Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, USA.
J Am Chem Soc. 2011 Sep 21;133(37):14593-603. doi: 10.1021/ja202754e. Epub 2011 Aug 29.
A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)(2)(IMes)H with KH and 18-crown-6 gives the tungsten anion CpW(CO)(2)(IMes)K(18-crown-6). Electrochemical oxidation of CpW(CO)(2)(IMes) in MeCN (0.2 M (n)Bu(4)N(+)PF(6)(-)) is fully reversible (E(1/2) = -1.65 V vs Cp(2)Fe(+•/0)) at all scan rates, indicating that CpW(CO)(2)(IMes)(•) is a persistent radical. Hydride transfer from CpW(CO)(2)(IMes)H to Ph(3)C(+)PF(6)(-) in MeCN affords cis-CpW(CO)(2)(IMes)(MeCN)PF(6)(-). Comproportionation of CpW(CO)(2)(IMes) with CpW(CO)(2)(IMes)(MeCN) gives the 17-electron tungsten radical CpW(CO)(2)(IMes)(•). This complex shows paramagnetically shifted resonances in the (1)H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(•) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(•) and CpW(CO)(2)(IMes) are reported. DFT calculations were carried out on CpW(CO)(2)(IMes)H, as well as on related complexes bearing NHC ligands with N,N' substituents Me (CpW(CO)(2)(IMe)H) or H (CpW(CO)(2)(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W-H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol(-1) for CpW(CO)(2)(IH)H, CpW(CO)(2)(IMe)H, and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(•) is 0.63. The W radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•) are calculated to form weak W-W bonds. The weakly bonded complexes CpW(CO)(2)(IMe) and CpW(CO)(2)(IH) are predicted to have W-W BDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•).
一种由钨阴离子、自由基和阳离子组成的系列，由 N-杂环卡宾 1,3-双(2,4,6-三甲基苯基)咪唑-2-亚基(IMes)支撑，并跨越形式氧化态 W(0)、W(I)和 W(II)，已经被合成、分离和表征。将氢化物 CpW(CO)(2)(IMes)H 与 KH 和 18-冠-6 反应，得到钨阴离子[CpW(CO)(2)(IMes)]- [K(18-冠-6)]+。在 MeCN(0.2 M(n)Bu4NPF6(-))中，[CpW(CO)(2)(IMes)]-的电化学氧化在所有扫描速率下都是完全可逆的(E1/2=-1.65 V 相对于 Cp2Fe(+•/0))，表明 CpW(CO)(2)(IMes)(•)是一种持久自由基。CpW(CO)(2)(IMes)H 向 Ph3C+PF6(-)的氢化物转移在 MeCN 中提供[cis-CpW(CO)(2)(IMes)(MeCN)]+PF6(-)。[CpW(CO)(2)(IMes)]-与[CpW(CO)(2)(IMes)(MeCN)]+的复分解反应得到 17 电子钨自由基 CpW(CO)(2)(IMes)(•)。该配合物在(1)H NMR 谱中显示出顺磁位移共振，已通过红外光谱、低温 EPR 光谱和 X 射线衍射进行了表征。CpW(CO)(2)(IMes)(•)相对于歧化和二聚化是稳定的。报道了 CpW(CO)(2)(IMes)(•)和CpW(CO)(2)(IMes)之间简并电子转移的 NMR 研究。对 CpW(CO)(2)(IMes)H 以及带有 NHC 配体的相关配合物进行了密度泛函理论(DFT)计算，这些配体带有 N,N'取代基 Me(CpW(CO)(2)(IMe)H)或 H(CpW(CO)(2)(IH)H)，以与实验研究的带有均三甲苯基取代基的 IMes 配合物进行比较。这些计算表明，W-H 均裂键解离能(BDE)随着 NHC 配体的空间位阻的增加而降低，从 67 到 64 到 63 kcal mol(-1)，分别为 CpW(CO)(2)(IH)H、CpW(CO)(2)(IMe)H 和 CpW(CO)(2)(IMes)H。对于 CpW(CO)(2)(IMes)(•)，计算出的 W 自旋密度为 0.63。CpW(CO)(2)(IMe)(•)和 CpW(CO)(2)(IH)(•)的 W 自由基被计算形成弱的 W-W 键。预测弱键合的配合物[CpW(CO)(2)(IMe)]2和[CpW(CO)(2)(IH)]2具有 6 和 18 kcal mol(-1)的 W-W BDE，并且容易解离为 W 中心自由基 CpW(CO)(2)(IMe)(•)和 CpW(CO)(2)(IH)(•)。
