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
