Comparisons of Photoinduced Oxidative Addition of B−H, B−B, and Si−H Bonds at Rhodium(η⁵-cyclopentadienyl)phosphine Centers

Ultraviolet irradiation of [Rh(η5-C5H5)(PMe3)(C2H4)] (1a), [Rh(η5-C5H5)(PPh3)(C2H4)] (1b), and [Rh(η5-C5H4CF3)(PMe3)(C2H4)] (1c) (collectively abbreviated as [Rh(Cp‘)(PR3)(C2H4)]) in the presence of HBpin (pinacolate = pin = 1,2-O2C2Me4) results in elimination of C2H4 and B−H oxidative addition, leading to the formation of boryl hydride complexes [Rh(Cp‘)(Bpin)(H)(PR3)]. Complete conversion is achieved in liquid HBpin or by photolysis in hexane at −10 °C. Similarly, photolysis of 1a−c in the presence of B2pin2 in hexane at −10 °C leads to B−B oxidative addition products, [Rh(Cp‘)(Bpin)2(PR3)]. Irradiation at room temperature leads to formation of [Rh(Cp‘)(PR3)2] in addition to the desired products. The rhodium boryl products were characterized by multinuclear NMR spectroscopy and, in the case of [Rh(η5-C5H5)(Bpin)(H)(PPh3)], by X-ray crystallography. The structure reveals a Rh−B distance of 2.0196(15) Å. The H···B separation of 2.09(2) Å together with the bond angles at the metal suggest some residual H···B interaction. Photolysis of 1a−c in the presence of tertiary and secondary silanes (HSiEt3, HSiiPr3, HSi(OMe)3, HSiMe2Et, HSiMeEt2, and H2SiEt2) results in rhodium silyl hydride complexes [Rh(Cp‘)(SiR‘2R‘ ‘)(H)(PR3)]. The structure of [Rh(η5-C5H5)(SiiPr3)(H)(PMe3)] was determined by single-crystal X-ray diffraction, yielding a Rh−Si bond length of 2.3617(3) Å and a Rh−H bond length of 1.508(17) Å. The H···Si distance of 2.278(17) Å and the very unequal H−Rh−P and H−Rh−Si angles suggest some residual H···Si interaction. Competition reactions were performed with 1b dissolved in hexane in the presence of HBpin and B2pin2 simultaneously. 31P NMR measurements, made after brief irradiation, showed a slight preference for B−B oxidative addition over B−H oxidative addition. Similar experiments with three-way competition among HBpin, HSiMe2Et, and HC6F5, analyzed by 1H NMR spectroscopy, showed negligible selectivity among H−B, H−C, and H−Si oxidative addition. Molecular structures were also determined by single-crystal X-ray diffraction for 1b, 1c, [Rh(η5-C5H5)(PPh3)2], and [Rh(η5-C5H5)Cl2(PPh3)].对[Rh(η⁵-环戊二烯基(η⁵-Cyclopentadienyl))(三甲基膦(PMe₃))(C₂H₄)]（1a）、[Rh(η⁵-环戊二烯基(η⁵-Cyclopentadienyl))(三苯基膦(PPh₃))(C₂H₄)]（1b）与[Rh(η⁵-(三氟甲基)环戊二烯基)(三甲基膦(PMe₃))(C₂H₄)]（1c）（统称缩写为[Rh(Cp′)(PR₃)(C₂H₄)]）在HBpin（频哪醇硼酸酯，pinacolate，缩写为pin = 1,2-O₂C₂Me₄）存在下进行紫外辐照，可引发乙烯（C₂H₄）消除与B-H氧化加成反应，最终生成硼基氢化物配合物[Rh(Cp′)(Bpin)(H)(PR₃)]。该反应在液态HBpin中可实现完全转化，或在正己烷溶剂中于-10 ℃下通过光解完成。 类似地，在-10 ℃的正己烷中对1a~1c进行光解且存在B₂pin₂时，会得到B-B氧化加成产物[Rh(Cp′)(Bpin)₂(PR₃)]。若在室温下进行辐照，则除目标产物外，还会生成副产物[Rh(Cp′)(PR₃)₂]。 上述铑硼基产物通过多核核磁共振波谱（multinuclear NMR spectroscopy）进行表征，其中[Rh(η⁵-环戊二烯基)(Bpin)(H)(三苯基膦(PPh₃))]还通过X射线晶体学完成结构解析，其Rh-B键长为2.0196(15) Å。结合H···B间距为2.09(2) Å以及金属中心处的键角数据，可推测体系中存在一定程度的残留H···B相互作用。 在叔硅烷与仲硅烷（HSiEt₃、HSiiPr₃、HSi(OMe)₃、HSiMe₂Et、HSiMeEt₂及H₂SiEt₂）存在下对1a~1c进行光解，可得到铑硅基氢化物配合物[Rh(Cp′)(SiR′₂R′′)(H)(PR₃)]。其中配合物[Rh(η⁵-环戊二烯基)(三异丙基硅基)(H)(三甲基膦(PMe₃))]的结构通过单晶X射线衍射确定，其Rh-Si键长为2.3617(3) Å，Rh-H键长为1.508(17) Å。H···Si间距为2.278(17) Å，且H-Rh-P与H-Rh-Si键角差异显著，表明体系中存在一定程度的残留H···Si相互作用。 竞争反应实验如下：将1b溶解于正己烷中，同时加入HBpin与B₂pin₂。短暂辐照后进行的³¹P NMR测量结果显示，相较于B-H氧化加成，反应对B-B氧化加成具有轻微的选择性偏好。类似地，在HBpin、HSiMe₂Et与HC₆F₅三者同时存在的条件下开展竞争实验，通过¹H NMR光谱分析后发现，H-B、H-C与H-Si氧化加成之间几乎无选择性差异。 此外，配合物1b、1c、[Rh(η⁵-环戊二烯基)(三苯基膦(PPh₃))₂]以及[Rh(η⁵-环戊二烯基)Cl₂(PPh₃)]的分子结构也通过单晶X射线衍射得到了解析。