Effective Control of Ligation and Geometric Isomerism: Direct Comparison of Steric Properties Associated with Bis-mesityl and Bis-diisopropylphenyl m-Terphenyl Isocyanides

摘要

A synthetic procedure for the sterically encumbered m-terphenyl isocyanide CNArDipp2 (Dipp = 2,6-diisopropylphenyl) is presented. In comparison to the less encumbering m-terphenyl isocyanide ligand CNArMes2, the steric attributes of the flanking Dipp groups effectively control the extent of CNArDipp2 ligation to monovalent Cu and Ag centers and zerovalent Mo centers. Direct structural comparisons of Cu(I) and Ag(I) complexes of both CNArDipp2 and CNArMes2 are made. It was found that only two CNArDipp2 ligands are accommodated by monovalent Cu and Ag centers, whereas three CNArMes2 units can readily bind. As demonstrated by both H-1 NMR and FTIR spectroscopic studies, addition of a third equivalent of CNArDipp2 to [(THF)(2)Cu(CNArDipp2)(2)]OTf in C6D6 solution results in slow isocyanide exchange. However, rapid isocyanide exchange is observed when an additional equivalent of CNArDipp2 is added to (TfO)Ag(CNArDipp2)(2). Three CNArMes2 ligands react smoothly with fac-Mo(CO)(3)(NCMe)(3) to afford the octahedral complex fac-Mo(CO)(3)(CNArMes2)(3) which can be converted irreversibly to the mer isomer upon heating in solution. Contrastingly, addition of CNArDipp2 to fac-Mo(CO)(3)(NCMe)(3) results in a mixture of both the tetracarbonyl and the tricarbonyl complexes trans-Mo(CO)(4)(CNArDipp2)(2) and trans-Mo(NCMe)(CO)(3)(CNArDipp2)(2), respectively, in which the encumbering CNArDipp2 ligands are in a traps-disposition. ultraviolet irradiation of the preceding mixture in NCMe/Et2O under an argon flow provides exclusively the tricarbonyl complex trans-Mo(NCMe)(CO)(3)(CNArDipp2)(2). Addition of free CNArDipp2 to trans-Mo(NCMe)(CO)(3)(CNArDipp2)(2) does not result in the binding of a third isocyanide unit by the Mo center as determined by H-1 NMR spectroscopy. Treatment of trans-Mo(NCMe)(CO)(3)(CNArDipp2)(2) with the Lewis base pyridine (py) affords the complex fac,cis-Mo(py)(CO)(3)(CNArDipp2)(2) as determined by X-ray diffraction. Notably, the encumbering nature of the CNArDipp2 units forces a cis C-iso-Mo-C-iso angle of about 100 degrees.