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

A simple exchange reaction between [(ArSn)-Sn-iPr4-(mu-Cl)](2) (1) and sodium azide afforded the doubly bridged Sn(II) azide, [(ArSn)-Sn-iPr4(mu-N-3)](2) (2) (Ar-iPr4 = C6H3-2,6(C6H3-2,6-Pr-i(2))(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-Pr-i(2))-6-(C6H3N-3,7-Pr-i(2))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, (ArSnN3BCF)-Sn-iPr4 (4) in which BCF coordinates the alpha-nitrogen, or (ArSn)-Sn-iPr4-(pyridine)N-3 (6) in which pyridine coordinates to the tin atom. Photolysis of 4 in diethyl ether for 12 h results in the insertion of the alpha-nitrogen of the azide group into one of the B-C bonds of the BCF acceptor to yield the tin(II) amide, (ArSnN)-Sn-iPr4(C6F3)B(C6F3)(2) (5). In contrast, photolysis of 6 for over 36 h afforded no apparent reaction. A highly reactive Sn nitride intermediate, (ArSn)-Sn-iPr4 equivalent to 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 [g(1), g(2), g(3)] = [2.029, 1.978, 1.933]. This radical intermediate could be related to the valence isomers of the nitride [-Sn-IV equivalent to N] intermediate, in isomeric equilibrium with the nitrene [-Sn-II-N] and nitridyl [-Sn-III=N center dot] 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 center dot=N(H).