Nitrogen abstraction from nitriles by osmium(IV) complexes
Nitrogen abstraction from nitriles by osmium(IV) complexes
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DOI：
10.1021/ja005645d
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发表时间：
2001-05-09
影响因子：
15
通讯作者：
Mayer, JM
中科院分区：
文献类型：
--
作者：
Bennett, BK;Lovell, S;Mayer, JM
与路易斯酸或氧化性金属中心的结合会强烈影响配体的反应性。例如，有机腈被缺乏电子的金属激活，趋向亲核攻击和水解。工业和酶促腈水解都使用金属中心作为路易斯酸催化剂。这里描述的是Os （IV）配合物TpOs (OTf) Cl2(1)对腈的水解歧化反应，生成锇（VI）一氧化氮配合物。沿着建议的途径的中间体已经独立制备，表明水解发生在Os （IV）中心，然后是胺配合物歧化为氮化产物。硝基化合物由于其作为胺化和叠氮化试剂的潜在用途而引起人们的兴趣。以前仅有的关于由腈形成氮配体的报道涉及高还原性的金属配合物和金属-金属键合化合物。TpOs (OTf) Cl2(1)是由TpOs (NPPh3)-Cl2(6)与过量的三酸在二氯甲烷中质子化制备的（图1）。它的1H NMR谱8是尖锐的，但顺磁位移(δ)+ 5至26 ppm)，这是典型的W， Re和Os的d4-八面体配合物。10 x射线晶体结构中的Os-OTf距离为2.053 (6)Å（图1）。8b在+ 0.28 V vs Cp2Fe+/0的MeCN中（~ + 0.9 V vs NHE）发生1的可逆还原，还原的Os （III）形式被分离为[Cp* 2Fe][TpOs-(OTf) Cl2]（1R[表示还原]）。1中的三氟酸配体具有很强的抗取代性。在干燥、脱气的MeCN中，在70℃下加热3天后，其收率为93%。可以分离出少量（< 4%）的取代产物，但它是还原锇（III）络合物TpOs-(NCMe) Cl2 (2R)，而不是预期的锇（IV）产物[TpOs (NCMe) Cl2]+（2，见下文）。同样，在干燥的MeCN中，1也不与[nBu4N] Cl反应。取代的缺乏是动力学上的而不是热力学上的，因为取代产物2和TpOsCl3 11是可分离的物质，在加入OTf-后不会还原为1。在CD2Cl2中加入HCl/Et2O或在湿MeCN中加入[nBu4N] Cl可迅速生成TpOsCl3，表明三氟酸盐取代是质子辅助的。相对于在干燥的MeCN中没有反应，在湿的乙腈中65℃加热3小时，可以生成锇（VI）氮络合物TpOs (N) Cl2(3) 6（产率25%）、2R（53%）和乙酰胺/乙酸（≈30%）。13 3与2R的1:2比例与式1中的平衡反应一致。用CH3C15N和H2O加热1
The reactivity of ligands can be strongly affected by binding to Lewis-acidic or oxidizing metal centers. Organonitriles, for instance, are activated by electron deficient metals toward nucleophilic attack and hydrolysis. 1-3 Both industrial and enzymatic nitrile hydrolyses utilize metal centers as Lewis acid catalysts. 1-3 Described here is the hydrolytic disproportionation of nitriles by the Os (IV) complex TpOs (OTf) Cl2 (1), yielding an osmium (VI) nitrido complex. Intermediates along the suggested pathway have been independently prepared, indicating that hydrolysis occurs on the Os (IV) center, followed by disproportionation of an ammine complex to the nitrido product. Nitrido compounds are of interest due to their potential use as amination and aziridination reagents. 4 The only previous reports of forming nitrido ligands from nitriles have involved highly reducing metal complexes and metal-metal bonded compounds. 5 TpOs (OTf) Cl2 (1) is prepared by protonation of TpOs (NPPh3)-Cl2 6 with excess triflic acid in methylene chloride (Figure 1). 7 Its 1H NMR spectra8 are sharp but paramagnetically shifted (δ)+ 5 to-26 ppm), as is typical of d4-octahedral complexes of W, Re, and Os. 6, 9 The 19F NMR chemical shift at-69.54 ppm and the IR band at 1360 cm-1 indicate a coordinated triflate ligand, 10 as also shown by the Os-OTf distance of 2.053 (6) Å in the X-ray crystal structure (Figure 1). 8b A reversible reduction of 1 occurs at+ 0.28 V vs Cp2Fe+/0 in MeCN (∼+ 0.9 V vs NHE) and the reduced Os (III) form has been isolated as [Cp* 2Fe][TpOs-(OTf) Cl2](1R [for Reduced]). 8bThe triflate ligand in 1 is quite resistant to substitution. It is recovered in 93% yield after heating in dried, degassed MeCN at 70 C for 3 d. A small amount (< 4%) of a substituted product can be isolated, but it is the reduced osmium (III) complex TpOs-(NCMe) Cl2 (2R) rather than the expected osmium (IV) product [TpOs (NCMe) Cl2]+(2, see below). Similarly, 1 does not react with [nBu4N] Cl in dry MeCN. The lack of substitution is kinetic rather than thermodynamic in origin, since the substitution products 2 and TpOsCl3 11 are isolable species that do not revert to 1 on addition of OTf-. TpOsCl3 is rapidly formed from 1 on addition of HCl/Et2O in CD2Cl2, or on addition of [nBu4N] Cl in wet MeCN, 12 indicating that triflate substitution is proton-assisted. In contrast to the lack of reaction in dry MeCN, heating 1 for 3 h at 65 C in wet acetonitrile produces the osmium (VI) nitrido complex TpOs (N) Cl2 (3) 6 (25% yield), 2R (53%), and acetamide/acetic acid (≈ 30%). 13 The 1: 2 ratio of 3 to 2R is consistent with the balanced reaction in eq 1. Heating 1 with CH3C15N and H2O