FACILE CONVERSION OF ORGANOMETALLIC HALIDE-COMPLEXES TO HALOMETHYL DERIVATIVES - SYNTHESIS, STRUCTURE, AND REACTIVITY OF THE (ETA-5-C5R5)CR(NO)2CH2X SERIES (R = H, CH3, X = CL, BR, I, OCH3, OCH2CH3, PPH3, CN, SO3C6H4CH3)

Treatment of (η5-C5R5)Cr(NO)2X with ethereal diazomethane in the presence of Cu powder provides access to the new (η5-C5R5)Cr(NO)2CH2X derivatives in 88-93% yield (R = H, CH3; X = Cl, Br). In contrast to the chemistry of the isoelectronic/isostructural (η5-C5H5)Fe(CO)2CH2X systems, the CH2 of the halomethyl ligand in the (η5-C5H5)Cr(NO)2CH2Cl complex undergoes a remarkably facile migration into a C‒H bond of the coordinated η5-C5H5 ring upon abstraction of the α-halide, giving the (η5-C5H4Me)Cr(NO)2+ cation in >80% yield. Deuterium labeling shows the process to be intramolecular. Methylene migration is attributed to the extremely electrophilic nature of the Cr-alkylidene species generated upon halide abstraction. Intermolecular CH2 transfer is only observed when η5-C5Me5 derivatives are treated with Ag+ in the presence of excess cyclohexene. The stable iodomethyl derivatives are obtained in high yield by simple Nal/THF metathesis from the chloromethyl complexes. The (η5-C5Me5)Cr(NO)2CH2I complex has been characterized by single-crystal X-ray diffraction methods: monoclinic space group P21/a, a = 10.366 (3) Å, b = 10.974 (2) Å, c = 12.818 (3) Å, β = 90.81 (2)°, Z = 4, final R/Rw = 7.47%/6.86%. The iodomethyl carbon‒Cr distance of 2.093 (11) Å can be considered short for a Cr0-C single bond. The halomethyl halide is irreversibly displaced by Y- (Y- = cyanide, tosylate, and alkoxides) to give stable (η5-C5R5)Cr(NO)2CH2Y derivatives. Reaction of (η5-C5H5)Cr(NO)2CH2I with PPh3 followed by NaBPh4 gives the cationic ylide (η5-C5H5)Cr(NO)2CH2PPn3+BPh4-, which has been characterized by X-ray crystallography: monoclinic space group P21/c, a = 10.038 (2) Å, b = 23.587 (4) Å, c = 17.150 (3) Å, β= 102.35 (1)°, Z = 4, final R/Rw = 4.81%/5.427o. A C‒P bond distance of 1.748 (3) Å indicates considerable ylide character for the CH2PPh3 ligand; the IR and NMR data show the CH2PPh3 ligand to be a stronger donor than the CH2I ligand. The Cr‒C distance for the CH2PPh3 ligand is 2.115 (3) Å in this case. © 1990, American Chemical Society. All rights reserved.