The chemistry of transition metal complexes containing group 16 donors has drawn considerable interest due to its relevance to metal-centered oxygen-transfer reactions in biological systems, metal-catalyzed oxidation and hydrodesulfurization processes for industrial catalysts, and the synthesis of high oxidation state organometallics. In particular, cyclopentadienyl ligands, among others, have been used very often as ancillary ligands because of their ability to stabilize both low and high formal oxidation states. On the other hand, the dicarbollide anion $[nido-7,8-C_2B_9H_{11}]^{2-}$, which is isolobal with the $η^5$-cyclopentadienyl ligands, has not been employed in synthesizing metal complexes containing group 16 donors in spite of its known ability to stabilize higher formal oxidation states in metallacarborane complexes. Consequently, a systematic synthetic search for metallacarboranes in high oxidation states was undertaken.
The oxidative decarbonylation reactions of $[C_2B_9H_{11})Mo(CO)_3]_{2-}$(1) with tetraethylthiuram disulfide, aldrithiol, elemental sulfur, and phenyl disulfide gave oxidized monomeric products $[(C_2B_9H_{11})Mo^{II}(CO)_2X_2]^{2-or 1-} (X_2 = S_2CNEt_2$ (2), 2-pyridine thiolate (3), 2SPh(4), $S_2$), respectively. The further oxidation of 4 with iodosylbenzene (PhIO) resulted in two new oxomolybdacarboranes $[(η^1-C_2B_9H_{11})Mo^{VI}(O)_3]^{2-}$(5) and $[(C_2B_9H_{11})O_2Mo^{VI}(μ-O)Mo^{VI} O_2(C_2B_9H_{11})]^{2-}$(6). In contrast to the stepwise conversion of 1 to 5 and 6 via 4, direct treatment of 1 with PhIO provided 5 as well as two new products $[(C_2B_9H_{11}Mo^VO(μ-O))_2^{2-}$(7) and $[(C_2B_9H_{11})Mo^{II}(CO)_2(η^2-CO_3)]^{2-}$ (8). Interestingly, The oxidation of 1 with trimethylamine N-oxide generated another useful synthon dimer $[(C_2B_9H_{11}) Mo(CO)_3]_2^{2-}$ which in turn gave a dinuclear linear complex 6 in the presence of PhIO. Furthermore, in the course of investigating the reactivities of these oxomolybdacarboranes, the halogenation of 5 and 7 with $Me_3SiCl$ gave potential synthons $[(C_2B_9H_{11})MoO_2Cl]^-$(9) and $[(C_2B_9H_{11})MoOCl_2]^-$(10), respectively. On the other hand, the oxidation of 4 with phenyl disulfide caused complete decarbonylation and carbon-carbon bond cleavage of the carborane cage, yielding $[(C_2B_9H_{11})Mo^Ⅳ(μ-SPh)_2]_2^{2-}$(11). The compound 11 showed two reversible oxidation potentials in cyclic voltammogram and one electron oxidation of 11, effected by use of PhIO, $Br_2$ or silver acetate, was accompanied by concomitant reformation of the carbon-carbon bond of the carborane cages, yielding a mixed valence species $[(C_2B_9H_{11})Mo^Ⅲ(μ-SPh)_4Mo^Ⅳ (C_2B_9H_{11})]^-$ (12). Interestingly, the synthon dimer $[(C_2B_9H_{11})Mo(CO)_3]_2^{2-}$ which gave sulfido analogue of 7, $[(C_2B_9H_{11})Mo^VS(μ-S)]_2^{2-}$(13) in the presence of elemental sulfur. Two terminal sulfido groups of 13 can be substituted by oxo groups with the use of excess PhIO, yielding $[(C_2B_9H_{11})Mo^VS(μ-O)]_2^{2-}$(14).
As part of our ongoing efforts in pursuing the exploitation of the high oxidation state metallacarboranes, the tungsten analogues of 5 and 6, $[(η^1-C_2B_9H_{11})W^Ⅵ(O)_3]^{2-}$(16) and $[(C_2B_9H_{11})O_2W^Ⅵ(μ-O)W^{VI}O_2(C_2B_9H_{11})]^{2-}$(17) have been also synthesized, respectively.
As another part of investigation, the metallacarborane analogues of the cyclopentadienyliron compounds which are among the most widely studied species in organometallic chemistry, particularly ferracarboranes containing sulfur donors, have been studied. The reactions of $[(C_2B_9H_{11})Fe^I(CO)_2]_2^{2-}$(18) with tetraethylthiuram disulfide, phenyl disulfide and iodine gave oxidized monomeric products $[(C_2B_9H_{11})Fe^Ⅱ(CO) (η^2-S_2CNEt_2)]^-$ (19), $[(C_2B_9H_{11})Fe^Ⅱ(CO)_2(X)]^-$ (X = SPh (20) and I (21)), respectively. In addition, through the reactions of the compound 21 with several nucleophiles via the method of salt-elemination, the compound 21 converted to the new ferracarboranes $[(C_2B_9H_{11})Fe^Ⅱ(9S3)]$ (22) and $[(C_2B_9H_{11})Fe^Ⅱ(CO)_2(SC_4H_8)]$ (23).
Rational synthesis, molecular structures and reactivities of the foregoing new metallacarboranes were discussed along with their physical properties.
