The synthesis of the m-terphenyl isocyanide ligand CNAr (Mes2) (Mes = 2,4,6-Me 3C 6H 2) is described. Isocyanide CNAr (Mes2) readily functions as a sterically encumbering supporting unit for several Cu(I) halide and pseudo halide fragments, fostering in some cases rare structural motifs. Combination of equimolar quantities of CNAr (Mes2) and CuX (X = Cl, Br and I) in tetrahydrofuran (THF) solution results in the formation of the bridging halide complexes (mu-X) 2[Cu(THF)(CNAr (Mes2))] 2. Addition of CNAr (Mes2) to cuprous chloride in a 2:1 molar ratio generates the complex ClCu(CNAr (Mes2)) 2 in a straightforward manner. Single-crystal X-ray diffraction has revealed ClCu(CNAr (Mes2)) 2 to exist as a three-coordinate monomer in the solid state. As determined by solution (1)H NMR and FTIR spectroscopic studies, monomer ClCu(CNAr (Mes2)) 2 resists tight binding of a third CNAr (Mes2) unit, resulting in rapid isocyanide exchange. Contrastingly, addition of 3 equiv of CNAr (Mes2) to cuprous iodide readily affords the tris-isocyanide species, ICu(CNAr (Mes2)) 3, as determined by X-ray diffraction. Similar coordination behavior is observed in the tris-isocyanide salt [(THF)Cu(CNAr (Mes2)) 3]OTf (OTf = O 3SCF 3), which is generated upon treatment of (C 6H 6)[Cu(OTf)] 2 with 6 equiv of CNAr (Mes2) in THF. The disparate coordination behavior of the [CuCl] fragment relative to both [CuI] and [CuOTf] is rationalized in terms of structure and Lewis acidity of the Cu-containing fragments. The putative triflate species [Cu(CNAr (Mes2)) 3]OTf itself serves as a good Lewis acid and is found to weakly bind C 6H 6 in an eta (1)- C manner in the solid-state. Density Functional Theory is used to describe the bonding and energetics of the eta (1)- C Cu-C 6H 6 interaction.

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.

Blocking the pass: Low-valent Ni centers readily bind Tl(I) ions in a synthetically reversible fashion. The Tl units, in turn, serve as coordination site protection agents for Ni with respect to incoming Lewis basic ligands. This synthetic sequence allows for the isolation of a reactive zero-valent Ni tris-isocyanide complex.

It is well established that the active intermediate in reactions mediated by nickel tetracarbonyl (Ni(CO) 4 ) is the three-coordinate complex Ni(CO) 3 . 1À4 This coordinatively unsaturated binary carbonyl is formed by CO dissociation from its fourcoordinate, 18-electron precursor and is active toward ligand association 1À4 and oxidative addition 5À7 reactions. While too reactive to be isolated or observed in the condensed phase, pioneering gas-phase and matrix-isolation studies by Turner, DeKock, and Burdett succeeded in photolytically synthesizing and fully characterizing Ni(CO) 3 . 8À10 Along with these experiments, early theoretical predictions based on molecular orbital and angular overlap methods corroborated the fact that d 10 Ni(CO) 3 adopts a trigonal-planar coordination geometry. 11À13 This geometry is similar to three-coordinate, Ni(0) trisphosphine complexes (i.e., Ni(PR 3 ) 3 ), which are long known 14 and have been isolated when encumbering phosphines are employed. 15,16 Despite sharing three-coordinate, trigonal-planar geometries, Ni(CO) 3 is electronically distinct from Ni(PR 3 ) 3 complexes. This is a result of the stabilization of Ni-based, π-symmetry orbitals (e 0 and e 00 in D 3h symmetry) by back-donation to the CO ligands. Accordingly, the HOMO of D 3h -symmetric Ni(CO) 3 is d z 2 (a 1 0 ) in parentage, rather than the degenerate e 0 set (x 2 À y 2 , xy), as is widely accepted for Ni(PR 3 ) 3 complexes (Figure 1). 17 Indeed, stabilization of both the e 0 and e 00 orbital sets by the cylindrically symmetric CO π* orbitals serves to energetically isolate the Ni d z 2 orbital in Ni(CO) 3 (Figure 1) and renders the complex isolobal to σ-type Lewis bases such as phosphines and amines. In this respect, Ni(CO) 3 is also differentiated from the "planar" form of trisethylene Ni(0) (Ni(C 2 H 4 ) 3 ), 18 which possesses only the in-plane, e 0 -symmetry π-back-bonding interaction. However, the weaker π-acidity of ethylene relative to CO does not allow for an e 0 orbital Figure 1. (top) Qualitative molecular orbital diagrams for the D 3h -symmetric NiL 3 complexes Ni(C 2 H 4 ) 3 (left), Ni(PMe 3 ) 3 (center), and Ni(CO) 3 (right), reflecting the effect of π-back-donation on the d-orbital splitting pattern. (bottom) DFT-calculated HOMO for Ni(CO) 3 (left) and Ni(CN Me) 3ABSTRACT: Details are presented regarding a convenient synthesis of the nickel trisisocyanide complex Ni(CNAr Dipp2 ) 3 (Ar Dipp2 = 2,6-(2,6-[i-Pr] 2 C 6 H 3 ) 2 C 6 H 3 ). A previous synthesis of a Ni tris-isocyanide complex relied on a Tl(I) coordination-site protection strategy to discourage the formation of a tetrakis-isocyano complex. However, protectinggroup-free access to Ni(CNAr Dipp2 ) 3 is enabled by the encumbering m-terphenyl isocyanide CNAr Dipp2 . Treatment of Ni(COD) 2 with CNAr Dipp2 affords Ni(COD)-(CNAr Dipp2 ) 2 , which is readily oxidized to NiI 2 (CNAr Dipp2 ) 2 upon addition of I 2 . Reduction of NiI 2 (CNAr Dipp2 ) 2 with Mg metal generates Ni(CNAr Dipp2 ) 3 and does not require the addition of a third equivalent of C...

Abgeschirmt: Niedervalente Nickelzentren binden leicht und synthetisch reversibel Thallium(I)‐Ionen, die wiederum die Koordinationsstellen des Nickelzentrums vor angreifenden Lewis‐basischen Liganden schützen. Diese Synthesesequenz ermöglicht die Isolierung eines reaktiven, nullwertigen Nickel‐ Tris(isocyanid)‐Komplexes.