, 26.6 C7H10 28.4, 29.1, 31.4, 38.4, 28.4, 29.0, 31.3, 38.4, - -

39.1, 48.2, 53.2 39.2, 48.2, 53.2 CsH s - 23.8, 30.0, 30.2, 43.9, 23.3, 29.2, 30.0, 44.2,

49.5 50.0 C a 116.5 (962 c, 14.4 a) 117.0 (1003 c, 13.5 ~ 115.4 (986 c, 14.8 a) 116.4 (974 ~, 14.8 a) Cp 99.4 (273 c) 98.9 (276 c) 99.4 (292 c) 104.2 (282 r C~ 136.1 136.3 133.5 134.1 C o 159.8 ~ 159.3 e 161.6 160.0 CO 209.3 209.0 211.0 210.0

aRecorded in CDCI 3, in p.p.m, with coupling constants in Hz in parentheses; bj(H--H); ~ dJ(P--C); eJ(H--C) = 165 Hz.

backbone were assigned on the basis of signal intensity and coupling constants (23). The e-carbon resonance of the acetylene chain is a triplet of triplets from coupling to one Pt nuclear (19spt, I = 1/2, 33.3% natural abundance) and to two equivalent 31p nuclei. Whilst the fl-carbon appears as a triplet due to only P t - - C coupling. On compar ing the spectra of ( I ) and (3), it appears that the Co2(CO)6 complexat ion of C 7 and C~ shifts the adjacent Ca and C a resonance downfield. A similar trend was observed in (5) and (6) when terminal alkyne units were conver ted to a 2-cyclopentenone group.

Conclusion

In summary, we have prepared the p l a t i n u m - a l k y n e - cobalt complexes from pla t inum bis(1,3-butadiynyl) com- plexes and Coz(CO)8. The C02(CO)6 groups appear to be very similar due to steric interactions and coordinates to alkyne units away from the pla t inum centre, regardless of the substi tut ion on these alkynyl bonds. A steric contrast was also found to be impor tan t in considering further reactions of these cobalt carbonyl substi tuted problems and only the complexes bearing free terminal alkyne units perform positive P a u s o n - K h a n d reactions with norbor - nylene or cyclopentene ligands.

Acknowledgements

We gratefully acknowledge Corpus Christi College and the SERC (B.L.) for financial support .

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(20) M. Karpf and A. S. Dreiding, Helv. Chim. Acta, 62, 852 (1979). (al)H. B. Abrahamson, C. C. Frazier, D. S. Ginley, H. B. Gray, J.

Lilienthal, D. R. Tyler and M. S. Wrighton, lnorg. Chem., 16, 1554 (1977).

(22)j. Lewis, M. S. Khan, A. K. Kakkar, B. F. G. Johnson, T. B. Marder, H. B. Fyfe, F. Wittmann, R. H. Friend and A. E. Dray, J. Organometal. Chem., 425, 165 (1992).

(a31(a) Sebald, B. Wrackmeyer, C. R. Theocharis, and W. Jones, J. Chem. Soc., Dalton Trans., 747 (1984); (b) A. Sebald, C. Stader, B. Wrackmeyer and W. Bensch, J. Organometal. Chem., 311, 233 (1986); (c) A. Sebald, B. Wraekmeyer and W. Beck, Z. Naturforsch, B, 38, 45 (1983); ibid., 46, 35 (1991).

(Received 1 June 1995) TMC 3508