Preferred Coordination Sites for Metal Fragments in σ,π-Bimetallic Complexes. Detailed Mechanistic Insight from Heteroatom, Bridging Ligand, Solvent, Temperature, and Pressure Effects on the Irreversible Exchange of Coordination Sites¹

Abstract

New heterobimetallic complexes with σ,π-bridging thiophene and selenophene ligands, (η 1 :η 5 -XCRCHCHCMn(CO) 5 )Cr(CO) 3 (X ) S, R ) H (1), R ) Me (2); X ) Se, R ) H (3)), were synthesized from (η 5 -XCRCHCHCLi)Cr(CO) 3 and Mn(CO) 5 Hal (Hal ) Cl -, Br -, CF 3 SO 3 -). The complexes 1-3 irreversibly convert at 0 °C in acetone into the complexes (η 1 :η 5 -XCRCHCHCCr(CO) 5 )Mn(CO) 3 (X ) S, R ) H (4), R ) Me (5); X ) Se, R ) H ( 6)) by exchanging coordination sites. The σ,π-exchange of coordination sites is a first-order process and rate constants for the reaction of 2 are (3.8 ( 0.1) × 10 -5 and (1.1 ( 0.1) × 10 -5 s -1 in acetone and cyclohexane at 15 °C, respectively. This reaction shows no significant pressure dependence. The activation entropies for the exchange process are -16 ( 6 and -30 ( 11 J/(mol K) for 1 and 2, respectively. The kinetic data suggest an intramolecular exchange mechanism involving bridging carbonyls without any direct involvement of the solvent. It is suggested that in the activated complex the metal centers are η 1 -bonded to the C2 of the thienyl ligand and that the free coordination sites are occupied by two bridging carbonyls. The bimetallic complexes (η 1 :η 5 -XCRCHCHCC(O)Mn(CO) 5 )Cr(CO) 3 (X ) S, R ) H (7), R ) Me (8); X ) Se, R ) H ( 9)) were also isolated from the reaction mixtures and could be obtained in higher yields by working under a CO atmosphere. The inserted carbonyl in the bridging ligand inhibits the metal fragments from exchanging coordination positions. Excess BuLi leads to the formation of the trimetallic five-membered ring complexes (µ-Hal){µ-(η 1 :η 1 :η 5 -SCRCHCHCC(O)Mn(CO) 4 )Cr(CO) 3 }Mn(CO) 4 (R ) H, Hal ) Cl (10); R ) Me, Hal ) Br ( 11)). The lithiated thiophene precursor exclusively attacks a carbonyl of Re(CO) 5 Br to give a bimetallic acylate, which after subsequent alkylation with Et 3 OBF 4 affords the bimetallic rhenium carbene complex (η 1 :η 5 -SCHCHCHCC(OEt)Re(CO) 4 Cl)Cr(CO) 3 (12). The target complex (η 1 :η 5 -SCHCHCHCRe(CO) 5 )Cr(CO) 3 (13), which could not be converted and did not insert a carbonyl, was obtained from Re(CO) 5 CF 3 SO 3 and the lithiated precursor. The structures of 7 and 10 were confirmed by single-crystal X-ray diffraction studies.

Ferrocenyl‐Functionalized η5‐Thiophene Cr(CO)3 Half‐Sandwich Compounds

“…The Cr(CO) 3 fragment exhibits a piano‐stool coordination environment, clarified by almost ideal carbonyl C–Cr–C angles of 86.9(4)–91.1(10) °. One carbonyl is directed antiperiplanar towards the sulfur atom by 172.8(10)–174.1(10) °, which is in accordance to literature reported solid‐state structures of Cr(CO) 3 complexed thiophenes . The cyclopentadienyls of one of each ferrocenyl substituent in 3a , b adopts an ecliptic conformation [1.6(10) to 8.9(7) °], whereas the other one reveals a rather staggered positioning [18.6(6)–31.6(12) °].…”

“…[10a] This approach is contrary to literature reported procedures, where a post‐functionalization of the η 5 ‐thiophene Cr(CO) 3 compounds is realized , ,. [10b], [10c], , Thiophenes 1a – g are accessible by cross‐coupling protocols that were recently reported by our group. [1a], [1c], [2c], , The complexation reactions of thiophenes 1a – g with 2 were performed in dibutyl ether at 90 °C for 15 h to give the respective Cr(CO) 3 compounds 3a – e in up to 65 % yield (Scheme ).…”

“…Within this family of compounds, η 5 ‐thiophene chromium tricarbonyl was first prepared by Fischer and Öfele as early as 1958 in a solvothermal metathesis reaction of thiophene and Cr(CO) 6 . Up to date, only a few Cr(CO) 3 complexes with η 5 ‐bonded thiophenes bearing additional organometallic fragments have been reported, including titanocene structures, Pt complexes, silyl groups, –M(CO) 5 (M = Mn/Cr,W). Although, basic knowledge of their reaction behavior and NMR properties were investigated, electrochemical studies are still pending.…”

New metal aminocarbene-substituted cyclopentadienyliron half sandwich complexes

“…Among the organometallic aromatic systems few reports about ferrocenylcarbene complexes have been published [18,19]. Prior to our work only one half sandwich cyclopentadienylcarbene complex [20] and a related thiophene complex had been reported [21]. Some bimetallic half sandwich iron cyclopentadienyl alkoxycarbene complexes were reported by our group [22,23], and these complexes showed interesting structural and electrochemical behavior.…”

Mono-, di- and tetrarhenium Fischer carbene complexes with thienothiophene substituents

“…However, unlike for the analogous [Mn(CO) 5 Br], it is possible to synthesize rhenium carbene bromido complexes in reasonable yields, when the lithiated thiophene is π-coordinated to a chromium tricarbonyl fragment acting as an electron-sink. 3 The corresponding reaction of manganese pentacarbonyl bromide by lithium-halogen exchange proceeded to give novel σ,π-Cr/Mn complexes with bridging thienyl and carboxythienyl ligands.…”