Electronic structures of methylated azaferrocenes and their borane adducts: Photoelectron spectroscopy and electronic structure calculations

Abstract

The preparation and crystal structure of a new 2,5-dimethylazaferrocene (DMAF) adduct of borane (DMAF-BH(3)) are reported. The electronic structures of 1,2,3,4,5-pentamethylazaferrocene (PMAF), its borane adduct 1,2,3,4,5-pentamethylazaferrocene-BH(3) (PMAF-BH(3)) and of DMAF-BH(3) have been studied by HeI-HeII UV photoelectron spectroscopy (UPS) and high-level DFT methods. UPS data reveal the influence of borane and methyl substitution on the electronic structures of azaferrocenes, which have been rationalized by inductive and hyperconjugative effects. UPS data also allow for the estimation of azaferrocene lone-pair stabilization upon coordination to borane, and suggest that pyridine is a slightly stronger sigma donor toward BH(3) than either azaferrocene studied. UV-Vis spectroscopic data for the radical cation of PMAF were also obtained and compared with the UPS data.

Synthesis and Reactivity of Sterically Encumbered Diazaferrocenes

“…The instability of [(η 5 -C 4 H 4 N) 2 Fe] was attributed to η 5 –κ 1 haptotropic shifts of the pyrrolyl ligand. , An interesting observation in this context is the reaction of meso -disubstituted pyrrole with [Fe­{N­(SiMe 3 ) 2 } 2 (thf)] to form a tetranuclear iron cluster, which combines two η 5 -diazaferrocene units and two iron centers with a distorted tetrahedral κ N coordination . Because of the problems encountered with diazaferrocene, research efforts have mainly focused on azaferrocenes and the synthesis of moderately substituted derivatives such as [(η 5 -C 5 Me 5 )­Fe­(η 5 -C 4 H 4 N)] and [(η 5 -C 5 H 5 ) 2 Fe­(η 5 -2,5-Me 2 C 4 H 2 N)]. , In the early 1990s Kuhn introduced several sterically encumbered pyrrolyls, which stabilize η 5 coordination by effectively blocking the κ N coordination mode. , With tetramethylpyrrolyl the formation of a thermally unstable [(κ 1 -C 4 Me 4 N) 2 Fe­(thf) 2 ] complex was reported, which decomposed in solution. However, in the presence of small quantities of water the structurally characterized tetramethylpyrrole adduct of octamethyldiazaferrocene, [(η 5 -C 4 Me 4 N) 2 Fe]­(C 4 Me 4 NH) 2 , was isolated.…”