Electron-rich metal complexes for C02 and CS2 incorporation

Pure & Appl. Chem., Vol. 61, No. 10, pp. 1701-1706, 1989. Printed in Great Britain. @ 1989 IUPAC Electron-rich metal complexes for C02 and CS2 incorporation Ernest0 Carmona, Juan Cimpora, Miguel A. MuRoz, Margarita Paneque and Manuel L. Poveda Departamento de Quimica Inorginica-Instituto de Ciencia de Materiales, Facultad de Quimica, Universidad-CSIC, Apartado 553, 41071 Sevilla, Spain Abstract - The dihapto coordination of two molecules of carbon dioxide to a square-planar MoP2(P-P) fragment (P=monodentate-, P-P=bidentate phosphine ligands) can occur in several ways giving rise to different conformers. Compounds of this type have been prepared and their spectroscopic properties investigated by variable temperature 'H and 31P NMR studies. The formation of ethylene complexes related to Mo(C2H4)2(PMe3)4, 6, and the study of their reactivity towards carbon dioxide and carbon disulphide is also described. In particular, compound 6 induces a head-to-tail dimerization of carbon disulphide with formation of a Mo(II)-C$4 linkage, which, in the presence of carbon monoxide, undergoes a symmetric, oxidative rupture, which produces a Mo(0) complex and two molecules of carbon disulphide. The insertion reactions of carbon dioxide and other unsaturated molecules into the nickel-carbon bonds of the nickelacyclopentene complexes 9 have been investigated and different organic products isolated by means of simple or sequential insertion reactions. INTRODUCTION The reactions of carbon dioxide with transition metal complexes have been extensively investigated in the last decade. These investigations have led to the isolation of some stable transition metal-carbon dioxide complexes, as well as to the observation of carbon-carbon coupling reactions between carbon dioxide and a variety of organic substrates (refs. 1-5). Considerable progress has been made in the understanding of the interaction of carbon dioxide with transition metals but many important aspects of this chemistry remain little explored or underdeveloped, so that carbon dioxide chemistry is still a promising area of research. In order to overcome the difficulties very often encountered in the study of the reactions of carbon dioxide with transition metals, carbon disulphide and other related molecules (ref. 6) have been used as models for carbon dioxide chemistry. However, the results obtained to date clearly show that these molecules exhibit a very distinct chemical behaviour which only very rarely parallels carbon dioxide chemistry. In particular, carbon disulphide displays a very rich chemistry of much intrinsic academic interest. In this contribution we wish to report some reactions of carbon dioxide with complexes of molybdenum and nickel, that lead to adduct formation or to insertion into a metal-carbon bond. The reactions of these complexes with CS2 are also described. CARBON DIOXIDE AND CARBON DISULPHIDE COMPLEXES OF MOLYBDENUM Carbon dioxide complexes containing chelating phosphine ligands Coordination of carbon dioxide in its intact form to a transition metal is a rare result and the number of structurally characterized complexes is therefore very limited. At variance with the behaviour found for other carbon dioxide complexes, the bis-carbon dioxide adduct trans-Mo(C02)2(PMe3)4, (l), has been reported to undergo a facile substitution of one of the coordinated PMe3 groups by an isocyanide ligand, with formation of truns,mer-Mo(C02)2(CNR)(P Me3)3 tempts to substitute a second phosphine by heating at 5O-6O0C, in the presence of an excess of the isocyanide, lead to loss of carbon dioxide. We have now found (ref. 8) that chelating diphosphine ligands effect substitution of two molecules of trimethylphosphine of complex 1, without change in the coordination mode of the carbon dioxide groups. Compounds,Za-Zb, are formed, as shown in reaction (1). The new complexes are yellow crystalline solids which display IR and NMR spectroscopic properties very similar to those reported for the parent compound 1. Our interest in complexes 2 was mainly concentrated in the following points: i) assuming that the coordinated C02 ligands are staggered with respect to one another and (ref, 7). Substitution of the carbon dioxide ligands is not observed, although at- 1701 Brought to you by | Biblioteca de la Universidad de Sevilla Authenticated Download Date | 9/10/19 10:14 AM 1702 E. CARMONA eta/. eclipsing the P-Mo-P vectors of the equatorial plane, as predicted theoretically for 1 (ref. 9) and demonstrated by X-ray studies for the mono-isocyanide derivative (ref. 7), there could be three different conformations in compounds 2, as represented in structures 3-5. Would any 3 4 5 of these conformations be favoured in solution and in the solid state?, ii) compounds 2, as the parent complex 1, are fluxional, and the process responsible for the observed changes in the NMR spectra is rotation around the molybdenum-carbon dioxide bond axis. Since there are two molecules of carbon dioxide coordinated to the molybdenum atom, the question arises as to what is the nature of the rotation process. Variable temperature NMR studies provide some clues to both questions. The room temperature 31P{1H1 NMR of complex 2a, consists of two very broad and featureless humps, which give no useful information with regards to the structure of the compound. Upon cooling some changes are observed, and at -9OOC four different sets of resonances (AMNX spin system) appear. This is in accord with structure 2 and not with 2 or 4. With respect to the second of the above questions, namely the nature of the fluxional process, a clear distinction between the possible mechanisms cannot be achieved with present data. In the low temperature limit (-75OC) the 'H NMR spectrum of 2a consists of two partially superimposed doublets for the PMe3 ligands, as well as of four doublets, one for each of the dmpm methyl groups, in accord with the rigid structure 5 proposed for these complexes. Upon warming the spectrum simplifies and at 4OoC it consists of one doublet for the trimethylphosphine groups and two doublets for the methyl protons of the chelating phosphine ligand, in agreement with a concerted, but not with a random rotation. Finally, in accord with these observations, the 31P{1H1 NMR spectrum of compounds 2 consists, at high temperatures (60-80°C), of a pattern of lines characteristic of an AA'XX' spin system (ref. 8). With the aim of obtaining new carbon dioxide complexes, the reactions of various dinitrogen complexes of molybdenum, prepared as summarized in reactions (2)-(4), with C02 have been stu- "Mo(N2 )3(PE~3 )3" + PME~ - =-Mo( N2 )2 (PME~) 3( PET^) + died. Reaction (3) produces in fact a mixture of the tris-dinitrogen complex and the dinitrogen bridged species [Mo(N2)2(PEt3)2]2(~-N2), written in a simplified form as shown. Action of C02 upon solutions of these complexes provide only carbonate complexes, i. e., the products of the so-called reductive disproportionation of carbon dioxide, and therefore their investigation has not been pursued any further (ref. 10). Brought to you by | Biblioteca de la Universidad de Sevilla Authenticated Download Date | 9/10/19 10:14 AM Electron rich metal complexes for CO, and CS, incorporation 1703 Reactions of tranaM~(C~H~)~(PMe~)~, (61, and related complexes with carbon dioxide and carbon disulphide We have recently reported that carbon dioxide reacts with one of the coordinated ethylene ligands of complex 6, forming a new C-C bond (ref. 11). The product of this reaction is a binuclear molybdenum complex which contains two bridging acrylate ligands (ref. 12), and the fa- P cile activation of carbon dioxide by complex 6 is probably a consequence of the electron-rich nature of the metal centre and of the availability of a vacant coordination position by facile dissociation of one of the trimethylphosphine ligands (ref. 11). In order to gain further information on this remarkable transformation other related systems have been investigated, reactions (5)-(7): P-P= DMPM, DMPE i) attempts to prepare the propylene analogue of 6 lead instead to a hydrido ally1 species (ref. 13) which reacts with carbon dioxide with evolution of propylene and formation of 1 and other products (ref. 10). ii) PMe3 substitution in 6 by the Il-acid ligands CO or CNR (ref. 14) affords new ethylene complexes as represented in reaction (6). Not unexpectedly, their reactivity towards carbon dioxide does not parallel that of 6. iii) compounds related to 6 containing chelating phosphine ligands can be prepared as depicted in equation (7). These compounds are surprisingly inert towards carbon dioxide. For instance, the dmpe derivative can be recovered unchanged after heating at 6OoC, for several hours, in tetrahydrofuran, under 2-3 atm of carbon dioxide. The lack of reactivity in this case reflects probably the non availability of a vacant coordination position. It was also of interest to ascertain if other heterocumulenes, in particular carbon disulphide, would undergo similar transformations. Addition of carbon disulphide to solutions of complex 6 produces a black solid of composition Mo(C~S,)(C,H,)(PM~~)~, (7), which has the PMe3 groups in a meridional distribution. The ethylene ligand anysochronous methylenic protons, while the two CS2 molecules give rise to a singlet and a quartet (I3C enriched sample of 7, 227 and 293 ppm, respectively), suggesting that one of the carbon disulphide groups is C-bonded to the metal atom. The bonding mode of the C2S4 unit has been determined by X-ray studies (ref. 15) which show the two molecules of CS2 have linked to a head-to-tail dimer. Formation of 7 can therefore be represented as in equation (8). has equivalent CH2 moieties but 7 Brought to you by | Biblioteca de la Universidad de Sevilla Authenticated Download Date | 9/10/19 10:14 AM 1704 E. CARMONA eta/. Metallocyclic complexes of this type are often proposed as intermediates in the metal-promo- ted reductive disproportionation of carbon dioxide and related molecules, although conclusive evidence has never been obtained (ref. 16). Furthermore, there is a surprising lack of studies aimed at the investigation of the chemical reactivity of these units. Compound 7 exhibits a high thermal stability and does not decompose, nor produces the disproportionation products, CS and CS32-, after heating at 120°C for several hours. An unexpected reaction occurs with carbon monoxide under relatively mild conditions (5OoC, 2 atm). As shown in Scheme I, symmetric oxidative rupture of the C2S4 unit is observed, with reduction to the molybdenum(0) complex 8, i. e., the reverse of the reductive dirnerization reaction. These results not only demonstrate a new type of reactivity for these metalacyclic units, but question their proposed intermediacy in the transition metal induced reductive disproportionation of heterocumulenes. INSERTION CHEMISTRY OF THE NICKELACYCLOPENTENE COMPLEX Ni[CH2C(Me)2-O-C6H41(PMeJ)2r 9a. REACTIONS WITH CARBON DIOXIDE, CARBON DlSULPHlDE AND OTHER UNSATURATED MOLECULES The nickelacyclopentene complex 9a has a rich insertion chemistry and reacts with carbon monoxide, carbon dioxide, carbon disulphide and other unsaturated molecules (ref. 17) with formation of a variety of organic and metallo-organic products. The interaction of 9a with CO and PhCaCPh, under ambient conditions, takes place as indicated in equations (9) and (10). As shown, insertion is followed by a rapid reductive elimination which yields the corresponding organic and metallo-organic products. A differentiation between the two possible reaction pathways, namely insertion into the nickel-alkyl or aryl carbon bonds, can be accomplished by carrying out, in either order, the sequential insertions depicted in reaction (11) (ref. 18). Formation of 10 unequivocally stablishes that the insertion of CO takes place exclusively into the nickel-carbon alkyl bond. 9a Brought to you by | Biblioteca de la Universidad de Sevilla Authenticated Download Date | 9/10/19 10:14 AM Electron rich metal complexes for CO, and CS, incorporation 1705 In the presence of wet C02, compound 9a undergoes a fast protolytic cleavage of the nickel- -aryl bond, with formation of the binuclear, carbonate-bridged species 11, reaction (12). In the absence of water, the nickelalactone derivative 12 is formed as the result of the inser- P PHCME CH 0 I 2\N~’ ‘C-O-N!-CH~C~~PH I (12) P Y p/ \O’ 11 (13) NI 9a P’ \o-c II 12 0 tion of carbon dioxide into the Ni-aryl bond (reaction 13). The existence in these compounds of Ni-C bonds, suggest further insertion chemistry can be pursued, and indeed, interaction with carbon monoxide (20°C, 1 atm) proceeds as indicated in reactions (14) and (15). While the former deserves no additional comments, the results of the latter reaction are rather unexpected, (ref. 17) since they correspond, in a formal sense, to a metal promoted oxidative conproportionation of carbonate and carbonyl ligands. Two molecules of carbon dioxide are formed, one of them is evolved as free C02 and the other becomes incorporated into the anhydride functionality. The reaction of 9a with 3-4 equiv of carbon disulphide (ref. 19) produces a red-brown microcrystalline precipitate of the known complex 13 (ref. 20), together with a red solution, from which red crystals of the dithiolactone 14 can be isolated (Scheme 11). The nature of 13 and 14 implies the intermediacy of a Ni(0) species, 15, which can, in fact, be isolated by the low temperature reaction of 9a with a slight defect of carbon disulphide. As expected, 15 reacts with excess CS2, with formation of 13 and 14. It seems likely, that the first step in this reaction is insertion of carbon disulphide into the nickel-aryl carbon bond, and this has been confirmed, as already indicated, by isolation of complex 10, as the result of the sequential insertion of carbon disulphide and carbon monoxide (see reaction 11). The resulting nickeladithiolactone intermediate species, which would in fact be the CS2 analogue of 12, undergoes reductive elimination, with concomitant intramolecular C-S bond formation, to yield the observed complex 15. Scheme I/ Brought to you by | Biblioteca de la Universidad de Sevilla Authenticated Download Date | 9/10/19 10:14 AM 1706 E. CARMONA eta/. A very different reaction is observed between carbon disulphide and complex 9b, the dmpe analogue of 9a. Upon addition of 2 equiv of carbon disulphide to a solution of 9b, the trithiocarbonate 16 and the thioketene 17 are formed (Scheme 111). This reaction corresponds formally to a metal-induced reductive disproportionation of carbon disulphide but, rather interestingly, it occurs without change in the oxidation state of the metal centre. Action of 1 equiv of carbon disulphide upon compound 9b affords the 1,l-dithiolate species 18, which, as expected, reacts with another equivalent of carbon disulphide, with formation of 16 and 17. Thus, the course of the reaction of carbon disulphide with the nickelacyclopentene derivatives 9 depends markedely on the nature of the phosphine co-ligands. It seems reasonable that the reaction of 9b with carbon disulphide proceeds also with initial insertion of the heterocumulene into the nickel-aryl bond. The chelating nature of the dmpe ligand prevents phosphi- Scheme Ill 2 cs2 9b \ ne dissociation and may induce C-C instead of C-S bond formation. Nevertheless, electronic factors must also play an important role and this is clearly manifested by the course of the reaction of the 2,2'-bipyridine derivative 9c (ref. 18) with carbon disulphide. As shown in equation (16), this yields a complex containing the coordinated dithiolactone 14, that is, a product analogous to the trimethylphosphine derivative 15. No evidence for the formation of a 1.1-dithiolate complex analogous to 18 has been found. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19 * 20. 9c Acknowledgement 1. The authors gratefully acknowledge the continuous support of the Ministerio de Educaci6n y Ciencia and the Agencia Nacional de Evaluaci6n y Prospectiva. We thank Professor Pierre Braunstein for a pre-print of reference REFERENCES P. Braunstein, D. Matt and D. Novel, Chem. Rev. 88, 747 (1988) A. Behr,,Anvew. Chem. Int. Ed. Engl. 27, 661 (1988). D. Walther, Coord. Chem. Rev. 79, 135 (1987). D.J. Darensbourg and R.A. Kudaroski, Adv. Organornet. Chem. 22, 129 (1983). R.P.A. Sneeden, "Comprehensive Organometallic Chemistry"; Pergamon Press: Oxford 1982. C. Floriani, Pure & Appl. Chem. 54, 59 (1982). R. Alvarez, E. Carmona, J.M. Marin, M.L. 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