Discrimination of pi-acids at Stereochemically different Axial Site of syn-Crown Thioether Complexes of Low-valent Transition Metals

顺式冠硫醚过渡金属配合物的两个轴向位置是立体化学上不同的环境;被环碳原子包围的位置是拥挤的，而相反的位置是不拥挤的。因此，具有不同空间位阻需求的两个π酸或阴离子配体有望在轴向配位上被区分。这是通过trans-Mo（eta^2-O，C-PhNCO）（eta^2-C，NPhNCO）（syn-Me_8[16]aneS_4）实现的，其中大体积的eta^2-O，C-PhNCO在非拥挤位点特异性地配位。通过EHMO和分子力学计算证明了PhNCO的C=O和C=N键的新识别是空间位阻的。用反式RuH（Cl）L（L=Me_4[14]aneS_4，Me_6[15]aneS_4，Me_8[16]aneS_4）证明了冠硫醚对轴向配体识别的环尺寸效应。在Me_4[14]aneS_4和Me_6[15]aneS_4中，C1和C2配体分别占据了轴向的拥挤位置，而在Me_8[16]aneS_4中，轴向位置不能区分这两个阴离子配体。前两种配合物的X-射线衍射结构表明，位于拥挤位置的轴向配体与轴向CH_2氢原子之间的吸引货车德瓦耳斯相互作用是导致识别的原因。通过对trans-Mo（S）_2（syn-Me_8[16]aneS_4）中位于轴向密集位的末端硫醚与MeI和PhCH_2Br的立体专一性烷基化反应，合成了一种新型的含硫配合物trans-Mo（S）（SR）（syn-Me_8[16]aneS_4）（R=Me，PhCH_2）。与HBF_4进行类似的质子化反应得到反式-（Mo（S）（F）（syn-Me_8[16]aneS_4））^+和（MO_2（S）_2（mu-S）（syn-Me_8[16]aneS_4）_2）^<2+>。X-射线衍射和电子光谱研究表明，后者含有一个新的扩展的线性S=Mo*S*Mo=S共轭键。

Two axial sites of transition metal complexes of syn-crown thioethers are stereochemically different environments ; the site surrounded by the ring carbon atoms is congested, while the opposite site is uncongested. Therefore, two Pi-acids or anionic ligands possessing different steric demands are expected to be discrimianted on axial coordination. This was achieved by trans-Mo (eta^2-O, C-PhNCO) (eta^2-C, NPhNCO) (syn-Me_8[16]aneS_4), where the bulky eta^2-O, C-PhNCO coordinates specifically at the uncongested site. The novel recognition of the C=O and C=N bondds of PhNCO was proved be steric in origin by EHMO and molecular mechanics calculations. The ring size effect of crown thioethers upon the discrimination of axial ligands was demonstrated cleary by trans-RuH (Cl) L (L=Me_4[14]aneS_4, Me_6[15]aneS_4, Me_8[16]aneS_4). The hydrido and Cl ligands in the Me_4[14]aneS_4 and Me_6[15]aneS_4, complexes occupy the congested axial site respectively, while the axial sites in the Me_8[16]aneS_4 complex failed to discriminate the two anionic ligands. The X-ray structures of the two former complexes suggest that the attractive van der Waals interactions between the axial ligand at the congested site and axial CH_2 hydrogen atoms are responsible for the recognition. Stereospecific alkylation of the terminal sulfide located at the congested axial site in trans-Mo (S)_2 (syn-Me_8[16]aneS_4) with Mel and PhCH_2Br took place readily to give novel MoS_6 complexes trans-Mo (S) (SR) (syn-Me_8[16]aneS_4) (R=Me, PhCH_2) containing the three distinctive types of sulfur donors, thiolate, terminal sulfide, and thioether. A protonlation with HBF_4 took place similarly to give trans-(Mo (S) (F) (syn-Me_8[16]aneS_4) )^+ and (MO_2 (S)_2 (mu-S) (syn-Me_8[16]aneS_4)_2)^<2+>. The x-ray structural study and electronic spectrum show that the latter complex contains a novel extended linear S=Mo*S*Mo=S conjugated linkage.