Binding Small Molecules to a cis-Dicarbonyl ⁹⁹Tc^I-PNP Complex via Metal–Ligand Cooperativity

Abstract

Metal−ligand cooperativity is a powerful tool for the activation of various bonds but has rarely, if ever, been studied with the radioactive transition metal 99 Tc. In this work, we explore this bond activation pathway with the dearomatized PNP complex cis-[ 99 Tc I ( Pyr PNP tBu *)(CO) 2 ] (4), which was synthesized by deprotonation of trans-[ 99 Tc I ( Pyr PNP tBu )(CO) 2 Cl] with KO t Bu. Analogous to its rhenium congener, the dearomatized compound reacts with CO 2 to form the carboxy complex cis-[ 99 Tc I ( Pyr PNP tBu −COO)(CO) 2 ] and with H 2 to form the mono-hydride complex cis-[ 99 Tc I ( Pyr PNP tBu )(CO) 2 H] (7). Substrates with weakly acidic protons are deprotonated by the Brønsted basic pincer backbone of 4, yielding a variety of intriguing complexes. Reactions with terminal alkynes enable the isolation of acetylide complexes. The deprotonation of an imidazolium salt results in the in situ formation and coordination of a carbene ligand. Furthermore, a study with heterocyclic substrates allowed for the isolation of pyrrolide and pyrazolide complexes, which is uncommon for Tc. The spectroscopic analyses and their solid-state structures are reported.

Existence and Properties of [TcH(CO)5]

“…No splitting of the 99 Tc signal due to Tc–H coupling is observed, probably owing to the high quadrupole moment of this nucleus. A similar behavior was observed for the 99 Tc NMR spectrum of a technetium carbonyl hydride complex with a pincer ligand; it was attributed to small value of the Tc–H coupling constant. It should be noted that 99 Tc NMR signals of technetium pentacarbonyl halides ([ 99 TcX­(CO) 5 ], X = Cl, Br, I) are in the range from −1700 to −2000 ppm.…”

“…Complexes of the fac -{Tc­(CO) 3 } + type have become a focus in research after [Tc­(CO) 3 (H 2 O) 3 ] + has been made available through a convenient 1 atm synthesis. − There is, however, still a knowledge gap in the fundamental chemistry of technetium carbonyl hydrides as compared to the rhenium and manganese analogues. The number of unambiguously characterized technetium carbonyl hydride compounds is limited: [ 99 Tc 3 H 3 (CO) 12 ], , [ 99 Tc 3 H­(CO) 14 ], [ 99 Tc 2 (μ-H)­(μ-NC 5 H 4 )­(NC 5 H 5 ) 2 (CO) 6 ], [ 99 TcH­(CO) 3 (PPh 3 ) 2 ], , [H 99 Tc­(CO)­(dppe) 2 ], [HTc­(CO)­(CN p -FAr DArF2 ) 4 ], and [ 99 Tc­( Pyr PNP tBu )­(CO) 2 H] . X-ray structural data are available only for the first four compounds.…”

Induced fac–mer rearrangements in {M(CO)3}+ complexes (M = Re, 99(m)Tc) by a PNP ligand

“…These tridentate PNP-based pincer systems are preorganised for meridional coordination with various d-elements, also with 99 Tc, as recently reported by us. 17,18 The distinct preference of pincer ligands such as 2,6-bis((di-tertbutylphosphino) methyl)pyridine ( Pyr PNP tBu ) for coordinating the metal centre in a meridional fashion prompted us to explore their interactions with Re and 99(m) Tc tricarbonyl cores. Of special interest was the question if the mer-{ 99m Tc(CO) 3 } + motif was accessible for radiopharmaceuticals as well.…”