Metal-Ligand Multiple Bonds in High-Spin Complexes

Abstract

The chemistry of late first row transition metals supported by dipyrromethane and dipyrromethene ligands bearing sterically bulky substituents was explored. Transition metal complexes (Mn, Fe, Co, Ni, Zn) of the dipyrromethane ligand 1,9-dimesityl-5,5-dimethyldipyrromethane (dpma) were prepared. Structural and magnetic characterization (SQUID, EPR) of the bis-pyridine adducts \((dpma)Mn(py)_2\), \((dpma)Fe(py)_2\), and \((dpma)Co(py)_2\) showed each tetrahedral divalent ion to be high-spin, while square planar \((dpma)Ni^{II}(py)_2\) and tetrahedral \((dpma)Zn(py)_2\) were shown to be diamagnetic. Electrochemical experiments revealed oxidative events at common potentials independent of metal identity or spin state, consistent with ligand-based oxidation. Dipyrromethene ligand scaffolds were synthesized bearing large aryl \((Ar = 2,4,6-Ph_{3}C_{6}H_{2}, Mes = 2,4,6-Me_{3}C_{6}H_{2})\) or alkyl \((^{t}Bu = CMe_3, Ad = 1-adamantyl)\) flanking groups to afford three new disubstituted ligands \((^{R}dpme, 1, 9-R_2-5-mesityldipyrromethene, R = Ar, Mes, ^{t}Bu, Ad)\). While high-spin \((S=2)\), four-coordinate iron complexes of the type \((^{R}dpme)FeCl(solv)\) were obtained when R was Mes, tBu, or Ad, use of the sterically encumbered aryl-substituted ligand gave a three-coordinate high-spin \((S=2)\) complex \((^{Ar}dpme)FeCl\). Intramolecular C−H amination was discovered in the reaction of organic azides with \((^{Mes}dpme)FeCl(thf)\), though no intermediate was observed by UV/Vis, IR, or \(^{1}H\) VT-NMR experiments. Reaction of \((^{Ad}dpme)FeCl(OEt_2)\) with alkyl azides resulted in the catalytic amination of C–H bonds or aziridination of olefins at room temperature. Reaction of \(p-^{t}BuC_{6}H_{4}N_{3}\) with \((^{Ar}dpme)FeCl\) permitted isolation of a high-spin \((S=2)\) iron complex \((^{Ar}dpme)FeCl(N(p-^{t}BuC_6H_4))\), featuring a terminal imidyl radical antiferromagnetically coupled to high-spin \(Fe^{III}\), as determined by \(^{1}H\) NMR, X-ray crystallography, and \(^{57}Fe\) Mössbauer. A three-coordinate CoI complex \((^{Ar}dpme)Co(py)\) was synthesized and characterized by \(^{1}H\) NMR, SQUID magnetometry, and X-ray crystallography. Reaction of \((^{Ar}dpme)Co(py)\) with \(^{t}BuN_3\) afforded an isolable three-coordinate Co imide complex \((^{Ar}dpme)Co(N^{t}Bu)\) that exhibits spin crossover from a singlet to a quintet. Reaction of \((^{Ar}dpme)Co(py)\) with mesityl azide produces a spectroscopically observed intermediate, consistent with an \(S=1\) terminal imide complex, that converted via benzylic C–H activation into the metallacycloindoline \((^{Ar}dome)Co(\kappa^{2}-NHC_{6}H_{2}-2,4-Me_{2}-6-CH_2)\).