Abstract
Three different aryl substrates thiophene (ThH), ferrocene (FcH) and p-bromodimethylaniline (p-DMABr) were lithiated and reacted with [Co(CO)4SnPh3], according to the Fischer carbene protocol. The products [CoSnPh3(CO)3{C(OEt)R}] (R =Th, 1, p-DMA, 2 and R = Fc, 3) were analysed to unravel the role of the aryl carbene substituent in stabilizing intermediates and final products. The ethoxy carbene complexes were aminolysed by in situ generated HNMe2 to afford [CoSnPh3(CO)3{C(NMe2)R}] (R =Th, 4, p-DMA, 5) and with N,N-dimethylethylenediamine [CoSnPh3(CO)3{C(NHCH2CH2NMe2)R}] (R = p-DMA, 6) in high yields. The trigonal Co(CO)3 in the equatorial plane is very stable and efforts to displace a carbonyl in 6 was unsuccessful, both by heating and irradiation. The role of the aryl carbene substituents in stabilizing the electrophilic carbene carbon was investigated and studied by NMR spectroscopy in solution and single crystal X-ray structure determinations.
Highlights
Inexpensive and earth-abundant first row transition metal complexes with N-heterocyclic carbene (NHC) ligands have proved attractive and viable alternatives to their 4d and 5d transition metal counterparts as, for example, homogeneous catalysts over the past two decades [1,2,3,4]
The first reports detailing the preparation of cobalt(I) Fischer carbene complexes (FCCs) involved nucleophilic attack of cobalt carbonyl precursors stabilized by triphenylstannyl, -germyl or -plumbyl ligands, to yield complexes of the type [Co(CO)3(YPh3){C(OR)R }] (Y = Sn, Ge, Pb; R = Et, Me and R = Me, Et, n-Pr, n-Bu, Ph) [9,10,11,12,13]
Challenges initially experienced during the syntheses are the insolubility of the metal carbonyl precursor during the nucleophilic attack step at low reaction temperatures (−78 °C) and later excessive decomposition during the purification of the FCCs
Summary
Inexpensive and earth-abundant first row transition metal complexes with N-heterocyclic carbene (NHC) ligands have proved attractive and viable alternatives to their 4d and 5d transition metal counterparts as, for example, homogeneous catalysts over the past two decades [1,2,3,4]. Examples of acyclic Fischer-type [5] carbene complexes of 3d metals in this context are virtually unknown. The first reports detailing the preparation of cobalt(I) FCCs involved nucleophilic attack of cobalt carbonyl precursors stabilized by triphenylstannyl, -germyl or -plumbyl ligands, to yield complexes of the type [Co(CO)3(YPh3){C(OR)R }] (Y = Sn, Ge, Pb; R = Et, Me and R = Me, Et, n-Pr, n-Bu, Ph) [9,10,11,12,13]. In the synthesis of Co(I) FCCs, the imperative of effective nucleophilic carbonyl attack needs to be facilitated by a bulky ancillary ligand on the cobalt precursor to electronically and structurally direct and stabilize acyl formation. In practise this means that for more basic metals, metal alkylation com-
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