Interconversion between Ruthenacyclohexadiene and Ruthenabenzene: A Combined Experimental and Theoretical Study

Abstract

Treatment of ruthenabenzene [(C 9 H 6 NO)Ru{CC-(PPh 3 )CHC(PPh 3 )CH}(C 9 H 6 NO)(PPh 3 )]Cl 2 (1) with NaBH 4 produces the first ruthenacyclohexa-1,4-diene [(C 9 H 6 NO)Ru{CC-(PPh 3 )CH 2 C(PPh 3 )CH}(C 9 H 6 NO)(PPh 3 )]Cl (2), which was fully characterized. Under an oxygen atmosphere, complex 2 can easily convert to complex 1. DFT calculations were carried out to rationalize the high regioselectivity in the reaction of the ruthenabenene 1 with NaBH 4 and the interconversion between 1 and 2. ■ INTRODUCTIONMetallacycles have attracted considerable attention because they can display unique properties and mediate various reactions to construct unsaturated molecules. 1 As one of the major models of six-membered metallacycles, the isolation and characterization of stable metallacyclohexadienes have been extensively investigated. 2−4 Most of the well-characterized stable metallacyclohexadienes are those with late transition metals, especially iridium. 2a−j One of the common strategies previously employed is the formation of metallacyclohexadienes from various metallabenzenes, including stibabenzene, 3a bismabenzenes, 3a iridabenzenes, 2b,d,f−h,j osmabenzenes, 2j and ruthenabenzenes. 4c A previously reported example of a ruthenacyclohexa-1,3-diene (I), 4a,b which was better formulated as a ruthenabenzofuran, 5 can convert to a tethered ruthenabenzene by protonation of a carbon atom of the five-membered ring of this ruthenabenzofuran. In our earlier study, we found that the nucleophilic addition reactions of metallabenzene [M{CHC-(PPh 3 )CHC(PPh 3 )CH}Cl 2 (PPh 3 ) 2 ]Cl (M = Os or Ru) produced the metallacyclohexa-1,3-diene and η 2 -allene-coordinated metallacycles. 4c In this contribution, we present the synthesis of the first example of ruthenacyclohexa-1,4-dienes (II), which could be obtained from the nucleophilic addition reaction of ruthenabenzene with NaBH 4 . The regioselectivity of the nucleophilic addition reaction and the interconversion between ruthenacyclohexadiene and ruthenabenzene have been studied with the aid of density functional theory (DFT) calculations. ■ RESULTS AND DISCUSSIONTreatment of complex 1 with excess sodium borohydride in dichloromethane led to the formation of complex 2, which can be isolated as a reddish-brown solid in 78% yield (Scheme 1). Complex 2 was characterized by NMR spectroscopy, and the structure was further confirmed by single-crystal X-ray diffraction.As complex 2 has good solubility in organic solvents, it is difficult to obtain a single crystal of 2 to determine its solidstate structure. Fortunately, the counteranion Cl − in 2 can be easily replaced with BPh 4 − by treatment of 2 with NaBPh 4 to give ruthenacyclohexa-1,4-diene 2′. The molecular structure of 2′ is shown in Figure 1, which reveals that it is a ruthenacyclohexa-1,4-diene complex. The Ru1−C1 (1.973(9) Å) and C1−C2 (1.349(11) Å) bond lengths compare well with

Recent Advances in Metallaaromatic Chemistry

“…[85][86][87] Metallabenzenes of one second row transition metal, ruthenium, are also known. [88][89][90][91][92][93][94] Although theoretically feasible, [16,18,95,96] the isolation of simple metallabenzenes containing other transition metals,p articularly those in the first row, has remained elusive.…”

“…They are dependent on (but do not have ac lear correlation with changes in) the metal, ligands, charge and ring substituents. [16,86,[100][101][102] Through other computational studies, metallabenzenes have been shown to exhibit negative nucleus-independent chemical shift (NICS) values, [60,94,96,97] diatropic ring currents [96,103,104] and relatively large absolute hardness, [100,105] each of whichi sc onsistent with aromaticity.I th as been noted, however,t hat many of these latter methods may have limited applicability to met-allabenzenes due to the influence of the ancillary ligands and metal anisotropy. [18,97] Although many metallabenzenes have an early planar structure, ac onsiderable number do not.…”

“…Nitration or bromination occurs at the C4 position, para to the -SMe substituent, following normal aromatic directing effects (see (a), Scheme 3). [45,77] Other "benzene-like" reactions that have been observed for metallabenzenes include nucleophilic aromatic substitution of hydrogen [52,54,92] (Scheme 3see (b)) nucleophilic aromatic addition to the ring, [57,66,94] and p-complexation to suitable metal substrates [84,[106][107][108] (Scheme 3see (c)). The reactions of metallabenzenes, however, do not always follow those classically observed for benzene.…”

“…The third row transition metals are well‐represented with examples incorporating osmium, iridium, platinum and, most recently, rhenium . Metallabenzenes of one second row transition metal, ruthenium, are also known . Although theoretically feasible, the isolation of simple metallabenzenes containing other transition metals, particularly those in the first row, has remained elusive.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…Addition of chloride to the vinylphosphonium salt group followed by elimination of triphenylphosphine affords the osmabenzene. The interconversion of ruthenabenzene 730 and ruthenacyclohexadiene 731 was studied experimentally and computationally[976]. Treatment of the dicationic complex 730 with sodium borohydride led to the ruthenacyclohexadiene complex 731 as the exclusive regioisomer in the reaction.…”