Preparation of (1,4,7-triazacyclononane)Rh(hydrocarbyl)3 compounds and their derivatives. Strong donor labilization of RhC1 bonds toward alkylation and preparation of unusually stable alkyl hydride complexes of rhodium

Article abstract of DOI:10.1021/om960623e

(tacn)Rh(R)₃ compounds (tacn=1,4,7-triazacyclononane; R=Me, Et, Ph, vinyl) have been prepared in 65-86% yields by treatment of (tacn)RhCl₃·H₂O with more than a 7-fold molar quantity of RLi in THF, followed by protonation by methanol. The product before protonation is (Li₃tacn)RhR₃ (Li₃tacn = 1,4,7-trilithio-1,4,7-triazacyclononane) which can be isolated. The alkylation is complete in a few minutes to a couple of hours, depending on R, which compares with 3-4 days for CnRhCl₃ with MeLi (Cn = 1,4,7-trimethyl-1,4,7-triazacy-clononane) and suggests that the presumably first-formed (Li₃tacn)RhCl₃ is highly activated toward Cl^- dissociation by the strong donor effect of the three LiNR₂ groups in the rhodium coordination sphere. Protonation of (Li₃tacn)RhR₃ by methanol gives the neutral products (tacn)RhR₃. X-ray structure determinations of (tacn)RhEt₃ and (tacn)RhPh₃ have been carried out. Protonolysis of (tacn)RhR₃ (R = Me, Et, Ph) by 2 HSO₃CF₃ (HOTf) affords (tacn)-RhR(OTf)₂. Dissolution of the latter in D₂O gives [(tacn)RhR(D₂O)₂](OTf)₂, which on standing for up to 2 days undergoes H/D exchange of the two NH groups trans to the aqua ligands. Treatment of (tacn)RhR(OTf)₂ with 1 equiv of PMe₃ followed by NaBH₄ or KBH₄ yields a mixture of [(tacn)Rh(H)R(PMe₃)](OTf) and [(tacn)Rh(H)₂(PMe₃))](OTf) which is separated by benzene extraction. Counterion exchange has been carried out where R = Me and Et with Na{B[C₆H₃-3,5-(CF₃)₂]₄} (NaBAr^F₄) yielding [(tacn)Rh(H)R(PMe₃)](BAr^F₄). Heating of [CnRh-(H)Et(PMe₃)IX (X = OTf or BAr^F₄), [(tacn)Rh(H)Et(PMe₃)]X, and [(tacn)Rh(H)Me(PMe₃)]X in C₆D₆ leads to formation of [(Cn/tacn)Rh(D)(C₆D₅)(L)]XL)]X in high yield in all cases. The half-lives of these cleanly first-order reactions at 80 °C are 1 min, 1.2 h, and 7.3 h, respectively, illustrating alkyl hydride thermal stabilities unprecedented in rhodium chemistry.