ORGANOTRANSITION-METAL METALLACARBORANES .21. TRANSITION-METAL SANDWICH COMPLEXES OF HETEROCYCLIC LIGANDS - CARBORANE-STABILIZED ETA-5-PYRROLYL AND ETA-5-PHOSPHOLYL DOUBLE-DECKER AND TRIPLE-DECKER COMPOUNDS - SYNTHESIS, STRUCTURE, AND ELECTROCHEMISTRY

This paper reports the controlled synthesis, characterization, and study of a series of pyrrolyl- and phospholyl-containing transition-metal sandwich complexes stabilized by cyclic Et2C2B3H(n)n-7 (n = 3, 5) carborane units, including the first triple-decker complexes of N- or P-containing heterocyclic ligands. The reaction of the nido-Et2C2B4H5- ion with CoCl2 and a dimethyl- or tetramethylpyrrolide ion in THF generated the sandwich complexes (eta-5-NC4Me2R2)Co(Et2C2B4H4) (1a/1b, R = Me, H) together with the corresponding ''decapitated'' species (eta-5-NC4Me2R2)Co(Et2C2B3H5) (2a/2b, R = Me, H); in each case, subsequent treatment of the mixture with TMEDA (tetramethylethylenediamine) generated pure 2a or 2b. The reaction of the cobaltacarborane anion Cp*Co(Et2C2B3H6)- with CoCl2 and a di- or tetramethylpyrrolide ion in THF yielded the dicobalt triple-deckers (eta-5-NC4Me2R2)Co(Et2C2B3H3)CoCp* (5a/5b, R = Me, H). Reaction of the cymene-ruthenium-carborane complex anion (MeC6H4CHMe2)Ru(Et2C2B3H4)- with CoCl2 and a di- or tetramethylpyrrolide ion afforded the cobalt-ruthenium triple-decker complexes (eta-5-NC4Me2R2)Co(Et2C2B3H3)Ru(eta-6-1,4-MeC6H4CHMe2) (8a/8b, R = Me, H). Treatment of the deprotonated anion of 2a with CoCl2 and NC4Me4- gave the bis(pyrrolyl) triple-decker complex (eta-5-NC4Me4)2Co2(Et2C2B3H3) (9). Although 5a reacted with CH3I to give the N-methyl cationic derivative 12, similar treatment of the dimethylpyrrolyl species 5b resulted only in iodo substitution at the central boron atom, B(5). Attempts at nucleophilic substitution at the ring nitrogen on the triple-decker pyrrolyl complexes, via reactions with alkyl halides, gave only B-halo derivatives. However, treatment of the monopyrrolyl and bis(pyrrolyl) triple-decker complexes 5a and 9 with BH3.THF formed N-BH3 and N,N'-(BH3)2 adducts. The phospholyl analogue of 1a was prepared via the reaction of the tetramethylphospholide anion with nido-Et2C2B4H5- and CoCl2, forming the desired species (eta-5-PC4Me4)Co(Et2C2B4H4) (25), which was readily converted to (eta-5-PC4Me4)Co(Et2C2B3H5) (26) via treatment with wet TMEDA. The cobaltacarborane anion Cp*Co(Et2C2B3H6)- underwent a similar reaction to give the phospholyl-containing triple-decker (eta-5-PC4Me4)-Co(Et2C2B3H3)CoCp* (27); a cobalt-ruthenium analogue, (PC4Me4)Co(Et2C2B3H3)Ru(MeC6H4CHMe2) (28), was synthesized from the (MeC6H4CHMe2)Ru(Et2C2B3H4)- anion, CoCl2, and the PC4Me4- ion. Reactions of the (eta-5-PC4Me4)Co(Et2C2B3H4)- anion (26-) with CoCl2 and with PC4Me4- or NC4Me4- ions gave, respectively, a bis(phospholyl) triple-decker, (PC4Me4)2Co2(Et2C2B3H3) (29), and a phospholyl-pyrrolyl triple-decker, (PC4Me4)Co(Et2C2B3H3)Co(NC4Me4) (30). All of the new double- and triple-decker sandwich species were isolated via column and/or plate chromatography as crystalline solids or oils and were characterized via multinuclear NMR, IR, UV-visible, and mass spectra, supported by X-ray diffraction data on 9 (to be reported elsewhere) and 30. Except for 22, 23, 27, and 28, all new complexes are air-stable. Cyclic voltammetry on 5a/5b, 8a/8b, 9, and 27-30 in dichloromethane and dimethoxyethane (DME) solutions gave reversible oxidations in both solvents and reversible 1-electron reductions in DME only; in several cases a second, irreversible oxidation was observed. The pyrrolyl and phospholyl triple-deckers exhibited higher oxidation and reduction potentials than are seen in analogous C2B3-bridged triple-decker complexes having Cp or Cp* end ligands, with the highest values observed in the case of the bis(pyrrolyl) species 9. These data are interpreted in terms of known electron-donor properties of pyrrolyl and phospholyl pi-bonded ligands. Crystal data for 30: mol wt 496.8; space group P1BAR; Z = 2; a = 9.017 (5) angstrom, b = 9.966 (7) angstrom, c = 15.115 (9) angstrom, alpha = 109.95 (5)-degrees, beta = 93.97 (4)-degrees, gamma = 71.55 (5)-degrees; V = 1210 angstrom-3; R = 0.069 for 3390 reflections having I > 2.5-sigma-(I).