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.