Thiophene bimetallic carbene complexes

Abstract

The syntheses of σ,σ-bimetallic biscarbene- and a, σ π -bimetallic monocarbene complexes, as well as the reactivity of the former, were investigated. The metal carbonyls Cr(C0)6, W(C0)6, Mn ( n5-C5H4Me)(C0)3 and Mn(n5-C5H5)(C0)3 were reacted with dilithiated thiophene to synthesize the novel σ,σ -bimetallic biscarbene complexes: (C0)2L3M{C(OEt)C4H2SC(OEt)}MLlC0)2, where M = Cr, W, Mn; L =CO; = n5-C5H4Me, n5-C5H5. With dilithiated 2,2'-methylene dithiophene; (C0)2L3M{C(OEt)C4H2SCH2C4H2SC( OEt)} ML3 CO )2, were obtained. These complexes were fully characterized and formulations were confirmed by ctystal structure determinations for the former. Spectroscopic data indicate the thiophene rings to act as sources of electron density which stabilize the electrophilic carbene carbons. Treatment ofmonolithiated (n5-thiophene)Cr(C0)3 with hexacarbonyls afforded in addition to the expected a,1t-bimetallic monocarbene complexes, (n1:n5-C4H3SC(OEt)M(C0)5)Cr(C0)3, the complexes (n5 : n5-C4H3SC( O(CH2}40Et)M(C0)5)Cr(C0)3 and M { C(O(CH2)40Et)C4H3S }(C0)5, where M = Cr, W. The formation of the latter two types is most unexpected and indicates that -rt-coordination of Cr(C0)3 to thiophene plays an important electronic and steric role which leads to an unique pathway in which a THF-ring is opened and incorporated into the carbene functionality. The a,1t-bimetallic monocarbene complex of (n6-benzo[b]thiophene) Cr(C0)3 was synthesized to enable a comparison with the former. Monolithiation of (n6-benzo[b]thiophene)Cr(C0)3 occured in the 2-position of the thiophene ring and (n1 : n6 C8H5SC(OEt)Cr(C0)5)Cr( C0)3 was afforded in a quantitative yield, thereby proving that -rt-coordination to the benzene ring is stronger and more distant from the a-bonded metal fragment. Related THF-incorporated complexes were not formed in this reaction. The stability of the thienylene biscarbene complexes at various temperatures, in different solvents and under two different inert atmospheres, led to the characterization of the following classes of complexes: I: (C0)2L3M{C(OEt)C4H2SC(O)OEt}; II: W{C(OEt)C4H2SC(OEt)C4H2SC(OEt)C4H2SC(O)OEt}(C0)5; III: (C0)2L3M{C(OEt)C4H2SC(S)OEt}; IV: M{C(OEt)C4H2S-3-[2,5-{C(S)OEt}2C4HS}(C0)2; V: Cr{C( OEt)C4H2SC(O)C(O)C4H2SC(O)OEt}(C0)5 . I was obtained from the decomposition in acetone under nitrogen and was presumably formed in the reaction between the carbene moiety and oxygen which had diffused into the system, as no proof of an oxygen transfer from acetone could be found. II was isolated from the analogous decomposition under argon, thus indicating that the complete elimination of oxygen from a better equiped system would lead to the formation of new and different types of oligomeric products. III was afforded by the reaction in CS2, with the exception of IV, which was yielded, for the chromium biscarbene in CS2, as well as, hexane. V was isolated from the decomposition in hexane. The result of these studies indicates that as opposed to monocarbene complexes, biscarbene complexes displayed enhanced reactivity and the reaction conditions control conversion. The reaction of the thienylene chromium and tungsten biscarbenes with hex-3-yne yielded analogous monocarbene products, M{C(OEt)CCHCC(OH)C(Et)C(Et)C(OEt)CS}(C0)5, lacking a TI-coordinated Cr(CO)3. This is a surprising result for tungsten as the expected product should be the cyclopentathienyl complex. The result suggests an alternative mechanism for the Dotz reaction and a possible reaction route is proposed. The reaction of biphenyl acetylene, however, afforded the expected cyclopentathienyl complex, M{C(OEt)CCHCC(Ph)C(Ph)CH(OEt)CS}(CO)5, in two isomeric forms. The analogous chromium reaction gave the related cyclopentathienyl product, as well as, the expected benzothienyl complex, as two isomers, with Cr(CO)3 TI-coordinated to either the benzene- or thiophene ring , and the end product lacking the Cr(CO)3-fragment. The isolation of the alkyne inserted intermediate, (CO)5Cr{C(OEt)C4H2SC(OEt)C(Ph)C(Ph) }Cr(CO)5, emphasises that the stabilization of intermediates by coordination to more than one metal fragment, highlights the value of bimetallic systems in studying reaction mechanisms.