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

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-[99TcI(PyrPNPtBu*)(CO)2] (4), which was synthesized by deprotonation of trans-[99TcI(PyrPNPtBu)(CO)2Cl] with KOtBu. Analogous to its rhenium congener, the dearomatized compound reacts with CO2 to form the carboxy complex cis-[99TcI(PyrPNPtBu–COO)(CO)2] and with H2 to form the mono-hydride complex cis-[99TcI(PyrPNPtBu)(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.


■ INTRODUCTION
Pincer-type ligands and their transition metal complexes have been established as highly versatile compounds in various catalytic processes. 1−6 Especially pyridine-and acridine-based PNP ligands, in combination with a multitude of metal centers, find applications in diverse bond activation processes. 7−15 Their chemistries in combination with 99 Tc is though limited due to its radioactive nature (β − decay with E max = 293 keV, half-life: ∼2 × 10 5 y). 16 Only recently we described 99 Tcdinitrogen complexes employing two structurally related PNP pincer ligands. 17 The Tc III complex (1), comprising the 2,6bis((di-tertbutylphosphino)methyl)pyridine ligand ( Pyr PNP tBu ), was reduced to Tc I , resulting in the coordinatively unsaturated compound [ 99 Tc I ( Pyr PNP tBu )Cl]. If this reduction is carried out under an N 2 atmosphere, the dinuclear [ 99 Tc I ( Pyr PNP tBu )(N 2 )Cl] 2 (μ-N 2 )] complex forms (Scheme 1). We highlighted distinct differences between PNP complexes of technetium and its higher homologue rhenium 18 in presence of N 2 . Rhenium pincer chemistry has been extensively explored, and benchmark catalysts for photochemical, electrochemical, and thermal N 2 fixation have been reported. 19 The radioactive nature of 99 Tc generates a knowledge gap in comparison to its nearest neighbors, especially for uncommon complexes such as those found in bond activation processes. The situation is different for the metastable nuclear isomer 99m Tc, which is an integral part of nuclear medicine in imaging. 20,21 Among the diverse efforts in the field of small molecule activation (SMA), the findings on metal−ligand cooperativity (MLC) represent an opportunity for alternative activation pathways. The first examples of dearomatization/rearomatization cooperativity were reported by Gunanathan and Milstein in 2011 with pyridine-based pincer ligands coordinated to Ru, Ir, and Pt. 22 This type of MLC offers potential for the design of catalysts for globally relevant chemical transformations in a greener fashion. In general, MLC allows for bond activation in substrates while the oxidation state of the metal center remains unchanged. Published results reveal activation of small substrate molecules such as H 2 , 23 C 6 D 6 , 12 alcohols 24 and amines. 25 The covalent binding of CO 2 to transition metal complexes has been reported via a variety of pathways, such as bifunctional reactivity, 26−28 ligand-centered reactivity, 29−31 C− H bond insertion, 32 MLC with a bidentate aminopyridine ligand, 33 and MLC with dearomatized pyridine PNP ligands. 34,35 The pyridine-based PNP ligand ( Pyr PNP tBu ) is prevalent in MLC chemistry, where dearomatization−rearomatization of the pyridine moiety forms the basis of the cooperativity. The rhenium complex cis-[Re( Pyr PNP tBu )-(CO) 2 Cl] has been employed as a platform for dearomatization and subsequent substrate activations by Milstein and coworkers. Deprotonation of this compound leads to the formation of the complex cis-[Re( Pyr PNP tBu *)(CO) 2 ] (the asterisk indicates the dearomatized PNP ligand), which reacts with nitriles via an addition reaction to form ketimido or enamido complexes, and its catalytic properties for Michael addition were established. 34 The loss of aromaticity (dearomatization in the following means conversion of the central pyridine ligand to N-deprotonated 1,2-dihydro-2-methylene pyridine) in the pyridine moiety upon deprotonation is evidenced by a large upfield shift of its protons, while the metal center stabilizes the dearomatized ligand.
We The deep red complex 3 is the second example of small molecules coordinating to the [ 99 Tc I ( Pyr PNP tBu )Cl] framework upon reduction from +III to +I (2, Scheme 1). The IR spectrum of 3 showed two main bands, ν CO at 1952 and 1867 cm −1 in a 1:2 ratio. The more intense band is assigned to the two trans CO ligands, while the presence of the second band suggests the cis isomer in the product. We did not attempt to separate the two complexes at this stage since the relative coordination of the two COs is not essential for the follow-up reactions; however, it gives an insight into the CO coordination mechanism after the reduction. The trans isomer 3 crystallized from benzene in the hexagonal space group P6 1 22. The X-ray crystal structure analysis shows a distorted octahedral geometry induced by the bite angle of 159.43(4)°o f the meridionally coordinated Pyr PNP tBu ligand ( Figure 1). The bite angle of the axially symmetric structure 3 is almost the same as the one of the starting material 1 (159.84(3)°). 17 Furthermore, the 99 Tc NMR spectrum of 3 shows a broad singlet at δ −1266 ppm (Δ 1/2 = 337 Hz), comparable to the chemical shift reported for the Tc I complex [ 99 TcBr-(CO) 3 (PPh 3 ) 2 ] at δ −1460 ppm (Δ 1/2 = 500 Hz). 36 Treatment of a solution of 3 in THF with KO t Bu (or potassium bis(trimethylsilyl)amide) at −20°C results in a color change from deep red to green over the course of 30 min. Removal of solvent, extraction with n-pentane, and crystallization yielded the dearomatized complex 4 as a dark green solid. The deprotonation at one of the methylene groups of the PNP ligand results in the disruption of aromaticity in the former pyridine moiety, accompanied by the loss of the chloride ligand and the formation of [ 99 Tc I ( Pyr PNP tBu *) -(CO) 2 ] (4, Scheme 2). Complex 4 adopts a structure similar to its heavier rhenium homologue. 34 The spectroscopic . The unscathed methylene group of the second pincer arm appears as a doublet at δ 2.74 ppm ( 2 J HP = 9.03 Hz, 1H). The three proton signals of the dearomatized pyridine moiety give rise to a multiplet at δ 6.38 ppm (2H, CH pyr (4,5) ) and a doublet at δ 5.41 ppm (1H, CH pyr (3) ). These signals corroborate the dearomatization of the pyridine moiety. In the 31 P{ 1 H} NMR spectrum, the two chemically inequivalent phosphorus nuclei were observed as two very broad signals at δ 80.44 (Δ 1/2 = 524 Hz) and 70.67 ppm (Δ 1/2 = 513 Hz). The line broadening of the signals is a result of the coupling of the phosphorus nuclei to 99 Tc with a nuclear spin of 9/2. The carbon nuclei of the methine and methylene groups were observed as doublets in the 13 C{ 1 H} NMR at δ 68.3 and 35.7 ppm, respectively. The resonances for the two cis carbonyl ligands were not observed due to the high quadrupolar moment and 9/2 spin of 99 Tc. Together with the coupling to both phosphorus nuclei, this leads to an enormously broadened signal. The 99 Tc resonance was observed as a very broad signal at δ −1103 ppm (Δ 1/2 = 3.4 kHz), in the same region as the cis-dicarbonyl compound [ 99 Tc(η 2 -O,S−Et 2 btu)(CO) 2 (PPh 3 ) 2 ] (δ −1119 ppm (Δ 1/2 = 2.5 kHz)). 38 Compound 4 was crystallized from a saturated pentane solution at −20°C and analyzed by single-crystal Xray diffraction. The structure is depicted in Figure 2 and compared to the homologous rhenium structure reported by Vogt et al. 34 The single-crystal structure of 4 reveals the mutual cis orientation of the two CO ligands (

Reactions of cis-[ 99 Tc( Pyr PNP tBu *)(CO) 2 ] (4) with CO 2 , CS 2 , and H 2 .
Key compound 4 enables the exploration of its reactivity toward small molecules and its comparison with rhenium. Milstein and co-workers reported on the activation of CO 2 via MLC with Ru and Re PNP complexes, 35,39 which inspired us to explore the equivalent reaction with 99 Tc. Dearomatized 4 was freshly prepared, dissolved in pentane, and exposed to CO 2 gas via a syringe at 23°C, which resulted in an immediate color change from green to pale yellow and precipitation of the product. Evaporation of the solvent afforded cis-[ 99 Tc I ( Pyr PNP tBu −COO)(CO) 2 ] (5, Scheme 2) as a pale yellow solid. The CO 2 binding proceeds as a [1,3]addition to the −Tc−P−CH− moiety. The reaction results in the formation of a C−C bond between the carbon dioxide molecule and the PNP backbone methine group, the formation of a Tc−O bond, and the rearomatization of the pyridine ring. The 1 H NMR spectrum provides evidence for the rearomatization and for the asymmetry of the PNP ligand as the pyridine protons appear as three separate signals at δ 6.78 ppm (t, 3 J HH = 7.78 Hz, 1H, CH pyr(4) ), 6.73 ppm (d, 3 J HH = 7.76 Hz, 1H), and 6.34 ppm (d, 3 J HH = 7.70 Hz, 1H). The asymmetry is further obvious in the 31 P{ 1 H} NMR where two strongly broadened signals are observed at δ 113.2 (Δ 1/2 = 5.3 kHz) and 91.1 ppm (Δ 1/2 = 1.8 kHz). These spectroscopic features coincide with the data reported for rhenium, and the 13 C{ 1 H} NMR resonance of the carboxylate carbon (Tc−O−CO−C)   The binding of CO 2 via MLC in complex 5 illustrates basic similarities in the reactivities of homologous Re I and Tc I complexes and is the first example of CO 2 directly binding to a Tc complex. Single crystals for X-ray diffraction could not be obtained. However, the reaction of 4 with CS 2 instead of CO 2 resulted in a rapid color change to dark yellow and the formation of a precipitate. Evaporation of the solvent, extraction with benzene, and subsequent crystallization gave dark yellow cis-[ 99 Tc I ( Pyr PNP tBu −CSS)(CO) 2 ] (6, Scheme 2). Spectroscopy of the dithiocarboxylate complex 6 evidenced a high similarity to 5 with CO 2 implying an equivalent [1,3]addition as with CO 2 . The pyridine protons appear as a set of two doublets and a triplet in the 1 H NMR spectrum, and the methine proton of the pincer backbone appears as a doublet of doublets at δ 5.89 ppm (dd, 2 J HP = 6.63 Hz, 4 J HP = 1.48 Hz, 1H). The phosphorus nuclei were observed in a similar region as in 5. The 13 C{ 1 H} NMR signal of the PNP−CS−S−Tc moiety shifted upfield as compared to 5, overlaid with the signal of a quaternary pyridine carbon, consistent with the lower electronegativity of sulfur relative to oxygen. In the 99 Tc NMR spectrum, the signal for 6 was found as an upfield-shifted resonance of δ −1282 ppm (Δ 1/2 = 1.9 kHz). The stronger color of 6 over 5 is evidenced by the more intense molar absorption coefficient at λ max 428 nm (ε = 4317) which is bathochromically shifted [5, λ max 397 nm (ε = 400)]. The IR spectrum shows ν CO in an approximate 1:1 ratio at 1934 and 1872 cm −1 . Crystals grown from a saturated toluene solution at −20°C allowed for a solid-state structure analysis of 6 ( Figure  3).
The solid-state structure of the dithiocarboxylate 6 reveals bond lengths for S1−C26 and S2−C26 of 1.6637(14) and 1.6982(14) Å, respectively. This difference of 0.03 Å suggests delocalization of the π electrons in the CSS moiety and matches the values found for a Cu complex comprising a CS 2 group in a similar binding motif. 40 The coordination geometry of 6 is a highly distorted octahedron, which is corroborated by the bond angle P1−Tc1−S2 of 79.714(12)°. The two CO ligands are still oriented in an approximate cis fashion (C24− Tc1−C25 = 91.39(7)°). The co-crystallized triphenylphosphine originates from the synthesis of starting material 1 and was not removed in the work-up.
The reaction of [Re( Pyr PNP tBu *)(CO) 2 ] with H 2 leads to a heterolytic cleavage. Following the binding of CO 2 and CS 2 to Tc, a pentane solution of 4 was treated with a gentle stream of dihydrogen via a syringe. The green solution immediately turned pale yellow. After evaporation of the solvent, the monohydride complex 7 (Scheme 2) was obtained in quantitative yield. The analysis of cis-[ 99 Tc I ( Pyr PNP tBu )(CO) 2 H] by 1 H NMR evidenced its homologous structure to the rhenium analogue. The heterolytic H 2 splitting presumably proceeds via initial side-on coordination, polarization, and deprotonation by the methine group. The pincer backbone is protonated, and the four protons appear as a not fully resolved triplet-like singlet at δ 3.03 ppm. The protons of the pyridine moiety appear as a triplet and doublet signal at δ 6.68 ppm (t, 3 J HH = 7.68 Hz, 1H, CH pyr(4) ) and 6.37 ppm (d, 3 J HH = 7.70 Hz, 2H, CH pyr (3,5) ), indicating a symmetry plane through the aromatic ring. The hydride appears as a triplet at a characteristic chemical shift for transition metal hydrides of δ −3.20 ppm (t, 2 J HP = 24.09 Hz, 1H, Figure 4) due to the coupling to the chemically equivalent phosphorus nuclei of the PNP ligand. The fact that the hydride signal can be observed in the 1 H NMR, in contrast to the lack of signals for CO ligands in the 13 C{ 1 H} spectra for all complexes, is attributed to a small scalar coupling constant J of the 1 H− 99 Tc pair (applied formulas can be found in the Supporting Information). The identical chemical environment of both phosphorus nuclei is also apparent in the observation of a single peak in the 31 P{ 1 H}   43 There, the alkynes were deprotonated with organo-lithium reagents prior to coordination, while in our reaction, complex 4 acts as an internal base and anion acceptor in one. In the 1 H NMR spectrum, the pyridine protons for 8 and 9 appear as triplets at roughly δ 6.7 ppm (1H, CH pyr (4) ) and doublets at δ 6.4 ppm (1H, CH pyr (3,5) ), indicating reestablished symmetry in the PNP ligand. Furthermore, the methylene and t Bu protons are observed at nearly identical chemical shifts in both complexes. The nine TMS protons of 9 appear as a singlet at δ 0.22 ppm and the phenyl protons of 8 as an overlaid doublet at δ 7.40 ppm (2H), a triplet at δ 7.11 ppm (2H), and a triplet at δ 6.94 ppm (1H). These signals indicate free rotation of both the phenyl and the TMS moieties. The 31 P{ 1 H} spectra revealed broad singlets at δ ≈ 92 ppm, corroborating the chemical equivalence of both phosphorus nuclei, comparable to what was found in the mono-hydride complex 7. The 29 Si NMR signal for the TMS group was observed at δ −29.31 ppm. The 99 Tc NMR resonance of the TMS acetylide complex was found as a strongly broadened signal at δ −1435 ppm (Δ 1/2 = 15.7 kHz), downfield shifted as compared to the published [ 99 Tc I (−C� C−TMS)(CO) 3 (PPh 3 ) 2 ] (δ −1939 ppm (Δ 1/2 = 8.2 kHz)). 43 The 99 Tc signal for the phenylacetylide complex 8 was not observed, likely due to a high field gradient induced by the ligands leading to a distinctly rapid quadrupolar relaxation of 99 Tc. Thus, the signal is broadened to the point of becoming unrecognizable on the NMR time scale. The ν C�C is found at 2070 (Tc−C�C−Ph) and 2010 cm −1 (Tc−C�C−TMS), respectively. The two respective bands for the CO ligands appeared at 1915 and 1841 (Tc−C�C−Ph) or 1844 (Tc− C�C−TMS) cm −1 in 1:1 ratios. Further structural evidence for the nature of the acetylide complexes was found in a singlecrystal structure analysis by XRD. Suitable crystals of 9 were grown from a saturated pentane solution at −20°C ( Figure 5).
The intercarbonyl angle (C24−Tc1−C25) is 88.9(2)°c omparable to 4 and 6. The carbon−carbon triple bond length is 1.211(7) Å, similar to C 2 H 2 (1.20(2) Å 44 and [ 99 Tc I (−C�C−TMS)(CO) 3 (PPh 3 ) 2 ] (1.213(6) Å). 43 Bond lengths and angles for the PNP ligand remain nearly unchanged compared to 4. We note that alkynes must be terminal, bearing an "acidic" proton (the pK a of phenylacetylene is 28.7 in DMSO 45 ). Diphenylacetylene did not react with 4, confirming the observation. Terminal alkenes (such as 1-hexene) did not react with 4 either, presumably due to the far higher pK a of alkenes (ca. 44 for, e.g., propene 45 ). The spectroscopic analysis of the carbene complex indicated a coordinated NHC carbene unit, hindered in its axial rotation due to the steric demands of the −P( t Bu) 2 groups. The methine in 4 deprotonated the [DMIM] cation (pK a of 23.0 in water), 46 thereby forming the NHC-carbene in situ. The carbene unit is virtually "locked in" between the phosphine substituents in contrast to the acetylides in 8 and 9, which are rotating freely. The 1 H NMR spectrum reflects this structural feature in the distinctly different chemical shifts of the methyl and methylene protons. The two CH 3 groups are observed as two singlets at δ 3.72 ppm (3H, Im CH 3 ) and δ 3.37 ppm (3H, Im CH 3 ), while the two broad singlets δ 5.89 ppm (1H, Im CH) and δ 5.73 ppm (1H, Im CH) are assigned to the CH protons of the carbene unit. Interestingly, the protons for the pyridine moiety appear as a set of three signals at δ 6.58 ppm (t, 3 J HH = 7.37 Hz, 1H, CH pyr(4) ), 6.38 ppm (d, 3 J HH = 8.65 Hz, 1H, CH pyr(5) ), and 5.63 ppm (d, 3 J HH = 6.30 Hz, 1H, CH pyr (3) ). Two broad signals were observed in the 31 P{ 1 H} NMR spectrum at δ 82.23 and 61.66 ppm. The 99 Tc NMR resonance for 10 was found at δ −1277 ppm (Δ 1/2 = 5.5 kHz). The carbon signals for the carbonyl ligands and the carbene (−N− C−N−) could not be observed due to coupling to 99 Tc. However, the carbonyl ligands gave rise to two bands at 1910 and 1822 cm −1 (1:1 ratio) in the IR spectrum, which are clearly shifted compared to 4.
As was found for 10, the 1 H NMR spectra of 11 and 12 evidenced coordination of the ligands and hindered rotation due to being "sandwiched" between the −P( t Bu) 2 groups. The pyrrolide protons appear as four distinct signals at δ 7.69 ppm (1H, CH (26) ), 6.69 ppm (1H, CH (27) ), 6.60 ppm (1H, CH (28) ), and 6.10 ppm (1H, CH (29) ). This asymmetry and the large down-field shift of the CH (26) proton is a result of its proximity to the aromatic pyridine ring of the PNP ligand and the

Inorganic Chemistry
pubs.acs.org/IC Article influence of the aromatic ring current (cf., solid-state structure in Figure 6). The 2D and correlation NMR spectra confirm the coordination of the pyrrolide. The protons of the PNP ligand appeared in comparable regions as for the other complexes in this study. The phosphorus resonances were observed as a broadened multiplet at δ 79.44 ppm in the 31 P{ 1 H} NMR spectrum (Δ 1/2 = 3.7 kHz) while the 99 Tc signal was found at δ −1000 ppm (Δ 1/2 = 4.0 kHz). NMR spectra of the pyrazolide complex 12 are comparable to those of the nearly identical pyrrolide compound (Figures S67−S73 and S79−S85). Three protons for the pyrazolide moiety were observed at 8.25 ppm (1H, CH (28) ), 7.83 ppm (1H, CH (27) ), and 6.50 ppm (1H, CH (26) ) in the 1 H NMR spectrum. The pyrazolide ligand is also hindered in its rotation. Interestingly, the observation of a single set of three protons clearly indicates that the anionic ligand selectively coordinates with the imine oriented in the direction of the pyridine ring. This strongly suggests that the reaction proceeds either via a concerted deprotonationcoordination pathway or a stepwise pathway via imine coordination and subsequent deprotonation. The 31 P(1H) and 99 Tc NMR resonances of 12 appear at virtually the same shifts compared to 11 at δ 76.95 ppm (Δ 1/2 = 3.8 kHz) and δ −1013 ppm (Δ 1/2 = 4.0 kHz), respectively. The structural similarity of 11 and 12 is confirmed by X-ray structure analyses. For example, both anionic ligands are bound to Tc with an approximately identical bond length (Tc1−N2) of 2.222(3) Å (11) and 2.2204(15) Å (12). The two structures reveal distorted octahedral geometries with bite angles defined by the PNP ligand (P1−Tc1−P2) of 158.06(4) Å for 11 and 157.359(17) Å for 12, respectively. The carbonyl ligands are cis oriented (C24−Tc1−C25: 86.67(17)°for 11, 89.61(9)°for 12), substantiating the 1:1 ratio of the carbonyl bands found in the IR spectrum, comparable to all other cis compounds in this work. Unsurprisingly, the general coordination geometry of the PNP ligand was determined to be relatively similar to that of the other discussed compounds. The solid-state structure further verifies that the rotation of the pyrrolide and pyrazolide ligands is rendered impossible due to the bulky t Bu groups. Especially in the pyrrolide complex 11, this leads to the largely differing chemical shifts of the four protons as they are governed by the relative surroundings of the pincer ligand. Due to the constraints of the phosphine substituents and hindered rotation of the heterocyclic anionic ligands, the proton signals are not observed with their expected multiplicities but rather as broadened singlets. Pyrrolide and pyrazolide complexes 11 and 12 with 99 Tc are the first of their kind. Only two pyrrolide complexes have been reported for Re 47,48 and one pyrazolide complex for Mn within the manganese triad. 49

■ CONCLUSIONS
The well-established activation of substrates via MLC with rhenium pincer complexes was translated to its lighter homologue, technetium. Reactions with H 2 and CO 2 substantiated equivalent reactivities of Re and 99 Tc in low oxidation states. We emphasize that this is not a necessity given the similarity between 4d and 5d elements, as distinct differences have been reported in the literature. 41,42 The concept of MLC was applied to an extended substrate scope with a focus on weakly acidic ligands. The dearomatized pincer backbone of cis-[ 99 Tc I ( Pyr PNP tBu *)(CO) 2 ] acts as the internal base in reactions with H 2 , CO 2 , terminal alkynes, an imidazolium cation, pyrrole, and pyrazole. The driving force for these reactions is the rearomatization of the highly basic (Lewis and Brønsted) pincer ligand, formally stabilized by the technetium center. The formation of 99 Tc acetylide complexes without initial deprotonation is the first of its kind for Tc. The fact that less acidic substrates such as 1-hexene did not react highlights the limitations of this reactivity pathway. Based on the pK a values of the substrates, the pK a of the dearomatized technetium complex can thus be roughly estimated as being between 28.7 and 32.2 (pK a of t BuOH in DMSO: 32.2). 50 This work contributes to the closing of a "knowledge gap" between Re and Mn. ■ ASSOCIATED CONTENT * sı Supporting Information