Binding Small Molecules to a cis-Dicarbonyl 99TcI-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 ⁹⁹Tc. In this work, we explore this bond activation pathway with the dearomatized PNP complex cis-[⁹⁹Tc^I(^PyrPNP^tBu*)(CO)₂] (4), which was synthesized by deprotonation of trans-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂Cl] with KO^tBu. Analogous to its rhenium congener, the dearomatized compound reacts with CO₂ to form the carboxy complex cis-[⁹⁹Tc^I(^PyrPNP^tBu-COO)(CO)₂] and with H₂ to form the mono-hydride complex cis-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂H] (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.

Scheme 1. Two-Electron Reduction of 1 to the Electronically Unsaturated Intermediate [⁹⁹Tc^I(^PyrPNP^tBu)Cl], which Coordinates N₂ to Yield 2 or CO for 3(17)

Scheme 2. Synthesis of Dearomatized [⁹⁹Tc(^PyrPNP^tBu*)(CO)₂] (4) by Deprotonation of 3 with KO^tBu and Its Reaction with CO₂, CS₂, and H₂ to Form the Tc–Y–CY–PNP Complexes (Y = O, 5; Y = S, 6) and Monohydride 7

Figure 1

Ellipsoid displacement plot of trans-[⁹⁹Tc(^PyrPNP^tBu)(CO)₂Cl] (3). Ellipsoids represent a 35% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Tc1–P1 2.4092(7), Tc1–N1 2.136(3), Tc1–C13 1.983(3), Tc1–Cl1 2.5261(10), C13–O1 1.109(4), C1–C2 1.498(4), C13–Tc1–C13¹ 176.76(19), P1–Tc1–P1¹ 159.43(4), Cl1–Tc1–P1 100.287(18), and Cl1–Tc1–C13 88.38(10).

Figure 2

Ellipsoid displacement plots of [⁹⁹Tc(^PyrPNP^tBu*)(CO)₂] (4, left) and the homologous [Re(^PyrPNP^tBu*)(CO)₂] (right). Ellipsoids represent a 35% probability. Hydrogen atoms, except for the methine and methylene groups, are omitted for clarity.

Figure 3

Ellipsoid displacement plot of [⁹⁹Tc(^PyrPNP^tBu–CSS)(CO)₂] × PPh₃ (6). Ellipsoids represent a 35% probability. Hydrogen atoms and the co-crystallized PPh₃ molecule are omitted for clarity. Selected bond lengths (Å) and angles (°): Tc1–P1 2.4296(4), Tc1–P2 2.4303(4), Tc1–N1 2.1743(12), Tc1–C24 1.9000(16), Tc1–C25 1.8918(17), C24–O1 1.158(2), C25–O2 1.161(2), Tc1–S2 2.4997(4), S2–C26 1.6982(14), S1–C26 1.6637(14), C24–Tc1–C25 91.39(7), S1–C26–S2 124.31(9), C1–C26–S2 116.26(10), Tc1–S2–C26 102.64(5), and P1–Tc1–S2 79.714(12).

Figure 4

Excerpt of the ¹H NMR spectrum of the mono-hydride complex 7, displaying the triplet signal for the Tc–H proton at δ −3.19 ppm (Δ_1/2 = 62 Hz, full spectra in Supporting Information). The multiplicity arises from the coupling to both chemically equivalent phosphorus nuclei, and the signal is broadened because of the direct coupling to Tc.

Scheme 3. Syntheses of Acetylide Complexes 8 and 9, Carbene Complex 10, Pyrrolide Complex 11, and Pyrazolide Complex 12 from Dearomatized [⁹⁹Tc(^PyrPNP^tBu*)(CO)₂] (4)

Figure 5

Ellipsoid replacement plot of cis-[⁹⁹Tc(^PyrPNP^tBu)(CO)₂(−C≡C–TMS)] (9). Ellipsoids represent a 35% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Tc1–P1 2.4299(15), Tc1–P2 2.4183(15), Tc1–N1 2.192(4), Tc1–C24 1.882(6), Tc1–C25 1.910(6), C24–O1 1.161(7), C25–O2 1.173(7), Tc1–C26 2.132(5), C26–C27 1.211(7), C24–Tc1–C25 88.9(2), Tc1–C26–C27 174.3(5), P1–Tc1–C26 90.63(15), and P2–Tc1–C26 85.22(15).

Figure 6

Ellipsoid replacement plots of cis-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂(NC₄H₄)] (11, left) and cis-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂(N₂C₃H₃)] (12, right). Ellipsoids represent a 35% probability. Hydrogen atoms are omitted for clarity.