Syntheses and structures of cyclopentadienyl arsenic compounds .2. Pentamethyl- and tetraisopropylcyclopentadienyl arsenic amido derivatives

A series of amido derivatives of arsenic cyclopentadienyls (Cp'AsNR)(n) (4-6) (4: Cp' = C5Me5, R = H, n = 4; 5: Cp' = C5Me5, R = Me; n = 2; 6: Cp' = C(5)i-Pr4H, R = Me, n = 2) has been synthesized by the reaction of Cp'AsX2 (1-2) (1: Cp' = C5Me5, X = Cl; 2: Cp' = C(5)i-Pr4H, X = I) with an excess of amine. The reaction proceeds via a diamido substituted intermediate which eliminates in vacuum one equivalent of free amine to give in situ imino arsanes which rapidly oligomerize to octa-or tetracyclic compounds in almost quantitative yield. In the case of bulky amines, viz. t-BuNH2 and (Me3Si)(2)NH, monoamido substituted arsanes Cp*AsCl((NRR2)-R-1) (7-8) (7: R-1 = H, R-2 = t-Bu; 8: R-1 = R-2 = SiMe3) have been obtained by treatment of I with an excess of t-BuNH2 or with one equivalent of NaN(SiMe3)(2). The fluorine substituted analogue of 8, Cp*AsF[N(SiMe3)(2)] (9), has been synthesized either by reaction of one equivalent of NaN(SiMe3)(2) with Cp*AsF2 (3) or by a substitution reaction between 8 and Cp2CoF in moderate yields. 7 reacts with strong bases, e.g. Li(Na)N(SiMe3)(2) or Me3SnNEt2, giving an imino arsane as an intermediate which quickly dimerizes to diazadiarsetane (Cp*AsNt-Bu)(2) (10). The reaction of 1 with Ph2C=NNH2 in the presence of Et3N as a base gives the disubstituted hydrazonato arsane Cp*As(NHN=CPh2)(2) (11), independent of the reagent ratio. All new compounds were characterized by spectroscopic methods (H-1, (CNMR)-C-13, MS) and elemental analyses. The crystal structures of 4-8 have been determined by X-ray diffraction methods. Bonding of the arsenic fragment to the cyclopentadienyl ligand can be described as a primary sigma-interaction with an additional pi-interaction between the cyclopentadienyl ligand and the arsenic atom, resulting in pseudo-eta(2) to eta(3)-coordination. Short intramolecular As-As contacts are found for 5 and 6. (C) 1997 Elsevier Science S.A.