PREPARATION, STRUCTURE, AND DIVERGENT FLUXIONAL BEHAVIOR OF CATIONIC DINUCLEAR IRON ACETYLIDES [FPSTAR2(C=C-H)]BF4, [FPSTAR2(C=C-PH)]BF4

Cationic diiron μ-acetylide complexes [FP2(C≡C.R)]BF4 [2a, FP2 = Fp2, R = H; 2b, FP2 = Fp*2, R = Ph; 2c, FP2 = Fp*Fp, R = Ph; 2d, Fp2, R = Ph; FP = (η5-C5Me5)Fe(CO)2 (Fp*), (η5-C5H5)Fe(CO)2 (Fp)] are prepared by the ligand-exchange reaction of [FP+(THF)]BF4 with FP—C≡C—R (1a, FP = Fp*, R = H; 1b, FP = Fp*, R = Ph; 1c, FP = Fp, R = Ph). Molecular structures of 1a, 1b, 2a, and 2b were determined by single-crystal X-ray diffraction studies. Spectroscopic analyses of 2 reveal the dominant contribution of a π-complex resonance form, (η2-M—C≡C—R)M+ (A), as well as fluxional behavior. For [Fp*2(C≡C—H)]BF4 (2a) the13C NMR absorptions of the two ethynyl carbon atoms, distinctively observed at lower temperature (<−60 °C), emerge as a single absorption with the averaged δ and JCH values for the α and β carbons in the limiting spectrum accompanied by coalescence of the Fp* signals at 50 °C. The H atom on the ethynyl ligand moves between the two ethynyl carbon atoms faster than the NMR time scale at higher temperature. On the other hand, [FP2(C≡C—Ph)]BF4 (2b-d) does not show any notable changes over the range of −80 to +30 °C except for the coalescence of the FP signals. The transition states of these processes correspond to the limiting resonance structures [inline formula omitted] for 2a and M—C(M)=C+—Ph (C) for 2b-d. As is evident from the molecular structure of 2a and 2b, the π-bonded metal atom lies closer to the β-carbon atom of the acetylide part than the α-carbon atom, in contrast to the previously reported μ-acetylide complexes. The structural feature arises from the additional contribution of a vinylidene complex resonance form, M+=C=C(M)R (B), as well as the absence of structural constraints owing to bridging ligands or a metal-metal bond. Thus, spectroscopic and crystallographic analyses reveal the contribution of B in addition to the dominant contribution of A, and through analyses of dynamic processes the contribution of C or D becomes apparent. The present work suggests that a 1,2-H shift mechanism similar to that proposed for the fluxional process of 2a operates for the formation of a vinylidene complex from a terminal acetylene and a metal complex. © 1990, American Chemical Society. All rights reserved.