An Unusual Conversion of Paramagnetic [3-Cl-3,3,8-{Ph2P(CH2)nPPh-µ-(H8] (n = 3 and 4) to Form the First 18-Electron P-Phenylene ortho-Cycloboronated closo-Ruthenacarboranes with a Dioxygen Ligand

Treatment of [3-Cl-3,3,8-[Ph2P(CH2)nPPh-µ-(H8] (1, n = 3; 2, n = 4) with an excess of KOH in a 1:1 benzene/methanol mixture at room temperature in air affords new dioxygen closo-ruthenacarborane complexes [3-(η2-O2)-3,3,8-[Ph2P(CH2)nPPh-µ-(H8] (3, n = 3) and (4, n = 4) in 42.5% and 45.8% yield respectively. The structures of dioxygen complexes 3 and 4 were established by single-crystal X-ray diffraction. The IR and multinuclear NMR data [1H, 13C{1H}, 31P{1H} and 11B{1H}] along with 2D HSQC correlation spectra for the new dioxygen closo-ruthenacarboranes are discussed.


Introduction
Boron-containing transition metal clusters exhibiting ortho-cycloboronation of the phosphine or diphosphine ligands on a metal atom are long known in metallaborane [1][2][3] and monocarbon OPEN ACCESS metallacarborane [4,5] chemistry. However, first examples of related 12-vertex dicarbon metallacarborane complexes were reported only recently [6][7][8][9][10][11], and all known complexes of these series are Ru III species having a 17-electron shell. In this regard it is of interest that these paramagnetic ortho-cycloboronated metallacarborane systems demonstrate the exceptionally high efficiency as catalyst precursors for the atom transfer radical polymerization (ATRP) of methyl methacrylate, as well as of other vinyl monomers [7][8][9][10][11][12][13][14]. On the other hand, such electron-deficient complexes hold considerable promise in a variety of ways for the development of ruthenacarborane chemistry. We now report an initial study on the reactivity of paramagnetic ortho-phenylenecycloboronated closo-ruthenacarboranes resulting in preparation of a short series of new dioxygen ruthenium-carborane complexes.

Results and Discussion
Polyhedral contraction reactions of transition metals metallacarboranes occurring under base-mediated conditions to produce smaller metallacarborane species have not been extensively reported [15][16][17][18]. Hence, we proposed to extend studies in this area starting from the previously untested ortho-cycloboronated P-phenylene paramagnetic species 3, (2); n = 4 [9]) with the hope of preparing either ortho-cycloboronated 11-vertex {RuC 2 B 8 } complexes or smaller clusters of these series. Accordingly, we examined the reaction of 1 and 2 with a fourfold molar excess of KOH at room temperature in a 1:1 benzene/methanol mixture in air.

Scheme 1.
Schematic representation of the reactions of 1 and 2 with an oxygen.
Single-crystal X-ray diffraction studies were carried out on both species 3 and 4. Their selected bond distances and angles are summarized in Table 1. Figure 1, on the top, shows the molecular structure of complex 3. In the crystal of complex 4, there are two independent molecules, which show only small differences in their geometry (see Table 1), and the molecular structure of only one of them is therefore shown in Figure 1, on the bottom.  (18) According to the crystallographic data, the ruthenium atom in both 3 and 4 is bound in a η 5 -fashion to the C 2 B 3 open face of C,C'-dimethylated nido-{C 2 B 9 }-carborane cage ligands, which have ortho-phenylenecycloboronated linkages connecting one of the P-phenyl rings of the ruthenium-bound dppp or dppb ligand, respectively, to the cluster B(8) atom. As expected, the presence of ortho-cycloboronated bridging fragment in both 3 and 4 accounts for the observed differences in the Ru-P(1) and Ru-P(2) bond lengths (2.3412(7) and 2.2928(7) Å in 3; av. 2.3637 (13) (1)) Å [20], however, is somewhat shorter as compared with that in the only structurally studied oxygen iridium-carborane species with the exo-polyhedrally bound metal atom, [(Ph 3 P) 2 IrO 2 (7,8-(MeS) 2 -7,8-C 2 B 9 H 10 )]•Me 2 CO (1.471(8) Å) [21]. The carborane C-C bond distances in 3 (1.621(4) Å) and 4 (1.633(7) and 1.640(6) Å), as well as the other cluster C-B (1.69-1.75 Å) and B-B (1.73-1.84 Å) connectivities, are typical of 12-vertex closo-metallacarboranes [22]. Analytical, IR and NMR spectroscopic data are entirely consistent with the formulation of complexes 3 and 4 made based on the above X-ray diffraction experiments. The Nujol mull infrared spectra of 3 and 4 show a broad band at ca. 940 cm −1 . Since no such bands were observed in the IR spectra of starting complexes 1 and 2, these bands in IR spectra of 3 and 4 were assigned to ν O-O . The detailed examination of the 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra of complexes 3 and 4 provided additional useful information on their structures. The 31 P{ 1 H} NMR spectra of complexes 3 and 4 each contains two resonances: two sharp doublets (δ P(1) = 18.6 ppm, δ P(2) = 52.9; J P(1)-P(2) = 75.3 Hz) for 3; one broadened and one sharp doublet (δ P(1) = 23.0 ppm, δ P(2) = 64.8; J P(1)-P(2) = 65.0 Hz) for 4. The low-field phosphorus resonances at δ 52.9 and 64.8 ppm were tentatively assigned to the "PPh-µ-(C 6 H 4 -ortho)" moiety of 3 and 4, which is connected to boron atom of the cage ligand. The 1 H-NMR spectra of 3 and 4, along with the resonances derived from the phenyl protons of the dppp and dppb ligands at δ between ca. 7.8 and 6.8 ppm, as well as the resonances of the cage CH 3 protons (δ, 1.14, 0.81 for 3 and δ, 1.09, 0.64 ppm for 4), are indicative of a set of non-equivalent protons of three and four methylene groups attributable to the chelating diphosphine ligands. The latter sets proved to be the only structurally significant and are particularly diagnostic of the presence the ortho-phenylenecycloboronated moiety in 3 and 4. Therefore, we strictly assigned all these resonances by the use of 1 H{ 31 P}, 13 C{ 1 H} NMR, as well as 2D [ 13 C-1 H]-HSQC correlation spectra. Thus, the fully 31 P-decoupled 1 H{ 31 P} NMR spectra of 3 and 4 each revealed two pairs of apparently simplified 1H multiplet resonances originating from diphosphine CH 2 groups substituted by the PPh 2 functions. In agreement with this, in the 2D [ 13 C-1 H]-HSQC spectrum of 3 (Figure 2, on the left) each pair of these 1H multiplets are connected by cross-peaks to one of the two carbon resonances that appear as doublet of doublets at δ 24.4 and 26.7 ppm with ca. J C,P = 35.3 and J C,P = 7.5 Hz. Similar cross-peaks between each pairs of 1H multiplets, which are susceptible to 31 P-decoupling, a broadened doublet at δ 30.8 ppm and a sharp doublet at 25.3 ppm both having ca. same J C,P = 32.0 Hz can be observed in the 2D [ 13 C-1 H]-HSQC spectrum of 4 ( Figure 2, on the right). The differences in the multiplicity observed for the carbon resonances derived from "Ph 2 P (1) CH 2 (CH 2 ) n CH 2 P (2) Ph-µ-(C 6 H 4 -ortho)" units in the 13 C{ 1 H} NMR spectra of 3 and 4 can be explained by a shorter carbon chain in the case of diphosphine ligand of 3, which results in additional splitting of resonances derived from CH 2 groups on the far-located 31 P nucleus.

Preparation of [3-(η 2 -O 2 )-3,3,8-[Ph 2 P(CH 2 ) 3 PPh-µ-(C 6 H 4 -ortho)]-1,2-Me 2 -closo-3,1,2-Ru II C 2 B 9 H 8 ] (3)
A mixture of 1 (113 mg, 0.16 mmol) and an excess of crushed KOH (45 mg, 0.804 mmol) in a 1:1 benzene-methanol mixture (20 mL) was stirred at ambient temperature (~10 min) until all starting material was dissolved and the color of the solution changes from deep red to pale yellow. The flask was opened on air, and the reaction mixture was allowed to stir additionally for 1 h. Solvent was evaporated under reduced pressure and the residue was dissolved in benzene (8-10 mL), filtered and treated by column chromatography on silica gel. The brownish band was eluted from the column using benzene, the solvent was removed, and the resultant solid was re-crystallized from a n-hexane-benzene mixture, affording 50.

X-ray Structure Determination for Complexes 3 and 4
Single-crystal X-ray diffraction study was carried out with a Bruker SMART 1000 [23] for 3 and Bruker SMART APEX II [24] for 4 diffractometers (graphite monochromated Mo-K α radiation, λ = 0.71073 Å, ω-scan technique). The SHELXTL software [25] was used for space group and structure determination, refinements, graphics, and structure reporting. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F 2 with the anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms of carborane moieties were located from difference Fourier maps, the rest hydrogen atoms were placed in the geometrically calculated positions. All hydrogen atoms were included in the structure factors calculations in the riding motion approximation. In the crystal structure of 4, the central atom C (16B) of dppb ligand in the one of two independent molecules is disordered over two positions with 0.8/0.2 occupancies. In both structures 3 and 4, the CH 2 Cl 2 solvent molecules are presented. The principal experimental and crystallographic parameters are listed in Table 2. CCDC 992977 (for 3) and 992978 (for 4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).