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

Abstract

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.

Protecting-Group-Free Access to a Three-Coordinate Nickel(0) Tris-isocyanide

“…26 For these latter metals, CNAr Dipp2 was shown to very effectively discourage the association of a third isocyanide ligand. 26 In the present case of Ni(0), it is demonstrated that CNAr Dipp2 can indeed form a stable, homoleptic tris-isocyanide complex and then discourage ligation of a fourth isocyanide ligand.…”

“…26 For these latter metals, CNAr Dipp2 was shown to very effectively discourage the association of a third isocyanide ligand. 26 In the present case of Ni(0), it is demonstrated that CNAr Dipp2 can indeed form a stable, homoleptic tris-isocyanide complex and then discourage ligation of a fourth isocyanide ligand. Importantly, this property enabled the synthesis of Ni-(CNAr Dipp2 ) 3 without the need for a coordination-site protection as had been employed previously for the synthesis of the dimesityl derivative, Ni(CNAr Mes2 ) 3 .…”

Regioselective Formation of (E)‐β‐Vinylstannanes with a Topologically Controlled Molybdenum‐Based Alkyne Hydrostannation Catalyst

“…[32][33][34] In addition, in our studies on Group 6m etal complexes featuring two CNAr Dipp2 ligands,w eh ad previously shown that the steric encumbrance of these isocyanides enforces am utually trans orientation through multiple redox transformations of the metal center. [18,19] We believe that to relieve steric pressure,this constrained, trans isocyanide environment around the metal center shifts the equilibrium between the two C-alkenyl insertion products to the configuration favoring production of the (E)-b-vinylstannane isomer ( Figure 2). This argument is analogous to that made for regioselective (E)-b-vinylstannane formation using palladium and encumbering phosphine ligands, [15] with the added feature that two cis-oriented ligands provide steric pressure in this sixcoordinate Mo system.…”

Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

“…Additionally, investigations into the one-electron activation chemistry of [Mn(CO) 5Àn L n ]s pecies with small molecule substrates are limited. [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17e À radical both isolable in the solid state and persistent in solution. [19][20][21] Accordingly,successful strategies leading to the kinetic stabilization and isolation of such [MnL 5 ] monoradicals,while explicitly preventing the formation of the common Mn À Mn single-bond motif,a re of importance in these processes.F urthermore,a ccess to long-lived [MnL 5 ] monoradicals offers ap otential avenue for the development of selective,well-defined metal-mediated one-electron transformations.H erein we describe the isolation, structure,a nd spectroscopic features of the neutral monoradical [Mn-(CO) 3 (CNAr Dipp2 ) 2 ]( Ar Dipp2 = 2,6-(2,6-(iPr) 2 C 6 H 3 ) 2 C 6 H 3 )f eaturing two m-terphenyl isocyanide ligands.…”