Coordinatively- and electronically-unsaturated square planar cobalt(iii) complexes of a pyridine dianionic pincer ligand

A series of low-valent Co(iii) square planar complexes supported by a dianionic pincer ligand bis(arylamido)pyridine ([NNN]^2-) was synthesized, including the first structurally characterized square planar hydroxide complex ([NNN]Co(OH), 4) of a 3d transition metal in the +3 oxidation state. The magnetic properties of the Co(iii) complexes were highly dependent on the charge of the complex and on the coordination environment of the metal. The diamagnetic cationic complex {[NNN]Co(py)}BF₄ (2) can be converted to neutral paramagnetic complexes [NNN]Co(OR) (R = Ph 3a, iPr 3b, and H 4) by a simple substitution of the ancillary pyridine ligand in 2. The double bond character between the metal and the anilido nitrogen was evident from short Co-N2,3 bond lengths in the solid-state structures, and further supported by density functional theory calculations. Complex 2 showed well-behaved redox processes at -0.93 and +0.70 V assigned to Co^II/III and [NNN]^2-/1- redox couples, respectively. In contrast, both complexes 3a and 4 showed some irreversibility in redox processes on either cobalt or ligands.

The electronic nature of terminal oxo ligands in transition-metal complexes: ambiphilic reactivity of oxorhenium species

The synthesis of the Lewis acid-base adducts of B(C6F5)3 and BF3 with [DAAmRe(O)(X)] DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C6F5 (X = Me, 1, COCH3, 2, Cl, 3) as well as their diamidopyridine (DAP) (DAP=(2,6-bis((mesitylamino)methyl)pyridine) analogues, [DAPRe(O)(X)] (X = Me, 4, Cl, 5, I, 6, and COCH3,7), are described. In these complexes the terminal oxo ligands act as nucleophiles. In addition we also show that stoichiometric reactions between 3 and triarylphosphine (PAr3) result in the formation of triarylphosphine oxide (OPAr3). The electronic dependence of this reaction was studied by comparing the rates of oxygen atom transfer for various para-substituted triaryl phosphines in the presence of CO. From these experiments a reaction constant ρ = -0.29 was obtained from the Hammett plot. This suggests that the oxygen atom transfer reaction is consistent with nucleophilic attack of phosphorus on an electrophilic metal oxo. To the best of our knowledge, these are the first examples of mono-oxo d(2) metal complexes in which the oxo ligand exhibits ambiphilic reactivity.