THE STRUCTURE AND PROPERTIES OF RED PERTECHNETIC ACID IN SOLID STATE AND SOLUTIONS AS PROBED BY NMR G.A. Kirakosyan*, K.E. German**, V.P. Tarasov*, and M.S. Grigoriev** * Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, ** Institute of Physical Chemistry, Russian Academy of Sciences

Hystorical aspect The structure of pertechnetic acid is still a matter of controversy for researchers engaged in studying basic aspects of technetium chemistry. Major issues that fueled controversy were the red color of this compound and some doubts about its composition As early as 1960, Boyd showed based on gravimetry data that this compound either does not contain hydration water so that it should be formulated as HTcO 4 or contains one water molecule per Tc 2 O 7 molecule and has the composition Tc 2 O 7  H 2 O Structures of M(VII) acids are much different depending on the element M(VII): Rulfs, J.Inorg.nucl. Chem..1967

STRUCTURE OF Re 2 O 7 *2H 2 O Composition differs from HTcO 4 by one H 2 O molecule

STRUCTURE OF HIO 4 ( Kraft, T. and Sansen, M ) Composition does not differ from HTcO 4Composition does not differ from HTcO 4 One type of iodine – atomsOne type of iodine – atoms Chain of cis-edge-sharing IO 6 octahedraChain of cis-edge-sharing IO 6 octahedra

Synthesis of pertechnetic acid HTcO 4 Pertechnetic acid HTcO 4 and its deuterated analog DTcO 4 were prepared by oxidizing Tc metal in O 2 at 500 o C, condensation of solid Tc 2 O 7 in quartz receiver, dissolving Tc 2 O 7 in H 2 O or D 2 O, followed by concentration of resulting solutions to complete dryness with P 4 O 5 or Mg(ClO 4 ) 2. No access to air dust was allowed, excluding the possibility of Tc(VII) reduction to Tc(V). The samples were dark red crystalline powders. Elemental analysis gave the ratio H/Tc  1.

Unit cell parameters for HTcO 4 In 1994, the first unit cell parameters for the acid HTcO 4 were reported: a = 11.26(2), b = 12.87(2), c = 14.16(4) Å;  = 71.0(2) ,  = 69.1(2) ,  = 74.1(2)  ; V = 1783(6) Å 3 [2]. The unit cell contains at least eight technetium atoms. The crystallographic data for pertechnetic acid differ dramatically from those for HClO 4  H 2 O [3] and anhydrous HIO 4 (a = (5), b = (4), c = (3) Å; b = (3) , space group C2/c; or from neutron diffraction data: a = (18), b = (13), c = 4.975(3) Å; b = (3) , space group C2/c) [4]. The IR spectrum of HTcO 4 (846, 901s, 934s cm  1 ) [5] is similar to that of HReO 4 [6]. The spectral pattern is explained by the lowering of the TcO 4  symmetry from T d to C 3v.

1 H NMR study of the HTcO 4 The 1 H NMR spectrum of a fresh HTcO 4 powder at 259 K showed two signals: - -a strong signal at 5.6 ppm with the width at half- maximum of 400 Hz - -a weak signal at  0 ppm (relative to the external H 2 O reference) - due to hygroscopic properties of solid HTcO 4. Decreasing temperature to 120 K resulted in the broadening of the spectrum to 4 kHz However, the expected fine structure of the spectrum (as the Pake doublet) was not observed è èThis permits a tentative conclusion that the compound is not a crystal hydrate and the lattice hydrogen is highly mobile.

99 Tc-NMR of red HTcO 4 (solid)   The spectrum of a fine powder of red pertechnetic acid (figure) showed two signals at  1 = +213(5) and  2 =  147(5) ppm (referenced to the external standard, a 0.1 M KTcO 4 solution)   the integrated intensity ratio was I 1 : I 2 = 2 : 3. Each of the signals had a multiplet structure due to first-order quadrupole interactions: the quadrupole coupling constants were C Q (1) = ~ 0.25(1) and C Q (2) = 0.43(1) MHz and the asymmetry parameter of the 99 Tc EFG tensor was  = 0 in both cases. è èThe spectral pattern and resonance parameters indicate that technetium has two states in the structure, none of them corresponding to the TcO 4  ion.

Structure of HTcO 4 r resulting from 99 Tc-NMR Most likely, the pertechnetic acid structure is represented by one of polymeric frameworks in which Tc(VII) has two types of oxygen surroundings: The signal at 213 ppm corresponds to four- coordinate technetium, The signal at  147 ppm, to six-coordinate technetium, as is illustrated by formula O 3 = Tc –(  -O) – Tc(=O)(OH) 2 (  -O) 2 or presented by the schemes below

Structure of HTcO 4 resulting from 99 Tc-NMR and 1 H - Spectra í Four-coordinate moieties are involved in polymerization with time and lose their mobility to produce a three- dimensional framework. í This scheme is the most probable one, although other schemes cannot be ruled out.

Some alternative structures S