Synthesis, Characterization, and Structural Studies of Multimetallic Ferrocenyl Carbene Complexes of Group VII Transition Metals

Abstract

Fischer carbene complexes of the group VII transition metals (Mn and Re) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized. The structural features and their relevance to bonding in the carbene multimetal compounds were investigated, as they represent indicators of possible reactivity sites in polymetallic carbene assemblies. For complexes of the type [ML(x){C(OR)R'}] (ML(x) = MnCp(CO)(2) or Re(2)(CO)(9)), ferrocenyl (Fc) was chosen as the R' substituent, while the OR substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1a (ML(x) = MnCp(CO)(2), R = Et, R' = Fc), 2a (ML(x) = MnCp(CO)(2), R = TiCp(2)Cl, R' = Fc), 3a (ML(x) = Re(2)(CO)(9), R = Et, R' = Fc), and 4a (ML(x) = Re(2)(CO)(9), R = TiCp(2)Cl, R' = Fc). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, resulted in formation of the novel biscarbene complexes with bridging ferrocen-1,1'-diyl (Fc') substituents [{π-Fe(C(5)H(4))(2)-C,C'}{C(OEt)ML(x)}(2)] (1b, ML(x) = MnCp(CO)(2); 3b, ML(x) = Re(2)(CO)(9)) or the unusual bimetallacyclic bridged biscarbene complexes [{π-TiCp(2)O(2)-O,O'}{π-Fe(C(5)H(4))(2)-C,C'}{CML(x)}(2)] (2b, ML(x) = MnCp(CO)(2); 4b, ML(x) = Re(2)(CO)(9)). The target compounds that were isolated displayed a variety of different geometric isomers and conformations. The greater reactivity of the binary dirhenium acylates in solution, compared to that of the cyclopentadienyl manganese acylate, resulted in a complex reaction mixture. Although the stabilization of hydroxycarbene or hydrido-acyl intermediates of dirhenium carbonyls could not be achieved, their existence in solution was confirmed by the isolation of [(π-H)(2)-(Re(CO)(4){C(O)Fc})(2)] (8), the unique dichloro-bridged biscarbene complex fac-[(π-Cl)(2)-(Re(CO)(3){C(OEt)Fc})(2)] (6), the known hydrido complex [Re(3)(CO)(14)H] (5), the acyl complex [Re(CO)(5){C(O)Fc}] (7), and the aldehyde-functionalized eq-[Re(2)(CO)(9){C(OTiCp(2)Cl)(Fc'CHO)}] (9).

Redox Behaviour of Cymantrene Fischer Carbene Complexes in Designing Organometallic Multi‐tags

“…Synthesis and characterization of 1 b ,12c 3 a 12b and 3 b 12b have been reported earlier. Novel complexes 1 a , 2 a and 2 b were prepared by using an adapted version of a previously reported method 24…”

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…Structure data suggest a higher contribution of the acyl resonance structure in Fig. 43 to the bonding than in the alkoxycarbene complexes, with shorter M´-O bond lengths [140,151,159,170,186,187].…”

Rhenium ethoxy- and hydroxycarbene complexes with thiophene substituents

“…The formation of hydroxycarbene intermediates is also supported by results found in our laboratories indicating the hydrolysis of a titanoxy substituent of a dirhenium ferrocenylcarbene complex. 12 In spite of not being able to isolate a hydroxycarbene or acyl complex with the exception of 4 (Fig. 8), the sequence from ethoxycarbene to hydroxycarbene to acylhydride to aldehyde was assumed.…”

“…Recently we reported the first examples of tetrarhenium biscarbene complexes with ferrocendiyl and titanocendioxy bridging spacer ligands. 12 All attempts to synthesize dirhenium biscarbene complexes from [Re 2 (CO) 10 ] precursor and lithiated thiophenes failed. We ascribe this to both electronic and steric factors.…”