Open problems of the $TcCl_2$ structure type: the missing technetium dibromide, structures of $ReI_2$ and $OsI_2$, the $\alpha$-$TcCl_2$ intergrowth disorder, and bonding-linked properties

Technetium dichloride, discovered in 2011, is built from infinite chains of face-sharing [$Tc_2Cl_8$] rectangular prisms held together by intraprism $Tc\equiv Tc$ triple bonds — a structure type previously unknown. Its discoverers posed a coherent set of open questions: whether this structure type also occurs for other transition metals (Mo, W, Re, Os) and heavier halides (Br, I), which requires determining the crystal structures of long-reported but structurally uncharacterized dihalides such as $\beta$-$MoCl_2$, $ReI_2$, and $OsI_2$; whether the missing technetium dibromide $TcBr_2$ can be synthesized; what the ~15% intergrowth/orientational disorder of $\alpha$-$TcCl_2$ is at the atomic scale and how it depends on synthesis conditions; and how metal–metal multiple bonding governs macroscopic properties such as aqueous solubility and long-term chemical stability, the criteria relevant to $TcCl_2$ as a $^{99}Tc$ waste form. Later work resolved parts: $\beta$-$MoCl_2$ was shown by EXAFS and first-principles calculations not to adopt the $TcCl_2$ type (it contains [$Mo_4Cl_{12}$] units instead), and a second, long-range-ordered modulated polymorph $\beta$-$TcCl_2$ was solved. However, the nature and accessibility of a technetium dibromide remain unresolved — decomposition of higher bromides gives, as documented in the 2014 review, a sodium-stabilized [$Tc_6Br_{12}$] cluster salt instead — and no structure determinations of $ReI_2$ or $OsI_2$, no atomic-scale study of the $\alpha$-$TcCl_2$ disorder, and no quantification of the bonding–property link have appeared. The problem remains open.

Background

Technetium (Tc, element 43) is the lightest radioelement; its only long-lived isotope, $^{99}Tc$ (half-life $2.1\times10^5$ years), is generated in large quantities in nuclear reactors, so its solid-state chemistry matters both for completing periodic trends among the transition metals and for nuclear-waste disposal, where mobile pertechnetate ($TcO_4^-$) is the dominant risk species. A metal–metal multiple bond is a direct bond between two transition-metal centers of order greater than one, the paradigm being the quadruply bonded dinuclear ion $[Re_2Cl_8]^{2-}$; dinuclear $[M_2X_8]$-type units, in which two metal atoms share a rectangular prism of eight halide ligands, are the classic carriers of such bonds. For half a century the binary halide chemistry of technetium consisted of only $TcF_6$, $TcF_5$, and $TcCl_4$; between 2009 and 2015 a UNLV/Los Alamos/Northwestern collaboration rebuilt it, determining the crystal structure of the long-known $TcCl_4$ (chains of edge-sharing $TcCl_6$ octahedra, isostructural to $PtCl_4$) and reporting the new phases $TcBr_4$ (isostructural to $PtBr_4$), $\alpha$/$\beta$-$TcCl_3$, $TcBr_3$ and $TcI_3$ (the latter two of the $TiI_3$ type, i.e. chains of face-sharing octahedra; $TcI_3$ was the first binary technetium iodide), and the two polymorphs of $TcCl_2$. In $\alpha$-$TcCl_2$ (Poineau et al., 2011), face-sharing rectangular prisms [$Tc_2Cl_8$] form infinite chains with a short intraprism separation of 2.127(2) Å, consistent with a $Tc\equiv Tc$ triple bond supported by electronic-structure calculations; the chains pack in a commensurate $2\times2\times1$ supercell (an enlarged crystallographic cell that is an integer multiple of the basic subcell), and single-crystal diffraction revealed roughly 15% intergrowth/orientational disorder — interleaved domains of the two possible chain orientations — manifesting as diffuse supercell reflections. The 2011 paper conjectured that this new structure type might be attainable for other metals that form molecular $M_2X_8$ complexes ($M$ = Mo, W, Re, Os; $X$ = Cl, Br, I), noting that the structures of several long-reported but never structurally characterized dihalides ($\beta$-$MoCl_2$, $ReI_2$, $OsI_2$) were the required tests, that the analogous dibromide $TcBr_2$ might be accessible by similar stoichiometric reactions, and that the relationship between metal–metal bonding and aqueous solubility/long-term chemical stability — the properties behind $TcCl_2$'s proposed consideration as a $^{99}Tc$ waste form — warranted further study. Two later results define the current frontier. First, the Mo test was answered negatively: extended X-ray absorption fine structure (EXAFS) spectroscopy, which measures the local neighbor distances around an absorbing atom, shows that $\beta$-$MoCl_2$ contains [$Mo_4Cl_{12}$] units built from two triply Mo–Mo-bonded [$Mo_2Cl_8$] units rather than $TcCl_2$-type chains, and first-principles calculations confirm that a $TcCl_2$-type arrangement of $MoCl_2$ is less stable than cluster alternatives (Poineau et al., 2012). Second, the second polymorph $\beta$-$TcCl_2$ was solved as a modulated crystal structure — a structure refined in (3+1)-dimensional superspace, in which atomic positions deviate periodically from an average lattice — exhibiting long-range-ordered perpendicular/parallel arrangements of adjacent $Tc\equiv Tc$ vectors; it is a diamagnetic p-type semiconductor with a 0.12(2) eV optical gap (Malliakas et al., 2013). Divalent technetium halide chemistry nonetheless remains the open frontier: thermal decomposition of $TcBr_3$/$TcBr_4$ in glass ampoules yields the sodium-stabilized cluster salt $\mathrm{Na}\{[Tc_6Br_{12}]_2Br\}$ — a trigonal-prismatic hexanuclear cluster with alternating Tc–Tc single and triple bonds — rather than binary $TcBr_2$, and divalent Tc–Tc bonded bromide species are otherwise known only as molecular complexes such as $trans$-$TcBr_2(PMe_3)_4$ and $[Tc_2Br_4(PMe_3)_4]$ ($PMe_3$ = trimethylphosphine); hydrothermal iodide chemistry likewise produced the molecular compound $Tc_2(O_2CCH_3)_4I$ rather than a binary diiodide.

Problem Statement

Overarching question, as posed by the 2011 discovery of $\alpha$-$TcCl_2$ and still open: does the $TcCl_2$ structure type — infinite chains of face-sharing [$M_2X_8$] rectangular prisms with intraprism metal–metal multiple bonds — extend beyond the two known technetium dichloride polymorphs to other transition metals and heavier halides, and how does the metal–metal multiple bonding govern the macroscopic properties of such solids? Concretely, with the Mo test already answered negatively ($\beta$-$MoCl_2$ is not $TcCl_2$-type), the following linked questions remain: (1) Can binary $TcBr_2$ be prepared, for example from technetium metal and $Br_2$ or by controlled decomposition or reduction of $TcBr_3$/$TcBr_4$ under conditions that avoid the sodium-stabilized cluster salt $\mathrm{Na}\{[Tc_6Br_{12}]_2Br\}$ and technetium metal, and what solid-state structure does it adopt: $TcCl_2$-type [$Tc_2Br_8$] chains, a 'naked' $Tc_6Br_{12}$ cluster motif, or another arrangement? (2) What are the crystal structures of the long-reported but structurally uncharacterized $ReI_2$ and $OsI_2$, can rhenium dichloride ($ReCl_2$), of unknown existence, be synthesized (e.g. by metallothermic reduction of $ReCl_3$, that is, reduction of $ReCl_3$ by another metal), and do any of these or the tungsten analogs adopt the $TcCl_2$ structure type — i.e., is the type unique to technetium? (3) What, at the atomic scale, is the ~15% intergrowth/orientational disorder that produces the diffuse supercell reflections of $\alpha$-$TcCl_2$, and how does it depend on synthesis conditions? (The 2013 solution of the ordered, modulated $\beta$-$TcCl_2$ identifies the two possible chain orientations but does not characterize the disorder of the $\alpha$ phase or its synthesis dependence.) (4) Quantitatively, how does the intraprism metal–metal multiple bonding in $TcCl_2$-type halides affect their aqueous solubility, dissolution kinetics, and long-term chemical stability — the properties that determine whether such halides are credible waste forms for $^{99}Tc$, given that $TcCl_2$ was reported to be insoluble in water but has never been quantitatively characterized in this respect? These four items jointly map the occurrence and the consequences of the $TcCl_2$ structure type: items (1) and (2) test its generality across metals and halides, item (3) completes the structural characterization of the founding compound, and item (4) connects the motif to the macroscopic property that motivated its waste-form relevance.

References

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