Insertion of a Transient Tin Nitride into Carbon-Carbon and Boron-Carbon Bonds

A simple exchange reaction between [Ar^iPr4Sn(μ-Cl)]₂ (1) and sodium azide afforded the doubly bridged Sn(II) azide, [Ar^iPr4Sn(μ-N₃)]₂ (2) (Ar^iPr4 = C₆H₃-2,6(C₆H₃-2,6-ⁱPr₂)₂) in 85% yield. Photolysis of a diethyl ether solution of 2 for ca. 16 h yielded an azepinyl-substituted insertion product, [C₆H₃-2-(C₆H₃-2,6-ⁱPr₂)-6-(C₆H₃N-3,7-ⁱPr₂)Sn]₂ (3). The reaction of the Lewis acid, B(C₆F₅)₃ (BCF), or the Lewis base, pyridine, with 2 dissociates the dimer to afford the corresponding complexed monomeric Sn(II) azide, Ar^iPr4SnN₃BCF (4) in which BCF coordinates the α-nitrogen, or Ar^iPr4Sn(pyridine)N₃ (6) in which pyridine coordinates to the tin atom. Photolysis of 4 in diethyl ether for 12 h results in the insertion of the α-nitrogen of the azide group into one of the B-C bonds of the BCF acceptor to yield the tin(II) amide, Ar^iPr4SnN(C₆F₅)B(C₆F₅)₂ (5). In contrast, photolysis of 6 for over 36 h afforded no apparent reaction. A highly reactive Sn nitride intermediate, Ar^iPr4Sn≡N, is proposed as part of the mechanistic pathway for the formation of 3 and 5 as a result of trapping the tin-centered radical isomers. This was effected by immediate freezing the samples of 2 or 4 after ca. 30 min of UV photolysis and recording their electron paramagnetic resonance spectra. These exhibited a rhombic g tensor of [g₁, g₂, g₃] = [2.029, 1.978, 1.933]. This radical intermediate could be related to the valence isomers of the nitride [-Sn^IV≡N] intermediate, in isomeric equilibrium with the nitrene [-Sn^II-N] and nitridyl [-Sn^III═N·] forms, but with the spin density on the nitrogen being quenched, possibly by the H atom abstraction to form an S = 1/2 species of formula -Sn·═N(H).