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Free and Metal‐Coordinated ( N ‐Isocyanimino)triphenylphosphorane: X‐ray Structures and Selected Reactions
Author(s) -
Stolzenberg Heribert,
Weinberger Bernd,
Fehlhammer Wolf Peter,
Pühlhofer Frank G.,
Weiss Robert
Publication year - 2005
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200500196
Subject(s) - chemistry , isomerization , bond length , lewis acids and bases , isocyanide , stereochemistry , metal , crystallography , medicinal chemistry , trans effect , crystal structure , catalysis , organic chemistry
An improved procedure for the synthesis of ( N ‐isocyanimino)triphenylphosphorane, C≡N–N=PPh 3 ( 3 ), is described. The X‐ray structure analysis reveals an unusually small N–N=P angle [115.2(2)°] and an N–N bond order of only about 1.5, which indicates considerable C≡N–N – –P + participation and electronically more‐isolated functional groups (CN, P=N) in the isocyanide than, for example, in the isomeric N≡C–N=PPh 3 ( 4 ) [C–N=P = 123.0(4)°, C–N bond order = 2.0]. In order to gain insight into the stereochemical consequences of metal coordination of 3 , an X‐ray structural study of [Cr(CO) 5 C≡N–N=PPh 3 ] ( 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)°] further decreased by 15σ, the elongated (by 7σ) P–N bond, the somewhat shortened (by 4σ) 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 Brønstedt and Lewis acids such as CF 3 COOH, HCl, COS, phosgene or, most efficiently, [PdCl 2 (1,5‐COD)] causes CN→NC isomerisation to give [M(CO) 5 N≡C–N=PPh 3 ] [M = Cr ( 6 ), W ( 7 )]. In solution, [PdCl 2 (CNNPh 3 ) 2 ] and Ph 3 BCNNPPh 3 ( 8 ) slowly isomerize even without additional acid to give both free and Pd‐coordinated 4 and Ph 3 BNCNPPh 3 ( 9 ), respectively. In the presence of catalytic amounts of [PdCl 2 (1,5‐COD)], 3 is converted into 4 and the dimer Ph 3 PN–C(CN)=N–NPPh 3 ( 10 ) in an almost 1:1 ratio. The optimised geometries of the methyl derivatives of 3 and 4 , namely Me 3 P=N–N≡C ( 3c ) and Me 3 P=N–C≡N ( 4c ), are in excellent agreement with the experimental data; major differences between the isomers (P–N–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 π‐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≡C A ( N A – C A )] cyclic transition state. The overall process is exothermic by 50 kcal mol –1 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)