Free and metal-coordinated (N-isocyanimino)triphenylphosphorane:: X-ray structures and selected reactions

An improved procedure for the synthesis of (N-isocyanimino)triphenylphosphorane, C equivalent to N-N=PPh3 (3), is described. The X-ray structure analysis reveals an unusually small N-N=P angle [115.2(2)degrees] and an N-N bond order of only about 1.5, which indicates considerable C equivalent to N-N--P+ participation and electronically more-isolated functional groups (CN, P=N) in the isocyanide than, for example, in the isomeric N equivalent to CN=PPh3 (4) [C-N=P = 123.0(4)degrees, C-N bond order = 2.0]. In order to gain insight into the stereochernical consequences of metal coordination of 3, an X-ray structural study of [Cr-(CO)(5)C equivalent to N-N=PPh3] (5) was also undertaken. Surprisingly, the central bond lengths (C-N, N-N) and angles (C-N-N) remain practically unchanged with noticeable coordination effects occurring only at the periphery of 5, with the N-N-P angle [112.3(2)degrees] further decreased by 15 sigma, the elongated (by 7 sigma) P-N bond, the somewhat shortened (by 4 sigma) P-C(Ph) bonds and even shorter C-H(Ph) bonds on the one side, and the well-known Cr-C(O)(trans) contraction on the other. Treatment of 5 or its tungsten derivative with anhydrous Bronstedt and Lewis acids such as CF3COOH, HCl, COS, phosgene or, most efficiently, [PdCl2(1,5-COD)] causes CN -> NC isomerisation to give [M(CO)(5)N equivalent to C-N=PPh3] [M = 6 (6), W (7)]. In solution, [PdCl2(CNNPh3)2] and Ph3BCNNPPh3 (8) Slowly isomerize even without additional acid to give both free and Pd-coordinated 4 and Ph3BNCNPPh3 (9), respectively. In the presence of catalytic amounts of [PdCl2(1,5-COD)], 3 is converted into 4 and the dimer Ph3PN-C(CN)=N-NPPh3 (10) in an almost 1:1 ratio. The optimised geometries of the methyl derivatives of 3 and 4, namely Me3P=N-N equivalent to C (3c) and Me3P=N-C equivalent to N (4c), are in excellent agreement with the experimental data; major differences between the isomers (PN-N angle, N-N bond length) are explained by the higher electronegativity of the isocyano group as compared to the CN substituent, which, in turn, is a better pi-acceptor). The reaction path of the isomerisation of 3 to 4 (3c to 4c) has also been studied computationally and been found to proceed via an [(P)=N-A-N equivalent to C-A(N-A-C-A)] cyclic transition state. The overall process is exothermic by 50 kcal mol(-1). ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).