Probing the Presence of Multiple Metal–Metal Bonds in Technetium Chlorides by X‑ray Absorption Spectroscopy: Implications for Synthetic Chemistry

The cesium salts of [Tc2X8]3– (X = Cl, Br), the reduction product of (n-Bu4N)­[TcOCl4] with (n-Bu4N)­BH4 in THF, and the product obtained from reaction
of Tc2(O2CCH3)4Cl2 with HCl­(g) at 300 °C have been characterized by extended
X-ray absorption fine structure (EXAFS) spectroscopy. For the [Tc2X8]3– anions, the Tc–Tc
separations found by EXAFS spectroscopy (2.12(2) Å for both X
= Cl and Br) are in excellent agreement with those found by single-crystal
X-ray diffraction (SCXRD) measurements (2.117[4] Å for X = Cl
and 2.1265(1) Å for X = Br). The Tc–Tc separation found
by EXAFS in these anions is slightly shorter than those found in the
[Tc2X8]2– anions (2.16(2)
Å for X = Cl and Br). Spectroscopic and SCXRD characterization
of the reduction product of (n-Bu4N)­[TcOCl4] with (n-Bu4N)­BH4 are
consistent with the presence of dinuclear species that are related
to the [Tc2Cl8]n− (n = 2, 3) anions. From these results, a new preparation
of (n-Bu4N)2[Tc2Cl8] was developed. Finally, EXAFS characterization of
the product obtained from reaction of Tc2(O2CCH3)4Cl2 with HCl­(g) at 300 °C
indicates the presence of amorphous α-TcCl3. The
Tc–Tc separation (i.e., 2.46(2) Å) measured in this compound
is consistent with the presence of TcTc double bonds in the
[Tc3]9+ core.

本研究通过扩展X射线吸收精细结构（extended X-ray absorption fine structure, EXAFS）光谱学，对三类样品开展了表征：分别为[Tc2X8]3–（X=Cl、Br）的铯盐、由(n-Bu4N)[TcOCl4]与(n-Bu4N)BH4在四氢呋喃（tetrahydrofuran, THF）中经还原反应得到的产物，以及Tc2(O2CCH3)4Cl2与气态氯化氢在300 ℃下反应生成的产物。针对[Tc2X8]3–阴离子，EXAFS光谱测得的Tc-Tc键长（X=Cl、Br时均为2.12(2) Å）与单晶X射线衍射（single-crystal X-ray diffraction, SCXRD）测试结果（X=Cl时为2.117[4] Å，X=Br时为2.1265(1) Å）高度吻合。本研究通过EXAFS测得的该类阴离子中的Tc-Tc键长，略短于[Tc2X8]2–阴离子中的对应键长（X=Cl、Br时均为2.16(2) Å）。对(n-Bu4N)[TcOCl4]与(n-Bu4N)BH4的还原产物进行光谱学与SCXRD表征的结果，与[Tc2Cl8]n−（n=2、3）类双核阴离子的存在特征相符。基于上述结果，我们开发了一种制备(n-Bu4N)2[Tc2Cl8]的新方法。最后，对Tc2(O2CCH3)4Cl2与气态氯化氢在300 ℃下反应得到的产物进行EXAFS表征的结果表明，该产物为无定形α-TcCl3。该化合物中测得的Tc-Tc键长（即2.46(2) Å）与[Tc3]9+核团中存在Tc=Tc双键的结论一致。