C2B3 AND C2B4 CARBORANE LIGANDS AS CYCLOPENTADIENYL ANALOGS - EARLY TRANSITION-METAL COMPLEXES

This paper reports the directed synthesis, characterization, and react ivity of a series of tantalum, niobium, and zirconium sandwich complex es incorporating small carborane or cobaltacarborane ligands, centered on the development of suitable families of reagents for eventual appl ication to organic synthesis. Complexes of the types (R(2)(1)C(2)B(4)H -4)MCl(2)Cp' and (Et(2)C(2)B(4)H-4)ZrCl . THFCp' (R(1) = Et, SiMe(3), or Me; Cp' = C5H5 or C(5)Me(5); M = Ta, Nb) were prepared from Cp'MCl( n) reagents (M = Ta, Nb, Zr) and the R(2)(1)C(2)B(4)H(5)(-) monoanion in THF. Similar treatment of the Cp()Co(Et(2)C(2)B(3)H(4))(-) cobalta carborane anion (Cp() = C(5)Me(5)) afforded the bent triple-decker sa ndwich complexes [Cp()Co(Et(2)C(2)B(3)H(3))]MCl(2)Cp' (M = Ta, Nb). B oth families of compounds were obtained generally in high yield as air -stable crystalline solids that are readily soluble in organic solvent s. In the Ta and Nb species, replacement of one or both chlorines with a variety of alkyl groups was effected via reactions with alkylating agents to generate (R(2)(1)C(2)B(4)H(4))Ta(L)ClCp' or (R(2)(1)C(2)B(4) H(4))ML(2)Cp' (M = Ta, Nb), and the corresponding alkylated triple-dec kers [CpCo(Et(2)C(2)B(3)H(3))]Ta(L)ClCp' and [Cp(*)Co(Et(2)C(2)B(3)H( 3))]-TaL(2)Cp' (L = Me, Et, Ph, CH(2)Ph, CH(2)CMe(3), or OPh). Yields of the mono- and dialkyl derivatives ranged from moderate to quantitat ive. The new complexes were characterized via H-1, C-13, and B-11 NMR, mass spectrometry, and elemental analysis supplemented by FTIR and UV -visible spectroscopic data for many compounds, electrochemical studie s on selected species, and crystal structure determinations on seven p roducts. Exploratory studies of the reactivities of these complexes re vealed significant differences from those of standard organometallic s pecies such as Cp(2)TiCl(2) or Cp(2)ZrR(2). Thus, tantalum and niobium C2B4 dichloro complexes on treatment with Al(2)Me(6) gave dimethyl de rivatives rather than methylidene compounds. The reaction of (Et(2)C(2 )B(4)H(4))TaMe(2)Cp with excess HBF4 in acetonitrile formed a single i solable product identified as a difluoro derivative, (Et(2)C(2)B(4)H(4 ))TaF(2)Cp. X-ray crystal structures were obtained for (Me(3)Si)(2)C2B 4H4]TaCl(2)Cp (1b), (Et(2)C(2)B(4)H(4))(TaCl(2)Cp() (1c), [Cp(*)Co(Et (2)C(2)B(3)H(3))]TaCl(2)Cp (4a), (Et(2)C(2)B(4)H(4))TaPh(2)Cp (6d), Cp ()Co(Et(2)C(2)B(3)H(3))TaMe(2)Cp (7b), Cp*Co(Et(2)C(2)B(3)H(3))Ta(CH( 2)Ph)ClCp (7c), and (Et(2)C(2)B(4)H(4))NbMe(2)Cp (8a). Crystal data fo r 1b: space group P2(1)/a; Z = 4; a = 14.292(4) Angstrom, b = 9.008(2) Angstrom, c = 17. 899(7) Angstrom, beta = 112.61(2)degrees; R = 0.043 for 2854 independent reflections. For 1c: space group P2(1)/c; Z = 4; a = 8.650(2) Angstrom, b = 12.362(5) Angstrom, c = 18.601(7) Angstrom , beta = 90.10(3)degrees R = 0.038 for 1831 independent reflections. F or 4a: space group P2(1)/n; Z = 4; a = 8.874(2) Angstrom, b = 14.303(4 ) Angstrom, c = 18.585(6) Angstrom, beta = 91.53(2)degrees; R = 0.036 for 3033 independent reflections. For 6d: space group P (1) over bar; Z = 2; a = 8.943(1) Angstrom, b = 15.726(2) Angstrom, c = 7.843(2) Ang strom, alpha = 90.58(2)degrees; beta = 102.78(2)degrees; gamma = 103.5 3(1)degrees; R = 0.024 for 3376 independent reflections. For 7b: space group P2(1)/n; Z = 4; a = 8.998(2) Angstrom, b = 14.374(2) Angstrom, c = 18.508(3) Angstrom, beta = 92.98(2)degrees; R = 0.027 for 3169 ind ependent reflections. For 7c: space group P2(1)/n; Z = 4; a = 12.780(2 ) Angstrom, b = 16.084(2) Angstrom, c = 13.442(2) Angstrom, beta = 104 .16(1)degrees; R = 0.030 for 3684 independent reflections. For 8a: spa ce group P2(1)/c; Z = 4; 14.148(3) Angstrom, b = 7.781(5) Angstrom, c = 15.315(2) Angstrom, beta = 116.32(1)degrees; R = 0.031 for 2297 inde pendent reflections.