CH-NH Tautomerism in the Products of the Reactions of the Methanide [HC(PPh2NSiMe3)2]- with Pnictogen and Tellurium Iodides

The reactions of K[HC(PPh2NSiMe3)(2)] (K[1]) and MI3 (M = As, Sb) or TeI4 gave as the major products the complexes [{MIn}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] (MIn = trans-AsI2, trans- or cis-SbI2, TeI3), which feature the ligand [C(PPh2NSiMe3)(PPh2NHSiMe3)](-) (2). This anion, the NH tautomer of [HC(PPh2NSiMe3)(2)](-), is formally generated by a 1,3-hydrogen shift to give complexes in which the ligand is N,C-chelated to the main-group-metalloid center. The M-C distances are slightly longer than the sum of the covalent radii for M and C in the case of the group IS metalloids but significantly shorter for M = Te. The arsenic derivative [{t-AsI2}2] is monomeric in the solid state, with As I distances that differ by ca. 0.55 angstrom, whereas the antimony analogues [{t-SbI2}2] and [{c-SbI2}2] are dimeric through bridging iodide ligands and the disparity in Sb-I distances of the SbI2 units is 0.10 and 0.33 angstrom, respectively. The tellurium derivative [{TeI3}2] is monomeric with a distorted-square-pyramidal geometry at the Te center and Te-I distances in the narrow range 2.9142(4)-3.0337(4) angstrom. In contrast to the lighter pnictogens, the bismuth complex [{t-BiI2}1] is comprised of the methanide 1 coordinated in a tridentate (N,C,N) mode to a BiI2+ cation. In the case of arsenic triiodide, the metathesis is accompanied by Si-N bond cleavage to give [{AsI2}{CH2(PPh2N)(PPh2NSiMe3)}] (3), which was characterized by P-31 NMR spectroscopy, and conversion to the corresponding salt [{AsI}{CH2(PPh2N)(PPh2NSiMe3)}][SbF6] (3A) by treatment with AgSbF6. The As-N distances in the six-membered CP2N2As ring in 3A differ by 0.20 angstrom due to the different coordination numbers (2 and 3) of the two N atoms in the novel N,N'-chelated [CH2(PPh2N)(PPh2NSiMe3)](-) anion. In contrast, reaction of [{t-SbI2}2] with AgSbF6 gives the expected salt as the dimer {[{SbI}2][SbF6]}(2). The two hydrolysis products [CH2(PPh2NSiMe3)(PPh2NHSiMe3)][SbF6] (4A) and [CH2(PPh2NSiMe3) (PPh2NHSiMe3)](2)[Te2I6] (4B) were also structurally characterized and shown to contain the same cation. DFT calculations indicate that the N-H tautomer 2 is stabilized by strong M-N and M-C bonding interactions which include a small degree of pi character. Weaker bonds, as in the Bi complex, favor the C-H tautomer 1 as the ligand.