d8···d10 RhI···AuI Interactions in Rh 2,6-Xylylisocyanide Complexes with [Au(CN)2]: Bond Analysis and Crystal Effects

Producción CientíficaThe well known [RhL4]n(anion)n structures, with RhI···RhI d8···d8 interactions, are replaced by others with RhI···AuI d8···d10 interactions such as [{RhL4}{Au(CN)2}] (L = 2,6-Xylylisocyanide) or [{RhL4}{Au(CN)2} {RhL4}{Au2(CN)3}·4(CHCl3)]∞ when the anion is [Au(CN)2]–. Orbital (Rh···Au), coulombic, and inter-unit π-π aryl stacking interactions stabilize these crystal structures.Ministerio de Economía, Industria y Competitividad (grant CTQ2017-89217-P)Junta de Castilla y León (project VA038G18

Reactivity of Fluorinated-Chalcone Phosphines, RPEWO-F4, Induced by C–F Activation upon Coordination to PdCl2

Producción CientíficaThe E phosphine ligands (R = Ph, o-Tol, Cy), abbreviated as RPEWO-F4, are stable in solution, but they develop a rich reactivity on coordination to PdCl2. The chelate P-olefin coordination to PdCl2 leads eventually to a Z conformation of the fluorinated-chalcone group o-C6F4CH═CHC(O)Ph. From there, a cyclization reaction occurs involving the C═O group and activation of a F atom, yielding a strongly chelated PdCl2(P-carbene) complex. The carbene carbon atom in the complex displays some electrophilicity, which is expressed in hydrolysis, ammonolysis, and oxidation (with peroxide) reactions, affording PdCl2 complexes with new P,C,O-pincer, P,C,N-pincer, or P,O-chelate fluorinated ligands. The C–F activation reactions are slow in comparison to the catalysis rates when the [PdCl2(RPEWO-F4)] complexes have been used in Negishi catalyses. Consequently, the reactivity discussed here is not expected to interfere with the interpretation of the data obtained in Pd-catalytic studies or processes, at least for fast transmetalating nucleophiles.Ministerio de Economía, Industria y Competitividad (projects CTQ2017-89217-P and CTQ2016-80913-P)Junta de Castilla y León (projects VA051P17 and VA062G18

4-Pyridylisocyanide Gold(I) and Gold(I)-plus-Silver(I) Luminescent and Mechanochromic Materials: The Silver Role

Producción CientíficaCrystallographic and DFT examination of the metalloligands [AuAr(CNPy-4)] (Ar = C6F5 (1), C6F3Cl2-3,5 (2)) and their silver complexes [Ag[AuAr(CNPy-4)]2](BF4) (3 and 4) support that the marked luminescence red-shifts observed on moving from 1 to 2, from 1,2 to 3,4, or upon grinding, are not caused by electronic differences (either by changing the aryls C6F5/C6F3Cl2, or by N coordination to silver), nor by non-existent Au···Ag interactions. They are always due to structural changes disturbing stronger π-π stackings in order to allow for shorter Au···Au interactions.Ministerio de Economía, Industria y Competitividad (grant CTQ2017-89217-P )Junta de Castilla y León (project VA038G18

E–Z Isomerization of Phosphine-Olefin (PEWO-F4) Ligands Revealed upon PdCl2 Capture: Facts and Mechanism

Producción CientíficaThe PEWO phosphines R2P(o-C6H4CH═CHC(O)Ph), R2P(o-C6H2F2CH═CHC(O)Ph), and R2P(o-C6F4CH═CHC(O)Ph) and their P-monodentate complexes trans-[PdCl2(P-monodentate)2] show, in solution and (when available) in the X-ray diffraction structures, an E configuration of the double bond. In contrast, the structures of [PdCl2(P-chelate)] display E and Z configurations. The E/Z isomerization of the latter requires first decoordination of the double bond, which then allows for easy rotation about the electron-deficient double bond. Thus, the E/Z equilibria exist for the free and the P-monodentate complexes as well but are not observed because they are extremely displaced toward the E isomer. Their capture in the form of [PdCl2(P-chelate)], with equilibrium constants on the order Keq ≈ 1–3, allows the two configurations to be observed and isolated. Evaluation of their ability to couple Pf–Pf from cis-[PdPf2(THF)2] (Pf = C6F5) affords values of their ΔG⧧(Pf–Pf)Pd parameters confirming that higher substitution of H by F produces lower coupling barriers and a double bond that is more electron deficient when it is free and more electron withdrawing when it is coordinated.Junta de Castilla y León (projects VA051P17 and VA062G18)Ministerio de Economía, Industria y Competitividad (projects CTQ2016-80913-P and CTQ2017-89217-P

Coordinatively Unsaturated [RhCp*Rf2] (Cp* = C5Me5; Rf = C6F3Cl2-3,5), General Precursor to Cp*-Diaryl and Cp*-Halo-Aryl RhIII Complexes. Observing and Testing the Effect of Cp* as Electronic Buffer

Producción CientíficaThe pentacoordinated [RhCp*Rf2] (Rf = C6F3Cl2-3,5) and the octahedral (μ-Cl)2[RhCp*Rf]2, obtained by stoichiometric rearrangement with (μ-Cl)2[RhCp*Cl]2, are general precursors of [RhCp*RfXL] (X = Rf, Cl; L = ligand) complexes, which were studied by NMR (L dissociation and fluxional processes) and X-ray diffraction (structural effects affecting the Rh–Cp* distances) techniques. The Rh–Cp*centroid distances decrease markedly for identical L in the order [RhCp*Rf2L] > [RhCp*RfClL] > [RhCp*Cl2L] and are further influenced regularly within each family by the trans influence of L (longer distances for higher trans influence of L). The structural effects observed reveal a remarkable capability of Cp* to act as an electron-density buffer, which attenuates the Rh electron density variations induced by the substituents in front of Cp* by releasing toward Rh or polarizing toward Cp*, on demand, the electron density of the Rh–Cp* bonds. This buffer effect explains the easy L dissociation from [RhCp*Rf2L] and the accessibility to formally 16e pentacoordinated [RhCp*Rf2].2019-09-092019-09-09Ministerio de Economía, Industria y Competitividad (Projects CTQ2016-80913-P, CTQ2014-52796-P, and CTQ2017-89217-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA051P17

Hidden aryl-exchange processes in stable 16e RhIII [RhCp*Ar2] complexes, and their unexpected transmetalation mechanism

Producción CientíficaExperiments mixing the stable 16e 5-coordinate complexes [RhCp*Ar2] (Cp* = C5Me5; Ar = C6F5, C6F3Cl2-3,5) uncover fast aryl transmetalations. Unexpectedly, as supported computationally, these exchanges are not spontaneous, but catalyzed by minute amounts of 18e (μ-OH)2[RhCp*Ar]2 as a source of 16e [RhCp*Ar(OH)]. The OH group is an amazingly efficient bridging partner to diminish the activation barrier of transmetalation.2019-04-04Ministerio de Economía, Industria y Competitividad (Projects CTQ2016-80913-P and CTQ2014-52796-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA051P17

Some Singular Features of Gold Catalysis: Protection of AuI Catalysts by Substoichiometric Agents, and Associated Phenomena

Producción CientíficaThis study deals with two striking phenomena: the complete protection against decomposition of hypothetically monocoordinated AuI intermediates [AuL]Y (L = strongly coordinating ligand; Y– = poorly coordinating anion) by addition of small substoichiometric amounts (5 mol% relative to Au) of not strongly coordinating ligands (e.g. AsPh3), and the fact that, in contrast, strongly coordinating ligands cannot provide this substoichiometric protection. The two phenomena are explained considering that: i) the existence of real monocoordinated [AuL]Y is negligible in condensed phases, and the kinetically efficient existing species are dicoordinated [AuL(W)]Y (W = any very weekly coordinating ligand existing in solution, including OH2, the solvent, or the Y– anion); and ii) these [AuL(W)]Y intermediates give rise to decomposition by a disproportionation mechanism, via polynuclear intermediates formed by associative oligomerization with release of some W ligands. It is also shown that very small concentrations of [AuL(W)]Y are still catalytically efficient, and can be stabilized by overstoichiometric adventitious water, so that full decomposition of the catalyst is hardly reached although eventually the stabilized concentration can result kinetically inefficient for the catalysis. These results suggest that, in cases of gold catalysis requiring the use of a significant quantity of gold catalyst, the turnover numbers can be increased, or the concentration of gold catalyst widely reduced, using substoichiometric protection properly tuned to the case.2017-12-30Ministerio de Economía, Industria y Competitividad (CTQ2014-52796-P)Ministerio de Economía, Industria y Competitividad (CTQ2013-48406-P)Junta de Castilla y León (programa de apoyo a proyectos de investigación – GR 169 and project VA256U13

Problematic ArF–Alkynyl Coupling with Fluorinated Aryls. From Partial Success with Alkynyl Stannanes to Efficient Solutions via Mechanistic Understanding of the Hidden Complexity

Producción CientíficaThe synthesis of aryl–alkynyl compounds is usually achieved via Sonogashira catalysis, but this is inefficient for fluorinated aryls. An alternative method reported by Shirakawa and Hiyama, using alkynylstannanes and hemilabile PN ligands, works apparently fine for conventional aryls, but it is also poor for fluorinated aryls. The revision of the unusual literature cycle reveals the existence and nature of unreported byproducts and uncovers coexisting cycles and other aspects that explain the reasons for the conflict. This knowledge provides a full understanding of the real complexity of these aryl/alkynylstannane systems and the deviations of their evolution from that of a classic Stille process, providing the clues to design several very efficient alternatives for the catalytic synthesis of the desired ArF–alkynyl compounds in almost quantitative yield. The same protocols are also very efficient for the catalytic synthesis of alkynyl–alkynyl’ hetero- and homocoupling.Ministerio de Asuntos Económicos y Transformación Digital (project PID2020- 118547GB-I00)Junta de Castilla y León (project VA224P20)Irish Research Council (GOIPD/2020/701)Universidad de Valladolid (Margarita Salas program, ref. CONVREC- 2021-221

Sodium Mediated Deprotonative Borylation of Arenes Using Sterically Demanding B(CH2SiMe3)3: Unlocking Polybasic Behaviour and Competing Lateral Borane Sodiation

Producción CientíficaThe deprotonative metalation of organic molecules has become a convenient route to prepare functionalised aromatic substrates. Amongst the different metallating reagents available, sodium bases have recently emerged as a more sustainable and powerful alternative to their lithium analogues. Here we report the study of the sterically demanding electrophilic trap B(CH2SiMe3)3 for the deprotonative borylation of arenes using NaTMP (TMP = 2,2,6,6-tetramethylpiperidide) in combination with tridentate Lewis donor PMDETA (PMDETA = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine). Using anisole and benzene as model substrates, unexpected polybasic behaviour has been uncovered, which enables the formal borylation of two equivalents of the relevant arene. The combination of X-ray crystallographic and NMR monitoring studies with DFT calculations has revealed that while the first B–C bond forming process takes place via a sodiation/borylation sequence to furnish [(PMDETA)NaB(Ar)(CH2SiMe3)3] (I) species, the second borylation step is facilitated by the formation of a borata-alkene intermediate, without the need of an external base. For non-activated benzene, it has aslo been found that under stoichimetric conditions the lateral sodiation of B(CH2SiMe3)3 becomes a competitive reaction pathway furnishing a novel borata-alkene complex. Showing a clear alkali-metal effect, the use of the sodium base is key to access this reactivity, while the metalation/borylation of the amine donor PMDETA is observed instead when LiTMP is usedUniversidad de Valladolid. Margarita Salas Postdoctoral Fellowship (CONVREC-2021-221)University of Bern and the Swiss National Science Foundation (Grant numbers 188573, 210608 and R'Equip 206021_177033)The Irish Research Council (M.M. GOIPG/2021/88

Experimental study of speciation and mechanistic implications when using chelating ligands in aryl-alkynyl Stille coupling

Producción CientíficaNeutral palladium(II) complexes [Pd(Rf)X(P–L)] (Rf = 3,5-C6Cl2F3, X = Cl, I, OTf) with P–P (dppe and dppf) and P–N (PPh2(bzN)) ligands have chelated structures in the solid-state, except for P–L = dppf and X = Cl, were chelated and dimeric bridged structures are found. The species present in solution in different solvents (CDCl3, THF, NMP and HMPA) have been characterised by 19F and 31P{1H} NMR and conductivity studies. Some [Pd(Rf)X(P–L)] complexes are involved in equilibria with [Pd(Rf)(solv)(P–L)]X, depending on the solvent and X. The ΔH° and ΔS° values of these equilibria explain the variations of ionic vs. neutral complexes in the range 183–293 K. Overall the order of coordination strength of solvents and anionic ligands is: HMPA ≫ NMP > THF and I−, Cl− > TfO−. This coordination preference is determining the complexes participating in the alkynyl transmetalation from PhC[triple bond, length as m-dash]CSnBu3 to [Pd(Rf)X(P–L)] (X = OTf, I) in THF and subsequent coupling. Very different reaction rates and stability of intermediates are observed for similar complexes, revealing neglected complexities that catalytic cycles have to deal with. Rich information on the evolution of these Stille systems after transmetalation has been obtained that leads to proposal of a common behaviour for complexes with dppe and PPh2(bzN), but a different evolution for the complexes with dppf: this difference leads the latter to produce PhC[triple bond, length as m-dash]CRf and black Pd, whereas the two former yield PhC[triple bond, length as m-dash]CRf and [Pd(C[triple bond, length as m-dash]CPh)(SnBu3)(dppe)] or [Pd(C[triple bond, length as m-dash]CPh)(SnBu3){PPh2(bzN)}].Ministerio de Economía, Industria y Competitividad (projects CTQ2016-80913-P and CTQ2017-89217-P)Junta de Castilla y León (project VA062G18, UIC 176