Complexes obtained by electrophilic attack on a dinitrogen-derived terminal molybdenum nitride: electronic structure analysis by solid state CP/MAS 15N NMR in combination with DFT calculations

Abstract

15N Solid state CP/MAS NMR spectroscopy has been used to study a dinitrogen-derived terminal nitride of molybdenum, 15NMo(N[ t Bu]Ar) 3 (Ar = 3,5-(CH 3) 2C 6H 3). A number of Lewis acid adducts, including X 3E–NMo(N[ t Bu]Ar) 3 (X = F, E = B; X = Cl, E = B, Al, Ga, In; X = Br, E = Al; X = I, E = Al) and Cl 2E–NMo(N[ t Bu]Ar) 3 (E = Ge, Sn), were prepared by the combination of 15NMo(N[ t Bu]Ar) 3 with 1 equiv. of Lewis acid. A series of cationic imido complexes, [RNMo(N[ t Bu]Ar) 3]X was prepared by the reaction of electrophiles, RX [R = CH 3, X = I; R = PhC(O) or Me 3Si, X = OTf (OTf = SO 3CF 3)], with NMo(N[ t Bu]Ar) 3. Deprotonation of [CH 3NMo(N[ t Bu]Ar) 3]I by LiN(SiMe 3) 2 afforded the ketimide complex H 2CNMo(N[ t Bu]Ar) 3, which has been shown to undergo a reaction with neat CH 3I to form [CH 3CH 2NMo(N[ t Bu]Ar) 3]I. 15N solid state CP/MAS NMR spectroscopy was employed in the characterization of each complex. Complementary density functional theory (DFT) studies of 15NMo(N[ t Bu]Ar) 3 and derivatives enabled a detailed examination of the experimental solid state NMR parameters in terms of electronic structure at the labeled N-atom. Computational analysis demonstrated that significant paramagnetic contributions to the perpendicular components of the chemical shielding tensor ( δ 11 and δ 22) were responsible for the huge span of the tensor measured for 15NMo(N[ t Bu]Ar) 3 ( Ω = 1187 ppm). Perturbation of the electronic structure in 15NMo(N[ t Bu]Ar) 3 upon coordination of a Lewis acid or formation of a cationic imido complex was attributed to stabilization of a σ-symmetric orbital. An upfield shift in the perpendicular components of the chemical shift tensor results from the reduced paramagnetic contribution to these tensor components upon increasing the energy gap between the magnetically coupled occupied and virtual orbitals ( e occ − e vir).