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

A simple exchange reaction between [Ar^iPr4Sn­(μ-Cl)]₂ (<b>1</b>) and sodium azide afforded the doubly bridged Sn­(II) azide, [Ar^iPr4Sn­(μ-N₃)]₂ (<b>2</b>) (Ar^iPr4 = C₆H₃-2,6­(C₆H₃-2,6-^<i>i</i>Pr₂)₂) in 85% yield. Photolysis of a diethyl ether solution of <b>2</b> for ca. 16 h yielded an azepinyl-substituted insertion product, [C₆H₃-2-(C₆H₃-2,6-^<i>i</i>Pr₂)-6-(C₆H₃N-3,7-^<i>i</i>Pr₂)­Sn]₂ (<b>3</b>). The reaction of the Lewis acid, B­(C₆F₅)₃ (BCF), or the Lewis base, pyridine, with <b>2</b> dissociates the dimer to afford the corresponding complexed monomeric Sn­(II) azide, Ar^iPr4SnN₃BCF (<b>4</b>) in which BCF coordinates the α-nitrogen, or Ar^iPr4Sn­(pyridine)­N₃ (<b>6</b>) in which pyridine coordinates to the tin atom. Photolysis of <b>4</b> 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 (<b>5</b>). In contrast, photolysis of <b>6</b> 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 <b>3</b> and <b>5</b> as a result of trapping the tin-centered radical isomers. This was effected by immediate freezing the samples of <b>2</b> or <b>4</b> after ca. 30 min of UV photolysis and recording their electron paramagnetic resonance spectra. These exhibited a rhombic <b>g</b> tensor of [<i>g</i>₁, <i>g</i>₂, <i>g</i>₃] = [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 <i>S</i> = 1/2 species of formula −Sn·N­(H)