Effective control of ligation and geometric isomerism: direct comparison of steric properties associated with bis-mesityl and bis-diisopropylphenyl m-terphenyl isocyanides.
A synthetic procedure for the sterically encumbered m-terphenyl isocyanide CNAr(Dipp2) (Dipp = 2,6-diisopropylphenyl) is presented. In comparison to the less encumbering m-terphenyl isocyanide ligand CNAr(Mes2), the steric attributes of the flanking Dipp groups effectively control the extent of CNAr(Dipp2) ligation to monovalent Cu and Ag centers and zero-valent Mo centers. Direct structural comparisons of Cu(I) and Ag(I) complexes of both CNAr(Dipp2) and CNAr(Mes2) are made. It was found that only two CNAr(Dipp2) ligands are accommodated by monovalent Cu and Ag centers, whereas three CNAr(Mes2) units can readily bind. As demonstrated by both (1)H NMR and FTIR spectroscopic studies, addition of a third equivalent of CNAr(Dipp2) to [(THF)(2)Cu(CNAr(Dipp2))(2)]OTf in C(6)D(6) solution results in slow isocyanide exchange. However, rapid isocyanide exchange is observed when an additional equivalent of CNAr(Dipp2) is added to (TfO)Ag(CNAr(Dipp2))(2). Three CNAr(Mes2) ligands react smoothly with fac-Mo(CO)(3)(NCMe)(3) to afford the octahedral complex fac-Mo(CO)(3)(CNAr(Mes2))(3), which can be converted irreversibly to the mer isomer upon heating in solution. Contrastingly, addition of CNAr(Dipp2) to fac-Mo(CO)(3)(NCMe)(3) results in a mixture of both the tetracarbonyl and the tricarbonyl complexes trans-Mo(CO)(4)(CNAr(Dipp2))(2) and trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2), respectively, in which the encumbering CNAr(Dipp2) ligands are in a trans-disposition. Ultraviolet irradiation of the preceding mixture in NCMe/Et(2)O under an argon flow provides exclusively the tricarbonyl complex trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2). Addition of free CNAr(Dipp2) to trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) does not result in the binding of a third isocyanide unit by the Mo center as determined by (1)H NMR spectroscopy. Treatment of trans-Mo(NCMe)(CO)(3)(CNAr(Dipp2))(2) with the Lewis base pyridine (py) affords the complex fac,cis-Mo(py)(CO)(3)(CNAr(Dipp2))(2) as determined by X-ray diffraction. Notably, the encumbering nature of the CNAr(Dipp2) units forces a cis C(iso)-Mo-C(iso) angle of about 100 degrees.
Ditri
Treffly B
TB
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093-0358, USA.
Fox
Brian J
BJ
Moore
Curtis E
CE
Rheingold
Arnold L
AL
Figueroa
Joshua S
JS